Interaction of glucose transporter 1 with anion exchanger 1 in vitro

The protein organization of a red blood cell

Red blood cells (RBCs) (erythrocytes) are the simplest primary human cells, lacking nuclei and major organelles and instead employing about a thousand proteins to dynamically control cellular function and morphology in response to physiological cues. In this study, we define a canonical RBC proteome and interactome using quantitative mass spectrometry and machine learning. Our data reveal an RBC interactome dominated by protein homeostasis, redox biology, cytoskeletal dynamics, and carbon metabolism. We validate protein complexes through electron microscopy and chemical crosslinking and, with these data, build 3D structural models of the ankyrin/Band 3/Band 4.2 complex that bridges the spectrin cytoskeleton to the RBC membrane. The model suggests spring-like compression of ankyrin may contribute to the characteristic RBC cell shape and flexibility. Taken together, our study provides an in-depth view of the global protein organization of human RBCs and serves as a comprehensive resource for future research.

Architecture of the human erythrocyte ankyrin-1 complex

The stability and shape of the erythrocyte membrane is provided by the ankyrin-1 complex, but how it tethers the spectrin-actin cytoskeleton to the lipid bilayer and the nature of its association with the band 3 anion exchanger and the Rhesus glycoproteins remains unknown. Here we present structures of ankyrin-1 complexes purified from human erythrocytes. We reveal the architecture of a core complex of ankyrin-1, the Rhesus proteins RhAG and RhCE, the band 3 anion exchanger, protein 4.2, glycophorin A and glycophorin B. The distinct T-shaped conformation of membrane-bound ankyrin-1 facilitates recognition of RhCE and, unexpectedly, the water channel aquaporin-1. Together, our results uncover the molecular details of ankyrin-1 association with the erythrocyte membrane, and illustrate the mechanism of ankyrin-mediated membrane protein clustering.

Insight into the Different Channel Proteins of Human Red Blood Cell Membranes Revealed by Combined dSTORM and AFM Techniques

Membrane proteins tend to interact with each other in the cell membranes to form protein clusters and perform the corresponding physiological functions. However, because channel proteins are involved in many biological functions, their distribution and nano-organization in these protein clusters are unclear. To study the distribution patterns and relationships between the different channel proteins, we identified the locations of glucose transporter 1 (Glut1) and Band3 (anion transporter 1) precisely in the topography of the cytoplasmic side of the human red blood cell (hRBC) membranes using combined atomic force microscopy (AFM) and single-molecule localization microscopy (SMLM). The AFM results revealed that membrane proteins interacted with each other and aggregated into protein islands. The SMLM results showed that Glut1 and Band3 tended to form protein clusters in the hRBC membranes, and there was a strong colocalization between the two proteins. The results of the combined AFM and SMLM method indicated that the protein clusters of Glut1 and Band3 were mainly located in the protein islands of topography, and the protein islands in topography also interacted with each other to assemble into larger protein clusters or functional microdomains.

Ankyrin G organizes membrane components to promote coupling of cell mechanics and glucose uptake

The response of cells to forces is critical for their function and occurs via rearrangement of the actin cytoskeleton¹. Cytoskeletal remodelling is energetically costly^2,3, yet how cells signal for nutrient uptake remains undefined. Here we present evidence that force transmission increases glucose uptake by stimulating glucose transporter 1 (GLUT1). GLUT1 recruitment to and retention at sites of force transmission requires non-muscle myosin IIA-mediated contractility and ankyrin G. Ankyrin G forms a bridge between the force-transducing receptors and GLUT1. This bridge is critical for enabling cells under tension to tune glucose uptake to support remodelling of the actin cytoskeleton and formation of an epithelial barrier. Collectively, these data reveal an unexpected mechanism for how cells under tension take up nutrients and provide insight into how defects in glucose transport and mechanics might be linked.

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.

