Converting Microglia to Neurons Has Therapeutic Potential Following Stroke
Post by Laura Maile
The takeaway
In adult mammals, most neurons cannot proliferate, which means that neuronal loss following stroke and other brain injuries is irreversible. Microglia, the immune cells of the brain, maintain their capacity to divide and can be converted into neurons in mice with stroke, leading to improved neurological function.
What's the science?
Neuronal loss is one of the major pathological effects of stroke that contributes to disability and poor health outcomes. The mammalian brain maintains limited ability for adult neurogenesis, adding to the negative effects of neuronal loss due to stroke and other brain injuries. The conversion of other cell types to neurons at the site of injury therefore presents a therapeutic opportunity that could improve functional recovery. Some researchers have had success in converting astrocytes to neurons, leading to functional improvement. In the most common cause of stroke, however, both astrocytes and neurons are depleted at the site of injury while microglia and macrophages infiltrate the injured area, making them a better target for conversion following ischemic stroke. This week in PNAS, Irie and colleagues converted microglia and macrophages into neurons in the striatum of stroke mice using a single transcription factor and measured their functional improvements over time.
How did they do it?
The authors induced ischemic stroke using transient middle cerebral artery occlusion (tMCAO), and performed immunohistochemistry to analyze the lesioned area and the cell types located throughout the injured tissue. A week following stroke, they injected a virus carrying the NeuroD1 (ND1) transcription factor driven by a microglia-specific promoter into the injured brain area to convert infiltrating microglia and macrophages into neurons. Next, to determine whether the ND1-converted neurons became functionally integrated into the striatum, they used immunohistochemistry to stain striatal projection neurons in control and tMCAO mice and patch-clamp electrophysiology to record neuronal activity from these cells. To quantify how many microglia/macrophages were effectively transformed into neurons, they utilized a transgenic mouse line expressing cre-inducible diphtheria toxin receptor (DTR). By injecting a Cre virus with a microglia-specific promoter, they could permanently express DTR in microglia, and then count the number of DTR-expressing cells that became neurons at different time points. The authors then tested whether the conversion of microglia into neurons had functional outcomes on stroke recovery, by comparing multiple motor behaviors in injured and control mice. Finally, they ablated the ND1-converted neurons to examine whether these cells were responsible for functional improvements.
What did they find?
They showed that following stroke, there is a significant neuronal loss in the striatum, and that while astrocytes remain at the border of the lesioned area, microglia and macrophages infiltrate into the core of the lesion. Two weeks following the injection of their virally packaged transcription factor, they showed a reduction in the microglial population and an increase in neuronal markers, which means that cells at the lesion site were reprogrammed from a microglial identity into a neuronal one. They also showed that when microglia were depleted prior to injection of ND1, they no longer observed the increase in neuronal cells at the lesion site, indicating it is likely mostly microglia that are successfully converted into neurons. Using immunohistochemistry, they demonstrated that striatal projection neurons, which are largely depleted in the lesioned area following tMCAO, show recovery following ND1 transduction. Their ND1-transduced cells became positive for a striatal neuron marker and showed functional activity that mimicked that of native striatal neurons.
They next found that one week after labeling microglia and initiating their conversion into neurons, very few labeled cells expressed neuronal markers, indicating they hadn’t yet converted to neurons at this early stage. At the eight week timepoint, however, a large number of the permanently labeled DTR cells also expressed striatal neuron markers and showed anatomically relevant connections with other neurons. This means that their strategy to convert microglia at the lesion site into neurons that would be integrated into the native circuitry was successful. Finally, they showed that following the conversion of microglia to neurons at the lesion site one week post-injury, mice showed improvements in multiple motor behaviors impacted by stroke. Damaging ND1-converted neurons blocked these improvements, suggesting that these newly converted neurons were responsible for the functional improvements observed.
What's the impact?
This study is the first to show successful in vivo conversion of microglia into functional neurons following ischemic stroke in mice. This treatment, which led to improved neurological function in injured mice, demonstrates a promising therapeutic strategy for stroke and other injuries resulting in the loss of neurons.
