The polypeptide binding conformation of calreticulin facilitates its cell-surface expression under conditions of endoplasmic reticulum stress

siRNA targeting Atp5a1 gene encoding ATPase α, the ligand of Peg fimbriae, reduced Salmonella Enteritidis adhesion

Salmonella enterica serovar Enteritidis (S. Enteritidis) is a zoonotic pathogen that can infect both humans and animals. Among the 13 types of fimbrial operons in S. Enteritidis, the highly conserved Peg fimbriae play a crucial role in the adhesion and invasion of S. Enteritidis into host cells but are not well studied. In this study, we identified the ATP synthase subunit alpha (ATPase α) as a ligand of Peg fimbriae using ligand blotting and mass spectrometry techniques. We confirmed the in vitro binding of ATPase α to the purified adhesion protein (PegD). Furthermore, we used siRNA to suppress the expression of ATPase α gene Atp5a1 in Leghorn male hepatoma (LMH) cells, which resulted in a significant reduction in the adhesion rate of S. Enteritidis to the cells (P < 0.05). The findings in this study provide insight into the mechanism of S. Enteritidis infection through Peg fimbriae and highlight the importance of ATPase α in the adhesion process.RESEARCH HIGHLIGHTS Ligand blotting was performed to screen the ligand of S. Enteritidis Peg fimbriae.Binding assay confirmed that ATPase α is the ligand of the Peg fimbriae.siRNA targeting ATPase α gene (Atp5a1) significantly reduced S. Enteritidis adhesion.

The role of cellular proteostasis in anti-tumor immunity

Immune checkpoint blockade therapy is perhaps the most important development in cancer treatment in recent memory. It is based on decades of investigation into the biology of immune cells and the role of the immune system in controlling cancer growth. While the molecular circuitry that governs the immune system in general - and anti-tumor immunity in particular - is intensely studied, far less attention has been paid to the role of cellular stress in this process. Proteostasis, intimately linked to cell stress responses, refers to the dynamic regulation of the cellular proteome and is maintained through a complex network of systems that govern the synthesis, folding, and degradation of proteins in the cell. Disruption of these systems can result in the loss of protein function, altered protein function, the formation of toxic aggregates, or pathologies associated with cell stress. However, the importance of proteostasis extends beyond its role in maintaining proper protein function; proteostasis governs how tolerant cells may be to mutations in protein coding genes and the overall half-life of proteins. Such gene expression changes may be associated with human diseases including neurodegenerative diseases, metabolic disease, and cancer and manifest at the protein level against the backdrop of the proteostasis network in any given cellular environment. In this review, we focus on the role of proteostasis in regulating immune responses against cancer as well the role of proteostasis in determining immunogenicity of cancer cells.

Endoplasmic reticulum stress-induced release and binding of calreticulin from human ovarian cancer cells

BACKGROUND

Calreticulin (CRT) is an endoplasmic reticulum (ER) chaperone, but can appear surface bound on cancers cells, including ovarian cancers (OC). We investigated at what stage of cell viability, CRT appeared associated with surface of human OC cells. CRT on pre-apoptotic tumour cells is thought to initiate their eradication via a process termed immunogenic cell death (ICD).

METHODS

We treated OC cells with the chemotherapeutic-doxorubicin (DX) known to induce translocation of CRT to some tumour cell surfaces, with and without the ER stressor-thapsigargin (TG)-and/or an ER stress inhibitor-TUDCA. We monitored translocation/release of CRT in pre-apoptotic cells by flow cytometry, immunoblotting and ELISA. We investigated the difference in binding of FITC-CRT to pre-apoptotic, apoptotic and necrotic cells and the ability of extracellular CRT to generate immature dendritic cells from THP-1 monocytes.

RESULTS

Dx-treatment increased endogenously released CRT and extracellular FITC_CRT binding to human pre-apoptotic OC cells. DX and TG also promoted cell death in OC cells which also increased CRT release. These cellular responses were significantly inhibited by TUDCA, suggesting that ER stress is partially responsible for the changes in CRT cellular distribution. Extracellular CRT induces maturation of THP-1 towards a imDC phenotype, an important component of ICD.

CONCLUSION

Collectively, these cellular responses suggest that ER stress is partially responsible for the changes in CRT cellular distribution. ER-stress regulates in part the release and binding of CRT to human OC cells where it may play a role in ICD.

Mechanism of mutant calreticulin-mediated activation of the thrombopoietin receptor in cancers

Myeloproliferative neoplasms (MPNs) are frequently driven by mutations within the C-terminal domain (C-domain) of calreticulin (CRT). CRT_Del52 and CRT_Ins5 are recurrent mutations. Oncogenic transformation requires both mutated CRT and the thrombopoietin receptor (Mpl), but the molecular mechanism of CRT-mediated constitutive activation of Mpl is unknown. We show that the acquired C-domain of CRT_Del52 mediates both Mpl binding and disulfide-linked CRT_Del52 dimerization. Cysteine mutations within the novel C-domain (C400A and C404A) and the conserved N-terminal domain (N-domain; C163A) of CRT_Del52 are required to reduce disulfide-mediated dimers and multimers of CRT_Del52. Based on these data and published structures of CRT oligomers, we identify an N-domain dimerization interface relevant to both WT CRT and CRT_Del52. Elimination of disulfide bonds and ionic interactions at both N-domain and C-domain dimerization interfaces is required to abrogate the ability of CRT_Del52 to mediate cell proliferation via Mpl. Thus, MPNs exploit a natural dimerization interface of CRT combined with C-domain gain of function to achieve cell transformation.

