Genetic control of dendrite morphogenesis in C. elegans

The shapes and attachments of cells determine the machinery of organs; for example, the shapes and attachments of neurons and glia establish the wiring of the brain. To understand how neuronal dendrites obtain their morphologies and make the appropriate connections, we used C. elegans sense organs as models. Previous work identified a requirement for the extracellular matrix protein DYF-7 in dendrite extension: DYF-7 anchors dendrites dendrite endings at the embryonic nose while neuronal cell bodies migrate away, and in its absence, dendrites fail to extend. Here, we show that these dendrites are part of a sensory epithelium composed of glial cells and neurons. The dendrites are ensheathed by glial cells, form adherens junctions onto glia, and are stabilized at their apical surfaces by the extracellular matrix protein DYF-7. In dyf-7 mutants, the pulling force of cell migration causes this sensory epithelium to rupture along the glia:glia junctions. By comparison, dendrites of the URX and BAG neurons are intimately connected to the external surface of glial cells but are not known to form adherens junctions and are not affected in dyf-7 mutants. To identify factors required for URX and BAG dendrite extension, we performed forward genetic screens for dendrite extension defects in these cells and identified mutations in the cytoplasmic protein GRDN-1/Girdin and the adhesion molecule SAX-7/L1CAM. We show that in wild-type embryos, URX and BAG dendrites also extend by attaching to the nose and then stretching during embryo elongation but, in grdn-1 embryos, they fail to remain attached. GRDN-1 can promote dendrite attachment by acting in glia—it localizes to glial endings and causes localized accumulation of SAX-7, creating an adhesive compartment where dendrites attach. Thus, GRDN-1 and SAX-7 determine dendrite length by positioning a neuron-glia attachment site that couples dendrite extension to embryonic growth. Finally, we identified several other mutants with URX dendrite morphogenesis defects, including overgrowth of the URX dendrite; some have been mapped to genes associated with the cytoskeleton. Together, these studies define genetic mechanisms that control morphogenesis of distinct classes of sensory dendrites through specific adhesive interactions with their glial neighbors.Medical Science