Illuminating the terminal nerve: Uncovering the link between GnRH-1 neuron and olfactory development

During embryonic development, the olfactory placode (OP) gives rise to various populations of neurons; these include putative olfactory pioneer neurons, different neurons of unknown identity and function, cells of the terminal nerve, and the Gonadotropin-releasing hormone-1 (GnRH-1) neurons. In mice, the GnRH-1 neurons are first detectable in the developing olfactory system around mid-gestation. From here, the GnRH-1 neurons migrate, along the axons of the terminal nerve (TN), to various regions of the developing brain. Once in the brain, the GnRH-1 neurons play a central role in controlling the hypothalamic-pituitary-gonadal (HPG) axis. Early migratory neurons forming from the olfactory placode have been proposed to play a vital role in inducing olfactory bulb morphogenesis. Kallmann syndrome is a condition characterized by defective development of the olfactory system and infertility. Murine studies have demonstrated the critical role of the Prokineticin 2-Prokinteicin Receptor 2 pathway in olfactory bulb morphogenesis and GnRH-1 neuronal migration. Loss-of-function Prokr2 mutations cause both Kallmann syndrome associated with bulb agenesis and normosmic idiopathic hypogonadotropic hypogonadism (nIHH). Following Prokr2 expression and lineage tracing, we found that Prokr2 is not expressed by the cells of the developing olfactory bulb but by migratory putative pioneer/terminal nerve neurons. Performing single-cell-RNA-sequencing, we identified genes enriched in the migratory cells of the putative terminal nerve. By integrating genetic lineage tracing and single-cell transcriptomics, we identified previously undescribed populations of migratory neurons that appear to be enriched in the expression of several genes related to olfactory defects, GnRH migratory deficiencies and infertility.