Protein kinase C of human erythrocytes phosphorylates bands 4.1 and 4.9

Signaling mechanisms in red blood cells: A view through the protein phosphorylation and deformability

Intracellular signaling mechanisms in red blood cells (RBCs) involve various protein kinases and phosphatases and enable rapid adaptive responses to hypoxia, metabolic requirements, oxidative stress, or shear stress by regulating the physiological properties of the cell. Protein phosphorylation is a ubiquitous mechanism for intracellular signal transduction, volume regulation, and cytoskeletal organization in RBCs. Spectrin-based cytoskeleton connects integral membrane proteins, band 3 and glycophorin C to junctional proteins, ankyrin and Protein 4.1. Phosphorylation leads to a conformational change in the protein structure, weakening the interactions between proteins in the cytoskeletal network that confers a more flexible nature for the RBC membrane. The structural organization of the membrane and the cytoskeleton determines RBC deformability that allows cells to change their ability to deform under shear stress to pass through narrow capillaries. The shear stress sensing mechanisms and oxygenation-deoxygenation transitions regulate cell volume and mechanical properties of the membrane through the activation of ion transporters and specific phosphorylation events mediated by signal transduction. In this review, we summarize the roles of Protein kinase C, cAMP-Protein kinase A, cGMP-nitric oxide, RhoGTPase, and MAP/ERK pathways in the modulation of RBC deformability in both healthy and disease states. We emphasize that targeting signaling elements may be a therapeutic strategy for the treatment of hemoglobinopathies or channelopathies. We expect the present review will provide additional insights into RBC responses to shear stress and hypoxia via signaling mechanisms and shed light on the current and novel treatment options for pathophysiological conditions.

Headpiece domain of dematin is required for the stability of the erythrocyte membrane

Dematin is an actin-binding and bundling protein of the erythrocyte membrane skeleton. Dematin is localized to the spectrin-actin junctions, and its actin-bundling activity is regulated by phosphorylation of cAMP-dependent protein kinase. The carboxyl terminus of dematin is homologous to the "headpiece" domain of villin, an actin-bundling protein of the microvillus cytoskeleton. The headpiece domain contains an actin-binding site, a cAMP-kinase phosphorylation site, plays an essential role in dematin self-assembly, and bundles F-actin in vitro. By using homologous recombination in mouse embryonic stem cells, the headpiece domain of dematin was deleted to evaluate its function in vivo. Dematin headpiece null mice were viable and born at the expected Mendelian ratio. Hematological evaluation revealed evidence of compensated anemia and spherocytosis in the dematin headpiece null mice. The headpiece null erythrocytes were osmotically fragile, and ektacytometry/micropore filtration measurements demonstrated reduced deformability and filterability. In vitro membrane stability measurements indicated significantly greater membrane fragmentation of the dematin headpiece null erythrocytes. Finally, biochemical characterization, including the vesicle/cytoskeleton dissociation, spectrin self-association, and chemical crosslinking measurements, revealed a weakened membrane skeleton evidenced by reduced association of spectrin and actin to the plasma membrane. Together, these results provide evidence for the physiological significance of dematin and demonstrate a role for the headpiece domain in the maintenance of structural integrity and mechanical properties of erythrocytes in vivo.

Regulation of the glycophorin C-protein 4.1 membrane-to-skeleton bridge and evaluation of its contribution to erythrocyte membrane stability

The band 3-ankyrin-spectrin bridge and the glycophorin C-protein 4.1-spectrin/actin bridge constitute the two major tethers between the erythrocyte membrane and its spectrin skeleton. Although a structural requirement for the band 3-ankyrin bridge is well established, the contribution of the glycophorin C-protein 4.1 bridge to red cell function remains to be defined. In order to explore this latter bridge further, we have identified and/or characterized five stimuli that sever the linkage in intact erythrocytes and have examined the impact of this rupture on membrane mechanical properties. We report here that elevation of cytosolic 2,3-bisphosphoglycerate, an increase in intracellular Ca(2+), removal of cell O(2), a decrease in intracellular pH, and activation of erythrocyte protein kinase C all promote dissociation of protein 4.1 from glycophorin C, leading to reduced retention of glycophorin C in detergent-extracted spectrin/actin skeletons. Significantly, where mechanical studies could be performed, we also observe that rupture of the membrane-to-skeleton bridge has little or no impact on the mechanical properties of the cell, as assayed by ektacytometry and nickel mesh filtration. We, therefore, suggest that, although regulation of the glycophorin C-protein 4.1-spectrin/actin bridge likely occurs physiologically, the role of the tether and the associated regulatory changes remain to be established.

