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How the Brain Heals After a Stroke

stroke simulation

You have all the brain cells you’ll ever have when you reach adulthood. That was the science lesson I was taught in high school from, maybe a misguided teacher, or maybe just misinformed, I do not know. That statement however is not true, we know that the brain is very plastic and ever changing. It’s resilience still amazes us, even today with all that we know about it. Now a previously unknown mechanism through which the brain produces new nerve cells after a stroke has been discovered, showing us just how resilient the brain truly is.

A stroke is caused by a blood clot blocking a blood vessel in the brain, this then leads to an interruption of blood flow and therefore a shortage of oxygen. Many nerve cells die, resulting in motor, sensory and cognitive problems. Typically more present on one half of the body, stroke patents can, in some cases, regain most of their functionality and now we know how.

The researchers have shown that following an induced stroke in mice, support cells, so-called astrocytes, start to form nerve cells in the injured part of the brain. Using genetic methods to map the fate of the cells, the scientists could demonstrate that astrocytes in this area formed immature nerve cells, which then developed into mature nerve cells.

“This is the first time that astrocytes have been shown to have the capacity to start a process that leads to the generation of new nerve cells after a stroke”, says Zaal Kokaia, Professor of Experimental Medical Research at Lund University.

The scientists were also able to identify the signalling mechanism that regulates the conversion of the astrocytes to nerve cells. In a healthy brain, this signalling mechanism is active and inhibits the conversion — consequently, the astrocytes do not generate nerve cells. Following a stroke however, the signalling mechanism is suppressed and astrocytes can start the process of generating new cells.

“Interestingly, even when we blocked the signalling mechanism in mice not subjected to a stroke, the astrocytes formed new nerve cells”, says Zaal Kokaia.

“This indicates that it is not only a stroke that can activate the latent process in astrocytes. Therefore, the mechanism is a potentially useful target for the production of new nerve cells, when replacing dead cells following other brain diseases or damage.”

The new nerve cells were found to form specialized contacts with other cells. It remains to be shown whether the nerve cells are functional and to what extent they contribute to the spontaneous recovery that is observed in a majority of experimental animals and patients after a stroke.

A decade ago the first research was published to show that stroke leads to the formation of new nerve cells from the adult brain’s own neural stem cells. The new findings further underscore that when the adult brain suffers a major blow such as a stroke, it makes a strong effort to repair itself using a variety of mechanisms.

The major advancement with the new study is that it demonstrates for the first time that self-repair in the adult brain involves astrocytes entering a process by which they change their identity to nerve cells.

“One of the major tasks now is to explore whether astrocytes are also converted to neurons in the human brain following damage or disease. Interestingly, it is known that in the healthy human brain, new nerve cells are formed in the striatum. The new data raise the possibility that some of these nerve cells derive from local astrocytes,”says Olle Lindvall, Senior Professor of Neurology.

“If the new mechanism also operates in the human brain and can be potentiated, this could become of clinical importance not only for stroke patients, but also for replacing neurons which have died, thus restoring function in patients with other disorders such as Parkinson’s disease and Huntington’s disease”, he continued.

At the end of the day, like so much research into the human brain, we find that the new pieces of the puzzle can help with much more than just what we originally were looking at. This could technically have implications that we haven’t even thought of yet and really that is much more exciting than the discovery itself in my opinion.

Sources
Magnusson, J., Goritz, C., Tatarishvili, J., Dias, D., Smith, E., Lindvall, O., Kokaia, Z., & Frisen, J. (2014). A latent neurogenic program in astrocytes regulated by Notch signaling in the mouse Science, 346 (6206), 237-241 DOI: 10.1126/science.346.6206.237

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