Beta-crystallin mRNAs: differential distribution in the developing chicken lens

Cell context reveals a dual role for Maf in oncogenesis

Maf b-Zip transcription factors are involved in both terminal differentiation and oncogenesis. To investigate this apparent contradiction, we used two different primary cell types and performed an extensive analysis of transformation parameters induced by Maf proteins. We show that MafA and c-Maf are potent oncogenes in chicken embryo fibroblasts, while MafB appears weaker. We also provide the first evidence that MafA can confer growth factor independence and promote cell division at low density. Moreover, using MafA as a model, we identified several parameters that are critical for Maf transforming activities. Indeed, MafA ability to induce anchorage-independent cell growth was sensitive to culture conditions. In addition, the transforming activity of MafA was dependent on its phosphorylation state, since mutation on Ser65 impaired its ability to induce growth at low density and anchorage-independent growth. We next examined transforming activity of large Maf proteins in embryonic neuroretina cells, where they are known to induce differentiation. Unlike v-Jun, MafA, MafB and c-Maf did not show oncogenic activity in these cells. Moreover, they counteracted transformation induced by constitutive activation of the Ras/Raf/MEK pathway. Taken together, our results show that Maf proteins could display antagonistic functions in oncogenesis depending on the cellular context, and support a dual role for Maf as both oncogenes and tumor suppressor-like proteins.

Developmental Regulation of the Direct Interaction between the Intracellular Loop of Connexin 45.6 and the C Terminus of Major Intrinsic Protein (Aquaporin-0)

The eye lens is dependent upon a network of gap junction-mediated intercellular communication to facilitate its homeostasis and development. Three gap junction-forming proteins are expressed in the lens of which two are in lens fibers, namely connexin (Cx) 45.6 and 56. Major intrinsic protein (MIP), also known as aquaporin-0 (AQP0), is the most abundant membrane protein in lens fibers. However, its role in the lens is not clear. Our previous studies show that MIP(AQP0) associates with gap junction plaques formed by Cx45.6 and Cx56 during the early stages of embryonic chick lens development but not in late embryonic and adult lenses. We report here that MIP(AQP0) directly interacts with Cx45.6 but not with Cx56. We further identified the intracellular loop of Cx45.6 as the interacting domain for the MIP(AQP0) C terminus. Surface plasmon resonance experiments indicated that the C-terminal domain of MIP(AQP0) interacts with two binding sites within the intracellular loop region of Cx45.6 with a K(D(app)) of 7.5 and 10.3 microm, respectively. The K(D(app)) for the full-length loop region is 7.7 microm. The cleavage at the intracellular loop of Cx45.6 was observed during lens development, and the C terminus of MIP(AQP0) did not interact with the loop-cleaved form of Cx45.6. Thus, the dissociation between these two proteins that occurs in the mature fibers of late lens development is likely caused by this cleavage. Finally this interaction had no impact on Cx45.6-mediated intercellular communication, suggesting that the Cx45.6-MIP(AQP0) interaction plays a novel unidentified role in lens fibers.

Phosphorylation of MafA is essential for its transcriptional and biological properties

We previously described the identification of quail MafA, a novel transcription factor of the Maf bZIP (basic region leucine zipper) family, expressed in the differentiating neuroretina (NR). In the present study, we provide the first evidence that MafA is phosphorylated and that its biological properties strongly rely upon phosphorylation of serines 14 and 65, two residues located in the transcriptional activating domain within a consensus for phosphorylation by mitogen-activated protein kinases and which are conserved among Maf proteins. These residues are phosphorylated by ERK2 but not by p38, JNK, and ERK5 in vitro. However, the contribution of the MEK/ERK pathway to MafA phosphorylation in vivo appears to be moderate, implicating another kinase. The integrity of serine 14 and serine 65 residues is required for transcriptional activity, since their mutation into alanine severely impairs MafA capacity to activate transcription. Furthermore, we show that the MafA S14A/S65A mutant displays reduced capacity to induce expression of QR1, an NR-specific target of Maf proteins. Likewise, the integrity of serines 14 and 65 is essential for the MafA ability to stimulate expression of crystallin genes in NR cells and to induce NR-to-lens transdifferentiation. Thus, the MafA capacity to induce differentiation programs is dependent on its phosphorylation.

