Heat shock-induced prompt glycosylation. Identification of P-SG67 as calreticulin

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.

Molecular characterization of calreticulin from Anopheles stephensi midgut cells and functional assay of the recombinant calreticulin with Plasmodium berghei ookinetes

Transmission blocking vaccines (TBVs) that target the antigens on the midgut epithelium of Anopheles mosquitoes are among the promising tools for the elimination of the malaria parasite. Characterization and analysis of effective antigens is the first step to design TBVs. Calreticulin (CRT), a lectin-like protein, from Anopheles albimanus midgut, has shown antigenic features, suggesting a promising and novel TBV target. CRT is a highly conserved protein with similar features in vertebrates and invertebrates including anopheline. We cloned the full-length crt gene from malaria vector, Anopheles stephensi (AsCrt) and explored the interaction of recombinant AsCrt protein, expressed in a prokaryotic system (pGEX-6p-1), with surface proteins of Plasmodium berghei ookinetes by immunofluorescence assay. The cellular localization of AsCrt was determined using the baculovirus expression system. Sequence analysis of the whole cDNA of AsCrt revealed that AsCrt contains an ORF of 1221 bp. The amino acid sequence of AsCrt protein obtained in this study showed 64% homology with similar protein in human. The AsCrt shares the most common features of CRTs from other species. This gene encodes a 406 amino-acid protein with a molecular mass of 46 kDa, which contains a predicted 16 amino-acid signal peptides, conserved cysteine residues, a proline-rich region, and highly acidic C-terminal domain with endoplasmic reticulum retrieval sequence HDEL. The production of GST-AsCrt recombinant protein was confirmed by Western blot analysis using an antibody against the GST protein. The FITC-labeled GST-AsCrt exhibited a significant interaction with P. berghei ookinete surface proteins. Purified recombinant GST-AsCrt, labeled with FITC, displayed specific binding to the surface of P. berghei ookinetes in comparison with control. Moreover, the expression of AsCrt in baculovirus expression system indicated that AsCrt was localized on the surface of Sf9 cells. Our results suggest that AsCrt could be utilized as a potential target for future studies in TBV area for malaria control.

Stimulation of N-Linked Glycosylation and Lipid-Linked Oligosaccharide Synthesis by Stress Responses in Metazoan Cells

Endoplasmic reticulum (ER) stress responses comprising the unfolded protein response (UPR) are activated by conditions that disrupt folding and assembly of proteins inside the ER lumenal compartment. Conditions known to be proximal triggers of the UPR include saturation of chaperones with misfolded protein, redox imbalance, disruption of Ca2+ levels, interference with N-linked glycosylation, and failure to dispose of terminally misfolded proteins. Potentially, ER stress responses can reprogram cells to correct all of these problems and thereby restore ER function to normal. This article will review literature on stimulation of N-linked glycosylation by ER stress responses, focusing on metazoan systems. The mechanisms involved will be contrasted with those mediating stimulation of N-linked glycosylation by cytoplasmic stress responses. This information will interest readers who study the biological roles of stress responses, the functions of N-linked glycans, and potential strategies for treatment of genetic disorders of N-linked glycosylation.

Glycobiology in the cytosol: the bitter side of a sweet world

Progress in glycobiology has undergone explosive growth over the past decade with more of the researchers now realizing the importance of glycan chains in various inter- and intracellular processes. However, there is still an area of glycobiology awaiting exploration. This is especially the case for the field of "glycobiology in the cytosol" which remains rather poorly understood. Yet evidence is accumulating to demonstrate that the glycoconjugates and their recognition molecules (i.e. lectins) are often present in this subcellular compartment.

LDL receptor-related protein 1: unique tissue-specific functions revealed by selective gene knockout studies

The LDL receptor-related protein (originally called LRP, but now referred to as LRP1) is a large endocytic receptor that is widely expressed in several tissues. LRP1 is a member of the LDL receptor family that plays diverse roles in various biological processes including lipoprotein metabolism, degradation of proteases, activation of lysosomal enzymes, and cellular entry of bacterial toxins and viruses. Deletion of the LRP1 gene leads to lethality in mice, revealing a critical, but as of yet, undefined role in development. Tissue-specific gene deletion studies reveal an important contribution of LRP1 in the vasculature, central nervous system, macrophages, and adipocytes. Three important properties of LRP1 dictate its diverse role in physiology: 1) its ability to recognize more than 30 distinct ligands, 2) its ability to bind a large number of cytoplasmic adaptor proteins via determinants located on its cytoplasmic domain in a phosphorylation-specific manner, and 3) its ability to associate with and modulate the activity of other transmembrane receptors such as integrins and receptor tyrosine kinases.

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.

