Dynamic Hippo pathway activity underlies mesenchymal differentiation during lung alveolar morphogenesis

Alveologenesis, the final stage in lung development, substantially remodels the distal lung, expanding the alveolar surface area for efficient gas exchange. Secondary crest myofibroblasts (SCMF) exist transiently in the neonatal distal lung and are crucial for alveologenesis. However, the pathways that regulate SCMF function, proliferation and temporal identity remain poorly understood. To address this, we purified SCMFs from reporter mice, performed bulk RNA-seq and found dynamic changes in Hippo-signaling components during alveologenesis. We deleted the Hippo effectors Yap/Taz from Acta2-expressing cells at the onset of alveologenesis, causing a significant arrest in alveolar development. Using single cell RNA-seq, we identified a distinct cluster of cells in mutant lungs with altered expression of marker genes associated with proximal mesenchymal cell types, airway smooth muscle and alveolar duct myofibroblasts. In vitro studies confirmed that Yap/Taz regulates myofibroblast-associated gene signature and contractility. Together, our findings show that Yap/Taz is essential for maintaining functional myofibroblast identity during postnatal alveologenesis.

Dysregulated alveolar epithelial cell progenitor function and identity in Hermansky-Pudlak syndrome pulmonary fibrosis

Hermansky-Pudlak syndrome (HPS) is a genetic disorder associated with pulmonary fibrosis in specific subtypes, including HPS-1 and HPS-2. Single mutant HPS1 and HPS2 mice display increased fibrotic sensitivity while double mutant HPS1/2 mice exhibit spontaneous fibrosis with aging, which has been attributed to HPS mutations in alveolar epithelial type II (AT2) cells. Unifying mechanisms of AT2 cell dysfunction in genetic and sporadic fibrotic lung diseases remain unknown. Incorporating AT2 cell lineage tracing in HPS mice, we observed a progressive decline in AT2 cell numbers with aging and aberrant differentiation with increased AT2-derived alveolar epithelial type I cells. HPS AT2 cell proliferation was impaired ex vivo and in vivo , suggesting an intrinsic progenitor defect. Transcriptomic analysis of HPS AT2 cells revealed elevated expression of genes associated with aberrant differentiation and cellular senescence. Through lineage tracing and organoid modeling, we demonstrated that HPS AT2 cells were primed to persist in a Krt8 + reprogrammed transitional state, mediated by p53 activity. These findings suggest that pulmonary fibrosis in HPS may be driven by AT2 cell progenitor dysfunction in the setting of p53-mediated senescence, highlighting a novel potential therapeutic target in HPS and suggesting unifying mechanisms underlying HPS and other forms of pulmonary fibrosis.

Quantitative Measures of Right Ventricular Size and Function by Echocardiogram Correlate with Cardiac Catheterization Hemodynamics in Congenital Diaphragmatic Hernia

OBJECTIVE

To evaluate associations between cardiac catheterization (cath) hemodynamics, quantitative measures of right ventricular (RV) function by echocardiogram, and survival in patients with congenital diaphragmatic hernia (CDH).

STUDY DESIGN

This single-center retrospective cohort study enrolled patients with CDH who underwent index cath from 2003 to 2022. Tricuspid annular plane systolic excursion z score, RV fractional area change, RV free wall and global longitudinal strain, left ventricular (LV) eccentricity index, RV/LV ratio, and pulmonary artery acceleration time were measured from preprocedure echocardiograms. Associations between hemodynamic values, echocardiographic measures, and survival were evaluated by Spearman correlation and Wilcoxon rank sum test, respectively.

RESULTS

Fifty-three patients (68% left-sided, 74% liver herniation, 57% extracorporeal membrane oxygenation, 93% survival) underwent cath (39 during index hospitalization, 14 later) including device closure of a patent ductus arteriosus in 5. Most patients (n = 31, 58%) were on pulmonary hypertension treatment at cath, most commonly sildenafil (n = 24, 45%) and/or intravenous treprostinil (n = 16, 30%). Overall, hemodynamics were consistent with precapillary pulmonary hypertension. Pulmonary capillary wedge pressure was >15 mm Hg in 2 patients (4%). Lower fractional area change and worse ventricular strain were associated with higher pulmonary artery pressure while higher LV eccentricity index and higher RV/LV ratio were associated with both higher pulmonary artery pressure and higher pulmonary vascular resistance. Hemodynamics did not differ based on survival status.

CONCLUSIONS

Worse RV dilation and dysfunction by echocardiogram correlate with higher pulmonary artery pressure and pulmonary vascular resistance on cath in this CDH cohort. These measures may represent novel, noninvasive clinical trial targets in this population.

Single Cell Multiomics Identifies Cells and Genetic Networks Underlying Alveolar Capillary Dysplasia

Rationale: Alveolar capillary dysplasia with misalignment of pulmonary veins (ACDMPV) is a lethal developmental disorder of lung morphogenesis caused by insufficiency of FOXF1 (forkhead box F1) transcription factor function. The cellular and transcriptional mechanisms by which FOXF1 deficiency disrupts human lung formation are unknown. Objectives: To identify cell types, gene networks, and cell-cell interactions underlying the pathogenesis of ACDMPV. Methods: We used single-nucleus RNA and assay for transposase-accessible chromatin sequencing, immunofluorescence confocal microscopy, and RNA in situ hybridization to identify cell types and molecular networks influenced by FOXF1 in ACDMPV lungs. Measurements and Main Results: Pathogenic single-nucleotide variants and copy-number variant deletions involving the FOXF1 gene locus in all subjects with ACDMPV (n = 6) were accompanied by marked changes in lung structure, including deficient alveolar development and a paucity of pulmonary microvasculature. Single-nucleus RNA and assay for transposase-accessible chromatin sequencing identified alterations in cell number and gene expression in endothelial cells (ECs), pericytes, fibroblasts, and epithelial cells in ACDMPV lungs. Distinct cell-autonomous roles for FOXF1 in capillary ECs and pericytes were identified. Pathogenic variants involving the FOXF1 gene locus disrupt gene expression in EC progenitors, inhibiting the differentiation or survival of capillary 2 ECs and cell-cell interactions necessary for both pulmonary vasculogenesis and alveolar type 1 cell differentiation. Loss of the pulmonary microvasculature was associated with increased VEGFA (vascular endothelial growth factor A) signaling and marked expansion of systemic bronchial ECs expressing COL15A1 (collagen type XV α 1 chain). Conclusions: Distinct FOXF1 gene regulatory networks were identified in subsets of pulmonary endothelial and fibroblast progenitors, providing both cellular and molecular targets for the development of therapies for ACDMPV and other diffuse lung diseases of infancy.

