Motor axons are long nerve cables, or processes, that start in brain cells, or neurons, and course the spinal cord to form synapses (connections) with muscle. Brain says move, message goes down the motor axon to the motor, muscle fires. Of course spinal cord injury destroys these axons; the mother nerve cell survives, up in the brain stem. But the broken cable cuts off brain-to-muscle control, which leads to paralysis.
A major question in neuroscience research: Can these motor axons be regenerated from above and make appropriate connections below the cord injury? On their own, no, they cannot. The area of damage is toxic; the axons are stuck and need a jump-start to head south, and then it seems they also need a chemical seduction from below the lesion so they can cross the tortured anatomy of the spinal cord.
There are indeed ways to help these axons. Using a combination of strategies, motor axons can be rebooted, they can grow along a cellular bridge across the scarred-up lesion site, they can overcome inhibitory roadblocks, and once they get a whiff of an irresistible chemical allure below them, they are indeed able to form synapses with muscle units. This finding is supported by a compelling paper that came out this week from the Mark Tuszynski lab at the University of California, San Diego, “Motor Axonal Regeneration after Partial and Complete Spinal Cord Transection
Let’s get the suboptimal news out of the way. Success in getting motor axons to rehook with muscle is challenging and an important accomplishment. Problem is, the animals in this set of experiments got worse after the treatment. Quoting the paper: “… in experimental animals with hemisections, or partial lesions, the motor outcomes worsened after partial cervical lesions and spasticity worsened after complete transection. These findings highlight the complexity of spinal cord repair and the need for additional control and shaping of axonal regeneration.”
I spoke with Pengzhe (Paul) Lu, Ph.D., the lead author of the paper. In the eyes of the research team, the paper is an unprecedented success. “A lot of people claim motor axon regeneration – most of them go around the lesion because it is an incomplete lesion. There may be some growth but nobody
has shown that a motor axon can penetrate the lesion site and go to the other side. With our motor axon bridge and combinatory treatment, we are the first to do that.”
Lu said it was important to use a complete transection model -- to remove any doubt that axons moving across the lesion were not there already. His results showed axons that came from the brain, crossed the damaged area, and hooked up below.
Lu is one of a handful of scientists working on spinal cord research who is informed by his own spinal cord injury. About 15 years ago he was a post-doc in molecular biology at UC Davis; he had a car accident and became paraplegic. Newly motivated, he looked for an SCI research lab, contacted Tuszynski and has been there 14 years. Most of that time has been spent studying motor axon regeneration.
In 2004 Lu was main author of a Tuszynski lab paper that got a lot of notice as a major advice in SCI research: “Combinatorial Therapy with Neurotrophins and cAMP Promotes Axonal Regeneration beyond Sites of Spinal Cord Injury
Said Tuszynski at the time: “This study shows unequivocally that axons can be stimulated to regenerate into a cell graft placed in a lesion site, and out again, into the spinal cord -- the potential basis for putting together a practical therapy.”
In 2009, Lu was on a team that showed motor axon regeration in a chronic model of SCI, as long as 15 months post-injury: “Combined Intrinsic and Extrinsic Neuronal Mechanisms Facilitate Bridging Axonal Regeneration One Year After Spinal Cord Injury
This made the news, as well it should have. Tuszynski, again: “Our findings indicate that there is potential for promoting repair of the injured spinal cord even in chronic stages of injury. While the regenerating axons grow for relatively short distances, even this degree of growth could be useful. For example, restoration of nerve function even one level below an injury in the neck might improve movement of a wrist or hand, providing greater quality of life or independence.”
Back to the new paper, which builds directly on the previous work. This one is an acute model but certainly has implications for the chronic SCI community.
A little bit of background: For many years scientists have known that if you cut a peripheral nerve first, then cut its related spinal cord axons, regeneration occurs much better than if you cut the spinal axon only. This is called a conditioning lesion and it has to do with neurochemical remodeling in the cord to make it more hospitable to recovery. In 2002, Marie Filbin, at Hunter College, City University of New York, published a paper
showing that she could mimic the effect of a conditioning lesion without cutting peripheral nerve. She elevated a molecule called cyclic AMP and thus enabled axons from the dorsal roots to regenerate.
This is where Lu joins the story. His group replicated Filbin’s work, then collaborated with her. Indeed, she was co-author of the 2004 regeneration-beyond-the-lesion paper mentioned earlier. Lu then moved from dorsal axons, which are sensory and easier to restore, to motor axons.
In the new study, the first part of the Lu's combination was cAMP, injected into the brain, which sensitizes motor axons to begin their new journey. Next, a matrix of bone marrow stromal cells was transplanted to the lesion site. This provides a scaffold or bridge for the axons to cross what is normally no-man’s land. Third, they inserted viral-vector cells that continuously produce BDNF, a growth factor (kind of a vitamin or nutrient) below the lesion. There were six experimental groupings of animals; some got a lesion only, some got one or two parts of the three-way combo, others got the whole array.
Result, from the paper:
Subjects that received full combination treatment with cell grafts, cAMP injections into the pontine reticular nucleus, and growth factor gradients within and beyond the lesion site, exhibited axonal regeneration across sites of complete thoracic spinal cord transection. … Regenerating axons crossed glial interfaces and sites of chondroitin sulfate proteoglycan deposition to penetrate the host spinal cord beyond the lesion.
What’s happening? Lu said the stromal cells may have helped enhance the spinal cord for growth, including modifying scar tissue. He thinks the BDNF attraction was stronger than the resistance of the scar, thus success in crossing the bridge.
Alas, it is BDNF that is the likely reason the animals fared worse than those with lesions and no full combo treatment. Said Lu, “We used gene therapy to deliver BDNF. The problem is that it is continued to be expressed. BDNF not only attracts axons to other side but may modify local spinal cord circuitry.” That explains the side effect, why hind and forelimbs were more spastic.
“The BDNF should be shut down once we make some connections; it is not necessary to always be there. That is our challenge for future studies. Using a viral vector for gene therapy, there is almost no short term expression. We need to develop a short term vector.”
Said Lu, “Anatomically this study was very successful. Other scientists have said they can grow motor axons but only along a ramp, as in a roadway. They are not able to get off the ramp. My paper, this study, I believe it is the first real experiment to show we can get an axon off the ramp. We achieved success with a motor axon bridge. Our next direction is how to maximize the effect.”