Capicua Refines Mossy Fiber-CA3 Axon Targeting in the Late Postnatal Hippocampus.

Proper brain wiring relies on the precise distribution of axonal projections to specific subcellular domains of their target neurons. These spatially confined connections establish the anatomical foundation for neural circuit assembly. The mossy fiber (MF)-CA3 pathway in the hippocampus is an excellent system to study the mechanisms underlying lamina-specific connectivity. In rodents, MF projections develop postnatally and reach their mature configuration by the end of the second postnatal week. MF axons synapse on the proximal segments of the dendrites but avoid the somas of CA3 pyramidal neurons. As dentate gyrus granule neurons are continuously generated and integrated into the existing hippocampal circuit throughout the postnatal period and adulthood, the mechanisms that guide MF axons to achieve lamina-specific targeting of these later-born granule neurons remain unclear. Here, we show that deletion of the neurodevelopmental disorder-associated protein capicua (CIC) results in abnormal MF targeting in the mouse hippocampus. Notably, this defect emerges after the second postnatal week and persists into adulthood, distinguishing it from classical MF guidance defects, which typically manifest during the first postnatal week. We also demonstrate that this miswiring is due to CIC loss in dentate gyrus granule neurons rather than CA3 pyramidal neurons. Single-nucleus transcriptomics and trajectory analysis reveal a loss of a mature granule neuron subtype and dysregulation of axon guidance genes that are normally downregulated as granule neurons mature. Our findings uncover a previously unrecognized role for CIC in hippocampus development and offer insights into the regulation of lamina-specific MF connectivity in the postnatal brain.