The Phagocytic Code Regulating Phagocytosis of Mammalian Cells

Mammalian phagocytes can phagocytose (i.e. eat) other mammalian cells in the body if they display certain signals, and this phagocytosis plays fundamental roles in development, cell turnover, tissue homeostasis and disease prevention. To phagocytose the correct cells, phagocytes must discriminate which cells to eat using a 'phagocytic code' - a set of over 50 known phagocytic signals determining whether a cell is eaten or not - comprising find-me signals, eat-me signals, don't-eat-me signals and opsonins. Most opsonins require binding to eat-me signals - for example, the opsonins galectin-3, calreticulin and C1q bind asialoglycan eat-me signals on target cells - to induce phagocytosis. Some proteins act as 'self-opsonins', while others are 'negative opsonins' or 'phagocyte suppressants', inhibiting phagocytosis. We review known phagocytic signals here, both established and novel, and how they integrate to regulate phagocytosis of several mammalian targets - including excess cells in development, senescent and aged cells, infected cells, cancer cells, dead or dying cells, cell debris and neuronal synapses. Understanding the phagocytic code, and how it goes wrong, may enable novel therapies for multiple pathologies with too much or too little phagocytosis, such as: infectious disease, cancer, neurodegeneration, psychiatric disease, cardiovascular disease, ageing and auto-immune disease.

Calreticulin Regulates Syncytialization Through Control of the Synthesis and Transportation of E-Cadherin in BeWo Cells

During placental development, mononuclear cytotrophoblasts differentiate and fuse to syncytiotrophoblasts (STBs) to form syncytia, which secrete human chorionic gonadotropin (hCG). Decreased maternal perfusion of the placenta, which leads to placental dysfunction, induces changes in trophoblast syncytialization. Our previous study showed that calreticulin (CRT), a Ca2+-binding molecular chaperone found in the endoplasmic reticulum, is expressed in the human placenta and is involved in regulating extravillous trophoblast invasion, although its role in villous trophoblasts remains unclear. In this study, we investigated the functional role of CRT in trophoblast differentiation using the human trophoblast-like cell line BeWo, in which CRT gene expression was knocked down. We found that CRT was highly expressed in human third-trimester placentas and mainly localized in STBs. The fetal growth restriction group exhibited significantly lower CRT expression levels than did the control group. In BeWo cells, CRT knockdown markedly suppressed forskolin-induced cell fusion and β-hCG secretion. As for the mechanism responsible for these effects, the cell surface expression of E-cadherin, a key adhesion molecule related to syncytialization, was decreased, and E-cadherin accumulated adjacent to the Golgi apparatus in the CRT-knockdown cells, which led to dysfunctional cell-to-cell adhesion. Additionally, metabolic labeling and a pulse-chase study revealed that the protein expression of E-cadherin was suppressed at the translational level in the CRT-knockdown cells. Collectively, these results demonstrate that CRT regulates syncytialization by ensuring appropriate control of both the synthesis and transportation of E-cadherin, suggesting that CRT expression is important for placental development during pregnancy.

Calreticulin Is Involved in Invasion of Human Extravillous Trophoblasts Through Functional Regulation of Integrin β1

Calreticulin (CRT), a molecular chaperone in the endoplasmic reticulum (ER), plays a variety of roles in cell growth, differentiation, apoptosis, immunity, and cancer biology. It has been reported that CRT is expressed in the human placenta, although its function in placental development is poorly understood. Appropriate invasion of extravillous trophoblasts (EVTs) into the maternal decidua is necessary for successful pregnancy. The objective of the present study was to investigate the expression and functional role of CRT in EVTs using the human EVT cell line HTR8/SVneo, in which CRT gene expression was knocked down. We found that CRT was highly expressed in the human placenta in the early stage of pregnancy and localized to the EVTs. CRT knockdown markedly suppressed the invasion ability of HTR8/SVneo cells. Furthermore, the adhesion to fibronectin was suppressed in the CRT-knockdown cells via the dysfunction of integrin α5β1. In the CRT-knockdown cells, terminal sialylation and fucosylation were decreased, and the core galactose-containing structure was increased in the N-glycans of integrin β1. In addition, the expression levels of several critical glycosyltransferases were changed in the CRT-knockdown cells, consistent with the changes in the N-glycans. These results showed that CRT regulates the function of integrin β1 by affecting the synthesis of N-glycans in HTR8/SVneo cells. Collectively, the results of the present study demonstrate that the ER chaperone CRT plays a regulatory role in the invasion of EVTs, suggesting the importance of CRT expression in placental development during early pregnancy.

[Study on Potential Role of Calreticulin in Pressure Overload Induced Cardiac Hypertrophy]

This study aims to investigate the role of calreticulin in(CRT)pressure overload induced cardiac hypertrophy.In our study,cardiac hypertrophy was induced by left ventricular pressure overload in male SD rats subjected to transverse aortic constriction(TAC)operation.Expression of gene and protein of calreticulin,markers of cardiac hypertrophy and endoplasmic reticulum stress(ERS)were measured with real-time qPCR and Western blot respectively.Meanwhile,atorvastatin(a known ERS inhibitor)and calreticulin-specific small interference ribonucleic acid(siRNA)were used to inhibit the expression of ERS and calreticulin respectively.The experimental data demonstrated that the gene and protein levels of calreticulin,hypertrophic and ERS markers were increased significantly in the heart tissues of TAC rat models after 4weeks.Moreover,atorvastatin administration improved the cardiac function and reduced the expression of calreticulin and ERS markers in TAC rats.In addition,cultured primary neonatal rat cardiomyocytes(NCMs)were treated with norepinephrine(NE),angiotensionⅡ(AngⅡ)or isoprenaline(ISO)to induce hypertrophic phenotype and ERS.The expression of hypertrophic markers was reduced in NCMs transfected with calreticulin-siRNA.The results suggested that calreticulin might be a promising target for the treatment of cardiac hypertrophy.

A new approach for investigating venom function applied to venom calreticulin in a parasitoid wasp

A new method is developed to investigate functions of venom components, using venom gene RNA interference knockdown in the venomous animal coupled with RNA sequencing in the envenomated host animal. The vRNAi/eRNA-Seq approach is applied to the venom calreticulin component (v-crc) of the parasitoid wasp Nasonia vitripennis. Parasitoids are common, venomous animals that inject venom proteins into host insects, where they modulate physiology and metabolism to produce a better food resource for the parasitoid larvae. vRNAi/eRNA-Seq indicates that v-crc acts to suppress expression of innate immune cell response, enhance expression of clotting genes in the host, and up-regulate cuticle genes. V-crc KD also results in an increased melanization reaction immediately following envenomation. We propose that v-crc inhibits innate immune response to parasitoid venom and reduces host bleeding during adult and larval parasitoid feeding. Experiments do not support the hypothesis that v-crc is required for the developmental arrest phenotype observed in envenomated hosts. We propose that an important role for some venom components is to reduce (modulate) the exaggerated effects of other venom components on target host gene expression, physiology, and survival, and term this venom mitigation. A model is developed that uses vRNAi/eRNA-Seq to quantify the contribution of individual venom components to total venom phenotypes, and to define different categories of mitigation by individual venoms on host gene expression. Mitigating functions likely contribute to the diversity of venom proteins in parasitoids and other venomous organisms.

