Alterations in gene expression of human vascular endothelial cells associated with nanotopographic cues

The influence of physical and spatial substrate characteristics on endothelial cells

Cardiovascular diseases are a main cause of death worldwide, leading to a growing demand for medical devices to treat this patient group. Central to the engineering of such devices is a good understanding of the biology and physics of cell-surface interactions. In existing blood-contacting devices, such as vascular grafts, the interaction between blood, cells, and material is one of the main limiting factors for their long-term durability. An improved understanding of the material's chemical- and physical properties as well as its structure all play a role in how endothelial cells interact with the material surface. This review provides an overview of how different surface structures influence endothelial cell responses and what is currently known about the underlying mechanisms that guide this behavior. The structures reviewed include decellularized matrices, electrospun fibers, pillars, pits, and grated surfaces.

Functionality of macrophages encapsulated in porcine decellularized adipose matrix hydrogels and interaction with Candida albicans

Extracellular matrix hydrogels are considered one of the most suitable biomaterials for tissue regeneration due to their similarity with the extracellular microenvironment of the native tissue. Their properties are dependent on their composition, material concentration, fiber density and the fabrication approaches, among other factors. The encapsulation of immune cells in this kind of hydrogels, both in absence or presence of a pathogen, represents a promising strategy for the development of platforms that mimic healthy and infected tissues, respectively. In this work, we have encapsulated macrophages in 3D hydrogels of porcine decellularized adipose matrices (pDAMs) without and with the Candida albicans fungus, as 3D experimental models to study the macrophage immunocompetence in a closer situation to the physiological conditions and to mimic an infection scenario. Our results indicate that encapsulated macrophages preserve their functionality within these pDAM hydrogels and phagocytose live pathogens. In addition, their behavior is influenced by the hydrogel pore size, inversely related to the hydrogel concentration. Thus, larger pore size promotes the polarization of macrophages towards M2 phenotype along the time and enhances their phagocytosis capability. It is important to point out that encapsulated macrophages in absence of pathogen showed an M2 phenotype, but macrophages coencapsulated with C. albicans can switch towards an M1 inflammatory phenotype to resolve the infection, depending on the fungus quantity. The present study reveals that pDAM hydrogels preserve the macrophage plasticity, demonstrating their relevance as new models for macrophage-pathogen interaction studies that mimic an infection scenario with application in regenerative medicine research.

A comprehensive review on methods for promotion of mechanical features and biodegradation rate in amniotic membrane scaffolds

Amniotic membrane (AM) is a biological tissue that surrounds the fetus in the mother's womb. It has pluripotent cells, immune modulators, collagen, cytokines with anti-fibrotic and anti-inflammatory effect, matrix proteins, and growth factors. In spite of the biological characteristics, some results have been released in preventing the adhesion on traumatized surfaces. Application of the AM as a scaffold is limited due to its low biomechanical resistance and rapid biodegradation. Therefore, for using the AM during surgery, its modification by different methods such as cross-linking of the membrane collagen is necessary, because the cross-linking is an effective way to reduce the rate of biodegradation of the biological materials. In addition, their cross-linking is likely an efficient way to increase the tensile properties of the material, so that they can be easily handled or sutured. In this regard, various methods related to cross-linking of the AM subsuming the composite materials, physical cross-linking, and chemical cross-linking with the glutraldehyde, carbodiimide, genipin, aluminum sulfate, etc. are reviewed along with its advantages and disadvantages in the current work.

Bioprinted microvasculature: progressing from structure to function

Three-dimensional (3D) bioprinting seeks to unlock the rapid generation of complex tissue constructs, but long-standing challenges with efficientin vitromicrovascularization must be solved before this can become a reality. Microvasculature is particularly challenging to biofabricate due to the presence of a hollow lumen, a hierarchically branched network topology, and a complex signaling milieu. All of these characteristics are required for proper microvascular-and, thus, tissue-function. While several techniques have been developed to address distinct portions of this microvascularization challenge, no single approach is capable of simultaneously recreating all three microvascular characteristics. In this review, we present a three-part framework that proposes integration of existing techniques to generate mature microvascular constructs. First, extrusion-based 3D bioprinting creates a mesoscale foundation of hollow, endothelialized channels. Second, biochemical and biophysical cues induce endothelial sprouting to create a capillary-mimetic network. Third, the construct is conditioned to enhance network maturity. Across all three of these stages, we highlight the potential for extrusion-based bioprinting to become a central technique for engineering hierarchical microvasculature. We envision that the successful biofabrication of functionally engineered microvasculature will address a critical need in tissue engineering, and propel further advances in regenerative medicine andex vivohuman tissue modeling.

Effects of Human and Porcine Adipose Extracellular Matrices Decellularized by Enzymatic or Chemical Methods on Macrophage Polarization and Immunocompetence

The decellularized extracellular matrix (ECM) obtained from human and porcine adipose tissue (AT) is currently used to prepare regenerative medicine bio-scaffolds. However, the influence of these natural biomaterials on host immune response is not yet deeply understood. Since macrophages play a key role in the inflammation/healing processes due to their high functional plasticity between M1 and M2 phenotypes, the evaluation of their response to decellularized ECM is mandatory. It is also necessary to analyze the immunocompetence of macrophages after contact with decellularized ECM materials to assess their functional role in a possible infection scenario. In this work, we studied the effect of four decellularized adipose matrices (DAMs) obtained from human and porcine AT by enzymatic or chemical methods on macrophage phenotypes and fungal phagocytosis. First, a thorough biochemical characterization of these biomaterials by quantification of remnant DNA, lipids, and proteins was performed, thus indicating the efficiency and reliability of both methods. The proteomic analysis evidenced that some proteins are differentially preserved depending on both the AT origin and the decellularization method employed. After exposure to the four DAMs, specific markers of M1 proinflammatory and M2 anti-inflammatory macrophages were analyzed. Porcine DAMs favor the M2 phenotype, independently of the decellularization method employed. Finally, a sensitive fungal phagocytosis assay allowed us to relate the macrophage phagocytosis capability with specific proteins differentially preserved in certain DAMs. The results obtained in this study highlight the close relationship between the ECM biochemical composition and the macrophage’s functional role.

