Week 13: cell signaling 2

Types of intracellular signaling

autocrine, paracrine, endocrine

Extracellular messenger molecules

transmit messages between cells

autocrine signaling

the target cell is also the secreting cell

paracrine signaling

Signal released from a cell has an effect on neighboring cells in the extracellular space

endocrine signaling

secreted molecules diffuse into the bloodstream and trigger responses in target cells anywhere in the body

Basic elements of cell signaling systems

Receptors
Second messengers (effectors)
Specific proteins activated by 2mes or proteins recruited to the intracellular domains of the surface receptors

Signaling Pathways

a series of protein interactions that initiate a cellular response

Each protein alters the conformation of the next (usually by phosphorylation)

Kinases

Add phosphate groups

Phosphatases

remove phosphate groups

target proteins

Receive a message transduced by a cascade to alter cell activity

Signal transduction

the transmission of molecular signals from a cell's exterior (receptor) to its interior (target protein)

Protein phosphorylation effects

• activation/inactivation of enzymes
• increase/decrease in protein-protein interactions
• change in the subcellular location of the protein
• triggering protein degradation

Extracellular messengers include

Small molecules (aa and their derivatives)
Gases (NO, CO)
Steroids
Eicosanoids (derivatives of arachidonic acid)
Various peptides and proteins

Receptor types

GPCRs
RTKs
ligand - gated channels
Steroid hormone receptors
Specific receptors (BCR, TCR)

GPCRs

Largest superfamily of proteins

Have 7 a-helical transmembrane domains, interact with G proteins

GPCR natural ligands

Hormones, neurotransmitters, opium derivatives, chemoattractants

signal transduction pathway

LIGANDS activate RECEPTORS that stimulate EFFECTORS to give rise to a PHYSIOLOGICAL RESPONSE

GPCR structure

7 transmembrane alpha helices that transverse the plasma membrane

N-terminus on the outside of the cell
C-terminus on the inside of the cell

Ligand binding to the GPCR extracellular domain

Increases the affinity for G protein

Signal transduction by GPCRs

1. Ligand binds
2. Conformational change in intracellular domain
3. Affinity for G protein increases, G protein binds
4. GDP -> GTP on Ga subunit = activating G protein
5. Gby dissociate, effector activation

ONE LIGAND-BOUND RECEPTOR CAN ACTIVATE MANY G PROTEINS

Termination of the GPCR response

• desensitization
• GRK
• arrestins
• RGSs

Desensitization of GPCRs

By blocking active receptors from turning on additional G proteins

GRK (G protein-coupled receptor kinase)

Activates a GPCR via phosphorylation

Arrestins

proteins that compete with G proteins to bind GPCRs

RGSs (Regulators of G Protein Signaling)

Accelerate the termination of GPCRs

GTP to GDP hydrolysis of Ga

Turns off the Ga, causing a decreased affinity for the effector

Causes increased affinity for Gby subunits

Heterotrimeric G proteins

Gs, Gi, Gq, G12/13

Gs of G protein

stimulates adenylate cyclase, which increases levels of cAMP in the cell

Gq of G protein

Activates phospholipase C, increasing PIP2, increasing DAG and IP3 (Ca2+ release) second messengers, activating protein kinase C

Gi of G protein

inhibits adenylate cyclase, which decreases levels of cAMP in the cell

G12/13 of G protein

Not well characterized

Gby complex function

Can couple to PLCb, K+ & Ca2+ ion channels, and adenylyl cyclase

Target of cholera toxin

Galpha-s

beta-adrenergic receptors

Stimulate G-alpha-s to activate adenylate cyclase

alpha-adrenergic receptors

Stimulate G-alpha-i to inhibit adenylate cyclase

target of pertussis toxin

G-alpha-i

adenylate cyclase signaling system

Gai/o -> AC decrease -> cAMP decrease
Gas -> AC increase -> cAMP increase

cyclic AMP (cAMP), Ca2+, phosphoinositides, inositol trisphosphate, diacylglycerol, cGMP, NO

An example of a second messenger
Diffuses to other sites in the cell

Second messengers

Second messengers enable cells to mount a large-scale, coordinated
response following stimulation by a single extracellular ligand.

