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We and others have reported that taste cells in taste buds express many peptides in common with cells in the gut and islets of Langerhans in the pancreas. Islets and taste bud cells express the hormones glucagon and ghrelin, the same ATP-sensitive potassium channel responsible for depolarizing the insulin-secreting β cell during glucose-induced insulin secretion, as well as the propeptide-processing enzymes PC1/3 and PC2. Given the common expression of functionally specific proteins in taste buds and islets, it is surprising that no one has investigated whether insulin is synthesized in taste bud cells. Using immunofluorescence, we demonstrated the presence of insulin in mouse, rat, and human taste bud cells. By detecting the postprocessing insulin molecule C-peptide and green fluorescence protein (GFP) in taste cells of both insulin 1-GFP and insulin 2-GFP mice and the presence of the mouse insulin transcript by in situ hybridization, we further proved that insulin is synthesized in individual taste buds and not taken up from the parenchyma. In addition to our cytology data, we measured the level of insulin transcript by quantitative RT-PCR in the anterior and posterior lingual epithelia. These analyses showed that insulin is translated in the circumvallate and foliate papillae in the posterior, but only insulin transcript was detected in the anterior fungiform papillae of the rodent tongue. Thus, some taste cells are insulin-synthesizing cells generated from a continually replenished source of precursor cells in the adult mammalian lingual epithelium.

FOP is a disabling disorder in which skeletal muscle is progressively replaced with bone. Lymphocytes, our model system for examining BMP signaling, cannot signal through the canonical Smad pathway unless exogenous Smad1 is supplied, providing a unique cell type in which the BMP–p38 MAPK pathway can be examined. FOP lymphocytes exhibit defects in the BMP–p38 MAPK pathway, suggesting that altered BMP signaling underlies ectopic bone formation in this disease.

FOP is a disabling disorder in which skeletal muscle is progressively replaced with bone. Lymphocytes, our model system for examining BMP signaling, cannot signal through the canonical Smad pathway unless exogenous Smad1 is supplied, providing a unique cell type in which the BMP–p38 MAPK pathway can be examined. FOP lymphocytes exhibit defects in the BMP–p38 MAPK pathway, suggesting that altered BMP signaling underlies ectopic bone formation in this disease.Introduction:Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by progressive heterotopic ossification of connective tissues. Whereas the primary genetic defect in this condition is unknown, BMP4 mRNA and protein and BMP receptor type IA (BMPRIA) protein are overexpressed in cultured lymphocytes from FOP patients, supporting that altered BMP signaling is involved in this disease. In this study, we examined downstream signaling targets to study the BMP–Smad and BMP–p38 mitogen‐activated protein kinase (MAPK) pathways in FOP.Materials and Methods:Protein phosphorylation was assayed by immunoblots, and p38 MAPK activity was measured by kinase assays. To examine BMP target genes, the mRNA expression of ID1, ID3, and MSX2was determined by quantitative real‐time PCR. Statistical analysis was performed using Student's t‐test or ANOVA.Results:FOP lymphocytes exhibited increased levels of p38 phosphorylation and p38 MAPK activity in response to BMP4 stimulation. Furthermore, in response to BMP4, FOP cells overexpressed the downstream signaling targets ID1by 5‐fold and ID3by 3‐fold compared with controls. ID1 and ID3 mRNA induction was specifically blocked with a p38 MAPK inhibitor, but not extracellular signal‐related kinase (ERK) or c‐Jun N‐terminal kinase (JNK) inhibitors. MSX2, a known Smad pathway target gene, is not upregulated in control or FOP cells in response to BMP, suggesting that lymphocytes do not use this limb of the BMP pathway. However, introduction of Smad1 into lymphocytes made the cells competent to regulate MSX2 mRNA after BMP4 treatment.Conclusions:Lymphocytes are a cell system that signals primarily through the BMP–p38 MAPK pathway rather than the BMP–Smad pathway in response to BMP4. The p38 MAPK pathway is dysregulated in FOP lymphocytes, which may play a role in the pathogenesis of FOP.

FOP is a disorder in which skeletal muscle is progressively replaced with bone. FOP lymphocytes, a model system for exploring the BMP pathway in these patients, exhibit a defect in BMPRIA internalization and increased activation of downstream signaling, suggesting that altered BMP receptor trafficking underlies ectopic bone formation in this disease.

FOP is a disorder in which skeletal muscle is progressively replaced with bone. FOP lymphocytes, a model system for exploring the BMP pathway in these patients, exhibit a defect in BMPRIA internalization and increased activation of downstream signaling, suggesting that altered BMP receptor trafficking underlies ectopic bone formation in this disease.Introduction:Fibrodysplasia ossificans progressiva (FOP) is a severely disabling disorder characterized by progressive heterotopic ossification of connective tissues. Whereas the genetic defect and pathophysiology of this condition remain enigmatic, BMP4 mRNA and protein are overexpressed, and mRNAs for a subset of secreted BMP antagonists are not synthesized at appropriate levels in cultured lymphocytes from FOP patients. These data suggest involvement of altered BMP signaling in the disease. In this study, we investigate whether the abnormality is associated with defective BMP receptor function in lymphocytes.Materials and Methods:Cell surface proteins were quantified by fluorescence‐activated cell sorting (FACS). Protein phosphorylation was assayed by immunoprecipitation and immunoblotting. Protein synthesis and degradation were examined by [35S]methionine labeling and pulse‐chase assays. mRNA was detected by RT‐PCR.Results:FOP lymphocytes expressed 6‐fold higher levels of BMP receptor type IA (BMPRIA) on the cell surface compared with control cells and displayed a marked reduction in ligand‐stimulated internalization and degradation of BMPRIA. Moreover, in control cells, BMP4 treatment increased BMPRIA phosphorylation, whereas BMPRIA showed ligand‐insensitive constitutive phosphorylation in FOP cells. Our data additionally support that the p38 mitogen‐activated protein kinase (MAPK) signaling pathway is a major BMP signaling pathway in these cell lines and that expression of inhibitor of DNA binding and differentiation 1 (ID‐1), a transcriptional target of BMP signaling, is enhanced in FOP cells.Conclusions:These data extend our previous observations of misregulated BMP4 signaling in FOP lymphocytes and show that cell surface overabundance and constitutive phosphorylation of BMPRIA are associated with a defect in receptor internalization. Altered BMP receptor trafficking may play a significant role in FOP pathogenesis.