Marginal cells of the stria vascularis of gerbils take up glucose via the facilitated transporter GLUT: application of autofluorescence

Modulation of Glucose Takeup by Glucose Transport on the Isolated OHCs

Glucose is a fundamental source of energy for mammalian cells; however, whether glucose is taken up through the lateral walls of cochlear outer hair cells (OHCs) is unknown. The OHC lateral wall is complex, composed of a plasma membrane, cortical lattice, and subsurface cisternae. This study assessed the uptake of glucose by OHCs using live-cell microscopy and examined the distribution of glucose transporter isoforms by immunohistochemistry. We found that glucose transporter-4 was mostly expressed on the lateral wall of OHCs. Glucose crossed the lateral walls of OHCs via glucose transporters-4 mainly, and this process could be modulated. These results suggest that the lateral walls are involved in modulating energy transport into OHCs.

Differential localizations of the myo-inositol transporters HMIT and SMIT1 in the cochlear stria vascularis

The cochlear stria vascularis produces endolymph and thereby plays an active role in inner ear homeostasis. We recently reported that the H⁺/myo-inositol cotransporter (HMIT) gene is expressed in the stria vascularis. Here, we examined the protein localization of HMIT and Na⁺/myo-inositol cotransporter 1 (SMIT1) in the stria vascularis by immunohistochemistry. HMIT and SMIT1 were detected in the lateral wall of the cochlear duct. HMIT was widely detected throughout the stria vascularis, while SMIT1 was enriched in the strial basal cells. To examine the localization of HMIT in the stria vascularis in more detail, dissociated strial cells were immunostained, which resulted in the detection of HMIT immunoreactivity in marginal cells. These results indicate that HMIT is expressed in marginal cells and basal cells of the stria vascularis, while SMIT1 expression is enriched in basal cells. We speculate that HMIT and SMIT1 may play important roles in the homeostasis of cochlear fluids, for example by participating in pH regulation and osmoregulation.

Sodium-glucose transporter-2 (SGLT2; SLC5A2) enhances cellular uptake of aminoglycosides

Aminoglycoside antibiotics, like gentamicin, continue to be clinically essential worldwide to treat life-threatening bacterial infections. Yet, the ototoxic and nephrotoxic side-effects of these drugs remain serious complications. A major site of gentamicin uptake and toxicity resides within kidney proximal tubules that also heavily express electrogenic sodium-glucose transporter-2 (SGLT2; SLC5A2) in vivo. We hypothesized that SGLT2 traffics gentamicin, and promotes cellular toxicity. We confirmed in vitro expression of SGLT2 in proximal tubule-derived KPT2 cells, and absence in distal tubule-derived KDT3 cells. D-glucose competitively decreased the uptake of 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose (2-NBDG), a fluorescent analog of glucose, and fluorescently-tagged gentamicin (GTTR) by KPT2 cells. Phlorizin, an SGLT2 antagonist, strongly inhibited uptake of 2-NBDG and GTTR by KPT2 cells in a dose- and time-dependent manner. GTTR uptake was elevated in KDT3 cells transfected with SGLT2 (compared to controls); and this enhanced uptake was attenuated by phlorizin. Knock-down of SGLT2 expression by siRNA reduced gentamicin-induced cytotoxicity. In vivo, SGLT2 was robustly expressed in kidney proximal tubule cells of heterozygous, but not null, mice. Phlorizin decreased GTTR uptake by kidney proximal tubule cells in Sglt2^+/− mice, but not in Sglt2^−/− mice. However, serum GTTR levels were elevated in Sglt2^−/− mice compared to Sglt2^+/− mice, and in phlorizin-treated Sglt2^+/− mice compared to vehicle-treated Sglt2^+/− mice. Loss of SGLT2 function by antagonism or by gene deletion did not affect gentamicin cochlear loading or auditory function. Phlorizin did not protect wild-type mice from kanamycin-induced ototoxicity. We conclude that SGLT2 can traffic gentamicin and contribute to gentamicin-induced cytotoxicity.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

Multiple expression of glucose transporters in the lateral wall of the cochlear duct studied by quantitative real-time PCR assay

We have investigated the gene expression of the facilitated glucose transporter (GLUT), H+-coupled myo-inositol cotransporter (HMIT), and Na+ glucose cotransporter (SGLT) in the lateral wall of the cochlear duct by conventional RT-PCR and quantitative real-time PCR. The isoforms GLUT1, -3, -4, -5, -8, -10, -12 and HMIT were detected in both the stria vascularis and the spiral ligament, whereas no SGLT isoforms could be detected in these tissues. Quantitative real-time PCR analysis revealed significant differences in the gene expression of GLUT1, -4, -5, -10, and HMIT isoforms between the stria vascularis and the spiral ligament. This result reflects the tissue-dependent distributions of GLUT isoforms. These findings strongly suggest that a number of GLUT isoforms participate in glucose transport in the stria vascularis and the spiral ligament.

