Isolation, cDNA cloning, and overexpression of a 33-kD cell surface glycoprotein that binds to the globular "heads" of C1q

Frequency and clinical characteristics of early-onset dementia in consecutive patients in a memory clinic

AIMS

To investigate the frequency, rate of causes of dementia, and clinical characteristics of early-onset dementia in consecutive patients of a memory clinic.

METHODS

A total of 668 consecutive demented patients were involved in this study. We examined the distribution of patients' diagnosis, differences in sex, education, dementia severity and cognitive function at the first visit, and the duration from onset to consultation. We also examined the changes in the proportion of subjects during the research period.

RESULTS

There were 185 early-onset patients, 28% of all demented patients. No significant differences were observed between the early-onset and late-onset dementia groups in Clinical Dementia Rating and Mini-Mental State Examination score at the first consultation, but the duration from onset to consultation was significantly longer in the early-onset group. In the early-onset group, the rates of patients with Alzheimer's disease and dementia with Lewy bodies were relatively low and the rate of patients with frontotemporal lobar degeneration was relatively high. There were no significant differences in the proportion between either demented subjects and nondemented subjects or early-onset dementia patients and late-onset dementia patients during the research period.

CONCLUSION

We conclude that early-onset dementia is not rare and its clinical characteristics and causes are different from late-onset dementia.

The autophagic inducer smARF interacts with and is stabilized by the mitochondrial p32 protein

The alternative reading frame (ARF) mRNA encodes two pro-death proteins, the nucleolar p19ARF and a shorter mitochondrial isoform, named smARF (hsmARF in human). While p19ARF can inhibit cell growth by causing cell cycle arrest or type I apoptotic cell death, smARF is able to induce type II autophagic cell death. Inappropriate proliferative signals generated by proto-oncogenes, such as c-Myc and E2F1, can elevate both p19ARF and smARF proteins. Here, we reveal a novel means of regulation of smARF protein steady state levels through its interactions with the mitochondrial p32. The p32 protein physically interacts with both human and murine smARF, and colocalizes with these short isoforms to the mitochondria. Remarkably, knocking down p32 protein levels significantly reduced the steady state levels of smARF by increasing its turn over. As a consequence, the ability of ectopically expressed smARF to induce autophagy and to cause mitochondrial membrane dissipation was significantly reduced. In contrast, the protein levels of full-length p19ARF, which mainly resides in the nucleolus, were not influenced by p32 depletion, suggesting that p32 exclusively stabilizes the mitochondrial smARF protein. Thus the interaction with p32 provides a means of specifically regulating the expression of the recently identified autophagic inducer, smARF, and adds yet another layer of complexity to the multifaceted regulation of the ARF gene.

A microarray analysis of the temporal response of liver to methylprednisolone: a comparative analysis of two dosing regimens

Microarray analyses were performed on livers from adrenalectomized male Wistar rats chronically infused with methylprednisolone (MPL) (0.3 mg/kg.h) using Alzet mini-osmotic pumps for periods ranging from 6 h to 7 d. Four control and 40 drug-treated animals were killed at 10 different times during drug infusion. Total RNA preparations from the livers of these animals were hybridized to 44 individual Affymetrix REA230A gene chips, generating data for 15,967 different probe sets for each chip. A series of three filters were applied sequentially. These filters were designed to eliminate probe sets that were not expressed in the tissue, were not regulated by the drug, or did not meet defined quality control standards. These filters eliminated 13,978 probe sets (87.5%) leaving a remainder of 1989 probe sets for further consideration. We previously described a similar dataset obtained from animals after administration of a single dose of MPL (50 mg/kg given iv). That study involved 16 time points over a 72-h period. A similar filtering schema applied to the single-bolus-dose dataset identified 1519 probe sets as being regulated by MPL. A comparison of datasets from the two different dosing regimens identified 358 genes that were regulated by MPL in response to both dosing regimens. Regulated genes were grouped into 13 categories, mainly on gene product function. The temporal profiles of these common genes were subjected to detailed scrutiny. Examination of temporal profiles demonstrates that current perspectives on the mechanism of glucocorticoid action cannot entirely explain the temporal profiles of these regulated genes.

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 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.

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

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

Hemopexin domains as multifunctional liganding modules in matrix metalloproteinases and other proteins

The heme-binding hemopexin consists of two, four-bladed propeller domains connected by a linker region. Hemopexin domains are found in different species on the phylogenetic tree and in the human species represented in hemopexin, matrix metalloproteinases (MMPs), vitronectin, and products of the proteoglycan 4 gene. Hemopexin and hemopexin domains of human proteins fulfill functions in activation of MMPs, inhibition of MMPs, dimerization, binding of substrates or ligands, cleavage of substrates, and endocytosis by low-density lipoprotein receptor-related protein-1 (LRP-1; CD91) and LRP-2 (megalin, GP330). Insights into the structures and functions of hemopexin (domains) form the basis for positive or negative interference with the formation of molecular complexes and hence, might be exploited therapeutically in inflammation, cancer, and wound healing.

