Role of high mobility group box 1 (HMGB1) in SCA17 pathogenesis.

Li-Ching Lee,Chun-Yen Chang,Ming-Tsan Su,Chiung-Mei Chen,Guey-Jen Lee-Chen,Pin-Rong Wang,Vladimir V Kalinichenko

doi:10.1371/journal.pone.0115809

Abstract

Spinocerebellar ataxia type 17 (SCA17) involves the expression of a polyglutamine (polyQ) expanded TATA-binding protein (TBP), a general transcription initiation factor. TBP interacts with other protein factors, including high mobility group box 1 (HMGB1), to regulate gene expression. Previously, our proteomic analysis of soluble proteins prepared from mutant TBP (TBP/Q61) expressing cells revealed a reduced concentration of HMGB1. Here, we show that HMGB1 can be incorporated into mutant TBP aggregates, which leads to reduced soluble HMGB1 levels in TBP/Q61∼79 expressing cells. HMGB1 overexpression reduced mutant TBP aggregation. HMGB1 cDNA and siRNA co-transfection, as well as an HSPA5 immunoblot and luciferase reporter assay demonstrated the important role of HMGB1 in the regulation of HSPA5 transcription. In starvation-stressed TBP/Q36 and TBP/Q79 cells, increased reactive oxygen species generation accelerated the cytoplasmic translocation of HMGB1, which accompanied autophagy activation. However, TBP/Q79 cells displayed a decrease in autophagy activation as a result of the reduction in the cytoplasmic HMGB1 level. In neuronal SH-SY5Y cells with induced TBP/Q61∼79 expression, HMGB1 expression was reduced and accompanied by a significant reduction in the total outgrowth and branches in the TBP/Q61∼79 expressing cells compared with the non-induced cells. The decreased soluble HMGB1 and impaired starvation-induced autophagy in cells suggest that HMGB1 may be a critical modulator of polyQ disease pathology and may represent a target for drug development.

Highlights

The TATA-binding protein (TBP) is a universal basal transcription factor that is involved in the expression of most eukaryotic genes [1]
To examine whether high mobility group box 1 (HMGB1) incorporates itself into mutant TBP aggregates, 293derived cells with inducible TBP/Q36,79 expression were examined for endogenous HMGB1 expression after 6 days, positive nuclei with punctuate inclusion bodies that co-localized with HMGB1 were visible in the TBP/Q61,79 cells (Fig. 1A)
Mutant polyQ proteins may impair the functions of nuclear factors through altered interactions with nuclear factors or sequestration into polyQ aggregates, which leads to transcriptional dysfunction

Summary

Introduction

The TATA-binding protein (TBP) is a universal basal transcription factor that is involved in the expression of most eukaryotic genes [1]. TBP contains a polymorphic polyglutamine (polyQ) domain in its N-terminus and a DNAbinding domain in its C-terminus. The highly conserved C-terminus mediates the transcriptionally relevant interactions that involve TBP, the evolutionarily divergent N-terminal region may play a role in transcriptional activation at TATA-containing promoters [2, 3]. The high mobility group box 1 (HMGB1) is a ubiquitous and highly conserved nuclear protein with a proposed role in the regulation of eukaryotic gene expression [9, 10]. The acidic C-terminus interacts with the Q-tract in the N-terminus of TBP to increase the affinity of TBP for the TATA element [13]

Methods

Results

Conclusion