Decreased ER-associated degradation of alpha-TCR induced by Grp78 depletion with the SubAB cytotoxin

Endoplasmic reticulum stress in T cell-mediated diseases

T cells synthesize a large number of proteins during their development, activation, and differentiation. The build-up of misfolded and unfolded proteins in the endoplasmic reticulum, however, causes endoplasmic reticulum (ER) stress. Thus, T cells can maintain ER homeostasis via endoplasmic reticulum-associated degradation, unfolded protein response, and autophagy. In T cell-mediated diseases, such as rheumatoid arthritis, systemic lupus erythematosus, Sjogren's syndrome, type 1 diabetes and vitiligo, ER stress caused by changes in the internal microenvironment can cause disease progression by affecting T cell homeostasis. This review discusses ER stress in T cell formation, activation, differentiation, and T cell-mediated illnesses, and may offer new perspectives on the involvement of T cells in autoimmune disorders and cancer.

Hydra-Elastin-like Polypeptides Increase Rapamycin Potency When Targeting Cell Surface GRP78

Rapalogues are powerful therapeutic modalities for breast cancer; however, they suffer from low solubility and dose-limiting side effects. To overcome these challenges, we developed a long-circulating multiheaded drug carrier called 5FA, which contains rapamycin-binding domains linked with elastin-like polypeptides (ELPs). To target these "Hydra-ELPs" toward breast cancer, we here linked 5FA with four distinct peptides which are reported to engage the cell surface form of the 78 kDa glucose-regulated protein (csGRP78). To determine if these peptides affected the carrier solubility, this library was characterized by light scattering and mass spectrometry. To guide in vitro selection of the most potent functional carrier for rapamycin, its uptake and inhibition of mTORC1 were monitored in a ductal breast cancer model (BT474). Using flow cytometry to track cellular association, it was found that only the targeted carriers enhanced cellular uptake and were susceptible to proteolysis by SubA, which specifically targets csGRP78. The functional inhibition of mTOR was monitored by Western blot for pS6K, whereby the best carrier L-5FA reduced mTOR activity by 3-fold compared to 5FA or free rapamycin. L-5FA was further visualized using super-resolution confocal laser scanning microscopy, which revealed that targeting increased exposure to the carrier by ∼8-fold. This study demonstrates how peptide ligands for GRP78, such as the L peptide (RLLDTNRPLLPY), may be incorporated into protein-based drug carriers to enhance targeting.

Genomic Epidemiology of Shiga Toxin-Producing Escherichia coli Isolated from the Livestock-Food-Human Interface in South America

Simple Summary

Shiga toxin-producing Escherichia coli (STEC) are zoonotic pathogens that cause food-borne diseases in humans, where cattle and derived products play a key role as reservoirs and vehicles. We analyzed the genomic data of STEC strains circulating at the livestock-food-human interface in South America, extracting clinically and epidemiologically relevant information (serotypes, virulome, resistance genes, sequence types, and phylogenomics). This study included 130 STEC genomes obtained from cattle (n = 51), beef (n = 48), and human (n = 31) samples. The successful expansion of O157:H7 (ST11) and non-O157 (ST16, ST21, ST223, ST443, ST677, ST679, ST2388) clones is highlighted, suggesting common activities, such as multilateral trade and travel. Circulating STEC strains analyzed exhibit high genomic diversity and harbor several genetic determinants associated with severe illness in humans, highlighting the need to establish official surveillance of this pathogen that should be focused on detecting molecular determinants of virulence and clonal relatedness, in the whole beef production chain.

Abstract

Shiga toxin-producing Escherichia coli (STEC) are zoonotic pathogens responsible for causing food-borne diseases in humans. While South America has the highest incidence of human STEC infections, information about the genomic characteristics of the circulating strains is scarce. The aim of this study was to analyze genomic data of STEC strains isolated in South America from cattle, beef, and humans; predicting the antibiotic resistome, serotypes, sequence types (STs), clonal complexes (CCs) and phylogenomic backgrounds. A total of 130 whole genome sequences of STEC strains were analyzed, where 39.2% were isolated from cattle, 36.9% from beef, and 23.8% from humans. The ST11 was the most predicted (20.8%) and included O-:H7 (10.8%) and O157:H7 (10%) serotypes. The successful expansion of non-O157 clones such as ST16/CC29-O111:H8 and ST21/CC29-O26:H11 is highlighted, suggesting multilateral trade and travel. Virulome analyses showed that the predominant stx subtype was stx2a (54.6%); most strains carried ehaA (96.2%), iha (91.5%) and lpfA (77.7%) genes. We present genomic data that can be used to support the surveillance of STEC strains circulating at the livestock-food-human interface in South America, in order to control the spread of critical clones “from farm to table”.

