Key mechanisms of early lung development

Pediatric Diffuse Lung Disease in Infants: Imaging Findings and Histopathologic Correlation

Childhood interstitial lung disease (chILD) encompasses a diverse group of genetic, infectious, and inflammatory conditions affecting infants and children. The recognition and understanding of these entities have highlighted the necessity for more accurate classification. This group of rare heterogeneous diseases comprises more than 200 different conditions and has a combined estimated prevalence of less than one patient per 100 000 children. Hence, a systematic diagnostic approach is crucial. This article describes a diagnostic approach for pediatric diffuse lung diseases in infancy, including an analysis of clinical presentations and imaging and histologic features to effectively distinguish among various chILD entities. Although they often have overlapping and nonspecific radiologic features, some chILD entities may exhibit typical imaging findings, resulting in a CT diagnosis or aiding in narrowing the differential diagnosis, thus guiding the clinician to the appropriate genetic tests, potentially limiting unnecessary biopsies. This approach aims to enhance the understanding and diagnosis of chILD in infants, thereby facilitating improved patient care.

The interplay of cells, polymers, and vascularization in three-dimensional lung models and their applications in COVID-19 research and therapy

Millions of people have been affected ever since the emergence of the corona virus disease of 2019 (COVID-19) outbreak, leading to an urgent need for antiviral drug and vaccine development. Current experimentation on traditional two-dimensional culture (2D) fails to accurately mimic the in vivo microenvironment for the disease, while in vivo animal model testing does not faithfully replicate human COVID-19 infection. Human-based three-dimensional (3D) cell culture models such as spheroids, organoids, and organ-on-a-chip present a promising solution to these challenges. In this report, we review the recent 3D in vitro lung models used in COVID-19 infection and drug screening studies and highlight the most common types of natural and synthetic polymers used to generate 3D lung models.

Gestational Diabetes Mellitus Impedes Fetal Lung Development Through Exosome-Dependent Crosstalk Between Trophoblasts and Lung Epithelial Cells

Background

Fetal lung underdevelopment (FLUD) is associated with neonatal and childhood severe respiratory diseases, among which gestational diabetes mellitus (GDM) play crucial roles as revealed by recent prevalence studies, yet mechanism underlying GDM-induced FLUD, especially the role of trophoblasts, is not all known.

Methods

From the perspective of trophoblast-derived exosomes, we established in vitro, ex vivo, in vivo and GDM trophoblast models. Utilizing placenta-derived exosomes (NUB-exos and GDMUB-exos) isolated from normal and GDM umbilical cord blood plasma and trophoblast-derived exosomes (NC-exos and HG-exos) isolated from HTR8/SVneo trophoblasts medium with/without high glucose treatment, we examined their effects on fetal lung development and biological functions.

Results

We found that, compared with the NUB-exos group, the exosome concentration increased in GDMUB-exos group, and the content of exosomes also changed evidenced by 61 dysregulated miRNAs. After applying these exosomes to A549 alveolar type II epithelial cells, the proliferation and biological functions were suppressed while the proportion of apoptotic cells was increased as compared to the control. In ex vivo studies, we found that GDMUB-exos showed significant suppression on the growth of the fetal lung explants, where the number of terminal buds and the area of explant surface decreased and shrank. Besides, the expression of Fgf10, Vegfa, Flt-1, Kdr and surfactant proteins A, B, C, and D was downregulated in GDMUB-exos group, whilst Sox9 was upregulated. For in vivo studies, we found significant suppression of fetal lung development in GDMUB-exos group. Importantly, we found consistent alterations when we used NC-exos and HG-exos, suggesting a dominant role of trophoblasts in placenta-derived exosome-induced FLUD.

Conclusion

In conclusion, GDM can adversely affect trophoblasts and alter exosome contents, causing crosstalk disorder between trophoblasts and fetal lung epithelial cells and finally leading to FLUD. Findings of this study will shine insight into the theoretical explanation for the pathogenesis of FLUD.

Hedgehog Signaling Pathway Orchestrates Human Lung Branching Morphogenesis

The Hedgehog (HH) signaling pathway plays an essential role in mouse lung development. We hypothesize that the HH pathway is necessary for branching during human lung development and is impaired in pulmonary hypoplasia. Single-cell, bulk RNA-sequencing data, and human fetal lung tissues were analyzed to determine the spatiotemporal localization of HH pathway actors. Distal human lung segments were cultured in an air-liquid interface and treated with an SHH inhibitor (5E1) to determine the effect of HH inhibition on human lung branching, epithelial-mesenchymal markers, and associated signaling pathways in vitro. Our results showed an early and regulated expression of HH pathway components during human lung development. Inhibiting HH signaling caused a reduction in branching during development and dysregulated epithelial (SOX2, SOX9) and mesenchymal (ACTA2) progenitor markers. FGF and Wnt pathways were also disrupted upon HH inhibition. Finally, we demonstrated that HH signaling elements were downregulated in lung tissues of patients with a congenital diaphragmatic hernia (CDH). In this study, we show for the first time that HH signaling inhibition alters important genes and proteins required for proper branching of the human developing lung. Understanding the role of the HH pathway on human lung development could lead to the identification of novel therapeutic targets for childhood pulmonary diseases.

Perspectives on the Structure and Function of the Avian Respiratory System: Functional Efficiency Built on Structural Complexity

Among the air-breathing vertebrates, regarding respiratory efficiency, the avian respiratory system rests at the evolutionary zenith. Structurally, it is separated into a lung that serves as a gas exchanger and air sacs that mechanically ventilate the lung continuously and unidirectionally in a caudocranial direction. Largely avascular, the air sacs are delicate, transparent, compliant and capacious air-filled spaces that are not meaningfully involved in gas exchange. The avian lungs are deeply and firmly attached to the vertebrae and the ribs on the dorsolateral aspects, rendering them practically rigid and inflexible. The attachment of the lung to the body wall allowed extreme subdivision of the exchange tissue into minuscule and stable terminal respiratory units, the air capillaries. The process generated a large respiratory surface area in small lungs with low volume density of gas exchange tissue. For the respiratory structures, invariably, thin blood-gas barrier, large respiratory surface area and large pulmonary capillary blood volume are the foremost adaptive structural features that confer large total pulmonary morphometric diffusing capacities of O2. At parabronchial level, the construction and the arrangement of the airway- and the vascular components of the avian lung determine the delivery, the presentation and the exposure of inspired air to capillary blood across the blood-gas barrier. In the avian lung, crosscurrent-, countercurrent- and multicapillary serial arterialization systems that stem from the organization of the structural parts of the lung promote gas exchange. The exceptional respiratory efficiency of the avian respiratory system stems from synergy of morphological properties and physiological processes, means by which O2 uptake is optimized and high metabolic states and capacities supported. Given that among the extant animal taxa insects, birds and bats (which accomplished volancy chronologically in that order) possess structurally much different respiratory systems, the avian respiratory system was by no means a prerequisite for evolution of powered flight but was but one of the adaptive solutions to realization of an exceptionally efficient mode of locomotion.

Complete lung agenesis caused by complex genomic rearrangements with neo-TAD formation at the SHH locus

During human organogenesis, lung development is a timely and tightly regulated developmental process under the control of a large number of signaling molecules. Understanding how genetic variants can disturb normal lung development causing different lung malformations is a major goal for dissecting molecular mechanisms during embryogenesis. Here, through exome sequencing (ES), array CGH, genome sequencing (GS) and Hi-C, we aimed at elucidating the molecular basis of bilateral isolated lung agenesis in three fetuses born to a non-consanguineous family. We detected a complex genomic rearrangement containing duplicated, triplicated and deleted fragments involving the SHH locus in fetuses presenting complete agenesis of both lungs and near-complete agenesis of the trachea, diagnosed by ultrasound screening and confirmed at autopsy following termination. The rearrangement did not include SHH itself, but several regulatory elements for lung development, such as MACS1, a major SHH lung enhancer, and the neighboring genes MNX1 and NOM1. The rearrangement incorporated parts of two topologically associating domains (TADs) including their boundaries. Hi-C of cells from one of the affected fetuses showed the formation of two novel TADs each containing SHH enhancers and the MNX1 and NOM1 genes. Hi-C together with GS indicate that the new 3D conformation is likely causative for this condition by an inappropriate activation of MNX1 included in the neo-TADs by MACS1 enhancer, further highlighting the importance of the 3D chromatin conformation in human disease.

