The cytosol of human erythrocytes contains a highly Ca2+-sensitive thiol protease (calpain I) and its specific inhibitor protein (calpastatin)

Revisiting the calpain hypothesis of learning and memory 40 years later

In 1984, Gary Lynch and Michel Baudry published in Science a novel biochemical hypothesis for learning and memory, in which they postulated that the calcium-dependent protease, calpain, played a critical role in regulating synaptic properties and the distribution of glutamate receptors, thereby participating in memory formation in hippocampus. Over the following 40 years, much work has been done to refine this hypothesis and to provide convincing arguments supporting what was viewed at the time as a simplistic view of synaptic biochemistry. We have now demonstrated that the two major calpain isoforms in the brain, calpain-1 and calpain-2, execute opposite functions in both synaptic plasticity/learning and memory and in neuroprotection/neurodegeneration. Thus, calpain-1 activation is required for triggering long-term potentiation (LTP) of synaptic transmission and learning of episodic memory, while calpain-2 activation limits the magnitude of LTP and the extent of learning. On the other hand, calpain-1 is neuroprotective while calpain-2 is neurodegenerative, and its prolonged activation following various types of brain insults leads to neurodegeneration. The signaling pathways responsible for these functions have been identified and involve local protein synthesis, cytoskeletal regulation, and regulation of glutamate receptors. Human families with mutations in calpain-1 have been reported to have impairment in motor and cognitive functions. Selective calpain-2 inhibitors have been synthesized and clinical studies to test their potential use to treat disorders associated with acute neuronal damage, such as traumatic brain injury, are being planned. This review will illustrate the long and difficult journey to validate a bold hypothesis.

Calcium Channels and Calcium-Regulated Channels in Human Red Blood Cells

Free Calcium (Ca²⁺) is an important and universal signalling entity in all cells, red blood cells included. Although mature mammalian red blood cells are believed to not contain organelles as Ca²⁺ stores such as the endoplasmic reticulum or mitochondria, a 20,000-fold gradient based on a intracellular Ca²⁺ concentration of approximately 60 nM vs. an extracellular concentration of 1.2 mM makes Ca²⁺-permeable channels a major signalling tool of red blood cells. However, the internal Ca²⁺ concentration is tightly controlled, regulated and maintained primarily by the Ca²⁺ pumps PMCA1 and PMCA4. Within the last two decades it became evident that an increased intracellular Ca²⁺ is associated with red blood cell clearance in the spleen and promotes red blood cell aggregability and clot formation. In contrast to this rather uncontrolled deadly Ca²⁺ signals only recently it became evident, that a temporal increase in intracellular Ca²⁺ can also have positive effects such as the modulation of the red blood cells O₂ binding properties or even be vital for brief transient cellular volume adaptation when passing constrictions like small capillaries or slits in the spleen. Here we give an overview of Ca²⁺ channels and Ca²⁺-regulated channels in red blood cells, namely the Gárdos channel, the non-selective voltage dependent cation channel, Piezo1, the NMDA receptor, VDAC, TRPC channels, Ca_V2.1, a Ca²⁺-inhibited channel novel to red blood cells and i.a. relate these channels to the molecular unknown sickle cell disease conductance P_sickle. Particular attention is given to correlation of functional measurements with molecular entities as well as the physiological and pathophysiological function of these channels. This view is in constant progress and in particular the understanding of the interaction of several ion channels in a physiological context just started. This includes on the one hand channelopathies, where a mutation of the ion channel is the direct cause of the disease, like Hereditary Xerocytosis and the Gárdos Channelopathy. On the other hand it applies to red blood cell related diseases where an altered channel activity is a secondary effect like in sickle cell disease or thalassemia. Also these secondary effects should receive medical and pharmacologic attention because they can be crucial when it comes to the life-threatening symptoms of the disease.

The human blood parasite Schistosoma mansoni expresses extracellular tegumental calpains that cleave the blood clotting protein fibronectin

Schistosomes are intravascular, parasitic flatworms that cause debilitating disease afflicting >200 million people. Proteins expressed at the host-parasite interface likely play key roles in modifying the worm's local environment to ensure parasite survival. Proteomic analysis reveals that two proteases belonging to the calpain family (SmCalp1 and SmCalp2) are expressed in the Schistosoma mansoni tegument. We have cloned both; while highly conserved in domain organization they display just 31% amino acid sequence identity. Both display high relative expression in the parasite's intravascular life forms. Immunolocalization and activity based protein profiling experiments confirm the presence of the enzymes at the host-parasite interface. Living parasites exhibit surface calpain activity that is blocked in the absence of calcium and in the presence of calpain inhibitors (E64c, PD 150606 and calpastatin). While calpains are invariably reported to be exclusively intracellular (except in diseased or injured tissues), our data show that schistosomes display unique, constitutive, functional extracellular calpain activity. Furthermore we show that the worms are capable of cleaving the host blood clotting protein fibronectin and that this activity can be inhibited by E64c. We hypothesize that SmCalp1 and/or SmCalp2 perform this cleavage function to impede blood clot formation around the worms in vivo.

