Endothelial cell-mediated type I collagen gel contraction is regulated by hemin

Differential effects of biocompatible peritoneal dialysis fluids on human mesothelial and endothelial cells in 2D and 3D phenotypes

INTRODUCTION

Peritoneal dialysis (PD) is a life maintaining treatment in patients with end-stage renal disease. Its chronic application leads to peritoneal mesothelial layer denudation and fibrotic transformation along with vascular activation of inflammatory pathways. The impact of different PD fluids (PDF) on mesothelial and endothelial cell function and repair mechanisms are not comprehensively described.

MATERIALS AND METHODS

Mesothelial (MeT-5A) and endothelial cells (EA.hy926) were cultured in 1:1 ratio with cell medium and different PDF (icodextrin-based, amino acid-based, and glucose-based). Cell adhesion, cell migration, and cell proliferation in 2D and spheroid formation and collagen gel contraction assays in 3D cell cultures were performed.

RESULTS

Cell proliferation and cell-mediated gel contraction were both significantly decreased in all conditions. 3D spheroid formation was significantly reduced with icodextrin and amino acid PDF, but unchanged with glucose PDF. Adhesion was significantly increased by amino acid PDF in mesothelial cells and decreased by icodextrin and amino acid PDF in endothelial cells. Migration capacity was significantly decreased in mesothelial cells by all three PDF, while endothelial cells remained unaffected.

CONCLUSIONS

In 3D phenotypes the effects of PDF are more uniform in both mesothelial and endothelial cells, mitigating spheroid formation and gel contraction. On the contrary, effects on 2D phenotypes are more uniform in the icodextrin and amino acid PDF as opposed to glucose ones and affect mesothelial cells more variably. 2D and 3D comparative assessments of PDF effects on the main peritoneal membrane cell barriers, the mesothelial and endothelial, could provide useful translational information for PD studies.

In vitro microvascular engineering approaches and strategies for interstitial tissue integration

The increasing gap between clinical demand for tissue or organ transplants and the availability of donated tissue highlights the emerging opportunities for lab-grown or synthetically engineered tissue. While the field of tissue engineering has existed for nearly half a century, its clinical translation remains unrealised, in part, due to a limited ability to engineer sufficient vascular supply into fabricated tissue, which is necessary to enable nutrient and waste exchange, prevent cellular necrosis, and support tissue proliferation. Techniques to develop anatomically relevant, functional vascular networks in vitro have made significant progress in the last decade, however, the challenge now remains as to how best incorporate these throughout dense parenchymal tissue-like structures to address diffusion-limited development and allow for the fabrication of large-scale vascularised tissue. This review explores advances made in the laboratory engineering of vasculature structures and summarises recent attempts to integrate vascular networks together with sophisticated in vitro avascular tissue and organ-like structures. STATEMENT OF SIGNIFICANCE: The ability to grow full scale, functional tissue and organs in vitro is primarily limited by an inability to adequately diffuse oxygen and nutrients throughout developing cellularised structures, which generally results from the absence of perfusable vessel networks. Techniques to engineering both perfusable vascular networks and avascular miniaturised organ-like structures have recently increased in complexity, sophistication, and physiological relevance. However, integrating these two essential elements into a single functioning vascularised tissue structure represents a significant spatial and temporal engineering challenge which is yet to be surmounted. Here, we explore a range of vessel morphogenic phenomena essential for tissue-vascular co-development, as well as evaluate a range of recent noteworthy approaches for generating vascularised tissue products in vitro.

Strategy for improving cell-mediated vascularized soft tissue formation in a hydrogen peroxide-triggered chemically-crosslinked hydrogel