Kell and Kx, two disulfide-linked proteins of the human erythrocyte membrane are phosphorylated in vivo

Kell and Kx are two quantitatively minor proteins from the human erythrocyte membrane which carry blood groups antigens and are thought to be a metalloprotease and a membrane transporter, respectively. In the red cell membrane, these proteins form a complex stabilized by disulfide bond(s). Phosphorylation status of these proteins was studied, in the presence or absence of effectors of several kinases, either on intact cells incubated with [32P]-orthophosphate or on ghosts incubated with [gamma-32P]ATP. Purification of Kell-Kx complex, by immunochromatography on an immobilized human monoclonal antibody of Kell blood group specificity allowed to establish that (i) neither protein is phosphorylated on tyrosine; (ii) the Kell protein is a putative substrate for Casein Kinase II (CKII) and Casein Kinase I (CKI) but not for protein kinase C (PKC), whereas Kx protein is phosphorylated by CKII and PKC but not by CKI; (iii) Protein Kinase A neither phosphorylates the Kell nor the Kx proteins.

Modulation of Protein Kinase C Activity in Plasmodium falciparum – Infected Erythrocytes

Infection of human erythrocytes with the malaria parasite Plasmodium falciparum induces many morphological and biochemical changes in the host cell. Host serine/threonine protein kinases could be involved in some of these processes. The aim of this study was to determine the effect of infection on red blood cell protein kinase C (PKC) and establish the importance of this enzyme in parasite growth and sexual stage differentiation. Phorbol myristate acetate (PMA)-induced translocation of erythrocyte PKC activity is impaired in erythrocytes enriched for mature asexual stage infected cells. Western blotting shows that this is due to a relative reduction in membrane PKC protein levels rather than inhibition of enzyme activity and analysis of PKC activity isolated from whole cell lysates by DE52 chromatography suggests that total activatable PKC levels are lower in infected erythrocytes. A reduction in PMA-induced activation is also observed in PKC assays performed in situ. Downregulation of erythrocyte PKC by overnight incubation with PMA before infection causes a significant decrease in the rate of the asexual growth, suggesting that the enzyme, although lost later in infection, may be important in the earlier development of the parasite. By contrast, the lack of PKC had no effect on the production of sexual stage parasites.

Modulation of Protein Kinase C Activity in Plasmodium falciparum – Infected Erythrocytes

Infection of human erythrocytes with the malaria parasite Plasmodium falciparum induces many morphological and biochemical changes in the host cell. Host serine/threonine protein kinases could be involved in some of these processes. The aim of this study was to determine the effect of infection on red blood cell protein kinase C (PKC) and establish the importance of this enzyme in parasite growth and sexual stage differentiation. Phorbol myristate acetate (PMA)-induced translocation of erythrocyte PKC activity is impaired in erythrocytes enriched for mature asexual stage infected cells. Western blotting shows that this is due to a relative reduction in membrane PKC protein levels rather than inhibition of enzyme activity and analysis of PKC activity isolated from whole cell lysates by DE52 chromatography suggests that total activatable PKC levels are lower in infected erythrocytes. A reduction in PMA-induced activation is also observed in PKC assays performed in situ. Downregulation of erythrocyte PKC by overnight incubation with PMA before infection causes a significant decrease in the rate of the asexual growth, suggesting that the enzyme, although lost later in infection, may be important in the earlier development of the parasite. By contrast, the lack of PKC had no effect on the production of sexual stage parasites.

