Stretch Axis Research Articles

Strain-dependent dynamic re-alignment of collagen fibers in skeletal muscle extracellular matrix

Collagen fiber architecture within the skeletal muscle extracellular matrix (ECM) is significant to passive muscle mechanics. While it is thought that collagen fibers re-orient themselves in response to changes in muscle length, this has not been dynamically visualized and quantified within a muscle. The goal of this study was to measure changes in collagen alignment across a range of muscle lengths and compare the corresponding alignment to muscle mechanics. We hypothesized that collagen fibers dynamically increase alignment in response to muscle stretching, and this change in alignment is related to passive muscle stiffness. Further, we hypothesized that digesting collagen fibers with collagenase would reduce the re-alignment response to muscle stretching. Using DBA/2J and D2.mdx mice, we isolated extensor digitorum longus (EDL), soleus, and diaphragm muscles for collagenase or sham treatment and decellularization to isolate intact or collagenase-digested decellularized muscles (DCMs). These DCMs were mechanically tested and imaged using second harmonic generation microscopy to measure collagen alignment across a range of strains. We found that collagen alignment increased in a strain-dependent fashion, but collagenase did not significantly affect the strain-dependent change in alignment. We also saw that the collagen fibers in the diaphragm epimysium (surface ECM) and perimysium (deep ECM) started at different angles, but still re-oriented in the same direction in response to stretching. These robust changes in collagen alignment were weakly related to passive DCM stiffness. Overall, we demonstrated that the architecture of muscle ECM is dynamic in response to strain and is related to passive muscle mechanics. Statement of significanceOur study presents a unique visualization and quantification of strain-induced changes in muscle collagen fiber alignment as they relate to passive mechanics. Using dynamic imaging of collagen in skeletal muscle we demonstrate that as skeletal muscle is stretched, collagen fibers re-orient themselves along the axis of stretch and increase their alignment. The degree of alignment and the increase in alignment are each weakly related to passive muscle stiffness. Collagenase treatments further demonstrate that the basis for muscle Extracellular matrix stiffness is dependent on factors beyond collagen crosslinking and alignment. Together the study contributes to the knowledge of the structure-function relationships of muscle extracellular matrix to tissue stiffness relevant to conditions of fibrosis and aberrant stiffness.

Differential sensitivity to longitudinal and transverse stretch mediates transcriptional responses in mouse neonatal ventricular myocytes.

To identify how cardiomyocyte mechanosensitive signaling pathways are regulated by anisotropic stretch, micropatterned mouse neonatal cardiomyocytes were stretched primarily longitudinally or transversely to the myofiber axis. Four hours of static, longitudinal stretch induced differential expression of 557 genes, compared with 30 induced by transverse stretch, measured using RNA-seq. A logic-based ordinary differential equation model of the cardiac myocyte mechanosignaling network, extended to include the transcriptional regulation and expression of 784 genes, correctly predicted measured expression changes due to anisotropic stretch with 69% accuracy. The model also predicted published transcriptional responses to mechanical load in vitro or in vivo with 63-91% accuracy. The observed differences between transverse and longitudinal stretch responses were not explained by differential activation of specific pathways but rather by an approximately twofold greater sensitivity to longitudinal stretch than transverse stretch. In vitro experiments confirmed model predictions that stretch-induced gene expression is more sensitive to angiotensin II and endothelin-1, via RhoA and MAP kinases, than to the three membrane ion channels upstream of calcium signaling in the network. Quantitative cardiomyocyte gene expression differs substantially with the axis of maximum principal stretch relative to the myofilament axis, but this difference is due primarily to differences in stretch sensitivity rather than to selective activation of mechanosignaling pathways.NEW & NOTEWORTHY Anisotropic stretch applied to micropatterned neonatal mouse ventricular myocytes induced markedly greater acute transcriptional responses when the major axis of stretch was parallel to the myofilament axis than when it was transverse. Analysis with a novel quantitative network model of mechanoregulated cardiomyocyte gene expression suggests that this difference is explained by higher cell sensitivity to longitudinal loading than transverse loading than by the activation of differential signaling pathways.