Topographic Guidance in Melt-Electrowritten Tubular Scaffolds Enhances Engineered Kidney Tubule Performance

Introduction: To date, tubular tissue engineering relies on large, non-porous tubular scaffolds (Ø > 2 mm) for mechanical self-support, or smaller (Ø 150-500 μm) tubes within bulk hydrogels for studying renal transport phenomena. To advance the engineering of kidney tubules for future implantation, constructs should be both self-supportive and yet small-sized and highly porous. Here, we hypothesize that the fabrication of small-sized porous tubular scaffolds with a highly organized fibrous microstructure by means of melt-electrowriting (MEW) allows the development of self-supported kidney proximal tubules with enhanced properties. Materials and Methods: A custom-built melt-electrowriting (MEW) device was used to fabricate tubular fibrous scaffolds with small diameter sizes (Ø = 0.5, 1, 3 mm) and well-defined, porous microarchitectures (rhombus, square, and random). Human umbilical vein endothelial cells (HUVEC) and human conditionally immortalized proximal tubular epithelial cells (ciPTEC) were seeded into the tubular scaffolds and tested for monolayer formation, integrity, and organization, as well as for extracellular matrix (ECM) production and renal transport functionality. Results: Tubular fibrous scaffolds were successfully manufactured by fine control of MEW instrument parameters. A minimum inner diameter of 1 mm and pore sizes of 0.2 mm were achieved and used for subsequent cell experiments. While HUVEC were unable to bridge the pores, ciPTEC formed tight monolayers in all scaffold microarchitectures tested. Well-defined rhombus-shaped pores outperformed and facilitated unidirectional cell orientation, increased collagen type IV deposition, and expression of the renal transporters and differentiation markers organic cation transporter 2 (OCT2) and P-glycoprotein (P-gp). Discussion and Conclusion: Here, we present smaller diameter engineered kidney tubules with microgeometry-directed cell functionality. Due to the well-organized tubular fiber scaffold microstructure, the tubes are mechanically self-supported, and the self-produced ECM constitutes the only barrier between the inner and outer compartment, facilitating rapid and active solute transport.

The basement membrane as a structured surface - role in vascular health and disease

The basement membrane (BM) is a thin specialized extracellular matrix that functions as a cellular anchorage site, a physical barrier and a signaling hub. While the literature on the biochemical composition and biological activity of the BM is extensive, the central importance of the physical properties of the BM, most notably its mechanical stiffness and topographical features, in regulating cellular function has only recently been recognized. In this Review, we focus on the biophysical attributes of the BM and their influence on cellular behavior. After a brief overview of the biochemical composition, assembly and function of the BM, we describe the mechanical properties and topographical structure of various BMs. We then focus specifically on the vascular BM as a nano- and micro-scale structured surface and review how its architecture can modulate endothelial cell structure and function. Finally, we discuss the pathological ramifications of the biophysical properties of the vascular BM and highlight the potential of mimicking BM topography to improve the design of implantable endovascular devices and advance the burgeoning field of vascular tissue engineering.

Endothelial Cell Morphology Regulates Inflammatory Cells Through MicroRNA Transferred by Extracellular Vesicles

Vascular inflammation plays an important role in the pathogenesis and the development of cardiovascular diseases such as arteriosclerosis and restenosis, and the dysfunction of endothelial cells (ECs) may result in the activation of monocytes and other inflammatory cells. ECs exhibit an elongated morphology in the straight part of arteries but a cobblestone shape near the pro-atherogenic region such as branch bifurcation. Although the effects of hemodynamic forces on ECs have been widely studied, it is not clear whether the EC morphology affects its own function and thus the inflammatory response of monocytes. Here we showed that elongated ECs cultured on poly-(dimethyl siloxane) membrane surface with microgrooves significantly suppressed the activation of the monocytes in co-culture, in comparison to ECs with a cobblestone shape. The transfer of EC-conditioned medium to monocytes had the same effect, suggesting that soluble factors were involved in EC-monocyte communication. Further investigation demonstrated that elongated ECs upregulated the expression of anti-inflammatory microRNAs, especially miR-10a. Moreover, miR-10a was found in the extracellular vesicles (EVs) released by ECs and transferred to monocytes, and the inhibition of EV secretion from ECs repressed the upregulation of miR-10a. Consistently, the inhibition of miR-10a expression in ECs reduced their anti-inflammatory effect on monocytes. These results reveal that the EC morphology can regulate inflammatory response through EVs, which provides a basis for the design and the optimization of biomaterials for vascular tissue engineering.

Micro/nano-hierarchical scaffold fabricated using a cell electrospinning/3D printing process for co-culturing myoblasts and HUVECs to induce myoblast alignment and differentiation

Human skeletal muscle is composed of intricate anatomical structures, including uniaxially arranged myotubes and widely distributed blood capillaries. In this regard, vascularization is an essential part of the successful development of an engineered skeletal muscle tissue to restore its function and physiological activities. In this paper, we propose a method to obtain a platform for co-culturing human umbilical vein endothelial cells (HUVECs) and C2C12 cells using cell electrospinning and 3D bioprinting. To elaborate, on the surface of mechanical supporters (polycaprolactone and collagen struts) with a topographical cue, HUVECs-laden alginate bioink was uniaxially electrospun. The electrospun HUVECs showed high cell viability (90%), homogeneous cell distribution, and efficient HUVEC growth. Furthermore, the myoblasts (C2C12 cells), which were seeded on the vascularized structure (HUVECs-laden fibers), were co-cultured to facilitate myoblast regeneration. As a result, the scaffold that included myoblasts and HUVECs represented a high degree of the myosin heavy chain (MHC) with striated patterns and enhanced myogenic-specific gene expressions (MyoD, troponin T, MHC and myogenin) as compared to the scaffold that included only myoblasts. STATEMENT OF SIGNIFICANCE: Cell electrospinning is an advanced electrospinning method that improves cell-matrix interactions by embedding cells directly into micro/nanofibers. Here, cell electrospinning was employed to achieve not only the homogeneous human umbilical vein endothelial cells (HUVECs) distribution with a high cell-viability (~90%), but also highly aligned topographical cue. Moreover, the uniaxially micropatterned PCL/collagen struts as a physical support were generated using three-dimensional (3D) printing, and was covered with HUVEC-laden micro/nanofibers. This hierarchical structure provided meaningful mechanical stability, homogeneous cell distribution, and HUVEC transformation into a narrow, elongated structure. Furthermore, the myoblasts (C2C12 cells) were seeded on the HUVECs-laden fibers and cocultured to facilitate myogenesis. In brief, a myosin heavy chain with striated patterns and enhanced myogenic specific gene expressions were represented.

Topography elicits distinct phenotypes and functions in human primary and stem cell derived endothelial cells

The effective deployment of arterial (AECs), venous (VECs) and stem cell-derived endothelial cells (PSC-ECs) in clinical applications requires understanding of their distinctive phenotypic and functional characteristics, including their responses to microenvironmental cues. Efforts to mimic the in-vivo vascular basement membrane milieu have led to the design and fabrication of nano- and micro-topographical substrates. Although the basement membrane architectures of arteries and veins are different, investigations into the effects of substrate topographies have so far focused on generic EC characteristics. Thus, topographical modulation of arterial- or venous-specific EC phenotype and function remains unknown. Here, we comprehensively evaluated the effects of 16 unique topographies on primary AECs, VECs and human PSC-ECs using a Multi Architectural (MARC) Chip. Gratings and micro-lenses augmented venous-specific phenotypes and depressed arterial functions in VECs; while AECs did not respond consistently to topography. PSC-ECs exhibited phenotypic and functional maturation towards an arterial subtype with increased angiogenic potential, NOTCH1 and Ac-LDL expression on gratings. Specific topographies could elicit different phenotypic and functional changes, despite similar morphological response in different ECs, demonstrating no direct correlation between the two responses.

