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International audience; The pathogenic bacterium Streptococcus pneumoniae (S. pneumoniae) can invade the cerebrospinal fluid (CSF) and cause meningitis with devastating consequences. Whether and how sensory cells in the central nervous system (CNS) become activated during bacterial infection, as recently reported for the peripheral nervous system, is not known. We find that CSF infection by S. pneumoniae in larval zebrafish leads to changes in posture and behavior that are reminiscent of pneumococcal meningitis, including dorsal arching and epileptic-like seizures. We show that during infection, invasion of the CSF by S. pneumoniae massively activates in vivo sensory neurons contacting the CSF, referred to as “CSF-cNs” and previously shown to detect spinal curvature and to control posture, locomotion, and spine morphogenesis. We find that CSF-cNs express orphan bitter taste receptors and respond in vitro to bacterial supernatant and metabolites via massive calcium transients, similar to the ones observed in vivo during infection. Upon infection, CSF-cNs also upregulate the expression of numerous cytokines and complement components involved in innate immunity. Accordingly, we demonstrate, using cell-specific ablation and blockade of neurotransmission, that CSF-cN neurosecretion enhances survival of the host during S. pneumoniae infection. Finally, we show that CSF-cNs respond to various pathogenic bacteria causing meningitis in humans, as well as to the supernatant of cells infected by a neurotropic virus. Altogether, our work uncovers that central sensory neurons in the spinal cord, previously involved in postural control and morphogenesis, contribute as well to host survival by responding to the invasion of the CSF by pathogenic bacteria during meningitis.

Chemoreception plays a central role in the survival and reproduction of many animals in aquatic environments. Fishes use three chemosensory systems, the olfactory system, the diffuse chemosensory system, and the taste (gustatory) system. While these are present in the sea lamprey, a majority of chemosensory research has focused on olfaction. This review summarizes current knowledge of the lamprey chemosensory systems, including stimuli, receptors, neural pathways and behavioural roles of the chemosensory stimuli. A description of the olfactory sensory neurons located deep within the nostril, of synaptic pathways within the brain and of modulatory mechanisms that underly the translation of olfaction to movement are included. The lamprey diffuse chemosensory system may enable solitary chemosensory cells on the skin of the mouth, nostril, gill, and tailfin to function as sentinels for rapid responses to chemical stimuli. While little is known abouth the lamprey taste system, taste buds are located within the pharynx, thus enabling chemosensory responses to material that has entered the body cavity. Knowledge and understanding of function of these three chemosensory systems supports the management of sea lamprey populations.

The sense of taste is associated with the evaluation of food and other environmental parameters such as salinity. In aquatic mammals, anatomic and behavioral evidence of the use of taste varies by species and genomic analysis of taste receptors indicates an overall reduction and, in some cases, complete loss of intact bitter and sweet taste receptors. However, the receptors used by taste buds in the oral cavity are found on cells in other areas of the body and play an important role in immune responses. In the respiratory tract, an example of such cells is solitary chemosensory cells (SCCs) which have bitter and sweet taste receptors. The bitter receptors detect chemicals given off by pathogens and initiate an innate immune response. Although many aquatic mammals may not have a role for taste in the assessment of food, they likely would benefit from the added protection that SCCs provide, especially considering respiratory diseases are a problem for many aquatic mammals. While evidence indicates that some species do not possess functional bitter receptors for taste, many do have intact bitter receptor genes and it is important for researchers to be aware of all roles for these receptors in homeostasis. Through a better understanding of the anatomy and physiology of aquatic mammal's respiratory systems, better treatment and management is possible.

