TRPM5-expressing microvillous cells in the main olfactory epithelium

A nasal cell atlas reveals heterogeneity of tuft cells and their role in directing olfactory stem cell proliferation

The olfactory neuroepithelium serves as a sensory organ for odors and forms part of the nasal mucosal barrier. Olfactory sensory neurons are surrounded and supported by epithelial cells. Among them, microvillous cells (MVCs) are strategically positioned at the apical surface, but their specific functions are enigmatic, and their relationship to the other specialized epithelial cells is unclear. Here, we establish that the family of MVCs comprises tuft cells and ionocytes in both mice and humans. Integrating analysis of the respiratory and olfactory epithelia, we define the distinct receptor expression of TRPM5+ tuft-MVCs compared with Gɑ-gustducinhigh respiratory tuft cells and characterize a previously undescribed population of glandular DCLK1+ tuft cells. To establish how allergen sensing by tuft-MVCs might direct olfactory mucosal responses, we used an integrated single-cell transcriptional and protein analysis. Inhalation of Alternaria induced mucosal epithelial effector molecules including Chil4 and a distinct pathway leading to proliferation of the quiescent olfactory horizontal basal stem cell (HBC) pool, both triggered in the absence of olfactory apoptosis. Alternaria- and ATP-elicited HBC proliferation was dependent on TRPM5+ tuft-MVCs, identifying these specialized epithelial cells as regulators of olfactory stem cell responses. Together, our data provide high-resolution characterization of nasal tuft cell heterogeneity and identify a function of TRPM5+ tuft-MVCs in directing the olfactory mucosal response to allergens.

A succinate/SUCNR1-brush cell defense program in the tracheal epithelium

Host-derived succinate accumulates in the airways during bacterial infection. Here, we show that luminal succinate activates murine tracheal brush (tuft) cells through a signaling cascade involving the succinate receptor 1 (SUCNR1), phospholipase Cβ2, and the cation channel transient receptor potential channel subfamily M member 5 (TRPM5). Stimulated brush cells then trigger a long-range Ca2+ wave spreading radially over the tracheal epithelium through a sequential signaling process. First, brush cells release acetylcholine, which excites nearby cells via muscarinic acetylcholine receptors. From there, the Ca2+ wave propagates through gap junction signaling, reaching also distant ciliated and secretory cells. These effector cells translate activation into enhanced ciliary activity and Cl- secretion, which are synergistic in boosting mucociliary clearance, the major innate defense mechanism of the airways. Our data establish tracheal brush cells as a central hub in triggering a global epithelial defense program in response to a danger-associated metabolite.

Tuft Cells: Context- and Tissue-Specific Programming for a Conserved Cell Lineage

Tuft cells are found in tissues with distinct stem cell compartments, tissue architecture, and luminal exposures but converge on a shared transcriptional program, including expression of taste transduction signaling pathways. Here, we summarize seminal and recent findings on tuft cells, focusing on major categories of function-instigation of type 2 cytokine responses, orchestration of antimicrobial responses, and emerging roles in tissue repair-and describe tuft cell-derived molecules used to affect these functional programs. We review what is known about the development of tuft cells from epithelial progenitors under homeostatic conditions and during disease. Finally, we discuss evidence that immature, or nascent, tuft cells with potential for diverse functions are driven toward dominant effector programs by tissue- or perturbation-specific contextual cues, which may result in heterogeneous mature tuft cell phenotypes both within and between tissues.

Isolation, Ex Vivo Culture, and Stimulation of Tracheal and Nasal Chemosensory Cells

Brush cells are chemosensory epithelial cells present at most mucosal surfaces.Brush cells are a dominant source of cysteinyl leukotrienes and IL-25 in the airway epithelium and are equipped with the machinery to generate prostaglandins and acetylcholine. Activation of innate type 2 lymphoid cells and dendritic cells triggered by brush cell-derived mediators skew the immune response in the airway to type 2 inflammation that underlies atopic disease such as asthma. This chapter describes an effective method of brush cell isolation from the mouse trachea for transcriptional analysis and from the nasal cavity for transcriptional analysis and ex vivo stimulation.The nasal or tracheal mucosa is first incubated in a dispase solution for easy mechanical separation of the epithelial layer from the underlying submucosa. The detached epithelium is then digested with a papain solution. This method provides high yields of viable brush cells in a single-cell suspension, which can be used for flow cytometric analysis, single-cell sorting, cell culture, and functional assays.In the nose, where brush cells are more abundant, we present two methods of isolation of brush cells: (1) using fluorescent reporter mice that mark brush cells or (2) using a combination of high expression of EpCAM and low expression of CD45 to obtain a population of cells that is enriched for nasal chemosensory brush cells.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Modulation of olfactory signal detection in the olfactory epithelium: focus on the internal and external environment, and the emerging role of the immune system

Detection and discrimination of odorants by the olfactory system plays a pivotal role in animal survival. Olfactory-based behaviors must be adapted to an ever-changing environment. Part of these adaptations includes changes of odorant detection by olfactory sensory neurons localized in the olfactory epithelium. It is now well established that internal signals such as hormones, neurotransmitters, or paracrine signals directly affect the electric activity of olfactory neurons. Furthermore, recent data have shown that activity-dependent survival of olfactory neurons is important in the olfactory epithelium. Finally, as olfactory neurons are directly exposed to environmental toxicants and pathogens, the olfactory epithelium also interacts closely with the immune system leading to neuroimmune modulations. Here, we review how detection of odorants can be modulated in the vertebrate olfactory epithelium. We choose to focus on three cellular types of the olfactory epithelium (the olfactory sensory neuron, the sustentacular and microvillar cells) to present the diversity of modulation of the detection of odorant in the olfactory epithelium. We also present some of the growing literature on the importance of immune cells in the functioning of the olfactory epithelium, although their impact on odorant detection is only just beginning to be unravelled.

Adhesion receptor ADGRG2/GPR64 is in the GI-tract selectively expressed in mature intestinal tuft cells

Objective

GPR64/ADGRG2 is an orphan Adhesion G protein-coupled receptor (ADGR) known to be mainly expressed in the parathyroid gland and epididymis. This investigation aimed to delineate the cellular expression of GPR64 throughout the body with focus on the gastrointestinal (GI) tract.

Methods

Transgenic Gpr64^mCherry reporter mice were histologically examined throughout the body and reporter protein expression in intestinal tuft cells was confirmed by specific cell ablation. The GPCR repertoire of intestinal Gpr64^mCherry-positive tuft cells was analyzed by quantitative RT-PCR analysis and in situ hybridization. The Gpr64^mCherry was crossed into the general tuft cell reporter Trpm5^GFP to generate small intestinal organoids for time-lapse imaging. Intestinal tuft cells were isolated from small intestine, FACS-purified and transcriptionally compared using RNA-seq analysis.

Results

Expression of the Gpr64^mCherry reporter was identified in multiple organs and specifically in olfactory microvillous cells, enteric nerves, and importantly in respiratory and GI tuft cells. In the small intestine, cell ablation targeting Gpr64-expressing epithelial cells eliminated tuft cells. Transcriptional analysis of small intestinal Gpr64^mCherry -positive tuft cells confirmed expression of Gpr64 and the chemo-sensors Sucnr1, Gprc5c, Drd3, and Gpr41/Ffar3. Time-lapse studies of organoids from Trpm5^GFP:Gpr64^mCherry mice revealed sequential expression of initially Trpm5^GFP and subsequently also Gpr64^mCherry in maturing intestinal tuft cells. RNA-seq analysis of small intestinal tuft cells based on these two markers demonstrated a dynamic change in expression of transcription factors and GPCRs from young to mature tuft cells.

