Abundance of miRNA during muscle growth is not influenced by dietary protein inclusion levels in neonatal pigs
Authors: Kroscher Kellie A. , Fausnacht Dane W. , Murray Robert L. , Zhang Wei , Stahl Chad H. , Rhoads Robert P. 
Published: April 2020

Postnatal muscle growth requires satellite cell (SC) activation and fusion with existing myofibers. Delay or absence of SC proliferation and differentiation leads to a reduction in muscle hypertrophy. Nutrition during neonatal life impacts developmental growth programing by altering SC activity. Growth as a percentage of body weight is highest during the neonatal period and restricting neonatal nutrition can lead to permanent muscle growth deficits. Reducing dietary intake of protein can improve gut function during weaning, however, this may limit amino acid availability for muscle growth. The feed‐induced protein synthesis response in muscle is highest in the neonatal period and subsequently progressively declines throughout life. The heightened responsiveness of protein synthesis during this period is mediated mainly by the activation of translation initiation factors. Post‐transcriptional regulation of RNA is mediated in part by micro‐RNAs (miRNA) by preventing translation and/or promoting degradation and are known to be important modulators of stem cell activity. Proliferation of SCs is promoted by miR‐133a, while differentiation is promoted by miR‐1, miR‐133b and miR‐206 through repression of Pax7. The objective of this study was to determine miRNA abundance during neonatal muscle growth in response to a protein deficient, adequate, and excess diets in pigs. Twenty‐eight piglets obtained at 1d of age received isocaloric dietary treatments of either an adequate protein diet (A), a 25% deficient protein diet (D), or a 25% excess protein diet (E) over a 17‐day trial. Piglet starting weights were 1.47kg±0.1 kg for D and 1.46±0.1 kg for E and A treatments. Final body weight and ADG were increased in E compared to D pigs (5.60±0.27kg vs 4.75±0.27kg and 0.27±0.01kg/d vs 0.21±0.01kg/d) despite an equal feed intake across treatments. Gene expression in the longissimus dorsi was evaluated by qPCR. There were no differences IGF1, or in myogenic genes MyoD and MyoG despite the increased growth rates of E treatment pigs. The SC marker Pax‐7 did not differ between treatments. Genes associated with lipid oxidation, PPARd and CPT1, were not different between treatments. There were no differences in degradation pathway genes muRF‐1, atrogin‐1, and FOXO despite the reduction in growth of D pigs. The differences in growth response without alterations in gene expression may be due to post‐transcriptional regulation, however, no differences were observed in miRNA expression. The differentiation promoting miRs‐1, 133b and 206 and proliferation promoting miR‐133a were not different between treatments. These data indicate that diets excess in protein increased growth rates which may lead a sustained increase in muscle mass throughout life. While miRNA expression was not different between treatments, miRNAs may still play a role in the differences observed in growth between D and E treatments through their activity and rate of binding to their target mRNAs.