Signalling pathways Flashcards
Beta-1 receptors, myocytes
Gs - increase cAMP, increase PKA
Positive isotropy (increased contractility) - increased Ca2+ entry
Po of L-type Ca channels - increased influx
Po of RyR2 - increases Po
Increased [Ca] on every heartbeat - increases contractility
Positive lusitrophy (increased rate of relaxation) - increased Ca removal Po of PLB (phospholamban) - removes inhibition of SERCA2A (pumps Ca into stores) Po of troponin I - decreases affinity for Ca, promotes muscle relaxation
Beta-2 receptors, smooth muscle
Gs, relaxation
Agonist = adrenaline
Po + inhibits myosin light chain kinase (MLCK)
P-MLCK - decreased affinity for CaM-Ca, cannot Po MLK
Unphosphorylated MLC = important in relaxtion
PKA metabolism
Increases metabolism
Po + activates phosphorylase kinase
Activates phosphorylase - important in glycogen breakdown/glycogenolysis
Po transcription factors such as CREB - binds to CRE regions of DNA, stimulates transcription of genes involved in glujconeogenesis
Alpha-1 receptors, smooth muscle cell
Gq
Agonist = adrenaline
Smooth muscle cell contraction - vasoconstriction
Activates Ca stores, increases Ca
Ca binds to calmodulin, conformational change
Ca-calmodulin complex binds + activates MLCK
MLCK Po MLC = important in contraction
ie. Salivary gland secretion
PKC activation
Ca binds to C2 region of PKC
cPKC has high affinity for lipids - translocates to DAG in the membrane
DAG binds, removes psedosubstrate motif out of binding site + activates
Exocrine secretion - ie. salivary gland
mAChR activates Gq
Ca = exocytosis of amylase-containing vesicles
Ca = activates Ca-activated Cl- channel - efflux of Cl-
Alpha-1
Gq
Beta-1
Gs
Beta-2
Gs
Alpha-2
Gi
Alpha-2, smooth muscle
Gi
Agonist = adrenaline
Blocks inhibitory pathway - blocks Po of MLCK
Contraction
Golf GPCR
Gs pathway - increases cAMP
Activates cAMP-gated channels
Olfaction
Gt GPCR
Transduction of light
Light - activates GPCR, GTP-bound-alpha subunit activates phosphodiesterase (PDE)
PDE cleaves cGMP –> CMP
No Na+ current - hyperpolarisation
Dark - GPCR not activated
cGMP activates cGMP-gated Na+ channels
Produces dark current
GIRK channels
Inwards rectifying K+ channels
Beta-gamma subunit mediated - Gi
GPCR - beta-gamma subunit activates GIRK channel
Hyperpolarisation
Receptor desensitisation
- Agonist mediated
* GPCR Po on the cytoplasmic tail by GRK (G-protein kinase)
* Po generates binding sites for arresting
* Arrestin binds - GPCR is internalised
* Signal stops, arresti detaches, re-inserted into cell membrane
* Pro-longed signal = internalised receptors in granules fuse with lysosomes + degraded - 2nd-messenger mediated
* Gs generates PKA
* PKA Po cytoplasmic tail of GPCR, inactivate signalling
* Depending on cell type/GPCR - can stimulate internalisation via arrestin binding
- Gq generates PKC
Direct: PKC Po cytoplasmic tails of the GPCR
Indirect: - PKC Po + activates GRK, Po tails
- PKC can Po regulatory proteins, bind to cytoplasmic tail, switch off GPCR signalling
Receptor tyrosine kinase
ie. RTK (growth factor receptor)
Growth factor binds
Dimerisation of receptors, autophosphorylation in the kinase domain, increases tyrosine kinase activity
Phosphorylation sites allow for the recruitment of signalling molecules, Po by the tyrosine kinase activity
PIP2
PLC, cleaved: PIP2 –> IP3 + DAG
PI3 kinase, phosphorylated: PIP2 –> PIP3
SH2 domain
src homology domain 2
src = non-receptor tyrosine kinase
SH2 = domain in signalling proteins that can bind to specific phosphorylated residues
Small G-proteins
Example: Ras, Rho, Rac
Ras mutations leads to tumours (Ras involved in control of the cell cycle)
GEF: stimulate GTP-binding (activate)
GAP: stimulate GTP hydrolysis (inactivate)
Adaptor proteins
ie. Insulin receptor substrate (IRS)
Insulin binds and activates insulin receptor
Recruitment and phosphorylation of IRS
PI3 kinase has SH2 domains (binds Po proteins) - binds to P-IRS, enables activation of PI3 kinase
PI3 kinase Po lipids (PIP2) in the membrane
PIP2 –> PIP3
PKB is recruited
PKB is phosphorylated + activated by PDK1 kinase- leads to fusion of vesicles containing GLUT4 transporters
Sos
GEF for Ras - activate Ras
MAP kinase pathway
Activated downstream of RTKs
Po and activated - recruit Grb-2 (adaptor)
Grb-2 Po and binds to Sos (GEF for Ras)
Activated Ras (binds GTP) activates Raf-1 kinase
Raf-1 Po Mek1/2 kinase
Mek1/2 kinase Po Erk1/2 k
Erk1/2 + Rsk mediates transduction of genes
MAP-K cascades have a mitogenic effect on cells - stimulate growth and proliferation
Cytokine receptors
Enzyme not intrinsic - associated (coupled) or bound to TK (not intrinsic to the receptor)
JAK-2 binds to cytokine receptor
Cytokine activates receptor - autophosphorylation and activation of JAK-2 kinase
JAK-2 kinase can Po transcription factors such as STATs
Po STATs, dimerise + enter nucleus
Cytoplasmic receptors
ie. Guanyl cyclase
Receptor in cytosol - ligand must be small + hydrophobic
Peptide bind to receptor
Activates intrinsic guanyl cyclase activity - stimulates an increase in cGMP, activates PKG
Leads to reabsorption of water
(Similar to how cAMP activates PKA)
- NO released by endothelial cells, bind to cytosolic guanyl cyclase (smooth muscle cells), increase cGMP, activates PLG
Smooth muscle cells Cytoplasmic receptor (guanyl cyclase)
NO released by endothelial cells, bind to cytosolic guanyl cyclase (smooth muscle cells), increase cGMP, activates PLG
Reduce IC [Ca]
- Po Ca2+ channels - reduce influx
- Po IP3 receptors (causes Ca release from IC stores) - less sensitive to IP3
cGMP activate PKG
- Po + activate MLC phosphatase - dephosphorylate MLC (important in relaxation)
- Po K+ channels leading to hyper polarisation
Cortisol receptor
Nuclear receptor
Ligand - small + hydrophobic to pass through membrane
Bind + translocate to the nucleus
Regulates transcription
- Cortisol receptor is bound to a chaperone protein, ligand binds + chaperone is released
Only nuclear hormone which binds in the nucleus - does not bind in cytosol + get translocated
Thyroid hormone