Circadian regulation of lung repair and regeneration

Optimal lung repair and regeneration are essential for recovery from viral infections, including influenza A virus (IAV). We have previously demonstrated that acute inflammation and mortality induced by IAV is under circadian control. However, it is not known whether the influence of the circadian clock persists beyond the acute outcomes. Here, we utilize the UK Biobank to demonstrate an association between poor circadian rhythms and morbidity from lower respiratory tract infections, including the need for hospitalization and mortality after discharge; this persists even after adjusting for common confounding factors. Furthermore, we use a combination of lung organoid assays, single-cell RNA sequencing, and IAV infection in different models of clock disruption to investigate the role of the circadian clock in lung repair and regeneration. We show that lung organoids have a functional circadian clock and the disruption of this clock impairs regenerative capacity. Finally, we find that the circadian clock acts through distinct pathways in mediating lung regeneration - in tracheal cells via the Wnt/β-catenin pathway and through IL-1β in alveolar epithelial cells. We speculate that adding a circadian dimension to the critical process of lung repair and regeneration will lead to novel therapies and improve outcomes.

Defining the spatial landscape of KRAS mutated congenital pulmonary airway malformations: a distinct entity with a spectrum of histopathologic features

The potential pathogenetic mechanisms underlying the varied morphology of congenital pulmonary airway malformations (CPAMs) have not been molecularly determined, but a subset have been shown to contain clusters of mucinous cells (MCC). These clusters are believed to serve as precursors for potential invasive mucinous adenocarcinoma, and they are associated with KRAS codon 12 mutations. To assess the universality of KRAS mutations in MCCs, we sequenced exon 2 of KRAS in 61 MCCs from 18 patients, and we found a KRAS codon 12 mutation in all 61 MCCs. Furthermore, all MCCs from a single patient always had the same KRAS mutation, and the same KRAS mutation was also found in non-mucinous lesional tissue. Next generation sequencing of seven MCCs showed no other mutations or copy number variations. Sequencing of 46 additional CPAMs with MCCs revealed KRAS mutations in non-mucinous lesional tissue in all cases. RNA in situ hybridization confirmed widespread distribution of cells with mutant KRAS RNA, even extending outside of the bronchiolar type epithelium. We identified 25 additional CPAMs with overall histologic architecture similar to CPAMs with KRAS mutations but without identifiable MCCs, and we found KRAS mutations in 17 (68%). The histologic features of these KRAS mutated CPAMs included type 1 and type 3 morphology, as well as lesions with an intermediate histologic appearance, and analysis revealed a strong correlation between the specific amino acid substitution and histomorphology. These findings, together with previously published model organism data, suggests that the formation of type 1 and 3 CPAMs is driven by mosaic KRAS mutations arising in the lung epithelium early in development and places them within the growing field of mosaic RASopathies. The presence of widespread epithelial mutation explains late metastatic disease in incompletely resected patients and reinforces the recommendation for complete resection of these lesions.

CXCL12 defines lung endothelial heterogeneity and promotes distal vascular growth

There is a growing amount of data uncovering the cellular diversity of the pulmonary circulation and mechanisms governing vascular repair after injury. However, the molecular and cellular mechanisms contributing to the morphogenesis and growth of the pulmonary vasculature during embryonic development are less clear. Importantly, deficits in vascular development lead to significant pediatric lung diseases, indicating a need to uncover fetal programs promoting vascular growth. To address this, we used a transgenic mouse reporter for expression of Cxcl12, an arterial endothelial hallmark gene, and performed single-cell RNA sequencing on isolated Cxcl12-DsRed+ endothelium to assess cellular heterogeneity within pulmonary endothelium. Combining cell annotation with gene ontology and histological analysis allowed us to segregate the developing artery endothelium into functionally and spatially distinct subpopulations. Expression of Cxcl12 is highest in the distal arterial endothelial subpopulation, a compartment enriched in genes for vascular development. Accordingly, disruption of CXCL12 signaling led to, not only abnormal branching, but also distal vascular hypoplasia. These data provide evidence for arterial endothelial functional heterogeneity and reveal conserved signaling mechanisms essential for pulmonary vascular development.

GSK3 inhibition rescues growth and telomere dysfunction in dyskeratosis congenita iPSC-derived type II alveolar epithelial cells

Dyskeratosis congenita (DC) is a rare genetic disorder characterized by deficiencies in telomere maintenance leading to very short telomeres and the premature onset of certain age-related diseases, including pulmonary fibrosis (PF). PF is thought to derive from epithelial failure, particularly that of type II alveolar epithelial (AT2) cells, which are highly dependent on Wnt signaling during development and adult regeneration. We use human induced pluripotent stem cell-derived AT2 (iAT2) cells to model how short telomeres affect AT2 cells. Cultured DC mutant iAT2 cells accumulate shortened, uncapped telomeres and manifest defects in the growth of alveolospheres, hallmarks of senescence, and apparent defects in Wnt signaling. The GSK3 inhibitor, CHIR99021, which mimics the output of canonical Wnt signaling, enhances telomerase activity and rescues the defects. These findings support further investigation of Wnt agonists as potential therapies for DC-related pathologies.

