Transcriptional analysis of renal dopamine-mediated Na+ homeostasis response to environmental salinity stress in Scatophagus argus

Salinity-induced virulence alteration of Aeromonas hydrophila isolated from Scatophagus argus: insights from transcriptomic profiling and phenotypic characterization

BACKGROUND

The emerging foodborne pathogen, Aeromonas hydrophila, co-infects humans and animals, especially fish, threatening aquacultural production and public health. Previously, we found that Scatophagus argus, a widely cultivated fish species with high economic value, exhibited enhanced growth but increased susceptibility to A. hydrophila infection under freshwater conditions compared to seawater conditions. However, the exact mechanisms involved remain unclear.

RESULTS

Our study demonstrated that the enhanced virulence of A. hydrophila 201416, isolated from S. argus, in response to increasing salinity was associated with altered quorum sensing-related gene expression and regulated behaviors. Results from virulence assays incorporating phenotypic characterization indicated that elevated salinity levels (from 0 to 35‰) significantly hindered Ah201416 infection of S. argus. This trend correlated with increased biofilm mass and swimming motility, yet was inversely related to bacterial growth. RNA-sequencing and quantitative reverse transcriptional PCR analysis confirmed significant upregulation of genes related to flagellar assembly (flgB, flgH, flgC, flgI, flhA, and fliA), bacterial secretion (HlyD and Ahh1), and quorum sensing (AhyR, LuxO, and LuxE) of Ah201416 in response to elevated salinity. These findings suggested that increased salinity not only enhanced the virulence of Ah201416 but also bolstered the resistance of S. argus, thereby mitigating its susceptibility.

CONCLUSIONS

This study provides deeper insights into the microbial risks associated with A. hydrophila in aquacultural production, which is critical to developing effective prevention and control strategies and ensuring a safe seafood supply.

CLINICAL TRIAL NUMBER

Not applicable.

Gut microbiota dysbiosis triggered by salinity stress enhances systemic inflammation in spotted scat (Scatophagus argus)

As an ecological disturbance, salinity changes substantially impact aquatic organism health. Gut microbiota plays a pivotal role in host health and exhibits heightened sensitivity to environmental salinity stress; however, the potential correlative mechanisms between gut microbiota dysbiosis triggered by salinity changes and host health remain unclear. The present study conducted a 4-week stress experiment to investigate the precise impact of gut microbiota on the inflammatory response in Scatophagus argus under different salinities (0 ‰ [hyposaline group, HO], 25 ‰ [control group, CT], and 40 ‰ [hypersaline group, HE]). Our results revealed that both HO and HE stress significantly changed the relative abundances of Gram-negative bacteria and the impairment of intestinal barrier function. Subsequently, the levels of lipopolysaccharide (LPS) in the serum exhibited a significant increase, and the expression levels of genes (tlrs, myd88, irak1, irak4, and traf6) involving TLRs/MyD88/NF-κB signaling pathway and pro-inflammatory cytokines (il-6, il-8, il-1β, and tnf-α) in the representative immune organs were significantly upregulated. Conversely, the abundance of the anti-inflammatory gene (tgf-β1) and its protein contents in serum were decreased. Transplantation of the gut microbiota from S. argus exposed to varying salinities into germ-free Oryzias latipes resulted in an enhanced inflammatory response. Our results suggested that both HO and HE stress increased the presence of Gram-negative bacteria and disrupted the intestinal barrier, leading to elevated serum LPS and subsequent systemic inflammation in fish. These findings provide innovative insights into the influence of salinity manipulation strategies on the health of aquatic organisms, contributing to the mariculture management in coastal areas.

Genome assembly, gene content, and plastic gene expression responses to salinity changes in the Brackishwater Clam (Corbicula japonica) from a dynamic estuarine environment

Estuaries are dynamic transition zones between marine and freshwater environments, where salinity varies greatly on spatial and temporal scales. The temporal salinity fluctuations of these habitats require organisms to rapidly regulate ionic concentrations and osmotic pressure to survive in these dynamic conditions. Understanding the extent of plasticity of euryhaline animals is vital for predicting their responses and resilience to salinity change. We generated the first high-resolution genome and transcriptome sequences of C. japonica. In comparison with 11 other molluscan genomes, the C. japonica genome displayed striking expansions of putative neuron-related genes and gene families. The involvement of these genes in the glutamate/GABA-glutamine and glycine cycle suggests a possible contribution to the excitation of neuronal networks, particularly under high salinity conditions. This study contributes to our understanding of mechanisms underlying the rapid responses of estuarine species to changing conditions and raises many intriguing hypotheses and questions for future investigation.

