As a postdoctoral fellow with Massimo Hilliard at the Queensland Brain Institute, I sought to address this knowledge gap by analyzing precisely how individual axons respond to injury and the mechanisms by which they can mount a regenerative response. To achieve this we exploited the experimental advantages afforded by the tiny nematode Caenorhabditis elegans (C. elegans), including the ability to visualize and sever the axons of individual neurons in living animals. Using a UV-laser to transect the axons of the C. elegans mechanosensory neurons, we identified a mechanism of repair known as axonal fusion (Neumann et al. Developmental Dynamics, 2011), a regenerative process in which severed axons spontaneously repair themselves by regrowing, reconnecting and fusing with their separated counterparts. Axonal fusion provides a highly efficient means of nervous system repair, as instead of recreating the entire axonal length beyond the site of damage, neuronal structure can be repaired by simply bridging the damaged zone. This means of repair is not limited to C. elegans, having been previously observed in several invertebrate species, including crayfish, earthworm, and leech, as well as in cultured murine neuroblastoma cells.
Following axonal transection, axonal fusion occurs if the regenerating axonal can reconnect and fuse with its separated segment to restore membrane and cytoplasmic continuity.
Axonal injury initiates a biological race between regeneration and degeneration, which must be won by regeneration if axonal fusion is to occur.
However, in addition to controlling the degeneration/regeneration balance, an understanding of the molecular mechanisms that mediate the axonal fusion process itself is vital for manipulating the outcome of the race. The observation of axonal fusion in the highly genetically amenable nematode provided the opportunity to characterize the process at a molecular level. Our analysis of a novel mutation that results in hyper-stable axonal microtubules revealed the importance of microtubules in the axonal fusion process, with their disruption strongly inhibiting the success rate of this repair mechanism (Kirszenblat, Neumann et al. Molecular Biology of the Cell 2013). In addition, we speculated that molecules involved in recognition and fusion events in different biological contexts may also be involved in axonal fusion, and therefore tested the ability of severed axons to undergo axonal fusion in the absence of such molecules. We discovered that regenerative axonal fusion shares much of its molecular machinery with that involved in the process of apoptosis (Neumann et al. Nature 2015). Following injury, the composition of the axonal membrane is altered, such that the phospholipid phosphatidylserine (PS), which is normally restricted to the cytoplasmic leaflet of the membrane, is flipped to the external surface to serve as a recognition, or ‘save-me’ signal for the regrowing axon. Dying cells also display this PS signal, in the form of an ‘eat-me’ signal for engulfment by surrounding phagocytes. This ‘eat-me’ signal is recognized by secreted ligands including the transthyretin TTR-52 and lipid binding molecule NRF-5 which, together with the ABC transporter CED-7, modulate the signal to promote engulfment. A critical component for the recognition process is the PS receptor PSR-1, which is expressed on the engulfing cell and directly binds the eat-me signal on the dying cell. We found that these mechanisms are largely shared with the process of axonal fusion, with the regrowing segment emulating the engulfing cell, and the separated axon analogous to the dying cell. Once reconnection between the axon segments is established, the two membranes are fused together by the actions of the fusogen molecule EFF-1.
Recognition between the regrowing axon and its separated segment occurs through similar mechanisms to those that drive apoptosis, with the severed axon exposing PS as a ‘save-me’ signal (red circles), which is bound by the secreted ligands TTR-52 (light blue) and NRF-5 (dark blue), and the membrane-bound receptor PSR-1 (gray). Bound ligands bridge the two membranes through interaction with membrane-bound receptors, which may include CED-1 (black).