Mechanical Cues Direct Focal Adhesion Dynamics

Designing Biomimetic Strain-Stiffening into Synthetic Hydrogels

Biological tissues are mechanoresponsive; that is, their properties dynamically change in response to mechanical stimuli. For example, in response to shear or elongational strain, collagen, fibrin, actin, and other filamentous biomaterials undergo dramatic strain-stiffening. Above a critical strain, their stiffness increases over orders of magnitude. While it is widely accepted that the stiffness of biological tissues impacts cell phenotype and several diseases, the biological impact of strain-stiffening remains understudied. Synthetic hydrogels that mimic the mechanoresponsive nature of biological tissues could serve as an in vitro platform for these studies. This review highlights recent efforts to mimic the strain-stiffening behavior of biological materials in synthetic hydrogels. We discuss the design principles for imparting synthetic hydrogels with biomimetic strain-stiffening, critically compare designs of strain-stiffening hydrogels that have been reported thus far, and discuss their use as in vitro platforms to probe how strain-stiffening impacts cell behavior, diseases, and other biological processes.

Temporal regulation of BMP2 growth factor signaling in response to mechanical loading is linked to cytoskeletal and focal adhesion remodeling

Biophysical cues have the ability to enhance cellular signaling response to Bone Morphogenetic Proteins, an essential growth factor during bone development and regeneration. Yet, therapeutic application of Bone Morphogenetic Protein 2 (BMP2) is restricted due to uncontrolled side effects. An understanding of the temporal characteristics of mechanically regulated signaling events and underlying mechanism is lacking. Using a 3D bioreactor system in combination with a soft macroporous biomaterial substrate, we mimic the in vivo environment that BMP2 is acting in. We show that the intensity and duration of BMP2 signaling increases with increasing loading frequency in synchrony with the number and size of focal adhesions. Long-term mechanical stimulation increases the expression of BMP receptor type 1B, specific integrin subtypes and integrin clustering. Together, this triggered a short-lived mechanical echo that enhanced BMP2 signaling even when BMP2 is administered directly after mechanical stimulation, but not when it is applied after a resting period of ≥30 min. Interfering with cytoskeletal remodeling hinders focal adhesion remodeling verifying its critical role in shifting cells into a state of high BMP2 responsiveness. The design of biomaterials that exploit this potential locally at the site of injury will help to overcome current limitations of clinical growth factor treatment.

Actin alpha 2, smooth muscle, a transforming growth factor-β1-induced factor, regulates collagen production in human periodontal ligament cells via Smad2/3 pathway

Background/purpose

Actin alpha 2, smooth muscle (ACTA2) is an actin isoform that forms the cytoskeleton. Actin plays a crucial role in numerous cellular functions. ACTA2 is a marker of functional periodontal ligament (PDL) fibroblasts and is upregulated by transforming growth factor-β1 (TGF-β1); however, the underlying function of ACTA2 in PDL tissue is unknown. We aimed to examine the localization and potential function of ACTA2 in PDL tissues and cells.

Materials and methods

RNA expression was determined using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) and quantitative RT-PCR. Protein expression was determined using immunofluorescence staining and Western blot analysis. Soluble and insoluble collagen production was examined using the Sircol collagen assay and picrosirius red staining, respectively. Small interfering RNA (siRNA) was used for knockdown assay to examine the effect of ACTA2 in human PDL cells.

Results

ACTA2 expression was observed in human primary PDL cells and PDL cell line (2-23 cells). TGF-β1 upregulated ACTA2, collagen type Ⅰ alpha1 chain (COL1A1), periostin (POSTN), and fibrillin-Ⅰ(FBN1) expression and soluble and insoluble collagen production in 2-23 cells. However, ACTA2 depletion by siRNA strongly suppressed PDL-related gene expression and collagen production compared with those of TGF-β1-stimulated control cells. Furthermore, ACTA2 knockdown significantly suppressed the phosphorylation of Smad2 and Smad3.

Conclusion

ACTA2 plays a crucial role in PDL-related marker expression and collagen production via Smad2/3 phosphorylation. Our findings might contribute to the development of novel and effective periodontal therapies.

