Signalwege – Fakultatives Material –

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G-Protein-gekoppelter Rezeptor aus Wikipedia, der freien Enzyklopädie Wechseln zu: Navigation , Suche

Der Begriff G-Protein-gekoppelter Rezeptor (kurz GPCR , für englisch G protein-coupled receptor ) wird in der Biologie für Rezeptoren in der Zellmembran verwendet, die Signale über GTP -bindende Proteine (kurz G-Proteine ) in das Zellinnere weiterleiten (Signaltransduktion ). Diese stellen die größte und vielseitigste Gruppe von Membranrezeptoren dar. In der Neurobiologie wird für G-Protein-gekoppelte Rezeptoren häufig der Begriff metabotrope Rezeptoren verwendet, um sie von einem anderen Rezeptortyp, den ligandengesteuerten Ionenkanälen (Ionotroper Rezeptor), zu unterscheiden.

G-Protein-gekoppelte Rezeptoren sind für die Verarbeitung von Licht -, Geruchs - und einer Vielzahl von Geschmacksreizen verantwortlich. Sie spielen eine entscheidende Rolle bei Entzündungsprozessen , der gezielten Zellbewegung (Chemotaxis ), dem Transport von Stoffen durch die Zellmembran (Endozytose und Exozytose ) sowie beim Zellwachstum und bei der Zelldifferenzierung . Sie sind darüber hinaus als Zielstrukturen für die Wirkung von Hormonen , wie Adrenalin oder Glucagon , und Neurotransmittern , wie Serotonin und Acetylcholin , verantwortlich. Auch einige Viren nutzen G-Protein-gekoppelte Rezeptoren als Bindungsstellen für den Eintritt in die Zelle (beispielsweise HIV ).

Definition

Als G-Protein-gekoppelte Rezeptoren im engeren Sinn werden alle heptahelikalen, das heißt mit 7 Helices in der Zellmembran verankerten Rezeptoren bezeichnet, die zur Bindung und Aktivierung von G-Proteinen befähigt sind. Zusätzlich zur Bindung von G-Proteinen sind viele dieser Rezeptoren in der Lage, auch mit anderen signalweiterleitenden Proteinen zu interagieren. Als G-Protein-gekoppelte Rezeptoren werden, wie im Fall vieler sogenannter "Orphan-GPCRs", auch Rezeptorproteine bezeichnet, für die eine Kopplung mit G-Proteinen lediglich vermutet wird. Bisweilen wird dieser Begriff auch auf Rezeptoren angewandt, die zwar nicht zur Bindung und Aktivierung von G-Proteinen befähigt sind, jedoch auf Grund ihrer phylogenetischen Verwandtschaft mit anderen, der klassischen Definition genügenden G-Protein-gekoppelten Rezeptoren dieser Familie zugerechnet werden können.

Dem gegenüber werden prokaryotische Rhodopsine , die zwar in ihrer Struktur eukaryotischen G-Protein-gekoppelten Rezeptoren ähneln, aber nicht zur Bindung von G-Proteinen befähigt sind und als Ionenkanäle fungieren, nicht der Familie der G-Protein-gekoppelten Rezeptoren zugeordnet.

Vorkommen

G-Protein-gekoppelte Rezeptoren kommen in fast allen Lebewesen vor, nicht nur in Wirbeltieren und Wirbellosen , sondern auch in Protozoen (z. B. Amöben ) und in Pilzen (beispielsweise in Hefen ). Auch im Pflanzenreich konnte das Vorkommen G-Protein-gekoppelter Rezeptoren am Beispiel der Acker-Schmalwand und des Reis nachgewiesen werden. Hier wird eine Rolle als Rezeptor für Phytohormone diskutiert. Einige Fotorezeptoren mit einer Struktur, die G-Protein-gekoppelten Rezeptoren ähnelt, können sogar in Archaeen gefunden werden (Bacteriorhodopsine ). Diese Rezeptoren haben jedoch keine Verwandtschaft zu Fotorezeptoren höherer Tiere und werden in der Regel nicht der Familie der G-Protein-gekoppelten Rezeptoren zugerechnet.

Beim Menschen konnten bisher etwa 800 G-Protein-gekoppelte Rezeptoren identifiziert werden. Diese werden durch etwa 3 % des menschlichen Genoms kodiert. Mehr als die Hälfte der G-Protein-gekoppelten Rezeptoren des Menschen werden den Geruchsrezeptoren (olfaktorischen Rezeptoren) zugeordnet. Bei über 140 der ca. 800 G-Protein-gekoppelten Rezeptoren ist der endogene Ligand nicht bekannt und sie werden deshalb als Orphan-GPCRs bezeichnet. [1]

Struktur

Auf Grund ihrer Struktur gehören G-Protein-gekoppelte Rezeptoren der Superfamilie der heptahelikalen Transmembranproteine (gebräuchliche Synonyme: Sieben-Transmembrandomänen-Rezeptoren, 7-TM-Rezeptoren und heptahelikale Rezeptoren) an. Sie bestehen aus Untereinheiten mit sieben (griechisch „ hepta “) die Zellmembran durchspannenden (transmembranären) Helixstrukturen , die durch drei intrazelluläre und drei extrazelluläre Schleifen miteinander verbunden sind. G-Protein-gekoppelte Rezeptoren besitzen eine extrazelluläre oder transmembranäre Bindungsdomäne für einen Liganden . Das G-Protein bindet an der zellinneren (intrazellulären) Seite des Rezeptors.

Lange Zeit war nur die Struktur von GPCRs bei Bakterien bekannt. Die Schematische Darstellung der Struktur eines GPCR. dreidimensionale Strukturaufklärung eines G-Protein-gekoppelten Rezeptors -Fakultativ- 1 Signalwege – Fakultatives Material – bei Vertebraten , des bovinen Rhodopsins , gelang im Jahr 2000 mit Hilfe der gilt die dreidimensionale Struktur zahlreicher G-Protein-gekoppelter [2] [4] Röntgenstrukturanalyse . Die Kristallisierung und Strukturaufklärung Rezeptoren, darunter die des β1-Adrenozeptors , des A 2A - [5] [6] anderer G-Protein-gekoppelter Rezeptoren ist hingegen wegen ihrer Adenosinrezeptors , des D 3-Dopaminrezeptors und des physikochemischen Eigenschaften und aufgrund der geringen Chemokinrezeptors CXCR4 [7] , als aufgeklärt. Zudem ist noch mit dem Rezeptordichte in der Membran erschwert. Daher konnte erst im Jahr 2007 Rhodopsin des japanischen Fliegenden Kalmars die Struktur eines G- die Kristallstruktur eines ligandenaktivierten G-Protein-gekoppelten Protein-gekoppelten Rezeptors bei Invertebraten bekannt. [8] [3] Rezeptors (menschlicher β2-Adrenozeptor ) aufgeklärt werden . Inzwischen Transmembrandomänen

Die sieben membrandurchspannenden helikalen Domänen G-Protein-gekoppelter Rezeptoren, die bei Sicht auf den Rezeptor von der extrazellulären Seite aus entgegen dem Uhrzeigersinn angeordnet sind, sind für die Verankerung des Rezeptors in der Zellmembran verantwortlich. Insbesondere die Transmembrandomänen III–VI beherbergen Bindungsstellen für einen Liganden. Den Transmembrandomänen I, II und IV kommt möglicherweise eine Rolle bei der Di- und Oligomerisierung von G-Protein-gekoppelten Rezeptoren zu.

Im Gegensatz zu den extrazellulären und intrazellulären Domänen sind die Transmembrandomänen innerhalb der Familie G-Protein-gekoppelter Rezeptoren stark konserviert. Einige Aminosäuresequenzen (Motive) innerhalb der Transmembrandomänen sind für viele G-Protein-gekoppelte Rezeptoren charakteristisch. Beispielsweise können das E/DRY-Motiv der Transmembrandomäne III und das NPxxY-Motiv der Transmembrandomäne VII in fast allen Rhodopsin-ähnlichen Rezeptoren gefunden werden. Ihnen wird eine wichtige Rolle bei der Rezeptoraktivierung zugeschrieben.

Extrazelluläre Domänen

Einige G-Protein-gekoppelte Rezeptoren, wie z. B. die metabotropen Glutamatrezeptoren , besitzen in ihrer extrazellulären N-terminalen Domäne ihre primäre Ligandenbindungsstelle. Diese Rezeptoren sind durch lange N-terminale Aminosäuresequenzen gekennzeichnet (bis 2800 Aminosäuren), während Rezeptoren mit intrazellulären Ligandenbindungsdomänen meist nur kurze Reste aufweisen (meist unter 30 Aminosäuren). Der N-Terminus und die extrazellulären Domänen des Rezeptors sind häufig glykosyliert . In der zweiten extrazellulären Schleife (EL 2) und am Anfang der dritten transmembranären Domäne des Rezeptors befinden sich zwei konservierte, zur Disulfidbrückenbildung befähigte Cysteine , welche die Struktur des Rezeptors stabilisieren, indem sie die Transmembrandomänen III bis V aneinander binden.

Intrazelluläre Domänen

Die intrazelluläre Seite des Rezeptors ist mit Bindungsstellen für G-Proteine und andere Signalmoleküle ausgestattet. An der Bindung von G-Proteinen sind insbesondere die transmembrandomänennahen Aminosäuren der zweiten (IL2) und dritten intrazellulären Schleife (IL 3) sowie der sich an die 7. transmembranäre Domäne anschließende C-terminale Rest, beteiligt. Der intrazelluläre C-terminale Anteil ist in der Regel sehr kurz (meist unter 50 Aminosäuren). Einigen G-Protein- gekoppelten Rezeptoren, wie beispielsweise dem Gonadotropin-Releasing-Hormon-Rezeptor , fehlt dieser Teil. Direkt an das intrazelluläre Ende der 7. transmembranären Domäne kann sich auch noch eine mit einem konservierten Cystein beginnende achte Helix (Hx 8) anschießen, die parallel zur Zellmembran verläuft.

Funktion

Die Hauptfunktion der G-Protein-gekoppelten Rezeptoren besteht in der Weiterleitung von Signalen in das Zellinnere. Diese Signalweiterleitung (Signaltransduktion) geschieht insbesondere über die Aktivierung von G-Proteinen. Einige G-Protein-gekoppelte Rezeptoren sind auch zu einer G-Protein- unabhängigen Signaltransduktion, beispielsweise über Arrestine , befähigt.

Aktivierung von G-Proteinen

Nahezu alle G-Protein-gekoppelten Rezeptoren sind zu einer direkten Aktivierung eines aus drei Untereinheiten ( α, β und γ) bestehenden (heterotrimeren) G-Proteins befähigt. Die Aktivierung eines G-Proteins ist ein mehrstufiger Prozess, der die Bindung eines Liganden an den Rezeptor, die Konformationsänderung des Rezeptors sowie die Bindung und Aktivierung eines G-Proteins einschließt und dabei den Gesetzen der Thermodynamik unterliegt.

-Fakultativ- 2 Signalwege – Fakultatives Material –

Aktivierungszyklus von G-Proteinen durch G-Protein-gekoppelte Rezeptoren. (1) Bindung des G-Proteins. (2) Ligandenbindung. (3) Aktivierung des Rezeptors. (4) Aktivierung des G-Proteins. (5) Dissoziation des G-Proteins und Signaltransduktion. (6) Inaktivierung des G-Proteins.

