Axon regeneration: Vaccinating against spinal cord injury

AbstractMyelin is a potent inhibitor of axon regeneration, but has been viewed as just one of many factors that prevent regeneration after injury. So it comes as a surprise that immunization against myelin has been found to allow extensive axon regeneration after injury, without apparent autoimmune-induced demyelination

Myelin-associated Glycoprotein Interacts with Neurons via a Sialic Acid Binding Site at ARG118 and a Distinct Neurite Inhibition Site

Inhibitory components in myelin are largely responsible for the lack of regeneration in the mammalian CNS. Myelin-associated glycoprotein (MAG), a sialic acid binding protein and a component of myelin, is a potent inhibitor of neurite outgrowth from a variety of neurons both in vitro and in vivo. Here, we show that MAG's sialic acid binding site is distinct from its neurite inhibitory activity. Alone, sialic acid–dependent binding of MAG to neurons is insufficient to effect inhibition of axonal growth. Thus, while soluble MAG-Fc (MAG extracellular domain fused to Fc), a truncated form of MAG-Fc missing Ig-domains 4 and 5, MAG(d1-3)-Fc, and another sialic acid binding protein, sialoadhesin, each bind to neurons in a sialic acid– dependent manner, only full-length MAG-Fc inhibits neurite outgrowth. These results suggest that a second site must exist on MAG which elicits this response. Consistent with this model, mutation of arginine 118 (R118) in MAG to either alanine or aspartate abolishes its sialic acid–dependent binding. However, when expressed at the surface of either CHO or Schwann cells, R118-mutated MAG retains the ability to inhibit axonal outgrowth. Hence, MAG has two recognition sites for neurons, the sialic acid binding site at R118 and a distinct inhibition site which is absent from the first three Ig domains

Epitope-Tagged P0Glycoprotein Causes Charcot-Marie-Tooth–Like Neuropathy in Transgenic Mice

In peripheral nerve myelin, the intraperiod line results from compaction of the extracellular space due to homophilic adhesion between extracellular domains (ECD) of the protein zero (P0) glycoprotein. Point mutations in this region of P0 cause human hereditary demyelinating neuropathies such as Charcot-Marie-Tooth. We describe transgenic mice expressing a full-length P0 modified in the ECD with a myc epitope tag. The presence of the myc sequence caused a dysmyelinating peripheral neuropathy similar to two distinct subtypes of Charcot-Marie-Tooth, with hypomyelination, altered intraperiod lines, and tomacula (thickened myelin). The tagged protein was incorporated into myelin and was associated with the morphological abnormalities. In vivo and in vitro experiments showed that P0myc retained partial adhesive function, and suggested that the transgene inhibits P0-mediated adhesion in a dominant-negative fashion. These mice suggest new mechanisms underlying both the pathogenesis of P0 ECD mutants and the normal interactions of P0 in the myelin sheath

A spatially specified systems pharmacology therapy for axonal recovery after injury

There are no known drugs or drug combinations that promote substantial central nervous system axonal regeneration after injury. We used systems pharmacology approaches to model pathways underlying axonal growth and identify a four-drug combination that regulates multiple subcellular processes in the cell body and axons using the optic nerve crush model in rats. We intravitreally injected agonists HU-210 (cannabinoid receptor-1) and IL-6 (interleukin 6 receptor) to stimulate retinal ganglion cells for axonal growth. We applied, in gel foam at the site of nerve injury, Taxol to stabilize growing microtubules, and activated protein C to clear the debris field since computational models predicted that this drug combination regulating two subcellular processes at the growth cone produces synergistic growth. Physiologically, drug treatment restored or preserved pattern electroretinograms and some of the animals had detectable visual evoked potentials in the brain and behavioral optokinetic responses. Morphology experiments show that the four-drug combination protects axons or promotes axonal regrowth to the optic chiasm and beyond. We conclude that spatially targeted drug treatment is therapeutically relevant and can restore limited functional recovery

PirB, a Second Receptor for the Myelin Inhibitors of Axonal Regeneration Nogo66, MAG, and OMgp: Implications for Regeneration In Vivo

SummaryInhibitors of axonal regeneration in myelin are believed to be major contributors to the lack of regeneration in the adult CNS. Three of the four known myelin inhibitors, although very different structurally, interact with the same receptor, NgR. However, the absence of NgR has no effect on inhibition of neurite outgrowth in culture, and there is no improvement in CST regeneration in vivo. In a recent issue of Science, a second receptor for these myelin inhibitors was described, PirB, a receptor first described in the immune system. Will PirB be the answer to CST regeneration in vivo

Recapitulate development to promote axonal regeneration: good or bad approach?

In the past decade there has been an explosion in our understanding, at the molecular level, of why axons in the adult, mammalian central nervous system (CNS) do not spontaneously regenerate while their younger counterparts do. Now a number of inhibitors of axonal regeneration have been described, some of the receptors they interact with to transduce the inhibitory signal are known, as are some of the steps in the signal transduction pathway that is responsible for inhibition. In addition, developmental changes in the environment and in the neurons themselves are also now better understood. This knowledge in turn reveals novel, putative sites for drug development and therapeutic intervention after injury to the brain and spinal cord. The challenge now is to determine which of these putative treatments are the most effective and if they would be better applied in combination rather than alone. In this review I will summarize what we have learnt about these molecules and how they signal. Importantly, I will also describe approches that have been shown to block inhibitors and encourage regeneration in vivo. I will also speculate on what the differences are between the neonatal and adult CNS that allow the former to regenerate and the latter not to

A role for cAMP in regeneration of the adult mammalian CNS

Injury to the adult mammalian central nervous system (CNS) often results in permanent loss of sensory and motor function. This is due to the failure of injured axons to regenerate. The inhibitory nature of the CNS can be attributed to several factors, including formation of the glial scar, the presence of several molecules, associated with myelin, which inhibit axonal regrowth, and the intrinsic growth state of these neurons. Encouraging regeneration in the adult mammalian CNS therefore will require targeting one or all of these factors following injury. Here we illustrate recent work from our laboratory that identifies some of the signalling components involved in modulation of the intrinsic growth state of adult neurons. When activated, these signalling pathways can induce axonal regeneration in the presence of the myelin-associated inhibitors both in vitro and in vivo