cell signalling Higher level, Post graduate
- 1. CELL SIGNALLING B.Sc (H) ZOOLOGY I SEM
- 2. Cell Signalling • Cells adjacent to one another frequently communicate through cell-cell contact • Survival depends on an elaborate intercellular communication network that coordinates growth, differentiation and cell death
- 3. General Principles of Cell Signalling Each cell responds to a complex profile of signaling molecules
- 4. Concept of Cell Signaling The process in which cells sense the extracellular stimuli through membranous or intracellular receptors, transduce the signals via intracellular molecules, and thus regulate the biological function of the cells
- 5. Cell-to-cell communication by extracellular signaling usually involves six steps • (1) synthesis of the signaling molecule by the signaling cell • (2) release of the signaling molecule by the signaling cell • (3) transport of the signal to the target cell • (4) detection of the signal by a specific receptor protein – receptor-ligand specificity • (5) a change in cellular metabolism, function, or development = cellular response – triggered by the receptor-ligand complex – specific to the ligand- receptor complex • (6) removal of the signal, which usually terminates the cellular response – degredation of ligand
- 7. Mode of Signalling – Endocrine • hormones – Paracrine • Neurotransmitters • Growth factors – Autocrine • Growth factors (cultured cells, tumor cells Contact-dependent signals: signals are not released but affect other cells in contact through protein-protein interactions
- 8. 8 • Mode of cell signaling Cell Signaling
- 9. Signaling molecules Signaling molecules come in many chemical forms: • Proteins: insulin, glucagon • Steroids et al.: testosterone, estradiol, cortisol • Amines: thyroxine, catecholamines, acetylcholine • Gases: nitric oxide, carbon mono oxide Typically released from one cell and recognized by another cell
- 10. Receptors Proteins that bind signals and initiate a signaling cascade Cell membrane receptors -integral membrane proteins that bind an extracellular signal and start a signal cascade Intracellular receptors -nuclear hormone receptors
- 11. Classification of receptors • Intracellular receptors (for lipid soluble messengers) • function in the nucleus as transcription factors to alter the rate of transcription of particular genes. • Cell membrane receptors (for lipid insoluble messengers) • Receptors function as ion channels • receptors function as enzymes or are closely associated with cytoplasmic enzymes • receptors that activate G proteins which in turn act upon effector proteins, either ion channels or enzymes, in the plasma membrane.
- 12. General terms • First messengers: Messengers bind to specific receptor. E.g. Hormone • Second messengers: non-protein substances enter the cytoplasm and diffuse throughout the cell and transmit signals e.g c-AMP • Protein kinase: any enzymes that phosphorylates other proteins by transferring phosphate group
- 13. The primary pathways of cell signalling G-protein-mediated pathway Adenylate cyclase mediated pathway Phospholipase mediated pathway Non-G-protein-mediated pathway Receptor tyrosine kinase mediated pathway Receptor serine/threonine kinase mediated pathway Receptor guanilate cyclase mediated pathway Intracellular (unclear) receptor mediated pathway
- 14. G-protein-mediated pathway Adenylate cyclase mediated pathway Phospholipase mediated pathway
- 15. G Protein-Coupled Receptors • A very large family of receptors coupled to trimeric G proteins • Activate or inhibit adenylate cyclase • All have seven membrane spanning region • Ligands include: – Hormones, neurotransmitters, light activated receptors (rhodopsins), thousands of odorant receptors
- 16. Structure: Embedded in the plasma membrane 7 transmembrane -helices Guanine nucleotide binding proteins which act as a Transducer between a receptor & an effector Discovered by Alfred Gilman & Martin Rodbell in 1990 Protein and peptide agonists bind to N terminus G-proteins bind either to second and third cytoplasmic loop -Adrenergic Receptor PDB 2RH1 Lysozyme insert ligand
- 18. A generalized picture of the G-Protein-linked or G-Protein-coupled receptor
- 19. G Proteins: αβγ = alpha, beta, gamma subunits • G Proteins are molecular switches whose on or off state depends on whether GDP or GTP is bound to the α subunit. (A smaller monomeric G protein is called Ras and is associated with tyrosine kinase receptors that mediate cell growth and movement.) • The G protein moves away from the receptor when GTP binds, and α dissociates from βγ (which are permanently linked). Both pieces of the G protein can interact with messenger systems, although in many cases the βγ subunit’s roles are not known. • When Gα locates its target, the process of activating the enzyme causes hydrolysis of GTP, leaving GDP, and then the αβγ subunits must reunite. This terminates the active response to the ligand.
