Identification and Characterization of Zebrafish Tlr4 Coreceptor Md-2

Illicit Drugs in Surface Waters: How to Get Fish off the Addictive Hook

The United Nations World Drug Report published in 2022 alarmed that the global market of illicit drugs is steadily expanding in space and scale. Substances of abuse are usually perceived in the light of threats to human health and public security, while the environmental aspects of their use and subsequent emissions usually remain less explored. However, as with other human activities, drug production, trade, and consumption of drugs may leave their environmental mark. Therefore, this paper aims to review the occurrence of illicit drugs in surface waters and their bioaccumulation and toxicity in fish. Illicit drugs of different groups, i.e., psychostimulants (methamphetamines/amphetamines, cocaine, and its metabolite benzoylecgonine) and depressants (opioids: morphine, heroin, methadone, fentanyl), can reach the aquatic environment through wastewater discharge as they are often not entirely removed during wastewater treatment processes, resulting in their subsequent circulation in nanomolar concentrations, potentially affecting aquatic biota, including fish. Exposure to such xenobiotics can induce oxidative stress and dysfunction to mitochondrial and lysosomal function, distort locomotion activity by regulating the dopaminergic and glutamatergic systems, increase the predation risk, instigate neurological disorders, disbalance neurotransmission, and produce histopathological alterations in the brain and liver tissues, similar to those described in mammals. Hence, this drugs-related multidimensional harm to fish should be thoroughly investigated in line with environmental protection policies before it is too late. At the same time, selected fish species (e.g., Danio rerio, zebrafish) can be employed as models to study toxic and binge-like effects of psychoactive, illicit compounds.

Effects of Antibiotic Residues on Fish Gut Microbiome Dysbiosis and Mucosal Barrier-Related Pathogen Susceptibility in Zebrafish Experimental Model

The symbiotic community of microorganisms in the gut plays an important role in the health of the host. While many previous studies have been performed on the interactions between the gut microbiome and the host in mammals, studies in fish are still lacking. In this study, we investigated changes in the intestinal microbiome and pathogen susceptibility of zebrafish (Danio rerio) following chronic antibiotics exposure. The chronic antibiotics exposure assay was performed on zebrafish for 30 days using oxytetracycline (Otc), sulfamethoxazole/trimethoprim (Smx/Tmp), or erythromycin (Ery), which are antibiotics widely used in the aquaculture industry. The microbiome analysis indicated that Fusobacteria, Proteobacteria, Firmicutes, and Bacteroidetes were the dominant phyla in the gut microbiome of the zebrafish used in this study. However, in Smx/Tmp-treated zebrafish, the compositions of Fusobacteria and Proteobacteria were changed significantly, and in Ery-treated zebrafish, the compositions of Proteobacteria and Firmicutes were altered significantly. Although alpha diversity analysis showed that there was no significant difference in the richness, beta diversity analysis revealed a community imbalance in the gut microbiome of all chronically antibiotics-exposed zebrafish. Intriguingly, in zebrafish with dysbiosis in the gut microbiome, the pathogen susceptibility to Edwardsiella piscicida, a representative Gram-negative fish pathogen, was reduced. Gut microbiome imbalance resulted in a higher count of goblet cells in intestinal tissue and an upregulation of genes related to the intestinal mucosal barrier. In addition, as innate immunity was enhanced by the increased mucosal barrier, immune and stress-related gene expression in the intestinal tissue was downregulated. In this study, we provide new insight into the effect of gut microbiome dysbiosis on pathogen susceptibility.

