Contraction-mediated pinocytosis of RGD-peptide by dermal fibroblasts: inhibition of matrix attachment blocks contraction and disrupts microfilament organisation

Spatial differences of cellular origins and in vivo hypoxia modify contractile properties of pulmonary artery smooth muscle cells: lessons for arterial tissue engineering

Tissue engineering of functional arteries is challenging. Within the pulmonary artery wall, smooth muscle cells (PASMCs) have site-specific developmental and functional phenotypes, reflecting differing contractile roles. The force generated by PASMCs isolated from the inner 25% and outer 50% of the media of intrapulmonary elastic arteries from five normal and eight chronically hypoxic (hypertensive) 14 day-old piglets was quantified in a three-dimensional (3D) collagen construct, using a culture force monitor. Outer medial PASMCs from normal piglets exerted more force (528 +/- 50 dynes) than those of hypoxic piglets (177 +/- 42 dynes; p < 0.01). Force generation by inner medial PASMCs from normal and hypoxic piglets was similar (349 +/- 35 and 239 +/- 60 dynes). In response to agonist (thromboxane) stimulation, all PASMCs from normal and hypoxic piglets contracted, but the increase in force generated by outer and inner hypoxic PASMCs (ranges 13-72 and 14-56 dynes) was less than by normal PASMCs (ranges 27-154 and 34-159 dynes, respectively; p < 0.05 for both). All hypoxic PASMCs were unresponsive to antagonist (sodium nitroprusside) stimulation, all normal PASMCs relaxed (range - 87 to - 494 dynes). Myosin heavy chain expression by both hypoxic PASMC phenotypes was less than normal (p < 0.05 for both), as was the activity of focal adhesion kinase, regulating contraction, in hypoxic inner PASMCs (p < 0.01). Chronic hypoxia resulted in the development of abnormal PASMC phenotypes, which in collagen constructs exhibited a reduction in contractile force and reactivity to agonists. Characterization of the mechanical response of spatially distinct cells and modification of their behaviour by hypoxia is critical for successful tissue engineering of major blood vessels.

Collagen stiffness regulates cellular contraction and matrix remodeling gene expression

Cell-level mechanical and 3D spatial cues are essential to the organization and architecture of new tissues that form during growth, repair or in bioreactors. Fibroblast-seeded 3D collagen constructs have been used as bioartifical extracellular matrix (ECM) providing a 3D environment to embedded resident cells. As cells attach to scaffold fibrils, they generate quantifiable contractile forces which depend on cell type, cell attachment, cell density, growth factors, and matrix stiffness. The aim of this study was to quantify the cytomechanical and molecular responses of human dermal (HDF) and neonatal foreskin fibroblasts (HNFF) seeded in constructs of increased stiffness. We also tested the effect of blocking early attachment using serum starvation on these outputs. Constructs were placed under uniaxial strains of 0-10% to increase scaffold stiffness, prior to gel contraction, and force generation was monitored using a tensional culture force monitor (t-CFM). Increased matrix stiffness reduced generation of quantifiable cellular force (up to 70%) over 24 h in both cell types and delayed the onset of measurable contraction (upto sevenfold). The delay of measurable force generation was cell lineage dependent but not FCS dependent. Gene expression of MMP-2, TIMP-2, and collagen type III expression in HDFs were significantly upregulated in constructs of increased stiffness. HNFFs did not show any significant changes in these gene expressions indicating a lineage specific response.

Cell responses to biomimetic protein scaffolds used in tissue repair and engineering

Basic science research in tissue engineering and regenerative medicine aims to investigate and understand the deposition, growth, and remodeling of tissues by drawing together approaches from a range of disciplines. This review discusses approaches that use biomimetic proteins and cellular therapies, both in the development of clinical products and of model platforms for scientific investigation. Current clinical approaches to repairing skin, bone, nerve, heart valves, blood vessels, ligaments, and tendons are described and their limitations identified. Opportunities and key questions for achieving clinical goals are discussed through commonly used examples of biomimetic scaffolds: collagen, fibrin, fibronectin, and silk. The key questions addressed by three-dimensional culture models, biomimetic materials, surface chemistry, topography, and their interaction with cells in terms of durotaxis, mechano-regulation, and complex spatial cueing are reviewed to give context to future strategies for biomimetic technology.

Novel spectroscopic technique for in situ monitoring of collagen fibril alignment in gels

Development of collagen fibril alignment in contracting fibroblast-populated and externally tensioned acellular collagen gels was studied using elastic scattering spectroscopy. Spectra of the backscattered light (320-860 nm) were acquired with a 2.75-mm source-detector separation probe placed perpendicular to the gel surface and rotated to achieve different angles to the collagen fibril alignment. Backscatter was isotropic for noncontracted/unloaded gels (disorganized matrix). As gels were contracted/externally loaded (collagen alignment developed), anisotropy of backscatter increased: more backscatter was detected perpendicular than parallel to the direction of the fibril alignment. An "anisotropy factor" (AF) was calculated to characterize this effect as the ratio of backscatter intensities at orthogonal positions. Before contraction (or zero strain) the AF was close to unity at all wavelengths. In contrast, at 72 h, backscatter anisotropy varied from AF(400 nm) = 2.14 +/- 0.29 to AF(700 nm) = 3.04 +/- 0.48. It also increased over threefold up to a strain of 20%. The AF strongly correlated with the contraction time/strain. Different directions of the backscatter were detected in gel zones with known differences in the matrix alignment. Thus, backscatter anisotropy allows in situ nondestructive determination of collagen fibril alignment and quantitative monitoring of its development.

