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

The infectious salmon anemia virus esterase prunes erythrocyte surfaces in infected Atlantic salmon and exposes terminal sialic acids to lectin recognition

Many sialic acid-binding viruses express a receptor-destroying enzyme (RDE) that removes the virus-targeted receptor and limits viral interactions with the host cell surface. Despite a growing appreciation of how the viral RDE promotes viral fitness, little is known about its direct effects on the host. Infectious salmon anemia virus (ISAV) attaches to 4-O-acetylated sialic acids on Atlantic salmon epithelial, endothelial, and red blood cell surfaces. ISAV receptor binding and destruction are effectuated by the same molecule, the haemagglutinin esterase (HE). We recently discovered a global loss of vascular 4-O-acetylated sialic acids in ISAV-infected fish. The loss correlated with the expression of viral proteins, giving rise to the hypothesis that it was mediated by the HE. Here, we report that the ISAV receptor is also progressively lost from circulating erythrocytes in infected fish. Furthermore, salmon erythrocytes exposed to ISAV ex vivo lost their capacity to bind new ISAV particles. The loss of ISAV binding was not associated with receptor saturation. Moreover, upon loss of the ISAV receptor, erythrocyte surfaces became more available to the lectin wheat germ agglutinin, suggesting a potential to alter interactions with endogenous lectins of similar specificity. The pruning of erythrocyte surfaces was inhibited by an antibody that prevented ISAV attachment. Furthermore, recombinant HE, but not an esterase-silenced mutant, was sufficient to induce the observed surface modulation. This links the ISAV-induced erythrocyte modulation to the hydrolytic activity of the HE and shows that the observed effects are not mediated by endogenous esterases. Our findings are the first to directly link a viral RDE to extensive cell surface modulation in infected individuals. This raises the questions of whether other sialic acid-binding viruses that express RDEs affect host cells to a similar extent, and if such RDE-mediated cell surface modulation influences host biological functions with relevance to viral disease.

The regulation roles of Ca2+ in erythropoiesis: What have we learned?

Calcium (Ca²⁺) is an important second messenger molecule in the body, regulating cell cycle and fate. There is growing evidence that intracellular Ca²⁺ levels play functional roles in the total physiological process of erythroid differentiation, including the proliferation and differentiation of erythroid progenitor cells, terminal enucleation, and mature red blood cell aging and clearance. Moreover, recent research on the pathology of erythroid disorders has made great progress in the past decades, indicating that calcium ion hemostasis is closely related to ineffective erythropoiesis and increased sensitivity to stress factors. In this review, we summarized what is known about the functional roles of intracellular Ca²⁺ in erythropoiesis and erythrocyte-related diseases, with an emphasis on the regulation of the intracellular Ca²⁺ homeostasis during erythroid differentiation. An understanding of the regulation roles of Ca²⁺ homeostasis in erythroid differentiation will facilitate further studies and eventually molecular identification of the pathways involved in the pathological process of erythroid disorders, providing new therapeutic opportunities in erythrocyte-related disease.

Red Blood Cells: Tethering, Vesiculation, and Disease in Micro-Vascular Flow

The red blood cell has become implicated in the progression of a range of diseases; mechanisms by which red cells are involved appear to include the transport of inflammatory species via red cell-derived vesicles. We review this role of RBCs in diseases such as diabetes mellitus, sickle cell anemia, polycythemia vera, central retinal vein occlusion, Gaucher disease, atherosclerosis, and myeloproliferative neoplasms. We propose a possibly unifying, and novel, paradigm for the inducement of RBC vesiculation during vascular flow of red cells adhered to the vascular endothelium as well as to the red pulp of the spleen. Indeed, we review the evidence for this hypothesis that links physiological conditions favoring both vesiculation and enhanced RBC adhesion and demonstrate the veracity of this hypothesis by way of a specific example occurring in splenic flow which we argue has various renderings in a wide range of vascular flows, in particular microvascular flows. We provide a mechanistic basis for membrane loss and the formation of lysed red blood cells in the spleen that may mediate their turnover. Our detailed explanation for this example also makes clear what features of red cell deformability are involved in the vesiculation process and hence require quantification and a new form of quantitative indexing.

Understanding quasi-apoptosis of the most numerous enucleated components of blood needs detailed molecular autopsy

Erythrocytes are the most numerous cells in human body and their function of oxygen transport is pivotal to human physiology. However, being enucleated, they are often referred to as a sac of molecules and their cellularity is challenged. Interestingly, their programmed death stands a testimony to their cell-hood. They are capable of self-execution after a defined life span by both cell-specific mechanism and that resembling the cytoplasmic events in apoptosis of nucleated cells. Since the execution process lacks the nuclear and mitochondrial events in apoptosis, it has been referred to as quasi-apoptosis or eryptosis. Several studies on molecular mechanisms underlying death of erythrocytes have been reported. The data has generated a non-cohesive sketch of the process. The lacunae in the present knowledge need to be filled to gain deeper insight into the mechanism of physiological ageing and death of erythrocytes, as well as the effect of age of organism on RBCs survival. This would entail how the most numerous cells in the human body die and enable a better understanding of signaling mechanisms of their senescence and premature eryptosis observed in individuals of advanced age.

Inhibitory efficiencies for mechanism-based inactivators of sialidases

Here we describe the measurement of the inactivation rate constants for the mechanism-based inactivator 2,3-difluorosialic acid acting upon the sialidase from Micromonospora viridifaciens. Using double mixing stopped-flow experiments conducted in a 3-(N-morpholino)propanesulfonic acid buffer (100 mmol/L, pH 7.00) at 25 °C, the derived kinetic parameters are kinact/Ki = (3.9 ± 0.8) × 106 (mol/L)–1 s–1 and Ki = 1.7 ± 0.4 μmol/L. We demonstrate that the inhibitory efficiency of the inactivation event is similar to the catalytic efficiency for this sialidase acting upon a typical substrate, 4-methylumbelliferone α-d-sialoside, kcat/Km = (7.2 ± 2.8) × 106 (mol/L)–1 s–1. Furthermore, we show that the catalytic efficiencies for inactivation and hydrolysis by the Kdnase from Aspergillus fumigatus are similar for the corresponding Kdn-analogues. We conclude that the deactivating effect of incorporating an axial 3-fluoro substituent onto the sialic acid scaffold is comparable to the enhanced activation that occurs when the 4-methylumbelliferone leaving group is changed to the more nucleofugal fluoride ion.

