beta-Crystallins of the adult chicken lens: relatedness of the polypeptides and their aggregates

General utility of the chicken betaB1-crystallin promoter to drive protein expression in lens fiber cells of transgenic mice

Transgenic mouse technology has been very valuable for the study of lens fiber cells since they can not be propagated in cell culture. The targeting of transgenes to the lens has traditionally been done with the alphaA-crystallin promoter. However, while lens-specific, transgenic lines made with the alphaA-crystallin promoter express the transgene at levels 100-300-fold lower than endogenous alphaA-crystallin. Here we propose an alternative, the chicken betaB1-crystallin promoter (-432/+30). Transgenic mice made with this promoter have successfully expressed CAT, d/n m-calpain, Weel, and betaB2-crystallin mRNA at levels comparable to the endogenous betaB1-crystallin gene and no eye abnormalities such as cataracts, have resulted. All of the transgenic lines made with the chicken betaB1-crystallin promoter have expressed the transgene in the lens fiber cells, and the best lines express at levels close to endogenous betaB1-crystallin. While RNA expression is very high, only moderate protein expression has been achieved, implying that the high protein expression of the crystallins is partially controlled at the level of translation. Thus, the chicken betaB1-crystallin promoter directs high level RNA expression to lens fiber cells, which may be especially useful for the expression of ribozyme and anti-sense RNAs in addition to ectopic proteins.

The chicken beta A4- and beta B1-crystallin-encoding genes are tightly linked

Analysis of the 5' flanking region of the chicken beta B1-crystallin-encoding gene (beta B1-cry) revealed regions of sequence homology with the bovine beta A4-crystallin-encoding gene (beta A4-cry). Subsequently, the chicken beta A4-cry cDNA sequence was determined, and it was demonstrated that beta A4- and beta B1-cry are linked head to head in the chicken chromosome with 2147 nucleotides (nt) of intergenic spacer. Chicken beta A4-cry contains six exons, with the first exon being noncoding. Chicken beta A4-cry is the smallest beta-cry ever described, due to the small size of its introns which range in length from 68 to 96 nt. While three polymorphisms were noted between some cDNA clones and the genomic sequence, Southern blot analysis demonstrated that beta A4-cry exists as a single copy in the chicken genome. Northern blot analysis indicated that beta A4-cry is a lens-specific transcript which is expressed at higher levels in the embryo than in the adult. The beta A4-cry mRNA is present at 400-fold lower levels than the beta B1-cry mRNA in the 14-day embryonic chicken lens, and at 2000-fold lower levels than the beta B1-cry mRNA in the adult lens. These results are consistent with the idea that the beta-cry family was once clustered in the chromosome as the gamma-cry family is today, and raises the possibility that the relatively low expression of beta A4-cry is mechanistically linked to the high expression of beta B1-cry in the chicken lens.

Structure and lens expression of the gene encoding chicken beta A3/A1-crystallin

The beta A1- and beta A3-crystallins are major polypeptides in the lenses of vertebrates. We present evidence that a single beta A3/A1 gene encodes these two proteins in the chicken. The beta A3/A1 gene has been sequenced and its functional promoter identified in transfection experiments. The chicken beta A3/A1 gene has the same structure as the human orthologue: six exons with standard splice sites and two alternative start codons from which the protein products are apparently translated. Northern analysis revealed an abundant 0.9-kb transcript in the lenses of 1-2-day-old chickens and no detectable transcripts in the rest of the eye, brain, heart, kidney, liver or skeletal muscle. The 5'-flanking sequence of the chicken beta A3/A1 gene is very similar to that of the human and mouse genes, suggesting conservation of important putative regulatory sequences in addition to the TATA box. A thymidine-rich element (bp -218 to -163) and a potential AP-1-binding site (bp -264 to -258) are present within the chicken 5'-flanking region. A DNA fragment from -382 to +22 of the chicken beta A3/A1 gene is sufficient to promote expression of the bacterial cat gene in transfected chicken primary lens epithelial cells, but not in transfected dermal fibroblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Accumulation of crystallin in developing chicken lens

Separation and quantitation of crystallin subunits in embryonic and post-hatched chicken lens were carried out by two-dimensional gel electrophoresis and an image analysing system in order to elucidate detail in the accumulation process of each crystallin subunit in lens differentiation. Complete separation of the subunits was possible when 7 M urea was included in the second dimension gel of the electrophoresis. In particular, beta-crystallin could be separated into more than 24 spots on the gel. These experiments showed that delta-crystallin accumulated rapidly during early development up to more than 80% of total crystallins, while beta-crystallin accumulated quickly only after hatching. In contrast with the contents of beta- and delta-crystallins, alpha-crystallin content in total crystallins was kept at approximately 18% throughout lens development. Therefore, it was concluded that crystallins accumulated in several different ways. This suggests that different regulation mechanisms work on the accumulation of each crystallin subunit and that the subunit composition of lens proteins is specific to each state of lens development.

