Osmolality/salinity-responsive enhancers (OSREs) control induction of osmoprotective genes in euryhaline fish

Transcriptional upregulation of the myo-inositol biosynthesis pathway is enhanced by NFAT5 in hyperosmotically stressed tilapia cells

Euryhaline fish experience variable osmotic environments requiring physiological adjustments to tolerate elevated salinity. Mozambique tilapia (Oreochromis mossambicus) possess one of the highest salinity tolerance limits of any fish. In tilapia and other euryhaline fish species, the myo-inositol biosynthesis (MIB) pathway enzymes, myo-inositol phosphate synthase (MIPS) and inositol monophosphatase 1 (IMPA1.1), are among the most upregulated mRNAs and proteins indicating the high importance of this pathway for hyperosmotic (HO) stress tolerance. These abundance changes must be precluded by HO perception and signaling mechanism activation to regulate the expression of MIPS and IMPA1.1 genes. In previous work using a O. mossambicus cell line (OmB), a reoccurring osmosensitive enhancer element (OSRE1) in both MIPS and IMPA1.1 was shown to transcriptionally upregulate these enzymes in response to HO stress. The OSRE1 core consensus (5'-GGAAA-3') matches the core binding sequence of the predominant mammalian HO response transcription factor, nuclear factor of activated T-cells (NFAT5). HO-challenged OmB cells showed an increase in NFAT5 mRNA suggesting NFAT5 may contribute to MIB pathway regulation in euryhaline fish. Ectopic expression of wild-type NFAT5 induced an IMPA1.1 promoter-driven reporter by 5.1-fold (P < 0.01). Moreover, expression of dominant negative NFAT5 in HO media resulted in a 47% suppression of the reporter signal (P < 0.005). Furthermore, reductions of IMPA1.1 (37-49%) and MIPS (6-37%) mRNA abundance were observed in HO-challenged NFAT5 knockout cells relative to control cells. Collectively, these multiple lines of experimental evidence establish NFAT5 as a tilapia transcription factor contributing to HO-induced activation of the MIB pathway.NEW & NOTEWORTHY In our study, we use a multi-pronged synthetic biology approach to demonstrate that the fish homolog of the predominant mammalian osmotic stress transcription factor nuclear factor of activated T-cells (NFAT5) also contributes to the activation of hyperosmolality inducible genes in cells of extremely euryhaline fish. However, in addition to NFAT5 the presence of other strong osmotically inducible signaling mechanisms is required for full activation of osmoregulated tilapia genes.

Metagenomic insights into microbial adaptation to the salinity gradient of a typical short residence-time estuary

BACKGROUND

Microbial adaptation to salinity has been a classic inquiry in the field of microbiology. It has been demonstrated that microorganisms can endure salinity stress via either the "salt-in" strategy, involving inorganic ion uptake, or the "salt-out" strategy, relying on compatible solutes. While these insights are mostly based on laboratory-cultured isolates, exploring the adaptive mechanisms of microorganisms within natural salinity gradient is crucial for gaining a deeper understanding of microbial adaptation in the estuarine ecosystem.

RESULTS

Here, we conducted metagenomic analyses on filtered surface water samples collected from a typical subtropical short residence-time estuary and categorized them by salinity into low-, intermediate-, and high-salinity metagenomes. Our findings highlighted salinity-driven variations in microbial community composition and function, as revealed through taxonomic and Clusters of Orthologous Group (COG) functional annotations. Through metagenomic binning, 127 bacterial and archaeal metagenome-assembled genomes (MAGs) were reconstructed. These MAGs were categorized as stenohaline-specific to low-, intermediate-, or high-salinity-based on the average relative abundance in one salinity category significantly exceeding those in the other two categories by an order of magnitude. Those that did not meet this criterion were classified as euryhaline, indicating a broader range of salinity tolerance. Applying the Boruta algorithm, a machine learning-based feature selection method, we discerned important genomic features from the stenohaline bacterial MAGs. Of the total 12,162 COGs obtained, 40 were identified as important features, with the "inorganic ion transport and metabolism" COG category emerging as the most prominent. Furthermore, eight COGs were implicated in microbial osmoregulation, of which four were related to the "salt-in" strategy, three to the "salt-out" strategy, and one to the regulation of water channel activity. COG0168, annotated as the Trk-type K+ transporter related to the "salt-in" strategy, was ranked as the most important feature. The relative abundance of COG0168 was observed to increase with rising salinity across metagenomes, the stenohaline strains, and the dominant Actinobacteriota and Proteobacteria phyla.

