Characterization of the calpastatin defect in erythrocytes from patients with essential hypertension

Calcium-dependent cleavage of the Na(+)/Ca(2+) exchanger by m-calpain in isolated endoplasmic reticulum

We have recently demonstrated the localization of associated m-calpain and calpastatin in the endoplasmic reticulum (ER) of bovine pulmonary artery smooth muscle. Herein, we sought to determine the role of m-calpain on calcium-dependent proteolytic cleavage of Na(+)/Ca(2+) exchanger (NCX) in the ER. Treatment of the ER with Ca(2+) (5 mM) dissociates m-calpain-calpastatin association leading to the activation of m-calpain, which subsequently cleaves the ER integral transmembrane protein NCX1 (116 kDa) to an 82 kDa fragment. Pre-treatment of the ER with calpain inhibitors, calpeptin (10 microM) or MDL28170 (10 microM), or Ca(2+) chelator, EGTA (10 mM) does not cleave NCX1. In vitro cleavage of the ER purified NCX1 by the ER purified m-calpain also supports our finding. Cleavage of NCX1 by m-calpain in the ER may be interpreted as the main cause of intracellular Ca(2+) overload in the smooth muscle, which could be important for the manifestation of pulmonary hypertension.

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.

Normal erythrocyte calpain I activity on membrane proteins under near-physiological conditions in patients with essential hypertension

CONTEXT

It has been reported that the equilibrium between the erythrocyte protease calpain I and its physiological inhibitor calpastatin is disrupted in patients with essential hypertension.

OBJECTIVE

To investigate the activity of non-purified calpain I in hemolysates against the erythrocytic membrane proteins, rather than against other substrates.

DESIGN

Evaluation of calpain I red cell activity upon its own physiological substrates in hypertensive patients, in a near-physiological environment.

SETTING

LIM-23 and LIM-40 of Hospital das Clinicas of the Faculty of Medicine of USP.

SAMPLE

Patients with moderate primary hypertension over 21 years of age who were given amlodipine (n:10) and captopril (n:10) for 8 weeks, plus normal controls (n:10).

MAIN MEASUREMENTS

Red cell membrane proteins were incubated with and without protease inhibitors and with and without calcium chloride and underwent polyacrylamide gel electrophoresis.

RESULTS

Digestion of bands 2.1 and 4.1 was observed, indicating calpain I activity. No statistical differences regarding bands 2.1 and 4.1 were observed before treatment, between the controls and the hypertensive patients, either in ghosts prepared without calcium or with increasing concentrations of calcium. Nor were statistical differences observed after treatment, between the controls and the patients treated with amlodipine and captopril, or between the patients before and after treatment with both drugs.

CONCLUSION

The final activity of non-purified calpain I upon its own physiological substrate, which was the approach utilized in this study, may more adequately reflect what happens in red cells. Under such conditions no imbalance favoring calpain I activity increase was observed. The protective factor provided by calpastatin against calpain I activity may diminish under hypertension.

Increased calpain expression is associated with apoptosis in rat spinal cord injury: calpain inhibitor provides neuroprotection

Calpain content was investigated in the lesion of rat spinal cord at 1, 4, 24, and 72 h following injury induced by the weight-drop (40 g-cm force) technique. Calpain content was increased in the lesion, and was highest at 24 h following injury. microCalpain mRNA level in the lesion was increased by 58.4% (p = 0.0135) at 24 h following trauma, compared to sham. Alterations in mRNA expression in the lesion increased bax/bcl-2 ratio by 20.8% (p = 0.0395) at this time point, indicating a commitment to apoptosis. Therapeutic effect of the calpain inhibitor E-64-d (1 mg/kg) was studied in SCI rats following administration for 24 h. Internucleosomal DNA fragmentation (apoptosis) was observed in SCI rats, but not in sham or E-64-d treated rats. These results indicate a new information that E-64-d has the therapeutic potential for inhibiting apoptosis in SCI.

Calpain upregulation in spinal cords of mice with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinson's disease

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a heroin analogue, is a neurotoxin that undergoes in vivo oxidation by monoamine oxidase-B (MAO-B) to 1-methyl-4-phenylpyridinium ion (MPP+) which preferentially exerts its toxic effects on the dopaminergic neurons of the substantia nigra in brain. Spinal interneuronal pathways are also likely to be affected in the course of MPP+ neurotoxicity. The primary effect of MPP+ is mediated by irreversible inhibition of mitochondrial complex I, releasing free radicals. MPP+ may also activate N-methyl-D-aspartate (NMDA) receptors, increasing the cytosolic concentration of free Ca2+. Intracellular free radicals indirectly and free Ca2+ directly can activate Ca2+-dependent proteases such as calpain. We investigated involvement of calpain in spinal cord degeneration due to neurotoxin by subjecting male C57BL/6N mice (17 months old) to MPTP administration (12.5 mg/kg for 0.5 h; 25 mg/kg for 0.25 h; and 50 mg/kg for 0.25, 0.5, 1, 2, and 24 h). RT-PCR and Western blot analysis were performed using the thoracic segment of spinal cords from control and MPTP-administered mice. The administration of MPTP caused calpain upregulation at the mRNA and protein levels to various extents, compared to control mice. Calpain activity was measured by 68 kDa neurofilament protein (NFP) degradation, which was increased in MPTP-induced PD mice. These results suggest that calpain may play a role in spinal cord degeneration in mice with MPTP-induced PD.

