High-resolution profiling and functional modulation of striatal grafts for Huntington’s Disease

Huntington’s disease (HD) is an inherited neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin gene. Early and prominent pathology involves degeneration of striatal medium spiny neurons (MSNs), leading to dysfunction of the direct and indirect pathways. Unfortunately, no disease-modifying therapy is currently available in the clinic, and existing or emerging approaches do not restore lost neurons or rebuild damaged striatal circuitry. This has motivated cell replacement as a strategy aimed at reconstructing circuit function rather than solely alleviating symptoms. However, a central unresolved problem is the lack of a clear definition of what is required for therapeutic efficacy, both in terms of optimal graft composition and the degree and precision of circuit reconstruction that must be achieved. This thesis addresses these gaps in the quinolinic acid rat model of HD by combining high-resolution graft profiling, analyses of graft-mediated circuit reconstruction, and causal interrogation of graft functional activity. First, single-nucleus transcriptomics defines the long-term cellular composition of hESC-derived striatal progenitor grafts, showing that in vivo maturation generates a heterogeneous ventral telencephalic mixture rather than a purely MSN-restricted product. Second, circuit-level analyses provide evidence of afferent integration and graft-derived projections to major downstream basal ganglia targets consistent with partial rebuilding of direct- and indirect-pathway-like outputs, while highlighting remaining uncertainty regarding the identity and specificity of projecting graft neurons and how wiring precision relates to efficacy. Finally, chemogenetic manipulation demonstrates that grafts become functionally embedded within host networks and can modulate striatum-relevant behaviours in a task-dependent manner, despite the absence of robust baseline therapeutic recovery. Overall, these findings support the view that graft survival and anatomical outgrowth are not sufficient predictors of benefit, and that therapeutic outcomes likely depend on the interplay between cellular composition (e.g., balance of MSN precursors, interneurons, and glia) and the accuracy and strength of reconstructed striatal output pathways. To enable pathway-resolved tests of these requirements, this thesis also lays methodological groundwork for developing customized MSN-selective, and potentially pathway-biased, AAV tools using the BRAVE platform (Davidsson et al., 2019) to link graft identity and circuit reconstruction to functional outcome.