Conclusions

GPR64 is expressed in chemosensory epithelial cells across a broad range of tissues; however, in the GI tract, GPR64 is remarkably selectively expressed in mature versus young immunoregulatory tuft cells.

•

GPR64-RFP is expressed mainly in chemosensory epithelial cells, including tuft cells.

•

Maturing intestinal tuft cells initially express Trpm5 and subsequently Gpr64.

•

Mature intestinal Gpr64+ tuft cells express Sucnr1, Gprc5c, Drd3, and Gpr41/Ffar3.

•

RNAseq analysis reveals dynamic transcriptional change of mature vs. young tuft cells.

Prevalence of Olfactory Dysfunction in Coronavirus Disease 2019 (COVID-19): A Meta-analysis of 27,492 Patients

OBJECTIVES/HYPOTHESIS

Olfactory dysfunction has been observed as one of the clinical manifestations in COVID-19 patients. We aimed to conduct a systematic review and meta-analysis to estimate the overall pooled prevalence of olfactory dysfunction in COVID-19 patients.

STUDY DESIGN

Systematic review and meta-analyses.

METHODS

PubMed, Scopus, Web of Science, Embase, and Google Scholar databases were searched to identify studies published between 1 December 2019 and 23 July 2020. We used random-effects model to estimate the pooled prevalence with 95% confidence intervals (CIs). Heterogeneity was assessed using the I2 statistic and Cochran's Q test. Robustness of the pooled estimates was checked by different subgroup and sensitivity analyses This study is registered with PROSPERO (CRD42020183768).

RESULTS

We identified 1162 studies, of which 83 studies (n = 27492, 61.4% female) were included in the meta-analysis. Overall, the pooled prevalence of olfactory dysfunction in COVID-19 patients was 47.85% [95% CI: 41.20-54.50]. We observed olfactory dysfunction in 54.40% European, 51.11% North American, 31.39% Asian, and 10.71% Australian COVID-19 patients. Anosmia, hyposmia, and dysosmia were observed in 35.39%, 36.15%, and 2.53% of the patients, respectively. There were discrepancies in the results of studies with objective (higher prevalence) versus subjective (lower prevalence) evaluations. The discrepancy might be due to false-negative reporting observed in self-reported health measures.

CONCLUSIONS

The prevalence of olfactory dysfunction in COVID-19 patients was found to be 47.85% based on high-quality evidence. Due to the subjective measures of most studies pooled in the analysis, further studies with objective measures are advocated to confirm the finding.

LEVEL OF EVIDENCE

2 Laryngoscope, 131:865-878, 2021.

Transcriptional profiling reveals potential involvement of microvillous TRPM5-expressing cells in viral infection of the olfactory epithelium

Background

Understanding viral infection of the olfactory epithelium is essential because the olfactory nerve is an important route of entry for viruses to the central nervous system. Specialized chemosensory epithelial cells that express the transient receptor potential cation channel subfamily M member 5 (TRPM5) are found throughout the airways and intestinal epithelium and are involved in responses to viral infection.

Results

Herein we performed deep transcriptional profiling of olfactory epithelial cells sorted by flow cytometry based on the expression of mCherry as a marker for olfactory sensory neurons and for eGFP in OMP-H2B::mCherry/TRPM5-eGFP transgenic mice (Mus musculus). We find profuse expression of transcripts involved in inflammation, immunity and viral infection in TRPM5-expressing microvillous cells compared to olfactory sensory neurons.

Conclusion

Our study provides new insights into a potential role for TRPM5-expressing microvillous cells in viral infection of the olfactory epithelium. We find that, as found for solitary chemosensory cells (SCCs) and brush cells in the airway epithelium, and for tuft cells in the intestine, the transcriptome of TRPM5-expressing microvillous cells indicates that they are likely involved in the inflammatory response elicited by viral infection of the olfactory epithelium.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07528-y.

Olfactory Rod Cells: A Rare Cell Type in the Larval Zebrafish Olfactory Epithelium With a Large Actin-Rich Apical Projection

We report the presence of a rare cell type, the olfactory rod cell, in the developing zebrafish olfactory epithelium. These cells each bear a single actin-rich rod-like apical projection extending 5–10 μm from the epithelial surface. Live imaging with a ubiquitous Lifeact-RFP label indicates that the olfactory rods can oscillate. Olfactory rods arise within a few hours of the olfactory pit opening, increase in numbers and size during larval stages, and can develop in the absence of olfactory cilia. Olfactory rod cells differ in morphology from the known classes of olfactory sensory neuron, but express reporters driven by neuronal promoters. A sub-population of olfactory rod cells expresses a Lifeact-mRFPruby transgene driven by the sox10 promoter. Mosaic expression of this transgene reveals that olfactory rod cells have rounded cell bodies located apically in the olfactory epithelium and have no detectable axon. We offer speculation on the possible function of these cells in the Discussion.

TRPM5-expressing Microvillous Cells Regulate Region-specific Cell Proliferation and Apoptosis During Chemical Exposure

The mammalian main olfactory epithelium (MOE) is exposed to a wide spectrum of external chemicals during respiration and relies on adaptive plasticity to maintain its structural and functional integrity. We previously reported that the chemo-responsive and cholinergic transient receptor potential channel M5 (TRPM5)-expressing-microvillous cells (MCs) in the MOE are required for maintaining odor-evoked electrophysiological responses and olfactory-guided behavior during two-week exposure to an inhaled chemical mixture. Here, we investigated the underlying factors by assessing the potential modulatory effects of TRPM5-MCs on MOE morphology and cell proliferation and apoptosis, which are important for MOE maintenance. In the posterior MOE of TRPM5-GFP mice, we found that two-week chemical exposure induced a significant increase in Ki67-expressing proliferating basal stem cells without a significant reduction in the thickness of the whole epithelium or mature olfactory sensory neuron (OSN) layer. This adaptive increase in stem cell proliferation was missing in chemical-exposed transcription factor Skn-1a knockout (Skn-1a^-/-) mice lacking TRPM5-MCs. In addition, a greater number of isolated OSNs from chemical-exposed Skn-1a^-/- mice displayed unhealthily high levels of resting intracellular Ca²⁺. Intriguingly, in the anterior MOE where we found a higher density of TRPM5-MCs, chemical-exposed TRPM5-GFP mice exhibited a time-dependent increase in apoptosis and a loss of mature OSNs without a significant increase in proliferation or neurogenesis to compensate for OSN loss. Together, our data suggest that TRPM5-MC-dependent region-specific upregulation of cell proliferation in the majority of the MOE during chemical exposure contributes to the adaptive maintenance of OSNs and olfactory function.

Epithelial cell function and remodeling in nasal polyposis

OBJECTIVE

To review the latest discoveries on airway epithelial cell diversity and remodeling in type 2 inflammation, including nasal polyposis.

DATA SOURCES

Reviews and primary research manuscripts were identified from PubMed, Google, and Bioarchives, using the search words airway epithelium, nasal polyposis, or chronic rhinosinusitis with nasal polyposis AND basal cell, ciliated cell, secretory cell, goblet cell, neuroendocrine cell, pulmonary neuroendocrine cell, ionocyte, brush cell, solitary chemosensory cell, microvillus cell, or tuft cell.

STUDY SELECTIONS

Studies were selected based on novelty and likely relevance to airway epithelial innate immune functions or the pathobiology of type 2 inflammation.

