Skin mucus metabolomics provides insights into the interplay between diet and wound in gilthead seabream (Sparus aurata)

The molecular processes underlying skin wound healing in several fish species have been elucidated in the last years, however, metabolomic insights are scarce. Here we report the skin mucus metabolome of wounded and non-wounded gilthead seabream (Sparus aurata) fed with silk fibroin microparticles, a functional additive considered to accelerate the wound healing process. The three experimental diets (commercial diet enriched with 0 mg (control), 50 mg or 100 mg of silk fibroin microparticles Kg^-1) were administered for 30 days and thereafter, a skin wound was inflicted. Skin mucus was collected on day 30 of feeding and 7 days post-wounding and subjected to metabolomic analysis by Ultra Performance Liquid Chromatography coupled with a high-resolution quadrupole-orbitrap mass spectrometry. The most enriched metabolite class was amino acids and derivatives, followed by nucleotides, nucleosides and analogues and carbohydrates and their derivatives. Metabolomic profiles revealed that the diet had a more profound effect than wounding in skin mucus. Metabolic pathway analysis of significantly affected metabolites revealed perturbations in the aminoacyl t-RNA biosynthesis in the skin. In particular, skin wound resulted in a decreased methionine level in mucus. Further, silk fibroin supplementation increased methionine level in skin mucus, which correlated with several wound morphometric parameters that characterized the epithelial healing capacity in seabream. The results provided new insight into the physiological consequences of skin wounds and how these processes could be influenced by dietary manipulation.

Sulphide donors affect the expression of mucin and sulphide detoxification genes in the mucosal organs of Atlantic salmon (Salmo salar)

Hydrogen sulphide (H₂S) is a gas that affects mucosal functions in mammals. However, its detrimental effects are less understood in fish despite being known to cause mass mortality. Here we used explant models to demonstrate the transcriptional responses of Atlantic salmon (Salmo salar) mucosa to the sulphide donor sodium hydrosulphide (NaHS). The study focused on two groups of genes: those encoding for sulphide detoxification and those for mucins. Moreover, we performed pharmacological studies by exposing the organ explants to mucus-interfering compounds and consequently exposed them to a sulphide donor. Exposure to NaHS significantly affected the expression of sulphide:quinone oxidoreductase (sqor1, sqor2) and mucin-encoding genes (muc5ac, muc5b). The general profile indicated that NaHS upregulated the expression of sulphide detoxification genes while a significant downregulation was observed with mucins. These expression profiles were seen in both organ explant models. Pharmacological stimulation and inhibition of mucus production used acetylcholine (ACh) and niflumic acid (NFA), respectively. This led to a significant regulation of the two groups of marker genes in the gills and olfactory rosette explants. Treatment of the mucosal organ explants with the mucus-interfering compounds showed that low dose NFA triggered more substantial changes while a dose-dependent response could not be established with ACh. Pharmacological interference demonstrated that mucins played a crucial role in mucosal protection against H₂S toxicity. These results offer insights into how a sulphide donor interfered with mucosal responses of Atlantic salmon and are expected to contribute to our understanding of the least explored H₂S-fish interactions-particularly at the mucosa.

Induction and characterization of extracellular traps by gilthead seabream (Sparus aurata L.) head-kidney leucocytes

The aim of this study was the induction and characterization of extracellular traps (ETs) produced by gilthead seabream (Sparus aurata L.) head-kidney leucocytes. The cells were incubated several times (10, 30, 60, 120, and 180 min) with different concentrations of the stimulants diluted in RPMI-1640 culture medium: RPMI-1640 (control), β-glucan from Saccharomyces cerevisiae (BG, 0-400 μg mL^-1), lipopolysaccharide from Escherichia coli (LPS, 0-10 μg mL^-1), calcium ionophore A23187 (CaI, 0-5 μg mL^-1), Phorbol 12-myristate 13-acetate (PMA, 0-1000 ng mL^-1) and polyinosinic-polycytidylic acid sodium salt (Poly I:C, 0-200 μg mL^-1). BG, LPS and CaI exerted only weak stimulatory activity, while PMA and poly I:C exerted a potent one. After stimulation of the leucocytes, ETs structures were quantified and visualised through staining of the chromatin with nucleic acid-specific dyes and immunocytochemical probing of characteristic proteins expected to decorate the structure. ETs structures had DNA and myeloperoxidase. The ETs morphology was studied by light and scanning electron microscopy. These data confirm that seabream leucocytes form ETs with different morphological properties, depending on the used stimulant. These results will be the basis for new studies to analyse the implication of this mechanism in fish immunity. All this new knowledge will have its application in fish farms when we learn to manipulate the innate immune response in order to mitigate microbial infections.