Stem cells and lung regeneration

The ability to replace defective cells in an airway with cells that can engraft, integrate, and restore a functional epithelium could potentially cure a number of lung diseases. Progress toward the development of strategies to regenerate the adult lung by either in vivo or ex vivo targeting of endogenous stem cells or pluripotent stem cell derivatives is limited by our fundamental lack of understanding of the mechanisms controlling human lung development, the precise identity and function of human lung stem and progenitor cell types, and the genetic and epigenetic control of human lung fate. In this review, we intend to discuss the known stem/progenitor cell populations, their relative differences between rodents and humans, their roles in chronic lung disease, and their therapeutic prospects. Additionally, we highlight the recent breakthroughs that have increased our understanding of these cell types. These advancements include novel lineage-traced animal models and single-cell RNA sequencing of human airway cells, which have provided critical information on the stem cell subtypes, transition states, identifying cell markers, and intricate pathways that commit a stem cell to differentiate or to maintain plasticity. As our capacity to model the human lung evolves, so will our understanding of lung regeneration and our ability to target endogenous stem cells as a therapeutic approach for lung disease.

Clinical, Histopathological, and Molecular Diagnostics in Lethal Lung Developmental Disorders

Lethal lung developmental disorders are a rare but important group of pediatric diffuse lung diseases presenting with neonatal respiratory failure. On the basis of histopathological appearance at lung biopsy or autopsy, they have been termed: alveolar capillary dysplasia with misalignment of the pulmonary veins, acinar dysplasia, congenital alveolar dysplasia, and other unspecified primary pulmonary hypoplasias. However, the histopathological continuum in these lethal developmental disorders has made accurate diagnosis challenging, which has implications for recurrence risk. Over the past decade, genetic studies in infants with alveolar capillary dysplasia with misalignment of the pulmonary veins have revealed the causative role of the dosage-sensitive FOXF1 gene and its noncoding regulatory variants in the distant lung-specific enhancer at chromosome 16q24.1. In contrast, the molecular bases of acinar dysplasia and congenital alveolar dysplasia have remained poorly understood. Most recently, disruption of the TBX4-FGF10-FGFR2 epithelial-mesenchymal signaling pathway has been reported in patients with these lethal pulmonary dysplasias. Application of next-generation sequencing techniques, including exome sequencing and whole-genome sequencing, has demonstrated their complex compound inheritance. These data indicate that noncoding regulatory elements play a critical role in lung development in humans. We propose that for more precise lethal lung developmental disorder diagnosis, a diagnostic pathway including whole-genome sequencing should be implemented.

Aberrant Epithelial Cell Proliferation in Peripheral Airways in Bronchiectasis

Dilation of bronchi and bronchioles caused by destruction and excessive epithelial remodeling is a characteristic feature of bronchiectasis. It is not known how epithelial progenitor cells contribute to these pathologic conditions in peripheral airways (bronchioles) in bronchiectasis. We aimed to explore the expression levels of signature airway progenitor cells in the dilated bronchioles in patients with bronchiectasis. We obtained the surgically resected peripheral lung tissues from 43 patients with bronchiectasis and 33 control subjects. Immunostaining was performed to determine the expression patterns of thyroid transcription factor-1 (TTF-1, for labeling progenitor cells in distal airways), P63 (basal cells), club cell 10 kDa protein (CC10, club cells), and surfactant protein C (SPC, alveolar type II epithelial cells) in epithelium or sub-epithelium. Here, we reported significantly lower percentage of TTF-1⁺ cells and CC10⁺ cells, and higher percentage of P63⁺ cells within the epithelium of dilated bronchioles compared with control bronchioles. In airway sub-epithelium of the dilated bronchioles, epithelial hyperplasia with disarrangement of TTF-1⁺ cells yielded cuboidal (100%) and columnar (93.0%) type among bronchiectasis patients. Most progenitor cell markers co-localized with TTF-1. The median (the 1st, 3rd quartile) percentage of P63⁺TTF-1⁺, CC10⁺TTF-1⁺, and SPC⁺TTF-1⁺ cells was 16.0% (8.9, 24.0%), 14.5% (7.1, 20.8%), and 52% (40.3, 64.4%), respectively. For cuboidal epithelial hyperplasia, 91.0% (86.5, 94.0%) of areas co-stained with SPC and TTF-1. Columnar epithelial hyperplasia was characterized by TTF-1 co-staining with P63⁺TTF-1⁺ and CC10⁺TTF-1⁺ cells. Taken together, aberrant proliferation of airway progenitor cells in both epithelium and sub-epithelium are implicated in bronchiectasis.

Comprehensive anatomic ontologies for lung development: A comparison of alveolar formation and maturation within mouse and human lung

BACKGROUND

Although the mouse is widely used to model human lung development, function, and disease, our understanding of the molecular mechanisms involved in alveolarization of the peripheral lung is incomplete. Recently, the Molecular Atlas of Lung Development Program (LungMAP) was funded by the National Heart, Lung, and Blood Institute to develop an integrated open access database (known as BREATH) to characterize the molecular and cellular anatomy of the developing lung. To support this effort, we designed detailed anatomic and cellular ontologies describing alveolar formation and maturation in both mouse and human lung.

DESCRIPTION

While the general anatomic organization of the lung is similar for these two species, there are significant variations in the lung's architectural organization, distribution of connective tissue, and cellular composition along the respiratory tract. Anatomic ontologies for both species were constructed as partonomic hierarchies and organized along the lung's proximal-distal axis into respiratory, vascular, neural, and immunologic components. Terms for developmental and adult lung structures, tissues, and cells were included, providing comprehensive ontologies for application at varying levels of resolution. Using established scientific resources, multiple rounds of comparison were performed to identify common, analogous, and unique terms that describe the lungs of these two species. Existing biological and biomedical ontologies were examined and cross-referenced to facilitate integration at a later time, while additional terms were drawn from the scientific literature as needed. This comparative approach eliminated redundancy and inconsistent terminology, enabling us to differentiate true anatomic variations between mouse and human lungs. As a result, approximately 300 terms for fetal and postnatal lung structures, tissues, and cells were identified for each species.

CONCLUSION

These ontologies standardize and expand current terminology for fetal and adult lungs, providing a qualitative framework for data annotation, retrieval, and integration across a wide variety of datasets in the BREATH database. To our knowledge, these are the first ontologies designed to include terminology specific for developmental structures in the lung, as well as to compare common anatomic features and variations between mouse and human lungs. These ontologies provide a unique resource for the LungMAP, as well as for the broader scientific community.