The role of exchange protein directly activated by cyclic AMP 2-mediated calreticulin expression in the decidualization of human endometrial stromal cells

Decidualization of human endometrial stromal cells (ESCs) accompanied by the production of prolactin (PRL) and IGF-binding protein (IGFBP) 1 and rounded-cell morphology is indispensable for the establishment and maintenance of pregnancy. Protein kinase A (PKA)-mediated cAMP signaling is known to be crucial for decidualization. We previously reported that activation of a cAMP mediator, called Exchange protein directly activated by cAMP (EPAC) promotes cAMP analog- or ovarian steroid-induced decidualization in cultured human ESCs. In addition, small interfering RNA-mediated knock-down of the EPAC subtypes, EPAC1 or EPAC2, or knock-down of Rap1, a downstream factor of EPAC signaling, blocked functional and morphological decidualization of ESCs. However, factors downstream of EPAC2 other than Rap1 have not been determined. The present study was undertaken to identify additional downstream targets of EPAC2 associated with decidualization. Using proteomic analysis, we identified calreticulin (CRT) as a potential target of EPAC2. Knock-down of CRT expression in cultured ESCs significantly inhibited PKA-selective cAMP analog- or PKA-selective cAMP analog plus EPAC-selective cAMP analog-induced PRL and IGFBP1 expression. Furthermore, CRT knock-down suppressed the ovarian steroid-stimulated PRL and IGFBP1 expression and morphological differentiation, and silencing of EPAC2 or CRT significantly increased senescence-associated β-galactosidase activity with enhanced p21 expression and decreased p53 expression. These results suggest that EPAC2 and CRT are associated with cellular senescence in ESCs. In conclusion, we demonstrate here that EPAC2-mediated CRT expression is essential for the functional and morphological differentiation of ESCs into decidual cells. Furthermore, both EPAC2 and CRT might prevent ESCs from undergoing abnormal cellular senescence during decidualization.

[Calreticulin-mediated thermal treatment on the adaptation change of calmodulin mechanism in rat skeletal muscle]

OBJECTIVE

To study the protection of stress protein calreticulin (CRT) in rat skeletal muscle during the adaptation mechanism of calmodulin in the course of heat treatment.

METHODS

Increased heat treatment program would be applied, 40 SD rats were randomly divided into the quiet control group C (n = 8) and heat-treated group H (n = 32), then the heat treatment group would be divided into immediately group (H1), 24-hour post-heat treatment group (H2), 48 -hour post-heat treatment group (H3) and six days post-heat treatment group (H4) (n = 8).

RESULTS

After heat treatment, the Ca(2+)-ATP activity in rat skeletal muscle sarcoplasmic reticulum in H2 group reached the highest value compared with that in the quiet control group C (P < 0.01), and the value in H1 group showed significant differences compared with control group C (P < 0.05); The Ca(2+)-ATP activity in mitochondrial had the highest value in H1 group, compared with the quiet control group C (P < 0.05), while the Ca2+ concentration in rat skeletal muscle sarcoplasmic reticulum had the highest in group H2, followed by H1 group, both showing significant difference compared with the quiet control group (P < 0.05); The Ca2+ concentration in mitochondrial was high in H1 and H2 group than that of the quiet control group C, and the value in H3 and H4 group was lower than that of the quiet control group C, which had no difference; After heat treatment, the expression of stress proteins of CRT from H1, H2 and H3 group in rat skeletal muscle increased significantly compared with quiet group C.

CONCLUSION

In the process of increased heat treatment, calreticulin played the regulatory role on the imbalance of calcium homeostasis in skeletal muscle cells, and the adaptation protection from the thermal stimulation could have the very good effect on muscle.

Aberrant calreticulin expression is involved in the dedifferentiation of dedifferentiated liposarcoma

Liposarcomas are a representative group of soft tissue sarcomas with variably hampered adipogenesis, which is most exemplified by its dedifferentiated subtype. However, the factor(s) responsible for inhibiting adipocyte differentiation remains unknown. A recent gene expression profiling study identified several unique genes that were highly expressed in dedifferentiated liposarcoma, and the gene encoding calreticulin (CALR), a major Ca(2+)-buffering protein that can inhibit adipocyte differentiation, was found to be overexpressed. Thus, we investigated the expression of calreticulin in 45 cases of liposarcomas, including 15 dedifferentiated tumors, at both the protein and mRNA levels. Immunohistochemically, calreticulin was consistently expressed in the dedifferentiated areas of dedifferentiated liposarcomas and commonly observed in atypical stromal cells and/or lipoblasts in the well-differentiated areas (87%), whereas large vacuolated adipocytic cells in either the tumors or normal fat were essentially negative. These results were further supported by the findings of Western blot and quantitative RT-PCR analyses. Although abnormalities in 19p13.1-13.2 where CALR is localized were uncommon in the dedifferentiated liposarcomas examined by fluorescence in situ hybridization, expression of miR-1257, a putative microRNA that targets calreticulin, was suppressed in the dedifferentiated subtype. The down-regulation of calreticulin by small-interfering RNA could induce adipogenesis in dedifferentiated liposarcoma cells and reduce cell proliferation. Our results therefore suggest that aberrantly expressed calreticulin in dedifferentiated liposarcoma is involved in its dedifferenitation and/or tumor progression.

Programmed cell removal: a new obstacle in the road to developing cancer

The development of cancer involves mechanisms by which aberrant cells overcome normal regulatory pathways that limit their numbers and their migration. The evasion of programmed cell death is one of several key early events that need to be overcome in the progression from normal cellular homeostasis to malignant transformation. Recently, we provided evidence in mouse and human cancers that successful cancer clones must also overcome programmed cell removal. In this Opinion article, we explore the role of programmed cell removal in both normal and neoplastic cells, and we place this pathway in the context of the initiation of programmed cell death.

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.

Higher plant calreticulins have acquired specialized functions in Arabidopsis

Background

Calreticulin (CRT) is a ubiquitous ER protein involved in multiple cellular processes in animals, such as protein folding and calcium homeostasis. Like in animals, plants have evolved divergent CRTs, but their physiological functions are less understood. Arabidopsis contains three CRT proteins, where the two CRTs AtCRT1a and CRT1b represent one subgroup, and AtCRT3 a divergent member.

Methodology/Principal Findings

Through expression of single Arabidopsis family members in CRT-deficient mouse fibroblasts we show that both subgroups have retained basic CRT functions, including ER Ca²⁺-holding potential and putative chaperone capabilities. However, other more general cellular defects due to the absence of CRT in the fibroblasts, such as cell adhesion deficiencies, were not fully restored. Furthermore, in planta expression, protein localization and mutant analyses revealed that the three Arabidopsis CRTs have acquired specialized functions. The AtCRT1a and CRT1b family members appear to be components of a general ER chaperone network. In contrast, and as recently shown, AtCRT3 is associated with immune responses, and is essential for responsiveness to the bacterial Pathogen-Associated Molecular Pattern (PAMP) elf18, derived from elongation factor (EF)-Tu. Whereas constitutively expressed AtCRT1a fully complemented Atcrt1b mutants, AtCRT3 did not.

