Protective roles of carbonic anhydrase 8 in Machado-Joseph Disease

Machado-Joseph disease (MJD)/Spinocerebellar ataxia type 3 (SCA3) is an inherited neurodegenerative disease that can lead to a regression of motor coordination and muscle control in the extremities. It is known that expansion of CAG repeats encodes abnormally long polyQ in mutant ataxin-3, the disease protein. It is also noted that mutant ataxin-3 interacts with 1,4,5-trisphosphate receptor type 1 (IP3R1) and induces abnormal Ca²⁺ release. Previously, we have shown a significant increase in the expression of carbonic anhydrase VIII (CA8) in SK-N-SH-MJD78 cells, which are human neuroblastoma cells overexpressing mutant ataxin-3 with 78 glutamine repeats. In the current study, we showed the presence of significantly increased CA8 expression in MJD mouse cerebellum in either early or late disease stage, with a gradual decrease in CA8 expression as the MJD mice naturally aged. By immunofluorescence and immunoprecipitation analysis, we also found that CA8 co-localized and interacted with mutant ataxin-3 in SK-N-SH-MJD78 cells harboring overexpressed CA8 (SK-MJD78-CA8). In addition, we found that SK-MJD78-CA8 cells, as well as cerebellar granule neurons (CGNs) of MJD transgenic (Tg) mouse with overexpressed CA8, were more resistant to reactive oxygen species (ROS) stress than the control cells. Importantly, overexpression of CA8 in SK-MJD78-CA8 cells and in MJD CGNs rescued abnormal Ca²⁺ release and caused an increase in cell survival. In summary, we demonstrate the protective function of CA8 in MJD disease models and speculate that the declining expression of CA8 following an initial increased expression may be related to the late onset phenomenon of MJD.