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G-Proteins and the Lung: Nuts and Bolts and out of the Gutter

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G-Proteins and the Lung: Nuts and Bolts and out of the Gutter Eur Respir J, 1995, 8, 1–4 Copyright ERS Journals Ltd 1995 DOI: 10.1183/09031936.95.08010001 European Respiratory Journal Printed in UK - all rights reserved ISSN 0903 - 1936 EDITORIAL G-proteins and the lung: nuts and bolts and out of the gutter C. Kroegel*, W. Luttmann, J.C. Virchow, H. Matthys The first documented description of inflammation as stimulating receptors and secondary messengers were a distinct phenomenon dates back several centuries discovered, it became clear that this receptor-secondary B.C. to preserved Mesopotamian clay tablets and Egyp- messenger model could not account for the diversity in tian medical papyri [1]. Since then, through the eminent exogenous signals on the one hand and the cellular work by J.F. Cohnheim, E. Mechnikoff, T. Leber and response on the other. It was left to the work of bio- many others thereafter, as well as the advances in mod- chemist M. Rodbell of the National Institute of Environ- ern molecular biology, the understanding of mechanisms mental Health Sciences in Research Triangle Park, North underlying inflammation has made much progress. At Carolina, and the pharmacologist A.G. Gilman of the the present time, inflammation may be adequately descri- University of Texas Southwestern Medical Center in bed as an extremely complex series of interdigitating Dallas to discover another missing piece in the puzzle, processes, in a wide variety of combinations and per- which bridges the multitude of surface receptors and mutations, which is presumably designed as a reparative intracellular signalling pathways [4, 5] ultimately de- and protective response to external noxae and tissue termining the individual cellular response in a given injury. context. The cornerstones of this dynamic pathophysiological The cellular mechanisms of the physiological regu- host reaction are a variety both of circulating and con- lation have been the subject of intense research for stitutive cells, which communicate through an even some time, and certain principles of cellular pathways more complex web of structural or soluble signals. To have emerged throughout the past decade or so. Utiliza- complicate matters further, it has become apparent that tion of intracellular calcium, cAMP and diacylglycerol the net biological effect of intercellular messengers, or (DAG), as well as the protein kinase cascades and the mediators, may be determined by microenvironmental inositol phosphate cycle, represent well-established factors and varies depending on the composition, as well examples [6]. In addition, the interactive crosstalk of as the state of degradation, of the surrounding matrix, the major cell signalling systems at different levels bet- the actual concentration of a particular signal achieved ween plasma membrane and gene transcription and cell at tissue level, and the presence or absence of co-secra- cycle control are beginning to be unveiled. Due to the ted mediators [2]. In addition, the biological outcome groundwork by Gilman and Rodbell, there is now sub- may ultimately depend on the state of activation and/or stantial evidence that an increasing number of ubiqui- maturation of the individual target cell, which, in turn, tous proteins, which are located close to the inner aspect may be regulated both by the cell itself (autocrine ac- of the cell membrane, may serve as a highly versatile tion), and other surrounding or distant cells (paracrine molecular switch that regulate the seemingly infinite action). However, the principal molecular mechanisms external signals and cellular signalling processes. Accor- by which the cell discriminates between the plethora of ding to their characteristic property of binding guanine synergistic or antagonistic incoming signals have long nucleotides, they have collectively been termed G- remained obscure. proteins. When, during the 1950s, Sutherland discovered that G-proteins belong to a larger superfamily of guano- epinephrine exerts its effects by stimulating the intra- sine triphosphases (GTPases) that include both factors cellular generation of a then unknown compound called controlling protein synthesis, of which the elongation cyclic adenosine monophosphate (cAMP), he unveiled factor-Tu (EF-Tu) has been studied most extensively, the first of many secondary messengers [3]. It was for and a group of "small" guanosine triphosphate (GTP)- this discovery, that Sutherland received the 1971 Nobel binding proteins with a molecular mass of 20–25 kD. prize. Although the precise mechanism by which epine- Members of the latter group include the ras gene phrine binding to its respective receptor might stimu- product, the adenosine diphosphate (ADP)-ribosylation late cAMP was unknown, it was generally assumed that factor (ARF) (a co-factor for cholera toxin catalysed the receptor directly signalled the cAMP-generating ADP-ribosylation of Gs), Gp (a GTP-binding protein enzyme adenylate cyclase. However, as more cAMP- purified from the placenta) the rho gene product, and *Correspondence: C. Kroegel, Dept of