Generalized Signaling Pathway

1. Generalized Signaling Pathway • Release signal molecule from signaling cell • Bind and activate receptor in target cell • Activate intracellular signaling pathway • Alter effector protein(s) to change cell behavior

2. Forms of Cell Signaling • Paracrine: signal molecule secreted and acts locally • Contact-dependent: signal molecule remains bound to signaling cell

3. Endocrine: signal molecule secreted into bloodstream • Synaptic: neurotransmitter acts at synaptic contacts

4. Autocrine: signaling cell signals to cells of the same type and to itself • Gap junctions: allow passage of small signaling molecules

5. Speed of Signaling Response • Alterations to proteins already present occurs rapidly • New gene expression and protein synthesis occurs slowly

6. Response to Combinations of Signals • Cell responds to set of signals based on its receptors • Signals act in different combinations to effect behavior of cell

7. Different Responses to Same Signal • Different receptors • Same receptor coupled to different intracellular signaling machinery

8. Types of Cell Surface Receptors • Ion-channel-linked: neurotransmitter controls opening of ion channel • G-protein-linked: intracellular signaling mediated by trimeric GTP-binding protein

9. Enzyme-linked: function as enzyme or directly associated with enzyme

10. Relaying Signal Within Cell • Chain of molecular interactions between receptor and target protein • Small intracellular mediators (second messengers) • Intracellular signaling proteins

11. Some Ways for Intracellular Signaling Proteins to Relay Signal • Generate small intracellular mediators • Induced to interact with other signaling proteins • Act as molecular switches • Phosphorylation and de-phosphorylation • GTP binding and hydrolysis • Other mechanisms

12. Induced Interactions of Signaling Proteins • Intracellular signaling propagated by inducing protein interactions • Interaction domains in signaling proteins • Adaptor proteins

13. Molecular Switches Regulated by Phosphorylation • Covalent attachment of phosphate by kinases, removal by phosphatases • Different signaling proteins can be activated by phosphorylation or de-phosphorylation

14. Molecular Switches Regulated by GTP Binding • Active when GTP-bound; inactive when GDP-bound • GTP binding by guanine nucleotide exchange factors, hydrolysis by GTPase activating proteins

15. Target Cell Desensitization • Decreased response to signal at particular level following prolonged exposure • Negative feedback with short delay • Signaling machinery modified, less responsive

16. Receptor-Mediated Activation of G Proteins • Trimeric G protein (abg) • Inactive when GDP bound to a • Activation of receptor causes dissociation of a from bg and exchange of GDP for GTP • Activated a mediates signaling

17. Turning off G proteins • Stimulation of GTPase activity of a by interaction with target signaling protein or specific RGS

18. Signaling by cAMP • Receptors linked to Gs activate adenylyl cyclase • Converts ATP to cAMP, which acts a small intracellular mediator • Activates cAMP-dependent protein kinase (PKA)

19. Cholera & Pertussis Toxins • Gsa subunit activates adenylyl cyclase • Cholera toxin: ADP ribosylation of Gs (a); cannot hydrolyze GTP • Gia subunit inhibits adenylyl cyclase • Pertussis toxin: ADP ribosylation of Gi (a); prevents binding to receptors

20. Signaling by Inositol Phospholipids • Receptors linked to Gq activate phospholipase C-b • PI(4,5)P2 cleaved to diacylglycerol and IP3 • IP3 opens channels in ER causing influx of Ca2+ into cytosol • Diacylglycerol activates protein kinase C (PKC)

21. Cleavage by Phospholipase C-b

22. Receptor Desensitization • G protein-linked receptor kinases phosphorylate activated receptor • Arrestin binding prevents G protein interaction, promotes endocytosis

23. Ca2+ as Small Intracellular Mediator • Involved in PKC activation • Binds calmodulin to regulate other proteins • Activates CaM-kinases • Activates calcineurin, a phosphatase, whose substrates include NFAT

24. Calcineurin-NFAT Signaling in T Cells • Calcineurin dephosphorylates, activates NFAT • NFAT turns on genes for T cell activation • Cyclosporin inhibits calcineurin and thereby NFAT

25. Signaling Through Enzyme-Linked Receptors • Ligand binds to extracellular side of receptor • Activates enzyme activity present or associated with cytoplasmic side • Classes: • Receptor tyrosine kinases • Tyrosine kinase associated receptors • Receptor tyrosine phosphatases • Receptor serine/threonine kinases • Receptor guanylyl cyclases

26. Receptor Tyrosine Kinases Examples of RTK subfamilies

27. Activation of Receptor Tyrosine Kinases • Ligand-induced receptor dimerization/oligomerization • Cross-phosphorylation of receptors

28. Recruiting Intracellular Signaling Proteins • Tyrosine phosphorylation generates binding sites for specific signaling proteins • Most proteins that bind to activated receptor have SH2 domains

29. Receptor-Mediated Activation of Ras • Ras is monomeric GTP binding protein • Grb-2 adaptor has SH2 domain that binds activated receptor • Grb-2 also binds Ras GEF, which activates Ras

30. MAP Kinase Pathway • Chain of three protein kinases that are sequentially activated • Ras activates first kinase in chain

31. Signaling by Nitric Oxide • Diffuses across plasma membrane from signaling cell into target cell • NO activates guanylyl cyclase in smooth muscle to cause relaxation • Viagra inhibits cGMP phosphodiesterase

32. Signaling by Nuclear Receptors • Ligand-activated gene regulatory proteins • Ligands diffuse through plasma membrane, include steroid hormones, thyroid hormone, retinoic acid

33. Selective Estrogen Receptor Modulators • Antagonists in some cell types and agonists in others • Tamoxifen as breast cancer therapy • Raloxifene to help prevent osteoporosis