The Id2+ distal tip lung epithelium contains individual multipotent embryonic progenitor cells

A critical role for miR-142 in alveolar epithelial lineage formation in mouse lung development

The respiratory epithelium arises from alveolar epithelial progenitors which differentiate into alveolar epithelial type 1 (AT1) and type 2 (AT2) cells. AT2 cells are stem cells in the lung critical for the repair process after injury. Mechanisms regulating AT1 and AT2 cell maturation are poorly defined. We report that the activation of the glucocorticoid pathway in an in vitro alveolar epithelial lineage differentiation assay led to increased AT2 marker Sftpc and decreased miR-142 expression. Using miR-142 KO mice, we demonstrate an increase in the AT2/AT1 cell number ratio. Overexpression of miR-142 in alveolar progenitor cells in vivo led to the opposite effect. Examination of the KO lungs at E18.5 revealed enhanced expression of miR-142 targets Apc, Ep300 and Kras associated with increased β-catenin and p-Erk signaling. Silencing of miR-142 expression in lung explants grown in vitro triggers enhanced Sftpc expression as well as increased AT2/AT1 cell number ratio. Pharmacological inhibition of Ep300-β-catenin but not Erk in vitro prevented the increase in Sftpc expression triggered by loss of miR-142. These results suggest that the glucocorticoid-miR-142-Ep300-β-catenin signaling axis controls pneumocyte maturation.

Cellular crosstalk in the development and regeneration of the respiratory system

The respiratory system, including the peripheral lungs, large airways and trachea, is one of the most recently evolved adaptations to terrestrial life. To support the exchange of respiratory gases, the respiratory system is interconnected with the cardiovascular system, and this interconnective nature requires a complex interplay between a myriad of cell types. Until recently, this complexity has hampered our understanding of how the respiratory system develops and responds to postnatal injury to maintain homeostasis. The advent of new single-cell sequencing technologies, developments in cellular and tissue imaging and advances in cell lineage tracing have begun to fill this gap. The view that emerges from these studies is that cellular and functional heterogeneity of the respiratory system is even greater than expected and also highly adaptive. In this Review, we explore the cellular crosstalk that coordinates the development and regeneration of the respiratory system. We discuss both the classic cell and developmental biology studies and recent single-cell analysis to provide an integrated understanding of the cellular niches that control how the respiratory system develops, interacts with the external environment and responds to injury.

Tissue-Resident Group 2 Innate Lymphoid Cells Differentiate by Layered Ontogeny and In Situ Perinatal Priming

The perinatal period is a critical window for distribution of innate tissue-resident immune cells within developing organs. Despite epidemiologic evidence implicating the early-life environment in the risk for allergy, temporally controlled lineage tracing of group 2 innate lymphoid cells (ILC2s) during this period remains unstudied. Using complementary fate-mapping approaches and reporters for ILC2 activation, we show that ILC2s appeared in multiple organs during late gestation like tissue macrophages, but, unlike the latter, a majority of peripheral ILC2 pools were generated de novo during the postnatal window. This period was accompanied by systemic ILC2 priming and acquisition of tissue-specific transcriptomes. Although perinatal ILC2s were variably replaced across tissues with age, the dramatic increases in tissue ILC2s following helminth infection were mediated through local expansion independent of de novo generation by bone marrow hematopoiesis. We provide comprehensive temporally controlled fate mapping of an innate lymphocyte subset with notable nuances as compared to tissue macrophage ontogeny.

From the pathophysiology of the human lung alveolus to epigenetic editing: Congress 2018 highlights from ERS Assembly 3 "Basic and Translational Science."

The European Respiratory Society (ERS) International Congress is the largest respiratory congress and brings together leading experts in all fields of respiratory medicine and research. ERS Assembly 3 shapes the basic and translational science aspects of this congress, aiming to combine cutting-edge novel developments in basic research with novel clinical findings. In this article, we summarise a selection of the scientific highlights from the perspective of the three groups within Assembly 3. In particular, we discuss new insights into the pathophysiology of the human alveolus, novel tools in organoid development and (epi)genome editing, as well as insights from the presented abstracts on novel therapeutic targets being identified for idiopathic pulmonary fibrosis.

The amount of basic and translational science presented at #ERSCongress is steadily increasing, showing novel cutting-edge technologies and models.http://bit.ly/2GgXIJi

Yap and its subcellular localization have distinct compartment-specific roles in the developing lung

Although the Hippo-yes-associated protein (Yap) pathway has been implicated in lung development, the specific roles for Yap and its nucleocytoplasmic shuttling in the developing airway and alveolar compartments remain elusive. Moreover, conflicting results from expression studies and differences in the lung phenotypes of Yap and Hippo kinase null mutants caused controversy over the dynamics and significance of Yap subcellular localization in the developing lung. Here, we show that the aberrant morphogenesis of Yap-deficient lungs results from the disruption of developmental events specifically in distal epithelial progenitors. We also show that activation of nuclear Yap is enough to fulfill the Yap requirements to rescue abnormalities in these lungs. Remarkably, we found that Yap nucleocytoplasmic shuttling is largely dispensable in epithelial progenitors for both branching morphogenesis and sacculation. However, if maintained transcriptionally active in airways, nuclear Yap profoundly alters proximal-distal identity and halts epithelial differentiation. Taken together, these observations provide novel insights into the crucial importance of Hippo-Yap signaling in the lung prenatally.

The proportion of alveolar type 1 cells decreases in murine hypoplastic congenital diaphragmatic hernia lungs

BACKGROUND

Pulmonary hypoplasia, characterized by incomplete alveolar development, remains a major cause of mortality and morbidity in congenital diaphragmatic hernia. Recently demonstrated to differentiate from a common bipotent progenitor during development, the two cell types that line the alveoli type 1 and type 2 alveolar cells have shown to alter their relative ratio in congenital diaphragmatic hernia lungs.

OBJECTIVE

We used the nitrofen/bisdiamine mouse model to induce congenital diaphragmatic hernia and accurately assess the status of alveolar epithelial cell differentiation in relation to the common bipotent progenitors.

STUDY DESIGN

Pregnant Swiss mice were gavage-fed with nitrofen/bisdiamine or vehicle at embryonic day 8.5. The administered dose was optimized by assessing the survival, congenital diaphragmatic hernia and facial abnormality rates of the exposed mouse pups. NanoCT was performed on embryonic day 11.5 and 16.5 to assess the embryonic and early canalicular stages of lung development. At embryonic day 17.5 corresponding to late canalicular stage, congenital diaphragmatic hernia lungs were characterized by measuring the lung weight/body weight ratio, morphometry, epithelial cell marker gene expression levels and alveolar cell type quantification.

