Second Messengers: Calcium

Second Messengers: Calcium

1. the role of calcium

   • first messenger = calcium sensing receptor
   • second messenger = receptor operated channel, transient receptor potential channel, voltage gated channel
   • third messenger = IP3/DAG signalling, IP3R
   • fertilization
   • gene transcription
   • differentiation
   • secretion of solutes
   • contraction of skeletal muscles and heart
   • metabolic pathways
   • cell death
     • increased Ca2+ in cytosol = mitochondrial Ca2+ overload = cell death

2. mechanisms through which calcium ions concentration in the cytosol is regulated

   • ion channels
     • at plasma membrane
       • receptor operated channels
         • non selective e.g. L-glutamate receptors
       • transient receptor potential channels
         • non selective, activated by a variety of stimuli e.g. temp and pH
       • voltage gated channels
         • open as a result of depolarisation of the plasma membrane
         • complex of 5 sububits
           • alpha1 forms the pore
           • alpha 2 is an extracellular subunit linked by disulfide bond
       • store operated Ca2+ entry channels
         • calcium release activated calcium channel
         • activated upon depletion of Ca2+ from the ER
         • STIM1 (stromal interaction molecule 1) binds Ca2+ in ER, ORAI1 is located in the PM
         • upon depletion of Ca2+ STIM1, it undergoes conformational change and ORAI1 are redistributed to specific ER-PM junctions
         • binding of STIM1 to ORAI1 opens the calcium release activated calcium channel
         • when store is refilled STIM1 and ORAI return to original states
     • at ER
       • IP3R
         • binds to suppressor domain and IP3 binding core domain
       • ryanodine receptor
         • activation of receptor in skeletal muscle
           • T tubule feet: protein complexes bring a voltage sensitive receptor in contact with the ryanodine receptor
             • voltage changes = conformational change of the receptor that is detected by the ryanodine receptor = opening of channel
         • activation of receptor in cardiac muscle
           • voltage gated Ca2+ channels will result in result in a small influx of Ca2+
             • detected by ryanodine receptor and result in its activation
   • CA2+ ATPases
     • Ca2+ pumps
       • are P type ATPases
       • use energy to remove Ca2+ from the cytosol
         • plasma membrane Ca2+ pumps
         • ER Ca2+ pumps
         • golgi membrane Ca2+ pumps
     • ATP driven pumps
       • Ca2+ is transported against its concentration gradient
       • hydrolysis of ATP and phosphorylation of aspartate 351
       • domains
         • N domain = nucleotide binding domain
         • P domain = phosphorylation domain
         • A domain = actuator domain : protein phosphatase that functions to dephosphorylate that phosphorylated P domain
       • steps in sarcoplasmic reticulum Ca2+ ATPase
         • 1. binding of ATP and two Ca2+ to the enzyme in its E1 state
         • 2. aspartate 351 is phosphorylated by ATP ( Ca2+ must be bound to enzyme for this to take place). Phosphorylation = conformation changes that shift the enzyme towards its E2 state
         • 3. the shift to the E2 state causes ion binding sites to 'evert' so that the ions can diisociate into the luminal side of the membrane
         • 4. in the E2-P state, the enzyme has a low affinity for the Ca2+ ions, so they are released
         • 5. with the release of the Ca2+, the phosphoaspartate residue is hydrolysed, and the phosphate group is released
         • 6. the enzyme, devoid of a covalently attahced phosphate group is not stable in the E2 state, It everts back to the E1 form, completing the cycle.
   • Ca2+ binding proteins
     • buffer proteins
       • calbindin
       • parvalbumin
     • calmodulin
       • intracellular calcium sensory/adaptor protein
       • single polypeptide
       • 4 high affinity Ca2+ binding sites (EF) hands
         • 2 in the N terminus and 2 in the C terminus
           • C terminus will bind Ca2+ first
       • no enzymatic activity
       • targets
         • plasma membrane Ca2+ pump
         • CaM kinases
       • calmodulin dependent kinase II
         • serine/threonine kinase
         • forms 2 rings of 6 polypeptide chains
           • each chain contains a kinase domain, hub domain and regulatory segment
         • dynamic equilibrium between active and inactive state
         • calmodulin binds regulatory segment and prevents it from inhibiting the kinase
         • is autophosphorylated and phosphatase can deactivate it

3. Ca2+ oscillation

   • persist for as long as the receptors at the cell surface are activated
   • reflect the strength of the extracellular stimulus

4. functions of calcium binding proteins

   • adaptor sensor proteins e.g. calmodulin
   • buffer proteins e.g. parvalbumin, calbindin

5. Ca2+ sensing receptor

   • involved in systemic calcium homeostasis
   • detects concentration of calcium in the blood
   • expressed in parathyroid cells and cells of the nephron
     • if an increase in Ca2+ is detected from the cells of the nephron, the reabsorption of Ca2+ is reduced
   • if an increase in Ca2+ is detected from the cells of the parathyroid gland, the secretion and production of the parathyroid hormone is blocked
     • parathyroid hormone is responsible for the increase of blood Ca2+ levels by reabsorption of Ca2+ from the kidneys