Complex Compound Inheritance of Lethal Lung Developmental Disorders Due to Disruption of the TBX-FGF Pathway

Primary defects in lung branching morphogenesis, resulting in neonatal lethal pulmonary hypoplasias, are incompletely understood. To elucidate the pathogenetics of human lung development, we studied a unique collection of samples obtained from deceased individuals with clinically and histopathologically diagnosed interstitial neonatal lung disorders: acinar dysplasia (n = 14), congenital alveolar dysplasia (n = 2), and other lethal lung hypoplasias (n = 10). We identified rare heterozygous copy-number variant deletions or single-nucleotide variants (SNVs) involving TBX4 (n = 8 and n = 2, respectively) or FGF10 (n = 2 and n = 2, respectively) in 16/26 (61%) individuals. In addition to TBX4, the overlapping ∼2 Mb recurrent and nonrecurrent deletions at 17q23.1q23.2 identified in seven individuals with lung hypoplasia also remove a lung-specific enhancer region. Individuals with coding variants involving either TBX4 or FGF10 also harbored at least one non-coding SNV in the predicted lung-specific enhancer region, which was absent in 13 control individuals with the overlapping deletions but without any structural lung anomalies. The occurrence of rare coding variants involving TBX4 or FGF10 with the putative hypomorphic non-coding SNVs implies a complex compound inheritance of these pulmonary hypoplasias. Moreover, they support the importance of TBX4-FGF10-FGFR2 epithelial-mesenchymal signaling in human lung organogenesis and help to explain the histopathological continuum observed in these rare lethal developmental disorders of the lung.

Transcriptional regulation of cell shape during organ morphogenesis

The emerging field of transcriptional regulation of cell shape changes aims to address the critical question of how gene expression programs produce a change in cell shape. Together with cell growth, division, and death, changes in cell shape are essential for organ morphogenesis. Whereas most studies of cell shape focus on posttranslational events involved in protein organization and distribution, cell shape changes can be genetically programmed. This review highlights the essential role of transcriptional regulation of cell shape during morphogenesis of the heart, lungs, gastrointestinal tract, and kidneys. We emphasize the evolutionary conservation of these processes across different model organisms and discuss perspectives on open questions and research avenues that may provide mechanistic insights toward understanding birth defects.

MicroRNA-200b regulates distal airway development by maintaining epithelial integrity

miR-200b plays a role in epithelial-to-mesenchymal transition (EMT) in cancer. We recently reported abnormal expression of miR-200b in the context of human pulmonary hypoplasia in congenital diaphragmatic hernia (CDH). Smaller lung size, a lower number of airway generations, and a thicker mesenchyme characterize pulmonary hypoplasia in CDH. The aim of this study was to define the role of miR-200b during lung development. Here we show that miR-200b-/- mice have abnormal lung function due to dysfunctional surfactant, increased fibroblast-like cells and thicker mesenchyme in between the alveolar walls. We profiled the lung transcriptome in miR-200b-/- mice, and, using Gene Ontology analysis, we determined that the most affected biological processes include cell cycle, apoptosis and protein transport. Our results demonstrate that miR-200b regulates distal airway development through maintaining an epithelial cell phenotype. The lung abnormalities observed in miR-200b-/- mice recapitulate lung hypoplasia in CDH.

Canonical Sonic Hedgehog Signaling in Early Lung Development

The canonical hedgehog (HH) signaling pathway is of major importance during embryonic development. HH is a key regulatory morphogen of numerous cellular processes, namely, cell growth and survival, differentiation, migration, and tissue polarity. Overall, it is able to trigger tissue-specific responses that, ultimately, contribute to the formation of a fully functional organism. Of all three HH proteins, Sonic Hedgehog (SHH) plays an essential role during lung development. In fact, abnormal levels of this secreted protein lead to severe foregut defects and lung hypoplasia. Canonical SHH signal transduction relies on the presence of transmembrane receptors, such as Patched1 and Smoothened, accessory proteins, as Hedgehog-interacting protein 1, and intracellular effector proteins, like GLI transcription factors. Altogether, this complex signaling machinery contributes to conveying SHH response. Pulmonary morphogenesis is deeply dependent on SHH and on its molecular interactions with other signaling pathways. In this review, the role of SHH in early stages of lung development, specifically in lung specification, primary bud formation, and branching morphogenesis is thoroughly reviewed.

The Role of Serotonin Transporter in Human Lung Development and in Neonatal Lung Disorders

Introduction. Failure of the vascular pulmonary remodeling at birth often manifests as pulmonary hypertension (PHT) and is associated with a variety of neonatal lung disorders including a uniformly fatal developmental disorder known as alveolar capillary dysplasia with misalignment of pulmonary veins (ACD/MPV). Serum serotonin regulation has been linked to pulmonary vascular function and disease, and serotonin transporter (SERT) is thought to be one of the key regulators in these processes. We sought to find evidence of a role that SERT plays in the neonatal respiratory adaptation process and in the pathomechanism of ACD/MPV. Methods. We used histology and immunohistochemistry to determine the timetable of SERT protein expression in normal human fetal and postnatal lungs and in cases of newborn and childhood PHT of varied etiology. In addition, we tested for a SERT gene promoter defect in ACD/MPV patients. Results. We found that SERT protein expression begins at 30 weeks of gestation, increases to term, and stays high postnatally. ACD/MPV patients had diminished SERT expression without SERT promoter alteration. Conclusion. We concluded that SERT/serotonin pathway is crucial in the process of pulmonary vascular remodeling/adaptation at birth and plays a key role in the pathobiology of ACD/MPV.

Organotypic culture in three dimensions prevents radiation-induced transformation in human lung epithelial cells

The effects of radiation in two-dimensional (2D) cell culture conditions may not recapitulate tissue responses as modeled in three-dimensional (3D) organotypic culture. In this study, we determined if the frequency of radiation-induced transformation and cancer progression differed in 3D compared to 2D culture. Telomerase immortalized human bronchial epithelial cells (HBECs) with shTP53 and mutant KRas expression were exposed to various types of radiation (gamma, ⁺H, ⁵⁶Fe) in either 2D or 3D culture. After irradiation, 3D structures were dissociated and passaged as a monolayer followed by measurement of transformation, cell growth and expression analysis. Cells irradiated in 3D produced significantly fewer and smaller colonies in soft agar than their 2D-irradiated counterparts (gamma P = 0.0004; ⁺H P = 0.049; ⁵⁶Fe P < 0.0001). The cell culture conditions did not affect cell killing, the ability of cells to survive in a colony formation assay, and proliferation rates after radiation—implying there was no selection against cells in or dissociated from 3D conditions. However, DNA damage repair and apoptosis markers were increased in 2D cells compared to 3D cells after radiation. Ideally, expanding the utility of 3D culture will allow for a better understanding of the biological consequences of radiation exposure.

Expression analysis of Shh signaling members in early stages of chick lung development

Lung organogenesis is guided by epithelial-mesenchymal interactions that coordinate cellular events responsible for the formation of the respiratory system. Several signaling pathways have been implicated in this process; among them, sonic hedgehog (Shh) signaling has emerged as a crucial regulator of branching morphogenesis in the mammalian lung. Canonical Shh signaling requires the presence of patched (Ptch) and smoothened (Smo) transmembrane receptors in order to induce the activation of glioblastoma (Gli) zinc finger transcription factors that are the true effectors of the pathway. Signal transduction is finely regulated by Ptch1, Gli, and Hhip (hedgehog-interacting protein). The present work characterizes, for the first time, the expression pattern of shh, ptch1, smo, gli1, and hhip in early stages of the embryonic chick lung. In situ hybridization studies revealed that these genes are expressed in the same cellular compartments as their mammalian counterparts, although their proximo-distal distribution is slightly changed. Moreover, the molecular interactions between fibroblast growth factor (FGF) and Shh signaling pathway were assessed, in vitro, by grafting beads soaked in SU5402 (an FGF receptor inhibitor). In the chick lung, Shh signaling seems to have some features that are species specific since shh is not a downstream target of FGF signaling. Nonetheless and despite the observed differences, these findings suggest a role for Shh signaling in the epithelial-mesenchymal interactions that control chick lung morphogenesis.