Cellular localization of facilitated glucose transporter 1 (GLUT-1) in the cochlear stria vascularis: its possible contribution to the transcellular glucose pathway

Immunoreactivity for the facilitated glucose transporter 1 (GLUT-1) has been found in the cochlear stria vascularis, but whether the strial marginal cells are immunopositive for GLUT-1 remains uncertain. To determine the cellular localization of GLUT-1 and to clarify the glucose pathway in the stria vascularis of rats and guinea pigs, immunohistochemistry was performed on sections, dissociated cells, and whole-tissue preparations. Immunoreactivity for GLUT-1 in sections was observed in the basal side of the strial tissue and in capillaries in both rats and guinea pigs. However, the distribution of the positive signals within the guinea pig strial tissue was more diffuse than that in rats. Immunostaining of dissociated guinea pig strial cells revealed GLUT-1 in the basal cells and capillary endothelial cells, but not in the marginal cells. These results indicated that GLUT-1 was not expressed in the marginal cells, and that another isoform of GLUT was probably expressed in these cells. Three-dimensional observation of whole-tissue preparations demonstrated that cytoplasmic prolongations from basal cells extended upward to the apical surface of the stria vascularis from rats and guinea pigs, and that the marginal cells were surrounded by these protrusions. We speculate that these upward extensions of basal cells have been interpreted as basal infoldings of marginal cells in previous reports from other groups. The three-dimensional relationship between marginal cells and basal cells might contribute to the transcellular glucose pathway from perilymph to intrastrial space.

Non-invasive glucose monitoring by NAD(P)H autofluorescence spectroscopy in fibroblasts and adipocytes: a model for skin glucose sensing

The aim of this study was to develop an in vitro cell-culture model of skin-component cells to test the hypothesis that glucose can be monitored non-invasively by measuring NAD(P)H-related fluorescence changes in tissues. 3T3-L1 fibroblasts and adipocytes were grown in culture, and the response to added glucose was assessed by changes in steady-state autofluorescence at 400-500 nm [excitation at 340 nm, an index of NAD(P)H]. We also studied glucose-related fluorescence changes in cells stained with the mitochondrial marker, rhodamine-123. Fibroblasts and adipocytes showed glucose-dependent increases in autofluorescence with both short- and long-term exposure. Spectral properties indicated that the fluorescence was due to NAD(P)H production. With 5-h exposure to glucose, the maximal response was at 10-15 mmol/L glucose. Cells stained with the fluorescent mitochondrial marker, rhodamine-123, showed an immediate and marked decrease in fluorescence when exposed to glucose. We conclude that glucose can be sensed non-invasively by cellular fluorescence changes in fibroblasts and adipocytes. This is a model for the further exploration of fluorescence-based non-invasive metabolic monitoring in human diabetes.

In vivo labeling of mitochondrial complex I (NADH:ubiquinone oxidoreductase) in rat brain using [(3)H]dihydrorotenone

Defects in mitochondrial energy metabolism have been implicated in several neurodegenerative disorders. Defective complex I (NADH:ubiquinone oxidoreductase) activity plays a key role in Leber's hereditary optic neuropathy and, possibly, Parkinson's disease, but there is no way to assess this enzyme in the living brain. We previously described an in vitro quantitative autoradiographic assay using [(3)H]dihydrorotenone ([(3)H]DHR) binding to complex I. We have now developed an in vivo autoradiographic assay for complex I using [(3)H]DHR binding after intravenous administration. In vivo [(3)H]DHR binding was regionally heterogeneous, and brain uptake was rapid. Binding was enriched in neurons compared with glia, and white matter had the lowest levels of binding. In vivo [(3)H]DHR binding was markedly reduced by local and systemic infusion of rotenone and was enhanced by local NADH administration. There was an excellent correlation between regional levels of in vivo [(3)H]DHR binding and the in vitro activities of complex II (succinate dehydrogenase) and complex IV (cytochrome oxidase), suggesting that the stoichiometry of these components of the electron transport chain is relatively constant across brain regions. The ability to assay complex I in vivo should provide a valuable tool to investigate the status of this mitochondrial enzyme in the living brain and suggests potential imaging techniques for complex I in humans.

Real-time Analysis of Glucose Metabolism by Microscopy

Glucose metabolism has traditionally been assayed via biochemical means. Fluorescence monitoring of NAD(P)H levels has provided a non-invasive method to assay glucose metabolism in cells and tissues. However, these measurements have traditionally been of low resolution (no subcellular information) because of limitations imposed by optical and cellular photodamage problems. The recent advent of two-photon excitation microscopy as a dependable tool for biological research has opened the possibility of real-time, high-resolution analysis of glucose metabolism in living cells. Such measurements have the potential to provide subcellular information from intact tissue that cannot be obtained by other techniques.

Ultrastructural localization of glucose transporter 1 (GLUT1) in guinea pig stria vascularis and vestibular dark cell areas: an immunogold study

Glucose transporter 1 (GLUT1) is one of the facilitated-diffusion glucose transporters, which plays a major role in the glucose transport across blood-tissue barriers. We demonstrated localization of GLUT1 in guinea pig stria vascularis (SV) and vestibular dark cell areas, using electron microscopy. GLUT1 immunoreactivity was seen mainly in the SV. Electron microscopically, immunogold particles were localized along the plasma membranes of the basal cells and the basolateral infoldings of the marginal cells. Erythrocyte membranes and the capillary walls in both the SV and the dark cell areas were also positive for GLUT1. These results indicate that GLUT1 is involved in glucose regulation for energy metabolism in the marginal cells and vestibular dark cells, and in glucose transport between blood and perilymph.