Conclusions/Significance

We conclude that the physiological functions of the two CRT subgroups in Arabidopsis have diverged, resulting in a role for AtCRT3 in PAMP associated responses, and possibly more general chaperone functions for AtCRT1a and CRT1b.

Characterization of calreticulin expression in mouse endometrium during embryo implantation

Calreticulin (CRT), a Ca(2+)-binding storage protein and chaperone in the endoplasmic reticulum, modulates cell adhesiveness and integrin-dependent Ca(2+) signaling. However, the role of CRT during implantation remains poorly understood. In the present study, we characterized the expression of CRT mRNA and the protein in mouse endometria from pregnancy DI to D7. Real-Time PCR and in situ hybridization results showed that the levels of CRT mRNA in the endometria of pregnant mice were significantly higher than those of non-pregnant mice (P<0.05), and increased gradually from pregnancy DI to D4, reaching the máximum level on D4, followed by a plateau from D4 to D7. Using immunofluorescence histochemistry and western blot, changes of CRT expression in the endometria of pregnant mice were consistent with the expression of CRT mRNA. Furthermore, antisense CRT oligodeoxynucleotide was injected into the uterus horns of pregnant mice (D3) to investígate its effect on embryo implantation. The result showed that the number of implanted embryos markedly decreased in the side of uterine horns receiving antisense CRT oligodeoxynucleotide(í(>)<0.05). These findings suggest that CRT may play an important role in embryo implantation in mice.

Calreticulin-independent regulation of the platelet integrin αIIbβ3by the KVGFFKR αIIb-cytoplasmic motif

The platelet integrin alphaIIbbeta3 alters conformation in response to platelet activation and ligand binding, although the molecular mechanisms involved are not known. We previously showed that a lipid modified peptide, corresponding to the membrane proximal 989KVGFFKR995 portion of the alphaIIb cytoplasmic tail, independently activates platelet alphaIIbbeta3. Calreticulin (CRT) is a potential integrin regulatory protein based on its interaction with the highly conserved alpha-integrin sequence KxGFFKR. We therefore examined the possible interaction of calreticulin and alphaIIbbeta3 in human platelets. We demonstrate that calreticulin in platelets is localised to the granulomere. In contrast, the known integrin-binding protein talin accumulates at the periphery of spreading platelets and colocalises with alphaIIbbeta3 during the process of adhesion. An interaction between calreticulin and alphaIIbbeta3 could not be demonstrated using co-immunoprecipitation techniques under various platelet activation states, even in the presence of covalent chemical crosslinkers. Thus, calreticulin does not functionally interact with the major integrin in human platelets. In order to identify proteins that interact with the integrin KVGFFKR motif we then used a peptide 'pull-down' assay from platelet lysates with biotinylated peptides and demonstrate that only the alphaIIb and beta3 subunits selectively and individually interact with this sequence. This interaction is divalent cation-dependent, has high-affinity, and occurs both with purified alphaIIbbeta3 complex and with electroeluted alpha and beta subunits. Thus, our data show that the conserved integrin KVGFFKR domain interacts primarily with the alpha and beta cytoplasmic tails and not with CRT in human platelets.

Calreticulin: conserved protein and diverse functions in plants

Calreticulin (CRT) is a key Ca2+-binding protein mainly resident in the endoplasmic reticulum (ER), which is highly conserved and extensively expressed in all eukaryotic organisms investigated. The protein plays important roles in a variety of cellular processes including Ca2+ signaling and protein folding. Although calreticulin has been well characterized in mammalian systems, increased investigations have demonstrated that plant CRTs have a number of specific properties different from their animal counterparts. Recent developments on plant CRTs have highlighted the significance of CRTs in plants growth and development as well as biotic and abiotic stress responses. There are at least two distinct groups of calreticulin isoforms in higher plants. Glycosylation of CRT was uniquely observed in plants. In this article, we will describe our current understanding of plant calreticulin gene family, protein structure, cellular localization, and diverse functions in plants. We also discuss the prospects of using this information for genetic improvements of crop plants.

Calreticulin: conserved protein and diverse functions in plants

Calreticulin (CRT) is a key Ca2+-binding protein mainly resident in the endoplasmic reticulum (ER), which is highly conserved and extensively expressed in all eukaryotic organisms investigated. The protein plays important roles in a variety of cellular processes including Ca2+ signaling and protein folding. Although calreticulin has been well characterized in mammalian systems, increased investigations have demonstrated that plant CRTs have a number of specific properties different from their animal counterparts. Recent developments on plant CRTs have highlighted the significance of CRTs in plants growth and development as well as biotic and abiotic stress responses. There are at least two distinct groups of calreticulin isoforms in higher plants. Glycosylation of CRT was uniquely observed in plants. In this article, we will describe our current understanding of plant calreticulin gene family, protein structure, cellular localization, and diverse functions in plants. We also discuss the prospects of using this information for genetic improvements of crop plants.

Subversion of complement by hematophagous parasites

The complement system is a crucial part of innate and adaptive immunity which exerts a significant evolutionary pressure on pathogens. It has selected for those pathogens, mainly microorganisms but also parasites, that have evolved countermeasures. The characterization of how pathogens evade complement attack is a rapidly developing field of current research. In recent years, multiple complement evasion strategies have been characterized. In this review, we focus on complement escape mechanisms expressed by hematophagous parasites, a heterogeneous group of metazoan parasites that share the property of ingesting the whole blood of their host. Complement inhibition is crucial for parasite survival within the host tissue or to facilitate blood feeding. Finally, complement inhibition by hematophagous parasites may also contribute to their success as pathogen vectors.

Functional characterization of Arabidopsis calreticulin1a: a key alleviator of endoplasmic reticulum stress

The chaperone calreticulin plays important roles in a variety of processes in the endoplasmic reticulum (ER) of animal cells, such as Ca2+ signaling and protein folding. Although the functions of calreticulin are well characterized in animals, only indirect evidence is available for plants. To increase our understanding of plant calreticulins we introduced one of the Arabidopsis isoforms, AtCRT1a, into calreticulin-deficient (crt-/-) mouse embryonic fibroblasts. As a result of calreticulin deficiency, the mouse crt-/- fibroblasts have decreased levels of Ca2+ in the ER and impaired protein folding abilities. Expression of the AtCRT1a in mouse crt-/- fibroblasts rescued these phenotypes, i.e. AtCRT1a restored the Ca2+-holding capacity and chaperone functions in the ER of the mouse crt-/- fibroblasts, demonstrating that the animal sorting machinery was also functional for a plant protein, and that basic calreticulin functions are conserved across the Kingdoms. Expression analyses using a beta-glucuronidase (GUS)-AtCRT1a promoter construct revealed high expression of CRT1a in root tips, floral tissues and in association with vascular bundles. To assess the impact of AtCRT1a in planta, we generated Atcrt1a mutant plants. The Atcrt1a mutants exhibited increased sensitivity to the drug tunicamycin, an inducer of the unfolded protein response. We therefore conclude that AtCRT1a is an alleviator of the tunicamycin-induced unfolded protein response, and propose that the use of the mouse crt-/- fibroblasts as a calreticulin expression system may prove useful to assess functionalities of calreticulins from different species.

