Tunable In Situ Synthesis of Ultrathin Extracellular Matrix-Derived Membranes in Organ-on-a-Chip Devices

Thin cell culture membranes in organ-on-a-chip (OOC) devices are used to model a wide range of thin tissues. While early and most current platforms use microporous or fibrous elastomeric or thermoplastic membranes, there is an emerging class of devices using extra-cellular matrix (ECM) protein-based membranes to improve their biological relevance. These ECM-based membranes present physiologically relevant properties, but they are difficult to integrate into OOC devices due to their relative fragility. Additionally, the specialized fabrication methods developed to date make comparison between methods difficult. This work presents the development and characterization of a method to produce ultrathin matrix-derived membranes (UMM) in OOC devices that requires only a preassembled thermoplastic device and a micropipette, decoupling the device and UMM fabrication processes. Control over the thickness and permeability of the UMM is demonstrated, along with integration of the UMM in a device enabling high-resolution on-chip microscopy. The reliability of the UMM fabrication method is leveraged to develop a medium-throughput well-plate format device with 32 independent UMM-integrated samples. Finally, proof-of-concept cell culture experiments are demonstrated. Due to its simplicity and controllability, the presented method has the potential to overcome technical barriers preventing wider adoption of physiologically relevant ECM-based membranes in OOC devices.

Fibroblasts and polymer composition are essential for bioengineering of airway epithelium on nonwoven scaffolds

To make tissue engineering a truly effective tool, it is necessary to understand how the patterns of specific tissue development are modulated by and depend on the artificial environment. Even the most advanced approaches still do not fully meet the requirements of practical engineering of tracheobronchial epithelium. This study aimed to test the ability of the synthetic and natural nonwoven scaffolds to support the formation of morphological sound airway epithelium including the basement membrane (BM). We also sought to identify the potential role of fibroblasts in this process. Our results showed that nonwoven scaffolds are generally suitable for producing well-differentiated tracheobronchial epithelium (with cilia and goblet cells), while the structure and functionality of the equivalents appeared to be highly dependent on the composition of the scaffolds. Unlike natural scaffolds, synthetic ones supported the formation of the epithelium only when epithelial cells were cocultured with fibroblasts. Fibroblasts also appeared to be obligatory for basal lamina formation, regardless of the type of the nonwoven material used. However, even in the presence of fibroblasts, the synthetic scaffolds were unable to support the formation of the epithelium and of the BM (in particular, basal lamina) as effectively as the natural scaffolds did.

A kidney proximal tubule model to evaluate effects of basement membrane stiffening on renal tubular epithelial cells

The kidney tubule consists of a single layer of epithelial cells supported by the tubular basement membrane (TBM), a thin layer of specialized extracellular matrix (ECM). The mechanical properties of the ECM are important for regulating a wide range of cell functions including proliferation, differentiation and cell survival. Increased ECM stiffness plays a role in promoting multiple pathological conditions including cancer, fibrosis and heart disease. How changes in TBM mechanics regulate tubular epithelial cell behavior is not fully understood. Here we introduce a cell culture system that utilizes in vivo-derived TBM to investigate cell-matrix interactions in kidney proximal tubule cells. Basement membrane mechanics was controlled using genipin, a biocompatibility crosslinker. Genipin modification resulted in a dose-dependent increase in matrix stiffness. Crosslinking had a marginal but statistically significant impact on the diffusive molecular transport properties of the TBM, likely due to a reduction in pore size. Both native and genipin-modified TBM substrates supported tubular epithelial cell growth. Cells were able to attach and proliferate to form confluent monolayers. Tubular epithelial cells polarized and assembled organized cell-cell junctions. Genipin modification had minimal impact on cell viability and proliferation. Genipin stiffened TBM increased gene expression of pro-fibrotic cytokines and altered gene expression for N-cadherin, a proximal tubular epithelial specific cell-cell junction marker. This work introduces a new cell culture model for cell-basement membrane mechanobiology studies that utilizes in vivo-derived basement membrane. We also demonstrate that TBM stiffening affects tubular epithelial cell function through altered gene expression of cell-specific differentiation markers and induced increased expression of pro-fibrotic growth factors.

