A re-assessment of the effects of a Nogo-66 receptor antagonist on regenerative growth of axons and locomotor recovery after spinal cord injury in mice
Introduction
Achieving a high level of recovery of function after spinal cord injury is likely to require the regeneration of the axons of the long ascending sensory and descending motor and autonomic tracts of the spinal cord. Accordingly, there is great interest in identifying the reasons for regeneration failure, and in developing interventions to promote regeneration that would otherwise not occur. In this regard, there has been considerable interest in the evidence for various inhibitory molecules in myelin, including Nogo, myelin-associated glycoprotein (MAG) and oligodendrocyte myelin glycoprotein (OmGP). These proteins are expressed by oligodendrocytes, and are thought to interact with the Nogo receptor on axons to trigger signaling events that lead to axon growth inhibition. These different molecular pathways that inhibit axon regeneration present reasonable targets for therapeutic interventions, for example by using antibodies to block Nogo or synthetic peptides to block the interaction between Nogo, OmGP, and MAG and the Nogo receptor (Lee et al., 2003, Xu et al., 2004).
Although a number of strategies show promise for enhancing regeneration, a barrier to translation is that promising findings are often not re-evaluated in independent replications to assess the robustness and reproducibility of the effects. To meet this need, the NINDS launched the “Facilities of Research—Spinal Cord Injury” (FOR-SCI) replication project, in which promising published studies are independently replicated. Here, we repeat an experiment reporting that delayed subcutaneous treatment with the NgR antagonist peptide NEP1-40 (Nogo extracellular protein, residue 1–40) results in enhanced growth of corticospinal (CST) axons, sprouting of serotonergic (5HT) fibers and enhanced locomotor recovery after thoracic spinal cord injury.
Section snippets
Materials and methods
Consistent with the requirements of the FOR-SCI replication project, our goal here was to repeat as closely as possible the key experiments reported in Li and Strittmatter (2003). All experimental procedures were as described in the original report. Where necessary, the original authors were contacted for clarification of procedural details. The original report described results from a total of eight different groups of animals (10–12 animals per group) treated in one of 3 ways: 1) Subcutaneous
Experiment 1A, early treatment
A total of 6 out of 31 animals died over the course of the experiment. Four of these were in the group that was receiving NEP1-40; these animals were found dead in the morning 2, 3, 6, and 11 days post-injury without having shown signs of illness or debilitation the day prior. The other two were receiving Vehicle. One of the mice died as the animal was receiving the Vehicle injection on day 3 post-injury. The other was found dead on the morning of day 2 without showing signs of illness or
Discussion
The present study assessed the reproducibility and robustness of the findings of Li and Strittmatter (2003) that treatment with the NgR antagonist peptide NEP1-40 results in enhanced growth of corticospinal and serotonergic axons and enhanced locomotor recovery after thoracic spinal cord injury in mice. Our anatomical analyses did not support the interpretation that treatment with NEP1-40 robustly enhanced regenerative growth of CST axons or 5HT axons. Similarly, the various functional
Conclusion
Our experiments overall failed to replicate the findings of Li and Strittmatter regarding enhanced sprouting and regeneration of the CST and the serotonergic system. Nevertheless, when taken together, our results do suggest that treatment with NEP1-40 created a situation that was slightly more conducive to a form of axon regeneration or sprouting that also occurs in un-treated mice. Similarly, we failed to replicate the findings of enhanced functional recovery, although there was a trend for
Acknowledgments
Supported by NO1-NS-3-2353 to O.S. Thanks to Dr. Stephen Strittmatter for open and frank discussions regarding the experiment and for supplying the NEP1-40 and control peptides. Thanks to Shuxin Li for performing the spinal cord injuries.
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