Microphysiological systems for recapitulating physiology and function of blood-brain barrier

Central nervous system (CNS) diseases are emerging as a major issue in an aging society. Although extensive research has focused on the development of CNS drugs, the limited transport of therapeutic agents across the blood-brain barrier (BBB) remains a major challenge. Conventional two-dimensional culture dishes do not recapitulate in vivo physiology and real-time observations of molecular transport are not possible in animal models. Recent advances in engineering techniques have enabled the generation of more physiologically relevant in vitro BBB models, and their applications have expanded from fundamental biological research to practical applications in the pharmaceutical industry. In this article, we provide an overview of recent advances in the development of in vitro BBB models, with a particular focus on the recapitulation of BBB function. The development of biomimetic BBB models is postulated to revolutionize not only fundamental biological studies but also drug screening.

Environmental Specification of Pluripotent Stem Cell Derived Endothelial Cells Toward Arterial and Venous Subtypes

Endothelial cells (ECs) are required for a multitude of cardiovascular clinical applications, such as revascularization of ischemic tissues or endothelialization of tissue engineered grafts. Patient derived primary ECs are limited in number, have donor variabilities and their in vitro phenotypes and functions can deteriorate over time. This necessitates the exploration of alternative EC sources. Although there has been a recent surge in the use of pluripotent stem cell derived endothelial cells (PSC-ECs) for various cardiovascular clinical applications, current differentiation protocols yield a heterogeneous EC population, where their specification into arterial or venous subtypes is undefined. Since arterial and venous ECs are phenotypically and functionally different, inappropriate matching of exogenous ECs to host sites can potentially affect clinical efficacy, as exemplified by venous graft mismatch when placed into an arterial environment. Therefore, there is a need to design and employ environmental cues that can effectively modulate PSC-ECs into a more homogeneous arterial or venous phenotype for better adaptation to the host environment, which will in turn contribute to better application efficacy. In this review, we will first give an overview of the developmental and functional differences between arterial and venous ECs. This provides the foundation for our subsequent discussion on the different bioengineering strategies that have been investigated to varying extent in providing biochemical and biophysical environmental cues to mature PSC-ECs into arterial or venous subtypes. The ability to efficiently leverage on a combination of biochemical and biophysical environmental cues to modulate intrinsic arterio-venous specification programs in ECs will greatly facilitate future translational applications of PSC-ECs. Since the development and maintenance of arterial and venous ECs in vivo occur in disparate physio-chemical microenvironments, it is conceivable that the application of these environmental factors in customized combinations or magnitudes can be used to selectively mature PSC-ECs into an arterial or venous subtype.

Biophysical restriction of growth area using a monodispersed gold sphere nanobarrier prolongs the mitotic phase in HeLa cells

Homogeneous 83 nm gold nanospheres with a human fibronectin-coated substrate surrounding the cells induce biophysical cues which result in a delay in the mitotic phase of the cell cycle.

Surface Patterning and Innate Physicochemical Attributes of Silk Films Concomitantly Govern Vascular Cell Dynamics

Functional impairment of vascular cells is associated with cardiovascular pathologies. Recent literature clearly presents evidence relating cell microenvironment and their function. It is crucial to understand the cell-material interaction while designing a functional tissue engineered vascular graft. Natural silk biopolymer has shown potential for various tissue-engineering applications. In the present work, we aimed to explore the combinatorial effect of variable innate physicochemical properties and topographies of silk films on functional behavior of vascular cells. Silk proteins from different varieties (mulberry Bombyx mori, BM; and non-mulberry Antheraea assama, AA) possess unique inherent amino acid composition that leads to variable surface properties (roughness, wettability, chemistry, and mechanical stiffness). In addition, we engineered the silk film surfaces and printed a microgrooved pattern to induce unidirectional cell orientation mimicking their native form. Patterned silk films induced unidirectional alignment of porcine vascular cells. Regardless of alignment, endothelial cells (ECs) proliferated favorably on AA films; however, it suppressed production of nitric oxide (NO), an endogenous vasodilator. Unidirectional alignment of smooth muscle cells (SMCs) encouraged contractile phenotype as indicated by minimal cell proliferation, increment of quiescent (G0) phase cells, and upregulation of contractile genes. Moderately hydrophilic flat BM films induced cell aggregation and augmented the expression of contractile genes (for SMCs) and endothelial nitric oxide synthase, eNOS (for ECs). Functional studies further confirmed SMCs' alignment improving collagen production, remodeling ability (matrix metalloproteinase, MMP-2 and MMP-9 production) and physical contraction. Altogether, this study confirms vascular cells' functional behavior is crucially regulated by synergistic effect of their alignment and cell-substrate interfacial properties.

Concise Review: Altered Versus Unaltered Amniotic Membrane as a Substrate for Limbal Epithelial Cells

Limbal stem cell deficiency (LSCD) can result from a variety of corneal disorders, including chemical and thermal burns, infections, and autoimmune diseases. The symptoms of LSCD may include irritation, epiphora, blepharospasms, photophobia, pain, and decreased vision. There are a number of treatment options, ranging from nonsurgical treatments for mild LSCD to various forms of surgery that involve different cell types cultured on various substrates. Ex vivo expansion of limbal epithelial cells (LEC) involves the culture of LEC harvested either from the patient, a living relative, or a cadaver on a substrate in the laboratory. Following the transfer of the cultured cell sheet onto the cornea of patients suffering from LSCD, a successful outcome can be expected in approximately three out of four patients. The phenotype of the cultured cells has proven to be a key predictor of success. The choice of culture substrate is known to affect the phenotype. Several studies have shown that amniotic membrane (AM) can be used as a substrate for expansion of LEC for subsequent transplantation in the treatment of LSCD. There is currently a debate over whether AM should be denuded (i.e., de-epithelialized) prior to LEC culture, or whether this substrate should remain intact. In addition, crosslinking of the AM has been used to increase the thermal and mechanical stability, optical transparency, and resistance to collagenase digestion of AM. In the present review, we discuss the rationale for using altered versus unaltered AM as a culture substrate for LEC. Stem Cells Translational Medicine 2018;7:415-427.

Functional differences between healthy and diabetic endothelial cells on topographical cues

The endothelial lining of blood vessels is severely affected in type II diabetes. Yet, there is still a paucity on the use of diabetic endothelial cells for study and assessment of implantable devices targeting vascular disease. This critically impairs our ability to determine appropriate topographical cues to be included in implantable devices that can be used to maintain or improve endothelial cell function in vivo. Here, the functional responses of healthy and diabetic human coronary arterial endothelial cells were studied and observed to differ depending on topography. Gratings (2 μm) maintained normal endothelial functions such as adhesiveness, angiogenic capacity and cell-cell junction formation, and reduced immunogenicity of healthy cells. However, a significant and consistent effect was not observed in diabetic cells. Instead, diabetic endothelial cells cultured on the perpendicularly aligned multi-scale hierarchical gratings (250 nm gratings on 2 μm gratings) drastically reduced the uptake of oxidized low-density lipoprotein, decreased immune activation, and accelerated cell migration. Concave microlens (1.8 μm diameter) topography was additionally observed to overwhelmingly deteriorate diabetic endothelial cell function. The results of this study support a new paradigm and approach in the design and testing of implantable devices and biomedical interventions for diabetic patients.

