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Functional regeneration after nervous system injury requires transected axons to reconnect with their original target tissue. Axonal fusion, a spontaneous regenerative mechanism identified in several species, provides an efficient means of achieving target reconnection as a regrowing axon is able to contact and fuse with its own separated axon fragment, thereby re-establishing the original axonal tract1-7. Here we report a molecular characterization of this process in Caenorhabditis elegans, revealing dynamic changes in the subcellular localization of the EFF-1 fusogen after axotomy, and establishing phosphatidylserine (PS) and the PS receptor (PSR-1) as critical components for axonal fusion.PSR-1 functions cell-autonomously in the regrowing neuron and, instead of acting in its canonical signalling pathway8, acts in a parallel phagocytic pathway that includes the transthyretin protein TTR-52, as well as CED-7, NRF-5 and CED-6 (refs 9-12). We show that TTR-52 binds toPS exposed on the injured axon, and can restore fusion several hours after injury. We propose that PS functions as a 'save-me' signal for the distal fragment, allowing conserved apoptotic cell clearance molecules to function in reestablishing axonal integrityduring regeneration of the nervous system.
Axonal fusion, which occurs spontaneously in several invertebrate species1-7, is a highly efficient means to re-establish the connection between an injured neuron and its target tissue; the proximal axon which is still attached to the cell body regrows towards, reconnects, and fuses with its separated distal fragment.We and others have shown that axonal fusion occurs in C. elegans mechanosensory neurons4,7. However, the genetic components of this process remain largely unidentified, with the nematode-specific fusogen, epithelial fusion failure 1 (EFF-1), the only protein known to be involved4.
Weexpressed green fluorescent protein (GFP) specifically within the six mechanosensory neurons of C. elegans (Fig. 1a) and performed laser axotomy of the posterior lateral mechanosensory (PLM) neurons. We then analysed severed axons that re-established a contact between the regrowing proximal axon and its separated distal fragment (Extended Data Fig. 1); maintenance, or inhibition of degeneration of the distal axonal fragment, was used as evidence of successful fusion, whereas degeneration indicated unsuccessful fusion7. In wild-type animals, 80%of axons that displayed proximal-distal reconnection underwent successful fusion (Fig. 1b, d). EFF-1, a trimeric fusogen similar to class-II viral fusion proteins13,14,mediates a wide range of fusion events inC. elegans15,16, including the...