Glia are abundant components of the nervous system, yet their contributions to neural circuits and animal behavior are poorly characterized. To study these contributions, we ablated the astrocyte-like CEPsh glia that ensheath synapses in the C. elegans nerve ring. We find that ablation results in enhanced locomotory quiescence preceding molting-associated lethargy (a sleep-like state), and molting-independent short-duration locomotory pausing. Synapses between the neuron ALA, which governs sleep-like behavior, and the locomotory interneuron AVE are ensheathed by CEPsh glia. Loss of ALA function suppress both precocious lethargus and locomotory pausing, suggesting that CEPsh blocks a locomotion-inhibiting signal from ALA. Several observations suggest this inhibition occurs at the ALA-AVE synapses. Inactivation of AVE promotes locomotory pausing independently of ALA or CEPsh glia states, suggesting it acts downstream of these two cells. In addition, CEPsh glia ablation results in prolonged calcium transients in AVE, which are rescued by ALA inactivation. Finally, AVE calcium levels normally correlate with backwards locomotion; in CEPsh-ablated animals, calcium induction in AVE is uncoupled from movement, and coupling is restored by ALA inactivation. These results suggest that CEPsh glia are essential for the functions of ALA-AVE synapses. To understand how CEPsh glia control synaptic activity, we developed a robust method for isolating these cells from larvae, and generated a list of CEPsh glia-enriched genes using RNAseq. RNAi studies have uncovered potential roles for some enriched genes in locomotion control.
CEPsh glia also ensheath the outer aspect of the nerve ring, where they are in close proximity to glutamatergic synapses. Studies in other labs showed that glutamatergic signaling levels correlate with reversal frequency. We found that CEPsh-disrupted animals have an increased reversal frequency, suggesting accumulation of glutamate at synapses. Analysis of the CEPsh transcriptome revealed enriched expression of glt-1, a homolog of the astrocytic glutamate clearance transporter (EAAT2). glt-1 mutants were previously shown to have increased reversal frequency, and we found that expression of glt-1 in CEPsh glia rescues this defect. Together, our studies establish functional and molecular homology between CEPsh glia and vertebrate astrocytes, and identify exciting roles for these cells in locomotory control in C. elegans.
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