CONCLUSIONS

We demonstrated that salinity exerts influences on both the taxonomic and functional profiles of the microbial communities inhabiting the estuarine ecosystem. Our findings shed light on diverse salinity adaptation strategies employed by the estuarine microbial communities, highlighting the crucial role of the "salt-in" strategy mediated by Trk-type K+ transporters for microorganisms thriving under osmotic stress in the short residence-time estuary. Video Abstract.

Study on the osmotic response and function of myo-inositol oxygenase in euryhaline fish nile tilapia (Oreochromis niloticus)

To understand the role of myo-inositol oxygenase (miox) in the osmotic regulation of Nile tilapia, its expression was analyzed in various tissues. The results showed that the expression of miox gene was highest in the kidney, followed by the liver, and was significantly upregulated in the kidney and liver under 1 h hyperosmotic stress. The relative luminescence efficiency of the miox gene transcription starting site (-4,617 to +312 bp) under hyperosmotic stress was measured. Two fragments (-1,640/-1,619 and -620/-599) could induce the luminescence activity. Moreover, the -1,640/-1,619 and -620/-599 responded to hyperosmotic stress and high-glucose stimulation by base mutation, suggesting that osmotic and carbohydrate response elements may exist in this region. Finally, the salinity tolerance of Nile tilapia was significantly reduced after the knocking down of miox gene. The accumulation of myo-inositol was affected, and the expression of enzymes in glucose metabolism was significantly reduced after the miox gene was knocked down. Furthermore, hyperosmotic stress can cause oxidative stress, and MIOX may help maintain the cell redox balance under hyperosmotic stress. In summary, MIOX is essential in osmotic regulation to enhance the salinity tolerance of Nile tilapia by affecting myo-inositol accumulation, glucose metabolism, and antioxidant performance.NEW & NOTEWORTHY Myo-inositol oxygenase (MIOX) is the rate-limiting enzyme that catalyzes the first step of MI metabolism and determines MI content in aquatic animals. To understand the role of miox in the osmotic regulation of Nile tilapia, we analyzed its expression in different tissues and its function under hyperosmotic stress. This study showed that miox is essential in osmotic regulation to enhance the salinity tolerance of Nile tilapia by affecting myo-inositol accumulation, glucose metabolism, and antioxidant performance.

Removal of evolutionarily conserved functional MYC domains in a tilapia cell line using a vector-based CRISPR/Cas9 system

MYC transcription factors have critical roles in facilitating a variety of cellular functions that have been highly conserved among species during evolution. However, despite circumstantial evidence for an involvement of MYC in animal osmoregulation, mechanistic links between MYC function and osmoregulation are missing. Mozambique tilapia (Oreochromis mossambicus) represents an excellent model system to study these links because it is highly euryhaline and highly tolerant to osmotic (salinity) stress at both the whole organism and cellular levels of biological organization. Here, we utilize an O. mossambicus brain cell line and an optimized vector-based CRISPR/Cas9 system to functionally disrupt MYC in the tilapia genome and to establish causal links between MYC and cell functions, including cellular osmoregulation. A cell isolation and dilution strategy yielded polyclonal myca (a gene encoding MYC) knockout (ko) cell pools with low genetic variability and high gene editing efficiencies (as high as 98.2%). Subsequent isolation and dilution of cells from these pools produced a myca ko cell line harboring a 1-bp deletion that caused a frameshift mutation. This frameshift functionally inactivated the transcriptional regulatory and DNA-binding domains predicted by bioinformatics and structural analyses. Both the polyclonal and monoclonal myca ko cell lines were viable, propagated well in standard medium, and differed from wild-type cells in morphology. As such, they represent a new tool for causally linking myca to cellular osmoregulation and other cell functions.

