gC1q-R/p33, a member of a new class of multifunctional and multicompartmental cellular proteins, is involved in inflammation and infection

Species specificity of the Listeria monocytogenes InlB protein

InlA and InlB mediate L. monocytogenes entry into eukaryotic cells. InlA is required for the crossing of the intestinal and placental barriers. InlA uses E-cadherin as receptor in a species-specific manner. The human E-cadherin but not the mouse E-cadherin is a receptor for InlA. In human cells, InlB uses Met and gC1qR as receptors. By studying the role of InlB in vivo, we found that activation of Met by InlB is species-specific. In mice, InlB is important for liver and spleen colonization, but not for the crossing of the intestinal epithelium. Strikingly, the virulence of a DeltainlB deletion mutant is not attenuated in guinea pigs and rabbits. Guinea pig and rabbit cell lines do not respond to InlB, although expressing Met and gC1qR, but support InlB-mediated responses upon human Met gene transfection, indicating that InlB does not recognize or stimulate guinea pig and rabbit Met. In guinea pig cells, the effect of human Met gene transfection on InlB-dependent entry is increased upon cotransfection with human gc1qr gene, showing the additive roles of gC1qR and Met. These results unravel a second L. monocytogenes species specificity critical for understanding human listeriosis and emphasize the need for developing new animal models for studying InlA and InlB functions in the same animal model.

SOCS1 and SOCS3 are targeted by hepatitis C virus core/gC1qR ligation to inhibit T-cell function

T cells play an important role in the control of hepatitis C virus (HCV) infection. We have previously demonstrated that the HCV core inhibits T-cell responses through interaction with gC1qR. We show here that core proteins from chronic and resolved HCV patients differ in sequence, gC1qR-binding ability, and T-cell inhibition. Specifically, chronic core isolates bind to gC1qR more efficiently and inhibit T-cell proliferation as well as gamma interferon (IFN-gamma) production more profoundly than resolved core isolates. This inhibition is mediated by the disruption of STAT phosphorylation through the induction of SOCS molecules. Silencing either SOCS1 or SOCS3 by small interfering RNA dramatically augments the production of IFN-gamma in T cells, thereby abrogating the inhibitory effect of core. Additionally, the ability of core proteins from patients with chronic infections to induce SOCS proteins and suppress STAT activation greatly exceeds that of core proteins from patients with resolved infections. These results suggest that the HCV core/gC1qR-induced T-cell dysfunction involves the induction of SOCS, a powerful inhibitor of cytokine signaling, which represents a novel mechanism by which a virus usurps the host machinery for persistence.

Receptors for complement C3 on T-lymphocytes: Relics of evolution or functional molecules?

Despite the fact that receptors for complement on T-cells have been described many years ago the function remains unclear as is the role of complement in the T-cell response. In this review we will evaluate how the accumulated wisdom concur with the current concepts of the adaptive T-cell response.

gC1q receptor ligation selectively down-regulates human IL-12 production through activation of the phosphoinositide 3-kinase pathway

gC1qR, a complement receptor for C1q, plays a pivotal role in the regulation of inflammatory and antiviral T cell responses. Several pathogens, including hepatitis C virus, exploit gC1qR-dependent regulatory pathways to manipulate host immunity. However, the molecular mechanism(s) of gC1qR signaling involved in regulating inflammatory responses remains unknown. We report the selective inhibition of TLR4-induced IL-12 production after cross-linking of gC1qR on the surface of macrophages and dendritic cells. Suppression of IL-12 did not result from increased IL-10 or TGF-beta, but was dependent on PI3K activation. Activation of PI3K and subsequent phosphorylation of Akt define an intracellular pathway mediating gC1qR signaling and cross-talk with TLR4 signaling. This is the first report to identify signaling pathways used by gC1qR-mediated immune suppression, and it establishes a means of complement-mediated immune suppression to inhibit Th1 immunity crucial for clearing pathogenic infection.

Retrotranslocation of the chaperone calreticulin from the endoplasmic reticulum lumen to the cytosol

Polypeptide folding and quality control in the endoplasmic reticulum (ER) are mediated by protein chaperones, including calreticulin (CRT). ER localization of CRT is specified by two types of targeting signals, an N-terminal hydrophobic signal sequence that directs insertion into the ER and a C-terminal KDEL sequence that is responsible for retention in the ER. CRT has been implicated in a number of cytoplasmic and nuclear processes, suggesting that there may be a pathway for generating cytosolic CRT. Here we show that CRT is fully inserted into the ER, undergoes processing by signal peptidase, and subsequently undergoes retrotranslocation to the cytoplasm. A transcription-based reporter assay revealed an important role for the C-terminal Ca(2+) binding domain in CRT retrotranslocation. Neither ubiquitylation nor proteasome activity was necessary for retrotranslocation, which indicates that the pathway is different from that used by unfolded proteins targeted for destruction. Forced expression of cytosolic CRT is sufficient to rescue a cell adhesion defect observed in mouse embryo fibroblasts from crt(-/-) mice. The ability of CRT to retrotranslocate from the ER lumen to the cytosol explains how CRT can change compartments and modulate cell adhesion, transcription, and translation.