Riding the wave of innovation: immunoinformatics in fish disease control

The spread of infectious illnesses has been a significant factor restricting aquaculture production. To maximise aquatic animal health, vaccination tactics are very successful and cost-efficient for protecting fish and aquaculture animals against many disease pathogens. However, due to the increasing number of immunological cases and their complexity, it is impossible to manage, analyse, visualise, and interpret such data without the assistance of advanced computational techniques. Hence, the use of immunoinformatics tools is crucial, as they not only facilitate the management of massive amounts of data but also greatly contribute to the creation of fresh hypotheses regarding immune responses. In recent years, advances in biotechnology and immunoinformatics have opened up new research avenues for generating novel vaccines and enhancing existing vaccinations against outbreaks of infectious illnesses, thereby reducing aquaculture losses. This review focuses on understanding in silico epitope-based vaccine design, the creation of multi-epitope vaccines, the molecular interaction of immunogenic vaccines, and the application of immunoinformatics in fish disease based on the frequency of their application and reliable results. It is believed that it can bridge the gap between experimental and computational approaches and reduce the need for experimental research, so that only wet laboratory testing integrated with in silico techniques may yield highly promising results and be useful for the development of vaccines for fish.

Interplay between a bacterial pathogen and its host in rainbow trout isogenic lines with contrasted susceptibility to cold water disease

Infectious diseases are a major constraint on aquaculture. Genetic lines with different susceptibilities to diseases are useful models to identify resistance mechanisms to pathogens and to improve prophylaxis. Bacterial cold-water disease (BCWD) caused by Flavobacterium psychrophilum represents a major threat for freshwater salmonid farming worldwide. A collection of rainbow trout (Oncorhynchus mykiss) isogenic lines was previously produced from a French domestic population. Here, we compared BCWD resistance phenotypes using a subset of isogenic lines chosen for their contrasted susceptibilities to F. psychrophilum. We applied individual monitoring to document the infection process, including time-course quantification of bacteremia and innate immune response. Strikingly, BCWD resistance was correlated with a lower bacterial growth rate in blood. Several immune genes were expressed at higher levels in resistant fish regardless of infection: the Type II arginase (arg2), a marker for M2 macrophages involved in anti-inflammatory responses and tissue repair, and two Toll-like receptors (tlr2/tlr7), responsible for pathogen detection and inflammatory responses. This study highlights the importance of innate and intrinsic defense mechanisms in determining the outcome of F. psychrophilum infections, and illustrates that non-lethal time-course blood sampling for individual monitoring of bacteremia is a powerful tool to resolve within-host pathogen behavior in bacterial fish diseases.

Evolution of LPS recognition and signaling: The bony fish perspective

Fish are the most diverse and successful group of vertebrate animals, with about 30,000 species. The study of fish immunity is of great importance for understanding the evolution of vertebrate immunity, as they are the first animals to show both innate and adaptive immune responses. Although fish immunity is similar to that of mammals, there are obvious differences, such as their dependence of ambient temperature, their poor antibody response, and lack of antibody switching and lymph nodes. In addition, several important differences have also been found between the innate immune responses of fish and mammals. Among these, we will discuss in this review the high resistance of fish to the toxic effects of lipopolysaccharide (LPS) which can be explained by the absence of a Toll-like receptor 4 (Tlr4) ortholog in most fish species or by the inability of the Tlr4/Md2 (Myeloid differentiation 2) complex to recognize LPS, together with the presence of a negative regulator of the LPS signaling complex formed by the TLR-like molecule Rp105 (Radioprotective 105) and Md1. Taken together, these data support the idea that, although TLR4 and RP105 arose from a common ancestor to fish and tetrapods, the TLR4/MD2 receptor complex for LPS recognition arose after their divergence about 450 million years ago.

Zebrafish: A Relevant Genetic Model for Human Primary Immunodeficiency (PID) Disorders?

Primary immunodeficiency (PID) disorders, also commonly referred to as inborn errors of immunity, are a heterogenous group of human genetic diseases characterized by defects in immune cell development and/or function. Since these disorders are generally uncommon and occur on a variable background profile of potential genetic and environmental modifiers, animal models are critical to provide mechanistic insights as well as to create platforms to underpin therapeutic development. This review aims to review the relevance of zebrafish as an alternative genetic model for PIDs. It provides an overview of the conservation of the zebrafish immune system and details specific examples of zebrafish models for a multitude of specific human PIDs across a range of distinct categories, including severe combined immunodeficiency (SCID), combined immunodeficiency (CID), multi-system immunodeficiency, autoinflammatory disorders, neutropenia and defects in leucocyte mobility and respiratory burst. It also describes some of the diverse applications of these models, particularly in the fields of microbiology, immunology, regenerative biology and oncology.

