Development of Purkinje cell degeneration in a knockin mouse model reveals lysosomal involvement in the pathogenesis of SCA6

Abstract

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disease caused by the expansion of a polyglutamine tract in the Ca v 2.1 voltage-gated calcium channel. To elucidate how the expanded polyglutamine tract in this plasma membrane protein causes the disease, we created a unique knockin mouse model that modestly overexpressed the mutant transcripts under the control of an endogenous promoter (MPI-118Q). MPI-118Q mice faithfully recapitulated many features of SCA6, including selective Purkinje cell degeneration. Surprisingly, analysis of inclusion formation in the mutant Purkinje cells indicated the lysosomal localization of accumulated mutant Ca v 2.1 channels in the absence of autophagic response. The lack of cathepsin B, a major lysosomal cysteine proteinase, exacerbated the loss of Purkinje cells and was accompanied by an acceleration of inclusion formation in this model. Thus, the pathogenic mechanism of SCA6 involves the endolysosomal degradation pathway, and unique pathological features of this model further illustrate the pivotal role of protein context in the pathogenesis of polyglutamine diseases.

A functional deficiency of TERA/VCP/p97 contributes to impaired DNA repair in multiple polyglutamine diseases

“…There are a number of evidences that activation of brain autophagy by chemicals attenuates toxicity of mutant Htt protein 47-49 and that a certain defect of autophagy contributes to other neurodegenerative disorders like Alzheimer’s disease 50 and Parkinson’s disease 51 . Meanwhile, a report showed that an mTOR inhibitor was effective for the muscle pathology but not brain pathology of HD 52 , and another group recently reported that a new mouse model of SCA6 did not show any change of autophagy in the brain 53 .…”

Pathogenesis of severe ataxia and tremor without the typical signs of neurodegeneration

“…Purkinje cells are the presumed source of these disorders, and they often degenerate (Unno et al, 2012, Orr, 2012, Prudente et al, 2013, Louis, 2014). Yet in some diseases, movement is obstructed before Purkinje cells degenerate (Shakkottai et al, 2011).…”

Autophagy in polyglutamine disease: Imposing order on disorder or contributing to the chaos?

“…As previously reported for SCA6 patient samples, MPI 118Q/118Q mice form inclusions, which are negative for ubiquitin, but were heavily stained for lysosomal markers cathepsin B and LAMP2 (TABLE 1). Ultrastructural immunogold analysis revealed that cytoplasmic nuclear inclusions of mutant Cav2.1 channels are also associated with LAMP1, and this finding was confirmed in SCA6 patient cerebellar tissue [80]. Interestingly, MPI 118Q/118Q mice had no detectable autophagic changes, suggesting that while there is lysosomal involvement in SCA6 aggregate turnover, the pathway may be independent of canonical macroautophagy.…”

“…Interestingly, whereas polyQ-inclusions in other polyQ disorders are heavily ubiquitinated, mutant Cav2.1 channels form inclusions in the cytoplasm of SCA6 Purkinje cells that typically lack ubiquitin immunoreactivity [79]. Recently, a novel knock-in mouse model of SCA6 was generated by introducing a splice site mutation in the CACNA1A locus that favors splicing of the polyQ-expanded isoform (MPI 118Q/118Q) of Cav2.1 [80]. These mice display motor impairments and gait ataxia, and they suffer an age-dependent specific loss of Purkinje cells [80].…”

“…Recently, a novel knock-in mouse model of SCA6 was generated by introducing a splice site mutation in the CACNA1A locus that favors splicing of the polyQ-expanded isoform (MPI 118Q/118Q) of Cav2.1 [80]. These mice display motor impairments and gait ataxia, and they suffer an age-dependent specific loss of Purkinje cells [80]. As previously reported for SCA6 patient samples, MPI 118Q/118Q mice form inclusions, which are negative for ubiquitin, but were heavily stained for lysosomal markers cathepsin B and LAMP2 (TABLE 1).…”