The likely role of proteolytic enzymes in unwanted differentiation of stem cells in culture

Tissue engineering aims at developing the necessary technological strategies for replacement or regeneration tissues. However, the number of cells required is much greater than the number obtained from a cell source. Expanding the cells' number in cell culture for a long period is required until the necessary amount of cells is obtained. While in culture, cells often go unwanted differentiation. Little attention has been given to the use of proteolytic enzymes in cell passage. Review the importance of extracellular matrix and surface proteins for cell behavior and the possible mechanisms of cellular changes that may occur due to the use of proteolytic enzymes is an essential issue. Possible alternative to replace these enzymes in cell passage has also been developed.

Connections between cadherin-catenin proteins, spindle misorientation, and cancer

Cadherin-catenin mediated adhesion is an important determinant of tissue architecture in multicellular organisms. Cancer progression and maintenance is frequently associated with loss of their expression or functional activity, which not only leads to decreased cell-cell adhesion, but also to enhanced tumor cell proliferation and loss of differentiated characteristics. This review is focused on the emerging implications of cadherin-catenin proteins in the regulation of polarized divisions through their connections with the centrosomes, cytoskeleton, tissue tension and signaling pathways; and illustrates how alterations in cadherin-catenin levels or functional activity may render cells susceptible to transformation through the loss of their proliferation-differentiation balance.

Phosphoregulation of the C. elegans cadherin-catenin complex

Adherens junctions play key roles in mediating cell-cell contacts during tissue development. In Caenorhabditis elegans embryos, the cadherin-catenin complex (CCC), composed of the classical cadherin HMR-1 and members of three catenin families, HMP-1, HMP-2 and JAC-1, is necessary for normal blastomere adhesion, gastrulation, ventral enclosure of the epidermis and embryo elongation. Disruption of CCC assembly or function results in embryonic lethality. Previous work suggests that components of the CCC are subject to phosphorylation. However, the identity of phosphorylated residues in CCC components and their contributions to CCC stability and function in a living organism remain speculative. Using mass spectrometry, we systematically identify phosphorylated residues in the essential CCC subunits HMR-1, HMP-1 and HMP-2 in vivo. We demonstrate that HMR-1/cadherin phosphorylation occurs on three sites within its β-catenin binding domain that each contributes to CCC assembly on lipid bilayers. In contrast, phosphorylation of HMP-2/β-catenin inhibits its association with HMR-1/cadherin in vitro, suggesting a role in CCC disassembly. Although HMP-1/α-catenin is also phosphorylated in vivo, phosphomimetic mutations do not affect its ability to associate with other CCC components or interact with actin in vitro. Collectively, our findings support a model in which distinct phosphorylation events contribute to rapid CCC assembly and disassembly, both of which are essential for morphogenetic rearrangements during development.

Molecular control of stress transmission in the microtubule cytoskeleton

In this article, we will summarize recent progress in understanding the mechanical origins of rigidity, strength, resiliency and stress transmission in the MT cytoskeleton using reconstituted networks formed from purified components. We focus on the role of network architecture, crosslinker compliance and dynamics, and molecular determinants of single filament elasticity, while highlighting open questions and future directions for this work.

Mechanobiology of mesenchymal stem cells: Perspective into mechanical induction of MSC fate

Bone marrow-derived mesenchymal stem and stromal cells (MSCs) are promising candidates for cell-based therapies in diverse conditions including tissue engineering. Advancement of these therapies relies on the ability to direct MSCs toward specific cell phenotypes. Despite identification of applied forces that affect self-maintenance, proliferation, and differentiation of MSCs, mechanisms underlying the integration of mechanically induced signaling cascades and interpretation of mechanical signals by MSCs remain elusive. During the past decade, many researchers have demonstrated that external applied forces can activate osteogenic signaling pathways in MSCs, including Wnt, Ror2, and Runx2. Besides, recent advances have highlighted the critical role of internal forces due to cell-matrix interaction in MSC function. These internal forces can be achieved by the materials that cells reside in through its mechanical properties, such as rigidity, topography, degradability, and substrate patterning. MSCs can generate contractile forces to sense these mechanical properties and thereby perceive mechanical information that directs broad aspects of MSC functions, including lineage commitment. Although many signaling pathways have been elucidated in material-induced lineage specification of MSCs, discovering the mechanisms by which MSCs respond to such cell-generated forces is still challenging because of the highly intricate signaling milieu present in MSC environment. However, bioengineers are bridging this gap by developing platforms to control mechanical cues with improved throughput and precision, thereby enabling further investigation of mechanically induced MSC functions. In this review, we discuss the most recent advances that how applied forces and cell-generated forces may be engineered to determine MSC fate, and overview a subset of the operative signal transduction mechanisms and experimental platforms that have emerged in MSC mechanobiology research. Our main goal is to provide an up-to-date view of MSC mechanobiology that is relevant to both mechanical loading and mechanical properties of the environment, and introduce these emerging platforms for tissue engineering use.

