Mechanosensitive Signaling Research Articles

Why exercise builds muscles: titin mechanosensing controls skeletal muscle growth under load

Muscles sense internally generated and externally applied forces, responding to these in a coordinated hierarchical manner at different timescales. The center of the basic unit of the muscle, the sarcomeric M-band, is perfectly placed to sense the different types of load to which the muscle is subjected. In particular, the kinase domain of titin (TK) located at the M-band is a known candidate for mechanical signaling. Here, we develop a quantitative mathematical model that describes the kinetics of TK-based mechanosensitive signaling and predicts trophic changes in response to exercise and rehabilitation regimes. First, we build the kinetic model for TK conformational changes under force: opening, phosphorylation, signaling, and autoinhibition. We find that TK opens as a metastable mechanosensitive switch, which naturally produces a much greater signal after high-load resistance exercise than an equally energetically costly endurance effort. Next, for the model to be stable and give coherent predictions, in particular for the lag after the onset of an exercise regime, we have to account for the associated kinetics of phosphate (carried by ATP) and for the nonlinear dependence of protein synthesis rates on muscle fiber size. We suggest that the latter effect may occur via the steric inhibition of ribosome diffusion through the sieve-like myofilament lattice. The full model yields a steady-state solution (homeostasis) for muscle cross-sectional area and tension and, a quantitatively plausible hypertrophic response to training, as well as atrophy after an extended reduction in tension.

Electrophysiological and Molecular Mechanisms of Sinoatrial Node Mechanosensitivity.

The understanding of the electrophysiological mechanisms that underlie mechanosensitivity of the sinoatrial node (SAN), the primary pacemaker of the heart, has been evolving over the past century. The heart is constantly exposed to a dynamic mechanical environment; as such, the SAN has numerous canonical and emerging mechanosensitive ion channels and signaling pathways that govern its ability to respond to both fast (within second or on beat-to-beat manner) and slow (minutes) timescales. This review summarizes the effects of mechanical loading on the SAN activity and reviews putative candidates, including fast mechanoactivated channels (Piezo, TREK, and BK) and slow mechanoresponsive ion channels [including volume-regulated chloride channels and transient receptor potential (TRP)], as well as the components of mechanochemical signal transduction, which may contribute to SAN mechanosensitivity. Furthermore, we examine the structural foundation for both mechano-electrical and mechanochemical signal transduction and discuss the role of specialized membrane nanodomains, namely, caveolae, in mechanical regulation of both membrane and calcium clock components of the so-called coupled-clock pacemaker system responsible for SAN automaticity. Finally, we emphasize how these mechanically activated changes contribute to the pathophysiology of SAN dysfunction and discuss controversial areas necessitating future investigations. Though the exact mechanisms of SAN mechanosensitivity are currently unknown, identification of such components, their impact into SAN pacemaking, and pathological remodeling may provide new therapeutic targets for the treatment of SAN dysfunction and associated rhythm abnormalities.

Strong as a Hippo’s Heart: Biomechanical Hippo Signaling During Zebrafish Cardiac Development

The heart is comprised of multiple tissues that contribute to its physiological functions. During development, the growth of myocardium and endocardium is coupled and morphogenetic processes within these separate tissue layers are integrated. Here, we discuss the roles of mechanosensitive Hippo signaling in growth and morphogenesis of the zebrafish heart. Hippo signaling is involved in defining numbers of cardiac progenitor cells derived from the secondary heart field, in restricting the growth of the epicardium, and in guiding trabeculation and outflow tract formation. Recent work also shows that myocardial chamber dimensions serve as a blueprint for Hippo signaling-dependent growth of the endocardium. Evidently, Hippo pathway components act at the crossroads of various signaling pathways involved in embryonic zebrafish heart development. Elucidating how biomechanical Hippo signaling guides heart morphogenesis has direct implications for our understanding of cardiac physiology and pathophysiology.

