Biophysical journal最新文献

In homoeostasis, the shape and sessility of untransformed epithelial cells are intricately linked together. Variations of this relationship in migrating cancer cells as they encounter different microenvironments are as yet ill understood. Here, we explore the interdependency of such traits in two morphologically distinct invasive ovarian cancer cell lines (OVCAR-3 and SK-OV-3) under mechanically variant contexts. We first established a metric toolkit that assessed traits associated with cell motion and shape, and rigorously measured their dynamical variation across trajectories of migration using a Shannon entropic distribution. Two stiffness conditions on polymerized collagen I with Young's moduli of 0.5 kPa (soft) and 20 kPa (stiff) were chosen. Both the epithelioid OVCAR-3 and mesenchymal SK-OV-3 cells on soft substrata exhibited slow and undirected migration. On stiff substrata, SK-OV-3 showed faster persistent directed motion. Surprisingly, OVCAR-3 cells on stiffer substrata moved even faster than SK-OV-3 cells but showed a distinct angular motion. The polarity of SK-OV-3 cells on stiff substrata was well correlated with their movement, whereas, for OVCAR-3, we observed an unusual "slip" behavior, wherein the axes of cell shape and movement were poorly correlated. Whereas SK-OV-3 and OVCAR-3 showed greater mean deformation on stiffer substrata, the latter was anticorrelated with variation in angular motion or the mean deviation between shape and motility axis for SK-OV-3 but poorly correlated for OVCAR-3. Moreover, on softer substrata OVCAR-3 and SK-OV-3 were relatively rigid but showed greater shape variation (with OVCAR-3 showing a higher fold change) on stiffer substrata. Our findings suggest that greater deformability on stiffer milieu allow epithelioid cells to overcome constraints on the congruence in axis of shape and motion seen for mesenchymal cells and display distinct motile behaviors across this phenotypic spectrum.

Living cells are known to exhibit power-law viscoelastic responses and localized stress relaxation behaviors in frequency spectrum. However, the precise interplay between molecular scale cytoskeletal dynamics and macroscale dynamical rheological responses remains elusive. Here, we propose a mechanism-based general theoretical model showing that cytoskeleton dissociation generates a peak in the loss modulus as a function of frequency, while the cytoplasmic viscosity promotes its recovery, producing a subsequent trough. We define two characteristic frequencies ( ω_c1 and ω_c2 ) related to the dissociation rate of crosslinkers and the viscosity of the cytoplasm, where the loss modulus (1) exhibits peak and trough values for ω_c1>ω_c2 , and (2) monotonically increases with frequency for ω_c1>ω_c2. Furthermore, the characteristic frequency ω_c1 exhibits a biphasic stress-dependent behavior, with a local minimum at sufficiently high stress due to the stress-dependent dissociation rate. Moreover, the characteristic frequency ω_c2 evolves with age, following a power-law relationship. The predictions of the DMM model align well with experimental observations. Our model provides a comprehensive description of the dynamical viscoelastic behaviors of cells and cell-like materials.

Structural biology relies on several powerful techniques, but these tend to be limited in their ability to characterize protein fluctuations and mobility. Over-reliance on structural approaches can lead to omission of critical information regarding biological function. Currently there is a need for complementary biophysical methods to visualize these mobile aspects of protein function. Here we review hydrostatic and osmotic pressure-based techniques to address this shortcoming for the paradigm of rhodopsin. Hydrostatic and osmotic pressure data contribute important examples which are interpreted in terms of an energy landscape for hydration-mediated protein dynamics. We find that perturbations of rhodopsin conformational equilibria by force-based methods are not unrelated phenomena; rather they probe various hydration states involving functional proton reactions. Hydrostatic pressure acts on small numbers of strongly interacting structural or solvent-shell water molecules with relatively high energies, while osmotic pressure acts on large numbers of weakly interacting bulk-like water molecules with low energies. Local solvent fluctuations due to the hydration shell and collective water interactions affect hydrogen-bonded networks and domain motions that are explained by a hierarchical energy landscape model for protein dynamics.

