Calpain II induced insolubilization of lens beta-crystallin polypeptides may induce cataract

Alterations in the lenticular protein profile in experimental selenite-induced cataractogenesis and prevention by ellagic acid

BACKGROUND

Accumulating evidence suggests that oxidative stress underlies age-related formation of cataract, and that antioxidants retard cataractogenesis. This study aimed to evaluate whether ellagic acid, a natural polyphenol with antioxidant properties, prevents alterations in the lenticular protein profile in an experimental model of selenite cataract.

METHODS

Alterations in lenticular protein were determined by two-dimensional electrophoresis (2DE) and image analysis. Eluted αA-crystallin spots were analyzed by mass spectrometry. Western blot analysis was also performed to confirm the differential expression of certain crystallins and cytoskeletal proteins.

RESULTS

In cataractous lenses, 2DE and image analysis revealed approximately 45 and 60 prominent spots in soluble and insoluble protein fractions respectively. Analysis of the pI and molecular weight of protein spots revealed differences in the expression of crystallin proteins in soluble and insoluble fractions. Western blot analysis confirmed changes in the expression of αA- and βB1- crystallins in both soluble and insoluble protein fractions, while mass spectrometry confirmed the degradation of αA-crystallin in selenite cataractous lenses. Western blot analysis also confirmed the occurrence of altered expression of certain cytoskeletal proteins in insoluble fractions. However, the lenticular protein profile in lenses from selenite-challenged, ellagic acid-treated rats was essentially similar to that noted in lenses from normal rats.

CONCLUSIONS

The present study confirms the importance of structural and cytoskeletal proteins in the maintenance of lenticular transparency; the results also suggest that ellagic acid prevents lenticular protein alterations induced by selenite in an experimental setting.

Calpain 10 is required for cell viability and is decreased in the aging kidney

Aging is associated with abnormalities in kidney function, but the exact mechanisms are unknown. We examined calpains 1, 2, and 10 protein levels in kidneys from rats, mice, and humans of various ages and determined whether calpain 10 is required for cell viability. Calpain 10 protein expression decreased in the kidney, but not in the liver, of aging Fischer 344 rats, and this decrease was attenuated with caloric restriction. There was no change in calpains 1 or 2 levels in the kidney or liver in control and caloric-restricted aging rats. Aging mice also exhibited decreased calpain 10 protein levels. Calpain 10 protein and mRNA levels decreased linearly in human kidney samples with age in the absence of changes in calpains 1 or 2. Our laboratory previously found calpain 10 to be expressed in both the cytosol and mitochondria of rabbit renal proximal tubular cells (RPTC). Adenoviral-delivered shRNA to rabbit RPTC decreased mitochondrial calpain 10 expression below detectable levels by 3 days while cytosolic calpain 10 levels remained unchanged at 3 days and decreased to approximately 20% of control by 5 days. Knockdown of mitochondrial calpain 10 resulted in nuclear condensation and cleaved procaspase 3, markers of apoptosis. In summary, mitochondrial calpain 10 is required for cell viability and calpain 10 levels specifically decrease in aging rat, mice, and human kidney tissues when renal function decreases, suggesting that calpain 10 is required for renal function and is a biomarker of the aging kidney.

Oxidative stress and calpain inhibition induce alpha B-crystallin phosphorylation via p38-MAPK and calcium signalling pathways in H9c2 cells