Lipopolysaccharide detection by the innate immune system may be an uncommon defence strategy used in nature

Since the publication of the Janeway's Pattern Recognition hypothesis in 1989, study of pathogen-associated molecular patterns (PAMPs) and their immuno-stimulatory activities has accelerated. Most studies in this area have been conducted in model organisms, which leaves many open questions about the universality of PAMP biology across living systems. Mammals have evolved multiple proteins that operate as receptors for the PAMP lipopolysaccharide (LPS) from Gram-negative bacteria, but LPS is not immuno-stimulatory in all eukaryotes. In this review, we examine the history of LPS as a PAMP in mammals, recent data on LPS structure and its ability to activate mammalian innate immune receptors, and how these activities compare across commonly studied eukaryotes. We discuss why LPS may have evolved to be immuno-stimulatory in some eukaryotes but not others and propose two hypotheses about the evolution of PAMP structure based on the ecology and environmental context of the organism in question. Understanding PAMP structures and stimulatory mechanisms across multi-cellular life will provide insights into the evolutionary origins of innate immunity and may lead to the discovery of new PAMP variations of scientific and therapeutic interest.

Tollip-deficient zebrafish display no abnormalities in development, organ morphology or gene expression in response to lipopolysaccharide

Tollip is a multifunctional adaptor protein implicated in innate immunity, lysosomal trafficking/autophagy of protein aggregates and various signaling processes in mammalian models. To verify evolutionary conservation of these functions, we used CRISPR/Cas9 editing to construct a zebrafish line bearing a stable tollip knockout. In contrast to previously reported tollip morphants, Tollip‐deficient fish display normal development until adulthood, are fertile, and have no apparent physiological defects. When challenged with lipopolysaccharide (LPS), inflammatory gene expression is unaffected. Moreover, Tollip deficiency does not aggravate swimming deficiency resulting from lysosomal dysfunction and proteotoxicity in a fish model of Gaucher disease. Thus, individual functions of Tollip may be organism‐specific or manifest only upon certain conditions/challenges or disease backgrounds.

Dietary supplementation of Bacillus velezensis improves Vibrio anguillarum clearance in European sea bass by activating essential innate immune mechanisms

Bacillus spp. supplementation as probiotics in cultured fish diets has a long history of safe and effective use. Specifically, B. velezensis show great promise in fine-tuning the European sea bass disease resistance against the pathogenicity caused by several members of the Vibrio family. However, the immunomodulatory mechanisms behind this response remain poorly understood. Here, to examine the inherent immune variations in sea bass, two equal groups were fed for 30 days with a steady diet, with one treatment supplemented with B. velezensis. The serum bactericidal capacity against live cells of Vibrio anguillarum strain 507 and the nitric oxide and lysozyme lytic activities were assayed. At the cellular level, the phagocytic response of peripheral blood leukocytes against inactivated Candida albicans was determined. Moreover, head-kidney (HK) total leukocytes were isolated from previously in vivo treated fish with LPS of V. anguillarum strain 507. Mechanistically, the expression of some essential proinflammatory genes (interleukin-1 (il1b), tumor necrosis factor-alpha (tnfa), and cyclooxygenase 2 (cox2) and the sea bass specific antimicrobial peptide (AMP) dicentracin (dic) expressions were assessed. Surprisingly, the probiotic supplementation significantly increased all humoral lytic and cellular activities assayed in the treated sea bass. In addition, time-dependent differences were observed between the control and probiotic treated groups for all the HK genes markers subjected to the sublethal LPS dose. Although the il1b was the fastest responding gene to a significant level at 48 h post-injection (hpi), all the other genes followed 72 h in the probiotic supplemented group. Finally, an in vivo bacteria challenge against live V. anguillarum was conducted. The probiotic fed fish observed a significantly higher survival. Overall, our results provide clear vertical evidence on the beneficial immune effects of B. velezensis and unveil some fundamental immune mechanisms behind its application as a probiotic agent in intensively cultured European sea bass.