Decreased calpain activity in chronic myeloid leukemia impairs apoptosis by increasing survivin in myeloid progenitors and xiap1 in differentiating granulocytes

Chronic Myeloid Leukemia (CML) is characterized by translocations between chromosomes 9 and 22, resulting in expression of Bcr-abl oncogenes. Although the clinical course of CML was revolutionized by development of Bcr-abl-directed tyrosine kinase inhibitors (TKIs), CML is not cured by these agents. Specifically, the majority of subjects relapsed in clinical trials attempting TKI discontinuation, suggesting persistence of leukemia stem cells (LSCs) even in molecular remission. Identifying mechanisms of CML-LSC persistence may suggest rationale therapeutic targets to augment TKI efficacy and lead to cure. Apoptosis resistance is one proposed mechanism. In prior studies, we identified increased expression of Growth Arrest Specific 2 (Gas2; a Calpain inhibitor) in Bcr-abl⁺ bone marrow progenitor cells. A number of previously described Calpain substrates might influence apoptosis in CML, including βcatenin and the X-linked Inhibitor of Apoptosis Protein 1 (Xiap1). We previously found Gas2/Calpain dependent stabilization of βcatenin in CML, and increased expression of βcatenin target genes, including Survivin (also an IAP). In the current work, we investigate contributions of Survivin and Xiap1 to Fas-resistance in Bcr-abl⁺ bone marrow cells. Inhibitors of these proteins are currently in clinical trials for other malignancies, but a role for either IAP in CML-LSC persistence is unknown.

Red Cell Properties after Different Modes of Blood Transportation

Transportation of blood samples is unavoidable for assessment of specific parameters in blood of patients with rare anemias, blood doping testing, or for research purposes. Despite the awareness that shipment may substantially alter multiple parameters, no study of that extent has been performed to assess these changes and optimize shipment conditions to reduce transportation-related artifacts. Here we investigate the changes in multiple parameters in blood of healthy donors over 72 h of simulated shipment conditions. Three different anticoagulants (K3EDTA, Sodium Heparin, and citrate-based CPDA) for two temperatures (4°C and room temperature) were tested to define the optimal transportation conditions. Parameters measured cover common cytology and biochemistry parameters (complete blood count, hematocrit, morphological examination), red blood cell (RBC) volume, ion content and density, membrane properties and stability (hemolysis, osmotic fragility, membrane heat stability, patch-clamp investigations, and formation of micro vesicles), Ca(2+) handling, RBC metabolism, activity of numerous enzymes, and O2 transport capacity. Our findings indicate that individual sets of parameters may require different shipment settings (anticoagulants, temperature). Most of the parameters except for ion (Na(+), K(+), Ca(2+)) handling and, possibly, reticulocytes counts, tend to favor transportation at 4°C. Whereas plasma and intraerythrocytic Ca(2+) cannot be accurately measured in the presence of chelators such as citrate and EDTA, the majority of Ca(2+)-dependent parameters are stabilized in CPDA samples. Even in blood samples from healthy donors transported using an optimized shipment protocol, the majority of parameters were stable within 24 h, a condition that may not hold for the samples of patients with rare anemias. This implies for as short as possible shipping using fast courier services to the closest expert laboratory at reach. Mobile laboratories or the travel of the patients to the specialized laboratories may be the only option for some groups of patients with highly unstable RBCs.

Calcium in red blood cells-a perilous balance

Ca2+ is a universal signalling molecule involved in regulating cell cycle and fate, metabolism and structural integrity, motility and volume. Like other cells, red blood cells (RBCs) rely on Ca2+ dependent signalling during differentiation from precursor cells. Intracellular Ca2+ levels in the circulating human RBCs take part not only in controlling biophysical properties such as membrane composition, volume and rheological properties, but also physiological parameters such as metabolic activity, redox state and cell clearance. Extremely low basal permeability of the human RBC membrane to Ca2+ and a powerful Ca2+ pump maintains intracellular free Ca2+ levels between 30 and 60 nM, whereas blood plasma Ca2+ is approximately 1.8 mM. Thus, activation of Ca2+ uptake has an impressive impact on multiple processes in the cells rendering Ca2+ a master regulator in RBCs. Malfunction of Ca2+ transporters in human RBCs leads to excessive accumulation of Ca2+ within the cells. This is associated with a number of pathological states including sickle cell disease, thalassemia, phosphofructokinase deficiency and other forms of hereditary anaemia. Continuous progress in unravelling the molecular nature of Ca2+ transport pathways allows harnessing Ca2+ uptake, avoiding premature RBC clearance and thrombotic complications. This review summarizes our current knowledge of Ca2+ signalling in RBCs emphasizing the importance of this inorganic cation in RBC function and survival.

Clinical severity of β-thalassaemia/Hb E disease is associated with differential activities of the calpain-calpastatin proteolytic system

Earlier observations in the literature suggest that proteolytic degradation of excess unmatched α-globin chains reduces their accumulation and precipitation in β-thalassaemia erythroid precursor cells and have linked this proteolytic degradation to the activity of calpain protease. The aim of this study was to correlate the activity of calpain and its inhibitor, calpastatin, with different degrees of disease severity in β-thalassaemia. CD34(+) cells were enriched from peripheral blood of healthy individuals (control group) and patients with mild and severe clinical presentations of β(0)-thalassaemia/Hb E disease. By ex vivo cultivation promoting erythroid cell differentiation for 7 days, proerythroblasts, were employed for the functional characterization of the calpain-calpastatin proteolytic system. In comparison to the control group, enzymatic activity and protein amounts of μ-calpain were found to be more than 3-fold increased in proerythroblasts from patients with mild clinical symptoms, whereas no significant difference was observed in patients with severe clinical symptoms. Furthermore, a 1.6-fold decrease of calpastatin activity and 3.2-fold accumulation of a 34 kDa calpain-mediated degradation product of calpastatin were observed in patients with mild clinical symptoms. The increased activity of calpain may be involved in the removal of excess α-globin chains contributing to a lower degree of disease severity in patients with mild clinical symptoms.