Type V Collagen Regulates the Structure and Biomechanics of TMJ Condylar Cartilage: A Fibrous-Hyaline Hybrid

This study queried the role of type V collagen in the post-natal growth of temporomandibular joint (TMJ) condylar cartilage, a hybrid tissue with a fibrocartilage layer covering a secondary hyaline cartilage layer. Integrating outcomes from histology, immunofluorescence imaging, electron microscopy and atomic force microscopy-based nanomechanical tests, we elucidated the impact of type V collagen reduction on TMJ condylar cartilage growth in the type V collagen haploinsufficiency and inducible knockout mice. Reduction of type V collagen led to significantly thickened collagen fibrils, decreased tissue modulus, reduced cell density and aberrant cell clustering in both the fibrous and hyaline layers. Post-natal growth of condylar cartilage involves the chondrogenesis of progenitor cells residing in the fibrous layer, which gives rise to the secondary hyaline layer. Loss of type V collagen resulted in reduced proliferation of these cells, suggesting a possible role of type V collagen in mediating the progenitor cell niche. When the knockout of type V collagen was induced in post-weaning mice after the start of physiologic TMJ loading, the hyaline layer exhibited pronounced thinning, supporting an interplay between type V collagen and occlusal loading in condylar cartilage growth. The phenotype in hyaline layer can thus be attributed to the impact of type V collagen on the mechanically regulated progenitor cell activities. In contrast, knee cartilage does not contain the progenitor cell population at post-natal stages, and develops normal structure and biomechanical properties with the loss of type V collagen. Therefore, in the TMJ, in addition to its established role in regulating the assembly of collagen I fibrils, type V collagen also impacts the mechanoregulation of progenitor cell activities in the fibrous layer. We expect such knowledge to establish a foundation for understanding condylar cartilage matrix development and regeneration, and to yield new insights into the TMJ symptoms in patients with classic Ehlers-Danlos syndrome, a genetic disease due to autosomal mutation of type V collagen.

Primary pulmonary vein stenosis during infancy: state of the art review

Primary pulmonary vein stenosis (PPVS) is an emerging problem among infants. In contrast to acquired disease, PPVS is the development of stenosis in the absence of preceding intervention. While optimal care approaches remain poorly characterized, over the past decade, understanding of potential pathophysiological mechanisms and development of novel therapeutic strategies are increasing. A multidisciplinary team of health care providers was assembled to review the available evidence and provide a common framework for the diagnosis, management, and treatment of PPVS during infancy. To address knowledge gaps, institutional and multi-institutional approaches must be employed to generate knowledge specific to ex-premature infants with PPVS. Within individual institutions, creation of a team comprised of dedicated health care providers from diverse backgrounds is critical to accelerate clinical learning and provide care for infants with PPVS. Multi-institutional collaborations, such as the PVS Network, provide the infrastructure and statistical power to advance knowledge for this rare disease.

Pathogenic LMNA variants disrupt cardiac lamina-chromatin interactions and de-repress alternative fate genes

Pathogenic mutations in LAMIN A/C (LMNA) cause abnormal nuclear structure and laminopathies. These diseases have myriad tissue-specific phenotypes, including dilated cardiomyopathy (DCM), but how LMNA mutations result in tissue-restricted disease phenotypes remains unclear. We introduced LMNA mutations from individuals with DCM into human induced pluripotent stem cells (hiPSCs) and found that hiPSC-derived cardiomyocytes, in contrast to hepatocytes or adipocytes, exhibit aberrant nuclear morphology and specific disruptions in peripheral chromatin. Disrupted regions were enriched for transcriptionally active genes and regions with lower LAMIN B1 contact frequency. The lamina-chromatin interactions disrupted in mutant cardiomyocytes were enriched for genes associated with non-myocyte lineages and correlated with higher expression of those genes. Myocardium from individuals with LMNA variants similarly showed aberrant expression of non-myocyte pathways. We propose that the lamina network safeguards cellular identity and that pathogenic LMNA variants disrupt peripheral chromatin with specific epigenetic and molecular characteristics, causing misexpression of genes normally expressed in other cell types.

Genomic, epigenomic, and biophysical cues controlling the emergence of the lung alveolus

The lung alveolus is the functional unit of the respiratory system required for gas exchange. During the transition to air breathing at birth, biophysical forces are thought to shape the emerging tissue niche. However, the intercellular signaling that drives these processes remains poorly understood. Applying a multimodal approach, we identified alveolar type 1 (AT1) epithelial cells as a distinct signaling hub. Lineage tracing demonstrates that AT1 progenitors align with receptive, force-exerting myofibroblasts in a spatial and temporal manner. Through single-cell chromatin accessibility and pathway expression (SCAPE) analysis, we demonstrate that AT1-restricted ligands are required for myofibroblasts and alveolar formation. These studies show that the alignment of cell fates, mediated by biophysical and AT1-derived paracrine signals, drives the extensive tissue remodeling required for postnatal respiration.

mTORC1 activation in lung mesenchyme drives sex- and age-dependent pulmonary structure and function decline

