Mechanism of action of a new component of the Ca(2+)-dependent proteolytic system in rat brain: the calpain activator

The Critical Role of Calpain in Cell Proliferation

Calpain is a conserved family of calcium-dependent, cytosolic, neutral cysteine proteases. The best characterized members of the family are the ubiquitously expressed calpain 1 and calpain 2. They perform controlled proteolysis of their target proteins. The regulation of these enzymes includes autolysis, calcium, phosphorylation as a posttranslational modification, and binding of calpastatin, phospholipids or activator proteins, respectively. Calpain are implicated in many physiological and pathological processes. They have significant role in the cell proliferation, differentiation and migration in a variety of mammalian cell types, contributing to the development of angiogenesis, vascular remodeling, and cancer. Therefore the knowledge of the precise mechanism of calpain signaling could provide therapeutic approaches in these processes.

Effect of radiation on the development of immature hippocampal neurons in vitro

Little is known about the direct biological effects of radiation on immature neurons, despite its relevance to the mental retardation caused by irradiation of the brains of fetuses and children. In this study, we investigated the effects of radiation using primary cultured hippocampal neuronal cells with exclusion of glial cells, focusing on cell survival and structural development. Primary neurons were prepared from the hippocampi of fetal rats at embryonic day 18 and cultured according to Banker's methods. After incubation for 7 days, cells were irradiated with X rays and incubated continuously for 7 or 14 days. The number of neurons, their rate of apoptosis, and the patterns of expression of synaptic proteins on the neural dendrites were investigated by immunohistochemical methods. The total numbers of neurons were the same regardless of whether they were irradiated. The number of TUNEL-positive neurons, which can be considered as undergoing apoptosis, increased significantly in a dose-dependent fashion at both 7 and 14 days after irradiation. The mean numbers of clusters of synaptic proteins on neural dendrites, which are considered to represent their developmental level, decreased dose-dependently at both 7 and 14 days after irradiation. These results suggest that radiation not only induces apoptosis but also produces structural defects in the surviving neurons that may directly suppress neural development.

Cellular responses to environmental contaminants in amoebic cells of the slime mould Dictyostelium discoideum

Amoebic Dictyostelium discoideum cells were employed in a bioassay to evaluate stress responses after exposures to the polyaromatic hydrocarbon benzo[a]pyrene (B[a]P) and two heavy metals (copper and mercury). Furthermore, we developed a recombinant cell line expressing a labile Green Fluorescent Protein (GFP) variant expressed under the control of an actin promoter to monitor stress-related protein degradation. Finally, cell viability was monitored to discriminate lethal exposure concentrations. The results demonstrated that exposure to sub-micromolar concentrations of mercury rendered significant changes in all studied physiological parameters, whereas B[a]P became toxic at low micromolar, and copper at high micromolar concentrations. Exposure to 0.5 microM mercury significantly reduced lysosomal membrane stability (LMS), endocytosis rate, GFP expression, and further resulted in the elevation of cytosolic free Ca(2+) ([Ca(2+)](i)). LMS in mercury-treated cells that had been pre-incubated with a specific Ca(2+)-dependent phospholipase A2 blocking agent was however not affected by the exposure, indicating that the toxic action of mercury is linked to the activation of phospholipase A2 via a Ca(2+)-signaling pathway. Exposure to 20 microM B[a]P significantly reduced LMS, endocytosis rate, and GFP expression, however without affecting [Ca(2+)](i), suggesting a calcium-independent route of toxicity for this compound. None of the physiological parameters were significantly affected by copper exposure at concentrations <400 microM, demonstrating a high resistance to this metal. Our results further showed that neither cell growth nor viability was affected by concentrations altering the studied physiological parameters. LMS, endocytosis rate, and [Ca(2+)](I), therefore, appear sensitive biomarkers of pollutant-related stress in amoebic cells.

Subcellular mobility of the calpain/calpastatin network: an organelle transient

Calpain (Cp) is a calcium (Ca(2+))-dependent cysteine protease. Activation of the major isoforms of Cp, CpI and CpII, are required for a number of important cellular processes including adherence, shape change and migration. The current concept that cytoplasmic Cp locates and associates with its regulatory subunit (Rs) and substrates as well as translocates throughout the cell via random diffusion is not compatible with the spatial and temporal constraints of cellular metabolism. The novel finding that Cp and Rs function relies upon tenacious hydrophobic interactions with organelle membranes offers a unifying explanation for the paradoxical and puzzling features of Cp activation and regulation such as how nM concentrations of intracellular Ca(2+) can activate Cp molecules requiring muM to mM concentrations of Ca(2+) for in vitro activation, and how this protease can spatially and temporally locate specific substrates and translocate throughout the cell. We hypothesize that Cp and its regulatory moieties associate with organelles to facilitate the activation of this protease resulting in the cleavage of substrates and aid in its translocation throughout the cell.