Differential Expression of Adipokines in Mechanically Stretched‐ Endothelial cells

IntroductionHypertension is considered as the major risk factor for cardiovascular diseases, which are the leading cause of morbidity and mortality worldwide. This disease can develop mainly due to the mechanical forces exerted by the blood on the vessels wall. The endothelium, lining the blood vessel, responds to these hemodynamic mechanical forces such as mechanical stretch leading to endothelial dysfunction. Leptin and adiponectin (APN) are two adipokines that are known to be associated with the pathogenesis of hypertension. However, how the level of these hormones is elevated in response to mechanical stretch in endothelial cells is yet to be elucidated.AimIn the present study, we investigated the expression of leptin and adiponectin in endothelial cells exposed to mechanical stretch.MethodsImmortalized human umbilical vein endothelial cells (EA. hy926) were subjected to mechanical stretch (15%, 60 cycle/min and 1Hz) using STREX cell stretching system (STREX Inc., California, USA) for 6, 16 and 24 h. Cultured cells not subjected to stretch (static condition) where used as controls. The mRNA expression of leptin and its receptor (OB‐Rb) as well as APN and adiponectin receptor 2 (APN‐R2) were assessed at different time points using real‐time PCR.ResultsMechanical stretch induced changes in cell morphology in which the cells were elongated and aligned perpendicular to the stretch axis. Stretching of endothelial cells significantly upregulated the expression of leptin after 16 h (0.9515 ± 0.4238‐fold, n=6, p<0.05) and after 24 h (1.577 ± 0.4503‐fold, n=6, p<0.05) of stretching but without affecting the expression of its receptor (OB‐Rb). The mRNA expression of APN also increased significantly in response to 6 h stretching (0.5457 ± 0.1890‐fold, n=6, p<0.05), 16 h stretching (0.9487 ± 0.4058‐fold, n=6, p<0.05) and 24 h stretching (2.664 ± 0.3447, n=6, p<0.0001) compared to unstretched cells, however no change in the expression of APN‐R2 was observed.ConclusionOur findings suggesting that the exposure of endothelial cells to mechanical stretch can result in changes in the expression of adipokines, leptin and adiponectin, will result in better understanding of how vascular biology is affected by mechanical forces and how these molecules can modulate endothelial function in cardiovascular diseases.

High‐precision and Scalable Fabrication of Biomimetic Culture Environments for Replicating the Extracellular Matrix In Vitro

In vivo, cell and tissue organization is driven by a complex interplay of cells and their environment, including the extracellular matrix (ECM). The ECM can provide varied cues such as structure, protein content, mechanical stiffness, stretch, and electrical stimulation to influence cell and tissue development and function. Traditional in vitro cell culture environments—typically composed of non-dynamic hard and unstructured glass or plastic—fail to fulfill the essential roles of the ECM in development. Consequently, many in vitro cell models fall short in correctly reproducing critical in vivo phenotypes as evidenced when cultured cells lose type-specific characteristics or express phenotypes indicative of an immature developmental stage. Considerable effort is directed at fabricating biomimetic culture environments that maintain or promote mature phenotypes. However, making biomimetic substrates typically involves costly or hard-to-reproduce techniques that are often incompatible with many standard assays. These challenges are further compounded when using high-throughput assays. In this study, our objective is to develop a host of techniques for recapitulating certain aspects of the ECM in vitro. To replicate the shape and stiffness of the extracellular niche, we first developed novel nanopatterning techniques based on high-precision photolithography methods that are highly reproducible, scalable, and amenable to integration with most industry-standard endpoint assays, including high-NA microscopy. Our data demonstrate that a variety of cell types—including cardiac, skeletal, endothelial, neuronal, and cancer—can cue off of patterns that mimic the fibrous structure of the native ECM. We then extend this biomimetic patterning technique to deformable elastomeres which permit the addition of mechanical strain to recapitulate mechanical signals present in the ECM of specific tissues. Our data show that mechanical strain can orient skeletal muscle cells and tissues along the axis of stretch, resulting in a polarized orientation that is similar to what is seen in vivo but is not recapitulated in traditional culture environments. We conclude that our approach is a viable method for recreating specific aspects of the ECM that are critical for driving the development and maturation of cells in culture in a cell, instrument, and assay-agnostic manner.

