Ran GTPase is a key regulator of macromolecular transport between nucleus and cytoplasm and has important role in several steps of cell division, including mitotic spindle assembly and nuclear envelope reformation at the exit from mitosis. Because RCC1, the guanine nucleotide exchange factor for Ran, binds to chromatin while RanGAP is cytoplasmic, the position of chromosomes is marked by the highest cellular concentration of RanGTP, the RanGTP gradient. The cellular functions of Ran are mediated by RanGTP interactions with importin beta-related nuclear transport receptors (NTRs). Ran and NTRs functionally interact with nucleoporins (Nups) the components of NPCs. In interphase, step-wise RanGTP gradient across nuclear envelope provides directionality to the Ran-regulated transport of cargos carried by NTRs through nuclear pore complexes. During mitosis, chromosomes are surrounded by a steep diffusional gradient of RanGTP which continues to regulate the loading and unloading of cargos on NTRs in the mitotic cytoplasm. Because some mitotic regulators are at the same time NTR cargos, their activities or binding to mitotic structures are controlled by RanGTP, contributing to mitotic spindle assembly and chromosome segregation to daughter cells. Previously, we focused on the role of Ran in mitotic spindle assembly and our goal was to elucidate differences in the contribution of Ran to mitosis in cancer cells vs. normal cells. To that end, we developed methods for quantitative measurements of Ran function in mitotic cells using fluorescence lifetime imaging microscopy (FLIM) of FRET biosensors expressed in live cells. Using this approach, we found that the mitotic RanGTP gradients were more robust in rapidly proliferating normal and cancer cells. Furthermore, we showed that increased RCC1 expression and increased number of mitotic chromosomes in aneuploid cells were the two most important factors determining the steepness of mitotic RanGTP gradients. As a follow up to this study, which was published in 2013, we pursued two approaches. First, we set out to determine the mechanisms of chromosome gain-driven activation of mitotic RanGTP gradients. We hypothesized that the underlining mechanism depends on the biophysical properties of the chromosome-cytoplasm interface in mitotic cells and involves the increased number of chromosome binding sites for RCC1 and reduced diffusion in the center of mitotic cells with increased chromosome number. To test this hypothesis, we planned combining live cell measurements with computational modeling of the mitotic RanGTP gradient, in collaboration with the laboratory of Dr. David Odde at the University of Minnesota. Although we soon obtained the initial set of measurements and developed the mitotic models, the progress was interrupted by the departure of Dr. Hasegawa from NIH at the end of 2013. In the second approach, we studied the role of Ran in the regulation of cell cycle. In the first phase of this project we analyzed role of Ran in the exit from cell cycle in senescent cells. We found that, as a result of irreversible cytoplasmic processing of chromatin, the permanent G1/S arrest in senescent cells was associated with a strong decline of RCC1 protein levels. That result was consistent with our initial hypothesis that the depletion of RanGTP levels enforces the stable exit from cell cycle via reducing the Ran-regulated nuclear-cytoplasmic transport of cell cycle factors. Unfortunately, the progress of this project, which was carried by Dr. Ryu in the lab, was disrupted by serious technical setback that prevented him to obtain definitive results before leaving the lab for a new job at the end of September 2013. As a result, both main projects of the lab were severely disrupted and unpublished at the end of 2013, about half a year before the Site Visit of the lab in June 2014. Dr. Cekan, the remaining recently hired postdoctoral researcher, and I have focused on the cell cycle project. We obtained evidence that RCC1-driven activation of Ran accelerates the reentry to cell cycle following DNA damage, leading to the evasion of cell senescence in normal and cancer cells. The underlining mechanism involved the activation of DNA damage repair, required exportin 1 function and involved the RCC1 expression-induced activation of prominent oncogenic transcriptional regulators. A manuscript describing these results will be submitted for publication in a few days. However, as a result of the unfavorable outcome of the Site Visit 2014 review, the lab will be closing in about a year from now.