The basement membrane in epidermal polarity, stemness, and regeneration

The epidermis is a specialized epithelium that constitutes the outermost layer of the skin, and it provides a protective barrier against environmental assaults. Primarily consisting of multilayered keratinocytes, the epidermis is continuously renewed by proliferation of stem cells and the differentiation of their progeny, which undergo terminal differentiation as they leave the basal layer and move upward toward the surface, where they die and slough off. Basal keratinocytes rest on a basement membrane at the dermal-epidermal junction that is composed of specific extracellular matrix proteins organized into interactive and mechanically supportive networks. Firm attachment of basal keratinocytes, and their dynamic regulation via focal adhesions and hemidesmosomes, is essential for maintaining major skin processes, such as self-renewal, barrier function, and resistance to physical and chemical stresses. The adhesive integrin receptors expressed by epidermal cells serve structural, signaling, and mechanosensory roles that are critical for epidermal cell anchorage and tissue homeostasis. More specifically, the basement membrane components play key roles in preserving the stem cell pool, and establishing cell polarity cues enabling asymmetric cell divisions, which result in the transition from a proliferative basal cell layer to suprabasal cells committed to terminal differentiation. Finally, through a well-regulated sequence of synthesis and remodeling, the components of the dermal-epidermal junction play an essential role in regeneration of the epidermis during skin healing. Here too, they provide biological and mechanical signals that are essential to the restoration of barrier function.

Engineering porous membranes mimicking in vivo basement membrane for organ-on-chips applications

Porous membrane-based microfluidic chips are frequently used for developing in vitro tissue-barrier models, the so-called tissue barriers-on-chips (TBoCs). The porous membrane in a TBoC plays a crucial role as an alternative to an in vivo basement membrane (BM). To improve the physiological relevance of an artificial porous membrane, it should possess complex BM-like characteristics from both biophysical and biochemical perspectives. For practical use, artificial membranes should have high mechanical robustness, and their fabrication processes should be conducive to mass production. There have been numerous approaches to accomplishing these requirements in BM-like porous membranes. Extracellular matrix (ECM) hydrogels have emerged as physiologically relevant materials for developing artificial BMs; they remarkably improve the phenotypes and functions of both cells and their layers when compared to previous synthetic porous membranes. However, for practical use, the poor mechanical robustness of ECM membranes needs to be improved. Recently, an advanced ECM membrane reinforced with a nanofiber scaffold has been introduced that possesses both BM-like characteristics and practical applicability. This advanced ECM membrane is expected to promote not only in vivo-like cellular functions but also cellular responses to drugs, which in turn further facilitates the practical applications of TBoCs.

Regenerating airway epithelium using fibrous biomimetic basement membranes

There are reciprocal interactions between epithelial cells and underlying basement membrane. The resemblance of biomaterials to native basement membrane is thus critical for their success when used to regenerate epithelium-containing organs. Particularly, the use of nanofibers and the incorporation of basement membrane proteins may mimic both biophysical and biochemical properties of basement membrane, respectively. Herein we tested how electrospun polycaprolactone/heparin fibers with and without adsorbed laminin and collagen IV proteins affect epithelial cell functions. We found that airway epithelial cells attached, migrated, and proliferated on all scaffolds but protein-functionalized fibers promoted higher attachment, quicker migration, and increased proliferation. Fibers were then integrated on polyethylene scaffolds and cultured at an air-liquid interface. The detection of secretory and ciliated cell markers was higher in cells on polyethylene with fibers. These findings demonstrate that electrospun fibers incite beneficial epithelial cell responses and can be used in the fabrication of bioengineered functional epithelia.

