Limited proteolysis of the erythrocyte membrane skeleton by calcium-dependent proteinases

Comparing the impact of an acute exercise bout on plasma amino acid composition, intraerythrocytic Ca(2+) handling, and red cell function in athletes and untrained subjects

The N-methyl d-aspartate receptors (NMDARs) mediating Ca(2+) uptake upon stimulation with glutamate and glycine were recently discovered in red blood cells (RBC) of healthy humans. Activation of these receptors with agonists triggered transient Ca(2+)-dependent decrease in hemoglobin oxygen affinity in RBC suspension. The aim of this study was to assess the potential physiological relevance of this phenomenon. Two groups formed by either healthy untrained volunteers or endurance athletes were subjected to a stepwise incremental cycling test to exhaustion. Plasma glutamate levels, activity of the NMDARs, and hemoglobin O2 affinity were measured in blood samples obtained before and after the exercise in both groups. Increase in plasma glutamate levels following exercise was observed in both groups. Transient Ca(2+) accumulation in response to the NMDAR stimulation with NMDA and glycine was followed by facilitated Ca(2+) extrusion from the RBC and compensatory decrease in cytosolic Ca(2+) levels. Short-term activation of the receptors triggered a transient decrease in O2 affinity of hemoglobin in both groups. These exercise-induced responses were more pronounced in athletes compared to the untrained subjects. Athletes were initially presented with lower basal intracellular Ca(2+) levels and hemoglobin oxygen affinity compared to non-trained controls. High basal plasma glutamate levels were associated with induction of hemolysis and formation of echinocytes upon stimulation with the receptor agonists. These findings suggest that glutamate release occurring during exhaustive exercise bouts may acutely facilitate O2 liberation from hemoglobin and improve oxygen delivery to the exercising muscle.

Protease inhibitors-based therapy induces acquired spherocytic-like anaemia and ineffective erythropoiesis in chronic hepatitis C virus patients

BACKGROUND & AIMS

The addition of protease inhibitors, boceprevir (BOC) or telaprevir (TRV), to peg-interferon and ribavirin (PR) increases the incidence of anaemia in patients with chronic hepatitis C virus (HCV) infection. Although genetic variants in inosine triphosphatase (ITPA) gene have been linked to the haemolytic anaemia induced by PR, the mechanism sustaining severe anaemia during triple therapy is still unknown. This study aims to elucidate the molecular mechanisms underlying anaemia in chronic HCV patients with combined therapy.

METHODS

We studied 59 patients with chronic HCV genotype-1: 29 treated with TRV/PR and 30 with BOC/PR. We evaluated biochemical and haematological parameters, red cell index at baseline, 4, 12, 16 and 24 weeks of treatment; in a subgroup, we performed functional studies: osmotic fragility, red cell membrane protein separation, mass spectrometry analysis, quantification of erythroid microparticles release. IL28B and ITPA polymorphisms were also evaluated.

RESULTS

We found early acute normochromic normocytic haemolytic anaemia (4-8 weeks) followed by a late macrocytic hypo-regenerative anaemia with inappropriate low reticulocyte count (12-24 weeks). Studies on red cells revealed: (i) presence of spherocytes; (ii) increased osmotic fragility; (iii) abnormalities in red cell membrane protein composition; (iv) reduced membrane-cytoskeleton stability; (v) increased release of erythroid microparticles. ITPA polymorphisms impacted only the early phase of anaemia.

CONCLUSIONS

The bimodal pattern of anaemia in chronic HCV patients on triple therapy might be because of acquired spherocytic-like anaemia in the early phase, followed by hyporegenerative anaemia, most likely related to the combined effects of PR and TRV or BOC on erythropoiesis.

Mechanisms of neuronal membrane sealing following mechanical trauma

Membrane integrity is crucial for maintaining the intricate signaling and chemically-isolated intracellular environment of neurons; disruption risks deleterious effects, such as unregulated ionic flux, neuronal apoptosis, and oxidative radical damage as observed in spinal cord injury and traumatic brain injury. This paper, in addition to a discussion of the current understanding of cellular tactics to seal membranes, describes two major factors involved in membrane repair. These are line tension, the hydrophobic attractive force between two lipid free-edges, and membrane tension, the rigidity of the lipid bilayer with respect to the tethered cortical cytoskeleton. Ca(2+), a major mechanistic trigger for repair processes, increases following flux through a membrane injury site, and activates phospholipase enzymes, calpain-mediated cortical cytoskeletal proteolysis, protein kinase cascades, and lipid bilayer microdomain modification. The membrane tension appears to be largely modulated through vesicle dynamics, cytoskeletal organization, membrane curvature, and phospholipase manipulation. Dehydration of the phospholipid gap edge and modification of membrane packaging, as in temperature variation, experimentally impact line tension. Due to the time-sensitive nature of axonal sealing, increasing the efficacy of axolemmal sealing through therapeutic modification would be of great clinical value, to deter secondary neurodegenerative effects. Better therapeutic enhancement of membrane sealing requires a complete understanding of its intricate underlying neuronal mechanism.

