Cell Signaling A. Types of Cell Signaling B. Intracellular Receptor Signaling: Steroid Hormones C. Cell Surface Receptors: Types D. Cell Surface Receptors: G-protein linked receptors

A. Types of Cell Signaling Direct cell-cell signaling vs. signaling via secretion

A. Types of Cell Signaling Cell surface receptors vs. Intracellular receptors

A. Types of Cell Signaling Three forms of signaling by secreted molecules Paracrine Synaptic Endocrine

A. Types of Cell Signaling Gap junctions

A. Types of Cell Signaling The same signal can produce different effects in different cells

A. Types of Cell Signaling Proper signaling generally depends on a signal molecule being degraded rapidly

A. Types of Cell Signaling Types of Signals based on Where they Bind To an intracellular receptor: e.g. Steroid hormones To a cell-surface receptor: G-protein linked, enzyme-linked, or ion channel-linked

B. Intracellular Receptor Signaling: Steroid Hormones Overview of Steroid Hormone Action: Steroid hormone (small, hydrophobic) is secreted by glandular cells and released into bloodstream (usually transported via shuttle proteins) The hormone enters the cytoplasm of cells In the cytoplasm of target cells, the hormone binds to steroid hormone receptor protein

B. Intracellular Receptor Signaling: Steroid Hormones Overview of Steroid . . . (cont.): The steroid-receptor conplex is translocated into the nucleus, where it activates the transcription of the primary response genes Transcription of the primary response genes, followed by translation, results in the production of primary response proteins

B. Intracellular Receptor Signaling: Steroid Hormones Overview of Steroid . . . (cont.): The primary response proteins usually inhibit further transcription of their own genes, and they may activate transcription of secondary response genes.

C. Cell Surface Receptors: Types Three known classes of cell surface receptors Ion channel-linked, G-protein-linked, Enzyme-linked An activated cell-surface receptor triggers a phosphorylation cascade

D. Cell Surface Receptors: G-protein linked receptors Largest family of cell surface receptors Different ones respond to a wide variety of mediators including different hormones, neurotransmitters, local mediators Examples: Receptors to epinephrine, acetylcholine, serotonin

B. G-linked Receptors What do G-protein receptors do? G-protein receptors activate trimeric G-protein Activated G-protein alters the cellular concentration of a “second messenger”: usually cyclic AMP or Ca2+ The second messenger activates a protein kinase enzyme The protein kinase phosphorylates another enzyme and alters its activity

B. G-linked Receptors Trimeric G-proteins disassemble when activated Three chains: a, b, and g a chain binds and hydrolyzes GTP b & g chains form a tight complex bg that anchors G-protein to the plasma membrane

B. G-linked Receptors Trimeric G-proteins disassemble when activated Inactive G-protein has a bound GDP When activated: GDP dissociates, new GTP is bound This causes a to dissociate from bg a binds to adenylate cyclase, altering its activity Gs protein stimulates activates adenylate cyclase, Gi inhibits it

B. G-linked Receptors Action of epinephrine on glycogen metabolism Epinephrine (adrenaline) is released from the adrenal gland in times of stress. It has two overall actions on glycogen metabolism: it inhibits glycogen synthesis, and it promotes glycogen breakdown This is mediated by a protein phosphorylation cascade