Isolation and in vitro translation of delta-crystallin mRNA from embryonic chick lens fibers

Overview of the Lens

In order to accomplish its function of transmitting and focusing light, the crystalline lens of the vertebrate eye has evolved a unique cellular structure and protein complement. These distinct adaptations have provided a rich source of scientific discovery ranging from biochemistry and genetics to optics and physics. In addition, because of these adaptations, lens cells persist for the lifetime of an organism, providing an excellent model of the aging process. The chapters dealing with the lens will demonstrate how the different aspects of lens biology and biochemistry combine in this singular refractive organ to accomplish its critical role in the visual system.

RNA stability in terminally differentiating fibre cells of the ocular lens

During terminal differentiation of lens fibre cells all cytoplasmic organelles are degraded abruptly. This process eliminates light-scattering elements from the optical axis of the lens and thereby ensures the transparency of the tissue. With the breakdown of the nucleus, transcription ceases, but the degree to which extant RNA is translated in the anucleated cells is uncertain. Previous studies indicated that fibre cell mRNA is unusually stable. For example, full-length delta-crystallin transcripts have been detected in core fibres months after transcription in these cells ceased. In the present study, we used the embryonic chicken lens as a model to examine the fate of RNA in the period immediately before and after organelle degradation. We mapped the tissue distribution of ribosomal RNA (rRNA) using acridine orange staining, in situ hybridization, and direct visualization of ribosomes by electron microscopy. These experiments suggested that rRNA decayed in the anucleated core fibre cells with a half-life of approximately 2.5 days. Similarly, in situ hybridization analysis of polyadenylated transcripts, beta-actin, or GAPDH mRNA indicated that these sequences were not stable in the core fibre cells. However, in agreement with earlier findings, we detected a strong in situ hybridization signal for delta-crystallin in the lens core, many days after transcription had ceased. We used quantitative PCR to compare the levels of GAPDH, L14 and delta-crystallin transcripts in the core region during development. Surprisingly, all three mRNAs decayed with indistinguishable kinetics. We conclude that the persistent delta-crystallin hybridization signal was not evidence of an unusually stable mRNA but, rather, reflected the extraordinary initial abundance of this transcript. Taken together, our data indicate that the half-life of both mRNA and the protein synthetic machinery in the lens core is only a few days. Given that, in vertebrate lenses, nuclei in this region of the lens are degraded during embryonic development, protein synthesis in central lens fibre cells is probably completed well before birth.

Reliability of delta-crystallin as a marker for studies of chick lens induction

Induction of a lens by the optic vesicle of the brain was the first demonstration of how tissue interactions could influence cell fate during development. However, recent work with amphibians has shown that the optic vesicle is not the primary inducer of lens formation. Rather, an earlier interaction between anterior neural plate and presumptive lens ectoderm appears to direct lens formation. One problem with many early experiments was the absence of an unambiguous assay for lens formation. Before being able to test whether the revised model of lens induction applies to chicken embryos, we examined the suitability of using delta-crystallin as a marker of lens formation. Although delta-crystallin is the major protein synthesized in the chick lens, one or both of the two delta-crystallin genes found in chickens is transcribed in many non-lens tissues as well. In studies of lens formation where appearance of the delta-crystallin protein is used as a positive assay, synthesis of delta-crystallin outside of the lens could make experiments difficult to interpret. Therefore, polyacrylamide gel electrophoresis, immunoblotting, and immunofluorescence were used to determine whether the delta-crystallin messenger RNA detected in non-lens tissues is translated into protein, as it is in the lens. On Coomassie-blue-stained gels of several tissues from stage-22 embryos, a prominent protein was observed that co-migrated with delta-crystallin. However, on immunoblots, none of the nonlens tissues tested contained detectable levels of delta-crystallin at this stage. By imunofluorescence, delta-crystallin was observed in Rathke's pouch and in a large area of oral ectoderm near Rathke's pouch, yet none of the cells in these non-lens tissues showed the typical elongated morphology of lens fiber cells. When presumptive lens ectoderm or other regions of ectoderm from stage-10 embryos were cultured and tested for lens differentiation, both cell elongation and delta-crystallin synthesis were observed, or neither were observed. The results suggest that delta-crystallin synthesis and cell elongation together serve as useful criteria for assessing a positive lens response.