Mechanisms tagging senescent red blood cells for clearance in healthy humans

This review focuses on the analysis and evaluation of the diverse senescence markers suggested to prime red blood cells (RBC) for clearance in humans. These tags develop in the course of biochemical and structural alterations accompanying RBC aging, as the decrease of activities of multiple enzymes, the gradual accumulation of oxidative damage, the loss of membrane in form of microvesicles, the redistribution of ions and alterations in cell volume, density, and deformability. The actual tags represent the penultimate galactosyl residues, revealed by desialylation of glycophorins, or the aggregates of the anion exchanger (band 3 protein) to which anti-galactose antibodies bind in the first and anti-band 3 naturally occurring antibodies (NAbs) in the second case. While anti-band 3 NAbs bind to the carbohydrate-free portion of band 3 aggregates in healthy humans, induced anti-lactoferrin antibodies bind to the carbohydrate-containing portion of band 3 and along with anti-band 3 NAbs may accelerated clearance of senescent RBC in patients with anti-neutrophil cytoplasmic antibodies (ANCA). Exoplasmically accessible phosphatidylserine (PS) and the alterations in the interplay between CD47 on RBC and its receptor on macrophages, signal regulatory protein alpha (SIRPalpha protein), were also reported to induce erythrocyte clearance. We discuss the relevance of each mechanism and analyze the strength of the data.

Longer storage of red blood cells is associated with increased in vitro erythrophagocytosis

BACKGROUND AND OBJECTIVES

Refrigerated storage of red blood cells (RBCs) induces numerous changes that may target the cells for erythrophagocytosis following transfusion. The influence of storage upon the phagocytosis of unseparated and fractionated young and old stored RBCs was investigated using two in vitro quantitative phagocytosis assays.

MATERIALS AND METHODS

Leucocyte-depleted RBC units were sampled at day 1 or 42 of storage. Young and old RBCs were fractionated at day 1 by density centrifugation and stored in paediatric packs for up to 42 days. RBCs were labelled with the fluorescent dye PKH26 and incubated with the human monocytic cell line THP-1. Erythrophagocytosis was quantified by flow cytometry and plate fluorometric assays.

RESULTS

A higher proportion of THP-1 cells phagocytosed RBCs stored for 42 days compared with 1 day (41% and 24% respectively; P<0·0001). This was associated with an increased mean number of RBCs phagocytosed per THP-1 cell (5·2±0·6 and 3·3±0·2 respectively; P<0·002). Erythrophagocytosis of fractionated young and old RBCs increased with longer storage duration up to 28 days (P<0·05). However, no significant differences were observed between erythrophagocytosis of young and old RBCs.

CONCLUSION

The susceptibility of stored RBCs to erythrophagocytosis significantly increased with longer storage time of the RBC units. Storage duration of RBCs had a greater influence on in vitro erythrophagocytosis than the chronological age of the RBCs at donation.

Identification of lysosomal sialidase NEU1 and plasma membrane sialidase NEU3 in human erythrocytes

The sialylation level of molecules, sialoglycoproteins and gangliosides, protruding from plasma membranes regulates multiple facets of erythrocyte function, from interaction with endothelium to cell lifespan. Our results demonstrate that: (a) Both sialidases NEU1 and NEU3 are present on erythrocyte plasma membrane; (b) NEU1 is kept on the plasma membrane in absence of the protective protein/cathepsin A (PPCA); (c) NEU1 and NEU3 are retained on the plasma membrane, as peripheral proteins, associated to the external leaflet and released by alkaline treatments; (d) NEU1 and NEU3 are segregated in Triton X-100 detergent-resistant membrane domains (DRMs); (e) NEU3 shows activity also at neutral pH; and (f) NEU1 and NEU3 are progressively lost during erythrocyte life. Interestingly, sialidase activity released from erythrocyte membranes after an alkaline treatment preserves its functionality and recognizes sialoglycoproteins and gangliosides. On the other hand, the weak anchorage of sialidases to the plasma membrane and their loss during erythrocyte life could be a tool to preserve the cellular sialic acid content in order to avoid the early ageing of erythrocyte and processes of cell aggregation in the capillaries.

Flow cytometric assessment of erythrocyte shape through analysis of FSC histograms: use of kurtosis and implications for longitudinal evaluation

Sphericity of erythrocytes can be estimated from analysis of FSC signal distribution in flow cytometry. Previously, Pearson's coefficient of dissymmetry (PCD) and spherical index (SphI) were applied to determine erythrocyte sphericity from the FSC histogram. The aim of the present study is to illustrate the application of kurtosis as an indicator of erythrocyte sphericity in flow cytometry in a broad range of FSC distributions. Moreover, the possibility of longitudinal evaluation of erythrocyte sphericity is studied. Change of erythrocyte sphericity of 10 healthy subjects was induced by variation of buffer osmolarity to validate applicability of sphericity measures. Agreement between the sphericity indicators was then studied in samples from 20 healthy donors taken at three time points, which were processed through density gradient centrifugation and incubated with FITC-labelled antibodies to induce a broad variation of erythrocyte form (1086 samples). SphI, PCD and kurtosis of FSC distribution were calculated. Correlation of the respective measures, standard error of measurement (SEM) and r ratio (intra- to interindividual variance) were determined to illustrate agreement between the sphericity indicators. In the first study part, all sphericity indicators illustrated change of erythrocyte shape as induced by osmolarity variation. In the second part, correlation between kurtosis and SphI was -0.97 and correlation between kurtosis and PCD was 0.58 (p<0.05). In isotype control samples, correlation between kurtosis and SphI was -0.98 and correlation between kurtosis and PCD was 0.48 (p<0.05). In these samples, mean kurtosis was -0.80 (SEM 0.03), mean SphI was 2.19 (SEM 0.04) and mean PCD was -0.31 (SEM 0.02). r ratios of all measures of sphericity were <0.6. Our results show that kurtosis is closely correlated with SphI in a broad range of erythrocyte FSC distributions. Moreover, all measures of sphericity feature r ratios <0.6, highlighting that erythrocyte sphericity appears as a feasible parameter for individual longitudinal data monitoring.

Reticulocyte count in red-blood-cell units stored in AS-1

BACKGROUND AND OBJECTIVES

Previous data that showed maintenance of reticulocyte percentage in whole blood stored in CPDA-1 have led to the assumption that reticulocyte maturation becomes arrested during refrigerated storage. However, reticulocyte behaviour in red-blood-cell units stored in additive solutions has not yet been studied. This study was thus aimed at determining reticulocyte count and reticulocyte subtypes in red-blood-cells units stored in AS-1.

MATERIALS AND METHODS

Reticulocyte percentage and subtypes were determined by flow cytometry with thiazole orange in six red-blood-cells units stored in AS-1.

RESULTS

Reticulocyte count was 26.8 ± 4.6 × 10(9) /l at week 0.5 and 8.2 ± 2.9 × 10(9) /l at week 6. Total haemolysis during storage was 0.19 ± 0.08%. High-fluorescence reticulocytes were 2.0 ± 3.2 × 10(9) /l at week 0.5 and decreased by weeks 2, 4 and 6. Low-fluorescence reticulocytes were 22.1 ± 3.1 × 10(9) /l at week 0.5 and decreased by weeks 4 and 6.