The partnership between renalase and ejection fraction as a risk factor for increased cardiac remodeling biomarkers in chronic heart failure patients

Objective: Heart failure (HF) represents a huge socio-economic burden. It has been demonstrated, experimentally, that renalase, a newly discovered protein, prevents cardiac hypertrophy and adverse remodeling, which is seen in HF. We postulated the following aims: to investigate associations of renalase with biomarkers of cardiac remodeling: galectin-3, soluble suppression of tumorigenicity, (sST2), growth differentiation factor 15 (GDF-15) and syndecan-1, myocardial stretch (BNP) and cardio-renal axis (cystatin C) in HF patients with reduced ejection fraction (HFrEF) and preserved ejection fraction (HFpEF) to determine whether renalase, in combination with left ventricular ejection fraction (LVEF), represents a risk factor for plasma elevation in biomarkers.Methods: We classified HF patients (n = 76) according to LVEF (preserved/reduced), applied a median plasma renalase (113 ng/mL) as a cut-off value (low/high) and created four subgroups of HF patients: HFpEF/low renalase (n = 19), HFrEF/low renalase (n = 19), HFrEF/high renalase (n = 32) and HFpEF/high renalase (n = 6). A control group (n = 35) consisted of healthy volunteers.Results: Plasma concentrations of evaluated biomarkers were determined using an ELISA technique and were highest in HF patients with reduced EF (p < .001, respectively), and renalase’s positive correlations were obtained relating to all biomarkers: galectin-3 (r = 0.913; p < .001), sST2 (r = 0.965; p < .001), GDF-15 (r = 0.887; p < .001), syndecan-1 (r = 0.922; p < .001), BNP (r = 0.527; p < .001) and cystatin C (r = 0.844; p < .001) and strong and negative correlation with LVEF (r = −0.456, p < .001). Increased renalase, regardless of the EF (preserved/reduced), was shown to be an independent risk factor for an increase in all evaluated cardiac remodeling biomarkers, p < .001, respectively. However, increased renalase and reduced EF was the only independent risk factor for BNP and cystatin C elevation, p < .001, respectively. Results after multivariable adjustments (age/gender) were identical.Conclusion: When elevated plasma renalase and HF are present, regardless of EF being reduced or preserved, that represents a significant risk factor for increase in cardiac remodeling biomarker plasma concentrations. However, only elevated renalase and reduced EF demonstrated significance as a risk factor for BNP and cystatin C plasma elevation. Renalase may be considered a promising molecule for the improved predictive abilities of conventional biomarkers and is worthy of further investigation.

As the endothelial cell reorients, its tensile forces stabilize

When adherent cells are subjected to uniaxial sinusoidal stretch at frequencies close to physiological, their body and their contractile stress fibers realign nearly perpendicularly to the stretch axis. A common explanation for this phenomenon is that stress fibers reorient along the direction where they are unaffected by the applied cyclic stretch and thus can maintain optimal (homeostatic) tensile force. The ability of cells to achieve tensional homeostasis in response to external disturbances is important for normal physiological functions of cells and tissues and it provides protection against diseases, including cancer and atherosclerosis. However, quantitative experimental data that support the idea that stretch-induced reorientation is associated with tensional homeostasis are lacking. We observed previously that in response to uniaxial cyclic stretch of 10% strain amplitudes, traction forces of single endothelial cells reorient in the direction perpendicular to the stretch axis. Here we carried out a secondary analysis of those data to investigate whether this reorientation of traction forces is associated with tensional homeostasis. Our analysis showed that stretch-induced reorientation of traction forces was accompanied by attenuation of temporal variability of the traction field to the level that was observed in the absence of stretch. These findings represent a quantitative experimental evidence that stretch-induced reorientation of cellular traction forces is associated with the cell’s tendency to achieve tensional homeostasis.

