Claudio Hetz

The unfolded protein response: controlling cell fate decisions under ER stress and beyond

Protein-folding stress at the endoplasmic reticulum (ER) is a salient feature of specialized secretory cells and is also involved in the pathogenesis of many human diseases. ER stress is buffered by the activation of the unfolded protein response (UPR), a homeostatic signalling network that orchestrates the recovery of ER function, and failure to adapt to ER stress results in apoptosis. Progress in the field has provided insight into the regulatory mechanisms and signalling crosstalk of the three branches of the UPR, which are initiated by the stress sensors protein kinase RNA-like ER kinase (PERK), inositol-requiring protein 1α (IRE1α) and activating transcription factor 6 (ATF6). In addition, novel physiological outcomes of the UPR that are not directly related to protein-folding stress, such as innate immunity, metabolism and cell differentiation, have been revealed.

In Vitro Generation of Infectious Scrapie Prions

Prions are unconventional infectious agents responsible for transmissible spongiform encephalopathy (TSE) diseases. They are thought to be composed exclusively of the protease-resistant prion protein (PrPres) that replicates in the body by inducing the misfolding of the cellular prion protein (PrPC). Although compelling evidence supports this hypothesis, generation of infectious prion particles in vitro has not been convincingly demonstrated. Here we show that PrPC --> PrPres conversion can be mimicked in vitro by cyclic amplification of protein misfolding, resulting in indefinite amplification of PrPres. The in vitro-generated forms of PrPres share similar biochemical and structural properties with PrPres derived from sick brains. Inoculation of wild-type hamsters with in vitro-produced PrPres led to a scrapie disease identical to the illness produced by brain infectious material. These findings demonstrate that prions can be generated in vitro and provide strong evidence in support of the protein-only hypothesis of prion transmission.

Targeting the unfolded protein response in disease

Stress induced by the accumulation of unfolded proteins in the endoplasmic reticulum (ER) is a feature of specialized secretory cells and is also observed in many diseases, including cancer, diabetes, autoimmune conditions, liver disorders, obesity and neurodegenerative disorders. Cellular adaptation to ER stress is achieved by the activation of the unfolded protein response, which is an integrated signal transduction pathway that modulates many aspects of ER physiology. When these mechanisms of adaptation are insufficient to handle the unfolded protein load, cells undergo apoptosis. Here, we discuss recent advances in the design of novel compounds and therapeutic strategies to manipulate levels of ER stress in disease.

XBP-1 deficiency in the nervous system protects against amyotrophic lateral sclerosis by increasing autophagy

Mutations in superoxide dismutase-1 (SOD1) cause familial amyotrophic lateral sclerosis (fALS). Recent evidence implicates adaptive responses to endoplasmic reticulum (ER) stress in the disease process via a pathway known as the unfolded protein response (UPR). Here, we investigated the contribution to fALS of X-box-binding protein-1 (XBP-1), a key UPR transcription factor that regulates genes involved in protein folding and quality control. Despite expectations that XBP-1 deficiency would enhance the pathogenesis of mutant SOD1, we observed a dramatic decrease in its toxicity due to an enhanced clearance of mutant SOD1 aggregates by macroautophagy, a cellular pathway involved in lysosome-mediated protein degradation. To validate these observations in vivo, we generated mutant SOD1 transgenic mice with specific deletion of XBP-1 in the nervous system. XBP-1-deficient mice were more resistant to developing disease, correlating with increased levels of autophagy in motoneurons and reduced accumulation of mutant SOD1 aggregates in the spinal cord. Post-mortem spinal cord samples from patients with sporadic ALS and fALS displayed a marked activation of both the UPR and autophagy. Our results reveal a new function of XBP-1 in the control of autophagy and indicate critical cross-talk between these two signaling pathways that can provide protection against neurodegeneration.

Caspase-12 and endoplasmic reticulum stress mediate neurotoxicity of pathological prion protein.

Prion diseases are characterized by accumulation of misfolded prion protein (PrPSc), and neuronal death by apoptosis. Here we show that nanomolar concentrations of purified PrPSc from mouse scrapie brain induce apoptosis of N2A neuroblastoma cells. PrPSc toxicity was associated with an increase of intracellular calcium released from endoplasmic reticulum (ER) and up‐regulation of several ER chaperones. Caspase‐12 activation was detected in cells treated with PrPSc, and cellular death was inhibited by overexpression of a catalytic mutant of caspase‐12 or an ER‐targeted Bcl‐2 chimeric protein. Scrapie‐infected N2A cells were more susceptible to ER‐stress and to PrPSc toxicity than non‐infected cells. In scrapie‐infected mice a correlation between caspase‐12 activation and neuronal loss was observed in histological and biochemical analyses of different brain areas. The extent of prion replication was closely correlated with the up‐regulation of ER‐stress chaperone proteins. Similar results were observed in humans affected with sporadic and variant Creutzfeldt–Jakob disease, implicating for the first time the caspase‐12 dependent pathway in a neurodegenerative disease in vivo, and thus offering novel potential targets for the treatment of prion disorders.

