45. MicroRNA Treatment Delivery Methods and Short-Term Effects on Growth in Juvenile Rainbow Trout (Oncorhynchus mykiss): A Pilot Study

Abstract MicroRNAs (miRNAs) are vital post-transcriptional regulators of muscle growth and development. In mammalian livestock, muscle fiber number is typically fixed at or shortly after birth, and postnatally, growth mainly occurs via muscle fiber hypertrophy. In contrast, fish maintain the capacity for muscle hyperplasia well beyond juvenile stages, offering a unique opportunity to explore strategies that may alter muscle fiber number. MiR-133a-3p has been associated with myogenic proliferation and may support hyperplastic growth in teleosts. This study aims to evaluate the short-term persistence and effects of miR-133a-3p supplementation in juvenile rainbow trout (Oncorhynchus mykiss). Forty trout (n = 40; 10 per treatment group) were randomly allocated across four recirculating raceways (10 fish per raceway), maintained at an average of 15 °C in a 2 × 2 factorial design. Fish individually received either phosphate-buffered saline (PBS, CON) or a synthetic miR-133a-3p AgomiR (Creative Biogene, TRT), delivered by immersion (IMM) or intraperitoneal (ITP) injection. Following supplementation, trout were reared for 14 days and reared under standard conditions. At the conclusion of the study, growth performance measurements (whole body weight, length, empty carcass weight, left filet weight) were collected. Red + pink and white skeletal muscle were collected for RNA extraction and RT-qPCR analysis of miR-133a-3p expression and target gene expression. Results display a trend (p < 0.10) toward differential miR-133a-3p expression among treatment groups. Growth parameters in supplemented trout were not statistically different (p > 0.05) from controls, but treated fish exhibited consistently higher numerical values for body weight, length, carcass yield, and filet weight compared to controls. This pilot study illuminates the feasibility of miR-133a-3p delivery by immersion and injection and provides valuable preliminary evidence that supplementation may support positive growth-related traits. Although not statistically significant, the consistent positive growth trends in miRNA-treated fish compared to controls suggest that future work using larger sample sizes, longer study periods, additional treatment timepoints, or direct histological measurements of muscle fiber hyperplasia may provide further insight into the potential growth benefits of miRNA treatments for rainbow trout.