Derivation of Airway Basal Stem Cells from Human Pluripotent Stem Cells

Mammalian tracheal development and reconstruction: insights from in vivo and in vitro studies

The trachea delivers inhaled air into the lungs for gas exchange. Anomalies in tracheal development can result in life-threatening malformations, such as tracheoesophageal fistula and tracheomalacia. Given the limitations of current therapeutic approaches, development of technologies for the reconstitution of a three-dimensional trachea from stem cells is urgently required. Recently, single-cell sequencing technologies and quantitative analyses from cell to tissue scale have been employed to decipher the cellular basis of tracheal morphogenesis. In this Review, recent advances in mammalian tracheal development and the generation of tracheal tissues from pluripotent stem cells are summarized.

3D Bioprinting for fabrication of tissue models of COVID-19 infection

Over the last few decades, the world has witnessed multiple viral pandemics, the current severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) pandemic being the worst and most devastating one, claiming millions of lives worldwide. Physicians, scientists, and engineers worldwide have joined hands in dealing with the current situation at an impressive speed and efficiency. One of the major reasons for the delay in response is our limited understanding of the mechanism of action and individual effects of the virus on different tissues and organs. Advances in 3D bioprinting have opened up a whole new area to explore and utilize the technology in fabricating models of these tissues and organs, recapitulating in vivo environment. These biomimetic models can not only be utilized in learning the infection pathways and drug toxicology studies but also minimize the need for animal models and shorten the time span for human clinical trials. The current review aims to integrate the existing developments in bioprinting techniques, and their implementation to develop tissue models, which has implications for SARS-CoV-2 infection. Future translation of these models has also been discussed with respect to the pandemic.

Developmental basis of trachea-esophageal birth defects

Trachea-esophageal defects (TEDs), including esophageal atresia (EA), tracheoesophageal fistula (TEF), and laryngeal-tracheoesophageal clefts (LTEC), are a spectrum of life-threatening congenital anomalies in which the trachea and esophagus do not form properly. Up until recently, the developmental basis of these conditions and how the trachea and esophagus arise from a common fetal foregut was poorly understood. However, with significant advances in human genetics, organoids, and animal models, and integrating single cell genomics with high resolution imaging, we are revealing the molecular and cellular mechanisms that orchestrate tracheoesophageal morphogenesis and how disruption in these processes leads to birth defects. Here we review the current understanding of the genetic and developmental basis of TEDs. We suggest future opportunities for integrating developmental mechanisms elucidated from animals and organoids with human genetics and clinical data to gain insight into the genotype-phenotype basis of these heterogeneous birth defects. Finally, we envision how this will enhance diagnosis, improve treatment, and perhaps one day, lead to new tissue replacement therapy.

Implications for Extracellular Matrix Interactions With Human Lung Basal Stem Cells in Lung Development, Disease, and Airway Modeling

The extracellular matrix (ECM) is not simply a quiescent scaffold. This three-dimensional network of extracellular macromolecules provides structural, mechanical, and biochemical support for the cells of the lung. Throughout life, the ECM forms a critical component of the pulmonary stem cell niche. Basal cells (BCs), the primary stem cells of the airways capable of differentiating to all luminal cell types, reside in close proximity to the basolateral ECM. Studying BC-ECM interactions is important for the development of therapies for chronic lung diseases in which ECM alterations are accompanied by an apparent loss of the lung's regenerative capacity. The complexity and importance of the native ECM in the regulation of BCs is highlighted as we have yet to create an in vitro culture model that is capable of supporting the long-term expansion of multipotent BCs. The interactions between the pulmonary ECM and BCs are, therefore, a vital component for understanding the mechanisms regulating BC stemness during health and disease. If we are able to replicate these interactions in airway models, we could significantly improve our ability to maintain basal cell stemness ex vivo for use in in vitro models and with prospects for cellular therapies. Furthermore, successful, and sustained airway regeneration in an aged or diseased lung by small molecules, novel compounds or via cellular therapy will rely upon both manipulation of the airway stem cells and their immediate niche within the lung. This review will focus on the current understanding of how the pulmonary ECM regulates the basal stem cell function, how this relationship changes in chronic disease, and how replicating native conditions poses challenges for ex vivo cell culture.