Regulation of cell attachment, spreading, and migration by hydrogel substrates with independently tunable mesh size

Hydrogels are widely used as substrates to investigate interactions between cells and their microenvironment as they mimic many attributes of the extracellular matrix. The stiffness of hydrogels is an important property that is known to regulate cell behavior. Beside stiffness, cells also respond to structural cues such as mesh size. However, since the mesh size of hydrogel is intrinsically coupled to its stiffness, its role in regulating cell behavior has never been independently investigated. Here, we report a hydrogel system whose mesh size and stiffness can be independently controlled. Cell behavior, including spreading, migration, and formation of focal adhesions is significantly altered on hydrogels with different mesh sizes but with the same stiffness. At the transcriptional level, hydrogel mesh size affects cellular mechanotransduction by regulating nuclear translocation of yes-associated protein. These findings demonstrate that the mesh size of a hydrogel plays an important role in cell-substrate interactions. STATEMENT OF SIGNIFICANCE: Hydrogels are ideal platforms with which to investigate interactions between cells and their microenvironment as they mimic many physical properties of the extracellular matrix. However, the mesh size of hydrogels is intrinsically coupled to their stiffness, making it challenging to investigate the contribution of mesh size to cell behavior. In this work, we use hydrogel-on-glass substrates with defined thicknesses whose stiffness and mesh size can be independently tuned. We use these substrates to isolate the effects of mesh size on cell behavior, including attachment, spreading, migration, focal adhesion formation and YAP localization in the nucleus. Our results show that mesh size has significant, yet often overlooked, effects, on cell behavior, and contribute to a further understanding of cell-substrate interactions.

Functional Coordination among the Golgi Complex, the Centrosome and the Microtubule Cytoskeleton during the Cell Cycle

The Golgi complex of mammalian cells is organized in a ribbon-like structure often closely associated with the centrosome during interphase. Conversely, the Golgi complex assumes a fragmented and dispersed configuration away from the centrosome during mitosis. The structure of the Golgi complex and the relative position to the centrosome are dynamically regulated by microtubules. Many pieces of evidence reveal that this microtubule-mediated dynamic association between the Golgi complex and centrosome is of functional significance in cell polarization and division. Here, we summarize findings indicating how the Golgi complex and the centrosome cooperate in organizing the microtubule network for the directional protein transport and centrosome positioning required for cell polarization and regulating fundamental cell division processes.

Focal adhesion dynamics in cellular function and disease

Acting as a bridge between the cytoskeleton of the cell and the extra cellular matrix (ECM), the cell-ECM adhesions with integrins at their core, play a major role in cell signalling to direct mechanotransduction, cell migration, cell cycle progression, proliferation, differentiation, growth and repair. Biochemically, these adhesions are composed of diverse, yet an organised group of structural proteins, receptors, adaptors, various enzymes including protein kinases, phosphatases, GTPases, proteases, etc. as well as scaffolding molecules. The major integrin adhesion complexes (IACs) characterised are focal adhesions (FAs), invadosomes (podosomes and invadopodia), hemidesmosomes (HDs) and reticular adhesions (RAs). The varied composition and regulation of the IACs and their signalling, apart from being an integral part of normal cell survival, has been shown to be of paramount importance in various developmental and pathological processes. This review per-illustrates the recent advancements in the research of IACs, their crucial roles in normal as well as diseased states. We have also touched on few of the various methods that have been developed over the years to visualise IACs, measure the forces they exert and study their signalling and molecular composition. Having such pertinent roles in the context of various pathologies, these IACs need to be understood and studied to develop therapeutical targets. We have given an update to the studies done in recent years and described various techniques which have been applied to study these structures, thereby, providing context in furthering research with respect to IAC targeted therapeutics.

A Novel Electroporation System for Living Cell Staining and Membrane Dynamics Interrogation

A novel electroporation system was developed to introduce transient membrane pores to cells in a spatially and temporally controlled manner, allowing us to achieve fast electrotransfection and live cell staining as well as to systematically interrogate the dynamics of the cell membrane. Specifically, using this platform, we showed that both reversible and irreversible electroporation could be induced in the cell population, with nano-sized membrane pores in the former case being able to self-reseal in ~10 min. In addition, green fluorescent protein(GFP)-vinculin plasmid and 543 phalloidin have been delivered successively into fibroblast cells, which enables us to monitor the distinct roles of vinculin and F-actin in cell adhesion and migration as well as their possible interplay during these processes. Compared to conventional bulk electroporation and staining methods, the new system offers advantages such as low-voltage operation, cellular level manipulation and testing, fast and adjustable transfection/staining and real-time monitoring; the new system therefore could be useful in different biophysical studies in the future.

The role of microtubules in secretory protein transport

Microtubules are part of the dynamic cytoskeleton network and composed of tubulin dimers. They are the main tracks used in cells to organize organelle positioning and trafficking of cargos. In this Review, we compile recent findings on the involvement of microtubules in anterograde protein transport. First, we highlight the importance of microtubules in organelle positioning. Second, we discuss the involvement of microtubules within different trafficking steps, in particular between the endoplasmic reticulum and the Golgi complex, traffic through the Golgi complex itself and in post-Golgi processes. A large number of studies have assessed the involvement of microtubules in transport of cargo from the Golgi complex to the cell surface. We focus here on the role of kinesin motor proteins and protein interactions in post-Golgi transport, as well as the impact of tubulin post-translational modifications. Last, in light of recent findings, we highlight the role microtubules have in exocytosis, the final step of secretory protein transport, occurring close to focal adhesions.