Cross-talk between the complement system and endothelial cells in physiologic conditions and in vascular diseases

The endothelial layer represents a continuous physical barrier that controls coagulation and allows selective passage of soluble molecules and circulating cells across the vessel wall into the tissue. The functional activity of the endothelial cells may be influenced by their interaction with components of the complement system. In this review we shall discuss the complex interplay that can be established between the endothelium and complement proteins or activation products. Endothelial cells may also secrete several complement components which contribute to the circulating pool. This process can be regulated by cytokines and other pro-inflammatory stimuli. In addition, complement activation products stimulate endothelial cells to acquire a pro-inflammatory and pro-coagulant status. Expression of regulatory molecules on the cell surface provides protection against an undesired attack by complement activation products. Unrestricted complement activation under pathological conditions may lead to structural and functional changes of the endothelium resulting in vascular disease.

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.

Upregulation of hyaluronan binding protein 1 (HABP1/p32/gC1qR) is associated with Cisplatin induced apoptosis

We have earlier reported that overexpression of HABP1 in fibroblast cells causes perturbed cell growth, extensive vacuolation and restricted entry to the S-phase, finally leading to apoptosis (Biochem Biophys Res Commun 2003; 300: 686-693). In the present study, we investigate the regulation of HABP1 expression in cisplatin induced apoptosis in HeLa cells. Apoptosis induced in HeLa cells at 24 h of cisplatin treatment was confirmed by nuclear fragmentation, increase in subdiploid population and the enhanced activation of ERK and upregulation of p53. In association with apoptosis induction, an upregulation of HABP1 expression was observed in HeLa cells at 18 and 24 h of cisplatin treatment. Quantification of HABP1 expression by flow cytometry confirmed a two-fold increase in total intracellular HABP1 expression at 24 h of cisplatin treatment. Under the same condition the HABP1 transcript level measured by semi quantitative RT PCR showed 2.5-fold increase ascertaining transcriptional regulation of HABP1 during cisplatin induced apoptosis. Further, in normal HeLa cells though a small amount of HABP1 can be detected in nucleus, but with apoptosis induction the protein is mainly concentrating around the nuclear periphery at 18 h of cisplatin treatment and is present both in the nucleus as well as in the cytosol at 24 h of treatment, suggesting its nuclear translocation during apoptosis. To substantiate our findings prior to the cisplatin treatment, the expression of HABP1 was reduced by small interfering RNA mediated knockdown. We observed a reduction in apoptotic cell population in cisplatin treated HeLa cells with disrupted HABP1 conferring resistance to cisplatin induced apoptosis. We report here that HABP1 upregulation in the cell is important for cisplatin induced apoptosis.

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.

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.

An acidic protein, YBAP1, mediates the release of YB-1 from mRNA and relieves the translational repression activity of YB-1

Eukaryotic Y-box proteins are nucleic acid-binding proteins implicated in a wide range of gene regulatory mechanisms. They contain the cold shock domain, which is a nucleic acid-binding structure also found in bacterial cold shock proteins. The Y-box protein YB-1 is known to be a core component of messenger ribonucleoprotein particles (mRNPs) in the cytoplasm. Here we disrupted the YB-1 gene in chicken DT40 cells. Through the immunoprecipitation of an epitope-tagged YB-1 protein, which complemented the slow-growth phenotype of YB-1-depleted cells, we isolated YB-1-associated complexes that likely represented general mRNPs in somatic cells. RNase treatment prior to immunoprecipitation led to the identification of a Y-box protein-associated acidic protein (YBAP1). The specific association of YB-1 with YBAP1 resulted in the release of YB-1 from reconstituted YB-1-mRNA complexes, thereby reducing the translational repression caused by YB-1 in the in vitro system. Our data suggest that YBAP1 induces the remodeling of YB-1-mRNA complexes.

Formation of Bradykinin: A Major Contributor to the Innate Inflammatory Response

The plasma kinin-forming cascade can be activated by contact with negatively charged macromolecules leading to binding and autoactivation of factor XII, activation of prekallikrein to kallikrein by factor XIIa, and cleavage of high molecular weight kininogen (HK) by kallikrein to release the vasoactive peptide bradykinin. Once kallikrein formation begins, there is rapid cleavage of unactivated factor XII to factor XIIa, and this positive feedback is favored kinetically over factor XII autoactivation. Examples of surface initiators that can function in this fashion are endotoxin, sulfated mucopolysaccharides, and aggregated Abeta protein. Physiological activation appears to occur along the surface of endothelial cells both by the aforementioned contact-initiated reactions as well as bypass pathways that are independent of factor XII. Factor XII binds primarily to cell surface u-PAR (urokinase plasminogen activator receptor); HK binds to gC1qR via its light chain (domain 5) and to cytokeratin 1 by its heavy chain (domain 3) and, to a lesser degree, by its light chain. Prekallikrein circulates bound to HK (as does coagulation factor XI), and prekallikrein is thereby brought to the surface as HK binds. All cell-binding reactions are dependent on zinc ion. Endothelial cells (HUVECs) have bimolecular complexes of u-PAR-cytokeratin 1 and gC1qR-cytokeratin 1 at the cell surface plus free gC1qR, which is present in substantial molar excess. Factor XII appears to interact primarily with the u-PAR-cytokeratin 1 complex, whereas HK binds primarily to the gC1qR-cytokeratin 1 complex and to free gC1qR. Release of endothelial cell heat shock protein 90 (Hsp90) or the enzyme prolylcarboxypeptidase leads to activation of the bradykinin-forming cascade by activating the prekallikrein-HK complex. In contrast to factor XIIa, neither will activate prekallikrein in the absence of HK, both reactions require zinc ion, and the stoichiometry suggests interaction of one molecule of Hsp90 (for example) with one molecule of prekallikrein-HK complex. The presence of factor XII, however, leads to a marked augmentation in reaction rate via the kallikrein feedback as well as to a change to classic enzyme-substrate kinetics. The circumstances in which activation is initiated by factor XII autoactivation or by these factor XII bypasses are yet to be defined. The pathologic conditions in which bradykinin generation appears important include hereditary and acquired C1 inhibitor deficiency, cough and angioedema due to ACE inhibitors, endotoxin shock, with contributions to conditions as diverse as Alzheimer's disease, stroke, control of blood pressure, and allergic diseases.

