Temperature modulates the immunological response of the sub-antarctic notothenioid fish Eleginops maclovinus injected with Piscirickettsia salmonis

Host-bacteria interactions: ecological and evolutionary insights from ancient, professional endosymbionts

Interactions between eukaryotic hosts and their bacterial symbionts drive key ecological and evolutionary processes, from regulating ecosystems to the evolution of complex molecular machines and processes. Over time, endosymbionts generally evolve reduced genomes, and their relationship with their host tends to stabilize. However, host-bacteria relationships may be heavily influenced by environmental changes. Here, we review these effects on one of the most ancient and diverse endosymbiotic groups, formed by-among others-Legionellales, Francisellaceae, and Piscirickettsiaceae. This group is referred to as Deep-branching Intracellular Gammaproteobacteria (DIG), whose last common ancestor presumably emerged about 2 Ga ago. We show that DIGs are globally distributed, but generally at very low abundance, and are mainly identified in aquatic biomes. Most DIGs harbour a type IVB secretion system, critical for host-adaptation, but its structure and composition vary. Finally, we review the different types of microbial interactions that can occur in diverse environments, with direct or indirect effects on DIG populations. The increased use of omics technologies on environmental samples will allow a better understanding of host-bacterial interactions and help unravel the definition of DIGs as a group from an ecological, molecular, and evolutionary perspective.

Discovery of BbX transcription factor in the patagonian blennie: Exploring expression changes following combined bacterial and thermal stress exposure

High-Mobility Group (HMG) proteins are involved in different processes such as transcription, replication, DNA repair, and immune response. The role of HMG proteins in the immune response of fish has been studied mainly for HMGB1, where its expression can be induced by the stimulation of viral/bacterial PAMPs and can act as a proinflammatory mediator and as a global regulator of transcription in response to temperature. However, for BbX this role remains to be discovered. In this work, we identified the BbX of E. maclovinus and evaluated the temporal expression levels after simultaneous challenge with P. salmonis and thermal stress. Phylogenetic analysis does not significantly deviate from the expected organismal relationships suggesting orthologous relationships and that BbX was present in the common ancestor of the group. BbX mRNA expression levels were very high in the intestinal tissue of E. maclovinus (foregut, midgut, and hindgut). Nevertheless, the protein levels analyzed by WB showed the highest levels of BbX protein in the liver (constitutive expression). On the other hand, the mRNA expression levels of BbX in the liver of E. maclovinus injected with P. salmonis and subjected to thermal stress showed an increase at days 16 and 20 in all treatments applied at 12 °C and 18 °C. Meanwhile, the protein levels quantified by WB showed a statistically significant increase in the HMG-Bbx at all experimental times (4, 8, 12, 16, and 20 dpi). However, at 4 dpi the HMG-Bbx protein levels were much higher than the other days evaluated. The results suggest that BbX protein may be implicated in the response mechanism to temperature and bacterial stimulation in the foregut, midgut, hindgut, and liver, according to our findings at the level of mRNA and protein. Furthermore, our WB analysis suggests an effect of P. salmonis on the expression of this protein that can be observed in condition C+ 12 °C compared to C- 12 °C. Then, there is an effect of temperature that can be evidenced in the condition AM 18 °C and SM 18 °C, compared to AB 18 °C and SB 18 °C at 4, 8, and 12 dpi. We found not differences in the levels of this protein if the thermal stress is achieved through acclimatization or shock. More research is necessary to clarify the importance of this type of HMG in the immune response and thermal tolerance in fish.