Osmosensitive transcription factors in the prolactin cell of a euryhaline teleost

In euryhaline fish, prolactin (Prl) plays a key role in freshwater acclimation. Prl release in the rostral pars distalis (RPD) of the pituitary is directly stimulated by a fall in extracellular osmolality. Recently, we identified several putative transcription factor modules (TFM) predicted to bind to the promoter regions of the two prl isoforms in Mozambique tilapia, Oreochromis mossambicus. We characterized the effects of extracellular osmolality on the activation of these TFMs from RPDs, in vivo and in vitro. OCT1_PIT1 01, CEBP_CEBP 01 and BRNF_RXRF 01 were significantly activated in freshwater (FW)- acclimated tilapia RPDs while SORY_PAX3 02 and SP1F_SP1F 06, SP1F_SP1F 09 were significantly activated in seawater (SW)- counterparts. Short-term incubation of SW- acclimated tilapia RPDs in hyposmotic media (280 mOsm/kg) resulted in activation of CAAT_AP1F 01, OCT1_CEBP 01, AP1F_SMAD 01, GATA_SP1F 01, SORY_PAX6 01 and CREB_EBOX 02, EBOX_AP2F 01, EBOX_MITF 01 while hyperosmotic media (420 mOsm/kg) activated SORY_PAX3 02 and AP1F_SMAD 01 in FW- tilapia. Short-term incubation of dispersed Prl cells from FW- acclimated fish exposed to hyperosmotic conditions decreased pou1f1, pou2f1b, stat3, stat1a and ap1b1 expression, while pou1f1, pou2f1b, and stat3 were inversely related to osmolality in their SW- counterparts. Further, in Prl cells of SW- tilapia, creb3l1 was suppressed in hyposmotic media. Collectively, our results indicate that multiple TFMs are involved in regulating prl transcription at different acclimation salinities and, together, they modulate responses of Prl cells to changes in extracellular osmolality. These responses reflect the complexity of osmosensitive molecular regulation of the osmoreceptive Prl cell of a euryhaline teleost.

Prediction and Experimental Validation of a New Salinity-Responsive Cis-Regulatory Element (CRE) in a Tilapia Cell Line

Transcriptional regulation is a major mechanism by which organisms integrate gene x environment interactions. It can be achieved by coordinated interplay between cis-regulatory elements (CREs) and transcription factors (TFs). Euryhaline tilapia (Oreochromis mossambicus) tolerate a wide range of salinity and thus are an appropriate model to examine transcriptional regulatory mechanisms during salinity stress in fish. Quantitative proteomics in combination with the transcription inhibitor actinomycin D revealed 19 proteins that are transcriptionally upregulated by hyperosmolality in tilapia brain (OmB) cells. We searched the extended proximal promoter up to intron1 of each corresponding gene for common motifs using motif discovery tools. The top-ranked motif identified (STREME1) represents a binding site for the Forkhead box TF L1 (FoxL1). STREME1 function during hyperosmolality was experimentally validated by choosing two of the 19 genes, chloride intracellular channel 2 (clic2) and uridine phosphorylase 1 (upp1), that are enriched in STREME1 in their extended promoters. Transcriptional induction of these genes during hyperosmolality requires STREME1, as evidenced by motif mutagenesis. We conclude that STREME1 represents a new functional CRE that contributes to gene x environment interactions during salinity stress in tilapia. Moreover, our results indicate that FoxL1 family TFs are contribute to hyperosmotic induction of genes in euryhaline fish.

