| Summary: | Previous work suggests N-methyl-D-aspartate receptor (NMDAR) activation may be
involved in degeneration of medium spiny striatal neurons in Huntington's disease (HD). Here,
we determined whether expression of huntingtin (htt) containing the polyglutamine expansion
augments NMDAR-mediated excitotoxicity. First, human embryonic kidney (HEK) 293 cells
co-expressing mutant huntingtin (htt-138Q) and either NR1A/NR2A- or NR1 A/NR2B-type
NMDARs exposed to 1 mM NMDA showed a significant increase in excitotoxic cell death
compared to controls [cells co-expressing htt-15Q or green fluorescent protein (GFP)], but the
difference was larger for NR1A/NR2B. Moreover, agonist-dependent cell death showed
apoptotic features for cells co-expressing htt-138Q and NR1A/NR2B, but not for cells
expressing htt-138Q and NR1A/NR2A. Furthermore, NRlA/NR2B-mediated apoptosis was not
seen with co-expression of an N-terminal fragment of mutant htt. Since NR1A/NR2B is the
predominant NMDAR subtype in neostriatal medium-sized spiny neurons (MSNs), we
hypothesized that enhancement of NMDA-induced apoptotic death in NRlA/NR2B-expressing
cells by full-length mutant htt may contribute to selective neurodegeneration in HD. To test this
hypothesis we compared NMDAR-induced cell death in striatal neurons from a yeast artificial
chromosome (YAC) transgenic mouse model of HD expressing full-length mutant huntingtin
with striatal neurons from the same strain of wild-type mice. Excitotoxic death of MSNs from
the transgenic mice was increased after NMDA but not AMPA. NMDAR-mediated cell death
was completely blocked by the NR2B subtype-selective antagonist ifenprodil, and was also
associated with increased caspase-3 activity relative to wildtype (WT) MSNs. Importantly, there
was no enhancement of NMDAR-mediated cell death in cerebellar granule neurons expressing
mutant huntingtin, demonstrating cell type specificity and also consistent with NMDAR subtype
specificity. Although caspase-3 can cleave huntingtin to form smaller fragments we did not
observe an increase in the proteolysis of huntingtin after exposure to NMDA in WT or mutant htt
expressing striatal cultures. Enhanced mitochondrial membrane depolarization and intracellular
calcium levels were observed upon NMDAR application in MSNs expressing mutant htt.
NMDAR excitotoxicity was partially inhibited by application of cyclosporin A in MSNs, and
inhibition was greater in MSNs from transgenic mice than WT mice. Together, our data support
a role for NR2B-subtype NMDAR activation as an upstream trigger for mitochondrial
dysfunction and caspase-3 activation. These results may help explain selective neuronal
degeneration in HD.
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