RESULTS

Airway epithelial cells are more diverse than previously appreciated, with specialized subsets, including ionocytes, solitary chemosensory cells, and neuroendocrine cells that contribute to important innate immune functions. In chronic rhinosinusitis with nasal polyposis, the composition of the epithelium is significantly altered. Loss of ciliated cells and submucosal glands and an increase in basal airway epithelial progenitors leads to loss of innate immune functions and an expansion of proinflammatory potential. Type 2 cytokines play a major role in driving this process.

CONCLUSION

Airway epithelial remodeling in chronic rhinosinusitis is extensive, leading to loss of innate immune function and enhanced proinflammatory potential. The mechanisms driving airway remodeling and its sequelae deserve further attention before restitution of epithelial differentiation can be considered a reasonable therapeutic target.

Airway brush cells generate cysteinyl leukotrienes through the ATP sensor P2Y2

Chemosensory epithelial cells (EpCs) are specialized cells that promote innate type 2 immunity and protective neurally mediated reflexes in the airway. Their effector programs and modes of activation are not fully understood. Here, we define the transcriptional signature of two choline acetyltransferase-expressing nasal EpC populations. They are found in the respiratory and olfactory mucosa and express key chemosensory cell genes including the transcription factor Pou2f3, the cation channel Trpm5, and the cytokine Il25 Moreover, these cells share a core transcriptional signature with chemosensory cells from intestine, trachea and thymus, and cluster with tracheal brush cells (BrCs) independently from other respiratory EpCs, indicating that they are part of the brush/tuft cell family. Both nasal BrC subsets express high levels of transcripts encoding cysteinyl leukotriene (CysLT) biosynthetic enzymes. In response to ionophore, unfractionated nasal BrCs generate CysLTs at levels exceeding that of the adjacent hematopoietic cells isolated from naïve mucosa. Among activating receptors, BrCs express the purinergic receptor P2Y2. Accordingly, the epithelial stress signal ATP and aeroallergens that elicit ATP release trigger BrC CysLT generation, which is mediated by the P2Y2 receptor. ATP- and aeroallergen-elicited CysLT generation in the nasal lavage is reduced in mice lacking Pou2f3, a requisite transcription factor for BrC development. Last, aeroallergen-induced airway eosinophilia is reduced in BrC-deficient mice. These results identify a previously undescribed BrC sensor and effector pathway leading to generation of lipid mediators in response to luminal signals. Further, they suggest that BrC sensing of local damage may provide an important sentinel immune function.

Neuropeptides in sensory signal processing

Peptides released from trigeminal fibers fulfill well-understood functions in neuroinflammatory processes and in the modulation of nociceptive signal processing. In particular, calcitonin gene-related peptide (CGRP) and substance P (SP), released from afferent nerve terminals, exert paracrine effects on the surrounding tissue and this has been recently highlighted by the prominent parcrine role of CGRP in the development of headache and migraine. Some recent communications suggest that these sensory neuropeptides may also modulate the workings of sensory organs and influence afferent signals from nose, tongue, eyes and ears. Here, we briefly review the evidence for modulatory effects of CGRP and SP in the sensory periphery.

Role of Taste Receptors as Sentinels of Innate Immunity in the Upper Airway

Evidence is emerging that shows taste receptors serve functions outside of taste sensation of the tongue. Taste receptors have been found in tissue across the human body, including the gastrointestinal tract, bladder, brain, and airway. These extraoral taste receptors appear to be important in modulating the innate immune response through detection of pathogens. This review discusses taste receptor signaling, focusing on the G-protein-coupled receptors that detect bitter and sweet compounds in the upper airway epithelium. Emphasis is given to recent studies which link the physiology of sinonasal taste receptors to clinical manifestation of upper airway disease.

Chemical Exposure-Induced Changes in the Expression of Neurotrophins and Their Receptors in the Main Olfactory System of Mice Lacking TRPM5-Expressing Microvillous Cells

Functional maintenance of the mammalian main olfactory epithelium (MOE) is challenging because of its direct exposure to a wide spectrum of environmental chemicals. We previously reported that transient receptor potential channel M5-expressing microvillous cells (TRPM5-MCs) in the MOE play an important role in olfactory maintenance. To investigate the underpinning mechanisms, we exposed transcription factor Skn-1a knockout (Skn-1a-/-) mice lacking TRPM5-MCs, and TRPM5-GFP mice to either vehicle (water) or a mixture of odorous chemicals and chitin for two weeks and analyzed the expression of olfactory signaling proteins using immunolabeling and neurotrophin (NT) and NT receptor (NTR) gene transcripts using real-time quantitative PCR. The chemical exposure did not significantly attenuate the immunolabeling of olfactory signaling proteins. Vehicle-exposed Skn-1a-/- and TRPM5-GFP mice expressed similar levels of NT and NTR gene transcripts in the MOE and olfactory bulb. Chemical exposure significantly increased MOE expression of p75NTR in Skn-1a-/- mice, while p75NTR expression was reduced in TRPM5-GFP mice, as compared to vehicle-exposed mice. Additionally, our RNA in situ hybridization analysis and immunolabeling confirmed MOE expression of most NTs and NTRs. Together, these results indicate that TRPM5-MCs and chemical exposure influence expression of some NTs and NTRs in the MOE and olfactory bulb (OB).

Structural organization of the olfactory organ in an amphihaline migratory fish Hilsa, Tenualosa ilisha

The histological as well as ultramicroscopic structures of olfactory system of an amphihaline migratory fish hilsa Tenualosa ilisha, were studied. The sexually matured riverine fish were collected from a common breeding habitat-the Hooghly, a tributary of river Ganga, West Bengal, India. This study revealed that the riverine hilsa has larger olfactory bulb compared to marine hilsa with the olfactory lobes well exposed through nostrils. The olfactory lamellae (OL) are 40-45 in number and posses three distinct layers of sensory cells across each lamellae, namely, outer receptor cells (RC), middle sensory cells, and inner basal cells (BC). Besides the above arrangement, the sensory part of olfactory epithelium (OE) also bears rich microvillous cells exposed to the surface of the OE. The sensory and non-sensory surfaces on OL are distinguishable, with clear dendritic cells on sensory epithelium and solitary chemosensory cells on non sensory OE. Abundance of both types of cells in the OE is an indication of its chemoattraction ability towards molecules of amino acid origin. The feature of having abundant, dense, and large dendritic knobs on the surface of OE describes resemblance to the typical morphology of the chemosensory septal organs neuron. The expression of four G protein subunits, like Gαs/olf, Gαq, Gαo, and Gαi-3 in OE indicate that its olfaction is a functional attributes of two olfactory systems, namely main olfactory system and Vomaronasal Olfactory System. Expression of ACIII and PLCβ2 in OE further confirms two signaling pathways involved in odorant reception in hilsa. RESEARCH HIGHLIGHTS: The olfactory bulb in the amphihaline migratory fish hilsa is big in size, with 40-45 lamellae. Its sensory areas showed multilayered cellular features with prominent sensory as well as microvillous cells, whereas non-sensory area possesses solitary chemosensory cells. The expression of four G protein subunits, Gαs/olf, Gαq, Gαo, and Gαi-3 in olfactory epithelium indicates that its olfaction is a functional attributes of two olfactory systems, namely main olfactory system and vomaronasal olfactory system.

