Cross-linking and proteolysis in Ca2(+)-treated lens homogenates

Ca2+ and βγ-crystallins: An affair that did not last?

BACKGROUND

During the last three decades, lens β- and γ-crystallins have found a huge number of kin from numerous taxonomical sources. Most of these proteins from invertebrates and microbes have been demonstrated or predicted to bind Ca2+ involving a distinct double-clamp motif, which is largely degenerated in lens homologues.

SCOPE OF REVIEW

The various aspects of transformation of βγ-crystallins from a quintessential Ca2+-binding protein into a primarily structural molecule have been reviewed.

MAJOR CONCLUSIONS

In lens members of βγ-crystallins, the residues involved in Ca2+ binding have diverged considerably from the classical consensus with consequent reduction in their Ca2+-binding properties. This evolutionary change is congenial to their new role as robust constituents of lens. The exact functions of the residual affinity for Ca2+ are yet to be established.

GENERAL SIGNIFICANCE

This review highlights the significance of reduction in Ca2+-binding ability of the βγ-crystallins for lens physiology and why this residual affinity may be functionally important. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

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.

Transglutaminases: nature's biological glues

Transglutaminases (Tgases) are a widely distributed group of enzymes that catalyse the post-translational modification of proteins by the formation of isopeptide bonds. This occurs either through protein cross-linking via epsilon-(gamma-glutamyl)lysine bonds or through incorporation of primary amines at selected peptide-bound glutamine residues. The cross-linked products, often of high molecular mass, are highly resistant to mechanical challenge and proteolytic degradation, and their accumulation is found in a number of tissues and processes where such properties are important, including skin, hair, blood clotting and wound healing. However, deregulation of enzyme activity generally associated with major disruptions in cellular homoeostatic mechanisms has resulted in these enzymes contributing to a number of human diseases, including chronic neurodegeneration, neoplastic diseases, autoimmune diseases, diseases involving progressive tissue fibrosis and diseases related to the epidermis of the skin. In the present review we detail the structural and regulatory features important in mammalian Tgases, with particular focus on the ubiquitous type 2 tissue enzyme. Physiological roles and substrates are discussed with a view to increasing and understanding the pathogenesis of the diseases associated with transglutaminases. Moreover the ability of these enzymes to modify proteins and act as biological glues has not gone unnoticed by the commercial sector. As a consequence, we have included some of the present and future biotechnological applications of this increasingly important group of enzymes.

Properties of purified lens transglutaminase and regulation of its transamidase/crosslinking activity by GTP

On account of its protein crosslinking activity, the Ca2+-dependent transglutaminase of the lens is likely to be involved in the formation of cataracts. We have now purified the rabbit lens enzyme to near homogeneity as judged by SDS-PAGE (Mr approximately 78 kDa), and a key feature of the procedure was the use of a highly selective affinity chromatographic step with a fibronectin fragment as ligand. The catalytic activity of the lens transglutaminase, measured by the incorporation of dansylcadaverine into dimethylcasein, was compared with those of two similar enzymes isolated from human red cells and from guinea pig liver, respectively. All three enzymes were inhibited by GTP, but the lens enzyme was most sensitive to inhibition by the nucleotide. Moreover, GTP was also shown to inhibit the formation of the approximately 55 kDa betacrystallin dimers in the Ca2+-treated rabbit lens homogenate, proving that the nucleotide is a negative regulator for the crosslinking activity of transglutaminase in this tissue.

Novel inhibitors against the transglutaminase-catalysed crosslinking of lens proteins

