Protein synthesis in bovine and human nuclear fiber cells

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.

Development of a macromolecular diffusion pathway in the lens

The mammalian lens consists of an aged core of quiescent cells enveloped by a layer of synthetically active cells. Abundant gap junctions within and between these cell populations ensure that the lens functions as an electrical syncytium and facilitates the exchange of small molecules between surface and core cells. In the present study, we utilized an in vivo mouse model to characterize the properties of an additional pathway, permeable to macromolecules, which co-exists with gap-junction-mediated communication in the lens core. The TgN(GFPU)5Nagy strain of mice carries a green fluorescent protein (GFP) transgene. In the lenses of hemizyous animals, GFP was expressed in a variegated fashion, allowing diffusion of GFP to be visualized directly. Early in development, GFP expression in scattered fiber cells resulted in a checkerboard fluorescence pattern in the lens. However, at E15 and later, the centrally located fiber cells became uniformly fluorescent. In the adult lens, a superficial layer of cells, approximately 100 microm thick, retained the original mosaic fluorescence pattern, but the remainder, and majority, of the tissue was uniformly fluorescent. We reasoned that at the border between the two distinct labeling patterns, a macromolecule-permeable intercellular pathway was established. To test this hypothesis, we microinjected 10 kDa fluorescent dextran into individual fiber cells and followed its diffusion by time-lapse microscopy. Injections at depths of >100 microm resulted in intercellular diffusion of dextran from injected cells. By contrast, when injections were made into superficial fiber cells, the injected cell invariably retained the dextran. Together, these data suggest that, in addition to being coupled by gap junctions, cells in the lens core are interconnected by a macromolecule-permeable pathway. At all ages examined, a significant proportion of the nucleated fiber cell population of the lens was located within this region of the lens.

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.

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.