Total sialic acid content of glycophorins during senescence of human red blood cells

Roles of bovine sialoglycoproteins for anti-skin aging and accelerating skin wound healing

BACKGROUND

Fibroblasts are the most predominant cell subpopulation in the dermal layer of human skin, they play an important role in maintaining skin architecture and function. The senescence of fibroblasts is one of major causes of skin aging and chronic wound in the elderly, which is accompanied with a reduction of α2,6-sialylation on the cell surface.

AIMS

In this study, we investigated the effects of the bovine sialoglycoproteins on normal human dermal fibroblasts (NHDF).

RESULTS

The results showed that bovine sialoglycoproteins could promote the proliferation and migration of NHDF cells, and accelerate the contraction of fibroblast-populated collagen lattice (FPCL). The average doubling time of NHDF cells treated with bovine sialoglycoproteins (0.5 mg/mL) was 31.1 ± 1.0 h whereas that was 37.9 ± 2.7 h for the control (p ˂ 0.05). Moreover, the expression of basic fibroblast growth factor (FGF-2) was upregulated, while that of transforming growth factor-beta 1 (TGF-β1) and human type I collagen (COL-I) were downregulated in treated NHDF cells. Furthermore, bovine sialoglycoproteins treatment significantly enhanced the α2,6-sialylation on the cell surfaces, which was consistent with the upregulation of α2,6-sialyltransferase I (ST6GAL1) expression.

CONCLUSIONS

These results indicated that the bovine sialoglycoproteins might be developed as a reagent against skin aging in the cosmetic industry, or as a new candidate for accelerating skin wound healing and inhibiting scar formation.

The Factor H protein family: The switchers of the complement alternative pathway

The factor H (FH) protein family is emerging as a complex network of proteins controlling the fate of the complement alternative pathway (AP) and dictating susceptibility to a wide range of diseases including infectious, inflammatory, autoimmune, and degenerative diseases and cancer. Composed, in man, of seven highly related proteins, FH, factor H-like 1, and 5 factor H-related proteins, some of the FH family proteins are devoted to down-regulating the AP, while others exert an opposite function by promoting AP activation. Recent findings have provided insights into the molecular mechanisms defining their biological roles and their pathogenicity, illustrating the relevance that the balance between the regulators and the activators within this protein family has in defining the outcome of complement activation on cell surfaces. In this review we will discuss the emerging roles of the factor H protein family, their impact in the complement cascade, and their involvement in the pathogenesis of complement-mediated diseases associated with the AP dysregulation.

Human erythrocyte surface fucose expression increases with age and hyperglycemia

Background: Reactive oxygen species and other free radicals, together with glucose and its metabolites are believed to play important roles in the aging process. The carbohydrate components of glycosylated proteins are important in mediating cell-cell interactions and a role has been suggested for them in the aging process. Erythrocytes are critical cells in the human body, heavily glycosylated and relatively easily available and so are good candidates to yield insights into how patterns of glycosylation change with age and disease. It has been claimed, based on a periodic acid Schiff assay, that human aging is associated with a decline of erythrocyte surface sialic acids. Plant lectins allow for more specific assays for glycans, including determining the linkage of sialic acids and analysis of single cells by flow cytometry. Methods: Plant lectins, including Maackia amurensis lectin II (MAL), binding to α-2,3 linked sialic acids and Sambucus nigra (SNA), α-2,6 sialic acids, were used in flow cytometry and western blot of erythrocyte surface membrane. N-glycomics mass spectrometry determines glycan structures. Donors varying in age and hyperglycemia, as indicated by HbA1c were analysed. Results: Erythrocyte surface sialic acids have no significant associations with donor age. A combination of storage and cellular aging produces a specific loss of α-2,6 sialic acids. By contrast, erythrocyte surface terminal fucoses increase significantly with donor age. In order to determine which aspects of aging are important in determining this change, we investigated whether this novel human aging biomarker is associated with higher plasma glucose values, assessed by glycated hemoglobin (HbA1c) and reactive oxygen species (ROS) generation. Fucose levels were associated with HbA1c levels, but not ROS generation. Conclusion: Our study identifies novel glycan-based biomarkers for human aging and disease. The simplicity of lectin-based assays provide an attractive cellular tool to study aging and disease processes.

Immune and Metabolic Interactions of Human Erythrocytes: A Molecular Perspective

Apart from their main function as oxygen carriers in vertebrates, erythrocytes are also involved in immune regulation. By circulating throughout the body, the erythrocytes are exposed and interact with tissues that are damaged as a result of a disease. In this study, we summarize the literature regarding the contribution of erythrocytes to immune regulation and metabolism. Under the circumstances of a disease state, the erythrocytes may lose their antioxidant capacity and release Damage Associated Molecular Patterns, resulting in the regulation of innate and adaptive immunity. In addition, the erythrocytes scavenge and affect the levels of chemokines, circulating cell-free mtDNA, and C3b attached immune complexes. Furthermore, through surface molecules, erythrocytes control the function of T lymphocytes, macrophages, and dendritic cells. Through an array of enzymes, red blood cells contribute to the pool of blood's bioactive lipids. Finally, the erythrocytes contribute to reverse cholesterol transport through various mechanisms. Our study is highlighting overlooked molecular interactions between erythrocytes and immunity and metabolism, which could lead to the discovery of potent therapeutic targets for immunometabolic diseases.

Age-Related Changes in O-Acetylation of Sialic Acids Bound to N-Glycans of Male Rat Serum Glycoproteins and Influence of Dietary Intake on Their Changes

O-Acetylation of sialic acids has been widely found in eukaryotic cells. Such modifications of sialic acids are tissue-specific and seem to be developmentally regulated. In this study, we performed comprehensive analysis of age-related changes in the serum N-glycans of male rats using capillary electrophoresis (CE) and investigated the changes in the O-acetylation of sialic acids bound to N-glycans with aging and different diets. The present method offered sufficient resolution to assess the degree of O-acetylation of the N-glycans and allowed for the determination of the age-related changes in O-acetylation of sialic acids. Using the CE-based method, we found that the relative abundance of disialo-biantennary N-glycans modified with 9-O-acetylated N-acetylneuraminic acid (Neu5,9Ac) significantly increased with aging. In addition, the relative abundances of N-glycans with two Neu5,9Ac reversed to those of N-glycans with only Neu5Ac during 12 weeks. Next, we evaluated the influence of high-fat diet and food restriction on age-related changes in O-acetylation. Although the total amount of disialo-biantennary N-glycans increased with aging, age-related O-acetylation of sialic acids was suppressed by a high-fat diet. On the other hand, food restriction enhanced the O-acetylation of sialic acids, and the relative abundance of N-glycans with two Neu5,9Ac residues at 15 weeks of age was higher than that observed in the standard diet group. These findings suggest that the O-acetylation of sialic acids is closely related to changes in energy metabolisms such as glycolysis or fatty acid metabolism.

