Lens differentiation in vertebrates. A review of cellular and molecular features

Overexpression of the vimentin gene in transgenic mice inhibits normal lens cell differentiation

To investigate the role of the intermediate filament protein vimentin in the normal differentiation and morphogenesis of the eye lens fiber cells, we generated transgenic mice bearing multiple copies of the chicken vimentin gene. In most cases, the vimentin transgene was overexpressed in the lenses of these animals, reaching up to 10 times the endogenous levels. This high expression of vimentin interfered very strongly with the normal differentiation of the lens fibers. The normal fiber cell denucleation and elongation processes were impaired and the animals developed pronounced cataracts, followed by extensive lens degeneration. The age of appearance and extent of these abnormalities in the different transgenic lines were directly related to the vimentin level. Electron microscopic analysis revealed that the accumulated transgenic protein forms normal intermediate filaments.

NCAM in the differentiation of embryonic lens tissue

The role of the neural cell adhesion molecule (NCAM)2 in ocular lens differentiation was investigated in chicken embryos. Changes in expression of NCAM were documented by immunohistology of frozen sections. This analysis revealed that NCAM diminished during lens fiber differentiation, in contrast to the gap junction-associated protein MP26 which became more abundant. The form of NCAM expressed was determined by Western blot analysis of proteins extracted from the different regions of the Embryonic Day 6 lenses. All regions expressed NCAM with an apparent molecular weight of 140 kDa and relatively low levels of polysialylation. The function of NCAM in lens differentiation was investigated using antibodies that inhibit NCAM-mediated adhesion. Two parameters that change during maturation of the lens epithelial cells were monitored: the thickness of the tissue, indicating the length of lens cells, and the particle arrangement of gap junctions, reflecting the state of junctional differentiation. When epithelial cell explants of Embryonic Day 6 lenses were cultured for 5 days, the cells elongated and displayed an increase in the loose, random intramembranous particle arrangements characteristic of maturing lens fiber gap junctions. When the explants were cultured in the presence of anti-NCAM Fabs, the epithelia were thinner than in matched controls and had particle arrangements characteristic of a less mature state. The expression of NCAM during lens differentiation and the effects of attenuating NCAM function suggest that adhesion mediated by NCAM is an essential event in lens cell differentiation.

Immunological studies on gamma crystallins from Xenopus: localization, tissue specificity and developmental expression of proteins

In an effort to understand the spatio-temporal regulation of crystallins and their genes during lens development, the gamma crystallins from the frog lens have been isolated, purified and characterized. Using an immunological approach, they were found to be localized exclusively in the lens fiber cells and were not detected in any other lens cells or non-lens tissues including mature oocytes. During embryogenesis, the antigens were first detected in stage 25 embryos (but not in stage 20 embryos). Their level first decreased and then increased during subsequent stages of development. A different member of the family was also found to be expressed during later stages of embryogenesis.

Developmental expression of creatine kinase isozymes in mammalian lens

Four different isoforms are thought to comprise the creatine kinase of enzymes which regulate energy metabolism through the interconversion of ADP and creatine phosphate. In addition to these well characterized isoforms, MM, MB, BB and mitochondrial creatine kinase, several uncharacterized variants with atypical electrophoretic mobility have been described. In mammalian lens, creatine kinase isoforms exhibit both a regional and developmental pattern of expression. In neonatal rat and human lens, the only isoform expressed is a variant cathodic creatine kinase. Near the time of sexual maturation (11-13 yr) there is a dramatic increase in the expression of BB creatine kinase in human lens. In rat lens, a similar pattern of isoenzyme expression is also seen near the time of sexual maturation (5-6 weeks). In the mature rat lens, in addition to the cathodic variant, there is expression of BB and, to a lesser extent, MM creatine kinase. Using a polyclonal antisera, we have localized BB creatine kinase to the cuboidal epithelial cells of the adult rat lens. This unique pattern of isoenzyme expression and developmental regulation suggests a more complex scheme for the regulation of creatine kinase gene expression than previously postulated.

Bovine lens 23, 21 and 19 kDa intrinsic membrane proteins have an identical amino-terminal amino acid sequence

We have isolated bovine lens intrinsic membrane proteins (MP) having molecular masses of 19, 21 and 23 kDa. Limited amino acid sequence analysis of the amino-terminal portion of each of these polypeptides revealed a 100% homology in sequence for the number of residues determined (20 amino acids). Northern blot analysis of bovine lens mRNA using a labeled antisense oligonucleotide probe common to the amino acid sequence of these three peptides revealed a single band having an apparent molecular size of 0.8 kb. Taken together, these findings suggest a genetic commonality between these polypeptides.

Perturbed development of the mouse lens by polyomavirus large T antigen does not lead to tumor formation

To study how the oncogenic process may involve effects on differentiation, we overexpressed an immortalizing oncogene in a developing tissue in transgenic mice. By use of a gene fusion of the alpha A-crystallin promoter to the viral immortalizing oncogene, polyoma large T antigen (PyLT), we created transgenic mice that express PyLT specifically in ocular lens. Expression of large T antigen during embryonic development led to a perturbation in lens development, specifically, an interference with the normal program of fiber cell differentiation. This resulted in microphthalmia, which persisted throughout the life of the animal. Histological analysis revealed impairment of cell elongation, denucleation, and mitotic senescence in both primary and secondary fiber cell differentiation. Strikingly, there was no evidence for hyperplasia or for tumor development in vivo, unlike the consequences of many immortalizing oncogenes on tissues in other transgenic mice. In vitro, however, the developmentally perturbed cells derived from the transgenic lens showed high proliferative capacity. Our results suggest that a primary effect of aberrant expression of an immortalizing gene is an interference with normal tissue development; however, this interference may not necessarily induce proliferation or lead to tumor formation.

