Role of Ca2+ channels in cell signalling Flashcards

1
Q

What are the 2 broad types of cell signalling mechanisms?

A

Electrical
= e.g. propagation of action potential

Second Messenger
= e.g. transduction of signals by ready diffusible cytosolic molecules = cytosolic Ca2+

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What are the 3 cytosolic Ca2+ dynamics? (+examples)

A

Ca2+ waves
= e.g. medaka fish eggs - synchronising activation of egg

Ca2+ gradient
= e.g. pollen tubes - tip high Ca2+ gradient to induce exocytosis in growing tip

Ca2+ oscillations / sparks
= e.g. smooth muscle - excitation / contraction coupling

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What are Ca2+ signals the result of?

A

Two opposing functions:

influx into
+
efflux out of

(the cytosol)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What are the energetics of Ca2+ transport?

A

cytosolic Ca2+ = maintained at approx. 100-200 nM at resting levels

inside cytosol of cell = negative membrane potential = -60 - -300 mV

= electrochemical potential gradient needed for Ca2+ influx into cytosol

= ALWAYS a driving force favouring Ca2+ INFLUX into cytosol

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What are the most important aspects of Ca2+ transport energetics?

A

INFLUX
= energetically downhill process
= ALWAYS via ion channels

EFFLUX
= energetically uphill process
= energised by ATP hydrolysis (pumps) or driver ion gradients (carriers)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What are the 2 pools of Ca2+ outside the cytosol?

A

Extracellular
= Ca2+ influx into cytosol across the plasma membrane

Intracellular
=. Ca2+ influx into cytosol from discrete compartments within the cell

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What are some examples of intracellular pools of Ca2+? (where are they found)

A

ER (ubiquitous)
SR (muscle cells)
Nucleus (ubiquitous)
Vacuoles (plants and fungi)
Chloroplasts (plants)
Mitochondria (ubiquitous)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What are some examples of Ca2+ permeable channel families in animal cells?

A

Endomembranes
= IP3R (inositol triphosphate receptor) - 3 members
= RyR (ryanodine receptor) - 3 members

Plasma membranes
= Voltage-gated Ca2+ permeable channel family - 10 members
= TRP - 32 members
= CNGCs - 6 members
= HCN - 4 members

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the structure / function and monomer organisation of IP3R (3)

A

Formed as a tetramer (4 monomers), total size over 1 MDa

(1) N-terminus forms ligand binding site

(2) Large central domain forms regulatory domain
= Ca2+, CAM, ATP binding
= phosphorylation by PKA and TK

(3) C-terminus contains 6 TMS domains
= channel pore

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are the biophysical properties of IP3 receptors?

A

Selectivity
= PCa: PK = 4.1
= the pore region formed between TMS 5 + 6

Pharmacology
= blocked by heparin

Single channel conductance
= 350 pS (can allow a lot of Ca2+ in at once)

Gating
= IP3-dependent
= IP3 binding affinity (+sensitivity of channel activation) is modulated by: Ca2+, ATP + kinases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What are the 3 isoforms of IP3R, where are they expressed?

How do they differ?

A

IP3R-1
= CNS

IP3R-2
= hepatocytes + lymphocytes

IP3R-3
= cardiomyocytes

= differ in their response to IP3

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is the result of different responses to IP3 of IP3R isoforms?

A

High Ca2+ = does NOT inhibit IP3R activity
= CICR (calcium induced calcium release)

Low Ca2+ = acts as antagonist = mechanism for rapid negative feedback = short, self limiting pulses of Ca2+ release

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What do different Ca2+ signals generated by different subtypes of IP3R indicate?

A

That different sensitives to IP3 and Ca2+ are physiologically relevant.

Cells expressing type 2 = have similar response in WT = suggests this subtype predominantly involved in cell response

e.g. study using different IP3Rs to generate different Ca2+ signatures in lymphocytes challenged with antigen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is the structure function relationship of voltage gated Ca2+ channels (VGCCs)?

A

alpha subunit
= 175 kDa
= main part forming channel pore

alpha-2 subunit
= 143 kDa
= extracellular and linked by disulphide linked to delta subunit

delta subunit
= 27 kDa
= fixed in membrane by one TMS domain

gamma subunit
= 32 kDa
= 4 TMS

beta subunit
= 52-78 kDa
= cytosolic
= elevates size of current in pore forming aplha subunit

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Give more detail about the structure of the alpha-subunit?

A

4 repeats of S4 family

S4 domain = voltage sensor

P-region = pore forming with conserved E

binding site for:
DHP, B-subunit, RyR

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What are the biophysical properties of voltage-gated calcium channels?

A

Activated by membrane depolarisation

Allow extracellualr Ca2+ influx across plasma membrane

Present in all EXCITABLE CELLS in the plasma membrane

Highly selective for Ca2+

17
Q

How do VGCCs act as signal transducers?

A

They convert electrical signals to Ca2+ signals.

Action Potential
Membrane Depolarisation
VGCC activation
Ca2+ influx
Elevation of cytosolic Ca2+
Downstream effect (response)

18
Q

What are the 3 most researched ‘responses’ from Ca2+ elevations?

A

Excitation - contraction coupling

Exocytosis - e.g. releasing neurotransmitter across synaptic gap

Gating ion channel activity

19
Q

What is an example of signal amplification?

A

Excitation-contraction coupling in skeletal muscle

20
Q

What are the (7) steps of Exocytosis at synaptic junctions?

A

(1) Action potential
(2) Membrane depolarisation
(3) VGCCs activation
(4) Ca2+ elevation
(5) Exocytosis and secretion of neurotransmitter
(6) Activation of ligand gated channels in post-synaptic membrane
(7) Depolarisation fires a new action potential

21
Q

What is an example of gating of ion channels?

A

Regulation of smooth muscle tone by BKCa channels

= controls vasoconstriction / dilation = blood pressure

22
Q

What are the 4 main isoforms of VGCCs (+where are they expressed)?

A

L-type / CaV1
= skeletal , cardiac
= EC - coupling
= sensitive to DHP
= 25 pS

N-type / CaV2
= neurons, endocrine cells
= neurotransmitter release at synaptic gaps
= sensitive to conotoxin
= 13 pS

P-type / CaV2
= purkinje cells
= sensitive to FTX (funnel spider toxin)
= 10-20 pS

T-type / CaV3
= heart cells
= single channel conductance tiny
= sensitive to mibefradil

23
Q

How do the different VGCC isoforms differ?

A

Have different sensitivities to membrane depolarisation.

e.g. T type (blue)
= LOW threshold for activation
= Ipeak = +0 mV
= activation potential = -50 mV

e.g. L, P , N types (red)
= HIGH threshold for activity
= Ipeak = +30 mV
= activation potential = -20 mV

24
Q

What is a summary of VGCCs

A

Have different isoforms with different biophysical properties
= different single channel conductances
= different sensitivities to membrane voltage / depolarisations

Have different expression patterns

Modulation of the alpha pore subunit by auxiliary subunits (beta, gamma , alpha2delta)

25
Q

What is a summary for IP3Rs?

A

Have different modulation of channel activity by Ca2+, ATP + phosphorylation

Have different isoforms with different biophysical properties

Have different expression of IP3R isoforms in different cell types (abundance and type)

Can be formed from heterotetramers
= different combinations of types = diversity