Transcriptional profiling of Bulinus globosus provides insights into immune gene families in snails supporting the transmission of urogenital schistosomiasis

Schistosomiasis, urogenital and intestinal, afflicts 251 million people worldwide with approximately two-thirds of the patients suffering from the urogenital form of the disease. Freshwater snails of the genus Bulinus (Gastropoda: Planorbidae) serve as obligate intermediate hosts for Schistosoma haematobium, the etiologic agent of human urogenital schistosomiasis. These snails also act as vectors for the transmission of schistosomiasis in livestock and wildlife. Despite their crucial role in human and veterinary medicine, our basic understanding at the molecular level of the entire Bulinus genus, which comprises 37 recognized species, is very limited. In this study, we employed Illumina-based RNA sequencing (RNAseq) to profile the genome-wide transcriptome of Bulinus globosus, one of the most important intermediate hosts for S. haematobium in Africa. A total of 179,221 transcripts (N50 = 1,235) were assembled and the benchmarking universal single-copy orthologs (BUSCO) was estimated to be 97.7%. The analysis revealed a substantial number of transcripts encoding evolutionarily conserved immune-related proteins, particularly C-type lectin (CLECT) domain-containing proteins (n = 316), Toll/Interleukin 1-receptor (TIR)-containing proteins (n = 75), and fibrinogen related domain-containing molecules (FReD) (n = 165). Notably, none of the FReDs are fibrinogen-related proteins (FREPs) (immunoglobulin superfamily (IgSF) + fibrinogen (FBG)). This RNAseq-based transcriptional profile provides new insights into immune capabilities of Bulinus snails, helps provide a framework to explain the complex patterns of compatibility between snails and schistosomes, and improves our overall understanding of comparative immunology.

A fibrinogen-related Lectin from Echinometra lucunter represents a new FReP family in Echinodermata phylum

Fibrinogen-related proteins (FREPs) have been identified in several animals. They are involved in the body's defense, acting as mediators of phagocytosis. Ficolins and intelectins are some of the most studied Fibrinogen-related Domain (FReD)-containing lectins. In this work, we have isolated a singular FReD-containing lectin, which cannot be classified as ficolin or intelectin. ELL (Echinometra lucunter lectin) was isolated from coelomic plasma by affinity chromatography on xanthan gum. Primary structure was determined by tandem mass spectrometry. Moreover, antimicrobial activity of ELL was evaluated against planktonic cells and biofilm of Escherichia coli, Staphylococcus aureus and S. epidermidis. ELL showed hemagglutinating activity in Ca²⁺ presence, which was inhibited by glycoprotein mucin and thyroglobulin. Complete amino acid sequence consisted of 229 residues, including a FReD in the N-terminal. Searches for similarity found that ELL was very close to putative proteins from Strongylocentrotus purpuratus. ELL showed moderate similarity with uncharacterized sea stars proteins and protochordate intelectins. ELL was able to inhibit the planktonic growth of the Gram-positive bacteria and significantly reduce the biofilm formation of all bacteria tested. In conclusion, we identified a new type of FReP-containing lectin with some structural and functional conservation towards intelectins.

Novel fibrinogen-related protein with single FReD contributes to the innate immunity of Macrobrachium rosenbergii

Fibrinogen-related proteins (FREPs) are widely found in vertebrates and invertebrates, and they play crucial roles in innate immunity. Here, a new FREP named as MrFREP was identified from giant freshwater prawn (Macrobrachium rosenbergii). The full-length cDNA of MrFREP measures 1649 bp in length and consists of a 1086 bp open reading frame encoding a polypeptide composed of 361 amino acids. The MrFREP sequence has a signal peptide with 20 amino acids and a fibrinogen-related domain (FReD) with 223 amino acids. Phylogenetic analysis showed that MrFREP was grouped with FREPs from Marsupenaeus japonicus and Litopenaeus vannamei. BLASTp results showed that it had 43% identity with a FREP from M. japonicus. The expression of MrFREP was higher in gills, intestine, and hepatopancreas than in hemocytes, heart, stomach, and muscles. The expression levels of MrFREP in gills and intestine were obviously upregulated after they were exposed to Vibrio parahaemolyticus or White spot syndrome virus infection. Recombinant MrFReD protein (rMrFReD) could bind to Gram-positive and Gram-negative bacteria and agglutinate the tested bacteria in the presence of calcium. rMrFReD demonstrated lipopolysaccharide and peptidoglycan binding activities. rMrFReD could accelerate the clearance of V. parahaemolyticus in vivo. These results suggested that MrFREP could function as a pattern recognition receptor contributing to the innate immunity of M. rosenbergii.

