Transcriptional maintenance of cortical somatostatin interneuron subtype identity during migration

Cortical interneuron diversity arises from the interplay of intrinsic developmental patterning and local extrinsic cues. While individual genetic programs underlying cardinal cell type identity are established in immature neurons prior to integration into cortical circuits, it remains unclear whether distinct interneuron subtype identities are pre-established, and if so, how their identity is maintained prior to circuit integration. Sox6 is a transcription factor with an established role in the maturation of interneurons derived from the medial ganglionic eminence and cell-type specification in other neuronal and non-neuronal cells. To determine a possible role in maintaining cortical somatostatin-expressing (Sst+) interneuron subtype identity, we conditionally removed Sox6 (Sox6-cKO) in migrating Sst+ interneurons and assessed the effects on their mature identity. In adolescent animals, five of eight molecular Sst+ subtypes were nearly absent in the cortex of Sox6-cKO mice. This reduced subtype diversity was not due to a decrease in the overall number of Sst+ interneurons and cells displayed electrophysiological maturity and expressed genes enriched within the broad class of Sst+ interneurons. Furthermore, we show that at embryonic day 18.5, prior to cortical integration, mature Sst+ cell subtype identity could already be inferred in both control and Sox6-cKO cortices, suggesting that the loss in subtype diversity observed in the mature cortex is due to a disrupted subtype maintenance. Importantly, Sox6 removal at postnatal day 7, after Sst+ interneurons have finished migrating and begun integration into the network, did not disrupt marker expression of Sst+ subtypes in the mature cortex. Therefore, Sox6 is necessary during this migratory phase for maintenance of Sst+ subtypes identity, indicating that subtype maintenance requires active transcriptional programs during migration. Overall design: Single-cell RNA sequencing on cortical somatostatin interneurons (SstCre::fluorescent reporter) from Sox6 control and Sox6 conditional knock-out mice. Cells were collected at two different time points, 1) after tangential migration but before neuronal network integration from neocortex of pups at embryonic day 18.5 and 2) mature somatostatin interneurons from primary somatosensory cortex of mice at postnatal day 21-28. We have also included a dataset of cortical parvalbumin expressing interneurons from control mice at postnatal day 21-28.