Sialoglycosylation of RBC in visceral leishmaniasis leads to enhanced oxidative stress, calpain-induced fragmentation of spectrin and hemolysis

Visceral leishmaniasis (VL) caused by the intracellular parasite Leishmania donovani accounts for an estimated 12 million cases of human infection. It is almost always associated with anemia, which severely complicates the disease course. However, the pathological processes leading to anemia in VL have thus far not been adequately characterized to date. In studying the glycosylation patterns of peripheral blood cells we found that the red blood cells (RBC) of VL patients (RBC(VL)) express eight 9-O-acetylated sialoglycoproteins (9-O-AcSGPs) that are not detected in the RBC of healthy individuals (RBC(N)). At the same time, the patients had high titers of anti-9-O-AcSGP IgG antibodies in their sera. These two conditions appear to be linked and related to the anemic state of the patients, as exposure of RBC(VL) but not RBC(N) to anti-9-O-AcSGPs antibodies purified from patient sera triggered a series of responses. These included calcium influx via the P/Q-type but not L-type channels, activation of calpain I, proteolysis of spectrin, enhanced oxidative stress, lipid peroxidation, externalization of phosphatidyl serine with enhanced erythrophagocytosis, enhanced membrane fragility and, finally, hemolysis. Taken together, this study suggests that the enhanced hemolysis is linked to an impairment of membrane integrity in RBC(VL) which is mediated by ligand-specific interaction of surface 9-O-AcSGPs. This affords a potential explanation for the structural and functional features of RBC(VL) which are involved in the hemolysis related to the anemia which develops in VL patients.

Degradation of C/EBPβ in cultured myotubes is calpain-dependent

Members of the C/EBP transcription factor family regulate cell differentiation and multiple other cellular functions. The cellular levels of C/EBPalpha, gamma, delta, epsilon, and Gadd153/CHOP are regulated in part by proteasome-dependent degradation. In contrast, mechanisms regulating the degradation of C/EBPbeta are poorly understood. We tested the hypothesis that the degradation of C/EBPbeta is calpain-dependent. Studies were performed in cultured L6 myotubes (a rat skeletal muscle cell line) because we have found previously that C/EBPbeta may be involved in the regulation of muscle proteolysis. Treatment of cultured L6 myotubes with the calpain inhibitors calpeptin and Calpain Inhibitor I and II resulted in increased C/EBPbeta concentrations but did not influence cellular levels of the other C/EBP transcription factor family members. Transfection of myoblasts with a plasmid expressing the endogenous calpain inhibitor calpastatin resulted in increased cellular levels of C/EBPbeta whereas the opposite result was observed in myoblasts overexpressing micro- or m-calpain. Co-immunoprecipitation provided evidence for protein-protein interaction between C/EBPbeta and micro- and m-calpain suggesting that C/EBPbeta may be a calpain substrate. This notion was supported by experiments in which immunoprecipitated C/EBPbeta was incubated with purified micro-calpain in a cell-free system. The increase in C/EBPbeta levels caused by inhibition of calpain activity was accompanied by increased C/EBPbeta DNA-binding and gene activation. The present results suggest that C/EBPbeta is degraded by a calpain-dependent mechanism in skeletal muscle cells and that the role of calpains is specific for C/EBPbeta among different members of the C/EBP transcription factor family.

Regulating cell migration: calpains make the cut

The calpain family of proteases has been implicated in cellular processes such as apoptosis, proliferation and cell migration. Calpains are involved in several key aspects of migration, including: adhesion and spreading; detachment of the rear; integrin- and growth-factor-mediated signaling; and membrane protrusion. Our understanding of how calpains are activated and regulated during cell migration has increased as studies have identified roles for calcium and phospholipid binding, autolysis, phosphorylation and inhibition by calpastatin in the modulation of calpain activity. Knockout and knockdown approaches have also contributed significantly to our knowledge of calpain biology, particularly with respect to the specific functions of different calpain isoforms. The mechanisms by which calpain-mediated proteolysis of individual substrates contributes to cell motility have begun to be addressed, and these efforts have revealed roles for proteolysis of specific substrates in integrin activation, adhesion complex turnover and membrane protrusion dynamics. Understanding these mechanisms should provide avenues for novel therapeutic strategies to treat pathological processes such as tumor metastasis and chronic inflammatory disease.

