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

Alteration of splice site selection in the LMNA gene and inhibition of progerin production via AMPK activation

Hutchinson-Gilford Progeria Syndrome (HGPS) is a rare genetic condition characterized by an accelerated aging phenotype and an average life span of 13years. Patients typically exhibit extensive pathophysiological vascular alterations, eventually resulting in death from stroke or myocardial infarction. A silent point mutation at position 1824 (C1824T) of the LMNA gene, generating a truncated form of lamin A (progerin), has been shown to be the cause of most cases of HGPS. Interestingly, this mutation induces the use of an internal 5' cryptic splice site within exon 11 of the LMNA pre-mRNA, leading to the generation of progerin via aberrant alternative splicing. The serine-arginine rich splicing factor 1 (SRSF1 or ASF/SF2) has been shown to function as an oncoprotein and is upregulated in many cancers and other age-related disorders. Indeed, SRSF1 inhibition results in a splicing ratio in the LMNA pre-mRNA favoring lamin A production over that of progerin. It is our hypothesis that activation of AMP-activated protein kinase (AMPK), a master regulator of cellular metabolism, may lead to a reduction in SRSF1 and thus a decrease in the use of the LMNA 5' cryptic splice site in exon 11 through upregulation of p32, a splicing factor-associated protein and putative mitochondrial chaperone that has been shown to inhibit SRSF1 and enhance mitochondrial DNA (mtDNA) replication and oxidative phosphorylation. AMPK activation by currently available compounds such as metformin, resveratrol, and berberine may thus have wide-ranging implications for disorders associated with increased production and accumulation of progerin.

cC1qR/CR and gC1qR/p33: observations in cancer

The survival and growth of a primary tumor depends, by and large, on three major events: immune evasion, angiogenesis and metastasis. Tumor cells are "modified self", and as such express a plethora of modified surface antigens capable of inducing antibody production. Anti-tumor cell antibodies should, in theory, activate complement resulting in cell destruction. But this is not the case. Akin to many pathogenic microorganisms whose survival depends on evading the immune system, cancer cells have also evolved diverse mechanisms to prevent host mediated cell destruction by either retaining critical regulatory molecules or by hijacking host proteins to ensure their survival. Although immune evasion, angiogenesis and metastasis are complex biological processes involving a myriad of tumor associated proteins, enzymes, and cytokines, C1qRs can, nonetheless play an important role in all or part of these processes. Although both cC1qR/CR and gC1qR are expressed by all somatic cells, with the exception of red blood cells, both are highly upregulated on almost all types of tumors. It is not surprising therefore that blockade of C1qR on tumor cells inhibits their proliferation suggesting the significance of C1qRs in tumor growth and progression. Interestingly, the two C1q receptors: cC1qR/CR and gC1qR play a differential role in carcinogenesis. While gC1qR promotes tumor cell survival by enhancing angiogenesis and metastasis and also by contributing to the hypercoagulable and prothrombotic microenvironment, cC1qR/CR expression represents a pro-phagocytic "eat-me" signal through which cC1qR/CR expressing tumor cells are tagged for destruction by macrophages. The data accumulated to date therefore identify gC1qR and cC1qR/CR as potential targets for the design of either protein-based, antibody-based or chemical based therapeutic intervention that could be used to enhance conventional anti-cancer therapy. The inhibition of tumor cell proliferation by monoclonal antibody recognizing the C1q site on gC1qR, as well as the identification of agents such as anthracyclin that enhance cC1qR/CR expression on tumor cells, are indeed steps in the right direction.

High mortality of juvenile gilthead sea bream (Sparus aurata) from photobacteriosis is associated with alternative macrophage activation and anti-inflammatory response: results of gene expression profiling of early responses in the head kidney

The halophilic bacterium Photobacterium damselae subsp. piscicida (Phdp) represents a substantial health problem for several fish species in aquaculture. Bacteria that reside free and inside phagocytes cause acute and chronic forms of photobacteriosis. Infections of juveniles rapidly kill up to 90-100% fish. Factors underlying failure of the immune protection against bacteria remain largely unknown. The reported study used a transcriptomic approach to address this issue. Juvenile sea breams (0.5 g) were challenged by immersion in salt water containing 2.89 × 10(8) CFU of a virulent Phdp and the head kidney was sampled after 24- and 48-h. Analyses were performed using the second version of a 44 k oligonucleotide DNA microarray that represents 19,734 sea bream unique transcripts and covers diverse immune pathways. Expression changes of selected immune genes were validated with qPCR. Results suggested rapid recognition of the pathogen, as testified by up-regulation of lectins and antibacterial proteins (bactericidal permeability-increasing protein lectins, lysozyme, intracellular and extracellular proteases), chemokines and chemokine receptors. Increased expression of proteins involved in iron and heme metabolism also could be a response against bacteria that are dependent on iron. However, negative regulators of immune/inflammatory response were preponderant among the up-regulated genes. A remarkable finding was the increased expression of IL-10 in concert with up-regulation of arginase I and II and proteins of the polyamine biosynthesis pathway that diverts the arginine flux from the production of reactive nitrogen species. Such expression changes are characteristic for alternatively activated macrophages that do not develop acute inflammatory responses. Immune suppression can be induced by the host to reduce tissue damages or by the pathogen to evade host response.

A calreticulin/gC1qR complex prevents cells from dying: a conserved mechanism from arthropods to humans

The crossroad between cell death and proliferation is a general target for viral infections because viruses need to obstruct apoptosis to use cells for their own replication. Inducing immunogenic cell death in proliferating cells is also an important aim of anticancer chemotherapy. The C1q-binding proteins calreticulin (CRT) and gC1qR are highly conserved ubiquitous proteins, which are putative targets for viral manipulation and are associated with cancer. Here we show that these proteins form a complex in the cytoplasm as a response to viral infection resulting in apoptosis prevention. The formation of a cytosolic CRT/gC1qR complex prevents cell death by reducing gC1qR translocation into the mitochondria, and we provide evidence that this mechanism is conserved from arthropods to human cancer cells. Furthermore, we show that it is possible to prevent this complex from being formed in cancer cells. When the peptides of the complex proteins are overexpressed in these cells, the cells undergo apoptosis. This finding shows a causal link between virus and cancer and may be used to develop new tools in anticancer or antiviral therapy.

Virus factories: biogenesis and structural design

Replication and assembly of many viruses occur in specific intracellular compartments known as ‘virus factories’. Our knowledge of the biogenesis and architecture of these unique structures has increased considerably in the last 10 years, due to technical advances in cellular, molecular and structural biology. We now know that viruses build replication organelles, which recruit cell and viral components in a macrostructure in which viruses assemble and mature. Cell membranes and cytoskeleton participate in the biogenesis of these scaffolds and mitochondria are present in many factories, where they might supply energy and other essential factors. New inter‐organelle contacts have been visualized within virus factories, whose structure is very dynamic, as it changes over time. There is increasing interest in identifying the factors involved in their biogenesis and functional architecture, and new microscopy techniques are helping us to understand how these complex entities are built and work. In this review, we summarize recent findings on the cell biology, biogenesis and structure of virus factories.

Bivariate Linkage between Acylation-Stimulating Protein and BMI and High-Density Lipoproteins

OBJECTIVE

Given the importance of visceral adiposity in the metabolic syndrome, whether levels of adipokines have shared genetic effects (pleiotropy) with aspects of the metabolic syndrome should be addressed. Acylation-stimulating protein (ASP), an adipose-derived protein, influences lipid metabolism, obesity, and glucose use. Therefore, our objective was to examine the genetic regulation of ASP and associated pleiotropic effects.

RESEARCH METHODS AND PROCEDURES

We assayed serum ASP levels in 435 Mexican Americans participating in the San Antonio Family Heart Study and performed univariate and bivariate variance components analysis.