The mouse beta B1-crystallin promoter: strict regulation of lens fiber cell specificity

Previous studies have shown that the chicken beta B1-crystallin promoter (-434/+30) contains all of the signals necessary to specifically direct high level expression of heterologous genes to the lens fiber cells of mice. In the present study, the mouse beta B1-crystallin gene was cloned, and its regulation was investigated to further elucidate the mechanisms controlling lens fiber cell-specific gene expression. Phylogenetic footprinting analysis of the 5' flanking sequence from the mouse, rat, human and chicken beta B1-crystallin genes identified several known and putative functional cis elements including the PL2 element which is required for lens-specific expression of the chicken beta B1 promoter. Surprisingly, however, all six mouse beta B1-crystallin/CAT constructs tested (-1493/+44, -1493/+30, -870/+30, -250/+30, -135/+30 and -98/+30) were inactive in three different mammalian lens-derived cell lines while only the -870/+30 and -98/+30 constructs were active in chicken primary patched lens epithelial cells. In contrast, the chicken beta B1-crystallin promoter (-434/+30) was transcriptionally active in all lens-derived cells tested. Transgenic mice harboring a mouse beta B1-crystallin -1493/+44 CAT construct did express the transgene specifically in lens fiber cells, however, at lower levels than that previously reported for a chicken -434/+30 CAT construct. These data suggest that, as in other crystallin genes, the regulatory signals controlling lens fiber cell-specific expression are conserved between chicken and mouse. However, the inability of the mouse beta B1-crystallin promoter to function in mammalian lens-derived cultured cells implies that this gene has acquired additional cis-regulatory elements to ensure lens fiber cell specificity.

Dual roles for Pax-6: a transcriptional repressor of lens fiber cell-specific beta-crystallin genes

It has been demonstrated previously that Pax-6, a paired domain (PD)/homeodomain (HD) transcription factor critical for eye development, contributes to the activation of the alphaB-, alphaA-, delta1-, and zeta-crystallin genes in the lens. Here we have examined the possibility that the inverse relationship between the expression of Pax-6 and beta-crystallin genes within the developing chicken lens reflects a negative regulatory role of Pax-6. Cotransfection of a plasmid containing the betaB1-crystallin promoter fused to the chloramphenicol acetyltransferase reporter gene and a plasmid containing the full-length mouse Pax-6 coding sequences into primary embryonic chicken lens epithelial cells or fibroblasts repressed the activity of this promoter by as much as 90%. Pax-6 constructs lacking the C-terminal activation domain repressed betaB1-crystallin promoter activity as effectively as the full-length protein, but the PD alone or Pax-6 (5a), a splice variant with an altered PD affecting its DNA binding specificity, did not. DNase footprinting analysis revealed that truncated Pax-6 (PD+HD) binds to three regions (-183 to -152, -120 to -48, and -30 to +1) of the betaB1-crystallin promoter. Earlier experiments showed that the betaB1-crystallin promoter sequence from -120 to -48 contains a cis element (PL2 at -90 to -76) that stimulates the activity of a heterologous promoter in lens cells but not in fibroblasts. In the present study, we show by electrophoretic mobility shift assay and cotransfection that Pax-6 binds to PL2 and represses its ability to activate promoter activity; moreover, mutation of PL2 eliminated binding by Pax-6. Taken together, our data indicate that Pax-6 (via its PD and HD) represses the betaB1-crystallin promoter by direct interaction with the PL2 element. We thus suggest that the relatively high concentration of Pax-6 contributes to the absence of betaB1-crystallin gene expression in lens epithelial cells and that diminishing amounts of Pax-6 in lens fiber cells during development allow activation of this gene.

