Modeling of the Nuclear Envelope and Mitotic Spindle in Closed Mitosis

The remodeling of the nuclear envelope during closed mitosis in the fission yeast Schizosaccharomyces pombe requires precise coordination of the mechanical forces generated within the mitotic spindle. We introduce a computational model that captures the dynamic coupling among spindle-generated forces, chromatin distribution, and the elastic response of the nuclear envelope. The envelope is modeled as a dynamically triangulated Helfrich membrane, incorporating bending elasticity, membrane tension, viscous hydrodynamic interactions, and thermal fluctuations. The mitotic spindle is represented by a quasi-rigid bundle of polymerizing microtubules subject to internal motor-driven forces that push apart opposite ends of the spindle. Extensile forces are transmitted to the nuclear envelope through rigid spindle pole bodies and associated chromatin clusters that maintain contact with the inner nuclear membrane near the spindle poles. Mechanical interactions between the envelope, spindle, and chromosomes regulate spindle growth and positioning, while simultaneously driving large-scale deformations of the envelope. Simulations reproduce hallmark features of S. Pombe closed mitosis, including the characteristic spherical-to-dumbbell nuclear shape transition.