We investigated the response of alphaB-crystallin to oxidative stress and calpain inhibition in an attempt to elucidate the signalling pathways mediating its phosphorylation. Given the high expression levels of alphaB-crystallin in cardiac muscle one can evaluate the significance of its participation in preservation of homeostasis under adverse conditions. H9c2 cardiac myoblasts were used as our experimental model since their response reflects the signal transduction pathways activated by stress conditions in the myocardium. Thus, in H9c2 cells treated with H2O2 the mechanism regulating alphaB-crystallin phosphorylation was found to involve p38-MAPK/MSK1 as well as intracellular free calcium levels. Our immunocytochemical experiments demonstrated phosphorylated alphaB-crystallin to be co-localized with tubulin, potentially preserving cytoskeletal architecture under these interventions. In H9c2 cells treated with calpain inhibitors (ALLN, ALLM) alphaB-crystallin exhibited a p38-MAPK- and [Ca 2+](i)-dependent phosphorylation pattern since the latter was ablated in the presence of the selective p38-MAPK inhibitor SB203580 and calcium chelator BAPTA-AM. Calpain activity repression ultimately led to apoptosis confirmed by PARP fragmentation and chromatin condensation. However, the apoptotic pathway activated by ALLM and ALLN differed, underlying the diverse transduction mechanisms stimulated. In addition to this, an anti-apoptotic role for phospho-alphaB-crystallin was verified by confirmation of its interaction with pro-caspase 3, hindering its cleavage and subsequent activation. Collectively, our findings underline alphaB-crystallin crucial role as a participant of cardiac cells early response to stressful stimuli compromising their survival.

Degradation of C-terminal truncated alpha A-crystallins by the ubiquitin-proteasome pathway

PURPOSE

Calpain-mediated C-terminal cleavage of alpha A-crystallins occurs during aging and cataractogenesis. The objective of the present study was to explore the role of the ubiquitin-proteasome pathway (UPP) in degrading C-terminal truncated alpha A-crystallins.

METHODS

Recombinant wild-type (wt) alpha A-crystallin and C-terminal truncated alpha A(1-168)-, alpha A(1-163)-, and alpha A(1-162)-crystallins were expressed in Escherichia coli and purified to homogeneity. The wt and truncated alpha A-crystallins were labeled with (125)I, and proteolytic degradation was determined using both lens fiber lysate and reticulocyte lysate as sources of ubiquitinating and proteolytic enzymes. Far UV circular dichroism, tryptophan fluorescence intensity, and binding to the hydrophobic fluorescence probe Bis-ANS were used to characterize the wt and truncated alpha A-crystallins. Oligomer sizes of these crystallins were determined by multiangle light-scattering.

RESULTS

Whereas wt alpha A-crystallin was degraded moderately in both lens fiber and reticulocyte lysates, alpha A(1-168)-crystallin was resistant to degradation. The susceptibility of alpha A(1-163)-crystallin to degradation was comparable to that of wt alpha A-crystallin. However, alpha A(1-162)-crystallin was much more susceptible than wt alpha A-crystallin to degradation in both lens fiber and reticulocyte lysates. The degradation of both wt and C-terminal truncated alpha A(1-162)-crystallins requires adenosine triphosphate (ATP) and was stimulated by addition of a ubiquitin-conjugating enzyme, Ubc4. The degradation was substantially inhibited by the proteasome inhibitor MG132 and a dominant negative mutant of ubiquitin, K6W-Ub, indicating that at least part of the proteolysis was mediated by the UPP. Spectroscopic analyses of wt and C-terminal truncated alpha A-crystallins revealed that C-terminal truncation of alpha A-crystallin resulted in only subtle changes in secondary structures. However, C-terminal truncations resulted in significant changes in surface hydrophobicity and thermal stability. Thus, these conformational changes may reveal or mask the signals for the ubiquitin-dependent degradation.

CONCLUSIONS

The present data demonstrate that C-terminal cleavage of alpha A-crystallin not only alters its conformation and thermal stability, but also its susceptibility to degradation by the UPP. The rapid degradation of alpha A(1-162) by the UPP may prevent its accumulation in the lens.

Cleavage of integrin by mu-calpain during hypoxia in human endometrial cells

PROBLEM

The distribution and activation of mu-calpain and possible cleavage of integrin in human endometrial cells under hypoxic condition were investigated.

METHOD OF STUDY

Human endometrial epithelial and stromal cells were subjected to hypoxia, and subsequently used for immunostaining and western blot analysis.