Hypoosmotic stress induced functional alternations of intestinal barrier integrity, inflammatory reactions, and neurotransmission along gut-brain axis in the yellowfin seabream (Acanthopagrus latus)

The gut-brain axis plays a major role in multiple metabolic regulation processes, but studies regarding its responses to environmental stress in fish are still limited. In this study, we performed transcriptome sequencing analysis and enzyme-linked immunosorbent assay (ELISA) in yellowfin seabream (Acanthopagrus latus) exposed to environments with different water salinity (freshwater: 0 ppt; low-saline water: 3 ppt; brackish water: 6 ppt). According to transcriptome analysis, 707 and 1477 genes were identified as differentially expressed genes (DEGs) between freshwater and brackish water treatments in the brain and gut, respectively. Brain DEGs were significantly enriched into a set of Gene Ontology terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways associated with signal transduction, most of which were downregulated. Gut DEGs were enriched into a neurotransmission-relevant KEGG pathway tryptophan metabolism, and the downregulated DEGs were enriched into the KEGG pathway focal adhesion. ELISA demonstrated significant physiological responses of the brain and gut across treatments, as determined by the concentrations of tight junction protein ZO-2, interleukin 1β, and serotonin. Under hypoosmotic stress, the functions of the gut-brain axis are altered via impairment of intestinal barrier integrity, by disturbance of gut-brain neurotransmission, and through tissue-damaging inflammatory reactions. Our work identified candidate genes which showed significantly differential expression in the gut-brain axis when yellowfin seabream encountered hypoosmotic stress, which could shed lights on the understanding of the potential osmotic regulation mechanisms of the gut-brain axis in teleost.

RNA-seq analyses of Marine Medaka (Oryzias melastigma) reveals salinity responsive transcriptomes in the gills and livers

Increasing salinity levels in marine and estuarine ecosystems greatly influence developmental, physiological and molecular activities of inhabiting fauna. Marine medaka (Oryzias melastigma), a euryhaline research model, has extraordinary abilities to survive in a wide range of aquatic salinity. To elucidate how marine medaka copes with salinity differences, the responses of Oryzias melastigma after being transferred to different salt concentrations [0 practical salinity units (psu), 15 psu, 30 psu (control), 45 psu] were studied at developmental, histochemical and transcriptome levels in the gill and liver tissues. A greater number of gills differentially expressed genes (DEG) under 0 psu (609) than 15 psu (157) and 45 psu (312), indicating transcriptomic adjustments in gills were more sensitive to the extreme hypotonic environment. A greater number of livers DEGs were observed in 45 psu (1,664) than 0 psu (87) and L15 psu (512), suggesting that liver was more susceptible to hypertonic environment. Further functional analyses of DEGs showed that gills have a more immediate response, mainly in adjusting ion balance, immune and signal transduction. In contrast, DEGs in livers were involved in protein synthesis and processing. We also identified common DEGs in both gill and liver and found they were mostly involved in osmotic regulation of amino sugar and nucleotide sugar metabolism and steroid biosynthesis. Additionally, salinity stresses showed no significant effects on most developmental and histochemical parameters except increased heartbeat with increasing salinity and decreased glycogen after transferred from stable conditions (30 psu) to other salinity environments. These findings suggested that salinity-stress induced changes in gene expressions could reduce the effects on developmental and histochemical parameters. Overall, this study provides a useful resource for understanding the molecular mechanisms of fish responses to salinity stresses.

Transcriptome Profiling Reveals a Divergent Adaptive Response to Hyper- and Hypo-Salinity in the Yellow Drum, Nibea albiflora

Simple Summary

Global warming and certain climate disasters (typhoon, tsunami, etc.) can lead to fluctuation in seawater salinity that causes salinity stress in fish. The aim of this study was to investigate the functional genes and relevant pathways in response to salinity stress in the yellow drum. Genes and pathways related to signal transduction, osmoregulation, and metabolism may be involved in the adaptive regulation to salinity in the yellow drum. Additionally, the genes under salinity stress were mainly divided into three expression trends. Our results provided novel insights into further study of the salinity adaptability of euryhaline fishes.