Stimulation of human red blood cells leads to Ca2+-mediated intercellular adhesion

Red blood cells (RBCs) are a major component of blood clots, which form physiologically as a response to injury or pathologically in thrombosis. The active participation of RBCs in thrombus solidification has been previously proposed but not yet experimentally proven. Holographic optical tweezers and single-cell force spectroscopy were used to study potential cell-cell adhesion between RBCs. Irreversible intercellular adhesion of RBCs could be induced by stimulation with lysophosphatidic acid (LPA), a compound known to be released by activated platelets. We identified Ca(2+) as an essential player in the signaling cascade by directly inducing Ca(2+) influx using A23187. Elevation of the internal Ca(2+) concentration leads to an intercellular adhesion of RBCs similar to that induced by LPA stimulation. Using single-cell force spectroscopy, the adhesion of the RBCs was identified to be approximately 100 pN, a value large enough to be of significance inside a blood clot or in pathological situations like the vasco-occlusive crisis in sickle cell disease patients.

Characteristic properties of endoplasmic reticulum membrane m-calpain, calpastatin and lumen m-calpain: a comparative study between membrane and lumen m-calpains

Previously, we reported that bovine pulmonary smooth muscle endoplasmic reticulum (ER) membrane possesses associated m-calpain and calpastatin and ER lumen contains only m-calpain. Herein, we report characteristic properties of ER membrane m-calpain (MCp), calpastatins and lumen m-calpain (LCp) and a brief comparative study between MCp and LCp. MCp containing 80 kDa large and 28 kDa small subunit is non-phosphorylated, whereas LCp containing only 80 kDa large subunit is phosphorylated. Optimum pH, Ca(2+) concentration and pI value of both MCp and LCp are 7.5, 5 mM and 4.5, respectively. MCp and LCp have similar kinetic parameters and circular dichroism (CD) spectra. Autolysis of MCp and LCp are different. Coimmunoprecipitation studies revealed that LCp is associated with ERp57 in the ER lumen, which suggests that the regulation of LCp differs from the regulation of MCp. In presence of Ca(2+), the activated LCp cleaves inositol 1,4,5-trisphosphate receptor-1 (IP(3)R1) in the ER lumen, whereas the activated MCp cleaves Na(+)/Ca(2+) exchanger-1 (NCX1) in the ER membrane. We have determined pI (4.6 and 4.7, respectively) and IC(50) (0.52 and 0.8 nM, respectively) values of 110 and 70 kDa calpastatins. For first time, we have determined the characteristic properties, regulation and functional activity of LCp in the ER lumen.

N terminus of calpain 1 is a mitochondrial targeting sequence

The ubiquitous m- and mu-calpains are thought to be localized in the cytosolic compartment, as is their endogenous inhibitor calpastatin. Previously, mu-calpain was found to be enriched in mitochondrial fractions isolated from rat cerebral cortex and SH-SY5Y neuroblastoma cells, but the submitochondrial localization of mu-calpain was not determined. In the present study, submitochondrial fractionation and digitonin permeabilization studies indicated that both calpain 1 and calpain small subunit 1, which together form mu-calpain, are present in the mitochondrial intermembrane space. The N terminus of calpain 1 contains an amphipathic alpha-helical domain, and is distinct from the N terminus of calpain 2. Calpain 1, but not calpain 2, was imported into mitochondria. Removal of the N-terminal 22 amino acids of calpain 1 blocked the mitochondrial calpain import, while addition of this N-terminal region to calpain 2 or green fluorescent protein enabled mitochondrial import. The N terminus of calpain 1 was not processed following mitochondrial import, but was removed by autolysis following calpain activation. Calpain small subunit 1 was not directly imported into mitochondria, but was imported in the presence of calpain 1. The presence of a mitochondrial targeting sequence in the N-terminal region of calpain 1 is consistent with the localization of mu-calpain to the mitochondrial intermembrane space and provides new insight into the possible functions of this cysteine protease.

Subcellular localization and in vivo subunit interactions of ubiquitous mu-calpain

Ubiquitously expressed calpains are Ca(2+)-dependent, intracellular cysteine proteases comprising a large catalytic subunit (domains DI-DIV) and a noncovalently bound small regulatory subunit (domains DV and DVI). It is unclear whether Ca(2+)-induced calpain activation is followed by subunit dissociation or not. Here, we have applied advanced fluorescence microscopy techniques to study calpain subunit interactions in living cells using recombinant calpain subunits or domains fused to enhanced cyan and enhanced yellow fluorescent reporter proteins. All of the overexpressed variants of the catalytic subunit (DI-IV, DI-III, and DI-IIb) were active and Ca(2+)-dependent. The intact large subunit, but not its truncated variants, associates with the small subunit under resting and ionomycin-activated conditions. All of the variants were localized in cytoplasm and nuclei, except DI-IIb, which accumulates in the nucleus and in nucleoli as shown by microscopy and cell fractionation. Localization studies with mutated and chimeric variants indicate that nuclear targeting of the DI-IIb variant is conferred by the two N-terminal helices of DI. Only those variants that contain DIII migrated to membranes upon the addition of ionomycin, suggesting that DIII is essential for membrane targeting. We propose that intracellular localization and in particular membrane targeting of activated calpain, but not dissociation of its intact subunits, contribute to regulate its proteolytic activity in vivo.

Calpain in the pathophysiology of spinal cord injury: neuroprotection with calpain inhibitors

Spinal cord injury (SCI) evokes an increase in intracellular free Ca(2+) level resulting in activation of calpain, a Ca(2+)-dependent cysteine protease, which cleaves many cytoskeletal and myelin proteins. Calpain is widely expressed in the central nervous system (CNS) and regulated by calpastatin, an endogenous calpain-specific inhibitor. Calpastatin degraded by overactivation of calpain after SCI may lose its regulatory efficiency. Evidence accumulated over the years indicates that uncontrolled calpain activity mediates the degradation of many cytoskeletal and membrane proteins in the course of neuronal death and contributes to the pathophysiology of SCI. Cleavage of the key cytoskeletal and membrane proteins by calpain is an irreversible process that perturbs the integrity and stability of CNS cells leading to cell death. Calpain in conjunction with caspases, most notably caspase-3, can cause apoptosis of the CNS cells following trauma. Aberrant Ca(2+) homeostasis following SCI inevitably activates calpain, which has been shown to play a crucial role in the pathophysiology of SCI. Therefore, calpain appears to be a potential therapeutic target in SCI. Substantial research effort has been focused upon the development of highly specific inhibitors of calpain and caspase-3 for therapeutic applications. Administration of cell permeable and specific inhibitors of calpain and caspase-3 in experimental animal models of SCI has provided significant neuroprotection, raising the hope that humans suffering from SCI may be treated with these inhibitors in the near future.

