Spatially selective myosin regulatory light chain regulation is absent in dedifferentiated vascular smooth muscle cells but is partially induced by fibronectin and Klf4

Fluoride-induced hypertension by regulating RhoA/ROCK pathway and phenotypic transformation of vascular smooth muscle cells: In vitro and in vivo evidence

Fluoride exposure has been implicated as a potential risk factor for hypertension, but the underlying mechanisms remain unclear. This study investigated the role of the RhoA/ROCK signaling pathway in fluoride-induced hypertension. Male Wistar rats were divided into different groups and exposed to varying concentrations of sodium fluoride (NaF) or sodium chloride (NaCl) via drinking water. The rats' blood pressure was measured, and their aortic tissue was utilized for high-throughput sequencing analysis. Additionally, rat and A7r5 cell models were established using NaF and/or Fasudil. The study evaluated the effects of fluoride exposure on blood pressure, pathological changes in the aorta, as well as the protein/mRNA expression levels of phenotypic transformation indicators (a-SMA, calp, OPN) in vascular smooth muscle cells (VSMCs), along with the RhoA/ROCK signaling pathway (RhoA, ROCK1, ROCK2, MLC/p-MLC). The results demonstrated that fluoride exposure in rats led to increased blood pressure. High-throughput sequencing analysis revealed differential gene expression associated with vascular smooth muscle contraction, with the RhoA/ROCK signaling pathway emerging as a key regulator. Pathological changes in the rat aorta, such as elastic membrane rupture and collagen fiber deposition, were observed following NaF exposure. However, fasudil, a ROCK inhibitor, mitigated these pathological changes. Both in vitro and in vivo models confirmed the activation of the RhoA/ROCK signaling pathway and the phenotypic transformation of VSMCs from a contractile to a synthetic state upon fluoride exposure. Fasudil effectively inhibited the activities of ROCK1 and ROCK2 and attenuated the phenotypic transformation of VSMCs. In conclusion, fluoride has the potential to induce hypertension through the activation of the RhoA/ROCK signaling pathway and phenotypic changes in vascular smooth muscle cells. These results provide new insights into the mechanism of fluoride-induced hypertension.

Asymmetric response emerges between creation and disintegration of force-bearing subcellular structures as revealed by percolation analysis

Cells dynamically remodel their internal structures by modulating the arrangement of actin filaments (AFs). In this process, individual AFs exhibit stochastic behavior without knowing the macroscopic higher-order structures they are meant to create or disintegrate, but the mechanism allowing for such stochastic process-driven remodeling of subcellular structures remains incompletely understood. Here we employ percolation theory to explore how AFs interacting only with neighboring ones without recognizing the overall configuration can nonetheless create a substantial structure referred to as stress fibers (SFs) at particular locations. We determined the interaction probabilities of AFs undergoing cellular tensional homeostasis, a fundamental property maintaining intracellular tension. We showed that the duration required for the creation of SFs is shortened by the increased amount of preexisting actin meshwork, while the disintegration occurs independently of the presence of actin meshwork, suggesting that the coexistence of tension-bearing and non-bearing elements allows cells to promptly transition to new states in accordance with transient environmental changes. The origin of this asymmetry between creation and disintegration, consistently observed in actual cells, is elucidated through a minimal model analysis by examining the intrinsic nature of mechano-signal transmission. Specifically, unlike the symmetric case involving biochemical communication, physical communication to sense environmental changes is facilitated via AFs under tension, while other free AFs dissociated from tension-bearing structures exhibit stochastic behavior. Thus, both the numerical and minimal models demonstrate the essence of intracellular percolation, in which macroscopic asymmetry observed at the cellular level emerges not from microscopic asymmetry in the interaction probabilities of individual molecules, but rather only as a consequence of the manner of the mechano-signal transmission. These results provide novel insights into the role of the mutual interplay between distinct subcellular structures with and without tension-bearing capability. Insight: Cells continuously remodel their internal elements or structural proteins in response to environmental changes. Despite the stochastic behavior of individual structural proteins, which lack awareness of the larger subcellular structures they are meant to create or disintegrate, this self-assembly process somehow occurs to enable adaptation to the environment. Here we demonstrated through percolation simulations and minimal model analyses that there is an asymmetry in the response between the creation and disintegration of subcellular structures, which can aid environmental adaptation. This asymmetry inherently arises from the nature of mechano-signal transmission through structural proteins, namely tension-mediated information exchange within cells, despite the stochastic behavior of individual proteins lacking asymmetric characters in themselves.