I'm Infected, Eat Me! Innate Immunity Mediated by Live, Infected Cells Signaling To Be Phagocytosed

Innate immunity against pathogens is known to be mediated by barriers to pathogen invasion, activation of complement, recruitment of immune cells, immune cell phagocytosis of pathogens, death of infected cells, and activation of the adaptive immunity via antigen presentation. Here, we propose and review evidence for a novel mode of innate immunity whereby live, infected host cells induce phagocytes to phagocytose the infected cell, thereby potentially reducing infection. We discuss evidence that host cells, infected by virus, bacteria, or other intracellular pathogens (i) release nucleotides and chemokines as find-me signals, (ii) expose on their surface phosphatidylserine and calreticulin as eat-me signals, (iii) release and bind opsonins to induce phagocytosis, and (iv) downregulate don't-eat-me signals CD47, major histocompatibility complex class I (MHC1), and sialic acid. As long as the pathogens of the host cell are destroyed within the phagocyte, then infection can be curtailed; if antigens from the pathogens are cross-presented by the phagocyte, then an adaptive response would also be induced. Phagocytosis of live infected cells may thereby mediate innate immunity.

Roles of Calreticulin in Protein Folding, Immunity, Calcium Signaling and Cell Transformation

The endoplasmic reticulum (ER) is an organelle that mediates the proper folding and assembly of proteins destined for the cell surface, the extracellular space and subcellular compartments such as the lysosomes. The ER contains a wide range of molecular chaperones to handle the folding requirements of a diverse set of proteins that traffic through this compartment. The lectin-like chaperones calreticulin and calnexin are an important class of structurally-related chaperones relevant for the folding and assembly of many N-linked glycoproteins. Despite the conserved mechanism of action of these two chaperones in nascent protein recognition and folding, calreticulin has unique functions in cellular calcium signaling and in the immune response. The ER-related functions of calreticulin in the assembly of major histocompatibility complex (MHC) class I molecules are well-studied and provide many insights into the modes of substrate and co-chaperone recognition by calreticulin. Calreticulin is also detectable on the cell surface under some conditions, where it induces the phagocytosis of apoptotic cells. Furthermore, mutations of calreticulin induce cell transformation in myeloproliferative neoplasms (MPN). Studies of the functions of the mutant calreticulin in cell transformation and immunity have provided many insights into the normal biology of calreticulin, which are discussed.

Calnexin cycle - structural features of the ER chaperone system

The endoplasmic reticulum (ER) is the major folding compartment for secreted and membrane proteins and is the site of a specific chaperone system, the calnexin cycle, for folding N-glycosylated proteins. Recent structures of components of the calnexin cycle have deepened our understanding of quality control mechanisms and protein folding pathways in the ER. In the calnexin cycle, proteins carrying monoglucosylated glycans bind to the lectin chaperones calnexin and calreticulin, which recruit a variety of function-specific chaperones to mediate protein disulfide formation, proline isomerization, and general protein folding. Upon trimming by glucosidase II, the glycan without an inner glucose residue is no longer able to bind to the lectin chaperones. For proteins that have not yet folded properly, the enzyme UDP-glucose:glycoprotein glucosyltransferase (UGGT) acts as a checkpoint by adding a glucose back to the N-glycan. This allows the misfolded proteins to re-associate with calnexin and calreticulin for additional rounds of chaperone-mediated refolding and prevents them from exiting the ERs. Here, we review progress in structural studies of the calnexin cycle, which reveal common features of how lectin chaperones recruit function-specific chaperones and how UGGT recognizes misfolded proteins.

Regulation of vitellogenin gene expression under the negative modulator, gonad-inhibiting hormone in Penaeus monodon

Vitellogenesis is a principal process during ovarian maturation in crustaceans. This process is negatively regulated by gonad-inhibiting hormone (GIH), a neuronal peptide hormone from eyestalks. However, the detailed mechanism through which GIH regulates Vg expression is still ambiguous. In this study, suppression subtractive hybridization (SSH) under specific GIH-knockdown condition was utilized to determine the expression of genes in the ovary that may act downstream of GIH to control vitellogenin synthesis in Penaeus monodon. The total of 102 and 82 positive clones of up-regulated and down-regulated genes in GIH- knockdown shrimp were identified from the forward and reverse SSH libraries, respectively. Determination of the expression profiles of these reproduction-related genes during ovarian development revealed that the expression of calreticulin (CALR) was significantly reduced in vitellogenic ovary suggesting its role in vitellogenesis. Suppression of CALR by specific dsRNA showed elevated vitellogenin (Vg) transcript level in the ovary at day 7 post-dsRNA injection. Since CALR can bind to steroid hormone receptors and prevents the binding of the receptor to its responsive element to regulate gene expression, it is possible that CALR is an inhibitory mediator of vitellogenin synthesis via steroidal pathway. Our results posted a possible novel pathway of GIH signaling that might interfere the steroid signaling cascade to mediate Vg synthesis in the shrimp.

