Extracellular Vesicles as Carriers of Neurotrophic Signals During Sympathetic Neuronal Circuit Development

Proper wiring of the peripheral nervous system relies on neurotrophic signaling via nerve growth factor (NGF). NGF secreted by target organs (i.e. eye) binds to the TrkA receptor expressed on the distal axons of postganglionic neurons. Upon binding, TrkA is internalized into a signaling endosome and retrogradely trafficked back to the soma and into the dendrites to promote cell survival and postsynaptic maturation, respectively. Postganglionic neurons that do not receive sufficient amounts of NGF will die off. Interestingly, the survival of presynaptic preganglionic neurons residing in the spinal cord is a corollary of NGF-dependent postganglionic survival. In the final circuit, preganglionic neurons are quantitatively matched to postganglionic neurons and, by extension, to the final target, even though TrkA is not expressed on preganglionic neurons. The trophic cue governing this presynaptic survival matching has yet to be identified. Here, we propose that extracellular vesicles (EVs) mediate this trophic cue between postganglionic and preganglionic neurons. Using the mouse superior cervical ganglion (SCG) as a model, we isolated EVs and confirmed their identity, morphology, and concentration using Western blot, cryo-electron microscopy, and nanoparticle tracking analysis. Furthermore, we have shown that EVs secreted from SCG neurons contain retrogradely trafficked TrkA that originated in the distal axon. Lastly, using a survival assay we have shown that the addition of SCG-derived EVs to developing spinal cord cells promotes their survival and this effect is abrogated by blocking TrkA EV signaling using an anti-NGF neutralizing antibody. These data point to a novel route of neurotrophic signaling through EVs.