Dopamine regulates renal osmoregulation during hyposaline stress via DRD1 in the spotted scat (Scatophagus argus)

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.

Inflammatory responses associated with hyposaline stress in gill epithelial cells of the spotted scat Scatophagus argus

The molecular processes of immune responses in mucosal tissues such as fish gills under environmental stress are poorly understood. In the present study, pro-inflammatory response under hyposaline stress and its regulation by cortisol/corticosteroid receptors (CRs) in gill epithelial cells of the spotted scat Scatophagus argus were analyzed. The fish were transferred to freshwater for 6 days (144 h) of acclimation. Following freshwater exposure, the cortisol concentration increased transiently before returning to the control level over time. mRNA expression of pro-inflammatory cytokines (TNF-a, IL-1b and IL-6) was stimulated by cortisol through CR signals at early stages of acclimation, but hyposaline stress inhibited their levels by the end of the experimental period. The transcriptional profile of anti-inflammatory cytokine IL-10 was quite different from these pro-inflammatory cytokines, and its value fluctuated within a narrow range during the experimental period. Full-length cDNAs of mineralocorticoid receptor (MR) and glucocorticoid receptor 1 (GR1) (different kinds of CRs) were cloned from the gills. Our results showed that MR and GR displayed mutually antagonistic effects during hyposaline stress. MR responded quickly at early stages, and its expression decreased with the drop of cortisol concentration. By contrast, GR expression was maintained at high levels after the acclimation of freshwater exposure. The tight coordination of GR and MR helps to shape the effects of stress on the immune system, which in turn, regulates the stress response. Our results confirm the interaction between endocrine and cytokine messengers and a clear difference in the sensitivity of GR and MR during the hyposaline challenge in gill epithelial cells of the spotted scat Scatophagus argus.

Molecular characterization of transient receptor potential vanilloid 4 (TRPV4) gene transcript variant mRNA of chum salmon Oncorhynchus keta in response to salinity or temperature changes

Transient receptor potential vanilloid 4 (TRPV4) is an osmosensory cation channel that respond to an increase in cell volume and participates in various physiological functions. Among organisms in aquatic environments, euryhaline teleost is are suitable experimental models to study ion channel proteins related to physiological functions involving osmosensing. Among the studies of various regulatory molecules that mediate osmotic regulation in fish, however, information is lacking, particularly on the TRP family. This study investigated the structural characteristics of theTRPV4 gene of chum salmon (Oncorhynchus keta) and their responses to changes in salinity and temperature. Interestingly, TRPV4 generates transcript variants of the intron-retention form through alternative splicing, resulting in a frameshift leading to the generation of transcripts of different structures. In particular, TRPV4 x1 and TRPV x2 mRNAs were predominant in the gill and skin including at the lateral line. The expression levels of chum salmon TRPV4 x1 were significantly increased with increase in salinity and temperature, whereas TRPV4 x2 mainly responded to temperature decrease. Overall, these results demonstrate for the first time the effects of salinity and temperature on the expression of two salmonid TRPV4 transcript variants, suggesting their contribution to the regulation of hydromineral balance.

Identification and Characterization of Osmoregulation Related MicroRNAs in Gills of Hybrid Tilapia Under Three Types of Osmotic Stress

Researchers have increasingly suggested that microRNAs (miRNAs) are small non-coding RNAs that post-transcriptionally regulate gene expression and protein translation in organs and respond to abiotic and biotic stressors. To understand the function of miRNAs in osmotic stress regulation of the gills of hybrid tilapia (Oreochromis mossambicus ♀ × Oreochromis urolepis hornorum ♂), high-throughput Illumina deep sequencing technology was used to investigate the expression profiles of miRNAs under salinity stress (S, 25‰), alkalinity stress (A, 4‰) and salinity-alkalinity stress (SA, S: 15‰, A: 4‰) challenges. The results showed that 31, 41, and 27 upregulated and 33, 42, and 40 downregulated miRNAs (P < 0.05) were identified in the salt stress, alkali stress, and saline-alkali stress group, respectively, which were compared with those in the control group (C). Fourteen significantly differently expressed miRNAs were selected randomly and then validated by a quantitative polymerase chain reaction. On the basis of Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analysis, genes related to osmoregulation and biosynthesis were enriched in the three types of osmotic stress. In addition, three miRNAs and three predicted target genes were chosen to conduct a quantitative polymerase chain reaction in the hybrid tilapia and its parents during 96-h osmotic stress. Differential expression patterns of miRNAs provided the basis for research data to further investigate the miRNA-modulating networks in osmoregulation of teleost.

