Herp enhances ER-associated protein degradation by recruiting ubiquilins

Hormesis and Cellular Quality Control: A Possible Explanation for the Molecular Mechanisms that Underlie the Benefits of Mild Stress

In contrast to the detrimental action of severe stress conditions, the beneficial effects of mild stress, known as hormesis, is increasingly discussed and studied. A variety of applications for hormesis in risk assessment processes, anti-ageing strategies and clinical therapies have been proposed. The molecular mechanisms underlying the phenomenon of hormesis, however, are not yet fully understood. A possible mechanism that has been proposed for hormesis, the homoeostasis overshoot hypothesis, assumes that an overshoot of repair- and self-recovery mechanisms in response to mild damage can be held responsible for the beneficial effects of hormesis. The present paper proposes 'cellular quality control' as a further explanation of the molecular mechanisms underlying the benefits observed after exposure to mild stress. The most important quality control mechanisms are outlined and their known and hypothesised actions in hormesis are discussed. As an example, different aspects of protein quality control will be described in more detail, which includes the reaction of the cell upon stress-induced protein damage and -aggregation. The regulation of Heat Shock Proteins and components from the ubiquitin proteasome system as part of cellular quality control is described in relation to its beneficial role in hormesis.

Derlin-1 deficiency is embryonic lethal, Derlin-3 deficiency appears normal, and Herp deficiency is intolerant to glucose load and ischemia in mice

Accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) causes a cellular condition called ER stress. To overcome ER stress, unfolded proteins are eliminated by an ER-associated degradation (ERAD) system. To explore the physiological requirements for ERAD-related membrane proteins in mammals, we generated Derlin-1-, Derlin-3-, and Herp-deficient mice by gene targeting. Complete loss of Derlin-1 caused embryonic lethality at around E7-E8 (early somite stages). In contrast, Derlin-3- and Herp-deficient mice were born alive with the expected Mendelian frequency, and were superficially indistinguishable from wild-type mice. However, in the Derlin-3- and Herp-deficient mouse organs, the expression levels of ERAD-related proteins were affected under both normal and ER stress conditions; specific effects differed among the organs. Degradation of ERAD substrates was reduced in the Herp-deficient liver, and Herp-deficient mice exhibited impaired glucose tolerance and vulnerability to brain ischemic injury, both of which are known to be implicated in ER stress. Our findings indicate that ERAD or uncharacterized functions involving Derlin-1 are essential in early embryonic development. Derlin-3- and Herp-deficient mice may become useful model animals for investigations of the physiological contribution of ERAD under stressful or pathological conditions.

The homocysteine-inducible endoplasmic reticulum (ER) stress protein Herp counteracts mutant α-synuclein-induced ER stress via the homeostatic regulation of ER-resident calcium release channel proteins

Endoplasmic reticulum (ER) stress has been implicated as an initiator or contributing factor in neurodegenerative diseases. The mechanisms that lead to ER stress and whereby ER stress contributes to the degenerative cascades remain unclear but their understanding is critical to devising effective therapies. Here we show that knockdown of Herp (Homocysteine-inducible ER stress protein), an ER stress-inducible protein with an ubiquitin-like (UBL) domain, aggravates ER stress-mediated cell death induced by mutant α-synuclein (αSyn) that causes an inherited form of Parkinson's disease (PD). Functionally, Herp plays a role in maintaining ER homeostasis by facilitating proteasome-mediated degradation of ER-resident Ca(2+) release channels. Deletion of the UBL domain or pharmacological inhibition of proteasomes abolishes the Herp-mediated stabilization of ER Ca(2+) homeostasis. Furthermore, knockdown or pharmacological inhibition of ER Ca(2+) release channels ameliorates ER stress, suggesting that impaired homeostatic regulation of Ca(2+) channels promotes a protracted ER stress with the consequent activation of ER stress-associated apoptotic pathways. Interestingly, sustained upregulation of ER stress markers and aberrant accumulation of ER Ca(2+) release channels were detected in transgenic mutant A53T-αSyn mice. Collectively, these data establish a causative link between impaired ER Ca(2+) homeostasis and chronic ER stress in the degenerative cascades induced by mutant αSyn and suggest that Herp is essential for the resolution of ER stress through maintenance of ER Ca(2+) homeostasis. Our findings suggest a therapeutic potential in PD for agents that increase Herp levels or its ER Ca(2+)-stabilizing action.