Phosphatidylinositol (PI) second messengers

Phospholipids converted into second messengers by phospholipases, phospholipid kinases, or phospholipid phosphatases

PH domains of PLC

Formed by phosphorylated phosphoinositides

Produce IP3 and DAG ) secondary messengers

DAG functions as a second messenger

Activates protein kinase C which Phosphorylates serie and threonine residues on target proteins

IP3

inositol triphosphate;
binds to ligand-gated channels in the ER and opens them, they release Ca2+ into the cytosol

Ca2+ functions

Smooth muscle cell contraction

isoforms

slightly different versions of the same protein

Can have different affinities for the ligand

May coexist in the same plasma membrane

Many GPCRs are photosensitive (color receptors)

Odorant receptors are GPCRs (smell)

taste receptors are also GPCRs

GPCRs play a role in our sensory perception

Types of protein-tyrosine kinases

Receptor protein-tyrosine kinases (RTKs) = MEMBRANE PROTEINS
Non-receptor protein/tyrosine kinases = CYTOPPLASMIC

RTKs activation

Extracellular growth and differentiation factors (EGF, PDGF)
Metabolic regulators (insulin)

Non-receptor tyrosine kinases

Control immune responses, cell adhesion, neuronalne cell migration

Protein kinase activation

Autophosphorylation on tyrosine residues
Binding of cytoplasmic signaling molecules

Phosphotyrosine-Dependent Protein-Protein Interactions

Phosphorylated tyrosines bind effector proteins that have either a Src-homology 2 (SH2) domain or a phosphotyrosine-binding (PTB) domain.

SH2 and PTB domain proteins include

-Adaptor proteins that bind other proteins.
-Docking proteins that supply receptors with other tyrosine phosphorylation sites.
-Signaling enzymes (kinases) that lead to changes in cell.
-Transcription factors

Grb2

an adapter protein with SH2 and SH3 domains.
SH2 domain binds phosphorylated RTKs
SH3 bind Sos

Termination of RTKs signal transduction

Usually by internalization of the receptor through clathrin-mediated endocytosis

Some RTKs bind to clathrin adaptor protein AP-2

Some are ubiquinated by ubiquitin ligases through SH2 domains or adaptor proteins

Fates of internalized RTKs

Degradation in lysosmes
Return to the plasma membrane
Become part of the endosomal signaling complexes (continue in intracellular signaling)

Two important downstream signaling pathways

Ras-MAP Kinase pathway
Insulin receptor-mediate cascade

Ras protein

A G protein
Involved in regulation of cell division, differentiation, gene expression, cytoskeletal organization, vesicle trafficking, nucleocytoplasmic transport

Its a small GTPase anchored at the inter surface of the plasma membrane

Ras protein forms

Active = GTP-bound
Inactive = GDP bound

Mutations in Ras

Lead to tumor formation
Prevent the protein from hydrolizing GDP to GTP

Ras/MAP kinase pathway

Results in activation of transcription factors
Control cell proliferation and differentiation

Glucose levels

Too high: loss of glucose, fluids, and electrolites in urine, Health problems

Too low:M loss of consciousness and coma

Insulin receptor

tyrosine kinase

The response of the insulin receptor to ligand binding

Conformational change and tyrosine phosphorylation

Insulin receptor structure

Alpha and beta chains linked by disulfide bonds

Associate with insulin receptor substrates (IRSs)

IRSs (insulin receptor substrate proteins)

Provide binding sites for SH2 domain-containing signaling proteins (PI3-kinase, Grb2)

diabetes mellitus (DM)

insulin is not secreted adequately or tissues are resistant to its effects

Diabetes type 2

A chronic increase in insulin secretion overstimulates target cells in the liver and elsewhere, which leads to insulin resistance

target cells stop responding to insulin

In result: chronic elevation of blood glucose levels, which stimulates the pancreas to secrete even more insulin.

Health risks of diabetes

Blindness, kidney disease, nerve damage, and cardiovascular disease, reduced circulation in the limbus

Convergence of signals

Multiple molecules can activate a common effector

Divergence of signals

One signal can activate a bariery of receptors

Cross-talk in signaling pathways

Signals can be passed back and forth between pathways

Apoptosis

ordered process of cell death, involving cell shrinkage, loss of adhesion to
other cells, dissection of chromatin, and engulfment by phagocytosis

Necroptosis

programmed necrosis/inflammatory cell death

Apoptosis - functions

• organ development (removing excess cells)
• normal maintenance (cycling out older/damaged cells)

Reduced or elevated apoptosis is linked to cancer, parkinsons, alzheimers, Huntingtons, diabetes I

Necrosis

Cell death resulting in the rupture of the plasma membrane, triggers inflammatory responses

Connect to Crohns disease and inflammatory bowel disease

Apoptosis vs. Necrosis

Apoptosis- active, orderly and organized; requires energy a plan and enzymatic degradation of proteins/dna; produces neatly packed cell fragments that are recycled

Necrosis; passive and disorderly; produces cell debris