The Endoplasmic Reticulum and Calcium Homeostasis in Pancreatic Beta Cells

The endoplasmic reticulum (ER) mediates the first steps of protein assembly within the secretory pathway and is the site where protein folding and quality control are initiated. The storage and release of Ca2+ are critical physiological functions of the ER. Disrupted ER homeostasis activates the unfolded protein response (UPR), a pathway which attempts to restore cellular equilibrium in the face of ER stress. Unremitting ER stress, and insufficient compensation for it results in beta-cell apoptosis, a process that has been linked to both type 1 diabetes (T1D) and type 2 diabetes (T2D). Both types are characterized by progressive beta-cell failure and a loss of beta-cell mass, although the underlying causes are different. The reduction of mass occurs secondary to apoptosis in the case of T2D, while beta cells undergo autoimmune destruction in T1D. In this review, we examine recent findings that link the UPR pathway and ER Ca2+ to beta cell dysfunction. We also discuss how UPR activation in beta cells favors cell survival versus apoptosis and death, and how ER protein chaperones are involved in regulating ER Ca2+ levels. Abbreviations: BiP, Binding immunoglobulin Protein ER; endoplasmic reticulum; ERAD, ER-associated protein degradation; IFN, interferon; IL, interleukin; JNK, c-Jun N-terminal kinase; KHE, proton-K+ exchanger; MODY, maturity-onset diabetes of young; PERK, PRKR-like ER kinase; SERCA, Sarco/Endoplasmic Reticulum Ca2+-ATPases; T1D, type 1 diabetes; T2D, type 2 diabetes; TNF, tumor necrosis factor; UPR, unfolded protein response; WRS, Wolcott-Rallison syndrome.

Calreticulin protects insulin against reductive stress in vitro and in MIN6 cells

Oxidative folding of proinsulin in the endoplasmic reticulum (ER) is critical for the proper sorting and secretion of insulin from pancreatic β-cells. Here, by using non-cell-based insulin aggregation assays and mouse insulinoma-derived MIN6 cells, we searched for a candidate molecular chaperone for (pro)insulin when its oxidative folding is compromised. We found that interaction between insulin and calreticulin (CRT), a lectin that acts as an ER-resident chaperone, was enhanced by reductive stress in MIN6 cells. Co-incubation of insulin with recombinant CRT prevented reductant-induced aggregation of insulin. Furthermore, lysosomal degradation of proinsulin, which was facilitated by dithiothreitol-induced reductive stress, depended on CRT in MIN6 cells. Together, our results suggest that CRT may be a protective molecule against (pro)insulin aggregation when oxidative folding is defective, e.g. under reductive stress conditions, in vitro and in cultured cells. Because CRT acts as a molecular chaperone for not only glycosylated proteins but also non-glycosylated polypeptides, we also propose that (pro)insulin is a novel candidate client of the chaperone function of CRT.

Aberrant Glycosylation Augments the Immuno-Stimulatory Activities of Soluble Calreticulin

Calreticulin (CRT), a luminal resident calcium-binding glycoprotein of the cell, is a tumor-associated antigen involved in tumorigenesis and also an autoantigen targeted by autoantibodies found in patients with various autoimmune diseases. We have previously shown that prokaryotically expressed recombinant murine CRT (rCRT) exhibits strong stimulatory activities against monocytes/macrophages in vitro and potent immunogenicity in vivo, which is partially attributable to self-oligomerization of soluble rCRT. However, even in oligomerized form native CRT (nCRT) isolated from mouse liver is much less active than rCRT, arguing against the possibility that self-oligomerization alone would license potent pro-inflammatory properties to nCRT. Since rCRT differs from nCRT in its lack of glycosylation, we wondered if aberrant glycosylation of eukaryotically expressed CRT (eCRT) would significantly enhance its immunological activity. In the present study, tunicamycin, an N-glycosyltransferase inhibitor, was employed to treat CHO cells (CHO-CRT) stably expressing full-length recombinant mouse CRT in secreted form for preparation of aberrantly glycosylated eCRT (tun-eCRT). Our biochemical and immunological analysis results indicate that eCRT produced by CHO-CRT cells is similar to nCRT in terms of glycosylation level, lack of self-oligomerization, relatively poor immunogenicity and weak macrophage-stimulatory activity, while tun-eCRT shows reduced glycosylation yet much enhanced ability to elicit specific humoral responses in mice and TNF-α and nitric oxide production by macrophages in vitro. Given that abberant glycosylation of proteins is a hallmark of cancer cells and also related to the development of autoimmune disorders in humans, our data may provide useful clues for better understanding of potentiating roles of dysregulated glycosylation of molecules such as CRT in tumorigenesis and autoimmunity.

The Novel Type 1 Fimbriae FimH Receptor Calreticulin Plays a Role in Salmonella Host Specificity

It was suggested that minor differences in the structure of FimH are most likely associated with differences in its adhesion specificities and may determine the tropism of various Salmonella serovars to different species and tissues. We have recently shown that FimH adhesins from host-adapted serovars, e.g., Salmonella Choleraesuis (SCh), bind to other glycoprotein receptors compared to FimH from host-unrestricted Salmonella Enteritidis (SE). Here we identify porcine calreticulin expressed by swine intestinal cells as a host-specific receptor for SCh FimH adhesin, suggesting that such an interaction may contribute to SCh host specificity. Calreticulin was identified by 2D electrophoresis and mass spectrometry as a glycoprotein that was bound specifically by recombinant SCh FimH protein, but not by FimH from SE. The functionality of calreticulin as a specific receptor of SCh FimH adhesin was further confirmed by adhesion and invasion of mutated strains of SCh carrying different variants of FimH proteins to IPEC-J2 cells with overexpression and silenced expression of calreticulin. It was found that SCh carrying the active variant of FimH adhered and invaded IPEC-J2 cells with calreticulin overexpression at significantly higher numbers than those of SCh expressing the non-active variant or SE variant of FimH. Moreover, binding of SCh carrying the active variant of FimH to IPEC-J2 with silenced calreticulin expression was significantly weaker. Furthermore, we observed that SCh infection induces translocation of calreticulin to cell membrane. All of the aforementioned results lead to the general conclusion that Salmonella host specificity requires not only special mechanisms and proteins expressed by the pathogen but also specifically recognized receptors expressed by a specific host.