Evidence for the extralenticular expression of members of the beta-crystallin gene family in the chick and a comparison with delta-crystallin during differentiation and transdifferentiation

The beta-crystallins are major water soluble proteins of vertebrate lens fibre cells and have previously been regarded as lens-specific proteins: however beta B2-and beta A3/A1-crystallin RNAs are transcribed and beta-crystallin polypeptides are detectable in the developing chick retina. The beta-crystallin RNA is transcribed in a subpopulation of retina cells and the number of transcribing cells and the level of beta-crystallin polypeptides increase during the differentiation of the retina. Several tissues express beta-crystallin polypeptides, but individual tissues are characterised by qualitative and quantitative differences in the beta- and delta-crystallin polypeptides expressed. The expression of beta-crystallins appears to be non-random as defined by tissue distribution, cellular localisation and ontogeny, implying a function for extralenticular beta-crystallins and a complex mechanism for the regulation of their expression.

Monoclonal antibodies to chick crystallins

Several monoclonal antibodies against chick crystallins were obtained. Immunoblot analysis indicated that the monoclonal antibodies distinguish not only the three classes of crystallin (alpha, beta and delta), but also their subclasses. Monoclonal antibodies alpha 2 and alpha 1 reacted exclusively with alpha A and alpha B, respectively, demonstrating that alpha-crystallin subclasses of the chicken are antigenically distinct. An immunohistological study utilizing these monoclonal antibodies showed that the two alpha-crystallin subclasses, alpha A and alpha B, are co-expressed in the same cells and are more concentrated in the epithelium than the fibers in 14-day-old chick embryo lenses. However, immunofluorescence suggested different distribution of alpha-crystallin subclasses within an epithelial cell. alpha A is distributed evenly in the cytoplasm, but alpha B is more concentrated in the fiber-proximal side. Using anti-beta monoclonal antibodies, it was shown that beta-crystallins are divided into three distinct subclasses according to their antigenicity: 27-kDa and 25-kDa beta-crystallins are recognized by beta 1 antibody, 26-kDa and 19-kDa beta-crystallins by beta 2 antibody, and 35-kDa beta-crystallins by beta 3 antibody. All of the anti-delta-crystallin monoclonal antibodies (delta 1 to delta 3) obtained here bound to all delta-crystallin molecular species separable by isoelectrofocusing.

Transdifferentiated embryonic neuroretina cells: an in vitro system to study crystallin aggregation process

Transdifferentiated embryonic quail neuroretina cells synthesize in vitro crystallins (the lens-specific proteins) and form lentoid bodies (structures that mimic lens fiber cells) which also contain crystallins. A comparative study on the size of crystallins is reported in 7-day-old embryonic quail lenses, in 7-day-old embryonic quail transdifferentiated neuroretina cells (normal and MH2 transformed), and in isolated lentoid bodies. Analyses are performed using Superose FPLC in combination with SDS-PAGE and Western blot procedures. In quail lenses, an apparent 560-580-kDa alpha crystallin homopolymer is found and delta crystallin, the major avian lens protein, is detected as a 180-kDa tetramer. beta Crystallins, present in low amount within the 180-kDa peak, are a heterogeneous population composed of subunits of molecular weight identical to those found in chick lenses. In addition, an apparent 46-kDa monomeric delta crystallin is found. Normal and MH2-transformed neuroretina cultures produce an alpha crystallin polymer of lower molecular weight (450 kDa) and delta crystallin in a monomeric or dimeric form. The Western blot pattern of beta crystallins from MH2-transformed neuroretina cultures is strictly identical to that of quail lens beta crystallins. In particular, the beta B1 crystallin, which is specific to lens fiber cell differentiation, and the major beta 25-kDa crystallin are present. However, analysis of isolated lentoid bodies from normal transdifferentiated quail neuroretina cultures showed alpha and delta crystallins of comparable size to those found in lens extract, in particular the delta crystallin in tetrameric form. The lentoid body lens-like structure could favour the crystallin aggregation process.

delta- and beta-Crystallin mRNA levels in the embryonic and posthatched chicken lens: temporal and spatial changes during development