The physically-crosslinked collagen hydrogels can provide suitable microenvironments for cell-based functional vascular network formation due to their biodegradability, biocompatibility, and good diffusion properties. However, encapsulation of cells into collagen hydrogels results in extensive contraction and rapid degradation of hydrogels, an effect known from their utilization as a pre-vascularized graft in vivo. Various types of chemically-crosslinked collagen-based hydrogels have been successfully synthesized to decrease volume contraction, retard the degradation rate, and increase mechanical tunability. However, these hydrogels failed to form vascularized tissues with uniformly distributed microvessels in vivo. Here, the enzymatically chemically-crosslinked collagen-Phenolic hydrogel was used as a model to determine and overcome the difficulties in engineering vascular networks. Results showed that a longer duration of inflammation and excessive levels of hydrogen peroxide limited the capability for blood vessel forming cells-mediated vasculature formation in vivo. Lowering the unreacted amount of crosslinkers reduced the densities of infiltrating host myeloid cells by half on days 2–4 after implantation, but blood vessels remained at low density and were mainly located on the edge of the implanted constructs. Co-implantation of a designed spacer with cell-laden hydrogel maintained the structural integrity of the hydrogel and increased the degree of hypoxia in embedded cells. These effects resulted in a two-fold increase in the density of perfused blood vessels in the hydrogel. Results agreed with computer-based simulations. Collectively, our findings suggest that simultaneous reduction of the crosslinker-induced host immune response and increase in hypoxia in hydrogen peroxide-triggered chemically-crosslinked hydrogels can effectively improve the formation of cell-mediated functional vascular networks.

Engineered aligned endothelial cell structures in tethered collagen hydrogels promote peripheral nerve regeneration

Vascularisation is important in nerve tissue engineering to provide blood supply and nutrients for long-term survival of implanted cells. Furthermore, blood vessels in regenerating nerves have been shown to serve as tracks for Schwann cells to migrate along and thus form Bands of Büngner which promote axonal regeneration. In this study, we have developed tissue-engineered constructs containing aligned endothelial cells, or co-cultures of both endothelial cells and Schwann cells to test whether these structures could promote regeneration across peripheral nerve gaps. Type I rat tail collagen gels containing HUVECs (Human Umbilical Vein Endothelial Cells, 4 × 10⁶ cells/ml) were cast in perforated tethering silicone conduits to facilitate cellular self-alignment and tube formation for 4 days of culture. For co-culture constructs, optimal tube formation and cellular alignment was achieved with a ratio of 4:0.5 × 10⁶ cells/ml (HUVECs:Schwann cells). An in vivo test of the engineered constructs to bridge a 10 mm gap in rat sciatic nerves for 4 weeks revealed that constructs containing only HUVECs significantly promoted axonal regeneration and vascularisation across the gap, as compared to conventional aligned Schwann cell constructs and those containing co-cultured HUVECs and Schwann cells. Our results suggest that tissue-engineered constructs containing aligned endothelial cells within collagen matrix could be good candidates to treat peripheral nerve injury. STATEMENT OF SIGNIFICANCE: Nerve tissue engineering provides a potential way to overcome the limitations associated with current clinical grafting techniques for the repair of severe peripheral nerve injuries. However, the therapeutic cells within engineered nerve tissue require effective vascularisation in order to survive. This work therefore aimed to develop engineered nerve constructs containing aligned tube-like structures made from endothelial cells. Not only did this provide a method to improve vascularisation, it demonstrated for the first time that aligned endothelial cells can outperform Schwann cells in promoting nerve regeneration in the rat sciatic nerve model. This has introduced the concept of developing pre-vascularised engineered nerve tissues, and indicated the potential usefulness of endothelial cell structures in tissue engineering for peripheral nerve repair.

Characterization of Tissue Engineered Endothelial Cell Networks in Composite Collagen-Agarose Hydrogels

Scaffolds constitute an important element in vascularized tissues and are therefore investigated for providing the desired mechanical stability and enabling vasculogenesis and angiogenesis. In this study, supplementation of hydrogels containing either Matrigel^TM and rat tail collagen I (Matrigel^TM/rCOL) or human collagen (hCOL) with SeaPlaque^TM agarose were analyzed with regard to construct thickness and formation and characteristics of endothelial cell (EC) networks compared to constructs without agarose. Additionally, the effect of increased rCOL content in Matrigel^TM/rCOL constructs was studied. An increase of rCOL content from 1 mg/mL to 3 mg/mL resulted in an increase of construct thickness by approximately 160%. The high rCOL content, however, impaired the formation of an EC network. The supplementation of Matrigel^TM/rCOL with agarose increased the thickness of the hydrogel construct by approximately 100% while supporting the formation of a stable EC network. The use of hCOL/agarose composite hydrogels led to a slight increase in the thickness of the 3D hydrogel construct and supported the formation of a multi-layered EC network compared to control constructs. Our findings suggest that agarose/collagen-based composite hydrogels are promising candidates for tissue engineering of vascularized constructs as cell viability is maintained and the formation of a stable and multi-layered EC network is supported.

