Cell Signaling A B C D Types of

Cell Signaling A. B. C. D. Types of Cell Signaling Intracellular Receptor Signaling: Steroid Hormones Cell Surface Receptors: Types 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, Enzymelinked – 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 Ca 2+ – 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