Developmental regulation of the chicken beta B1-crystallin promoter in transgenic mice

The cis-elements responsible for the high-level, lens-specific expression of the chicken beta B1-crystallin gene were investigated by generating mice harboring beta B1-crystallin promoter/chloramphenicol acetyl transferase (CAT) transgenes. Deletion of promoter sequences -434/-153 and -152/-127 as well as site-directed mutagenesis of the PL1 (-116/-102) and Pl2 (-90/-76) elements significantly decreased CAT gene expression in the lenses of adult transgenic mice. Transfection studies using multimerized PL1 and PL2 elements fused to the chicken beta-actin basal promoter indicated that PL1 is a general activating element while PL2 is involved in the lens-specificity of the chicken beta B1-crystallin promoter. CAT histochemistry demonstrated that the chicken beta B1-crystallin promoter (-434/+30) was active in both primary and secondary lens fiber cells from 12.5 days post coitum (dpc) until adulthood. Activity of the -152/+30/CAT transgene was relatively low and confined to the primary lens fiber cells of 16.5 dpc mice. Together, these data suggest that the reduced activity of this promoter in the adult lens is due both to this developmentally restricted expression pattern and a reduction in promoter activity. RNA hybridization studies demonstrated that the chicken beta B1-crystallin/CAT (-434/+30) transgene was expressed at similar levels in the same cells as the endogenous mouse beta B1-crystallin gene in 16.5 dpc transgenic mouse embryos. These data show a strict conservation of the lens-specific spatial and temporal regulation of the chicken and mouse beta B1-crystallin genes.

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)

Temporal regulation of six crystallin transcripts during mouse lens development

Using the polymerase chain reaction (PCR) and RNA blot analysis, we have examined the differential expression patterns of the gamma-crystallins during lens development. Since only four of these genes had been previously characterized, the cDNAs for the remaining two genes, gamma C and gamma F, were isolated and sequenced. The steady-state mRNA profiles were then determined by RNA blot analysis of samples from embryonic stages to 180 days after birth, with gene-specific probes for gamma A, gamma B, gamma C, and gamma D, and a common probe for gamma E and gamma F. Due to the paucity of mismatches between the gamma E and gamma F-crystallin genes, the PCR technique was exploited to determine their relative abundance. The data showed that while all six gamma-crystallin genes were expressed in the embryonic lens, they were differentially regulated during development. At early stages, the levels of gamma B and gamma C mRNAs were found to be relatively low in comparison to those for gamma A, gamma D, gamma E and gamma F. After 30-40 days, however, the levels of gamma A, gamma E, and gamma F mRNAs declined rapidly, and the gamma B, gamma C and gamma D transcripts became the major gamma-crystallin mRNA species. The utility of the PCR technique in studying the relative abundance of steady-state gamma-crystallin mRNAs was also investigated.

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.

Presence of hybridizing DNA sequences homologous to bovine acidic and basic beta-crystallins in all classes of vertebrates

The eye lens beta-crystallins in cow and chicken are encoded by a family of at least six genes. In order to assess the distribution of the corresponding genes among other vertebrates we hybridized beta-crystallin sequences (beta A2, beta A3/A1, beta A4, beta B1, beta B2, beta B3), isolated from a bovine lens cDNA library, to Southern blots on which EcoR1-digested chromosomal DNA was blotted from different vertebrate species. These included human, chimpanzee, calf, rat, pigeon, duck, monitor lizard, toad, trout, and lamprey. Positive hybridization signals were found in the representatives of virtually all classes of vertebrates. The basic beta B-crystallins gave hybridization signals in more species than the acidic beta A ones. In monitor lizard and toad the weakest hybridization signals for basic crystallin probes were found. For acidic crystallin probes the distribution pattern was more simple; among cold-blooded vertebrates a signal for beta A2 was found in trout and lamprey, for beta A4 in trout, and for beta A3/A1 only in toad. The results demonstrate that the duplications leading to the beta-crystallin gene family occurred before or during the earliest stages of vertebrate evolution.

