The Mechanics of Nuclear Shaping

Yuan Li ¹, David Lovett¹, Qiao Zhang¹, Ram Kuchibhotla¹, Srujana Neelam², Ruijun Zhu³,

Gregg G. Gundersen³, Richard B. Dickinson¹, Tanmay P. Lele^1*

¹ Department of Chemical Engineering, College of Engineering, University of Florida, 1006 Center Drive, Gainesville, FL 32611, USA

² Department of Biomedical Engineering, College of Engineering, University of Florida, 1275 Center Drive, Gainesville, FL 32611, USA

³ Department of Pathology and Cell Biology, College of Physicians and Surgeons, Columbia University,  630 West 168th Street, New York, NY 10032, USA

The nuclei in cultured cells are extremely flat (~ 3 microns in height, ~15 microns in width). How the nucleus is flattened in cells is not understood. We performed x-z laser scanning confocal fluorescence microscopy of NIH 3T3 fibroblasts expressing GFP-histone as they settled from suspension onto a fibronectin-coated glass dish. Three distinct nuclear behaviors could in general be discerned during the spreading. First, the nucleus settled toward the base of the cell and the lower surface of the nucleus began to spread in the first few minutes of attachment. Next, the top surface of the nucleus collapsed dramatically while the length of the flattened bottom surface stayed roughly constant. This large change in the nuclear shape happened in 5-6 minutes. In the third phase, the collapsed nucleus increased in width at near constant height. Consistent with these findings, the volume of the nucleus decreased during initial cell spreading correlating with the collapse of the top surface, followed by an increase as the nucleus and the cell spread. Surprisingly, nuclear flattening does not require myosin activity; rather the degree of cell spreading is a key predictor of nuclear flattening. We present simulations that explain how the nucleus may be flattened in spreading cells by the intracellular flow of F-actin without requiring myosin activity.