Azopolymer photopatterning for directional control of angiogenesis

Understanding cellular behavior in response to microenvironmental stimuli is central to tissue engineering. An increasing number of reports emphasize the high sensitivity of cells to the physical characteristics of the surrounding milieu and in particular, topographical cues. In this work, we investigated the influence of dynamic topographic signal presentation on sprout formation and the possibility to obtain a space-time control over sprouting directionality without growth factors, in order to investigate the contribution of just topography in the angiogenic process. To test our hypothesis, we employed a 3D angiogenesis assay based on the use of spheroids derived from human umbilical vein endothelial cells (HUVECs). We then modulated the in situ presentation of topographical cues during early-stage angiogenesis through real-time photopatterning of an azobenzene-containing polymer, poly (Disperse Red 1 methacrylate) (pDR1m). Pattern inscription on the polymer surface was made using the focused laser of a confocal microscope. We demonstrate that during early-stage angiogenesis, sprouts followed the pattern direction, while spheroid cores acquired a polarized shape. These findings confirmed that sprout directionality was influenced by the photo-inscribed pattern, probably through contact guidance of leader cells, thus validating the proposed platform as a valuable tool for understanding complex processes involved in cell-topography interactions in multicellular systems.

STATEMENT OF SIGNIFICANCE

The complex relationship between endothelial cells and the surrounding environment that leads to formation of a newly formed vascular network during tissue repair is currently unknown. We have developed an innovative in vitro platform to study these mechanisms in a space and time controlled fashion simulating what happens during regeneration. In particular, we combine a "smart" surface, namely a polymer film, with a three-dimensional living cell aggregate. The polymer is activated by light through which we can design a path to guide cells toward the formation of a new vessel. Our work lies at the intersection of stimuli-responsive biointerfaces and cell biology and may be particularly inspiring for those interested in designing biomaterial surface related to angiogenesis.

Biological responses to immobilized microscale and nanoscale surface topographies

Cellular responses are highly influenced by biochemical and biomechanical interactions with the extracellular matrix (ECM). Due to the impact of ECM architecture on cellular responses, significant research has been dedicated towards developing biomaterials that mimic the physiological environment for design of improved medical devices and tissue engineering scaffolds. Surface topographies with microscale and nanoscale features have demonstrated an effect on numerous cellular responses, including cell adhesion, migration, proliferation, gene expression, protein production, and differentiation; however, relationships between biological responses and surface topographies are difficult to establish due to differences in cell types and biomaterial surface properties. Therefore, it is important to optimize implant surface feature characteristics to elicit desirable biological responses for specific applications. The goal of this work was to review studies investigating the effects of microstructured and nanostructured biomaterials on in vitro biological responses through fabrication of microscale and nanoscale surface topographies, physico-chemical characterization of material surface properties, investigation of protein adsorption dynamics, and evaluation of cellular responses in specific biomedical applications.

Nanoporous Gold Biointerfaces: Modifying Nanostructure to Control Neural Cell Coverage and Enhance Electrophysiological Recording Performance

Nanostructured neural interface coatings have significantly enhanced recording fidelity in both implantable and in vitro devices. As such, nano-porous gold (np-Au) has shown promise as a multifunctional neural interface coating due, in part, to its ability to promote nanostructure-mediated reduction in astrocytic surface coverage while not affecting neuronal coverage. The goal of this study is to provide insight into the mechanisms by which the np-Au nanostructure drives the differential response of neurons versus astrocytes in an in vitro model. Utilizing microfabricated libraries that display varying feature sizes of np-Au, it is demonstrated that np-Au influ-ences neural cell coverage through modulating focal adhesion formation in a feature size-dependent manner. The results here show that surfaces with small (≈30 nm) features control astrocyte spreading through inhibition of focal adhesion formation, while surfaces with large (≈170 nm and greater) features control astrocyte spreading through other mechanotransduction mechanisms. This cellular response combined with lower electrical impedance of np-Au electrodes significantly enhances the fidelity and stability of electrophysiological recordings from cortical neuronglia co-cultures relative to smooth gold electrodes. Finally, by leveraging the effect of nanostructure on neuronal versus glial cell attachment, the use of laser-based nanostructure modulation is demonstrated for selectively patterning neurons with micrometer spatial resolution.

Nano- and microstructured materials for in vitro studies of the physiology of vascular cells

The extracellular environment of vascular cells in vivo is complex in its chemical composition, physical properties, and architecture. Consequently, it has been a great challenge to study vascular cell responses in vitro, either to understand their interaction with their native environment or to investigate their interaction with artificial structures such as implant surfaces. New procedures and techniques from materials science to fabricate bio-scaffolds and surfaces have enabled novel studies of vascular cell responses under well-defined, controllable culture conditions. These advancements are paving the way for a deeper understanding of vascular cell biology and materials-cell interaction. Here, we review previous work focusing on the interaction of vascular smooth muscle cells (SMCs) and endothelial cells (ECs) with materials having micro- and nanostructured surfaces. We summarize fabrication techniques for surface topographies, materials, geometries, biochemical functionalization, and mechanical properties of such materials. Furthermore, various studies on vascular cell behavior and their biological responses to micro- and nanostructured surfaces are reviewed. Emphasis is given to studies of cell morphology and motility, cell proliferation, the cytoskeleton and cell-matrix adhesions, and signal transduction pathways of vascular cells. We finalize with a short outlook on potential interesting future studies.

The effects of nanotopography and coculture systems to promote angiogenesis for wound repair

Insufficient angiogenesis in severe wounds delays wound repair because of a lack of blood supply to the wound site. Therefore, pro-angiogenic therapeutics may enhance wound repair. Many studies have investigated various physical and biochemical cues to improve angiogenesis, such as biocompatible materials, surface modifications, angiogenic factors and coculture systems using various cell types. However, the present capability to mimic the micro- and nanostructure of the natural microenvironment, particularly its porous, fibrous features, is limited. Nanotopography may represent a promising tool to overcome these limitations. Here, we discuss various approaches to the use of nanotopography to enhance angiogenesis and consider the combination of coculture systems with nanotopography to mimic the native environment for promotion of angiogenesis in wound healing and repair.

Mechanisms of Reduced Astrocyte Surface Coverage in Cortical Neuron-Glia Co-cultures on Nanoporous Gold Surfaces

Nanoporous gold (np-Au) is a promising multifunctional material for neural electrodes. We have previously shown that np-Au nanotopography reduces astrocyte surface coverage (linked to undesirable gliosis) while maintaining high neuronal coverage in a cortical primary neuron-glia co-culture model as long as two weeks in vitro. Here, we investigate the potential influence of secreted soluble factors from cells grown on np-Au on the cell type-specific surface coverage of cells grown on conventional tissue culture plastic and test the hypothesis that secretion of factors is responsible for inhibiting astrocyte coverage on np-Au. In order to assess whether factors secreted from cells grown on np-Au surfaces reduced surface coverage by astrocytes, we seeded fresh primary rat neuron-glia co-cultures on conventional polystyrene culture dishes, but maintained the cells in conditioned media from co-cultures grown on np-Au surfaces. After one week in vitro, a preferential reduction in astrocyte surface coverage was not observed, suggesting that soluble factors are not playing a role. In contrast, four hours after cell seeding there were a significant number of non-adhered, yet still viable, cells for the cultures on np-Au surfaces. We hypothesize that the non-adherent cells are mainly astrocytes, because: (i) there was no difference in neuronal cell coverage between np-Au and pl-Au for long culture durations and (ii) neurons are post-mitotic and not expected to increase in number upon attaching to the surface. Overall, the results suggest that the np-Au topography leads to preferential neuronal attachment shortly after cell seeding and limits astrocyte-specific np-Au surface coverage at longer culture durations.