Microvillous cells in the olfactory epithelium express elements of the solitary chemosensory cell transduction signaling cascade

The nasal cavity hosts an array of chemoresponsive cells, including the extended olfactory system and several other cells involved in detection of and responses to irritants. Solitary chemosensory cells (SCCs), which respond to irritants and bacteria, express the transient receptor potential channel TRPM5 an essential element of the taste transduction-signaling cascade. Microvillous cells (MVCs), non-neuronal cells situated in the apical layer of the main olfactory epithelium, also express TRPM5, but their function has not yet been clarified. TRPM5-positive MVCs, like SCCs, show a cholinergic phenotype expressing choline acetyl transferase (ChAT), but none of the other elements of the bitter taste transduction cascade could be detected. We reexamined TRPM5-positive MVCs with more sensitive gene expression and staining techniques to clarify whether they rely only on TRPM5 and ChAT or express other elements of the taste/SCC transduction cascade. Analyzing existing RNA sequencing data from whole olfactory mucosa and isolated olfactory sensory neurons, we determined that several elements of the taste/SCC transduction cascade, including taste receptors, are expressed in the olfactory mucosa in cells other than olfactory sensory neurons. Immunostaining confirmed the presence TRPM5 and ChAT in a subset of cells of the olfactory mucosa, which also showed the expression of PLCB2, gustducin, and T1R3. Specifically, these cells were identified as TRPM5-positive MVCs. Furthermore, we examined whether MVCs are innervated by trigeminal fibers, similarly to SCCs. Using antibodies against trigeminal nerve markers calcitonin gene-related peptide and substance P, we determined that, despite the cholinergic phenotype, most MVCs in the olfactory mucosa lacked consistent trigeminal innervation. Our findings indicate that MVCs, like SCCs, express all the elements of the bitter taste transduction cascade but that, unlike SCCs, they possess only sparse trigeminal innervation. The cholinergic phenotype of MVCs suggests a modulatory function of the surrounding olfactory epithelium, through the release of acetylcholine.

Role of neuropeptide Y (NPY) in the differentiation of Trpm-5-positive olfactory microvillar cells

The mouse olfactory neuroepithelium (ON) is comprised of anatomically distinct populations of cells in separate regions; apical (sustentacular and microvillar), neuronal (olfactory sensory neurons) and basal (horizontal and globose basal cells). The existence of microvillar cells (MVCs) is well documented but their nature and function remains unclear. An important transcription factor for the differentiation of MVCs is Skn-1a, with loss of function of Skn-1a in mice resulting in a complete loss of Trpm-5 expressing MVCs, while olfactory sensory neuron differentiation is normal. Our previous research has shown that neuropeptide Y (NPY) is expressed in MVCs and is important in the neuroproliferation of olfactory precursors. This study showed that following X-ray irradiation of the snout of wildtype mice, which decreases the proliferation of basal precursor cells, the numbers of Trpm-5-positive MVCs is increased at 2 and 5 weeks post-irradiation compared to controls. Skn-1a expression in the ON following X-ray irradiation also increases at 2 weeks post-irradiation in a regionally specific manner matching the expression pattern of Trpm-5-positive MVCs. In parallel, NPYCre knock-in mice were used to examine the expression of Skn-1a following activation of NPY unilaterally in the ON (unilateral nasal irrigation of AAV-NPY-FLEX). These experiments demonstrated that Skn-1a is only expressed when NPY is activated in MVCs. Therefore the expression of NPY is necessary for the transcription factor-mediated differentiation of olfactory MVCs.

ATP and Odor Mixture Activate TRPM5-Expressing Microvillous Cells and Potentially Induce Acetylcholine Release to Enhance Supporting Cell Endocytosis in Mouse Main Olfactory Epithelium

The main olfactory epithelium (MOE) functions to detect odor molecules, provide an epithelial surface barrier, and remove xenobiotics from inhaled air. Mechanisms coordinating the activities of different cell types within the MOE to maintain these functions are poorly understood. Previously, we showed that superficially located microvillous cells (MCs) in the MOE expressing transient receptor potential channel M5 (TRPM5) are cholinergic and chemoresponsive and that they play an important role in maintaining odor responses and olfactory-guided behavior under challenging chemical environment. Here we investigated TRPM5-MC activation and subsequent paracrine regulation. Ca²⁺ imaging showed that TRPM5-MCs dose-dependently increase their intracellular Ca²⁺ levels in response to ATP, an important signaling molecule for airway mucociliary movement, and to an odor mixture. Pharmacological examination showed that the ATP responses are primarily mediated by P2X purinergic receptors. Interestingly, using the endocytosis dye pHrodo Red dextran, we found that chemical-activated TRPM5-MCs significantly increase the number of pHrodo-labeled puncta compared to controls without stimulation and compared to cells that do not respond to ATP or to the odor mixture. These results indicate potential vesicle recycling after release of the signaling molecule acetylcholine (ACh). Interestingly, TRPM5 knockout (KO) results in a decrease in ATP-induced pHrodo internalization. We further investigated cholinergic regulation of neighboring supporting cells (SCs). We found that ACh strongly elevates intracellular Ca²⁺ and potentiates pHrodo endocytosis in SCs. The ACh effects are diminished in the presence of atropine or M3 muscarinic receptor antagonist and in SCs lacking M3 receptors. Collectively, these data suggest that TRPM5-MCs may regulate the MOE’s multicellular network activity via cholinergic paracrine signaling for functional maintenance and adaptive plasticity.

Transcriptional and Epigenetic Control of Mammalian Olfactory Epithelium Development

The postnatal mammalian olfactory epithelium (OE) represents a major aspect of the peripheral olfactory system. It is a pseudostratified tissue that originates from the olfactory placode and is composed of diverse cells, some of which are specialized receptor neurons capable of transducing odorant stimuli to afford the perception of smell (olfaction). The OE is known to offer a tractable miniature model for studying the systematic generation of neurons and glia that typify neural tissue development. During OE development, stem/progenitor cells that will become olfactory sensory neurons and/or non-neuronal cell types display fine spatiotemporal expression of neuronal and non-neuronal genes that ensures their proper proliferation, differentiation, survival, and regeneration. Many factors, including transcription and epigenetic factors, have been identified as key regulators of the expression of such requisite genes to permit normal OE morphogenesis. Typically, specific interactive regulatory networks established between transcription and epigenetic factors/cofactors orchestrate histogenesis in the embryonic and adult OE. Hence, investigation of these regulatory networks critical for OE development promises to disclose strategies that may be employed in manipulating the stepwise transition of olfactory precursor cells to become fully differentiated and functional neuronal and non-neuronal cell types. Such strategies potentially offer formidable means of replacing injured or degenerated neural cells as therapeutics for nervous system perturbations. This review recapitulates the developmental cellular diversity of the olfactory neuroepithelium and discusses findings on how the precise and cooperative molecular control by transcriptional and epigenetic machinery is indispensable for OE ontogeny.

Lack of TRPM5-Expressing Microvillous Cells in Mouse Main Olfactory Epithelium Leads to Impaired Odor-Evoked Responses and Olfactory-Guided Behavior in a Challenging Chemical Environment

The mammalian main olfactory epithelium (MOE) modifies its activities in response to changes in the chemical environment. This process is essential for maintaining the functions of the olfactory system and the upper airway. However, mechanisms involved in this functional maintenance, especially those occurring via paracrine regulatory pathways within the multicellular MOE, are poorly understood. Previously, a population of non-neuronal, transient receptor potential M5-expressing microvillous cells (TRPM5-MCs) was identified in the MOE, and the initial characterization of these cells showed that they are cholinergic and responsive to various xenobiotics including odorants at high concentrations. Here, we investigated the role of TRPM5-MCs in maintaining olfactory function using transcription factor Skn-1a knockout (Skn-1a^-/-) mice, which lack TRPM5-MCs in the MOE. Under our standard housing conditions, Skn-1a^-/- mice do not differ significantly from control mice in odor-evoked electro-olfactogram (EOG) responses and olfactory-guided behaviors, including finding buried food and preference reactions to socially and sexually relevant odors. However, after a 2-wk exposure to high-concentration odor chemicals and chitin powder, Skn-1a^-/- mice exhibited a significant reduction in their odor and pheromone-evoked EOG responses. Consequently, their olfactory-guided behaviors were impaired compared with vehicle-exposed Skn-1a^-/- mice. Conversely, the chemical exposure did not induce significant changes in the EOG responses and olfactory behaviors of control mice. Therefore, our physiological and behavioral results indicate that TRPM5-MCs play a protective role in maintaining the olfactory function of the MOE.

