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

Abstract

Background: In the past, ciliated receptor neurons, basal cells, and supporting cells were considered the principal components of the main olfactory epithelium. Several studies reported the presence of microvillous cells but their function is unknown. A recent report showed cells in the main olfactory epithelium that express the transient receptor potential channel TrpM5 claiming that these cells are chemosensory and that TrpM5 is an intrinsic signaling component of mammalian chemosensory organs. We asked whether the TrpM5-positive cells in the olfactory epithelium are microvillous and whether they belong to a chemosensory system, i.e. are olfactory neurons or trigeminally-innervated solitary chemosensory cells.

Results: 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.

Conclusion: 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.

Figure 1

Cell types in the MOE of mice. A Different types of microvillous cells (MV) (arrows) in the MOE labeled by GFP. Note that the olfactory epithelium varies in thickness. B Higher magnification of TrpM5a MV cells. C ORNs retrogradely labeled from the olfactory bulb by DiI. D Typical long slender ciliated ORNs retrogradely labeled by DiI.

Figure 2

Electron micrographs of different MV cells in the MOE of wild type mice. A Apices of cells in the MOE. Thick arrow: ciliated ORN, thin arrows: different types of MV cells. B TrpM5a MV cell with "stiff" microvilli. nu-mv – nucleus of MV cell; nu-sc – nucleus of supporting cell (SC). C Higher magnification of apex of TrpM5a MV cell in B. tw: The terminal web does not continue in the TrpM5a MV cell (arrows). (*)"spine"; ci – ciliated ORN. D TrpM5b MV cell. ci – ciliated ORN; nu-mv – nucleus of MV cell. E Higher magnification of cell apex in D. F Higher magnification of basal part of TrpM5b MV cell. Small protrusions interdigitate with other cells (arrowheads). G A non-TrpM5 MV cell with a tapering basal part. nu-sc – nucleus of SC. H Higher magnification of a non-TrpM5 MV cell. Arrow – centriole with rootlet; ci – ciliated ORN.

Figure 3

Electron micrographs of different MV cells in the MOE of transgenic mice. A TrpM5a MV cell labeled with the GFP antibody. Note how the neighboring SC "wraps" around the GFP-+ cell. B Higher magnification of a GFP-+ TrpM5a MV cell. The microvilli radiate from the apex giving the impression of being "stiff". C GFP-+ TrpM5b MV cell. Note the difference in cell size compared to the TrpM5a cell in A. D Higher magnification of the apex of a TrpM5b MV cell. E Basal portion of a TrpM5b MV cell. Small protrusions extend into the interstitium (arrow). F Non-TrpM5 MV cell in a TrpM5-GFP mouse. Arrow: nerve fiber profile adjacent to MV cell. G A non-TrpM5 MV cell in a TrpM5-GFP mouse with different morphology than that in F suggesting that non-TrpM5 MV cells comprise several subpopulations. ci – ciliated ORN.

Figure 4

A CGRP-positive nerve fibers are present but do not contact TrpM5-+ cells. B SUS-1 labels SCs in the MOE but not TrpM5-+ cells. C GFP-+ TrpM5a cells show villin immunoreactivity in their microvilli and in their cell membranes. D Microvilli and the cell membrane of a GFP-+ TrpM5b cell labeled with villin antisera. E GFP-+ TrpM5a type cells show espin immunoreactivity in their microvilli. In addition to TrpM5-GFP-+ cells some non-TrpM5 cells show espin label (arrow). Some ORNs are slightly GFP-+ but are espin-negative. F Microvilli of GFP-+ TrpM5b cells labeled with espin antisera.

Figure 5

A P2X3 receptor antiserum labels cells with nuclei high in the epithelium but does not label TrpM5-GFP-+ cells (arrow). B Cells located high in the epithelium labeled for TrpC6 show no TrpM5-GFP label. C GFP-+ TrpM5a cells do not double-label with antisera against Gαq/11. Other presumably MV cells (non-TrpM5) are Gαq/11-+ (arrowheads). D GFP-+ TrpM5a cells do not express PDE2A. Other cells in the vicinity are PDE2A-+ (arrows).