Phorbol 12-myristate 13-acetate-stimulated phosphorylation of erythrocyte membrane skeletal proteins is blocked by calpain inhibitors: possible role of protein kinase M

Human erythrocytes contain cytosolic protein kinase C (PKC) which, when activated by phorbol 12-myristate 13-acetate (PMA), induces the phosphorylation of the membrane skeletal proteins band 4.1, band 4.9 and adducin. We found that brief treatments of erythrocytes with PMA resulted in a decrease in cytosolic PKC content and in the transient appearance in the cytosol of a Ca(2+)- and phospholipid-independent 55 kDa fragment of PKC, called PKM. Prolonged treatment with PMA resulted in the complete and irreversible loss of erythrocyte PKC. To investigate the possible role of calpain in this process, the calpain inhibitors leupeptin and E-64 were sealed inside erythrocytes by reversible haemolysis. Both inhibitors prolonged the lifetime of PKC in PMA-treated cells, and leupeptin was shown to block the PMA-stimulated appearance of PKM in the cytosol. Significantly, leupeptin also completely blocked PMA-stimulated phosphorylation of membrane and cytosolic substrates. This effect was mimicked by other calpain inhibitors (MDL-28170 and calpain inhibitor I), but did not occur when other protease inhibitors such as phenylmethanesulphonyl fluoride, pepstatin A or chymostatin were used. In addition, the phosphorylation of exogenous histone sealed inside erythrocytes was also blocked by leupeptin. Immunoblotting showed that leupeptin did not prevent the PMA-induced translocation of PKC to the erythrocyte membrane. Thus inhibition of PKC phosphorylation of membrane skeletal proteins by calpain inhibitors was not due to inhibition of PKC translocation to the membrane. Our results suggest that PMA treatment of erythrocytes results in the translocation of PKC to the plasma membrane, followed by calpain-mediated cleavage of PKC to PKM. This cleavage, or some other leupeptin-inhibitable process, is a necessary step for the phosphorylation of membrane skeletal substrates, suggesting that the short-lived PKM may be responsible for membrane skeletal phosphorylation. Our results suggest a potential mechanism whereby erythrocyte PKC may be subject to continual down-regulation during the lifespan of the erythrocyte due to repeated activation events, possibly related to transient Ca2+ influx. Such down-regulation may play an important role in erythrocyte survival or pathophysiology.

Cloning of human erythroid dematin reveals another member of the villin family

Dematin is an actin-bundling protein originally identified in the human erythroid membrane skeleton. Its actin-bundling activity is abolished upon phosphorylation by the cAMP-dependent protein kinase and is restored after dephosphorylation. Here we report the complete primary structure of human erythroid dematin, whose sequence includes a homologue of the "headpiece" sequence found at the C terminus of villin. This headpiece is essential for villin function in inducing microvillar development and actin redistribution. The widespread expression of dematin transcripts in human tissues suggests that dematin and its homologues may substitute for villin in villin-negative tissues to regulate actin reorganization by a phosphorylation-regulated mechanism.

Erythrocyte protein 4.1 binds and regulates myosin

Myosin was recently identified in erythrocytes and was shown to partition both with membrane and cytosolic fractions, suggesting that it may be loosely bound to membranes [Fowler, V. M., Davis, J. Q. & Bennett, V. (1985) J. Cell Biol. 100, 47-55, and Wong, A. J., Kiehart, D. P. & Pollard, T. D. (1985) J. Biol. Chem. 260, 46-49]; however, the molecular basis for this binding was unclear. The present studies employed immobilized monomeric myosin to examine the interaction of myosin with erythrocyte protein 4.1. In human erythrocytes, protein 4.1 binds to integral membrane proteins and mediates spectrin-actin assembly. Protein 4.1 binds to rabbit skeletal muscle myosin with a Kd = 140 nM and a stoichiometry consistent with 1:1 binding. Heavy meromyosin competes for protein 4.1 binding with Ki = 36-54 nM; however, the S1 fragment (the myosin head) competes less efficiently. Affinity chromatography of partial chymotryptic digests of protein 4.1 on immobilized myosin identified a 10-kDa domain of protein 4.1 as the myosin-binding site. In functional studies, protein 4.1 partially inhibited the actin-activated Mg2+-ATPase activity of rabbit skeletal muscle myosin with Ki = 51 nM. Liver cytosolic and erythrocyte myosins preactivated with myosin light-chain kinase were similarly inhibited by protein 4.1. These studies show that protein 4.1 binds, modulates, and thus may regulate myosin. This interaction might serve to generate the contractile forces involved in Mg2+-ATP-dependent shape changes in erythrocytes and may additionally serve as a model for myosin organization and regulation in non-muscle cells.