Lack of TRPM5-Expressing Microvillous Cells in Mouse Main Olfactory Epithelium Leads to Impaired Odor-Evoked Responses and Olfactory-Guided Behavior in a Challenging Chemical Environment

The mammalian main olfactory epithelium (MOE) modifies its activities in response to changes in the chemical environment. This process is essential for maintaining the functions of the olfactory system and the upper airway. However, mechanisms involved in this functional maintenance, especially those occurring via paracrine regulatory pathways within the multicellular MOE, are poorly understood. Previously, a population of non-neuronal, transient receptor potential M5-expressing microvillous cells (TRPM5-MCs) was identified in the MOE, and the initial characterization of these cells showed that they are cholinergic and responsive to various xenobiotics including odorants at high concentrations. Here, we investigated the role of TRPM5-MCs in maintaining olfactory function using transcription factor Skn-1a knockout (Skn-1a^-/-) mice, which lack TRPM5-MCs in the MOE. Under our standard housing conditions, Skn-1a^-/- mice do not differ significantly from control mice in odor-evoked electro-olfactogram (EOG) responses and olfactory-guided behaviors, including finding buried food and preference reactions to socially and sexually relevant odors. However, after a 2-wk exposure to high-concentration odor chemicals and chitin powder, Skn-1a^-/- mice exhibited a significant reduction in their odor and pheromone-evoked EOG responses. Consequently, their olfactory-guided behaviors were impaired compared with vehicle-exposed Skn-1a^-/- mice. Conversely, the chemical exposure did not induce significant changes in the EOG responses and olfactory behaviors of control mice. Therefore, our physiological and behavioral results indicate that TRPM5-MCs play a protective role in maintaining the olfactory function of the MOE.

Trpm5 expression in the olfactory epithelium

The Ca²⁺-activated monovalent cation channel Trpm5 is a key element in chemotransduction of taste receptor cells of the tongue, but the extent to which Trpm5 channels are expressed in olfactory sensory neurons (OSNs) of the main olfactory epithelium (MOE) of adult mice as part of a specific pheromonal detection system is debated. Here, we used a novel Trpm5-IRES-Cre knockin strain to drive Cre recombinase expression, employed previously validated Trpm5 antibodies, performed in situ hybridization experiments to localize Trpm5 RNA, and searched extensively for Trpm5 splice variants in genetically-labeled, Trpm5-expressing MOE cells. In contrast to previous reports, we find no evidence for the existence in adult mouse OSNs of the classical Trpm5 channel known from taste cells. We show that Trpm5-expressing adult OSNs express a novel Trpm5 splice variant, Trpm5-9, that is unlikely to form a functional cation channel by itself. We also demonstrate that Trpm5 is transiently expressed in a subpopulation of mature OSNs in the embryonic olfactory epithelium, indicating that Trpm5 channels could play a specific role in utero during a narrow developmental time window. Ca²⁺ imaging with GCaMP3 under the control of the Trpm5-IRES-Cre allele using a newly developed MOE wholemount preparation of the adult olfactory epithelium reveals that Trpm5-GCaMP3 OSNs comprise a heterogeneous group of sensory neurons many of which can detect general odorants. Together, these studies are essential for understanding the role of transient receptor potential channels in mammalian olfaction.

Ascl3 transcription factor marks a distinct progenitor lineage for non-neuronal support cells in the olfactory epithelium

The olfactory epithelium (OE) is composed of olfactory sensory neurons (OSNs), sustentacular supporting cells, and several types of non-neuronal cells. Stem and progenitor cells are located basally, and are the source of all cell types needed to maintain OE homeostasis. Here, we report that Ascl3, a basic helix-loop-helix transcription factor, is expressed in the developing OE. Lineage tracing experiments demonstrate that the non-neuronal microvillar cells and Bowman's glands are exclusively derived from Ascl3+ progenitor cells in the OE during development. Following chemically-induced injury, Ascl3 expression is activated in a subset of horizontal basal cells (HBCs), which repopulate all microvillar cells and Bowman's glands during OE regeneration. After ablation of Ascl3-expressing cells, the OE can regenerate, but lacks the non-neuronal microvillar and Bowman's gland support cells. These results demonstrate that Ascl3 marks progenitors that are lineage-committed strictly to microvillar cells and Bowman's glands, and highlight the requirement for these cell types to support OE homeostasis.

Expression of homeobox genes in the mouse olfactory epithelium

Homeobox genes constitute a large family of genes widely studied because of their role in the establishment of the body pattern. However, they are also involved in many other events during development and adulthood. The main olfactory epithelium (MOE) is an excellent model to study neurogenesis in the adult nervous system. Analyses of homeobox genes during development show that some of these genes are involved in the formation and establishment of cell diversity in the MOE. Moreover, the mechanisms of expression of odorant receptors (ORs) constitute one of the biggest enigmas in the field. Analyses of OR promoters revealed the presence of homeodomain binding sites in their sequences. Here we characterize the expression patterns of a set of 49 homeobox genes in the MOE with in situ hybridization. We found that seven of them (Dlx3, Dlx5, Dlx6, Msx1, Meis1, Isl1, and Pitx1) are zonally expressed. The homeobox gene Emx1 is expressed in three guanylate cyclase(+) populations, two located in the MOE and the third one in an olfactory subsystem known as Grüneberg ganglion located at the entrance of the nasal cavity. The homeobox gene Tshz1 is expressed in a unique patchy pattern across the MOE. Our findings provide new insights to guide functional studies that aim to understand the complexity of transcription factor expression and gene regulation in the MOE. J. Comp. Neurol. 524:2713-2739, 2016. © 2016 Wiley Periodicals, Inc.

The Role of Bitter and Sweet Taste Receptors in Upper Airway Immunity

Over the past several years, taste receptors have emerged as key players in the regulation of innate immune defenses in the mammalian respiratory tract. Several cell types in the airway, including ciliated epithelial cells, solitary chemosensory cells, and bronchial smooth muscle cells, all display chemoresponsive properties that utilize taste receptors. A variety of bitter products secreted by microbes are detected with resultant downstream inflammation, increased mucous clearance, antimicrobial peptide secretion, and direct bacterial killing. Genetic variation of bitter taste receptors also appears to play a role in the susceptibility to infection in respiratory disease states, including that of chronic rhinosinusitis. Ongoing taste receptor research may yield new therapeutics that harness innate immune defenses in the respiratory tract and may offer alternatives to antibiotic treatment. The present review discusses taste receptor-protective responses and analyzes the role these receptors play in mediating airway immune function.

