Characterization of a functional domain of human calpastatin

Exploring Intrinsic Disorder in Human Synucleins and Associated Proteins

In this work, we explored the intrinsic disorder status of the three members of the synuclein family of proteins-α-, β-, and γ-synucleins-and showed that although all three human synucleins are highly disordered, the highest levels of disorder are observed in γ-synuclein. Our analysis of the peculiarities of the amino acid sequences and modeled 3D structures of the human synuclein family members revealed that the pathological mutations A30P, E46K, H50Q, A53T, and A53E associated with the early onset of Parkinson's disease caused some increase in the local disorder propensity of human α-synuclein. A comparative sequence-based analysis of the synuclein proteins from various evolutionary distant species and evaluation of their levels of intrinsic disorder using a set of commonly used bioinformatics tools revealed that, irrespective of their origin, all members of the synuclein family analyzed in this study were predicted to be highly disordered proteins, indicating that their intrinsically disordered nature represents an evolutionary conserved and therefore functionally important feature. A detailed functional disorder analysis of the proteins in the interactomes of the human synuclein family members utilizing a set of commonly used disorder analysis tools showed that the human α-synuclein interactome has relatively higher levels of intrinsic disorder as compared with the interactomes of human β- and γ- synucleins and revealed that, relative to the β- and γ-synuclein interactomes, α-synuclein interactors are involved in a much broader spectrum of highly diversified functional pathways. Although proteins interacting with three human synucleins were characterized by highly diversified functionalities, this analysis also revealed that the interactors of three human synucleins were involved in three common functional pathways, such as the synaptic vesicle cycle, serotonergic synapse, and retrograde endocannabinoid signaling. Taken together, these observations highlight the critical importance of the intrinsic disorder of human synucleins and their interactors in various neuronal processes.

Formation of the calpain-1/calpastatin complex promotes activation of calpain-1 under oxidizing conditions

Calpains are cysteine proteinases responsible for many biological roles in muscle, including protein degradation, muscle growth, and myoblast fusion. Calpains are inhibited by calpastatin, an endogenous inhibitor. Other factors, such as variations in pH, ionic strength, and oxidation influence calpain activity. This study aimed to determine the extent to which oxidation influences calpastatin inhibition of calpain-1. A series of order of addition assays were used to determine calpain-1 calcium activation and autolysis after exposure to an oxidizing agent (n-ethylmaleimide [NEM] or hydrogen peroxide [H2O2]. In the first series, purified calpastatin was added to the assay before or after oxidizing exposure at 165 mM NaCl, pH 6.5. In the second series, incubation buffer ionic strength (165 mM or 295 mM NaCl) was evaluated. The inhibitory activities of purified porcine calpastatin, purified human calpastatin domain I, or a subdomain B inhibitor peptide were evaluated in the third series. In the fourth series, a maleimide-polyethylene glycol molecule (MAL-PEG; MW = 5,000 Dalton) was used to evaluate the accessibility of free sulfhydryl groups and tagging of calpain-1 under each condition through a molecular weight shift assay. Results from this study indicate that autolysis of calpain-1, when used as an indicator of activation, occurred when the calpain-1/calpastatin complex was exposed to an oxidant or cysteine modifier such as NEM. However, when calpain-1 was exposed to the cysteine modifier before calpastatin, autolysis of calpain-1 did not occur or was significantly decreased (P < 0.05). Irreversible modification of cysteine residues by NEM prevented activation of calpain-1 in the absence of calpastatin, but if the cysteine modification is potentially reversible (H2O2), calpain-1 activity can be recovered. Results from this study indicate that when calpastatin is bound to calpain-1, calpain-1 activation can occur even after being exposed to a cysteine modifier (NEM) or hydrogen peroxide (H2O2). Calpain-1 is not tagged with maleimide-polyethylene glycol (MAL-PEG) in the presence of calpastatin, indicating that calpastatin blocks or covers free cysteines on calpain-1 from modification. Moreover, exposure to calpain-1/calpastatin complex with a cysteine modifier allows activation of calpain-1, indicating that the inhibitory action of calpastatin is compromised. These results indicate a regulatory role for calpastatin that is not inhibitory but protective for calpain-1.

Cysteine proteases as therapeutic targets: does selectivity matter? A systematic review of calpain and cathepsin inhibitors

Cysteine proteases continue to provide validated targets for treatment of human diseases. In neurodegenerative disorders, multiple cysteine proteases provide targets for enzyme inhibitors, notably caspases, calpains, and cathepsins. The reactive, active-site cysteine provides specificity for many inhibitor designs over other families of proteases, such as aspartate and serine; however, a) inhibitor strategies often use covalent enzyme modification, and b) obtaining selectivity within families of cysteine proteases and their isozymes is problematic. This review provides a general update on strategies for cysteine protease inhibitor design and a focus on cathepsin B and calpain 1 as drug targets for neurodegenerative disorders; the latter focus providing an interesting query for the contemporary assumptions that irreversible, covalent protein modification and low selectivity are anathema to therapeutic safety and efficacy.