RESULTS

Nitrofen/bisdiamine associated congenital diaphragmatic hernia lungs showed delayed development, hypoplasia with morphologic immaturity and thickened alveolar walls. Expression levels of distal epithelial progenitor marker Id2 increased, alveolar type 1 cell markers Pdpn and Hopx decreased, while type 2 cell markers pro-SPC and Muc1 remained constant during the canalicular stage. The number of Pdpn+ type 1 alveolar cells also decreased in congenital diaphragmatic hernia lungs.

CONCLUSION

The mouse nitrofen/bisdiamine model is a potential model allowing the study of congenital diaphragmatic hernia lung development from early stages using a wide array of methods. Based on this model, the alveolar epithelium showed a decrease in the number of alveolar type 1 cell in congenital diaphragmatic hernia lungs while type 2 cell population remains unchanged.

Generation of lung organoids from human pluripotent stem cells in vitro

The lung epithelium is derived from the endodermal germ layer, which undergoes a complex series of endoderm-mesoderm-mediated signaling events to generate the final arborized network of conducting airways (bronchi, bronchioles) and gas-exchanging units (alveoli). These stages include endoderm induction, anterior-posterior and dorsal-ventral patterning, lung specification, lung budding, branching morphogenesis, and, finally, maturation. Here we describe a protocol that recapitulates several of these milestones in order to differentiate human pluripotent stem cells (hPSCs) into ventral-anterior foregut spheroids and further into two distinct types of organoids: human lung organoids and bud tip progenitor organoids. The resulting human lung organoids possess cell types and structures that resemble the bronchi/bronchioles of the developing human airway surrounded by lung mesenchyme and cells expressing alveolar-cell markers. The bud tip progenitor organoids possess a population of highly proliferative multipotent cells with in vitro multilineage differentiation potential and in vivo engraftment potential. Human lung organoids can be generated from hPSCs in 50-85 d, and bud tip progenitor organoids can be generated in 22 d. The two hPSC-derived models presented here have been benchmarked with human fetal tissue and found to be representative of human fetal-like tissue. The bud tip progenitor organoids are thus ideal for exploring epithelial fate decisions, while the human lung organoids can be used to model epithelial-mesenchymal cross-talk during human lung development. In addition to their applications in developmental biology, human lung organoids and bud tip progenitor organoids may be implemented in regenerative medicine, tissue engineering, and pharmaceutical safety and efficacy testing.

An Id2RFP-Reporter Mouse Redefines Innate Lymphoid Cell Precursor Potentials

Innate lymphoid cell (ILC) development proposes that ILC precursors (ILCPs) segregate along natural killer (NK) cell versus helper cell (ILC1, ILC2, ILC3) pathways, the latter depending on expression of Id2, Zbtb16, and Gata3. We have developed an Id2-reporter strain expressing red fluorescent protein (RFP) in the context of normal Id2 expression to re-examine ILCP phenotype and function. We show that bone-marrow ILCPs were heterogeneous and harbored extensive NK-cell potential in vivo and in vitro. By multiplexing Id2RFP with Zbtb16CreGFP and Bcl11btdTomato strains, we made a single-cell dissection of the ILCP compartment. In contrast with the current model, we have demonstrated that Id2+Zbtb16+ ILCPs included multi-potent ILCPs that retained NK-cell potential. Late-stage ILC2P and ILC3P compartments could be defined by differential Zbtb16 and Bcl11b expression. We suggest a revised model for ILC differentiation that redefines the cell-fate potential of helper-ILC-restricted Zbtb16+ ILCPs.

Repairing the lungs one breath at a time: How dedicated or facultative are you?

In this review, Leach and Morrisey focus on lung regeneration to explore the importance of facultative regeneration controlled by functional and differentiated cell lineages as well as how they are positioned and regulated by distinct tissue niches.

Tissue regeneration involves various types of cellular and molecular responses depending on the type of tissue and the injury or disease that is inflicted. While many tissues contain dedicated stem/progenitor cell lineages, many others contain cells that, during homeostasis, are considered physiologically functional and fully differentiated but, after injury or in disease states, exhibit stem/progenitor-like activity. Recent identification of subsets of defined cell types as facultative stem/progenitor cells has led to a re-examination of how certain tissues respond to injury to mount a regenerative response. In this review, we focus on lung regeneration to explore the importance of facultative regeneration controlled by functional and differentiated cell lineages as well as how they are positioned and regulated by distinct tissue niches. Additionally, we discuss the molecular signals to which cells respond in their differentiated state during homeostasis and those signals that promote effective regeneration of damaged or lost cells and structures after injury.

Fibrillin-2 is a key mediator of smooth muscle extracellular matrix homeostasis during mouse tracheal tubulogenesis

Epithelial tubes, comprised of polarised epithelial cells around a lumen, are crucial for organ function. However, the molecular mechanisms underlying tube formation remain largely unknown. Here, we report on the function of fibrillin (FBN)2, an extracellular matrix (ECM) glycoprotein, as a critical regulator of tracheal tube formation.We performed a large-scale forward genetic screen in mouse to identify regulators of respiratory organ development and disease. We identified Fbn2 mutants which exhibit shorter and narrowed tracheas as well as defects in tracheal smooth muscle cell alignment and polarity.We found that FBN2 is essential for elastic fibre formation and Fibronectin accumulation around tracheal smooth muscle cells. These processes appear to be regulated at least in part through inhibition of p38-mediated upregulation of matrix metalloproteinases (MMPs), as pharmacological decrease of p38 phosphorylation or MMP activity partially attenuated the Fbn2 mutant tracheal phenotypes. Analysis of human tracheal tissues indicates that a decrease in ECM proteins, including FBN2 and Fibronectin, is associated with tracheomalacia.Our findings provide novel insights into the role of ECM homeostasis in mesenchymal cell polarisation during tracheal tubulogenesis.

Early lineage specification defines alveolar epithelial ontogeny in the murine lung

During the stepwise specification and differentiation of tissue-specific multipotent progenitors, lineage-specific transcriptional networks are activated or repressed to orchestrate cell specification. The gas-exchange niche in the lung contains two major epithelial cell types, alveolar type 1 (AT1) and AT2 cells, and the timing of lineage specification of these cells is critical for the correct formation of this niche and postnatal survival. Integrating cell-specific lineage tracing studies, spatially specific mRNA transcript and protein expression, and single-cell RNA-sequencing analysis, we demonstrate that specification of alveolar epithelial cell fate begins concomitantly with the proximal-distal specification of epithelial progenitors and branching morphogenesis earlier than previously appreciated. By using a newly developed dual-lineage tracing system, we show that bipotent alveolar cells that give rise to AT1 and AT2 cells are a minor contributor to the alveolar epithelial population. Furthermore, single-cell assessment of the transcriptome identifies specified AT1 and AT2 progenitors rather than bipotent cells during sacculation. These data reveal a paradigm of organ formation whereby lineage specification occurs during the nascent stages of development coincident with broad tissue-patterning processes, including axial patterning of the endoderm and branching morphogenesis.

