Abstract
BackgroundHuntington's disease is caused by aggregation of mutant huntingtin (mHtt) protein containing more than a 36 polyQ repeat. Upregulation of macroautophagy was suggested as a neuroprotective strategy to degrade mutant huntingtin. However, macroautophagy initiation has been shown to be highly efficient in neurons whereas lysosomal activities are rate limiting. The role of the lysosomal and other proteases in Huntington is not clear. Some studies suggest that certain protease activities may contribute to toxicity whereas others are consistent with protection. These discrepancies may be due to a number of mechanisms including distinct effects of the specific intermediate digestion products of mutant huntingtin generated by different proteases. These observations suggested a critical need to investigate the consequence of upregulation of individual lysosomal enzyme in mutant huntingtin accumulation and toxicity.ResultsIn this study, we used molecular approaches to enhance lysosomal protease activities and examined their effects on mutant huntingtin level and toxicity. We found that enhanced expression of lysosomal cathepsins D and B resulted in their increased enzymatic activities and reduced both full-length and fragmented huntingtin in transfected HEK cells. Furthermore, enhanced expression of cathepsin D or B protected against mutant huntingtin toxicity in primary neurons, and their neuroprotection is dependent on macroautophagy.ConclusionsThese observations demonstrate a neuroprotective effect of enhancing lysosomal cathepsins in reducing mutant huntingtin level and toxicity in transfected cells. They highlight the potential importance of neuroprotection mediated by cathepsin D or B through macroautophagy.
Highlights
Huntington’s disease is caused by aggregation of mutant huntingtin protein containing more than a 36 polyQ repeat
Overexpression of cathepsins D (CathD) and B (CathB) reduce mutant huntingtin (mHtt) level in human embryonic kidney (HEK) cells Consistent with prior studies [24], we found that overexpressing full-length Htt protein with a short polyQ repeat (23QHtt) or mHtt protein with a long polyQ repeat (145QmHtt) did not induce cell death assessed by three independent methods (Calcein AM, MTS (3(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)2-(4-sulfophenyl)-2H-tetrazolium) colorimetric cell survival, or trypan blue exclusion assays)
Real-time RT-PCR results showed that the mRNA levels of Cathepsin D (CathD) or CathB are greatly increased in CathD or CathB transfected cells with or without 23QHtt or 145QmHtt (Additional file 1, Figure S1B)
Summary
Huntington’s disease is caused by aggregation of mutant huntingtin (mHtt) protein containing more than a 36 polyQ repeat. Some studies suggest that certain protease activities may contribute to toxicity whereas others are consistent with protection These discrepancies may be due to a number of mechanisms including distinct effects of the specific intermediate digestion products of mutant huntingtin generated by different proteases. These observations suggested a critical need to investigate the consequence of upregulation of individual lysosomal enzyme in mutant huntingtin accumulation and toxicity. Investigating the impact of enhancing lysosomal proteins on mutant huntingtin accumulation and toxicity is of particular importance
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