Partial Spinal Regeneration in Rats Using a Two Part Process

This is rather clever. Houle et al at Case Western show in the Journal of Neuroscience that you can use a bacterial enzyme called chondroitinase to degrade scars in spinal cord lesions and enable regeneration of axons.

Just for background, there is some interesting neurobiology when it comes to spinal cord regeneration. If you sever peripheral nerve axons and then reconnect it, the axons will regrow to find their old targets leading to a functional restoration. However, something different happens in the central nervous system (CNS). Instead of regrowing, the axons just stop.

For many years, researchers believes that this difference between the behavior of axons in the PNS and in the CNS was because of some intrinsic difference in capacity for the axons. They believed that the PNS axons had a capacity for regeneration that the CNS axons lacked. It turns out that the reverse is the case. Instead, axons in the CNS are actually tonically inhibited from regrowth by the presence of discrete signaling pathways. An example of these pathways is the binding of a protein called Nogo to a protein called Nogo-receptor (incidentally the protein was named for this effect). Nogo is a protein present in CNS myelin that binds to its receptor on the axon and inhibits axon regrowth. If you infuse a Nogo inhibitor into an animal with a spinal cord hemisection, you can improve axon regrowth and lead to a functional recovery. (There are actually other pathways that do this same thing -- not just Nogo -- so it is a little bit more complicated, but that is basically the story.) Any interesting side note is why the nervous system would be organized in this matter. Why would the nervous system evolve to prevent regeneration? That is a good question. It has been speculated that in order to ensure stability of function you can't just have neurons growing axons willy-nilly, but evidence for that speculation has not to my knowledge been forthcoming.

Anyway, getting back to the paper, these guys show that an additional impediment to CNS regrowth is the formation of a glial scar. Whenever you have like a stroke or a transection -- any injury -- astrocytes in the CNS proliferate and form a glial scar. Scars in the nervous system -- like scars anywhere else -- are composed of a bunch of gunk including cells but also components of what is called ground substance. Ground substance -- as you learn in medical school -- is the stuff between cells. Pathologists tend to talk about it like it is some kind of ether, but really it is composed of glycoproteins like a protein called chondroitin sulfate. As an interesting aside, bacteria often create proteins to degrade stuff like chondroitin, and we can use these bacterial proteins to degrade scars.

This paper shows that if you use a two part strategy you can get regrowth around a hemisection of the spinal cord in a rat.

First, you use part of a peripheral nerve -- a so-called PN graft -- to get the axons regrowing around the damaged area. This overcomes that first issue I talked about related to the tonic inhibition of axon regrowth in the CNS. Second, you use chondroitinase to degrade the scar around the graft-spine interface -- the area where the regrowing axons re-enter the spinal cord. This overcomes the issue of physical obstruction of the regrowing axons.

And they show the animals get functionally better off -- this is an important thing that you should always check when reading a paper like this.

Below is a picture of rats treated with chondroitinase (ChABC) as opposed to those treated with saline. Between the two is a rat that was treated with chondroitinase but then had the PN graft cut to show that it reverts back to the level of the saline treated rat (indicating the functional recovery was because of the axons in the graft).

Here is a chart of the functional recovery comparing the chondroitinase-treated (ChABC) with saline treated rats:

As you can see the level of recovery is nothing to necessarily write home about, but it is an improvement. When you are talking about spinal recovery 1) even a little bit of improvement can make a big difference in someone's quality of life and 2) we need to work on incremental improvements before we can hope to achieve the holy grail of complete recovery. Baby steps...but we are getting there:

Money quote:

Our observation that ~20% of the fibers in the bridge exit into the CNS raises two important questions. Why is axonal outgrowth limited even after enzyme-induced CSPG remodeling, and how can we improve the extent of regeneration? One likely reason for regeneration failure at PNS/CNS interfaces is an addictive-like behavior that develops between axons and PNS glia (Grimpe et al., 2005Go), in which the strong interaction of the two somehow reduces the capacity for axonal exploration beyond the graft. Another possibility may be the long-term, de novo accumulation of heavily glycosylated proteoglycans at the graft-host interface after termination of the enzyme treatment. In the face of increasing CSPGs, perhaps only the most vigorously growing fibers would have access to the cord. Strategies that might enhance the exodus of axons would include the modification of the integrin repertoire of the regenerating growth cones (Condic, 2001Go), the delivery of classic (Kobayashi et al., 1997Go) or novel (Yin et al., 2006Go) neurotrophins to the region of supraspinal cell bodies or to the lesion site (Ye and Houle, 1997Go; Ramer et al., 2000Go; Romero et al., 2001Go), or the use of a combinatorial approach to target other inhibitory molecules that are not affected by ChABC (Filbin, 2003Go). Techniques that allow for the sustained release of ChABC even after distal graft insertion also may be advantageous. Enhancement of the rate and efficacy of axonal elongation in the bridge (Dusart et al., 2005Go; Hu et al., 2005Go) as well as increasing the magnitude of axonal exit into and synaptogenesis within the CNS is going to be especially important in larger animals in which distances across the lesion and into the relevant spinal gray matter regions are much greater than in the rat. It also will be important to ascertain whether a conditioning effect can be identified and used to advantage in larger animals and human injury situations.

It is noteworthy to discuss the obvious advantages as well as the disadvantages of PN grafts compared with other potentially useful transplantation techniques. Schwann cells have evolved to provide severed axons with an outstanding support system for rapid regeneration via growth-supporting adhesion and trophic molecules (Mirsky and Jessen, 1999Go; Sherman and Brophy, 2005Go). As the axons journey through the bridge toward the CNS, bands of Bungner support growth along the longitudinal axis of the bridge and curtail excess sprouting in ectopic directions. Another critical biological feature of the Schwann cell is its capacity to provide new myelin for the fibers they encapsulate. Indeed, all of the structural elements necessary for maintenance of a regenerated axonal tract are assembled in a peripheral nerve with connective tissue and vascular elements in the epineuria and perineuria conferring proper tensile and elastic characteristics and a well nourished environment to the tract formed within them. Given that there is an abundant supply of PNS segments, especially in larger animals, that can be used for autografting without major declinations in function (unlike the rat), there is no need for immunosuppressive therapy or an embryonic donor. A clinical disadvantage of long-distance PN grafting might be the need to route the bridge outside of the cord dura mater, making it vulnerable to long-term mechanical trauma. Perhaps in larger animals, a course under the dura may be feasible. Finally, the PNS bridging strategy, although capable of stimulating relatively long-distance regeneration of efferent axon systems caudally, has yet to be demonstrated to be capable of guiding sensory axons rostrally. Nonetheless, our results demonstrate that, once allowed back into the spinal cord, even a relatively small number of regenerated supraspinal axons targeted to a specific spinal cord segment can promote a significant measure of functional improvement after cervical spinal cord injury.

Hat-tip: EurekAlert.