Gene Therapy: A Possible Alternative to CFTR Modulators?

Cystic fibrosis (CF) is a rare genetic disease that affects several organs, but lung disease is the major cause of morbidity and mortality. The gene responsible for CF, the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene, has been discovered in 1989. Since then, gene therapy i.e., defective gene replacement by a functional one, remained the ultimate goal but unfortunately, it has not yet been achieved. However, patients care and symptomatic treatments considerably increased CF patients’ life expectancy ranging from 5 years old in the 1960s to 40 today. In the last decade, research works on CFTR protein structure and activity led to the development of new drugs which, by readdressing CFTR to the plasma membrane (correctors) or by enhancing its transport activity (potentiators), allow, alone or in combination, an improvement of CF patients’ lung function and quality of life. While expected, it is not yet known whether taking these drugs from an early age and for years will improve the quality of life of CF patients in the long term and further increase their life expectancy. Besides, these molecules are not available (specific variants of CFTR) or accessible (national health policies) for all patients and there is still no curative treatment. Another alternative that could benefit from new technologies, such as gene therapy, is therefore still attractive, although it is not yet offered to patients. Faced with the development of new CFTR correctors and potentiators, the question arises as to whether there is still a place for gene therapy and this is discussed in this perspective.

Discovery of CFTR modulators for the treatment of cystic fibrosis

INTRODUCTION

Cystic fibrosis (CF) is a life-threatening inherited disease caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR) protein, an anion channel expressed at the apical membrane of secretory epithelia. CF leads to multiorgan dysfunction with progressive deterioration of lung function being the major cause of untimely death. Conventional CF therapies target only symptoms and consequences downstream of the primary genetic defect and the current life expectancy and quality of life of these individuals are still very limited.

AREA COVERED

CFTR modulator drugs are novel-specialized therapies that enhance or even restore functional expression of CFTR mutants and have been approved for clinical use for individuals with specific CF genotypes. This review summarizes classical approaches used for the pre-clinical development of CFTR correctors and potentiators as well as emerging strategies aiming to accelerate modulator development and expand theratyping efforts.

EXPERT OPINION

Highly effective CFTR modulator drugs are expected to deeply modify the disease course for the majority of individuals with CF. A multitude of experimental approaches have been established to accelerate the development of novel modulators. CF patient-derived specimens are valuable cell models to predict therapeutic effectiveness of existing (and novel) modulators in a precision medicine approach.

Engineering Tissue-Informed Biomaterials to Advance Pulmonary Regenerative Medicine

Biomaterials intentionally designed to support the expansion, differentiation, and three-dimensional (3D) culture of induced-pluripotent stem cells (iPSCs) may pave the way to cell-based therapies for chronic respiratory diseases. These conditions are endured by millions of people worldwide and represent a significant cause of morbidity and mortality. Currently, there are no effective treatments for the majority of advanced lung diseases and lung transplantation remains the only hope for many chronically ill patients. Key opinion leaders speculate that the novel coronavirus, COVID-19, may lead to long-term lung damage, further exacerbating the need for regenerative therapies. New strategies for regenerative cell-based therapies harness the differentiation capability of human iPSCs for studying pulmonary disease pathogenesis and treatment. Excitingly, biomaterials are a cell culture platform that can be precisely designed to direct stem cell differentiation. Here, we present a closer look at the state-of-the-art of iPSC differentiation for pulmonary engineering, offer evidence supporting the power of biomaterials to improve stem cell differentiation, and discuss our perspective on the potential for tissue-informed biomaterials to transform pulmonary regenerative medicine.