CALRETICULIN DOWNREGULATION IS ASSOCIATED WITH FGF-2-INDUCED ANGIOGENESIS THROUGH CALCINEURIN PATHWAY IN ISCHEMIC MYOCARDIUM

Fibroblast growth factor 2 (FGF-2) plays an integral role in therapeutic angiogenesis associated with myocardial infarct healing. Calcium (Ca(2+)) is one of the most universal important signaling molecules that affect cell proliferation and angiogenesis. Calreticulin (CRT), a 46-kd (Ca(2+)) -binding chaperone found mainly in the endoplasmic reticulum, plays an important role in regulating calcium homeostasis. The role of CRT in FGF-2-induced angiogenesis and its signaling pathways in ischemic myocardium are not clear. For this study, two-dimensional gel electrophoresis and matrix-assisted laser desorption ionization mass spectrometry were used to analyze CRT's differential expression in myocardial microvascular endothelial cells treated with or without FGF-2. Western blotting analysis was used to detect the expression of CRT and calcineurin (CaN) in sham-operated, FGF-2-, or saline intramyocardially injected myocardium. It is found that FGF-2 induced angiogenesis after sustained ischemia with downregulation of CRT expression and upregulation of CaN expression in myocardium. The CRT expression was negatively correlated to angiogenesis. Furthermore, overexpression of CRT or inhibition of CaN with cyclosporine A abolishes FGF-2-induced microvascular endothelial cells proliferation and CaN expression. The results indicate that intramyocardial administration of FGF-2 decreases myocardial CRT expression in parallel with myocardial angiogenesis in ischemic myocardium. The study further indicates that Ca(2+)/CaN signaling pathway may be involved in CRT-related angiogenesis.

Calreticulin, a molecular chaperone in the endoplasmic reticulum, modulates radiosensitivity of human glioblastoma U251MG cells

Radiotherapy is the primary and most important adjuvant therapy for malignant gliomas. Although the mechanism of radiation resistance in gliomas has been studied for decades, it is still not clear how the resistance is related with functions of molecular chaperones in the endoplasmic reticulum. Calreticulin (CRT) is a Ca(2+)-binding molecular chaperone in the endoplasmic reticulum. Recently, it was reported that changes in intracellular Ca(2+) homeostasis play a role in the modulation of apoptosis. In the present study, we found that the level of CRT was higher in neuroglioma H4 cells than in glioblastoma cells (U251MG and T98G), and was well correlated with the sensitivity to gamma-irradiation. To examine the role of CRT in the radiosensitivity of malignant gliomas, the CRT gene was introduced into U251MG cells, which express low levels of CRT, and the effect of overexpression of CRT on the radiosensitivity was examined. The cells transfected with the CRT gene exhibited enhanced radiation-induced apoptosis compared with untransfected control cells. In CRT-overexpressing cells, cell survival signaling via Akt was markedly suppressed. Furthermore, the gene expression of protein phosphatase 2Ac alpha (PP2Ac alpha), which is responsible for the dephosphorylation and inactivation of Akt, was up-regulated in CRT-overexpressing cells, and the regulation was dependent on Ca(2+). Thus, overexpression of CRT modulates radiation-induced apoptosis by suppressing Akt signaling through the up-regulation of PP2Ac alpha expression via altered Ca(2+) homeostasis. These results show the novel mechanism by which CRT is involved in the regulation of radiosensitivity and radiation-induced apoptosis in malignant glioma cells.

[Calreticulin is involved in ischemic postconditioning-induced attenuation of ischemia/reperfusion injury in rat skeletal muscle]

The present study was aimed to investigate the effect of ischemic postconditioning (I-postC) on ischemia/reperfusion (I/R) injury and whether calreticulin (CRT) is involved in its intracellular signal transduction both in vivo and in cultured skeletal muscle cells. I/R injury in the right hind limb of healthy male Wistar rats was induced by clamping the right femoral artery, and the rats were randomly divided into 3 groups (n=16): I/R group (4-hour ischemia/12- or 24-hour reperfusion), ischemic preconditioning (IPC) group (3 cycles of 1-minute ischemia/1-minute reperfusion) and I-postC group (3 cycles of 5-minute reperfusion/5-minute ischemia). The left hind limb was used as control. Lactate dehydrogenase (LDH) activity in blood plasma, wet/dry weight ratio (W/D) and ultramicrostructure of skeletal muscle were detected 12 h or 24 h after reperfusion. Cultured skeletal muscle cells from neonatal Sprague-Dawley (SD) rat were divided into 6 groups: hypoxia/reoxygenation (H/R) group, hypoxic postconditioning (H-postC) group, hypoxic preconditioning (HPC) group, cyclosporine A (CsA) + H-postC group, CsA + H/R group and control group. H/R was produced by 2-hour hypoxia/24-hour reoxygenation. The survival rate and apoptotic rate of skeletal muscle cells in each group were measured. Western blot was used to detect the expressions of CRT and calcineurin (CaN). The results were as follows: (1) During in vivo experiment, compared with I/R, I-postC significantly decreased LDH activity and W/D, attenuated the ultramicrostructure injury of skeletal muscle and the apoptosis of nucleolus. 12 h and 24 h after reperfusion, compared with that in I/R group, the expression of CRT in I-postC group increased by 439% and 102%, respectively (P<0.05), and the expression of CaN increased by 196% and 63%, respectively (P<0.05). Correlation analysis indicated a positive correlation between CRT and CaN expressions (r=0.865, P<0.01). (2) In cultured skeletal muscle cells, H-postC attenuated cell injury induced by H/R. Compared with those in H/R group, CRT and CaN expressions in H-postC increased by 31.8% (P<0.05) and 6.02%, respectively. The protection of H-postC and CaN up-regulation were eliminated when CsA, the inhibitor of CaN, was added before H-postC. Both in vivo and in vitro results indicate that I-postC, similar as IPC, can protect the skeletal muscle against I/R injury, and its effects may be mediated by CRT and CaN up-regulation. The inhibition of CaN expression may also attenuate the protective effects of I-postC.

Analysis of the suitability of calreticulin inducible HEK cells for adhesion studies: microscopical and biochemical comparisons

Calreticulin is a Ca(2+)-buffering ER chaperone that also modulates cell adhesiveness. In order to study the effect of calreticulin on the expression of adhesion-related genes, we created a calreticulin inducible Human Embryonic Kidney (HEK) 293 cell line. We found that fibronectin mRNA and both intra- and extra-cellular fibronectin protein levels increased following calreticulin induction. However, despite this increase in fibronectin, HEK293 cells did not assemble an extracellular fibrillar fibronectin matrix regardless of the level of calreticulin expression. Furthermore, HEK293 cells exhibited a poorly organized actin cytoskeleton, did not have clustered fibronectin receptors at the cell surface, and did not form focal contacts. This likely accounts for the lack of fibronectin matrix deposition by these cells regardless of calreticulin expression level. Vinculin abundance did not appreciably increase upon calreticulin induction and the level of active c-Src, a regulatory kinase of focal contacts, was found to be abundant and unregulated by calreticulin induction in these cells. The inability to form stable focal contacts and to commence fibronectin fibrillogenesis due to high c-Src activity may be responsible for the poor adhesive phenotype of HEK 293 cells. Thus, we show here that HEK293 cells are not suitable for microscopical studies of cell-substratum adhesions, but are best suited for biochemical studies.

