egl-20/wnt is required for axonal tiling between dorsal D-type 5 (DD5) and dorsal D-type 6 (DD6) neurons. (A) Schematic of axonal, dendritic, and presynaptic tiling between DD5 and DD6 in wild type. (B) Representative image of axonal tiling in wild type animals. Yellow line represents region of axonal overlap between DD5 and DD6. Green arrow indicates the end of DD5 posterior axon (top panel). Magenta arrow indicates the end of DD6 anterior axon (middle panel). (C) Representative image of presynaptic tiling in wild type animals. The magnified straightened image of the presynaptic tiling border, indicated by the dashed box, is shown above. Green arrow indicates the most posterior DD5 synapse (top panel). Magenta arrow indicates the most anterior DD6 synapse (middle panel). (D) Schematic of axonal and dendritic overlap between DD5 and DD6 of the egl-20(n585) mutant. (E) Representative image of axonal tiling in egl-20(n585) mutant animals. Yellow line represents region of axonal overlap between DD5 and DD6. Green arrow indicates the end of DD5 posterior axon (top panel). Magenta arrow indicates the end of DD6 anterior axon (middle panel). (F) Representative image of presynaptic tiling in the egl-20(n585) mutant. Yellow line represents region of presynaptic overlap between DD5 and DD6. The magnified straightened image of the presynaptic tiling border, indicated by the dashed box, is shown above. Green arrow indicates the most posterior DD5 synapse (top panel). Magenta arrow indicates the most anterior DD6 synapse (middle panel). Asterisks: DD5 and DD6 cell bodies. Scale bar: 20 µm. (G) Quantification of axonal overlap between DD5 and DD6. See Figure 1—source data 1. (H) Quantification of presynaptic overlap between DD5 and DD6. See Figure 1—source data 2. (I) Quantification of DD6 axonal length. See Figure 1—source data 3. (J) Quantification of DD5 posterior axonal length. See Figure 1—source data 4. See Figure 1—figure supplement 1A for the definition of the DD5 posterior axon. Each dot represents a single animal. See source data for sample size and individual value. Black bars indicate mean ± SEM. **p<0.01; ***p<0.001.
Ardalan Hendi, Long-Gang Niu, Andrew William Snow, Richard Ikegami, Zhao-Wen Wang, Kota Mizumoto. 2022. Channel-independent function of UNC-9/Innexin in spatial arrangement of GABAergic synapses in C. elegans. eLife
Abstract
Precise synaptic connection of neurons with their targets is essential for the proper functioning of the nervous system. A plethora of signaling pathways act in concert to mediate the precise spatial arrangement of synaptic connections. Here we show a novel role for a gap junction protein in controlling tiled synaptic arrangement in the GABAergic motor neurons in Caenorhabditis elegans, in which their axons and synapses overlap minimally with their neighboring neurons within the same class. We found that while EGL-20/Wnt controls axonal tiling, their presynaptic tiling is mediated by a gap junction protein UNC-9/Innexin, that is localized at the presynaptic tiling border between neighboring dorsal D-type GABAergic motor neurons. Strikingly, the gap junction channel activity of UNC-9 is dispensable for its function in controlling tiled presynaptic patterning. While gap junctions are crucial for the proper functioning of the nervous system as channels, our finding uncovered the novel channel-independent role of UNC-9 in synapse patterning.