The calpain-calpastatin system in rheumatoid arthritis

Calpain-calpastatin system and cancer progression

The calpain system is required by many important physiological processes, including the cell cycle, cytoskeleton remodelling, cellular proliferation, migration, cancer cell invasion, metastasis, survival, autophagy, apoptosis and signalling, as well as the pathogenesis of a wide range of disorders, in which it may function to promote tumorigenesis. Calpains are intracellular conserved calcium-activated neutral cysteine proteinases that are involved in mediating cancer progression via catalysing and regulating the proteolysis of their specific substrates, which are important signalling molecules during cancer progression. μ-calpain, m-calpain, and their specific inhibitor calpastatin are the three molecules originally identified as comprising the calpain system and they contain several crucial domains, specific motifs, and functional sites. A large amount of data supports the roles of the calpain-calpastatin system in cancer progression via regulation of cellular adhesion, proliferation, invasion, metastasis, and cellular survival and death, as well as inflammation and angiogenesis during tumorigenesis, implying that the inhibition of calpain activity may be a potential anti-cancer intervention strategy targeting cancer cell survival, invasion and chemotherapy resistance.

Proteomic characterization of platelet gel releasate from adult peripheral and cord blood

PURPOSE

Activated platelet gel (PG) derived from adult peripheral blood (APB) has been extensively used for topical therapy of various clinical conditions. Conversely, few observations on PG from umbilical cord blood (CB) have been reported so far. Although PG preparations are known to contain a high concentration of a large number of biological factors involved in inflammation and tissue repair, their comprehensive characterization is still missing. The innovative goal of our research was to use proteomics technologies in order to profile biologically active components in these blood derivatives.

EXPERIMENTAL DESIGN

Supernatants recovered from three independent APB and CB-derived PGs, prepared using batroxobin, were enriched for low-abundance proteins with ProteoMiner and subsequently analyzed by GeLC-MS/MS.

RESULTS

The 751 and 760 proteins were identified in the APB and CB-derived PG releasates, respectively. A core dataset including only proteins found in 2/3 and 3/3 biological replicates was generated and functionally characterized by gene ontology. Searching against Vesiclepedia database showed that 33% of our dataset consists of novel releasate proteins. Comparison between the two types of PG secretomes revealed that 117 proteins are present only in the APB-derived samples, 104 proteins are distinctive of the CB-derived samples, and 229 are in common.

CONCLUSION AND CLINICAL RELEVANCE

Our study highlighted a differential content of proteins supporting tissue repair and regeneration between APB and CB-derived PGs. These findings may help better identifying future appropriate clinical applications.

Reducing C-terminal-truncated alpha-synuclein by immunotherapy attenuates neurodegeneration and propagation in Parkinson's disease-like models

Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are common neurodegenerative disorders of the aging population, characterized by progressive and abnormal accumulation of α-synuclein (α-syn). Recent studies have shown that C-terminus (CT) truncation and propagation of α-syn play a role in the pathogenesis of PD/DLB. Therefore, we explored the effect of passive immunization against the CT of α-syn in the mThy1-α-syn transgenic (tg) mouse model, which resembles the striato-nigral and motor deficits of PD. Mice were immunized with the new monoclonal antibodies 1H7, 5C1, or 5D12, all directed against the CT of α-syn. CT α-syn antibodies attenuated synaptic and axonal pathology, reduced the accumulation of CT-truncated α-syn (CT-α-syn) in axons, rescued the loss of tyrosine hydroxylase fibers in striatum, and improved motor and memory deficits. Among them, 1H7 and 5C1 were most effective at decreasing levels of CT-α-syn and higher-molecular-weight aggregates. Furthermore, in vitro studies showed that preincubation of recombinant α-syn with 1H7 and 5C1 prevented CT cleavage of α-syn. In a cell-based system, CT antibodies reduced cell-to-cell propagation of full-length α-syn, but not of the CT-α-syn that lacked the 118-126 aa recognition site needed for antibody binding. Furthermore, the results obtained after lentiviral expression of α-syn suggest that antibodies might be blocking the extracellular truncation of α-syn by calpain-1. Together, these results demonstrate that antibodies against the CT of α-syn reduce levels of CT-truncated fragments of the protein and its propagation, thus ameliorating PD-like pathology and improving behavioral and motor functions in a mouse model of this disease.

