Tension Causes Unfolding of Intracellular Vimentin Intermediate Filaments

Abstract

and intermediate filaments (IFs). While the roles of actin-based networks and MT in cell mechanics and cell function have been extensively studied (reviewed, e.g., in refs. [2, 3]), the contribution of IFs to cell mechanics is still relatively opaque, beyond their function in bearing large tensile forces. Of the cytoskeletal proteins, IF constituent proteins have the unique feature of being able to undergo molecular structural changes in response to external loads, at least in vitro, as shown by single-molecule experiments and molecular dynamics simulations. [4-7] This structural polymorphism observed outside of a cellular context has led to the hypothesis that IFs could play a role in intracellular mechanotransduction-relaying information about the cell's mechanical state into biochemical changes that influence cell shape [8-10] and phenotype. [11] IF proteins consist dimeric building blocks ≈45 nm in length that assemble into ≈60 nm antiparallel tetrameters (due to dimer overlap), which laterally associate into unit length filaments that assemble longitudinally to make apolar filaments. These filaments can further bundle into fibers and form networks. [7,12,13] Crystal structures show that IF protein are at least 67% α-helical. [14,15] Individual IFs self-assemble into quite diverse networks with location-specific architectures and composition in cells. [8] IFs proteins can form both ionic and hydrophobic molecular contacts resulting in multiple stabilizing interactions. In addition, proteins such as plectin help bundling and cross-bridging between IFs and to stress fibers and MT. [8,16] Vimentin is one IF protein that forms cytoplasmic IF networks and is particularly interesting because of its important role in cell adhesion, mechanical stability, [17-19] and cell migration, as well as intracellular signaling. [8,20] Specific demonstrations of laser ablation of super-stretched cells showed that the IF network is load bearing; however, this effect was not observed on relaxed cells grown on very soft substrates. [21] Moreover, vimentin has been established as a marker of the epithelial-to-mesenchymal transitions (EMT) in embryogenesis and in tumor metastasis, as the cytoplasmic IF network in epithelial cells mostly consists of keratin that is converted to a vimentin-rich IF network during EMT. [10,22-24] A complex multi-regime response of IF protein structure to deformation has been revealed via computational [7,25,26] and in vitro experimental [4,5] studies of IF protein mechanics.

Vimentin Intermediate Filaments Can Enhance or Abate Active Cellular Forces in a Microenvironmental Stiffness-Dependent Manner

“…In addition to non-physical interactions through biochemical signaling, intermediate filaments also interact with actin filaments through physical contact mediated by cross-linkers, direct binding, and steric effects ( 24 ) as we have previously shown in our in vitro studies ( 44 ). A large portion of VIFs that interact with contractile actin filaments are expected to experience tensile stresses (Figure 1C) ( 49 ). We thus add an element of intermediate filaments in parallel with the actin element, thereby experiencing tensile stresses (Figure 1 A).…”

Intermediate Filaments from Tissue Integrity to Single Molecule Mechanics

“…In relaxed cells, vimentin is in α-helical conformation. In contrast, when cells are under tension, β-sheet conformations are mostly observed [93]. This implies that single filaments display similar mechanical behavior in cells as in vitro.…”

“…Recently, non-vibrational spectroscopy experiments have shown the α-to-β sheet conformational change in living cells under different cellular tension [93]. In relaxed cells, vimentin is in α-helical conformation.…”

“…Different sets of experiments point towards a decrease in the lateral coupling of monomers due to phosphorylation-induced changes in charges and electrostatic interactions, leading to the softening of the filaments [100]. Interestingly, plating cells on soft substrates induces vimentin phosphorylation at pSer55 [93]. How phosphorylation changes the mechanical response of the IF network to stresses is currently unknown.…”

Intermediate Filaments in Cellular Mechanoresponsiveness: Mediating Cytoskeletal Crosstalk From Membrane to Nucleus and Back

“…These physical effects identified in simplified reconstituted systems likely contribute to mechanical co-dependencies observed in cells, such as toughening by stress dissipation in the vimentin network ( Hu et al, 2019 ), protection against compressive forces by the vimentin network ( Mendez et al, 2014 ), and vimentin-dependent modulation of actin-myosin contractility ( De Pascalis et al, 2018 ). Raman imaging recently showed that actomyosin forces are transmitted to the intermediate filament cytoskeleton: cells on rigid substrates, where myosin contractility is high ( Gupta et al, 2019 ), contained more unfolded vimentin than on soft substrates, where tension is low ( Fleissner et al, 2020 ). In epithelial monolayers a similar mechanical interplay between the actin and intermediate filament networks was found ( Latorre et al, 2018 ), where cell stretching dilutes the actin cortex and hence decreases tension, while keratin filaments that bear tension re-stiffen the cells.…”