Rett Syndrome, caused by mutations of the DNA binding protein MeCP2, is one of the most common causes for mental retardation in females. It is part of the broader spectrum of pervasive developmental disorders, and affects patients beginning in infancy. Patients feature dramatic traits shared with autism spectrum disorders. In Rett syndrome a single gene is affected: there is hope that the mechanisms may be easier to dissect than for multifactorial disorders, yet may shed light on common forms of autism. Most of the studies of Rett Syndrome, since the discovery of the causal gene in 1999, have relied heavily on mouse models one of which was developed in the Jaenisch lab), and focused on neurons, the electrically active cells of the brain. However, recent evidence suggest that other brain cells, the so-called glia, far from being merely structural players, actively participate in the electrical and chemical balance, and contribute to the development of Rett Syndrome. For example, specialized cells of the immune system known as microglia are lifelong residents, constantly surveying the integrity of the tissue. They play a role in establishing proper connections between nerve cells, and may also be involved during early development. For the past few years, we have learned to be constant gardeners, and managed to generate all these cell types in the culture dish, deriving these cells from human patients. We have also learned to generate genetically-matched controls for any mutant cells we study, in order to validate mutation-related functional variations. We recently applied these efforts to the study of Rett Syndrome, and described novel roles for MeCP2 in neurons. In this proposal we want to shift our focus to glia, and study the effects of mutant glia on normal neurons, or normal glia on mutant neurons. The former will shed further light on the mechanisms of Rett Syndrome, while the latter may also suggest therapeutic strategies. In any case, the ambitious platform we propose will allow better study of glia-neuron dialogue in the normal brain, and in pathologies ranging from stroke to Alzheimer's disease. Many facets of brain function are better studied in vivo, but we have a unique opportunity to study aspects of communications between cells which can only be dissected in vitro.