Calpain-1 inhibitors for selective treatment of rheumatoid arthritis: what is the future?

Effective small-molecule treatment of inflammatory diseases remains an unmet need in medicine. Current treatments are either limited in effectiveness or invasive. The latest biologics prevent influx of inflammatory cells to damaged tissue. Calpain-1 is a calcium-activated cysteine protease that plays an important role in neutrophil motility. It is, therefore, a potential target for intervention in inflammatory disease. Many inhibitors of calpains have been developed but most are unselective and so unsuitable for drug use. However, recent series of α-mercaptoacrylate inhibitors target regulatory domains of calpain-1 and are much more specific. These compounds are effective in impairing the cell spreading mechanism of neutrophils in vitro and raise the possibility of treating rheumatoid arthritis with a pill; however, challenges still remain. Improved bioavailability is needed and solution of their precise mode of action should prompt the development of specific calpain-1 screens for novel classes of inhibitors.

The calpain inhibitor calpeptin prevents bleomycin-induced pulmonary fibrosis in mice

Pulmonary fibrosis is characterized by progressive worsening of pulmonary function leading to a high incidence of death. Currently, however, there has been little progress in therapeutic strategies for pulmonary fibrosis. There have been several reports on cytokines being associated with lung fibrosis, including interleukin (IL)-6 and transforming growth factor (TGF)-β1. We reported recently that two substances (ATRA and thalidomide) have preventive effects on pulmonary fibrosis by inhibiting IL-6-dependent proliferation and TGF-β1-dependent transdifferentiation of lung fibroblasts. Rheumatoid arthritis is a chronic autoimmune disorder, and its pathogenesis is also characterized by an association with several cytokines. It has been reported that calpain, a calcium-dependent intracellular cysteine protease, plays an important role in the progression of rheumatoid arthritis. In this study, we examined the preventive effect of Calpeptin, a calpain inhibitor, on bleomycin-induced pulmonary fibrosis. We performed histological examinations and quantitative measurements of IL-6, TGF-β1, collagen type Iα1 and angiopoietin-1 in bleomycin-treated mouse lung tissues with or without the administration of Calpeptin. Calpeptin histologically ameliorated bleomycin-induced pulmonary fibrosis in mice. Calpeptin decreased the expression of IL-6, TGF-β1, angiopoietin-1 and collagen type Iα1 mRNA in mouse lung tissues. In vitro studies disclosed that Calpeptin reduced (i) production of IL-6, TGF-β1, angiopoietin-1 and collagen synthesis from lung fibroblasts; and (ii) both IL-6-dependent proliferation and angiopoietin-1-dependent migration of the cells, which could be the mechanism underlying the preventive effect of Calpeptin on pulmonary fibrosis. These data suggest the clinical use of Calpeptin for the prevention of pulmonary fibrosis.

HLA-DRB1*0404 is strongly associated with anticalpastatin antibodies in rheumatoid arthritis

OBJECTIVE

To test whether HLA-DR alleles influence the production of particular autoantibodies in rheumatoid arthritis (RA) patients, we screened synovial proteins with sera of RA patients homozygous for different HLA-DR alleles by using 2D blots. We found that sera of RA patients homozygous for HLA-DRB1*0404 recognised a 100-kDa synovial protein identified as calpastatin. We studied B and T cell epitopes on calpastatin and their association with HLA-DRB1*0404.

METHODS

The frequency of positive sera in patients expressing different RA-associated HLA-DR allele combinations was calculated by inhouse ELISA using purified synovial calpastatin or calpastatin peptides encompassing the entire calpastatin protein as immunosorbent. Interaction between calpastatin peptides and HLA-DR alleles was tested by a direct binding assay. T cell responses to calpastatin were measured in RA patients and controls.