Host response to the subtilase cytotoxin produced by locus of enterocyte effacement-negative Shiga-toxigenic Escherichia coli

Shiga-toxigenic Escherichia coli (STEC) is a major bacterium responsible for disease resulting from foodborne infection, including bloody diarrhea and hemolytic uremic syndrome. STEC produces important virulence factors such as Shiga toxin (Stx) 1 and/or 2. In the STEC family, some locus of enterocyte effacement-negative STEC produce two different types of cytotoxins, namely, Stx2 and subtilase cytotoxin (SubAB). The Stx2 and SubAB cytotoxins are structurally similar and composed of one A subunit and pentamer of B subunits. The catalytically active A subunit of SubAB is a subtilase-like serine protease and specifically cleaves an endoplasmic reticulum (ER) chaperone 78-kDa glucose-regulated protein (GRP78/BiP), a monomeric ATPase that is crucial in protein folding and quality control. The B subunit binds to cell surface receptors. SubAB recognizes sialic carbohydrate-modified cell surface proteins as a receptor. After translocation into cells, SubAB is delivered to the ER, where it cleaves GRP78/BiP. SubAB-catalyzed BiP cleavage induces ER stress, which causes various cell events including inhibition of protein synthesis, suppression of nuclear factor-kappa B activation, apoptotic cell death, and stress granules formation. In this review, we describe SubAB, the SubAB receptor, and the mechanism of cell response to the toxin.

Valosin-containing protein promotes metastasis of osteosarcoma through autophagy induction and anoikis inhibition via the ERK/NF-κβ/beclin-1 signaling pathway

Valosin-containing protein (VCP) promotes the development of metastasis in osteosarcoma (OS) via the PI3K/Akt signaling pathway. However, inhibition of the PI3K/Akt pathway does not completely reverse VCP-mediated invasion and migration of OS, suggesting that VCP-mediated OS invasion and migration involves additional mechanisms. In the present study, a positive correlation between the expression of VCP and cell autophagy was observed among OS tissues. Inhibiting VCP may decrease the survival of malignant cells; however, an autophagy stimulator may compensate for VCP inhibition and promote malignant cell survival. Altering the level of autophagy did not affect cell invasiveness or migration. ERK, NF-κβ and beclin-1 protein expression levels were markedly decreased following VCP inhibition. These findings indicated that VCP may induce autophagy and enhance anoikis resistance without affecting cell invasiveness or migration. Via anoikis resistance, VCP may promote metastasis in OS. Therefore, targeting of the ERK/NF-κβ/beclin-1 signaling pathway may be an effective therapeutic strategy for the management of OS.

Systemic effects of Subtilase cytotoxin produced by Escherichia coli O113:H21

Subtilase cytotoxin (SubAB) is a member of the AB₅ cytotoxin family and is produced by certain strains of Shiga toxigenic Escherichia coli. The toxin is known to be lethal to mice, but the pathological mechanisms that contribute to Uremic Hemolytic Syndrome (HUS) are poorly understood. In this study we show that intraperitoneal injection of a sublethal dose of SubAB in rats triggers a systemic response, with ascitic fluid accumulation, heart hypertrophy and damage to the liver, colon and kidney. SubAB treated rats presented microalbuminuria 20 days post inoculation. At this time we found disruption of the glomerular filtration barrier and alteration of the protein reabsorption mechanisms of the proximal tubule. In the kidney, SubAB also triggered an epithelial to mesenchymal transition (Wuyts et al., 1996). These findings indicate that apart from direct cytotoxic effects on renal tissues, SubAB causes significant damage to the other organs, with potential consequences for HUS pathogenesis.

