M&R Session 7 Flashcards
By what two mechanisms can cell surface receptors act?
Directly alter cellular activity
Transduction of initial binding event via other intracellular components
What tends to happens to cells if they do not receive extracellular signals?
They die
What cellular activity can be controlled by transduction of initial binding event?
Contraction
Secretion
Proliferation
Differentiation
What are the three superfamilies of cell-surface receptors?
Ligand-gated (receptor operated) ion channels
Receptors w/intrinsic enzymatic activity
GPCRs
Which superfamily of cell-surface receptors is heterogenous?
Receptors w/ intrinsic enzyme activity
Describe the mechanism of receptors w/intrinsic enzyme activity.
Ligand binding activates an enzyme
Enzyme phosphorylates the receptor itself and other substrates
Which superfamily of cell-surface receptors do insulin receptors belong?
Receptors w/intrinsic enzyme activity
How do GPCRs have specificity of action?
Each receptor subtype is specific for one/ limited number of endogenous ligands
What is an agonist?
A molecule that binds to a GPCR and activates it causing intracellular signal transduction events
What drugs are used to treat asthma?
Beta-2 adrenoreceptor agonists
E.g. Salbutamol, Salmeterol
What type of molecules are used for analgesia/anaesthesia?
Mu-opioid receptor agonists
E.g. Morphine, Fentanyl
What are antagonists?
Molecules that bind to a receptor but do not activate it; they block the effects of agonists
What type of molecules are used to treat hypertension?
Beta-adrenoreceptor antagonists
E.g. Propanolol, Atenolol
What are Haloperidol and Sulpiride examples of?
Anti-schizophrenic D2 dopamine receptor antagonists
What effect do antagonists have on certain mechanisms?
Damp-down hyper activation
Give the key features of GPCR structure.
300-1200 a.a. chain 7TMD 2 regions for ligand binding: in TMD or N-terminal region Extracellular N-terminal Intracellular C-terminal
What can drugs targeting GPCRs be used to treat?
Parkinson's disease Congestive heart failure Thrombosis Benign prostatic hyperplasia Acid reflux
What two types of mutation can occur to cause a disease associated w/signal transduction?
Loss of function
Gain of function
What causes retinitis pigmentosa?
Loss of function rhodopsin mutation
What causes nephrogenic diabetes insipidus?
Love of function mutation of V2 vasopressin receptor
What happens in a gain of function mutation?
Receptor becomes independent of ligand –> endocrine changes
What causes familial male precocious puberty?
LH receptor mutation
What stimuli can GPCRs respond to?
Light, odour, taste Ions Neurotransmitters Hormones Large glycoproteins
How do GPCRs cause a change in cellular activity?
Ligand binds –> conformational change –> GPCR activated –> interacts w/guanine-nucleotide binding protein –> activates G-protein –> GDP replaced by GTP on alpha-subunit
What is the structure of a guanine-nucleotide binding protein?
Heterotrimeric
Alpha, beta and gamma subunits
Why is a G-protein functionally dimeric?
Once the beta and game subunits are synthesised they stick together
What is the function of the alpha subunit of a G-protein?
Bind to guanine nucleotide
GTPase activity
When does GTP have a natural tendency to bind to the alpha subunit?
When the binding site is empty
How do you activate a G-protein?
Guanine nucleotide exchange by replacing GDP w/GTP
How do you turn off a G-protein?
Hydrolyse GTP
Which subunits can interact w/effector proteins?
Alpha
Beta-gamma
What effector proteins can the G-protein subunits interact with?
2nd messenger generating enzymes
Ion channels
What is amplification governed by?
Timer function of the G-protein unit
What is timer function?
Capacity of how long the signal can be passed on for by a GPCR
Why can efficiency of signal transmission be targeted therapeutically?
In different pathways it can be altered
What is the primary determinant of receptor G-protein selection?
The ~20 different G-alpha proteins in the human genome
Does a GPCR have its own unique G-protein combination?
No
How many possible G-alpha-beta-gamma protein combinations are possible in the human genome?
> 1000
How is a specific cellular response brought about by a GPCR?
An extracellular signal working via a specific GPCR will activate a single/small sub-population of G-proteins and effectors in the cell
What is the ligand, G-protein and effector for beta-adrenoreceptors?
Adrenaline/NA
Gs-alpha
+ adenylyl cyclase
What is the ligand, G-protein and effector for alpha-2-adrenoreceptors?
Adrenaline/NA
Gi-alpha
- adenylyl cyclase
What GPCR, ligand and effector are used for Gq-alpha proteins?
Alpha-1-adrenoreceptors
Adrenaline/NA
+ phospholipase C
What ligand, GPCR and G protein are used to stimulate cyclic GMP phosphodiester Awe?
Light
Rhodopsin
Gt-alpha
What ligand acts at M2/4 muscarinic GPCRs?
ACh
What G-proteins are used for M2/4 muscarinic receptors?
Gi-alpha
What effector does Gi-alpha activation act on?
-adenylyl cyclase
What ligand, G-protein and effector do M1/3 GPCRs use?
ACh
Gq-alpha
+ phospholipase C
Which pathway do even numbered muscarinic receptors follow the same as?