RESULTS

We found that RA-associated HLA-DR alleles are associated with presence of autoantibodies to synovial calpastatin in RA patients' sera. HLA-DRB1*0404 is strongly associated with antisynovial calpastatin in RA sera. One linear B cell epitope is preferentially associated with HLA-DRB1*0404. Multiple peptides from calpastatin bind every tested HLA-DR allele associated or not with RA. Peptides from domain 1 and 4 of calpastatin are the best HLA-DR allele binders. The T cell response to calpastatin is frequent in RA patients and independent of the HLA-DR background.

CONCLUSIONS

HLA-DRB1*0404 is strongly associated with anticalpastatin antibodies in rheumatoid arthritis.

Auto-antibodies and autoreactive T-cells in rheumatoid arthritis

Rheumatoid arthritis (RA) is not only the most severe of all joint diseases but also the most common systemic autoimmune disease affecting approximately 1% of the world-wide adult population. RA is characterized by the presence of autoantibodies in serum and synovial fluid distinguishing the disease from other arthritides such reactive arthritis or osteoarthritis. Since the historical description of rheumatoid factor (RF), which is an autoantibody directed to immunoglobulin G, numerous additional autoantibodies have been discovered in sera of RA patients. These antibodies may be directed to cartilage components, stress proteins, enzymes, nuclear proteins and, most importantly, citrullinated proteins such as fibrin or vimentin. In contrast to other antibodies including RF, anti-citrullinated protein antibodies are targeted almost exclusively by RA patients thus being the most specific serological markers of RA. Even though most other antibodies are not used for diagnostics, they may contribute to the patholophysiology of RA by forming immune complexes in the joint. Furthermore, autoreactive T-cells in serum and synovial fluid may initiate or enhance the disease process via production of proinflammatory cytokines leading to autoantibody secretion, stimulation of macrophages and activation of bone resorbing osteoclasts. Identification of novel autoantigens, particularly citrullinated proteins, and the characterization of the cellular and molecular processes underlying the autoimmune reactions against them has provided new insights into the complex pathogenesis of RA. This has made possible the development of novel therapeutic concepts that may allow to treat the disease more effectively in its early stages where the chances are highest to interrupt the deleterious processes.

Involvement of the ERK/MAP kinase signalling pathway in milli-calpain activation and myogenic cell migration

Recent research carried out in our laboratory has shown that IGF-1, TGF-beta1, and insulin were able to strongly stimulate myoblast migration by increasing milli-calpain expression and activity. However, the signalling pathways involved in these phenomena remain unknown. The aim of this study was to identify the signalling pathway(s) responsible for the effects of IGF-1, TGF-beta1, and insulin on myoblast migration and on milli-calpain expression and activity. For this purpose, wound healing assays were carried out in the presence of growth factors with or without specific inhibitors of ERK/MAP kinase and PI3K/Akt pathways. The results clearly showed that the inhibition of the ERK/MAP kinase pathway prevents the effects of growth factors on myoblast migration. Secondly, the expression and the activity of milli-calpain were studied in cells treated with growth factor, alone or with ERK/MAP kinase inhibitor. The results demonstrated that the up-regulation of milli-calpain expression and activity was mediated by the ERK/MAP kinase pathway. Finally, the possible implication of MyoD and myogenin, myogenic regulatory factors able to regulate milli-calpain expression, was studied. Taken together our results clearly showed that the ERK/MAP kinase signalling pathway is responsible for the effects of the three growth factors on myoblast migration and on milli-calpain expression and activity. On the opposite, the PI3K/Akt signalling pathway, MyoD and myogenin seem to be not implicated in these phenomena.

Autoantibodies in rheumatoid arthritis: a review

Emerging insights into the importance of B cells in the pathogenesis of rheumatoid arthritis (RA) as highlighted by the efficacy of B cell depletion is one factor that has contributed to the upsurge of interest in the potential role of autoantibodies both as disease markers and with respect to a pathogenic role. Since the initial description of rheumatoid factor (RF), a large number of both disease-specific and non-specific autoantibodies have been described in patients with RA including antibodies to type II collagen (CII), immunoglobulin binding protein (BiP) and antibodies directed at citrullinated peptides (anti-CCP) and other citrullinated proteins such as vimentin (anti-Sa) . Despite some overlap the serological profile of RA does appear to be distinct from other diseases such as SLE . Although the precise mechanisms responsible for the formation of these antibodies have not been well defined their presence must reflect the interaction between T and B cells believed to be relevant to the pathogenesis of RA. The specificity of the association of such factors as anti-CCP and anti-BiP with RA may reflect unique pathogenic events leading to the processing and presentation of the "cryptic self" . Ease of measurement and stability make autoantibodies attractive diagnostic and prognostic markers particularly in early disease when it may be difficult to distinguish self-limiting synovitis from persistent disease . The purpose of this article is to provide an overview of the current state of knowledge of the spectrum of autoantibodies thus far characterised in individuals with rheumatoid arthritis, and discuss their diagnostic, prognostic and pathogenetic relevance.