Elasticity Modulation of Fibroblast-Derived Matrix for Endothelial Cell Vascular Morphogenesis and Mesenchymal Stem Cell Differentiation

Biophysical properties of the microenvironment, including matrix elasticity and topography, are known to affect various cell behaviors; however, the specific role of each factor is unclear. In this study, fibroblast-derived matrix (FDM) was used as cell culture substrate and physically modified to investigate the influence of its biophysical property changes on human umbilical vein endothelial cells (HUVECs) and human mesenchymal stem cells (hMSCs) behavior in vitro. These FDMs were physically modified by simply storing them at different temperatures: the one stored at 4°C, maintained its original properties, was considered natural FDM, whereas the ones stored at -20°C or -80°C, exhibited a distinct surface morphology, were considered physically modified FDM. Physical modification induced matrix fiber rearrangement in FDM, forming different microstructures on the surface as characterized by focused ion beam (FIB)-cryoSEM. A significant increase of matrix elasticity was found with physically modified FDMs as determined by atomic force microscopy. HUVEC and hMSC behaviors on these natural and physically modified FDMs were observed and compared with each other and with gelatin-coated coverslips. HUVECs showed a similar adhesion level on these substrates at 3 h, but exhibited different proliferation rates and morphologies at 24 h; HUVECs on natural FDM proliferated relatively slower and assembled to capillary-like structures (CLSs). It is observed that HUVECs assembled to CLSs on natural FDMs are independent on the exogenous growth factors and yet dependent on nonmuscle myosin II activity. This result indicates the important role of matrix mechanical properties in regulating HUVECs vascular morphogenesis. As for hMSCs multilineage differentiation, adipogenesis is improved on natural FDM that with lower matrix elasticity, while osteogenesis is accelerated on physically modified FDMs that with higher matrix elasticity, these results further confirm the crucial role of matrix elasticity on cell fate determination.

Comparing the endothelialisation of extracellular matrix bioscaffolds with coated synthetic vascular graft materials

INTRODUCTION

Existing synthetic vascular grafts have unacceptably high failure rates when replacing below knee arteries. In vitro endothelialisation is a technique, which has been shown to enhance the patency rates of below knee vascular grafts. Synthetic materials are however poor cellular substrates and must be combined with coatings to promote cellular growth and attachment. The most common coating clinically is fibrin-coated ePTFE. The aim of our study was to compare the endothelialisation of fibrin-coated ePTFE with novel extracellular matrix (ECM) biomaterials that we hypothesise will provide a superior substrate for cell growth.

METHODS

Human endothelial cells were cultured on ECM scaffolds and fibrin-coated ePTFE. Uncoated Dacron and ePTFE acted as controls. The cells were examined for viability, phenotype, adhesion and proliferation. Cell morphology was accessed using scanning electron microscopy.

RESULTS

Cells remained viable and produced von Willebrand factor on all substrates tested. ECM scaffolds and fibrin-modified ePTFE achieved statistically higher attachment efficiency when compared to both uncoated synthetic graft materials (p ≤ 0.001). At 90 min 80 ± 3.6% of cells had attached to the ECM scaffold compared to Dacron (30 ± 4.5%, n = 3) and ePTFE (33 ± 2.5%, n = 3). There was no difference in adhesion rates between ECM scaffolds and fibrin-coated ePTFE (p = 1.00). Endothelial cells proliferated fastest on ECM scaffolds when compared to all other materials tested (p < 0.001) and reached confluency on day seven.

CONCLUSION

ECM bioscaffolds offer an improved substrate for promoting rapid endothelialisation compared to fibrin-coated ePTFE by combining firm cellular anchorage and superior cell expansion.

Dual-Microstructured Porous, Anisotropic Film for Biomimicking of Endothelial Basement Membrane

Human endothelial basement membrane (BM) plays a pivotal role in vascular development and homeostasis. Here, a bioresponsive film with dual-microstructured geometries was engineered to mimic the structural roles of the endothelial BM in developing vessels, for vascular tissue engineering (TE) application. Flexible poly(ε-caprolactone) (PCL) thin film was fabricated with microscale anisotropic ridges/grooves and through-holes using a combination of uniaxial thermal stretching and direct laser perforation, respectively. Through optimizing the interhole distance, human mesenchymal stem cells (MSCs) cultured on the PCL film's ridges/grooves obtained an intact cell alignment efficiency. With prolonged culturing for 8 days, these cells formed aligned cell multilayers as found in native tunica media. By coculturing human umbilical vein endothelial cells (HUVECs) on the opposite side of the film, HUVECs were observed to build up transmural interdigitation cell-cell contact with MSCs via the through-holes, leading to a rapid endothelialization on the PCL film surface. Furthermore, vascular tissue construction based on the PCL film showed enhanced bioactivity with an elevated total nitric oxide level as compared to single MSCs or HUVECs culturing and indirect MSCs/HUVECs coculturing systems. These results suggested that the dual-microstructured porous and anisotropic film could simulate the structural roles of endothelial BM for vascular reconstruction, with aligned stromal cell multilayers, rapid endothelialization, and direct cell-cell interaction between the engineered stromal and endothelial components. This study has implications of recapitulating endothelial BM architecture for the de novo design of vascular TE scaffolds.

Sub-micron lateral topography affects endothelial migration by modulation of focal adhesion dynamics

Through the interaction with topographical features, endothelial cells tune their ability to populate target substrates, both in vivo and in vitro. Basal textures interfere with the establishment and maturation of focal adhesions (FAs) thus inducing specific cell-polarization patterns and regulating a plethora of cell activities that govern the overall endothelial function. In this study, we analyze the effect of topographical features on FAs in primary human endothelial cells. Reported data demonstrate a functional link between FA dynamics and cell polarization and spreading on structured substrates presenting variable lateral feature size. Our results reveal that gratings with 2 µm lateral periodicity maximize contact guidance. The effect is linked to the dynamical state of FAs. We argue that these results are readily applicable to the rational design of active surfaces at the interface with the blood stream.

