Chapter 20: Cell to Cell signaling

Extracellular Signaling:

∑ Stages in extracellular communication ≠ 1) Synthesis of signaling molecule; 2) Release of signaling molecule; 3) Transport of signaling molecule to the target; 4) Detection of signal by a receptor; 5) Response of cell to the signal; and 6) Termination of signaling.

∑ Signaling distances (20-1) ≠ Endocrine signaling is carried through the bloodstream; Paracrine signaling is release of signal to other cells in the local area; Autocrine signaling is the same cell releasing and receiving signal; Signaling of neighboring cells through contact.

∑ Properties of receptors - They must bind only to specific signaling molecules; They must effect only specific cellular response pathways.

∑ Classifying hormones ≠ 1) Lipophilic molecules that bind to intracellular receptors (ex. Steroids); 2) Water soluble molecules that bind to cell surface receptors (ex. Catecholamines and peptides); and 3) Lipophilic that bind to cell surface receptors (ex. Prostaglandins).

∑ Classifying receptors (20-3) ≠ 1) G-protein coupled; 2) Ion channel receptors; 3) Tyrosine kinase linked; 4) Intrinsic enzyme activity (ex. Guanylate cyclase and receptor tyrosine kinase).

∑ Second messengers (20-4) ≠ Transmit information from many types of receptors; major types of second messengers are cAMP, cGMP, diacylglycerol (DAG), inositol trisphophate (IP3) and Ca⁺²; can work together with signal transduction pathways.

∑ Signal transduction pathways (20-5, 20-6)- Transmit information from many types of receptors; major types include GTPase switch proteins (ex. Ras), Protein kinases (ex. MAP kinase) and Adapter proteins (ex. GRB); can work together with second messengers.

∑ Regulation of hormone levels (T20-1) ≠ Peptides and catecholamines are for rapid responses, thus they are released quickly and have a short half-life; Lipophilic ≠ released slowly, long half-life, present for hours or days; Feedback control ≠ hormones can feedback positively or negatively to regulate their own levels or the levels of other hormones.

Seven-pass G protein linked receptors:

∑ fl and a adrenergic receptors - Stimulation of these receptors generates the fight or flight response; Bound by the catecholamines adrenaline and noradrenalin; fl receptors are found in liver and fat cells, heart and intestinal endothelia; a receptors are found in smooth muscles lining blood vessels in intestine, skin and kidney; binding of hormone to receptor leads to a rise in cAMP levels.

∑ Structure (20-14) - 7 transmembrane domains; Mapping regions of the receptor that bind to hormone and to G protein.

∑ Pharmacology - Agonists and antagonists can bind to the receptor (T20-2); fl₁-adrenergic antagonists used as åbeta blockersπ for cardiac angina and fl₂-adrenergic agonists used to relax muscles around bronchiole passages to treat asthma.

∑ G protein signaling (20-16) - G_s is linked to fl-adrenergic receptors and activates adenylate cyclase; G proteins are trimeric, and the a subunit is bound to GDP or GTP; GTP binding and Ga dissociation from Gflg provides the signal to activate adenylate cyclase upon receptor stimulation; Termination of signaling occurs when GTP is hydrolyzed; Cholera toxin attacks the body by constitutively activating Ga by ribosylation (20-17); Adenylate cyclase can also be inactivated by an inhibitory Ga subunit at a different binding site (20-18); cAMP degradation is also controlled by phosphodiesterases activated by Ca⁺².

Receptor Tyrosine Kinases and Ras:

∑ Ligands for RTK receptors ≠ Soluble or membrane bound peptides or proteins such as growth factors and insulin.

∑ Response of RTKs to ligand binding (20-21) ≠ Receptors dimerize then phosphorylate each other at tyrosine residues through an innate kinase activity.

∑ GTPase Switch proteins Ras and Ga - The activity of G-proteins and Ras are dependent on binding to GTP; Cycling of Ras between active and inactive states (20-22).

∑ Linking RTKs to Ras (20-23) ≠ Contributions of accessory factors GRB2 (an adapter protein) and Sos (a guanine exchange factor ≠ GEF); Function of SH2 and SH3 domains.

∑ RTKs in eye development ≠ Drosophila eye structure; R7 cell determination cascade (20-25).

