alpha A-crystallin is expressed in non-ocular tissues

alpha-crystallin stabilizes actin filaments and prevents cytochalasin-induced depolymerization in a phosphorylation-dependent manner

alpha-crystallin, a major lens protein of approximately 800 kDa with subunits of about 20 kDa has previously been shown to act as a chaperone protecting other proteins from stress-induced damage and to share sequence similarity with small heat-shock proteins, sHsp. It is now demonstrated that this chaperone effect extends to protection of the intracellular matrix component actin. It was found that the powerful depolymerization effect of cytochalasin D could be almost completely blocked by alpha-crystallin, alpha A-crystallin or alpha B-crystallin. However, phosphorylation of alpha-crystallin markedly decreased its protective effect. It is suggested that phosphorylation of alpha-crystallin may contribute to changes in actin structure observed during cellular remodeling that occurs with the terminal differentiation of a lens epithelial cell to a fiber cell and contributes to cellular remodeling in other cell types that contain alpha-crystallin species. This communication presents biochemical evidence clearly demonstrating that alpha-crystallin is involved in actin polymerization-depolymerization dynamics. It is also shown that alpha-crystallin prevented heat-induced aggregation of actin filaments. alpha-crystallin was found to stabilize actin polymers decreasing dilution-induced depolymerization rates up to twofold while slightly decreasing the critical concentration from 0.23 microM to 0.18 microM. Similar results were found with either alpha-crystallin or its purified subunits alpha A-crystallin and alpha B-crystallin. In contrast to the experiments with cytochalasin D, phosphorylation had no effect. There does not appear to be an interaction between alpha-crystallin and actin monomers since the effect of alpha-crystallin in enhancing actin polymerization does not become apparent until some polymerization has occurred. Examination of the stoichiometry of the alpha-crystallin effect indicates that 2-3 alpha-crystallin monomers/actin monomer give maximum actin polymer stabilization.

Spatial and temporal activity of the alpha B-crystallin/small heat shock protein gene promoter in transgenic mice

In order to study the spatial and temporal activity of the mouse alpha B-crystallin/small heat shock gene promoter during embryogenesis, we generated mice harboring a transgene consisting of approximately 4 kbp of alpha B-crystallin promoter sequence fused to the Escherichia coli lacZ reporter gene. beta-galactosidase activity was first observed in the heart rudiment of 8.5 days post coitum (d.p.c.) embryos. An identical expression pattern was obtained for the endogenous alpha B-crystallin gene by whole mount in situ hybridization. At 9.5 d.p.c., beta-galactosidase activity was detected in the lens placode, in the myotome of the somites, in Rathke's pouch (future anterior pituitary), and in some regions of oral ectoderm. We also examined the stress inducibility of the alpha B-crystallin promoter in vivo. Injection of sodium arsenite into mice resulted in increased endogenous alpha B-crystallin expression in the adrenal gland and possibly the liver. Our results indicate that visualization of beta-galactosidase activity provides an accurate reflection of endogenous alpha B-crystallin expression and demonstrate that the complex developmental pattern of mouse alpha B-crystallin gene expression is regulated at the transcriptional level. This expression pattern, coupled with the present literature which addresses functions of the protein, suggests a role for the alpha B-crystallin/small heat shock protein in intermediate filament turnover and cellular remodeling which occur during normal development and differentiation.

Conformational properties of substrate proteins bound to a molecular chaperone alpha-crystallin

alpha-Crystallin, the major protein of the ocular lens, acts as a molecular chaperone by suppressing the nonspecific aggregation of damaged proteins. To investigate the mechanism of the interaction between alpha-crystallin and substrate proteins, we prepared a tryptophan-free mutant of human alpha A-crystallin and assessed the conformation of thermally destabilized proteins captured by this chaperone using fluorescence spectroscopy. The fluorescence emission characteristics of bound substrates (rhodanese and gamma-crystallin) and the results of fluorescence quenching experiments indicate that the proteins captured by alpha-crystallin are characterized by a very low degree of unfolding. In particular, the structure of rhodanese bound to alpha A-crystallin appears to be considerably more native-like compared to that of the enzyme bound to the chaperonin GroEL. We postulate that alpha-crystallin (and likely other small heat shock proteins) recognize preferentially the aggregation-prone conformers that occur very early on the denaturation pathway. With its ability to capture and stabilize these early non-native structures, alpha-crystallin appears to be uniquely well suited to chaperone the transparency properties of the ocular lens.

