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Mutant Huntingtin-mediated alterations of N-Methyl-D-Aspartate receptor currents and downstream events underlying enhanced excitotoxicity in Huntington's disease Fernandes, Herman Brian


Expansion of a CAG repeat in the Huntington disease (HD) gene results in progressive neuronal loss, particularly of medium-sized spiny neurons of the striatum (MSNs). Previous studies suggest that increased activity of N-methyl-D-aspartate (NMDA) receptors (NMDARs) and altered mitochondrial function contribute to selective neuronal degeneration. Here I utilized MSNs obtained from transgenic mice expressing human huntingtin (htt) on a yeast artificial chromosome (YAC) to study alterations in NMDAR function and possible downstream consequences, particularly in regards to altered calcium (Ca²⁺) handling and mitochondrial function. I examined the relationship between the length of the polyglutamine (polyQ) repeat in mutant htt (mhtt) and NMDAR currents in MSNs in vitro. I found that NMDAR current density was significantly enhanced in YAC72 MSNs compared to wild type (WT), YAC18, YAC46 and YAC128 MSNs, and that there was a trend toward larger NMDAR current density with increasing polyQ length for YAC 18, YAC46 and YAC72 MSNs. This increase was selective for NMDARs and occurred in a neuronal type-specific fashion. The mechanism for increased NMDAR current in YAC72 MSNs appears due to a mhtt-mediated increase in trafficking of NMDARs to the plasma membrane. While NMDA-induced apoptosis is enhanced in YAC128 MSNs, I found the initial steps in the intrinsic apoptotic pathway, including NMDAR current and cytosolic Ca²⁺ loading, are similar to those observed in WT MSNs. In contrast, the NMDAR mediated Ca²⁺ load triggered a significantly greater depolarization of mitochondria in YAC 128 MSNs, suggesting that NMDAR signaling via the mitochondrial apoptotic pathway is altered. YAC128 MSNs demonstrated impaired cytosolic Ca²⁺ clearance following NMDA application, a difference that was not apparent following depolarization-evoked Ca²⁺ entry. Inhibitors of the mitochondrial permeability transition (mPT) reduced peak cytosolic Ca²⁺ and mitochondrial depolarization evoked by NMDA in YAC128, but not WT, MSNs. Hence, in contrast to YAC models with moderate CAG expansions, enhanced NMDA-induced apoptosis in YAC 128 MSNs is largely due to augmented mitochondrial sensitivity to Ca²⁺-induced activation of the mPT. These findings suggest that the CAG repeat length can influence the mechanism by which mhtt enhances NMDAR-mediated excitotoxicity. However, as signaling pathways activated by strong NMDAR stimulation converge on mitochondria in all YAC HD models, bolstering mitochondrial function and/or inhibiting the mPT represent possible therapeutic targets.

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