Characterization of Erythrocytes in the Sickle Cell Trait
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Characterization of Erythrocytes in the Sickle Cell Trait Jamie L. Maciaszek and George Lykotrafitis Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, U.S.A. ABSTRACT Atomic force microscopy (AFM) allows for high-resolution topography studies of biological cells, measurement of their mechanical properties, and quantification of proteinprotein interactions in physiological conditions. In this work, AFM was employed to investigate morphological, material, and chemomechanical properties of red blood cells from human subjects with sickle cell trait. We measured the stiffness of the cells and demonstrated that the Young’s modulus of pathological erythrocytes was three times greater than in normal cells. A single molecule AFM method was employed to report that erythrocytes from human subjects with the sickle cell trait express a greater number of the laminin receptors BCAM/Lu (p < 0.05) than erythrocytes from normal human subjects. Observed differences indicate the effect of sickle hemoglobin in the erythrocyte and possible changes in the organization of the cell cytoskeleton and membrane proteins associated with the sickle cell trait. INTRODUCTION Sickle cell trait (SCT) is characterized by the presence of both normal adult hemoglobin (HbA) and abnormal sickle hemoglobin (HbS) in the red blood cell (RBC, erythrocyte). SCT affects over three million people in the United States, about 40 to 50 times more than sickle cell disease (SCD). In HbS, the normal sequence Val-His-Leu-Thr-Pro-Glu-Glu-Lys is changed to Val-His-Leu-Thr-Pro-Val-Glu-Lys, with the amino acid valine substituted for glutamic acid in the β6 site. The replacement of two charged groups by two hydrophobic ones leads to polymerization of deoxygenated Hb and to the formation of stiff HbS fibers comprising a fibrous gel [1]. Between 40-42% of the hemoglobin content of most individuals with SCT is HbS [2]. In comparison to the homozygous SCD, the heterozygous SCT is traditionally regarded as a benign condition. However, in extreme conditions including hypoxia, acidosis, and dehydration, individuals with SCT can develop a syndrome resembling SCD with vaso-occlusive sequelae [3]. It has been hypothesized that increased association of HbS with the membrane proteins contributes to the change in the mechanical properties of erythrocytes. HbS could alter the mechanical properties of RBCs not only by extending the spectrin filaments or by the interaction between HbS filaments and the lipid bilayer, but also by altering the functionality of the membrane proteins. There is considerable evidence that HbS is associated with the inner membrane of RBCs and in particular with the cytoplasmic tail of the band 3 protein [4]. Furthermore, interactions of receptor molecule cytoplasmic domains with the cytoskeleton can play critical roles in regulating receptor function. It has been determined that Lutheran group (BCAM/Lu) has a high degree of connectivity to the erythrocyte membrane cytoskeleton via spectrin. Importantly, disruption of the BCAM/Lu-s
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