Mechanical signals and IGF-I gene splicing in vitro in relation to development of skeletal muscle

It has been shown that the insulin-like growth factor (IGF-I) gene is spliced in response to mechanical signals producing forms of IGF-I which have different actions. In order to study how mechanical signals influence this gene splicing in developing muscle, C2C12 cells were grown in three-dimensional (3D) culture and subjected to different regimens of mechanical strain. IGF-IEa which initiates the fusion of myoblasts to form myotubes was found to be constitutively expressed in myoblasts and myotubes (held under endogenous tension) and its expression upregulated by a single ramp stretch of 1-h duration but reduced by repeated cyclical stretch. In contrast, mechano growth factor (MGF), which is involved in the proliferation of mononucleated myoblasts that are required for secondary myotube formation and to establish the muscle satellite (stem) cell pool, showed no significant constitutive expression in static cultures, but was upregulated by a single ramp stretch and by cycling loading. The latter types of force simulate those generated in myoblasts by the first contractions of myotubes. These data indicate the importance of seeking to understand the physiological signals that determine the ratios of splice variants of some growth factor/tissue factor genes in the early stages of development of skeletal muscle.

A multi-station dynamic-culture force monitor system to study cell mechanobiology

To study mechanobiological responses of cells, a dynamic-culture force monitor (D-CFM) system has been developed. The D-CFM extends our previous work to measure contractile forces of a cell-populated collagen gel (CPCG) using a cantilever beam with semiconductor strain gauges. Linear actuators are used in the system and are computer controlled using a LabVIEW interface to independently apply precise motion waveforms to multiple CPCGs. The feasibility tests showed that the new system can detect the differences in force patterns resulting from different motion waveforms imparted to the CPCG. This new system will facilitate the study of the effects of dynamic mechanical loading on cells, remodeling of extracellular matrix, and cell-matrix interactions in vitro.

Evidence for sequential utilization of fibronectin, vitronectin, and collagen during fibroblast-mediated collagen contraction

Contraction plays a major role in wound healing and is inevitably mediated through the mechanical interaction of fibroblast cytoskeleton and integrins with their extracellular matrix ligands. Cell-matrix attachment is critical for such events. In human dermal fibroblasts most such interactions are mediated by the beta1-type integrins. This study investigated the role played by key components in this system, notably fibronectin, vitronectin, and integrin subcomponents alpha2 and alpha5, which recognize collagen and fibronectin. Inhibition of adhesion through these ligands was studied either by antibody blocking or with fibronectin and/or vitronectin depletion. Functional effects of inhibition were monitored as force generation in collagen-glycosaminoglycan (IntegraTM) sponges, over 20 hours using a culture force monitor. Dose and time-course inhibition studies indicated that initial attachment and force generation (approx. 0-5 hours postseeding) was through fibronectin receptors and this was followed by vitronectin ligand and receptor utilization (4 hours onward). Utilization of the collagen integrin subcomponent alpha2 appeared to be increasingly important between 6 and 16 hours and dominant thereafter. Additionally, there was evidence for functional interdependence between the three ligand systems fibronectin, vitronectin, and collagen. We propose that there is a short cascade of sequential integrin-ligand interactions as cells attach to, extend through, and eventually contract their matrix. (WOUND REP REG 2002;10:-408)

Development of magnetic particle techniques for long-term culture of bone cells with intermittent mechanical activation

Magnetic particles were coated with RGD and adhered to primary human osteoblasts. During a 21-day culture, the osteoblasts plus adhered magnetic particles underwent a daily exposure to a time-varying magnetic field via a permanent NdFeB magnet, thus applying a direct mechanical stress to the cells (Bmax approximately 60 mT). After 21 days, preliminary results show that the cells plus magnetic particles were viable and had proliferated. A von-kossa stain showed mineralized bone matrix produced at 21 days in the experimental group whereas the control groups showed no mineralized matrix production. Real-time reverse transcription-polymerase chain reaction at 21 days showed an upregulation of osteopontin from the experimental group in comparison to the control group of cells with adhered particles and no magnet applied. These preliminary results indicate that adherence of RGD-coated 4.5 microm ferromagnetic particles to primary human osteoblasts does not initiate cell necrosis up to 21 days in vitro. Also, mechanical stimulation of human osteoblasts by magnetic particle technology appears to have an influence on osteoblastic activity.