CONCLUSION

A significant decrease in reticulocytes occurred during red-blood-cells units' storage in AS-1. Even if it were assumed that all of haemolysed cells during storage were reticulocytes, there are a number of them whose disappearance cannot be explained by this mechanism. Changes observed in reticulocyte subtypes suggest that they mature during storage.

Electrochemical lectin based biosensors as a label-free tool in glycomics

Glycans and other saccharide moieties attached to proteins and lipids, or present on the surface of a cell, are actively involved in numerous physiological or pathological processes. Their structural flexibility (that is based on the formation of various kinds of linkages between saccharides) is making glycans superb "identity cards". In fact, glycans can form more "words" or "codes" (i.e., unique sequences) from the same number of "letters" (building blocks) than DNA or proteins. Glycans are physicochemically similar and it is not a trivial task to identify their sequence, or - even more challenging - to link a given glycan to a particular physiological or pathological process. Lectins can recognise differences in glycan compositions even in their bound state and therefore are most useful tools in the task to decipher the "glycocode". Thus, lectin-based biosensors working in a label-free mode can effectively complement the current weaponry of analytical tools in glycomics. This review gives an introduction into the area of glycomics and then focuses on the design, analytical performance, and practical utility of lectin-based electrochemical label-free biosensors for the detection of isolated glycoproteins or intact cells.

Carbohydrate clearance receptors in transfusion medicine

BACKGROUND

Complex carbohydrates play important functions for circulation of proteins and cells. They provide protective shields and refraction from non-specific interactions with negative charges from sialic acids to enhance circulatory half-life. For recombinant protein therapeutics carbohydrates are especially important to enhance size and reduce glomerular filtration loss. Carbohydrates are, however, also ligands for a large number of carbohydrate-binding lectins exposed to the circulatory system that serve as scavenger receptors for the innate immune system, or have more specific roles in targeting of glycoproteins and cells.

SCOPE OF REVIEW

Here we provide an overview of the common lectin receptors that play roles for circulating glycoproteins and cells, and present a discussion of ways to engineer glycosylation of recombinant biologics and cells to improve therapeutic effects.

MAJOR CONCLUSIONS

While the pharmaceutical industry has learned how to exploit carbohydrates to improve pharmacokinetic properties of recombinant therapeutics, our understanding of how to improve cell-based therapies by manipulation of complex carbohydrates is still at its infancy. Progress with the latter has recently been achieved with cold-stored platelets, where exposure of uncapped glycans lead to rapid clearance from circulation by several lectin-mediated pathways.

GENERAL SIGNIFICANCE

Understanding lectin-mediated clearance pathways is essential for progress in development of biological pharmaceuticals.

Desialylation accelerates platelet clearance after refrigeration and initiates GPIbα metalloproteinase-mediated cleavage in mice

When refrigerated platelets are rewarmed, they secrete active sialidases, including the lysosomal sialidase Neu1, and express surface Neu3 that remove sialic acid from platelet von Willebrand factor receptor (VWFR), specifically the GPIbα subunit. The recovery and circulation of refrigerated platelets is greatly improved by storage in the presence of inhibitors of sialidases. Desialylated VWFR is also a target for metalloproteinases (MPs), because GPIbα and GPV are cleaved from the surface of refrigerated platelets. Receptor shedding is inhibited by the MP inhibitor GM6001 and does not occur in Adam17(ΔZn/ΔZn) platelets expressing inactive ADAM17. Critically, desialylation in the absence of MP-mediated receptor shedding is sufficient to cause the rapid clearance of platelets from circulation. Desialylation of platelet VWFR therefore triggers platelet clearance and primes GPIbα and GPV for MP-dependent cleavage.

PP13, maternal ABO blood groups and the risk assessment of pregnancy complications

BACKGROUND

Placental Protein 13 (PP13), an early biomarker of preeclampsia, is a placenta-specific galectin that binds beta-galactosides, building-blocks of ABO blood-group antigens, possibly affecting its bioavailability in blood.

METHODS AND FINDINGS

We studied PP13-binding to erythrocytes, maternal blood-group effect on serum PP13 and its performance as a predictor of preeclampsia and intrauterine growth restriction (IUGR). Datasets of maternal serum PP13 in Caucasian (n = 1078) and Hispanic (n = 242) women were analyzed according to blood groups. In vivo, in vitro and in silico PP13-binding to ABO blood-group antigens and erythrocytes were studied by PP13-immunostainings of placental tissue-microarrays, flow-cytometry of erythrocyte-bound PP13, and model-building of PP13--blood-group H antigen complex, respectively. Women with blood group AB had the lowest serum PP13 in the first trimester, while those with blood group B had the highest PP13 throughout pregnancy. In accordance, PP13-binding was the strongest to blood-group AB erythrocytes and weakest to blood-group B erythrocytes. PP13-staining of maternal and fetal erythrocytes was revealed, and a plausible molecular model of PP13 complexed with blood-group H antigen was built. Adjustment of PP13 MoMs to maternal ABO blood group improved the prediction accuracy of first trimester maternal serum PP13 MoMs for preeclampsia and IUGR.

CONCLUSIONS

ABO blood group can alter PP13-bioavailability in blood, and it may also be a key determinant for other lectins' bioavailability in the circulation. The adjustment of PP13 MoMs to ABO blood group improves the predictive accuracy of this test.

The human Cas1 protein: a sialic acid-specific O-acetyltransferase?

Sialic acids are important sugars at the reducing end of glycoproteins and glycolipids. They are among many other functions involved in cell-cell interactions, host-pathogen recognition and the regulation of serum half-life of glycoproteins. An important modification of sialic acids is O-acetylation, which can alter or mask the biological properties of the parent sialic acid molecule. The nature of mammalian sialate-O-acetyltransferases (EC 2.3.1.45) involved in their biosynthesis is still unknown. We have identified the human CasD1 (capsule structure1 domain containing 1) gene as a candidate to encode the elusive enzyme. The human CasD1 gene encodes a protein with a serine-glycine-asparagine-histidine hydrolase domain and a hydrophobic transmembrane domain. Expression of the Cas1 protein tagged with enhanced green fluorescent protein in mammalian and insect cells directed the protein to the medial and trans-cisternae of the Golgi. Overexpression of the Cas1 protein in combination with α-N-acetyl-neuraminide α-2,8-sialyltransferase 1 (GD3 synthase) resulted in an up to 40% increased biosynthesis of 7-O-acetylated ganglioside GD3. By quantitative real-time polymerase chain reaction, we found up to 5-fold increase in CasD1 mRNA in tumor cells overexpressing O-Ac-GD3. CasD1-specific small interfering RNA reduced O-acetylation in tumor cells. These results suggest that the human Cas1 protein is directly involved in O-acetylation of α2-8-linked sialic acids.

