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Cell membrane is made up of:
- phospholipids
- cholesterol
- proteins
-
membrane transports
- passive (facilitated)
- active (primary, secondary)
-
types of communication
- intercellular signaling
- intracellular signaling
-
adrenergic receptors
- alpha 1 (vasoconstriction)
- alpha 2
- beta 1 (increase heart rate)
- beta 2 (bronchodilation)
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Cellular signaling
intercellular signaling
- cell-to-cell/direct
- autocrine
- paracrine
- nervous/synaptic
- endocrine
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Cellular signaling
intracellular signaling
Ligand
aka chemical messenger or 1st messenger
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intracellular signaling
ligand
- extracellular mol that binds receptor
- can activate or inhibit intracell.signal.
2 types: endogenous, exogenous
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Endogenous ligand
produced by body
ex. hormones, paracrine factors, etc.
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Exogenous ligand
administered to body
ex. drugs, toxins
-
Ligand activity
Efficacy
ability of ligand to initiate cellular effect
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Ligand activity
Agonist
most are agonists!
ligand that has both receptor affinity & efficacy - (binds to receptor and initiates cell effect)
endo/exogenous
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Ligand activity
antagonist
- ligand that has receptor affinity but lacks efficacy
- (blocks access of agonist) -->competitive antagonist
endo/exogenous
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Receptors
- located in cm or intracell. mol
- binds ligand
- mods signal transduction pathway (can turn on/off)
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signal transduction pathway
aka intracell. signaling or 2nd msgr
molecular chain of events, mods cell activity
-
modifying cell activity via signal trans.pthwy
enzymes
- kinase (phosphorylation)
- phosphotase (dephosphorylation)
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modifying cell activity via signal trans.pthwy
cell activities
- opening ion channels
- initiate gene transcription
- initiate gene translation
- protein synthesis
- secretion of proteins
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Receptor regulation by ligands
Desensitization
due to continuous or repeated exposure
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Types of desensitization
- receptor sequestration
- receptor down-regulation
- receptor inactivation
- inactivation of signaling protein
- production of inhibitory protein
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receptor sequestration
if not as many receptors on cell surface then not much activity
-
receptor down-regulation
- decrease gene transcription of receptor
- due to prolonged increased levels of ligand signaling
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receptor inactivation
- turns off!
- receptors get phosphorylated
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inactivation of signaling protein
- turns off!
- something binds to receptor bug signal is turned off
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production of inhibitory protein
signaling causes synthesis of some protein that inhibits signal
similar to neg.fdbk
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up-regulation
- increase gene transcripton of receptor
- due to prolonged decreased lvls of ligand
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is activation of receptor leading to down-regulation?
- OVERACTIVATION does!
- (or underactivation)
form of neg.fdbk
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Receptor classes
- cell membrane receptors
- intracellular receptors
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Types of cell membrane receptors
- GPCRs
- Enzyme-linked receptors/kinase receptors
- ion channel-linked receptors
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types of intracellular receptors
- steroid receptors
- thyroid receptors
located on cytoplasm or nucleus
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ion channel-linked receptor's ligand
neurotransmitters
takes milliseconds to reach target
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GPCR's ligand
- a mix
- any type of signaling molecule
- activation of enz act. other proteins
- takes seconds to happen
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ligand for kinase-linked receptors
- can act as paracrine hormone, etc. depends where in body
- takes hours to reach target
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ligand for intracellular receptors
hormones
- ex. steroid hormones
- takes longest to reach target (hours)
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endogenous macromolecules that bind drug to elicit therapeutic outcome
- receptors
- enzymes
- nucleic acids
- ligands
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drug targets
receptors
CM or intracellular receptor
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drug targets
enzymes
- cm
- intracellular
- extracellular
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drug targets
nucleic acids
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GPCRs
- aka metabotropic receptors (nerv.sys)
- aka 7-transmembrane spanning receptor
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GPCRs
signaling
- couples (associates) with G proteins (Gs, Gi, Gq)
- leads to production of 2nd msgrs
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GPCRs: intracell. signal.
General scheme
- receptors
- 1st msgrs (ligands)
- transducers/effectors (trans- usually Gproteins)
- 2nd msgrs
- physiological response
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How do GPCRs work?
- GPCRs couple w/G proteins
- G protein acts as transducer, activates effector (Ac)
- 2nd msgrs produced (amplification)
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G proteins are heterotrimeric
(contains 3 subunits)
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How do G proteins work?
activated when bound to GTP
- have intrinsic GTPase activity
- (converts GTP->GDP)
- (Gprotein inactive when GDP bound)
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GPCR signaling
Gs
- -Ligands bind receptor
- -Gs activated
- -alpha subunit dissociates away from beta, gamma
-AC stimulated, converts ATP->cAMP
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GPCR signaling: Gs
cAMP
activates cAMP-dependent PKA
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GPCR signaling: Gs
PKA
- 2 catalytic subunits & 2 regulatory subunits
- phosphorylates other mol
- activates PDE
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GPCR signaling: Gs
activated PKA
- mediates effects in cell, occur in seconds
- can enter nucleus and regulate gene trans. by activating CREB
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GPCR signaling: Gs
CREB
- cAMP response element binding protein
- recognizes cAMP response element which stimulates transcrptn of target genes
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GPCR signaling: Gs
stimulates AC
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GPCR signaling: Gi
inhibits AC
-
GPCR signaling
PDEs
phosphodiesterases
- - breakdown cAMP (to 5' AMP, so cant act as 2nd msgr)
- - activated by PKA
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GPCR signaling: Gq
- diff signal transduction pthwy
- effector = Phospholipase C (PLC
- IP3 or Ca2+ can be 2nd msgrs
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Gq
- -signal mol binds
- -GPCR activated
- -GDP->GTP
- -activated Gq protein
- -binds and activates effector (PLC)
- -phos. PLC --> PI 4,5bisphosphate and Ip3
-IP3 binds to receptor on ER, Ca in cell & increased
- -PI4,5bisphos. -> DAG
- -DAG binds act. protein kinase C
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example of Gq
- smooth muscle cells in blood vessels
- increase in calcium causes contraction
- bc intracellular signals increase!
