Mechanisms tailoring the expression of heat shock proteins to proteostasis challenges

Abstract

All cells possess an internal stress response to cope with environmental and pathophysiological challenges. Upon stress, cells reprogram their molecular functions to activate a survival mechanism known as the heat shock response, which mediates the rapid induction of molecular chaperones such as the heat shock proteins (HSPs). This potent production overcomes the general suppression of gene expression and results in high levels of HSPs to subsequently refold or degrade misfolded proteins. Once the damage or stress is repaired or removed, cells terminate the production of HSPs and resume regular functions. Thus, fulfillment of the stress response requires swift and robust coordination between stress response activation and completion that is determined by the status of the cell. In recent years, single-cell fluorescence microscopy techniques have begun to be used in unravelling HSP-gene expression pathways, from DNA transcription to mRNA degradation. In this review, we will address the molecular mechanisms in different organisms and cell types that coordinate the expression of HSPs with signaling networks that act to reprogram gene transcription, mRNA translation, and decay and ensure protein quality control.

Keywords: acclimation; gene expression; heat shock factor 1; heat shock proteins; heat shock response; mRNA decay; proteostasis; stress-regulated translation.

Figure 1

Overview of the cellular response to heat stress. Cells under nonstress conditions keep the transcription of the inducible HSPs inactive. A paused polymerase occupies their promoter, and the transcription factor HSF1 is sequestered in monomeric form by constitutive chaperones HSC70/HSP90 in the cytoplasm. Constitutive chaperones also assist in protein folding and preserving protein homeostasis. Under physiological conditions, nonstress–regulated genes are transcribed, and their mRNAs undergo canonical cap-dependent translation. Exposure to heat stress induces protein misfolding, which titrates out the HSC70/HSP90 and allows HSF1 to trimerize and translocate to the nucleus, where it binds to the HSE in the promoter of HSPs and activates transcription. Concomitant to the HSR activation, there is a global transcriptional and translational repression. The translation is repressed by (1) phosphorylation of eIF2α (2); inhibition of eIF4F complex formation (3); recruitment of untranslated mRNAs and regulatory proteins in stress granules (SGs) and processing bodies (PBs) (4); and translation arrest at the stage of elongation. The inducible HSP mRNAs, especially HSP70, skip translation repression and are translated through a cap-independent mechanism to increase the number of available chaperones needed to cope with the abundant misfolded proteins and prevent their toxic aggregation. Once the temperature returns to being permissive, the newly synthesized HSPs favor recovering proteostasis and functionality by folding misfolded proteins and disabling SGs. The resumption of regular translation and transcription coincides with the decay of HSP mRNAs and silencing of their transcription. HSEs, heat shock elements; HSF1, heat shock factor 1; HSP, heat shock protein.

Figure 2

The function of HSC70/HSP70 in retaining the cellular proteostasis. The illustration depicts the significant tasks of the HSP70 chaperone network inside the cell to maintain proteostasis. (Starting from the top left tile) Under nonstress conditions, HSC70 provides cotranslational folding of the nascent polypeptide to obtain native conformation; helps to refold misfolded proteins; transports nascent polypeptide from the cytoplasm to the mitochondria where it is assisted by mitochondrial HSP70 (mtHSP70) and HSP60 to attain functional conformation; involved in protein complex assembly and/or disassembly; and leads specific proteins for their degradation by the lysosome through chaperone-mediated autophagy (236, 237). (Continuing bottom left tile) During stress, the lack of HSP70 at the exit of the ribosome tunnel represses the translation at the elongation stage. HSP70 and HSP90 prevent protein aggregation, and HSP70 also resolves stress granules so that the sequestered mRNAs can resume their translation during recovery from stress; targets terminally misfolded protein for proteasomal degradation; and mediates autophagy by autophagosome. HSP, heat shock protein.

Figure 3

Chromatin remodeling and transcriptional activation of HSP genes. The figure represents the changes in the chromatin region and the promoter of heat shock genes under nonstress and stress conditions in mammalian cells. Under physiological conditions, HSF1 is sequestered in the cytoplasm by constitutive chaperones HSP90 and HSC70. RNAPII is bound to the open promoter region of HSP genes and remains paused/transcriptionally inactive, and the HSP70 gene locus is located close to the membrane. Under stress, the HSP70 locus moves to the nuclear speckle. The chaperones bound to HSF1 now bind misfolded protein, thereby releasing HSF1, which trimerizes and localizes to the nucleus where it binds to the heat shock elements (HSEs) in the HSP gene promoter. Multiple posttranslational modifications activate the HSF1 trimer, resulting in the recruitment of transcription factors (P-TEFb) and nucleosome removal factors (FACT, SWI/SNF, Spt6) to the site causing chromatin remodeling and favoring transcription elongation. HSF1, heat shock factor 1; HSP, heat shock protein; RNAPII, RNA polymerase II.

Figure 4

Milestones on the discovery of HSP70 mRNA translation. Timeline of the discoveries made toward elucidating the translation mechanism of HSP70 mRNA. Since the discovery of internal ribosome entry site (IRES)–mediated cap-independent translation, several studies have attempted to characterize an IRES in 5′ UTR of HSP70 mRNA. While no studies have reported an IRES so far, they have emphasized the significance of the 5′ UTR of HSP70 mRNA. It is now widely accepted that HSP70 mRNA undergoes IRES-independent noncanonical translation. UTR, untranslated region.

Figure 5

Milestones on the discovery of HSP70 mRNA degradation. Stress stabilizes the HSP70 mRNA. However, soon after the removal of stress stimulus, the cells rapidly and selectively degrade the HSP70 mRNA. The figure indicates the crucial discoveries made toward elucidating the mechanism of degradation of HSP70 mRNA. Various studies have reported that the 3′ UTR of HSP70 mRNA coordinates its stability or turnover. AREs, AU-rich elements; HSP, heat shock protein; UTR, untranslated region.