4th Nov - Control of cytosolic calcium Flashcards
Calcium regulates many cellular processes, give some examples
Gene expression Apoptosis Neurotransmission Learning and memory Metabolism Necrosis Intracellular signalling
What is the equilibrium potential for calcium?
+123 mV
What are the advantages of a large inward calcium current?
Changes in [calcium i] occur rapidly with little movement of calcium
Little calcium has to be removed in order to re-establish resting conditions
What are the disadvantages of a large inward calcium current?
Extrusion of calcium and maintenance of the gradient is energy expensive
If lose the ability to control calcium quickly leads to calcium overload and cell death
How is the calcium gradient set up and maintained?
Relative impermeability of the plasma membrane
Expulsion of calcium across the plasma membrane by Ca2+ ATPase and Na/Ca exchanger
Calcium buffers
Intracellular calcium stores
Describe the Ca2+ ATPase
Extrudes calcium by ATP hydrolysis
Has a positive feedback loop
High affinity
Low capacity
Outline the positive feedback loop for calcium ATPase
[Calcium int] increases –> binds calmodulin –> binds Calcium ATPase –> calcium removal
Describe the Na+/Ca2+ exchanger
An antiporter which exchanges 1 calcium ion for 3 sodium ions
Low affinity
High capacity
How does an antiporter work?
It works electrogenically, using the potential of one ion to transport another
When is the Na+/Ca2+ exchanger most effective?
When the cell is at resting membrane potential as it is reliant upon the sodium current. Therefore it is best used to get rid of the bulk of calcium at the beginning of extrusion
What are calcium buffers?
Calcium binding proteins which regulate calcium concentration such as paralbumin, alreticulin, calbindin and calsequesterin and ATP, thus how far calcium travels is dependent on these buffers (Calcium travels roughly 0.5µm before binding)
What is a trigger protein?
Calcium binding proteins which alter their function upon calcium binding
How does the activity of synaptotagmin alter depending upon where calcium binds?
Binding to Ca2A domain –> increases affinity for syntaxin
Binding to Ca2B domain –> dimerization
What are the two types of intracellular calcium stores?
Rapidly releasable
Non-rapidly releasable
How is calcium elevated and returned to basal levels?
- Calcium influx across the pm by voltage operated calcium channels and ionotropic receptors
- Calcium release from rapidly releasable intracellular calcium stores e.g. ER
- Calcium release from non-rapidly releasable intracellular calcium stores e.g. mitochondria
- Termination of signal
- Calcium removal
- Calcium store refilling
Give an example of a voltage operated calcium channel involved in calcium influx across the pm
L -type N-type R-type P/Q-type T- type
Give an example of an ionotropic receptor involved in calcium influx across the pm
NMDA receptors
AMPA receptors
ACh receptors
What are the potential roles of calcium release from rapidly releasable intracellular stores?
Synaptic plasticity e.g. LTP
Gene expression
Post-synaptic signalling
What is calcium induced calcium release?
A process important in cardiac myocytes in which small Ca2+ influx from outside of the cell leads to a massive release of calcium from the sarcoplasmic reticulum
How is release from rapidly releasable calcium stores mediated?
Through VOCC –> RyR and IP3 receptor stimulation (CICR)
Through Gq –> IP3
If mitochondrial uptake only occurs when [Ca2+i] is very high, why is it relevant in normal signalling?
Because signalling microdomains mean that the overal calcium domain could be within normal physiological range however at the mitochondria the calcium concentration could exceed that required for calcium uptake
What is the transport mechanism for mitochondrial uptake of calcium?
A uniporter driven by respiratory chain proton production, which has relatively low affinity and high capacity
What is the role of mitochondrial calcium uptake in the cell?
Calcium buffering - regulate the pattern and extent of calcium signalling
Apoptosis - mitochondrial calcium overload –> cell dear
Stimulation of mitochondrial metabolism
How are the intracellular calcium stores refilled?
Through recycling of released calcium
VOCC and/or capacitative calcium entry
What is capacitative calcium entry?
When there is a low calcium signal in the store –> opening of a store operated calcium channel (SOC).
This is done by stromal interacting molecule (STIM) 1, which aggregates on store emptying, as Calcium is no longer bound to it’s EF hand domain, opening up it’s coiled-coil domain for oligomerisation. STIM1 aggregates then interact with calcium release activated channels (CRAC) causing calcium entry.
How can calcium levels be monitored?
Radioisotopes - use 45 Ca2+
Electrophysiology
Fluorescent calcium indicators
What are fluorescent calcium indicators?
Dyes or genetically encoded sensors which measure calcium levels in cells in real time
How are cells loaded with calcium dyes?
Microinjection - but difficult to inject multiple cells
Use fluo-4-acetoxymethyl which allows it to pass through the plasma membrane by masking the charge.
What is the excitation wavelength of fluo-4?
494nm
How can calcium regulate so many different processes within the cell?
Spatial regulation - dependent on influx and microdomains
Temporal regualation - nature of transients provides information about the nature of the signal and the events to be regulated e.g. NT release requires a transient signal and gene transcription requires repetitive global transients
Amplitude of the signal