Hydrogels with enhanced protein conjugation efficiency reveal stiffness-induced YAP localization in stem cells depends on biochemical cues

Polyacrylamide hydrogels have been widely used in stem cell mechanotransduction studies. Conventional conjugation methods of biochemical cues to polyacrylamide hydrogels suffer from low conjugation efficiency, which leads to poor attachment of human pluripotent stem cells (hPSCs) on soft substrates. In addition, while it is well-established that stiffness-dependent regulation of stem cell fate requires cytoskeletal tension, and is mediated through nuclear translocation of transcription regulator, Yes-associated protein (YAP), the role of biochemical cues in stiffness-dependent YAP regulation remains largely unknown. Here we report a method that enhances the conjugation efficiency of biochemical cues on polyacrylamide hydrogels compared to conventional methods. This modified method enables robust hPSC attachment, proliferation and maintenance of pluripotency across varying substrate stiffness (3 kPa-38 kPa). Using this hydrogel platform, we demonstrate that varying the types of biochemical cues (Matrigel, laminin, GAG-peptide) or density of Matrigel can alter stiffness-dependent YAP localization in hPSCs. In particular, we show that stiffness-dependent YAP localization is overridden at low or high density of Matrigel. Furthermore, human mesenchymal stem cells display stiffness-dependent YAP localization only at intermediate fibronectin density. The hydrogel platform with enhanced conjugation efficiency of biochemical cues provides a powerful tool for uncovering the role of biochemical cues in regulating mechanotransduction of various stem cell types.

Structural basis of the target-binding mode of the G protein-coupled receptor kinase-interacting protein in the regulation of focal adhesion dynamics

Focal adhesions (FAs) are specialized sites where intracellular cytoskeleton elements connect to the extracellular matrix and thereby control cell motility. FA assembly depends on various scaffold proteins, including the G protein-coupled receptor kinase-interacting protein 1 (GIT1), paxillin, and liprin-α. Although liprin-α and paxillin are known to competitively interact with GIT1, the molecular basis governing these interactions remains elusive. To uncover the underlying mechanisms of how GIT1 is involved in FA assembly by alternatively binding to liprin-α and paxillin, here we solved the crystal structures of GIT1 in complex with liprin-α and paxillin at 1.8 and 2.6 Å resolutions, respectively. These structures revealed that the paxillin-binding domain (PBD) of GIT1 employs distinct binding modes to recognize a single α-helix of liprin-α and the LD4 motif of paxillin. Structure-based design of protein variants produced two binding-deficient GIT1 variants; specifically, these variants lost the ability to interact with liprin-α only or with both liprin-α and paxillin. Expressing the GIT1 variants in COS7 cells, we discovered that the two PBD-meditated interactions play different roles in either recruiting GIT1 to FA or facilitating FA assembly. Additionally, we demonstrate that, unlike for the known binding mode of the FAT domain to LD motifs, the PBD of GIT1 uses different surface patches to achieve high selectivity in LD motif recognition. In summary, our results have uncovered the mechanisms by which GIT1's PBD recognizes cognate paxillin and liprin-α structures, information we anticipate will be useful for future investigations of GIT1-protein interactions in cells.

Live cell imaging reveals focal adhesions mechanoresponses in mammary epithelial cells under sustained equibiaxial stress

Mechanical stimuli play a key role in many cell functions such as proliferation, differentiation and migration. In the mammary gland, mechanical signals such as the distension of mammary epithelial cells due to udder filling are proposed to be directly involved during lactation and involution. However, the evolution of focal adhesions -specialized multiprotein complexes that mechanically connect cells with the extracellular matrix- during the mammary gland development, as well as the influence of the mechanical stimuli involved, remains unclear. Here we present the use of an equibiaxial stretching device for exerting a sustained normal strain to mammary epithelial cells while quantitatively assessing cell responses by fluorescence imaging techniques. Using this approach, we explored changes in focal adhesion dynamics in HC11 mammary cells in response to a mechanical sustained stress, which resembles the physiological stimuli. We studied the relationship between a global stress and focal adhesion assembly/disassembly, observing an enhanced persistency of focal adhesions under strain as well as an increase in their size. At a molecular level, we evaluated the mechanoresponses of vinculin and zyxin, two focal adhesion proteins postulated as mechanosensors, observing an increment in vinculin molecular tension and a slower zyxin dynamics while increasing the applied normal strain.