Truncated variants of hyaluronan-binding protein 1 bind hyaluronan and induce identical morphological aberrations in COS-1 cells

Hyaluronan (HA)-binding protein 1 (HABP1) is multifunctional in nature and exists as a trimer through coiled-coil interaction between alpha-helices at its N- and C-termini. To investigate the importance of trimeric assemblage and HA-binding ability of HABP1, we generated and overexpressed variants of HABP1 by truncating the alpha-helices at its termini. Subsequently, these variants were transiently expressed in COS-1 cells to examine the influence of these structural variations on normal cell morphology, as compared with those imparted by HABP1. Substantiating the centrality of coiled-coil interaction for maintaining the trimeric assembly of HABP1, we demonstrate that disruption of trimerization does not alter the affinity of variants towards its ligand HA. Transient expression of HABP1 altered the morphology of COS-1 cells by generating numerous cytoplasmic vacuoles along with disruption of the f-actin network. Interestingly, the truncated variants also imparted identical morphological changes. Characterization of the cytoplasmic vacuoles revealed that most of these vacuoles were autophagic in nature, resembling those generated under stress conditions. The identical morphological changes manifested in COS-1 cells on transient expression of HABP1 or its variants is attributed to their comparable HA-binding ability, which in concert with endogenous HABP1, may deplete the cellular HA pool. Such quenching of HA below a threshold level in the cellular milieu could generate a stress condition, manifested through cytoplasmic vacuoles and a disassembly of the f-actin network.

The FK506 binding protein 13 kDa (FKBP13) interacts with the C-chain of complement C1q

Background

The pharmacological action of specific immunosuppressants is mediated by immunophilins. While cyclosporin A binds to cyclophilins, FK506/tacrolimus, rapamycin, and others bind to FK506 binding proteins (FKBPs). Different physiological actions of immunophilins were described but their genuine function, however, remains elusive and is still under investigation. A yeast two-hybrid screen was performed using the FK506 binding protein 13 kDa (FKBP13) as a bait and a fetal liver expression library as a prey.

Results

The C-chain of complement C1q (C1q-C) was detected to interact with FKBP13 in the yeast two-hybrid system and in a protein complementation assay. Neither FKBP12, FKBP25, FKBP52 nor the unrelated immunophilin CypA did react with C1q-C in the yeast system stressing the specificity of the interaction. Binding of C1q-C to FKBP13 could not be prevented in the presence of FK506, demonstrating that possibly other regions than the binding pocket of the drug are responsible for the interaction of the two proteins.

Conclusion

It is concluded that exclusively FKBP13 but no other FKBPs tested so far interact with the C-chain of complement C1q in the two different assays and further work will be initiated to investigate the physiological relevance of the interaction.

A Comprehensive Screen for Chicken Proteins that Interact with Proteins Unique to Virulent Strains of Marek's Disease Virus

Genetic resistance to Marek's disease (MD) has been proposed as a method to augment current vaccinal control of MD. Although it is possible to identify QTL and candidate genes that are associated with MD resistance, it is necessary to integrate functional screens with linkage analysis to confirm the identity of true MD resistance genes. To help achieve this objective, a comprehensive 2-hybrid screen was conducted using genes unique to virulent Marek's disease virus (MDV) strains. Potential MDV-host protein interactions were tested by an in vitro binding assay to confirm the initial two-hybrid results. As a result, 7 new MDV-chicken protein interactions were identified and included the chicken proteins MHC class II beta (BLB) and invariant (Ii) chain (CD74), growth-related translationally controlled tumor protein (TPT1), complement component Clq-binding protein (C1QBP), retinoblastoma-binding protein 4 (RBBP4), and alpha-enolase (ENO1). Mapping of the encoding chicken genes suggests that BLB, the gene for MHC class II beta chain, is a positional candidate gene. In addition, the known functions of the chicken proteins suggest mechanisms that MDV might use to evade the chicken immune system and alter host gene regulation. Taken together, our results indicate that integrated genomic methods provide a powerful strategy to gain insights on complex biological processes and yield a manageable number of genes and pathways for further characterization.

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.

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.

In vitro and in vivo interactions between the hepatitis B virus protein P22 and the cellular protein gC1qR

gC1qR, a mitochondrial matrix protein, was identified as the main cellular partner of the hepatitis B virus P22 protein. We demonstrated by immunofluorescence studies that some P22 molecules were colocalized with the endogenous gC1qR in both the cytoplasm and the nucleus but never in the mitochondria. We also showed that the last 34 amino acids of P22 were involved in the association with gC1qR.