IMPORTANCE

Uremic Hemolytic Syndrome is an endemic disease in Argentina, with over 400 hundred new cases each year. We have previously described renal effects of Shiga Toxin and its ability to alter renal protein handling. Bearing in mind that Subtilase Cytotoxin is an emerging pathogenic factor, that it is not routinely searched for in patients with HUS, and that to the date its systemic effects have not been fully clarified we decided to study both its systemic effects, and its renal effects to assess whether SubAB could be contributing to pathology seen in children.

The opposite role of two UBA-UBX containing proteins, p47 and SAKS1 in the degradation of a single ERAD substrate, α-TCR

The UBA-UBX domain-containing proteins can interact with ubiquitinated substrates and p97 during endoplasmic reticulum-associated degradation (ERAD). Here, we found that the expressions of all UBA-UBX genes p47, SAKS1, UBXD8, FAF1, and UBXD7 were elevated upon ER stress, albeit with different levels. Of which p47, SAKS1, and UBXD8 are 'immediate' respondents whereas FAF1 and UBXD7 were 'late' respondents to ER stress. Interestingly, the expression of specific UBA-UBX genes were altered in cells stably expressing three different ERAD substrates such as α-TCR, α1-antitrypsin, and δCD3. We first found that p47 and UBXD8 expression levels were increased in α-TCR and α1-antitrypsin stable cell lines, respectively, whereas SAKS1 expression level was reduced in all the three ERAD substrates tested. Of note, we also found p47 promotes, whereas SASK1 delays the degradation of a single ERAD substrate, α-TCR. Additionally, we found that SAKS1 selectively inhibits the degradation of ERAD substrates without affecting cytosolic proteasomal substrates. Taken together, our results identified that UBA-UBX proteins possess substrate selectivity and opposite role of two different UBA-UBX proteins in the degradation of a single ERAD substrate.

Modulation of adipocyte differentiation by omega-3 polyunsaturated fatty acids involves the ubiquitin-proteasome system

We have evaluated the effects of three different omega-3 polyunsaturated fatty acids (ω-3 PUFAs) – docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPA) on fat accumulation and expression of adipogenic and inflammatory markers using both 3T3-L1 pre-adipocytes and differentiated 3T3-L1 adipocytes. Our results indicate that ω-3 PUFAs induce the degradation of fatty acid synthase through the ubiquitin-proteasome system, which is likely to have beneficial metabolic effect on adipose cells. Omega-3 PUFAs also increase overall levels of polyubiquitinated proteins, at least in part through decreasing the expression of proteasome subunits. Moreover, adipocytes are resistant to proteasome inhibition, which induces adipophilin while decreasing perilipin expression. On the other hand, ω-3 PUFAs decrease expression of SREBP1 while inducing expression of adipophilin and GLUT4. Moreover, all three ω-3 PUFAs appear to induce tumour necrosis factor-α without affecting NFκB levels. All three ω-3 PUFAs appear to have overall similar effects. Further research is needed to elucidate their mechanism of action.

Human ASPL/TUG interacts with p97 and complements the proteasome mislocalization of a yeast ubx4 mutant, but not the ER-associated degradation defect

Background

In mammalian cells, ASPL is involved in insulin-stimulated redistribution of the glucose transporter GLUT4 and assembly of the Golgi apparatus. Its putative yeast orthologue, Ubx4, is important for proteasome localization, endoplasmic reticulum-associated protein degradation (ERAD), and UV-induced degradation of RNA polymerase.

Results

Here, we show that ASPL is a cofactor of the hexameric ATPase complex, known as p97 or VCP in mammals and Cdc48 in yeast. In addition, ASPL interacts in vitro with NSF, another hexameric ATPase complex. ASPL localizes to the ER membrane. The central area in ASPL, containing both a SHP box and a UBX domain, is required for binding to the p97 N-domain. Knock-down of ASPL does not impair degradation of misfolded secretory proteins via the ERAD pathway. Deletion of UBX4 in yeast causes cycloheximide sensitivity, while ubx4 cdc48-3 double mutations cause proteasome mislocalization. ASPL alleviates these defects, but not the impaired ERAD.

