Dopamine neurons from astrocytes

Reprogramming of cells in the brain of a live mouse relieved the symptoms of Parkinson's disease

Polina Loseva, N+1

American and Chinese scientists have proposed another way to repair damage in Parkinson's disease: to turn astrocytes into neurons by reprogramming in vivo, that is, in a living brain. They found that it was enough to block the production of a single protein. At the same time, they managed to replenish the number of neurons in the brain of mice so much that the symptoms of motor disorders associated with degeneration disappeared in animals. The work was published in the journal Nature (Qian et al., Reversing a model of Parkinson's disease with in situ converted nigral neurons).

There is still no good medicine for Parkinson's disease that would stop neurodegeneration, and not just compensate for the symptoms of the disease. One way to solve this problem is to learn how to supply new neurons to the brain. But since there are few neural stem cells in humans and it is not easy to extract them, scientists are looking for ways to turn cells of some other types into them.

In 2017, we wrote about how with the help of four substances – three transcription factors and one microRNA – Swedish scientists learned how to turn astrocytes (auxiliary cells of nervous tissue) into real neurons. Now Hao Qian from the University of California, together with colleagues from China and the US have moved even further: they decided to reprogram the cells inside the brain of a live mouse (similar to how the mouse spleen was recently turned into a liver).

As raw materials for future neurons, Qian and colleagues, like their predecessors, chose astrocytes – because there are quite a lot of them in the brain, they actively divide, and they are relatively easy to turn into another type of cell. As a means for reprogramming, the scientists chose the RNA-binding protein PTB1. It is known that a decrease in its expression causes the expression of its neuronal variant nPTB1, and that, in turn, suppresses the work of genes responsible for the maturation of neurons.

After the expression of PTB1 was suppressed in mouse and human astrocytes, within a month 50-80 percent of the cells in the culture began to look like neurons. In reprogrammed cells, it was also possible to detect the expression of proteins characteristic of neurons and register action potentials – the main sign of functional fitness of neurons.

After making sure that the method works in vitro, the researchers moved on to in vivo experiments. They worked with transgenic mice that express cre recombinase in astrocytes. This makes it possible to target the virus carrying the PTB1 blocker directly on astrocytes. To make sure that the virus reaches its target, a red fluorescent protein gene was embedded in it. It turned out that after the virus was injected into mice into the black substance of the brain – one of the main targets of Parkinson's disease – glowing red cells, similar in shape to astrocytes, actually appeared in it. After three weeks, 20 percent of these cells had neuronal markers, and after ten weeks, 80 percent of the glowing cells became neurons.

After that, the authors repeated their experiments with other areas of the brain – the cerebral cortex and the striatum. They noticed that the efficiency of reprogramming astrocytes in different regions of the brain is about the same. But the result was different: the newfound neurons expressed different markers depending on where they appeared. This is probably due to the fact that astrocytes in different areas of the brain initially differed in gene expression, but it may also be a consequence of differences in the microenvironment. However, new neurons are embedded in existing brain structures. To test this, scientists injected glowing granules into the striatum, and a day later found them in the black substance – hence, neurons sprouted from one structure to another and were able to capture granules in one part of the brain and carry them to another.

Finally, the researchers tried to apply reprogramming to combat Parkinson's disease. Its symptoms are caused in mice by a toxic analog of dopamine, which causes the death of dopaminergic neurons. A month after the injection of the toxin, the number of neurons in the striatum decreased by 90 percent: from almost three thousand to about 266, according to the calculations of the author of the work. Then a reprogramming virus was injected into the mice's brain, and after 10-12 weeks it helped restore more than 600 cells. Thus, about 30 percent of the initial number of dopaminergic neurons remained at the output of the animals.

Reprogrammed astrocytes turned out to be active producers of dopamine: if after injection of the toxin its concentration decreased to about a quarter of the norm, then after therapy it rose again to 65 percent. In addition, reprogramming made it possible to restore motor functions. In this model of Parkinson's disease, mice were injected with the toxin only in one of the halves of the striatum, and the degeneration of neurons turned out to be one-sided: after that, the animals preferred to use their limbs only on one side to touch objects. However, after therapy, this selectivity disappeared, and the animals moved all their limbs evenly.

The brain of a mouse with "unilateral Parkinson's disease" before (top) and after (bottom) reprogramming. Dopaminergic neurons glow green. UC San Diego Health Sciences.

The authors of the work note that several important unresolved problems separate their methodology from human use. For example, it is necessary to decide what to do with other cells that accidentally turn out to be reprogrammed, and how to limit the number of astrocytes susceptible to therapy so that they are not in short supply. In addition, in older people, who mainly suffer from Parkinson's disease, astrocytes divide worse and are less plastic than in young people, so reprogramming may be less effective.

From the editor However, no one promises yet that such a remedy will help get rid of the disease for good.
In mice, you can only create an analogue of Parkinson's disease: neurodegeneration does not progress, because the cells die once. So it is unknown whether reprogramming will be able to stop the development of Parkinson's disease in humans, or it will need to be used as a permanent supportive therapy.

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