Endoplasmic reticulum stress in the absence of calnexin

Impact of ovariectomy on neurotransmitter receptors BDNF/TrkB and endoplasmic reticulum molecular chaperones in rat hypoglossal nucleus

Currently hypoglossal nerve-genioglossus axis is the major research core of OSA pathogenesis. The pathogenesis of OSA incidence changes before and after menopause needs to be clarified further. Little is known about the influences of ovariectomy on hypoglossal motoneurons. In the research, we utilized a rat ovariectomy model to evaluate the expression changes of 5-HT2A and α1-Adrenergic receptors in the hypoglossal nucleus and to explore the involvement of BDNF/TrkB signaling and endoplasmic reticulum molecular chaperones in the hypoglossal nucleus. Results indicated that the expression of 5-HT2A and α1-Adrenergic receptors reduced dramatically in the hypoglossal nucleus of ovariectomized rats. The apoptosis level of hypoglossal motor neurons increased markedly in the OVX groups. The up-regulated expression of BDNF and down-regulated expression of TrkB were found in the OVX groups. Ovarian insufficiency resulted in the activation of UPR and the loss of CANX-CALR cycle. Estrogen replacement could restore these changes partially. Estrogen level influences the expression of neurotransmitter receptors, and regulates BDNF/TrkB signaling compensation and endoplasmic reticulum homeostasis, which might be one of the pathogenesis of menopausal female OSA. The results reveal a new perspective for studying female OSA from the view of hypoglossal nerve and hormonal changes and attempt to propel 17β-estradiol toward a feasible therapy for female OSA.

Supplementary Information

The online version contains supplementary material available at 10.1007/s41105-024-00520-5.

Specific heterozygous variants in MGP lead to endoplasmic reticulum stress and cause spondyloepiphyseal dysplasia

Matrix Gla protein (MGP) is a vitamin K-dependent post-translationally modified protein, highly expressed in vascular and cartilaginous tissues. It is a potent inhibitor of extracellular matrix mineralization. Biallelic loss-of-function variants in the MGP gene cause Keutel syndrome, an autosomal recessive disorder characterized by widespread calcification of various cartilaginous tissues and skeletal and vascular anomalies. In this study, we report four individuals from two unrelated families with two heterozygous variants in MGP, both altering the cysteine 19 residue to phenylalanine or tyrosine. These individuals present with a spondyloepiphyseal skeletal dysplasia characterized by short stature with a short trunk, diffuse platyspondyly, midface retrusion, progressive epiphyseal anomalies and brachytelephalangism. We investigated the cellular and molecular effects of one of the heterozygous deleterious variants (C19F) using both cell and genetically modified mouse models. Heterozygous 'knock-in' mice expressing C19F MGP recapitulate most of the skeletal anomalies observed in the affected individuals. Our results suggest that the main underlying mechanism leading to the observed skeletal dysplasia is endoplasmic reticulum stress-induced apoptosis of the growth plate chondrocytes. Overall, our findings support that heterozygous variants in MGP altering the Cys19 residue cause autosomal dominant spondyloepiphyseal dysplasia, a condition distinct from Keutel syndrome both clinically and molecularly.

The regulatory role of endoplasmic reticulum chaperone proteins in neurodevelopment

The endoplasmic reticulum (ER) is the largest tubular reticular organelle spanning the cell. As the main site of protein synthesis, Ca2+ homeostasis maintenance and lipid metabolism, the ER plays a variety of essential roles in eukaryotic cells, with ER molecular chaperones participate in all these processes. In recent years, it has been reported that the abnormal expression of ER chaperones often leads to a variety of neurodevelopmental disorders (NDDs), including abnormal neuronal migration, neuronal morphogenesis, and synaptic function. Neuronal development is a complex and precisely regulated process. Currently, the mechanism by which neural development is regulated at the ER level remains under investigation. Therefore, in this work, we reviewed the recent advances in the roles of ER chaperones in neural development and developmental disorders caused by the deficiency of these molecular chaperones.

Role of Ca2+, Calnexin and Calreticulin in Platelet from Adult Patients with Chronic Immune Thrombocytopenic Purpura

BACKGROUND

Adult chronic immune thrombocytopenia (chronic ITP) is a common autoimmune hemorrhagic disease characterized by decreased platelet production and increased platelet destruction, leading to thrombocytopenia. In this study, Ca²⁺, calnexin (CNX) and calreticulin (CRT) within platelets from adult patients with chronic ITP were investigated.

METHODS

Platelets were isolated from blood specimen collected from 20 adult patients with chronic ITP and 20 healthy volunteers. Ca²⁺, CNX and CRT were determined by flow cytometry, and the results were analyzed with EXPO32 ADC software.

RESULTS

Flow cytometry showed the expressions of Ca²⁺ (74.19±19.40% vs 22.79±10.47%) was elevated (P<0.05). However, CNX (15.10±7.32% vs 41.79±14.45%) and CRT (25.11±12.66% vs 38.58±12.02%) were decreased markedly in platelets from adult patients with chronic ITP (P<0.05 compared with healthy volunteers).

CONCLUSION

Based on enhanced expression of Ca²⁺ and attenuated expression of CNX and CRT in patients with chronic ITP, Ca²⁺ concentration and its associated down-regulated proteins may be important regulatory signals in the pathogenesis of chronic ITP.

Sequencing of Argonaute-bound microRNA/mRNA hybrids reveals regulation of the unfolded protein response by microRNA-320a

MicroRNAs (miRNA) are short non-coding RNAs widely implicated in gene regulation. Most metazoan miRNAs utilize the RNase III enzymes Drosha and Dicer for biogenesis. One notable exception is the RNA polymerase II transcription start sites (TSS) miRNAs whose biogenesis does not require Drosha. The functional importance of the TSS-miRNA biogenesis is uncertain. To better understand the function of TSS-miRNAs, we applied a modified Crosslinking, Ligation, and Sequencing of Hybrids on Argonaute (AGO-qCLASH) to identify the targets for TSS-miRNAs in HCT116 colorectal cancer cells with or without DROSHA knockout. We observed that miR-320a hybrids dominate in TSS-miRNA hybrids identified by AGO-qCLASH. Targets for miR-320a are enriched for the eIF2 signaling pathway, a downstream component of the unfolded protein response. Consistently, in miR-320a mimic- and antagomir- transfected cells, differentially expressed gene products are associated with eIF2 signaling. Within the AGO-qCLASH data, we identified the endoplasmic reticulum (ER) chaperone calnexin as a direct miR-320a down-regulated target, thus connecting miR-320a to the unfolded protein response. During ER stress, but not amino acid deprivation, miR-320a up-regulates ATF4, a critical transcription factor for resolving ER stress. In summary, our study investigates the targetome of the TSS-miRNAs in colorectal cancer cells and establishes miR-320a as a regulator of unfolded protein response.

