Tlr22 structure and expression characteristic of barbel chub, Squaliobarbus curriculus provides insights into antiviral immunity against infection with grass carp reovirus

Unveiling the multifaceted role of toll-like receptors in immunity of aquatic animals: pioneering strategies for disease management

The pattern recognition receptor (PRR), which drives innate immunity, shields the host against invasive pathogens. Fish and other aquatic species with poorly developed adaptive immunity mostly rely on their innate immunity, regulated by PRRs such as inherited-encoded toll-like receptors (TLRs). The discovery of 21 unique TLR variations in various aquatic animals over the past several years has sparked interest in using TLRs to improve aquatic animal's immune response and disease resistance. This comprehensive review provides an overview of the latest investigations on the various characteristics of TLRs in aquatic animals. It emphasizes their categorization, insights into 3D architecture, ligand recognition, signaling pathways, TLRs mediated immune responses under biotic and abiotic stressors, and expression variations during several developmental stages. It also highlights the differences among aquatic animals' TLRs and their mammal counterparts, which signifies the unique roles that TLRs play in aquatic animal's immune systems. This article summarizes current aquaculture research to enhance our understanding of fish immune systems for effective aquaculture -related disease management.

A comprehensive review on the dynamic role of toll-like receptors (TLRs) in frontier aquaculture research and as a promising avenue for fish disease management

Toll-like receptors (TLRs) represent a conserved group of germline-encoded pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and play a crucial role in inducing the broadly acting innate immune response against pathogens. In recent years, the detection of 21 different TLR types in various fish species has sparked interest in exploring the potential of TLRs as targets for boosting immunity and disease resistance in fish. This comprehensive review offers the latest insights into the diverse facets of fish TLRs, highlighting their history, classification, architectural insights through 3D modelling, ligands recognition, signalling pathways, crosstalk, and expression patterns at various developmental stages. It provides an exhaustive account of the distinct TLRs induced during the invasion of specific pathogens in various fish species and delves into the disparities between fish TLRs and their mammalian counterparts, highlighting the specific contribution of TLRs to the immune response in fish. Although various facets of TLRs in some fish, shellfish, and molluscs have been described, the role of TLRs in several other aquatic organisms still remained as potential gaps. Overall, this article outlines frontier aquaculture research in advancing the knowledge of fish immune systems for the proper management of piscine maladies.

TLR22-Induced Pro-Apoptotic mtROS Abets UPRmt-Mediated Mitochondrial Fission in Aeromonas hydrophila-Infected Headkidney Macrophages of Clarias gariepinus

Toll-like receptors (TLRs) are epitomized as the first line of defense against pathogens. Amongst TLRs, TLR22 is expressed in non-mammalian aquatic vertebrates, including fish. Using headkidney macrophages (HKM) of Clarias gariepinus, we reported the pro-apoptotic and microbicidal role of TLR22 in Aeromonas hydrophila infection. Mitochondria act as a central scaffold in the innate immune system. However, the precise molecular mechanisms underlying TLR22 signaling and mitochondrial involvement in A. hydrophila-pathogenesis remain unexplored in fish. The aim of the present study was to investigate the nexus between TLR22 and mitochondria in pro-apoptotic immune signaling circuitry in A. hydrophila-infected HKM. We report that TLR22-induced mitochondrial-Ca²⁺ [Ca²⁺]_mt surge is imperative for mtROS production in A. hydrophila-infected HKM. Mitigating mtROS production enhanced intracellular bacterial replication implicating its anti-microbial role in A. hydrophila-pathogenesis. Enhanced mtROS triggers hif1a expression leading to prolonged chop expression. CHOP prompts mitochondrial unfolded protein response (UPR^mt) leading to the enhanced expression of mitochondrial fission marker dnml1, implicating mitochondrial fission in A. hydrophila pathogenesis. Inhibition of mitochondrial fission reduced HKM apoptosis and increased the bacterial burden. Additionally, TLR22-mediated alterations in mitochondrial architecture impair mitochondrial function (ΔΨ_m loss and cytosolic accumulation of cyt c), which in turn activates caspase-9/caspase-3 axis in A. hydrophila-infected HKM. Based on these findings we conclude that TLR22 prompts mtROS generation, which activates the HIF-1α/CHOP signalosome triggering UPR^mt-induced mitochondrial fragmentation culminating in caspase-9/-3-mediated HKM apoptosis and bacterial clearance.