RNA-Seq of Single Fish Cells – Seeking Out the Leukocytes Mediating Immunity in Teleost Fishes

The immune system is a complex and sophisticated biological system, spanning multiple levels of complexity, from the molecular level to that of tissue. Our current understanding of its function and complexity, of the heterogeneity of leukocytes, is a result of decades of concentrated efforts to delineate cellular markers using conventional methods of antibody screening and antigen identification. In mammalian models, this led to in-depth understanding of individual leukocyte subsets, their phenotypes, and their roles in health and disease. The field was further propelled forward by the development of single-cell (sc) RNA-seq technologies, offering an even broader and more integrated view of how cells work together to generate a particular response. Consequently, the adoption of scRNA-seq revealed the unexpected plasticity and heterogeneity of leukocyte populations and shifted several long-standing paradigms of immunology. This review article highlights the unprecedented opportunities offered by scRNA-seq technology to unveil the individual contributions of leukocyte subsets and their crosstalk in generating the overall immune responses in bony fishes. Single-cell transcriptomics allow identifying unseen relationships, and formulating novel hypotheses tailored for teleost species, without the need to rely on the limited number of fish-specific antibodies and pre-selected markers. Several recent studies on single-cell transcriptomes of fish have already identified previously unnoticed expression signatures and provided astonishing insights into the diversity of teleost leukocytes and the evolution of vertebrate immunity. Without a doubt, scRNA-seq in tandem with bioinformatics tools and state-of-the-art methods, will facilitate studying the teleost immune system by not only defining key markers, but also teaching us about lymphoid tissue organization, development/differentiation, cell-cell interactions, antigen receptor repertoires, states of health and disease, all across time and space in fishes. These advances will invite more researchers to develop the tools necessary to explore the immunology of fishes, which remain non-conventional animal models from which we have much to learn.

Ctenopharyngodon idella Tollip regulates MyD88-induced NF-κB activation

Toll-interacting protein (Tollip) and MyD88 are key components of the TLR/IL-1R signaling pathway in mammals. MyD88 is known as a universal adaptor protein involving in TLR/IL-1R-induced NF-κB activation. Tollip is a crucial negative regulator of TLR-mediated innate immune responses. Previous studies have demonstrated that teleost Tollip served as a negative regulator of MyD88-dependent TLR signaling pathway. However, the mechanism is still unclear. In particular, the effect of TBD, C2, and CUE domains of Tollip on MyD88-NF-κB signaling pathway remains to be elucidated. In this study, we found that the response of grass carp Tollip (CiTollip) to LPS stimulation was faster and stronger than that of poly I:C treatment, and CiTollip diminished the expression of tnf-α induced by LPS. Further assays indicated that except for the truncated mutant of △CUE2 (1-173 aa), wild type CiTollip and other truncated mutants (△N-(52-276 aa), △C2-(173-276 aa) and △CUE1-(1-231 aa)) could associate with MyD88 and negatively regulate MyD88-induced NF-κB activation. It suggested that the C-terminal (173-276 aa), in particular the connection section between C2 and CUE domains (173-231 aa), played a pivotal role in suppressing MyD88-induced activation of NF-κB.

