Calcium Homeostasis 2 Flashcards
What activates the ryanodine receptor
Calcium - when high calcium conc is cytoplasm this opens the ryanodine receptor to trigger more calcium release from cytoplasmic reticulum and increase the cytosolic calcium
Where is the ryanodine receptor mostly found
Cardiac and muscle cells
How does an action potential trigger the ryanodine receptor to open
AP –> voltage-gated calcium channels open in T tubule of cell membrane –> T-tubule interacts directly with ryanodine receptor in ER/SR –> Opening of Ryr-sensitive calcium release channels –> calcium release into cytoplasm
Essentially a positive feedback mechanism as initiated by calcium and increases calcium
What happens when calcium levels get too high (mM conc)
This inactivates RyR so the signal is turned off - a protective mechanism to prevent extremely high calcium levels
What is the ryanodine receptor similar to
IP3R - tetramer and large with a central pour.
Ryanodine twice as big though
What effect does ryanodine have on the ryanodine receptor and what is important about this
Low conc - permanently open but subconductance state
High conc - fully block channel
THIS IS NOT PHYSIOLOGICAL
Where is ryanodine receptors important
Malignant hyperthermia
Give some features of malignant hyperthermia
Inherited disorder - 1 in 15,000
Caused by volatile anaesthetic (halothane)
Due to defect in RYR1 calcium channel release - causes increase sensitivity to caffeine and calcium and greater channel conductance and longer opening times of channels
What is the pathophysiology of malignant hyperthermia
Uncontrolled increase in sarcoplasmic reticulum calcium concentration which increases the aerobic and anaerobic metabolism.
As a result, this causes an increased body temperature, acidosis and muscle rigidity leading to death
How is malignant hyperthermia treated
Dantrolene - a RYR channel blocker
What are the different features of IP3 and ryanodine when considering skeletal muscle
IP3 - minor
Ryanodine - major
What are the different features of IP3 and ryanodine when considering smooth muscle
IP3 - dominant
Ryanodine - minor
What are the different features of IP3 and ryanodine when considering caffiene
IP3 - no action
Ryanodine - opener
What are the different features of IP3 and ryanodine when considering ruthenium red
IP3 - no action
Ryanodine - blocker
What are the different features of IP3 and ryanodine when considering heparin
IP3 - inhibitor
Ryanodine - no action
What are the different features of IP3 and ryanodine when considering dantrolene
IP3 - no action
Ryanodine - blocker
What is the role of cyclic ADP-ribose (cADPR)
Finds and binds to ryanodine receptors on the endoplasmic reticulum for calcium release
How does NO activate cADPR
Produced by nitric oxide activated G-cyclase to form cGMP which activates the ADP-ribosyl cyclase to form the cADPribose which binds to RyR to release calcium from the ryanodine store
What are calcium microdomains
Rapid, small and localised different concentrations of calcium across the cell
How do calcium microdomains occur
Via localized one or a few calcium release or calcium intracellular entry channels
What are unitary events
Sparks of calcium rising which are due to RyR receptors
These are spontaneous with no other channels involved, due to local balance of cytoplasmic and SR calcium concentrations
What are evoked events
Due to RyR activation induced via calcium entry into cells via voltage gated calcium channels (as spoke about previously)
What is the role of these calcium sparks
Unsure but may prime or initiate larger calcium events
What is calmodulin
When calcium binds it activates it and mediates many calcium ion signalling functions.
