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The C1q-domain-containing (C1qDC) proteins are a family of proteins characterized by a globular C1q (gC1q) domain in their C-terminus which hold the potential function in the shell formation as shell matrix proteins. In this study, a C1qDC protein was identified and characterized in pearl oyster (Pinctada fucata martensii) (PmC1qDC) to explore its function in nacre formation. The PmC1qDC-deduced protein sequence carried a typical globular C1q (gC1q) domain that possessed the typical 10-stranded β-sandwich fold with a jelly-roll topology common to all C1qDC family members and shared high homology with other gC1q domains. Homologous analysis of PmC1qDC presented it contained conserved secondary structure and Phe135, Phe155, Tyr166, Phe173, Tyr181, Phe183, and Phe256 amino acid residues. Expression pattern analysis showed that PmC1qDC expressed in all the detected tissues and exhibited a significantly higher expression level in nacre formation-associated tissues. After the shell notching, the expression level of PmC1qDC showed significantly up-regulation after 12 h in the central zone of mantle (MC). PmC1qDC expression significantly decreased in the MC after RNA interference (RNAi). Furthermore, disordered crystals with evident rough surface and irregular crystal tablets were observed in the nacre after RNAi. Results suggested that PmC1qDC affects the shell nacre formation, which is significant to improve the pearl production of pearl oyster., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The serum complement component C1q mediates a variety of immune regulatory functions. Herein, we identified a globular head C1q (ghC1q) gene in razor clam Sinonovacula constricta. The complete Sc-ghC1q gene was 872 bp long included an 81 bp 5'-untranslated region (UTR), a 95 bp 3'-UTR with a poly(A) tail, and an open reading frame (ORF) of 696 bp. The mRNA expression of Sc-ghC1q was upregulated in hepatopancreas and hemocytes. After Staphylococcus aureus or Vibrio anguillarum challenge, Sc-ghC1q mRNA transcript abundance was significantly upregulated in hemolymph. Recombinant Sc-ghC1q protein could bind lipopolysaccharide (LPS) and lipoteichoic acid (LTA), and it could agglutinate both Gram-positive and Gram-negative bacteria. Additionally, flow cytometry revealed that Sc-ghC1q strongly promoted phagocytosis in hemocytes. Together, these results demonstrated that Sc-ghC1q played an important role in innate immunity in S. constricta., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

C1q-domain-containing (C1qDC) proteins, which are involved in a series of immune responses, are important pattern recognition receptors in innate immunity in vertebrates and invertebrates. Functional studies of C1qDC proteins in vertebrates are scarce. In the present study, a C1qDC protein (SsC1qDC) from the teleost black rockfish (Sebastes schlegelii) was identified and examined at expression and functional levels. The open reading frame of SsC1qDC is 636 bp, and the predicted amino acid sequence of SsC1qDC shares 62%-69% overall identity with the C1qDC proteins of several fish species. SsC1qDC possesses conserved C1qDC features, including a signal sequence and a C1q domain. SsC1qDC was expressed in different tissues and its expression was up-regulated by bacterial and viral infection. Recombinant SsC1qDC (rSsC1qDC) exhibited apparent binding activities against PAMPs including LPS and PGN. rSsC1qDC had antibacterial activity against Vibrio parahaemolyticus, and was able to enhance the phagocytic activity of macrophages towards Vibrio anguillarum. rSsC1qDC interacted with human heat-aggregated IgG. Furthermore, in the presence of rSsC1qDC, fish exhibited enhanced resistance against bacterial infection. Collectively, these results indicated that SsC1qDC serves as a pattern recognition receptor and plays a vital role in the defense system of black rockfish., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The receptor for the globular head of complement component C1q, gC1qR, is a multifunctional and multiligand binding protein with a crucial role in host defense. In the present study, a full-length cDNA sequence of a gC1qR homolog (PtgC1qR) in Portunus trituberculatus was identified. PtgC1qR was a 268-amino-acid polypeptide with a conserved MAM33 domain and a mitochondrial targeting sequence in the first 56 amino acids. The transcripts of PtgC1qR were detected in all examined tissues with the highest level detected in the hepatopancreas. Compared with other early embryonic stages, PtgC1qR was highly expressed in the fertilized eggs and embryos at the cleavage stage, which suggest PtgC1qR may be a maternal gene. The transcripts of PtgC1qR in hemocytes exhibited time-dependent response expression pattern after challenged with bacteria (Vibrio alginolyticus, Micrococcus luteus) and fungi (Pichia pastoris). Moreover, the recombinant PtgC1qR (rPtgC1qR) exhibited strong antibacterial activity and microbial-binding activity, suggesting its crucial role in immune defense and recognition. Further phenoloxidase (PO) assay showed that rPtgC1qR could suppress the crab PO activity in vitro in a dose-dependent manner, and it could result in nearly 100% inhibition of PO activity under the concentration of 11.65 μM. Knockdown of PtgC1qR could significantly enhance the expression of serine protease related genes (PtSP1-3 and PtSPH), proPO-associated genes (PtproPO and PtPPAF) and C3-like genes (Ptα2M1 and PtTEP). However, the phagocytosis related genes (PtMyosin, PtRab5 and PtArp) and Ptα2M2 were significantly down-regulated in the PtgC1qR silenced crabs. These findings together demonstrate that PtgC1qR might function in crab immune response via its antibacterial activity, immune recognition or regulating the proPO system, complement pathway and phagocytosis., (Copyright © 2018. Published by Elsevier Ltd.)

