Dynamic Functional Pathway Development in Type 1 Spinal Interneurons: Stage-specific roles of retinoic acid activity

The development of dI1 spinal interneurons is orchestrated by an interplay of transcriptional and signaling programs whose dynamics remains to be elucidated. By single-cell RNA sequencing of the E4 quail neural tube under both normal and retinoic acid (RA) activity-deprived conditions, we provide a high-resolution atlas delineating a continuous developmental trajectory from proliferative progenitors to differentiated neurons, each marked by distinct gene expression profiles and spatial positioning. Early stages are dominated by proliferation and cell cycle regulators, while differentiation involves waves of genes controlling cytoskeletal remodeling and migration. Signaling pathways, including BMP and RA, exhibit stage-specific activity: BMP signaling is prominent in early progenitors and antagonized as differentiation proceeds, with RA modulating both BMP and Wnt pathways in a context-dependent manner. RA exerts dual roles, promoting cell cycle progression in progenitors but restraining differentiation at later stages, and is essential for proper dI1 interneuron migration and localization. Loss of RA signaling disrupts ventral migration, leading to aberrant positioning and increased cell numbers, linked to altered expression of genes governing cell adhesion, cytoskeletal dynamics, and axon guidance. These findings illuminate the multifaceted, stage-dependent functions of RA in coordinating the specification, differentiation, and migration of dI1 interneurons, advancing our understanding of neural circuit formation in the developing spinal cord.