Squamous Odontogenic Tumor: Review of the Literature and Report of a New Case.

PURPOSE: Squamous odontogenic tumor (SOT) is a rare, benign, locally infiltrative odontogenic tumor of the gnathic bones. It is composed of islands of bland, well-differentiated squamous epithelium of varying shape and size. Because of histologic overlap, SOT has often been overdiagnosed as ameloblastoma and squamous cell carcinoma. It thus becomes important to understand the clinical, radiologic, histopathologic, and treatment characteristics of this tumor. MATERIALS AND METHODS: Using the PubMed and Google Scholar databases, we searched for reported cases of SOT published in the English-language literature. We were able to retrieve 49 acceptable cases and perform a comprehensive literature review of the intraosseous SOTs, with emphasis on their clinical, radiographic, and pathologic characteristics, as well as treatment strategies. In addition, we present an additional case of SOT affecting the posterior mandible in a 44-year-old female patient. RESULTS: The tumor in the posterior mandible in our patient was accompanied by acute pain and treated by enucleation. Histopathologic evaluation showed variably sized islands of benign squamous epithelium scattered in a fibrous stroma, consistent with the diagnosis of an SOT. Uneventful healing was noted at the 1-month postoperative appointment. However, the patient was lost to long-term follow-up. Our literature review showed that the average age at the time of diagnosis of SOT is 34.2 years. Men and women are equally affected, and the tumor does not show a predilection for either jaw bone. The most common locations are the anterior maxilla and posterior mandible. Most SOTs are treated conservatively by enucleation or curettage, whereas aggressive or recurrent tumors require radical resection. CONCLUSIONS: Careful evaluation of the excised specimen, with immunohistochemical investigations, may prove rewarding in differentiating an SOT from other odontogenic neoplasms and thus minimize the patient's chances of undergoing an unnecessary aggressive intervention.

Peripheral Ameloblastoma: A Study of 18 Cases and Usage of Ber-EP4 Immunohistochemistry to Rule out a Diagnosis of Intraoral Basal Cell Carcinoma.

PURPOSE: Peripheral ameloblastoma (PA) is a rare odontogenic tumor arising in the mucosa of tooth-bearing areas of the jaws that typically shows no radiographic evidence of bone involvement. It bears close histologic resemblance to intraoral basal cell carcinoma (IOBCC), an extremely rare entity. In our experience from previous published data, 3 cases of IOBCC were initially misdiagnosed as PA and were later differentiated from PA on the basis of Ber-EP4 protein expression. This unusual but significant experience set the premise for us to rule out a diagnosis of IOBCC by evaluating Ber-EP4 expression in all previously diagnosed cases of PA from the University of Florida Oral Pathology (UFOP) biopsy service archives. MATERIALS AND METHODS: With institutional review board approval, 18 cases of PA were retrieved from the UFOP biopsy service archives. We describe the clinicopathologic features of these cases and discuss the Ber-EP4 immunohistochemical staining performed to rule out a potential diagnosis of IOBCC. In addition, we conducted calretinin and epithelial membrane antigen staining for 1 case of PA. RESULTS: Most PAs presented in the lingual gingiva of the posterior mandible. Men were affected twice as often as women, and the average age at the diagnosis was 59 ± 21.5 years. Of the 18 lesions, 13 showed no reactivity to Ber-EP4, 4 displayed patchy membranous immunoreactivity, and 1 demonstrated nonspecific reactivity. CONCLUSIONS: We have concluded that all cases of PA that present with histologic overlap with basal cell carcinoma, especially those from incisional biopsies, those that appear significantly infiltrative, and those that appear ulcerated and/or demonstrate recurrence should be evaluated with Ber-EP4 to rule out IOBCC.

Analysis of the expression of human bitter taste receptors in extraoral tissues.