Lymphangioleiomyomatosis (LAM) is a rare fatal cystic lung disease due to bi-allelic inactivating mutations in tuberous sclerosis complex (TSC1/TSC2) genes coding for suppressors of the mechanistic target of rapamycin complex 1 (mTORC1). The origin of LAM cells is still unknown. Here, we profile a LAM lung compared to an age- and sex-matched healthy control lung as a hypothesis-generating approach to identify cell subtypes that are specific to LAM. Our single-cell RNA sequencing (scRNA-seq) analysis reveals novel mesenchymal and transitional alveolar epithelial states unique to LAM lung. This analysis identifies a mesenchymal cell hub coordinating the LAM disease phenotype. Mesenchymal-restricted deletion of Tsc2 in the mouse lung produces a mTORC1-driven pulmonary phenotype, with a progressive disruption of alveolar structure, a decline in pulmonary function, increase of rapamycin-sensitive expression of WNT ligands, and profound female-specific changes in mesenchymal and epithelial lung cell gene expression. Genetic inactivation of WNT signaling reverses age-dependent changes of mTORC1-driven lung phenotype, but WNT activation alone in lung mesenchyme is not sufficient for the development of mouse LAM-like phenotype. The alterations in gene expression are driven by distinctive crosstalk between mesenchymal and epithelial subsets of cells observed in mesenchymal Tsc2-deficient lungs. This study identifies sex- and age-specific gene changes in the mTORC1-activated lung mesenchyme and establishes the importance of the WNT signaling pathway in the mTORC1-driven lung phenotype.

STAT3-BDNF-TrkB signalling promotes alveolar epithelial regeneration after lung injury

Alveolar epithelial regeneration is essential for recovery from devastating lung diseases. This process occurs when type II alveolar pneumocytes (AT2 cells) proliferate and transdifferentiate into type I alveolar pneumocytes (AT1 cells). We used genome-wide analysis of chromatin accessibility and gene expression following acute lung injury to elucidate repair mechanisms. AT2 chromatin accessibility changed substantially following injury to reveal STAT3 binding motifs adjacent to genes that regulate essential regenerative pathways. Single-cell transcriptome analysis identified brain-derived neurotrophic factor (Bdnf) as a STAT3 target gene with newly accessible chromatin in a unique population of regenerating AT2 cells. Furthermore, the BDNF receptor tropomyosin receptor kinase B (TrkB) was enriched on mesenchymal alveolar niche cells (MANCs). Loss or blockade of AT2-specific Stat3, Bdnf or mesenchyme-specific TrkB compromised repair and reduced Fgf7 expression by niche cells. A TrkB agonist improved outcomes in vivo following lung injury. These data highlight the biological and therapeutic importance of the STAT3-BDNF-TrkB axis in orchestrating alveolar epithelial regeneration.

Lung disease and pulmonary hypertension in the premature infant

The premature infant is born into the world unprepared to naturally thrive in a foreign environment. Lung development entails immense growth, structural remodeling and differentiation of specialized cells during the normal term perinatal and postnatal periods. Thus, the premature infant presents with a lung deficient for appropriate respiration. Disruption of lung development seen in bronchopulmonary dysplasia (BPD) and chronic lung disease (CLD) results in not only impaired airway growth but also a deficiency in the accompanying vasculature including the capillary system required for gas exchange. Deficient vascular area can lead to elevated pulmonary vascular resistance and the development of pulmonary hypertension (PH). Unlike PH seen in children and adults with pulmonary arterial hypertension (PAH), treatment with conventional pulmonary vasodilators can be limited in developmental lung disease-associated PH because there are fewer blood vessels to dilate. In this brief review, we highlight some of the knowledge on PH in the premature infant presented at the Proceedings of the 22^nd Annual Update on Pediatric and Congenital Cardiovascular Disease.

Variation in the use of pulmonary vasodilators in children and adolescents with pulmonary hypertension: a study using data from the MarketScan® insurance claims database

Despite progress in pharmacotherapy in pediatric pulmonary hypertension, real-world patterns of directed pulmonary hypertension therapy have not been studied in the current era. A retrospective observational study of children (≤18 years) with pulmonary hypertension was performed using data from the MarketScan® Commercial and Medicaid claims databases. Associations between etiology of pulmonary hypertension and pharmaceutical regimen were evaluated, as were the associations between subject social and geographic characteristics (insurance-type, race, and/or census region) and regimen. Annualized costs of single- and multi-class regimens were calculated. In total, 873 subjects were studied, of which 94% received phosphodiesterase-5 inhibitors, 31% endothelin receptor antagonist, 9% prostacyclin analogs, and 7% calcium channel blockers. Monotherapy was used in 72% of subjects. Phosphodiesterase-5 inhibitors monotherapy was the most common regimen (93%). Subjects with idiopathic pulmonary hypertension, congenital heart disease, and unclassified pulmonary hypertension receive more than one agent and were more likely to receive both endothelin receptor antagonist and prostacyclin analogs than other forms of pulmonary hypertension. Compared to recipients of public insurance, subjects with commercial insurance were more likely to receive more intense therapy (p = 0.003), which was confirmed in multivariable analysis (OR: 1.4, p = 0.03). Receipt of commercial insurance was also associated with increased annual costs across all subjects (p < 0.001) and for the most common specific regimens. The majority of children with pulmonary hypertension receive phosphodiesterase monotherapy, followed by phosphodiesterase–endothelin receptor antagonist two drug regimens, and finally the addition of prostacyclin analogs for three-drug therapy. However, even after adjustment for measurable confounders, commercial insurance was associated with higher intensity care and higher costs (even within specific classes of pulmonary vasodilators). The effect of these associations on clinical outcome cannot be discerned from the current data set, but patterns of treatment deserve further attention.