ID2 and GJB2 promote early-stage breast cancer progression by regulating cancer stemness

PURPOSE

Ductal carcinoma in situ (DCIS) is a non-invasive form of breast cancer which could progress to or recur as invasive breast cancer. The underlying molecular mechanism of DCIS progression is yet poorly understood, and appropriate biomarkers to distinguish benign form of DCIS from potentially invasive tumor are urgently needed.

METHODS

To identify the key regulators of DCIS progression, we performed gene-expression analysis of syngeneic breast cancer cell lines MCF10A, DCIS.com, and MCF10CA and cross-referenced the targets with patient cohort data.

RESULTS

We identified ID2 as a critical gene for DCIS initiation and found that ID2 promoted DCIS formation by enhancing cancer stemness of pre-malignant cells. ID2 also plays a pivotal role in survival of the aggressive cancer cells. In addition, we identified INHBA and GJB2 as key regulators for the transition of benign DCIS to aggressive phenotype. These two genes regulate migration, colonization, and stemness of invasive cancer cells. Upregulation of ID2 and GJB2 predicts poor prognosis after breast-conserving surgery. Finally, we found a natural compound Helichrysetin as ID2 inhibitor which suppresses DCIS formation in vitro and in vivo.

CONCLUSION

Our results indicate that ID2 is a key driver of DCIS formation and therefore is considered to be a potential target for prevention of DCIS, while INHBA and GJB2 play vital roles in progression of DCIS to IDC and they may serve as potential prognosis markers.

Building and Regenerating the Lung Cell by Cell

The unique architecture of the mammalian lung is required for adaptation to air breathing at birth and thereafter. Understanding the cellular and molecular mechanisms controlling its morphogenesis provides the framework for understanding the pathogenesis of acute and chronic lung diseases. Recent single-cell RNA sequencing data and high-resolution imaging identify the remarkable heterogeneity of pulmonary cell types and provides cell selective gene expression underlying lung development. We will address fundamental issues related to the diversity of pulmonary cells, to the formation and function of the mammalian lung, and will review recent advances regarding the cellular and molecular pathways involved in lung organogenesis. What cells form the lung in the early embryo? How are cell proliferation, migration, and differentiation regulated during lung morphogenesis? How do cells interact during lung formation and repair? How do signaling and transcriptional programs determine cell-cell interactions necessary for lung morphogenesis and function?

Alveolus-like organoid from isolated tip epithelium of embryonic mouse lung

Embryonic lungs were obtained from embryonic day 13.5 ICR mice. The lung-tip epithelium isolated using dispase treatment was embedded in low-growth factor Matrigel, cultured in DMEM/F12 medium containing 0.1% bovine serum albumin, supplemented with insulin, transferrin, and selenium (ITS), with or without fibroblast growth factor 7 (FGF7), and were observed for 14 days. With the addition of FGF7, the tip epithelium grew to form a cyst by culture day 7. Then, tubular tufts-like alveolus appeared around the cyst surface. Reverse transcription-polymerase chain reaction revealed that, with the addition of FGF7, the cultured lung explants expressed alveolar-type 1 cell markers, such as HopX and Aquaporin5, and type 2 cell markers, such as Lamp3 and Surfactant apoproteins (Sftp) C and D. Paraffin-embedded sections were stained with hematoxylin and eosin, and alveolar structures at culture day 14 were composed of squamous and cuboidal epithelial cells. Immunohistochemical studies showed that the squamous epithelial cells were positive for HopX, and the cuboidal epithelial cells were positive for pro-SftpC. Furthermore, transmission electron microscopic observation confirmed that the squamous epithelial cells were alveolar-type 1 cells and the cuboidal cells were type 2 cells, because they had many lamellar inclusion bodies. Embryonic lung-tip epithelium forms an alveolus-like organoid through the self organization with the aid of Matrigel, ITS, and FGF7. This method to make alveolus-like organoid in vitro is easy, reproducible, and economical. This method could have potential to solve many issues in alveolar epithelial cells in normal and pathological conditions.

Use of three-dimensional organoids and lung-on-a-chip methods to study lung development, regeneration and disease

Differences in lung anatomy between mice and humans, as well as frequently disappointing results when using animal models for drug discovery, emphasise the unmet need for in vitro models that can complement animal studies and improve our understanding of human lung physiology, regeneration and disease. Recent papers have highlighted the use of three-dimensional organoids and organs-on-a-chip to mimic tissue morphogenesis and function in vitro Here, we focus on the respiratory system and provide an overview of these in vitro models, which can be derived from primary lung cells and pluripotent stem cells, as well as healthy or diseased lungs. We emphasise their potential application in studies of respiratory development, regeneration and disease modelling.

Multiple roles of lymphatic vessels in peripheral lymph node development

This work shows how blood and lymphatic vessels contribute to lymph node organogenesis. Both vessel types transport lymphoid tissue inducer cells, while lymphatics also generate interstitial flow, important for mechanical stromal activation and further lymph node expansion.

The mammalian lymphatic system consists of strategically located lymph nodes (LNs) embedded into a lymphatic vascular network. Mechanisms underlying development of this highly organized system are not fully understood. Using high-resolution imaging, we show that lymphoid tissue inducer (LTi) cells initially transmigrate from veins at LN development sites using gaps in venous mural coverage. This process is independent of lymphatic vasculature, but lymphatic vessels are indispensable for the transport of LTi cells that egress from blood capillaries elsewhere and serve as an essential LN expansion reservoir. At later stages, lymphatic collecting vessels ensure efficient LTi cell transport and formation of the LN capsule and subcapsular sinus. Perinodal lymphatics also promote local interstitial flow, which cooperates with lymphotoxin-β signaling to amplify stromal CXCL13 production and thereby promote LTi cell retention. Our data unify previous models of LN development by showing that lymphatics intervene at multiple points to assist LN expansion and identify a new role for mechanical forces in LN development.

Anxa4 mediated airway progenitor cell migration promotes distal epithelial cell fate specification

Genetic studies have shown that FGF10/FGFR2 signaling is required for airway branching morphogenesis and FGF10 functions as a chemoattractant factor for distal epithelial cells during lung development. However, the detail downstream cellular and molecular mechanisms have not been fully characterized. Using live imaging of ex vivo cultured lungs, we found that tip airway epithelial progenitor cells migrate faster than cleft cells during airway bud formation and this migration process is controlled by FGFR2-mediated ERK1/2 signaling. Additionally, we found that airway progenitor cells that migrate faster tend to become distal airway progenitor cells. We identified that Anxa4 is a downstream target of ERK1/2 signaling. Anxa4^-/- airway epithelial cells exhibit a "lag-behind" behavior and tend to stay at the stalk airways. Moreover, we found that Anxa4-overexpressing cells tend to migrate to the bud tips. Finally, we demonstrated that Anxa4 functions redundantly with Anxa1 and Anxa6 in regulating endoderm budding process. Our study demonstrates that ERK1/2/Anxa4 signaling plays a role in promoting the migration of airway epithelial progenitor cells to distal airway tips and ensuring their distal cell fate.

