Glutamate is a major excitatory neurotransmitter, and impaired glutamate clearance following synaptic release promotes spillover, inducing extra-synaptic signaling. The effects of glutamate spillover on animal behavior and its neural correlates are poorly understood. We developed a glutamate spillover model in Caenorhabditis elegans by inactivating the conserved glial glutamate transporter GLT-1. GLT-1 loss drives aberrant repetitive locomotory reversal behavior through uncontrolled oscillatory release of glutamate onto AVA, a major interneuron governing reversals. Repetitive glutamate release and reversal behavior require the glutamate receptor MGL-2/mGluR5, expressed in RIM and other interneurons presynaptic to AVA. mgl-2 loss blocks oscillations and repetitive behavior; while RIM activation is sufficient to induce repetitive reversals in glt-1 mutants. Repetitive AVA firing and reversals require EGL-30 / Gαq, an mGluR5 effector. Our studies reveal that cyclic autocrine presynaptic activation drives repetitive reversals following glutamate spillover. That mammalian GLT1 and mGluR5 are implicated in pathological motor repetition suggests a common mechanism controlling repetitive behaviors.

Glia in the central nervous system engulf neuron fragments to remodel synapses and recycle photoreceptor outer-segments. Whether glia passively clear shed neuronal debris, or actively prune neuron fragments is unknown. How pruning of single-neuron endings impacts animal behavior is also unclear. Here we report our discovery of glia-directed neuron pruning in C. elegans. Adult C. elegans AMsh glia engulf sensory endings of the AFD thermosensory neuron by repurposing components of the conserved apoptotic corpse phagocytosis machinery. The phosphatidylserine (PS) flippase TAT-1/ATP8A, functions with glial PS-receptor PSR-1/PSR and PAT-2/α-integrin to initiate engulfment. This activates glial CED-10/Rac1 GTPase through the ternary GEF complex of CED-2/CrkII, CED-5/DOCK180, CED-12/ELMO. Execution of phagocytosis uses the actin-remodeler WSP-1/nWASp. This process dynamically tracks AFD activity and is regulated by temperature, the AFD sensory input. Importantly, glial CED-10 levels regulate engulfment rates downstream of neuron activity, and engulfment-defective mutants exhibit altered AFD-ending shape and thermosensory behavior. Our findings reveal a molecular pathway underlying glia-dependent engulfment in a peripheral sense-organ, and demonstrate that glia actively engulf neuron-fragments, with profound consequences on neuron shape and animal sensory behavior.

Citation: Howard, M. M., A. Bae, and M. Königer. 2019. The importance of chloroplast movement, nonphotochemical quenching, and electron transport rates in light acclimation and tolerance to high light in PREMISE: While essential for photosynthesis, excess light can damage plants. We investigated how growth light conditions affect two photoprotective strategies, chloroplast movement and nonphotochemical quenching (NPQ), as well as electron transport rates (ETR), and the relative importance of these processes in the short-term stress tolerance of Arabidopsis thaliana. METHODS:We grew wild-type (WT) and mutant plants with impaired chloroplast movement (phot1, phot2, phot1 phot2, chup1) or NPQ (npq1) at low (160 μmol photons m −2 s −1 ) or intermediate light (400 μmol photons m −2 s −1 ) before quantifying transmission changes due to chloroplast movement, NPQ, ETR, and the ability to recover from a short-term high-light treatment. RESULTS:Plants with impaired chloroplast avoidance movement (phot2, phot1 phot2, chup1) did not recover as well from a short-term high light treatment as the WT or npq1 and phot1 mutants. Plants grown at intermediate light recovered more completely from the same stress treatment regardless of their genotype and despite reduced degrees of transmission changes due to chloroplast movement. This result was due in part to all genotypes having up to a 2-fold increase in ETR max and a slight increase in NPQ max . CONCLUSIONS:Growth light conditions affect which mechanisms are important in dealing with short-term high-light stress. The chloroplast avoidance response is important for low-light-grown plants, while increases in ETR max and NPQ max allow plants grown at intermediate light intensities to avoid being damaged.

Glia in the central nervous system engulf neuron fragments during synapse remodeling and recycling of photoreceptor outer-segments. Whether glia passively clear shed neuronal debris, or actively remove neuron fragments is unknown. How pruning of single-neuron endings impacts animal behavior is also unclear. Here we report that adult C. elegans AMsh glia engulf sensory endings of the AFD thermosensory neuron. Engulfment is regulated by temperature, AFD’s sensory input, and tracks AFD activity. Phosphatidylserine (PS) flippase TAT-1/ATP8A, functions with glial PS-receptor PSR-1/PSR and PAT-2/α-integrin to initiate engulfment. Glial CED-10/Rac1 GTPase, acting through a conserved GEF complex, executes phagocytosis using the actin-remodeler WSP-1/nWASp and the membrane-sealing factor EFF-1 fusogen. CED-10 levels determine engulfment rates, and engulfment-defective mutants exhibit altered AFD-ending shape and thermosensory behavior. Our findings reveal a molecular pathway underpinning glia-dependent phagocytosis in a peripheral sense-organ, and demonstrate that glia actively engulf neuron-fragments, with profound consequences on neuron shape and animal behavior.