Cytoskeletal disruption and small heat shock protein translocation immediately after lengthening contractions

The purposes of this study were to determine whether, immediately after lengthening contractions, 1) levels of specific force-transmitting cytoskeletal elements are reduced in skeletal muscle cells and 2) cytosolic small heat shock proteins (HSPs) translocate to structures prone to disruption. Western blot analysis demonstrated decreased concentrations of z-disk proteins alpha-actinin and plectin and membrane scaffolding proteins dystrophin and beta-spectrin in muscle exposed to lengthening contractions compared with contralateral control muscle. Lengthening contractions also resulted in immediate translocation of constitutively expressed HSP25 and alphaB-crystallin from the soluble to the insoluble fraction of muscle homogenates, and cryosections showed translocation from a diffuse, cytosolic localization to striations that corresponded to z-disks. Lengthening contraction-induced translocation of HSP25 and alphaB-crystallin was associated with phosphorylation of these small HSPs, which may trigger their protective activity. In summary, these findings demonstrate loss of z-disk and membrane scaffolding proteins immediately after lengthening contractions, and concomitant translocation of HSP25 and alphaB-crystallin to the z-disk, which may help to stabilize or repair cytoskeletal elements at this site.

Short window of opportunity for calpain induced growth cone formation after axotomy of Aplysia neurons

Our laboratory has established that local activation of calpain by a transient elevation of the free intracellular calcium concentration is crucial for the induction of growth cone (GC) formation in cultured Aplysia neurons. The mechanisms and stages in which calpain is involved in the formation of a GC are not known. We began to study these questions by determining the nature of calpain's action and the stages in which calpain activity affects the cascade of events that leads to the formation of the GC and its extension. We report that the calpain-dependent transformation of an axonal segment into a GC occurs within a narrow window of opportunity that lasts approximately 5 min. If calpain is inhibited during this window of opportunity, GC formation does not occur. Inhibition of calpain after the window of opportunity slows down the rate of lamellipodial extension but doesn't arrest it. The proteolysis of spectrin, a calpain substrate and a major component of the membrane skeleton, occurs within this window of opportunity, in agreement with the hypothesis that spectrin proteolysis is an early step in the formation of the GC. If the onset of proteolysis is deferred, spectrin remains unchanged and GC formation is compromised. We suggest that calpain participates in two different processes: it is critical for the triggering of GC formation and plays a modulatory role during the extension of the GC's lamellipodia.

Evidence for involvement of calpain in c-Myc proteolysis in vivo

Precise control of the level of c-Myc protein is important to normal cellular homeostasis, and this is accomplished in part by degradation through the ubiquitin-proteasome pathway. The calpains are a family of calcium-dependent proteases that play important roles in proteolysis of some proteins, and their possible participation in degradation of intracellular c-Myc was therefore investigated. Activation of calpain with the cell-permeable calcium ionophore A23187 in Rat1a-myc or ts85 cells in culture induced rapid cleavage of c-Myc. This degradation was both calpain- and calcium-dependent since it was inhibited by preincubation with either the calpain-inhibitory peptide calpeptin or the calcium-chelating agent EGTA. A23187-induced c-Myc cleavage occurred in a time-dependent manner comparable to that of FAK, a known calpain substrate, and while calpeptin was able to significantly protect c-Myc from degradation, inhibitors of the proteasome or caspase proteases could not. Exposure of Rat1a-myc or ts85 cells in culture to calpeptin, or to the thiol-protease inhibitor E64d, resulted in the accumulation of c-Myc protein without an impact on ubiquitin-protein conjugates. Using an in vitro assay, calpain-mediated degradation occurred rapidly with wild-type c-Myc as the substrate, but was significantly prolonged in some c-Myc mutants with increased transforming activity derived from lymphoma patients. Those mutants with a prolonged half-life in vitro were also more resistant to A23187-induced cleavage in intact cells. These studies support a role for calpain in the control of c-Myc levels in vivo, and suggest that mutations impacting on sensitivity to calpain may contribute to c-Myc-mediated tumorigenesis.