Progresses on three pattern recognition receptor families (TLRs, RLRs and NLRs) in teleost

Pattern recognition receptors (PRRs) are a class of immune sensors that play crucial roles in detecting and responding to the conserved patterns of microorganisms. To date, many PRRs, such as TLRs, RLRs and NLRs, as well as their downstream molecules have been identified and characterized in teleost, while their ligands and immunoregulatory mechanisms remain largely unknown. In the present review, we described and discussed the main members of TLR/RLR/NLR families, including their expression profiles, signaling transductions and functions in teleost. And some splicing isoforms from TLR/RLR/NLR families were also addressed, which play synergistic and/or antagonistic roles in response to pathogen infections in teleost. TLRs sense different pathogens by forming homodimer and/or heterodimer. Beyond, functions of TLRs can also be affected by migrating. And some endolysosomal TLRs undergo proteolytic cleavage and in a pH-dependent mechanism to attain a mature functional form that mediate ligand recognition and downstream signaling. Until now, more than 80 members in TLR/RLR/NLR families have been identified in teleost, while only TLR5, TLR9, TLR19, TLR21, TLR22, MDA5, LGP2, NOD1 and NOD2 have direct evidence of ligand recognition in teleost. Meanwhile, new ligands as well as signaling pathways do occur during evolution of teleost. This review summarizes progresses on the TLRs/RLRs/NLRs in teleost. We attempt to insight into the ligands recognition and signaling transmission of TLRs/RLRs/NLRs in teleost.

Zebrafish larvae as experimental model to expedite the search for new biomarkers and treatments for neonatal sepsis

Neonatal sepsis is a major cause of death and disability in newborns. Commonly used biomarkers for diagnosis and evaluation of treatment response lack sufficient sensitivity or specificity. Additionally, new targets to treat the dysregulated immune response are needed, as are methods to effectively screen drugs for these targets. Available research methods have hitherto not yielded the breakthroughs required to significantly improve disease outcomes, we therefore describe the potential of zebrafish (Danio rerio) larvae as preclinical model for neonatal sepsis. In biomedical research, zebrafish larvae combine the complexity of a whole organism with the convenience and high-throughput potential of in vitro methods. This paper illustrates that zebrafish exhibit an immune system that is remarkably similar to humans, both in terms of types of immune cells and signaling pathways. Moreover, the developmental state of the larval immune system is highly similar to human neonates. We provide examples of zebrafish larvae being used to study infections with pathogens commonly causing neonatal sepsis and discuss known limitations. We believe this species could expedite research into immune regulation during neonatal sepsis and may hold keys for the discovery of new biomarkers and novel treatment targets as well as for screening of targeted drug therapies.

Identification and expression analysis of sixteen Toll-like receptor genes, TLR1, TLR2a, TLR2b, TLR3, TLR5M, TLR5S, TLR7-9, TLR13a-c, TLR14, TLR21-23 in mandarin fish Siniperca chuatsi

Toll-like receptors (TLRs), as a family of pattern recognition receptors (PRRs), possess specific pathogen-related molecular pattern (PAMP) recognition spectrum in inducing immune responses. In this study, sixteen TLRs were identified and characterized in mandarin fish (Siniperca chuatsi). All these TLRs consist of leucine-rich repeats (LRRs), a transmembrane domain and a Toll/interleukin-I receptor (TIR) domain, with the exception of TLR5S which lacks TIR domain, and they can be clustered into five branches, i.e. TLR1 subfamily, TLR3 subfamily, TLR5 subfamily, TLR7 subfamily and TLR11 subfamily in phylogenetic tree. These TLR genes were expressed in all tested tissues and had high expression levels in immune-related tissues such as head-kidney and spleen or mucosa-related tissues such as intestine and pyloric caecum. The transcripts of TLR2a, TLR2b, TLR3, TLR13a, TLR14, TLR22 and TLR23 were all significantly up-regulated after stimulation with poly(I:C); TLR1, TLR2a, TLR2b, TLR3, TLR5M, TLR5S, TLR13a and TLR13b transcripts were all significantly up-regulated after stimulation with PGN; and TLR2a, TLR2b, TLR5M, TLR5S, TLR7, TLR8, TLR9, TLR13c, TLR14 and TLR22 transcripts were all significantly up-regulated after stimulation with LPS in isolated head kidney lymphocytes of mandarin fish. The findings in this study may provide a valuable basis for functional study on TLR genes in mandarin fish.