Continuous noninvasive glucose monitoring; water as a relevant marker of glucose uptake in vivo

With diabetes set to become the number 3 killer in the Western hemisphere and proportionally growing in other parts of the world, the subject of noninvasive monitoring of glucose dynamics in blood remains a "hot" topic, with the involvement of many groups worldwide. There is a plethora of techniques involved in this academic push, but the so-called multisensor system with an impedance-based core seems to feature increasingly strongly. However, the symmetrical structure of the glucose molecule and its shielding by the smaller dipoles of water would suggest that this option should be less enticing. Yet there is enough phenomenological evidence to suggest that impedance-based methods are truly sensitive to the biophysical effects of glucose variations in the blood. We have been trying to answer this very fundamental conundrum: "Why is impedance or dielectric spectroscopy sensitive to glucose concentration changes in the blood and why can this be done over a very broad frequency band, including microwaves?" The vistas for medical diagnostics are very enticing. There have been a significant number of papers published that look seriously at this problem. In this review, we want to summarize this body of research and the underlying mechanisms and propose a perspective toward utilizing the phenomena. It is our impression that the current world view on the dielectric response of glucose in solution, as outlined below, will support the further evolution and implementation toward practical noninvasive glucose monitoring solutions.

Evidence for three populations of the glucose transporter in the human erythrocyte membrane

Glucose transporter 1 (GLUT1) is one of 13 members of the human equilibrative glucose transport protein family and the only glucose transporter thought to be expressed in human erythrocyte membranes. Although GLUT1 has been shown to be anchored to adducin at the junctional spectrin-actin complex of the membrane through interactions with multiple proteins, whether other populations of GLUT1 also exist in the human erythrocyte membrane has not been examined. Because GLUT1 plays such a critical role in erythrocyte biology and since it comprises 10% of the total membrane protein, we undertook to evaluate the subpopulations of erythrocyte GLUT1 using single particle tracking. By monitoring the diffusion of individual AlexaFluor 488-labeled GLUT1 molecules on the surfaces of intact erythrocytes, we are able to identify three distinct subpopulations of GLUT1. While the mobility of the major subpopulation is similar to that of the anion transporter, band 3, both a more mobile and more anchored subpopulation also exist. From these studies, we conclude that ~65% of GLUT1 resides in similar or perhaps the same protein complex as band 3, while the remaining 1/3rd are either freely diffusing or interacting with other cytoskeletally anchored membrane protein complexes.

Dielectric Response of Cytoplasmic Water and Its Connection to the Vitality of Human Red Blood Cells: I. Glucose Concentration Influence

The vitality of red blood cells depends on the process control of glucose homeostasis, including the membrane's ability to "switch off" d-glucose uptake at the physiologically specific concentration of 10-12 mM. We present a comprehensive study of human erythrocytes suspended in buffer solutions with varying concentrations of d-glucose at room temperature, using microwave dielectric spectroscopy (0.5 GHz-50 GHz) and cell deformability characterization (the Elongation ratio). By use of mixture formulas the contribution of the cytoplasm to the dielectric spectra was isolated. It reveals a strong dependence on the concentration of buffer d-glucose. Tellingly, the concentration 10-12 mM is revealed as a critical point in the behavior. The dielectric response of cytoplasm depends on dipole-matrix interactions between water structures and moieties, like ATP, produced during glycolysis. Subsequently, it is a marker of cellular health. One would hope that this mechanism could provide a new vista on noninvasive glucose monitoring.

Severe Ankyrin-R deficiency results in impaired surface retention and lysosomal degradation of RhAG in human erythroblasts