Integrated metabolomic and transcriptomic responses to heat stress in a high-altitude fish, Triplophysa siluroides

Species in Triplophysa display strong adaptability to the extreme environment of the plateau, thus offering an ideal model to study the molecular mechanism of fish adaptation to environmental stress. In the present study, we conducted integrated analysis of the transcriptome and metabolism of liver tissue in Triplophysa siluroides under heat stress (28 °C) and control (10 °C) conditions to identify heat stress-induced genes, metabolites and pathways. RNA-Seq identified 2373 differentially expressed genes, which consisted of 1360 upregulated genes and 1013 downregulated genes, in the heat stress group vs. the control group. Genes in the heat shock protein (Hsp) family, including Hsp40, Hsp70, Hsp90 and other Hsps, were strongly upregulated by heat stress. Pathway enrichment analysis revealed that the PI3K/AKT/mTOR and protein processing in the endoplasmic reticulum (ER) pathways were significantly affected by heat stress. Metabolism sequencing identified a total of 155 differentially abundant metabolites, including 118 significantly upregulated metabolites and 37 downregulated metabolites. Combined analysis of the transcriptome and metabolism results showed that ubiquitin-dependent proteolysis and purine metabolism pathways were enhanced in response to acute heat stress to protect cells from damage under stress conditions. The results of this study may contribute to our understanding of the underlying molecular mechanism of the heat stress response in cold-water fish.

High water temperature-mediated immune gene expression of olive flounder, Paralichthys olivaceus according to pre-stimulation at high temperatures

Increased ocean temperature due to global warming affects the health and immunity of fish. In this study, juvenile Paralichthys olivaceus were exposed to high temperature after pre-heat (Acute: Acute heat shock at 32 °C, AH-S: Acquired heat shock at 28 °C & short recovery (2h) and heat shock at 32 °C, AH-L: acquired heat shock at 28 °C and long recovery (2 days), AH-LS: acquired heat shock at 28 °C & long (2 days) + short (2h) recovery). Heat shock after pre-heat significantly upregulated various immune-related genes, including interleukin 8 (IL-8), c-type lysozyme (c-lys), immunoglobulin M (IgM), Toll-like receptor 3 (tlr3), major histocompatibility complex IIα (mhcIIα) and cluster of differentiation 8α (cd8α) in the liver and brain of P. olivaceus. This study showed pre-exposure to high temperatures below the critical temperature can activate fish immunity and increase tolerance to high temperatures.

Effects of temperature on the innate immune response on Antarctic and sub-Antarctic fish Harpagifer antarcticus and Harpagifer bispinis challenged with two immunostimulants, LPS and Poly I:C: In vivo and in vitro approach

Rising ocean temperatures due to climate change combined with the intensification of anthropogenic activity can drive shifts in the geographic distribution of species, with the risks of introducing new diseases. In a changing environment, new host-pathogen interactions or changes to existing dynamics represent a major challenge for native species at high latitudes. Notothenioid fish constitute a unique study system since members of this group are found inside and outside Antarctica, are highly adapted to cold and particularly sensitive to temperature increments. However, data about their immune response remains scarce. Here, we aimed to evaluate the innate immune response under thermal stress in two species of Notothenioid fish, Harpagifer antarcticus and Harpagifer bispinis. Adult individuals from both species were collected on King George Island (Antarctica), and Punta Arenas (Chile), respectively. Specimens were assigned to a control group or injected with one of two agents (LPS and Poly I:C) to simulate either a bacterial or viral infection, and subjected to three different temperatures 2, 5 and 8 °C for 1 week. In parallel, we established leukocytes primary cell cultures from head kidney, which were also subjected to the immunostimulants at the same three temperatures, and incubated for 0.5, 1, 3, 6, 12, 24, and 48 h. We evaluated the relative gene expression of genes involved in the innate immune response (TLR1, TLR3, NF-kB, MYD88, IFNGR e IL-8) through real time qPCR. We found differences between species mainly in vivo, where H. antarcticus exhibited upregulation at high temperatures and H. bispinis seemed to have reached their physiological minimum at 2 °C. Although temperature had a strong effect during the in vivo assay for both species, it was negligible for primary cell cultures, which responded primarily to condition and time. Moreover, while leukocytes responded with fluctuations across time points, in vivo both species manifested strong and clear patterns of gene expression. These results highlight the importance of evaluating the effect of multiple stressors and set a precedent for future research.