- 20. G Proteins: One possible target of G protein signal cascades is adenyl cyclase, the enzyme that catalyzes the formation of cyclic AMP
- 27. Different G-protein families are coupled to different 2nd messenger pathways • Gi inhibits the operation of adenyl cyclase • Gs stimulates the operation of the same enzyme • Gq stimulates phospholipase C, resulting in formation of inositol trisphosphate (IP3) and diacylglycerol (DAG) from a common membrane phospholipid, inositol bisphosphate.
- 29. Second messengers • produced by the activation of GPCRs • Hormone stimulation of Gs protein-coupled receptors leads to activation of adenylyl cyclase and synthesis of the second messenger cAMP – most commonly studied second messenger • cAMP has a wide variety of effects depending on the cell type and the downstream PKAs and other kinases – in adipocytes, increased cAMP activates a PKA that stimulates production of fatty acids – in ovarian cells another PKA will respond to cAMP by increase estrogen synthesis • second messenger systems allow for amplification of an extracellular signal – one epinephine molecule can bind one GPCR – this can result in the synthesis of multiple cAMP molecules which can go on to activate and amplified number of PKAs • a blood level as low as 10-10 M epinephrine can raise blood glucose levels by 50%
- 30. Second Messengers….. Molecules that relay signals from receptors on the cell surface to target molecules inside the cell i.e. cytoplasm or nucleus Relay the signals of hormones like epinephrine, growth factors & others; causing some kind of change in the activity of the cell The term was coined upon the discovery of these substances in order to distinguish them from hormones & other molecules that function outside the cell as “first messengers” in the transmission of biological information
- 31. Earl Wilbur Sutherland Jr. discovered second messengers won the 1971 Nobel Prize in Medicine He saw that epinephrine would stimulate the liver to convert glycogen to glucose in liver cells, but epinephrine alone would not convert glycogen to glucose He found that epinephrine had to trigger a second messenger, cyclic AMP for the liver to convert glycogen to glucose
- 32. Second messenger amplification increases the ligand’s effect, but this takes time…
- 33. cAMP
- 34. Adenyl Cyclase- cAMP Pathway
- 36. cAMP has following major targets: 1.cAMP dependent Protein Kinase A (PKA) 2.CREB (cAMP responsive element binding protein)
- 37. cAMP vs PKA
- 38. Protein Phosphorylation→ most common form of post- translational modification in nature Protein function altered by addition of a negatively charged phosphate group to a Ser, Thr or Tyr residue by Protein Kinase: • Binding properties • Enzymatic activity if a catalytic protein P ro te in O H + A T P P ro te in O P O O O + A D P P i H 2 O P rotein K inase P rotein P hosphatase
- 39. Cell Type Stimulators Inhibitors Effects Hepatocyte Epinephrine(β), Glucagon Produce glucose: stimulate glycogenolysis, inhibit glycogenesis, stimulate gluconeogenesis, inhibit glycolysis. Skeletal - Myocyte Epinephrine(β) Produce glucose: stimulate glycogenolysis, inhibit glycogenesis, stimulate glycolysis Cardio- myocyte Norepinephrine (β) Sequester Ca2+ in sarcoplasmic reticulum, Phosphorylates phospholamban Actions of Protein kinase A
- 40. Smooth muscle myocyte β2 agonist (β2) Histamine (H2) Prostacyclin Prostaglandin D2/E2 Muscarinic (M2) Vasodilation Adipocyte Epinephrine (β), Glucagon Enhance lipolysis Neurons in Nucleus accumens Dopamine (D3) Activate reward system Principal Cells Vasopressin (V2) Synthesis & Exocytosis of Aquaporin 2 Juxtaglomerular Cells Adrenergic (β1) Dopamine(D1) Glucagon Renin secretion
- 41. O UT IN G protein adenylate cyclase cAMP adrenalin MEM BRANE adrenalin receptor first messenger receptor transducer amplifier second messenger
- 42. Phospholipase C mediated pathway G-protein-mediated pathway
Editor's Notes
- #36: GEF- Guanine exchange factors
- #39: The protein is an inhibitor of cardiac muscle sarcoplasmic reticulum Ca++-ATPase (SERCA) in the unphosphorylated state, but inhibition is relieved upon phosphorylation of the protein. The subsequent activation of the Ca++ pump leads to shorter intervals between contractions, thereby contributing to the inotropic response
- #40: stimulate lipase[