Microglia play a role in the regulation of metabolism and pathogenesis of obesity. Microglial activity is altered in response to changes in diet and the body’s metabolic state. Solute carrier family 2 member 5 (Slc2a5) that encodes glucose transporter 5 (GLUT5) is a fructose transporter primarily expressed in microglia within the central nervous system. However, little is known about the nutritional regulation of Slc2a5 expression in microglia and its role in the regulation of metabolism. The present study aimed to address the hypothesis that nutrients affect microglial activity by altering the expression of glucose transporter genes. Murine microglial cell line SIM-A9 cells and primary microglia from mouse brain were exposed to different concentrations of glucose and levels of microglial activation markers and glucose transporter genes were measured. High concentration of glucose increased levels of the immediate-early gene product c-Fos, a marker of cell activation, Slc2a5 mRNA, and pro-inflammatory cytokine genes in microglial cells in a time-dependent manner, while fructose failed to cause these changes. Glucose-induced changes in pro-inflammatory gene expression were partially attenuated in SIM-A9 cells treated with the GLUT5 inhibitor. These findings suggest that an increase in local glucose availability leads to the activation of microglia by controlling their carbohydrate sensing mechanism through both GLUT5-dependent and –independent mechanisms.

Feeding-regulating neurotrophic factors are expressed in both neurons and glial cells. However, nutritional regulation of anorexigenic glial cell line-derived neurotrophic factor (GDNF) and orexigenic mesencephalic astrocyte-derived neurotrophic factor (MANF) expression in specific cell types remains poorly understood. Hypothalamic glucose sensing plays a critical role in the regulation of food intake. It has been theorized that local glucose concentration modulates microglial activity partially via glucose transporter 5 (GLUT5). We hypothesized that an increased local glucose concentration stimulates GDNF expression while inhibiting MANF expression in the hypothalamus and microglia via GLUT5. The present study investigated the effect of glucose on Gdnf and Manf mRNA expression in the mouse hypothalamus and murine microglial cell line SIM-A9. Intracerebroventricular glucose treatment significantly increased Gdnf mRNA levels in the hypothalamus without altering Manf mRNA levels. Exposure to high glucose caused a significant increase in Gdnf mRNA expression and a time-dependent change in Manf mRNA expression in SIM-A9 cells. GLUT5 inhibitor treatment did not block glucose-induced Gdnf mRNA expression in these cells. These findings suggest that microglia are responsive to changes in the local glucose concentration and increased local glucose availability stimulates the expression of microglial GNDF through a GLUT5-independent mechanism, contributing to glucose-induced feeding suppression.

Brain-derived neurotrophic factor (BDNF) is expressed in both hypothalamic neurons and microglia, and plays a critical role in the regulation of metabolism. Although hypothalamic expression of BDNF is regulated by metabolic signals such as nutrients and hormones, it remains unknown whether these signals differentially regulate BDNF expression in different cell types. The present study aimed to determine whether glucose and fructose regulate BDNF expression in microglia via the specific glucose transporter. To determine the effect of glucose and fructose on Bdnf mRNA and protein expression, murine microglial cell line SIM-A9 cells were exposed to the maintenance concentration of glucose (17.5 mmol/l), high glucose (25 mmol/l), or fructose (7.5 mmol/l) for 40 min to 24 h. To determine whether the blockade of glucose transporter 5 (GLUT5) negates the effect of glucose on Bdnf mRNA expression, cells were exposed to 25 mmol/l glucose in the presence or absence of the GLUT5 inhibitor for 4 h. Levels of Bdnf mRNA and protein were measured by real-time PCR and ELISA, respectively. High glucose caused a significant increase in both pan-Bdnf and long-form Bdnf (L-Bdnf) mRNA as well as protein levels when compared with the maintenance concentration of glucose in a time-dependent manner. Fructose treatment also increased L-Bdnf mRNA expression. Pharmacological blockade of GLUT5 did not affect glucose-induced Bdnf mRNA expression. These findings suggest that glucose and fructose directly stimulate Bdnf mRNA expression in microglia and these responses may mediate the metabolic actions of glucose and fructose.