The teleost head kidney: Integrating thyroid and immune signalling

Dietary exposure to a binary mixture of polybrominated diphenyl ethers alters innate immunity and disease susceptibility in juvenile Chinook salmon (Oncorhynchus tshawytscha)

Polybrominated diphenyl ethers (PBDEs) have been used as flame retardants in consumer products and are now found in the aquatic environment. The presence of PBDEs puts the health and survival of aquatic species at risk due to the various toxic effects associated with exposure to these compounds. The effects of a binary dietary mixture of PBDEs on innate immunity and disease susceptibility of juvenile Chinook salmon (Oncorhynchus tshawytscha) were examined in the present study. Salmon were fed roughly 1:1 mixtures of two environmentally predominant PBDE congeners, BDE-47 and BDE-99. The six resulting whole body total PBDE concentrations ranged from less than the limit of quantification to 184 ng/g, wet weight (ww). The innate immune system was assessed by using two in vitro macrophage function assays. Specifically, assays that examined the ability of head kidney macrophages to: (1) engulf sheep red blood cells (SRBCs); and (2) produce a respiratory burst, as determined by the production of a reactive oxygen species, superoxide anion. Macrophages from salmon fed the BDE-47/99 mixture diets engulfed more SRBCs and produced greater superoxide anion than salmon fed the control diet. An increase in macrophage function was observed in fish with whole body total PBDE concentrations ranging from 2.81 ng/g, ww to 184 ng/g, ww. The mechanism for this increase in macrophage function due to PBDE exposure is currently unknown, but may be due to the ability of PBDEs to act as an endocrine receptor agonist and/or antagonist. Salmon exposed to the BDE-47/99 mixture diets were also challenged with the pathogenic bacteria, Vibrio (Listonella) anguillarum to determine disease susceptibility. Kaplan-Meier survival curves of fish exposed to the BDE-47/99 mixture and control diets were significantly different. The Cox proportional hazard risk ratios of disease-induced mortality in juvenile Chinook salmon with whole body concentrations of total PBDEs of 10.9, 36.8, and 184 ng/g, ww were significantly greater than the fish fed the control diet by 1.56, 1.83 and 1.50 times, respectively. Not all concentrations of the binary mixture diets had significant hazard ratios relative to the control diet, due to a non-monotonic concentration response curve. The mixture of PBDE congeners resulted in interactive effects that were generally non-additive and dependent upon the congener concentrations and metric examined. Consequently, predicting the interactive effects in juvenile Chinook salmon exposed to mixtures of PBDE congeners on innate immunity and disease susceptibility cannot be readily determined from the adverse effects of individual PBDE congeners.

Streptococcus agalactiae infection kills red tilapia with chronic Francisella noatunensis infection more rapidly than the fish without the infection

In this study we examined the effect that a Francisella noatunensis (Fno) infection had on hybrid red tilapia (Oreochromis niloticus × Oreochromis mossambicus) subsquently infected with Streptococcus agalactiae. A variety of hemato-immunological parameters (haematocrit, total red blood cell count, mean corpuscular volume, total white blood and differential cell counts, total plasma protein, plasma lysozyme and plasma peroxidase activities, and respiratory burst and phagocytic activities of head-kidney macrophages) were measured in hybrid red tilapia that had been previously exposed to an Fno outbreak in a tilapia grow-out farm. The head-kidneys of these apparently healthy survivors, when checked by PCR were found to be Fno-positive with hemato-immunological parameters that were similar to fish without an a priori infection. The only exception was the percentage lymphocyte count in the peripheral blood, which was slightly, but significantly, lower in the Fno-infected fish, compared to those without the infection. When experimentally infected with S. agalactiae, the Fno-infected fish died more rapidly and at a significantly higher rate than fish without the infection. During the challenge, the hemato-immunological parameters of both groups of fish were very similar, although the Fno-infected fish, challanged with S. agalactiae expressed significantly higher plasma lysozyme and peroxidase activities, and their head kidney macrophages had significantly higher respiratory burst activity compared to non-Fno-infected fish challanged with S. agalactiae. The only two parameters for which Fno-infected fish showed significantly lower expressions than that of their non-infected counterparts were haematocrit and total red blood cell count. The cause of the rapidity and higher rates of mortality observed in the Fno-infected fish when challenged with S. agalactiae is unknown; but it may be due to a reduced erythropoiesis capability within the head-kidney because of the presence of Fno.

