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

Abstract

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)

Rapid regulation of microtubule-associated protein 2 in dendrites of nucleus laminaris of the chick following deprivation of afferent activity

“…and adult brain. This idea is supported by dynamic regulation of MAP2 expression and its involvement in dynamic changes in neuronal morphology caused by excitotoxins (Siman and Noszek, 1988;Bigot et al, 1991;Felipo et al, 1993;Arias et al, 1997;Faddis et al, 1997;Hoskison and Shuttleworth, 2006;Hoskison et al, 2007), traumatic brain injury (Taft et al, 1992;Folkerts et al, 1998), or focal ischemia (Pettigrew et al, 1996;Schwab et al, 1998). In addition, manipulations of synaptic activity alter the immunoreactivity for MAP2 in dendrites (Hendry and Bhandari, 1992;Steward and Halpain, 1999;Vaillant et al, 2002), further suggesting that synaptic activity either directly or indirectly regulates MAP2 expression.…”

“…Among a number of regulatory mechanisms controlling MAP2 function (see review, Sánchez et al, 2000), changes in calcium signaling, especially rises in intracellular calcium concentration ([Ca 2+ ] i ), have been most extensively studied following manipulations of synaptic inputs. Excessive glutamate receptor stimulation induced by physiological stimulation or exposure of glutamate or agonists of its receptors causes dendritic injury and/or MAP2 loss (Siman and Noszek, 1988;Bigot et al, 1991;Felipo et al, 1993;Arias et al, 1997;Faddis et al, 1997;Steward and Halpain, 1999;Swann et al, 2000;Vaillant et al, 2002). Recent studies in hippocampal slices further demonstrated that these changes are calcium-dependent (Hoskison and Shuttleworth, 2006;Hoskison et al, 2007).…”

H7, a protein kinase C inhibitor, increases the glutathione content of neuroblastoma cells

“…This effect was reversed by the PKC activator 12-0-tetradecanoyl-13.phorbol-acetate (TPA), which confirms the involvement of PKC [3]. It has been claimed that PKC is sensitive to thiol-disulflde exchange; hence, its activity may be sensitive to the redox state of the cellular environment [4].…”

“…was obtainccl from the American Type Culture Collection, Rockvillr, MD, and grown at 37°C in Eagle's minimum csscntial medium. supplemented with 10% feral bovine serum, 100 IU penicillin/ml, and 1OOyg of streptomycin/ml as previously described [2,3]. H7, TPA, staurosporinc and sphingosine were from Sigma Chemical Co. (St. Louis, MO).…”

Effect of L-carnitine on postischemic inhibition of protein synthesis in the rat brain

“…L-carnitine, which protects rats against glutamate toxicity, also prevented MAP-2 degradation by cytosolic Ca 2+ -dependent protease, which has been identified tentatively as calpain I (Felipo et al 1993). Kuzin and Kolesnikova (1999) found that L-carnitine after cerebral hypoxia inhibits development of apoptosis, limits the area of damage and restores structure of nervous tissue.…”