Roles and occurrences of microbiota in the osmoregulatory organs, gills and gut, in marine medaka upon hypotonic stress

Gills and gut are the two primary osmoregulatory organs in fish. Recently, studies have expanded beyond the osmoregulatory mechanisms of these organs to explore the microbiota communities inhabiting them. It is now known that microbial communities in both organs shift in response to osmotic stress. However, there are limited studies identifying the major contributors and co-occurrence among these microbiota in both organs under seawater and freshwater transfer conditions. The current data mining report performed a bioinformatics analysis on two previous published datasets from our group, aiming to provide insights into host-bacteria relationships under osmotic stress. We divided the samples into four groups: control seawater gills (LSW); control seawater gut (TSW); freshwater transfer gills (LFW); and freshwater transfer gut (TFW). Our results showed that LSW had higher diversities, richness, and evenness compared to TSW. However, both the LFW and LSW did not show any significant differences after the freshwater transfer experiment. We further applied co-occurrence network analysis and, for the first time, reported on the interactions of taxa shaping the community structure in these two organs. Moreover, we identified enriched ectoine biosynthesis in seawater samples, suggesting its potential role in seawater environments. Increased mRNA expression levels of Na+/K+-atpase, and cftr, were observed in gills after 6 h of ectoine treatment. These findings provide a foundation for future studies on host-bacteria interactions under osmotic stress.

Identification of mapk genes, and their expression profiles in response to low salinity stress, in cobia (Rachycentron canadum)

Mitogen-activated protein kinases (MAPKs) are a class of protein kinases that regulate various physiological processes, and play a crucial role in maintaining the osmotic equilibrium of fish. The objective of this study was to identify and characterize the mapk family genes in cobia (Rachycentron canadum) and examine their expression profiles under different low salinity stress regimes (acute: from 30‰ to 10‰ in 1 h, sub-chronic: from 30‰ to 10‰ over 4 d). A total of 12 cobia mapk genes (Rcmapks) were identified and cloned, including six erk subfamily genes (Rcmapk1/3/4/6/7/15), three jnk subfamily genes (Rcmapk8/9/10) and three p38 mapk subfamily genes (Rcmapk 11/13/14). Domain analysis indicated that the RcMAPKs possessed the typical domains including S_TKc and PKc_like domain. Phylogenetic analysis revealed that the Rcmapks were most closely related to those of the turbot (Scophthalmus maximus). The tissue distribution of mapk genes in adult cobia and the expression patterns of Rcmapks under different low salinity stress regimes were investigated using quantitative real-time PCR (qRT-PCR). The results revealed that Rcmapk3/9/10/11/13/14 exhibited a relatively broad expression distribution across 14 different tissues. For all these genes the highest expression level was in the brain, except for Rcmapk14 (highly expressed in the stomach, gill, and skin). The genes Rcmapk1/6/15 showed significantly higher expression in the testis. Under acute low salinity stress, expression of Rcmapk1/3/6/7/9/11/13/14 was significantly altered in the gill, intestine, and trunk kidney, however, the aforementioned genes exhibited very different expression patterns among the three tissues. In the gill, most of the genes from the erk (Rcmapk3/6/7) and p38 mapk subfamily (Rcmapk11/13/14) were significantly up-regulated at almost all the time points (P < 0.05); Similarly, the expression of Rcmapk3/9/11/13/14 genes were significantly increased in the trunk kidney; while in the intestine, most of the altered genes (Rcmapk6/7/9/11/13/14) were significantly down-regulated at 1 h. Following the sub-chronic low salinity stress, expression of Rcmapk1/3/6/7/9/11/13/14 genes were significantly altered in all three tissues. These findings provide important reference data for elucidating the roles of cobia mapk family genes in response to low salinity stress.

Reproductive endocrine disruption and gonadal intersex induction in male Japanese medaka chronically exposed to betamethasone at environmentally relevant levels

The broad utilization of betamethasone in medical treatments may pose a significant ecotoxicological risk to aquatic organisms, yet its potential reproductive toxicity remains unclear. The present study examined the impacts of environmental exposure on male reproduction using Japanese medaka (Oryzias latipes). After 110 days of betamethasone exposure at environmentally relevant concentrations (0, 20 and 200 ng/L), LH/FSH synthesis and release in the pituitary was inhibited, and the production of sex hormones and their signaling pathways in the gonads of male medaka were greatly influenced. This synthetic glucocorticoid restrained testosterone (T) synthesis and gave rise to a significant increase in E₂/T and E₂/11-KT ratios. Furthermore, chronic betamethasone exposure (20 and 200 ng/L) led to the suppression of androgen receptor (AR) signaling and enhancement of estrogen receptors (ERs) signaling. An increase in hepatic vitellogenin contents was also detected, and testicular oocytes were observed in both 20 and 200 ng/L betamethasone-treated groups. It showed that 20 and 200 ng/L betamethasone could induce male feminization and even intersex, triggering abnormal spermatogenesis in medaka males. With its adverse effects on male fertility, betamethasone could potentially influence the fishery productivity and population dynamics in aquatic ecosystems.

