Novel Mechanisms of Axonal Growth Inhibition Following Central Nervous System Injury.

Abstract

Depending on location and severity, injury to the adult mammalian central nervous system (CNS) typically results in neurological deficits ranging from mild motor impairment to complete paralysis. As the regenerative capacity of severed axons is extremely limited, there is a great unmet need for strategies that promote neural tissue growth and repair. There are two major extrinsic barriers to axon regeneration in the injured CNS: myelin-associated inhibitors (MAIs) and chondroitin sulfate proteoglycans (CSPGs). MAIs and CSPGs bind to axonal receptors to prevent significant regenerative growth; thus, these receptors represent potential targets for therapeutic intervention. The Nogo receptors (NgR1, NgR2, and NgR3) form a small subfamily of proteins, of which NgR1 and NgR2 have been shown to act as receptors for MAIs. Here we report on a novel interaction between select members of the Nogo receptor family and CSPGs. NgR1 and NgR3 bind with high affinity and selectivity to the sugar moiety of CSPGs. In vitro, primary neurons isolated from mice lacking both NgR1 and NgR3 (NgR13–/–) grow longer neurites on substrate-adsorbed CSPGs than neurons isolated from controls. NgR13–/– double mutants, but not single mutants, show enhanced axonal regeneration of retinal ganglion cell (RGC) fibers following optic nerve crush injury in vivo. When combined with activation of RGC intrinsic growth programs, genetic manipulations result in a further increase in regeneration. These results thus provide unexpected evidence for shared mechanisms of MAI and CSPG inhibition. We have also identified the low-density lipoprotein receptor-related protein 1 (LRP1) as a novel receptor for one of the MAIs, the myelin-associated glycoprotein (MAG). In addition to its established role in neuronal growth inhibition, MAG has recently been shown to protect neurons from axonal degeneration and excitotoxicity. Here we show that LRP1 mediates MAG-induced neurite outgrowth inhibition, whereas NgR1 mediates MAG-induced neuroprotection. Loss of NgR1 results in increased vulnerability to acrylamide toxicity, as determined by behavioral analysis and electrophysiological recordings, as well as increased susceptibility to kainic acid-induced epileptic seizures. Collectively, this dissertation work provides novel insights aimed at the development of new treatment strategies for CNS disease, including brain and spinal cord injury.