The C-terminal domain of ERp29 mediates polyomavirus binding, unfolding, and infection

Regulatory role of endoplasmic reticulum resident chaperone protein ERp29 in anti-murine β-coronavirus host cell response

Gap junctional intercellular communication (GJIC) involving astrocytes is important for proper CNS homeostasis. As determined in our previous studies, trafficking of the predominant astrocyte GJ protein, Connexin43 (Cx43), is disrupted in response to infection with a neurotropic murine β-coronavirus (MHV-A59). However, how host factors are involved in Cx43 trafficking and the infection response is not clear. Here, we show that Cx43 retention due to MHV-A59 infection was associated with increased ER stress and reduced expression of chaperone protein ERp29. Treatment of MHV-A59-infected astrocytes with the chemical chaperone 4-sodium phenylbutyrate increased ERp29 expression, rescued Cx43 transport to the cell surface, increased GJIC, and reduced ER stress. We obtained similar results using an astrocytoma cell line (delayed brain tumor) upon MHV-A59 infection. Critically, delayed brain tumor cells transfected to express exogenous ERp29 were less susceptible to MHV-A59 infection and showed increased Cx43-mediated GJIC. Treatment with Cx43 mimetic peptides inhibited GJIC and increased viral susceptibility, demonstrating a role for intercellular communication in reducing MHV-A59 infectivity. Taken together, these results support a therapeutically targetable ERp29-dependent mechanism where β-coronavirus infectivity is modulated by reducing ER stress and rescuing Cx43 trafficking and function.

Spike S1 domain interactome in non-pulmonary systems: A role beyond the receptor recognition

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes Coronavirus Disease 2019 (COVID-19), which, since 2019 in China, has rapidly become a worldwide pandemic. The aggressiveness and global spread were enhanced by the many SARS-CoV-2 variants that have been isolated up to now. These mutations affect mostly the viral glycoprotein Spike (S), the capsid protein mainly involved in the early stages of viral entry processes, through the recognition of specific receptors on the host cell surface. In particular, the subunit S1 of the Spike glycoprotein contains the Receptor Binding Domain (RBD) and it is responsible for the interaction with the angiotensin-converting enzyme 2 (ACE2). Although ACE2 is the primary Spike host receptor currently studied, it has been demonstrated that SARS-CoV-2 is also able to infect cells expressing low levels of ACE2, indicating that the virus may have alternative receptors on the host cells. The identification of the alternative receptors can better elucidate the pathogenicity and the tropism of SARS-CoV-2. Therefore, we investigated the Spike S1 interactomes, starting from host membrane proteins of non-pulmonary cell lines, such as human kidney (HK-2), normal colon (NCM460D), and colorectal adenocarcinoma (Caco-2). We employed an affinity purification-mass spectrometry (AP-MS) to pull down, from the membrane protein extracts of all cell lines, the protein partners of the recombinant form of the Spike S1 domain. The purified interactors were identified by a shotgun proteomics approach. The lists of S1 potential interacting proteins were then clusterized according to cellular localization, biological processes, and pathways, highlighting new possible S1 intracellular functions, crucial not only for the entrance mechanisms but also for viral replication and propagation processes.

The chaperone ERp29 is required for tunneling nanotube formation by stabilizing MSec

Tunneling nanotubes (TNTs) are membrane conduits that mediate long-distance intercellular cross-talk in several organisms and play vital roles during development, pathogenic transmission, and cancer metastasis. However, the molecular mechanisms of TNT formation and function remain poorly understood. The protein MSec (also known as TNFα-induced protein 2 (TNFAIP2) and B94) is essential for TNT formation in multiple cell types. Here, using affinity protein purification, mass spectrometric identification, and confocal immunofluorescence microscopy assays, we found that MSec interacts with the endoplasmic reticulum (ER) chaperone ERp29. siRNA-mediated ERp29 depletion in mammalian cells significantly reduces TNT formation, whereas its overexpression induces TNT formation, but in a strictly MSec-dependent manner. ERp29 stabilized MSec protein levels, but not its mRNA levels, and the chaperone activity of ERp29 was required for maintaining MSec protein stability. Subcellular ER fractionation and subsequent limited proteolytic treatment suggested that MSec is associated with the outer surface of the ER. The ERp29-MSec interaction appeared to require the presence of other bridging protein(s), perhaps triggered by post-translational modification of ERp29. Our study implicates MSec as a target of ERp29 and reveals an indispensable role for the ER in TNT formation, suggesting new modalities for regulating TNT numbers in cells and tissues.