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Gs
- alpha subunit
- activates AC
- activates calcium channels
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Gi
- beta/gamma subunit
- activate potassium channels
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Gq
- alpha subunit
- activates PLC
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Why are GPCRs good drug targets?
- -->location
- cell surface receptor- no need to go inside to effect
- -->ratio
- 1 ligand bind= multiple G proteins can activate
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GPCRs and Drug Industry
most successful drug target
- 25-30% top 100 Rx target GPCRs
- 50-70% of Rxs target GPCRs
- 75% of all CNS Rxs target GPCRs
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GPCRs and Drug Industry
Saturated?
- 20-80% diff GPCRs r targeted by Rxs
- ~80% GPCRs w/known ligands r not Rx targets
- 140 orphan receptors still w/o ligands
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Types of GPCR disorders
- mutations
- processing error
- antibodies
- toxins
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GPCR disorders
mutations
- - incorrect gene seq = incorrect protein seq & receptor - shape is altered
- - can result in permanent OFF or ON
- ex. inherited hypocalciuric hypercalcemia
- (receptor lose ability to sense Ca properly)
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GPCR disorders
processing error
- - gene is OK
- - protein misfolds during processing
- - receptor shape altered
- - can result in permanent OFF or ON
- ex. blindness caused by altered rhodopsin receptor
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GPCR disorders
antibodies
- - structure of receptor is OK
- - body producing antibodies that stick to it
- - can turn receptor OFF or ON
ex. graves disease
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GPCR disorders
toxins
- struct./fn receptor is OK
- toxins interfering w/signaling of G Proteins
- Can turn receptor sig. OFF or ON
- ex. cholera vs. pertussis
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Bacterial toxins that alter G protein activity
Cholera toxin
- (intestinal infection)
- abolish GTPase activity in Gs
- (increased cAMP lvl)
- excess Cl lost in lumen, H2O follows
- Diarrhea, dehydration
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Bacterial toxins that alter G protein activity
Pertussis toxin
- (whooping cough)
- locks Gi in GDP state
- Decrease activity of phagocytic immune cells
- paralyzes respiratory cilia
- results: infection, inflam., coughing
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Enzyme-linked receptors
or Kinase receptors
- aka Tyrosine Kinase Receptor
- aka Serine/Threonine Kinase Receptors
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Enzyme-linked receptors
Receptor structure
- binding domain (on extracell. side)
- transmembrane region
- kinase domain
- autophosphorylated region
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Enzyme-linked receptors
Receptor activation
- ligand binds
- receptors dimerize
- receptors autophosphorylate
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Enzyme-linked receptors
endogenous ligands
- insulin (hormone)
- GFs (paracrine, hormone)
- Cytokines (paracrine, hormone)
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endogenous ligands
insulin
- promotes glu & aa uptake into cells
- promotes DNA rep & protein synthesis
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endogenous ligands
growth factors
- +/- cell replication
- +/- cell differentiation
-
endogenous ligands
cytokines
- +/- cell replication
- +/- cell differentiation
- +/- inflammation
-
enzyme linked receptor signaling
- kinase receptor
- kinase receptor signaling
- abnormal kinase receptor signaling
-
enzyme linked receptor signaling
kinase receptor
- dimerization
- autophosphorylation
-
enzyme linked receptor signaling
kinase receptor signaling
- activation of intracell. kinases
- (lots of phosphorylation, often cascades)
- activation of intracell. proteins
- (Ras activation)
-
enzyme linked receptor signaling
abnormal kinase receptor signaling
- can result in cancer
- mutated ras found in 25% of all cancers
-
RAS
- -small G protein
- -made of 1 subunit (monomeric)
- -has no intrinsic GTPase activity
- -activates MAP kinase cascade
- -result: turns on gene trnscrptn/translatn
--> cell growth & proliferation
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ion channel-linked receptors
- aka ligand-gated ion channel
- aka ionotropic receptor
-
ion channel-linked receptors
signaling
- ligand binds
- opens channel
- ion diffusion allowed
-
ion channel-linked receptors
endogenous ligands
- Ach = Na+ in
- Serotonin (5HT3) = cations
- glutamate (NMDA) = cations
- GABAA = Cl in
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intracellular receptors
- aka nuclear hormone receptors (NHR)
- aka cytosolic receptors
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intracellular receptors
signaling
- ligand diffuse thru membrane
- bind in cytosol/nucleus
- complex migrates to nucleus
- affects gene transcrp/transltn
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intracellular receptors
endogenous ligands
- glucocoritcoids (corticosteroids)
- estrogen, progesterone, androgens
- thyroid hormones
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What is a ligand?
primary messenger
-
what is the diff btwn agonist and antagonist?
- agonist = has receptor affinity & efficacy
- antag = has only affinity
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what are the main types of intercellular signaling?
- autocrine
- paracrine
- nervous
- cell-to-cell
- endocrine (neuroendorcrine)
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what are the 3 types of cell membrane receptors?
- GPCRs
- ion channel receptors
- enzyme-linked receptors
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what are the 3 main classes of G proteins?
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what is the mechanism of cholera toxin?
- inhibits GTPase activty
- increase cAMP
- -Gs protein
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why is overactivation of Ras associated with cancer?
ras inovolved in activation of growth pathway
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what types of molecules bind to intracellular receptors?
lipophillic
ex. steroid hormones
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