Distinct SERT Function in Feeding and Obesity HFD- and ND-fed SERT2/2 mice was not statistically significant (p = 0.06, Student’s t-test). H. Whole-body fat content at age of 6 months. Fat content was increased in SERT2/2 mice fed ND and HFD compared to corresponding WT mice. I. Absolute lean mass at age of 6 months. Lean mass was reduced in HFD-fed SERT2/2 mice, as compared to HFD-fed WT mice. Data in panels A–I are presented as means 6 SEM. *p,0.05, **p,0.01, ***p,0.001, Student’s t-test. The number of animals analyzed is indicated in parentheses. J and K. Lipid content detected by Oil Red O staining of liver sections of WT and SERT2/2 mice. Scale bar, 50 mm. doi:10.1371/journal.pone.0032511.g002 HFD-fed WT mice during ITT (Figure 3D). Remarkably, 3- month old SERT2/2 mice did not exhibit an increase in body weight and body fat content (Figure 2E, F) but they did manifest glucose and insulin intolerance, suggesting that the metabolic deficits develop prior to a significant increase in the adiposity. SERT-deficient mice exhibit hyperinsulinemia and hypertrophic pancreatic islets The poor glucose handling in SERT-deficient mice could be completely ascribed to impaired insulin sensitivity. Alternatively, SERT deficiency could result in reduced pancreatic beta-cell secretion of insulin. To test the second possibility, we analyzed insulin secretion in SERT-deficient mice. We found that fasting plasma insulin levels were elevated in both SERT2/2 and SERT+/2 mice at the age of 3 and 6 months kept on ND, although the increase in 3-month old SERT+/2 mice was not statistically significant (Figure 4A, B). To evaluate the impact of SERT deficiency on glucose-induced insulin secretion, we measured plasma insulin levels following bolus injection of glucose. Glucose induced insulin in both SERT-deficient and WT mice as measured at 15 min and 30 min post glucose injection (Figure 4C). While the absolute values of plasma insulin in SERT2/2 and SERT+/2 mice were higher than that in WT mice, the fold- increase of insulin was similar among tested strains. Thus, SERT deficiency does not attenuate insulin secretion, but it causes hyperinsulinemia. Figure 3. Characterization of glucose homeostasis in SERT-deficient mice. A. GTT (i.p., 1 g/kg) of 3-month-old mice after 16 h fast. B. GTT (i.p., 1 g/kg) of 6-month-old mice after 16 h fast. C. ITT (i.p., 1 U/kg) of 3-month-old mice after 6 h fast. D. ITT (i.p., 1 U/kg) of 6-month-old mice after 6 h fast. N, number of mice analyzed for each genotype. Data represent mean 6 SEM. Student’s t-test was used to analyze statistical significance of the difference between the following groups: SERT2/2 vs. WT mice on ND feeding, *p,0.05, **p,0.01, ***p,0.001; SERT2/2 vs. WT mice on HFD feeding, #p,0.05, ##p,0.01, Student’s t-test. At 3 months of age, the differences in GTT of SERT+/2 vs. WT mice were not statistically significant; however, there were significant differences between SERT+/2 vs. WT mice in ITT. At 6 months of age, both SERT2/2 and SERT+/2 mice differed significantly from WT mice in GTT and ITT. doi:10.1371/journal.pone.0032511.g003 PLoS ONE | www.plosone.org 4 March 2012 | Volume 7 | Issue 3 | e32511