Live imaging of alveologenesis in precision-cut lung slices reveals dynamic epithelial cell behaviour

Dysregulation of cell migration by matrix metalloproteinases in geleophysic dysplasia

Geleophysic dysplasia (GD) is characterized by short stature, brachydactyly, joint limitations, a distinctive facial appearance, as well as cardiac and respiratory dysfunction that can be life-threatening. GD is caused by pathogenic variants in the ADAMTSL2, FBN1, or LTBP3 genes. While dermal fibroblasts derived from affected individuals have shown poor organization of the extracellular matrix (ECM), it remains elusive how the disorganized ECM contributes to GD pathogenesis. To understand the molecular mechanisms in GD, we isolated and characterized primary human dermal fibroblasts from affected individuals with ADAMTSL2 and FBN1 variants. We found that the secretion of ECM proteins including ADAMTSL2, FBN1, and Fibronectin were impaired in GD fibroblasts. Increased cell migration was observed in GD fibroblasts carrying ADAMTSL2 or FBN1 variants, which was associated with up-regulation of MMP-1 and MMP-14, two proteases related to cell mobility. The enhanced cell migration and up-regulation of MMP-1 and MMP-14 were corroborated in mouse primary dermal fibroblasts carrying pathogenic variants in Adamtsl2 and in lung and heart tissues from Adamtsl2-knockout mice. A pan MMP inhibitor, GM6001, inhibited the migration of GD fibroblasts. Overall, our results suggest that MMP-1/-14 up-regulation play a role in the development of GD and may be utilized as a treatment target.

Development of novel technology for the visualization and quantitation of angiogenesis and the alveolar-capillary network in a mouse model of fibrosis

Adaptive angiogenesis can drive repair or underlie the pathogenesis of tissue remodeling. Pulmonary vascular dysfunction is a major manifestation of chronic lung disease (CLD), but the role of angiogenesis in the development of CLD is not well defined. Microvascular capillaries in the alveolar-capillary network are the vessels most affected by pruning and remodeling in the lung, resulting in reduced capillary length and diameter with subsequent loss of gas exchange surfaces. Our lab has previously demonstrated that microvascular endothelial progenitor cells (mvEPCs) drive reparative angiogenesis. We hypothesize that visualization of the alveolar-capillary microvasculature in three-dimensions is essential to define the mechanisms governing repair versus progression to the pathogenesis of CLD. To address this gap in knowledge, we have developed a simple and reliable fluorescent perfusion technique that will allow the quantitation of microvessel structure in the alveolar-capillary network using mouse models of lung injury. This approach may be used in various organ systems to visualize microvasculature structure and its role in disease.NEW & NOTEWORTHY We developed and validated a fluorescent technology to visualize and quantify the alveolar-capillary network in three-dimensions in mouse lung for modeling of the microvasculature in models of lung disease.

Protective effects of 10 °C preservation on donor lungs with lipopolysaccharide-induced acute lung injury

OBJECTIVES

Hypothermic lung preservation at 10 °C has been recently shown to enhance quality of healthy donor lungs during ischemia. This study aims to show generalizability of the 10 °C lung preservation using an endotoxin-induced lung injury with specific focus on the benefits of post-transplant lung function and mitochondrial preservation.

METHODS

Lipopolysaccharide (3 mg/kg) was injected intratracheally in rats to induce lung injury. Injured lungs were flushed with preservation solution and allocated to 3 groups (n = 6 each): minimum cold storage, 6-hour storage on ice (ice), and 6-hour storage at 10 °C (10 °C). Left lungs were transplanted and reperfused for 2 hours. After storage, lung tissue was used to evaluate the effects of hypothermic storage on the mitochondrial function: mitochondrial membrane potential was assessed by JC-1 staining; mitochondrial oxygen consumption was assessed using high-resolution respirometry.

RESULTS

Two hours after reperfusion, the oxygen tension/inspired oxygen fraction ratio from the graft was significantly greater in the 10 °C group than in the Ice group (P = .015), whereas the wet-to-dry weight ratio was significantly lower (P = .041). Levels of interleukin-8 in lung tissues were significantly lower in the 10 °C group than in the Ice group (P = .004). Mechanistically, we noted greater mitochondrial membrane potential and elevated state III respiration in the 10 °C group than in the Ice group (P = .015 and P = .002, respectively), implying higher metabolic activities may be maintained during 10 °C preservation.

CONCLUSIONS

Favorable metabolism during 10 °C preservation prevented ischemia-induced mitochondrial damages in injured lungs, leading to better post-transplant outcomes.

Non-canonical Wnt signaling promotes epithelial fluidization in the repairing airway

Concerted migration of basal stem cells (BCs) in the airway, also known as epithelial fluidization, has been implicated in epithelial repair after injury. How BC migration is regulated, and how it influences the success of epithelial repair, remains unclear. Here we have identified non-canonical Wnt signaling through Ptk7, Fzd7, and YAP as a critical regulator of BC migration in the mouse trachea. Using live imaging and genetic studies in the mouse, we find that Ptk7 is required for the concerted movement of BCs after injury, and that this requirement extends to BC proliferation and subsequent restoration of epithelial homeostasis after injury. We demonstrate that Ptk7 exerts this function in conjunction with Wnt5a and Fzd7, and through YAP activation in BCs. Our data provide mechanistic insight into the regulation of epithelial repair in the airway.

Tissue-Informed Biomaterial Innovations Advance Pulmonary Regenerative Engineering

Irreversible progressive pulmonary diseases drastically reduce the patient quality of life, while transplantation remains the only definitive cure. Research into lung regeneration pathways holds significant potential to expand and promote the discovery of new treatment options. Polymeric biomaterials designed to replicate key tissue characteristics (i.e., biochemical composition and mechanical cues) show promise for creating environments in which to study chronic lung diseases and initiate lung tissue regeneration. In this Viewpoint, we explore how naturally derived materials can be employed alone or combined with engineered polymer systems to create advanced tissue culture platforms. Pulmonary tissue models have historically leveraged natural materials, including basement membrane extracts and a decellularized extracellular matrix, as platforms for lung regeneration studies. Here, we provide an overview of the progression of pulmonary regenerative engineering, exploring how innovations in the growing field of tissue-informed biomaterials have the potential to advance lung regeneration research by bridging the gap between biological relevance and mechanical precision.

Aging shapes infection profiles of influenza A virus and SARS-CoV-2 in human precision-cut lung slices

BACKGROUND

The coronavirus disease 2019 (COVID-19) outbreak revealed the susceptibility of elderly patients to respiratory virus infections, showing cell senescence or subclinical persistent inflammatory profiles and favoring the development of severe pneumonia.

METHODS

In our study, we evaluated the potential influence of lung aging on the efficiency of replication of influenza A virus (IAV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), as well as determining the pro-inflammatory and antiviral responses of the distal lung tissue.

RESULTS

Using precision-cut lung slices (PCLS) from donors of different ages, we found that pandemic H1N1 and avian H5N1 IAV replicated in the lung parenchyma with high efficacy. In contrast to these IAV strains, SARS-CoV-2 Early isolate and Delta variant of concern (VOC) replicated less efficiently in PCLS. Interestingly, both viruses showed reduced replication in PCLS from older compared to younger donors, suggesting that aged lung tissue represents a suboptimal environment for viral replication. Regardless of the age-dependent viral loads, PCLS responded to H5N1 IAV infection by an induction of IL-6 and IP10/CXCL10, both at the mRNA and protein levels, and to H1N1 IAV infection by induction of IP10/CXCL10 mRNA. Finally, while SARS-CoV-2 and H1N1 IAV infection were not causing detectable cell death, H5N1 IAV infection led to more cytotoxicity and induced significant early interferon responses.

CONCLUSIONS

In summary, our findings suggest that aged lung tissue might not favor viral dissemination, pointing to a determinant role of dysregulated immune mechanisms in the development of severe disease.