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.

Human lung development: recent progress and new challenges

Recent studies have revealed biologically significant differences between human and mouse lung development, and have reported new in vitro systems that allow experimental manipulation of human lung models. At the same time, emerging clinical data suggest that the origins of some adult lung diseases are found in embryonic development and childhood. The convergence of these research themes has fuelled a resurgence of interest in human lung developmental biology. In this Review, we discuss our current understanding of human lung development, which has been profoundly influenced by studies in mice and, more recently, by experiments using in vitro human lung developmental models and RNA sequencing of human foetal lung tissue. Together, these approaches are helping to shed light on the mechanisms underlying human lung development and disease, and may help pave the way for new therapies.

Summary: This Review describes how recent technological advances have shed light on the mechanisms underlying human lung development and disease, and outlines the future challenges in this field.

Chronic lung disease, lung regeneration and future therapeutic strategies

Chronic lung diseases have been recognized as one of the world's leading causes of death in recent decades. Lacking effective treatments brings the patients not only bad quality of life but also higher risk for lung cancer development. By increasing the understanding of deeper mechanism of how lung develops and regenerates, researchers now focus on studying lung regenerative medicine, aiming to apply different and more efficient therapies to treat chronic lung diseases. This review will provide a wide picture of both basic lung developmental, regeneration mechanism and different designed strategies for treating chronic lung diseases in the future decades.

Dynamic changes in intrathymic ILC populations during murine neonatal development

Members of the innate lymphoid cell (ILC) family have been implicated in the development of thymic microenvironments and the recovery of this architecture after damage. However, a detailed characterization of this family in the thymus is lacking. To better understand the thymic ILC compartment, we have utilized multiple in vivo models including the fate mapping of inhibitor of DNA binding‐2 (Id2) expression and the use of Id2 reporter mice. Our data demonstrate that ILCs are more prominent immediately after birth, but were rapidly diluted as the T‐cell development program increased. As observed in the embryonic thymus, CCR6⁺NKp46⁻ lymphoid tissue inducer (LTi) cells were the main ILC3 population present, but numbers of these cells swiftly declined in the neonate and ILC3 were barely detectable in adult thymus. This loss of ILC3 means ILC2 are the dominant ILC population in the thymus. Thymic ILC2 were able to produce IL‐5 and IL‐13, were located within the medulla, and did not result from ILC3 plasticity. Furthermore, in WT mice, thymic ILC2 express little RANKL (receptor activator of nuclear factor kappa‐B ligand) arguing that functionally, these cells provide different signals to LTi cells in the thymus. Collectively, these data reveal a dynamic switch in the ILC populations of the thymus during neonatal development.

Characterization of Adult Vestibular Organs in 11 CreER Mouse Lines

Utricles are vestibular sense organs that encode linear head movements. They are composed of a sensory epithelium with type I and type II hair cells and supporting cells, sitting atop connective tissue, through which vestibular nerves project. We characterized utricular Cre expression in 11 murine CreER lines using the ROSA26^tdTomato reporter line and tamoxifen induction at 6 weeks of age. This characterization included Calbindin2^CreERT2, Fgfr3-iCreER^T2, GFAP-A-CreER™, GFAP-B-CreER™, GLAST-CreER^T2, Id2^CreERT2, Otoferlin^CreERT2, Parvalbumin^CreERT2, Prox1^CreERT2, Sox2^CreERT2, and Sox9-CreER^T2. Otoferlin^CreERT2 mice had inducible Cre activity specific to hair cells. GLAST-CreER^T2, Id2^CreERT2, and Sox9-CreER^T2 had inducible Cre activity specific to supporting cells. Sox2^CreERT2 had inducible Cre activity in supporting cells and most type II hair cells. Parvalbumin^CreERT2 mice had small numbers of labeled vestibular nerve afferents. Calbindin2^CreERT2 mice had labeling of most type II hair cells and some type I hair cells and supporting cells. Only rare (or no) tdTomato-positive cells were detected in utricles of Fgfr3-iCreER^T2, GFAP-A-CreER™, GFAP-B-CreER™, and Prox1^CreERT2 mice. No Cre leakiness (tdTomato expression in the absence of tamoxifen) was observed in Otoferlin^CreERT2 mice. A small degree of leakiness was seen in GLAST-CreER^T2, Id2^CreERT2, Sox2^CreERT2, and Sox9-CreER^T2 lines. Calbindin2^CreERT2 mice had similar tdTomato expression with or without tamoxifen, indicating lack of inducible control under the conditions tested. In conclusion, 5 lines-GLAST-CreER^T2, Id2^CreERT2, Otoferlin^CreERT2, Sox2^CreERT2, and Sox9-CreER^T2-showed cell-selective, inducible Cre activity with little leakiness, providing new genetic tools for researchers studying the vestibular periphery.

Spatial-Temporal Lineage Restrictions of Embryonic p63+ Progenitors Establish Distinct Stem Cell Pools in Adult Airways

Basal cells (BCs) are p63-expressing multipotent progenitors of skin, tracheoesophageal and urinary tracts. p63 is abundant in developing airways; however, it remains largely unclear how embryonic p63⁺ cells contribute to the developing and postnatal respiratory tract epithelium, and ultimately how they relate to adult BCs. Using lineage-tracing and functional approaches in vivo, we show that p63⁺ cells arising from the lung primordium are initially multipotent progenitors of airway and alveolar lineages but later become restricted proximally to generate the tracheal adult stem cell pool. In intrapulmonary airways, these cells are maintained immature to adulthood in bronchi, establishing a rare p63⁺Krt5^- progenitor cell population that responds to H1N1 virus-induced severe injury. Intriguingly, this pool includes a CC10 lineage-labeled p63⁺Krt5^- cell subpopulation required for a full H1N1-response. These data elucidate key aspects in the establishment of regionally distinct adult stem cell pools in the respiratory system, potentially with relevance to other organs.

β-Catenin maintains lung epithelial progenitors after lung specification

The entire lung epithelium arises from SRY box 9 (SOX9)-expressing progenitors that form the respiratory tree and differentiate into airway and alveolar cells. Despite progress in understanding their initial specification within the embryonic foregut, how these progenitors are subsequently maintained is less clear. Using inducible, progenitor-specific genetic mosaic mouse models, we showed that β-catenin (CTNNB1) maintains lung progenitors by promoting a hierarchical lung progenitor gene signature, suppressing gastrointestinal (GI) genes, and regulating NK2 homeobox 1 (NKX2.1) and SRY box 2 (SOX2) in a developmental stage-dependent manner. At the early, but not later, stage post-lung specification, CTNNB1 cell-autonomously maintained normal NKX2.1 expression levels and suppressed ectopic SOX2 expression. Genetic epistasis analyses revealed that CTNNB1 is required for fibroblast growth factor (Fgf)/Kirsten rat sarcoma viral oncogene homolog (Kras)-mediated promotion of the progenitors. In silico screening of Eurexpress and translating ribosome affinity purification (TRAP)-RNAseq identified a progenitor gene signature, a subset of which depends on CTNNB1. Wnt signaling also maintained NKX2.1 expression and suppressed GI genes in cultured human lung progenitors derived from embryonic stem cells.

