title: Proneurotrophins require endocytosis and intracellular proteolysis to induce TrkA activation.
abstract: The uncleaved, pro-form of nerve growth factor (proNGF) functions as a pro-apoptotic ligand for the p75 neurotrophin receptor (p75NTR). However, some reports have indicated that proneurotrophins bind and activate Trk receptors. In this study, we have examined proneurotrophin receptor binding and activation properties in an attempt to reconcile these findings. We show that proNGF readily binds p75NTR expressed in HEK293T cells but does not interact with TrkA expressed under similar circumstances. Importantly, proNGF activates TrkA tyrosine phosphorylation, induces Erk and Akt activation, and causes PC12 cell differentiation. We show that inhibiting endocytosis or furin activity reduced TrkA activation induced by proNGF but not that induced by mature NGF and that proNGF123, a mutant form of NGF lacking dibasic cleavage sites in the prodomain, does not induce TrkA phosphorylation in PC12 cells. Therefore, endocytosis and cleavage appear to be prerequisites for proNGF-induced TrkA activity. We also found that proBDNF induces activation of TrkB in cerebellar granule neurons and that proBDNF cleavage by furin and metalloproteases facilitates this effect. Taken together, these data indicate that under physiological conditions, proneurotrophins do not directly bind or activate Trk receptors. However, endocytosis and cleavage of proneurotrophins produce processed forms of neurotrophins that are capable of inducing Trk activation.
authors.affiliation: Department of Neurology and Neurosurgery, McGill University, Montreal Neurological Institute, Montreal H3A 2B4, Canada.
authors.forename: Jacqueline
authors.initials: J
authors.lastname: Boutilier
authors.forename: Claire
authors.initials: C
authors.lastname: Ceni
authors.forename: Promila C
authors.initials: PC
authors.lastname: Pagdala
authors.forename: Alison
authors.initials: A
authors.lastname: Forgie
authors.forename: Kenneth E
authors.initials: KE
authors.lastname: Neet
authors.forename: Philip A
authors.initials: PA
authors.lastname: Barker
mesh_terms: D000818:Animals; D019208:Brain-Derived Neurotrophic Factor / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D004705:Endocytosis; D004789:Enzyme Activation; D005786:Gene Expression Regulation; D006801:Humans; D020932:Nerve Growth Factor / Q000235:genetics / Q000378:metabolism*; D010447:Peptide Hydrolases / Q000378:metabolism*; D011485:Protein Binding; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D020917:Receptor, trkA / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D019208:Brain-Derived Neurotrophic Factor; D011498:Protein Precursors; D020932:Nerve Growth Factor; D020917:Receptor, trkA; D010447:Peptide Hydrolases