Evidence for high-capacity bidirectional glucose transport across the endoplasmic reticulum membrane by genetically encoded fluorescence resonance energy transfer nanosensors

Tanycytes release glucose using the glucose-6-phosphatase system during hypoglycemia to control hypothalamic energy balance

OBJECTIVE

The liver releases glucose into the blood using the glucose-6-phosphatase (G6Pase) system, a multiprotein complex located in the endoplasmic reticulum (ER). Here, we show for the first time that the G6Pase system is also expressed in hypothalamic tanycytes, and it is required to regulate energy balance.

METHODS

Using automatized qRT-PCR and immunohistochemical analyses, we evaluated the expression of the G6Pase system. Fluorescent glucose analogue (2-NBDG) uptake was evaluated by 4D live-cell microscopy. Glucose release was tested using a glucose detection kit and high-content live-cell analysis instrument, Incucyte s3. In vivo G6pt knockdown in tanycytes was performed by AAV1-shG6PT-mCherry intracerebroventricular injection. Body weight gain, adipose tissue weight, food intake, glucose metabolism, c-Fos, and neuropeptide expression were evaluated at 4 weeks post-transduction.

RESULTS

Tanycytes sequester glucose-6-phosphate (G6P) into the ER through the G6Pase system and release glucose in hypoglycaemia via facilitative glucose transporters (GLUTs). Strikingly, in vivo tanycytic G6pt knockdown has a powerful peripheral anabolic effect observed through decreased body weight, white adipose tissue (WAT) tissue mass, and strong downregulation of lipogenesis genes. Selective deletion of G6pt in tanycytes also decreases food intake, c-Fos expression in the arcuate nucleus (ARC), and Npy mRNA expression in fasted mice.

CONCLUSIONS

The tanycyte-associated G6Pase system is a central mechanism involved in controlling metabolism and energy balance.

Transcellular Barriers to Glucose Delivery in the Body

Glucose is the universal fuel of most mammalian cells, and it is largely replenished through dietary intake. Glucose availability to tissues is paramount for the maintenance of homeostatic energetics and, hence, supply should match demand by the consuming organs. In its journey through the body, glucose encounters cellular barriers for transit at the levels of the absorbing intestinal epithelial wall, the renal epithelium mediating glucose reabsorption, and the tight capillary endothelia (especially in the brain). Glucose transiting through these cellular barriers must escape degradation to ensure optimal glucose delivery to the bloodstream or tissues. The liver, which stores glycogen and generates glucose de novo, must similarly be able to release it intact to the circulation. We present the most up-to-date knowledge on glucose handling by the gut, liver, brain endothelium, and kidney, and discuss underlying molecular mechanisms and open questions. Diseases associated with defects in glucose delivery and homeostasis are also briefly addressed. We propose that the universal problem of sparing glucose from catabolism in favor of translocation across the barriers posed by epithelia and endothelia is resolved through common mechanisms involving glucose transfer to the endoplasmic reticulum, from where glucose exits the cells via unconventional cellular mechanisms.

Dynamic glucose uptake, storage, and release by human microvascular endothelial cells

Endothelia determine blood-to-tissue solute delivery, yet glucose transit is poorly understood. To illuminate mechanisms, we tracked [³H]-2-deoxyglucose (2-DG) in human adipose-tissue microvascular endothelial cells. 2-DG uptake was largely facilitated by the glucose transporters GLUT1 and GLUT3. Once in the cytosol, >80% of 2-DG became phosphorylated and ∼20% incorporated into glycogen, suggesting that transported glucose is readily accessible to cytosolic enzymes. Interestingly, a fraction of intracellular 2-DG was released over time (15-20% over 30 min) with slower kinetics than for uptake, involving GLUT3. In contrast to intracellular 2-DG, the released 2-DG was largely unphosphorylated. Glucose release involved endoplasmic reticulum-resident translocases/phosphatases and was stimulated by adrenaline, consistent with participation of glycogenolysis and glucose dephosphorylation. Surprisingly, the fluorescent glucose derivative 2-NBD-glucose (2-NBDG) entered cells largely via fluid phase endocytosis and exited by recycling. 2-NBDG uptake was insensitive to GLUT1/GLUT3 inhibition, suggesting poor influx across membranes. 2-NBDG recycling, but not 2-DG efflux, was sensitive to N-ethyl maleimide. In sum, by utilizing radioactive and fluorescent glucose derivatives, we identified two parallel routes of entry: uptake into the cytosol through dedicated glucose transporters and endocytosis. This reveals the complex glucose handling by endothelial cells that may contribute to glucose delivery to tissues.

Visualizing physiological parameters in cells and tissues using genetically encoded indicators for metabolites

The study of metabolism is undergoing a renaissance. Since the year 2002, over 50 genetically-encoded fluorescent indicators (GEFIs) have been introduced, capable of monitoring metabolites with high spatial/temporal resolution using fluorescence microscopy. Indicators are fusion proteins that change their fluorescence upon binding a specific metabolite. There are indicators for sugars, monocarboxylates, Krebs cycle intermediates, amino acids, cofactors, and energy nucleotides. They permit monitoring relative levels, concentrations, and fluxes in living systems. At a minimum they report relative levels and, in some cases, absolute concentrations may be obtained by performing ad hoc calibration protocols. Proper data collection, processing, and interpretation are critical to take full advantage of these new tools. This review offers a survey of the metabolic indicators that have been validated in mammalian systems. Minimally invasive, these indicators have been instrumental for the purposes of confirmation, rebuttal and discovery. We envision that this powerful technology will foster metabolic physiology.

Luseogliflozin, a SGLT2 Inhibitor, Does Not Affect Glucose Uptake Kinetics in Renal Proximal Tubules of Live Mice

Proximal tubules (PTs) take up most of the glucose in the glomerular filtrate and return it to peritubular capillary blood. Sodium-glucose cotransporter 2 (SGLT2) at the apical membrane takes up glucose into the cell. Glucose then flows across the cells and is transported to the interstitium via glucose transporter 2 (GLUT2) at the basolateral membrane. However, glucose transport under SGLT2 inhibition remains poorly understood. In this study, we evaluated the dynamics of a fluorescent glucose analog, 2-NBDG, in the PTs of live mice treated with or without the SGLT2 inhibitor, luseogliflozin. We employed real-time multiphoton microscopy, in which insulin enhanced 2-NBDG uptake in skeletal muscle. Influx and efflux of 2-NBDG in PT cells were compared under hypo-, normo-, and hyperglycemic conditions. Luseogliflozin did not exert significant effects on glucose influx parameters under any level of blood glucose. Our results suggest that blood glucose level per se does not alter glucose influx or efflux kinetics in PTs. In conclusion, neither SGLT2 inhibition nor blood glucose level affect glucose uptake kinetics in PTs. The former was because of glucose influx through basolateral GLUT2, which is an established bidirectional transporter.

