Age changes in bovine lens endopeptidase activity

Changes in three types of ubiquitin mRNA and ubiquitin-protein conjugate levels during lens development

Ubiquitin is a small, highly conserved protein that covalently attaches to other proteins to form a unique branched protein structure. The best characterized function of this post-translational modification is to mark the modified protein for degradation by the proteasome. To investigate whether ubiquitin genes are regulated in lens development, the authors analyzed the levels of three ubiquitin mRNAs (UbA(52), UbB and UbC) in freshly dissected fiber and epithelial cells, and in epithelial explants induced to differentiate ex vivo. Explants, comprising the capsule and adherent epithelial cells, were dissected from lenses of 3 day old Sprague Dawley rats and cultured +/-bFGF to induce differentiation. Quantitative competitive RT-PCR was used to determine the mRNA levels in fresh and cultured cells. UbA(52), UbB and UbC mRNAs were 3.2 (P < 0.0001), 5.0 (P < 0.0001) and 6.8 (P < 0.0001) fold higher, respectively, in freshly dissected epithelial cells than in differentiated fiber cells. Immunological spot assays showed that ubiquitin protein is over two fold as high in rat pup lens epithelial cells as in fiber cells. The ubiquitin protein in fiber cells of adult rat is lower than that in adult epithelium and in pup fiber cells, indicating that ubiquitin content further decreased during lens fiber maturation. Western blots showed a greater amount of protein-conjugated ubiquitin (MW > 81 kD) in epithelial cells than in fiber cells, demonstrating a parallel pattern between the expression of ubiquitin mRNA, the level of ubiquitin protein and the level of conjugates in the cells. Epithelial cell explant cultures permit study of cells initiating differentiation. In contrast to fully differentiated fiber cells, explant cultures induced to initiate differentiation underwent differential up-regulation of ubiquitin gene expression. UbA(52) and UbB mRNA levels in +bFGF (differentiating) explant cultures were 2.6 (P < 0.001) and 1.4 (P < 0.001) fold higher, respectively, than those of -bFGF cultures. UbC mRNA content was similar in explants cultured with or without bFGF. Dissection of the isolated epithelial cells into regions representing distinct populations gave results consistent with this observation of the explant results. UbA(52), UbB and UbC mRNAs are 2.0, 2.2 and 1.76 fold higher, respectively, in the peripheral (initiating differentiation) than in the central (undifferentiated) region of epithelial cells. These results together indicate that UbA(52) and UbB mRNAs are transiently increased during the initiation and early stages of differentiation. However, UbC mRNA appears to be relatively unaffected at the earliest stage in this differentiation model and may have a different distribution than UbA(52) and UbB in the anterior lens cells. These data are consistent with an important role for ubiquitin during the early stages of lens differentiation. The selective expression indicates that the three genes have specific differentiation related functions.

Studies on trypsin-modified bovine and human lens acylpeptide hydrolase

Acylpeptide hydrolase removes the N -acetylated amino acids from the peptide substrates but not from intact proteins. Cleavage between amino acid residues 203--204 of the native acylpeptide hydrolase results in the formation of a 55 kDa truncated active enzyme in the bovine lens, in vivo. In this study we explored the hydrolytic properties of the truncated enzyme using lens beta- and gamma-crystallins as substrates. SDS--PAGE analysis indicated that the beta B2-crystallin was cleaved by truncated acylpeptide hydrolase into several protein fragments (10--26 kDa). No cleavage of the gamma-crystallins was observed under similar conditions. Both the acylpeptide hydrolase activity and the protease activity of the 55 kDa enzyme were completely inhibited by diisopropylfluorophosphate, p -chloromercuribenzoate and ebelactone, and moderately inhibited by N -tosyl phenylalanine chloromethyl ketone. SDS--PAGE analysis followed by fluorography of ((3)H) diisopropylfluorophosphate labeled human lens acylpeptide hydrolase preparation showed the presence of the 55 kDa truncated form of the enzyme, as observed in the bovine lens. The peptide (d)-AIKGDQFL-NH(2)--the amino acid sequence 200--207 of the native bovine acylpeptide hydrolase with an in vivo cleavage site of native protein--was hydrolysed by the lens protease(s) suggesting that the in vivo generation of the 55 kDa acylpeptide hydrolase may be mediated through a proteolytic processing. The protease(s) responsible for the cleavage of this peptide was inhibited by diisopropylfluorophosphate and p -chloromercuribenzoate.

