Calcium Signalling

• Calcium Signalling


  The coordination chemistry of Ca²⁺ allows rapid, selective, reversible binding to proteins

  
• Why Ca²⁺ is used in signalling


  • Slightly lower hydration energy than magnesium, so calcium bonds can be broken more readily
  • Ca²⁺ is more polarised than magnesium, meaning the bonds are positioned all over the place and the bond lengths are quite different
  • Ca²⁺ can accommodate itself into irregularly shaped pockets within proteins
  • Magnesium has a much lower polarizability, meaning it's very difficult to pull the electrons out, so it has a very rigid octahedron shape
  • Ca²⁺ can coordinate multiple ligands (typically 7-8, but up to 12), enabling it to cross link multiple segments of a protein to produce large conformational changes
• Extracellular Ca²⁺ concentration is 2.5mM, about half of which is free Ca²⁺
• Intracellular Ca²⁺ concentration is 50-100nM of free Ca²⁺
• Intracellular Ca²⁺ stores


  Formed by membrane compartments called the sarco/endoplasmic reticulum, which contain about 1-2mM of free calcium

  
• Considerations of Ca²⁺ as a second messenger

  • Diffusion of Ca²⁺ in cytosol is very slow (100x slower than predicted for free solution) due to calcium binding proteins
  • Barriers to diffusion arise due to the highly complex sub-cellular architecture of cells
  • Ca²⁺ acts as a local messenger, so tends to act close where it enters the cytosol
• Ca²⁺ signalling is the most widespread signalling pathway used in nature and is important in a huge variety of processes
• How Ca²⁺ enters/exits the cell cytosol

  1. Across the plasma membrane, using ion channels and Ca exchangers (active)

  2. From internal Ca stores, using ion channels and Ca pumps (active)

  
• 1. type Ca²⁺ channel


  A multi-subunit protein in the plasma membrane of smooth muscle cells, with a voltage sensor component that opens the channel when the membrane depolarises, allowing rapid Ca²⁺ influx

  
• Inositol 1,4,5-Triphosphate (IP₃)


  Diffuses rapidly in cytosol and acts as a global messenger, coupling events at the plasma membrane to intracellular Ca²⁺ release

  
• IP₃ generation


  From the plasma membrane phospholipid PIP₂ by the action of phospholipase C

  
• Phospholipase C (PLC) family

  • PLCβ (coupled via G-proteins)
  • PLCγ (interacts with activated tyrosine kinase receptors)
  • PLCδ (activated directly by high free Ca²⁺)
  • PLCζ (constitutively active, found in sperm head)

  
• InsP₃ receptor and ryanodine receptor (RYR)

  Regulated by free Ca²⁺ in a biphasic fashion - potentiated at low [Ca²⁺], inhibited at high [Ca²⁺]
• Smooth muscle cells utilise both plasma membrane and ER/SR Ca²⁺ entry mechanisms

  
• Calcium signalling in smooth, skeletal and cardiac muscle

  • Smooth muscle: GPC receptors linked to IP₃ production, calmodulin sensor, voltage-sensitive and ligand-gated Ca²⁺ channels
  • Skeletal/cardiac muscle: Plasma membrane has voltage-gated Ca²⁺ channels, SR/ER has ligand/physiomechanical-gated Ca²⁺ channels
  • Cardiac muscle: Mixture of triggers and channels

  
• Cytosolic calcium buffers


  • EF-hand proteins (e.g. parvalbumin, calbindin)
  • Lipids (e.g. phosphatidylserine)

  
• Mitochondrial Ca²⁺ buffering


  Outer membrane is permeable, inner membrane is impermeable - Ca²⁺ enters through the mitochondrial calcium uniporter down its electrochemical gradient

  
• Ca²⁺ removal from the cytoplasm

  1. Plasma membrane to extracellular space: Active Ca²⁺ pump, facilitated transporters

  2. Cytosol to SR/ER: Active Ca²⁺ pump
• Ca²⁺ signalling to effectors


  Intracellular Ca²⁺ elevation is sensed by Ca²⁺ sensors/switches like calmodulin, which then activate effectors like Ca²⁺-calmodulin-dependent protein kinases, ion channels, troponin C