Cholinergic neurotransmission links solitary chemosensory cells to nasal inflammation

Channel synapse mediates neurotransmission of airway protective chemoreflexes

Neural reflexes to chemicals in the throat protect the airway from aspiration and infection. Mechanistic understanding of these reflexes remains premature, exemplified by chronic cough-a sensitized cough reflex-being a prevalent unmet clinical need. Here, in mice, a whole-body search for channel synapses-featuring CALHM1/3 channel-mediated neurotransmitter release-and single-cell transcriptomics uncovered subclasses of the Pou2f3+ chemosensory cell family in the throat communicating with vagal neurons via this synapse. They express G protein-coupled receptors (GPCRs) for noxious chemicals, T2Rs, which upon stimulation trigger swallow and cough-like expulsive reflexes in the hypopharynx and larynx, respectively. These reflexes were abolished by Calhm3 and Pou2f3 knockout and could be triggered by targeted optogenetic stimulation. Furthermore, aeroallergen exposure augmented CALHM3-dependent expulsive reflex. This study identifies Pou2f3+ epithelial cells with channel synapses as chemosensory end organs of airway protective reflexes and sites of their hyperresponsiveness, advancing mechanistic understanding of airway defense programs with distinct therapeutic potential.

The Impact of Montelukast Duration on the Risk of Neuropsychiatric Disorders in Children with Asthma: A Population-Based Cohort Study

Background/Objectives: Asthma is one of the most common chronic diseases in children, and montelukast is widely prescribed to manage symptoms. However, concerns have emerged regarding its potential association with neuropsychiatric disorders. This study aims to investigate the impact of montelukast duration on neuropsychiatric risks in children with asthma. Methods: A cohort study was conducted using Taiwan's National Health Insurance Research Database (NHIRD), including children diagnosed with asthma between 2004 and 2007. A total of 14,606 children in the montelukast cohort and 8432 in the non-montelukast cohort were analyzed, with propensity score matching applied to reduce confounding bias. Neuropsychiatric outcomes, including Tics/Tourette's syndrome, were evaluated using Cox proportional hazard models. Results: Overall, montelukast use did not increase the risk of neuropsychiatric disorders. However, among children aged 6-15 years, prolonged use beyond 63 days was associated with a significantly elevated risk of Tics/Tourette's syndrome, with a 2.6-fold increase observed in girls and a 1.8-fold increase in boys. Conversely, shorter montelukast use in children aged 0-6 years was linked to a lower risk of neuropsychiatric disorders. Conclusions: Although montelukast generally does not elevate neuropsychiatric risks, extended use in older children may increase the likelihood of developing Tics/Tourette's syndrome. These findings highlight the importance of cautious prescribing in pediatric asthma management. Further research is necessary to validate these associations and inform clinical decision making.

The roles of bitter and sweet taste receptors in food allergy: Where are we now?

Food allergy (FA) is a growing global concern, which contributes significantly to anaphylaxis and severe allergic reactions. Despite advancements in treatments like allergen immunotherapy and biologics, current approaches have notable limitations and there is a pressing need for new therapeutic strategies. Recent research into taste receptors has unveiled their potential role in FA, offering fresh perspectives for understanding and managing this condition. Taste receptors, particularly type 1 taste receptors (TAS1Rs/T1Rs, sweet taste receptors) and type 2 taste receptors (TAS2Rs/T2Rs, bitter taste receptors), are distributed not only in the oral cavity but also in various extra-oral tissues, and their interactions with immune responses are increasingly recognized. This review highlights the connections between taste receptors and FA, exploring how taste receptor mechanisms might contribute to FA pathogenesis and treatment. Taste receptors, especially TAS2Rs, which include multiple subtypes with varying ligand specificities, have been implicated in modulating allergic responses and could serve as targets for novel FA therapies. Additionally, compounds such as bitter agents and sweeteners that interact with taste receptors show promise in influencing FA outcomes. This review emphasizes the need for further research into the mechanisms of taste receptor involvement in FA and suggests that targeting these receptors could provide new avenues for therapeutic intervention in the future.

Tracheal tuft cells release ATP and link innate to adaptive immunity in pneumonia

Tracheal tuft cells shape immune responses in the airways. While some of these effects have been attributed to differential release of either acetylcholine, leukotriene C4 and/or interleukin-25 depending on the activating stimuli, tuft cell-dependent mechanisms underlying the recruitment and activation of immune cells are incompletely understood. Here we show that Pseudomonas aeruginosa infection activates mouse tuft cells, which release ATP via pannexin 1 channels. Taste signaling through the Trpm5 channel is essential for bacterial tuft cell activation and ATP release. We demonstrate that activated tuft cells recruit dendritic cells to the trachea and lung. ATP released by tuft cells initiates dendritic cell activation, phagocytosis and migration. Tuft cell stimulation also involves an adaptive immune response through recruitment of IL-17A secreting T helper cells. Collectively, the results provide a molecular framework defining tuft cell dependent regulation of both innate and adaptive immune responses in the airways to combat bacterial infection.

Extragustatory bitter taste receptors in head and neck health and disease

Taste receptors, first described for their gustatory functions within the oral cavity and oropharynx, are now known to be expressed in many organ systems. Even intraoral taste receptors regulate non-sensory pathways, and recent literature has connected bitter taste receptors to various states of health and disease. These extragustatory pathways involve previously unexplored, clinically relevant roles for taste signaling in areas including susceptibility to infection, antibiotic efficacy, and cancer outcomes. Among other physicians, otolaryngologists who manage head and neck diseases should be aware of this growing body of evidence and its relevance to their fields. In this review, we describe the role of extragustatory taste receptors in head and neck health and disease, highlighting recent advances, clinical implications, and directions for future investigation. Additionally, this review will discuss known TAS2R polymorphisms and the associated implications for clinical prognosis.

The Remarkable Diversity of Vertebrate Bitter Taste Receptors: Recent Advances in Genomic and Functional Studies

Bitter taste perception is crucial for animal survival. By detecting potentially harmful substances, such as plant secondary metabolites, as bitter, animals can avoid ingesting toxic compounds. In vertebrates, this function is mediated by taste receptors type 2 (T2Rs), a family of G protein-coupled receptors (GPCRs) expressed on taste buds. Given their vital roles, T2Rs have undergone significant selective pressures throughout vertebrate evolution, leading to frequent gene duplications and deletions, functional changes, and intrapopulation differentiation across various lineages. Recent advancements in genomic and functional research have uncovered the repertoires and functions of bitter taste receptors in a wide range of vertebrate species, shedding light on their evolution in relation to dietary habits and other ecological factors. This review summarizes recent research on bitter taste receptors and explores the mechanisms driving the diversity of these receptors from the perspective of vertebrate ecology and evolution.

