Extrinsic repair of injured dendrites as a paradigm for regeneration by fusion

Abstract Injury triggers regeneration of axons and dendrites. Research identified factors required for axonal regeneration outside the CNS, but little is known about regeneration triggered by dendrotomy. Here we study neuronal plasticity triggered by dendrotomy and determine the fate of complex PVD arbors following laser surgery of dendrites. We find that severed primary dendrites grow towards each other and reconnect via branch fusion. Simultaneously, terminal branches lose self-avoidance and grow towards each other, meeting and fusing at the tips via an AFF-1-mediated process. Ectopic branch growth is identified as a step in the regeneration process required for bypassing the lesion site. Failure of reconnection to the severed dendrites results in degeneration of the distal end of the neuron. We discover pruning of excess branches via EFF-1 that acts to recover the original wild-type arborization pattern in a cell-autonomous process. In contrast, AFF-1 activity during dendritic auto-fusion is derived from the lateral seam cells and not autonomously from the PVD neuron. We propose a model in which AFF-1-vesicles derived from the epidermal seam cells fuse neuronal dendrites from without. Thus, EFF-1 and AFF-1 fusion proteins emerge as new players in neuronal arborization and maintenance of arbor connectivity following injury in C. elegans . Our results demonstrate that there is a genetically determined multi-step pathway to repair broken dendrites in which EFF-1 and AFF-1 act on different steps of the pathway. Intrinsic EFF-1 is essential for dendritic pruning after injury and extrinsic AFF-1 mediates dendrite fusion to bypass injuries. Author summary Neurons in the central nervous system have very limited regenerative ability, they fail to remodel following amputation and only in some invertebrates, axons can repair themselves by fusion. Some genetic pathways have been identified for axonal regeneration but few studies exist on dendrite regeneration following injury. To determine how neurons regenerate dendrites following injury we study the C. elegans PVD polymodal neurons that display an arborized pattern of repetitive menorah-like structures. We injure dendrites by laser microsurgery, follow their fate and show that broken primary dendrites often regenerate via fusion. We describe how PVD dendrites regenerate and present roles for EFF-1 and AFF-1 proteins in fusion and remodeling of menorahs. Menorahs lose self-avoidance and AFF-1 fuses them, bypassing the injury site. Branch sprouting, EFF-1-mediated pruning, and arbor simplification completes regeneration. When auto-fusion fails the distal arbor degenerates. Surprisingly, AFF-1 acts non-cell autonomously to mediate dendrite fusion. We propose that extracellular vesicles derived from the lateral epidermis fuse severed dendrites in a process reminiscent of enveloped virus-mediated cell fusion without infection.