Accounting for red blood cell accessibility reveals distinct invasion strategies in Plasmodium falciparum strains

The growth of the malaria parasite Plasmodium falciparum in human blood causes all the symptoms of malaria. To proliferate, non-motile parasites must have access to susceptible red blood cells, which they invade using pairs of parasite ligands and host receptors that define invasion pathways. Parasites can switch invasion pathways, and while this flexibility is thought to facilitate immune evasion, it may also reflect the heterogeneity of red blood cell surfaces within and between hosts. Host genetic background affects red blood cell structure, for example, and red blood cells also undergo dramatic changes in morphology and receptor density as they age. The in vivo consequences of both the accessibility of susceptible cells, and their heterogeneous susceptibility, remain unclear. Here, we measured invasion of laboratory strains of P. falciparum relying on distinct invasion pathways into red blood cells of different ages. We estimated invasion efficiency while accounting for red blood cell accessibility to parasites. This approach revealed different tradeoffs made by parasite strains between the fraction of cells they can invade and their invasion rate into them, and we distinguish "specialist" strains from "generalist" strains in this context. We developed a mathematical model to show that generalist strains would lead to higher peak parasitemias in vivo compared to specialist strains with similar overall proliferation rates. Thus, the ecology of red blood cells may play a key role in determining the rate of P. falciparum parasite proliferation and malaria virulence.

Fisetin, a potential caloric restriction mimetic, attenuates senescence biomarkers in rat erythrocytes

An imbalanced redox status is a hallmark of the aging process. Caloric restriction mimetics (CRMs) are compounds that produce caloric restriction benefits at the molecular, cellular, and physiological level, translating into health-promoting effects. Fisetin is the least explored CRM, and its role in modulating oxidative stress during aging is not clearly known. This study investigated the antioxidative and protective potential of fisetin in a rat model of d-galactose (D-gal)-induced accelerated senescence, and in naturally aged rat erythrocytes. Young rats (4 months), aged D-gal-induced rats [24 months; 500 mg/kg body mass (b.m.); subcutaneous injection] and naturally aged D-gal-induced rats [24 months; 500 mg/kg b.m.; subcutaneous injection] were supplemented with fisetin (15 mg/kg b.m.; orally) for 6 weeks. The resulting data indicated that supplementation with fisetin suppresses aging-induced increases in the levels of reactive oxygen species, eryptosis, lipid peroxidation, and protein oxidation. Our data also show that fisetin significantly increases the levels of antioxidants and activates the plasma membrane redox system. Taken together, the findings show that a fisetin-rich diet could be an anti-aging intervention strategy.

Bacterial neuraminidase-mediated erythrocyte desialylation provokes cell surface aminophospholipid exposure

BACKGROUND

Surface desialylation is associated with erythrocyte aging and mediates phagocytic recognition and clearance of senescent erythrocytes. Neuraminidases, a family of glycohydrolytic enzymes, cleave the glycosidic linkages between sialic acid and mucopolysaccharides and have previously been implicated in erythrocyte dysfunction associated with sepsis. Erythrocytes in septic patients further display a phenotype of accelerated eryptosis characterized by membrane phospholipid scrambling resulting in phosphatidylserine (PS) externalization. Herein, we examined the impact of artificial erythrocyte desialylation on eryptosis.

METHODS

Using flow cytometry and/or fluorescence microscopy, we analyzed desialylation patterns and eryptotic alterations in erythrocytes exposed to Clostridium perfringens-derived neuraminidase.

RESULTS

Exogenous bacterial neuraminidase significantly augmented membrane PS exposure and cytosolic Ca²⁺ levels in a dose- and time-dependent manner. Neuraminidase treatment significantly reduced fluorescence-tagged agglutinin binding, an effect temporally preceding the increase in PS externalization. Neuraminidase-induced PS exposure was significantly curtailed by pretreatment with the pan-sialidase inhibitor N-acetyl-2,3-dehydro-2-deoxyneuraminic acid. Neuraminidase treatment further induced hemolysis but did not significantly impact erythrocyte volume, ceramide abundance, or the generation of reactive oxygen species.

CONCLUSION

Collectively, our data reveal that alteration of erythrocyte sialylation status by bacterial neuraminidase favors eryptotic cell death, an effect potentially contributing to reduced erythrocyte lifespan and anemia in sepsis.

Molecular mechanism for the red blood cell senescence clock

Lacking protein synthesis machinery and organelles necessary for autophagy or apoptosis, aged red blood cells (RBCs) are marked by circulating auto-antibodies for macrophage-mediated clearance. The antigen recognized by these auto-antibodies is the major protein of the RBC membrane, Band 3. To ensure regulation and specificity in clearance, the molecular "clock" must mark senescent cells in a way that differentiates them from younger cells, to prevent premature clearance. Predominant models of Band 3 senescence signaling are reviewed, and merits are discussed in light of the recently published crystal structure of the Band 3 membrane domain. © 2017 IUBMB Life, 70(1):32-40, 2018.

Crosstalk between red blood cells and the immune system and its impact on atherosclerosis

Atherosclerosis is a chronic multifactorial disease of the arterial wall characterized by inflammation, oxidative stress, and immune system activation. Evidence exists on a pathogenic role of oxidized red blood cells (RBCs) accumulated in the lesion after intraplaque hemorrhage. This review reports current knowledge on the impact of oxidative stress in RBC modifications with the surface appearance of senescent signals characterized by reduced expression of CD47 and glycophorin A and higher externalization of phosphatidylserine. The review summarizes findings indicating that oxidized, senescent, or stored RBCs, due to surface antigen modification and release of prooxidant and proinflammatory molecules, exert an impaired modulatory activity on innate and adaptive immune cells and how this activity contributes to atherosclerotic disease. In particular RBCs from patients with atherosclerosis, unlike those from healthy subjects, fail to control lipopolysaccharide-induced DC maturation and T lymphocyte apoptosis. Stored RBCs, accompanied by shedding of extracellular vesicles, stimulate peripheral blood mononuclear cells to release proinflammatory cytokines, augment mitogen-driven T cell proliferation, and polarize macrophages toward the proinflammatory M1 activation pathway. Collectively, literature data suggest that the crosstalk between RBCs with immune cells represents a novel mechanism by which oxidative stress can contribute to atherosclerotic disease progression and may be exploited for therapeutic interventions.