The epithelial cell types and their multi-phased defenses against fungi and other pathogens

Mucus and its movements are essential to epithelial tissue immune defenses against pathogens, including fungal pathogens, which can infect respiratory, gastrointestinal or the genito-urinary tracts. Several epithelial cell types contribute to their immune defense. This review focuses on the respiratory tract because of its paramount importance, but the observations will apply to epithelial cell defenses of other mucosal tissue, including the gastrointestinal and genito-urinary tracts. Mucus and its movements can enhance or degrade the immune defenses of the respiratory tract, particularly the lungs. The enhancements include inhaled pathogen entrapments, including fungal pathogens, pollutants and particulates, for their removal. The detriments include smaller lung airway obstructions by mucus, impairing the physical removal of pathogens and impairing vital transfers of oxygen and carbon dioxide between the alveolar circulatory system and the pulmonary air. Inflammation, edema and/or alveolar cellular damage can also reduce vital transfers of oxygen and carbon dioxide between the lung alveolar circulatory system and the pulmonary air. Furthermore, respiratory tract defenses are affected by several fatty acid mediators which activate cellular receptors to manipulate neutrophils, macrophages, dendritic cells, various innate lymphoid cells including the natural killer cells, T cells, γδ T cells, mucosal-associated invariant T cells, NKT cells and mast cells. These mediators include the inflammatory and frequently immunosuppressive prostaglandins and leukotrienes, and the special pro-resolving mediators, which normally resolve inflammation and immunosuppression. The total effects on the various epithelial cell and immune cell types, after exposures to pathogens, pollutants or particulates, will determine respiratory tract health or disease.

Nasal solitary chemosensory cells govern daily rhythm in mouse model of allergic rhinitis

BACKGROUND

While the daily rhythm of allergic rhinitis (AR) has long been recognized, the molecular mechanism underlying this phenomenon remains enigmatic.

OBJECTIVE

We aimed to investigate the role of circadian clock in AR development and to clarify the mechanism by which the daily rhythm of AR is generated.

METHODS

AR was induced in mice with ovalbumin. Toluidine blue staining, liquid chromatography-tandem mass spectrometry analysis, real-time quantitative PCR, and immunoblotting were performed with AR and control mice.

RESULTS

Ovalbumin-induced AR is diurnally rhythmic and associated with clock gene disruption in nasal mucosa. In particular, Rev-erbα is generally downregulated and its rhythm retained, but with a near-12-hour phase shift. Furthermore, global knockout of core clock gene Bmal1 or Rev-erbα increases the susceptibility of mice to AR and blunts AR rhythmicity. Importantly, nasal solitary chemosensory cells (SCCs) are rhythmically activated, and inhibition of the SCC pathway leads to attenuated AR and a loss of its rhythm. Moreover, rhythmic activation of SCCs is accounted for by diurnal expression of ChAT (an enzyme responsible for the synthesis of acetylcholine) and temporal generation of the neurotransmitter acetylcholine. Mechanistically, Rev-erbα trans-represses Chat through direct binding to a specific response element, generating a diurnal oscillation in this target gene.

CONCLUSION

SCCs, under the control of Rev-erbα, are a driver of AR rhythmicity; targeting SCCs should be considered as a new avenue for AR management.

Weep and sweep and the broom: tuft cell acetylcholine limits the worm

Two studies defined how tuft cell acetylcholine promotes parasite expulsion. Billip et al. demonstrated that acetylcholine increases water secretion, to promote the 'weep' response. Ndjim et al. found that tuft cell acetylcholine has a direct effect on worm fecundity. Both processes are only effective in the remodeled epithelium when the rare tuft cells have become abundant.

Mechanisms and Functions of Sweet Reception in Oral and Extraoral Organs

The oral detection of sugars relies on two types of receptor systems. The first is the G-protein-coupled receptor TAS1R2/TAS1R3. When activated, this receptor triggers a downstream signaling cascade involving gustducin, phospholipase Cβ2 (PLCβ2), and transient receptor potential channel M5 (TRPM5). The second type of receptor is the glucose transporter. When glucose enters the cell via this transporter, it is metabolized to produce ATP. This ATP inhibits the opening of KATP channels, leading to cell depolarization. Beside these receptor systems, sweet-sensitive taste cells have mechanisms to regulate their sensitivity to sweet substances based on internal and external states of the body. Sweet taste receptors are not limited to the oral cavity; they are also present in extraoral organs such as the gastrointestinal tract, pancreas, and brain. These extraoral sweet receptors are involved in various functions, including glucose absorption, insulin release, sugar preference, and food intake, contributing to the maintenance of energy homeostasis. Additionally, sweet receptors may have unique roles in certain organs like the trachea and bone. This review summarizes past and recent studies on sweet receptor systems, exploring the molecular mechanisms and physiological functions of sweet (sugar) detection in both oral and extraoral organs.

Tuft cell acetylcholine is released into the gut lumen to promote anti-helminth immunity

Upon parasitic helminth infection, activated intestinal tuft cells secrete interleukin-25 (IL-25), which initiates a type 2 immune response during which lamina propria type 2 innate lymphoid cells (ILC2s) produce IL-13. This causes epithelial remodeling, including tuft cell hyperplasia, the function of which is unknown. We identified a cholinergic effector function of tuft cells, which are the only epithelial cells that expressed choline acetyltransferase (ChAT). During parasite infection, mice with epithelial-specific deletion of ChAT had increased worm burden, fitness, and fecal egg counts, even though type 2 immune responses were comparable. Mechanistically, IL-13-amplified tuft cells release acetylcholine (ACh) into the gut lumen. Finally, we demonstrated a direct effect of ACh on worms, which reduced their fecundity via helminth-expressed muscarinic ACh receptors. Thus, tuft cells are sentinels in naive mice, and their amplification upon helminth infection provides an additional type 2 immune response effector function.

