Cells Two categories: (i) simple, non-nucleated prokaryotic cells

(ii) complex, nucleated eukaryotic cells.

Cell Degradation • Normally worn-out cells are replaced through division.

• In humans, after 52 divisions cell division comes to a halt: “Hayflick” limit -referred to as senescent.

• Cancer cells, do not degrade in this way. Telomerase, present in large quantities in cancerous cells, rebuilds the telomeres, allowing division to continue indefinitely. (GF)

Is a signalling molecules capable of stimulating cellular growth, proliferation and .

• Usually it is a or a steroid .

• GF is important for regulating a variety of cellular processes- cell differentiation and maturation.

e.g. and

• GF is used in the treatment of hematologic and oncologic diseases and cardiovascular diseases like:

e.g. Neutropenia, myelodysplastic syndrome,leukemias, aplastic anaemia, bone marrow transplantation etc. • GF is sometimes used interchangeably with the term .

• While GF implies a (+) ve effect on cell division, cytokine is a neutral term with respect to whether a molecule affects proliferation.

e.g. some cytokines can be GF: G-CSF and GM-CSF,

but Fas is used as "death" signals causing . Classes of GF Individual GF tend to occur as members of larger families of structurally and evolutionarily related proteins. e.g.

• Bone morphogenetic proteins (BMPs) • Platelet-derived growth factor (PDGF)

• Epidermal growth factor (EGF) • Thrombopoietin (TPO)

• Erythropoietin (EPO) • Transforming growth factor alpha(TGF-α)

• Fibroblast growth factor (FGF) • Transforming growth factor beta (TGF-β)

• Hepatocyte growth factor (HGF) • Vascular endothelial growth factor (VEGF)

• Insulin-like growth factor (IGF)

• Myostatin (GDF-8)

• Nerve growth factor (NGF) and other neurotrophins Growth Factor Range of Specificity Effects

Epidermal growth Broad Stimulates cell proliferation in many factor (EGF) tissues; plays a key role in regulating embryonic development

Required for proliferation of red blood cell Erythropoietin Narrow precursors and their maturation into erythrocytes (red blood cells)

Fibroblast growth Broad Initiates the proliferation of many cell factor (FGF) types; inhibits maturation of many types of stem cells; acts as a signal in embryonic development

Insulin-like Broad Stimulates metabolism of many cell types; growth factor potentiates the effects of other growth factors in promoting cell proliferation

Interleukin-2 Narrow Triggers the division of activated T lymphocytes during the immune response Characteristics of Growth Factors

• Over 50 different GF have been isolated . A specific cell surface “recognizes” each growth factor, its shape fitting that growth factor precisely. When the growth factor binds with its receptor, the receptor reacts by triggering events within the cell.

• Some growth factors, like PDGF and epidermal growth factor (EGF), affect a broad range of cell types, while others affect only specific types. Growth

• Implies development, from the time of birth to the time of maturity and for many species, beyond maturity to eventual senescence or death.

• Increase in size, height and mass resulting from cell multiplication and cell expansion, as well as maturation of tissues.

• Also necessitates programmed cell death, leading to the production of the final body form. Cell division

• Growth is a steady, continuous process, interrupted only briefly at M phase when the nucleus and then the cell divide in two.

• Cell division/, has four major parts

– G1 phase , S phase, G2 phase and M phase Phases in a 1 h Cell cycle

12 h

Phases of the cell cycle other than mitosis, is often termed as 12 h interphase (12 - 36h)

7 h Cell cycle

The cell cycle of eukaryotic cells can be divided into four successive

phases:

(i) M phase (mitosis): (1-2 h)

• The nucleus and the cytoplasm divide

• Cell growth and protein production stop at this stage

• Nuclear division (karyokinesis) and cytoplasmic division (cytokinesis),

accompanied by the formation of a new .

• Physical division of "mother" and "daughter" cells.

• M phase has - prophase, prometaphase, metaphase, anaphase and

telophase leading to cytokinesis. • Focused on the complex and orderly division into two similar daughter cells.

• There is a Metaphase Checkpoint that ensures the cell is ready to complete cell division. (ii) S phase (DNA synthesis):

The DNA in the nucleus replicates (once only) Cell cycle

(iii & iv) Two gap phases, G1 & G2.

– The G1 phase is a critical stage, allowing either commitment to a further round of cell division or withdrawal from the cell cycle

(G0) to embark on a differentiation pathway Cell cycle

• G1 phase is also involved in the control of DNA integrity before the onset of DNA replication.