RESULTS

Additive genetic heritability of ASP was 26% (p = 0.0004). Bivariate genetic analysis detected significant genetic correlations between ASP and several lipid measures but not between ASP and adiposity or diabetes measures. We detected two potential quantitative trait loci influencing ASP levels. The strongest signal was on chromosome 17 near marker D17S1303 [log of the odds ratio (LOD) = 2.7]. The signal on chromosome 15 reached its peak near marker D15S641 (LOD = 2.1). Both signals localize in regions reported to harbor quantitative trait loci influencing obesity and lipid phenotypes in this population. Bivariate linkage analysis yielded LODs of 4.7 for ASP and BMI on chromosome 17 and 3.2 for ASP and high-density lipoprotein2a on chromosome 15.

DISCUSSION

Given these findings, there seems to be a significant genetic contribution to variation in circulating levels of ASP and an interesting pattern of genetic correlation (i.e., pleiotropy) with other risk factors associated with the metabolic syndrome.

gC1qR expression in normal and pathologic human tissues: differential expression in tissues of epithelial and mesenchymal origin

The gC1qR (i.e., gC1q receptor, gC1q binding protein, p32, p33) is a multifunctional cellular protein that interacts with components of the complement, kinin, and coagulation cascades and select microbial pathogens. Enhanced gC1qR expression has been reported in adenocarcinomas arising in a variety of organs. The present study compared gC1qR expression in normal, inflammatory, dysplastic, and malignant tissue of epithelial and mesenchymal origin. gC1qR expression was visualized in tissue sections by immunohistochemistry using the 60.11 monoclonal antibody (i.e., IgG(1) mouse monoclonal antibody directed against gC1qR) and the UltraVision LP Detection System. Sections were counterstained with hematoxylin and examined by light microscopy. Strongest gC1qR expression was noted in epithelial tumors of breast, prostate, liver, lung, and colon, as well as in squamous and basal cell carcinoma of the skin. However, increased gC1qR staining was appreciated also in inflammatory and proliferative lesions of the same cell types, as well as in normal continuously dividing cells. In contrast, tumors of mesenchymal origin generally stained weakly, with the exception of osteoblasts, which stained in both benign and malignant tissues. The data suggest that increased gC1qR expression may be a marker of benign and pathologic cell proliferation, particularly in cells of epithelial origin, with potential diagnostic and therapeutic applications.

A novel pathogen-binding gC1qR homolog, FcgC1qR, in the Chinese white shrimp, Fenneropenaeus chinensis

In vertebrates, the globular "head" of complement component C1q receptor (gC1qR) is a versatile, multiligand binding protein. However, research on its function in invertebrates is limited. In the present study, a full-length cDNA sequence of a novel gC1qR homolog, FcgC1qR, from the Chinese white shrimp Fenneropenaeus chinensis was cloned. Semi-quantitative polymerase chain reaction (PCR) detected FcgC1qR in all examined tissues, with the highest level detected in the intestine. Western blot assay further revealed that the FcgC1qR protein was distributed in all tested tissues except the cell-free hemolymph of normal Chinese white shrimp. In the expression pattern study, quantitative real-time PCR demonstrated that the transcripts of FcgC1qR were up-regulated when challenged with bacteria (Vibrio anguillarum or Staphylococcus aureus) and white spot syndrome virus. Subsequently, FcgC1qR was over-expressed in Escherichia coli, and the polyclonal antibody was prepared with the purified recombinant protein. Microorganism binding was examined using Western blot assay, and revealed that FcgC1qR could bind to Bacillus cereus, Bacillus thuringiensis, S. aureus, V. anguillarum, Vibrioharveyi, and Candida albicans. FcgC1qR was also proven able to bind to S. aureus in a concentration-dependent manner, and this binding activity was partly inhibited by the polyclonal antibody. These results suggest that FcgC1qR may be involved in defending against bacterial infections in the Chinese white shrimp.

The role of humoral innate immunity in hepatitis C virus infection

Infection with Hepatitis C Virus (HCV) causes chronic disease in approximately 80% of cases, resulting in chronic inflammation and cirrhosis. Current treatments are not completely effective, and a vaccine has yet to be developed. Spontaneous resolution of infection is associated with effective host adaptive immunity to HCV, including production of both HCV-specific T cells and neutralizing antibodies. However, the supporting role of soluble innate factors in protection against HCV is less well understood. The innate immune system provides an immediate line of defense against infections, triggering inflammation and playing a critical role in activating adaptive immunity. Innate immunity comprises both cellular and humoral components, the humoral arm consisting of pattern recognition molecules such as complement C1q, collectins and ficolins. These molecules activate the complement cascade, neutralize pathogens, and recruit antigen presenting cells. Here we review the current understanding of anti-viral components of the humoral innate immune system that play a similar role to antibodies, describing their role in immunity to HCV and their potential contribution to HCV pathogenesis.

The C1q family of proteins: insights into the emerging non-traditional functions

Research conducted over the past 20 years have helped us unravel not only the hidden structural and functional subtleties of human C1q, but also has catapulted the molecule from a mere recognition unit of the classical pathway to a well-recognized molecular sensor of damage-modified self or non-self antigens. Thus, C1q is involved in a rapidly expanding list of pathological disorders – including autoimmunity, trophoblast migration, preeclampsia, and cancer. The results of two recent reports are provided to underscore the critical role C1q plays in health and disease. First is the observation by Singh et al. (2011) showing that pregnant C1q−/− mice recapitulate the key features of human preeclampsia that correlate with increased fetal death. Treatment of the C1q−/− mice with pravastatin restored trophoblast invasiveness, placental blood flow, and angiogenic balance and, thus, prevented the onset of preeclampsia. Second is the report by Hong et al. (2009) which showed that C1q can induce apoptosis of prostate cancer cells by activating the tumor suppressor molecule WW-domain containing oxydoreductase (WWOX or WOX1) and destabilizing cell adhesion. Downregulation of C1q on the other hand, enhanced prostate hyperplasia and cancer formation due to failure of WOX1 activation. C1q belongs to a family of structurally and functionally related TNF-α-like family of proteins that may have arisen from a common ancestral gene. Therefore C1q not only shares the diverse functions with the tumor necrosis factor family of proteins, but also explains why C1q has retained some of its ancestral “cytokine-like” activities. This review is intended to highlight some of the structural and functional aspects of C1q by underscoring the growing list of its non-traditional functions.

Genetic analysis in Drosophila reveals a role for the mitochondrial protein p32 in synaptic transmission

Mitochondria located within neuronal presynaptic terminals have been shown to play important roles in the release of chemical neurotransmitters. In the present study, a genetic screen for synaptic transmission mutants of Drosophila has identified the first mutation in a Drosophila homolog of the mitochondrial protein P32. Although P32 is highly conserved and has been studied extensively, its physiological role in mitochondria remains unknown and it has not previously been implicated in neural function. The Drosophila P32 mutant, referred to as dp32^EC1, exhibited a temperature-sensitive (TS) paralytic behavioral phenotype. Moreover, electrophysiological analysis at adult neuromuscular synapses revealed a TS reduction in the amplitude of excitatory postsynaptic currents (EPSC) and indicated that dP32 functions in neurotransmitter release. These studies are the first to address P32 function in Drosophila and expand our knowledge of mitochondrial proteins contributing to synaptic transmission.

A proteomic approach to identification of plutonium-binding proteins in mammalian cells

Plutonium can enter the body through different routes and remains there for decades; however its specific biochemical interactions are poorly defined. We, for the first time, have studied plutonium-binding proteins using a metalloproteomic approach with rat PC12 cells. A combination of immobilized metal ion chromatography, 2D gel electrophoresis, and mass spectrometry was employed to analyze potential plutonium-binding proteins. Our results show that several proteins from PC12 cells show affinity towards Pu(4+)-NTA (plutonium bound to nitrilotriacetic acid). Proteins from seven different spots in the 2D gel were identified. In contrast to the previously known plutonium-binding proteins transferrin and ferritin, which bind ferric ions, most identified proteins in our experiment are known to bind calcium, magnesium, or divalent transition metal ions. The identified plutonium interacting proteins also have functional roles in downregulation of apoptosis and other pro-proliferative processes. MetaCore™ analysis based on this group of proteins produced a pathway with a statistically significant association with development of neoplastic diseases.