Micromechanics of lung capillaries across mouse lifespan and in positive- vs negative-pressure ventilation

The lung undergoes continuous remodeling throughout normal development and aging, including changes to alveolar and capillary structure and function. While histological methods allow static analysis of these age-related changes, characterizing the changes that occur in response to mechanical stimuli remains difficult, particularly over a dynamic, physiologically relevant range in a functioning lung. Alveolar and capillary distension - the change in diameter of alveoli and capillaries, respectively, in response to pressure changes - is one such process, where dynamically controlling and monitoring the diameter of the same capillary or alveolus is essential to infer its mechanical properties. We overcome these limitations by utilizing the recently developed crystal ribcage to image the alveoli and vasculature of a functional mouse lung across the lifespan in postnatal (6-7 days), young adult (12-18 weeks), and aged (20+ months) mice. Using a range of biologically relevant vascular (0-15 cmH2O) and transpulmonary (3-12 cmH2O) pressures, we directly quantify vascular and alveolar distention in the functional lung as we precisely adjust pulmonary pressures. Our results show differences in age-related alveolar and vascular distensibility: when we increase transpulmonary alveolar or vascular pressure, vessels in postnatal lungs expand less and undergo less radial and axial strain, under each respective pressure type, suggesting stiffer capillaries than in older lungs. However, while vessels in young adult and aged lungs respond similarly to variations in vascular pressure, differences in elasticity start to emerge at the alveolar scale in response to transpulmonary alveolar pressure changes. Our results further indicate that differing effects of ventilation mode (i.e., positive vs negative) present themselves at the capillary level, with vessels under positive pressure undergoing more compression than when under negative-pressure conditions. These findings contribute both to the understanding of the functional changes that occur within the lung across the lifespan, as well as to the debate of ventilation effects on lung microphysiology.

Intrinsic electrical activity drives small-cell lung cancer progression

Elevated or ectopic expression of neuronal receptors promotes tumour progression in many cancer types1,2; neuroendocrine (NE) transformation of adenocarcinomas has also been associated with increased aggressiveness3. Whether the defining neuronal feature, namely electrical excitability, exists in cancer cells and impacts cancer progression remains mostly unexplored. Small-cell lung cancer (SCLC) is an archetypal example of a highly aggressive NE cancer and comprises two major distinct subpopulations: NE cells and non-NE cells4,5. Here we show that NE cells, but not non-NE cells, are excitable, and their action potential firing directly promotes SCLC malignancy. However, the resultant high ATP demand leads to an unusual dependency on oxidative phosphorylation in NE cells. This finding contrasts with the properties of most cancer cells reported in the literature, which are non-excitable and rely heavily on aerobic glycolysis. Additionally, we found that non-NE cells metabolically support NE cells, a process akin to the astrocyte-neuron metabolite shuttle6. Finally, we observed drastic changes in the innervation landscape during SCLC progression, which coincided with increased intratumoural heterogeneity and elevated neuronal features in SCLC cells, suggesting an induction of a tumour-autonomous vicious cycle, driven by cancer cell-intrinsic electrical activity, which confers long-term tumorigenic capability and metastatic potential.

Epithelial outgrowth through mesenchymal rings drives lung alveologenesis

Determining how alveoli are formed and maintained is critical to understanding lung organogenesis and regeneration after injury. To study the cellular dynamics of this critical stage of lung development, we have used scanned oblique-plane illumination microscopy of living lung slices to observe alveologenesis in real time at high resolution over several days. Contrary to the prevailing notion that alveologenesis occurs by airspace subdivision via ingrowing septa, we found that alveoli form by ballooning epithelial outgrowth supported by contracting mesenchymal ring structures. Systematic analysis has produced a computational model of finely timed cellular structural changes that drive normal alveologenesis. With this model, we can now quantify how perturbing known regulatory intercellular signaling pathways and cell migration processes affects alveologenesis. In the future, this paradigm and platform can be leveraged for mechanistic studies and screening for therapies to promote lung regeneration.

LungVis 1.0: an automatic AI-powered 3D imaging ecosystem unveils spatial profiling of nanoparticle delivery and acinar migration of lung macrophages

Targeted (nano-)drug delivery is essential for treating respiratory diseases, which are often confined to distinct lung regions. However, spatio-temporal profiling of drugs or nanoparticles (NPs) and their interactions with lung macrophages remains unresolved. Here, we present LungVis 1.0, an AI-powered imaging ecosystem that integrates light sheet fluorescence microscopy with deep learning-based image analysis pipelines to map NP deposition and dosage holistically and quantitatively across bronchial and alveolar (acinar) regions in murine lungs for widely-used bulk-liquid and aerosol-based delivery methods. We demonstrate that bulk-liquid delivery results in patchy NP distribution with elevated bronchial doses, whereas aerosols achieve uniform deposition reaching distal alveoli. Furthermore, we reveal that lung tissue-resident macrophages (TRMs) are dynamic, actively patrolling and redistributing NPs within alveoli, contesting the conventional paradigm of TRMs as static entities. LungVis 1.0 provides an advanced framework for exploring pulmonary delivery dynamics and deepening insights into TRM-mediated lung immunity.

Precision Cut Lung Slices: Emerging Tools for Preclinical and Translational Lung Research. An Official American Thoracic Society Workshop Report

The urgent need for effective treatments for acute and chronic lung diseases underscores the significance of developing innovative preclinical human research tools. The 2023 ATS Workshop on Precision Cut Lung Slices (PCLS) brought together 35 experts to discuss and address the role of human tissue-derived PCLS as a unique tool for target and drug discovery and validation in pulmonary medicine. With increasing interest and usage, along with advancements in methods and technology, there is a growing need for consensus on PCLS methodology and readouts. The current document recommends standard reporting criteria and emphasizes the requirement for careful collection and integration of clinical metadata. We further discuss current clinically relevant readouts that can be applied to PCLS and highlight recent developments and future steps for implementing novel technologies for PCLS modeling and analysis. The collection and correlation of clinical metadata and multiomic analysis will further advent the integration of this preclinical platform into patient endotyping and the development of tailored therapies for lung disease patients.

Skeletal Muscle SIRT3 Deficiency Contributes to Pulmonary Vascular Remodeling in Pulmonary Hypertension Due to Heart Failure With Preserved Ejection Fraction

BACKGROUND

Pulmonary hypertension (PH) is a major complication linked to adverse outcomes in heart failure with preserved ejection fraction (HFpEF), yet no specific therapies exist for PH associated with HFpEF (PH-HFpEF). We have recently reported on the role of skeletal muscle SIRT3 (sirtuin-3) in modulation of PH-HFpEF, suggesting a novel endocrine signaling pathway for skeletal muscle modulation of pulmonary vascular remodeling.

METHODS

Using skeletal muscle-specific Sirt3 knockout mice (Sirt3skm-/-) and mass spectrometry-based comparative secretome analysis, we attempted to define the processes by which skeletal muscle SIRT3 defects affect pulmonary vascular health in PH-HFpEF.

RESULTS

Sirt3skm-/- mice exhibited reduced pulmonary vascular density accompanied by pulmonary vascular proliferative remodeling and elevated pulmonary pressures. Comparative analysis of secretome by mass spectrometry revealed elevated secretion levels of LOXL2 (lysyl oxidase homolog 2) in SIRT3-deficient skeletal muscle cells. Elevated circulation and protein expression levels of LOXL2 were also observed in plasma and skeletal muscle of Sirt3skm-/- mice, a rat model of PH-HFpEF, and humans with PH-HFpEF. In addition, expression levels of CNPY2 (canopy fibroblast growth factor signaling regulator 2), a known proliferative and angiogenic factor, were increased in pulmonary artery endothelial cells and pulmonary artery smooth muscle cells of Sirt3skm-/- mice and animal models of PH-HFpEF. CNPY2 levels were also higher in pulmonary artery smooth muscle cells of subjects with obesity compared with nonobese subjects. Moreover, treatment with recombinant LOXL2 protein promoted pulmonary artery endothelial cell migration/proliferation and pulmonary artery smooth muscle cell proliferation through regulation of CNPY2-p53 signaling. Last, skeletal muscle-specific Loxl2 deletion decreased pulmonary artery endothelial cell and pulmonary artery smooth muscle cell expression of CNPY2 and improved pulmonary pressures in mice with high-fat diet-induced PH-HFpEF.

