Determinants of mammalian nucleolar architecture

Abstract

The nucleolus is responsible for the production of ribosomes, essential machines which synthesize all proteins needed by the cell. The structure of human nucleoli is highly dynamic and is directly related to its functions in ribosome biogenesis. Despite the importance of this organelle, the intricate relationship between nucleolar structure and function remains largely unexplored. How do cells control nucleolar formation and function? What are the minimal requirements for making a functional nucleolus? Here we review what is currently known regarding mammalian nucleolar formation at nucleolar organizer regions (NORs), which can be studied by observing the dissolution and reformation of the nucleolus during each cell division. Additionally, the nucleolus can be examined by analyzing how alterations in nucleolar function manifest in differences in nucleolar architecture. Furthermore, changes in nucleolar structure and function are correlated with cancer, highlighting the importance of studying the determinants of nucleolar formation.

Fig. 1

An electron micrograph of a HeLa cell demonstrates that the nucleolus is comprised of three subcompartments: the fibrillar center (FC), dense fibrillar component (DFC) and granular component (GC). Bar, 0.5 µm. Reprinted with permission from (Sirri et al. 2002)

Fig. 2

The nucleolus forms around nucleolar organizer regions (NORs), located on the short arms of the acrocentric chromosomes. NORs are made of rDNA repeats. One rDNA repeat contains the transcript for the pre-rRNA. In human cells, the 47S pre-rRNA consists of the 18S, 5.8S, and 28S rRNAs flanked by external transcribed spacers (5’ETS and 3’ETS) and internal transcribed spacers (ITS1 and ITS2). The 18S, 5.8S, and 28S rRNAs are included into the small and large subunits of the forming ribosome

Fig. 3

Nucleolar number varies greatly among tissue culture cells. The indicated cell lines were fixed with paraformaldehyde and stained with an antibody to the nucleolar protein fibrillarin (72B9; Reimer et al. 1987). Cells were also stained with HOECHST for visualization of the nucleus. Images were analyzed using a CellProfiler pipeline which counts the number of nucleoli per cell (Carpenter et al. 2006). Representative images for each cell line are depicted with a frequency distribution of the number of nucleoli per nucleolus shown to the right