Novel collectin/C1q receptor mediates mast cell activation and innate immunity

Mast cells play a critical role in innate immunity, allergy, and autoimmune diseases. The receptor/ligand interactions that mediate mast cell activation are poorly defined. The alpha2beta1 integrin, a receptor for collagens, laminins, decorin, E-cadherin, matrix metalloproteinase-1 (MMP-1), endorepellin, and several viruses, has been implicated in normal developmental, inflammatory, and oncogenic processes. We recently reported that alpha2 integrin subunit-deficient mice exhibited markedly diminished neutrophil and IL-6 responses during Listeria monocytogenes- and zymosan-induced peritonitis. Peritoneal mast cells require alpha2beta1 integrin expression for activation in response to pathogens, yet the ligand and molecular mechanisms by which the alpha2beta1 integrin induces activation and cytokine secretion remain unknown. We now report that the alpha2beta1 integrin is a novel receptor for multiple collectins and the C1q complement protein. We demonstrate that the alpha2beta1 integrin provides a costimulatory function required for mast cell activation and cytokine secretion. This finding suggests that the alpha2beta1 integrin is not only important for innate immunity but may serve as a critical target for the regulation of autoimmune/allergic disorders.

Localization of mitochondrial DNA encoded cytochrome c oxidase subunits I and II in rat pancreatic zymogen granules and pituitary growth hormone granules

Cytochrome c oxidase (COX) complex is an integral part of the electron transport chain. Three subunits of this complex (COX I, COX II and COX III) are encoded by mitochondrial (mit-) DNA. High-resolution immunogold electron microscopy has been used to study the subcellular localization of COX I and COX II in rat tissue sections, embedded in LR Gold resin, using monoclonal antibodies for these proteins. Immunofluorescence labeling of BS-C-1 monkey kidney cells with these antibodies showed characteristic mitochondrial labeling. In immunogold labeling studies, the COX I and COX II antibodies showed strong and specific mitochondrial labeling in the liver, kidney, heart and pancreas. However, in rat pancreatic acinar tissue, in addition to mitochondrial labeling, strong and specific labeling was also observed in the zymogen granules (ZGs). In the anterior pituitary, strong labeling with these antibodies was seen in the growth hormone secretory granules. In contrast to these compartments, the COX I or COX II antibodies showed only minimal labeling (five- to tenfold lower) of the cytoplasm, endoplasmic reticulum and the nucleus. Strong labeling with the COX I or COX II antibodies was also observed in highly purified ZGs from bovine pancreas. The observed labeling, in all cases, was completely abolished upon omission of the primary antibodies. These results provide evidence that, similar to a number of other recently studied mit-proteins, COX I and COX II are also present outside the mitochondria. The presence of mit-DNA encoded COX I and COX II in extramitochondrial compartments, provides strong evidence that proteins can exit, or are exported, from the mitochondria. Although the mechanisms responsible for protein exit/export remain to be elucidated, these results raise fundamental questions concerning the roles of mitochondria and mitochondrial proteins in diverse cellular processes in different compartments.

Structural and functional anatomy of the globular domain of complement protein C1q

C1q is the first subcomponent of the classical pathway of the complement system and a major connecting link between innate and acquired immunity. As a versatile charge pattern recognition molecule, C1q is capable of engaging a broad range of ligands via its heterotrimeric globular domain (gC1q) which is composed of the C-terminal regions of its A (ghA), B (ghB) and C (ghC) chains. Recent studies using recombinant forms of ghA, ghB and ghC have suggested that the gC1q domain has a modular organization and each chain can have differential ligand specificity. The crystal structure of the gC1q, molecular modeling and protein engineering studies have combined to illustrate how modular organization, charge distribution and the spatial orientation of the heterotrimeric assembly offer versatility of ligand recognition to C1q. Although the biochemical and structural studies have provided novel insights into the structure-function relationships within the gC1q domain, they have also raised many unexpected issues for debate.

Adhesins and invasins of pathogenic bacteria: a structural view

Adhesion and invasion of pathogenic bacteria represent the important initial step of infection. Pathogens utilize surface-located adhesins/invasins for specific interaction with host cell receptors. The three-dimensional structures of a number of adhesins/invasins show that many are elongated molecules containing domains commonly found in eukaryotic proteins. Similar folds are employed repeatedly to target different receptors.