Regulation of expression of c-myc protoocogene in a clonal line of mouse lens epithelial cells by serum growth factors

Steady-state levels of c-myc mRNA were determined in a clonal line of mouse lens epithelial cells in quiescent and growth-stimulated states. Steady-state mRNA levels for c-myc increased rapidly from an undetectable amount in quiescent cells to the maximum level (8-fold) in growth-stimulated cells. In contrast to its steady-state mRNA levels, its rate of transcription increased by only 3.4-fold in serum-stimulated cells versus quiescent cells, indicating that the abundance of c-myc transcripts in lens epithelial cells during the serum-induced transition from quiescence to proliferation is regulated by both transcriptional and post-transcriptional mechanisms. Serum stimulation in combination with cycloheximide caused superinduction in the steady-state levels of c-myc mRNA in lens epithelial cells. These additive increases in c-myc mRNA levels in the presence of cycloheximide could be due to a decrease in the apparent turnover rate of c-myc mRNA, which, in fact, was observed in actively growing cells. DNA synthesis, as revealed by [3H]thymidine uptake, began 18 h after the addition of serum to quiescent cells and peaked at 24 h. From these results it is concluded that the expression of c-myc gene in mouse lens epithelial cells in response to serum induction is growth dependent.

The protein concentration gradient within eye lens might originate from constant osmotic pressure coupled to differential interactive properties of crystallins

A protein concentration gradient exists from the center to the periphery of most lenses, the origin of which is still a matter of debate. The gradient, which contributes to the lens optical quality, seems to be accompanied by an uneven distribution of the crystallin classes, with the nucleus usually enriched in gamma- and the cortex in alpha-crystallins. Since the osmotic pressure within the lens seems to be constant and since a rather different interaction behaviour of alpha- and gamma-crystallins was demonstrated in previous studies, we propose that the maintenance of a constant osmotic pressure through the lens is sufficient to induce and stabilize a protein concentration gradient. The theoretical treatment has been worked out and the validity of the hypothesis has been demonstrated with colloidal osmotic pressure measurements of lens cortical and nuclear cytoplasmic extracts as a function of protein concentration. To account for the observed lens concentration gradient, however, a small additional concentration gradient in the opposite direction, involving an ion or small molecule, might be required.

Stage-dependent expression of the chicken delta-crystallin gene in transgenic fish embryos

To study the regulation of gene expression of vertebrate crystallin genes, the chicken delta-crystallin gene was introduced into a small freshwater fish, medaka (Oryzias latipes), which lacks this gene, and its expression was examined immunohistologically at several developmental stages before hatching. The gene expression was detected in the central fiber cells of the lens at an early stage, showing a stage-dependent expression. In non-lens tissues, the expression was barely detectable before tissue differentiation. It first became substantial mainly in mesodermal tissues and then later in a greater variety of tissues, including ectodermal and endodermal ones. Thus, the non-lens expression of delta-crystallin was also stage-dependent, with the stage being dependent on the tissue type. These results from lens and non-lens tissues are discussed in relation to tissue differentiation and two categories of delta-crystallin expression.

Structure of the bovine eye lens gamma s-crystallin gene (formerly beta s)

The organization of a number of crystallin genes has already been resolved. One of the remaining genes of which the structure was hitherto unknown is the gamma s gene (formerly beta s). We determined the complete sequence of the bovine gamma s-crystallin-coding gene, apart from the middle region of the first intron. Since it contains three exons and two introns, we conclude that the former beta s, also at the gene level is gamma-crystallin-like. However, it is located on chromosome 3, in contrast to other gamma genes which occur in tandem on the human chromosome 2.

Alpha B-crystallin is expressed in kidney epithelial cell lines and not in fibroblasts

We have recently shown the presence of alpha B-crystallin in non-ocular tissues of diverse embryological origins such as the heart, brain, spinal cord, kidney, retina, etc. Using an alpha B-crystallin-specific antiserum and immunofluorescence, immunoblotting, immunoprecipitation and peptide mapping with Staphylococcus aureus protease, we demonstrate differential expression of alpha B-crystallin in epithelial and fibroblast cell lines. alpha B-Crystallin was detectable only in epithelial cell lines such as MDBK, MDCK, LLCPK1 and JTC-12, and was not observed in two kidney fibroblast cell lines, one skin fibroblast cell line, and one corneal fibroblast cell line. Differential expression of the alpha B-crystallin gene was also confirmed by Northern blot analysis of the RNAs isolated from these cell lines. These data suggest a cell-type-specific role for alpha B.

Nuclear run-on transcription from primary embryonic lens tissue

We have devised an in vitro RNA elongation assay (nuclear "run-on" transcription) that is suitable for use with small amounts of primary embryonic tissue. The assay is sensitive enough to detect transcription of single-copy genes in 8 X 10(5) nuclei isolated from embryonic chicken lens epithelia, and gives no detectable hybridization to unrelated DNAs, such as phi X or pBR322. We have used this assay to examine transcription of delta-crystallin and six proto-oncogenes in lens epithelia of 6-day-old embryonic chickens. The results indicate that delta-crystallin, c-myc, p53, and c-fos are actively transcribed in these cells, while c-myb, N-ras, and c-mil are not transcribed at detectable levels.

Human fetal lens epithelial cells in culture: an in vitro model for the study of crystallin expression and lens differentiation

We have cultured and passaged human fetal lens epithelial cells. Cultured cells exhibited hexagonal, cuboidal shape typical of epithelial cells. Unlike previous observations made with cultured mammalian lens epithelial cells, indirect immunofluorescence and temporal analysis of 35S-labeled proteins demonstrated undiminished levels of alpha B-crystallin in primary, secondary, and tertiary cultures. Among the alpha-crystallins only alpha B synthesis was detected. Two dimensional gel electrophoresis indicated the presence of alpha B2 and no alpha B1. beta B2-crystallin, a fiber cell specific protein hardly detectable in primary cultures, increased significantly upon passaging. Human fetal lens epithelial cell cultures, described in this report, thus present a useful in vitro model for the study of lens epithelial cell differentiation and its pathological manifestations.