Molecular mechanisms involved in intestinal iron absorption

Iron is an essential trace metal in the human diet due to its obligate role in a number of metabolic processes. In the diet, iron is present in a number of different forms, generally described as haem (from haemoglobin and myoglobin in animal tissue) and non-haem iron (including ferric oxides and salts, ferritin and lactoferrin). This review describes the molecular mechanisms that co-ordinate the absorption of iron from the diet and its release into the circulation. While many components of the iron transport pathway have been elucidated, a number of key issues still remain to be resolved. Future work in this area will provide a clearer picture regarding the transcellular flux of iron and its regulation by dietary and humoral factors.

Adiponectin modulates inflammatory reactions via calreticulin receptor-dependent clearance of early apoptotic bodies

Obesity and type 2 diabetes are associated with chronic inflammation. Adiponectin is an adipocyte-derived hormone with antidiabetic and antiinflammatory actions. Here, we demonstrate what we believe to be a previously undocumented activity of adiponectin, facilitating the uptake of early apoptotic cells by macrophages, an essential feature of immune system function. Adiponectin-deficient (APN-KO) mice were impaired in their ability to clear apoptotic thymocytes in response to dexamethasone treatment, and these animals displayed a reduced ability to clear early apoptotic cells that were injected into their intraperitoneal cavities. Conversely, adiponectin administration promoted the clearance of apoptotic cells by macrophages in both APN-KO and wild-type mice. Adiponectin overexpression also promoted apoptotic cell clearance and reduced features of autoimmunity in lpr mice whereas adiponectin deficiency in lpr mice led to a further reduction in apoptotic cell clearance, which was accompanied by exacerbated systemic inflammation. Adiponectin was capable of opsonizing apoptotic cells, and phagocytosis of cell corpses was mediated by the binding of adiponectin to calreticulin on the macrophage cell surface. We propose that adiponectin protects the organism from systemic inflammation by promoting the clearance of early apoptotic cells by macrophages through a receptor-dependent pathway involving calreticulin.

Identification of calreticulin as a marker for phagocytosis of apoptotic cells in Drosophila

Apoptotic cell phagocytosis is initiated through the specific interaction between markers for phagocytosis present at the surface of targets and their receptors of phagocytes. Although many molecules have been proposed to be phagocytosis markers and receptors in mammals, information as to the identity of those molecules is limited for invertebrate animals. Calreticulin, a molecular chaperone that functions in the lumen of the endoplasmic reticulum, was recently reported to be the second general marker, the membrane phospholipid phosphatidylserine being the first, for mammalian apoptotic cells to be recognized by phagocytes. We here asked whether or not calreticulin serves as a marker for phagocytosis in Drosophila. Phagocytosis of apoptotic S2 cells by Drosophila hemocyte-derived l(2)mbn cells, which we previously showed to occur independent of phosphatidylserine, was inhibited by the addition of anti-calreticulin antibody. This inhibition was observed when the target cells, but not phagocytes, were pre-incubated with the antibody. In addition, RNA interference-mediated reduction of calreticulin expression in apoptotic S2 cells, but not in l(2)mbn cells, reduced the level of phagocytosis. An immunocytochemical analysis revealed that calreticulin is widely distributed at the surface of viable S2 cells. After the induction of apoptosis, cell surface calreticulin seemed to form aggregates, with no change in its amount. Furthermore, in embryos of a mutant Drosophila strain that expresses calreticulin at a reduced level, the level of phagocytosis of apoptotic cells was about a half of that observed in embryos of a wild-type strain. These results collectively indicate that calreticulin is the first molecule to be identified as a marker for phagocytosis of apoptotic cells by Drosophila phagocytes.

Calreticulin as a new histone binding protein in mitotic chromosomes

Calreticulin (CRT) is a multifunctional Ca(2+)-binding protein that mainly functions in the endoplasmic reticulum as a molecular chaperone for newly synthesized proteins. Recently we reported the protein composition of human metaphase chromosomes (Uchiyama et al., 2004), which included CRT. Here we describe new characteristics of CRT in vitro as well as its localization on the surface of metaphase chromosomes in vivo. CRT was detected in the chromosomal fraction by Western blotting and its binding partners were identified as core and linker histones by ligand overlay assay. Surface plasmon resonance sensor analyses revealed that CRT is bound to chromatin fibers. Moreover, we found that CRT has both supercoiling activity, which assists core histone assembly into chromatin fibers, and binding ability to histone H2A/H2B dimers and histone H3/H4 tetramers. Unlike the chromosome scaffold proteins, indirect immunofluorescent staining revealed that CRT is located on the surface of metaphase chromosomes. These results suggest that CRT plays a role which involves chromatin dynamics on the surface of mitotic chromosomes.

Overview of the role for calreticulin in the enhancement of wound healing through multiple biological effects

Calreticulin (CRT), an intracellular chaperone protein crucial for the proper folding and transport of proteins through the endoplasmic reticulum, has more recent acclaim as a critical regulator of extracellular functions, particularly in mediating cellular migration and as a requirement for phagocytosis of apoptotic cells. Consistent with these functions, we show that the topical application of CRT has profound effects on the process of wound healing by causing a dose-dependent increase in epithelial migration and granulation tissue formation in both murine and porcine normal and impaired animal models of skin injury. These effects of CRTare substantiated, in vitro, as we show that CRT strongly induces cell migration/wound closure of human keratinocytes and fibroblasts, using a wound/scratch plate assay, and stimulates cellular proliferation of human keratinocytes, fibroblasts, and vascular endothelial cells, providing mechanistic insight into how CRT functions in repair. Similarly, in both animal models, the histology of the wounds show marked proliferation of basal keratinocytes and dermal fibroblasts, dense cellularity of the dermis with notably increased numbers of macrophages and well-organized collagen fibril deposition. Thus, CRT profoundly affects the wound healing process by recruiting cells essential for repair into the wound, stimulating cell growth, and increasing extracellular matrix production.

Complement proteins C1q and MBL are pattern recognition molecules that signal immediate and long-term protective immune functions

C1q and mannose binding lectin, members of the "defense collagen" family, are pattern recognition molecules that can trigger rapid enhanced phagocytosis resulting in efficient containment of pathogens or clearance of cellular debris, apoptotic cells and immune complexes. In addition, interaction of C1q and mannose binding lectin with the phagocyte alters subsequent phagocyte cytokine synthesis, and thus may have important implications in directing acute inflammation as well as long-term protective immunity. The importance of the role of defense collagens in phagocytosis of apoptotic cells is highlighted by studies in vivo of mice deficient in C1q, pulmonary surfactant D and mannose binding lectin in which there is delayed clearance of apoptotic cells. Indeed, deficiency of C1q is a risk factor for the development of autoimmunity in both humans and mice, consistent with the hypothesis that inefficient clearance of apoptotic cells results in release of autoantigens and contributes to the pathology associated with autoimmune diseases such as systemic lupus erythematosus. Further understanding of the importance of C1q and mannose binding lectin in the clearance of apoptotic cells and regulation of cytokine synthesis and identification of the receptors implicated in mediating these processes should provide novel targets for therapeutic intervention in the control and manipulation of the immune response in terms of both host defense against infectious disease and tissue repair and remodeling.