MAP kinase pathways:

∑ MAP kinase ≠ A downstream effector protein of the Ras pathway; MAP kinase activated at the membrane as a result of sequential activation of 1) Raf kinase by binding to Ras, 2) activation of MEK kinase by Raf dependent phosphorylation at serine/threonine, 3) Activation of MAP kinase by MEK dependent phosphorylation at tyrosine and threonine (20-28); 14-3-3 protein sequesters Raf in an inactive conformation and Ksr provides a scaffold for the multikinase complex that forms at the membrane; MAP kinase can respond to signals transmitted by other receptors on the membrane; Different MAP kinases are activated depending on the receptor and the cell type.

Second Messengers:

∑ cAMP ≠ Mediates its effects on the cell through the action of protein kinases (cAMP kinases); cAMP kinase structure (3-27).

∑ Glycogen metabolism ≠ cAMP kinase dependent process that is initiated with adrenalin stimulation of fl adrenergic receptor; Synthesis and breakdown of glycogen (20-34); Regulation of glycogen breakdown by cAMP kinase (20-35); Regulation of glycogen synthesis by cAMP kinase (20-36, 20-35); Amplification of hormonal signals through second messengers, kinases and enzymes (20-37); Cellular responses to cAMP depends on the effector molecules activated in different cell types; AAP proteins anchor cAMP kinases to regions of the cell where they are needed.

∑ PIP2 (20-38) ≠ Generates two second messengers, IP3 and DAG, from its regulated cleavage by membrane associated phospholipase C (PLC); Both G protein coupled receptors and RTKs can activates different forms of PLC; IP3 releases Ca⁺² from their mitochondrial and ER stores and indirectly from outside the cell (20-39); Ryanodine receptors lead to Ca⁺² release from the SR in muscle cells due to a change in membrane potential.

∑ Ca⁺² ≠ Release of Ca⁺² leads to cellular responses such as contraction of muscles, release of insulin from the pancreas, degradation of glycogen in liver and muscle (T20-4); Effects are mediated through calmodulin, which binds Ca⁺² and activates effector enzymes.

∑ DAG ≠ Activates protein kinase C (PKC); PKC is soluble until there is an increase in Ca⁺², which promotes PKC movement to the membrane where it can bind to, and be activated by, DAG.

∑ cGMP ≠ Generated by soluble guanylate cyclase through nitric oxide gas (NO) signaling or by guanylate cyclase linked receptors that are activated by peptide hormones.

Interaction and Regulation of Signaling Pathways:

∑ RTK linked to different signaling pathways ≠ Can activate the MAP kinase pathway (described earlier) and the IP3/DAG pathway by activating PLC; can activate different signaling pathways in a tissue specific manner.

∑ Multiple G proteins transduce signals to different effectors ≠ Different G protein subunits link G protein coupled receptors to different effector molecules (T20-5); Like the Ga subunit, the Gb/g subunits can also regulate effector proteins.

∑ Glyconeogenesis ≠ Glycogen metabolism can be regulated by different second messengers in different cell types (20-44); Activity of the different second messenger pathways is synergistic.

∑ Insulin (20-45) ≠ Stimulates the activity of Ras dependent (MAP kinase) and Ras-independent (protein kinase B) signaling pathways; The Ras dependent pathway has an additional component, IRS1, which is activated by the receptor and binds to GRB2; The Ras independent pathway is also mediated by IRS1, which binds to PI-3 kinase and ultimately activates protein kinase B.

∑ Blood glucose regulation (20-46) ≠ Insulin and glucagon are hormones released from the islets of Langerhans cells in the pancreas that control blood glucose; Insulin allows for uptake of blood glucose and synthesis of glycogen, while glucagon leads to release of glucose into the blood and breakdown of glycogen.

∑ Regulating cell surface receptors ≠ Receptors can be downregulated by reducing their numbers via receptor-mediated endocytosis due to binding of ligand to the receptor; Receptor activity can be modulated by phosphorylation; Long term b-adrenergic receptor stimulation leads to the activity of cAMP kinase and beta adrenergic receptor kinase (BARK), which phosphorylate and desensitize the receptor.

Hormone Mediated Regulation of Gene Expression:

∑ cAMP mediated gene regulation ≠ Cyclic AMP Response Element Binding Protein (CREB) is a transcriptional activator that is activated by cAMP kinase phosphorylation (20-48).

∑ Ras mediated gene regulation ≠ MAP kinase leads to the phosphorylation and activation of SRF and TCF, two transcriptional activators that bind serum response elements and mediate growth factor dependent activation (20-48).