Naturally occurring anti-band 3 antibodies in clearance of senescent and oxidatively stressed human red blood cells

SUMMARY

Naturally occurring anti-band 3 antibodies (anti-band 3 NAbs) are directed against the 55-kDa chymotryptic fragment of the anion transport protein (band 3) of red blood cells (RBCs). They bind to senescent and oxidatively stressed RBCs and induce their selective clearance. These IgG NAbs exist at low concentrations, and have a weak affinity that prevents them from actively recruiting second binding sites. Cellular senescence or oxidative damage induces a cascade of biochemical events that results in the detachment of band 3 from the cytoskeleton and in clustering of band 3 protein by bound hemichromes and Syk kinase. Clustered band 3 proteins allow bivalent binding of anti-band 3 NAbs. Bivalently bound anti-band 3 NAbs have the unique capacity to stimulate C3b deposition by preferentially generating C3b2-IgG complexes, which act as potent C3 convertase precursors of the alternative complement pathway. Antibody binding not only to clustered, but also to oligomerized band 3 protein further increases if the human plasma also contains induced anti-lactoferrin antibodies. These bind to the polylactosaminyl oligosaccharide, a carbohydrate that exists in lactoferrin and in the 38-kDa fragment of band 3 protein. Anti-lactoferrin antibodies are found primarily in plasma of patients with autoimmune diseases and who have anti-neutrophil cytoplasmic antibodies (ANCA).

Naturally occurring autoantibodies in mediating clearance of senescent red blood cells

Germline-encoded naturally occurring autoantibodies (NAbs) developed about 400 to 450 million years ago to provide specificity for clearance ofbody waste in animals with 3 germ layers. Such NAbs became a necessity to selectively clear aged red blood cells (RBC) surviving 60 to 120 d in higher vertebrates. IgG NAbs to senescent RBC are directed to the most abundant integral membrane protein, the anion-transport protein or band 3 protein, but only bind firmly upon its oligomerization, which facilitates bivalent binding. The main constituent of RBC, the oxygen-carrying hemoglobin, is susceptible to oxidative damage. Oxidized hemoglobin forms hemichromes (a form of aggregates) that bind to the cytoplasmic portion of band 3 protein, induces their clustering on the cytoplasmic, as well as the exoplasmic side and thereby provides the prerequisites for the low affinity IgG anti-band 3 NAbs to bind bivalently. Bound anti-band 3 NAbs overcome their low numbers per RBC by stimulating complement amplification. An affinity for C3 outside the antigen binding region is responsible for a preferential formation of C3b(2)-IgG complexes from anti-band 3 NAbs. These complexes first bind oligomeric properdin, which enhances their affinity for factor B in assembling an alternative C3 convertase.

Unique expression pattern of human nicotinamide mononucleotide adenylyltransferase isozymes in red blood cells

Nicotinamide mononucleotide adenylyltransferase (NMNAT) catalyzes the formation of nicotinamide adenine dinucleotide (NAD). In humans, three isozymes have been identified: NMNAT1, which is widely expressed in all tissues, NMNAT2 and NMNAT3, which show a tissue-specific expression and whose mRNA levels are generally lower compared to NMNAT1. In the present study we determined the individual NMNAT isozymes activity in human red blood cells (RBCs) by using a biochemical discrimination assay based on the distinctive catalytic properties of the three proteins. We found that isozyme 3 predominates over isozyme 1, whereas isozyme 2 is absent. This high prevalence of NMNAT3 is cell-aging independent and was also confirmed by analyzing the mRNA and protein levels. RBC represent the first human cell type with a remarkable predominance of NMNAT3, and this unique expression pattern is discussed in light of the catalytic properties of the isozymes and in consideration of the biochemical microenvironment of RBC.

Eryptosis in hereditary spherocytosis and thalassemia: role of glycoconjugates

The present work is aimed to study the mechanism of faster erythrocyte clearance in hereditary spherocytosis (HS), a heterogeneous disorders characterized by alterations in the proteins of the red cell membrane skeleton along with different kinds of thalassemia. The maximum exposure of phosphatidylserine (PS) is found in HS compared to those in both α- and β-thalassemia. Interestingly, in HS more PS exposed cells were found in younger erythrocytes compared to normal and the thalassemics where aged cells showed higher loss of PS asymmetry. Loss of sialic acid and GlcNAc bearing glycoconjugates, presumably the glycophorins, was also found upon aging. The loss of PS asymmetry together with the cell surface glycoproteins mediated by membrane vesiculation, seemed to play key role in early clearance of erythrocytes from circulation following a mechanism similar to HbEβ-thalassemia.

Loss of phospholipid membrane asymmetry and sialylated glycoconjugates from erythrocyte surface in haemoglobin E beta-thalassaemia

This study aimed to investigate any correlation between the extent of phosphatidylserine (PS) asymmetry and sialylated glycoconjugate levels with the faster clearance of circulating erythrocytes in haemoglobin E (HbE) beta-thalassaemia. Erythrocytes from peripheral blood samples of different HbEbeta-thalassaemia patients showed loss of PS asymmetry measured by annexin V binding using flow cytometry. Maximum PS exposure was found when HbE was 50-60% and HbF was <20% indicating a possible correlation with severity of the disease. Separation of erythrocytes into aged and younger cells showed higher loss of PS asymmetry in the younger erythrocytes of HbEbeta-thalassaemia patients when compared with normal blood, where PS asymmetry was lost only in the older cells. Sialylated glycoconjugate measurement using the lectins wheatgerm agglutinin and pokeweed mitogen showed loss of sialic acid and N-acetyl-D-glucosamine-bearing glycoproteins in the order normal<homozygous E<HbEbeta-thalassaemic upon ageing. A possible correlation was found between the loss of PS asymmetry with HbE level and the reduction of glycophorins from the cell surface, mediated by membrane vesiculation. A more facilitated vesiculation process in HbEbeta-thalassaemic erythrocytes could lead to faster shedding of glycophorin-containing microvesicles, leaving highly PS-exposed erythrocytes accessible to phagocytes.