Abstract

The yellow drum (Nibea albiflora) is an important marine economic fish that is widely distributed in the coastal waters of the Northwest Pacific. In order to understand the molecular regulatory mechanism of the yellow drum under salinity stress, in the present study, transcriptome analysis was performed under gradients with six salinities (10, 15, 20, 25, 30, and 35 psu). Compared to 25 psu, 907, 1109, 1309, 18, and 243 differentially expressed genes (DEGs) were obtained under 10, 15, 20, 30, and 35 psu salinities, respectively. The differential gene expression was further validated by quantitative real-time PCR (qPCR). The results of the tendency analysis showed that all DEGs of the yellow drum under salinity fluctuation were mainly divided into three expression trends. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that the PI3K-Akt signaling pathway, Jak-STAT signaling pathway as well as the glutathione metabolism and steroid biosynthesis pathways may be the key pathways for the salinity adaptive regulation mechanism of the yellow drum. G protein-coupled receptors (GPCRs), the solute carrier family (SLC), the transient receptor potential cation channel subfamily V member 6 (TRPV6), isocitrate dehydrogenase (IDH1), and fructose-bisphosphate aldolase C-B (ALDOCB) may be the key genes in the response of the yellow drum to salinity stress. This study explored the transcriptional patterns of the yellow drum under salinity stress and provided fundamental information for the study of salinity adaptability in this species.

Nonlinear effects of environmental salinity on the gill transcriptome versus proteome of Oreochromis niloticus modulate epithelial cell turnover

A data-independent acquisition (DIA) assay library for targeted quantitation of thousands of Oreochromis niloticus gill proteins using a label- and gel-free workflow was generated and used to compare protein and mRNA abundances. This approach generated complimentary rather than redundant data for 1899 unique genes in gills of tilapia acclimated to freshwater and brackish water. Functional enrichment analyses identified mitochondrial energy metabolism, serine protease and immunity-related functions, and cytoskeleton/ extracellular matrix organization as major processes controlled by salinity in O. niloticus gills. Non-linearity in salinity-dependent transcriptome versus proteome regulation was revealed for specific functional groups of genes. The relationship was more linear for other molecular functions/ cellular processes, suggesting that the salinity-dependent regulation of O. niloticus gill function relies on post-transcriptional mechanisms for some functions/ processes more than others. This integrative systems biology approach can be adopted for other tissues and organisms to study cellular dynamics for many changing ecological contexts.

Alleviation of the Adverse Effect of Dietary Carbohydrate by Supplementation of Myo-Inositol to the Diet of Nile Tilapia (Oreochromis niloticus)

This study investigated the effect of dietary myo-inositol (MI) on alleviating the adverse effect of the high carbohydrate diet in Nile tilapia (Oreochromis niloticus). Six diets contained either low carbohydrate (LC 30%) or high carbohydrate (HC 45%) with three levels of MI supplementation (0, 400 and 1200 mg/kg diet) to each level of the carbohydrate diet. After an 8-week trial, the fish fed 400 mg/kg MI under HC levels had the highest weight gain and fatness, but the fish fed 1200 mg/kg MI had the lowest hepatosomatic index, visceral index and crude lipid in the HC group. The diet of 1200 mg/kg MI significantly decreased triglyceride content in the serum and liver compared with those fed the MI supplemented diets regardless of carbohydrate levels. Dietary MI decreased triglyceride accumulation in the liver irrespective of carbohydrate levels. The content of malondialdehyde decreased with increasing dietary MI at both carbohydrate levels. Fish fed 1200 mg/kg MI had the highest glutathione peroxidase, superoxide dismutase, aspartate aminotransferase and glutamic-pyruvic transaminase activities. The HC diet increased the mRNA expression of key genes involved in lipid synthesis (DGAT, SREBP, FAS) in the fish fed the diet without MI supplementation. Dietary MI significantly under expressed fatty acid synthetase in fish fed the HC diets. Moreover, the mRNA expression of genes related to lipid catabolism (CPT, ATGL, PPAR-α) was significantly up-regulated with the increase of dietary MI levels despite dietary carbohydrate levels. The gene expressions of gluconeogenesis, glycolysis and MI biosynthesis were significantly down-regulated, while the expression of the pentose phosphate pathway was up-regulated with the increase of MI levels. This study indicates that HC diets can interrupt normal lipid metabolism and tend to form a fatty liver in fish. Dietary MI supplement can alleviate lipid accumulation in the liver by diverging some glucose metabolism into the pentose phosphate pathway and enhance the antioxidant capacity in O. niloticus.