Proteomics-based identification of autoantibodies in the sera of healthy Chinese individuals from Beijing

The identification of panels of tumor antigens that elicit an antibody response may have utility in cancer screening, diagnosis and in establishing prognosis. However, autoantibodies normally exist in sera of healthy individuals and are enormously diversified. To explore the reservoir of autoantibody in healthy population, we performed a proteomics investigation of autoantibody profiles in the sera of 36 healthy Chinese individuals from Beijing, which may provide valuable reference information to the identification of disease-specific autoantibodies. The results showed that autoantibody profiles varied individually, but some autoantibodies were identified at a high frequency in the healthy population. The autoantibodies against alpha-enolase and those against heterogeneous nuclear ribonucleoprotein L were positive in more than 50% of the sera samples. The autoantibodies identified in more than 20% of samples included those against annexin II, F-actin capping protein beta subunit and calreticulin. Some of these autoantibodies have been previously reported to be involved in autoimmune conditions and cancers. Autoantibodies in the healthy population are important as a foundation from which disease-specific autoantibodies can be defined. Thus our report on autoantibodies in healthy individuals may be useful as a reference for defining new autoantibody biomarkers.

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.

Role of calreticulin from parasites in its interaction with vertebrate hosts

Although parasites range from protozoan to complex, evolutionary advanced arthropods, in general, a hallmark of parasite life cycles is their ability to adapt to changes in temperature, pH and host defense strategies. Calreticulin, a calcium-binding protein, highly conserved and multifunctional, is present in every cell of higher organisms, except erythrocytes. The surprising array of calreticulin-associated functions include lectin-like chaperoning, calcium storage and signaling, modulation of gene expression, cell adhesion, enhancement of phagocytosis of C1q or collectin opsonized apoptotic cells, inhibition of angiogenesis and tumoral growth, inhibition of perforin pore formation in T and NK cells, and inhibition of C1q-dependent complement activation. Likewise, calreticulin is present in a wide spectrum of sub cellular compartments. Parasite calreticulin shows a surprisingly high degree of conservation within the framework of its functional domains. Its role within the parasite/host relationship needs to be assessed further, in particular with regard to its impact on parasite infectivity, by helping to evade from its hosts' immune response. With special emphasis on calreticulin from Trypanosoma cruzi, the intracellular protozoan agent of American trypanosomiasis (Chagas' disease), we wish to exemplify and highlight the various implications of parasite calreticulin, within the pathophysiology of parasite-mediated human and animal disease.

Phylogenetic analyses and expression studies reveal two distinct groups of calreticulin isoforms in higher plants

Calreticulin (CRT) is a multifunctional protein mainly localized to the endoplasmic reticulum in eukaryotic cells. Here, we present the first analysis, to our knowledge, of evolutionary diversity and expression profiling among different plant CRT isoforms. Phylogenetic studies and expression analysis show that higher plants contain two distinct groups of CRTs: a CRT1/CRT2 group and a CRT3 group. To corroborate the existence of these isoform groups, we cloned a putative CRT3 ortholog from Brassica rapa. The CRT3 gene appears to be most closely related to the ancestral CRT gene in higher plants. Distinct tissue-dependent expression patterns and stress-related regulation were observed for the isoform groups. Furthermore, analysis of posttranslational modifications revealed differences in the glycosylation status among members within the CRT1/CRT2 isoform group. Based on evolutionary relationship, a new nomenclature for plant CRTs is suggested. The presence of two distinct CRT isoform groups, with distinct expression patterns and posttranslational modifications, supports functional specificity among plant CRTs and could account for the multiple functional roles assigned to CRTs.

N-Glycosylation and conserved cysteine residues in RAMP3 play a critical role for the functional expression of CRLR/RAMP3 adrenomedullin receptor

The calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein-3 (RAMP3) can assemble into a CRLR/RAMP3 heterodimeric receptor that exhibits the characteristics of a high affinity adrenomedullin receptor. RAMP3 participates in adrenomedullin (AM) binding via its extracellular N-terminus characterized by the presence of six highly conserved cysteine residues and four N-glycosylation consensus sites. Here, we assessed the usage of these conserved residues in cotranslational modifications of RAMP3 and addressed their role in functional expression of the CRLR/RAMP3 receptor. Using a Xenopus oocyte expression system, we show that (i) RAMP3 is assembled with CRLR as a multiple N-glycosylated species in which two, three, or four consensus sites are used; (ii) elimination of all N-glycans in RAMP3 results in a significant inhibition of receptor [(125)I]AM binding and an increase in the EC(50) value for AM; (iii) several lines of indirect evidence indicate that each of the six cysteines is involved in disulfide bond formation; (iv) when all cysteines are mutated to serines, RAMP3 is N-glycosylated at all four consensus sites, suggesting that disulfide bond formation inhibits N-gylcosylation; and (v) elimination of all cysteines abolishes adrenomedullin binding and leads to a complete loss of receptor function. Our data demonstrate that cotranslational modifications of RAMP3 play a critical role in the function of the CRLR/RAMP3 adrenomedullin receptor.