Impact of Nanotopography, Heparin Hydrogel Microstructures, and Encapsulated Fibroblasts on Phenotype of Primary Hepatocytes

Hepatocytes, the main epithelial cell type in the liver, perform most of the biochemical functions of the liver. Thus, maintenance of a primary hepatocyte phenotype is crucial for investigations of in vitro drug metabolism, toxicity, and development of bioartificial liver constructs. Here, we report the impact of topographic cues alone and in combination with soluble signals provided by encapsulated feeder cells on maintenance of the primary hepatocyte phenotype. Topographic features were 300 nm deep with pitches of either 400, 1400, or 4000 nm. Hepatocyte cell attachment, morphology and function were markedly better on 400 nm pitch patterns compared with larger scale topographies or planar substrates. Interestingly, topographic features having biomimetic size scale dramatically increased cell adhesion whether or not substrates had been precoated with collagen I. Albumin production in primary hepatocytes cultured on 400 nm pitch substrates without collagen I was maintained over 10 days and was considerably higher compared to albumin synthesis on collagen-coated flat substrates. In order to investigate the potential interaction of soluble cytoactive factors supplied by feeder cells with topographic cues in determining cell phenotype, bioactive heparin-containing hydrogel microstructures were molded (100 μm spacing, 100 μm width) over the surface of the topographically patterned substrates. These hydrogel microstructures either carried encapsulated fibroblasts or were free of cells. Hepatocytes cultured on nanopatterned substrates next to fibroblast carrying hydrogel microstructures were significantly more functional than hepatocytes cultured on nanopatterned surfaces without hydrogels or stromal cells significantly elevated albumin expression and cell junction formation compared to cells provided with topographic cues only. The simultaneous presentation of topographic biomechanical cues along with soluble signaling molecules provided by encapsulated fibroblasts cells resulted in optimal functionality of cultured hepatocytes. The provision of both topographic and soluble signaling cues could enhance our ability to create liver surrogates and inform the development of engineered liver constructs.

Directional cell elongation through filopodia-steered lamellipodial extension on patterned silk fibroin films

Micropatterned biomaterials have been used to direct cell alignment for specific tissue engineering applications. However, the understanding of how cells respond to guidance cues remains limited. Plasticity in protrusion formation has been proposed to enable cells to adapt their motility mode to microenvironment. In this study, the authors investigated the key role of protrusion response in cell guidance on patterned silk fibroin films. The results revealed that the ability to transform between filopodia and small lamellipodia played important roles in directional cell guidance. Filopodia did not show directional extension on patterned substrates prior to spreading, but they transduced topographical cues to the cell to trigger the formation of small lamellipodia along the direction of a microgrooved or parallel nanofiber pattern. The polar lamellipodia formation provided not only a path with directionality, but a driving force for directional cell elongation. Moreover, aligned nanofibers coating provided better mechanical support for the traction of filopodia and lamellipodia, promoting cell attachment, spreading, and migration. This study provides new insight into how cells respond to guidance cues and how filopodia and lamellipodia control cell contact guidance on micropatterned biomaterial surfaces.

Combined effects of substrate topography and stiffness on endothelial cytokine and chemokine secretion

Endothelial physiology is regulated not only by humoral factors, but also by mechanical factors such as fluid shear stress and the underlying cellular matrix microenvironment. The purpose of the present study was to examine the effects of matrix topographical cues on the endothelial secretion of cytokines/chemokines in vitro. Human endothelial cells were cultured on nanopatterned polymeric substrates with different ratios of ridge to groove widths (1:1, 1:2, and 1:5) and with different stiffnesses (6.7 MPa and 2.5 GPa) in the presence and absence of 1.0 ng/mL TNF-α. The levels of cytokines/chemokines secreted into the conditioned media were analyzed with a multiplexed bead-based sandwich immunoassay. Of the nanopatterns tested, the 1:1 and 1:2 type patterns were found to induce the greatest degree of endothelial cell elongation and directional alignment. The 1:2 type nanopatterns lowered the secretion of inflammatory cytokines such as IL-1β, IL-3, and MCP-1, compared to unpatterned substrates. Additionally, of the two polymers tested, it was found that the stiffer substrate resulted in significant decreases in the secretion of IL-3 and MCP-1. These results suggest that substrates with specific extracellular nanotopographical cues or stiffnesses may provide anti-atherogenic effects like those seen with laminar shear stresses by suppressing the endothelial secretion of cytokines and chemokines involved in vascular inflammation and remodeling.

Comparative endothelial cell response on topographically patterned titanium and silicon substrates with micrometer to sub-micrometer feature sizes

In this work, we evaluate the in vitro response of endothelial cells (EC) to variation in precisely-defined, micrometer to sub-micrometer scale topography on two different substrate materials, titanium (Ti) and silicon (Si). Both substrates possess identically-patterned surfaces composed of microfabricated, groove-based gratings with groove widths ranging from 0.5 to 50 µm, grating pitch twice the groove width, and groove depth of 1.3 µm. These specific materials are chosen due to their relevance for implantable microdevice applications, while grating-based patterns are chosen for the potential they afford for inducing elongated and aligned cellular morphologies reminiscent of the native endothelium. Using EA926 cells, a human EC variant, we show significant improvement in cellular adhesion, proliferation, morphology, and function with decreasing feature size on patterned Ti substrates. Moreover, we show similar trending on patterned Si substrates, albeit to a lesser extent than on comparably patterned Ti substrates. Collectively, these results suggest promise for sub-micrometer topographic patterning in general, and sub-micrometer patterning of Ti specifically, as a means for enhancing endothelialization and neovascularisation for novel implantable microdevice applications.

Biophysical cues and cell behavior: the big impact of little things

The extracellular matrix is composed of a variety of proteins, polysaccharides, and glycosaminoglycans that self-assemble into a hierarchical order of nanometer- to micrometer-scale fibrils and fibers. The shapes, sizes, and elasticity present within this highly ordered meshwork regulate behaviors in most cell types. It has been well documented that cellular migration, proliferation, differentiation, and tissue development are all influenced by matrix geometries and compliance, but how these external biophysical cues are translated into activated intracellular signaling cascades remains poorly understood. Fortunately, technological improvements in artificial substrate fabrication have provided biologists with tools to test cellular interactions within controlled three-dimensional environments. Here, we review cellular responses to biophysical cues and discuss their clinical relevancy and application. We focus especially on integrative approaches that aim to first characterize the properties of specific extracellular matrices and then precisely fabricate biomimetic materials to elucidate how relevant cells respond to the individual biophysical cues present in their native tissues. Through these types of comprehensive studies, biologists have begun to understand and appreciate how exceedingly small features can have a significant impact on the regulation, development, and homeostasis of cells and tissues.

Influence of extracellular matrix proteins and substratum topography on corneal epithelial cell alignment and migration

The basement membrane (BM) of the corneal epithelium presents biophysical cues in the form of topography and compliance that can impact the phenotype and behaviors of cells and their nuclei through modulation of cytoskeletal dynamics. In addition, it is also well known that the intrinsic biochemical attributes of BMs can modulate cell behaviors. In this study, the influence of the combination of exogenous coating of extracellular matrix proteins (ECM) (fibronectin-collagen [FNC]) with substratum topography was investigated on cytoskeletal architecture as well as alignment and migration of immortalized corneal epithelial cells. In the absence of FNC coating, a significantly greater percentage of cells aligned parallel with the long axis of the underlying anisotropically ordered topographic features; however, their ability to migrate was impaired. Additionally, changes in the surface area, elongation, and orientation of cytoskeletal elements were differentially influenced by the presence or absence of FNC. These results suggest that the effects of topographic cues on cells are modulated by the presence of surface-associated ECM proteins. These findings have relevance to experiments using cell cultureware with biomimetic biophysical attributes as well as the integration of biophysical cues in tissue-engineering strategies and the development of improved prosthetics.