Disturbance of endoplasmic reticulum proteostasis in neurodegenerative diseases

The unfolded protein response (UPR) is a homeostatic mechanism by which cells regulate levels of misfolded proteins in the endoplasmic reticulum (ER). Although it is well characterized in non-neuronal cells, a proliferation of papers over the past few years has revealed a key role for the UPR in normal neuronal function and as an important driver of neurodegenerative diseases. A complex scenario is emerging in which distinct UPR signalling modules have specific and even opposite effects on neurodegeneration depending on the disease context. Here, we provide an overview of the most recent findings addressing the biological relevance of ER stress in the nervous system.

The Unfolded Protein Response: Integrating Stress Signals Through the Stress Sensor IRE1α

Stress induced by accumulation of unfolded proteins at the endoplasmic reticulum (ER) is a classic feature of secretory cells and is observed in many tissues in human diseases including cancer, diabetes, obesity, and neurodegeneration. Cellular adaptation to ER stress is achieved by the activation of the unfolded protein response (UPR), an integrated signal transduction pathway that transmits information about the protein folding status at the ER to the nucleus and cytosol to restore ER homeostasis. Inositol-requiring transmembrane kinase/endonuclease-1 (IRE1α), the most conserved UPR stress sensor, functions as an endoribonuclease that processes the mRNA of the transcription factor X-box binding protein-1 (XBP1). IRE1α signaling is a highly regulated process, controlled by the formation of a dynamic scaffold onto which many regulatory components assemble, here referred to as the UPRosome. Here we provide an overview of the signaling and regulatory mechanisms underlying IRE1α function and discuss the emerging role of the UPR in adaptation to protein folding stress in specialized secretory cells and in pathological conditions associated with alterations in ER homeostasis.

Fine-Tuning of the Unfolded Protein Response: Assembling the IRE1α Interactome

Endoplasmic reticulum (ER) stress is a hallmark feature of secretory cells and many diseases, including cancer, neurodegeneration, and diabetes. Adaptation to protein-folding stress is mediated by the activation of an integrated signal transduction pathway known as the unfolded protein response (UPR). The UPR signals through three distinct stress sensors located at the ER membrane-IRE1alpha, ATF6, and PERK. Although PERK and IRE1alpha share functionally similar ER-luminal sensing domains and both are simultaneously activated in cellular paradigms of ER stress in vitro, they are selectively engaged in vivo by the physiological stress of unfolded proteins. The differences in terms of tissue-specific regulation of the UPR may be explained by the formation of distinct regulatory protein complexes. This concept is supported by the recent identification of adaptor and modulator proteins that directly interact with IRE1alpha. In this Review, we discuss recent evidence supporting a model where IRE1alpha signaling emerges as a highly regulated process, controlled by the formation of a dynamic scaffold onto which many regulatory components assemble.

BAX Inhibitor-1 Is a Negative Regulator of the ER Stress Sensor IRE1α

Adaptation to endoplasmic reticulum (ER) stress depends on the activation of an integrated signal transduction pathway known as the unfolded protein response (UPR). Bax inhibitor-1 (BI-1) is an evolutionarily conserved ER-resident protein that suppresses cell death. Here we have investigated the role of BI-1 in the UPR. BI-1 expression suppressed IRE1alpha activity in fly and mouse models of ER stress. BI-1-deficient cells displayed hyperactivation of the ER stress sensor IRE1alpha, leading to increased levels of its downstream target X-box-binding protein-1 (XBP-1) and upregulation of UPR target genes. This phenotype was associated with the formation of a stable protein complex between BI-1 and IRE1alpha, decreasing its ribonuclease activity. Finally, BI-1 deficiency increased the secretory activity of primary B cells, a phenomenon regulated by XBP-1. Our results suggest a role for BI-1 in early adaptive responses against ER stress that contrasts with its known downstream function in apoptosis.

Proteostasis control by the unfolded protein response

Stress induced by accumulation of misfolded proteins in the endoplasmic reticulum is observed in many physiological and pathological conditions. To cope with endoplasmic reticulum stress, cells activate the unfolded protein response, a dynamic signalling network that orchestrates the recovery of homeostasis or triggers apoptosis, depending on the level of damage. Here we provide an overview of recent insights into the mechanisms that cells employ to maintain proteostasis and how the unfolded protein response determines cell fate under endoplasmic reticulum stress.