Monitoring endoplasmic reticulum-to-Golgi traffic of a plant calreticulin by protein glycosylation analysis

Calreticulin is a ubiquitous and highly conserved Ca(2+)-binding protein that is involved in intracellular Ca(2+) homeostasis and molecular chaperoning in the endoplasmic reticulum (ER). Plant calreticulin, in contrast to its animal counterpart, is often glycosylated: its N-glycans have been shown so far to be of the high-mannose type, typical of ER-resident glycoproteins. During the characterization of calreticulin from vegetative and reproductive tissues of Liriodendron tulipifera L., we gained some biochemical evidence that prompted us to investigate the monosaccharide composition and primary structure of the calreticulin N-glycans isolated from the ovary of this dicotyledon tree. The structures of the components of the N-glycan pool were elucidated by HPLC analysis and exoglycosidase sequencing, and further confirmed by matrix-assisted laser desorption/ionization mass spectrometry. The 16 identified oligosaccharide structures, which consisted of both the high-mannose and complex type, are indicative of calreticulin glycan processing through the ER-to-Golgi pathway up to the medial and trans Golgi stacks. Approximately 45% of calreticulin glycan chains are of the complex type, always containing beta(1,2)-xylose, and approximately a third of these also contain alpha(1,3)-fucose in the core. The most complex glycoform harbors the Lewis-a epitope Gal(beta)1-3[Fuc(alpha)1-4]GlcNAc. Immunolocalization of calreticulin with anti-calreticulin antibodies was consistent with protein transit through the Golgi. Thus, although it contains the tetrapeptide HDEL ER retention signal, the reticuloplasmin calreticulin possesses the competence to transit from the ER compartment to the distal Golgi stacks. The final fate of the protein after its complete maturation is still obscure.

Drosophila segment polarity gene product porcupine stimulates the posttranslational N-glycosylation of wingless in the endoplasmic reticulum

Wnt is a family of cysteine-rich secreted glycoproteins, which controls the fate and behavior of the cells in multicellular organisms. In the absence of Drosophila segment polarity gene porcupine (porc), which encodes an endoplasmic reticulum (ER) multispanning transmembrane protein, the N-glycosylation of Wingless (Wg), one of Drosophila Wnt family, is impaired. In contrast, the ectopic expression of porc stimulates the N-glycosylation of both endogenously and exogenously expressed Wg. The N-glycosylation of Wg in the ER occurs posttranslationally, while in the presence of dithiothreitol, it efficiently occurs cotranslationally. Thus, the cotranslational disulfide bond formation of Wg competes with the N-glycosylation by an oligosaccharyl transferase complex. Porc binds the N-terminal 24-amino acid domain (residues 83-106) of Wg, which is highly conserved in the Wnt family and stimulates the N-glycosylation at surrounding sites. Porc is also necessary for the processing of Drosophila Wnt-3/5 in both embryos and cultured cells. Thus, Porc binds the N-terminal specific domain of the Wnt family and stimulates its posttranslational N-glycosylation by anchoring them at the ER membrane possibly through acylation.

CABYR, a novel calcium-binding tyrosine phosphorylation-regulated fibrous sheath protein involved in capacitation

To reach fertilization competence, sperm undergo an incompletely understood series of morphological and molecular maturational processes, termed capacitation, involving, among other processes, protein tyrosine phosphorylation and increased intracellular calcium. Hyperactivated motility and an ability to undergo the acrosome reaction serve as physiological end points to assess successful capacitation. We report here that acidic (pI 4.0) 86-kDa isoforms of a novel, polymorphic, testis-specific protein, designated calcium-binding tyrosine phosphorylation-regulated protein (CABYR), were tyrosine phosphorylated during in vitro capacitation and bound (45)Ca on 2D gels. Acidic 86-kDa calcium-binding forms of CABYR increased during in vitro capacitation, and calcium binding to these acidic forms was abolished by dephosphorylation with alkaline phosphatase. Six variants of CABYR containing two coding regions (CR-A and CR-B) were cloned from human testis cDNA libraries, including five variants with alternative splice deletions. A motif homologous to the RII dimerization domain of PK-A was present in the N-terminus of CR-A in four CABYR variants. A single putative EF handlike motif was noted in CR-A at aas 197-209, while seven potential tyrosine phosphorylation-like sites were noted in CR-A and four in CR-B. Pro-X-X-Pro (PXXP) modules were identified in the N- and C-termini of CR-A and CR-B. CABYR localizes to the principal piece of the human sperm flagellum in association with the fibrous sheath and is the first demonstration of a sperm protein that gains calcium-binding capacity when phosphorylated during capacitation.