Microinjection of calpastatin inhibits fusion in myoblasts

Rat satellite cells (RSC) were microinjected with purified calpastatin or m-calpain, and myoblasts from a C2C12 mouse line were microinjected with purified calpastatin. Microinjection with calpastatin completely prevented fusion of myoblasts from both sources, whereas microinjection with m-calpain significantly increased the rate of fusion of cultured RSC; 44% of the nuclei of RSC cultures were in multinucleated myotubes within 48 h after microinjection with m-calpain plus labeled dextran, whereas only 15% of the nuclei were in multinucleated myotubes after microinjection with dextran alone. Western analyses indicated that neither RSC nor C2C12 myoblasts contained detectable amounts of mu-calpain before fusion. The levels of calpastatin in C2C12 myoblasts increased as cells passed from the proliferative stage to the onset of fusion, and these levels increased substantially in both the C2C12 and the RSC cells as they progressed to the late or postfusion stage. Both RSC and C2C12 myoblasts contained an 80-kDa polypeptide that was labeled with an anti-m-calpain antibody in Western blots. The results are consistent with a role of the calpain system (m-calpain in these myoblast lines) in remodeling of the cytoskeletal/plasma membrane interactions during cell fusion.

Selective coupling of mu-calpain activation with the NMDA receptor is independent of translocation and autolysis in primary cortical neurons

Excessive mu-calpain activation has been linked to several cellular pathologies including excitotoxicity and ischemia. In erythrocytes and other non-central nervous system (CNS) cells, calpain activation is thought to occur following a Ca2+-induced translocation of inactive cytosolic enzyme to membranes and subsequent autolysis. In the present report, we show that transiently exposing primary rat cortical neurons to lethal (50 microM) N-methyl-D-aspartic acid (NMDA) caused protracted calpain activation, measured as increased spectrin hydrolysis, but this was independent of translocation or autolysis of the protease. An anti-mu-calpain antibody showed that calpain was largely membrane associated in cortical neurons, and, consequently, neither translocation nor autolysis of the protease was observed following ionomycin or lethal NMDA treatment. By contrast, in rat erythrocytes, calpain was largely cytosolic and underwent rapid translocation and autolysis in response to ionomycin. Calpain-mediated spectrin hydrolysis was specifically coupled to Ca2+ entry through the NMDA receptor because nonspecific Ca2+ influx via ionomycin or KCl-mediated depolarization failed to activate the enzyme. Thus, calpain appears selectively linked to glutamate receptors in cortical neurons and regulated by mechanisms distinct from that occurring in many non-CNS cells. The data suggest that intracellular signals coupled to the NMDA receptor are responsible for activating calpain already associated with cellular membranes in cortical cells.

Is calpain activity regulated by membranes and autolysis or by calcium and calpastatin?

Although the Ca(2+)-dependent proteinase (calpain) system has been found in every vertebrate cell that has been examined for its presence and has been detected in Drosophila and parasites, the physiological function(s) of this system remains unclear. Calpain activity has been associated with cleavages that alter regulation of various enzyme activities, with remodeling or disassembly of the cell cytoskeleton, and with cleavages of hormone receptors. The mechanism regulating activity of the calpain system in vivo also is unknown. It has been proposed that binding of the calpains to phospholipid in a cell membrane lowers the Ca2+ concentration, [Ca2+], required for the calpains to autolyze, and that autolysis converts an inactive proenzyme into an active protease. Recent studies, however, show that the calpains bind to specific proteins and not to phospholipids, and that binding to cell membranes does not affect the [Ca2+] required for autolysis. It seems likely that calpain activity is regulated by binding of Ca2+ to specific sites on the calpain molecule, with binding to each site eliciting a response (proteolytic activity, calpastatin binding, etc.) specific for that site. Regulation must also involve an, as yet, undiscovered mechanism that increases the affinity of the Ca(2+)-binding sites for Ca2+.