By binding SIRPalpha or calreticulin/CD91, lung collectins act as dual function surveillance molecules to suppress or enhance inflammation

Surfactant proteins A and D (SP-A and SP-D) are lung collectins composed of two regions, a globular head domain that binds PAMPs and a collagenous tail domain that initiates phagocytosis. We provide evidence that SP-A and SP-D act in a dual manner, to enhance or suppress inflammatory mediator production depending on binding orientation. SP-A and SP-D bind SIRPalpha through their globular heads to initiate a signaling pathway that blocks proinflammatory mediator production. In contrast, their collagenous tails stimulate proinflammatory mediator production through binding to calreticulin/CD91. Together a model is implied in which SP-A and SP-D help maintain a non/anti-inflammatory lung environment by stimulating SIRPalpha on resident cells through their globular heads. However, interaction of these heads with PAMPs on foreign organisms or damaged cells and presentation of the collagenous tails in an aggregated state to calreticulin/CD91, stimulates phagocytosis and proinflammatory responses.

Cell surface expression of C1qRP/CD93 is stabilized by O-glycosylation

C1qRP/CD93 is a cell surface receptor predominantly expressed on monocytes, neutrophils, endothelial cells, and early stem cell precursors. In phagocytic cells, it has been characterized as contributing to the enhancement of FcR- and CR1-induced phagocytosis triggered by innate immune system defense collagens such as C1q and mannose binding lectin (MBL). Previously, we demonstrated a high level of glycosylation on C1qRP/CD93 that was predominantly O-linked. In this study, we investigate the role of glycosylation in C1qRP/CD93 stability first by inhibiting O-glycosylation by addition of benzyl 2-acetamido-2-deoxy-alpha-D-galactopyranoside (BAG) to the human histiocytic cell line U937, and secondly, by expression of C1qRP/CD93 in the CHO-derived cell line ldlD which has a reversible defect in protein glycosylation. In both U937 cells and in ldlD cells transfected to express C1qRP/CD93, glycosylation deficiency caused cell surface expression levels of C1qRP/CD93 to decrease, concomitant with the detection of C1qRP/CD93 reactivity in the culture media. Metabolic labeling studies show that when glycosylation is absent, C1qRP/CD93 is synthesized and rapidly released into the culture supernatant or degraded. These studies demonstrate that O-glycosylation is important in the stable cell surface expression of C1qRP/CD93 .

Structural flexibility of multifunctional HABP1 may be important for regulating its binding to different ligands

Hyaluronan-binding protein 1 (HABP1)/p32/gC1qR was characterized as a highly acidic and oligomeric protein, which binds to different ligands like hyaluronan, C1q, and mannosylated albumin. It exists as trimer in high ionic and reducing conditions as shown by crystal structure. In the present study, we have examined the structural changes of HABP1 under a wide range of ionic environments. HABP1 exhibits structural plasticity, which is influenced by the ionic environment under in vitro conditions near physiological pH. At low ionic strength HABP1 exists in a highly expanded and loosely held trimeric structure, similar to that of the molten globule-like state, whereas the presence of salt stabilizes the trimeric structure in a more compact fashion. It is likely that the combination of the high net charge asymmetrically distributed along the faces of the molecule and the relatively low intrinsic hydrophobicity of HABP1 result in its expanded structure at neutral pH. Thus, the addition of counter ions in the molecular environment minimizes the intramolecular electrostatic repulsion in HABP1 leading to its stable and compact conformations, which reflect in its differential binding toward different ligands. Whereas the binding of HABP1 toward HA is enhanced on increasing the ionic strength, no significant effect was observed with the two other ligands, C1q and mannosylated albumin. Thus, although HA interacts only with compact HABP1, C1q and mannosylated albumin can bind to loosely held oligomeric HABP1 as well. In other words, structural changes in HABP1 mediated by changes in the ionic environment are responsible for recognizing different ligands.

Aromatic amino acids at the surface of InlB are essential for host cell invasion by Listeria monocytogenes

The surface protein InlB of the pathogen Listeria monocytogenes promotes invasion of this bacterium into host cells by binding to and activating the receptor tyrosine kinase Met. The curved leucine-rich repeat (LRR) domain of InlB, which is essential for this process, contains a string of five surface-exposed aromatic amino acid residues positioned along its concave face. Here, we show that the replacement of four of these residues (F104, W124, Y170 or Y214) by serine leads to a complete loss of uptake of latex beads coated with InlB', a truncated functional variant of InlB. The mutants correspondingly display severely reduced binding to Met. To abrogate fully invasion of bacteria expressing full-length InlB, exchange of at least four aromatic amino acids is required. We conclude that InlB binds to Met through its concave surface of the LRR domain, and that aromatic amino acids are critical for binding and signalling before invasion.

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.

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.

Inhibition of contact activation by a kininogen peptide (HKH20) derived from domain 5

Contact activation can be initiated by interaction of Factor XII, prekallikrein (PK) and high molecular weight kininogen (HK) with inorganic negatively charged biologic macromolecules, or upon cell surfaces, or interaction with membrane protein derivatives such as aggregated beta amyloid. The latter two examples are zinc-dependent. The interaction with cells is dependent on peptides derived from HK domains 3 and 5 termed LDC27 and HKH20, respectively. We have tested the ability of each of these peptides to inhibit HK-dependent contact activation. HKH20 inhibited activation of prekallikrein when a mixture containing HK, prekallikrein and Factor XII was incubated with dextran sulfate, gC1qR, amyloid beta or endothelial cells. Comparable quantities of LDC27 had no effect. The binding of biotinylated HK or biotinylated Factor XII was inhibited in a dose response fashion by increasing concentrations of HKH20 while LDC27, again had no effect. The N-terminal region of HKH20 (amino acids 475-485) is of particular importance for binding and histidine 485 prominently enhances the reaction as assessed employing overlapping and deleted peptides. Since there is a role for HK heavy chain in binding to endothelial cells and LDC27 can be employed as an affinity ligand to isolate the binding proteins, we increased the LDC27 concentration from 10-fold to 250-fold to determine whether it is functional. Inhibition of endothelial cell-dependent prekallikrein activation required 100-fold greater concentration of LDC27 when compared to HKH20 to achieve significant inhibition. We conclude that the interactions of the light chain of HK via HKH20 is of particular importance for activation of the bradykinin forming cascade in zinc-dependent or independent reactions and is true for all "surface" initiators tested thus far.

