A new tool in C. elegans reveals changes in secretory protein metabolism in ire-1-deficient animals

HLH-30/TFEB Rewires the Chaperone Network to Promote Proteostasis Upon Perturbations to the Coenzyme A and Iron-Sulfur Cluster Biosynthesis Pathways

The maintenance of a properly folded proteome is critical for cellular function and organismal health, and its age-dependent collapse is associated with a wide range of diseases. Here, we find that despite the central role of Coenzyme A as a molecular cofactor in hundreds of cellular reactions, inhibition of the first and rate-limiting step in CoA biosynthesis can be beneficial and promote proteostasis. Impairment of the cytosolic iron-sulfur cluster formation pathway, which depends on Coenzyme A, similarly promotes proteostasis and acts in the same pathway. Proteostasis improvement by interference with the Coenzyme A/iron-sulfur cluster biosynthesis pathways is dependent on the conserved HLH-30/TFEB transcription factor. Strikingly, under these conditions, HLH-30 promotes proteostasis by potentiating the expression of select chaperone genes, providing a chaperone-mediated proteostasis shield, rather than by its established role as an autophagy and lysosome biogenesis-promoting factor. This reflects the versatile nature of this conserved transcription factor, which can transcriptionally activate a wide range of protein quality control mechanisms, including chaperones and stress response genes alongside autophagy and lysosome biogenesis genes. These results highlight TFEB as a key proteostasis-promoting transcription factor and underscore it and its upstream regulators as potential therapeutic targets in proteostasis-related diseases.

Unveiling the Intercompartmental Signaling Axis: Mitochondrial to ER Stress Response (MERSR) and its Impact on Proteostasis

Maintaining protein homeostasis is essential for cellular health. Our previous research uncovered a cross-compartmental Mitochondrial to Cytosolic Stress Response, activated by the perturbation of mitochondrial proteostasis, which ultimately results in the improvement of proteostasis in the cytosol. Here, we found that this signaling axis also influences the unfolded protein response of the endoplasmic reticulum (UPR ER ), suggesting the presence of a Mitochondria to ER Stress Response (MERSR). During MERSR, the IRE1 branch of UPR ER is inhibited, introducing a previously unknown regulatory component of MCSR. Moreover, proteostasis is enhanced through the upregulation of the PERK-eIF2α signaling pathway, increasing phosphorylation of eIF2α and improving the ER's ability to handle proteostasis. MERSR activation in both polyglutamine and amyloid-beta peptide-expressing C. elegans disease models also led to improvement in both aggregate burden and overall disease outcome. These findings shed light on the coordination between the mitochondria and the ER in maintaining cellular proteostasis and provide further evidence for the importance of intercompartmental signaling.

HLH-30/TFEB rewires the chaperone network to promote proteostasis under conditions of Coenzyme A and Iron-Sulfur Cluster Deficiency

The maintenance of a properly folded proteome is critical for cellular function and organismal health, and its age-dependent collapse is associated with a wide range of diseases. Here, we find that despite the central role of Coenzyme A as a molecular cofactor in hundreds of cellular reactions, limiting Coenzyme A levels in C. elegans and in human cells, by inhibiting the conserved pantothenate kinase, promotes proteostasis. Impairment of the cytosolic iron-sulfur clusters formation pathway, which depends on Coenzyme A, similarly promotes proteostasis and acts in the same pathway. Proteostasis improvement by Coenzyme A/iron-sulfur cluster deficiencies are dependent on the conserved HLH-30/TFEB transcription factor. Strikingly, under these conditions, HLH-30 promotes proteostasis by potentiating the expression of select chaperone genes providing a chaperone-mediated proteostasis shield, rather than by its established role as an autophagy and lysosome biogenesis promoting factor. This reflects the versatile nature of this conserved transcription factor, that can transcriptionally activate a wide range of protein quality control mechanisms, including chaperones and stress response genes alongside autophagy and lysosome biogenesis genes. These results highlight TFEB as a key proteostasis-promoting transcription factor and underscore it and its upstream regulators as potential therapeutic targets in proteostasis-related diseases.

