Abstract
The lack of axonal growth after injury in the adult central nervous system (CNS) is
due to several factors including the formation of a glial scar, the absence of neurotrophic
factors, the presence of growth-inhibitory molecules associated with myelin and the
intrinsic growth-state of the neurons. To date, three inhibitors have been identified
in myelin: Myelin-Associated Glycoprotein (MAG), Nogo-A, and Oligodendrocyte-Myelin
glycoprotein (OMgp). In previous studies we reported that MAG inhibits axonal regeneration
by high affinity interaction (KD 8 nM) with the Nogo66 receptor (NgR) and activation of a p75 neurotrophin receptor
(p75NTR)-mediated signaling pathway.
Similar to other axon guidance molecules, MAG is bifunctional. When cultured on MAG-expressing
cells, dorsal root ganglia neurons (DRG) older than post-natal day 4 (PND4) extend
neurites 50% shorter on average than when cultured on control cells. In contrast,
MAG promotes neurite outgrowth from DRG neurons from animals younger than PND4. The
response switch, which is also seen in retinal ganglia (RGC) and Raphe nucleus neurons,
is concomitant with a developmental decrease in the endogenous neuronal cAMP levels.
We report that artificially increasing cAMP levels in older neurons can alter their
growth-state and induce axonal growth in the presence of myelin-associated inhibitors.
Keywords
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© 2005 Elsevier B.V. Published by Elsevier Inc. All rights reserved.
