Decrease in Na(+)-K(+)-ATPase associated with maturation of sheep reticulocytes

Exosomes in erythropoiesis

Multivesicular endosomes contain membrane vesicles which can be released into the extracellular environment as exosomes. This review describes the role of exosome secretion in the remodeling of the red cell plasma membrane during the last stage of erythropoietic differentiation. Herein, we propose that the exosome biogenesis involves several mechanisms of protein sorting and leads to partial or complete loss of membrane activities, in some cases in a regulated way.

Red blood cell Na pump: Insights from species differences

The red blood cell membrane is specialized to exchange chloride and bicarbonate; usually the pH gradient, the chloride ratio, and the membrane potential are tightly coupled. We review the evidence that led to the ability to separately vary inside and outside pH in red cells. The effect of pH on Na pump activity and on the selectivity of the inside and the outside transport sites is reviewed. In red blood cells, at high pH, the outside site is not selective. An increase in protons leads to an increase in K(+) affinity, thus making the site more selective. The pK for this site is different in rats and humans; because of the high conservation of residues in these two species, there are only a few possible residues that can account for this difference. On the inside, work from unsided preparations suggests that, at high pH, the transport site is highly selective for Na(+). Once again, an increase in protons leads to an increase in K(+) affinity, but now the result is a less selective site. During their maturation, reticulocytes lose many membrane proteins. The type and fractional loss is species dependent. For example, most reticulocytes lose most of their Na pumps, retaining about 100 pumps per cell, but animals from the order Carnivora lose all their pumps. We review some of the evidence that PKC phosphorylation of N-terminus serines is responsible for endocytosis in other cell types and species variation in this region.

Changes in Na,K-adenosine triphosphatase (ATPase) concentration and Na,K-ATPase-dependent adenosine triphosphate turnover in human erythrocytes in diabetes

The concentration of Na,K-adenosine triphosphatase (ATPase) and Na,K-ATPase-dependent adenosine triphosphate (ATP) turnover was measured in fasting blood samples of 20 subjects with insulin-dependent diabetes mellitus (IDDM), 22 subjects with non-insulin-dependent diabetes mellitus (NIDDM), and 20 nondiabetic subjects. [3H]ouabain binding was used to determine Na,K-ATPase concentration. There were 471 +/- 70 (mean +/- SD) ouabain binding sites per erythrocyte, normally distributed in the nondiabetic subjects. The number of ouabain sites per cell was lognormally distributed in the two populations of diabetic subjects. The mean of lognormal distributions of ouabain sites per cell was significantly lower in the IDDM group. The mean of the lognormal distribution for the NIDDM group was not significantly different from that of the nondiabetic subjects. Na,K-ATPase-dependent ATP turnover (molar activity) was 9,580 +/- 742 mol/mol minute (mean +/- SD) normally distributed in the nondiabetic population. A lognormal distribution was observed in the diabetic population. Means of the lognormal distributions were significantly different: 3.98 +/- 0.05 for the nondiabetic population and 3.13 +/- 0.48 for both diabetic populations. Changes in the concentration of Na,K-ATPase (ouabain sites per cell) and Na,K-ATPase-dependent ATP turnover did not correlate with hemoglobin A1C (HbA1C) or with blood glucose. This would suggest that elevated glucose concentrations do not directly cause decreased Na,K-ATPase function in the diabetic erythrocyte.

Effects of deoxygenation on active and passive Ca2+ transport and on the cytoplasmic Ca2+ levels of sickle cell anemia red cells

