Adequate levels of dietary sulphur amino acids impart improved liver and gut health in juvenile yellowtail kingfish (Seriola lalandi)

Abstract

The sulphur amino acids methionine (Met) and cysteine (Cys) and their derivative taurine (Tau) are metabolically active molecules with interlinked roles in nutritional requirements. Deficiencies in these nutrients are linked to poor growth and health; however, the impacts of these deficiencies on organ structure and function are largely unknown. This study examined the effects of dietary Met, Cys and Tau fed at different levels on yellowtail kingfish (YTK) liver histology and surface colour, plasma biochemistry and posterior intestine histology. Samples were collected from two dose-response feeding trials that quantified (1) the Tau requirement and sparing effect of Met by feeding YTK diets containing one of seven levels of Tau at one of two levels of Met and (2) the Met requirement and sparing effect of Cys by feeding YTK diets containing one of five levels of Met at one of two levels of Cys. YTK fed inadequate levels of dietary Met, Cys and Tau exhibited thicker bile ducts, less red livers, more intestinal acidic goblet cell mucus and supranuclear vacuoles and less posterior intestinal absorptive surface area. Further, thicker bile ducts correlated with less red livers (a*, R), whereas increased hepatic fat correlated with a liver yellowing (b*). Our results indicate a shift towards histological properties and functions indicative of improved intrahepatic biliary condition, posterior intestinal nutrient absorption and homoeostasis of YTK fed adequate amounts of Met, Cys and Tau. These findings may assist in formulating aquafeed for optimised gastrointestinal and liver functions and maintaining good health in YTK.

Keywords: Aquaculture; Goblet cells; Liver histology; Methionine; Posterior intestine histology; Taurine; Yellowtail kingfish.

Fig. 1.

Semi-quantitative scoring of yellowtail kingfish (Seriola lalandi) liver collected from the TauMet study for the presence of lipid vacuoles, indicating fattiness/steatosis. The four levels are (a) 0 – normal; (b) 1 – mild; (c) 2 – moderate; (d) 3 – severe (Haematoxylin–eosin stain, scale bar = 50 µm).

Fig. 2.

Histological features measured in yellowtail kingfish (Seriola lalandi) liver (TauMet study) fed one of seven dietary tauriness and one of two methionine levels. YTK liver was measured and quantified for (a) bile duct wall thickness (fibrous wall + epithelium), (b) necrotic hepatocytes, lipid vacuoles (semi-quantitative, see Fig. 1), (c) large nucleus of hepatocytes, cytoplasm eosinophilia in hepatocytes and marginated chromatin. (Haematoxylin–eosin stain, scale bar = 50 µm (a, b) and 20 µm (c)).

Fig. 3.

Posterior intestinal structures of juvenile yellowtail kingfish (Seriola lalandi). A (M) following descriptions indicate that structure was measured and statistically analysed. (a): VA, villus area; LPA, lamina propria area (yellow area); VL, villus length; (b): TVH, total villus height (M); MU, muscularis; ME, muscularis externa; MI, muscularis interna (M); SC, stratum compactum; SG, stratum granulosum; S, submucosa (M); M, mucosa; TIW, total intestinal wall thickness (M); (c): LE, lamina epithelial; SV, supranuclear vacuoles (M); GC, goblet cells (M); (d): AB+, mucus that stained blue with Alcian blue (blue) (M); PAS+, mucus that stained with periodic acid-Schiff’s (magenta) (M); AB + PAS+, Alcian blue – periodic acid-Schiff’s positive stain mucus (purple) (M); S-PAS+, small periodic acid-Schiff’s dense bullet-shaped bodies (magenta and 18·6 (se 0·7) µm) (M). (AB-PAS stain, scale bar = 800 µm (a–b), 200 µm (c), 100 µm (d)).

Fig. 4.

Liver histology of juvenile yellowtail kingfish (Seriola lalandi) (TauMet study), fed one of six taurines and one of two methionine levels. (a) Bile duct wall thickness (µm), (b) number of necrotic cells per 0·094 mm², (c) count of large nuclei per 0·094 mm², (d) count of cytoplasm eosinophilia in hepatocytes per 0·094 mm². Data expressed as mean values with their standard errors.

Fig. 5.

Plasma chemistry of juvenile yellowtail kingfish (Seriola lalandi) fed diets containing one of six different taurine levels and one of two methionine levels. Panels (a–b) show results on lipoproteins; panels (c–d) show results on liver function tests. ALP, alkaline phosphatase (c); AST, aspartate transaminase (d); LD, lactate dehydrogenase (e); panels (f–h) show results on solutes that form electrolytes and panels (i–k) show results on other plasma chemistry results. Red lines are the low met series (10·9 g Met/kg diet), and blue lines are the high methionine series (17·2 g Met/kg diet) at varying levels of taurine. The range bars indicate the two collected values.

Fig. 6.

(a) Distribution of juvenile yellowtail kingfish (Seriola lalandi) liver surface colours across dietary treatments from the TauMet study. Grey sequences are the collection of colours that were each ≤ 3 % present in the total liver surface colour composition (n 6) within a diet. Coloured sequences are tinted in the respective hex colour code and represented ≥ 3 % of the total liver colour composition (n 6). (b) Average RGB liver surface colour of each dietary treatment from the TauMet study. Average RGB values were converted to a single-colour square that represents the average liver colour of liver tissue (n 6).

Fig. 7.

PCoA using the colour distance method and clustered by similarity of individual juvenile yellowtail kingfish (Seriola lalandi) liver surface colours from the TauMet study. A dot represents an individual liver, and numbers correspond to the respective diet. Ellipses (dashed lines) indicate distribution at 95 % confidence level of Tau/Met levels at high/high (h/h in red), high/low (h/l in green), low/high (l/h in blue) and low/low (l/l in purple). PCoA, principal coordinate analysis.

Fig. 8.

Barplots on the histochemical analysis per villus area of the PI of juvenile yellowtail kingfish (Seriola lalandi) fed one of six taurine-methionine combinations (TauMet). (a) Neutral goblet cell mucus (PAS+ per villus area), acid goblet cell mucus (AB+ per villus area), or mixed goblet cell mucus (AB + PAS+ per villus area), total goblet cell mucus (TGC per villus area) and bullet-shaped PAS+ mucus (S-PAS+ per villus area). (b) Supranuclear vacuole density (SV per villus area). Error bars indicate standard error.

Fig. 9.

Barplots on the histochemical analysis of juvenile yellowtail kingfish (Seriola lalandi) posterior intestine, fed one of five methionine-cysteine combinations (MetCys). (a) Neutral goblet cell mucus (PAS+ per villus area), acid goblet cell mucus (AB+ per villus area), or mixed goblet cell mucus (AB + PAS+ per villus area), total goblet cell mucus (TGC per villus area) and bullet-shaped PAS+ mucus (S-PAS+ per villus area). (b) Supranuclear vacuole density (SV per villus area). Error bars indicate standard error.