Engineering three-dimensional pulmonary tissue constructs

Recent Advances in Graphene Oxide-Based on Organoid Culture as Disease Model and Cell Behavior - A Systematic Literature Review

Due to their ability to replicate the in vivo microenvironment through cell interaction and induce cells to stimulate cell function, three-dimensional cell culture models can overcome the limitations of two-dimensional models. Organoids are 3D models that demonstrate the ability to replicate the natural structure of an organ. In most organoid tissue cultures, matrigel made of a mouse tumor extracellular matrix protein mixture is an essential ingredient. However, its tumor-derived origin, batch-to-batch variation, high cost, and safety concerns have limited the usefulness of organoid drug development and regenerative medicine. Its clinical application has also been hindered by the fact that organoid generation is dependent on the use of poorly defined matrices. Therefore, matrix optimization is a crucial step in developing organoid culture that introduces alternatives as different materials. Recently, a variety of substitute materials has reportedly replaced matrigel. The purpose of this study is to review the significance of the latest advances in materials for cell culture applications and how they enhance build network systems by generating proper cell behavior. Excellence in cell behavior is evaluated from their cell characteristics, cell proliferation, cell differentiation, and even gene expression. As a result, graphene oxide as a matrix optimization demonstrated high potency in developing organoid models. Graphene oxide can promote good cell behavior and is well known for having good biocompatibility. Hence, advances in matrix optimization of graphene oxide provide opportunities for the future development of advanced organoid models.

Effectiveness of extracellular vesicles derived from hiPSCs in repairing hyperoxia-induced injury in a fetal murine lung explant model

BACKGROUND

Despite advances in neonatal care, the incidence of Bronchopulmonary Dysplasia (BPD) remains high among preterm infants. Human induced pluripotent stem cells (hiPSCs) have shown promise in repairing injury in animal BPD models. Evidence suggests they exert their effects via paracrine mechanisms. We aim herein to assess the effectiveness of extracellular vesicles (EVs) derived from hiPSCs and their alveolar progenies (diPSCs) in attenuating hyperoxic injury in a preterm lung explant model.

METHODS

Murine lung lobes were harvested on embryonic day 17.5 and maintained in air-liquid interface. Following exposure to 95% O2 for 24 h, media was supplemented with 5 × 106 particles/mL of EVs isolated from hiPSCs or diPSCs by size-exclusion chromatography. On day 3, explants were assessed using Hematoxylin-Eosin staining with mean linear intercept (MLI) measurements, immunohistochemistry, VEGFa and antioxidant gene expression. Statistical analysis was conducted using one-way ANOVA and Multiple Comparison Test. EV proteomic profiling was performed, and annotations focused on alveolarization and angiogenesis signaling pathways, as well as anti-inflammatory, anti-oxidant, and regenerative pathways.

RESULTS

Exposure of fetal lung explants to hyperoxia induced airspace enlargement, increased MLI, upregulation of anti-oxidants Prdx5 and Nfe2l2 with decreased VEGFa expression. Treatment with hiPSC-EVs improved parenchymal histologic changes. No overt changes in vasculature structure were observed on immunohistochemistry in our in vitro model. However, VEGFa and anti-oxidant genes were upregulated with diPSC-EVs, suggesting a pro-angiogenic and cytoprotective potential. EV proteomic analysis provided new insights in regard to potential pathways influencing lung regeneration.

CONCLUSION

This proof-of-concept in vitro study reveals a potential role for hiPSC- and diPSC-EVs in attenuating lung changes associated with prematurity and oxygen exposure. Our findings pave the way for a novel cell free approach to prevent and/or treat BPD, and ultimately reduce the global burden of the disease.

Advances in lung bioengineering: Where we are, where we need to go, and how to get there

Lung transplantation is the only potentially curative treatment for end-stage lung failure and successfully improves both long-term survival and quality of life. However, lung transplantation is limited by the shortage of suitable donor lungs. This discrepancy in organ supply and demand has prompted researchers to seek alternative therapies for end-stage lung failure. Tissue engineering (bioengineering) organs has become an attractive and promising avenue of research, allowing for the customized production of organs on demand, with potentially perfect biocompatibility. While breakthroughs in tissue engineering have shown feasibility in practice, they have also uncovered challenges in solid organ applications due to the need not only for structural support, but also vascular membrane integrity and gas exchange. This requires a complex engineered interaction of multiple cell types in precise anatomical locations. In this article, we discuss the process of creating bioengineered lungs and the challenges inherent therein. We summarize the relevant literature for selecting appropriate lung scaffolds, creating decellularization protocols, and using bioreactors. The development of completely artificial lung substitutes will also be reviewed. Lastly, we describe the state of current research, as well as future studies required for bioengineered lungs to become a realistic therapeutic modality for end-stage lung disease. Applications of bioengineering may allow for earlier intervention in end-stage lung disease and have the potential to not only halt organ failure, but also significantly reverse disease progression.

Protocol-Dependent Morphological Changes in Human Embryonic Stem Cell Aggregates during Differentiation toward Early Pancreatic Fate

Cell therapy is one of the promising approaches used against type 1 diabetes. Efficient generation of human embryonic stem cell (hESC)-derived pancreatic progenitors (PPs) is of great importance. Since signaling pathways underlying human pancreas development are not yet fully understood, various differentiation protocols are conducted, each considering variable duration, timing, and concentrations of growth factors and small molecules. Therefore, we compared two PP differentiation protocols in static suspension culture. We tested modified protocols developed by Pagliuca et al. (protocol 1) and Royan researchers (protocol 2) until early PP stage. The morphological changes of hESC aggregates during differentiation, and also gene and protein expression after differentiation, were evaluated. Different morphological structures were formed in each protocol. Quantitative gene expression analysis, flow cytometry, and immunostaining revealed a high level of PDX1 expression on day 13 of Royan's differentiation protocol compared to protocol 1. Our data showed that using protocol 2, cells were further differentiated until day 16, showing higher efficiency of early PPs. Moreover, protocol 2 is able to produce hESCs-PPs in a static suspension culture. Since protocol 2 is inexpensive in terms of media, growth factors, and chemicals, it can be used for massive production of PPs using static and dynamic suspension cultures.

