Cell Biology: Chapter 15 - Mechanism of Cell Communication

types of intracellular signaling

1. contact-dependent

2. paracrine

3. synaptic

4. endocrine

contact-dependent

- requires cells to be in direct membrane-membrane contact

- important during development and immune response

paracrine

- signals (local mediators) are released into the extracellular space and act locally on neighboring cells

synaptic

- performed by neurons that transmit signals electrically along their axons and release neurotransmitters at synapses, which are often located far away from the neuronal cell body

endocrine

- depends on endocrine cells, which secrete hormones into the bloodstream for distribution throughout the body

location of receptors

1. cell surface receptors

2. intracellular receptors

gap junctions

- allow neighboring cells to share signaling information

- directly connect the cytoplasms of adjacent epithelial cells

- allow exchange of inorganic ions and other water soluble molecules but not macromolecules (proteins or nucleic acids)

signal molecules

1. bind to specific receptors either on the cell surface or inside the cell

2. each cell send information via a cascade

3. cells response in different ways via effector molecules

NO path

- activated nerve terminal releases acethylcholine

- Ach binds to extracellular receptor on endothelial cells

- activates NO synthase (NOS) and arganine produces NO

- NO diffuses across the membrane and into smooth muscle cells

- NO binds to guanylyl cyclase and produces cGMP from GTP

- this results in smooth muscle relaxation

intracellular signaling proteins

- function as molecular switches

- enhance speed, efficiency and specificity of the response

- activated by phosphorylation or GTP binding

nuclear receptors

- are regulatory gene transcription proteins

- ligands include small hydrophobic signal molecules that diffuse across the plasma membrane

cell surface receptor proteins classes

1. ion-channel coupled receptor

2. G-protein coupled receptor

3. enzyme coupled receptors

ion-channel coupled receptor

- transmitter-gated ion channels or ionotropic receptors

- involved in rapid synaptic signaling between nerve cells and other electrically excitable target (muscles or nerve cells)

- changes ion permeability of plasma membrane

G-protein coupled receptor

- indirectly regulate activity of a separate plasma membrane bound target protein

- trimeric GTP binding protein (G protein) mediates interaction between the activated receptor and target protein

- activation changes concentration of target proteins

enzyme-coupled receptors

- function directly as enzymes or associate directly with enzyme that they activate

- ligand binding on the surface and catalytic or enzyme binding site is inside the cell

- protein kinases or associate with protein that phosphorylate proteins when activated

signal transduction

- cell surface receptor convert extracellular signals into intracellular ones

signals are relayed via

- second messengers

second messengers

- small intracellular mediators

- small intracellular molecules

ex. cAMP and Ca2+ - water soluble and diffuse in the cytosol

DAG (diacyglycerol) - lipid soluble and diffuses into plasma membrane

- second messengers bind and alter conformation and behavior of selected signaling protei or effector proteins

signaling molecules are activated via

1. phosphorylation

2. GTP binding

phosphorylation

- signaling pathways depend on gain or loss of phosphate groups for activation or inactivation

protein kinase

- covalently adds one or more phosphate groups to the signaling protein

protein phosphatase

- removes phosphate groups

types of protein kinases

1. serine/threonin kinase - phosphorylate proteins on serines and (less often) threonines

2. tyrosine kinase - phosphorylate proteins on tyrosin

GTP binding proteins

- can gain or loss phosphate groups

- active when GTP is bound

- inactive when GDP is bound

- they have intrinsic GTPase activity when 'on' state and shut themselves off by hydrating their bound GTP to GDP

types of GTP binding proteins

1. larger trimeric GTP-binding proteins (G-protein)

2. monomeric GTPases

GAP

- GTPase activating proteins

- inactivates GTP

GEF

- guanine nucleotide exchange factors

- activate monomeric GTPase by promoting the release of bound GDP in exchange for GTP

GPCR signaling molecules

- adrenaline

- acetylcholine

- neurotransmitter

rhodopsin (light activated protein in eye)

G-protein

- consist of alpha, beta and gamma subunits

- inactive, GDP is bound to alpha subunit

- active, GTP is bound to alpha subunit

GPCR Pathway 1 (cAMP)