Transcriptional modulation of immune genes in gut of Sub-Antarctic notothenioid fish Eleginops maclovinus challenged with Francisella noatunensis subsp. noatunensis

The search for functional foods that improve the immune response has traditionally been focused on lymphoid tissue and the intestinal mucosa. However, it is unknown whether there is a different immune response in different portions of the gut following exposure to a bacterial pathogen. We challenged Eleginops maclovinus intraperitoneally (i.p) with Francisella noatunensis subsp. noatunensis and measured mRNA transcripts related to innate and adaptive immune responses in different parts of the gut (foregut, midgut and hindgut). We used control (i.p only with bacterial culture medium), low dose (i.p of F. noatunensis at 1 × 10¹ bact/μL), medium dose (i.p of F. noatunensis at 1 × 10⁵ bact/μL) and high dose (i.p of F. noatunensis at 1 × 10¹⁰ bact/μL) groups in our experiments. We sampled fish at days 1, 3, 7, 14, 21, and 28 post-injection. We observed tissue-specific expression of TLR1, TLR5, TLR8, MHCI, MHCII and IgM, and transcription of these immune markers was lower in foregut and higher in midgut and hindgut. We detected Francisella genetic material (DNA) in fish stimulated with a high dose from day 1-28 in foregut, midgut, and hindgut. However, we could only detect Francisella DNA in fish stimulated the medium and low dose at later timepoints in the foregut (21-28 days post injection "dpi") and hindgut (low dose from day 7-28 dpi). Our results suggest that the immune responses to bacterial pathogens occur throughout the gut, but certain segments may be more susceptible to infection because of their cellular morphology (anterior, middle and posterior).

Temperature increase and its effects on fish stress physiology in the context of global warming

The capacity of fishes to cope with environmental variation is considered to be a main determinant of their fitness and is partly determined by their stress physiology. By 2100, global ocean temperature is expected to rise by 1-4°C, with potential consequences for stress physiology. Global warming is affecting animal populations worldwide through chronic temperature increases and an increase in the frequency of extreme heatwave events. As ectotherms, fishes are expected to be particularly vulnerable to global warming. Although little information is available about the effects of global warming on stress physiology in nature, multiple studies describe the consequences of temperature increases on stress physiology in controlled laboratory conditions, providing insight into what can be expected in the wild. Chronic temperature increase constitutes a physiological load that can alter the ability of fishes to cope with additional stressors, which might compromise their fitness. In addition, rapid temperature increases are known to induce acute stress responses in fishes and might be of ecological relevance in particular situations. This review summarizes knowledge about effects of temperature increases on the stress physiology of fishes and discusses these in the context of global warming.

Growth hormone (GH) and growth hormone release factor (GRF) modulate the immune response in the SHK-1 cell line and leukocyte cultures of head kidney in Atlantic salmon

To clarify the role of growth hormone (GH) in the immune system of fish, we examine the comparative effect of GH and Growth Hormone Release Factor (GRF) on leukocytes culture of the head kidney of Atlantic salmon and the SHK-1 cell line. There are studies that associate the growth hormone (GH) / insulin-like growth factor (IGF) axis with the immune regulation of fish. However, there is no evidence that GH and GRF stimulate Atlantic salmon leukocyte cell lines, where there areńt reports on expression changes in different immune response markers. Thus, we investigated the effect of GH and GRF in Atlantic salmon leukocytes extracted from head kidney and the SHK-1 cell line on the different immune response markers such as: NLRC5, NLRC3, IL-1β, TNF-α, and IL-8 through qPCR. Our data suggest that GH increases the expression of NLRC5, NLRC3, and IL-1β mainly at 16 h post-stimulation in Atlantic salmon leukocytes. This indicates differential regulation between the two models used, helping us to better understand the independent action of GH on the immune system and the GH / IGF axis for future research.