Links between electrophysiological and molecular pathology of amyotrophic lateral sclerosis

Multiple deficits have been described in amyotrophic lateral sclerosis (ALS), from the first changes in normal functioning of the motoneurons and glia to the eventual loss of spinal and cortical motoneurons. In this review, current results, including changes in size, and electrical properties of motoneurons, glutamate excitotoxicity, calcium buffering, deficits in mitochondrial and cellular transport, impediments to proteostasis which lead to stress of the endoplasmic reticulum (ER), and glial contributions to motoneuronal vulnerability are recapitulated. Results are mainly drawn from the mutant SOD1 mouse model of ALS, and emphasis is placed on early changes that precede the onset of symptoms and the interplay between molecular and electrical processes.

The endoplasmic reticulum protein folding factory and its chaperones: new targets for drug discovery?

Cytosolic heat shock proteins have received significant attention as emerging therapeutic targets. Much of this excitement has been triggered by the discovery that HSP90 plays a central role in the maintenance and stability of multifarious oncogenic membrane receptors and their resultant tyrosine kinase activity. Numerous studies have dealt with the effects of small molecules on chaperone- and stress-related pathways of the endoplasmic reticulum (ER). However, unlike cytosolic chaperones, relatively little emphasis has been placed upon translational avenues towards targeting of the ER for inhibition of folding/secretion of disease-promoting proteins. Here, we summarise existing small molecule inhibitors and potential future targets of ER chaperone-mediated inhibition. Client proteins of translational relevance in disease treatment are outlined, alongside putative future disease treatment modalities based on ER-centric targeted therapies. Particular attention is paid to cancer and autoimmune disorders via the effects of the GRP94 inhibitor geldanamycin and its population of client proteins, overloading of the unfolded protein response, and inhibition of members of the IL-12 family of cytokines by celecoxib and non-coxib analogues.

Assays for detecting the unfolded protein response

The endoplasmic reticulum (ER) is the site for folding of membrane and secreted proteins in the cell. Physiological or pathological processes that disturb protein folding in the ER cause ER stress and activate a set of signaling pathways termed the unfolded protein response (UPR). The UPR leads to transcriptional activation of genes encoding ER-resident chaperones, oxidoreductases, and ER-associated degradation (ERAD) components. Thus, UPR promotes cellular repair and adaptation by enhancing protein-folding capacity, reducing the secretory protein load, and promoting degradation of misfolded proteins. In mammalian cells, the UPR also triggers apoptosis, perhaps when adaptive responses fail. Research into ER stress and the UPR continues to grow at a rapid rate as many new investigators are entering the field. Here, we describe the experimental methods that we have used to study UPR in tissue culture cells. These methods can be used by researchers to plan and interpret experiments aimed at evaluating whether the UPR and related processes are activated or not. It is important to note that these are general guidelines for monitoring the UPR and not all assays will be appropriate for every model system.

Herp regulates Hrd1-mediated ubiquitylation in a ubiquitin-like domain-dependent manner

Accumulation of aberrant proteins in the endoplasmic reticulum (ER) triggers the unfolded protein response pathway that helps the cell to survive under these stress conditions. Herp is a mammalian ubiquitin domain protein, which is strongly induced by the unfolded protein response. It is involved in ER-associated protein degradation (ERAD) and interacts directly with the ubiquitin ligase Hrd1, which is found in high molecular mass complexes of the ER membrane. Here we present the first evidence that Herp regulates Hrd1-mediated ubiquitylation in a ubiquitin-like (UBL) domain-dependent manner. We found that upon exposure of cells to ER stress, elevation of Herp steady state levels is accompanied by an enhanced association of Herp with pre-existing Hrd1. Hrd1-associated Herp is rapidly degraded and substituted by de novo synthesized Herp, suggesting a continuous turnover of the protein at Hrd1 complexes. Further analysis revealed the presence of multiple Hrd1 copies in a single complex enabling binding of a variable number of Herp molecules. Efficient ubiquitylation of the Hrd1-specific ERAD substrate α1-antitrypsin null Hong Kong (NHK) required the presence of the Herp UBL domain, which was also necessary for NHK degradation. In summary, we propose that binding of Herp to Hrd1-containing ERAD complexes positively regulates the ubiquitylation activity of these complexes, thus permitting survival of the cell during ER stress.