Tuft cell-derived acetylcholine promotes epithelial chloride secretion and intestinal helminth clearance

Epithelial cells secrete chloride to regulate water release at mucosal barriers, supporting both homeostatic hydration and the "weep" response that is critical for type 2 immune defense against parasitic worms (helminths). Epithelial tuft cells in the small intestine sense helminths and release cytokines and lipids to activate type 2 immune cells, but whether they regulate epithelial secretion is unknown. Here, we found that tuft cell activation rapidly induced epithelial chloride secretion in the small intestine. This response required tuft cell sensory functions and tuft cell-derived acetylcholine (ACh), which acted directly on neighboring epithelial cells to stimulate chloride secretion, independent of neurons. Maximal tuft cell-induced chloride secretion coincided with immune restriction of helminths, and clearance was delayed in mice lacking tuft cell-derived ACh, despite normal type 2 inflammation. Thus, we have uncovered an epithelium-intrinsic response unit that uses ACh to couple tuft cell sensing to the secretory defenses of neighboring epithelial cells.

Structures and functions of the normal and injured human olfactory epithelium

The olfactory epithelium (OE) is directly exposed to environmental agents entering the nasal cavity, leaving OSNs prone to injury and degeneration. The causes of olfactory dysfunction are diverse and include head trauma, neurodegenerative diseases, and aging, but the main causes are chronic rhinosinusitis (CRS) and viral infections. In CRS and viral infections, reduced airflow due to local inflammation, inflammatory cytokine production, release of degranulated proteins from eosinophils, and cell injury lead to decreased olfactory function. It is well known that injury-induced loss of mature OSNs in the adult OE causes massive regeneration of new OSNs within a few months through the proliferation and differentiation of progenitor basal cells that are subsequently incorporated into olfactory neural circuits. Although normal olfactory function returns after injury in most cases, prolonged olfactory impairment and lack of improvement in olfactory function in some cases poses a major clinical problem. Persistent inflammation or severe injury in the OE results in morphological changes in the OE and respiratory epithelium and decreases the number of mature OSNs, resulting in irreversible loss of olfactory function. In this review, we discuss the histological structure and distribution of the human OE, and the pathogenesis of olfactory dysfunction associated with CRS and viral infection.

G protein subunit Gγ13-mediated signaling pathway is critical to the inflammation resolution and functional recovery of severely injured lungs

Tuft cells are a group of rare epithelial cells that can detect pathogenic microbes and parasites. Many of these cells express signaling proteins initially found in taste buds. It is, however, not well understood how these taste signaling proteins contribute to the response to the invading pathogens or to the recovery of injured tissues. In this study, we conditionally nullified the signaling G protein subunit Gγ13 and found that the number of ectopic tuft cells in the injured lung was reduced following the infection of the influenza virus H1N1. Furthermore, the infected mutant mice exhibited significantly larger areas of lung injury, increased macrophage infiltration, severer pulmonary epithelial leakage, augmented pyroptosis and cell death, greater bodyweight loss, slower recovery, worsened fibrosis and increased fatality. Our data demonstrate that the Gγ13-mediated signal transduction pathway is critical to tuft cells-mediated inflammation resolution and functional repair of the damaged lungs.To our best knowledge, it is the first report indicating subtype-specific contributions of tuft cells to the resolution and recovery.

Epithelial-neuronal-immune cell interactions: Implications for immunity, inflammation, and tissue homeostasis at mucosal sites

The epithelial lining of the respiratory tract and intestine provides a critical physical barrier to protect host tissues against environmental insults, including dietary antigens, allergens, chemicals, and microorganisms. In addition, specialized epithelial cells communicate directly with hematopoietic and neuronal cells. These epithelial-immune and epithelial-neuronal interactions control host immune responses and have important implications for inflammatory conditions associated with defects in the epithelial barrier, including asthma, allergy, and inflammatory bowel diseases. In this review, we discuss emerging research that identifies the mechanisms and impact of epithelial-immune and epithelial-neuronal cross talk in regulating immunity, inflammation, and tissue homeostasis at mucosal barrier surfaces. Understanding the regulation and impact of these pathways could provide new therapeutic targets for inflammatory diseases at mucosal sites.

Mast cell-sensory neuron crosstalk in allergic diseases

Mast cells (MCs) are tissue-resident immune cells, well-positioned at the host-environment interface for detecting external antigens and playing a critical role in mobilizing innate and adaptive immune responses. Sensory neurons are afferent neurons innervating most areas of the body but especially in the periphery, where they sense external and internal signals and relay information to the brain. The significance of MC-sensory neuron communication is now increasingly becoming recognized, especially because both cell types are in close physical proximity at the host-environment interface and around major organs of the body and produce specific mediators that can activate each other. In this review, we explore the roles of MC-sensory neuron crosstalk in allergic diseases, shedding light on how activated MCs trigger sensory neurons to initiate signaling in pruritus, shock, and potentially abdominal pain in allergy, and how activated sensory neurons regulate MCs in homeostasis and atopic dermatitis associated with contact hypersensitivity and type 2 inflammation. Throughout the review, we also discuss how these 2 sentinel cell types signal each other, potentially resulting in a positive feedback loop that can sustain inflammation. Unraveling the mysteries of MC-sensory neuron crosstalk is likely to unveil their critical roles in various disease conditions and enable the development of new therapeutic approaches to combat these maladies.

Cholinergic sensing of allergen exposure by airway epithelium promotes type 2 immunity in the lungs

BACKGROUND

Nonneuronal cells, including epithelial cells, can produce acetylcholine (ACh). Muscarinic ACh receptor antagonists are used clinically to treat asthma and other medical conditions; however, knowledge regarding the roles of ACh in type 2 immunity is limited.

OBJECTIVE

Our aim was to investigate the roles of epithelial ACh in allergic immune responses.

METHODS

Human bronchial epithelial (HBE) cells were cultured with allergen extracts, and their ACh production and IL-33 secretion were studied in vitro. To investigate immune responses in vivo, naive BALB/c mice were treated intranasally with different muscarinic ACh receptor antagonists and then exposed intranasally to allergens.

RESULTS

At steady state, HBE cells expressed cellular components necessary for ACh production, including choline acetyltransferase and organic cation transporters. Exposure to allergens caused HBE cells to rapidly release ACh into the extracellular medium. Pharmacologic or small-interfering RNA-based blocking of ACh production or autocrine action through the M3 muscarinic ACh receptors in HBE cells suppressed allergen-induced ATP release, calcium mobilization, and extracellular secretion of IL-33. When naive mice were exposed to allergens, ACh was quickly released into the airway lumen. A series of clinical M3 muscarinic ACh receptor antagonists inhibited allergen-induced IL-33 secretion and innate type 2 immune response in the mouse airways. In a preclinical murine model of asthma, an ACh receptor antagonist suppressed allergen-induced airway inflammation and airway hyperreactivity.

