Validating cGAS-STING pathway as drug target in Huntington disease mouse model

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Project summary Huntington disease (HD) is a fatal neurodegenerative disorder caused by the huntingtin (HTT) CAG expansion mutation (coding for polyglutamine, mHTT). mHTT is a ubiquitously expressed gene, yet it prominently damages the striatum and cortex, followed by widespread peripheral defects as the disease progresses. Prior works have linked innate and adaptive inflammatory responses in HD. Microglia, a cellular indicator of inflammation, is also increased in the striatum of HD animal and cell culture models and HD patients. Increased levels of reactive monocytes, inflammatory cytokines, chemokines, and the n-kynurenine/tryptophan ratio, an indicator of persistent inflammation, have all been observed in pre-manifest HD patients and correlated with HD progression. Furthermore, RNA-seq analysis of tissue obtained from human HD patients and HD monkey models reveals extensive transcriptional dysregulation associated with proinflammatory pathway activation. Inflammation is also closely linked to autophagy, a catabolic process that is dysregulated in HD. Despite these studies, the mechanisms or treatment targets for the reduction of inflammatory responses remain less clear. To fill this knowledge gap, this proposal tests the hypothesis that cyclic GMP–AMP synthase (cGAS)–stimulator of interferon genes (STING), a major innate immunity response pathway, is a disease modifier in HD pathogenesis. This hypothesis was formulated based on our recent finding that the cGAS-STING pathway is upregulated in HD patients’ tissue and cell models and mediates autophagy and inflammatory responses. We found high ribosome occupancy in exon 1 of cGAS mRNA and cGAS-dependent inflammatory transcription factors (Irf3, Irf7), and inflammatory chemokine (Ccl5, and Cxcl10) are upregulated in HD. Depletion of cGAS diminishes downstream effector STING activation and also inhibits autophagy flux in HD cells. The latter is consistent with the primordial function of the cGAS-STING pathway in autophagy induction. These data underline an important role of cGAS-STING signaling in the pathological mechanisms of HD. Subsequently, an independent study confirmed activation of cGAS-STING in HD models. Yet, it is unclear whether the cGAS- STING pathway is a passive or active contributor to HD development. In Aim 1, we will determine whether the genetic deletion of cGAS alters the behavioral and pathological hallmarks of HD KI (z175HD) mice. We will determine age-related behavioral alterations and pathological changes in cGAS–/–;HD KI mice. In Aim 2, we will determine whether the small molecule STING inhibitor diminishes the HD phenotype in HD KI mice. We will treat z175HD mice with the small molecule STING inhibitor, H-151 and determine changes in the HD-like deficits compared vehicle treated z175HD. The proposed experiments will clarify the role of the cGAS–STING pathway as anti-inflammatory therapeutic agents in HD.