Different expression patterns for ubiquitous calpains and Capn3 splice variants in monkey ocular tissues

Calpains as mechanistic drivers and therapeutic targets for ocular disease

Ophthalmic neurodegenerative diseases encompass a wide array of molecular pathologies unified by calpain dysregulation. Calpains are calcium-dependent proteases that perpetuate cellular death and inflammation when hyperactivated. Calpain inhibition trials in other organs have faced pharmacological challenges, but the eye offers many advantages for the development and testing of targeted molecular therapeutics, including small molecules, peptides, engineered proteins, drug implants, and gene-based therapies. This review highlights structural mechanisms underlying calpain activation, distinct cellular expression patterns, and in vivo models that link calpain hyperactivity to human retinal and developmental disease. Optimizing therapeutic approaches for calpain-mediated eye diseases can help accelerate clinically feasible strategies for treating calpain dysregulation in other diseased tissues.

CAPN3: A muscle‑specific calpain with an important role in the pathogenesis of diseases (Review)

Calpains are a family of Ca²⁺‑dependent cysteine proteases that participate in various cellular processes. Calpain 3 (CAPN3) is a classical calpain with unique N‑terminus and insertion sequence 1 and 2 domains that confer characteristics such as rapid autolysis, Ca²⁺‑independent activation and Na⁺ activation of the protease. CAPN3 is the only muscle‑specific calpain that has important roles in the promotion of calcium release from skeletal muscle fibers, calcium uptake of sarcoplasmic reticulum, muscle formation and muscle remodeling. Studies have indicated that recessive mutations in CAPN3 cause limb‑girdle muscular dystrophy (MD) type 2A and other types of MD; eosinophilic myositis, melanoma and epilepsy are also closely related to CAPN3. In the present review, the characteristics of CAPN3, its biological functions and roles in the pathogenesis of a number of disorders are discussed.

Redox Regulation of Calpains: Consequences on Vascular Function

SIGNIFICANCE

Calpains (CAPNs) are a family of calcium-activated cysteine proteases. The ubiquitous isoforms CAPN1 and CAPN2 have been involved in the maintenance of vascular integrity, but uncontrolled CAPN activation plays a role in the pathogenesis of vascular diseases. Recent Advances: It is well accepted that chronic and acute overproduction of reactive oxygen species (ROS) is associated with the development of vascular diseases. There is increasing evidence that ROS can also affect the CAPN activity, suggesting CAPN as a potential link between oxidative stress and vascular disease.

CRITICAL ISSUES

The physiopathological relevance of ROS in regulating the CAPN activity is not fully understood but seems to involve direct effects on CAPNs, redox modifications of CAPN substrates, as well as indirect effect on CAPNs via changes in Ca²⁺ levels. Finally, CAPNs can also stimulate ROS production; however, data showing in which context ROS are the causes or the consequences of CAPN activation are missing.

FUTURE DIRECTIONS

Detailed characterization of the molecular mechanisms underlying the regulation of the different members of the CAPN system by specific ROS would help understanding the pathophysiological role of CAPN in the modulation of the vascular function. Moreover, given that CAPNs have been found in different cellular compartments such as mitochondria and nucleus as well as in the extracellular space, identification of new CAPN targets as well as their functional consequences would add new insights in the function of these enigmatic proteases.

Calpain-5 Expression in the Retina Localizes to Photoreceptor Synapses

Purpose

We characterize calpain-5 (CAPN5) expression in retinal and neuronal subcellular compartments.

Methods

CAPN5 gene variants were classified using the exome variant server, and RNA-sequencing was used to compare expression of CAPN5 mRNA in the mouse and human retina and in retinoblastoma cells. Expression of CAPN5 protein was ascertained in humans and mice in silico, in mouse retina by immunohistochemistry, and in neuronal cancer cell lines and fractionated central nervous system tissue extracts by Western analysis with eight antibodies targeting different CAPN5 regions.