CONCLUSIONS

ACh is released quickly by airway epithelial cells on allergen exposure, and it plays an important role in type 2 immunity. The epithelial ACh system can be considered a therapeutic target in allergic airway diseases.

Intestinal Tuft Cells: Morphology, Function, and Implications for Human Health

Tuft cells are a rare and morphologically distinct chemosensory cell type found throughout many organs, including the gastrointestinal tract. These cells were identified by their unique morphologies distinguished by large apical protrusions. Ultrastructural data have begun to describe the molecular underpinnings of their cytoskeletal features, and tuft cell-enriched cytoskeletal proteins have been identified, although the connection of tuft cell morphology to tuft cell functionality has not yet been established. Furthermore, tuft cells display variations in function and identity between and within tissues, leading to the delineation of distinct tuft cell populations. As a chemosensory cell type, they display receptors that are responsive to ligands specific for their environment. While many studies have demonstrated the tuft cell response to protists and helminths in the intestine, recent research has highlighted other roles of tuft cells as well as implicated tuft cells in other disease processes including inflammation, cancer, and viral infections. Here, we review the literature on the cytoskeletal structure of tuft cells. Additionally, we focus on new research discussing tuft cell lineage, ligand-receptor interactions, tuft cell tropism, and the role of tuft cells in intestinal disease. Finally, we discuss the implication of tuft cell-targeted therapies in human health and how the morphology of tuft cells may contribute to their functionality.

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.

The Sympathetic-Immune Milieu in Metabolic Health and Diseases: Insights from Pancreas, Liver, Intestine, and Adipose Tissues

Sympathetic innervation plays a crucial role in maintaining energy balance and contributes to metabolic pathophysiology. Recent evidence has begun to uncover the innervation landscape of sympathetic projections and sheds light on their important functions in metabolic activities. Additionally, the immune system has long been studied for its essential roles in metabolic health and diseases. In this review, the aim is to provide an overview of the current research progress on the sympathetic regulation of key metabolic organs, including the pancreas, liver, intestine, and adipose tissues. In particular, efforts are made to highlight the critical roles of the peripheral nervous system and its potential interplay with immune components. Overall, it is hoped to underscore the importance of studying metabolic organs from a comprehensive and interconnected perspective, which will provide valuable insights into the complex mechanisms underlying metabolic regulation and may lead to novel therapeutic strategies for metabolic diseases.

Human airway tuft cells influence the mucociliary clearance through cholinergic signalling

BACKGROUND

Airway tuft cells, formerly called brush cells have long been described only morphologically in human airways. More recent RNAseq studies described a chemosensory cell population, which includes tuft cells, by a distinct gene transcription signature. Yet, until which level in the tracheobronchial tree in native human airway epithelium tuft cells occur and if they function as regulators of innate immunity, e.g., by regulating mucociliary clearance, remained largely elusive.

METHODS

We performed immunohistochemistry, RT-PCR and immunoblotting analyses for various tuft cell markers to confirm the presence of this cell type in human tracheal samples. Immunohistochemistry was conducted to study the distribution of tuft cells along the intrapulmonary airways in humans. We assessed the influence of bitter substances and the taste transduction pathway on mucociliary clearance in mouse and human tracheal samples by measuring particle transport speed.

RESULTS

Tuft cells identified by the expression of their well-established marker POU class 2 homeobox 3 (POU2F3) were present from the trachea to the bronchioles. We identified choline acetyltransferase in POU2F3 expressing cells as well as the transient receptor potential melastatin 5 (TRPM5) channel in a small population of tracheal epithelial cells with morphological appearance of tuft cells. Application of bitter substances, such as denatonium, led to an increase in mucociliary clearance in human tracheal preparations. This was dependent on activation of the TRPM5 channel and involved cholinergic and nitric oxide signalling, indicating a functional role for human tuft cells in the regulation of mucociliary clearance.

CONCLUSIONS

We were able to detect tuft cells in the tracheobronchial tree down to the level of the bronchioles. Moreover, taste transduction and cholinergic signalling occur in the same cells and regulate mucociliary clearance. Thus, tuft cells are potentially involved in the regulation of innate immunity in human airways.

[WITHDRAWN] Neuron-dependent tuft cell expansion initiates sinonasal allergic Type 2 inflammation

The authors have withdrawn this manuscript owing to inaccuracies in the calculation of tuft cell numbers and errors in the selection of immunofluorescence images used to support our claims. Therefore, the authors do not wish this work to be cited as reference for the project. If you have any questions, please contact the corresponding author.

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.

Trigeminal Function in Sino-Nasal Health and Disease

The upper airway (nasal passages, paranasal sinuses, pharynx, and glottis) provides the sentinel portion of the human respiratory tract, with the combined senses of olfaction (cranial nerve I) and trigeminal sensation (cranial nerve V) signaling the quality of inspired air. Trigeminal function also complements the sense of taste (in turn mediated by cranial nerves VII, IX and X), and participates in the genesis of taste aversions. The ability of trigeminal stimulation in the upper aero-digestive tract to trigger a variety of respiratory and behavioral reflexes has long been recognized. In this context, the last three decades has seen a proliferation of observations at a molecular level regarding the mechanisms of olfaction, irritation, and gustation. Concurrently, an ever-widening network of physiological interactions between olfaction, taste, and trigeminal function has been uncovered. The objective of this review is to summarize the relatively recent expansion of research in this sub-field of sensory science, and to explore the clinical and therapeutic implications thereof.

Interkingdom Detection of Bacterial Quorum-Sensing Molecules by Mammalian Taste Receptors

Bitter and sweet taste G protein-coupled receptors (known as T2Rs and T1Rs, respectively) were originally identified in type II taste cells on the tongue, where they signal perception of bitter and sweet tastes, respectively. Over the past ~15 years, taste receptors have been identified in cells all over the body, demonstrating a more general chemosensory role beyond taste. Bitter and sweet taste receptors regulate gut epithelial function, pancreatic β cell secretion, thyroid hormone secretion, adipocyte function, and many other processes. Emerging data from a variety of tissues suggest that taste receptors are also used by mammalian cells to "eavesdrop" on bacterial communications. These receptors are activated by several quorum-sensing molecules, including acyl-homoserine lactones and quinolones from Gram-negative bacteria such as Pseudomonas aeruginosa, competence stimulating peptides from Streptococcus mutans, and D-amino acids from Staphylococcus aureus. Taste receptors are an arm of immune surveillance similar to Toll-like receptors and other pattern recognition receptors. Because they are activated by quorum-sensing molecules, taste receptors report information about microbial population density based on the chemical composition of the extracellular environment. This review summarizes current knowledge of bacterial activation of taste receptors and identifies important questions remaining in this field.

