Sex differences in microRNA expression during developmentin rat cortex

There are important sex differences in the risk and outcome of conditions and diseases between males and females. For example, stroke occurs with greater frequency in men than in women across diverse ethnic backgrounds and nationalities. Work from our lab and others have revealed a sex-specific sensitivity to cerebral ischemia whereby males exhibit a larger extent of brain damage following an ischemic event compared to females. Studies suggest that the difference in male and female susceptibility to ischemia may be triggered by innate variations in gene regulation and protein expression between the sexes that are independent of post-natal exposure to sex hormones. We have shown that there are differences in microRNA (miRNA) expression in adult male and female brain following focal cerebral ischemia in mouse cortex. Herein we examine a role for differential expression of miRNAs during development in male and female rat cortex as potential effectors of the phenotype that leads to sex differences to ischemia. Expression studies in male and female cortices isolated from postnatal day 0 (P0), postnatal day 7 (P7), and adult rats using TaqMan Low Density miRNA arrays and NanoString nCounter analysis revealed differential miRNA levels between males and females at each developmental stage. We focused on the miR-200 family of miRNAs that showed higher levels in females at P0, but higher levels in males at P7 that persisted into adulthood, and validated the expression of miR-200a, miR-200b, and miR-429 by individual qRT-PCR as these are clustered on chromosome 5 and may be transcriptionally co-regulated. Prediction analysis of the miR-200 miRNAs revealed that genes within the Gonadotropin releasing hormone receptor pathway are the most heavily targeted. These studies support that developmental changes in miRNA expression may influence phenotypes in adult brain that underlie sexually dimorphic responses to disease, including ischemia.