5HTR3A‐driven GFP labels immature olfactory sensory neurons

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The olfactory epithelium generates new receptor neurons throughout adult life in all vertebrates including mammals. In a normal, resting adult olfactory epithelium, new neurons arise by proliferation and differentiation of a population of mitotically active globose basal cells situated near the basal lamina of the epithelium. In the adult epithelium, the globose basal cells are not homogeneously distributed throughout the epithelium, but assume a patchy distribution suggesting that neuronal addition to the epithelium is nonuniform. A second population of basal cells, the horizontal basal cells, serve as a reserve stem cell population becoming activated under conditions of extreme damage (Schwob et al., 2017).
The newly generated neurons extend a dendrite to the surface of the epithelium and an axon to the olfactory bulb as they differentiate into mature olfactory sensory neurons (OSNs). During this process of maturation, the new‐born OSNs undergo a stereotyped molecular and morphological maturation as their cell body migrates radially outward from the basal lamina to assume an intermediate position within the stratified epithelium (Rodriguez‐Gil et al., 2015). Shortly after their terminal cell division, immature OSNs express markers of a neuronal phenotype (Jang, Chen, Flis, Harris, & Schwob, 2014; Packard, Schnittke, Romano, Sinha, & Schwob, 2011), for example, NCAM and PGP9.5, as well as growth associated protein‐43 (GAP43) which is indicative of neuronal axon extension. After 4–5 days, the OSNs mature (Rodriguez‐Gil et al., 2015), expressing specific odorant receptors (OR), shortly followed by expression of elements of the olfactory transduction cascade and the characteristic olfactory marker protein (OMP). Molecular maturation is roughly coincident with the axon reaching the olfactory bulb (Rodriguez‐Gil et al., 2015) and the dendrite extending long cilia along the surface of the epithelium.
In adult mammals, a limited number of neurogenic zones persist in addition to the olfactory epithelium, chiefly in the dentate gyrus of the hippocampal formation and in the subventricular zone of the telencephalon (Lepousez, Nissant, & Lledo, 2015). The newly generated cells of the subventricular zone migrate tangentially along the rostral migratory stream to the olfactory bulb where they integrate as subpopulations of interneurons. In the hippocampal formation, cells generated in the subgranular zone of the dentate gyrus migrate radially into the granule cell layer where they differentiate into dentate gyrus granule cells. Immature neurons in several neuronal systems, including those in the rostral migratory stream, express the 5‐HT3 receptor (Engel, Smidt, & van Hooft, 2013). Accordingly, in these neurogenic systems, many neuroblasts and new‐born neurons can be identified by expression of green fluorescent protein (GFP) under control of the 5‐HT3A promoter (Chen, Lin, You, & Liu, 2012; Inta et al., 2008; Vucurovic et al., 2010). Here, we demonstrate the expression of 5‐HT3A mRNA in the olfactory epithelium, and using 5HT3AGFP mice, characterize the perdurance of 5HTR3A‐driven GFP after cessation of expression the 5‐HT3A mRNA. We show that 5HTR3A‐driven GFP is an effective marker for immature OSNs in the main olfactory epithelium, septal organ, and to a lesser extent in the vomeronasal organ.
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