Tuft cells - the immunological interface and role in disease regulation

Tuft cells, also known as taste chemosensory cells, accumulate during parasite colonization or infection and have powerful immunomodulatory effects on substances that could be detrimental, as well as possible anti-inflammatory or antibacterial effects. Tuft cells are the primary source of interleukin (IL)-25. They trigger extra Innate lymphoid type-2 cells (ILC2) in the intestinal lamina propria to create cytokines (type 2); for instance, IL-13, which leads to an increase in IL-25. As tuft cells can produce biological effector molecules, such as IL-25 and eicosanoids involved in allergy (for example, cysteinyl leukotrienes and prostaglandin D₂) and the neurotransmitter acetylcholine. Following parasite infection, tuft cells require transient receptor potential cation channel subfamily M member 5 (TRPM5)-dependent chemosensation to produce responses. Secretory tuft cells provide a physical mucus barrier against the external environment and therefore have vital defensive roles against diseases by supporting tissue maintenance and repair. In addition to recent research on tuft cells, more studies are required to understand the distribution, cell turnover, molecular characteristics, responses in various species, involvement in immunological function across tissues, and most importantly, the mechanism involved in the control of various diseases.

Type 2 neuroimmune circuits in the shaping of physiology

Type 2 immune responses drive a broad range of biological processes including defense from large parasites, immunity to allergens, and non-immunity-related functions, such as metabolism and tissue homeostasis. The symptoms provoked by type 2 immunity, such as vomiting, coughing or itching, encompass nervous system triggering. Here, we review recent findings that place type 2 neuroimmune circuits at the center stage of immunity at barrier surfaces. We emphasize the homeostatic functions of these circuitries and how deregulation may drive pathology and impact disease outcomes, including in the context of cancer. We discuss a paradigm wherein type 2 neuroimmune circuits are central regulators of organismal physiology.

Tuft cell-derived acetylcholine regulates epithelial fluid secretion

Tuft cells are solitary chemosensory epithelial cells that can sense lumenal stimuli at mucosal barriers and secrete effector molecules to regulate the physiology and immune state of their surrounding tissue. In the small intestine, tuft cells detect parasitic worms (helminths) and microbe-derived succinate, and signal to immune cells to trigger a Type 2 immune response that leads to extensive epithelial remodeling spanning several days. Acetylcholine (ACh) from airway tuft cells has been shown to stimulate acute changes in breathing and mucocilliary clearance, but its function in the intestine is unknown. Here we show that tuft cell chemosensing in the intestine leads to release of ACh, but that this does not contribute to immune cell activation or associated tissue remodeling. Instead, tuft cell-derived ACh triggers immediate fluid secretion from neighboring epithelial cells into the intestinal lumen. This tuft cell-regulated fluid secretion is amplified during Type 2 inflammation, and helminth clearance is delayed in mice lacking tuft cell ACh. The coupling of the chemosensory function of tuft cells with fluid secretion creates an epithelium-intrinsic response unit that effects a physiological change within seconds of activation. This response mechanism is shared by tuft cells across tissues, and serves to regulate the epithelial secretion that is both a hallmark of Type 2 immunity and an essential component of homeostatic maintenance at mucosal barriers.

CSF-contacting neurons respond to Streptococcus pneumoniae and promote host survival during central nervous system infection

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.

Initiation of type 2 immunity at barrier surfaces

Although seemingly unrelated, parasitic worms, venoms, and allergens all induce a type 2 immune response. The effector functions and clinical features of type 2 immunity are well-defined, but fundamental questions about the initiation of type 2 immunity remain unresolved. How are these enormously diverse type 2 stimuli first detected? How are type 2 helper T cells primed and regulated? And how do mechanisms of type 2 initiation vary across tissues? Here, we review the common themes governing type 2 immune sensing and explore aspects of T cell priming and effector reactivation that make type 2 helper T cells a unique T helper lineage. Throughout the review, we emphasize the importance of non-hematopoietic cells and highlight how the unique anatomy and physiology of each barrier tissue shape mechanisms of type 2 immune initiation.

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.

The correlation between tic disorders and allergic conditions in children: A systematic review and meta-analysis of observational studies

Aim

To evaluate the correlation between tic disorders and allergies and to inform strategies for the treatment and prevention of tic disorders.

Methods

We conducted online searches of the MEDLINE, Embase, Cochrane, CNKI, CBM, WanFang, and VIP Information databases. Case-control studies and cohort studies related to tic disorders and allergic conditions were searched. Two researchers screened the literature, extracted data, and evaluated quality in strict accordance with the predetermined retrieval strategy and inclusion criteria. Finally, RevMan 5.4 software was used to conduct a meta-analysis. We used the Grading of Recommendations, Assessment, Development, and Evaluations (GRADE) approach to rating the certainty of evidence about each allergy outcome as high, moderate, low, or very low.

Results

We obtained seven eligible studies involving eight allergic conditions. The following allergic conditions were significantly associated with the presence of a tic disorder: asthma (OR = 1.90, 95% CI = 1.57-2.30, P < 0.001), allergic rhinitis (OR = 2.61, 95% CI = 1.90-3.57, P < 0.001), allergic conjunctivitis (OR = 3.65, 95% CI = 1.53-8,67, P = 0.003), eczema (OR = 3.87, 95% CI = 2.24-6.67, P < 0.001) and food allergy (OR = 2.79, 95% CI = 1.56-4.99, P < 0.001). There was no significant correlation between atopic dermatitis, urticaria, drug allergy, and tic disorder.

Conclusion

The occurrence of tic disorders may be associated with the presence of certain allergic disorders. However, whether allergy is one of the causes of tic disorders remains unclear.

Systematic review registration

The registration number for this systematic review is PROSPERO: CRD42021231658.

Duodenal cholinergic tuft cell number is increased in functional dyspepsia

BACKGROUND

Low-grade duodenal inflammation has recently been identified in patients with functional dyspepsia (FD). Chemosensory tuft cells were reported to be associated with gastrointestinal diseases. We therefore assessed duodenal tuft cell density and microinflammation in patients with FD to determine whether these measures could serve as useful biomarkers, and also correlated tuft cell density and microinflammation in FD patients.