Conclusions

In conclusion, ASPL and Ubx4 are homologous proteins with only partially overlapping functions. Both interact with p97/Cdc48, but while Ubx4 is important for ERAD, ASPL appears not to share this function.

BiP prevents rod opsin aggregation

Mutations in rod opsin-the light-sensitive protein of rod cells-cause retinitis pigmentosa. Many rod opsin mutations lead to protein misfolding, and therefore it is important to understand the role of molecular chaperones in rod opsin biogenesis. We show that BiP (HSPA5) prevents the aggregation of rod opsin. Cleavage of BiP with the subtilase cytotoxin SubAB results in endoplasmic reticulum (ER) retention and ubiquitylation of wild-type (WT) rod opsin (WT-green fluorescent protein [GFP]) at the ER. Fluorescence recovery after photobleaching reveals that WT-GFP is usually mobile in the ER. By contrast, depletion of BiP activity by treatment with SubAB or coexpression of a BiP ATPase mutant, BiP(T37G), decreases WT-GFP mobility to below that of the misfolding P23H mutant of rod opsin (P23H-GFP), which is retained in the ER and can form cytoplasmic ubiquitylated inclusions. SubAB treatment of P23H-GFP-expressing cells decreases the mobility of the mutant protein further and leads to ubiquitylation throughout the ER. Of interest, BiP overexpression increases the mobility of P23H-GFP, suggesting that it can reduce mutant rod opsin aggregation. Therefore inhibition of BiP function results in aggregation of rod opsin in the ER, which suggests that BiP is important for maintaining the solubility of rod opsin in the ER.

Regulation of subtilase cytotoxin-induced cell death by an RNA-dependent protein kinase-like endoplasmic reticulum kinase-dependent proteasome pathway in HeLa cells

Shiga-toxigenic Escherichia coli (STEC) produces subtilase cytotoxin (SubAB), which cleaves the molecular chaperone BiP in the endoplasmic reticulum (ER), leading to an ER stress response and then activation of apoptotic signaling pathways. Here, we show that an early event in SubAB-induced apoptosis in HeLa cells is mediated by RNA-dependent protein kinase (PKR)-like ER kinase (PERK), not activating transcription factor 6 (ATF6) or inositol-requiring enzyme 1(Ire1), two other ER stress sensors. PERK knockdown suppressed SubAB-induced eIF2α phosphorylation, activating transcription factor 4 (ATF4) expression, caspase activation, and cytotoxicity. Knockdown of eIF2α by small interfering RNA (siRNA) or inhibition of eIF2α dephosphorylation by Sal003 enhanced SubAB-induced caspase activation. Treatment with proteasome inhibitors (i.e., MG132 and lactacystin), but not a general caspase inhibitor (Z-VAD) or a lysosome inhibitor (chloroquine), suppressed SubAB-induced caspase activation and poly(ADP-ribose) polymerase (PARP) cleavage, suggesting that the ubiquitin-proteasome system controls events leading to caspase activation, i.e., Bax/Bak conformational changes, followed by cytochrome c release from mitochondria. Levels of ubiquitinated proteins in HeLa cells were significantly decreased by SubAB treatment. Further, in an early event, some antiapoptotic proteins, which normally turn over rapidly, have their synthesis inhibited, and show enhanced degradation via the proteasome, resulting in apoptosis. In PERK knockdown cells, SubAB-induced loss of ubiquitinated proteins was inhibited. Thus, SubAB-induced ER stress is caused by BiP cleavage, leading to PERK activation, not by accumulation of ubiquitinated proteins, which undergo PERK-dependent degradation via the ubiquitin-proteasome system.

Modulation of the unfolded protein response by GRP78 in prostate cancer

The unfolded protein response (UPR) is an adaptive survival mechanism through which cells can weather the stress of misfolded protein accumulation induced by a wide variety of pathophysiologic and pharmacologic insults. The ER chaperone GRP78 is a central modulator of the UPR both through its protein-binding capacity and its direct regulation of the UPR signaling molecules IRE1α, PERK, and ATF6. Recent reports have revealed the presence of GRP78 on the surface of cancer cells. Biological roles for cell-surface GRP78 include competing NH(2)-domain and COOH-domain agonist receptor activities that induce opposite effects on proliferation and apoptosis. Modulation of the UPR impacts both of these processes directly and indirectly. Here, we outline methods that we use to investigate UPR modulation via direct ligation of cell-surface GRP78. Specifically, we review methods of cell culture, cell-signaling analysis with emphasis on UPR components, and ultimately, the impact that these have on cell proliferation, survival, and apoptosis.

