Cell Communication.docx

Cell Communication  general concepts o endocrine / paracrine / neural o receptor protein, target cell, signal transduction o fast vs slow responses o intracellular vs cell-surface receptors  intracellular receptors o mechanism of action, nuclear receptor family of transcription factors  cell-surface receptors o second messengers o molecular switches  phosphorylation, kinases / phosphatases, phosphorylation cascades  GTP-binding proteins (G-proteins) o 3 classes of cell-surface receptors: 1) ion channel-linked 2) G-protein coupled receptors (GPCRs), a, b, g subunits 3) enzyme-linked receptors o a closer look at GPCRs and enzyme-linked receptors:  GPCRs - downstream targets of activated G-proteins 1) ion channels e.g. Ach-R in cardiac muscle, K channels 2) activate enzymes associated with cytosolic face of membrane 2a) adenylyl cyclase  cAMP  PKA 2b) phospholipase C  IP3+ DAG  Ca  PKC 2+ Ca as an intracellular signal; calmodulin, CaM-kinases  enzyme-linked receptors  receptor tyrosine kinases (RTKs) and Ra o RTK, Ras and cancer Why Cells Communicate  single celled organisms o ‘social life’ – yeast mating  multicellular organisms, coordination of o development (from single cell to trillions of cells) o growth o coordinating ‘whole body’ growth and development with environment o day to day physiology Long vs. Short Range Communication  Endocrine o if you have the receptor, you can hear the signal (hormone carried in blood stream)  Paracrine o if you have the receptor and are in close distance, you can hear the signal (local mediator)  Neuronal o Long distances (down axon to neurotransmitter, across synapse)  Contact-Dependant o Cells touch (membrane bound signal molecule) Signalling Pathways  Pathway o signalling molecule synthesized and released by signalling cell o signal molecule travels to target cell o signal binds to receptor protein on / in target cell o signal transduction o change in target cell behaviour  cell shape, movement  metabolism, secretion  gene expression  Same signal can cause different responses, depending on the target cell. o acetyl choline (signal)  Heart muscle cell  decreased rate and force of contraction  Salivary gland cell  Secretion  Skeletal muscle cell  contraction  Fast/Slow Responses o Slow (minutes to hours)  When signal binds to receptor, it can affect nucleus, which will result in RNA being transcribed which will alter protein synthesis, causing altered cytoplasmic machinery and eventually altered cell behaviour o Fast (seconds to minutes)  When signal binds to cell-surface receptor protein, it can follow the intracellular signalling pathway, cause altered protein function, altered cytoplasmic machinery and eventually altered cell behaviour Receptors  Cell Surface Receptor (on plasma membrane) o Signal molecule is hydrophilic  Bind to receptor protein on plasma membrane  Relay, transduction and amplification of message through secondary messengers  small (non-protein) molecules that relay signals from cell surface receptors to target molecules within cell (through cytoplasm)  integration  distribution of message to effector proteins (metabolic enzyme, cytoskeletal enzyme, transcription regulator)  causes cell response o Molecular Switches (intracellular signal molecules)  turning off a signal is just as important as turning it on  Turn on: protein kinase (transfer phosphate group from ATP to phosphate)  Turning off: protein phosphatase (remove phosphate group)  each activation step in a cascade needs to be inactivated  Signalling by Protein Phosphorylation  activity of a protein regulated by phosphorylation depends on balance between activities of its kinases and phosphatases  many proteins regulated by phosphorylation are themselves kinases o phosphorylation cascades o Signal molecule binds to receptor, activating a relay molecule, which activates protein kinase 1, which changes ATP to ADP, activating protein kinase 2, etc.. o Eventually activated protein, which causes cellular response  phosphorylation of proteins is not random o serine / threonine kinases  phosphorylate hydroxyl groups of serine and threonine in particular sequences o tyrosine kinases  phosphorylate hydroxyl groups of tyrosine  Signalling by GTP binding proteins  switch from active to inactive state depending on whether they are bound to GTP or GDP  once bound to GTP, they have intrinsic GTPase activity, and turn themselves off by hydrolyzing GTP to GDP (lose phosphate group)  dissociation of GDP and replacement with a ‘fresh’ GTP is often in response to a signal  active form will activate downstream steps in the cascade  Turning on: GTP binding  Turning off: GTP hydrolysis o Classes of Cell-surface Receptors  Ion Channel-Coupled Receptors  binding of ligand opens (or closes) ion channel  flow of ions (inward or outward) changes voltage across membrane o ‘neurotransmitter-gated’ channels o ‘ligand-gated’ channels  G-protein Coupled Receptors (GPCRs)  binding of ligand activates trimeric GTP-binding proteins (G-proteins), which in turn activate an enzyme or ion channel in membrane to set off cascade  about half of all known drugs act via GPCRs  Activation of a G-Protein Coupled Receptor o Signal molecule binds to G-coupled receptor o Activation of trimeric G-protein o Gα subunit exchanges GDP for GTP o Separation of α and βγ subunits  Self-Inactivating o α subunit activates target protein o Hydrolysis of GTP by α subunit inactivates itself and causes it to dissociate from target protein o Inactive α subunit reassembles itself with βγ complex to re-form inactive G- protein  Length of a G-protein signal o which is as long as the Gα-subunit is bound to GTP (α and βγ subunits are free) o normally it hydrolyzes GTP  GDP within a few seconds and re-associates with βγ-subunits  Consequences of disrupted the Gα-subunit function o cholera  bacterium produces toxin that enters intestinal cells and alters Gα subunit