High‐precision and Scalable Fabrication of Biomimetic Culture Environments for Replicating the Extracellular Matrix In Vitro

In vivo, cell and tissue organization is driven by a complex interplay of cells and their environment, including the extracellular matrix (ECM). The ECM can provide varied cues such as structure, protein content, mechanical stiffness, stretch, and electrical stimulation to influence cell and tissue development and function. Traditional in vitro cell culture environments—typically composed of non‐dynamic hard and unstructured glass or plastic—fail to fulfill the roles of the ECM in development. Consequently, many in vitro cell models fall short in correctly reproducing critical in vivo phenotypes as evidenced when cultured cells lose type‐specific characteristics or express phenotypes indicative of an immature developmental stage. Considerable effort is directed at fabricating biomimetic culture environments that maintain or promote mature phenotypes. However, making biomimetic substrates typically involves costly or hard‐to‐reproduce techniques that are often incompatible with many standard assays. These challenges are further compounded when using high‐throughput assays. In this study, our objective is to develop a host of techniques for recapitulating certain aspects of the ECM in vitro. To replicate the shape and stiffness of the extracellular niche, we first developed novel nanopatterning techniques based on high‐precision photolithography methods that are highly reproducible, scalable, and amenable to integration with most industry‐standard endpoint assays, including high‐NA microscopy. Our data demonstrate that a variety of cell types‐‐including cardiac, skeletal, endothelial, neuronal, and cancer‐‐can cue off of patterns that mimic the fibrous structure of the native ECM. We then extended this biomimetic patterning technique to deformable elastomeres which permitted the addition of mechanical strain to recapitulate mechanical signals present in the ECM of specific tissues. Our data show that mechanical strain can orient skeletal muscle cells and tissues along the axis of stretch, resulting in a polarized orientation that is similar to what is seen in vivo but is not recapitulated in traditional culture environments. Last, we demonstrate that a combination of ECM patterning, mechanical stretch, and electrical pacing can reproduce the extracellular environment of the intact myocardium to facilitate studies of induced pluripotent stem cell‐derived cardiomyocytes. We conclude that our approach is a viable method for recreating specific aspects of the ECM that are critical for driving the development and maturation of cells in culture in a cell, instrument, and assay‐agnostic manner.Support or Funding InformationNational Institutes of Health Small Business Innovation Research Grant.

Cortical contraction drives the 3D patterning of epithelial cell surfaces.

Cellular protrusions create complex cell surface topographies, but biomechanical mechanisms regulating their formation and arrangement are largely unknown. To study how protrusions form, we focused on the morphogenesis of microridges, elongated actin-based structures that are arranged in maze-like patterns on the apical surfaces of zebrafish skin cells. Microridges form by accreting simple finger-like precursors. Live imaging demonstrated that microridge morphogenesis is linked to apical constriction. A nonmuscle myosin II (NMII) reporter revealed pulsatile contractions of the actomyosin cortex, and inhibiting NMII blocked apical constriction and microridge formation. A biomechanical model suggested that contraction reduces surface tension to permit the fusion of precursors into microridges. Indeed, reducing surface tension with hyperosmolar media promoted microridge formation. In anisotropically stretched cells, microridges formed by precursor fusion along the stretch axis, which computational modeling explained as a consequence of stretch-induced cortical flow. Collectively, our results demonstrate how contraction within the 2D plane of the cortex can pattern 3D cell surfaces.

Adaptation of Patterns of Motile Filaments under Dynamic Boundary Conditions.

Boundary conditions are important for pattern formation in active matter. However, it is still not well-understood how alterations in the boundary conditions (dynamic boundary conditions) impact pattern formation. To elucidate the effect of dynamic boundary conditions on the pattern formation by active matter, we investigate an in vitro gliding assay of microtubules on a deformable soft substrate. The dynamic boundary conditions were realized by applying mechanical stress through stretching and compression of the substrate during the gliding assay. A single cycle of stretch-and-compression (relaxation) of the substrate induces perpendicular alignment of microtubules relative to the stretch axis, whereas repeated cycles resulted in zigzag patterns of microtubules. Our model shows that the orientation angles of microtubules correspond to the direction to attain smooth movement without buckling, which is further amplified by the collective migration of the microtubules. Our results provide an insight into understanding the rich dynamics in self-organization arising in active matter subjected to time-dependent boundary conditions.