Identification and pharmacological correction of a membrane trafficking defect associated with a mutation in the sulfonylurea receptor causing familial hyperinsulinism

Persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is a genetic disorder characterized by excess secretion of insulin and hypoglycemia. In most patients, the disease is caused by mutations in sulfonylurea receptor-1 (SUR1), which, in association with Kir6.2, constitutes the functional ATP-sensitive potassium (K(ATP)) channel of the pancreatic beta-cell. Previous studies reported that coexpression of the PHHI mutant R1394H-SUR1 with Kir6.2 in COS cells produces no functional channels. To investigate if the loss of function could be due to impaired trafficking of mutant channels to the cell membrane, we have cotransfected wild-type and mutant SUR1 subunits with Kir6.2 into HEK293 cells and examined their cellular localization by immunofluorescent staining. Our results show that unlike the wild-type subunits, which showed fluorescence at the cell surface, the mutant subunits displayed fluorescence in punctate structures. Co-immunostaining with antibodies against organelle-specific marker proteins identified these structures as the trans-Golgi network. Limited localization in clathrin-positive, but transferrin receptor-negative vesicles was also observed. The post-endoplasmic reticulum localization suggests that the mutation does not impair the folding and assembly of the channels so as to cause its retention by the endoplasmic reticulum. Diazoxide, a K(ATP) channel opener drug that is used in the treatment of PHHI, restored the surface expression in a manner that could be prevented by the channel blocker glibenclamide. When expressed in Xenopus oocytes, R1394H-SUR1 formed functional channels with Kir6.2, indicating that the primary consequence of the mutation is impairment of trafficking rather than function. Thus, our data uncover a novel mechanism underlying the therapeutic action of diazoxide in the treatment of PHHI, i.e. its ability to recruit channels to the membrane. Furthermore, this is the first report to describe a trafficking disorder effecting retention of mutant proteins in the trans-Golgi network.

Analysis of N-glycans of pathological tau: possible occurrence of aberrant processing of tau in Alzheimer's disease

In a previous study [Wang et al. (1996) Nat. Med. 2, 871-875], Wang et al. found (i) that abnormally hyperphosphorylated tau (AD P-tau) isolated from Alzheimer's disease (AD) brain as paired helical filaments (PHF)-tau and as cytosolic AD P-tau but not tau from normal brain were stained by lectins, and (ii) that on in vitro deglycosylation the PHF untwisted into sheets of thin straight filaments, suggesting that tau only in AD brains is glycosylated. To elucidate the primary structure of N-glycans, we comparatively analyzed the N-glycan structures obtained from PHF-tau and AD P-tau. More than half of N-glycans found in PHF-tau and AD P-tau were different. High mannose-type sugar chains and truncated N-glycans were found in both taus in addition to a small amount of sialylated bi- and triantennary sugar chains. More truncated glycans were richer in PHF-tau than AD P-tau. This enrichment of more truncated glycans in PHF might be involved in promoting the assembly and or stabilizing the pathological fibrils in AD.

Post-translational N-glycosylation of a truncated form of a peptide processing enzyme

Peptidylglycine alpha-amidating monooxygenase (PAM) catalyzes the carboxyl-terminal amidation of bioactive peptides through a two-step reaction involving the monooxygenase and lyase domains. PAM undergoes endoproteolytic cleavage in neuroendocrine cells in the lyase domain. To determine which of the two possible paired basic sites is utilized, truncated PAM proteins ending at these sites were stably expressed in Chinese hamster ovary cells. While characterizing the truncation mutants, it became apparent that N-glycosylation occurred post-translationally at the single site localized near the carboxyl terminus of the lyase domain. The post-translational N-glycosylation of this site does not require the newly synthesized protein to remain tightly bound to membranes. Both malfolded, secretion incompetent proteins and fully active, secreted proteins were subject to post-translational N-glycosylation.

Calreticulin associates with stress proteins: implications for chaperone function during heat stress

Acute heat stress leads to the glycosylation of a "prompt" stress glycoprotein, P-SG67/64, identified as calreticulin. In the present study, we used immunoprecipitation to investigate the interactions of P-SG/calreticulin with other proteins during cellular recovery from heat stress. In heat-stressed CHO and M21 cells, both glycosylated and unglycosylated P-SGs interact with HSP90, GRP94, GRP78, and the other prompt stress glycoprotein, P-SG50, in an ATP-independent manner. Specificity of HSP-P-SG interactions was determined by chemical cross-linking with the homo-bifunctional agent DSP (3,3'-dithiobis[succinimidyl propionate]). Characterization of the cross-linked complexes involving calreticulin and heat shock proteins (HSPs) showed an average mass of 400-600 kDa by gel filtration chromatography. Overall, the consistent association of glycosylated and unglycosylated calreticulin with P-SG50 and unglycosylated HSPs suggests that P-SG/calreticulin is an active member of the cast of glycone/aglycone chaperones that cooperate to achieve cellular recovery from acute heat stress.