A cytometric study of the red blood cells in Gaucher disease reveals their abnormal shape that may be involved in increased erythrophagocytosis

BACKGROUND

Gaucher disease is a sphingolipidosis caused by a deficiency of the enzyme glucocerebrosidase. Macrophages transform into pathogenic Gaucher cells following the phagocytosis of red blood cells (RBCs) and subsequent accumulation of glucosylceramide. Enhanced erythrophagocytosis is one feature of the disease indicating abnormal macrophage-RBC interactions. We hypothesized that the erythrophagocytosis observed in Gaucher disease may be at least partly due to abnormalities in the RBCs themselves.

METHODS

To investigate this hypothesis, we used flow cytometry FSC/SSC to study RBCs sampled from seven patients with Gaucher disease in terms of their shape and the expression of markers of senescence and phagocytosis. Cells from two of the seven patients were evaluated before and 9 months after the start of enzyme-replacement therapy.

RESULTS

Untreated patients were found to have abnormal flow-cytometry profiles suggesting an alteration of Gaucher RBC morphology. Scanning electron microscopy confirmed this finding by revealing many abnormally shaped RBCs. Whereas there was no evidence of desialylation of membrane glycoconjugates or phosphatidylserine exposure, RBC viability (calcein-AM test) and CD47 expression were reduced. These anomalies found in RBCs sampled from two patients before treatment, were no longer present after a 9 month-long enzyme-replacement therapy.

CONCLUSIONS

We report on previously overlooked alterations of Gaucher RBCs that may facilitate erythrophagocytosis in untreated patients. Their potential role in the anemia, the excess of aggregation and rheological anomalies associated with Gaucher disease must now be addressed. RBC anomalies may take part in the abnormal crosstalk between RBCs and macrophages leading to the accumulation of Gaucher cells.

Novel clearance mechanisms of platelets

PURPOSE OF REVIEW

Blood platelets are involved in primary and secondary hemostasis and thus maintain the integrity of the vasculature. They circulate with an average lifespan of 5-9 days in humans. Thus, the body must generate and clear platelets daily to maintain normal physiological blood platelet counts. Known platelet clearance mechanisms include antibody-mediated clearance by spleen macrophages, as in immune thrombocytopenia, and platelet consumption due to massive blood loss.

RECENT FINDINGS

New concepts in the clearance mechanisms of platelets have recently emerged. New evidence shows that platelets desialyted due to chilling or sepsis are cleared in the liver by macrophages, that is Kupffer cells, as well as hepatocytes, through lectin-mediated recognition of platelet glycans. On the contrary, platelet-associated antibodies normalize the clearance of platelets in a mouse model for Wiskott-Aldrich syndrome.

SUMMARY

The goal of this review is to summarize the latest findings in platelet clearance mechanisms with a focus on lectin-mediated recognition of platelet glycans. Transfusion medicine and treatments of hematopoietic disorders associated with severe thrombocytopenia may benefit from a better understanding of these mechanisms.

Red cell survival: relevance and mechanism involved

Red blood cells (RBCs) often have a short circulating half-life in hemodialysis patients, which increases the difficulty of achieving a stable hemoglobin level. Fluctuations in erythropoietin (EPO) levels contribute to this increased RBC turnover because a decline in the level of EPO triggers the preferential destruction of newly-formed RBC, a process termed neocytolysis. The RBCs that are released during the treatment of renal anemia are often hypochromic, with a low content of iron; these RBCs are vulnerable to rapid turnover because iron-deficiency affects RBCs in several ways, such as, increased exposure of the phagocytic signaling molecule phosphatidylserine, loss of deformability, and increased oxidative stress. Both EPO fluctuation and the release of iron-deficient RBCs are characteristic events occurring during the management of renal anemia, and the shorter RBC lifetime is a component of the large fluctuations in hemoglobin level seen in patients on hemodialysis.

Stored red blood cells: a changing universe waiting for its map(s)

The availability of stored red blood cells (RBCs) for transfusion remains an important aspect of the treatment of polytrauma, acute anemia or major bleedings. RBCs are prepared by blood banks from whole blood donations and stored in the cold in additive solutions for typically six weeks. These far from physiological storage conditions result in the so-called red cell storage lesion that is of importance both to blood bankers and to clinical practitioners. Here we review the current state of knowledge about the red cell storage lesion from a proteomic perspective. In particular, we describe the current models accounting for RBC aging and response to lethal stresses, review the published proteomic studies carried out to uncover the molecular basis of the RBC storage lesion, and conclude by suggesting a few possible proteomic studies that would provide further knowledge of the molecular alterations carried by RBCs stored in the cold for six weeks.

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.

Role of sialic acid for platelet life span: exposure of beta-galactose results in the rapid clearance of platelets from the circulation by asialoglycoprotein receptor-expressing liver macrophages and hepatocytes

Although surface sialic acid is considered a key determinant for the survival of circulating blood cells and glycoproteins, its role in platelet circulation lifetime is not fully clarified. We show that thrombocytopenia in mice deficient in the St3gal4 sialyltransferase gene (St3Gal-IV(-/-) mice) is caused by the recognition of terminal galactose residues exposed on the platelet surface in the absence of sialylation. This results in accelerated platelet clearance by asialoglycoprotein receptor-expressing scavenger cells, a mechanism that was recently shown to induce thrombocytopenia during Streptococcus pneumoniae sepsis. We now identify platelet GPIbalpha as a major counterreceptor on ST3Gal-IV(-/-) platelets for asialoglycoprotein receptors. Moreover, we report data that establish the importance of sialylation of the von Willebrand factor in its function.

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.

Age-dependent increase in green autofluorescence of blood erythrocytes

Green auto-fluorescence (GAF) of different age groups of mouse blood erythrocytes was determined by using a double in vivo biotinylation (DIB) technique that enables delineation of circulating erythrocytes of different age groups. A significant increase in GAF was seen for erythrocytes of old age group (age in circulation more than 40 days) as compared to young erythrocytes (age less than 15 days). Erythrocytes are removed from blood circulation by macrophages in the reticulo-endothelial system and depletion of macrophages results in an increased proportion of aged erythrocytes in the blood. When mice were depleted of macrophages for 7 days by administration of clodronate loaded liposomes, the overall GAF of erythrocytes increased significantly and this increase could be ascribed to an increase in GAF of the oldest population of erythrocytes. Using the DIB technique, the GAF of a cohort of blood erythrocyte generated during a 5 day window was tracked in vivo. GAF of the defined cohort of erythrocytes remained low till 40 days of age in circulation and then increased steeply till the end of the life span of erythrocytes. Taken together our results provide evidence for an age dependent increase in the GAF of blood erythrocytes that is accentuated by depletion of macrophages. Kinetics of changes in GAF of circulating erythrocytes with age has also been defined.

