Hippocampus single-nucleus transcriptomics reveals coordinated regulation of social and spatial representation development by perinatal SERT expression in CA3 pyramidal neurons

The hippocampal formation (HPF) provides neural substrates integrating disparate sensory cues into episodic memories and coherent action. Whereas HPF structures are formed by birth, the functional circuits evolve over postnatal development. Our previous studies showed that transient perinatal expression of the serotonin (5-HT) transporter SERT/Slc6a4 in CA3 pyramidal neurons, which do not synthesize 5-HT but take up extracellular 5-HT thus termed "5-HT-absorbing neurons", exerts sex-biased effects on long-term activity-dependent HPF synaptic plasticity and behavior in mice. This study investigates SERT impact on circuit development, through single-nucleus transcriptomics of postnatal HPF from CA3-pyramidal neuron SERT knockout (SERTPyramid{Delta}) mice. We demonstrate that SERTPyramid{Delta} mice preserve cell identities across the HPF but alter gene expression in specific neuronal types in a sex-biased manner. We observed SERTPyramid{Delta} male-biased upregulation of genes preferentially in glutamatergic neurons, particularly affecting the CA2 and parasubiculum (PaS) when they develop social novelty and spatial representations, respectively. In both the CA2 and PaS, altered genes center on two categories -- modulators of gene expression patterning including chromatin plasticity, RNA processing and ubiquitin-dependent protein degradation, and aspects of synaptic transmission. >20% of the dysregulated genes in the CA2 and PaS are associated with Autism and engaged in cell-type distinct functional networks, showing CA3 SERT regulation of ASD-vulnerable genes in intersecting biological processes in specific neurons during social and spatial circuits development. The data, available at https://scviewer.shinyapps.io/hippocampus_sertKO, provide an entry map for further deducing anatomical neuronal origin and the molecular and cellular pathways impaired by 5-HT dysfunction during HPF circuits development leading to lifetime cognitive deficits.