DUK114, the Drosophila orthologue of bovine brain calpain activator protein, is a molecular chaperone

UK114, the goat liver tumour antigen, is a member of a widely distributed family of conserved low-molecular-mass proteins (YER057c/YjgF/UK114), the function of which is ill understood. To the various orthologues diverse functions have been ascribed, such as translation inhibition, regulation of purine repressor or calpain activation. Owing to a limited sequence similarity to Hsp90 (heat-shock protein 90), they have also been proposed to be molecular chaperones; however, this has never been tested. In the present paper, we report the cloning and characterization of the Drosophila orthologue, DUK114. In brief, DUK114 had no effect that would have qualified it as a calpain activator. In contrast, it proved to be a very potent molecular chaperone in in vitro assays. In a heat-aggregation test, it significantly decelerated the formation of citrate synthase aggregates. In a reverse assay, the recovery of the enzyme from urea- and heat-induced denatured states was accelerated almost 3-fold. On a molar basis, the chaperone activity of the 15-kDa DUK114 is comparable with that of Hsp90, the almost 6-times-larger archetypal molecular chaperone. In similar assays, DUK114 was ineffective with Drosophila calpain A or calpain B. To test for its chaperone activity in vivo, DUK114 was transfected into Schneider (S2) cells; after heat shock, the number of viable non-transfected cells started to increase after a lag time; in the presence of DUK114, cell proliferation started at once. Our work is the first experimental evidence that DUK114, and possibly other members of this family, are molecular chaperones.

The intriguing Ca2+ requirement of calpain activation

Mammalian ubiquitous micro- and m-calpains, as well as their Drosophila homologs, Calpain A and Calpain B, are Ca(2+)-activated cytoplasmic proteases that act by limited proteolysis of target proteins. Calpains are thought to be part of many cellular signaling pathways. These enzymes, however, require such high Ca(2+) concentration for half-maximal activation in vitro, [Ca(2+)](0.5), that hardly ever occurs in intact cells. This major dilemma has pervaded the literature on calpains for decades. In this paper several considerations are put forward that challenge the orthodox view and envisage mechanisms that may govern calpain action in vivo. The "unphysiologically" high Ca(2+) demand for activation may turn out to be an evolutionarily adjusted safety device.

Differential Compartmentalization of the Calpain/Calpastatin Network with the Endoplasmic Reticulum and Golgi Apparatus

Calpain, a calcium-activated cysteine protease, is involved in modulating a variety of cell activities such as shape change, mobility, and apoptosis. The two ubiquitous isoforms of this protease, calpain I and II, are considered to be cytosolic proteins that can translocate to various sites in the cell. The activity of calpain is modulated by two regulatory proteins, calpastatin, the specific endogenous inhibitor of calpain, and the 28-kDa regulatory subunit. Using velocity gradient centrifugation, the results of this study confirm and greatly expand upon our previous finding that the calpain/calpastatin network is associated with the endoplasmic reticulum and Golgi apparatus in cells. Moreover, confocal microscopy demonstrates that calpain II colocalizes with specific proteins found in these organelles. Additional experiments reveal that hydrophobic rather than electrostatic interactions are responsible for the association of the calpain/calpastatin network with these organelles. Treatment of the organelles with Na2CO3 or deoxycholate reveal that calpain I, 78-kDa calpain II, and the regulatory subunit are "embedded" within the organelle membranes similar to integral membrane proteins. Proteinase K treatment of the organelles shows that calpain I and II, calpastatin, and the regulatory subunit localize to the cytosolic surface of the organelle membranes, and a subset of calpain II and the regulatory subunit are also found within the lumen of these organelles. These results provide a new and novel explanation for how the calpain/calpastatin network is organized in the cell.

The role of calpain in oncotic cell death

Numerous lines of evidence demonstrate that calpains, a family of 14 Ca(2+)-activated neutral cysteine proteases, are involved in oncotic cell death in a variety of models. At this time, the biochemistry of most calpains and the specific roles of different calpains in physiology and pathology remain to be determined. A number of calpain substrates have been identified in cellular systems, including cytoskeletal proteins, and recent studies suggest that calpains mediate the increase in plasma membrane permeability to ions and the progressive breakdown of the plasma membrane observed in oncosis through the proteolysis of cystokeletal and plasma membrane proteins. Further, a number of reports provide evidence that the mitochondrial dysfunction observed in oncosis may be mediated by a mitochondrial calpain of unknown identity. Finally, a number of diverse calpain inhibitors have been developed that show cytoprotective properties in cellular systems and in vivo following diverse insults. It is suggested that future research be directed toward elucidation of the role(s) of specific calpain isozymes in physiological and pathological conditions; identifying and linking specific calpain substrates with altered cellular functions; and developing cell-permeable, potent, isozyme-selective calpain inhibitors.