An alternative pathway for sweet sensation: possible mechanisms and physiological relevance

Sweet substances are detected by taste-bud cells upon binding to the sweet-taste receptor, a T1R2/T1R3 heterodimeric G protein-coupled receptor. In addition, experiments with mouse models lacking the sweet-taste receptor or its downstream signaling components led to the proposal of a parallel "alternative pathway" that may serve as metabolic sensor and energy regulator. Indeed, these mice showed residual nerve responses and behavioral attraction to sugars and oligosaccharides but not to artificial sweeteners. In analogy to pancreatic β cells, such alternative mechanism, to sense glucose in sweet-sensitive taste cells, might involve glucose transporters and K_ATP channels. Their activation may induce depolarization-dependent Ca²⁺ signals and release of GLP-1, which binds to its receptors on intragemmal nerve fibers. Via unknown neuronal and/or endocrine mechanisms, this pathway may contribute to both, behavioral attraction and/or induction of cephalic-phase insulin release upon oral sweet stimulation. Here, we critically review the evidence for a parallel sweet-sensitive pathway, involved signaling mechanisms, neural processing, interactions with endocrine hormonal mechanisms, and its sensitivity to different stimuli. Finally, we propose its physiological role in detecting the energy content of food and preparing for digestion.

Differences in cytosolic glucose dynamics in astrocytes and adipocytes measured by FRET-based nanosensors

The cellular response to fluctuations in blood glucose levels consists of integrative regulation of cell glucose uptake and glucose utilization in the cytosol, resulting in altered levels of glucose in the cytosol. Cytosolic glucose is difficult to be measured in the intact tissue, however recently methods have become available that allow measurements of glucose in single living cells with fluorescence resonance energy transfer (FRET) based protein sensors. By studying the dynamics of cytosolic glucose levels in different experimental settings, we can gain insights into the properties of plasma membrane permeability to glucose and glucose utilization in the cytosol, and how these processes are modulated by different environmental conditions, agents and enzymes. In this review, we compare the cytosolic regulation of glucose in adipocytes and astrocytes - two important regulators of energy balance and glucose homeostasis in whole body and brain, respectively, with particular emphasis on the data obtained with FRET based protein sensors as well as other biochemical and molecular approaches.

Glucose Transport and Transporters in the Endomembranes

Glucose is a basic nutrient in most of the creatures; its transport through biological membranes is an absolute requirement of life. This role is fulfilled by glucose transporters, mediating the transport of glucose by facilitated diffusion or by secondary active transport. GLUT (glucose transporter) or SLC2A (Solute carrier 2A) families represent the main glucose transporters in mammalian cells, originally described as plasma membrane transporters. Glucose transport through intracellular membranes has not been elucidated yet; however, glucose is formed in the lumen of various organelles. The glucose-6-phosphatase system catalyzing the last common step of gluconeogenesis and glycogenolysis generates glucose within the lumen of the endoplasmic reticulum. Posttranslational processing of the oligosaccharide moiety of glycoproteins also results in intraluminal glucose formation in the endoplasmic reticulum (ER) and Golgi. Autophagic degradation of polysaccharides, glycoproteins, and glycolipids leads to glucose accumulation in lysosomes. Despite the obvious necessity, the mechanism of glucose transport and the molecular nature of mediating proteins in the endomembranes have been hardly elucidated for the last few years. However, recent studies revealed the intracellular localization and functional features of some glucose transporters; the aim of the present paper was to summarize the collected knowledge.

Effective Glucose Uptake by Human Astrocytes Requires Its Sequestration in the Endoplasmic Reticulum by Glucose-6-Phosphatase-β

After its uptake into the cytosol, intracellular glucose is phosphorylated to glucose-6-phosphate (G6P), trapping it within the cell and preparing it for metabolism. In glucose-exporting tissues, like liver, G6P is transported into the ER, where it is dephosphorylated by G6Pase-α. The glucose is then returned to the cytosol for export [1, 2]. Defects in these pathways cause glycogen storage diseases [1]. G6Pase-β, an isozyme of G6Pase-α, is widely expressed [3, 4]. Its role in cells that do not export glucose is unclear, although mutations in G6Pase-β cause severe and widespread abnormalities [5, 6, 7]. Astrocytes, the most abundant cells in the brain, provide metabolic support to neurons, facilitated by astrocytic endfeet that contact blood capillaries or neurons [8, 9, 10, 11, 12]. Perivascular endfeet are the main site of glucose uptake by astrocytes [13], but in human brain they may be several millimeters away from the perineuronal processes [14]. We show that cultured human fetal astrocytes express G6Pase-β, but not G6Pase-α. ER-targeted glucose sensors [15, 16] reveal that G6Pase-β allows the ER of human astrocytes to accumulate glucose by importing G6P from the cytosol. Glucose uptake by astrocytes, ATP production, and Ca²⁺ accumulation by the ER are attenuated after knockdown of G6Pase-β using lentivirus-delivered shRNA and substantially rescued by expression of G6Pase-α. We suggest that G6Pase-β activity allows effective uptake of glucose by astrocytes, and we speculate that it allows the ER to function as an intracellular “highway” delivering glucose from perivascular endfeet to the perisynaptic processes.

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Glucose-6-phosphatase-β (G6Pase-β) is expressed in human astrocytes

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G6P is sequestered by ER and dephosphorylated to glucose in the lumen by G6Pase-β

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Loss of G6Pase-β reduces glucose uptake, intracellular ATP, and ER Ca²⁺ content

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ER may provide a protected highway for long-range glucose transport in astrocytes

Glucose Transporters and Virulence in Leishmania mexicana

Glucose transport plays important roles for in vitro growth of insect-stage promastigotes and especially for viability of intramacrophage mammalian host-stage amastigotes of Leishmania mexicana. However, the roles of the three distinct glucose transporters, GT1, GT2, and GT3, in parasite viability inside macrophages and virulence in mice have not been fully explored. Parasite lines expressing GT1 or GT2 alone were strongly impaired in growth inside macrophages, but lines expressing GT3 alone infected macrophages and caused lesions in mice as robustly as wild-type parasites. Notably, GT3 localizes to the endoplasmic reticulum of intracellular amastigotes, suggesting a potential role for salvage of glucose from that organelle for viability of infectious amastigotes. This study establishes the unique role of GT3 for parasite survival inside host macrophages and for robust virulence in infected animals.

