A C-TERMINAL ATAXIN-2 DISORDERED REGION PROMOTES HUNTINGTIN PROTEIN AGGREGATION AND NEURODEGENERATION IN DROSOPHILA MODELS OF HUNTINGTON'S DISEASE

The Ataxin-2 (Atx2) protein contributes to the progression of neurodegenerative phenotypes in animal models of amyotrophic lateral sclerosis (ALS), type 2 spinocerebellar ataxia (SCA-2), Parkinson’s Disease (PD) and Huntington’s Disease (HD). However, because the Atx2 protein contains multiple separable activities, deeper understanding requires experiments to address the exact mechanisms by which Atx2 modulates neurodegeneration (ND) progression. Recent work on two ALS models, C9ORF72 and FUS, in Drosophila has shown that a C-terminal intrinsically disordered region (cIDR) of Atx2 protein, required for assembly of ribonucleoprotein (RNP) granules, is essential for the progression of neurodegenerative phenotypes as well as for accumulation of protein inclusions associated with these ALS models. Here we show that the Atx2-cIDR also similarly contributes to the progression of degenerative phenotypes and accumulation of Huntingtin protein aggregates in Drosophila models of HD. Because Huntingtin is not an established component of RNP granules, these observations support a recently hypothesised, unexpected protein-handling function for RNP granules, which could contribute to the progression of Huntington’s disease and, potentially, other proteinopathies.

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

ABSTRACTThe Ataxin-2 (Atx2) protein contributes to the progression of neurodegenerative phenotypes in animal models of amyotrophic lateral sclerosis (ALS), type 2 spinocerebellar ataxia (SCA-2), Parkinson’s Disease (PD) and Huntington’s Disease (HD). However, because the Atx2 protein contains multiple separable activities, deeper understanding requires experiments to address the exact mechanisms by which Atx2 modulates neurodegeneration (ND) progression. Recent work on two ALS models, C9ORF72 and FUS, in Drosophila has shown that a C-terminal intrinsically disordered region (cIDR) of Atx2 protein, required for assembly of ribonucleoprotein (RNP) granules, is essential for the progression of neurodegenerative phenotypes as well as for accumulation of protein inclusions associated with these ALS models. Here we show that the Atx2-cIDR also similarly contributes to the progression of degenerative phenotypes and accumulation of Huntingtin protein aggregates in Drosophila models of HD. Because Huntingtin is not an established component of RNP granules, these observations support a recently hypothesised, unexpected protein-handling function for RNP granules, which could contribute to the progression of Huntington’s disease and, potentially, other proteinopathies.

Regulation of Blos1 by IRE1 prevents the accumulation of Huntingtin protein aggregates

Huntington's Disease is characterized by accumulation of the aggregation-prone mutant Huntingtin (mHTT) protein. Here, we show that expression of mHTT in mouse cultured cells activates IRE1, the transmembrane sensor of stress in the endoplasmic reticulum, leading to degradation of the Blos1 mRNA and repositioning of lysosomes and late endosomes toward the microtubule organizing center. Overriding Blos1 degradation results in accumulation of larger mHTT aggregates and increased cell death. Although mHTT is degraded by macroautophagy when highly expressed, we show that prior to the formation of large aggregates, mHTT is degraded via an ESCRT-dependent, endosomal microautophagy pathway. This pathway is enhanced by Blos1 degradation and appears to protect cells from a toxic, less aggregated form of mHTT.

The structural basis of huntingtin (Htt) fibril polymorphism, revealed by cryo-EM of exon 1 Htt fibrils

The lack of detailed insight into the structure of aggregates formed by the huntingtin protein has hampered efforts to develop therapeutics and diagnostics targeting pathology formation in the brain of patients with Huntington's disease. To address this knowledge gap, we investigated the structural properties of in vitro generated fibrils from exon1 of the huntingtin protein by electron cryo-microscopy and single- particle analysis. We show that wildtype and mutant exon1 of the huntingtin protein form non-helical fibrils with a polygultamine amyloid core composed of β-hairpins with unique characteristics that have not been previously observed with other amyloid filaments. The stacks of β-hairpins form long planar β- sheets (protofilaments) with variable stacking angle and occasional out-of-register state of individual β-hairpins. These features and the propensity of protofilament to undergo lateral association results in a high degree of fibril polymorphism, including fibrils composed of varying numbers of protofilaments. Our results also represent the first direct observation of how the flanking domains are organized around the polyglutamine core of the fibril and provide insight into how they might affect huntingtin fibril structure, polymorphism, and stacking of β-hairpins within its core structure. Removal of the first 17 amino acids at the N-terminus resulted in surprising intra-fibril structural heterogeneity and reduced fibril's propensity to lateral associations. Overall, this work provides valuable insights that could guide future mechanistic studies to elucidate the sequence and structural determinants of huntingtin aggregation, as well as cryo- EM and structural studies of fibrils derived from huntingtin proteins and other disease-associated polyglutamine-containing proteins.