Multiple Cell Cultures for MRI Analysis

Magnetic resonance imaging (MRI) is an imaging method that enables diagnostics. In recent years, this technique has been widely used for research using cell cultures used in pharmaceutical science to understand the distribution of various drugs in a variety of biological samples, from cellular models to tissues. MRI's dynamic development in recent years, in addition to diagnostics, has allowed the method to be implemented to assess response to applied therapies. Conventional MRI imaging provides anatomical and pathological information. Due to advanced technology, MRI provides physiological information. The use of cell cultures is very important in the process of testing new synthesized drugs, cancer research, and stem cell research, among others. Two-dimensional (2D) cell cultures conducted under laboratory conditions, although they provide a lot of information, do not reflect the basic characteristics of the tumor. To replicate the tumor microenvironment in science, a three-dimensional (3D) culture of tumor cells was developed. This makes it possible to reproduce in vivo conditions where, in addition, there is a complex and dynamic process of cell-to-cell communication and cell-matrix interaction. In this work, we reviewed current research in 2D and 3D cultures and their use in MRI studies. Articles for each section were collected from PubMed, ScienceDirect, Web of Science, and Google Scholar.

Organoid transduction using recombinant adeno-associated viral vectors: Challenges and opportunities

Cellular 3D structures, for example, organoids, are an excellent model for studying and developing treatments for various diseases, including hereditary ones. Therefore, they are increasingly being used in biomedical research. From the point of view of safety and efficacy, recombinant adeno-associated viral (rAAV) vectors are currently most in demand for the delivery of various transgenes for gene replacement therapy or other applications. The delivery of transgenes using rAAV vectors to various types of organoids is an urgent task, however, it is associated with a number of problems that are discussed in this review. Cellular heterogeneity and specifics of cultivation of 3D structures determine the complexity of rAAV delivery and are sometimes associated with low transduction efficiency. This review surveys the main ways to solve emerging problems and increase the efficiency of transgene delivery using rAAVs to organoids. A clear understanding of the stage of development of the organoid, its cellular composition and the presence of surface receptors will allow obtaining high levels of organoid transduction with existing rAAV vectors.

Additive manufacturing in respiratory sciences - Current applications and future prospects

Additive Manufacturing (AM) comprises a variety of techniques that enable fabrication of customised objects with specific attributes. The versatility of AM procedures and constant technological improvements allow for their application in the development of medicinal products and medical devices. This review provides an overview of AM applications related to respiratory sciences. For this purpose, both fields of research are briefly introduced and the potential benefits of integrating AM to respiratory sciences at different levels of pharmaceutical development are highlighted. Tailored manufacturing of microstructures as a particle design approach in respiratory drug delivery will be discussed. At the dosage form level, we exemplify AM as an important link in the iterative loop of data driven inhaler design, rapid prototyping and in vitro testing. This review also presents the application of bioprinting in the respiratory field for design of biorelevant in vitro cellular models, followed by an overview of AM-related processes in preventive and therapeutic care. Finally, this review discusses future prospects of AM as a component in a digital health environment.

Autonomously Propelled Colloids for Penetration and Payload Delivery in Complex Extracellular Matrices

For effective treatment of diseases such as cancer or fibrosis, it is essential to deliver therapeutic agents such as drugs to the diseased tissue, but these diseased sites are surrounded by a dense network of fibers, cells, and proteins known as the extracellular matrix (ECM). The ECM forms a barrier between the diseased cells and blood circulation, the main route of administration of most drug delivery nanoparticles. Hence, a stiff ECM impedes drug delivery by limiting the transport of drugs to the diseased tissue. The use of self-propelled particles (SPPs) that can move in a directional manner with the application of physical or chemical forces can help in increasing the drug delivery efficiency. Here, we provide a comprehensive look at the current ECM models in use to mimic the in vivo diseased states, the different types of SPPs that have been experimentally tested in these models, and suggest directions for future research toward clinical translation of SPPs in diverse biomedical settings.

SARS-CoV-2 and tissue damage: current insights and biomaterial-based therapeutic strategies

The effect of SARS-CoV-2 infection on humanity has gained worldwide attention and importance due to the rapid transmission, lack of treatment options and high mortality rate of the virus. While scientists across the world are searching for vaccines/drugs that can control the spread of the virus and/or reduce the risks associated with infection, patients infected with SARS-CoV-2 have been reported to have tissue/organ damage. With most tissues/organs having limited regenerative potential, interventions that prevent further damage or facilitate healing would be helpful. In the past few decades, biomaterials have gained prominence in the field of tissue engineering, in view of their major role in the regenerative process. Here we describe the effect of SARS-CoV-2 on multiple tissues/organs, and provide evidence for the positive role of biomaterials in aiding tissue repair. These findings are further extrapolated to explore their prospects as a therapeutic platform to address the tissue/organ damage that is frequently observed during this viral outbreak. This study suggests that the biomaterial-based approach could be an effective strategy for regenerating tissues/organs damaged by SARS-CoV-2.

3D printed hydrogels with oxidized cellulose nanofibers and silk fibroin for the proliferation of lung epithelial stem cells

A novel biomaterial ink consisting of regenerated silk fibroin (SF) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized bacterial cellulose (OBC) nanofibrils was developed for 3D printing lung tissue scaffold. Silk fibroin backbones were cross-linked using horseradish peroxide/H₂O₂ to form printed hydrogel scaffolds. OBC with a concentration of 7wt% increased the viscosity of inks during the printing process and further improved the shape fidelity of the scaffolds. Rheological measurements and image analyses were performed to evaluate inks printability and print shape fidelity. Three-dimensional construct with ten layers could be printed with ink of 1SF-2OBC (SF/OBC = 1/2, w/w). The composite hydrogel of 1SF-1OBC (SF/OBC = 1/1, w/w) printed at 25 °C exhibited a significantly improved compressive strength of 267 ± 13 kPa and a compressive stiffness of 325 ± 14 kPa at 30% strain, respectively. The optimized printing parameters for 1SF-1OBC were 0.3 bar of printing pressure, 45 mm/s of printing speed and 410 μm of nozzle diameter. Furthermore, OBC nanofibrils could be induced to align along the print lines over 60% degree of orientation, which were analyzed by SEM and X-ray diffraction. The orientation of OBC nanofibrils along print lines provided physical cues for guiding the orientation of lung epithelial stem cells, which maintained the ability to proliferate and kept epithelial phenotype after 7 days' culture. The 3D printed SF-OBC scaffolds are promising for applications in lung tissue engineering.

