Martin L. Duennwald

HOP is a cochaperone that facilitates client transfer between two major chaperones, Hsp90 and Hsp70. Emerging evidence, however, suggests that HOP plays additional roles in coordinating complex proteostasis networks. Upon exposure to proteostatic stress, HOP rapidly sequesters soluble misfolded proteins into cytoplasmic foci in a Hsp90 independent manner, thereby facilitating their clearance through the ubiquitin proteasome system. We demonstrate here that stress-dependent HOP foci are biomolecular condensates formed by liquid-liquid phase separation. Purified HOP forms protein droplets that closely resemble the foci observed in cells. Our biophysical analyses show that the phase separation of HOP is driven by electrostatic interactions between its tandem TPR domains, with a critical role of its TPR2A domain. Of note, Hsp90 and Hsp70 regulate the extent of HOP phase separation, with Hsp70 driving HOP droplet formation and Hsp90 reversing it. Finally, we find that the Y354E phosphomimetic variant of HOP impairs phase separation and sensitizes cells to acute misfolding stress, suggesting a key role of HOP condensation in mitigating protein misfolding stress. Our work thus identifies a new mechanism by which HOP phase separation mitigates protein misfolding stress in eukaryotic cells and is regulated by Hsp70 and Hsp90.

Oxidative stress and the progressive degeneration of dopaminergic neurons are key features of Parkinsons disease (PD). The intrinsically disordered structure of the transcription factor Nuclear factor erythroid 2-related factor 2 (Nrf2), which coordinates the main cellular antioxidant response of the body, makes it highly susceptible to misfolding and aggregation under severe oxidative stress, compromising cellular survival. Cannabidiol (CBD) has potent neuroprotective properties, but its exact molecular mechanism within the dopaminergic redox environment remains unclear. In this study, we investigated the protective effects of CBD against 6-hydroxydopamine (6-OHDA)-induced toxicity in both undifferentiated and mature, post-mitotic differentiated SH-SY5Y cells. We found that CBD confers robust Nrf2-dependent neuroprotection against 6-OHDA. Importantly, we uncover a previously unexplored mechanism of neuroprotection by which CBD actively prevents the stress-induced sequestration of Nrf2 into insoluble cytoplasmic inclusions under oxidative stress. We find that CBD keeps Nrf2 in a soluble, functional state, increases Ser40 phosphorylation, restores nuclear localization, and drives the robust transcriptional upregulation of antioxidant enzymes. This targeted activation of Nrf2 effectively reduces intracellular ROS, significantly attenuates mitochondrial fragmentation, and decreases aberrant mitophagic activity. Overall, our results show that rather than merely scavenging reactive oxygen species, CBD directly increases Nrf2 activity during oxidative stress, enabling a sustained cytoprotective response. We thus identify CBD as a highly specific, targeted molecule with a high potential for neuroprotective therapy in PD.

Cells employ a bevy of transcriptional and post-translational stress responses to tolerate the burden of misfolded proteins induced by stress. In particular, the heat shock response facilitates the upregulation of molecular chaperones and protein remodeling factors that mediate proteostasis in response to accumulated misfolded proteins in the nucleus and cytosol. However, in response to stress neurons struggle to induce a canonical heat shock response, highlighting our poor understanding of how neurons maintain proteostasis. Specifically, the ability of post-mitotic respiring cells to regulate the heat shock response in comparison to their rapidly dividing, predominantly glycolytic counterparts has been under-studied. In this study, we employ yeast models that are easily manipulated to generate energy via glycolysis or mitochondrial respiration by changing the carbon source in the media. Using this model, we demonstrate that Hsf1 activity, the heat shock response and proteostasis are impaired in respiring cells. Interestingly, our data show that reduced Hsf1 activity regulates viability of respiring cells, with respiring cells poorly tolerating constitutively activated Hsf1. Finally, we describe alternative post-translational programming of the molecular chaperones Hsp70 and Hsp104 that plausibly enables respiring cells to mediate proteostasis despite a dampened heat shock response. Our findings offer new insights into possible proteostatic strategies employed by cells in different metabolic conditions.