Thin and stretchable extracellular matrix (ECM) membrane reinforced by nanofiber scaffolds for developing in vitro barrier models

An extracellular matrix (ECM) membrane made up of ECM hydrogels has great potentials to develop a physiologically relevant organ-on-a-chip because of its biochemical and biophysical similarity to in vivo basement membranes (BMs). However, the limited mechanical stability of the ECM hydrogels makes it difficult to utilize the ECM membrane in long-term and dynamic cell/tissue cultures. This study proposes an ultra-thin but robust and transparent ECM membrane reinforced with silk fibroin (SF)/polycaprolactone (PCL) nanofibers, which is achieved by in situ self-assembly throughout a freestanding SF/PCL nanofiber scaffold. The SF/PCL nanofiber-reinforced ECM (NaRE) membrane shows biophysical characteristics reminiscent of native BMs, including small thickness (< 5 μm), high permeability (< 9 × 10−5 cm s-1), and nanofibrillar architecture (~10 to 100 nm). With the BM-like characteristics, the nanofiber reinforcement ensured that the NaRE membrane stably supported the construction of various types of in vitro barrier models, from epithelial or endothelial barrier models to complex co-culture models, even over two weeks of cell culture periods. Furthermore, the stretchability of the NaRE membrane allowed emulating the native organ-like cyclic stretching motions (10 to 15%) and was demonstrated to manipulate the cell and tissue-level functions of the in vitro barrier model.

Condensed ECM-based nanofilms on highly permeable PET membranes for robust cell-to-cell communications with improved optical clarity

The properties of a semipermeable porous membrane, including pore size, pore density, and thickness, play a crucial role in creating a tissue interface in a microphysiological system (MPS) because it dictates multicellular interactions between different compartments. The small pore-sized membrane has been preferentially used in an MPS for stable cell adhesion and the formation of tissue barriers on the membrane. However, it limited the applicability of the MPS because of the hindered cell transmigration via sparse through-holes and the optical translucence caused by light scattering through pores. Thus, there remain unmet challenges to construct a compartmentalized MPS without those drawbacks. Here we report a submicrometer-thickness (∼500 nm) fibrous extracellular matrix (ECM) film selectively condensed on a large pore-sized track-etched (TE) membrane (10µm-pores) in an MPS device, which enables the generation of functional tissue barriers simultaneously achieving optical transparency, intercellular interactions, and transmigration of cells across the membrane. The condensed ECM fibers uniformly covering the surface and 10µm-pores of the TE membrane permitted sufficient surface areas where a monolayer of the human induced pluripotent stem cell-derived brain endothelial cells is formed in the MPS device. The functional maturation of the blood-brain barrier (BBB) was proficiently achieved due to astrocytic endfeet sheathing the brain endothelial cells through 10µm pores of the condensed-ECM-coated TE (cECMTE) membrane. We also demonstrated the extravasation of human metastatic breast tumor cells through the human BBB on the cECMTE membrane. Thus, the cECMTE membrane integrated with an MPS can be used as a versatile platform for studying various intercellular communications and migration, mimicking the physiological barriers of an organ compartment.

A Biomimetic In Vitro Model of the Kidney Filtration Barrier Using Tissue-Derived Glomerular Basement Membrane

The glomerular filtration barrier (GFB) filters the blood to remove toxins while retaining high molecular weight proteins in the circulation. The glomerular basement membrane (GBM) and podocytes, highly specialized epithelial cells, are critical components of the filtration barrier. The GBM serves as a physical barrier to passage of molecules into the filtrate. Podocytes adhere to the filtrate side of the GBM and further restrict passage of high molecular weight molecules into the filtrate. Here, a 3D cell culture model of the glomerular filtration barrier to evaluate the role of the GBM and podocytes in mediating molecular diffusion is developed. GBM is isolated from mammalian kidneys to recapitulate the composition and mechanics of the in vivo basement membrane. The GFB model exhibits molecular selectivity that is comparable to the in vivo filtration barrier. The GBM alone provides a stringent barrier to passage of albumin and Ficoll. Podocytes further restrict molecular diffusion. Damage to the GBM that is typical of diabetic kidney disease is simulated using hypochlorous acid and results in increased molecular diffusion. This system can serve as a platform to evaluate the effects of GBM damage, podocyte injury, and reciprocal effects of altered podocyte-GBM interactions on kidney microvascular permeability.

[Models of glomerular filtration barrier : New developments]

In this article, we present the latest innovations to generate in vitro models of the glomerular filtration barrier. There is currently a growing interest for such model systems that allow to reduce the use of animal models. Methodologies to improve their physiological relevance have taken advantage of the development of induced pluripotent stem cells and of bioengineering, particularly tissue engineering. Here, we first introduce the methods to overcome the limitations of the currently used glomerular cells based on the use of stem cells. The different approaches to obtain podocytes, the most important cells in the glomerulus, are presented. Finally, we emphasize the importance of the glomerular microenvironment in maintaining the glomerular cell phenotype, which can be achieved by co-culturing different glomerular cells, integration of biomaterials mimicking the extracellular matrix and introduction of flows with microfluidics.