The role of calpain and calpastatin in the catabolism of erythrocyte-membrane proteins during anaemia in hamsters (Mesocricetus auretus) infected with Leishmania donovani

The anaemia associated with visceral leishmaniasis is accompanied by altered Ca(2+) homeostasis and degradation of the cytoskeletal and integral proteins of the erythrocytic membrane. In the present study, such changes were followed in hamsters that were anaemic as the result of their experimental infection with Leishmania donovani. At each stage of the infection, the blood concentration of haemoglobin was found to be negatively correlated with the concentration of Ca(2+) (R(2) = 0.91), the percentage of erythrocytes with Heinz bodies (R(2) = 0.98) and thiol depletion (R(2) = 0.96) in the erythrocytes. Calpain (Ca(2+)-activated protease; EC 3.4.22.17) and its natural inhibitor calpastatin are known to regulate the catabolism of membrane structural proteins. Densitometric scanning of SDS-PAGE gels showed that erythrocytic membranes from infected hamsters contained less calpain and calpastatin than those from control animals. The level of calpain autolysis was found to increase as the infection progressed. The addition of purified calpain (from control hamsters) to erythrocyte ghosts caused greater degradation of the membranes of erythrocytes from infected animals than of the corresponding membranes from control animals. Calpastatin from the control hamsters was more effective, at inhibiting calpain-induced membrane proteolysis, than calpastatin from the infected animals. The results indicate that the Ca(2+)-activated protease and its inhibitor are involved in the degradation of erythrocytic membranes observed during visceral leishmaniasis.

The purification and characterization of mu-calpain and calpastatin from ostrich brain

Calcium-activated neutral proteinases (CANPs) and their endogenous specific inhibitor calpastatin are found in a wide variety of vertebrate and invertebrate tissues. The CANPs are cysteine proteinases that have an absolute requirement for Ca(2+) for activity. mu-Calpain and calpastatin were purified by successive chromatographic steps on Toyopearl-Super Q 650S and Pharmacia Mono Q HR 5/5 columns. The enzyme has a M(r) of 84KDa using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), a M(min) of 79KDa from amino acid analysis and an pI of 5.2. Calpastatin has a M(r) of 323KDa using denaturing gradient PAGE and a pI of 4.7. The amino acid composition of mu-calpain revealed 689 residues and the pH and temperature optima were found to be 7.5 and 37 degrees C, respectively. mu-Calpain underwent a Ca(2+)-dependent autoproteolysis producing a fragment of 82KDa. The N-terminal sequence of mu-calpain showed 24 and 18% sequence identity with human and bovine mu-calpain.

The calpain system in human placenta

The calpain system is involved in a number of human pathologies ranging from the muscular dystrophies to Alzheimer's disease. It is important, therefore, to be able to obtain and to characterize both mu-calpain and m-calpain from human tissue. Although human mu-calpain can be conveniently obtained from either erythrocytes or platelets, no readily available source of human m-calpain has been described. Human placenta extracts contain both mu-calpain and m-calpain in nearly equal proportions and in significant quantities (3-4 mg mu-calpain and 4-5 mg m-calpain/1000 g placenta tissue). Placenta also contains calpastatin that elutes off ion-exchange columns over a wide range of KCl concentrations completely masking the mu-calpain activity eluting off these columns and even partly overlapping m-calpain elution. Placenta mu-calpain requires 50-70 microM Ca2+ and placenta m-calpain requires 450-460 microM Ca2+ for half-maximal proteolytic activity. Western analysis of washed placenta tissue shows that placenta contains both mu- and m-calpain, although some of the mu-calpain in whole placenta extracts likely originates from the erythrocytes that are abundant in the highly vascularized placenta. Placenta calpastatin could not be purified with conventional methods. The most prominent form of calpastatin in Western analyses of placenta obtained as soon as possible after birth was approximately 48-51 kDa; partly purified preparations of placenta calpastatin also contained 48-51 and 70 kDa polypeptides. Human placenta extracts likely contain two different calpastatin isoforms, a 48-51 kDa "placenta calpastatin" and a 70 kDa erythrocyte calpastatin.

Characterization of membrane-bound serine protease related to degradation of oxidatively damaged erythrocyte membrane proteins

It has been shown that erythrocyte membrane proteins become susceptible to degradation by membrane-bound serine protease activity after oxidative modification of the membranes (M. Beppu, M. Inoue, T. Ishikawa, K. Kikugawa, Biochim. Biophys. Acta 1196 (1994) 81-87). The aim of the present study was to clarify the presence of the serine protease in oxidized erythrocyte membranes and to characterize the selectivity of the enzyme to oxidized proteins. Human erythrocytes were oxidized in vitro with xanthine/xanthine oxidase/Fe(III) and oxidized membranes isolated. Proteolytic activity of the membranes toward spectrin obtained from oxidized membranes and bovine serum albumin oxidized with H2O2/horseradish peroxidase was increased by membrane oxidation, and the degradability of the substrates was increased by substrate oxidation. The proteolytic activity was inhibited by the serine protease inhibitor diisopropyl fluorophosphate (DFP). The 72 kDa and 80 kDa proteins in the membranes were labeled by [3H]DFP when detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions and subsequent fluorography. The 72 kDa protein was found to be a serine enzyme, acetylcholine esterase. The 80 kDa protein appeared to be responsible for the degradation of oxidatively damaged proteins. The 80 kDa protein was loosely bound to membranes and readily solubilized into a 0.1% NP-40 detergent solution. The presence of the same 80 kDa protease in intact erythrocyte cytosol was suggested. The increased serine protease activity in oxidized membranes can result from the increased adherence of the cytosolic 80 kDa serine protease to the membranes due to oxidation.