The 25 bitter taste receptors (T2Rs) in humans perform a chemosensory function. However, very little is known about the level of expression of these receptors in different tissues. In this study, using nCounter gene expression we analyzed the expression patterns of human TAS2R transcripts in cystic fibrosis bronchial epithelial (CuFi-1), normal bronchial epithelial (NuLi-1), airway smooth muscle (ASM), pulmonary artery smooth muscle (PASM), mammary epithelial, and breast cancer cells. Our results suggest a specific pattern of TAS2R expression with TAS2R3, 4, 5, 10, 13, 19, and 50 transcripts expressed at moderate levels and TAS2R14 and TAS2R20 (or TASR49) at high levels in the various tissues analyzed. This pattern of expression is mostly independent of tissue origin and the pathological state, except in cancer cells. To elucidate the expression at the protein level, we pursued flow cytometry analysis of select T2Rs from CuFi-1 and NuLi-1 cells. The expression levels observed at the gene level by nCounter analysis correlate with the protein levels for the T2Rs analyzed. Next, to assess the functionality of the expressed T2Rs in these cells, we pursued functional assays measuring intracellular calcium mobilization after stimulation with the bitter compound quinine. Using PLC inhibitor, U-73122, we show that the calcium mobilized in these cells predominantly takes place through the Quinine-T2R-Gαßγ-PLC pathway. This report will accelerate studies aimed at analyzing the pathophysiological function of T2Rs in different extraoral tissues.

The structure-function role of C-terminus in human bitter taste receptor T2R4 signaling.

Bitter taste, in humans, is sensed by 25 G protein-coupled receptors, referred to as bitter taste receptors (T2Rs). The diverse roles of T2Rs in various extraoral tissues have implicated them as a potential target for therapeutic intervention. Structure-function studies have provided insights into the role of transmembrane and loop regions in the activation mechanism of T2Rs. However, studies aimed at deciphering the role of their carboxyl-terminus (C-terminus) are limited. In this study, we identified a KLK/R motif in the C-terminus that is conserved in 19 of the 25 T2Rs. Using site-directed mutagenesis we studied the role of 16 residues in the C-terminus of T2R4. The C-terminus of T2R4 is polybasic with 6 of the 16 residues consisting of lysines, constituting two separate KK motifs. We analyzed the effect of the C-terminus mutations on plasma membrane trafficking, and characterized their function in response to the T2R4 agonist quinine. The majority of the mutants showed defective receptor trafficking with ≤50% expression on the cell surface. Interestingly, mutation of the distal Lys296 of the KLK motif in T2R4 resulted in constitutive activity. The K296A mutant displayed five-fold basal activity over wild type T2R4, while the conservative substitution K296R showed wild type characteristics. The Lys294, Leu295 and Lys296 of the KLK motif in T2R4 were found to perform crucial roles, both, in receptor trafficking and function. Results from this study provide unique mechanistic insights into the structure-function role of the C-terminus in T2R signaling.

The third intracellular loop plays a critical role in bitter taste receptor activation.

Bitter taste receptors (T2Rs) belong to the superfamily of G protein-coupled receptors (GPCRs). T2Rs are chemosensory receptors with important therapeutic potential. In humans, bitter taste is perceived by 25 T2Rs, which are distinct from the well-studied Class A GPCRs. The activation mechanism of T2Rs is poorly understood and none of the structure-function studies are focused on the role of the important third intracellular loop (ICL3). T2Rs have a unique signature sequence at the cytoplasmic end of fifth transmembrane helix (TM5), a highly conserved LxxSL motif. Here, we pursue an alanine scan mutagenesis of the ICL3 of T2R4 and characterize the functionality of 23 alanine mutants. We identify four mutants, H214A, Q216A, V234A and M237A, that exhibit constitutive activity. To our surprise, the H214A mutant showed very high constitutive activity over wild type T2R4. Interestingly, His214 is highly conserved (96%) in T2Rs and is present two amino acids below the LxxSL motif in TM5. Molecular modeling shows a dynamic network of interactions involving residues in TM5-ICL3-TM6 that restrain the movement of the helices. Changes in this network, as in the case of H214A, Q216A, V234A and M237A mutants, cause the receptor to adopt an active conformation. The conserved LxxSL motif in TM5 performs both structural and functional roles in this process. These results provide insight into the activation mechanism of T2Rs, and emphasize the unique functional role of ICL3 even within the GPCR subfamilies.

