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

Once dismissed as "junk", transposable elements (TE) have recently gained recognition for their regulatory roles, notably in the brain and in 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 others 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. It is unknown, however, if B2 SINE ncRNA is necessary for the proper development of hippocampal neurons, and furthermore, if there is a sex difference in this development. This work investigates the difference in 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 treated primary hippocampal neurons with dihydrotestosterone (DHT) to test if there is a difference in dendritic complexity. We used Sholl analysis to quantify the branching, number of tips, and Sholl mean. We found a sex difference in both B2 SINE, higher in males compared to females, and {beta}-actin, lower in males compared to females. Additionally, knocking down B2 SINE RNA results in a reduction of dendritic complexity in male but not in female neurons. Taken together, this work suggests that B2 SINE RNA is expressed differentially and 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 a transposable elements RNA has in the regulation of the development of neurons and the first to show that differential regulation by sex. Summary ParagraphLong considered "junk DNA", transposable elements have gained recognition as regulators in the genome. Some of these elements are transcriptionally active and have been known to control the expression of downstream protein coding genes. Recently, murine B2 SINE RNA has been shown to regulate glucocorticoid responsive genes in the rodent hippocampus. However, it is unknown if B2 SINE RNA is necessary for the proper development of hippocampal neuron structure and if there is a sex difference in this development. Here we show that B2 SINE is higher in the male hippocampus compared to females, and knocking down B2 SINE RNA results in a reduction of dendritic complexity in male but not in female neurons. This work shows that B2 SINE RNA is expressed differentially across sexes, and that this noncoding RNA plays an important role in the development of hippocampal neurons. These findings are the first to demonstrate the role for a transposable elements RNA in the development of neuronal structure and the first to show that this is differential across sex. These findings support the identification of a sex specific biomarker that could lead to individual treatments of neurological disorders influenced by sex.