Integration of actin dynamics and cell adhesion by a three-dimensional, mechanosensitive molecular clutch

During cell migration, the forces generated in the actin cytoskeleton are transmitted across transmembrane receptors to the extracellular matrix or other cells through a series of mechanosensitive, regulable protein-protein interactions termed the molecular clutch. In integrin-based focal adhesions, the proteins forming this linkage are organized into a conserved three-dimensional nano-architecture. Here we discuss how the physical interactions between the actin cytoskeleton and focal-adhesion-associated molecules mediate force transmission from the molecular clutch to the extracellular matrix.

Structural Determinants of the Mechanical Stability of α-Catenin

α-Catenin plays a crucial role in cadherin-mediated adhesion by binding to β-catenin, F-actin, and vinculin, and its dysfunction is linked to a variety of cancers and developmental disorders. As a mechanotransducer in the cadherin complex at intercellular adhesions, mechanical and force-sensing properties of α-catenin are critical to its proper function. Biochemical data suggest that α-catenin adopts an autoinhibitory conformation, in the absence of junctional tension, and biophysical studies have shown that α-catenin is activated in a tension-dependent manner that in turn results in the recruitment of vinculin to strengthen the cadherin complex/F-actin linkage. However, the molecular switch mechanism from autoinhibited to the activated state remains unknown for α-catenin. Here, based on the results of an aggregate of 3 μs of molecular dynamics simulations, we have identified a dynamic salt-bridge network within the core M region of α-catenin that may be the structural determinant of the stability of the autoinhibitory conformation. According to our constant-force steered molecular dynamics simulations, the reorientation of the MII/MIII subdomains under force may constitute an initial step along the transition pathway. The simulations also suggest that the vinculin-binding domain (subdomain MI) is intrinsically much less stable than the other two subdomains in the M region (MII and MIII). Our findings reveal several key insights toward a complete understanding of the multistaged, force-induced conformational transition of α-catenin to the activated conformation.

Mechanosensitive Adaptation of E-Cadherin Turnover across adherens Junctions

In the natural and technological world, multi-agent systems strongly depend on how the interactions are ruled between their individual components, and the proper control of time-scales and synchronization is a key issue. This certainly applies to living tissues when multicellular assemblies such as epithelial cells achieve complex morphogenetic processes. In epithelia, because cells are known to individually generate actomyosin contractile stress, each individual intercellular adhesive junction line is subjected to the opposed stresses independently generated by its two partner cells. Contact lines should thus move unless their two partner cells mechanically match. The geometric homeostasis of mature epithelia observed at short enough time-scale thus raises the problem to understand how cells, if considered as noisy individual actuators, do adapt across individual intercellular contacts to locally balance their time-average contractile stress. Structural components of adherens junctions, cytoskeleton (F-actin) and homophilic bonds (E-cadherin) are quickly renewed at steady-state. These turnovers, if they depend on forces exerted at contacts, may play a key role in the mechanical adaptation of epithelia. Here we focus on E-cadherin as a force transducer, and we study the local regulation and the mechanosensitivity of its turnover in junctions. We show that E-cadherin turnover rates match remarkably well on either side of mature intercellular contacts, despite the fact that they exhibit large fluctuations in time and variations from one junction to another. Using local mechanical and biochemical perturbations, we find faster turnover rates with increased tension, and asymmetric rates at unbalanced junctions. Together, the observations that E-cadherin turnover, and its local symmetry or asymmetry at each side of the junction, are mechanosensitive, support the hypothesis that E-cadherin turnover could be involved in mechanical homeostasis of epithelia.