The in vivo genetic program of murine primordial lung epithelial progenitors

Multipotent Nkx2-1-positive lung epithelial primordial progenitors of the foregut endoderm are thought to be the developmental precursors to all adult lung epithelial lineages. However, little is known about the global transcriptomic programs or gene networks that regulate these gateway progenitors in vivo. Here we use bulk RNA-sequencing to describe the unique genetic program of in vivo murine lung primordial progenitors and computationally identify signaling pathways, such as Wnt and Tgf-β superfamily pathways, that are involved in their cell-fate determination from pre-specified embryonic foregut. We integrate this information in computational models to generate in vitro engineered lung primordial progenitors from mouse pluripotent stem cells, improving the fidelity of the resulting cells through unbiased, easy-to-interpret similarity scores and modulation of cell culture conditions, including substratum elastic modulus and extracellular matrix composition. The methodology proposed here can have wide applicability to the in vitro derivation of bona fide tissue progenitors of all germ layers.

Paradigms that define lung epithelial progenitor cell fate in development and regeneration

Purpose of Review

Throughout the lifespan, lung injury impedes the primary critical function essential for life-respiration. To repair quickly and efficiently is critical and is orchestrated by a diverse repertoire of progenitor cells and their niche. This review incorporates knowledge gained from early studies in lung epithelial morphogenesis and cell fate and explores its relevance to more recent findings of lung progenitor and stem cells in development and regeneration.

Recent Findings

Cell fate in the lung is organized into an early specification phase and progressive differentiation phase in lung development. The advent of single cell analysis combined with lineage analysis and projections is uncovering new functional cell types in the lung providing a topographical atlas for progenitor cell lineage commitment during development, homeostasis, and regeneration.

Summary

Lineage commitment of lung progenitor cells is spatiotemporally regulated during development. Single cell sequencing technologies have significantly advanced our understanding of the similarities and differences between developmental and regenerative cell fate trajectories. Subsequent unraveling of the molecular mechanisms underlying these cell fate decisions will be essential to manipulating progenitor cells for regeneration.

Histopathologic and Genetic Features of Alveolar Capillary Dysplasia with Atypical Late Presentation and Prolonged Survival

Alveolar capillary dysplasia typically presents with neonatal pulmonary hypertension and early mortality. However, there is growing evidence for a subset of disease with atypical late onset and/or prolonged survival. Here, we present the variable clinical, genetic, and pathology findings of 4 such patients.

Aicardi goutières syndrome is associated with pulmonary hypertension

While pulmonary hypertension (PH) is a potentially life threatening complication of many inflammatory conditions, an association between Aicardi Goutières syndrome (AGS), a rare genetic cause of interferon (IFN) overproduction, and the development of PH has not been characterized to date. We analyzed the cardiac function of individuals with AGS enrolled in the Myelin Disorders Bioregistry Project using retrospective chart review (n = 61). Additional prospective echocardiograms were obtained when possible (n = 22). An IFN signature score, a marker of systemic inflammation, was calculated through the measurement of mRNA transcripts of type I IFN-inducible genes (interferon signaling genes or ISG). Pathologic analysis was performed as available from autopsy samples. Within our cohort, four individuals were identified to be affected by PH: three with pathogenic gain-of-function mutations in the IFIH1 gene and one with heterozygous TREX1 mutations. All studied individuals with AGS were noted to have elevated IFN signature scores (Mann-Whitney p < .001), with the highest levels in individuals with IFIH1 mutations (Mann-Whitney p < .0001). We present clinical and histologic evidence of PH in a series of four individuals with AGS, a rare interferonopathy. Importantly, IFIH1 and TREX1 may represent a novel cause of PH. Furthermore, these findings underscore the importance of screening all individuals with AGS for PH.

Midgut Laterality Is Driven by Hyaluronan on the Right

For many years, biologists have focused on the role of Pitx2, expressed on the left side of developing embryos, in governing organ laterality. Here, we identify a different pathway during left-right asymmetry initiated by the right side of the embryo. Surprisingly, this conserved mechanism is orchestrated by the extracellular glycosaminoglycan, hyaluronan (HA) and is independent of Pitx2 on the left. Whereas HA is normally synthesized bilaterally as a simple polysaccharide, we show that covalent modification of HA by the enzyme Tsg6 on the right triggers distinct cell behavior necessary to drive the conserved midgut rotation and to pattern gut vasculature. HA disruption in chicken and Tsg6^-/- mice results in failure to initiate midgut rotation and perturbs vascular development predisposing to midgut volvulus. Our study leads us to revise the current symmetry-breaking paradigm in vertebrates and demonstrates how enzymatic modification of HA matrices can execute the blueprint of organ laterality.

Novel Molecular and Phenotypic Insights into Congenital Lung Malformations

RATIONALE

Disruption of normal pulmonary development is a leading cause of morbidity and mortality in infants. Congenital lung malformations are a unique model to study the molecular pathogenesis of isolated structural birth defects, as they are often surgically resected.

OBJECTIVES

To provide insight into the molecular pathogenesis of congenital lung malformations through analysis of cell-type and gene expression changes in these lesions.

METHODS

Clinical data, and lung tissue for DNA, RNA, and histology, were obtained from 58 infants undergoing surgical resection of a congenital lung lesion. Transcriptome-wide gene expression analysis was performed on paired affected and unaffected samples from a subset of infants (n = 14). A three-dimensional organoid culture model was used to assess isolated congenital lung malformation epithelium (n = 3).

MEASUREMENTS AND MAIN RESULTS

Congenital lung lesions express higher levels of airway epithelial related genes, and dysregulated expression of genes related to the Ras and PI3K-AKT-mTOR (phosphatidylinositol 3-kinase-AKT-mammalian target of rapamycin) signaling pathways. Immunofluorescence confirmed differentiated airway epithelial cell types throughout all major subtypes of congenital lung lesions, and three-dimensional cell culture demonstrated a cell-autonomous defect in the epithelium of these lesions.

CONCLUSIONS

This study provides the first comprehensive analysis of the congenital lung malformation transcriptome and suggests that disruptions in Ras or PI3K-AKT-mTOR signaling may contribute to the pathology through an epithelial cell-autonomous defect.