MiR-449a Affects Epithelial Proliferation during the Pseudoglandular and Canalicular Phases of Avian and Mammal Lung Development

Congenital diaphragmatic hernia is associated with pulmonary hypoplasia and respiratory distress, which result in high mortality and morbidity. Although several transgenic mouse models of lung hypoplasia exist, the role of miRNAs in this phenotype is incompletely characterized. In this study, we assessed microRNA expression levels during the pseudoglandular to canalicular phase transition of normal human fetal lung development. At this critical time, when the distal respiratory portion of the airways begins to form, microarray analysis showed that the most significantly differentially expressed miRNA was miR-449a. Prediction algorithms determined that N-myc is a target of miR-449a and identified the likely miR-449a:N-myc binding sites, confirmed by luciferase assays and targeted mutagenesis. Functional ex vivo knock-down in organ cultures of murine embryonic lungs, as well as in ovo overexpression in avian embryonic lungs, suggested a role for miR-449a in distal epithelial proliferation. Finally, miR-449a expression was found to be abnormal in rare pulmonary specimens of human fetuses with Congenital Diaphragmatic Hernia in the pseudoglandular or canalicular phase. This study confirms the conserved role of miR-449a for proper pulmonary organogenesis, supporting the delicate balance between expansion of progenitor cells and their terminal differentiation, and proposes the potential involvement of this miRNA in human pulmonary hypoplasia.

Initial Suppression of Transforming Growth Factor-β Signaling and Loss of TGFBI Causes Early Alveolar Structural Defects Resulting in Bronchopulmonary Dysplasia

Septation of the gas-exchange saccules of the morphologically immature mouse lung requires regulated timing, spatial direction, and dosage of transforming growth factor (TGF)-β signaling. We found that neonatal hyperoxia acutely initially diminished saccular TGF-β signaling coincident with alveolar simplification. However, sustained hyperoxia resulted in a biphasic response and subsequent up-regulation of TGF-β signaling, ultimately resulting in bronchopulmonary dysplasia. Significantly, we found that the TGF-β-induced matricellular protein (TGFBI) was similarly biphasically altered in response to hyperoxia. Moreover, genetic ablation revealed that TGFBI was required for normal alveolar structure and function. Although the phenotype was not neonatal lethal, Tgfbi-deficient lungs were morphologically abnormal. Mutant septal tips were stunted, lacked elastin-positive tips, exhibited reduced proliferation, and contained abnormally persistent alveolar α-smooth muscle actin myofibroblasts. In addition, Tgfbi-deficient lungs misexpressed TGF-β-responsive follistatin and serpine 1, and transiently suppressed myofibroblast platelet-derived growth factor α differentiation marker. Finally, despite normal lung volume, Tgfbi-null lungs displayed diminished elastic recoil and gas exchange efficiency. Combined, these data demonstrate that initial suppression of the TGF-β signaling apparatus, as well as loss of key TGF-β effectors (like TGFBI), underlies early alveolar structural defects, as well as long-lasting functional deficits routinely observed in chronic lung disease of infancy patients. These studies underline the complex (and often contradictory) role of TGF-β and indicate a need to design studies to associate alterations with initial appearance of phenotypical changes suggestive of bronchopulmonary dysplasia.

Organoids as a model system for studying human lung development and disease

The lung is a complex organ comprising multiple cell types that perform a variety of vital processes, including immune defense and gas exchange. Diseases of the lung, such as chronic obstructive pulmonary disease, asthma and lung cancer, together represent one of the largest causes of patient suffering and mortality. Logistical barriers that hamper access to embryonic, normal adult or diseased lung tissue currently hinder the study of lung disease. In vitro lung modeling represents an attractive and accessible avenue for investigating lung development, function and disease pathology, but accurately modeling the lung in vitro requires a system that recapitulates the structural features of the native lung. Organoids are stem cell-derived three-dimensional structures that are supported by an extracellular matrix and contain multiple cell types whose spatial arrangement and interactions mimic those of the native organ. Recently, organoids representative of the respiratory system have been generated from adult lung stem cells and human pluripotent stem cells. Ongoing studies are showing that organoids may be used to model human lung development, and can serve as a platform for interrogating the function of lung-related genes and signalling pathways. In a therapeutic context, organoids may be used for modeling lung diseases, and as a platform for screening for drugs that alleviate respiratory disease. Here, we summarize the organoid-forming capacity of respiratory cells, current lung organoid technologies and their potential use in future therapeutic applications.

Neonatal hyperoxic lung injury favorably alters adult right ventricular remodeling response to chronic hypoxia exposure

The development of pulmonary hypertension (PH) requires multiple pulmonary vascular insults, yet the role of early oxygen therapy as an initial pulmonary vascular insult remains poorly defined. Here, we employ a two-hit model of PH, utilizing postnatal hyperoxia followed by adult hypoxia exposure, to evaluate the role of early hyperoxic lung injury in the development of later PH. Sprague-Dawley pups were exposed to 90% oxygen during postnatal days 0-4 or 0-10 or to room air. All pups were then allowed to mature in room air. At 10 wk of age, a subset of rats from each group was exposed to 2 wk of hypoxia (Patm = 362 mmHg). Physiological, structural, and biochemical endpoints were assessed at 12 wk. Prolonged (10 days) postnatal hyperoxia was independently associated with elevated right ventricular (RV) systolic pressure, which worsened after hypoxia exposure later in life. These findings were only partially explained by decreases in lung microvascular density. Surprisingly, postnatal hyperoxia resulted in robust RV hypertrophy and more preserved RV function and exercise capacity following adult hypoxia compared with nonhyperoxic rats. Biochemically, RVs from animals exposed to postnatal hyperoxia and adult hypoxia demonstrated increased capillarization and a switch to a fetal gene pattern, suggesting an RV more adept to handle adult hypoxia following postnatal hyperoxia exposure. We concluded that, despite negative impacts on pulmonary artery pressures, postnatal hyperoxia exposure may render a more adaptive RV phenotype to tolerate late pulmonary vascular insults.

Acellular lung scaffolds direct differentiation of endoderm to functional airway epithelial cells: requirement of matrix-bound HS proteoglycans

Efficient differentiation of pluripotent cells to proximal and distal lung epithelial cell populations remains a challenging task. The 3D extracellular matrix (ECM) scaffold is a key component that regulates the interaction of secreted factors with cells during development by often binding to and limiting their diffusion within local gradients. Here we examined the role of the lung ECM in differentiation of pluripotent cells in vitro and demonstrate the robust inductive capacity of the native lung matrix alone. Extended culture of stem cell-derived definitive endoderm on decellularized lung scaffolds in defined, serum-free medium resulted in differentiation into mature airway epithelia, complete with ciliated cells, club cells, and basal cells with morphological and functional similarities to native airways. Heparitinase I, but not chondroitinase ABC, treatment of scaffolds revealed that the differentiation achieved is dependent on heparan sulfate proteoglycans and its bound factors remaining on decellularized scaffolds.

•

Lung scaffolds direct ESC-derived endoderm differentiation to airway epithelia

•

ESC-derived airway epithelial cells are functional and resemble native airways

•

Differentiation by scaffolds is dependent on matrix heparan sulfate proteoglycans

Relationship of structural to functional impairment during alveolar-capillary membrane development

Bronchopulmonary dysplasia is a chronic lung disease of extreme preterm infants and results in impaired gas exchange. Although bronchopulmonary dysplasia is characterized histologically by alveolar-capillary simplification in animal models, it is clinically defined by impaired gas diffusion. With the use of a developmentally relevant model, we correlated alveolar-capillary structural simplification with reduced functional gas exchange as measured by the diffusing factor for carbon monoxide (DFCO). Neonatal mouse pups were exposed to >90% hyperoxia or room air during postnatal days 0 to 7, and then all pups were returned to room air from days 7 to 56. At day 56, DFCO was measured as the ratio of carbon monoxide uptake to neon dilution, and lungs were fixed for histologic assessment of alveolar-capillary development. Neonatal hyperoxia exposure inhibited alveolar-capillary septal development as evidenced by significantly increased mean linear intercept, increased airspace-to-septal ratio, decreased nodal density, and decreased pulmonary microvasculature. Importantly, alveolar-capillary structural deficits in hyperoxia-exposed pups were accompanied by a significant 28% decrease in DFCO (0.555 versus 0.400; P < 0.0001). In addition, DFCO was highly and significantly correlated with structural measures of reduced alveolar-capillary growth. Simplification of alveolar-capillary structure is highly correlated with impaired gas exchange function. Current mechanistic and therapeutic animal models of inhibited alveolar development may benefit from application of DFCO as an alternative physiologic indicator of alveolar-capillary development.