The accumulation of alpha-synuclein (a-Syn) as toxic oligomers, and subsequently in Lewy bodies, is a pathological hallmark of Parkinson's disease (PD) and other synucleinopathies. Molecular chaperones and cochaperones are expected to act in concert to maintain physiological activities of proteins, including a-Syn, but in neurodegeneration this process can become mal-adaptive. Transcript levels of Stress inducible phosphoprotein 1 (STIP1), a co-chaperone of Hsp90/Hsp70, are elevated in brain samples from PD patients. In synucleinopathy mouse models, STIP1 has unexpected bidirectional effects on a-Syn, with overexpression of STIP1 aggravating a-Syn toxicity, whereas knockdown of STIP1 improves toxicity and behavioural phenotypes. However, it is unclear how STIP1 enhances the toxicity of a-Syn. Here we investigate the direct impact of the interaction between STIP1 and a-Syn on the aggregation kinetics of a-Syn using a diverse and integrated set of techniques, including Nuclear Magnetic Resonance (NMR), molecular dynamics simulation, aggregation kinetics assays, electron microscopy, atomic force microscopy, and dynamic light scattering. The toxicity of a-Syn aggregates formed in the presence of STIP1 was assessed using yeast models and SH-SY5Y cell assays. We unravel the mechanisms by which STIP1/HOP regulates the neurotoxicity of a-Syn. Specifically, two binding motifs in the C-terminus of a-Syn directly interact with the TPR2A domain of STIP1/HOP in a dynamic manner, competing for a shared interface on TPR2A. Binding of STIP1/HOP to a-Syn attenuates the formation of a-Syn fibrils while promoting the accumulation of high molecular weight amorphous a-Syn species. Samples of a-Syn aggregated in the presence of STIP1/HOP contain significantly more A11-positive oligomeric species and cause a greater reduction in cell viability than a-Syn aggregated in the absence of STIP1/HOP in neuronal cells. Our results provide a mechanism by which the direct interaction between STIP1/HOP and the C-terminus of a-Syn promotes the formation of cytotoxic, non-amyloidogenic, high molecular weight a-Syn species. Our model offers an explanation for the unexpected pathological link between STIP1 and a-Syn toxicity, thus opening new therapeutic avenues for the treatment of synucleinopathies. Classification: Biological Sciences - Biochemistry The online version contains supplementary material available at 10.1186/s44477-026-00030-3.

Protein aggregation is indicative of the loss of proteostasis associated with neurodegenerative diseases, including Amyotrophic Lateral Sclerosis (ALS) and

All cellular functions rely on accurate protein biosynthesis. Yet, many variants of transfer RNA (tRNA) genes that induce amino acid misincorporation are found in human genomes. Mistranslation induces pleiotropic effects on proteostasis, ranging from protein misfolding to impaired protein biosynthesis and degradation. We employ Saccharomyces cerevisiae (budding yeast), a genetically and biochemically tractable model that facilitates quantitative analysis of how specific proteostasis pathways interact with mistranslating tRNAs. We tested two mistranslating tRNASer variants, one inducing proline to serine (P > S), the other arginine to serine (R > S) misincorporation. We found that P > S misincorporation impairs cellular fitness and sensitizes cells to protein misfolding to a greater extent than R > S misincorporation. Of note, we also show that, even though both tRNA variants induce misincorporation of serine, they result in the accumulation of misfolded proteins by distinct mechanisms. Specifically, R > S misincorporation reduces that association of Hsp70 with misfolded proteins, while P > S misincorporation impairs the degradation of nascent polypeptides. Our findings reveal that different mistranslating tRNASer variants impair specific branches of proteostasis and thus compromise cellular fitness by distinct mechanisms.