Post-translational modifications by transglutaminase may contribute to the remodeling of cellular architecture in the development of lens fiber cells, and there is evidence that the enzyme may also play a role in cataract formation. It catalyses hydrolytic deamidations as well as amide exchanges on select glutamine side chains at endo positions in a small subset of proteins of the lens. N epsilon(gamma-glutamyl)lysine crosslinks, the characteristic hallmarks of transglutaminase activity, were identified in polymers isolated from human cataract. Following up on our earlier studies relating to the inhibition of protein crosslinking by the Ca(2+)-activated transglutaminase in the lens, we have now examined the effects of 2-[(2-oxopropyl)thio]-imidazolium derivatives, recently described as active site-directed inhibitors for this family of enzymes. First, we have shown that the compounds at concentrations of 1-2 microM were effective in blocking the transamidating activities of partially purified lens transglutaminase. Then we focused on their efficacy in preventing the formation of the ca. 55 kDa beta crystallin dimers in the whole lens tissue. The production of these dimers, crosslinked by N epsilon(gamma-glutamyl)lysine isopeptide bridges, is an early sign of transglutaminase action in rabbit lens, and it can be readily documented by the SDS-PAGE analysis of proteins remaining in the soluble phase after brief exposure of the homogenate to Ca2+. The new compounds proved to be potent inhibitors of transglutaminase also in this preparation, preventing the crosslinking event at ca. 1 microM concentration. Moreover, even when applied at a 1,000-fold greater concentration (2 mM), they did not interfere with the action of calpain which, similarly to the activation of the transglutaminase system, is triggered by the addition of Ca2+. The high selectivity of the new compounds for differentially blocking only the transglutaminase and not the calpain of the lens, is all the more remarkable because these two enzymes share several mechanistic and structural similarities.

Isolation of transglutaminase-reactive sequences from complex biological systems: a prominent lysine donor sequence in bovine lens

The transglutaminase (protein-glutamine: amine gamma-glutamyltransferase, EC 2.3.2.13)-catalyzed cross-linking of proteins in biological systems can often be inhibited by inclusion of small primary amines or glutamine-containing peptides, which act as site-specific blockers of the relevant acceptor (i.e., glutamine) and donor (i.e., lysine) functionalities of the natural substrates. Compounds such as dansylcadaverine and dansyl-epsilon-aminocaproyl-Gln-Gln-Ile-Val are particularly useful in sorting out acceptor-donor relationships among lens crystallins. Apart from its fluorescent properties, the dansyl hapten offered special advantages as a "handle" for the rapid isolation of transglutaminase targets even in the complex system of lens cortical homogenate. The dansylated peptide was incorporated into bovine lens proteins under the influence of the Ca(2+)-activated intrinsic transglutaminase and, after digestion by endoproteinase Glu-C, the tracer-containing fragments were isolated by affinity chromatography on an anti-dansyl antibody column. The major fluorescent peak was isolated by HPLC and sequenced by Edman degradation, which yielded phenylthiohydantoin amino acid derivatives for the first 10 cycles, EKPAVTAAPK, and none for the next 2. The sequence, corresponding to residues 165-174 of alpha B-crystallin, unambiguously identifies the known carboxyl-terminal domain, EK-PAVTAAPKK, as the prominent lysine-donating fragment in bovine lens.

Labeling of epsilon-lysine crosslinking sites in proteins with peptide substrates of factor XIIIa and transglutaminase

Peptides patterned on the N-terminal sequence of fibronectin were synthesized and tested for amine acceptor qualities in reactions with dansylcadaverine catalyzed either by coagulation factor XIIIa or intracellular transglutaminase (protein-glutamine:amine gamma-glutamyltransferase, EC 2.3.2.13). On the basis of inverse half-saturations of the enzymes, the order of acceptor substrate affinity for factor XIIIa was pEAQQIV much greater than Boc-AQQIV greater than Boc-QQIV, and for transglutaminase, Boc-QQIV greater than Boc-AQQIV greater than pEAQQIV (amino acid residues are shown in one-letter code; pE, pyroglutamic acid; Boc, tert-butyloxycarbonyl). Sequence analysis of dansylcadaverine-substituted pEAQQIV indicated that the first of the two adjacent glutamine residues was the target of enzymatic modification. Boc-QIV showed no substrate activity with either enzyme. Crosslinking of crystallins in Ca2(+)-treated rabbit lens homogenate was readily inhibited by Boc-QQIV, Boc-AQQIV, and pEAQQIV, as was the formation of alpha-chain polymers in human fibrin by pEAQQIV in the presence of human factor XIIIa. SDS/PAGE analysis suggested that the inhibitory peptides selectively blocked the electron donor functionalities in these enzymatic crosslinking reactions.