Determination of GLUT1 Oligomerization Parameters using Bioluminescent Förster Resonance Energy Transfer

Living Cell as a Self-Synchronized Chemical Reactor

Thermal fluctuations power all processes inside living cells. Therefore, these processes are inherently random. However, myriad multistep chemical reactions act in concerto inside a cell, finally leading to this chemical reactor's self-replication. We speculate that an underlying mechanism in nature must exist that allows all of these reactions to synchronize at multiple time and length scales, overcoming in this way the random nature of any single process in a cell. This Perspective discusses what type of research is needed to understand this undiscovered synchronization law.

Cellular binding and uptake of fluorescent glucose analogs 2-NBDG and 6-NBDG occurs independent of membrane glucose transporters

The classical methods for determining glucose uptake rates in living cells involve the use of isotopically labeled 2-deoxy-d-glucose or 3-O-methyl-d-glucose, which enter cells via well-characterized membrane transporters of the SLC2A and SLC5A families, respectively. These classical methods, however, are increasingly being displaced by high-throughput assays that utilize fluorescent analogs of glucose. Among the most commonly used of these analogs are 2-NBDG and 6-NBDG, which contain a bulky 7-nitro-2,1,3-benzoxadiazol-4-yl-amino moiety in place of a hydroxy group on d-glucose. This fluorescent group significantly alters both the size and shape of these molecules compared to glucose, calling into question whether they actually enter cells by the same transport mechanisms. In this study, we took advantage of the well-defined glucose uptake mechanism of L929 murine fibroblasts, which rely exclusively on the Glut1/Slc2a1 membrane transporter. We demonstrate that neither pharmacologic inhibition of Glut1 nor genetic manipulation of its expression has a significant impact on the binding or uptake of 2-NBDG or 6-NBDG by L929 cells, though both approaches significantly impact [³H]-2-deoxyglucose uptake rates. Together these data indicate that 2-NBDG and 6-NBDG can bind and enter mammalian cells by transporter-independent mechanisms, which calls into question their utility as an accurate proxy for glucose transport.

Visualization of GLUT1 Trafficking in Live Cancer Cells by the Use of a Dual-Fluorescence Reporter

Glucose metabolism is an essential process for energy production and cell survival for both normal and abnormal cellular metabolism. Several glucose transporter/solute carrier 2A (GLUT/SLC2A) superfamily members, including glucose transporter 1 (GLUT1), have been shown to mediate the cellular uptake of glucose in diverse cell types. GLUT1-mediated glucose uptake is a transient and rapid process; thus, the real-time monitoring of GLUT1 trafficking is pivotal for a better understanding of GLUT1 expression and GLUT1-dependent glucose uptake. In the present study, we established a rapid and effective method to visualize the trafficking of GLUT1 between the plasma membrane (PM) and endolysosomal system in live cells using an mCherry-EGFP-GLUT1 tandem fluorescence tracing system. We found that GLUT1 localized at the PM exhibited both red (mCherry) and green (EGFP) fluorescence (yellow when overlapping). However, a significant increase in red punctate fluorescence (mCherry is resistant to acidic pH), but not green fluorescence (EGFP is quenched by acidic pH), was observed upon glucose deprivation, indicating that the mCherry-EGFP-GLUT1 functional protein was trafficked to the acidic endolysosomal system. Besides, we were able to calculate the relative ratio of mCherry to EGFP by quantification of the translocation coefficient, which can be used as a readout for GLUT1 internalization and subsequent lysosomal degradation. Two mutants, mCherry-EGFP-GLUT1-S226D and mCherry-EGFP-GLUT1-ΔC4, were also constructed, which indirectly confirmed the specificity of mCherry-EGFP-GLUT1 for monitoring GLUT1 trafficking. By using a series of endosomal (Rab5, Rab7, and Rab11) and lysosomal markers, we were able to define a model of GLUT1 trafficking in live cells in which upon glucose deprivation, GLUT1 dissociates from the PM and experiences a pH gradient from 6.8-6.1 in the early endosomes to 6.0-4.8 in the late endosomes and finally pH 4.5 in lysosomes, which is appropriate for degradation. In addition, our proof-of-concept study indicated that the pmCherry-EGFP-GLUT1 tracing system can accurately reflect endogenous changes in GLUT1 in response to treatment with the small molecule, andrographolide. Since targeting GLUT1 expression and GLUT1-dependent glucose metabolism is a promising therapeutic strategy for diverse types of cancers and certain other glucose addiction diseases, our study herein indicates that pmCherry-EGFP-GLUT1 can be utilized as a biosensor for GLUT1-dependent functional studies and potential small molecule screening.

Erythroid glucose transport in health and disease

Glucose transport is intimately linked to red blood cell physiology. Glucose is the unique energy source for these cells, and defects in glucose metabolism or transport activity are associated with impaired red blood cell morphology and deformability leading to reduced lifespan. In vertebrate erythrocytes, glucose transport is mediated by GLUT1 (in humans) or GLUT4 transporters. These proteins also account for dehydroascorbic acid (DHA) transport through erythrocyte membrane. The peculiarities of glucose transporters and the red blood cell pathologies involving GLUT1 are summarized in the present review.

