Calcium Homeostasis 1 Flashcards
What are the calcium stores called within cells
Normal cells - endoplasmic reticulum
Muscle cells - sarcoplasmic reticulum
Where is the calcium concentration the highest
Inside the cell (20-300nM) compared to outside (2nM)
What does a persistent high intracellular calcium suggest
Apoptosis
How do calcium enter the cell
Down concentrated gradient into cell via voltage gated calcium channels
How can the calcium levels increase inside the cell
Via release of calcium from the endoplasmic reticulum
OR
Down concentration gradient into cells via voltage-gated calcium channels
How is calcium released from the endoplasmic reticulum
IP3 (produced by phospholipase C via classical G protien-coupled receptor. The IP3 binds to the IP3 receptor on the membrane of calcium stores which opens the IP3R as also a channel which therefore releases calcium
OR
Ryanodine receptor opening
How is calcium decreased inside the cell (4 ways)
Calcium-sodium exchanger
Buffering calcium proteins
ATP-dependent pumps
Endoplasmic reticulum pumps - take calcium into ER to refill the calcium vesicles
Where is calcium found in the blood
50% on protein-bound
Conc same as phosphorus
Give the 3 main calcium pools
Inside ER
Blood - plasma proteins
Bone - most is mineral but 1% is free
How does calcium get into ER or SR
Via high-affinity calcium uptake through calcium-ATPase transporters
Form of active transport
These pumps are called SERCA - sarco/endoplasmic reticulum Ca-ATPase
What are the name of the calcium channels that pump calcium ions back out of cell when desired concentration is achieved
PMCA - transports calcium out of the cell via high-affinity calcium uptake through calcium-ATPase transporters
How do sodium-calcium exchangers work
Get calcium out of the cell in exchange for sodium into the cell.
Work at high concentration - (low affinity) but
High capacity (large exchange so fast)
Works first to get bulk of calcium out
How does the ATP-dependent calcium pump differ in its capacity and affinity
Works at low concentration so (Affinity is high)
Low capacity (small exchange so slow)
Basically fine tunes the calcium level
How is calcium brought back into the calcium vesicles in the ER without increasing cytoplasmic calcium levels
STIM1 and Orai1 are proteins as part of a store-operated calcium entry -
IP3 opens IP3R to release calcium which empties the stores
The store emptying triggers Orai1 (Cell membrane) and STIM1 (ER membrane) to align and form a single channel, allowing calcium to flow into the endoplasmic reticulum
Give 4 calcium binding sites
Membranes
Amino acids
Nucleotides
Binding proteins
Give some features of calcium binding proteins
Rapid with high affinity
Some are free and some restricted
Free - parvalbumin, calbinding, calretinin, calmodulin
Restricted - calreticulin and calsequestrin
What is the purpose of calcium binding proteins/sites
Keep enough extra “inactivated” calcium present to refill calcium vesicles
What is the mitochondria’s role in calcium uptake
When calcium is of high conc then the mitochondria will be activated, so much so that if the calcium conc is too high then the mitochondria stops ATP synthesis and puts all efforts into bringing calcium back down to prevent sustained toxic periods of high calcium ion conc intracellularly
How is IP3 activated to work on IP3R to release calcium from ER (2 ways)
Tyrosine kinase linked receptor e.g. epidermal growth factor
OR
G-protein coupled receptor e.g. glutamate receptor and olfactory receptors
What is the purpose of different IP3 isoforms
Theres 4 types -
All have different affinity and sensitivity for calcium
What can block IP3 formation
Lithium
What can block IP3 binding
Heparin
How does the IP3R open
IP3 and calcium bind to cytosolic side of receptor to open it
Why is calcium binding to the IP3R important
Promotes channel closure via feedback inhibition - high cystolic calcium and calcium ATPase pump activity turns off the IP3R - causes calcium oscillations
Why are these calcium oscillations important
Believed that specific oscillations may be able to turn gene expression on etc
What is the structure of IP3
A tetramer - each with 6 membrane spanning domains
Central pore for calcium
Each tetramer has 4 IP3 binding sites which activates its opening
How do IP3 channels actually open
Conformational change on IP3 binding, this hinges suppressor region away from gatekeeper region and allows channel to open.
What role does STIM1/ORAI1 have in store-operated calcium entry in endothelial cells
WIDER READING - Abdullaev 2008
Knockdown of orai1, stim1 and stim2 showed that it inhibited EC proliferation and caused cell cycle arrest. Therefore store-operated calcium (Ca2+) entry is crucial for endothelial cell proliferation in mice.
Also shows that SOCE through STIM1-ORAI1 not TRPC1 as previously believed
What role did STIM1 and ORAI1 have in T cells
WIDER READING - lioudyno et al 2008
STIM1 and Orai1 are recruited to the immunological synapse between primary human T cells and autologous dendritic cells. Both STIM1 and Orai1 accumulated in the area of contact between either resting or superantigen (SEB)-pretreated T cells and SEB-pulsed dendritic cells, where they were colocalized with T cell receptor (TCR)
Also showed area of higher calcium concentration at the interface
What joins mitochondria and ER and how much is joined
WIDER READING - Rizzuto et al 1998
20% - mitochondria-associated membranes
How does calcium enter the mitochondria once released by the IP3Rs
WIDER READING - De Pinto 1992
The Ca2+ released through IP3Rs is transferred to the mitochondrial intermembrane space by a class of mitochondrial porins known as voltage-dependent anion channels (VDACs), which form very abundant and large voltage-gated pores in the outer mitochondrial membrane at the ER-mitochondria contacts
How does the tumour suppressor gene PTEN regulate apoptosis
WIDER READING - Bononi et al 2013
PTEN enhances ER-Mitochondria calcium transfer to cause apoptosis. It does this by dephosphorylating IP3Rs which causes increased calcium release to the mitochondria to kill the cell and prevent a tumour
What was shown regarding calcium oscillations and gene expression
WIDER READING
- Doletsch et al 1998
We have developed a Ca2+ clamp technique to investigate the roles of oscillation amplitude and frequency in regulating gene expression driven by the proinflammatory transcription factors NF-AT, Oct/OAP and NF-κB. Here we report that oscillations reduce the effective Ca2+ threshold for activating transcription factors, thereby increasing signal detection at low levels of stimulation. In addition, specificity is encoded by the oscillation frequency: rapid oscillations stimulate all three transcription factors, whereas infrequent oscillations activate only NF-κB.