RESULTS

The proform of mu-calpain was detected in the cytoplasm of normal cells, and displayed a substantial decrease after hypoxia. Conversely, the active form of mu-calpain was not detected in normal cells, but was abundant after hypoxia. The cytoplasmic domain of integrin beta3 was also detected in the cytoplasm of endometrial cells. Western blot analysis confirmed that both the proform of mu-calpain and the integrin beta3 cytoplasmic domain decreased during hypoxia.

CONCLUSIONS

Mu-calpain is activated in human endometrial cells during hypoxia and that subsequent cleavage of the integrin beta3 cytoplasmic domain may give some adverse effects to the function of human endometrium.

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.

Differential influence of proteolysis by calpain 2 and Lp82 on in vitro precipitation of mouse lens crystallins

The purpose of the present study was to compare the susceptibility of crystallins proteolyzed by ubiquitous calpain 2 and by lens-specific calpain Lp82 to insolubilization. To test this, transgenic (TG) mice expressing a calpain 2, in which the active site cysteine 105 was mutated to alanine, were produced. Expression of mutated calpain 2 was driven in lens by coupling the mutated gene to the betaB1-crystallin promoter. Light scattering was measured in solutions of lens proteins after activation of endogenous calpain 2 and/or Lp82. Mass spectrometric analysis was performed to determine the cleavage sites and the calpain responsible for insolubilization of crystallins. Lens proteins from TG mice incubated in vitro with calcium showed higher light scattering compared to proteins from wild type (WT) mice. alphaA-crystallin from TG mice was proteolyzed by Lp82. In contrast, alphaA-crystallin in lenses from WT mice were proteolyzed by both calpain 2 and Lp82. These results suggested that Lp82-induced proteolysis of crystallins caused increased susceptibility of truncated crystallins to in vitro precipitation. Since Lp82 is highest in young animals, Lp82-induced proteolysis and precipitation may be one of the factors responsible for the cataract formation in young rodents.

Protein expression patterns for ubiquitous and tissue specific calpains in the developing mouse lens

Calcium activated proteases (calpains) have been implicated in the processing of lens crystallins during lens maturation and cataract formation. Ubiquitous type calpain 2 and calpain 10 and lens specific Lp82 and Lp85 protein distribution were determined using immunohistochemistry and immunoblotting in embryonic and post-natal mouse eyes. Calpain 2 was first expressed late in embryonic development and localized to the lens epithelium and transition zone. Lp82 was expressed at E9.5 in the lens placode, head ectoderm, and throughout the fiber cells during embryonic lens maturation. Lp82 co-localized at sites of crystallin modification in the juvenile lens. In the adult lens, Lp82 protein was maintained in cortical fibers but could not be detected in the lens nucleus. Lp85, the slightly larger splice variant of Lp82, was first observed at E9.5 and throughout early embryonic lens development. Abundant localization of this enzyme was observed in the cell nuclei of lens epithelium, elongating fibers, and undifferentiated mesoderm. Robust peri-nuclear localization of calpain 10 was observed in the head ectoderm, lens placode, and optic vesicle during early eye induction. Further, calpain 10 protein was maintained in the lens epithelium of pre- and post-natal lens. These data support the hypothesis that Lp82 in rodent lens has an important role in crystallin proteolysis during normal lens maturation. In contrast, calpain 2, Lp85, and calpain 10 may have roles in cell signaling pathways.

Identification and properties of anti-chaperone-like peptides derived from oxidized bovine lens betaL-crystallins