Transcriptional regulation of prolactin in a euryhaline teleost: Characterisation of gene promoters through in silico and transcriptome analyses

The sensitivity of prolactin (Prl) cells of the Mozambique tilapia (Oreochromis mossambicus) pituitary to variations in extracellular osmolality enables investigations into how osmoreception underlies patterns of hormone secretion. Through the actions of their main secretory products, Prl cells play a key role in supporting hydromineral balance of fishes by controlling the major osmoregulatory organs (ie, gill, intestine and kidney). The release of Prl from isolated cells of the rostral pars distalis (RPD) occurs in direct response to physiologically relevant reductions in extracellular osmolality. Although the particular signal transduction pathways that link osmotic conditions to Prl secretion have been identified, the processes that underlie hyposmotic induction of prl gene expression remain unknown. In this short review, we describe two distinct tilapia gene loci that encode Prl177 and Prl188 . From our in silico analyses of prl177 and prl188 promoter regions (approximately 1000 bp) and a transcriptome analysis of RPDs from fresh water (FW)- and seawater (SW)-acclimated tilapia, we propose a working model for how multiple transcription factors link osmoreceptive processes with adaptive patterns of prl177 and prl188 gene expression. We confirmed via RNA-sequencing and a quantitative polymerase chain reaction that multiple transcription factors emerging as predicted regulators of prl gene expression are expressed in the RPD of tilapia. In particular, gene transcripts encoding pou1f1, stat3, creb3l1, pbxip1a and stat1a were highly expressed; creb3l1, pbxip1a and stat1a were elevated in fish acclimated to SW vs FW. Combined, our in silico and transcriptome analyses set a path for resolving how adaptive patterns of Prl secretion are achieved via the integration of osmoreceptive processes with the control of prl gene transcription.

An osmolality/salinity-responsive enhancer 1 (OSRE1) in intron 1 promotes salinity induction of tilapia glutamine synthetase

Euryhaline tilapia (Oreochromis mossambicus) are fish that tolerate a wide salinity range from fresh water to > 3× seawater. Even though the physiological effector mechanisms of osmoregulation that maintain plasma homeostasis in fresh water and seawater fish are well known, the corresponding molecular mechanisms that control switching between hyper- (fresh water) and hypo-osmoregulation (seawater) remain mostly elusive. In this study we show that hyperosmotic induction of glutamine synthetase represents a prominent part of this switch. Proteomics analysis of the O. mossambicus OmB cell line revealed that glutamine synthetase is transcriptionally regulated by hyperosmolality. Therefore, the 5' regulatory sequence of O. mossambicus glutamine synthetase was investigated. Using an enhancer trapping assay, we discovered a novel osmosensitive mechanism by which intron 1 positively mediates glutamine synthetase transcription. Intron 1 includes a single, functional copy of an osmoresponsive element, osmolality/salinity-responsive enhancer 1 (OSRE1). Unlike for conventional enhancers, the hyperosmotic induction of glutamine synthetase by intron 1 is position dependent. But irrespective of intron 1 position, OSRE1 deletion from intron 1 abolishes hyperosmotic enhancer activity. These findings indicate that proper intron 1 positioning and the presence of an OSRE1 in intron 1 are required for precise enhancement of hyperosmotic glutamine synthetase expression.