Differential degradation of calpastatin by mu- and m-calpain in Ca(2+)-enriched human neuroblastoma LAN-5 cells

In neuroblastoma LAN-5 cells during calpain activation, in addition to the two expressed 70 kDa and 30 kDa calpastatin forms, other inhibitory species are produced, having molecular masses of 50 kDa and 15 kDa. At longer times of incubation, both native and new calpastatin species disappear. The formation of these new calpastatins as well as the decrease in intracellular total calpastatin activity are mediated by calpain itself, as indicated by the effect of the synthetic calpain inhibitor I, which prevents both degradative processes. Analysis of the calcium concentrations required for the two processes indicates that the first conservative proteolytic event is mediated by micro-calpain, whereas the second one is preferentially carried out by m-calpain. The appearance of the 15 kDa form, containing only the calpastatin repetitive inhibitory domain and identified also in red cells of hypertensive rats as the major inhibitor form, can be considered a marker of intracellular calpain activation, and it can be used for the monitoring of the involvement of calpain in pathological situations.

E-64-d prevents both calpain upregulation and apoptosis in the lesion and penumbra following spinal cord injury in rats

Calpain, a Ca(2+)-dependent cysteine protease, has been implicated in cytoskeletal protein degradation and neurodegeneration in the lesion and adjacent areas following spinal cord injury (SCI). To attenuate apoptosis or programmed cell death (PCD) in SCI, we treated injured rats with E-64-d, a cell permeable and selective inhibitor of calpain. SCI was induced on T12 by the weight-drop (40 g-cm force) method. Within 15 min, E-64-d (1 mg/kg) in 1.5% DMSO was administered i.v. to the SCI rats. Following 24 h treatment, a 5-cm long spinal cord section with the lesion in the center was collected. The spinal cord section was divided equally into five 1-cm segments (S1: distant rostral, S2: near rostral, S3: lesion or injury, S4: near caudal and S5: distant caudal) for analysis. Determination of mRNA levels by reverse transcriptase-polymerase chain reaction (RT-PCR) indicated that ratios of bax/bcl-2 and calpain/calpastatin were increased in spinal cord segments from injured rats compared to controls. Degradation of the 68-kD neurofilament protein and internucleosomal DNA fragmentation were also increased. All of these changes were maximally increased in the lesion and gradually decreased in the adjacent areas of SCI rats, while largely undetectable in E-64-d treated rats and absent in sham controls. The results indicate that apoptosis in rat SCI appears to be associated with calpain activity which can be attenuated by the calpain inhibitor E-64-d.

Calpeptin and methylprednisolone inhibit apoptosis in rat spinal cord injury

Intracellular free Ca2+ and free radicals are increased following spinal cord injury (SCI). These can activate calpain to degrade cytoskeletal proteins leading to apoptotic and necrotic cell death. Primary injury triggers a cascade of secondary injury, which spreads to rostral and caudal areas. We tested calpain involvement in apoptosis in five 1-cm segments of rat spinal cord with injury (40 g-cm) induced at T12 by weight-drop. Animals were immediately treated with calpeptin (250 micrograms/kg) and methylprednisolone (165 mg/kg) and sacrificed at 48 hr. Untreated SCI rats manifested 68-kD neurofilament protein (NFP) degradation (indicating calpain activity), and internucleosomal DNA fragmentation (indicating apoptosis). Both calpain activity and apoptosis were highest in the lesion, and decreased with increasing distance from the lesion. Treatment decreased 68-kD NFP degradation with reduction in apoptosis in all five areas. Thus, calpeptin and methylprednisolone are found to be neuroprotective in SCI.

Oxidative stress and Ca2+ influx upregulate calpain and induce apoptosis in PC12 cells

Calpain, a Ca2+-dependent cysteine protease, has previously been implicated in apoptosis or programmed cell death (PCD) in immune cells. Although oxidative stress and intracellular free Ca2+ are involved in neurodegenerative diseases, the mechanism of neuronal cell death in the central nervous system (CNS) due to these agents has not yet been defined. To explore a possible role for calpain in neuronal PCD under oxidative stress and Ca2+ influx, we examined the effects of H2O2 and A23187 on PC12 cells. Treatments caused PCD (light microscopy and TUNEL assay) with altered mRNA expression (RT-PCR) of bax (pro-apoptotic) and bcl-2 (anti-apoptotic) genes, resulting in a high bax/bcl-2 ratio. Control cells expressed 1.3-fold more microcalpain (requiring microM Ca2+) than mcalpain (requiring mM Ca2+). Expression of mcalpain was significantly increased following exposure to oxidative stress and Ca2+ influx. The mRNA levels of calpastatin (endogenous calpain inhibitor) and beta-actin (house-keeping) genes were not changed. Western analysis indicated degradation of 68 kDa neurofilament protein (NFP), a calpain substrate. Pretreatment of cells with MDL28170 (a cell permeable and selective inhibitor of calpain) prevented increase in bax/bcl-2 ratio, upregulation of calpain, degradation of 68 kDa NFP, and occurrence of PCD. These results suggest a role for calpain in PCD of PC12 cells due to oxidative stress and Ca2+ influx.

