Shp2 plays a crucial role in cell structural orientation and force polarity in response to matrix rigidity

Shp2 contributes to the regulation of nuclear shape and cellular viscoelasticity in response to substrate spatial cues

Cell polarization can be guided by substrate topology through space constraints and adhesion induction, which are part of cellular mechanosensing pathways. Here, we demonstrated that protein tyrosine phosphatase Shp2 plays a crucial role in mediating the response of cells to substrate spatial cues. When compared to cells spreading on surfaces coated uniformly with fibronectin (FN), cells attached to 10 μm-width FN-strip micropattern (MP), which provides spatial cues for uniaxial spreading, exhibited elongated focal adhesions (FAs) and aligned stress fibers in the direction of the MP. As a result of uniaxial cell spreading, nuclei became elongated, dependent on ROCK-mediated actomyosin contractility. Additionally, intracellular viscoelasticity also increased. Shp2-deficient cells did not display elongated FAs mediated by MP, well-aligned stress fibers, or changes in nuclear shape and intracellular viscoelasticity. Overall, our data suggest that Shp2 is involved in regulating FAs and the actin cytoskeleton to modulate nuclear shape and intracellular physical properties in response to substrate spatial cues.

Mechanical factors driving cancer progression

A fundamental step of tumor metastasis is tumor cell migration away from the primary tumor site. One mode of migration that is essential but still understudied is collective invasion, the process by which clusters of cells move in a coordinated fashion. In recent years, there has been growing interest to understand factors regulating collective invasion, with increasing number of studies investigating the biomechanical regulation of collective invasion. In this review we discuss the dynamic relationship between tumor microenvironment cues and cell response by first covering mechanical factors in the microenvironment and second, discussing the mechanosensing pathways utilized by cells in collective clusters to dynamically respond to mechanical matrix cues. Finally, we discuss model systems that have been developed which have increased our understanding of the mechanical factors contributing to tumor progression.

Microgroove Cues Guiding Fibrogenesis of Stem Cells via Intracellular Force

Groove patterns are widely used in material surface modifications. However, the independent role of ditches/ridges in regulating fibrosis of soft tissues is not well-understood, especially the lack of linkage evidence in vitro and in vivo. Herein, two kinds of combinational microgroove chips with the gradient ditch/ridge width were fabricated by photolithography technology, termed R and G groups, respectively. In group R, the ridge width was 1, 5, 10, and 30 μm, with a ditch width of 30 μm; in group G, the groove width was 5, 10, 20, and 30 μm, and the ridge width was 5 μm. The effect of microgrooves on the morphology, proliferation, and expression of fibrous markers of stem cells was systematically investigated in vitro. Moreover, thicknesses of fibrous capsules were evaluated after chips were implanted into the muscular pouches of rats for 5 months. The results show that microgrooves have almost no effect on cell proliferation but significantly modulate the morphology of cells and focal adhesions (FAs) in vitro, as well as fibrosis differentiation. In particular, the differentiation of stem cells is attenuated after the intracellular force caused by stress fibers and FAs is interfered by drugs, such as rotenone and blebbistatin. Histological analysis shows that patterns of high intracellular force can apparently stimulate soft tissue fibrosis in vivo. This study not only reveals the specific rules and mechanisms of ditch/ridge regulating stem cell behaviors but also offers insight into tailoring implant surface patterns to induce controlled soft tissue fibrosis.

Heparin-mimicking polymer-based hydrogel matrix regulates macrophage polarization by controlling cell adhesion

The physicochemical properties of biomaterials influence cell adhesion, shape, and polarization of macrophages. In this study, we aimed to evaluate the polarization of macrophages in terms of the regulation of cell adhesion and how synthetic mimics for heparin and poly(sodium-4-styrenesulfonate) can regulate macrophage polarization by modulating cell shape, focal adhesion, cell traction force, and intracellular tension. Our initial findings showed that macrophages cultured with heparin-mimicking polymer-based hydrogel matrix showed reduced expression of cell adhesion markers such as integrins, vinculin, RhoA, and ROCK1/2 and reduced cell shape, elongation, cell-matrix traction force, and intracellular tension. Furthermore, we observed a significant decrease in cell adhesion in cells cultured on the hydrogel, resulting in the promotion of M1 polarization. These findings offer insights into the important roles of cell-matrix interactions in macrophage polarization and offer a platform for heparin-mimicking polymer-based hydrogel matrices to induce M1 polarization by inducing cell adhesion without classical activators.