Regeneration of the lung alveolus by an evolutionarily conserved epithelial progenitor

Functional tissue regeneration is required for the restoration of normal organ homeostasis after severe injury. Some organs, such as the intestine, harbour active stem cells throughout homeostasis and regeneration; more quiescent organs, such as the lung, often contain facultative progenitor cells that are recruited after injury to participate in regeneration. Here we show that a Wnt-responsive alveolar epithelial progenitor (AEP) lineage within the alveolar type 2 cell population acts as a major facultative progenitor cell in the distal lung. AEPs are a stable lineage during alveolar homeostasis but expand rapidly to regenerate a large proportion of the alveolar epithelium after acute lung injury. AEPs exhibit a distinct transcriptome, epigenome and functional phenotype and respond specifically to Wnt and Fgf signalling. In contrast to other proposed lung progenitor cells, human AEPs can be directly isolated by expression of the conserved cell surface marker TM4SF1, and act as functional human alveolar epithelial progenitor cells in 3D organoids. Our results identify the AEP lineage as an evolutionarily conserved alveolar progenitor that represents a new target for human lung regeneration strategies.

Hemodynamic Forces Sculpt Developing Heart Valves through a KLF2-WNT9B Paracrine Signaling Axis

Hemodynamic forces play an essential epigenetic role in heart valve development, but how they do so is not known. Here, we show that the shear-responsive transcription factor KLF2 is required in endocardial cells to regulate the mesenchymal cell responses that remodel cardiac cushions to mature valves. Endocardial Klf2 deficiency results in defective valve formation associated with loss of Wnt9b expression and reduced canonical WNT signaling in neighboring mesenchymal cells, a phenotype reproduced by endocardial-specific loss of Wnt9b. Studies in zebrafish embryos reveal that wnt9b expression is similarly restricted to the endocardial cells overlying the developing heart valves and is dependent upon both hemodynamic shear forces and klf2a expression. These studies identify KLF2-WNT9B signaling as a conserved molecular mechanism by which fluid forces sensed by endothelial cells direct the complex cellular process of heart valve development and suggest that congenital valve defects may arise due to subtle defects in this mechanotransduction pathway.

Hedgehog actively maintains adult lung quiescence and regulates repair and regeneration

Postnatal tissue quiescence is thought to be a default state in the absence of a proliferative stimulus such as injury. Previous studies have demonstrated that certain embryonic development programs are reactivated aberrantly in adult organs to drive repair and regeneration^1–3, it is not well understood how quiescence is maintained in organs such as the lung which displays a remarkably low level of cellular turnover^4,5. We now demonstrate that quiescence in the adult lung is an actively maintained state and is regulated by hedgehog signaling. Epithelial-specific deletion of sonic hedgehog during postnatal homeostasis in the lung results in a proliferative expansion of the adjacent lung mesenchyme. Hedgehog signaling is initially down-regulated during the acute phase of epithelial injury as the mesenchyme proliferates in response, but returns to baseline during injury resolution as quiescence is restored. Activation of hedgehog during acute epithelial injury attenuates the proliferative expansion of the lung mesenchyme, whereas inactivation of hedgehog signaling prevents the restoration of quiescence during injury resolution. Finally, we show that hedgehog also regulates epithelial quiescence and regeneration in response to injury via a mesenchymal feedback mechanism. These results demonstrate that epithelial-mesenchymal interactions coordinated by hedgehog actively maintains postnatal tissue homeostasis, and deregulation of hedgehog during injury leads to aberrant repair and regeneration in the lung.

Trends in pediatric pulmonary hypertension-related hospitalizations in the United States from 2000-2009

There are few data on the epidemiology of pulmonary hypertension (PH)-related hospitalizations in children in the United States. Our aim was to determine hospital mortality, length of hospitalization, and hospital charges pertaining to PH-related hospitalizations and also the effects of codiagnoses and comorbidities. A retrospective review of the Kids' Inpatient Database during the years 2000, 2003, 2006, and 2009 was analyzed for patients ≤20 years of age with a diagnosis of PH by ICD-9 (International Classification of Diseases, Ninth Revision) codes, along with associated diagnoses and comorbidities. Descriptive statistics, including Rao-Scott χ(2), ANOVA, and logistic regression models, were utilized on weighted values with survey analysis procedures. The number of PH-related hospital admissions is rising, from an estimated 7,331 (95% confidence interval [CI]: 5,556-9,106) in 2000 to 10,792 (95% CI: 8,568-13,016) in 2009. While infant age and congenital heart disease were most commonly associated with PH-related hospitalizations, they were not associated with mortality. Overall mortality for PH-related hospitalizations was greater than that for hospitalizations not associated with PH, 5.7% versus 0.4% (odds ratio: 16.22 [95% CI: 14.78%-17.8%], P < 0.001), but mortality is decreasing over time. Sepsis, respiratory failure, acute renal failure, hepatic insufficiency, arrhythmias, and the use of extracorporeal membrane oxygenation are associated with mortality. The number of PH-related hospitalizations is increasing in the United States. The demographics of PH in this study are evolving. Despite the increasing prevalence, mortality is improving.

Pulmonary arterial hypertension associated with congenital heart disease and Eisenmenger syndrome: current practice in pediatrics

Pulmonary arterial hypertension (PAH) is an uncommon but serious disease characterized by severe pulmonary vascular disease and significant morbidity and mortality. PAH associated with congenital heart disease (APAH-CHD) is one etiology of PAH that has innate characteristics delineating it from other forms of PAH. The patient with APAH-CHD presents with unique challenges consisting of not only pulmonary vascular disease but also the complexity of the cardiac lesion. Eisenmenger syndrome (ES) represents the severe end of the spectrum for disease in APAH-CHD. Over time, systemic-to-pulmonary shunting through cardiac defects increases pulmonary vascular resistance to levels significant enough to reverse shunting across the defect. Historically, ES patients have been reported to have better outcomes than IPAH despite similarities in pulmonary vascular disease. However, recent studies are challenging this notion. Nonetheless, APAH-CHD survival has improved with the advent of modern PAH targeted therapies. New therapeutic options have allowed us to reconsider the dogma of inoperability in APAH-CHD patients with unrepaired defects. Certainly advances have been made, however, investigators must continue to advance the field through controlled clinical trials in both adult and pediatric APAH-CHD patients.