Alteration in protein kinase C activity and subcellular distribution in sickle erythrocytes

In agreement with previous data, membrane protein phosphorylation was found to be altered in intact sickle cells (SS) relative to intact normal erythrocytes (AA). Similar changes were observed in their isolated membranes. The involvement of protein kinase C (PKC) in this process was investigated. The membrane PKC content in SS cells, measured by [3H]phorbol ester binding, was about 6-times higher than in AA cells. In addition, the activity of the enzyme, measured by histone phosphorylation was also found to be increased in SS cell membranes but decreased in their cytosol compared to the activity in AA cell membranes and cytosol. The increase in membrane PKC activity was observed mostly in the light fraction of SS cells, fractionated by density gradient, whereas the decrease in cytosolic activity was only observed in the dense fraction. PKC activity, measured in cells from the blood of reticulocyte-rich patients, exhibited an increase in both membranes and cytosol, thus explaining some of the effects observed in the SS cell light fraction, which is enriched in reticulocytes. The increase in PKC activity in the membranes of SS cells is partly explained by their young age but the loss of PKC activity in their cytosol, particularly in that of the dense fraction, seems to be specific to SS erythrocytes. The relative decrease in membrane PKC activity between the dense and the light fractions of SS cells might be related to oxidative inactivation of the enzyme.

Volume-dependent regulation of ion transport and membrane phosphorylation in human and rat erythrocytes

Osmotic swelling of human and rat erythrocytes does not induce regulatory volume decrease. Regulatory volume increase was observed in shrunken erythrocytes of rats only. This reaction was blocked by the inhibitors of Na+/H+ exchange. Cytoplasmic acidification in erythrocytes of both species increases the amiloride-inhibited component of 22Na influx by five- to eight-fold. Both the osmotic and isosmotic shrinkage of rat erythrocytes results in the 10- to 30-fold increase of amiloride-inhibited 22Na influx and a two-fold increase of furosemide-inhibited 86Rb influx. We failed to indicate any significant changes of these ion transport systems in shrunken human erythrocytes. The shrinking of quin 2-loaded human and rat erythrocytes results in the two- to threefold increase of the rate of 45Ca influx, which is completely blocked by amiloride. The dependence of volume-induced 22Na influx in rat erythrocytes and 45Ca influx in human erythrocytes on amiloride concentration does not differ. The rate of 45Ca influx in resealed ghosts was reduced by one order of magnitude when intravesicular potassium and sodium were replaced by choline. It is assumed that the erythrocyte shrinkage increases the rate of a nonselective Cao2+/(Nai+, Ki+) exchange. Erythrocyte shrinking does not induce significant phosphorylation of membrane protein but increases the 32P incorporation in diphosphoinositides. The effect of shrinkage on the 32P labeling of phosphoinositides is diminished after addition of amiloride. It is assumed that volume-induced phosphoinositide response plays an essential role in the mechanism of the activation of transmembrane ion movements.

The spectrin-actin junction of erythrocyte membrane skeletons

High-resolution electron microscopy of erythrocyte membrane skeletons has provided striking images of a regular lattice-like organization with five or six spectrin molecules attached to short actin filaments to form a sheet of five- and six-sided polygons. Visualization of the membrane skeletons has focused attention on the (spectrin)5,6-actin oligomers, which form the vertices of the polygons, as basic structural units of the lattice. Membrane skeletons and isolated junctional complexes contain four proteins that are stable components of this structure in the following ratios: 1 mol of spectrin dimer, 2-3 mol of actin, 1 mol of protein 4.1 and 0.1-0.5 mol of protein 4.9 (numbers refer to mobility on SDS gels). Additional proteins have been identified that are candidates to interact with the junction, based on in vitro assays, although they have not yet been localized to this structure and include: tropomyosin, tropomyosin-binding protein and adducin. The spectrin-actin complex with its associated proteins has a key structural role in mediating cross-linking of spectrin into the network of the membrane skeleton, and is a potential site for regulation of membrane properties. The purpose of this article is to review properties of known and potential constituent proteins of the spectrin-actin junction, regulation of their interactions, the role of junction proteins in erythrocyte membrane dysfunction, and to consider aspects of assembly of the junctions.

