From a pool of cells, the single-cell model we developed is representative of average b-cell behavior, neglecting the intrinsic heterogeneity of the cell population. Stochastic noise is also averaged in the experimental cell population, further justifying our deterministic modeling approach. The full model includes the regulation of GLUT1 and GLUT2 genes by the transcription factors HNF1A and FOXA2. MedChemExpress BIBS39 Specifically, we described the POR8 site production and degradation of HNF1A and FOXA2 at the RNA and protein level, and the translocationModeling Glucose Transport in Pancreatic b-CellsFigure 2. Comparison of glucose transport and phosphorylation in health and T2D b-cells. (A) GLUT-1 and GLUT-2 outwards rates (v{G1 and v{G2 , respectively), and GK kinetics as a function of intra-cellular glucose concentration for normal (e 1) glucose transporters’ expression and reduced GLUT-2 expression (e 0:20). (B) Steady-state GK rate calculated at 16.8 mM extra-cellular glucose concentration as a function of concerted GLUT-1 and GLUT-2 deficiency. e1 and e2 are the fractions of GLUT-1 and GLUT-2, respectively, compared to normal. The solid line indicates the threshold between transport- and phosphorylation-limited G6P formation (derived in (A)), whereas asterisks indicate the positions of T2D patients bcells. Inset represents an enlargement of the lower left region. doi:10.1371/journal.pone.0053130.gof the two proteins to the nucleus where they transactivate their target genes (modules I and II in Figure 3) [8,19]. Besides GLUT1 and GLUT2, HNF1A and FOXA2 also regulate MGAT4A, another gene particularly relevant for b-cell glucose entry, as discussed below. The transcription of these three genes, GLUT1, GLUT2 and MGAT4A, includes two layers of regulation: first, HNF1A induces histone hyperacetylation at target gene promoter nucleosomes [7,8,19,20]; and second, HNF1A and FOXA2 bind to target gene promoter sequences and promote transcription [7,8,21] (modules III and IV). GLUT-1 and GLUT-2 are regulated also at the posttranslational level, by protein glycosylation. In particular, glucose transporter residency at the b-cell plasma membrane requires a specific N-glycan structure produced on both transporters by the Golgi-resident GNT-4A glycosyltransferase enzyme, the 24195657 product of MGAT4A gene, [7,8,22] (module V). This post-translational modification promotes GLUT-1 and GLUT-2 interaction with one or more lectins at the plasma membrane and maintains their residency at the membrane by a mechanism competing with normal endocytic internalization and degradation rates. Thus, despite cycles of production and degradation, GLUT-1 and GLUT-2 glycoproteins are steadily present at the b-cell plasma membrane in healthy individuals [23]. The model includes these post-translational regulation steps, as schematically shown in Figure 3, where GLUT-1 and GLUT-2 are simply identified as `glycosylated’ or `unglycosylated’, according to the presence or absence of the 11967625 GNT-4A-dependent N-glycan modification, although N-glycosylation comprises multiple different N-glycan structures on the glucose transporters. We assumed the same kinetic rates for GLUT-1 and GLUT-2 interactions with MGAT4A and lectins. Thus, in the model, differences in the concentration of the two transporters at the membrane are the result of differences in transcription. Within the network considered, our previous experimental work showed that both b-cells from T2D donors and b-cells from healthy donors treated with pal.From a pool of cells, the single-cell model we developed is representative of average b-cell behavior, neglecting the intrinsic heterogeneity of the cell population. Stochastic noise is also averaged in the experimental cell population, further justifying our deterministic modeling approach. The full model includes the regulation of GLUT1 and GLUT2 genes by the transcription factors HNF1A and FOXA2. Specifically, we described the production and degradation of HNF1A and FOXA2 at the RNA and protein level, and the translocationModeling Glucose Transport in Pancreatic b-CellsFigure 2. Comparison of glucose transport and phosphorylation in health and T2D b-cells. (A) GLUT-1 and GLUT-2 outwards rates (v{G1 and v{G2 , respectively), and GK kinetics as a function of intra-cellular glucose concentration for normal (e 1) glucose transporters’ expression and reduced GLUT-2 expression (e 0:20). (B) Steady-state GK rate calculated at 16.8 mM extra-cellular glucose concentration as a function of concerted GLUT-1 and GLUT-2 deficiency. e1 and e2 are the fractions of GLUT-1 and GLUT-2, respectively, compared to normal. The solid line indicates the threshold between transport- and phosphorylation-limited G6P formation (derived in (A)), whereas asterisks indicate the positions of T2D patients bcells. Inset represents an enlargement of the lower left region. doi:10.1371/journal.pone.0053130.gof the two proteins to the nucleus where they transactivate their target genes (modules I and II in Figure 3) [8,19]. Besides GLUT1 and GLUT2, HNF1A and FOXA2 also regulate MGAT4A, another gene particularly relevant for b-cell glucose entry, as discussed below. The transcription of these three genes, GLUT1, GLUT2 and MGAT4A, includes two layers of regulation: first, HNF1A induces histone hyperacetylation at target gene promoter nucleosomes [7,8,19,20]; and second, HNF1A and FOXA2 bind to target gene promoter sequences and promote transcription [7,8,21] (modules III and IV). GLUT-1 and GLUT-2 are regulated also at the posttranslational level, by protein glycosylation. In particular, glucose transporter residency at the b-cell plasma membrane requires a specific N-glycan structure produced on both transporters by the Golgi-resident GNT-4A glycosyltransferase enzyme, the 24195657 product of MGAT4A gene, [7,8,22] (module V). This post-translational modification promotes GLUT-1 and GLUT-2 interaction with one or more lectins at the plasma membrane and maintains their residency at the membrane by a mechanism competing with normal endocytic internalization and degradation rates. Thus, despite cycles of production and degradation, GLUT-1 and GLUT-2 glycoproteins are steadily present at the b-cell plasma membrane in healthy individuals [23]. The model includes these post-translational regulation steps, as schematically shown in Figure 3, where GLUT-1 and GLUT-2 are simply identified as `glycosylated’ or `unglycosylated’, according to the presence or absence of the 11967625 GNT-4A-dependent N-glycan modification, although N-glycosylation comprises multiple different N-glycan structures on the glucose transporters. We assumed the same kinetic rates for GLUT-1 and GLUT-2 interactions with MGAT4A and lectins. Thus, in the model, differences in the concentration of the two transporters at the membrane are the result of differences in transcription. Within the network considered, our previous experimental work showed that both b-cells from T2D donors and b-cells from healthy donors treated with pal.
