Calcium Signaling Flashcards
Ca2+ gradient
STEEP gradient across cell membrane; 20,000 fold lower in cytoplasm than in extracellular fluid; plus cells have membrane potential of -60mV favoring Ca2+ influx increasing gradient further; PM VERY impermeable for Ca2+
Ca2+ cell signals
non excitable cells -fertalization - proliferation - metabolism - secretion excitable cells - neurons - B cells - skeletal muscle - cardiac muscle
Ca2+ On reactions
- no spontaneous transport across cell; can be voltage or ligand gated channel
- GPCR and enzyme linked receptors -> activators/ agonists -> bind with Ca2+ channel -> Ca2+ released from intracellular stores in SR/ER to cytoplasm
Ca2+ Off reactions
- Ca2+ pump: SR, ER, PM; use ATP to pump Ca2+ against [ ] gradient
- Na+/ Ca2+ ; use Na+ gradient to move Ca2+ across cell membrane
Ca2+ enters cytoplasm
small amount binds to calcium effector proteins rest is bound to buffers in cytoplasm or pumped into SR or ER via SERCA pump; in stores bound to calcium-binding proteins (calsequestrin in SR) allowing lg amounts of calcium accumulation; if high [Ca2+] mitochondria will accumulate calcium
Name of calcium binding protein for SR
calsequestrin
SERCA stands for
sarco(end)plasmic reticulum Ca2+-ATPase
SR
ER in muscle cells
intracellular Ca2+ stores
SR, ER, mitochondria
Ca2+ extrusion
Removing Ca2+ from cell
Ca2+ in plasma membrane (Ca2+-ATPase) in all cells
Na+/Ca2+ exchanger (NCX) in excitable cells; couples Ca2+ to Na+ moving down electrochemical gradient
Ca2+ signal ranges
depends on cell type; depends on combination of on and off reactions and effectors; Amplitude, Duration, Oscillation Frequency, Spatial
Amplitude
Ca2+ signals can be v large (nerve cells) or v small (non-excitable cells)
duration
brief (microseconds) (nerve cells) or persist (hours) (transcription and cell proliferation)
oscillation frequency
oscillate (or spike) in cell types, duration of spike and their frequency determine strength and nature of response; responses can be oscillatory but Ca2+ oscillations may elicit a sustained response
spatial
may be throughout whole cell or in small region of cell
global Ca2+ wave intacellular
signal spreads throughout cell b/c coordinate activity numerous elementary events-> signal spreading throughout cell ex muscle contraction and gene transcription
global Ca2+ wave intercellular
signal spreads to neighboring cell; ex wound healing and insulin secretion from pancreatic islets
Elementary events
localized within cell (Ca2+ enters cytosol from external store)
ex. vesicle secretion, opening Ca2+-activated channels, and mitochondria metabolism
Ca2+ activates Ca2+ effectors
Ca2+ binding protein Ca2+/ calmodulin related enzyme Ca2+ regulated enzyme Ca2+ responsive transcription factor Ca2+ sensitive ion channel
Ca2+ signaling in excitable cells
- usually initiated by voltage-sensitive calcium channels in PM = activated by cell depolarization (some also have ligand-gated or receptor-operated channels which open bc binding of extracellular transmitter)
Nerve terminal
exocytic release of synaptic vesicles activated by influx Ca2+ through voltage-sensitive calcium channel; channel bound to syntax, SNAP-25, and synaptobrevin which carry out fusion event initiated by synaptotagmin
Cardiac muscle cell
depolarized; l-type voltage sensitive calcium channels in T tubule membrane activated -> Ca2+ in cell -> ryanodine receptors activated -> SR release Ca2+ -> amplified signal -> Ca2+ binding to troponin C -> myofibril contraction
ryanodine receptor
intracellular Ca2+ release channel
mitochondria Ca2+
Ca2+ influx into mitochondria stimulates mitochondrial metabolism to provide ATP necessary to sustain contraction
phospholamban
inhibitor of SERCA pump, inactivated by cyclic AMP-dependent phosphorylation -> Ca2+ -> SR
Ca2+ signaling in non-excitable cells
involves receptor activation and generation of second messengers that ultimately -> increase in intracellular Ca2+ via several mechanisms
-Inositol Phospholipid Signaling
Inositol Phospholipid Signalling