Dietary administration of silk microparticles improves the epidermal and dermal regeneration after a skin wounding in gilthead seabream (Sparus aurata L.)

The impact of dietary supplementation with silk fibroin (SF) microparticles on the wound healing process in gilthead seabream (Sparus aurata) skin was studied. A control diet was enriched with different SF levels: 0 (control), 50 (SF50 diet), and 100 (SF100 diet) mg Kg^-1 to form three experimental diets and was fed to seabream for 30 days. Experimental wounds were performed and after 7 days post-wounding (dpw) skin mucus immunity, macroscopic wound closure, and skin regeneration were studied at a microscopic and genetic level. Results indicated that fish fed SF100 did not suffer the decreases in protease and IgM levels observed in the skin mucus of wounded fish fed with the control diet. Macroscopic findings illustrated that dietary SF100 significantly improved the wound closure ratio compared to those reared in the control group. At a microscopic level, changes in the shape of keratocyte cells were evident in the wounded fish. In addition, the intercellular spaces present between epidermal cells and their proliferation in the epidermis, as well as the presence of blood vessels in the dermis were significantly statistically higher in the skin of fish fed the SF100 diet and sampled at 7 dpw compared to those observed in the skin of fish fed the control or SF50 diets. Moreover, regarding the RNA: DNA ratio, statistically significant increases and decreases were observed in fish fed the control and SF100 diet, respectively, in non-wounded and wounded fish. Interestingly, dietary SF100 supplementation improved skin cell proliferation, enhanced the inflammatory phase, and increased the expression of important genes involved in tissue repair and extracellular matrix formation. In conclusion, the SF100 diet can be considered as an appropriate feed additive to improve wound healing in gilthead seabream.

Inclusion of dietary Ulva ohnoi 5% modulates Solea senegalensis immune response during Photobacterium damselae subsp. piscicida infection

Macroalgae represent valuable sources of functional ingredients for fish diets, and the influence of supplemented aquafeeds on growth performance has been studied for some fish and seaweed species. In the present work, the potential immunomodulation exerted by U. ohnoi (5%) as dietary ingredient was investigated in Senegalese sole. After feeding with the experimental diets for 90 d, fish immune response before and after challenge with Photobacterium damselae subsp. piscicida (Phdp) was assessed. In absence of infection, systemic immune response was not modified by 5% U. ohnoi dietary inclusion for 90 d. Thus, no differences in liver and head kidney immune gene transcription or serum lysozyme, peroxidase, antiprotease and complement activities were observed based on the diet received by Senegalese sole specimens. Regarding mucosal immune parameters, no changes in gene transcription were detected in the skin and gills, whilst only tnf, cd4 and cd8 were significantly up-regulated in the intestine of fish fed with U. ohnoi, compared to the values obtained with control diet. On the contrary, when S. senegalensis specimens were challenged with Phdp, modulation of the immune response consisting in increased transcription of genes encoding complement (c1q4, c3, c9), lysozyme g (lysg), tumor necrosis factor alpha (tnfα) as well as those involved in the antioxidant response (gpx, sodmn) and iron metabolism (ferrm, hamp-1) was observed in the liver of fish fed with U. ohnoi. In parallel, decreased inflammatory cytokine and complement encoding gene transcription was displayed by the spleen of fish receiving the algal diet. Though mortality rates due to Phdp challenge were not affected by the diet received, lower pathogen loads were detected in the liver of soles receiving U. ohnoi diet. Further research to investigate the effects of higher inclusion levels of this seaweed in fish diets, feeding during short periods as wells as to assess the response against other pathogens needs to be carried out.