Tuning of Liver Sieve: The Interplay between Actin and Myosin Regulatory Light Chain Regulates Fenestration Size and Number in Murine Liver Sinusoidal Endothelial Cells

Liver sinusoidal endothelial cells (LSECs) facilitate the efficient transport of macromolecules and solutes between the blood and hepatocytes. The efficiency of this transport is realized via transcellular nanopores, called fenestrations. The mean fenestration size is 140 ± 20 nm, with the range from 50 nm to 350 nm being mostly below the limits of diffraction of visible light. The cellular mechanisms controlling fenestrations are still poorly understood. In this study, we tested a hypothesis that both Rho kinase (ROCK) and myosin light chain (MLC) kinase (MLCK)-dependent phosphorylation of MLC regulates fenestrations. We verified the hypothesis using a combination of several molecular inhibitors and by applying two high-resolution microscopy modalities: structured illumination microscopy (SIM) and scanning electron microscopy (SEM). We demonstrated precise, dose-dependent, and reversible regulation of the mean fenestration diameter within a wide range from 120 nm to 220 nm and the fine-tuning of the porosity in a range from ~0% up to 12% using the ROCK pathway. Moreover, our findings indicate that MLCK is involved in the formation of new fenestrations—after inhibiting MLCK, closed fenestrations cannot be reopened with other agents. We, therefore, conclude that the Rho-ROCK pathway is responsible for the control of the fenestration diameter, while the inhibition of MLCK prevents the formation of new fenestrations.

A statistical mechanics model for determining the length distribution of actin filaments under cellular tensional homeostasis

Tensional homeostasis is a cellular process whereby nonmuscle cells such as fibroblasts keep a constant level of intracellular tension and signaling activities. Cells are allowed thanks to tensional homeostasis to adapt to mechanical stress, but the detailed mechanism remains unclear. Here we address from a theoretical point of view what is required for maintaining cellular tensional homeostasis. A constrained optimization problem is formulated to analytically determine the probability function of the length of individual actin filaments (AFs) responsible for sustaining cellular tension. An objective function composed of two entropic quantities measuring the extent of formation and dispersion of AFs within cells is optimized under two constraint functions dictating a constant amount of actin molecules and tension that are arguably the two most salient features of tensional homeostasis. We then derive a specific probability function of AFs that is qualitatively consistent with previous experimental observations, in which short AF populations preferably appear. Regarding the underlying mechanism, our analyses suggest that the constraint for keeping the constant tension level makes long AF populations smaller in number because long AFs have a higher chance to be involved in bearing larger forces. The specific length distribution of AFs is thus required for achieving the constrained objectives, by which individual cells are endowed with the ability to stably maintain a homeostatic tension throughout the cell, thereby potentially allowing cells to locally detect deviation in the tension, keep resulting biological functions, and hence enable subsequent adaptation to mechanical stress. Although minimal essential factors are included given the actual complexity of cells, our approach would provide a theoretical basis for understanding complicated homeostatic and adaptive behavior of the cell.