Volume-sensitive K influx in human red cell ghosts

K influx into resealed human red cell ghosts increases when the ghosts are swollen. The influx demonstrates properties similar to volume-sensitive K fluxes present in other cells. The influx is, for the most part, insensitive to the nature of the major intracellular cation and therefore is not a K-K exchange. The influx is much greater when the major anion is Cl than when the major anion is NO3; Cl stimulates the flux and, at constant Cl, NO3 inhibits it. Increase in the influx rate is rapid when shrunken ghosts are swollen or when NO3 is replaced by Cl. The volume-sensitive K influx requires intracellular MgATP at low concentrations, and ATP cannot be replaced by nonhydrolyzable ATP analogues. The volume-sensitive influx is inhibited by Mg2+ and by high concentrations of vanadate, but is stimulated by low concentrations of vanadate. It is not modified by cAMP, the removal of Ca2+ by EGTA, substances that activate protein kinase C, or by inhibition of phosphatidylinositol kinase. The influx is inhibited by neomycin and by trifluoperazine.

Erythrocyte membrane skeleton phosphoproteins: identification of two unrelated phosphoproteins in band 4.9

Human erythrocyte membrane band 4.9 is phosphorylated by several erythrocyte protein kinases. Chromatography of erythrocyte membrane skeleton proteins on DEAE-Sephacel produces two proteins with relative mobilities, on gel electrophoresis, similar to that of band 4.9. The first, with a molecular mass of 49 kDa, is quite basic (pI greater than 8) while the second, 50.5 kDa, is slightly acidic (pI = 6.2). Comparative two-dimensional peptide mapping reveals that both proteins are present in band 4.9 on one-dimensional gels of total erythrocyte membrane proteins and membrane skeleton proteins. The 49 kDa protein, but not the 50.5 kDa protein, binds to actin filaments in a sedimentation assay. In intact erythrocytes metabolically labeled with [32P]orthophosphate, the 49 kDa protein is phosphorylated by protein kinase C, cAMP-dependent protein kinase, and protein kinases which are active in the absence of exogenous kinase activators. In contrast, the 50.5 kDa protein is phosphorylated by protein kinase C but not by the other protein kinases examined. Finally, two-dimensional peptide mapping was employed to compare the 49 kDa protein and a 57 kDa protein which copurifies with, and has many characteristics of, the 49 kDa protein. Significant similarities were found in both 125I-labeled chymotryptic peptide maps and 32P-labeled tryptic peptide maps, suggesting that the 49 kDa and 57 kDa proteins are closely related.

Abolition of actin-bundling by phosphorylation of human erythrocyte protein 4.9

Protein 4.9, first identified as a component of the human erythrocyte membrane skeleton, binds to and bundles actin filaments. Protein 4.9 is a substrate for various kinases, including a cyclic AMP(cAMP)-dependent one, in vivo and in vitro. We show here that phosphorylation of protein 4.9 by the catalytic subunit of cAMP-dependent protein kinase reversibly abolishes its actin-bundling activity, but phosphorylation by protein kinase C has no such effect. A quantitative immunoassay showed that human erythrocytes contain 43,000 trimers of protein 4.9 per cell, which is equivalent to one trimer for each actin oligomer in these red blood cells. As analogues of protein 4.9 have been identified together with analogues of other erythroid skeletal proteins in non-erythroid tissues of numerous vertebrates, phosphorylation and dephosphorylation of protein 4.9 may be the basis for a mechanism that regulates actin bundling in many cells.