Visualising and understanding cGMP signals in the cardiovascular system.

cGMP is an important signalling molecule in humans. Fluorescent cGMP biosensors have emerged as powerful tools for the sensitive analysis of cGMP pathways at the single-cell level. Here, we briefly outline cGMP's multifaceted role in (patho)physiology and pharmacotherapy. Then we summarise what new insights cGMP imaging has provided into endogenous cGMP signalling and drug action, with a focus on the cardiovascular system. Indeed, the use of cGMP biosensors has led to several conceptual advances, such as the discovery of local, intercellular and mechanosensitive cGMP signals. Importantly, single-cell imaging can provide valuable information about the heterogeneity of cGMP signals within and between individual cells of an isolated cell population or tissue. We also discuss current challenges and future directions of cGMP imaging, such as the direct visualisation of cGMP microdomains, simultaneous monitoring of cGMP and other signalling molecules and, ultimately, cGMP imaging in tissues and animals under close-to-native conditions. LINKED ARTICLES: This article is part of a themed issue on cGMP Signalling in Cell Growth and Survival. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.11/issuetoc.

Mechanotransduction assays for neural regeneration strategies: A focus on glial cells.

Glial cells are mechanosensitive, and thus, engineered systems have taken a step forward to design mechanotransduction platforms in order to impart diverse mechanical stresses to cells. Mechanical strain encountered in the central nervous system can arise from diverse mechanisms, such as tissue reorganization, fluid flow, and axon growth, as well as pathological events including axon swelling or mechanical trauma. Biomechanical relevance of the in vitro mechanical testing requires to be placed in line with the physiological and mechanical changes in central nervous tissues that occur during the progression of neurodegenerative diseases. Mechanotransduction signaling utilized by glial cells and the recent approaches intended to model altered microenvironment adapted to pathological context are discussed in this review. New insights in systems merging substrate's stiffness and topography should be considered for further glial mechanotransduction studies, while testing platforms for drug discoveries promise great advancements in pharmacotherapy. Potential leads and strategies for clinical outcomes are expected to be developed following the exploration of these glial mechanosensitive signaling pathways.

Recursive feedback between matrix dissipation and chemo-mechanical signaling drives oscillatory growth of cancer cell invadopodia.

Most extracellular matrices (ECMs) are known to be dissipative, exhibiting viscoelastic and often plastic behaviors. However, the influence of dissipation, in particular mechanical plasticity in 3D confining microenvironments, on cell motility is not clear. In this study, we develop a chemo-mechanical model for dynamics of invadopodia, the protrusive structures that cancer cells use to facilitate invasion, by considering myosin recruitment, actin polymerization, matrix deformation, and mechano-sensitive signaling pathways. We demonstrate that matrix dissipation facilitates invadopodia growth by softening ECMs over repeated cycles, during which plastic deformation accumulates via cyclic ratcheting. Our model reveals that distinct protrusion patterns, oscillatory or monotonic, emerge from the interplay of timescales for polymerization-associated extension and myosin recruitment dynamics. Our model predicts the changes in invadopodia dynamics upon inhibition of myosin, adhesions, and the Rho-Rho-associated kinase (ROCK) pathway. Altogether, our work highlights the role of matrix plasticity in invadopodia dynamics and can help design dissipative biomaterials to modulate cancer cell motility.

Matrix stiffness and shear stresses modulate hepatocyte functions in a fibrotic liver sinusoidal model.

