Context-dependent Dysregulation of Transcription by Mutant Huntingtin

Jonathan Cornett,Lauren Smith,Meyer Friedman,Ji-Yeon Shin,Xiao-Jiang Li,Shi-Hua Li

doi:10.1074/jbc.m607839200

Abstract

Huntington disease (HD) is an adult-onset neurodegenerative disease caused by expansion of a polyglutamine (poly(Q) tract in the N-terminal region of huntingtin (htt). Although the precise mechanisms leading to neurodegeneration in HD have not been fully elucidated, transcriptional dysregulation has been implicated in disease pathogenesis. In HD, multiple N-terminal mutant htt fragments smaller than the first 500 amino acids have been found to accumulate in the nucleus and adversely affect gene transcription. It is unknown whether different htt fragments in the nucleus can differentially bind transcription factors and affect transcription. Here, we report that shorter N-terminal htt fragments, which are more prone to misfolding and aggregation, are more competent to bind Sp1 and inhibit its activity. These effects can be reversed by Hsp40, a molecular chaperone that reduces the misfolding of mutant htt. Our results provide insight into the beneficial effects of molecular chaperones and suggest that context dependent transcriptional dysregulation may contribute to differential toxicity of various N-terminal htt fragments.

Highlights

In the nucleus, mutant htt abnormally interacts with a number of transcription factors [6]
Biochemical analysis of Huntington disease (HD) knock-in mice that express a 150-glutamine repeat in the endogenous mouse htt [17] revealed the presence of multiple N-terminal htt fragments smaller than the first 508 amino acids in the nucleus [18], consistent with the notion that cleavage of mutant htt is a key event in HD pathology [19, 20]
Generation and Expression of N-terminal Htt Constructs— To study whether protein length can alter the ability of htt to affect gene transcription, we used three N-terminal htt constructs encoding exon-1 htt (72Q-67), htt amino acids 1–212 (72Q-212), or 1–508 (72Q-508) with a 72Q repeat

Summary

Introduction

Mutant htt abnormally interacts with a number of transcription factors [6]. Transcription Reporter Assays—To examine the effect of different fragments of intranuclear htt on the NGFR promoter, we transfected 70 – 80% confluent HEK293T cells in a 12-well plate for 48 h with pRK vector, NLS-72Q-67, NLS-72Q-212, or NLS72Q-508 (1 ␮g/well) and NGFR-DsRed reporter constructs (0.33 ␮g/well).

Results

Conclusion