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.
The proposed research is relevant to the public health because it interrogates the mechanisms underlying neural developmental deficits with key pathogenic roles in Huntington?s disease. More specifically, it will define what processes adversely influence the elaboration of interneurons, how their reductions impact the maturation of relevant brain circuits, and whether these deficits yield neuronal species more susceptible to stressors. Accordingly, this research would uncover mechanistic cascades that will provide the substrate for the development of therapeutic paradigms aimed at modifying disease progression, which is relevant to the goals of NIH?s overall strategic mission.