PROJECT SUMMARY/ABSTRACT All major mammalian glial subtypes of the central nervous system (CNS) make direct contacts with neuronal cell bodies; however, how glia support and communicate with neuronal somas is vastly understudied compared to glial interactions at synapses or axons. The overarching goal of this project is to gain a deep mechanistic understanding of glial development, communication, and function at neuronal cell bodies to begin to fill in the gaps of how these associations regulate CNS health and dysfunction. Drosophila glia demonstrate remarkable similarity to a number of mammalian glial subtypes measured by morphological, functional, and molecular criteria. Among these is cortex glia, a glial subclass that forms a lace-like meshwork to individually ensheath nearly every neuronal cell body in the CNS. We recently developed new genetic tools to manipulate gene function with remarkable specificity in Drosophila cortex glia, and now have a powerful system in which to study glial cell development and neuron-glia interactions at neuronal cell bodies in vivo. In addition to regulating neuronal health and behavior, cortex glia provide metabolic support to neurons, regulate neuronal ion and nutrient balance, engulf neuronal debris, and can therefore inform the interrogation of multiple vertebrate glial cells that interact with neuronal cell bodies. We previously demonstrated that when cortex glia lack a single secreted neurotrophin, Spätzle 3 (Spz3), they take on a globular appearance and no longer wrap neuronal cell bodies. The loss of Spz3 and these glial-somal interactions leads to widespread nervous system dysfunction, including increased neuronal cell death, locomotor impairment, and aberrant growth of surrounding healthy glial cells. We propose to use powerful in vivo genetic tools available in Drosophila, along with a variety of techniques in cellular and molecular biology, biochemistry, and imaging to elucidate the mechanisms of glial-somal interactions that maintain neuronal health in a live, intact nervous system. Specifically, we will dissect the mechanisms that regulate the maturation and distribution of this neurotrophin to maintain glial contact at neuronal cell bodies (Aim 1), define how this neurotrophin signals to its receptor to support glial morphology, somal interactions, and neuronal health (Aim 2), and finally, we will determine how nearby glial cells compensate when glial-somal signaling and associations are impaired (Aim 3). These findings will begin to shed light on an understudied, yet important phenomenon by providing a foundation for elucidating the cellular and molecular underpinnings of glial interactions with neuronal cell bodies in the healthy and diseased CNS.