Computational Modelling of the Role of GLUT2 in Glucose Uptake by Intestinal Epithelial Cells

Abstract

I. INTRODUCTION There is controversy over the role of GLUT2 in glucose uptake in the small intestine. For many years there was only one mechanism suggested for the glucose absorption which was through sodium/glucose cotransporter (SGLT1) in the apical side of the epithelial cells and extrusion via the glucose transporter GLUT2 in the basolateral membrane [1]. Kellett in 2000 [2] suggested that the non-saturable component of glucose uptake at high luminal glucose concentrations is because of existence of GLUT2 in the apical membrane of the epithelial cell. Zheng et al. measured glucose uptake in enterocyte-like cell culture models in response to varying apical glucose stimuli and concluded that apical GLUT2 is required to explain the data [3]. Other studies have suggested that only the sodium-dependent glucose cotransporter SGLT1 is involved in apical uptake [4]. We have developed a computational model of glucose uptake in order to study the characteristics of transport in the presence and absence of apical GLUT2. II. MATERIAL AND METHOD The model was implemented in CellML. The transporters included are: ENaC, CFTR, BK, GLUT2, SGLT1, NHE3, AE1, Na/K pump, NBC and NKCC1 which have been detected in intestinal epithelial cells [5]. Existing mathematical models of each transporter from the Physiome Model Repository (models.physiomeproject.org) were combined to create a composite model of an intestinal epithelial cell. The model was simulated in the OpenCOR environment (www.opencor.ws) to determine intracellular concentrations, membrane potentials and fluxes through the transporters.We simulated the experiments of Zheng et al [4] with and without apical GLUT2 in order to determine the contribution of each pathway to glucose uptake. Model predictions were compared against measurements in the CaCo2 and IEC6 cell lines at different time points and glucose stimuli. III. RESULTS Figure 1A presents the experiment results at 600 seconds on CaCo2 along with model results with and without GLUT2. The results indicate that the measurements are consistent with glucose absorption through both SGLT1 and GLUT2. According to Figure 1B SGLT1 is saturated at concentrations around 15 mM and glucose uptake not increased with higher value of extracellular glucose, whereas GLUT2 flux increases with higher concentrations. This suggests that the majority of glucose uptake at low concentrations (< 10 mM) is due to SGLT1 while GLUT2 dominates at high concentrations. Fig. 1. (A) Intracellular glucose concentrations after 600 s exposure to varying glucose stimuli. (B) Model predictions of steady state glucose flux through SGLT1 and GLUT2. IV. DISCUSSION AND CONCLUSION The model used reported values on transporter densities and kinetic constants and hence did not require any fitting to the experiments. This improves confidence in the ability of the model to capture the relevant physiological mechanisms. We have also found that the model matches well with the other conditions reported by Zheng et al [4]. Those results are not presented here due to space constraints. The model currently can only describe transport under isotonic conditions which is sufficient for comparison with Zheng et al [4]. A future goal is to model non-isotonic scenarios which could be used to look at animal experiments reported in the literature. REFERENCES [1] Kellett, G. L., & Helliwell, P. A. (2000). Biochemical Journal, 350(Pt 1), 155. [2] Thorens, B. E. R. N. A. R. D., Cheng, Z. Q., Brown, D. E. N. N. I. S., & Lodish, H. F. (1990). American Journal of Physiology-Cell Physiology, 259(6), 1-1. [3] R¨oder, P. V., Geillinger, K. E., Zietek, T. S., Thorens, B., Koepsell, H., & Daniel, H. (2014). PloS one, 9(2), e89977. [4] Zheng, Y., Scow, J. S., Duenes, J. A., & Sarr, M. G. (2012). Surgery, 151(1), 13-25. [5] Barrett, K. E., & Keely, S. J. (2015). Yamada’s Textbook of Gastroenterology, 420-449. ACKNOWLEDGMENT Nima Afshar acknowledges PhD funding from the Riddet CoRE.