Ataxin2 functions via CrebA to mediate Huntingtin toxicity in circadian clock neurons

Abstract

Disrupted circadian rhythms is a prominent and early feature of neurodegenerative diseases including Huntington’s disease (HD). In HD patients and animal models, striatal and hypothalamic neurons expressing molecular circadian clocks are targets of mutant Huntingtin (mHtt) pathogenicity. Yet how mHtt disrupts circadian rhythms remains unclear. In a genetic screen for modifiers of mHtt effects on circadian behavior in Drosophila, we discovered a role for the neurodegenerative disease gene Ataxin2 (Atx2). Genetic manipulations of Atx2 modify the impact of mHtt on circadian behavior as well as mHtt aggregation and demonstrate a role for Atx2 in promoting mHtt aggregation as well as mHtt-mediated neuronal dysfunction. RNAi knockdown of the Fragile X mental retardation gene, dfmr1, an Atx2 partner, also partially suppresses mHtt effects and Atx2 effects depend on dfmr1. Atx2 knockdown reduces the cAMP response binding protein A (CrebA) transcript at dawn. CrebA transcript level shows a prominent diurnal regulation in clock neurons. Loss of CrebA also partially suppresses mHtt effects on behavior and cell loss and restoration of CrebA can suppress Atx2 effects. Our results indicate a prominent role of Atx2 in mediating mHtt pathology, specifically via its regulation of CrebA, defining a novel molecular pathway in HD pathogenesis.

Ataxin-2 Disordered Region Promotes Huntingtin Protein Aggregation And Neurodegeneration In Drosophila Models Of Huntington’s Disease

“…Taken together with additional observations that the Atx2-cIDR is also required for the aggregation of FUS protein and C9-associated dipeptide repeats in cell culture, these results support a model in which RNP granules, perhaps by concentrating aggregation-prone proteins within membrane-less organelles, enhance the efficiency with which initial pathogenic aggregates associated with ALS/FTS can be nucleated (Becker and Gitler 2018). A more recent study of Huntington's Disease (HD) in Drosophila showed a connection between polyQ expanded Huntingtin (Htt) and Ataxin-2 (Xu et al 2019b). In Huntington's disease, pathogenic expansions occur within an endogenous polyQ domain starting at amino acid 17 of Htt.…”

“…A recent study on Huntington's Disease showed that overexpression of different mutant polyQ Huntington transgenes in circadian clock neurons induces circadian arrhythmicity and aggregation of Htt-polyQ in small ventral lateral neurons (sLNVs) as well as reduction in cell numbers indicative of cytotoxicity (Xu et al 2019a(Xu et al , 2019b. Reduction of Atx2 via RNAi knockdown alleviates both of these effects (Xu et al 2019b). As removal of the Atx2-cIDR has a similar effect in larval photoreceptor cells as the knock down of Atx2 in adult clock neurons, it is likely, that the formation of further aggregates and their subsequent relocalisation to the nucleus in adult flies is also inhibited.…”

“…Interestingly in adult flies Htt-polyQ protein predominantly re-locates to the nucleus of photoreceptor neurons with increased age (Jackson et al 1998). A recent study on Huntington's Disease showed that overexpression of different mutant polyQ Huntington transgenes in circadian clock neurons induces circadian arrhythmicity and aggregation of Htt-polyQ in small ventral lateral neurons (sLNVs) as well as reduction in cell numbers indicative of cytotoxicity (Xu et al 2019a(Xu et al , 2019b. Reduction of Atx2 via RNAi knockdown alleviates both of these effects (Xu et al 2019b).…”

“…Expression of pathogenic forms of Htt-polyQ in Drosophila resulted in the formation of Htt-polyQ aggregates most easily visualized in cells of the eye imaginal disc, death of adult photoreceptors or, if appropriately expressed, in death of cells that control the circadian clock in Drosophila (Jackson et al 1998;Romero et al 2008;Weiss et al 2012;Schilling et al 2019;Xu et al 2019b). Remarkably RNAi mediated knockdown of Atx2, in clock cells expressing Htt-polyQ, reduced both Htt-polyQ aggregate formation and cytotoxicity in these cells (Xu et al 2019b). Given that Htt is not an established component of neuronal RNP granules, we were interested in examining whether Atx2's RNP-granule forming ability was also required for its role in promoting Htt-polyQ induced protein aggregation and neurodegeneration.…”

Mutant Huntingtin stalls ribosomes and represses protein synthesis in a cellular model of Huntington disease

“…Although Fmrp upregulation did not affect translation in HD cultured striatal cells, we cannot rule out its role in HD pathogenesis. This notion is supported by evidence suggesting that deletion of Fmrp (dfmr1) suppresses the mHTT-mediated toxicity in the Drosophila model of HD 110 . In the HD brain, hypothetically, Fmrp may regulate the translation of selected mRNA transcripts involved in synaptic functions 104 , 111 , 112 .…”