A functional interplay between 5-lipoxygenase and μ-calpain affects survival and cytokine profile of human Jurkat T lymphocyte exposed to simulated microgravity

A growing body of evidence strongly indicates that both simulated and authentic weightlessness exert a broad range of effects on mammalian tissues and cells, including impairment of immune cell function and increased apoptotic death. We previously reported that microgravity-dependent activation of 5-lipoxygenase (5-LOX) might play a central role in the initiation of apoptosis in human T lymphocytes, suggesting that the upregulation of this enzyme might be (at least in part) responsible for immunodepression observed in astronauts during space flights. Herein, we supplement novel information about the molecular mechanisms underlying microgravity-triggered apoptotic cell death and immune system deregulation, demonstrating that under simulated microgravity human Jurkat T cells increase the content of cytosolic DNA fragments and cytochrome c (typical hallmarks of apoptosis) and have an upregulated expression and activity of µ-calpain. These events were paralleled by the unbalance of interleukin- (IL-) 2 and interferon- (INF-) γ, anti- and proapoptotic cytokines, respectively, that seemed to be dependent on the functional interplay between 5-LOX and µ-calpain. Indeed, we report unprecedented evidence that 5-LOX inhibition reduced apoptotic death, restored the initial IL-2/INF-γ ratio, and more importantly reverted µ-calpain activation induced by simulated microgravity.

Effects of genetic variants for the calpastatin gene on calpastatin activity and meat tenderness in Hanwoo (Korean cattle)

This study was designed to investigate the effects of calpastatin genotypes determined by PCR-SSCP (polymerase chain reaction-single strand conformation polymorphism) on calpastatin activity (CAC) and Warner-Bratzler Shear Force (WBS). Longissimus muscles were prepared from 379 Hanwoo bulls aged approximately 20months. The selection of PCR primers was based on exons (27 and 28) of the bovine calpastatin cDNA sequences, and genetic variants were detected by SSCP analysis using Taq I restriction enzymes. Sequencing analysis confirmed 4 restriction sites (nucleotide positions 52, 67, 796, and 1369), and a genetic variant was verified at a nucleotide position 641 (C/T substitutions) based on sequences (AF281256). The CAST28 genotypes showing allele frequencies of C (0.429) and T (0.571) were significantly associated with CAC and WBS. A significant positive residual correlation (r=0.121, P=0.02) between CAC and WBS was obtained.

Inhibition of human μ-calpain by conformationally constrained calpastatin peptides

The 27-mer peptide CP1B-[1–27] derived from exon 1B of calpastatin stands out among the known inhibitors for μ- and m-calpain due to its high potency and selectivity. By systematical truncation, a 20-mer peptide, CP1B-[4–23], was identified as the core sequence required to maintain the affinity/selectivity profile of CP1B-[1–27]. Starting with this peptide, the turn-like region Glu10(i)-Leu11(i+1)-Gly12(i+2)-Lys13(i+3) was investigated. Sequence alignment of subdomains 1B, 2B, 3B and 4B from different mammalians revealed that the amino acid residues in position i+1 and i+2 are almost invariably flanked by oppositely charged residues, pointing towards a turn-like conformation stabilized by salt bridge/H-bond interaction. Accordingly, using different combinations of acidic and basic residues in position i and i+3, a series of conformationally constrained variants of CP1B-[4–23] were synthesized by macrolactamization utilizing the side chain functionalities of these residues. With the combination of Glu(i)/Dab(i+3), the maximum of conformational rigidity without substantial loss in affinity/selectivity was reached. These results clearly demonstrate that the linear peptide chain corresponding to subdomain 1B reverses its direction in the region Glu10-Lys13upon binding to μ-calpain, and thereby adopts a loop-like rather than a tight turn conformation at this site.

Calpastatin simultaneously binds four calpains with different kinetic constants

Calpastatin is the endogenous, specific protein inhibitor of the calcium-dependent protease, calpain. Using an active site knock-out m-calpain mutant we have studied the enzyme's calcium-dependent binding to calpastatin by surface plasmon resonance without the complication of proteolysis. Calpastatin was capable of simultaneously binding four molecules of calpain. Its four inhibitory domains (CAST1, 2, 3, and 4) were individually expressed in Escherichia coli and the kinetics of their interaction with calpain was separately compared. Their K(d) values ranged from picomolar to nanomolar in the order CAST1>4>3>2. They have similar k(on) values but the k(off) values ranged over three orders of magnitude and can account for the differences in affinity.

Calpain activity in retinal degeneration

Retinal degenerations such as retinitis pigmentosa (RP) or glaucoma are a major cause of blindness in humans. Understanding the mechanisms underlying the various types of retinal degeneration is a pre-requisite for the development of rational therapies for these diseases. Activation of the calcium dependent protease, calpain, has been suggested to play an important role in cell death in various neuronal tissues including the retina. Improved detection and analysis of calpain activity during degenerative processes is likely to expand the list of pathological conditions with calpain involvement. We give a short overview of the methods available for the detection of calpain activity, and briefly discuss properties of calpain inhibitors. We then discuss the role of calpains in different cell death mechanisms and review existing work on retinal degeneration and the possible involvement of calpains therein. The implication of calpains in retinal cell death raises the possibility to use calpain inhibitors to prevent or delay retinal degeneration.