Elevated [Ca2+]i in deoxygenated sickle cell anemia (SS) red cells (RBCs) could trigger a major dehydration pathway via the Ca(2+)-sensitive K+ channel. But apart from an increase in calcium permeability, the effects of deoxygenation on the Ca2+ metabolism of sickle cells have not been previously documented. With the application of 45Ca(2+)-tracer flux methods and the combined use of the ionophore A23187, Co2+ ions, and intracellular incorporation of the Ca2+ chelator benz-2, in density-fractionated SS RBCs, we show here for the first time that upon deoxygenation, the mean [Ca2+]i level of SS discocytes was significantly increased, two- to threefold, from a normal range of 9.4 to 11.4 nM in the oxygenated cells, to a range of 21.8 to 31.7 nM in the deoxygenated cells, closer to K+ channel activatory levels. Unlike normal RBCs, deoxygenated SS RBCs showed a two- to fourfold increase in pump-leak Ca2+ turnover. Deoxygenation of the SS RBCs reduced their Ca2+ pump Vmax, more so in reticulocyte- and discocyte-rich than in dense cell fractions, and decreased their cytoplasmic Ca2+ buffering. Analysis of these results suggests that both increased Ca2+ influx and reduced Ca2+ pump extrusion contribute to the [Ca2+]i elevation.

Expression and loss of the transferrin receptor in growing and differentiating HD3 cells

During induced differentiation and maturation of HD3 cells (a chicken erythroblast cell line infected with a temperature-sensitive mutant of the avian erythroblastosis virus), the levels of transferrin receptor (TFR) and nucleoside transporter increase. Both these activities increase before elevated levels of hemoglobin are detected. Shortly after induction, as cellular TFR levels rise, a native-size TFR is detected in the cell-free culture medium, associated with an exosome fraction (100,000 xg pellet). Nucleoside transporter (measured as NBMPR-binding activity) is not increased in this pellet with induction. Previous studies have suggested that exosome formation in peripheral reticulocytes may be a significant route for loss of specific membrane proteins (Johnstone et al., 1991). Although the present experiments in HD3 cells do not address the quantitative importance of exosome formation, the studies suggest that exosome formation is an early event in commitment to the red cell lineage and is not a phenomenon restricted to the terminal stages of red cell maturation.

Effect of cell age and phenylhydrazine on the cation transport properties of rabbit erythrocytes

We studied the effect of cell age on the cation transport systems of rabbit erythrocytes by increasing the proportion of circulating young erythrocytes with either repeated bleeding or with phenylhydrazine (PHZ) treatment. We found that when the reticulocyte content of rabbit blood is increased by bleeding (from 1 to 40-50% of the circulating red cells), the response of the various transport pathways differs. The largest increase (fivefold) was found in the activity of K-Cl cotransport, which peaked 3 days after the last bleeding. The Na-K pump activity peaked at a similar time, but the % increase was twofold less than the K-Cl cotransport. There was a very small increase in the activity of the Na-Li exchange, whereas the Na-H exchange reached peak values 10 days after the last bleeding (twofold increase), when activities of K-Cl cotransport and Na-K pump had returned to almost normal levels. In vivo PHZ treatment resulted in anemia and marked reticulocytosis (80-90% of circulating cells). Transport rates were markedly increased (Na-K pump 9.6-fold, Na-H exchange 6.8-fold, Na-Li exchange 2.75-fold; K-Cl cotransport: 10-20-fold). When blood from PHZ-treated rabbits was incubated in vitro for 24-48 hours, red cell volume and K content decreased. This process was associated with a 70% reduction in the activity of the K-Cl cotransport after 24 hours and a 90% reduction after 48 hours. The activity of the other systems also declined and approached baseline values after 48 hours. Loss of transport activity was not affected by 10 microM E-64, whereas 10 mM methylamine reduced the inactivation of the Na-H exchange and of the Na-Li exchange. PHZ treatment of rabbit red cells in vitro resulted in marked increase of the K-Cl cotransport and inhibition of Na-K pump, Na-H exchange, and Na-Li exchange. These effects were abolished by DTT, with the exception of the Na-K pump inhibition, which was DTT insensitive. Thus both cell age and oxidative damage are important determinants of cation transport in rabbit red cells.

