RNA and protein synthesis in the primate lens

Mapping ribosomal RNA transcription activity in the mouse eye

All cells must make ribosomes, in which rRNA transcription is the rate-limiting step; however, some cells may require more ribosomes than others. Cell-type specific regulation of rRNA synthesis has been largely ignored in the past, because of the inability to measure rRNA transcription rate in situ. Here we map rRNA transcription activity in individual cells in mouse ocular tissues detected by a novel in situ hybridization technique, which detects the full-length transcripts (47S pre-rRNA) as well as various rRNA processing intermediates. In the adult mouse eye, the corneal and lens epithelia and some retinal neurons contain a higher level of 47S pre-rRNA and rRNA processing intermediates, which are regulated developmentally in neonates prior to eye opening. In the cornea and lens epithelia, the higher rRNA level of 47S rRNA correlates with cell proliferation, which is consistent with the notion that dividing cells require more protein synthesis. Interestingly, in some retinal neurons, the high level of 47S pre-rRNA does not correlate with mature rRNA accumulation or protein synthesis, suggesting the existence of unappreciated biochemical needs of these cells.

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.

Changes in three types of ubiquitin mRNA and ubiquitin-protein conjugate levels during lens development

Ubiquitin is a small, highly conserved protein that covalently attaches to other proteins to form a unique branched protein structure. The best characterized function of this post-translational modification is to mark the modified protein for degradation by the proteasome. To investigate whether ubiquitin genes are regulated in lens development, the authors analyzed the levels of three ubiquitin mRNAs (UbA(52), UbB and UbC) in freshly dissected fiber and epithelial cells, and in epithelial explants induced to differentiate ex vivo. Explants, comprising the capsule and adherent epithelial cells, were dissected from lenses of 3 day old Sprague Dawley rats and cultured +/-bFGF to induce differentiation. Quantitative competitive RT-PCR was used to determine the mRNA levels in fresh and cultured cells. UbA(52), UbB and UbC mRNAs were 3.2 (P < 0.0001), 5.0 (P < 0.0001) and 6.8 (P < 0.0001) fold higher, respectively, in freshly dissected epithelial cells than in differentiated fiber cells. Immunological spot assays showed that ubiquitin protein is over two fold as high in rat pup lens epithelial cells as in fiber cells. The ubiquitin protein in fiber cells of adult rat is lower than that in adult epithelium and in pup fiber cells, indicating that ubiquitin content further decreased during lens fiber maturation. Western blots showed a greater amount of protein-conjugated ubiquitin (MW > 81 kD) in epithelial cells than in fiber cells, demonstrating a parallel pattern between the expression of ubiquitin mRNA, the level of ubiquitin protein and the level of conjugates in the cells. Epithelial cell explant cultures permit study of cells initiating differentiation. In contrast to fully differentiated fiber cells, explant cultures induced to initiate differentiation underwent differential up-regulation of ubiquitin gene expression. UbA(52) and UbB mRNA levels in +bFGF (differentiating) explant cultures were 2.6 (P < 0.001) and 1.4 (P < 0.001) fold higher, respectively, than those of -bFGF cultures. UbC mRNA content was similar in explants cultured with or without bFGF. Dissection of the isolated epithelial cells into regions representing distinct populations gave results consistent with this observation of the explant results. UbA(52), UbB and UbC mRNAs are 2.0, 2.2 and 1.76 fold higher, respectively, in the peripheral (initiating differentiation) than in the central (undifferentiated) region of epithelial cells. These results together indicate that UbA(52) and UbB mRNAs are transiently increased during the initiation and early stages of differentiation. However, UbC mRNA appears to be relatively unaffected at the earliest stage in this differentiation model and may have a different distribution than UbA(52) and UbB in the anterior lens cells. These data are consistent with an important role for ubiquitin during the early stages of lens differentiation. The selective expression indicates that the three genes have specific differentiation related functions.

Quantification and regulation of mRNAs encoding beaded filament proteins in the chick lens

PURPOSE

To quantify the expression of beaded filament protein mRNA levels in regions of the chick lens and to examine the in vitro regulation of message and protein levels using cell culture techniques.

METHODS

RNase protection assays and Northern blotting were used to quantify beaded filament protein mRNA levels in dissected lenses. Cultured cells were assayed for mRNA with RNase protection and for protein with Western blotting and ELISA techniques after treatment with cAMP analogs.

RESULTS

Beaded filament protein message levels were greatly up-regulated in cortical fiber cells compared to annular pad cells. Full length messages were also detected in nuclear fiber cells. The presence of an unusual form of the CP49 message with a lamin-like insert, CP49INS, was also established. Both message and protein levels were subject to regulation in response to elevated intracellular cAMP levels.

CONCLUSIONS

The accumulation of beaded filament protein levels during fiber cell development may be due to the increased cAMP-mediated transcription of message. The presence of CP49INS may lend new insight into mechanisms of intermediate filament assembly.