α-Actinin-4 recruits Shp2 into focal adhesions to potentiate ROCK2 activation in podocytes

α-Actinin-4 is crucial in the regulation of Shp2 FA targeting to enhance ROCK2-mediated actomyosin contractility and thereby strengthening cell adhesion and cytoskeletal architecture in podocytes.

Cell–matrix adhesions are mainly provided by integrin-mediated focal adhesions (FAs). We previously found that Shp2 is essential for FA maturation by promoting ROCK2 activation at FAs. In this study, we further delineated the role of α-actinin-4 in the FA recruitment and activation of Shp2. We used the conditional immortalized mouse podocytes to examine the role of α-actinin-4 in the regulation of Shp2 and ROCK2 signaling. After the induction of podocyte differentiation, Shp2 and ROCK2 were strongly activated, concomitant with the formation of matured FAs, stress fibers, and interdigitating intracellular junctions in a ROCK-dependent manner. Gene knockout of α-actinin-4 abolished the Shp2 activation and subsequently reduced matured FAs in podocytes. We also demonstrated that gene knockout of ROCK2 impaired the generation of contractility and interdigitating intercellular junctions. Our results reveal the role of α-actinin-4 in the recruitment of Shp2 at FAs to potentiate ROCK2 activation for the maintenance of cellular contractility and cytoskeletal architecture in the cultured podocytes.

Dissecting protein tyrosine phosphatase signaling by engineered chemogenetic control of its activity

Protein tyrosine phosphatases (PTPases) are critical mediators of dynamic cell signaling. A tool capable of identifying transient signaling events downstream of PTPases is essential to understand phosphatase function on a physiological time scale. We report a broadly applicable protein engineering method for allosteric regulation of PTPases. This method enables dissection of transient events and reconstruction of individual signaling pathways. Implementation of this approach for Shp2 phosphatase revealed parallel MAPK and ROCK II dependent pathways downstream of Shp2, mediating transient cell spreading and migration. Furthermore, we show that the N-SH2 domain of Shp2 regulates MAPK-independent, ROCK II-dependent cell migration. Engineered targeting of Shp2 activity to different protein complexes revealed that Shp2-FAK signaling induces cell spreading whereas Shp2-Gab1 or Shp2-Gab2 mediates cell migration. We identified specific transient morphodynamic processes induced by Shp2 and determined the role of individual signaling pathways downstream of Shp2 in regulating these events. Broad application of this approach is demonstrated by regulating PTP1B and PTP-PEST phosphatases.

The role and therapeutic implication of protein tyrosine phosphatases in Alzheimer's disease

Protein tyrosine phosphatases (PTPs) are important regulator of neuronal signal transduction and a growing number of PTPs have been implicated in Alzheimer's disease (AD). In the brains of patients with AD, there are a variety of abnormally phosphorylated proteins, which are closely related to the abnormal expression and activity of PTPs. β-Amyloid plaques (Aβ) and hyperphosphorylated tau protein are two pathological hallmarks of AD, and their accumulation ultimately leads to neurodegeneration. Studies have shown that protein phosphorylation signaling pathways mediates intracellular accumulation of Aβ and tau during AD development and are involved in synaptic plasticity and other stress responses. Here, we summarized the roles of PTPs related to the pathogenesis of AD and analyzed their therapeutic potential in AD.

Integrin molecular tension required for focal adhesion maturation and YAP nuclear translocation

Focal adhesions (FAs) provide the cells linkages to extracellular matrix (ECM) at sites of integrins binding and transmit mechanical forces between the ECM and the actin cytoskeleton. Cells sense and respond to physical stimuli from their surrounding environment through the activation of mechanosensitive signaling pathways, a process called mechanotransduction. In this study, we used RGD-peptide conjugated DNA tension gauge tethers (TGTs) with different tension tolerance (T_tol) to determine the molecular forces required for FA maturation in different sizes and YAP nuclear translocation. We found that the limitation of FA sizes in cells seeded on TGTs with different T_tol were less than 1 μm, 2 μm, 3 μm, and 6 μm for T_tol values of 43 pN, 50 pN, 54 pN, and 56 pN, respectively. This suggests that the molecular tension across integrins increases gradually as FA size increases throughout FA maturation. For YAP nuclear translocation, significant YAP nuclear localization was observed only in the cells seeded on the TGTs with T_tol ≥ 54 pN, but not on TGTs with T_tol ≤ 50 pN, suggesting a threshold of molecular force across integrins for YAP nuclear translocation lies in the range of 50 pN–54 pN.