A novel mechanism of erythrocyte capture from circulation in humans

OBJECTIVE

The capture of blood cells from the circulation is mediated by highly specialized adhesion molecules. These molecules contribute to the specificity of recruitment for various subsets. Here, we used a simple substrate of hyaluronic acid to investigate the specificity of CD44-mediated recruitment from human whole blood under shear conditions.

MATERIALS AND METHODS

Human whole blood was perfused through a parallel-plate flow chamber, which mimics intravascular conditions. Microscopy was used to directly observe blood-cell interactions with adhesion molecule substrates.

RESULTS

Erythrocytes, but not leukocytes, efficiently tethered to and rolled on the hyaluronic acid substrate. These interactions were demonstrated to be mediated by CD44 and regulated by the sialic acid content of the cells. Inflammatory stimuli did not result in enhanced erythrocyte rolling. Rather, interactions were restricted to aged erythrocytes approaching senescence. This mechanism of erythrocyte capture from the blood flow was found to be restricted to primates and not conserved across mammalian species.

CONCLUSION

This is the first report of erythrocyte tethering and rolling under shear conditions, a behavior, until now, thought to be exclusive to leukocytes. It may represent an important mechanism to identify, capture, and clear old erythrocytes during normal homeostasis or clot formation.

Nouveaux critères d’évaluation de la viabilité des hématies destinées à la transfusion

In light of recent results on the mechanism of programmed cell death of human red blood cells (RBC), the aim of the present study was to solve the enigma of the rapid clearance of transfused RBCs.

MATERIALS AND METHODS

We describe new criteria of RBC viability founded on the use of flow cytometry. They were applied, in association with the classical ones: ATP and hemolysis measurements, to RBCs stored in SAGM medium for 42 days.

RESULTS AND CONCLUSIONS

Application of an original method of flow cytometric quantitation of in vitro erythrophagocytosis showed that an important proportion of stored RBCs were phagocytized although the following classical signals for phagocytosis were absent, i.e.: desialylation, phosphatidylserine exposure in the outer leaflet of the RBC membrane, loss of CD47 receptor, an antiphagocytosis signal. In addition, ATP was still present and hemolysis was very low. This enigma was solved by the use of scanning electron microscopy, which showed the disappearance of discocytes and the presence of an important proportion of spheroechinocytes, which are the phagocytable forms of RBCs. The mechanism of this dramatic morphological transformation remains to be elucidated.

Reduced expression of CD47 during murine red blood cell (RBC) senescence and its role in RBC clearance from the circulation

BACKGROUND

Almost 2 percent of murine blood red blood cells (RBCs) are destroyed each day and are replaced by fresh RBCs generated through the process of erythropoiesis. RBCs to be destroyed are phagocytosed by macrophages in the reticuloendothelial system, especially in the spleen. CD47 molecules on RBCs may regulate the susceptibility of RBC to destruction by phagocytosis because its recognition by inhibitory receptor (signal regulatory protein alpha) on macrophages sends a negative signal, which if sufficiently strong, may abort the phagocytic response altogether. The aim of this study was to investigate whether age-dependent changes in CD47 expression on circulating RBCs have a role in destruction of senescent RBCs by macrophages.

STUDY DESIGN AND METHODS

A two-step in vivo biotinylation method for labeling mouse RBCs in vivo was used to track the CD47 expression levels as well as the turnover of circulating RBCs of defined age groups.

RESULTS

Our results indicate that CD47 expression levels decrease on circulating RBCs throughout their life span in circulation. The oldest RBCs in circulation have 30 percent lower mean expression of CD47 than the youngest RBCs. Depletion of macrophages by administration of clodronate-loaded liposomes resulted in a significant decrease in the mean expression of CD47 on RBCs of all age groups and a significant accumulation of senescent RBCs in blood and spleen. A decrease in mean expression of CD47 and accumulation of senescent RBCs in macrophage-depleted mice were significantly higher for spleen RBCs compared to blood RBCs.

CONCLUSIONS

Our results provide supportive evidence for a role of decreasing CD47 expression on aging circulating RBCs in their destruction by macrophages.

Red blood cell (RBC) age at collection and storage influences RBC membrane-associated carbohydrates and lectin binding

BACKGROUND

Membrane-associated carbohydrate changes act as signals for removal of senescent and damaged red blood cells (RBCs) from the circulation and could play a role in the RBC storage lesion and RBC survival after transfusion. In this study, a panel of lectins was used to investigate the expression of carbohydrates on RBCs that had been separated before storage into young and old RBCs.

STUDY DESIGN AND METHODS

Leukodepleted RBCs were separated before storage into young and old RBCs (n = 9 paired units) by centrifugation and sampled at nominated time points during 42 days of storage. Changes to carbohydrate expression at the RBC membrane during storage were determined by flow cytometry with a panel of fluorescein-labeled lectins.

RESULTS

Old RBCs showed lower fluorescence intensity throughout storage, suggesting reduced binding of lectins compared to young RBCs. Progressively increased binding of lectins specific for galactose and N-acetylglucosamine residues was observed during storage of young and old RBCs.

CONCLUSION

Changes to lectin binding during storage of RBCs suggest that significant changes occur to the carbohydrate structures at the RBC membrane. These findings provide further insight into the mechanisms of the RBC storage lesion and potential influence on RBC survival after transfusion.

Enhanced erythro-phagocytosis in polycythemic mice overexpressing erythropoietin

Adaptive mechanisms to hematocrit levels of 0.9 in our erythropoietin-overexpressing mice (tg6) include increased plasma nitric oxide levels and erythrocyte flexibility. Doubled reticulocyte counts in tg6 suggest an increased erythrocyte turnover. Here we show that compared with wild-type (wt) animals, erythrocyte lifespan in tg6 is 70% lower in tg6 mice. Transgenic mice have a younger erythrocyte population as indicated by higher intercellular water and potassium content, higher flexibility, decreased density, increased surface to volume ratio, and decreased osmotic fragility. Interestingly, despite being younger, the tg6 erythrocyte population also harbors characteristics of accelerated aging such as an increased band 4.1a to 4.1b ratio, signs of oxidative stress, or decreased surface CD47 and sialic acids. In tg6, in vivo tracking of PKH26-labeled erythrocytes revealed dramatically increased erythrocyte incorporation by their liver macrophages. In vitro experiments showed that tg6 macrophages are more active than wt macrophages and that tg6 erythrocytes are more attractive for macrophages than wt ones. In conclusion, in tg6 mice erythrocyte aging is accelerated, which results, together with an increased number and activity of their macrophages, in enhanced erythrocyte clearance. Our data points toward a new mechanism down-regulating red cell mass in excessive erythrocytosis in mice.