Targeting GRP78 to enhance melanoma cell death

Targeting endoplasmic reticulum stress-induced apoptosis may offer an alternative therapeutic strategy for metastatic melanoma. Fenretinide and bortezomib induce apoptosis of melanoma cells but their efficacy may be hindered by the unfolded protein response, which promotes survival by ameliorating endoplasmic reticulum stress. The aim of this study was to test the hypothesis that inhibition of GRP78, a vital unfolded protein response mediator, increases cell death in combination with endoplasmic reticulum stress-inducing agents. Down-regulation of GRP78 by small-interfering RNA increased fenretinide- or bortezomib-induced apoptosis. Treatment of cells with a GRP78-specific subtilase toxin produced a synergistic enhancement with fenretinide or bortezomib. These data suggest that combining endoplasmic reticulum stress-inducing agents with strategies to down-regulate GRP78, or other components of the unfolded protein response, may represent a novel therapeutic approach for metastatic melanoma.

Subtilase cytotoxin induces apoptosis in HeLa cells by mitochondrial permeabilization via activation of Bax/Bak, independent of C/EBF-homologue protein (CHOP), Ire1alpha or JNK signaling

Subtilase cytotoxin (SubAB) is an AB(5) cytotoxin produced by some strains of Shiga-toxigenic Escherichia coli. The A subunit is a subtilase-like serine protease and cleaves an endoplasmic reticulum (ER) chaperone, BiP, leading to transient inhibition of protein synthesis and cell cycle arrest at G(1) phase, and inducing caspase-dependent apoptosis via mitochondrial membrane damage in Vero cells. Here we investigated the mechanism of mitochondrial permeabilization in HeLa cells. SubAB-induced cytochrome c release into cytosol did not depend on mitochondrial permeability transition pore (PTP), since cyclosporine A did not suppress cytochrome c release. SubAB did not change the expression of anti-apoptotic Bcl-2 or Bcl-XL and pro-apoptotic Bax or Bak, but triggered Bax and Bak conformational changes and association of Bax with Bak. Silencing using siRNA of both bax and bak genes, but not bax, bak, or bim alone, resulted in reduction of cytochrome c release, caspase-3 activation, DNA ladder formation and cytotoxicity, indicating that Bax and Bak were involved in apoptosis. SubAB activated ER transmembrane transducers, Ire1alpha, and cJun N-terminal kinase (JNK), and induced C/EBF-homologue protein (CHOP). To investigate whether these signals were involved in cytochrome c release by Bax activation, we silenced ire1alpha, jnk or chop; however, silencing did not decrease SubAB-induced cytochrome c release, suggesting that these signals were not necessary for SubAB-induced mitochondrial permeabilization by Bax activation.

Structure, biological functions and applications of the AB5 toxins

AB(5) toxins are important virulence factors for several major bacterial pathogens, including Bordetella pertussis, Vibrio cholerae, Shigella dysenteriae and at least two distinct pathotypes of Escherichia coli. The AB(5) toxins are so named because they comprise a catalytic A-subunit, which is responsible for disruption of essential host functions, and a pentameric B-subunit that binds to specific glycan receptors on the target cell surface. The molecular mechanisms by which the AB(5) toxins cause disease have been largely unravelled, including recent insights into a novel AB(5) toxin family, subtilase cytotoxin (SubAB). Furthermore, AB(5) toxins have become a valuable tool for studying fundamental cellular functions, and are now being investigated for potential applications in the clinical treatment of human diseases.