Extracellular matrix (ECM) rigidity has important effects on cell behaviors and increases sharply in liver fibrosis and cirrhosis. Hepatic blood flow is essential in maintaining hepatocytes' (HCs) functions. However, it is still unclear how matrix stiffness and shear stresses orchestrate HC phenotype in concert. A fibrotic three-dimensional (3-D) liver sinusoidal model is constructed using a porous membrane sandwiched between two polydimethylsiloxane (PDMS) layers with respective flow channels. The HCs are cultured in collagen gels of various stiffnesses in the lower channel, whereas the upper channel is pre-seeded with liver sinusoidal endothelial cells (LSECs) and accessible to shear flow. The results reveal that HCs cultured within stiffer matrices exhibit reduced albumin production and cytochrome P450 (CYP450) reductase expression. Low shear stresses enhance synthetic and metabolic functions of HC, whereas high shear stresses lead to the loss of HC phenotype. Furthermore, both mechanical factors regulate HC functions by complementing each other. These observations are likely attributed to mechanically induced mass transport or key signaling molecule of hepatocyte nuclear factor 4α (HNF4α). The present study results provide an insight into understanding the mechanisms of HC dysfunction in liver fibrosis and cirrhosis, especially from the viewpoint of matrix stiffness and blood flow.NEW & NOTEWORTHY A fibrotic three-dimensional (3-D) liver sinusoidal model was constructed to mimic different stages of liver fibrosis in vivo and to explore the cooperative effects of matrix stiffness and shear stresses on hepatocyte (HC) functions. Mechanically induced alterations of mass transport mainly contributed to HC functions via typical mechanosensitive signaling.

The cellular mechanobiology of aging: from biology to mechanics.

Aging is a chronic, complicated process that leads to degenerative physical and biological changes in living organisms. Aging is associated with permanent, gradual physiological cellular decay that affects all aspects of cellular mechanobiological features, including cellular cytoskeleton structures, mechanosensitive signaling pathways, and forces in the cell, as well as the cell's ability to sense and adapt to extracellular biomechanical signals in the tissue environment through mechanotransduction. These mechanobiological changes in cells are directly or indirectly responsible for dysfunctions and diseases in various organ systems, including the cardiovascular, musculoskeletal, skin, and immune systems. This review critically examines the role of aging in the progressive decline of the mechanobiology occurring in cells, and establishes mechanistic frameworks to understand the mechanobiological effects of aging on disease progression and to develop new strategies for halting and reversing the aging process. Our review also highlights the recent development of novel bioengineering approaches for studying the key mechanobiological mechanisms in aging.

Abstract 16123: In Vitro Valve-on-chip Platform to Assess the Effects of Hemodynamic and Mechanical Stimuli on Early Calcific Disease Progression

Introduction: Calcific aortic valve disease (CAVD) involves mechanical stress, endothelial dysfunction, matrix remodeling, and mineral deposition. Complex mechanical and hemodynamic environment around the valve closely affects its function, biology, and disease. However, the exact role of these forces in regulating cellular dysfunction during disease initiation is yet to be elucidated. Methods and Results: We developed a 3D valve-on-chip (VOC) model that encompasses cell-cell and cell-matrix interactions, flow shear stresses and cyclic strains, better mimicking the native valve. The multilayered VOC construct was built in house using photolithography and soft lithography and a silicon based polymer (Fig.1A). Valve endothelial cells (VECs) are seeded in a monolayer on a porous PDMS membrane (Fig.B) which separates the VECs from the a matrigel-collagen semi interpenetrating hybrid gel housing valve interstitial cells (VICs) (Fig.3C). We simulated either static or 20% (pathological) strain under quiescent or osteogenic conditions. Osteogenic VICs demonstrated nodule formation (Fig.1D) and had reduced redox ratios in 20% stretch conditions suggesting disease progression (Fig.1E,F). Effect of mechanical and hemodynamic forces on structure, function, and phenotypic changes in VECs and VICs particularly during disease initiation will be further assessed. Conclusions: This VOC model for VEC-VIC co-culture better mimics the native valve environment and can be used to study the early disease mechanisms. The VOC platform can be used as a drug testing tool and aid in development of treatment strategies through efficient employment in the laboratory. Further exploring the effect of mechanical and hemodynamic forces may reveal mechanosensitive signaling cascades involved in disease progression, which can be targeted for CAVD therapeutics.