Mechanobiology of myofibroblast adhesion in fibrotic cardiac disease

Fibrotic cardiac disease, a leading cause of death worldwide, manifests as substantial loss of function following maladaptive tissue remodeling. Fibrosis can affect both the heart valves and the myocardium and is characterized by the activation of fibroblasts and accumulation of extracellular matrix. Valvular interstitial cells and cardiac fibroblasts, the cell types responsible for maintenance of cardiac extracellular matrix, are sensitive to changing mechanical environments, and their ability to sense and respond to mechanical forces determines both normal development and the progression of disease. Recent studies have uncovered specific adhesion proteins and mechano-sensitive signaling pathways that contribute to the progression of fibrosis. Integrins form adhesions with the extracellular matrix, and respond to changes in substrate stiffness and extracellular matrix composition. Cadherins mechanically link neighboring cells and are likely to contribute to fibrotic disease propagation. Finally, transition to the active myofibroblast phenotype leads to maladaptive tissue remodeling and enhanced mechanotransductive signaling, forming a positive feedback loop that contributes to heart failure. This Commentary summarizes recent findings on the role of mechanotransduction through integrins and cadherins to perpetuate mechanically induced differentiation and fibrosis in the context of cardiac disease.

The mechanotransduction machinery at work at adherens junctions

The shaping of a multicellular body, and the maintenance and repair of adult tissues require fine-tuning of cell adhesion responses and the transmission of mechanical load between the cell, its neighbors and the underlying extracellular matrix. A growing field of research is focused on how single cells sense mechanical properties of their micro-environment (extracellular matrix, other cells), and on how mechanotransduction pathways affect cell shape, migration, survival as well as differentiation. Within multicellular assemblies, the mechanical load imposed by the physical properties of the environment is transmitted to neighboring cells. Force imbalance at cell-cell contacts induces essential morphogenetic processes such as cell-cell junction remodeling, cell polarization and migration, cell extrusion and cell intercalation. However, how cells respond and adapt to the mechanical properties of neighboring cells, transmit forces, and transform mechanical signals into chemical signals remain open questions. A defining feature of compact tissues is adhesion between cells at the specialized adherens junction (AJ) involving the cadherin super-family of Ca(2+)-dependent cell-cell adhesion proteins (e.g., E-cadherin in epithelia). Cadherins bind to the cytoplasmic protein β-catenin, which in turn binds to the filamentous (F)-actin binding adaptor protein α-catenin, which can also recruit vinculin, making the mechanical connection between cell-cell adhesion proteins and the contractile actomyosin cytoskeleton. The cadherin-catenin adhesion complex is a key component of the AJ, and contributes to cell assembly stability and dynamic cell movements. It has also emerged as the main route of propagation of forces within epithelial and non-epithelial tissues. Here, we discuss recent molecular studies that point toward force-dependent conformational changes in α-catenin that regulate protein interactions in the cadherin-catenin adhesion complex, and show that α-catenin is the core mechanosensor that allows cells to locally sense, transduce and adapt to environmental mechanical constrains.

Molecular Mechanoneurobiology: An Emerging Angle to Explore Neural Synaptic Functions

Neural synapses are intercellular asymmetrical junctions that transmit biochemical and biophysical information between a neuron and a target cell. They are very tight, dynamic, and well organized by many synaptic adhesion molecules, signaling receptors, ion channels, and their associated cytoskeleton that bear forces. Mechanical forces have been an emerging factor in regulating axon guidance and growth, synapse formation and plasticity in physiological and pathological brain activity. Therefore, mechanical forces are undoubtedly exerted on those synaptic molecules and modulate their functions. Here we review current progress on how mechanical forces regulate receptor-ligand interactions, protein conformations, ion channels activation, and cytoskeleton dynamics and discuss how these regulations potentially affect synapse formation, stabilization, and plasticity.

Lamins at the crossroads of mechanosignaling

The intermediate filament proteins, A- and B-type lamins, form the nuclear lamina scaffold adjacent to the inner nuclear membrane. Lamins also contribute to chromatin regulation and various signaling pathways affecting gene expression. In this review, Osmanagic-Myers et al. focus on the role of nuclear lamins in mechanosensing and also discuss how disease-linked lamin mutants may impair the response of cells to mechanical stimuli and influence the properties of the extracellular matrix.

The intermediate filament proteins, A- and B-type lamins, form the nuclear lamina scaffold adjacent to the inner nuclear membrane. B-type lamins confer elasticity, while A-type lamins lend viscosity and stiffness to nuclei. Lamins also contribute to chromatin regulation and various signaling pathways affecting gene expression. The mechanical roles of lamins and their functions in gene regulation are often viewed as independent activities, but recent findings suggest a highly cross-linked and interdependent regulation of these different functions, particularly in mechanosignaling. In this newly emerging concept, lamins act as a “mechanostat” that senses forces from outside and responds to tension by reinforcing the cytoskeleton and the extracellular matrix. A-type lamins, emerin, and the linker of the nucleoskeleton and cytoskeleton (LINC) complex directly transmit forces from the extracellular matrix into the nucleus. These mechanical forces lead to changes in the molecular structure, modification, and assembly state of A-type lamins. This in turn activates a tension-induced “inside-out signaling” through which the nucleus feeds back to the cytoskeleton and the extracellular matrix to balance outside and inside forces. These functions regulate differentiation and may be impaired in lamin-linked diseases, leading to cellular phenotypes, particularly in mechanical load-bearing tissues.

