Erythrocyte Glut1 triggers dehydroascorbic acid uptake in mammals unable to synthesize vitamin C.

Of all cells, human erythrocytes express the highest level of the Glut1 glucose transporter. However, the regulation and function of Glut1 during erythropoiesis are not known. Here, we report that glucose transport actually decreases during human erythropoiesis despite a >3-log increase in Glut1 transcripts. In contrast, Glut1-mediated transport of L-dehydroascorbic acid (DHA), an oxidized form of ascorbic acid (AA), is dramatically enhanced. We identified stomatin, an integral erythrocyte membrane protein, as regulating the switch from glucose to DHA transport. Notably though, we found that erythrocyte Glut1 and associated DHA uptake are unique traits of humans and the few other mammals that have lost the ability to synthesize AA from glucose. Accordingly, we show that mice, a species capable of synthesizing AA, express Glut4 but not Glut1 in mature erythrocytes. Thus, erythrocyte-specific coexpression of Glut1 with stomatin constitutes a compensatory mechanism in mammals that are unable to synthesize vitamin C.

Drug-induced endovesiculation of erythrocytes is modulated by the dynamics in the cytoskeleton/membrane interaction.

Recent studies on erythrocyte membrane fluctuations revealed that the erythrocyte cytoskeleton actively modulates its membrane association thereby regulating crucial membrane properties. Cationic amphiphilic drugs like chlorpromazine are known to induce a cup-like cell shape and vesicle formation into the cell interior, effectors of this process, however, are largely unknown. Using flow cytometry, this study explored conditions that influence endovesiculation induced by chlorpromazine. We found that inhibitors of membrane fluctuations, like ATP depletion, vanadate or fluoride, also inhibited endovesiculation whereas activation of PKC, known to decrease cytoskeleton association and increase membrane fluctuations, also enhanced endovesicle formation. This indicates that endovesicle formation and membrane fluctuations are modulated by the same cytoskeleton-regulated membrane properties. Further, acanthocytic erythrocytes of chorea acanthocytosis (ChAc) patients that lack the VPS13A/chorein protein - likely a crucial organizer at the erythrocyte cytoskeleton/membrane interface - showed a strong decrease in chlorpromazine-induced endovesiculation. The responses of ChAc erythrocytes to effectors of endovesiculation were similar to that of control erythrocytes, yet at drastically reduced levels. This suggests a more rigid and less dynamic interaction at the membrane-cytoskeleton interphase of ChAc erythrocytes.

Stomatin interacts with GLUT1/SLC2A1, band 3/SLC4A1, and aquaporin-1 in human erythrocyte membrane domains.

The widely expressed, homo-oligomeric, lipid raft-associated, monotopic integral membrane protein stomatin and its homologues are known to interact with and modulate various ion channels and transporters. Stomatin is a major protein of the human erythrocyte membrane, where it associates with and modifies the glucose transporter GLUT1; however, previous attempts to purify hetero-oligomeric stomatin complexes for biochemical analysis have failed. Because lateral interactions of membrane proteins may be short-lived and unstable, we have used in situ chemical cross-linking of erythrocyte membranes to fix the stomatin complexes for subsequent purification by immunoaffinity chromatography. To further enrich stomatin, we prepared detergent-resistant membranes either before or after cross-linking. Mass spectrometry of the isolated, high molecular, cross-linked stomatin complexes revealed the major interaction partners as glucose transporter-1 (GLUT1), anion exchanger (band 3), and water channel (aquaporin-1). Moreover, ferroportin-1 (SLC40A1), urea transporter-1 (SLC14A1), nucleoside transporter (SLC29A1), the calcium-pump (Ca-ATPase-4), CD47, and flotillins were identified as stomatin-interacting proteins. These findings are in line with the hypothesis that stomatin plays a role as membrane-bound scaffolding protein modulating transport proteins.

Brain, blood, and iron: perspectives on the roles of erythrocytes and iron in neurodegeneration.