METHODS

Duodenal biopsy specimens were obtained from patients with FD and from controls. Tuft cells, eosinophils, and mast cells were immunochemically stained with specific antibodies. Tuft cells were identified by immunostaining for doublecortin-like kinase 1 (DCLK1); cholinergic tuft cells were assessed by double staining for choline acetyltransferase (ChAT) and DCLK1. Immune-type tuft cells were assessed by IL-25 mRNA expression using real-time PCR.

KEY RESULTS

The density of intramucosal eosinophils and mast cells was significantly higher in the duodenum of FD patients than in controls. The density of tuft cells was significantly higher in the duodenum of FD patients compared with controls, and significantly correlated with eosinophil density in the duodenum of FD patients and controls. Moreover, a fraction of ChAT-positive cells was DCLK1 positive; all duodenal DCLK1+ tuft cells were ChAT-immunoreactive in FD and in control subjects.

CONCLUSIONS AND INFERENCES

Cholinergic tuft cell density was higher in the duodenum of patients with FD and significantly correlated with eosinophil density. Further studies are needed to investigate the pathophysiological significance of tuft cells in FD and may provide valuable clues to the pathophysiology of FD.

Immunological Aspects of Chronic Rhinosinusitis

Chronic rhinosinusitis (CRS) is related to persistent inflammation with a dysfunctional relationship between environmental agents and the host immune system. Disturbances in the functioning of the sinus mucosa lead to common clinical symptoms. The major processes involved in the pathogenesis of CRS include airway epithelial dysfunctions that are influenced by external and host-derived factors which activate multiple immunological mechanisms. The molecular bases for CRS remain unclear, although some factors commonly correspond to the disease: bacterial, fungal and viral infections, comorbidity diseases, genetic dysfunctions, and immunodeficiency. Additionally, air pollution leads increased severity of symptoms. CRS is a heterogeneous group of sinus diseases with different clinical courses and response to treatment. Immunological pathways vary depending on the endotype or genotype of the patient. The recent knowledge expansion into mechanisms underlying the pathogenesis of CRS is leading to a steadily increasing significance of precision medicine in the treatment of CRS. The purpose of this review is to summarize the current state of knowledge regarding the immunological aspects of CRS, which are essential for ensuring more effective treatment strategies.

Transient Receptor Potential (TRP) Channels in Airway Toxicity and Disease: An Update

Our respiratory system is exposed to toxicants and pathogens from both sides: the airways and the vasculature. While tracheal, bronchial and alveolar epithelial cells form a natural barrier in the airways, endothelial cells protect the lung from perfused toxic compounds, particulate matter and invading microorganism in the vascular system. Damages induce inflammation by our immune response and wound healing by (myo)fibroblast proliferation. Members of the transient receptor potential (TRP) superfamily of ion channel are expressed in many cells of the respiratory tract and serve multiple functions in physiology and pathophysiology. TRP expression patterns in non-neuronal cells with a focus on TRPA1, TRPC6, TRPM2, TRPM5, TRPM7, TRPV2, TRPV4 and TRPV6 channels are presented, and their roles in barrier function, immune regulation and phagocytosis are summarized. Moreover, TRP channels as future pharmacological targets in chronic obstructive pulmonary disease (COPD), asthma, cystic and pulmonary fibrosis as well as lung edema are discussed.

Taste Receptors beyond Taste Buds

Taste receptors are responsible for detecting their ligands not only in taste receptor cells (TRCs) but also in non-gustatory organs. For several decades, many research groups have accumulated evidence for such “ectopic” expression of taste receptors. More recently, some of the physiologic functions (apart from taste) of these ectopic taste receptors have been identified. Here, we summarize our current understanding of these ectopic taste receptors across multiple organs. With a particular focus on the specialized epithelial cells called tuft cells, which are now considered siblings of type II TRCs, we divide the ectopic expression of taste receptors into two categories: taste receptors in TRC-like cells outside taste buds and taste receptors with surprising ectopic expression in completely different cell types.

Taste Receptor Activation in Tracheal Brush Cells by Denatonium Modulates ENaC Channels via Ca2+, cAMP and ACh

Mucociliary clearance is a primary defence mechanism of the airways consisting of two components, ciliary beating and transepithelial ion transport (I_SC). Specialised chemosensory cholinergic epithelial cells, named brush cells (BC), are involved in regulating various physiological and immunological processes. However, it remains unclear if BC influence I_SC. In murine tracheae, denatonium, a taste receptor agonist, reduced basal I_SC in a concentration-dependent manner (EC₅₀ 397 µM). The inhibition of bitter taste signalling components with gallein (G_βγ subunits), U73122 (phospholipase C), 2-APB (IP3-receptors) or with TPPO (Trpm5, transient receptor potential-melastatin 5 channel) reduced the denatonium effect. Supportively, the I_SC was also diminished in Trpm5^−/− mice. Mecamylamine (nicotinic acetylcholine receptor, nAChR, inhibitor) and amiloride (epithelial sodium channel, ENaC, antagonist) decreased the denatonium effect. Additionally, the inhibition of G_α subunits (pertussis toxin) reduced the denatonium effect, while an inhibition of phosphodiesterase (IBMX) increased and of adenylate cyclase (forskolin) reversed the denatonium effect. The cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor CFTR_inh172 and the KCNQ1 potassium channel antagonist chromanol 293B both reduced the denatonium effect. Thus, denatonium reduces I_SC via the canonical bitter taste signalling cascade leading to the Trpm5-dependent nAChR-mediated inhibition of ENaC as well as G_α signalling leading to a reduction in cAMP-dependent I_SC. Therefore, BC activation contributes to the regulation of fluid homeostasis.

Role of Taste Receptors in Innate Immunity and Oral Health

Taste receptors are receptor proteins that detect ligands belonging to the 5 taste modalities: sweet, bitter, sour, salty, and umami. Taste receptors are not restricted to taste cells in taste buds; rather, they are distributed throughout the entire body. For example, solitary chemosensory cells (SCCs) and tuft cells express taste signal proteins and are present in several mucosae. In the airways, SCCs sense bacteria, allergens, viruses, and noxious stimuli and drive evasive behavior, neuroinflammation, and antibacterial responses. In the gut, tuft cells detect helminth infection and bacterial dysbiosis and initiate type II immune responses characterized by tissue remodeling. In the gingiva, SCCs detect oral pathogenic bacteria, evoke innate immune responses and release antimicrobial compounds in the epithelium, and regulate the microbiome composition. This review summarizes the most recent research on extragustatory taste receptors and their function in antibacterial defense. We also discuss how these findings have provided insights into the development of potential therapeutic strategies for mucosal bacterial infection and dental diseases.