Age-Related Decline in Expression of Molecular Chaperones Induces Endoplasmic Reticulum Stress and Chondrocyte Apoptosis in Articular Cartilage

Aging is a major risk factor for the development of osteoarthritis (OA). One hallmark of aging is loss of proteostasis resulting in increased cellular stress and cell death. However, its effect on the development of OA is not clear. Here, using knee articular cartilage tissue from young and old cynomolgus monkeys (Macaca fascicularis), we demonstrate that with aging there is loss of molecular chaperone expression resulting in endoplasmic reticulum (ER) stress and cell death. Chondrocytes from aged articular cartilage showed decreased expression of molecular chaperones, including protein disulfide isomerase, calnexin, and Ero1-like protein alpha, and increased immunohistochemical staining for ER stress markers (phosphorylated IRE1 alpha, spliced X-box binding protein-1, activating transcription factor 4 and C/EBP homologous protein), and apoptotic markers [cleaved caspase 3 and cleaved poly(ADP-ribose) polymerase], suggesting that decreased expression of molecular chaperone during aging induces ER stress and chondrocyte apoptosis in monkey articular cartilage. Apoptosis induced by aging-associated ER stress was further confirmed by TUNEL staining. Aged monkey cartilage also showed increased expression of nuclear protein 1 (Nupr1) and tribbles related protein-3 (TRB3), known regulators of apoptosis and cell survival pathways. Treatment of cultured monkey chondrocytes with a small molecule chemical chaperone, 4-phenylbutyric acid (PBA, a general ER stress inhibitor) or PERK Inhibitor I (an ER stress inhibitor specifically targeting the PERK branch of the unfolded protein response pathway), decreased the expression of ER stress and apoptotic markers and reduced the expression of Nupr1 and TRB3. Consistent with the above finding, knockdown of calnexin expression induces ER stress and apoptotic markers in normal human chondrocytes in vitro. Taken together, our study clearly demonstrates that aging promotes loss of proteostasis and induces ER stress and chondrocyte apoptosis in articular cartilage. Thus, restoring proteostasis using chemical/molecular chaperone or ER stress inhibitor could be a therapeutic option to treat aged-linked OA.

Understanding the mammalian TRAP complex function(s)

In eukaryotic cells, about one-third of the synthesized proteins are translocated into the endoplasmic reticulum; they are membrane or lumen resident proteins and proteins direct to the Golgi apparatus. The co-translational translocation takes place through the heterotrimeric protein-conducting channel Sec61 which is associated with the ribosome and many accessory components, such as the heterotetrameric translocon-associated protein (TRAP) complex. Recently, microscopic techniques, such as cryo-electron microscopy and cryo-electron tomography, have enabled the determination of the translocation machinery structure. However, at present, there is a lack of understanding regarding the roles of some of its components; indeed, the TRAP complex function during co-translational translocation needs to be established. In addition, TRAP may play a role during unfolded protein response, endoplasmic-reticulum-associated protein degradation and congenital disorder of glycosylation (ssr4 CDG). In this article, I describe the current understanding of the TRAP complex in the light of its possible function(s).

SMAR1 favors immunosurveillance of cancer cells by modulating calnexin and MHC I expression

Down-regulation or loss of MHC class I expression is a major mechanism used by cancer cells to evade immunosurveillance and increase their oncogenic potential. MHC I mediated antigen presentation is a complex regulatory process, controlled by antigen processing machinery (APM) dictating immune response. Transcriptional regulation of the APM that can modulate gene expression profile and their correlation to MHC I mediated antigen presentation in cancer cells remain enigmatic. Here, we reveal that Scaffold/Matrix-Associated Region 1- binding protein (SMAR1), positively regulates MHC I surface expression by down-regulating calnexin, an important component of antigen processing machinery (APM) in cancer cells. SMAR1, a bonafide MAR binding protein acts as a transcriptional repressor of several oncogenes. It is down-regulated in higher grades of cancers either through proteasomal degradation or through loss of heterozygosity (LOH) at the Chr.16q24.3 locus where the human homolog of SMAR1 (BANP) has been mapped. It binds to a short MAR region of the calnexin promoter forming a repressor complex in association with GATA2 and HDAC1. A reverse correlation between SMAR1 and calnexin was thus observed in SMAR1-LOH cells and also in tissues from breast cancer patients. To further extrapolate our findings, influenza A (H1N1) virus infection assay was performed. Upon viral infection, the levels of SMAR1 significantly increased resulting in reduced calnexin expression and increased MHC I presentation. Taken together, our observations establish that increased expression of SMAR1 in cancers can positively regulate MHC I surface expression thereby leading to higher chances of tumor regression and elimination of cancer cells.

Endoplasmic reticulum and the microRNA environment in the cardiovascular system 1

Stress responses are important to human physiology and pathology, and the inability to adapt to cellular stress leads to cell death. To mitigate cellular stress and re-establish homeostasis, cells, including those in the cardiovascular system, activate stress coping response mechanisms. The endoplasmic reticulum, a component of the cellular reticular network in cardiac cells, mobilizes so-called endoplasmic reticulum stress coping responses, such as the unfolded protein response. MicroRNAs play an important part in the maintenance of cellular and tissue homeostasis, perform a central role in the biology of the cardiac myocyte, and are involved in pathological cardiac function and remodeling. In this paper, we review a link between endoplasmic reticulum homeostasis and microRNA with an emphasis on the impact on stress responses in the cardiovascular system.

Identification of calnexin as a diacylglycerol acyltransferase-2 interacting protein

Triacylglycerol synthesis is catalyzed by acyl CoA:diacylglycerol acyltransferase-2 (DGAT2). DGAT2 is an integral membrane protein that is localized to the endoplasmic reticulum and interacts with lipid droplets. Using BioId, a method to detect proximal and interacting proteins, we identified calnexin as a DGAT2-interacting protein. Co-immunoprecipitation and proximity ligation assays confirmed this finding. We found that calnexin-deficient mouse embryonic fibroblasts had reduced intracellular triacylglycerol levels and fewer large lipid droplets (>1.0 μm² area). Despite the alterations in triacylglycerol metabolism, in vitro DGAT2 activity, localization and protein stability were not affected by the absence of calnexin.