Molecular cloning of human ornithine aminotransferase mRNA

The isolation and characterization of a cDNA clone for the mRNA of human ornithine aminotransferase (OATase; ornithine-oxo-acid aminotransferase; L-ornithine:2-oxo-acid aminotransferase, EC 2.6.1.13), a nonabundant mitochondrial matrix enzyme that is severely deficient in a hereditary chorioretinal degenerative disease (gyrate atrophy), is described. Human liver, retina, and retinoblastoma (Y79) mRNAs were prepared and tested for the OATase mRNA content by in vitro translation, immunoprecipitation, and NaDodSO4/PAGE. The retinoblastoma cells were found to be expressing this enzyme at a relatively high level. The primary translation product of the OATase mRNA is larger than the pure OATase protein on NaDodSO4/PAGE by approximately equal to 4 kDa, suggesting a precursor protein. lambda gt11 cDNA libraries were prepared from the human mRNAs, and the recombinant clones were immunoscreened as plaques with two different preparations of rabbit anti-human OATase antibodies. A clone (lambda gtRB315) was isolated from the retinoblastoma library that reacts with both of the antibody preparations, and the DNA sequence of its 2.1-kilobase-pair cDNA insert was obtained. An open reading frame consisting of 1371 nucleotides is present in the sequence, and a putative translational initiation methionine codon is identified at position 55. A putative leader sequence consisting of 32 amino acid residues is identified, resulting in a precursor protein of 439 amino acid residues and a molecular mass of 48,534 Da and a mature protein of 407 residues and 45,136 Da. The amino acid sequences of seven tryptic peptides (115 amino acid residues) of the pure human OATase were obtained by microsequencing. When the tryptic peptide and cDNA-derived amino acid sequences were compared, homologies in 111 of 115 residues, including a match of 20 consecutive residues, were observed. An RNA blot hybridization of 32P-labeled OATase cDNA to normal human retina and retinoblastoma mRNAs demonstrated an OATase mRNA species of approximately equal to 2.2 kilobases. The level of OATase mRNA in the normal human retina is approximately equal to 1/100th the level of rhodopsin mRNA and 1/5th to 1/10th the level present in the retinoblastoma cells.

Delta crystallins and their nucleic acids

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

Deficiency of functional messenger RNA for a developmentally regulated beta-crystallin polypeptide in a hereditary cataract

The messenger RNA for a beta-crystallin polypeptide with a molecular size of 27 kilodaltons, first detected 5 to 10 days after birth in the normal mouse lens and the Nakano mouse cataract, was not detected in the Philly mouse cataract with translation in vitro. The heterozygous Philly lens had intermediate levels of the 27-kilodalton beta-crystallin polypeptide and exhibited delayed onset of the cataract. The deficiency of functional 27-kilodalton beta-crystallin messenger RNA is the earliest lesion reported yet for the Philly lens and points to a transcriptional or posttranscriptional developmental defect in this hereditary cataract.

Multiple gamma-crystallins of the mouse lens: fractionation of mRNAs by cDNA cloning

cDNAs made from polyadenylylated RNAs of the mouse lens were cloned by the G.C tailing procedure in the bacterial plasmid pBR322. Four recombinant DNAs containing gamma-crystallin sequences were identified by hybrid selection and translation. Sequence analysis of the in vivo-labeled gamma-crystallin polypeptides that cofocused isoelectrically with the hybrid-selected translation products established that the four cloned cDNAs were derived from mRNAs encoding gamma-crystallin polypeptides with similar NH2 termini. The cDNA clones had different restriction maps and could discriminate among the different gamma-crystallin mRNAs under stringent hybridization conditions. Under relaxed hybridization conditions, the cDNA clones cross-hybridized with all gamma-crystallin mRNAs, and even slightly with beta-crystallin mRNAs, as judged by in vitro translation. RNA blot hybridization showed that the mouse lens gamma-crystallin mRNAs are 840 +/- 100 nucleotides long. These data indicate that there are at least four similar gamma-crystallin mRNAs and suggest (but do not establish) the existence of a closely related family of gamma-crystallin genes.