Complement Protein C1q Recognizes a Conformationally Modified Form of the Prion Protein

Several studies have suggested the implication of the classical complement pathway in the early stages of prion disease pathogenesis. To explore this hypothesis, surface plasmon resonance spectroscopy was used to test the ability of human C1q to recognize mouse PrP immobilized on a sensor chip. In this configuration, C1q bound avidly to PrP, with a K(D) of 5.4 nM (k(on) = 2.4 x 10(5) M(-1) s(-1); k(off) = 1.3 x 10(-3) s(-1)). The isolated C1q globular domain also bound to immobilized PrP, although with a higher K(D) (238 nM), due to a decreased k(on) (4.2 x 10(3) M(-1) s(-1)). Interaction was strongly enhanced by Cu(2+) ions, with a 10-fold increase in overall binding in the presence of 10 microM CuSO(4), without significant modification of the kinetic parameters. In contrast, using the same technique, no interaction was detected between immobilized C1q and soluble PrP. Likewise, gel filtration and chemical cross-linking analyses yielded no evidence for an interaction between these proteins in solution. Comparative analysis of the antigenic reactivity of soluble and immobilized PrP was performed by ELISA and surface plasmon resonance spectroscopy, respectively, using anti-PrP monoclonal antibodies. This analysis provides evidence that immobilized PrP undergoes a major conformational change in the sequence stretch 141GNDWEDRYYRENMYRYPNQ159 located in its C-terminal globular domain. It is concluded that immobilized PrP undergoes structural modifications that possibly mimic the conformational changes occurring during conversion to the pathological isoform and that C1q represents a natural sensor of these changes. Pathological implications of this recognition property are discussed in light of recent reports.

Exploitation of host cell cytoskeleton and signalling during Listeria monocytogenes entry into mammalian cells

Deciphering how Listeria monocytogenes exploits the host cell machinery to invade mammalian cells during infection isa key issue for the understanding how this food-borne pathogen causes a pleiotropic disease ranging from gastro-enteritis to meningitis and abortions. Using multidisciplinary approaches, essentially combining bacterial genetics and cell biology, we have identified two bacterial proteins critical for entry into target cells, InlA and InlB. Their cellular ligands have been also identified: InlA interacts with the adhesion molecule E-cadherin, while InlB interacts with the receptor for the globular head of the complement factor Clq (gClq-R), with the hepatocyte growth factor receptor (c-Met) and with glycosaminoglycans(including heparan sulphate). The dynamic interaction between these cellular receptors and the actin cytoskeleton is currently under investigation. Several intracellular molecules have been recognized as key effectors for Listeria entry into target cells,including catenins (implicated in the connection of E-cadherin to actin) and the actin depolymerising factor/cofilin (involved in the rearrangement of the cytoskeleton in the InlB-dependent internalisation pathway). At the organism level, species specificity has been discovered concerning the interaction between InlA and E-cadherin, leading to the generation of transgenic mice expressing the human E-cadherin, in which the critical role of InlA in the crossing of the intestinal barrier has been clearly determined. Listeria appears as an instrumental model for addressing critical questions concerning both the complex process of bacterial pathogenesis and also fundamental molecular processes, such as phagocytosis.

Specific interactions between gC1qR and alpha1-adrenoceptor subtypes

The multi-functional protein gC1qR has been reported to interact with an arginine-rich motif in the C-tail of hamster alpha1B-adrenoceptors (ARs), controlling their expression and subcellular localization. Since a similar motif is present in alpha1D-, but not alpha1A-ARs, we studied the specificity of this interaction. Human alpha1-ARs, tagged at their amino termini with Flag epitopes, were coexpressed in HEK293 cells with gC1qR containing a hemaglutinin (HA) tag at its carboxy terminus. Immunoprecipitation studies showed that Flag-alpha1B- or alpha1D-, but not alpha1A-ARs, caused coimmunoprecipitation of HA-gC1qR, while immunoprecipitation of HA-gC1qR caused coimmunoprecipitation of Flag-alpha1B- or alpha1D-, but not alpha1A-ARs, supporting specific interactions between subtypes. C-terminal truncation of Flag-alpha1-ARs prevented interaction with HA-gC1qR, supporting previous conclusions about the role of the C-terminal arginine-rich motif. These studies suggest that gC1qR interacts specifically with alpha1B- and alpha1D-, but not alpha1A-ARs, and this interaction depends on the presence of an intact C-tail.