Solitary chemosensory cells and chemosensory cell clusters are distributed in the pharynx and larynx. In the present study, the morphology and reflexogenic function of solitary chemosensory cells and chemosensory cell clusters in the nasal cavity and pharynx were examined using immunofluorescence for GNAT3 and electrophysiology. In the nasal cavity, GNAT3-immunoreactive solitary chemosensory cells were widely distributed in the nasal mucosa, particularly in the cranial region near the nostrils. Solitary chemosensory cells were also observed in the nasopharynx. Solitary chemosensory cells in the nasopharyngeal cavity were barrel like or slender in shape with long lateral processes within the epithelial layer to attach surrounding ciliated epithelial cells. Chemosensory cell clusters containing GNAT3-immunoreactive cells were also detected in the pharynx. GNAT3-immunoreactive cells gathered with SNAP25-immunoreactive cells in chemosensory clusters. GNAT3-immunoreactive chemosensory cells were in close contact with a few SP- or CGRP-immunoreactive nerve endings. In the pharynx, GNAT3-immunoreactive chemosensory cells were also attached to P2X3-immunoreactive nerve endings. Physiologically, the perfusion of 10 mM quinine hydrochloride (QHCl) solution induced ventilatory depression. The QHCl-induced reflex was diminished by bilateral section of the glossopharyngeal nerve, suggesting autonomic reflex were evoked by chemosensory cells in pharynx but not in nasal mucosa. The present results indicate that complex shape of nasopharyngeal solitary chemosensory cells may contribute to intercellular communication, and pharyngeal chemosensory cells may play a role in respiratory depression.

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.

Background Solitary chemosensory cells (SCCs) in the murine nasal epithelium are discrete specialized cells that respond to irritants and activate trigeminal nerve fibers through the release of acetylcholine (ACh), resulting in local neurogenic inflammation. In addition to releasing ACh, SCCs are the exclusive epithelial source of interleukin (IL)-25. In humans, SCCs are significantly expanded in sinonasal polyps (NPs). However, the SCC-trigeminal synapse has yet to be demonstrated in human sinonasal epithelium. Methods Immunofluorescence for trigeminal nerve fiber markers, nicotinic ACh receptors (nChR), and SCC markers was performed in vibratome sections from polyp and healthy turbinate tissue. Quantitative polymerase chain reaction and immunofluorescence of cultured epithelial cells were used to evaluate the expansion of SCCs. Last, intracellular calcium imaging was used to demonstrate cholinergic signaling in sinonasal epithelial cells. Results Calcitonin gene-related peptide (CGRP) immunostaining was used to identify cholinergic nerve endings, which were only evident in sections from the inferior turbinate and intertwined with SCCs (α-gustducin-positive cells). CGRP-positive nerve endings were not identified in sections from NPs. Human SCCs expressed nChR as well as the ACh synthetic enzyme choline acetyltransferase. Live cell calcium imaging demonstrated functionally active cholinergic signaling in discrete sinonasal epithelial cells, consistent with SCCs. Finally, SCC-specific genes were dramatically upregulated with pretreatment with IL-13 and nicotinic agonists. Conclusion SCCs are innervated by trigeminal nerve endings in healthy turbinate tissue but not in NPs. SCCs express ACh receptors as well as choline acetyltransferase and, in the setting of a type 2 inflammatory environment, denervated SCCs dramatically expand with nicotinic stimulation.

Chemosensory systems (olfaction, taste, trigeminus nerve, solitary chemoreceptor cells, neuroendocrine pulmonary cells, and carotid body, etc.) detect molecules outside or inside our body and may share common molecular markers. In addition to the impairment of taste and olfaction, the detection of the internal chemical environment may also be incapacitated by COVID-19. If this is the case, different consequences can be expected. (1) In some patients, hypoxia does not trigger distressing dyspnea (“silent” hypoxia): Long-term follow-up may determine whether silent hypoxia is related to malfunctioning of carotid body chemoreceptors. Moreover, taste/olfaction and oxygen chemoreceptors may be hit simultaneously: Testing olfaction, taste, and oxygen chemoreceptor functions in the early stages of COVID-19 allows one to unravel their connections and trace the recovery path. (2) Solitary chemosensory cells are also involved in the regulation of the innate mucosal immune response: If these cells are affected in some COVID-19 patients, the mucosal innate immune response would be dysregulated, opening one up to massive infection, thus explaining why COVID-19 has lethal consequences in some patients. Similar to taste and olfaction, oxygen chemosensory function can be easily tested with a non-invasive procedure in humans, while functional tests for solitary chemosensory or pulmonary neuroendocrine cells are not available, and autoptic investigation is required to ascertain their involvement.