Mechanosensitive Notch-Dll4 and Klf2-Wnt9 Signaling Pathways Intersect in Guiding Valvulogenesis in Zebrafish

In the zebrafish embryo, the onset of blood flow generates fluid shear stress on endocardial cells, which are specialized endothelial cells that line the interior of the heart. This mechanical stimulation activates signaling pathways that play crucial roles in atrioventricular valvulogenesis. High levels of fluid shear stress activate both Notch and Klf2 signaling in endocardial cells, causing some to ingress into the cardiac jelly and initiate valvulogenesis. But it remains unclear why only specific, individual cells respond this way, and how the mechanosensitive Notch and Klf2 pathways contribute to the process. Here, we show that lateral inhibition between endocardial cells, mediated by Notch, singles out Delta-like 4-positive endocardial cells. These cells ingress into the cardiac jelly where they form an abluminal cell population. High Notch activity prevents this cell behavior. We also find that Delta-like 4-positive cells only ingress in response to Wnt9a, which is produced in parallel through an Erk5-Klf2-Wnt9a signaling cascade also activated by blood flow. Our work explains how mechanical stimulation activates parallel mechanosensitive signaling pathways that cooperate and produce binary effects by driving endocardial cells towards either luminal or abluminal fates. Ultimately, these cell fate decisions sculpt cardiac valve leaflets.

Fluid flow as a driver of embryonic morphogenesis.

Fluid flow is a powerful morphogenic force during embryonic development. The physical forces created by flowing fluids can either create morphogen gradients or be translated by mechanosensitive cells into biological changes in gene expression. In this Primer, we describe how fluid flow is created in different systems and highlight the important mechanosensitive signalling pathways involved for sensing and transducing flow during embryogenesis. Specifically, we describe how fluid flow helps establish left-right asymmetry in the early embryo and discuss the role of flow of blood, lymph and cerebrospinal fluid in sculpting the embryonic cardiovascular and nervous system.

Abstract 351: Focal Adhesion Kinase Promotes Tissue Destruction in Aortic Dissection in Mice

Aortic dissection (AD) is a fatal disease due to the sudden destruction of the aortic medial layer. Currently, Molecular pathogenesis of AD is unknown. We investigated the role of focal adhesion kinase (FAK), a mechanosensitive signal transducer, in AD pathogenesis. We created a mouse model of AD with a continuous infusion of beta-aminopropionitrile (150 mg/kg/day), a collagen crosslink inhibitor, and angiotensin II (1,000 ng/kg/min) (BAPN + Ang II) by osmotic pumps. This AD model showed about 60% mortality within 2 weeks due to AD rupture. Immunohistochemical staining for activated FAK revealed that FAK was inactive in normal mouse aorta, but was strongly activated in the aortic walls after BAPN + Ang II infusion. Immunofluorescence staining showed that FAK was activated mainly in smooth muscle cells after the BAPN + Ang II challenge. Western blot analysis revealed that FAK was activated in 3 days after BAPN + Ang II infusion before AD development, followed by transient reduction at day 7, and re-activation after AD development at day 14. We examined the effect of PND-1186, an orally available FAK inhibitor, on the severity as determined by the AD lesion length and the mortality of AD. Mice were administered with either vehicle or PND-1186 (150 mg/kg twice daily by oral gavage) during the BAPN + Ang II challenge (n=20 for each group). Administration of PND-1186 resulted in significant reduction in the lesion length of AD (vehicle; 12.5 ± 1.65 mm, PND; 7.46 ± 1.88 mm, P<0.05). The suppressive effect of PND-1186 was most significant in the aortic arch (vehicle; 2.13 ± 0.29 mm, PND; 0.85 ± 0.22 mm, P<0.01). Furthermore, PND-1186 significantly improved the survival rate of mice from 40.0% to 80.0% (P<0.01). Transcriptome analysis indicated that destruction and inflammation of tissue were suppressed by PND-1186 administration. These findings indicated that FAK plays an important role in AD pathogenesis, possibly by transducing the pathological stress to the tissue destructive response in the aortic walls. We propose that FAK is a potential therapeutic target to limit the fatal destruction of aortic walls in AD.

Controlling Fibronectin Fibrillogenesis Using Visible Light.

