Previously, we showed that taurine protects neurons against glutamate-induced excitotoxicity by inhibiting the glutamate-induced increase of [Ca2+](i). In this study, we report that taurine prevents glutamate-induced chromosomal condensation, indicating that taurine inhibits glutamate-induced apoptosis. We found that Bcl-2 was down-regulated while Bax was up-regulated by glutamate treatment, and these changes were prevented in the presence of taurine. We have also shown that taurine inhibits glutamate-induced activation of calpain. Furthermore, calpastatin, a specific calpain inhibitor, also prevented glutamate-induced cell death. Here we propose the mechanisms underlying glutamate-induced apoptosis and taurine's inhibition of glutamate-induced apoptosis to be as follows: glutamate stimulation induces [Ca2+](i) elevation, which in turn activates calpain; activation of calpain leads to a reduction of Bcl-2:Bax ratios; with decreased Bcl-2:Bax ratios Bax homodimers form, Bax homodimerization, and translocation to the mitochondria result in the release of cytochrome c; released cytochrome c in turn activates a downstream caspase cascade leading to apoptosis. The antiapoptotic function of taurine is due to its inhibition of glutamate-induced membrane depolarization.

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by an expanded polyglutamine repeat within the N-terminus of the huntingtin protein. It is characterized by a selective loss of medium spiny neurons in the striatum. It has been suggested that impaired proteasome function and ER stress play important roles in mutant huntingtin (mHtt) induced cell death. However, the molecular link involved is poorly understood. In the present study, we identified the essential role of the extra long form of Bim (Bcl-2 interacting mediator of cell death), BimEL, in mHtt-induced cell death. BimEL protein expression level was significantly increased in cell lines expressing the N-terminus of mHtt and in a mouse model of HD. Although quantitative RT-PCR analysis indicated that BimEL mRNA was increased in cells expressing mHtt, we provided evidence showing that, at the post-translational level, phosphorylation of BimEL played a more important role in regulating BimEL expression. Up-regulation of BimEL facilitated the translocation of Bax to the mitochondrial membrane, which further led to cytochrome c release and cell death. On the other hand, knocking down BimEL expression prevented mHtt-induced cell death. Taken together, these findings suggest that BimEL is a key element in regulating mHtt-induced cell death. A model depicting the role of BimEL in linking mHtt-induced ER stress and proteasome dysfunction to cell death is proposed.

Synopsis Huntington disease (HD) is caused by an expanded polyglutamine repeat in the huntingtin protein. GABAergic medium spiny neurons in the striatum are mostly affected in HD. However, mutant huntingtin (mhtt)-induced molecular changes in these neurons remain largely unknown. This study focuses on the effect of mhtt on the subcellular localization of glutamic acid decarboxylase (GAD), the enzyme responsible for synthesizing GABA. In this study, we report that the subcellular distribution of GAD is significantly altered in two neuronal cell lines that express either the N-terminus or full length mhtt. GAD65 is predominantly associated with Golgi membrane in cells expressing normal htt. However, it diffuses in the cytosol of cells expressing mhtt. As a result, vesicle-associated GAD65 trafficking is impaired. Since palmitoylation of GAD65 is required for GAD65 trafficking, we then demonstrate that palmitoylation of GAD65 is reduced in the HD model. Furthermore, overexpression of huntingtin-interacting protein 14, the enzyme responsible for palmitoylating GAD65 in vivo, could rescue GAD65 palmitoylation and vesicle-associated GAD65 trafficking. Taken together, our data support the idea that GAD65 palmitoylation is important for the delivery of GAD65 to inhibitory synapses and suggest that impairment of GAD65 palmitoylation by mhtt may lead to altered inhibitory neurotransmission in HD.

The subventricular zone (SVZ) is one of the two major neurogenic regions in the adult mammalian brain. Its close proximity to the striatum suggests that a cell-based therapeutic strategy for the treatment of Huntington’s disease (HD) is possible. To achieve this, it is important to understand how adult cell production, migration and differentiation may be altered in the HD brain. In this study, we quantified the number of adult-born striatal cells and characterized their fate in the R6/2 transgenic mouse model of HD. We found that the number of new striatal cells was approximately two-fold greater in R6/2 versus wild type mice, while SVZ cell proliferation was not affected. Using cell-type specific markers, we demonstrated that the majority of new striatal cells were mature oligodendrocytes or oligodendroglial precursors that were intrinsic to the striatum. We also detected a significant increase in the number of migrating neuroblasts that appeared to be recruited from the SVZ to the striatum. However, these neuroblasts did not mature into neurons and most were lost between 1 and 2 weeks of cell age. Crossing the R6/2 mice with mice over-expressing brain-derived neurotrophic factor in the striatum increased the numbers of neuroblasts that survived to 2 weeks, but did not promote their differentiation. Together, our data indicate that the potential treatment of HD based on manipulating endogenous progenitor cells should take into consideration the apparent enhancement in striatal oligodendrogliogenesis and the limited ability of recruited SVZ neuroblasts to survive long-term and differentiate in the diseased striatum.