Biomechanical Cues Research Articles

The Role of Mechanical Loading in Ligament Tissue Engineering

Tissue-engineered ligaments have received growing interest as a promising alternative for ligament reconstruction when traditional transplants are unavailable or fail. Mechanical stimulation was recently identified as a critical component in engineering load-bearing tissues. It is well established that living tissue responds to altered loads through endogenous changes in cellular behavior, tissue organization, and bulk mechanical properties. Without the appropriate biomechanical cues, new tissue formation lacks the necessary collagenous organization and alignment for sufficient load-bearing capacity. Therefore, tissue engineers utilize mechanical conditioning to guide tissue remodeling and improve the performance of ligament grafts. This review provides a comparative analysis of the response of ligament and tendon fibroblasts to mechanical loading in current bioreactor studies. The differential effect of mechanical stimulation on cellular processes such as protease production, matrix protein synthesis, and cell proliferation is examined in the context of tissue engineering design.

Cell–matrix interactions and dynamic mechanical loading influence chondrocyte gene expression and bioactivity in PEG-RGD hydrogels

The pericellular matrix (PCM) surrounding chondrocytes is thought to play an important role in transmitting biochemical and biomechanical signals to the cells, which regulates many cellular functions including tissue homeostasis. To better understand chondrocytes interactions with their PCM, three-dimensional poly(ethylene glycol) (PEG) hydrogels containing Arg–Gly–Asp (RGD), the cell-adhesion sequence found in fibronectin and which is present in the PCM of cartilage, were employed. RGD was incorporated into PEG hydrogels via tethers at 0.1, 0.4 and 0.8 mM concentrations. Bovine chondrocytes were encapsulated in the hydrogels and subjected to dynamic compressive strains (0.3 Hz, 18% amplitude strain) for 48 h, and their response assessed by cell morphology, ECM gene expression, cell proliferation and matrix synthesis. Incorporation of RGD did not influence cell morphology under free swelling conditions. However, the level of cell deformation upon an applied strain was greater in the presence of RGD. In the absence of dynamic loading, RGD appears to have a negative effect on chondrocyte phenotype, as seen by a 4.7-fold decrease in collagen II/collagen I expressions in 0.8 mM RGD constructs. However, RGD had little effect on early responses of chondrocytes (i.e. cell proliferation and matrix synthesis/deposition). When isolating RGD as a biomechanical cue, cellular response was very different. Chondrocyte phenotype (collagen II/collagen I ratio) and proteoglycan synthesis were enhanced with higher concentrations of RGD. Overall, our findings demonstrate that RGD ligands enhance cartilage-specific gene expression and matrix synthesis, but only when mechanically stimulated, suggesting that cell–matrix interactions mediate chondrocyte response to mechanical stimulation.

Designing 3D Photopolymer Hydrogels to Regulate Biomechanical Cues and Tissue Growth for Cartilage Tissue Engineering

Synthetic hydrogels fabricated from photopolymerization are attractive for tissue engineering for their controlled macroscopic properties, the ability to incorporate biological functionalities, and cell encapsulation. The goal of the present study was to exploit the attractive features of synthetic hydrogels to elucidate the role of gel structure and chemistry in regulating biomechanical cues. Cartilage cells were encapsulated in poly(ethylene glycol) (PEG) hydrogels with different crosslinking densities. Cellular deformation was examined as a function of gel crosslinking. The effects of continuous versus intermittent dynamic loading regimens were examined. RGD, a cell adhesion peptide, was incorporated into PEG gels and subjected to mechanical loading. Chondrocyte morphology and activity was assessed by anabolic and catabolic ECM gene expression and matrix production by collagen and glycosaminoglycan production. Cell deformation was mediated by gel crosslinking. In the absence of loading, anabolic activity was moderately upregulated while catabolic activity was significantly inhibited regardless of gel crosslinking. Dynamic loading enhanced anabolic activities, but continuous loading inhibited catabolic activity, while intermittent loading stimulated catabolic activity. RGD acted as a mechanoreceptor to influence tissue deposition. We demonstrate the ability to regulate biomechanical cues through manipulations in the gel structure and chemistry and cartilage tissue engineering.