Pulmonary alveolar type I cell population consists of two distinct subtypes that differ in cell fate

Pulmonary alveolar type I (AT1) cells are essential for the gas-exchange function of lungs. AT1 cells retain their cellular plasticity during injury-induced alveolar regeneration. However, we know very little about the developmental heterogeneity of the AT1 cell population. Our study identified a robust genetic marker of postnatal AT1 cells, insulin-like growth factor-binding protein 2 (Igfbp2). We use this marker to demonstrate that the postnatal AT1 cell population actually consists of two AT1 cell subtypes (Hopx⁺Igfbp2⁺ and Hopx⁺Igfbp2⁻ AT1 cells) with distinct cell fates during alveolar regeneration. The large majority of adult AT1 cells expresses Igfbp2 and cannot transdifferentiate into AT2 cells during post pneumonectomy formation of new alveoli. Therefore, Hopx⁺Igfbp2⁺ AT1 cells represent the terminally differentiated population of AT1 cells.

Pulmonary alveolar type I (AT1) cells cover more than 95% of alveolar surface and are essential for the air–blood barrier function of lungs. AT1 cells have been shown to retain developmental plasticity during alveolar regeneration. However, the development and heterogeneity of AT1 cells remain largely unknown. Here, we conducted a single-cell RNA-seq analysis to characterize postnatal AT1 cell development and identified insulin-like growth factor-binding protein 2 (Igfbp2) as a genetic marker specifically expressed in postnatal AT1 cells. The portion of AT1 cells expressing Igfbp2 increases during alveologenesis and in post pneumonectomy (PNX) newly formed alveoli. We found that the adult AT1 cell population contains both Hopx⁺Igfbp2⁺ and Hopx⁺Igfbp2⁻ AT1 cells, which have distinct cell fates during alveolar regeneration. Using an Igfbp2-CreER mouse model, we demonstrate that Hopx⁺Igfbp2⁺ AT1 cells represent terminally differentiated AT1 cells that are not able to transdifferentiate into AT2 cells during post-PNX alveolar regeneration. Our study provides tools and insights that will guide future investigations into the molecular and cellular mechanism or mechanisms underlying AT1 cell fate during lung development and regeneration.

The Strength of Mechanical Forces Determines the Differentiation of Alveolar Epithelial Cells

The differentiation of alveolar epithelial type I (AT1) and type II (AT2) cells is essential for the lung gas exchange function. Disruption of this process results in neonatal death or in severe lung diseases that last into adulthood. We developed live imaging techniques to characterize the mechanisms that control alveolar epithelial cell differentiation. We discovered that mechanical forces generated from the inhalation of amniotic fluid by fetal breathing movements are essential for AT1 cell differentiation. We found that a large subset of alveolar progenitor cells is able to protrude from the airway epithelium toward the mesenchyme in an FGF10/FGFR2 signaling-dependent manner. The cell protrusion process results in enrichment of myosin in the apical region of protruded cells; this myosin prevents these cells from being flattened by mechanical forces, thereby ensuring their AT2 cell fate. Our study demonstrates that mechanical forces and local growth factors synergistically control alveolar epithelial cell differentiation.

Flt3 ligand expands bona fide innate lymphoid cell precursors in vivo

A common helper-like innate lymphoid precursor (CHILP) restricted to the innate lymphoid cells (ILC) lineage has been recently characterized. While specific requirements of transcription factors for CHILPs development has been partially described, their ability to sense cytokines and react to peripheral inflammation remains unaddressed. Here, we found that systemic increase in Flt3L levels correlated with the expansion of Lineage (Lin)^negα4β7⁺ precursors in the adult murine bone marrow. Expanded Lin^negα4β7⁺ precursors were bona fide CHILPs as seen by their ability to differentiate into all helper ILCs subsets but cNK in vivo. Interestingly, Flt3L-expanded CHILPs transferred into lymphopenic mice preferentially reconstituted the small intestine. While we did not observe changes in serum Flt3L during DSS-induced colitis in mice or plasma from inflammatory bowel disease (IBD) patients, elevated Flt3L levels were detected in acute malaria patients. Interestingly, while CHILP numbers were stable during the course of DSS-induced colitis, they expanded following increased serum Flt3L levels in malaria-infected mice, hence suggesting a role of the Flt3L-ILC axis in malaria. Collectively, our results indicate that Flt3L expands CHILPs in the bone marrow, which might be associated with specific inflammatory conditions.

Nanotechnologies for early diagnosis, in situ disease monitoring, and prevention

Nanotechnology is an enabling technology with great potential for applications in stem cell research and regenerative medicine. Fluorescent nanodiamond (FND), an inherently biocompatible and nontoxic nanoparticle, is well suited for such applications. We had developed a prospective isolation method using CD157, CD45, and CD54 to obtain lung stem cells. Labeling of CD45⁻CD54⁺CD157⁺ cells with FNDs did not eliminate their abilities for self-renewal and differentiation. The FND labeling in combination with cell sorting, fluorescence lifetime imaging microscopy, and immunostaining identified transplanted stem cells allowed tracking of their engraftment and regenerative capabilities with single-cell resolution. Time-gated fluorescence (TGF) imaging in mouse tissue sections indicated that they reside preferentially at the bronchoalveolar junctions of lungs, especially in naphthalene-injured mice. Our results presented in Subchapter 1.1 demonstrate not only the remarkable homing capacity and regenerative potential of the isolated stem cells, but also the ability of finding rare lung stem cells in vivo using FNDs.

The topical use of antiretroviral-based microbicides, namely of a dapivirine ring, has been recently shown to partially prevent transmission of HIV through the vaginal route. Among different formulation approaches, nanotechnology tools and principles have been used for the development of tentative vaginal and rectal microbicide products. Subchapter 1.2 provides an overview of antiretroviral drug nanocarriers as novel microbicide candidates and discusses recent and relevant research on the topic. Furthermore, advances in developing vaginal delivery platforms for the administration of promising antiretroviral drug nanocarriers are reviewed. Although mostly dedicated to the discussion of nanosystems for vaginal use, the development of rectal nanomicrobicides is also addressed.