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Defining forces required for FA maturation and YAP nuclear translocation.

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Integrin tension enhances gradually with the increase of FA size.

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Forces required for YAP nuclear translocation lies in the range of 50–54 pN.

Snail Augments Nuclear Deformability to Promote Lymph Node Metastasis of Head and Neck Squamous Cell Carcinoma

Up to 50% of head and neck squamous cell carcinoma (HNSCC) patients have lymph node (LN) metastasis, resulting in poor survival rate. Numerous studies have supported the notion that the alterations of gene expression and mechanical properties of cancer cells play an important role in cancer metastasis. However, which genes and how they regulate the biomechanical properties of HNSCC cells to promote LN metastasis remains elusive. In this study, we used an LN-metastatic mouse model in vivo to generate an LN-metastatic head and neck squamous cell carcinoma cell line and compared the differences in the biomolecular and biomechanical properties of LN-metastatic and non-metastatic cells. Our results showed that LN-metastatic cells had a higher level of Snail expression compared to non-LN-metastatic cells. The higher Snail expression promoted the cellular invasion capability in confined environments, mainly by increasing the longitudinal strain of the cell nuclei, which could be attributed to the stronger cell traction force and softer nuclear stiffness. These two biomechanical changes were correlated, respectively, to a larger amount of focal adhesion and less amount of nuclear lamins. Taken together, our works revealed not only the biomechanical profiles of LN-metastatic cells but also the corresponding biomolecular expressions to pinpoint the key process in LN metastasis.

Protein Tyrosine Phosphatases: A new paradigm in an old signaling system?

Protein Tyrosine Phosphatases reverse cellular signals initiated by growth factors receptors and other tyrosine kinases by dephosphorylating phosphotyrosine on target proteins. The activity of these enzymes is crucial for maintaining cell homeostasis, yet these enzymes have been often dismissed as humble house-keeping proteins. Understandably, mutations and changes in expression patterns of Protein Tyrosine Phosphatases are implicated in tumorigenesis and various carcinomas. The conserved nature of their catalytic domains makes drug discovery a challenging pursuit. In this review, we focus on describing the various classes of Protein Tyrosine Phosphatases and their catalytic domains. We also summarize their role in cancer and neurodegenerative diseases using specific members as the model system. Finally, we explain the dichotomy in the biological role of catalytically active vs the pseudoenzyme forms of Protein Tyrosine Phosphatases in the context of their membrane bound receptor forms. This chapter aims to provide a current understanding of these proteins, in the background of their foundational past research.

Inflammatory Cytokines Alter Mesenchymal Stem Cell Mechanosensing and Adhesion on Stiffened Infarct Heart Tissue After Myocardial Infarction

Mesenchymal stem cell (MSC) transplantation has demonstrated its potential in repairing infarct heart tissue and recovering heart function after myocardial infarction (MI). However, its therapeutic effect is still limited due to poor MSC engraftment at the injury site whose tissue stiffness and local inflammation both dynamically and rapidly change after MI. Whether and how inflammatory cytokines could couple with stiffness change to affect MSC engraftment in the infarct zone still remain unclear. In this study, we characterized dynamic stiffness changes of and inflammatory cytokine expression in the infarct region of rat heart within a month after MI. We found that the tissue stiffness of the heart tissue gradually increased and peaked 21 days after MI along with the rapid upregulation of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and interleukin-1β (IL-1β) in the first 3 days, followed by a sharp decline. We further demonstrated in vitro that immobilized inflammatory cytokine IL-6 performed better than the soluble form in enhancing MSC adhesion to stiffened substrate through IL-6/src homology 2 (SH2) domain-containing tyrosine phosphatase-2 (SHP2)/integrin signaling axis. We also confirmed such mechano-immune coupling of tissue stiffness and inflammatory cytokines in modulating MSC engraftment in the rat heart after MI in vivo. Our study provides new mechanistic insights of mechanical–inflammation coupling to improve MSC mechanosensing and adhesion, potentially benefiting MSC engraftment and its clinical therapy for MI.