Escherichia coli Subtilase Cytotoxin

Subtilase cytotoxin (SubAB) is the prototype of a new AB(5) toxin family produced by a subset of Shiga toxigenic Escherichia coli (STEC) strains. Its A subunit is a subtilase-like serine protease and cytotoxicity for eukaryotic cells is due to a highly specific, single-site cleavage of BiP/GRP78, an essential Hsp70 family chaperone located in the endoplasmic reticulum (ER). This cleavage triggers a severe and unresolved ER stress response, ultimately triggering apoptosis. The B subunit has specificity for glycans terminating in the sialic acid N-glycolylneuraminic acid. Although its actual role in human disease pathogenesis is yet to be established, SubAB is lethal for mice and induces pathological features overlapping those seen in the haemolytic uraemic syndrome, a life-threatening complication of STEC infection. The toxin is also proving to be a useful tool for probing the role of BiP and ER stress in a variety of cellular functions.

Endoplasmic reticulum-associated degradation of a degron-containing polytopic membrane protein

The presence of two basic amino acids strategically located within a single spanning transmembrane region has previously been shown to act as a signal for the endoplasmic reticulum associated degradation (ERAD) of several polypeptides. In contrast, the functionality of this degron motif within the context of a polytopic membrane protein has not been established. Using opsin as a model system, we have investigated the consequences of inserting the degron motif in the first of its seven transmembrane (TM) spans. Whilst these basic residue reduce the binding of the targeting factor, signal recognition particle, to the first TM span, this has no effect on membrane integration in vitro or in vivo. This most likely reflects the presence of multiple TM spans that can act as targeting signals within in the nascent opsin chain. We find that the degron motif leads to the efficient retention of mutant opsin chains at the endoplasmic reticulum. The mutant opsin polypeptides are degraded via a proteasomal pathway that involves the actions of the E3 ubiquitin ligase HRD1. In contrast, wild-type opsin remains stable for a prolonged period even when artificially accumulated at the endoplasmic reticulum. We conclude that a single dibasic degron motif is sufficient to initiate both the ER retention and subsequent degradation of ospin via an ERAD pathway.

Chaperone-targeting cytotoxin and endoplasmic reticulum stress-inducing drug synergize to kill cancer cells

Diverse physiological and therapeutic insults that increase the amount of unfolded or misfolded proteins in the endoplasmic reticulum (ER) induce the unfolded protein response, an evolutionarily conserved protective mechanism that manages ER stress. Glucose-regulated protein 78/immunoglobulin heavy-chain binding protein (GRP78/BiP) is an ER-resident protein that plays a central role in the ER stress response and is the only known substrate of the proteolytic A subunit (SubA) of a novel bacterial AB(5) toxin. Here, we report that an engineered fusion protein, epidermal growth factor (EGF)-SubA, combining EGF and SubA, is highly toxic to growing and confluent epidermal growth factor receptor-expressing cancer cells, and its cytotoxicity is mediated by a remarkably rapid cleavage of GRP78/BiP. Systemic delivery of EGF-SubA results in a significant inhibition of human breast and prostate tumor xenografts in mouse models. Furthermore, EGF-SubA dramatically increases the sensitivity of cancer cells to the ER stress-inducing drug thapsigargin, and vice versa, demonstrating the first example of mechanism-based synergism in the action of a cytotoxin and an ER-targeting drug.

Differences in endoplasmic-reticulum quality control determine the cellular response to disease-associated mutants of proteolipid protein

Missense mutations in human PLP1, the gene encoding myelin proteolipid protein (PLP), cause dysmyelinating Pelizaeus-Merzbacher disease of varying severity. Although disease pathology has been linked to retention of misfolded PLP in the endoplasmic reticulum (ER) and induction of the unfolded protein response (UPR), the molecular mechanisms that govern phenotypic heterogeneity remain poorly understood. To address this issue, we examined the cellular response to missense mutants of PLP that are associated with distinct disease phenotypes. We found that the mild-disease-associated mutants, W162L and G245A, were cleared from the ER comparatively quickly via proteasomal degradation and/or ER exit. By contrast, the more ;aggressive' A242V mutant, which causes severe disease, was significantly more stable, accumulated at the ER and resulted in a specific activation of the UPR. On the basis of these findings, we propose that the rate at which mutant PLP proteins are cleared from the ER modulates disease severity by determining the extent to which the UPR is activated.