The mitochondrial protein C1qbp promotes cell proliferation, migration and resistance to cell death

Complement 1q-Binding Protein (C1qbp) is a mitochondrial protein reported to be upregulated in cancer. However, whether C1qbp plays a tumor suppressive or tumorigenic role in the progression of cancer is controversial. Moreover, the exact effects of C1qbp on cell proliferation, migration, and death/survival have not been definitely proven. To this end, we comprehensively examined the effects of C1qbp on mitochondrial-dependent cell death, proliferation, and migration in both normal and breast cancer cells using genetic gain- and loss-of-function approaches. In normal fibroblasts, overexpression of C1qbp protected the cells against staurosporine-induce apoptosis, increased proliferation, decreased cellular ATP, and increased cell migration in a wound-healing assay. In contrast, the opposite effects were observed in fibroblasts depleted of C1qbp by RNA interference. C1qbp expression was found to be markedly elevated in 4 different human breast cancer cell lines as well as in ductal and adenocarcinoma tumors from breast cancer patients. Stable knockdown of C1qbp by shRNA in the aggressive MDA-MB-231 breast cancer cell line greatly reduced cell proliferation, increased ATP levels, and decreased cell migration compared to control shRNA-transfected cells. Moreover, C1qbp knockdown elicited a significant increase in doxorubicin-induced apoptosis in the MDA-MB-231 cells. Finally, C1qbp upregulation was not restricted to breast cancer cells and tumors, as levels of C1qbp were also found to be significantly elevated in both human lung and colon cancer cell lines and carcinomas. Together, these results establish a pro-tumor, rather than anti-tumor, role for C1qbp, and indicate that C1qbp could serve as a molecular target for cancer therapeutics.

Novel roles of complement in T effector cell regulation

Our understanding of the complement system has markedly evolved from its early beginnings as a protein system merely detecting and tagging a pathogen for further clearance. For example, the repertoire of danger that complement recognizes covers currently a wide range of distinct self and non-self danger signals. Further, complement is now firmly established as instructor of adaptive B and T cell immunity. This review focuses on two the recent emerging paradigms in the field. Firstly, that complement is not only vitally required for the induction of Th1 immunity but also for the timely contraction of this protective response and therefore for prevention of autoimmunity and immune homeostasis. Secondly, that local rather than systemic complement is impacting on immune modulation during a T cell response.

Tim-3 negatively regulates IL-12 expression by monocytes in HCV infection

T cell immunoglobulin and mucin domain-containing protein 3 (Tim-3) is a newly identified negative immunomodulator that is up-regulated on dysfunctional T cells during viral infections. The expression and function of Tim-3 on human innate immune responses during HCV infection, however, remains poorly characterized. In this study, we report that Tim-3 is constitutively expressed on human resting CD14⁺ monocyte/macrophages (M/M_Ø) and functions as a cap to block IL-12, a key pro-inflammatory cytokine linking innate and adaptive immune responses. Tim-3 expression is significantly reduced and IL-12 expression increased upon stimulation with Toll-like receptor 4 (TLR4) ligand - lipopolysaccharide (LPS) and TLR7/8 ligand - R848. Notably, Tim-3 is over-expressed on un-stimulated as well as TLR-stimulated M/M_Ø, which is inversely associated with the diminished IL-12 expression in chronically HCV-infected individuals when compared to healthy subjects. Up-regulation of Tim-3 and inhibition of IL-12 are also observed in M/M_Ø incubated with HCV-expressing hepatocytes, as well as in primary M/M_Ø or monocytic THP-1 cells incubated with HCV core protein, an effect that mimics the function of complement C1q and is reversible by blocking the HCV core/gC1qR interaction. Importantly, blockade of Tim-3 signaling significantly rescues HCV-mediated inhibition of IL-12, which is primarily expressed by Tim-3 negative M/M_Ø. Tim-3 blockade reduces HCV core-mediated expression of the negative immunoregulators PD-1 and SOCS-1 and increases STAT-1 phosphorylation. Conversely, blocking PD-1 or silencing SOCS-1 gene expression also decreases Tim-3 expression and enhances IL-12 secretion and STAT-1 phosphorylation. These findings suggest that Tim-3 plays a crucial role in negative regulation of innate immune responses, through crosstalk with PD-1 and SOCS-1 and limiting STAT-1 phosphorylation, and may be a novel target for immunotherapy to HCV infection.

Cell-surface receptor for complement component C1q (gC1qR) is a key regulator for lamellipodia formation and cancer metastasis

We previously demonstrated that the receptor for the complement component C1q (gC1qR) is a lipid raft protein that is indispensable for adipogenesis and insulin signaling. Here, we provide the first report that gC1qR is an essential component of lamellipodia in human lung carcinoma A549 cells. Cell-surface gC1qR was concentrated in the lamellipodia along with CD44, monosialoganglioside, actin, and phosphorylated focal adhesion kinase in cells stimulated with insulin, IGF-1, EGF, or serum. The growth factor-induced lamellipodia formation and cell migration were significantly decreased in gC1qR-depleted cells, with a concomitant blunt activation of the focal adhesion kinase and the respective receptor tyrosine kinases. Moreover, the gC1qR-depleted cells exhibited a reduced proliferation rate in culture as well as diminished tumorigenic and metastatic activities in grafted mice. We therefore conclude that cell-surface gC1qR regulates lamellipodia formation and metastasis via receptor tyrosine kinase activation.

Structure-function studies using deletion mutants identify domains of gC1qR/p33 as potential therapeutic targets for vascular permeability and inflammation

The endothelial cell receptor complex for kininogen (HK) comprises gC1qR, cytokeratin 1, and urokinase-type plasminogen activator receptor and is essential for activation of the kinin system that leads to bradykinin (BK) generation. Of these, gC1qR/p33 constitutes a high affinity site for HK – the BK precursor – and is therefore critical for the assembly of the kinin-generating cascade. Previous studies have identified a putative HK site within the C-terminal domain (residues 204–218) of gC1qR recognized by mAb 74.5.2. In these studies, we used information from the crystal structure of gC1qR, to engineer several deletion (Δ) mutants and test their ability to bind and/or support BK generation. While deletion of residues 204–218 (gC1qRΔ204–218), showed significantly reduced binding to HK, BK generation was not affected when tested by a sensitive bradykinin immunoassay. In fact, all of the gC1qR deletion mutants supported BK generation with the exception of gC1qRΔ154–162 and a point mutation in which Trp 233 was substituted with Gly. Binding studies also identified the existence of two additional sites at residues 144–162 and 190–202. Moreover, binding of HK to a synthetic peptide 190–202 was inhibited by mAbs 48 and 83, but not by mAb 74.5.2. Since a single residue separates domains 190–202 and 204–218, they may be part of a highly stable HK binding pocket and therefore a potential target for drug design to prevent vascular permeability and inflammation.

Complement 1q-binding protein inhibits the mitochondrial permeability transition pore and protects against oxidative stress-induced death

Opening of the MPT (mitochondrial permeability transition) pore is a critical event in mitochondrial-mediated cell death. However, with the exception of CypD (cyclophilin D), the exact molecular composition of the MPT pore remains uncertain. C1qbp (complement 1q-binding protein) has recently been hypothesized to be an essential component of the MPT pore complex. To investigate whether C1qbp indeed plays a critical role in MPT and cell death, we conducted both gain-of-function and loss-of-function experiments in MEFs (mouse embryonic fibroblasts). We first confirmed that C1qbp is a soluble protein that localizes to the mitochondrial matrix in mouse cells and tissues. Similarly, overexpression of C1qbp in MEFs using an adenovirus resulted in its exclusive localization to mitochondria. To our surprise, increased C1qbp protein levels actually suppressed H2O2-induced MPT and cell death. Antithetically, knockdown of endogenous C1qbp with siRNA (small interfering RNA) sensitized the MEFs to H2O2-induced MPT and cell death. Moreover, we found that C1qbp could directly bind to CypD. Therefore C1qbp appears to act as an endogenous inhibitor of the MPT pore, most likely through binding to CypD, and thus protects cells against oxidative stress.

