Approaches to study differentiation and repair of human airway epithelial cells

Microphysiological Systems Evaluation: Experience of TEX-VAL Tissue Chip Testing Consortium

Much has been written and said about the promise and excitement of microphysiological systems, miniature devices that aim to recreate aspects of human physiology on a chip. The rapid explosion of the offerings and persistent publicity placed high expectations on both product manufacturers and regulatory agencies to adopt the data. Inevitably, discussions of where this technology fits in chemical testing paradigms are ongoing. Some end-users became early adopters, while others have taken a more cautious approach because of the high cost and uncertainties of their utility. Here, we detail the experience of a public-private collaboration established for testing of diverse microphysiological systems. Collectively, we present a number of considerations on practical aspects of using microphysiological systems in the context of their applications in decision-making. Specifically, future end-users need to be prepared for extensive on-site optimization and have access to a wide range of imaging and other equipment. We reason that cells, related reagents and the technical skills of the research staff, not the devices themselves, are the most critical determinants of success. Extrapolation from concentration-response effects in microphysiological systems to human blood or oral exposures, difficulties with replicating the whole organ, and long-term functionality remain as critical challenges. Overall, we conclude that it is unlikely that a rodent- or human-equivalent model is achievable through a finite number of microphysiological systems in the near future; therefore, building consensus and promoting the gradual incorporation of these models into tiered approaches for safety assessment and decision-making is the sensible path to wide adoption.

A model of human small airway on a chip for studies of sub-acute effects of inhalation toxicants

Testing for acute inhalation hazards is conducted in animals; however, a number of robust in vitro human cell-based alternatives were developed and tested. These models range in complexity from cultures of cell lines or primary cells in air-liquid interface on trans-wells, to more complex and physiologically-relevant flow- and mechanical stimulation-enabled tissue chips. While the former models are relatively straightforward to establish and can be tested in medium/high-throughput, the latter require specialized equipment and lack in throughput. In this study, we developed a device that can be easily manufactured while allowing for the production of a differentiated lung tissue. This multilayered microfluidic device enables co-culture of primary human small airway epithelial cells and lung microvascular endothelial cells under physiological conditions for up to 14 days and recreates the parenchymal-vascular interface in the distal lung. To explore the potential of this airway-on-a-chip for applications in inhalation toxicology, we also devised a system that allows for direct gas/aerosol exposures of the engineered airway epithelium to noxious stimuli known to cause adverse respiratory effects, including dry flowing air, lipopolysaccharide, particulate matter, and iodomethane. This study generated quantitative, high-content data that were indicative of aberrant changes in biochemical (lactate dehydrogenase), barrier (dextran permeability), functional (ciliary beating), and molecular (imaging for various markers) phenotypes of the small airway epithelium due to inhalational exposures. This study is significant because it established an in vitro model of human small airway on a chip that can be used in medium/high-throughput studies of sub-acute effects of inhalation toxicants.

Pathophysiology of Lung Disease and Wound Repair in Cystic Fibrosis

Cystic fibrosis (CF) is an autosomal recessive, life-threatening condition affecting many organs and tissues, the lung disease being the chief cause of morbidity and mortality. Mutations affecting the CF Transmembrane Conductance Regulator (CFTR) gene determine the expression of a dysfunctional protein that, in turn, triggers a pathophysiological cascade, leading to airway epithelium injury and remodeling. In vitro and in vivo studies point to a dysregulated regeneration and wound repair in CF airways, to be traced back to epithelial CFTR lack/dysfunction. Subsequent altered ion/fluid fluxes and/or signaling result in reduced cell migration and proliferation. Furthermore, the epithelial-mesenchymal transition appears to be partially triggered in CF, contributing to wound closure alteration. Finally, we pose our attention to diverse approaches to tackle this defect, discussing the therapeutic role of protease inhibitors, CFTR modulators and mesenchymal stem cells. Although the pathophysiology of wound repair in CF has been disclosed in some mechanisms, further studies are warranted to understand the cellular and molecular events in more details and to better address therapeutic interventions.

