LncRNA THOR regulates spermatogenesis by interacting with and stabilizing Sycp3 protein in zebrafish.

THOR (testis-associated highly conserved oncogenic long non-coding RNA) is a highly conserved and testis-enriched lncRNA across vertebrates that plays diverse roles in various cancers. However, its physiological function and regulatory mechanism in testes remain largely unknown. Here, we investigated the genomic location and expression pattern of THOR in the model organism zebrafish, and generated a homozygous THOR knockout model using CRISPR-Cas9 technology. Loss of THOR in zebrafish impaired spermatogenesis, leading to oligospermia (38.7% reduction in sperm count), reduced sperm motility, sperm ultrastructural defects, and decreased fertilization rates. RNA-seq analysis of WT and THOR knockout testes revealed dysregulation of cell cycle-related genes, including cdkn1d, foxo1a, tsc1a, tsc2, atrx, and rad21b. RNA pulldown assays in zebrafish testes identified 486 potential THOR-interacting proteins primarily involved in ribosome biogenesis, RNA splicing, chromatin architecture, and meiotic progression. Notably, the core synaptonemal complex components Sycp1, Sycp2, and Sycp3 were all captured as THOR-binding partners. We further demonstrated that THOR directly interacts with Sycp3 and positively regulates its protein levels. Immunostaining assays on chromosome spreads revealed a significantly higher frequency of discontinuous Sycp3 signals in THOR-/- testes, suggesting the presence of meiosis defects caused by Sycp3 downregulation. Our findings expand the understanding of lncRNA-mediated control of spermatogenesis and male infertility by providing the first evidence that lncRNA THOR interacts with the synaptonemal complex to regulate meiosis progression.