Evidence that alpha-crystallin prevents non-specific protein aggregation in the intact eye lens

The ocular lens is a transparent organ comprised of a highly concentrated and highly ordered matrix of structural proteins, called crystallins, which are probably the longest lived proteins of the body. Lens transparency is dependent upon maintenance of the short range order of the crystallin matrix. This transparency must be maintained for decades in the absence of normal protein synthesis or repair capacity. We present evidence here that alpha-crystallin, one of the major lens proteins, plays a central role in vivo in stabilizing the other crystallins and preventing uncontrolled aggregation of these progressively modified and aging molecules. alpha-Crystallin has previously been shown to suppress non-specific aggregation of denaturing proteins in simple binary systems through a chaperone-like activity. Our studies using soluble homogenates of monkey lenses demonstrate a strong resistance to heat induced non-specific aggregation when the complete complement of crystallins is present; in contrast, if alpha-crystallin is selectively removed prior to heating, the remaining crystallins undergo extensive non-specific aggregation as indicated by light scattering. When alpha-crystallin is present it complexes with denaturing proteins forming a soluble heavy molecular weight (HMW) fraction but no insolubilization is observed, while when alpha-crystallin is absent there is heavy insolubilization and no HMW formed. When intact monkey lenses were heated it could be demonstrated that soluble HMW was generated. Similar HMW protein appears in vivo in the human lens as a function of age. These findings suggest that the soluble HMW protein present in the human lens is the product of the chaperone-like function of alpha-crystallin and that under physiological conditions alpha-crystallin inhibits the uncontrolled aggregation of damaged proteins, thereby preventing the formation of light scattering centers and opacification of the lens.

Beta B2-crystallin in the mammalian retina

beta-crystallins are abundant lens proteins in most, if not all vertebrate species. We have previously reported the presence of low levels of beta-crystallins in chick non-lens tissues, both ocular and extra-ocular, including the expression of beta B2-crystallin in the retina. Here we report that extralenticular beta-crystallin expression is also found in mammals. beta B2-crystallin is expressed in mouse and cat neural and pigmented retinas and in cat iris. Although present at levels lower than those found in the lens, the appearance and accumulation of beta B2-crystallin in the neural retina coincides with the functional maturation of this tissue.

Effect of cross-linking on the chaperone-like function of alpha crystallin

Alpha crystallin can function as a molecular chaperone in suppressing the heat-induced aggregation of other crystallins and proteins. During cataractogenesis, alpha-crystallin becomes a water-insoluble, high-molecular-weight, cross-linked aggregate. To determine whether the chaperone activity of alpha crystallin is lost during this age-related modification, extracts were prepared by sonication of water-insoluble proteins isolated from aged bovine lenses and human cataract lenses. All the preparations were tested for chaperone-like activity using beta L-crystallin as the target protein and the percentage of alpha-crystallin in water-insoluble sonicated supernatant (WISS) was determined by slot blot immunoassay. The WISS from bovine as well as human lenses were still effective in protecting beta L-crystallin aggregation at 56 degrees C. The bovine cortical WISS with 50% immunoreactive alpha-crystallin showed 62% of the chaperone-like activity displayed by native alpha-crystallin. The WISS from bovine lens nucleus and human lenses with 17% and 5% immunoreactive alpha-crystallin showed 19% and 4% chaperone-like activity compared to native alpha-crystallin. Prior treatment of the WISS of both bovine and human lenses with dithiothreitol resulted in nearly 50% increase in chaperone-like activity suggesting possible loss of chaperone-like activity due to disulfide cross-links. To see if the chaperone-like activity of alpha-crystallin can be altered by non-disulfide cross-linking, native alpha-crystallin isolated from bovine lenses was cross-linked with dimethylsuberimidate (DMS) and dimethyl 3,3'-dithiobispropionimidate (DTBP) and tested for chaperone-like activity. The DMS cross-linked alpha-crystallin was effective in inhibiting the aggregation of beta L-crystallins at 56 degrees C, but required a two- to five-fold higher concentration than the native alpha-crystallin. alpha-Crystallin with higher degree of cross-linking showed lower chaperone-like activity. alpha-Crystallin cross-linked with DTBP, a cleavable cross-linking agent, also showed a 80% loss in chaperone-like activity. However, when the DTBP cross-linked alpha-crystallin was treated with dithiothreitol to cleave the cross-links there was a 50% recovery in the chaperone-like activity. These data suggest that the age-related cross-linking, which restricts the molecular flexibility of alpha-crystallin decreases its chaperone-like function.