Differential inhibition of three peptidase activities of the proteasome in human lens epithelium by heat and oxidation

The proteasome is a large protease complex that is thought to be responsible for proteolytic removal of damaged proteins. We have previously shown that the level of proteolytic activity due to the proteasome is lower in lens epithelium from human cataractous lenses compared to the activity in epithelium from clear donor lenses. This study aimed to characterize the three main peptidase activities of the proteasome in human lens epithelium with respect to kinetic properties and sensitivity to heat and oxidation. Human lens epithelia were obtained from cataract surgery and analysis performed on pools of epithelial cell cytoplasm. Using the fluorogenic peptide substrates Suc-Leu-Leu-Val-Tyr-AMC (LLVY), Boc-Val-Gly-Arg-AMC (VGR) and Z-Leu-Leu-Glu-betaNA (LLE), Km-values of 56, 678 and 108 micrometers were obtained. All peptidase activities were inhibited by lactacystin, a specific proteasome inhibitor, but at very different rates; with LLVY-hydrolysing activity being the most sensitive (Ki50%=0.15 micrometers). Thermostability was investigated by performing the proteolytic assay at 20 degrees, 37 degrees and 53 degrees C. The trypsin-like activity, as measured by VGR, was completely stable at 53 degrees C for at least 24 hr whereas hydrolysis of LLVY and LLE declined after a few hours at 37 degrees C. Oxidative inhibition was induced by incubation of the samples in 0.5 m m H2O2for 1 or 24 hr. One hour exposure to H2O2caused moderate inhibition of all peptidase activities. The activity could be partially restored by adding 1 m m dithiotreitol, indicating the dependency on intact SH-groups. After 24 hr, peptidase activities were decreased to 25% (LLVY), 73% (VGR) and 44% (LLE) of corresponding control. This inhibition was irreversible for VGR and LLE, but could be partly prevented by the presence of heat shock protein 90 (LLVY and VGR) or alpha-crystallin (LLVY). These data show that the peptidase activities of the human lens proteasome can be modulated by metabolites, such as reactive oxygen species, and by endogenous proteins such as alpha-crystallin and heat shock protein 90.

Gene expression of the proteasome in rat lens development

To study the involvement of the proteasome in ocular lens cell proliferation and differentiation, a partial cDNA encoding rat S7, a subunit of the ATPase complex that regulates the 20S proteasome (multicatalytic proteinase complex), and RC3, a subunit of the 20S proteasome moiety, were cloned and used to compare relative levels of S7 and RC3 mRNAs. mRNA was measured, using a competitive RT-PCR assay, in isolated lens cells or explant cultures induced to differentiate or proliferate. During differentiation, S7 mRNA levels increased (1.7 fold) and RC3 mRNA levels remained the same compared to mRNA in quiescent cells. During proliferation, RC3 mRNA levels were elevated (2 fold) and S7 mRNA levels remained the same. This demonstrated that representative proteasome and ATPase complex mRNA levels are regulated differentially during differentiation and proliferation. The maintenance of proteasome subunit mRNA and increase in ATPase complex subunit mRNA observed in differentiating lens cells is in contrast to the patterns of expression that have been reported for other differentiating cells, which down-regulate the 20S and/or 26S proteasome. This suggests that the role of the proteasome in cell development is cell specific.

Peptide hydrolysis in lens: role of leucine aminopeptidase, aminopeptidase III, prolyloligopeptidase and acylpeptidehydrolase