Protein glycosylation in rat fibroblast cells expressing deletion variants of the human hsp70 gene

Elevated heat resistance is often associated with high cellular levels of hsp70. In the rat cell line, M21, increased heat resistance is associated with both the expression of an intact exogenous human hsp70 gene, and of an exogenous stress glycoprotein, GP62. In this study, we examined heat-stress induced alterations in protein glycosylation in two variant lines of M21 that contain specific deletions in the exogenous human hsp70 gene. The deletion mutant, MVH delta Sm, lacks the nucleolar localization sequence for human hsp70 gene, whereas the other mutant, MVH delta Bg, is characterized by deletion of the ATP binding domain in the human hsp70 gene. After heat induction, the MVH delta Sm mutants exhibited constitutive and heat-induced endogenous hsp70 mRNA levels, protein glycosylation, as well as hsp70 and GP62 protein levels comparable to those in the parent cell line, Rat-1. Cellular heat sensitivity of MVH delta Sm mutants was also similar to that of Rat-1 cells, although the mutant cells had a reduced capacity for thermotolerance development. On the other hand, MVH delta Bg mutants before thermotolerance induction, exhibited constitutive endogenous and human exogenous hsp70 mRNA levels similar to those in MVH delta Sm mutants. However, after thermotolerance induction, MVH delta Bg mutants showed lower levels of heat-induced endogenous hsp70 mRNA than MVH delta Sm mutants, an overall reduction in protein glycosylation, low hsp70 and GP62 levels, and increased heat sensitivity when compared to the parent Rat-1 cell line. Incorporation of D-[2-3H]mannose into oligosaccharide precursor pools and glycoproteins was consistent with protein glycosylation pattern for each cell line. Acute heat stress resulted in the selective glycosylation of the 'prompt' glycoprotein, P-SG64, in the two deletion mutants, similar to that in M21 cells expressing the intact human hsp70 gene. The data indicate that both protein glycosylation and hsp70 expression generally correlate with cellular heat resistance and thermotolerance expression. The presence of full or partially deleted copies of human hsp70 modulates thermotolerance and protein glycosylation in a complex manner that is not yet fully understood.

Clinical relevance of calreticulin in systemic lupus erythematosus

Calreticulin is an abundant intracellular protein which is proposed to have numerous biological functions. However, there is increasing evidence to suggest that calreticulin plays a multifunctional role as an autoantigen present in patients with systemic lupus erythematosus. In this review we detail some of the recent evidence which indicate that calreticulin may play a supportive role in the formation of the autoantigen complex-Ro/SS-A. In addition, several proposed mechanisms of release and surface expression of calreticulin are described in relation to SLE mediated responses to the autoantigen. In particular, the generation of autoantibodies to specific regions of the protein and the ability of calreticulin to interfere with complement mediated inflammatory processes.

Intracellular distribution of stress glycoproteins in a heat-resistant cell model expressing human HSP70

Heat stress results in the cellular accumulation of heat-shock proteins (HSP) and increased protein glycosylation. Among known stress glycoproteins, GP50 and GP62 are associated with the expression of thermotolerance. In the present study, we characterized subcellular localization and redistribution of GP62 and GP50 in a rodent cell line, M21, before and after cellular heat-stress. M21 cells are heat-resistant cells that overexpress human HSP70 and also have concomitantly high GP62 levels. Cellular fractionation by differential centrifugation showed that GP62 and GP50 was present in each subcellular fraction. However, each stress glycoprotein exhibited a characteristic kinetic pattern of redistribution during cellular recovery after heat stress. For example, glycosylated GP62 was seen predominantly in the mitochondria before heat-stress. Immediately after heat-stress, its presence in the mitochondrial fraction was dramatically reduced, while it increased in lysosomes, microsomes and cytosol. By 1 h after heat stress, it had largely disappeared from microsomal and cytosolic fractions. After 24 h, all subcellular fractions showed only trace amounts of residual GP62. By comparison, GP50 was also highest in the mitochondrial fraction before heat-stress, redistributed like GP62 immediately after heat stress, but remained relatively unchanged thereafter. In contrast to GP62, GP50 showed little redistribution during 24 h after heat-stress and remained at high concentrations in all cell fractions, including microsomes. Distribution of GP50 and GP62 before and after heat stress, based on differential centrifugation, was consistent with immunolocalization data. Following heat stress, both GP50 and GP62 showed a partial overlap in distribution with HSP70. The above results indicate that each stress glycoprotein has a specific subcellular location, both before and after heat stress. The presence of GP62 in virtually all cell fractions is consistent with a multifunctional role for GP62 in the cellular stress response.

