Ca2+ transport activities of inside-out vesicles prepared from density-separated erythrocytes from rat and human

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.

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.

Erythrocyte ion channels in regulation of apoptosis

Erythrocytes lack mitochondria and nuclei, key organelles in the regulation of apoptosis. Until recently, erythrocytes were thus not considered subject to this type of cell death. However, exposure of erythrocytes to the Ca2+ ionophore ionomycin was shown to induce cell shrinkage, cell membrane blebbing and breakdown of phosphatidylserine asymmetry with subsequent phosphatidylserine exposure at the cell surface, all typical features of apoptosis. Further studies revealed the participation of ion channels in the regulation of erythrocyte "apoptosis." Osmotic shock, oxidative stress and energy depletion all activate a Ca2(+)-permeable non-selective cation channel in the erythrocyte cell membrane. The subsequent increase of Ca2+ concentration stimulates a scramblase leading to breakdown of cell membrane phosphatidylserine asymmetry and activates Ca2+ sensitive K+ (Gardos) channels leading to KCl loss and (further) cell shrinkage. Phosphatidylserine exposure and cell shrinkage are blunted in the nominal absence of extracellular Ca2+, in the presence of the cation channel inhibitors amiloride or ethylisopropylamiloride, at increased extracellular K+ or in the presence of the Gardos channel inhibitors clotrimazole or charybdotoxin. Thus, increase of cytosolic Ca2+ and cellular loss of K+ participate in the triggering of erythrocyte scramblase. Nevertheless, phosphatidylserine exposure is not completely abrogated in the nominal absence of Ca2+, pointing to additional Ca2(+)-independent pathways. One of those is activation of sphingomyelinase with subsequent formation of ceramide which in turn leads to stimulation of erythrocyte scramblase. The exposure of phosphatidylserine at the extracellular face of the cell membrane stimulates phagocytes to engulf the apoptotic erythrocytes. Thus, sustained activation of the cation channels eventually leads to clearance of affected erythrocytes from peripheral blood. Erythropoietin inhibits the non-selective cation channel and thus interferes with erythrocyte "apoptosis." Susceptibility to scramblase activation is enhanced in thalassemia, sickle cell disease and glucose-6-phosphate dehydrogenase deficiency. Infection with Plasmodium falciparum leads to activation of the cation channel eventually triggering erythrocyte "apoptosis."

Cation channels, cell volume and the death of an erythrocyte

Similar to a variety of nucleated cells, human erythrocytes activate a non-selective cation channel upon osmotic cell shrinkage. Further stimuli of channel activation include oxidative stress, energy depletion and extracellular removal of Cl-. The channel is permeable to Ca2+ and opening of the channel increases cytosolic [Ca2+]. Intriguing evidence points to a role of this channel in the elimination of erythrocytes by apoptosis. Ca2+ entering through the cation channel stimulates a scramblase, leading to breakdown of cell membrane phosphatidylserine asymmetry, and stimulates Ca(2+)-sensitive K+ channels, thus leading to KCl loss and (further) cell shrinkage. The breakdown of phosphatidylserine asymmetry is evidenced by annexin binding, a typical feature of apoptotic cells. The effects of osmotic shock, oxidative stress and energy depletion on annexin binding are mimicked by the Ca2+ ionophore ionomycin (1 microM) and blunted in the nominal absence of extracellular Ca2+. Nevertheless, the residual annexin binding points to additional mechanisms involved in the triggering of the scramblase. The exposure of phosphatidylserine at the extracellular face of the cell membrane stimulates phagocytes to engulf the apoptotic erythrocytes. Thus, sustained activation of the cation channels eventually leads to clearance of affected erythrocytes from peripheral blood. Susceptibility to annexin binding is enhanced in several genetic disorders affecting erythrocyte function, such as thalassaemia, sickle-cell disease and glucose-6-phosphate dehydrogenase deficiency. The enhanced vulnerability presumably contributes to the shortened life span of the affected erythrocytes. Beyond their role in the limitation of erythrocyte survival, cation channels may contribute to the triggering of apoptosis in nucleated cells exposed to osmotic shock and/or oxidative stress.

