Translocation of the dense nucleus along a gravity vector initiates mechanical remodeling of a cell, but the underlying mechanisms of cytoskeletal network and focal adhesion complex (FAC) reorganization in a mammalian cell remain unclear. We quantified the remodeling of an MC3T3-E1 cell placed in upward-, downward-, or edgeon-orientated substrate. Nucleus longitudinal translocation presents a high value in downward orientation at 24 h or in edge-on orientation at 72 h, which is consistent with orientation-dependent distribution of perinuclear actin stress fibers and vimentin cords. Redistribution of total FAC area and fractionized super mature adhesion number coordinates this dependence at short duration. This orientation-dependent remodeling is associated with nucleus flattering and lamin A/C phosphorylation. Actin depolymerization or Rho-associated protein kinase signaling inhibition abolishes the orientation dependence of nucleus translocation, whereas tubulin polymerization inhibition or vimentin disruption reserves the dependence. A biomechanical model is therefore proposed for integrating the mechanosensing of nucleus translocation with cytoskeletal remodeling and FAC reorganization induced by a gravity vector.-Zhang, C., Zhou, L., Zhang, F., Lü, D., Li, N., Zheng, L., Xu, Y., Li, Z., Sun, S., Long, M. Mechanical remodeling of normally-sized mammalian cells under gravity vector. FASEB J. 31, 000-000 (2017). www.fasebj.org KEY WORDS: gravity-directed • mechanosensing • nucleus translocation • cytoskeletal remodeling • FAC reorganization Cells on Earth are subject to a variety of mechanical forces, including gravity. Statolith, or amyloplast, has long been assumed to be a gravity receptor for plant species to sense, transduce, and respond to altered gravity (1); however, little is known about mechanisms that underlie gravisensing and gravitransduction in mammalian cells, even though a body of evidence confirms the effect of gravity on their biological responses (2). Conceptual studies have indicated that the spreading and mitosis of normally sized (;10 1 mm) mammalian cells are sensitive to the change in gravity vector. For example, randomizing the direction of gravity has no effect on the division orientation of Chinese hamster ovary cells that are point-attached in a vertical plane (3). Inversion of culture substrate cannot alter the number and function of attached osteoblasts, even though an immediate response-diminished viable osteoblast number-is observed in sparse, early cultures (4). Cell area, nucleus translocation, cell cycle, and F-actin reorganization vary significantly in a duration-dependent manner when Ros 17/2.8 cells make up a monolayer and are spread on downward-or edge-on-orientated substrate compared with those on an upward orientation (5). More quantitative analysis is observed in large-sized Xenopus oocytes (;1 mm), where the nucleolus sedimentation is dominant compared with the thermal fluctuation at gravity potential . thermal energy scale (6), which implies that the sediment...