In vitro model reveals a role for mechanical stretch in the remodeling response of lymphatic muscle cells.

Using primary LMCs in vitro, we sought to characterize the impact of LMC remodeling on their functional and molecular response to mechanical loading and culture conditions. Primary "wounded leg" LMCs were derived from the hindlimb of three sheep who underwent lymphatic injury 6weeks prior, while "control leg" LMCs were derived from the contralateral, unwounded, limb. Function of the LMCs was characterized in response to media of variable levels of serum (10% vs 0.2%) and glucose (4.5 vs 1g/L). Functional and proteomic data were evaluated in LMCs exposed to cyclic stretch (0.1Hz, 7.5% elongation) for 1week. LMCs were sensitive to changes in serum levels, significantly reducing overall activity and collagen synthesis under low serum conditions. LMCs from the remodeled vessel had higher baseline levels of metabolic activity but not collagen synthesis. Cyclic loading induced cellular alignment perpendicular to the axis of stretch and alterations in signaling pathways associated with metabolism. Remodeled LMCs had consistently higher levels of metabolic activity and were more resistant to strain-induced apoptosis. LMCs exist on a functional spectrum, becoming more active in response to stretching and maintaining phenotypic remodeling in response to local lymphatic/tissue damage.

E-cadherin and LGN align epithelial cell divisions with tissue tension independently of cell shape

Tissue morphogenesis requires the coordinated regulation of cellular behavior, which includes the orientation of cell division that defines the position of daughter cells in the tissue. Cell division orientation is instructed by biochemical and mechanical signals from the local tissue environment, but how those signals control mitotic spindle orientation is not fully understood. Here, we tested how mechanical tension across an epithelial monolayer is sensed to orient cell divisions. Tension across Madin-Darby canine kidney cell monolayers was increased by a low level of uniaxial stretch, which oriented cell divisions with the stretch axis irrespective of the orientation of the cell long axis. We demonstrate that stretch-induced division orientation required mechanotransduction through E-cadherin cell-cell adhesions. Increased tension on the E-cadherin complex promoted the junctional recruitment of the protein LGN, a core component of the spindle orientation machinery that binds the cytosolic tail of E-cadherin. Consequently, uniaxial stretch triggered a polarized cortical distribution of LGN. Selective disruption of trans engagement of E-cadherin in an otherwise cohesive cell monolayer, or loss of LGN expression, resulted in randomly oriented cell divisions in the presence of uniaxial stretch. Our findings indicate that E-cadherin plays a key role in sensing polarized tensile forces across the tissue and transducing this information to the spindle orientation machinery to align cell divisions.

Stepwise morphological changes and cytoskeletal reorganization of human mesenchymal stem cells treated by short-time cyclic uniaxial stretch.

This study aimed to investigate stepwise remodeling of human mesenchymal stem cells (hMSCs) in response to cyclic stretch through rearrangement and alignment of cells and cytoskeleton regulation toward smooth muscle cell (SMC) fate in different time spans. Image analysis techniques were utilized to calculate morphological parameters. Cytoskeletal reorganization was observed by investigating F-actin filaments using immunofluorescence staining, and expression level of contractile SMC markers was followed by a quantitative polymerase chain reaction method. Applying cyclic uniaxial stretch on cultured hMSCs, utilizing a costume-made device, led to alteration in fractal dimension (FD) and cytoskeleton structure toward continuous alignment and elongation of cells by elevation of strain duration. Actin filaments became more aligned perpendicular to the axis of mechanical stretch by increasing uniaxial loading duration. At first, FD met a significant decrease in 4h loading duration then increased significantly by further loading up to 16h, followed by another decrease up to 1d of uniaxial stretching. HMSCs subjected to 24h cyclic uniaxial stretching significantly expressed early and intermediate contractile SM markers. It was hypothesized that the increase in FD after 4h while cells continuously became more aligned and elongated was due to initiation of change in phenotype that influenced arrangement of cells. At this point, change in cell phenotype started leading to change in morphology while mechanical loading still caused cell alignment and rearrangement. Results can be helpful when optimized engineered cells are needed based on mechanical condition for functional engineered tissue and cell therapy.