An unfolded protein response is the initial cellular response to the expression of mutant matrilin-3 in a mouse model of multiple epiphyseal dysplasia

Multiple epiphyseal dysplasia (MED) can result from mutations in matrilin-3, a structural protein of the cartilage extracellular matrix. We have previously shown that in a mouse model of MED the tibia growth plates were normal at birth but developed a progressive dysplasia characterised by the intracellular retention of mutant matrilin-3 and abnormal chondrocyte morphology. By 3 weeks of age, mutant mice displayed a significant decrease in chondrocyte proliferation and dysregulated apoptosis. The aim of this current study was to identify the initial post-natal stages of the disease. We confirmed that the disease phenotype is seen in rib and xiphoid cartilage and, like tibia growth plate cartilage is characterised by the intracellular retention of mutant matrilin-3. Gene expression profiling showed a significant activation of classical unfolded protein response (UPR) genes in mutant chondrocytes at 5 days of age, which was still maintained by 21 days of age. Interestingly, we also noted the upregulation of arginine-rich, mutated in early stage of tumours (ARMET) and cysteine-rich with EGF-like domain protein 2 (CRELD2) are two genes that have only recently been implicated in the UPR. This endoplasmic reticulum (ER) stress and UPR did not lead to increased chondrocyte apoptosis in mutant cartilage by 5 days of age. In an attempt to alleviate ER stress, mutant mice were fed with a chemical chaperone, 4-sodium phenylbutyrate (SPB). SPB at the dosage used had no effect on chaperone expression at 5 days of age but modestly decreased levels of chaperone proteins at 3 weeks. However, this did not lead to increased secretion of mutant matrilin-3 and in the long term did not improve the disease phenotype. We performed similar studies with a mouse model of Schmid metaphyseal chondrodysplasia, but again this treatment did not improve the phenotype.

The Herp protein pathway is not involved in the pro-amyloidogenic effect of hyperhomocysteinemia

Diet-induced high circulating levels of homocysteine, also known as hyper-homocysteinemia (HHcy), is associated with an acceleration of Alzheimer's disease-like amyloidosis. Herp is a homocysteine-responsive stress protein, which has been shown to increase the formation of amyloid-beta (Abeta) via interaction with presenilins in vitro. The aim of our paper was to investigate the functional role that Herp plays in HHcy-induced amyloidosis. Amyloidosis secondary to diet-induced HHcy in Tg2576 mice is associated with an increase of Herp protein and mRNA levels. By contrast, no other stress-related proteins are altered by the same diet regimen. Compared to wild type animals, brains from a genetically induced HHcy mouse model did not manifest any significant change in Herp levels. Cells stably over-expressing human AbetaPP Swedish mutant incubated with high levels of homocysteine had an increase in Abeta formation, but no change in Herp level. Finally, over-expression of Herp did not result in any significant modification of Abeta levels. We conclude that the Herp protein pathway is unlikely to be directly involved in the pro-amyloidotic effect of HHcy.