alpha A-crystallin messenger RNA of the mouse lens: more noncoding than coding sequences

The 14S messenger RNA (1300 to 1500 nucleotides) for the alpha A chain of alpha-crystallin of the mammalian lens is nearly three times larger than required to code for the polypeptide that contains 173 amino acids. As a means of accounting for this anomaly, a complementary DNA clone for the mouse alpha A-crystallin messenger RNA was constructed in pBR322 and sequenced. Derivation of the protein sequence from the nucleic acid sequence showed that mouse alpha A-crystallin is similar to that of other organisms. The messenger RNA contains 536 nucleotides located on the 3' side of the coding region, excluding the polyadenylate stretch. This 3' sequence does not encode any other crystallin and has multiple termination codons in the three possible reading frames.

Structural and functional similarities of delta-crystallin messenger ribonucleic acids from duck and chicken lenses

delta-Crystallin of the embryonic duck lens was compared with that of the embryonic chicken lens with respect to polypeptide composition, synthesis, and messenger ribonucleic acid (mRNA) sequences. Labeling experiments with [35S]methionine revealed that the duck delta-crystallin is composed of minor amounts of polypeptides with molecular weights near 50000 (50K) and 49000 (49K) and much greater amounts of polypeptides with molecular weights near 48000 (48K) and 47000 (47K), as judged by sodium dodecyl sulfate-urea-polyacrylamide gel electrophoresis. All four sizes of polypeptides were synthesized in similar relative proportions as found in vivo in a rabbit reticulocytes lysate supplemented with delta-crystallin mRNA isolated from the embryonic duck lens. Synthesis of the 48K and 47K delta-crystallin polypeptides was differentially reduced in duck lenses cultured in the presence of ouabain. This is similar to the differential reduction of synthesis of the lower molecular weight delta-crystallin peptides in embryonic chicken lenses demonstrated previously. R loops formed between duck or chicken delta-crystallin mRNA and a cloned chicken delta-crystallin cDNA and heteroduplexes formed between duck or chicken delta-crystallin mRNA and cloned chicken genomic DNAs containing delta-crystallin sequences showed that, except for the putative 5' leader sequence, the duck and chicken delta-crystallin mRNAs have extremely similar nucleotide sequences. These data indicate considerable conservation of delta-crystallin throughout the approximately 100 million years of divergence between ducks and chickens. The findings also suggest a possible relationship between the structure of delta-crystallin mRNA and the differential reduction in synthesis of the lower molecular weight delta-crystallin polypeptides in ouabain-treated lenses of ducks and chickens.

Persistence of crystallin messenger RNA's with reduced translation in hereditary cataracts in mice

In vitro translation experiments showed that the lens fiber cells of two hereditary cataracts in mice (Nakano and Philly) possessed a full complement of crystallin messenger RNA's, despite severely reduced synthesis of crystallin in these cells. The reduction in synthesis in the lens fiber cells correlated with the increase in Na+ and the decrease in K+, which occurs during cataractogenesis. In contrast to the fiber cells, the epithelial cells continued to synthesize crystallins in the cataractous lenses. Crystallin synthesis was stimulated in the fiber cells by raising the K+ concentration and lowering the Na+ concentration in the cultured lenses. The reduction in crystallin synthesis in the initial stages of cataractogenesis in the Nakano and Philly lenses thus appears to be due to poor utilization of crystallin messenger RNA's in the fiber cells.

Chicken lens crystallin DNA sequences show at least two delta-crystallin genes

Restriction analysis of chicken DNA using probes derived from a cloned cDNA and analysis of cloned genomic DNA fragments containing delta-crystallin gene sequences have indicated the presence of at least two non-allelic genes for delta-crystallin, the first and principal crystallin synthesised in the embryonic chicken lens. Electron microscopic analyses of three cloned genomic fragments revealed that the delta-crystallin mRNA gene sequences are interrupted at least 14 times in one of the delta-crystallin genes.