CONCLUSIONS

This study demonstrates a systemic pathogenic impact of skeletal muscle SIRT3 deficiency in remote pulmonary vascular remodeling and PH-HFpEF. This study suggests a new endocrine signaling axis that links skeletal muscle health and SIRT3 deficiency to remote CNPY2 regulation in the pulmonary vasculature through myokine LOXL2. Our data also identify skeletal muscle SIRT3, myokine LOXL2, and CNPY2 as potential targets for the treatment of PH-HFpEF.

Local photo-crosslinking of native tissue matrix regulates cell function

Within most tissues, the extracellular microenvironment provides mechanical cues that guide cell fate and function. Changes in the extracellular matrix such as aberrant deposition, densification and increased crosslinking are hallmarks of late-stage fibrotic diseases that often lead to organ dysfunction. Biomaterials have been widely used to mimic the mechanical properties of the fibrotic matrix and study cell function. However, the initiation of fibrosis has largely been overlooked, due to the challenges in recapitulating early fibrotic lesions within the native extracellular microenvironment. Using visible light mediated photochemistry, we induced local crosslinking and stiffening of extracellular matrix proteins within ex vivo murine and human tissue. In ex vivo lung tissue of epithelial cell lineage-traced mice, local matrix crosslinking mimicked early fibrotic lesions that increased alveolar epithelial cell spreading, differentiation and extracellular matrix remodeling. However, inhibition of cytoskeletal tension or integrin engagement reduced epithelial cell spreading and differentiation, resulting in alveolar epithelial cell dedifferentiation and reduced extracellular matrix deposition. Our findings emphasize the role of local extracellular matrix crosslinking and remodeling in early-stage tissue fibrosis and have implications for ex vivo disease modeling and applications to other tissues.

Van Gogh-like 2 is essential for the architectural patterning of the mammalian biliary tree

BACKGROUND & AIMS

In the developing liver, bipotent epithelial progenitor cells undergo lineage segregation to form hepatocytes, which constitute the bulk of the liver parenchyma, and biliary epithelial cells (cholangiocytes), which comprise the bile duct (a complex tubular network that is critical for normal liver function). Notch and TGFβ signalling promote the formation of a sheet of biliary epithelial cells, the ductal plate, that organises into discontinuous tubular structures. How these structures elongate and connect to form a continuous duct remains undefined. We aimed to define the mechanisms by which the ductal plate transitions from a simple sheet of epithelial cells into a complex and connected bile duct.

METHODS

By combining single-cell RNA sequencing of embryonic mouse livers with genetic tools and organoid models we functionally dissected the role of planar cell polarity in duct patterning.

RESULTS

We show that the planar cell polarity protein VANGL2 is expressed late in intrahepatic bile duct development and patterns the formation of cell-cell contacts between biliary cells. The patterning of these cell contacts regulates the normal polarisation of the actin cytoskeleton within biliary cells and loss of Vangl2 function results in the abnormal distribution of cortical actin remodelling, leading to the failure of bile duct formation.

CONCLUSIONS

Planar cell polarity is a critical step in the post-specification sculpture of the bile duct and is essential for establishing normal tissue architecture.

IMPACT AND IMPLICATIONS

Like other branched tissues, such as the lung and kidney, the bile ducts use planar cell polarity signalling to coordinate cell movements; however, how these biochemical signals are linked to ductular patterning remains unclear. Here we show that the core planar cell polarity protein VANGL2 patterns how cell-cell contacts form in the mammalian bile duct and how ductular cells transmit confluent mechanical changes along the length of a duct. This work sheds light on how biological tubes are patterned across mammalian tissues (including within the liver) and will be important in how we promote ductular growth in patients where the duct is mis-patterned or poorly formed.

Precision cut lung slices: an integrated ex vivo model for studying lung physiology, pharmacology, disease pathogenesis and drug discovery

Precision Cut Lung Slices (PCLS) have emerged as a sophisticated and physiologically relevant ex vivo model for studying the intricacies of lung diseases, including fibrosis, injury, repair, and host defense mechanisms. This innovative methodology presents a unique opportunity to bridge the gap between traditional in vitro cell cultures and in vivo animal models, offering researchers a more accurate representation of the intricate microenvironment of the lung. PCLS require the precise sectioning of lung tissue to maintain its structural and functional integrity. These thin slices serve as invaluable tools for various research endeavors, particularly in the realm of airway diseases. By providing a controlled microenvironment, precision-cut lung slices empower researchers to dissect and comprehend the multifaceted interactions and responses within lung tissue, thereby advancing our understanding of pulmonary pathophysiology.

A Novel Method for Floxed Gene Manipulation Using TAT-Cre Recombinase in Ex Vivo Precision-Cut Lung Slices (PCLS)

Precision-cut lung slices (PCLS), ex vivo 3D lung tissue models, have been widely used for various applications in lung research. PCLS serve as an excellent intermediary between in vitro and in vivo models because they retain all resident cell types within their natural niche while preserving the extracellular matrix environment. This protocol describes the TReATS (TAT-Cre recombinase-mediated floxed allele modification in tissue slices) method that enables rapid and efficient gene modification in PCLS derived from adult floxed animals. Here, we present detailed protocols for the TReATS method, consisting of two simple steps: PCLS generation and incubation in a TAT-Cre recombinase solution. Subsequent validation of gene modification involves live staining and imaging of PCLS, quantitative real-time PCR, and cell viability assessment. This four-day protocol eliminates the need for complex Cre-breeding, circumvents issues with premature lethality related to gene mutation, and significantly reduces the use of animals. The TReATS method offers a simple and reproducible solution for gene modification in complex ex vivo tissue-based models, accelerating the study of gene function, disease mechanisms, and the discovery of drug targets. Key features • Achieve permanent ex vivo gene modifications in complex tissue-based models within four days. • Highly adaptable gene modification method that can be applied to induce gene deletion or activation. • Allows simple Cre dosage testing in a controlled ex vivo setting with the advantage of using PCLS generated from the same animal as true controls. • With optimisation, this method can be applied to precision-cut tissue slices of other organs.

In vivo tracking of mesenchymal stem cell dynamics and therapeutics in LPS-induced acute lung injury models

Mesenchymal stem cells (MSCs) have been widely used to treat various inflammatory and immune-related diseases in preclinical and clinical settings. Intravital microscopy (IVM) is considered the gold standard for investigating pathophysiological conditions in living animals. However, the potential for real-time monitoring of MSCs in the pulmonary microenvironment remains underexplored. In this study, we first constructed a lung window and captured changes in the lung at the cellular level under both inflammatory and noninflammatory conditions with a microscope. We further investigated the dynamics and effects of MSCs under two different conditions. Meanwhile, we assessed the alterations in the adhesive capacity of vascular endothelial cells in vitro to investigate the underlying mechanisms of MSC retention in an inflammatory environment. This study emphasizes the importance of the "lung window" for live imaging of the cellular behavior of MSCs by vein injection. Moreover, our results revealed that the upregulation of vascular cell adhesion molecule 1 (VCAM1) in endothelial cells post-inflammatory injury could enhance MSC retention in the lung, further ameliorating acute lung injury. In summary, intravital microscopy imaging provides a practical method to investigate the therapeutic effects of MSCs in acute lung injury.