Isolation and characterization of cDNAs encoding beta A2- and beta A4-crystallins: heterologous interactions in the predicted beta A4-beta B2 heterodimer

Except for the two acidic chains, beta A2 and beta A4, the primary structures of all bovine beta-crystallins have previously been elucidated, either by direct protein sequencing or prediction from cDNA sequencing. Both beta A2 and beta A4 were found to be synthesized in half-year-old calf lenses and are therefore likely to be present in a cDNA bovine library constructed from mRNA isolated from lenses of that age. A large number of cDNA clones was screened with all available crystallin, actin, vimentin and lens membrane protein MP26 probes and finally with a randomly primed mRNA probe. Clones positive for the latter, but negative for known lens proteins, were isolated and sequenced. beta A2, comprising 197 aa, and beta A4, comprising 209 aa, were identified. Both proteins have a conserved two-domain structure and an N-terminal extension which is variable. A three-dimensional model of the structure of beta A4 was made based on the coordinates of one subunit from the beta B2 dimer which has recently been solved using x-ray diffraction techniques. The resulting heterodimer structure, together with the compiled bovine beta-crystallin sequences, was used to indicate those regions of the sequences which distinguish acidic from basic beta-crystallins with a view to defining structural features necessary for subunit recognition in beta-crystallin aggregates. With the aid of the present data, the complete evolutionary tree of the bovine beta-crystallin family has been constructed, which confirms the early separation of the genes encoding the three acidic and the three basic beta-crystallins.

Anomalous behavior of beta B1-crystallin subunits from avian lenses

In soluble extracts of bird lenses, crystallin subunits of apparent sizes between 30kDa and 40kDa show considerable variability in both abundance and mobility in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE). This is only partly due to the taxon-specific distribution of lactate dehydrogenase-B (LDH-B)/epsilon-crystallin. Here we show that, in spite of high conservation in primary sequence, beta B1-crystallin subunits of avian lenses have a markedly slower migration in SDS PAGE than those of mammals and that the apparent subunit size of beta B1-crystallin varies among birds. This may be at least partly due to unusual post-translational modifications, since beta B1-crystallins of adult birds react strongly in a glycan detection procedure. No other crystallins in birds or mammals share this strong reaction. Modification of beta B1-crystallin could have interesting consequences for the formation of high molecular weight beta-crystallin aggregates in bird lenses.

Chicken beta B1-crystallin gene expression: presence of conserved functional polyomavirus enhancer-like and octamer binding-like promoter elements found in non-lens genes

Expression of the chicken beta B1-crystallin gene was examined. Northern (RNA) blot and primer extension analyses showed that while abundant in the lens, the beta B1 mRNA is absent from the liver, brain, heart, skeletal muscle, and fibroblasts of the chicken embryo, suggesting lens specificity. Promoter fragments ranging from 434 to 126 bp of 5'-flanking sequence (plus 30 bp of exon 1) of the beta B1 gene fused to the bacterial chloramphenicol acetyltransferase gene functioned much more efficiently in transfected embryonic chicken lens epithelial cells than in transfected primary muscle fibroblasts or HeLa cells. Transient expression of recombinant plasmids in cultured lens cells, DNase I footprinting, in vitro transcription in a HeLa cell extract, and gel mobility shift assays were used to identify putative functional promoter elements of the beta B1-crystallin gene. Sequence analysis revealed a number of potential regulatory elements between positions -126 and -53 of the beta B1 promoter, including two Sp1 sites, two octamer binding sequence-like sites (OL-1 and OL-2), and two polyomavirus enhancer-like sites (PL-1 and PL-2). Deletion and site-specific mutation experiments established the functional importance of PL-1 (-116 to -102), PL-2 (-90 to -76), and OL-2 (-75 to -68). DNase I footprinting using a lens or a HeLa cell nuclear extract and gel mobility shifts using a lens nuclear extract indicated the presence of putative lens transcription factors binding to these DNA sequences. Competition experiments provided evidence that PL-1 and PL-2 recognize the same or very similar factors, while OL-2 recognizes a different factor. Our data suggest that the same or closely related transcription factors found in many tissues are used for expression of the chicken beta B1-crystallin gene in the lens.

