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Is TrpM5 a reliable marker for chemosensory cells? Multiple types of microvillous cells in the main olfactory epithelium of mice

DOI: 10.1186/1471-2202-9-115

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Abstract:

We investigated the main olfactory epithelium of mice at the light and electron microscopic level and describe several subpopulations of microvillous cells. The ultrastructure of the microvillous cells reveals at least three morphologically different types two of which express the TrpM5 channel. None of these cells have an axon that projects to the olfactory bulb. Tests with a large panel of cell markers indicate that the TrpM5-positive cells are not sensory since they express neither neuronal markers nor are contacted by trigeminal nerve fibers.We conclude that TrpM5 is not a reliable marker for chemosensory cells. The TrpM5-positive cells of the olfactory epithelium are microvillous and may be chemoresponsive albeit not part of the sensory apparatus. Activity of these microvillous cells may however influence functionality of local elements of the olfactory system.Traditionally, the main olfactory epithelium (MOE) of mammals was said to contain only basal cells, supporting cells, and ciliated olfactory receptor neurons (ORNs) that utilize OR-type receptor molecules and the canonical G-protein-coupled transduction pathway via Gαolf, adenylyl cyclase III (ACIII), and cAMP [1]. However, a review of the literature suggests that this conventional view is too simplistic, e.g. microvillous ORNs are present in the olfactory epithelium of fishes and in the vomeronasal organ of mammals. Also, microvillous cells have been reported for the MOE of some mammals including humans [2-5]. A study by Rowley et al. utilizing HRP tracing claimed that at least some microvillous cells project directly to the olfactory bulb [6]. Braun and Zimmermann [4], utilizing ecto-5'-nucleotidase as a marker, detected microvillous cells in the MOE and suggested a mechanosensory function for these elements. Carr et al. reported microvillous cells in rats and concluded that these cells were non-sensory cells [7]. Functional studies revealed that mice with a disrupted cAMP pathway of ciliated ORNs are s

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