Glycophorins and red cell invasion by Plasmodium falciparum

The major red cell sialoglycoproteins, the glycophorins, play a central role in the invasion of human red cells by Plasmodium falciparum. En(a-) cells deficient in glycophorin A (alpha) and S-s-U- cells deficient in glycophorin B (delta) are relatively resistant to invasion, while trypsin treatment of S-s-U- cells, which removes most of the remaining sialoglycoprotein, renders these cells almost totally resistant to invasion. Parasites inside these glycophorin-deficient cells develop normally. Invasion of erythroid precursors in vitro by merozoites of P. falciparum parallels the appearance of glycophorins on the surface of these nucleated cells, even though parasites fail to develop inside them. However, another type of cell from an erythroleukaemic line (K562) which expresses glycophorins on its surface is resistant to invasion. Furthermore, the observed increased invasion of young cells as opposed to an older cell population is not related quantitatively to the presence of glycophorins on the cell surface. Thus, although the role of glycophorins is both specific and important in the invasion of cells by P. falciparum, it is clearly only part of a complex process.

Two different pathways are involved in peroxynitrite-induced senescence and apoptosis of human erythrocytes

CO(2) changes the biochemistry of peroxynitrite basically in two ways: (i) nitrating species is the CO(3)(-) / ()NO(2) radical pair, and (ii) peroxynitrite diffusion distance is significantly reduced. For peroxynitrite generated extracellularly this last effect is particularly dramatic at low cell density because CO(3)(-) and ()NO(2) are short-lived and decay mostly in the extracellular space or at the cell surface/membrane. This study was aimed to distinguish between peroxynitrite-induced extra- and intracellular modifications of red blood cells (RBC). Our results show that at low cell density and in the presence of CO(2) peroxynitrite induced the oxidation of surface thiols, the formation of 3-nitrotyrosine and DMPO-RBC adducts, and the down-regulation of glycophorins A and C (biomarkers of senescence). Reactivation of glycolysis reversed only the oxidation of surface thiols. Without CO(2) peroxynitrite also induced the oxidation of hemoglobin and glutathione, the accumulation of lactate, a decrease in ATP, the clustering of band 3, the externalization of phosphatidylserine, and the activation of caspases 8 and 3 (biomarkers of apoptosis). The latter biomarkers were all reversed by reactivation of glycolysis. We hypothesize that cell senescence could (generally) be derived by irreversible radical-mediated oxidation of membrane targets, while the appearance of apoptotic biomarkers could be bolstered by oxidation of intracellular targets. These results suggest that, depending on extracellular homolysis or diffusion to the intracellular space, peroxynitrite prompts RBCs toward either senescence or apoptosis through different oxidation mechanisms.

Preferential reduction of the α-2-6-sialylation from cell surface N-glycans of human diploid fibroblastic cells by in vitro aging

Human diploid fibroblastic cell line, TIG-3, has a finite life span of about 80 population doubling levels (PDL), and is used for in vitro aging studies. Young cells (PDL 23) grew to higher cell densities at a higher growth rate than aged cells (PDL 77). When the electrophoretic mobility of cells was determined, the negative surface charge of the aged cells decreased significantly when compared to that of young cells. Lectin blot analysis of membrane glycoproteins showed that the alpha-2-6-sialylation but not the alpha-2-3-sialylation of N-glycans decreases markedly in the aged cells when compared to the young cells. In support of this observation, the cDNA microarray assay and reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that the gene expression of the alpha-2,6-sialyltransferase I (ST6Gal I), which transfers sialic acid to galactose residues of N-glycans, decreases in the aged cells. These results indicate that the concordant decrease of the alpha-2,6-sialylation of N-glycans with the ST6Gal I gene expression is induced in TIG-3 cells by in vitro aging.

Caspase 3-mediated proteolysis of the N-terminal cytoplasmic domain of the human erythroid anion exchanger 1 (band 3)

The N-terminal cytoplasmic domain of the anion exchanger 1 (AE1 or band 3) of the human erythrocyte associates with peripheral membrane proteins to regulate membrane-cytoskeleton interactions, with glycolytic enzymes such as glyceraldehyde-3-phosphate dehydrogenase and aldolase, with the protein-tyrosine kinase p72syk, with hemoglobin and with hemichromes. We have demonstrated that the N-terminal cytoplasmic domain of band 3 (CDB3) is a substrate of the apoptosis executioner caspase 3 (1). CDB3 has two non-conventional caspase 3 cleavage sites, TATD45 and EQGD205 (2). In vitro treatment of recombinant CDB3 with caspase 3 generated two fragments, which could be blocked by pretreatment with the caspase 3 inhibitor Z-DEVD-fmk (3). Recombinant CDB3 in which the caspase 3 cleavage sites Asp45 and Asp205 were mutated, was resistant to proteolysis (4). Proteolytically derived fragments crossreactive with polyclonal anti-band 3 antibody appeared with simultaneous cleavage of poly (ADP-ribose) polymerase and procaspase 3 in staurosporine (STS)-treated HEK293 cells transiently transfected with CDB3 (5). In vivo cleavage of CDB3 could be blocked by pretreatment of cells with Z-DEVD-fmk or in cells transfected with mutant CDB3 (D45A, D205A) (6). Co-transfection experiments showed that STS-mediated cleavage of CDB3 diminished its interaction with the N-terminal domain of protein 4.2, confirming that such cleavage interferes with the interaction of CDB3 with cytoskeletal proteins (7). Active caspase 3 was observed in aged red cells but not in young cells. This red cell caspase 3 could cleave band 3 present in inside-out vesicles prepared from young erythrocytes arguing in favor of a physiological role of caspase 3 in aged erythrocytes.

Cell-cell affinity of senescent human erythrocytes

During their 120-day life span, human red blood cells (RBC) undergo several physicochemical changes, including an increased tendency to aggregate in plasma or polymer solutions. This study was designed to examine potential associations between age-related differences in RBC mobility, aggregation, and membrane glycocalyx properties for cells suspended in buffer and in 3 g/dl solutions of 70.3 kDa dextran. A recent model for depletion-mediated RBC aggregation was employed to calculate the changes of glycocalyx properties that were consistent with experimental electrophoretic mobility (EPM) and aggregation data. Young and old cells were obtained by density separation, after which aggregation and EPM were determined versus ionic strength; old cells exhibited a two- to threefold greater aggregation in dextran. EPM of old cells was identical to young cells in polymer-free media yet was 4% greater in dextran. The greater EPM for old RBC indicates a larger polymer depletion layer, which could be explained either by a 10-15% decrease of their glycocalyx thickness or a similar percentage decrease of polymer penetration into their glycocalyx. The larger depletion layer leads to markedly elevated cell-cell affinities for old cells, with the computed affinity increases consistent with enhanced old RBC aggregation. These results provide a rational explanation for the aggregation and EPM behavior of old RBC, and raise the possibility of depletion-mediated interactions contributing to senescent cell removal from the circulation.

