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

Hansen A, Finger TE - BMC Neurosci (2008)

Bottom Line: 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.Activity of these microvillous cells may however influence functionality of local elements of the olfactory system.

View Article: PubMed Central - HTML - PubMed

Affiliation: Rocky Mountain Taste and Smell Center, Dept. of Cell and Developmental Biology, School of Medicine, University of Colorado Denver, Aurora, 80045, USA. Anne.Hansen@uchsc.edu

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.

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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.
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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.

Mentions: Since the light microscopic level does not allow for detailed description of cell features we processed wild-type and TrpM5-GFP mouse OE for electron microscopy. These experiments revealed multiple microvillous cell types. As a preliminary classification we call these microvillous cells TrpM5a type, TrpM5b type, and a non-TrpM5 type on the basis of GFP-experiments and ultrastructural features. All 3 cell types occur in the main olfactory epithelium between ciliated ORNs and supporting cells (Fig 2A). The nuclei of the 3 microvillous cell types show a checkerboard pattern that typically has been used to distinguish ORN nuclei from the more homogenous nuclei of the supporting cells (Fig. 2B, D, G; 3C). In none of the 3 cell types described here did serial ultrathin sections reveal an axon penetrating the basal lamina. As our immunohistochemical experiments reveal (see below), the population of non-TrpM5 type microvillous cells consists most likely of several subsets with different molecular features, and it is possible that one or more of these subpopulations might have an axon that escaped our detection.


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

Hansen A, Finger TE - BMC Neurosci (2008)

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.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2629774&req=5

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.
Mentions: Since the light microscopic level does not allow for detailed description of cell features we processed wild-type and TrpM5-GFP mouse OE for electron microscopy. These experiments revealed multiple microvillous cell types. As a preliminary classification we call these microvillous cells TrpM5a type, TrpM5b type, and a non-TrpM5 type on the basis of GFP-experiments and ultrastructural features. All 3 cell types occur in the main olfactory epithelium between ciliated ORNs and supporting cells (Fig 2A). The nuclei of the 3 microvillous cell types show a checkerboard pattern that typically has been used to distinguish ORN nuclei from the more homogenous nuclei of the supporting cells (Fig. 2B, D, G; 3C). In none of the 3 cell types described here did serial ultrathin sections reveal an axon penetrating the basal lamina. As our immunohistochemical experiments reveal (see below), the population of non-TrpM5 type microvillous cells consists most likely of several subsets with different molecular features, and it is possible that one or more of these subpopulations might have an axon that escaped our detection.

Bottom Line: 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.Activity of these microvillous cells may however influence functionality of local elements of the olfactory system.

View Article: PubMed Central - HTML - PubMed

Affiliation: Rocky Mountain Taste and Smell Center, Dept. of Cell and Developmental Biology, School of Medicine, University of Colorado Denver, Aurora, 80045, USA. Anne.Hansen@uchsc.edu

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.

Show MeSH
Related in: MedlinePlus