alpha-Crystallin quaternary structure: molecular basis for its chaperone activity

alpha-Crystallin, the major protein in all vertebrate lenses, functions as a chaperone. In the present analysis, an 'open' micellar structure composed of alpha A subunits is used to simulate chaperoning of partially heat denatured soluble gamma-crystallin. The interaction is both electrostatic and hydrophobic and satisfies experimental evidence for a 1:1 alpha/gamma molar ratio, a doubling of molecular mass and a minimal increase in the dimensions of the complex [J. Biol. Chem. (1994) 269, 13601-13608; Invest. Opthalmol. Vis. Sci. (1995) 36, 311-21]. These data are also in accord with Farahbaksh et al. [Biochemistry (1995) 34, 509-16]; i.e. the bound gamma-crystallin monomers are not in a central cavity, but are separated by alpha A subunits.

The mutation Asp69-->Ser affects the chaperone-like activity of alpha A-crystallin

alpha-Crystallins are members of the family of small heat-shock proteins. The conformation and mode of action of these 'junior chaperones' are unknown. To investigate the structure and chaperone-like activity, four mutants of bovine alpha A-crystallin were generated by site-directed mutagenesis. In comparison with wild-type alpha A-crystallin, the D69S mutant, in which a highly conserved charged residue has been replaced, forms larger multimers and displays a threefold reduced heat-protection capacity. The conformation and thermal stability of this mutant are not noticeably affected. Three other mutations, replacing hydrophobic by uncharged hydrophilic residues, were aimed at disturbing hydrophobic intersubunit interactions. None of these mutations resulted in major structural perturbations and only minor differences in heat-protective capacity were observed. Although it is assumed that small heat-shock proteins interact with denaturing proteins via their hydrophobic surfaces, this study clearly shows that charged residues in alpha-crystallin can also influence the efficiency of substrate binding.

Conversion from oligomers to tetramers enhances autophosphorylation by lens alpha A-crystallin. Specificity between alpha A- and alpha B-crystallin subunits

Previously we showed that alpha-crystallins are autophosphorylated (Kantorow, M., and Piatigorsky, J. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 3112-3116). Here we report that addition of 1% deoxycholate converted alpha A-crystallin aggregates into 80-kDa tetramers which were 10-fold more active for autophosphorylation. Circular dichroism (CD) spectra of alpha-crystallin revealed little or no change in secondary and tertiary structures in 1% deoxycholate, alpha A2D, a truncated form of bovine alpha A that exists as a tetramer, was as active for autophosphorylation in the absence of deoxycholate as intact alpha A was in the presence of deoxycholate. At least one serine between amino acids 131 and 145 of bovine alpha A was autophosphorylated in peptide mapping experiments. Chicken alpha A-crystallin, which lacks the Ser-122 cAMP-dependent kinase site of bovine alpha A, was also autophosphorylated in the presence of deoxycholate. In contrast to alpha A-crystallin, autophosphorylation by alpha B-crystallin was not activated by deoxycholate despite its conversion to a tetrameric form, and alpha B was also more efficiently phosphorylated by cAMP-dependent kinase than alpha A. These data suggest metabolic differences between the alpha-crystallin subunits that may be related to specific expression of alpha A in the lens and ubiquitous expression of alpha B in numerous normal and diseased tissues.