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.

Comparative transcriptome analysis of male and female gonads reveals sex-biased genes in spotted scat (Scatophagus argus)

Scatophagus argus is a new emerging aquaculture fish in East and Southeast Asia. To date, research on reproductive development and regulation in S. argus is lacking. Additionally, genetic and genomic information about reproduction, such as gonadal transcriptome data, is also lacking. Herein, we report the first gonadal transcriptomes of S. argus and identify genes potentially involved in reproduction and gonadal development. A total of 136,561 unigenes were obtained by sequencing of testes (n = 3) and ovaries (n = 3) at stage III. Genes upregulated in males and females known to be involved in gonadal development and gametogenesis were identified, including male-biased dmrt1, amh, gsdf, wt1a, sox9b, and nanos2, and female-biased foxl2, gdf9, bmp15, sox3, zar1, and figla. Serum estradiol-17β and 11-ketotestosterone levels were biased in female and male fish, respectively. Sexual dimorphism of serum steroid hormone levels were interpreted after expression analysis of 20 steroidogenesis-related genes, including cyp19a1a and cyp11b2. This gonadal transcript dataset will help investigate functional genes related to reproduction in S. argus.

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

Background

To control the osmotic pressure in the body, physiological adjustments to salinity fluctuations require the fish to regulate body fluid homeostasis in relation to environmental change via osmoregulation. Previous studies related to osmoregulation were focused primarily on the gill; however, little is known about another organ involved in osmoregulation, the kidney. The salinity adaptation of marine fish involves complex physiological traits, metabolic pathways and molecular and gene networks in osmoregulatory organs. To further explore of the salinity adaptation of marine fish with regard to the role of the kidney, the euryhaline fish Scatophagus argus was employed in the present study. Renal expression profiles of S. argus at different salinity levels were characterized using RNA-sequencing, and an integrated approach of combining molecular tools with physiological and biochemical techniques was utilized to reveal renal osmoregulatory mechanisms in vivo and in vitro.

Results

S. argus renal transcriptomes from the hyposaline stress (0‰, freshwater [FW]), hypersaline stress (50‰, hypersaline water [HW]) and control groups (25‰) were compared to elucidate potential osmoregulatory mechanisms. In total, 19,012 and 36,253 differentially expressed genes (DEGs) were obtained from the FW and HW groups, respectively. Based on the functional classification of DEGs, the renal dopamine system-induced Na⁺ transport was demonstrated to play a fundamental role in osmoregulation. In addition, for the first time in fish, many candidate genes associated with the dopamine system were identified. Furthermore, changes in environmental salinity affected renal dopamine release/reuptake by regulating the expression of genes related to dopamine reuptake (dat and nkaα1), vesicular traffic-mediated dopamine release (pink1, lrrk2, ace and apn), DAT phosphorylation (CaMKIIα and pkcβ) and internalization (akt1). The associated transcriptional regulation ensured appropriate extracellular dopamine abundance in the S. argus kidney, and fluctuations in extracellular dopamine produced a direct influence on Na⁺/K⁺-ATPase (NKA) expression and activity, which is associated with Na⁺ homeostasis.

Conclusions

These transcriptomic data provided insight into the molecular basis of renal osmoregulation in S. argus. Significantly, the results of this study revealed the mechanism of renal dopamine system-induced Na⁺ transport is essential in fish osmoregulation.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5795-x) contains supplementary material, which is available to authorized users.