Neurotrophins and neurotrophin receptors in adult primary sensory neurons
Karchewski, Laurie Ann
Neurotrophins regulate sensory neuron survival, differentiation and phenotype, yet much remains to be discovered. The role of neurotrophins and neurotrophin receptors in adult primary sensory neurons was investigated along three main avenues 'in vivo'. First, to better predict neurotrophin responsiveness in DRG, the localization of messenger RNA for neurotrophin receptors was examined using 'in situ' hybridization. Results indicate extensive overlap in trk expression in lumbar sensory neurons, such that expression of multiple trk receptor mRNAs in neurons is more prevalent than the expression of single trk mRNAs. These findings provide the anatomical substrate to explain the differential modulatory effects of neurotrophins on individual neurons. Like trkA, neuronal expression of trkB and trkC mRNA is reduced following axotomy, suggesting a neurotrophin-deficient state and a role for these molecules in promoting normal phenotype. Secondly, neuronal BDNF was characterized in intact, axotomized and NGF-infusedDRG using 'in situ' hybridization and immunohistochemistry. BDNF is expressed in a subpopulation of small to medium-sized sensory neurons that overlaps largely with trkA mRNA, and less so with BDNF receptor mRNA, trkB. This suggests that BDNF serves a largely paracrine role in DRG, which may include a role in nociception. Injury increases BDNF transiently in small neurons and chronically in larger neurons, many of which express trkB, thus implicating BDNF in a greater autocrine function that may be neuroprotective. NGF upregulates BDNF expression in trkA positive neurons which implicates it in nociceptive responses. The downregulation of trkB expression corresponding with high BDNF levels suggests novel receptor regulation not seen before with neurotrophins in the PNS. Lastly, the role of p75 in sensory neurons was examined by conducting a detailed analysis of DRG in p75 knock-out mice. Compared to wildtype controls, p75 knock-out mice show diminished NGF binding affinity, reduced retrograde transport of low quantities of NGF and a more rapid phenotypic switch following injury, suggesting that p75 is important in the ability of sensory neurons to bind and transport limited quantities of NGF. Such a role for p75 in high-amity NGF binding in sensory neurons may explain developmental deficits in p75 knock-out mice, which were found to include fewer lumbar sensory neurons and unmyelinated sensory nerve fibers and were reflected in reduced sensory nerve conduction velocities. The findings of this thesis research both strengthen and extend that predicted by the classic neurotrophic hypothesis which states that only developing neurons that successfully compete for limited amount of target derived neurotrophic factor will survive a period of naturally occurring cell death. An absence of p75 may render many neurons incapable of adequately binding and transporting limiting quantities of neurotrophins as predicted by this theory. This thesis also extends this theory to implicate neurotrophins in the complex modulation of phenotype in intact adult sensory neurons, the switch in function and phenotype that occurs with injury, and a role for neuronal expression of a neurotrophin in nociception in the intact state and mechanoreception in the injured state.