Astrocytic glutamate transport regulates a Drosophila CNS synapse that lacks astrocyte ensheathment

Abstract

Anatomical, molecular, and physiological interactions between astrocytes and neuronal synapses regulate information processing in the brain. The fruit fly Drosophila melanogaster has become a valuable experimental system for genetic manipulation of the nervous system and has enormous potential for elucidating mechanisms that mediate neuron glia interactions. Here, we show the first electrophysiological recordings from Drosophila astrocytes and characterize their spatial and physiological relationship with particular synapses. Astrocyte intrinsic properties were found to be strongly analogous to those of vertebrate astrocytes, including a passive current-voltage relationship, low membrane resistance, high capacitance, and dye-coupling to local astrocytes. Responses to optogenetic stimulation of glutamatergic pre-motor neurons were correlated directly with anatomy using serial electron microscopy reconstructions of homologous identified neurons and surrounding astrocytic processes. Robust bidirectional communication was present: neuronal activation triggered astrocytic glutamate transport via Eaat1, and blocking Eaat1 extended glutamatergic interneuron-evoked inhibitory post-synaptic currents in motor neurons. The neuronal synapses were always located within a micron of an astrocytic process, but none were ensheathed by those processes. Thus, fly astrocytes can modulate fast synaptic transmission via neurotransmitter transport within these anatomical parameters. Graphical AbstractCorresponding Author: Sarah E. MacNamee, University of Arizona Dept. of Neuroscience, 1040 E 4 th St GS Rm 611, Tucson AZ 85721, (520) 621 6671, Smac3@email.arizona.edu. Role of AuthorsAll authors had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design: SEM, LAO. Acquisition of data: SEM, KEL, SG, CTT, RDF, AC. Analysis and interpretation of data: SEM, KEL, CTT. Drafting of the manuscript: SEM. Critical revision of the manuscript for important intellectual content: LAO, LPT, AC. Statistical analysis: SEM. Obtained funding: LAO, LPT, AC. Administrative, technical, and material support: none. Study supervision: LAO, LPT. Conflict of Interest StatementThe authors have no conflict of interest to disclose. Data AccessibilityData generated in the lab and the subsequent analyses will be made available upon request to the Oland/Tolbert laboratory by qualified individuals as long as doing so would not compromise intellectual property interests or interfere with publication. Any shared data would include notations, standards, and the like that are necessary for interpretation of the data. HHMI Author Manuscript HHMI Author Manuscript HHMI Author ManuscriptThe authors show the first recordings from Drosophila astrocytes and find that their electrophysiological properties are strongly analogous to those of vertebrate astrocytes. Using correlative electron microscopy and physiology, they found no glial ensheathment of glutamatergic synap...

The glia of the adult Drosophila nervous system

“…This web of contacts may simply provide physical cohesion for the anatomical structure, but it is interesting to speculate that they might also be used to communicate cellular signals between neighbors as well as more distant regions. Coupling of glia by gap junctions is suspected for several glial subtypes but has been demonstrated for SPG, where they are required to translate metabolic signals into synchronized calcium pulses and insulin secretion (Speder and Brand, ), for CG in the lamina (Chaturvedi et al, ) and most recently in ALG in the larva (MacNamee et al, ), where they are required for neurotransmitter recycling.…”

“…The strict negative correlation of astrocyte and synaptic density we observe in Drosophila is contrasted by a more variable relationship observed in vertebrates (“tripartite synapse”), where some neuropils, such as the cerebellum, show high coverage of synapses by astrocytes while others, such as the hippocampus, show lower coverage (Araque et al, ; Ventura and Harris, ). This suggests that, in lieu of increasing astrocytic structural density, some compensatory mechanisms must exist to deal with high synaptic activity, perhaps by changing their morphology in response to neuronal activity (MacNamee et al, ).…”

Drosophila glia: Few cell types and many conserved functions

“…Thus, subperineurial glial cells, presumably together with cortex glial cells, provide a niche regulating neuroblast proliferation (Spéder & Brand, ). Gap junctional coupling has been also shown for astrocyte‐like glial cells, but no functional gap junctional coupling has been demonstrated between different glial cell types (MacNamee et al, ). The latter is also supported by the fact that gap junctional coupling is only marginally required for glucose distribution in the brain (Volkenhoff, Hirrlinger, Kappel, Klämbt, & Schirmeier, ).…”

“…Drosophila astrocyte‐like glial cells resemble their vertebrate counterparts in terms of morphological, molecular and functional aspects (Freeman, ; Ma et al, ). These cells are highly polarized and extend a variable number of fine and highly branched processes into the neuropil (MacNamee et al, ; Omoto et al, ; Peco et al, ; Stork, Sheehan, Tasdemir‐Yilmaz, & Freeman, ; Figure ). Their large and rounded cell bodies reside between the ensheathing glial sheaths covering the neuropil (Omoto et al, ; Stork et al, ).…”

“…Once specified, astrocyte‐like glial cells dynamically and progressively invade the neuropil in an FGF‐signaling dependent manner (Stork et al, ). During the infiltration process, they tile with one another with only limited overlap (MacNamee et al, ; Peco et al, ; Stork et al, ).…”