Evidence for the presence of HABP1 pseudogene in multiple locations of mammalian genome

The gene encoding hyaluronan-binding protein 1 (HABP1) is expressed ubiquitously in different rat tissues, and is present in eukaryotic species from yeast to humans. Fluorescence in situ hybridization indicates that this is localized in human chromosome 17p13.3. Here, we report the presence of homologous sequences of HABP1 cDNA, termed processed HABP1 pseudogene in humans. This is concluded from an additional PCR product of ~0.5 kb, along with the expected band at approximately 5 kb as observed by PCR amplification of human genomic DNA with HABP1-specific primers. Partial sequencing of the 5-kb PCR product and comparison of the HABP1 cDNA with the sequence obtained from Genbank accession number AC004148 indicated that the HABP1 gene is comprised of six exons and five introns. The 0.5-kb additional PCR product was confirmed to be homologous to HABP1 cDNA by southern hybridization, sequencing, and by a sequence homology search. Search analysis with HABP1 cDNA sequence further revealed the presence of similar sequence in chromosomes 21 and 11, which could generate ~0.5 kb with the primers used. In this report, we describe the presence of several copies of the pseudogene of HABP1 spread over different chromosomes that vary in length and similarity to the HABP1 cDNA sequence. These are 1013 bp in chromosome 21 with 85.4% similarity, 1071 bp in chromosome 11 with 87.2% similarity, 818 bp in chromosome 15 with 82.3% similarity, and 323 bp in chromosome 4 with 84% similarity to HABP1 cDNA. We have also identified similar HABP1 pseudogenes in the rat and mouse genome. The human pseudogene sequence of HABP1 possesses a 10 base pair direct repeat of "AGAAAAATAA" in chromosome 21, a 12-bp direct repeat of "AG/CAAATTA/CAA/TTA" in chromosome 4, a 8-bp direct repeat of "ACAAAG/TCT" in chromosome 15. In the case of chromosome 11, there is an inverted repeat of "AGCCTGGGCGACAGAGCGAGA" ~50 bp upstream of the HABP1 pseudogene sequence. All of the HABP1 pseudogene sequences lack 5' promoter sequence and possess multiple mutations leading to the insertion of premature stop codons in all three reading frames. Rat and mouse homologs of the HABP1 pseudogene also contain multiple mutations, leading to the insertion of premature stop codons confirming the identity of a processed pseudogene.

The cytoplasmic tail peptide sequence of membrane type-1 matrix metalloproteinase (MT1-MMP) directly binds to gC1qR, a compartment-specific chaperone-like regulatory protein

Membrane type-1 matrix metalloproteinase (MT1-MMP), a key enzyme in cell locomotion, is known to be primarily recruited to the leading edge of migrating cells. This raises a possibility that the C-terminal cytoplasmic tail of MT1-MMP interacts with intracellular regulatory proteins, which modulate translocations of the protease across the cell. Here, we demonstrated that MT1-MMP via its cytoplasmic tail directly associates with a chaperone-like compartment-specific regulator gC1qR. Although a direct functional link between these two proteins remains uncertain, our observations suggest that the transient associations of gC1qR with the cytoplasmic tail of MT1-MMP are likely to be involved in the mechanisms regulating presentation of the protease at the tumor cell surface.

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.

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.

Stage-specific expression of proprotein form of hyaluronan binding protein 1 (HABP1) during spermatogenesis in rat

The presence of the 34-kDa hyaluronan binding protein 1 (HABP1) on sperm surface and its role in fertilization is already established (Ranganathan et al., 1994: Mol Reprod Dev 38:69-76). In the present communication, we examined the expression of HABP1 in adult rat testis during spermatogenesis. Interestingly, using anti-rHABP1 antibody, we detected a protein of 55 kDa which was present only in testis, but not in other somatic tissues like spleen and liver. However, even in testis, only one transcript of HABP1 mRNA of 1.63 kb was observed. In addition, we confirm that this testis-specific 55 kDa protein was immunologically identical with proprotein form of HABP1 using antibody raised against a decapeptide present in the proprotein region of HABP1. Comparative immunohistochemistry of testis, spleen, and liver tissues using both the antibodies supported the observation that the proprotein form of HABP1 is present only in testis. Higher mRNA expression of HABP1 in testis as compared to that of liver and spleen could be speculated from the RT-PCR product. Finally, detailed study of the immunohistochemical staining of the seminiferous tubules revealed the expression of the HABP1 proprotein in specific stages of germ cells, like pachytene spermatocytes and round spermatids, but not in elongated ones, suggesting a possible role of HABP1 proprotein in spermatogenic differentiation.