A gC1qR prevents white spot syndrome virus replication in the freshwater crayfish Pacifastacus leniusculus

The gC1qR/p32 protein is a multiple receptor for several proteins and pathogens. We cloned a gC1qR homologue in a crustacean, Pacifastacus leniusculus, and analyzed the expression of P. leniusculus C1qR (PlgC1qR) in various tissues. The gC1qR/p32 transcript was significantly enhanced by white spot syndrome virus (WSSV) infection 6 h after viral infection both in vitro in a hematopoietic tissue cell culture (Hpt) and in vivo compared to appropriate controls. Moreover, PlgC1qR silencing in both the Hpt cell culture and live crayfish enhanced the WSSV replication. In addition, by making a recombinant PlgC1qR protein we could show that if this recombinant protein was injected in a crayfish, Pacifastacus leniusculus, followed by injection of WSSV, this significantly reduced viral replication in vivo. Furthermore, if the recombinant PlgC1qR was incubated with Hpt cells and then WSSV was added, this also reduced viral replication. These experiments clearly demonstrate that recombinant PlgC1qR reduce WSSV replication both in vivo and in vitro. The results from a far-Western overlay and glutathione S-transferase pull-down assays showed that PlgC1qR could bind to VP15, VP26, and VP28. Altogether, these results demonstrate a role for PlgC1qR in antiviral activity against WSSV.

Three-dimensional structure of Rubella virus factories

Viral factories are complex structures in the infected cell where viruses compartmentalize their life cycle. Rubella virus (RUBV) assembles factories by recruitment of rough endoplasmic reticulum (RER), mitochondria and Golgi around modified lysosomes known as cytopathic vacuoles or CPVs. These organelles contain active replication complexes that transfer replicated RNA to assembly sites in Golgi membranes. We have studied the structure of RUBV factory in three dimensions by electron tomography and freeze-fracture. CPVs contain stacked membranes, rigid sheets, small vesicles and large vacuoles. These membranes are interconnected and in communication with the endocytic pathway since they incorporate endocytosed BSA-gold. RER and CPVs are coupled through protein bridges and closely apposed membranes. Golgi vesicles attach to the CPVs but no tight contacts with mitochondria were detected. Immunogold labelling confirmed that the mitochondrial protein p32 is an abundant component around and inside CPVs where it could play important roles in factory activities.

HIV gp41 engages gC1qR on CD4+ T cells to induce the expression of an NK ligand through the PIP3/H2O2 pathway

CD4⁺ T cell loss is central to HIV pathogenesis. In the initial weeks post-infection, the great majority of dying cells are uninfected CD4⁺ T cells. We previously showed that the 3S motif of HIV-1 gp41 induces surface expression of NKp44L, a cellular ligand for an activating NK receptor, on uninfected bystander CD4⁺ T cells, rendering them susceptible to autologous NK killing. However, the mechanism of the 3S mediated NKp44L surface expression on CD4⁺ T cells remains unknown. Here, using immunoprecipitation, ELISA and blocking antibodies, we demonstrate that the 3S motif of HIV-1 gp41 binds to gC1qR on CD4⁺ T cells. We also show that the 3S peptide and two endogenous gC1qR ligands, C1q and HK, each trigger the translocation of pre-existing NKp44L molecules through a signaling cascade that involves sequential activation of PI3K, NADPH oxidase and p190 RhoGAP, and TC10 inactivation. The involvement of PI3K and NADPH oxidase derives from 2D PAGE experiments and the use of PIP3 and H2O2 as well as small molecule inhibitors to respectively induce and inhibit NKp44L surface expression. Using plasmid encoding wild type or mutated form of p190 RhoGAP, we show that 3S mediated NKp44L surface expression on CD4⁺ T cells is dependent on p190 RhoGAP. Finally, the role of TC10 in NKp44L surface induction was demonstrated by measuring Rho protein activity following 3S stimulation and using RNA interference. Thus, our results identify gC1qR as a new receptor of HIV-gp41 and demonstrate the signaling cascade it triggers. These findings identify potential mechanisms that new therapeutic strategies could use to prevent the CD4⁺ T cell depletion during HIV infection and provide further evidence of a detrimental role played by NK cells in CD4⁺ T cell depletion during HIV-1 infection.

HIV infected individuals suffer from a loss of CD4⁺ lymphocytes. Initially, dying CD4⁺ lymphocytes are mainly infected ones. Afterward, the great majority of dying CD4⁺ lymphocytes are uninfected. The cause of uninfected CD4⁺ lymphocyte death during HIV infection is still under debate. We previously showed that one of the HIV-1 envelop proteins, gp41, induces the expression of a stress molecule called NKp44L on the surface of uninfected CD4⁺ lymphocytes. Uninfected CD4⁺ lymphocytes expressing NKp44L are killed, in vitro and in vivo, by cells of the immune system called NK cells. In this report, we study the CD4⁺ lymphocyte's proteins involved in the expression of NKp44L. To do so, we used several techniques to identify interacting or differentially expressed proteins and to inhibit or monitor enzymes activity. We also induce NKp44L using the product of some of the proteins involved in NKp44L expression. We found that HIV-1 gp41 binds to its receptor gC1qR on CD4⁺ lymphocytes. This interaction respectively activates the PI3K, the NADPH oxidase and p190 RhoGAP which inactivates TC10. Using the obtained data we build a model of the protein cascade involved in NKp44L surface expression.

Rubella virus capsid protein: a small protein with big functions

Virus replication occurs in the midst of a life or death struggle between the virus and the infected host cell. To limit virus replication, host cells can activate a number of antiviral pathways, the most drastic of which is programmed cell death. Whereas large DNA viruses have the luxury of encoding accessory proteins whose main function is to interfere with host cell defences, the genomes of RNA viruses are not large enough to encode proteins of this type. Recent studies have revealed that proteins encoded by RNA viruses often play multiple roles in the battles between viruses and host cells. In this article, we discuss the many functions of the rubella virus capsid protein. This protein has well-defined roles in virus assembly, but recent research suggests that it also functions to modulate virus replication and block host cell defences.

Mechanisms of oral bacteria-induced platelet activation

The oral cavity is inhabited by over 500 different bacterial species that normally exist in ecological balance both with each other and with the host. When this equilibrium is disturbed, an overgrowth of individual organisms can occur, which, in turn, can lead to the onset of pathological processes, notably dental caries and periodontitis. Generally, bacteraemias occur more frequently in individuals with periodontal disease, and these bacteraemias have been implicated in the development of a range of systemic diseases, including atherothrombotic disorders. The mechanism underlying this relationship remains to be precisely defined, although studies have shown a link between bacteria of oral origin and platelet activation. Several orally derived species of bacteria interact with platelets, including those of the Streptococcus ( Streptococcus sanguinis , Streptococcus mutans , Streptococcus agalactiae , Streptococcus pyogenes , Streptococcus gordonii , Streptococcus pneumoniae , Streptococcus mitis ) and Staphylococcus ( Staphylococcus epidermidis , Staphylococcus capitis ) genera, as well as Pseudomonas aeruginosa and Porphyromonas gingivalis . In addition, some members of both the Streptococcus and the Staphylococcus genera, as well as Porphyromonas gingivalis , can activate platelets in vitro. The current review describes the heterogeneous mechanisms of platelet activation employed by individual bacterial species. The pathological and clinical implications of platelet activation by orally derived bacteria are discussed.