The effect of partial replacement of fish meal by soy protein concentrate on growth performance, immune responses, gut morphology and intestinal inflammation for juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂)

An 8-week feeding trial was conducted to investigate the effects of partial replacement of fish meal by soy protein concentrate (SPC) on the growth performance, immune responses, intestine morphology and relation gene expression of intestinal inflammation for juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) (initial weight 12.5 ± 0.00 g). Eight isonitrogenous and isolipidic diets (48.61% protein and 11.17% lipid) were formulated by replacing 0% (the control), 11%, 22%, 33%, 44%, 55%, 66%, and 77% of fish meal (FM) with SPC, respectively (the eight dietary be named FM, S11, S22, S33, S44, S55, S66, and S77, respectively). With the replacement level increased, the final body weight, weight gain ratio (WGR), specific growth rate (SGR), and survival rate of fish were significantly decreased (P < 0.05) compared with the group FM. By contrast, the feed conversion ratio (FCR) of fish was significantly increased (P < 0.05) when the replacement level up to 44%. Partial FM replacement by SPC (ranging from 11% to 77%) substantially reduced (P < 0.05) the serum total protein, albumin, and total cholesterol contents compared with the group FM. Liver total superoxide dismutase, glutathione peroxidase, catalase activities, and total antioxidant capacity showed the same trend of gradual increase first and then decrease. Their highest values were found in the replacement levels of 55%, 33%, 22%, and 55% and were significantly higher (P < 0.05) than the control group. The lowest malondialdehyde content was observed in group S77 and was significantly lower (P < 0.05) than that of the control group. The complements C3 and C4 contents of fish fed with experimental diets (replacement level ranged from 11% to 66%) were significantly higher (P < 0.05) than the group FM. The liver lysozyme activity of the control group was the lowest and was significantly lower than that of other dietary treatments (P < 0.05). Villus length and muscle thickness in the intestine of fish were significantly lower (P < 0.05) than other groups when the replacement level exceeded 44%. With dietary replacement levels increased, the TLR22, MyD88, p65, pro-inflammatory cytokines (IL-1β, TNF-α, IL-12P40 and INF-γ) and anti-inflammatory cytokines (TGF-β, IL-10, epinecidin, MHCIIβ and hepcidin) mRNA levels in the proximal intestine were significantly up-regulated (P < 0.05). The TLR22, MyD88, p65, pro-inflammatory cytokines (IL-1β, TNF-α, IL-12P40 and INF-γ) and anti-inflammatory cytokines (TGF-β, IL-10, MHCIIβ and hepcidin) mRNA levels in the mid intestine were significantly up-regulated (P < 0.05). The mRNA levels of TLR22, anti-inflammatory cytokines (IL-1β, TNF-α, IL-12P40, INF-γ) and anti-inflammatory cytokines (TGF-β, IL-10, epinecidin, MHCIIβ and hepcidin) in the distal intestine were significantly down-regulated (P < 0.05). The mRNA expression of MyD88 and p65 mRNA were showed a tend increased first and then decreased, and the highest values were observed in group S33 and S55 (P < 0.05), respectively. Based on the present work, the correlation between WGR and FM replacement level with SPC was described using the broken-line model, which estimated the optimum FM replacement to 37.23% for juvenile hybrid grouper dietary.

