SGLT3A AND GLP-1 DISPLAY A DIURNAL RHYMICITY OF MRNA EXPRESSION IN MOUSE PROXIMAL SMALL BOWEL

Introduction A diurnal cycle is one that recurs every 24 hours. Many physiological processes such as blood sugar levels exhibit diurnal variation. Such processes are under the control of central and peripheral clock genes that have an endogenous rhythmicity, but are entrained (synchronised) by external light and food input cues. Diurnal rhythmicity of gene expression has previously been described in intestinal nutrient/energy transporters such as Sodium Glucose co- transporter-1 (SGLT-1) and Glucose transporter 5 (GLUT-5). SGLT-1 mediates the glucose induced release of glucose-dependent insulinotropic peptide (GIP) and Glucagon like peptide 1 (GLP-1) and therefore has an additional sugar sensing role. Mouse SGLT-3a does not transport sugar and is thus postulated to be purely a sugar sensor. SGLT- 3a, GLP-1 or GIP have not previously been demonstrated to have a diurnal rhythmicity of expression. Methods Sixteen C57BL/6J mice were fed ad libitum under conditions of 12-hour light/dark cycles. Half the animals were randomly euthanized in the morning and half were euthanized in the evening. Duodenal and jejunal tissues were isolated from the carcasses and messenger RNA (mRNA) extracted. Complementary DNA (cDNA) was synthesised from mRNA and underwent real-time (quantitative) PCR. Expression levels for each gene were expressed as a ratio to two housekeeping genes (HMBS and HPRT-1) Relative quantification of gene expression was done using the comparative CT (2-∆∆CT) method. Results In keeping with previous studies the sugar transporters GLUT-5 and SGLT-1 (p < 0.005) and the clock genes Cry- 2 and Bmal-1 (p < 0.01) displayed a diurnal rhythmicity of expression in both tissues. For the first time SGLT-3a was shown to display a marked (more than double) up-regulation of mRNA expression in the evening compared to the morning in both duodenum and jejunum (p < 0.005). GLP-1 exhibited approximately twice the levels of expression in the evening than in the morning but this was not statistically significant. GIP failed to show any diurnal rhythmicity of expression. Conclusion Demonstrated for the first time was a diurnal rhythmicity of SGLT-3a and GLP-1 expression. It is postulated that sugar sensing by SGLT-3a has an important role in mediating beneficial downstream sequelae such as gut peptide hormone release. Dysregulation of such mechanisms may play important roles in metabolic diseases such as diabetes.