Targeting HPV-16 antigens to the endoplasmic reticulum induces an endoplasmic reticulum stress response

Very promising results have been observed with a deoxyribonucleic acid (DNA) vaccine based on human papillomavirus type-16 (HPV-16) antigen retention and delivery system in the endoplasmic reticulum (ER). However, the mechanism by which these antigens are processed once they reach this organelle is still unknown. Therefore, we evaluated whether this system awakens a stress response in the ER. Different DNA constructs based on E6 and E7 mutant antigens fused to an ER signal peptide (SP), a signal for retention in the ER (KDEL), or both signals (SPK), were transfected into HEK-293 cells. Overexpression of E6 and E7 antigens targeted to the ER (SP, and SPK constructs) induced ER stress, which was indicated by an increase of the ER-stress markers GRP78/BiP and CHOP. Additionally, the ER stress response was mediated by the ATF4 transcription factor, which was translocated into the nucleus. Besides, the overexpressed antigens were degraded by the proteasome. Through a cycloheximide-chase assay, we demonstrated that when both protein synthesis and proteasome were inhibited, the overexpressed antigens were degraded. Interestingly, when proteasome was blocked autophagy was increased and the ER stress response decreased. Taken together, these results indicate that the antigens are initially degraded by the ERAD pathway, and autophagy degradation pathway can be induced to compensate the proteasome inhibition. Therefore, we provided a new insight into the mechanism by which E6 and E7 mutant antigens are processed once they reach the ER, which will help to improve the development of more effective vaccines against cancer.

Calnexin is necessary for T cell transmigration into the central nervous system

In multiple sclerosis (MS), a demyelinating inflammatory disease of the CNS, and its animal model (experimental autoimmune encephalomyelitis; EAE), circulating immune cells gain access to the CNS across the blood-brain barrier to cause inflammation, myelin destruction, and neuronal damage. Here, we discovered that calnexin, an ER chaperone, is highly abundant in human brain endothelial cells of MS patients. Conversely, mice lacking calnexin exhibited resistance to EAE induction, no evidence of immune cell infiltration into the CNS, and no induction of inflammation markers within the CNS. Furthermore, calnexin deficiency in mice did not alter the development or function of the immune system. Instead, the loss of calnexin led to a defect in brain endothelial cell function that resulted in reduced T cell trafficking across the blood-brain barrier. These findings identify calnexin in brain endothelial cells as a potentially novel target for developing strategies aimed at managing or preventing the pathogenic cascade that drives neuroinflammation and destruction of the myelin sheath in MS.

Calcium signaling at the endoplasmic reticulum: fine-tuning stress responses

Endoplasmic reticulum (ER) calcium signaling is implicated in a myriad of coordinated cellular processes. The ER calcium content is tightly regulated as it allows a favorable environment for protein folding, in addition to operate as a major reservoir for fast and specific release of calcium. Altered ER homeostasis impacts protein folding, activating the unfolded protein response (UPR) as a rescue mechanism to restore proteostasis. ER calcium release impacts mitochondrial metabolism and also fine-tunes the threshold to undergo apoptosis under chronic stress. The global coordination between UPR signaling and energetic demands takes place at mitochondrial associated membranes (MAMs), specialized subdomains mediating interorganelle communication. Here we discuss current models explaining the functional relationship between ER homeostasis and various cellular responses to coordinate proteostasis and metabolic maintenance.

Dysregulated expression of proteins associated with ER stress, autophagy and apoptosis in tissues from nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is categorized into nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH) and has emerged as a risk factor for more critical clinical conditions. However, the underlying mechanisms of NAFLD pathogenesis are not fully understood. In this study, expression of proteins associated with endoplasmic reticulum (ER) stress, apoptosis and autophagy were analyzed in normal, NAFL and NASH human livers by western blotting. Levels of some ER stress-transducing transcription factors, including cleaved activating transcription factor 6, were higher in NASH than in the normal tissues. However, the expression of a majority of the ER chaperones and foldases analyzed, including glucose-regulated protein 78 and ER protein 44, was lower in NASH than in the normal tissues. Levels of apoptosis markers, such as cleaved poly (ADP-ribose) polymerase, were also lower in NASH tissues, in which expression of some B-cell lymphoma-2 family proteins was up- or down-regulated compared to the normal tissues. The level of the autophagy substrate p62 was not different in NASH and normal tissues, although some autophagy regulators were up- or down-regulated in the NASH tissues compared to the normal tissues. Levels of most of the proteins analyzed in NAFL tissues were either similar to those in one of the other two types, NASH and normal, or were somewhere in between. Together, these findings suggest that regulation of certain important tissues processes involved in protein quality control and cell survival were broadly compromised in the NAFLD tissues.

The hepatocyte proteome in organotypic rat liver models and the influence of the local microenvironment

BACKGROUND

Liver models that closely mimic the in vivo microenvironment are useful for understanding liver functions, capabilities, and intercellular communication processes. Three-dimensional (3D) liver models assembled using hepatocytes and liver sinusoidal endothelial cells (LSECs) separated by a polyelectrolyte multilayer (PEM) provide a functional system while also permitting isolation of individual cell types for proteomic analyses.

METHODS

To better understand the mechanisms and processes that underlie liver model function, hepatocytes were maintained as monolayers and 3D PEM-based formats in the presence or absence of primary LSECs. The resulting hepatocyte proteomes, the proteins in the PEM, and extracellular levels of urea, albumin and glucose after three days of culture were compared.

RESULTS

All systems were ketogenic and found to release glucose. The presence of the PEM led to increases in proteins associated with both mitochondrial and peroxisomal-based β-oxidation. The PEMs also limited production of structural and migratory proteins associated with dedifferentiation. The presence of LSECs increased levels of Phase I and Phase II biotransformation enzymes as well as several proteins associated with the endoplasmic reticulum and extracellular matrix remodeling. The proteomic analysis of the PEMs indicated that there was no significant change after three days of culture. These results are discussed in relation to liver model function.

CONCLUSIONS

Heterotypic cell-cell and cell-ECM interactions exert different effects on hepatocyte functions and phenotypes.

Endoplasmic Reticulum Malfunction in the Nervous System

Neurodegenerative diseases often have multifactorial causes and are progressive diseases. Some are inherited while others are acquired, and both vary greatly in onset and severity. Impaired endoplasmic reticulum (ER) proteostasis, involving Ca²⁺ signaling, protein synthesis, processing, trafficking, and degradation, is now recognized as a key risk factor in the pathogenesis of neurological disorders. Lipidostasis involves lipid synthesis, quality control, membrane assembly as well as sequestration of excess lipids or degradation of damaged lipids. Proteostasis and lipidostasis are maintained by interconnected pathways within the cellular reticular network, which includes the ER and Ca²⁺ signaling. Importantly, lipidostasis is important in the maintenance of membranes and luminal environment that enable optimal protein processing. Accumulating evidence suggest that the loss of coordinate regulation of proteostasis and lipidostasis has a direct and negative impact on the health of the nervous system.