Glucose transporters are important for viability and infectivity of the disease-causing amastigote stages of Leishmania mexicana. The Δgt1-3 null mutant, in which the 3 clustered glucose transporter genes, GT1, GT2, and GT3, have been deleted, is strongly impaired in growth inside macrophages in vitro. We have now demonstrated that this null mutant is also impaired in virulence in the BALB/c murine model of infection and forms lesions considerably more slowly than wild-type parasites. Previously, we established that amplification of the PIFTC3 gene, which encodes an intraflagellar transport protein, both facilitated and accompanied the isolation of the original Δgt1-3 null mutant generated in extracellular insect-stage promastigotes. We have now isolated Δgt1-3 null mutants without coamplification of PIFTC3. These amplicon-negative null mutants are further impaired in growth as promastigotes, compared to the previously described null mutants containing the PIFTC3 amplification. In contrast, the GT3 glucose transporter plays an especially important role in promoting amastigote viability. A line that expresses only the single glucose transporter GT3 grows as well inside macrophages and induces lesions in animals as robustly as do wild-type amastigotes, but lines expressing only the GT1 or GT2 transporters replicate poorly in macrophages. Strikingly, GT3 is restricted largely to the endoplasmic reticulum in intracellular amastigotes. This observation raises the possibility that GT3 may play an important role as an intracellular glucose transporter in the infectious stage of the parasite life cycle.

IMPORTANCE Glucose transport plays important roles for in vitro growth of insect-stage promastigotes and especially for viability of intramacrophage mammalian host-stage amastigotes of Leishmania mexicana. However, the roles of the three distinct glucose transporters, GT1, GT2, and GT3, in parasite viability inside macrophages and virulence in mice have not been fully explored. Parasite lines expressing GT1 or GT2 alone were strongly impaired in growth inside macrophages, but lines expressing GT3 alone infected macrophages and caused lesions in mice as robustly as wild-type parasites. Notably, GT3 localizes to the endoplasmic reticulum of intracellular amastigotes, suggesting a potential role for salvage of glucose from that organelle for viability of infectious amastigotes. This study establishes the unique role of GT3 for parasite survival inside host macrophages and for robust virulence in infected animals.

Occludin regulates glucose uptake and ATP production in pericytes by influencing AMP-activated protein kinase activity

Energetic regulation at the blood-brain barrier is critical for maintaining its integrity, transport capabilities, and brain demands for glucose. However, the underlying mechanisms that regulate these processes are still poorly explored. We recently characterized the protein occludin as a NADH oxidase and demonstrated its influence on the expression and activation of the histone deacetylase SIRT-1. Because SIRT-1 works in concert with AMP-activated protein kinase (AMPK) (AMPK), we investigated the impact of occludin on this metabolic switch. Here we show that in blood-brain barrier pericytes, occludin promotes AMPK expression and activation, influencing the expression of glucose transporters GLUT-1 and GLUT-4, glucose uptake, and ATP content. Furthermore, occludin expression, AMP-dependent protein kinase activity, and glucose uptake were altered under inflammatory (TNFα) and infectious (HIV) conditions. We also show that pericytes share glucose and mitochondria with astrocytes, and that occludin levels modify the ability of pericytes to share those energetic resources. In addition, we demonstrate that murine mitochondria can be transferred from live brain microvessels to energetically impaired human astrocytes, promoting their survival. Our findings demonstrate that occludin plays an important role in blood-brain barrier pericyte metabolism by influencing AMPK protein kinase activity, glucose uptake, ATP production, and by regulating the ability of pericytes to interact metabolically with astrocytes.

Glucose-Modified Silicon Nanoparticles for Cellular Imaging

Luminescent silicon nanoparticles have recently attracted attention due to their remarkable stability, covalent functionalisation and tunable photoemission properties. Owing to their biocompatibility, low toxicity, and the small particle size that can be achieved by different synthetic approaches, these nanomaterials are candidates as cellular probes in the field of bioimaging, and potentially for in vivo applications. Tailoring the surface of the particles with active biomolecules such as sugar moieties can be an interesting strategy to increase the kinetics of internalisation or to vary the localisation of nanosystems in living cells. In this study, we synthesised and modified ultrasmall silicon nanoparticles with glucose covalently linked on their surface. Moreover, by varying the ratio between the amount of silicon nanoparticles and the saccharide groups, the amount of glucose, as a capping moiety, can be well controlled. FTIR spectroscopy, NMR spectroscopy, zeta potential measurements and anisotropy decay analysis confirmed the covalent binding of glucose to the nanoparticles. The photophysical behaviour of the surface-functionalised silicon quantum dots was not significantly different to that of the unmodified nanoparticles. In vitro studies demonstrated faster internalisation of the glucose-functionalised nanoparticles into HeLa cells. Different localisation and uptake kinetics of the glucose-modified particles compared to the unmodified particles are discussed in order to reveal the role played by the sugar molecules.

Exploring cells with targeted biosensors

Cellular signaling networks are composed of multiple pathways, often interconnected, that form complex networks with great potential for cross-talk. Signal decoding depends on the nature of the message as well as its amplitude, temporal pattern, and spatial distribution. In addition, the existence of membrane-bound organelles, which are both targets and generators of messages, add further complexity to the system. The availability of sensors that can localize to specific compartments in live cells and monitor their targets with high spatial and temporal resolution is thus crucial for a better understanding of cell pathophysiology. For this reason, over the last four decades, a variety of strategies have been developed, not only to generate novel and more sensitive probes for ions, metabolites, and enzymatic activity, but also to selectively deliver these sensors to specific intracellular compartments. In this review, we summarize the principles that have been used to target organic or protein sensors to different cellular compartments and their application to cellular signaling.

Mitoenergetic Dysfunction Triggers a Rapid Compensatory Increase in Steady-State Glucose Flux

ATP can be produced in the cytosol by glycolytic conversion of glucose (GLC) into pyruvate. The latter can be metabolized into lactate, which is released by the cell, or taken up by mitochondria to fuel ATP production by the tricarboxylic acid cycle and oxidative phosphorylation (OXPHOS) system. Altering the balance between glycolytic and mitochondrial ATP generation is crucial for cell survival during mitoenergetic dysfunction, which is observed in a large variety of human disorders including cancer. To gain insight into the kinetic properties of this adaptive mechanism we determined here how acute (30 min) inhibition of OXPHOS affected cytosolic GLC homeostasis. GLC dynamics were analyzed in single living C2C12 myoblasts expressing the fluorescent biosensor FLII(12)Pglu-700μδ6 (FLII). Following in situ FLII calibration, the kinetic properties of GLC uptake (V1) and GLC consumption (V2) were determined independently and used to construct a minimal mathematical model of cytosolic GLC dynamics. After validating the model, it was applied to quantitatively predict V1 and V2 at steady-state (i.e., when V1 = V2 = Vsteady-state) in the absence and presence of OXPHOS inhibitors. Integrating model predictions with experimental data on lactate production, cell volume, and O2 consumption revealed that glycolysis and mitochondria equally contribute to cellular ATP production in control myoblasts. Inhibition of OXPHOS induced a twofold increase in Vsteady-state and glycolytic ATP production flux. Both in the absence and presence of OXPHOS inhibitors, GLC was consumed at near maximal rates, meaning that GLC consumption is rate-limiting under steady-state conditions. Taken together, we demonstrate here that OXPHOS inhibition increases steady-state GLC uptake and consumption in C2C12 myoblasts. This activation fully compensates for the reduction in mitochondrial ATP production, thereby maintaining the balance between cellular ATP supply and demand.

