Neurotrophic Factor Signaling Mechanisms Underlying the Development of Peripheral Neural Circuits

Abstract

The first discovery of a growth-regulating signaling molecule was more than 65 years ago, when the work of Rita Levi-Montalcini, Viktor Hamburger, and Stanley Cohen led to the identification and purification of nerve growth factor (NGF). As the prototypical growth factor, the identification of NGF was a milestone in developmental biology, leading to the subsequent discovery of hundreds of additional secreted growth factors. Many decades later, NGF is now recognized as one of many neurotrophic factors (NTFs) and the founding member of the neurotrophin family, which includes NGF, brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT-4), and neurotrophin-3 (NT-3). The physiologic functions of these factors are mediated by the tropomyosin-related kinase (Trk) receptor family as well as the p75 neurotrophin receptor (p75). A more recently identified family of neurotrophic factors is the the glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs), which are potent growth factors that promote the survival of numerous populations of neurons in both the peripheral nervous system and central nervous system. The GFLs consist of four homologous ligands: GDNF, neurturin, artemin, and persephin, and signal by binding to one of four GDNF family co-receptors (GFRαs), which subsequently bind to and activate the tyrosine kinase Ret to initiate downstream signaling.

This thesis work collectively sought to understand the functions of Ret signaling in peripheral nervous system development. In the first investigation (Chapter 2), we explored the function of a newly identified p75-Ret receptor complex in the development of nociceptive sensory neurons of the dorsal root ganglion (DRG), known to be dependent on GFL-Ret signaling for their survival and maintenance. In our investigation we found that p75 is required for cell surface localization of Ret. In the absence of p75, GFL-dependent, but not NGF-dependent, nociceptors are specifically reduced, with sensory populations that normally express lower levels of Ret being most greatly affected by p75 deletion. Based on these data, we conclude that p75 has a surprising role in augmenting GFL signaling, and collectively serves to promote the establishment of postnatal sensory neuron diversity.

In the second study (Chapter 3), we investigated the function of this p75-Ret receptor complex in developing sympathetic neurons of the superior cervical ganglion (SCG). Interestingly, p75 signaling regulates programmed cell death (PCD) during perinatal development. In this investigation, we found that Ret expression is restricted to a subpopulation of apoptotic neurons that are rapidly eliminated. Ret and p75 form a complex induced by pro-apoptotic stimuli both in vitro and in vivo. Importantly, p75 deletion specifically within Ret-expressing neurons, and Ret deletion specifically during PCD, result in a significant abrogation in apoptosis. These studies collectively revealed a surprising non-canonical function of Ret in augmenting apoptotic signaling through p75 during PCD in vivo.

In the last study (Chapter 4), we explored the function of Ret in the development of the peripheral taste system, focusing our studies on the geniculate ganglion (GG) which afferently innervates taste buds within fungiform papillae on the anterior 2/3 of the dorsal tongue. We identified a novel, biphasic function for GDNF-Ret signaling in the peripheral taste system, initially acting to promote the chemosensory phenotype of all GG neurons, while acting postnatally to define a unique subpopulation of lingual mechanoreceptors. These findings collectively broaden our understanding of the cues responsible for taste neuron development, and bring to light new information regarding taste neuron heterogeneity.