Novel fluorescence assay using calcein-AM for the determination of human erythrocyte viability and aging

BACKGROUND

A highly sensitive, fast, and simple flow cytometric assay to assess human red blood cell (RBCs) viability and aging is reported.

METHODS

The assay described in this report is based on the use of acetoxymethyl ester of calcein (calcein-AM), a fluorescein derivative and nonfluorescent vital dye that passively crosses the cell membrane of viable cells and is converted by cytosolic esterases into green fluorescent calcein, which is retained by cells with intact membranes and inactive multidrug resistance protein. The loss of calcein can be easily determined by flow cytometry, and the cytosolic localization of esterases was demonstrated by spectrofluorometric analyses.

RESULTS

We found that RBCs incubated with Ca(2+), which induces a rapid and modulated self-death that shares several features with apoptosis (Bratosin et al., Cell Death Differ 2001;8:1143-1156), externalized phosphatidylserine and lost calcein staining and cytosolic adenosine triphosphate content. Double labeling using phycoerythrin-labeled annexin-V and calcein-AM showed that the decrease of esterase activity is an early event that precedes the externalization of phosphatidylserine residues. In addition, this assay allowed us to distinguish young and aged RBCs isolated by ultracentrifugation in a self-forming Percoll gradient and can be considered as a reliable marker of RBC aging.

CONCLUSIONS

Calcein-AM assay may represent a wide application for assessing RBC viability, particularly in blood banks.

Victor Ginsburg's influence on my research of the role of sialic acids in biological recognition

Sialic acids are monosaccharides with relatively strong acidity which belong to the most important molecules of higher animals and also occur in some microorganisms. They are bound to complex carbohydrates and occupy prominent positions, especially in cell membranes. Their structural diversity is high and, correspondingly, the mechanisms for their biosynthesis complex. Sialic acids are involved in a great number of cell functions. Due to their cell surface location these acidic molecules shield macromolecules and cells from enzymatic and immunological attacks and thus contribute to innate immunity. In contrast to this masking role, enabling, for example, blood cells and serum glycoproteins a longer life-time, sialic acids also represent recognition sites for various physiological receptors, such as the selectins and siglecs, as well as for toxins and microorganisms and thus allow their colonization. The recognition function of sialic acids can again be masked by O-acetylation, which modifies the interaction with receptors. Many viruses use sialic acids for the infection of cells. As sialic acids play also a decisive role in tumor biology, they prove to be rather versatile molecules that modulate biological and pathological cellular events in a sensitive way. Thus, they are most prominent representatives of mediators of molecular and cellular recognition.

Flow cytometric approach to the study of erythrophagocytosis: evidence for an alternative immunoglobulin-independent pathway in agammaglobulinemic mice

Neuraminidase treatment of red blood cells (RBCs) is believed to induce changes similar to RBC senescence, and leads to a rapid clearance of RBCs from the circulation in vivo. The objective of this study using immunodeficient SCID mice and the lipophilic fluorescent probe PKH-26 was to ascertain whether antibodies are required as the final signal allowing the phagocytosis of neuraminidase-treated murine RBCs. All of the methods we applied are based on flow cytometry analysis using fluorescent probes: fluoresceinyl isothiocyanate (FITC)-labeled lectins for membrane carbohydrate identification and PKH-26-labeled RBCs for in vitro phagocytosis and in vivo clearance studies. The results can be summarized as follows: (i) the rate of neuraminidase-induced desialylation of RBCs from normal and immunodeficient mice is identical as ascertained with FITC-labeled lectins (wheat germ agglutinin (WGA) and Ricinus communis agglutinin (RCA(120))); (ii) the rate of clearance of enzyme-treated RBCs from both types of mice is also similar, as is their localization in spleen, liver and lung; (iii) the rates of in vitro phagocytosis of untreated and neuraminidase-treated PKH-26-labeled RBCs from both species of mice are very similar in the presence of homologous sera. In the absence of serum or in the presence of heterologous sera, the rate of phagocytosis is markedly decreased but not totally abolished. These data suggest that neuraminidase-treated RBCs can be cleared via an alternative pathway that is antibody-independent. This pathway exists in immunocompetent mice but with a very low activity and is the only one active in immunodeficient mice. In accordance with results reported by Connor et al. [J. Biol. Chem. 269 (1994) 2399], it is possible that this antibody-independent mechanism is involved in the clearance of circulating senescent RBCs. Finally, the methods described here may also be of interest for the investigation of the mechanisms involved in the phagocytosis of apoptotic cells.

Characterization of a sialate pyruvate-lyase in the cytosol of human erythrocytes

Sialate pyruvate-lyases, also known as sialate aldolases (EC 4.1.3.3), reversibly catalyse the cleavage of free N-acetylneuraminic acids to form pyruvate and N-acetylmannosamine. These enzymes are widely distributed and are present in numerous pro- and eukaryotic cells, in which they are localized only in the cytosol. They play an important role in the regulation of sialic acid metabolism by controlling the intracellular concentration of sialic acids of biosynthetic or exogenous origin, thus preventing the accumulation of toxic levels of this sugar. Application of an original colorimetric micromethod for N-acetylmannosamine determination, as well as the use of [4,5,6,7,8,9-14C]N-acetylneuraminic acid, led us to evidence a cytosolic neuraminate aldolase activity in human red blood cells (RBCs) and then to define the main characteristics of this enzyme: Michaelis-Menten type, K(m:) 1.4 +/- 0.05 mM, optimal pH: 7.6 +/- 0.2, optimal temperature: 70 +/- 2 degrees C, inhibition by heavy metals: Ag(+) and Hg(++). These enzyme parameters are close to those of the bacterial and mammalian aldolases described up to now. At the moment, the presence of sialate pyruvate-lyase in the cytosol of red blood cells remains an enigma.

Nuclear localization signal of murine CMP-Neu5Ac synthetase includes residues required for both nuclear targeting and enzymatic activity

5-N-Acetylneuraminic acid (Neu5Ac) is the major sialic acid derivative found in animal cells. As a component of cell surface glycoconjugates, Neu5Ac is pivotal to numerous cellular recognition and communication processes including host-parasite interactions. A prerequisite for the synthesis of sialylated glycoconjugates is the activation of Neu5Ac to cytidine-monophosphate N-acetylneuraminic acid (CMP-Neu5Ac). The reaction is catalyzed by CMP-Neu5Ac-synthetase (syn), which, for unknown reasons, resides in the nucleus. Sequence analysis of the cloned murine CMP-Neu5Ac synthetase identified three clusters of basic amino acids (BC1-BC3) that might function as nuclear localization signals (NLS). In the present study chimeric protein and mutagenesis strategies were used to show that BC1 and BC2 are active NLS sequences when attached to the green fluorescent protein (enhanced GFP), but only BC2 is necessary and sufficient to mediate the nuclear import of CMP-Neu5Ac synthetase. Site-directed mutations identified the residues K(198)RXR to be essential for nuclear transport and Arg(202) to be necessary to complete the transport process. Cytoplasmic forms of CMP-Neu5Ac synthetase generated by single site mutations in BC2 demonstrated that (i) enzyme activity is independent of nuclear localization, and (ii) Arg(199) and Arg(202) are involved in both nuclear transport and synthetase activity. Comparison of all known and predicted CMP-sialic acid synthetases reveals Arg(202) and Gln(203) as highly conserved in evolution and critically important for optimal synthetase activity but not for nuclear localization. Combined, the data demonstrate that nuclear transport and enzyme activity are independent functions that share some common amino acid requirements in CMP-Neu5Ac synthetase.