Control of cholesterol synthesis through regulated ER-associated degradation of HMG CoA reductase

Multiple mechanisms for feedback control of cholesterol synthesis converge on the rate-limiting enzyme in the pathway, 3-hydroxy-3-methylglutaryl coenzyme A reductase. This complex feedback regulatory system is mediated by sterol and nonsterol metabolites of mevalonate, the immediate product of reductase activity. One mechanism for feedback control of reductase involves rapid degradation of the enzyme from membranes of the endoplasmic reticulum (ER). This degradation results from the accumulation of sterols in ER membranes, which triggers binding of reductase to ER membrane proteins called Insig-1 and Insig-2. Insig binding leads to the recruitment of a membrane-associated ubiquitin ligase called gp78 that initiates ubiquitination of reductase. Ubiquitinated reductase then becomes extracted from ER membranes and is delivered to cytosolic 26S proteasomes through an unknown mechanism that is mediated by the gp78-associated ATPase Valosin-containing protein/p97 and appears to be augmented by nonsterol isoprenoids. Here, we will highlight several advances that have led to the current view of mechanisms for sterol-accelerated, ER-associated degradation of reductase. In addition, we will discuss potential mechanisms for other aspects of the pathway such as selection of reductase for gp78-mediated ubiquitination, extraction of the ubiquitinated enzyme from ER membranes, and the contribution of Insig-mediated degradation to overall regulation of reductase in whole animals.

Ubiquilin functions in autophagy and is degraded by chaperone-mediated autophagy

Autophagy is the process by which organelles and portions of the cytoplasm are degraded in lysosomes. Several different forms of autophagy are known that are distinguishable chiefly by the mode in which cargo is delivered to the lysosome for degradation. Ubiquilin was recently reported to regulate macroautophagy, the form of autophagy in which cytosolic cargo is packaged in a double-membrane structure or autophagosome that fuses with lysosomes for degradation. We confirm here using different morphological and biochemical procedures that ubiquilin is present in autophagosomes in HeLa cells and in brain and liver tissue of mouse. Coimmunoprecipitation studies indicated that ubiquilin binds the autophagosome marker LC3 in a complex and that reduction of ubiquilin expression reduces autophagosome formation, which correlates with a reduction in maturation of LC3-I to the LC3-II form of the protein. We found that ubiquilin is degraded during both macroautophagy and during chaperone-mediated autophagy (CMA), the latter of which involves the active transport of proteins into lysosomes. We discuss the implication of this degradation in mediating cross-talk between macroautophagy and CMA. Finally, we demonstrate that ubiquilin protects cells against starvation-induced cell death propagated by overexpression of mutant Alzheimer's disease PS2N141I protein and green fluorescent protein (GFP)-huntingtin exon-1 fusion protein containing 74 polyglutamines.

Emerging role of Alzheimer's disease-associated ubiquilin-1 in protein aggregation

Abnormal protein aggregation and intracellular or extracellular accumulation of misfolded and aggregated proteins are key events in the pathogenesis of different neurodegenerative diseases. Furthermore, endoplasmic reticulum stress and impairment of the ubiquitin-proteasome system probably contribute to neurodegeneration in these diseases. A characteristic feature of AD (Alzheimer's disease) is the abnormal accumulation of Abeta (amyloid beta-peptide) in the brain. Evidence shows that the AD-associated PS (presenilin) also forms aggregates under certain conditions and that another AD-associated protein, ubiquilin-1, controls protein aggregation and deposition of aggregated proteins. Here, we review the current knowledge of ubiquilin-1 and PS in protein aggregation and related events that potentially influence neurodegeneration.

Gene regulation of alpha4beta2 nicotinic receptors: microarray analysis of nicotine-induced receptor up-regulation and anti-inflammatory effects

alpha4beta2 Nicotinic acetylcholine receptors play an important role in the reward pathways for nicotine. We investigated whether receptor up-regulation of alpha4beta2 nicotinic acetylcholine receptors involves expression changes for non-receptor genes. In a microarray analysis, 10 muM nicotine altered expression of 41 genes at 0.25, 1, 8 and 24 h in halpha4beta2 SH-EP1 cells. The maximum number of gene changes occurred at 8 h, around the initial increase in (3)[H]-cytisine binding. Quantitative RT-PCR corroborated gene induction of endoplasmic reticulum proteins CRELD2, PDIA6, and HERPUD1, and suppression of the pro-inflammatory cytokines IL-1beta and IL-6. Nicotine suppresses IL-1beta and IL-6 expression at least in part by inhibiting NFkappaB activation. Antagonists dihydro-beta-erythroidine and mecamylamine blocked these nicotine-induced changes showing that receptor activation is required. Antagonists alone or in combination with nicotine suppressed CRELD2 message while increasing alpha4beta2 binding. Additionally, small interfering RNA knockdown of CRELD2 increased basal alpha4beta2 receptor expression, and antagonists decreased CRELD2 expression even in the absence of alpha4beta2 receptors. These data suggest that endoplasmic reticulum proteins such as CRELD2 can regulate alpha4beta2 expression, and may explain antagonist actions in nicotine-induced receptor up-regulation. Further, the unexpected finding that nicotine suppresses inflammatory cytokines suggests that nicotinic alpha4beta2 receptor activation promotes anti-inflammatory effects similar to alpha7 receptor activation.

