Regulating Intracellular Calcium Flashcards
(36 cards)
In what time frames do the following events occur?
- Synaptic vesicle release
- Excitation-contraction coupling
- Smooth muscle relaxation
- Excitation-transcription coupling
- Gene transcription
- Fertilisation
- Synaptic vesicle release (ms)
- Excitation-contraction coupling (ms)
- Smooth muscle relaxation (ms-sec)
- Excitation-transcription coupling (min-hours)
- Gene transcription (hours)
- Fertilisation (hours)
What happens if there is too little intracellular calcium activity?
Isolation of cell, decreased survival chance
What happens if there is too much intracellular calcium activity?
Seizures, toxicity, cell death
What is the distribution of calcium in the body?
Ca comprises 1.5%-2% of total body mass
99% is in the bone, but this can change significantly
Serum Ca is <0.1% of total
What are the 3 kinds of Ca homeostasis, and the amount of Ca implicated?
Cellular Calcium (1-10 to 50 nM)
Serum Calcium (1.3 mM)
Bone calcium (continuously changes)
How does calcium transmit information?
- The intracellular calcium concentration goes from nanomolar to micromolar
- Ca can then bind to specific proteins in the cell (at a lower concentration they wouldn’t have)
- Ca occupancy results in 3D conformational change and changes in activity
Intracellular calcium concentration at rest?
< 10^-7 M
Extracellular calcium concentration at rest?
~ 10^-3 M
ER calcium concentration at rest?
~ 10^-3 M
Why and how do cells maintain a low intracellular Ca concentration?
So that a little Ca entering the cytosol will cause a large increase in concentration
They maintain this by actively pumping Ca out of the cytosol (Na/Ca exchanger, PM Ca ATPase), and pumping Ca into the ER and mitchondria (SERCA, Ca binding molecules in cytoplasm, Ca pump on mitochondria)
Is free or stored Ca regulated?
Free
What are the 6 major pathways for cellular Ca increase?
- Voltage gated Ca channels (VGCC)
- Receptor operated Ca channels (ROCC)
- Na/Ca exchanger
- Ca ATPase of sarco(endo)plasmic reticulum
- Ca ATPase of plasma membranes (in cell)
- Ca releasing channels of the SR or ER (in cell)
How is calcium released from intracellular stores?
Ligand binds to GPCR linked to G-alpha-q -> activates PLC -> PLC catalyses PIP2 into IP3 and DAG > IP3 binds to IP3 Receptors (IP3R) on ER surface > calcium is released into cytosol
What is the structure of the IP3 Receptor?
- A macromolecular complex: monomers form functional tatramers (4 subunits)
- Have ligand binding domain, regulatory domain, and channel domain
- 6 membrane spanning regions: M1-M6
- Four binding sites for IP3 (one on each subunit)
- Has an IP3 sensor
- Has a calcium release channel
- At least 3 subtypes exist (1, 2, and 3)
- IP3R1 is the main type in the brain (cerebellum)
What is ‘store-operated calcium entry’?
Depletion of intracellular calcium stores activates calium entry to subsequently refill the emptied calcium stores
How does store-operated calcium entry work?
Two key components; a sensor of calcium store depletion (STIM - stromal interacting molecule) and a channel that when activated by the sensor allows Ca into the cell (ORAI proteins)
STIM proteins on the ER membrane oligomerize and move to areas of the ER that are close to the plasma membrane, where they activate ORAI channels and allow Ca entry
What are agents that can prevent Ca resequestration and thereby cause store depletion?
Thapsigargin and cyclopiazonic acid (CPA)
What are ORAI proteins?
Form Ca pore on plasma membrane
Tetrameric proteins
Coded for by 3 genes; ORAI1, ORAI2, ORAI3
What are STIM proteins?
Proteins that sense when the intracellular Ca store is depleted
Coded for by 2 genes; STIM1 (responds to agonist-mediated Ca store depletion) and STIM2 (sensor of basal ER Ca levels)
Ca binds with low affinity to the EF-hand domain of STIM proteins when the ER store is full, and releases itself when depleted, leading to activation of the CRAC channel (unbound STIM leads to CRAC channel activation + entry of Ca regardless of whether the ER store is depleted)
STIM1 binds to the microtubule plus-end tracking protein EB1 (end bnding protein 1) and associates with tubulin at the growing plus-end of microtubules. STIM1 can thus cover long distances within the cell
What happens if there is an increase in mitochondrial Ca?
Increase in mitochondrial Ca causes mitochondria to stimulate ATP production
What happens when the mitochondrial Ca capacity is exceeded?
The opening of a high-conductance channel (the mitochondrial permeability transition pore (PTP)) is induced, which leads to collapse of the proton gradient, and ATP production, and to a loss of mitochondrial membrane integrity and some forms of cell death
Mitochondrial Ca homeostasis and cell death?
Ca accumulation by mitochondria is common in necrotic and apoptotic cell death, and may induce apoptosis
Entry and exit of Ca into/out of the mitochondria
Mitochondria can maintain low matrix Ca concentrations at rest, but can also quickly increase conc when cytosolic Ca is increased
Outer mitochondrial membrane (OMM) is highly permeable to Ca due to abundant expression of voltage-dependent anion channels (VDAC)
Ca entry across the IMM is dependent on electrical + chemical gradients
Ca exit acroos the IMM mediated by Na dependent and independent pathways
Mitochondria as cytosolic Ca buffers
- Mitochondrial Ca buffering regulates the activity of Ca channels
- Mitochondria quickly remove Ca ions from mouths of open channels on the ER, SR, or plasma membrane
- Coordinates subsequent IP3-induced rises in cellular Ca into single propagating Ca waves of lower frequency and higher amplitude
- Removal of Ca from the vicinity of IP3 inhibits channel opening, but relieves Ca-mediated inhibition of open channels - Control of Ca gradients through mitochondrial positioning
- Mitochondria may block off a certain area in a cell before which there isn’t much Ca and after which there is