Results

Most CAPN5 genetic variation occurs outside its protease core; and searches of cancer and epilepsy/autism genetic databases found no variants similar to hyperactivating retinal disease alleles. The mouse retina expressed one transcript for CAPN5 plus those of nine other calpains, similar to the human retina. In Y79 retinoblastoma cells, the level of CAPN5 transcript was very low. Immunohistochemistry detected CAPN5 expression in the inner and outer nuclear layers and at synapses in the outer plexiform layer. Western analysis of fractionated retinal extracts confirmed CAPN5 synapse localization. Western blots of fractionated brain neuronal extracts revealed distinct subcellular patterns and the potential presence of autoproteolytic CAPN5 domains.

Conclusions

CAPN5 is moderately expressed in the retina and, despite higher expression in other tissues, hyperactive disease mutants of CAPN5 only manifest as eye disease. At the cellular level, CAPN5 is expressed in several different functional compartments. CAPN5 localization at the photoreceptor synapse and with mitochondria explains the neural circuitry phenotype in human CAPN5 disease alleles.

An association between the calpastatin (CAST) gene and keratoconus

PURPOSE

Keratoconus (KC) is a genetically heterogeneous corneal dystrophy. Previously, we performed 2 genome-wide linkage scans in a 4-generation autosomal dominant pedigree and repeatedly mapped a KC locus to a genomic region located on chromosome 5q overlapping the gene encoding the inhibitor of calpains, calpastatin (CAST). To test whether variants in CAST gene are involved in genetic susceptibility to KC, we performed genetic testing of polymorphic markers in CAST gene in family and case-control panels of patients with KC.

METHODS

We genotyped single-nucleotide polymorphisms (SNPs) located in CAST gene in 262 patients in 40 white KC families and in a white case-control panel with 304 cases and 518 controls. Generalized estimating equation models accounting for familial correlations implemented in GWAF program were used for association testing in families. Logistic regression models implemented in PLINK were performed to test the associations in case-control samples.

RESULTS

Genetic testing of the first set of 7 SNPs in familial samples revealed 2 tentative nominally significant markers (rs4869307, P = 0.03; rs27654, P = 0.07). Additional genotyping of 12 tightly spaced SNPs identified CAST SNP rs4434401 to be associated with KC in both familial and case-control panels with P values of 0.005 and 0.05, respectively, and with combined meta P value of familial and case-control cohorts of 0.002 or after Bonferroni correction of 0.04.

CONCLUSIONS

Linkage analysis and genetic association support involvement of CAST gene in the genetic susceptibility to KC. In silico analysis of CAST expression suggests differential regulation of calpain/calpastatin system in cornea as a potential mechanism of functional defect.

Decreased chaperone activity of alpha-crystallins in naphthalene-induced cataract possibly results from C-terminal truncation

Naphthalene-induced cataract has been extensively used to test potential anticataract drugs. Because the morphology as well as the toxic manifestations of naphthalene-induced cataract are reported to be similar to that of age-related cataract, naphthalene cataractogenesis in rats has been used as a valuable animal model to study the aetiology of age-related cataract in humans. This study aimed to determine whether the molecular chaperone activity of the alpha-crystallins was altered in naphthalene-induced cataract, and to clarify the possible mechanism for these changes. The data showed that the chaperone activity of the alpha-crystallins decreased in naphthalene-induced cataract. By mass spectrometry, C-terminal truncation of 16 amino acids and other post-translational modifications such as acetylation, phosphorylation, oxidation and carbamylation of the alpha-crystallins were detected. Furthermore, the results suggested that, at the proteomics level, naphthalene-induced cataract is a valuable animal model for the study of age-related cataract in humans.

Calpain inhibitor SNJ-1945 attenuates events prior to angiogenesis in cultured human retinal endothelial cells

PURPOSE

Vascular endothelial growth factor (VEGF) is an important regulator of angiogenesis and microvascular permeability. VEGF-induced cytoskeletal reorganization plays a crucial role in angiogenesis. Cytoskeletal organization in endothelial cells is regulated by calpain proteases (EC 3.4.22.17). Calpains are a family of 14 calcium-regulated, intracellular cysteine proteases, which modulate cellular functions by limited, specific proteolysis. Calpain 1 (mu-calpain) and calpain 2 (m-calpain) are the 2 major typical calpain isoforms and are responsible for most calpain activity in endothelial cells. The purpose of the present study was to determine if an orally available form of calpain inhibitor, SNJ-1945, prevented angiogenesis induced by VEGF in cultured retinal endothelial cells.