• Synthesis of enzymes for nucleotide and nucleic acid biosynthesis takes place in this phase Cell cycle

– G2 phase: The cell checks the completion of DNA replication and the genomic integrity before cell division starts. Protein and RNA synthesis that takes place in the S

phase continues to the G2 Phase significant protein synthesis occurs during this phase, mainly involving the production of microtubules, which are required during the process of division, called mitosis. Control of cell cycle

Note the 3 main sites Control of the Cell Cycle

Check points: Quality Control of the Cell Cycle • Eukaryotic cells have gene products that govern the transition from one phase to the other. • These are family of proteins in the cytoplasm e.g. Cyclins • Their levels in the cell rise and fall with the stages of the cell cycle. Check points

• They turn on different cyclin dependent Kinase (CDKs) and Cell division cyclin kinase (CdCK) that phosphorylates substrate essential for progression through the cycle.

• These ensure that all phases of the cell cycle are executed in the correct order. Cyclins and CDKs involved in cell cycle progression Phase Cyclin Kinase Function

G1 Cyclin D CDK 4 & 6 Cell cycle progression- passing G1/S boundary S Cyclin E & A CDK2 Initiation of DNA synthesis in early S phase M Cyclin B & A CDK1 Transition from G2 to M

Check points

• CDK levels in the cell remain fairly stable, but each must bind the appropriate cyclin (whose levels fluctuate) in order to be activated.

• CDK act by adding phosphate groups to a variety of protein substrates that control processes in the cell cycle.

Cyclin • Cell cycle is controlled by various proteins regulated by the Genes catalytic and target (Cyclin) unit

• The cyclin units appear transiently at various sites and disintegrate after passing. G1 Start S G2 M G1

CDK S PHASE CYCLIN

MITOTIC CYCLIN G1 CYCLIN G1 Start S G2 M G1

Rb, P53, p16 CDK S PHASE CYCLIN

TYROSINE PHOSPHORYLATION

MITOTIC CYCLIN G1 CYCLIN G0- quiescent • • Anaphase-promoting complex (APC)

– triggers the events leading to destruction of the cohesins thus allowing the sister chromatids to separate;

– degrades the mitotic cyclin B promoting exit from mitosis Check points & mechanism of DNA repair:

Cell has several systems to interrupt the cell cycle if something goes wrong.

(i) DNA damage checkpoints. These sense DNA damage.

- G1 checkpoint: Damage to DNA inhibits the action of CDK thus stopping the progression of the cell cycle until the damage can be repaired Checkpoints & mechanism of DNA repair

- The common repairing mechanisms are

mismatch repair, base excision repair, nucleotide excision repair, double strand break repair

- If the damage is so severe that it cannot be repaired, the cell self-destructs by apoptosis.

(ii) Check point for successful replication of DNA

is present at S phase Checkpoints & mechanism of DNA repair

(iii) Spindle checkpoints.

– detect any failure of spindle fibers that attach to kinetochores and arrest the cell in metaphase until all the kinetochores are attached correctly (M checkpoint )

– detect improper alignment of the spindle itself and block cytokinesis

– trigger apoptosis if the damage is irreparable. Cancer and Oncogenes

• All the checkpoints examined require the services of a complex of proteins.

• Mutations in the genes encoding some of these have been associated with cancer: oncogenes

• Checkpoint failures allow the cell to continuously divide despite damage to its integrity?? “developing cancer” Growth Factors and Cancer

• Two main genes – Tumor-suppressor Genes – Proto-oncogenes. Proto-oncogenes.

• PDGF and many other growth factors, stimulate cell division by triggering G1 checkpoint by aiding the formation of cyclins

• Genes that normally stimulate cell division are sometimes called proto-oncogenes because mutations that cause them to be overexpressed or hyperactive convert them into oncogenes (Greek onco, “cancer”).

• Even a single mutation (creating a heterozygote) can lead to cancer if the other cancer preventing genes are non-functional.

– E.g. myc, fos, and jun, Tumour-suppressor Genes

• They block passage through the G1 checkpoint by preventing cyclins from binding to Cdk, thus inhibiting cell division.

• When mutated, they can also lead to unrestrained cell division, but only if both copies of the gene are mutant.

• Hence, these cancer-causing mutations are recessive. • Rate of cancer cell growth:

The proportion of cancer cells growing and making new cells varies. If more than 6 -10% of the cells are making new cells, the rate of growth is considered unfavourably high.