The terms "neuroacanthocytosis" (NA) and "neurodegeneration with brain iron accumulation" (NBIA) both refer to groups of genetically heterogeneous disorders, classified together due to similarities of their phenotypic or pathological findings. Even collectively, the disorders that comprise these sets are exceedingly rare and challenging to study. The NBIA disorders are defined by their appearance on brain magnetic resonance imaging, with iron deposition in the basal ganglia. Clinical features vary, but most include a movement disorder. New causative genes are being rapidly identified; however, the mechanisms by which mutations cause iron accumulation and neurodegeneration are not well understood. NA syndromes are also characterized by a progressive movement disorder, accompanied by cognitive and psychiatric features, resulting from mutations in a number of genes whose roles are also basically unknown. An overlapping feature of the two groups, NBIA and NA, is the occurrence of acanthocytes, spiky red cells with a poorly-understood membrane dysfunction. In this review we summarise recent developments in this field, specifically insights into cellular mechanisms and from animal models. Cell membrane research may shed light upon the significance of the erythrocyte abnormality, and upon possible connections between the two sets of disorders. Shared pathophysiologic mechanisms may lead to progress in the understanding of other types of neurodegeneration.

Stomatin-like protein-1 interacts with stomatin and is targeted to late endosomes.

The human stomatin-like protein-1 (SLP-1) is a membrane protein with a characteristic bipartite structure containing a stomatin domain and a sterol carrier protein-2 (SCP-2) domain. This structure suggests a role for SLP-1 in sterol/lipid transfer and transport. Because SLP-1 has not been investigated, we first studied the molecular and cell biological characteristics of the expressed protein. We show here that SLP-1 localizes to the late endosomal compartment, like stomatin. Unlike stomatin, SLP-1 does not localize to the plasma membrane. Overexpression of SLP-1 leads to the redistribution of stomatin from the plasma membrane to late endosomes suggesting a complex formation between these proteins. We found that the targeting of SLP-1 to late endosomes is caused by a GYXXPhi (Phi being a bulky, hydrophobic amino acid) sorting signal at the N terminus. Mutation of this signal results in plasma membrane localization. SLP-1 and stomatin co-localize in the late endosomal compartment, they co-immunoprecipitate, thus showing a direct interaction, and they associate with detergent-resistant membranes. In accordance with the proposed lipid transfer function, we show that, under conditions of blocked cholesterol efflux from late endosomes, SLP-1 induces the formation of enlarged, cholesterol-filled, weakly LAMP-2-positive, acidic vesicles in the perinuclear region. This massive cholesterol accumulation clearly depends on the SCP-2 domain of SLP-1, suggesting a role for this domain in cholesterol transfer to late endosomes.

Myristoylation of human LanC-like protein 2 (LANCL2) is essential for the interaction with the plasma membrane and the increase in cellular sensitivity to adriamycin.

Human LANCL2, also known as Testis-specific Adriamycin Sensitivity Protein (TASP), is a member of the highly conserved and widely distributed lanthionine synthetase component C-like (LANCL) protein family. Expression studies of tagged LANCL2 revealed the major localization to the plasma membrane, juxta-nuclear vesicles, and the nucleus, in contrast to the homologue LANCL1 that was mainly found in the cytosol and nucleus. We identified the unique N-terminus of LANCL2 to function as the membrane anchor and characterized the relevant N-terminal myristoylation and a basic phosphatidylinositol phosphate-binding site. Interestingly, the non-myristoylated protein was confined to the nucleus indicating that the myristoylation targets LANCL2 to the plasma membrane. Cholesterol depletion by methyl-beta-cyclodextrin caused the partial dissociation of overexpressed LANCL2 from the plasma membrane in vitro, whereas in vivo we observed an enhanced cell detachment from the matrix. We found that overexpressed LANCL2 interacts with the cortical actin cytoskeleton and therefore may play a role in cytoskeleton reorganization and in consequence to cell detachment. Moreover, we confirmed previous data that LANCL2 overexpression enhances the cellular sensitivity to the anticancer drug adriamycin and found that this sensitivity is dependent on the myristoylation and membrane association of LANCL2.

Stomatocytosis of Standard Schnauzers is not associated with stomatin deficiency.

