The involvement of CgCaspase-8-2 in regulating the expressions of cytokines, antibacterial peptide and autophagy-related genes in oysters

Study on the mechanism of miR-7562 regulating ATG5 and ATG12 genes in Penaeus monodon under Vibrio harveyi infection

MicroRNAs (miRNAs) play a fundamental role in the post-transcriptional regulation of genes and are pivotal in modulating immune responses in marine species, particularly during pathogen assaults. This study focused on the function of miR-7562 and its regulatory effects on autophagy against Vibrio harveyi infection in the black tiger shrimp (Penaeus monodon), an economically important aquatic species. We successfully cloned and characterized two essential autophagy-related genes (ATGs) from P. monodon, PmATG5 and PmATG12, and then identified the miRNAs potentially involved in co-regulating these genes, which were notably miR-7562, miR-8485, and miR-278. Subsequent bacterial challenge experiments and dual-luciferase reporter assays identified miR-7562 as the principal regulator of both genes, particularly by targeting the 3'UTR of each gene. By manipulating the in vivo levels of miR-7562 using mimics and antagomirs, we found significant differences in the expression of PmATG5 and PmATG12, which corresponded to alterations in autophagic activity. Notably, miR-7562 overexpression resulted in the downregulation of PmATG5 and PmATG12, leading to a subdued autophagic response. Conversely, miR-7562 knockdown elevated the expression levels of these genes, thereby enhancing autophagic activity. Our findings further revealed that during V. harveyi infection, miR-7562 continued to influence the autophagic pathway by specifically targeting the ATG5-ATG12 complex. This research not only sheds light on the miRNA-dependent mechanisms governing autophagic immunity in shrimp but also proposes miR-7562 as a promising target for therapeutic strategies intended to strengthen disease resistance within the crustacean aquaculture industry.

An immunoglobulin superfamily member (CgIgIT2) functions as immune inhibitory receptor to inhibit the inflammatory cytokine expressions in Crassostrea gigas

Immune inhibitory receptors are increasingly acknowledged as potent regulators of immune response, which inhibit the overactivation of immune system and play an important role in maintaining immune homeostasis. In the present study, a novel immunoglobulin superfamily member (CgIgIT2) was identified from the Pacific oyster, Crassostrea gigas. The protein sequence of CgIgIT2 contained one signal peptide, four Ig domains, one fibronectin type III domain, one transmembrane domain, and a cytoplasmic tail with two intracellular immunoreceptor tyrosine-based inhibitory motifs (ITIMs) and one immunoreceptor tyrosine-based switch motif (ITSM). The mRNA transcripts of CgIgIT2 were widely expressed in all the tested tissues, including haemolymph, gill, mantle, adductor muscle, labial palp, gonad and hepatopancreas, with the highest expression in haemolymph. The mRNA expressions of CgIgIT2 in haemocytes increased significantly at 24, 48 and 72 h after Vibrio splendidus stimulation. The positive green signals of CgIgIT2 protein were mainly detected in granulocytes of haemocytes, which were 1.27-fold and 2.15-fold (p < 0.05) higher than that of semi-granulocytes and agranulocytes, respectively. And CgIgIT2 was mainly located in the membrane and cytoplasm of haemocytes. The recombinant protein of CgIgIT2-4 × Ig (rCgIgIT2-4 × Ig) exhibited binding activity towards multiple pathogen-associated molecular patterns (PAMPs), including lipopolysaccharides (LPS), peptidoglycan (PGN), mannose (MAN) and polyinosinic-polycytidylic acid (Poly (I: C)) with the highest affinity for LPS. rCgIgIT2-4 × Ig could also bind Gram-negative bacteria (V. splendidus, V. anguillarum, Escherichia coli), Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis), and fungi (Pichia pastoris). In the blocking assay with anti-CgIgIT2 antibody, the mRNA expressions of interleukins (CgIL17-1, CgIL17-3 and CgIL17-6) and tumor necrosis factors (CgTNF-1 and CgTNF-2) in haemocytes all increased significantly at 12 h after V. splendidus stimulation. These results suggested that CgIgIT2 could function as an inhibitor receptor to bind different PAMPs and microbes, as well as inhibit the mRNA expressions of multiple inflammatory cytokines in oysters.

Species-specific CgCaspase-Cg-5 in the pacific oyster induces haemocyte apoptosis by regulating the mRNA expression of apoptosis-related genes in the early stage of immune response

Caspases are cysteinyl aspartate-specific proteinases, playing critical roles in apoptotic pathway to induce apoptosis and inflammatory response. In this study, the expanded repertoire of Caspases was revealed in the Pacific oyster Crassostrea gigas, and a total of 30 Caspases were identified from the genomic and stress-induced transcriptomic databases of the Pacific oyster. They were clustered into CgCaspase-2/9, CgCaspase-8/10, CgCaspase-3/6/7, CgCaspase-Cg, and CgCaspase-L. CgCaspase-Cg subgroup was found to be specifically expanded after a positive selection in oyster with average Ka/Ks of 0.50. The mRNA expression of CgCaspase-Cg-5 was found to be obviously induced against various bacterial and viral stimulations or environmental stresses. The relative expression level of CgCaspase-Cg-5 in haemocytes increased and reached the peak at 6 h after Vibrio splendidus stimulation, which was 5.57-fold of that in the control group (p < 0.01). In the oysters whose CgCaspase-Cg-5 expression was knocked down, the mRNA expression of apoptosis-related genes including CgBcl2, CgBax, CgCaspase3 and CgCaspase9 changed significantly at 12 h after V. splendidus stimulation. The expression of CgBax, CgCaspase3 and CgCaspase9 decreased, which was 0.64-fold (p < 0.05), 0.53-fold (p < 0.05) and 0.62-fold (p < 0.01), while the expression of CgBcl2 increased, which was 2.81-fold (p < 0.01) of that in the EGFP-dsRNA group, respectively. Meanwhile, the apoptotic rate of haemocytes (1.90 ± 0.71%) significantly decreased compared to that in the EGFP-dsRNA group (5.40 ± 0.72%) (p < 0.05), and the histological damages of widened cell spacing, gill filament swelling and loose cytoplasm were observed in the CgCaspase-Cg-5-knockdown oysters after V. splendidus stimulation. Collectively, CgCaspase-Cg subgroup was specifically expanded in oyster and some bivalve species, and species-specific CgCaspase-Cg-5 regulated the mRNA expression of the apoptosis-related genes to induce haemocyte apoptosis in the early stage of immune response. This provided insight into the evolutionary and functional characteristics of Caspase repertoire in the Pacific oyster and highlighted the important role of CgCaspase-Cg-5 in the response to pathogen infection and environmental stresses.