Growth Factors, and Cytokines; Understanding the Role of Tyrosine Phosphatase SHP2 in Gametogenesis and Early Embryo Development

Growth factors and cytokines have vital roles in germ cell development, gamete maturation, and early embryo development. Cell surface receptors are present for growth factors and cytokines to integrate with and trigger protein signaling in the germ and embryo intracellular milieu. Src-homology-2-containing phosphotyrosine phosphatase (SHP2) is a ubiquitously expressed, multifunctional protein that plays a central role in the signaling pathways involved in growth factor receptors, cytokine receptors, integrins, and G protein-coupled receptors. Over recent decades, researchers have recapitulated the protein signaling networks that influence gamete progenitor specification as well as gamete differentiation and maturation. SHP2 plays an indispensable role in cellular growth, survival, proliferation, differentiation, and migration, as well as the basic events in gametogenesis and early embryo development. SHP2, a classic cytosolic protein and a key regulator of signal transduction, displays unconventional nuclear expression in the genital organs. Several observations provided shreds of evidence that this behavior is essential for fertility. The growth factor and cytokine-dependent roles of SHP2 and its nuclear/cytoplasmic presence during gamete maturation, early embryonic development and embryo implantation are fascinating and complex subjects. This review is intended to summarize the previous and recent knowledge about the SHP2 functions in gametogenesis and early embryo development.

PTPN11 (SHP2) Is Indispensable for Growth Factors and Cytokine Signal Transduction During Bovine Oocyte Maturation and Blastocyst Development

This study was aimed to investigate the role of SHP2 (Src-homology-2-containing phosphotyrosine phosphatase) in intricate signaling networks invoked by bovine oocyte to achieve maturation and blastocyst development. PTPN11 (Protein Tyrosine Phosphatase, non-receptor type 11) encoding protein SHP2, a positive transducer of RTKs (Receptor Tyrosine Kinases) and cytokine receptors, can play a significant role in bovine oocyte maturation and embryo development, but this phenomenon has not yet been explored. Here, we used different growth factors, cytokines, selective activator, and a specific inhibitor of SHP2 to ascertain its role in bovine oocyte developmental stages in vitro. We found that SHP2 became activated by growth factors and cytokines treatment and was highly involved in the activation of oocyte maturation and embryo development pathways. Activation of SHP2 triggered MAPK (mitogen-activated protein kinases) and PI3K/AKT (Phosphoinositide 3-kinase/Protein kinase B) signaling cascades, which is not only important for GVBD (germinal vesical breakdown) induction but also for maternal mRNA translation. Inhibition of phosphatase activity of SHP2 with PHPS1 (Phenylhydrazonopyrazolone sulfonate 1) reduced oocytes maturation as well as bovine blastocyst ICM (inner cell mass) volume. Supplementation of LIF (Leukemia Inhibitory Factor) to embryos showed an unconventional direct relation between p-SHP2 and p-STAT3 (Signal transducer and activator of transcription 3) for blastocyst ICM development. Other than growth factors and cytokines, cisplatin was used to activate SHP2. Cisplatin activated SHP2 modulate growth factors effect and combine treatment significantly enhanced quality and rate of developed blastocysts.

Early stage mechanical remodeling of collagen surrounding head and neck squamous cell carcinoma spheroids correlates strongly with their invasion capability

Mechanical remodeling of stromal collagen, such as reorientation and deformation of collagen matrix, generated by invading cancer cells, plays an important role in the progression of cancer invasion and metastasis. In this study, we applied time-lapse microscopy in conjunction with particle displacement mapping to analyze time-dependent contraction and expansion deformations of collagen surrounding individual spheroids of head and neck squamous cell carcinoma cells (HNSCC), OECM-1 & SAS, as the cancer cells detached from the spheroid and invaded into the surrounding 3D collagen matrix. Our results revealed that highly-invasive HNSCC spheroids, stimulated by epidermal growth factor (EGF), generated a strong contraction deformation of the surrounding collagen in the very early stage, and aligned the collagen fibers radially with respect to the center of the spheroid. This initial collagen contraction deformation generated by the HNSCC spheroid bears a strong positive correlation with the overall extent of subsequent cancer cells invasion; hence, it may serve as an early indicator of the invasion capability of the HNSCC spheroids. STATEMENT OF SIGNIFICANCE: Mechanical remodeling of extracellular matrix (ECM) generated by cancer cells plays an important role in the progression of cancer invasion and metastasis. We observed that the extent of initial contraction deformation of collagen surrounding a head and neck squamous cell carcinoma cell (HNSCC) spheroid played an indispensable role in early stage to promote cancer cells invasion into the surrounding ECM. Our results revealed that more invasive HNSCC spheroids generated a larger extent of initial collagen contraction to align the surrounding collagen and to promote cancer cells invasion. This initial collagen contraction deformation generated by the HNSCC spheroids bears a strong positive correlation with the overall extent of cancer cells invasion; hence, it may serve as an early indicator of the invasion capability of the HNSCC spheroids.

