It is well known that damaged axons from the central nervous system of mammals cannot regrow. Except perhaps they can, with the right genotype. In the May 21 Neuron online, researchers from Boston Children’s Hospital report that a mouse line known as CAST, derived from Asian forebears, possess the capacity for stunning axon regeneration following injury or stroke. Genetic analysis showed this was due in part to their high expression of the signaling protein Activin. 

Axon Regeneration: Following optic nerve crush, the axons of standard C57 Black mice make feeble attempts at regrowth (top), while the axons of CAST mice (bottom) forge ahead. [Image courtesy of Neuron, Omura et al.]

While injured neurons in the periphery can sprout anew, those in the central nervous system are stumped by the myelin milieu, which produces factors that block growth (reviewed in Filbin, 2003). Takao Omura, first author on the Neuron paper and a surgeon at the Hamamatsu University School of Medicine in Japan, was frustrated that treatments for CNS injuries have not advanced in decades. He and senior author Clifford Woolf hatched a plan to search for new genes that might help design treatments. They began by comparing genetically different mice to see if some were better at axon regeneration than others. Omura collected nine strains and cultured their dorsal root ganglion neurons on myelin. Most of those neurons extended piddling axons, but the CAST cultures stood out, growing neurites five times longer than the others. “It was amazing,” Omura said. “At first, I thought it was a mistake.” 

Omura’s collaborators, in Boston and at the David Geffen School of Medicine at the University of California, Los Angles, tested the CAST mice in classic paradigms of axonal damage—stroke, spinal-cord injury, and optic-nerve crush. In each case, axon regrowth in CAST mice outpaced that of other strains, such as C57 Black. “You see crazy amounts of regeneration,” said Larry Benowitz of Boston Children’s Hospital, whose lab performed the optic nerve experiments (see image above). However, the CAST effect was limited to the CNS; in the periphery, regeneration rates were like those of other mice. The CAST genotype must allow the neurons to ignore the inhibitory signals coming from myelin, reasoned co-senior author Michael Costigan, also at Children’s.

The scientists told Alzforum that CAST mice differ from most mouse lines used in research, which are descended from the “pocket pets” of European mouse fanciers of the Victorian era. The CAST founders were not so dainty—they were wild mice trapped while gorging on grains in a warehouse in Thailand. Costigan’s group profiled the mRNA of the different strains, and determined the CAST mice made unusually large amounts of the gene Activin, a signaling molecule in the TGF-β pathway. Treating CAST dorsal root ganglion neurons with SB-431542, a small molecule inhibitor of the Activin receptor ALK, diminished their neurite growth ability. Adding Activin to C57 Black cultures enhanced axon growth.

Bruce Dobkin of the David Geffen School of Medicine, who was not involved in the study, thought that a pro-Activin treatment might have clinical potential for stroke and spinal-cord injury. He was less optimistic about neurodegenerative diseases, but speculated that activating sprouting might slow Alzheimer’s or ALS. Costigan was also skeptical about treating ALS this way, pointing out that motor neurons project into the periphery, where the CAST genotype made little difference.—Amber Dance


  1. Unlike the axons in the peripheral nervous system, the axons in the central nervous system do not readily regenerate after injury. In this study by Omura et al., the investigators have screened several different mouse strains and discovered that one strain, namely the CAST/Ei, shows exceptional regenerative phenotypes. Furthermore, the investigators carried out a series of experiments to examine what genes/proteins are differentially expressed in the CAST/Ei mice, and revealed that a protein called Activin is particularly active, and is responsible for the remarkable axon regeneration in these animals.

    Interestingly, this protein is known to regulate tissue regeneration in lower-vertebrate species such as fish and gecko. While some studies in the past have indicated differential regenerative and cell survival capacities among different mouse strains, this current study went the distance and performed comprehensive analyses, revealing key growth regulators. Given that the lack of axon regeneration/plasticity poses great challenges to the healing of neurodegenerative conditions including ALS, and CNS trauma (e.g., spinal cord injury), uncovering such mouse strains and proteins will help us further understand mechanisms controlling this process, and ultimately develop therapies.

  2. Thanks, Kevin, for the encouraging words in summarizing this article. We are thrilled with the amount of regeneration observed in the CAST animals, and also that the molecular mechanisms we identify parallel those in lower species that can readily regenerate body parts. We are confident that this is a major discovery in the field of CNS regenerative medicine. We plan to continue to develop these mechanisms and hope that others also attempt to elucidate the keys that this mouse strain offers us toward achieving sustained effective axonal growth in the CNS. Clearly our aim in unraveling these mechanisms is to one day allow therapies for patients with neurodegenerative conditions in the CNS.

  3. First, I want to congratulate Dr. Cliff Woolf, his colleagues and collaborators for this beautifully executed study in Neuron. Speaking as someone who was trained as a mouse geneticist, the current work is a tour de force, using the power of mouse genetics to identify new genes that regulate an important biological process. Scientists have long suspected - and even had some fragmentary evidence - that different mouse strains have different regenerative abilities and that this is a trait that can be traced to discover new genes to promote axon regeneration. However, to my knowledge, the current study is the first of its kind to bring this idea to fruition.

    What is particularly impressive here is that a single gene was identified as a major contributor to the high regenerative ability of Mus musculus castaneus. In retrospect, the inclusion of castaneus was crucial to the success of the project. For ease of handling, castaneus may not have been an obvious choice. These mice are not easy to work with: they try to jump out of the cage at any instance. However, casteneus is genetically divergent from common laboratory mouse strains and the high level of polymorphisms between the two has been employed to facilitate genetic mapping studies. This was probably the original plan and it would have taken years just to finish the mapping, let alone identify new genes. Instead, Woolf and colleagues took advantage of new tools in genomics in their search for candidate genes. Gene expression profiling allowed them to quickly narrow a list of candidate regenerative genes down to just over a dozen genes, with one (Inhba, which encodes Inhibin beta A, a subunit of Activin as well as Inhibin) topping the list. Pharmacological experiments showed the functional relevance of this molecule.

    In an ideal world, one would also like to test the function of Inhba by conditionally deleting the gene in castaneus. However, that would simply be too much to ask for in this case, given the tremendous amount of work already done by the researchers. In fact, if an assistant professor were to start a lab with this project, he/she would likely run out of funding before a tenure decision is made. The repertoire of expertise involved in this study is unparalleled by any other in neural regeneration research - the long list of authors, including many recognized scientists, is a testimony to this fact.

    In the end, we learned that Activin is a new important player in CNS axon regeneration, especially under conditions that prime the neurons for regeneration. We are reaffirmed - more strongly than ever - that mouse genetics has a place in neural regeneration research, and that classical mouse genetics can be combined with modern genomic tools to speed up discoveries even for something as seemingly intractable as regeneration in the brain and spinal cord.

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Paper Citations

  1. . Myelin-associated inhibitors of axonal regeneration in the adult mammalian CNS. Nat Rev Neurosci. 2003 Sep;4(9):703-13. PubMed.

Further Reading


  1. . Metallothionein-I/II Promotes Axonal Regeneration in the Central Nervous System. J Biol Chem. 2015 Jun 26;290(26):16343-56. Epub 2015 May 6 PubMed.
  2. . Axonal regeneration. Systemic administration of epothilone B promotes axon regeneration after spinal cord injury. Science. 2015 Apr 17;348(6232):347-52. Epub 2015 Mar 12 PubMed.

Primary Papers

  1. . Robust Axonal Regeneration Occurs in the Injured CAST/Ei Mouse CNS. Neuron. 2015 May 20; PubMed.