Phosphorylation of alpha-crystallin in rat lenses is stimulated by H2O2 but phosphorylation has no effect on chaperone activity

Alpha crystallin (alpha), a phosphorylated structural protein of the lens, has been shown to be a chaperone preventing other lens proteins from aggregating. It is now demonstrated that with oxidative stress imposed on cultured rat lenses, the incorporation of labeled phosphate into the alpha polypeptide chains increased by two to four times over a 90-min period in comparison to control experiments. The phosphorylation rate of the B chain, alpha B, was twice that of the A chain, alpha A. However, phosphorylation of the alpha chains has an insignificant effect on the chaperone activity of alpha or the individual alpha A and alpha B chains as measured by suppressing the thermally induced aggregation of beta low or gamma crystallins. It was also found that the alpha A aggregates are more effective chaperones than the alpha B aggregates. The size of the macromolecules resulting from reaggregation of the isolated non-phosphorylated or phosphorylated alpha B chains are not markedly effected by phosphorylation. However, phosphorylation of the alpha A chain leads to a heterogeneous population with two major species, one similar in size to alpha A and another approximately twice as large. It is concluded that the phosphorylation of alpha is associated with some other function of the protein than that of chaperone activity and that this function may be linked to a protective response to oxidative stress.

Reduced chaperone-like activity of alpha A(ins)-crystallin, an alternative splicing product containing a large insert peptide

alpha-Crystallin is a multimeric protein complex which is constitutively expressed at high levels in the vertebrate eye lens, where it serves a structural role, and at low levels in several non-lenticular tissues. Like other members of the small heat shock protein family, alpha-crystallin has a chaperone-like activity in suppressing nonspecific aggregation of denaturing proteins in vitro. Apart from the major alpha A- and alpha B-subunits, alpha-crystallin of rodents contains an additional minor subunit resulting from alternative splicing, alpha A(ins)-crystallin. This polypeptide is identical to normal alpha A-crystallin except for an insert peptide of 23 residues. To explore the structural and functional consequences of this insertion, we have expressed rat alpha A- and alpha A(ins)-crystallin in Escherichia coli. The multimeric particles formed by alpha A(ins) are larger and more disperse than those of alpha A, but they are native-like and display a similar thermostability and morphology, as revealed by gel permeation chromatography, tryptophan fluorescence measurements, and electron microscopy. However, as compared with alpha A, the alpha A(ins)-particles display a diminished chaperone-like activity in the protection of heat-induced aggregation of beta low-crystallin. Our experiments indicate that alpha A(ins)-multimers have a 3-4-fold reduced substrate binding capacity, which might be correlated to their increased particle size and to a shielding of binding sites by the insert peptides. The structure-function relationship of the natural mutant alpha A(ins)-crystallin may shed light on the mechanism of chaperone-like activity displayed by all small heat shock proteins.

Alpha A-crystallin confers cellular thermoresistance

The bovine eye lens protein alpha A-crystallin has been overexpressed both by stable transfection of HeLa cells and by transient transfection of NIH 3T3 cells. In both experimental systems alpha A-crystallin overexpression results in an increased cellular thermoresistance as judged by different clonal survival assays. In contrast, similar overexpression of another stable lens protein, beta B2-crystallin, does not confer thermoresistance. These results indicate that the structural relationship of alpha A-crystallin to the small heat shock proteins HSP25/27 and to alpha B-crystallin is sufficient for the shared thermoprotective function of all of these molecules and strongly suggests that the chaperone-like properties that they have in common are responsible for the conferred cellular thermoresistance.