Calreticulin attenuated microwave radiation-induced human microvascular endothelial cell injury through promoting actin acetylation and polymerization

Recent work reveals that actin acetylation modification has been linked to different normal and disease processes and the effects associated with metabolic and environmental stressors. Herein, we highlight the effects of calreticulin on actin acetylation and cell injury induced by microwave radiation in human microvascular endothelial cell (HMEC). HMEC injury was induced by high-power microwave of different power density (10, 30, 60, 100 mW/cm2, for 6 min) with or without exogenous recombinant calreticulin. The cell injury was assessed by lactate dehydrogenase (LDH) activity and Cell Counting Kit-8 in culture medium, migration ability, intercellular junction, and cytoskeleton staining in HMEC. Western blotting analysis was used to detected calreticulin expression in cytosol and nucleus and acetylation of globular actin (G-actin). We found that HMEC injury was induced by microwave radiation in a dose-dependent manner. Pretreatment HMEC with calreticulin suppressed microwave radiation-induced LDH leakage and increased cell viability and improved microwave radiation-induced decrease in migration, intercellular junction, and cytoskeleton. Meanwhile, pretreatment HMEC with exogenous calreticulin upregulated the histone acetyltransferase activity and the acetylation level of G-actin and increased the fibrous actin (F-actin)/G-actin ratio. We conclude that exogenous calreticulin protects HMEC against microwave radiation-induced injury through promoting actin acetylation and polymerization.

Contributions of the Lectin and Polypeptide Binding Sites of Calreticulin to Its Chaperone Functions in Vitro and in Cells

Calreticulin is a lectin chaperone of the endoplasmic reticulum that interacts with newly synthesized glycoproteins by binding to Glc1Man9GlcNAc2 oligosaccharides as well as to the polypeptide chain. In vitro, the latter interaction potently suppresses the aggregation of various non-glycosylated proteins. Although the lectin-oligosaccharide association is well understood, the polypeptide-based interaction is more controversial because the binding site on calreticulin has not been identified, and its significance in the biogenesis of glycoproteins in cells remains unknown. In this study, we identified the polypeptide binding site responsible for the in vitro aggregation suppression function by mutating four candidate hydrophobic surface patches. Mutations in only one patch, P19K/I21E and Y22K/F84E, impaired the ability of calreticulin to suppress the thermally induced aggregation of non-glycosylated firefly luciferase. These mutants also failed to bind several hydrophobic peptides that act as substrate mimetics and compete in the luciferase aggregation suppression assay. To assess the relative contributions of the glycan-dependent and -independent interactions in living cells, we expressed lectin-deficient, polypeptide binding-deficient, and doubly deficient calreticulin constructs in calreticulin-negative cells and monitored the effects on the biogenesis of MHC class I molecules, the solubility of mutant forms of α1-antitrypsin, and interactions with newly synthesized glycoproteins. In all cases, we observed a profound impairment in calreticulin function when its lectin site was inactivated. Remarkably, inactivation of the polypeptide binding site had little impact. These findings indicate that the lectin-based mode of client interaction is the predominant contributor to the chaperone functions of calreticulin within the endoplasmic reticulum.

The C-Terminal Acidic Region of Calreticulin Mediates Phosphatidylserine Binding and Apoptotic Cell Phagocytosis

Calreticulin is a calcium-binding chaperone that is normally localized in the endoplasmic reticulum. Calreticulin is detectable on the surface of apoptotic cells under some apoptosis-inducing conditions, where it promotes the phagocytosis and immunogenicity of dying cells. However, the precise mechanism by which calreticulin, a soluble protein, localizes to the outer surface of the plasma membrane of dying cells is unknown, as are the molecular mechanisms that are relevant to calreticulin-induced cellular phagocytosis. Calreticulin comprises three distinct structural domains: a globular domain, an extended arm-like P-domain, and a C-terminal acidic region containing multiple low-affinity calcium binding sites. We show that calreticulin, via its C-terminal acidic region, preferentially interacts with phosphatidylserine (PS) compared with other phospholipids and that this interaction is calcium dependent. Additionally, exogenous calreticulin binds apoptotic cells via a higher-affinity calcium-dependent mode that is acidic region dependent. Exogenous calreticulin also binds live cells, including macrophages, via a second, lower-affinity P-domain and globular domain-dependent, but calcium-independent binding mode that likely involves its generic polypeptide binding site. Truncation constructs lacking the acidic region or arm-like P-domain of calreticulin are impaired in their abilities to induce apoptotic cell phagocytosis by murine peritoneal macrophages. Taken together, the results of this investigation provide the first molecular insights into the phospholipid binding site of calreticulin as a key anchor point for the cell surface expression of calreticulin on apoptotic cells. These findings also support a role for calreticulin as a PS-bridging molecule that cooperates with other PS-binding factors to promote the phagocytosis of apoptotic cells.

Mutant calreticulin-expressing cells induce monocyte hyperreactivity through a paracrine mechanism

Mutations in the calreticulin gene (CALR) were recently identified in approximately 70-80% of patients with JAK2-V617F-negative essential thrombocytosis and primary myelofibrosis. All frameshift mutations generate a recurring novel C-terminus. Here we provide evidence that mutant calreticulin does not accumulate efficiently in cells and is abnormally enriched in the nucleus and extracellular space compared to wildtype calreticulin. The main determinant of these findings is the loss of the calcium-binding and KDEL domains. Expression of type I mutant CALR in Ba/F3 cells confers minimal IL-3-independent growth. Interestingly, expression of type I and type II mutant CALR in a nonhematopoietic cell line does not directly activate JAK/STAT signaling compared to wildtype CALR and JAK2-V617F expression. These results led us to investigate paracrine mechanisms of JAK/STAT activation. Here we show that conditioned media from cells expressing type I mutant CALR exaggerate cytokine production from normal monocytes with or without treatment with a toll-like receptor agonist. These effects are not dependent on the novel C-terminus. These studies offer novel insights into the mechanism of JAK/STAT activation in patients with JAK2-V617F-negative essential thrombocytosis and primary myelofibrosis.