Systems Mechanobiology of Cardiac Myocytes

Cardiac myocytes are sensitive to specific and distinct mechanical stimuli. We examined the effect of the major axis of biaxial mechanical stretch on cardiac myocyte gene expression and reconstructed a stretch signaling model to identify pathways and transcription factors regulating these responses. Neonatal cardiac myocytes were cultured on a micropatterned substrate, and the primary stretch axis was applied either parallel or transverse to the myofibril direction. RNA sequencing (RNA-Seq) after acute myocyte stretch showed a more robust response to longitudinal than to transverse stretch. After 30 minutes of stretch, 53 and 168 genes were significantly up- or down-regulated by transverse or longitudinal stretch, respectively. After 4 hours, the number of differentially expressed genes increased to 795 with longitudinal stretch but only 35 with transverse stretch. Genes regulated by both stretches showed significant enrichment of transcription factor activity and protein kinase activity, whereas longitudinal but not transverse stretch specifically activated genes involved in sarcomere organization and cytoskeletal protein binding. Network analysis using logic-based signaling model constructed from published data identified serum response factor (SRF) and myocyte enhancer factor-2 (MEF2) as critical regulators of longitudinal stretch-induced changes mediated by protein kinase C (PKC). cAMP response element-binding protein (CREB) was found to be activated by both longitudinal and transverse stretch. Inhibitor experiments were used to validate these model predictions. Cardiac myocytes activate different pathways in response to transverse and longitudinal mechanical strain; these responses may underlie differential whole organ responses to pressure versus volume overload.

Peak Stress in the Annulus Fibrosus Under Cyclic Biaxial Tensile Loading.

Numerous in vitro studies have examined the initiation and propagation of fatigue injury pathways in the annulus fibrosus (AF) using isolated motion segments; however, the cycle-varying changes to the AF under cyclic biaxial tensile loading conditions have yet to be examined. Therefore, the primary objective of this study was to characterize the cycle-varying changes in peak tensile stress in multilayer AF tissue samples within a range of physiologically relevant loading conditions at subacute magnitudes of tissue stretch up to 100 loading cycles. A secondary aim was to examine whether the stress-relaxation response would be different across loading axes (axial and circumferential) and whether this response would vary across regions of the intervertebral disk (IVD) (anterior and posterior-lateral). The results from the study demonstrate that several significant interactions emerged between independent factors that were examined in the study. Specifically, a three-way interaction between the radial location, magnitude of peak tissue stretch, and cycle rate (p = 0.0053) emerged. Significant two-way interactions between the magnitude of tissue stretch and cycle number (p < 0.0001) and the magnitude of tissue stretch and loading axis (p < 0.0001) were also observed. These findings are discussed in the context of known mechanisms for structural damage, which have been linked to fatigue loading in the IVD (e.g., cleft formation, radial tearing, increased neutral zone, disk bulging, and loss of intradiscal pressure).

Oriented cell division affects the global stress and cell packing geometry of a monolayer under stretch

Cell division plays a vital role in tissue morphogenesis and homeostasis, and the division plane is crucial for cell fate. For isolated cells, extensive studies show that the orientation of divisions is sensitive to cell shape and the direction of extrinsic mechanical forces. However, it is poorly understood that how the cell divides within a cell monolayer and how the local stress change, due to the division, affects the global stress of epithelial monolayers. Here, we use the vertex dynamics models to investigate the effects of division orientation on the configurations and mechanics of a cell monolayer under stretch. We examine three scenarios of the divisions: dividing along the stretch axis, dividing along the geometric long axis of cells, and dividing at a random angle. It is found that the division along the long cell axis can induce the minimal energy difference, and the global stress of the monolayer after stretch releases more rapidly in this case. Moreover, the long-axis division can result in more random cell orientations and more isotropic cell shapes within the monolayer, comparing with other two cases. This study helps understand the division orientation of cells within a monolayer under mechanical stimuli, and may shed light on linking individual cell׳s behaviors to the global mechanics and patterns of tissues.