The N-terminal conserved domain of rubella virus capsid interacts with the C-terminal region of cellular p32 and overexpression of p32 enhances the viral infectivity

Cellular 'defense collagens' are produced to launch virus-specific responses to clear the invading viruses. Cellular p32, the C1q binding protein is one such protein. In this report, we identified the interaction of p32 derived from a human lung diploid cell line (WI-38) with rubella virus capsid (RVCP from Therien strain) N-terminal 28-amino acid domain, which is conserved among several RV strains including the vaccine strains. We further identified that the C-terminal 69 aa of the mature p32 is sufficient to interact with the CP. In addition, we observed that in three independent Vero 76-derived cell lines constitutively overexpressing p32, the RV infectivity was enhanced. Our results suggest that RV has evolved a strategy whereby one of its proteins is recruited to interact with, and exploit the cellular defense machinery to its advantage.

Antibody-mediated activation of the classical complement pathway in xenograft rejection

Transplant rejection is a multifactorial process involving complex interactions between components of the innate and the acquired immune system. In view of the shortage of donor organs available for transplantation, xenotransplantation of pig organs into man has been considered as a potential solution. However, in comparison to allografts, xenografts are subject to extremely potent rejection processes that are currently incompletely defined. Consequently, an appropriate and safe treatment protocol ensuring long-term graft survival is not yet available. The first barrier that has to be taken for a xenograft is hyperacute rejection, a rapid process induced by the binding of pre-formed antibodies from the host to the graft endothelium, followed by activation of the classical complement pathway. The present review concentrates on the role of antibodies and complement in xenograft rejection as well as on the approaches for treatment that target these components. The first part focuses on porcine xenoantigens that are recognized by human xenoreactive antibodies and the different treatment strategies that aim on interference in antibody binding. The second part of the review deals with complement activation by xenoreactive antibodies, and summarizes the role of complement in the induction of endothelial cell damage and cell activation. Finally, various options that are currently under development for complement inhibition are discussed, with special reference to the specific inhibition of the classical complement pathway by soluble complement inhibitors.

The hemopexin-like C-terminal domain of membrane type 1 matrix metalloproteinase regulates proteolysis of a multifunctional protein, gC1qR

Matrix metalloproteinases (MMPs) including membrane type 1 MMP (MT1-MMP) can degrade extracellular matrix and cell surface receptor molecules and have an essential function in malignancy. Recently, we established a functional link between MT1-MMP and the receptor of complement component 1q (gC1qR). The gC1qR is known as a compartment-specific regulator of diverse cellular and viral proteins. Once released by proliferating cells, soluble gC1qR may inhibit complement component 1q hemolytic activity and play important roles in vivo in assisting tumor cells to evade destruction by complement. Here, we report that gC1qR is susceptible to MT1-MMP proteolysis in vitro and in cell cultures. The major MT1-MMP cleavage site (Gly(79) down arrow Gln(80)) is localized within the structurally disordered loop connecting the beta(3) and the beta(4) strands of gC1qR. The recombinant MT1-MMP construct that included the catalytic domain but lacked the hemopexin-like domain lost the proteolytic capacity; however, it retained the ability to bind gC1qR. Inhibition of MT1-MMP activity by a hydroxamate inhibitor converted the protease into a cell surface receptor of gC1qR and promoted co-precipitation MT1-MMP with the soluble gC1qR protein. It is tempting to hypothesize that these novel mechanisms may play important roles in vivo and have to be taken into account in designing hydroxamate-based cancer therapy.

Hyaluronan binding protein 1 (HABP1)/C1QBP/p32 is an endogenous substrate for MAP kinase and is translocated to the nucleus upon mitogenic stimulation

The role of hyaluronan binding protein 1 (HABP1) in cell signaling was investigated and in vitro kinase assay demonstrated that it is a substrate for MAP kinase. Phosphorylation of endogenous HABP1 was also observed following treatment of J774 cells with PMA. HABP1 was coimmunoprecipitated with activated ERK, confirming their physical interaction in the cellular context. Upon PMA stimulation of normal rat fibroblast (F111) and transformed (HeLa) cells, the HABP1 level in the cytoplasm gradually decreased with a parallel increase in the nucleus. In HeLa cells, within 6 h of PMA treatment, HABP1 was completely translocated to the nucleus, which was prevented by PD98059, a selective inhibitor of ERK. We also observed that the nuclear translocation of HABP1 is concurrent with that of ERK, suggesting that ERK activation is a requirement for the translocation of HABP1. It is thus established for the first time that HABP1 is a substrate for ERK and an integral part of the MAP kinase cascade.

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.

p32 (gC1qBP) is a general protein kinase C (PKC)-binding protein; interaction and cellular localization of P32-PKC complexes in ray hepatocytes