• S-phase fraction and Ki-67 tests may be required to measure rates of cell growth but treatment decisions are made on other more reliable cancer characteristics. • "Grade" of cancer cell growth: Patterns of cell growth are rated on a scale from 1 to 3 (also referred to as low, medium, and high instead of 1, 2 or 3).

– Calm, well-organized growth with few cells reproducing is considered grade 1. Disorganized, irregular growth patterns in which many cells are in the process of making new cells is called grade 3. The lower the grade, the more favourable the expected outcome.

• Dead cells within the tumour: necrosis (or dead tumour cells) is one of several signs of excessive tumour growth. It means that a tumour is growing so fast that some tumour cells die. Cell growth disorders

• A series of growth disorders can occur at the cellular level and these underpins much of the subsequent course in cancer,

– group of cells display uncontrolled growth and division beyond the normal limits,

– invasion (intrusion on and destruction of adjacent tissues),

– metastasis (spread to other locations in the body via lymph or blood). LABILE CELL

– cells that multiply constantly throughout . They spend little or no time in the quiescent G0 phase of the cell cycle, but regularly performs cell division .

– E.g. skin cells, cells in the gastrointestinal tract and blood cells in the bone marrow.

– It is mainly not the segments of the cell cycle that go faster (i.e., but rather a short or absent G0 phase.

– higher risk of becoming malignant and develop cancer. Note:

• Cytotoxic drugs inhibit the proliferation of dividing cells, with the malignant cells as the desired target.

• This has adverse effect against the cells normally dividing in the body, and thus impairing normal body function of skin, GI tract and bone marrow. Positive Growth Regulators: Promoting Cell Division

• Rous discovered that he could grind up sarcomas and extract an unidentified substance that, when injected into healthy chickens, caused cancer.

– not because it was carefully filtered,

• so something much smaller that could pass through the filter.

– first tumor virus, which was named the Rous sarcoma virus in honour of its discovery and the type of tumor from which it was obtained. Negative Growth Regulators: Inhibiting Cell Division

• For a cell to divide, proto-oncogenes must be activated to promote the process, and tumor suppressor genes must be inactivated to allow the process to happen. Example p53 • This is known as tumour suppressor gene

• The p53 protein senses DNA damage and can halt progression of the cell cycle in G1 (by blocking the activity of CDK2).

• If both copies (as mutations in p53 are recessive) of the p53 gene is mutated the above mechanism fails

• The p53 protein is also a key player in apoptosis, forcing "bad" cells to commit suicide. p53

Note : • More than half of all human cancers do, in fact, harbor p53 mutations and have non functioning p53 protein. Note: • Inappropriate division of a clone of cells at the inappropriate time lead to hypertrophy/ hyperplasia/ neoplasia • Cell cycle is controlled by Genes, which secretes growth and inhibitory stimuli with contact inhibition (cell to cell) • Any errors in the entry or exit in the cell cycle could cause a tumour Control at different stages of the cell cycle G0Phase • A cell may leave the cell cycle, temporarily or permanently.

i.e. it exits the cycle at G1 and enters a

stage designated G0

• G0 cell is often called "quiescent“-not proliferating. • Many G0 cells are in resting stage and does not divide; but they carry out their functions in the organism. e.g. secretion, attacking pathogens.

• G0 to G1 requires growth factor G0Phase

• If G0 cells are terminally differentiated: they will never reenter the cell cycle but instead will carry out their function in the organism until they die.

e.g. Terminally differentiated cannot undergo cell-cycle re-entry.

Note: Epithelial cells divide more than twice a day,

Liver cells divide only once every year or two,

spending most of their time in G0 phase • Most of the lymphocytes in human blood

are in G0. However, with proper stimulation, such as encountering the appropriate antigen, they can be stimulated to reenter the cell cycle and proceed on to new cycle. G0Phase

• G0 represents not simply the absence of signals for mitosis but an active repression of the genes needed for mitosis.

• Cancer cells cannot enter G0 and are destined to repeat the cell cycle indefinitely.