Stomatocytosis resembles human overhydrated hereditary stomatocytosis (OHSt), a disease characterised by a reduced or absent stomatin expression. The objective of this report was to investigate the expression level of stomatin in erythrocytes from Standard Schnauzers with stomatocytosis. Routine haematology, intraerythrocytic Na(+)/K(+) concentration and stomatin expression were evaluated in blood from twelve Standard Schnauzers and from three controls. SDS-PAGE and Western blotting on isolated integral membrane proteins were used to investigate stomatin expression. Circulating stomatocytes, macrocytosis, anisocytosis, increased erythrocyte fragility and high intracellular sodium and potassium concentrations were found in 10/12 dogs from the same breeding line although stomatin levels were similar to those of controls. In spite of the clinico-pathological similarities between human and canine stomatocytosis, erythrocytes from affected dogs do not lack stomatin and the expression level of this protein cannot therefore be used to diagnose hereditary stomatocytosis in Standard Schnauzers.

Structure-function analysis of human stomatin: A mutation study.

Stomatin is an ancient, widely expressed, oligomeric, monotopic membrane protein that is associated with cholesterol-rich membranes/lipid rafts. It is part of the SPFH superfamily including stomatin-like proteins, prohibitins, flotillin/reggie proteins, bacterial HflK/C proteins and erlins. Biochemical features such as palmitoylation, oligomerization, and hydrophobic "hairpin" structure show similarity to caveolins and other integral scaffolding proteins. Recent structure analyses of the conserved PHB/SPFH domain revealed amino acid residues and subdomains that appear essential for the structure and function of stomatin. To test the significance of these residues and domains, we exchanged or deleted them, expressed respective GFP-tagged mutants, and studied their subcellular localization, molecular dynamics and biochemical properties. We show that stomatin is a cholesterol binding protein and that at least two domains are important for the association with cholesterol-rich membranes. The conserved, prominent coiled-coil domain is necessary for oligomerization, while association with cholesterol-rich membranes is also involved in oligomer formation. FRAP analyses indicate that the C-terminus is the dominant entity for lateral mobility and binding site for the cortical actin cytoskeleton.

The presence of stomatin in detergent-insoluble domains of neutrophil granule membranes.

Neutrophil azurophil granules, traditionally regarded as the neutrophil counterpart to lysosomes, lack the lysosomal marker lysosome-associated membrane glycoprotein and have recently been suggested to be nonlysosomal secretory organelles. The membrane of the azurophil granules is poorly characterized-CD63 and CD68 are the only membrane proteins identified so far. Here, azurophil granule membranes were isolated by Percoll gradient subcellular fractionation. Using matrix-assisted laser desorption ionization time of flight mass spectrometry of tryptic peptides from an isolated protein, stomatin was identified in these membranes. Using immunoelectron microscopy and immunoblot analysis of isolated organelles, stomatin was found to be subcellularly localized, not only to the azurophil granules but also by a major part to the specific granules and by a minor part to the secretory vesicles/plasma membrane. We also show the presence of detergent-insoluble, low-density membrane domains in the plasma membrane and the granule membranes and found stomatin to be localized to these domains.

Acanthocytosis and the c.680 A>G Mutation in the PANK2 Gene: A Study Enrolling a Cohort of PKAN Patients from the Dominican Republic.

Pantothenate Kinase-Associated Neurodegeneration (PKAN) is a form of Neurodegeneration with Brain Iron Accumulation (NBIA) associated with mutations in the pantothenate kinase 2 gene (PANK2). Pantothenate kinases catalyze the rate-limiting step of coenzyme A synthesis and Pank2 is the only pantothenate kinase isoform in humans that is localized to mitochondria. Acanthocytosis, the occurrence of spiculated erythrocytes, is observed in about 10% of the PKAN patients. Therefore PKAN is also classified together with other rare neurodegenerative diseases like Chorea Acanthocytosis (ChAc) and McLeod syndrome (MLS) into the Neuroacanthocytosis (NA) syndromes. It has not been investigated yet whether acanthocytosis in PKAN is associated with a specific subset of Pank2 mutations. In this study, we analyzed acanthocytosis of a cohort of 25 PKAN patients from the Dominican Republic that are homozygous for the c.680 A>G mutation in the PANK2 gene as compared to control donors that are heterozygous or wild-type with respect to this mutation. 3D modeling of this mutation indicated that the replacement of a tyrosine by a cysteine at position 227 in Pank2 disrupts a polar interaction within the A domain of the enzyme. Mean acanthocyte count was elevated in the cohort of patients, however, acanthocytosis varied among the patients with nearly half of them showing high (>20%) or elevated acanthocytosis and the rest showing mild (6-10%) or no (<6%) acanthocytosis. Heterozygous control donors revealed a tendency to mild acanthocytosis. Based on the insight that Pank2 is a normal constituent of red blood cells and de novo biosynthesis of coenzyme A is likely to take place in the erythrocyte cytosol we propose a hypothetical model that accounts for the variability in the occurrence of acanthocytic cells in PKAN.

