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Dynamic Cell > GPCR 2 - Gq + IP3 > Flashcards

GPCR 2 - Gq + IP3 Flashcards

1
Q

examples of G1-activating signals + responses

A

Liver
- vasopressin
= glycogen breakdown

Smooth muscle
- thrombin
= platelet aggregation

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2
Q

Gq proteins activate inositol phospholipid pathway

A

GPCR activated
->activates alpha subunit

  • > activates phospholipase C-beta
  • > cleaves PIP2
  • > IP3 + DAG
  • > initiate downstream events
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3
Q

IP3 role

A

increases cytoplasmic [Ca2+] from ER

  • > activates calmodulin
  • > activates CAM kinases
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4
Q

DAG role

A

activates protein kinase C

-> actives IkB/NF-k-B pathway

OR

-> MAP-kinase pathway

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5
Q

Gq + Ras signalling pathways

A

cross-talk between the 2

Raf->Mek->Erk->
nucelar proteins

C-kinase activates Raf by phosphorylation

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6
Q

protein kinase C inactivates inhibitor of NFk-B

A

Ik-B bound to NF-kB

PKC phosphorylates IkB

  • > Ik-B degraded through proteasome
  • > releases Nf-kB
  • > enters nucleus and laters transcription
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7
Q

IP3 increases cystolic [Ca2+] + activates PKC

A
  1. activated Gq activates phospholipase C-beta
  2. converts PIP2 to IP3 + DAG
  3. IP3 binds to gated Ca2+ channels on ER
  4. Ca2+ release
  5. Ca2+ and DAG bind to and activate protein kinase C
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8
Q

treating cells with TPA + Ionomycin

- induces..?

A

proliferation

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9
Q

treating cells with TPA + Ionomycin

-mechanism

A

TPA mimics DAG action
- binds to PKC

Ionomycin mimics IP3-gated channels
- binds + transports Ca2+ across ER membrane

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10
Q

Ca2+

  • role
  • effects mediated by?
A

triggers embryo development post-fertilisation

muscle contraction

secretion in nerves + other secretory cells

mediated by Ca2+ response proteins

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11
Q

mechanisms that keep [Ca2+] low

A

Ca2+ pumps on PM re-establish Ca2+ levels in resting cells

Organellar Ca2+ pumps

Ca2+ binding proteins in cytosol reduce free [Ca2+]

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12
Q

Ca2+ pumps on PM re-establish Ca2+ levels in resting cells

A

secretory cells, nerves + muscles:

Na+/Ca2+ antiport in the PM

all cells:

Ca2+ ATPase in PM

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13
Q

Organellar Ca2+ pumps re-establish resting [Ca2+]

A

Ca2+ ATPase in ER membrane
-> directs Ca2+ into ER from cytoplasm

active Ca2+ import in mitochondria
(emergency mechanism e.g. membrane breached)

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14
Q

Ca2+ release due to positive feedback

A
  1. Ca2+ binds to IP3-gated channels
  2. Ca2+ binds to ryanodine channels alos on ER

= rapid release of Ca2+ in cytoplasm

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15
Q

reversal of IP3 signal involves negative feedback

A

enzymes that turn over IP3 are activated by Ca2+

IP3
-> lipid phosphatase removes P 
-> IP2
-> can't bind to IP3-gated channels
= no Ca2+ released
IP3
-> lipid kinase adds P
-> IP4
-> Ca2+ ATPase activity 
= negative feedback
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16
Q

feedback generates Ca2+ waves + oscillations

A

local signalling events open IP3-gated channels

  • > Ca2+ out of ER
  • > Ca2+ opens other channels
  • > propagates the wave

reversed by high levels

  • > channels start to close
  • > resting state
17
Q

hepatocyte response to increasing [vasoperessin]

A

Ca2+ amplitude is constant

BUT frequency increases with vasopressin concentration

18
Q

intracellular Ca2+ levels oscillate due to feedback

- positive

A

Ca2+ induced Ca2+ release

via IP3 and ryanodine-gated channels

19
Q

intracellular Ca2+ levels oscillate due to feedback

- negative

A

high Ca2+ stops further Ca2+ release

high Ca2+ stimulates lipid kinase -> IP4

IP4 -> promotes Ca2+ removal from cytosol

20
Q

Ca2+ oscillation frequency-dependent responses

A

release pituitary hormones is pulsatile
-> pulses follow Ca2+ spike freq

some cells activate gene transcription in response to spike freq

21
Q

calmodulin and Ca2+

A

has x4 Ca2+ binding sites

-> Ca2+ causes conformational change (kink)

  • > can then recognise specific target proteins
    e. g. Ca2+ ATPase in PM as part of -ve feedback
22
Q

CaM-kinases

A

serine/threonine kinases

an activate CREB
- cross-talk with Gs/cAMP signalling

cell type specific response to increasing [Ca2+]

23
Q

CaM-kinase specificity

A

Narrow:
myosin light-chain kinase
- smooth muscle contraction

phosphorylase kinase
- glycogen breakdown in liver

Broad:
CaM-KII
- multifunctional

24
Q

CaM-kinase II

- molecular memory

A

activity continues after [Ca2+] drop
and Ca2+/calmodulin dissociates from it

due to autophosphorylation

25
Q

CaM-kinase II

- hexameric complex

A

functions as a homo-hexameric complex

6 subunits bound

Ca2+/Calmodulin activates each subunit

26
Q

CaM-KII activity based on Ca2+ oscillation freq

  • low freq
  • high freq
A

CAMK-II dephosphorylated after each Ca2+ spike

CAM-II activity ratchets up as more molecules phosphorylated

27
Q

when Cam-KII activity reaches max…?

A

increasingly difficult for phosphates to compete

when all subunits of each CaMK-II complex are phosphorylated

-> full activity maintained at relatively low Ca2+ levels

28
Q

desensitisation of GPCR

A

phosphorylation by PKA, PKC or GRK (=GPCR regulatory kinase)
of serine/threonine reissues on cyto loops

arrestin binds to phosphorylated GPCR
- blocks GPCR from receptor

Dynamic Cell (15 decks)

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