Studies on TRP channels have significantly extended our knowledge of the molecular events underlying the sensation of temperature, mechanical force, smell and taste. Thus, TRPM5 was recently shown to be enriched in TRC and is essential for the perception of sweet, bitter and umami taste compounds [7
]. However, it is well documented in the scientific literature that the expression pattern of TRPM5 is not restricted to taste buds. These findings prompted us to test the hypothesis that TRPM5 may play a broader functional role in chemosensation than initially suggested by experiments with TRPM5-deficient mice.
To this end, we raised and tested anti-TRPM5 antibodies whose staining pattern of taste receptor cells is fully congruent with previous work by other researchers [7
]. However, performing a more detailed analysis, we observed in addition that the basolateral plasma membrane of TRC is readily stained, whereas the apical part, the gustatory pore, is spared – a principle applying to all other TRPM5 positive cells investigated with the notable exception of cells in the sensory epithelium of the VNO. These observations provide further insight into the function of TRPM5 in TRC. Most other TRP channels that are involved in sensory signaling are eo ipso receptors of extra- or intracellular messengers and are found at the exact location of primary signal detection. This notion holds true for both direct signal detectors like heat- or cold-sensing TRPs [44
] as well as for TRPC channels that are activated by a receptor and G protein-dependent signaling cascade like TRP and TRPL in the insect rhabdomere [45
], the TRPC2 channel expressed in the dendritic knob of rodent VNO [3
] or TRPC6 expressed in microvillar cells of the main olfactory epithelium [46
]. Our data suggest that TRPM5 is not directly involved in the process of taste recognition. Activation of TRPM5 may depolarize the basolateral plasma membrane resulting in the modulation of other voltage-dependent ion channels and subsequent neurotransmitter release. Alternatively, the TRPM5-mediated depolarization of the basolateral membrane could modulate the function of gap junctions, and, thus change the physiological status of neighboring cells [47
Our studies with other sensory organs strongly indicate that in addition to taste perception TRPM5 is also involved in the processing of olfactory stimuli. Accordingly, we identified TRPM5 in two chemosensory organs, VNO and olfactory epithelia.
We detected TRPM5 in solitary cells within the non-sensory epithelium and apically in the sensory epithelium of the VNO. Interestingly, another member of the TRP gene family, the Ca2+
-permeable TRPC2 is also present on the apical cell surface of the sensory epithelium of the VNO [3
]. Thus, it is imaginable that in conjunction with TRPC2, TRPM5 is required for the detection of pheromones. In this context it should be recalled that TRPM5 cDNA was isolated from the VNO [12
]. Furthermore, CAN channel activity with electrophysical properties similar to TRPM5 or TRPM4 was measured in freshly isolated VNO neurons [12
]. Preliminary findings of our group using an anti-TRPM4 antibody illustrate that TRPM4 is highly expressed in the sensory epithelium of the VNO where it localizes to the apical region of sensory cells like TRPM5 (data not shown). These findings raise the possibility that in the VNO, TRPM4 and TRPM5 are co-expressed and co-localized and might therefore assemble to form heteromultimeric channels whose subcellular trafficking and functional properties could differ from either channel expressed alone in other cells.
In the olfactory epithelium, TRPM5 was found in scattered cells. The morphology and distribution of TRPM5-expressing cells are reminiscent of microvillar cells which are thought to be involved in sensing of odorants [46
]. By means of immunohistochemical criteria, it was recently shown that microvillar cells can be classified into at least two subtypes, one of which was denoted brush cells [51
] that were also found to be localized in the non-sensory epithelium of the VNO [52
]. The physiological relevance of TRPM5 expression in the VNO and the olfactory epithelium still remains to be clarified by a meticulous analysis of odorant and pheromone sensing in TRPM5-deficient mice. While this manuscript was under review, Lin W. et al. [53
] reported expression data similar to ours for TRPM5 in the main olfactory epithelium. Furthermore, these authors provided evidence for an involvement of TRPM5 in the sensing of semiochemicals, compatible with our hypothesis that TRPM5 may take part in chemosensation.
It was recently postulated that a population of specialized cells of the respiratory epithelium and gastrointestinal tract forming the so-called diffuse chemosensory system participates in the recognition and processing of diverse environmental cues [30
]. This assumption was recently substantiated by the immunolocalization of signaling proteins such as Gαgust
, T2R, PLCβ2 and the IP3
III receptor in solitary intestinal epithelial cells and in cells of the respiratory system.)[23
]. In accordance with these earlier findings, Bezençon et al. took advantage of TRPM5 promoter-based eGFP transgenic mice and identified GFP-positive cells in the gastrointestinal tract which partially co-express a set of taste signaling proteins mentioned above [56
These observations are in line with our data demonstrating TRPM5 expression in scattered epithelial cells of the respiratory system and gastrointestinal tract. The cell body of TRPM5-enriched cells is bottle-shaped. The apical elongated narrow cell pole directly contacts the lumen and protrudes beyond the neighboring epithelial cells. On the basolateral cell pole, TRPM5-expressing cells have a long process contacting the lamina propria. Thus, the distribution and morphological characteristics of TRPM5-immunopositive cells conspicuously share numerous features with brush cells [30
]. To identify the cellular origin of TRPM5-immunopositive cells, we resorted to double immunofluorescence staining using antibodies directed against villin or CK18, proteins known to be enriched in brush cells [41
]. We observed an invariable co-localization of these marker proteins with TRPM5 in the respiratory system and a predominant co-localization in the duodenum.
Brush cells were initially defined by morphological criteria, and a clear biological role of this cell population still remains elusive. Hypotheses about these cells' functional role range from absorptive processes [57
], mechanosensation [58
], secretion [59
] or a contribution to the local immunological defense barrier [62
]. In the respiratory system and the gastrointestinal tract, brush cells may play a role in sensation of chemical compounds in the lumen [23
]. The latter notion is derived from two salient observations: Firstly, brush cells display morphological features similar to TRC. Both cell types are characterized by an apical tuft of stiff microvilli with long rootlets into the cytoplasm lacking classical exocytotic vesicles or synapses [8
]. Secondly, several signaling proteins pertinent to taste perception were identified in a subset of brush cells, including α-gustducin [23
] and NO-synthase [70
]. To our knowledge, TRPM5 is the first protein identified to be specifically expressed in brush cells of the gastrointestinal tract and the respiratory system. These results lend further support to the view that taste cells and brush cells share a common functional principle, i.e. chemosensation, and that brush cells are an important cellular correlate for chemosensation in the periphery.