Sympathetic neurons secrete retrogradely transported TrkA on extracellular vesicles

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. Much progress has been made in recent years to define the fate of the retrogradely trafficked TrkA signaling endosome, yet it has not been fully characterized. Here we investigate extracellular vesicles (EVs) as a novel route of neurotrophic signaling. Using the mouse superior cervical ganglion (SCG) as a model, we isolate EVs derived from sympathetic cultures and characterize them using immunoblot assays, nanoparticle tracking analysis, and cryo-electron microscopy. Furthermore, using a compartmentalized culture system, we find that TrkA derived from endosomes originating in the distal axon can be detected on EVs secreted from the somatodendritic domain. In addition, inhibition of classic TrkA downstream pathways, specifically in somatodendritic compartments, greatly decreases TrkA packaging into EVs. Our results suggest a novel trafficking route for TrkA: it can travel long distances to the cell body, be packaged into EVs, and be secreted. Secretion of TrkA via EVs appears to be regulated by its own downstream effector cascades, raising intriguing future questions about novel functionalities associated with TrkA(+) EVs.

Automated summary

Proper wiring of the peripheral nervous system relies on neurotrophic signaling via nerve growth factor, in which NGF secreted by target organs binds to the TrkA receptor on postganglionic neurons and promotes cell survival and postsynaptic maturation. This study investigates extracellular vesicles as a novel route of neurotrophic signaling and finds that TrkA derived from endosomes can be detected on EVs secreted from the somatodendritic domain. Inhibition of classic TrkA downstream pathways decreases TrkA packaging into EVs. These findings suggest a novel trafficking route for TrkA and raise questions about the functionality of TrkA(+) EVs.