The amino-terminal segment of the catalytic subunit of kidney Na,K-ATPase regulates the potassium deocclusion pathway of the reaction cycle

Tryptic cleavage of the catalytic subunit of kidney Na,K-ATPase in the E1 conformation effects a change in kinetic behavior apparent at low ATP concentration. Thus, at < or = 10 microM ATP, K+ inhibits Na(+)-dependent ATPase activity of the undigested enzyme but activates activity of the digested enzyme. With time of trypsinolysis, a transient increase followed by a decrease in activity is observed at low [ATP], whereas at high [ATP] (1 mM), activity is progressively reduced. At low [ATP], the trypsin-treated/control activity ratio was > or = 3-fold higher with K+ compared to the ratio observed with the K+ congener Li+. Also, the relative Na/K exchange activity (22Na+ influx into K(+)-loaded inside-out vesicles from erythrocytes) with either 0.01 mM ATP or 1 mM CTP compared to 1 mM ATP was greater for the trypsin-treated than for the control enzyme. The kinetic change is correlated with the initial rapid cleavage of the N-terminal tryptic fragment (< or = 30 residues) from the catalytic subunit. It is concluded that this segment regulates the K+ deocclusion pathway of the reaction; removal of this fragment produces a modified active species having an increased rate of K+ deocclusion.

Calmodulin increases Ca-dependent inhibition of the Na,K-ATPase in human red blood cells

Proteins in human red cell hemolysate were purified to determine which of them increase inhibition of the Na,K-ATPase in the presence of 2 microM free Ca. Samples purified 600,000-fold inhibited the Na,K-ATPase of human red cells in a Ca-dependent manner and stimulated the (Ca+Mg)-ATPase. These samples contained two proteins as analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE): calmodulin (18,000 Mr), which comprised most (greater than 90%) of the total protein, and an unidentified protein of approximately 13,000 Mr. Both proteins were a distinctive light yellow when stained with silver. Calmodulin from bovine testes also inhibited the Na,K-ATPase and stimulated the (Ca+Mg)-ATPase. This preparation also contained two proteins as analyzed by SDS-PAGE: calmodulin (95 to 99% of the total protein) and another protein of approximately 13,000 Mr (1 to 5% of the total protein). Both were light yellow when stained with silver. Since the amount of red cell protein was limited, the remainder of the study was carried out with the bovine testes preparation. Heating the testes preparation decreased, but did not abolish, inhibition of the Na,K-ATPase and reduced stimulation of the (Ca+Mg)-ATPase. When corrected for denatured calmodulin, both heated and unheated proteins increased inhibition of the Na,K-ATPase to the same extent. The Na,K-ATPase was inhibited at 2 microM free Ca in a dose-dependent manner over a range of 15 to 100 nM calmodulin. To establish if the inhibition was due to the calmodulin or the 13,000 Mr protein, both were electroeluted after SDS-PAGE. Electroeluted calmodulin stimulated the (Ca+Mg)-ATPase and increased Ca inhibition of the Na,K-ATPase. Electroeluted amounts of the smaller Mr protein slightly stimulated the (Ca+Mg)-ATPase, but had no effect on the Na,K-ATPase. This protein was digested with cyanogen bromide, partially sequenced, and thereby identified as a fragment of calmodulin. We conclude that intact calmodulin increases inhibition of the Na,K-ATPase at 2 microM free Ca. We suggest that calmodulin is part of a mechanism mediating the effects of physiological free Ca on the Na,K-ATPase.

Exosome formation during maturation of mammalian and avian reticulocytes: evidence that exosome release is a major route for externalization of obsolete membrane proteins

We have assessed whether exosome formation is a significant route for loss of plasma membrane functions during sheep reticulocyte maturation in vitro. Although the recovery of transferrin binding activity in exosomes is at best approximately 25-30% of the lost activity, recoveries of over 50% of the lost receptor can be obtained if 125I-labelled transferrin receptor is measured using an that receptor instability may contribute to the less than quantitative recovery of the transferrin receptor. Significantly higher (75-80%) levels of the nucleoside transporter can be recovered in exosomes during red cell maturation using 3H-nitrobenzylthioinosine binding to measure the nucleoside transporter. These data suggest that exosome formation is a major route for removal of plasma membrane proteins during reticulocyte maturation and plasma membrane remodelling. We have also shown that both in vivo and in vitro, embryonic chicken reticulocytes form exosomes which contain the transferrin receptor. Thus, exosome formation is not restricted to mammalian red cells, but also occurs in red cells, which retain organelles, such as nuclei and mitochondria, into the mature red cell stage.