METHODS

Human retinal microvascular endothelial cells (HRMEC) were incubated with VEGF (60-100 ng/mL) for 24 h. Calcium uptake was measured with Fluo8. Total calpain activity was measured using fluorescent-labeled casein substrate, and separate activities for calpains 1 and 2 were assessed by casein zymography. Proteolysis of endogenous calpain substrate alpha-spectrin in situ was analyzed by immunoblotting. Angiogenesis in vitro was evaluated by measuring cell migration and tube formation into Matrigel.

RESULTS

Incubation of HRMEC with VEGF resulted in calcium uptake, increased activity of mainly calpain 2, and increased calpain proteolysis of alpha-spectrin. Treatment of endothelial cells with calpain inhibitor SNJ-1945 reversed VEGF-mediated tube formation and cell motility.

CONCLUSIONS

Inhibition of angiogenesis by specific calpain inhibitor in the presence of VEGF supported our hypothesis that calpains may be involved in VEGF-mediated angiogenesis in retinal endothelial cells. Therefore, manipulating calpain activity by calpain inhibitor SNJ-1945 might provide a promising therapy for management of pathological angiogenesis, such as that occurring in proliferative retinopathy and age-related macular degeneration with neovascularization.

Calpain expression and activity during lens fiber cell differentiation

In animal models, the dysregulated activity of calcium-activated proteases, calpains, contributes directly to cataract formation. However, the physiological role of calpains in the healthy lens is not well defined. In this study, we examined the expression pattern of calpains in the mouse lens. Real time PCR and Western blotting data indicated that calpain 1, 2, 3, and 7 were expressed in lens fiber cells. Using controlled lysis, depth-dependent expression profiles for each calpain were obtained. These indicated that, unlike calpain 1, 2, and 7, which were most abundant in cells near the lens surface, calpain 3 expression was strongest in the deep cortical region of the lens. We detected calpain activities in vitro and showed that calpains were active in vivo by microinjecting fluorogenic calpain substrates into cortical fiber cells. To identify endogenous calpain substrates, membrane/cytoskeleton preparations were treated with recombinant calpain, and cleaved products were identified by two-dimensional difference electrophoresis/mass spectrometry. Among the calpain substrates identified by this approach was alphaII-spectrin. An antibody that specifically recognized calpain-cleaved spectrin was used to demonstrate that spectrin is cleaved in vivo, late in fiber cell differentiation, at or about the time that lens organelles are degraded. The generation of the calpain-specific spectrin cleavage product was not observed in lens tissue from calpain 3-null mice, indicating that calpain 3 is uniquely activated during lens fiber differentiation. Our data suggest a role for calpains in the remodeling of the membrane cytoskeleton that occurs with fiber cell maturation.

The effect of acetyl-L-carnitine on lenticular calpain activity in prevention of selenite-induced cataractogenesis

The present study sought to determine whether acetyl-L-carnitine (ALCAR) prevents selenite cataractogenesis by mechanisms involving lenticular calpain activity, Wistar rat pups were divided into 3 groups of 15 each. Group I (normal) rats received an intraperitoneal (i.p.) injection of normal saline on postpartum day 10; Group II (cataract-untreated) rats received a single subcutaneous (s.c.) injection of sodium selenite (19micromol/kg body weight) on postpartum day 10; Group III (cataract-treated) pups received a single s.c. injection of sodium selenite on postpartum day 10 and intraperitoneal injections of acetyl-L-carnitine (200mg/kg body weight) on postpartum days 9-14. At the end of the study period (postpartum day 16), both eyes of each rat pup were examined by slit-lamp biomicroscopy. There was dense lenticular opacification in all Group II rats, minimal lenticular opacification in 33% of Group III rats, and no lenticular opacification in 67% of Group III and in all Group I rats. Group II lenses exhibited significantly lower mean values of calpain activity and Lp82 (lens-specific calpain) protein expression, decreases in relative transcript level of m-calpain mRNA and significantly higher mean Ca(2+) concentrations than Group I or Group III lenses; the values of these parameters in Group III rat lenses (ALCAR-treated) approximated those in Group I rat lenses. The results suggest that, in addition to its already-described antioxidant potential, ALCAR prevents selenite cataractogenesis by maintaining calpain activity at near normal levels. These findings may stimulate further efforts to develop ALCAR as a novel drug for prevention of cataract.

