Role of myelin-associated inhibitors in axonal repair after spinal cord injury

Abstract

Myelin-associated inhibitors of axon growth, including Nogo, MAG and OMgp, have been the subject of intense research. A myriad of experimental approaches have been applied to investigate the potential of targeting these molecules to promote axonal repair after spinal cord injury. However, there are still conflicting results on their role in axon regeneration and therefore a lack of a cohesive mechanism on how these molecules can be targeted to promote axon repair. One major reason may be the lack of a clear definition of axon regeneration in the first place. Nevertheless, recent data from genetic studies in mice indicate that the roles of these molecules in CNS axon repair may be more intricate than previously envisioned.

Figure 1

Comparisons of Nogo, MAG and OMgp mutant alleles used in the two triple knockout studies. A) Our Nogo targeting strategy utilized Cre-loxP technology to delete a region between the N- and C-terminus, resulting in a null allele that lacks all three Nogo isoforms. Strittmatter’s Nogo mutant allele was the result of a gene trap strategy using a retroviral insertion targeting the Nogo-A isoform, which also affected Nogo-B expression to result in a Nogo-A,B knockout mouse. B) Our OMgp targeting strategy utilized loxP and frt sites to delete both the entire coding sequence as well as the reporter/selection cassette. Strittmatter’s OMgp mutant allele similarly has the coding sequence deleted, but retaining the reporter gene (rep). C) Both labs used the same MAG mutation generated by Roder and colleagues where a neomycin coding sequence is inserted into the fifth exon to disrupt MAG expression.

Figure 2

Different injury models to assess axon regeneration and sprouting. A) To investigate regeneration of corticospinal tract (CST) axons, we use a mid-thoracic dorsal hemisection model of spinal cord injury that disrupts virtually all CST input into the caudal spinal cord. B) To investigate regeneration of serotonergic axon, we used a mid-thoracic complete transection model, which is necessary to cut serotonergic axons that are located throughout the spinal cord. C) To investigate sprouting of CST axons, we used a pyramidotomy model where descending CST axons are lesioned at the brainstem level before the decussation. The contralateral intact CST axons are traced and then detected at the level of the cervical spinal cord to assess sprouting across the midline. D) To investigate serotonergic axon sprouting, we used a mid-thoracic lateral hemisection, which lesions one side of the spinal cord. Serotonergic axon sprouting from the intact side is then detected immunohistochemically at the level of the lumbar spinal cord (i.e. below the level of injury). Areas shaded in gray in (A, B, D) were injured.