We previously developed a surface-assisted assay to image early steps of cell-induced plasma fibronectin (FN) fibrillogenesis by timelapse atomic force microscopy (AFM). Unexpectedly, complementary attempts to visualize FN fibrillogenesis using fluorescently labeled FN (Alexa Fluor 488 or 568) and live-cell light microscopy initially failed consistently. Further analysis revealed that fibrillar remodeling was inhibited efficiently in the focal area illuminated during fluorescence imaging, but progressed normally elsewhere on the substrate, suggesting photo sensitivity of the FN fibrillogenesis process. In agreement, active cell-driven fibrillar extension of FN could be stopped by transient illumination with visible light during AFM timelapse scanning. Phototoxic effects on the cells could be ruled out, because pre-illuminating the FN layer before cell seeding also blocked subsequent fibrillar formation. Varying the illumination wavelength range between 400 and 640 nm revealed strong inhibition across the visible spectrum up to 560 nm, and a decreasing inhibitory effect at longer wavelengths. The photo effect also affected unlabeled FN, but was enhanced by fluorophore labeling of FN. The inhibitory effect could be reduced when reactive oxygen species (ROS) were removed for the cell imaging medium. Based on these findings, FN fibrillogenesis could be imaged successfully using a labeling dye with a long excitation wavelength (Alexa Fluor 633, excitation at 632 nm) and ROS scavengers, such as oxyrase, in the imaging medium. Fibrillar remodeling of exposed cell-free FN layers by AFM scanning required higher scan forces compared to non-exposed FN, consisting with mechanical stiffing of the FN layer after illumination. In agreement with changes in FN mechanics, cells spreading on pre-exposed FN showed reduced migration speeds, altered focal adhesion arrangement, and changes in mechanosensitive signaling pathways, including reduced FAK (Y397) and paxillin (Y118) phosphorylation. Pre-exposure of FN to visible light prior to cell seeding thus provides a useful tool to delineate mechanosensitive signaling pathway related to FN fibrillogenesis. When using FN-coated cell adhesion substrates, care should be taken when comparing experimental results obtained on non-exposed FN layers in cell culture incubators, or during live-cell fluorescence imaging, as FN fibrillogenesis and mechanosensitive cellular signaling pathways may be affected differently.

Antagonistic Activities of Vegfr3/Flt4 and Notch1b Fine-tune Mechanosensitive Signaling during Zebrafish Cardiac Valvulogenesis

The formation of cardiac valves depends on mechanical forces exerted by blood flow. Endocardial cells lining the interior of the heart are sensitive to these stimuli and respond by rearranging into luminal cells subjected to shear stress and abluminal cells not exposed to it. The mechanisms by which endocardial cells sense these dynamic biomechanical stimuli and how they evoke different cellular responses are largely unknown. Here, we show that blood flow activates two parallel mechanosensitive pathways, one mediated by Notch and the other by Klf2a. Both pathways negatively regulate the angiogenesis receptor Vegfr3/Flt4, which becomes restricted to abluminal endocardial cells. Its loss disrupts valve morphogenesis and results in the occurrence of Notch signaling within abluminal endocardial cells. Our work explains how antagonistic activities by Vegfr3/Flt4 on the abluminal side and by Notch on the luminal side shape cardiac valve leaflets by triggering unique differences in the fates of endocardial cells.

The actin-bundling protein L-plastin regulates NLRP3 inflammasome activation in macrophages

Abstract In macrophages, inflammasome pathways induce IL-1β release and pyroptotic cell death to clear infections. For example, NLRP3 inflammasome activation in lung resident alveolar macrophages (AMs) is crucial in controlling pneumonia. We have found that mice lacking the actin-bundling protein L-plastin (LPL−/−) mice are highly susceptible to Streptococcus pnuemoniae lung infection and produce less IL-1β. LPL is also required for podosome formation. Podosomes are integrin-based sites of adhesion that anchor the actin cytoskeleton of AMs to the extracellular environment. As such, AMs can sense their mechanical environment via podosomes. We have found that AMs lacking LPL exhibit defective NLRP3 inflammasome activation in ex-vivo stimulation. Specifically, LPL is required for ASC oligomerization and downstream IL-1β processing after NLRP3 assembly. Since LPL also supports mechanosensitive integrin signaling, we hypothesized that LPL might link contact-based mechanosensation to inflammasome activation in macrophages. To test this hypothesis, we challenged LPL−/− mice with bleomycin, as bleomycin-induced lung injury and fibrosis is thought to be dependent upon NLRP3 activation. LPL−/− mice were resistant to bleomycin-induced lung injury, suggesting a previously unrecognized mechanosensitive mechanism regulating inflammasome activation in macrophages. We propose that the increased lung stiffness in fibrotic lung exerts mechanical stress on AMs and thus may contribute in inflammasome signaling. Increased understanding how mechanical signaling regulates may thus aid in developing therapies to treat lung pathologies i.e. acute respiratory distress syndrome (ARDS), COPD and lung fibrosis.