Receptor for the globular heads of C1q (gC1q-R, p33, hyaluronan-binding protein) is preferentially expressed by adenocarcinoma cells

Combinatorial Ig libraries with phage display allow in vitro generation of human Ig fragments without the need to maintain hybridomas in ongoing cell culture or to select circulating Ig from human serum. Identifying tumor-associated antigens on the surface of intact tumor cells, as opposed to purified proteins, presents a challenge due to the difficulty of preserving complex 3-D epitopic sites on the cell surface, the variable expression of antigens on different malignant cell types and the stereotactic interference of closely associated proteins on the intact membrane surface limiting accessibility to antigenic sites. A combinatorial Ig library of 10(10) clones was generated from the cDNA of PBMCs derived from patients with breast adenocarcinoma. Following subtractive panning, the library was enriched for Ig (Fab fragment) binding to intact adenocarcinoma cells and the resultant Fabs were screened against a cDNA expression library, itself generated from breast cancer cells. Using this approach, we isolated clones from the cDNA library expressing gC1q-R, a glycoprotein comprising the major structure of C1, the first component of the complement system. gC1q-R is a 33 kDa glycoprotein expressed not only on the cell surface but also intracellularly, with motifs that target it to mitochondria and complete homology with HABP and human HeLa cell protein p32, which is copurified with pre-mRNA SF2. Sequencing of the gene encoding tumor-associated gC1q-R did not reveal any consistent tumor-specific mutations. However, histochemical staining with anti-gC1q-R MAb demonstrated marked differential expression of gC1q-R in thyroid, colon, pancreatic, gastric, esophageal and lung adenocarcinomas compared to their nonmalignant histologic counterparts. In contrast, differential expression was not seen in endometrial, renal and prostate carcinomas. Despite high expression in breast carcinoma, gC1q-R was also expressed in nonmalignant breast tissue. Although the precise relation of gC1q-R to carcinogenesis remains unclear, our finding of tumor overexpression and the known multivalent binding of gC1q-R to not only C1q itself but also a variety of circulating plasma proteins as well as its involvement in cell-to-cell interactions suggest that gC1q-R may have a role in tumor metastases and potentially serve in molecule-specific targeting of malignant cells.

Direct binding of hepatitis C virus core to gC1qR on CD4+ and CD8+ T cells leads to impaired activation of Lck and Akt

Complement plays a pivotal role in the regulation of innate and adaptive immunity. It has been shown that the binding of C1q, a natural ligand of gC1qR, on T cells inhibits their proliferation. Here, we demonstrate that direct binding of the hepatitis C virus (HCV) core to gC1qR on T cells leads to impaired Lck/Akt activation and T-cell function. The HCV core associates with the surface of T cells specifically via gC1qR, as this binding is inhibited by the addition of either anti-gC1qR antibody or soluble gC1qR. The binding affinity constant of core protein for gC1qR, as determined by BIAcore analysis, is 3.8 x 10(-7) M. The specificity of the HCV core-gC1qR interaction is confirmed by reduced core binding on Molt-4 T cells treated with gC1qR-silencing small interfering RNA and enhanced core binding on GPC-16 guinea pig cells transfected with human gC1qR. Interestingly, gC1qR is expressed at higher levels on CD8(+) than on CD4(+) T cells, resulting in more severe core-induced suppression of the CD8(+)-T-cell population. Importantly, T-cell receptor-mediated activation of the Src kinases Lck and ZAP-70 but not Fyn and the phosphorylation of Akt are impaired by the HCV core, suggesting that it inhibits the very early events of T-cell activation.

pH and Cation-induced Thermodynamic Stability of Human Hyaluronan Binding Protein 1 Regulates Its Hyaluronan Affinity

Hyaluronan-binding protein 1 (HABP1) is a trimeric protein with high negative charges distributed asymmetrically along the faces of the molecule. Recently, we have reported that HABP1 exhibits a high degree of structural flexibility, which can be perturbed by ions under in vitro conditions near physiological pH (Jha, B. K., Salunke, D. M., and Datta, K. (2003) J. Biol. Chem. 278, 27464-27472). Here, we report the effect of ionic strength and pH on thermodynamic stability of HABP1. Trimeric HABP1 was shown to unfold reversibly upon dissociation ruling out the possibility of existence of folded monomer. An increase in ionic concentration (0.05-1 M) or decrease in pH (pH 8.0-pH 5.0) induced an unusually high thermodynamic stability of HABP1 as reflected in the gradual increase in transition midpoint temperature, enthalpy of transition, and conformational entropy. Our studies suggest that the presence of counter ions in the molecular environment of HABP1 leads to dramatic reduction of the intramolecular electrostatic repulsion either by de-ionizing the charged amino acid residues or by direct binding leading to a more stable conformation. A regulation on cellular HA-HABP1 interaction by changes in pH and ionic strength may exist, because the more stable conformation attained at higher ionic strength or at acidic pH showed maximum affinity toward HA as probed either in solid phase binding assay on HA-immobilized plates or an in-solution binding assay using intrinsic fluorescence of HABP1.