Spectroscopic Approaches to the Conformation of Tau Protein in Solution and in Paired Helical Filaments

The abnormal aggregation of the microtubule-associated protein tau into paired helical filaments is one the hallmarks of Alzheimer's disease. This aggregation is based in the partial formation of beta-structure. In contrast, the soluble protein shows a mostly random coil structure, as judged by circular dichroism, Fourier transform infrared, X-ray scattering and biochemical assays. Here, we review the basis of the natively unstructured character of tau, as well as recent studies of residual structure and long-range interactions between different domains of the protein. Analysis of the primary structure reveals a very low content of hydrophobic amino acids and a high content of charged residues, both of which tend to counteract a well-folded globular state of proteins. In the case of tau, the low overall hydrophobicity is sufficient to explain the lack of folding. This is in contrast to other proteins which also carry an excess charge at physiological pH. By tryptophan scanning mutagenesis and fluorimetry we found that most of the sequence is solvent exposed. Analysis of the hydrodynamic radii confirms a mostly random coil structure of various tau isoforms and tau domains. The proteins can be further expanded by denaturation with GdHCl which indicates some global folding. This was substantiated by a FRET-based approach where the distances between different domains of tau were determined. The combined data show that tau is mostly disordered and flexible but tends to assume a hairpin-like overall fold which may be important in the transition to a pathological aggregate.

Disruption of excitation-contraction coupling and titin by endogenous Ca2+-activated proteases in toad muscle fibres

This study investigated the effects of elevated, physiological levels of intracellular free [Ca(2+)] on depolarization-induced force responses, and on passive and active force production by the contractile apparatus in mechanically skinned fibres of toad iliofibularis muscle. Excitation-contraction (EC) coupling was retained after skinning and force responses could be elicited by depolarization of the transverse-tubular (T-) system. Raising the cytoplasmic [Ca(2+)] to approximately 1 microm or above for 3 min caused an irreversible reduction in the depolarization-induced force response by interrupting the coupling between the voltage sensors in the T-system and the Ca(2+) release channels in the sarcoplasmic reticulum. This uncoupling showed a steep [Ca(2+)] dependency, with 50% uncoupling at approximately 1.9 microm Ca(2+). The uncoupling occurring with 2 microm Ca(2+) was largely prevented by the calpain inhibitor leupeptin (1 mm). Raising the cytoplasmic [Ca(2+)] above 1 microm also caused an irreversible decline in passive force production in stretched skinned fibres in a manner graded by [Ca(2+)], though at a much slower relative rate than loss of coupling. The progressive loss of passive force could be rapidly stopped by lowering [Ca(2+)] to 10 nm, and was almost completely inhibited by 1 mm leupeptin but not by 10 microm calpastatin. Muscle homogenates preactivated by Ca(2+) exposure also evidently contained a diffusible factor that caused damage to passive force production in a Ca(2+)-dependent manner. Western blotting showed that: (a) calpain-3 was present in the skinned fibres and was activated by the Ca(2+)exposure, and (b) the Ca(2+) exposure in stretched skinned fibres resulted in proteolysis of titin. We conclude that the disruption of EC coupling occurring at elevated levels of [Ca(2+)] is likely to be caused at least in part by Ca(2+)-activated proteases, most likely by calpain-3, though a role of calpain-1 is not excluded.

Interaction of calpastatin with calpain: a review

Calpastatin is a multiheaded inhibitor capable of inhibiting more than one calpain molecule. Each inhibitory domain of calpastatin has three subdomains, A, B, and C; A binds to domain IV and C binds to domain VI of the calpains. Crystallographic evidence shows that binding of C to domain VI involves hydrophobic interactions at a site near the first EF-hand in domain VI. Sequence homology suggests that binding of A to calpain domain IV also involves hydrophobic interactions near the EF1-hand of domain IV. Neither subdomain A nor C have inhibitory activity without subdomain B, but both increase the inhibitory activity of B. Subdomain B peptides have no inhibitory activity unless they contain at least 13 amino acids, and inhibitory activity increases with the number of amino acid residues, suggesting that inhibition requires interaction over a large area of the calpain molecule. Although subdomain B inhibition kinetically is competitive in nature, subdomain B does not seem to interact with the active site of the calpains directly, but may bind to domain III of the calpains and act to block access to the active site. It is possible that subdomain B binds to calpain only after it has been activated by Ca2+.