ILK supports RhoA/ROCK-mediated contractility of human intestinal epithelial crypt cells by inducing the fibrillogenesis of endogenous soluble fibronectin during the spreading process

BackgroundFibronectin (FN) assembly into an insoluble fibrillar matrix is a crucial step in many cell responses to extracellular matrix (ECM) properties, especially with regards to the integrin-related mechanosensitive signaling pathway. We have previously reported that the silencing of expression of integrin-linked kinase (ILK) in human intestinal epithelial crypt (HIEC) cells causes significant reductions in proliferation and spreading through concomitantly acquired impairment of soluble FN deposition. These defects in ILK-depleted cells are rescued by growth on exogenous FN. In the present study we investigated the contribution of ILK in the fibrillogenesis of FN and its relation to integrin-actin axis signaling and organization.ResultsWe show that de novo fibrillogenesis of endogenous soluble FN is ILK-dependent. This function seemingly induces the assembly of an ECM that supports increased cytoskeletal tension and the development of a fully spread contractile cell phenotype. We observed that HIEC cell adhesion to exogenous FN or collagen-I (Col-I) is sufficient to restore fibrillogenesis of endogenous FN in ILK-depleted cells. We also found that optimal engagement of the Ras homolog gene family member A (RhoA) GTPase/Rho-associated kinase (ROCK-1, ROCK-2)/myosin light chain (MLC) pathway, actin ventral stress fiber formation, and integrin adhesion complex (IAC) maturation rely primarily upon the cell’s capacity to execute FN fibrillogenesis, independent of any significant ILK input. Lastly, we confirm the integrin α5β1 as the main integrin responsible for FN assembly, although in ILK-depleted cells αV-class integrins expression is needed to allow the rescue of FN fibrillogenesis on exogenous substrate.ConclusionOur study demonstrates that ILK specifically induces the initiation of FN fibrillogenesis during cell spreading, which promotes RhoA/ROCK-dependent cell contractility and maturation of the integrin-actin axis structures. However, the fibrillogenesis process and its downstream effect on RhoA signaling, cell contractility and spreading are ILK-independent in human intestinal epithelial crypt cells.

Mechanical Forces and Their Effect on the Ribosome and Protein Translation Machinery.

Mechanical forces acting on biological systems, at both the macroscopic and microscopic levels, play an important part in shaping cellular phenotypes. There is a growing realization that biomolecules that respond to force directly applied to them, or via mechano-sensitive signalling pathways, can produce profound changes to not only transcriptional pathways, but also in protein translation. Forces naturally occurring at the molecular level can impact the rate at which the bacterial ribosome translates messenger RNA (mRNA) transcripts and influence processes such as co-translational folding of a nascent protein as it exits the ribosome. In eukaryotes, force can also be transduced at the cellular level by the cytoskeleton, the cell’s internal filamentous network. The cytoskeleton closely associates with components of the translational machinery such as ribosomes and elongation factors and, as such, is a crucial determinant of localized protein translation. In this review we will give (1) a brief overview of protein translation in bacteria and eukaryotes and then discuss (2) how mechanical forces are directly involved with ribosomes during active protein synthesis and (3) how eukaryotic ribosomes and other protein translation machinery intimately associates with the mechanosensitive cytoskeleton network.

Lateral induction limits the impact of cell connectivity on Notch signaling in arterial walls.