Pneumology, Medical Clinic, products of yeast YPT1 and SEC4 genes. Albert-Ludwigs University, Hugstetter Str. 55, D-79106 Freiburg, In contrast to the small G-proteins, classical G-proteins Germany. are heterotrimers composed of three distinct subunits 2 C. KROEGEL ET AL. termed α, β and γ, in order of decreasing mass [7]. The phospholipase C, resulting in phosphatidylositol turnover subunits β and γ are tightly associated and function as and the release of calcium from intracellular calcium a unit. In the basal state, this subunit complex favours stores. In contrast, the M2Cho receptor is located at the the association with a guanosine diphosphate (GDP)- terminals of cholinergic nerves and is connected to a α α carrying -subunit. During activation of G-proteins, GTP G protein containing 2 as the GTP-binding unit. Stimu- α i i is substituted for GDP at the binding site on the -sub- lation of M2Cho receptors inhibit adenylate cyclase and + ++ unit, a site that is structurally conserved across the en- modulate ion channels for both K and Ca . M2Cho tire superfamily, which results in the dissociation of the receptors serve as autoreceptors and attenuate acetyl- heterotrimer leaving GTP-α and free Βγ-subunits. Whilst choline release from neurons via a negative feedback the βγ-subunit forms a new heterotrimer with another loop. They are believed to limit cholinergic broncho- GDP−α unit, the original α-subunit slowly converts GTP constriction following inhalation of sulphur dioxide to GDP via the action of its intrinsic hydrolytic acti- (SO2). In asthma, there is some evidence that M2Cho vity, eventually returning the G-protein to its inactive receptors may be dysfunctional [8], a finding which is heterotrimeric form. believed to contribute to the pathogenesis in β-blocker- The family of G-protein α-units can be divided accord- induced asthma [9]. Thus, the molecular defect in this ing to their functional or genetic structural relationships. condition may reside within the G-protein-dependent sig- As yet, 21 distinct α−, four β-, and six γ-subunits have nal transduction. been cloned, and this number is sure to rise [7]. Accor- Whilst major scientific interest focuses around the α- ding to their structural homologies, α-subunits can be subunits of G-proteins as the nucleotide binding units subclassified into four subfamilies Gs, Gi, Gq, and G12. with intrinsic GTPase enzyme activity, there is some Functionally, each member of these subfamilies link circumstantial evidence that the βγ-subunit complex receptors to different intracellular signalling cascades in also participates in functions other than stabilization of different cell types, but members of different subfami- the GDP-α-subunit, receptor presentation and mem- lies may also propagate the signal to the same secondary brane anchor for the obligomer. For instance, in mam- messenger system. For instance, whereas hormone or malian brain cells, several types of adenylate cyclase βγ odorant receptors interact with members of the Gs fam- are inhibited by -dimers, albeit solely in the presence ily to stimulate adenylate cyclase, members of the G of G α [7]. This constellation may come about when the i s α family are involved both in inhibition of adenylate dimer would arise by dissociation of Gi following 2- cyclase and activation or inhibition of various ion chan- adrenergic stimulation, whilst G would be liberated β sa nels following exposure to several hormones. In addi- by activation of Gs-linked -adrenergic receptors. More tion, members of the Gq family transduce the actions of importantly, such a situation highlights the possibility a broad spectrum of hormones, neurotransmitters and of crosstalk between different signalling pathways via growth hormones to phospholipase C in various organs. G-proteins. In addition, the fact that βγ-subunits can G-Protein-linked receptors have been identified for bind to several members of α-unit subclasses possibly several classes of drugs and mediators believed to play provide another differential communication pathway a role in respiratory physiology, pathophysiology and between second messenger systems. pharmacology, such as neurotransmitters, neuropeptides, A new family of GTP-binding proteins has recently and the phospholipid-derived mediators. For instance, been identified. The prototype for this family is the there is ample evidence that G-proteins may play a role ras oncoprotein, p21ras, but well over 30 related pro- in regulating the neurovegetative regulation of the air- teins are now known, and that figure is rising. All are way muscle tone, and other functions such as mucus monomeric and around 21–25 kD in size, bind GDP/ secretion and mucociliary clearance. G-proteins have GTP, and have an instrinsic GTPase activity. The ras β β β been identified and cloned for the 1, 2, 3 as well as protein is localized at the inner side of the plasma α α 1 and 2 adrenoreceptors, the five subtypes of mus- membrane, and besides transducing extracellular signals carinic receptors M1 to M5, and for the A1 and A2 ade- into cellular responses it is essentially involved in regu- nosine-receptors. Beta-agonist receptors are coupled to lating normal cell growth [10].
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