A high-throughput microtissue platform to probe endothelial function in vitro

A critical role of vascular endothelium is as a semi-permeable barrier, dynamically regulating the flux of solutes between blood and the surrounding tissue. Existing platforms that quantify endothelial function in vitro are either significantly throughput limited or overlook physiologically relevant extracellular matrix (ECM) interactions and thus do not recapitulate in vivo function. Leveraging droplet microfluidics, we developed a scalable platform to measure endothelial function in nanoliter-volume, ECM-based microtissues. In this study, we describe our high-throughput method for fabricating endothelial-coated collagen microtissues that incorporate physiologically relevant cell-ECM interactions. We showed that the endothelial cells had characteristic morphology, expressed tight junction proteins, and remodeled the ECM via compaction and deposition of basement membrane. We also measured macromolecular permeability using two optical modalities, and found the cell layers: (1) had permeability values comparable to in vivo measurements and (2) were responsive to physiologically-relevant modulators of endothelial permeability (TNF-α and TGF-β). This is the first demonstration, to the authors' knowledge, of high-throughput assessment (n > 150) of endothelial permeability on natural ECM. Additionally, this technology is compatible with standard cell culture equipment (e.g. multi-well plates) and could be scaled up further to be integrated with automated liquid handling systems and automated imaging platforms. Overall, this platform recapitulates the functions of traditional transwell inserts, but extends application to high-throughput studies and introduces new possibilities for interrogating cell-cell and cell-matrix interactions.

Collagen hydrogels incorporated with surface-aminated mesoporous nanobioactive glass: Improvement of physicochemical stability and mechanical properties is effective for hard tissue engineering

Collagen (Col) hydrogels have poor physicochemical and mechanical properties and are susceptible to substantial shrinkage during cell culture, which limits their potential applications in hard tissue engineering. Here, we developed novel nanocomposite hydrogels made of collagen and mesoporous bioactive glass nanoparticles (mBGns) with surface amination, and addressed the effects of mBGn addition (Col:mBG = 2:1, 1:1 and 1:2) and its surface amination on the physicochemical and mechanical properties of the hydrogels. The amination of mBGn was shown to enable chemical bonding with collagen molecules. As a result, the nanocomposite hydrogels exhibited a significantly improved physicochemical and mechanical stability. The hydrolytic and enzymatic degradation of the Col-mBGn hydrogels were slowed down due to the incorporation of mBGn and its surface amination. The mechanical properties of the hydrogels, specifically the resistance to loading as well as the stiffness, significantly increased with the addition of mBGn and its aminated form, as assessed by a dynamic mechanical analysis. Mesenchymal stem cells cultivated within the Col-mBGn hydrogels were highly viable, with enhanced cytoskeletal extensions, due to the addition of surface aminated mBGn. While the Col hydrogel showed extensive shrinkage (down to ∼20% of initial size) during a few days of culture, the shrinkage of the mBGn-added hydrogel was substantially reduced, and the aminated mBGn-added hydrogel had no observable shrinkage over 21 days. Results demonstrated the effective roles of aminated mBGn in significantly improving the physicochemical and mechanical properties of Col hydrogel, which are ultimately favorable for applications in stem cell culture for bone tissue engineering.