Modulation of hypoxia-induced calpain activity in rat renal proximal tubules

The effect of the newly developed, nonpeptide, calpain inhibitor, PD 150606, on hypoxia and ionomycin-induced increases in calpain activity in rat proximal tubules (PT) was determined. PD150606 inhibited both hypoxia and ionomycin-induced calpain activity as determined by the fluorescent substrate N-succinyl-Leu-Leu-Val-Tyr-7-amido-4-methyl coumarin (N-succinyl-Leu-Leu-Val-Tyr-AMC). This decrease in calpain activity was accompanied by dose-dependent cytoprotection against hypoxia and ionomycin-induced cell membrane damage. PD150606 had no effect on cathepsin B and L activity in PT as measured by the fluorescent substrate, benzyloxycarbonyl-L-phenylalanyl-L-arginine-7-amido-4-methyl coumarin (Z-Phe-Arg-AMC). The effects of low intracellular pH (pHi) or low free cytosolic calcium [Ca2+]i on this hypoxia-induced calpain activity were also determined. Both low pHi and low [Ca2+]i attenuated the hypoxia-induced increase in calpain activity. This attenuation of calpain activity was observed early before hypoxia-induced membrane damage and was associated with marked reduction in the typical pattern of hypoxia-induced cell membrane damage observed in this model. To identify the isoform of calpain activated in rat proximal tubules, normoxic, hypoxic and ionomycin treated tubules were fractionated by MONO-Q anion exchange chromatography and the fractions were assayed for calpain activity. A single peak of calpain activity characteristic of mu-calpain was found. The calcium dependency of the calpain activity was in the nanomolar range, further confirming that the activity was the low Ca(2+)-sensitive mu-calpain. The present study suggests that in rat proximal tubules: (1) PD 150606 is a specific inhibitor of calpain and not cathepsins B and L; (2) the cytoprotective effects of low pHi and low [Ca2+]i are mediated, at least in part, by inhibition of calpain activity; and (3) the predominant active form of calpain is the isoenzyme mu-calpain.

Proteolytic processing of nuclear factor kappa B by calpain in vitro

Nuclear factor kappaB (NF-kappaB) is a transcription factor that is critical for the inducible expression of multiple cellular and viral genes. Using the electrophoretic mobility shift assay, we demonstrated that DNA binding activity of NF-kappaB was abolished by proteolysis with mu- and m-calpains in vitro. The proteolysis of NF-kappaB by calpains and hence the abolition of its DNA binding was prevented by calpastatin, calpain inhibitor I and proteasome inhibitor. We also provided evidence that calpains degrade the C-terminal domain of NF-kappaB by Western blot using anti-NF-kappaB (p65) C-terminal antibody. These observations indicate that calpains regulate gene expression through processing of NF-kappaB.

Biologically active monomeric and heterodimeric recombinant human calpain I produced using the baculovirus expression system

Calpain I is a heterodimeric protein that is part of a family of calcium-activated intracellular cysteine proteases presumed to play a role in mediating signals transduced by calcium. Expression of bioactive recombinant human calpain I has been achieved using the baculovirus expression system, by either co-infection with two viruses, each expressing one of the subunits, or infection with a single virus containing both subunits. The approximately 80 kDa catalytic subunit exhibited calcium-dependent proteolytic activity when expressed alone or with the approximately 30 kDa regulatory subunit. Baculoviral recombinant calpain I appeared fully active in that the catalytic subunit in unpurified cell extracts exhibited calcium-dependent autocatalytic cleavage at the correct locus. The amount of approximately 80 kDa subunit accumulated at steady state was greatly increased by co-expression of the approximately 30 kDa subunit, suggesting a possible role for enzyme stabilization by the latter subunit. The recombinant human calpain I was purified to near homogeneity and compared with purified native human erythrocyte calpain I. The recombinant and native enzymes had equivalent inhibition constants for structurally diverse calpain inhibitors, identical calcium activation profiles, and similar specific activities, demonstrating the suitability of using the recombinant protein for studies of the native enzyme.

Calcium-dependent neutral proteinase (calpain) in fracture healing in rats

Calpain refers to Ca(2+)-dependent neutral cysteine proteinase, which originally was thought to be an intracellular proteinase but recently has been shown to function extracellularly as well. This report describes the immunohistochemical demonstration of calpain and biochemical changes in the amount of calpain during fracture healing in rats. The tibiae of 6-week-old Wistar rats were fractured, and calluses were obtained 5-28 days after fracture. A frozen section of the fracture callus was stained by the immunoperoxidase method with use of polyclonal antibodies of calpains I and II. Positive staining was noted with the anti-calpain II antibody in the perivascular areas, chondrocytes, and cartilage matrix in calluses at 5, 7, and 10 days. Less intense staining was seen in older calluses. The caseinolytic activity of calpain II reached its maximum on the 5th day, was high on the 7th and 10th days, and decreased rapidly thereafter. The quantity of calpain II was dependent on the process of fracture healing. It was concluded that calpain was working as one of the matrix proteinases in fracture callus.

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.

Calcium-dependent cysteine proteinase (calpain) in human arthritic synovial joints

OBJECTIVE

To study the roles of calpains in the synovial joint in rheumatoid arthritis (RA) and osteoarthritis (OA) and to verify the hypothesis that calpains present in the synovial fluid come from the synovium.

METHODS

We performed immunohistochemical, biochemical, and immunoblotting analyses for calpains in synovial tissues, synovial cell cultures, and synovial fluids.