Stem cell migration drives lung repair in living mice

Tissue repair requires a highly coordinated cellular response to injury. In the lung, alveolar type 2 cells (AT2s) act as stem cells to replenish both themselves and alveolar type 1 cells (AT1s); however, the complex orchestration of stem cell activity after injury is poorly understood. Here, we establish longitudinal imaging of AT2s in murine intact tissues ex vivo and in vivo in order to track their dynamic behavior over time. We discover that a large fraction of AT2s become motile following injury and provide direct evidence for their migration between alveolar units. High-resolution morphokinetic mapping of AT2s further uncovers the emergence of distinct motile phenotypes. Inhibition of AT2 migration via genetic depletion of ArpC3 leads to impaired regeneration of AT2s and AT1s in vivo. Together, our results establish a requirement for stem cell migration between alveolar units and identify properties of stem cell motility at high cellular resolution.

Bronchoconstriction damages airway epithelia by crowding-induced excess cell extrusion

Asthma is deemed an inflammatory disease, yet the defining diagnostic feature is mechanical bronchoconstriction. We previously discovered a conserved process called cell extrusion that drives homeostatic epithelial cell death when cells become too crowded. In this work, we show that the pathological crowding of a bronchoconstrictive attack causes so much epithelial cell extrusion that it damages the airways, resulting in inflammation and mucus secretion in both mice and humans. Although relaxing the airways with the rescue treatment albuterol did not affect these responses, inhibiting live cell extrusion signaling during bronchoconstriction prevented all these features. Our findings show that bronchoconstriction causes epithelial damage and inflammation by excess crowding-induced cell extrusion and suggest that blocking epithelial extrusion, instead of the ensuing downstream inflammation, could prevent the feed-forward asthma inflammatory cycle.

The alveolus: Our current knowledge of how the gas exchange unit of the lung is constructed and repaired

The mammalian lung completes its last step of development, alveologenesis, to generate sufficient surface area for gas exchange. In this process, multiple cell types that include alveolar epithelial cells, endothelial cells, and fibroblasts undergo coordinated cell proliferation, cell migration and/or contraction, cell shape changes, and cell-cell and cell-matrix interactions to produce the gas exchange unit: the alveolus. Full functioning of alveoli also involves immune cells and the lymphatic and autonomic nervous system. With the advent of lineage tracing, conditional gene inactivation, transcriptome analysis, live imaging, and lung organoids, our molecular understanding of alveologenesis has advanced significantly. In this review, we summarize the current knowledge of the constituents of the alveolus and the molecular pathways that control alveolar formation. We also discuss how insight into alveolar formation may inform us of alveolar repair/regeneration mechanisms following lung injury and the pathogenic processes that lead to loss of alveoli or tissue fibrosis.

Integrating mechanical cues with engineered platforms to explore cardiopulmonary development and disease

Mechanical forces provide critical biological signals to cells during healthy and aberrant organ development as well as during disease processes in adults. Within the cardiopulmonary system, mechanical forces, such as shear, compressive, and tensile forces, act across various length scales, and dysregulated forces are often a leading cause of disease initiation and progression such as in bronchopulmonary dysplasia and cardiomyopathies. Engineered in vitro models have supported studies of mechanical forces in a number of tissue and disease-specific contexts, thus enabling new mechanistic insights into cardiopulmonary development and disease. This review first provides fundamental examples where mechanical forces operate at multiple length scales to ensure precise lung and heart function. Next, we survey recent engineering platforms and tools that have provided new means to probe and modulate mechanical forces across in vitro and in vivo settings. Finally, the potential for interdisciplinary collaborations to inform novel therapeutic approaches for a number of cardiopulmonary diseases are discussed.

On the path to predicting immune responses in the lung: Modeling the pulmonary innate immune system at the air-liquid interface (ALI)

Chronic respiratory diseases and infections are among the largest contributors to death globally, many of which still have no cure, including chronic obstructive pulmonary disorder, idiopathic pulmonary fibrosis, and respiratory syncytial virus among others. Pulmonary therapeutics afford untapped potential for treating lung infection and disease through direct delivery to the site of action. However, the ability to innovate new therapeutic paradigms for respiratory diseases will rely on modeling the human lung microenvironment and including key cellular interactions that drive disease. One key feature of the lung microenvironment is the air-liquid interface (ALI). ALI interface modeling techniques, using cell-culture inserts, organoids, microfluidics, and precision lung slices (PCLS), are rapidly developing; however, one major component of these models is lacking-innate immune cell populations. Macrophages, neutrophils, and dendritic cells, among others, represent key lung cell populations, acting as the first responders during lung infection or injury. Innate immune cells respond to and modulate stromal cells and bridge the gap between the innate and adaptive immune system, controlling the bodies response to foreign pathogens and debris. In this article, we review the current state of ALI culture systems with a focus on innate immune cells and suggest ways to build on current models to add complexity and relevant immune cell populations.

A method for TAT-Cre recombinase-mediated floxed allele modification in ex vivo tissue slices

Precision-cut lung slices (PCLS) are used for a variety of applications. However, methods to manipulate genes in PCLS are currently limited. We developed a new method, TAT-Cre recombinase-mediated floxed allele modification in tissue slices (TReATS), to induce highly effective and temporally controlled gene deletion or activation in ex vivo PCLS. Treatment of PCLS from Rosa26-flox-stop-flox-EYFP mice with cell-permeant TAT-Cre recombinase induced ubiquitous EYFP protein expression, indicating successful Cre-mediated excision of the upstream loxP-flanked stop sequence. Quantitative real-time PCR confirmed induction of EYFP. We successfully replicated the TReATS method in PCLS from Vangl2flox/flox mice, leading to the deletion of loxP-flanked exon 4 of the Vangl2 gene. Cre-treated Vangl2flox/flox PCLS exhibited cytoskeletal abnormalities, a known phenotype caused by VANGL2 dysfunction. We report a new method that bypasses conventional Cre-Lox breeding, allowing rapid and highly effective gene manipulation in ex vivo tissue models.

The RNA binding proteins ZFP36L1 and ZFP36L2 are dysregulated in airway epithelium in human and a murine model of asthma

Introduction: Asthma is the most common chronic inflammatory disease of the airways. The airway epithelium is a key driver of the disease, and numerous studies have established genome-wide differences in mRNA expression between health and asthma. However, the underlying molecular mechanisms for such differences remain poorly understood. The human TTP family is comprised of ZFP36, ZFP36L1 and ZFP36L2, and has essential roles in immune regulation by determining the stability and translation of myriad mRNAs encoding for inflammatory mediators. We investigated the expression and possible role of the tristetraprolin (TTP) family of RNA binding proteins (RBPs), poorly understood in asthma. Methods: We analysed the levels of ZFP36, ZFP36L1 and ZFP36L2 mRNA in several publicly available asthma datasets, including single cell RNA-sequencing. We also interrogated the expression of known targets of these RBPs in asthma. We assessed the lung mRNA expression and cellular localization of Zfp36l1 and Zfp36l2 in precision cut lung slices in murine asthma models. Finally, we determined the expression in airway epithelium of ZFP36L1 and ZFP36L2 in human bronchial biopsies and performed rescue experiments in primary bronchial epithelium from patients with severe asthma. Results: We found ZFP36L1 and ZFP36L2 mRNA levels significantly downregulated in the airway epithelium of patients with very severe asthma in different cohorts (5 healthy vs. 8 severe asthma; 36 moderate asthma vs. 37 severe asthma on inhaled steroids vs. 26 severe asthma on oral corticoids). Integrating several datasets allowed us to infer that mRNAs potentially targeted by these RBPs are increased in severe asthma. Zfp36l1 was downregulated in the lung of a mouse model of asthma, and immunostaining of ex vivo lung slices with a dual antibody demonstrated that Zfp36l1/l2 nuclear localization was increased in the airway epithelium of an acute asthma mouse model, which was further enhanced in a chronic model. Immunostaining of human bronchial biopsies showed that airway epithelial cell staining of ZFP36L1 was decreased in severe asthma as compared with mild, while ZFP36L2 was upregulated. Restoring the levels of ZFP36L1 and ZFP36L2 in primary bronchial epithelial cells from patients with severe asthma decreased the mRNA expression of IL6, IL8 and CSF2. Discussion: We propose that the dysregulation of ZFP36L1/L2 levels as well as their subcellular mislocalization contributes to changes in mRNA expression and cytoplasmic fate in asthma.