Glucose transporter type 10-lacking in arterial tortuosity syndrome-facilitates dehydroascorbic acid transport

Loss-of-function mutations in the gene encoding GLUT10 are responsible for arterial tortuosity syndrome (ATS), a rare connective tissue disorder. In this study GLUT10-mediated dehydroascorbic acid (DAA) transport was investigated, supposing its involvement in the pathomechanism. GLUT10 protein produced by in vitro translation and incorporated into liposomes efficiently transported DAA. Silencing of GLUT10 decreased DAA transport in immortalized human fibroblasts whose plasma membrane was selectively permeabilized. Similarly, the transport of DAA through endomembranes was markedly reduced in fibroblasts from ATS patients. Re-expression of GLUT10 in patients' fibroblasts restored DAA transport activity. The present results demonstrate that GLUT10 is a DAA transporter and DAA transport is diminished in the endomembranes of fibroblasts from ATS patients.

Computational Analysis of AMPK-Mediated Neuroprotection Suggests Acute Excitotoxic Bioenergetics and Glucose Dynamics Are Regulated by a Minimal Set of Critical Reactions

Loss of ionic homeostasis during excitotoxic stress depletes ATP levels and activates the AMP-activated protein kinase (AMPK), re-establishing energy production by increased expression of glucose transporters on the plasma membrane. Here, we develop a computational model to test whether this AMPK-mediated glucose import can rapidly restore ATP levels following a transient excitotoxic insult. We demonstrate that a highly compact model, comprising a minimal set of critical reactions, can closely resemble the rapid dynamics and cell-to-cell heterogeneity of ATP levels and AMPK activity, as confirmed by single-cell fluorescence microscopy in rat primary cerebellar neurons exposed to glutamate excitotoxicity. The model further correctly predicted an excitotoxicity-induced elevation of intracellular glucose, and well resembled the delayed recovery and cell-to-cell heterogeneity of experimentally measured glucose dynamics. The model also predicted necrotic bioenergetic collapse and altered calcium dynamics following more severe excitotoxic insults. In conclusion, our data suggest that a minimal set of critical reactions may determine the acute bioenergetic response to transient excitotoxicity and that an AMPK-mediated increase in intracellular glucose may be sufficient to rapidly recover ATP levels following an excitotoxic insult.

A novel probe based on phenylboronic acid functionalized carbon nanotubes for ultrasensitive carbohydrate determination in biofluids and semi-solid biotissues

An ultrasensitive SPME probe based on phenylboronic acid functionalized CNTs is applied for direct in vitro or in vivo recognition of carbohydrates in biofluids as well as semi-solid biotissues.

Carbohydrates are known to be involved in a wide range of biological and pathological processes. However, due to the presence of multiple hydroxyl groups, carbohydrate recognition is a particular challenge. Herein, we reported an ultrasensitive solid-phase microextraction (SPME) probe based on phenylboronic acid (PBA) functionalized carbon nanotubes (CNTs) for direct in vitro or in vivo recognition of carbohydrates in biofluids as well as semi-solid biotissues. The coating of the proposed probe possessed a 3D interconnected porous architecture formed by the stacking of CNTs. As a result, the binding capacity toward carbohydrates was excellent. The proposed approach was demonstrated to be much superior to most carbohydrate sensors, including higher sensitivity, wider linear range, and excellent qualitative ability in multi-carbohydrate systems. Thus, this approach opens up new avenues for the facile and efficient recognition of carbohydrates for important applications such as glycomics.

Glucose transport machinery reconstituted in cell models

Here we demonstrate the production of a functioning cell model by formation of giant vesicles reconstituted with the GLUT1 glucose transporter and a glucose oxidase and hydrogen peroxidase linked fluorescent reporter internally. Hence, a simplified artificial cell is formed that is able to take up glucose and process it.

Metabolic engineering to enhance the value of plants as green factories

The promise of plants to serve as the green factories of the future is ever increasing. Plants have been used traditionally for construction, energy, food and feed. Bioactive compounds primarily derived from specialized plant metabolism continue to serve as important scaffold molecules for pharmaceutical drug production. Yet, the past few years have witnessed a growing interest on plants as the ultimate harvesters of carbon and energy from the sun, providing carbohydrate and lipid biofuels that would contribute to balancing atmospheric carbon. How can the metabolic output from plants be increased even further, and what are the bottlenecks? Here, we present what we perceive to be the main opportunities and challenges associated with increasing the efficiency of plants as chemical factories. We offer some perspectives on when it makes sense to use plants as production systems because the amount of biomass needed makes any other system unfeasible. However, there are other instances in which plants serve as great sources of biological catalysts, yet are not necessarily the best-suited systems for production. We also present emerging opportunities for manipulating plant genomes to make plant synthetic biology a reality.

An evaluation of genetically encoded FRET-based biosensors for quantitative metabolite analyses in vivo

A broad range of genetically-encoded fluorescence biosensors has been developed, allowing the detection of signaling intermediates and metabolites in real time. Many of these biosensors are based on Foerster Resonance Energy Transfer (FRET). The two biosensors of the well-known "Venus-flytrap" type exemplarily studied in this work are composed of a central sugar binding protein flanked by two green fluorescent protein derivatives, namely ECFP as well as Citrine and EYFP, respectively. In order to evaluate FRET-based biosensors as an in vivo tool for quantitative metabolite analyses, we have thoroughly studied the effects of pH, buffer salts, ionic strength, temperature and several intracellular metabolites on the signal intensity of both biosensors and both fluorescence proteins. Almost all micro-environmental variations led to considerably different FRET signals, because either the fluorescent proteins or the metabolite binding domains were affected by the tested parameters. This resulted not only in altered FRET ratios between the apo state and the saturated state but also in significant shifts of the apparent binding constant. This underlines the necessity of careful controls in order to allow reliable quantitative measurements in vivo.

Single-cell imaging tools for brain energy metabolism: a review

Neurophotonics comes to light at a time in which advances in microscopy and improved calcium reporters are paving the way toward high-resolution functional mapping of the brain. This review relates to a parallel revolution in metabolism. We argue that metabolism needs to be approached both in vitro and in vivo, and that it does not just exist as a low-level platform but is also a relevant player in information processing. In recent years, genetically encoded fluorescent nanosensors have been introduced to measure glucose, glutamate, ATP, NADH, lactate, and pyruvate in mammalian cells. Reporting relative metabolite levels, absolute concentrations, and metabolic fluxes, these sensors are instrumental for the discovery of new molecular mechanisms. Sensors continue to be developed, which together with a continued improvement in protein expression strategies and new imaging technologies, herald an exciting era of high-resolution characterization of metabolism in the brain and other organs.

