LIGHT-INDUCIBLE PROXIMITY LABELLING <EM>IN VIVO</EM> CAPTURES SEX-SPECIFIC RNA AT EXCITATORY SYNAPSES

Local RNA regulation is essential for synaptic plasticity, yet the full repertoire of RNA species and associated isoforms within specific synaptic compartments in vivo have yet to be determined. Existing RNA profiling approaches lack the spatial and temporal precision needed to resolve RNA repertoires restricted to cell-type specific synaptic compartments. To overcome this challenge, we developed PSD-95–Halo-seq, a light-induced proximity labelling technique that, when combined with long-read sequencing, selectively captures all post-synaptic full length RNA species in the excitatory post-synaptic compartment. Here we applied this approach to investigate sex differences in RNA expression within excitatory synapses following exposure to an associative fear learning task in C57/Bl6 mice. We found dramatic sex differences in pseudogene and protein coding RNA expression, which are most abundant in females, with males exhibiting multiple noncoding RNA classes including lncRNA, snoRNA, and rRNA. Females generally showed more 3′ UTR expression, and there was widespread differential exon usage following fear conditioning, including 179 isoforms in males, 69 in females, with no significant gene-level differential expression, indicating that behavioral state modifies sex-specific isoform usage rather than overall transcript abundance. Our discovery that synaptic RNA composition is dynamic, sexually dimorphic, and profoundly shaped by experience, offers new insight into previously inaccessible mechanisms underlying sex differences in fear-related learning and memory. PSD-95-Halo-seq is therefore a powerful method for the precise spatiotemporal identification of compartment- and cell-type–specific RNA.