In vitro maturation of large-scale cardiac patches based on a perfusable starter matrix by cyclic mechanical stimulation

The ultimate goal of tissue engineering is the generation of implants similar to native tissue. Thus, it is essential to utilize physiological stimuli to improve the quality of engineered constructs. Numerous publications reported that mechanical stimulation of small-sized, non-perfusable, tissue engineered cardiac constructs leads to a maturation of immature cardiomyocytes like neonatal rat cardiomyocytes or induced pluripotent stem cells/embryonic stem cells derived self-contracting cells. The aim of this study was to investigate the impact of mechanical stimulation and perfusion on the maturation process of large-scale (2.5×4.5cm), implantable cardiac patches based on decellularized porcine small intestinal submucosa (SIS) or Biological Vascularized Matrix (BioVaM) and a 3-dimensional construct containing neonatal rat heart cells. Application of cyclic mechanical stretch improved contractile function, cardiomyocyte alignment along the stretch axis and gene expression of cardiomyocyte markers. The development of a complex network formed by endothelial cells within the cardiac construct was enhanced by cyclic stretch. Finally, the utilization of BioVaM enabled the perfusion of the matrix during stimulation, augmenting the beneficial influence of cyclic stretch. Thus, this study demonstrates the maturation of cardiac constructs with clinically relevant dimensions by the application of cyclic mechanical stretch and perfusion of the starter matrix. Statement of significanceConsidering the poor endogenous regeneration of the heart, engineering of bioartificial cardiac tissue for the replacement of infarcted myocardium is an exciting strategy. Most techniques for the generation of cardiac tissue result in relative small-sized constructs insufficient for clinical applications. Another issue is to achieve cardiomyocytes and tissue maturation in culture. Here we report, for the first time, the effect of mechanical stimulation and simultaneous perfusion on the maturation of cardiac constructs of clinical relevant dimensions, which are based on a perfusable starter matrix derived from porcine small intestine. In response to these stimuli superior organization of cardiomyocytes and vascular networks was observed in contrast to untreated controls. The study provides substantial progress towards the generation of implantable cardiac patches.

Variability in vascular smooth muscle cell stretch-induced responses in 2D culture.

The pulsatile nature of blood flow exposes vascular smooth muscle cells (VSMCs) in the vessel wall to mechanical stress, in the form of circumferential and longitudinal stretch. Cyclic stretch evokes VSMC proliferation, apoptosis, phenotypic switching, migration, alignment, and vascular remodeling. Given that these responses have been observed in many cardiovascular diseases, a defined understanding of their underlying mechanisms may provide critical insight into the pathophysiology of cardiovascular derangements. Cyclic stretch-triggered VSMC responses and their effector mechanisms have been studied in vitro using tension systems that apply either uniaxial or equibiaxial stretch to cells grown on an elastomer-bottomed culture plate and ex vivo by stretching whole vein segments with small weights. This review will focus mainly on VSMC responses to the in vitro application of mechanical stress, outlining the inconsistencies in acquired data, and comparing them to in vivo or ex vivo findings. Major discrepancies in data have been seen in mechanical stress-induced proliferation, apoptosis, and phenotypic switching responses, depending on the stretch conditions. These discrepancies stem from variations in stretch conditions such as degree, axis, duration, and frequency of stretch, wave function, membrane coating, cell type, cell passage number, culture media content, and choice of in vitro model. Further knowledge into the variables that cause these incongruities will allow for improvement of the in vitro application of cyclic stretch.

Static and cyclic mechanical loading of mesenchymal stem cells on elastomeric, electrospun polyurethane meshes.