Infectious diseases are currently responsible for over 8 million deaths per year. Efficient treatments require accurate recognition of pathogens at low concentrations, which in the case of blood infection (septicemia) can go as low as 1 mL^–1. Detecting and quantifying bacteria at such low concentrations is challenging and typically demands cultures of large samples of blood (∼1 mL) extending over 24–72 h. This delay seriously compromises the health of patients and is largely responsible for the death toll of bacterial infections. Recent advances in nanoscience, spectroscopy, plasmonics, and microfluidics allow for the development of optical devices capable of monitoring minute amounts of analytes in liquid samples. In Subchapter 1.3 we critically discuss these recent developments that will, in the future, enable the multiplex identification and quantification of microorganisms directly on their biological matrix with unprecedented speed, low cost, and sensitivity.

Radiolabeled nanoparticles (NPs) are finding an increasing interest in a broad range of biomedical applications. They may be used to detect and characterize diseases, to deliver relevant therapeutics, and to study the pharmacokinetic/pharmacodynamic parameters of nanomaterials. The use of radiotracer techniques in the research of novel NPs offers many advantages, but there are still some limitations. The binding of radionuclides to NPs has to be irreversible to prevent their escape to other tissues or organs. Due to the short half-lives of radionuclides, the manufacturing process is time limited and difficult, and there is also a risk of contamination. Subchapter 1.4 presents the main selection criteria for radionuclides and applicable radiolabeling procedures used for the radiolabeling of various NPs. Also, an overview of different types of NPs that have so far been labeled with radionuclides is presented.

Cysts mark the early stage of metastatic tumor development in non-small cell lung cancer

Identifying metastatic tumor growth at an early stage has been one of the biggest challenges in the treatment of lung cancer. By genetic lineage tracing approach in a conditional model of Non-Small Cell Lung Cancer (NSCLC) in mice, we demonstrate that cystic lesions represent an early stage of metastatic invasion. We generated a mouse model for NSCLC which incorporated a heritable DsRed fluorescent tag driven by the ubiquitous CAG promoter in the alveolar type II cells of the lung. We found early cystic lesions in a secondary organ (liver) that lacked the expression of bona fide lung makers namely Scgb1a1 and surfactant protein C Sftpc and were DsRed positive hence identifying lung as their source of origin. This demonstrates the significant potential of alveolar type II cells in orchestrating the process of metastasis, rendering it as one of the target cell types of the lung of therapeutic importance in human NSCLC.

Distinct populations of embryonic epithelial progenitors generate Lgr5+ intestinal stem cells

The adult intestinal stem cells (ISCs) are transcriptionally heterogeneous. As the mechanisms governing their developmental specification are still poorly understood, whether this heterogeneity reflects an early determination of distinct cellular sub-types with potentially distinct physiological functions remains an open question. We investigate the cellular heterogeneity within the mouse embryonic midgut epithelium at the molecular and functional levels. Cell fate mapping analysis revealed that multiple early embryonic epithelial progenitors give rise to Lgr5⁺ ISCs. The origin of the molecularly distinct early precursors along the anterior-posterior axis defines the transcriptional signature of embryonic Lgr5⁺ ISC progenitors. We further show that the early epithelial progenitors have different capacity to generate Lgr5⁺ ISC progenitors and Axin2⁺ early precursors display the highest potential.

In Vitro Induction and In Vivo Engraftment of Lung Bud Tip Progenitor Cells Derived from Human Pluripotent Stem Cells

The current study aimed to understand the developmental mechanisms regulating bud tip progenitor cells in the human fetal lung, which are present during branching morphogenesis, and to use this information to induce a bud tip progenitor-like population from human pluripotent stem cells (hPSCs) in vitro. We identified cues that maintained isolated human fetal lung epithelial bud tip progenitor cells in vitro and induced three-dimensional hPSC-derived organoids with bud tip-like domains. Bud tip-like domains could be isolated, expanded, and maintained as a nearly homogeneous population. Molecular and cellular comparisons revealed that hPSC-derived bud tip-like cells are highly similar to native lung bud tip progenitors. hPSC-derived epithelial bud tip-like structures survived in vitro for over 16 weeks, could be easily frozen and thawed, maintained multilineage potential, and successfully engrafted into the airways of immunocompromised mouse lungs, where they persisted for up to 6 weeks and gave rise to several lung epithelial lineages.

Retinoic acid regulates avian lung branching through a molecular network

Retinoic acid (RA) is of major importance during vertebrate embryonic development and its levels need to be strictly regulated otherwise congenital malformations will develop. Through the action of specific nuclear receptors, named RAR/RXR, RA regulates the expression of genes that eventually influence proliferation and tissue patterning. RA has been described as crucial for different stages of mammalian lung morphogenesis, and as part of a complex molecular network that contributes to precise organogenesis; nonetheless, nothing is known about its role in avian lung development. The current report characterizes, for the first time, the expression pattern of RA signaling members (stra6, raldh2, raldh3, cyp26a1, rarα, and rarβ) and potential RA downstream targets (sox2, sox9, meis1, meis2, tgfβ2, and id2) by in situ hybridization. In the attempt of unveiling the role of RA in chick lung branching, in vitro lung explants were performed. Supplementation studies revealed that RA stimulates lung branching in a dose-dependent manner. Moreover, the expression levels of cyp26a1, sox2, sox9, rarβ, meis2, hoxb5, tgfβ2, id2, fgf10, fgfr2, and shh were evaluated after RA treatment to disclose a putative molecular network underlying RA effect. In situ hybridization analysis showed that RA is able to alter cyp26a1, sox9, tgfβ2, and id2 spatial distribution; to increase rarβ, meis2, and hoxb5 expression levels; and has a very modest effect on sox2, fgf10, fgfr2, and shh expression levels. Overall, these findings support a role for RA in the proximal-distal patterning and branching morphogenesis of the avian lung and reveal intricate molecular interactions that ultimately orchestrate branching morphogenesis.

Deciphering the Innate Lymphoid Cell Transcriptional Program

Innate lymphoid cells (ILCs) are enriched at mucosal surfaces, where they provide immune surveillance. All ILC subsets develop from a common progenitor that gives rise to pre-committed progenitors for each of the ILC lineages. Currently, the temporal control of gene expression that guides the emergence of these progenitors is poorly understood. We used global transcriptional mapping to analyze gene expression in different ILC progenitors. We identified PD-1 to be specifically expressed in PLZF⁺ ILCp and revealed that the timing and order of expression of the transcription factors NFIL3, ID2, and TCF-1 was critical. Importantly, induction of ILC lineage commitment required only transient expression of NFIL3 prior to ID2 and TCF-1 expression. These findings highlight the importance of the temporal program that permits commitment of progenitors to the ILC lineage, and they expand our understanding of the core transcriptional program by identifying potential regulators of ILC development.

Inhibitor of DNA binding 2 is a novel therapeutic target for stemness of head and neck squamous cell carcinoma

Background:

Head and neck squamous cell carcinomas (HNSCCs) are highly lethal epithelial tumours containing self-renewal cancer stem cells (CSCs). CSCs in HNSCCs are strongly associated with tumour initiation, invasion, and chemoradiation resistance. However, the important factors regulating stemness in HNSCCs remain unclear. Here, we investigated the molecular roles and clinical significance of inhibitor of DNA binding 2 (Id2) protein to determine if it constitutes a novel therapeutic target for ablating HNSCC cells with stemness.