Lung Organotypic Slices Enable Rapid Quantification of Acute Radiotherapy Induced Toxicity

To rapidly assess healthy tissue toxicities induced by new anti-cancer therapies (i.e., radiation alone or in combination with drugs), there is a critical need for relevant and easy-to-use models. Consistent with the ethical desire to reduce the use of animals in medical research, we propose to monitor lung toxicity using an ex vivo model. Briefly, freshly prepared organotypic lung slices from mice were irradiated, with or without being previously exposed to chemotherapy, and treatment toxicity was evaluated by analysis of cell division and viability of the slices. When exposed to different doses of radiation, this ex vivo model showed a dose-dependent decrease in cell division and viability. Interestingly, monitoring cell division was sensitive enough to detect a sparing effect induced by FLASH radiotherapy as well as the effect of combined treatment. Altogether, the organotypic lung slices can be used as a screening platform to rapidly determine in a quantitative manner the level of lung toxicity induced by different treatments alone or in combination with chemotherapy while drastically reducing the number of animals. Translated to human lung samples, this ex vivo assay could serve as an innovative method to investigate patients' sensitivity to radiation and drugs.

Piezo1 activates noncanonical EGFR endocytosis and signaling

EGFR-ERK signaling controls cell cycle progression during development, homeostasis, and disease. While EGF ligand and mechanical inputs can activate EGFR-ERK signaling, the molecules linking mechanical force to this axis have remained mysterious. We previously found that stretch promotes mitosis via the stretch-activated ion channel Piezo1 and ERK signaling. Here, we show that Piezo1 provides the missing link between mechanical signals and EGFR-ERK activation. While both EGF- and Piezo1-dependent activation trigger clathrin-mediated EGFR endocytosis and ERK activation, EGF relies on canonical tyrosine autophosphorylation, whereas Piezo1 involves Src-p38 kinase-dependent serine phosphorylation. In addition, unlike EGF, ex vivo lung slices treated with Piezo1 agonist promoted cell cycle re-entry via nuclear ERK, AP-1 (FOS and JUN), and YAP accumulation, typical of regenerative and malignant signaling. Our results suggest that mechanical activation via Piezo1, Src, and p38 may be more relevant to controlling repair, regeneration, and cancer growth than tyrosine kinase signaling via canonical EGF signaling, suggesting an alternative therapeutic approach.

Bronchoconstriction damages airway epithelia by excess crowding-induced extrusion

Asthma is deemed an inflammatory disease, yet the defining diagnostic symptom is mechanical bronchoconstriction. We previously discovered a conserved process that drives homeostatic epithelial cell death in response to mechanical cell crowding called cell extrusion(1, 2). Here, we show that the pathological crowding of a bronchoconstrictive attack causes so much epithelial cell extrusion that it damages the airways, resulting in inflammation and mucus secretion. While relaxing airways with the rescue treatment albuterol did not impact these responses, inhibiting live cell extrusion signaling during bronchoconstriction prevented all these symptoms. Our findings propose a new etiology for asthma, dependent on the mechanical crowding of a bronchoconstrictive attack. Our studies suggest that blocking epithelial extrusion, instead of ensuing downstream inflammation, could prevent the feed-forward asthma inflammatory cycle.

Variations in mechanical stiffness alter microvascular sprouting and stability in a PEG hydrogel model of idiopathic pulmonary fibrosis

OBJECTIVE

Microvascular remodeling is governed by biomechanical and biochemical cues which are dysregulated in idiopathic pulmonary fibrosis. Understanding how these cues impact endothelial cell-pericyte interactions necessitates a model system in which both variables can be independently and reproducibly modulated. In this study we develop a tunable hydrogel-based angiogenesis assay to study how varying angiogenic growth factors and environmental stiffness affect sprouting and vessel organization.

METHODS

Lungs harvested from mice were cut into 1 mm long segments then cultured on hydrogels having one of seven possible stiffness and growth factor combinations. Time course, brightfield, and immunofluorescence imaging were used to observe and quantify sprout formation.

RESULTS

Our assay was able to support angiogenesis in a comparable manner to Matrigel in soft 2 kPa gels while enabling tunability to study the effects of stiffness on sprout formation. Matrigel and 2 kPa groups contained significantly more samples with sprouts when compared to the stiffer 10 and 20 kPa gels. Growth factor treatment did not have as obvious an effect, although the 20 kPa PDGF + FGF-treated group had significantly longer vessels than the vascular endothelial growth factor-treated group.

CONCLUSIONS

We have developed a novel, tunable hydrogel assay for the creation of lung explant vessel organoids which can be modulated to study the impact of specific environmental cues on vessel formation and maturation.

Perspectives on precision cut lung slices-powerful tools for investigation of mechanisms and therapeutic targets in lung diseases

Precision cut lung slices (PCLS) have emerged as powerful experimental tools for respiratory research. Pioneering studies using mouse PCLS to visualize intrapulmonary airway contractility have been extended to pulmonary arteries and for assessment of novel bronchodilators and vasodilators as therapeutics. Additional disease-relevant outcomes, including inflammatory, fibrotic, and regenerative responses, are now routinely measured in PCLS from multiple species, including humans. This review provides an overview of established and innovative uses of PCLS as an intermediary between cellular and organ-based studies and focuses on opportunities to increase their application to investigate mechanisms and therapeutic targets to oppose excessive airway contraction and fibrosis in lung diseases.

Application of Precision-Cut Lung Slices as an In Vitro Model for Research of Inflammatory Respiratory Diseases

The leading cause of many respiratory diseases is an ongoing and progressive inflammatory response. Traditionally, inflammatory lung diseases were studied primarily through animal models, cell cultures, and organoids. These technologies have certain limitations, despite their great contributions to the study of respiratory diseases. Precision-cut lung slices (PCLS) are thin, uniform tissue slices made from human or animal lung tissue and are widely used extensively both nationally and internationally as an in vitro organotypic model. Human lung slices bridge the gap between in vivo and in vitro models, and they can replicate the living lung environment well while preserving the lungs' basic structures, such as their primitive cells and trachea. However, there is no perfect model that can completely replace the structure of the human lung, and there is still a long way to go in the research of lung slice technology. This review details and analyzes the strengths and weaknesses of precision lung slices as an in vitro model for exploring respiratory diseases associated with inflammation, as well as recent advances in this field.

Epithelial coxsackievirus adenovirus receptor promotes house dust mite-induced lung inflammation

Airway inflammation and remodelling are important pathophysiologic features in asthma and other respiratory conditions. An intact epithelial cell layer is crucial to maintain lung homoeostasis, and this depends on intercellular adhesion, whilst damaged respiratory epithelium is the primary instigator of airway inflammation. The Coxsackievirus Adenovirus Receptor (CAR) is highly expressed in the epithelium where it modulates cell-cell adhesion stability and facilitates immune cell transepithelial migration. However, the contribution of CAR to lung inflammation remains unclear. Here we investigate the mechanistic contribution of CAR in mediating responses to the common aeroallergen, House Dust Mite (HDM). We demonstrate that administration of HDM in mice lacking CAR in the respiratory epithelium leads to loss of peri-bronchial inflammatory cell infiltration, fewer goblet-cells and decreased pro-inflammatory cytokine release. In vitro analysis in human lung epithelial cells confirms that loss of CAR leads to reduced HDM-dependent inflammatory cytokine release and neutrophil migration. Epithelial CAR depletion also promoted smooth muscle cell proliferation mediated by GSK3β and TGF-β, basal matrix production and airway hyperresponsiveness. Our data demonstrate that CAR coordinates lung inflammation through a dual function in leucocyte recruitment and tissue remodelling and may represent an important target for future therapeutic development in inflammatory lung diseases.

