Mechanical Behaviors Of Mitotic Spindles In Eukaryocyte

Cell proliferation is an important characteristic of biological phenomena,and is the basis of growth,development,heredity and reproduction.The most general and com-mon type of cell proliferation is the mitosis,and the mitotic spindle is the most important organelle which plays an essential role in the DNA separation and the positioning of the division.Mistakes in the spindle can induce gene errors,failures of cell division,cell cancerization or death,and thus may lead to developmental malformation or tumorige-nesis.A growing body of research suggests that mechanical factors are also crucial for the assembly and function of mitotic spindles.Therefore,to investigate the mechani-cal behaviors and mechanically regulating mechanisms of spindles is significant in the study of the cell division,development,tumorigenesis and cancer therapy.In this thesis,we used computational simulations combined with experiments to investigate the key mechanical behaviors of mitotic spindles.Firstly,we developed a general computational model for mitotic spindles,which completely considered cen-trosomes,chromosomes,microtubules,cortex,and various molecular motors,and their interactions through the forces generated by microtubules and motors.For the first time,we successfully used simulations to reproduce the self-assembly,positioning and ori-entation of spindles,and the aligning and separation of chromosomes.By using the model,we systematically studied the mechanical questions of spindles as following.(1)We investigated the control mechanism of spindle length and why there is an upper limit of spindle length.We found the length of the in vitro spindle without the interaction with the droplet boundary is determined by the number of building blocks,which scales with the cell volume.In contrast,the length of the in vivo spindle is mainly influenced by the cell boundary through cortical microtubules.However,these two mechanisms both break down in large cells due to the almost unlimited cytoplasm and few cortical microtubules,and the geometric asymmetry of the spindle structure naturally induces the upper limit of spindle length.(2)The spindle can be positioned to the cell center and oriented along the long axis of the cell shape.We found the positioning and orientation are mainly by the cortical force in small cells and by the cytoplasmic force in large cells.The characteristic time of positioning and orientation processes increases with the cell size.We also found that more slender cells have a faster orientation process,and the final orientation is not necessarily along the longest axis but is determined by the radial profile and the symmetry of the cell shape.(3)Through loading simulations,we also studied the axial mechanical properties of mitotic spindles,and found they have the viscoelasticity and tension-compression asym-metry.Spindles can transiently soften or harden,and then recover,because the sudden stretching or compressing results in the different speeds of binding and unbinding pro-cesses of microtubules.Based on the viscoelastic responses,we proposed a minimal constitutive model for mitotic spindles.(4)We also investigated the competitively orientation-controlling mechanism be-tween the cell shape and adhesion in epithelial cells.We found when the anisotropy of cell shape is small,the intercellular adhesion dominates the spindle orientation,and the division is planar.In contrast,when the anisotropy of cell shape is large,the cell shape dominates,and the division is orthogonal.A strong adhesion and moderate adhesive size can ensure the planar division of epithelial cells with large apico-basal elongation.We also showed the competitive mechanism between the unilateral adhesion and cell compression,or between the tricellular junctions and cell elongation,and generalized the competitive mechanism to various intercellular microenvironments.(5)We combined simulations and experiments to study the influences of cell geom-etry on the multipolar spindle.Multipolar spindles have the structure of regular polygon in round cells,but we found that the multipolar structure can be compressed with the change of cell shape,and finally to a straight line shape when the the aspect ratio of the cell shape is very large.We verified this phenomenon in experiments and explained it by the mechanism of cell shape-regulated force balance.Taken together,we not only provided a new and general tool for the mitotic spin-dle,but also used it to study many key questions,explained some past experiences,and provided many predictions and experimental verifications.Our research may contribute to the further study of the mechanical behaviors and control mechanism of mitotic spin-dles,and the further understanding of cell division,and provide potential values for related application in mechanobiology.