Impaired recognition of apoptotic neutrophils by the C1q/calreticulin and CD91 pathway in systemic lupus erythematosus

OBJECTIVE

A deficiency in a subcomponent of C1q can result in increased susceptibility to autoimmune diseases such as systemic lupus erythematosus (SLE). The monocyte endocytic receptor CD91 is implicated in the endocytosis of apoptotic neutrophils via interactions with C1q and calreticulin. In this clinical study, we studied the binding of C1q to leukocytes and determined whether C1q bound specifically to calreticulin and CD91 on cells undergoing apoptosis in SLE.

METHODS

Proximal antibody phage display, calreticulin-transfected cells, and immunocytochemical and confocal techniques were used in a comprehensive analysis of direct binding of C1q to apoptotic neutrophils that were obtained from healthy individuals and from patients with SLE. In addition, apoptotic cellular systems were assessed in vitro.

RESULTS

C1q appeared to colocalize to apoptotic blebs on the surface of leukocytes in association with both calreticulin and CD91, as determined by phage display and transfected cell studies. However, C1q did not bind to apoptotic cells isolated from SLE patients, despite the positivity of the cells for both calreticulin and CD91. Surface expression of calreticulin decreased on neutrophils as they aged, but increased on monocytes. In an apoptotic phagocytic assay, the addition of C1q and calreticulin significantly enhanced the phagocytosis of apoptotic cell debris by monocyte-derived cells.

CONCLUSION

These observations indicate that neutrophils from SLE patients have a reduced ability to be recognized and removed by the C1q/calreticulin/CD91-mediated apoptotic pathway, despite the presence of main apoptotic recognition partners. This suggests that an additional component, as yet unidentified, acts as a C1q binding partner on apoptotic cells, and this component may be lacking in cells isolated from SLE patients.

Alternative roles for CD59

CD59 was first identified as a regulator of the terminal pathway of complement, which acts by binding to the C8/C9 components of the assembling membrane attack complex (MAC), to inhibit formation of the lytic pore. Structurally, CD59 is a small, highly glycosylated, GPI-linked protein, with a wide expression profile. Functionally, the role of CD59 in complement regulation is well-defined but studies have also shown clear evidence for signalling properties, which are linked to its glycophosphatidyl inositol (GPI) anchor and its location within lipid rafts. Cross-linking of CD59 using specific monoclonal antibodies drives both calcium release and activation of lipid-raft associated signalling molecules such as tyrosine kinases. These observations clearly show that CD59 exhibits roles independent of its function as a complement inhibitor. In this review, we examine the progression of research in this area and explore the alternative functions of CD59 that have recently been defined.

[Research on physiological and pathophysiological functions of calreticulin]

Calreticulin (CRT) is an essential Ca2+-binding /storage chaperone resident protein of endoplasmic reticulum or sarcoplasmic reticulum found across a diverse range of species. The protein is involved in the regulation of intracellular Ca2+ homeostasis and endoplasmic reticulum Ca2+ storage capacity, and is also an important molecular chaperone involved in "quality control" within secretory pathways. Playing an important role in apoptosis, cell adhesion, gene expression and autoimmunity, calreticulin is involved in the genesis, development and prognosis of many diseases. Now we summarized the physiological function of CRT and its expression changes during myocardial hypertrophy, heart failure, angiogenesis, stresses and many other pathological conditions.

Calreticulin induces delayed cardioprotection through mitogen-activated protein kinases

Hypoxic preconditioning (HPC) attenuates tissue injury caused by ischemia/reperfusion. The protective mechanisms of HPC involve up-regulation of the protective proteins and mitigation of cellular calcium overload. Calreticulin (CRT), a Ca(2+)-binding chaperone, plays an important role in regulating calcium homeostasis and folding of proteins. The role of CRT in cardioprotection of HPC and the pathways determining CRT expression during HPC are not clear. In this work, 2-DE and MALDI-MS were employed to analyze CRT differential expression in cardiomyocytes subjected to transient hypoxia. Western blotting analysis was used to detect the CRT expression and activities of p38 mitogen-activated protein kinase (p38 MAPK) and c-Jun NH(2)-terminal kinase (JNK) in myocardium subjected to ischemia with and without HPC and sham operation. The hearts from HPC group were more resistant to sustained ischemia and had much stronger phosphorylation of p38 MAPK, with a reduced phosphorylation of JNK, than controls. The CRT expression was positively correlated with the phosphorylation of p38 MAPK and negatively correlated with the level of JNK phosphorylation. Furthermore, inhibition of the p38 MAPK with SB202190 abolished, while inhibition of the JNK with SP600125 enhanced the CRT up-regulation in cardiomyocytes induced by HPC. The results indicate that HPC up-regulates CRT expression through the MAPK signaling pathways.

Reaction diffusion modeling of calcium dynamics with realistic ER geometry

We describe a finite-element model of mast cell calcium dynamics that incorporates the endoplasmic reticulum's complex geometry. The model is built upon a three-dimensional reconstruction of the endoplasmic reticulum (ER) from an electron tomographic tilt series. Tetrahedral meshes provide volumetric representations of the ER lumen, ER membrane, cytoplasm, and plasma membrane. The reaction-diffusion model simultaneously tracks changes in cytoplasmic and ER intraluminal calcium concentrations and includes luminal and cytoplasmic protein buffers. Transport fluxes via PMCA, SERCA, ER leakage, and Type II IP3 receptors are also represented. Unique features of the model include stochastic behavior of IP3 receptor calcium channels and comparisons of channel open times when diffusely distributed or aggregated in clusters on the ER surface. Simulations show that IP3R channels in close proximity modulate activity of their neighbors through local Ca2+ feedback effects. Cytoplasmic calcium levels rise higher, and ER luminal calcium concentrations drop lower, after IP3-mediated release from receptors in the diffuse configuration. Simulation results also suggest that the buffering capacity of the ER, and not restricted diffusion, is the predominant factor influencing average luminal calcium concentrations.

Regulation of peripheral T cell activation by calreticulin

Regulated expression of positive and negative regulatory factors controls the extent and duration of T cell adaptive immune response preserving the organism's integrity. Calreticulin (CRT) is a major Ca2+ buffering chaperone in the lumen of the endoplasmic reticulum. Here we investigated the impact of CRT deficiency on T cell function in immunodeficient mice reconstituted with fetal liver crt-/- hemopoietic progenitors. These chimeric mice displayed severe immunopathological traits, which correlated with a lower threshold of T cell receptor (TCR) activation and exaggerated peripheral T cell response to antigen with enhanced secretion of inflammatory cytokines. In crt-/- T cells TCR stimulation induced pulsatile cytosolic elevations of Ca2+ concentration and protracted accumulation of nuclear factor of activated T cells in the nucleus as well as sustained activation of the mitogen-activated protein kinase pathways. These observations support the hypothesis that CRT-dependent shaping of Ca2+ signaling critically contributes to the modulation of the T cell adaptive immune response.

