Enhancing NeuroD1 Expression to Convert Lineage-Traced Astrocytes into Neurons

Regenerating functional new neurons in adult mammalian brains has been proven a difficult task for decades. Recent advancement in direct glia-to-neuron conversion in vivo opens a new field for neural regeneration and repair. However, this emerging new field is facing serious challenges from misuse of viral vectors to misinterpretation of conversion data. Here, we employ a variety of AAV vectors with different promoters and enhancers to demonstrate that astrocytes can be converted into neurons in a NeuroD1 dose-dependent manner in both wildtype (WT) and transgenic mice. Notably, astrocytes in WT mice were relatively easy to convert with higher conversion efficiency, whereas lineage-traced astrocytes in Aldh1l1-CreERT2 mice showed high resistance to reprogramming but were still converted into neurons after enhancing NeuroD1 expression with CMV enhancer. Furthermore, under two-photon microscope, we observed direct astrocyte-to-neuron conversion within 3 weeks of serial live imaging in the mouse cortex. We also demonstrated that high titre AAV reaching 1013 GC/ml caused severe neuronal leakage using a variety of AAV GFAP::GFP vectors, highlighting the necessity to inject low titre AAV into healthy brains to avoid artifactual results. Together, our studies suggest that lineage-traced astrocytes can be converted into neurons but require stronger conversion force such as enhanced NeuroD1 expression. Failure to recognize the difference between WT astrocytes and lineage-traced astrocytes in terms of conversion barrier will lead to misinterpretation of data.