Schritt 1: Bindung des G-Proteins [Bearbeiten ]

Aktivierte Rezeptoren interagieren mit heterotrimeren G-Proteinen, die dadurch selbst aktiviert werden. Der Rezeptor zeigt dabei eine Selektivität für ein (beispielsweise β1-Adrenozeptor : G s) oder für mehrere verschiedene (z. B. β2-Adrenozeptor: G s und G i/o ) G-Proteinsubtypen. Dieser Komplex aus Rezeptor und heterotrimeren G-Protein ist dabei ein Bestandteil eines größeren Netzwerks, an dem auch weitere signalweiterleitende Proteine, wie z. B. GIRK-Kanäle , Phospholipase C und Proteinkinase C beteiligt sind [9] . Mit Hilfe dieses Netzwerks kann eine schnelle, oft nur Millisekunden bis wenige Sekunden dauernde Aktivierung der Signaltransduktionskaskade G-Protein-gekoppelter Rezeptoren erklärt werden. Ob der Rezeptor mit dem G-Protein durch Kollisionskopplung interagiert, oder ob Rezeptoren auch im inaktiven Zustand mit dem G-Protein assoziiert sind ist Gegenstand aktueller Forschung [10] .

Schritt 2: Ligandenbindung [Bearbeiten ]

Ligandenbindung an G-Protein-gekoppelte Rezeptoren

Abhängig von der Art des Liganden erfolgt die Bindung an den Rezeptor an seine extrazellulären, transmembranären oder intrazellulären Domänen: -Fakultativ- 3 Signalwege – Fakultatives Material –

• Der Ligand All-trans-Retinal ist fester transmembranärer Bestandteil des Lichtrezeptors Rhodopsin (Abb. a). • Amine (z. B. Acetylcholin , Adrenalin , Histamin und Serotonin ), Nucleotide (z. B. ATP ), Eikosanoide (z. B. Prostacyclin ) und einige Lipide (z. B. Ceramide ) binden an transmembranäre Bindungsstellen ihrer Rezeptoren (Abb. a). • Neuropeptide (z. B. Oxytocin und Vasopressin ) besetzen mehrere transmembranäre und extrazelluläre Bindungsstellen ihrer Rezeptoren gleichzeitig (Abb. b). • Proteinasen (z. B. Thrombin und Trypsin ) spalten spezifisch ein kurzes N-terminales Fragment ihrer Rezeptoren, den Protease-aktivierten Rezeptoren , ab. Durch die proteolytische Spaltung wird ein intrinsischer Ligand (" tethered ligand ") am neu entstehenden Rezeptor-N-Terminus freigelegt, welcher sich in einem zweiten Schritt an eine transmembranäre Bindungsstelle anlagert (Abb. c) [11] . • Proteohormone (z. B. Glucagon ) binden primär an extrazelluläre Domänen des Rezeptors. Nach einer Konformationsänderung des Rezeptors erfolgt eine sekundäre Bindung des Peptidhormons an transmembranäre Bindungsstellen (Abb. d). • Einige kleine Moleküle (z. B. Glutamat und Calcium ) binden ausschließlich an extrazellulären Rezeptordomänen. Durch die Anbindung des Liganden ändert sich die Konformation der extrazellulären Domänen, so dass diese mit intrazellulären Domänen in Kontakt kommen (Abb. e).

Schritt 3: Aktivierung des Rezeptors [Bearbeiten ]

Bindet an einen Rezeptor ein Ligand und wird dieser durch den Liganden aktiviert (Agonist ), so führt diese Anlagerung meist zu einer Sprengung der Salzbrücke zwischen der 3. und der 7. transmembranären Domäne des G-Protein-gekoppelten Rezeptors. Dieser so aktivierte Rezeptor erhält mehr Flexibilität und ändert seine dreidimensionale Struktur. Durch die Änderung der Konformation des Rezeptors kann dieser jetzt als GTP-Austauschfaktor (GTP exchange factor, GEF) für das gebundene G-Protein fungieren.

Viele G-Protein-gekoppelte Rezeptoren befinden sich bereits in Abwesenheit eines Agonisten in einem Gleichgewicht zwischen inaktivem und aktiviertem Zustand. Die Anbindung eines Agonisten verschiebt das Gleichgewicht in Richtung aktiver Zustand, während inverse Agonisten das Gleichgewicht in Richtung inaktiver Zustand verschieben. Rezeptoren, die sich auch in Abwesenheit eines Agonisten in einem aktivierten Zustand befinden können nennt man konstitutiv aktiv . Auch Mutationen im Rezeptor können eine konstitutive Aktivität bewirken („ Constitutive active mutants “, CAM). Derartig mutierte G-Protein-gekoppelte Rezeptoren werden mit bestimmten Erkrankungen (z. B. bestimmte Formen von Retinopathia pigmentosa durch konstitutiv aktives Rhodopsin) in Verbindung gebracht und kommen in bestimmten Herpesviren vor. Weiterhin werden in der experimentellen Pharmakologie inzwischen G-Protein-gekoppelte Rezeptoren auch gezielt zu CAMs verändert, um diese bei der Suche nach neuen Medikamenten zu verwenden.

Schritt 4: Aktivierung der G-Proteine [Bearbeiten ]

Heterotrimere G-Proteine können GTP und GDP binden, die GDP-gebundene Form ist inaktiv. Die Aktivierung des Rezeptors sorgt für den Austausch von GDP gegen GTP an der α-Untereinheit des G-Proteins. Der G-Protein-Komplex wird durch die Bindung des GTP instabil. Als Folge ändert sich die Konformation des heterotrimeren G-Proteins und es kann in die α- und die βγ -Untereinheit dissoziieren. Die aktivierten G-Proteine können nun die exogenen, durch den Liganden übertragenen Signale in das Zellinnere weiterleiten (Signaltransduktion).

Schritt 5: Signaltransduktion [Bearbeiten ]

Die aktivierten Untereinheiten des G-Proteins sind für die weitere Signaltransduktion verantwortlich. Je nach Untereinheit werden weitere zell- oder membranständige Proteine aktiviert oder deaktiviert. So modulieren beispielsweise die α-Untereinheiten der G s/olf die Aktivität der Adenylylcyclase , während α- Untereinheiten der G q/11 -Proteine die Phospholipase C aktivieren. Diese Enzyme sind dann an der Bildung eines Second Messengers beteiligt.

Schritt 6: G-Protein-Inaktivierung [Bearbeiten ]

Die intrinsische GTPase-Aktivität der α-Untereinheit des G-Proteins spaltet nach einer Zeit und unter Mithilfe von Proteinen, die die GTPase-Aktivität erhöhen (GTPase aktivierendes Protein , GAP), das gebundene GTP in GDP + P i. Die α-Untereinheit des G-Proteins kann somit wieder mit der βγ -Untereinheit reassoziieren und erneut an einen Rezeptor binden (siehe Schritt 1). Es findet also eine Selbstregulierung statt.

Alternative Signaltransduktionswege [Bearbeiten ]

Viele G-Protein-gekoppelte Rezeptoren sind nicht nur zu einer Aktivierung von G-Proteinen, sondern auch zu einer Bindung und Aktivierung anderer, Zellprozesse steuernder Moleküle befähigt. Auf diese Weise können G-Protein-gekoppelte Rezeptoren G-Protein-unabhängig alternative Signaltransduktionswege steuern. Beispiele hierfür sind die Frizzled-Rezeptoren, die über den Wnt-Singnalweg β-Catenin aktivieren können, der β2-Rezeptor, der G-Protein-unabhängig die Funktion + + des Na /H -Austauschers modulieren kann und der 5-HT 2B -Rezeptor, dessen eNOS aktivierende Funktion zumindest teilweise ebenfalls G-Protein-unabhängig ist. Durch G-Protein-unabhängige Aktivierungen von Arrestin und Homer-Proteinen kann darüber hinaus die Funktion G-Protein-gekoppelter Rezeptoren selbst reguliert werden.

Regulierung der Rezeptorfunktion [Bearbeiten ]

Für die Funktion von G-Protein-gekoppelten Rezeptoren ist deren Lokalisation an der Zelloberfläche Voraussetzung. Zelluläre Prozesse, die zu einer Erhöhung der Rezeptorzahl an der Zellmembran und damit der Rezeptorfunktion führen, werden als Up-Regulation bezeichnet. Im Gegensatz dazu stellt die Entfernung funktionstüchtiger Rezeptoren von der Zellmembran (Down-Regulation ) einen Mechanismus der Beendigung der Rezeptorfunktion dar.

Up-Regulation [Bearbeiten ]

-Fakultativ- 4 Signalwege – Fakultatives Material –

Die Heraufregulierung („Up-Regulation“) G-Protein-gekoppelter Rezeptoren ist ein mehrstufiger Prozess. Ausgangspunkt ist die Proteinbiosynthese des Rezeptors am endoplasmatischen Reticulum , die unter anderem durch G-Protein-gekoppelte Rezeptoren indirekt reguliert werden kann.

Der Transport des so synthetisierten Rezeptors über Golgi-Vesikel zur Zellmembran kann durch Chaperone und Chaperon-ähnliche Proteine einschließlich Rezeptoraktivität-modifizierender Proteine gesteuert werden. Eine Regulation des Membrantransports ist ebenfalls durch eine Glykosylierung des Rezeptors möglich. Eine Bedeutung der für viele G-Protein-gekoppelte Rezeptoren beobachteten Di- und Oligomerisierung als Voraussetzung für deren Transport zur Zellmembran und somit für deren Funktionalität wird ebenfalls diskutiert. Der Einbau der G-Protein-gekoppelten Rezeptoren in die Zellmembran kann beispielsweise durch Homer-Proteine, PSD-95 und Spinophilin gesteuert werden. [12]

Down-Regulation [Bearbeiten ]

Die durch G-Protein-gekoppelte Rezeptoren vermittelten Effekte nehmen nach längerer Zeit der Aktivierung ab. Eine Schlüsselrolle dieser Herabregulierung („Down-Regulation“) spielt dabei die Phosphorylierung intrazellulärer Domänen des Rezeptors (C-terminale Serin - oder Threonin -Reste) durch Proteinkinasen .

Phosphorylierung durch Second-Messenger-aktivierte Kinasen [Bearbeiten ]

Proteinkinasen, die über G-Protein-vermittelte Produktion von Second Messengern (sekundären Botenstoffen) aktiviert werden (wie beispielsweise die durch cyclisches Adenosinmonophosphat (cAMP) aktivierte Proteinkinase A oder die durch Diacylglycerol und Calcium aktivierte Proteinkinase C ), können viele G- Protein-gekoppelte Rezeptoren phosphorylieren. Häufig wird dadurch die Signaltransduktion über den Rezeptor unterbrochen, da der phosphorylierte Rezeptor eine geringere Affinität zu G-Proteinen besitzt. Diese Regulation kann demzufolge als ein negativer Rückkopplungs -Mechanismus wirken.

Phosphorylierung durch G-Protein-gekoppelte Rezeptorkinasen [Bearbeiten ]

G-Protein-gekoppelte Rezeptorkinasen (kurz GRKs ) haben im Wesentlichen zwei Funktionen. Erstens können sie mit den G α- und G βγ -Untereinheiten der heterotrimeren G-Proteine interagieren, womit diese nicht mehr zur Signaltransduktion beitragen. Außerdem können sie als GAP (GTPase-aktivierendes Protein) wirken und die Umwandlung von GTP*G α zu GDP*G α beschleunigen. Zweitens sind sie Serin-/Threonin-Kinasen und können als solche G-Protein-gekoppelte Rezeptoren phosphorylieren.