The globular C1q domain containing (C1qDC) proteins are a family of versatile pattern recognition receptors (PRRs) to bind various ligands by their globular C1q (gC1q) domain. In the present study, a novel globular C1qDC (CgC1qDC-7) was characterized from Pacific oyster Crassostrea gigas. The open reading frame of CgC1qDC-7 was of 555 bp, encoding a polypeptide of 185 amino acids. Phylogenetic analysis indicated that CgC1qDC-7 shared high homology with C1qDCs from Crassostrea virginica, Mytilus galloprovincialis, and Mizuhopecten yessoensis. The mRNA transcripts of CgC1qDC-7 were widely expressed in all the tested tissues including mantle, gonad, gills, adductor muscle, hemocytes, hepatopancreas and labial palps, with the highest expression level in hemocytes and gills. The recombinant protein of CgC1qDC-7 (rCgC1qDC-7) exhibited binding activity towards Gram-negative bacteria (Vibrio splendidus, V. anguillarum, Escherichia coli, V. alginolyticus, and Aeromonas hydrophila), Gram-positive bacteria (Micrococcus luteus and Staphylococcus aureus) and fungi (Pichia pastoris and Yarrowia lipolytica), and displayed strongest binding affinity towards Gram-negative bacteria V. splendidus and V. anguillarum. It also exhibited affinity to vital pathogen-associated molecular patterns (PAMPs), such as lipopolysaccharide (LPS), peptidoglycan (PGN), mannan (MAN) and Poly (I:C) with high affinity towards LPS and PGN, and low affinity to MAN and Poly (I:C). These results collectively indicated that CgC1qDC-7 was a novel PRR in C. gigas with high binding affinity towards LPS and PGN as well as Gram-negative bacteria., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

C1q is the target recognition sequence of the classical complement pathway and a major link that connects innate and acquired immunity. In this study, a C1qDC homolog, HcC1qDC5, from the triangle-shell pearl mussel (Hyriopsis cumingii) was identified. The complete nucleotide sequence of HcC1qDC5 cDNA consists of a 5'-untranslated terminal region (UTR) of 123 bp, a 3'-UTR of 105 bp with a poly(A) tail, and an open reading frame (ORF) of 1344 bp, which encodes a polypeptide of 447 amino acids. HcC1qDC5 contains a signal peptide and three typical C1q domains. The HcC1qDC5 gene was expressed in all tested tissues, with the highest expression in the mantle. Staphylococcus aureus or Vibrio parahaemolyticus infection increased the mRNA transcript levels of HcC1qDC5 in the hepatopancreas and mantle. The recombinant HcC1qDC5 protein could bind to Gram-negative and Gram-positive bacteria as well as to different PAMPs (LPS and PGN). RNAi results showed that HcC1qDC5 was involved in V. parahaemolyticus-induced HcTNF and HcWAP expression. The combined results demonstrated that HcC1qDC5 participates in the innate immunity of H. cumingii., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

