Unveiling the Cell Composition of the Olfactory Migratory Mass: Perspectives on Terminal Nerve Formation, Olfactory Development, and Migration of the GnRH-1 Neurons

Puberty is a postnatal developmental process required to initiate key sexual maturation events that are necessary for the propagation of species. Signals to initiate these events first begin with the Gonadotropin releasing hormone-1 (GnRH-1) neurons within the hypothalamus. The hormone GnRH-1 released by these neurons is crucial for puberty, fertility and reproductive processes by establishing the hypothalamic-pituitary-gonadal (HPG) axis, regulating release of sex hormones (Progesterone, Estrogen, Testosterone). Conditions related to inability of the GnRH-1 neurons to correctly release GnRH-1 can lead to aberrant or delayed puberty, a condition known as hypogonadotropic hypogonadism (HH). Interestingly, some patients with the hormonal disorder seen in HH have been found to have impaired or no sense of smell, this classification is known as Kallmann syndrome (KS). The link between olfaction and puberty can be found during early embryonic development, the GnRH-1 neurons are not found within the brain and instead, the nose. The GnRH-1 neurons first arise within the olfactory placode (OP), migrating out of the nasal area to the brain as development progresses. How exactly the GnRH-1 neurons make this migration is still unknown. We believe that the GnRH-1 neurons are supported by a long forgotten and elusive structure known as the terminal nerve (TN) rather than olfactory neurons. The TN is the only cranial nerve (XIII) known to project from the nose to basal forebrain. This is known from dissection studies over a century ago identifying the TN in various species such as sharks, fish, rodents and humans at both embryonic and adult stages. The characterization and function of the TN remains to be elucidated. My doctoral research aimed to learn more about TN development and its role in GnRH-1 migration. My second chapter highlights the first genetic lineage tracing experiments of the TN using the gene Prokineticin receptor 2 (Prokr2), a gene that has been found to have mutations associated with KS. We discovered that the GnRH-1 neurons and TN neurons are two distinct structures that closely associate, and that the neurons of the TN may be pioneer neurons, which initiate olfactory bulb morphogenesis. Additionally, we performed the first single-cell RNA sequencing of the TN, acquiring the first transcriptomic data of the TN. 26 genes associated with HH/KS were enriched in the TN sequencing data. In chapter 3, I discuss my work characterizing knockout mice for Fezf1, an important transcription factor in the correct differentiation of the olfactory neurons. We described the adverse effects to overall olfactory system development, and defects in the migratory mass, including reduced olfactory ensheathing cells (OECs), glial cells that support the GnRH-1 and TN neurons during their migration. Finally, in chapter 4, I describe my work on mice with mutations for Gli3, an effector in the Sonic Hedgehog (Shh) signaling and a critical molecule in OEC development. We acquired the first single-cell sequencing for two mutants of Gli3, Gli3XT/XT knockouts and Gli3PDN/PDN hypomorphs, and illustrated the dose-dependent effects of Gli3 on the development of the olfactory system at both the tissue and molecular level. This work uncovers developmental aspects of the TN, to gain a better understanding of its role in GnRH-1 migration and KS.