The Endoplasmic Reticulum Chaperone Calnexin Is a NADPH Oxidase NOX4 Interacting Protein

Within the family of NADPH oxidases, NOX4 is unique as it is predominantly localized in the endoplasmic reticulum, has constitutive activity, and generates hydrogen peroxide (H₂O₂). We hypothesize that these features are consequences of a so far unidentified NOX4-interacting protein. Two-dimensional blue native (BN) electrophorese combined with SDS-PAGE yielded NOX4 to reside in macromolecular complexes. Interacting proteins were screened by quantitative SILAC (stable isotope labeling of amino acids in cell culture) co-immunoprecipitation (Co-IP) in HEK293 cells stably overexpressing NOX4. By this technique, several interacting proteins were identified with calnexin showing the most robust interaction. Calnexin also resided in NOX4-containing complexes as demonstrated by complexome profiling from BN-PAGE. The calnexin NOX4 interaction could be confirmed by reverse Co-IP and proximity ligation assay, whereas NOX1, NOX2, or NOX5 did not interact with calnexin. Calnexin deficiency as studied in mouse embryonic fibroblasts from calnexin^−/− mice or in response to calnexin shRNA reduced cellular NOX4 protein expression and reactive oxygen species formation. Our results suggest that endogenous NOX4 forms macromolecular complexes with calnexin, which are needed for the proper maturation, processing, and function of NOX4 in the endoplasmic reticulum.

Hepatic Macrosteatosis Is Partially Converted to Microsteatosis by Melatonin Supplementation in ob/ob Mice Non-Alcoholic Fatty Liver Disease

Background

Obesity is a common risk factor for non-alcoholic fatty liver disease (NAFLD). Currently, there are no specific treatments against NAFLD. Thus, examining any molecule with potential benefits against this condition emerged melatonin as a molecule that influences metabolic dysfunctions. The aim of this study was to determine whether melatonin would function against NAFDL, studying morphological, ultrastuctural and metabolic markers that characterize the liver of ob/ob mice.

Methods

Lean and ob/ob mice were supplemented with melatonin in the drinking water for 8 weeks. Histology and stereology were performed to assess hepatic steatosis and glycogen deposition. Ultrastructural features of mitochondria, endoplasmic reticulum (ER) and their juxtapositions were evaluated in livers of all experimental groups. Furthermore, hepatic distribution and expression of markers of ER and mitochondria (calnexin, ATP sintase β, GRP78 and CHOP) and metabolic dysfunction (RPB4, β-catenin) and cellular longevity (SIRT1) were analyzed.

Results

Melatonin significantly reduced glycemia, identified also by a decrease of hepatic RBP4 expression, reversed macrosteatosis in microsteatosis at the hepatic pericentral zone, enlarged ER-mitochondrial distance and ameliorated the morphology and organization of these organelles in ob/ob mouse liver. Furthermore, in ob/ob mice, calnexin and ATP synthase β were partially restored, GRP78 and CHOP decreased in periportal and midzonal hepatocytes and β-catenin expression was, in part, restored in peripheral membranes of hepatocytes. Melatonin supplementation to ob/ob mice improves hepatic morphological, ultrastructural and metabolic damage that occurs as a result of NAFLD.

Conclusions

Melatonin may be a potential adjuvant treatment to limit NAFLD and its progression into irreversible complications.

N-Myristoyltransferase 1 interacts with calnexin at the endoplasmic reticulum

Calnexin is a type 1 integral endoplasmic reticulum (ER) membrane molecular chaperone with a highly conserved C-terminal domain oriented to the cytoplasm. Protein N-myristoylation plays an important role in a wide variety of cellular signal transduction pathways and it is catalyzed by N-myristoyltransferase (NMT), a cytoplasmic and ER associated enzyme. Here using yeast two-hybrid screen, Western blot analysis, immunoprecipitation, immunolocalization and cellular fractionation we discovered that N-myristoyltransferase 1 interacts with calnexin at the ER. These observations point at a previously unrecognized contribution of calnexin to the retention of NMT1 at the ER membrane.

Endoplasmic reticulum quality control and dysmyelination

Dysmyelination contributes to several human diseases including multiple sclerosis, Charcot-Marie-Tooth, leukodystrophies, and schizophrenia and can result in serious neurological disability. Properly formed, compacted myelin sheaths are required for appropriate nerve conduction velocities and the health and survival of neurons. Many different molecular mechanisms contribute to dysmyelination and many of these mechanisms originate at the level of the endoplasmic reticulum. The endoplasmic reticulum is a critical organelle for myelin biosynthesis and maintenance as the site of myelin protein folding quality control, Ca2+ homeostasis, cholesterol biosynthesis, and modulation of cellular stress. This review paper highlights the role of the endoplasmic reticulum and its resident molecules as an upstream and dynamic contributor to myelin and myelin pathologies.

Patient-derived heavy chain antibody targets cell surface HSP90 on breast tumors

Background

Monoclonal antibodies have been used to effectively treat various tumors. We previously established a unique strategy to identify tumor specific antibodies by capturing B-cell response against breast tumor antigens from patient-derived sentinel lymph nodes. Initial application of this approach led to identification of a tumor specific single domain antibody. In this paper we optimized our previous strategy by generating heavy chain antibodies (HCAbs) to overcome the deficiencies of single domain antibodies. Here we identified and characterized a heavy chain antibody (HCAb2) that targets cell surface HSP90 antigen on breast tumor cells but not normal cells.

Methods

Eight HCAbs derived from 4 breast cancer patients were generated using an in vitro expression system. HCAbs were screened against normal breast cells (MCF10A, HMEC) and tumor cell lines (MCF7, MDA-MB-231) to identify cell surface targeting and tumor specific antibodies using flow cytometry and immunofluorescence. Results observed with cell lines were validated by screening a cohort of primary human breast normal and tumor tissues using immunofluorescence. Respective antigens for two HCAbs (HCAb1 and HCAb2) were identified using immunoprecipitation followed by mass spectrometry. Finally, we generated MDA-MB-231 xenograft tumors in NOD scid gamma mice and performed in vivo tumor targeting analysis of HCAb1 and HCAb2.