Identification and characterization of a novel PRR of fibrinogen-related protein in Apostichopus japonicus

Fibrinogen-related proteins (FREPs) play important roles in innate immunity by recognizing pathogen associated molecular patterns on pathogenic bacteria surfaces via conserved fibrinogen-like domain (FBG). In this paper, the full-length cDNA of Apostichopus japonicus FREP (designated as AjFREP) was cloned combined with rapid amplification of cDNA ends (RACE) and transcriptome sequencing. The full-length cDNA of AjFREP was of 2110 bp with an open reading frame (ORF) of 1659 bp. SMART analysis revealed that the AjFREP contained a typical signal peptide of 19 amino acid residues, a FBG and two unusual epidermal growth factor-like domains (EGFs). Multiple sequence alignments suggested that FBG domain shared a remarkably high structural conservation in polypeptide binding site and Ca²⁺ binding site. Tissue distribution analysis revealed that AjFREP was constitutively expressed in all examined tissues with the largest magnitude in coelomocytes, indicating AjFREP might play an important role in immune defense. The mRNA level of AjFREP in coelomocytes was sharply up-regulated by Vibrio splendidus challenge, and reached its peak expression at 48 h. Knock-down AjFREP by specific siRNA could significantly repress the coelomocyte phagocytosis rate. Meantime, the survival number of V. splendidus in the coelomic fluid was promoted. All these current results indicated that AjFREP might be involved in pathogen clearance through mediating coelomocytes phagocytosis activity.

A hemocyte-expressed fibrinogen-related protein gene (LvFrep) from the shrimp Litopenaeus vannamei: Expression analysis after microbial infection and during larval development

Fibrinogen-related proteins (FREPs) comprise a large family of microbial recognition proteins involved in many biological functions in both vertebrate and invertebrate animals. By taking advantage of publicly accessible databases, we have identified a FREP-like homolog in the most cultivated penaeid shrimp, Litopenaeus vannamei (LvFrep). The obtained sequence showed a conserved fibrinogen-related domain (FReD) and displayed significant similarities to FREP-like proteins from other invertebrates and to ficolins from crustaceans. The expression of LvFrep appeared to be limited to circulating hemocytes. Interestingly, LvFrep gene expression was induced in shrimp hemocytes only in response to a Vibrio infection but not to the White spot syndrome virus (WSSV). Moreover, LvFrep transcript levels were detected early in fertilized eggs, suggesting the participation of this immune-related gene in the antimicrobial defenses during shrimp development.

A fibrinogen-related protein identified from hepatopancreas of crayfish is a potential pattern recognition receptor

Fibrinogen-related protein (FREP) family is a large group of proteins containing fibrinogen-like (FBG) domain and plays multiple physiological roles in animals. However, their immune functions in crayfish are not fully explored. In the present study, a novel fibrinogen-like protein (designated as PcFBN1) was identified and characterized from hepatopancreas of red swamp crayfish Procambarus clarkii. The cDNA sequence of PcFBN1 contains an open reading frame (ORF) of 1353 bp encoding a protein of 450 amino acids. Sequence and structural analysis indicated that PcFBN1 contains an FBG domain in C-terminal and a putative signal peptide of 19 amino acids in N-terminal. Semi-quantitative PCR revealed that the main expression of PcFBN1 was observed in hepatopancreas and hemocyte. Temporal expression analysis exhibited that PcFBN1 expression could be significantly induced by heat-killed Aeromonas hydrophila. Tissue distribution and temporal change of PcFBN1 suggested that PcFBN1 may be involved in immune responses of red swamp crayfish. Recombinant PcFBN1 protein binds and agglutinates both gram-negative bacteria Escherichia coli and gram-positive bacteria Micrococcus lysodeikticus. Moreover, binding and agglutination is Ca(2+) dependent. Further analysis indicated that PcFBN1 recognizes some acetyl group-containing substance LPS and PGN. RNAi experiment revealed that PcFBN1 is required for bacterial clearance and survival from A. hydrophila infection. Reduction of PcFBN1 expression significantly decreased the survival and enhanced the number of A. hydrophila in the hemolymph. These results indicated that PcFBN1 plays an important role in the innate immunity of red swamp crayfish as a potential pattern recognition receptor.

