Specialized components of the immune system present in the brain, such as microglia, are integral to many of the processes involved in or degraded by neurodegenerative conditions. For example, microglia may be a primary cause of the chronic inflammation found in older brain tissue, and which contributes to the pathology of a range of conditions, including Alzheimer's disease. The study noted here focuses on a different aspect of the role of microglia, a way in which they participate in the normal operation of the brain in conjunction with neurons: the researchers involved show that microglia play a necessary role in altering the connections between neurons. This will no doubt be of interest to the field of aging research, as the plasticity of neural connections diminishes with age, and it will be interesting and potentially useful to know the degree to which this is a problem of neurons versus a problem of the immune system.
A new study shows that cells normally associated with protecting the brain from infection and injury also play an important role in rewiring the connections between nerve cells. While this discovery sheds new light on the mechanics of neuroplasticity, it could also help explain diseases like dementia, which may arise when this process breaks down and connections between brain cells are not formed or removed correctly. "We have long considered the reorganization of the brain's network of connections as solely the domain of neurons. These findings show that a precisely choreographed interaction between multiple cells types is necessary to carry out the formation and destruction of connections that allow proper signaling in the brain."
The study is another example of a dramatic shift in scientists' understanding of the role that the immune system, specifically cells called microglia, plays in maintaining brain function. Microglia have been long understood to be the sentinels of the central nervous system, patrolling the brain and spinal cord and springing into action to stamp out infections or gobble up dead cell tissue. However, scientists are now beginning to appreciate that, in addition to serving as the brain's first line of defense, these cells also have a nurturing side, particularly as it relates to the connections between neurons. The formation and removal of the physical connections between neurons is a critical part of maintaining a healthy brain and the process of creating new pathways and networks among brain cells enables us to absorb, learn, and memorize new information.
While this constant reorganization of neural networks - called neuroplasticity - has been well understood for some time, the basic mechanisms by which connections between brain cells are made and broken has eluded scientists. Performing experiments in mice, the researchers employed a well-established model of measuring neuroplasticity by observing how cells reorganize their connections when visual information received by the brain is reduced from two eyes to one. The researchers found that in the mice's brains microglia responded rapidly to changes in neuronal activity as the brain adapted to processing information from only one eye. They observed that the microglia targeted the synaptic cleft - the business end of the connection that transmits signals between neurons. The microglia "pulled up" the appropriate connections, physically disconnecting one neuron from another, while leaving other important connections intact. "These findings demonstrate that microglia are a dynamic and integral component of the complex machinery that allows neurons to reorganize their connections in the healthy mature brain. While more work needs to be done to fully understand this process, this study may help us understand how genetics or disruption of the immune system contributes to neurological disorders."