Biomaterial substrates composed of semi-aligned electrospun fibers are attractive supports for the regeneration of connective tissues because the fibers are durable under cyclic tensile loads and can guide cell adhesion, orientation, and gene expression. Previous studies on supported electrospun substrates have shown that both fiber diameter and mechanical deformation can independently influence cell morphology and gene expression. However, no studies have examined the effect of mechanical deformation and fiber diameter on unsupported meshes. Semi-aligned large (1.75 μm) and small (0.60 μm) diameter fiber meshes were prepared from degradable elastomeric poly(esterurethane urea) (PEUUR) meshes and characterized by tensile testing and scanning electron microscopy (SEM). Next, unsupported meshes were aligned between custom grips (with the stretch axis oriented parallel to axis of fiber alignment), seeded with C3H10T1/2 cells, and subjected to a static load (50 mN, adjusted daily), a cyclic load (4% strain at 0.25 Hz for 30 min, followed by a static tensile loading of 50 mN, daily), or no load. After 3 days of mechanical stimulation, confocal imaging was used to characterize cell shape, while measurements of deoxyribonucleic acid (DNA) content and messenger ribonucleic acid (mRNA) expression were used to characterize cell retention on unsupported meshes and expression of the connective tissue phenotype. Mechanical testing confirmed that these materials deform elastically to at least 10%. Cells adhered to unsupported meshes under all conditions and aligned with the direction of fiber orientation. Application of static and cyclic loads increased cell alignment. Cell density and mRNA expression of connective tissue proteins were not statistically different between experimental groups. However, on large diameter fiber meshes, static loading slightly elevated tenomodulin expression relative to the no load group, and tenascin-C and tenomodulin expression relative to the cyclic load group. These results demonstrate the feasibility of maintaining cell adhesion and alignment on semi-aligned fibrous elastomeric substrates under different mechanical conditions. The study confirms that cell morphology is sensitive to the mechanical environment and suggests that expression of select connective tissue genes may be enhanced on large diameter fiber meshes under static tensile loads.

Emergence of homeostatic epithelial packing and stress dissipation through divisions oriented along the long cell axis

Cell division plays an important role in animal tissue morphogenesis, which depends, critically, on the orientation of divisions. In isolated adherent cells, the orientation of mitotic spindles is sensitive to interphase cell shape and the direction of extrinsic mechanical forces. In epithelia, the relative importance of these two factors is challenging to assess. To do this, we used suspended monolayers devoid of ECM, where divisions become oriented following a stretch, allowing the regulation and function of epithelial division orientation in stress relaxation to be characterized. Using this system, we found that divisions align better with the long, interphase cell axis than with the monolayer stress axis. Nevertheless, because the application of stretch induces a global realignment of interphase long axes along the direction of extension, this is sufficient to bias the orientation of divisions in the direction of stretch. Each division redistributes the mother cell mass along the axis of division. Thus, the global bias in division orientation enables cells to act collectively to redistribute mass along the axis of stretch, helping to return the monolayer to its resting state. Further, this behavior could be quantitatively reproduced using a model designed to assess the impact of autonomous changes in mitotic cell mechanics within a stretched monolayer. In summary, the propensity of cells to divide along their long axis preserves epithelial homeostasis by facilitating both stress relaxation and isotropic growth without the need for cells to read or transduce mechanical signals.

Multicellular Regulation of Tensional Homeostasis

Vascular endothelial cells are exposed to routine and directed mechanical forces due to the pulsatile nature of blood flow. As a result, these cells undergo cytoskeletal remodeling and elicit adaptive changes in cell signaling that protect them against atherosclerosis. A main purpose of this remodeling is to minimize alterations in intracellular mechanical tension caused by external forces and to maintain it at a preferred (homeostatic) level. It has been purported that normal physiological functions of the endothelium requires tensional homeostasis across a broad range of length scales, from the subcellular level to the tissue level. Our recent study challenged this view, however. We measured reorientation of cellular traction forces in isolated endothelial cells in response to periodic uniaxial stretch. We found that within ∼40-50 min from the onset of stretch the traction field reorients in the direction perpendicular to the stretch axis, but exhibits a highly dynamic and erratic behavior long after reorientation is completed. Moreover, even in the absence of external stretch the traction field remains dynamic and erratic while the cell spreading area changes very little and cells do not migrate. Here, we hypothesized that tensional homeostasis of the endothelium requires intercellular cooperation. We measured cellular traction forces in two-dimensional endothelial cell clusters of different sizes and at different length scales over an extended time period and analyzed traction dynamics. Our analysis revealed that dynamic fluctuation of traction forces diminished with increasing cluster size and with increasing strength of the traction field, suggesting that cell-cell cooperation might promote tensional homeostasis in the endothelium. Using a mathematical model, we showed that the cell clusters might regulate the homeostatic state of stress through mechanical interdependence between adjacent cells.