Binding-induced folding transitions in calpastatin subdomains A and C

Calpastatin, the endogenous inhibitor of calpain, is an intrinsically unstructured protein proposed to undergo folding transitions upon binding to the enzyme. As this feature has never been experimentally tested, we have set out to characterize the conformation of two peptides corresponding to its conserved subdomains, A and C, known to interact with calpain in a Ca(2+)-dependent manner. The peptides are disordered in water but show a high propensity for alpha-helical conformation in the presence of trifluoroethanol. The conformational transition is sensitive to Ca(2+), and is clearly seen upon binding of the peptides to the enzyme. Secondary-structure prediction of all calpastatin sequences shows that the helix-forming potential within these regions is a conserved feature of the inhibitor. Furthermore, quantitative data on the binding strength of calpastatin fragments reveal that binding of the inhibitor is accompanied by a large decrease in its configurational entropy. Taken together, these observations point to significant binding-induced local folding transitions in calpastatin, in a way that ensures highly specific, yet reversible, action of the inhibitor.

The calpain system

The calpain system originally comprised three molecules: two Ca2+-dependent proteases, mu-calpain and m-calpain, and a third polypeptide, calpastatin, whose only known function is to inhibit the two calpains. Both mu- and m-calpain are heterodimers containing an identical 28-kDa subunit and an 80-kDa subunit that shares 55-65% sequence homology between the two proteases. The crystallographic structure of m-calpain reveals six "domains" in the 80-kDa subunit: 1). a 19-amino acid NH2-terminal sequence; 2). and 3). two domains that constitute the active site, IIa and IIb; 4). domain III; 5). an 18-amino acid extended sequence linking domain III to domain IV; and 6). domain IV, which resembles the penta EF-hand family of polypeptides. The single calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85 kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. Since 1989, cDNA cloning has identified 12 additional mRNAs in mammals that encode polypeptides homologous to domains IIa and IIb of the 80-kDa subunit of mu- and m-calpain, and calpain-like mRNAs have been identified in other organisms. The molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in cells is still unclear, but the calpains ostensibly participate in a variety of cellular processes including remodeling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma.

A structural model for the inhibition of calpain by calpastatin: crystal structures of the native domain VI of calpain and its complexes with calpastatin peptide and a small molecule inhibitor

The Ca(2+)-dependent cysteine protease calpain along with its endogenous inhibitor calpastatin is widely distributed. The interactions between calpain and calpastatin have been studied to better understand the nature of calpain inhibition by calpastatin, which can aid the design of small molecule inhibitors to calpain. Here we present the crystal structure of a complex between a calpastatin peptide and the calcium-binding domain VI of calpain. DIC19 is a 19 residue peptide, which corresponds to one of the three interacting domains of calpastatin, which is known to interact with domain VI of calpain. We present two crystal structures of DIC19 bound to domain VI of calpain, determined by molecular replacement methods to 2.5A and 2.2A resolution. In the process of crystallizing the inhibitor complex, a new native crystal form was identified which had the homodimer 2-fold axis along a crystallographic axis as opposed to the previously observed dimer in the asymmetric unit. The crystal structures of the native domain VI and its inhibitor PD150606 (3-(4-iodophenyl)-2-mercapto-(Z)-2-propenoic acid) complex were determined with the help of molecular replacement methods to 2.0A and 2.3A resolution, respectively. In addition, we built a homology model for the complex between domain IV and DIA19 peptide of calpastatin. Finally, we present a model for the calpastatin-inhibited calpain.

Structural determinants of the calpain inhibitory activity of calpastatin peptide B27-WT

Calpastatin is the natural specific inhibitor of calpain. Recent research has linked uncontrolled calpain activation to tissue damage after neuronal and cardiac ischemias, traumatic spine and brain injuries, as well as Alzheimer's disease and cataract formation. An imbalance between the activities of calpain and calpastatin is believed to be responsible for the pathological role of calpain. An important key to understanding calpain regulation by calpastatin is to determine, at the molecular level, how calpastatin interacts with calpain to inhibit its enzymatic activity. A 27-residue peptide (DPMSSTYIEELGKREVTIPPKYRELLA) derived from subdomain 1B of the repetitive domains of calpain, named peptide B27-WT, was previously shown to be a potent inhibitor of mu- and m-calpain. In this report, a combination of beta-alanine scanning mutagenesis and kinetic measurements was used to probe, in a quantitative, systematic, and simultaneous fashion, the relative contribution of the amino acid side chain and backbone functionalities to the overall calpain-inhibitory activity of B27-WT. The study identified two "hot spots," Leu(11)-Gly(12) and Thr(17)-Ile(18)-Pro(19), in B27-WT within which the residues critical for inhibitory function are clustered. Mutation of any one of the key residues in either of the two hot spots resulted in a dramatic loss of inhibitory activity. Furthermore, it was shown that a restricted conformation of the Leu(11)-Gly(12) and Thr(17)-Ile(18)-Pro(19) backbones is required for the peptide inhibitory function. These results suggest a plausible model in which the two hot spots are situated at or near the interface(s) of the calpain-calpastatin complex and act in a concerted fashion to inhibit calpain. The information on the specific contribution of the amide bond and side chain of each key residue to the bioactivity of B27-WT will contribute to a better understanding of the mechanism of calpain inhibition and lead to novel and effective therapies based on the specific inhibition of dysregulated or overactivated calpain.