Identification of a proximal progenitor population from murine fetal lungs with clonogenic and multilineage differentiation potential

Lung development-associated diseases are major causes of morbidity and lethality in preterm infants and children. Access to the lung progenitor/stem cell populations controlling pulmonary development during embryogenesis and early postnatal years is essential to understand the molecular basis of such diseases. Using a Nkx2-1^mCherry reporter mouse, we have identified and captured Nkx2-1-expressing lung progenitor cells from the proximal lung epithelium during fetal development. These cells formed clonal spheres in semisolid culture that could be maintained in vitro and demonstrated self-renewal and expansion capabilities over multiple passages. In-vitro-derived Nkx2-1-expressing clonal spheres differentiated into a polarized epithelium comprised of multiple cell lineages, including basal and secretory cells, that could repopulate decellularized lung scaffolds. Nkx2-1 expression thus defines a fetal lung epithelial progenitor cell population that can be used as a model system to study pulmonary development and associated pediatric diseases.

•

Nkx2-1 expression can be used to isolate proximal lung progenitors

•

The fetal proximal lung progenitors are distinct from currently known progenitors

•

The fetal proximal lung progenitors are clonogenic and self-renewing

•

The fetal proximal lung progenitors are multipotent for airway lineages

Branching morphogenesis of immortalized human bronchial epithelial cells in three-dimensional culture

While mouse models have contributed in our understanding of lung development, repair and regeneration, inherent differences between the murine and human airways requires the development of new models using human airway epithelial cells. In this study, we describe a three-dimensional model system using human bronchial epithelial cells (HBECs) cultured on reconstituted basement membrane. HBECs form complex budding and branching structures on reconstituted basement membrane when co-cultured with human lung fetal fibroblasts. These structures are reminiscent of the branching epithelia during lung development. The HBECs also retain markers indicative of epithelial cell types from both the central and distal airways suggesting their multipotent potential. In addition, we illustrate how the model can be utilized to understand respiratory diseases such as lung cancer. The 3D novel cell culture system recapitulates stromal-epithelial interactions in vitro that can be utilized to understand important aspects of lung development and diseases.

Generation of multiciliated cells in functional airway epithelia from human induced pluripotent stem cells

Despite therapeutic advancement, pulmonary disease still remains a major cause of morbidity and mortality around the world. Opportunities to study human lung disease either in vivo or in vitro are currently limited. Using induced pluripotent stem cells (iPSCs), we generated mature multiciliated cells in a functional airway epithelium. Robust multiciliogenesis occurred when notch signaling was inhibited and was confirmed by (i) the assembly of multiple pericentrin-stained centrioles at the apical surface, (ii) expression of transcription factor forkhead box protein J1, and (iii) presence of multiple acetylated tubulin-labeled cilia projections in individual cells. Clara, goblet, and basal cells were all present, confirming the generation of a complete polarized epithelial-cell layer. Additionally, cAMP-activated and cystic fibrosis transmembrane regulator inhibitor 172-sensitive cystic fibrosis transmembrane regulator currents were recorded in isolated epithelial cells. Our report demonstrating the generation of mature multiciliated cells in respiratory epithelium from iPSCs is a significant advance toward modeling a number of human respiratory diseases in vitro.

Vascular mediators in chronic lung disease of infancy: role of endothelial monocyte activating polypeptide II (EMAP II)

Bronchopulmonary dysplasia (BPD) is a chronic lung disease of prematurity. Over the years, the BPD phenotype has evolved, but despite various advances in neonatal management approaches, the reduction in the BPD burden is minimal. With the advent of surfactant, glucocorticoids, and new ventilation strategies, BPD has evolved from a disease of structural injury into a new BPD, marked by an arrest in alveolar growth in the lungs of extremely premature infants. This deficient alveolar growth has been associated with a diminution of pulmonary vasculature. Several investigators have described the epithelial / vascular co-dependency and the significant role of crosstalk between vessel formation, alveologenesis, and lung dysplasia's; hence identification and study of factors that regulate pulmonary vascular emergence and inflammation has become crucial in devising effective therapeutic approaches for this debilitating condition. The potent antiangiogenic and proinflammatory protein Endothelial Monocyte Activating Polypeptide II (EMAP II) has been described as a mediator of pulmonary vascular and alveolar formation and its expression is inversely related to the periods of vascularization and alveolarization in the developing lung. Hence the study of EMAP II could play a vital role in studying and devising appropriate therapeutics for diseases of aberrant lung development, such as BPD. Herein, we review the vascular contribution to lung development and the implications that vascular mediators such as EMAP II have in distal lung formation during the vulnerable stage of alveolar genesis.

Assessment of inhibited alveolar-capillary membrane structural development and function in bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a chronic lung disease of extreme prematurity and is defined clinically by dependence on supplemental oxygen due to impaired gas exchange. Optimal gas exchange is dependent on the development of a sufficient surface area for diffusion. In the mammalian lung, rapid acquisition of distal lung surface area is accomplished in neonatal and early adult life by means of vascularization and secondary septation of distal lung airspaces. Extreme preterm birth interrupts secondary septation and pulmonary capillary development and ultimately reduces the efficiency of the alveolar-capillary membrane. Although pulmonary health in BPD infants rapidly improves over the first few years, persistent alveolar-capillary membrane dysfunction continues into adolescence and adulthood. Preventative therapies have been largely ineffective, and therapies aimed at promoting normal development of the air-blood barrier in infants with established BPD remain largely unexplored. The purpose of this review will be: (1) to summarize the histological evidence of aberrant alveolar-capillary membrane development associated with extreme preterm birth and BPD, (2) to review the clinical evidence assessing the long-term impact of BPD on alveolar-capillary membrane function, and (3) to discuss the need to develop and incorporate direct measurements of functional gas exchange into clinically relevant animal models of inhibited alveolar development.

Heterogeneous pulmonary phenotypes associated with mutations in the thyroid transcription factor gene NKX2-1

BACKGROUND

Mutations in the gene encoding thyroid transcription factor, NKX2-1, result in neurologic abnormalities, hypothyroidism, and neonatal respiratory distress syndrome (RDS) that together are known as the brain-thyroid-lung syndrome. To characterize the spectrum of associated pulmonary phenotypes, we identified individuals with mutations in NKX2-1 whose primary manifestation was respiratory disease.

METHODS

Retrospective and prospective approaches identified infants and children with unexplained diffuse lung disease for NKX2-1 sequencing. Histopathologic results and electron micrographs were assessed, and immunohistochemical analysis for surfactant-associated proteins was performed in a subset of 10 children for whom lung tissue was available.

RESULTS

We identified 16 individuals with heterozygous missense, nonsense, and frameshift mutations and five individuals with heterozygous, whole-gene deletions of NKX2-1. Neonatal RDS was the presenting pulmonary phenotype in 16 individuals (76%), interstitial lung disease in four (19%), and pulmonary fibrosis in one adult family member. Altogether, 12 individuals (57%) had the full triad of neurologic, thyroid, and respiratory manifestations, but five (24%) had only pulmonary symptoms at the time of presentation. Recurrent respiratory infections were a prominent feature in nine subjects. Lung histopathology demonstrated evidence of disrupted surfactant homeostasis in the majority of cases, and at least five cases had evidence of disrupted lung growth.

