Probes for INSULIN

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 (KATP) responsible for depolarizing the insulin secreting beta (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 until now. Using immunofluorescence, we demonstrate the presence of insulin in mouse, rat and human taste bud cells. We further prove that insulin is synthesized in individual taste buds and not taken up from the parenchyma by: detection of the post-processing insulin molecule C-peptide and green fluorescence protein (GFP) in taste cells of both insulin 1- and insulin 2-GFP mice, and the presence of the mouse insulin transcript by in situ hybridization (ISH). In addition to our cytology data we measured the level of insulin transcript by qRT-PCR in the anterior and posterior lingual epithelium. These analyses show insulin is translated in the circumvallate and foliate papillae in the posterior but only insulin transcript was detected in the anterior fungiform papillae of rodent tongue. Thus, some taste cells are insulin synthesizing cells generated from a continually replenished source of precursor cells in adult mammalian lingual epithelium.

Inactivating mutations in the insulin receptor results in extreme insulin resistance. The resulting hyperglycemia is very difficult to treat, and patients are at risk for early morbidity and mortality from complications of diabetes. We used the insulin receptor antagonist S961 to induce severe insulin resistance, hyperglycemia, and ketonemia in mice. Using this model, we show that glucagon receptor (GCGR) inhibition with a monoclonal antibody normalized blood glucose and β-hydroxybutyrate levels. Insulin receptor antagonism increased pancreatic β-cell mass threefold. Normalization of blood glucose levels with GCGR-blocking antibody unexpectedly doubled β-cell mass relative to that observed with S961 alone and 5.8-fold over control. GCGR antibody blockage expanded α-cell mass 5.7-fold, and S961 had no additional effects. Collectively, these data show that GCGR antibody inhibition represents a potential therapeutic option for treatment of patients with extreme insulin-resistance syndromes.

SLC30A8 encodes a zinc transporter that is primarily expressed in the pancreatic islets of Langerhans. In β-cells it transports zinc into insulin-containing secretory granules. Loss-of-function (LOF) mutations in SLC30A8 protect against type 2 diabetes in humans. In this study, we generated a knockin mouse model carrying one of the most common human LOF mutations for SLC30A8, R138X. The R138X mice had normal body weight, glucose tolerance, and pancreatic β-cell mass. Interestingly, in hyperglycemic conditions induced by the insulin receptor antagonist S961, the R138X mice showed a 50% increase in insulin secretion. This effect was not associated with enhanced β-cell proliferation or mass. Our data suggest that the SLC30A8 R138X LOF mutation may exert beneficial effects on glucose metabolism by increasing the capacity of β-cells to secrete insulin under hyperglycemic conditions.