Anti-Malignant Effect of Tensile Loading to Adherens Junctions in Cutaneous Squamous Cell Carcinoma Cells

Actomyosin contractility regulates various cellular processes including proliferation and differentiation while dysregulation of actomyosin activity contributes to cancer development and progression. Previously, we have reported that actomyosin-generated tension at adherens junctions is required for cell density-dependent inhibition of proliferation of normal skin keratinocytes. However, it remains unclear how actomyosin contractility affects the hyperproliferation ability of cutaneous squamous cell carcinoma (cSCC) cells. In this study, we find that actomyosin activity is impaired in cSCC cells both in vitro and in vivo. External application of tensile loads to adherens junctions by sustained mechanical stretch attenuates the proliferation of cSCC cells, which depends on intact adherens junctions. Forced activation of actomyosin of cSCC cells also inhibits their proliferation in a cell-cell contact-dependent manner. Furthermore, the cell cycle arrest induced by tensile loading to adherens junctions is accompanied by epidermal differentiation in cSCC cells. Our results show that the degree of malignant properties of cSCC cells can be reduced by applying tensile loads to adherens junctions, which implies that the mechanical status of adherens junctions may serve as a novel therapeutic target for cSCC.

Analysis of senescence-responsive stress fiber proteome reveals reorganization of stress fibers mediated by elongation factor eEF2 in HFF-1 cells

Stress fibers (SFs), which are actomyosin structures, reorganize in response to various cues to maintain cellular homeostasis. Currently, the protein components of SFs are only partially identified, limiting our understanding of their responses. Here we isolate SFs from human fibroblasts HFF-1 to determine with proteomic analysis the whole protein components and how they change with replicative senescence (RS), a state where cells decline in the ability to replicate after repeated divisions. We found that at least 135 proteins are associated with SFs, and 63 of them are up-regulated with RS, by which SFs become larger in size. Among them, we focused on eEF2 (eukaryotic translation elongation factor 2) as it exhibited on RS the most significant increase in abundance. We show that eEF2 is critical to the reorganization and stabilization of SFs in senescent fibroblasts. Our findings provide a novel molecular basis for SFs to be reinforced to resist cellular senescence.

Postnatal development alters functional compartmentalization of myosin light chain kinase in ovine carotid arteries

The rate-limiting enzyme for vascular contraction, myosin light chain kinase (MLCK), phosphorylates regulatory myosin light chain (MLC₂₀) at rates that appear faster despite lower MLCK abundance in fetal compared with adult arteries. This study explores the hypothesis that greater apparent tissue activity of MLCK in fetal arteries is due to age-dependent differences in intracellular distribution of MLCK in relation to MLC₂₀. Under optimal conditions, common carotid artery homogenates from nonpregnant adult female sheep and near-term fetuses exhibited similar values of V_max and K_m for MLCK. A custom-designed, computer-controlled apparatus enabled electrical stimulation and high-speed freezing of arterial segments at exactly 0, 1, 2, and 3 s, calculation of in situ rates of MLC₂₀ phosphorylation, and measurement of time-dependent colocalization between MLCK and MLC₂₀. The in situ rate of MLC₂₀ phosphorylation divided by total MLCK abundance averaged to values 147% greater in fetal (1.06 ± 0.28) than adult (0.43 ± 0.08) arteries, which corresponded, respectively, to 43 ± 10% and 31 ± 3% of the V_max values measured in homogenates. Confocal colocalization analysis revealed in fetal and adult arteries that 33 ± 6% and 20 ± 5% of total MLCK colocalized with pMLC₂₀, and that MLCK activation was greater in periluminal than periadventitial regions over the time course of electrical stimulation in both age groups. Together, these results demonstrate that the catalytic activity of MLCK is similar in fetal and adult arteries, but that the fraction of total MLCK in the functional compartment involved in contraction is significantly greater in fetal than adult arteries.

Mechanosensitive myosin II but not cofilin primarily contributes to cyclic cell stretch-induced selective disassembly of actin stress fibers