Patients infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019, suffer from respiratory and non-respiratory symptoms. Among these symptoms, the loss of smell has attracted considerable attention. The objectives of this study were to determine which cells are infected, what happens in the olfactory system after viral infection, and how these pathologic changes contribute to olfactory loss. For this purpose, Syrian golden hamsters were used. First, we verified the olfactory structures in the nasal cavity of Syrian golden hamsters, namely the main olfactory epithelium, the vomeronasal organ, and their cellular components. Second, we found angiotensin-converting enzyme 2 expression, a receptor protein of SARS-CoV-2, in both structures and infections of supporting, microvillar, and solitary chemosensory cells. Third, we observed pathological changes in the infected epithelium, including reduced thickness of the mucus layer, detached epithelia, indistinct layers of epithelia, infiltration of inflammatory cells, and apoptotic cells in the overall layers. We concluded that a structurally and functionally altered microenvironment influences olfactory function. We observed the regeneration of the damaged epithelium, and found multilayers of basal cells, indicating that they were activated and proliferating to reconstitute the injured epithelium.

BACKGROUND: Severe acute respiratory syndrome-associated coronavirus-2 (SARS-CoV-2) has been identified as the pathogen causing the outbreak of coronavirus disease-2019 (COVID-19) commencing in Wuhan, China, in December 2019. Multiple reports have shown subjective loss of taste and smell as an early and hallmark symptom for COVID-19. METHODS: A retrospective study was performed in our clinical practice during July 2020 on patients positive for SARS-CoV-2 via polymerase chain reaction. All patients were categorized into 3 groups (supertasters, tasters, and nontasters) via taste sensitivity to phenylthiocarbamide, thiourea, and sodium benzoate with taste strip testing. The results of the taste strip tests were correlated with clinical course. RESULTS: A total of 100 patients (mean, 51 [range, 24-82] years of age; 44 [44%] women) were assessed. We found that 21 of 100 (21%) were nontasters, 79 of 100 (79%) were tasters, and 0 of 100 (0%) were supertasters (p < 0.001). Twenty-one of 21 (100%) (p < 0.001) of the patients requiring inpatient admission were classified as nontasters. All 79 (100%) (p < 0.001) of the patients who displayed mild to moderate symptoms not requiring admission were classified as tasters. CONCLUSION: Our results show objective data that taste disturbance, specifically global loss of taste, appears to correlate with the clinical course specific to each individual, because 100% of the patients requiring inpatient admission were classified as nontasters.

Molecules present in an animal's environment can indicate the presence of predators, food, or sexual partners and consequently, induce migratory, reproductive, foraging, or escape behaviors. Three sensory systems, the olfactory, gustatory, and solitary chemosensory cell (SCC) systems detect chemical stimuli in vertebrates. While a great deal of research has focused on the olfactory and gustatory system over the years, it is only recently that significant attention has been devoted to the SCC system. The SCCs are microvillous cells that were first discovered on the skin of fish, and later in amphibians, reptiles, and mammals. Lampreys also possess SCCs that are particularly numerous on cutaneous papillae. However, little is known regarding their precise distribution, innervation, and function. Here, we show that sea lampreys (Petromyzon marinus L.) have cutaneous papillae located around the oral disk, nostril, gill pores, and on the dorsal fins and that SCCs are particularly numerous on these papillae. Tract-tracing experiments demonstrated that the oral and nasal papillae are innervated by the trigeminal nerve, the gill pore papillae are innervated by branchial nerves, and the dorsal fin papillae are innervated by spinal nerves. We also characterized the response profile of gill pore papillae to some chemicals and showed that trout-derived chemicals, amino acids, and a bile acid produced potent responses. Together with a companion study (Suntres et al., Journal of Comparative Neurology, this issue), our results provide new insights on the function and evolution of the SCC system in vertebrates.

Solitary chemosensory cells (SCCs) are epithelial sentinels that utilize bitter Tas2r receptors and coupled taste transduction elements to detect pathogenic bacterial metabolites, triggering host defenses to control the infection. Here we report that SCCs are present in mouse gingival junctional epithelium, where they express several Tas2rs and the taste signaling components α-gustducin (Gnat3), TrpM5, and Plcβ2. Gnat3−/− mice have altered commensal oral microbiota and accelerated naturally occurring alveolar bone loss. In ligature-induced periodontitis, knockout of taste signaling molecules or genetic absence of gingival SCCs (gSCCs) increases the bacterial load, reduces bacterial diversity, and renders the microbiota more pathogenic, leading to greater alveolar bone loss. Topical treatment with bitter denatonium to activate gSCCs upregulates the expression of antimicrobial peptides and ameliorates ligature-induced periodontitis in wild-type but not in Gnat3−/− mice. We conclude that gSCCs may provide a promising target for treating periodontitis by harnessing innate immunity to regulate the oral microbiome., Solitary chemosensory cells (SCCs) expressing taste receptors reside in mucosal epithelia including the gingiva. Here the authors show that ablation of taste-sensing by SSCs leads to outgrowth of pathogenic oral bacteria and periodontitis, whereas bitter taste receptor stimulation promotes antimicrobial peptide production and ameliorates periodontitis.

