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.

Glial cell-line derived neurotrophic factor is essential for electroconvulsive shock-induced neuroprotection in an animal model of Parkinson's disease.

Sustained motor improvement in human patients with idiopathic Parkinson's disease has been described following electroconvulsive shock (ECS) treatment. In rats, ECS stimulates the expression of various trophic factors (TFs), some of which have been proposed to exert neuroprotective actions. We previously reported that ECS protects the integrity of the rat nigrostriatal dopaminergic system against 6-hydroxydopamine (6-OHDA)-induced toxicity; in order to shed light into its neuroprotective mechanism, we studied glial cell-line derived neurotrophic factor (GDNF) levels (the most efficient TF for dopaminergic neurons) in the substantia nigra (SN) and striatum of 6-OHDA-injected animals with or without ECS treatment. 6-OHDA injection decreased GDNF levels in the SN control animals, but not in those receiving chronic ECS, suggesting that changes in GDNF expression may participate in the ECS neuroprotective mechanism. To evaluate this possibility, we inhibit GDNF by infusion of GDNF function blocking antibodies in the SN of 6-OHDA-injected animals treated with ECS (or sham ECS). Animals were sacrificed 7 days after 6-OHDA infusion, and the integrity of the nigrostriatal system was studied by tyrosine hydroxylase immunohistochemistry and Cresyl Violet staining. Neuroprotection observed in ECS-treated animals was inhibited by GDNF antibodies in the SN. These results robustly demonstrate that GDNF is essential for the ECS neuroprotective effect observed in 6-OHDA-injected animals.