Don’t lose your nerve: choosing the right path during axon regeneration

From the Moens Lab, Basic Sciences Division

Superman’s X-ray vision. The Flash’s super speed. Wolverine’s super healing. Who among us hasn’t fantasized about having superpowers, only to rue that fact that they’re just that – fantasies? Except, perhaps, for that last one. In the animal kingdom, super healing, also known as regeneration, abounds. Cut off a salamander’s arm, it’ll grow right back. Crush a zebrafish’s spinal cord, it’ll be back up and swimming in no time. And don’t even get me started on planarians – those things regenerate so well they use it as a reproductive strategy, tearing themselves in half and allowing each half to regrow its missing parts. Humans, unfortunately, are rather lacking in this capacity. Sure, we can heal cuts and scrapes. We’re even pretty good at regrowing a liver, if need be. But if you’ve got a severed arm or a crushed spine, I’m afraid you’re out of luck.

Scientists have long wondered at the mystery of why some animals are so much better at regenerating than we are, whether we possess the latent genetic potential to regenerate, and how such potential can be unlocked. Answering these questions generally involves examining how regeneration is controlled in animals that do it well, such as those described above, and then attempting to apply those strategies in mammals to enhance the healing process. Such is the goal of Drs. Adam Isabella (that’s me!) and Cecilia Moens in Fred Hutch’s Basic Sciences Division. In a new paper published in Development, Drs. Isabella and Moens have begun to work out the process by which the vagus nerve is rebuilt after injury.

Like the twisted roots of a tree, the vagus nerve extends axon branches out from the brain to touch several tissues in our bodies.  “The mammalian vagus nerve regenerates poorly after injury, which can result in loss of speech, difficulty swallowing, abnormal heart rate, and gastroparesis [improper stomach emptying],” the authors explain. In zebrafish, on the other hand, nerve regeneration is generally very successful. Thus, they turned to the fish to understand what it takes to get it right. To examine vagus regeneration, the authors used a laser to sever the nerve. Amazingly, the whole branched network grew back within a few days of injury. Moreover, it regained its function – the authors examined the fish’s ability to swallow, a key behavior controlled by the vagus nerve, and saw that this ability was lost after nerve severing and that it recovered coincident with nerve’s regrowth.

zebrafish vagus nerve
The many branches of the zebrafish vagus nerve (green). Image provided by Dr. Adam Isabella

But rebuilding a functional nerve is more difficult than simply regrowing its branches. The vagus, like most nerves, is a precisely organized structure – while it innervates many different tissues in the body, each of its neurons, based on where that neuron sits in the nerve, is tasked with extending an axon to just one specific site. Thus, the authors surmised, correct regeneration would mean that each neuron extend back to not just a target, but to the correct target, lest the nerve’s careful organization get jumbled in the process. To determine whether the vagus has this level of precision, the group set to the painstaking task of labeling individual neurons and tracking where they went during regeneration. Sure enough, they found that 89% of neurons regrew to the correct target.

How is this level of precision possible? How does the nerve convey to its constituent neurons exactly where they should extend to?  To dig deeper, the authors developed a new technique in which they could injure a single neuron, by plucking it out of the brain and putting it back into the brain of another fish, and then challenge the injured neuron in a variety of ways to see if they could disturb its regeneration.  Perturbing regeneration turned out to be a difficult task. They tried taking away the molecular signal that’s required for the neurons to find their targets during embryonic development – the neurons didn’t seem to care; they re-extended axons back to the correct target just fine. They tried moving the neurons to a different position in the brain, since position usually corresponds with which target a neuron extends to – again, no problem, the neurons found their correct target anyway. The only way they found to disturb regeneration was to have injured neurons regenerate in a situation where their intended tissue had never been innervated – without a path to follow back, an injured neuron would miss its mark and regrow to a different target.

The lesson, it seems, is that vagus regeneration is very robust. Injured neurons seem to have a strong sense of which tissue they need to regrow to, and they can overcome a number of obstacles to get there. Dr. Moens was also quick to point out the significance of the fact that the regenerating vagus nerve does not rely on the mechanisms that guided its formation during development: “Most studies of axon regeneration to date either describe general principles of how a new axon is initiated and grows, or demonstrate how developmental guidance mechanisms are re-used during regeneration. In vagus motor axon regeneration, the developmental cues we know about are gone, so axons need to use other information to find their correct targets.” Therefore, she explains, “this paper introduces a new model system in which to discover regeneration-specific axon guidance mechanisms”. As for the next big question the group plans to address, she notes, “The paper identifies general principles of vagus motor axon regeneration, but not the underlying molecular mechanisms…that these general principles imply.” And regarding the broader implications of this work: “The underlying regenerative mechanisms are likely to be more relevant to human stem-cell based therapies for motor neuron disease than the developmental mechanisms, since stem cell-based therapies necessarily introduce new neurons into a post-developmental context.” Super healing may remain the domain of superheroes for now, but if we can learn enough from animals like the zebrafish, this is one fantasy that may someday come true.

This work was supported by the National Institutes of Health.

UW/Fred Hutch Cancer Consortium member Cecilia Moens contributed to this work.

Isabella AJ, Stonick JA, Dubrulle J, Moens CB. (2021) Intrinsic positional memory guides target-specific axon regeneration in the zebrafish vagus nerve. Development 148 (18): dev199706.