Calpain

2022-05-20: reference:

Calpain #

Protease. Proteolyzes: Spectrin

• Leads to microtubule degradation via:
  • Calpain-1 inhibits RhoA.
  • Calpain-10 regulates actin dynamics by proteolysis of microtubule-associated protein 1B
    • Directly cleaves MAP1B (which generates a heavy chain and light chain complex)
• Proteolyzes Protein Kinase C and Phospholipase C, cleaving their regulatory domains from the active (is this catalytic?) domains. R
• Calpain-2 inhibits PTEN.

NMDAR #

• Calcium-dependent. Thus it is activated by NMDAR.

• $\ce{Ca^2+}$ -> Calpain -> p35 cleavage p25 -> binds to CDK5 -> degeneration transcription stuff.

• $\ce{Ca^2+}$ -> Calpain -> STEP cleavage -> Fyn up -> NR2B expression.

• Selective Activation Induced Cleavage of the NR2B Subunit by Calpain.

  • Directly cleaves NR2 - at C-terminal. Guess what? Only NR2B.
    • As a consequence, all phosphorylation sites and PDZ-binding sites are cleaved and just chills in the cell, probably to only be degraded.
    • However, they can still be attached to the other subunits in the tetramer. I suppose it otherwise still maintains unmodulated function
      • …creat(ing) heteromeric receptors with potentially complex modulation and turnover properties (Sheng et al., 1994).
  • we incubated neuronal cultures in glutamate and glycine…
  • NMDAR-mediated calpain activity was not significantly inhibited by 10 μM ifenprodil, suggesting NR2B-containing is not necessary for activating calpain (to adequate degrees)
  • (After cleavage) An NR2B-specific antibody demonstrated a 45% decrease of full-length NR2B immunoreactivity with the same time course but greater in amount than the decrease of full-length NR2A/2B immunoreactivity. In contrast, no change was seen in NR2A levels using a subunit-selective antibody to the C terminus of NR2A. This identifies NR2B as the major subunit cleaved after NMDA receptor activation.
    • Why? Not sure. This experiment is just empirical measurements
  • In transfected cell models, as in cultured neurons, NR2B can be cleaved by calpain, but only if the NR2A subunit is present to facilitate calpain activation.
    • This may reflect the greater rise in calcium and greater opening probability of NR2A-containing receptors compared with NR2B-containing receptors in transfected cells
  • When neuronal calpain activity has been measured, it is typically highest early in development, with subsequent decline. This does not clearly match the developmental pattern of NR2 subunits… Though, I suppose it does match the fact that NR2B levels decline.

• The question is, what activates calpain? The other question is, how does it preferentially cleave NR2B?

  • Calpain I activation in rat hippocampal neurons in culture is NMDA receptor selective and not essential for excitotoxic cell death
    • Substantial Ca2+ rise, did not by itself activate calpain I.
    • Calpain activation and Na+/Ca2+ exchanger degradation occur downstream of calcium deregulation in hippocampal neurons exposed to excitotoxic glutamate
      • The initial increase in cytosolic Ca(2+) that precedes delayed calciumm dysregulation (DCD) is insufficient for sustained calpain activation, which most likely occurs downstream of DCD - consequence, not the cause.