Molecular cloning and partial characterization of delta-crystallin cDNA sequences in a bacterial plasmid

Double-stranded cDNA synthesized from delta-crystallin mRNA isolated from lens fiber cells of 15-day-old embryonic chicken was cloned in Escherichia coli chi 1776 in the Pst I site of the plasmid pBR322 by using the oligo(dC) . oligo(dG) joining procedure. Twelve Amps Tetr transformants contained sequences complementary to purified delta-crystallin [32P]cDNA. One of the recombinant clones (p delta Cr-2) had an insert of 1241 +/- 240 base pairs, as judged by R-looping analysis with purified delta-crystallin mRNA. The inserted cDNA represents at least 69% of the delta-crystallin coding sequences. p delta Cr-2 was further characterized by restriction analysis, protection of delta-crystallin [3H]cDNA from digestion by S1 nuclease, and hybrid-mediated arrest of delta-crystallin mRNA translation in vitro. p delta Cr-2 provides an invaluable probe for additional analysis of the primary structure, gene organization, and regulated synthesis of delta-crystallin, the principal protein synthesized during lens differentiation in the chicken embryo.

Differential synthesis of lens proteins in the presence of m7G(5')pppG and cleavage product m7GMP in an embryonic chick lens cell lysate

The effect of m7GMP release from m7GMP-containing mRNA cap sequence m7GpppG by the embryonic chick lens m7GpppN-pyrophosphatase activity on the synthesis of lens proteins was examined in a newly developed homologous translation system derived from 15-day embryonic chick lenses. The synthesis of total lens polypeptides and delta-crystallin polypeptides, the major translation product, was inhibited 84% and 88%, respectively, by 0.5 mM m7GpppG; m7GMP (0.5 mM) inhibited total synthesis by 63% but was 33% less inhibitory toward delta-crystallin synthesis; GpppG and GMP were not inhibitors, m7GpppG inhibited met-tRNAfmet incorporation into 80S initiation complexes.

Receptors for glucocorticoids in the lens epithelium of the calf

The calf lens epithelium contains a specific cytoplasmic receptor for glucocorticoids. This binding protein has a high affinity for dexamethasone (average dissociation constant, 8 x 10(-9) mole per liter), a low capacity (average, 550 femtomoles per milligram of protein), extreme heat sensitivity, and exhibits a pattern of competition similar to that of glucocorticoid receptors in other tissues. This provides direct biochemical evidence that these tissues may function as a target organ for glucocorticoids.

Cleavage of pm7G from mRNA 5' terminal cap structures by pyrophosphatase activity in embryonic chick lens cells

The presence of pyrophosphatase activity in embryonic lens cells which cleaves pm7G and ppGm from m7G(5')pppGm was demonstrated. It was also found that m7G(5')pppG, but not G(5')pppG, was hydrolyzed, and conversion of m7GpppG to m7G*pppG, in which the 5-membered ring of the m7G moiety is open, abolished its hydrolysis. For the caps hydrolyzed, pm7G was released only in the presence of lens cellular fraction; pm7G inhibited cap hydrolysis.

Translation of mRNA for glutamate dehydrogenase and spectrophotometric procedure to follow the enzyme biosynthesis

Heterogeneous poly (A)-mRNA fraction was isolated from rat liver microsomes using phenol-chloroform extraction, millipore filtration and poly (U)-agarose affinity chromatography. Obtained fractions were characterized with respect to their secondary structure and poly (A) content. Isolated poly (A)-mRNA fraction contained high template activity for glutamate dehydrogenase in cell-free systems with microsomes or polysomes. A spectrophotometric procedure to follow enzyme biosynthesis was also developed.

Detection of methyltransferase activities which modify Gppp G to m7GpppGm in embryonic chick lens

GpppG was modified to m7 GpppGm by a cytoplasmic extract, prepared from embryonic lens cells, in a reaction mixture which contained S-adenosyl-methionine as methyl group donor. The appearance of m7 GpppGm was a function of time and lens extract concentration. S-adenosyl-homocysteine inhibited both the m7G and Gm modification reactions. Analogues of GpppG, pG, ppG and pppG were relatively ineffective as substrates.