Mechanical properties of single electrospun drug-encapsulated nanofibres

The mechanical and structural properties of a surface play an important role indetermining the morphology of attached cells, and ultimately their cellular functions. Assuch, mechanical and structural integrity are important design parameters for a tissuescaffold. Electrospun fibrous meshes are widely used in tissue engineering. When in contactwith electrospun scaffolds, cells see the individual micro- or nanofibres as their immediatemicroenvironment. In this study, tensile testing of single electrospun nanofibres composed ofpoly(ε-caprolactone) (PCL), and its copolymer, poly(caprolactone-co-ethyl ethylenephosphate) (PCLEEP), revealed a size effect in the Young’s modulus,E, and tensilestrength, σT. Both strength and stiffness increase as the fibre diameter decreases from bulk(∼5 µm) into the nanometre region (200–300 nm). In particular,E and σT of individual PCL nanofibres were at least two-fold and an order of magnitude higher thanthat of PCL film, respectively. PCL films were observed to have more pronouncedcrystallographic texture than the nanofibres; however no difference in crystalline fraction,perfection, or texture was detected among the various fibres. When drugs wereencapsulated into single PCLEEP fibres, mechanical properties were enhanced with1–20 wt% of loaded retinoic acid, but weakened by 10–20 wt% of encapsulated bovine serumalbumin. This understanding of the effect of size and drug and protein encapsulation onthe mechanical properties of electrospun fibres may help in the optimization oftissue scaffold design that combines biochemical and biomechanical cues for tissueregeneration.

Vascular Endothelial Cadherin-mediated Cell-cell Adhesion Regulated by a Small GTPase, Rap1

Vascular endothelial cadherin (VE-cadherin), which belongs to the classical cadherin family, is localized at adherens junctions exclusively in vascular endothelial cells. Biochemical and biomechanical cues regulate the VE-cadherin adhesive potential by triggering the intracellular signals. VE-cadherin-mediated cell adhesion is required for cell survival and endothelial cell deadhesion is required for vascular development. It is therefore crucial to understand how VE-cadherin-based cell adhesion is controlled. This review summarizes the inter-endothelial cell adhesions and introduces our recent advance in Rap1-regulated VE-cadherin adhesion. A further analysis of the VE-cadherin recycling system will aid the understanding of cell adhesion/deadhesion mechanisms mediated by VE-cadherin in response to extracellular stimuli during development and angiogenesis.

Why Are There NIMBYs?

<h3>Abstract</h3> Biomechanical cues are instrumental in guiding embryonic development and cell differentiation. Understanding how these physical stimuli translate into transcriptional programs could provide insight into mechanisms underlying mammalian pre-implantation development. Here, we explore this by exerting microenvironmental control over mouse embryonic stem cells (ESCs). Microfluidic encapsulation of ESCs in agarose microgels stabilized the naïve pluripotency network and specifically induced expression of Plakoglobin (<i>Jup</i>), a vertebrate homologue of β-catenin. Indeed, overexpression of Plakoglobin was sufficient to fully re-establish the naïve pluripotency gene regulatory network under metastable pluripotency conditions, as confirmed by single-cell transcriptome profiling. Finally, we found that in the epiblast, Plakoglobin was exclusively expressed at the blastocyst stage in human and mouse embryos – further strengthening the link between Plakoglobin and naïve pluripotency <i>in vivo</i>. Our work reveals Plakoglobin as a mechanosensitive regulator of naïve pluripotency and provides a paradigm to interrogate the effects of volumetric confinement on cell-fate transitions. <h3>Highlights</h3> 3D agarose spheres stabilize the naïve pluripotency network in mouse ESCs. Volumetric confinement induces expression of Plakoglobin, a vertebrate homologue of β-catenin. Plakoglobin expression in the epiblast is specific to pre-implantation human and mouse embryos. Plakoglobin overexpression maintains naïve pluripotency independently of β-catenin.