The ability of biological systems to withstand and recover from various disruptions, such as spontaneous genetic mutations and environmental damage, largely relies on intricate feedback mechanisms. We theoretically study the mechanical response of an epithelial tissue facing damage in the form of a circular wound. Our model describes a feedback loop between the generation of active forces in the actomyosin and tissue mechanics, described by the vertex model. While the exact dynamics of wound closure may be influenced by several biophysical mechanisms that interplay in a nontrivial way, our findings suggest that the closure may initiate as an active instability, triggered by a reduced myosin turnover rate at the wound's perimeter. We explore the interplay between myosin dynamics and the elastic properties of the tissue, elucidating their collective role in determining a wound's loss of stability, leading to the initiation of the closure process.

Under physiological conditions, mammalian PIEZO channels (PIEZO1 and PIEZO2) elicit transient currents mostly carried by monovalent and divalent cations. PIEZO1 is also known to permeate chloride ions, with a Cl^-/Na⁺ permeability ratio of about 0.2. Yet, little is known about how anions permeate PIEZO channels. Here, by separately measuring sodium and chloride currents using nonpermanent counterions, we show that both PIEZO1 and PIEZO2 rectify chloride currents outwardly, favoring entry of chloride ions at voltages above their reversal potential, whereas little to no rectification was observed for sodium currents. Interestingly, chloride currents elicited by 9K, an anion-selective PIEZO1 mutant harboring multiple positive residues along intracellular pore fenestrations, also rectify but in the inward direction. Molecular dynamics simulations reveal that the inward rectification of chloride currents in 9K correlates with the presence of a large positive electrostatic potential at intracellular pore fenestrations, suggesting that rectification can be tuned by the electrostatic polarity of the pore. These results demonstrate that the pore of mammalian PIEZO channels inherently rectifies chloride currents.

Cell-cell communication through direct contact, or juxtacrine signaling, is important in development, disease, and many areas of physiology. Synthetic forms of juxtacrine signaling can be precisely controlled and operate orthogonally to native processes, making them a powerful reductionist tool with which to address fundamental questions in cell-cell communication in vivo. Here we investigate how cell-cell contact length and tissue growth dynamics affect juxtacrine signal responses through implementing a custom synthetic gene circuit in Drosophila wing imaginal discs alongside mathematical modeling to determine synthetic Notch (synNotch) activation patterns. We find that the area of contact between cells largely determines the extent of synNotch activation, leading to the prediction that the shape of the interface between signal-sending and signal-receiving cells will impact the magnitude of the synNotch response. Notably, synNotch outputs form a graded spatial profile that extends several cell diameters from the signal source, providing evidence that the response to juxtacrine signals can persist in cells as they proliferate away from source cells, or that cells remain able to communicate directly over several cell diameters. Our model suggests the former mechanism may be sufficient, since it predicts graded outputs without diffusion or long-range cell-cell communication. Overall, we identify that cell-cell contact area together with output synthesis and decay rates likely govern the pattern of synNotch outputs in both space and time during tissue growth, insights that may have broader implications for juxtacrine signaling in general.