The distribution of leucine aminopeptidase, aminopeptidase III, prolyloligopeptidase and acylpeptidehydrolase activities in different regions of a bovine lens was determined and correlated with the distribution of crystallin fragments (measured as < 18 kDa protein) and water-insoluble proteins in the same lens. A gradient of activity was observed for all the peptidases tested, with the highest specific activity present in the cortical fibers which decreased to one half or below in the inner cortical fibers and nucleus. An inverse correlation between peptidase activities and the amount of crystallin fragments was observed in different regions of the lens. However, a direct correlation between the water-insoluble protein content and the crystallin fragments was observed in all fibers of the same lens. The amount of crystallin fragments and the amount of water-insoluble proteins increased from 2.7% and 8% in the outer cortical fibers to 13% and 68% in the nucleus of the same lens. The water-insoluble fraction from both cortical and nuclear fibers however displayed 4-5 fold more crystallin fragments compared to that present in the water-soluble fraction of the same preparation. When the bovine lens cortical and nuclear extracts were tested for their ability to hydrolyze the peptide substrate, Ile-Ser-bradykinin, the cortical extract was found to be at least ten times superior to the nuclear extract. Prior inactivation of prolyloligopeptidase and other serine proteases by diisopropylfluorophosphate however diminished the ability of the cortical extract to hydrolyze peptide substrates. Bovine lens cortical extract was able to completely hydrolyze alpha-melanocyte stimulating hormone as well as N-Acetyl-Met-Asp-Arg-Val-Leu-Ser-Arg-Tyr showing the presence of active acylpeptidehydrolase facilitating the complete hydrolysis of N-terminally blocked peptides. The human lens extract was found to contain both diisopropylfluorophosphate sensitive and resistant enzymes capable of hydrolyzing peptide substrates.

Anatomy and morphology of the cornea of bovine eyes from a slaughterhouse

Over a 1-year period, isolated bovine eyes were obtained from a slaughterhouse and assessed within 3 h post mortem. The gross disposition (including damage or disease) and corneal surface characteristics (overall appearance and wettability) were assessed by visual inspection and light and scanning electron microscopy. The results from 315 eyes (assessed to be free of gross abnormalities or damage) showed that the corneal thickness was 1015 ± 104 μm (mean ± SD; n = 315). Measures on 100 of these eyes revealed corneal dimensions averaging 29.8 ± 1.3 mm horizontally and 23.9 ± 1.5 mm vertically. The horizontal corneal diameter was greater in eyes with thicker corneas (r = 0.917). Regardless of thickness, corneas evaluated within 3 h post mortem had a uniform thickness within ± 3% from center to edge. Histology and scanning electron microscopy revealed that even corneas subjectively assessed to be in good condition had relatively large numbers of exfoliating cells at the epithelial surface, indicating that the corneal surface of bovine eyes from the slaughterhouse is likely to be slightly compromised. Scanning electron microscopy showed the endothelium to be a mosaic of uniformly sized polygons of which 67.1 ± 2.7% were six-sided cells (hexagons).

Structure and modifications of the junior chaperone alpha-crystallin. From lens transparency to molecular pathology

alpha-Crystallin is a high-molecular-mass protein that for many decades was thought to be one of the rare real organ-specific proteins. This protein exists as an aggregate of about 800 kDa, but its composition is simple. Only two closely related subunits termed alpha A- and alpha B-crystallin, with molecular masses of approximately 20 kDa, form the building blocks of the aggregate. The idea of organ-specificity had to be abandoned when it was discovered that alpha-crystallin occurs in a great variety of nonlenticular tissues, notably heart, kidney, striated muscle and several tumors. Moreover alpha B-crystallin is a major component of ubiquinated inclusion bodies in human degenerative diseases. An earlier excitement arose when it was found that alpha B-crystallin, due to its very similar structural and functional properties, belongs to the heat-shock protein family. Eventually the chaperone nature of alpha-crystallin could be demonstrated unequivocally. All these unexpected findings make alpha-crystallin a subject of great interest far beyond the lens research field. A survey of structural data about alpha-crystallin is presented here. Since alpha-crystallin has resisted crystallization, only theoretical models of its three-dimensional structure are available. Due to its long life in the eye lens, alpha-crystallin is one of the best studied proteins with respect to post-translational modifications, including age-induced alterations. Because of its similarities with the small heat-shock proteins, the findings about alpha-crystallin are illuminative for the latter proteins as well. This review deals with: structural aspects, post-translational modifications (including deamidation, racemization, phosphorylation, acetylation, glycation, age-dependent truncation), the occurrence outside of the eye lens, the heat-shock relation and the chaperone activity of alpha-crystallin.