The effect of eye-derived-growth-factor (EDGFs) on methionine incorporation in the different cell populations of bovine adult lens in organ culture

When adult bovine lenses were cultured in vitro, the purified retina-derived growth factors EDGF I or EDGF II, as well as the soluble fraction of the retina RE, increased the rate of incorporation of [35S]methionine into protein in cells belonging to different populations in the anterior epithelium as well as in fibers from the most superficial region of the cortex. These fiber cells were the most sensitive to stimulation by the retinal factors as they exhibit a significant increase of total protein synthesis 24 hr after addition of the factors to the culture medium. The epithelial cells studied--central epithelial cells and germinative cells--appeared stimulated only 1 day later. The stimulation of incorporation was not directed towards a particular subset of proteins but to all major polypeptides constituting the electrophoretic pattern of each cell population. It is suggested that this type of ocular signal, which stimulates the expression of a definite program, may act as a permissive signal.

Regional distribution of the enzymes and substrates mediating the action of cAMP in the mammalian lens

Localization of adenylate cyclase activity in the outer cortical regions of the bovine lens correlates with the restriction of the Gs and Gi guanine nucleotide regulatory subunits of this enzyme to these same regions of the lens. In contrast, the major membrane substrates for cAMP-dependent protein kinase (cAMP-PK) (molecular masses of 18, 26 and 28 kDa) were identified in both the inner nuclear and the outer cortical regions of the lens. However, there were differences in the relative amounts of Pi incorporated into the 18 kDa and 28 kDa components in different lens regions. The three major membrane substrates for cAMP-PK were also phosphorylated when homogenates of lens cortex were incubated with [gamma-32P]ATP plus activators of the lens adenylate cyclase. In contrast, there was no incorporation of 32P into these substrates when homogenates of lens nucleus were used. When exogenous cAMP was added to homogenates of lens nucleus or cortex, 32P was incorporated into the membrane substrates for cAMP-PK in both regions of the lens, indicating that cAMP-PK was present in both regions. Interestingly, cAMP phosphodiesterase activity was at least 10-times greater in lens cortex than in the lens nucleus. These results indicate that while the major membrane substrates for cAMP-PK could be phosphorylated in all regions of the lens, there is a restriction of those enzymes that synthesize and degrade cAMP to the outer cortical regions of this organ.

alpha B subunit of lens-specific protein alpha-crystallin is present in other ocular and non-ocular tissues

alpha-Crystallin, a tissue specific structural protein of the ocular lens, is known to be composed of two subunits, alpha A and alpha B. By using a specific antibody in an immunoblotting procedure we have found that one of the subunits, alpha B is present in a number of non-lenticular tissues including the retina, heart, skeletal muscle, skin, brain, spinal cord and lungs. Interestingly, in the rat, this protein is present in significantly higher concentrations in adult than in fetal tissues and, with the exception of the lens, fetal and adult heart has the highest concentration among the tissues examined. That the protein in question is, in fact, alpha B, was confirmed a) by the remarkable similarity of Staphylococcus aureus protease peptide maps of the protein in the heart and purified alpha-crystallin and b) by the sequence analysis of a rat heart cDNA clone identified by the alpha B antibody. Based on these observations we conclude that while alpha A has a tissue-specific role, alpha B is a polypeptide of independent function not restricted to the ocular lens.

Induction of lens fibre differentiation by acidic and basic fibroblast growth factor (FGF)

Explants of epithelial cells from newborn rat lenses undergo changes characteristic of fibre differentiation when cultured with neural retina or retina-conditioned medium. Here we show that similar changes occur when acidic and basic fibroblast growth factor (FGF) are used instead of retina-conditioned medium. When cultured without FGF, epithelial explants contained negligible amounts of beta-crystallin, a lens protein found only in fibre cells. However, at saturating concentrations of FGF, about 20 micrograms beta-crystallin was produced per explant in 5 days. The response was dose-dependent, half maximal response requiring 55 and 290 ng/ml of basic and acidic FGF, respectively. FGF also stimulated cell proliferation and cell migration. All three responses to basic FGF were blocked by an antibody specific for basic FGF. The concentration of FGF required to produce a maximal response was lower for cell proliferation and migration than for beta-crystallin accumulation. The results suggest a possible role for FGF in the control of events in lens development.

An immunocytochemical comparison of Müller cells and astrocytes in the cat retina

Immunocytochemical localization, at the light and electron microscopic levels, of five different known glial proteins was used to compare Müller cells with astrocytes in the adult cat retina. Retina from two different areas of the eye was examined. A region of retina on the border of the optic nerve was used because of its large population of astrocytes, and a region away from the optic nerve was used to examine Müller cells (astrocytes are sparse in this region). Antibodies to cellular retinaldehyde binding protein and glutamine synthetase labeled the Müller cells but not the astrocytes, while labeling with anti-carbonic anhydrase C, anti-alpha crystallin and anti-glial fibrillary acidic protein was found in both Müller cells and astrocytes.

An immunocytochemical study of cat retinal Müller cells in culture

Müller cells, the specialized radial glia found in vertebrate retinas, were enzymatically dissociated from adult cat retinas and grown in culture. The cells were processed for immunofluorescence microscopy at times ranging from 4 hr to 3 months in culture. Labeling with antibodies specific to glial fibrillary acidic protein, cellular retinaldehyde binding protein, glutamine synthetase, carbonic anhydrase C and alpha crystallin, all proteins known to be found in Müller cells, was detected in the cultured cells. Immunoblot analysis of the cultured cells showed single protein bands corresponding to the appropriate molecular weights of the antigens.