Interleukin-6-dependent epithelial fluidization initiates fibrotic lung remodeling

Chronic disease results from the failure of tissues to maintain homeostasis. In the lung, coordinated repair of the epithelium is essential for preserving homeostasis. In animal models and human lung disease, airway epithelial cells mobilize in response to lung injury, resulting in the formation of airway-like cysts with persistent loss of functional cell types and parenchymal architecture. Using live-cell imaging of human lung epithelial cultures and mouse precision-cut lung slices, we demonstrated that distal airway epithelia are aberrantly fluidized both after injury and in fibrotic lung disease. Through transcriptomic profiling and pharmacologic stimulation of epithelial cultures, we identified interleukin-6 (IL-6) signaling as a driver of tissue fluidization. This signaling cascade occurred independently of canonical Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling but instead was dependent on a downstream SRC family kinase (SFK)-yes-associated protein (YAP) axis. Airway epithelial-fibroblast cocultures revealed that the fibrotic mesenchyme acts as a source of IL-6 family cytokines, which drive airway fluidization. Inhibition of the IL-6-SFK-YAP cascade was sufficient to prevent fluidization in both in vitro and ex vivo models. Last, we demonstrated a reduction in fibrotic lung remodeling in mice through genetic or pharmacologic targeting of IL-6-related signaling. Together, our findings illustrate the critical role of airway epithelial fluidization in coordinating the balance between homeostatic lung repair and fibrotic airspace remodeling.

A Barrier to Defend - Models of Pulmonary Barrier to Study Acute Inflammatory Diseases

Pulmonary diseases represent four out of ten most common causes for worldwide mortality. Thus, pulmonary infections with subsequent inflammatory responses represent a major public health concern. The pulmonary barrier is a vulnerable entry site for several stress factors, including pathogens such as viruses, and bacteria, but also environmental factors e.g. toxins, air pollutants, as well as allergens. These pathogens or pathogen-associated molecular pattern and inflammatory agents e.g. damage-associated molecular pattern cause significant disturbances in the pulmonary barrier. The physiological and biological functions, as well as the architecture and homeostatic maintenance of the pulmonary barrier are highly complex. The airway epithelium, denoting the first pulmonary barrier, encompasses cells releasing a plethora of chemokines and cytokines, and is further covered with a mucus layer containing antimicrobial peptides, which are responsible for the pathogen clearance. Submucosal antigen-presenting cells and neutrophilic granulocytes are also involved in the defense mechanisms and counterregulation of pulmonary infections, and thus may directly affect the pulmonary barrier function. The detailed understanding of the pulmonary barrier including its architecture and functions is crucial for the diagnosis, prognosis, and therapeutic treatment strategies of pulmonary diseases. Thus, considering multiple side effects and limited efficacy of current therapeutic treatment strategies in patients with inflammatory diseases make experimental in vitro and in vivo models necessary to improving clinical therapy options. This review describes existing models for studyying the pulmonary barrier function under acute inflammatory conditions, which are meant to improve the translational approaches for outcome predictions, patient monitoring, and treatment decision-making.

Nrf2 Activation Protects Against Organic Dust and Hydrogen Sulfide Exposure Induced Epithelial Barrier Loss and K. pneumoniae Invasion

Agriculture workers report various respiratory symptoms owing to occupational exposure to organic dust (OD) and various gases. Previously, we demonstrated that pre-exposure to hydrogen sulfide (H₂S) alters the host response to OD and induces oxidative stress. Nrf2 is a master-regulator of host antioxidant response and exposures to toxicants is known to reduce Nrf2 activity. The OD exposure-induced lung inflammation is known to increase susceptibility to a secondary microbial infection. We tested the hypothesis that repeated exposure to OD or H₂S leads to loss of Nrf2, loss of epithelial cell integrity and that activation of Nrf2 rescues this epithelial barrier dysfunction. Primary normal human bronchial epithelial (NHBE) cells or mouse precision cut-lung slices (PCLS) were treated with media, swine confinement facility organic dust extract (ODE) or H₂S or ODE+H₂S for one or five days. Cells were also pretreated with vehicle control (DMSO) or RTA-408, a Nrf2 activator. Acute exposure to H₂S and ODE+H₂S altered the cell morphology, decreased the viability as per the MTT assay, and reduced the Nrf2 expression as well as increased the keap1 levels in NHBE cells. Repeated exposure to ODE or H₂S or ODE+H₂S induced oxidative stress and cytokine production, decreased tight junction protein occludin and cytoskeletal protein ezrin expression, disrupted epithelial integrity and resulted in increased Klebsiella pneumoniae invasion. RTA-408 (pharmacological activator of Nrf2) activated Nrf2 by decreasing keap1 levels and reduced ODE+H₂S-induced changes including reversing loss of barrier integrity, inflammatory cytokine production and microbial invasion in PCLS but not in NHBE cell model. We conclude that Nrf2 activation has a partial protective function against ODE and H₂S.

The benefits, limitations and opportunities of preclinical models for neonatal drug development

Increased research to improve preclinical models to inform the development of therapeutics for neonatal diseases is an area of great need. This article reviews five common neonatal diseases - bronchopulmonary dysplasia, retinopathy of prematurity, necrotizing enterocolitis, perinatal hypoxic-ischemic encephalopathy and neonatal sepsis - and the available in vivo, in vitro and in silico preclinical models for studying these diseases. Better understanding of the strengths and weaknesses of specialized neonatal disease models will help to improve their utility, may add to the understanding of the mode of action and efficacy of a therapeutic, and/or may improve the understanding of the disease pathology to aid in identification of new therapeutic targets. Although the diseases covered in this article are diverse and require specific approaches, several high-level, overarching key lessons can be learned by evaluating the strengths, weaknesses and gaps in the available models. This Review is intended to help guide current and future researchers toward successful development of therapeutics in these areas of high unmet medical need.

Inflammatory blockade prevents injury to the developing pulmonary gas exchange surface in preterm primates

Perinatal inflammatory stress is associated with early life morbidity and lifelong consequences for pulmonary health. Chorioamnionitis, an inflammatory condition affecting the placenta and fluid surrounding the developing fetus, affects 25 to 40% of preterm births. Severe chorioamnionitis with preterm birth is associated with significantly increased risk of pulmonary disease and secondary infections in childhood, suggesting that fetal inflammation may markedly alter the development of the lung. Here, we used intra-amniotic lipopolysaccharide (LPS) challenge to induce experimental chorioamnionitis in a prenatal rhesus macaque (Macaca mulatta) model that mirrors structural and temporal aspects of human lung development. Inflammatory injury directly disrupted the developing gas exchange surface of the primate lung, with extensive damage to alveolar structure, particularly the close association and coordinated differentiation of alveolar type 1 pneumocytes and specialized alveolar capillary endothelium. Single-cell RNA sequencing analysis defined a multicellular alveolar signaling niche driving alveologenesis that was extensively disrupted by perinatal inflammation, leading to a loss of gas exchange surface and alveolar simplification, with notable resemblance to chronic lung disease in newborns. Blockade of the inflammatory cytokines interleukin-1β and tumor necrosis factor-α ameliorated LPS-induced inflammatory lung injury by blunting stromal responses to inflammation and modulating innate immune activation in myeloid cells, restoring structural integrity and key signaling networks in the developing alveolus. These data provide new insight into the pathophysiology of developmental lung injury and suggest that modulating inflammation is a promising therapeutic approach to prevent fetal consequences of chorioamnionitis.