Note: Normal cell exist in G0 more than cancer cells Sexual reproduction

• Notice that when meiosis starts, the two copies of sister chromatids number 2 are adjacent to each other. During this time, there can be genetic recombination events. Parts of the chromosome 2 DNA gained from one parent will swap over to the chromosome 2 DNA molecule that received from the other parent

• It is these new combinations of parts of chromosomes that provide the major advantage for sexually reproducing organisms by allowing for new combinations of genes and more efficient evolution. Specialization & Communication of Cells

• Larger the organism, the greater the need for different types of cells with different structures and functions. • Specialized types of cells that are especially suited to specific duties ensures that all the processes necessary for the life of the organism are carried out quickly and efficiently. • Specialized cells fulfil a wide variety of needs in multicellular organisms. • Secondly principle that applies to all multicellular organisms is that of cell communication. • With the diversity of cells & tissues found in a , they must have some way to coordinate their activities, and must be able to communicate with one another. Cell – cell Interactions

• Cell-cell commu•nication allows an individual cell to determine its position in the body, to adjust its metabolism to suit its particular func•tion, and to grow and divide at the proper time, in concert with its neighbours • Cells must be ready to respond to essential signals in their environment by releasing small signalling molecules, which are received by target cells. Ligand-Receptor Interactions

• Molecules that activate (or, in some cases, inhibit) receptors : – hormones – – cytokines – growth factors but all of these are called receptor ligands Types of Signalling

• distant locations in a multicellular organism e.g. endocrine signalling by hormones;

• nearby cells

e.g. paracrine stimulation by cytokines;

• secreted by themselves ( = autocrine stimulation)

• also respond to molecules on the surface of adjacent cells

e.g. producing contact inhibition

• Some cell-to-cell communication requires direct cell-cell contact.

• Some cells can form gap junctions that connect their cytoplasm to the cytoplasm of adjacent cells.

– E.g. gap junctions between adjacent cells allows for action potential propagation from the cardiac pacemaker region of the heart to spread and co-ordinately cause contraction of the heart.

• Notch signalling mechanism -juxtacrine signalling (also known as contact dependent signalling) in which two adjacent cells must make physical contact in order to communicate. Synaptic Signalling • Neurons communicate with distant cells but they are not

carried to the responding cells by the circula•tory

system.

• The site at which the neurotransmitters are released is

called a . While paracrine signals cross

interstitial fluid between cells, neurotransmitters cross

the synapse and persist only. • Adjacent cells can signal others by direct contact, while nearby cells that are not touching can communi•cate by the release of paracrine signals. Two sys•tems mediate communica•tion over much longer dis•tances: the releases hormones that are carried by circulating body fluids to distant cells; in ani•mals, the nervous system se•cretes neurotransmitters from long cellular extensions that end close to the re•sponding cells. • Signalling molecules may trigger: an immediate change in the metabolism of the cell (e.g., increased glycogenolysis when a liver cell detects adrenaline); • an immediate change in the electrical charge across the plasma membrane (e.g., the source of action potentials); • a change in the gene expression — transcription — within the nucleus. (These responses take more time.) Pathways by which a Chemical Signal Turns on Gene Expression

• Two categories of signalling molecules (i) steroids and nitric oxide diffuse into the cell & bind internal receptors. (ii) Proteins & peptides bind to receptors displayed at the surface of the cell. – These are transmembrane proteins: extracellular portion binds the ligand & intracellular portion activates proteins in the cytosol that eventually regulate gene transcription in the nucleus. Receptors

• When a signalling molecule reaches a target cell, cell have a specific means of receiving it and acting on its message.

• This responsibility is carried out by a class of proteins called receptors. • Cell surface receptors are that are embedded or otherwise attached to the cell's plasma membrane and have a binding site for specific ligands (cytokines, hormones, growth factors, neurotransmitters, adhesion molecules, etc.), exposed to the extracellular environment. • Ligand binding to a generally leads to a biological signal Protein binding diagram

• The steroid–receptor complex acts on specific genes, activating the production of the proteins they encode

e.g. genes that are activated by progesterone in target cells in the uterus, for example, encode proteins that are necessary for the proper growth of the uterine lining. • How does nitric oxide serve as a signal to lower blood pressure?

• State five molecules that act intra-cellularly to alter gene expression

Cortisol, oestrogen, progesterone, vitamin D &

thyroid hor•mone

• State five molecules that act on the surface receptors.

Insulin, glucagon, GH, adrenalin, PDGF • How Cell Surface Receptors Initiate Changes inside the Cell • specific chemical reactions are triggered inside a cell by an external signal, causing a series of protein activations known as a signal cascade- .