Transdifferentiation of ocular tissues in larval Xenopus laevis

Transdifferentiation phenomena offer a useful opportunity to study experimentally the mechanisms on which cell phenotypic stability depends. The capacities of vertebrate eye tissues to reprogram cell differentiation are well known in avian and mammalian embryos, and in larval and adult newt. From research into the capacity of anuran eye tissues to reprogram differentiation into a new pathway, considerable data have accumulated concerning the transdifferentiative capacities of eye tissues in larval Xenopus laevis. This work reviews the data concerning the transdifferentiative phenomena of eye tissues in that species and, based on these, aims to establish the extent of our knowledge about the mechanism controlling these processes. In larval Xenopus laevis the outer cornea can regenerate a lens by a lens-transdifferentiation process triggered and substained by a factor(s), probably of a protein nature, produced by the neural retina. In a normal eye phenotypic stability of the outer cornea is guaranteed by the presence of the inner cornea and lens, which prevent the spread of retinal factor(s). The stimulus for lens transdifferentiation of the outer cornea can be supplied by other tissues as well, but this capacity is not widely distributed. The iris and retinal pigmented epithelium can transdifferentiate into neural retina if isolated from the surrounding tissues and implanted in the vitreous chamber. As for lens transdifferentiation of the outer cornea, retinal transdifferentiation of the iris can be stimulated by certain nonocular tissues as well.

Relaxometry of calf lens homogenates, including cross-relaxation by crystallin NH groups

We studied the magnetic field dependence of the longitudinal relaxation rates of water protons (1/T1 nuclear magnetic relaxation dispersion (NMRD) profiles) in transparent homogenates of calf lens. The samples included nuclear homogenates with total (heterogeneous) crystallin contents between 34% (v/v) (native) and 14% (diluted) as well as cortical homogenate, 21% (native) and 34% (concentrated). The NMRD profiles had two components: a monotonic dispersive component (analogous to that of both globular protein solutions and diamagnetic tissue) and "14N quadrupolar peaks." 14N peaks have never been reported for protein solutions, only for tissues and dehydrated proteins. These peaks occur between 0.5 and 5 MHz proton Larmor frequency and arise from interactions of solvent water protons with NH moieties of proteins. The 14N peaks in lens cytoplasm are very large and may correlate with the crystallin structure and interactions required to maintain short-range order and lens transparency. The monotonic and 14N quadrupolar components were largest in concentrated samples, but with different concentration dependencies. The dispersive components of samples above approximately 19% protein concentration had a fixed functional form, the amplitude of which varied with protein volume fraction, f, by the multiplicative factor f/(1 - f), suggesting spatial organization and dynamics of the solute proteins that are relatively independent of water content. In contrast, at concentrations less than 19%, the NMRD profiles are concentration dependent, indicating a dependence of the orientational relaxation time of the proteins on protein-protein interactions seen previously in other globular proteins at these concentrations. The 14N peaks are not resolved below approximately 19% protein and increase linearly with incremental volume fraction at protein concentrations above 19%. In addition, the 14N peaks in nuclear homogenates are 50-100% larger than those of cortical homogenates at the same concentrations. Partial substitution of solvent D2O for H2O decreases the peak heights, indicating that an exchangeable proton mediates the interaction between solvent protons and protein 14N nuclei.

The ontogeny of gamma-crystallin mRNAs in CatFraser mice

Mice which are either homozygous or heterozygous for the CatFraser mutation have ocular cataracts accompanied by selective reduction of the γ-crystallins, a homologous family of proteins present in the lens and encoded by a family of tightly linked genes. We measured the concentrations of four different mRNAs, each encoding a different γ-crystallin, in the lenses of homozygous CatFraser mice and in normal controls at various stages of development by preparing Northern blots from lens RNA, probing with RNAs complementary to each of the four messages and densitometry of the bands thus generated. The results show that, for each of these messages, the ontogenetic patterns observed in normal mice are retained in the mutant, but at much lower concentrations.

Simultaneous separation of all lens crystallin subunits by chromatofocusing

The total population of water-soluble lens crystallin subunits has been separated in one run on a single chromatofocusing column. Dissociation of the polymeric alpha- and beta-crystallin subunits occurred simultaneously on the column. Two components not described previously in the literature have been found in the gamma-crystallin region.

Evidence for positive and negative regulation in the promoter of the chicken delta 1-crystallin gene

We investigated the role of sequences flanking the transcription initiation site of the delta 1-crystallin gene in transient transfection assays of primary embryonic chicken lens epithelial cells or fibroblasts. Varying lengths of the 5' flanking sequence of the delta 1-crystallin gene (containing some untranslated sequence from exon 1) were fused to the bacterial chloramphenicol acetyltransferase (CAT) gene in the pSVOCAT plasmid. A plasmid carrying the bacterial beta-galactosidase gene driven by the Rous sarcoma virus (RSV) promoter was used as an internal control. Standardized results showed that the sequence located between -120 to -43 exhibited strong promoter activity; however, the promoter activity was markedly reduced (20-fold) when the upstream sequence between -603 and -120 was included in the construct. The delta 1-crystallin promoter displayed little lens preference. This upstream sequence did not reduce the activity of the Simian virus 40 (SV40) early promoter (with or without its enhancer) or the Herpes thymidine kinase promoter in transfection tests, indicating some specificity in its effect. Evidence for a delta 1-crystallin negative trans-acting factor was provided by competition experiments. Our data raise the possibility that expression of the delta 1-crystallin gene involves a negative cis-acting transcription element, a speculation which may deserve further attention in view of the gradual decrease in delta-crystallin synthesis in the developing lens.

Expression of the delta-crystallin genes in the embryonic chicken lens

The amounts of lens mRNA derived from the delta 1- and delta 2-crystallin genes of the chicken were determined by primer extension experiments using gene-specific synthetic oligonucleotides. The primer extended products were sequenced to establish the identity of the resulting cDNAs. The results indicated that most of the delta-crystallin mRNA in the 14-day-old embryonic lens contained transcripts derived from the delta 1 gene. Importantly, however, about 1-2% of the extended products were derived from delta 2 mRNA. Although not quantitative, the primer extension experiments suggested that the delta 1/delta 2 mRNA ratio may differ in the lens fiber cells during development between 6 days of embryogenesis and 3 weeks after hatching. These data provide the first demonstration for the presence of delta 2-crystallin mRNA in the chicken lens and raise the possibility that the two linked, extremely similar delta-crystallin genes are differentially regulated during development.

