Selective enrichment of TCF4 in GABAergic neurons during postnatal primate development

The transcription factor 4 (TCF4) gene is essential for brain development, and its disruption causes Pitt-Hopkins syndrome. Common genetic variants in TCF4 also confer risk for schizophrenia and related psychiatric disorders. While the developmental roles of TCF4 are well established, its postnatal functions remain poorly defined, particularly during the prolonged maturation of the primate neocortex, when many neuropsychiatric symptoms first emerge. Here, we mapped cell-type-specific TCF4 expression across postnatal development in rhesus macaque neocortex using immunofluorescence, and in situ hybridization. We also re-analyzed public bulk-tissue and single-nucleus transcriptomic datasets spanning 16 brain regions in both humans and macaques. TCF4 was predominantly neuronal, with minimal expression in microglia, oligodendrocytes, and astrocytes. In late gestation, TCF4 was broadly expressed in excitatory and inhibitory neurons, but expression declined with maturation and became selectively enriched in inhibitory populations. By young adulthood, nuclear TCF4 levels were highest in somatostatin-positive and vasoactive intestinal peptide-positive interneurons, intermediate in parvalbumin interneurons (~6-fold above non-GABAergic cells), and lowest in cholecystokinin interneurons (~4-fold above non-GABAergic cells). This interneuron-subtype hierarchy was independently reproduced by single-nucleus RNA sequencing across all neocortical regions. Together, these findings position mature GABAergic interneurons as the principal site of TCF4 function in the primate neocortex, providing a cellular framework for linking TCF4 dysfunction to cortical circuit imbalance in Pitt-Hopkins syndrome and psychiatric disease.