Protein post-translational modifications, such as phosphorylation, are important regulatory signals for diverse cellular functions. In particular, intrinsically disordered protein regions (IDRs) are subject to phosphorylation as a means to modulate their interactions and functions. Toward understanding the relationship between phosphorylation in IDRs and specific functional outcomes, we must consider how phosphorylation affects the IDR conformational ensemble. Various experimental techniques are suited to interrogate the features of IDR ensembles; molecular simulations can provide complementary insights and even illuminate ensemble features that may be experimentally inaccessible. Therefore, we sought to expand the tools available to study phosphorylated IDRs by all-atom Monte Carlo simulations. To this end, we implemented parameters for phosphoserine (pSer) and phosphothreonine (pThr) into the OPLS version of the continuum solvent model, ABSINTH, and assessed their performance in all-atom simulations compared with published findings. We simulated short (<20 residues) and long (>80 residues) phospho-IDRs that, collectively, survey both local and global phosphorylation-induced changes to the ensemble. Our simulations of four well-studied phospho-IDRs show near-quantitative agreement with published findings for these systems via metrics including changes to radius of gyration, transient helicity, and persistence length. We also leveraged the inherent advantage of sequence control in molecular simulations to explore the conformational effects of diverse combinations of phospho-sites in two multiphosphorylated IDRs. Our results support and expand on previous observations that connect phosphorylation to changes in the IDR conformational ensemble. Herein, we describe phosphorylation as a means to alter sequence chemistry, net charge and charge patterning, and intramolecular interactions, which can collectively modulate the local and global IDR ensemble features.

Synaptic vesicle clusters or pools are functionally important constituents of chemical synapses. In the so-called reserve and the active pools, neurotransmitter-loaded synaptic vesicles (SVs) are stored and conditioned for fusion with the synaptic membrane and subsequent neurotransmitter release during synaptic activity. Vesicle clusters can be considered as so-called membraneless compartments, which form by liquid-liquid phase separation. Synapsin as one of the most abundant synaptic proteins has been identified as a major driver of pool formation. It has been shown to induce liquid-liquid phase separation and form condensates on its own in solution, but also has been shown to integrate vesicles into condensates in vitro. In this process, the intrinsically disordered region of synapsin is believed to play a critical role. Here, we first investigate the solution structure of synapsin and SVs separately by small-angle x-ray scattering. In the limit of low momentum transfer q, the scattering curve for synapsin gives clear indication for supramolecular aggregation (condensation). We then study mixtures of SVs and synapsin-forming condensates, aiming at the morphology and intervesicle distances, i.e., the structure of the condensates in solution. To obtain the structure factor S(q) quantifying intervesicle correlation, we divide the scattering curve of condensates by that of pure SV suspensions. Analysis of S(q) in combination with numerical simulations of cluster aggregation indicates a noncompact fractal-like vesicular fluid with rather short intervesicle distances at the contact sites.

The ability of cells to sense and respond to mechanical forces is crucial for navigating their environment and interacting with neighboring cells. Myosin II and cortexillin I form complexes known as contractility kits (CKs) in the cytosol, which facilitate a cytoskeletal response by accumulating locally at the site of inflicted stress. Here, we present a computational model for mechanoresponsiveness in Dictyostelium, analyzing the role of CKs within the mechanoresponsive mechanism grounded in experimentally measured parameters. Our model further elaborates on the established distributions and channeling of contractile proteins before and after mechanical force application. We rigorously validate our computational findings by comparing the responses of wild-type cells, null mutants, overexpression mutants, and cells deficient in CK formation to mechanical stresses. Parallel in vivo experiments measuring myosin II cortical distributions at equilibrium provide additional validation. Our results highlight the essential functions of CKs in cellular mechanosensitivity and suggest new insights into the regulatory dynamics of mechanoresponsiveness.

T cell receptor (TCR) and peptide-major histocompatibility complex (pMHC) interactions that result in T cell activation are complex and have been distinguished by their equilibrium affinity and kinetic profiles. While prior affinity-based models can successfully predict meaningful TCR-pMHC interactions in many cases, they occasionally fail at identifying TCR-pMHC interactions with low binding affinity. This study analyzes TCR-pMHC systems for which empirical kinetic and affinity data exist and prior affinity-based predictions have failed. We identify criteria for TCR-pMHC systems with available kinetic information where the introduction of a correction factor improves energy-based model predictions. This kinetic correction factor offers a means to refine existing models with additional data and offers molecular insights to help reconcile previously conflicting reports concerning the influence of TCR-pMHC binding kinetics and affinity on T cell activation.