Oral Lesions Associated with Systemic Disease.

Oral manifestations may be the first sign of a systemic disease, or represent lesions associated with an established or recurrent disease. Oral health care providers are often the first to recognize these signs. Some lesions have characteristic features that allow for early detection and intervention. On the contrary, clinical manifestations may be diverse and require a comprehensive evaluation to establish a definitive diagnosis. This article reviews the oral manifestations of select systemic diseases to help clinicians develop a differential diagnosis that leads to early diagnosis and timely intervention.

Structural basis of activation of bitter taste receptor T2R1 and comparison with Class A G-protein-coupled receptors (GPCRs).

The human bitter taste receptors (T2Rs) are non-Class A members of the G-protein-coupled receptor (GPCR) superfamily, with very limited structural information. Amino acid sequence analysis reveals that most of the important motifs present in the transmembrane helices (TM1-TM7) of the well studied Class A GPCRs are absent in T2Rs, raising fundamental questions regarding the mechanisms of activation and how T2Rs recognize bitter ligands with diverse chemical structures. In this study, the bitter receptor T2R1 was used to systematically investigate the role of 15 transmembrane amino acids in T2Rs, including 13 highly conserved residues, by amino acid replacements guided by molecular modeling. Functional analysis of the mutants by calcium imaging analysis revealed that replacement of Asn-66(2.65) and the highly conserved Asn-24(1.50) resulted in greater than 90% loss of agonist-induced signaling. Our results show that Asn-24(1.50) plays a crucial role in receptor activation by mediating an hydrogen bond network connecting TM1-TM2-TM7, whereas Asn-66(2.65) is essential for binding to the agonist dextromethorphan. The interhelical hydrogen bond between Asn-24(1.50) and Arg-55(2.54) restrains T2R receptor activity because loss of this bond in I27A and R55A mutants results in hyperactive receptor. The conserved amino acids Leu-197(5.50), Ser-200(5.53), and Leu-201(5.54) form a putative LXXSL motif which performs predominantly a structural role by stabilizing the helical conformation of TM5 at the cytoplasmic end. This study provides for the first time mechanistic insights into the roles of the conserved transmembrane residues in T2Rs and allows comparison of the activation mechanisms of T2Rs with the Class A GPCRs.

Structural and functional roles of small group-conserved amino acids present on helix-H7 in the ß(2)-adrenergic receptor.

Sequence analysis of the class A G protein-coupled receptors (GPCRs) reveals that most of the highly conserved sites are located in the transmembrane helices. A second level of conservation exists involving those residues that are conserved as a group characterized by small and/or weakly polar side chains (Ala, Gly, Ser, Cys, Thr). These positions can have group conservation levels of up to 99% across the class A GPCRs and have been implicated in mediating helix-helix interactions in membrane proteins. We have previously shown that mutation of group-conserved residues present on transmembrane helices H2-H4 in the ß(2)-adrenergic receptor (ß(2)-AR) can influence both receptor expression and function. We now target the group-conserved sites, Gly315(7.42) and Ser319(7.46), on H7 for structure-function analysis. Replacing Ser319(7.46) with smaller amino acids (Ala or Gly) did not influence the ability of the mutant receptors to bind to the antagonist dihydroalprenolol (DHA) but resulted in ~15-20% agonist-independent activity. Replacement of Ser319(7.46) with the larger amino acid leucine lowered the expression of the S319L mutant and its ability to bind DHA. Both the G315A and G315S mutants also exhibited agonist-independent signaling, while the G315L mutant did not show specific binding to DHA. These data indicate that Gly315(7.42) and Ser319(7.46) are stabilizing ß(2)-AR in an inactive conformation. We discuss our results in the context of van der Waals interactions of Gly315(7.42) with Trp286(6.48) and hydrogen bonding interactions of Ser319(7.46) with amino acids on H1-H2-H7 and with structural water.