Exploitation of host cell cytoskeleton and signalling during Listeria monocytogenes entry into mammalian cells

Deciphering how Listeria monocytogenes exploits the host cell machinery to invade mammalian cells during infection is a key issue for the understanding how this food-borne pathogen causes a pleiotropic disease ranging from gastro-enteritis to meningitis and abortions. Using multidisciplinary approaches, essentially combining bacterial genetics and cell biology, we have identified two bacterial proteins critical for entry into target cells, InlA and InlB. Their cellular ligands have been also identified: InlA interacts with the adhesion molecule E-cadherin, while InlB interacts with the receptor for the globular head of the complement factor C1q (gC1q-R), with the hepatocyte growth factor receptor (c-Met) and with glycosaminoglycans (including heparan sulphate). The dynamic interaction between these cellular receptors and the actin cytoskeleton is currently under investigation. Several intracellular molecules have been recognized as key effectors for Listeria entry into target cells, including catenins (implicated in the connection of E-cadherin to actin) and the actin depolymerising factor/cofilin (involved in the rearrangement of the cytoskeleton in the InlB-dependent internalisation pathway). At the organism level, species specificity has been discovered concerning the interaction between InlA and E-cadherin, leading to the generation of transgenic mice expressing the human E-cadherin, in which the critical role of InlA in the crossing of the intestinal barrier has been clearly determined. Listeria appears as an instrumental model for addressing critical questions concerning both the complex process of bacterial pathogenesis and also fundamental molecular processes, such as phagocytosis.

Targeting and crossing of the human maternofetal barrier by Listeria monocytogenes: role of internalin interaction with trophoblast E-cadherin

Listeria monocytogenes produces severe fetoplacental infections in humans. How it targets and crosses the maternofetal barrier is unknown. We used immunohistochemistry to examine the location of L. monocytogenes in placental and amniotic tissue samples obtained from women with fetoplacental listeriosis. The results raised the possibility that L. monocytogenes crosses the maternofetal barrier through the villous syncytiotrophoblast, with secondary infection occurring via the amniotic epithelium. Because epidemiological studies indicate that the bacterial surface protein, internalin (InlA), may play a role in human fetoplacental listeriosis, we investigated the cellular patterns of expression of its host receptor, E-cadherin, at the maternofetal interface. E-cadherin was found on the basal and apical plasma membranes of syncytiotrophoblasts and in villous cytotrophoblasts. Established trophoblastic cell lines, primary trophoblast cultures, and placental villous explants were each exposed to isogenic InlA+ or InlA- strains of L. monocytogenes, and to L. innocua expressing or not InlA. Quantitative assays of cellular invasion demonstrated that bacterial entry into syncytiotrophoblasts occurs via the apical membrane in an InlA-E-cadherin dependent manner. In human placental villous explants, bacterial invasion of the syncytiotrophoblast barrier and underlying villous tissue and subsequent replication produces histopathological lesions that mimic those seen in placentas of women with listeriosis. Thus, the InlA-E-cadherin interaction that plays a key role in the crossing of the intestinal barrier in humans is also exploited by L. monocytogenes to target and cross the placental barrier. Such a ligand-receptor interaction allowing a pathogen to specifically cross the placental villous trophoblast barrier has not been reported previously.

Analysis of a high-throughput yeast two-hybrid system and its use to predict the function of intracellular proteins encoded within the human MHC class III region

High-throughput (HTP) protein-interaction assays, such as the yeast two-hybrid (Y2H) system, are enormously useful in predicting the functions of novel gene-products. HTP-Y2H screens typically do not include all of the reconfirmation and specificity tests used in small-scale studies, but the effects of omitting these steps have not been assessed. We performed HTP-Y2H screens that included all standard controls, using the predicted intracellular proteins expressed from the human MHC class III region, a region of the genome associated with many autoimmune diseases. The 91 novel interactions identified provide insight into the potential functions of many MHC genes, including C6orf47, LSM2, NELF-E (RDBP), DOM3Z, STK19, PBX2, RNF5, UAP56 (BAT1), ATP6G2, LST1/f, BAT2, Scythe (BAT3), CSNK2B, BAT5, and CLIC1. Surprisingly, our results predict that 1/3 of the proteins may have a role in mRNA processing, which suggests clustering of functionally related genes within the human genome. Most importantly, our analysis shows that omitting standard controls in HTP-Y2H screens could significantly compromise data quality.

Subversion of immune responses by hepatitis C virus: immunomodulatory strategies beyond evasion?

