Here’s a new paper this week on the topic of the growth-blocking glial scar that forms in the area of spinal cord damage. There has been much discussion in recent years about ways to chemically degrade this scar, using an enzyme called chondroitinase, now stuck with the nickname chase. Chase dissolves certain sugar chains that characterize the cartilage-like scar material, called condroitin sulfate proteoglycans (CSGPs).
It is always a noteworthy paper if the words "improves" and "locomotor function" appear in the title. The new work, “Neutralization of Inhibitory Molecule NG2 Improves Synaptic Transmission, Retrograde Transport, and Locomotor Function after Spinal Cord Injury in Adult Rats
,” comes from the lab of Victor Arvanian at the State University of New York, Stony Brook. Arvanian is well known to the Reeve Foundation, having been set along his career path as a post-doc Associate in the International Research Consortium on Spinal Cord Injury, part of the Lorne Mendell group at SUNY Stony Brook. Arvanian has his own lab now with strong grant support to continue his work. This new paper is in collaboration with a group at the Brain Research Institute in Zurich, including Lisa Schnell, also well known here as part of the Consortium’s Martin Schwab lab in Zurich.
On to the paper: The researchers found that in a thoracic hemisection animal model (partially severed spinal cord injury), blocking just one part of the CSGP, called NG2, with an injected antibody allowed axon growth, improved formation of synapses and improved locomotor function. Says the paper:
Neutralizing antibodies against NG2 may be an excellent way to promote axonal conduction after SCI.
They used the NG2-Ab (the rather lackluster name of the antibody) acutely but more clinically relevant, in a chronic model too, using an implanted pump to infuse the growth-promoting molecules in the site of injury.
Also from the paper:
Results of the current study demonstrate that chronic administration of NG2-Ab improved diminished synaptic transmission to lumbar L5 motoneurons through spared VLF axons [ventrolateral funiculus], most probably by enhancing excitability of these axons and by promoting anatomical plasticity after chronic hemisection injury. The effects of NG2-Ab on synaptic plasticity in lumbar segments of the damaged spinal cord were tightly correlated with the effects of NG2-Ab on anatomical plasticity.
These results suggest that neutralization of NG2 may either prevent cell death, partially preserve axonal connections between segments L1 and L5, or restore/maintain axonal transport in these nerve fibers. These anatomical results are in a good agreement with electrophysiological recordings demonstrating the enhanced synaptic plasticity in the L1–L5 segment of the injured spinal cord….
Consistent with the beneficial effects of NG2-Ab on synaptic transmission in damaged spinal cord, NG2-Ab treatment increased the excitability of these axons, promoted anatomical plasticity in lumbar segments, and improved locomotor recovery.
The scientists had to measure function compared to the natural recovery lab rats typically show:
During recovery phase at weeks 2 and 3 post injury, NG2- Ab-treated rats showed significantly better recovery of locomotor function versus control-Ab-treated rats. However, beginning at 4 weeks and continuing through week 8, no significant differences were observed between groups in BBB locomotor score. It is important to note that the quasiquantitative BBB scoring has been widely used for evaluating the loss of function and recovery following injury and is particularly useful for locomotor evaluation during the initial days postinjury, when other challenging tests cannot be performed because of lack of adequate locomotor function. However, the BBB protocol was not sensitive enough to assess the subtle improvements that result from treatments after thoracic HX [hemisection] injury in rodents because of the robust spontaneous recovery of locomotor function that takes place after this type of injury. Therefore all animals were also tested in more sensitive tests.
The paper explains why they chose to deal with just one component of CSPGs:
Because CSPGs are essential components of the extracellular matrix, neutralization of a single CSPG family member has been proposed to be more beneficial than degrading all CSPGs Among the CSPGs elevated in the vicinity of the glial scar, NG2 has been implicated as a major obstacle to axonal regeneration following brain and spinal cord injury. In addition to its function as a key inhibitory molecule of axonal regeneration and sprouting, NG2 was found to block axonal conduction. The results of the current study revealed that acute intraspinal injections of NG2-Ab, before injections of NG2, prevent the ability of NG2 to block axonal conduction. These results strongly suggest that delivery of NG2-Ab may be a potent tool for the neutralization of the inhibitory function of NG2 on axonal conduction leading to increased synaptic plasticity in spinal circuits where levels of NG2 are elevated.
According to the paper, “The importance of improved plasticity in the lumbar circuitry cannot be underestimated,” due to the minimal amount of regeneration from above, and due to the possible connection to the central pattern generator, the basis of activity-based rehab training. Verbatim:
In damaged spinal cord, supraspinal regeneration is absent or minimal. Thus, the goal of enhancing the regeneration of damaged propriospinal fibers around the lesion or sprouting of spared propriospinal axons caudal to the injury epicenter are more achievable alternatives for recovery of function after injury. L5 motoneurons in the adult rat lumbar spinal cord directly innervate the hindlimb muscles. …
Thus, improved synaptic and anatomical plasticity of the short propriospinal neurons within lumbar segments induced by NG2-Ab treatment may have translational potential for improving function after partial SCI.
But the authors of the paper don’t expect NG2 alone will be the answer. Arvanian has anchored other collaborations on the combinatorial approach: Chondroitinase ABC Combined with Neurotrophin NT-3 Secretion and NR2D Expression Promotes Axonal Plasticity and Functional Recovery in Rats with Lateral Hemisection of the Spinal Cord
; and also Combined Delivery of Nogo-A Antibody, neurotrophin-3 and the NMDA-NR2d Subunit Establishes a Functional ‘Detour’ in the Hemisected Spinal Cord
. From the latest paper:
… treatment with NG2-Ab alone did not affect the negligible number cells in thoracic T4–T7 segments projecting to L5 and induced minor (although sufficient) improvement of locomotor function. Consistent with these limited effects of NG2-Ab alone on locomotor function, treatment with Ch-ABC alone, although found to improve axonal sprouting, induced little or no recovery of locomotor function following contusion SCI and lateral HX injury at thoracic and cervical levels. More promising therapies for spinal cord repair can be expected when strategies incorporating inhibition of CSPGs are combined with neurotrophic support, peripheral nerve graft implantation or rehabilitation. Restoring function after spinal cord injury through the multipronged approach of combining NG2-Ab with neurotrophin delivery and/or rehabilitation is our next task.
The wrap up:
In conclusion, our results demonstrate that prolonged treatment with NG2-Ab may be an excellent tool for improving synaptic and anatomical plasticity after partial midthoracic SCI. These results provide a solid foundation for suggesting that NG2-Ab can be a very important and useful part of a combinatorial treatment. We expect that therapies based on combining neutralization of NG2 molecules with appropriate neurotrophic support will be an effective strategy to promote recovery of locomotor function.