In vitro digestion of dystrophin by calcium-dependent proteases, calpains I and II

Dystrophin is a cytoskeletal protein which is thought to play an important role in membrane physiology since its absence (due to gene deficiency) leads to the symptoms of Duchenne muscular dystrophy (DMD). Some disruption in the regulation of intracellular free Ca2+ levels could lead to DMD-like symptoms. In this study, calpains, which are very active calcium-dependent proteases, were examined for their capacity to hydrolyse dystrophin in vitro. The results show that calpains are able to split dystrophin and produce breakdown products of different sizes (the degree of cleavage being dependent on the incubation time with proteases). The time-course of protease degradation was examined by Western immunoblot using three polyclonal sera which were characterized as being specific to the central (residues 1173-1728) and two distal parts of the molecule ie specific to the N-terminal (residues 43-760) or the C-terminal (residues 3357-3660) extremities of the dystrophin molecule. The cleavage patterns of dystrophin showed an accumulation of some major protease-resistant fragments of high relative molecular mass (250-370 kDa). These observations demonstrate that calpains digest dystrophin very rapidly when the calcium concentration is compatible with their activation. For instance, it is clear that calpains first give rise to large dystrophin products in which the C-terminal region is lacking. These observations suggest that dystrophin antibodies specific to the central domain of the molecule should be used to detect dystrophin for diagnostic purposes and before any conclusion as to the presence or absence of dystrophin can be deduced from results obtained using immunoanalyses of muscle biopsies.

Calcium is required for binding of Escherichia coli hemolysin (HlyA) to erythrocyte membranes

The calcium requirement for hemolytic activity of Escherichia coli hemolysin was investigated by using hemolytic assays and immunoblotting of toxin-treated erythrocytes. The hemolytic activity of cell culture supernatants obtained during growth of E. coli in Luria-Bertani (LB) broth or calcium-free LB broth was calcium dependent. The hemolytic activity of culture supernatants obtained during growth in LB broth supplemented with calcium was calcium independent. Osmotic protection experiments using Dextran 4 to prevent cell lysis indicated that calcium was required for the binding of hemolysin to erythrocytes at both 4 and 37 degrees C. The binding efficiency at 4 degrees C was 50% of that occurring at 37 degrees C. The calcium-dependent binding was confirmed by immunoblotting saline-washed, toxin-treated erythrocytes with a monoclonal antibody after sodium dodecyl sulfate-polyacrylamide gel electrophoresis separation of membrane proteins. Bound hemolysin increased the calcium permeability of the cell membranes as evidenced by calcium-induced membrane protein alterations. The alterations in membrane proteins did not directly cause lysis of the cells. The results were consistent with a mechanism of lysis involving the formation of cation-selective pores in the membranes of target cells.

In vitro digestion of spectrin, protein 4.1 and ankyrin by erythrocyte calcium dependent neutral protease (calpain I)

1. In whole ghosts, ankyrin, protein 4.1, protein band 3 and spectrin are lysed by purified calpain I in the presence of calcium. 2. Limited calpain lysis of purified ankyrin results in several peptides, including a 85 kD peptide bearing the ankyrin interaction site for the protein band 3 internal fragment (43 kD), and a 55 kD peptide carrying the ankyrin-spectrin interaction site. 3. These peptides are differently phosphorylated: the 85 kD by cytosol casein kinase, and the 55 kD by membrane casein kinase. 4. Protein 4.1 lysis mainly produces a 30 kD peptide resistant to proteolysis. 5. The spectrin beta-chain is more sensitive to calpain cleavage than the alpha chain; both chains seem to be cleaved in a similar sequential manner. 6. Limited proteolysis of spectrin dimer does not impede tetramerization in vitro.

Calpains (intracellular calcium-activated cysteine proteinases): structure-activity relationships and involvement in normal and abnormal cellular metabolism

1. Calpains (calcium-activated cysteine proteinases) have evolved by gene fusion events involving calmodulin-like genes, cysteine proteinase genes and other sequences of unknown origin. 2. The enzymes are composed of two non-identical subunits, each of which contains functional calcium-binding sequences. 3. Calpains are inhibited by the endogenous protein inhibitor, calpastatin and some calmodulin antagonists are also inhibitors of calpain. A number of synthetic proteinase inhibitors also inhibit calpains. 4. Calpains can be activated by phospholipids, an endogenous protein activator and some amino acid derivatives. 5. Various protein substrates for calpains have been recognized in vitro, but the identity of in situ substrates remains unclear. 6. Proposals have been made for calpain function, including involvement in signal transduction, platelet activation, cell fusion, mitosis and cytoskeleton and contractile protein turnover. 7. Calpain and calpastatin expression is altered in a number of abnormal states including muscular dystrophy, muscle denervation and tenotomy, hypertension and platelet abnormalities.

