Huntington’s disease (HD) is a deadly neurodegenerative disorder whose pathogenesis remains unknown. The
disease is caused by aberrant polyglutamine expansion in Huntingtin (Htt), a pleiotropic protein with essential
functions during development. Using genetically modified mouse models to temporally regulate gene expression
during the developmental period, previous works from Molero et al., revealed that either exposure to mutant Htt,
or loss of normal Htt elicits HD during midlife. These findings demonstrate that events taking place during neural
development play pathogenic roles in the disease. Interestingly, both mutant and loss-of-function models exhibit
early interneuron deficits. This application presents evidence that the early ontogenic rescue of interneurons
delayed disease onset and ameliorated disease progression in a model of HD, which supports the role of
interneurons in disease pathogenesis. The overall objective of this proposal is to define the mechanisms
disrupting interneuron production, and the ensuing pathogenic cascade mediated by these cells. Our central
hypothesis is that changes in subpallial cell-cycle dynamics and migration dampen the neurogenic output of
interneurons, resulting in the disruption of corticostriatal connectivity, circuit maldeveloment, activation of stress
responses and the generation of “metastable” cells with lower reserves to cope with stress. This hypothesis will
be interrogated with three specific aims: (1) elucidate the underpinnings of the deficient interneuron neurogenic
output; (2) determine the effects of interneuron deficits on corticostriatal circuit maturation; (3) identify and
characterize developmentally vulnerable cells. The first aim will employ: a cell-cycle phase biosensor to define
cell-cycle length and checkpoints; organotypic cultures, fate mapping techniques and time-lapse microscopy to
define interneuron migration; and molecular studies to define DNA damage within interneuron germinative
domains. For the second aim, whole-cell patch clamping in acute slides and in-vivo optogenetic techniques will
be employed to define the establishment of the of thalamocortical feedforward circuit and the functional
maturation of the striatum. Lastly, the third aim will employ a cellular stress reporter system to map
“developmentally stressed” cells, define their survival throughout ontogenesis, and determine their molecular
signature. The proposed research is significant because it will elucidate the primary mechanisms underlying
developmental deficits with key pathogenic roles in disease occurrence, uncovering a novel window for
therapeutic interventions encompassing a potential array of novel disease-relevant targets. Moreover, these
studies have important implications for our understanding of HD comorbidities and would provide an original
methodological approach to interrogate related neurological disorders, particularly those involving polyglutamine
expansions. The application is innovative because upon confirming of our hypothesis, it would shift the focus of
current research efforts from mechanistic processes acting on the mature brain to events operating in the earliest
incipient stages of the disease prodrome, thereby introducing a paradigm shift in the field.