Calpastatin exon 1B-derived peptide, a selective inhibitor of calpain: enhancing cell permeability by conjugation with penetratin

The ubiquitous calpains, mu- and m-calpain, have been implicated in essential physiological processes and various pathologies. Cell-permeable specific inhibitors are important tools to elucidate the roles of calpains in cultivated cells and animal models. The synthetic N-acetylated 27-mer peptide derived from exon B of the inhibitory domain 1 of human calpastatin (CP1B) is unique as a potent and highly selective reversible calpain inhibitor, but is poorly cell-permeant. By addition of N-terminal cysteine residues we have generated a disulfide-conjugated CP1B with the cell-penetrating 16-mer peptide penetratin derived from the third helix of the Antennapedia homeodomain protein. The inhibitory potency and selectivity of CP1B for calpain versus cathepsin B and L, caspase 3 and the proteasome was not affected by the conjugation with penetratin. The conjugate was shown to efficiently penetrate into living LCLC 103H cells, since it prevents ionomycin-induced calpain activation at 200-fold lower concentration than the non-conjugated inhibitor and is able to reduce calpain-triggered apoptosis of these cells. Penetratin-conjugated CP1B seems to be a promising alternative to the widely used cell-permeable peptide aldehydes (e.g. calpain inhibitor 1) which inhibit the lysosomal cathepsins and partially the proteasome as well or even better than the calpains.

Autoantibodies directed against the amino-terminal domain I of human calpastatin (ACAST-DI Ab) in connective tissue diseases. High levels of ACAST-DI Ab are associated with vasculitis in lupus

To identify new autoantibody populations in patients with rheumatic diseases, a cDNA expression library was immunoscreened with a rheumatoid arthritis (RA) patient's serum which contains autoantibodies binding to uncharacterized polypeptides by Western-blotting. One clone encoding the amino-terminal region (Nt) [domain L and half of domain I] of human calpastatin was selected. Different fragments of the selected cDNA were prepared and the corresponding recombinant polypeptides were produced by in vitro translation and analysed by Western blotting. Most RA sera bound to recombinant amino-terminal region and domain I but not to domain L. This prompted us to use a recombinant polypeptide corresponding to the domain I of calpastatin as the antigen in a solid-phase ELISA to test sera from patients with various systemic rheumatic diseases and healthy controls.Anti-calpastatin domain I antibodies (ACAST-DI Ab), were detected by ELISA in RA, systemic lupus erythematosus (SLE), Sjögren's syndrome and control sera at respective frequencies of 10, 9, 0 and 1%. These Ab did not have prognostic value in early RA; high levels were significantly associated with vasculitis in SLE. Antibodies reacting with the calpastatin amino-terminal region are produced during systemic rheumatic diseases and are predominantly directed against domain I. High levels of these Ab may constitute a marker of vasculitis in SLE.

Immunoaffinity purification of calpastatin and calpastatin constructs

It has been difficult to purify calpastatin without using a step involving heating to 90-100 degrees C. Preparations of calpastatin obtained after heating often contain several polypeptides that have been ascribed to proteolytic degradation. Because calpastatin is highly susceptible to proteolytic degradation and several different calpastatin isoforms can be produced by using different start sites of transcription/translation and/or alternative splicing from the single calpastatin gene, it is not clear whether the different polypeptides observed in purified calpastatin preparations are proteolytic fragments or calpastatin isoforms. It would be useful, therefore, to have a method for purifying calpastatin that does not involve heating. At low ionic strength, calpastatin from skeletal muscle extracts binds quantitatively to an immunoaffinity column made by coupling a monoclonal antibody (MAb) to the C-terminal end of calpastatin (epitope between amino acids 707 and 786) to agarose; the bound calpastatin can be eluted at pH 2.5. The C-terminal end of the calpastatin polypeptide was used because the known isoforms of calpastatin all contain domain IV. The eluted calpastatin, which retains all its calpain inhibitory activity, consists largely of a 125 kDa polypeptide (70%), and several smaller polypeptides that are labeled with a MAb to calpastatin. Expressed calpastatin constructs representing the full-length XL-IV calpastatin and domains L-IV, II-IV, III-IV, and IV also bind to the immunoaffinity column and can be purified. The immunoaffinity column is especially useful for purifying calpastatin from small tissue samples in a single step.

Epidermal growth factor induces acute matrix contraction and subsequent calpain-modulated relaxation

