In recent years, researchers have produced what looked like promising results in reprogramming supporting cells in the brain into neurons via neuroD1 gene therapy. A way to do this, to produce new neurons that can integrate into existing neural circuits, would provide a road to regeneration of the brain. Unfortunately, and as sometimes happens, this may all be a dead end, and the early promise was based on misinterpretation of the data. This will likely be hashed out further in the next few years; science often proceeds in this way, and this is one of the many reasons as to why independent replication is vital to scientific progress.
In 2019, researchers in Japan published breakthrough results detailing how NeuroD1, a protein involved in cell differentiation, could coax microglia into new neurons. Now, researchers in China have found that not only does NeuroD1 not induce microglia-to-neuron conversion, but also that the protein induces microglia death. The team set out to investigate the molecular mechanisms underpinning the original finding, since microglia and neurons descend from different cellular lineages.
The researchers applied a rigid lineage tracing protocol to follow the cellular differentiation progression in mice, as well as to monitor the effect of lentiviral vectors - an inert virus package used to carry NeuroD1 to the central nervous system - on the process. They validated their observations through live cell imaging and pharmacological approaches. "Disappointingly, our results do not support the 'microglia-to-neuron conversion. Instead, our data strongly indicate that the previously observed conversion was actually due to the experimental artifacts from viral leakage."
The assumed finding was likely due to NeuroD1's actual role: triggering microglial cell death. Neurons are unaffected by NeuroD1 so their numbers will stay the same, while microglia cell numbers decrease. However, due to the low purity of the microglia and the viral leakage, it could appear that while microglia cells were decreasing, non-microglia cells were increasing, leading to the conclusion in vitro that microglia were converting to neurons.
"The 'microglia-to-neuron' conversion should be verified following three principles: 1) unambiguous microglial-based lineage tracing and lack of lentiviral leakage, along with well-designed controls; 2) unambiguous live cell imaging to show how an individual microglial cell converts to a neuron; and 3) upon microglial depletion, there should be no or few microglia-converted neurons." The last point appeared to be supported in the original paper, but when researchers replicated the experiment, they found that even when 98.9% of microglia cells were killed, numerous "microglia-converted neurons" were still observed. Such a finding suggests that the converted neurons were mislabeled cells rather than the desired neurons.