Hepatitis C virus (HCV) is an important human pathogen that causes mild to severe liver disease worldwide. This positive-strand RNA virus is remarkably efficient at establishing persistent infection. In order for a non-cytopathic virus such as HCV to persist, the virus must escape immune recognition or inhibit the host immune responses. Immune escape via mutations in antigenic sites may occur under selective pressure during B-cell or T-cell responses to HCV infection, and may serve as a mechanism for the establishment HCV persistence. In addition to antigenic variation, HCV is able to subvert the host immune response by encoding specific viral gene product(s). An understanding of the mechanisms behind HCV persistence will provide a basis for the rational design of vaccines and novel therapeutic agents targeting human HCV infection.

Maturation-dependent expression of C1q-binding proteins on the cell surface of human monocyte-derived dendritic cells

The expression and cell surface levels of many important receptors are dependent on the maturation stage of dendritic cells (DCs), and related to the unique function of immature and mature DCs. In this report, we show for the first time that human monocyte-derived DCs express two types of C1q-receptors, gC1qR and cC1qR. Furthermore, immature DCs secrete detectable amount of C1q into the culture supernatant. Immature DCs express higher cell surface levels of both C1qRs than mature ones, while the total C1qR protein and mRNA levels remain the same. The following experimental evidence supports this conclusion. (1) Inflammatory cytokines and LPS, which induce maturation of DCs, downregulate surface expression of both C1qR molecules. (2) Cytokines and drugs (IL-10, IFNalpha, dexamethasone) that keep DCs phenotypically and functionally immature significantly upregulate the cell surface expression of both C1qRs. (3) Neither of these treatments changed the intracellular gC1qR level nor the gC1qR mRNA levels measured by real-time RT-PCR. The elevated surface expression of C1qRs on DCs has been found not to be due to increased apoptosis or cell death as the result of DC treatment. Taken together, these data show that human monocyte-derived DCs express gC1qR and cC1qR, their expression on the cell surface is maturation dependent and imature DCs secrete C1q. These data strongly suggest the role of C1qRs in immature DC function and in the regulation of immune processes.

beta 1 Integrin-dependent cell adhesion to EMILIN-1 is mediated by the gC1q domain

EMILIN-1 (Elastin Microfibril Interface Located ProteIN), the prototype of the EMILIN family, consists of a cysteine-rich domain (EMI domain) at the N terminus, an extended region with a high potential coiled-coil structure, a short collagenous stalk, and a self-interacting globular gC1q-l domain. EMILIN-1 is an adhesive extracellular matrix constituent associated with elastic fibers, detected also in the proximity of cell surfaces. To localize the cell attachment site(s), monoclonal antibodies (mAbs) against EMILIN-1 or the gC1q-1 domain were used to inhibit cell attachment to EMILIN-1. Thus, one mAb mapping to the gC1q-1 domain caused complete inhibition of cell attachment. EMILIN-1 and gC1q-1 displayed a comparable dose-dependent ability to promote cell adhesion. Adhesion kinetics was similar to that of fibronectin (FN), reaching the maximum level of attachment at 20 min, but in the absence of cations adhesion was negligible. The relative adhesion strength to detach 50% of the cells was similar for EMILIN-1 and gC1q-1 (250-270 x g) but lower than that for FN (>>500). Cell adhesion to EMILIN-1 or gC1q-1 was completely blocked by a function-blocking beta(1) integrin subunit mAb. In contrast, adhesion to the complement C1q component was totally unaffected. Among the various function-blocking mAbs against the alpha integrin subunits only the anti-alpha(4) fully abrogated cell adhesion to gC1q-1 and up to 70% to EMILIN-1. Furthermore, only K562 cells transfected with the alpha(4) integrin chain, but not wild type K562, were able to adhere to EMILIN-1 and were specifically inhibited by anti-alpha(4) function-blocking mAb. Finally, cells attached to EMILIN-1 or gC1q-1, compared with cells plated on FN or vitronectin, which appeared well spread out on the substrate with prominent stress fibers and focal contacts, were much smaller with wide ruffles and a different organization status of the actin cytoskeleton along the cell periphery. This pattern was in accord with the ability of EMILIN-1 to promote cell movement.

Rules for gene usage inferred from a comparison of large-scale gene expression profiles of T and B lymphocyte development

Ribonucleic acid expression profiles of seven consecutive stages of mouse thymocyte development were generated on high density oligonucleotide arrays. Previously known expression patterns of several genes were confirmed. Ten percent (1,304 of more than 13,000) of the monitored genes were found with 99% confidence to be differentially expressed across all T cell developmental stages. When compared with 1,204 genes differentially expressed in five consecutive B lineage developmental stages of bone marrow, >40% (546 genes) appeared to be shared by both lineages. However, when four pools of functionally distinct cell stages were compared between B and T cell development, DJ-rearranged precursor cells and resting immature precursor cells before and after surface Ag receptor expression shared less than 10%, mature resting lymphocytes between 15 and 20%, and only cycling precursors responding to precursor lymphocyte receptor deposition shared >50% of these differentially expressed genes. Three general rules emerge from these results: 1) proliferation of cells at comparable stages is in majority executed by the same genes; 2) intracellular signaling and intercellular communication are effected largely by different genes; and 3) most genes are not used strictly at comparable, but rather at several, stages, possibly in different functional contexts.