Folgen der Phosphorylierung [Bearbeiten ]

1. Konformationsänderung des Rezeptors : durch die stark negative Ladung des Phosphatrests und damit einhergehende elektrostatische Wechselwirkungskräfte ändert sich die Konformation des Rezeptors. Die neue Konformation ist oft ungünstiger für die Rezeptor-G-Protein-Interaktion oder beeinflusst die Affinität des Rezeptors, so dass es zu einer Abschwächung des Rezeptorsignals kommt. Diese Art der Desensitivierung erfolgt oft durch die Second-Messenger-abhängigen Proteinkinasen A und C. 2. Interaktion mit beta-Arrestinen : Durch Bindung von Arrestin an den vor allem durch GRKs phosphorylierten Rezeptor wird sterisch eine Anbindung der G-Proteine verhindert (= Kurzzeitregulation innerhalb weniger Minuten). Außerdem dienen die beta-Arrestine als „scaffold“-Moleküle für eine Vielzahl weiterer Proteine, besonders hervorzuheben sind dabei Clathrin und die MAP-Kinasen . 3. Internalisierung : Die Bindung des Rezeptor-Arrestin-Komplexes an Clathrin führt zur Entfernung des phosphorylierten Rezeptors von der Zelloberfläche ins Zellinnere in Form von Membranvesikeln (clathrin-coated pits). Nachfolgend kann der Rezeptor intrazelluär abgebaut, recycelt und damit an die Oberfläche zurückgebracht werden oder auch als intrazellulärer Rezeptor fortbestehen. Diese Regulation erfolgt meist innerhalb von 10 bis 30 Minuten und wird nach Entfernung des Rezeptorstimulus oft innerhalb von 30 bis 60 Minuten rückgängig gemacht. Eine langfristige Regulation über Tage oder Monate ist oft auf Transkriptionsregulation zurückzuführen.

Die Bindung von MAP-Kinasen an den Rezeptor-Arrestin-Komplex führt dazu, dass diese nicht mehr über G-Proteine, sondern direkt vom Rezeptor stimuliert werden. Dieser Wechsel des Signaltransduktionsmechanismus erfolgt ebenfalls innerhalb von etwa zehn Minuten.

Einteilung [Bearbeiten ]

Klassifizierung nach Funktion [Bearbeiten ]

Eine erste systematische Klassifizierung der G-Protein-gekoppelten Rezeptoren erfolgte Anfang der 1990er Jahre anhand funktioneller Merkmale. Anhand dieses Systems wurden die G-Protein-gekoppelten Rezeptoren von Wirbeltieren und Wirbellosen in fünf Gruppen (A-E) unterteilt. Die Gruppe A repräsentierten mit Rhodopsin verwandte Rezeptoren, Glycoproteinrezeptoren wurden in die Gruppe B und die metabotropen Glutamatrezeptoren in die Gruppe C eingeteilt. Rezeptoren der Gruppen D und E kommen nicht bei Wirbeltieren vor. Sie fungieren als Pheromonrezeptoren in Hefen bzw. als cAMP-Rezeptoren in Nematoden .

Mit der Entdeckung neuer G-Protein-gekoppelter Rezeptoren wurde dieses System in den letzten Jahren erweitert. Außerhalb des oben beschriebenen ABCDE- Systems wurden eigene Gruppen für die MLO-Rezeptoren der Pflanzen, für die Geruchsrezeptoren der Insekten, für die Chemorezeptoren der Nematoden und für die „Frizzeled/Smoothened“-Rezeptoren höherer Tiere etabliert. [13]

Ein neues System der Klassifizierung der humanen G-Protein-gekoppelten Rezeptoren wurde basierend auf phylogenetischen Untersuchungen vorgeschlagen (GRAFS- oder Fredriksson-System). Diesem System zufolge werden G-Protein-gekoppelte Rezeptoren in fünf Hauptgruppen unterteilt: in die Glutamat-, Rhodopsin-, Adhäsions-, Frizzled/Taste2- und Secretin-Gruppe [14] .

Einteilung nach intrinsischer Wirkung [Bearbeiten ]

-Fakultativ- 5 Signalwege – Fakultatives Material –

Vor allem in der Pharmakologie werden G-Protein-gekoppelte Rezeptoren nach ihrer intrinsischen Aktivität in 3 Klassen eingeteilt:

1. Gs-gekoppelte Rezeptoren : Bei Aktivierung wird durch das G-Protein die Adenylatcyclase aktiviert und es wird vermehrt cAMP gebildet.

1. Gi-gekoppelte Rezeptoren : Bei Aktivierung wird durch das G-Protein die Adenylatcyclase gehemmt und es wird weniger cAMP gebildet.

1. Gq-gekoppelte Rezeptoren : Bei Aktivierung wird durch das G-Protein die Phospholipase C aktiviert, welche wiederum ein Phospholipd in Inositoltriphosphat (IP3) und Diacylglycerol (DAG) spaltet.

Bedeutung der G-Protein-gekoppelten Rezeptoren für die Medizin [Bearbeiten ]

Arzneistoffe [Bearbeiten ]

In der modernen Medizin nehmen G-Protein-gekoppelte Rezeptoren eine Schlüsselposition ein: etwa 60 % aller verschreibungspflichtigen Medikamente , die derzeit auf dem Markt sind, wirken auf G-Protein-gekoppelte Rezeptoren ein. Unter diesen Medikamenten befinden sich unter anderem die häufig verschriebenen Betablocker , Neuroleptika , Antihistaminika , Opioide und Sympathomimetika . Neue, über G-Protein-gekoppelte Rezeptoren wirkende Medikamente, wie beispielsweise die Triptane , Setrone und Sartane , haben in den letzten Jahren ebenfalls einen hohen Stellenwert erreicht.

Beispiele für den therapeutischen Einsatz von Arzneimitteln, die an G-Protein-gekoppelten Rezeptoren wirken Arzneistoff Beispiele Indikation Rezeptor(en) Kopplung Erläuterungen

Senken als Agonisten an α2-Adrenozeptoren im α2-Agonisten Clonidin Arterielle Hypertonie α2-Adrenozeptoren Gi Zentralnervensystem die Aktivität des Sympathikus und reduzieren darüber den Blutdruck. Prazosin , Arterielle Hypertonie , Senken als Antagonisten an α -Adrenozeptoren den Tonus der Alphablocker α -Adrenozeptoren Gq 1 Tamsulosin Prostatahyperplasie 1 glatten Muskulatur in Blutgefäßen und im Urogenitaltrakt . Harninkontinenz , Senken als Antagonisten an Muskarin-Rezeptoren den Tonus der Asthma bronchiale , Muscarinische Anticholinergika Atropin Gi & Gq glatten Muskulatur in Bronchien und im Urogenitaltrakt. Hemmen bradykarde Acetylcholinrezeptoren die Herzfrequenz -senkende Wirkung von Acetylcholin . Herzrhythmusstörungen Arterielle Hypertonie , Atenolol , Herzinsuffizienz , Senken als Antagonisten an β -Adrenozeptoren unter anderem die Betablocker β -Adrenozeptoren Gs 1 Metoprolol koronare 1 Herzfrequenz. Herzkrankheit , Migräne Pergolid , Dopamin- Cabergolin , Parkinson-Krankheit , Imitieren als Agonisten an Dopaminrezeptoren die Wirkung des Dopamin-Rezeptoren Gi & Gs Agonisten Pramipexol , Restless-Legs-Syndrom Dopamins. Ropinirol Diphenhydramin , H - Hemmen als Antagonisten die Wirkung von Histamin , das bei 1 Loratadin , Allergische Reaktionen H -Rezeptoren Gq Antihistaminika 1 allergischen Reaktionen ausgeschüttet wird, an H -Rezeptoren. Cetirizin 1 Kontrolle der Ranitidin , H - Magensäureproduktion Hemmen als Antagonisten an H -Rezeptoren die Histamin- 2 Famotidin , H -Rezeptoren Gs 2 Antihistaminika (Refluxkrankheiten , 2 vermittelte Freisetzung von Magensäure . Cimetidin Magengeschwür ) Haloperidol , Führen über eine vorrangige Hemmung von D -Rezeptoren Risperidon , D -Rezeptoren , 5- 2 Neuroleptika Schizophrenie 2 Gi & Gq (typische Neuroleptika) oder 5-HT -Rezeptoren (atypische Clozapin , HT -Rezeptoren 2A 2A Neuroleptika) zu einer antipsychotischen Wirkung. Olanzapin Morphin , Führen als Agonisten an den Opioidrezeptoren µ,κ und δ zu einer Codein , Schmerzen , Anästhesie , Opioide Opioidrezeptoren Gi spinalen und supraspinalen Analgesie sowie zu einer zentralen Fentanyl , Husten , Durchfall Hemmung des Hustenreizes. Loperamid Losartan , Candesartan , Hypertonie, AT - Senken als Antagonisten des Angiotensins II an AT -Rezeptoren 1 Irbesartan , Herzinsuffizienz, AT -Rezeptoren Gq 1 Antagonisten 1 den Tonus der glatten Muskulatur in Blutgefäßen. Valsartan , koronare Herzkrankheit Telmisartan Sumatriptan , Migränetherapeutische Wirkung über eine Stimulation von 5-HT - Triptane Naratriptan , Migräne 5-HT -Rezeptoren Gi 1B 1B Rezeptoren in zerebralen Blutgefäßen und Neuronen . Zolmitriptan

Forschung [Bearbeiten ]

-Fakultativ- 6 Signalwege – Fakultatives Material –

Das Prinzip der Signalübertragung von Hormonen mittels Rezeptoren und sekundärer Botenstoffe wurde 1960 erstmals postuliert, die Übertragung durch G-Proteine wurde in den 70er bis 80er Jahren des 20. Jahrhunderts erarbeitet. Für diese Arbeiten wurden 1971 Earl W. Sutherland Jr. „für seine Entdeckungen über die Wirkungsmechanismen von Hormonen “ und 1994 Alfred G. Gilman und Martin Rodbell „für die Entdeckung der Zellkommunikation und im speziellen der Entdeckung der G-Proteine “ mit dem Nobelpreis für Medizin geehrt.

G-Protein-gekoppelte Rezeptoren gehören nach wie vor zu den am intensivsten untersuchten Zielen für die Entwicklung neuer Medikamente in der Arzneimittelindustrie . Dabei rücken insbesondere neue, innerhalb der letzten 20 Jahre entdeckte Rezeptoren, wie beispielsweise Cannabinoid-Rezeptoren , CGRP - Rezeptoren, Chemokin -Rezeptoren, Endothelin -Rezeptoren, Leptin -Rezeptoren, Neurokinin -Rezeptoren und Neuropeptid-Y-Rezeptoren, in das Interesse der Forschung.