Utility of electrocardiogram in the assessment and monitoring of pulmonary hypertension (idiopathic or secondary to pulmonary developmental abnormalities) in patients≤18 years of age

Electrocardiograms have utility in disease stratification and monitoring in adult pulmonary arterial hypertension (PAH). We examined the electrocardiographic findings that are common in pediatric PAH and assessed for correlation with disease severity and progression. We retrospectively identified patients aged≤18 years followed at a single institution from January 2001 to June 2012 with catheterization-confirmed diagnosis of idiopathic PAH and PAH secondary to pulmonary developmental abnormalities. Patients with an electrocardiography performed within 60 days of catheterization were included. Primary and secondary outcomes are the prevalence of abnormal electrocardiographic findings at the time of catheterization and the association between electrocardiographic and hemodynamic findings and electrocardiographic changes with disease progression on follow-up catheterization, respectively. Of the 100 electrocardiography-catheterization pairs derived from the 46 patients identified, 93% had an electrocardiographic abnormality: 78% had right ventricular hypertrophy (RVH) and 52% had right axis deviation (RAD) for age. In patients with idiopathic PAH, the presence of RVH and RAD correlated with pulmonary vascular resistance and transpulmonary gradient. RAD and RVH on baseline electrocardiogram was associated with an increased risk of disease progression on subsequent catheterization (odds ratio 11.0, 95% confidence interval 1.3 to 96.2, p=0.03) after adjusting for PAH subgroup. The sensitivity, specificity, and positive and negative predictive values of RAD and RVH on baseline electrocardiogram for disease progression were 92%, 48%, 33%, and 95%, respectively. In conclusion, electrocardiographic abnormalities are common in pediatric PAH. RAD and RVH on electrocardiogram were associated with worse hemodynamics, whereas their absence is suggestive of a lack of disease progression.

Junctional tachycardia in a child with Lyme carditis

A 3-year-old girl presented with fatigue, fever, and rash. Tachycardia was noted and an ECG showed junctional tachycardia. Lyme titers were positive and the arrhythmia resolved with intravenous ceftriaxone.

An autonomous BMP2 regulatory element in mesenchymal cells

BMP2 is a morphogen that controls mesenchymal cell differentiation and behavior. For example, BMP2 concentration controls the differentiation of mesenchymal precursors into myocytes, adipocytes, chondrocytes, and osteoblasts. Sequences within the 3'untranslated region (UTR) of the Bmp2 mRNA mediate a post-transcriptional block of protein synthesis. Interaction of cell and developmental stage-specific trans-regulatory factors with the 3'UTR is a nimble and versatile mechanism for modulating this potent morphogen in different cell types. We show here, that an ultra-conserved sequence in the 3'UTR functions independently of promoter, coding region, and 3'UTR context in primary and immortalized tissue culture cells and in transgenic mice. Our findings indicate that the ultra-conserved sequence is an autonomously functioning post-transcriptional element that may be used to modulate the level of BMP2 and other proteins while retaining tissue specific regulatory elements.

Chorioamnionitis stimulates angiogenesis in saccular stage fetal lungs via CC chemokines

The fetal lung vasculature forms in tandem with developing airways. Whereas saccular airway morphogenesis is arrested in bronchopulmonary dysplasia (BPD), the potential vascular phenotype in BPD at this stage of development is less well-understood. As inflammation increases the risk of BPD and induces arrest of saccular airway morphogenesis, we tested the effects of Escherichia coli LPS on fetal mouse lung vascular development. Injecting LPS into the amniotic fluid of Tie2-lacZ endothelial reporter mice at embryonic day 15 stimulated angiogenesis in the saccular stage fetal lung mesenchyme. LPS also increased the number of endothelial cells in saccular stage fetal mouse lung explants. Inflammation appeared to directly promote vascular development, as LPS stimulated pulmonary microvascular endothelial cell angiogenesis, cell migration, and proliferation in vitro. Whereas LPS did not increase expression of VEGF, angiopoietin-1 (Ang-1), Tie2, fetal liver kinase-1 (Flk-1), fms-like tyrosine kinase-1 (Flt-1), PDGFA, PDGFB, heparin-binding EGF-like growth factor (HB-EGF), or connective tissue growth factor (CTGF), LPS did stimulate the production of the angiogenic CC chemokines macrophage inflammatory protein-1α (MIP-1α) and monocyte chemoattractant protein-1 (MCP-1). Both MIP-1α and MCP-1 increased angiogenesis in fetal mouse lung explants. In addition, inhibitory antibodies against MIP-1α and MCP-1 blocked the effects of LPS on fetal lung vascular development, suggesting these chemokines are downstream mediators of LPS-induced angiogenesis. We speculate that an inflammation-mediated surge in angiogenesis could lead to formation of aberrant alveolar capillaries in the lungs of patients developing BPD.

