The long-term goal of the proposed research is to reveal how electrically non-excitable glial cells modulate synaptic transmission in response to neuronal inputs using fly visual system as a model. Previous studies on vision mostly have been focused on the visual transduction cascades and the neuronal circuits, whereas our knowledge about the role of glial cells in vision is very limited. Neurobiological research in the last decade has found that, In addition to their supportive role in neuron survival and function, the central neuropil glia astrocytes can respond to various neurotransmitters, and likely modulate neuronal synaptic transmissions. Neurotransmitter receptors are also detected in visual glia including Mller cells. However, it remains to be elucidated how glia modulate synaptic transmission in visual system, and how this glial function is controlled conversely by neurons. To study this function of perisynaptic glia, we plan to use the Drosophila visual system as a novel model, which allows genetic manipulation of signaling molecules specifically in glia or neuron, as well as subsequent observation of neuronal activities via reporters in live animals with intact glial networks. In the first visual neuropil region (lamina) of fly, photoreceptor axons release histamine upon light stimulation to hyperpolarize projective large monopolar cells (LMC). All neuronal processes are wrapped laterally by three epithelial glia cells (EG) in each laminar cartridge. We have previous found that EG concentrate a glutamate-gated chloride channel GluCl in special membrane processes abutting terminals of T1 interneuron. Our preliminary study showed that loss of GluCl diminished the Ca2+ response of LMC to light change, and impaired fly locomotion vision in dim conditions. Both dark-vision and electroretinogram defects of the GluCl mutant were phenocopied by downregulation of a glutamate transporter EAAT1 in T1, suggesting the involvement of T1 neuron and EAAT1 in the stimulation of GluCl. Finally, a cation channel NA in T1 appeared to function upstream of GluCl as well. Based on these observations, we propose to test a voltage-dependent, non-vesicular mechanism of neuron-glia communication, in which T1 neuron releases glutamate through EAAT1 to open a GluCl-gated Cl- pool in EG, thereby facilitating the inhibition of LMC by photoreceptors. This model may represent a general mechanism for interneuron to modulate synaptic weight through glia in both flies and mammals, and may occur in central brain as well. Using a combination of molecular and cell biological, genetic, histological, electrophysiological and in vivo imaging approaches, we will specifically demonstrate that 1) a GluCl-gated glial Cl- pool is essential for the inhibitory visual transmission; 2) T1 neuron releases glutamate through EAAT1 to open glial GluCl channel; and 3) depolarization of T1 is required for EAAT1 to release glutamate.
Glial cells support neuronal function and survival in both physiological and pathological conditions. In this proposal we plan to use the Drosophila visual sytem as a genetic model to study how glial cells directly modulate neuronal synaptic transmission and how this glial function is conversely controlled by neurons. Findings from this study will reveal novel, general mechanisms of neuron-glia interaction, and help to advance pathological understanding of retinopathies and psychiatric disorders.