Variation in the relative abundance of gamma-crystallin gene transcripts during development and ageing

The transcripts of gamma-crystallin mRNA were examined during adulthood. The mRNA transcripts were detected by Northern blot technique. Although the total RNA per lens measured remains constant during adulthood, the mRNA transcript size was observed to decline with ageing, specifically for the gamma-crystallin and not for alpha- and beta-crystallins. We could show that the relative amount of mRNA transcripts detected with the four probes decreased with ageing, with gamma 1 and gamma 2 transcripts being found at a higher level than the gamma 3 and gamma 4 transcripts.

Expression of the gene for main intrinsic polypeptide (MIP): separate spatial distributions of MIP and beta-crystallin gene transcripts in rat lens development

The main intrinsic polypeptide (MIP) is the major protein present in the lens fiber cell membrane and is the product of a gene which, as far as is known, is expressed only in the lens. We have used in situ hybridization and immunofluorescence microscopy to characterize the expression of this gene during the course of development in the rat. At progressive stages of lens morphogenesis, we find that synthesis of the protein is closely tied to the accumulation of MIP mRNA in cells that are committed to terminal differentiation, first in the elongating presumptive primary lens fibers and later in the secondary fibers as they differentiate from the anterior epithelial cells. The transcripts accumulate in the basal cytoplasm of the primary fibers and in the cytoplasm which surrounds the cell nucleus in the secondary fibers. We have compared this pattern of expression with that of a gene for a cytoplasmic protein, beta-crystallin beta-A1/A3. In sharp contrast to the localized concentrations seen for the MIP mRNA, beta-A1/A3 transcripts are relatively uniformly distributed throughout the cytoplasm. Neither MIP nor crystallin gene appears to be transcriptionally active in the undifferentiated epithelial cell, but transcripts from the beta-A1/A3 gene appear earlier in fiber cell differentiation than do those from the gene for MIP.

Rat lens gamma-crystallins. Characterization of the six gene products and their spatial and temporal distribution resulting from differential synthesis

We have isolated, purified and characterized six individual gamma-crystallin polypeptides present in the rat lens. Comparison of their amino acid compositions with the known structure of the six gamma-crystallin genes permits a one-to-one correspondence to be made between each protein synthesized and the encoding gene. This demonstrates that each of the six genes is actually expressed in vivo. Two classes of three gamma-crystallins each, which we have designated classes gamma ABC and gamma DEF, are known to exist, on the basis of internal sequence homology. We have measured the temperature-dependent phase-separation characteristics of solutions of the six purified gamma-crystallins, and find that the three members of the gamma DEF class (gamma 2-2, gamma 3-1 and gamma 4-1) are all cryo-proteins with relatively high phase-separation temperatures, whereas the three gamma ABC crystallins (gamma 1-1, gamma 1-2 and gamma 2-1) do not show phase separation above -7 degrees C. We have measured the spatial distribution in rat lens of each of the alpha-, beta- and gamma-crystallins as a function of age from 1 to 420 days, using size-exclusion and ion-exchange high-pressure liquid chromatography (HPLC). Our findings in the cortical layer permit us to establish the differential synthesis of each of the crystallins during lens development. Particular attention has been devoted to the spatial and temporal distribution of the six individual gamma-crystallins. Up to birth, synthesis of the three components of the gamma DEF class predominates, and in particular that of gamma 2-2. In subsequent development the three components of the gamma ABC class assume a greater proportion of monomeric crystallins synthesized, while beta s-crystallin synthesis predominates in late development. Our analysis of different layers within single lenses provides novel information on spatial gradients of the water-soluble and water-insoluble protein fractions as a function of age. We consider the consequences of these findings for lens transparency and opacity in both rat and mouse lens. We show that the high concentrations of gamma DEF-crystallins appear to be responsible for the opacity known to occur in young rat lenses. We conclude from these observations that close control of the differential synthesis of gamma-crystallins plays an important role in maintaining lens transparency during development.

The murine cataractogenic mutation, Cat Fraser, segregates independently of the gamma crystallin genes

The murine mutation, Cat Fraser (CatFr), causes dominantly inherited ocular cataracts. Lenses of adult mice bearing this mutation contain reduced amounts of all seven γ-crystallin proteins and their corresponding transcripts. Levels of other lens proteins and transcripts appear normal and no extra-ocular effects of the mutation have been observed. The selective effect of this mutation on the γ-crystallins is consistent with the possibility that the site at which it occurs is involved in the coordinated regulation of the family of genes which encodes them. We have shown that several restriction fragment length polymorphisms in the γ-crystallin genes segregate independently of the CatFr mutation. Therefore, despite its selective effect on the expression of the γ-crystallin genes, the mutation is not linked to them. This observation rules out the possibility that the mutation is in a cis-acting regulatory site.

Homology of delta crystallin and argininosuccinate lyase

1. Delta crystallin, a major lens protein characteristic of birds and reptiles, is homologous to argininosuccinate lyase; 57% of the residues in chicken delta crystallin and human lyase are identical. 2. Even more similar (62% identical residues) to the human lyase is the sequence translated from the presumably inactive delta-2 gene of the delta crystallin locus. 3. As both delta crystallin and lyase are synthesized in birds only during the embryonic and juvenile stages, the persistence of delta crystallin in the adult lens appears to be paedomorphic. 4. Possible correlations of the origins of delta crystallin with other events in sauropsid evolution are proposed.

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.

