Mechanisms underlying probiotic effects on neurotransmission and stress resilience in fish via transcriptomic profiling.

In recent years, studies have highlighted the significant impact of probiotic treatment on the central nervous system (brain) and stress regulation through the microbiota-gut-brain axis, yet there have been limited knowledge on this axis in fish. Therefore, this study aimed to enhance the current understanding of the mechanisms underlying probiotic effects on neurotransmission and stress alleviation in fish through transcriptomic profiling. In this study, olive flounders (Paralichthys olivaceus) were subjected to two trial setups: a 1-month lab-scale trial and a 6-month field-scale trial, with and without the probiotic strain Lactococcus lactis WFLU12. RNA-Seq analysis was performed using liver samples collected from fish at one-month post-feeding (mpf) in both trials. Additionally, fish growth was monitored monthly, and serological parameters were measured at one mpf in the field-scale experiment. The results of the lab-scale trial showed that probiotic administration significantly upregulated genes related to neurotransmission, such as htr3a, mao, ddc, ntsr1, and gfra2. These findings highlight the impact of probiotics on modulating neurotransmission via the microbiota-gut-brain axis. In the field-scale experiment, fish growth was significantly promoted and the sera levels of AST, LDH, and cortisol were significantly higher in the control group compared to the probiotics group. Furthermore, genes involved in stress responses (e.g. hsp70, hsp90B1, hspE1, prdx1, and gss) and transcriptional regulators (e.g. fos, dusp1, and dusp2) exhibited significant upregulation in the control group compared to the probiotics group, indicating that probiotic administration can alleviate stress levels in fish. Overall, this study provides valuable insights into the mechanisms underlying the beneficial effects of probiotics in fish, specifically regarding their impact on neurotransmission and stress alleviation.

Molecular mechanisms underlying the vulnerability of Pacific abalone (Haliotis discus hannai) to Vibrio harveyi infection at higher water temperature.

Climate change is one of the most important threats to farmed abalone worldwide. Although abalone is more susceptible to vibriosis at higher water temperatures, the molecular mode of action underlying this has not been fully elucidated. Therefore, this study aimed to address the high susceptibility of Halitotis discus hannai to V. harveyi infection using abalone hemocytes exposed to low and high temperatures. Abalone hemocytes were divided into four groups, 20C, 20 V, 25C, and 25 V, depending on co-culture with (V)/without (C) V. harveyi (MOI = 12.8) and incubation temperature (20 °C or 25 °C). After 3 h of incubation, hemocyte viability and phagocytic activity were measured, and RNA sequencing was performed using Illumina Novaseq. The expression of several virulence-related genes in V. harveyi was analyzed using real-time PCR. The viability of hemocytes was significantly decreased in the 25 V group compared to cells in the other groups, whereas phagocytic activity at 25 °C was significantly higher than at 20 °C. Although a number of immune-associated genes were commonly upregulated in abalone hemocyte exposed to V. harveyi, regardless of temperature, pathways and genes regarding pro-inflammatory responses (interleukin-17 and tumor necrosis factor) and apoptosis were significantly overexpressed in the 25 V group compared to the 25C group. Notably, in the apoptosis pathway, genes encoding executor caspases (casp3 and casp7) and pro-apoptotic factor, bax were significantly up-regulated only in the 25 V group, while the apoptosis inhibitor, bcl2L1 was significantly up-regulated only in the 20 V group compared to the control group at the respective temperatures. The co-culture of V. harveyi with abalone hemocytes at 25 °C up-regulated several virulence-related genes involved in quorum sensing (luxS), antioxidant activity (katA, katB, and sodC), motility (flgI), and adherence/invasion (ompU) compared to those at 20 °C. Therefore, our results showed that H. discus hannai hemocytes exposed to V. harveyi at 25 °C were highly stressed by vigorously activated inflammatory responses and that the bacterial pathogen overexpressed several virulence-related genes at the high temperature tested. The transcriptomic profile of both abalone hemocytes and V. harveyi in the present study provide insight into differential host-pathogen interactions depending on the temperature conditions and the molecular backgrounds related to increased abalone vulnerability upon global warming.