Calpain activation and secretion promote glomerular injury in experimental glomerulonephritis: evidence from calpastatin-transgenic mice

Glomerular injury and albuminuria in acute glomerulonephritis are related to the severity of inflammatory process. Calpain, a calcium-activated cysteine protease, has been shown to participate in the development of the inflammatory process. Therefore, for determination of the role of calpain in the pathophysiology of acute glomerulonephritis, transgenic mice that constitutively express high levels of calpastatin, a calpain-specific inhibitor protein, were generated. Wild-type mice that were subjected to anti-glomerular basement membrane nephritis exhibited elevated levels of calpain activity in kidney cortex at the heterologous phase of the disease. This was associated with the appearance in urine of calpain activity, which originated potentially from inflammatory cells, abnormal transglomerular passage of plasma proteins, and tubular secretion. In comparison with nephritic wild-type mice, nephritic calpastatin-transgenic mice exhibited limited activation of calpain in kidney cortex and limited secretion of calpain activity in urine. This was associated with less severe glomerular injury (including capillary thrombi and neutrophil activity) and proteinuria. There was a reduction in NF-kappaB activation, suggesting that calpain may participate in inflammatory lesions through NF-kappaB activation. There also was a reduction in nephrin disappearance from the surface of podocytes, indicating that calpain activity would enhance proteinuria by affecting nephrin expression. Exposure of cultured podocytes to calpain decreased nephrin expression, and, conversely, exposure of these cells to calpastatin prevented TNF-alpha from decreasing nephrin expression, demonstrating a role for the secreted form of calpain. Thus, both activation and secretion of calpains participate in the development of immune glomerular injury.

Involvement of calpains in growth factor-mediated migration

Previous research in our laboratory has already shown the importance of the role played by ubiquitous calpains during myoblast migration. The aim of this study was to investigate calpain expression during myoblast migration and, to enhance this phenomenon via calpain stimulation. Ubiquitous calpains are members of a large family of calcium-dependent cysteine proteases. They play an important role in numerous biological and pathological phenomena, such as signal transduction, apoptosis, cell-cycle regulation, cell spreading, adhesion, invasion, myogenesis, and motility. Myoblast migration is a crucial step in myogenesis, as it is necessary for myoblast alignment and fusion to form myotubes. This study started by examining changes in calpain expression during migration, then investigated the possibility of activating myoblast migration via the stimulation of calpain expression and/or activity. The migration rate of myoblasts overexpressing mu- or milli-calpain was quantified. The results showed that calpain overexpression dramatically inhibited myoblast migration. Growth-factor treatments were then used to enhance myoblast migration. The results showed that treatment with IGF-1, TGF-beta1, or insulin induced a major increase in migration and caused a significant increase in m-calpain expression and activity. The increase in migration was totally inhibited by adding calpeptin, a calpain-specific inhibitor. These findings suggest that milli-calpain is involved in growth factor-mediated migration.

Analysis of in vivo role of alpha-fodrin autoantigen in primary Sjogren's syndrome