Ankyrin-R provides a key link between band 3 and the spectrin cytoskeleton that helps to maintain the highly specialized erythrocyte biconcave shape. Ankyrin deficiency results in fragile spherocytic erythrocytes with reduced band 3 and protein 4.2 expression. We use in vitro differentiation of erythroblasts transduced with shRNAs targeting ANK1 to generate erythroblasts and reticulocytes with a novel ankyrin-R 'near null' human phenotype with less than 5% of normal ankyrin expression. Using this model, we demonstrate that absence of ankyrin negatively impacts the reticulocyte expression of a variety of proteins, including band 3, glycophorin A, spectrin, adducin and, more strikingly, protein 4.2, CD44, CD47 and Rh/RhAG. Loss of band 3, which fails to form tetrameric complexes in the absence of ankyrin, alongside GPA, occurs due to reduced retention within the reticulocyte membrane during erythroblast enucleation. However, loss of RhAG is temporally and mechanistically distinct, occurring predominantly as a result of instability at the plasma membrane and lysosomal degradation prior to enucleation. Loss of Rh/RhAG was identified as common to erythrocytes with naturally occurring ankyrin deficiency and demonstrated to occur prior to enucleation in cultures of erythroblasts from a hereditary spherocytosis patient with severe ankyrin deficiency but not in those exhibiting milder reductions in expression. The identification of prominently reduced surface expression of Rh/RhAG in combination with direct evaluation of ankyrin expression using flow cytometry provides an efficient and rapid approach for the categorization of hereditary spherocytosis arising from ankyrin deficiency.

SNPs altering ammonium transport activity of human Rhesus factors characterized by a yeast-based functional assay

Proteins of the conserved Mep-Amt-Rh family, including mammalian Rhesus factors, mediate transmembrane ammonium transport. Ammonium is an important nitrogen source for the biosynthesis of amino acids but is also a metabolic waste product. Its disposal in urine plays a critical role in the regulation of the acid/base homeostasis, especially with an acid diet, a trait of Western countries. Ammonium accumulation above a certain concentration is however pathologic, the cytotoxicity causing fatal cerebral paralysis in acute cases. Alteration in ammonium transport via human Rh proteins could have clinical outcomes. We used a yeast-based expression assay to characterize human Rh variants resulting from non synonymous single nucleotide polymorphisms (nsSNPs) with known or unknown clinical phenotypes and assessed their ammonium transport efficiency, protein level, localization and potential trans-dominant impact. The HsRhAG variants (I61R, F65S) associated to overhydrated hereditary stomatocytosis (OHSt), a disease affecting erythrocytes, proved affected in intrinsic bidirectional ammonium transport. Moreover, this study reveals that the R202C variant of HsRhCG, the orthologue of mouse MmRhcg required for optimal urinary ammonium excretion and blood pH control, shows an impaired inherent ammonium transport activity. Urinary ammonium excretion was RHcg gene-dose dependent in mouse, highlighting MmRhcg as a limiting factor. HsRhCG(R202C) may confer susceptibility to disorders leading to metabolic acidosis for instance. Finally, the analogous R211C mutation in the yeast ScMep2 homologue also impaired intrinsic activity consistent with a conserved functional role of the preserved arginine residue. The yeast expression assay used here constitutes an inexpensive, fast and easy tool to screen nsSNPs reported by high throughput sequencing or individual cases for functional alterations in Rh factors revealing potential causal variants.

Protein distribution during human erythroblast enucleation in vitro

Enucleation is the step in erythroid terminal differentiation when the nucleus is expelled from developing erythroblasts creating reticulocytes and free nuclei surrounded by plasma membrane. We have studied protein sorting during human erythroblast enucleation using fluorescence activated cell sorting (FACS) to obtain pure populations of reticulocytes and nuclei produced by in vitro culture. Nano LC mass spectrometry was first used to determine the protein distribution profile obtained from the purified reticulocyte and extruded nuclei populations. In general cytoskeletal proteins and erythroid membrane proteins were preferentially restricted to the reticulocyte alongside key endocytic machinery and cytosolic proteins. The bulk of nuclear and ER proteins were lost with the nucleus. In contrast to the localization reported in mice, several key erythroid membrane proteins were detected in the membrane surrounding extruded nuclei, including band 3 and GPC. This distribution of key erythroid membrane and cytoskeletal proteins was confirmed using western blotting. Protein partitioning during enucleation was investigated by confocal microscopy with partitioning of cytoskeletal and membrane proteins to the reticulocyte observed to occur at a late stage of this process when the nucleus is under greatest constriction and almost completely extruded. Importantly, band 3 and CD44 were shown not to restrict specifically to the reticulocyte plasma membrane. This highlights enucleation as a stage at which excess erythroid membrane proteins are discarded in human erythroblast differentiation. Given the striking restriction of cytoskeleton proteins and the fact that membrane proteins located in macromolecular membrane complexes (e.g. GPA, Rh and RhAG) are segregated to the reticulocyte, we propose that the membrane proteins lost with the nucleus represent an excess mobile population of either individual proteins or protein complexes.

