Abstract
Huntington's disease (HD) is caused by the expansion of N-terminal polymorphic poly Q stretch of the protein huntingtin (HTT). Deregulated microRNAs and loss of function of transcription factors recruited to mutant HTT aggregates could cause characteristic transcriptional deregulation associated with HD. We observed earlier that expressions of miR-125b, miR-146a and miR-150 are decreased in STHdhQ111/HdhQ111 cells, a model for HD in comparison to those of wild type STHdhQ7/HdhQ7 cells. In the present manuscript, we show by luciferase reporter assays and real time PCR that decreased miR-146a expression in STHdhQ111/HdhQ111 cells is due to decreased expression and activity of p65 subunit of NFkB (RelA/NFkB). By reporter luciferase assay, RT-PCR and western blot analysis, we also show that both miR-150 and miR-125b target p53. This partially explains the up regulation of p53 observed in HD. Elevated p53 interacts with RelA/NFkB, reduces its expression and activity and decreases the expression of miR-146a, while knocking down p53 increases RelA/NFkB and miR-146a expressions. We also demonstrate that expression of p53 is increased and levels of RelA/NFkB, miR-146a, miR-150 and miR-125b are decreased in striatum of R6/2 mice, a mouse model of HD and in cell models of HD. In a cell model, this effect could be reversed by exogenous expression of chaperone like proteins HYPK and Hsp70. We conclude that (i) miR-125b and miR-150 target p53, which in turn regulates RelA/NFkB and miR-146a expressions; (ii) reduced miR-125b and miR-150 expressions, increased p53 level and decreased RelA/NFkB and miR-146a expressions originate from mutant HTT (iii) p53 directly or indirectly regulates the expression of miR-146a. Our observation of interplay between transcription factors and miRNAs using HD cell model provides an important platform upon which further work is to be done to establish if such regulation plays any role in HD pathogenesis.
Highlights
Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder caused by the expansion of polymorphic CAG repeats in exon1 of Huntingtin (HTT ) gene
We have shown earlier that expressions of several Micro RNA (miRNA) are altered in STHdhQ111/HdhQ111 cells in comparison with STHdhQ7/ HdhQ7 cells
We present evidences to show that (i) in STHdhQ111/HdhQ111 cells decreased expression of miR-146a is mediated through decreased expression and activity of RelA/ NFkB, (ii) increased expression of p53 in the same cells could be due to decreased expression of miR-125b and miR-150, (iii) p53 and RelA/NFkB regulate the expression of miR-146a and (iv) neuronal cells expressing N-terminal HTT with 83Q coded by exon1 exhibit decreased miR-125b and miR-150 expressions, increased p53 expression and reduced RelA/NFkB expression and activity and miR-146a expression
Summary
Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder caused by the expansion of polymorphic CAG repeats in exon of Huntingtin (HTT ) gene. Among various molecular and cellular dysfunctions originated from mutations to HTT gene, which eventually lead to neuronal loss from striatal regions in HD patients, transcriptional deregulation is considered to be one of the important events [1,2] Such deregulation of genes has been attributed, at least partially, to interactions and recruitments of several transcription factors to the mutant HTT aggregates [2,3]. Similar decrease in NFkB activity after 72 hours of induction of mutant HTT was observed in a cell model of HD, while in early stage of induction, NFkB activity was increased [13,14] This dual role of mutant HTT on NFkB activity could be due to initial protective action of NFkB, which is suppressed at a later stage by the recruitment of NFkB into the aggregates. Alteration of NFkB activity may result in altered expression of NFkB regulated genes
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