The alpha-fodrin N-terminal portion (AFN) autoantigen mediates in vivo immunoregulation of autoimmune responses in primary Sjögren's syndrome (SS). We further examined this process and found that cleavage products of AFN were frequently detected in the salivary gland duct cells of SS patients. In in vitro studies using human salivary gland HSY cells, anti-Fas-induced apoptosis resulted in specific cleavage of alpha-fodrin into the 120-kd fragment, in association of alpha-fodrin with mu-calpain, and activation of caspase 3. Significant proliferative responses against AlphaFN autoantigen were observed in the peripheral blood mononuclear cells (PBMCs) from SS patients with higher pathological score (grade 4) and with short duration from onset (within 5 years). In vivo roles of AFN peptides were investigated using PBMCs from patients with SS, systemic lupus erythematosus, and rheumatoid arthritis. Significant proliferative T-cell responses of PBMCs to AFN peptide were detected in SS but not in systemic lupus erythematosus or rheumatoid arthritis. AFN peptide induced Th1-immune responses and accelerated down-regulation of Fas-mediated T-cell apoptosis in SS. Our data further elucidate the in vivo role of AFN autoantigen on the development of SS and suggest that the AFN autoantigen is a novel participant in peripheral tolerance.

Amelioration of experimental arthritis by a calpain-inhibitory compound: regulation of cytokine production by E-64-d in vivo and in vitro

Calpain, a calcium-dependent cysteine proteinase, has been reported to participate in the pathophysiology of rheumatoid arthritis (RA). The aim of this study is to investigate the therapeutic efficacy of calpain-inhibitory compounds in an animal model of RA and to clarify the underlying mechanisms in vivo and in vitro. Arthritis was induced in BALB/c mice with anti-type II collagen mAbs and LPS, and the mice were treated intra-peritoneally with a high dose (9 mg kg(-1) per day) or low dose (3 mg kg(-1) per day) of E-64-d (a membrane-permeable cysteine proteinase inhibitor) or control diluent. As a result, a high dose of E-64-d significantly alleviated the clinical arthritis and the histopathological findings, compared with the control diluent, although a low dose of E-64-d did not have a significant effect. Next, we evaluated the effects of E-64-d on cytokine mRNA expression at the inflamed joints by quantitative reverse transcription-PCR. High dose of E-64-d significantly decreased IL-6 and IL-1beta mRNA levels at the inflamed joints. The regulatory effects of E-64-d on cytokine production were also confirmed in vitro, using a synovial cell line (E11) and crude synoviocytes derived from RA patients. These results suggest the key roles of calpain in the pathophysiology of arthritis and that calpain-inhibitory compounds might be applicable to the treatment of arthritic diseases such as RA.

Rheumatoid arthritis specific anti-Sa antibodies target citrullinated vimentin

Antibodies directed to the Sa antigen are highly specific for rheumatoid arthritis (RA) and can be detected in approximately 40% of RA sera. The antigen, a doublet of protein bands of about 50 kDa, is present in placenta and in RA synovial tissue. Although it has been stated that the Sa antigen is citrullinated vimentin, experimental proof for this claim has never been published. In this study, we investigated the precise nature of the antigen. Peptide sequences that were obtained from highly purified Sa antigen were unique to vimentin. Recombinant vimentin, however, was not recognized by anti-Sa reference sera. In vivo, vimentin is subjected to various post-translational modifications, including citrullination. Since antibodies to citrullinated proteins are known to be highly specific for RA, we investigated whether Sa is citrullinated and found that Sa indeed is citrullinated vimentin. Anti-Sa antibodies thus belong to the family of anticitrullinated protein/peptide antibodies. The presence of the Sa antigen in RA synovial tissue, and the recent observation that vimentin is citrullinated in dying human macrophages, make citrullinated vimentin an interesting candidate autoantigen in RA and may provide new insights into the potential role of citrullinated synovial antigens and the antibodies directed to them in the pathophysiology of RA.

The calpains in aging and aging-related diseases

Calpains are a family of calcium-dependent cysteine proteases under complex cellular regulation. By making selective limited proteolytic cleavages, they modulate the activity of enzymes, including key signaling molecules, and induce specific cytoskeletal rearrangements, accounting for their roles in cell motility, signal transduction, vesicular trafficking and structural stabilization. Calpain activation has been implicated in various aging phenomena and diseases of late life, including cataract formation, erythrocyte senescence, diabetes mellitus type 2, hypertension, arthritis, and neurodegenerative disorders. The early and pervasive involvement of calpains in Alzheimer's disease potentially influences the development of beta-amyloid and tau disturbances and their consequences for neurodegeneration and neuronal cell loss.

