Glial Glucokinase Expression in Adult and Post-Natal Development of the Hypothalamic Region

Abstract

It has recently been proposed that hypothalamic glial cells sense glucose levels and release lactate as a signal to activate adjacent neurons. GK (glucokinase), the hexokinase involved in glucose sensing in pancreatic β-cells, is also expressed in the hypothalamus. However, it has not been clearly determined if glial and/or neuronal cells express this protein. Interestingly, tanycytes, the glia that cover the ventricular walls of the hypothalamus, are in contact with CSF (cerebrospinal fluid), the capillaries of the arcuate nucleus and adjacent neurons; this would be expected for a system that can detect and communicate changes in glucose concentration. Here, we demonstrated by Western-blot analysis, QRT–PCR [quantitative RT–PCR (reverse transcription–PCR)] and in situ hybridization that GK is expressed in tanycytes. Confocal microscopy and immunoultrastructural analysis revealed that GK is localized in the nucleus and cytoplasm of β1-tanycytes. Furthermore, GK expression increased in these cells during the second week of post-natal development. Based on this evidence, we propose that tanycytes mediate, at least in part, the mechanism by which the hypothalamus detects changes in glucose concentrations.

Glial hypothalamic inhibition of GLUT2 expression alters satiety, impacting eating behavior

“…Similarly, comparison of protein extracts obtained from cells transduced with Ad‐shβgal (Supporting Information Figure S1B, lane 1) versus those transduced with Ad‐shGLUT2 (Supporting Information Figure S1B, lane 2) indicated that GLUT2 levels were reduced by 60%. We also evaluated whether GK expression, which is also present in tanycytes (Millan et al, 2010), was altered by GLUT2 inhibition. We detected no significant differences in protein levels between both animal groups, demonstrating the specificity of the GLUT2 shRNA (Supporting Information Figure S1K).…”

“…Although the expression of several proteins involved in the detection and response to glucose has been demonstrated in tanycytes (Cortes‐Campos et al, 2011; Garcia et al, 2003; Millan et al, 2010), there was no evidence of their involvement in the regulation of food intake. We used a molecular tool for in vivo GLUT2 knockdown in specific cells of the rat brain.…”

Brain glucose sensing in homeostatic and hedonic regulation

“…a Tanycytes can be distinguished from b tanycytes by the histochemical markers ATP and esterase [33,34]. Morphologically, a tanycytes can be recognized by their apical microvilli, and b tanycytes by apical bleb projections [35]. Tanycytes now emerge as major players in the transfer of hormonal signals to the hypothalamus, such as leptin [36] and thyroid hormones [9].…”

“…They express GLUT2, GCK, and the K ATP channel [35,[38][39][40]; they also express monocarboxylate transporters (MCTs) 1 and 4 [41]. This suggests that tanycytes can sense glucose using the GLUT2/ GCK/K ATP signaling pathway; the presence of the MCTs also suggests that, as proposed for astrocytes [42], these cells may convert glucose into lactate and transfer it to neighboring neurons to activate their metabolism [41].…”

Monocarboxylate Transporters (MCTs) and their Role in Hypothalamic Glucosensing

“…Therefore, the presence of MCT1 and MCT4 in tanycytic basal processes and MCT2 expression in AN neurons suggest that lactate could be transferred between both cellular types as a signal derived from glucose for regulating energy balance and food intake. In support of this model, it has been demonstrated that glucose increases intracellular Ca 2+ concentration ([Ca 2+ ] i ) in cultured tanycytes, a response that is oxidative phosphorylation-independent but can be diminished through the inhibition of glucokinase, a critical glucosensor enzyme expressed in tanycytes [32][33][34]. Interestingly, this mechanism involves the opening of hemi channels (HC), and pharmacological inhibitors of HC abolish the response to glucose, but do not affect the efflux of lactate [6,33,35].…”