Targeting neurite growth inhibitors to induce CNS regeneration

Prominent among the several endogenous inhibitors known to limit recovery and plasticity after CNS injury are Nogo (neurite outgrowth inhibitor) and MAG (myelin associated glycoprotein). The effects of these inhibitors on axonal regeneration can be reduced by administration of specific antagonists, some of which are commercially available for experimental investigation. There are three aspects of therapeutic manipulations: targeting the inhibitory proteins, antagonizing the known receptor, and inhibiting the intracellular signal transduction of these inhibitory molecules. Infusion of an antibody against Nogo improves behavioral deficits and enhances corticospinal tract regeneration in animals after stroke and spinal cord injury (SCI). Similarly, peripheral injection of a mouse monoclonal antibody directed against MAG results in dramatic preferential motor reinnervation in mice after transection of the femoral nerve, indicating that interference with the repellant function of MAG facilitates reinnervation of correct pathways by motor neurons. Further, antagonism of the Nogo receptor by the peptide NEP 1-40 (Nogo extracellular peptide residues 1-40) can promote axonal regeneration in rats after SCI. Blockade of signal transduction also can be effective. The p75 neurotrophin receptor probably represents the signaling part of the receptor complex for neurite growth inhibitors. There is evidence in vitro that the inhibitory actions of MAG and myelin are blocked if neurons are primed with a variety of neurotrophins. Thus, there are several therapeutic approaches to overcome the actions of endogenous neurite growth inhibitors so as to promote CNS regeneration.