| dcterms.abstract | Nerve growth factor (NGF) is a member of the nerurotrophin family that plays a crucial role during development. Although NGF was discovered because of its actions during development, it is now known to function throughout the life of the animal. NGF plays a profound role in nociception because its high affinity receptor, TrkA, is expressed in nociceptors. In mammals, NGF causes thermal hyperalgesia that develops within minutes (acute hyperalgesia) and lasts for several days (chronic hyperalgesia). While NGF-induced acute peripheral sensitization/hyperalgesia occurs via TRPV1 receptor activation, chronic hyperalgesia is thought to take place with a delay, caused by upregulation of genes. This delay would involve 1) retrograde transport of the NGF-TrkA complex from the axon terminals (DAs) to the cell bodies (CBs) and 2) modulation of gene expression at CBs. If altered gene expression can sensitize nociceptors, chronic hyperalgesia could thus develop. To test this idea, I looked at the expression of two genes, VGF, a neuropeptide, and Nav1.7, a sodium channel. Both proteins are widely expressed in primary sensory neurons including nociceptors, have been functionally linked to pain perception, and can be upregulated by NGF in some peripheral neurons. I first asked whether NGF can upregulate these proteins in sensory dorsal root ganglion (DRG) neurons and whether upregulation can be mediated by retrograde signaling. Here I show that NGF treatment increased Nav1.7 and VGF protein levels, and by using microfluidic devices to biochemically separate CBs from DAs, I show NGF stimulation at the DA retrogradely upregulated both mRNA and protein levels of these genes. Since Pincher, a member of the EHD protein family, is required for NGF/TrkA internalization and retrograde transport of NGF-TrkA endosomes in cultured sympathetic neurons, I investigated the role of Pincher in mediating this upregulation. NGF-induced retrograde upregulation in DRG neurons, indeed, was abolished in the neurons expressing a dominant negative form of Pincher, PincherG68E. In mice, NGF injection into the paw upregulated both VGF and Nav1.7 expression in DRG cell bodies through retrograde signaling. However, NGF failed to retrogradely upregulate these proteins in PincherG68E expressing DRG neurons. Finally, I show that mice injected with virus to express PincherG68E didn't develop NGF-induced chronic thermal hyperalgesia (tested 24 hours after NGF injection). Thus, Pincher-mediated retrograde NGF signaling may be a means by which NGF mediates a transition from acute to chronic hyperalgesia through changes in gene expression. | |