Transcription factor p63 controls the reserve status but not the stemness of horizontal basal cells in the olfactory epithelium

Adult tissue stem cells can serve two broad functions: to participate actively in the maintenance and regeneration of a tissue or to wait in reserve and participate only when activated from a dormant state. The adult olfactory epithelium, a site for ongoing, life-long, robust neurogenesis, contains both of these functional stem cell types. Globose basal cells (GBCs) act as the active stem cell population and can give rise to all the differentiated cells found in the normal tissue. Horizontal basal cells (HBCs) act as reserve stem cells and remain dormant unless activated by tissue injury. Here we show that HBC activation following injury by the olfactotoxic gas methyl bromide is coincident with the down-regulation of protein 63 (p63) but anticipates HBC proliferation. Gain- and loss-of-function studies show that this down-regulation of p63 is necessary and sufficient for HBC activation. Moreover, activated HBCs give rise to GBCs that persist for months and continue to act as bona fide stem cells by participating in tissue maintenance and regeneration over the long term. Our analysis provides mechanistic insight into the dynamics between tissue stem cell subtypes and demonstrates that p63 regulates the reserve state but not the stem cell status of HBCs.

Observations on the effects of odours on the homeopathic response

Samuel Hahnemann described incidences where the homeopathic response was disrupted by noxious smells in the environment. An earlier paper proposed that homeopathic medicines may be sensed by vomeronasal cells (VNCs) i.e. microvillus or brush cells in the vomeronasal organ (VNO), the taste buds and associated with the trigeminal nerve and nervus terminalis. This paper proposes an extension to the theory and suggests that a subset of solitary chemosensory cells (SCCs) in the diffuse chemosensory system (DCS) that is morphologically similar to VNCs might also be receptive to homeopathic medicines. The types of odours that may interfere with this process are described. Two clinical cases of disruption of the homeopathic response are given as examples, showing that successful re-establishment of remedy action can be produced by timely repetition of the medicine. The ramifications on clinical homeopathic practice are discussed.

Evolution of spatially coexpressed families of type-2 vomeronasal receptors in rodents

The vomeronasal organ (VNO) is an olfactory structure for the detection of pheromones. VNO neurons express three groups of unrelated G-protein-coupled receptors. Type-2 vomeronasal receptors (V2Rs) are specifically localized in the basal neurons of the VNO and are believed to sense protein pheromones eliciting specific reproductive behaviors. In murine species, V2Rs are organized into four families. Family-ABD V2Rs are expressed monogenically and coexpress with family-C V2Rs of either subfamily C1 (V2RC1) or subfamily C2 (V2RC2), according to a coordinate temporal diagram. Neurons expressing the phylogenetically ancient V2RC1 coexpress family-BD V2Rs or a specific group of subfamily-A V2Rs (V2RA8-10), whereas a second neuronal subset (V2RC2-positive) coexpresses a recently expanded group of five subfamily-A V2Rs (V2RA1-5) along with vomeronasal-specific Major Histocompatibility Complex molecules (H2-Mv). Through database mining and Sanger sequencing, we have analyzed the onset, diversification, and expansion of the V2R-families throughout the phylogeny of Rodentia. Our results suggest that the separation of V2RC1 and V2RC2 occurred in a Cricetidae ancestor in coincidence with the evolution of the H2-Mv genes; this phylogenetic event did not correspond with the origin of the coexpressing V2RA1-5 genes, which dates back to an ancestral myomorphan lineage. Interestingly, the evolution of receptors within the V2RA1-5 group may be implicated in the origin and diversification of some of the V2R putative cognate ligands, the exocrine secreting peptides. The establishment of V2RC2, which probably reflects the complex expansion and diversification of family-A V2Rs, generated receptors that have probably acquired a more subtle functional specificity.

Sensational placodes: neurogenesis in the otic and olfactory systems

For both the intricate morphogenetic layout of the sensory cells in the ear and the elegantly radial arrangement of the sensory neurons in the nose, numerous signaling molecules and genetic determinants are required in concert to generate these specialized neuronal populations that help connect us to our environment. In this review, we outline many of the proteins and pathways that play essential roles in the differentiation of otic and olfactory neurons and their integration into their non-neuronal support structures. In both cases, well-known signaling pathways together with region-specific factors transform thickened ectodermal placodes into complex sense organs containing numerous, diverse neuronal subtypes. Olfactory and otic placodes, in combination with migratory neural crest stem cells, generate highly specialized subtypes of neuronal cells that sense sound, position and movement in space, odors and pheromones throughout our lives.

Chemosensors in the nose: guardians of the airways

The G-protein-coupled receptor molecules and downstream effectors that are used by taste buds to detect sweet, bitter, and savory tastes are also utilized by chemoresponsive cells of the airways to detect irritants. Here, we describe the different cell types in the airways that utilize taste-receptor signaling to trigger protective epithelial and neural responses to potentially dangerous toxins and bacterial infection.

Skn-1a/Pou2f3 is required for the generation of Trpm5-expressing microvillous cells in the mouse main olfactory epithelium

Background

The main olfactory epithelium (MOE) in mammals is a specialized organ to detect odorous molecules in the external environment. The MOE consists of four types of cells: olfactory sensory neurons, supporting cells, basal cells, and microvillous cells. Among these, development and function of microvillous cells remain largely unknown. Recent studies have shown that a population of microvillous cells expresses the monovalent cation channel Trpm5 (transient receptor potential channel M5). To examine functional differentiation of Trpm5-expressing microvillous cells in the MOE, we investigated the expression and function of Skn-1a, a POU (Pit-Oct-Unc) transcription factor required for functional differentiation of Trpm5-expressing sweet, umami, and bitter taste bud cells in oropharyngeal epithelium and solitary chemosensory cells in nasal respiratory epithelium.

Results

Skn-1a is expressed in a subset of basal cells and apical non-neuronal cells in the MOE of embryonic and adult mice. Two-color in situ hybridization revealed that a small population of Skn-1a-expressing cells was co-labeled with Mash1/Ascl1 and that most Skn-1a-expressing cells coexpress Trpm5. To investigate whether Skn-1a has an irreplaceable role in the MOE, we analyzed Skn-1a-deficient mice. In the absence of Skn-1a, olfactory sensory neurons differentiate normally except for a limited defect in terminal differentiation in ectoturbinate 2 of some of MOEs examined. In contrast, the impact of Skn-1a deficiency on Trpm5-expressing microvillous cells is much more striking: Trpm5, villin, and choline acetyltransferase, cell markers previously shown to identify Trpm5-expressing microvillous cells, were no longer detectable in Skn-1a-deficient mice. In addition, quantitative analysis demonstrated that the density of superficial microvillous cells was significantly decreased in Skn-1a-deficient mice.

Conclusion

Skn-1a is expressed in a minority of Mash1-positive olfactory progenitor cells and a majority of Trpm5-expressing microvillous cells in the main olfactory epithelium. Loss-of-function mutation of Skn-1a resulted in complete loss of Trpm5-expressing microvillous cells, whereas most of olfactory sensory neurons differentiated normally. Thus, Skn-1a is a critical regulator for the generation of Trpm5-expressing microvillous cells in the main olfactory epithelium in mice.

TRPM5

TRPM5 is a Ca(2+)-activated cation channel that mediates signaling in taste and other chemosensory cells. Within taste cells, TRPM5 is the final element in a signaling cascade that starts with the activation of G protein-coupled receptors by bitter, sweet, or umami taste molecules and that requires the enzyme PLCβ2. PLCβ2 breaks down PIP2 into DAG and IP3, and the ensuing release of Ca(2+) from intracellular stores activates TRPM5. Since its initial discovery in the taste system, TRPM5 has been found to be distributed in sparse chemosensory cells located throughout the digestive track, in the respiratory system, and in the olfactory system. It is also found in pancreatic islets, where it contributes to insulin secretion. This review highlights recent work on the mechanisms of the activation of the TRPM5 channel and its regulation by voltage, phosphoinositides, temperature, and pH. The distribution of the channel in the body and its functional contribution to various sensory and nonsensory processes are discussed.