Bioengineering lungs - current status and future prospects

INTRODUCTION

Once pulmonary disease progresses to end-stage pulmonary disease, treatment options are very limited. An important advance in the field is the development of a bioartificial lung derived from a generic matrix scaffold populated with patients' own cells. Significant progress has already been made in the engineering of bioartificial lungs.

AREAS COVERED

This review explains how previous and current research contributes to the goal of creating a successful bioartificial lung, and the barriers faced in doing so. We will also highlight some of the design considerations being explored to optimize bioartificial lungs and considerations for clinical translation.

EXPERT OPINION

While current bioartificial lungs are able to provide short-term gas exchange in large-animal studies, much work is still required to combine the disciplines of cell biology, materials science, and tissue engineering to create such clinically useful and functioning artificial lungs.

Synthetic alternatives to Matrigel

Matrigel, a basement-membrane matrix extracted from Engelbreth-Holm-Swarm mouse sarcomas, has been used for more than four decades for a myriad of cell culture applications. However, Matrigel is limited in its applicability to cellular biology, therapeutic cell manufacturing and drug discovery owing to its complex, ill-defined and variable composition. Variations in the mechanical and biochemical properties within a single batch of Matrigel - and between batches - have led to uncertainty in cell culture experiments and a lack of reproducibility. Moreover, Matrigel is not conducive to physical or biochemical manipulation, making it difficult to fine-tune the matrix to promote intended cell behaviours and achieve specific biological outcomes. Recent advances in synthetic scaffolds have led to the development of xenogenic-free, chemically defined, highly tunable and reproducible alternatives. In this Review, we assess the applications of Matrigel in cell culture, regenerative medicine and organoid assembly, detailing the limitations of Matrigel and highlighting synthetic scaffold alternatives that have shown equivalent or superior results. Additionally, we discuss the hurdles that are limiting a full transition from Matrigel to synthetic scaffolds and provide a brief perspective on the future directions of synthetic scaffolds for cell culture applications.

High-Throughput Manufacture of 3D Fiber Scaffolds for Regenerative Medicine

Engineered scaffolds used to regenerate mammalian tissues should recapitulate the underlying fibrous architecture of native tissue to achieve comparable function. Current fibrous scaffold fabrication processes, such as electrospinning and three-dimensional (3D) printing, possess application-specific advantages, but they are limited either by achievable fiber sizes and pore resolution, processing efficiency, or architectural control in three dimensions. As such, a gap exists in efficiently producing clinically relevant, anatomically sized scaffolds comprising fibers in the 1-100 μm range that are highly organized. This study introduces a new high-throughput, additive fibrous scaffold fabrication process, designated in this study as 3D melt blowing (3DMB). The 3DMB system described in this study is modified from larger nonwovens manufacturing machinery to accommodate the lower volume, high-cost polymers used for tissue engineering and implantable biomedical devices and has a fiber collection component that uses adaptable robotics to create scaffolds with predetermined geometries. The fundamental process principles, system design, and key parameters are described, and two examples of the capabilities to create scaffolds for biomedical engineering applications are demonstrated. Impact statement Three-dimensional melt blowing (3DMB) is a new, high-throughput, additive manufacturing process to produce scaffolds composed of highly organized fibers in the anatomically relevant 1-100 μm range. Unlike conventional melt-blowing systems, the 3DMB process is configured for efficient use with the relatively expensive polymers necessary for biomedical applications, decreasing the required amounts of material for processing while achieving high throughputs compared with 3D printing or electrospinning. The 3DMB is demonstrated to make scaffolds composed of multiple fiber materials and organized into complex shapes, including those typical of human body parts.

Biomimetic collagen biomaterial induces in situ lung regeneration by forming functional alveolar

In situ restoration of severely damaged lung remains difficult due to its limited regeneration capacity after injury. Artificial lung scaffolds are emerging as potential substitutes, but it is still a challenge to reconstruct lung regeneration microenvironment in scaffold after lung resection injury. Here, a 3D biomimetic porous collagen scaffold with similar structure characteristics as lung is fabricated, and a novel collagen binding hepatocyte growth factor (CBD-HGF) is tethered on the collagen scaffold for maintaining the biomimetic function of HGF to improve the lung regeneration microenvironment. The biomimetic scaffold was implanted into the operative region of a rat partial lung resection model. The results revealed that vascular endothelial cells and endogenous alveolar stem cells entered the scaffold at the early stage of regeneration. At the later stage, inflammation and fibrosis were attenuated, the microvascular and functional alveolar-like structures were formed, and the general morphology of the injured lung was restored. Taken together, the functional 3D biomimetic collagen scaffold facilitates recovery of the injured lung, alveolar regeneration, and angiogenesis after acute lung injury. Particularly, this is the first study of lung regeneration in vivo guided by biomimetic collagen scaffold materials, which supports the concept that tissue engineering is an effective strategy for alveolar regeneration.

Characterization and bioactivity of self-assembled anti-angiogenic chondroitin sulfate-ES2-AF nanoparticle conjugate

BACKGROUND

In the past few years, significant progress has been made in inhibiting neovascularization at the tumor site, cutting off the nutrient supply of the tumor, and inhibiting tumor growth and metastasis. However, many proteins/peptides have the disadvantage of poor stability, short half-life, and uncertain targeting ability. Chemical modification can be used to overcome these disadvantages; many polyethylene glycol-modified proteins/peptides have been approved by US FDA. The purpose of this study was to obtain a novel anti-angiogenic chondroitin sulfate (CS)-peptide nanoparticle conjugate with efficient anti-neovascularization and tumor targeting ability and an acceptable half-life.

MATERIALS AND METHODS

The CS-ES2-AF nanoparticle conjugate was synthesized and characterized using 1H-nuclear magnetic resonance spectroscopy, transmission electron microscopy, and particle size and zeta potential analyzer. The anti-angiogenic ability was studied using MTT, migration, tube formation, and chick chorioallantoic membrane assays. The targeting ability of CS-ES2-AF was studied by ELISA, surface plasmon resonance, and bioimaging. The pharmacokinetics was also studied.

RESULTS

The CS-ES2-AF could self-assemble into stable nanoparticles in aqueous solution, which significantly enhances its anti-neovascularization activity, tumor targeting more explicit, and prolongs its half-life.