A human coronavirus OC43 variant harboring persistence-associated mutations in the S glycoprotein differentially induces the unfolded protein response in human neurons as compared to wild-type virus

We have reported that human respiratory coronavirus OC43 (HCoV-OC43) is neurotropic and neuroinvasive in humans and mice, and that neurons are the primary target of infection in mice, leading to neurodegenerative disabilities. We now report that an HCoV-OC43 mutant harboring two persistence-associated S glycoprotein point mutations (H183R and Y241H), induced a stronger unfolded protein response (UPR) and translation attenuation in infected human neurons. There was a major contribution of the IRE1/XBP1 pathway, followed by caspase-3 activation and nuclear fragmentation, with no significant role of the ATF6 and eIF2-alpha/ATF4 pathways. Our results show the importance of discrete molecular viral S determinants in virus–neuronal cell interactions that lead to increased production of viral proteins and infectious particles, enhanced UPR activation, and increased cytotoxicity and cell death. As this mutant virus is more neurovirulent in mice, our results also suggest that two mutations in the S glycoprotein could eventually modulate viral neuropathogenesis.

Ubiquilin and p97/VCP bind erasin, forming a complex involved in ERAD

Unwanted proteins in the endoplasmic reticulum (ER) are exported into the cytoplasm and degraded by the proteasome through the ER-associated protein degradation pathway (ERAD). Disturbances in ERAD are linked to ER stress, which has been implicated in the pathogenesis of several human diseases. However, the composition and organization of ERAD complexes in human cells is still poorly understood. In this paper, we describe a trimeric complex that we propose functions in ERAD. Knockdown of erasin, a platform for p97/VCP and ubiquilin binding, or knockdown of ubiquilin in human cells slowed degradation of two classical ERAD substrates. In Caenorhabditis elegans, ubiquilin and erasin are ER stress-response genes that are regulated by the ire-1 branch of the unfolded protein response pathway. Loss of ubiquilin or erasin resulted in activation of ER stress, increased accumulation of polyubiquitinated proteins, and shortened lifespan in worms. Our results strongly support a role for this complex in ERAD and in the regulation of ER stress.

Insig-mediated, sterol-accelerated degradation of the membrane domain of hamster 3-hydroxy-3-methylglutaryl-coenzyme A reductase in insect cells

Sterol-accelerated degradation of the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A reductase is one of several mechanisms through which cholesterol synthesis is controlled in mammalian cells. This degradation results from sterol-induced binding of the membrane domain of reductase to endoplasmic reticulum membrane proteins called Insig-1 and Insig-2, which are carriers of a ubiquitin ligase called gp78. The ensuing gp78-mediated ubiquitination of reductase is a prerequisite for its rapid, 26 S proteasome-mediated degradation from endoplasmic reticulum membranes, a reaction that slows a rate-limiting step in cholesterol synthesis. Here, we report that the membrane domain of hamster reductase is subject to sterol-accelerated degradation in Drosophila S2 cells, but only when mammalian Insig-1 or Insig-2 are co-expressed. This degradation mimics the reaction that occurs in mammalian cells with regard to its absolute requirement for the action of Insigs, sensitivity to proteasome inhibition, augmentation by nonsterol isoprenoids, and sterol specificity. RNA interference studies reveal that this degradation requires the Drosophila Hrd1 ubiquitin ligase and several other proteins, including a putative substrate selector, which associate with the enzyme in yeast and mammalian systems. These studies define Insigs as the minimal requirement for sterol-accelerated degradation of the membrane domain of reductase in Drosophila S2 cells.