Mouse Grueneberg ganglion neurons share molecular and functional features with C. elegans amphid neurons

The mouse Grueneberg ganglion (GG) is an olfactory subsystem located at the tip of the nose close to the entry of the naris. It comprises neurons that are both sensitive to cold temperature and play an important role in the detection of alarm pheromones (APs). This chemical modality may be essential for species survival. Interestingly, GG neurons display an atypical mammalian olfactory morphology with neurons bearing deeply invaginated cilia mostly covered by ensheathing glial cells. We had previously noticed their morphological resemblance with the chemosensory amphid neurons found in the anterior region of the head of Caenorhabditis elegans (C. elegans). We demonstrate here further molecular and functional similarities. Thus, we found an orthologous expression of molecular signaling elements that was furthermore restricted to similar specific subcellular localizations. Calcium imaging also revealed a ligand selectivity for the methylated thiazole odorants that amphid neurons are known to detect. Cellular responses from GG neurons evoked by chemical or temperature stimuli were also partially cGMP-dependent. In addition, we found that, although behaviors depending on temperature sensing in the mouse, such as huddling and thermotaxis did not implicate the GG, the thermosensitivity modulated the chemosensitivity at the level of single GG neurons. Thus, the striking similarities with the chemosensory amphid neurons of C. elegans conferred to the mouse GG neurons unique multimodal sensory properties.

An effective manual deboning method to prepare intact mouse nasal tissue with preserved anatomical organization

The mammalian nose is a multi-functional organ with intricate internal structures. The nasal cavity is lined with various epithelia such as olfactory, respiratory, and squamous epithelia which differ markedly in anatomical locations, morphology, and functions. In adult mice, the nose is covered with various skull bones, limiting experimental access to internal structures, especially those in the posterior such as the main olfactory epithelium (MOE). Here we describe an effective method for obtaining almost the entire and intact nasal tissues with preserved anatomical organization. Using surgical tools under a dissecting microscope, we sequentially remove the skull bones surrounding the nasal tissue. This procedure can be performed on both paraformaldehyde-fixed and freshly dissected, skinned mouse heads. The entire deboning procedure takes about 20-30 min, which is significantly shorter than the experimental time required for conventional chemical-based decalcification. In addition, we present an easy method to remove air bubbles trapped between turbinates, which is critical for obtaining intact thin horizontal or coronal or sagittal sections from the nasal tissue preparation. Nasal tissue prepared using our method can be used for whole mount observation of the entire epithelia, as well as morphological, immunocytochemical, RNA in situ hybridization, and physiological studies, especially in studies where region-specific examination and comparison are of interest.

Potential role of transient receptor potential channel M5 in sensing putative pheromones in mouse olfactory sensory neurons

Based on pharmacological studies of chemosensory transduction in transient receptor potential channel M5 (TRPM5) knockout mice it was hypothesized that this channel is involved in transduction for a subset of putative pheromones in mouse olfactory sensory neurons (OSNs). Yet, in the same study an electroolfactogram (EOG) in the mouse olfactory epithelium showed no significant difference in the responses to pheromones (and odors) between wild type and TRPM5 knockout mice. Here we show that the number of OSNs expressing TRPM5 is increased by unilateral naris occlusion. Importantly, EOG experiments show that mice lacking TRPM5 show a decreased response in the occluded epithelia to putative pheromones as opposed to wild type mice that show no change upon unilateral naris occlusion. This evidence indicates that under decreased olfactory sensory input TRPM5 plays a role in mediating putative pheromone transduction. Furthermore, we demonstrate that cyclic nucleotide gated channel A2 knockout (CNGA2-KO) mice that show substantially decreased or absent responses to odors and pheromones also have elevated levels of TRPM5 compared to wild type mice. Taken together, our evidence suggests that TRPM5 plays a role in mediating transduction for putative pheromones under conditions of reduced chemosensory input.

Peripheral modulation of smell: fact or fiction?

Despite studies dating back 30 or more years showing modulation of odorant responses at the level of the olfactory epithelium, most descriptions of the olfactory system infer that odorant signals make their way from detection by cilia on olfactory sensory neurons to the olfactory bulb unaltered. Recent identification of multiple subtypes of microvillar cells and identification of neuropeptide and neurotransmitter expression in the olfactory mucosa add to the growing body of literature for peripheral modulation in the sense of smell. Complex mechanisms including perireceptor events, modulation of sniff rates, and changes in the properties of sensory neurons match the sensitivity of olfactory sensory neurons to the external odorant environment, internal nutritional status, reproductive status, and levels of arousal or stress. By furthering our understanding of the players mediating peripheral olfaction, we may open the door to novel approaches for modulating the sense of smell in both health and disease.

A conditioned aversion study of sucrose and SC45647 taste in TRPM5 knockout mice

Previously, published studies have reported mixed results regarding the role of the TRPM5 cation channel in signaling sweet taste by taste sensory cells. Some studies have reported a complete loss of sweet taste preference in TRPM5 knockout (KO) mice, whereas others have reported only a partial loss of sweet taste preference. This study reports the results of conditioned aversion studies designed to motivate wild-type (WT) and KO mice to respond to sweet substances. In conditioned taste aversion experiments, WT mice showed nearly complete LiCl-induced response suppression to sucrose and SC45647. In contrast, TRPM5 KO mice showed a much smaller conditioned aversion to either sweet substance, suggesting a compromised, but not absent, ability to detect sweet taste. A subsequent conditioned flavor aversion experiment was conducted to determine if TRPM5 KO mice were impaired in their ability to learn a conditioned aversion. In this experiment, KO and WT mice were conditioned to a mixture of SC45647 and amyl acetate (an odor cue). Although WT mice avoided both components of the stimulus mixture, they avoided SC45647 more than the odor cue. The KO mice also avoided both stimuli, but they avoided the odor component more than SC45647, suggesting that while the KO mice are capable of learning an aversion, to them the odor cue was more salient than the taste cue. Collectively, these findings suggest the TRPM5 KO mice have some residual ability to detect SC45647 and sucrose, and, like bitter, there may be a TRPM5-independent transduction pathway for detecting these substances.

Gastric tuft cells express DCLK1 and are expanded in hyperplasia

Epithelial tuft cells are named after their characteristic microtubule bundles located at the cell apex where these are exposed to the luminal environment. As such, tuft cells are found in multiple organs, including the gastrointestinal (GI) tract where the apical "tuft" is hypothesized to detect and transmit environmental signals. Thus, the goal of our study was to characterize gastric tuft cells during GI tract development, then subsequently in the normal and metaplastic adult stomach. GI tracts from mouse embryos, and newborn and postnatal mice were analyzed. Tuft cells were identified by immunohistochemistry using acetylated-α-tubulin (acTub) antibody to detect the microtubule bundle. Additional tuft cell markers, e.g., doublecortin-like kinase 1 (DCLK1), were used to co-localize with acTub. Tuft cells were quantified in human gastric tissue arrays and in mouse stomachs with or without inflammation. In the developing intestine, tuft cells in both the crypts and villi expressed all markers by E18.5. In the stomach, acTub co-localized with DCLK1 and other established tuft cell markers by E18.5 in the antrum, but not until postnatal day 7 in the corpus, with the highest density of tuft cells clustered at the forestomach ridge. Tuft cell numbers increased in hyperplastic human and mouse stomachs. In the adult GI tract, the tuft cell marker acTub co-expressed with DCKL1 and chemosensory markers, e.g.,TRPM5. In summary, tuft cells appear in the gastric antrum and intestine at E18.5, but their maximal numbers in the corpus are not achieved until after weaning. Tuft cell numbers increase with inflammation, hyperplasia, and metaplasia.