Cloning and sequencing of a carp beta s-crystallin cDNA

The mRNAs were extracted from common carp (Cyprinus carpio) lenses, purified, reverse transcribed, dC tailed and cloned into Escherichia coli with pBR322 as vector. The cloning efficiency was around 1 X 10(7) colonies per micrograms of mRNA. A clone (pC20) was found by hybrid-arrested to contain the cDNA related to carp crystallins. However, comparison of the derived amino-acid sequence with bovine gamma-II and beta s-crystallins indicates that this carp crystallin sequence resembles closely the bovine beta s-crystallin and should be better classified as such except that this fish sequence does not contain the N-terminal 'arm' of four amino-acid residues present in bovine beta s-crystallin.

Lens-specific enhancer in the third intron regulates expression of the chicken delta 1-crystallin gene

We have previously shown that the tissue specificity determinant of the chicken delta 1-crystallin gene lies 3' of position -100 (Hayashi et al. 1985). Since the promoter of the gene (delta 1-crystallin promoter) did not show any tissue specificity, we examined various segments of the delta 1-crystallin gene for a tissue-specific enhancer activity by placing each segment downstream of a heterologous transcriptional unit coding for chloramphenicol acetyltransferase (CAT) and by transfecting chicken tissues in primary culture. We found that a segment spanning the third intron bears a strong lens-specific enhancer activity. This "delta 1-crystallin enhancer" activates transcription from the delta 1-crystallin promoter 20- to 40-fold in lens cells and to various degrees with other promoters. Deletion analysis of the enhancer region indicated that it covered nearly 1 kb but did not indicate clear-cut boundaries. For its enhancer effect the core region of 120 bp and associations with certain adjoining regions were required. Removal of the enhancer from the gene totally abolished delta 1-crystallin expression, and reinsertion of the enhancer in either upstream, internal, or downstream positions restored expression. We conclude that the delta 1-crystallin enhancer is an essential and major determinant for lens-specificity of delta 1-crystallin expression.

Developmental expression of crystallin genes: in situ hybridization reveals a differential localization of specific mRNAs

The time and place of the accumulation of alpha A-, beta B1- and gamma-crystallin RNA in the developing rat lens have been studied by in situ hybridization. alpha A- and gamma-crystallin RNA were first detected in the lens vesicle, while beta B1-crystallin RNA could be seen only after elongation of the primary fiber cells. Both beta B1- and gamma-crystallin RNA were confined to the fiber cells of fetal lenses, while alpha A-crystallin mRNA could also be detected in the epithelial cells. A quantification of the hybridization pattern obtained in the differentiation zone of the newborn rat lens showed that alpha A-crystallin RNA is concentrated in the cortical zone. alpha B-crystallin mRNA has the same distribution pattern. beta B1-crystallin RNA was relatively poorly detectable by in situ hybridization in both fetal and newborn rat lenses. The grain densities obtained with this probe increased from the periphery of the lens toward the interior, indicating that beta B1-crystallin RNA accumulated during differentiation of the secondary fiber cells. A similar accumulation pattern was obtained for gamma-crystallin mRNA, but, unexpectedly, this RNA could also be detected in the elongating epithelial cells. Our results show that gamma-crystallin RNA starts to accumulate as soon as visible elongation of epithelial cells occurs, during differentiation of the primary as well as the secondary fiber cells.

Junctions between lens cells in differentiating cultures: structure, formation, intercellular permeability, and junctional protein expression

We previously described cultures of chick embryo lens cells which displayed a marked degree of differentiation. In this report, the junctions found between the lens fiber-like cells in the differentiated "lentoids" are characterized in several ways. Thin-section methods with electron microscopy first demonstrated that numerous, large junctions between lentoid cells accompanied the other differentiated features of these cells. Freeze-fracture techniques, including quantitative analysis, then revealed that (a) junctional particles were loosely arranged as is typical of fiber cells, (b) the population of individual junctional areas in culture was indistinguishable from that found in 10- to 12-day chick embryo lenses, and (c) apparent junction formation occurred during the development of the lens cells, with lacy arrays of particles being associated with fiber-like junctions. In addition, gap junctions with hexagonally packed particles, typical of lens epithelial cells, largely disappeared during the course of differentiation. Injection of tracer dyes into lentoid cells resulted in rapid intercellular movement of dye, consistent with functional cell-to-cell channels connecting lentoid cells. During the development of the lens cells in culture, as junction formation occurred, an increase of approximately eight-fold in MP28 protein was observed within the cells. These combined results indicate that (a) extensive lens fiber junctions and functional cell-to-cell channels are found between differentiated lentoid lentoid cells in vitro, (b) lens fiber junctions appear to form during the course of lens cell differentiation in culture, (c) a significant increase occurs in the putative junctional protein before the cultures are highly developed, (d) the increased levels of MP28 and junction formation may be required for the full expression of the differentiated state in the lens fiber cell, and (e) this culture system should prove to be valuable for additional experiments on lens junctions and for other studies requiring the development of lens fiber cells in vitro.

The mouse eye lens obsolescence (Elo) mutant: studies on crystallin gene expression and linkage analysis between the mutant locus and the gamma-crystallin genes

Previous studies showed that the mouse Elo (eye lens obsolescence) mutation is located on chromosome 1, at a site near the Len-1 locus, which is defined by a set of polymorphic gamma-crystallin proteins. To investigate further the relationship between Elo and the gamma-crystallins, we have examined the steady-state levels of gamma-crystallin transcripts in normal and mutant eyes and analyzed the linkage relationship between the Elo locus and the gamma-crystallin genes. Our data showed that, while gamma-crystallin mRNA levels are preferentially reduced in the mutant eyes, the mutation does not seem to map within the gamma-crystallin gene cluster. The distance between Elo and the gamma 6 gene (the most proximal gamma-crystallin gene member) is estimated to be 1.4 +/- 0.9 cM, whereas that between gamma 6 and the distantly linked gamma 2 gene is 2.7 +/- 1.3 cM. Our data also suggest the possibility of recombination hot spots with the gamma-crystallin gene cluster.

