Cholinergic neurons trigger epithelial Ca2+ currents to heal the gut

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.

Neuroimmunophysiology of the gastrointestinal tract

Gut physiology is the epicenter of a web of internal communication systems (i.e., neural, immune, hormonal) mediated by cell-cell contacts, soluble factors, and external influences, such as the microbiome, diet, and the physical environment. Together these provide the signals that shape enteric homeostasis and, when they go awry, lead to disease. Faced with the seemingly paradoxical tasks of nutrient uptake (digestion) and retarding pathogen invasion (host defense), the gut integrates interactions between a variety of cells and signaling molecules to keep the host nourished and protected from pathogens. When the system fails, the outcome can be acute or chronic disease, often labeled as "idiopathic" in nature (e.g., irritable bowel syndrome, inflammatory bowel disease). Here we underscore the importance of a holistic approach to gut physiology, placing an emphasis on intercellular connectedness, using enteric neuroimmunophysiology as the paradigm. The goal of this opinion piece is to acknowledge the pace of change brought to our field via single-cell and -omic methodologies and other techniques such as cell lineage tracing, transgenic animal models, methods for culturing patient tissue, and advanced imaging. We identify gaps in the field and hope to inspire and challenge colleagues to take up the mantle and advance awareness of the subtleties, intricacies, and nuances of intestinal physiology in health and disease by defining communication pathways between gut resident cells, those recruited from the circulation, and "external" influences such as the central nervous system and the gut microbiota.

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.

Sensory neurons: An integrated component of innate immunity

The sensory nervous system possesses the ability to integrate exogenous threats and endogenous signals to mediate downstream effector functions. Sensory neurons have been shown to activate or suppress host defense and immunity against pathogens, depending on the tissue and disease state. Through this lens, pro- and anti-inflammatory neuroimmune effector functions can be interpreted as evolutionary adaptations by host or pathogen. Here, we discuss recent and impactful examples of neuroimmune circuitry that regulate tissue homeostasis, autoinflammation, and host defense. Apparently paradoxical or conflicting reports in the literature also highlight the complexity of neuroimmune interactions that may depend on tissue- and microbe-specific cues. These findings expand our understanding of the nuanced mechanisms and the greater context of sensory neurons in innate immunity.

Nicotinic acetylcholine receptor signaling maintains epithelial barrier integrity

Disruption of epithelial barriers is a common disease manifestation in chronic degenerative diseases of the airways, lung, and intestine. Extensive human genetic studies have identified risk loci in such diseases, including in chronic obstructive pulmonary disease (COPD) and inflammatory bowel diseases. The genes associated with these loci have not fully been determined, and functional characterization of such genes requires extensive studies in model organisms. Here, we report the results of a screen in Drosophila melanogaster that allowed for rapid identification, validation, and prioritization of COPD risk genes that were selected based on risk loci identified in human genome-wide association studies (GWAS). Using intestinal barrier dysfunction in flies as a readout, our results validate the impact of candidate gene perturbations on epithelial barrier function in 56% of the cases, resulting in a prioritized target gene list. We further report the functional characterization in flies of one family of these genes, encoding for nicotinic acetylcholine receptor (nAchR) subunits. We find that nAchR signaling in enterocytes of the fly gut promotes epithelial barrier function and epithelial homeostasis by regulating the production of the peritrophic matrix. Our findings identify COPD-associated genes critical for epithelial barrier maintenance, and provide insight into the role of epithelial nAchR signaling for homeostasis.