Astrocytes are crucial for survival and maturation of embryonic hippocampal neurons in a neuron-glia cell-insert coculture assay.

Synapses represent specialized cell-cell contact sites between nerve cells. These structures mediate the rapid and efficient transmission of signals between neurons and are surrounded by glial cells. Previous investigations have shown that astrocytes are important for the formation, maintenance, and function of CNS synapses. To study effects of glial-derived molecules on synaptogenesis, we have established an in vitro cell-insert coculture system for E18 rat hippocampal neurons and various glial cell types. Neurons were cultured without direct contact with glial cells for distinct time periods. First, it was confirmed that astrocytes are essential to promote survival of E18 hippocampal neurons. Beginning with 10 days in culture, the concurrent expression of pre- and postsynaptic proteins was observed. Moreover, the colocalization of the presynaptic marker Bassoon and the postsynaptic protein ProSAP1/Shank2 indicated the formation of synapses. A technique was developed that permits the semiautomated quantitative determination of the number of synaptic puncta per neuron. The culture system was used to assess effects of pharmacological treatments on synapse formation by applying blockers and activators of small GTPases. In particular, treatment with lysophosphatidic acid enhanced synaptogenesis in the coculture system.