Receptor tyrosine kinase

From Wikipedia, the free encyclopedia Jump to: navigation , search

receptor protein-tyrosine kinase

Identifiers

EC number 2.7.10.1

Databases

IntEnz IntEnz view

BRENDA BRENDA entry

ExPASy NiceZyme view

KEGG KEGG entry

MetaCyc metabolic pathway

PRIAM profile

PDB structures

Gene Ontology AmiGO / EGO

[show ]Search

Receptor tyrosine kinases (RTK)s are the high-affinity cell surface receptors for many polypeptide growth factors , cytokines , and hormones . Of the 90 unique tyrosine kinase genes identified in the human genome , 58 encode receptor tyrosine kinase proteins. [1] Receptor tyrosine kinases have been shown not only to be key regulators of normal cellular processes but also to have a critical role in the development and progression of many types of cancer .[2]

[edit ] Receptor tyrosine kinase classes

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Approximately 20 different RTK classes have been identified. [3]

1. RTK class I (EGF receptor family )(ErbB family) 10. RTK class X (LTK receptor family) 2. RTK class II (Insulin receptor family) 11. RTK class XI (TIE receptor family) 3. RTK class III (PDGF receptor family) 12. RTK class XII (ROR receptor family) 4. RTK class IV (FGF receptor family) 13. RTK class XIII (DDR receptor family) 5. RTK class V (VEGF receptors family) 14. RTK class XIV (RET receptor family) 6. RTK class VI (HGF receptor family) 15. RTK class XV (KLG receptor family) 7. RTK class VII (Trk receptor family) 16. RTK class XVI (RYK receptor family) 8. RTK class VIII (Eph receptor family) 17. RTK class XVII (MuSK receptor family) 9. RTK class IX (AXL receptor family) [edit ] Structure

Most RTKs are single subunit receptors but some exist as multimeric complexes , e.g., the insulin receptor that forms disulfide-linked dimers in the absence of hormone; moreover, ligand binding to the extracellular domain induces formation of receptor dimers. [4] Each monomer has a single hydrophobic transmembrane - spanning domain composed of 25-38 amino acids , an extracellular N-terminal region, and an intracellular C-terminal region. The extracellular N-terminal region exhibits a variety of conserved elements including immunoglobulin (Ig)-like or epidermal growth factor (EGF)-like domains, fibronectin type III repeats, or cysteine- rich regions that are characteristic for each subfamily of RTKs; these domains contain primarily a ligand-binding site, which binds extracellular ligands, e.g., a particular growth factor or hormone .[5] The intracellular C-terminal region displays the highest level of conservation and comprises catalytic domains responsible for the kinase activity of these receptors, which catalyses receptor autophosphorylation and tyrosine phosphorylation of RTK substrates. [5] .

[edit ] Kinase activity

In biochemistry , a kinase is a type of enzyme that transfers phosphate groups (see below) from high-energy donor molecules, such as ATP (see below) to specific target molecules (substrates ); the process is termed phosphorylation . The opposite, an enzyme that removes phosphate groups from targets, is known as a phosphatase . Kinase enzymes that specifically phosphorylate tyrosine amino acids are termed tyrosine kinases .

•

Phosphate

•

ATP

•

Tyrosine

When a growth factor binds to the extracellular domain of an RTK, its dimerization is triggered with other adjacent RTKs. Dimerization leads to a rapid activation of the protein's cytoplasmic kinase domains, the first substrate for these domains being the receptor itself. The activated receptor as a result then becomes autophosphorylated on multiple specific intracellular tyrosine residues .

[edit ] Signal transduction

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The phosphorylation of specific tyrosine residues within the activated receptor creates binding sites for Src homology 2 (SH2) domain- and phosphotyrosine binding (PTB) domain-containing proteins. [6] Specific proteins containing these domains include Src and phospholipase C γ, the phosphorylation and activation of these two proteins on receptor binding, leading to the initiation of signal transduction pathways. Other proteins that interact with the activated receptor act as adaptor proteins and have no intrinsic enzymatic activity of their own. These adaptor proteins link RTK activation to downstream signal transduction pathways, such as the MAP kinase signalling cascade .[2]

[edit ] Families

[edit ] Epidermal growth factor receptor family

For more details on this topic, see ErbB .

The ErbB protein family or epidermal growth factor receptor (EGFR) family is a family of four structurally related receptor tyrosine kinases. Insufficient ErbB signaling in humans is associated with the development of neurodegenerative diseases , such as multiple sclerosis and Alzheimer's Disease .[7] In mice, loss of signaling by any member of the ErbB family results in embryonic lethality with defects in organs including the lungs , skin , heart , and brain . Excessive ErbB signaling is associated with the development of a wide variety of types of solid tumor . ErbB-1 and ErbB-2 are found in many human cancers and their excessive signaling may be critical factors in the development and malignancy of these tumors .[8]

[edit ] Fibroblast growth factor receptor (FGFR) family

For more details on this topic, see fibroblast growth factor receptor .

Fibroblast growth factors comprise the largest family of growth factor ligands at 23 members. [9] The natural alternate splicing of four fibroblast growth factor receptor (FGFR) genes results in the production of over 48 different isoforms of FGFR. [10] These isoforms vary in their ligand binding properties and kinase domains; however, all share a common extracellular region composed of three immunoglobulin (Ig) like domains (D1-D3), and thus belong to the immunoglobulin superfamily .[11] Interactions with FGFs occur via FGFR domains D2 and D3. Each receptor can be activated by several FGFs. In many cases the FGFs themselves can also activate more than one receptor, this is not the case with FGF-7, however, which can activate only FGFR2b. [10] A gene for a fifth FGFR protein, FGFR5, has also been identified. In contrast to FGFRs 1-4 it lacks a cytoplasmic tyrosine kinase domain and one isoform, FGFR5 γ, only contains the extracellular domains D1 and D2. [12]

[edit ] Vascular endothelial growth factor receptor (VEGFR) family

For more details on this topic, see Vascular endothelial growth factor .

Vascular endothelial growth factor (VEGF) is one of the main inducers of endothelial cell proliferation and permeability of blood vessels . Two RTKs bind to VEGF at the cell surface, VEGFR-1 (Flt-1) and VEGFR-2 (KDR/Flk-1). [13]

The VEGF receptors have an extracellular portion consisting of seven Ig -like domains so, like FGFRs, belong to the immunoglobulin superfamily. They also possess a single transmembrane spanning region and an intracellular portion containing a split tyrosine-kinase domain. VEGF-A binds to VEGFR-1 (Flt-1) and VEGFR-2 (KDR/Flk-1). VEGFR-2 appears to mediate almost all of the known cellular responses to VEGF. The function of VEGFR-1 is less well defined, although it is thought to modulate VEGFR-2 signaling. Another function of VEGFR-1 may be to act as a dummy/decoy receptor, sequestering VEGF from VEGFR-2 binding (this appears to be particularly important during vasculogenesis in the embryo). A third receptor has been discovered (VEGFR-3); however, VEGF-A is not a ligand for this receptor. VEGFR-3 mediates lymphangiogenesis in response to VEGF-C and VEGF-D.

[edit ] RET receptor family

For more details on this topic, see RET proto-oncogene .

The natural alternate splicing of the RET gene results in the production of 3 different isoforms of the protein RET. RET51, RET43, and RET9 contain 51, 43, and 9 amino acids in their C-terminal tail, respectively. [14] The biological roles of isoforms RET51 and RET9 are the most well studied in-vivo as these are the most common isoforms in which RET occurs.

RET is the receptor for members of the glial cell line-derived neurotrophic factor (GDNF) family of extracellular signalling molecules or ligands (GFLs). [15]

In order to activate RET, first GFLs must form a complex with a glycosylphosphatidylinositol (GPI)-anchored co-receptor . The co-receptors themselves are classified as members of the GDNF receptor-α (GFR α) protein family. Different members of the GFR α family (GFR α1-GFR α4) exhibit a specific binding activity for a specific GFLs. [16] Upon GFL-GFR α complex formation, the complex then brings together two molecules of RET, triggering trans-autophosphorylation of specific tyrosine residues within the tyrosine kinase domain of each RET molecule. Phosphorylation of these tyrosines then initiates intracellular signal transduction processes. [17]

[edit ] Eph receptor Family

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For more details on this topic, see Eph_receptor .

Ephrin and Eph receptors are the largest subfamily of RTKs.

Cytokine receptor

From Wikipedia, the free encyclopedia Jump to: navigation , search

Key steps of the JAK-STAT pathway for type 1 and 2 cytokine receptors

Signal transduction. (Cytokine receptor at center left.)

Cytokine receptor s are receptors that bind cytokines .[1]

In recent years, the cytokine receptors have come to demand the attention of more investigators than cytokines themselves, partly because of their remarkable characteristics, and partly because a deficiency of cytokine receptors has now been directly linked to certain debilitating immunodeficiency states. In this regard, and also because the redundancy and pleiotropy of cytokines are, in fact, a consequence of their homologous receptors, many authorities are now of the opinion that a classification of cytokine receptors would be more clinically and experimentally useful.

Classification

A classification of cytokine receptors based on their three-dimensional structure has been attempted. (It must be noted that such a classification, though seemingly cumbersome, provides several unique perspectives for attractive pharmacotherapeutic targets.)

• Type I cytokine receptors , whose members have certain conserved motifs in their extracellular amino-acid domain. The IL-2 receptor belongs to this chain, whose γ-chain (common to several other cytokines) deficiency is directly responsible for the x-linked form of Severe Combined Immunodeficiency (X-SCID ). • Type II cytokine receptors , whose members are receptors mainly for interferons . -Fakultativ- 10 Signalwege – Fakultatives Material –

• Immunoglobulin (Ig) superfamily , which are ubiquitously present throughout several cells and tissues of the vertebrate body • Tumor necrosis factor receptor family , whose members share a cysteine -rich common extracellular binding domain, and includes several other non- cytokine ligands like CD40 , CD27 and CD30 , besides the ligands on which the family is named (TNF). • Chemokine receptors , two of which acting as binding proteins for HIV (CXCR4 and CCR5 ). They are G protein coupled receptors . • TGF beta receptors

Comparison

Type Examples Structure Mechanism • Type 1 interleukin receptors • Erythropoietin receptor • GM-CSF receptor • G-CSF receptor • growth hormone type I cytokine receptor receptor • prolactin receptor • Oncostatin M Certain conserved motifs in their extracellular amino- JAK phosphorylate and activate receptor acid domain. Connected to Janus kinase (JAK) family downstream proteins involved in their • Leukemia inhibitory of tyrosine kinases signal transduction pathways factor receptor

• Type II interleukin receptors • interferon-alpha/beta type II cytokine receptor receptor • interferon-gamma receptor

• Interleukin-1 receptor • CSF1 Many members of the Share structural homology with immunoglobulins • C-kit receptor immunoglobulin (antibodies ), cell adhesion molecules , and even some superfamily • Interleukin-18 cytokines. receptor

• CD27 • CD30 • CD40 Tumor necrosis factor • cysteine -rich common extracellular binding domain receptor family CD120 • Lymphotoxin beta receptor

• Interleukin-8 receptor • CCR1 • CXCR4 chemokine receptors Seven transmembrane helix G protein -coupled • MCAF receptor • NAP-2 receptor

• TGF beta receptor 1 TGF beta receptors • TGF beta receptor 2

Solubility

Cytokine receptors may be both membrane-bound and soluble. Soluble cytokine receptors are extremely common regulators of cytokine function.

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TGF beta signaling pathway

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The Transforming growth factor beta (TGF β) signaling pathway is involved in many cellular processes in both the adult organism and the developing embryo including cell growth , cell differentiation , apoptosis , cellular homeostasis and other cellular functions. In spite of the wide range of cellular processes that the TGF β signaling pathway regulates, the process is relatively simple. TGF β superfamily ligands bind to a type II receptor, which recruits and phosphorylates a type I receptor. The type I receptor then phosphorylates receptor-regulated SMADs (R-SMADs ) which can now bind the coSMAD SMAD4 . R-SMAD/coSMAD complexes accumulate in the nucleus where they act as transcription factors and participate in the regulation of target gene expression.