Bmp2 and Bmp4 exert opposing effects in hypoxic pulmonary hypertension

The bone morphogenetic protein (BMP) type 2 receptor ligand, Bmp2, is upregulated in the peripheral pulmonary vasculature during hypoxia-induced pulmonary hypertension (PH). This contrasts with the expression of Bmp4, which is expressed in respiratory epithelia throughout the lung. Unlike heterozygous null Bmp4 mice (Bmp4(LacZ/+)), which are protected from the development of hypoxic PH, mice that are heterozygous null for Bmp2 (Bmp2(+/-)) develop more severe hypoxic PH than their wild-type littermates. This is associated with reduced endothelial nitric oxide synthase (eNOS) expression and activity in the pulmonary vasculature of hypoxic Bmp2(+/-) but not Bmp4(LacZ/+) mutant mice. Furthermore, exogenous BMP2 upregulates eNOS expression and activity in intrapulmonary artery and pulmonary endothelial cell preparations, indicating that eNOS is a target of Bmp2 signaling in the pulmonary vasculature. Together, these data demonstrate that Bmp2 and Bmp4 exert opposing roles in hypoxic PH and suggest that the protective effects of Bmp2 are mediated by increasing eNOS expression and activity in the hypoxic pulmonary vasculature.

Increased susceptibility to hypoxic pulmonary hypertension in Bmpr2 mutant mice is associated with endothelial dysfunction in the pulmonary vasculature

Patients with familial pulmonary arterial hypertension inherit heterozygous mutations of the type 2 bone morphogenetic protein (BMP) receptor BMPR2. To explore the cellular mechanisms of this disease, we evaluated the pulmonary vascular responses to chronic hypoxia in mice carrying heterozygous hypomorphic Bmpr2 mutations (Bmpr2 delta Ex2/+). These mice develop more severe pulmonary hypertension after prolonged exposure to hypoxia without an associated increase in pulmonary vascular remodeling or proliferation compared with wild-type mice. This is associated with defective endothelial-dependent vasodilatation and enhanced vasoconstriction in isolated intrapulmonary artery preparations. In addition, there is a selective decrease in hypoxia-induced, BMP-dependent, endothelial nitric oxide synthase expression and Smad signaling in the intact lungs and in cultured pulmonary microvascular endothelial cells from Bmpr2 delta Ex2/+ mutant mice. These findings indicate that the pulmonary endothelium is a target of abnormal BMP signaling in Bmpr2 delta Ex2/+ mutant mice and suggest that endothelial dysfunction contributes to their increased susceptibility to hypoxic pulmonary hypertension.

Bone Morphogenetic Protein 4 Promotes Pulmonary Vascular Remodeling in Hypoxic Pulmonary Hypertension

We show that 1 of the type II bone morphogenetic protein (BMP) receptor ligands, BMP4, is widely expressed in the adult mouse lung and is upregulated in hypoxia-induced pulmonary hypertension (PH). Furthermore, heterozygous null Bmp4 lacZ/+ mice are protected from the development of hypoxia-induced PH, vascular smooth muscle cell proliferation, and vascular remodeling. This is associated with a reduction in hypoxia-induced Smad1/5/8 phosphorylation and Id1 expression in the pulmonary vasculature. In addition, pulmonary microvascular endothelial cells secrete BMP4 in response to hypoxia and promote proliferation and migration of vascular smooth muscle cells in a BMP4-dependent fashion. These findings indicate that BMP4 plays a dominant role in regulating BMP signaling in the hypoxic pulmonary vasculature and suggest that endothelium-derived BMP4 plays a direct, paracrine role in promoting smooth muscle proliferation and remodeling in hypoxic PH.

Sorting nexin 6, a novel SNX, interacts with the transforming growth factor-beta family of receptor serine-threonine kinases

Sorting nexins (SNX) comprise a family of proteins with homology to several yeast proteins, including Vps5p and Mvp1p, that are required for the sorting of proteins to the yeast vacuole. Human SNX1, -2, and -4 have been proposed to play a role in receptor trafficking and have been shown to bind to several receptor tyrosine kinases, including receptors for epidermal growth factor, platelet-derived growth factor, and insulin as well as the long form of the leptin receptor, a glycoprotein 130-associated receptor. We now describe a novel member of this family, SNX6, which interacts with members of the transforming growth factor-beta family of receptor serine-threonine kinases. These receptors belong to two classes: type II receptors that bind ligand, and type I receptors that are subsequently recruited to transduce the signal. Of the type II receptors, SNX6 was found to interact strongly with ActRIIB and more moderately with wild type and kinase-defective mutants of TbetaRII. Of the type I receptors, SNX6 was found to interact only with inactivated TbetaRI. SNXs 1-4 also interacted with the transforming growth factor-beta receptor family, showing different receptor preferences. Conversely, SNX6 behaved similarly to the other SNX proteins in its interactions with receptor tyrosine kinases. Strong heteromeric interactions were also seen among SNX1, -2, -4, and -6, suggesting the formation in vivo of oligomeric complexes. These findings are the first evidence for the association of the SNX family of molecules with receptor serine-threonine kinases.

Transforming growth factor-beta receptor-associated protein 1 is a Smad4 chaperone

Members of the transforming growth factor-beta (TGF-beta) superfamily signal through unique cell membrane receptor serine-threonine kinases to activate downstream targets. TRAP1 is a previously described 96-kDa cytoplasmic protein shown to bind to TGF-beta receptors and suggested to play a role in TGF-beta signaling. We now fully characterize the binding properties of TRAP1, and show that it associates strongly with inactive heteromeric TGF-beta and activin receptor complexes and is released upon activation of signaling. Moreover, we demonstrate that TRAP1 plays a role in the Smad-mediated signal transduction pathway, interacting with the common mediator, Smad4, in a ligand-dependent fashion. While TRAP1 has only a small stimulatory effect on TGF-beta signaling in functional assays, deletion constructs of TRAP1 inhibit TGF-beta signaling and diminish the interaction of Smad4 with Smad2. These are the first data to identify a specific molecular chaperone for Smad4, suggesting a model in which TRAP1 brings Smad4 into the vicinity of the receptor complex and facilitates its transfer to the receptor-activated Smad proteins.