Summary Proper morphogenesis of dendrites plays a fundamental role in the establishment of neural circuits. The molecular mechanism by which dendrites grow highly complex branches is not well understood. Here, using the C. elegans PVD neuron, we demonstrate that high-order dendritic branching requires actin polymerization driven by coordinated interactions between two membrane proteins, DMA-1 and HPO-30, and with their cytoplasmic interactors, the RacGEF TIAM-1 and the actin nucleation promotion factor WAVE Regulatory Complex (WRC). The dendrite branching receptor DMA-1 directly binds to the PDZ domain of TIAM-1, while the claudin-like protein HPO-30 directly interacts with the WRC. On dendrites, DMA-1 and HPO-30 form a receptor-associated signaling complex to bring TIAM-1 and the WRC to close proximity, leading to elevated assembly of F-actin needed to drive high-order dendrite branching. The synergistic activation of F-actin assembly by scaffolding distinct actin regulators might represent a general mechanism in promoting complex dendrite arborization.

A polarized arrangement of neuronal microtubule arrays is the foundation of membrane trafficking and subcellular compartmentalization. Conserved among both invertebrates and vertebrates, axons contain exclusively 'plus-end-out' microtubules while dendrites contain a high percentage of 'minus-end-out' microtubules, the origins of which have been a mystery. Here we show that in Caenorhabditis elegans the dendritic growth cone contains a non-centrosomal microtubule organizing center, which generates minus-end-out microtubules along outgrowing dendrites and plus-end-out microtubules in the growth cone. RAB-11-positive endosomes accumulate in this region and co-migrate with the microtubule nucleation complex γ-TuRC. The MTOC tracks the extending growth cone by kinesin-1/UNC-116-mediated endosome movements on distal plus-end-out microtubules and dynein clusters this advancing MTOC. Critically, perturbation of the function or localization of the MTOC causes reversed microtubule polarity in dendrites. These findings unveil the endosome-localized dendritic MTOC as a critical organelle for establishing axon-dendrite polarity.

Sensory dendrites innervate peripheral tissues through cell-cell interactions that are poorly understood. The proprioceptive neuron PVD in C. elegans extends regular terminal dendritic branches between muscle and hypodermis. We found that the PVD branch pattern was instructed by adhesion molecule SAX-7/L1CAM, which formed regularly spaced stripes on the hypodermal cell. The regularity of the SAX-7 pattern originated from the repeated and regularly spaced dense body of the sarcomeres in the muscle. The extracellular proteoglycan, UNC-52/Perlecan, links the dense body to the hemidesmosome on the hypodermal cells, which in turn instructed the SAX-7 stripes and PVD dendrites. Both UNC-52 and hemidesmosome components exhibited highly regular stripes that interdigitated with the SAX-7 stripe and PVD dendrites, reflecting the striking precision of subcellular patterning between muscle, hypodermis and dendrites. Hence, the muscular contractile apparatus provides the instructive cues to pattern proprioceptive dendrites.