The aim of this study was to identify cellular proteins that bind protein kinase C (PKC) and may influence its activity and its localization. A 32-kDa PKC-binding protein was purified to homogeneity from the Triton X-100-insoluble fraction obtained from hepatocytes homogenates. The protein was identified by NH(2)-terminal amino acid sequencing as the previously described mature form of p32 (gC1qR). Recombinant p32 was expressed as a glutathione S-transferase fusion protein, affinity-purified, and tested for an in vitro interaction with PKC using an overlay assay approach. All PKC isoforms expressed in rat hepatocytes interacted in vitro with p32, but the binding dependence on PKC activators was different for each one. Whereas PKCdelta only binds to p32 in the presence of PKC activators, PKCzeta and PKCalpha increase their binding when they are in the activated form. Other PKC isoforms such as beta, epsilon, and theta bind equally well to p32 regardless of the presence of PKC activators, and PKCmu binds even better in their absence. It was also found that p32 is not a substrate for any of the PKC isoforms tested, but interestingly, its presence had a stimulatory effect (2-fold for PKCdelta) on PKC activity. We also observed in vivo interaction between PKC and p32 by immunofluorescence and confocal microscopy. A time course of phorbol ester treatment of cultured rat hepatocytes (C9 cells) showed that PKCtheta and p32 are constitutively associated in vivo, whereas PKCdelta activation is required for its association with p32. Our data also showed that phorbol ester treatment induces a transient translocation of p32 from the cytoplasm to the cell nucleus. Together, these findings suggest that p32 may be a regulator of PKC location and function.

Disulfide bond formation through Cys186 facilitates functionally relevant dimerization of trimeric hyaluronan-binding protein 1 (HABP1)/p32/gC1qR

Hyaluronan-binding protein 1 (HABP1), a ubiquitous multifunctional protein, interacts with hyaluronan, globular head of complement component 1q (gC1q), and clustered mannose and has been shown to be involved in cell signalling. In vitro, this recombinant protein isolated from human fibroblast exists in different oligomeric forms, as is evident from the results of various independent techniques in near-physiological conditions. As shown by size-exclusion chromatography under various conditions and glutaraldehyde cross-linking, HABP1 exists as a noncovalently associated trimer in equilibrium with a small fraction of a covalently linked dimer of trimers, i.e. a hexamer. The formation of a covalently-linked hexamer of HABP1 through Cys186 as a dimer of trimers is achieved by thiol group oxidation, which can be blocked by modification of Cys186. The gradual structural transition caused by cysteine-mediated disulfide linkage is evident as the fluorescence intensity increases with increasing Hg(2+) concentration until all the HABP1 trimer is converted into hexamer. In order to understand the functional implication of these transitions, we examined the affinity of the hexamer for different ligands. The hexamer shows enhanced affinity for hyaluronan, gC1q, and mannosylated BSA compared with the trimeric form. Our data, analyzed with reference to the HABP1/p32 crystal structure, suggest that the oligomerization state and the compactness of its structure are factors that regulate its function.

Specific inhibition of the classical complement pathway by C1q-binding peptides

Undesired activation of the complement system is a major pathogenic factor contributing to various immune complex diseases and conditions such as hyperacute xenograft rejection. We aim for prevention of complement-mediated damage by specific inhibition of the classical complement pathway, thus not affecting the antimicrobial functions of the complement system via the alternative pathway and the lectin pathway. Therefore, 42 peptides previously selected from phage-displayed peptide libraries on basis of C1q binding were synthesized and examined for their ability to inhibit the function of C1q. From seven peptides that showed inhibition of C1q hemolytic activity but no inhibition of the alternative complement pathway, one peptide (2J) was selected and further studied. Peptide 2J inhibited the hemolytic activity of C1q from human, chimpanzee, rhesus monkey, rat, and mouse origin, all with a similar dose-response relationship (IC(50) 2-6 microM). Binding of C1q to peptide 2J involved the globular head domain of C1q. In line with this interaction, peptide 2J dose-dependently inhibited the binding of C1q to IgG and blocked activation of C4 and C3 and formation of C5b-9 induced via classical pathway activation, as assessed by ELISA. Furthermore, the peptide strongly inhibited the deposition of C4 and C3 on pig cells following their exposure to human xenoreactive Abs and complement. We conclude that peptide 2J is a promising reagent for the development of a therapeutic inhibitor of the earliest step of the classical complement pathway, i.e., the binding of C1q to its target.

The trypanosome homolog of human p32 interacts with RBP16 and stimulates its gRNA binding activity

RBP16 is a guide RNA (gRNA)-binding protein that was shown through immunoprecipitation experiments to interact with approximately 30% of total gRNAs in Trypanosoma brucei mitochondria. To gain insight into the biochemical function of RBP16, we used affinity chromatography and immunoprecipitation to identify RBP16 protein binding partners. By these methods, RBP16 does not appear to stably interact with the core editing machinery. However, fractionation of mitochondrial extracts on MBP-RBP16 affinity columns consistently isolated proteins of 12, 16, 18 and 22 kDa that were absent from MBP control columns. We describe here our analysis of one RBP16-associated protein, p22. The predicted p22 protein has significant sequence similarity to a family of multimeric, acidic proteins that includes human p32 and Saccharomyces cerevisiae mam33p. Glutaraldehyde crosslinking of recombinant p22 identified homo-multimeric forms of the protein, further substantiating its homology to p32. We confirmed the p22-RBP16 interaction and demonstrated that the two proteins bind each other directly by ELISA utilizing recombinant p22 and RBP16. p32 family members have been reported to modulate viral and cellular pre-mRNA splicing, in some cases by perturbing interaction of their binding partners with RNA. To determine whether p22 similarly affects the gRNA binding properties of RBP16, we titrated recombinant p22 into UV crosslinking assays. These experiments revealed that p22 significantly stimulates the gRNA binding capacity of RBP16. Thus, p22 has the potential to be a regulatory factor in T.brucei mitochondrial gene expression by modulating the RNA binding properties of RBP16.