The accumulation of alpha-synuclein (a-Syn) as toxic oligomers, and subsequently in Lewy bodies, is a pathological hallmark of Parkinson’s disease (PD) and other synucleinopathies. Molecular chaperones and co-chaperones are expected to act in concert to maintain physiological activities of proteins, including a-Syn, but in neurodegeneration this process can become mal-adaptive. Transcript levels of Stress inducible phosphoprotein 1 (STIP1), a co-chaperone of Hsp90/Hsp70, are elevated in brain samples from PD patients. In synucleinopathy mouse models, STIP1 has unexpected bidirectional effects on a-Syn, with overexpression of STIP1 aggravating a-Syn toxicity, whereas knockdown of STIP1 improves toxicity and behavioural phenotypes. However, it is unclear how STIP1 enhances the toxicity of a-Syn. Here we unravel the mechanisms by which the direct interaction between STIP1/HOP and a-Syn regulates the neurotoxicity of a-Syn. Specifically, two binding motifs in the C-terminus of a-Syn directly interact with the TPR2A domain of STIP1/HOP in a dynamic manner, competing for a shared interface on TPR2A. Binding of STIP1/HOP to a-Syn attenuates the formation of a-Syn fibrils while promoting the accumulation of high molecular weight amorphous a-Syn species. Samples of a-Syn aggregated in the presence of STIP1/HOP contain significantly more A11-positive oligomeric species and cause a greater reduction in cell viability than a-Syn aggregated in the absence of STIP1/HOP in neuronal cells. Our results provide a mechanism by which the direct interaction between STIP1/HOP and the C-terminus of a-Syn promotes the formation of cytotoxic, non-amyloidogenic, high molecular weight a-Syn species. Our model offers an explanation for the unexpected pathological link between STIP1 and a-Syn toxicity, thus opening new therapeutic avenues for the treatment of synucleinopathies.

Co‐chaperones are key elements of cellular protein quality control. They cooperate with the major heat shock proteins Hsp70 and Hsp90 in folding proteins and preventing the toxic accumulation of misfolded proteins upon exposure to stress. Hsp90 interacts with the co‐chaperone stress‐inducible phosphoprotein 1 (Sti1/Stip1/Hop) and activator of Hsp90 ATPase protein 1 (Aha1) among many others. Sti1 and Aha1 control the ATPase activity of Hsp90, but Sti1 also facilitates the transfer of client proteins from Hsp70 to Hsp90, thus connecting these two major branches of protein quality control. We find that misbalanced expression of Sti1 and Aha1 in yeast and mammalian cells causes severe growth defects. Also, deletion of STI1 causes an accumulation of soluble misfolded ubiquitinated proteins and a strong activation of the heat shock response. We discover that, during proteostatic stress, Sti1 forms cytoplasmic inclusions in yeast and mammalian cells that overlap with misfolded proteins. Our work indicates a key role of Sti1 in proteostasis independent of its Hsp90 ATPase regulatory functions by sequestering misfolded proteins during stress.

Organisms are continually exposed to exogenous and endogenous sources of reactive oxygen species (ROS) and other oxidants that have both beneficial and deleterious effects on the cell. ROS have important roles in a wide range of physiological processes; however, high ROS levels are associated with oxidative stress and disease progression. Oxidative stress has been implicated in nearly all major human diseases, from neurogenerative diseases and neuropsychiatric disorders to cardiovascular disease, diabetes, and cancer. Antioxidant defence systems have evolved as a means of protection against oxidative stress, with the transcription factor Nrf2 as the key regulator. Nrf2 is responsible for regulating an extensive panel of antioxidant enzymes involved in the detoxification and elimination of oxidative stress and has been extensively studied in the disease contexts. This review aims to provide the reader with a general overview of oxidative stress and Nrf2, including basic mechanisms of Nrf2 activation and regulation, and implications in various major human diseases.