The levels of delta- and beta-crystallin mRNAs were examined by cDNA hybridization in the embryonic and posthatched chicken eye lens. Four different cloned beta-crystallin cDNAs were used, allowing discrimination among different members of the beta-crystallin family. Each crystallin mRNA displayed a characteristic temporal and spatial pattern in the developing lens. delta-Crystallin mRNA accumulated rapidly during early embryonic development; by contrast, the beta-crystallin mRNAs began to accumulate rapidly near the end of embryogenesis. Both delta- and beta-crystallin mRNAs increased in the lens for the first month after hatching and began to decrease 3 months after hatching. The levels of the delta- and the different beta-crystallin mRNAs were also differentially regulated in cultured embryonic lens epithelia. The most fiber cell specific crystallin gene product in the differentiating lens was the beta 35 mRNA. These experiments provide a quantitative basis for exploring the differential expression of the delta- and beta-crystallin gene families in the chicken lens.

Characterization and crystallization of delta-crystallin from the eye lens of adult turkey: comparison with delta-crystallin from adult quail and young chick

delta-Crystallin, purified from either adult turkey or quail lenses, and characterized by amino acid composition and circular dichroism spectroscopy, is shown to be similar to delta-crystallin purified from 3-day-old chick lenses. Crystallization of turkey delta-crystallin is reported along with the characterization of the crystal unit cell by X-ray diffraction techniques. Variation in unit cell parameters is observed and is correlated with hydration.

Immunochemical characterization and quantitative distribution of crystallins in the epithelium and differentiating fibre cell populations of chick embryonic lens

Lenses from 19-day chick embryos are fractionated by a double punch method to obtain the epithelium-annular pad complex (EP), outer fibres (OF), middle fibres (MF) and central fibres (CF). Water-soluble crystallins are characterized by SDS PAGE, isoelectric focusing (IEF) and two-dimensional IEF-SDS PAGE. Crystallins are also characterized by immunoelectrophoresis (IE), rocket IE, IEF-immunoblotting, and quantified by two-dimensional antigen-antibody crossed electrophoresis using antibodies to total 19-day embryonic as well as adult crystallins. In the adult lens, alpha-, beta- and delta-crystallins are 19%, 67% and 14%, respectively, while these are present at concentrations of 9%, 27% and 64%, respectively, in 19-day embryonic lens. In absolute amounts, delta-crystallin increases only by 1.23-fold between 19-day embryonic age and 6 months post-hatching, while total lens protein increases 12.5-fold. The predominance of delta-crystallin in central fibres, located along the optical axis, suggests that this protein is of embryonic origin. delta-Crystallin from fibres is electrofocused as 12 distinct molecular classes (pI 5.2-5.42) which react against anti-delta-crystallin on an immunoblot. Of these, the three most anodal species are not detected in EP. Fibres contain 50 000, 48 000 and 45 000 dalton delta-crystallin subunits while only 50 000 and 48 000 dalton subunits are present in EP.

Homology between the primary structures of the major bovine beta-crystallin chains

Partial amino acid sequences of six major subunits of bovine beta-crystallin have been determined by automatic liquid-phase Edman degradation and the dansyl-Edman procedure, complemented by amino acid analyses of peptides. The results show that, including the previously established beta Bp sequence [H. P. C. Driessen et al. (1981) Eur. J. Biochem. 121, 83-91], there exist at least seven primary gene products in bovine beta-crystallin, which exhibit 40% or more sequence homology. Two of the gene products are completely identical except for the presence in one of them of 17 additional residues at the N terminus, possibly caused by differential splicing of the same primary RNA transcript. The rate of evolutionary change of the beta chains (4% sequence change per 100 X 10(6) years) is about equally slow as that of alpha-crystallin, and the gene duplications giving rise to the different chains must have occurred very early in vertebrate evolution. The beta chains can be divided into two groups, according to sequence homology and presence of deletions/insertions and C-terminal extension, on which basis a new, rational nomenclature for the beta subunits is introduced. The N-terminal extensions of all beta chains are very different in length and sequence, even between homologous beta chains in different species. Possible explanations for this finding are discussed.