A second binding site revealed by C-terminal truncation of calpain small subunit, a penta-EF-hand protein

The subunits in calpain and in the related penta-EF-hand (PEF) proteins are bound through contacts between the unpaired EF-hand 5 from each subunit. To study subunit binding further, a tetra-EF-hand 18 kDa N- and C-terminally truncated form of the calpain small subunit was prepared (18k). This protein does not combine with the calpain large subunit to form active calpain, but forms homodimers in solution, as shown by ultracentrifugation. The X-ray structure of the 18k protein in the presence of cadmium was solved to a resolution of 2.0 A. The structure of the monomer is almost identical to the known structure of the calpain small subunit, but the 18k protein forms an oligomer in the crystal by the use of two binding sites. One of these sites is an artefact arising from the C-terminal truncation, but the other is a naturally occurring site that is fully exposed to water in intact purified calpain. The characteristics of this site suggest that it may be important in binding other protein modulators involved in the regulation of calpain and of PEF proteins.

Association of the calpain/calpastatin network with subcellular organelles

The calcium-activated cysteine protease calpain is intimately involved in modulating cell adhesion and migration. The two ubiquitous isoforms of this protease, calpain I and II, are considered to be cytosolic proteins that can translocate to both focal complexes/adhesions or the plasma membrane. Using confocal microscopy and isopycnic density centrifugation, the results demonstrate that calpain I and II, the 30kDa regulatory subunit, and calpastatin associate with the endoplasmic reticulum and Golgi apparatus. Confocal microscopy reveals that calpain II colocalizes with the subcellular proteins calnexin and Rab6 in cells bound to laminin. To further verify this association, cell lysates prepared from laminin stimulated and unstimulated cells were subjected to isopycnic density centrifugation. The results reveal an increased association of calpain I, II, calpastatin, and the 30kDa regulatory subunit with the endoplasmic reticulum and Golgi apparatus as evidenced by their position in the gradient relative to calnexin, Rab6, caveolin, and beta1 integrin after laminin stimulation. This correlates with the accumulation of inducible calpain activity at the endoplasmic reticulum-Golgi apparatus interface. Further experiments established that calpain II colocalizes with phosphatidylinositol 4,5-bisphosphate. Finally, calpain II associates with membrane lipid rafts. These results provide new insights into how the calpain/calpastatin network is spatially and temporally regulated in cells binding to the extracellular matrix.

Distinct cleavage patterns of normal and pathologic forms of alpha-synuclein by calpain I in vitro

Parkinson's disease (PD) is characterized by fibrillary neuronal inclusions called Lewy bodies (LBs) consisting largely of alpha-synuclein (alpha-syn), the protein mutated in some patients with familial PD. The mechanisms of alpha-syn fibrillization and LB formation are unknown, but may involve aberrant degradation or turnover. We examined the ability of calpain I to cleave alpha-syn in vitro. Calpain I cleaved wild-type alpha-syn predominantly after amino acid 57 and within the non-amyloid component (NAC) region. In contrast, calpain I cleaved fibrillized alpha-syn primarily in the region of amino acid 120 to generate fragments like those that increase susceptibility to dopamine toxicity and oxidative stress. Further, while calpain I cleaved wild-type alpha-syn after amino acid 57, this did not occur in mutant A53T alpha-syn. This paucity of proteolysis could increase the stability of A53T alpha-syn, suggesting that calpain I might protect cells from forming LBs by specific cleavages of soluble wild-type alpha-syn. However, once alpha-syn has polymerized into fibrils, calpain I may contribute to toxicity of these forms of alpha-syn by cleaving at aberrant sites within the C-terminal region. Elucidating the role of calpain I in the proteolytic processing of alpha-syn in normal and diseased brains may clarify mechanisms of neurodegenerative alpha-synucleinopathies.

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.