Chicken beta B1-crystallin gene expression: presence of conserved functional polyomavirus enhancer-like and octamer binding-like promoter elements found in non-lens genes

Expression of the chicken beta B1-crystallin gene was examined. Northern (RNA) blot and primer extension analyses showed that while abundant in the lens, the beta B1 mRNA is absent from the liver, brain, heart, skeletal muscle, and fibroblasts of the chicken embryo, suggesting lens specificity. Promoter fragments ranging from 434 to 126 bp of 5'-flanking sequence (plus 30 bp of exon 1) of the beta B1 gene fused to the bacterial chloramphenicol acetyltransferase gene functioned much more efficiently in transfected embryonic chicken lens epithelial cells than in transfected primary muscle fibroblasts or HeLa cells. Transient expression of recombinant plasmids in cultured lens cells, DNase I footprinting, in vitro transcription in a HeLa cell extract, and gel mobility shift assays were used to identify putative functional promoter elements of the beta B1-crystallin gene. Sequence analysis revealed a number of potential regulatory elements between positions -126 and -53 of the beta B1 promoter, including two Sp1 sites, two octamer binding sequence-like sites (OL-1 and OL-2), and two polyomavirus enhancer-like sites (PL-1 and PL-2). Deletion and site-specific mutation experiments established the functional importance of PL-1 (-116 to -102), PL-2 (-90 to -76), and OL-2 (-75 to -68). DNase I footprinting using a lens or a HeLa cell nuclear extract and gel mobility shifts using a lens nuclear extract indicated the presence of putative lens transcription factors binding to these DNA sequences. Competition experiments provided evidence that PL-1 and PL-2 recognize the same or very similar factors, while OL-2 recognizes a different factor. Our data suggest that the same or closely related transcription factors found in many tissues are used for expression of the chicken beta B1-crystallin gene in the lens.

Multiple regulatory elements of the murine gamma 2-crystallin promoter

Crystallins are the major water-soluble proteins of the vertebrate eye lens. These lens-specific proteins are encoded by several multi-gene families whose expression is differentially regulated during development. Our previous studies showed that the mouse gamma 2-crystallin promoter is active on transfection into lens-explant cultures derived from 14-day-old chick embryos but not on transfection into a variety of non-lens cells. In this study, transient expression data show that a sequence of 226 nucleotides upstream from the transcription start site is sufficient for activity of this promoter in the chicken lens cells. This sequence can be further divided into two domains, A and B, both of which are required for promoter function. Domain A (nucleotide -68 to -18) contains the TATA box and sequence motifs that are conserved in all gamma-crystallin promoters. Domain B (-226 to -120) consists of three regions. One of these regions contains an element with dyad symmetry and a sequence similar to the octamer motif. The second region contains an enhancer core consensus sequence. Two "enhancer-like" activities have been detected, one in Domain B and a second in a more distal region (-392 to -278) that does not appear to be required for promoter activity in transfection assays.

The cellular eye lens and crystallins of cubomedusan jellyfish

The ultrastructure and major soluble proteins of the transparent eye lens of two cubomedusan jellyfish, Tripedalia cystophora and Carybdea marsupialis, have been examined. Each species has two complex eyes (one large and one small) on four sensory structures called rhopalia. The lenses consist of closely spaced cells with few organelles. The lens is situated next to the retina, with only an acellular layer separating it from the photoreceptors. SDS-PAGE showed that the large lens of C. marsupialis has only two crystallin polypeptide bands (with molecular masses of approximately 20,000 and 35,000 daltons), while that of T. cystophora has three bands (two with a molecular mass near 20,000 daltons and one with a molecular mass near 35,000 daltons). Interestingly, the small lens of T. cystophora appears to be markedly deficient in or lack the lower molecular weight proteins. The crystallins behaved as monomeric proteins by FPLC and showed no immunological reaction with antisera of the major squid crystallin, chicken delta-crystallin or mouse gamma-crystallin in western immunoblots. Very weak reactions were found with antimouse alpha- and beta-crystallin sera. The 35,000 dalton crystallin of T. cystophora was purified and called J1-crystallin. It contained relatively high leucine (13%) and tyrosine (9%) and low methionine (2%). Several tryptic peptides were sequenced. Weak sequence similarities were found with alpha- and beta-crystallins, which may account for some of the apparent weak immunological cross-reactivity with these vertebrate crystallins. A polyclonal antiserum made in rabbits from a synthetic peptide of J1-crystallin reacted strongly with J1-crystallin of T. cystophora and C. marsupialis in immunoblots; by contrast, no reaction was obtained with the lower molecular weight crystallins from these jellyfish, with the squid crystallin, or with any crystallins from the frog or human lens. Thus, despite the structural similarities between the cubomedusan, squid and vertebrate lenses, their crystallins appear very different.

