Hsp27 overexpression in the R6/2 mouse model of Huntington's disease: chronic neurodegeneration does not induce Hsp27 activation

Huntington's disease (HD) is caused by an expanded polyglutamine tract in the huntingtin protein. Mitochondrial dysfunction and free radical damage occur in both R6/2 mice and HD patient brains and might play a role in disease pathogenesis. In cell culture systems, heat-shock protein 27 (Hsp27), a small molecular chaperone, suppresses mutant huntingtin-induced reactive oxygen species formation and cell death. To investigate this in vivo, we conducted an extensive phenotypic characterization of mice arising from a cross between R6/2 mice and Hsp27 transgenic mice but did not observe an improvement of the R6/2 phenotype. Hsp27 overexpression had no effect in reducing oxidative stress in the R6/2 brain, assessed by measuring striatal aconitase activity and protein carbonylation levels. Native protein gel analysis revealed that transgenic Hsp27 forms active, large oligomeric species in heat-shocked brain lysates, demonstrating that it is efficiently activated upon stress. In contrast, Hsp27 in double transgenic brains exists predominantly as a low molecular weight, inactive species. This suggests that Hsp27, which is otherwise activatable upon heat shock, remains inactive in the R6/2 model of chronic neurodegeneration. Hsp27 transgenics had been previously shown to be protected from acute stresses such as kainate administration, ischemia/reperfusion heart injury and neonatal nerve injury. Our study is the first to suggest a differential modulation of Hsp27 activation in vivo and, importantly, it illustrates the diverse effect of Hsp27 on acute versus chronic models of disease.