A novel microRNA shapes development of cortical projection neurons

Do you or anyone you know experience physical paralysis? Paralysis can be caused by a number of different reasons, but the most common reason is related to damage of the neurons that control movement. In humans, these neurons are called corticospinal motor neurons, and they are found in the motor cortex of the brain. Doctors and scientists are currently working towards treating paralysis by turning stem cells into motor neurons. By studying how corticospinal motor neurons naturally develop in an animal’s brain, scientists can mimic this developmental process in the lab to turn stem cells into this specific type of neuron to replace the damaged ones. This paper contributes to learning more about corticospinal motor neuron development by identifying a novel microRNA that promotes the development of corticospinal motor neurons early in development.

Messenger RNA is the intermediate molecule between DNA and proteins – it provides the sequence used to determine the amino acids that make up a gene’s protein product. MicroRNA (miRNA) are small pieces of RNA that can block the expression of genes to proteins by binding and degrading their specific messenger RNA. Manipulating miRNAs is a promising strategy for increasing or decreasing specific protein levels to drive the development of a cell type. To find a candidate miRNA that can promote the development of mouse corticospinal motor neurons, they compared the miRNA expression profiles of corticospinal motor neurons with a different type of neuron that arises from the same progenitor—callosal projection neurons. The time window of production of callosal projection neurons overlap significantly with corticospinal motor neurons, yet these two categories of cortical projection neurons serve very different functions and project their axons to different parts of the central nervous system (Figure 1), so the authors hypothesized that a difference in miRNA expression may explain why two different types of neurons can be produced at the same time from the same progenitor. They found 19 miRNAs that are more abundant in corticospinal motor neurons compared to callosal projection neurons. From that list of miRNAs, miR-409-3p stood out for its predicted potential to repress LMO4, a transcription factor that promotes callosal projection neuron development. Using immunohistochemistry, the authors found that embryonic cortical progenitor cells that were transfected with miR-409-3p showed less LMO4-positive cells than embryonic cortical progenitors that were transfected with a control miRNA. Similarly, the authors also found that embryonic cortical progenitor cells that were transfected with miR-409-3p were more characteristic of corticospinal motor neurons in their protein expression than they were of callosal projection neurons. This suggested that miR-409-3p is able to promote corticospinal motor neuron development through the repression of LMO4 (Figure 2).

Figure 1. Corticospinal motor neurons and callosal projection neurons arise from the same progenitor but project their axons to different locations. Peak production of corticospinal motor neurons occurs on embryonic day 13.5. Peak production of callosal projection neurons occurs on embryonic day 15.5, but the production of callosal projection neurons overlaps significantly with corticospinal motor neurons on E13.5. After birth, by postnatal day 4, callosal projection neurons create the corpus callosum by projecting their axons to the opposite hemisphere and corticospinal motor neurons create the corticospinal tract by projecting their axons towards the spinal cord. E = embryonic day, P = postnatal day, CSMN = corticospinal motor neurons, CPN = callosal projection neurons.

Figure 2. miR-409-3p promotes corticospinal motor neurons by repressing LMO4. LMO4 is a transcription factor that promotes callosal projection neuron development. miR-409-3p is known to be highly expressed in corticospinal motor neurons relative to callosal projection neurons by postnatal day 1. The overexpression of miR-409-3p decreases LMO4 expression and promotes corticospinal motor neuron expression. IPC = intermediate progenitor cells, RGC = radial glial cells, CSMN = corticospinal motor neurons, CPN = callosal projection neurons.

The first few experiments that the authors performed were conducted on cortical progenitor cells removed from brains of developing mice. To determine whether their results would replicate within mouse brain, the authors used in utero electroporation to conduct their final experiment. In utero electroporation enables the insertion of miRNA into embryonic stem cells of the fetal brain so that researchers can determine its effect on neuronal development. To determine whether miR-409-3p could promote corticospinal motor neuron development in a living mouse, the authors conducted in utero electroporation during peak production of corticospinal motor neurons and were able to significantly increase the number of corticospinal motor neurons that were produced. This experiment shows that miR-409-3p is a powerful enough regulator of the genome to double the number of corticospinal motor neurons that get produced during development.

In summary, the authors identified a miRNA, miR-409-3p, that plays an important role in the development of corticospinal motor neurons. According to the results of the study, miR-409-3p promotes corticospinal motor neuron development by repressing LMO4, a transcription factor that promotes callosal projection neuron development. This discovery of miR-409-3p’s role in development suggests that miRNAs can be used to increase the abundance of corticospinal motor neurons from cortical progenitor cells, and that ability will help researcherspotentially leading to therapies that turn stem cells into corticospinal motor neurons and restore motor function in patients experiencing paralysis.

Edited by Tamara Chan.

References

Diaz, Jessica L., et al. "An evolutionarily acquired microRNA shapes development of mammalian cortical projections." Proceedings of the National Academy of Sciences 117.46 (2020): 29113-29122.