Methods:

We performed in vitro and in vivo studies of Id2 function and its effects on stemness using HNSCC cells. We also examined whether Id2 expression could be used as a prognostic indicator through immunohistochemical staining of 119 human HNSCC tumours.

Results:

Expression of Id2 was higher in HNSCC cells with stemness compared with differentiated HNSCC cells. Overexpression of Id2 increased proliferation, self-renewal, and expression of the putative stemness marker CD44 in HNSCC cells in vitro and in vivo. In contrast, silencing of Id2 using short hairpin RNA attenuated the stemness phenotype of HNSCC cells by reducing self-renewal, CD44 expression, cisplatin chemoresistance, and xenograft tumourigenicity. Most importantly, increased expression of Id2 was closely associated with poorer post-treatment survival rates in HNSCC patients.

Conclusions:

Inhibitor of DNA binding2 represents a novel and promising therapeutic target for treating and improving the clinical outcomes for patients with HNSCC.

Plasticity in the lung: making and breaking cell identity

In contrast to a prior emphasis on the finality of cell fate decisions in developmental systems, cellular plasticity is now emerging as a general theme in the biology of multiple adult organ systems. In the lung, lineage tracing has been used to identify distinct epithelial stem and progenitor cell populations. These cells, together with their differentiated progeny, maintain a stable identity during steady state conditions, but can display remarkable lineage plasticity following injury. This Review summarizes our current understanding of the different cell lineages of the adult mammalian lung and their responses to injury. In the lung, which is constantly exposed to infection and aerosolized toxins, epithelial plasticity might be more of a rule than an exception, and it is likely that different injuries elicit different facultative responses.

The neuropeptide neuromedin U stimulates innate lymphoid cells and type 2 inflammation

The type 2 cytokines interleukin (IL)-4, IL-5, IL-9 and IL-13 have important roles in stimulating innate and adaptive immune responses that are required for resistance to helminth infection, promotion of allergic inflammation, metabolic homeostasis and tissue repair. Group 2 innate lymphoid cells (ILC2s) produce type 2 cytokines, and although advances have been made in understanding the cytokine milieu that promotes ILC2 responses, how ILC2 responses are regulated by other stimuli remains poorly understood. Here we demonstrate that ILC2s in the mouse gastrointestinal tract co-localize with cholinergic neurons that express the neuropeptide neuromedin U (NMU). In contrast to other haematopoietic cells, ILC2s selectively express the NMU receptor 1 (NMUR1). In vitro stimulation of ILC2s with NMU induced rapid cell activation, proliferation, and secretion of the type 2 cytokines IL-5, IL-9 and IL-13 that was dependent on cell-intrinsic expression of NMUR1 and Gαq protein. In vivo administration of NMU triggered potent type 2 cytokine responses characterized by ILC2 activation, proliferation and eosinophil recruitment that was associated with accelerated expulsion of the gastrointestinal nematode Nippostrongylus brasiliensis or induction of lung inflammation. Conversely, worm burden was higher in Nmur1-/- mice than in control mice. Furthermore, use of gene-deficient mice and adoptive cell transfer experiments revealed that ILC2s were necessary and sufficient to mount NMU-elicited type 2 cytokine responses. Together, these data indicate that the NMU-NMUR1 neuronal signalling circuit provides a selective mechanism through which the enteric nervous system and innate immune system integrate to promote rapid type 2 cytokine responses that can induce anti-microbial, inflammatory and tissue-protective type 2 responses at mucosal sites.

ILC2s regulate adaptive Th2 cell functions via PD-L1 checkpoint control

Group 2 innate lymphoid cells (ILC2s) are important effector cells driving the initiation of type 2 immune responses leading to adaptive T helper 2 (Th2) immunity. Here we show that ILC2s dynamically express the checkpoint inhibitor molecule PD-L1 during type 2 pulmonary responses. Surprisingly, PD-L1:PD-1 interaction between ILC2s and CD4⁺ T cells did not inhibit the T cell response, but PD-L1-expressing ILC2s stimulated increased expression of GATA3 and production of IL-13 by Th2 cells both in vitro and in vivo. Conditional deletion of PD-L1 on ILC2s impaired early Th2 polarization and cytokine production, leading to delayed worm expulsion during infection with the gastrointestinal helminth Nippostrongylus brasiliensis Our results identify a novel PD-L1-controlled mechanism for type 2 polarization, with ILC2s mediating an innate checkpoint to control adaptive T helper responses, which has important implications for the treatment of type 2 inflammation.

Human embryonic lung epithelial tips are multipotent progenitors that can be expanded in vitro as long-term self-renewing organoids

The embryonic mouse lung is a widely used substitute for human lung development. For example, attempts to differentiate human pluripotent stem cells to lung epithelium rely on passing through progenitor states that have only been described in mouse. The tip epithelium of the branching mouse lung is a multipotent progenitor pool that self-renews and produces differentiating descendants. We hypothesized that the human distal tip epithelium is an analogous progenitor population and tested this by examining morphology, gene expression and in vitro self-renewal and differentiation capacity of human tips. These experiments confirm that human and mouse tips are analogous and identify signalling pathways that are sufficient for long-term self-renewal of human tips as differentiation-competent organoids. Moreover, we identify mouse-human differences, including markers that define progenitor states and signalling requirements for long-term self-renewal. Our organoid system provides a genetically-tractable tool that will allow these human-specific features of lung development to be investigated.

DOI:http://dx.doi.org/10.7554/eLife.26575.001

Degenerative lung disease occurs when the structure of the lungs breaks down, which makes it harder to get enough oxygen into the bloodstream. Most, but not all, cases occur in smokers and ex-smokers or people who have been exposed to a lot of air pollution. Currently, there is no way to reverse the damage, and even slowing the progress of the disease is extremely difficult. Some researchers are looking for ways to treat patients with degenerative lung diseases by regenerating the surface of their lungs. However, it is still not clear what the most effective route towards this long-term goal will be.

One approach to lung regeneration is to use findings from developmental biology to understand how embryos normally build the gas exchange surfaces in the lungs. This knowledge may allow scientists to trigger a similar process in an adult lung to renew or replace any diseased tissue. Alternatively, cells could be collected from patients, reprogrammed and then coaxed into becoming a gas exchange surface in the laboratory. Such a “lung-in-a-dish” could be used to understand how degenerative diseases develop, to discover and test new drugs, or even to treat the patient directly via a transplant.

To date, the embryonic development of lungs has mostly been studied using mouse lungs as a model system. However, it was not clear if human lungs actually develop in similar ways to mouse lungs, and whether using mice is a valid research strategy.