Senescent erythrocytes: factors affecting the aging of red blood cells

Human red blood cells (RBC) have a well-defined lifespan of 120 days affected by many cellular parameters. The aim of the present study was to investigate through a functional assay the effect of some factors in the interaction of erythrocytes with monocytes: heat rigidification, equilibration at different pH and desialyzation. We also studied the interaction between stored RBC and peripheral blood monocytes with this functional erythrophagocytosis assay. Blood samples from 30 volunteer donors were investigated. 1) Senescent (Se) and Young (Y) RBC were obtained by differential centrifugation. 2) Erythrocyte suspensions: Aliquots of each sample were subjected to the following treatments: a) Rigidification by heat (RRBC), b) Equilibration at different pH (5.34, 6.30, 7.33, 9.20) and c) Desialyzation with neuraminidase and trypsin. The functional assay was performed incubating monocytes obtained by glass adherence with these suspensions of RBC. Whole blood samples (n = 20) were stored during different periods of time (0, 7, 14, 21, 28, 35 and 42 days). The erythrophagocytosis assay was performed during six weeks incubating isologous monocytes with RBC from every unit. Negative and positive controls were performed using non sensitized (NSRBC) and sensitized with IgG anti-RhD (SRBC) red cells. The percentage of active monocytes (AM) obtained were: 1) YRBC: 2.8 +/- 0.9 and SeRBC: 17.5 +/- 2.1; 2a) RRBC: 3.0 +/- 0.9; 2b) 10.9 +/- 0.9, 15.5 +/- 0.8, 3.1 +/- 1.0, 4.0 +/- 1.1; 2c) 11.1 +/- 1.4 and 3.9 +/- 1.0; SRBC 32.1 +/- 1.7 and NSRBC: 2.8 +/- 1.5. The % of AM with SeRBC was higher (p < 0.001) than those obtained with NSRBC. The data of AM with RRBC were significantly lower (p < 0.001) than those obtained with SeRBC and SBRC, indicatingthat heat rigidification of RBC does not increase phagocytosis by monocytes. The values of AM obtained from the suspensions of erythrocytes equilibrated at different pH indicate that the acidification of RBC increases the interaction with monocytes. The % AM with neuraminidase treated RBC was higher than those observed with YRBC and NSRBC (p < 0.001). No modifications were observed with trypsin treated RBC. These results suggest that the loss of sialic acid may be involved in the physiological phagocytosis. The values of AM of stored whole blood were: 2.3 +/- 1.3, 2.7 +/- 1.3, 4.4 +/- 1.6, 6.7 +/- 1.2, 9.6 +/- 1.0, 11.7 +/- 0.8 and 13.0 +/- 1.2. The results showed a significant increase in the % of AM as a function of the preservation time from 2,3 +/- 1,3 for the first day to 13,0 +/- 1,2 for the 42nd day (p < 0.001). The data obtained in this ex vivo model show a significant increase (p<0.001) in the phagocytosis of RBC equilibrated at low pH, desialinized (greater than 80%) with neuraminidase and stored for over 28 days. These factors would be involved in erythrocyte removal via phagocytosis during tissular homeostasis.

Improved storage of erythrocytes by prior leukodepletion: flow cytometric evaluation of stored erythrocytes

In vivo phagocytosis of senescent red blood cells (RBCs) by macrophages occurs 120 days after their release into the circulation. It depends on two sequential signals that trigger phagocytosis: (1) desialylation of membrane glycoconjugates with the exposure of the penultimate beta-galactosyl residues and (2) exposure of phosphatidylserine in the membrane outer leaflet. Leukodepleted and nonleukodepleted RBCs were compared using flow cytometric procedures to determine whether the in vitro deterioration of RBCs during storage might be attributable to an identical mechanism of desialylation induced by leukocyte neuraminidases, resulting in exposure of beta-galactosyl and subsequently phosphatidylserine residues - signals of senescent RBCs. Without prior leukodepletion, stored RBCs showed an increased population of senescent RBCs (using light scatter measurements), extensive desialylation with the exposure of beta-galactosyl residues (using specific fluorescein isothiocyanate [FITC]-lectins), significant exposure of phosphatidylserine in the outer leaflet of the RBC membrane (using FITC-annexin V), and extensive in vitro phagocytosis (using PKH-26-labeled RBCs). There were minimal changes observed with the leukodepleted RBCs. These results lead to the conclusion that leukocyte enzymes, including neuraminidases, are definitive contributers to the desialylation of RBCs during storage and to the exposure of phosphatidylserine residues. These deleterious effects resulting from highly active leukocyte enzymes are preventable by prior leukodepletion of the stored RBCs. Previously developed flow cytometric procedures to detect in vivo "RBC senescence" have been applied and proved to be reliable criteria to monitor the viability of stored RBCs.

Alpha-2,6-sialylation regulation in CD34+ progenitor cells in the human bone marrow and granulocyte colony-stimulating factor mobilization

The use of peripheral blood (PB) progenitor cells to reconstitute hematopoiesis after high-dose chemotherapy is now common practice in the treatment of malignancies. Growth factors such as granulocyte colony-stimulating factor (G-CSF) induce high proliferation and self-renewal within the stem cell compartment of the bone marrow. This is followed by the release of a very high number of progenitor cells into the peripheral blood, the mechanism of which remains unclear. Previously, we have showed that high cell-surface sialylation reduces matured myeloid attachment to the bone marrow stroma, which is likely to help their release into the peripheral blood. Consequently, we decided to investigate the possibility that sialylation may also contribute to the CD34(+) cell release into the peripheral blood during G-CSF treatment. We found that G-CSF-mobilized PB CD34(+) cells exhibit an increase in the sialyltransferase ST6Gal-I mRNA level. The analysis of the heterogeneity of ST6Gal-I transcripts showed a predominance of the Y form in both bone marrow (BM) and G-CSF PB CD34(+) cells. Higher levels in ST6Gal-I mRNA resulted in a higher expression of alpha-2,6-sialylation on the surface of G-CSF PB CD34(+) cells when compared to noninduced PB CD34(+) cells but not higher to BM CD34(+) cells. Our present results, together with recent observations, suggest that ST6Gal-I and alpha-2,6-sialylation expression in myeloid cells is tightly dependent on the presence of G-CSF in their environment. Up-regulation of ST6Gal-I together with alpha-2,6-sialylation of cell surface glycoproteins is likely to modulate cellular adherence and survival of progenitor cells.