CONCLUSIONS

Patients with mutations in NKX2-1 may present with pulmonary manifestations in the newborn period or during childhood when thyroid or neurologic abnormalities are not apparent. Surfactant dysfunction and, in more severe cases, disrupted lung development are likely mechanisms for the respiratory disease.

A mutation in TTF1/NKX2.1 is associated with familial neuroendocrine cell hyperplasia of infancy

BACKGROUND

Neuroendocrine cell hyperplasia of infancy (NEHI) is a childhood diffuse lung disease of unknown etiology. We investigated the mechanism for lung disease in a subject whose clinical, imaging, and lung biopsy specimen findings were consistent with NEHI; the subject's extended family and eight other unrelated patients with NEHI were also investigated.

METHODS

The proband's lung biopsy specimen (at age 7 months) and serial CT scans were diagnostic of NEHI. Her mother, an aunt, an uncle, and two first cousins had failure to thrive in infancy and chronic respiratory symptoms that improved with age. Genes associated with autosomal-dominant forms of childhood interstitial lung disease were sequenced.

RESULTS

A heterozygous NKX2.1 mutation was identified in the proband and the four other adult family members with histories of childhood lung disease. The mutation results in a nonconservative amino acid substitution in the homeodomain in a codon extensively conserved through evolution. None of these individuals have thyroid disease or movement disorders. NKX2.1 mutations were not identified by sequence analysis in eight other unrelated subjects with NEHI.

CONCLUSIONS

The nature of the mutation and its segregation with disease support that it is disease-causing. Previously reported NKX2.1 mutations have been associated with "brain-thyroid-lung" syndrome and a spectrum of more severe pulmonary phenotypes. We conclude that genetic mechanisms may cause NEHI and that NKX2.1 mutations may result in, but are not the predominant cause of, this phenotype. We speculate that altered expression of NKX2.1 target genes other than those in the surfactant system may be responsible for the pulmonary pathophysiology of NEHI.

Generation of Lung Epithelium from Pluripotent Stem Cells

The understanding of key processes and signaling mechanisms in lung development has been mainly demonstrated through gain and loss of function studies in mice, while human lung development remains largely unexplored due to inaccessibility. Several recent reports have exploited the identification of key signaling mechanisms that regulate lineage commitment and restriction in mouse lung development, to direct differentiation of both mouse and human pluripotent stem cells towards lung epithelial cells. In this review, we discuss the recent advances in the generation of respiratory epithelia from pluripotent stem cells and the potential of these engineered cells for novel scientific discoveries in lung diseases and future translation into regenerative therapies.

Periostin downregulation is an early marker of inhibited neonatal murine lung alveolar septation

BACKGROUND

Extreme preterm birth exposes the saccular lung to multiple teratogens, which ultimately retard alveolar development. Specifically, therapeutic high level oxygen supplementation adversely affects the premature lungs and results in blunted alveolarization. Prolonged hyperoxic lung injury has previously been shown to upregulate the matricellular protein Periostin (Postn) and stimulate ectopic accumulation of alpha smooth muscle actin (αSMA) myofibroblasts. Therapies that promote lung septation are lacking largely due to a lack of reliable early biomarkers of injury. Thus, we determined if Postn expression correlated with the initial appearance of myofibroblasts in the saccular lung and was required for early alveolar development.

METHODS

Lung development in C57BL/6J mice following room-air (RA, 21%-O₂) or continuous hyperoxia (85%-O₂) from birth (P0) through postnatal day P14 was correlated with Postn and αSMA expression. Alveolarization in Postn knockout mice exposed to room-air, 60%-, and 85%-O₂ was also examined.

RESULTS

Postn was widely expressed in distal lung septa through P2 to P4 and peak expression coincided with accumulation of saccular myofibroblasts. Initially, 85%-O₂ prematurely downregulated Postn and αSMA expression and suppressed proliferation before the first evidence of distal lung simplification at P4. By P14, chronic 85%-O₂ resulted in secondary upregulation of Postn and αSMA in blunted septa. Myofibroblast differentiation and alveolar development was unaffected in Postn null mice and acute 85%-O₂ exposure equally inhibited septal formation in Postn null and wild-type littermates.

CONCLUSION

Postn expression is tightly correlated with the presence of αSMA-myofibroblasts and is a novel early biomarker of acutely inhibited alveolar septation during a crucial window of lung development.

Cellular and molecular basis of liver development

Liver is a prime organ responsible for synthesis, metabolism, and detoxification. The organ is endodermal in origin and its development is regulated by temporal, complex, and finely balanced cellular and molecular interactions that dictate its origin, growth, and maturation. We discuss the relevance of endoderm patterning, which truly is the first step toward mapping of domains that will give rise to specific organs. Once foregut patterning is completed, certain cells within the foregut endoderm gain competence in the form of expression of certain transcription factors that allow them to respond to certain inductive signals. Hepatic specification is then a result of such inductive signals, which often emanate from the surrounding mesenchyme. During hepatic specification bipotential hepatic stem cells or hepatoblasts become apparent and undergo expansion, which results in a visible liver primordium during the stage of hepatic morphogenesis. Hepatoblasts next differentiate into either hepatocytes or cholangiocytes. The expansion and differentiation is regulated by cellular and molecular interactions between hepatoblasts and mesenchymal cells including sinusoidal endothelial cells, stellate cells, and also innate hematopoietic elements. Further maturation of hepatocytes and cholangiocytes continues during late hepatic development as a function of various growth factors. At this time, liver gains architectural novelty in the form of zonality and at cellular level acquires polarity. A comprehensive elucidation of such finely tuned developmental cues have been the basis of transdifferentiation of various types of stem cells to hepatocyte-like cells for purposes of understanding health and disease and for therapeutic applications.

Directed differentiation of human pluripotent stem cells into mature airway epithelia expressing functional CFTR protein

Cystic fibrosis (CF) is a fatal genetic disease caused by mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene that regulates chloride and water transport across all epithelia and affects multiple organs including the lungs. Here we report an in vitro directed differentiation protocol for generating functional CFTR-expressing airway epithelia from human embryonic stem cells. Carefully timed treatment by exogenous growth factors that mimic endoderm developmental pathways in vivo followed by air-liquid interface culture results in maturation of patches of tight junction-coupled differentiated airway epithelial cells that demonstrate active CFTR transport function. As a proof-of-concept, treatment of CF patient induced pluripotent stem cells (iPSC)-derived epithelial cells with a novel small molecule compound to correct for the common CF-processing mutation resulted in enhanced plasma membrane localization of mature CFTR protein. Our study provides a method for generating patient-specific airway epithelial cells for disease modeling and in vitro drug testing.

Endogenous lung regeneration: potential and limitations

The exploration of the endogenous regenerative potential of the diseased adult human lung represents an innovative and exciting task. In this pulmonary perspective, we discuss three major components essential for endogenous lung repair and regeneration: epithelial progenitor populations, developmental signaling pathways that regulate their reparative and regenerative potential, and the surrounding extracellular matrix in the human diseased lung. Over the past years, several distinct epithelial progenitor populations have been discovered within the lung, all of which most likely respond to different injuries by varying degrees. It has become evident that several progenitor populations are mutually involved in maintenance and repair, which is highly regulated by developmental pathways, such as Wnt or Notch signaling. Third, endogenous progenitor cells and developmental signaling pathways act in close spatiotemporal synergy with the extracellular matrix. These three components define and refine the highly dynamic microenvironment of the lung, which is altered in a disease-specific fashion in several chronic lung diseases. The search for the right mixture to induce efficient and controlled repair and regeneration of the diseased lung is ongoing and will open completely novel avenues for the treatment of patients with chronic lung disease.