Development and evaluation of cross-linked collagen-hydroxyapatite scaffolds for tissue engineering

This study examines the tissue engineering potential of type I collagen cross-linked in the presence of hydroxyapatite (HAp). Scaffolds were prepared by controlled freezing followed by lyophilization of composite mixtures of collagen and HAp in acetic acid, followed by cross-linking with 0.3% glutaraldehyde. Scaffolds of three ratios were prepared, corresponding to collagen/HAp ratios of 1:2, 1:4, and 1:6. The scaffolds were evaluated for their microstructure, chemical and physical properties, swelling behavior, mechanical strength, biodegradability hemocompatability, cytocompatibility, and histopathology following subcutaneous implantation in Sprague Dawley rats. The collagen/HAp matrices showed a smaller pore size of 10-40 μm compared to 50-100 μm for pure collagen scaffolds. Pure collagen showed a mechanical strength of 0.25 MPa, and the value almost doubled for cross-linked composites with collagen/HAp ratio 1:6. The improvement in mechanical strength corresponded to a decrease in swelling and enzymatic degradation (measured by resistance to collagenases). FTIR spectra results in conjunction with scanning electron micrographs showed that cross-linking in the presence of HAp did not significantly alter the structure of collagen. MTT assay and calcein AM staining revealed prominent and healthy growth of mesenchymal stem cells in both the pure collagen as well as collagen:HAp composites of ratio 1:2. In vivo implantation in Sprague Dawley rats showed an initial acute inflammatory response during days 3 and 7, followed by a chronic, macrophage-mediated inflammatory response on days 14 and 28. Overall, a cross-linked collagen/HAp composite scaffold of ratio 1:2 was identified as having potential for further development in tissue engineering.

Bisphosphonate-decorated lipid nanoparticles designed as drug carriers for bone diseases

A conjugate of distearoylphosphoethanolamine-polyethylene glycol with 2-(3-mercaptopropylsulfanyl)-ethyl-1,1-bisphosphonic acid (thiolBP) was synthesized and incorporated into micelles and liposomes to create mineral-binding nanocarriers for therapeutic agents. The micelles and liposomes were used to encapsulate the anticancer drug doxorubicin (DOX) and a model protein lysozyme (LYZ) by using lipid film hydration (LFH) and reverse-phase evaporation vesicle (REV) methods. The results indicated that the micelles and LFH-derived liposomes were better at DOX loading than the REV-derived liposomes, while the REV method was preferable for encapsulating LYZ. The affinity of the micellar and liposomal formulations to hydroxyapatite (HA) was assessed in vitro, and the results indicated that all the thiolBP-incorporated nanocarriers had stronger HA affinity than their counterparts without thiolBP. The thiolBP-decorated liposomes also displayed a strong binding to a collagen/HA composite scaffold in vitro. More importantly, thiolBP-decorated liposomes gave increased retention in the collagen/HA scaffolds after subcutaneously implantation in rats. The designed liposomes were able to entrap the bone morphogenetic protein-2 in a bioactive form, indicating that the proposed nanocarriers could deliver bioactive factors locally in mineralized scaffolds for bone tissue engineering.

ATP induces contraction mediated by the P2Y(2) receptor in rat intestinal subepithelial myofibroblasts

Intestinal subepithelial myofibroblasts (IMFs) exist just under the epithelial membrane directly facing the mucosal microvascular capillary surface distributed in the lamina propria. In the gastrointestinal tract, ATP is released from epithelial and endothelial cells in response to mechanical stimuli. Although it has been reported that mechanical stimuli evoke synchronized Ca(2+) waves in cultured IMFs, the contractile responses by ATP stimulation have not been examined. The aim of this study was to clarify the mechanism of the contraction of IMFs in response to ATP. ATP (1-30μM) induced contraction in a concentration-dependent manner. These contractions were inhibited by LaCl(3) (100-300μM) and by Ca(2+)-free solution (0.5mM EGTA). Fura-2/Ca(2+) signals indicated that ATP (1-10μM) elicited transient increases in intracellular Ca(2+) concentration ([Ca(2+)](i)). In addition, αβ-methylene-ATP (10, 30 and 300μM), a broad spectrum P2X agonist at a concentration higher than 100μM, induced neither contraction nor [Ca(2+)](i) rise. UTP (1-30μM), a selective P2Y(2) and P2Y(4) agonist in rodent, induced concentration-dependent contractions and [Ca(2+)](i) increases, whereas ADP and UDP (10μM) did not induce contractions. Pretreatment with suramin (30-100μM), a relatively selective P2Y(2) antagonist, strongly inhibited ATP- and UTP-induced contractions and [Ca(2+)](i) increases. However, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS: 10-30μM), a receptor antagonist for several P2X and P2Y but less effective to P2Y(2) receptor, failed to inhibit ATP- and UTP-induced contractions and [Ca(2+)](i) increases. By RT-PCR, mRNA expressions of the P2Y(1) and P2Y(2) receptors, but not P2Y(4) or P2Y(6), were detected in IMFs. These results suggest that ATP induces [Ca(2+)](i)-dependent contraction in IMFs, which is mediated through the P2Y(2) receptor.