Tuft cells—rare solitary chemosensory cells in mucosal epithelia—are undergoing intense scientific scrutiny fueled by recent discovery of unsuspected connections to type 2 immunity. These cells constitute a conduit by which ligands from the external space are sensed via taste-like signaling pathways to generate outputs unique among epithelial cells: the cytokine IL-25, eicosanoids associated with allergic immunity, and the neurotransmitter acetylcholine. The classic type II taste cell transcription factor POU2F3 is lineage defining, suggesting a conceptualization of these cells as widely distributed environmental sensors with effector functions interfacing type 2 immunity and neural circuits. Increasingly refined single-cell analytics have revealed diversity among tuft cells that extends from nasal epithelia and type II taste cells to ex- Aire-expressing medullary thymic cells and small-intestine cells that mediate tissue remodeling in response to colonizing helminths and protists.

Solitary chemosensory epithelial cells are scattered in most mucosal surfaces. They are referred to as tuft cells in the intestinal mucosa, brush cells in the trachea, and solitary chemosensory and microvillous cells in the nasal mucosa. They are the primary source of IL-25 in the epithelium and are also engaged in acetylcholine generation. We recently demonstrated that nasal solitary chemosensory (brush) cells can generate robust levels of cysteinyl leukotrienes in response to stimulation with calcium ionophore, aeroallergens, and danger-associated molecules, such as ATP and UTP, and this mechanism depends on brush cell expression of the purinergic receptor P2Y2. This protocol describes an effective method of nasal brush cell isolation in the mouse. The method is based on physical separation of the mucosal layer of the nasal cavity and pre-incubation with dispase, followed by digestion with papain solution. The single cell suspension obtained this way contains a high yield of brush cells for fluorescence-activated cell sorting (FACS), RNA-sequencing, and ex vivo assays. Graphic abstract: [Image: see text] Workflow of nasal digestion for brush cell isolation.

BACKGROUND Chronic rhinosinusitis (CRS) with nasal polyps (CRSwNP) and CRS without nasal polyps (CRSsNP), and aspirin-exacerbated respiratory disease (AERD) have varying levels of inflammation and disease severity. Solitary chemosensory cells (SCCs) are enriched in nasal polyps, are the primary source of interleukin 25 (IL-25) in upper airways, leading to type 2 inflammation, and are activated by bitter-tasting denatonium benzoate (DB). Thus, we sought to evaluate DB taste perception at a range of concentrations in order to identify 1 that most differentiates CRS subgroups from controls. METHODS CRSsNP (n = 25), CRSwNP (n = 26), and AERD (n = 27) patients as well as controls (n = 25) tasted 6 DB concentrations in a fixed, random order, rating on a category scale of 0 (no intensity) to 12 (extremely intense). Sinonasal epithelial cultures were treated with and without denatonium and analyzed for IL-25 via flow cytometry. RESULTS CRSsNP patients rated DB as significantly less intense than did controls at all concentrations: 5.62 × 10-9 M, 1.00 × 10-8 M, 1.78 × 10-8 M, 3.16 × 10-8 M, 5.62 × 10-8 M, and 1.00 × 10-7 M (all p < 0.0083). CRSwNP patients did not show significant differences from controls. AERD patients rated DB as significantly more intense than did controls at concentrations of 1.00 × 10-8 M and 3.16 × 10-8 M (p < 0.0083). In vitro data demonstrated significant increase in IL-25-positive cells after denatonium stimulation (n = 5), compared to control (n = 5) (p = 0.012). CONCLUSION Our findings link in vitro DB stimulation of sinonasal tissue with increased IL-25 and show differential DB taste perception in CRS subgroups relative to the control group, with CRSsNP being hyposensitive and AERD being hypersensitive. We propose a concentration of 3.16 × 10-8 M for future study of clinical utility.

