Cell Signalling (2) Flashcards
(27 cards)
What are some important pathways in intracellular signalling pathways?
1) cAMP
2) cGMP
3) Ca2+
4) Ras/Raf/MAP Kinase
5) JAK/Stat
6) Inositol Triphosphate (IP3)
7) Diacylgylcerol (DAG)
What’s an important mechanism in intracellular signalling?
Phosphorylation
How is amplification important during transduction?
1) More molecules can be activated in fewer steps
2) Signal is amplified in cell
3) Reduces signal errors during transduction
4) Signal needs to be repeated several times before cell can respond
5) Signal would be too small for cells to recognise without 2nd messengers
What is chonotropy?
Refers to rate
What is inotropy?
Refers to force
What is the process of the cAMP pathway?
1) cAMP usually activates protein kinase A, which phosphorylates various other proteins
2) Regulation of cell metabolism is provided by G-protein systems that inhibit adenylyl cyclase
How is cAMP synthesised?
Adenylyl cyclase is stimulated by Gas to synthesise cAMP from ATP (and inhibited by Gai to prevent cAMP synthesis)
How is cAMP degraded?
Hydrolysis by phosphodiesterase to form AMP
What is cAMP mediated by?
Protein Kinase A (PKA)
What is the importance of cAMP as a signalling molecule?
1) Cell growth and differentiation
2) Gene transcription
3) Protein expression
4) Regulation of metabolism
5) Transfers information regarding cellular status
What are the functions of cAMP as a signalling molecule?
1) Transferring into cells the effects of hormones such as glucagon and adrenaline
2) Activation of protein kinases
What is Protein Kinase A (PKA)?
1) Inactive PKA - 2x regulatory subunits, 2x catalytic subunits
2) cAMP binds to R subunits which release active C subunits which phosphorylate target proteins
What target proteins are regulated by PKA-mediated phosphorylation?
1) Enzymes - those involved in glycogen metabolism
2) Channels - cystic fibrosis channel (CFTR)
3) Transcription Factors - CREB (cAMP response element binding protein)
What is Ca2+ role in contraction and dilation?
1) 2nd messengers (PLC leading IP3 generation), releases Ca2+ from within cell and increases intracellular Ca2+ levels - leads to contraction
2) 2nd messengers (cAMP and cGMP) lead to a decrease in intracellular Ca2+ - dilation
What is myosin light chain kinase (MLCK) in contraction?
1) Influx of Ca2+ into muscle via Ca2+ ion channels - contraction of smooth muscle fibres may begin
2) Ca2+ binds to calmodulin
3) Ca2+-calmodulin complex activates MLCK - phosphorylates MLCK
4) Enables myosin cross bridge to bind to actin filament - all contraction begins
5) Smooth muscles does not contain troponin complex - main pathway for regulating smooth muscle contraction
How does reversing contraction happen?
1) Reducing intracellular Ca2+ conc. inactivates MLCK but does not stop smooth muscle contraction - myosin light chain modified through phosphorylation
2) Dephosphorylation of myosin light chain occurs through activity of 2nd enzyme (MLCP, myosin light chain phosphatase)
How does cAMP affect Ca channels?
1) Catecholamines in system bind to + activate beta1 adrenoreceptors, leads to increase intracellular cAMP
2) cAMP activates PKA which phosphorylates sites on alpha1 subunits of Ca channels
3) Increases probability that channels will open, increasing inward Ca2+ current
What is the reason for having GPCR subunits?
1) 20 alpha, 6 beta, 12 gamma
2) Provides array of different G-proteins such as G(alpha)s and G(alpha)i which associate with different receptors and targets giving rise to variety of responses
What are the 4 families of G-protein alpha subunits?
1) G(alpha)s - stimulates adenylyl cyclase
2) G(alpha)i - inhibits adenylyl cyclase
3) G(alpha)q/11 - interacts with phospholipase C; PLC
4) G(alpha)12/13 - less well characterised, involved in cell proliferation and malignancy
What are the functions of cardiac alpha2 - adrenoceptors?
1) Reduce sympathetic outflow to heart, decreasing cardiac output by decreasing heart rate and contractility
2) Reduced sympathetic output to vasculature decreases sympathetic vascular tone - causes vasodilation and reduced systemic vascular resistance, decreases arterial pressure
What does G(alpha)s signal in cardiac cells?
1) Adrenaline/noradrenaline bind to beta1 adrenoreceptor - coupled to G(alpha)s
2) G(alpha)s activates adenylyl cyclase
3) Adenylyl cyclase (AC) dephosphorylates ATP to form cyclic AMP (cAMP)
4) cAMP activates PKA - causes increased influx of Ca2+ via phosphorylation and activation of L-Type Ca channels
5) Increased Ca2+ leads to increased inotropy and chonotropy
What is the role of G(alpha)i in cardiac cells?
1) Ach binds to muscarinic receptor - coupled to G(alpha)i
2) G(alpha)i inhibits adenylyl cyclase
3) Decreases cAMP
4) Inactivates PKA and decreases Ca2+ to cell, increases outward K+
What is the role of G(alpha)q/11 signals in cardiac cells?
1) Stimulates formation of IP3 from phosphatidylinositol biphosphate (PIP2)
2) Increased IP3 stimulates Ca2+ release by sarcoplasmic reticulum in heart, increases inotropy
What is the role of cardiac beta1 adrenoreceptors?
1) Activated beta1 adrenoreceptors increase Ca2+ sensitivity of contractile machinery by phosphorylating troponin C
2) Activated beta1 adrenoreceptors facilities Ca2+ capture by sarcoplasmic reticulum - increases amount of Ca2+ available for release by APs
3) Increase in HR results from increased slope of pacemaker potential
4) Increased Ca2+ entry also increases automaticity because of effect of Ca2+ on transient inward current - train of APs follows single stimulus