Optimization of long-term cold storage of rat precision-cut lung slices with a tissue preservation solution

Precision-cut lung slices (PCLS) are used as ex vivo model of the lung to fill the gap between in vitro and in vivo experiments. To allow optimal utilization of PCLS, possibilities to prolong slice viability via cold storage using optimized storage solutions were evaluated. Rat PCLS were cold stored in DMEM/F-12 or two different preservation solutions for up to 28 days at 4°C. After rewarming in DMEM/F-12, metabolic activity, live/dead staining, and mitochondrial membrane potential was assessed to analyze overall tissue viability. Single-cell suspensions were prepared and proportions of CD45⁺, EpCAM⁺, CD31⁺, and CD90⁺ cells were analyzed. As functional parameters, TNF-α expression was analyzed to detect inflammatory activity and bronchoconstriction was evaluated after acetylcholine stimulus. After 14 days of cold storage, viability and mitochondrial membrane potential were significantly better preserved after storage in solution 1 (potassium chloride rich) and solution 2 (potassium- and lactobionate-rich analog) compared with DMEM/F-12. Analysis of cell populations revealed efficient preservation of EpCAM⁺, CD31⁺, and CD90⁺ cells. Proportion of CD45⁺ cells decreased during cold storage but was better preserved by both modified solutions than by DMEM/F-12. PCLS stored in solution 1 responded substantially longer to inflammatory stimulation than those stored in DMEM/F-12 or solution 2. Analysis of bronchoconstriction revealed total loss of function after 14 days of storage in DMEM/F-12 but, in contrast, a good response in PCLS stored in the optimized solutions. An improved base solution with a high potassium chloride concentration optimizes cold storage of PCLS and allows shipment between laboratories and stockpiling of tissue samples.

The Challenge of Long-Term Cultivation of Human Precision-Cut Lung Slices

Human precision-cut lung slices (PCLS) have proven to be an invaluable tool for numerous toxicologic, pharmacologic, and immunologic studies. Although a cultivation period of <1 week is sufficient for most studies, modeling of complex disease mechanisms and investigating effects of long-term exposure to certain substances require cultivation periods that are much longer. So far, data regarding tissue integrity of long-term cultivated PCLS are incomplete. More than 1500 human PCLS from 16 different donors were cultivated under standardized, serum-free conditions for up to 28 days and the viability, tissue integrity, and the transcriptome was assessed in great detail. Even though viability of PCLS was well preserved during long-term cultivation, a continuous loss of cells was observed. Although the bronchial epithelium was well preserved throughout cultivation, the alveolar integrity was preserved for about 2 weeks, and the vasculatory system experienced significant loss of integrity within the first week. Furthermore, ciliary beat in the small airways gradually decreased after 1 week. Interestingly, keratinizing squamous metaplasia of the alveolar epithelium with significantly increasing manifestation were found over time. Transcriptome analysis revealed a significantly increased immune response and significantly decreased metabolic activity within the first 24 hours after PCLS generation. Overall, this study provides a comprehensive overview of histomorphologic and pathologic changes during long-term cultivation of PCLS.

Alveologenesis: What Governs Secondary Septa Formation

The simplification of alveoli leads to various lung pathologies such as bronchopulmonary dysplasia and emphysema. Deep insight into the process of emergence of the secondary septa during development and regeneration after pneumonectomy, and into the contribution of the drivers of alveologenesis and neo-alveolarization is required in an efficient search for therapeutic approaches. In this review, we describe the formation of the gas exchange units of the lung as a multifactorial process, which includes changes in the actomyosin cytoskeleton of alveocytes and myofibroblasts, elastogenesis, retinoic acid signaling, and the contribution of alveolar mesenchymal cells in secondary septation. Knowledge of the mechanistic context of alveologenesis remains incomplete. The characterization of the mechanisms that govern the emergence and depletion of αSMA will allow for an understanding of how the niche of fibroblasts is changing. Taking into account the intense studies that have been performed on the pool of lung mesenchymal cells, we present data on the typing of interstitial fibroblasts and their role in the formation and maintenance of alveoli. On the whole, when identifying cell subpopulations in lung mesenchyme, one has to consider the developmental context, the changing cellular functions, and the lability of gene signatures.

Precision-cut lung slices: A powerful ex vivo model to investigate respiratory infectious diseases

Respiratory infections are a leading cause of mortality worldwide. Most of the research on the underlying disease mechanisms is based on cell culture, organoid, or surrogate animal models. Although these provide important insights, they have limitations. Cell culture models fail to recapitulate cellular interactions in the lung and animal models often do not permit high‐throughput analysis of drugs or pathogen isolates; hence, there is a need for improved, scalable models. Precision‐cut lung slices (PCLS), small, uniform tissue slices generated from animal or human lungs are increasingly recognized and employed as an ex vivo organotypic model. PCLS retain remarkable cellular complexity and the architecture of the lung, providing a platform to investigate respiratory pathogens in a near‐native environment. Here, we review the generation and features of PCLS, their use to investigate the pathogenesis of viral and bacterial pathogens, and highlight their potential to advance respiratory infection research in the future.

Recent advances in human respiratory epithelium models for drug discovery

The respiratory epithelium is intimately associated with the pathophysiologies of highly infectious viral contagions and chronic illnesses such as chronic obstructive pulmonary disorder, presently the third leading cause of death worldwide with a projected economic burden of £1.7 trillion by 2030. Preclinical studies of respiratory physiology have almost exclusively utilised non-humanised animal models, alongside reductionistic cell line-based models, and primary epithelial cell models cultured at an air-liquid interface (ALI). Despite their utility, these model systems have been limited by their poor correlation to the human condition. This has undermined the ability to identify novel therapeutics, evidenced by a 15% chance of success for medicinal respiratory compounds entering clinical trials in 2018. Consequently, preclinical studies require new translational efficacy models to address the problem of respiratory drug attrition. This review describes the utility of the current in vivo (rodent), ex vivo (isolated perfused lungs and precision cut lung slices), two-dimensional in vitro cell-line (A549, BEAS-2B, Calu-3) and three-dimensional in vitro ALI (gold-standard and co-culture) and organoid respiratory epithelium models. The limitations to the application of these model systems in drug discovery research are discussed, in addition to perspectives of the future innovations required to facilitate the next generation of human-relevant respiratory models.

Decellularized Human Lung Scaffolds as Complex Three-Dimensional Tissue Culture Models to Study Functional Behavior of Fibroblasts

In vitro culturing of cells in two-dimensional (2D) environments is a widespread used methodology in biomedical research. Most commonly, cells are cultured on artificial plastic dish surfaces, which lead to abnormal functional behaviors, as plastic does not reflect the native microenvironment found in vivo or in situ. Therefore, a multitude of three-dimensional (3D) cell culture systems were developed in the past years, which aim to bridge the gap between 2D cell culture dishes and the in vivo situation. One of the more recent development in the field, the generation of viable precision-cut tissue slices from various organs emerged as an exciting approach to study complex interactions and biological processes ex vivo in 3D. Decellularization of such tissue slices leads to the removal of all functional cells, and leaves behind a scaffold of extracellular matrix (ECM), which closely recapitulates the molecular composition, mechanical properties, topology, and microarchitecture of native ECM. Subsequently, decellularized precision-cut lung slices (PCLS), also called 3D lung tissue culture (3D-LTCs), can be successfully reseeded with a variety of cell types, including fibroblasts, which attach to and engraft into the matrix. Here, we describe the generation of PCLS from resected human lung tissue and their decellularization and recellularization with primary human fibroblasts. This novel 3D tissue culture model allows for various functional studies of fibroblast behavior on native ECM composition and topology.