Tuft cells, rare solitary chemosensory cells, are distributed in mucosal epithelium throughout mammalian organs. Their nomenclatures are various in different organs and may be confused with other similar cells. Current studies mainly focus on their chemosensory ability and immune functions in type 2 inflammation. Several state-of-the-art reviews have already systematically discussed their role in immune responses. However, given that tuft cells are one of the crucial components of non-neuronal cholinergic system, the functions of tuft cell derived acetylcholine (ACh) and the underlying mechanisms remain intricate. Existing evidence demonstrated that tuft cell derived ACh participates in maintaining epithelial homeostasis, modulating airway remodeling, regulating reflexes, promoting muscle constriction, inducing neurogenic inflammation, initiating carcinogenesis and producing ATP. In this review, the ACh biosynthesis pathways and potential clinical applications of tuft cells have been proposed. More importantly, the main pathophysiological roles and the underlying mechanisms of tuft cell derived ACh are summarized and discussed.

Cholinergic chemosensory cells (CCC) are infrequent epithelial cells with immunosensor function, positioned in mucosal epithelia preferentially near body entry sites in mammals including man. Given their adaptive capacity in response to infection and their role in combatting pathogens, we here addressed the time points of their initial emergence as well as their postnatal development from first exposure to environmental microbiota (i.e., birth) to adulthood in urethra and trachea, utilizing choline acetyltransferase (ChAT)-eGFP reporter mice, mice with genetic deletion of MyD88, toll-like receptor-2 (TLR2), TLR4, TLR2/TLR4, and germ-free mice. Appearance of CCC differs between the investigated organs. CCC of the trachea emerge during embryonic development at E18 and expand further after birth. Urethral CCC show gender diversity and appear first at P6-P10 in male and at P11-P20 in female mice. Urethrae and tracheae of MyD88- and TLR-deficient mice showed significantly fewer CCC in all four investigated deficient strains, with the effect being most prominent in the urethra. In germ-free mice, however, CCC numbers were not reduced, indicating that TLR2/4-MyD88 signaling, but not vita-PAMPs, governs CCC development. Collectively, our data show a marked postnatal expansion of CCC populations with distinct organ-specific features, including the relative impact of TLR2/4-MyD88 signaling. Strong dependency on this pathway (urethra) correlates with absence of CCC at birth and gender-specific initial development and expansion dynamics, whereas moderate dependency (trachea) coincides with presence of first CCC at E18 and sex-independent further development.

Tuft (or brush) cells are solitary chemosensory cells scattered throughout the epithelia of the respiratory and alimentary tract. The actin-binding protein villin (Vil1) is used as a marker of tuft cells and the villin promoter is frequently used to drive expression of the Cre recombinase in tuft cells. While there is widespread agreement about the expression of villin in tuft cells there are several disagreements related to tuft cell lineage commitment and function. We now show that many of these inconsistencies could be resolved by our surprising finding that intestinal tuft cells, in fact, do not express villin protein. Furthermore, we show that a related actin-binding protein, advillin which shares 75% homology with villin, has a tuft cell restricted expression in the gastrointestinal epithelium. Our study identifies advillin as a marker of tuft cells and provides a mechanism for driving gene expression in tuft cells but not in other epithelial cells of the gastrointestinal tract. Our findings fundamentally change the way we identify and study intestinal tuft cells.

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.

Bitter (T2R) and sweet (T1R) taste receptors have been implicated in sinonasal innate immunity and in the pathophysiology of chronic rhinosinusitis (CRS). Taste receptors are expressed on several sinonasal cell types including ciliated epithelial cells and solitary chemosensory cells. Bitter agonists released by pathogenic microbes elicit a T2R dependent signaling cascade which induces the release of bactericidal nitric oxide, increases mucociliary clearance, and promotes secretion of antimicrobial peptides. Genetic variation conferred by polymorphisms in T2R related genes is associated with differential CRS susceptibility, symptomatology and post-treatment outcomes. More recently, based on our understanding of T1R and T2R function, investigators have discovered novel potential therapeutics in T2R agonists and T1R antagonists. This review will discuss bitter and sweet taste receptor function in sinonasal immunity, explore the emerging diagnostic and therapeutic implications stemming from the most recent findings, and suggest directions for future research. Keywords: Chronic rhinosinusitis, Taste Receptor Family 1 (T1R), Taste Receptor Family 2 (T2R), Sweet taste receptors, Bitter taste receptors, Innate immunity, Solitary chemosensory cells, Polymorphism