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.

The influence of the genotype on the process of ageing of chick lens cells in vitro

We reported previously that changes in crystallin expression in differentiating long-term primary cultures of lens cells from five different chick genotypes are similar to those which occur in vivo between hatching and the 8-week-old adult. These changes followed a similar program in all genotypes but occurred more rapidly in cells from the fast-growing than from the slow-growing genotypes. The present study examines ageing changes in lens cell populations from the same five genotypes, over a 4-6 month period, using long-term serial subcultures. The capacity for lentoid differentiation was progressively lost, but the rate of loss was inversely related to the intrinsic growth rate of the cells of these genotypes, occurring at the first passage in the slowest-growing strain, while fifth passage cells of the fastest-growing strain still retained some lentoid-forming capacity. The rate of loss of crystallin expression was also inversely related to the genetic growth rate, but the sequence of changes appears to be nonrandom, since it was broadly similar in all genotypes, starting with a preferential loss of delta-crystallin, as occurs in vivo; although alpha- and beta-crystallins were undetectable in late dedifferentiated cultures, the capacity of the cells for their synthesis was still present. Cultures from both fast-growing genotypes eventually showed senescence, but those from all three slow-growing genotypes underwent transformation. The major cell component in late cultures of all genotypes was actin.

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.

Patterns of crystallin expression during differentiation in vitro of several chick genotypes with different effects on lens cell growth rate

We have recently reported that chick lens cells during differentiation in long-term culture show a programme of change in crystallin expression which mimics events during lens development in vivo. The aim of the present work was to examine the stability of the programme by testing the response to genetic influences and exposure to a carcinogen. Five genetically distinct inbred strains of chick, differing in the intrinsic growth rates of lens epithelial cells in vitro, were used to study the effects of the rate of mitosis on crystallin expression, both during lens development and in long-term cell culture. The time of appearance of lentoids, their size and abundance and the rate of change in crystallin expression were all modified in a genotype-specific way, related to the rate of mitosis, but the programme of changes in crystallin expression was the same for all genotypes. Genetic differences were also found in the patterns of response to treatment of cultures during the logarithmic growth phase with a nitrosoguanidine compound known to affect cell differentiation in lens cultures and several other systems. The changes in crystallin expression and fibre differentiation were delayed, but cultures of the faster growing genotypes were least affected. With further culture, crystallin expression tended to recover to control values although levels of fibre differentiation and cell growth remained depressed. The results indicate that genetic differences in intrinsic growth rate moderate but do not change the programme of crystallin expression shown by lens epithelial cells in culture, and that this programme shows resistance to change.

Alterations in crystallin gene expression during subculture of chick lens cells

We report here on the changes in crystallin gene expression during serial subculture of lens epithelial cells derived from day-old post-hatch chicks. Total cellular RNA from mass cultures were analysed by in vitro cell-free translation and by RNA blot (Northern) hybridization using a cloned delta-crystallin cDNA. Our results indicate that following subculture, lens epithelial cells which still retain the capacity for lens fibre differentiation (lentoid body formation) show a selective loss of delta-crystallin synthesis, and that this is related to the loss of delta-crystallin mRNA. The data suggest that older epithelial-cell populations give rise to lentoid bodies which in terms of crystallin gene expression closely resemble the later-formed cortical fibres of the adult chick lens. Tertiary cultures had an accelerated growth rate, formed no lentoids, contained no translatable alpha- or delta-crystallin mRNAs but still contained translatable beta-crystallin mRNAs.