CONCLUSION

CS is an effective protein/peptide modifier, and CS-ES2-AF displayed good potential in tumor targeting therapy.

Lung tissue engineering: An update

Pulmonary disease is a worldwide public health problem that reduces the life quality and increases the need for hospital admissions as well as the risk of premature death. A common problem is the significant shortage of lungs for transplantation as well as patients must also take immunosuppressive drugs for the rest of their lives to keep the immune system from attacking transplanted organs. Recently, a new strategy has been proposed in the cellular engineering of lung tissue as decellularization approaches. The main components for the lung tissue engineering are: (1) A suitable biological or synthetic three-dimensional (3D) scaffold, (2) source of stem cells or cells, (3) growth factors required to drive cell differentiation and proliferation, and (4) bioreactor, a system that supports a 3D composite biologically active. Although a number of synthetic as well biological 3D scaffold suggested for lung tissue engineering, the current favorite scaffold is decellularized extracellular matrix scaffold. There are a large number of commercial and academic made bioreactors, the favor has been, the one easy to sterilize, physiologically stimuli and support active cell growth as well as clinically translational. The challenges would be to develop a functional lung will depend on the endothelialized microvascular network and alveolar-capillary surface area to exchange gas. A critical review of the each components of lung tissue engineering is presented, following an appraisal of the literature in the last 5 years. This is a multibillion dollar industry and consider unmet clinical need.

How to build a lung: latest advances and emerging themes in lung bioengineering

Chronic respiratory diseases remain a major cause of morbidity and mortality worldwide. The only option at end-stage disease is lung transplantation, but there are not enough donor lungs to meet clinical demand. Alternative options to increase tissue availability for lung transplantation are urgently required to close the gap on this unmet clinical need. A growing number of tissue engineering approaches are exploring the potential to generate lung tissue ex vivo for transplantation. Both biologically derived and manufactured scaffolds seeded with cells and grown ex vivo have been explored in pre-clinical studies, with the eventual goal of generating functional pulmonary tissue for transplantation. Recently, there have been significant efforts to scale-up cell culture methods to generate adequate cell numbers for human-scale bioengineering approaches. Concomitantly, there have been exciting efforts in designing bioreactors that allow for appropriate cell seeding and development of functional lung tissue over time. This review aims to present the current state-of-the-art progress for each of these areas and to discuss promising new ideas within the field of lung bioengineering.

The long and winding road: stem cells for cystic fibrosis

INTRODUCTION

Cystic fibrosis (CF) is a genetic syndrome with a high mortality rate due to severe lung disease. Despite having several drugs targeting specific mutated CFTR proteins already in clinical trials, new therapies, based on stem cells, are also emerging to treat those patients.

AREAS COVERED

The authors review the main sources of stem cells, including embryonic stem cells (ESCs), induced-pluripotent stem cells (iPSCs), gestational stem cells, and adult stem cells, such as mesenchymal stem cells (MSCs) in the context of CF. Furthermore, they describe the main animal and human models of lung physiology and pathology, involved in the optimization of these stem cell-applied therapies in CF.

EXPERT OPINION

ESCs and iPSCs are emerging sources for disease modeling and drug discovery purposes. The allogeneic transplant of healthy MSCs, that acts independently to specific mutations, is under intense scrutiny due to their secretory, immunomodulatory, anti-inflammatory and anti-bacterial properties. The main challenge for future developments will be to get exogenous stem cells into the appropriate lung location, where they can regenerate endogenous stem cells and act as inflammatory modulators. The clinical application of stem cells for the treatment of CF certainly warrants further insight into pre-clinical models, including large animals, organoids, decellularized organs and lung bioengineering.

When stem cells meet graphene: Opportunities and challenges in regenerative medicine

Recent advances in stem cell research and nanotechnology have significantly influenced the landscape of tissue engineering and regenerative medicine. Precise and reproducible control of the fate of stem cells and their lineage specification have, therefore, become more crucial than ever for the success of stem cell-based technologies. Extensive research has been geared towards developing materials that are capable of mimicking the physiological microenvironment of stem cells and at the same time, controlling their eventual fate. An interesting example of these materials is two-dimensional graphene and its related derivatives. A high specific surface area coupled with superior chemical stability, biocompatibility, and flexibility in functionalization render graphene-based nanomaterials one of the most exciting platforms for tissue engineering and regenerative medicine applications, especially for stem cell growth, proliferation, and differentiation. In this review, we discuss the love-hate relationship between stem cells and graphene-based nanomaterials in tissue engineering and regenerative medicine. We first discuss the role and importance of stem cells in tissue engineering and regenerative medicine. We then highlight the use of nanomaterials for stem cell control, the interaction between stem cells and graphene nanomaterials as well as their biocompatibility, biodistribution, and biodegradability considerations. We also offer our perspectives on the various challenges and opportunities facing the use of graphene and its derivatives for stem cell growth and differentiation.

Engineering epithelial-stromal interactions in vitro for toxicology assessment

Crosstalk between epithelial and stromal cells drives the morphogenesis of ectodermal organs during development and promotes normal mature adult epithelial tissue homeostasis. Epithelial-stromal interactions (ESIs) have historically been examined using mammalian models and ex vivo tissue recombination. Although these approaches have elucidated signaling mechanisms underlying embryonic morphogenesis processes and adult mammalian epithelial tissue function, they are limited by the availability of tissue, low throughput, and human developmental or physiological relevance. In this review, we describe how bioengineered ESIs, using either human stem cells or co-cultures of human primary epithelial and stromal cells, have enabled the development of human in vitro epithelial tissue models that recapitulate the architecture, phenotype, and function of adult human epithelial tissues. We discuss how the strategies used to engineer mature epithelial tissue models in vitro could be extrapolated to instruct the design of organotypic culture models that can recapitulate the structure of embryonic ectodermal tissues and enable the in vitro assessment of events critical to organ/tissue morphogenesis. Given the importance of ESIs towards normal epithelial tissue development and function, such models present a unique opportunity for toxicological screening assays to incorporate ESIs to assess the impact of chemicals on mature and developing epidermal tissues.