Synergy between the N- and C-terminal domains of InlB for efficient invasion of non-phagocytic cells by Listeria monocytogenes

InlB is a Listeria monocytogenes protein promoting entry in non-phagocytic cells, and has been shown recently to activate the hepatocyte growth factor receptor (HGFR or Met). The N-terminal domain of InlB (LRRs) binds and activates Met, whereas the C-terminal domain of InlB (GW modules) mediates loose attachment of InlB to the listerial surface. As HGF activation of Met is tightly controlled by glycosaminoglycans (GAGs), we tested if GAGs also modulate the Met-InlB interactions. We show that InlB-dependent invasion of non-phagocytic cells decreases up to 10 times in the absence of GAGs, and that soluble heparin releases InlB from the bacterial surface and promotes its clustering. Furthermore, we demonstrate that InlB binds cellular GAGs by its GW modules, and that this interaction is required for efficient InlB-mediated invasion. Therefore, GW modules have an unsuspected dual function: they attach InlB to the bacterial surface and enhance entry triggered by the LRRs domain. Our results thus provide the first evidence for a synergy between two host factor-binding domains of a bacterial invasion protein, and reinforce similarities between InlB and mammalian growth factors.

C1q-bearing immune complexes induce IL-8 secretion in human umbilical vein endothelial cells (HUVEC) through protein tyrosine kinase- and mitogen-activated protein kinase-dependent mechanisms: evidence that the 126 kD phagocytic C1q receptor mediates immune complex activation of HUVEC

Endothelial cells play a pivotal role in the initiation and perpetuation of inflammation. C1q, the first component of the classical pathway of complement, is a potent stimulus leading to endothelial cell activation and cytokine production. The specific cellular mechanisms through which endothelial cells are stimulated by C1q are not known. We stimulated human umbilical vein endothelial cells (HUVEC) with either monomeric C1q or C1q-bearing immune complexes (C1q-IC) in the presence or absence of inhibitors of protein tyrosine kinases (PTK) or mitogen-activated protein kinases (MAPK). C1q-IC, but not monomeric C1q, induced IL-8 production in dose- and time-dependent fashion. R3, a cross-linking monoclonal IgM antibody against the 126 kD phagocytic C1q receptor (C1qR), also stimulated IL-8 production. IL-8 mRNA accumulation was detected by Northern blot analysis within 2 h of stimulation by the immune complexes and was enhanced by the addition of cycloheximide. Secretion of IL-8 by C1q-IC stimulated HUVEC was completely blocked by the PTK inhibitor, genistein or the MAPK inhibitor, UO126. These experiments demonstrate that C1q-IC-induced production of IL-8 in HUVEC is dependent upon the activation of PTK and MAPK. These findings also support a role for the phagocytic C1qR as an important activator of HUVEC by immune complexes.

Interaction between GABAA Receptor β Subunits and the Multifunctional Protein gC1q-R

gamma-Aminobutyric acid type A (GABA(A)) receptors were immunopurified from bovine brain using a monoclonal antibody directed against the alpha1 subunit. Of the several proteins that copurified, a 34-kDa protein was analyzed further. After enrichment and tryptic proteolysis, the resulting fragments were sequenced, and the protein was identified as gC1q-R. Using anti-gC1q-R and anti-GABA(A) receptor antibodies, mutual coimmunoprecipitation could be demonstrated from solubilized rat brain membranes. The stability of this interaction was estimated to be very high. Using the yeast two-hybrid system, various GABA(A) receptor subunit intracellular loop constructs were tested for an interaction with gC1q-R. All beta subunits, but not alpha 1 and gamma 2 subunits, were found to bind to gC1q-R. NH(2)- and COOH-terminally truncated beta 2 subunit loops were used to find the region responsible for the interaction with gC1q-R. A stretch of 15 amino acids containing 7 positively charged residues was identified (amino acids 399--413). This region contains residue Ser-410, which is a protein kinase substrate, and it is known that phosphorylation of this residue leads to an alteration in receptor activity. Localization studies suggested a predominantly intracellular localization. Our observations therefore suggest a tight interaction between gC1q-R and the GABA(A) receptor which might be involved in receptor biosynthesis or modulation of the mature function.

Retargeting of the mitochondrial protein p32/gC1Qr to a cytoplasmic compartment and the cell surface

p32/gC1qR is a small acidic protein that has been reported to have a broad range of distinct functions and to associate with a wide array of cellular, viral and bacterial proteins. It has been found in each of the main cellular compartments including mitochondria, nucleus and cytoplasm and is also thought to be located at the plasma membrane and secreted into the extracellular matrix. The true physiological role(s) of p32 remains controversial because it has been difficult to reconcile all of the findings on protein interactions and the seemingly disparate observations on compartmentalisation. However, it has been proposed that p32 is somehow involved in transport processes connecting diverse cellular compartments and the cell surface. Here we show that native p32 appears to be localised mainly in the mitochondria and is not detectable on the cell surface. However, addition of a short tag to the N-terminus of p32 appears to block its mitochondrial targeting, resulting in redirection into a cytoplasmic vesicular pattern, overlapping with the endoplasmic reticulum. The redirection of p32 results in an alteration in and co-localisation with ER markers including calreticulin, a lumenal ER chaperone. Furthermore, we show both by immunofluorescence and cross-linking studies that this also results in cell-surface expression of p32. These results indicate that, at least under certain circumstances, p32 can be retargeted and may help to provide an explanation for the diverse observations on its localization.