The Apoptotic Engulfment Machinery Regulates Axonal Degeneration in C. elegans Neurons

Abstract

Axonal degeneration is a characteristic feature of neurodegenerative disease and nerve injury. Here, we characterize axonal degeneration in Caenorhabditis elegans neurons following laser-induced axotomy. We show that this process proceeds independently of the WLDS and Nmnat pathway, and requires the axonal clearance machinery that includes the conserved transmembrane receptor CED-1/Draper, the adaptor protein CED-6, the guanine nucleotide exchange factor complex Crk/Mbc/dCed-12 (CED-2/CED-5/CED-12) and the small GTPase Rac1 (CED-10). We demonstrate that CED-1 and CED-6 function non-cell-autonomously in the surrounding hypodermis, which we show acts as the engulfing tissue for the severed axon. Moreover, we establish a function in this process for CED-7, an ATP-binding cassette (ABC) transporter, and NRF-5, a lipid-binding protein, both associated with release of lipid-vesicles during apoptotic cell clearance. Thus, our results reveal the existence of a WLDS /Nmnat-independent axonal degeneration pathway, conservation of the axonal clearance machinery, and a function for CED-7 and NRF-5 in this process.

Aberrant Information Transfer Interferes with Functional Axon Regeneration

“…We found that 12 hours after axotomy, DA9 axons had initiated regeneration and regenerated past the injury site (Figure 1C), similar to the kinetics of regeneration in other neurons (Chuang et al, 2014; Hammarlund and Jin, 2014; Yanik et al, 2004). However, even after 12 hours, the distal axon fragment still was present (Figure 1C), similar to slow removal of fragments after injury in other C. elegans neurons (Nichols et al, 2016). Presence of this distal fragment raised the possibility that regenerating DA9 axons might restore connectivity simply by fusion with the fragment, as previously observed in C. elegans (Abay et al, 2017; Ghosh-Roy et al, 2010; Neumann et al, 2015; Neumann et al, 2011).…”

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

“…The distal fragment can exhibit transient filopodial sprouting but slowly degenerates (Wu et al 2007), and is phagocytosed by the surrounding epidermis (Nichols et al 2016). Degeneration and phagocytosis of axonal fragments occur rapidly in early larval stages, but are slower in adults.…”

“…Degeneration and phagocytosis of axonal fragments occur rapidly in early larval stages, but are slower in adults. The phagocytic engulfment of axon fragments involves pathways defined from studies of engulfment of apoptotic cell corpses, and suggests that the anucleate axonal fragments are recognized as dying cells (Nichols et al 2016).…”

“…Axon fragments display membrane phosphatidylserine (PS), required for the fusion pathway (Neumann et al 2015). Despite acting as an "eat-me" signal for dying cells or axon fragments, axonal PS may also act as a "save-me" signal, leading to degeneration and engulfment or fusion depending on the context (Nichols et al 2016). Axon-axon fusion has also been observed in the unc-70 fragile axon mutant, where it can occur between neighboring axons at a low frequency (Neumann et al 2015).…”

RAB-5 regulates regenerative axonal fusion by controlling EFF-1 endocytosis

“…However, if there is instead molecular machinery for active secretion of these vesicles, one strong candidate is the ABC transporter CED-7, which is known to generate extracellular vesicles containing phosphatidylserine during apoptotic cell clearance 42 . We have shown that CED-7 functions in regeneration of the PLM neuron 8 as well as its degeneration 43 , possibly through vesicle generation. It is therefore possible that CED-7 activity in the neuron is required for the secretion of EFF-1 vesicles.…”