Sialic acid, intercellular adhesion molecule-1 and rheumatoid arthritis: a study on the erythrocyte membrane

We measured serum levels of soluble intercellular adhesion molecule-1 and erythrocyte membrane sialic acid in patients with rheumatoid arthritis and studied their correlation with the disease activity. Serum soluble intercellular adhesion molecule-1 level was determined with sandwich ELISA and sialic acid level with the method of Shamberger in 42 patients with rheumatoid arthritis and in 30 healthy controls. Significantly lower erythrocyte membrane sialic acid and higher serum levels of soluble intercellular adhesion molecule-1 were found in patients with rheumatoid arthritis compared with healthy controls (p<0.001 for both). Significant negative correlation between soluble intercellular adhesion molecule-1 level and erythrocyte membrane sialic acid concentration (r=-0.49, p<0.001) and positive correlations between soluble intercellular adhesion molecule-1 level and Ritche Articular Index score and C-reactive protein (r=0.32, p<0.05; r=0.44, p<0.01, respectively) were observed. From these data we conclude that decreases in erythrocyte membrane sialic acid concentration and increases in soluble intercellular adhesion molecule-1, erythrocyte sedimentation rate and C-reactive protein levels are present in rheumatoid arthritis, and that the increased soluble intercellular adhesion molecule-1 in rheumatoid arthritis might be due to the decreased erythrocyte membrane sialic acid concentration. The levels of soluble intercellular adhesion molecule-1 may be a novel marker for the disease status and the activity of rheumatoid arthritis.

Membrane anchoring and surface distribution of glycohydrolases of human erythrocyte membranes

The membrane anchoring of the following glycohydrolases of human erythrocyte plasma membranes was investigated: alpha- and beta-D-glucosidase, alpha- and beta-D-galactosidase, beta-D-glucuronidase, N-acetyl-beta-D-glucosaminidase, alpha-D-mannosidase, and alpha-L-fucosidase. Optimized fluorimetric methods for the assay of these enzymes were set up. Treatment of the ghost preparation with 1.0 mol/l (optimal concentration) NaCl caused release ranging from 4.2% of alpha-D-glucosidase to 70% of beta-D-galactosidase; treatment with 0.4% (optimal concentration) Triton X-100 liberated 5.1% of beta-D-galactosidase to 89% of alpha-D-glucosidase; treatment with 1.75% (optimal concentration) octylglucoside yielded solubilization from 6.3% of beta-D-galactosidase to 85% of alpha-D-glucosidase. Treatment with phosphoinositide-specific phospholipase C caused no liberation of any of the studied glycohydrolases. These results are consistent with the notion that the above glycohydrolases are differently anchored or associated with the erythrocyte plasma membrane, and provide the methodological basis for inspecting the occurrence of these enzymes in different membrane microdomains.

Sialic acid, transketolase and Na+, K+, ATPase in patients with rheumatoid arthritis

We studied transketolase activity of red blood cell hemolysates, and Na+, K+, ATPase activity and sialic acid concentration in red blood cell membranes from 52 patients with rheumatoid arthritis and 24 control subjects. Decreased red blood cell membrane Na+, K+, ATPase activity and sialic acid concentration and decreased transketolase in red blood cell hemolysates were observed in rheumatoid arthritis patients compared with control subjects (p < 0.001). Erythrocyte sedimentation rate and C-reactive protein values were increased in rheumatoid arthritis patients compared with control subjects (p < 0.0001). Significant correlations between sialic acid and Na+, K+, ATPase (r = 0.65, p < 0.001) and between sialic acid and transketolase (r = 0.58, p < 0.001) were observed. Erythrocyte sedimentation rate and C-reactive protein levels did not correlate with Na+, K+, ATPase activity or with sialic acid or transketolase in rheumatoid arthritis patients. These data show that decreases in Na+, K+, ATPase, and transketolase activities and sialic acid concentration are present in rheumatoid arthritis patients, and that the decrease in Na+, K+, ATPase and transketolase activities in rheumatoid arthritis might be due to decreased sialic acid.

Presence in Human Erythrocyte Membranes of a Novel Form of Sialidase Acting Optimally at Neutral pH

The feature of intact human erythrocytes and erythrocyte white ghosts is a unique sialidase activity with acidic optimal pH (acidic sialidase). The treatment of white ghosts with mildly alkaline isotonic solutions at 37°C, like that used to produce resealed ghosts, is accompanied by the expression, together with the acidic sialidase, of a novel sialidase with a pH optimum of 7.2 (neutral sialidase) that remained masked in the inside-out vesicles prepared from white ghosts. Exhaustive treatment of resealed ghosts with Bacillus Thuringiensis phosphatidylinositol-specific phospholipase C causes an almost complete release of the acidic sialidase, with the neutral enzyme remaining totally unaffected. The treatment of resealed ghosts with 1.2% Triton X-100 resulted in the solubilization of only the neutral sialidase, whereas 3.6% octylglucoside also solubilized the acidic sialidase. The neutral enzyme affected not only the artificial substrate but also any sialoderivatives of a ganglioside, glycoprotein, and oligosaccharide nature; the acidic enzyme did not affect sialoglycoproteins. Erythrocyte endogenous gangliosides were hydrolyzed by both sialidases, whereas the endogenous sialoglycoproteins responded to only the neutral enzyme. It was definitely proved that the acidic sialidase is located on the outer erythrocyte membrane surface, so presumably the neutral enzyme has the same location. It could be that the newly discovered neutral sialidase has a physiologic role in the releasing of sialic acid from erythrocytes during the erythrocyte aging process, leading to eventual phagocytosis by macrophages.

The elimination of aged glycoproteins by the isolated perfused rat liver

The isolated livers of Sprague-Dawley rats aged ten months were perfused with nine different glycoproteins (coeruloplasmine, haptoglobin, transferrin, alpha2-mocroglobulin, apolipoprotein A, plasminogen, acid alpha1-glycoprotein, alpha1-antitrypsin, beta2-glycoprotein), which were suspended in a basic medium free of hemoglobin and other proteins. Besides a control group two other groups were formed to give a model for aging: one group of glycoproteins was desialysed, another was desilysed and incubated with immunoglobulins before the perfusion. In spite of the application of two procedures which are supposed to produce a model of old glycoproteins, neither treatment resulted in the recognition, binding and elimination of the glycoproteins from the perfusion medium.

