The biased ligands NGF and NT-3 differentially stabilize Trk-A dimers.

Trk-A is a receptor tyrosine kinase (RTK) which plays an essential role in the development and functioning of the nervous system. Trk-A is expressed in neurons and signals in response to two ligands, NGF and NT-3, with very different functional consequences. Thus, NGF and NT-3 are "biased" ligands for Trk-A. Since it has been hypothesized that biased RTK ligands induce differential stabilization of RTK dimers, here we seek to test this hypothesis for NGF and NT-3. In particular, we use Förster Resonance Energy Transfer (FRET) and Fluorescence Intensity Fluctuation (FIF) spectroscopy to assess the strength of Trk-A interactions and Trk-A oligomer size in the presence of the two ligands. While the difference in Trk-A behavior in response to the two ligands has been previously attributed to differences in their binding to Trk-A in the endosomes at low pH, here we further show differences in the stabilities of the NGF and NT-3 bound Trk-A dimers in the plasma membrane and at neutral pH. We discuss the biological significance of these new findings and their implications for the design of Trk-A ligands with novel functionalities.