ERp29 controls invasion and metastasis of gastric carcinoma by inhibition of epithelial-mesenchymal transition via PI3K/Aktsignaling pathway

Background

Gastric cancer (GC) accounts for the fourth most occurring malignancy and the third major cause of cancer death. Identifying novel molecular signaling pathways participating in gastric tumorigenesis and progression is pivotal for rational design of targeted therapies to improve advanced GC outcome. Recently, the endoplasmic reticulum (ER) protein 29 (ERp29) has been shown to inversely associate with primary tumor development and function as a tumor suppressor in breast cancer. However, the role of ERp29 in GC patients’ prognosis and its function in GC progression is unknown.

Methods

Clinical importance of ERp29 in the prognosis of GC patients was assessed by examining its expression in 148 GC tumor samples and correlation with clinicopathological characteristics and survival of the patients. The function and underlying mechanisms of ERp29 in GC growth, invasion and metastasis were explored both in vitro and in vivo.

Results

Downregulation of ERp29 was commonly found in GC tissues and highly correlated with more aggressive phenotypes and poorer prognosis. Functional assays demonstrated that knockdown of ERp29 increased GC cell migration and invasion and promoted metastasis. Conversely, ectopic overexpression of ERp29 produced opposite effects. Mechanistic studies revealed that loss of ERp29 induced an epithelial-to-mesenchymal transition (EMT) in the GC cells through activation of PI3K/Akt pathway signaling.

Conclusion

These findings suggest that downregulation of ERp29 is probably one of the key molecular mechanisms responsible for the development and progression of GC.

Electronic supplementary material

The online version of this article (10.1186/s12885-017-3613-x) contains supplementary material, which is available to authorized users.

How Polyomaviruses Exploit the ERAD Machinery to Cause Infection

To infect cells, polyomavirus (PyV) traffics from the cell surface to the endoplasmic reticulum (ER) where it hijacks elements of the ER-associated degradation (ERAD) machinery to penetrate the ER membrane and reach the cytosol. From the cytosol, the virus transports to the nucleus, enabling transcription and replication of the viral genome that leads to lytic infection or cellular transformation. How PyV exploits the ERAD machinery to cross the ER membrane and access the cytosol, a decisive infection step, remains enigmatic. However, recent studies have slowly unraveled many aspects of this process. These emerging insights should advance our efforts to develop more effective therapies against PyV-induced human diseases.

Endoplasmic reticulum protein 29 (ERp29) confers radioresistance through the DNA repair gene, O(6)-methylguanine DNA-methyltransferase, in breast cancer cells

Resistance of cancer cells to radiotherapy is a major clinical problem in cancer treatment. Therefore, understanding the molecular basis of cellular resistance to radiotherapy and identification of novel targets are essential for improving treatment efficacy for cancer patients. Our previous studies have demonstrated a significant role of ERp29 in breast cancer cell survival against doxorubicin-induced genotoxic stress. We here reported that ERp29 expression in the triple negative MDA-MB-231 breast cancer cells significantly increased cell survival against ionizing radiation. Methylation PCR array analysis identified that ERp29 expression increased promoter hypomethylation of the DNA repair gene, O⁶-methylguanine DNA-methyltransferase (MGMT), by downregulating DNA methyltransferase 1. Knockdown of MGMT in the ERp29-transfected cancer cells increased radiosensitivity, leading to a decreased post-irradiation survival. In addition, radiation treatment in the MGMT-knockdown cells elevated phosphorylation of γ-H2AX and cleavage of caspase 3, indicating that depletion of MGMT facilitates DNA double strands breaks and increases cell apoptosis. Hence, our studies prove a novel function of ERp29\MGMT in cancer cell survival against radiation. Targeting ERp29\MGMT axis may be useful for providing better treatment efficacy in combination with radiotherapy in breast cancer.