RESULTS

Immunohistochemical staining of RA synovium demonstrated specific cytoplasmic staining of cells in the synovial lining layer, storomal fibroblasts, and endothelial cells. OA synovium showed almost the same intensity and distribution of calpain staining. DEAE-cellulose chromatography of RA and OA synovial extracts and synovial fluids showed a peak of caseinolytic activity attributable to calpain, as well as an inhibitory peak of calpastatin, a specific inhibitor protein of calpains. Immunoblotting using the anticalpain antibody from the calpain peak of RA and OA synovium and synovial fluid showed identity with the heavy subunit of calpain (80 kd). Similarly, calpain existed in the same form (80 kd) in conditioned media (supernatant) obtained from synovial cell cultures, as well as in the synoviocytes. The total specific activity of the 2 calpains in the synovial fluid of RA patients was higher than that of calpastatin.

CONCLUSION

The findings suggest that the extracellular appearance of calpains could be due to the secretion of these proteins from the synovial cells and that calpains may play a role in cartilage damage of RA and OA that occurs in synovial joints.

Effect of intra-erythrocytic magnesium ions on invasion by Plasmodium falciparum

Exclusion of magnesium ions from resealed ghosts or their extraction from intact human red cells by means of an ionophore results in a reversible drop in susceptibility to invasion by Plasmodium falciparum merozoites in vitro. Resealed ghosts, containing magnesium-ATP and diluted cytosol, are invaded with high efficiency only when the original hypotonic lysis is carried out in the presence of magnesium ions. This effect is not related to the loss of membrane-associated constituents when magnesium ions are absent. Ghosts containing calcium ions, together with the protective agent, flunarizine, were essentially resistant to invasion; this effect is again at least partially reversible. A possible explanation of these phenomena is that entry of the merozoite may be inhibited by breakdown of the host cell phospholipid asymmetry, with the appearance of aminophospholipids at the outer cell surface.

Characterization of proteoglycan degradation by calpain

Degradation of cartilage proteoglycans was investigated under neutral conditions (pH 7.5) by using pig kidney calpain II (EC 3.4.22.17; Ca(2+)-dependent cysteine proteinase). Aggregate and monomer degradation reached a maximum in 5 min at 30 degrees C when the substrate/enzyme ratio was less than 1000:1. The mode of degradation was limited proteolysis of the core protein; the size of the products was larger than that of papain-digested products and comparable with that of trypsin-digested products. The hyaluronic acid-binding region was lost from the major glycosaminoglycan-bearing region after incubation with calpain II. Calpains thus may affect the form of proteoglycans in connective tissue. Ca(2+)-dependent proteoglycan degradation was unique in that proteoglycans adsorb large amounts of Ca2+ ions rapidly before activation of calpain II: 1 mg of pig cartilage proteoglycan monomer adsorbed 1.3-1.6 mu equiv. of Ca2+ ions before activation of calpain II, which corresponds to half the sum of anion groups in glycosaminoglycan side chains. This adsorption of Ca2+ was lost after solvolysis of proteoglycan monomer with methanol/50 mM-HCl, which was used to desulphate glycosaminoglycans. Therefore cartilage proteoglycans are not merely the substrates of proteolysis, but they may regulate the activation of Ca(2+)-dependent enzymes including calpains through tight chelation of Ca2+ ions between glycosaminoglycan side chains.

Variations in Ca(2+)-mediated activation of Ca(2+)-ATPase and its associated inhibitor in erythrocyte membrane

1. Two distinct patterns of Ca(2+)-mediated activation of Ca(2+)-ATPase were identified in calmodulin-depleted membranes. 2. In membranes showing no activation (type A), preincubation with micromolar concentration of cyclic AMP and ATP made possible stimulation of the enzyme while in membranes already exhibiting activation (type B), preincubation with cyclic AMP and ATP abolished the activation. 3. ATPase stimulation in type A membranes was suppressible by leupeptin. 4. Triton extractable inhibitor isolated from type A membranes was as active as that derived from type B membranes only after preincubating the membranes with cyclic AMP and ATP. 5. The inhibitor could be inactivated by alkaline phosphatase.

Immunoassay and activity of calcium-activated neutral proteinase (mCANP): distribution in soluble and membrane-associated fractions in human and mouse brain

The millimolar form of calcium-activated neutral proteinase (mCANP) is generally regarded as a cytosolic enzyme in nonneuronal systems, although its subcellular localization in brain is less well established. To resolve conflicting reports on the localization of mCANP based on activity measurements, we developed an immunoassay for CANP and compared the content and activity of the molecule in soluble and membrane fractions of mouse and human brain. Western blot immunoassays, using two different antibodies specific for mCANP, demonstrated that mCANP content is 4.5 ng/g in human or mouse brain, about 0.0005% of the total protein. More than 95% of the total immunoreactive mCANP remained in the soluble fraction after 15,000 g centrifugation of the whole homogenate. mCANP activity was determined with [14C]azocasein as substrate after removing endogenous CANP inhibitor(s) by ion-exchange chromatography on DEAE-cellulose. Caseinolytic activity was detected only in fractions derived from the supernatant extract. The distribution of mCANP content and enzyme activity were unchanged when tissues were extracted with different concentrations of Triton X-100. These findings establish the usefulness and validity of the CANP immunoassay and demonstrate that mCANP in mouse and human brain is localized predominantly within the cytosol.