Transcriptomic profile of cystic fibrosis airway epithelial cells undergoing repair

Pathological remodeling of the airway epithelium is commonly observed in Cystic Fibrosis (CF). The different cell types that constitute the airway epithelium are regenerated upon injury to restore integrity and maintenance of the epithelium barrier function. The molecular signature of tissue repair in CF airway epithelial cells has, however, not well been investigated in primary cultures. We therefore collected RNA-seq data from well-differentiated primary cultures of bronchial human airway epithelial cells (HAECs) of CF (F508del/F508del) and non-CF (NCF) origins before and after mechanical wounding, exposed or not to flagellin. We identified the expression changes with time of repair of genes, the products of which are markers of the different cell types that constitute the airway epithelium (basal, suprabasal, intermediate, secretory, goblet and ciliated cells as well as ionocytes). Researchers in the CF field may benefit from this transcriptomic profile, which covers the initial steps of wound repair and revealed differences in this process between CF and NCF cultures.

Non-woven bilayered biodegradable chitosan-gelatin-polylactide scaffold for bioengineering of tracheal epithelium

OBJECTIVES

The conversion of tissue engineering into a routine clinical tool cannot be achieved without a deep understanding of the interaction between cells and scaffolds during the process of tissue formation in an artificial environment. Here, we have investigated the cultivation conditions and structural features of the biodegradable non-woven material in order to obtain a well-differentiated human airway epithelium.

MATERIALS AND METHODS

The bilayered scaffold was fabricated by electrospinning technology. The efficiency of the scaffold has been evaluated using MTT cell proliferation assay, histology, immunofluorescence and electron microscopy.

RESULTS

With the use of a copolymer of chitosan-gelatin-poly-l-lactide, a bilayered non-woven scaffold was generated and characterized. The optimal structural parameters of both layers for cell proliferation and differentiation were determined. The basal airway epithelial cells differentiated into ciliary and goblet cells and formed pseudostratified epithelial layer on the surface of the scaffold. In addition, keratinocytes formed a skin equivalent when seeded on the same scaffold. A comparative analysis of growth and differentiation for both types of epithelium was performed.

CONCLUSIONS

The structural parameters of nanofibres should be selected experimentally depending on polymer composition. The major challenges on the way to obtain the well-differentiated equivalent of respiratory epithelium on non-woven scaffold include the following: the balance between scaffold permeability and thickness, proper combination of synthetic and natural components, and culture conditions sufficient for co-culturing of airway epithelial cells and fibroblasts. For generation of skin equivalent, the lack of diffusion is not so critical as for pseudostratified airway epithelium.

A Modular Strategy to Engineer Complex Tissues and Organs

Currently, there are no synthetic or biologic materials suitable for long-term treatment of large tracheal defects. A successful tracheal replacement must (1) have radial rigidity to prevent airway collapse during respiration, (2) contain an immunoprotective respiratory epithelium, and (3) integrate with the host vasculature to support epithelium viability. Herein, biopolymer microspheres are used to deliver chondrogenic growth factors to human mesenchymal stem cells (hMSCs) seeded in a custom mold that self-assemble into cartilage rings, which can be fused into tubes. These rings and tubes can be fabricated with tunable wall thicknesses and lumen diameters with promising mechanical properties for airway collapse prevention. Epithelialized cartilage is developed by establishing a spatially defined composite tissue composed of human epithelial cells on the surface of an hMSC-derived cartilage sheet. Prevascular rings comprised of human umbilical vein endothelial cells and hMSCs are fused with cartilage rings to form prevascular-cartilage composite tubes, which are then coated with human epithelial cells, forming a tri-tissue construct. When prevascular- cartilage tubes are implanted subcutaneously in mice, the prevascular structures anastomose with host vasculature, demonstrated by their ability to be perfused. This microparticle-cell self-assembly strategy is promising for engineering complex tissues such as a multi-tissue composite trachea.

Animal and model systems for studying cystic fibrosis

The cystic fibrosis (CF) field is the beneficiary of five species of animal models that lack functional cystic fibrosis transmembrane conductance regulator (CFTR) channel. These models are rapidly informing mechanisms of disease pathogenesis and CFTR function regardless of how faithfully a given organ reproduces the human CF phenotype. New approaches of genetic engineering with RNA-guided nucleases are rapidly expanding both the potential types of models available and the approaches to correct the CFTR defect. The application of new CRISPR/Cas9 genome editing techniques are similarly increasing capabilities for in vitro modeling of CFTR functions in cell lines and primary cells using air-liquid interface cultures and organoids. Gene editing of CFTR mutations in somatic stem cells and induced pluripotent stem cells is also transforming gene therapy approaches for CF. This short review evaluates several areas that are key to building animal and cell systems capable of modeling CF disease and testing potential treatments.