MALDI tissue profiling of integral membrane proteins from ocular tissues

MALDI tissue profiling and imaging have become valuable tools for rapid, direct analysis of tissues to investigate spatial distributions of proteins, potentially leading to an enhanced understanding of the molecular basis of disease. Sample preparation methods developed to date for these techniques produce protein expression profiles from predominantly hydrophilic, soluble proteins. The ability to obtain information about the spatial distribution of integral membrane proteins is critical to more fully understand their role in physiological processes, including transport, adhesion, and signaling. In this article, a sample preparation method for direct tissue profiling of integral membrane proteins is presented. Spatially resolved profiles for the abundant lens membrane proteins aquaporin 0 (AQP0) and MP20, and the retinal membrane protein opsin, were obtained using this method. MALDI tissue profiling results were validated by analysis of dissected tissue prepared by traditional membrane protein processing methods. Furthermore, direct tissue profiling of lens membrane proteins revealed age related post-translational modifications, as well as a novel modification that had not been detected using conventional tissue homogenization methods.

The role of calcium-activated protease calpain in experimental retinal pathology

The purpose of this review is to present the recent evidence linking the family of ubiquitous proteases called calpains (EC 3.4.22.17) to neuropathologies of the retina. The hypothesis being tested in such studies is that over-activation of calpains by elevated intracellular calcium contributes to retinal cell death produced by conditions such as elevated intraocular pressure and hypoxia. Recent x-ray diffraction studies have provided insight into the molecular events causing calpain activation. Further, x-ray diffraction data has provided details on how side chains on calpain inhibitors affect docking into the active site of calpain 1. This opens the possibility of testing calpain-specific inhibitors, such as SJA6017 and SNJ1945, for human safety and as a site-directed form of treatment for retinal pathologies.

Calpains and their multiple roles in diabetes mellitus

Type 2 diabetes mellitus (T2DM) can lead to death without treatment and it has been predicted that the condition will affect 215 million people worldwide by 2010. T2DM is a multifactorial disorder whose precise genetic causes and biochemical defects have not been fully elucidated, but at both levels, calpains appear to play a role. Positional cloning studies mapped T2DM susceptibility to CAPN10, the gene encoding the intracellular cysteine protease, calpain 10. Further studies have shown a number of noncoding polymorphisms in CAPN10 to be functionally associated with T2DM while the identification of coding polymorphisms, suggested that mutant calpain 10 proteins may also contribute to the disease. Here we review recent studies, which in addition to the latter enzyme, have linked calpain 5, calpain 3, and its splice variants, calpain 2 and calpain 1 to T2DM-related metabolic pathways along with T2DM-associated phenotypes, such as obesity and impaired insulin secretion, and T2DM-related complications, such as epithelial dysfunction and diabetic cataract.

Regulation of Calpain Activity in Rat Brain with Altered Ca2+ Homeostasis

Activation of calpain occurs as an early event in correlation with an increase in [Ca2+]i induced in rat brain upon treatment with a high salt diet for a prolonged period of time. The resulting sequential events have been monitored in the brain of normal and hypertensive rats of the Milan strain, diverging for a constitutive alteration in the level of [Ca2+]i found to be present in nerve cells of hypertensive animals. After 2 weeks of treatment, the levels of the plasma membrane Ca2+-ATPase and of native calpastatin are profoundly decreased. These degradative processes, more pronounced in the brain of hypertensive rats, are progressively and efficiently compensated in the brain of both rat strains by different incoming mechanisms. Along with calpastatin degradation, 15-kDa still-active inhibitory fragments are accumulated, capable of efficiently replacing the loss of native inhibitor molecules. A partial return to a more efficient control of Ca2+ homeostasis occurs in parallel, assured by an early increase in the expression of Ca2+-ATPase and of calpastatin, both producing, after 12 weeks of a high salt (sodium) diet, the restoration of almost original levels of the Ca2+ pump and of significant amounts of native inhibitor molecules. Thus, conservative calpastatin fragmentation, associated with an increased expression of Ca2+-ATPase and of the calpain natural inhibitor, has been demonstrated to occur in vivo in rat brain. This represents a sequential adaptive response capable of overcoming the effects of calpain activation induced by a moderate long term elevation of [Ca2+]i.

