Ammonium injection induces an N-methyl-D-aspartate receptor-mediated proteolysis of the microtubule-associated protein MAP-2

J Neurochem. 1993 May;60(5):1626-30. doi: 10.1111/j.1471-4159.1993.tb13384.x.


We have shown previously that chronic hyperammonemia increases, in brain, the polymerization of microtubules that is regulated mainly by the level and state of phosphorylation of microtubule-associated protein 2 (MAP-2). Activation of the N-methyl-D-aspartate (NMDA) receptor dephosphorylates MAP-2. Because we have found that acute ammonia toxicity is mediated by the NMDA receptor, we have tested the effect of high ammonia levels on MAP-2 in brain. Microtubules isolated from rats injected intraperitoneally with 6 mmol/kg ammonium acetate showed a marked decrease of MAP-2. Also, the amount of MAP-2 in brain homogenates, determined by immunoblotting, was markedly reduced, presumably by proteolysis. The content of MAP-2 was decreased by approximately 75% 1-2 h after ammonium injection and returned to normal values after 4 h. Proteolysis of MAP-2 was prevented completely by injection of 2 mg/kg MK-801, a specific antagonist of the NMDA receptor, suggesting that proteolysis is mediated by activation of this receptor. L-Carnitine, which protects rats against ammonia toxicity, also prevented MAP-2 degradation. Because activation of the NMDA receptor increases [Ca2+]i, we determined whether rat brain contains a Ca(2+)-dependent protease that selectively degrades MAP-2. We show that there is a cytosolic Ca(2+)-dependent protease that degrades MAP-2, but not other brain proteins. The protease has been identified tentatively as calpain I, for it is inhibited by a specific inhibitor of this protease. Our results suggest that ammonium injection activates the NMDA receptor, leading to an increase in [Ca2+]i, which activates calpain I. This, in turn, selectively degrades MAP-2.(ABSTRACT TRUNCATED AT 250 WORDS)

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Ammonia / pharmacology*
  • Animals
  • Calcium / physiology
  • Calpain / antagonists & inhibitors
  • Carnitine / pharmacology
  • Dizocilpine Maleate / pharmacology
  • Injections, Intraperitoneal
  • Male
  • Microtubule-Associated Proteins / metabolism*
  • Peptide Hydrolases / metabolism*
  • Protease Inhibitors / pharmacology
  • Rats
  • Rats, Wistar
  • Receptors, N-Methyl-D-Aspartate / physiology*


  • Microtubule-Associated Proteins
  • Protease Inhibitors
  • Receptors, N-Methyl-D-Aspartate
  • Dizocilpine Maleate
  • Ammonia
  • Peptide Hydrolases
  • Calpain
  • Carnitine
  • Calcium