Sequence comparison of gamma-crystallins from the reptilian and other vertebrate species

Lens crystallins were isolated from homogenates of reptilian eye lenses (Caiman crocodylus apaporiensis) by gel-permeation chromatography and characterized by gel electrophoresis, and amino acid and N-terminal sequence analyses. Four fractions corresponding to alpha-, delta/epsilon/beta-, beta- and gamma-crystallins were identified on the basis of their electrophoretic patterns as revealed by SDS gel electrophoresis. Comparison of the amino acid contents of reptilian crystallins with those of mammals suggests that each orthologous class of crystallins from the evolutionarily distant species still exhibits similarity in their amino acid compositions and probably sequence homology as well. All fractions except that of gamma-crystallin were found to be N-terminally blocked. N-terminal sequence analysis of the purified gamma-crystallin subfractions showed extensive homology between the reptilian gamma-crystallin polypeptides themselves and also those from other vertebrate species, suggesting the existence of a multigene family and their close relatedness to gamma-crystallins of other vertebrates.

Confirmation of assignment of the human alpha 1-crystallin gene (CRYA1) to chromosome 21 with regional localization to q22.3

The crystallins are highly conserved structural proteins universally found in the eye lens of all vertebrate species. In mammals, three immunologically distinct classes are present, alpha-, beta-, and gamma-crystallins, and each class represents a multigene family. The alpha-crystallin gene family consists of alpha 1-crystallin (CRYA1) and alpha 2-crystallin (CRYA2) genes (previously designated alpha A- and alpha B-crystallin, respectively), which show extensive sequence homology. We constructed a synthetic oligonucleotide probe of 25 bases corresponding to a specific region of the human alpha 1-crystallin gene sequence. This 25-mer probe bears little sequence homology to human alpha 2-crystallin gene and does not cross-hybridize to alpha 2-crystallin sequences in Southern blot analysis. Using this unique synthetic probe, we have demonstrated the identity of the alpha 1-crystallin gene in human genomic DNA. In addition, we have also confirmed its chromosomal location on human chromosome 21. Finally, we have regionally localized the gene to q22.3 by using both Southern blot analysis of a panel of cell hybrids containing different parts of human chromosome 21, and in situ hybridization to metaphase chromosomes. The use of synthetic oligonucleotide probes specific for individual genes should be useful in identifying and mapping members of multigene families.

Accumulation and distribution of sulfated materials in the maturing mouse lens capsule

Lenses of late gestational and postnatal normal-eyed mice were tested for accumulated sulfated materials by using Spicer's high-iron-diamine staining method and also for newly incorporated sulfate autoradiographically following administration of 35SO4 either in vivo or in isolated and organ-cultured lenses. Accumulated and newly incorporated sulfate was observed in all lenses for each age group tested. Discrete regional differences were seen in histochemical staining patterns for sulfate on the lens capsule in specimens of all ages, and distinct laminar zonations were seen in the various regions of the capsule in older specimens. Typically, the anterior and equatorial regions of the capsule demonstrated three histochemically distinct laminar zones while the posterior capsule usually demonstrated two laminar zones. Autoradiographic results indicated that sulfate was indeed being incorporated into these regions, and in the same general pattern as seen with histochemistry. The materials were largely insensitive to testicular hyaluronidase but were preferentially sensitive to nitrous acid digestion, indicating the presence of capsular heparan sulfates. Autoradiographic results from organ-cultured lenses indicated that this tissue itself is a primary source of these materials.

A gene location for the inheritance of the cataract Fraser (CatFr) mouse congenital cataract

Animal models which emulate defects similar to those in man are required for medical research. Many investigations on the cellular, developmental and molecular aspects of cataractogenesis use the cataract Fraser (CatFr) mouse. This report shows that the CatFT and Lop lens abnormalities are linked, and are probably allelic genes on chromosome 10. It also shows that the CatFT gene is maintained on an inbred genetic background which differs from 79 other strains; it is proposed that this strain be named CAT.

Collagen synthesis by long-lived mRNA in embryonic chicken lens

Lens capsule collagen synthesis by epithelial and fiber cells was examined by immunoprecipitation and collagenase digestion in embryonic and posthatch chicken eye lens. Epithelial cells and lens fibers in the process of terminal differentiation produce alpha 1 and alpha 2 type IV collagen chains. At 6 days of embryonic development in addition to the alpha 1 (IV) and alpha 2 (IV) collagen chains, lens cells produce high molecular weight collagenase-sensitive proteins not immunologically related to type IV collagen. Lens capsule collagen components have been identified in central and outer fibers isolated from 18-day embryos and from 10-day posthatch chicken eyes. At these stages, fibers which have an increasing number of picnotic nuclei still show collagen synthesis due to long-lived mRNA. Analysis of collagen synthesis by lens cells incubated with actinomycin D suggests that stabilization of collagen mRNA occurs in lens fiber cells and to a lesser extent in epithelial cells as early as 6 days of embryonic development.

Phosphorylation of ribosomal protein S6 during lens cell differentiation: correlation with translational efficiency

In vitro differentiation of embryonic chicken lens epithelial explants to form lens fiber cells is accompanied by an increase in protein synthesis without a corresponding increase in mRNA levels. This apparent increase in translational efficiency is correlated with a specific enhancement of phosphorylation of a 32K protein, which we identify as ribosomal protein S6 by two dimensional gel electrophoresis of purified ribosomal proteins. Serum, insulin, and chicken vitreous humor, three agents known to initiate differentiation in this system, all lead to enhanced S6 phosphorylation. Maximal enhancement of phosphorylation is reached within the first hour after the onset of differentiation, and is not blocked by inhibitors of RNA and protein synthesis.