The role of fibrinogen-related proteins in the gastropod immune response

Fibrinogen-related proteins or FREPs constitute a large family of molecules, defined by the presence of a fibrinogen-related domain (FReD). These molecules are found in all animals and are diverse in both form and function. Here, we review the current understanding of gastropod FREPs, which are characterized by the presence of a fibrinogen domain connected to one or two immunoglobulin superfamily domains by way of a short interceding region. We present a historical perspective on the discovery of FREPs in gastropods followed by a summary of advances made in the nearly two decades of research focused on the characterization of FREPs in Biomphalaria glabrata (BgFREPs). Topics covered include BgFREP genomic architecture, predicted structure and known functions, structural comparisons between BgFREPs, and evidence of somatic diversification. Also examined are the expression patterns of BgFREPs during snail development and immunological challenges. Recent functional characterization of the role BgFREPs play in the defence response against digenean trematodes is also presented, as well as new data investigating the nucleotide-level genomic conservation of FREPs among Pulmonate gastropods. Finally, we identify areas in need of further research. These include confirming and identifying the specific binding targets of BgFREPs and elucidating how they later engage snail haemocytes to elicit an immunological response, precise mechanisms and importance of BgFREP diversification, characterizing the tissue expression patterns of BgFREPs, as well as addressing whether gastropod FREPs retain immunological importance in alternative snail-trematode associations or more broadly in snail-pathogen interactions.

A fibrinogen-related protein (FREP) is involved in the antibacterial immunity of Marsupenaeus japonicus

Fibrinogen-related proteins (FREPs) in invertebrates have important functions in innate immunity. In this study, the cDNA of FREP was identified from the kuruma shrimp Marsupenaeus japonicus (MjFREP2). The full-length cDNA of MjFREP2 is 1138 bp with an open reading frame of 954 bp that encodes a 317-amino acid protein comprising a signal peptide and a fibrinogen-like domain. MjFREP2 could be detected in hemocytes, heart, hepatopancreas, gills, stomach, and intestines. MjFREP2 could also be upregulated in hemocytes after Vibrio anguillarum and Staphylococcus aureus challenge. Agglutination and binding assay results revealed that the recombinant MjFREP2 bound to bacteria and polysaccharides. Immunocytochemical analysis results showed that MjFREP2 proteins were mainly distributed in the cytoplasm of hemocytes from unchallenged shrimp and transported to the membrane or secreted out of the cell after V. anguillarum or S. aureus challenge. The secreted MjFREP2 bound to the bacteria presented in shrimp hemolymph. The overexpression of MjFREP2 could enhance bacterial clearance by inducing the phagocytosis of hemocytes. This ability was impaired by knockdown of MjFREP2 with RNA interference. The cumulative mortality of MjFREP2-silenced shrimp was significantly higher than that of the control shrimp. These results suggested that MjFREP2 has an important function in the antibacterial immunity of M. japonicus.

A fibrinogen-related protein (TfFREP2) gene involving in the immune response of Trachidermus fasciatus against Vibrio anguillarum

Fibrinogen-related proteins play important roles in the immune responses. We have obtained a cDNA encoding a novel fibrinogen-related protein from roughskin sculpin Trachidermus fasciatus (T. fasciatus) and named it as TfFREP2. The N and C terminus of TfFREP2 contain a putative 21-amino acid signal peptide and a typical 217-amino acid fibrinogen-like domain, which is conserved in all fibrinogen-related proteins. TfFREP2 has three glycosylation sites and two potential calcium-binding sites that are possibly involved in calcium coordination. The results of tissue specific checking showed that the mRNA and protein of TfFREP2 were particularly abundant in skin and gill among all the tested tissues. TfFREP2 mRNA and protein expression changed significantly after being challenged by Vibrio anguillarum pathogen in those immune-barrier tissues, such as skin and gill. Furthermore, recombinant TfFREP2 is able to agglutinate and bind V. anguillarum in the presence of calcium ion. The above results suggest that TfFREP2 might be involved in the host defense of fish against V. anguillarum infection.