Microglial control of neuronal death and synaptic properties.

Microglia have long been characterized by their immune function in the nervous system and are still mainly considered in a beneficial versus detrimental dialectic. However a review of literature enables to shed novel lights on microglial function under physiological conditions. It is now relevant to position these cells as full time partners of neuronal function and more specifically of synaptogenesis and developmental apoptosis. Indeed, microglia can actively control neuronal death. It has actually been shown in retina that microglial nerve growth factor (NGF) is necessary for the developmental apoptosis to occur. Similarly, in cerebellum, microglia induces developmental Purkinje cells death through respiratory burst. Furthermore, in spinal cord, microglial TNFalpha commits motoneurons to a neurotrophic dependent developmental apoptosis. Microglia can also control synaptogenesis. This is suggested by the fact that a mutation in KARAP/DAP12, a key protein of microglial activation impacts synaptic functions in hippocampus, and synapses protein content. In addition it has been now demonstrated that microglial brain-derived neurotrophin factor (BDNF) directly regulates synaptic properties in spinal cord. In conclusion, microglia can control neuronal function under physiological conditions and it is known that neuronal activity reciprocally controls microglial activation. We will discuss the importance of this cross-talk which allows microglia to orchestrate the balance between synaptogenesis and neuronal death occurring during development or injuries.