Alpha-2-adrenoreceptors
Which pathway do odd numbered muscarinic receptors follow the same as?
Alpha-1-adrenoreceptors
Name two ADP-ribosylate specific G-proteins.
Cholera toxin
Pertussis toxin
What mechanism of action does cholera toxin use?
Eliminates GTPase activity of Gs-alpha
Irreversibly activates Gs-alpha
How does cholera toxin irreversibly activate Gs-alpha?
ADP-ribosylation of the unit prevents deactivation of Gs protein mediated signalling
Cells in which area of the body are infected by cholera toxin?
Small intestine
What happens to the toxin complex in both cholera and pertussis toxin action?
It binds to the cell and the emzyme is ‘injected’ into the cell
What type of cell does Bordetella pertussis bacteria colonise?
Tracheal epithelial
What characteristic symptom does pertussis toxin cause?
Whooping cough
By what method of action does pertussin toxin cause whooping cough?
Interferes w/ GDP-GTP exchange so Gi-alpha is irreversibly inactivated
How does pertussis toxin cause irreversible inactivation of Gi-alpha?
ADP-ribosylation of the subunit prevents Gi protein activation by GPCRs
What is PIP3?
A more phosphorylated form of the membrane phospholipid PIP2 that regulates a variety of processes - often survival signals
How is adenylyl cyclase regulated by agonist stimulation?
Ligand on receptor activates G-protein –> guanine nucleotide transfer –> activated Gs-alpha –> stimulates adenylyl cyclase –> secondary messenger cAMP formed
Which receptors are Gs coupled receptors?
Beta-adrenoreceptors
D1 dopamine receptors
H2 histamine receptors
Which receptors are Gi coupled receptors?
Alpha 2-adrenoreceptors
D2 dopamine receptors
My-opioid receptors
What does cAMP act on?
cAMP-dependent protein kinase A (PKA)
Epacs (GEFs)
Cyclic-nucleotide-gated ion channels (CNGs)
Describe the structure of PKA.
2 regulatory subunits sat in active site of 2 catalytic subunits
How does cAMP activate PKA?
Binds to regulatory subunits –> catalytic subunits dissociate exposing active site –> catalytic subunits phosphorylate specific target proteins in the cell that have serine or threonine residues
How is phospholipase C regulated by agonist stimulation?
Ligand on receptor activates G-protein –> guanine nucleotide transfer –> activated Gq-alpha –> stimulates PLC –> PIP2 into IP3 and DAG
Which part of the PIP2 molecule forms IP3?
Polar head group
Which part of the PIP2 molecule forms DAG?
Lipid moiety
What intracellular affect does IP3 have?
It releases calcium ions
What affect does DAG have?
Acts on PKC which can act on lots of different molecules to affect activity
Which receptors are Gq coupled receptors?
Alpha 1-adrenoreceptors
M1 muscarinic receptors
H1 histamine receptors
What is the advantage of signal amplification?
Modest concentration changes cause significant effects within cell
What affect does a few hundred molecules of adrenaline binding to cell surface beta-adrenoceptors have?
Glycogenolysis in the liver causing milimolar changes in blood glucose concentration
What magnitude of amplification does adenylyl cyclase itself cause?
Little
What follows activation of adenyly cyclase?
Lots of cAMP molecules released which activate PKA
Which two molecules cause ventricular mass to contract more forcefully?
Blood-borne adrenaline
Sympathetically released NA
Which receptors are activated to cause increased for of ventricular contraction?
Beta 1-adrenoreceptors
What is an increase in the force of contraction also called?
+ve inotropy
What causes an increase in the force of contraction?
VOCC phosphorylation by PKA –> each depolarisation allows in slightly more calcium than usual –> triggers CICR
What do sympathetically released NA and some blood-borne adrenaline interact with to cause vasoconstriction?
Alpha 1-adrenoreceptors
What causes bronchoconstriction?
Parasympathetic Ach action on M3-muscarinic receptors
What causes GI and GU smooth muscle contraction?
A variety of agents on GPCRs
What mechanism do all methods of regulation of smooth muscle tone utilise?
Gq-phospholipase C-IP3/Ca2+, DAG/protein kinase C pathways
What allows for coordinated increase in contractility of smooth muscle?
Utilisation of the same pathways
What often modulates neurotransmitter release in the CNS and PNS?
Pre-synoptic GPCRs
What is encephalin?
An endogenous ligand
What receptors do morphine and encephalin bind to?
Mu-opioid
Which subunit has most affect in the activation of mu-opioid receptors?
G-beta-gamma
What action does the g-beta-gamma subunit have?
Causes modulating inhibition of specific types of VOCC (very different to those in the heart)
What causes turned down neurotransmitter release?
Decreased calcium influx from specific VOCC modulating inhibition causing reduced vesicle docking
What allows for diversity in effector mechanisms of signal transduction?
Range of stimuli
Receptors
G-proteins
Effectors
What gives specificity of effector mechanisms in signal transduction?
Specific ligand-receptor interactions
Specific G-alpha subunits recruited coupled to particular effector pathways - different between cells but hard wired so that in the same cell type they have the same response
What benefit does amplification give to signal transduction?
Allows for gain of control on signalling pathways