Quality control system of the endoplasmic reticulum and related diseases

The quality control (QC) system of the endoplasmic reticulum (ER) is an important monitoring mechanism in the protein maturation process, which ensures export of properly folded proteins from the ER. Incorrectly or incompletely folded proteins are retained in the ER for refolding or degradation by the ER-residing proteasome. The calnexin/calreticulin cycle and ER-associated degradation are the key elements in QC. These two mechanisms work together to allow incorrectly folded proteins have additional opportunities to achieve their native conformations. The QC dysfunction is involved in many diseases caused by mutant proteins, many of which are causes of neurodegenerative disorders. A better understanding of molecular regulation in the QC system will uncover the molecular pathogenic mechanisms of many diseases caused by protein misfolding and help discover novel strategies for preventing or treating these diseases.

Differential CD91 dependence for calreticulin and Pseudomonas exotoxin-A endocytosis

Calreticulin (CRT) interaction with cell-surface receptors is integral to its function in escorting associated peptides into the antigen-presenting cell (APC) antigen presentation pathway. Additionally, extracellular CRT is proposed to be required for lung APC interaction with collectins. In both cases, CD91 has been proposed to act as the APC cell-surface receptor requisite for mediating these processes. However, the evidence for a CRT interaction with CD91 is indirect, predicated on partial competition of cellular binding by gp96, of which CD91 has been proposed as the unique endocytic receptor, and by the CD91 ligand alpha2-macroglobulin. Here, we directly investigate the function of CD91 in binding and trafficking CRT. We find that the ability of CRT to interact with APC does not correlate with cellular CD91 expression or function. Additionally, in the first genetic test of CD91 function regarding CRT, CD91 expression neither conferred CRT association nor did CD91-deficient (CD91-/-) and CD91-expressing cells differ in their ability to traffic CRT. Finally, cellular CRT trafficking did not parallel that of Pseudomonas exotoxin-A, an obligate CD91 ligand, by the criteria of CD91 dependence, cell-type specificity and endocytic itinerary. These data identify that CRT trafficking is not, as previously hypothesized, CD91 dependent and indicate usage of alternative cellular trafficking pathways.

Functional and phenotypic relevance of differentially expressed proteins in calcineurin mutants ofCaenorhabditis elegans

Calcineurin is a heterodimeric serine/threonine protein phosphatase, important for many cellular processes such as T-cell regulation, cardiac hypertrophy and kidney development. We previously reported the characterization of Caenorhabditis elegans calcineurin mutants as providing a simple but excellent genetic model system for studying in vivo functions of calcineurin. Calcineurin loss-of-function mutants, cnb-1(lf), and gain-of-function mutants, tax-6(gf), show certain opposite phenotypes as well as some similar phenotypes. In order to explain the phenotypic similarity observed in both loss-of-function and gain-of-function mutants, we examined the proteins that followed similar trends in both mutants relative to wild-type worms by using 2-DE. Interestingly, VHA-13, HSP-6 and phosphoenolpyruvate carboxykinase are down-regulated in both mutants. A total of 96 differentially regulated proteins were identified by MALDI-TOF/MS. Among these, 42 proteins are up-regulated and 54 proteins are down-regulated in calcineurin mutants. Furthermore, knock-down of about 30% of the genes, which are down-regulated in calcineurin mutants, showed some of the phenotypes of calcineurin-null mutants. This analysis suggests the functional relevance of these proteins to calcineurin activity in C. elegans.

Assembly of MHC Class I Molecules within the Endoplasmic Reticulum

MHC class I molecules bind cytosolically derived peptides within the endoplasmic reticulum (ER) and present them at the cell surface to cytotoxic T cells. A major focus of our laboratory has been to understand the functions of the diverse proteins involved in the intracellular assembly of MHC class I molecules. These include the molecular chaperones calnexin and calreticulin, which enhance the proper folding and subunit assembly of class I molecules and also retain assembly intermediates within the ER; ERp57, a thiol oxidoreductase that promotes heavy chain disulfide formation and proper assembly of the peptide loading complex; tapasin, which recruits class I molecules to the TAP peptide transporter and enhances the loading of high affinity peptide ligands; and Bap31, which is involved in clustering assembled class I molecules at ER exit sites for export along the secretory pathway. This review describes our contributions to elucidating the functions of these proteins; the combined effort of many dedicated students and postdoctoral fellows.

Calreticulin Destabilizes Glucose Transporter-1 mRNA in Vascular Endothelial and Smooth Muscle Cells Under High-Glucose Conditions

Substrate autoregulation of glucose transporter-1 (GLUT-1) mRNA and protein expression provides vascular endothelial and smooth muscle cells a sensitive mechanism to adapt their rate of glucose transport in response to changing glycemic conditions. Hyperglycemia-induced downregulation of glucose transport is particularly important in protecting these cells against an excessive influx of glucose and consequently increased intracellular protein glycation and generation of free radicals; both are detrimental in the development of vascular disease in diabetes. We aimed to investigate the molecular mechanism of high glucose–induced downregulation of GLUT-1 mRNA expression in primary bovine aortic vascular endothelial (VEC) and smooth muscle (VSMC) cell cultures. Using RNA mobility shift, UV cross-linking, and in vitro degradation assays, followed by mass-spectrometric analysis, we identified calreticulin as a specific destabilizing trans -acting factor that binds to a 10-nucleotide cis -acting element (CAE 2181-2190 ) in the 3′-untranslated region of GLUT-1 mRNA. Pure calreticulin accelerated the rate of GLUT-1 mRNA-probe degradation in vitro, whereas overexpression of calreticulin in vascular cells decreased significantly the total cell content of GLUT-1 mRNA and protein. The expression of calreticulin was augmented in vascular cells exposed to high glucose in comparison with low-glucose conditions. Similarly, increased expression of calreticulin was observed in aortae of diabetic Psammomys obesus in comparison with normoglycemic controls. These data suggest that CAE 2181-2190 –calreticulin complex, which is formed in VSMC and VEC exposed to hyperglycemic conditions, renders GLUT-1 mRNA susceptible to degradation. This interaction underlies the process of downregulation of glucose transport in vascular cells under high-glucose conditions.