Pharmacologic inhibition of N-linked glycan trimming with kifunensine disrupts GLUT1 trafficking and glucose uptake

The facilitative glucose transport GLUT1 (SLC2A1) is a constitutively expressed membrane protein involved in basal uptake of blood glucose. GLUT1 modification by N-linked glycosylation at a single asparagine residue (N45) appears to play multiple roles in the trafficking, stability and transport activity of this protein. Here we examine the role of complex N-glycosylation on GLUT1 function in renal epithelial cells by arresting this modification at the high-mannose stage with the mannosidase I inhibitor kifunensine. Consistent with prior work in which GLUT1 glycosylation was completely inhibited, we find that kifunensine treatment results in a time-dependent decrease of up to 40% in cellular glucose uptake. We further demonstrate that this effect is primarily a result of deficient GLUT1 trafficking to the cell membrane due to quality control mechanisms that instead direct GLUT1 to the ER-associated degradation (ERAD) pathway. Unlike tunicamycin, which inhibits the first step in N-glycosyl transfer and causes dramatic cell cycle arrest, kifunensine causes only a modest decrease in GLUT1 levels and cell cycle progression in both normal and transformed renal cells. The effect of kifunensine on the cell cycle appears to be independent of its effect on GLUT1, since all renal cell types in this study displayed decreased proliferation regardless of their dependence on glucose uptake for growth and survival. Together these results indicate that proper N-glycan processing plays an important role in directing GLUT1 to the cell surface and that disruption of mannosidase activity results in aberrant degradation of GLUT1 by the ERAD pathway.

Mechanistic Insights into Protein Stability and Self-aggregation in GLUT1 Genetic Variants Causing GLUT1-Deficiency Syndrome

Human sodium-independent glucose cotransporter 1 (hGLUT1) has been studied for its tetramerization and multimerization at the cell surface. Homozygous or compound heterozygous mutations in hGLUT1 elicit GLUT1-deficiency syndrome (GLUT1-DS), a metabolic disorder, which results in impaired glucose transport into the brain. The reduced cell surface expression or loss of function have been shown for some GLUT1 mutants. However, the mechanism by which deleterious mutations affect protein structure, conformational stability and GLUT1 oligomerization is not known and require investigation. In this review, we combined previous knowledge of GLUT1 mutations with hGLUT1 crystal structure to analyze native interactions and several natural single-point mutations. The modeling of native hGLUT1 structure confirmed the roles of native residues in forming a range of side-chain interactions. Interestingly, the modeled mutants pointed to the formation of a variety of non-native novel interactions, altering interaction networks and potentially eliciting protein misfolding. Self-aggregation of the last part of hGLUT1 was predicted using protein aggregation prediction tool. Furthermore, an increase in aggregation potential in the aggregation-prone regions was estimated for several mutants suggesting increased aggregation of misfolded protein. Protein stability change analysis predicted that GLUT1 mutant proteins are unstable. Combining GLUT1 oligomerization behavior with our modeling, aggregation prediction, and protein stability analyses, this work provides state-of-the-art view of GLUT1 genetic mutations that could destabilize native interactions, generate novel interactions, trigger protein misfolding, and enhance protein aggregation in a disease state.

Enhanced Molecular Tension Sensor Based on Bioluminescence Resonance Energy Transfer (BRET)

Molecular tension sensors measure piconewton forces experienced by individual proteins in the context of the cellular microenvironment. Current genetically encoded tension sensors use FRET to report on extension of a deformable peptide encoded in a cellular protein of interest. Here, we present the development and characterization of a new type of molecular tension sensor based on bioluminescence resonance energy transfer (BRET), which exhibits more desirable spectral properties and an enhanced dynamic range compared to other molecular tension sensors. Moreover, it avoids many disadvantages of FRET measurements in cells, including autofluorescence, photobleaching, and corrections of direct acceptor excitation. We benchmark the sensor by inserting it into the canonical mechanosensing focal adhesion protein vinculin, observing highly resolved gradients of tensional changes across focal adhesions. We anticipate that the BRET tension sensor will expand the toolkit available to study mechanotransduction at a molecular level and allow potential extension to an in vivo context.

Caffeine inhibition of GLUT1 is dependent on the activation state of the transporter

Caffeine has been shown to be a robust uncompetitive inhibitor of glucose uptake in erythrocytes. It preferentially binds to the nucleotide-binding site on GLUT1 in its tetrameric form and mimics the inhibitory action of ATP. Here we demonstrate that caffeine is also a dose-dependent, uncompetitive inhibitor of 2-deoxyglucose (2DG) uptake in L929 fibroblasts. The inhibitory effect on 2DG uptake in these cells was reversible with a rapid onset and was additive to the competitive inhibitory effects of glucose itself, confirming that caffeine does not interfere with glucose binding. We also report for the first time that caffeine inhibition was additive to inhibition by curcumin, suggesting distinct binding sites for curcumin and caffeine. In contrast, caffeine inhibition was not additive to that of cytochalasin B, consistent with previous data that reported that these two inhibitors have overlapping binding sites. More importantly, we show that the magnitude of maximal caffeine inhibition in L929 cells is much lower than in erythrocytes (35% compared to 90%). Two epithelial cell lines, HCLE and HK2, have both higher concentrations of GLUT1 and increased basal 2DG uptake (3-4 fold) compared to L929 cells, and subsequently display greater maximal inhibition by caffeine (66-70%). Interestingly, activation of 2DG uptake (3-fold) in L929 cells by glucose deprivation shifted the responsiveness of these cells to caffeine inhibition (35%-70%) without a change in total GLUT1 concentration. These data indicate that the inhibition of caffeine is dependent on the activity state of GLUT1, not merely on the concentration.