A Novel Circular RNA circSPEF2 Regulates Testis Development in Crucian Carp

Circular RNAs (circRNAs) exhibit significant sex- and development stage-specific expression patterns in the gonads of various fish species, yet their functions and regulatory mechanisms in male reproductive development remain largely unexplored in crucian carp (Carassius auratus). In this study, we characterized the expression features and biological functions of circSPEF2, a circular RNA derived from the reproduction-related gene spef2. Our results showed that circSPEF2 expression was markedly elevated in mature testes and progressively upregulated during gonadal maturation. Functional studies suggested that circSPEF2 likely does not act through a ceRNA-dependent mechanism. Transcriptome sequencing following circSPEF2 overexpression identified 45 upregulated and 70 downregulated differentially expressed genes, with GO and KEGG enrichment analyses revealing significant alterations in multiple gonadal development-related genes and signaling pathways. Subsequent siRNA-mediated knockdown of circSPEF2, combined with qRT-PCR validation, confirmed that circSPEF2 positively regulates the expression of genes associated with cell maturation and differentiation, including prdm1a, lamc2, and slc25a27, while concurrently suppressing that of proliferation- and apoptosis-related genes such as wnt8b, cpeb3, and bcl2l11. Furthermore, RNA pull-down combined with mass spectrometry identified three candidate circSPEF2-binding proteins, namely, hnRNP A/B, SRSF2, and CFAP263. Collectively, these findings indicate that circSPEF2 plays an important role in male gonadal development in fish and provide new insights into the post-transcriptional regulatory mechanisms underlying vertebrate male reproduction.