In vitro and in vivo assays to assess the functions of calnexin and calreticulin in ER protein folding and quality control

Newly synthesized polypeptides entering the endoplasmic reticulum (ER) encounter a large array of molecular chaperones and folding factors that facilitate proper folding as well as assess folding status, retaining non-native proteins within the ER. Calnexin (CNX), an ER membrane protein, and its soluble homologue, calreticulin (CRT), are two important molecular chaperones that contribute to both processes. They are highly unusual chaperones in that they act as lectins, binding the Asn-linked oligosaccharides of newly synthesized glycoproteins, as well as recognizing the polypeptide segments of glycoproteins. Furthermore, they associate with ERp57, a thiol oxidoreductase, that is thought to enhance the oxidative folding of glycoproteins bound to CNX/CRT. These characteristics of CNX and CRT as well as their mode of action have been elucidated though the use of multiple in vitro and in vivo approaches. This chapter will focus on the description of a number of in vitro assays that have been used to characterize the lectin and ERp57-binding functions of CNX/CRT and also their abilities to act as molecular chaperones to suppress protein aggregation. In addition, we will describe insect and mammalian expression systems in which major histocompatibility complex class I molecules are used as model glycoprotein substrates for CNX and CRT. These systems have been valuable in assessing folding and quality control events in vivo that are influenced by CNX or CRT as well as in characterizing the spectrum of substrates that are recognized by these chaperones.

Effects of calreticulin on viral cell-to-cell movement

Cell-to-cell tobacco mosaic virus movement protein (TMV MP) mediates viral spread between the host cells through plasmodesmata. Although several host factors have been shown to interact with TMV MP, none of them coresides with TMV MP within plasmodesmata. We used affinity purification to isolate a tobacco protein that binds TMV MP and identified it as calreticulin. The interaction between TMV MP and calreticulin was confirmed in vivo and in vitro, and both proteins were shown to share a similar pattern of subcellular localization to plasmodesmata. Elevation of the intracellular levels of calreticulin severely interfered with plasmodesmal targeting of TMV MP, which, instead, was redirected to the microtubular network. Furthermore, in TMV-infected plant tissues overexpressing calreticulin, the inability of TMV MP to reach plasmodesmata substantially impaired cell-to-cell movement of the virus. Collectively, these observations suggest a functional relationship between calreticulin, TMV MP, and viral cell-to-cell movement.

Neuronal toxicity in Caenorhabditis elegans from an editing site mutant in glutamate receptor channels

Ionotropic glutamate receptors (iGluRs) in Caenorhabditis elegans are predicted to have high permeability for Ca2+ because of glutamine (Q) residues in the pore loop. This contrasts to the low Ca2+ permeability of similar iGluRs in principal neurons of mammals, because of an edited arginine (R) at the critical pore position in at least one channel subunit. Here, we introduced the R residue into the pore loop of a glutamate receptor subunit, GLR-2, in C. elegans. GLR-2(R) participated in channel formation, as revealed by decreased rectification of kainate-evoked currents in electrophysiological recordings when GLR-2(R) and the wild-type GLR-2(Q) were coexpressed in worms. Notably, the transgenic worms exhibited, at low penetrance, strong phenotypic impairments including uncoordination, neuronal degeneration, developmental arrest, and lethality. Penetrance of adverse phenotypes could be enhanced by transgenic expression of an optimal GLR-2(Q)/(R) ratio, implicating channel activity as the cause. In direct support, a mutation in eat-4, which prevents glutamatergic transmission, suppressed adverse phenotypes. Suppression was also achieved by mutation in calreticulin, which is necessary for maintainance of intracellular Ca2+ stores in the endoplasmic reticulum. Thus, synaptically activated GLR-2(R)-containing iGluR channels appear to trigger inappropriate, neurotoxic Ca2+ release from intracellular stores.

CBFB-SMMHC is correlated with increased calreticulin expression and suppresses the granulocytic differentiation factor CEBPA in AML with inv(16)

The pericentric inversion of chromosome 16, inv(16)(p13q22), is associated with acute myeloid leukemia (AML) subtype M4Eo that is characterized by the presence of myelomonocytic blasts and atypical eosinophils. This rearrangement fuses the CBFB and MYH11 genes, with the latter encoding the smooth muscle myosin heavy chain (SMMHC). The myeloid transcription factor CCAAT/enhancer-binding protein alpha (CEBPA) is crucial for normal granulopoiesis. Alterations of structure and expression of CEBPA have been implicated in particular subtypes of AML. Here, we found that conditional expression of core-binding factor beta (CBFB)-SMMHC in U937 cells suppresses CEBPA protein expression and binding activity. However, CEBPA mRNA levels remained unchanged. No differences were detected in CEBPA mRNA levels in patients with inv(16) AML-M4Eo (n = 12) compared to patients with AML with a normal karyotype and M4 subtype (n = 6), whereas CEBPA protein and binding activity were significantly reduced in patients with CBFB-SMMHC. Furthermore, calreticulin, an inhibitor of CEBPA translation, was induced on mRNA and protein level in CBFB-SMMHC patients with AML and after expression of CBFB-SMMHC in the U937-cell system. Inhibition of calreticulin by siRNA restored CEBPA levels. Our results suggest that modulation of CEBPA by calreticulin represents a novel mechanism involved in the differentiation block in CBFB-SMMHC AML.

Surface calreticulin mediates muramyl dipeptide-induced apoptosis in RK13 cells

Calreticulin (CRT) is a binding protein for apoptotic N-acetylmuramyl-L-alanyl-D-isoglutamine (L,D-MDP) or peptidoglycan in RK(13) cells. CRT on RK(13) cell surface (srCRT) forms complex(es) with tumor necrosis factor receptor 1 (TNFR1) and TNFR-associated death domain (TRADD) protein of the cell membrane. CRT polyclonal or monoclonal antibody binding to RK(13) srCRT dose-dependently inhibited L,D-MDP-induced apoptosis. In RK(13) cells, L,D-MDP up-regulated the TNFR1.TRADD complex of the plasma membrane and subsequently induced cytosolic TRADD-Fas-associated death domain protein complex. Biotinylated srCRT was capable of calcium-dependent binding of Sepharose-immobilized L,D-MDP or peptidoglycan. However, Toll-like receptors TLR-2 and TLR-4, Nod2, and CD14 of RK(13) cells did not specifically bind Sepharose-immobilized L,D-MDP. High concentrations (5-40 mm) of EGTA dose-dependently inhibited free L,D-MDP binding to purified RK(13) cell CRT and promoted free L,D-MDP dissociation from RK(13) cell CRT.MDP complex. Different concentrations of EGTA (0-40 mm) added to Dulbecco's modified essential medium with 1.8 mm calcium or phosphate-buffered saline with 0.18 mm calcium have different effects on medium free calcium concentrations but have identical inhibiting effects on L,D-MDP-induced apoptosis. More inhibition of the L,D-MDP-induced apoptotic DNA ladders and caspase-3 activity in RK(13) cells was obtained with EGTA pretreatment (83%) than just EGTA + L,D-MDP (47%). The knocking down of srCRT by antisense oligonucleotide CRTAS121 (250 nmol/ml) and stealth small interfering RNA CRT_siR479 (150 pm/ml) for 2 days (44 and 66%, respectively), resulted in the inhibition of L,D-MDP-induced caspase-3 activity (47 and 65%, respectively). The results suggest that (a) the binding of L,D-MDP to srCRT is calcium-dependent, i.e. on srCRT-bound calcium, and (b) it is srCRT, not TLR-2, TLR-4, Nod2 or CD14, that mediates L,D-MDP-induced RK(13) cell apoptosis through activating the TNFR1. TRADD-Fas-associated death domain protein apoptotic pathway.