Longitudinal Behavioral Evaluation of a Knock-In Mouse Model of Huntington Disease and Mechanisms of Pathogenesis.

Abstract

Huntington’s disease (HD) is a dominantly inherited neurodegenerative disorder caused by expanded CAG repeat/polyglutamine repeats within the huntingtin locus resulting in a pathogenic huntingtin protein (htt). Early and prominent neurodegeneration of striatal GABAergic medium spiny neurons (MSNs) is a pathologic hallmark of HD. HD is incurable and death follows inexorably approximately 15 – 20 years after diagnosis. The molecular and cellular mechanisms underlying the selective degeneration of MSNs in HD are not yet understood. Murine genetic models are crucial for identifying mechanisms of neurodegeneration in HD and for preclinical evaluation of possible therapies for HD. Here we report longitudinal characterization of the murine Hdh(CAG)150 knock-in model of HD. A series of behavioral tests at five different time points (20, 40, 50, 70, and 100 weeks) demonstrate an age-dependent, late onset behavioral phenotype with significant motor abnormalities at 70 and 100 weeks of age. Pathologic analysis demonstrated loss of striatal dopamine D1 and D2 receptor binding sites at 70 and 100 weeks of age, and stereological analysis showed significant loss of striatal neuron number at 100 weeks. Late onset behavioral abnormalities, decrease in striatal dopamine receptors, and diminished striatal neuron number observed in this mouse model recapitulate key features of HD. The Hdh(CAG)150 knock-in mouse is a valid model to evaluate early events in the pathogenesis of neurodegeneration in HD. N-methyl-D-aspartate receptor (NMDAR) mediated excitotoxicity has long been implicated as a proximate cause of neurodegeneration in Huntington Disease (HD), however this hypothesis has not been tested rigorously in vivo. NMDAR NR2B-subunits are the predominant NR2 subunit expressed by the striatal medium spiny neurons that degenerate in HD. We crossed a well validated murine genetic model of HD - Hdh(CAG)150 - with a transgenic line overexpressing NMDAR NR2B-subunits. In the resulting double mutant line, we show exacerbation of selective striatal neuron degeneration. These results provide the first direct in vivo evidence of NR2B-NMDA recpetor mediated excitotoxicity in the context of HD. Our results are consistent with prior suggestions that direct and/or indirect interactions of mutant huntingtin with NMDARs are a proximate cause of neurodegeneration in HD.