Kidney Organoids: A Translational Journey

Human pluripotent stem cells (hPSCs) are attractive sources for regenerative medicine and disease modeling in vitro. Directed hPSC differentiation approaches have derived from knowledge of cell development in vivo rather than from stochastic cell differentiation. Moreover, there has been great success in the generation of 3D organ-buds termed 'organoids' from hPSCs; these consist of a variety of cell types in vitro that mimic organs in vivo. The organoid bears great potential in the study of human diseases in vitro, especially when combined with CRISPR/Cas9-based genome-editing. We summarize the current literature describing organoid studies with a special focus on kidney organoids, and discuss goals and future opportunities for organoid-based studies.

Genome engineering of stem cell organoids for disease modeling

Precision medicine emerges as a new approach that takes into account individual variability. Successful realization of precision medicine requires disease models that are able to incorporate personalized disease information and recapitulate disease development processes at the molecular, cellular and organ levels. With recent development in stem cell field, a variety of tissue organoids can be derived from patient specific pluripotent stem cells and adult stem cells. In combination with the state-of-the-art genome editing tools, organoids can be further engineered to mimic disease-relevant genetic and epigenetic status of a patient. This has therefore enabled a rapid expansion of sophisticated in vitro disease models, offering a unique system for fundamental and biomedical research as well as the development of personalized medicine. Here we summarize some of the latest advances and future perspectives in engineering stem cell organoids for human disease modeling.

Nano-Therapeutics for the Lung: State-of-the-Art and Future Perspectives

Inhalation of aerosolized compounds is a popular, non-invasive route for the targeted delivery of therapeutic molecules to the lung. Various types of nanoparticles have been used as carriers to facilitate drug uptake and intracellular action in order to treat lung diseases and/or to facilitate lung repair and growth. These include polymeric nanoparticles, liposomes, and dendrimers, among many others. In addition, nanoparticles are sometimes used in combination with small molecules, cytokines, growth factors, and/or pluripotent stem cells. Here we review the rationale and state-of-the-art nanotechnology for pulmonary drug delivery, with particular attention to new technological developments and approaches as well as the challenges associated with them, the emerging advances, and opportunities for future development in this field.

Organoids as an in vitro model of human development and disease

The in vitro organoid model is a major technological breakthrough that has already been established as an essential tool in many basic biology and clinical applications. This near-physiological 3D model facilitates an accurate study of a range of in vivo biological processes including tissue renewal, stem cell/niche functions and tissue responses to drugs, mutation or damage. In this Review, we discuss the current achievements, challenges and potential applications of this technique.

Toxicological Considerations, Toxicity Assessment, and Risk Management of Inhaled Nanoparticles

Novel engineered nanoparticles (NPs), nanomaterial (NM) products and composites, are continually emerging worldwide. Many potential benefits are expected from their commercial applications; however, these benefits should always be balanced against risks. Potential toxic effects of NM exposure have been highlighted, but, as there is a lack of understanding about potential interactions of nanomaterials (NMs) with biological systems, these side effects are often ignored. NPs are able to translocate to the bloodstream, cross body membrane barriers effectively, and affect organs and tissues at cellular and molecular levels. NPs may pass the blood–brain barrier (BBB) and gain access to the brain. The interactions of NPs with biological milieu and resulted toxic effects are significantly associated with their small size distribution, large surface area to mass ratio (SA/MR), and surface characteristics. NMs are able to cross tissue and cell membranes, enter into cellular compartments, and cause cellular injury as well as toxicity. The extremely large SA/MR of NPs is also available to undergo reactions. An increased surface area of the identical chemical will increase surface reactivity, adsorption properties, and potential toxicity. This review explores biological pathways of NPs, their toxic potential, and underlying mechanisms responsible for such toxic effects. The necessity of toxicological risk assessment to human health should be emphasised as an integral part of NM design and manufacture.

Coming to terms with tissue engineering and regenerative medicine in the lung

Lung diseases such as emphysema, interstitial fibrosis, and pulmonary vascular diseases cause significant morbidity and mortality, but despite substantial mechanistic understanding, clinical management options for them are limited, with lung transplantation being implemented at end stages. However, limited donor lung availability, graft rejection, and long-term problems after transplantation are major hurdles to lung transplantation being a panacea. Bioengineering the lung is an exciting and emerging solution that has the ultimate aim of generating lung tissues and organs for transplantation. In this article we capture and review the current state of the art in lung bioengineering, from the multimodal approaches, to creating anatomically appropriate lung scaffolds that can be recellularized to eventually yield functioning, transplant-ready lungs. Strategies for decellularizing mammalian lungs to create scaffolds with native extracellular matrix components vs. de novo generation of scaffolds using biocompatible materials are discussed. Strengths vs. limitations of recellularization using different cell types of various pluripotency such as embryonic, mesenchymal, and induced pluripotent stem cells are highlighted. Current hurdles to guide future research toward achieving the clinical goal of transplantation of a bioengineered lung are discussed.

Synthetic Extracellular Microenvironment for Modulating Stem Cell Behaviors

The innate ability of stem cells to self-renew and differentiate into multiple cell types makes them a promising source for tissue engineering and regenerative medicine applications. Their capacity for self-renewal and differentiation is largely influenced by the combination of physical, chemical, and biological signals found in the stem cell niche, both temporally and spatially. Embryonic and adult stem cells are potentially useful for cell-based approaches; however, regulating stem cell behavior remains a major challenge in their clinical use. Most of the current approaches for controlling stem cell fate do not fully address all of the complex signaling pathways that drive stem cell behaviors in their natural microenvironments. To overcome this limitation, a new generation of biomaterials is being developed for use as three-dimensional synthetic microenvironments that can mimic the regulatory characteristics of natural extracellular matrix (ECM) proteins and ECM-bound growth factors. These synthetic microenvironments are currently being investigated as a substrate with surface immobilization and controlled release of bioactive molecules to direct the stem cell fate in vitro, as a tissue template to guide and improve the neo-tissue formation both in vitro and in vivo, and as a delivery vehicle for cell therapy in vivo. The continued advancement of such an intelligent biomaterial system as the synthetic extracellular microenvironment holds the promise of improved therapies for numerous debilitating medical conditions for which no satisfactory cure exists today.

