Bitter taste receptor T2R14 detects quorum sensing molecules from cariogenic Streptococcus mutans and mediates innate immune responses in gingival epithelial cells

Abstract

Host-pathogen interactions play an important role in defining the outcome of a disease. Recent studies have shown bacterial quorum sensing molecules (QSM) can interact with host cell membrane proteins, mainly G protein-coupled receptors (GPCRs), and induce innate immune responses. However, few studies have examined QSM-GPCR interactions and their influence on oral innate immune responses. In this study, we examined the role of bitter taste receptor T2R14 in sensing competence stimulating peptides (CSPs) secreted by cariogenic bacterium Streptococcus mutans and in mediating innate immune responses in gingival epithelial cells (GECs). Transcriptomic and western blot analyses identify T2R14 to be highly expressed in GECs. Our data shows that only CSP-1 from S. mutans induces robust intracellular calcium mobilization compared to CSP-2 and CSP-3. By using CRISPR-Cas9, we demonstrate that CSP-1 induced calcium signaling and secretion of cytokines CXCL-8/IL-8, TNF-α and IL-6 is mediated through T2R14 in GECs. Interestingly, the NF-kB signaling activated by CSP-1 in GECs was independent of T2R14. CSP-1 primed GECs attracted differentiated HL-60 immune cells (dHL-60) and this effect was abolished in T2R14 knock down GECs and also in cells primed with T2R14 antagonist 6-Methoxyflavone (6-MF). Our findings identify S. mutans CSP-1 as a peptide ligand for the T2R family. Our study establishes a novel host-pathogen interaction between cariogenic S. mutans CSP-1 and T2R14 in GECs leading to an innate immune response. Collectively, these findings suggest T2Rs as potential therapeutic targets to modulate innate immune responses upon oral bacterial infections.

Bitter Taste Receptor T2R14 Modulates Gram-Positive Bacterial Internalization and Survival in Gingival Epithelial Cells

“…Oral epithelial cells are known to secrete IL-8 chemokine, a potent neutrophil attractant upon bacterial insult. Previously, we have shown an increased IL-8 secretion in GECs upon treatment with T2R14 agonists, including S. mutans CSP-1, as well as a decreased secretion of IL-8 upon knockdown of T2R14 [ 2 ]. However, T2R14 dependent IL-8 secretion profile upon bacterial insult has not been investigated.…”

“…To characterize the bacterial growth inhibitory effect of T2R14, the OKF6 WT, MOCK and T2R14 KD cells were treated with the recently identified T2R14 agonist CSP-1 secreted by S. mutans [ 2 ], and bitter agonists DPH and apigenin. The CM from the abovementioned treated GECs was used to test the bactericidal effect on S. aureus and S. mutans.…”

“…PAMPs are bacterial cell-wall components, such as lipoteichoic acid in Gram-positive organisms and lipopolysaccharide in Gram-negative organisms [ 1 ]. Apart from these above-mentioned molecules, bacteria secrete quorum-sensing molecules (QSMs) that are recognized by bitter taste receptors (T2Rs) [ 2 , 3 , 4 ].…”

“…However, several studies have shown both extra-oral expression and extra-gustatory functions for T2Rs in physiological, as well as patho-physiological, conditions [ 7 , 8 ]. T2Rs have now been identified as important modulators of innate immune responses in several cell types, including sinonasal epithelial cells; upper airway epithelial cells; immune cells, such as macrophages; and gingival epithelial cells (GECs) [ 2 , 9 , 10 , 11 ]. The stimulation of bronchial airway epithelial cells by bitter compounds resulted in an increase in intracellular Ca 2+ , resulting in increased ciliary beat frequency [ 12 ], nitric oxide (NO) production and increased bactericidal effects in sinonasal epithelial cells [ 10 ] and regulation of type-2 immunity against gut pathogens [ 13 , 14 ].…”

“…To test this hypothesis, we have used opportunistic pathogen S. aureus , which is often associated with oral disorders, such as mucositis, angular cheilitis and parotitis. We also followed up on our recent studies, where we analyzed the effects of the competence stimulating peptides (CSPs) secreted by the cariogenic S. mutans in GECs [ 2 , 18 ]. S. mutans CSP-1 is shown to activate T2R14, leading to an innate immune response in GECs.…”