Synthesis of delta crystallin from embryonic chick lens messenger ribonucleoprotein complex

A single, major 21 S messenger ribonucleoprotein (mRNP complex) was isolated and purified by sucrose gradient centrifugation after EDTA treatment of high salt washed polysomes from 15 day embryonic chick lenses. A 17 S mRNA was released from the 21 S subunit of delta crystallin. Similar results were obtained with the 17 S mRNA released from the 21 S mRNP complex.

Isolation and chracterization of rat-lens messenger RNAs. Comparison of lens proteins, synthesized in lens culture and in homologous and heterologous cell-free systems

Messenger RNA has been isolated from rat lens tissue. The mRNA species which codes for the A2 chain of alpha-crystallin, revealed the same extremely high sedimentation value (14S) as the corresponding messenger from calf lens. However, it has been shown that in rat lens the 14-S messenger preparation directs the synthesis of an additional alpha-crystallin chain, designated as alpha-X with an approximate molecular weight of 24000. For comparative purpose crystallin synthesis has also been studied as well in cultured rat lens cells as in the rat lens cell-free system.

Quantitation of delta-crystallin messenger RNA during lens induction in chick embryos

Embryonic chick cells of the presumptive lens ectoderm are induced to differentiate into a lens by the optic vesicle, an outgrowth from the developing brain. This lens induction involves formation of the lens placode by cell elongation between about 44 and 50 hr of development, followed by invagination of the placode between about 50 and 55 hr of development. The amount of delta-crystallin messenger RNA (mRNA) in heads of embryos between 48 and 72 hr of development was determined by molecular hybridization with [3H]DNA complementary to purified delta-crystallin mRNA. An average of approximately 0.5 pg of delta-crystallin mRNA was found per embryonic head at 48 hr of development. This leads to approximately 265 molecules of delta-crystallin mRNA per lens cell, if one assumes that the mRNA is confined to the lens rudiment, as is indicated by immunofluorescence studies of delta-crystallin performed by other investigators. Pulse-labeling experiments with [35S]methionine indicated that the delta-crystallin mRNA is being translated already at this time. delta-Crystallin mRNA accumulated at an average rate of 1 molecule per cell per minute between 48 and 72 hr of development. Thus, by extrapolation, the initiation of delta-crystallin mRNA accumulation coincides with the initiation of lens placode formation at approximately 43-44 hr of development, which is some 8-9 hr after the initiation of lens induction by the optic vesicle. These data suggest that delta-crystallin synthesis is regulated by the accumulation of delta-crystallin mRNA during lens induction in chick embryos.

Cellular protein synthesis shutoff by mengovirus: translation of nonviral and viral mRNA's in extracts from uninfected and infected Ehrlich ascites tumor cells

The mechanism whereby picornaviruses inhibit host protein synthesis while their own synthetic processes proceed unabated has remained elusive. One of our approaches to this problem was to study the ability of cell-free extracts derived from uninfected and mengovirus-infected Ehrlich ascites tumor cells to translate viral and nonviral mRNA's under various conditions of incubation. Our results indicate that viral messengers (from mengovirus and encephalomyocarditis virus) and cellular messengers [L cell and Ehrlich ascites tumor poly(A)-containing mRNA's, rabbit globin mRNA, and chicken embryo lens crystallin mRNA] are translated equally well in both extracts. We also examined the simultaneous translation of viral and nonviral mRNA's in extracts from uninfected Ehrlich ascites tumor cells. Our results indicate that under certain conditions mengovirus RNA can suppress completely the translation of globin mRNA. The significance of these results in terms of the shutoff of host protein synthesis is discussed.

Cleavage of adenovirus messenger RNA and of 28S and 18S ribosomal RNA by RNase III

Escherichia coli ribonuclease III cleaves adenovirus messenger RNA and mammalian 28S and 18S ribosomal RNA. Fragmentation is not random, but in each case a specific collection of products is generated. This points to the potential use of the enzyme as a tool for specific fragmentation of RNA. Cleavage by RNase III abolishes the capability of adenovirus messenger RNA to direct cell-free synthesis of virus polypeptides.