Making New Neurons When You Need Them

Western societies are aging societies, and the incidence of dementias, Alzheimer’s disease, and other diseases of the aged are on the rise. Treatments for these conditions are largely supportive, but being able to make new neurons to replace the ones that have died is almost certainly where it’s at.

At INSERM and CEA in Marseille, France, researchers have shown that chemicals that block the activity of a growth factor called TGF-beta improves the generation of new neurons in aged mice. These findings have spurred new investigations into compounds that can enable new neuron production in order to mitigate the symptoms of neurodegenerative diseases. Such treatments could also restore the cognitive abilities of those who have suffered neuron loss as a result of radiation therapy or a stroke.

The brain forms new neurons regularly to maintain our cognitive abilities, but aging or radiation therapy to treat tumors can greatly perturb this function. Radiation therapy is the adjunctive therapy of choice for brain tumors in children and adults.

Various studies suggest that the reduction in our cache of neurons contributes to cognitive decline. For example, exposure of mice to 15 Grays of radiation is accompanied by disruption to the olfactory memory and reduction in neuron production. A similar event occurs as a result of aging, but in human patients undergoing radiation treatment, cognitive decline is accelerated and seems to result from the death of neurons.

How then, can we preserve the cache of neurons in our brains? The first step is to determine the factors responsible for the decline is neuron production. In contrast to contemporary theory, neither heavy doses of radiation nor aging causes completely destruction of the neural stem cells that can replenish neurons. Even after doses of radiation and aging, neuron stem cell activity remains highly localized in the subventricular zone (a paired brain structure located in the outer walls of the lateral ventricles), but they do not work properly.

Subventricular Zone
Subventricular Zone

Experiments at the INSERM and CEA strongly suggest that in response to aging and high doses of radiation, the brain makes high levels of a signaling molecule called TGF-beta, and this signaling molecule pushes neural stem cell populations into dormancy. This dormancy also increases the susceptibility of neural stem cells into apoptosis.

Marc-Andre Mouthon, one of the main authors of this research, explained his results in this manner: “Our study concluded that although neurogenesis is reduced in aging and after a high dose of radiation, many stem cells survive for several months, retaining their ‘stem’ characteristics.”

Part two of this project showed that blocking TGFbeta with drugs restored the production of new neurons in aging or irradiated mice.

Thus targeted therapies that block TGFbeta in the brains of older patients or cancer patients who have undergone high dose radiation for a brain tumor might reduce the impact of brain lesions caused by such events in elderly patients who show distinct signs of cognitive decline.