Comparative molecular developmental aspects of the mammalian- and the avian lungs, and the insectan tracheal system by branching morphogenesis: recent advances and future directions

Gas exchangers fundamentally form by branching morphogenesis (BM), a mechanistically profoundly complex process which derives from coherent expression and regulation of multiple genes that direct cell-to-cell interactions, differentiation, and movements by signaling of various molecular morphogenetic cues at specific times and particular places in the developing organ. Coordinated expression of growth-instructing factors determines sizes and sites where bifurcation occurs, by how much a part elongates before it divides, and the angle at which branching occurs. BM is essentially induced by dualities of factors where through feedback- or feed forward loops agonists/antagonists are activated or repressed. The intricate transactions between the development orchestrating molecular factors determine the ultimate phenotype. From the primeval time when the transformation of unicellular organisms to multicellular ones occurred by systematic accretion of cells, BM has been perpetually conserved. Canonical signalling, transcriptional pathways, and other instructive molecular factors are commonly employed within and across species, tissues, and stages of development. While much still remain to be elucidated and some of what has been reported corroborated and reconciled with rest of existing data, notable progress has in recent times been made in understanding the mechanism of BM. By identifying and characterizing the morphogenetic drivers, and markers and their regulatory dynamics, the elemental underpinnings of BM have been more precisely explained. Broadening these insights will allow more effective diagnostic and therapeutic interventions of developmental abnormalities and pathologies in pre- and postnatal lungs. Conservation of the molecular factors which are involved in the development of the lung (and other branched organs) is a classic example of nature's astuteness in economically utilizing finite resources. Once purposefully formed, well-tested and tried ways and means are adopted, preserved, and widely used to engineer the most optimal phenotypes. The material and time costs of developing utterly new instruments and routines with every drastic biological change (e.g. adaptation and speciation) are circumvented. This should assure the best possible structures and therefore functions, ensuring survival and evolutionary success.

Aberrant signaling pathways of the lung mesenchyme and their contributions to the pathogenesis of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a chronic lung disease in infants born extremely preterm, typically before 28 weeks' gestation, characterized by a prolonged need for supplemental oxygen or positive pressure ventilation beyond 36 weeks postmenstrual age. The limited number of autopsy samples available from infants with BPD in the postsurfactant era has revealed a reduced capacity for gas exchange resulting from simplification of the distal lung structure with fewer, larger alveoli because of a failure of normal lung alveolar septation and pulmonary microvascular development. The mechanisms responsible for alveolar simplification in BPD have not been fully elucidated, but mounting evidence suggests that aberrations in the cross-talk between growth factors of the lung mesenchyme and distal airspace epithelium have a key role. Animal models that recapitulate the human condition have expanded our knowledge of the pathology of BPD and have identified candidate matrix components and growth factors in the developing lung that are disrupted by conditions that predispose infants to BPD and interfere with normal vascular and alveolar morphogenesis. This review focuses on the deviations from normal lung development that define the pathophysiology of BPD and summarizes the various candidate mesenchyme-associated proteins and growth factors that have been identified as being disrupted in animal models of BPD. Finally, future areas of research to identify novel targets affected in arrested lung development and recovery are discussed.

c-Kit is essential for alveolar maintenance and protection from emphysema-like disease in mice

RATIONALE

Previously, we demonstrated a candidate region for susceptibility to airspace enlargement on mouse chromosome 5. However, the specific candidate genes within this region accounting for emphysema-like changes remain unrecognized. c-Kit is a receptor tyrosine kinase within this candidate gene region that has previously been recognized to contribute to the survival, proliferation, and differentiation of hematopoietic stem cells. Increases in the percentage of cells expressing c-Kit have previously been associated with protection against injury-induced emphysema.

OBJECTIVES

Determine whether genetic variants of c-Kit are associated with spontaneous airspace enlargement.

METHODS

Perform single-nucleotide polymorphism association studies in the mouse strains at the extremes of airspace enlargement phenotype for variants in c-Kit tyrosine kinase. Characterize mice bearing functional variants of c-Kit compared with wild-type controls for the development of spontaneous airspace enlargement. Epithelial cell proliferation was measured in culture.

MEASUREMENTS AND MAIN RESULTS

Upstream regulatory single-nucleotide polymorphisms in the divergent mouse strains were associated with the lung compliance difference observed between the extreme strains. c-Kit mutant mice (Kit(W-sh)/(W-sh)), when compared with genetic controls, developed altered lung histology, increased total lung capacity, increased residual volume, and increased lung compliance that persist into adulthood. c-Kit inhibition with imatinib attenuated in vitro proliferation of cells expressing epithelial cell adhesion molecule.

CONCLUSIONS

Our findings indicate that c-Kit sustains and/or maintains normal alveolar architecture in the lungs of mice. In vitro data suggest that c-Kit can regulate epithelial cell clonal expansion. The precise mechanisms that c-Kit contributes to the development of airspace enlargement and increased lung compliance remain unclear and warrants further investigation.

Atopy history and the genomics of wheezing after influenza vaccination in children 6-59 months of age

BACKGROUND

A multinational clinical trial compared the safety and efficacy of intranasal trivalent live attenuated influenza vaccine (LAIV) with intramuscular trivalent inactivated vaccine (TIV) in very young children prior to the 2004-5 influenza season [1]. Wheezing was noted more often in recipients of LAIV and laboratory-confirmed influenza infection was noted more often in recipients of TIV. We sought to determine whether epidemiologic or genetic factors were associated with these outcomes.

METHODS

Atopy surveys and DNA collections were performed in trial participants at two United States sites, Nashville, TN and Boston, MA. DNA samples were genotyped on Illumina Infinium 610 or 660-Quad. Standard allelic tests of association were performed.

RESULTS

At the Nashville and Boston sites, a total of 99 children completed the trial, 6 (1 TIV, 5 LAIV) developed medically attended wheezing within 42 days following vaccination, and 8 (5 TIV, 3 LAIV) developed laboratory-confirmed influenza during the season. Eighty-one surveys and 70 DNA samples were collected. Family history of asthma (p=0.001) was associated with wheezing after vaccination. Of 468,458 single nucleotide polymorphisms tested in the genome-wide association study (GWAS), none achieved genome-wide significance for either wheezing after vaccination or laboratory-confirmed influenza infection.

CONCLUSIONS

Family history of asthma appears to be a risk factor for wheezing after influenza vaccination. Given the limitations of the sample size, our pilot study demonstrated the feasibility of performing a GWAS but was not able to determine genetic polymorphisms associated with wheezing after influenza immunization.

Vitamin A metabolism: an update

Retinoids are required for maintaining many essential physiological processes in the body, including normal growth and development, normal vision, a healthy immune system, normal reproduction, and healthy skin and barrier functions. In excess of 500 genes are thought to be regulated by retinoic acid. 11-cis-retinal serves as the visual chromophore in vision. The body must acquire retinoid from the diet in order to maintain these essential physiological processes. Retinoid metabolism is complex and involves many different retinoid forms, including retinyl esters, retinol, retinal, retinoic acid and oxidized and conjugated metabolites of both retinol and retinoic acid. In addition, retinoid metabolism involves many carrier proteins and enzymes that are specific to retinoid metabolism, as well as other proteins which may be involved in mediating also triglyceride and/or cholesterol metabolism. This review will focus on recent advances for understanding retinoid metabolism that have taken place in the last ten to fifteen years.