Hypoxia modulates the transcriptional immunological response in Oncorhynchus kisutch

Oncorhynchus kisutch is the third most cultivated salmonid species in the Chilean salmon industry and its farming conditions are characterised by high stocking density leading to the generation of high levels of organic matter (food - feces) and decomposition. In addition to the increasingly frequent hypoxic oceanographic events, these inappropriate farming conditions increase the demand for oxygen within the fish farm pen and lead to the appearance of hypoxic events that are harmful to fish.This study aimed to evaluate the stress response (cortisol) and transcription of genes involved in the immune response in head kidney and spleen of Oncorhynchus kisutch subjected to chronic hypoxic stress conditions. The fish were exposed to 100%, 60%, 50%, 35%, and 25% of DO for 28 days, then the blood (plasma), head kidney and spleen were removed. We observed mortality in the 25% DO group at days 15 and 20. Plasma cortisol increased significantly under 35% and 25% DO conditions compared to control. Transcription of Toll-like receptors (TLR1, TLR5M, TLR8, and TLR9) and cytokines (IL-1β, IL6, IL10, TNF-α) increased in the head kidney only in the 50% DO group, while in spleen there was an increase of these markers in the conditions of 60%, 35%, and 25% DO. As for the markers involved in cell-mediated immunity, CD4-MHCII and CD8-MHCI do not have a clear expression pattern, although there was down-regulation in MHCII transcription in the head kidney, in all the hypoxia conditions evaluated. Finally, IgM transcription was increased in the spleen in all hypoxia conditions, although it wasn't always statistically significant compared to the control. These results indicate that chronic hypoxia induces the stress response, increasing plasma cortisol levels and modulating the transcription of genes involved in the innate and adaptive immune response. The expression patterns were tissue-specific, indicating that the degree of hypoxia differentially affects the transcription of genes involved in the immune response of Oncorhynchus kisutch.

Damselfish face climate change: Impact of temperature and habitat structure on agonistic behavior

Oceans absorb a huge part of the atmospheric heat, leading to the rise in water temperature. Reefs are among the most affected ecosystems, where the complex behavioral repertoire of fishes is usually an indicator of environmental impacts. Here, we examined whether temperature (28 and 34°C) and habitat complexity (high and low) interact to affect the agonistic behavior (mirror test) of the dusky damselfish (Stegastes fuscus), a key species in Brazilian reefs because of its gardening capacity and territorial behavior. Higher temperatures altered basal behavior in both high and low-complexity conditions. Fish kept at 28°C under the high-complexity condition were more aggressive than those at a higher temperature (34°C) and in a low-complexity condition, which also exhibited lower dispersion. Our data show that changes in behavior of coral reef fish is associated to fluctuations in environmental conditions. Thus, it is important to implement management or conservation strategies that could mitigate global change effects.

Intermediary metabolic response and gene transcription modulation on the Sub-Antarctic notothenioid Eleginops maclovinus (Valenciennes, 1930) injected with two strains of Piscirickettsia salmonis

Pathogen interactions with cultured fish populations are well studied, but their effects on native fishes have not been characterized. In Chile, the disease caused by bacterial species Piscirickettsia salmonis represents one of the main issues and is considered to be one of the important pathogens in the field of aquaculture. They have been found to infect native fish. Therefore, it is necessary to understand the impact of P. salmonis on native species of local commercial value, as well as the potential impact associated with the emergence of antibiotic-resistant strains of P. salmonis. Due to this purpose, the native fish Eleginops maclovinus was used in our study. Fish were randomly distributed in tanks and intraperitoneally inoculated with two strains of P. salmonis. No mortality was recorded during the experiment. Cortisol, glucose and total α-amino acid levels increased in fish injected with AUSTRAL-005 strain compared to sham-injected and LF-89-inoculated fish. Moreover, results showed an increase in the activity of carbohydrates and lipids metabolism in liver; and an increase in the carbohydrates, lipids and total α-amino acid metabolism in muscle after injection with AUSTRAL-005. Our results suggest that P. salmonis modulates the physiology of E. maclovinus and the physiological impact increase in the presence of the antibiotic-resistant strain AUSTRAL-005.