Analysis of Thickness and Roughness Effects of Artificial Basement Membranes on Endothelial Cell Functions

Various cells and tissues are highly organized in vivo by basement membranes (BMs) and thus promising artificial BMs (A-BMs) constructed by electrospinning and layer-by-layer (LbL) assembly have recently attracted much attention in the tissue engineering field. However, control of cell adhesion, morphology, and migration of the attached cells on the A-BMs has not been reported yet. In this study, we investigated both thickness and roughness-dependent effects of A-BMs on the functions of endothelial cells (ECs), which resulted from different assembly concentrations. The results indicated that the roughness of A-BMs increased gradually with the increase of nanofilm thickness. EC adhesion, spreading and proliferation were inhibited on thicker A-BM surfaces with larger roughness, while interendothelial junctions and the barrier effect of confluent EC monolayers on thicker A-BM surfaces were compensated by increasing seeding cell number and expanding culture time. Our study highlights the influence of LbL assembly conditions on endothelial functions, which offers a new criterion for the design of A-BMs in well-organized 3D tissues.

In Situ Cross-Linking of Artificial Basement Membranes in 3D Tissues and Their Size-Dependent Molecular Permeability

In the human body, highly organized tissues rely on the compartmentalization effect of basement membranes (BMs) that separate different types of cells. We recently reported an artificial basement membrane (A-BM) composed of type-IV collagen and laminin (Col-IV/LM), which are the main components of natural BMs, for cell compartmentalization in three-dimensional (3D) tissues. However, such compartmentalized structures can be maintained only for 3 days, probably due to the degradation issues. In this study, a robust A-BM was fabricated by in situ cross-linking the Col-IV/LM layer-by-layer (LbL) nanofilms in 3D tissues by transglutaminase. The effects of molecular size and configuration on the permeability of obtained A-BMs were comprehensively studied using polystyrene nanoparticles (PS NPs) and dextran with various hydrodynamic diameters, as well as albumin. The findings agreed well with the known size-selective behavior of the glomerular basement membrane. Cross-linked Col-IV/LM nanofilms demonstrate improved stability and a more powerful barrier effect to maintain cell compartmentalization for organized 3D tissues. This in vitro A-BM exhibit great potentials for the design of more complex compartmentalized 3D tissues, for understanding the unique cell-cell cross talk through BMs, and for providing a more reliable 3D tissue model for new drug screening and other in vitro physiological studies.

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.

Fabrication of Artificial Nanobasement Membranes for Cell Compartmentalization in 3D Tissues

In recent decades, tissue engineering techniques have attracted much attention in the construction of 3D tissues or organs. However, even though precise control of cell locations in 3D has been achieved, the organized cell locations are easily destroyed because of the cell migration during the cell culture period. In human body, basement membranes (BMs) maintain the precise cell locations in 3D (compartmentalization). Constructing artificial BMs that mimic the structure and biofunctions of natural BMs remains a major challenge. Here, a nanometer-sized artificial BM through layer-by-layer assembly of collagen type IV (Col-IV) and laminin (LM), chosen because they are the main components of natural BMs, is reported. This multilayered Col-IV/LM nanofilm imitates natural BM structure closely, showing controllable and similar components, thickness, and fibrous network. The Col-IV/LM nanofilms have high cell adhesion properties and maintain the spreading morphology effectively. Furthermore, the barrier effect of preventing cell migration but permitting effective cell-cell crosstalk between fibroblasts and endothelial cells demonstrates the ability of Col-IV/LM nanofilms for cell compartmentalization in 3D tissues, providing more reliable tissue models for evaluating drug efficacy, nanotoxicology, and implantation.

Layer-by-layer assembly of nanofilms to control cell functions

Control of cell functions by layer-by-layer assembly has a great challenge in tissue engineering and biomedical applications. We summarize current hot approaches in this review.