Direct chemical measurement of DNA synthesis and net rates of differentiation of rat lens epithelial cells in vivo: applied to the selenium cataract

This report describes a direct chemical method for rapidly estimating DNA synthesis and net rates of epithelial cell differentiation in the ocular lens in vivo. DNA synthesis in the lens of control and selenium-treated rats (12- or 13 days of age) was estimated by chemically isolating and measuring trichloroacetic acid (TCA)-insoluble 3H from the lens following injection of [3H]thymidine. Labeled substrate for DNA synthesis peaked in the lens at 1 hr after injection, decreased markedly by the third hour and was essentially gone by hour 12. Synthesis of labeled DNA in the lens was largely complete by about 3 hr. The [3H]DNA content of the whole lens, measured as TCA-insoluble 3H, remained constant for at least 4 months. The distribution of labeled epithelial cells between the epithelial-cell layer and fiber-cell mass was followed for up to 1 month after injection by measuring the ratio of [3H]DNA in the capsule (epithelial-cell layer) to lens body. Between days 2-3 and day 14 after injection, the ratio of [3H]DNA in the epithelial-cell layer to lens fiber cells decreased linearly in a semilogarithmic plot of the ratio vs. time; i.e. the rate of change of the ratio followed first-order kinetics. Thus, the rate constant (k) for the rate of change in the ratio of [3H]DNA in the capsule layer to lens body can provide an estimate of the percentage of the labeled epithelial cells which leave the capsule per day through differentiation into fiber cells. An apparent rate constant of 0.27 day-1 was estimated from the mean of five experiments; i.e. 27% of labeled epithelial cells were differentiating into cortical fiber cells per day. Therefore, about 70% of the germinative epithelial cells would be replaced every 4 days in these rats. This value is in good agreement with results of studies using autoradiographic technics. The selenium cataract is reported to involve rapid damage to lens epithelial cells. Incorporation of [3H]thymidine into DNA was decreased by at least 60% in the lens of selenium-treated rats. Selenium did not decrease the availability of substrate in the lens for DNA synthesis. The cause of the decreased incorporation of [3H]thymidine into the selenium-exposed lenses remains to be determined.

Lentropin, a protein that controls lens fiber formation, is related functionally and immunologically to the insulin-like growth factors

Lentropin, a factor present in the vitreous humor of the eye, stimulates lens fiber differentiation from chicken embryo lens epithelial cells in vitro. Lentropin has been partially purified but has not been isolated in sufficient quantity or purity for direct comparison with other growth and differentiation factors. Previous studies have shown that insulin and fetal bovine serum share with lentropin the ability to stimulate lens fiber formation from cultured epithelial cells. In the present study, a number of hormones and growth factors were assayed for lentropin activity. Of those tested, the only substances that had this activity were the insulin-like growth factors (IGFs) somatomedin C (Sm-C/IGF-I) and multiplication-stimulating activity (MSA/IGF-II). Sm-C/IGF-I was approximately 30 times more potent than insulin or MSA/IGF-II in promoting fiber cell formation. A monoclonal antibody to human Sm-C/IGF-I inhibited purified Sm-C/IGF-I, fetal bovine serum, and chicken vitreous humor from stimulating fiber cell differentiation in vitro. This antibody has been shown not to crossreact with insulin and did not block insulin-stimulated lens fiber formation. These findings indicate that lentropin is related to the IGFs and that these factors may play important roles in controlling cell differentiation, in addition to their better-known ability to stimulate cell division.

Transient appearance of and regional differences in apical cell surface materials during early morphogenesis of the chicken lens

Apical cell surface materials were analysed with staining and lectin histochemistry in the chicken lens, from the earliest stages of lens morphogenesis through the completion of primary fibre cell elongation. Acidic materials were found to accumulate on the apical cell surface of the presumptive lens fibres from the mid cup stage through the early stages of lens vesicle formation, peaking just before lens fibre cell elongation. These materials labelled strongly with concanavalin A, but not with soybean lectin. By the completion of fibre cell elongation, these materials were gone. Conversely, the apical surface of the future lens epithelial cells demonstrated neutral materials, which were also largely removed by the completion of primary fibre cell elongation. These materials labelled with both concanavalin A and soybean lectin. The identity of these materials is not known, but their location prior to and during chicken lens morphogenesis suggests that they may be involved in establishing polarity during elongation of the primary lens fibre cells.

Differential expression of crystallin genes during development of the rat eye lens

The concentrations of alpha A-, beta B1-, and total gamma-crystallin mRNAs were measured during development of the rat eye lens, using Northern blot and dot blot analysis. After 14 days of fetal growth a sharp increase in the concentration of all three mRNA species was observed. After birth, the concentration of alpha A-crystallin transcripts remains high until 6 months of age, the concentration of gamma-crystallin transcripts decreases gradually, while the concentration of beta B1-crystallin transcripts decreases sharply. The composition of the gamma-crystallin mRNA pool was determined by measuring the relative amount of the transcripts of each of the six gamma-crystallin genes using primer extension and S1-nuclease mapping. The transcripts of all six genes are found until the third month after birth. Thereafter the transcripts of the gamma 1-1, the gamma 3-1, and gamma 4-1 crystallin genes are no longer detectable. Later on the transcripts of the gamma 2-1 and gamma 2-2 genes also disappear leaving only the transcripts of the gamma 1-2 crystallin gene at the age of 8 months. The concentration of the six different gamma-crystallin mRNAs is thus regulated differentially.

Oncogenesis of the lens in transgenic mice

Neoplastic tumors of the ocular lens of vertebrates do not naturally occur. Transgenic mice carrying a hybrid gene comprising the murine alpha A-crystallin promoter (-366 to +46) fused to the coding sequence of the SV40 T antigens developed lens tumors, which obliterated the eye cavity and even invaded neighboring tissue, thus establishing that the lens is not refractive to oncogenesis. Large-T antigen was detected early in lens development; it elicited morphological changes and specifically interfered with differentiation of lens fiber cells. Both alpha- and beta-crystallins persisted in many of the lens tumor cells, while gamma-crystallin was selectively reduced. Accessibility, characteristic morphology, and defined protein markers make this transparent epithelial eye tissue a potentially useful system for testing tumorigenicity of oncogenes and for studying malignant transformation from its inception until death of the animal.