- extracellular signal molecules bind to GPCR that activates G protein

- this leads to activation of adenylyl cyclase

- adenylyl cyclase produces cAMP

- cAMP activates PKA for cellular responses such as glycogen breakdown and water resorption

PKA

- cyclic-AMP dependent protein kinase

PKA inactive state

- consists of a complex of two catalytic subunits and two regulatory subunits

PKA active state

- cAMP binds to regulatory subunits altering conformation and causing them to dissociates

- release subunits activate to phosphorylate specifc target proteins

phosphodiesterase

- keeps cAMP concentration low so PKA is inactive

GPCR Pathway 2 - PLC

- G-protein activates phospholipace C-beta (PLC beta)

- PLC-beta acts on PIP2 (phosphatidylinoositol bisphosphate)

- cleaved PICP2 produces IP3 (inositol triphosphate) and DAG (diacylglycerol)

IP3 pathway

- IP3 diffuses through cytosol and binds to IP3 receptors on the endoplasmic reticulum

- Ca2+ stored in ER is released

- Ca2+ binds to calmodulin and changes its conformation

- this results in either activation of Ca2+ pump on the plasma membrane or activate CaM-kinase to phosphorylate gene regulatory proteins

DAG pathway

- DAG can either

1. cleave to release arachidonic acid

- can act as signal or synthesize eicosonaids such as prostalglandins

2. activate protein kinase C (PKC)

enzyme coupled cell surface receptor

1. transmembrane proteins with their ligand-binding domain on the outer surface of the plasma membrane

- extracellula receptor

2. cytosolic domain either has intrinsic enzyme activity or associates directly with an enzyme

- intracellular receptor

enzyme coupled receptor on the surface of the cell

- act through RTP (receptor tyrosine kinase)

- signal proteins: EGF, PDGF, FGF, HGF, IGF-1, VEGF, MCGF, NGF (growth factors)

- binding of ligand to RTK causes dimerization so they can be activated and cross-phosphorylate each other on multiple tyrosines (transautophosphorylation)

RTK contributes to the receptor activation via

1. phosphorylation of tyrosine within the kinase domain increases the kinase activity of the enzyme

2. phosphorylation of tyrosine outside the kinase domain creates high-affinity docking sites for the binding of specific intracellular signaling proteins

ex. PLC-gamma that cleaves PIP2 to produce IP3 and DAG

RTK Pathway 1 - PLC

- RTK is activated

- PLC-gamma is phosphorylated

- PLC-gamma cleaves PIP2, which produces IP3 and DAG

Ras superfamily

- monomeric GTPases

- contain one or more covalently attached lipid groups that help anchor the protein to the cytoplasmic face of the membrane

- function as a molecular switch: active when GTP is bound and inactive when GDP is bound

Ras GEF

- stimulate dissociation of GDP and uptake of GTP from cytosol

Ras GAP

- hydrolyze bound GTP by Ras, which inactives Ras

RTK Pathway 2 - Ras

- signal protein binds to RTK

- adapter protein, Grb2, docks at the phosphorylated RTK

- Ras GEF (SOS) is activated

- Ras GEF stimulates dissociation of GDP and uptake of GTP and Ras is activated

- Ras activates MAPK signaling cascade (MAPKKK - Raf > MAPKK - Mek > MAPK - Erk)

- genes activated induce genes for cell proliferation

PI-3 kinase Akt

- activated by insulin

- promote growth and survival of cells

- inhibit apoptosis

RTK Pathway 3 - PI-3 kinase

- insulin binds to RTK

- RTK activates

- PI-3 kinase is activated (phosphoinositide 3 kinase)

- PI-3 kinase produces lipid docking sites in the plasma membrane and activates PIP3

- intracellular signaling proteins bind to PIP3 via interaction domain such as PH (pleckstrin homology)

- this results in activation of genes that promote growth and survival of cells by inhibiting apoptosis

serine/threonine protein kinase

- RTK are structurally related

- signals: members of TGFbeta superfamily

- signaling molecules: intracellular signal