proNT-3 (the precursor form of neurotrophin-3) functions through p75NTR and sortilin and induces neuronal apoptosis. In contrast,mature NT-3 interacts with Trk receptors and selectively promotes the survival,growth and differentiation of neurons. Since axon retrograde transport is crucial for neurotrophin signaling,proNT-3 also has an important role in the mechanism of retrograde signaling.