Topography of protein kinase C substrates analyzed by membrane splitting

We have used the methods of planar cell and membrane monolayer formation and monolayer splitting to study structural details of the transmembrane signaling process mediated by protein kinase C. We analyzed human red cell membrane proteins phosphorylated by phorbol ester activation of protein kinase C. Planar single membrane preparations, extraction procedures, and gel electrophoresis coupled with silver staining and autoradiography confirmed that two bands in the 100 kDa region, and bands 4.1, and 4.9, were peripheral and phosphorylated by treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA). TPA also stimulated minor incorporation of [32 P]Pi into most integral membrane proteins, including band 3, glycophorin A, the band 4.5 region (glucose transporter) and band 7. Planar cell and membrane-splitting methods revealed that neither integral nor peripheral phosphorylated polypeptides were cleaved by freeze fracture, that all phosphorylated peripheral proteins partitioned intact with the cytoplasmic side of the membrane, and that the percentages of [32P]Pi-labeled peripheral proteins were the same in split membrane cytoplasmic leaflets as in intact membranes. As a unique approach to examining protein topographies membrane splitting provides strong evidence that the major phosphorylated products of the polyphosphatidylinositide pathway are topographically associated with the cytoplasmic leaflet of the human erythrocyte plasma membrane. We further conclude that TPA-induced phosphorylation of red cell peripheral proteins does not significantly alter their transbilayer partitioning patterns after membrane splitting.

Stimulation of polyphosphoinositide turnover upon activation of protein kinases in human erythrocytes

Activation of protein kinase C in erythrocytes by 4-beta-phorbol 12-myristate 13-acetate (PMA) resulted in a parallel stimulation (time course and dose response) of the phosphorylation of both membrane proteins (heterodimers of 107 kDa and 97 kDa, protein 4.1 and 4.9, respectively) and of phosphatidylinositol 4-phosphate (PIP) and, to a lesser extent, of phosphatidylinositol 4,5-bisphosphate (PIP2). Evidence that the effect on lipid was mediated by protein kinase C activation and not by a direct action of PMA was provided by (1) the lack of effect of a phorbol ester that did not activate protein kinase C or of PMA addition on isolated membranes from control erythrocytes, (2) the reversal of the effect in the presence of protein kinase C inhibitors (alpha-cobrotoxin, H-7 (1-(5-isoquinolinesulfonyl)-2-methylpiperazine) or trifluoperazine). PMA treatment did not change the specific activity of ATP or the content of PIP2, but increased the content of PIP and decreased that of PI, indicating that the phosphorylation or dephosphorylation reactions linking PI and PIP were the target for the action of PMA. PMA treatment had no effect on the Ca2+-dependent PIP/PIP2 phospholipase C activity measured in isolated membranes. Mezerein, another protein kinase activator, had similar effects on both protein and lipid phosphorylation, when added with alpha-cobrotoxin. Activation of protein kinase A by cAMP also produced increases in phosphorylation, although quantitatively different from those induced by protein kinase C, in proteins and PIP. Simultaneous addition of PMA and cAMP at maximal doses resulted in only a partially additive effect on PIP labelling. These results show that inositol lipid turnover can be modulated by a protein kinase C and protein kinase A-dependent process involving the phosphorylation of a common protein. This could be PI kinase or PIP phosphatase or another protein regulating the activity of these enzymes.

Spectrin and related molecules

This review begins with a complete discussion of the erythrocyte spectrin membrane skeleton. Particular attention is given to our current knowledge of the structure of the RBC spectrin molecule, its synthesis, assembly, and turnover, and its interactions with spectrin-binding proteins (ankyrin, protein 4.1, and actin). We then give a historical account of the discovery of nonerythroid spectrin. Since the chicken intestinal form of spectrin (TW260/240) and the brain form of spectrin (fodrin) are the best characterized of the nonerythroid spectrins, we compare these molecules to RBC spectrin. Studies establishing the existence of two brain spectrin isoforms are discussed, including a description of the location of these spectrin isoforms at the light- and electron-microscope level of resolution; a comparison of their structure and interactions with spectrin-binding proteins (ankyrin, actin, synapsin I, amelin, and calmodulin); a description of their expression during brain development; and hypotheses concerning their potential roles in axonal transport and synaptic transmission.