Polyglutamine aggregates stimulate ER stress and trigger apoptosis by activating BH-3 only protein Bim

Huntington's disease (HD) is an inherited neurological disorder characterized by a
selective loss of neurons in the striatum and cortex leading to involuntary movement,
dementia and eventually cell death. HD is caused by an expanded polyglutamine (PolyQ)
repeat in Huntingtin (Htt) protein. It is well known that misfolded mutant Htt could form
intracellular aggregates, trigger ER stress and ultimately lead to apoptosis. However, the
molecular link between ER stress and apoptosis in mitochondria is poorly understood. In
the present study, we identified Bim (Bcl-2 interacting mediator of cell death) as the
essential protein. We first established a cellular model of HD by over expressing the Nterminus
of wild type and mutant Htt into HEK293 cell lines. We showed that the
accumulation and aggregation of misfolded mutant Htt protein triggers ER stress and
apoptosis. The Bim protein expression level was greatly increased in mutant Htt
transfected cells and this increase was partially due to up-regulation of Bim mRNA as analyzed using quantitative RT-PCR. We further showed that Bim phosphorylation
also played an important role in regulating Bim expression. Moreover, up-regulation of
Bim facilitates the translocation of Bax to mitochondrial membrane, which lead to
cytochrome c release and apoptosis. We also silenced Bim using siRNA to further
investigate the essential role of Bim in mutant Htt induced ER stress and apoptosis.
Identifying the Bim pathway that is altered in response to the mutant Htt protein is
important for understanding the cellular processes impacted by the disease.