Calmodulin-binding proteins as calpain substrates

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Spectrin, fodrin and protein 4.1-like proteins in differentiating rat germ cells

The presence and the distribution of proteins of the membrane skeleton in differentiating germ cells of the rat has been investigated. Immunofluorescence and immunoblotting analysis, performed using monoclonal and polyclonal antibodies to human erythroid alpha-spectrin and protein 4.1 and to brain spectrin (fodrin), demonstrated the presence of analogues of spectrin and fodrin in spermatocytes and round spermatids and of protein 4.1-like molecules in spermatocytes, spermatids and spermatozoa. Spectrin and fodrin showed molecular weights comparable to those of their analogues in erythrocytes but a distinct intracellular distribution. Fodrin was localized along the plasma membrane while spectrin appeared associated with the regions of the Golgi apparatus and of the developing acrosome. Antibodies to protein 4.1 recognized molecules with a molecular weight not comparable with that in erythrocytes, and their presence in spermatozoa was confined to specific regions of the head and of the tail.

Degradation of skeletal muscle plasma membrane proteins by calpain

Observations described here provide the first demonstration that calpain (Ca2+-dependent cysteine protease) can degrade proteins of skeletal muscle plasma membranes. Frog muscle plasma membrane vesicles were incubated with calpain preparations and alterations of protein composition were revealed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Calpain II (activated by millimolar concentrations of Ca2+) was isolated from frog skeletal muscle, but the activity of calpain I (activated by micromolar concentrations of Ca2+) was lost during attempts at fractionation. Calpain I obtained from skeletal muscle and erythrocytes of rats was tested instead, and exerted effects similar to those of frog muscle calpain on the membrane proteins. All of the calpain preparations caused striking losses of a major membrane protein of molecular mass of approximately 97 kDa, designated band c, and diminution of a thinner band of approximately 200 kDa. There were concomitant increases in 83- and 77-kDa polypeptides. These effects were absolutely dependent on the presence of free Ca2+, and were completely blocked by calpastatin, a specific inhibitor of calpain action. Frog muscle calpain differed only in being relatively more active at 0 degree C than were the calpains from rat tissues. Experimental observations suggest that calpain acts at the cytoplasmic surface of the plasma membrane.

Proteolytic modification of calcium-dependent protease 1 in erythrocytes treated with ionomycin and calcium

In vitro, limited proteolytic cleavage of the subunits of the purified calcium-dependent proteases [also known as calpains (EC 3.4.22.17) or calcium-activated neutral proteinases (CANPs)] appears to be required for enzyme activity. It has not yet been demonstrated if similar processing of the protease subunits occurs in vivo. To directly assess proteolytic modification of these proteases in cells, we have measured the loss of the proenzyme form of the regulatory subunit (a 26-kDa protein) and/or the appearance of the modified regulatory subunit (a 17-kDa protein) by densitometric analysis of immunoblots. In rat erythrocytes, proteolytic modification of the endogenous calcium-dependent protease (calcium-dependent protease 1, mu CANP) occurs in vivo in response to ionomycin and calcium. The extent of enzyme modification was dependent on time, ionomycin concentration, and calcium concentration, suggesting that in this cellular model Ca2+ regulates proteolytic modification of the enzyme.

Effects of long-chain acyl carnitine on membrane fluidity of human erythrocytes

Amphiphilic compounds such as long-chain acyl carnitines accumulate in ischemic myocardium and potentially contribute to the myocardial damage. To characterize alterations in membrane molecular dynamics produced by palmitoylcarnitine, human erythrocytes were spin-labeled with 5-doxylstearic acid, and membrane fluidity was quantified by measuring the changes in the order parameter derived from ESR spectra. Palmitoylcarnitine induced triphasic alterations in membrane fluidity of human erythrocytes. The membrane fluidity increased for 5 min, then decreased in a concentration-dependent manner. At higher concentrations (100 and 150 microM) of palmitoylcarnitine, membrane fluidity increased again after 30 and 20 min of the incubation, respectively. Addition of 2.8 mM CaCl2 resulted in a significant decrease in membrane fluidity and enhanced the alterations in membrane fluidity caused by palmitoylcarnitine. The results suggest that alterations in molecular dynamics are one mechanism through which long-chain acyl carnitine could play an important role in ischemic injury.