FIBS-enabled noninvasive metabolic profiling

In the era of computational biology, new high throughput experimental systems are necessary in order to populate and refine models so that they can be validated for predictive purposes. Ideally such systems would be low volume, which precludes sampling and destructive analyses when time course data are to be obtained. What is needed is an in situ monitoring tool which can report the necessary information in real-time and noninvasively. An interesting option is the use of fluorescent, protein-based in vivo biological sensors as reporters of intracellular concentrations. One particular class of in vivo biosensors that has found applications in metabolite quantification is based on Förster Resonance Energy Transfer (FRET) between two fluorescent proteins connected by a ligand binding domain. FRET integrated biological sensors (FIBS) are constitutively produced within the cell line, they have fast response times and their spectral characteristics change based on the concentration of metabolite within the cell. In this paper, the method for constructing Chinese hamster ovary (CHO) cell lines that constitutively express a FIBS for glucose and glutamine and calibrating the FIBS in vivo in batch cell culture in order to enable future quantification of intracellular metabolite concentration is described. Data from fed-batch CHO cell cultures demonstrates that the FIBS was able in each case to detect the resulting change in the intracellular concentration. Using the fluorescent signal from the FIBS and the previously constructed calibration curve, the intracellular concentration was accurately determined as confirmed by an independent enzymatic assay.

High glucose potentiates L-FABP mediated fibrate induction of PPARα in mouse hepatocytes

Although liver fatty acid binding protein (L-FABP) binds fibrates and PPARα in vitro and enhances fibrate induction of PPARα in transformed cells, the functional significance of these findings is unclear, especially in normal hepatocytes. Studies with cultured primary mouse hepatocytes show that: 1) At physiological (6mM) glucose, fibrates (bezafibrate, fenofibrate) only weakly activated PPARα transcription of genes in LCFA β-oxidation; 2) High (11-20mM) glucose, but not maltose (osmotic control), significantly potentiated fibrate-induction of mRNA of these and other PPARα target genes to increase LCFA β-oxidation. These effects were associated with fibrate-mediated redistribution of L-FABP into nuclei-an effect prolonged by high glucose-but not with increased de novo fatty acid synthesis from glucose; 3) Potentiation of bezafibrate action by high glucose required an intact L-FABP/PPARα signaling pathway as shown with L-FABP null, PPARα null, PPARα inhibitor-treated WT, or PPARα-specific fenofibrate-treated WT hepatocytes. High glucose alone in the absence of fibrate was ineffective. Thus, high glucose potentiation of PPARα occurred through FABP/PPARα rather than indirectly through other PPARs or glucose induced signaling pathways. These data indicated L-FABP's importance in fibrate-induction of hepatic PPARα LCFA β-oxidative genes, especially in the context of high glucose levels.

Functional analyse of GLUT1 and GLUT12 in glucose uptake in goat mammary gland epithelial cells

Glucose transport, mediated by glucose transporters, is necessary for mammary gland development and lactation. GLUT1 and GLUT12 could both be expressed in the pregnant and lactating mammary gland to participate in the glucose uptake process. In this study, the goat GLUT1 and GLUT12 genes were cloned from Saanen dairy goats and transfected into goat mammary gland epithelial cells to assess their biological functions and distributions. The results showed that both goat GLUT1 and GLUT12 had 12 predicted membrane-spanning helices. Goat GLUT1 and GLUT12 each influenced the mRNA expression of the other transporter and increased the glucose consumption and lactose yield in GLUT1- and GLUT12-transfected goat mammary gland epithelial cells, respectively. The overexpression of GLUT1 or GLUT12 also increased the expression of amino acid transporters SLC1A5, SLC3A2 and SLC7A5 and affected genes expressions in GMGE cells. Using immunofluorescence staining, GLUT1 was detected throughout the cytoplasm and localized to the Golgi apparatus around the nuclear membrane, whereas GLUT12 was mainly distributed in the perinuclear region and cytoplasm. This study contributes to the understanding of how GLUT1 and GLUT12 cooperate in the incorporation of nutrient uptake into mammary gland epithelial cells and the promotion of milk synthesis in the goat mammary gland during lactation.

A genetically encoded metabolite sensor for malonyl-CoA

Malonyl-CoA is the rate-determining metabolite for long chain de novo fatty acid synthesis and allosterically inhibits the rate-setting step in long chain fatty acid β-oxidation. We developed a cell-based genetically encoded biosensor based on the malonyl-CoA responsive Bacillus subtilis transcriptional repressor, FapR, for living mammalian cells. Here, we show that fluctuations in malonyl-CoA, in mammalian cells, can regulate the transcription of a FapR-based malonyl-CoA biosensor. The biosensor reflects changes in malonyl-CoA flux regulated by malonyl-CoA decarboxylase and AMP-activated protein kinase in a concentration-dependent manner. To gain further insight into the regulatory mechanisms that affect fatty acid metabolism, we used the malonyl-CoA sensor to screen and identify several kinases. LIMK1 was identified and its expression was shown to alter both fatty acid synthesis and oxidation rates. This simple genetically encoded biosensor can be used to study the metabolic properties of live mammalian cells and enable screens for novel metabolic regulators.

Impact of L-FABP and glucose on polyunsaturated fatty acid induction of PPARα-regulated β-oxidative enzymes

Liver fatty acid binding protein (L-FABP) is the major soluble protein that binds very-long-chain n-3 polyunsaturated fatty acids (n-3 PUFAs) in hepatocytes. However, nothing is known about L-FABP's role in n-3 PUFA-mediated peroxisome proliferator activated receptor-α (PPARα) transcription of proteins involved in long-chain fatty acid (LCFA) β-oxidation. This issue was addressed in cultured primary hepatocytes from wild-type, L-FABP-null, and PPARα-null mice with these major findings: 1) PUFA-mediated increase in the expression of PPARα-regulated LCFA β-oxidative enzymes, LCFA/LCFA-CoA binding proteins (L-FABP, ACBP), and PPARα itself was L-FABP dependent; 2) PPARα transcription, robustly potentiated by high glucose but not maltose, a sugar not taken up, correlated with higher protein levels of these LCFA β-oxidative enzymes and with increased LCFA β-oxidation; and 3) high glucose altered the potency of n-3 relative to n-6 PUFA. This was not due to a direct effect of glucose on PPARα transcriptional activity nor indirectly through de novo fatty acid synthesis from glucose. Synergism was also not due to glucose impacting other signaling pathways, since it was observed only in hepatocytes expressing both L-FABP and PPARα. Ablation of L-FABP or PPARα as well as treatment with MK886 (PPARα inhibitor) abolished/reduced PUFA-mediated PPARα transcription of these genes, especially at high glucose. Finally, the PUFA-enhanced L-FABP distribution into nuclei with high glucose augmentation of the L-FABP/PPARα interaction reveals not only the importance of L-FABP for PUFA induction of PPARα target genes in fatty acid β-oxidation but also the significance of a high glucose enhancement effect in diabetes.

Fluorescence lifetime imaging microscopy of intracellular glucose dynamics

BACKGROUND

One of the major hurdles in studying diabetes pathophysiology is the lack of adequate methodology that allows for direct and real-time determination of glucose transport and metabolism in cells and tissues. In this article, we present a new methodology that adopts frequency-domain fluorescence lifetime imaging microscopy (FD-FLIM) to visualize and quantify the dynamics of intracellular glucose within living cells using a biosensor protein based on fluorescence resonance energy transfer (FRET).