Lower fluidity of supported lipid bilayers promotes neuronal differentiation of neural stem cells by enhancing focal adhesion formation

Extensive studies have been performed to understand how the mechanical properties of a stem cell's microenvironment influence its behaviors. Supported lipid bilayers (SLBs), a well-known biomimetic platform, have been used to mimic the dynamic characteristics of the extracellular matrix (ECM) because of their fluidity. However, the effect of the fluidity of SLBs on stem cell fate is unknown. We constructed SLBs with different fluidities to explore the influence of fluidity on the differentiation of neural stem cells (NSCs). The results showed that the behavior of NSCs was highly dependent on the fluidity of SLBs. Low fluidity resulted in enhanced focal adhesion formation, a dense network of stress fibers, stretched and elongated cellular morphology and increased neuronal differentiation, while high fluidity led to less focal adhesion formation, immature stress fibers, round cellular morphology and more astrocyte differentiation. Mechanistic studies revealed that low fluidity may have enhanced focal adhesion formation, which activated FAK-MEK/ERK signaling pathways and ultimately promoted neuronal differentiation of NSCs. This work provides a strategy for manipulating the dynamic matrix surface for the development of culture substrates and tissue-engineered scaffolds, which may aid the understanding of how the dynamic ECM influences stem cell behaviors as well as improve the efficacy of stem cell applications.

"Feeling the force" in reproduction: Mechanotransduction in reproductive processes

Reproductive biologists are well-versed in many types of biochemical signaling, and indeed, there are almost innumerable examples in reproduction, including steroid and peptide hormone signaling, receptor-ligand and secondary messenger-mediated signaling, signaling regulated by membrane channels, and many others. Among reproductive scientists, a perhaps lesser-known but comparably important mode of signaling is mechanotransduction: the concept that cells can sense and respond to externally applied or internally generated mechanical forces. Given the cell shape changes and tissue morphogenesis events that are components of many phenomena in reproductive function, it should be no surprise that mechanotransduction has major impacts in reproductive health and pathophysiology. The conference on "Mechanotransduction in the Reproductive Tract" was a valuable launch pad to bring this hot issue in development, cell biology, biophysics, and tissue regeneration to the realm of reproductive biology. The goal of the meeting was to stimulate interest and increased mechanotransduction research in the reproductive field by presenting a broad spectrum of responses impacted by this process. The meeting highlighted the importance of convening expert investigators, students, fellows, and young investigators from a number of research areas resulting in cross-fertilization of ideas and suggested new avenues for study. The conference included talks on tissue engineering, stem cells, and several areas of reproductive biology, from uterus and cervix to the gametes. Specific reproductive health-relevant areas, including uterine fibroids, gestation and parturition, and breast tissue morphogenesis, received particular attention.

The many ways adherent cells respond to applied stretch

Cells in various tissues are subjected to mechanical stress and strain that have profound effects on cell architecture and function. The specific response of the cell to applied strain depends on multiple factors, including cell contractility, spatial and temporal strain pattern, and substrate dimensionality and rigidity. Recent work has demonstrated that the cell response to applied strain depends on a complex combination of these factors, but the way these factors interact to elicit a specific response is not intuitive. We submit that an understanding of the integrated response of a cell to these factors will provide new insight into mechanobiology and contribute to the effective design of deformable engineered scaffolds meant to provide appropriate mechanical cues to the resident cells.

Mechanotransduction Mechanisms for Intraventricular Diastolic Vortex Forces and Myocardial Deformations: Part 2

Epigenetic mechanisms are fundamental in cardiac adaptations, remodeling, reverse remodeling, and disease. A primary goal of translational cardiovascular research is recognizing whether disease-related changes in phenotype can be averted by eliminating or reducing the effects of environmental epigenetic risks. There may be significant medical benefits in using gene-by-environment interaction knowledge to prevent or reverse organ abnormalities and disease. This survey proposes that "environmental" forces associated with diastolic RV/LV rotatory flows exert important, albeit still unappreciated, epigenetic actions influencing functional and morphological cardiac adaptations. Mechanisms analogous to Murray's law of hydrodynamic shear-induced endothelial cell modulation of vascular geometry are likely to link diastolic vortex-associated shear, torque and "squeeze" forces to RV/LV adaptations. The time has come to explore a new paradigm in which such forces play a fundamental epigenetic role, and to work out how heart cells react to them. Findings from various imaging modalities, computational fluid dynamics, molecular cell biology and cytomechanics are considered. The following are examined, among others: structural dynamics of myocardial cells (endocardium, cardiomyocytes, and fibroblasts), cytoskeleton, nucleoskeleton, and extracellular matrix; mechanotransduction and signaling; and mechanical epigenetic influences on genetic expression. To help integrate and focus relevant pluridisciplinary research, rotatory RV/LV filling flow is placed within a working context that has a cytomechanics perspective. This new frontier in cardiac research should uncover versatile mechanistic insights linking filling vortex patterns and attendant forces to variable expressions of gene regulation in RV/LV myocardium. In due course, it should reveal intrinsic homeostatic arrangements that support ventricular myocardial function and adaptability.