• Cell surface receptors transduce ligand signals by a variety of mechanisms such as receptor clustering, activation of a hidden enzymatic activity, opening of ion channels, etc 1. Cell surface receptors trigger signal cascades by binding external signalling molecules, changing shape, and then activating or inactivating specific proteins inside the cell. e.g. G proteins and enzyme-catalyzed phosphorylation or specific nucleotides such as GTP. 2. biological signal that is propagated towards the cell interior, result in proliferation, differentiation, apoptosis, degranulation, etc. Cell surface receptors

• Chemically gated ion channels are multipass transmembrane proteins that form a pore in the cell mem•brane. This pore is opened or closed by chemical signals.

• Enzymic receptors are single-pass transmembrane proteins that bind the signal on the extracellular surface and contain a catalytic region on their cytoplasmic portion that initiates enzymatic activity inside the cell.

• -linked receptors bind to the signal outside the cell and to G proteins inside the cell. (~ The G protein-linked receptor is a seven-pass transmembrane protein.

•

Chemically Gated Ion Channels

• Integral protein has a pore that con•nects the extracellular fluid with the cytoplasm where ions pass through it, so the protein func•tions as an . • Ion chan•nels open or close when molecules bind to the protein. Enzyme Receptors

• Binding of a signal molecule to the receptor activates the enzyme. In almost all cases, these enzymes are protein kinases, that add phosphate groups. • Each receptor is a single-pass transmembrane protein (the amino acid chain passes through the plasma membrane only once); the portion that binds to the signal molecule is outside the cell, and the por•tion that carries out the enzyme activity is exposed to the cytoplasm. G Protein-linked Receptors

• Receptors in this category are members of the largest superfamily of surface receptors. Each is a seven-pass trans- • signal molecule binds the receptor protein changes shape, causing the associated G protein to bind GTP and become activated. • The activated G protein then diffuses away from the recep•tor, starting a chain of events that ultimately brings about the response of the cell. G Protein-linked Receptors

How Cell Surface Receptors Initiate Changes inside the Cell • Cell response to an external signal is known as signal transduction. • The signal is transferred, or transduced, from the outside to the inside of the cell. • It triggers a series of chemical reactions that activate proteins inside the cell-referred to as a signal cascade. • Each type of signalling molecule binds to a specific type of receptor on the cell surface, causing a specific signal cascade that activates specific proteins. G proteins

• Bind to and are regulated by nucleotides with a guanine base: • When a G protein is bound to GDP, it is inactive. When an inactive G protein binds to an activated receptor, however, it is able to release GDP and bind GTP from the cytosol. G-Protein-Coupled Receptors (GPCRs)

• Many ligands that alter gene expression by binding GPCRs: – protein and peptide hormones e.g. TSH, ACTH. – Serotonin and GABA (which affect gene expression in addition to their role as neurotransmitters) Turning GPCRs Off

• A cell must also be able to stop responding to a signal. Several mechanisms cooperate in turning GPCRs off. When activated, the Gα subunit of the G protein swaps GDP for GTP. However, the Gα subunit is a GTPase and quickly converts GTP back to GDP restoring the inactive state of the receptor. The receptor itself is phosphorylated by a kinase, which not only reduces the ability of the receptor to respond to its ligand but recruits a protein; β- arrestin, which further desensitizes the receptor, and triggers the breakdown of the second messengers of the GPCRs: cAMP for some GPCRs, DAG for others. G protein diagram Cytokine Receptors

• Most of these fall into one or the other of two major families:

1. Receptor Tyrosine Kinases (RTKs) and

2. Receptors that trigger a JAK-STAT pathway.

• Receptor Tyrosine Kinases (RTKs)

The receptors are transmembrane proteins that span the plasma membrane just once.

• Some ligands that trigger RTKs:

e.g. Insulin, Vascular Endothelial Growth Factor , PDGF,EGF, FGF, M-CSF Amplifying and Combining Signals inside the Cell

• The activation of many proteins at each step of a signal cascade greatly amplifies the original signal. Signal cascades that modify existing proteins inside the cell occur in a matter of seconds; those that activate genes to produce new proteins can take several hours. Certain proteins can participate in multiple signal cascades, allowing the cell to integrate different external signals. Defeating Deadly Bacterial Toxins • Knowing how signal cascades are affected by deadly bacterial toxins will allow biologists to design drug therapies that specifically block their effects. Signal Transducers Signal Amplification & Adhering Cell

• Both enzyme-linked and G protein-linked receptors receive signals at the surface of the cell

• signals are relayed to the cytoplasm or the nucleus by second messengers, which influence the activity of one or more enzymes or genes and so alter the behavior of the cell.

