BDNF and TRiC-inspired Reagents Rescue Cortical Synaptic Deficits in a Mouse Model of Huntington's Disease

Abstract

Huntingtons disease (HD) results from a CAG repeat expansion in the gene for Huntington (HTT) resulting in expansion of the polyglutamine (Q) tract in the mutant protein (mHTT). Synaptic changes are early manifestations of neuronal dysfunction in HD. However, the mechanism(s) by which mHTT impacts synapse formation and function is not well defined. Herein we explored HD pathogenesis in the BACHD and the {Delta}N17-BACHD mouse models of HD by examining cortical synapse formation and function in primary cultures maintained for up to 35 days (DIV35). We identified synapses by immunostaining with antibodies against pre-synaptic (Synapsin 1) and a post-synaptic (PSD95) marker. Consistent with earlier studies, cortical neurons from both WT and the HD models began to form synapses at DIV14; at this age there were no genotypic differences in synapse numbers. However, from DIV21 through DIV35 BACHD neurons showed progressively smaller numbers of synapses relative to WT neurons. Remarkably, BACHD synaptic deficits were completely rescued by treating cultures with BDNF. Building on earlier studies using reagents inspired by the chaperonin TRiC, we found that addition of the recombinant apical domain of CCT1 partially rescued synapse number. Unexpectedly, unlike BACHD cultures, synapses in {Delta}N17-BACHD cultures showed a progressive increase in number as compared to WT neurons, thus distinguishing synaptic changes in these HD models. Using multielectrode arrays, we discovered age-related functional deficits in BACHD cortical cultures with significant differences present by DIV28. As for synapse number, BDNF treatment prevented most synaptic deficits, including mean firing rate, spikes per burst, inter-burst interval, and synchrony. The apical domain of CCT1 showed similar, albeit less potent effects. These data are evidence that deficits in HD synapse number and function can be replicated in vitro and that treatment with either BDNF or a TRiC-inspired reagent can prevent them. Our findings support the use of cellular models to further explicate HD pathogenesis and its treatments.