In vitro model alveoli from photodegradable microsphere templates

Recreating the 3D cyst-like architecture of the alveolar epithelium in vitro has been challenging to achieve in a controlled fashion with primary lung epithelial cells. Here, we demonstrate model alveoli formed within a tunable synthetic biomaterial platform using photodegradable microspheres as templates to create physiologically relevant, cyst structures. Poly(ethylene glycol) (PEG)-based hydrogels were polymerized in suspension to form microspheres on the order of 120 μm in diameter. The gel chemistry was designed to allow erosion of the microspheres with cytocompatible light doses (≤15 min exposure to 10 mW cm(-2) of 365 nm light) via cleavage of a photolabile nitrobenzyl ether crosslinker. Epithelial cells were incubated with intact microspheres, modified with adhesive peptide sequences to facilitate cellular attachment to and proliferation on the surface. A tumor-derived alveolar epithelial cell line, A549, completely covered the microspheres after only 24 hours, whereas primary mouse alveolar epithelial type II (ATII) cells took ∼3 days. The cell-laden microsphere structures were embedded within a second hydrogel formulation at user defined densities; the microsphere templates were subsequently removed with light to render hollow epithelial cysts that were cultured for an additional 6 days. The resulting primary cysts stained positive for cell-cell junction proteins (β-catenin and ZO-1), indicating the formation of a functional epithelial layer. Typically, primary ATII cells differentiated in culture to the alveolar epithelial type I (ATI) phenotype; however, each cyst contained ∼1-5 cells that stained positive for an ATII marker (surfactant protein C), which is consistent with ATII cell numbers in native mouse alveoli. This biomaterial-templated alveoli culture system should be useful for future experiments to study lung development and disease progression, and is ideally suited for co-culture experiments where pulmonary fibroblasts or endothelial cells could be presented in the hydrogel surrounding the epithelial cysts.

Respiratory Tissue Engineering: Current Status and Opportunities for the Future

Currently, lung disease and major airway trauma constitute a major global healthcare burden with limited treatment options. Airway diseases such as chronic obstructive pulmonary disease and cystic fibrosis have been identified as the fifth highest cause of mortality worldwide and are estimated to rise to fourth place by 2030. Alternate approaches and therapeutic modalities are urgently needed to improve clinical outcomes for chronic lung disease. This can be achieved through tissue engineering of the respiratory tract. Interest is growing in the use of airway tissue-engineered constructs as both a research tool, to further our understanding of airway pathology, validate new drugs, and pave the way for novel drug therapies, and also as regenerative medical devices or as an alternative to transplant tissue. This review provides a concise summary of the field of respiratory tissue engineering to date. An initial overview of airway anatomy and physiology is given, followed by a description of the stem cell populations and signaling processes involved in parenchymal healing and tissue repair. We then focus on the different biomaterials and tissue-engineered systems employed in upper and lower respiratory tract engineering and give a final perspective of the opportunities and challenges facing the field of respiratory tissue engineering.

Bioactive glasses beyond bone and teeth: emerging applications in contact with soft tissues

The applications of bioactive glasses (BGs) have to a great extent been related to the replacement, regeneration and repair of hard tissues, such as bone and teeth, and there is an extensive bibliography documenting the role of BGs as bone replacement materials and in bone tissue engineering applications. Interestingly, many of the biochemical reactions arising from the contact of BGs with bodily fluids, in particular the local increase in concentration of various ions at the glass-tissue interface, are also relevant to mechanisms involved in soft tissue regeneration. An increasing number of studies report on the application of BGs in contact with soft tissues, aiming at exploiting the well-known bioactive properties of BGs in soft tissue regeneration and wound healing. This review focuses on research, sometimes involving preliminary in vitro studies but also in vivo evidence, that demonstrates the suitability of BGs in contact with tissues outside the skeletal system, which includes studies investigating vascularization, wound healing and cardiac, lung, nerve, gastrointestinal, urinary tract and laryngeal tissue repair using BGs in various forms of particulates, fibers and nanoparticles with and without polymer components. Potentially active mechanisms of interaction of BGs and soft tissues based on the surface bioreactivity of BGs and on biomechanical stimuli affecting the soft tissue-BG collagenous bonding are discussed based on results in the literature.

Concise review: current status of stem cells and regenerative medicine in lung biology and diseases

Lung diseases remain a significant and devastating cause of morbidity and mortality worldwide. In contrast to many other major diseases, lung diseases notably chronic obstructive pulmonary diseases (COPDs), including both asthma and emphysema, are increasing in prevalence and COPD is expected to become the third leading cause of disease mortality worldwide by 2020. New therapeutic options are desperately needed. A rapidly growing number of investigations of stem cells and cell therapies in lung biology and diseases as well as in ex vivo lung bioengineering have offered exciting new avenues for advancing knowledge of lung biology as well as providing novel potential therapeutic approaches for lung diseases. These initial observations have led to a growing exploration of endothelial progenitor cells and mesenchymal stem (stromal) cells in clinical trials of pulmonary hypertension and COPD with other clinical investigations planned. Ex vivo bioengineering of the trachea, larynx, diaphragm, and the lung itself with both biosynthetic constructs as well as decellularized tissues have been used to explore engineering both airway and vascular systems of the lung. Lung is thus a ripe organ for a variety of cell therapy and regenerative medicine approaches. Current state-of-the-art progress for each of the above areas will be presented as will discussion of current considerations for cell therapy-based clinical trials in lung diseases.

Hypoxia enhances differentiation of mouse embryonic stem cells into definitive endoderm and distal lung cells

We investigated the effects of hypoxia on spontaneous (SP)- and activin A (AA)-induced definitive endoderm (DE) differentiation of mouse embryonic stem cells (mESCs) and their subsequent differentiation into distal pulmonary epithelial cells. SP differentiation for 6 days of mESCs toward endoderm at hypoxia of 1% O2, but not at 3% or 21% (normoxia), increased the expression of Sox17 and Foxa2 by 31- and 63-fold above maintenance culture, respectively. Treatment of mESCs with 20 ng/mL AA for 6 days under hypoxia further increased the expression of DE marker genes Sox17, Foxa2, and Cxcr4 by 501-, 1,483-, and 126-fold above maintenance cultures, respectively. Transient exposure to hypoxia, as short as 24 h, was sufficient to enhance AA-induced endoderm formation. The involvement of hypoxia-inducible factor (HIF)-1α and reactive oxygen species (ROS) in the AA-induced endoderm enrichment was assessed using HIF-1α(-/-) mESCs and the ROS scavenger N-acetylcysteine (NAC). Under SP conditions, HIF-1α(-/-) mESCs failed to increase the expression of endodermal marker genes but rather shifted toward ectoderm. Hypoxia induced only a marginal potentiation of AA-induced endoderm differentiation in HIF-1α(-/-) mESCs. Treatment of mESCs with AA and NAC led to a dose-dependent decrease in Sox17 and Foxa2 expression. In addition, the duration of exposure to hypoxia in the course of a recently reported lung differentiation protocol resulted in differentially enhanced expression of distal lung epithelial cell marker genes aquaporin 5 (Aqp5), surfactant protein C (Sftpc), and secretoglobin 1a1 (Scgb1a1) for alveolar epithelium type I, type II, and club cells, respectively. Our study is the first to show the effects of in vitro hypoxia on efficient formation of DE and lung lineages. We suggest that the extent of hypoxia and careful timing may be important components of in vitro differentiation bioprocesses for the differential generation of distal lung epithelial cells from pluripotent progenitors.

