The NGF/TrkA trafficking and signaling cascades are differentially regulated by early endosomal Rab GTPases, Rab5 and Rab22, and the phosphatase, PTP-MEG2

The neurotrophin, nerve growth factor (NGF) and its high affinity receptor, TrkA, promote the survival and differentiation of neurons in mammals. It is well known that the signaling events of activated TrkA on the cell surface ultimately lead to changes in transcription. However, appropriate trafficking of TrkA in intracellular vesicles is critically important for proper TrkA biosynthesis, signaling, and function. My work focuses on two major pathways of TrkA trafficking, retrograde (movement toward the cell body) and anterograde (movement away from the cell body) trafficking, and the proteins that regulate these processes.;First, early endocytic Rab GTPases govern the endocytosis and retrograde trafficking of activated receptors. In particular, Rab5 and Rab22 differentially regulate TrkA trafficking and signaling in cell culture. These differences are highlighted by the differential regulation of autophagic progression and NGF-mediated differentiation by overexpression of Rab5 and Rab22 in PC12 cells. By producing chimeric Rab5/Rab22 protein constructs, we have identified key regulatory residues which led to the production of novel substitution point mutants of Rab5 and Rab22 that modulated the rates of GTPhydrolysis and effector binding between these similar Rabs. These data have broadened our understanding of previously unidentified residues that modulate GTP-binding and hydrolysis in Rab5 and Rab22, and may also apply to Rabs in general.;Second, de novo synthesis and anterograde transport of TrkA to the plasma membrane is a vital process for the development, function, and survival of neurons. We have identified a novel regulator of TrkA export to the plasma membrane, the protein tyrosine phosphatase - MEG2 (PTP-MEG2), which is not only a regulator of anterograde 10 trafficking of newly synthesized TrkA, but also dephosphorylates TrkA. In PC12 cells, appropriate TrkA biosynthesis and anterograde transport is inhibited by the overexpression of inactive PTP-MEG2, while the phosphorylation and signaling of TrkA is substantially decreased by overexpression of wild-type PTP-MEG2. Cortical neuron cultures were also deleteriously affected by PTP-MEG2 overexpression. Our findings demonstrate that PTPMEG2 is a novel regulator of both TrkA anterograde trafficking and signaling.;Taken together, my work has expanded the field's understanding of basic Rab GTPase biology and demonstrates that Rab5, Rab22, and PTP-MEG2 are important players in TrkA function in the health of neurons.