Project Summary/Abstract. Nerve growth factor (NGF) is required for the proper wiring of the sympathetic
nervous system during development. NGF secreted by target organs binds to its receptor, TrkA, on the distal
axons of innervating postganglionic neurons. Upon binding, the TrkA receptor is internalized into a signaling
endosome (SE) where it can either signal locally in the distal axon or it is retrogradely trafficked to the cell body
of the postganglionic neuron. Trafficking of the TrkA-SE from the final target to the cell body is critical for many
developmental processes including survival and synapse formation. Interestingly, the survival of presynaptic
preganglionic neurons residing in the spinal cord quantitatively matches that of sympathetic postganglionic
neurons and, by extension, the final target, even though TrkA is not expressed on preganglionic neurons. The
trophic cue governing this presynaptic matching has yet to be identified.
We now have preliminary data that TrkA can be secreted from the somatodendritic domain after
retrograde transport by inclusion in extracellular vesicles (EVs). In this proposal, we ask how this intracellular
long-distance signal (TrkA+ SE) regulates the secretion of an intercellular signal via extracellular vesicle (TrkA+
EV) production. We also ask whether these TrkA+ EVs are capable of neurotrophic signaling. We speculate that
TrkA+ EVs may constitute a novel trophic signal for preganglionic neurons and regulate their differentiation and
survival. EV biology is a nascent field, but a range of EV functions have been described mainly in non-neuronal
systems. EVs have a demonstrated role in tissue repair, immune surveillance, transportation of miRNAs, and
activation of signaling cascades. Although there have been a handful of recent EV studies focusing on neurons,
EVs have not previously been shown to participate in trophic neurodevelopmental processes.
We will test the hypothesis that retrogradely transported TrkA can be secreted in EVs from sympathetic
neurons and transferred to preganglionic neurons at nascent synapses to support trophic signaling. In Aim 1, we
will molecularly define TrkA+ EVs from sympathetic postganglionic neurons and determine if signaling
downstream of TrkA affects their production. In Aim 2, we will determine what cell types in the superior cervical
ganglia internalize TrkA+ EVs and if TrkA+ EVs are neurotrophic for preganglionic neurons. These experiments
will determine the form, function, and locus of action of this potentially novel mode of trophic signaling. Our long-
term goal is to explore a new type of neuron-neuron communication that may be critical for systems matching
during the development of a functional circuit: secretion and long-distance action of neurotrophic EVs.