What is an EF-hand
Calmodulin binds 4 calcium ions and at each site, oxygen atoms bind from glutamate and aspartate to form a helix-loop-helix motif called an EF-hand
What is the structure of calmodulin
Dumbell shaped - 4 helix-loop-helix motifs 2 at each end of calmodulin
How does calmodulin work
Calcium-binding promotes a conformational change that exposes hydrophobic pockets along the 2 lobes which provide binding sites for the target enzymes
Give some calmodulin target proteins
Calcineurin - a phosphatase
NOS
Calpains
Calcium-magnesium ATPase pump
CaM-kinase - transcription factor
Give the three methods for handling larger calcium loads in calcium homeostasis
Mitochondria
Calcium binding proteins
Na/Ca exchanger
Give the three methods for handling smaller calcium loads in calcium homeostasis
ER/SR uptake
Calcium binding proteins
ATP-dependent transport
What are calcium puffs
Wider reading - Yao et al 1995
Similar to calcium sparks but from IP3R in the ER - Localised calcium increase inside the cell
What is the link of microdomains and ROS
Wider reading - Brookes et al 2004
Microdomains in the mitochondria can increase resp activty and therefore increase ROS production by respiratory chain
What happens to the RYR in redox stress
WIDER READING - Xia et al 2000
Its sulfhydryl groups were shown to have a well-defined redox potential in skeletal muscle SR. Thus, under mild oxidative stress, relatively small changes in the cellular redox potential can contribute to significant stimulation of the ryanodine receptor
What is central core disease and why is it important
WIDER READING - McCarthy et al 2000
Autosomal dominant congenital myopathy with variable presentation but progressive proximal weakness and delayed motor development is common.
Patients more susceptible to milignant hyperthermia too.
90% linked to RYR1 mutation - causes impaired calcium influx so impaired intracellularl signalling and muscle contraction
How was dantrolene shown to help in polymorphic ventricular tachycardia
WIDER READING - Kobayashi et al 2010
Stopped leaky calcium RyR2 cardiac receptors from leaking in arrhythmic effect knock in mice (leaky RyR2) which in turn significantly decreased polymorphic ventricular tachycardia when these mice exercised with ephinephrine.
What is the role of magnesium in dantrolene
WIDER READING - Choi et al 2017
Key to working - high affinity of Mg to RyR causes RyR to shut therefore its needed for dantrolene
Believed that dantrolene helps Mg binding.
Explains why it works in malignant hypothermia (as increase in Mg during this time through MgATP hydrolysis) but not on individual RyR1 receptors
How does caffeine effect the RyR
WIDER READING - Kong et al 2008
Monitoring of endoplasmic reticulum luminal Ca2+ revealed that caffeine reduced the luminal Ca2+ threshold at which spontaneous Ca2+ release occurs by RyR.
Therefore caffeine reduces luminal calcium threshold but had no effect on cytosolic threshold.
Important as this decrease in threshold in luminal calcium trigger release is also seen in pro-arhythmic methylxanthines linke theophylline so it may share same arrhythmogenic mechanism
Why is calmodulin so flexible
WIDER READING - Yamniuk et al 2004
Structural variabilty when bound to different targets
Flexible center allows for wrapping
Hydrophobic so attracts many targets
How many target proteins can bind to calmodulin
WIDER READING - no reference
300
What is the role of calmodulin in smooth muscle
WIDER READING -Tansey 1994
Calmodulin activates myosin light chain kinase which phosphorylates the myosin light chain to cause cross-briding and smooth muscle contraction.
Calmodulin relies on calcium so this is why smooth muscle contraction is calcium dependent
What is the role of calmodulin on the RyR and how does this effect muscle contraction
WIDER READING - Walsh 1994
Controlling movement of Ca2+ across sarcoplasmic reticiulum membranes.
the ryanodine receptor of the sarcoplasmic reticulum, can be inhibited by calmodulin bound to calcium, thus affecting the overall levels of calcium in the cell
What role does calmodulin have in glucose metabolism
WIDER READING - Nishizawa et al 1988
Calmodulin plays an important role in the activation of phosphorylase kinase, which ultimately leads to glucose being cleaved from glycogen by glycogen phosphorylase.
What role does calmodulin have in memory
WIDER READING - Lledo 1995
Ca2+/calmodulin-dependent protein kinase II (CaMKII) plays a crucial role in a type of synaptic plasticity known as long-term potentiation (LTP) which requires the presence of calcium/calmodulin. CaMKII contributes to the phosphorylation of an AMPA receptor which increases the sensitivity of AMPA receptors.[38] Furthermore, research shows that inhibiting CaMKII interferes with LTP.
