Astrocytes Release Soluble Factors that Influence Synapse Formation

Introduction Glial cells comprise 90% of the human brain.1 Glia are divided into two subtypes, the microglia which function largely as scavengers to engulf apoptotic cell debris and the macroglia comprised of oligodendrocytes that myelinate axons, and astrocytes.2 Here we focus on the astrocyte which has many well-defined functions; astrocytes control ion homeostasis, uptake glutamate at the synapse and provide metabolic support for neighboring neurons.1-3 The past several decades have seen an expansion in our understanding of astrocyte roles in the brain, specifically at the synapse where they influence formation and function.3 Here, we review in vitro experiments that led to the identification of astrocyte secreted factors and astrocyte contact effects that influence synapse formation. We then shift to a discussion of in vivo data, where we focus on the developmental role of astrocytes. Recent imaging studies have expanded our understanding of how neurons and astrocytes interact during these initial contacts, when synapses are extensively forming. We highlight recent work from our lab where we demonstrate that astrocyte contact is necessary for the synaptic maturation of developing neurons. Finally, we contrast these developmental interactions with astrocyte organization in the adult brain and discuss what these differences might mean in the context of plasticity.