• They uses a chain of other protein messengers to amplify the sig•nal as it is being relayed to the nucleus. Amplifying & Combining Signals inside the Cell

• Imagine a situation in which a single hormone or growth factor binds to a cell surface receptor. If this binding event resulted in the activation or phosphorylation of only one protein inside the cell, many binding events at the cell surface would be needed in order to bring about any significant change in the cell. To avoid such an inefficient arrangement, most signal cascades greatly amplify the initial signal. A signal cascade is like a huge avalanche on a snow-covered mountain that is started by one small icicle falling off a tree at the top of the slope. •

Communicating Cell Junctions

• cell recognition proteins allow specific kinds of cells to bind to each other to make direct physical contact called . • three types of cell junctions – tight junctions – desmosomes – gap junctions • Cell junctions can be classified into three functional groups: • 1. Occluding junctions seal cells together in an epithelium in a way that prevents even small molecules from leaking from one side of the sheet to the other. • E.g. tight junctions (vertebrates only) & septate junctions ( mainly) 2. Anchoring junctions mechanically attach cells (and their cytoskeletons) to their neighbors or to the . • E.g cell-cell junctions (adherens junctions) • cell-matrix junctions (focal adhesions) 3. Communicating junctions mediate the passage of chemical or electrical signals from one interacting cell to its partner. • The major kinds of intercellular junctions within each group are listed in Table 16.1. We discuss each of them in turn, except for chemical synapses, which are formed exclusively by nerve cells and are considered in other course unit. • gap junctions, chemical synapses, plasmodesmata ( only) Cell recognition and adhesion involve proteins at the cell surface

• Membrane (80% sugar) that is partly embedded in the plasm is responsible for cell recognition. • protein has specific chemical groups exposed on its surface where they can interact with other substances, including other proteins. • Two types. (i) homotypic: The same molecule sticks out of both cells, & exposed surfaces bind to each other. (ii)heterotypic: binding of cells with different proteins. e.g. Male and female cells recognize each other Gap Junctions

• Allow small molecules to pass directly from Cell to Cell • In EM appears as 2 adjacent cells separated by a narrow gap (2–4 nm)

• The gap is spanned by channel-forming proteins () to form

(connexons)

• allow inorganic ions and other small water-soluble molecules to pass

directly from the cytoplasm of one cell to the other,

• couple the cells both electrically and metabolically to share small

molecules. But not macromolecules (e.g proteins, NA) Function of gap junctions

• In nerve cells - electrically coupled, allowing action potentials to spread rapidly from cell to cell, without the delay that occurs at chemical synapses. e.g when speed and reliability are crucial-in certain escape responses in fish and insects. • Synchronizes the contractions – heart muscle cells in heart – smooth muscle cells involved in the peristaltic movements of the intestine. • Gap junctions also occur in tissues that do not contain

electrically excitable cells.

– e.g. Release of noradrenaline from sympathetic nerve endings in

response to a fall in blood glucose levels stimulates hepatocytes

to increase glycogen breakdown and release glucose into the

blood.

• Note: Not all the hepatocytes are innervated by

sympathetic nerves, when glucose level fall, however by

means of the gap junctions that connect hepatocytes, the

signal is transmitted . • The normal development of ovarian follicles also depends on gap-junction-mediated.

• Cell coupling via gap junctions also seems to be important in embryogenesis. Permeability of Gap Junctions Can Be Regulated

• Like ion channels, individual gap-junction channels do not remain continuously open; instead, they flip between open and closed states. • The permeability of gap junctions is rapid (within seconds) and reversibly reduced by decrease cytosolic pH (not known) / increase cytosolic [free Ca2+ ] to very high levels. • They are dynamic structures that can undergo a reversible conformational change that closes the channel in response to changes in the cell. • Purpose of Ca2+ control: when a cell is damaged, its plasma membrane become leaky, fluid, such as Ca2+ and Na+, and then move into the cell, and valuable metabolites leak out.

• If the cell were to remain coupled to its healthy neighbours, the large influx of Ca2+ into the damaged cell causes its gap-junction channels to close immediately, effectively isolating the cell and preventing the damage from spreading to other cells. • Gap-junction communication can also be regulated by extracellular signals.

E.g. neurotransmitter dopamine, for example, reduces gap- junction communication between a class of neurons in the retina in response to an increase in light intensity. This reduction in gap-junction permeability helps the retina switch from using rod photoreceptors, which are good detectors of low light, to cone photoreceptors, which detect color and fine detail in bright light.