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

Effect of Salinity Stress on Gut-Brain-Gonad Axis in the Red-Eared Slider (Trachemys scripta elegans)

The red-eared slider (Trachemys scripta elegans) can adapt to brackish water, which can endanger its biodiversity. Spermatogonial stem cells (SSCs) are essential for establishing and maintaining spermatogenesis and are regulated by the gut-brain-gonad axis. However, the effect of salinity on SSCs is unclear. We investigated the influence of salinity stress on the composition of the gut microbiota in T. s. elegans to determine whether it regulates SSC self-renewal and differentiation via the gut-brain-gonad axis. Three salinity groups (0‰, 5‰, and 10‰) were used in this study, and samples were obtained after 6 months of feeding. The mRNA expression of self-renewing genes (GFRα-1, RAS, and ERK) and meiotic initiation genes (RARα, NRG3, and ERBB4) in SSCs decreased with increasing salinity, indicating that salinity affects renewal and differentiation. In addition, harmful bacteria such as Enterococcus and Clostridium were increased in the S10 group, and lower levels of g_norank_f_Eubacteriaceae were negatively associated with γ-aminobutyric acid (GABA), whereas higher Turicibacter levels were positively associated with GABA levels, resulting in increased GABA content in the S5 group. The results show that salinity affects the secretion of neurotransmitters in the brain and negatively regulates the synthesis of reproductive hormones by changing the composition of intestinal microorganisms and metabolites, which affect SSC function. In conclusion, salinity influences the reproductive ability of T. s. elegans through the gut-brain-gonad axis. This study provides a new perspective for understanding the adaptation of T. s. elegans to brackish water.

Transcriptome and DNA Methylation Responses in the Liver of Yellowfin Seabream Under Starvation Stress

Fish suffer from starvation due to environmental risks such as extreme weather in the wild and due to insufficient feedings in farms. Nutrient problems from short-term or long-term starvation conditions can result in stress-related health problems for fish. Yellowfin seabream (Acanthopagrus latus) is an important marine economic fish in China. Understanding the molecular responses to starvation stress is vital for propagation and culturing yellowfin seabream. In this study, the transcriptome and genome-wide DNA methylation levels in the livers of yellowfin seabream under 14-days starvation stress were analyzed. One hundred sixty differentially expressed genes (DEGs) by RNA-Seq analysis and 737 differentially methylated-related genes by whole genome bisulfite sequencing analysis were identified. GO and KEGG pathway enrichment analysis found that energy metabolism-related pathways such as glucose metabolism and lipid metabolism were in response to starvation. Using bisulfite sequencing PCR, we confirmed the presence of CpG methylation differences within the regulatory region of a DEG ppargc1a in response to 14-days starvation stress. This study revealed the molecular responses of livers in response to starvation stress at the transcriptomic and whole genome DNA methylation levels in yellowfin seabream.

Transcriptomic Analysis in Marine Medaka Gill Reveals That the Hypo-Osmotic Stress Could Alter the Immune Response via the IL17 Signaling Pathway

Fish gills are the major osmoregulatory tissue that contact the external water environment and have developed an effective osmoregulatory mechanism to maintain cellular function. Marine medaka (Oryzias melastigma) has the ability to live in both seawater and fresh water environments. The present study performed a seawater (SW) to 50% seawater (SFW) transfer, and the gill samples were used for comparative transcriptomic analysis to study the alteration of hypo-osmotic stress on immune responsive genes in this model organism. The result identified 518 differentiated expressed genes (DEGs) after the SW to SFW transfer. Various pathways such as p53 signaling, forkhead box O signaling, and the cell cycle were enriched. Moreover, the immune system was highlighted as one of the top altered biological processes in the enrichment analysis. Various cytokines, chemokines, and inflammatory genes that participate in the IL-17 signaling pathway were suppressed after the SW to SFW transfer. On the other hand, some immunoglobulin-related genes were up-regulated. The results were further validated by real-time qPCR. Taken together, our study provides additional gill transcriptome information in marine medaka; it also supports the notion that osmotic stress could influence the immune responses in fish gills.

Effects of seawater and freshwater challenges on the Gh/Igf system in the saline-tolerant blackchin tilapia (Sarotherodon melanotheron)

Prolactin (Prl) and growth hormone (Gh) as well as insulin-like growth factor 1 (Igf1) are involved in the physiological adaptation of fish to varying salinities. The Igfs have been also ascribed other physiological roles during development, growth, reproduction and immune regulation. However, the main emphasis in the investigation of osmoregulatory responses has been the endocrine, liver-derived Igf1 route and local regulation within the liver and osmoregulatory organs. Few studies have focused on the impact of salinity alterations on the Gh/Igf-system within the neuroendocrine and immune systems and particularly in a salinity-tolerant species, such as the blackchin tilapia Sarotherodon melanotheron. This species is tolerant to hypersalinity and saline variations, but it is confronted by severe climate changes in the Saloum inverse estuary. Here we investigated bidirectional effects of increased salinity followed by its decrease on the gene regulation of prl, gh, igf1, igf2, Gh receptor and the tumor-necrosis factor a. A mixed population of sexually mature 14-month old blackchin tilapia adapted to freshwater were first exposed to seawater for one week and then to fresh water for another week. Brain, pituitary, head kidney and spleen were excised at 4 h, 1, 2, 3 and 7 days after both exposures and revealed differential expression patterns. This investigation should give us a better understanding of the role of the Gh/Igf system within the neuroendocrine and immune organs and the impact of bidirectional saline challenges on fish osmoregulation in non-osmoregulatory organs, notably the complex orchestration of growth factors and cytokines.