Marginal linear gingival leukoplakia progressing to "ring around the collar"-An ominous sign of proliferative verrucous leukoplakia.

BACKGROUND: Potentially malignant lesions of the gingiva may frequently present as well-demarcated white lesions confined to the marginal gingiva. These lesions often become thick and verrucoid and spread along the marginal gingiva to encircle the tooth. Some cases of marginal gingival leukoplakia, over time, progress to extensively involve the gingiva fulfilling the criteria for proliferative verrucous leukoplakia (PVL). The objective of this study is to raise awareness of this pattern of leukoplakia by reporting a series of cases of marginal gingival leukoplakia. METHODS: An Institutional Review Board approved retrospective search of University of Florida and University of Nebraska Medical Center oral biopsy services was performed for all gingival biopsies. Inclusion criteria included cases exhibiting marginal gingival leukoplakia, and with accompanying clinical images. RESULTS: A total of 30 cases of marginal gingival leukoplakia were included. All cases presented as well-demarcated leukoplakias, either on the buccal or lingual gingival margin, or circumferentially forming a "ring around the collar" of single or multiple teeth. Eight patients had recurrent lesions and 12 had multifocal involvement. Six of the 12 patients with multifocal involvement presented with a "ring around the collar." The histopathologic diagnoses were representative of benign lesions in seven cases, premalignant in 13, and malignant or suggestive of malignancy in 10 cases. Seven patients had carcinoma at the time of first biopsy, whereas 6 cases showed progression at time of follow-up. CONCLUSION: This study aims to raise awareness that marginal gingival leukoplakia may represent potentially malignant lesions, and if circumferential and/or thick, may be the first manifestation of PVL.

Proliferative Verrucous Leukoplakia: An Expert Consensus Guideline for Standardized Assessment and Reporting.

The many diverse terms used to describe the wide spectrum of changes seen in proliferative verrucous leukoplakia (PVL) have resulted in disparate clinical management. The objective of this study was to produce an expert consensus guideline for standardized assessment and reporting by pathologists diagnosing PVL related lesions. 299 biopsies from 84 PVL patients from six institutions were selected from patients who had multifocal oral leukoplakic lesions identified over several years (a minimum follow-up period of 36 months). The lesions demonstrated the spectrum of histologic features described in PVL, and in some cases, patients developed oral cavity squamous cell carcinoma (SCC). An expert working group of oral and maxillofacial and head and neck pathologists reviewed microscopic features in a rigorous fashion, in combination with review of clinical photographs when available. The working group then selected 43 single slide biopsy cases for whole slide digital imaging (WSI) review by members of the consensus conference. The digital images were then reviewed in two surveys separated by a washout period of at least 90 days. Five non-PVL histologic mimics were included as controls. Cases were re-evaluated during a consensus conference with 19 members reporting on the cases. The best inter-observer diagnostic agreement relative to PVL lesions were classified as "corrugated ortho(para)hyperkeratotic lesion, not reactive" and "SCC" (chi-square p = 0.015). There was less than moderate agreement (kappa < 0.60) for lesions in the "Bulky hyperkeratotic epithelial proliferation, not reactive" category. There was ≥ moderate agreement (> 0.41 kappa) for 35 of 48 cases. This expert consensus guideline has been developed with support and endorsement from the leadership of the American Academy of Oral and Maxillofacial Pathology and the North American Society of Head and Neck Pathologists to recommend the use of standardized histopathologic criteria and descriptive terminology to indicate three categories of lesions within PVL: (1) "corrugated ortho(para)hyperkeratotic lesion, not reactive;" (2) "bulky hyperkeratotic epithelial proliferation, not reactive;" and (3) "suspicious for," or "squamous cell carcinoma." Classification of PVL lesions based on a combination of clinical findings and these histologic descriptive categories is encouraged in order to standardize reporting, aid in future research and potentially guide clinical management.