Molecular and cellular basis of calpainopathy (limb girdle muscular dystrophy type 2A)

Limb girdle muscular dystrophy type 2A results from mutations in the gene encoding the calpain 3 protease. Mutations in this disease are inherited in an autosomal recessive fashion and result in progressive proximal skeletal muscle wasting but no cardiac abnormalities. Calpain 3 has been shown to proteolytically cleave a wide variety of cytoskeletal and myofibrillar proteins and to act upstream of the ubiquitin-proteasome pathway. In this review, we summarize the known biochemical and physiological features of calpain 3 and hypothesize why mutations result in disease.

Altered expression, intracellular distribution and activity of lymphocyte calpain II in Duchenne muscular dystrophy

BACKGROUND

Calpain II is an calcium-dependent cysteine protease involved in essential regulatory or processing functions of the cell, mediated by physiological concentrations of Ca(2+). However, in an environment of abnormal intracellular calcium as in Duchenne muscular dystrophy (DMD), calpain is suggested to cause membrane alterations.

METHODS

Twelve individuals with dystrophin gene deletion and an equal number of age and sex matched controls were chosen for the study. The expression pattern of calpain II (both at RNA and protein levels), its cellular location upon activation and its activity in lymphocytes were specifically assessed to know if our earlier report of increased calpain activity in DMD lymphocytes is a result of de novo synthesis or is due to basic defect in calcium handling.

RESULTS

We found a significant increase in the expression, alteration in calpain II distribution and increased activity of this enzyme.

CONCLUSION

Membrane abnormalities and altered signaling pathways observed in DMD lymphocytes may be due to increased association of calpain II onto membrane and cytosol.

Low activity by the calpain system in primate lenses causes resistance to calcium-induced proteolysis

The human genome contains 14 genes for 80 kDa catalytic subunit of the calcium-activated protease calpain (EC 34.22.17), yet no calpain-like cleavage sites have been detected on human lens crystallins in vivo. The purpose of the present study was to provide a comprehensive study of calpain activation in human and macaque lenses developing experimental cataract due to lens culture in ionophore A23187. Zymography was used to measure calpain activity; SDS-PAGE and immunoblotting were used to detect hydrolysis of potential lens protein substrates. Quantitative PCR was used to measure transcripts for calpains and the endogenous inhibitor calpastatin. We found that the lack of appreciable calpain-induced proteolysis in primate lenses is most likely due to relatively low levels of endogenous calpain activity compared to the high levels of endogenous calpain inhibitor, calpastatin.

Proteases in eye development and disease

The eye is one of the classical systems in developmental biology. Furthermore, diseases of the eye, many of which have a developmental basis, have devastating effects that often result in blindness. Proteases have diverse roles in ocular physiology and pathophysiology. Here, a broad overview is provided of the recent literature pertaining to the involvement of proteases in various aspects of eye development and disease: lens development (focusing on apoptosis and lens fiber cell denucleation and organelle loss) and cataract progression, cornea development and disease, retina development and degeneration, sclera development and myopia, and the trabecular meshwork and glaucoma. Proteases discussed include caspases, calpains, matrix metalloproteases (MMPs), a disintegrin and metalloproteinases (ADAMs) and ADAM with thrombospondin motifs (ADAMTS), the ubiquitin-proteasome pathway (UPP), tissue plasminogen activator (tPA), and secretases. It is clear that proteases have diverse and important roles in ocular development and disease, and represent, in many cases, useful therapeutic targets for treating ocular conditions, which would otherwise lead to visual impairment.