Alterations of red cell membrane properties in neuroacanthocytosis.

Neuroacanthocytosis (NA) refers to a group of heterogenous, rare genetic disorders, namely chorea acanthocytosis (ChAc), McLeod syndrome (MLS), Huntington's disease-like 2 (HDL2) and pantothenate kinase associated neurodegeneration (PKAN), that mainly affect the basal ganglia and are associated with similar neurological symptoms. PKAN is also assigned to a group of rare neurodegenerative diseases, known as NBIA (neurodegeneration with brain iron accumulation), associated with iron accumulation in the basal ganglia and progressive movement disorder. Acanthocytosis, the occurrence of misshaped erythrocytes with thorny protrusions, is frequently observed in ChAc and MLS patients but less prevalent in PKAN (about 10%) and HDL2 patients. The pathological factors that lead to the formation of the acanthocytic red blood cell shape are currently unknown. The aim of this study was to determine whether NA/NBIA acanthocytes differ in their functionality from normal erythrocytes. Several flow-cytometry-based assays were applied to test the physiological responses of the plasma membrane, namely drug-induced endocytosis, phosphatidylserine exposure and calcium uptake upon treatment with lysophosphatidic acid. ChAc red cell samples clearly showed a reduced response in drug-induced endovesiculation, lysophosphatidic acid-induced phosphatidylserine exposure, and calcium uptake. Impaired responses were also observed in acanthocyte-positive NBIA (PKAN) red cells but not in patient cells without shape abnormalities. These data suggest an "acanthocytic state" of the red cell where alterations in functional and interdependent membrane properties arise together with an acanthocytic cell shape. Further elucidation of the aberrant molecular mechanisms that cause this acanthocytic state may possibly help to evaluate the pathological pathways leading to neurodegeneration.

Characterization of the stomatin domain involved in homo-oligomerization and lipid raft association.

The cytoplasmically oriented monotopic integral membrane protein stomatin forms high-order oligomers and associates with lipid rafts. To characterize the domains that are involved in oligomerization and detergent-resistant membrane (DRM) association, we expressed truncation and point mutants of stomatin and analyzed their size and buoyancy by ultracentrifugation methods. A small C-terminal region of stomatin that is largely hydrophobic, Ser-Thr-Ile-Val-Phe-Pro-Leu-Pro-Ile (residues 264-272), proved to be crucial for oligomerization, whereas the N-terminal domain (residues 1-20) and the last 12 C-terminal amino acids (residues 276-287) were not essential. The introduction of alanine substitutions in the region 264-272 resulted in the appearance of monomers. Remarkably, only three of these residues, Ile-Val-Phe (residues 266-268), were found to be indispensable for the DRM association. Interestingly, the exchange of Pro-269 and to some extent the residues 270-272, which are essential for oligomerization, did not affect the DRM association of stomatin. This suggests that the formation of oligomers is not necessary for the association of stomatin with DRMs. Internal deletions near the membrane anchoring domain resulted in the formation of intermediate size oligomers suggesting a conformational interdependence of large parts of the C-terminal region. Fluorescence recovery after photobleaching analysis of the tagged, monomeric, non-DRM mutant ST-(1-262)-green fluorescent protein and wild type stomatin StomGFP showed a significantly higher lateral mobility of the truncation mutant in the plasma membrane suggesting a membrane interaction of the respective C-terminal region also in vivo.