Contents [edit ] Ligand Binding

[hide ]

• 1 Mechanism o 1.1 Ligand Binding o 1.2 Receptor recruitment and phosphorylation o 1.3 SMAD phosphorylation o 1.4 CoSMAD binding o 1.5 Transcription • 2 Pathway regulation o 2.1 Ligand agonists/antagonists o 2.2 Receptor regulation o 2.3 R-SMAD regulation
2.3.1 Role of inhibitory SMADs
2.3.2 R-SMAD ubiquitination • 3 Summary table • 4 External links • 5 References

[edit ] Mechanism

The TGF Beta superfamily of ligands include: Bone morphogenetic proteins (BMPs) , Growth and differentiation factors (GDFs) , Anti-müllerian hormone (AMH) , Activin , Nodal and TGF β's [1] . Signalling begins with the binding of a TGF beta superfamily ligand to a TGF beta type II receptor. The type II receptor is a serine/threonine receptor kinase, which catalyses the phosphorylation of the Type I receptor. Each class of ligand binds to a specific type II receptor [2] .In mammals there are seven known type I receptors and five type II receptors [3] .

There are three activins: Activin A , Activin B and Activin AB . Activins are involved in embryogenesis and osteogenesis. They also regulate many hormones including pituitary , gonadal and hypothalamic hormones as well as insulin . They are also nerve cell survival factors.

The BMPs bind to the Bone morphogenetic protein receptor type-2 (BMPR2). They are involved in a multitude of cellular functions including osteogenesis, cell differentiation , anterior/posterior axis specification, growth, and homeostasis.

The TGF beta family include: TGF β1, TGF β2, TGF β3. Like the BMPS, TGF betas are involved in embryogenesis and cell differentiation, but they are also involved in apoptosis, as well as other functions. They bind to TGF-beta receptor type-2 (TGFBR2).

Nodal binds to activin A receptor, type IIB ACVR2B . It can then either form a receptor complex with activin A receptor, type IB (ACVR1B ) or with activin A receptor, type IC (ACVR1C )[3] .

[edit ] Receptor recruitment and phosphorylation

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The TGF beta ligand binds to a type II receptor dimer, which recruits a type I There are five receptor regulated SMADs: SMAD1 , SMAD2 , SMAD3 , receptor dimer forming a hetero-tetrameric complex with the ligand [4] . These SMAD5 , and SMAD9 (sometimes referred to as SMAD8). There are receptors are serine/threonine kinase receptors . They have a cysteine rich essentially two intracellular pathways involving these R-SMADs . TGF extracellular domain , a transmembrane domain and a cytoplasmic beta's, Activins, Nodals and some GDFs are mediated by SMAD2 and serine/threonine rich domain. The GS domain of the type I receptor consists SMAD3, while BMPs, AMH and a few GDFs are mediated by SMAD1 , of a series of about thirty serine -glycine repeats [5] . The binding of a TGF SMAD5 and SMAD9 . The binding of the R-SMAD to the type I receptor is beta family ligand causes the rotation of the receptors so that their mediated by a zinc double finger FYVE domain containing protein. Two cytoplasmic kinase domains are arranged in a catalytically favorable such proteins that mediate the TGF beta pathway include SARA (The orientation. The Type II receptor phosphorylates serine residues of the Type SMAD anchor for receptor activation) and HGS (Hepatocyte growth factor- I receptor, which activates the protein. regulated tyrosine kinase substrate).

[edit ] SMAD phosphorylation SARA is present in an early endosome which, by clathrin-mediated endocytosis , internalizes the receptor complex [6] . SARA recruits an R-SMAD . SARA permits the binding of the R-SMAD to the L45 region of the Type I receptor [7] . SARA orients the R-SMAD such that serine residue on its C-terminus faces the catalytic region of the Type I receptor. The Type I receptor phosphorylates the serine residue of the R-SMAD. Phosphorylation induces a conformational change in the MH2 domain of the R-SMAD and its subsequent dissociation from the receptor complex and SARA [8] .

[edit ] CoSMAD binding

The phosphorylated RSMAD has a high affinity for a coSMAD (e.g. SMAD4 ) and forms a complex with one. The phosphate group does not act as a docking site for coSMAD, rather the phosphorylation opens up an amino acid stretch allowing interaction.

[edit ] Transcription

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The phosphorylated RSMAD/coSMAD complex enters the nucleus where it binds transcription promoters/cofactors and causes the transcription of DNA.

Bone morphogenetic proteins cause the transcription of mRNAs involved in osteogenesis , neurogenesis , and ventral mesoderm specification.

TGF betas cause the transcription of mRNAs involved in apoptosis , extracellular matrix neogenesis and immunosuppression . It is also involved in G1 arrest in the cell cycle .

Activin causes the transcription of mRNAs involved in gonadal growth, embryo differentiation and placenta formation.

Nodal causes the transcription of mRNAs involved in left and right axis specification, and mesoderm and endoderm induction.

[edit ] Pathway regulation

The TGF beta signaling pathway is involved in a wide range of cellular process and subsequently is very heavily regulated. There are a variety of mechanisms that the pathway is modulated both positively and negatively: There are agonists for ligands and R-SMADs; there are decoy receptors; and R-SMADs and receptors are ubiquitinated.

[edit ] Ligand agonists/antagonists

Both chordin and noggin are antagonists of BMPs. They bind BMPs preventing the binding of the ligand to the receptor. [9] It has been demonstrated that Chordin and Noggin dorsalize mesoderm . They are both found in the dorsal lip of Xenopus and convert otherwise epidermis specified tissue into neural tissue (see neurulation ). Noggin plays a key role in cartilage and bone patterning. Mice Noggin-/- have excess cartilage and lacked joint formation. [9]

Members of the DAN family of proteins also antagonize TGF beta family members. They include Cerberus , DAN , and Gremlin . These proteins contain nine conserved cysteines which can form disulfide bridges. It is believed that DAN antagonizes GDF5 , GDF6 and GDF7 .

Follistatin inhibits Activin, which it binds. It directly affects follicle-stimulating hormone (FSH) secretion. Follistatin also is implicated in prostate cancers where mutations in its gene may preventing it from acting on activin which has anti-proliferative properties. [9]

Lefty is a regulator of TGF β and is involved in the axis patterning during embryogenesis. It is also a member of the TGF superfamily of proteins. It is asymmetrically expressed in the left side of murine embryos and subsequently plays a role in left-right specification. Lefty acts by preventing the phosphorylation of R-SMADs. It does so through a constitutively active TGF β type I receptor and through a process downstream of its activation. [10]

Drug-based antagonists have also been identified, such as SB431542, [11] which selectively inhibits ALK4, ALK5, and ALK7.

[edit ] Receptor regulation

The Transforming growth factor receptor 3 (TGFBR3) is the most abundant of the TGF-β receptors yet [12] , it has no known signaling domain [13] . It however may serve to enhance the binding of TGF beta ligands to TGF beta type II receptors by binding TGF β and presenting it to TGFBR2. One of the downstream targets of -Fakultativ- 14 Signalwege – Fakultatives Material –

TGF β signaling, GIPC , binds to its PDZ domain, which prevents its proteosomal degradation, which subsequently increases TGF β activity. It may also serve as an inhibin coreceptor to ActivinRII [9] .

BMP and Activin membrane bound inhibitor (BAMBI), has a similar extracellular domain as type I receptors. It lacks an intracellular serine/threonine protein kinase domain and hence is a pseudoreceptor. It binds to the type I receptor preventing it from being activated. It serves as a negative regulator of TGF beta signaling and may limit tgf-beta expression during embryogeneis. It requires BMP signaling for its expression

FKBP12 binds the GS region of the type I receptor preventing phosphorylation of the receptor by the type II receptors. It is believed that FKBP12 and its homologs help to prevent type I receptor activation in the absence of a ligands, since ligand binding causes its dissociation.

[edit ] R-SMAD regulation

[edit ] Role of inhibitory SMADs

There are two other SMADs which complete the SMAD family, the inhibitory SMADs (I-SMADS ), SMAD6 and SMAD7 . They play a key role in the regulation of TGF beta signaling and are involved in negative feeback. Like other SMADs they have an MH1 and an MH2 domain. SMAD7 competes with other R-SMADs with the Type I receptor and prevents their phosphorylation [9][14] . It resides in the nucleus and upon TGF beta receptor activation translocates to the cytoplasm where it binds the type I receptor. SMAD6 binds SMAD4 preventing the binding of other R-SMADs with the coSMAD. The levels of I-SMAD increase with TGF beta signaling suggesting that they are downstream targets of TGF-beta signaling.

[edit ] R-SMAD ubiquitination

The E3 ubiquitin-protein ligases SMURF1 and SMURF2 regulate the levels of SMADs. They accept ubiquitin from a E2 conjugating enzyme where they transfer ubiquitin to the RSMADs which causes their ubiquitination and subsequent proteosomal degradation. SMURF1 binds to SMAD1 and SMAD5 while SMURF2 binds SMAD1 , SMAD2 , SMAD3 , SMAD6 and SMAD7 . It enhances the inhibitory action of SMAD7 while reducing the transcriptional activities of SMAD2.

[edit ] Summary table

TGF Beta superfamily ligand Type II Receptor Type I receptor R-SMADs coSMAD Ligand inhibitors Activin A ACVR2A ACVR1B (ALK4) SMAD2 , SMAD3 SMAD4 Follistatin GDF1 ACVR2A ACVR1B (ALK4) SMAD2 , SMAD3 SMAD4 GDF11 ACVR2B ACVR1B (ALK4), TGF βRI (ALK5) SMAD2 , SMAD3 SMAD4 Bone morphogenetic proteins BMPR2 BMPR1A (ALK3), BMPR1B (ALK6) SMAD1 SMAD5 , SMAD8 SMAD4 Noggin , Chordin , DAN Nodal ACVR2B ACVR1B (ALK4), ACVR1C (ALK7) SMAD2 , SMAD3 SMAD4 Lefty TGF βs TGF βRII TGF βRI (ALK5) SMAD2 , SMAD3 SMAD4 LTBP1 , THBS1 , Decorin

Tumor necrosis factor receptor

From Wikipedia, the free encyclopedia (Redirected from TNF receptor ) Jump to: navigation , search

TNFR/NGFR cysteine-rich region

Structure of the soluble human 55 kd TNF receptor-human TNF beta

complex [1] .

Identifiers

Symbol TNFR_c6

Pfam PF00020

-Fakultativ- 15 Signalwege – Fakultatives Material –

InterPro IPR011614

PROSITE PDOC00561

SCOP 1tnr

[show ]Available PDB structures:

A tumor necrosis factor receptor (TNFR), or death receptor , is a cytokine receptor that binds tumor necrosis factors (TNF).

