Sex Differences in B2 SINE RNA Expression and Their Role in Hippocampal Development

Once dismissed as "junk", transposable elements (TEs) have recently gained recognition for their regulatory roles, notably in the brain and during development. The brain is hormone-responsive and the hippocampus in particular is sensitive to circulating gonadal hormones. While transcriptionally active, TE function remains poorly understood, especially in the brain. We and other researchers have shown that one particular TE RNA, B2 SINE ncRNA, is a regulator in the rodent hippocampus, especially after a psychologically stressful event like acute restraint stress. However, it is unknown if B2 SINE ncRNA is necessary for the proper development of hippocampal neurons, and, furthermore, if there are sex differences in this development. This work investigates the differences in the expression of B2 SINE RNA across sexes and its role in the development of primary hippocampal neurons. We utilized pooled locked nucleic acid (LNA) GapmeRs to knock down the expression of B2 SINE RNA, and we treated primary hippocampal neurons with dihydrotestosterone (DHT) to test if there is a difference in dendritic complexity. We used Sholl analysis to quantify branching, number of tips, and Sholl mean. We found a sex difference in both B2 SINE, higher in males compared to females, and ß-actin, lower in males compared to females. Additionally, knocking down B2 SINE RNA results in a reduction in dendritic complexity in male but not in female neurons. Taken together, this work suggests that B2 SINE RNA is expressed differentially and that it plays an important role in the proper development of hippocampal neurons in a sex-dependent manner. Our findings support the identification of a sex-specific biomarker that may enable individualized treatment of conditions influenced by sex. This is the first evidence of the role B2 SINE RNA may play in the regulation of the development of neuronal dendritic structure and the first to show differential regulation by sex.