Project description: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.
Project description:The olfactory sensory system is formed by the coordinated morphogenesis and differentiation of the peripheral olfactory epithelium (OE) and the anterior forebrain. At early stages, immature olfactory receptor neurons (ORN) elongate their axons to penetrate the brain basement membrane, contact and form synapses with projection neurons of the olfactory bulb primordium. Axonal elongation is accompanied by migration of the GnRH+ neurons, followed by their ingression in the septo-hypothalamic area of the forebrain. This process is specifically impaired in the Kallmann’s syndrome (KS), a disorder characterized by anosmia and central hypogonadism. A set of transcription factors are master regulators of olfactory connectivity and GnRH neuron migration. We explored the transcriptional network underlying this process, by profiling the OE and adjacent mesenchyme at distinct embryonic ages. We also profiled the OE from embryos null for Dlx5, a homeogene essential for olfactory development, that causes a KS-like phenotype when deleted. We also applied analysis of conserved co-expression to integrate the obtained data with information on KS disease genes. The prevalent categories of genes differentially expressed during development are neuronal differentiation, extracellular remodelling and cell adhesion. From the analysis of Dlx5 mutant tissues we identify about 120 genes with a prevalence of intermediate filaments, cell signalling, epithelial and neuronal differentiation. Filtering for true OE expression and for the presence of Dlx5 binding sites, yielded twenty genes, of the following categories: 1) transmembrane adhesion/receptor molecules, 2) axon-glia interaction molecules, 3) synaptic proteins, 4) scaffold/adapter for signalling molecules. To functionally analyze these genes in vivo, we used three zebrafish fluorescent reporter zebrafish strains, in which we monitored early phases of olfactory/GnRH development upon gene downmodulation. The depletion of three (of five) Dlx5 targets affected axonal extension and targeting, while two (of two) altered GnRH neuron position and neurite organization.