Thermal aggregation of betaL-crystallin was higher in the presence of peptide fragments generated from oxidized and trypsin-digested betaL-crystallin compared with thermal aggregation of the control proteins without oxidized betaL-crystallin fragments. Increased aggregation of betaL-crystallin was also observed despite the presence of alpha-crystallin (which has anti-aggregating properties) in the system. Self-aggregation of the oxidized betaL-crystallin fragments per se was not observed under the experimental conditions. Reverse-phase HPLC analysis of the precipitate obtained after heating a mixture of betaL-crystallin and oxidized betaL-crystallin fragments revealed that more than one peptide co-precipitates with betaL-crystallin. Electrospray mass spectrometry analysis of the peptides revealed that the molecular weight(s) of the peptides ranged from 1400-1800. Tandem mass spectrometry and a data base search revealed that two of the peptides originated from betaA4-crystallin (LTIFEQENFLGR, residues 121-132) and betaB3-crystallin (AINGTWVGYEFPGYR, residues 153-167) respectively. Oxidized synthetic peptides representing the same sequence were also found to enhance the aggregation of betaL-crystallin in a manner similar to oxidized lens betaL-crystallin peptides. These data suggest that the polypeptides generated after oxidation and proteolysis of betaL-crystallins interact with denaturing proteins and facilitate their aggregation and light scattering, thus behaving like anti-chaperones.

Purification and characterization of lens specific calpain (Lp82) from bovine lens

Ubiquitous type m-calpain and lens specific Lp82 calpain were separated and partially purified from fetal bovine lens and the enzymatic characteristics were compared. Lens m-calpain required 200 microM calcium for 1/2 maximal activity, while Lp82 required 30 microM. Both types of calpains were inhibited by 0.1 mM E64, and 5 mM iodoacetamide, but not by 1 mM phenylmethylsulfonyl fluoride. Lp82 was insensitive to 1 microM calpastatin peptide while m-calpain was effectively inhibited. In the presence of calcium, m-calpain lost most of its activity within 2 hr, while Lp82 was continually active for 18 hr. Both calpains cleaved the natural substrates betaA3 and alphaB crystallins in a similar manner. However, incubation of alphaA crystallin with m-calpain removed ten amino acid residues from its C-terminus, while incubation with Lp82 removed only five residues. The latter truncation product of alphaA was also found in vivo. These data suggested that Lp82 may have a more important role than m-calpain in modification of crystallins during lens maturation.

Calpain-induced light scattering in young rat lenses is enhanced by UV-B

The purpose of this study is to determine if UV-B enhances light scattering after proteolysis of crystallins by calpains, and to determine if lens-specific calpain Lp82 is involved, along with m-calpain, in the mechanism of in vitro precipitation. Lens soluble proteins from young rats were hydrolyzed for 24 hr by endogenous lens calpains, and the proteins were further incubated for up to 7 days with periodic irradiation by UV-B. Light scattering was measured daily at 405 nm. SDS-PAGE and immunoblotting assessed proteolysis of crystallins, activation of calpains, and formation of high molecular weight aggregations. Appreciable light scattering occurred in lens soluble proteins after proteolysis of crystallins by m-calpain and Lp82. UV-B markedly enhanced this light scattering and the formation of higher molecular weight aggregates consisting of proteolyzed alpha- and beta- and intact gamma-crystallins. Calpain inhibitor E64 and antioxidants DTE or GSH prevented the light scattering. These results show that calpain-induced light scattering is enhanced by the natural oxidant UV-B. Activation of Lp82, along with m-calpain, contributed to the light scattering. The linkage between proteolysis and oxidation is important because both oxidation and truncation of crystallins are found in aged human lenses, which are constantly exposed to UV irradiation.

Calpain-induced light scattering by crystallins from three rodent species

The purpose of the present investigation was to compare in vitro light scattering in the soluble proteins from rodent lenses after hydrolysis by the calcium-activated protease, m-calpain (EC 3.4.22.17). Light scattering was measured in solutions of lens proteins from mice, rats, and guinea pigs after activation of endogenous m-calpain or after addition of purified m-calpain. We found for the first time that, in addition to rat, crystallins from another rodent lens, young mouse, were susceptible to calpain-induced light scattering. As in rats, aging of mouse lens prevented calpain-induced light scattering. Although crystallins from guinea pig lens were also partially hydrolysed by calpain, appreciable light scattering did not occur. Limited proteolysis may cause common changes in the biophysical properties of mouse and rat crystallins to decrease their solubility. Discovery of the nature of these biophysical changes may help our understanding as to why crystallins precipitate under cataractous conditions.