Calreticulin Biosynthesis and Processing in Human Myeloid Cells: Demonstration of Signal Peptide Cleavage and N-Glycosylation

Calreticulin is a soluble endoplasmic reticulum protein comprising the major storage reservoir for inositol trisphosphate-releasable calcium. Although its highly conserved primary structure and a wide range of functions have been well described, less attention has been paid to its biosynthesis, particularly in human tissues. We report analyses of synthesis, proteolytic processing and glycosylation of human calreticulin. In both HL-60 and PLB-985 myeloid cell lines calreticulin was immunoprecipitated as a single 60-kD species without evidence of precursor forms. However, in vitro cell-free synthesis produced a 62-kD primary translation product, which in the presence of microsomal membranes, was processed by cotranslational signal peptide cleavage to a 60-kD species that comigrated with mature calreticulin produced in myeloid cells. Neither tunicamycin treatment of the cells nor endoglycosidase digestion of calreticulin resulted in any forms other than the 60-kD protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, suggesting that the potential site for N-glycosylation at asparagine-327 was unmodified. However, oxidative derivatization of carbohydrate components with digoxigenin showed that human calreticulin produced in either HL-60 cells or Sf9 insect cells is glycosylated, indicating that glycosylated and nonglycosylated human calreticulin have indistinguishable electrophoretic mobilities. Direct measurement by phenol-H2SO4 confirmed the presence of carbohydrate on recombinant human calreticulin. These data show that human myeloid calreticulin undergoes cotranslational signal peptide cleavage and posttranslational N-linked glycosylation. Although glycosylation of calreticulin has been shown in rat liver and bovine liver and brain, it has been reported to be lacking in other tissues including human lymphocytes.

Calreticulin Biosynthesis and Processing in Human Myeloid Cells: Demonstration of Signal Peptide Cleavage and N-Glycosylation

Calreticulin is a soluble endoplasmic reticulum protein comprising the major storage reservoir for inositol trisphosphate-releasable calcium. Although its highly conserved primary structure and a wide range of functions have been well described, less attention has been paid to its biosynthesis, particularly in human tissues. We report analyses of synthesis, proteolytic processing and glycosylation of human calreticulin. In both HL-60 and PLB-985 myeloid cell lines calreticulin was immunoprecipitated as a single 60-kD species without evidence of precursor forms. However, in vitro cell-free synthesis produced a 62-kD primary translation product, which in the presence of microsomal membranes, was processed by cotranslational signal peptide cleavage to a 60-kD species that comigrated with mature calreticulin produced in myeloid cells. Neither tunicamycin treatment of the cells nor endoglycosidase digestion of calreticulin resulted in any forms other than the 60-kD protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, suggesting that the potential site for N-glycosylation at asparagine-327 was unmodified. However, oxidative derivatization of carbohydrate components with digoxigenin showed that human calreticulin produced in either HL-60 cells or Sf9 insect cells is glycosylated, indicating that glycosylated and nonglycosylated human calreticulin have indistinguishable electrophoretic mobilities. Direct measurement by phenol-H2SO4 confirmed the presence of carbohydrate on recombinant human calreticulin. These data show that human myeloid calreticulin undergoes cotranslational signal peptide cleavage and posttranslational N-linked glycosylation. Although glycosylation of calreticulin has been shown in rat liver and bovine liver and brain, it has been reported to be lacking in other tissues including human lymphocytes.

Partial homology of stress glycoprotein GP62 with HSP70

Thermotolerance and heat resistance are often associated with elevated levels of heat shock proteins (HSPs) and a selective increase in protein glycosylation. In the present study, we have characterized heat stress-induced protein glycosylation in M21 cells, derived from the rat fibroblast line, Rat-1. M21 cells are characterized by constitutive overexpression of human HSP70 gene and show increased heat resistance without loss of its normal capacity for thermotolerance development after heat conditioning (Li et al., 1991, Proc. Natl. Acad. Sci. USA 88, 1681-1685). The data presented here show that the elevated heat resistance in these cells is associated not only with the constitutive overexpression of human HSP70, but also with increased glycosylation of a major stress glycoprotein, GP62 (Mr of 62,000). We further purified GP62 by sequential preparative isoelectric focusing and two-dimensional isoelectric focusing/SDS-polyacrylamide gel electrophoresis. The purified protein was digested and partially characterized by microsequencing of two peptide fragments, comprising of 14-15 amino acids each. These fragments had a 100% sequence homology with HSP70 and a 71-100% sequence homology with HSC70 from various species. Western blotting using both HSP70 and HSC70 antibodies showed positive reactivity of GP62 with HSP70. Affinity characterizations showed strong binding of GP62 to wheat germ agglutinin and concanavalin A, consistent with the presence of both alpha-D-mannosyl/glucosyl and N-acetyl-beta-D-glucosylaminyl/glucosamine oligomer residues in GP62. These data confirm the glycosylated status of GP62 and indicate that GP62 is a heat stress-induced glycoprotein with partial homology to HSP70.