Ubiquitin and ubiquitin conjugates in human lens

Ubiquitin, an 8.5 kDa polypeptide found almost universally in plants and animals, is a normal component in the lens. The best documented function for ubiquitin involves its conjugation to proteins as a signal to initiate degradation. Conjugates for ubiquitin-dependent degradation tend to be of very high molecular mass and are rapidly degraded. Another role of ubiquitin conjugation may be as a stabilizer during stress for protection of constituent proteins, resulting in ubiquitin conjugates that are long-lived. Examination of clear and cataractous human lenses of < 1 to > 50 years revealed the dramatic accumulation of ubiquitin and ubiquitin conjugates with age, beginning at approximately 10 years. Epithelial tissue contained predominantly conjugates of > 250 kDa, although ubiquitin conjugates were found at 98 and 40-60 kDa in tissues from older donors. The water-soluble, urea-soluble and urea-insoluble fractions of lens cortex and core also contain ubiquitin conjugates that accrue with age. High molecular mass conjugates (> 250 kDa) are particularly prominent in older lens tissue. Cataractous lenses, as compared with normal lenses of the same age, show more of these high molecular mass conjugates in the urea-soluble and urea-insoluble fractions of cortex and core. Heterogeneous conjugates in the 20-85 kDa range accumulate in an age-related fashion in all lens cortex and core fractions. While levels of free ubiquitin are significant in the epithelium and the water-soluble cortex and core for all ages, there is no detectable free ubiquitin in the urea-soluble and urea-insoluble fraction under conditions used in this study.(ABSTRACT TRUNCATED AT 250 WORDS)

Description of an acylpeptide hydrolase from lens

Acylpeptide hydrolase, which catalyses the hydrolysis of blocked N-terminal amino acids from peptide substrates, has been identified in the extracts from beef, human, rabbit and rat lens. In bovine lens sections, lower activity was observed in nuclear and inner cortical regions compared to the outer cortical region. The enzyme from bovine lens showed a high molecular weight nature, eluting between alpha and beta crystallins during Sephadex G-200 chromatography. The activity has a pH optimum around 7.8 when assayed with N-acetyl-Ala-p-NA as substrate. The enzyme was capable of hydrolyzing a variety of blocked peptides including N-acetyl-(Ala)2, Me-O-Suc-Ala-Ala-Pro-Val-p-NA, N-Acetyl-Met-Leu-Phe, Acetyl-Ser-Gln-Asn-Tyr and N-formyl-Met-p-NA. In each case the enzyme released an N-blocked amino acid and exposed a free amino group as judged by thin layer chromatography. Neither Ala-p-NA nor N-acetyl-Ala were hydrolysed by the same enzyme preparation. The enzyme activity from human and bovine lens was completely inhibited by DFP, and partially inhibited by PMSF, penicillin-G and ampicillin. These preliminary results show that lens tissue has an active acylpeptide hydrolase, however, a partially purified enzyme preparations was not able to cleave the acetyl-Met- from native alpha A-crystallin in vitro suggesting that the N-terminus of native crystallins is not accessible to the enzymes.

Post-translational arginylation in the bovine lens

This study demonstrates post-translational arginylation of bovine serum albumin and endogenous lens proteins by bovine lens arginyl-tRNA:protein transferase. This reaction has been proposed to be the first step in marking specific proteins for degradation by the non-lysosomal, ATP-dependent, ubiquitin-mediated proteolytic pathway. The transferase was obtained by the method used for isolation of the same enzyme from reticulocytes (Ferber and Ciechanover, 1987, Nature 326, 808-11). Incorporation of [3H]Arg was linear for at least 2 hr at 37 degrees C. The amount of incorporation was directly proportional to the amount of lens enzyme or substrate added. Arginylation was ATP-dependent. A requirement for tRNA was demonstrated by inhibition upon pretreatment of the enzyme preparation with nuclease to hydrolyse endogenous tRNA, and restoration of activity upon replacement of tRNA. [3H]Leu, [3H]Lys and [3H]His were not incorporated, demonstrating specificity of the reaction for arginine. This is the first demonstration of post-translational modification of proteins by arginylation in the lens.