Results

Flow cytometry screen revealed that HCAb2 selectively bound to the surface of MDA-MB-231 cells in comparison to MCF10A and MCF7 cells. HCAb2 showed punctate membrane staining on MDA-MB-231 cells and preferential binding to human breast tumor tissues in comparison to normal breast tissues. In primary breast tumor tissues, HCAb2 showed positive binding to both E-cadherin positive and negative tumor cells. We identified and validated the target antigen of HCAb2 as Heat shock protein 90 (HSP90). HCAb2 also selectively targeted MDA-MB-231 xenograft tumor cells in vivo with little targeting to mouse normal tissues. Finally, HCAb2 specifically targeted calnexin negative xenograft tumor cells.

Conclusions

From our screening methodology, we identified HCAb2 as a breast tumor specific heavy chain antibody targeting cell surface HSP90. HCAb2 also targeted MDA-MB-231 tumor cells in vivo suggesting that HCAb2 could be an ideal tumor targeting antibody.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1608-z) contains supplementary material, which is available to authorized users.

Identification of AMP-activated protein kinase targets by a consensus sequence search of the proteome

Background

AMP-activated protein kinase (AMPK) is a heterotrimeric serine/threonine protein kinase that is activated by cellular perturbations associated with ATP depletion or stress. While AMPK modulates the activity of a variety of targets containing a specific phosphorylation consensus sequence, the number of AMPK targets and their influence over cellular processes is currently thought to be limited.

Results

We queried the human and the mouse proteomes for proteins containing AMPK phosphorylation consensus sequences. Integration of this database into Gaggle software facilitated the construction of probable AMPK-regulated networks based on known and predicted molecular associations. In vitro kinase assays were conducted for preliminary validation of 12 novel AMPK targets across a variety of cellular functional categories, including transcription, translation, cell migration, protein transport, and energy homeostasis. Following initial validation, pathways that include NAD synthetase 1 (NADSYN1) and protein kinase B (AKT2) were hypothesized and experimentally tested to provide a mechanistic basis for AMPK regulation of cell migration and maintenance of cellular NAD⁺ concentrations during catabolic processes.

Conclusions

This study delineates an approach that encompasses both in silico procedures and in vitro experiments to produce testable hypotheses for AMPK regulation of cellular processes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0156-0) contains supplementary material, which is available to authorized users.

The role of N-glycan in folding, trafficking and pathogenicity of myelin oligodendrocyte glycoprotein (MOG)

Myelin oligodendrocyte glycoprotein (MOG) is a type I integral membrane protein that is expressed in the central nervous system. MOG has a single N-glycosylation site within its extracellular domain. MOG has been linked with pathogenesis of multiple sclerosis; anti-MOG antibodies have been detected in the sera of multiple sclerosis patients. N-glycosylation is an important post-translational modification of protein that might impact their folding, localization and function. However, the role of sugar in the biology of MOG is not well understood. In this study, we created a mutant MOG lacking N-linked glycan and tested its properties. We concluded that the lack of sugar did not impact on MOG abundance in the absence of endoplasmic reticulum molecular chaperone calnexin. We also show that the absence of N-glycan did not interfere with MOG's subcellular localization and it did not result in activation of endoplasmic reticulum stress. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Nitrosative stress induces peroxiredoxin 1 ubiquitination during ischemic insult via E6AP activation in endothelial cells both in vitro and in vivo

AIMS

Although there is accumulating evidence that increased formation of reactive nitrogen species in cerebral vasculature contributes to the progression of ischemic damage, but the underlying molecular mechanisms remain elusive. Peroxiredoxin 1 (Prx1) can initiate the antioxidant response by scavenging free radicals. Therefore, we tested the hypothesis that Prx1 regulates the susceptibility to nitrosative stress damage during cerebral ischemia in vitro and in vivo.

RESULTS

Proteomic analysis in endothelial cells revealed that Prx1 was upregulated after stress-related oxygen-glucose deprivation (OGD). Although peroxynitrite upregulated Prx1 rapidly, this was followed by its polyubiquitination within 6 h after OGD mediated by the E3 ubiquitin ligase E6-associated protein (E6AP). OGD colocalized E6AP with nitrotyrosine in endothelial cells. To assess translational relevance in vivo, mice were studied after middle cerebral artery occlusion (MCAO). This was accompanied by Prx1 ubiquitination and degradation by the activation of E6AP. Furthermore, brain delivery of a lentiviral vector encoding Prx1 in mice inhibited blood-brain barrier leakage and neuronal damage significantly following MCAO.

INNOVATION AND CONCLUSIONS

Nitrosative stress during ischemic insult activates E6AP E3 ubiquitin ligase that ubiquitinates Prx1 and subsequently worsens cerebral damage. Thus, targeting the Prx1 antioxidant defense pathway may represent a novel treatment strategy for neurovascular protection in stroke.

Rescue of functional CFTR channels in cystic fibrosis: a dramatic multivalent effect using iminosugar cluster-based correctors

Cystic fibrosis is caused by a mutation in the gene for the cystic fibrosis transmembrane conductance regulator (CFTR) protein. N-butyl 1-deoxynojirimycin (N-Bu DNJ), a clinical candidate for the treatment of cystic fibrosis, is able to act as a CFTR corrector by overcoming the processing defect of the mutant protein. To explore the potential of multivalency on CFTR correction activity, a library of twelve DNJ click clusters with valencies ranging from 3 to 14 were synthesized. Significantly, the trivalent analogues were found to be up to 225-fold more potent than N-Bu DNJ and up to 1000-fold more potent than the corresponding monovalent models. These results provide the first description of a multivalent effect for correcting protein folding defects in cells and should have application for the treatment of a number of protein folding disorders. Preliminary mechanistic studies indicated that CFTR correction activity enhancement was not due to a multivalent effect in ER-glucosidase inhibition or to a different mode of action of the multivalent iminosugars.

Palmitoylation is the switch that assigns calnexin to quality control or ER Ca2+ signaling

The palmitoylation of calnexin serves to enrich calnexin on the mitochondria-associated membrane (MAM). Given a lack of information on the significance of this finding, we have investigated how this endoplasmic reticulum (ER)-internal sorting signal affects the functions of calnexin. Our results demonstrate that palmitoylated calnexin interacts with sarcoendoplasmic reticulum (SR) Ca(2+) transport ATPase (SERCA) 2b and that this interaction determines ER Ca(2+) content and the regulation of ER-mitochondria Ca(2+) crosstalk. In contrast, non-palmitoylated calnexin interacts with the oxidoreductase ERp57 and performs its well-known function in quality control. Interestingly, our results also show that calnexin palmitoylation is an ER-stress-dependent mechanism. Following a short-term ER stress, calnexin quickly becomes less palmitoylated, which shifts its function from the regulation of Ca(2+) signaling towards chaperoning and quality control of known substrates. These changes also correlate with a preferential distribution of calnexin to the MAM under resting conditions, or the rough ER and ER quality control compartment (ERQC) following ER stress. Our results have therefore identified the switch that assigns calnexin either to Ca(2+) signaling or to protein chaperoning.