Bitter taste receptor T2R1 is activated by dipeptides and tripeptides.

Bitter taste signaling in humans is mediated by a group of 25 bitter receptors (T2Rs) that belong to the G-protein coupled receptor (GPCR) family. Previously, several bitter peptides were isolated and characterized from bitter tasting food protein derived extracts, such as pea protein and soya bean extracts. However, the molecular targets or receptors in humans for these bitter peptides were poorly characterized and least understood. In this study, we tested the ability of the bitter tasting tri- and di-peptides to activate the human bitter receptor, T2R1. In addition, we tested the ability of peptide inhibitors of the blood pressure regulatory protein, angiotensin converting enzyme (ACE) to activate T2R1. Using a heterologous expression system, T2R1 gene was transiently expressed in C6-glioma cells and changes in intracellular calcium was measured following addition of the peptides. We found that the bitter tasting tri-peptides are more potent in activating T2R1 than the di-peptides tested. Among the peptides examined, the bitter tri-peptide Phe-Phe-Phe (FFF), is the most potent in activating T2R1 with an EC50 value in the micromolar range. Furthermore, to elucidate the potential ligand binding pocket of T2R1 we used homology molecular modeling. The molecular models showed that the bitter peptides bind within the same binding pocket on the receptor. The ligand binding pocket in T2R1 is present on the extracellular surface of the receptor, and is formed by the transmembrane helices 1, 2, 3 and 7 and with extracellular loops 1 and 2 forming a cap like structure on the binding pocket.

Pigmented lateral periodontal cyst: A case report and review of pigmented odontogenic cysts.

Pigmented odontogenic cysts are uncommon and to date, only 37 cases have been reported in the English literature. Here, we report a case of a pigmented lateral periodontal cyst (LPC) in the maxilla of a 48-year-old female. The patient presented with clinical swelling in the maxillary anterior region. Microscopic features of the biopsied specimen were consistent with a diagnosis of LPC. The epithelial cyst lining exhibited numerous coarse granules of melanin pigment, which was confirmed by S-100 immunohistochemistry and Fontana-Masson bleach histochemical method. Almost all documented cases of pigmented odontogenic cysts have occurred in Asians and African-Americans, with only three cases in white patients. Racial pigmentation may have a role in the pathogenesis of these lesions. Although the origin and pathologic significance of melanocytes in these pigmented intraosseous lesions cannot be explained, it may be something to consider for investigation in future.

Inter-observer Variability in the Diagnosis of Proliferative Verrucous Leukoplakia: Clinical Implications for Oral and Maxillofacial Surgeon Understanding: A Collaborative Pilot Study.

The use of diverse terminology may lead to inconsistent diagnosis and subsequent mistreatment of lesions within the proliferative verrucous leukoplakia (PVL) spectrum. The objectives of this study were: (a) to measure inter-observer variability between a variety of pathologists diagnosing PVL lesions; and (b) to evaluate the impact of diverse terminologies on understanding, interpretation, and subsequent treatment planning by oral and maxillofacial surgeons (OMFS). Six oral pathologists (OP) and six head and neck pathologists (HNP) reviewed 40 digitally scanned slides of PVL-type lesions. Inter-observer agreement on diagnoses was evaluated by Fleiss' kappa analysis. The most commonly used diagnostic terminologies were sent to ten OMFS to evaluate their resulting interpretations and potential follow-up treatment approaches. The overall means of the surgeons' responses were compared by Student t test. There was poor inter-observer agreement between pathologists on the diagnosis of PVL lesions (κ = 0.270), although there was good agreement (κ = 0.650) when diagnosing frankly malignant lesions. The lowest agreement was in diagnosing verrucous hyperplasia (VH) with/without dysplasia, atypical epithelial proliferation (AEP), and verrucous carcinoma (VC). The OMFS showed the lowest agreement on identical categories of non-malignant diagnoses, specifically VH and AEP. This study demonstrates a lack of standardized terminology and diagnostic criteria for the spectrum of PVL lesions. We recommend adopting standardized criteria and terminology, proposed and established by an expert panel white paper, to assist pathologists and clinicians in uniformly diagnosing and managing PVL spectrum lesions.