C-type lectins (CTLs) are found in a wide number of invertebrates, and have been reported to participate in immune responses, such as the activation of prophenoloxidase, cell adhesion, bacterial clearance and phagocytosis. Previous studies on CTLs focused on the function of their carbohydrate recognition domains (CRDs). Currently, studies on lectins with multi-CRDs are limited. In this study, a lectin with four CRDs was cloned from Hyriopsis cumingii, and called HcLec4. HcLec4 was widely distributed in several tissues and was significantly down-regulated at the early stage (2 h) of bacterial infection. We further analyzed the bacteria and carbohydrate binding activities of HcLec4. The results showed that HcLec4 could bind to several bacteria, lipopolysaccharide (LPS) and peptidoglycan (PGN). In HcLec4 knockdown mussels, the bacterial clearance rate was increased, and the expression level of antimicrobial peptides (AMPs) was up-regulated. This study reveals that HcLec4 exerts its antibacterial effect by regulating the expression of AMPs at the early stage of bacterial infection., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

gC1qR, as a multicompartmental and a multifunctional protein, plays an important role in innate immunity. In this study, a gC1qR homolog (MrgC1qR) in the giant freshwater prawn, Macrobrachium rosenbergii was identified. MrgC1qR, a 258-amino-acid polypeptide, shares high identities with gC1qR from other species. MrgC1qR gene was expressed in different tissues and was highest expressed in the hepatopancreas. In addition, the MrgC1qR transcript was significantly enhanced after 6 h of white spot syndrome virus (WSSV) infection or post 2 h, 24 h of Vibrio anguillarum challenge compared to appropriate controls. Moreover, recombinant MrgC1qR (rMrgC1qR) had bacterial binding activity, the result also revealed that rMrgC1qR could bind pathogen-associated molecular patterns (PAMPs) such as LPS or PGN, suggesting that MrgC1qRmight function as a pathogen-recognition receptor (PRR). Furthermore, glutathione S-transferase (GST) pull-down assays showed that rMrgC1qR with GST-tag could bind to rMrFicolin1 or rMrFicolin2 with His-tag. Altogether, these results may demonstrate a role for MrgC1qR in innate immunity in the giant freshwater prawns., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Complement component 1q (C1q) is a subcomponent of the C1 complex and the key protein that recognizes and binds to a broad range of immune and non-immune ligands to initiate the classical complement pathway. In the present study, we identified and characterized three novel C1q family members from rock bream, Oplegnathus fasciatus. The full-length cDNAs of C1q A-like (RbC1qAL), C1q B-like (RbC1qBL), and C1q C-like (RbC1qCL) consist of 780, 720 and 726 bp of nucleotide sequence encoding polypeptides of 260, 240 and 242 amino acids, respectively. All three RbC1qs possess a leading signal peptide and collagen-like region(s) (CLRs) in the N-terminus, and a C1q domain at the C-terminus. The C1q characteristic Gly-X-Y repeats are present in all three RbC1qs, while the CLR-associated sequence that enhances phagocytic activity is present in RbC1qAL ((49)GEKGEP(54)) and RbC1qCL ((70)GEKGEP(75)). Moreover, the coding region was distributed across six exons in RbCqAL and RbC1qCL, but only five exons in RbC1qBL. Phylogenetic analysis revealed that the three RbC1qs tightly cluster with the fish clade. All three RbC1qs are most highly expressed in the spleen and liver, as indicated by qPCR tissue profiling. In addition, all three are transcriptionally responsive to immune challenge, with liver expression being significantly up-regulated in the early phase of infection with intact, live bacteria (Edwardsiella tarda and Streptococcus iniae) and virus (rock bream iridovirus) and in the late phase of exposure to purified endotoxin (lipopolysaccharide). These data collectively suggest that the RbC1qs may play defense roles as an innate immune response to protect the rock bream from bacterial and viral infections., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

The receptor for the globular heads of C1q, C1qBP/gC1qR/p33, is a multicompartmental, multifunctional cellular protein with an important role in infection and in inflammation. In the present study, we identified and characterized the complement component 1q subcomponent binding protein (C1qBP) from the tiger shrimp Penaeus monodon (designated as PmC1qBP). The open reading frame of PmC1qBP encodes 262 amino acid residues with a conserved MAM33 domain, an arginine-glycine-aspartate cell adhesion motif, and a mitochondrial targeting sequence in the first 53 amino acids. PmC1qBP shares 32%-81% similarity with known C1qBPs and clusters with lobster gC1qR under phylogenetic analysis. The temporal PmC1qBP mRNA expression in the hepatopancreas was significantly enhanced at 9 h after Vibrio vulnificus challenge. The native PmC1qBP was expressed in the gills, hepatopancreas, ovaries, and intestines as a precursor (38 kDa) and the active peptide (35 kDa). The recombinant PmC1qBP protein was expressed in Escherichia coli BL21, and was purified using nickel-nitrilotriacetic acid agarose. A complement 1q binding assay indicated that the rC1qBP protein competitively binds to C1q in mouse serum. The data reveal that PmC1qBP is not only involved in shrimp immune responses to pathogenic infections, but also cross-binding to the mouse C1q., (Copyright © 2012 Elsevier Ltd. All rights reserved.)