Three-dimensional organotypic culture: experimental models of mammalian biology and disease

Mammalian organs are challenging to study as they are fairly inaccessible to experimental manipulation and optical observation. Recent advances in three-dimensional (3D) culture techniques, coupled with the ability to independently manipulate genetic and microenvironmental factors, have enabled the real-time study of mammalian tissues. These systems have been used to visualize the cellular basis of epithelial morphogenesis, to test the roles of specific genes in regulating cell behaviours within epithelial tissues and to elucidate the contribution of microenvironmental factors to normal and disease processes. Collectively, these novel models can be used to answer fundamental biological questions and generate replacement human tissues, and they enable testing of novel therapeutic approaches, often using patient-derived cells.

Application of Nanoscaffolds in Mesenchymal Stem Cell-Based Therapy

Regenerative medicine is an alternative solution for organ transplantation. Stem cells and nanoscaffolds are two essential components in regenerative medicine. Mesenchymal stem cells (MSCs) are considered as primary adult stem cells with high proliferation capacity, wide differentiation potential, and immunosuppression properties which make them unique for regenerative medicine and cell therapy. Scaffolds are engineered nanofibers that provide suitable microenvironment for cell signalling which has a great influence on cell proliferation, differentiation, and biology. Recently, application of scaffolds and MSCs is being utilized in obtaining more homogenous population of MSCs with higher cell proliferation rate and greater differentiation potential, which are crucial factors in regenerative medicine. In this review, the definition, biology, source, characterization, and isolation of MSCs and current report of application of nanofibers in regenerative medicine in different lesions are discussed.

Engineering de novo assembly of fetal pulmonary organoids

Induction of morphogenesis by competent lung progenitor cells in a 3D environment is a central goal of pulmonary tissue engineering, yet little is known about the microenvironmental signals required to induce de novo assembly of alveolar-like tissue in vitro. In extending our previous reports of alveolar-like tissue formation by fetal pulmonary cells stimulated by exogenous fibroblast growth factors (FGFs), we identified some of the key endogenous mediators of FGF-driven morphogenesis (organoid assembly), for example, epithelial sacculation, endothelial network assembly, and epithelial-endothelial interfacing. Sequestration of endogenously secreted vascular endothelial growth factor-A (VEGF-A) potently inhibited endothelial network formation, with little or no effect on epithelial morphogenesis. Inhibition of endogenous sonic hedgehog (SHH) partially attenuated FGF-driven endothelial network formation, while the addition of exogenous SHH in the absence of FGFs was able to induce epithelial and endothelial morphogenesis, although with distinct morphological characteristics. Notably, SHH-induced endothelial networks exhibited fewer branch points, reduced sprouting behavior, and a periendothelial extracellular matrix (ECM) virtually devoid of tenascin-C (TN-C). By contrast, focal deposition of endogenous TN-C was observed in the ECM-surrounding endothelial networks of FGF-induced organoids, especially around sprouting tips. In the FGF-induced organoids, TN-C was also observed in the clefts of sacculated epithelium and at the epithelial-endothelial interface. In support of a critical role in the formation of alveolar-like tissue in vitro, TN-C blocking inhibited endothelial network formation and epithelial sacculation. Upon engraftment of in-vitro-generated pulmonary organoids beneath the renal capsule of syngeneic mice, robust neovascularization occurred in 5 days with a large contribution of patent vessels from engrafted organoids, providing proof of principle for exploring intrapulmonary engraftment of prevascularized hydrogel constructs. Expression of proSpC, VEGF-A, and TN-C following 1 week in vivo mirrored the patterns observed in vitro. Taken together, these findings advance our understanding of endogenous growth factor and ECM signals important for de novo formation of pulmonary tissue structures in vitro and demonstrate the potential of an organoid-based approach to lung tissue augmentation.

Modeling the lung: Design and development of tissue engineered macro- and micro-physiologic lung models for research use

Respiratory tract specific cell populations, or tissue engineered in vitro grown human lung, have the potential to be used as research tools to mimic physiology, toxicology, pathology, as well as infectious diseases responses of cells or tissues. Studies related to respiratory tract pathogenesis or drug toxicity testing in the past made use of basic systems where single cell populations were exposed to test agents followed by evaluations of simple cellular responses. Although these simple single-cell-type systems provided good basic information related to cellular responses, much more can be learned from cells grown in fabricated microenvironments which mimic in vivo conditions in specialized microfabricated chambers or by human tissue engineered three-dimensional (3D) models which allow for more natural interactions between cells. Recent advances in microengineering technology, microfluidics, and tissue engineering have provided a new approach to the development of 2D and 3D cell culture models which enable production of more robust human in vitro respiratory tract models. Complex models containing multiple cell phenotypes also provide a more reasonable approximation of what occurs in vivo without the confounding elements in the dynamic in vivo environment. The goal of engineering good 3D human models is the formation of physiologically functional respiratory tissue surrogates which can be used as pathogenesis models or in the case of 2D screening systems for drug therapy evaluation as well as human toxicity testing. We hope that this manuscript will serve as a guide for development of future respiratory tract model systems as well as a review of conventional models.