Structure of the Trypanosoma brucei p22 protein, a cytochrome oxidase subunit II-specific RNA-editing accessory factor

Kinetoplastid RNA (k-RNA) editing is a complex process in the mitochondria of kinetoplastid protozoa, including Trypanosoma brucei, that involves the guide RNA-directed insertion and deletion of uridines from precursor-mRNAs to produce mature, translatable mRNAs. k-RNA editing is performed by multiprotein complexes called editosomes. Additional non-editosome components termed k-RNA-editing accessory factors affect the extent of editing of specific RNAs or classes of RNAs. The T. brucei p22 protein was identified as one such accessory factor. Here we show that p22 contributes to cell growth in the procyclic form of T. brucei and functions as a cytochrome oxidase subunit II-specific k-RNA-editing accessory factor. To gain insight into its functions, we solved the crystal structure of the T. brucei p22 protein to 2.0-A resolution. The p22 structure consists of a six-stranded, antiparallel beta-sheet flanked by five alpha-helices. Three p22 subunits combine to form a tight trimer that is primarily stabilized by interactions between helical residues. One side of the trimer is strikingly acidic, while the opposite face is more neutral. Database searches show p22 is structurally similar to human p32, which has a number of functions, including regulation of RNA splicing. p32 interacts with a number of target proteins via its alpha1 N-terminal helix, which is among the most conserved regions between p22 and p32. Co-immunoprecipitation studies showed that p22 interacts with the editosome and the k-RNA accessory protein, TbRGG2, and alpha1 of p22 was shown to be important for the p22-TbRGG2 interaction. Thus, these combined studies suggest that p22 mediates its role in k-RNA editing by acting as an adaptor protein.

The rubella virus capsid protein inhibits mitochondrial import

The rubella virus (RV) capsid is an RNA-binding protein that functions in nucleocapsid assembly at the Golgi complex, the site of virus budding. In addition to its role in virus assembly, pools of capsid associate with mitochondria, a localization that is not consistent with virus assembly. Here we examined the interaction of capsid with mitochondria and showed that this viral protein inhibits the import and processing of mitochondrial precursor proteins in vitro. Moreover, RV-infected cells were found to contain lower intramitochondrial levels of matrix protein p32. In addition to inhibiting the translocation of substrates into mammalian mitochondria, capsid efficiently blocked import into yeast mitochondria, thereby suggesting that it acts by targeting a highly conserved component of the translocation apparatus. Finally, mutation of a cluster of five arginine residues in the amino terminus of capsid, though not interfering with its binding to mitochondria, abrogated its ability to block protein import into mitochondria. This is the first report of a viral protein that affects the import of proteins into mitochondria.

Hepatic gene expression profiling reveals protective responses in Atlantic salmon vaccinated against furunculosis

Background

Furunculosis, a disease caused with gram negative bacteria Aeromonas salmonicida produces heavy losses in aquaculture. Vaccination against furunculosis reduces mortality of Atlantic salmon but fails to eradicate infection. Factors that determine high individual variation of vaccination efficiency remain unknown. We used gene expression analyses to search for the correlates of vaccine protection against furunculosis in Atlantic salmon.

Results

Naïve and vaccinated fish were challenged by co-habitance. Fish with symptoms of furunculosis at the onset of mass mortality (LR - low resistance) and survivors (HR - high resistance) were sampled. Hepatic gene expression was analyzed with microarray (SFA2.0 - immunochip) and real-time qPCR. Comparison of LR and HR indicated changes associated with the protection and results obtained with naïve fish were used to find and filter the vaccine-independent responses. Genes involved in recruitment and migration of immune cells changed expression in both directions with greater magnitude in LR. Induction of the regulators of immune responses was either equal (NFkB) or greater (Jun) in LR. Expression levels of proteasome components and extracellular proteases were higher in LR while protease inhibitors were up-regulated in HR. Differences in chaperones and protein adaptors, scavengers of reactive oxygen species and genes for proteins of iron metabolism suggested cellular and oxidative stress in LR. Reduced levels of free iron and heme can be predicted in LR by gene expression profiles with no protection against pathogen. The level of complement regulation was greater in HR, which showed up-regulation of the components of membrane attack complex and the complement proteins that protect the host against the auto-immune damages. HR fish was also characterized with up-regulation of genes for proteins involved in the protection of extracellular matrix, lipid metabolism and clearance of endogenous and exogenous toxic compounds. A number of genes with marked expression difference between HR and LR can be considered as positive and negative correlates of vaccine protection against furunculosis.

Conclusion

Efficiency of vaccination against furunculosis depends largely on the ability of host to neutralize the negative impacts of immune responses combined with efficient clearance and prevention of tissue damages.

Role of the receptor for the globular domain of C1q protein in the pathogenesis of hepatitis C virus-related cryoglobulin vascular damage

Mixed cryoglobulinemia (MC) is a lymphoproliferative disorder observed in approximately 10 to 15% of hepatitis C virus (HCV)-infected patients. Circulating, nonenveloped HCV core protein, which has been detected in cryoprecipitable immune complexes, interacts with immunocytes through the receptor for the globular domain of C1q protein (gC1q-R). In this study, we have evaluated circulating gC1q-R levels in chronically HCV-infected patients, with and without MC. These levels were significantly higher in MC patients than in those without MC and in healthy controls and paralleled specific mRNA expression in PBL. Soluble gC1q-R circulates as a complexed form containing both C1q and HCV core proteins. Higher serum gC1q-R levels negatively correlated with circulating concentrations of the C4d fragment. The presence of sequestered C4d in the vascular bed of skin biopsies from MC patients was indicative of in situ complement activation. In vitro studies showed that release of soluble gC1q-R is regulated by HCV core-mediated inhibition of cell proliferation. Our results indicate that up-regulation of gC1q-R expression is a distinctive feature of MC, and that dysregulated shedding of C1q-R molecules contributes to vascular cryoglobulin-induced damage via the classic complement-mediated pathway.

T-786C polymorphism of the NOS-3 gene and the endothelial cell response to fluid shear stress-a proteome analysis

Endothelial dysfunction is a common denominator of cardiovascular disease. Central to endothelial dysfunction is a decrease in the bioavailability of nitric oxide (NO) synthesized by endothelial NO synthase (NOS-3). In vivo, the level of fluid shear stress (FSS) exerted by the flowing blood determines NOS-3 expression. However, in contrast to the -786T variant of the nos-3 gene, the -786C variant is not sensitive to shear stress. Consequently, cells homozygous for this variant have an inadequate capacity to synthesize NO. Therefore, we have compared shear stress-induced protein expression in human primary cultured endothelial cells with TT or CC genotype. Cells with the CC genotype exhibited a greatly reduced FSS-induced NOS-3 expression as well as a diminished NO synthesis capacity when compared to TT genotype cells. Proteome changes in response to FSS (30 dyn/cm(2) for 24 h) were monitored by 2D-gel electrophoresis/densitometry/mass spectrometry. Of a total of 14 FSS-sensitive proteins, 8 were identically expressed in all cells. Four proteins, all of them part of the NO-dependent endoplasmic reticulum-stress response, were up-regulated by FSS only in cells with TT genotype. In contrast, CC genotype cells responded to FSS with a unique increase in manganese-containing superoxide dismutase expression. These differences in protein expression may (i) reflect the low bioavailability of NO in cells homozygous for the -786C variant of the nos-3 gene and (ii) point to a mechanism by which this deficit is counterbalanced by protecting the less abundant NO from rapid degradation.

Evidence that a C1q/C1qR system regulates monocyte-derived dendritic cell differentiation at the interface of innate and acquired immunity

Growing evidence shows that C1q modulates the growth and function of cells committed to the monocyte-derived dendritic cell (DC) lineage. Because C1q regulates both innate and acquired immune responses, we postulated that C1q modulates the transition from monocytes to DCs, i.e. the interface between innate and acquired immunity. Human peripheral blood monocytes cultured with soluble C1q and DC growth factors (granulocyte-macrophage colony-stimulating factor + Interleukin-4) failed to down-regulate monocyte-associated (CD14, CD16) and up-regulate DC-associated (CD83, CD86) markers. Impaired DC differentiation was not due to apoptosis; further analysis revealed the development of CD14(hi)CD11c(hi)CD16 (+/-) cells that have previously been associated with both innate and acquired immunity. Monocyte-DC precursors expressed gC1qR, the receptor for globular heads of C1q, from the outset, while cC1qR, the receptor for the collagen tails of C1q, was expressed at low levels. Notably, the binding pattern of monoclonal antibodies specific to the globular heads of C1q indicated that C1q is bound to monocytes via globular heads, presumably through gC1qR. Moreover, gC1qR levels decreased, while cC1qR levels were dramatically amplified as monocytes differentiated into immature DC. Thus, specific C1q/C1q receptor (R) interactions may control the transition from the monocyte state (innate immunity) toward the professional antigen-presenting cell state (adaptive immunity).