SHP-2 is activated in response to force on E-cadherin and dephosphorylates vinculin Y822

The response of cells to mechanical inputs is a key determinant of cell behavior. In response to external forces, E-cadherin initiates signal transduction cascades that allow the cell to modulate its contractility to withstand the force. Much attention has focused on identifying the E-cadherin signaling pathways that promote contractility, but the negative regulators remain undefined. In this study, we identify SHP-2 as a force-activated phosphatase that negatively regulates E-cadherin force transmission by dephosphorylating vinculin Y822. To specifically probe a role for SHP-2 in E-cadherin mechanotransduction, we mutated vinculin so that it retains its phosphorylation but cannot be dephosphorylated. Cells expressing the mutant vinculin have increased contractility. This work provides a mechanism for inactivating E-cadherin mechanotransduction and provides a new method for specifically targeting the action of phosphatases in cells.

Epithelial-mesenchymal transition softens head and neck cancer cells to facilitate migration in 3D environments

The biological impact and signalling of epithelial‐mesenchymal transition (EMT) during cancer metastasis has been established. However, the changes in biophysical properties of cancer cells undergoing EMT remain elusive. Here, we measured, via video particle tracking microrheology, the intracellular stiffness of head and neck cancer cell lines with distinct EMT phenotypes. We also examined cells migration and invasiveness in different extracellular matrix architectures and EMT‐related signalling in these cell lines. Our results show that when cells were cultivated in three‐dimensional (3D) environments, the differences in cell morphology, migration speed, invasion capability and intracellular stiffness were more pronounced among different head and neck cancer cell lines with distinct EMT phenotypes than those cultivated in traditional plastic dishes and/or seated on top of a thick layer of collagen. An inverse correlation between intracellular stiffness and invasiveness in 3D culture was revealed. Knock‐down of the EMT regulator Twist1 or Snail or inhibition of Rac1 which is a downstream GTPase of Twist1 increased intracellular stiffness. These results indicate that the EMT regulators, Twist1 and Snail and the mediated signals play a critical role in reducing intracellular stiffness and enhancing cell migration in EMT to promote cancer cells invasion.

Lis1 dysfunction leads to traction force reduction and cytoskeletal disorganization during cell migration

Cell migration is a critical process during development, tissue repair, and cancer metastasis. It requires complex processes of cell adhesion, cytoskeletal dynamics, and force generation. Lis1 plays an important role in the migration of neurons, fibroblasts and other cell types, and is essential for normal development of the cerebral cortex. Mutations in human LIS1 gene cause classical lissencephaly (smooth brain), resulting from defects in neuronal migration. However, how Lis1 may affect force generation in migrating cells is still not fully understood. Using traction force microscopy (TFM) with live cell imaging to measure cellular traction force in migrating NIH3T3 cells, we showed that Lis1 knockdown (KD) by RNA interference (RNAi) caused reductions in cell migration and traction force against the extracellular matrix (ECM). Immunostaining of cytoskeletal components in Lis1 KD cells showed disorganization of microtubules and actin filaments. Interestingly, focal adhesions at the cell periphery were significantly reduced. These results suggest that Lis1 is important for cellular traction force generation through the regulation of cytoskeleton organization and focal adhesion formation in migrating cells.

The assembly and function of perinuclear actin cap in migrating cells

Stress fibers are actin bundles encompassing actin filaments, actin-crosslinking, and actin-associated proteins that represent the major contractile system in the cell. Different types of stress fibers assemble in adherent cells, and they are central to diverse cellular processes including establishment of the cell shape, morphogenesis, cell polarization, and migration. Stress fibers display specific cellular organization and localization, with ventral fibers present at the basal side, and dorsal fibers and transverse actin arcs rising at the cell front from the ventral to the dorsal side and toward the nucleus. Perinuclear actin cap fibers are a specific subtype of stress fibers that rise from the leading edge above the nucleus and terminate at the cell rear forming a dome-like structure. Perinuclear actin cap fibers are fixed at three points: both ends are anchored in focal adhesions, while the central part is physically attached to the nucleus and nuclear lamina through the linker of nucleoskeleton and cytoskeleton (LINC) complex. Here, we discuss recent work that provides new insights into the mechanism of assembly and the function of these actin stress fibers that directly link extracellular matrix and focal adhesions with the nuclear envelope.

