Long Non-Coding RNA 74687 Regulates Meiotic Progression and Gonadal Development in Rainbow Trout (Oncorhynchus mykiss) via the miR-15a-5p–ccne1 Regulatory Axis

High-throughput transcriptomic analyses have identified numerous candidate miRNA-mRNA and long non-coding RNA (lncRNA) regulatory networks in teleosts, but most remain without systematic functional validation or mechanistic definition. Here, by interrogating miRNA-lncRNA networks in rainbow trout (<i>Oncorhynchus mykiss</i>) gonads, we define their roles in meiotic progression and gonadal development. From preliminary screening, we identified <i>lncRNA74687</i> as a central node and characterised its function. Subcellular localisation showed predominant nuclear enrichment of <i>lncRNA74687</i> in gonadal cells. Dual-luciferase assays confirmed miR-15a-5p targeting of Cyclin E (<i>CCNE1</i>) and <i>lncRNA74687</i>. Functional studies showed that concurrent overexpression of <i>lncRNA74687</i> and inhibition of miR-15a-5p synergistically increased the CCNE1 protein to maximal levels. 5-ethynyl-2'-deoxyuridine (EdU) assays showed that knockdown of <i>lncRNA74687</i> and <i>CCNE1</i> in rainbow trout gonadal (RTG-2) cells reduced proliferation by 36.4% and 41.2%, respectively (<i>p</i> < 0.05). Immunofluorescence indicated that <i>lncRNA74687</i> increased Synaptonemal Complex Protein 1 (SYCP1) signalling 6.93-fold in gonadal cells via CCNE1. In vivo, <i>lncRNA74687</i> knockdown increased miR-15a-5p expression 6.34-fold relative to the wild-type controls (<i>p</i> < 0.01). Transcriptomic profiling revealed broad downregulation of meiosis-related genes in <i>lncRNA74687</i>-deficient gonads, with the strongest reduction in <i>mstrg1</i> expression, indicating a key role of <i>lncRNA74687</i> in germ-cell meiotic progression. Together, these data show that <i>lncRNA74687</i> enhances <i>CCNE1</i> mRNA and the CCNE1 protein in rainbow trout by competitively binding miR-15a-5p. This <i>lncRNA74687</i>-miR-15a-5p-<i>CCNE1</i> axis regulates gonadal cell proliferation and meiotic gene expression during gonadal development.