Friend or foe: Endoplasmic reticulum protein 29 (ERp29) in epithelial cancer

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ERp29 regulates epithelial cell plasticity and the mesenchymal–epithelial transition.

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ERp29 shows a tumor suppressive function in primary tumor development.

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ERp29 is potentially associated with distant metastasis in cancer.

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ERp29 modulates cell survival against genotoxic stress.

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Thus, ERp29 displays dual functions as a “friend or foe” in epithelial cancer.

The endoplasmic reticulum (ER) protein 29 (ERp29) is a molecular chaperone that plays a critical role in protein secretion from the ER in eukaryotic cells. Recent studies have also shown that ERp29 plays a role in cancer. It has been demonstrated that ERp29 is inversely associated with primary tumor development and functions as a tumor suppressor by inducing cell growth arrest in breast cancer. However, ERp29 has also been reported to promote epithelial cell morphogenesis, cell survival against genotoxic stress and distant metastasis. In this review, we summarize the current understanding on the biological and pathological functions of ERp29 in cancer and discuss the pivotal aspects of ERp29 as “friend or foe” in epithelial cancer.

Three-dimensional protein fold determination from backbone amide pseudocontact shifts generated by lanthanide tags at multiple sites

Site-specific attachment of paramagnetic lanthanide ions to a protein generates pseudocontact shifts (PCS) in the nuclear magnetic resonance (NMR) spectra of the protein that are easily measured as changes in chemical shifts. By labeling the protein with lanthanide tags at four different sites, PCSs are observed for most amide protons and accurate information is obtained about their coordinates in three-dimensional space. The approach is demonstrated with the chaperone ERp29, for which large differences have been reported between X-ray and NMR structures of the C-terminal domain, ERp29-C. The results unambiguously show that the structure of rat ERp29-C in solution is similar to the crystal structure of human ERp29-C. PCSs of backbone amides were the only structural restraints required. Because these can be measured for more dilute protein solutions than other NMR restraints, the approach greatly widens the range of proteins amenable to structural studies in solution.

Molecular biology, epidemiology, and pathogenesis of progressive multifocal leukoencephalopathy, the JC virus-induced demyelinating disease of the human brain

Progressive multifocal leukoencephalopathy (PML) is a debilitating and frequently fatal central nervous system (CNS) demyelinating disease caused by JC virus (JCV), for which there is currently no effective treatment. Lytic infection of oligodendrocytes in the brain leads to their eventual destruction and progressive demyelination, resulting in multiple foci of lesions in the white matter of the brain. Before the mid-1980s, PML was a relatively rare disease, reported to occur primarily in those with underlying neoplastic conditions affecting immune function and, more rarely, in allograft recipients receiving immunosuppressive drugs. However, with the onset of the AIDS pandemic, the incidence of PML has increased dramatically. Approximately 3 to 5% of HIV-infected individuals will develop PML, which is classified as an AIDS-defining illness. In addition, the recent advent of humanized monoclonal antibody therapy for the treatment of autoimmune inflammatory diseases such as multiple sclerosis (MS) and Crohn's disease has also led to an increased risk of PML as a side effect of immunotherapy. Thus, the study of JCV and the elucidation of the underlying causes of PML are important and active areas of research that may lead to new insights into immune function and host antiviral defense, as well as to potential new therapies.

