Investigating the pathogenic role of calpain proteases and the therapeutic potential of their inhibition in mice modelling Machado-Joseph disease

Machado-Joseph disease (MJD, also known as spinocerebellar ataxia type-3) is a fatal disease characterised by motor impairments and the presence of aggregated ataxin-3, the protein affected in MJD, in degenerating brain regions. Ataxin-3 protein aggregates have previously been reported to contain both full-length ataxin-3 protein and shorter protein fragments, highlighting proteolytic cleavage as a pathogenic mechanism. Calpains, calcium-activated proteases, have been reported to cleave ataxin-3 and have been implicated in MJD pathogenesis. This study aimed to explore whether calpain proteases were overactive at early, pathogenesis-relevant timepoints in male transgenic CMVMJD135 mice modelling MJD and identify the timepoint of calpain overactivation through obtaining longitudinal plasma samples. We detected increased levels of cleaved αII-spectrin in plasma from MJD mice as early as 12 weeks of age, shortly after the onset of neurological symptoms. Cerebellar and brainstem tissue from 15-week-old mice was immunoblotted, revealing a trend towards increased levels of calpain 1, and increased cleavage of calpain substrates such as αII-spectrin, beclin-1 and TAR DNA binding protein 43 (TDP-43) within the cerebellum. Further, we found that short-term treatment of male MJD mice (from 10 to 12 weeks of age) with the calpain inhibitor compound calpeptin yielded improvements in neurological symptoms and reduced the presence of cleaved αII-spectrin in plasma and cerebellum tissue when compared to vehicle treated MJD males. Our findings suggest that calpain overactivity may be an early disease phenotype that contributes to neurodegeneration in transgenic CMVMJD135 mice modelling MJD, and that calpeptin warrants further investigation as a potential treatment for MJD.