Myelin-associated glycoprotein protects motoneurons from programmed cell death

ANABELA PALANDRI; VICTORIA ROZÉS SALVADOR; MARIA JOSE VIRGOLINI; RONALD L. SCHNAAR; PABLO HECTOR HORACIO LOPEZ

Potomac

Congreso; Biennial meeting of the Peripheral Nerve Society; 2011

Peripheral nerve society

Myelin-associated glycoprotein (MAG) is a lectin present in the periaxonal layer of myelin that engages several axonal receptors, including Nogo-receptors (NgRs). It has been postulated that pharmacological activation of NgRs have a modulatory role on p75NTR-dependent programmed cell death (PCD) of motoneurons (MNs). It was also described that small GTPase RhoA regulates diverse cellular processes including apoptosis of MNs during embryonic development via one of the effector proteins, Rho-Kinase (ROCK). The aim of this study was to analyze a possible modulatory role of MAG on PCD of MNs via NgRs during postnatal development and study the possible involvement of RhoA activation with this effect. A time course study showed that early after birth Mag-null mice has a reduction in the number of MNs. Also Mag-null mice exhibit increased susceptibility in an in vivo model of PCD induced by a sciatic nerve crush. Interestingly pre-treatment with a soluble form of MAG (MAG-Fc) prevented MNs apoptosis in this model. Studies using an in vitro model of p75NTR-dependent PCD on spinal cord organotypic cultures and a MN cell line (MN1) confirmed the modulatory role of MAG. Infection of MN1 cells with lentiviral particles carrying shRNA sequences targeting NgRs abolished the protective effect of MAG. In addition studies on spinal cords from triple NgR1/2/3-null mice show differences in MNs count respect to Wt mice, confirming the role of these receptors. We further report a role of RhoA signaling pathway in the protective effect of MAG against apoptosis of MNs. Treatment of MN1 cells with ROCK inhibitor Y27632 blocked MAG protection against apoptosis. Also spatial/temporal analysis of RhoA activity was performed in these cells by using a RhoA biosensor based on fluorescence resonance energy transfer-FRET. Treatment with MAg-Fc showed a transient activation of RhoA preferentially in axons that peaks at 60min after treatment. The role of RhoA signaling was further confirmed in the nerve-crush model, where treatment with Y27632 abolished the protection induced by MAG-Fc treatment. Overall these findings identify a new protective role of MAG as a modulator of apoptosis of MNs during postnatal development by a mechanism involving NgR-mediated RhoA signaling activation.