Duplication of toll-like receptor 22 in teleost fishes

The TLRs of teleost fishes have distinct features and are highly diverse, but the duplication characteristics and expression patterns of the tlr22 gene remain unclear. Here, we identified paralogous tlr22 genes in 13 teleost fishes by screening available fish genomic resources and using molecular cloning. We then conducted comprehensive bioinformatics analyses and investigated spatiotemporal differences in the expression patterns of the tlr22 genes in G. eckloni. The results indicated that more than three paralogous tlr22 genes were possessed by some teleost fishes. Of these, tlr22c is specific to some subfamilies of the Cyprinidae (e.g., Barbinae, Cyprininae, Schizothoracinae, and Leuciscinae). Phylogenetic and syntenic analyses showed that the paralogous tlr22 genes originated from two single-gene duplication events. Molecular clock calculations dated the two gene duplication events at 49.5 and 39.3 MYA, which is before the common carp-specific genome duplication event and well after the fish-specific genome duplication. Gene duplication of tlr22 was followed by gene loss or pseudogene events in certain lineages. Spatiotemporal expression differences between the three duplicated tlr22 genes from G. eckloni suggested that these genes diverged functionally after gene duplication.

IFN Signaling in Inflammation and Viral Infections: New Insights from Fish Models

The overarching structure of the type I interferon (IFN) system is conserved across vertebrates. However, the variable numbers of whole genome duplication events during fish evolution offer opportunities for the expansion, diversification, and new functionalization of the genes that are involved in antiviral immunity. In this review, we examine how fish models provide new insights about the implication of virus-driven inflammation in immunity and hematopoiesis. Mechanisms that have been discovered in fish, such as the strong adjuvant effect of type I IFN that is used with DNA vaccination, constitute good models to understand how virus-induced inflammatory mechanisms can interfere with adaptive responses. We also comment on new discoveries regarding the role of pathogen-induced inflammation in the development and guidance of hematopoietic stem cells in zebrafish. These findings raise issues about the potential interferences of viral infections with the establishment of the immune system. Finally, the recent development of genome editing provides new opportunities to dissect the roles of the key players involved in the antiviral response in fish, hence enhancing the power of comparative approaches.

Toll-Like Receptors, Associated Biological Roles, and Signaling Networks in Non-Mammals

The innate immune system is the first line of defense against pathogens, which is initiated by the recognition of pathogen-associated molecular patterns (PAMPs) and endogenous damage-associated molecular patterns (DAMPs) by pattern recognition receptors (PRRs). Among all the PRRs identified, the toll-like receptors (TLRs) are the most ancient class, with the most extensive spectrum of pathogen recognition. Since the first discovery of Toll in Drosophila melanogaster, numerous TLRs have been identified across a wide range of invertebrate and vertebrate species. It seems that TLRs, the signaling pathways that they initiate, or related adaptor proteins are essentially conserved in a wide variety of organisms, from Porifera to mammals. Molecular structure analysis indicates that most TLR homologs share similar domain patterns and that some vital participants of TLR signaling co-evolved with TLRs themselves. However, functional specification and emergence of new signaling pathways, as well as adaptors, did occur during evolution. In addition, ambiguities and gaps in knowledge still exist regarding the TLR network, especially in lower organisms. Hence, a systematic review from the comparative angle regarding this tremendous signaling system and the scenario of evolutionary pattern across Animalia is needed. In the current review, we present overview and possible evolutionary patterns of TLRs in non-mammals, hoping that this will provide clues for further investigations in this field.

MicroRNA regulation of Toll-like receptor signaling pathways in teleost fish

The innate immune system is the first line defense mechanism that recognizes, responds to, controls or eliminates invading pathogens. Toll-like receptors (TLRs) are a critical family of pattern recognition receptors (PRRs) tightly regulated by complex mechanisms involving many molecules to ensure a beneficial outcome in response to foreign invaders. MicroRNAs (miRNAs), a transcriptional and posttranscriptional regulator family in a wide range of biological processes, have been identified as new molecules related to the regulation of TLR-signaling pathways in immune responses. To date, at least 22 TLR types have been identified in more than a dozen different fish species. However, the functions and underlying mechanisms of miRNAs in the regulation of inflammatory responses related to the TLR-signaling pathway in fish is lacking. In this review, we summarize the regulation of miRNA expression profiles in the presence of TLR ligands or pathogen infections in teleost fish. We focus on the effects of miRNAs in regulating TLR-signaling pathways by targeting multiple molecules, including TLRs themselves, TLR-associated signaling proteins, and TLR-induced cytokines. An understanding of the relationship between the TLR-signaling pathways and miRNAs may provide new insights for drug intervention to manipulate immune responses in fish.