Nikolić et al. compared embryonic lungs from humans and mice and showed that they are indeed very similar in terms of the cell types that they contain and how they mature. However, some key differences were identified that can only be explored in human cells and tissue. Nikolić et al. went on to identify conditions that allowed them to grow cells from human embryonic lungs indefinitely in a dish. These cells can now be used to investigate the aspects of lung development that are specific to humans.

Together these findings provide a useful guide to allow scientists to coax human cells growing in a laboratory to become lung cells. Further improvements to this process will make the lungs-in-a-dish more true to the real organs, meaning that they could be used to better understand lung disease and identify new medicines. In the longer term, Nikolić et al. hope to gain enough insight from the human lung-in-a-dish model to eventually be able to regenerate the lungs of patients with degenerative lung disease. However, this possibility is still many years away.

DOI:http://dx.doi.org/10.7554/eLife.26575.002

Sin3a regulates epithelial progenitor cell fate during lung development

Mechanisms that regulate tissue-specific progenitors for maintenance and differentiation during development are poorly understood. Here, we demonstrate that the co-repressor protein Sin3a is crucial for lung endoderm development. Loss of Sin3a in mouse early foregut endoderm led to a specific and profound defect in lung development with lung buds failing to undergo branching morphogenesis and progressive atrophy of the proximal lung endoderm with complete epithelial loss at later stages of development. Consequently, neonatal pups died at birth due to respiratory insufficiency. Further analysis revealed that loss of Sin3a resulted in embryonic lung epithelial progenitor cells adopting a senescence-like state with permanent cell cycle arrest in G1 phase. This was mediated at least partially through upregulation of the cell cycle inhibitors Cdkn1a and Cdkn2c. At the same time, loss of endodermal Sin3a also disrupted cell differentiation of the mesoderm, suggesting aberrant epithelial-mesenchymal signaling. Together, these findings reveal that Sin3a is an essential regulator for early lung endoderm specification and differentiation.

In Vitro Models to Study Human Lung Development, Disease and Homeostasis

The main function of the lung is to support gas exchange, and defects in lung development or diseases affecting the structure and function of the lung can have fatal consequences. Most of what we currently understand about human lung development and disease has come from animal models. However, animal models are not always fully able to recapitulate human lung development and disease, highlighting an area where in vitro models of the human lung can compliment animal models to further understanding of critical developmental and pathological mechanisms. This review will discuss current advances in generating in vitro human lung models using primary human tissue, cell lines, and human pluripotent stem cell derived lung tissue, and will discuss crucial next steps in the field.

Lung Organoids and Their Use To Study Cell-Cell Interaction

Purpose of Review

The lung research field has pioneered the use of organoids for the study of cell-cell interactions.

Recent Findings

The use of organoids for airway basal cells is routine. However, the development of organoids for the other regions of the lung is still in its infancy. Such cultures usually rely on cell-cell interactions between the stem cells and a putative niche cell for their growth and differentiation.

Summary

The use of co-culture organoid systems has facilitated the in vitro cultivation of previously inaccessible stem cell populations, providing a novel method for dissecting the molecular requirements of these cell-cell interactions. Future technology development will allow the growth of epithelial-only organoids in more defined media and also the introduction of specific non-epithelial cells for the study of cell interactions. These developments will require an improved understanding of the epithelial and non-epithelial cell types present in the lung and their lineage relationships.

Evolution of the hypoxia-sensitive cells involved in amniote respiratory reflexes

The evolutionary origins of the hypoxia-sensitive cells that trigger amniote respiratory reflexes - carotid body glomus cells, and 'pulmonary neuroendocrine cells' (PNECs) - are obscure. Homology has been proposed between glomus cells, which are neural crest-derived, and the hypoxia-sensitive 'neuroepithelial cells' (NECs) of fish gills, whose embryonic origin is unknown. NECs have also been likened to PNECs, which differentiate in situ within lung airway epithelia. Using genetic lineage-tracing and neural crest-deficient mutants in zebrafish, and physical fate-mapping in frog and lamprey, we find that NECs are not neural crest-derived, but endoderm-derived, like PNECs, whose endodermal origin we confirm. We discover neural crest-derived catecholaminergic cells associated with zebrafish pharyngeal arch blood vessels, and propose a new model for amniote hypoxia-sensitive cell evolution: endoderm-derived NECs were retained as PNECs, while the carotid body evolved via the aggregation of neural crest-derived catecholaminergic (chromaffin) cells already associated with blood vessels in anamniote pharyngeal arches.

Efficient Derivation of Functional Human Airway Epithelium from Pluripotent Stem Cells via Temporal Regulation of Wnt Signaling

Effective derivation of functional airway organoids from induced pluripotent stem cells (iPSCs) would provide valuable models of lung disease and facilitate precision therapies for airway disorders such as cystic fibrosis. However, limited understanding of human airway patterning has made this goal challenging. Here, we show that cyclical modulation of the canonical Wnt signaling pathway enables rapid directed differentiation of human iPSCs via an NKX2-1⁺ progenitor intermediate into functional proximal airway organoids. We find that human NKX2-1⁺ progenitors have high levels of Wnt activation but respond intrinsically to decreases in Wnt signaling by rapidly patterning into proximal airway lineages at the expense of distal fates. Using this directed approach, we were able to generate cystic fibrosis patient-specific iPSC-derived airway organoids with a defect in forskolin-induced swelling that is rescued by gene editing to correct the disease mutation. Our approach has many potential applications in modeling and drug screening for airway diseases.

Transcription factor Etv5 is essential for the maintenance of alveolar type II cells

Alveolar type II (AT2) cell dysfunction contributes to a number of significant human pathologies including respiratory distress syndrome, lung adenocarcinoma, and debilitating fibrotic diseases, but the critical transcription factors that maintain AT2 cell identity are unknown. Here we show that the E26 transformation-specific (ETS) family transcription factor Etv5 is essential to maintain AT2 cell identity. Deletion of Etv5 from AT2 cells produced gene and protein signatures characteristic of differentiated alveolar type I (AT1) cells. Consistent with a defect in the AT2 stem cell population, Etv5 deficiency markedly reduced recovery following bleomycin-induced lung injury. Lung tumorigenesis driven by mutant KrasG12D was also compromised by Etv5 deficiency. ERK activation downstream of Ras was found to stabilize Etv5 through inactivation of the cullin-RING ubiquitin ligase CRL4^COP1/DET1 that targets Etv5 for proteasomal degradation. These findings identify Etv5 as a critical output of Ras signaling in AT2 cells, contributing to both lung homeostasis and tumor initiation.

Un(MaSC)ing Stem Cell Dynamics in Mammary Branching Morphogenesis

The properties of stem cells that participate in mammary gland branching morphogenesis remain contested. Reporting in Nature, Scheele et al. (2017) establish a model for post-pubertal mammary branching morphogenesis in which position-dependent, lineage-restricted stem cells undergo cell mixing in order to contribute to long-term growth.