Response of filopodia and lamellipodia to surface topography on micropatterned silk fibroin films

Cell-microstructure surface interactions play a significant role in tissue engineering to guide cell spreading and migration. However, the mechanisms underlying cell-topography interactions are complex and remain elusive. To address this topic, microsphere array patterns were prepared on silk fibroin films through polystyrene microsphere self-assembly, followed by culturing rat bone marrow derived mesenchymal stem cells on the films to study cell-substrate interactions. Filopodia sensed and anchored to the microspheres to form initial attachments before spreading. Importantly, the anchored filopodia converted into lamellipodia, and this conversion initiated the directional formation of lamellipodia. Therefore, the conversion of exploratory filopodia into lamellipodia was the main driving force for directional extension of the lamellipodia. Correspondingly, cell spreading, morphology, and migration were modulated by pseudopodial recognition and conversion. This finding demonstrated that filopodia not only act as an antenna to detect microenvironment but also serve as skeleton to guide lamellipodial extension for directing cell motions. The micropatterned films promoted cell adhesion and proliferation due to accelerated lamellipodia formation and cell spreading, with recognition and conversion of filopodia into lamellipodia as a critical role in cell response to surface topography.

The basement membrane of the isolated rat colonic mucosa. A light, electron and atomic force microscopy study

Basement membranes (BM) are structures of the extracellular matrix (ECM), which are involved in epithelial barriers, but also play an important role in processes such as cell adhesion, cell growth and tissue healing. The aim of this study was to investigate possible effects of cell removal on the structure of the BM of the colonic mucosa. The superficial epithelium was removed with EDTA and the samples were then mechanically fixed for immunohistochemistry, TEM, SEM and AFM. For SEM and AFM, some samples were also prepared according to the OTO method. BM marker proteins were detected after cell removal by immunohistochemistry, indicating that BM remains. However, a lamina lucida (LL) was no longer visible in TEM, it disappeared and the BM became slightly thinner. The surface topography of the BM is characterized by the presence of globules, fenestrations and pore-like structures, which were visualized with SEM and AFM. Noteworthy is the visualization for the first time with AFM of a 3D network of fine fibers and filaments ("cords"), which very much resembled that described with TEM by Inoue (1994). An unresolved question is whether the pore-like structures observed in this study, especially with SEM, actually correspond to the pores of the BM whose existence has been demonstrated functionally. In conclusion, the structural patterns and changes described could be considered as a reference to evaluate the effects of other decellularization protocols on BMs, such as those used in tissue engineering.

The influence of substrate topography on the migration of corneal epithelial wound borders

Currently available artificial corneas can develop post-implant complications including epithelial downgrowth, infection, and stromal melting. The likelihood of developing these disastrous complications could be minimized through improved formation and maintenance of a healthy epithelium covering the implant. We hypothesize that this epithelial formation may be enhanced through the incorporation of native corneal basement membrane biomimetic chemical and physical cues onto the surface of the keratoprosthesis. We fabricated hydrogel substrates molded with topographic features containing specific bio-ligands and developed an in vitro wound healing assay. In our experiments, the rate of corneal epithelial wound healing was significantly increased by 50% in hydrogel surfaces containing topographic features, compared to flat surfaces with the same chemical attributes. We determined that this increased healing is not due to enhanced proliferation or increased spreading of the epithelial cells, but to an increased active migration of the epithelial cells. These results show the potential benefit of restructuring and improving the surface of artificial corneas to enhance epithelial coverage and more rapidly restore the formation of a functional epithelium.

Endothelial cells derived from embryonic stem cells respond to cues from topographical surface patterns

The generation of micro- and nano-topography similar to those found in the extra cellular matrix of three-dimensional tissues is one technique used to recapitulate the cell-tissue physiology found in the native tissues. Despite the fact that ample studies have been conducted on the physiological significance of endothelial cells alignment parallel to shear stress, as this is the normal physiologic arrangement for healthy arterial EC, very few studies have examined the use of topographical signals to initiate endothelial cell alignment. Here, we have examined the ability for our mouse embryonic stem cell-derived endothelial cells (ESC-EC) to align on various microchip topographical systems. Briefly, we generated metal molds with 'wrinkled' topography using 1) 15 nm and 2) 30 nm of gold coating on the pre-strained polystryene (PS) sheets. After thermal-induced shrinkage of the PS sheets, polydimethylsiloxane (PDMS) microchips were then generated from the wrinkled molds. Using similar Shrink™-based technology, 3) larger selectively crazed acetone-etched lines in the PS sheets, and 4) fully crazed acetone-treated PS sheets of stochastic topographical morphology were also generated. The 15 nm and 30 nm gold coating generated 'wrinkles' of uniaxial anisotropic channels at nano-scaled widths while the crazing generated micron-sized channels. The ESC-EC were able to respond and align on the 320 nm, 510 nm, and the acetone-etched 10.5 μm channels, but not on the fully 'crazed' topographies. Moreover, the ESC-EC aligned most robustly on the wrinkles, and preferentially to ridge edges on the 10.5 μm-sized channels. The ability to robustly align EC on topographical surfaces enables a variety of controlled physiological studies of EC-EC and EC-ECM contact guidance, as well as having potential applications for the rapid endothelialization of stents and vascular grafts.

Early responses of vascular endothelial cells to topographic cues

Vascular endothelial cells in vivo are exposed to multiple biophysical cues provided by the basement membrane, a specialized extracellular matrix through which vascular endothelial cells are attached to the underlying stroma. The importance of biophysical cues has been widely reported, but the signaling pathways that mediate cellular recognition and response to these cues remain poorly understood. Anisotropic topographically patterned substrates with nano- through microscale feature dimensions were fabricated to investigate cellular responses to topographic cues. The present study focuses on early events following exposure of human umbilical vein endothelial cells (HUVECs) to these patterned substrates. In serum-free medium and on substrates without protein coating, HUVECs oriented parallel to the long axis of underlying ridges in as little as 30 min. Immunocytochemistry showed clear differences in the localization of the focal adhesion proteins Src, p130Cas, and focal adhesion kinase (FAK) in HUVECs cultured on topographically patterned surfaces and on planar surfaces, suggesting involvement of these proteins in mediating the response to topographic features. Knockdown experiments demonstrated that FAK was not necessary for HUVEC alignment in response to topographic cues, although FAK knockdown did modulate HUVEC migration. These data identify key events early in the cellular response to biophysical stimuli.

Shear- vs. nanotopography-guided control of growth of endothelial cells on RGD-nanoparticle-nanowell arrays

Endothelialization of therapeutic cardiovascular implants is essential for their intravascular hemocompatibility. We previously described a novel nanowell-RGD-nanoparticle ensemble, which when applied to surfaces led to enhanced endothelialization and retention under static conditions and low flow rates. In the present study we extend our work to determine the interrelated effects of flow rate and the orientation of ensemble-decorated surface arrays on the growth, adhesion and morphology of endothelial cells. Human umbilical vascular endothelial cells (HUVECs) were grown on array surfaces with either 1 μm × 5 μm spacing (“parallel to flow”) and 5 μm × 1 μm spacing (“perpendicular to flow”) and were exposed to a range of shear stress of (0 to 4.7 ± 0.2 dyn·cm^-2 ), utilizing a pulsatile flow chamber. Under physiological flow (4.7 ± 0.2 dyn·cm^-2), RGD-nanoparticle-nanowell array patterning significantly enhanced cell adhesion and spreading compared with control surfaces and with static conditions. Furthermore, improved adhesion coincided with higher alignment to surface patterning, intimating the importance of interaction and response to the array surface as a means of resisting flow detachment. Under sub-physiological condition (1.7 ± 0.3 dyn·cm^-2; corresponding to early angiogenesis), nanowell-nanoparticle patterning did not provide enhanced cell growth and adhesion compared with control surfaces. However, it revealed increased alignment along the direction of flow, rather than the direction of the pattern, thus potentially indicating a threshold for cell guidance and related retention. These results could provide a cue for controlling cell growth and alignment under varying physiological conditions.