The calpain-calpastatin system in mammalian cells: properties and possible functions

All mammalian cells contain a calcium-dependent proteolytic system, composed by a proteinase, calpain, and an inhibitor, calpastatin. In some cell types an activator protein has also been identified. Moreover, two calpain isoforms, distinguishable on the basis of a different calcium requirement, can be present in a single cell. Both calpain forms are heterodimers composed of a heavy subunit (80 kDa) that contains the catalytic site and a smaller (regulatory?) subunit (30 kDa). Calpain I expresses full activity at 10-50 microM Ca2+, whereas calpain II requires calcium concentrations in the millimolar range. The removal by autoproteolysis of a fragment from the N-terminus of both calpain subunits generates a proteinase form that can express catalytic activity at concentrations of Ca2+ close to the physiological range. This process is significantly accelerated in the presence of cell membranes or phospholipid vesicles. Calpastatin, the specific inhibitor of calpain, prevents activation and the expression of catalytic activity of calpain. It is in itself a substrate of the proteinase and undergoes a degradation process which correlates with the general mechanism of regulation of the intracellular proteolytic system. The natural calpain activator specifically acts on calpain II isoform, by reducing the Ca2+ required for the autoproteolytic activation process. Based on the general properties of the calpain-calpastatin system and on the substrate specificity, its role in the expression of specific cell functions can be postulated.

Calpain activation is essential for membrane fusion of erythrocytes in the presence of exogenous Ca2+

The membrane mobility agent, 2-(methoxyethoxy)ethyl-cis-8-(2-octylcyclopropyl)octanoate (A2C) promotes the fusion of rat, rabbit, and human erythrocytes in the presence of exogenous Ca2+. Under these conditions, the high sensitivity form of calcium-activated neutral protease (mu-calpain) in erythrocytes is activated autolytically. mu-Calpain is activated in accordance with fusion; that is, both erythrocyte fusion and autolytic activation of mu-calpain are induced in rat erythrocytes at 30 min, in rabbit erythrocytes at 150 min, and in human erythrocytes at 240 min after the addition of A2C and Ca2+. When erythrocytes are preincubated with the Ca2+ ionophore A23187, both fusion and autolytic activation start earlier. A leupeptin analogue, Cbz-Leu-Leu-Leu-aldehyde (ZLLLal), inhibits both the autolytic activation of mu-calpain and fusion induced by A2C and Ca2+. These results indicate that treatment of erythrocytes with A2C and Ca2+, results in first an influx of Ca2+ into the cells, followed by autolytic activation of mu-calpain, proteolysis of membrane proteins, exposure of fusion-sites, and, finally, fusion of erythrocytes.

Activation of intracellular calcium-activated neutral proteinase in erythrocytes and its inhibition by exogenously added inhibitors

Intracellular calcium-activated neutral proteinase (CANP) in rabbit erythrocytes was activated by an influx of Ca2+ into the cells. The catalytic large subunit changed from the original 79 kDa from to the 77 kDa and 76 kDa forms on activation just in the same manner as occurs in the autolytic activation of purified CANP in vitro. The activation required both extracellular Ca2+ and A23187, and was accompanied by the degradation of some membrane proteins and morphological changes in erythrocyte shape from discocytes to echinodisks, echinocytes, and spherocytes. Exogenously added Cbz-Leu-Leu-Leu-aldehyde inhibited the activation of intracellular CANP as well as the degradation of membrane proteins and the morphological changes indicating that the latter two processes are due to the action of CANP. Leupeptin and E64d were without effect on intracellular CANP.

Immunohistochemical and biochemical demonstration of calcium-dependent cysteine proteinase (calpain) in calcifying cartilage of rats

Calpain is a Ca2(+)-dependent cysteine proteinase that has neutral pH optima. There are two classes of calpains that differ in their optimal calcium ion concentration for enzymatic activity. Calpain I requires a low concentration of Ca2+ for activation, and calpain II requires a much higher Ca2+ concentration. This report describes the immunohistochemical and biochemical demonstration of calpain II in calcifying cartilage in rats and also the degradation of the cartilage proteoglycan subunit by calpain II. Immunoperoxidase (peroxidase-antiperoxidase) staining of the frozen sections of the knee joint from 3-day-old and 6-day-old Wistar rats, using polyclonal antibodies against the respective heavy subunits of calpains I and II, showed positive staining only with the anti-calpain II antibody in the hypertrophic chondrocytes and surrounding cartilaginous matrix of the growth cartilage. Diethylaminoethyl-cellulose chromatography of the cartilaginous extract from 3-day-old rats showed a peak of caseinolytic activity attributable to calpain as well as an inhibitory peak of calpastatin, a specific inhibitor protein of calpains. Immunoblotting using the anti-calpain II antibody of the calpain peak demonstrated identity with the heavy subunit of calpain II (80 kDa). Proteoglycan-degrading activity of calpain was assessed using porcine kidney calpain II and the porcine articular cartilage proteoglycan subunit. After incubation in the presence of Ca2+, degradation of proteoglycan was demonstrated by the change of the elution position on Sepharose-2B chromatography. It is possible that calpain functions as one of the proteoglycan-degrading proteolytic enzymes of growth cartilage. Intracellular localization of calpain in hypertrophic chondrocytes also suggests a role in the hypertrophic process of the chondrocyte in growth cartilage.

Characterization of a functional domain of human calpastatin

Expression plasmids were constructed from the cDNA of human calpastatin to examine the contribution to the inhibition of calpain of highly conserved sequences in each of four repetitive domains. A series of deletion derivatives of domain 1 proteins, truncated at either the amino or carboxy terminus, were produced in E. coli. Deletion from the amino terminus past the amino terminal conserved sequence decreased the inhibition. When the middle conserved sequence, the M-sequence, was further deleted, no inhibition was detected, but deletion from the carboxy terminus past the carboxy terminal conserved sequence did not decrease the inhibition until the M-sequence was reached. Nuclear magnetic resonance and circular dichroism spectra showed that domain 1 has an unfolded structure. Peptides that contained the M-sequence and some neighboring sequences were synthesized to measure the minimum size of the inhibitory peptide, which was the M-sequence with the next six residues on the amino terminal side.