Because "TNF" is often used to describe TNF alpha , "TNFR" is often used to describe the receptors that bind to TNF alpha - namely, CD120 . However, there are several other members of this family that bind to the other TNFs. [2][3]

Contents

[hide ]

• 1 Members • 2 References • 3 Further reading • 4 External links

[edit ] Members

Family members include: [2]

Type Protein Aliases Gene CD120a TNFRSF1A 1 (CD120 ) CD120b TNFRSF1B 3 Lymphotoxin β receptor TNFRSF3 LTBR 4 CD134 TNFRSF4 5 CD40 TNFRSF5 CD40 FAS TNFRSF6 FAS 6 TNFRSF6B TNFRSF6B 7 CD27 TNFRSF7 CD27 8 CD30 TNFRSF8 9 CD137 TNFRSF9 TNFRSF10A DR4 TNFRSF10A TNFRSF10B DR5 TNFRSF10B 10 TNFRSF10C TNFRSF10C TNFRSF10D TNFRSF10D RANK TNFRSF11A 11 Osteoprotegerin TNFRSF11B 12 TNFRSF12A TNFRSF12A TNFRSF13B TNFRSF13B 13 TNFRSF13C TNFRSF13C

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14 TNFRSF14 TNFRSF14 16 Nerve growth factor receptor TNFRSF16 NGFR 17 TNFRSF17 TNFRSF17 18 TNFRSF18 TNFRSF18 19 TNFRSF19 TNFRSF19 21 TNFRSF21 DR6 TNFRSF21 25 TNFRSF25 TNFRSF25 27 Ectodysplasin A2 receptor TNFRSF27 EDA2R

Notch signaling pathway

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Notch-mediated juxtacrine signal between

Structure of the ligand binding region of the adjacent cells. human NOTCH-1 receptor

Notch signaling steps

The notch signaling pathway is a highly conserved cell signaling system present in most multicellular organisms. [1] Notch is present in all metazoans , and mammals possess four different notch receptors , referred to as NOTCH1 , NOTCH2 , NOTCH3 , and NOTCH4 . The notch receptor is a single-pass transmembrane receptor protein. It is a hetero-oligomer composed of a large extracellular portion, which associates in a calcium -dependent, non-covalent interaction with a smaller piece of the notch protein composed of a short extracellular region, a single transmembrane-pass, and a small intracellular region. [2]

[edit ] Discovery

In 1914, John S. Dexter noticed the appearance of a notch in the wings of the fruit fly Drosophila melanogaster . The alleles of the gene were identified in 1917 by Thomas Hunt Morgan .[3][4] Its molecular analysis and sequencing was independently undertaken in the 1980s by Spyros Artavanis-Tsakonas and Michael W. Young. [5][6]

[edit ] Mechanism of action

The notch protein sits like a trigger spanning the cell membrane , with part of it inside and part outside. Ligand proteins binding to the extracellular domain induce proteolytic cleavage and release of the intracellular domain, which enters the cell nucleus to alter gene expression .[7]

Because most ligands are also transmembrane proteins, the receptor is normally triggered only from direct cell-to-cell contact. In this way, groups of cells can organise themselves, such that, if one cell expresses a given trait, this may be switched off in neighbour cells by the intercellular notch signal. In this way, groups of cells influence one another to make large structures.

The notch cascade consists of notch and notch ligands , as well as intracellular proteins transmitting the notch signal to the cell's nucleus. The Notch/Lin-12/Glp-1 receptor family [8] was found to be involved in the specification of cell fates during development in Drosophila and C. elegans .[9]

[edit ] Function

The notch signaling pathway is important for cell-cell communication, which involves gene regulation mechanisms that control multiple cell differentiation processes during embryonic and adult life. Notch signaling also has a role in the following processes:

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• neuronal function and development [10][11][12][13] • stabilization of arterial endothelial fate and angiogenesis [14] • regulation of crucial cell communication events between endocardium and myocardium during both the formation of the valve primordial and ventricular development and differentiation [15] • cardiac valve homeostasis, as well as implications in other human disorders involving the cardiovascular system [16] • timely cell lineage specification of both endocrine and exocrine pancreas [17] • influencing of binary fate decisions of cells that must choose between the secretory and absorptive lineages in the gut [18] • expansion of the hematopoietic stem cell compartment during bone development and participation in commitment to the osteoblastic lineage, suggesting a potential therapeutic role for notch in bone regeneration and osteoporosis [19] • T cell lineage commitment from common lymphoid precursor [20] • regulation of cell-fate decision in mammary glands at several distinct development stages [21] • possibly some non-nuclear mechanisms, such as control of the actin cytoskeleton through the tyrosine kinase Abl [22]

Notch signaling is dysregulated in many cancers, [23] and faulty notch signaling is implicated in many diseases including T-ALL (T-cell acute lymphoblastic leukemia ), [24] CADASIL (Cerebral Autosomal Dominant Arteriopathy with Sub-cortical Infarcts and Leukoencephalopathy), MS (Multiple Sclerosis ), Tetralogy of Fallot , Alagille syndrome , and many other disease states.

Inhibition of notch signaling has been shown to have anti-proliferative effects on T-ALL in cultured cells and in a mouse model. [25][26][27]

[edit ] Pathway

Maturation of the notch receptor involves cleavage at the prospective extracellular side during intracellular trafficking in the Golgi complex. [28] This results in a bipartite protein, composed of a large extracellular domain linked to the smaller transmembrane and intracellular domain. Binding of ligand promotes two proteolytic processing events; as a result of proteolysis, the intracellular domain is liberated and can enter the nucleus to engage other DNA-binding proteins and regulate gene expression.

Notch and most of its ligands are transmembrane proteins, so the cells expressing the ligands typically must be adjacent to the notch expressing cell for signaling to occur. [citation needed ] The notch ligands are also single-pass transmembrane proteins and are members of the DSL (Delta/Serrate/LAG-2) family of proteins. In Drosophila melanogaster (the fruit fly), there are two ligands named Delta and Serrate . In mammals, the corresponding names are Delta-like and Jagged . In mammals there are multiple Delta-like and Jagged ligands, as well as possibly a variety of other ligands, such as F3/contactin. [22]

In the nematode Caenorhabditis elegans , two genes encode homologous proteins, glp-1 and lin-12 . There has been at least one report that suggests that some cells can send out processes that allow signaling to occur between cells that are as much as four or five cell diameters apart. [citation needed ]

The notch extracellular domain is composed primarily of small cysteine knot motifs called EGF -like repeats. [29]

Notch 1, for example, has 36 of these repeats. Each EGF-like repeat is composed of approximately 40 amino acids, and its structure is defined largely by six conserved cysteine residues that form three conserved disulfide bonds. Each EGF-like repeat can be modified by O-linked glycans at specific sites. [30] An O-glucose sugar may be added between the first and second conserved cysteines, and an O-fucose may be added between the second and third conserved cysteines. These sugars are added by an as-yet-unidentified O-glucosyltransferase , and GDP-fucose Protein O-fucosyltransferase 1 (POFUT1 ), respectively. The addition of O-fucose by POFUT1 is absolutely necessary for notch function, and, without the enzyme to add O-fucose, all notch proteins fail to function properly. As yet, the manner by which the glycosylation of notch affects function is not completely understood.

The O-glucose on notch can be further elongated to a trisaccharide with the addition of two xylose sugars by xylosyltransferases , and the O-fucose can be elongated to a tetrasaccharide by the ordered addition of an N-acetylglucosamine (GlcNAc) sugar by an N-Acetylglucosaminyltransferase called Fringe , the addition of a galactose by a galactosyltransferase , and the addition of a sialic acid by a sialyltransferase .[31]

To add another level of complexity, in mammals there are three Fringe GlcNAc-transferases, named lunatic fringe, manic fringe, and radical fringe. These enzymes are responsible for something called a "fringe effect" on notch signaling. [32] If Fringe adds a GlcNAc to the O-fucose sugar, then the subsequent addition of a galactose and sialic acid will occur. In the presence of this tetrasaccharide, notch signals strongly when it interacts with the Delta ligand, but has markedly inhibited signaling when interacting with the Jagged ligand. [33] The means by which this addition of sugar inhibits signaling through one ligand, and potentiates signaling through another is not clearly understood.

Once the notch extracellular domain interacts with a ligand, an ADAM-family metalloprotease called TACE (Tumor Necrosis Factor Alpha Converting Enzyme ) cleaves the notch protein just outside the membrane. [2] This releases the extracellular portion of notch, which continues to interact with the ligand. The ligand plus the notch extracellular domain is then endocytosed by the ligand-expressing cell. There may be signaling effects in the ligand-expressing cell after endocytosis; this part of notch signaling is a topic of active research. After this first cleavage, an enzyme called γ-secretase (which is implicated in Alzheimer's disease ) cleaves the remaining part of the notch protein just inside the inner leaflet of the cell membrane of the notch-expressing cell. This releases the intracellular domain of the notch protein, which then moves to the nucleus , where it can regulate gene expression by activating the transcription factor CSL. Other proteins also participate in the intracellular portion of the notch signaling cascade.

[edit ] External links

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• Diagram: notch signaling pathway in Homo sapiens • Diagram: Notch signaling in Drosophila • Netpath - A curated resource of signal transduction pathways in humans • MeSH Notch+Receptors

The Notch signaling pathway is an evolutionarily conserved, intercellular signaling mechanism essential for proper embryonic development in all metazoan organisms in the Animal kingdom. The Notch proteins (Notch1-Notch4 in vertebrates) are single-pass receptors that are activated by the Delta (or Delta-like) and Jagged/Serrate families of membrane-bound ligands. They are transported to the plasma membrane as cleaved, but otherwise intact polypeptides. Interaction with ligand leads to two additional proteolytic cleavages that liberate the Notch intracellular domain (NICD) from the plasma membrane. The NICD translocates to the nucleus, where it forms a complex with the DNA binding protein CSL, displacing a histone deacetylase (HDAc)-co-repressor (CoR) complex from CSL. Components of an activation complex, such as MAML1 and histone acetyltransferases (HATs), are recruited to the NICD-CSL complex, leading to the transcriptional activation of Notch target genes.

Hedgehog signaling pathway

From Wikipedia, the free encyclopedia Jump to: navigation , search

In a growing embryo, cells develop differently in the head or tail end of the embryo, the left or right, and other positions. They also form segments which develop into different body parts. The hedgehog signaling pathway gives cells information that they need to make the embryo develop properly. Different parts of the embryo have different concentrations of hedgehog signaling proteins. The pathway also has roles in the adult. When the pathway malfunctions, it can result in diseases like basal cell carcinoma . [1]

The hedgehog signaling pathway is one of the key regulators of animal development conserved from flies to humans (meaning it was present in the common ancestor of both). The pathway takes its name from its polypeptide ligand, an intercellular signaling molecule called Hedgehog ( Hh ) found in fruit flies of the genus Drosophila . Hh is one of Drosophila's segment polarity gene products, involved in establishing the basis of the fly body plan . The molecule remains important during later stages of embryogenesis and metamorphosis .

Mammals have three Hedgehog homologues, of which Sonic hedgehog is the best studied. The pathway is equally important during vertebrate embryonic development. In knockout mice lacking components of the pathway, the brain , skeleton , musculature , gastrointestinal tract and lungs fail to develop correctly. Recent studies point to the role of hedgehog signaling in regulating adult stem cells involved in maintenance and regeneration of adult tissues . The pathway has also been implicated in the development of some cancers . Drugs that specifically target hedgehog signaling to fight this disease are being actively developed by a number of pharmaceutical companies .

[edit ] Discovery

Figure 1. Normal and Hedgehog mutant larvae.

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In the 1970s, a fundamental problem in developmental biology was to understand how a relatively simple egg can give rise to a complex segmented body plan. In the late 1970s Christiane Nüsslein-Volhard and Eric Wieschaus isolated mutations in genes that control development of the segmented anterior-posterior body axis of the fly; [2] their "saturation mutagenesis" technique resulted in the discovery of a group of genes involved in the development of body segmentation . In 1995, they shared the Nobel Prize with Edward B. Lewis for their work studying genetic mutations in Drosophila embryogenesis .[3]

The Drosophila hedgehog ( hh ) gene was identified as one of several genes important for creating the differences between the anterior and posterior parts of individual body segments. The fly hh gene was independently cloned in 1992 by the labs of Jym Mohler, Philip Beachy , and Thomas B. Kornberg . Some hedgehog mutants result in abnormally-shaped embryos that are unusually short and stubby compared to wild type embryos. The function of the hedgehog segment polarity gene has been studied in terms of its influence on the normally polarized distribution of larval cuticular denticles as well as features on adult appendages such as legs and antennae. [4] Rather than the normal pattern of denticles, hedgehog mutant larvae tend to have "solid lawns" of denticles (Figure 1). The appearance of the stubby and "hairy" larvae inspired the name 'hedgehog' (see: Hedgehog, the animal ).