Delta crystallins and their nucleic acids

delta-Crystallin is a major structural protein of avian and reptilian lenses that is absent from the lenses of fish, amphibia and mammals. It appears to be a tetrameric protein with a native molecular weight near 200 000 (200K) and polypeptide molecular weight near 50K and 48K) (see Note added in proof). The alpha-crystallin polypeptides are extremely similar, associate in various combinations of four and are held together by hydrophobic interactions. Although principally cytoplasmic, delta-crystallin may associate with the cell membrane. delta-Crystallin differs from other lens crystallins in its alpha-helical content, native and subunit molecular weights, antigenicity, low wavelength of maximum fluorescence emission (315 nm) after excitation at 280 nm and amino acid composition (high in leucine; low in aromatic residues en no cysteine). Analyses of peptides, native and subunit molecular weights, and circular dichroism spectra indicate that the primary, secondary, tertiary and subunit structures of delta-crystallin have been generally conserved during evolution. There are at least two tandemly arranged delta-crystallin containing 13-15 introns in the chicken; a similar structure exists for a cloned delta-crystallin gene in the duck. Experiments with chicken show that delta-crystallin synthesis occurs principally in the embryo, especially during lens fiber cell differentiation. delta-Crystallin synthesis also takes place during lens fiber cell differentiation in culture. There is evidence for both transcriptional and post-transcriptional regulation of delta-crystallin synthesis. Current studies on the crystallographic and primary structures of delta-crystallin, on the structure, evolution and expression of the delta-crystallin genes, and on the translation of delta-crystallin mRNAs make this specialized lens protein an active area of investigation.

beta-Crystallin: endogenous substrate of lens transglutaminase. Characterization of the acyl-donor site in the beta Bp chain

Incubation of calf lens cortex homogenate with [14C]putrescine or dansylcadaverine, followed by two-dimensional gel electrophoresis and fluorography, enabled the identification of three different beta-crystallin chains as the endogenous substrates of Ca2+-dependent lens transglutaminase (R-glutaminyl-peptide:amine-gamma-glutamylyltransferase, EC 2.3.2.13). One of these is beta Bp, the predominant subunit of beta-crystallin, of which the amino acid sequence is known. The site of amine-labeling in beta Bp could be located, by limited proteolysis, in the N-terminal domain of this chain. Tryptic digestion of the N-terminal domain and subdigestion with elastase of the N-terminal tryptic peptide identified glutamine-7 as the single residue to which the amines are bound. This is the first example of an endogenous substrate of intracellular transglutaminase in which the site of the acyl-donor glutamine residue has been established. Tryptic digestion of the putrescine-labeled beta-crystallin aggregate, followed by high-voltage paper electrophoresis, provided a preliminary characterization of the labeled peptides originating from the other two labeled beta subunits.

Two-dimensional gel analysis of chick lens proteins

Two-dimensional analyses of the chick lens water-soluble and water-insoluble proteins were conducted according to the method of O'Farrell (1975). The results define the isoelectric properties of the water-soluble and the urea-soluble polypeptides and demonstrate differences in composition for cortical and nuclear proteins. Chick lens vimentin consists of at least two isoelectric variants, and its breakdown products were identified. Chick lens actin is primarily of the gamma-type. The 47 K polypeptide specific for fiber cells shows considerable charge heterogeneity, and its most acidic component is found primarily in the nuclear fiber cells. This study also shows that apparently single bands resolved by one-dimensional SDS-polyacrylamide gel electrophoresis of the urea-soluble fraction consists of different proteins, and that the composition of such bands may further be altered by ph. This is especially relevant to the composition of the 47 and 50 K bands.

Primary gene products of bovine beta-crystallin and reassociation behavior of its aggregates

beta-Crystallin from calf lens cortex was fractionated in three different aggregates of increasing size: beta L2- beta L1 and beta H. of which the subunit composition was revealed by 2-dimensional gel electrophoresis. While beta L2 mainly consists of beta Bp (the major polypeptide chain in all three aggregates). beta L1 is characterized by the addition of a neutral and two acidic chains, and beta H contains moreover two basic chains. Translation of calf lens polyribosomes in a reticulocyte cell-free system allowed the identification of six beta-crystallin subunits as primary gene products. The distribution of these newly synthesized polypeptides over the three aggregates was established after gel filtration in the presence of carrier lens proteins. The aggregation behavior of the beta-crystallin chains was studied by dissociation reassociation experiments. The three separate aggregates could be reversibly dissociated. Reassociation of basic, neutral and acidic polypeptides, isolated by ion-exchange chromatography of beta-crystallin, produced a beta H-like aggregate. The neutral and acidic polypeptides reassociated into a beta L1-like aggregate, while the neutral polypeptides gave dimers like beta L2. A beta H-like aggregate could also be obtained by reaggregation of beta L2 with the acidic and basic chains of beta H. On the basis of these results a preliminary model for the formation of beta-crystallin aggregates is discussed.