Autoantigenic posttranslational modifications of proteins: does it apply to rheumatoid arthritis?

There are many posttranslational modifications of proteins of which all are homeostatically important either to carry out a particular structural or functional role or to allow efficient recycling of the amino acid constituents. An important feature of the modified proteins is the acquisition of autoantigenicity. That notion should have been recognized for years with the modifications of immunoglobulin G that constitute new targets for rheumatoid factors. Citrullination or the deimination of arginine residues in proteins creates epitopes that are targeted by rheumatoid autoantibodies with a diagnostic sensitivity and specificity of 40% to 70% and 92% to 99%, respectively. The how, when, and why of the responsible break in tolerance are largely speculative but apoptosis, multiple genetic and environmental influences are likely required. Identifying citrullinated proteins as autoantigens has resulted in new diagnostic and prognostic autoantibody markers for RA and studying the citrullination process and its nature and role in cell biology has provided new insights into its pathogenesis.

Autoantibody systems in rheumatoid arthritis: specificity, sensitivity and diagnostic value

The diagnosis of rheumatoid arthritis (RA) is primarily based on clinical symptoms, so it is often difficult to diagnose RA in very early stages of the disease. A disease-specific autoantibody that could be used as a serological marker would therefore be very useful. Most autoimmune diseases are characterized by a polyclonal B-cell response targeting multiple autoantigens. These immune responses are often not specific for a single disease. In this review, the most important autoantibody/autoantigen systems associated with RA are described and their utility as a diagnostic and prognostic tool, including their specificity, sensitivity and practical application, is discussed. We conclude that, at present, the antibody response directed to citrullinated antigens has the most valuable diagnostic and prognostic potential for RA.

Possible involvement of EBV-mediated alpha-fodrin cleavage for organ-specific autoantigen in Sjogren's syndrome

A cleavage product of alpha-fodrin may be an important organ-specific autoantigen in the pathogenesis of Sjogren's syndrome (SS), but the mechanisms of alpha-fodrin cleavage remain unclear. Since EBV has been implicated in the pathogenesis of SS, we determined whether EBV activation could induce the SS-specific 120-kDa autoantigen alpha-fodrin. ZEBRA mRNA expression, a marker for activation of the lytic cycle of EBV, was found in the salivary gland tissues from SS patients, but not in those from control individuals. ZEBRA-expressing lymphoid cells were also found in the SS glands in double-stained immunohistochemistry. Furthermore, a significant link between production of Abs against 120-kDa alpha-fodrin and reactivated EBV Ag was found in sera from patients with SS, but not in those from control individuals. EBV-activated lymphoid cells showed specific alpha-fodrin cleavage to the expected 120-kDa fragments in vitro. Pretreatment with caspase inhibitors inhibited cleavage of alpha-fodrin. Thus, an increase in apoptotic protease activities induced by EBV reactivation may be involved in the progression of alpha-fodrin proteolysis in the development of SS.

Insights into rheumatoid arthritis derived from the Sa immune system

The Sa system is a recently described immune system that has a specificity and positive predictive value of nearly 100% for rheumatoid arthritis (RA) in Asia, Europe and the Americas. Its sensitivity of 30-40% suggests that it identifies a subset of RA patients. Anti-Sa antibodies are present from disease onset and are predictive of disease severity. The immune reactants are plentiful in the target tissue: antigen is present in the synovium, IgG antibody in the fluid. Immunologically, Sa is a hapten-carrier antigen in which vimentin is the carrier and citrulline is the hapten. The citrullination of vimentin is closely related to apoptosis, and citrullinated vimentin is extremely sensitive to digestion by the ubiquitous calpains. Nevertheless, Sa is found in only a few cell lines. Calpastatin, the natural specific inhibitor of calpains, is also a RA-associated, albeit non-specific, autoimmune system. Is it possible that calpain-related apoptotic pathways could be prominent in cells containing Sa? The task is to reconcile the specificity of Sa/citrullinated proteins in a multifactorial and polygenic disease such as RA.

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.

