Overexpression of heat shock factor 1 maintains TAR DNA binding protein 43 solubility via induction of inducible heat shock protein 70 in cultured cells

Abstract

TAR DNA binding protein 43 (TDP-43) is a nuclear protein that has been shown to have altered homeostasis in the form of neuronal nuclear and cytoplasmic aggregates in some familial and almost all cases of sporadic amyotrophic lateral sclerosis as well as 51% of frontotemporal lobar degeneration and 57% of Alzheimer's disease cases. Heat shock proteins (HSPs), such as HSP70, recognize misfolded or aggregated proteins and refold, disaggregate, or turn them over and are upregulated by the master transcription factor heat shock factor 1 (HSF1). Here, we explore the effect of HSF1 overexpression on proteotoxic stress-related alterations in TDP-43 solubility, proteolytic processing, and cytotoxicity. HSF1 overexpression reduced TDP-43-positive puncta concomitantly with upregulating HSP70 and HSP90 protein levels. HSF1 overexpression or pharmacological activation sustained TDP-43 solubility and significantly reduced truncation of TDP-43 in response to inhibition of the proteasome with Z-Leu-Leu-Leu-al, and this was reversed by HSF1 inhibition. HSF1 activation conferred protection against toxicity associated with TDP-43 C-terminal fragments without globally increasing the activity of the ubiquitin proteasome system (UPS) while concomitantly reducing the induction of autophagy, suggesting that HSF1 protection is an early event. In support of this, inhibition of HSP70 ATPase activity further reduced TDP-43 solubility. HSF1 knockout significantly increased TDP-43 insolubility and accelerated TDP-43 fragmentation in response to proteotoxic stress. Overall, this study shows that HSF1 overexpression protects against TDP-43 pathology by upregulation of chaperones, especially HSP70, rather than enhancing autophagy or the UPS during times of proteotoxic stress. © 2016 Wiley Periodicals, Inc.

Acetylation-induced TDP-43 pathology is suppressed by an HSF1-dependent chaperone program

“…In addition, HSF1 activating compounds including arimoclomol reduced SOD1 aggregation and were neuroprotective 68 . Our results suggest that TDP-43 is also targeted by an endogenous refolding program, which is consistent with two recent studies 43, 44 . However, HSF1 may be kept partially inactive in tissues that are uniquely vulnerable to TDP-43 pathology including skeletal muscle and CNS tissues.…”

“…Since sustained HSF1 activity is associated with tumor metastasis and progression 69 , an alternative approach could target critical downstream chaperones including small HSPs (Hsp27) and Hsp40 that are most highly induced by HSF1 and capable of suppressing TDP-43 aggregates (Fig. 8) 43, 44 . While the roles of these HSPs in TDP-43 proteinopathies are not fully understood, genetic mutations in DNAJB1 and HSPB1 that encode Hsp40 and Hsp27 are associated with limb-girdle muscular dystrophy and motor neuropathies 59, 60 .…”

“…However, HSF1 activation may not be readily achieved in neurons due to a higher threshold requirement for sustained HSF1 activity 40, 41 . Nonetheless, the HSF-1 targets Hsp40 and Hsp70 were recruited to full-length TDP-43 foci and phosphorylated C-terminal fragments 42 and also were sufficient to partially suppress TDP-43 aggregation 43, 44 . Whether increased chaperone activity is a viable strategy to alleviate aggregate burden in TDP-43 proteinopathies is not clear.…”

Heat Shock-induced Phosphorylation of TAR DNA-binding Protein 43 (TDP-43) by MAPK/ERK Kinase Regulates TDP-43 Function

“…The dramatic up-regulation of TDP-43 phosphorylation by heat shock, along with recent findings on the reduction of TDP-43 aggregation upon heat shock factor 1 (HSF1) activation (40,41), point to a previously unappreciated role of HSR in TDP-43 homeostasis and posttranslational regulation. In addition to hyperthermic stress, HSR is activated by other types of insults that compromise protein homeostasis (42,43), and defects in this response are commonly found in human disease including neurodegeneration.…”

Different aggregation states of a nuclear localization signal‐tagged 25‐kDa C‐terminal fragment of TAR RNA/DNA‐binding protein 43 kDa

“…As an inhibitor of HSP70 ATPase activity during inhibition of proteasome activity increased insoluble TDP‐43 and its CTF, HSP70 is known as a major contributor to TDP‐43 proteostasis (Lin et al . ).…”