An integrated multiomic and quantitative label-free microscopy-based approach to study pro-fibrotic signalling in ex vivo human precision-cut lung slices

Fibrosis can affect any organ, resulting in the loss of tissue architecture and function with often life-threatening consequences. Pathologically, fibrosis is characterised by the expansion of connective tissue due to excessive deposition of extracellular matrix (ECM) proteins, including the fibrillar forms of collagen. A significant limitation for discovering cures for fibrosis is the availability of suitable human models and techniques to quantify mature fibrillar collagen deposition as close as possible to human physiological conditions.Here we have extensively characterised an ex vivo cultured human lung tissue-derived, precision-cut lung slices (hPCLS) model using label-free second harmonic generation (SHG) light microscopy to quantify fibrillar collagen deposition and mass spectrometry-based techniques to obtain a proteomic and metabolomic fingerprint of hPCLS in ex vivo culture.We demonstrate that hPCLS are viable and metabolically active, with mesenchymal, epithelial, endothelial and immune cell types surviving for at least 2 weeks in ex vivo culture. Analysis of hPCLS-conditioned supernatants showed a strong induction of pulmonary fibrosis-related ECM proteins upon transforming growth factor-β1 (TGF-β1) stimulation. This upregulation of ECM proteins was not translated into an increased deposition of fibrillar collagen. In support of this observation, we revealed the presence of a pro-ECM degradation activity in our ex vivo cultures of hPCLS, inhibition of which by a metalloproteinase inhibitor resulted in increased collagen deposition in response to TGF-β1 stimulation.Together the data show that an integrated approach of measuring soluble pro-fibrotic markers alongside quantitative SHG-based analysis of fibrillar collagen is a valuable tool for studying pro-fibrotic signalling and testing anti-fibrotic agents.

Conserved Mechanisms in the Formation of the Airways and Alveoli of the Lung

Branching is an intrinsic property of respiratory epithelium that can be induced and modified by signals emerging from the mesenchyme. However, during stereotypic branching morphogenesis of the airway, the relatively thick upper respiratory epithelium extrudes through a mesenchymal orifice to form a new branch, whereas during alveologenesis the relatively thin lower respiratory epithelium extrudes to form sacs or bubbles. Thus, both branching morphogenesis of the upper airway and alveolarization in the lower airway seem to rely on the same fundamental physical process: epithelial extrusion through an orifice. Here I propose that it is the orientation and relative stiffness of the orifice boundary that determines the stereotypy of upper airway branching as well as the orientation of individual alveolar components of the gas exchange surface. The previously accepted dogma of the process of alveologenesis, largely based on 2D microscopy, is that alveoli arise by erection of finger-like interalveolar septae to form septal clefts that subdivide pre-existing saccules, a process for which the contractile properties of specialized alveolar myofibroblasts are necessary. Here I suggest that airway tip splitting and stereotypical side domain branching are actually conserved processes, but modified somewhat by evolution to achieve both airway tip splitting and side branching of the upper airway epithelium, as well as alveologenesis. Viewed in 3D it is clear that alveolar “septal tips” are in fact ring or purse string structures containing elastin and collagen that only appear as finger like projections in cross section. Therefore, I propose that airway branch orifices as well as alveolar mouth rings serve to delineate and stabilize the budding of both airway and alveolar epithelium, from the tips and sides of upper airways as well as from the sides and tips of alveolar ducts. Certainly, in the case of alveoli arising laterally and with radial symmetry from the sides of alveolar ducts, the mouth of each alveolus remains within the plane of the side of the ductal lumen. This suggests that the thin epithelium lining these lateral alveolar duct buds may extrude or “pop out” from the duct lumen through rings rather like soap or gum bubbles, whereas the thicker upper airway epithelium extrudes through a ring like toothpaste from a tube to form a new branch.

Lung Marginated and Splenic Murine Resident Neutrophils Constitute Pioneers in Tissue-Defense During Systemic E. coli Challenge

The rapid response of neutrophils throughout the body to a systemic challenge is a critical first step in resolution of bacterial infection such as Escherichia coli (E. coli). Here we delineated the dynamics of this response, revealing novel insights into the molecular mechanisms using lung and spleen intravital microscopy and 3D ex vivo culture of living precision cut splenic slices in combination with fluorescent labelling of endogenous leukocytes. Within seconds after challenge, intravascular marginated neutrophils and lung endothelial cells (ECs) work cooperatively to capture pathogens. Neutrophils retained on lung ECs slow their velocity and aggregate in clusters that enlarge as circulating neutrophils carrying E. coli stop within the microvasculature. The absolute number of splenic neutrophils does not change following challenge; however, neutrophils increase their velocity, migrate to the marginal zone (MZ) and form clusters. Irrespective of their location all neutrophils capturing heat-inactivated E. coli take on an activated phenotype showing increasing surface CD11b. At a molecular level we show that neutralization of ICAM-1 results in splenic neutrophil redistribution to the MZ under homeostasis. Following challenge, splenic levels of CXCL12 and ICAM-1 are reduced allowing neutrophils to migrate to the MZ in a CD29-integrin dependent manner, where the enlargement of splenic neutrophil clusters is CXCR2-CXCL2 dependent. We show directly molecular mechanisms that allow tissue resident neutrophils to provide the first lines of antimicrobial defense by capturing circulating E. coli and forming clusters both in the microvessels of the lung and in the parenchyma of the spleen.

Three-Dimensional Bioprinting of Anatomically Realistic Tissue Constructs for Disease Modeling and Drug Testing

Three-dimensional (3D) bioprinting is an emerging tissue engineering technology, already with several remarkable accomplishments and with more promises to fulfill. Besides the enduring goal of making tissues for implantation, it could also become an essential tool in the worldwide trend to replace animal experimentation with improved in vitro models for disease mechanism studies, or with new high-throughput pharmacological and toxicology assays. All these require the speed, reproducibility, and standardization that bioprinting could easily provide. However, originating from additive manufacturing with its top-down approach of "filling" a virtual volume with a semifluid (hydrogel) material, the finer internal anatomic structure of the tissues, as well as vascularization and innervation, has remained difficult to implement. Thus, the next frontier in bioprinting is the generation of more anatomically realistic models, needed for ascending to the functionality of living tissues. In this study, I discuss the conceptual and practical barriers still hampering the attainment of this goal and suggest solutions to overcome them. In this regard, I introduce two workflows that combine existing methods in new operational sequences: (1) bioprinting guided by images of histological sections assembled in 3D constructs and (2) bioprinting of bidimensional vascular patterns implemented among stackable cellular layers. While more sophisticated methods to capture the tissue structure in 3D constructs certainly exist, I contend that extrusion bioprinting may still offer a simple, practical, and affordable option. Impact statement Paucity of anatomic structural details is one of the limitations of three-dimensional bioprinting toward fulfilling its potential for tissue engineering, drug testing, and toxicological assays. The origins of this problem can be tracked back to derivation of bioprinting from inorganic additive manufacturing, making it more adept to render the shapes of the objects than their content. As solutions, I suggest two simple workflows that can be implemented by most current bioprinters, based on the import into the construct design of anatomically realistic structural information. If more largely adopted, these and similar approaches may significantly improve the applicability of bioprinted constructs.

The Biological Significance and Implications of Planar Cell Polarity for Nephrology

The orientation of cells in two-dimensional and three-dimensional space underpins how the kidney develops and responds to disease. The process by which cells orientate themselves within the plane of a tissue is termed planar cell polarity. In this Review, we discuss how planar cell polarity and the proteins that underpin it govern kidney organogenesis and pathology. The importance of planar cell polarity and its constituent proteins in multiple facets of kidney development is emphasised, including ureteric bud branching, tubular morphogenesis and nephron maturation. An overview is given of the relevance of planar cell polarity and its proteins for inherited human renal diseases, including congenital malformations with unknown aetiology and polycystic kidney disease. Finally, recent work is described outlining the influence of planar cell polarity proteins on glomerular diseases and highlight how this fundamental pathway could yield a new treatment paradigm for nephrology.