Bio- chemical and physical characterizations of mesenchymal stromal cells along the time course of directed differentiation

Cellular biophysical properties are novel biomarkers of cell phenotypes which may reflect the status of differentiating stem cells. Accurate characterizations of cellular biophysical properties, in conjunction with the corresponding biochemical properties could help to distinguish stem cells from primary cells, cancer cells, and differentiated cells. However, the correlated evolution of these properties in the course of directed stem cells differentiation has not been well characterized. In this study, we applied video particle tracking microrheology (VPTM) to measure intracellular viscoelasticity of differentiating human mesenchymal stromal/stem cells (hMSCs). Our results showed that osteogenesis not only increased both elastic and viscous moduli, but also converted the intracellular viscoelasticity of differentiating hMSCs from viscous-like to elastic-like. In contrast, adipogenesis decreased both elastic and viscous moduli while hMSCs remained viscous-like during the differentiation. In conjunction with bio- chemical and physical parameters, such as gene expression profiles, cell morphology, and cytoskeleton arrangement, we demonstrated that VPTM is a unique approach to quantify, with high data throughput, the maturation level of differentiating hMSCs and to anticipate their fate decisions. This approach is well suited for time-lapsed study of the mechanobiology of differentiating stem cells especially in three dimensional physico-chemical biomimetic environments including porous scaffolds.

Two-point particle tracking microrheology of nematic complex fluids

Many biological and technological complex fluids exhibit tight microstructural alignment that confers them nematic mechanical properties. Among these we count liquid crystals and biopolymer networks, which are often available in microscopic amounts. However, current microrheological methods cannot measure the directional viscoelastic coefficients that appear in the constitutive relation of nematic complex fluids. This article presents directional two-point particle-tracking microrheology (D2PTM) - a novel microrheology technique to determine these coefficients. We establish the theoretical foundation for D2PTM by analyzing the motion of a probing microscopic particle embedded in a nematic complex fluid, and the mutual hydrodynamic interactions between pairs of distant particles. From this analysis, we generalize the formulation of two-point particle tracking microrheology for nematic complex fluids, and demonstrate that the new formulation provides sufficient information to fully characterize the anisotropic viscoelastic coefficients of such materials. We test D2PTM by simulating the Brownian motion of particles in nematic viscoelastic fluids with prescribed directional frequency-dependent shear moduli, showing that D2PTM accurately recovers the prescribed shear moduli. Furthermore, we experimentally validate D2PTM by applying it to a lyotropic nematic liquid crystal, and demonstrate that this new microrheology method provides results in agreement with dynamic light scattering measurements. Lastly, we illustrate the experimental application of the new technique to characterize nematic F-actin solutions. These experiments constitute the first microrheological measurement of the directional viscoelastic coefficients of an anisotropic soft material.

Cooperative cell motility during tandem locomotion of amoeboid cells

Tandem pairs of Dictyostelium cells migrate synchronously with an ~54-s time delay between the formation of their frontal protrusions. Each cell establishes two active adhesions, with the trailing cell reusing the location of the adhesions of the leading cell. This coordinated motility is mechanically driven and aided by cell–cell adhesions.

Streams of migratory cells are initiated by the formation of tandem pairs of cells connected head to tail to which other cells subsequently adhere. The mechanisms regulating the transition from single to streaming cell migration remain elusive, although several molecules have been suggested to be involved. In this work, we investigate the mechanics of the locomotion of Dictyostelium tandem pairs by analyzing the spatiotemporal evolution of their traction adhesions (TAs). We find that in migrating wild-type tandem pairs, each cell exerts traction forces on stationary sites (∼80% of the time), and the trailing cell reuses the location of the TAs of the leading cell. Both leading and trailing cells form contractile dipoles and synchronize the formation of new frontal TAs with ∼54-s time delay. Cells not expressing the lectin discoidin I or moving on discoidin I–coated substrata form fewer tandems, but the trailing cell still reuses the locations of the TAs of the leading cell, suggesting that discoidin I is not responsible for a possible chemically driven synchronization process. The migration dynamics of the tandems indicate that their TAs’ reuse results from the mechanical synchronization of the leading and trailing cells’ protrusions and retractions (motility cycles) aided by the cell–cell adhesions.