Demographic, Clinical and Histopathological Features of Oral Neural Neoplasms: A Retrospective Study.

Intraoral neural neoplasms though unusual may be clinically significant. The aim of this study was to categorize and evaluate oral neural tumors in a large oral pathology biopsy service. With IRB approval, a retrospective search of all neural neoplasms of the oral cavity in the archives of the University of Florida Oral Pathology Biopsy Service spanning from 1994 to 2015 was performed. Extraoral cases as well as cases with insufficient patient information were excluded. A total of 340 out of 164,578 submitted specimens in a 22 year period (0.2%) were included with a mean age of 43.3 years (range: 6-89), and 44% male and 56% female. The most commonly affected locations were: tongue (37.5%), palate (22%), lip (19%), and gingiva (14%). The microscopic diagnoses rendered, in descending order of frequency were: neurofibromas (NFs): 123 (36%), granular cell tumor (GCT): 108 (32%), schwannomas: 61 (17%), palisaded encapsulated neuromas: 39 (11%), benign neural lesion not otherwise specified: 8 (2%), and mucosal neuroma c/w multiple endocrine neoplasia type 2B (MEN 2B): 1 (< 0.5%). Six cases of NF reported a history of neurofibromatosis Type 1 (NF 1). Four cases showed multifocal lesions. Immunohistochemical staining was performed on equivocal cases (25% of the lesions) and all were confirmed by their S-100 positivity. Intraoral neural neoplasms, though uncommon should be in the differential diagnosis of oral soft tissue entities and specific consideration to syndromal linkage is paramount as this may impact patient management.

Hybrid Central Odontogenic Fibroma with Giant Cell Granuloma like Lesion: A Report of Three Additional Cases and Review of the Literature.

Central odontogenic fibroma (COF) is an uncommon intraosseous neoplasm of the gnathic bones which is composed of fibrous connective tissue, with or without calcifications, and variable amounts of inactive odontogenic epithelium. It makes up less than 5% of odontogenic tumors and is more commonly seen in females. Central giant cell granuloma (CGCG) is a locally destructive but benign lesion of the jaws containing osteoclast-like multinucleated giant cells in a fibrovascular stroma. CGCG makes up approximately 10% of all benign jaw tumors and typically occurs in females younger than 30 years of age. A hybrid lesion with histologic features of both COF and CGCG is very rare and was first described in 1992. To date, fewer than 50 cases of this lesion have been reported. In this study, we present three additional cases of COF developing in conjunction with giant cell granuloma-like lesion, as well as provide a comprehensive literature review. Two of the lesions presented in our study were located in the posterior mandible and one occurred in the anterior mandible. Buccal and/or lingual expansion was noted in two patients and no recurrence was reported. Histologically, all three lesions demonstrated a blend of odontogenic epithelial islands with numerous multinucleated giant cells in a highly cellular connective tissue stroma. Immunohistochemical staining with CK19 and CD68 highlighted the odontogenic epithelium and multinucleated giant cells respectively. The precise nature of these hybrid lesions remains obscure and additional molecular studies may be of help in understanding their pathogenesis.

A Retrospective 20-Year Analysis of Proliferative Verrucous Leukoplakia and Its Progression to Malignancy and Association with High-risk Human Papillomavirus.