Refined views of multi-protein complexes in the erythrocyte membrane

The erythrocyte membrane has been extensively studied, both as a model membrane system and to investigate its role in gas exchange and transport. Much is now known about the protein components of the membrane, how they are organised into large multi-protein complexes and how they interact with each other within these complexes. Many links between the membrane and the cytoskeleton have also been delineated and have been demonstrated to be crucial for maintaining the deformability and integrity of the erythrocyte. In this study we have refined previous, highly speculative molecular models of these complexes by including the available data pertaining to known protein-protein interactions. While the refined models remain highly speculative, they provide an evolving framework for visualisation of these important cellular structures at the atomic level.

Anion exchanger 1 in red blood cells and kidney: Band 3's in a pod

The bicarbonate/chloride exchanger 1 (AE1, Band 3) is abundantly expressed in the red blood cell membrane, where it is involved in gas exchange and functions as a major site of cytoskeletal attachment to the erythrocyte membrane. A truncated kidney isoform (kAE1) is highly expressed in type A intercalated cells of the distal tubules, where it is vital for urinary acidification. Recently, kAE1 has emerged as a novel physiologically significant protein in the kidney glomerulus. This minireview will discuss the known interactions of kAE1 in the podocytes and the possible mechanisms whereby this important multispanning membrane protein may contribute to the function of the glomerular filtration barrier and prevent proteinuria.

The role of GLUT1 in the sugar-induced dielectric response of human erythrocytes

We propose a key role for the glucose transporter 1 (GLUT1) in mediating the observed changes in the dielectric properties of human erythrocyte membranes as determined by dielectric spectroscopy. Cytochalasin B, a GLUT1 transport inhibitor, abolished the membrane capacitance changes in glucose-exposed red cells. Surprisingly, D-fructose, known to be transported primarily by GLUT5, exerted similar membrane capacitance changes at increasing D-fructose concentrations. In order to evaluate whether the glucose-mediated membrane capacitance changes originated directly from intracellularly bound adenosine triphosphate (ATP) or other components of the glycolysis process, we studied the dielectric responses of swollen erythrocytes with a decreased ATP content and of nucleotide-filled ghosts. Resealed ghosts containing physiological concentrations of ATP yielded the same glucose-dependent capacitance changes as biconcave intact red blood cells, further supporting the finding that ATP is the effector of the glucose-mediated dielectric response where the ATP concentration is also the mediating factor in swollen red blood cells. The results suggest that ATP binding to GLUT1 elicits a membrane capacitance change that increases with the applied concentration gradient of D-glucose. A simplified model of the membrane capacitance alteration with glucose uptake is proposed.

Amyloid-beta peptide affects the oxygen dependence of erythrocyte metabolism: a role for caspase 3

Human erythrocyte metabolism is modulated by the cell oxygenation state. Among other mechanisms, competition of deoxyhemoglobin and some glycolytic enzymes for the cytoplasmic domain of band 3 is probably involved in modulation. This metabolic modulation is connected to variations in intracellular NADPH and ATP levels as a function of the oxygenation state of the cell, and, consequently, it should have physiologic relevance. The present study investigates the effect of amyloid-beta peptide exposure on this metabolic modulation and its relationship with the activity of erythrocyte caspase 3. Metabolic differences between erythrocytes incubated at high and low oxygen saturation disappear following to 24 h exposure to amyloid-beta peptide. Western blotting analysis shows that caspase 3 is concurrently activated. Pre-incubation of amyloid-beta peptide-treated erythrocytes with a specific inhibitor of caspase 3, partially restores the oxygen-dependent modulation. This finding suggests that human erythrocytes following to exposure to amyloid-beta peptide show a complete loss of the oxygen-dependent metabolic modulation, which is partially restored by caspase 3 inhibitor-treatment.