Brush cells fine-tune neurogenic inflammation in the airways

Airway epithelial cells, once considered a simple barrier layer, are now recognized as providing an active site for antigen sensing and immune response initiation. Most mucosal sites contain chemosensory epithelial cells, rare and specialized cells gaining recognition for their unique functions in sensing and directing the immune response symphony. In this issue of the JCI, Hollenhorst, Nandigama, et al. demonstrated that tracheal chemosensory brush cells detected bitter-tasting substances, including quorum-sensing molecules (QSMs) generated by pathogenic Pseudomonas aeruginosa. The authors used various techniques, including genetic deletion of brush cells, genetic manipulation of brush cell signaling, deletion of sensory neurons, in vivo imaging, and infection models with P. aeruginosa, to show that QSMs increased vascular permeability and innate immune cell influx into the trachea. These findings link the recognition of bacterial QSMs to the innate immune response in the airways, with translational implications for airway inflammation and infectious pathology.

Tas2R activation relaxes airway smooth muscle by release of Gαt targeting on AChR signaling

Both chronic obstructive pulmonary disease (COPD) and asthma are severe respiratory diseases. Bitter receptor-mediated bronchodilation is a potential therapy for asthma, but the mechanism underlying the agonistic relaxation of airway smooth muscle (ASM) is not well defined. By exploring the ASM relaxation mechanism of bitter substances, we observed that pretreatment with the bitter substances nearly abolished the methacholine (MCh)-induced increase in the ASM cell (ASMC) calcium concentration, thereby suppressing the calcium-induced contraction release. The ASM relaxation was significantly inhibited by simultaneous deletion of three Gα_t proteins, suggesting an interaction between Tas2R and AChR signaling cascades in the relaxation process. Biochemically, the Gα_t released by Tas2R activation complexes with AChR and blocks the Gα_q cycling of AChR signal transduction. More importantly, a bitter substance, kudinoside A, not only attenuates airway constriction but also significantly inhibits pulmonary inflammation and tissue remodeling in COPD rats, indicating its modulation of additional Gα_q-associated pathological processes. Thus, our results suggest that Tas2R activation may be an ideal strategy for halting multiple pathological processes of COPD.

Neuronal-Immune Cell Units in Allergic Inflammation in the Nose

Immune cells and immune-derived molecules, endocrine glands and hormones, the nervous system and neuro molecules form the combined tridirectional neuroimmune network, which plays a significant role in the communication pathways and regulation at the level of the whole organism and local levels, in both healthy persons and patients with allergic rhinitis based on an allergic inflammatory process. This review focuses on a new research paradigm devoted to neuronal-immune cell units, which are involved in allergic inflammation in the nose and neuroimmune control of the nasal mucociliary immunologically active epithelial barrier. The categorization, cellular sources of neurotransmitters and neuropeptides, and their prevalent profiles in constituting allergen tolerance maintenance or its breakdown are discussed. Novel data on the functional structure of the nasal epithelium based on a transcriptomic technology, single-cell RNA-sequencing results, are considered in terms of neuroimmune regulation. Notably, the research of pathogenesis and therapy for atopic allergic diseases, including recently identified local forms, from the viewpoint of the tridirectional interaction of the neuroimmune network and discrete neuronal-immune cell units is at the cutting-edge.

"Every cell is an immune cell; contributions of non-hematopoietic cells to anti-helminth immunity"

Helminths are remarkably successful parasites that can invade various mammalian hosts and establish chronic infections that can go unnoticed for years despite causing severe tissue damage. To complete their life cycles, helminths migrate through multiple barrier sites that are densely populated by a complex array of hematopoietic and non-hematopoietic cells. While it is clear that type 2 cytokine responses elicited by immune cells promote worm clearance and tissue healing, the actions of non-hematopoietic cells are increasingly recognized as initiators, effectors and regulators of anti-helminth immunity. This review will highlight the collective actions of specialized epithelial cells, stromal niches, stem, muscle and neuroendocrine cells as well as peripheral neurons in the detection and elimination of helminths at mucosal sites. Studies dissecting the interactions between immune and non-hematopoietic cells will truly provide a better understanding of the mechanisms that ensure homeostasis in the context of helminth infections.

Nicotinic acetylcholine receptor stimulation: A new approach for stimulating tear secretion in dry eye disease

Tear secretion is regulated by the lacrimal functional unit consisting of afferent and efferent nerve innervation. The afferent arm consists of trigeminal nociceptors on the ocular surface and nasal mucosa. When stimulated by agonists, nicotinic acetylcholine receptors on nerve endings in the nose initiate a reflex arc resulting in instantaneous tear secretion. Pharmacologic nasal neural stimulation to increase endogenous tear production is a novel approach to treating dry eye disease.

Bitter taste signaling in tracheal epithelial brush cells elicits innate immune responses to bacterial infection

Constant exposure of the airways to inhaled pathogens requires efficient early immune responses protecting against infections. How bacteria on the epithelial surface are detected and first-line protective mechanisms are initiated are not well understood. We have recently shown that tracheal brush cells (BCs) express functional taste receptors. Here we report that bitter taste signaling in murine BCs induces neurogenic inflammation. We demonstrate that BC signaling stimulates adjacent sensory nerve endings in the trachea to release the neuropeptides CGRP and substance P that mediate plasma extravasation, neutrophil recruitment, and diapedesis. Moreover, we show that bitter tasting quorum-sensing molecules from Pseudomonas aeruginosa activate tracheal BCs. BC signaling depends on the key taste transduction gene Trpm5, triggers secretion of immune mediators, among them the most abundant member of the complement system, and is needed to combat P. aeruginosa infections. Our data provide functional insight into first-line defense mechanisms against bacterial infections of the lung.