The round goby genome provides insights into mechanisms that may facilitate biological invasions

Background

The invasive benthic round goby (Neogobius melanostomus) is the most successful temperate invasive fish and has spread in aquatic ecosystems on both sides of the Atlantic. Invasive species constitute powerful in situ experimental systems to study fast adaptation and directional selection on short ecological timescales and present promising case studies to understand factors involved the impressive ability of some species to colonize novel environments. We seize the unique opportunity presented by the round goby invasion to study genomic substrates potentially involved in colonization success.

Results

We report a highly contiguous long-read-based genome and analyze gene families that we hypothesize to relate to the ability of these fish to deal with novel environments. The analyses provide novel insights from the large evolutionary scale to the small species-specific scale. We describe expansions in specific cytochrome P450 enzymes, a remarkably diverse innate immune system, an ancient duplication in red light vision accompanied by red skin fluorescence, evolutionary patterns of epigenetic regulators, and the presence of osmoregulatory genes that may have contributed to the round goby’s capacity to invade cold and salty waters. A recurring theme across all analyzed gene families is gene expansions.

Conclusions

The expanded innate immune system of round goby may potentially contribute to its ability to colonize novel areas. Since other gene families also feature copy number expansions in the round goby, and since other Gobiidae also feature fascinating environmental adaptations and are excellent colonizers, further long-read genome approaches across the goby family may reveal whether gene copy number expansions are more generally related to the ability to conquer new habitats in Gobiidae or in fish.

Electronic supplementary material

The online version of this article (10.1186/s12915-019-0731-8) contains supplementary material, which is available to authorized users.

Evolution of cellular stress response mechanisms

The cellular stress response (CSR) is pervasive to all domains of life. It has shaped the interaction between organisms and their environment since the origin of the first cell. Although the CSR has been subject to a myriad of nuanced modifications in the various branches of life present today, its core features remain preserved. The scientific literature covering the CSR is enormous and the broad scope of this brief overview was challenging. However, it is critical to conceptually understand how cells respond to stress in a holistic sense and to point out how fundamental aspects of the CSR framework are integrated. It was necessary to be extremely selective and not feasible to even mention many interesting and important developments in this expansive field. The purpose of this overview is to sketch out general and emerging CSR concepts with an emphasis on the initial cellular strain resulting from stress (macromolecular damage) and the evolutionarily most highly conserved elements of the CSR. Examples emphasize fish and aquatic invertebrates to highlight what is known in organisms beyond mammals, yeast, and other common models. Nonetheless, select pioneering studies using canonical models are also considered and the concepts discussed are applicable to all cells. More detail on important aspects of the CSR in aquatic animals is provided in the accompanying articles of this special issue.

Novel zebrafish behavioral assay to identify modifiers of the rapid, nongenomic stress response

When vertebrates face acute stressors, their bodies rapidly undergo a repertoire of physiological and behavioral adaptations, which is termed the stress response. Rapid changes in heart rate and blood glucose levels occur via the interaction of glucocorticoids and their cognate receptors following hypothalamic-pituitary-adrenal axis activation. These physiological changes are observed within minutes of encountering a stressor and the rapid time domain rules out genomic responses that require gene expression changes. Although behavioral changes corresponding to physiological changes are commonly observed, it is not clearly understood to what extent hypothalamic-pituitary-adrenal axis activation dictates adaptive behavior. We hypothesized that rapid locomotor response to acute stressors in zebrafish requires hypothalamic-pituitary-interrenal (HPI) axis activation. In teleost fish, interrenal cells are functionally homologous to the adrenocortical layer. We derived eight frameshift mutants in genes involved in HPI axis function: two mutants in exon 2 of mc2r (adrenocorticotropic hormone receptor), five in exon 2 or 5 of nr3c1 (glucocorticoid receptor [GR]) and two in exon 2 of nr3c2 (mineralocorticoid receptor [MR]). Exposing larval zebrafish to mild environmental stressors, acute changes in salinity or light illumination, results in a rapid locomotor response. We show that this locomotor response requires a functioning HPI axis via the action of mc2r and the canonical GR encoded by nr3c1 gene, but not MR (nr3c2). Our rapid behavioral assay paradigm based on HPI axis biology can be used to screen for genetic and environmental modifiers of the hypothalamic-pituitary-adrenal axis and to investigate the effects of corticosteroids and their cognate receptor interactions on behavior.