Molecular modeling of sialyloligosaccharide fragments into the active site of influenza virus N9 neuraminidase

Molecular modeling studies have been carried out to investigate the interactions between substrate sialyloligosaccharide (SOS) fragments bearing different glycosidic linkages and influenza virus N9 neuraminidase, a surface glycoprotein of influenza virus subtype N9. The studies revealed that the allowed orientation for sialic acid (SA) is less than 1% in the Eulerian space at the active site. The active site of this enzyme has enough space to accommodate various SOS fragments, NeuNAcalpha(2-3)Gal, NeuNAcalpha(2-6)Gal, NeuNAcalpha(2-8)NeuNAc and NeuNAcalpha(2-9)NeuNAc, but on specific conformations. In the bound conformation, among these substrates there exists a conformational similarity leading to a structural similarity, which may be an essential requirement for the cleavage activity of the neuraminidases irrespective of the type of glycosidic linkage.

Different behavior of ghost-linked acidic and neutral sialidases during human erythrocyte ageing

Acidic and neutral sialidases (pH optimum 4.7 and 7.2, respectively) were assayed on human circulating erythrocytes during ageing. The assays were performed on intact erythrocytes and resealed erythrocyte ghost membranes. From young to senescent erythrocytes the acidic sialidase featured a 2.7-fold and 2.5-fold decrease in specific activity when measured on intact cells or resealed ghost membranes, whereas the neutral sialidase a 5-fold and 7-fold increase, respectively. The Ca2+-loading procedure was employed to mimic the vesiculation process occurring during erythrocyte ageing. Under these conditions the released vesicles displayed an elevated content of acidic sialidase, almost completely linked through a glycan phosphoinositide (GPI) anchor but no neutral sialidase activity, that was completely retained by remnant erythrocytes together with almost all the starting content of sialoglycoconjugates. The loss with vesiculation of acidic sialidase with a concomitant relative increase of neutral sialidase was more marked in young than senescent erythrocytes. The data presented suggest that during ageing erythrocytes loose acidic sialidase, and get enriched in the neutral enzyme, the vesiculation process, possibly involving GPI-anchors-rich membrane microdomains, being likely responsible for these changes. The enhanced neutral sialidase activity might account for the sialic acid loss occurring during erythrocyte ageing.

Role of linkage specific 9-O-acetylated sialoglycoconjugates in activation of the alternate complement pathway in mammalian erythrocytes

Substitution of the -OH group at C-9 of sialic acid by an O-acetyl ester has been suggested to modify various biological phenomena that are regulated by sialic acids. Amongst them, enhancement of erythrocyte lysis by 9-O-acetylated sialic acid determinants through modulation of the alternate pathway of complement has been extensively studied on murine erythrocytes [1]. A variable expression of linkage specific 9-O-acetylated sialoglycoconjugates as defined by the lectinogenic epitope of Achatinin-H namely 9-O-acetylated sialic acid alpha2-->6Gal NAc was identified on rabbit, guinea pig, hamster, rat, mouse and human erythrocytes. This differential expression of linkage specific 9-O-acetylated sialoglycoconjugates strongly correlated with the susceptibility of mammalian erythrocytes to lysis by the alternate pathway of complement. Additionally, low levels of antibodies directed against O-acetylated sialic acids in these mammalian species suggested that these constitutively present determinants have low immunogenicity. Taken together, our results indicate that complement mediated hemolysis depends not simply upon the extent of surface 9-O-acetylated sialic acids present but more importantly upon the specific linkage.

Selection of anionic dopant for quantifying desialylation reactions with MALDI-FTMS

Alkylsulfonates are examined as anion dopants for the matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) of neutral oligosaccharides. The anion dopants allow neutral oligosaccharides to be examined in the same mixture as acidic oligosaccharides. The alkylsulfonate dopants interact strongly with the oligosaccharide to produce a quasimolecular ion composed of the oligosaccharides and the deprotonated alkylsulfonates. Compounds as small as disaccharides to large branched oligosaccharides can all be examined. The new series of alkylsulfonates works as well as HSO4- in terms of signal-to-noise ratio, sensitivity, and ease of preparation in the negative ion mode. The alkylsulfonates are used to construct a calibration curve for obtaining the relative amounts of varying neutral and acidic oligosaccharide mixtures. With the anion dopants, the MALDI-FTMS signals are shown to have a linear relationship with the molar ratios of the neutral and sialylated oligosaccharides. The method is also used to monitor the product of a desialylation reaction by acid hydrolysis.

Sialic acids in fungi: a minireview

The increasing number of reports on the presence of sialic acids in fungi (N-acetyl-, N-glycolyl- and 5,9-N,O-diacetylneuraminic acids) based on direct and indirect evidence warrants the present review. Formerly suggested as sialidase-sensitive sources of anionic groups at the cell surface of fungal species grown in chemically defined media (e.g., Fonsecaea pedrosoi), sialic acids have also been found in Sporothrix schenckii, Paracoccidioides brasiliensis, Cryptococcus neoformans and recently, in Candida albicans. Methods used involved adequate hydrolysis and extraction procedures, HPTLC, gas-chromatography, colorimetry, mass spectroscopy, lectin and influenza virus binding. Apart from protecting fungal cells against phagocytosis (S. schenckii, C. neoformans) and playing a cellular structural role (F. pedrosoi), other biological functions of sialic acids are still being investigated.

The 1.8 A structures of leech intramolecular trans-sialidase complexes: evidence of its enzymatic mechanism

Intramolecular trans-sialidase from leech (Macrobdella decora) is the first member of the sialidase superfamily found to exhibit strict specificity towards the cleavage of terminal Neu5Acalpha2-->3Gal linkage in sialoglycoconjugates. Its release of 2,7-anhydro-Neu5Ac instead of Neu5Ac indicates that it catalyzes an intramolecular trans-sialosyl reaction. Crystal structures of its complexes with an inactive substrate analogue 2-propenyl-Neu5Ac, and with the product 2,7-anhydro-Neu5Ac, have been determined to 1.8 A resolution. The boat conformation of the pyranose observed in the complexes supports the proposed enzymatic mechanism that O7 of an axial 6-glycerol group attacks the positively charged C2 of the intermediate. A generalized mechanism is proposed for the sialidase superfamily.