Astrocyte Ca²⁺Influx Negatively Regulates Neuronal Activity

Abstract

Maintenance of neural circuit activity requires appropriate regulation of excitatory and inhibitory synaptic transmission. Recently, glia have emerged as key partners in the modulation of neuronal excitability; however, the mechanisms by which glia regulate neuronal signaling are still being elucidated. Here, we describe an analysis of how Ca2+ signals within Drosophila astrocyte-like glia regulate excitability in the nervous system. We find that Drosophila astrocytes exhibit robust Ca2+ oscillatory activity manifested by fast, recurrent microdomain Ca2+ fluctuations within processes that infiltrate the synaptic neuropil. Unlike the enhanced neuronal activity and behavioral seizures that were previously observed during manipulations that trigger Ca2+ influx into Drosophila cortex glia, we find that acute induction of astrocyte Ca2+ influx leads to a rapid onset of behavioral paralysis and a suppression of neuronal activity. We observe that Ca2+ influx triggers rapid endocytosis of the GABA transporter (GAT) from astrocyte plasma membranes, suggesting that increased synaptic GABA levels contribute to the neuronal silencing and paralysis. We identify Rab11 as a novel regulator of GAT trafficking that is required for this form of activity regulation. Suppression of Rab11 function strongly offsets the reduction of neuronal activity caused by acute astrocyte Ca2+ influx, likely by inhibiting GAT endocytosis. Our data provide new insights into astrocyte Ca2+ signaling and indicate that distinct glial subtypes in the Drosophila brain can mediate opposing effects on neuronal excitability.

Drosophila glia: Few cell types and many conserved functions

“…Cortex glial cells exhibit spontaneous and activity‐dependent fluctuations in intracellular Ca 2+ levels which depend on the glial‐specific K + ‐dependent Na 2+ /Ca 2+ exchanger Zydeco (Melom & Littleton, ). Currently, the precise role of these oscillations is not known but possibly they affect the neighboring astrocyte‐like glial cells, whose Ca 2+ dynamics clearly influence neuronal activity (Ma, Stork, Bergles, & Freeman, ; Zhang, Ormerod, & Littleton, ). A direct connection between cortex glial cells and astrocyte‐like glial cells was also demonstrated following inhibition of the Drosophila Amyloid precursor protein homolog Appl in cortex glial cells.…”

“…Moreover, GAT levels are elevated in an activity dependent manner demonstrating neuron‐glia crosstalk (Muthukumar et al, ). Most likely, this is due to Ca 2+ influx into astrocyte‐like glial cells which triggers rapid endocytosis of GAT from astrocytic plasma membrane (Zhang et al, ). Ca 2+ influx might be regulated by neurotransmitter receptors which are expressed by astrocytes (Bazargani & Attwell, ).…”

Disruption of Glial Ca2+Oscillations at theDrosophilaBlood-Brain Barrier Predisposes to Seizure-Like Behavior

“…Hence, one possible mechanism by which PG Ca 2+ could alter neuronal activity is through regulation of transport across the BBB. First, PG Ca 2+ activity could modulate exocytotic/endocytotic cycling of membrane proteins within the PG layer itself, similar to its role in regulating surface levels of K + channels in cortex glia 24 and GABA transporters in astrocytelike glia 22 . Second, PG Ca 2+ may regulate transport in SPG cells via a cell non-autonomous mechanism.…”

“…We recently found that chronic Ca 2+ increase in Drosophila cortex glia predispose animals to stimulation-induced seizures 24 , while acute increases in intracellular Ca 2+ in Drosophila astrocyte-like glia drives neuronal silencing 22 . To identify glial signaling pathways that may modulate neuronal excitability, we performed a genetic screen using the pan-glial driver repo-gal4 to drive expression of RNA interference (RNAi) targeting ~850 genes encoding membrane receptors, secreted ligands, ion channels and transporters, vesicular trafficking proteins and known cellular Ca 2+ homeostasis and Ca 2+ signaling pathway components 24 .…”

“…Several Drosophila glial subtypes have been found to influence neuronal function via distinct mechanisms downstream of their glial Ca 2+ dynamics. Astrocytic Ca 2+ regulates neurotransmitter uptake 22 and secretion of neuromodulators 23 , while disruption of Ca 2+ signaling in Drosophila cortex glia impairs K + buffering capacity 24 . Synchronized Ca 2 + waves in Drosophila SPG cells control nutrient-dependent reactivation of neural stem cells and subsequent brain growth 25,26 .…”