It is well known that arteries grow and remodel in response to mechanical stimuli. Vascular smooth muscle cells are the main mediators of this process, as they can switch phenotype from contractile to synthetic, and vice‐versa, based on the surrounding bio‐chemo‐mechanical stimuli. A correct regulation of this phenotypic switch is fundamental to obtain and maintain arterial homeostasis. Notch, a mechanosensitive signaling pathway, is one of the main regulators of the vascular smooth muscle cell phenotype. Therefore, understanding Notch dynamics is key to elucidate arterial growth, remodeling, and mechanobiology. We have recently developed a one‐dimensional agent‐based model to investigate Notch signaling in arteries. However, due to its one‐dimensional formulation, the model cannot be adopted to study complex nonsymmetrical geometries and, importantly, it cannot capture the realistic “cell connectivity” in arteries, here defined as the number of cell neighbors. Notch functions via direct cell‐cell contact; thus, the number of cell neighbors could be an essential feature of Notch dynamics. Here, we extended the agent‐based model to a two‐dimensional formulation, to investigate the effects of cell connectivity on Notch dynamics and cell phenotypes in arteries. The computational results, supported by a sensitivity analysis, indicate that cell connectivity has marginal effects when Notch dynamics is dominated by the process of lateral induction, which induces all cells to have a uniform phenotype. When lateral induction is weaker, cells exhibit a nonuniform phenotype distribution and the percentage of synthetic cells within an artery depends on the number of neighbors.

Boolean model of anchorage dependence and contact inhibition points to coordinated inhibition but semi-independent induction of proliferation and migration

Epithelial cells respond to their physical neighborhood with mechano-sensitive behaviors required for development and tissue maintenance. These include anchorage dependence, matrix stiffness-dependent proliferation, contact inhibition of proliferation and migration, and collective migration that balances cell crawling with the maintenance of cell junctions. While required for development and tissue repair, these coordinated responses to the microenvironment also contribute to cancer metastasis. Predictive models of the signaling networks that coordinate these behaviors are critical in controlling cell behavior to halt disease. Here we propose a Boolean regulatory network model that synthesizes mechanosensitive signaling that links anchorage to a matrix of varying stiffness and cell density sensing to contact inhibition, proliferation, migration, and apoptosis. Our model can reproduce anchorage dependence and anoikis, detachment-induced cytokinesis errors, the effect of matrix stiffness on proliferation, and contact inhibition of proliferation and migration by two mechanisms that converge on the YAP transcription factor. In addition, we offer testable predictions related to cell cycle-dependent anoikis sensitivity, the molecular requirements for abolishing contact inhibition, and substrate stiffness dependent expression of the catalytic subunit of PI3K. Moreover, our model predicts heterogeneity in migratory vs. non-migratory phenotypes in sub-confluent monolayers, and co-inhibition but semi-independent induction of proliferation vs. migration as a function of cell density and mitogenic stimulation. Our model serves as a stepping-stone towards modeling mechanosensitive routes to the epithelial to mesenchymal transition, capturing the effects of the mesenchymal state on anoikis resistance, and understanding the balance between migration versus proliferation at each stage of the epithelial to mesenchymal transition.

Mechanoflammation in osteoarthritis pathogenesis

Mechanical injury is the most important risk factor in osteoarthritis (OA) development. Although once considered a passive disease of mechanical attrition, injury drives active mechanosensitive intracellular signalling which affects the structural and symptomatic course of disease. Mechanosensitive signalling in cartilage has been elucidated over the years and two principal responses emerge: those that cause the release of growth factors from the matrix and which stimulate repair, and those that drive inflammatory signalling, a process that we have termed “mechanoflammation”. The up-stream activator of mechanoflammation remains unknown, but it results in rapid activation of NFkB and the inflammatory mitogen activated protein (MAP) kinases and this controls the bioavailability of aggrecanase and regulation of nerve growth factor (NGF), causing pain. The precise relationship between mechanoflammation and cartilage repair is currently unclear but it is likely that chronic mechanoflammation will contribute to disease by also suppressing intrinisic tissue repair.