Investigating the Role of Glia in Activity-Dependent Synapse Elimination

During development, neural circuitry undergoes a remodeling process in which excess synapses are eliminated or pruned and the remaining synapses are strengthened. While it is clear that this is an activity-dependent process, the precise molecular mechanisms have not been elucidated. We recently discovered that components of the classical complement cascade (C1q and C3), traditionally associated with innate immune system function, are necessary for synapse elimination in the normal, developing central nervous system (CNS). Furthermore, C1q and C3 were found to localize to synapses in the developing brain and during early stages of neuro-degenerative disease. One of the major questions arising from these findings, and the main objective of the current study, is to determine by what mechanism complement is mediating synapse removal. In the innate immune system, complement proteins coat or opsonize debris for removal. A common pathway for complementopsonized debris removal is through phagocytosis by macrophages. We hypothesize that complementopsonized synapses are phagocytosed by resident microglia, the primary phagocytic cells in the brain and the only known resident CNS cell to express the C3 receptor (also known as alphaM beta2 integrin, cd11b/cd18, mac1).

Using a combination of high resolution Array Tomography (AT), serial electron microscopy, and two-photon in vivo time lapse imaging, we will address the following questions:

1. Do microglia actively phagocytose developing synapses?

2. Are microglia required for CNS synapse elimination, and if so, what is the molecular mechanism?

In addition, we will use a combination of transgenic and pharmacological approaches to selectively disrupt microglia-synapse interactions, in order to determine whether microglia are required for synapse elimination, and whether microglia are a key mechanism underlying complement-mediated synapse elimination.