Proliferative verrucous leukoplakia (PVL) is defined as an aggressive, relentless and recalcitrant form of leukoplakia that has a high propensity for malignant transformation. The aim of this study was to evaluate the malignant potential of PVL and determine its possible association with high-risk human papillomavirus (HPV). Twenty cases with a clinical and biopsy proven diagnosis of PVL were collected from the University of Florida Oral Medicine clinic database. Immunohistochemistry was performed to evaluate the expression of p16INK4A and p53 genes in the PVL lesions. The lesions were also tested for high-risk HPV by DNA in-situ hybridization. The average age of the patients at the time of first biopsy was 62.7 years. Most patients had multiple sites of involvement, gingiva being the most common location. The lesions progressed to malignancy in approximately 50% of patients. The expression of p16INK4A gene was considered negative, with at least a 50-65% immunoreactivity observed in only three cases that progressed to malignancy. No expression of high-risk HPV was detected, whereas p53 staining was positive in less than 25% of the cells demonstrating gene expression. No definite association between PVL and high-risk HPV infection could be established. Due to the high transformation potential of PVL, early recognition with aggressive treatment, including multiple biopsies, and continued close clinical follow-up, remain the mainstay of favorable management of this condition.

Papillon-Lefèvre syndrome: A series of three cases in the same family and a literature review.

Papillon-Lefèvre syndrome (PLS) is a rare autosomal recessive disorder that exhibits palmoplantar keratosis and early severe periodontitis. The oral disease affects both the primary and permanent dentitions leading to premature exfoliation of teeth. Various etiologic factors, such as genetic mutations, immunologic alterations, and bacteria have been implicated in PLS. Genetic mutations leading to the loss of function of cathepsin C (CTSC) gene, located on chromosome 11q14, is considered pivotal in this condition. The present case series describes PLS in three siblings, with consanguineously married parents, who live in a remote area of Yemen. The affected children presented with prominent palmoplantar keratosis and early periodontitis with only a few remaining teeth. The severity of skin lesions in all patients exhibited seasonal variations. Based on their clinical findings, a diagnosis of PLS was made. Dentists have a significant role in the early diagnosis and management of PLS patients.

Bitter taste receptors: Novel insights into the biochemistry and pharmacology.

Bitter taste receptors (T2Rs) belong to the super family of G protein-coupled receptors (GPCRs). There are 25 T2Rs expressed in humans, and these interact with a large and diverse group of bitter ligands. T2Rs are expressed in many extra-oral tissues and can perform diverse physiological roles. Structure-function studies led to the identification of similarities and dissimilarities between T2Rs and Class A GPCRs including amino acid conservation and novel motifs. However, the efficacy of most of the T2R ligands is not yet elucidated and the biochemical pharmacology of T2Rs is poorly understood. Recent studies on T2Rs characterized novel ligands including blockers for these receptors that include inverse agonist and antagonists. In this review we discuss the techniques used for elucidating bitter blockers, concept of ligand bias, generic amino acid numbering, the role of cholesterol, and conserved water molecules in the biochemistry and pharmacology of T2Rs.

The Pharmacochaperone Activity of Quinine on Bitter Taste Receptors.

Bitter taste is one of the five basic taste sensations which is mediated by 25 bitter taste receptors (T2Rs) in humans. The mechanism of bitter taste signal transduction is not yet elucidated. The cellular processes underlying T2R desensitization including receptor internalization, trafficking and degradation are yet to be studied. Here, using a combination of molecular and pharmacological techniques we show that T2R4 is not internalized upon agonist treatment. Pretreatment with bitter agonist quinine led to a reduction in subsequent quinine-mediated calcium responses to 35 ± 5% compared to the control untreated cells. Interestingly, treatment with different bitter agonists did not cause internalization of T2R4. Instead, quinine treatment led to a 2-fold increase in T2R4 cell surface expression which was sensitive to Brefeldin A, suggesting a novel pharmacochaperone activity of quinine. This phenomenon of chaperone activity of quinine was also observed for T2R7, T2R10, T2R39 and T2R46. Our results suggest that the observed action of quinine for these T2Rs is independent of its agonist activity. This study provides novel insights into the pharmacochaperone activity of quinine and possible mechanism of T2R desensitization, which is of fundamental importance in understanding the mechanism of bitter taste signal transduction.