During wound healing, dermal fibroblasts switch from a migratory, repopulating phenotype to a contractile, matrix-reassembling phenotype. The mechanisms controlling this switch are unknown. A possible explanation is suggested by the finding that chemokines that appear late in wound repair prevent growth factor-induced cell-substratum de-adhesion by blocking calpain activation. In this study, we tested the specific hypothesis that fibroblast contraction of the matrix is promoted by a pro-repair growth factor, epidermal growth factor, and is modulated by calpain-mediated release of adhesions. We employed an isometric force transduction system designed to measure the contraction of a collagen matrix under tension by a population of NR6 fibroblasts transfected with the human epidermal growth factor receptor. By maintaining a fixed level of strain, we could monitor both the initial contraction and subsequent relaxation of the matrix. Epidermal growth factor stimulated a transient, dose-dependent increase in matrix contraction that peaked within 60 minutes and then decayed over the ensuing 3 to 6 hours. Calpain inhibitor I (ALLN) prevented epidermal growth factor-stimulated cell de-adhesion and resulted in a significantly slower decay of matrix contraction, with only a slight decrease of the peak magnitude of contraction. The mitogen-activated protein kinase kinase-1-selective inhibitor PD 98059 that blocks signaling through the extracellular signal-regulated kinase/mitogen-activated protein kinase pathway, required for epidermal growth factor receptor-mediated activation of calpain and de-adhesion, does not significantly affect the magnitude of matrix contraction within minutes of epidermal growth factor addition, but slows the decay similarly to calpain inhibition. Epidermal growth factor receptor signaling thus stimulates the complementary mechanisms of intracellular contractile force generation and calpain-mediated de-adhesion, which are known to coordinately facilitate cell migration. These findings suggest that calpain can act as a functional switch for transmission of intracellular contractile force to the surrounding matrix, with calpain-mediated de-adhesion reducing this transmission and corresponding matrix contraction. Countervailing processes that down-regulate calpain activation can, accordingly, direct the transition of cell function from locomotion to matrix contraction.

Cystatins as calpain inhibitors: engineered chicken cystatin- and stefin B-kininogen domain 2 hybrids support a cystatin-like mode of interaction with the catalytic subunit of mu-calpain

Within the cystatin superfamily, only kininogen domain 2 (KD2) is able to inhibit mu- and m-calpain. In an attempt to elucidate the structural requirements of cystatins for calpain inhibition, we constructed recombinant hybrids of human stefin B (an intracellular family 1 cystatin) with KD2 and deltaL110 deletion mutants of chicken cystatin-KD2 hybrids. Substitution of the N-terminal contact region of stefin B by the corresponding KD2 sequence resulted in a calpain inhibitor of Ki = 188 nM. Deletion of L110, which forms a beta-bulge in family 1 and 2 cystatins but is lacking in KD2, improved inhibition of mu-calpain 4- to 8-fold. All engineered cystatins were temporary inhibitors of calpain due to slow substrate-like cleavage of a single peptide bond corresponding to Gly9-Ala10 in chicken cystatin. Biomolecular interaction analysis revealed that, unlike calpastatin, the cystatin-type inhibitors do not bind to the calmodulin-like domain of the small subunit of calpain, and their interaction with the mu-calpain heterodimer is completely prevented by a synthetic peptide comprising subdomain B of calpastatin domain 1. Based on these results we propose that (i) cystatin-type calpain inhibitors interact with the active site of the catalytic domain of calpain in a similar cystatin-like mode as with papain and (ii) the potential for calpain inhibition is due to specific subsites within the papain-binding regions of the general cystatin fold.

A new alternative method to quantify residual structure in 'unfolded' proteins

Pig (pCSD1) and human (hCSD1) calpastatin domain 1 proteins were studied to characterize common features of the denatured state of proteins. These proteins were chosen for the present investigation, because pCSD1 was suggested previously to be unstructured in water even at 25 degrees C (1) [T. Konno et al., Biochim. Biophys. Acta 1342 (1997) 73-82]. hCSD1 could be expected to exhibit similar features on the basis of preliminary spectroscopic studies. In the present study, the experimental grounds for the estimate of residual structure in the unfolded state were differential scanning calorimetry heat capacity and circular dichroism (CD) measurements over the temperature range 10-80 degrees C. At selected temperatures, we studied also the effect of guanidinium hydrochloride (GdnHCl) which is known to promote further unfolding of the polypeptide chain. All other measurements were performed at pH 6 in pure water. The present results support the conclusion that the comparison of the experimentally obtained heat capacity data with theoretical heat capacity values calculated on the basis of a newly established increment system gives insight into the degree of hydration of the unfolded polypeptide chain. The percentage by which the experimental heat capacity of the unfolded polypeptide chain differs from the calculated heat capacity permits a quantitative estimate of the residual structure. This estimate is in good agreement with that based on CD absorption. The heat capacity approach has the advantage of comparing fully hydrated and partially hydrated residues in the same aqueous environment, whereas for example spectroscopic measurements, such as CD, are generally referred to the fully unfolded chain in concentrated urea or GdnHCl solutions. As the unfolded chains of pCSD1 and hCSD1 exhibit a smaller heat capacity than that calculated on the new peptide-based increment system [M. Häckel et al., J. Mol. Biol. 291 (1999) 197-213], we conclude that the residues in the unfolded polypeptide chain are less hydrated than the same residues in oligopeptides. This suboptimal hydration is the result of residual structure in the chain as observed in both CD and heat capacity measurements.