Immunocytochemical localization of the 27K beta-crystallin polypeptide in the mouse lens during development using a specific monoclonal antibody: implications for cataract formation in the Philly mouse

The Philly mouse develops a hereditary cataract about 5 weeks after birth. Although the causative agent is not known, data suggest that there is a correlation between cataract formation and the selective absence of a 27 kilodalton (27K) beta-crystallin lens polypeptide. The ontogeny of the 27K beta-crystallin polypeptide was examined in normal mice in order to evaluate its role in normal development and determine what impact its absence may have on the Philly mouse lens. A monoclonal antibody was used with the PAP method to immunocytochemically localize the 27K polypeptide in lenses of normal mice during development. beta-Crystallins detected with polyclonal antisera were found in differentiated fiber cells throughout the lens. In contrast, the 27K beta-crystallin polypeptide detected with a specific monoclonal antibody was not found in the fiber cells of the inner part of the lens (nucleus), but was specifically localized in the fiber cells of the outer part of the lens called the cortex. The polypeptide was found only in elongating and differentiated fiber cells and not in mitotically active epithelial cells. Although a minor component of the 2-day-old lens, the 27K polypeptide comprised a large portion of the 16-day-old lens including the anterior and posterior poles. These data show that the 27K polypeptide is a minor component of the embryonic lens, but becomes a major contributor to the postnatal lens. The 27K beta-crystallin lens polypeptide is abundant in the fiber cells of the normal postnatal mouse lens. The absence of the 27K polypeptide in the Philly mouse may contribute to the observed failure of fiber cells to differentiate in the Philly mouse after birth or may be deleterious in some other manner to normal lens development. The selective absence of the 27K beta-crystallin polypeptide, a defect which precedes cataract formation in the Philly mouse, is intriguing since it suggests a relationship between this major lens polypeptide and lens clarity.

Ontogeny of the 38K epsilon-polypeptide during lens development of the duck Anas platyrhynchos

The epsilon-crystallin polypeptide is a recently described protein in the eye lens of the peking duck, Anas platyrhynchos. It does not cross react with alpha-, beta-, delta-, and gamma-crystallins. It has a molecular weight of 120K and consists of 3 identical 38K polypeptides. It is found in some reptiles and birds. The ontogeny of this polypeptide has been investigated in the developing A. platyrhynchos lens via the indirect immunofluorescence staining using a homologous antibody. The 38K polypeptide was extracted from 13% Tris-SDS acrylamide gels, lyophilized and injected into a young rabbit to raise an antibody. The purity of the isolated 38K polypeptide and and the specificity of the antibody were checked by Tris-SDS gel electrophoresis and immunoblotting, respectively. The first positive reaction is detected at 80 h (stage 18) incubated lens. It is confined to a few elongating early primary fibres. Until the 9th day of development the reaction is confined to the primary and secondary lens fibres. The first positive reaction in the annular pad area is observed in the "day 10" lens. In the anterior epithelium the first positive reaction is detectable in the "day 12" lens. At the beginning it is confined to a few cells in the center of the epithelium and gradually the reaction spreads to other cells. A strong and uniform reaction in the entire epithelium is noted for the first time in the lens of a just-hatched duckling. The 38K epsilon-polypeptide is detectable after the alpha-, beta-, and delta-crystallins, which, in the duck, appear simultaneously from 66 h (stage 15/16).

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.

Lens-specific expression of the chloramphenicol acetyltransferase gene promoted by 5' flanking sequences of the murine alpha A-crystallin gene in explanted chicken lens epithelia

We have developed a system using explanted embryonic chicken lens epithelia to express foreign recombinant genes containing crystallin DNA regulatory sequences introduced by calcium phosphate transfection. Optimal results were obtained with lens epithelia from 14-day embryos transfected 1 day after explantation and assayed 3 days later. When DNA sequences (-364 to +45) of the murine alpha A-crystallin gene were inserted in the pSVO-CAT expression vector of Gorman et al. [Gorman, C. M., Moffat, L. F. & Howard, B. H. (1982) Mol. Cell. Biol. 2, 1044-1051] in the same orientation as in the crystallin gene, they promoted chloramphenicol acetyltransferase (CAT; EC 2.3.1.28) activity in the transfected epithelia. Sequences 87 to 364 base pairs upstream from the murine gene cap site were required for CAT gene expression. These crystallin gene regulatory sequences did not promote CAT expression in primary cultures of embryonic chicken fibroblasts or other nonlens cells. By contrast, the long terminal repeat of Rous sarcoma virus and the early promoter of simian virus 40 promoted CAT activity in lens and nonlens cells. Our experiments thus demonstrate that the explanted embryonic chicken lens epithelium is an advantageous recipient for identifying lens-cell-specific regulatory sequences of crystallin genes and implicate a DNA region upstream of the "TATA box" for regulation of the murine alpha A-crystallin gene. These experiments also suggest that explanted epithelia from other tissues may be useful for studying the expression of foreign genes.