Protein glycosylation in a heat-resistant rat fibroblast cell model expressing human HSP70

Thermotolerance and heat resistance are frequently associated with elevated levels of heat shock proteins (HSPs). Elevated heat resistance is also found to be associated with the overexpression of high levels of HSP70, as seen in M21 cells, derived from the Rat-1 line. In the present study, we report that M21 cells also feature an increase in general protein glycosylation and specific expression of the stress glycoprotein, GP62, both of which correlate with cellular heat resistance. The expression of GP50, a major stress glycoprotein in cell lines such as CHO, however, did not correlate with cellular heat resistance in M21 cells. Protein glycosylation that occurs during acute heat stress ("prompt" glycosylation) was associated with the glycosylation of a major "prompt" stress glycoprotein, P-SG64 (M(r) of 64,000), that was identified by immunoblotting as a glycosylated form of calreticulin. The higher level of protein glycosylation in M21 cells correlated well with increased D-[2-3H]mannose incorporation into precursor pools of dolichyl phosphomannose and dolichyl pyrophosphoryl oligosaccharides and into glycoproteins. Thus, heat resistance in M21 cells is associated not only with expression of high levels of HSP70, but also with a concomitant increase in protein glycosylation. These data support the hypothesis that stress-induced protein glycosylation is a component of cellular stress response, either in association with HSPs or as an independent mechanism.

A subpopulation of WIL-2 cell calreticulin molecules is associated with RO/SS-A ribonucleoprotein particles

A subpopulation of human calreticulin (CR) molecules that is reactive with human Ro/SS-A autoimmune sera was identified in a nucleic acid- enriched Wil-2 cell fraction derived by anion exchange column chromatography. Further resolution of this fraction by gel filtration size separation demonstrated that the appearance of CR (true mol. weight 46 kD) coincided with the emergence of Ro/SS-A ribonucleoprotein (mol. weight > 250 kD) antigenic activity and increasing 260 nm ultraviolet absorbance. This high nucleic acid fraction could be further partitioned into four small RNA-containing Ro/SS-A antigenic subfractions by a second passage over the anion exchange column. CR was enriched in one subfraction and present in the other three subfractions as well. No CR was found in the RNA-free fraction of the repartition eluate. These results represent the first direct demonstration that CR, a high-affinity calcium binding protein, exists in a form that is directly associated with all four varieties of native, human Ro/SS-A ribonucleoprotein particles (hY1-5).

Effect of the D178N mutation and the codon 129 polymorphism on the metabolism of the prion protein

Prion diseases are thought to be caused by the conversion of the normal, or cellular, prion protein (PrPC)(PrPres). There are three familial forms of human prion disease, Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler-Scheinker syndrome, and fatal familial insomnia (FFI) which are all expressed at advanced age despite the congenital presence of the mutant prion protein (PrPM). The cellular mechanisms that result in the age-dependent conversion of PrPM into PrPres and the unique phenotypes associated with each PrPM are unknown. FFI and a familial type of Creutzfeldt-Jakob disease (CJD178), share the D178N mutation in the PrP gene but have distinct phenotypes linked to codon 129, the site of a methionine/valine polymorphism (129M/V). We analyzed PrP processing in cells transfected with constructs reproducing the FFI and CJD178 genotypes. The D178N mutation results in instability of the mutant PrP which is partially corrected by N-glycosylation. Hence, only the glycosylated forms of PrPM reach the cell surface whereas the unglycosylated PrPM is also under-represented in the brain of FFI patients validating the cell model. These results offer new insight into the effect of the D178N mutation on the metabolism of the prion protein.

UDP-glucose deficiency in a mutant cell line protects against glucosyltransferase toxins from Clostridium difficile and Clostridium sordellii

We have previously isolated a fibroblast mutant cell with high resistance to the two Rho-modifying glucosyltransferase toxins A and B of Clostridium difficile. We demonstrate here a low level of UDP-glucose in the mutant, which explains its toxin resistance since: (i) to obtain a detectable toxin B-mediated Rho modification in lysates of mutant cells, addition of UDP-glucose was required, and it promoted the Rho modification dose-dependently; (ii) high pressure liquid chromatography analysis of nucleotide extracts of cells indicated that the level of UDP-glucose in the mutant (0.8 nmol/10(6) cells) was lower than in the wild type (3.7 nmol/10(6) cells); and (iii) sensitivity to toxin B was restored upon microinjection of UDP-glucose. Using the mutant as indicator cell we also found that the related Clostridium sordellii lethal toxin is a glucosyltransferase which requires UDP-glucose as a cofactor. Like toxin B it glucosylated 21-23-kDa proteins in cell lysates, but Rho was not a substrate for lethal toxin.

