A decade ago, Zaal Kokaia and Olle Lindvall of the Lund Stem Cell Center in Sweden revealed that neural stem cells can respond to emergency traumas, such as a stroke, and differentiate into neural cells to replace those that have been damaged or killed. They have now contributed to another study which reveals another mechanism by which the brain attempts to repair itself and regain function following a stroke. The principal investigator on the study was Jonas Frisén of Lund University, and the paper was published in Science.
Strokes, or cerebrovascular accidents, affect 795,000 people per year in the United States. They occur when there is a blockage in the blood supply to the brain, resulting in death of brain cells and a loss of motor, neural, and sensory function. After a stroke, the brain can undergo neurogenesis and begin to grow new cells to replace those that have been lost.
The discovery was made when the team studied mice who experienced a stroke. Contrary to the previous study involving neural stem cells, they noticed that astrocytes--star shaped glial cells in the central nervous system--changed their identity and function and began to form nerve cells. Genetic mapping of the cells revealed this conversion.
"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," Kokaia said in a press release.
Of course, the body doesn't always have a need for astrocytes to develop into nerve cells. The team identified that Notch1 is the signaling mechanism that essentially tells the astrocytes that all is well, and there's no need to change. However, following an emergency like a stroke, the Notch1 signal becomes suppressed. Astrocytes then respond to the situation by creating the immature nerve cells that will eventually mature.
"Interestingly, even when we blocked the signaling mechanism in mice not subjected to a stroke, the astrocytes formed new nerve cells," Kokaia continued. "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."
What the researchers aren't sure of quite yet is how functional these astrocytes-turned-nerve-cells actually are and how significant their contribution is to healing the brain after experiencing a trauma such as a stroke. They did find that these cells do connect with other cells, but further research will be required to fully understand the scope of the astrocytes' influence.
"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," concluded Lindvall. "The new data raise the possibility that some of these nerve cells come from local astrocytes. 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 that have died, thus restoring function in patients with other disorders such as Parkinson's disease and Huntington's disease."