Calreticulin induces dilated cardiomyopathy

BACKGROUND

Calreticulin, a Ca(2+)-buffering chaperone of the endoplasmic reticulum, is highly expressed in the embryonic heart and is essential for cardiac development. After birth, the calreticulin gene is sharply down regulated in the heart, and thus, adult hearts have negligible levels of calreticulin. In this study we tested the role of calreticulin in the adult heart.

METHODOLOGY/PRINCIPAL FINDINGS

We generated an inducible transgenic mouse in which calreticulin is targeted to the cardiac tissue using a Cre/loxP system and can be up-regulated in adult hearts. Echocardiography analysis of hearts from transgenic mice expressing calreticulin revealed impaired left ventricular systolic and diastolic function and impaired mitral valve function. There was altered expression of Ca(2+) signaling molecules and the gap junction proteins, Connexin 43 and 45. Sarcoplasmic reticulum associated Ca(2+)-handling proteins (including the cardiac ryanodine receptor, sarco/endoplasmic reticulum Ca(2+)-ATPase, and cardiac calsequestrin) were down-regulated in the transgenic hearts with increased expression of calreticulin.

CONCLUSIONS/SIGNIFICANCE

We show that in adult heart, up-regulated expression of calreticulin induces cardiomyopathy in vivo leading to heart failure. This is due to an alternation in changes in a subset of Ca(2+) handling genes, gap junction components and left ventricle remodeling.

Gastrodia elata Blume (tianma) mobilizes neuro-protective capacities

Tianma (Gastrodia elata Blume) is a traditional Chinese medicine (TCM) often used for the treatment of headache, convulsions, hypertension and neurodegenerative diseases. Tianma also modulates the cleavage of the amyloid precursor protein App and cognitive functions in mice. The neuronal actions of tianma thus led us to investigate its specific effects on neuronal signalling. Accordingly, this pilot study was designed to examine the effects of tianma on the proteome metabolism in differentiated mouse neuronal N2a cells using an iTRAQ (isobaric tags for relative and absolute quantitation)-based proteomics research approach. We identified 2178 proteins, out of which 74 were found to be altered upon tianma treatment in differentiated mouse neuronal N2a cells. Based on the observed data obtained, we hypothesize that tianma could promote neuro-regenerative processes by inhibiting stress-related proteins and mobilizing neuroprotective genes such as Nxn, Dbnl, Mobkl3, Clic4, Mki67 and Bax with various regenerative modalities and capacities related to neuro-synaptic plasticity.

Neuroinflammation and endoplasmic reticulum stress are coregulated by crocin to prevent demyelination and neurodegeneration

Endoplasmic reticulum (ER) stress is a homeostatic mechanism, which is used by cells to adapt to intercellular and intracellular changes. Moreover, ER stress is closely linked to inflammatory pathways. We hypothesized that ER stress is an integral component of neuroinflammation and contributes to the development of neurological diseases. In autopsied brain specimens from multiple sclerosis (MS) and non-MS patients, XBP-1 spliced variant (XBP-1/s) was increased in MS brains (p < 0.05) and was correlated with the expression of the human endogenous retrovirus-W envelope transcript, which encodes the glycoprotein, Syncytin-1 (p < 0.05). In primary human fetal astrocytes transfected with a Syncytin-1-expressing plasmid, XBP-1/s, BiP, and NOS2 were induced, which was suppressed by crocin treatment (p < 0.05). Crocin also protected oligodendrocytes exposed to cytotoxic supernatants derived from Syncytin-1-expressing astrocytes (p < 0.05) and NO-mediated oligodendrocytotoxicity (p < 0.05). During experimental autoimmune encephalomyelitis (EAE), the transcript levels of the ER stress genes XBP-1/s, BiP, PERK, and CHOP were increased in diseased spinal cords compared with healthy littermates (p < 0.05), although CHOP expression was not involved in the EAE disease phenotype. Daily treatment with crocin starting on day 7 post-EAE induction suppressed ER stress and inflammatory gene expression in spinal cords (p < 0.05), which was accompanied by preserved myelination and axonal density, together with reduced T cell infiltration and macrophage activation. EAE-associated neurobehavioral deficits were also ameliorated by crocin treatment (p < 0.05). These findings underscored the convergent roles of pathogenic ER stress and immune pathways in neuroinflammatory disease and point to potential therapeutic applications for crocin.

Enhanced clathrin-dependent endocytosis in the absence of calnexin

Background

Calnexin, together with calreticulin, constitute the calnexin/calreticulin cycle. Calnexin is a type I endoplasmic reticulum integral membrane protein and molecular chaperone responsible for the folding and quality control of newly-synthesized (glyco)proteins. The endoplasmic reticulum luminal domain of calnexin is responsible for lectin-like activity and interaction with nascent polypeptide chains. The role of the C-terminal, cytoplasmic portion of calnexin is not clear.

Methodology/Principal Findings

Using yeast two hybrid screen and immunoprecipitation techniques, we showed that the Src homology 3-domain growth factor receptor-bound 2-like (Endophilin) interacting protein 1 (SGIP1), a neuronal specific regulator of endocytosis, forms complexes with the C-terminal cytoplasmic domain of calnexin. The calnexin cytoplasmic C-tail interacts with SGIP1 C-terminal domains containing the adaptor complexes medium subunit (Adap-Comp-Sub) region. Calnexin-deficient cells have enhanced clathrin-dependent endocytosis in neuronal cells and mouse neuronal system. This is reversed by expression of full length calnexin or calnexin C-tail.

Conclusions/Significance

We show that the effects of SGIP1 and calnexin C-tail on clathrin-dependent endocytosis are due to modulation of the internalization of the receptor-ligand complexes. Enhanced clathrin-dependent endocytosis in the absence of calnexin may contribute to the neurological phenotype of calnexin-deficient mice.