Re-epithelialization: a key element in tracheal tissue engineering

Trachea-tissue engineering is a thriving new field in regenerative medicine that is reaching maturity and yielding numerous promising results. In view of the crucial role that the epithelium plays in the trachea, re-epithelialization of tracheal substitutes has gradually emerged as the focus of studies in tissue-engineered trachea. Recent progress in our understanding of stem cell biology, growth factor interactions and transplantation immunobiology offer the prospect of optimization of a tissue-engineered tracheal epithelium. In addition, advances in cell culture technology and successful applications of clinical transplantation are opening up new avenues for the construction of a tissue-engineered tracheal epithelium. Therefore, this review summarizes current advances, unresolved obstacles and future directions in the reconstruction of a tissue-engineered tracheal epithelium.

Targeted omics analyses, and metabolic enzyme activity assays demonstrate maintenance of key mucociliary characteristics in long term cultures of reconstituted human airway epithelia

3D reconstituted respiratory epithelia have emerged as better in vitro models for toxicological testing compared to cell lines due to the conservation of key morphological features and functions. MucilAir™ is a commercially available human airway epithelia system that can potentially maintain functional attributes for up to a year, however, detailed mucociliary characteristics and xenobiotic metabolism relevant to inhaled pro-toxicant bioactivation is lacking. Here, we assessed in MucilAir™ some key biomarkers that are characteristic of the respiratory epithelia including morphology, function and xenobiotics metabolism. The end points that were measured included targeted proteomics using a panel of 243 airway surface liquid (ASL) proteins, cilia beat frequency (CBF), a qRT-PCR screen of xenobiotic metabolizing enzymes, and CYP2A6/13, CYP1A1/1B1 activity. Comparison of ASL proteomics with human sputum identified key proteins common to both matrices, but present at different levels. Xenobiotic metabolism gene profiling demonstrated strong similarities with the normal human lung and did not reveal any consistent changes when assessed over a 6 month period. Inducibility and activity of CYP1A1/1B1 and activity of CYP2A6/2A13 were present at one month in culture and maintained in one tested MucilAir™ donor for several months. In conclusion, MucilAir™ presented important morphological and metabolic characteristics of a mucociliary epithelium in short and long term culture. MucilAir™ is therefore a potentially useful model to test repeated sub-cytotoxic doses of toxicants.

Engineering airway epithelium

Airway epithelium is constantly presented with injurious signals, yet under healthy circumstances, the epithelium maintains its innate immune barrier and mucociliary elevator function. This suggests that airway epithelium has regenerative potential (I. R. Telford and C. F. Bridgman, 1990). In practice, however, airway regeneration is problematic because of slow turnover and dedifferentiation of epithelium thereby hindering regeneration and increasing time necessary for full maturation and function. Based on the anatomy and biology of the airway epithelium, a variety of tissue engineering tools available could be utilized to overcome the barriers currently seen in airway epithelial generation. This paper describes the structure, function, and repair mechanisms in native epithelium and highlights specific and manipulatable tissue engineering signals that could be of great use in the creation of artificial airway epithelium.

Disruption of cytokeratin-8 interaction with F508del-CFTR corrects its functional defect

We have previously reported an increased expression of cytokeratins 8/18 (K8/K18) in cells expressing the F508del mutation of cystic fibrosis transmembrane conductance regulator (CFTR). This is associated with increased colocalization of CFTR and K18 in the vicinity of the endoplasmic reticulum, although this is reversed by treating cells with curcumin, resulting in the rescue of F508del-CFTR. In the present work, we hypothesized that (i) the K8/K18 network may interact physically with CFTR, and that (ii) this interaction may modify CFTR function. CFTR was immunoprecipitated from HeLa cells transfected with either wild-type (WT) CFTR or F508del-CFTR. Precipitates were subjected to 2D-gel electrophoresis and differential spots identified by mass spectrometry. K8 and K18 were found significantly increased in F508del-CFTR precipitates. Using surface plasmon resonance, we demonstrate that K8, but not K18, binds directly and preferentially to the F508del over the WT human NBD1 (nucleotide-binding domain-1). In vivo K8 interaction with F508del-CFTR was confirmed by proximity ligation assay in HeLa cells and in primary cultures of human respiratory epithelial cells. Ablation of K8 expression by siRNA in F508del-expressing HeLa cells led to the recovery of CFTR-dependent iodide efflux. Moreover, F508del-expressing mice topically treated with K8-siRNA showed restored nasal potential difference, equivalent to that of WT mice. These results show that disruption of F508del-CFTR and K8 interaction leads to the correction of the F508del-CFTR processing defect, suggesting a novel potential therapeutic target in CF.