Cells adapt to applied cyclic stretch (CS) to circumvent chronic activation of proinflammatory signaling. Currently, the molecular mechanism of the selective disassembly of actin stress fibers (SFs) in the stretch direction, which occurs at the early stage of the cellular response to CS, remains controversial. Here, we suggest that the mechanosensitive behavior of myosin II, a major cross-linker of SFs, primarily contributes to the directional disassembly of the actomyosin complex SFs in bovine vascular smooth muscle cells and human U2OS osteosarcoma cells. First, we identified that CS with a shortening phase that exceeds in speed the inherent contractile rate of individual SFs leads to the disassembly. To understand the biological basis, we investigated the effect of expressing myosin regulatory light-chain mutants and found that SFs with less actomyosin activities disassemble more promptly upon CS. We consequently created a minimal mathematical model that recapitulates the salient features of the direction-selective and threshold-triggered disassembly of SFs to show that disassembly or, more specifically, unbundling of the actomyosin bundle SFs is enhanced with sufficiently fast cell shortening. We further demonstrated that similar disassembly of SFs is inducible in the presence of an active LIM-kinase-1 mutant that deactivates cofilin, suggesting that cofilin is dispensable as opposed to a previously proposed mechanism.

Phosphorylation of myosin regulatory light chain at Ser17 regulates actomyosin dissociation

Phosphorylation of myosin regulatory light chain (MRLC) can regulate muscle contraction and thus affect actomyosin dissociation and meat quality. The objective of this study was to explore the mechanism by how MRLC phosphorylation regulates actomyosin dissociation and thus develop strategies for improving meat quality. Here, the phosphorylation status of MRLC was modulated by myosin light chain kinase and myosin light chain kinase inhibitor. MRLC phosphorylation at Ser17 decreased the kinetic energy and total energy of actomyosin, thus stabilized the structure, facilitating the interaction between myosin and actin; this was one possible way that MRLC phosphorylation at Ser17 negatively affects actomyosin dissociation. Moreover, MRLC phosphorylation at Ser17 was beneficial to the formation of ionic bonds, hydrogen bonds, and hydrophobic interaction between myosin and actin, and was the second possible way that MRLC phosphorylation at Ser17 negatively affects actomyosin dissociation.

What factors determine the number of nonmuscle myosin II in the sarcomeric unit of stress fibers?

Actin stress fibers (SFs), a contractile apparatus in nonmuscle cells, possess a contractile unit that is apparently similar to the sarcomere of myofibrils in muscles. The function of SFs has thus often been addressed based on well-characterized properties of muscles. However, unlike the fixed number of myosin molecules in myofibrils, the number of nonmuscle myosin II (NMII) within the contractile sarcomeric unit in SFs is quite low and variable for some reason. Here we address what factors may determine the specific number of NMII in SFs. We suggest with a theoretical model that the number lies just in between the function of SFs for bearing cellular tension under static conditions and for promptly disintegrating upon forced cell shortening. We monitored shortening-induced disintegration of SFs in human osteosarcoma U2OS cells expressing mutants of myosin regulatory light chain that virtually regulates the interaction of NMII with actin filaments, and the behaviors observed were indeed consistent with the theoretical consequences. This situation-specific nature of SFs may allow nonmuscle cells to respond adaptively to mechanical stress to circumvent activation of pro-inflammatory signals as previously indicated, i.e., a behavior distinct from that of muscles that are basically specialized for exhibiting contractile activity.

Helical structure of actin stress fibers and its possible contribution to inducing their direction-selective disassembly upon cell shortening

Mechanisms of the assembly of actin stress fibers (SFs) have been extensively studied, while those of the disassembly-particularly cell shortening-induced ones-remain unclear. Here, we show that SFs have helical structures composed of multi-subbundles, and they tend to be delaminated upon cell shortening. Specifically, we observed with atomic force microscopy delamination of helical SFs into their subbundles. We physically caught individual SFs using a pair of glass needles to observe rotational deformations during stretching as well as ATP-driven active contraction, suggesting that they deform in a manner reflecting their intrinsic helical structure. A minimal analytical model was then developed based on the Frenet-Serret formulas with force-strain measurement data to suggest that helical SFs can be delaminated into the constituent subbundles upon axial shortening. Given that SFs are large molecular clusters that bear cellular tension but must promptly disassemble upon loss of the tension, the resulting increase in their surface area due to the shortening-induced delamination may facilitate interaction with surrounding molecules to aid subsequent disintegration. Thus, our results suggest a new mechanism of the disassembly that occurs only in the specific SFs exposed to forced shortening.