The Ran GTPase Gradient Protects the Nucleolus from Aging-Associated Morphological Changes

In the context of its regulatory function for nucleocytoplasmic transport, the Ran GTPase undergoes cycles of nuclear import, GTP loading, nuclear export, and GTP hydrolysis. These reactions give rise to a nuclear:cytoplasmic (N:C) Ran gradient. In Hutchinson-Gilford Progeria Syndrome, disruption of the Ran gradient suppresses nuclear import of high molecular mass complexes by reducing the nuclear concentration of Ran. Here, we report that cells undergoing senescence, as a consequence of passage number, chemical induction, and altered nuclear lamina structure, all display a Ran gradient disruption quantitatively similar to that observed in Progeria patient cells. We found that the Ran gradient is critical for maintenance of nucleolar structure, as its disruption increases the size and decreases the average number of nucleoli per cell. Nucleolar number and size are biomarkers of longevity in diverse organisms, thus the nuclear level of Ran may be important for the nucleolar morphology in aging. The contribution of the Ran gradient includes regulating import of nucleolin and nucleophosmin, nucleolar proteins that assemble into high molecular mass complexes. The steepness of the Ran gradient is highly dependent on nuclear heterochromatin, which is reduced by passage number and chemical induction of senescence in cultured cells, and is known to decline during normal aging. Our data suggest that the Ran gradient senses nuclear heterochromatin, and through its function as a transport regulator, helps maintain the protein composition and structure of the nucleolus.