Constitutive expression of hyaluronan binding protein 1 (HABP1/p32/gC1qR) in normal fibroblast cells perturbs its growth characteristics and induces apoptosis

Hyaluronan binding protein 1 (HABP1) is a ubiquitously expressed multifunctional phospho-protein that interacts with a wide range of ligands and is implicated in cell signalling. Recently, we have reported that HABP1 is an endogenous substrate for MAP kinase and upon mitogenic stimulation it is translocated to the nucleus in a MAP kinase-dependent manner (Biochem. Biophys. Res. Commun. 291(4) (2002) 829-837). This prompted us to investigate the role of HABP1 in cell growth or otherwise in low MAP kinase background. We demonstrate that HABP1, when overexpressed in normal rat skin fibroblasts, remained in the cytosol, primarily concentrated around the nuclear periphery. However, HABP1 overexpressing cells showed extensive vacuolation and reduced growth rate, which was corrected by frequent medium replenishment. Further investigation revealed that HABP1 overexpressing cells undergo apoptosis, as detected by TUNEL assay, induction of Bax expression, and FACS analysis, and they failed to enter into the S-phase. Periodic medium supplementation prevented these cells from undergoing apoptotic death. We also demonstrate that upon induction of apoptosis in HeLa cells by cisplatin, HABP1 level is upregulated, indicating a correlation between HABP1 and cell death in a normal cellular environment.

Maturation-dependent expression of C1q binding proteins on the cell surface of human monocyte-derived dendritic cells

The expression and cell surface levels of many important receptors are dependent on the maturation stage of dendritic cells (DCs), and related to the unique function of immature and mature DCs. In this report, we show, for the first time, that human monocyte-derived DCs express two types of C1q receptors, gC1qR and cC1qR. Furthermore, immature DCs secrete detectable amount of C1q into the culture supernatant. Immature DCs express higher cell surface levels of both C1qRs than mature ones, while the total C1qR protein and mRNA levels remain the same. The following experimental evidence support this conclusion: (1) Inflammatory cytokines and LPS, which induce maturation of DCs, downregulate surface expression of both C1qR molecules. (2) Cytokines and drugs (IL-10, IFN-alpha, Dexamethasone), which keep DCs phenotypically and functionally immature, significantly upregulate the cell surface expression of both C1qRs. (3) Neither of these treatments changed the intracellular gC1qR level nor the gC1qR mRNA levels measured by real time RT-PCR. The elevated surface expression of C1qRs on DCs has been found to be not due to increased apoptosis or cell death as the result of DC treatment. Taken together, these data show that human monocyte-derived DCs express gC1qR and cC1qR, their expression on the cell surface is maturation dependent, and immature DCs secrete C1q. These data strongly suggest the role of C1qRs in immature DC function and in the regulation of immune processes.

Zinc induces exposure of hydrophobic sites in the C-terminal domain of gC1q-R/p33

Endothelial cells and platelets are known to express gC1q-R on their surface. In addition to C1q, endothelial cell gC1q-R has been shown to bind high molecular weight kininogen (HK) and factor XII (FXII). However, unlike C1q, whose interaction with gC1q-R does not require divalent ions, the binding of HK to gC1q-R is absolutely dependent on the presence of zinc. However, the mechanism by which zinc modulates this interaction is not fully understood. To investigate the role of zinc, binding studies were done using the hydrophobic dye, bis-ANS. The fluorescence intensity of bis-ANS, greatly increases and the emission maximum is blue-shifted from 525 to 485nm upon binding to hydrophobic sites on proteins. In this report, we show that a blue-shift in emission maximum is also observed when bis-ANS binds to gC1q-R in the presence but not in the absence of zinc suggesting that zinc induces exposure of hydrophobic sites in the molecule. The binding of bis-ANS to gC1q-R is specific, dose-dependent, and reversible. In the presence of zinc, this binding is abrogated by monoclonal antibody 74.5.2 directed against gC1q-R residues 204-218. This segment of gC1q-R, which corresponds to the beta6 strand in the crystal structure, has been shown previously to be the binding site for HK. A similar trend in zinc-induced gC1q-R binding was also observed using the hydrophobic matrix octyl-Sepharose. Taken together, our data suggest that zinc can induce the exposure of hydrophobic sites in the C-terminal domain of gC1q-R involved in binding to HK/FXII.

