br Results br Materials and
Materials and method
Discussion Mitochondrial dysfunction is one of the main pathological mechanisms of HD. Perturbation in mitochondrial function causes mitochondrial stress and activates a series of mitochondrial unfolded protein response (mtUPR) signaling events, including CHOP up-regulation and HSP60 and Clpp transactivation, which in turn reduces the excessive mitochondrial proteotoxic stress [22,23]. Therefore, control of the mtUPR may be important for mitochondrial homeostasis. Upon mtUPR activation, CHOP is induced and recruited to the promoter of HSP60 and Clpp for their induction [22,26,27]. Furthermore, the misfolded and/or unfolded mitochondrial protein will be re-folded or degraded by mitochondrial chaperones and proteases [, , , ]. A recent study reported that ABCB10 is required for maintaining mitochondrial chaperone and protease expression, suggesting its critical role in controlling the mtUPR. Although the physiological function of the mtUPR has been reported in several diseases [36,37], little is known about the relationship between the mtUPR and HD. In this study, we demonstrated that the protein levels of the mitochondrial inner-membrane ABC transporter ABCB10 were reduced in HD cell cultures and HD R6/2 mice, which in turn led to downregulation of CHOP, HSP60, and Clpp. Notably, overexpression of ABCB10 rescued the levels of the mtUPR related genes, improved mitochondrial function, and attenuated cell death in HdhQ111 mutant striatal cells. Moreover, we found that mtHtt inhibited the mtUPR pathway by promoting ABCB10 mRNA degradation. Thus, inhibition of the mtUPR by mtHtt may be added to the list of proposed mechanisms of mitochondrial dysfunction in HD. Multiple studies report the disturbance of mtUPR machinery in neurodegenerative diseases. Polyglutamine diseases, also known as PolyQ diseases, are autosomal dominantly inherited diseases, including spinocerebellar ataxia (SCA) , dentatorubral pallidoluysian atrophy (DRPLA) , Machado–Joseph disease (MJD) , and HD [41,42]. Expansion of the PolyQ is the main cause of these diseases . In the neurons of the model organism Caenorhabditis elegans, PolyQ40 expression is sufficient to activate the mtUPR, but the activated mtUPR can be inhibited by CY7-SE of ATFS-1 [44,45]. However, we observed that the mtHtt carrying 111 CAG repeats suppressed the mtUPR machinery and induced striatal cell death. It is possible that the length of PolyQ expressed in the neuronal cells disrupts the mtUPR with different mechanisms. In Alzheimer's disease, Beck et al. revealed that specific markers of the mtUPR are upregulated in both sporadic and familial AD . Expression of ATFS-1ΔMTS exhibits dopaminergic neurodegenerative phenotypes in C. elegans. Hyperactivation of the mtUPR was found in α-synuclein over-expressing animals . In addition, we also found that Clpp was selectively elevated in the LRRK2 mutant expressing HEK293 cells (data not shown). These findings establish a link between the regulation of the mtUPR pathway and Parkinson's disease. Interestingly, α-synuclein and its mutant induced the mtUPR in a life span dependent manner, implying that the activity of the mtUPR associates with the development of disease and progression of age. Thus, it will of interest to investigate the changes of the UPRmt in a chronic animal model of HD, such as YAC128 mice, which exhibit progressive mitochondrial deficits. The role of ABCB10 in controlling the mtUPR in HD pathogenesis raises many questions. For instance, silencing of ABCB10 by RNAi reduces expression of mitochondrial chaperones (HSPD1, DNAJA3) as well as mitochondrial protease (LONP), which leads to increased ROS production . Consistently, we found that higher ROS level was detected in both ABCB10 deficient HD mouse striatal cells and in ABCB10 knocked-down wild-type striatal cells. However, mitochondrial protease Clpp, but not LONP, was downregulated significantly in ABCB10 deficient HD mouse striatal cells. It is possible that specific mechanisms might occur in different cell models. Transcription factors such as GATA-1 and those belonging to the E2F family have been reported to regulate the expression of ABCB10 in G1E-ER2 and HEK293 cells [48,49]. mtHtt suppresses gene expression by blocking the association of transcription factor to response element . In the present study, we examined ABCB10 promoter activity and found no significant changes between HdhQ7 and HdhQ111 cells (data not shown). Interestingly, the half-life of ABCB10 mRNA was greatly reduced in the HD mutant striatal cells, compared with wild-type striatal cells. Thus, a further investigation on how mtHtt promotes ABCB10 mRNA reduction is worthy. In addition, HAF-1 serves as a short peptide transfer channel under mtUPR activation in C. elegans , but ABCB10, the mammalian ortholog of HAF-1, has no effect on peptide export from the mitochondria matrix to the cytosol in mammalian cells . How ABCB10 regulates the mtUPR should be more carefully investigated in future studies.