GANGLIOSIDE GM1 BINDS TO THE TRK PROTEIN AND REGULATES RECEPTOR FUNCTION

Several lines of evidence have suggested that ganglioside GM1 stimulat es neuronal sprouting and enhances the action of nerve growth factor ( NGF), but its precise mechanism is yet to be elucidated. We report her e that GM1 directly and tightly associates with Trk, the high-affinity tyrosine kinase-type receptor for NGF, and strongly enhances neurite outgrowth and neurofilament expression in rat PC12 cells elicited by a low dose of NGF that alone is insufficient to induce neuronal differe ntiation. The potentiation of NGF activity by GM1 appears to involve t yrosine-autophosphorylation of Trk, which contains intrinsic tyrosine kinase activity that has been localized to the cytoplasmic domain. In the presence of GM1 in culture medium, there is a >3-fold increase in NGF-induced autophosphorylation of Trk as compared with NGF alone. We also found that GM1 could directly enhance NGF-activated autophosphory lation of immunoprecipitated Trk in vitro. Monosialoganglioside GM1, b ut not polysialogangliosides, is tightly associated with immunoprecipi tated Trk Furthermore, such tight association of GM1 with Trk appears to be specific, since a similar association was not observed with othe r growth factor receptors, such as low-affinity NGF receptor (p75(NGFR )) and epidermal growth factor receptor (EGFR). Thus, these results st rongly suggest that GM1 functions as a specific endogenous activator o f NGF receptor function, and these enhanced effects appear to be due, at least in part, to tight association of GM1 with Trk.