Differentiation of embryonic stem cells into fibroblast-like cells in three-dimensional type I collagen gel cultures

Fibroblasts are heterogeneous mesenchymal cells that play important roles in the production and maintenance of extracellular matrix. Although their heterogeneity is recognized, progenitor progeny relationships among fibroblasts and the factors that control fibroblast differentiation are poorly defined. The current study was designed to develop a reliable method that would permit in vitro differentiation of fibroblast-like cells from human and murine embryonic stem cells (ESCs). Undifferentiated ESCs were differentiated into embryoid bodies (EBs) with differentiation media. EBs were then cast into type I collagen gels and cultured for 21 d with basal media. The spindle-shaped cells that subsequently grew from the EBs were released from the gels and subsequently cultured as monolayers in basal media supplemented with serum. Differentiated cells showed a characteristic spindle-shaped morphology and had ultrastructural features consistent with fibroblasts. Immunocytochemistry showed positive staining for vimentin and alpha-smooth muscle actin but was negative for stage-specific embryonic antigens and cytokeratins. Assays of fibroblast function, including proliferation, chemotaxis, and contraction of collagen gels demonstrated that the differentiated cells, derived from both human and murine ESCs, responded to transforming growth factor-β1 and prostaglandin E(2) as would be expected of fibroblasts, functions not expected of endothelial or epithelial cells. The current study demonstrates that cells with the morphologic and functional features of fibroblasts can be reliably derived from human and murine ESCs. This methodology provides a means to investigate and define the mechanisms that regulate fibroblast differentiation.

Mesenchymal stem cell-seeded collagen matrices for bone repair: effects of cyclic tensile strain, cell density, and media conditions on matrix contraction in vitro

Type I collagen is the most abundant extracellular matrix protein in bone and contains arginine- glycine-aspartic acid sequences that promote cell adhesion and proliferation. We have previously shown that human mesenchymal stem cells (hMSCs) seeded in three-dimensional (3D) collagen gels upregulate BMP-2 mRNA expression in response to tensile strain, indicative of osteogenesis. Therefore, collagen could be a promising scaffold material for functional bone tissue engineering using hMSCs. However, high contraction of the collagen gels by hMSCs poses a challenge to creating large, tissue-engineered bone constructs. The effects of cyclic tensile strain, medium (with and without dexamethasone), and hMSC seeding density on contraction of collagen matrices have not been investigated. hMSCs were seeded in 3D collagen gels and subjected to cyclic tensile strain of 10% or 12% for 4 h/day at a frequency of 1 Hz in osteogenic-differentiating or complete MSC growth media for up to 14 days. Viability of hMSCs was not affected by strain or media conditions. While initial seeding density affected matrix contraction alone, there was a high interdependence of strain and medium on matrix contraction. These findings suggest a correlation between hMSC proliferation and osteogenic differentiation on collagen matrix contraction that is affected by media, cell-seeding density, and cyclic tensile strain. It is vital to understand the effects of culture conditions on collagen matrix contraction by hMSCs in order to consider hMSC-seeded collagen constructs for functional bone tissue engineering in vitro.