Identification of negative-acting and protein-binding elements in the mouse alpha A-crystallin -1556/-1165 region

The mouse alpha A-crystallin-encoding gene (alpha A-cry) is expressed in a highly lens-preferred manner. To date, it has been shown that this lens-preferred expression is controlled by four proximal positive-acting transcriptional regulatory elements: DE1 (-111/-97), alpha A-CRYBP1 (-66/-57), PE1/TATA (-35/-19) and PE2 (+24/+43). The present study extends our knowledge of mouse alpha A-cry transcriptional regulatory elements to the far upstream region of that gene by demonstrating that the -1556 to -1165 region contains negative-acting sequence elements which function in transfected lens cells derived from mouse, rabbit and chicken. This is the first negative-acting regulatory region identified in mouse alpha A-cry. The -1556 to -1165 region contains sequences similar to repressor/silencer elements identified in other genes, including those highly expressed in the lens, such as the delta 1-crystallin (delta 1-cry) and vimentin (vim) genes. The -1480 to -1401 region specifically interacts with nuclear proteins isolated from the alpha TN4-1 mouse lens cell line. Contained within this protein-binding region and positioned at -1453 to -1444 is a sequence (RS1) similar to the chicken delta 1-cry intron 3 repressor, and which competes for the formation of -1480 to -1401 DNA-protein complexes. Our findings suggest that lens nuclear proteins bind to the mouse alpha A-cry RS1 region. We demonstrate that the chicken delta 1-cry intron repressor binds similar nuclear proteins in chicken embryonic lens cells and mouse alpha TN4-1 lens cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Complete structure and expression of the rat alpha B-crystallin gene

alpha B-Crystallin, a member of the small heat shock family of proteins, is synthesized as a component of various developmental programs, in response to stress and in a number of pathological states. We have determined the complete structure of the alpha B-crystallin gene (6,806 bp encompassing 2,299 bp upstream from ATG and 859 bp at the 3' end, past the first polyadenylation signal). Comparison of the rat and the human alpha B-crystallin genes reveals significant conservation of the nucleotide sequences in almost all regions except in intron 2. The 1-kb region immediately upstream of ATG shows about 75% overall homology. A 78-bp sequence in the intron 1 and sequences in the 3' untranslated region show about 95% and 85% sequence identity, respectively. Characterization of the expression of this gene in different tissues in the rat by extensive analyses, utilizing primer extension. RNase protection, and rapid amplification of cDNA ends (RACE) revealed a predominant transcription initiation site 44 bp upstream of ATG. Northern analyses with "coding-only" and upstream "noncoding" probes did not support the thesis that heterogeneity in the alpha B-crystallin mRNAs arises from variations in the sequences immediately upstream of the predominant transcription initiation site. Importantly, the known relative levels of alpha B-crystallin protein in different tissues correlate best with the presence of transcripts starting from this initiation site.

Coordinate and independent regulation of alpha B-crystallin and hsp27 expression in response to physiological stress

alpha-Crystallins share structural and functional properties with the stress protein hsp27. These polypeptides are expressed at low constitutive levels in many tissues including brain, and alpha B-crystallin and hsp27 can accumulate in central nervous system glia in a variety of neurological conditions. We report here that heat shock and exposure to transition metals result in an increase in the steady state mRNA level of alpha B-crystallin and hsp27 in primary cultures of rat forebrain astrocytes. Both exposure to tumour necrosis factor-alpha and hypertonic conditions result in alpha B-crystallin mRNA accumulation but no change in the hsp27 mRNA level. Under some of these conditions increased synthesis and accumulation of alpha B-crystallin and hsp27 protein are also evident. We are unable to detect alpha A-crystallin mRNA in resting or stressed astrocytes. A novel phenomenon involving a transitory change in stress protein mRNA mobility in Northern blots during induction is reported, which is stress type and cell type independent. The results demonstrate multiple stress regulation of alpha B-crystallin and hsp27 in cultured astrocytes, suggesting that they can legitimately be regarded as stress proteins in the central nervous system.