Calpastatin subdomains A and C are activators of calpain

The inhibitory domains of calpastatin contain three highly conserved regions, A, B, and C, of which A and C bind calpain in a strictly Ca(2+)-dependent manner but have no inhibitory activity whereas region B inhibits calpain on its own. We synthesized the 19-mer oligopeptides corresponding to regions A and C of human calpastatin domain I and tested their effect on human erythrocyte mu-calpain and rat m-calpain. The two peptides significantly activate both calpains: the Ca(2+) concentration required for half-maximal activity is lowered from 4.3 to 2.4 microm for mu-calpain and from 250 to 140 microm for m-calpain. The EC(50) concentration of the peptides is 7.5 microm for mu-calpain and 25 microm for m-calpain. It is noteworthy that at low Ca(2+) concentrations (1-2 microm for mu-calpain and 70-110 microm for m-calpain) both enzymes are activated about 10-fold by the peptides. Based on these findings, it is suggested that calpastatin fragments may have a role in calpain activation in vivo. Furthermore, these activators open new avenues to cell biological studies of calpain function and eventually may alleviate pathological states caused by calpain malfunction.

Calpains mediate acute renal cell death: role of autolysis and translocation

The goals of this study were to determine 1) the expression of calpain isoforms in rabbit renal proximal tubules (RPT); 2) calpain autolysis and translocation, and calpastatin levels during RPT injury; and 3) the effect of a calpain inhibitor (PD-150606) on calpain levels, mitochondrial function, and ion transport during RPT injury. RT-PCR, immunoblot analysis, and FITC-casein zymography demonstrated the presence of only mu- and m-calpains in rabbit RPT. The mitochondrial inhibitor antimycin A decreased RPT mu- and m-calpain and calpastatin levels in conjunction with cell death and increased plasma membrane permeability. No increases in either mu- or m-calpain were observed in the membrane nor were increases observed in autolytic forms of either mu- or m-calpain in antimycin A-exposed RPT. PD-150606 blocked antimycin A-induced cell death, preserved calpain levels in antimycin A-exposed RPT, and promoted the recovery of mitochondrial function and active Na+ transport in RPT after hypoxia and reoxygenation. The present study suggests that calpains mediate RPT injury without undergoing autolysis or translocation, and ultimately they leak from cells subsequent to RPT injury/death. Furthermore, PD-150606 allows functional recovery after injury.

Domain II of m-calpain is a Ca(2+)-dependent cysteine protease

Calpain, a Ca(2+)-dependent cytosolic cysteine protease, proteolytically modulates specific substrates involved in Ca(2+)-mediated intracellular events, such as signal transduction, cell cycle, differentiation, and apoptosis. The 3D structure of m-calpain, in the absence of Ca(2+), revealed that the two subdomains (domains IIa and IIb) of the protease domain (II) have an 'open' conformation, probably due to interactions with other domains. Although the presence of an EF-hand structure was once predicted in the protease domain, no explicit Ca(2+)-binding structure was identified in the 3D structure. Therefore, it is predicted that if the protease domain is excised from the calpain molecule, it will have a Ca(2+)-independent protease activity. In this study, we have characterized a truncated human m-calpain that consists of only the protease domain. Unexpectedly, the proteolytic activity was Ca(2+)-dependent, very weak, and not effectively inhibited by calpastatin, a calpain inhibitor. Ca(2+)-dependent modification of the protease domain by the cysteine protease inhibitor, E-64c, was clearly observed as a SDS-PAGE migration change, indicating that the conformational changes of this domain are a result of Ca(2+) binding. These results suggest that the Ca(2+) binding to domain II, as well as to domains III, IV, and VI, is critical in the process of complete activation of calpain.

Matrix vesicles and media vesicles as nonclassical pathways for the secretion of m-Calpain from MC3T3-E1 cells

Calpain was generally believed to exist and function only in the cytoplasm. However, m-calpain has now been detected in the extracellular spaces of some kinds of tissue. In this study, we demonstrated the existence of m-calpain in the medium surrounding MC3T3-E1 cultures, and its activity by zymography. At the same time, the amount of lactate dehydrogenase in medium of MC3T3-E1 culture was extremely low compared with other cell cultures, suggesting that m-calpain found in the culture medium of MC3T3-E1 cells originated mainly from active secretion. Moreover, the secretion of m-calpain was not blocked by brefeldin A, implying that m-calpain may be secreted by a nonclassical pathway. Recently, MC3T3-E1 has been reported to produce matrix vesicles and media vesicles, and we demonstrated m-calpain in these vesicles produced by MC3T3-E1 cultures. We therefore concluded that these vesicles are partly responsible for the secretion of m-calpain into the culture medium of MC3T3-E1 cells.