Implication of proteasome in estrogen receptor degradation

In MCF-7 breast cancer cells, estradiol (E2) and pure antiestrogen RU 58668 down-regulate the estrogen receptor (ER). Interestingly, the protein synthesis inhibitor cycloheximide (CHX) abrogated solely the effect of E2 suggesting a selective difference in the degradation of the receptor induced by estrogenic and antiestrogenic stimulations. A panel of lysosome inhibitors (i.e. bafilomycin, chloroquine, NH4Cl, and monensin), calpain inhibitors (calpastatin and PD 150606) and proteasome inhibitors (lactacystin and proteasome inhibitor I) were tested to assess this hypothesis. Among all inhibitors tested, lactacystin and proteasome inhibitor I were the sole inhibitors to abrogate the elimination of the receptor induced by both E2 and RU 58668; this selective effect was also recorded in cells prelabeled with [3H]tamoxifen aziridine before exposure to these ligands. Hence, differential sensitivity to CHX seems to be linked to the different mechanisms which target proteins for proteasome-mediated destruction. Moreover, the two tested proteasome inhibitors produced a slight increase of ER concentration in cells not exposed to any ligand, suggesting also the involvement of proteasome in receptor turnover.

Caspase-mediated fragmentation of calpain inhibitor protein calpastatin during apoptosis

Two cysteine protease families (caspase and calpain) participate in apoptosis. Here we report that the endogenous calpain inhibitor calpastatin is fragmented by caspase(s) to various extents during early apoptosis in two cell types. In anti-fas or staurosporine-treated Jurkat T-cells, the high-molecular-weight form (HMW) of calpastatin (apparent Mr 110 K) was extensively degraded to immunoreactive fragments of Mr 75 K and 30 K In apoptotic SH-SY5Y human neuroblastoma cells, HMW calpastatin was degraded to a major immunoreactive fragment of 75 K. In both cell types, fragmentation of HMW calpastatin was blocked by a caspase-specific inhibitor carbobenzoxy-Asp-CH2OC(O)-2,6-dichlorobenzene. In vitro translated HMW calpastatin was sensitive to proteolysis by recombinant caspase-1, -3, and -7. By contrast, in vitro translated LMW calpastatin (which lacks domains L and I) was cleaved into multiple fragments only by caspase-1 and was relatively resistant to caspase-3, -7, and other caspases tested. Consistently with that, purified erythroid LMW calpastatin was also highly susceptible to caspase-1 digestion. Recombinant human calpastatin spanning domain I through III (CAST(DI-III)) was found cleaved by caspase-1 at at least three sites, located in either the A or the C helix of domains I and III (ALDD137*L, LSSD203*F and ALAD404*S), while only a single site (ALDD137*L) was cleaved by caspase-3. These findings suggest that both HMW and LMW calpastatins are more vulnerable to caspase-1 than to caspase-3. Surprisingly, both erythroid LMW calpastatin and recombinant CAST(DI-III) fragmented by caspase-1 suffered only a less than twofold reduction of inhibitory activity toward calpain. We propose that the proteolysis of calpastatin in early apoptosis might have yet unidentified effects on the cross-talk between the two protease systems.

Structure and physiological function of calpains

For a long time now, two ubiquitously expressed mammalian calpain isoenzymes have been used to explore the structure and function of calpain. Although these two calpains, mu- and m-calpains, still attract intensive interest because of their unique characteristics, various distinct homologues to the protease domain of mu- and m-calpains have been identified in a variety of organisms. Some of these 'novel' calpain homologues are involved in important biological functions. For example, p94 (also called calpain 3), a mammalian calpain homologue predominantly expressed in skeletal muscle, is genetically proved to be responsible for limb-girdle muscular dystrophy type 2A. Tra-3, a calpain homologue in nematodes, is involved in the sex determination cascade during early development. PalB, a key gene product involved in the alkaline adaptation of Aspergillus nidulans, is the first example of a calpain homologue present in fungi. These findings indicate various important functional roles for intracellular proteases belonging to the calpain superfamily.

Calpains: intact and active?

Calpains are a family of calcium-dependent thiol-proteases which are proposed to be involved in many physiological processes as well as pathological conditions. Calpains are likely to be involved in processing of numerous enzymes and cytoskeletal components, thereby linking their activity to a variety of intracellular events. Although widely studied, the precise mechanism(s) involved in calpain activation and activity in vivo remain poorly understood. Initial studies suggested that calpain exists primarily as an inactive proenzyme that required autolytic cleavage for activation. It was also hypothesized that calpain associated with membrane phospholipids, serving to increase calcium sensitivity, facilitating autolytic conversion and thus activating the enzyme. These hypotheses, however, have not been universally accepted and there is increasing evidence that intact, non-autolyzed calpain is the physiologically active calpain form.