ERp29 induces breast cancer cell growth arrest and survival through modulation of activation of p38 and upregulation of ER stress protein p58IPK

Endoplasmic reticulum protein 29 (ERp29) is an ER luminal protein that has a role in protein unfolding and secretion, but its role in cancer is unclear. Recently, we reported that overexpression of ERp29 significantly inhibited cell proliferation and prevented tumorigenesis in highly proliferative MDA-MB-231 breast cancer cells. Here, we show that ERp29-induced cancer cell growth arrest is modulated by the interplay between the concomitant phosphorylation of p38 and upregulation of the inhibitor of the interferon-induced, double-stranded RNA-activated protein kinase, p58(IPK). In this cell model, ERp29 overexpression significantly downregulates modulators of cell proliferation, namely urokinase plasminogen activator receptor, β(1)-integrin and epidermal growth factor receptor. Furthermore, ERp29 significantly (P<0.001) increases phosphorylation of p38 (p-p38) and reduces matrix metalloproteinase-9 secretion. The role of ERp29 in upregulating cyclin-dependent kinase inhibitors (p15 and p21) and in downregulating cyclin D(2) is demonstrated in slowly proliferating ERp29-overexpressing MDA-MB-231 cells, whereas the opposite response was observed in ERp29-knockdown MCF-7 cells. Pharmacological inhibition of p-p38 downregulates p15 and p21 and inhibits eIF2α phosphorylation, indicating a role for p-p38 in this process. Furthermore, p58(IPK) expression was increased in ERp29-overexpressing MDA-MB-231 cells and highly decreased in ERp29-knockdown MCF-7 cells. This upregulation of p58(IPK) by ERp29 suppresses the activation of p-p38/p-PERK/p-eIF2α by repressing eIF2α phosphorylation. In fact, reduction of p58(IPK) expression by RNA interference stimulated eIF2α phosphorylation. The repression of eIF2α phosphorylation by p58(IPK) prevents ERp29-transfected cells from undergoing ER-dependent apoptosis driven by the activation of ATF4/CHOP/caspase-3. Hence, the interplay between p38 phosphorylation and p58(IPK) upregulation has key roles in modulating ERp29-induced cell-growth arrest and survival.

Protein disulfide isomerases contribute differentially to the endoplasmic reticulum-associated degradation of apolipoprotein B and other substrates

ER-associated degradation (ERAD) rids the early secretory pathway of misfolded or misprocessed proteins. Some members of the protein disulfide isomerase (PDI) family appear to facilitate ERAD substrate selection and retrotranslocation, but a thorough characterization of PDIs during the degradation of diverse substrates has not been undertaken, in part because there are 20 PDI family members in mammals. PDIs can also exhibit disulfide redox, isomerization, and/or chaperone activity, but which of these activities is required for the ERAD of different substrate classes is unknown. We therefore examined the fates of unique substrates in yeast, which expresses five PDIs. Through the use of a yeast expression system for apolipoprotein B (ApoB), which is disulfide rich, we discovered that Pdi1 interacts with ApoB and facilitates degradation through its chaperone activity. In contrast, Pdi1's redox activity was required for the ERAD of CPY* (a misfolded version of carboxypeptidase Y that has five disulfide bonds). The ERAD of another substrate, the alpha subunit of the epithelial sodium channel, was Pdi1 independent. Distinct effects of mammalian PDI homologues on ApoB degradation were then observed in hepatic cells. These data indicate that PDIs contribute to the ERAD of proteins through different mechanisms and that PDI diversity is critical to recognize the spectrum of potential ERAD substrates.

Gadolinium tagging for high-precision measurements of 6 nm distances in protein assemblies by EPR

Double electron-electron resonance (DEER) distance measurements of a protein complex tagged with two Gd(3+) chelates developed for rigid positioning of the metal ion are shown to deliver outstandingly accurate distance measurements in the 6 nm range. The accuracy was assessed by comparison with modeled distance distributions based on the three-dimensional molecular structures of the protein and the tag and further comparison with paramagnetic NMR data. The close agreement between the predicted and experimentally measured distances opens new possibilities for investigating the structure of biomolecular assemblies. As an example, we show that the dimer interface of rat ERp29 in solution is the same as that determined previously for human ERp29 in the single crystal.