Structure of the third intracellular loop of the vasopressin V2 receptor and conformational changes upon binding to gC1qR

The V2 vasopressin receptor is a G-protein-coupled receptor that regulates the renal antidiuretic response. Its third intracellular loop is involved in the coupling not only with the GalphaS protein but also with gC1qR, a potential chaperone of G-protein-coupled receptors. In this report, we describe the NMR solution structure of the V2 i3 loop under a cyclized form (i3_cyc) and characterize its interaction with gC1qR. i3_cyc formed a left-twisted alpha-helical hairpin structure. The building of a model of the entire V2 receptor including the i3_cyc NMR structure clarified the side-chain orientation of charged residues, in agreement with literature mutagenesis reports. In the model, the i3 loop formed a rigid helical column, protruding deep inside the cytoplasm, as does the i3 loop in the recently elucidated structure of squid rhodopsin. However, its higher packing angle resulted in a different structural motif at the intracellular interface, which may be important for the specific recognition of GalphaS. Moreover, we could estimate the apparent K(d) of the i3_cyc/gC1qR complex by anisotropy fluorescence. Using a shorter and more soluble version of i3_cyc, which encompassed the putative site of gC1qR binding, we showed by NMR saturation transfer difference spectroscopy that the binding surface corresponded to the central arginine cluster. Binding to gC1qR induced the folding of the otherwise disordered short peptide into a spiral-like path formed by a succession of I and IV turns. Our simulations suggested that this folding would rigidify the arginine cluster in the entire i3 loop and would alter the conformation of the cytosolic extensions of TM V and TM VI helices. In agreement with this conformational rearrangement, we observed that binding of gC1qR to the full-length receptor modifies the intrinsic tryptophan fluorescence binding curves of V2 to an antagonist.

Pancreatic expression and mitochondrial localization of the progestin-adipoQ receptor PAQR10

Steroid hormones induce changes in gene expression by binding to intracellular receptors that then translocate to the nucleus. Steroids have also been shown to rapidly modify cell function by binding to surface membrane receptors. We identified a candidate steroid membrane receptor, the progestin and adipoQ receptor (PAQR) 10, a member of the PAQR family, in a screen for genes differentially expressed in mouse pancreatic beta-cells. PAQR10 gene expression was tissue restricted compared with other PAQRs. In the mouse embryonic pancreas, PAQR10 expression mirrored development of the endocrine lineage, with PAQR10 protein expression confined to endocrine islet-duct structures in the late embryo and neonate. In the adult mouse pancreas, PAQR10 was expressed exclusively in islet cells except for its reappearance in ducts of maternal islets during pregnancy. PAQR10 has a predicted molecular mass of 29 kDa, comprises seven transmembrane domains, and, like other PAQRs, is predicted to have an intracellular N-terminus and an extracellular C-terminus. In silico analysis indicated that three members of the PAQR family, PAQRs 9, 10, and 11, have a candidate mitochondrial localization signal (MLS) at the N-terminus. We showed that PAQR10 has a functional N-terminal MLS and that the native protein localizes to mitochondria. PAQR10 is structurally related to some bacterial hemolysins, pore-forming virulence factors that target mitochondria and regulate apoptosis. We propose that PAQR10 may act at the level of the mitochondrion to regulate pancreatic endocrine cell development/survival.

Differential regulation of SOCS-1 signalling in B and T lymphocytes by hepatitis C virus core protein

Hepatitis C virus (HCV) infection is characterized by a strong propensity toward chronicity, autoimmune phenomena and lymphomagenesis, supporting a role for lymphocyte dysregulation during persistent viral infection. We have shown that HCV core protein inhibits T-cell functions through interaction with a complement receptor, gC1qR. Here, we further report that B cells also express gC1qR that can be bound by HCV core protein. Importantly, using flow cytometry, we demonstrated differential regulation of B and T lymphocytes by the HCV core-gC1qR interaction, with down-regulation of CD69 activation in T cells but up-regulation of CD69 activation and cell proliferation in B cells. HCV core treatment led to decreased interferon-gamma production in CD8+ T cells but to increased immunoglobulin M and immunoglobulin G production as well as cell surface expression of costimulatory and chemokine receptors, including CD86 (B7-2), CD154 (CD40L) and CD195 (CCR5), in CD20+ B cells. Finally, we showed down-regulation of suppressor of cytokine signalling-1 (SOCS-1) using real-time reverse transcription-polymerase chain reaction, accompanied by up-regulation of signal transducer and activator of transcription-1 (STAT1) phosphorylation in B cells in response to HCV core protein, with the opposite pattern observed in HCV core-treated T cells. This study demonstrates differential regulation of B and T lymphocytes by HCV core and supports a mechanism by which lymphocyte dysregulation occurs in the course of persistent HCV infection.

Crosstalk between the alpha2beta1 integrin and c-met/HGF-R regulates innate immunity

Data from several investigators suggest that the alpha2beta1 integrin, a receptor for collagens, laminins, decorin, E-cadherin, matrix metalloproteinase-1, endorepellin, and several viruses, is required for innate immunity and regulation of autoimmune/allergic disorders. We demonstrated that the innate immune response to Listeria monocytogenes required alpha2beta1 integrin expression by peritoneal mast cells (PMCs). Ligation of the alpha2beta1 integrin by C1q contained in immune complexes comprised of Listeria and antibody was required for PMC activation in vitro and in vivo. However, ligation of the alpha2beta1 integrin alone was insufficient to activate cytokine secretion, suggesting that one or more additional signals emanating from a coreceptor were required for PMC activation. Here, we demonstrate that C1q, but neither other complement proteins nor FcRgamma, is required for early innate immune response to Listeria. The binding of Listeria's Internalin B (InlB) to hepatocyte growth factor receptor (HGF-R)/c-met provides the costimulatory function required for PMC activation. Either HGF or Listeria InlB bound to c-met and either C1q or type I collagen bound to alpha2beta1 integrin stimulates PMC activation. These findings suggest that crosstalk between c-met and the alpha2beta1 integrin may contribute to mast-cell activation in autoimmune and inflammatory disorders.

Platelet mediated complement activation

The complement system comprises a series of proteases and inhibitors that are activated in cascade-like fashion during host defense (Makrides 1998). A growing body of evidence supports the hypothesis that immune mechanisms, including complement activation, are involved in inflammatory conditions associated with vascular injury (Acostan et al. 2004; Giannakopoulos et al. 2007), and disseminated intravascular coagulation associated with massive trauma (Huber-Lang, this volume). We propose that platelets and platelet derived microparticles focus complement to sites of vascular injury where regulated complement activation participates in clearing terminally activated platelets and microparticles from the circulation, and dysregulated complement activation contributes to inflammation and thrombosis. Given the central role of platelets in hemostasis and thrombosis, it is not surprising that activated complement components have been demonstrated in many types of atherosclerotic and thrombotic vascular lesions (Torzewsjki et al. 2007; Niculescu et al. 2004).

Identification of p32 as a novel substrate for ATM in heart

Chemotherapeutic agents to induce DNA damage have been limited to use due to severe side effects of cardiotoxicity. ATM (Ataxia-telangiectasia mutated) is an essential protein kinase in triggering DNA damage responses. However, it is unclear how the ATM-mediated DNA damage responses are involved in the cardiac cell damage. To elucidate these functions in heart, we searched for specific substrates of ATM from mouse heart homogenate. Combining an in vitro phosphorylation following anion-exchange chromatography with purification by reverse-phase high-performance liquid chromatography (HPLC), we successfully identified p32, an ASF/SF2-associated protein, as a novel substrate for ATM. An in vitro kinase assay using recombinant p32 revealed that ATM directly phosphorylated p32. Furthermore, we determined Ser 148 of p32 as an ATM phosphorylation site. Since p32 is known to regulate mRNA splicing and transcription, p32 phosphorylation by ATM might be a new transcriptional regulatory pathway for specific DNA damage responses in heart.