A comparison of the changing patterns of crystallin expression in vivo, in long-term primary cultures in vitro and in response to a carcinogen

Changes in the differentiation of day-old chick lens epithelium in long-term primary culture conditions were investigated by sodium dodecyl sulphate-polyacrylamide electrophoresis, using integrating densitometry to assess the relative levels of accumulated crystallin and non-crystallin polypeptides and fluorography to assess their relative levels of synthesis. The main changes during the culture period included a relative decline in the proportion of actin and other non-crystallins, an initial increase in 48K delta-crystallin expression followed by a decline and a shift in beta-crystallin expression from a relative preponderance of the 24K and 23K polypeptides to a relative preponderance of the 24K and 22K polypeptides. At all stages the level of the 19K alpha-crystallin was higher than that of the 20K alpha-crystallin polypeptide. In general, the changes in the pattern of expression of these polypeptides in culture were similar to those observed in vivo in the post-hatch chick, suggesting an intrinsic programme of crystallin expression. The changes in gene expression were also tested indirectly by brief exposure of the cells in vitro to a carcinogen, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) which is known to produce, in some systems, effects related to the status of the cell at the time of treatment. The effects were found to depend on the stage of differentiation of the culture at the time of treatment. Treatment on day 1 of culture prevented later lentoid formation and severely reduced the expression of all crystallins with the exception of the 34K beta-crystallin polypeptide. Actin was the most abundant soluble cell component, and a proportion of the cells acquired a fibroblast-like morphology. Treatment with MNNG on day 7 led to a delay in lentoid formation and a differential reduction of the synthesis of crystallin polypeptides, whereas the treatment of already differentiated cultures on day 18 and to lesser extents on days 27, 45 and 55, respectively, led to an increase in crystallin synthesis relative to controls. These results suggest that this programme of crystallin gene expression becomes more resistant to change with increasing epithelial differentiation.

Differential synthesis of rat lens proteins during development

Developmental regulation of crystallin protein synthesis was observed in rat lenses between embryonic day 19 and postnatal day 21. Studies on lenses incubated in [35S]-methionine and on lens messenger RNAs translated in a reticulocyte lysate showed that several new polypeptides were synthesized in the lens beginning approximately 1 week after birth. One new polypeptide which had a molecular weight of 27 000 comigrated with the beta crystallins on SDS-PAGE and became a predominant component in older lenses. By crossed rocket immunoelectrophoresis and isoelectric focusing, synthesis of several native beta crystallins and one gamma crystallin was detected only in the postnatal lens. Many crystallin proteins were synthesized in the embryonic and the postnatal lens and did not change during the time period studied. These data suggest a differential regulation of the crystallin proteins during development. It appears that the lens undergoes a transition from embryonic to adult crystallin expression during the first weeks after birth. Factors such as maturation of the retina may be necessary for this transition.

Developmental acquisition of basement membrane heterogeneity: type IV collagen in the avian lens capsule

To investigate potential heterogeneity and developmental changes in basement membranes during embryogenesis, we performed immunohistochemical analyses on lens capsules in chicken embryos of different ages using domain-specific monoclonal antibodies against type IV collagen. We found that the capsule of the newly formed lens stained uniformly with antibodies against this component of basement membranes, but with increasing age and differentiation of the lens cells the anterior lens capsule remained brightly fluorescent while staining of the posterior capsule became relatively much less intense. This antero-posterior gradient of anti-type IV collagen antibody reactivity demonstrated that developmentally-regulated changes can occur within a single, continuous basement membrane.