Crystallins from rat lens are especially susceptible to calpain-induced light scattering compared to other species

PURPOSE

To compare the susceptibility of crystallins from various animal species to formation of light scattering elements after proteolysis by calpain II enzyme (EC 3.4.22.17).

METHODS

Lens, total soluble proteins from: 12-day and 4-week old rat, fetal and adult bovine, 16-day embryonic and 10-week chicken, and young human cortex and nucleus were proteolyzed by either endogenous lens calpain or addition of purified calpain II for 24 h followed by incubation for up to 11 days. Absorbance of light at 405 nm estimated light scattering by crystallins; SDS-PAGE and 2D-electrophoresis assessed proteolysis on the crystallins.

RESULTS

Most rapid light scattering occurred with total soluble proteins from young rat lens, either after adding purified calpain or by activating endogenous lens calpain with calcium. (Only rat lens showed activation of endogenous calpain II.) beta-crystallin polypeptides from rat, bovine, human, and to a more limited extent, chick lens were partially proteolyzed by addition of purified calpain II. In spite of this proteolysis, total soluble proteins from chicken, bovine, and human lenses showed no obvious light scattering by action of calpain. Crystallins from older rat lens showed approximately 50% of the light scattering displayed by crystallins from younger rats after 3 days, but only when purified calpain was added.

CONCLUSIONS

Our results indicate an unusually high susceptibility of crystallin polypeptides from young rat lens to formation of light scattering elements after limited proteolysis. Thus, young rat lens provides a unique opportunity to investigate how properties of crystallins influence the development of light scattering found in cataract.

Cataractogenesis in transgenic mice containing the HIV-1 protease linked to the lens alpha A-crystallin promoter

Several lines of transgenic mice were generated with either active or inactive forms of the human immunodeficiency virus type 1 (HIV-1) protease gene under the control of the mouse lens alpha A-crystallin promoter. Mice bearing the inactive protease coding sequence displayed no gross abnormalities in the lens, while mice with the active protease developed time-dependent bilateral cataracts. One line, TG61, developed cataracts in utero while the second line, TG72, developed cataracts postnatally. TG61 mice, homozygous for the transgene, developed severe microphthalmia and were significantly smaller than the control mice at postnatal day 30. two-dimensional-polyacrylamide gel electrophoresis analysis of the protein profiles of TG72 and TG61 lenses revealed extensive modifications in the lens crystallins. Proteolysis in the homozygous TG72 mouse lenses began at postnatal day 20 with the disappearance or partial loss of beta B1-, beta B3-, and beta A3-crystallins and the appearance of crystallin fragments. Protein leakage and the gradual breakdown of cytoskeletal elements also occurred. In contrast, the opacification of the homozygous TG61 lenses appeared to have been influenced by differentiation and developmental processes. It appears that HIV-1 protease expression activates other proteases, and these enzymes, in concert with HIV-1 protease, are responsible for the protein modifications that eventually result in the opacification of the lens.

Precipitation of crystallins from young rat lens by endogenous calpain

The purpose of these experiments was to study the mechanism for precipitation of lens crystallins in cataract. An in vitro model was developed to activate the endogenous protease calpain II in the soluble proteins from young rat lens by addition of calcium in the presence of 120 mM KCl. Light-scattering, insoluble proteins were produced approximately 4-6 days after calpain II activation. Results showed that proteolysis was caused by activation of lens calpain II, proteolysis preceded precipitation by several days, and alpha-crystallin acted as a molecular chaperone against precipitation of crystallins caused by proteolysis. These data supported our hypothesis that calpain-induced proteolysis of the N-terminal arms of beta-crystallin polypeptides leads to a loss of normal oligomerization of beta-crystallin polypeptides and formation of abnormal insoluble aggregates, possibly stabilized by hydrophobic interactions.