AAV exploits subcellular stress associated with inflammation, endoplasmic reticulum expansion, and misfolded proteins in models of cystic fibrosis

Barriers to infection act at multiple levels to prevent viruses, bacteria, and parasites from commandeering host cells for their own purposes. An intriguing hypothesis is that if a cell experiences stress, such as that elicited by inflammation, endoplasmic reticulum (ER) expansion, or misfolded proteins, then subcellular barriers will be less effective at preventing viral infection. Here we have used models of cystic fibrosis (CF) to test whether subcellular stress increases susceptibility to adeno-associated virus (AAV) infection. In human airway epithelium cultured at an air/liquid interface, physiological conditions of subcellular stress and ER expansion were mimicked using supernatant from mucopurulent material derived from CF lungs. Using this inflammatory stimulus to recapitulate stress found in diseased airways, we demonstrated that AAV infection was significantly enhanced. Since over 90% of CF cases are associated with a misfolded variant of Cystic Fibrosis Transmembrane Conductance Regulator (ΔF508-CFTR), we then explored whether the presence of misfolded proteins could independently increase susceptibility to AAV infection. In these models, AAV was an order of magnitude more efficient at transducing cells expressing ΔF508-CFTR than in cells expressing wild-type CFTR. Rescue of misfolded ΔF508-CFTR under low temperature conditions restored viral transduction efficiency to that demonstrated in controls, suggesting effects related to protein misfolding were responsible for increasing susceptibility to infection. By testing other CFTR mutants, G551D, D572N, and 1410X, we have shown this phenomenon is common to other misfolded proteins and not related to loss of CFTR activity. The presence of misfolded proteins did not affect cell surface attachment of virus or influence expression levels from promoter transgene cassettes in plasmid transfection studies, indicating exploitation occurs at the level of virion trafficking or processing. Thus, we surmised that factors enlisted to process misfolded proteins such as ΔF508-CFTR in the secretory pathway also act to restrict viral infection. In line with this hypothesis, we found that AAV trafficked to the microtubule organizing center and localized near Golgi/ER transport proteins. Moreover, AAV infection efficiency could be modulated with siRNA-mediated knockdown of proteins involved in processing ΔF508-CFTR or sorting retrograde cargo from the Golgi and ER (calnexin, KDEL-R, β-COP, and PSMB3). In summary, our data support a model where AAV exploits a compromised secretory system and, importantly, underscore the gravity with which a stressed subcellular environment, under internal or external insults, can impact infection efficiency.

Misfolded proteins have been associated with a variety of disorders such as cystic fibrosis, diabetes insipidus, alpha-antitrypsin deficiency, Parkinson's disease, and cancer. In this study, by using cellular models of events in cystic fibrosis lung disease we have revealed an effect of misfolded proteins on increasing susceptibility to infection with a parvovirus. Infection efficiency was an order of magnitude higher in cells expressing misfolded Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) mutant proteins than in cells expressing the correctly folded protein. During infection, virus capsids accumulated near cellular factors that normally process misfolded proteins and are involved in retrograde trafficking from the Golgi to endoplasmic reticulum. Furthermore, we have demonstrated that infection efficiency can be attenuated by restoring correct protein folding or augmented by siRNA-mediated knockdown of secretory pathway components. Taken together our results indicate that converging cellular systems operate to clear misfolded proteins and virus capsids from an infected cell. We raise the possibility that parvoviruses and perhaps other viruses exploit congested cellular secretory pathways during entry, and that viral infection could be a contributing factor in the progression of diseases associated with misfolded proteins.

Expression of ER quality control-related genes in response to changes in BiP1 levels in developing rice endosperm

Binding protein (BiP) is the key chaperone involved in folding of secretory proteins such as seed storage proteins in the ER lumen. To obtain functional information about BiP1, a gene that is predominantly expressed during rice seed maturation, we generated several transgenic rice plants in which various levels of BiP1 protein accumulated in an endosperm-specific manner. Severe suppression (BiP1 KD) or significant over-expression (BiP1 OEmax) of BiP1 not only altered seed phenotype and the intracellular structure of endosperm cells, but also reduced seed storage protein content, starch accumulation and grain weight. Microarray and RT-PCR analyses indicated that expression of many chaperone and co-chaperone genes was induced in transgenic plants, with more prominent expression in the BiP1 KD line than in the BiP1 OEmax line. Transcriptional induction of most chaperones was observed in calli treated with dithiothreitol or tunicamycin, treatments that trigger ER stress, indicating that induction of the chaperone genes in transgenic rice was caused by an ER stress response. In transient assays using rice protoplasts, the ortholog (Os06g0622700) of the AtbZIP60 transcription factor was shown to be involved in activation of some chaperone genes. Slight increases in the BiP1 level compared with wild-type, accompanied by increased levels of calnexin and protein disulfide isomerase-like proteins, resulted in significant enhancement of seed storage protein content, without any change in intracellular structure or seed phenotype. Judicious modification of BiP1 levels in transgenic rice can provide suitable conditions for the production of secretory proteins by alleviating ER stress.

Cell surface targeting of myelin oligodendrocyte glycoprotein (MOG) in the absence of endoplasmic reticulum molecular chaperones

Myelin oligodendrocyte glycoprotein (MOG) is a type I integral membrane glycoprotein that localizes to myelin sheaths in the central nervous system. MOG has important implications in multiple sclerosis, as pathogenic anti-MOG antibodies have been detected in the sera of multiple sclerosis patients. As a membrane protein, MOG achieves its native structure in the endoplasmic reticulum where its folding is expected to be controlled by endoplasmic reticulum chaperones. Calnexin, calreticulin, and ERp57 are essential components of the endoplasmic reticulum quality control where they assist in the proper folding of newly synthesized glycoproteins. In this study, we show that expression of MOG is not affected by the absence of the endoplasmic reticulum quality control proteins calnexin, calreticulin, or ERp57. We also show that calnexin forms complexes with MOG and these interactions might be glycan-independent. Importantly, we show that cell surface targeting of MOG is not disrupted in the absence of the endoplasmic reticulum chaperones. This article is part of a special issue entitled: 11th European Symposium on Calcium.