Cellular stress responses of Eleginops maclovinus fish injected with Piscirickettsia salmonis and submitted to thermal stress

Fluctuations in ambient temperature along with the presence of pathogenic microorganisms can induce important cellular changes that alter the homeostasis of ectothermic fish. The aim of this study was to evaluate how sudden or gradual changes in environmental temperature together with the administration of Piscirickettsia salmonis modulate the transcription of genes involved in cellular stress response in the liver of Eleginops maclovinus. Fish were subjected to the following experimental conditions in duplicate: C- 12 °C: Injection only with culture medium, C+ 12 °C: Injection with P. salmonis, AM 18 °C: Injection only with culture medium under acclimation at 18 °C, AB 18 °C: Injection with P. salmonis under acclimation at 18 °C, SM 18 °C: Injection only with culture medium and thermal shock at 18 °C and SB 18 °C: Injection with P. salmonis and thermal shock at 18 °C and sampling at 4-, 8-, 12-, 16- and 20-day post injection (dpi). The genes implied in the heat shock response (HSP70, HSC70, HSP90, and GRP78), apoptosis pathway (BAX and SMAC/Diablo), ubiquitination (E2, E3, ubiquitin, and CHIP), and 26 proteasome complex (PSMB7, PSMC1, and PSMA2) showed expression profiles dependent on time and type of injection applied. All the genes greatly increased their expression levels at day 16 and showed moderate increases at day 20, except for PSMA2 which showed a higher increase between 4- and 12-day post challenges. Our results suggest that the changes observed at the final days of the experiment are due to temperature more than P. salmonis.

Brain and Pituitary Response to Vaccination in Gilthead Seabream (Sparus aurata L.)

Vaccination is a widely used therapeutical strategy in aquaculture, but whether vaccination elicits stress responses in the central neuroendocrine system and enhances the crosstalk between the immune and endocrine systems in the brain or pituitary after vaccination is unclear. To answer this question two experiments using two different vaccine exposure routes, i.e., bath or intraperitoneal (i.p.) injection, were carried out on gilthead seabream (Sparus aurata L.). In the first one, the stress responses of fish subjected to waterborne Vibrio anguillarum bacterin were compared with responses after air exposure or their combination. In the second experiment, fish were subjected to an intraperitoneal injection of Lactococcus garvieae bacterin and we assessed the central stress response and also whether or not a significant immune response was induced in brain and pituitary. In both experiments, blood, brain and pituitary tissues were collected at 1, 6, and 24 h post stress for plasma hormone determination and gene expression analysis, respectively. Results indicated that bath vaccination induced a decreased central stress response compared to air exposure which stimulated both brain and pituitary stress genes. In the second experiment, injection vaccination kept unchanged plasma stress hormones except cortisol that raised at 6 and 24 h. In agreement, non-significant or slight changes on the transcription of stress-related genes were recorded, including the hormone genes of the hypothalamic pituitary interrenal (HPI) axis and other stress markers such as hsp70, hsp90, and mt genes in either brain or pituitary. Significant changes were observed, however, in crhbp and gr. In this second experiment the immune genes il1β, cox2, and lys, showed a strong expression in both brain and pituitary after vaccination, notably il1β which showed more than 10 fold raise. Overall, vaccination procedures, although showing a cortisol response, did not induce other major stress response in brain or pituitary, regardless the administration route. Other than main changes, the alteration of crhbp and gr suggests that these genes could play a relevant role in the feedback regulation of HPI axis after vaccination. In addition, from the results obtained in this work, it is also demonstrated that the immune system maintains a high activity in both brain and pituitary after vaccine injection.