Clinical potentials of human pluripotent stem cells in lung diseases

Lung possesses very limited regenerative capacity. Failure to maintain homeostasis of lung epithelial cell populations has been implicated in the development of many life-threatening pulmonary diseases leading to substantial morbidity and mortality worldwide, and currently there is no known cure for these end-stage pulmonary diseases. Embryonic stem cells (ESCs) and somatic cell-derived induced pluripotent stem cells (iPSCs) possess unlimited self-renewal capacity and great potential to differentiate to various cell types of three embryonic germ layers (ectodermal, mesodermal, and endodermal). Therapeutic use of human ESC/iPSC-derived lung progenitor cells for regeneration of injured or diseased lungs will have an enormous clinical impact. This article provides an overview of recent advances in research on pluripotent stem cells in lung tissue regeneration and discusses technical challenges that must be overcome for their clinical applications in the future.

Can stem cells be used to generate new lungs? Ex vivo lung bioengineering with decellularized whole lung scaffolds

For patients with end-stage lung diseases, lung transplantation is the only available therapeutic option. However, the number of suitable donor lungs is insufficient and lung transplants are complicated by significant graft failure and complications of immunosuppressive regimens. An alternative to classic organ replacement is desperately needed. Engineering of bioartificial organs using either natural or synthetic scaffolds is an exciting new potential option for generation of functional pulmonary tissue for human clinical application. Natural organ scaffolds can be generated by decellularization of native tissues; these acellular scaffolds retain the native organ ultrastructure and can be seeded with autologous cells towards the goal of regenerating functional tissues. Several decellularization strategies have been employed for lungs; however, there is no consensus on the optimal approach. A variety of cell types have been investigated as potential candidates for effective recellularization of acellular lung scaffolds. Candidate cells that might be best utilized are those which can be easily and reproducibly isolated, expanded in vitro, seeded onto decellularized matrices, induced to differentiate into pulmonary lineage cells, and which survive to functional maturity. Whole lung cell suspensions, endogenous progenitor cells, embryonic and adult stem cells and induced pluripotent stem (iPS) cells have been investigated for their applicability to repopulate acellular lung matrices. Ideally, patient-derived autologous cells would be used for lung recellularization as they have the potential to reduce the need for post-transplant immunosuppression. Several studies have performed transplantation of rudimentary bioengineered lung scaffolds in animal models with limited, short-term functionality but much further study is needed.

Toxicological perspectives of inhaled therapeutics and nanoparticles

INTRODUCTION

The human respiratory system is an important route for the entry of inhaled therapeutics into the body to treat diseases. Inhaled materials may consist of gases, vapours, aerosols and particulates. In all cases, assessing the toxicological effect of inhaled therapeutics has many challenges.

AREAS COVERED

This article provides an overview of in vivo and in vitro models for testing the toxicity of inhaled therapeutics and nanoparticles implemented in drug delivery. Traditionally, inhalation toxicity has been performed on test animals to identify the median lethal concentration of airborne materials. Later maximum tolerable concentration denoted by LC0 has been introduced as a more ethically acceptable end point. More recently, in vitro methods have been developed, allowing the direct exposure of airborne material to cultured human target cells on permeable porous membranes at the air-liquid interface.

EXPERT OPINION

Modifications of current inhalation therapies, new pulmonary medications for respiratory diseases and implementation of the respiratory tract for systemic drug delivery are providing new challenges when conducting well-designed inhalation toxicology studies. In particular, the area of nanoparticles and nanocarriers is of critical toxicological concern. There is a need to develop toxicological test models, which characterise the toxic response and cellular interaction between inhaled particles and the respiratory system.

Advances in tissue engineering through stem cell-based co-culture

Stem cells are the future in tissue engineering and regeneration. In a co-culture, stem cells not only provide a target cell source with multipotent differentiation capacity, but can also act as assisting cells that promote tissue homeostasis, metabolism, growth and repair. Their incorporation into co-culture systems seems to be important in the creation of complex tissues or organs. In this review, critical aspects of stem cell use in co-culture systems are discussed. Direct and indirect co-culture methodologies used in tissue engineering are described, along with various characteristics of cellular interactions in these systems. Direct cell-cell contact, cell-extracellular matrix interaction and signalling via soluble factors are presented. The advantages of stem cell co-culture strategies and their applications in tissue engineering and regenerative medicine are portrayed through specific examples for several tissues, including orthopaedic soft tissues, bone, heart, vasculature, lung, kidney, liver and nerve. A concise review of the progress and the lessons learned are provided, with a focus on recent developments and their implications. It is hoped that knowledge developed from one tissue can be translated to other tissues. Finally, we address challenges in tissue engineering and regenerative medicine that can potentially be overcome via employing strategies for stem cell co-culture use.

Salivary gland cell differentiation and organization on micropatterned PLGA nanofiber craters

There is a need for an artificial salivary gland as a long-term remedy for patients suffering from salivary hypofunction, a leading cause of chronic xerostomia (dry mouth). Current salivary gland tissue engineering approaches are limited in that they either lack sufficient physical cues and surface area needed to facilitate epithelial cell differentiation, or they fail to provide a mechanism for assembling an interconnected branched network of cells. We have developed highly-ordered arrays of curved hemispherical "craters" in polydimethylsiloxane (PDMS) using wafer-level integrated circuit (IC) fabrication processes, and lined them with electrospun poly-lactic-co-glycolic acid (PLGA) nanofibers, designed to mimic the three-dimensional (3-D) in vivo architecture of the basement membrane surrounding spherical acini of salivary gland epithelial cells. These micropatterned scaffolds provide a method for engineering increased surface area and were additionally investigated for their ability to promote cell polarization. Two immortalized salivary gland cell lines (SIMS, ductal and Par-C10, acinar) were cultured on fibrous crater arrays of various radii and compared with those grown on flat PLGA nanofiber substrates, and in 3-D Matrigel. It was found that by increasing crater curvature, the average height of the cell monolayer of SIMS cells and to a lesser extent, Par-C10 cells, increased to a maximum similar to that seen in cells grown in 3-D Matrigel. Increasing curvature resulted in higher expression levels of tight junction protein occludin in both cell lines, but did not induce a change in expression of adherens junction protein E-cadherin. Additionally, increasing curvature promoted polarity of both cell lines, as a greater apical localization of occludin was seen in cells on substrates of higher curvature. Lastly, substrate curvature increased expression of the water channel protein aquaporin-5 (Aqp-5) in Par-C10 cells, suggesting that curved nanofiber substrates are more suitable for promoting differentiation of salivary gland cells.