Stress induced subcellular distribution of ALG-2, RBM22 and hSlu7

ALG-2 is a highly conserved calcium binding protein in the cytoplasm which belongs to the family of penta-EF hand proteins. Recently, we showed that ALG-2 is interacting with RBM22, a highly conserved spliceosomal nuclear protein (Montaville et al. Biochim. Biophys. Acta 1763, 1335, 2006; Krebs, Biochim. Biophys. Acta 1793, 979, 2009). In NIH 3T3 cells expressing both proteins a significant amount of ALG-2mRFP is translocated to the nucleus due to the interaction with RBM22-EGFP. hSlu7, another spliceosomal nuclear protein, known to interact with RBM22 in yeast, has been shown to translocate to the cytoplasm under cellular stress conditions. Here we provide evidence that the 2 spliceosomal proteins differ significantly in their subcellular distributions under stress conditions, and that RBM22 enhances the cytoplasmic translocation of hSlu7 under stress, especially a stress induced by thapsigargin. On the other hand, in NIH 3T3 cells expressing RBM22-EGFP and ALG-2-mRFP, ALG-2 remains translocated into the nucleus under both stress conditions, i.e. heat shock or treatment with thapsigargin. We could further demonstrate that these stress conditions had a different influence on the splicing pattern of XBP-1, a marker for the unfolded protein response indicating that ER stress may play a role in stress-induced translocation of spliceosomal proteins. The article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Calcineurin interacts with PERK and dephosphorylates calnexin to relieve ER stress in mammals and frogs

BACKGROUND

The accumulation of misfolded proteins within the endoplasmic reticulum (ER) triggers a cellular process known as the Unfolded Protein Response (UPR). One of the earliest responses is the attenuation of protein translation. Little is known about the role that Ca2+ mobilization plays in the early UPR. Work from our group has shown that cytosolic phosphorylation of calnexin (CLNX) controls Ca2+ uptake into the ER via the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) 2b.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we demonstrate that calcineurin (CN), a Ca2+ dependent phosphatase, associates with the (PKR)-like ER kinase (PERK), and promotes PERK auto-phosphorylation. This association, in turn, increases the phosphorylation level of eukaryotic initiation factor-2 alpha (eIF2-alpha) and attenuates protein translation. Data supporting these conclusions were obtained from co-immunoprecipitations, pull-down assays, in-vitro kinase assays, siRNA treatments and [35S]-methionine incorporation measurements. The interaction of CN with PERK was facilitated at elevated cytosolic Ca2+ concentrations and involved the cytosolic domain of PERK. CN levels were rapidly increased by ER stressors, which could be blocked by siRNA treatments for CN-Aalpha in cultured astrocytes. Downregulation of CN blocked subsequent ER-stress-induced increases in phosphorylated elF2-alpha. CN knockdown in Xenopus oocytes predisposed them to induction of apoptosis. We also found that CLNX was dephosphorylated by CN when Ca2+ increased. These data were obtained from [gamma32P]-CLNX immunoprecipitations and Ca2+ imaging measurements. CLNX was dephosphorylated when Xenopus oocytes were treated with ER stressors. Dephosphorylation was pharmacologically blocked by treatment with CN inhibitors. Finally, evidence is presented that PERK phosphorylates CN-A at low resting levels of Ca2+. We further show that phosphorylated CN-A exhibits decreased phosphatase activity, consistent with this regulatory mechanism being shut down as ER homeostasis is re-established.

CONCLUSIONS/SIGNIFICANCE

Our data suggest two new complementary roles for CN in the regulation of the early UPR. First, CN binding to PERK enhances inhibition of protein translation to allow the cell time to recover. The induction of the early UPR, as indicated by increased P-elF2alpha, is critically dependent on a translational increase in CN-Aalpha. Second, CN dephosphorylates CLNX and likely removes inhibition of SERCA2b activity, which would aid the rapid restoration of ER Ca2+ homeostasis.

ERp57 modulates STAT3 signaling from the lumen of the endoplasmic reticulum

ERp57 is an endoplasmic reticulum (ER) resident thiol disulfide oxidoreductase. Using the gene trap technique, we created a ERp57-deficient mouse model. Targeted deletion of the Pdia3 gene, which encodes ERp57, in mice is embryonic lethal at embryonic day (E) 13.5. Beta-galactosidase reporter gene analysis revealed that ERp57 is expressed early on during blastocyst formation with the highest expression in the inner cell mass. In early stages of mouse embryonic development (E11.5) there is a relatively low level of expression of ERp57. As the embryos developed, ERp57 became highly expressed in both the brain and the lungs (E15.5 and E18.5). The absence of ERp57 has no impact on ER morphology; expression of ER-associated chaperones and folding enzymes, ER stress, or apoptosis. ERp57 has been reported to interact with STAT3 (signal transducer and activator of transcription)-DNA complexes. We show here that STAT3-dependent signaling is increased in the absence of ERp57 and this can be rescued by expression of ER-targeted ERp57 but not by cytoplasmic-targeted protein, indicating that ERp57 affects STAT3 signaling from the lumen of the ER. ERp57 effects on STAT3 signaling are enhanced by ER luminal complex formation between ERp57 and calreticulin. In conclusion, we show that ERp57 deficiency in mouse is embryonic lethal at E13.5 and ERp57-dependent modulation of STAT3 signaling may contribute to this phenotype.

Calnexin is not essential for mammalian rod opsin biogenesis

PURPOSE

Misfolding mutations in rod opsin are a major cause of the inherited blindness retinitis pigmentosa. Therefore, understanding the role of molecular chaperones in facilitating rod opsin biogenesis and the response to mutant rod opsin is important for retinal disease and fundamental retinal cell biology. A recent report has shown that Drosophila rhodopsin Rh1 requires calnexin (Cnx) for its maturation and correct localization to R1-6 rhabdomeres. In this report, we investigate the role of Cnx in the processing of wild-type and mutant mammalian rod opsin.

METHODS

Mouse embryonic fibroblasts (MEFs) from control mice (WT) and mice that express a truncated dysfunctional version of Cnx (sCnx) were used to assess the role of Cnx in the biogenesis, maturation, degradation, and aggregation of mutant and wild-type rod opsin. The mutant P23H rod opsin was used as a prototypical class II misfolding mutant as it is retained in the endoplasmic reticulum (ER) and is either degraded by ER associated degradation (ERAD) or forms aggregates that coalesce to form intracellular inclusions.

RESULTS

Wild-type rod opsin protein translocated normally to the plasma membrane in both cell lines. In contrast, P23H rod opsin was retained in the ER in both cell lines. The only difference observed in rod opsin processing between the WT and sCnx MEFs was a small increase in the incidence of P23H intracellular inclusions in the sCnx cells. This did not appear to be specific for rod opsin, however, as non-rod opsin-expressing sCnx cells also had an increased incidence of ubiquitylated inclusions.

CONCLUSIONS

Our data show that, unlike Drosophila Rh1, mammalian rod opsin biogenesis does not appear to have an absolute requirement for Cnx. Other chaperones are likely to be more important for mammalian rod opsin biogenesis and quality control.