Linear and nonlinear mechanics of mammalian cytoplasm

Abstract

The mechanical behavior of mammalian cytoplasm decides the ability of a cell to resist deformation, to transport intracellular cargo and to change shape during movement. Although the mechanical properties of mammalian cells have been measured by various methods such as atomic force microscope (AFM) indentation and micropipette aspiration, the mechanics of cytoplasm remains elusive, because the methods used before can only measure the coupling mechanical response of cell membrane, actomyosin cortex and cytoplasm. In this thesis, I identify the linear and nonlinear mechanics of mammalian cytoplasm by using optical tweezers to perform direct micromechanical measurements in living cytoplasm. Although the driving force generated by motor proteins to deliver intracellular cargos is widely studied, the mechanical nature of cytoplasm, which is also important for intracellular processes by providing mechanical resistance, remains unclear.

We use optical tweezers to directly characterize the resistance to transport in living mammalian cytoplasm. Using scaling analysis, we successfully distinguish between the underlying mechanisms governing the resistance to mechanical deformation, i.e., among viscosity, viscoelasticity, poroelasticity, or pure elasticity, depending on the speed and size of the probe. Moreover, a cytoplasmic state diagram is obtained to illustrate different mechanical behaviors as a function of two dimensionless parameters; with this, the underlying mechanics of various cellular processes over a broad range of speed and size scales is revealed. In many developmental and pathological processes, including cellular migration during normal development and invasion in cancer metastasis, cells are required to withstand severe deformations.

The structural integrity of eukaryotic cells under small deformations has been known to depend on the cytoskeleton, which is a main component of the cytoplasm, including actin filaments (F-actin), microtubules (MT) and intermediate filaments (IFs). However, it remains unclear how cells resist severe deformations since both F-actin and microtubules fluidize or disassemble under moderate strains. In the second project, using vimentin containing IFs (VIFs) as a model for studying the large family of IF proteins, we demonstrate that they dominate cytoplasmic mechanics and maintain cell viability at large deformations. Our results show that cytoskeletal VIFs form a stretchable, hyperelastic network in living cells. This network works synergistically with other cytoplasmic components, substantially enhancing the strength, stretchability, resilience and toughness of cells.

Moreover, we find the hyperelastic VIF network, together with other quickly recoverable cytoskeletal components, form a mechanically robust structure with a self-healing nature.