Increased numbers of nucleoli in a genome-wide RNAi screen reveal proteins that link the cell cycle to RNA polymerase I transcription

The nucleolus is a dynamic nuclear condensate and site of ribosome biogenesis. Using wide-field fluorescence microscopy, we screened for proteins that when depleted cause an increase in nucleolar number. Our results uncovered an unexpected subset of proteins that link the nucleolus, cell cycle regulation, and RNA polymerase I transcription.

NOL12 Repression Induces Nucleolar Stress-Driven Cellular Senescence and Is Associated with Normative Aging

ABSTRACT The nucleolus is a subnuclear compartment with key roles in rRNA synthesis and ribosome biogenesis, complex processes that require hundreds of proteins and factors. Alterations in nucleolar morphology and protein content have been linked to the control of cell proliferation and stress responses and, recently, further implicated in cell senescence and ageing. In this study, we report the functional role of NOL12 in the nucleolar homeostasis of human primary fibroblasts. NOL12 repression induces specific changes in nucleolar morphology, with increased nucleolar area but reduced nucleolar number, along with nucleolar accumulation and increased levels of fibrillarin and nucleolin. Moreover, NOL12 repression leads to stabilization and activation of p53 in an RPL11-dependent manner, which arrests cells at G2 phase and ultimately leads to senescence. Importantly, we found NOL12 repression in association with nucleolar stress-like responses in human fibroblasts from elderly donors, disclosing it as a biomarker in human chronological aging.

Role(s) of nucleoli and phosphorylation of ribosomal protein S6 and/or HSP27 in the regulation of muscle mass

Effects of 14 days of hindlimb unloading or synergist ablation-related overloading with or without deafferentation on the fiber cross-sectional area, myonuclear number, size, and domain, the number of nucleoli in a single myonucleus, and the levels in the phosphorylation of the ribosomal protein S6 (S6) and 27-kDa heat shock protein (HSP27) were studied in rat soleus. Hypertrophy of fibers (+24%), associated with increased nucleolar number (from 1–2 to 3–5) within a myonucleus and myonuclear domain (+27%) compared with the preexperimental level, was induced by synergist ablation. Such phenomena were associated with increased levels of phosphorylated S6 (+84%) and HSP27 (+28%). Fiber atrophy (−52%), associated with decreased number (−31%) and domain size (−28%) of myonuclei and phosphorylation of S6 (−98%) and HSP27 (−63%), and with increased myonuclear size (+19%) and ubiquitination of myosin heavy chain (+33%, P > 0.05), was observed after unloading, which inhibited the mechanical load. Responses to deafferentation, which inhibited electromyogram level (−47%), were basically similar to those caused by hindlimb unloading, although the magnitudes were minor. The deafferentation-related responses were prevented and nucleolar number was even increased (+18%) by addition of synergist ablation, even though the integrated electromyogram level was still 30% less than controls. It is suggested that the load-dependent maintenance or upregulation of the nucleolar number and/or phosphorylation of S6 and HSP27 plays the important role(s) in the regulation of muscle mass. It was also indicated that such regulation was not necessarily associated with the neural activity.

Nucleolar variation in a pigeon pea intergeneric hybrid: evidence for allosyndetic recombination

Cytological study of a hybrid between Cajanus cajan and Atylosia albicans revealed regular bivalent formation and disjunction. Nevertheless, high pollen sterility and low seed set were evident. An examination of pollen mother cells revealed variation in nucleolar number at telophase-I (four to eight) and at telophase-II (zero to four in daughter nuclei), although each genome contained two nucleolar organizers. Variation was also recorded for nucleolar size and distribution at telophase-II. Variation in nucleolar number and distribution are interpreted to have originated from pairing and recombination between nucleolar organizer chromosome(s) of one parental species with the nonnucleolar organizer chromosome(s) of the other. Size variation is attributed to nucleolar dominance. These results explain the high degree of pollen sterility in the hybrid in spite of normal meiosis, and also suggest that the karyotypes of C. cajan and A. albicans have differentiated through structural heterozygosity.Key words: Cajanus cajan, Atylosia albicans, nucleolar variation, structural heterozygosity, allosyndetic recombination.