New role for brain's immune cells discovered.

It has long been thought that it was only our brain cells or neurons that were actively engage in brain signalling. It now appears that immune cells in the brain, called microglia have an important role in the creation and elimination of brain cells synapses. Synapses are the junctions where brain cells or neurons can pass chemical signals to each other.

Microglia in their role as immune cells are known to protect the brain against infection and injury. When activated, they become able to release various inflammatory molecules, which can influence which brain cells survive. They can engulf or phagocytose and dispose of cellular debris. Now it is thought they are also involved in eliminating synapses. This new discovery suggests they have an additional role involving learning and memory.

The findings from researchers based at the University of Rochester have just been published the November 2nd edition of PLoS Biology. Quantitative electron microscopy and two photon in-vivo imaging were used to see how microglia interact with brain synapses in the visual cortex of mice.

The researchers discovered that when there is no infection or injury for the microglia to be concerned with, they nevertheless remain very active. Microglia are mobile and travel between the brain cell circuits. The researchers showed how the microglia were constantly touching and wrapping around synapses, and appear to have a role in determining which stay persist and which are eliminated.

Nerve cells have what are called dendritic spines, which allow them to grow new dendrites. If was observed that if microglia touched these spines then there was a far greater chance that they would be eliminated.

The researchers also looked at the effect of modifying the external visual environment. In the absence of light, the microglia contacted more synapses and were more likely to steer towards larger dendrites, a behaviour which was reversed when the light exposure was reintroduced.

These findings are adding to our growing understanding of the role of different types of brain cells and how they interact together. The role of microglia is now being looked at in relation to disease processes such as Parkinson’s disease, Alzheimer’s, schizophrenia, obsessive compulsive disorder and possibly even autism.

Tremblay M-E, Lowery RL, Majewska AK. Microglial Interactions with Synapses Are Modulated by Visual Experience. PLoS Biology, 2010; 8 (11): e1000527 DOI: 10.1371/journal.pbio.1000527