Myelin-associated glycoprotein modulates apoptosis of motoneurons during early postnatal development via NgR/p75(NTR) receptor-mediated activation of RhoA signaling pathways.

Myelin-associated glycoprotein (MAG) is a minor constituent of nervous system myelin, selectively expressed on the periaxonal myelin wrap. By engaging multiple axonal receptors, including Nogo-receptors (NgRs), MAG exerts a nurturing and protective effect the axons it ensheaths. Pharmacological activation of NgRs has a modulatory role on p75(NTR)-dependent postnatal apoptosis of motoneurons (MNs). However, it is not clear whether this reflects a physiological role of NgRs in MN development. NgRs are part of a multimeric receptor complex, which includes p75(NTR), Lingo-1 and gangliosides. Upon ligand binding, this multimeric complex activates RhoA/ROCK signaling in a p75(NTR)-dependent manner. The aim of this study was to analyze a possible modulatory role of MAG on MN apoptosis during postnatal development. A time course study showed that Mag-null mice suffer a loss of MNs during the first postnatal week. Also, these mice exhibited increased susceptibility in an animal model of p75(NTR)-dependent MN apoptosis induced by nerve-crush injury, which was prevented by treatment with a soluble form of MAG (MAG-Fc). The protective role of MAG was confirmed in in vitro models of p75(NTR)-dependent MN apoptosis using the MN1 cell line and primary cultures. Lentiviral expression of shRNA sequences targeting NgRs on these cells abolished protection by MAG-Fc. Analysis of RhoA activity using a FRET-based RhoA biosensor showed that MAG-Fc activates RhoA. Pharmacological inhibition of p75(NTR)/RhoA/ROCK pathway, or overexpression of a p75(NTR) mutant unable to activate RhoA, completely blocked MAG-Fc protection against apoptosis. The role of RhoA/ROCK signaling was further confirmed in the nerve-crush model, where pretreatment with ROCK inhibitor Y-27632 blocked the pro-survival effect of MAG-Fc. These findings identify a new protective role of MAG as a modulator of apoptosis of MNs during postnatal development by a mechanism involving the p75(NTR)/RhoA/ROCK signaling pathway. Also, our results highlight the relevance of the nurture/protective effects of myelin on neurons.

High affinity as a disease determinant factor in anti-GM(1) antibodies: comparative characterization of experimentally induced vs. disease-associated antibodies.

Elevated titers of serum anti-GM(1) antibodies of IgG isotype are found frequently in patients with Guillain-Barré syndrome. Much evidence indicates that these autoantibodies are involved in disease progression, but their exact function and the mechanism of their appearance are still unclear. In an attempt to reproduce "ganglioside syndrome", the experimental model of neuropathy developed by Nagai et al. (Neurosci. Lett. 2 (1976) 107), rabbits were intensively immunized with GM(1) in complete Freund adjuvant (CFA). High titers of anti-GM(1) antibodies were produced, with class switch and affinity maturation indicating an elaborate immune response. Unexpectedly, the rabbits did not show any clinical symptoms of neuropathy. Relatively affinities of both IgM and IgG antibodies were significantly lower than those of similar antibodies from neuropathy patients. These results suggest the existence of a threshold value above which affinity of anti-GM(1) antibodies becomes an important factor in disease induction. The absence of neuropathy symptoms in rabbits may be explained by absence of these high-affinity anti-GM(1) antibodies.