Epigenetic Contributions to Different Developmental Trajectories of Male and Female Hippocampus

Abstract

Embryonic neurogenesis is recognized as a key process in brain formation and maturation. Discovery of ongoing neurogenesis in the adult brain was a transformative observation and has garnered much investigation. However, the nexus between embryonic and adult phases of neurogenesis have largely gone ignored. During the period immediately following birth, male rats generate more new cells than female littermates. This is also a time when hippocampal androgen or estrogen content does not differ between males and females, suggesting steroids do not mediate the observed sex difference. Instead, we hypothesized that sexually differentiated epigenetic regulation is responsible for the sex difference in cell proliferation. Administration of the DNMT inhibitor and demethylating agent, Zebularine (ZEB; 300ng ICV), or the histone deacetylase (HDAC) inhibitor, Trichostatin A (TSA; 0.5 mg/Kg, IP) on PN0 and PN1 exerted sex specific effects on proliferation within the dentate gyrus (DG) of the hippocampus, as assessed by BrdU quantification. ZEB treatment significantly reduced proliferation in males, but caused only a minimal decline in females. Males also had higher levels of global DNA methylation in the DG compared to females, likely as a result of limited demethylation established by reduced GADD45α expression. Conversely, administration of TSA increased cell genesis in females, but not males. HDAC activity was relatively higher in the DG of females. Expression of the pro-proliferative gene, BDNF, was up regulated in the male DG, by increased permissive methylation. No pro-proliferative target genes were found to be regulated by histone acetylation. Together this data suggests that sexually dimorphic epigenetic regulation mediates the difference in neonatal cell genesis within the DG, whereby, elevated methylation promotes proliferation in males and HDAC activity represses proliferation in females. A novel transgenic approach, adapted for the first time for use in the neonate, provided the means to blunt proliferation in the male DG and induce a feminized proliferative profile. Using this technique, greater proliferation in the male DG was found to mediate suppression of behaviorally inhibited responses. Identification of the functional significance of greater neonatal proliferation in the developing male DG provides insight into how hippocampally-dependent behaviors may develop differently in males and females.