Calcium cataract: a model for optical anisotropy fluctuations

Young rat lenses were incubated in organ culture media enriched with 20 mM calcium. Lenses in the calcium rich medium developed cataracts and were characterized by the absence of vimentin in the urea soluble protein fractions. Sections from the same lenses were studied by polarized light scattering. The I+/Iparallel scattering intensity ratios were higher from the lenses in calcium-rich media than from the control lenses. This indicated an increase in the optical anisotropy fluctuations during cataractogenesis. The turbidity that developed due to these fluctuations was caused partly by the disappearance of vimentin and which in turn caused the enhancement of birefringence of the lens.

Proteolysis by calpain is an underlying mechanism for formation of sugar cataract in rat lens

To confirm the effect of a new aldose reductase inhibitor (ARI), rat lenses were cultured with xylose. ARI prevented opacities and reduced lens hydration caused by xylose. Next, cataract was produced by feeding a diet containing 50% galactose. ARI was tested for amelioration of cataract. On day 19 after feeding of galactose, nuclear cataracts were visible in 75% of the animals receiving only galactose, while nuclear cataracts were not observed in animals treated with ARI. In galactose cataract, lens hydration and calcium were significantly increased. Calpain in soluble and insoluble fractions was decreased. Alpha- and beta-crystallins were proteolyzed. These changes were inhibited by administration of ARI. These results suggested that proteolysis by calpain is an underlying mechanism in formation of sugar cataract in rat lens.

Calcium-activated neutral proteinase (calpain) system in aging and Alzheimer's disease

Calpains (CANPs) are a family of calcium-dependent cysteine proteases under complex cellular regulation. By making selective limited proteolytic cleavages, they activate or alter the regulation of certain enzymes, including key protein kinases and phosphatases, and induce specific cytoskeletal rearrangements, accounting for their suspected involvement in intracellular signaling, vesicular trafficking, and structural stabilization. Calpain activity has been implicated in various aging phenomena, including cataract formation and erythrocyte senescence. Abnormal activation of the large stores of latent calpain in neurons induces cell injury and is believed to underlie neurodegeneration in excitotoxicity, Wallerian degeneration, and certain other neuropathologic states involving abnormal calcium influx. In Alzheimer's disease, we found the ratio of activated calpain I to its latent precursor isoform in neocortex to be threefold higher than that in normal individuals and those with Huntington's or Parkinson's disease. Immunoreactivity toward calpastatin, the endogenous inhibitor of calpain, was also markedly reduced in layers II-V of the neocortex in Alzheimer's disease. The excessive calpain system activation suggested by these findings represents a potential molecular basis for synaptic loss and neuronal cell death in the brain in Alzheimer's disease given the known destructive actions of calpain I and its preferential neuronal and synaptic localization. In surviving cells, persistent calpain activation may also contribute to neurofibrillary pathology and abnormal amyloid precursor protein trafficking/processing through its known actions on protein kinases and the membrane skeleton. The degree of abnormal calpain activation in the brain in Alzheimer's disease strongly correlated with the extent of decline in levels of secreted amyloid precursor protein in brain. Cytoskeletal proteins that are normally good calpain substrates become relatively calpain resistant when they are hyperphosphorylated, which may contribute to their accumulation in neurofibrillary tangles. As a major effector of calcium signals, calpain activity may mirror disturbances in calcium homeostasis and mediate important pathologic consequences of such disturbances.

Beta-crystallins insolubilized by calpain II in vitro contain cleavage sites similar to beta-crystallins insolubilized during cataract

Incubation of soluble proteins from rat lens with the protease calpain II caused the precipitation of beta-crystallin polypeptides. Two-dimensional electrophoresis and sequence analysis identified beta-crystallin polypeptides both before and after their precipitation by calpain II. beta-crystallin polypeptides precipitated by calpain were cleaved at their NH2-terminal extensions. These cleavage sites were similar to cleavage sites occurring in beta-crystallin polypeptides precipitated during formation of experimental cataract induced by an overdose of selenite. These data suggested that calpain II caused beta-crystallin insolubilization during cataract formation, and indicated that the process can be mimicked in vitro.