Nanotopography-guided tissue engineering and regenerative medicine

Human tissues are intricate ensembles of multiple cell types embedded in complex and well-defined structures of the extracellular matrix (ECM). The organization of ECM is frequently hierarchical from nano to macro, with many proteins forming large scale structures with feature sizes up to several hundred microns. Inspired from these natural designs of ECM, nanotopography-guided approaches have been increasingly investigated for the last several decades. Results demonstrate that the nanotopography itself can activate tissue-specific function in vitro as well as promote tissue regeneration in vivo upon transplantation. In this review, we provide an extensive analysis of recent efforts to mimic functional nanostructures in vitro for improved tissue engineering and regeneration of injured and damaged tissues. We first characterize the role of various nanostructures in human tissues with respect to each tissue-specific function. Then, we describe various fabrication methods in terms of patterning principles and material characteristics. Finally, we summarize the applications of nanotopography to various tissues, which are classified into four types depending on their functions: protective, mechano-sensitive, electro-active, and shear stress-sensitive tissues. Some limitations and future challenges are briefly discussed at the end.

Influence of membrane cholesterol and substrate elasticity on endothelial cell spreading behavior

Interactions between implanted materials and the surrounding host cells critically affect the fate of bioengineered materials. In this study, the biomechanical response of bovine aortic endothelial cells (BAECs) with different membrane cholesterol levels to polyacrylamide (PA) gels was investigated by measuring cell adhesion and spreading behaviors at varying PA elasticity. The elasticity of gel substrates was manipulated by cross-linker content. Type I collagen (COL1) was coated on PA gel to provide a biologically functional environment for cell spreading. Precise quantitative characterization of changes in cell area and perimeter of cells across two treatments and three bioengineered substrates were determined using a customized software developed for computational image analysis. We found that the initial response of endothelial cells to changes in substrate elasticity was determined by membrane cholesterol levels, and that the extent of endothelial cell spreading increases with membrane cholesterol content. All of the BAECs with different cholesterol levels showed little growth on substrates with elasticity below 20 kPa, but increased spreading at higher substrate elasticity. Cholesterol-depleted cells were consistently smaller than control and cholesterol-enriched cells regardless of substrate elasticity. These observations indicate that membrane cholesterol plays an important role in cell spreading on soft biomimetic materials constructed with appropriate elasticity.

Focal adhesion kinase knockdown modulates the response of human corneal epithelial cells to topographic cues

A rapidly expanding literature broadly documents the impact of biophysical cues on cellular behaviors. In spite of increasing research efforts in this field, the underlying signaling processes are poorly understood. One of the candidate molecules for being involved in mechanotransduction is focal adhesion kinase (FAK). To examine the role of FAK in the response of immortalized human corneal epithelial (hTCEpi) cells to topographic cues, FAK was depleted by siRNA transfection. Contrary to expectations, FAK knockdown resulted in an enhanced response with a greater number of hTCEpi cells aligned to the long axis of anisotropically ordered surface ridges and grooves. Both underlying topographic features and FAK depletion modulated the migration of corneal epithelial cells. The impact of FAK knockdown on both migration and alignment varied depending on the topographic cues to which the cells were exposed, with the most significant change observed on the biologically relevant size scale (400nm). Additionally, a change in expression of genes encoding perinuclear Nesprins 1 and 2 (SYNE1, 2) was observed in response to topographic cues. SYNE1/2 expression was also altered by FAK depletion, suggesting that these proteins might represent a link between cytosolic and nuclear signaling processes. The data presented here have relevance to our understanding of the fundamental processes involved in corneal cell behavior to topographic cues. These results highlight the importance of incorporating biophysical cues in the conduction of in vitro studies and into the design and fabrication of implantable prosthetics.

Collagen-nanofiber hydrogel composites promote contact guidance of human lymphatic microvascular endothelial cells and directed capillary tube formation

Collagen and fibronectin matrices are known to stimulate migration of microvascular endothelial cells and the process of tubulogenesis, but the physical, chemical, and topographical cues for directed vessel formation have yet to be determined. In this study, growth, migration, elongation, and tube formation of human lymphatic microvascular endothelial cells (LECs) were investigated on electrospun poly(D,L-lactic-co-glycolic acid) (PLGA) and poly(L-lactic-co-D-lactic acid) (PLDL) nanofiber-coated substrates, and correlated with fiber density and diameter. Directed migration of LECs was observed in the presence of aligned nanofibers, whereas random fiber alignment slowed down migration and growth of LECs. Cell guidance was significantly enhanced in the presence of more hydrophobic PLDL polymer nanofibers compared to PLGA (10:90). Subsequent experiments with tube-forming assays reveal the ability of resorbable hydrophobic nanofibers >300 nm in diameter to promote cell guidance in collagen gels without direct cell-fiber contact, in contrast to the previously reported contact-guidance phenomena. Our results show that endothelial cell guidance is possible within nanofiber/collagen-gel constructs that mimic the native extracellular matrix in terms of size and orientation of fibrillar components.

Photopolymerized microfeatures for directed spiral ganglion neurite and Schwann cell growth

Cochlear implants (CIs) provide auditory perception to individuals with severe hearing impairment. However, their ability to encode complex auditory stimuli is limited due, in part, to poor spatial resolution caused by electrical current spread in the inner ear. Directing nerve cell processes towards target electrodes may reduce the problematic current spread and improve stimulatory specificity. In this work, photopolymerization was used to fabricate micro- and nano-patterned methacrylate polymers to probe the extent of spiral ganglion neuron (SGN) neurite and Schwann cell (SGSC) contact guidance based on variations in substrate topographical cues. Micropatterned substrates are formed in a rapid, single-step reaction by selectively blocking light with photomasks which have parallel line-space gratings with periodicities of 10-100 μm. Channel amplitudes of 250 nm-10 μm are generated by modulating UV exposure time, light intensity, and photoinitiator concentration. Gradual transitions are observed between ridges and grooves using scanning electron and atomic force microscopy. The transitions stand in contrast to vertical features generated via etching lithographic techniques. Alignment of neural elements increases significantly with increasing feature amplitude and constant periodicity, as well as with decreasing periodicity and constant amplitude. SGN neurite alignment strongly correlates (r = 0.93) with maximum feature slope. Multiple neuronal and glial types orient to the patterns with varying degrees of alignment. This work presents a method to fabricate gradually-sloping micropatterns for cellular contact guidance studies and demonstrates spatial control of inner ear neural elements in response to micro- and nano-scale surface topography.