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- neurons conversion, we employed virus-free genetic lineage tracing systems 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 physical and injury conditions. Instead, sustained expression of NeuroD1 facilitated microglia apoptosis in vivo . Collectively, our findings provide strong evidence that NeuroD1 alone is sufficient to induce microglia-to neuron conversion. This study further highlighted the importance of rigorous lineage-tracing and cell fate mapping strategies for validating in situ glia-to-neuron conversion.