Limb girdle muscular dystrophy in a sibling pair with a homozygous Ser606Leu mutation in the alternatively spliced IS2 region of calpain 3

Previous family studies revealed a large number of calpain 3 ( CAPN3 ) mutations that cause recessive forms of limb girdle muscular dystrophy (LGMD2A) with selective atrophy of the proximal limb muscles. Correlations between the nature and site of a particular mutation and its corresponding phenotype, however, can only be established from homozygous mutations, which are particularly rare in the alternatively spliced NS, IS1 and IS2 regions of CAPN3. Here we identified a sibling pair with LGMD2A-type muscular dystrophy caused by a homozygous Ser606Leu (S606L) substitution in the IS2 linker domain. Normal protein levels, unaltered myofibrillar targeting and conserved calcium-induced autocatalytic activity of the mutated protein could be demonstrated in muscle biopsies from one patient. Despite this inconspicuous modification of the IS2 linker between domains III and IV, both patients developed signs and symptoms of the disease within their second decade of life. The unexpected severity of the clinical manifestation points to the high relevance of the calpain 3-specific IS2 segment between domains III and IV. We conclude that the structural motif around the Ser606 residue represents an important functional site that may regulate the transient activation and limited proteolysis of calpain 3.

Calpain-related diseases

Calpains are calcium-modulated proteases which respond to Ca2+ signals by removing limited portions of protein substrates, thereby irreversibly modifying their function(s). Members of this protease family are present in a variety of organisms ranging from mammals to plants; some of them are ubiquitously expressed, while others are tissue specific. Although calpains are apparently involved in a multitude of physiological and pathological events, their functions are still poorly understood. In two cases, however, the alteration of a member of the calpain family has been clearly identified as being responsible for a human disease: the loss of function of calpain 3 causes limb girdle muscular dystrophy type 2A, and mutations in the gene coding for calpain 10 have been shown to correlate with non-insulin-dependent diabetes.

Calpains: targets of cataract prevention?

There is emerging evidence to suggest that the unregulated Ca(2+)-mediated proteolysis of essential lens proteins by calpains might be a major contributor to some forms of cataract in both animals and humans. Moreover, recently solved calpain structures have revealed molecular-level details of the activation mechanism used by these proteases, enabling the structure-based design of potent calpain inhibitors with the potential to act as anti-cataract agents. These agents offer the first real hope of an urgently needed alternative to the surgical treatment of at least some forms of cataract and relief from a life-depreciating condition on a global scale.

Insertion sequence 1 of muscle-specific calpain, p94, acts as an internal propeptide

The physiological role of the skeletal muscle-specific calpain 3, p94, is presently unknown, but defects in its gene cause limb girdle muscular dystrophy type 2A. This calcium-dependent cysteine protease resembles the large subunit of m-calpain but with three unique additional sequences: an N-terminal region (NS), and two insertions (IS1 and IS2). The latter two insertions have been linked to the chronic instability of the whole enzyme both in vivo and in vitro. We have shown previously that the core of p94 comprising NS, domains I and II, and IS1 is stable as a recombinant protein in the absence of Ca(2+) and undergoes autolysis in its presence. Here we show that p94I-II cannot hydrolyze an exogenous substrate before autolysis but is increasingly able to do so when autolysis proceeds for several hours. This gain in activity is caused by cleavage of IS1 during autolysis because a deletion mutant lacking the NS region (p94I-II DeltaNS) shows the same activation profile. Similarly, the calpain inhibitors E-64 and leupeptin have almost no inhibitory effect on substrate hydrolysis by p94I-II soon after calcium addition but cause complete inhibition when autolysis progresses for several hours. As autolysis proceeds, there is release of the internal IS1 peptide, but the two portions of the core remain tightly associated. Modeling of p94I-II suggests that IS1 contains an amphipathic alpha-helix flanked by extended loops. The latter are the targets of autolysis and limited digestion by exogenous proteases. The presence and location of the alpha-helix in recombinant IS1 were confirmed by circular dichroism and by the introduction of a L286P helix-disrupting mutation. Within p94I-II, L286P caused premature autoproteolysis of the enzyme. IS1 is an elaboration of a loop in domain II near the active site, and it acts as an internal autoinhibitory propeptide, blocking the active site of p94 from substrates and inhibitors.

