Steviol glycosides (SGs), such as stevioside and rebaudioside A, are natural, non-caloric sweet-tasting organic molecules, present in extracts of the scrub plant Stevia rebaudiana, which are widely used as sweeteners in consumer foods and beverages. TRPM5 is a Ca2+-activated cation channel expressed in type II taste receptor cells and pancreatic β-cells. Here we show that stevioside, rebaudioside A and their aglycon steviol potentiate the activity of TRPM5. We find that SGs potentiate perception of bitter, sweet and umami taste, and enhance glucose-induced insulin secretion in a Trpm5-dependent manner. Daily consumption of stevioside prevents development of high-fat-diet-induced diabetic hyperglycaemia in wild-type mice, but not in Trpm5−/− mice. These results elucidate a molecular mechanism of action of SGs and identify TRPM5 as a potential target to prevent and treat type 2 diabetes.

The active form of vitamin D, 1,25(OH)2D3, can prevent various forms of experimentally induced autoimmune disorders. The aim of this study was to confirm these findings in NOD mice that spontaneously develop an autoimmune type of diabetes mellitus. Therefore, the effect of a long-term 1,25(OH)2D3 treatment on the incidence of insulitis, the histological lesion preceding diabetes, was studied. Forty-three NOD mice were treated with 1,25(OH)2D3 (5 micrograms/kg) i.p. every other day from age 21 days on, when no insulitis was present yet. At day 100, 16 control mice receiving the treatment vehicle (arachis oil) had an incidence of insulitis of 75%, whereas only 41% of the 1,25(OH)2D3-treated animals developed insulitis (P < 0.025). Calcemia, determined 24 h after the last 1,25(OH)2D3 injection was 2.5 +/- 0.1 mM, which was higher than in control animals (2.3 +/- 0.1 mM), but was well tolerated. Cellular immunity, as assessed with the mixed lymphocyte reaction performed at day 100, was not impaired significantly. This study demonstrates that long-term treatment with high doses of 1,25(OH)2D3 is able to decrease the incidence of insulitis in spontaneous autoimmune diabetes without major side effects.

Early graft failure, graft rejection, and autoimmune recurrence remain unresolved issues in islet xenotransplantation in type 1 diabetes. The first aim of this study was to examine the existence of early graft failure in spontaneously diabetic autoimmune NOD mice after rat islet transplantation under technically controlled circumstances. The second aim was to examine the mediators of this early xenograft dysfunction. First, we demonstrated a higher percentage of early xenograft failure (48%) in spontaneously diabetic NOD mice as compared with chemically diabetic old NOD (13%, P < 0.05) and C57Bl/6 (7%, P < 0.01) mice. In addition, in spontaneously diabetic NOD mice, xenogeneic islets displayed early graft failure more frequently than allogeneic (23%, P ≤ 0.05) or isogeneic islets (7%, P < 0.01). No early graft failure was observed in allotransplantation or isotransplantation in chemically diabetic mice. Reverse transcriptase-polymerase chain reaction analysis of cytokine mRNA in islet xenografts 8 h after transplantation showed higher levels of interleukin (IL)-1 mRNA in autoimmune diabetic mice compared with chemically diabetic old NOD mice (1.40 ± 0.32 vs. 0.90 ± 0.14 IL-1 copies/␤-actin copies, P < 0.05). In contrast, mRNA levels of transforming growth factor (TGF)-␤ were lower in spontaneously diabetic NOD mice than in chemically diabetic old NOD mice (0.67 ± 0.16 vs. 1.36 ± 0.50 TGF-␤ copies/␤-actin copies, P < 0.05). No differences in tumor necrosis factor-␣, IL-6, and inducible nitric oxide synthase were seen between autoimmune and nonautoimmune diabetic mice. T-cell cytokines (IL-2, IL-4, IL-10, and ␥-interferon) were absent in all mice until 48 h after transplantation. These data suggest that early islet xenograft failure is more common in spontaneously diabetic NOD mice and could be due to a nonspecific inflammatory reaction locally in the grafts.

+ T cells in pancreatic draining lymph nodes. In conclusion, this study shows for the first time that high doses of regular dietary vitamin D 3 can safely prevent diabetes in NOD mice when administered lifelong, although caution is warranted with regards to administering equivalently high doses in humans.