Cell elongation and -crystallin synthesis without RNA synthesis in cultured early embryonic chick lens epithelia

RNA-binding proteins and post-transcriptional regulation in lens biology and cataract: Mediating spatiotemporal expression of key factors that control the cell cycle, transcription, cytoskeleton and transparency

Development of the ocular lens - a transparent tissue capable of sustaining frequent shape changes for optimal focusing power - pushes the boundaries of what cells can achieve using the molecular toolkit encoded by their genomes. The mammalian lens contains broadly two types of cells, the anteriorly located monolayer of epithelial cells which, at the equatorial region of the lens, initiate differentiation into fiber cells that contribute to the bulk of the tissue. This differentiation program involves massive upregulation of select fiber cell-expressed RNAs and their subsequent translation into high amounts of proteins, such as crystallins. But intriguingly, fiber cells achieve this while also simultaneously undergoing significant morphological changes such as elongation - involving about 1000-fold length-wise increase - and migration, which requires modulation of cytoskeletal and cell adhesion factors. Adding further to the challenges, these molecular and cellular events have to be coordinated as fiber cells progress toward loss of their nuclei and organelles, which irreversibly compromises their potential for harnessing genetically hardwired information. A long-standing question is how processes downstream of signaling and transcription, which may also participate in feedback regulation, contribute toward orchestrating these cellular differentiation events in the lens. It is now becoming clear from findings over the past decade that post-transcriptional gene expression regulatory mechanisms are critical in controlling cellular proteomes and coordinating key processes in lens development and fiber cell differentiation. Indeed, RNA-binding proteins (RBPs) such as Caprin2, Celf1, Rbm24 and Tdrd7 have now been described in mediating post-transcriptional control over key factors (e.g. Actn2, Cdkn1a (p21Cip1), Cdkn1b (p27Kip1), various crystallins, Dnase2b, Hspb1, Pax6, Prox1, Sox2) that are variously involved in cell cycle, transcription, cytoskeleton maintenance and differentiation in the lens. Furthermore, deficiencies of these RBPs have been shown to result in various eye and lens defects and/or cataract. Because fiber cell differentiation in the lens occurs throughout life, the underlying regulatory mechanisms operational in development are expected to also be recruited for the maintenance of transparency in aged lenses. Indeed, in support of this, TDRD7 and CAPRIN2 loci have been linked to age-related cataract in humans. Here, I will review the role of key RBPs in the lens and their importance in understanding the pathology of lens defects. I will discuss advances in RBP-based gene expression control, in general, and the important challenges that need to be addressed in the lens to define the mechanisms that determine the epithelial and fiber cell proteome. Finally, I will also discuss in detail several key future directions including the application of bioinformatics approaches such as iSyTE to study RBP-based post-transcriptional gene expression control in the aging lens and in the context of age-related cataract.

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.

Messenger RNA stabilization in chicken lens development: a reexamination

Previous studies, using actinomycin D, suggested that the rate of mRNA degradation in chicken embryo lens epithelial cells decreased sharply between Days 11 and 13 of development (Yoshida and Katoh, Exp. Eye Res. 11, 1971; Exp. Cell Res. 71, 1972). We repeated these studies and directly measured the effects of actinomycin on lens mRNAs. Although actinomycin decreased [3H]uridine incorporation greater than 90%, only modest decreases in methionine incorporation were detected. Treatment for 6 hr had no detectable effect on delta-crystallin mRNA levels or on the relative amounts of proteins synthesized in an in vitro translation system. Unlike the previous studies, we did not find significant changes in the stability of lens epithelial mRNAs during development.

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

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

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

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

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.

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.

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.

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

Of the protein synthesized and accumulated during differentiation of embryonic chick lens fibers, 70-80% is the tissue specific protein delta-crystallin. We have isolated and partially characterized the total cytoplasmic mRNA from purified lens fibers of 15-day-old embryos as an initial step toward understanding the regulated expression of delta-crystallin during development. Each lens fiber mass contained an average of 10 mug of cytoplasmic RNA; approximately 0.1 mug per fiber mass was recovered in the mRNA fraction by oligo(dT)-cellulose chromatography. The mRNA electrophoresed primarily as a single peak on a polyacrylamide-agarose gel with an apparent molecular weight of about 9 x 10(5) estimated by comparison with 28S and 18S rRNA markers. Of the protein synthesized in response to the mRNA in cell-free systems derived from Krebs II ascites tumor cells or rabbit reticulocytes, 70-80% comigrated with delta-crystallin on sodium dodecyl sulfatepolyacrylamide-agarose gels. Comparison of the tryptic peptides of delta-crystallin with those of the in vitro products from both heterologous systems established that the lens fiber mRNA contained delta-crystallin mRNA, and that no other functional mRNAs were present in detectable quantities. Thus, the specialization of protein synthesis in embryonic chick lens fibers apparently results from an accumulation of delta-crystallin mRNA in the cytoplasm.