Review ArticleAxon regeneration in C. elegans: Worming our way to mechanisms of axon regeneration
Section snippets
General features of the C. elegans axon regeneration model
The adult C. elegans hermaphrodite is a transparent cylinder approximately 1 mm long that contains 302 neurons (Fig. 1). The worm's nervous system includes motor, sensory, interneuron, and polymodal neurons and can be divided into 118 distinct classes of neurons, including GABAergic, cholinergic, chemosensory, mechanosensory, oxygen sensing, osmoceptors and proprioceptors (White et al., 1986). The development and positions of individual neurons are invariant from worm to worm. Together with the
Techniques for axon injury: lasers and genetics
Traditional mechanical techniques to injure axons, such as crushing nerves with forceps or severing nerves with a scalpel, have not been applied in C. elegans, due to the small size of the worm. Instead, optical or genetic techniques are used to sever axons. The first investigations of axon regeneration in C. elegans were carried out on axons that had been severed with amplified Ti-sapphire lasers that produce femtosecond pulses of near infrared light (780–800 nm) (Yanik et al., 2004). These
Genetic screens identify regulators of axon regeneration
A prominent benefit of the C. elegans regeneration model is the ability to conduct forward screens to identify genes involved in axon regeneration. To date, multiple screens of various types and scales have been undertaken, including RNAi screens, mutant screens, automated chemical screens, and candidate screens. These screens have made significant contributions to our understanding of axon regeneration, and have generated large amounts of data that awaits detailed analysis.
A large screen for
Current areas of investigation
Current investigations focus on the regulation and role of injury sensing, signal transduction, cytoskeletal dynamics, aging, and axon fusion in axon regeneration in C. elegans. In many cases, the investigations are motivated by the identification and characterization of genes in the screens described above. The emerging understanding of axon regeneration in C. elegans is that, as in the mammalian system, axon regeneration is a complex, orchestrated process regulated by the concerted function
Dual leucine zipper kinase-1
The MAP kinase kinase kinase dlk-1 (dual leucine zipper kinase) is the best characterized intrinsic regulator of axon regeneration in C. elegans (Nakata et al., 2005, Hammarlund et al., 2009, Yan et al., 2009). In dlk-1 loss of function mutants, axon regeneration is critically compromised, and in animals that overexpress dlk-1, axon regeneration is improved compared to axon regeneration in wild type animals. dlk-1's function in axon growth is specific to injury; dlk-1 is not required for
Cytoskeletal dynamics
In addition to affecting calcium signaling, MAP kinase signaling, and gene transcription, injury also affects cytoskeletal dynamics. A primary component of the cytoskeleton is the microtubule, which regulates cell shape, axonal transport, and axon growth. Microtubules are relatively dynamic in the growing or regenerating axon and stable in the mature axon. Upon injury, neurons must destabilize their microtubules in order to regenerate. In injured C. elegans mechanosensory neurons, DLK-1
Age vs. axon regeneration
As in the maturing mammalian central and peripheral nervous systems, C. elegans axons lose their ability to regenerate as they age (Pestronk et al., 1980, Tanaka et al., 1992, Verdu et al., 1995, Verdu et al., 2000, Wu et al., 2007, Gabel et al., 2008, Hammarlund et al., 2009, Nix et al., 2011, Zou et al., 2013, Byrne et al., 2014). In C. elegans, the loss of regenerative ability occurs in largely two phases: the first phase of decline occurs as the worm develops to adulthood (Wu et al., 2007,
Axon fusion
An active area of C. elegans regeneration focuses on a fusion event that joins a regenerating axon to the distal segment of the severed axon. When mechanosensory axons are severed, the regenerating axon can fuse with the severed distal axon segment (Gabel et al., 2008, Neumann et al., 2011, Neumann et al., 2015). Reconnecting the severed axon segment to the cell body prevents degeneration of the severed axon (Yanik et al., 2006, Ghosh-Roy et al., 2010, Neumann et al., 2011). After injury,
Environment
Although most research in C. elegans to date has focused on intrinsic mechanisms that mediate regeneration, axon regeneration in the worm is also affected by extracellular cues. For example, as in the mammalian nervous system, axon guidance pathways including netrin, ephrin, WNT, and SLT/ROBO play an important role in the ability of severed axons to regenerate towards their target (Benson et al., 2005, Fabes et al., 2007, Wu et al., 2007, Gabel et al., 2008, Liu et al., 2008, Low et al., 2008,
Future areas of investigation
Research in the next decade is likely to yield great advances in understanding axon regeneration, as synergistic findings are emerging from multiple diverse approaches. With its strong knowledge base and robust research community, as well as its particular experimental advantages discussed above, C. elegans is positioned to make important contributions. Conservation of function between molecules that regulate axon regeneration in C. elegans and those that regulate axon regeneration in mammals,
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