There are inherent biological differences between males and females that contribute to sex differences in brain function and to many sex-specific illnesses and disorders. Traditionally, it has been thought that such differences are largely due to hormonal regulation; however, there are also genetic and epigenetic effects caused by the inheritance and unequal dosage of genes located on the X- and Y-chromosomes. Here we discuss the evidence in favor of a genetic and epigenetic basis for sexually dimorphic behavior, as a consequence of underlying differences in the regulation of genes that drive brain function. A better understanding of sex-specific molecular processes in the brain will provide further insight for the development of novel therapeutic approaches for the treatment of neuropsychiatric disorders characterized by gender/sex differences.

DNA modification is known to regulate experience-dependent gene expression. However, beyond cytosine methylation and its oxidated derivatives, very little is known about the functional importance of chemical modifications on other nucleobases in the brain. Here we report that in adult mice trained in fear extinction the DNA modification N6-methyl-2’-deoxyadenosine (m6dA) accumulates along promoters and coding sequences in activated prefrontal cortical neurons. The deposition of m6dA is associated with increased genome-wide occupancy of the mammalian m6dA methyltransferase, N6amt1, and this correlates with extinction-induced gene expression. The accumulation of m6dA is associated with transcriptional activation at the brain-derived neurotrophic factor (Bdnf) P4 promoter, which is required for Bdnf exon IV mRNA expression and for the extinction of conditioned fear. These results expand the scope of DNA modifications in the adult brain and highlight changes in m6dA as an epigenetic mechanism associated with activity-induced gene expression and the formation of fear extinction memory.

Gene editing with clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9 (CRISPR/Cas9) has shown promise in models of Duchenne muscular dystrophy (DMD); however, nonviral strategies to deliver CRISPR to muscle have not been widely explored or optimized. Most studies have relied on viral vectors, which are likely limited to single dosing due to their immunogenicity, thus reducing their therapeutic potential. Therefore, there is a need to develop nonviral approaches that allow for delivery and repeat dosing of CRISPR/Cas9 therapies to skeletal muscle. Here, biocompatible multi-arm polyrotaxane (PRX) nanocarriers, are iteratively optimized for packaging large plasmid DNA for delivery to muscle cells. The PRXs are optimized by addition of a disulfide-responsive linker that enhances plasmid release. Furthermore, conjugation of peptides leads to quicker uptake and improved transfection efficiency in humanized dystrophic muscle cells in vitro. Finally, in vitro delivery of PRXs complexed with a CRISPR/Cas9 platform demonstrates effective deletion of DMD exons 45-55, a therapeutic strategy with potential to restore the reading frame for half of DMD patients. This work represents the first PRX platform that is optimized and designed for delivery of large plasmid DNA, such as CRISPR/Cas9, to dystrophic muscle cells.