[edit ] Fruit fly

Figure 2 . Production of the CiR transcriptional repressor when Hh is not bound to Patched. In the diagram, "P" represents phosphate .

Figure 3 . When Hh is bound to Patched (PTCH), Ci protein is able to act as a transcription factor in the nucleus.

[edit ] Mechanism

Insect cells express a full size zinc-finger transcription factor Cubitus interruptus (Ci), which forms a complex with the kinesin - like protein Costal-2 (Cos2) and is localized in the cytoplasm bound to cellular microtubules (Figure 2). The complex targets the 155 kb full length Ci protein for proteosome - dependent cleavage, which generates a 75 kb fragment (CiR). CiR builds up in the cell and diffuses into the nucleus , where it acts as a co-repressor for Hh target genes. [5] The steps leading to Ci protein proteolysis include phosphorylation of Ci protein by several protein kinases ; PKA , GSK3 β and CK1 (Figure 2). [6] The Drosophila protein Slimb is part of an SCF complex that targets proteins for ubiquitylation . Slimb binds to phosphorylated Ci protein.

In the absence of Hh (Figure 3), a cell-surface transmembrane protein called Patched (PTCH) acts to prevent high expression and activity of a 7 membrane spanning receptor [7] called Smoothened (SMO). Patched has sequence similarity to known membrane transport proteins. When extracellular Hh is present (Figure 3), it binds to and inhibits Patched, allowing Smoothened to accumulate and inhibit the proteolytic cleavage of the Ci protein. This process most likely involves the direct interaction of Smoothened and Costal-2 and may involve sequestration of the Ci protein-containing complex to a microdomain where the steps leading to Ci protein -Fakultativ- 20 Signalwege – Fakultatives Material – proteolysis are disrupted [5] . The mechanism by which Hh binding to Patched leads to increased levels of Smoothened is not clear (Step 1 in Figure 3). Following binding of Hh to Patched, Smoothened levels increase greatly over the level maintained in cells when Patched is not bound to Hh. [8] It has been suggested that phosphorylation of Smoothened plays a role in Hh-dependent regulation of Smoothened levels. [9]

In cells with Hh-activated Patched (Figure 3), the intact Ci protein accumulates in the cell cytoplasm and levels of CiR decrease, allowing transcription of some genes such as decapentaplegic (dpp, a member of the BMP growth factor family). For other Hh-regulated genes, expression requires not only loss of CiR but also the positive action of uncleaved Ci acting as a transcriptional activator [6] . Costal-2 is normally important for holding Ci protein in the cytoplasm, but interaction of Smoothened with Costal-2 allows some intact Ci protein to go to the nucleus. The Drosophila protein Fused (Fu in Figure 3) is a protein kinase that binds to Costal- 2. Fused can inhibit Suppressor of Fused (SUFU), which in turn interacts with Ci to regulate gene transcription in some cell types. [10]

[edit ] Role

Figure 4. Interactions between Wingless and Hedgehog.

Hedgehog has roles in larval body segment development and in formation of adult appendages. During the formation of body segments in the developing Drosophila embryo, stripes of cells that synthesize the transcription factor Engrailed can also express the cell-to-cell signaling protein Hedgehog (green in Figure 4). Hedgehog is not free to move very far from the cells that make it and so it only activates a thin stripe of cells adjacent to the Engrailed-expressing cells. Only cells to one side of the Engrailed-expressing cells are competent to respond to Hedgehog following interaction of Hh with the receptor protein Patched (blue in Figure 4).

Cells with Hh-activated Patched receptor synthesize the Wingless protein (red in Figure 4). If a Drosophila embryo is altered so as to produce Hh in all cells, all of the competent cells respond and form a broader band of Wingless-expressing cells in each segment. The wingless gene has an upstream transcription regulatory region that binds the Ci transcription factor in a Hh-dependent fashion resulting in an increase in wingless transcription (interaction 2 in Figure 3) in a stripe of cells adjacent to the stripe of Hh-producing cells. [11]

Wingless protein acts as an extracellular signal and patterns the adjacent rows of cells by activating its cell surface receptor Frizzled . Wingless acts on Engrailed- expressing cells to stabilize the stripes of Engrailed expression. Wingless is a member of the Wnt family of cell-to-cell signaling proteins. The reciprocal signaling by Hedgehog and Wingless stabilizes the boundary between parasegments (Figure 4, top). The effects of Wingless and Hedgehog on other stripes of cells in each segment establishes a positional code that accounts for the distinct anatomical features along the anterior-posterior axis of the segments [12]

The Wingless protein is called "wingless" because of the phenotype of some wingless fly mutants. Wingless and Hedgehog functioned together during metamorphosis to coordinate wing formation. Hedgehog is expressed in the posterior part of developing Drosophila limbs. Hedgehog also participates in the coordination of eye, brain, gonad, gut and tracheal development.

[edit ] Annelids hedgehog is also involved in segmentation in the annelid worms; because parallel evolution seems unlikely, this suggests a common origin of segmentation between the two phyla. [13] Whilst Hh does not induce the formation of segments, it seems to act to stabilize the segmented fields once they have appeared. [13]

[edit ] Vertebrates

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[edit ] Mechanism

Figure 5. Overview of Sonic hedgehog signaling. Click here for a more detailed diagram

Sonic hedgehog (SHH) is the best studied ligand of the vertebrate pathway. Most of what is known about hedgehog signaling has been established by studying SHH. It is translated as a ~45kDa precursor and undergoes autocatalytic processing to produce an ~20kDa N-terminal signaling domain (referred to as SHH-N) and a ~25kDa C-terminal domain with no known signaling role (1 on figure 5). During the cleavage, a cholesterol molecule is added to the carboxyl end of the N-terminal domain, which is involved in trafficking, secretion and receptor interaction of the ligand. SHH can signal in an autocrine fashion, affecting the cells in which it is produced. Secretion and consequent paracrine hedgehog signaling require the participation of Dispatched protein(2).

When SHH reaches its target cell, it binds to the Patched-1 (PTCH1) receptor(3). In the absence of ligand, PTCH1 inhibits Smoothened (SMO), a downstream protein in the pathway(4). It has been suggested that SMO is regulated by a small molecule, the cellular localisation of which is controlled by PTCH. [14] PTCH1 has homology to Niemann-Pick disease, type C1 (NPC1 ) that is known to transport lipophilic molecules across a membrane. [15] PTCH1 has a sterol sensing domain (SSD), which has been shown to be essential for suppression of Smo activity. [16] A current theory suggests that PTCH regulates SMO by removing oxysterols from SMO. PTCH acts like a sterol pump and removes oxysterols that have been created by 7-dehydrocholesterol reductase. [17] Upon binding of a Hh protein or a mutation in the SSD of PTCH the pump is turned off allowing oxysterols to accumulate around SMO.

Suggested regulation pathway for Smo via Hedgehog and Ptch1

This accumulation of sterols allows SMO to become active or stay on the membrane for a longer period of time. This hypothesis is supported by the existence of a number of small molecule agonists and antagonists of the pathway that act on SMO. The binding of SHH relieves SMO inhibition, leading to activation of the GLI transcription factors(5): the activators Gli1 and Gli2 and the repressor Gli3 . The sequence of molecular events that connect SMO to GLIs is poorly understood. Activated GLI accumulates in the nucleus(6) and controls the transcription of hedgehog target genes(7). PTCH1 has recently been reported to repress transcription of hedgehog target genes through a mechanism independent of Smoothened .[18]

In addition to PTCH1, mammals have another hedgehog receptor PTCH2 whose sequence identity with PTCH1 is 54%. [19] All three mammalian hedgehogs bind both receptors with similar affinity, so PTCH1 and PTCH2 cannot discriminate between the ligands. They do, however, differ in their expression patterns. PTCH2 is -Fakultativ- 22 Signalwege – Fakultatives Material – expressed at much higher levels in the testis and mediates desert hedgehog signaling there. [19] It appears to have a distinct downstream signaling role from PTCH1. In the absence of ligand binding PTCH2 has a decreased ability to inhibit the activity of SMO. [20] Furthermore, overexpression of PTCH2 does not replace mutated PTCH1 in basal cell carcinoma .[21]

In invertebrates, just as in Drosophila , the binding of hedgehog to PTCH leads to internalisation and sequestration of the ligand. [22] Consequently in vivo the passage of hedgehog over a receptive field that expresses the receptor leads to attenuation of the signal, an effect called ligand-dependent antagonism (LDA). In contrast to Drosophila , vertebrates possess another level of hedgehog regulation through LDA mediated by Hh-interacting protein 1 (HHIP1). HHIP1 also sequesters hedgehog ligands, but unlike PTCH, it has no effect on the activity of SMO. [23]

[edit ] Role

Members of the hedgehog family play key roles in a wide variety of developmental processes. [12] One of the best studied examples is the action of Sonic hedgehog during development of the vertebrate limb. The classic experiments of Saunders and Gasseling in 1968 on the development of the chick limb bud formed the basis of the morphogen concept. They showed that identity of the digits in the chick limb was determined by a diffusible factor produced by the zone of polarizing activity (ZPA), a small region of tissue at the posterior margin of the limb. Mammalian development appeared to follow the same pattern. This diffusible factor was later shown to be Sonic hedgehog . However, precisely how SHH determines digit identity remained elusive until recently. The current model, proposed by Harfe et al. ,[24] states that both the concentration and the time of exposure to SHH determines which digit the tissue will develop into in the mouse embryo (figure 6).

Figure 6. Sonic hedgehog specifies digit identity in mammalian development.

Digits V, IV and part of III arise directly from cells that express SHH during embryogenesis . In these cells SHH signals in an autocrine fashion and these digits develop correctly in the absence of DISP, which is required for extracellular diffusion of the ligand. These digits differ in the length of time that SHH continues to be expressed. The most posterior digit V develops from cells that express the ligand for the longest period of time. Digit IV cells express SHH for a shorter time, and digit III cells shorter still. Digit II develops from cells that are exposed to moderate concentrations of extracellular SHH. Finally, digit I development does not require SHH. It is, in a sense, the default program of limb bud cells.