Focal adhesion kinase-dependent focal adhesion recruitment of SH2 domains directs SRC into focal adhesions to regulate cell adhesion and migration

Directed cell migration requires dynamical control of the protein complex within focal adhesions (FAs) and this control is regulated by signaling events involving tyrosine phosphorylation. We screened the SH2 domains present in tyrosine-specific kinases and phosphatases found within FAs, including SRC, SHP1 and SHP2, and examined whether these enzymes transiently target FAs via their SH2 domains. We found that the SRC_SH2 domain and the SHP2_N-SH2 domain are associated with FAs, but only the SRC_SH2 domain is able to be regulated by focal adhesion kinase (FAK). The FAK-dependent association of the SRC_SH2 domain is necessary and sufficient for SRC FA targeting. When the targeting of SRC into FAs is inhibited, there is significant suppression of SRC-mediated phosphorylation of paxillin and FAK; this results in an inhibition of FA formation and maturation and a reduction in cell migration. This study reveals an association between FAs and the SRC_SH2 domain as well as between FAs and the SHP2_N-SH2 domains. This supports the hypothesis that the FAK-regulated SRC_SH2 domain plays an important role in directing SRC into FAs and that this SRC-mediated FA signaling drives cell migration.

Oscillatory shear stress mediates directional reorganization of actin cytoskeleton and alters differentiation propensity of mesenchymal stem cells

Shear stress stimuli differentially regulate cellular functions based on the pattern, magnitude as well as duration of the flow. Shear stress can modify intracellular kinase activities and cytoskeleton reorganization to result in changes of cell behavior. Mesenchymal stem cells (MSCs) are mechano-sensitive cells, but little is known about the effects of oscillatory shear stress (OS). In this study, we demonstrate that OS of 0.5 ± 4 dyn/cm(2) induces directional reorganization of F-actin to mediate the fate choice of MSCs through the regulation of β-catenin. We also found that intercellular junction molecules are the predominant mechanosensors of OS in MSCs to deliver the signals that result in directional rearrangement of F-actin, as well as the increase of phosphorylated β-catenin (pβ-catenin) after 30 minutes of OS stimulation. Depolymerization of F-actin and increase in pβ-catenin also lead to the upregulation of Wnt inhibitory factors sclerostin and dickkopf-1. Inhibition of β-catenin/Wnt signaling pathway is accompanied by the upregulation of sex determining region Y-box2 and NANOG to control self-renewal. In conclusion, the reorganization of actin cytoskeleton and increase in β-catenin phosphorylation triggered by OS regulate the expression of pluripotency genes via the β-catenin/Wnt signaling pathway to differentially direct fate choices of MSCs at different time points. Results from this study have provided new information regarding how MSCs respond to mechanical cues from their microenvironment in a time-dependent fashion, and such biophysical stimuli could be administered to guide the fate and differentiation of stem cells in addition to conventional biochemical approaches.

EGF augments TGFβ-induced epithelial-mesenchymal transition by promoting SHP2 binding to GAB1

In many epithelial cells, epidermal growth factor (EGF) augments the epithelial-mesenchymal transition (EMT) that occurs when cells are treated with transforming growth factor β (TGFβ). We demonstrate that this augmentation requires activation of SH2 domain-containing phosphatase-2 (SHP2; also known as PTPN11), a proto-oncogene. In lung and pancreatic cancer cell lines, reductions in E-cadherin expression, increases in vimentin expression and increases in cell scatter rates were larger when cells were treated with TGFβ and EGF versus TGFβ or EGF alone. SHP2 knockdown promoted epithelial characteristics basally and antagonized EMT in response to TGFβ alone or in combination with EGF. Whereas EGF promoted SHP2 binding to tyrosine phosphorylated GAB1, which promotes SHP2 activity, TGFβ did not induce SHP2 association with phosphotyrosine-containing proteins. Knockdown of endogenous SHP2 and reconstitution with an SHP2 mutant with impaired phosphotyrosine binding ability eliminated the EGF-mediated EMT augmentation that was otherwise restored with wild-type SHP2 reconstitution. These results demonstrate roles for basal and ligand-induced SHP2 activity in EMT and further motivate efforts to identify specific ways to inhibit SHP2, given the role of EMT in tumor dissemination and chemoresistance.