Dynamic visualization of α-catenin reveals rapid, reversible conformation switching between tension states

The cytosolic protein α-catenin is a postulated force transducer at cadherin complexes. The demonstration of force activation, identification of consequent downstream events in live cells, and development of tools to study these dynamic processes in living cells are central to elucidating the role of α-catenin in cellular mechanics and tissue function. Here we demonstrate that α-catenin is a force-activatable mechanotransducer at cell-cell junctions by using an engineered α-catenin conformation sensor based on fluorescence resonance energy transfer (FRET). This sensor reconstitutes α-catenin-dependent functions in α-catenin-depleted cells and recapitulates the behavior of the endogenous protein. Dynamic imaging of cells expressing the sensor demonstrated that α-catenin undergoes immediate, reversible conformation switching in direct response to different mechanical perturbations of cadherin adhesions. Combined magnetic twisting cytometry with dynamic FRET imaging revealed rapid, local conformation switching upon the mechanical stimulation of specific cadherin bonds. At acutely stretched cell-cell junctions, the immediate, reversible conformation change further reveals that α-catenin behaves like an elastic spring in series with cadherin and actin. The force-dependent recruitment of vinculin—a principal α-catenin effector—to junctions requires the vinculin binding site of the α-catenin sensor. In cells, the relative rates of force-dependent α-catenin conformation switching and vinculin recruitment reveal that α-catenin activation and vinculin recruitment occur sequentially, rather than in a concerted process, with vinculin accumulation being significantly slower. This engineered α-catenin sensor revealed that α-catenin is a reversible, stretch-activatable sensor that mechanically links cadherin complexes and actin and is an indispensable player in cadherin-specific mechanotransduction at intercellular junctions.

Polycystin-1 and Gα12 regulate the cleavage of E-cadherin in kidney epithelial cells

Interaction of polycystin-1 (PC1) and Gα12 is important for development of kidney cysts in autosomal dominant polycystic kidney disease (ADPKD). The integrity of cell polarity and cell-cell adhesions (mainly E-cadherin-mediated adherens junction) is altered in the renal epithelial cells of ADPKD. However, the key signaling pathway for this alteration is not fully understood. Madin-Darby canine kidney (MDCK) cells maintain the normal integrity of epithelial cell polarity and adherens junctions. Here, we found that deletion of Pkd1 increased activation of Gα12, which then promoted the cystogenesis of MDCK cells. The morphology of these cells was altered after the activation of Gα12. By using liquid chromatography-mass spectrometry, we found several proteins that could be related this change in the extracellular milieu. E-cadherin was one of the most abundant peptides after active Gα12 was induced. Gα12 activation or Pkd1 deletion increased the shedding of E-cadherin, which was mediated via increased ADAM10 activity. The increased shedding of E-cadherin was blocked by knockdown of ADAM10 or specific ADAM10 inhibitor GI254023X. Pkd1 deletion or Gα12 activation also changed the distribution of E-cadherin in kidney epithelial cells and caused β-catenin to shift from cell membrane to nucleus. Finally, ADAM10 inhibitor, GI254023X, blocked the cystogenesis induced by PC1 knockdown or Gα12 activation in renal epithelial cells. Our results demonstrate that the E-cadherin/β-catenin signaling pathway is regulated by PC1 and Gα12 via ADAM10. Specific inhibition of this pathway, especially ADAM10 activity, could be a novel therapeutic regimen for ADPKD.

Force transmission at cell-cell and cell-matrix adhesions

All cells are subjected to mechanical forces throughout their lifetimes. These forces are sensed by cell surface adhesion receptors and trigger robust actin cytoskeletal rearrangements and growth of the associated adhesion complex to counter the applied force. In this review, we discuss how integrins and cadherins sense force and transmit these forces into the cell interior. We focus on the complement of proteins each adhesion complex recruits to bear the force and the signal transduction pathways activated to allow the cell to tune its contractility. A discussion of the similarities, differences, and crosstalk between cadherin- and integrin-mediated force transmission is also presented.