Ammonium Injection Induces an N‐Methyl‐ d ‐Aspartate Receptor‐Mediated Proteolysis of the Microtubule‐Associated Protein MAP‐2

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 ∼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 [Ca 2+ ] 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 no 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 [Ca 2+ ] i , which activates calpain I. This, in turn, selectively degrades MAP‐2. Possible implications in chronic hyperammonemic states and in the mechanism of ammonia toxicity are discussed.