Continuous nucleolar ribosomal RNA synthesis in differentiating lens fiber cells until abrupt nuclear degradation required for ocular lens transparency

Abstract

Cellular differentiation requires highly coordinate action of all three transcriptional systems to produce rRNAs, mRNAs, and various "short" and "long" non-coding RNAs by RNA Polymerase I, II, and III systems, respectively. The RNA Polymerase I catalyzes transcription of about 400 copies of rDNA genes generating 18S, 5.8S, and 28S rRNA molecules from the individual primary transcript. Lens fiber cell differentiation is a unique process to study transcriptional mechanisms of individual crystallin genes as their very high transcriptional outputs are directly comparable only to globin genes in erythrocytes. Importantly, both terminally differentiated lens fiber cells and mammalian erythrocytes degrade their nuclei though by different mechanisms. In lens, generation of organelle-free zone (OFZ) includes degradation of mitochondria, endoplasmic reticulum, Golgi apparatus, and nuclei; nevertheless, very little is known about their nucleoli and rRNA transcription. Here, using RNA fluorescence in situ hybridization (FISH) we evaluated nascent rRNA transcription during the entire process of lens fiber cell differentiation. The lens fiber cell nuclei undergo morphological changes prior their denucleation, including chromatin condensation; remarkably, the nascent rRNA transcription persists in all nuclei next to the OFZ. The changes in both nuclei and nucleoli shape and microarchitecture were evaluated by immunofluorescence to detect fibrillarin, nucleolin, UBF, and other nuclear proteins. These studies demonstrate for the first time that highly condensed lens fiber cell nuclei have the capacity to support rRNA transcription. Thus, "late" production of rRNA molecules and consequently the ribosomes contribute to the terminal translational mechanisms to produce maximal quantities of the crystallin proteins.