Characterization of a new p94-like calpain form in human lymphocytes

Human circulating PBMC (peripheral blood mononuclear cells) contain three calpain isoforms distinguishable on the basis of their chromatographic properties. Two of these proteases belong to the ubiquitous calpain subfamily, corresponding to the classical mu- and m-calpain forms. The third, which shows peculiar activating and regulatory properties, is an alternatively spliced calpain 3 (p94) form. This new calpain differs from calpain 3 in that it has lost IS1 insertion and exon 15, a lysine-rich sequence regarded as a nuclear translocation signal. PBMC p94-calpain undergoes activation and inactivation without the accumulation of a low-Ca2+-requiring form that is typical of the classical activation processes of mu- and m-calpain. Furthermore, it differs from the ubiquitous forms in that it displays a lower sensitivity to calpastatin. On the basis of these selective properties, it can be postulated that PBMC p94-calpain can be activated in response to specific stimuli that are not effective on the other calpain isoenzymes. The enzyme is preferentially expressed in B- and T-lymphocytes, whereas it is poorly expressed in natural killer cells and almost undetectable in polymorphonuclear cells. This distribution might reflect the specific function of this protease, which is preferentially present in cells devoted to the production of the humoral, rather than to the cellular, immune response.

Contribution of ubiquitous calpains to cataractogenesis in the spontaneous diabetic WBN/Kob rat

To determine the involvement of calpains in human cataractogenesis, studies in aged animal models are needed. Aged, male WBN/Kob rats spontaneously develop cataract along with severe, persistent diabetes with hyperglycemia and nephropathy. The purpose of present experiments was to provide a biochemical mechanism for the involvement of ubiquitous calpains in cataractogenesis in WBN/Kob rats. Serum and urinary glucose were measured to confirm diabetes, and cataracts were observed by slit lamp biomicroscopy. Calcium determinations were performed on lens samples from several ages of WBN/Kob and Wistar rats. Casein zymography, immunoblot analysis for alpha-spectrin, calpain 2, and calpain 10 were performed to detect activation of calpain in lens samples. Serum glucose levels increased and cortical cataract developed in male WBN/Kob rats within 1 year, indicating diabetic cataract. Cataract was accompanied by several presumptive biochemical indicators of calpain activation, including increased calcium, proteolysis of alpha-spectrin, and decreased caseinolytic activity for calpains suggesting calpain activation followed by autolytic degradation. Activation of ubiquitous calpains may contribute to biochemical mechanism of cataractogenesis in spontaneously diabetic WBN/Kob rats. The WBN/Kob model may be useful for elucidating the roles of calpain 2 and calpain 10 in human cataractogenesis.

Characterization and regulation of lens-specific calpain Lp82

Eye tissues contain splice variants of muscle-preferred p94 (calpain 3), such as lens-specific Lp82 and Lp85, retina-specific Rt88, and cornea-specific Cn94. The purpose of the present experiment was to analyze the activation and regulation of the best characterized p94 splice variant, Lp82. Recombinant rat Lp82 (rLp82) was expressed using the baculovirus system, purified with Ni-NTA affinity and DEAE-ion exchange chromatographies, and characterized by SDS-PAGE, casein zymography, and immunoblotting. After incubation with calcium, rLp82 autolyzed into two major fragments at approximately 60 and 22 kDa. Sequencing of the autolytic fragments showed loss of three amino acids from the N terminus and cleavage near the IS2 region. Also, Lp82 and calpain 2 were found to hydrolyze each other. Calpastatin inhibited calpain 2 activity, but not Lp82. Homology modeling suggested that the lack of inhibition of Lp82 by calpastatin was due to molecular clashes at the unique AX1 region of Lp82. Lp82 also hydrolyzed calpastatin. These results suggested that Lp82 might regulate other calpain activities and cause hydrolysis of substrates such as crystallins during lens cataract formation.