UCLA

Abnormal cell excitability is commonly seen in neurodegenerative diseases. However, the roles of excitability in the pathogenic pathways are not fully understood. Through studying spinocerebellar ataxia type 13 (SCA13), we provide evidence in support of a direct causal link between altered excitability and neuronal degeneration. SCA13 is an autosomal dominant disease caused by mutations in the KCNC3 gene encoding Kv3.3. Kv3.3 has been studied extensively, and plays crucial roles in regulating cell excitability. Three causative mutations have been identified: R3Ha, R4Hi and FLi. They have differential effects on the channel biophysics. Depending on the mutations, SCA13 emerges during either infancy or adulthood. R3Ha is a dominant negative mutation that causes the adult-onset form of SCA13, whereas the R4Hi and FLi mutations are gating modifiers that alter the voltage-dependence of channel activation and cause the infant-onset form of SCA13. R4Hi also shows a dominant-negative effect over wildtype subunits. We hypothesize that infant- and adult-onset SCA13 mutations change cell excitability in distinct ways, which in turn generate the two clinical forms of the disease. To test this hypothesis, we developed a novel in vivo recording method in awake larval zebrafish. By employing this method, we characterized the electrical properties and functional maturation of cerebellar Purkinje cells, and investigated the impact of SCA13 mutations during early development. We found that while R3Ha shows frequency-dependent inhibition of excitability, R4Hi induces hyperexcitability and severe degeneration in developing Purkinje cells. Preliminary experiments suggest that this degeneration can be rescued by the addition of NS13001, a SK channel agonist that decreases excitability. Our findings argue strongly that the distinct biophysical properties of the SCA13 mutants generate different functional consequences in cell excitability, and that hyperexcitability is tightly related to the rapid morphological degeneration of Purkinje cells early in development. This degeneration is reminiscent of the early-onset atrophy of the cerebellum seen in children with infant-onset SCA13, suggesting that hyperexcitability may contribute to pathogenesis in this form of the disease. Our results suggest that there is a direct causal relationship between altered excitability and neuronal degeneration in SCA13 and potentially in other neurodegenerative diseases.