The central nervous system (CNS) is mainly composed of neurons and glial cells. Neurons perform the function of transmitting and integrating neural signals, while glial cells play an important role in support and nutrition. Unlike peripheral tissues and organs, neurons in the central nervous system of mammals can hardly be regenerated after adulthood. In neurodegenerative diseases, such as Alzheimer’s disease and Parkinson’s disease, a large number of neurons will die and the dead neurons cannot be regenerated, resulting in severe, irreversible brain damage. Unlike static neurons, glial cells can be regenerated to a certain degree.
The researchers proposed that by manipulating a single gene, glial cells can be induced to undergo reprogramming or conversion and finally be differentiated into neurons. Following this hypothesis, researchers used a type of regenerable cell (glial cells) to supplement the lost non-renewable cells (neurons) to achieve the in-situ regeneration of neurons and thereby treat neurodegenerative diseases. In 2002, the Max Planck Institute of Neurobiology in Germany reported that PAX6 can induce the reprogramming of astrocytes into neurons. Although astrocytes are regenerable, their regenerative capacity is weak. Therefore, researchers are continuing to explore whether they can induce other glial cells with stronger regenerative capacity that can be reprogrammed into neurons.
In 2018, the team of Professor Peng Bo of Fudan University cooperated with Professor Yuan Tifei, TCCI® investigator and researcher at Shanghai Mental Health Center and Dr. Rao Yanxia to discover that microglia are the fastest proliferating cell type in the brain which can quickly reproduce, recover and regain normal function after being eliminated. These findings shed new light on the treatment of brain diseases: Can we manipulate brain function by using microglia cells as the carriers to intervene in neuropsychiatric diseases? To achieve this, an urgent question that needs to be answered is the stability of the cell carrier function.
In 2019, a research team led by Kinichi Nakashima from Japan discovered that the heterologous expression of NeuroD1 through a lentivirus can induce the reprogramming of microglia into neurons. However, the feasibility and functionality of transdifferentiating glial cells is controversial. For example, there is the question of whether the NeuroD1-mediated reprogramming of astrocytes into neurons is an experimental artifact.
In December 2021, Dr. Rao Yanxia of the Shanghai Mental Health Center worked with the team of Professor Mao Ying, director of the Tianqiao and Chrissy Chen Institute for Translational Research and president of Huashan Hospital, the team of Professor Peng Bo of Fudan University, and Professor Yuan Tifei, TCCI® investigator and researcher at the Shanghai Mental Health Center in a joint research project published an article titled NeuroD1 induces microglial apoptosis and cannot induce microglia-to-neuron cross-lineage reprogramming in Neuron, a top journal in the field of neuroscience.
The researchers have used multiple methods such as live cell imaging, lineage tracking, and pharmacology to systematically explore the phenomenon of NeuroD1-mediated microglia-to-neuron reprogramming. They rigorously proved that NeuroD1 cannot transdifferentiate microglia into neurons.
With stringent verification, the investigators have responded to all sorts of doubts concerning this finding and proposed the following three principles for research on “microglia-to-neuron” reprogramming: 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 research results have provided important experimental evidence and theoretical support to the precise manipulation of microglia cells in intervening in neuropsychiatric diseases.
The team found that the NeuroD1-mediated microglia-to-neuron reprogramming reported by the Japanese team was not authentic, but an artifact caused by an invalid experimental design. Such experimental artifacts caused by non-specific leakage of viruses and other reasons are not rare in related research fields. In view of this, the most important point of this research is to the three basic principles to verify that glial cell-to-neuron transdifferentiation, based on endogenous nerve regeneration, can be scientifically proven.
Dr. Rao Yanxia from Shanghai Mental Health Center is the first author and co-corresponding author of the paper. Professor Peng Bo from the Institute of Transformation of Brain Science of Fudan University, Professor Mao Ying, director of the Translational Center of Tianqiao and Chrissy Chen Institute (TCCI®) and president of Huashan Hospital, and Professor Yuan Tifei, TCCI® researcher from Shanghai Mental Health Center are the co-corresponding authors of this article.