Calculation of a Gap restoration in the membrane skeleton of the red blood cell: possible role for myosin II in local repair

Human red blood cells contain all of the elements involved in the formation of nonmuscle actomyosin II complexes (V. M. Fowler. 1986. J. Cell. Biochem. 31:1-9; 1996. Curr. Opin. Cell Biol. 8:86-96). No clear function has yet been attributed to these complexes. Using a mathematical model for the structure of the red blood cell spectrin skeleton (M. J. Saxton. 1992. J. Theor. Biol. 155:517-536), we have explored a possible role for myosin II bipolar minifilaments in the restoration of the membrane skeleton, which may be locally damaged by major mechanical or chemical stress. We propose that the establishment of stable links between distant antiparallel actin protofilaments after a local myosin II activation may initiate the repair of the disrupted area. We show that it is possible to define conditions in which the calculated number of myosin II minifilaments bound to actin protofilaments is consistent with the estimated number of myosin II minifilaments present in the red blood cells. A clear restoration effect can be observed when more than 50% of the spectrin polymers of a defined area are disrupted. It corresponds to a significant increase in the spectrin density in the protein free region of the membrane. This may be involved in a more complex repair process of the red blood cell membrane, which includes the vesiculation of the bilayer and the compaction of the disassembled spectrin network.

Differences in Ca2+ pumping activity between sub-populations of human red cells

It has been shown recently that the free Ca2+ content of human red cells rises during ageing in vivo. With the aim of determining the mechanisms involved in such a change, we have investigated some aspects of Ca2+ homeostasis. Both the initial rate of Ca2+ influx and some kinetic parameters of the Ca2+ pump of human red cells were studied in light and dense sub-populations obtained through stringent, self-formed Percoll gradients. At 37 degrees C and pH 7.4, no differences in Ca2+ entry were found. By contrast, either at pH 7.0 or 7.4, the maximal Ca2+ extrusion rate of the approximately 10% heaviest cells was one-half of the corresponding lighter ones. The results demonstrate that the elevated free Ca2+ concentration distinctive of senescent cells, arises from a reduction in Ca2+ extrusion capacity during ageing. The possible physiological significance of this finding is discussed.

The effects of pentoxifylline on the plasma membrane Ca2+ ATPase in age-separated rat and human erythrocytes

Pentoxifylline, a dimethyl xanthine derivative given to patients with peripheral vascular disorders, increases erythrocyte deformability, diminishes Ca2+ entry, inhibits Ca(2+)-dependent transglutaminase activity and elevates ATP levels. The present study examined the effects of pentoxifylline on the Ca2+ pump ATPase, an enzyme which regulates intracellular Ca2+ levels. Studies were carried out with inside-out vesicles (IOVs) prepared from young (Ey) and old (Eo) human and rat erythrocytes. The pumping of Ca2+ depends on the concomitant hydrolysis of ATP; the two processes were measured using radiolabeled substrates. The catalytic properties of IOVs from young and old erythrocytes were stimulated by pentoxifylline when added directly to the assay medium. Pentoxifylline (0.5 to 5.0 mM) significantly activated the rates of Ca2+ dependent ATP hydrolysis in Ey and Eo IOVs of rat erythrocytes. The percent of activation was greater in the IOVs from older erythrocytes. The Ca2+ translocation was also affected by pentoxifylline. The early burst of Ca2+ uptake into IOVs decreased in the presence of pentoxifylline in Ey IOVs prepared from either species whereas steady state rates of Ca2+ transport only declined at 5.0 mM pentoxifylline. This pattern was not evident in the corresponding Eo IOVs. The response of Ey and Eo IOVs to pentoxifylline may be the basis of the difference in sensitivity of young and old erythrocytes to the drug in regulating intracellular Ca2+ concentrations.