Cell Signaling

A. Types of Cell SignalingB. 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 secretedmolecules– 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 degradedrapidly

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: and chain binds and hydrolyzes GTP & chains form a tight complex

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 to dissociate from 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