Glial-Neuronal Signaling

We have recently begun to use Drosophila as a model organism to characterize how changes in glial Ca2+ oscillatory activity modify neuronal excitability, and how dysfunction of these pathways may contribute to epilepsy. Epilepsy is defined by incapacitating episodes of hypersynchronous neuronal firing, affecting 3 million Americans and over 50 million worldwide. Although a primary disruption of neuronal membrane excitability has been implicated in seizure pathogenesis in a subset of idiopathic epilepsy cases, the cellular events that trigger seizure initiation in most epilepsy patients are not well understood. A non-neuronal origin for seizure induction has been suggested in some cases, with in situ studies correlating elevated glial Ca2+ oscillations with seizure initiation and in vivo work demonstrating several anti-epileptic drugs reduce glial Ca2+ oscillations. Although increased glial activity has been associated with abnormal neuronal excitability, the role of glia in the development and maintenance of seizures is poorly understood. Using an unbiased genetic screen for temperature-sensitive seizure mutants in Drosophila, we have identified and characterized a glial-specific gene that is required for microdomain glial Ca2+ oscillations. Acute disruption of this protein (termed Zydeco – an NCKX transporter), or acute induction of Ca2+ influx into glia via conditional TRP channels, induces the immediate onset of neuronal seizure activity. This is the first finding of conditional genetic manipulations of glial Ca2+ triggering neuronal seizures, indicating a Ca2+-dependent glial derived signal is important for maintaining appropriate neuronal excitability in the brain. Zydeco function is required specifically in cortex glia, which exhibit spatial segregation in Drosophila reminiscent of mammalian astrocytes, with each glial cell ensheathing multiple neuronal soma. Zydeco is not required in surface glial that constitute the blood-brain barrier or in ensheathing glia that insulate axons, and we have excluded a defect in ion balance as the cause of epileptic seizures in the mutant. These findings indicate that glial Ca2+ regulation is acutely involved in seizure generation, and that mutation of a glial-specific NCKX alters submembrane microdomain glial Ca2+ oscillations while increasing neuronal seizure susceptibility. We expect our studies will have significant impact on the current controversy concerning the physiological importance of glial Ca2+ signaling in the regulation of neuronal excitability. This debate on the significance of glial Ca2+ signaling is linked to the functional distinction between ER-mediated somatic Ca2+ oscillations and small, near-membrane Ca2+ oscillations in glial processes. Somatic glial Ca2+ waves are primarily mediated by the release of intracellular Ca2+ stores, and their relative importance in normal brain physiology has been disputed. Our findings indicate that microdomain glial Ca2+ activity is likely to be the more critical mode for glial modulation of neuronal function. We are currently characterizing the significance of both modes of glial Ca2+ oscillatory behavior to provide new insights into glial-neuronal communication and how non-neuronal triggers may contribute to epilepsy.