Sialic acid, diabetes, and aging: a study on the erythrocyte membrane

Sialic acid (SA) content and Na+/K+-ATPase activity of red blood cell (RBC) membranes were studied in 26 normoalbuminuric patients with insulin-dependent diabetes mellitus (IDDM), 25 normoalbuminuric patients with non-insulin-dependent diabetes mellitus (NIDDM), and 40 healthy nondiabetic subjects with a negative family history for diabetes. A decrease in RBC membrane SA content and Na+/K+-ATPase activity was observed in older control subjects compared with younger controls. A significant correlation between age, Na+/K+-ATPase activity, and SA content was also found. No difference was observed in RBC membrane SA content between IDDM and NIDDM subjects, but Na+/K+-ATPase activity was significantly lower in IDDM patients. SA content was increased in NIDDM subjects compared with healthy subjects of similar age, whereas Na+/K+-ATPase activity was significantly lower in both IDDM and NIDDM subjects compared with controls. In NIDDM, Na+/K+-ATPase activity was significantly correlated with age, whereas both Na+/K+-ATPase activity and SA content were significantly correlated in IDDM and NIDDM patients. Hemoglobin A1c, (HbA1c) levels did not show any significant correlation either with Na+/K+-ATPase or with SA content in diabetic patients. The modified SA content and Na+/K+-ATPase activity in elderly subjects described in the present study indicate a similar behavior of the erythrocyte membrane during both RBC senescence and aging of subjects.

Flow cytofluorimetric analysis of young and senescent human erythrocytes probed with lectins. Evidence that sialic acids control their life span

Comparing the properties of 'young' and senescent ('aged') O+ erythrocytes isolated by applying ultracentrifugation in a self-forming Percoll gradient, we demonstrate that the sialic acids of membrane glycoconjugates control the life span of erythrocytes and that the desialylation of glycans is responsible for the clearance of the aged erythrocytes. This capture is mediated by a beta-galactolectin present in the membrane of macrophages. The evidence supporting these conclusions is as follows: (1) Analysis by flow cytofluorimetry of the binding of fluorescein isothiocyanate labelled lectins specific for sialic acids shows that the aged erythrocytes bind less WGA, LPA, SNA and MAA than young erythrocytes. The binding of DSA and LCA is not modified. On the contrary, the number of binding sites of UEA-I specific for O antigen and of AAA decreases significantly. PNA and GNA do not bind to erythrocytes. (2) RCA120 as well as Erythrina cristagalli and Erythrina corallodendron lectins specific for terminal beta-galactose residues lead to unexpected and unexplained results with a decrease in the number of lectin binding sites associated with increasing desialylation. (3) The glycoconjugates from the old erythrocytes incorporate more sialic acid than the young cells. This observation results from the determination of the rate of transfer by alpha-2,6-sialyltransferase of fluorescent or radioactive N-acetylneuraminic acid, using as donors CMP-9-fluoresceinyl-NeuAc and CMP-[14C]-NeuAc, respectively. (4) Microscopy shows that the old erythrocytes are captured preferentially by the macrophages relative to the young ones. Fixation of erythrocytes by the macrophage membrane is inhibited by lactose, thus demonstrating the involvement of a terminal beta-galactose specific macrophage lectin. (5) Comparative study of the binding of WGA, LPA, SNA and MAA to the aged erythrocytes and to the in vitro enzymatically desialylated erythrocytes shows that the desialylation rate of aged cells is low but sufficient to lead to their capture by the macrophages.

Density separation of human red blood cells on self forming Percoll gradients: correlation with cell age

Human red blood cells were density separated on self-forming Percoll gradients. Redistribution of density fractionated red blood cells was studied by recentrifugation on self-forming Percoll gradients. A protocol that avoids centrifugation of red cells prior to removal of white cells and introduces EDTA before red cell pelleting completely avoided redistribution. Dense red cells separated according to this method were senescent on the basis of a biochemical and a physical criterion: the increase in the band 4.1a:4.1b ratio (Mueller, T., Jackson, C.W., Dockter, M.E. and Morrison, M. (1987) J. Clin. Invest. 79, 492-499) and the loss of maximum deformability. Characterization also included the relative content of two surface proteins (complement receptor 1, CR1 (Ripoche, J. and Sim, R.B. (1986) Biochem. J. 235, 815-821); decay accelerating factor, DAF) on density fractionated red cells. Unlike cytoplasmic proteins, these proteins face similar conditions, whether located on circulating reticulocytes or aging red cells. Both components were lost linearly within experimental errors with cell density and were lower by 60 and 40% in dense than light cells, respectively.

Peripheral catabolism of CR1 (the C3b receptor, CD35) on erythrocytes from healthy individuals and patients with systemic lupus erythematosus (SLE)

The present study investigated the rate of catabolism of CR1 (the C3b receptor, CD35) on erythrocytes (E) in vivo, in relationship with the expressed number of CR1/E, the CR1.1 HindIII quantitative CR1 polymorphism, and cell age. The relationship between the number of CR1/E and cell age was analysed by measuring G6PDH activity in E that had been sorted according to high or low expression of CR1 (CD35), by assessing the expression of CR1 (CD35) on E separated according to cell density, and by comparing the number of CR1 (CD35) antigenic sites on reticulocytes and on E. A physiological catabolism of CR1 (CD35) manifested by a reduction in the number of CR1 (CD35) antigenic sites/E with cell ageing was consistently observed in healthy individuals. The number of CR1/E decreased with ageing of E according to a complex pattern that associated an exponential decay and an offset. Calculated half-lives of CR1 (CD35) ranged between 11 and 32 days in healthy individuals. A more rapid loss of CR1 (CD35) with cell ageing occurred on cells from individuals expressing high numbers of CR1/E. In patients with systemic lupus erythematosus (SLE), half-lives of CR1 (CD35) on E were in the same range as those of healthy individuals with a similar quantitative CR1 genotype; the number of CR1 (CD35) on reticulocytes was reduced and linearly related to the number of CR1/E, independently of the patients' quantitative CR1 genotype. Transfusion experiments with E bearing high or low amounts of CR1/E indicated the lack of preferential removal of E bearing high numbers of CR1 (CD35) in patients with SLE. These results indicate that the rate of loss of CR1 (CD35) from E with cell ageing is directly related to the quantitative CR1 phenotype and suggest that enhanced peripheral catabolism is not the sole mechanism of the acquired loss of CR1 (CD35) on E in patients with SLE.