Regulation of calreticulin-major histocompatibility complex (MHC) class I interactions by ATP

The MHC class I peptide loading complex (PLC) facilitates the assembly of MHC class I molecules with peptides, but factors that regulate the stability and dynamics of the assembly complex are largely uncharacterized. Based on initial findings that ATP, in addition to MHC class I-specific peptide, is able to induce MHC class I dissociation from the PLC, we investigated the interaction of ATP with the chaperone calreticulin, an endoplasmic reticulum (ER) luminal, calcium-binding component of the PLC that is known to bind ATP. We combined computational and experimental measurements to identify residues within the globular domain of calreticulin, in proximity to the high-affinity calcium-binding site, that are important for high-affinity ATP binding and for ATPase activity. High-affinity calcium binding by calreticulin is required for optimal nucleotide binding, but both ATP and ADP destabilize enthalpy-driven high-affinity calcium binding to calreticulin. ATP also selectively destabilizes the interaction of calreticulin with cellular substrates, including MHC class I molecules. Calreticulin mutants that affect ATP or high-affinity calcium binding display prolonged associations with monoglucosylated forms of cellular MHC class I, delaying MHC class I dissociation from the PLC and their transit through the secretory pathway. These studies reveal central roles for ATP and calcium binding as regulators of calreticulin-substrate interactions and as key determinants of PLC dynamics.

Calreticulin translocation aggravates endoplasmic reticulum stress-associated apoptosis during cardiomyocyte hypoxia/reoxygenation

Background:

Calreticulin (CRT) is major Ca²⁺-binding chaperone mainly resident in the endoplasmic reticulum (ER) lumen. Recently, it has been shown that non-ER CRT regulates a wide array of cellular responses. We previously found that CRT was up-regulated during hypoxia/reoxygenation (H/R) and this study was aimed to investigate whether CRT nuclear translocation aggravates ER stress (ERS)-associated apoptosis during H/R injury in neonatal rat cardiomyocytes.

Methods:

Apoptosis rate and lactate dehydrogenase (LDH) leakage in culture medium were measured as indices of cell injury. Immunofluorescence staining showed the morphological changes of ER and intracellular translocation of CRT. Western blotting or reverse transcription polymerase chain reaction was used to detect the expression of target molecules.

Results:

Compared with control, H/R increased apoptosis rate and LDH activity. The ER became condensed and bubbled, and CRT translocated to the nucleus. Western blotting showed up-regulation of CRT, Nrf2, activating transcription factor 4 (ATF4), CHOP and caspase-12 expression after H/R. Exogenous CRT overexpression induced by plasmid transfection before H/R increased cell apoptosis, LDH leakage, ER disorder, CRT nuclear translocation and the expression of ERS-associated molecules. However, administration of the ERS inhibitor, taurine, or CRT siRNA alleviated cell injury, ER disorder, and inhibited ERS-associated apoptosis.

Conclusions:

Our results indicated that during H/R stress, CRT translocation increases cell apoptosis and LDH leakage, aggravates ER disorder, up-regulates expression of nuclear transcription factors, Nrf2 and ATF4, and activates ERS-associated apoptosis.

Cytosolic calreticulin inhibits microwave radiation-induced microvascular endothelial cell injury through the integrin-focal adhesion kinase pathway

OBJECTIVE

To determine the effects of cytosolic CRT on MR-induced MMEC injury, and the underlying mechanism.

METHODS

MMECs were randomized into eight groups: control, AdCRT (infected with pAdCMV/V5-DEST-CRT adenovirus), stCRT (transfected with rCRT-siRNAs), Mock (transfected with scrambled siRNAs), MR (exposed to MR for six minutes), AdCRT + MR, stCRT + MR, and Mock + MR. The magnitude of cell injury were assessed by Annexin V-PI staining, LDH activity in culture medium, MMEC migration ability, ultrastructure and cytoskeletal stability. Subcellular colocalization of CRT and ConA or integrin were evaluated by immunocytochemistry. The mRNA and protein expression levels of target genes were examined by qRT-PCR and western blotting, respectively.

RESULTS

MR-induced cytotoxicity was dose-dependent. Overexpression of cytosolic CRT suppressed MR injury, shown as decreased cell apoptosis, reduced LDH activity, enhanced cell migration capability, and maintenance of ultrastructure and cytoskeleton integrity. Conversely, CRT deficiency aggravated MR-induced injury. Exposure of AdCRT MMECs to MR promoted membrane translocation of CRT and the interaction of CRT-integrin-α. Correlation analysis revealed that integrin-α expression or FAK phosphorylation was positively associated with cytosolic CRT expression.

CONCLUSIONS

Cytosolic CRT inhibits MR-induced MMEC injury through activation of the integrin-FAK pathway.