Alleviating chronic ER stress by p38-Ire1-Xbp1 pathway and insulin-associated autophagy in C. elegans neurons

ER stress occurs in many physiological and pathological conditions. However, how chronic ER stress is alleviated in specific cells in an intact organism is an outstanding question. Here, overexpressing the gap junction protein UNC-9 (Uncoordinated) in C. elegans neurons triggers the Ire1-Xbp1-mediated stress response in an age-dependent and cell-autonomous manner. The p38 MAPK PMK-3 regulates the chronic stress through IRE-1 phosphorylation. Overexpressing gap junction protein also activates autophagy. The insulin pathway functions through autophagy, but not the transcription of genes encoding ER chaperones, to counteract the p38-Ire1-Xbp1-mediated stress response. Together, these results reveal an intricate cellular regulatory network in response to chronic stress in a subset of cells in multicellular organism.

The accumulation of unfolded proteins triggers the ER stress response (UPR), which allows cells to fight against fluctuations in protein expression under both physiological and pathological conditions. Severe acute ER stress responses can be induced by drug treatment. However, such intense ER stress rarely occurs ubiquitously in every cell type in vivo. Here, we designed a genetic system in the nematode C. elegans, which allows us to induce ER stress in specific cells, without drug treatment or any other external stimuli, and then to monitor the stress response. The p38 MAPK directly acts on the phosphorylation of IRE-1 to promote the stress response. Meanwhile, the insulin receptor function through autophagy activation to counteract the p38-IRE-1-XBP-1 pathway. Together, these results reveal an intricate cellular regulatory network in response to chronic stress in multicellular organism.

Application of Caenorhabditis elegans for Research on Endoplasmic Reticulum Stress

Caenorhabditis elegans is a versatile model organism that has been applied to research involving obesity, aging, and neurodegenerative diseases. C. elegans has many advantages over traditional animal models, including ease of handling, a short lifespan, a fully sequenced genome, ease of genetic manipulation, and a high similarity to human disease-related genes. With established C. elegans models of human disease, C. elegans provides a great platform for studying disease pathologies, including endoplasmic reticulum (ER) stress, which is characterized by the accumulation of unfolded and misfolded proteins involved in the pathologies of many diseases. ER stress can lead to activation of the unfolded and misfolded protein response, a mechanism that attenuates ER stress and recovers ER homeostasis. The current review gives an introduction to C. elegans and ER stress, along with the pathological role of ER stress in disease and the application of worm models in ER stress-related research.

PEK-1 is crucial for hormesis induced by inhibition of the IRE-1/XBP-1 pathway in the Caenorhabditis elegans mev-1 mutant

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes an imbalance of proteostasis and is related to many pathological conditions. In answer to this ER stress cells activate a network of three integrated signaling pathways consolidated as the unfolded protein response of the ER (UPR(ER)), which is also present in the stress-sensitive Caenorhabditis elegans mutant mev-1. Whereas inhibition of one of those pathways by RNA-interference (RNAi) versus xbp-1 results in reduced survival of mev-1 nematodes under heat stress, additional knockdown of the xbp-1 splicing activator ire-1 results in a PEK-1-dependent hormetic response. In contrast, increased survival under ire-1/xbp-1 double RNAi was found to be independent of the presence of HSP-4, an UPR(ER)-specific chaperone, as evidenced under ire-1/xbp-1/hsp-4 triple knockdown conditions. Moreover, ire-1/xbp-1 double-RNAi significantly increased chymotrypsin-like proteasomal activity, which was completely blocked under additional RNAi versus pek-1. In conclusion, we identified PEK-1 as a mediator of hormesis in the mev-1 mutant of C. elegans which is induced by simultaneous inhibition of XBP-1 and its splicing activator IRE-1 and mediated through activation of the proteasome.

The FOXO transcription factor DAF-16 bypasses ire-1 requirement to promote endoplasmic reticulum homeostasis

The unfolded protein response (UPR) allows cells to adjust the capacity of the endoplasmic reticulum (ER) to the load of ER-associated tasks. We show that activation of the Caenorhabditis elegans transcription factor DAF-16 and its human homolog FOXO3 restore secretory protein metabolism when the UPR is dysfunctional.We show that DAF-16 establishes alternative ER-associated degradation systems that degrade misfolded proteins independently of the ER stress sensor ire-1 and the ER-associated E3 ubiquitin ligase complex sel-11/sel-1. This is achieved by enabling autophagy-mediated degradation and by increasing the levels of skr-5, a component of an ER associated ubiquitin ligase complex. These degradation systems can act together with the conserved UPR to improve ER homeostasis and ER stress resistance, beyond wild-type levels. Because there is no sensor in the ER that activates DAF-16 in response to intrinsic ER stress, natural or artificial interventions that activate DAF-16 may be useful therapeutic approaches to maintain ER homeostasis.