Peihua Jiang

Taste buds undergo continuous cell turnover throughout life, and taste cell replenishment relies strictly on innervation, a phenomenon first described almost 150 years ago. Recently, we provided evidence that R-spondin 2 (Rspo2) may be the long-sought gustatory neuron-supplied factor that regulates taste stem cell activity, via its interaction with taste stem/progenitor cell-expressed receptor Rnf43/Znrf3. Yet, whether gustatory-neuron-supplied Rspo2 is strictly required for taste tissue maintenance has not been resolved. Here, we set out to determine the necessity of gustatory-neuron-supplied Rspo2 in taste tissue homeostasis using genetic approaches. We used a mouse line that harbors the neomycin-resistance gene ( NeoR ) in one of the intron regions of the Rspo2 gene, which results in reduced expression of Rspo2. The number of taste buds is significantly reduced in these mice, compared to wild-type mice, in both anterior and posterior tongue. This phenotypic change was completely reversed by removing NeoR from the Rspo2 gene, thus making it normal. We also combined adeno-associated virus (AAV)-based delivery of Cre recombinase with a mouse line amenable to Cre-based ablation of the Rspo2 exons encoding the receptor-binding domains. Such deletion of Rspo2 in the nodose-petrosal-jugular ganglion complex led to nearly complete loss of taste buds in the circumvallate papilla. Thus, we demonstrate that Rspo2 is the long-sought gustatory-neuron-supplied factor that acts on taste stem cells to maintain taste tissue homeostasis. Significance: We have known for 150 years that innervation is required to induce and maintain cell replacement in taste buds. Until recently, the identity of the inducing factor produced by neurons was unknown. We have shown that R-spondin alone is sufficient to substitute for neuronal input to induce taste bud regeneration. Using a genetic loss-of-function approach, we now demonstrate that gustatory-neuron-expressed Rspo2 is required to maintain taste tissue homeostasis. Altogether, our work reveals that Rspo2 is the long-sought neuron-supplied factor that regulates the activity of taste stem/progenitor cells.

Significance: Despite improved understanding of cellular and molecular events mediating type 2 immune responses in the gut, how these changes facilitate worm expulsion remains largely unknown. We identified pyroptosis-effector gasdermin C genes ( Gsdmc s) as type 2 cytokine-regulated target genes in intestinal epithelial cells. Furthermore, we showed that overexpression of Gsdmc2 triggered pyroptosis. Organoid cultures and in vivo mouse models were used to show that activating type 2 immunity leads to lytic cell death in gut epithelium and cleavage of Gsdmcs. We thus revealed a role of gasdermin-mediated lytic cell death in worm-induced type 2 immunity. Parasitic infection often leads to malnutrition and retarded growth; since this may involve villus atrophy, gasdermin activity may provide a pharmaceutical target for treatment.

Children have difficulty swallowing capsules. Yet, when presented with liquid formulations, children often reject oral medications due to their intense bitterness. Presently, effective strategies to identify methods, reagents, and tools to block bitterness remain elusive. For a specific bitter-tasting drug, identification of the responsible bitter receptors and discovery of antagonists for those receptors can provide a method to block perceived bitterness. We have identified a compound (6-methylflavone) that can block responses to an intensely bitter-tasting anti–human immunodeficiency virus (HIV) drug, tenofovir alafenamide (TAF), using a primary human taste bud epithelial cell culture as a screening platform. Specifically, TAS2R39 and TAS2R1 are the main type 2 taste receptors responding to TAF observed via heterologously expressing specific TAS2R receptors into HEK293 cells. In this assay, 6-methylflavone blocked the responses of TAS2R39 to TAF. In human sensory testing, 8 of 16 subjects showed reduction in perceived bitterness of TAF after pretreating (or “prerinsing”) with 6-methylflavone and mixing 6-methylflavone with TAF. Bitterness was completely and reliably blocked in two of these subjects. These data demonstrate that a combined approach of human taste cell culture–based screening, receptor-specific assays, and human psychophysical testing can successfully discover molecules for blocking perceived bitterness of pharmaceuticals, such as the HIV therapeutic TAF. Our hope is to use bitter taste blockers to increase medical compliance with these vital medicines. SIGNIFICANCE STATEMENT Identification of a small molecule that inhibits bitter taste from tenofovir alafenamide may increase the compliance in treating children with human immunodeficiency virus infections.