GPCR and TK linked receptors stimulated -> activation phospholipase C (PLC) -> cleave phosphatidyl inositol 4,5-bisphosphate (PIP2) from diacylglycerol and inositol triphosphate (IP3)
->diacylgycerol recruits protein kinase C (PKC) to PM -> activates PKC
and
-> IP3 binds IP3 gated Ca2+ release channels (IP3 receptors) -> release Ca2+
overall
-> increase cytoplasmic Ca2+
protein kinase C
feedback loop for Ca2+; diacylgylcerol provides landing site for this
IP3
inositol triphosphate; MAJOR signaling hub modulating this can -> bispectrum of signals
How do non excitable cells maintain Ca2+ signals for prolonged time
activate calcium channels in PM allowing Ca2+ influx into cells; most common channels= store-operated calcium channels (SOCs)
Store operated calcium channels (SOCs)
IP3 -> intracellular calcium stores empty -> activation store-operated calcium channels
include CRAC channel
Orali
Orai1 form CRAC channel pore
CRAC channel opening
Depletion Ca2+ stores leads to activation and accumulation of STIM1 in regions ER close to PM; Orali accumulates in apposed regions PM; molecular interaction these 2 proteins -> Ca2+ channel opening
STIM1
ER Ca2+ sensor; unusual bc inhibited by Ca2+ so when Ca2+ levels in cytoplasm high this is inhibited which in turn stops it from activating CRAC channel preventing Ca2+ release
Restoration resting Ca2+ levels
receptor stops signaling -> no more IP3 produced -> IP3 channels close -> Ca2+ pumped back into stores by SERCA -> stores refill -> termination signal for PM calcium channel activation
At same time Ca2+ pumped out cell via PM Ca2+-ATPase (PMCA) pump
How does calcium exert its effect
binding to calcium effector proteins Ca2+ binds to - calcium binding proteins - calmodulin-sensitive kinases - Ca2+/ calmodulin-regulated enzymes - Calcium sensitive transcription factors - Calcium-sensitive ion channels
Calcium binding proteins
Activated when bind calcium and interact with other proteins; include calmodulin (euk cells), troponin C (skeletal and cardiac muscle), synatotagmin (synaptic vesicles)
Calmodulin-Sensitive kinases
multifunctional calmodulin-dependent protein kinases (CaM-kinases), smooth muscle myosin light chain kinase
Ca2+/ calmodulin-regulated enzymes
PM Ca2+-ATPase, calcineurin (calcium-senstive phosphatase)
Calcium sensitive ion channels
- Ca2+ activated K+ channels
- Ca2+ activated chloride channels
Calmodulin
dumbbell shaped protein 2 globular ends connected by alpha helix; each end 2 Ca2+ biding domains, those in carboxyl-terminal part have tenfold higher affinity for Ca2+
Ca2+/ Calmodulin complex
when binds to target protein undergoes conf change wraps around region target protein and activates it
CaM kinase II
Ca2+/ Calmodulin- Dependent Protein Kinase II; multifunctional protein kinase; acts as frequency detector, can decode oscillatory Ca2+ signals; molecular memory device (remains active under certain conditions even after Ca2+ signal terminated); contains 12 homologous subunits
Molecular memory
inactive CaM-kinase subunit binds Ca2+/ Calmodulin its conformation changes -> exposure catalytic domain enzyme -> phosphorylation inhibitory domain neighboring subunit
Autophosphorylation CaM-kinase
prolongs enzyme activity two ways
- Traps bound Ca2+/ calmodulin so doesn’t dissociate from enzyme until Ca2+ returns to resting levels for 10 sec min
- Coverts enzyme to Ca2+ independent form so remains partially active after Ca2+/Calmodulin dissociates from it; activity remains until autophosphorylation enzyme overwhelmed by protein phosphatase activity
CaM-kinase II low frequency Ca2+ spikes
CaM-kinase II inactivates after each spike bc completely dephosphorylated before next Ca2+ spike arrives
CaM-kinase II spike frequency increase
spike frequency increases more subunits (12 subunits total) of enzyme stay phosphorylated between spieks
CaM-kinase II spike frequency V high
spike frequency high enough enzyme autophosphrylatd on all subunits and maximally active; once enough subunits phosphorylated enzyme can be maintained in highly activated state with low frequency Ca2+ spikes