The PCP genes Celsr1 and Vangl2 are required for normal lung branching morphogenesis

The lungs are generated by branching morphogenesis as a result of reciprocal signalling interactions between the epithelium and mesenchyme during development. Mutations that disrupt formation of either the correct number or shape of epithelial branches affect lung function. This, in turn, can lead to congenital abnormalities such as cystadenomatoid malformations, pulmonary hypertension or lung hypoplasia. Defects in lung architecture are also associated with adult lung disease, particularly in cases of idiopathic lung fibrosis. Identifying the signalling pathways which drive epithelial tube formation will likely shed light on both congenital and adult lung disease. Here we show that mutations in the planar cell polarity (PCP) genes Celsr1 and Vangl2 lead to disrupted lung development and defects in lung architecture. Lungs from Celsr1(Crsh) and Vangl2(Lp) mouse mutants are small and misshapen with fewer branches, and by late gestation exhibit thickened interstitial mesenchyme and defective saccular formation. We observe a recapitulation of these branching defects following inhibition of Rho kinase, an important downstream effector of the PCP signalling pathway. Moreover, epithelial integrity is disrupted, cytoskeletal remodelling perturbed and mutant endoderm does not branch normally in response to the chemoattractant FGF10. We further show that Celsr1 and Vangl2 proteins are present in restricted spatial domains within lung epithelium. Our data show that the PCP genes Celsr1 and Vangl2 are required for foetal lung development thereby revealing a novel signalling pathway critical for this process that will enhance our understanding of congenital and adult lung diseases and may in future lead to novel therapeutic strategies.

Canonical Notch signaling in the developing lung is required for determination of arterial smooth muscle cells and selection of Clara versus ciliated cell fate

Lung development is the result of complex interactions between four tissues: epithelium, mesenchyme, mesothelium and endothelium. We marked the lineages experiencing Notch1 activation in these four cellular compartments during lung development and complemented this analysis by comparing the cell fate choices made in the absence of RBPjkappa, the essential DNA binding partner of all Notch receptors. In the mesenchyme, RBPjkappa was required for the recruitment and specification of arterial vascular smooth muscle cells (vSMC) and for regulating mesothelial epithelial-mesenchymal transition (EMT), but no adverse affects were observed in mice lacking mesenchymal RBPjkappa. We provide indirect evidence that this is due to vSMC rescue by endothelial-mesenchymal transition (EnMT). In the epithelium, we show that Notch1 activation was most probably induced by Foxj1-expressing cells, which suggests that Notch1-mediated lateral inhibition regulates the selection of Clara cells at the expense of ciliated cells. Unexpectedly, and in contrast to Pofut1-null epithelium, Hes1 expression was only marginally reduced in RBPjkappa-null epithelium, with a corresponding minimal effect on pulmonary neuroendocrine cell fate selection. Collectively, the primary roles for canonical Notch signaling in lung development are in selection of Clara cell fate and in vSMC recruitment. These analyses suggest that the impact of gamma-secretase inhibitors on branching in vitro reflect a non-cell autonomous contribution from endothelial or vSMC-derived signals.

Morphogenesis of the thyroid gland

Congenital hypothyroidism is mainly due to structural defects of the thyroid gland, collectively known as thyroid dysgenesis. The two most prevalent forms of this condition are abnormal localization of differentiated thyroid tissue (thyroid ectopia) and total absence of the gland (athyreosis). The clinical picture of thyroid dysgenesis suggests that impaired specification, proliferation and survival of thyroid precursor cells and loss of concerted movement of these cells in a distinct spatiotemporal pattern are major causes of malformation. In normal development the thyroid primordium is first distinguished as a thickening of the anterior foregut endoderm at the base of the prospective tongue. Subsequently, this group of progenitors detaches from the endoderm, moves caudally and ultimately differentiates into hormone-producing units, the thyroid follicles, at a distant location from the site of specification. In higher vertebrates later stages of thyroid morphogenesis are characterized by shape remodeling into a bilobed organ and the integration of a second type of progenitors derived from the caudal-most pharyngeal pouches that will differentiate into C-cells. The present knowledge of thyroid developmental dynamics has emerged from embryonic studies mainly in chicken, mouse and more recently also in zebrafish. This review will highlight the key morphogenetic steps of thyroid organogenesis and pinpoint which crucial regulatory mechanisms are yet to be uncovered. Considering the co-incidence of thyroid dysgenesis and congenital heart malformations the possible interactions between thyroid and cardiovascular development will also be discussed.

Variation in the GST mu locus and tobacco smoke exposure as determinants of childhood lung function

RATIONALE

The glutathione S-transferases (GSTs) are important detoxification enzymes.

OBJECTIVES

To investigate effects of variants in GST mu genes on lung function and assess their interactions with tobacco smoke exposure.

METHODS

In this prospective study, 14,836 lung function measurements were collected from 2,108 children who participated in two Southern California cohorts. For each child, tagging single nucleotide polymorphisms in GSTM2, GSTM3, GSTM4, and GSTM5 loci were genotyped. Using principal components and haplotype analyses, the significance of each locus in relation to level and growth of FEV1, maximum midexpiratory flow rate (MMEF), and FVC was evaluated. Interactions between loci and tobacco smoke on lung function were also investigated.

MEASUREMENTS AND MAIN RESULTS

Variation in the GST mu family locus was associated with lower FEV1 (P = 0.01) and MMEF (0.04). Two haplotypes of GSTM2 were associated with FEV1 and MMEF, with effect estimates in opposite directions. One haplotype in GSTM3 showed a decrease in growth for MMEF (-164.9 ml/s) compared with individuals with other haplotypes. One haplotype in GSTM4 showed significantly decreased growth in FEV1 (-51.3 ml), MMEF (-69.1 ml/s), and FVC (-44.4 ml), compared with all other haplotypes. These results were consistent across two independent cohorts. Variation in GSTM2 was particularly important for FVC and FEV(1) among children whose mothers smoked during pregnancy.

CONCLUSIONS

Genetic variation across the GST mu locus is associated with 8-year lung function growth. Children of mothers who smoked during pregnancy and had variation in GSTM2 had lower lung function growth.

MASH1: a useful marker in differentiating pulmonary small cell carcinoma from Merkel cell carcinoma

Merkel cell carcinoma is the cutaneous counterpart of small cell carcinoma, and the most important differential diagnosis is cutaneous metastasis of small cell carcinoma of the lung. There have been a handful of studies reporting on the utility of a variety of immunohistochemical markers that distinguish between the two entities. Achaete-scute complex-like 1 (MASH1, ASCL1) is important in the development of the brain and the diffuse neuroendocrine system including pulmonary neuroendocrine cells. A recent study, using a cDNA array, identified Mash1 as one of the best classifier genes to differentiate pulmonary small cell carcinoma from Merkel cell carcinoma. We immunohistochemically applied this finding to the diagnostic setting. A total of 30 cases of Merkel cell carcinoma and 59 cases of small cell carcinoma of the lung were immunostained with anti-MASH1 and TTF-1 antibodies. Of 59 small cell carcinomas, 49 (83%) expressed MASH1 in nuclear staining whereas out of 59 small cell carcinomas, 43 (73%) expressed TTF-1 in nuclear staining. MASH1 was completely negative in all 30 Merkel cell carcinomas whereas TTF-1 expression was seen in 1 of the 30 Merkel cell carcinomas (3%). MASH1 is a useful adjunct marker for differentiating small cell carcinoma of the lung from Merkel cell carcinoma.

Chick pulmonary Wnt5a directs airway and vascular tubulogenesis

Wnt5a is an important factor patterning many aspects of early development, including the lung. We find pulmonary non-canonical Wnt5a uses Ror2 to control patterning of both distal air and vascular tubulogenesis (alveolarization). Lungs with mis/overexpressed Wnt5a develop with severe pulmonary hypoplasia associated with altered expression patterns of Shh, L-CAM, fibronectin, VEGF and Flk1. This hypoplastic phenotype is rescued by either replacement of the Shh protein or inhibition of fibronectin function. We find that the effect of Wnt5a on vascular patterning is likely to be through fibronectin-mediated VEGF signaling. These results demonstrate the pivotal role of Wnt5a in directing the essential coordinated development of pulmonary airway and vasculature, by affecting fibronectin levels directly, and by affecting the fibronectin pattern of expression through its regulation of Shh. Data herein suggest that Wnt5a functions in mid-pulmonary patterning (during alveolarization), and is distinct from the Wnt canonical pathway which is more important in earlier lung development.