A PDI family network acts distinctly and coordinately with ERp29 to facilitate polyomavirus infection

Endoplasmic reticulum (ER)-to-cytosol membrane transport is a decisive infection step for the murine polyomavirus (Py). We previously determined that ERp29, a protein disulfide isomerase (PDI) member, extrudes the Py VP1 C-terminal arm to initiate ER membrane penetration. This reaction requires disruption of Py's disulfide bonds. Here, we found that the PDI family members ERp57, PDI, and ERp72 facilitate virus infection. However, while all three proteins disrupt Py's disulfide bonds in vitro, only ERp57 and PDI operate in concert with ERp29 to unfold the VP1 C-terminal arm. An alkylated Py cannot stimulate infection, implying a pivotal role of viral free cysteines during infection. Consistent with this, we found that although PDI and ERp72 reduce Py, ERp57 principally isomerizes the virus in vitro, a reaction that requires viral free cysteines. Our mutagenesis study subsequently identified VP1 C11 and C15 as important for infection, suggesting a role for these residues during isomerization. C11 and C15 also act together to stabilize interpentamer interactions for a subset of the virus pentamers, likely because some of these residues form interpentamer disulfide bonds. This study reveals how a PDI family functions coordinately and distinctly to promote Py infection and pinpoints a role of viral cysteines in this process.

Endoplasmic reticulum protein 29 (ERp29): An emerging role in cancer

The endoplasmic reticulum protein 29 (ERp29) is a molecule that facilitates processing and transport of proteins in the early secretory pathway. Structural and functional analyses have suggested a biological role as a putative chaperone in the endoplasmic reticulum. The N-terminal domain of ERp29 resembles the thioredoxin domain of protein disulfide isomerase, but lacks its redox-active function due to the absence of an active motif consisting of double cysteines. In the context of carcinogenesis, the role of ERp29 in cancer progression has not been fully elucidated. However, recent studies indicate that high expression of ERp29 inversely correlates to tumor progression. In addition, over-expression of ERp29 significantly inhibits proliferation and suppresses tumorigenesis by modulating ER stress signaling and the mesenchymal-epithelial transition in breast cancer cells. In this review, we summarize the biological properties of ERp29 and its novel function as a tumor suppressor.

Over-expression of ERp29 attenuates doxorubicin-induced cell apoptosis through up-regulation of Hsp27 in breast cancer cells

The endoplasmic reticulum protein 29 (ERp29) has a critical role in regulating protein folding, maturation and secretion. However, its role in carcinogenesis remains elusive. Recently, we reported that ERp29 is a novel tumor suppressor and regulates mesenchymal-epithelial transition in MDA-MB-231 breast cancer cells. Here, we investigated whether ERp29 plays a role in the response of breast cancer cells to chemotherapeutic agents. We found that expression of ERp29 increased the resistance to doxorubicin, but not cisplatin and paclitaxel, and decreased the doxorubicin-induced cell apoptosis in MDA-MB-231 cells, whereas knockdown of ERp29 in MCF-7 cells increased the doxorubicin cytotoxicity. A proteomics study identified up-regulation of Hsp27 and down-regulation of stathmin-1, galectin and prohibitin in the doxorubicin-resistant, ERp29 over-expressing MDA-MB-231 cells. Further, we demonstrated that ERp29 up-regulated expression of Hsp27 by down-regulating eukaryotic translational initiation factor 2α (eIF2α). When Hsp27 was knocked down by siRNA in the doxorubicin-resistant, ERp29 over-expressing MDA-MB-231 cells and parental MCF-7 cells, cell viability was significantly decreased and doxorubicin-induced cell apoptosis was enhanced. These results indicate that Hsp27 is involved in the ERp29-mediated resistance to doxorubicin. Therefore, targeting of Hsp27, with a combination of other chemotherapeutic agents, is a rational strategy in treating doxorubicin-resistant cancer cells.

Endoplasmic reticulum protein 29 (ERp29), a protein related to sperm maturation is involved in sperm-oocyte fusion in mouse

BACKGROUND

Sperm-oocyte fusion is a critical step in fertilization, which requires a series of proteins from both spermatozoa and oocyte to mediate membrane adhesion and subsequent fusion. A rat spermatozoa membrane protein is endoplasmic reticulum protein 29 (ERp29), which significantly increases on the sperm surface as well as in the cytoplasm of epididymal epithelia from caput to cauda as the sperm undergo epididymal maturation. Moreover, ERp29 facilitates viral infection via mediating membrane penetration. We determined if in addition to promoting sperm maturation ERp29 may also play a role in facilitating gamete fusion during the fertilization process.