InlB, a surface protein ofListeria monocytogenesthat behaves as an invasin and a growth factor

Molecules from some pathogenic bacteria mimic natural host cell ligands and trigger engulfment of the bacterium after specifically interacting with cell-surface receptors. The leucine-rich repeat (LRR)-containing protein InlB of Listeria monocytogenes is one such molecule. It triggers bacterial entry by interacting with the hepatocyte growth factor receptor (HGF-R or Met)and two other cellular components: gC1q-R and proteoglycans. Recent studies point to significant similarities between the molecular mechanisms underlying InlB-mediated entry into cells and classic phagocytosis. In addition, InlB, in common with HGF, activates signaling cascades that are not involved in bacterial entry. Therefore, studies of InlB may help us to analyze the previously noticed similarities between growth factor receptor activation and phagocytosis.

Hepatitis C virus: immunosuppression by complement regulatory pathway

Hepatitis C virus (HCV) infection in humans is almost invariably associated with viral persistence and chronic hepatitis. HCV-induced chronic hepatitis is a major risk factor for the development of hepatocellular carcinoma. The high incidence of HCV persistence suggests that this virus has evolved one or more mechanisms to evade and possibly suppress host immune responses. To understand the mechanism(s) involved in the establishment of HCV persistence, we have identified an HCV core protein as an immunomodulatory molecule to suppress host immune response. We have further determined a molecular mechanism of HCV core-mediated immune suppression by searching for a potential host protein(s) capable of associating with the HCV core protein. Interestingly, the Clq complement receptor, gC1qR, can bind to the HCV core. Clq is a ligand of gClqR and is involved in the early defense against viral infection as well as regulation of adaptive immune response. Similar to Clq, the HCV core can inhibit human T-lymphocyte proliferative response through its interaction with the gC1qR. It implicates that HCV core/gClqR-induced immune suppression may play a critical role in the establishment of persistent infection.

Cooperation of C1q receptors and integrins in C1q-mediated endothelial cell adhesion and spreading

The interaction of C1q with endothelial cells elicits a multiplicity of biologic responses. Although these responses are presumed to be mediated by the interaction of C1q with endothelial cell surface proteins, the identity of the participants is not known. In this study we examined the roles of two C1q binding proteins, cC1q-R/calreticulin and gC1q-R/p33, in C1q-mediated adhesion and spreading of human dermal microvascular endothelial cells (HDMVEC). When HDMVEC were cultured in microtiter plate wells coated with concentrations of C1q ranging from 0 to 50 microg/ml, a specific and dose-dependent adhesion and spreading was observed. The extent of adhesion and spreading was similar to the adhesion seen on collagen-coated wells. Spreading (68 +/- 12%) and to a moderate extent adhesion (47 +/- 9%) were inhibited by anti-gC1q-R mAb 60.11. Similar effects were noted with polyclonal anti-cC1q-R but not with control nonimmune IgG. The two Abs had a slight additive effect (75 +/- 13% inhibition) when mixed together in the proportion of 100 microg/ml anti-gC1q-R and 30 microg/ml anti-cC1q-R. More importantly, a 100% inhibition of spreading, but not adhesion, to C1q-coated wells was observed when HDMVEC were cultured in the presence of 30 microM of the peptide GRRGDSP but not GRRGESP. Furthermore, while anti-beta(1) integrin Ab blocked both adhesion and spreading, anti-alpha(5) integrin blocked only spreading and not adhesion. Ag capture ELISA of endothelial cell membrane proteins using polyclonal anti-gC1q-R showed the presence of not only beta(1) and alpha(5) integrins but also CD44. Taken together these results suggest that endothelial cell adhesion and spreading require the cooperation of both C1qRs and beta(1) integrins and possibly other membrane-spanning molecules.

Surface expression of complement receptor gC1q-R/p33 is increased on the plasma membrane of human spermatozoa after capacitation

Evidence is increasing that complement components might play a role in fertilization. C1q, the first component of the classical complement cascade, has the ability to promote sperm agglutination in a capacitation-dependent manner as well as an effect on sperm-oolemma binding and fusion. We have previously detected gC1qR, the receptor for the globular head portion of C1q, on the surface of capacitated sperm. In this study, we examined the expression of gC1qR in both fresh and capacitated human spermatozoa. We performed immunoprecipitation for gC1qR and analyzed biotinylated sperm membrane by Western blot to illustrate an increase in receptor density after overnight capacitation. These results were confirmed by flow cytometric analysis of spermatozoa using fluorescein isothiocyanate-labeled monoclonal anti-gC1qR antibody. Confocal, indirect immunofluorescence microscopy revealed an increase in receptor expression over the rostral portion of the sperm head after capacitation. In addition, the ability of live spermatozoa to bind to monoclonal anti-gC1qR antibody-coated microtiter wells was also increased after capacitation. These results suggest that gC1qR may play a role in human fertilization.