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Striatal neuronal apoptosis is preferentially enhanced by N-methyl-D-aspartate receptor activation in a YAC transgenic mouse model of huntington's disease Shehadeh, Jacqueline

Abstract

Huntington's disease (HD) is a neurodegenerative disorder caused by an expansion of polyglutamine (polyQ) repeats in the huntington gene affecting medium spiny neurons (MSNs) of the striatum. One hypothesis put forward to explain neuronal degeneration in HD is that glutamate excitotoxicity results from increased sensitivity of N-methyl-D-aspartate receptors (NMDARs). Here, we tested the hypothesis that poor calcium handling and mitochondrial dysfunction play a role in the pathogenesis of HD, but to a lesser degree than activation of NMDARs in striatal MSNs. MSNs were isolated and cultured from yeast artificial chromosome (YAC) transgenic mice carrying the full-length HD gene with different CAG repeat lengths. Levels of apoptosis were determined after MSNs (either expressing wild-type, 18, 72, or 128 CAG repeats - WT, YAC18, YAC72 or YAC128 respectively) were exposed to different toxins that did not activate NMDA receptors, but increased intracellular calcium or disrupted mitochondrial function. YAC18 MSNs were used as a control for the YAC transgene, as well as the over expression of huntingtin. Surprisingly, YAC18 MSNs showed significantly less cell death than WT or HD MSNs for most toxins tested. For the YAC72 and YAC128 MSNs, it was found that in all treatments for which cytoplasmic calcium was increased via influx from the extracellular space, but not by release from intracellular stores, the level of apoptosis was increased in HD MSNs compared with WT; this is to a lesser degree than found for NMDA-induced apoptosis. Furthermore, inhibiting mitochondrial function in HD MSNs also produced higher levels of apoptosis than for WT MSNs, but once again the difference was not as dramatic as found after NMDA exposure. In addition, when NMDAR currents were partially inhibited by DL-2-amino-5-phosphonopentanoic acid (APV) in YAC72 MSNs to a level equivalent to that found in WT MSNs, levels of apoptosis were also decreased to WT levels. These data support the hypothesis that excitotoxicity, specifically through NMDAR activation, plays a major role in the pathogenesis of HD. Interestingly, NMDAR activation did not produce greater toxicity in YAC128 MSNs compared to YAC72 MSNs, leading to the hypothesis that there is a decrease in NMDARs in the YAC128 mouse model. Western blot analysis of 10 week and 12 month old animals showed a decrease in the NR2 B subunit of the NMDAR in the striatum, but in no other brain region tested, which has been previously shown to enhance current and toxicity in the 72 QMSNs. These results support the NMDA-mediated excitotoxicity theory behind the pathogenesis of this devastating disease.

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