The nucleolus under stress

Abstract

Cells typically respond quickly to stress, altering their metabolism to compensate. In mammalian cells, stress signaling usually leads to either cell-cycle arrest or apoptosis, depending on the severity of the insult and the ability of the cell to recover. Stress also often leads to reorganization of nuclear architecture, reflecting the simultaneous inhibition of major nuclear pathways (e.g., replication and transcription) and activation of specific stress responses (e.g., DNA repair). In this review, we focus on how two nuclear organelles, the nucleolus and the Cajal body, respond to stress. The nucleolus senses stress and is a central hub for coordinating the stress response. We review nucleolar function in the stress-induced regulation of p53 and the specific changes in nucleolar morphology and composition that occur upon stress. Crosstalk between nucleoli and CBs is also discussed in the context of stress responses.

Figure 1

Overview of Nucleolar Organization under Physiological Conditions in the Mammalian Cell Nucleus, and Visualization by Immunofluorescence of Stress-Induced Changes to Nucleolar and Cajal Body Organization (A) Differential interference contrast (DIC) image of live HeLa cells: nucleoli are readily observed as phase-dense structures. Scale bar, 15 μm (left panel). Schematic representation of nucleolar tripartite internal organization, formed by the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC) (right panel). (B) Fluorescence microscopy images showing the three subnucleolar compartments in human U2OS cells. FC is visualized using antibodies against UBF, DFC using antibodies against Nop58, and GC using antibodies against B23/NPM. Scale bar, 10 μm. (C) Examples of stress-induced changes in nucleolar and CB organization in U2OS cells. (Left panel) Untreated cells. (Middle panel) UV-C-treated cells (6 hr postirradiation, 30 J/m²). (Right panel) DRB-treated cells (3 hr, 25 μg/mL). All images show UBF (nucleolar fibrillar center) in red and coilin (CB) in green. UV-C treatment induces nucleolar segregation and relocalization of coilin to nucleoplasmic microfoci. In contrast, DRB treatment induces nucleolar fragmentation and unravelling of the FC, as well as CB disruption and association of coilin with the nucleolus in cap-like structures. Scale bar, 5 μm.

Figure 2

“Spatial Proteomics” Approach to Study Nucleolar Proteome Dynamics in Response to Stress (A) Typical “spatial proteomics” protocol. Cells are labeled with heavy-isotope containing amino acids (SILAC-based labeling) prior to fractionation into cytoplasm, nucleoplasm, and nucleolus. A whole-cell extract is created by recombining differentially labeled subcellular fractions and analyzed by LC-MS/MS. This can be used to measure the relative distribution of cellular proteins between the three compartments (medium/light [M/L] ratio, nucleoplasm/cytoplasm; heavy/medium [H/L] ratio, nucleolus/cytoplasm ratio) and analyze the stress-induced changes in protein localization, as visualized in (B).

Figure 3

Overview of the Nucleolar-Related, Stress-Induced Mechanisms that Result in Increased p53 Activity The different mechanisms have been separated into three broad categories, namely those that primarily involve either (A) alterations of protein-protein interactions, (B) changes to the translational profile, or (C) prevention of coribosomal export of p53 and Hdmd2. The size of each quadrant represents their relative importance according to current literature. Grayscale text and objects indicate processes that occur during normal physiological conditions, whereas orange text and objects show the changes that occur following stress that result in increased p53 levels and activity. The outcomes of increases p53 are summarized in (D), namely the upregulation of p53 target genes which ultimately leads to either cell-cycle arrest or apoptosis, and the inhibition of ribosome biogenesis via the inhibition of rRNA transcription. Refer to text for detailed explanations of these pathways and for references.