Dexamethasone-induced cellular tension requires a SGK1-stimulated Sec5-GEF-H1 interaction

Dexamethasone, a synthetic glucocorticoid, is often used to induce osteoblast commitment of mesenchymal stem cells (MSCs), and this process requires RhoA-dependent cellular tension. The underlying mechanism is unclear. In this study, we show that dexamethasone stimulates expression of fibronectin and integrin α5 (ITGA5), accompanied by an increase in the interaction of GEF-H1 (also known as ARHGEF2) with Sec5 (also known as EXOC2), a microtubule (MT)-regulated RhoA activator and a component of the exocyst, respectively. Disruption of this interaction abolishes dexamethasone-induced cellular tension and GEF-H1 targeting to focal adhesion sites at the cell periphery without affecting dexamethasone-induced levels of ITGA5 and fibronectin, and the extracellular deposition of fibronectin at adhesion sites is specifically inhibited. We demonstrate that dexamethasone stimulates the expression of serum-glucocorticoid-induced protein kinase 1 (SGK1), which is necessary and sufficient for the induction of the Sec5-GEF-H1 interaction. Given the function of SGK1 in suppressing MT growth, our data suggest that the induction of SGK1 through treatment with dexamethasone alters MT dynamics to increase Sec5-GEF-H1 interactions, which promote GEF-H1 targeting to adhesion sites. This mechanism is essential for the formation of fibronectin fibrils and their attachment to integrins at adhesion sites in order to generate cellular tension.

T cell activation is reduced by the catalytically inactive form of protein tyrosine phosphatase SHP-2

Src-homology 2 domain-containing tyrosine phosphatase-2 (SHP-2) is a ubiquitously expressed cytosolic tyrosine phosphatase implicated in many different signaling pathways involving cytokine receptors and T and B cell receptors; however, the precise functional role of SHP-2 in T cell signaling is not entirely clear. In this study, we overexpressed a catalytically inactive form of SHP-2 with a classic cysteine 459-to-serine mutation (dnSHP-2) to elucidate the in vivo effects of SHP-2 on T cells. We found that mice overexpressing dnSHP-2 showed reduced T cell activation, presumably due to increased tyrosine phosphorylation of Grb2-binding protein (Gab2) and inhibition of mitogen-activated protein kinase (MAPK) activity. SHP-2 appears to be a positive regulator of the MAPK pathway in T cells, likely through coupling of the multimeric complex to the Ras/MAPK pathway. However, SHP-2 does not appear to affect T cell antigen receptor (TCR)-evoked calcium mobilization, stress-activated protein kinase/c-jun N-terminal kinases (SAPK/JNKs) activation, or overall tyrosine phosphorylation.

GEF-H1 controls focal adhesion signaling that regulates mesenchymal stem cell lineage commitment

Focal adhesions (FAs) undergo maturation that culminates in size and composition changes that modulate adhesion, cytoskeleton remodeling and differentiation. Although it is well recognized that stimuli for osteogenesis of mesenchymal stem cells (MSCs) drive FA maturation, actin organization and stress fiber polarization, the extent to which FA-mediated signals regulated by the FA protein composition specifies MSC commitment remains largely unknown. Here, we demonstrate that, upon dexamethasone (osteogenic induction) treatment, guanine nucleotide exchange factor H1 (GEF-H1, also known as Rho guanine nucleotide exchange factor 2, encoded by ARHGEF2) is significantly enriched in FAs. Perturbation of GEF-H1 inhibits FA formation, anisotropic stress fiber orientation and MSC osteogenesis in an actomyosin-contractility-independent manner. To determine the role of GEF-H1 in MSC osteogenesis, we explore the GEF-H1-modulated FA proteome that reveals non-muscle myosin-II heavy chain-B (NMIIB, also known as myosin-10, encoded by MYH10) as a target of GEF-H1 in FAs. Inhibition of targeting NMIIB into FAs suppresses FA formation, stress fiber polarization, cell stiffness and osteogenic commitments in MSCs. Our data demonstrate a role for FA signaling in specifying MSC commitment.

Protein tyrosine phosphatase PTP1B in cell adhesion and migration

Cell migration requires a highly coordinated interplay between specialized plasma membrane adhesion complexes and the cytoskeleton. Protein phosphorylation/dephosphorylation modifications regulate many aspects of the integrin-cytoskeleton interdependence, including their coupling, dynamics, and organization to support cell movement. The endoplasmic reticulum-bound protein tyrosine phosphatase PTP1B has been implicated as a regulator of cell adhesion and migration. Recent results from our laboratory shed light on potential mechanisms, such as Src/FAK signaling through Rho GTPases and integrin-cytoskeletal coupling.