Oxidative stress induced heat shock factor phosphorylation and HSF-dependent activation of yeast metallothionein gene transcription

Metallothioneins (MTs) are a class of low-molecular-weight, cysteine- rich metal-binding proteins that function in metal detoxification and oxidative stress protection. We demonstrate that transcription of the Saccharomyces cerevisiae MT gene CUP1 is strongly activated by the superoxide anion generator menadione. This activation is exacerbated in a strain lacking the gene encoding Co, Zn superoxide dismutase (SOD1). CUP1 transcriptional activation by oxidative stress is dependent on a functional CUP1 promoter heat shock element (HSE) and the carboxy-terminal trans-activation domain of heat shock transcription factor (HSF). Furthermore, protection against oxidative stress conferred by CUP1 in a (sod1)delta strain requires HSF-mediated CUP1 transcription. Although in response to heat, HSF-mediated CUP1 transcription and HSF phosphorylation are transient, both CUP1 gene expression and HSF phosphorylation are sustained in response to oxidative stress. Moreover, the patterns of tryptic phosphopeptides resolved from HSF derived from cells subjected to heat shock or oxidative stress are distinct. These results demonstrate that transcription of the S. cerevisiae metallothionein gene under conditions of oxidative stress is mediated by HSF and that in response to distinct activation stimuli, HSF is differentially phosphorylated in a manner that parallels metallothionein gene transcription.

Heat-shock induced protein modifications and modulation of enzyme activities

Upon heat stress, the cell physiology is profoundly altered. The extent of the alterations depends on the severity of the stress and may lead to cell death. The heat shock response is an array of metabolic changes characterized by the impairment of major cellular functions and by an adaptative reprogramming of the cell metabolism. The enhanced synthesis of the HSPs is a spectacular manifestation of this reprogramming. Numerous post translational modifications of proteins occur in response to heat stress and can be related to altered cellular functions. Some proteins are heat-denatured and temporarily inactivated. Heat-denaturation is reversible, chaperones may contribute to the repair. The extent of heat-denaturation depends on the cell metabolism: (a) it is attenuated in thermotolerant cells or in cells overexpressing the appropriate chaperones (b) it is enhanced in energy-deprived cells. Covalent modifications may also rapidly alter protein function. Changes in protein glycosylation, methylation, acetylation, farnesylation, ubiquitination have been found to occur during stress. But protein phosphorylation is the most studied modification. Several protein kinase cascades are activated, among which the various mitogen activated protein kinase (MAP kinase) cascades which are also triggered by a wide range of stimuli. As a possible consequence, stress modifies the phosphorylation status and the activity of components from the transcriptional and translational apparatuses. The same kinases also target key enzymes of the cellular metabolism. Protein denaturation results in constitutive hsp titration, this titration is a signal to trigger the heat-shock gene transcription and to activate some of the protein kinase cascades.

Heat shock-sensitive expression of calreticulin. In vitro and in vivo up-regulation

Calreticulin (CRT) is an ubiquitous, highly conserved, Ca(2+)-binding protein of the sarcoplasmic and endoplasmic reticulum. The precise function(s) of CRT is unknown. However, based on sequence analyses and observations that it may bind to steroid receptors and integrins and store Ca2+ within the cell, it has been postulated to play a "housekeeping" role. To determine whether the level of expression of CRT is affected by stress, we examined the heat shock response of CRT from a variety of cultured cells, including vascular endothelial, lung epithelial, and lung fibroblasts. Following exposure of the cells to 42 degrees C, CRT mRNA transiently accumulated 2.5-4.2-fold at 1-6 h. Nuclear run-on studies and mRNA stability experiments confirmed that the predominant mechanism of augmentation was transcriptional. Chloramphenicol acetyltransferase assays further indicated that the promoter region, containing a putative heat shock element between -172 and -158 of the human CRT gene, is heat shock-sensitive. Finally, we demonstrated the in vivo significance of these findings by exposing rats to hyperthermia. This resulted in accumulation of CRT mRNA and an augmentation of CRT protein in lung tissue. We hypothesize that this stress-induced up-regulation of CRT contributes to the mechanism(s) by which the vascular endothelium and lung tissue, and possibly other organ systems, maintain homeostasis when exposed to a variety of pathophysiological conditions.