The exosporium of B. cereus contains a binding site for gC1qR/p33: implication in spore attachment and/or entry

B. cereus, is a member of a genus of aerobic, gram-positive, spore-forming rod-like bacilli, which includes the deadly, B. anthracis. Preliminary experiments have shown that gC1qR binds to B. cereus spores that have been attached to microtiter plates. The present studies were therefore undertaken, to examine if cell surface gC1qR plays a role in B. cereus spore attachment and/or entry. Monolayers of human colon carcinoma (Caco-2) and lung cells were grown to confluency on 6 mm coverslips in shell vials with gentle swirling in a shaker incubator. Then, 2 microl of a suspension of strain SB460 B. cereus spores (3x10(8)/ml, in sterile water), were added and incubated (1-4 h; 36 degrees C) in the presence or absence of anti-gC1qR mAb-carbon nanoloops. Examination of these cells by EM revealed that: (1) When B. cereus endospores contacted the apical Caco-2 cell surface, or lung cells, gC1qR was simultaneously detectable, indicating upregulation of the molecule. (2) In areas showing spore contact with the cell surface, gC1qR expression was often adjacent to the spores in association with microvilli (Caco-2 cells) or cytoskeletal projections (lung cells). (3) Furthermore, the exosporia of the activated and germinating spores were often decorated with mAb-nanoloops. These observations were further corroborated by experiments in which B.cereus spores were readily taken up by monocytes and neutrophils, and this uptake was partially inhibited by mAb 60.11, which recognizes the C1q binding site on gC1qR. Taken together, the data suggest a role, for gC1qR at least in the initial stages of spore attachment and/or entry.

T cell dysfunction by hepatitis C virus core protein involves PD-1/PDL-1 signaling

Reports have shown that a negative T cell costimulatory pathway mediated by PD-1 (programmed death-1) and PDL-1 (programmed death ligand-1) is associated with T cell exhaustion and persistent viral infection. Persistent hepatitis C virus (HCV) infection in humans is also characterized by impaired T lymphocyte function, but the role of the PD-1 and PDL-1 pathway in HCV infection is unknown. Here we report that T cells isolated from chronically HCV-infected patients express significantly higher levels of PD-1 when compared with healthy donors. In addition, PD-1 and PDL-1 expression is upregulated on healthy donor T cells exposed to HCV core, a nucleocapsid protein that is immunosuppressive; upregulation of PD-1 is mediated through interaction of HCV core with the complement receptor, gC1qR. Importantly, T cell functions that are dysregulated by HCV core, including T cell activation, proliferation, and apoptosis, can be restored by blocking PD-1 and PDL-1 engagement. Our results indicate that HCV core can upregulate a key negative T cell signaling pathway associated with viral persistence and highly expressed on the T cells of persistently infected individuals. This upregulation of the PD-1 and PDL-1 pathway in humans represents a novel and perhaps common mechanism by which a virus usurps host machinery to facilitate persistence.

Potential ligands of DmP29, a putative juvenile hormone esterase binding protein of Drosophila melanogaster

We previously reported the identification of a putative juvenile hormone esterase (JHE) binding protein DmP29 in Drosophila melanogaster and its primary localization to the mitochondria [Liu, Z., Ho, L., Bonning, B.C., 2007. Localization of a Drosophila melanogaster homolog of the putative juvenile hormone esterase binding protein of Manduca sexta. Insect Biochem. Mol. Biol. 37(2), 155-163]. To further characterize DmP29, we identified potential ligands of this protein. Recombinant DmP29 was shown by ligand blot and co-immunoprecipitation analyses to bind recombinant JHE as well as to larval serum proteins (LSP). The possible biological relevance of the in vitro DmP29-JHE interaction is provided by detection of JHE activity in D. melanogaster mitochondrial fractions; 0.48 nmol JH hydrolyzed/min/mg mitochondrial protein, 97% of which was inhibited by the JHE-specific inhibitor OTFP. However, the DmP29-LSP interactions may not be biologically relevant. Given the high abundance, and "sticky" nature of these proteins, interaction of DmP29 with LSP may result from non-specific associations. No DmP29 interactions with non-specific esterases were detected by co-immunoprecipitation analyses. The potential role of DmP29 as a chaperone of JHE is discussed.

gC1qR/p33 serves as a molecular bridge between the complement and contact activation systems and is an important catalyst in inflammation

The receptor for the globular heads of C1q, gC1qR/p33, is a ubiquitously expressed protein, which is distributed both intracellularly and on the cell-surface protein. In addition to C1q, this molecule also is able to bind several other biologically important plasma ligands, including high-molecular-weight kininogen (HK), factor XII (FXII), and multimeric vitronectin. Previous studies have shown that incubation of FXII, prekallikrein, and HK with gC1qR leads to a zinc-dependent and FXII-dependent conversion of prekallikrein to kallikrein, a requisite for kinin generation. In addition, these studies showed that normal plasma, but not plasma deficient in FXII, PK, or HK, activate upon binding to endothelial cells (EC), and that this activation could be inhibited by antibody to gClqR. In these studies, we show that incubation of serum with microtiter plate bound gC1qR results in complement activation, as evidenced by the binding and activation of C1 and generation of C4d. However, neither Clq-deficient serum nor a truncated form of gC1qR (gC1qRA74-96), supported complement activation. Taken together, the data strongly suggest that at sites of inflammation, such as vasculitis and atherosclerosis, where gC1qR as well as its two important plasma ligands, C1q and HK, have been shown to be simultaneously present, soluble or cell-surface-expressed gC1qR may contribute to the inflammatory process by modulating complement activation, kinin generation, and perhaps even initiation of clotting via the contact system. Based on these and other published data, we propose a model of inflammation in which atherogenic factors (e.g., immune complexes, virus, or bacteria) are perceived not only to convert the endothelium into a procoagulant and proinflammatory surface, but also to induce enhanced expression of cell surface molecules such as gC1qR. Enhanced expression of gC1qR in turn leads to: (i) high-affinity C1q binding and cell production of proinflammatory factors, and (ii) high-affinity HK binding and facilitation of the assembly of contact activation proteins leading to generation of bradykinin and possibly coagulation through activation of FXI.

The contribution of gC1qR/p33 in infection and inflammation

Human gC1qR/p33 is a multi-compartmental and multi-functional cellular protein expressed on a wide range of tissues and cell types including lymphocytes, endothelial cells, dendritic cells, and platelets. Although originally isolated as a receptor for C1q by virtue of its affinity (K(d)=15-50 nM), and specificity for the globular heads of this molecule, a large body of evidence has now been accumulated which shows that in addition to C1q, gC1qR can serve as a receptor for diverse proinflammatory ligands including proteins of the plasma kinin-forming system, most notably high molecular weight kininogen (HK; K(d)=9 nM). In addition, gC1qR has been reported to recognize and bind a number of functional antigens of viral and bacterial origin. It is its ability to interact with microbial antigens and its potential to serve as a cellular protein for bacterial attachment and/or entry that has been the focus of our laboratory in the past few years. On the surface of activated platelets, gC1qR has been shown to serve as a binding site for Staphylococcus aureus and this binding is mediated by protein A. Since the binding of S. aureus to platelets is postulated to play a major role in the pathogenesis of endocarditis, gC1qR may provide a suitable surface for the initial adhesion of the bacterium. Recent data also demonstrate that the exosporium of Bacillus cereus, a member of a genus of aerobic, Gram-positive, spore-forming rod-like bacilli, which includes the deadly Bacillus anthracis, contains a binding site for gC1qR. Therefore, by virtue of its ability to recognize plasma proteins such as C1q and HK, as well as bacterial and viral antigens, cell-surface gC1qR not only is able to generate proinflammatory byproducts from the complement and kinin/kallikrein systems, but also can be an efficient vehicle and platform for a plethora of pathogenic microorganisms.

