Research Projects

Spinocerebellar ataxia (SCA) is an autosomal dominant, adult-onset, progressive neurodegenerative disorder. In India, SCA12 is one of the prevalent subtypes, accounting for 25% of the SCA cases, with more than 50% of the SCA cases in the Agarwal community (Bhanushali et al., 2020). In SCA12, action tremor in the upper extremities is a clinically recognized distinguishing symptom, which precedes the onset of the canonical SCA symptoms like bradykinesia, ataxia, and hyperreflexia (Choudhury et al., 2018). Neuropathologically, SCA12 is characterized by a distinct pattern of generalized brain atrophy, including the cerebrum and cerebellum, along with Purkinje cell degeneration and motor cortical impairment, in contrast with other SCA types, where atrophy is localized to the cerebellum (Agarwal et al., 2021). The genetic basis of SCA12 is a CAG trinucleotide repeat expansion in the 5’ untranslated region of the PPP2R2B gene, which forms the brain-specific regulatory component (Bβ) of the protein phosphatase 2A (Holmes et al., 1999; Mayer et al., 1991). While normal alleles contain 7-32 CAG repeats, SCA12 is found in patients with more than 40 repeats (Ganaraja et al., 2022). However, how the CAG repeat expansion alters the function of the PPP2R2B protein has not been established in the in vivo model of SCA12, since the mutation is in the non-coding region. The widespread expression of PPP2R2B protein in the whole brain may be involved in the generalized atrophy of SCA12, yet how the broad expression translates to cerebellar circuit impairments and the hallmark action-tremor remains unknown. Also, despite the understanding of the genetic cause and the neuropathology, the molecular mechanisms driving SCA12 pathophysiology remain elusive. Post-mortem human brains and mouse models are limited because longitudinal whole-brain analysis of neural circuit dynamics during disease progression is not possible. These limitations can be overcome in the zebrafish model, due to its genetic tractability, well-established and conserved neuronal circuits, and its optical transparency. This allows us to perform molecular manipulations and visualization in addition to whole-brain as well as circuit-level in vivo imaging at single-cell resolution. In this study, I will generate transgenic lines overexpressing normal and pathogenic human PPP2R2B in zebrafish. I will vary the CAG repeat length, covering the normal and pathogenic range in different neurons. This strategy will allow us to dissect the direct and indirect effects of the CAG repeat expansion in PPP2R2B on the cerebellum. Further, we will analyse the effect of overexpression on RNA foci formation, cerebellar-mediated behaviours, Purkinje neurons morphology and activity. This study will provide crucial insights into the molecular and neuronal circuit mechanisms of SCA12. It will also establish a vertebrate model for this disease, which can be used for high-throughput screening for therapeutics development.