Epidermal sensory structures of adults and juveniles of amphihaline migratory fish hilsa Tenualosa ilisha were studied from two habitats, i.e., freshwater (FW) and marine water (MW). Every year, adults and sexually mature hilsa migrate upstream from marine habitat to riverine freshwater habitat for breeding. This report provides evidences of chemoreception on their upstream migration through several characteristic features on their body, especially on the head and oral cavity. Scanning electron microscopy (SEM) reveals that freshwater adult hilsa (FH) has abundant solitary chemosensory cells (SCCs) on the snout epidermis (around the openings of the epidermal pit) and upper lip, whereas marine water adult hilsa (MH) moderately possesses such sensory structures. The juveniles returning to marine water completely lack SCCs. Immunohistochemical studies revealed the expression of PLC β2 on the snout of FH and tongue of both FH and MH. Further analysis (immunofluorescence, immunoblot and densitometry) of the epidermis confirms the presence of chemosensory structures through strong expression and localization of G-proteins (Gαq and Gα s/olf) from the snout as well as tongue in freshwater hilsa. The SEM also confirms the presence of two types of taste buds in FH, viz. type I (TB I) and type III (TB III). Whereas TB I and TB III are observed on the upper palatine and lips, most of the TB III are located on the tongue region of freshwater and marine hilsa. The juvenile hilsa are devoid of such structures. The presence of dense and rich SCCs and taste sensory cells in adults could be a characteristic feature for strong sensory reception to recognize odour and food-related environmental cues from habitats where they often migrate.

Taste receptors, first identified on the tongue, are best known for their role in guiding our dietary preferences. The expression of taste receptors for umami, sweet, and bitter have been demonstrated in tissues outside of the oral cavity, including in the airway, brain, gastrointestinal tract, and reproductive organs. The extra-oral taste receptor chemosensory pathways and the endogenous taste receptor ligands are generally unknown, but there is increasing data suggesting that taste receptors are involved in regulating some aspects of innate immunity, and may potentially control the composition of the nasal microbiome in healthy individuals or patients with upper respiratory diseases like chronic rhinosinusitis (CRS). For this reason, taste receptors may serve as potential therapeutic targets, providing alternatives to conventional antibiotics. This review focuses on the physiology of sweet (T1R) and bitter (T2R) taste receptors in the airway and their activation by secreted bacterial products. There is particular focus on T2R38 in sinonasal ciliated cells, as well as the sweet and bitter receptors found on specialized sinonasal solitary chemosensory cells. Additionally, this review explores the impact of genetic variations in these receptors on the differential susceptibility of patients to upper airway infections, such as CRS.

Tracheal brush cells are cholinergic chemosensory epithelial cells poised to transmit signals from the airway lumen to the immune and nervous systems. They are part of a family of chemosensory epithelial cells which include tuft cells in the intestinal mucosa, brush cells in the trachea, and solitary chemosensory and microvillous cells in the nasal mucosa. Chemosensory cells in different epithelial compartments share key intracellular markers and a core transcriptional signature, but also display significant transcriptional heterogeneity, likely reflective of the local tissue environment. Isolation of tracheal brush cells from single cell suspensions is required to define the function of these rare epithelial cells in detail, but their isolation is challenging, potentially due to the close interaction between tracheal brush cells and nerve endings or due to airway-specific composition of tight and adherens junctions. Here, we describe a procedure for isolation of brush cells from mouse tracheal epithelium. The method is based on an initial separation of tracheal epithelium from the submucosa, allowing for a subsequent shorter incubation of the epithelial sheet with papain. This procedure offers a rapid and convenient solution for flow cytometric sorting and functional analysis of viable tracheal brush cells.