The pathogenic activation of calpain: a marker and mediator of cellular toxicity and disease states

Over-activation of calpain, a ubiquitous calcium-sensitive protease, has been linked to a variety of degenerative conditions in the brain and several other tissues. Dozens of substrates for calpain have been identified and several of these have been used to measure activation of the protease in the context of experimentally induced and naturally occurring pathologies. Calpain-mediated cleavage of the cytoskeletal protein spectrin, in particular, results in a set of large breakdown products (BDPs) that are unique in that they are unusually stable. Over the last 15 years, measurements of BDPs in experimental models of stroke-type excitotoxicity, hypoxia/ischemia, vasospasm, epilepsy, toxin exposure, brain injury, kidney malfunction, and genetic defects, have established that calpain activation is an early and causal event in the degeneration that ensues from acute, definable insults. The BDPs also have been found to increase with normal ageing and in patients with Alzheimer's disease, and the calpain activity may be involved in related apoptotic processes in conjunction with the caspase family of proteases. Thus, it has become increasingly clear that regardless of the mode of disturbance in calcium homeostasis or the cell type involved, calpain is critical to the development of pathology and therefore a distinct and powerful therapeutic target. The recent development of antibodies that recognize the site at which spectrin is cleaved has greatly facilitated the temporal and spatial resolution of calpain activation in situ. Accordingly, sensitive spectrin breakdown assays now are utilized to identify potential toxic side-effects of compounds and to develop calpain inhibitors for a wide range of indications including stroke, cerebral vasospasm, and kidney failure.

Properties and intracellular localization of calpain activator protein

In this paper, we have further analyzed the properties of calpain activator (CA) in order to better define its physiological function. The activator shows a pH optimum approximately 7.8-8.0, independently of the nature of the buffer used. Although the maximal activity is observed with human acid-denatured globin, the effect of CA is detectable with other protein substrates, such as casein and insulin. A comparable activating effect is observed also with the synthetic substrate Succ-Leu-Tyr-AMC. The activatory effect has been evaluated in a reconstructed system, using plasma membrane Ca(2+)-ATPase as substrate. CA is localized in erythrocyte precursor cells on the inner surface of the plasma membrane in very high amount and its level profoundly decreases up to 10% of the original value when cells reach the terminal differentiated state.

Elevated subsarcolemmal Ca2+ in mdx mouse skeletal muscle fibers detected with Ca2+-activated K+ channels

Duchenne muscular dystrophy results from the lack of dystrophin, a cytoskeletal protein associated with the inner surface membrane, in skeletal muscle. The cellular mechanisms responsible for the progressive skeletal muscle degeneration that characterizes the disease are still debated. One hypothesis suggests that the resting sarcolemmal permeability for Ca(2+) is increased in dystrophic muscle, leading to Ca(2+) accumulation in the cytosol and eventually to protein degradation. However, more recently, this hypothesis was challenged seriously by several groups that did not find any significant increase in the global intracellular Ca(2+) in muscle from mdx mice, an animal model of the human disease. In the present study, using plasma membrane Ca(2+)-activated K(+) channels as subsarcolemmal Ca(2+) probe, we tested the possibility of a Ca(2+) accumulation at the restricted subsarcolemmal level in mdx skeletal muscle fibers. Using the cell-attached configuration of the patch-clamp technique, we demonstrated that the voltage threshold for activation of high conductance Ca(2+)-activated K(+) channels is significantly lower in mdx than in control muscle, suggesting a higher subsarcolemmal [Ca(2+)]. In inside-out patches, we showed that this shift in the voltage threshold for high conductance Ca(2+)-activated K(+) channel activation could correspond to a approximately 3-fold increase in the subsarcolemmal Ca(2+) concentration in mdx muscle. These data favor the hypothesis according to which an increased calcium entry is associated with the absence of dystrophin in mdx skeletal muscle, leading to Ca(2+) overload at the subsarcolemmal level.

Acyl-CoA-binding protein is a potent m-calpain activator

Acyl-CoA-binding protein, a 20-kDa homodimer that exerts many physiological functions, promotes activation of the classic calpain forms, most markedly that of the m-isozyme. This protein factor was purified from rat skeletal muscle and was also expressed in Escherichia coli. Both native and recombinant acyl-CoA-binding proteins show the same molecular properties and an identical capacity to decrease the [Ca(2+)] required for m-calpain activity. The binding of long-chain acyl-CoAs to acyl-CoA-binding protein does not modify the activating effect on calpains. Acyl-CoA-binding protein seems to be involved in the m-calpain regulation process, whereas the previously identified UK114 activator is a specific modulator of micro-calpain. Acyl-CoA-binding protein is proposed as a new component of the Ca(2+)-dependent proteolytic system. A comparative analysis among levels of classic calpains and their activator proteins is also reported.