Mechanisms of Translocation of ER Chaperones to the Cell Surface and Immunomodulatory Roles in Cancer and Autoimmunity

Endoplasmic reticulum (ER) chaperones (e.g., calreticulin, heat shock proteins, and isomerases) perform a multitude of functions within the ER. However, many of these chaperones can translocate to the cytosol and eventually the surface of cells, particularly during ER stress induced by e.g., drugs, UV irradiation, and microbial stimuli. Once on the cell surface or in the extracellular space, the ER chaperones can take on immunogenic characteristics, as mostly described in the context of cancer, appearing as damage-associated molecular patterns recognized by the immune system. How ER chaperones relocate to the cell surface and interact with other intracellular proteins appears to influence whether a tumor cell is targeted for cell death. The relocation of ER proteins to the cell surface can be exploited to target cancer cells for elimination by immune mechanism. Here we evaluate the evidence for the different mechanisms of ER protein translocation and binding to the cell surface and how ER protein translocation can act as a signal for cancer cells to undergo killing by immunogenic cell death and other cell death pathways. The release of chaperones can also exacerbate underlying autoimmune conditions, such as rheumatoid arthritis and multiple sclerosis, and the immunomodulatory role of extracellular chaperones as potential cancer immunotherapies requires cautious monitoring, particularly in cancer patients with underlying autoimmune disease.

Radiation-induced immunogenic modulation of tumor enhances antigen processing and calreticulin exposure, resulting in enhanced T-cell killing

Radiation therapy (RT) is used for local tumor control through direct killing of tumor cells. Radiation-induced cell death can trigger tumor antigen-specific immune responses, but these are often noncurative. Radiation has been demonstrated to induce immunogenic modulation (IM) in various tumor types by altering the biology of surviving cells to render them more susceptible to T cell-mediated killing. Little is known about the mechanism(s) underlying IM elicited by sub-lethal radiation dosing. We have examined the molecular and immunogenic consequences of radiation exposure in breast, lung, and prostate human carcinoma cells. Radiation induced secretion of ATP and HMGB1 in both dying and surviving tumor cells. In vitro and in vivo tumor irradiation induced significant upregulation of multiple components of the antigen-processing machinery and calreticulin cell-surface expression. Augmented CTL lysis specific for several tumor-associated antigens was largely dictated by the presence of calreticulin on the surface of tumor cells and constituted an adaptive response to endoplasmic reticulum stress, mediated by activation of the unfolded protein response. This study provides evidence that radiation induces a continuum of immunogenic alterations in tumor biology, from immunogenic modulation to immunogenic cell death. We also expand the concept of immunogenic modulation, where surviving tumor cells recovering from radiation-induced endoplasmic reticulum stress become more sensitive to CTL killing. These observations offer a rationale for the combined use of radiation with immunotherapy, including for patients failing RT alone.

Immunological activity difference between native calreticulin monomers and oligomers

We have recently demonstrated that the greatly increased immunological activities of recombinant murine calreticulin (rCRT) are largely attributed to its self-oligomerization. Although native CRT (nCRT) can also oligomerize under stress conditions in vitro, whether this phenomenon could occur inside cells and the immunological activity difference between nCRT monomers and oligomers remained unclear. In this study, we illustrated the formation of CRT oligomers in tranfectant cells under “heat & low pH” (42°C/pH 6.5) condition. The mixture of nCRT oligomers and monomers (OnCRT) was obtained after 3 hr treatment of murine monomeric nCRT (MnCRT) under similar condition (42°C/pH 5.0) in vitro. The OnCRT thus obtained was better recognized by 2 monoclonal Abs from mice that had been immunized with oligomeric rCRT. Unlike MnCRT, OnCRT was able to elicit CRT-specific IgG production in mice. OnCRT also stimulated bone-marrow derived dendritic cells (BMDCs) to secrete significantly higher levels of TNF-α, IL-6 and IL-12p40 than did MnCRT in vitro. We postulate that oligomerization of soluble CRT may occur under certain pathophysiological conditions (e.g. ultrahyperpyrexia) and the resultant oligomers may exhibit exaggerated immunostimulating activities, thereby affiliating the inflammatory responses in vivo.

Immunolocalization of skeletal matrix proteins in tissue and mineral of the coral Stylophora pistillata

The precipitation and assembly of calcium carbonate skeletons by stony corals is a precisely controlled process regulated by the secretion of an ECM. Recently, it has been reported that the proteome of the skeletal organic matrix (SOM) contains a group of coral acid-rich proteins as well as an assemblage of adhesion and structural proteins, which together, create a framework for the precipitation of aragonite. To date, we are aware of no report that has investigated the localization of individual SOM proteins in the skeleton. In particular, no data are available on the ultrastructural mapping of these proteins in the calcification site or the skeleton. This information is crucial to assessing the role of these proteins in biomineralization. Immunological techniques represent a valuable approach to localize a single component within a calcified skeleton. By using immunogold labeling and immunohistochemical assays, here we show the spatial arrangement of key matrix proteins in tissue and skeleton of the common zooxanthellate coral, Stylophora pistillata. To our knowledge, our results reveal for the first time that, at the nanoscale, skeletal proteins are embedded within the aragonite crystals in a highly ordered arrangement consistent with a diel calcification pattern. In the tissue, these proteins are not restricted to the calcifying epithelium, suggesting that they also play other roles in the coral's metabolic pathways.

