Dietary protein complexity modulates growth, protein utilisation and the expression of protein digestion-related genes in Senegalese sole larvae

Given its complex metamorphosis and digestive system ontogeny, Senegalese sole larvae capacity to digest and utilize dietary protein is likely to change throughout development. In the present study, we hypothesized that the manipulation of dietary protein complexity may affect Senegalese sole larvae capacity to digest, absorb and retain protein during metamorphosis, as well as the mRNA expression of genes encoding for the precursors of proteolytic enzymes of the digestive tract and the enterocyte peptide transporter PepT1, which may have further impact on somatic growth. Three diets were formulated using approximately the same practical ingredients, except for the main protein source. The Intact diet protein content was mostly based on intact plant protein where the target peptide molecular weight (MW) would be >70kDa. The PartH diet protein fraction was mostly based on a protein hydrolysate with a high content of 5–70kDa peptides. The HighH diet protein fraction was mostly based on a protein hydrolysate with a high content of 5kDa peptides. A growth trial was performed with larvae reared at 19°C under a co-feeding regime from mouth opening. The transcription of pga, tryp1c, ialp, ampn and pepT1 (encoding respectively for PepsinogenA, Trypsinogen1C, Intestinal alkaline phosphatase, Aminopeptidase N and for the enterocyte peptide transporter 1) was quantified by qPCR, during the metamorphosis climax (16DAH) and after the metamorphosis was completed (28DAH). An in vivo method of controlled tube-feeding was used to assess the effect on the larvae capacity to utilize polypeptides with different MW (1.0 and 7.2kDa) representing a typical peptide MW of each of the hydrolysates included in the diets. The PartH diet stimulated growth in metamorphosing larvae (16DAH), whereas the Intact diet stimulated growth after 36DAH. The Intact diet stimulated the larvae absorption capacity for 1.0kDa peptides at 16DAH, which may have contributed for enhanced growth in later stages. The PartH diet stimulated the transcription of tryp1c and pept1 at 28DAH, which seemed to reflect on increased post-larvae capacity to retain dietary 7.2kDa polypeptides. That may indicate a possible strategy to optimize the digestion and utilisation of the PartH dietary protein, though it did not reflect into increased growth. The Intact diet promoted the transcription of pepsinogenA, which may reflect a reduced gastrointestinal transit time, which could have enhanced the dietary nutrients assimilation, ultimately improving growth. The present results suggest that, whereas pre-metamorphic sole larvae utilize better dietary protein with a moderate degree of hydrolysis, post-metamorphic sole make a greater use of intact protein.