The alpha2beta1 integrin: a novel collectin/C1q receptor

Our laboratory focuses on the alpha2beta1 integrin, a receptor for a number of matrix and non-matrix ligands, including collagens, laminins, decorin, E-cadherin, matrix metalloproteinase-1 (MMP-1), endorepellin, and several viruses. The alpha2beta1 integrin is expressed on numerous different cell types, including epithelial cells, endothelial cells, fibroblasts, and hematopoietic elements, including platelets and specific subsets of leukocytes. Although alpha2beta1 integrin expression is widespread, it is not ubiquitous. Rather, it is expressed in a differentiation-dependent and activation-dependent manner. Interactions between the alpha2beta1 integrin and extracellular matrix ligands have been implicated in important biological processes including inflammation and immunity. Studies from a number of laboratories have demonstrated a role for the alpha2beta1 integrin during the immune response. Our laboratory generated an alpha2beta1 integrin-deficient mouse to define the role of the alpha2beta1 integrin in vivo. Our studies demonstrated that the alpha2-null mice have a profound defect in the innate immune response. We have recently reported the identification of a novel family of ligands for the alpha2beta1 integrin, which include C1q and the collectins. The goal of this article is to review the important role that the interaction between the alpha2beta1 integrin and C1q plays in the innate immune response. The identification of C1q and the collectins as ligands for the alpha2beta1 integrin suggests that the integrin may play important roles in a number of immunological responses.

Subcellular localization of fumarase in mammalian cells and tissues

Fumarase, a mitochondrial matrix protein, is previously indicated to be present in substantial amounts in the cytosol as well. However, recent studies show that newly synthesized human fumarase is efficiently imported into mitochondria with no detectable amount in the cytosol. To clarify its subcellular localization, the subcellular distribution of fumarase in mammalian cells/tissues was examined by a number of different methods. Cell fractionation using either a mitochondria fraction kit or extraction with low concentrations of digitonin, detected no fumarase in a 100,000 g supernatant fraction. Immunofluorescence labeling with an affinity-purified antibody to fumarase and an antibody to the mitochondrial Hsp60 protein showed identical labeling pattern with labeling seen mainly in mitochondria. Detailed studies were performed using high-resolution immunogold electron microscopy to determine the subcellular localization of fumarase in rat tissues, embedded in LR White resin. In thin sections from kidney, liver, heart, adrenal gland and anterior pituitary, strong and specific labeling due to fumarase antibody was only detected in mitochondria. However, in the pancreatic acinar cells, in addition to mitochondria, highly significant labeling was also observed in the zymogen granules and endoplasmic reticulum. The observed labeling in all cases was completely abolished upon omission of the primary antibody indicating that it was specific. In a western blot of purified zymogen granules, a fumarase-antibody cross-reactive protein of the same molecular mass as seen in the mitochondria was present. These results provide evidence that fumarase in mammalian cells/tissues is mainly localized in mitochondria and significant amounts of this protein are not present in the cytosol. However, these studies also reveal that in certain tissues, in addition to mitochondria, this protein is also present at specific extramitochondrial sites. Although the cellular function of fumarase at these extramitochondrial locations is not known, the appearance/localization of fumarase outside mitochondria may help explain how mutations in this mitochondrial protein can give rise to a number of different types of cancers.

Blood platelets activate the classical pathway of human complement

OBJECTIVE

Activation of the complement system plays a key role in inflammation associated with vascular injury. Recently, platelet P-selectin was shown to activate C3 via the alternative pathway of human complement. As platelets also posses binding sites for C1q, the recognition unit of the classical complement pathway, the present study examined classical pathway activation on platelets.

METHODS

Complement activation was assessed by either a solid phase enzyme-linked immunosorbent assay (ELISA) or flow cytometry.

RESULTS

Using the ELISA approach, 2- to 10-fold increases (P < 0.001) in C1q and C4d deposition were demonstrated on adherent platelets following exposure (60 min 37 degrees C) to diluted (1/10) human plasma or serum. Similar results were obtained by flow cytometry using activated platelets in suspension. C1q and C4d deposition on platelets was accompanied by an approximately 4-fold increase in fluid phase C4d and C3a generation. Consistent with activation of the classical complement pathway, C4 cleavage failed to occur in serum depleted of C1q but was unchanged in factor B deficient serum. C4 activation was enhanced by platelet stimulation using chemical (SFLLRN peptide) or mechanical (shear) means, and decreased following platelet exposure to plasmin. These treatments were accompanied by changes in platelet surface gC1qR/p33 expression, a cellular C1q binding protein. In purified systems, recombinant gC1qR/p33 supported C4 activation, in a C1q dependent manner.

CONCLUSION

These data provide the first evidence for C1q dependent classical complement pathway activation on platelets, and support a role for gC1qR/p33 in this process. However, monoclonal antibodies (mAb) to gC1qR/p33 produced only modest (20% +/- 8%, mean +/- SD, n = 5) reductions in C4 activation on platelets. Thus, further studies are required to investigate the involvement of additional platelet membrane constituents in classical complement pathway activation.

gC1qR/p33 blockade reduces Staphylococcus aureus colonization of target tissues in an animal model of infective endocarditis

gC1qR/p33 (gC1qR) is a ubiquitously expressed cellular protein that is also found in plasma and the extracellular matrix. In addition to its role in modulating the activation of complement and kinin cascades, gC1qR has been identified as a putative host ligand for endovascular pathogens, including Staphylococcus aureus. The present study provides evidence of the ability of soluble gC1qR to enhance S. aureus-fibrinogen interactions via simultaneously binding fibrinogen and S. aureus. This interaction was inhibited in vitro by two monoclonal antibodies (MAbs 74.5.2 and 60.11) recognizing distinct structural and functional domains of gC1qR. To evaluate the in vivo role of gC1qR, MAbs 74.5.2 and 60.11 were used in an experimental rat model of S. aureus endocarditis. Each MAb (100 mg/kg of body weight, given intraperitoneally) reached sustained (>60 h) and high (100 to 200 microg/ml) serum levels. Prophylaxis with MAb 60.11 or 74.5.2 caused substantial reductions in S. aureus colonization of aortic valves, kidneys, and the spleen compared to untreated controls. However, only MAb 74.5.2 prophylaxis therapy reached statistical significance, and only sera from animals protected with MAb 74.5.2 inhibited gC1qR-mediated S. aureus interactions with fibrinogen. Although not statistically significant, the reductions in bacterial colonization achieved with MAb 60.11 alone and in combination with MAb 74.5.2 (versus MAb 74.5.2 alone) suggest that there are effects of gC1qR blockade on S. aureus infective endocarditis in addition to blocking gC1qR-mediated S. aureus binding to fibrinogen. Such impacts may include direct modulation of complement (MAb 60.11) and kinin cascades (MAb 74.5.2) and/or activation of immune and inflammatory responses via localized immune complex formation.

Protruding disordered loop of gC1qR is specifically exposed and related to antiapoptotic property in germ cell lineage

We established a monoclonal antibody (MAb), 5G9, with the use of a fixed seminoma tissue from an archival paraffin-embedded specimen, as an immunogen. Without antigen retrieval, positive 5G9-immunohistochemical staining was confined mostly to primordial germ cells, spermatogonia and various germ cell tumors. 5G9 recognized a mitochondrial 32-kD protein with an isoelectric point of pH 4.2, identified as a multifunctional ubiquitous protein, receptor for globular head of C1q (gC1qR), whose epitope was mapped in a disordered loop connecting the beta3 and the beta4 strands. Reflecting the ubiquitous distribution of gC1qR, with antigen retrieval, 5G9 was found reactive to a wide range of normal and tumor tissues. Since several co-precipitated and phosphorylated bands were observed in various human cell lines but not in germ cell tumor cell lines by in vitro phosphorylation assay, we speculate that the epitope of gC1qR is specifically unmasked in the germ cell lineage. By reducing gC1qR by siRNA, a significant increase was observed in the number of apoptotic cells in ITO-II and TCam-2 cell lines, but to a lesser extent in the Colo201 colon cancer cell line, showing an antiapoptotic property of gC1qR in the germ cells. Since protein-protein interaction is partially preserved by fixation, archival paraffin-embedded specimens can be a valuable source of immunogens for generating monoclonal antibodies (MAbs) that recognize tissue-specific protein conformation.