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

Abstract

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.

Keywords: IL-25; acetylcholine; cysteinyl leukotriene; mucociliary clearance; tuft cell; type 2 immunity; type II taste transduction.

Figure 1.. Major functional outputs of tuft cells.

Despite the similarity in tuft cell gene expression programs and structure across distinct tissues, tuft cell functions observed in vivo appear tissue and/or context specific. Tuft cell roles can be classified by dominant effector programs and their resulting impact on tissue physiology. Major roles for tuft cells in promoting type 2 cytokine responses, specifically from innate/innate-like lymphocytes (group 2 innate lymphoid cells – ILC2s, or type 2 natural killer T cells – NKT2s) have been found in gut, lung and thymus. In the gut, tuft cell-mediated production of IL-25 and cysteinyl leukotrienes was critical for anti-helminth responses, while IL-25 alone promoted adaptive responses to protist colonization downstream of succinate sensing (recently reviewed in (19)). Roles for tuft cells in tissue regeneration or response to injury have been demonstrated in models of colitis and intestinal stem cell loss (14; 128; 141) and in pancreatitis-induced pancreatic cancer (31). In the injured pancreatic duct, tuft cell PGD₂ repressed inflammatory gene expression in stromal cells and slowed tumorigenesis. Antimicrobial defensive function is best characterized in the conducting airways, where bacterial-derived formylated peptides (FMet) or TAS2R ligands induced tuft cell-mediated production of acetylcholine (ACh) and/or lateral calcium release, increasing ciliary beat frequency of neighboring epithelial cells and promoting antimicrobial peptide release (53; 54). Forthcoming work also supports antibacterial roles in the small intestine, via vomeronasal receptor signaling (29), in which tuft cell production of PGD₂ promoted increased mucus release. We suggest that these divergent roles for tuft cells arise as a product of environmental cues (i.e.: presence or absence of activating ligands such as succinate or FMet peptides), promoting environmental-driven maturation of effector functions from nascent tuft cells.

Figure 2.. IL-25+ tuft cells in olfactory epithelium.

Neuroepithelial tissues have critical roles in chemosensation that directs attractive and aversive behaviors. Tuft cells in the olfactory epithelium, taste bud, and vomeronasal organ are in direct contact with presynaptic neurons. (A) Confocal image of transverse section from immersion-fixed and decalcified mouse nasal cavity, posterior. NL = nasal lumen, T = turbinate, S = septum. (B) Inset from (A). OE tuft cells, aka “microvillus cells,” are IL-25+ (red) cells in the apical epithelial layer (marked by white arrow heads), above and in direct contact with olfactory sensory neurons (OSNs). OSNs are observed as a pseudostratified array of nuclei (DAPI, white) outlined by neural cell adhesion molecule 1 (NCAM, green) processes/cytoplasm, denoted between white dashed lines. A thin layer of keratin 5 (KRT5, red)-stained horizontal basal cells are also delineated by white dashed line. An NCAM+ nerve fiber is denoted with a white asterisk.

Figure 3.. Tuft cell differentiation at homeostasis and in injury.

During homeostasis, most tissue tuft cells arise from dedicated local stem cells. For example, tuft cells arise from LGR5+ stem cells in both the mouse colon and small intestine. In the small intestine, this process can proceed in ATOH1-dependent or -independent fashion, with both pathways operating under homeostatic conditions. ATOH1-independent tuft cell differentiation may be driven by type 2 cytokine signaling in a SOX4-dependent fashion. Analogous to the LGR5+ cells in the intestine, tuft cells in the conducting airways can be traced to KRT5+ basal cells, while those in the olfactory epithelium can be traced to globose basal cells. Local epithelial progenitors remain to be identified in some tuft cell-containing tissues (e.g.: extrahepatic biliary tree). After injury of tuft-cell-containing tissues, reserve stem cell populations can be mobilized to repopulate tuft cells. Such is the case in methimazole-induced ablation of olfactory epithelium, where otherwise quiescent horizontal basal cells are activated to renew all OE cells including tuft cells (–91). Injury to tissues where tuft cells are typically absent can also promote de novo emergence of tuft cells. Recent work in injury- and oncogene-induced mouse models of pancreatic ductal adenocarcinoma (103; 104) indicates that under severe injury, fully differentiated acinar cells de-differentiate or transdifferentiate into tuft cells, passing through a mucinous intermediate. Whether this could be the process driving emergence of tuft cells in severe lung injury has not been examined. In all cases, the fully differentiated tuft cells are remarkably similar in gene expression and structure.

Figure 4.. Heterogeneity of tuft cells across space and time.

Recent studies using single cell sequencing and variations on this technique have described heterogeneous gene expression profiles for tuft cells within a single tissue. The biological relevance of this transcriptional heterogeneity remains unknown. We suggest that tuft cell gene expression heterogeneity could represent tuft cell maturation through both space and time, related both to local signaling and environmental cues and to temporal maturation. In the small intestine (A) tuft cell gene expression profiles change along on the crypt-villus axis, which is concordant with cellular age and increasing exposure to luminal contents, including known tuft cell ligands like succinate. Many transcripts associated with immune function were enriched in tuft cells toward the villus tip (135), while transcripts previously associated with a “neuronal” phenotype were associated with physical position (peri-cryptal) or cellular age (newly differentiated), comprising a “nascent” tuft cell gene signature. (B) Tuft cell heterogeneity may also relate more globally to position in the tissue, driven by local environmental cues (i.e.: niche-specific stromal cells) and distinct luminal contents. In the small intestine, tuft cells could vary along the proximal to distal axis from stomach to cecum/colon, which have highly distinct luminal contents and physiologic functions.