Axon-specific microtubule regulation drives asymmetric regeneration of sensory neuron axons

Abstract

Sensory dorsal root ganglion (DRG) neurons have a unique pseudo-unipolar morphology in which a stem axon bifurcates into a peripheral and a central axon, with different regenerative abilities. Whereas peripheral DRG axons regenerate, central axons are unable to regrow. Central axon regeneration can however be elicited by a prior conditioning lesion to the peripheral axon. How DRG axon asymmetry is established remains unknown. Here we developed a rodent in vitro system replicating DRG pseudo-unipolarization and asymmetric axon regeneration. Using this model, we observed that from early development, central DRG axons have a higher density of growing microtubules. This asymmetry was also present in vivo and was abolished by a conditioning lesion that decreased microtubule polymerization of central DRG axons. An axon-specific microtubule-associated protein (MAP) signature, including the severases spastin and katanin and the microtubule regulators CRMP5 and tau, was found and shown to adapt upon conditioning lesion. Supporting its significance, interfering with the DRG MAP signature either in vitro or in vivo readily abolished central-peripheral asymmetries in microtubule dynamics and regenerative ability. In summary, our data unveil that axon-specific microtubule regulation drives asymmetric regeneration of sensory neuron axons.When nerves in our body are damaged, their ability to repair themselves depends on where they are. Some nerves, like those in the arms and legs, can heal, while the ones in the spinal cord cannot. This difference is particularly striking in dorsal root ganglion (DRG) neurons, which have a unique structure. Unlike other neurons, which transmit signals along a single long projection called an axon, DRG neurons branch into two axons – one connecting to the body and the other to the spinal cord. While the branch leading to the body can heal, the one connecting to the spinal cord is unable to regenerate.It is not clear how DRG neurons develop axons with these differing abilities. Researchers have found that an injury to the body side branch, known as the peripheral axon, can stimulate regrowth in the stretch leading to the spinal cord, known as the central axon. The damage increases the transport of molecules along both axons, boosting the repair of the whole neuron. This suggests that microtubules, the internal highways for transporting materials through cells, may contribute to the difference between the regenerative ability of the two axons of the DRG neuron.To explore this, Costa et al. studied DRG neurons grown in the laboratory and rodents. Powerful microscopes revealed that the central axons contain more actively growing microtubules than the peripheral axons. However, when the peripheral axon was damaged, the central axon reduced microtubule growth, making it more capable of regeneration. Costa et al. also identified that injury caused changes in levels of microtubule-associated proteins (MAPs), which regulate microtubule behaviour. Reducing the amount of one of these MAPs prevented axon repair in both cell cultures and animal models.These findings help explain why some nerve fibres regenerate while others do not, highlighting the role of microtubules in this process. Further research is needed to determine whether targeting MAPs may lead to new treatments for spinal cord injury or other nervous system damage in humans.