Not Only COVID-19: Involvement of Multiple Chemosensory Systems in Human Diseases

Chemosensory systems are deemed marginal in human pathology. In appraising their role, we aim at suggesting a paradigm shift based on the available clinical and experimental data that will be discussed. Taste and olfaction are polymodal sensory systems, providing inputs to many brain structures that regulate crucial visceral functions, including metabolism but also endocrine, cardiovascular, respiratory, and immune systems. Moreover, other visceral chemosensory systems monitor different essential chemical parameters of “milieu intérieur,” transmitting their data to the brain areas receiving taste and olfactory inputs; hence, they participate in regulating the same vital functions. These chemosensory cells share many molecular features with olfactory or taste receptor cells, thus they may be affected by the same pathological events. In most COVID-19 patients, taste and olfaction are disturbed. This may represent only a small portion of a broadly diffuse chemosensory incapacitation. Indeed, many COVID-19 peculiar symptoms may be explained by the impairment of visceral chemosensory systems, for example, silent hypoxia, diarrhea, and the “cytokine storm”. Dysregulation of chemosensory systems may underlie the much higher mortality rate of COVID-19 Acute Respiratory Distress Syndrome (ARDS) compared to ARDSs of different origins. In chronic non-infectious diseases like hypertension, diabetes, or cancer, the impairment of taste and/or olfaction has been consistently reported. This may signal diffuse chemosensory failure, possibly worsening the prognosis of these patients. Incapacitation of one or few chemosensory systems has negligible effects on survival under ordinary life conditions but, under stress, like metabolic imbalance or COVID-19 pneumonia, the impairment of multiple chemosensory systems may lead to dire consequences during the course of the disease.

Identification of cholinergic cells with chemosensory traits in the porcine uterus

Chemosensory cells are specialized epithelial cells that act as sentinels near body entry sites. The majority of these cells express a cholinergic phenotype and utilize the taste signaling system to monitor the mucosal environment for potentially harmful substances, triggering protective reflexes. We report the identification of cells with a putative chemosensory role in the uterus. Presumptive chemosensory cells were immunoreactive to key components of the taste transduction, including the transient receptor potential channel M5 (TRPM5) and the phospholipase Cβ2 (PLCB2). These cells localized to endometrial glandular and luminal epithelia, while absent from myometrium and perimetrium. Double immunofluorescence revealed co-expression of chemosensory cell markers with the acetylcholine (ACh) synthesizing enzyme, choline acetyltransferase (ChAT). Further, we investigated the regional distribution and expression of chemosensory cells at different stages of the estrous cycle. Uteri were collected postmortem from gilts and stages of the ovarian cycle were determined macroscopically. The uteri were classified into three groups: prepubertal (PB), follicular (FOL), or luteal (LUT). The number of ChAT-immunoreactive cells was increased in the luminal epithelium in the caudal compartment compared to the cranial region of the uterine horn, and at the LUT compared to PB and FOL stages. An increase in ChAT protein abundance in LUT uterine homogenates was noted, although not followed by an increase in ACh content. In summary, our study has identified a hitherto unrecognized cholinergic cell in the uterus that has chemosensory traits and may be involved in a multitude of biological processes.

Cysteinyl leukotrienes and acetylcholine are biliary tuft cell cotransmitters

The gallbladder stores bile between meals and empties into the duodenum upon demand and is thereby exposed to the intestinal microbiome. This exposure raises the need for antimicrobial factors, among them, mucins produced by cholangiocytes, the dominant epithelial cell type in the gallbladder. The role of the much less frequent biliary tuft cells is still unknown. We here show that propionate, a major metabolite of intestinal bacteria, activates tuft cells via the short-chain free fatty acid receptor 2 and downstream signaling involving the cation channel transient receptor potential cation channel subfamily M member 5. This results in corelease of acetylcholine and cysteinyl leukotrienes from tuft cells and evokes synergistic paracrine effects upon the epithelium and the gallbladder smooth muscle, respectively. Acetylcholine triggers mucin release from cholangiocytes, an epithelial defense mechanism, through the muscarinic acetylcholine receptor M3. Cysteinyl leukotrienes cause gallbladder contraction through their cognate receptor CysLTR1, prompting emptying and closing. Our results establish gallbladder tuft cells as sensors of the microbial metabolite propionate, initiating dichotomous innate defense mechanisms through simultaneous release of acetylcholine and cysteinyl leukotrienes.

Tuft cells are key mediators of interkingdom interactions at mucosal barrier surfaces

Although tuft cells were discovered over 60 years ago, their functions have long been enigmatic, especially in human health. Nonetheless, tuft cells have recently emerged as key orchestrators of the host response to diverse microbial infections in the gut and airway. While tuft cells are epithelial in origin, they exhibit functions akin to immune cells and mediate important interkingdom interactions between the host and helminths, protists, viruses, and bacteria. With broad intra- and intertissue heterogeneity, tuft cells sense and respond to microbes with exquisite specificity. Tuft cells can recognize helminth and protist infection, driving a type 2 immune response to promote parasite expulsion. Tuft cells also serve as the primary physiologic target of persistent murine norovirus (MNV) and promote immune evasion. Recently, tuft cells were also shown to be infected by rotavirus. Other viral infections, such as influenza A virus, can induce tuft cell–dependent tissue repair. In the context of coinfection, tuft cells promote neurotropic flavivirus replication by dampening antiviral adaptive immune responses. Commensal and pathogenic bacteria can regulate tuft cell abundance and function and, in turn, tuft cells are implicated in modulating bacterial infiltration and mucosal barrier integrity. However, the contribution of tuft cells to microbial sensing in humans and their resulting effector responses are poorly characterized. Herein, we aim to provide a comprehensive overview of microbial activation of tuft cells with an emphasis on tuft cell heterogeneity and differences between mouse and human tuft cell biology as it pertains to human health and disease.

Tuft Cells and Their Role in Intestinal Diseases

The interests in intestinal epithelial tuft cells, their basic physiology, involvement in immune responses and relevance for gut diseases, have increased dramatically over the last fifteen years. A key discovery in 2016 of their close connection to helminthic and protozoan infection has further spurred the exploration of these rare chemosensory epithelial cells. Although very sparse in number, tuft cells are now known as important sentinels in the gastrointestinal tract as they monitor intestinal content using succinate as well as sweet and bitter taste receptors. Upon stimulation, tuft cells secrete a broad palette of effector molecules, including interleukin-25, prostaglandin E2 and D2, cysteinyl leukotriene C4, acetylcholine, thymic stromal lymphopoietin, and β-endorphins, some of which with immunomodulatory functions. Tuft cells have proven indispensable in anti-helminthic and anti-protozoan immunity. Most studies on tuft cells are based on murine experiments using double cortin-like kinase 1 (DCLK1) as a marker, while human intestinal tuft cells can be identified by their expression of the cyclooxygenase-1 enzyme. So far, only few studies have examined tuft cells in humans and their relation to gut disease. Here, we present an updated view on intestinal epithelial tuft cells, their physiology, immunological hub function, and their involvement in human disease. We close with a discussion on how tuft cells may have potential therapeutic value in a clinical context.