Dietary Supplementation With ω6 LC-PUFA-Rich Algae Modulates Zebrafish Immune Function and Improves Resistance to Streptococcal Infection

Arachidonic acid (ARA, 20:4n-6) and dihomo-γ-linolenic acid (DGLA, 20:3n-6) are omega-6 long-chain polyunsaturated fatty acids (LC-PUFA), which are key precursors for lipid mediators of the immune system and inflammatory response. The microalga Lobosphaera incisa (WT) and its Δ5-desaturase mutant P127 (MUT) are unique photosynthetic sources for ARA and DGLA, respectively. This study explores the effect of dietary supplementation with L. incisa and P127 biomass on tissue fatty acid composition, immune function, and disease resistance in zebrafish (Danio rerio). The broken microalgal biomass was added to commercial fish feed at 7.5 and 15% (w/w), providing 21.8 mg/g feed ARA for the WT-supplemented group and 13.6 mg/g feed DGLA for the MUT-supplemented group at the 15% inclusion levels. An unsupplemented group was used as the control. After 1 month of feeding, fish were challenged with Streptococcus iniae. Fish were sampled before the challenge and 1 week after the challenge for various analyses. Tissue ARA and DGLA levels significantly increased in the liver, corresponding to microalgal supplementation levels. The elevated expression of specific immune-related genes was evident in the kidneys in all treatment groups after 1 month of feeding, including genes related to eicosanoid synthesis, lysozyme, and NF-κB. In the liver, microalgal supplementation led to the upregulation of genes related to immune function and antioxidant defense while the expression of examined genes involved in ARA metabolism was downregulated. Importantly, fish fed with 15% of both WT- and MUT-supplemented feed showed significantly (p < 0.05) higher survival percentages (78 and 68%, respectively, as compared to only 46% in the control group). The elevated expression of genes related to inflammatory and immune responses was evident post-challenge. Collectively, the results of the current study demonstrate the potential of microalgae-derived dietary ARA and DGLA in improving immune competence and resistance to bacterial infection in zebrafish as a model organism.

Histological development of the long-snouted seahorse Hippocampus guttulatus during ontogeny

The objective of the present study was to describe histological development of the European long-snouted seahorse Hippocampus guttulatus, to increase understanding of the biology and physiology of the species. Most vital organs were present in juveniles by the time of their release from the male's pouch. Digestive tract specialization occurred at 89 effective day-degrees (D°_eff ), corresponding to 15 days post partum (dpp), with development of the first intestinal loop and mucosal folding. At 118 D°_eff (20 dpp), lipids were being mobilized from the liver and oocytes attained the perinuclear stage. The fovea emerged at 177 D°_eff (30 dpp), contemporaneous with the shift from pelagic to benthic behaviour in juveniles. At this stage, the most interesting feature was the formation of the second intestinal loop. Male gonads were never observed during the study (from 0 to 354 D°_eff ; 0-60 dpp), but the first oogonia were present at 30 D°_eff (5 dpp). In 354 D°_eff (60 dpp) juveniles, oocytes were observed in a cortical alveoli stage, indicating maturity. Low digestive efficiency was observed at early stages, which was due to a poorly developed gastrointestinal tract and an immature digestive tract prior to 89 D°_eff . The present study demonstrates that approximately 89 and 177 D°_eff represent two important transitional stages in the early development of H. guttulatus. At a temperature of approximately 19 ± 1°C and an age of 1 month (177 D°_eff ), main organs were fully functional, suggesting that the adult phenotype was largely established by that age, with females becoming mature at the age of 2 months (354 D°_eff ).

Physiological response and microRNA expression profiles in head kidney of genetically improved farmed tilapia (GIFT, Oreochromis niloticus) exposed to acute cold stress

Cold stress has a serious impact on the overwintering survival and yield of genetically improved farmed tilapia (GIFT, Oreochromis niloticus). Understanding the physiological and molecular regulation mechanisms of low-temperature adaptation is necessary to help breed new tolerant strains. The semi-lethal low temperature of juvenile GIFT at 96 h was determined as 9.4 °C. We constructed and sequenced two small RNA libraries from head kidney tissues, one for the control (CO) group and one for the 9.4 °C-stressed (LTS) group, and identified 1736 and 1481 known microRNAs (miRNAs), and 164 and 152 novel miRNAs in the CO and LTS libraries, respectively. We verify the expression of nine up-regulated miRNAs and eight down-regulation miRNAs by qRT-PCR, and found their expression patterns were consistent with the sequencing results. We found that cold stress may have produced dysregulation of free radical and lipid metabolism, decreased superoxide dismutase activity, reduced respiratory burst and phagocytic activity of macrophages, increased malondialdehyde content, and adversely affected the physiological adaptation of GIFT, eventually leading to death. This study revealed interactions among miRNAs and signal regulated pathways in GIFT under cold stress that may help to understand the pathways involved in cold resistance.