Biochemical characterization of size-separated human red blood cells

Human red blood cells (RBC) are heterogeneous with respect to their size; the physiological significance of this heterogeneity has not yet been fully elucidated. To further investigate this problem, some characteristics of human RBC fractionated according to their mean corpuscular volume (MCV) by counterflow centrifugation were determined. Larger RBC are more prone to hypotonic lysis. The membrane protein content per cell increases with the MCV, but no obvious difference in the distribution of the major proteins can be demonstrated. The lipid content per cell also rises with the RBC size, while the percentages of the main lipid components do not significantly vary. However, the variations of sialic acid content per RBC according to MCV are more important than those of protein or lipid; thus, the sialic acid-to-protein ratio gradually increases with the MCV. This indicates that, in spite of the lack of major changes in the membrane composition, some qualitative differences exist between large and small cells.

Alteration of the aminophospholipid translocase activity during in vivo and artificial aging of human erythrocytes

Human erythrocytes were separated into three density groups representing different age groups. Phospholipid outside-inside translocation rates and equilibrium distribution were determined in each group with spin-labeled phosphatidylserine (PS*), phosphatidylethanolamine (PE*), and phosphatidylcholine (PC*), at 37 degrees C and 4 degrees C. At both temperatures, the initial velocity of aminolipid translocation was reduced in the more dense (older) cells. The equilibrium distribution was not significantly modified for PS*, but a larger fraction of PE* remained on the outer monolayer of the more dense cells. PC* transmembrane diffusion was identical in the three fractions. Cytosolic ATP, which is required for aminophospholipid translocation, was not responsible for the variability of the density separated cells since ATP enrichment did not cancel the differences between top and bottom fractions, although it equalized the ATP concentration of the various fractions. Variations in the level of intracellular Ca2+ could also be excluded. Thus, the enzyme aminophospholipid translocase seemed to be directly altered in aged cells, possibly due to oxidation caused by lipid peroxidation products. Experiments with malonyldialdehyde or H2O2 treated cells confirmed this interpretation and suggest that defects in endogenous lipid asymmetry observed in aged human erythrocytes may be due to altered activity of the translocase.

Covalent binding of detergent-solubilized membrane glycoproteins to 'Chemobond' plates for ELISA

An ELISA method is presented which is based on covalent binding of detergent-solubilized membrane proteins to surface-modified polystyrene plates (Chemobond plates). These plates carried 0.52-0.65 nmol of aldehyde groups per well (150 microliters) and allowed coupling of protein by Schiff base formation either at high pH and subsequent reduction with NaBH4 or by trapping reduced imines at pH 6-6.8 with cyanoborohydride. They bound 15 times the amount of normal plates. Sodium chloride (0.5 M) increased binding 2-3-fold. Binding was essentially resistant to elution by 1% sodium dodecyl sulfate. Reduction of uncoated plates with NaBH4 eliminated the high extent of binding. ELISA tests on Chemobond plates with a rabbit anti-band 3 antibody gave a ten-fold higher signal than plates to which band 3 protein was merely adsorbed. The use of an antigen-enzyme conjugate to detect bound antibody allowed to perform antibody binding and detection of bound antibody simultaneously in the presence of 0.05% Triton X-100. A competitive, one step ELISA system allowed determination of rabbit anti-band 3 antibodies in diluted serum with a sensitivity range of 0.02-0.4 microgram/ml.

Erythrocyte rheology

Two main subjects of erythrocyte rheology, deformation and aggregation, are discussed in detail, on the basis of biochemical structure. The close relationship between the life span (or cell aging) and the rheology of individual erythrocytes is also briefly described. A currently important problem is emphasized, that is, the molecular aspect of the dynamic cytoskeletal structure and the mechanism of its regulation. This concerns not only the rheological function and the survival of circulating erythrocytes, but also the pathophysiology of abnormal erythrocytes.

Binding of lectins to "young" and "old" human erythrocytes

"Old" human erythrocytes showed a 21.2% decrease in cell surface area and a 2% decrease in the number of WGA receptor sites, but a 27% increase in the distribution density of the WGA (lectin) receptor site, when compared with "young" human erythrocytes. For a list of lectin abbreviations, see Materials and methods). Both "young" and "old" erythrocytes exhibited very weak binding activity for 125I-labeled PNA, but there was no difference in binding activity for PNA between "young" erythrocytes and "old" ones. Compared with "young" erythrocytes, decreases in the number and distribution density of receptor sites for five lectins including LPA, Con A, RCA-II, SBA and BPA on the cell surface were observed in aged erythrocytes. "Old" erythrocytes also showed a decrease in the number of PHA-E receptor sites, while the distribution density of the same receptor site remained unchanged. In view of these and other observations, it is thought that human erythrocyte aging is accompanied by elimination of some glycoconjugates which have affinity for six lectins, LPA, Con A, RCA-II, PHA-E, SBA and BPA, whereas no WGA receptor-containing glycoconjugates are released from erythrocyte membranes. Elimination of the glycoconjugates results in shrinkage of erythrocytes to reduce their cell surface areas.

Comparative study on the main membrane-surface sialoglycopeptides released from young and old human erythrocytes with trypsin

1. The amount of sialoglycopeptide (SGP) mixture released from young (Y) erythrocytes by trypsin treatment was significantly larger than that from old (O) ones. 2. The two main membrane-surface sialoglycopeptides (Y-SGP and O-SGP) were isolated in yields of about 45% and 42% from the mixtures by a combination of Sephadex G-50 superfine gel filtration with DEAE-cellulose chromatography, respectively. Molecular weights of 12,000 and 12,100, respectively, were found for the Y-SGP and the O-SGP. 3. The Y-SGP showed a greater proportion of sialic acid than the O-SGP. The contents of galactose, N-acetylglucosamine and N-acetylgalactosamine of the Y-SGP were significantly lower than those of the O-SGP. 4. Both the SGPs exhibited the same reactivities against the MN reagents, but the reactivities against the lectins of Limulus polyphemus and Phaseolus vulgaris (E) were stronger in the Y-SGP than in the O-SGP. Neither the Y-SGP nor the O-SGP showed the reactivity to Arachis hypogaea (anti-T) lectin.