Syk regulates the haemocyte autophagy through inducing the mRNA expressions of autophagy-related genes and the cleavage of CgLC3 in oyster antibacterial immunity

Spleen tyrosine kinase (Syk) is reported to be involved in activating the autophagy. Recently, a homologue of Syk was identified from Pacific oyster Crassostrea gigas (defined as CgSyk). In the present study, the molecular characteristics of CgSyk and its regulation mechanism in autophagy were investigated in oyster C. gigas. The full-length cDNA of CgSyk was of 4566 bp with an open reading frame (ORF) of 1989 bp. CgSyk encoded a polypeptide of 662 amino acids, containing two Src homology 2 (SH2) domains and one tyrosine kinase catalytic (TyrKc) domain. The deduced amino acid sequence of CgSyk shared low similarity with the previously identified Syks from other species. In the phylogenetic tree, CgSyk was first clustered with Crassostrea virginica CvSyk, and then classified into a branch of invertebrate Syks. In CgSyk-RNAi oysters, the mRNA expressions of CgLC3, CgP62, CgBeclin-1 and CgATG5 in haemocytes decreased significantly at 12 h after Vibrio splendidus stimulation. At the same time, the abundance of CgLC3Ⅱ in haemocytes, and the autophagy rate of haemocytes in CgSyk-RNAi oysters decreased significantly at 12 h after V. splendidus stimulation. All the results collectively suggested that CgSyk regulated the autophagy through inducing the mRNA expressions of autophagy-related genes and the cleavage of CgLC3 to defend against bacterial invasion in oysters.

CgCaspase-3 activates the translocation of CgGSDME in haemocytes of Pacific oyster Crassostrea gigas

Cysteinyl aspartate specific proteinase-3 (Caspase-3) is an important protein involved in the apoptosis and gasdermin E (GSDME)-mediated cell pyroptosis pathways in vertebrates. A Caspase-3 homologue (designated as CgCaspase-3) was previously identified as an immune receptor specific for lipopolysaccharide (LPS) to regulate apoptosis in the Pacific oyster Crassostrea gigas. In the present study, the binding activity of CgCaspase-3 to different pathogen associated molecular patterns (PAMPs) and its effects on CgGSDME translocation in haemocytes were further investigated in C. gigas. The mRNA expression of CgCaspase-3 could be detected in all the tested tissues, including hepatopancreas, labial palp, adductor muscle, gonad, gill, mantle and haemocytes, and it was highly expressed in labial palp, gonad, haemocytes, and adductor muscle. The mRNA expression of CgCaspase-3 in haemocytes increased significantly at 3, 24, 48 and 72 h after LPS stimulation, and it increased significantly at 6, 12, 24 and 48 h after Vibrio splendidus stimulation. The recombinant CgCaspase-3 displayed binding activity towards LPS, mannose (MAN), peptidoglycan (PGN), and polyinosinic-polycytidylic acid potassium salt (Poly (I:C)). The positive signals of CgGSDME on haemocyte membrane became stronger at 3 h after V. splendidus stimulation, compared with that of Seawater group, and the co-localization of CgCaspase-3 and CgGSDME was observed in the haemocyte membrane. After the injection of dsCgCaspase-3, the positive signals of CgGSDME on haemocyte membrane became weaker compared with that of EGFP-RNAi group at 24 h after V. splendidus stimulation. The results suggested that CgCaspase-3 was able to bind diverse PAMPs and activate the translocation of CgGSDME in haemocytes of oyster response against pathogen invasion.

Bivalve Haemocyte Subpopulations: A Review

Bivalve molluscs stand out for their ecological success and their key role in the functioning of aquatic ecosystems, while also constituting a very valuable commercial resource. Both ecological success and production of bivalves depend on their effective immune defence function, in which haemocytes play a central role acting as both the undertaker of the cellular immunity and supplier of the humoral immunity. Bivalves have different types of haemocytes, which perform different functions. Hence, identification of cell subpopulations and their functional characterisation in immune responses is essential to fully understand the immune system in bivalves. Nowadays, there is not a unified nomenclature that applies to all bivalves. Characterisation of bivalve haemocyte subpopulations is often combined with 1) other multiple parameter assays to determine differences between cell types in immune-related physiological activities, such as phagocytosis, oxidative stress and apoptosis; and 2) immune response to different stressors such as pathogens, temperature, acidification and pollution. This review summarises the major and most recent findings in classification and functional characterisation of the main haemocyte types of bivalve molluscs.