H1N1 influenza virus infection induces dramatic and permanent alveolar remodeling mediated by p63+ progenitor cell expansion in both mice and some patients with acute respiratory distress syndrome. This persistent lung epithelial dysplasia is accompanied by chronic inflammation, but the driver(s) of this pathology are unknown. This work identified de novo appearance of solitary chemosensory cells (SCCs), as defined by the tuft cell marker doublecortin-like kinase 1, in post-influenza lungs, arising in close proximity with the dysplastic epithelium, whereas uninjured lungs are devoid of SCCs. Interestingly, fate mapping demonstrated that these cells are derived from p63-expressing lineage-negative progenitors, the same cell of origin as the dysplastic epithelium. Direct activation of SCCs with denatonium + succinate increased plasma extravasation specifically in post-influenza virus-injured lungs. Thus we demonstrate the previously unrecognized development and activity of SCCs in the lung following influenza virus infection, implicating SCCs as a central feature of dysplastic remodeling.

Solitary chemosensory cells (SCCs) and their innervating fibers are located in the respiratory system of many vertebrates, including papillae on lamprey gill pores. In order to gain stronger insight for the role of these chemosensory cells, we examined immunocytochemical and innervation characteristics, as well as abundance at the different stages of the lamprey life cycle. The SCCs were distinguished from the surrounding epithelial cells by calretinin and phospholipase C140 immunoreactivity. Nerve fibers extended into the gill pore papillae, as far as the SCCs and serotonergic fibers extended from the underlying dermis into the papillar base. Gill pore papillae were absent and SCCs were sparse during the larval stage and in newly transformed lamprey. Few SCCs were located on small nub-like papillae during the parasitic juvenile stage, but SCCs were abundant on prominent papillae in migrating and in spawning adults. These findings show similarities between the SCCs in lampreys and other vertebrates and suggest that gill SCC function may be important during the feeding juvenile and the adult stages of the lamprey life cycle.

Taste receptors in the oral cavity guide our preferences for foods, preventing toxic ingestions and encouraging proper nutrient consumption. More recently, expression of taste receptors has been demonstrated in other locations throughout the body, including the airway, gastrointestinal tract, pancreas, and brain. The extent and specific roles of extraoral taste receptors are largely unknown, but a growing body of evidence suggests that taste receptors in the airway serve a critical role in sensing bacteria and regulating innate immunity. This review will focus on the function of bitter and sweet taste receptors in the human airway, with particular emphasis on T2R38, a bitter taste receptor found in sinonasal ciliated cells, and the bitter and sweet receptors found on specialized sinonasal solitary chemosensory cells. The importance of these novel taste receptor‐immune circuits in the human airway and their clinical relevance in airway disease will also be reviewed.

Tuft cells help contain parasites Trillions of microbes inhabit our guts, including worms and other parasites. Epithelial cells that line the gut orchestrate parasite-targeted immune responses. Howitt et al. now identify a key cellular player in immunity to parasites: tuft cells (see the Perspective by Harris). Tuft cells make up a small fraction of gut epithelial cells but expand when parasites colonize or infect the gut. Parasites cause tuft cells to secrete large amounts of interleukin-25, a key cytokine for parasite clearance that also indirectly feeds back on tuft cells to expand their numbers. Tuft cells express chemosensory signaling machinery: disrupting this blocked parasite-triggered tuft cell expansion and weakened the ability of mice to control a parasitic infection. Science , this issue p. 1329 ; see also p. 1264

BACKGROUND Solitary chemosensory cells (SCCs) are rare epithelial cells enriched in nasal polyps and are the primary source of interleukin-25 (IL-25), an innate cytokine eliciting T-helper 2 (Th2) immune response. Although it is proposed that SCCs are stimulated by antigens released by upper airway pathogens, the exogenous triggers of human SCCs remain elusive. We studied patients with noninvasive fungal rhinosinusitis to determine whether extracts of Aspergillus fumigatus and Alternaria alternata stimulate SCC proliferation as an early event in type 2 inflammation. METHODS Multicolor flow cytometry, immunofluorescence, and enzyme-linked immunoassay were used to interrogate mucosa from patients with mycetomas and allergic fungal rhinosinusitis (AFRS) for SCCs and IL-25. Primary sinonasal epithelial cells from AFRS patients and noninflamed inferior turbinates were stimulated with fungal extracts for 72 hours, and SCC population frequency as well as mitotic activity were quantified using flow cytometry. RESULTS SCCs producing IL-25 are enriched in inflamed mucosa compared with intrapatient noninflamed control tissue (38.6% vs 6.5%, p = 0.029). In cultured sinonasal epithelial cells from AFRS nasal polyps, Aspergillus fumigatus and Alternaria alternata stimulated higher SCC frequency compared with controls (27.4% vs 10.6%, p = 0.002; 18.1% vs 10.6%, p = 0.046), which led to increased IL-25 secretion in culture media (75.5 vs 3.3 pg/mL, p < 0.001; 32.3 vs 3.3 pg/mL, p = 0.007). Ki-67 expression was higher in SCCs grown in fungal stimulation conditions compared with controls. CONCLUSION Although fungal antigens are known to potentiate immune response through innate cytokines, including IL-25, the early expansion of SCCs in the presence of fungus has not been described. This early event in the pathogenesis of noninvasive fungal rhinosinusitis may represent a target for intervention.