METHOD

The biosensor protein was developed by fusing a FRET pair, an AcGFP1 donor and a mCherry acceptor to N- and C- termini of a mutant glucose-binding protein (GBP), respectively. The probe was expressed and biosynthesized inside the cells, offering continuous monitoring of glucose dynamics in real time through fluorescence lifetime imaging microscopy (FLIM) measurement.

RESULTS

We transfected the deoxyribonucleic acid of the AcGFP1-GBP-mCherry sensor into murine myoblast cells, C2C12, and continuously monitored the changes in intracellular glucose concentrations in response to the variation in extracellular glucose, from which we determined glucose uptake and clearance rates. The distribution of intracellular glucose concentration was also characterized. We detected a high glucose concentration in a region close to the cell membrane and a low glucose concentration in a region close to the nucleus. The monoexponential decay of AcGFP1 was distinguished using FD-FLIM.

CONCLUSIONS

This work enables continuous glucose monitoring (CGM) within living cells using FD-FLIM and a biosensor protein. The sensor protein developed offers a new means for quantitatively analyzing glucose homeostasis at the cellular level. Data accumulated from these studies will help increase our understanding of the pathology of diabetes.

Loss of intracellular lipid binding proteins differentially impacts saturated fatty acid uptake and nuclear targeting in mouse hepatocytes

The liver expresses high levels of two proteins with high affinity for long-chain fatty acids (LCFAs): liver fatty acid binding protein (L-FABP) and sterol carrier protein-2 (SCP-2). Real-time confocal microscopy of cultured primary hepatocytes from gene-ablated (L-FABP, SCP-2/SCP-x, and L-FABP/SCP-2/SCP-x null) mice showed that the loss of L-FABP reduced cellular uptake of 12-N-methyl-(7-nitrobenz-2-oxa-1,3-diazo)-aminostearic acid (a fluorescent-saturated LCFA analog) by ∼50%. Importantly, nuclear targeting of the LCFA was enhanced when L-FABP was upregulated (SCP-2/SCP-x null) but was significantly reduced when L-FABP was ablated (L-FABP null), thus impacting LCFA nuclear targeting. These effects were not associated with a net decrease in expression of key membrane proteins involved in LCFA or glucose transport. Since hepatic LCFA uptake and metabolism are closely linked to glucose uptake, the effect of glucose on L-FABP-mediated LCFA uptake and nuclear targeting was examined. Increasing concentrations of glucose decreased cellular LCFA uptake and even more extensively decreased LCFA nuclear targeting. Loss of L-FABP exacerbated the decrease in LCFA nuclear targeting, while loss of SCP-2 reduced the glucose effect, resulting in enhanced LCFA nuclear targeting compared with control. Simply, ablation of L-FABP decreases LCFA uptake and even more extensively decreases its nuclear targeting.

Mining the Sinorhizobium meliloti transportome to develop FRET biosensors for sugars, dicarboxylates and cyclic polyols

BACKGROUND

Förster resonance energy transfer (FRET) biosensors are powerful tools to detect biologically important ligands in real time. Currently FRET bisosensors are available for twenty-two compounds distributed in eight classes of chemicals (two pentoses, two hexoses, two disaccharides, four amino acids, one nucleobase, two nucleotides, six ions and three phytoestrogens). To expand the number of available FRET biosensors we used the induction profile of the Sinorhizobium meliloti transportome to systematically screen for new FRET biosensors.

METHODOLOGY/PRINCIPAL FINDINGS

Two new vectors were developed for cloning genes for solute-binding proteins (SBPs) between those encoding FRET partner fluorescent proteins. In addition to a vector with the widely used cyan and yellow fluorescent protein FRET partners, we developed a vector using orange (mOrange2) and red fluorescent protein (mKate2) FRET partners. From the sixty-nine SBPs tested, seven gave a detectable FRET signal change on binding substrate, resulting in biosensors for D-quinic acid, myo-inositol, L-rhamnose, L-fucose, β-diglucosides (cellobiose and gentiobiose), D-galactose and C4-dicarboxylates (malate, succinate, oxaloacetate and fumarate). To our knowledge, we describe the first two FRET biosensor constructs based on SBPs from Tripartite ATP-independent periplasmic (TRAP) transport systems.

CONCLUSIONS/SIGNIFICANCE

FRET based on orange (mOrange2) and red fluorescent protein (mKate2) partners allows the use of longer wavelength light, enabling deeper penetration of samples at lower energy and increased resolution with reduced back-ground auto-fluorescence. The FRET biosensors described in this paper for four new classes of compounds; (i) cyclic polyols, (ii) L-deoxy sugars, (iii) β-linked disaccharides and (iv) C4-dicarboxylates could be developed to study metabolism in vivo.

In situ monitoring of intracellular glucose and glutamine in CHO cell culture

The development of processes to produce biopharmaceuticals industrially is still largely empirical and relies on optimizing both medium formulation and cell line in a product-specific manner. Current small-scale (well plate-based) process development methods cannot provide sufficient sample volume for analysis, to obtain information on nutrient utilization which can be problematic when processes are scaled to industrial fermenters. We envision a platform where essential metabolites can be monitored non-invasively and in real time in an ultra-low volume assay in order to provide additional information on cellular metabolism in high throughput screens. Towards this end, we have developed a model system of Chinese Hamster Ovary cells stably expressing protein-based biosensors for glucose and glutamine. Herein, we demonstrate that these can accurately reflect changing intracellular metabolite concentrations in vivo during batch and fed-batch culture of CHO cells. The ability to monitor intracellular depletion of essential nutrients in high throughput will allow rapid development of improved bioprocesses.

Combinatorial insulin secretion dynamics of recombinant hepatic and enteroendocrine cells

One of the most promising cell-based therapies for combating insulin-dependent diabetes entails the use of genetically engineered non-β cells that secrete insulin in response to physiologic stimuli. A normal pancreatic β cell secretes insulin in a biphasic manner in response to glucose. The first phase is characterized by a transient stimulation of insulin to rapidly lower the blood glucose levels, which is followed by a second phase of insulin secretion to sustain the lowered blood glucose levels over a longer period of time. Previous studies have demonstrated hepatic and enteroendocrine cells to be appropriate hosts for recombinant insulin expression. Due to different insulin secretion kinetics from these cells, we hypothesized that a combination of the two cell types would mimic the biphasic insulin secretion of normal β cells with higher fidelity than either cell type alone. In this study, insulin secretion experiments were conducted with two hepatic cell lines (HepG2 and H4IIE) transduced with 1 of 3 adenoviruses expressing the insulin transgene and with a stably transfected recombinant intestinal cell line (GLUTag-INS). Insulin secretion was stimulated by exposing the cells to glucose only (hepatic cells), meat hydrolysate only (GLUTag-INS), or to a cocktail of the two secretagogues. It was found experimentally that the recombinant hepatic cells secreted insulin in a more sustained manner, whereas the recombinant intestinal cell line exhibited rapid insulin secretion kinetics upon stimulation. The insulin secretion profiles were computationally combined at different cell ratios to arrive at the combinatorial kinetics. Results indicate that combinations of these two cell types allow for tuning the first and second phase of insulin secretion better than either cell type alone. This work provides the basic framework in understanding the secretion kinetics of the combined system and advances it towards preclinical studies.