Hedgehog signaling remains important in the adult. Sonic hedgehog has been shown to promote the proliferation of adult stem cells from various tissues, including primitive hematopoietic cells, [25] mammary [26] and neural [27] stem cells. Activation of the hedgehog pathway is required for transition of the hair follicle from the resting to the growth phase. [28] Curis Inc. together with Procter & Gamble are developing a hedgehog agonist to be used as a drug for treatment of hair growth disorders. [29] This failed due to toxicities found in animal models. [30]

[edit ] Human disease

Disruption of hedgehog signaling during embryonic development, either through deleterious mutation or consumption of teratogens by the gestating mother, can lead to severe developmental abnormalities. Holoprosencephaly , the failure of the embryonic prosencephalon to divide to form cerebral hemispheres, occurs with a frequency of about 1 in 16,000 live births and about 1 in 200 spontaneous abortions in humans and is commonly linked to mutations in genes involved in the hedgehog pathway, including SHH and PTCH .[31] Cyclopia , one of the most severe defects of holoprosencephaly , results if the pathway inhibitor cyclopamine is consumed by gestating mammals. [32]

Activation of the hedgehog pathway has been implicated in the development of cancers in various organs, including brain , lung , mammary gland , prostate and skin . Basal cell carcinoma , the most common form of cancerous malignancy , has the closest association with hedgehog signaling. Loss-of-function mutations in Patched and activating mutations in Smoothened have been identified in patients with this disease. [33] Abnormal activation of the pathway probably leads to development of disease through transformation of adult stem cells into cancer stem cells that give rise to the tumor. Cancer researchers hope that specific inhibitors of hedgehog signaling will provide an efficient therapy for a wide range of malignancies. [34]

Biotech companies are also attempting to turn this pathway on after a patient has a stroke or heart attack. Since the pathway has been implicated in a number of lethal cancers Curis and Wyeth have devised a stable hedgehog protein that can cross the blood brain barrier.[35] In pre-clinical animal models it has shown that the pathway is up regulated upon a stroke or heart attack event. The pathway provides a protective barrier against cell death and ischemia. Agonizing the pathway this way allows the PTCH to be up regulated providing a negative feedback system. This might help minimize the side effects.

Targeting the Hedgehog Pathway

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The most common way to target this pathway is modulate SMO. Antagonist and agonist of SMO have already shown to effect the pathway regulation downstream. PTCH [36] and Gli3 (5E1) [37] antibodies are also a way to regulate the pathway. A downstream effector and strong transcriptional activator siRNA Gli1 has been used to inhibit cell growth and promote apoptosis. [38]

Hedgehog Pathway and Metastasis

Activation of the Hedgehog pathway leads to an increase in Snail protein expression and a decrease in E-cadherin and Tight Junctions .[39] Hedgehog signaling also appears to be a crucial regulator of angiogenesis and thus metastasis. [40]

Hedgehog Pathway and Tumor Regulation

Activation of the Hedgehog pathway leads to an increase in Angiogenic Factors (angiopoietin-1 and angiopoietin-2), [41] Cyclins (cyclin D1 and B1)), [42] anti- apoptotic genes and a decrease in apoptotic genes (Fas). [43]

Clinical Trials GDC-0449 in Treating Patients With Locally Advanced or Metastatic Solid Tumors [2] A Study of Systemic Hedgehog Antagonist With Concurrent Chemotherapy and Bevacizumab As First-Line Therapy for Metastatic Colorectal Cancer[3] Video Presentation from AACR.org [4]

[edit ] Evolution

Figure 7 . Phylogenetic relationship of hedgehog ligands (based on Ingham and McMahon, 2001).

Hedgehog-like genes, 2 Patched homologs and Patched-related genes exist in the worm C. elegans .[44][45] These genes have been shown to code for proteins that have roles in C. elegans development [44] . The hedgehog-like and Patched-related gene families are very large and function without the need for a Smoothened homolog, suggesting a distinct pattern of selection for cholesterol modification and sensing mechanisms in coelomate and pseudo-coelomate lineages [45] .

Lancelets , which are primitive chordates , possess only one homologue of Drosophila Hh (figure 7). Vertebrates, on the other hand, have several hedgehog ligands that fall within three subgroups - desert , Indian and sonic , each represented by a single mammalian gene. This is probably a consequence of the two genome duplications that occurred early in the vertebrate evolutionary history. [46] Two such events would have produced four homologous genes, one of which must have been lost. Desert hedgehogs are the most closely related to Drosophila Hh . Additional gene duplications occurred within some species [12] such as the zebrafish Danio rerio , which has an additional tiggywinkle hedgehog gene in the sonic group. Various vertebrate lineages have adapted hedgehogs to unique developmental processes. For example, a homologue of the X.laevis banded hedgehog is involved in regeneration of the salamander limb. [47] shh has undergone accelerated evolution in the primate lineage leading to humans. [48] Dorus et al. hypothesise that this allowed for more complex regulation of the protein and may have played a role in the increase in volume and complexity of the human brain.

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The frizzled family of WNT receptors have some sequence similarity to Smoothened. [49] However, G proteins have been difficult to link to the function Smoothened. Smoothened seems to be a functionally divergent member of the G protein coupled receptor super family. Other similarities between the WNT and Hh signaling pathways have been reviewed. [50] Nusse observed that, "a signaling system based on lipid-modified proteins and specific membrane translocators is ancient, and may have been the founder of the Wnt and Hh signaling systems".

It has been suggested that invertebrate and vertebrate signaling downstream from Smoothened has diverged significantly. [51] The role of Suppressor of Fused (SUFU) has been enhanced in vertebrates compared to Drosophila where its role is relatively minor. Costal-2 is particularly important in Drosophila . The protein kinase Fused is a regulator of SUFU in Drosophila , but may not play a role in the Hh pathway of vertebrates. [52] In vertebrates, Hh signalling has been heavily coupled to cilia [53]

[edit ] See also

• Sonic hedgehog , best studied ligand of the vertebrate pathway • Smoothened , the conserved GPCR component of the pathway • Cyclopamine , a small molecule inhibitor of Hh signaling

[edit ] External links

• http://hedgehog.sfsu.edu (Hedgehog Pathway Database) • Netpath - A curated resource of signal transduction pathways in humans

Netpath

From Wikipedia, the free encyclopedia Jump to: navigation , search

NetPath [1] is a manually curated resource of human signal transduction pathways. It is a joint effort between Pandey Lab at the Johns Hopkins University and the Institute of Bioinformatics (IOB), Bangalore, India ,[2] and is also worked on by other parties.

A screenshot of the homepage of NetPath .

NetPath hosts 20 signaling pathways including 10 pathways with a major role in the regulation of immune system and 10 pathways with relevance to regulation of cancer .

[edit ] Overview

The 20 pathways contain information pertaining to protein-protein interactions , enzyme-protein substrate reactions which bring about post translational modifications (PTMs) and also a catalogue of genes which are differentially regulated upon activation of specific ligand mediated receptor pathways. The molecules which localises to different cellular organelles due to their PTMs or specific protein-protein interactions which occur downstream of ligand-receptor mediated pathway are available under translocation events. Recently, NetPath has also curated the molecules involved in the transcriptional regulation of genes in the context of immune signaling pathways. The reactions in NetPath are curated by PhD level scientists from experimental evidence available in published research articles. NetPath also contains textual description of its reactions with information on PTMs, dependence of PTMs on various signaling reactions, subcellular location, protein interaction domains or motifs and the cell type or cell line in which reactions are proved. The information in NetPath is linked to their corresponding research articles and are frequently updated. Each pathway is subjected to different level of internal quality checks and peer-review by the pathway experts and authorities.

[edit ] Development

NetPath was developed using PathBuilder, an open source software for annotating and developing pathway resources. [3] PathBuilder enables annotation of molecular events including protein-protein interactions, enzyme-substrate relationships and protein translocation events via manual or automatic methods. The features of PathBuilder include automatic validation of data formats, built-in modules for visualizing pathways, automated import of data from other pathway resources, export of data in several standard data exchange formats and an application programming interface for retrieving pathway datasets.

[edit ] Data availability

All the 20 pathways are freely downloadable in BioPAX , PSI-MI and SBML formats. BioPAX is an emerging standard for pathway data exchange. The pathways are made available under an adaptive Creative Commons License 2.5 which stipulates that the pathways may be used if adequate credit is given to the authors.

[edit ] Immune signaling pathways

The following immune signaling pathways are hosted by Netpath:

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• B cell receptor pathway • Interleukin-4 pathway • T cell receptor pathway • Interleukin-5 pathway • Interleukin-1 pathway • Interleukin-6 pathway • Interleukin-2 pathway • Interleukin-7 pathway • Interleukin-3 pathway • Interleukin-9 pathway

[edit ] Cancer signaling pathways

The cancer signaling pathways were developed in collaboration with the Computational Biology Center at Memorial Sloan-ettering Cancer Center and with Bader Lab at the University of Toronto for the "Cancer Cell Map". The following cancer signaling pathways are hosted by Netpath:

• Epidermal growth factor receptor Pathway • Hedgehog pathway • Transforming growth factor beta receptor pathway • Notch pathway • Tumor necrosis factor alpha pathway • Wnt pathway • Alpha6 Beta4 Integrin pathway • Androgen receptor pathway • Inhibitor of DNA binding pathway • Kit receptor pathway

[edit ] Current statistics

Curated pathways 20 Molecules involved 1,682 Physical interactions 1,800 Genes transcriptionally regulated 6,582 Transport 201 Enzyme catalysis 1,218 PubMed citations 11,739

[edit ] Community participation programme

The community participation programme is aimed at training the students in various universities from India on curation of pathway reactions. This is a joint programme led by the Institute of Bioinformatics, Bangalore, India with active participation from Dr. Akhilesh Pandey's laboratory at the Johns Hopkins University (USA) and Gary Bader's lab at the University of Toronto, Canada. Currently, students from 3 major Indian Universities namely Pondicherry University , University of Pune and University of Mysore are participants of this community effort.

J Clin Invest. 2008 Jul;118(7):2404-14.

Hedgehog signaling is critical for maintenance of the adult coronary vasculature in mice.

Lavine KJ , Kovacs A , Ornitz DM .

Department of Developmental Biology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.

Abstract Hedgehog (HH) signaling has emerged as a critical pathway involved in the pathogenesis of a variety of tumors. As a result, HH antagonists are currently being evaluated as potential anticancer therapeutics. Conversely, activation of HH signaling in the adult heart may be beneficial, as HH agonists have been shown to increase coronary vessel density and improve coronary function after myocardial infarction. To investigate a potential homeostatic role for HH signaling in the adult heart, we ablated endogenous HH signaling in murine myocardial and perivascular smooth muscle cells. HH signaling was required for proangiogenic gene expression and maintenance of the adult coronary vasculature in mice. In the absence of HH signaling, loss of coronary blood vessels led to tissue hypoxia, cardiomyocyte cell death, heart failure, and subsequent lethality. We further showed that HH signaling specifically controlled the survival of small coronary arteries and capillaries. Together, these data demonstrate that HH signaling is essential for cardiac function at the level of the coronary vasculature and caution against the use of HH antagonists in patients with prior or ongoing heart disease.

A Prickly Problem: Hedgehog Signaling In Heart's Blood Vessels

ScienceDaily (June 26, 2008) — New data, generated by David Ornitz and colleagues, at Washington University School of Medicine, St. Louis, have indicated a crucial role for signaling pathways that involve the protein sonic hedgehog in maintaining the blood vessels that supply the mouse heart and keep it beating.

See Also: Health & Medicine -Fakultativ- 26 Signalwege – Fakultatives Material –

Heart Disease Anemia Vioxx Diseases and Conditions Blood Clots Immune System Reference Human physiology Skeletal muscle Blood pressure Apoptosis These data have implications for drug development as they suggest that antagonists of hedgehog signaling pathways, such as those being developed as anticancer therapeutics, might have unwanted side effects.

In the study, mice lacking the ability to mediate hedgehog signaling in cells that form part of the blood vessels that supply the heart were found to die of heart failure. This was because in the absence of hedgehog signaling the blood vessels of the heart were lost, meaning that the heart cells were no longer supplied with enough oxygen and died.

Although these data indicate a need for caution when developing clinical antagonists of hedgehog signaling, it is possible that the degree of inhibition needed to have a clinical effect on tumor development might not have the effect on blood vessels of the heart that completely eliminating expression of the protein does.

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