Dietary amino acid supplementation affects temporal expression of amino acid transporters and metabolic genes in selected and commercial strains of rainbow trout (Oncorhynchus mykiss)

Highlights

• Genetically selected and non-selected rainbow trout were compared to identify physiological differences in with nutrient utilization

• Both strains of trout were force-fed a plant-protein blend with and without amino acid supplementation

• Results showed that expression levels of intestinal amino acid transporters were affected by strain, diet and time

• Improved weight gain and protein retention in the selected (vegetarian) trout strain is the result of nutritional adaptation to an all plant-protein diet

Abstract

Replacement of fishmeal as the major protein source in feeds is critical for continued growth and sustainability of the aquaculture industry. However, numerous studies have shown suboptimal fish growth performance and reduced protein retention efficiency when carnivorous fish species are fed low fishmeal-high plant protein feeds. A study was conducted using a commercial strain and a genetically improved strain of rainbow trout selected for improved performance when fed an all plant protein diet to identify physiological differences associated with growth performance in the selected trout strain. Fifty individuals per strain (average weight ~ 580 g) were force-fed a plant-protein blend with and without amino acid supplementation (lysine, methionine and threonine) at 0.5% body weight and sampled at intervals over 24 h. Samples from intestine and liver were analyzed for specific gene expression analysis related to amino acid transporters, digestive process control, protein degradation and amino acid metabolism. The results showed that expression levels of various intestinal amino acid transporters (SLC1A1, SLC7A9, SLC15A, SLC1A5 SLC6A19 and SLC36A1) were affected by strain, diet and time. Moreover, significant interactions were found regarding the temporal expression levels of cholecystokinin (CCK-L), Krüppel-like factor 15 (KLF15) and aspartate aminotransferase (GOT) transcripts in the examined tissues. The results provide evidence that improved growth and protein retention of the selected strain fed an all-plant protein diet is a result of nutritional adaptation and an overall change in physiological homeostatic control.

Introduction

Aquaculture production is playing an increasing role in providing fish for human consumption (FAO, 2018). It has become the fastest growing animal production sector with an annual average growth of almost 6% worldwide (FAO, 2018). Production increases are expected to continue, with a concomitant increase in fish feed production. Fish feed represents up to 60% of production costs, with dietary protein accounting for nearly half of that cost (NRC, 2011; Arru et al., 2019). The most desirable protein source for fish feeds is fishmeal, but annual fishmeal production is limited and insufficient to supply more than a portion of the protein need to support global fish feed production. Plant protein concentrates are widely used in fish feeds due to their commercial availability, favorable amino acid profiles, high protein digestibility coefficients, and low costs compared to fishmeal. During the last several decades, a considerable amount of research was employed towards fishmeal replacement by alternative protein sources of plant origin. However, numerous studies have shown suboptimal fish growth performance and reduced protein retention efficiency when carnivorous fish species are fed low fishmeal - high plant protein feeds, even when all known essential nutrients, including amino acids, are present in the diet above required levels (Gomes et al., 1995; Davies and Morris, 1997; Refstie et al., 2000; Martin et al., 2003; Gómez-Requeni et al., 2004).

Diets comprised of plant protein blends are routinely supplemented with amino acids to increase levels of limiting amino acids. However, studies suggest that this may cause a temporal imbalance of amino acids in blood plasma following a meal as a result of asynchronous digestion and absorption of plant and free amino acids (Boirie et al., 1997; Ambardekar et al., 2009; Larsen et al., 2012). Supplemental crystalline amino acids are rapidly absorbed by trout and other fish species whereas absorption of amino acids from plant proteins is slower (Ambardekar et al., 2009). Asynchronous amino acid intestinal absorption may also alter protein synthesis activity in cells which requires all essential amino acids to be available when proteins are being made. If one essential amino acid is not present in sufficient amounts, the remaining amino acids are quickly metabolized for energy (NRC, 2011). This results in lower protein retention efficiency and increased protein turnover, a common finding when fish are fed plant-based feeds (Davies and Morris, 1997; Refstie et al., 2000; Martin et al., 2003; Ambardekar et al., 2009).

A rainbow trout (Oncorhynchus mykiss) strain has been developed using selective breeding based on growth performance when fed an all-plant protein feed for 12 years (six generations) at the University of Idaho in collaboration with the US Department of Agriculture's Agricultural Research Service (Overturf et al., 2013). The selected strain grows rapidly and efficiently when fed all plant-protein feeds containing 45% soy products, unlike non-selected trout that exhibited 10–25% lower growth and feed efficiency (Overturf et al., 2013). Brezas and Hardy (2020) showed that synchronous amino acid uptake (intestinal absorption) and disappearance in systemic blood of the selected rainbow trout strain after a single meal is likely a nutritional adaptation in the selected trout strain to high plant protein diets. Furthermore, transcriptomic studies using the same strains and diets determined that genes related to strain are differentially regulated in multiple metabolic-related pathways (Abernathy et al., 2017). The mechanisms of adaptation of nutrient absorption rates by the small intestine have been found to vary between as well as within species (Karasov, 1988). The two potential adaptations that could account for differences in performance of selected and non-selected trout strains are anatomical changes in the intestine and changes in specific amino acid transport systems (Karasov, 1988). However, differences between the strains in intestine relative length index, indicating anatomical intestinal adaptation, is not likely to be responsible for increased performance of the selected trout strain (Brezas and Hardy, 2020; unpublished data). The present study was therefore conducted to examine specific amino acid transport systems in the selected and non-selected trout strains that might explain differences in growth performance when fish were fed the all plant-protein test diet. Intestinal and hepatic samples were analyzed for gene expression related to digestive process control, amino acid transporter systems and amino acid metabolism.