A circular dichroism study of preferential hydration and alcohol effects on a denatured protein, pig calpastatin domain I

Pig calpastatin domain I (CSD1), a proteinase inhibitor that specifically blocks activity of calpain I and II, is a good candidate protein for studying conformational variations in the denatured form of protein. An extensive structural characterization of CSD1 reported in the first part of this work has shown that CSD1 at neutral pH is in an expanded and flexible conformation without secondary and tertiary structures. Next, we further studied cosolvent effects of protein-stabilizers, polyols and sulfate salts, as well as protein-destabilizers, alcohols, on the conformation of CSD1 monitored by far- and near-UV CD spectroscopy. We found that both groups of cosolvents at high concentration induce highly helical structures of CSD1, but without specific tertiary interactions. Based on the results on the polyols and the sulfate salts, we have suggested that the preferential hydration is one of the thermodynamic forces to induce secondary structures in the denatured state of protein. Variations in isodichroic points of changes in far-UV CD spectrum as functions of cosolvent species and their concentration have exhibited complexity of the processes. The present study implies that protein stability in the presence of cosolvents is often determined by free energy difference between the folded and the highly structured denatured state, not between the folded and the random state.

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.

Preference of calcium-dependent interactions between calmodulin-like domains of calpain and calpastatin subdomains

Calpastatin molecule contains four repeated inhibition domains, each having highly conserved internal regions A, B and C. The synthetic oligopeptides of regions A and C had no calpain inhibition activity while region B oligopeptide showed weak inhibition activity. Real-time biomolecular interaction analysis using a BIAcore instrument revealed that the bacterially expressed calmodulin-like domain of the calpain large subunit (L-CaMLD) and that of the small subunit (S-CaMLD) interacted, in a Ca(2+)-dependent fashion, preferentially with the immobilized synthetic oligopeptide of region A and that of region C, respectively. Calmodulin showed no specific binding to these oligopeptides. The tripartite structure of the calpastatin functional domain may confer the specific interactions with the protease domain and the two CaMLDs of calpain.

Unphosphorylated and tyrosine-phosphorylated forms of a focal adhesion protein, paxillin, are substrates for calpain II in vitro: implications for the possible involvement of calpain II in mitosis-specific degradation of paxillin

Cell-to-substratum adhesion becomes weakened during mitosis of the cell cycle in fibroblasts. The level of one focal adhesion protein, paxillin, is greatly reduced in mitotic-arrested cells. We show here the possible involvement of calpain II, known to be localized in focal adhesion plaques, in the degradation of paxillin. Paxillin is tyrosine-phosphorylated during interphase of the cell cycle by protein tyrosine kinases (PTK) such as c-Src and Csk, and becomes dephosphorylated during mitosis. Our data, however, indicate that tyrosine phosphorylation of paxillin does not affect the rate of paxillin degradation by calpain in vitro.

Additional peptidyl diazomethyl ketones, including biotinyl derivatives, which affinity-label calpain and related cysteinyl proteinases

Calpain, the calcium-activated cysteinyl proteinase, can be irreversibly inactivated by peptidyl diazomethyl ketones in which the peptide portion contains a penultimate leucine residue. Some new derivatives of this type have been synthesized and examined for their rates of inactivation of chicken gizzard and human platelet calpain. Two derivatives containing a C-terminal biotin residue, Biot-Aca-Leu-TyrCHN2 and Biot-Aca-Leu-Leu-TyrCHN2, have also been prepared in the expectation that their application to the study of the function of calpain and related proteases will prove fruitful.

Structure of the active 27-residue fragment of human calpastatin

A synthetic 27-residue peptide corresponding to exon 1B of the endogenous inhibitor calpastatin contains a well-conserved region and has an ability to inhibit the cysteine endopeptidase calpain specifically. We examined the solution structure of this peptide in DMSO-d6 by two-dimensional 1H NMR spectroscopy. Although regular secondary structures such as alpha-helix and beta-sheet were not found, the region from Ile18 to Arg23 formed a well-defined structure with a type I beta-turn. This region coincided well with the highly conserved region of calpastatin. The result strongly suggests that this turn structure is essential for the inhibitory activity of calpastatin.

Direct measurement of calpastatin subtypes by sandwich enzyme immunoassay using monoclonal antibodies

Six stable hybridoma cell lines secreting monoclonal antibodies to human calpastatin were established. All monoclonal antibodies belong to the IgG1 subclass and recognized different epitopes on calpastatin. At least two groups were distinguished; the first group was specific for muscle-type (M-) calpastatin and the second group recognized not only M-calpastatin but also erythrocyte-type (E-) calpastatin. The inhibitory effect of all monoclonal antibodies on calpastatin activity was relatively low even at high concentrations of antibodies. Enzyme immunoassay systems were developed for direct determination of calpastatin subtypes in human cells requiring no other sample treatment than the disruption of the cells. The assay methods were, in principle, based on the sandwich enzyme immunoassay using epitope-specific monoclonal antibodies. The enzyme immunoassay system for M-calpastatin was specific for M-calpastatin and could not detect E-calpastatin. The enzyme immunoassay system for total calpastatin detected not only M-calpastatin but also E-calpastatin. The sensitivity of these assay systems was 10 pmol l-1 of calpastatins. Antigenicity of calpastatins was found to be unchanged in the presence of EDTA and haemoglobin. Good reproducibilities of within-and between-assay series and excellent recovery of exogenous calpastatins from cell lysates were observed. From these results, it seems that our newly developed subtype-specific enzyme immunoassay systems for calpastatins are useful in biochemical studies and clinical testing for determination of calpastatin subtypes.