Optical sensors for monitoring dynamic changes of intracellular metabolite levels in mammalian cells

Knowledge of the in vivo levels, distribution and flux of ions and metabolites is crucial to our understanding of physiology in both healthy and diseased states. The quantitative analysis of the dynamics of ions and metabolites with subcellular resolution in vivo poses a major challenge for the analysis of metabolic processes. Genetically encoded Förster resonance energy transfer (FRET) sensors can be used for real-time in vivo detection of metabolites. FRET sensor proteins, for example, for glucose, can be targeted genetically to any cellular compartment, or even to subdomains (e.g., a membrane surface), by adding signal sequences or fusing the sensors to specific proteins. The sensors can be used for analyses in individual mammalian cells in culture, in tissue slices and in intact organisms. Applications include gene discovery, high-throughput drug screens or systematic analysis of regulatory networks affecting uptake, efflux and metabolism. Quantitative analyses obtained with the help of FRET sensors for glucose or other ions and metabolites provide valuable data for modeling of flux. Here we provide a detailed protocol for monitoring glucose levels in the cytosol of mammalian cell cultures through the use of FRET glucose sensors; moreover, the protocol can be used for other ions and metabolites and for analyses in other organisms, as has been successfully demonstrated in bacteria, yeast and even intact plants. The whole procedure typically takes ∼4 d including seeding and transfection of mammalian cells; the FRET-based analysis of transfected cells takes ∼5 h.

Semisynthesis of fluorescent metabolite sensors on cell surfaces

Progress in understanding signal transduction and metabolic pathways is hampered by a shortage of suitable sensors for tracking metabolites, second messengers, and neurotransmitters in living cells. Here we introduce a class of rationally designed semisynthetic fluorescent sensor proteins, called Snifits, for measuring metabolite concentrations on the cell surface of mammalian cells. Functional Snifits are assembled on living cells through two selective chemical labeling reactions of a genetically encoded protein scaffold. Our best Snifit displayed fluorescence intensity ratio changes on living cells significantly higher than any previously reported cell-surface-targeted fluorescent sensor protein. This work establishes a generally applicable and rational strategy for the generation of cell-surface-targeted fluorescent sensor proteins for metabolites of interest.

Quantitative glucose and ATP sensing in mammalian cells

The functioning and survival of mammalian cells requires an active energy metabolism. Metabolic dysfunction plays an important role in many human diseases, including diabetes, cancer, inherited mitochondrial disorders, and metabolic syndrome. The monosaccharide glucose constitutes a key source of cellular energy. Following its import across the plasma membrane, glucose is converted into pyruvate by the glycolysis pathway. Pyruvate oxidation supplies substrates for the ATP-generating mitochondrial oxidative phosphorylation (OXPHOS) system. To gain cell-biochemical knowledge about the operation and regulation of the cellular energy metabolism in the healthy and diseased state, quantitative knowledge is required about (changes in) metabolite concentrations under (non) steady-state conditions. This information can, for instance, be used to construct more realistic in silico models of cell metabolism, which facilitates understanding the consequences of metabolic dysfunction as well as on- and off-target effects of mitochondrial drugs. Here we review the current state-of-the-art live-cell quantification of two key cellular metabolites, glucose and ATP, using protein-based sensors. The latter apply the principle of FRET (fluorescence resonance energy transfer) and allow measurements in different cell compartments by fluorescence microscopy. We further summarize the properties and applications of the FRET-based sensors, their calibration, pitfalls, and future perspectives.

Glucose transporters in the uterus: an analysis of tissue distribution and proposed physiological roles

Facilitative glucose transport molecules (glucose transporters, GLUTs) are responsible for glucose transport across cellular membranes. Of the 14 family members, expression of nine has been reported in the murine uterus and seven in the human uterus. Some studies reveal that adequate glucose uptake and metabolism are essential for the proper differentiation of the uterine endometrium toward a receptive state capable of supporting embryo implantation. However, the mechanistic role of GLUTs in endometrial function remains poorly understood. This review aims to present the current knowledge about GLUT expression in the uterus and distribution among the different cell types within the endometrium. In addition, it analyzes the available data in the context of roles GLUTs may play in normal uterine physiology as well as the pathological conditions of infertility, endometrial cancer, and polycystic ovarian syndrome.

Dynamic imaging of glucose flux impedance using FRET sensors in wild-type Arabidopsis plants

Quantitative and dynamic analysis of metabolites and signalling molecules is limited by technical challenges in obtaining temporally resolved information at the cellular and compartmental level. Real-time information on signalling and metabolite levels with subcellular granularity can be obtained with the help of genetically encoded FRET (Förster resonance energy transfer) nanosensors. FRET nanosensors represent powerful tools for gene discovery, and analysis of regulatory networks, for example by screening mutants. However, RNA silencing has impaired our ability to express FRET nanosensors functionally in Arabidopsis plants. This drawback was overcome here by expressing the nanosensors in RNA silencing mutants. However, the use of silencing mutants requires the generation of homozygous lines deficient in RNA silencing as well as the mutation of interest and co-expression of the nanosensor. Here it is shown that dynamic changes in cytosolic glucose levels can readily be quantified in wild-type Arabidopsis plants at early stages of development (7-15 d) before silencing had a major effect on fluorescence intensity. A detailed protocol for screening 10-20 mutant seedlings per day is provided. The detailed imaging protocol provided here is suitable for analysing sugar flux in young wild-type plants as well as mutants affected in sugar signalling, metabolism, or transport using a wide spectrum of FRET nanosensors.

Mannose efflux from the cells: a potential source of mannose in blood

All mammals have 50-100 μM mannose in their blood. However, the source of the dynamic pool of mannose in blood is unknown. Most of it is thought to be derived from glucose in the cells. We studied mannose uptake and release by various cell types. Interestingly, our results show that mannose taken up by the cells through transporters is handled differently from the mannose released within the cells due to glycan processing of protein-bound oligosaccharides. Although more than 95% of incoming mannose is catabolized, most of the mannose released by intracellular processing is expelled from the cells as free mannose predominantly via a nocodazole-sensitive sugar transporter. Under physiological conditions, incoming mannose is more accessible to hexokinase, whereas mannose released within the cells is protected from HK and therefore has a different fate. Our data also suggest that generation of free mannose due to the processing of glycoconjugates composed of glucose-derived mannose and its efflux from the cells can account for most of the mannose found in blood and its steady state maintenance.