Deciphering the effect of the different N-glycosylation sites on the secretion, activity, and stability of cellobiohydrolase I from Trichoderma reesei

N-linked glycosylation modulates and diversifies the structures and functions of the eukaryotic proteome through both intrinsic and extrinsic effects on proteins. We investigated the significance of the three N-linked glycans on the catalytic domain of cellobiohydrolase I (CBH1) from the filamentous fungus Trichoderma reesei in its secretion and activity. While the removal of one or two N-glycosylation sites hardly affected the extracellular secretion of CBH1, eliminating all of the glycosylation sites did induce expression of the unfolded protein response (UPR) target genes, and secretion of this CBH1 variant was severely compromised in a calnexin gene deletion strain. Further characterization of the purified CBH1 variants showed that, compared to Asn270, the thermal reactivity of CBH1 was significantly decreased by removal of either Asn45 or Asn384 glycosylation site during the catalyzed hydrolysis of soluble substrate. Combinatorial loss of these two N-linked glycans further exacerbated the temperature-dependent inactivation. In contrast, this thermal labile property was less severe when hydrolyzing insoluble cellulose. Analysis of the structural integrity of CBH1 variants revealed that removal of N-glycosylation at Asn384 had a more pronounced effect on the integrity of regular secondary structure compared to the loss of Asn45 or Asn270. These data implicate differential roles of N-glycosylation modifications in contributing to the stability of specific functional regions of CBH1 and highlight the potential of improving the thermostability of CBH1 by tuning proper interactions between glycans and functional residues.

Glycan-dependent and -independent interactions contribute to cellular substrate recruitment by calreticulin

Calreticulin is an endoplasmic reticulum chaperone with specificity for monoglucosylated glycoproteins. Calreticulin also inhibits precipitation of nonglycosylated proteins and thus contains generic protein-binding sites, but their location and contributions to substrate folding are unknown. We show that calreticulin binds glycosylated and nonglycosylated proteins with similar affinities but distinct interaction kinetics. Although both interactions involve the glycan-binding site or its vicinity, the arm-like proline-rich (P-) domain of calreticulin contributes to binding non/deglycosylated proteins. Correspondingly, ensemble FRET spectroscopy measurements indicate that glycosylated and nonglycosylated proteins induce "open" and "closed" P-domain conformations, respectively. The co-chaperone ERp57 influences substrate-binding kinetics and induces a closed P-domain conformation. Together with analysis of the interactions of calreticulin with cellular proteins, these findings indicate that the recruitment of monoglucosylated proteins to calreticulin is kinetically driven, whereas the P-domain and co-chaperone contribute to stable substrate binding. Substrate sequestration in the cleft between the glycan-binding site and P-domain is a likely mechanism for calreticulin-assisted protein folding.

Calreticulin-dimerization induced by post-translational arginylation is critical for stress granules scaffolding

Protein arginylation mediated by arginyl-tRNA protein transferase is a post-translational modification that occurs widely in biology, it has been shown to regulate protein and properties and functions. Post-translational arginylation is critical for embryogenesis, cardiovascular development and angiogenesis but the molecular effects of proteins arginylated in vivo are largely unknown. In the present study, we demonstrate that arginylation reduces CRT (calreticulin) thermostability and induces a greater degree of dimerization and oligomerization. R-CRT (arginylated calreticulin) forms disulfide-bridged dimers that are increased in low Ca(2+) conditions at physiological temperatures, a similar condition to the cellular environment that it required for arginylation of CRT. Moreover, R-CRT self-oligomerizes through non-covalent interactions that are enhanced at temperatures above 40 °C, condition that mimics the heat shock treatment where R-CRT is the only isoespecies of CRT that associates in cells to SGs (stress granules). We show that in cells lacking CRT the scaffolding of larger SGs is impaired; the transfection with CRT (hence R-CRT expression) restores SGs assembly whereas the transfection with CRT mutated in Cys146 does not. Thus, R-CRT disulfide-bridged dimers (through Cys146) are essential for the scaffolding of larger SGs under heat shock, although these dimers are not required for R-CRT association to SGs. The alteration in SGs assembly is critical for the normal cellular recover of cells after heat induced stress. We conclude that R-CRT is emerging as a novel protein that has an impact on the regulation of SGs scaffolding and cell survival.

Calreticulin in the immune system: ins and outs

Calreticulin is a calcium-binding chaperone that has several functions in the immune response. In the endoplasmic reticulum (ER), calreticulin facilitates the folding of major histocompatibility complex (MHC) class I molecules and their assembly factor tapasin, thereby influencing antigen presentation to cytotoxic T cells. Although calreticulin is normally ER-resident, it is found at the cell surface of living cancer cells and dying cells. Here, calreticulin promotes cellular phagocytic uptake. In tumor vaccine models, drugs that induce cell surface calreticulin confer enhanced tumor protection in an extracellular calreticulin-dependent manner. Much remains to be understood about the roles of calreticulin in these distinct functions. Further investigations are important towards advancing basic knowledge of glycoprotein-folding pathways, and towards developing new cancer therapeutic strategies.

Respiratory epithelial cell responses to cigarette smoke: the unfolded protein response

Cigarette smoking exposes the respiratory epithelium to highly toxic, reactive oxygen nitrogen species which damage lung proteins in the endoplasmic reticulum (ER), the cell organelle in which all secreted and membrane proteins are processed. Accumulation of damaged or misfolded proteins in the ER, a condition termed ER stress, activates a complex cellular process termed the unfolded protein responses (UPR). The UPR acts to restore cellular protein homeostasis by regulating all aspects of protein metabolism including: protein translation and syntheses; protein folding; and protein degradation. However, activation of the UPR may also induce signaling pathways which induce inflammation and cell apoptosis. This review discusses the role of UPR in the respiratory epithelial cell response to cigarette smoke and the pathogenesis of lung diseases like COPD.