Curvature-dependent lateral distribution of raft markers in the human erythrocyte membrane.

The distribution of raft markers in curved membrane exvaginations and invaginations, induced in human erythrocytes by amphiphile-treatment or increased cytosolic calcium level, was studied by fluorescence microscopy. Cholera toxin subunit B and antibodies were used to detect raft components. Ganglioside GM1 was enriched in membrane exvaginations (spiculae) induced by cytosolic calcium and amphiphiles. Stomatin and the cytosolic proteins synexin and sorcin were enriched in spiculae when induced by cytosolic calcium, but not in spiculae induced by amphiphiles. No enrichment of flotillin-1 was detected in spiculae. Analyses of the relative protein content of released exovesicles were in line with the microscopic observations. In invaginations induced by amphiphiles, the enrichment of ganglioside GM1, but not of the integral membrane proteins flotillin-1 and stomatin, was observed. Based on the experimental results and theoretical considerations we suggest that membrane skeleton-detached, laterally mobile rafts may sort into curved or flat membrane regions dependent on their intrinsic molecular shape and/or direct interactions between the raft elements.

Ca(++)-dependent vesicle release from erythrocytes involves stomatin-specific lipid rafts, synexin (annexin VII), and sorcin.

Cytosolic Ca(++) induces the shedding of microvesicles and nanovesicles from erythrocytes. Atomic force microscopy was used to determine the sizes of these vesicles and to resolve the patchy, fine structure of the microvesicle membrane. The vesicles are highly enriched in glycosyl phosphatidylinositol-linked proteins, free of cytoskeletal components, and depleted of the major transmembrane proteins. Both types of vesicles contain 2 as-yet-unrecognized red cell proteins, synexin and sorcin, which translocate from the cytosol to the membrane upon Ca(++) binding. In nanovesicles, synexin and sorcin are the most abundant proteins after hemoglobin. In contrast, the microvesicles are highly enriched in stomatin. The membranes of both microvesicles and nanovesicles contain lipid rafts. Stomatin is the major protein of the microvesicular lipid rafts, whereas synexin and sorcin represent the major proteins of the nanovesicular rafts in the presence of Ca(++). Interestingly, the raft proteins flotillin-1 and flotillin-2 are not found in the vesicles but remain in the red cell membrane. These data indicate the presence of different types of lipid rafts in the erythrocyte membrane with distinct fates after Ca(++) entry. Synexin, which is known to be vital to the process of membrane fusion, is suggested to be a key component in the process of vesicle release from erythrocytes.

Association of stomatin with lipid bodies.

The oligomeric lipid raft-associated integral protein stomatin normally localizes to the plasma membrane and the late endosomal compartment. Similar to the caveolins, it is targeted to lipid bodies (LBs) on overexpression. Endogenous stomatin also associates with LBs to a small extent. Green fluorescent protein-tagged stomatin (StomGFP) and the dominant-negative caveolin-3 mutant DGV(cav3)HA occupy distinct domains on LB surfaces but eventually intermix. Studies of StomGFP deletion mutants reveal that the region for membrane association but not oligomerization and raft association is essential for LB targeting. Blocking protein synthesis leads to the redistribution of StomGFP from LBs to LysoTracker-positive vesicles indicating a connection with the late endosomal/lysosomal pathway. Live microscopy of StomGFP reveals multiple interactions between LBs and microtubule-associated vesicles possibly representing signaling events and/or the exchange of cargo. Proteomic analysis of isolated LBs identifies adipophilin and TIP47, various lipid-specific enzymes, cytoskeletal components, chaperones, Ras-related proteins, protein kinase D2, and other regulatory proteins. The association of the Rab proteins 1, 6, 7, 10, and 18 with LBs indicates various connections to other compartments. Our data suggest that LBs are not only involved in the storage of lipids but also participate actively in the cellular signaling network and the homeostasis of lipids.

Stomatin is a major lipid-raft component of platelet alpha granules.