Rat brain contains multiple mRNAs for calpastatin

This work was undertaken to establish the forms of the calpain inhibitor, calpastatin, expressed in the brain tissue. Five cDNA clones were obtained and the corresponding amino acid sequences were deduced. Three of these proteins contain an N-terminal domain (domain L) and four inhibitory repeats typical of the calpastatin molecule. The other two are truncated forms, containing the domain L, free or associated with a single inhibitory repeat. Other differences, due to exon skipping, produce calpastatin forms with different susceptibility to posttranslational modifications. The more represented mRNA form corresponds to a calpastatin molecule containing the four inhibitory domains. These results may be useful to understand the involvement of calpain in the onset of acute and degenerative disorders of the central nervous system.

Purification of active calpain by affinity chromatography on an immobilized peptide inhibitor

Most purification schemes of calpain (CANP) involve a number of chromatographic steps. The final preparations often contain impurities, including degradation fragments. Two peptide-affinity columns were developed, using peptides of 27 amino acids and 30 amino acids, corresponding to the products of exons 1B and 1C, respectively, of the natural inhibitor (calpastatin) gene, coupled to CNBr-activated Sepharose 4B. Crude preparations of calpain, isolated by anion-exchange chromatography on a DEAE-Sepharose column, were incubated with a reversible or an irreversible synthetic inhibitor which blocks the catalytic subunit of the enzyme in the inactive 80-kDa form. The crude preparation was then loaded onto the peptide column in the presence of calcium. Calpain was eluted with an EGTA-containing buffer. Using the two peptide-affinity columns connected in tandem, calpain was isolated with a high degree of purity, suitable for structural and mechanistic studies, i.e. as an 80/30-kDa heterodimer or in the form of dissociated monomers.

PEST sequences do not influence substrate susceptibility to calpain proteolysis

Mutations lowering the PEST score of domains surrounding the calmodulin (CaM)-binding region of the plasma membrane Ca(2+)-ATPase failed to influence the susceptibility of the enzyme to mu-calpain (mu-CANP). Synthetic peptides corresponding to the high PEST score C-terminal sequences A18 and B28 had no effect on the rate of pump proteolysis by mu-CANP, i.e. the peptides did not compete for a putative high PEST score recognition site for mu-CANP in the pump molecule. An accessible CaM-binding region appears to be critical for substrate (i.e. the Ca2+ pump) proteolysis and probably also for its recognition by mu-CANP; phosphorylation of the CaM-binding domain of the pump or its occupation by CaM significantly decreased the rate of proteolysis.

Different susceptibility of red cell membrane proteins to calpain degradation

The presence of low levels of calpastatin activity in erythrocytes of hypertensive rats affects regulation of calpain activity so it is highly susceptible to activation within physiological fluctuations in [Ca2+]. Under identical conditions, in red cells of normotensive rats, calpain activation is efficiently controlled by the high levels of calpastatin activity, and a progressive increase in proteinase activity can only be observed in parallel with a decrease in the level of calpastatin. In intact erythrocytes from hypertensive rats exposed to small variations in [Ca2+], degradation of anion transport protein (band 3) and Ca(2+)-ATPase appears as a primary event indicating that these two transmembrane proteins are probably early recognized as targets of intracellular calpain activity. Furthermore, band 3 protein seems to be structurally modified in erythrocytes from hypertensive rats, as indicated by its increased susceptibility to degradation in the presence of 10-50 microM Ca2+. In addition, when exposed to progressive and limited increases in [Ca2+], erythrocytes from hypertensive rats, but not those from normotensive rats, show a high degree of fragility that can be restored to normal values by inhibition of calpain. These results indicate that, within fluctuations in [Ca2+] close to physiological values, regulation of calpain activity is efficiently accomplished in normal erythrocytes but is completely lost in cells from hypertensive animals. Regulation is of critical importance in maintaining normal structural and functional properties of selective red cell membrane and cytoskeletal proteins, among which band 3 and Ca(2+)-ATPase appear to be the substrates with highest susceptibility to digestion by calpain.

Respiratory burst in activated neutrophils is directly correlated to the intracellular level of protein kinase C

The production of superoxide anion in human and rat neutrophils is directly correlated to the level of protein kinase C. Such correlation has been established on a comparative basis by analysis of neutrophils from normal and hypertensive subjects, characterized by an increased amount of protein kinase C, and of neutrophils from normal and genetically hypertensive rats characterized by low amounts of the kinase. Protein kinase C activity in all these different populations of neutrophils is modulated by specific inhibitors in an identical dose-dependent fashion which results in a linearly correlated decrease in O2- production. Taken together, these results provide a direct demonstration that in neutrophils the intracellular level of protein kinase C represents one of the determinants of the rate and extent of O2- production.

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.