Subtilase cytotoxin cleaves newly synthesized BiP and blocks antibody secretion in B lymphocytes

Shiga-toxigenic Escherichia coli (STEC) use subtilase cytotoxin (SubAB) to interfere with adaptive immunity. Its inhibition of immunoglobulin secretion is both rapid and profound. SubAB favors cleavage of the newly synthesized immunoglobulin heavy chain–binding protein (BiP) to yield a C-terminal fragment that contains BiP’s substrate-binding domain. In the absence of its regulatory nucleotide-binding domain, the SubAB-cleaved C-terminal BiP fragment remains tightly bound to newly synthesized immunoglobulin light chains, resulting in retention of light chains in the endoplasmic reticulum (ER). Immunoglobulins are thus detained in the ER, making impossible the secretion of antibodies by SubAB-treated B cells. The inhibitory effect of SubAB is highly specific for antibody secretion, because other secretory proteins such as IL-6 are released normally from SubAB-treated B cells. Although SubAB also causes BiP cleavage in HepG2 hepatoma cells, (glyco)protein secretion continues unabated in SubAB-exposed HepG2 cells. This specific block in antibody secretion is a novel means of immune evasion for STEC. The differential cleavage of newly synthesized versus “aged” BiP by SubAB in the ER provides insight into the architecture of the ER compartments involved.

VCP mutations causing frontotemporal lobar degeneration disrupt localization of TDP-43 and induce cell death

Frontotemporal lobar degeneration (FTLD) with inclusion body myopathy and Paget disease of bone is a rare, autosomal dominant disorder caused by mutations in the VCP (valosin-containing protein) gene. The disease is characterized neuropathologically by frontal and temporal lobar atrophy, neuron loss and gliosis, and ubiquitin-positive inclusions (FTLD-U), which are distinct from those seen in other sporadic and familial FTLD-U entities. The major component of the ubiquitinated inclusions of FTLD with VCP mutation is TDP-43 (TAR DNA-binding protein of 43 kDa). TDP-43 proteinopathy links sporadic amyotrophic lateral sclerosis, sporadic FTLD-U, and most familial forms of FTLD-U. Understanding the relationship between individual gene defects and pathologic TDP-43 will facilitate the characterization of the mechanisms leading to neurodegeneration. Using cell culture models, we have investigated the role of mutant VCP in intracellular trafficking, proteasomal function, and cell death and demonstrate that mutations in the VCP gene 1) alter localization of TDP-43 between the nucleus and cytosol, 2) decrease proteasome activity, 3) induce endoplasmic reticulum stress, 4) increase markers of apoptosis, and 5) impair cell viability. These results suggest that VCP mutation-induced neurodegeneration is mediated by several mechanisms.

The Endoplasmic Reticulum-associated Degradation of Transthyretin Variants Is Negatively Regulated by BiP in Mammalian Cells

Amyloid fibril formation of mutant transthyretin (TTR) that causes familial amyloid polyneuropathy occurs in the extracellular space. Thus, secretion of TTR variants contributes to the pathogenesis of amyloidosis. However, the molecular mechanisms underlying the endoplasmic reticulum (ER) exit or retention and subsequent degradation of TTR variants remain unclear. Here, we demonstrated that the nonsecreted TTR variants, such as D18G TTR and amyloidogenic TTRs with introduced monomeric mutation (M-TTRs), stably interact with the ER chaperone BiP in mammalian cells. These proteins were co-secreted with the secreted form of BiP in which the KDEL signal was removed, indicating that BiP partially contributes to the ER retention of nonsecreted TTR variants. More interestingly, the degradation efficiency of nonsecreted TTRs was increased when BiP was down-regulated by small interfering RNA. Thus, BiP protects the TTR variants from immediate degradation. Additionally, we showed that the stability of nonsecreted TTR variants is not disturbed in the coat complex II-deficient conditions, which are enough to inhibit the ER export of secreted TTR variants, including wild-type TTR. Therefore, the post-ER retrieval mechanism might not contribute to the ER-associated degradation of nonsecreted TTR variants. These findings suggest that the affinity to the ER-resident protein BiP regulates the fate of TTR variants in the ER.