ER Stress Proteins in Autoimmune and Inflammatory Diseases

Over the past two decades, heat shock proteins (HSPs) have been implicated in inflammatory responses and autoimmunity. HSPs were originally believed to maintain protein quality control in the cytosol. However, they also exist extracellularly and appear to act as inflammatory factors. Recently, a growing body of evidence suggested that the other class of stress proteins such as, endoplasmic reticulum (ER) stress proteins, which originally act as protein quality control factors in the secretory pathway and are induced by ER stress in inflammatory lesions, also participate in inflammation and autoimmunity. The immunoglobulin heavy-chain binding protein (Bip)/glucose-regulated protein 78 (GRP78), calnexin, calreticulin, glucose-regulated protein 94 (GRP94)/gp96, oxygen regulated protein 150 (ORP150)/glucose-regulated protein 170 (GRP170), homocysteine-induced ER protein (Herp) and heat shock protein 47 (hsp47)/Serpin H1, which are expressed not only in the ER but also occasionally at the cell surface play pathophysiological roles in autoimmune and inflammatory diseases as pro- or anti-inflammatory factors. Here we describe the accumulating evidence of the participation of ER stress proteins in autoimmunity and inflammation and discuss the critical differences between the two classes of stress proteins.

Thrombospondin1 in tissue repair and fibrosis: TGF-β-dependent and independent mechanisms

Thrombospondin 1 (TSP1) plays major roles in both physiologic and pathologic tissue repair. TSP1 through its type 1 repeats is a known regulator of latent TGF-β activation and plays a role in wound healing and fibrosis. Binding of the TSP N-terminal domain to cell surface calreticulin in complex with LDL-receptor related protein 1 stimulates intermediate cell adhesion, cell migration, anoikis resistance, collagen expression and matrix deposition in an in vivo model of the foreign body response. There is also emerging evidence that TSP EGF-like repeats alter endothelial cell-cell interactions and stimulate epithelial migration through transactivation of EGF receptors. The mechanisms underlying these functions of TSP1 and the implications for physiologic and pathologic wound repair and fibrosis will be discussed.

Involvement of damage-associated molecular patterns in tumor response to photodynamic therapy: surface expression of calreticulin and high-mobility group box-1 release

Damage-associated molecular patterns (DAMPs), danger signal molecules expressed after injury or infection, have become recognized as prerequisite for orchestrating effective anti-tumor host response. The expression of two prototypical DAMPs, calreticulin and high-mobility group box-1 (HMGB1) protein, was examined following Photofrin-photodynamic therapy (PDT) of Lewis lung carcinoma (LLC) cells in vitro and LLC tumors growing in syngeneic mice. Cell surface expression of calreticulin was found to be highly increased at 1 h after PDT treatment both in vitro and in vivo. Increased exposure of calreticulin was also detected on the surface of macrophages from PDT-treated LLC tumors. At the same time interval, a rise in serum HMGB1 was detected in host mice. Intracellular staining of macrophages co-incubated for 16 h with PDT-treated LLC cells revealed elevated levels of HMGB1 in these cells. The knowledge of the involvement of these DAMPs uncovers important mechanistic insights into the development of host response induced by PDT.

Endoplasmic reticulum calcium depletion impacts chaperone secretion, innate immunity, and phagocytic uptake of cells

A number of immunological functions are ascribed to cell surface-expressed forms of the endoplasmic reticulum (ER) chaperone calreticulin (CRT). In this study, we examined the impact of ER stress-inducing drugs upon cell surface CRT induction and the resulting immunological consequences. We showed that cell surface expression of CRT and secretion of CRT, BiP, gp96, and PDI were induced by thapsigargin (THP) treatment, which depletes ER calcium, but not by tunicamycin treatment, which inhibits protein glycosylation. Surface expression of CRT in viable, THP-treated fibroblasts correlated with their enhanced phagocytic uptake by bone marrow-derived dendritic cells. Incubation of bone marrow-derived dendritic cells with THP-treated fibroblasts enhanced sterile IL-6 production and LPS-induced generation of IL-1β, IL-12, IL-23, and TNF-α. However, extracellular CRT is not required for enhanced proinflammatory responses. Furthermore, the pattern of proinflammatory cytokine induction by THP-treated cells and cell supernatants resembled that induced by THP itself and indicated that other ER chaperones present in supernatants of THP-treated cells also do not contribute to induction of the innate immune response. Thus, secretion of various ER chaperones, including CRT, is induced by ER calcium depletion. CRT, previously suggested as an eat-me signal in dead and dying cellular contexts, can also promote phagocytic uptake of cells subject to ER calcium depletion. Finally, there is a strong synergy between calcium depletion in the ER and sterile IL-6, as well as LPS-dependent IL-1β, IL-12, IL-23, and TNF-α innate responses, findings that have implications for understanding inflammatory diseases that originate in the ER.

Structural and functional relationships between the lectin and arm domains of calreticulin

Calreticulin and calnexin are key components in maintaining the quality control of glycoprotein folding within the endoplasmic reticulum. Although their lectin function of binding monoglucosylated sugar moieties of glycoproteins is well documented, their chaperone activity in suppressing protein aggregation is less well understood. Here, we use a series of deletion mutants of calreticulin to demonstrate that its aggregation suppression function resides primarily within its lectin domain. Using hydrophobic peptides as substrate mimetics, we show that aggregation suppression is mediated through a single polypeptide binding site that exhibits a K(d) for peptides of 0.5-1 μM. This site is distinct from the oligosaccharide binding site and differs from previously identified sites of binding to thrombospondin and GABARAP (4-aminobutyrate type A receptor-associated protein). Although the arm domain of calreticulin was incapable of suppressing aggregation or binding hydrophobic peptides on its own, it did contribute to aggregation suppression in the context of the whole molecule. The high resolution x-ray crystal structure of calreticulin with a partially truncated arm domain reveals a marked difference in the relative orientations of the arm and lectin domains when compared with calnexin. Furthermore, a hydrophobic patch was detected on the arm domain that mediates crystal packing and may contribute to calreticulin chaperone function.