Ca2+-mediated activation of human erythrocyte membrane Ca2+-ATPase

Ca2+-ATPase of human erythrocyte membranes, after being washed to remove Ca2+ after incubation with the ion, was found to be activated. Stimulation of the ATPase was related neither to fluidity change nor to cytoskeletal degradation of the membranes mediated by Ca2+. Activation of the transport enzyme was also unaffected by detergent treatment of the membrane, but was suppressed when leupeptin was included during incubation of the membranes with Ca2+. Stimulation of the ATPase by a membrane-associated Ca2+-dependent proteinase was thus suggested. Much less 138 kDa Ca2+-ATPase protein could be harvested from a Triton extract of membranes incubated with Ca2+ than without Ca2+. Activity of the activated enzyme could not be further elevated by exogenous calpain, even after treatment of the membranes with glycodeoxycholate. There was also an overlap in the effect of calmodulin and the Ca2+-mediated stimulation of membrane Ca2+-ATPase. While Km(ATP) of the stimulated ATPase remained unchanged, a significant drop in the free-Ca2+ concentration for half-maximal activation of the enzyme was observed.

Activation of the Ca2+-ATPase of human erythrocyte membrane by an endogenous Ca2+-dependent neutral protease

Limited proteolysis of the plasma membrane calcium transport ATPase (Ca2+-ATPase) from human erythrocytes by trypsin produces a calmodulin-like activation of its ATP hydrolytic activity and abolishes its calmodulin sensitivity. We now demonstrate a similar kind of activation of the human erythrocyte membrane Ca2+-ATPase by calpain (calcium-dependent neutral protease) isolated from the human red cell cytosol. Upon incubation of red blood cell membranes with purified calpain in the presence of Ca2+ the membrane-bound Ca2+-ATPase activity was increased and its sensitivity to calmodulin was lost. In contrast to the action of other proteases tested, proteolysis by calpain favors activation over inactivation of the Ca2+-ATPase activity, except at calpain concentrations more than 2 orders of magnitude higher. Exogenous calmodulin protects the Ca2+-ATPase against calpain-mediated activation at concentrations which also activate the Ca2+-ATPase activity. Calcium-dependent proteolytic modification of the Ca2+-ATPase could provide a mechanism for the irreversible activation of the membrane-bound enzyme.

Activation of erythrocyte membrane Ca2+-ATPase by calpain

Ca2+-ATPase of erythrocyte membranes, prepared from erythrocytes substantially removed of contaminating leukocytes, was found to be activated by calpain isolated from the same source. Saponin or glycodeoxycholate treatment of membranes was essential for elicitation of the calpain response. Unlike the membrane bound ATPase, solubilized ATPase was inactivated by calpain. Digestion of membranes with the protease did not affect the Km (ATP) of Ca2+-ATPase though stimulation of the membrane ATPase by calmodulin could be partially substituted by calpain treatment. As compared with control, Ca2+-ATPase of calpain-digested membranes attained maximal activity at a lower free Ca2+ concentration.

Modulation of red cell vesiculation by protease inhibitors

Release of vesicles from human red cell membranes was induced either by ATP-depletion or by incubation of the cells in presence of sonicated dimyristoylphosphatidylcholine (DMPC) vesicles. Vesicles released from ATP-depleted red cells but not the DMPC-induced vesicles contained degradation products of band 3 protein. Furthermore, in ATP-depleted erythrocytes proteolytic breakdown products could be demonstrated that were not detected in cells incubated with DMPC. Proteolysis was neither significantly affected by the protease inhibitor N-alpha-tosyl-L-lysine chloromethyl ketone (TLCK) nor by other protease inhibitors tested in this study (diisopropylfluorophosphate, N-ethylmaleimide and phenylmethylsulfonyl fluoride). Both vesiculation processes were inhibited in a concentration dependent way by TLCK while other protease inhibitors did not significantly influence membrane vesiculation. Phase contrast microscopy showed that TLCK diminished the DMPC-induced formation of echinocytes which is known to precede vesicle release. These results suggest that the influence of TLCK on membrane vesiculation is not primarily due to inhibition of proteolysis but to a direct interaction of the inhibitor with the intrinsic domain of the erythrocyte membrane.

