In vivo virus-free lineage tracing and live imaging reveal that NeuroD1 does not reprogram microglia into neurons.

In situ glia-to-neuron conversion induced by a single transcription factor is a promising regenerative strategy for treating central nervous system injuries and neurodegenerative diseases. Previous studies reported that NeuroD1 can induce microglia-to-neuron cross-lineage conversion and improve recovery after brain injury. However, these findings remain controversial. To examine the ability of NeuroD1 in inducing microglia-to-neuron conversion, we employed virus-free genetic lineage-tracing systems in mice to specifically induce NeuroD1 expression in microglia and track the cell fate of NeuroD1-expressing cells at multiple timepoints. Meanwhile, we longitudinally monitored NeuroD1-expressing microglia via two-photon imaging and characterized their transcriptional profile by single-cell RNA sequencing. All observations across these methods revealed that NeuroD1-expressing cells retained their microglia identity and cannot convert into neurons, no matter under physiological and injury conditions. Instead, sustained expression of NeuroD1 facilitated microglia apoptosis in vivo. Collectively, our findings provide strong evidence that NeuroD1 alone is not sufficient to induce microglia-to-neuron conversion. This study further highlights the importance of rigorous lineage-tracing and cell fate-mapping strategies for validating in situ glia-to-neuron conversion.