Construction of a novel glucose-sensing molecule based on a substrate-binding protein for intracellular sensing

A novel transcriptional regulator responding to glucose was designed with a substrate-binding protein (SBP) as a probe towards intracellular sensing system for glucose in mammalian cells. A chimeric protein of an SBP for glucose (GBP) and a LacI-type regulator, LacI (SLCP(GL) ), was designed, constructed and characterized using Escherichia coli recombinant protein. We report that SLCP(GL) has a glucose-specific binding ability and an operator-sequence specific DNA-binding ability. The loss of its DNA-binding ability in the presence of glucose suggests a role as a transcriptional regulator in vitro. The glucose-dependent gene regulation function of SLCP(GL) in cells was investigated using mammalian cells co-transfected with SLCP(GL) and Lac operator-fused luciferase gene constructs. The luciferase activity of the transfected cells increased with the glucose concentration in the medium, showing that the expression of the luciferase gene is regulated by SLCP(GL) , which can dissociate from DNA in a glucose concentration-dependent manner. Therefore, we demonstrated that SLCP(GL) functions as a glucose-sensitive transcriptional regulator in mammalian cells. These results reveal the possibility of developing an SBP-based regulator as a probe of intracellular sensing and gene regulation system for mammalian cells in response to a desired ligands depending on the SBP ligand specificity.

Sugar transporters for intercellular exchange and nutrition of pathogens

Sugar efflux transporters are essential for the maintenance of animal blood glucose levels, plant nectar production, and plant seed and pollen development. Despite broad biological importance, the identity of sugar efflux transporters has remained elusive. Using optical glucose sensors, we identified a new class of sugar transporters, named SWEETs, and show that at least six out of seventeen Arabidopsis, two out of over twenty rice and two out of seven homologues in Caenorhabditis elegans, and the single copy human protein, mediate glucose transport. Arabidopsis SWEET8 is essential for pollen viability, and the rice homologues SWEET11 and SWEET14 are specifically exploited by bacterial pathogens for virulence by means of direct binding of a bacterial effector to the SWEET promoter. Bacterial symbionts and fungal and bacterial pathogens induce the expression of different SWEET genes, indicating that the sugar efflux function of SWEET transporters is probably targeted by pathogens and symbionts for nutritional gain. The metazoan homologues may be involved in sugar efflux from intestinal, liver, epididymis and mammary cells.

Sugar transporters for intercellular exchange and nutrition of pathogens

Sugar efflux transporters are essential for the maintenance of animal blood glucose levels, plant nectar production, and plant seed and pollen development. Despite broad biological importance, the identity of sugar efflux transporters has remained elusive. Using optical glucose sensors, we identified a new class of sugar transporters, named SWEETs, and show that at least six out of seventeen Arabidopsis, two out of over twenty rice and two out of seven homologues in Caenorhabditis elegans, and the single copy human protein, mediate glucose transport. Arabidopsis SWEET8 is essential for pollen viability, and the rice homologues SWEET11 and SWEET14 are specifically exploited by bacterial pathogens for virulence by means of direct binding of a bacterial effector to the SWEET promoter. Bacterial symbionts and fungal and bacterial pathogens induce the expression of different SWEET genes, indicating that the sugar efflux function of SWEET transporters is probably targeted by pathogens and symbionts for nutritional gain. The metazoan homologues may be involved in sugar efflux from intestinal, liver, epididymis and mammary cells.

Facilitative plasma membrane transporters function during ER transit

Although biochemical studies suggested a high permeability of the endoplasmic reticulum (ER) membrane for small molecules, proteomics identified few specialized ER transporters. To test functionality of transporters during ER passage, we tested whether glucose transporters (GLUTs, SGLTs) destined for the plasma membrane are active during ER transit. HepG2 cells were characterized by low-affinity ER transport activity, suggesting that ER uptake is protein mediated. The much-reduced capacity of HEK293T cells to take up glucose across the plasma membrane correlated with low ER transport. Ectopic expression of GLUT1, -2, -4, or -9 induced GLUT isoform-specific ER transport activity in HEK293T cells. In contrast, the Na(+)-glucose cotransporter SGLT1 mediated efficient plasma membrane glucose transport but no detectable ER uptake, probably because of lack of a sufficient sodium gradient across the ER membrane. In conclusion, we demonstrate that GLUTs are sufficient for mediating ER glucose transport en route to the plasma membrane. Because of the low volume of the ER, trace amounts of these uniporters contribute to ER solute import during ER transit, while uniporters and cation-coupled transporters carry out export from the ER, together potentially explaining the low selectivity of ER transport. Expression levels and residence time of transporters in the ER, as well as their coupling mechanisms, could be key determinants of ER permeability.

The protein family of glucose transport facilitators: It's not only about glucose after all

The protein family of facilitative glucose transporters comprises 14 isoforms that share common structural features such as 12 transmembrane domains, N- and C-termini facing the cytoplasm of the cell, and a N-glycosylation side either within the first or fifth extracellular loop. Based on their sequence homology, three classes can be distinguished: class I includes GLUT1-4 and GLUT14, class II the "odd transporters" GLUT5, 7, 9, 11, and class III the "even transporters" GLUT6, 8, 10, 12 and the proton driven myoinositol transporter HMIT (or GLUT13). With the cloning and characterization of the more recent class II and III isoforms, it became apparent that despite their structural similarities, the different isoforms not only show a distinct tissue-specific expression pattern but also show distinct characteristics such as alternative splicing, specific (sub)cellular localization, and affinities for a spectrum of substrates. This review summarizes the current understanding of the physiological role for the various transport facilitators based on human genetically inherited disorders or single-nucleotide polymorphisms and knockout mice models. The emphasis of the review will be on the potential functional role of the more recent isoforms.

Single-walled carbon nanotube-based chemiresistive affinity biosensors for small molecules: ultrasensitive glucose detection

We report for the first time single-walled carbon nanotube (SWNT)-based chemiresistive affinity sensors for highly sensitive and selective detection of small and/or weakly charged or uncharged molecules using a displacement format. The detection of glucose, a small, weakly charged molecule, by displacement of plant lectin (concavalin A) bound to a polysaccharide (dextran) immobilized on SWNTs with picomolar sensitivity and selectivity over other sugars and human serum proteins is demonstrated as a proof of concept.

Imaging approach for monitoring cellular metabolites and ions using genetically encoded biosensors

The spatiotemporal patterns of ion and metabolite levels in living cells are important in understanding signal transduction and metabolite flux. Imaging approaches using genetically encoded sensors are ideal for detecting such molecule dynamics, which are hard to capture otherwise. Recent years have seen iterative improvements and evaluations of sensors, which in turn are starting to make applications in more challenging experimental settings possible. In this review, we will introduce recent progress made in the variety and properties of biosensors, and how biosensors are used for the measurement of metabolite and ion in live cells. The emerging field of applications, such as parallel imaging of two separate molecules, high-resolution transport studies and high-throughput screening using biosensors, will be discussed.