Osmoregulatory strategies of estuarine fish Scatophagus argus in response to environmental salinity changes

BACKGROUND

Scatophagus argus, an estuarine inhabitant, can rapidly adapt to different salinity environments. However, the knowledge of the molecular mechanisms underlying its strong salinity tolerance remains unclear. The gill, as the main osmoregulatory organ, plays a vital role in the salinity adaptation of the fish, and thus relative studies are constructive to reveal unique osmoregulatory mechanisms in S. argus.

RESULTS

In the present study, iTRAQ coupled with nanoLC-MS/MS techniques were employed to explore branchial osmoregulatory mechanisms in S. argus acclimated to different salinities. Among 1,604 identified proteins, 796 differentially expressed proteins (DEPs) were detected. To further assess osmoregulatory strategies in the gills under different salinities, DEPs related to osmoregulatory (22), non-directional (18), hypo- (52), and hypersaline (40) stress responses were selected. Functional annotation analysis of these selected DEPs indicated that the cellular ion regulation (e.g. Na⁺-K⁺-ATPase [NKA] and Na⁺-K⁺-2Cl^- cotransporter 1 [NKCC1]) and ATP synthesis were deeply involved in the osmoregulatory process. As an osmoregulatory protein, NKCC1 expression was inhibited under hyposaline stress but showed the opposite trend in hypersaline conditions. The expression levels of NKA α1 and β1 were only increased under hypersaline challenge. However, hyposaline treatments could enhance branchial NKA activity, which was inhibited under hypersaline environments, and correspondingly, reduced ATP content was observed in gill tissues exposed to hyposaline conditions, while its contents were increased in hypersaline groups. In vitro experiments indicated that Na⁺, K⁺, and Cl^- ions were pumped out of branchial cells under hypoosmotic stress, whereas they were absorbed into cells under hyperosmotic conditions. Based on our results, we speculated that NKCC1-mediated Na⁺ influx was inhibited, and proper Na⁺ efflux was maintained by improving NKA activity under hyposaline stress, promoting the rapid adaptation of branchial cells to the hyposaline condition. Meanwhile, branchial cells prevented excessive loss of ions by increasing NKA internalization and reducing ATP synthesis. In contrast, excess ions in cells exposed to the hyperosmotic medium were excreted with sufficient energy supply, and reduced NKA activity and enhanced NKCC1-mediated Na⁺ influx were considered a compensatory regulation.

CONCLUSIONS

S. argus exhibited divergent osmoregulatory strategies in the gills when encountering hypoosmotic and hyperosmotic stresses, facilitating effective adaptabilities to a wide range of environmental salinity fluctuation.

Effects of environmental salinity on the immune response of the coastal fish Scatophagus argus during bacterial infection

The coastal aquaculture is characterized with environmental salinity fluctuation, and the effects of salinity stress on the immunity of cultured fish are needed to be further explored. Scatophagus argus is an important species in the wild fisheries and aquaculture industry, it would be of great value to reveal the impact of salinity change on the immune response in this species. Understanding the effects of salinity stress on immune response can provide valuable insights into salinity management in the aquacultural process. The head kidney, which is an organ unique for teleost fish, functions not only as a central immune organ but also as a crucial role in the stress response during which the secretion of immunoregulatory molecules i.e. cytokines is facilitated. In the present study, Individuals of S. argus acclimated to 3 different salinities [0‰ (FW), 10‰ (BW), and 25‰ (SW)] were injected intraperitoneally with A. hydrophila, and then monitored throughout one week. The effects of environmental salinity on the immune response in S. argus stimulated by A. hydrophila infection were investigated. mRNA expression profiles of cytokine genes IL-1β, IL-6, IL-10 and TNF-α in different salinity groups was quite different. mRNA expression of cytokine genes in BW group and SW group rose more quickly and significantly higher than FW group (p < 0.05) at early stages (6-24 hpi) after bacterial injection, and before 96 hpi, the highest value of cytokine expression at each time point was recorded in SW group. Immune parameters such as lysozyme level, complement C3 activity and IgM content in BW and FW groups were lower than SW group at each time point from 24 to 144 hpi after bacterial injection. In addition, leukocyte profiles in the head kidney and blood were also investigated. Although hypoosmotic acclimation could temporarily stimulate monocyte and neutrophil proliferation, it was observed that the number of monocytes, neutrophils and lymphocytes of the head kidney and blood in SW group increased more quickly than BW and FW groups after bacterial infection. Our results indicate that hypoosmotic stress due to the decrease of environmental salinity has suppressive immunoregulatory effects on the immune response of S. argus.