Distributional and developmental variations of multiple forms of calpastatin in mouse brain

DEAE-cellulose chromatography of mouse brain extract demonstrated the occurrence of two calpastatin fractions, CS-0.1 and CS-0.2, with distinctly higher content of the latter. CS-0.1 emerged from the column at 0.1 M NaCl, inhibited calpain II more strongly than calpain I, and identified also immunologically with hitherto known calpastatin. CS-0.2 emerged at 0.2 M NaCl, inhibited calpain I more strongly than calpain II, and did not crossreact with anti-calpastatin antibody used. Fairly consistent amounts of CS-0.2 and calpain II were found in the brain of mice from 10 days to 10 weeks after birth, while CS-0.1 became measurable only after 4-week growth. In adult mice, CS-0.1 was highest in specific activity in brainstem, lower in cerebellum, and not detectable in cerebral hemisphere. Physiological significance of multiple forms of calpastatin and their variations found is not known.

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.

Identification of a new heparin-binding protein localized within chick basement membranes

A protein with a molecular mass of 19 kDa has been purified to homogeneity from 11-day-old chick embryos using a procedure involving chromatography on heparin - Sepharose, immunoaffinity resin and C4 reversed-phase. Indirect immunofluorescence studies, using polyclonal and monoclonal antibodies raised against this protein, indicate that it is essentially localized within the basement membranes in early embryonic tissues. After the 18th day of embryonic life and in post-hatched chicken, this protein could only be detected in some eye basement membranes. It appears to be bound to heparan sulfate chains of the proteoglycan present in these structures. Thus, the protein exhibits similar properties to those previously described for fibroblast growth factors (FGF), such as heparin affinity, molecular mass and localization in the basement membranes. In contrast, this protein is present in much larger amounts than FGFs, at least in 11-day-old embryos. Furthermore, the first 17 amino acid residues of the N-terminal sequence show that it does not strictly correspond to any previously described protein.

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.

Extracellular appearance of calpain and calpastatin in the synovial fluid of the knee joint

Extracellular location of calpain and calpastatin was demonstrated in the cell-free synovial fluid obtained from the knee joint of healthy adult humans and several patients with rheumatoid arthritis. Calpains I and II and a few molecular species of calpastatin were identified by chromatographies on DEAE-cellulose and on Ultrogel AcA 34 columns as well as by immunoelectrophoretic blot analysis. Calpains I and II in the synovial fluid of the patients increased 6.7 times and 3.5 times, respectively, compared with those of the control subjects. With the patients, shortening of the heavy subunits of calpains was noted. Calpastatin also increased in the patients, but it showed rather extensive fragmentation.

Proteolysis of N-ethylmaleimide-modified aldolase loaded into erythrocyte ghosts: prevention by inhibitors of calpain

1. When rabbit muscle aldolase labelled with tritium and inactivated by N-ethylmaleimide (NEM) was loaded into erythrocyte ghosts, significant proteolysis of the loaded protein occurred. The major product of this proteolysis, separated by electrophoresis under dissociating conditions, was found to be approx. 2 kDa smaller than the parent protein. 2. Proteolysis was detectable during erythrocyte ghost loading at 0 degrees C, reaching a plateau after approx. 12 min. Subsequent incubation at 37 degrees C to allow resealing of the ghosts resulted in additional proteolysis, and up to 20% of the loaded protein was converted to the smaller 38 kDa derivative. 3. EDTA, EGTA, leupeptin and chymostatin, each inhibitors of calcium-activated neutral proteinases (calpains), were the most effective inhibitors of the proteolysis of NEM-inactivated aldolase in ghosts. Other proteinase inhibitors were ineffective, while phenylmethanesulphonyl fluoride was only partially effective. 4. Inhibition of the proteolysis by EGTA was prevented by CaCl2, supporting the involvement of erythrocyte calpain. 5. Pretreatment of ghosts with EGTA prior to loading of NEM-modified aldolase followed by microinjection of the protein into HeLa cells did not result in a different rate of its overall breakdown to acid-soluble products. EGTA is suggested as a useful agent for the erythrocyte ghost-mediated microinjection of calpain-sensitive proteins.

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.

Comparison of calpain I and calpain II from carp muscle

1. The content of calpain II is 3.4 times more than that of calpain I when estimated by the elution profiles from a column of DEAE-cellulose. 2. Calpain I required 1 mM Ca2+ and calpain II required 5 mM Ca2+ to show the full activities. These data demonstrated that Ca2+-sensitivities of both calpains were lower than those of mammalian calpains, respectively. 3. The optimum caseinolytic activity was pH 7.2 for calpain I and pH 7.5 for calpain II. 4. The molecular weight of calpain I was estimated to be 110 k and that of calpain II to be 120 k by gel filtration. 5. Calpain I was much more heat-stable than calpain II around 50-60 degrees C. 6. Both calpains were sensitive to calpastatin, an endogenous inhibitor for calpain.

Ca2+-mediated catabolism of human erythrocyte band 3 protein

Catabolism of human erythrocyte membrane band 3 protein in the presence of Ca2+ was studied. An increase in the amount of a 30 kDa amino terminal fragment of band 3 was observed when erythrocyte membranes were incubated for 30 min with 1 mM Ca2+ in the presence of whole erythrosol. Incubation of the membranes with Ca2+ alone did not result in band 3 breakdown. Generation of the 30 kDa fragment from band 3 was related to the action of a leupeptin-sensitive Ca2+-dependent proteinase in the cytosol. This proteinase was also responsible for the increased production of a 52 kDa and a 70 kDa transmembrane carboxyl terminal fragment of band 3. From the size of the generated fragments, it is deduced that in the presence of Ca2+ and Ca2+-dependent proteinase, band 3 protein is cleaved at the cytoplasm/membrane interface and along its cytoplasmic domain.