Recruitment of enzymes and stress proteins as lens crystallins

The major water-soluble proteins--or crystallins--of the eye lens are either identical to or derived from proteins with non-refractive functions in numerous tissues. In general, the recruitment of crystallins has come from metabolic enzymes (usually with detoxification functions) or stress proteins. Some crystallins have been recruited without duplication of the original gene (i.e., lactate dehydrogenase B and alpha-enolase), while others have incurred one (i.e., argininosuccinate lyase and a small heat shock protein) or several (i.e., glutathione S-transferase) gene duplications. Enzyme (or stress protein)-crystallins often maintain their non-refractive function in the lens and/or other tissues as well as their refractive role, a process we call gene sharing. alpha-Crystallin/small heat shock protein/molecular chaperone is of special interest since it is the major crystallin of humans. There are two alpha-crystallin genes (alpha A and alpha B), with alpha B retaining the full functions of a small heat shock protein. Here we describe recent evidence indicating that alpha A and alpha B have kinase activity, which would make them members of the enzyme-crystallins. We also describe various regulatory elements of the mouse alpha-crystallin genes responsible for their expression in the lens and, for alpha B, in skeletal muscle. Delineating the control elements for gene expression of these multifunctional protective proteins provides the foundations for their eventual use in gene therapy. Finally, comparison of the mouse and chicken alpha A-crystallin genes reveals similarities and differences in their functional cis-acting elements, indicative of evolution at the level of gene regulation.

Defective lens fiber differentiation and pancreatic tumorigenesis caused by ectopic expression of the cellular retinoic acid-binding protein I

All-trans retinoic acid, a metabolite of retinol, is a possible morphogen in vertebrate development. Two classes of cellular proteins, which specifically bind all-trans retinoic acid, are thought to mediate its action: the nuclear retinoic acid receptors (RAR alpha, beta, gamma), and the cytoplasmic binding proteins known as cellular retinoic acid-binding proteins I and II (CRABP I and II). The function of the retinoic acid receptors is to regulate gene transcription by binding to DNA in conjunction with the nuclear retinoid X receptors (RXR alpha, beta, gamma), which in turn have 9-cis retinoic acid as a ligand. Several lines of evidence suggest that the role of the cellular retinoic acid-binding proteins is to control the concentration of free retinoic acid reaching the nucleus in a given cell. Here, we have addressed the role of the cellular retinoic acid-binding protein I in development by ectopically expressing it in the mouse lens, under the control of the alpha A-crystallin promoter. We show that this ectopic expression interferes with the development of the lens and with the differentiation of the secondary lens fiber cells, causing cataract formation. These results suggest that correct regulation of intracellular retinoic acid concentration is required for normal eye development. In addition, the generated transgenic mice also present expression of the transgene in the pancreas and develop pancreatic carcinomas, suggesting that overexpression of the cellular retinoic acid-binding protein is the cause of the tumors. These results taken together provide evidence for a role of the cellular retinoic acid-binding protein in development and cell differentiation. The relevance of these findings to the possible role of the cellular retinoic acid-binding proteins in the transduction of the retinoic acid signal is discussed.

Binding of tissue-specific forms of alpha A-CRYBP1 to their regulatory sequence in the mouse alpha A-crystallin-encoding gene: double-label immunoblotting of UV-crosslinked complexes

The alpha A-CRYBP1 regulatory sequence (alpha A-CRYBP1RS), at nucleotides -66 to -57 of the mouse alpha A-crystallin-encoding gene (alpha A-CRY) promoter, is an important control element involved in the regulation of mouse alpha A-CRY expression. The gene encoding a protein (alpha A-CRYBP1) that specifically binds to the alpha A-CRYBP1RS sequence has been cloned from a cultured mouse lens cell line. In the present study, we have used an antibody (specific to the alpha A-CRYBP1 protein and made against a synthetic peptide) to directly identify UV-crosslinked protein-DNA complexes via a double-label immunoblotting technique. Multiple alpha A-CRYB1 antigenically related proteins interacted with alpha A-CRYBP1RS in nuclear extracts from both a cloned mouse lens cell line (alpha TN4-1) that expresses alpha A-CRY and a mouse fibroblast line (L929) that does not express the gene. Two sizes (50 kDa and 90 kDa) of proteins reacting with the alpha A-CRYBP1-specific Ab were detected in both cell lines and, in addition, a > 200-kDa protein reacting with the Ab was unique to the fibroblast line. Thus, alpha A-CRYBP1 antigenically related proteins interact with alpha A-CRYBP1RS regardless of alpha A-CRY expression. Moreover, differential processing of the alpha A-CRYBP1 protein and/or alternative splicing of the alpha A-CRY transcript may affect expression of alpha A-CRY.