De Novo Transcriptomic Approach to Study Thyroid Hormone Receptor Action in Non-mammalian Models

Thyroid hormones are pleiotropic hormones involved in chordates physiology. Understanding their functions and mechanisms is also instrumental to diagnose dys-regulations and get a predictive power that can applied to medicine, ecology, etc. Today, high-throughput sequencing technologies offer the opportunity to address this issue not only in model organisms but also in non-model organisms. Here, we describe a method that makes use of RNA-seq to address differential expression analysis in non-model organism.

Zymosan-induced immune challenge modifies the stress response of hypoxic air-breathing fish (Anabas testudineus Bloch): Evidence for reversed patterns of cortisol and thyroid hormone interaction, differential ion transporter functions and non-specific immune response

Fishes have evolved physiological mechanisms to exhibit stress response, where hormonal signals interact with an array of ion transporters and regulate homeostasis. As major ion transport regulators in fish, cortisol and thyroid hormones have been shown to interact and fine-tune the stress response. Likewise, in fishes many interactions have been identified between stress and immune components, but the physiological basis of such interaction has not yet delineated particularly in air-breathing fish. We, therefore, investigated the responses of thyroid hormones and cortisol, ion transporter functions and non-specific immune response of an obligate air-breathing fish Anabas testudineus Bloch to zymosan treatment or hypoxia stress or both, to understand how immune challenge modifies the pattern of stress response in this fish. Induction of experimental peritonitis in these fish by zymosan treatment (200ngg^-1) for 24h produced rise in respiratory burst and lysozomal activities in head kidney phagocytes. In contrast, hypoxia stress for 30min in immune-challenged fish reversed these non-specific responses of head kidney phagocytes. The decline in plasma cortisol in zymosan-treated fish and its further suppression by hypoxia stress indicate that immune challenge suppresses the cortisol-driven stress response of this fish. Likewise, the decline in plasma T₃ and T₄ after zymosan-treatment and the rise in plasma T₄ after hypoxia stress in immune-challenged fish indicate a critical role for thyroid hormone in immune-stress response due to its differential sensitivity to both immune and stress challenges. Further, analysis of the activity pattern of ion-dependent ATPases viz. Na⁺/K⁺-ATPase, H⁺/K⁺-ATPase and Na⁺/NH₄⁺-ATPase indicates a functional interaction of ion transport system with the immune response as evident in its differential and spatial modifications after hypoxia stress in immune-challenged fish. The immune-challenge that produced differential pattern of mRNA expression of Na⁺/K⁺-ATPase α-subunit isoforms; nkaα1a, nkaα1b and nkaα1c and the shift in nkaα1a and nkaα1b isoforms expression after hypoxia stress in immune-challenged fish, presents transcriptomic evidence for a modified Na⁺/K⁺ ion transporter system in these fish. Collectively, our data thus provide evidence for an interactive immune-stress response in an air-breathing fish, where the patterns of cortisol-thyroid hormone interaction, the ion transporter functions and the non-specific immune responses are reversed by hypoxia stress in immune-challenged fish.

Molecular cloning and expression analysis of toll-like receptor genes (TLR7, TLR8 and TLR9) of golden pompano (Trachinotus ovatus)

Toll like receptor (TLR) 7, 8 and 9 are intracellular TLRs which play important roles in host immune defense against bacterial or virus pathogens. In this study, TLR7, 8 and 9 were identified from golden pompano (Trachinotus ovatus), a marine teleost with great economic values. Sequence analysis revealed that the three TLRs contained several conserved characteristic features, including signal peptides, 25 leucine-rich repeat (LRR) motifs, a transmembrane domain and a TIR motif. These three TLRs shared high sequence identity and similarity with their counterparts from other teleosts. The phylogenetic tree analysis showed the three TLRs were clustered well with their piscine counterparts, confirming the correctness of their nomenclatures and closed relationships during evolution. Quantitative real-time PCR revealed that the three TLRs were ubiquitously expressed in all the tested tissues from normal pompano, with high expression in spleen and head kidney, indicating their role in immune reaction. Further, pompano TLR7 and TLR8 was up-regulated in spleen and head kidney from 12 h to 48 h following polyI:C challenge, but remained no changes to Vibrio alginilyticus infection. In contrast, pompano TLR9 could be induced by V. alginilyticus infection but remained apathetic to polyI:C challenge. These results indicated that pompano TLR7, 8 and 9 might have distinct roles in response to bacterial or virus pathogens. Our results provided the basis for further study on ligand specificity and signaling pathways of fish TLRs which are required for elucidating the immune functions of fish TLRs.