Significance: A long-standing observation is that taste bud cell replenishment requires neuronal input. The molecular basis for neuronal regulation of taste tissue homeostasis is unclear. We demonstrate that Rspo2 is selectively expressed in sensory ganglion neurons. We further demonstrate that exogenous R-spondin can substitute for neuronal input to maintain taste tissue homeostasis by promoting generation of differentiated taste cells in vivo and that R-spondin is required for generation of differentiated taste cells in an ex vivo organoid culture system. We thus propose that Rspo2 may be the long-sought neuron-supplied niche factor for maintaining taste tissue homeostasis. The ability of R-spondin to promote generation of differentiated taste cells may provide a therapeutic approach for those who suffer from taste loss.

Divalent and trivalent salts exhibit a complex taste profile. They are perceived as being astringent/drying, sour, bitter, and metallic. We hypothesized that human bitter-taste receptors may mediate some taste attributes of these salts. Using a cell-based functional assay, we found that TAS2R7 responds to a broad range of divalent and trivalent salts, including zinc, calcium, magnesium, copper, manganese, and aluminum, but not to potassium, suggesting TAS2R7 may act as a metal cation receptor mediating bitterness of divalent and trivalent salts. Molecular modeling and mutagenesis analysis identified 2 residues, H94^3.37 and E264^7.32, in TAS2R7 that appear to be responsible for the interaction of TAS2R7 with metallic ions. Taste receptors are found in both oral and extraoral tissues. The responsiveness of TAS2R7 to various mineral salts suggests it may act as a broad sensor, similar to the calcium-sensing receptor, for biologically relevant metal cations in both oral and extraoral tissues.

By generating raw genetic material and diverse biological functions, gene duplication represents a major evolutionary mechanism that is of fundamental importance in ecological adaptation. The lineage‐specific duplication events of bitter taste receptor genes (Tas2rs) have been identified in a number of vertebrates, but functional evolution of newTas2rcopies after duplication remains largely unknown. Here, we present the largest data set of batTas2rs to date, identified from existing genome sequences of 15 bat species and newly sequenced from 17 bat species, and demonstrate lineage‐specific duplications ofTas2r16,Tas2r18andTas2r41that only occurred inMyotisbats.Myotisbats are highly speciose and represent the only mammalian genus that is naturally distributed on every continent except Antarctica. The occupation of such diverse habitats might have driven theTas2rgene expansion. New copies ofTas2rs inMyotisbats have shown molecular adaptation and functional divergence. For example, three copies ofTas2r16inMyotis davidiishowed differential sensitivities to arbutin and salicin that may occur in their insect prey, as suggested by cell‐based functional assays. We hypothesize that functional differences amongTas2rcopies inMyotisbats would increase their survival rate through preventing the ingestion of an elevated number of bitter‐tasting dietary toxins from their insect prey, which may have facilitated their adaptation to diverse habitats. Our study demonstrates functional changes of newTas2rcopies after lineage‐specific duplications inMyotisbats and highlights the potential role of taste perception in exploiting new environments.

The hallmark features of type 2 mucosal immunity include intestinal tuft and goblet cell expansion initiated by tuft cell activation. How infectious agents that induce type 2 mucosal immunity are detected by tuft cells is unknown. Published microarray analysis suggested that succinate receptor 1 (Sucnr1) is specifically expressed in tuft cells. Thus, we hypothesized that the succinate–Sucnr1 axis may be utilized by tuft cells to detect certain infectious agents. Here we confirmed that Sucnr1 is specifically expressed in intestinal tuft cells but not in other types of intestinal epithelial cells, and demonstrated that dietary succinate induces tuft and goblet cell hyperplasia via Sucnr1 and the tuft cell-expressed chemosensory signaling elements gustducin and Trpm5. Conventional mice with a genetic Sucnr1 deficiency (Sucnr1^−/−) showed diminished immune responses to treatment with polyethylene glycol and streptomycin, which are known to enhance microbiota-derived succinate, but responded normally to inoculation with the parasitic worm Nippostrongylus brasiliensis that also produces succinate. Thus, Sucnr1 is required for microbiota-induced but not for a generalized worm-induced type 2 immunity. Significance Tuft cells in the intestine are known to act as sentinels for infectious agents [e.g., helminths (worms) and bacterial microbiota] and express taste-signaling elements. In this work, the G protein-coupled receptor Sucnr1 was shown to be expressed specifically in tuft cells but not in other intestinal epithelial cells. Dietary succinate and perturbations in the microbiota activate tuft cells, and subsequently type 2 immunity, via tuft cell-expressed Sucnr1. Modulating this pathway using dietary succinate or specific Sucnr1 agonists may be a strategy for fighting bacterial and parasitic infections or other type 2 immune-related metabolic disorders such as obesity.