Membrane phospholipid organization in calcium-loaded human erythrocytes

Intracellular Ca2+ levels in human erythrocytes were increased by incubating them with variable concentrations of Ca2+ in the presence of ionophore A23187. Experiments were done to confirm that the Ca2+ loading did induce changes in the cell shape and membrane protein composition. The effect of the increased cytoplasmic Ca2+ levels on the membrane phospholipid organization was analysed using bee venom and pancreatic phospholipases A2, Merocyanine 540 and fluorescamine as the external membrane probes. About 20% phosphatidylethanolamine (PE) and 0% phosphatidylserine (PS) were hydrolysed by the phospholipases in intact control cells, whereas in identical conditions these enzymes readily degraded, 20-30% PE and 7-30% PS, in Ca2+-loaded erythrocytes, depending on the cytoplasmic Ca2+ concentration. Also, Merocyanine 540 failed to stain the fresh or control erythrocytes, but it labeled the cells loaded with Ca2+. Furthermore, fluorescamine labeled approx. 20% PE in fresh or control erythrocytes while in identical conditions, significantly higher amounts of PE were modified in intact Ca2+-loaded cells. These results demonstrate that Ca2+ loading in human erythrocytes leads to loss of the transbilayer phospholipid asymmetry, and suggest that, together with spectrin, polypeptides 2.1 and 4.1 may also play an important role in maintaining the asymmetric distribution of various phospholipids across the erythrocyte membrane bilayer.

Changes in pulmonary calpain activity following treatment of mice with butylated hydroxytoluene

The antioxidant, butylated hydroxytoluene (BHT), causes lung toxicity in mice followed by regenerative repair, and can also modulate the development of carcinogen-induced lung adenomas. We are investigating changes in pulmonary biochemistry following BHT treatment in order to understand the mechanisms of BHT-induced pulmonary regenerative repair. BHT administration lowered cytosolic Ca2+-activated neutral protease (calpain) activity, increased the activity of the endogenous calpain inhibitor, calpastatin, increased the extent of photoincorporation of 8-N3-[32P]cAMP into a Mr 37,000 proteolytic product derived from cAMP-dependent protein kinase regulatory (R) subunits, and increased membrane-associated protease activity. All of these changes were dependent on the BHT dosage; the altered proteolytic activities occurred at a dose lower than that which caused observable lung toxicity as assessed by the lung weight/body weight ratio. Decreased cytosolic calpain activity was detectable within 1 day after BHT administration, was lowest at 4-7 days, and had not returned to control levels by Day 21, a time when normal lung morphology had been regained. The decrease in calpain activity cannot fully be accounted for by increased calpastatin activity; upon separation of these proteins by DEAE chromatography, the amount of calpain activity from BHT-treated mice remained lower than the corresponding peak from control mice. Increased photolabeling of the Mr 37,000 protein began at 1 day and continued to increase up to 4 days after BHT. All of the cytosolic changes preceded the increased particulate proteolytic activity by 1-2 days. R-subunits which have dissociated from their catalytic subunits are more susceptible to degradation by calpain, but BHT treatment did not enhance subunit dissociation as determined by the elution profile of 8-N3-[32P]cAMP-labeled R-subunits following DEAE chromatography. A large percentage of the particulate protease activity was inhibited by calpastatin, leupeptin, and E-64, all of which are known to inhibit calpain activity; this suggested that calpain accounted for most of this activity. Changes in the activities of proteases which catalyze limited proteolysis reactions may play an important role in the repair of acute lung injury.

Beta spectrin bestows protein 4.1 sensitivity on spectrin-actin interactions

The ability of protein 4.1 to stimulate the binding of spectrin to F-actin has been compared by cosedimentation analysis for three avian (erythrocyte, brain, and brush border) and two mammalian (erythrocyte and brain) spectrin isoforms. Human erythroid protein 4.1 stimulated actin binding of all spectrins except the brush border isoform (TW 260/240). These results suggested that the beta subunit determined the protein 4.1 sensitivity of the heterodimer, since all avian alpha subunits are encoded by a single gene. Tissue-specific posttranslational modification of the alpha subunit was excluded by examining the properties of hybrid spectrins composed of the purified alpha subunit from avian erythrocyte or brush border spectrin and the beta subunit of human erythrocyte spectrin. A hybrid composed of avian brush border alpha and human erythroid beta spectrin ran on nondenaturing gels as a discrete band, migrating near human erythroid spectrin tetramers. The actin-binding activity of this hybrid was stimulated by protein 4.1, while either chain alone was devoid of activity. Therefore, although both subunits were required for actin binding, the sensitivity of the spectrin-actin interaction to protein 4.1 is a property uniquely bestowed on the heterodimer by the beta subunit. The singular insensitivity of brush border spectrin to stimulation by erythroid protein 4.1 was also consistent with the absence of proteins in avian intestinal epithelial cells which were immunoreactive with polyclonal antisera sensitive to all of the known avian and human erythroid 4.1 isoforms.