Prostaglandin D(2) induces contraction via thromboxane A(2) receptor in rat liver myofibroblasts

Increased intrahepatic resistance is one of the major characteristics of cirrhotic liver, in which extravascular cells including liver myofibroblasts (MFs) abnormally contract. Although several studies provided evidence that various prostaglandins (PG) are involved in liver cirrhosis, the role of PGD(2) remains unknown. In this study, we investigated the effect of PGD(2) on the contractile properties of liver MFs. Cultured rat liver MFs were used at passages 4-7. A collagen gel contraction assay was used for the evaluation of the MFs contraction. mRNA expression was assessed by semi-quantitative RT-PCR. Intracellular Ca(2+) concentrations ([Ca(2+)](i)) were measured by monitoring the fluorescence intensity of fura-2. PGD(2) (1-10 microM) induced liver MF contraction in a dose-dependent manner with [Ca(2+)](i) elevation. Pretreatment with 300 nM LaCl(3), a nonselective Ca(2+) channel blocker abolished the 10 microM PGD(2)-induced MFs contraction. RT-PCR revealed that three distinct PGD(2) responsive receptors, prostanoid DP receptor, chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2) and thromboxane A(2) receptor (prostanoid TP receptor), were expressed in liver MFs. While prostanoid DP receptor agonist and CRTH2 agonist didn't induce contraction, 0.01-1 microM U46619 (11alpha, 9alpha-epoxymethano-PGH(2), prostanoid TP receptor agonist) caused robust contraction with [Ca(2+)](i) elevation. Furthermore, pretreatment with prostanoid TP receptor antagonists ramatroban (1 microM) or SQ29548 ([1S-[1alpha, 2alpha(Z), 3alpha, 4alpha]]-7-[3-[[2-[(phenyl amino)carbonyl]hydrazino]methyl]-7-oxabicyclo[2.2.1]hept-2-yl]-5-heptenoic acid, 1 microM) completely suppressed PGD(2)-induced contraction and [Ca(2+)](i) elevation. Additionally, we observed that BW245C (1-10 microM) decreased basal MF contraction. These results suggest that PGD(2) induces rat liver MF contraction with an increase in [Ca(2+)](i) through prostanoid TP receptor.

The relative magnitudes of endothelial force generation and matrix stiffness modulate capillary morphogenesis in vitro

When suspended in collagen gels, endothelial cells elongate and form capillary-like networks containing lumens. Human blood outgrowth endothelial cells (HBOEC) suspended in relatively rigid 3 mg/ml floating collagen gels, formed in vivo-like, thin, branched multi-cellular structures with small, thick-walled lumens, while human umbilical vein endothelial cells (HUVEC) formed fewer multi-cellular structures, had a spread appearance, and had larger lumens. HBOEC exert more traction on collagen gels than HUVEC as evidenced by greater contraction of floating gels. When the stiffness of floating gels was decreased by decreasing the collagen concentration from 3 to 1.5 mg/ml, HUVEC contracted gels more and formed thin, multi-cellular structures with small lumens, similar in appearance to HBOEC in floating 3 mg/ml gels. In contrast to floating gels, traction forces exerted by cells in mechanically constrained gels encounter considerable resistance. In constrained collagen gels (3 mg/ml), both cell types appeared spread, formed structures with fewer cells, had larger, thinner-walled lumens than in floating gels, and showed prominent actin stress fibers, not seen in floating gels. These results suggest that the relative magnitudes of cellular force generation and apparent matrix stiffness modulate capillary morphogenesis in vitro and that this balance may play a role in regulating angiogenesis in vivo.

Microfluidic patterning of cells in extracellular matrix biopolymers: effects of channel size, cell type, and matrix composition on pattern integrity

The organization of cells within an extracellular matrix is critical to promote appropriate cellular interactions and tissue function in vivo. The ability to design and create biologically relevant cellular arrangements via microfluidic patterning on surfaces provides new capabilities for tissue engineering and biomimetics. The purpose of this article is to describe techniques using microfluidic patterning of three-dimensional biopolymer matrices to improve cellular pattern integrity and to provide microscale control over cellular microenvironments. Results demonstrated that the incorporation of extracellular matrix biopolymers in cell microfluidic patterning results in a more stable pattern of adherent human endothelial cells than patterning without matrix components after several days in vitro. This may be important for carrying out long-term biological experiments and tissue engineering in vitro. Moreover, chemical components in the patterned biopolymer matrices, such as collagen, chitosan, and fibronectin, influenced the ability of the matrices to control cell migration and pattern stability over time. Thus, microfluidic patterning of cells in extracellular matrix biopolymers was shown to be useful in patterning multiple cell types in well-defined three-dimensional geometries.