METHODS

Laser scanning confocal microscopy (LSCM) and Western blot analysis were employed to probe for ERp29 protein in BALB/c mouse epididymal and acrosome-reacted spermatozoa. We prepared rabbit polyclonal antibodies against mouse recombinant ERp29 (rERp29) to characterize: 1) fertilization rate (FR); 2) fertilization index (FI); 3) sperm motility and 4) acrosome reaction (AR).

RESULTS

Confocal microscopy indicated that ERp29 was partially localized at the sperm head of the epididymal caput as well as over the whole head and part of the principal piece of the tail region from the epididymal cauda. However, when the acrosome reacted, ERp29 remained in the equatorial and post-acrosomal regions of the sperm head, which is the initial site of sperm-oocyte membrane fusion. Such localization changes were confirmed based on the results of Western blot analysis. Furthermore, the antibodies against mouse rERp29 inhibited the spermatozoa from penetrating into the zona pellucida (ZP)-free oocytes. The functional blocking antibodies reduced both mouse sperm-oocyte FR and FI at concentrations of 100 and 200 micro g/ml compared with pre-immunized rabbit IgG or with anti-mouse recombinant bactericidal/permeability-increasing protein (BPI, a sperm surface protein unrelated to sperm-oocyte fusion) antibodies (100 micro g/ml), but they had no effect on sperm motility and AR.

CONCLUSION

This study demonstrates that ERp29 on mouse spermatozoa membrane changes during epididymal transit and AR. Accordingly, in mice this protein may be one of the important factors involved in sperm fertilization by facilitating sperm-oocyte membrane fusion.

Protein disulfide isomerase: a critical evaluation of its function in disulfide bond formation

Disulfide bond formation is probably involved in the biogenesis of approximately one third of human proteins. A central player in this essential process is protein disulfide isomerase or PDI. PDI was the first protein-folding catalyst reported. However, despite more than four decades of study, we still do not understand much about its physiological mechanisms of action. This review examines the published literature with a critical eye. This review aims to (a) provide background on the chemistry of disulfide bond formation and rearrangement, including the concept of reduction potential, before examining the structure of PDI; (b) detail the thiol-disulfide exchange reactions that are catalyzed by PDI in vitro, including a critical examination of the assays used to determine them; (c) examine oxidation and reduction of PDI in vivo, including not only the role of ERo1 but also an extensive assessment of the role of glutathione, as well as other systems, such as peroxide, dehydroascorbate, and a discussion of vitamin K-based systems; (d) consider the in vivo reactions of PDI and the determination and implications of the redox state of PDI in vivo; and (e) discuss other human and yeast PDI-family members.

ERp29 restricts Connexin43 oligomerization in the endoplasmic reticulum

Connexin43 (Cx43) is a gap junction protein that forms multimeric channels that enable intercellular communication through the direct transfer of signals and metabolites. Although most multimeric protein complexes form in the endoplasmic reticulum (ER), Cx43 seems to exit from the ER as monomers and subsequently oligomerizes in the Golgi complex. This suggests that one or more protein chaperones inhibit premature Cx43 oligomerization in the ER. Here, we provide evidence that an ER-localized, 29-kDa thioredoxin-family protein (ERp29) regulates Cx43 trafficking and function. Interfering with ERp29 function destabilized monomeric Cx43 oligomerization in the ER, caused increased Cx43 accumulation in the Golgi apparatus, reduced transport of Cx43 to the plasma membrane, and inhibited gap junctional communication. ERp29 also formed a specific complex with monomeric Cx43. Together, this supports a new role for ERp29 as a chaperone that helps stabilize monomeric Cx43 to enable oligomerization to occur in the Golgi apparatus.