GM1-MEDIATED NEURODIFFERENTIATION IS PROMOTED BY OLIGOGM1-TRKA INTERACTION

The present study proposes a clarification on the molecular mechanism by which ganglioside GM1 promotes neurodifferentiation, demonstrating in vitro that neurotrophic functions are exerted by an interaction between the oligosaccharide portion (OligoGM1) and an extracellular domain of TrkA receptor. Similarly to the entire molecule, the oligosaccharide portion of ganglioside GM1, rather than ceramide, is responsible for neurodifferentiation by augmenting neurite elongation and by increasing the expression of neurofilament proteins in mouse neuroblastoma cell line Neuro2a (N2a). Conversely, the single components of OligoGM1 (asialo-OligoGM1, OligoGM2, OligoGM3, sialic acid or galactose) are not able to induce a neuro-like morphology. The neurodifferentiative effect is exerted instead by fucosyl-OligoGM1. Contrarily to GM1, exogenous OligoGM1 never integrates in the plasma membrane composition and does not belong to the intracellular metabolism: the unique interaction with N2a is characterized by a weak non-covalent association to the plasma membrane that suggests the existence of an OligoGM1-stimulated target on the cell surface. In fact, the neurotrophic properties of GM1 oligosaccharide are exerted by activating TrkA receptor and the following cascade leading to neurodifferentiation event. The second part of this study elucidates the interaction between GM1 and TrkA, revealing a direct association of OligoGM1 to an extracellular domain of the receptor. Photolabeling experiments, performed employing nitrogen azide radiolabeled GM1 derivatives, show a direct association of the oligosaccharide chain to TrkA. Moreover, a bioinformatics study reveals that OligoGM1 fits perfectly in a pocket of the TrkA-NGF complex, stabilizing and favoring their intermolecular interactions as revealed by the increase in energy associated to the new complex TrkA-NGF-OligoGM1. A precise molecular recognition process between OligoGM1 and a specific extracellular domain of the TrkA receptor is supposed. According to the weak association of OligoGM1 to the cell surface, no covalent bounds between OligoGM1 and TrkA-NGF complex were found. For the first time the molecular mechanism by which GM1 exerts its neurodifferentiative potential was identified, finding out a direct interaction between the oligosaccharide portion and an extracellular domain of TrkA receptor responsible for enhancing the signal transduction related to the neurodifferentiation pathway.