Development of a Gill Assay Library for Ecological Proteomics of Threespine Sticklebacks (Gasterosteus aculeatus)

A data-independent acquisition (DIA) assay library for quantitative analyses of proteome dynamics has been developed for gills of threespine sticklebacks (Gasterosteus aculeatus). A raw spectral library was generated by data-dependent acquisition (DDA) and annotation of tryptic peptides to MSMS spectra and protein database identifiers. The assay library was constructed from the raw spectral library by removal of low-quality, ambiguous, and low-signal peptides. Only unique proteins represented by at least two peptides are included in the assay library, which consists of 1506 proteins, 5074 peptides, 5104 precursors, and 25,322 transitions. This assay library was used with DIA data to identify biochemical differences in gill proteomes of four populations representing different eco- and morpho-types of threespine sticklebacks. The assay library revealed unique and reproducible proteome signatures. Warm-adapted, low-plated, brackish-water fish from Laguna de la Bocana del Rosario (Mexico) show elevated HSP47, extracellular matrix, and innate immunity proteins whereas several immunoglobulins, interferon-induced proteins, ubiquitins, proteolytic enzymes, and nucleic acid remodeling proteins are reduced. Fully-plated, brackish-water fish from Westchester Lagoon (Alaska) display elevated ion regulation, GTPase signaling, and contractile cytoskeleton proteins, altered abundances of many ribosomal, calcium signaling and immunity proteins, and depleted transcriptional regulators and metabolic enzymes. Low-plated freshwater fish from Lake Solano (California) have elevated inflammasomes and proteolytic proteins whereas several iron containing and ion regulatory proteins are reduced. Gills of fully-plated, marine fish from Bodega Harbor (California) have elevated oxidative metabolism enzymes and reduced transglutaminase 2, collagens, and clathrin heavy chains. These distinct proteome signatures represent targets for testing ecological and evolutionary influences on molecular mechanisms of gill function in threespine sticklebacks. Furthermore, the gill assay library represents a model for other tissues and paves the way for accurate and reproducible network analyses of environmental context-dependent proteome dynamics in complex organisms.

Higher susceptibility to osmolality of the medaka (Oryzias latipes) mutants in orthologue genes of mammalian skin transglutaminases

Transglutaminase (TG) is an essential enzyme to catalyze cross-linking reactions of epidermal proteins. Recently, we biochemically characterized human skin TG orthologues for medaka (Oryzias latipes), a model fish. By genome editing, gene-modified fishes for the two orthologues were obtained, both of which lack the ordinal enzymes. These fish appeared to exhibit higher susceptibility to osmolality at the period of larvae.

Identifying a Major QTL Associated with Salinity Tolerance in Nile Tilapia Using QTL-Seq

Selection of new lines with high salinity tolerance allows for economically feasible production of tilapias in brackish water areas. Mapping QTLs and identifying the markers linked to salinity-tolerant traits are the first steps in the improvement of the tolerance in tilapia through marker-assisted selection techniques. By using QTL-seq strategy and linkage-based analysis, two significant QTL intervals (chrLG4 and chrLG18) on salinity-tolerant traits were firstly identified in the Nile tilapia. Fine mapping with microsatellite and SNP markers suggested a major QTL region that located at 23.0 Mb of chrLG18 and explained 79% of phenotypic variation with a LOD value of 95. Expression analysis indicated that at least 10 genes (e.g., LACTB2, KINH, NCOA2, DIP2C, LARP4B, PEX5R, and KCNJ9) near or within the QTL interval were significantly differentially expressed in intestines, brains, or gills under 10, 15, or 20 ppt challenges. Our findings suggest that QTL-seq can be effectively utilized in QTL mapping of salinity-tolerant traits in fish. The identified major QTL is a promising locus to improve our knowledge on the genetic mechanism of salinity tolerance in tilapia.