Ubiquitin-like and ubiquitin-associated domain proteins: significance in proteasomal degradation

The ubiquitin-proteasome pathway of protein degradation is one of the major mechanisms that are involved in the maintenance of the proper levels of cellular proteins. The regulation of proteasomal degradation thus ensures proper cell functions. The family of proteins containing ubiquitin-like (UbL) and ubiquitin-associated (UBA) domains has been implicated in proteasomal degradation. UbL-UBA domain containing proteins associate with substrates destined for degradation as well as with subunits of the proteasome, thus regulating the proper turnover of proteins.

The homocysteine-inducible endoplasmic reticulum stress protein counteracts calcium store depletion and induction of CCAAT enhancer-binding protein homologous protein in a neurotoxin model of Parkinson disease

The endoplasmic reticulum (ER) is a key organelle regulating intracellular Ca(2+) homeostasis. Oxidants and mitochondria-derived free radicals can target ER-based Ca(2+) regulatory proteins and cause uncontrolled Ca(2+) release that may contribute to protracted ER stress and apoptosis. Several ER stress proteins have been suggested to counteract the deregulation of ER Ca(2+) homeostasis and ER stress. Here we showed that knockdown of Herp, an ubiquitin-like domain containing ER stress protein, renders PC12 and MN9D cells vulnerable to 1-methyl-4-phenylpyridinium-induced cytotoxic cell death by a mechanism involving up-regulation of CHOP expression and ER Ca(2+) depletion. Conversely, Herp overexpression confers protection by blocking 1-methyl-4-phenylpyridinium-induced CHOP up-regulation, ER Ca(2+) store depletion, and mitochondrial Ca(2+) accumulation in a manner dependent on a functional ubiquitin-proteasomal protein degradation pathway. Deletion of the ubiquitin-like domain of Herp or treatment with a proteasomal inhibitor abolished the central function of Herp in ER Ca(2+) homeostasis. Thus, elucidating the underlying molecular mechanism(s) whereby Herp counteracts Ca(2+) disturbances will provide insights into the molecular cascade of cell death in dopaminergic neurons and may uncover novel therapeutic strategies to prevent and ameliorate Parkinson disease progression.

Potentiation of amyotrophic lateral sclerosis (ALS)-associated TDP-43 aggregation by the proteasome-targeting factor, ubiquilin 1

TDP-43 (43-kDa TAR DNA-binding domain protein) is a major constituent of ubiquitin-positive cytoplasmic aggregates present in neurons of patients with fronto-temporal lobular dementia and amyotrophic lateral sclerosis (ALS). The pathologic significance of TDP-43 aggregation is not known; however, dominant mutations in TDP-43 cause a subset of ALS cases, suggesting that misfolding and/or altered trafficking of TDP-43 is relevant to the disease process. Here, we show that the presenilin-binding protein ubiquilin 1 (UBQLN) plays a role in TDP-43 aggregation. TDP-43 interacted with UBQLN both in yeast and in vitro, and the carboxyl-terminal ubiquitin-associated domain of UBQLN was both necessary and sufficient for binding to polyubiquitylated forms of TDP-43. Overexpression of UBQLN recruited TDP-43 to detergent-resistant cytoplasmic aggregates that colocalized with the autophagosomal marker, LC3. UBQLN-dependent aggregation required the UBQLN UBA domain, was mediated by non-overlapping regions of TDP-43, and was abrogated by a mutation in UBQLN previously linked to Alzheimer disease. Four ALS-associated alleles of TDP-43 also coaggregated with UBQLN, and the extent of aggregation correlated with in vitro UBQLN binding affinity. Our findings suggest that UBQLN is a polyubiquitin-TDP-43 cochaperone that mediates the autophagosomal delivery and/or proteasome targeting of TDP-43 aggregates.