BACKGROUND: IL-25 can function as an early signal for the respiratory Type 2 response characteristic of allergic asthma and chronic rhinosinusitis with nasal polyps (CRSwNP). In the mouse gut, tuft cells are the epithelial source of IL-25. However, the source of human airway epithelial IL-25 has remained elusive. OBJECTIVE: In this study, we sought to determine if the solitary chemosensory cell (SCC) is the predominant source of IL-25 in the sinonasal epithelium. METHOD: Flow cytometry and immunofluorescence for SCCs and IL-25 were used to interrogate polyp and turbinate tissue from patients with chronic rhinosinusitis with nasal polyps (CRSwNP). Mucus was collected during acute inflammatory exacerbations from patients with CRSwNP or CRS without nasal polyps and IL-25 levels determined by Enzyme Linked Immuno-Sorbent Assay (ELISA). Lastly, sinonasal epithelial cultures derived from polyp and turbinate tissue were stimulated with IL-13 and analyzed for SCC proliferation and IL-25 production. RESULTS: This study demonstrates that a discrete cell type, likely a solitary chemosensory cell (SCC), characterized by expression of the taste-associated G-protein, gustducin, and the intestinal tuft cell marker, doublecortin-like kinase 1, DCAMKL1, is the predominant source of IL-25 in the human upper airway. Additionally, we show that patients with CRSwNP have increased numbers of SCCs in nasal polyp tissue and in-vitro IL-13 exposure both increased proliferation and induced apical secretion of IL-25 into the mucus layer. CONCLUSIONS: Inflammatory sinus polyps but not adjacent turbinate tissue have an expansion of a solitary chemosensory cell population that is the source of epithelial IL-25. CLINICAL IMPLICATIONS: Nasal polyps have an expansion of a “taster” solitary chemosensory cell (SCC) population that apically secretes the pro-Type 2 cytokine IL-25. Over abundance of upper airway SCC’s together with prolonged epithelial barrier breakdown may be a driver towards Type 2 inflammation. Quantifying IL-25 from sinus mucus may be a useful biomarker for type-2 inflammatory status.

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.

In the present study, we used immunohistochemistry for α - gustducin and Growth Associated Protein-43 (GAP-43) to examine the spatial distribution of the solitary chemosensory cells primarily in the nasopalatine ducts of rat at the time of weaning, which is lack in the literature. Methods: We found abundant solitary cells labeled with α- gustducin in the nasopalatine duct and vomeronasal organ of rats. In the nasopalatine duct, these cells were more frequent in the medial wall epithelium; meanwhile appreciable number of α-gustducin labeled cells were localized only in the neuroepithelium portion of the vomeronasal organ. We found the number of these cells increased toward the entries of the nasopalatine and vomeronasal ducts into the nasal cavity. We also found GAP-43 heavily expressed in the core of nasopalatine duct, close to the basement membrane and around the blood vessels and cavernous spaces of the vomeronasal organ. Results: GAP-43 labeled axons apposed the solitary chemosensory cells closely, either coursing along or wrapping the solitary chemosensory cells. Individual cells were apposed by one or a few intraepithelial nerve fibers and a single fiber sometimes contacted a few solitary chemosensory cells. Intraepithelial GAP-43 labeled fibers were more frequent toward the nasal cavity and the entry of nasopalatine and vomeronasal ducts in close association with the solitary chemosensory cells. Conclusion: We conclude that α -gustducin-expressing cells alongside the GAP-43 intraepithelial nerves in the nasopalatine and vomeronasal ducts suggests that they share the same transduction mechanisms.