Protein oxidation and proteolytic degradation. General aspects and relationship to cataract formation

1) Intracellular proteins are subject to oxidative and photooxidative denaturation. 2) Proteolytic systems recognize and selectively degrade oxidatively denatured, and photooxidatively denatured proteins. By degrading mildly denatured proteins these proteolytic systems prevent further oxidative/photooxidative damage which could otherwise result in the formation of cross-linked (undigestible) proteins, or protein fragments with toxic biological activities. Proteolytic systems also provide amino acids for the synthesis of new (replacement) proteins. 3) A 700,000 dalton neutral endoproteinase, which we have called macroxyproteinase or M.O.P., appears to be mostly responsible for the degradation of oxidatively denatured proteins. M.O.P. has been shown to function in red blood cells and in the eye lens, and appears to also exist in many other mammalian cell types. 4) Cataract is a disease associated with aging, and with photooxidative denaturation (and cross-linking) of lens crystallins and other proteins. 5) Both cataract and aging of lens cells are associated with declining proteolytic capacity and diminished antioxidant protection. 6) Lens aging and in vivo photooxidative stress can cause opacity ("cataract"), cross-linking of crystallins, and diminished proteolytic capacity. 7) High levels of dietary ascorbate increase ascorbate concentrations in lens tissue, and are associated with greater resistance of lens proteins and lens proteolytic enzymes to oxidative/photooxidative stress in vitro.

Protease activities in cultured beef lens epithelial cells peak and then decline upon progressive passage

Beef lens cells in culture are readily obtained and provide many opportunities to study phenomena related to cell differentiation and maturation, environmental stress, disease, and perhaps mechanisms of transformation. Although altered rates of proteolysis are known to accompany these phenomena, the proteolytic activities available in cultured beef lens epithelial cells have not been documented. In this work are documented the specific activities, based on protein and DNA content, of neutral exo- and endopeptidase, cathepsins B- and D-like enzymes and acid phosphatase in lens epithelial cortical and core tissue and in cultured epithelial cells at passages 1-43. Maximal activity of each protease occurs almost routinely at passage 5 or 9, reaching values of approx. 1400-, 0.77-, 4520-nmol min-1 per mg protein for neutral exopeptidase (passage 5), neutral endopeptidase (passage 5) and cathepsin B (passage 5) respectively, and 7.1 micrograms trichloroacetic acid soluble peptide min-1 per mg protein for cathepsin D (passage 15). On a microgram-1 DNA basis, the maximal specific activities for the same enzymes were 48 (passage 5), 0.03 (passage 5), 283 (passage 9), and 0.5 (passage 9) respectively. In subsequent passages, the specific activities declined to values which were similar to or lower than the specific activities observed for these proteases in lens epithelial tissue.

Protein oxidation and loss of protease activity may lead to cataract formation in the aged lens

Over 95% of the dry mass of the eye lens consists of specialized proteins called crystallins. Aged lenses are subject to cataract formation, in which damage, cross-linking, and precipitation of crystallins contribute to a loss of lens clarity. Cataract is one of the major causes of blindness, and it is estimated that over 50,000,000 people suffer from this disability. Damage to lens crystallins appears to be largely attributable to the effects of UV radiation and/or various active oxygen species (oxygen radicals, 1O2, H2O2, etc.). Photooxidative damage to lens crystallins is normally retarded by a series of antioxidant enzymes and compounds. Crystallins which experience mild oxidative damage are rapidly degraded by a system of lenticular proteases. However, extensive oxidation and cross-linking severely decrease proteolytic susceptibility of lens crystallins. Thus, in the young lens the combination of antioxidants and proteases serves to prevent crystallin damage and precipitation in cataract formation. The aged lens, however, exhibits diminished antioxidant capacity and decreased proteolytic capabilities. The loss of proteolytic activity may actually be partially attributable to oxidative damage which proteases (like any other protein) can sustain. We propose that the rate of crystallin damage increases as antioxidant capacity declines with age. The lower protease activity of aged lens cells may be insufficient to cope with such rates of crystallin damage, and denatured crystallins may begin to accumulate. As the concentration of oxidatively denatured crystallins rises, cross-linking reactions may produce insoluble aggregates which are refractive to protease digestion. Such a scheme could explain many events which are known to contribute to cataract formation, as well as several which have appeared to be unrelated.(ABSTRACT TRUNCATED AT 250 WORDS)