Lipid rafts are detergent-resistant, cholesterol- and sphingolipid-rich membrane domains that are involved in important cellular processes such as signal transduction and intracellular trafficking. Stomatin, a major lipid-raft component of erythrocytes and epithelial cells, is also an abundant platelet protein. Microscopical methods and subcellular fractionation showed that stomatin is located mainly at the alpha-granular membrane. The lipid-raft marker proteins flotillin-1 and flotillin-2 were also present in platelets but excluded from alpha granules. Stomatin and the flotillins were associated with Triton X-100-insoluble lipid rafts. Whereas stomatin was partly soluble in Triton X-100, it was insoluble in the detergents Lubrol and 3-[(3-cholamidopropyl)dimethylamonio]-1-propyl sulfonate (CHAPS). Flotation experiments after CHAPS lysis of platelets revealed a distinct set of lipid-raft-associated proteins, which were identified by matrix-assisted laser desorption/ionization mass spectrometry as stomatin, flotillin-1, flotillin-2, CD36, CD9, integrin alpha(IIb)beta(3), and the glucose transporter GLUT-3. Stomatin, the flotillins, and CD36 were exclusively present in this lipid-raft fraction. Activation of platelets by calcium ionophore A23187 or thrombin led to translocation of stomatin to the plasma membrane, cleavage by calpain, and specific sorting into released microvesicles. In conclusion, this study demonstrated the existence of alpha-granular lipid rafts and suggests an important role for stomatin in the organization and function of alpha granules.

Flotillin-1/reggie-2 traffics to surface raft domains via a novel golgi-independent pathway. Identification of a novel membrane targeting domain and a role for palmitoylation.

Flotillins are lipid raft-associated proteins, which have been implicated in neuronal regeneration and insulin signaling. We now show that newly synthesized flotillin-1 reaches the plasma membrane via a Sar1-independent and brefeldin A-resistant targeting pathway. Consistent with post-translational membrane association of flotillin, protease sensitivity experiments suggest that flotillin-1 is not a transmembrane protein but is associated with the cytoplasmic face of the plasma membrane. The N terminus of flotillin contains a prohibitin-like domain (PHB), which shows homology to a number of proteins associated with raft domains including stomatin, podocin, and prohibitin. We show that the PHB domain of flotillin can efficiently target a heterologous protein, green fluorescent protein, to the plasma membrane. Another PHB-containing protein, stomatin, traffics to the plasma membrane via the conventional secretory pathway. Plasma membrane association of both full-length flotillin and the green fluorescent protein-tagged PHB domain of flotillin is dependent on palmitoylation and requires a conserved cysteine residue, Cys-34, in the PHB domain. The results identify a novel targeting mechanism for plasma membrane association of flotillin-1 involving a Golgi-independent trafficking pathway, the PHB domain, and palmitoylation.

Erythrocyte detergent-resistant membrane proteins: their characterization and selective uptake during malarial infection.

Infection of human erythrocytes by the apicomplexan malaria parasite Plasmodium falciparum results in endovacuolar uptake of 4 host proteins that reside in erythrocyte detergent-resistant membranes (DRMs). Whether this vacuolar transport reflects selective uptake of host DRM proteins remains unknown. A further complication is that DRMs of vastly different protein and cholesterol contents have been isolated from erythrocytes. Here we show that isolated DRMs containing the highest cholesterol-to-protein ratio have low protein mass. Liquid chromatography, mass spectrometry, and antibody-based studies reveal that the major DRM proteins are band 3, flotillin-1 and -2, peroxiredoxin-2, and stomatin. Band 3 and stomatin, which reflect the bulk mass of erythrocyte DRM proteins, and all tested non-DRM proteins are excluded from the vacuolar parasite. In contrast, flotillin-1 and -2 and 8 minor DRM proteins are recruited to the vacuole. These data suggest that DRM association is necessary but not sufficient for vacuolar recruitment and there is active, vacuolar uptake of a subset of host DRM proteins. Finally, the 10 internalized DRM proteins show varied lipid and peptidic anchors indicating that, contrary to the prevailing model of apicomplexan vacuole formation, DRM association, rather than lipid anchors, provides the preferred criteria for protein recruitment to the malarial vacuole.