Gill Transcriptome Analysis Revealed the Difference in Gene Expression Between Freshwater and Seawater Acclimated Guppy (Poecilia reticulata)

Guppy (Poecilia reticulata) can adapt to a wide range of salinity changes. To investigate the gene expression changes in the guppy exposed to seawater, we characterized its gill transcriptome using RNA sequencing. Experimental fish were exposed to salinity increase from 0 to 30‰ within 4 days, while control fish were cultured in freshwater (0‰ salinity). Seven days after salinity exposure, the gills were sampled and the mortality within 2 weeks was recorded. No significant difference in the cumulative mortality at the second week was found between the two groups. Transcriptomic analysis identified 3477 differentially expressed genes (DEGs), including 1067 upregulated and 2410 downregulated genes. These DEGs were enriched in several biological processes, including ion transport, ion homeostasis, ATP biosynthetic process, metabolic process, and immune system process. Oxidative phosphorylation was the most activated pathway. DEGs involved in the pathway "endoplasmic reticulum (ER)-mediated phagocytosis," "starch and sucrose metabolism," and "steroid biosynthesis" were mainly downregulated; chemokines and interleukins involved in "cytokine-cytokine receptor interaction" were differentially expressed. The present results suggested that oxidative phosphorylation had essential roles in osmoregulation in the gills of seawater acclimated guppy, during which the decline in the expression of genes encoding V-ATPases and calreticulin had a negative effect on the phagocytosis and immune response. Besides, several metabolic processes including "starch and sucrose metabolism" and "steroid biosynthesis" were affected. This study elucidates transcriptomic changes in osmotic regulation, metabolism, and immunity in seawater acclimated guppy.

Modulation of inflammatory response by cortisol in the kidney of spotted scat (Scatophagus argus) in vitro under different osmotic stresses

Salinity changes on renal osmoregulation have often been investigated while the immune response of the kidney under osmotic stress is poorly understood in teleosts. Acute stress is generally associated with enhancement of circulating cortisol. The effects of osmotic stress on renal immune response and its regulation by cortisol deserve more attention. In the present study, the effects of exogenous cortisol treatment on the lipopolysaccharide (LPS)-induced immune response were analyzed in renal masses of Scatophagus argus under different osmotic stresses in vitro. mRNA expression of pro-inflammatory cytokines (TNF-α, IL1-β and IL-6) and immune-regulatory related genes (GR and SOCS1) was measured over a short course (15 h). Comprehensive analysis reveals that transcript abundances of pro-inflammatory cytokine genes such as TNF-α, IL-1β, and IL-6 induced by LPS, alone or in the combination of cortisol, are tightly associated with osmoregulation under acute osmotic stress. Our results showed that osmotic challenge could significantly enhance mRNA expression levels of pro-inflammatory cytokines in renal masses in vitro. Based on our analysis, it can be inferred that cortisol suppresses the magnitude of renal inflammatory response and attenuates LPS-induced immune response through GR signaling in the face of challenging environmental conditions.

Hypoosmotic stress induced tissue-specific immune responses of yellowfin seabream (Acanthopagrus latus) revealed by transcriptomic analysis

Salinity is a limiting factor for many marine organisms, including fishes. The shift in the ambient salinity can cause osmotic stress and arouse immune responses in fish. In this study, yellowfin seabream (Acanthopagrus latus), a euryhaline marine teleost, was used to investigate immune responses of different tissues (gill, liver, and muscle) under hypoosmotic stress. Comparative transcriptomic and physiological analyses of three tissues were conducted after fish exposed to the fresh water (FW, salinity = 0 ppt), low-saline water (LW, salinity = 3 ppt), and brackish water (BW, salinity = 6 ppt) for 8 days. The results showed that hypoosmotic stress dramatically altered the gene expression of three tissues in yellowfin seabream; The investigation of differentially expressed genes (DEGs) related to osmoregulation and immune response indicated that T cell-mediate immunity pathways were essential to tackle such stress. In terms of tissues, gill was found to be the most sensitive tissue under hypoosmotic stress by enhancing of Na⁺K⁺-ATPase activity and preventing the loss of Na⁺ and K⁺; Liver, on the other hand, was under the most sever oxidative stress indicated by the fluctuation of SOD, CAT activities and the MDA content; In contrast, muscle had the least osmoregulation and immune related response. We also identified several potential candidate genes, which may serve as gene indicators to identify the stressor. Overall, this study provides preliminary mechanistic insights into hypoosmotic stress adaption of aquatic organism.