The calcium-activated neutral protease calpain I is present in normal foetal skin and is decreased in neonatal harlequin ichthyosis

Calcium concentration is a critical factor for epidermal differentiation and is implicated in the expression and post-translational modification of numerous proteins in suprabasal cells of the epidermis. Calpains (calcium-activated neutral proteases) are believed to participate in signal transduction via highly regulated cytoplasmic protease activity. Here we investigate the expression of calpain I in normal human skin development and in neonatal harlequin ichthyosis (HI), a disorder of altered epidermal differentiation, especially the transition from the granular to the fully differentiated cornified layer. Calpain I was detected in developing foetal epidermis at 54 days estimated gestational age in the basal layer and the periderm of the developing foetal epidermis. By 125 days, calpain I was also detected in the granular layer. This pattern was maintained in newborn skin, but expression was significantly weaker in HI biopsies (n = 7). Reduced expression of calpain was specific to HI and was not observed in other skin diseases. Calpain was also normally expressed in the outer root sheath of hair follicles, in sebaceous glands and in sweat ducts and glands. Immunoblots of epidermal and keratinocyte extracts showed that the 78-kDa and 76-kDa active forms were generated via limited proteolysis of the 80-kDa inactive subunit; however, all forms were diminished in HI, consistent with findings in tissue sections. Our results show that calpain is present throughout the epidermis and is expressed from the early stages of development. These findings implicate calcium-mediated signalling events in the alteration of differentiation that occurs in HI.

CAPN 8: isolation of a new mouse calpain-isoenzyme

Recent molecular biological approaches indicate that calpain, also named CANP for calcium-activated neutral protease and originally characterized as an intracellular cytoplasmatic nonlysosomal cysteine protease that requires calcium ions for activity, constitutes a large superfamily consisting of ubiquitous and tissue specific homologues, which are widely distributed in cells of various organisms from human to fungus. Due to the increasing number of substrates along with the involvement of calpain isoenzymes in mammalian diseases, especially in malignancies, members of the calpain superfamily seem to be important biomodulators in physiological as well as pathological cell function. Here we report the characterisation of a new calpain, named CAPN 8 with a different C-terminal domain, implicating a putative new regulatory mechanism. Northern blot analysis revealed an ubiquitous expression with different RNA levels in all tissues examined. Highest levels were found in brain, kidney, and digestive tract, suggesting a specific regulatory function of CAPN 8 in these tissues.

A novel aspect of calpain activation

Calpain, a Ca2+-dependent biomodulator, alters the properties of substrate proteins by cleaving them at a limited number of specific sites. Recent studies of the structure-function relationship of calpain and X-ray analysis of its Ca2+-binding domain have revealed hitherto unknown features of the regulation of calpain activity. A novel dissociation/autolysis mechanism for the activation of calpain at the membrane is proposed, which incorporates recent findings from structure-function studies of calpain, and its implications are discussed.

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.

A new subfamily of vertebrate calpains lacking a calmodulin-like domain: implications for calpain regulation and evolution

Calpains are calcium-dependent intracellular nonlysosomal proteases that are believed to participate in signal transduction. In vertebrates, five different calpains have so far been identified, of which three, mu-, m-, and mu/m-calpain, are ubiquitously expressed while the other two, nCL-1 (p94) and nCL-2, exhibit a restricted tissue distribution. We have identified two new vertebrate calpain genes, Capn5 and Capn6. The human and mouse amino acid sequences of these new calpains are the most divergent of the vertebrate calpains identified. They possess most of the residues conserved in calpain family members but the C-terminal region lacks any homology to the calmodulin-like domain of other vertebrate calpains. They both exhibit significant homology over the entire coding region to the protein encoded by the gene tra-3, involved in nematode sex determination, and Capn5 may represent its vertebrate orthologue. The predicted Capn6 protein lacks critical active site residues and may not be proteolytically active. Both genes are differentially expressed in human tissues with highest RNA levels for Capn5 occurring in the testis, liver, trachea, colon, and kidney, while Capn6 is highly expressed only in the placenta sample of the 50 tissues examined. Phylogenetic analysis suggests that the vertebrate calpains arose through a series of gene duplication events that began before the initial divergence of the vertebrate and invertebrate lineages. The discovery of these two new calpains highlights a hitherto unknown complexity of the calpain family with subclasses perhaps possessing different modes of regulation.