Cholinergic microvillous cells in the mouse main olfactory epithelium and effect of acetylcholine on olfactory sensory neurons and supporting cells

The mammalian olfactory epithelium is made up of ciliated olfactory sensory neurons (OSNs), supporting cells, basal cells, and microvillous cells. Previously, we reported that a population of nonneuronal microvillous cells expresses transient receptor potential channel M5 (TRPM5). Using transgenic mice and immunocytochemical labeling, we identify that these cells are cholinergic, expressing the signature markers of choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter. This result suggests that acetylcholine (ACh) can be synthesized and released locally to modulate activities of neighboring supporting cells and OSNs. In Ca(2+) imaging experiments, ACh induced increases in intracellular Ca(2+) levels in 78% of isolated supporting cells tested in a concentration-dependent manner. Atropine, a muscarinic ACh receptor (mAChR) antagonist suppressed the ACh responses. In contrast, ACh did not induce or potentiate Ca(2+) increases in OSNs. Instead ACh suppressed the Ca(2+) increases induced by the adenylyl cyclase activator forskolin in some OSNs. Supporting these results, we found differential expression of mAChR subtypes in supporting cells and OSNs using subtype-specific antibodies against M(1) through M(5) mAChRs. Furthermore, we found that various chemicals, bacterial lysate, and cold saline induced Ca(2+) increases in TRPM5/ChAT-expressing microvillous cells. Taken together, our data suggest that TRPM5/ChAT-expressing microvillous cells react to certain chemical or thermal stimuli and release ACh to modulate activities of neighboring supporting cells and OSNs via mAChRs. Our studies reveal an intrinsic and potentially potent mechanism linking external stimulation to cholinergic modulation of activities in the olfactory epithelium.

Expression of taste receptors in solitary chemosensory cells of rodent airways

Background

Chemical irritation of airway mucosa elicits a variety of reflex responses such as coughing, apnea, and laryngeal closure. Inhaled irritants can activate either chemosensitive free nerve endings, laryngeal taste buds or solitary chemosensory cells (SCCs). The SCC population lies in the nasal respiratory epithelium, vomeronasal organ, and larynx, as well as deeper in the airway. The objective of this study is to map the distribution of SCCs within the airways and to determine the elements of the chemosensory transduction cascade expressed in these SCCs.

Methods

We utilized a combination of immunohistochemistry and molecular techniques (rtPCR and in situ hybridization) on rats and transgenic mice where the Tas1R3 or TRPM5 promoter drives expression of green fluorescent protein (GFP).

Results

Epithelial SCCs specialized for chemoreception are distributed throughout much of the respiratory tree of rodents. These cells express elements of the taste transduction cascade, including Tas1R and Tas2R receptor molecules, α-gustducin, PLCβ2 and TrpM5. The Tas2R bitter taste receptors are present throughout the entire respiratory tract. In contrast, the Tas1R sweet/umami taste receptors are expressed by numerous SCCs in the nasal cavity, but decrease in prevalence in the trachea, and are absent in the lower airways.

Conclusions

Elements of the taste transduction cascade including taste receptors are expressed by SCCs distributed throughout the airways. In the nasal cavity, SCCs, expressing Tas1R and Tas2R taste receptors, mediate detection of irritants and foreign substances which trigger trigeminally-mediated protective airway reflexes. Lower in the respiratory tract, similar chemosensory cells are not related to the trigeminal nerve but may still trigger local epithelial responses to irritants. In total, SCCs should be considered chemoreceptor cells that help in preventing damage to the respiratory tract caused by inhaled irritants and pathogens.

Microvillous cells expressing IP3 receptor type 3 in the olfactory epithelium of mice

Microvillous cells of the main olfactory epithelium have been described variously as primary olfactory neurons, secondary chemosensory cells or non-sensory cells. Here we generated an IP3R3(tm1(tauGFP)) mouse in which the coding region for a fusion protein of tau and green fluorescent protein replaces the first exon of the Itpr3 gene. We provide immunohistochemical and functional characterization of the cells expressing IP3 receptor type 3 in the olfactory epithelium. These cells bear microvilli at their apex, and we therefore termed them IP3R3 MV cells. The cell body of these IP3R3 MV cells lies in the upper third of the main olfactory epithelium; a long thick basal process projects towards the base of the epithelium without penetrating the basal lamina. Retrograde labeling and unilateral bulbectomy corroborated that these IP3R3 MV cells do not extend axons to the olfactory bulb and therefore are not olfactory sensory neurons. The immunohistochemical features of IP3R3 MV cells varied, suggesting either developmental stages or the existence of subsets of these cells. Thus, for example, subsets of the IP3R3 MV cells make contact with substance P fibers or express the purinergic receptor P2X3. In addition, in recordings of intracellular calcium, these cells respond to ATP and substance P as well as to a variety of odors. The characterization of IP3R3 MV cells as non-neuronal chemoresponsive cells helps to explain the differing descriptions of microvillous cells in the literature.

A TRP channel contributes to insulin secretion by pancreatic β-cells

The release of insulin by pancreatic beta cells involves a complex interplay of conductances that generate oscillations and drive secretion. A recent report identifies a new player in this process, the ion channel TRPM5. TRPM5 was originally identified in taste cells, where it forms a Ca(2+)-activated cation channel that is required for sensory responses to bitter and sweet tastes. New research now shows that TRPM5 is expressed within the pancreatic islets of Langerhans, where it regulates the frequency of Ca(2+) oscillations and contributes to insulin release by β-cells.

Chemoreception regulates chemical access to mouse vomeronasal organ: role of solitary chemosensory cells

Controlling stimulus access to sensory organs allows animals to optimize sensory reception and prevent damage. The vomeronasal organ (VNO) detects pheromones and other semiochemicals to regulate innate social and sexual behaviors. This semiochemical detection generally requires the VNO to draw in chemical fluids, such as bodily secretions, which are complex in composition and can be contaminated. Little is known about whether and how chemical constituents are monitored to regulate the fluid access to the VNO. Using transgenic mice and immunolabeling, we found that solitary chemosensory cells (SCCs) reside densely at the entrance duct of the VNO. In this region, most of the intraepithelial trigeminal fibers innervate the SCCs, indicating that SCCs relay sensory information onto the trigeminal fibers. These SCCs express transient receptor potential channel M5 (TRPM5) and the phospholipase C (PLC) beta2 signaling pathway. Additionally, the SCCs express choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) for synthesizing and packaging acetylcholine, a potential transmitter. In intracellular Ca2+ imaging, the SCCs responded to various chemical stimuli including high concentrations of odorants and bitter compounds. The responses were suppressed significantly by a PLC inhibitor, suggesting involvement of the PLC pathway. Further, we developed a quantitative dye assay to show that the amount of stimulus fluid that entered the VNOs of behaving mice is inversely correlated to the concentration of odorous and bitter substances in the fluid. Genetic knockout and pharmacological inhibition of TRPM5 resulted in larger amounts of bitter compounds entering the VNOs. Our data uncovered that chemoreception of fluid constituents regulates chemical access to the VNO and plays an important role in limiting the access of non-specific irritating and harmful substances. Our results also provide new insight into the emerging role of SCCs in chemoreception and regulation of physiological actions.