Neurons derived from NeuroD1-expressing astrocytes transition through transit-amplifying intermediates but lack functional maturity

In vivo conversion of nonneuronal cells into neurons is a proposed strategy to replace neurons lost to CNS injury or disease. Glia-to-neuron trans-differentiation by viral vector–mediated GFAP mini-promoter–driven NeuroD1 remains hotly debated. Developing inducible, lineage-traceable transgenic mice, we find that astrocyte-to-neuron conversion is restricted to a specific time window within the lesion core of injured spinal cord and brain. Spatiotemporal lineage-mapping combined with single-cell transcriptomics reveals that ectopic NeuroD1 induces astrocyte-to-neuron conversion specifically in lesion cores via transit-amplifying OLIG2⁺ progenitors during early injury phase, but not in late phases or in nonreactive astrocytes. Neither a loss-of-function NeuroD1 mutant nor stemness-reprogramming factor SOX2 induces astrocyte-to-neuron conversion. However, contrary to previous reports, the neuronal-like cells generated by NeuroD1 lack mature neuroelectrical properties, limiting their functional integration into neural circuits. Together, our findings establish a spatiotemporal framework for NeuroD1-driven glia-to-neuron conversion, revealing a mechanistic shift from direct astrocyte conversion toward transit-amplifying intermediates and highlighting the functional immaturity of NeuroD1-converted neurons.