Treatment of Cystic Fibrosis: From Gene- to Cell-Based Therapies

Prognosis of patients with cystic fibrosis (CF) varies extensively despite recent advances in targeted therapies that improve CF transmembrane conductance regulator (CFTR) function. Despite being a multi-organ disease, extensive lung tissue destruction remains the major cause of morbidity and mortality. Progress towards a curative treatment strategy that implements a CFTR gene addition-technology to the patients’ lungs has been slow and not yet developed beyond clinical trials. Improved delivery vectors are needed to overcome the body’s defense system and ensure an efficient and consistent clinical response before gene therapy is suitable for clinical care. Cell-based therapy–which relies on functional modification of allogenic or autologous cells ex vivo, prior to transplantation into the patient–is now a therapeutic reality for various diseases. For CF, pioneering research has demonstrated proof-of-principle for allogenic transplantation of cultured human airway stem cells into mouse airways. However, applying a cell-based therapy to the human airways has distinct challenges. We review CF gene therapies using viral and non-viral delivery strategies and discuss current advances towards autologous cell-based therapies. Progress towards identification, correction, and expansion of a suitable regenerative cell, as well as refinement of pre-cell transplant lung conditioning protocols is discussed.

An NKX2-1GFP and TP63tdTomato dual fluorescent reporter for the investigation of human lung basal cell biology

Basal cells are multipotent stem cells responsible for the repair and regeneration of all the epithelial cell types present in the proximal lung. In mice, the elusive origins of basal cells and their contribution to lung development were recently revealed by high-resolution, lineage tracing studies. It however remains unclear if human basal cells originate and participate in lung development in a similar fashion, particularly with mounting evidence for significant species-specific differences in this process. To address this outstanding question, in the last several years differentiation protocols incorporating human pluripotent stem cells (hPSC) have been developed to produce human basal cells in vitro with varying efficiencies. To facilitate this endeavour, we introduced tdTomato into the human TP63 gene, whose expression specifically labels basal cells, in the background of a previously described hPSC line harbouring an NKX2-1GFP reporter allele. The functionality and specificity of the NKX2-1GFP;TP63tdTomato hPSC line was validated by directed differentiation into lung progenitors as well as more specialised lung epithelial subtypes using an organoid platform. This dual fluorescent reporter hPSC line will be useful for tracking, isolating and expanding basal cells from heterogenous differentiation cultures for further study.

Epithelial Stem and Progenitor Cells in Lung Repair and Regeneration

The mammalian lung epithelium is composed of a wide array of specialized cells that have adapted to survive environmental exposure and perform the tasks necessary for respiration. Although the majority of these cells are remarkably quiescent during adult lung homeostasis, a growing body of literature has demonstrated the capacity of these epithelial lineages to proliferate in response to injury and regenerate lost or damaged cells. In this review, we focus on the regionally distinct lung epithelial cell types that contribute to repair after injury, and we address current controversies regarding whether elite stem cells or frequent facultative progenitors are the predominant participants. We also shed light on the newly emerging approaches for exogenously generating similar lung epithelial lineages from pluripotent stem cells.

Leveraging 3D Model Systems to Understand Viral Interactions with the Respiratory Mucosa

Respiratory viruses remain a significant cause of morbidity and mortality in the human population, underscoring the importance of ongoing basic research into virus-host interactions. However, many critical aspects of infection are difficult, if not impossible, to probe using standard cell lines, 2D culture formats, or even animal models. In vitro systems such as airway epithelial cultures at air-liquid interface, organoids, or 'on-chip' technologies allow interrogation in human cells and recapitulate emergent properties of the airway epithelium-the primary target for respiratory virus infection. While some of these models have been used for over thirty years, ongoing advancements in both culture techniques and analytical tools continue to provide new opportunities to investigate airway epithelial biology and viral infection phenotypes in both normal and diseased host backgrounds. Here we review these models and their application to studying respiratory viruses. Furthermore, given the ability of these systems to recapitulate the extracellular microenvironment, we evaluate their potential to serve as a platform for studies specifically addressing viral interactions at the mucosal surface and detail techniques that can be employed to expand our understanding.