For the past several decades, aquaculture has been the fastest growing sector of animal agriculture, with worldwide production of fish and shellfish tripling in volume produced since 1995. This growth has put significant stress of the availability and economic viability of using fishmeal and fish oils, the primary components of aquafeeds. Great strides have been made to decrease the inclusion rate of fishmeal in aquafeeds, while increasing the use of alternative protein sources, like plant proteins, as more sustainable and economical solutions to fishmeal. The most prevalent alternative protein sources are from plant products, which are inherently low in methionine . Therefore, supplementation of methionine, and other essential amino acids, is required for plant-based diets to match the nutritional and amino acid requirements of fish.
When dietary methionine levels are inadequate, nutrient partitioning between fat and protein deposition changes leading to decreases in visceral fat deposition. In addition, IGF-I levels in circulation decline and overall growth is reduced. With the increasing need for more plant-based diets, understanding the role methionine plays in fish growth is paramount. This study focused on the mechanistic effects of methionine on trout muscle cells directly, by using a methionine depletion/replenishment protocol. We hypothesize that methionine, because of its role in cellular methylation pathways , regulates cell differentiation via epigenetic mechanisms. We tested the role of methionine on 1) muscle-specific microRNAs and 2) global DNA methylation. Outcomes were analyzed in trout myogenic cells cultured under three different conditions: presence of methionine for 72h (ctl ), absence of methionine for 72h (Meth-), and absence of methionine for 48h followed by 24h of methionine replenishment (Meth-/+).
MicroRNA array analysis revealed three clusters: cluster I corresponds to miRNA upregulated only in Meth -/+ conditions; cluster II corresponds to miRNA downregulated only in Meth-/+ conditions; and cluster III corresponds to miRNAs with high expression in control, low expression in Meth- conditions and intermediate expression after methionine replenishment (Meth-/+). Cluster III aligned with previous data supporting an involvement with differentiation and identified seven miRNAs with muscle-related function (including miR-210 and -133a). Global methylation analysis revealed 348 bases hypo- or hyper-methylated in response to methionine restriction, and 2 methylated sites rescued by methionine replenishment. These data support epigenetic mechanisms regulating decreased muscle growth in response to inadequate dietary methionine intake, and suggest further investigation is needed to optimize supplemental feeding strategies to support efficient and efficacious growth.