Decreased deformability of erythrocytes in hyperglycaemic non-inbred ob/ob mice

The deformability of erythrocytes from non-inbred ob/ob mice and lean controls was analyzed by filtration through Nuclepore polycarbonate under constant pressure. At the age of 1-2 months there was no difference in erythrocyte filtrability between the two types of mice, whereas from 3 months the ob/ob mouse erythrocytes exhibited a markedly decreased deformability. The filtrability of erythrocytes was sensitive to osmotic pressure (NaCl or glucose). However, the difference between normal and ob/ob mouse erythrocytes was not due to acute osmotic effects of the hyperglycaemia in the ob/ob mice. When filtration was performed in the same glucose concentration as that recorded in the blood of the erythrocyte-donor animal, the difference in filtrability between adult normal and ob/ob mice remained large and significant (p less than 0.01). Moreover, the most pronounced hyperglycaemia occurred in young ob/ob mice with normal erythrocyte filtrability. It is suggested that non-inbred ob/ob mice are a useful model for studying the damaging influence of diabetes on erythrocyte deformability.

Qualitative and quantitative analysis of erythrocyte surface membrane sialyl residues using affinity cytochemistry with special reference to diabetic patients

Erythrocyte surface membrane sialyl residues were investigated by means of affinity cytochemistry using the avidin-biotin complex technique. Mild oxidation with the periodate (MO)-biotin hydrazide (BHZ)-ferritin avidin conjugate (FAv) sequence revealed numerous ferritin particles on erythrocytes from healthy donors. The ferritin particles attached on the perpendicularly sectioned membrane were seen at an average distance of 10 to 12 nm from the outer dense leaflet of the cell membrane. Pretreatment with neuraminidase followed by the MO-BHZ-FAv sequence almost eliminated erythrocyte ferritin labeling. Erythrocytes from diabetic patients showed less dense ferritin labeling compared with those from healthy donors. Quantiative analysis of sialyl residues demonstrated a marked reduction in ferritin labeling of erythrocytes from diabetic patients which was significantly less (p less than 0.01) than that of erythrocytes from healthy donors. This observation supports previous biochemical data demonstrating lower levels of surface membrane negative charge and sialyl residues on erythrocytes from patients with diabetes mellitus.

Effects of shape and size on red blood cell deformability: a static bending analysis

When flowing down a tapered tube, such as a narrow capillary, red blood cells (RBCs) are subject to deformation, the first event of which is folding in a pancake manner. The RBC deformability is reduced during cell ageing, a phenomenon that may reflect alterations in intracellular viscosity, membrane rigidity or RBC shape. Age related shape changes and their importance for increased RBC rigidity were theoretically analysed. The average empirically observed RBC profile is shown to offer little resistance to bending as compared to other, theoretically possible profiles of the same membrane area and RBC volume. Because of a decrease in projected area (diameter size), and therefore in pressure load, the pressure needed to initiate folding of an old RBC is between 20 and 55% higher than that required to fold a young one if, during RBC ageing, membrane area to cell volume ratio is constant as empirically observed. This difference exists whether the RBC is mathematically treated as a solid body or as a membrane shell.

Properties of the leak permeability induced by a cytotoxic protein from Pseudomonas aeruginosa (PACT) in rat erythrocytes and black lipid membranes

A cytotoxic protein, isolated from Pseudomonas aeruginosa (PACT), was tested on red blood cells of rats and on black lipid membranes for changes of membrane permeability. In rat erythrocytes PACT induces lysis indicative of the formation of a leak permeable to monovalent ions. The dose response curve for the PACT-induced hemolysis demonstrates that the rate of lysis as well as the fraction of lytic cells increases with increasing toxin concentration. Furthermore, the leak pathway discriminates hydrophilic non-electrolytes according to their molecular weight. The findings indicate formation by PACT of a pore with an apparent radius of about 1.2 nm. In pure lipid membranes PACT forms hydrophilic pathways with moderate selectivity for small cations over small anions. The presence of cholesterol is a prerequisite for the occurrence of these PACT-induced permeability changes.

Aged human erythrocytes exhibit increased anion exchange

Young and old erythrocytes show different rate constants of anion exchange as measured by 35SO4(2-) efflux at 37 degrees C. Results indicate that the rate constant for 35SO4(2-) efflux (SO2-4-Cl- exchange) from old cells is approximately 20% greater than from young less dense cells. The cell water volume of older cells is also decreased. Based on these results and previously reported decreases of cell membrane area in aged cells we conclude that anion exchange (35SO4(2-)) is increased in older, more dense human erythrocytes.

Cell surface alterations during blood-storage characterized by artificial aggregation of washed red blood cells

Aggregation measurement of washed human erythrocytes (RBC) were carried out in a NaCl-PBS solution under laminar shear conditions. Artificial aggregation of fresh and stored erythrocytes was caused by decreased pH and reduced ionic strength and characterized by collision efficiency alpha. Generally, the collision efficiency alpha of stored erythrocytes rises with the increased storage time. Such an aggregation technique might be useful to detect and quantify changes of the membrane and/or the surface structure due to aging and/or storage.

The relationship between sialic acid content and peanut agglutinin binding on senescent and enzyme treated human erythrocytes

Young, old and neuraminidase treated human red blood cells (RBC) were investigated with peanut agglutinin (PNA), a lectin with a specificity similar to that of serum T-agglutinin. The effect of serum agglutinins on this interaction was also investigated. The density and distribution of PNA receptors were evaluated by agglutination with PNA and binding of ferritin-conjugated PNA (PNA-F), or PNA labeled with radioactive iodine [( 131I] PNA). The results were correlated with the distribution of membrane bound sialic acids, as evaluated by chemical analysis and rate of agglutination with poly-L-lysine (PLL). Untreated RBC of all ages did not agglutinate with PNA and failed to bind PNA-F and [131I] PNA. Treatment of young RBC with neuraminidase, which resulted in reduction of membrane-bound sialic acids to an extent similar to that of physiologically aged RBC, resulted in the concomitant exposure of PNA binding sites and in the agglutination of these cells by autologous serum. Pretreatment of the neuraminidase treated RBC with autologous serum resulted in partial inhibition of the binding capacity of PNA on the RBC. The results indicate that the normal age-related loss of sialic acids in circulating RBC is not identical with enzymatic removal of sialic acids by neuraminidase. The observations suggest that different mechanisms are functional in the recognition and sequestration of old RBC and of RBC treated with neuraminidase.