The Development of Kisspeptin Circuits in the Mammalian Brain - Kisspeptin Signaling in Reproductive Biology - page 225

postnatal period in mice blocks the sexual differentiation of the size of the BNST
and also alters sexually dimorphic vasopressin fi ber projections [ 73 , 88 ]. Postnatal
HDAC inhibition also alters the sexual differentiation of male sexual behavior in
rats [ 87 ]. It is likely that these postnatal alterations in HDAC activity directly or
indirectly affected apoptosis in the BNST, as sex differences in this region are
known to be governed specifi cally by Bax -dependent apoptosis [ 77 ]. Other sexual
differentiation studies have addressed the role of DNA methylation, which occurs at
CpG sites or on CpG islands in a gene or its promoter, and which is generally asso-
ciated with the repression of gene expression. Studies investigating the involvement
of DNA methylation in neural sex differences have found that DNA methylation
levels in the hypothalamus correlate with sexually differentiated expression of sex
steroid receptor genes in this region [ 85 , 86 ]. In addition, the expression of DNA
methyl transferase 3a (DNMT3a) in newborn rats was found to be sexually dimor-
phic in the amygdala, which is a known sexually dimorphic brain region [ 90 ].
With all the recent evidence implicating the involvement of epigenetics in the
sexual differentiation of various brain parameters, we investigated whether either
histone deacetylation and/or DNA methylation contributes to the AVPV/PeN Kiss1
sex difference [ 83 ]. We pharmacologically blocked histone deacetylation during the
postnatal period by administering an HDAC inhibitor, valproic acid (VPA), or vehi-
cle to mice on PND 1 and PND 2 and then analyzed AVPV/PeN Kiss1 expression in
adulthood. This postnatal HDAC inhibitor treatment signifi cantly increased the
number of detectable Kiss1 cells in the adult AVPV in each sex. However, the sex
difference in Kiss1 expression was not eliminated, indicating that histone acetyla-
tion is not a key process for inducing the Kiss1 sex difference [ 83 ]. Although the sex
difference was still robust, the fact that overall Kiss1 levels were higher in mice
treated with HDAC inhibitor suggests that the level of histone H3 acetylation during
the critical period may be involved in modulating the development of Kiss1 neurons
in the AVPV.
Interestingly, we found signifi cant sex differences in the CpG methylation status
of the AVPV/PeN Kiss1 gene, predominantly in the putative promoter region
(Fig. 11.5 ). In all cases, these sexually dimorphic Kiss1 CpG sites were more meth-
ylated in females than males. Methylation of CpG sites can have multiple modes of
affecting gene activity. We tested if methyl-CpG binding protein-2 (MeCP2) was
involved in the sex difference by assessing AVPV/PeN Kiss1 levels in male and
female Mecp2 mutant mice. The AVPV/PeN sex difference was not eliminated in
Mecp2 mutant mice, suggesting that if DNA methylation infl uences the AVPV/PeN
Kiss1 sex difference, it likely does so via non-Mecp2 mechanisms, possibly by
blocking the binding of transcriptional repressors [ 83 ]. Although these experiments
increase our knowledge about the involvement (or lack of involvement) of several
epigenetic processes in the development of the AVPV/PeN Kiss1 sex difference,
more work is needed to elucidate the exact extent that certain processes, like DNA
methylation, are involved. In fact, the AVPV/PeN Kiss1 sex difference may be
induced by several epigenetic processes affected by the postnatal sex steroid milieu,
causing a silencing of the Kiss1 gene in males while simultaneously allowing
increased transcriptional activity in females.