Niche-mediated BMP/SMAD signaling regulates lung alveolar stem cell proliferation and differentiation

Consequences of PDGFRα+ fibroblast reduction in adult murine hearts

Fibroblasts produce the majority of collagen in the heart and are thought to regulate extracellular matrix (ECM) turnover. Although fibrosis accompanies many cardiac pathologies and is generally deleterious, the role of fibroblasts in maintaining the basal ECM network and in fibrosis in vivo is poorly understood. We genetically ablated fibroblasts in mice to evaluate the impact on homeostasis of adult ECM and cardiac function after injury. Fibroblast-ablated mice demonstrated a substantive reduction in cardiac fibroblasts, but fibrillar collagen and the ECM proteome were not overtly altered when evaluated by quantitative mass spectrometry and N-terminomics. However, the distribution and quantity of collagen VI, microfibrillar collagen that forms an open network with the basement membrane, was reduced. In fibroblast-ablated mice, cardiac function was better preserved following angiotensin II/phenylephrine (AngII/PE)-induced fibrosis and myocardial infarction (MI). Analysis of cardiomyocyte function demonstrated altered sarcomere shortening and slowed calcium decline in both uninjured and AngII/PE-infused fibroblast-ablated mice. After MI, the residual resident fibroblasts responded to injury, albeit with reduced proliferation and numbers immediately after injury. These results indicate that the adult mouse heart tolerates a significant degree of fibroblast loss with a potentially beneficial impact on cardiac function after injury. The cardioprotective effect of controlled fibroblast reduction may have therapeutic value in heart disease.

Comparative Evaluation of Inducible Cre Mouse Models for Fibroblast Targeting in the Healthy and Infarcted Myocardium

Several Cre recombinase transgenic mouse models have been generated for cardiac fibroblast (CF) tracking and heart regulation. However, there is still no consensus on the ideal mouse model to optimally identify and/or regulate these cells. Here, a comparative evaluation of the efficiency and specificity of the indirect reporter Cre-loxP system was carried out in three of the most commonly used fibroblast reporter transgenic mice (Pdgfra-CreERT2, Col1a1-CreERT2 and PostnMCM) under healthy and ischemic conditions, to determine their suitability in in vivo studies of cardiac fibrosis. We demonstrate optimal Cre recombinase activity in CF (but also, although moderate, in endothelial cells (ECs)) derived from healthy and infarcted hearts in the PDGFRa-creERT2 mouse strain. In contrast, no positive reporter signal was found in CF derived from the Col1a1-CreERT2 mice. Finally, in the PostnMCM line, fluorescent reporter expression was specifically detected in activated CF but not in EC, which leads us to conclude that it may be the most reliable model for future studies on cardiovascular disease. Importantly, no lethality or cardiac fibrosis were induced after tamoxifen administration at the established doses, either in healthy or infarcted mice of the three fibroblast reporter lineages. This study lays the groundwork for future efficient in vivo CF tracking and functional analyses.

The homeodomain transcription factor Ventx2 regulates respiratory progenitor cell number and differentiation timing during Xenopus lung development

Ventx2 is an Antennapedia superfamily/NK-like subclass homeodomain transcription factor best known for its roles in the regulation of early dorsoventral patterning during Xenopus gastrulation and in the maintenance of neural crest multipotency. In this work we characterize the spatiotemporal expression pattern of ventx2 in progenitor cells of the Xenopus respiratory system epithelium. We find that ventx2 is directly induced by BMP signaling in the ventral foregut prior to nkx2-1, the earliest epithelial marker of the respiratory lineage. Functional studies demonstrate that Ventx2 regulates the number of Nkx2-1/Sox9+ respiratory progenitor cells induced during foregut development, the timing and level of surfactant protein gene expression, and proper tracheal-esophageal separation. Our data suggest that Ventx2 regulates the balance of respiratory progenitor cell expansion and differentiation. While the ventx gene family has been lost from the mouse genome during evolution, humans have retained a ventx2-like gene (VENTX). Finally, we discuss how our findings might suggest a possible function of VENTX in human respiratory progenitor cells.

Promises and Challenges of Cell-Based Therapies to Promote Lung Regeneration in Idiopathic Pulmonary Fibrosis

The lung epithelium is constantly exposed to harmful agents present in the air that we breathe making it highly susceptible to damage. However, in instances of injury to the lung, it exhibits a remarkable capacity to regenerate injured tissue thanks to the presence of distinct stem and progenitor cell populations along the airway and alveolar epithelium. Mechanisms of repair are affected in chronic lung diseases such as idiopathic pulmonary fibrosis (IPF), a progressive life-threatening disorder characterized by the loss of alveolar structures, wherein excessive deposition of extracellular matrix components cause the distortion of tissue architecture that limits lung function and impairs tissue repair. Here, we review the most recent findings of a study of epithelial cells with progenitor behavior that contribute to tissue repair as well as the mechanisms involved in mouse and human lung regeneration. In addition, we describe therapeutic strategies to promote or induce lung regeneration and the cell-based strategies tested in clinical trials for the treatment of IPF. Finally, we discuss the challenges, concerns and limitations of applying these therapies of cell transplantation in IPF patients. Further research is still required to develop successful strategies focused on cell-based therapies to promote lung regeneration to restore lung architecture and function.

Temporal control of PDGFRα regulates the fibroblast-to-myofibroblast transition in wound healing

Fibroblasts differentiate into myofibroblasts by acquiring new contractile function. This is important for tissue repair, but it also contributes to organ fibrosis. Platelet-derived growth factor (PDGF) promotes tissue repair and fibrosis, but the relationship between PDGF and myofibroblasts is unclear. Using mice with lineage tracing linked to PDGF receptor α (PDGFRα) gene mutations, we examine cell fates during skin wound healing. Elevated PDGFRα signaling increases proliferation but unexpectedly delays the fibroblast-to-myofibroblast transition, suggesting that PDGFRα must be downregulated for myofibroblast differentiation. In contrast, deletion of PDGFRα decreases proliferation and myofibroblast differentiation by reducing serum response factor (SRF) nuclear localization. Consequences of SRF deletion resemble PDGFRα deletion, but deletion of two SRF coactivators, MRTFA and MRTFB, specifically eliminates myofibroblasts. Our findings suggest a scenario where PDGFRα signaling initially supports proliferation of fibroblast progenitors to expand their number during early wound healing but, later, PDGFRα downregulation facilitates fibroblast differentiation into myofibroblasts.

Modeling diverse genetic subtypes of lung adenocarcinoma with a next-generation alveolar type 2 organoid platform

Lung cancer is the leading cause of cancer-related death worldwide. Lung adenocarcinoma (LUAD), the most common histological subtype, accounts for 40% of all cases. While existing genetically engineered mouse models (GEMMs) recapitulate the histological progression and transcriptional evolution of human LUAD, they are time-consuming and technically demanding. In contrast, cell line transplant models are fast and flexible, but these models fail to capture the full spectrum of disease progression. Organoid technologies provide a means to create next-generation cancer models that integrate the most advantageous features of autochthonous and transplant-based systems. However, robust and faithful LUAD organoid platforms are currently lacking. Here, we describe optimized conditions to continuously expand murine alveolar type 2 (AT2) cells, a prominent cell of origin for LUAD, in organoid culture. These organoids display canonical features of AT2 cells, including marker gene expression, the presence of lamellar bodies, and an ability to differentiate into the AT1 lineage. We used this system to develop flexible and versatile immunocompetent organoid-based models of KRAS, BRAF, and ALK mutant LUAD. Notably, organoid-based tumors display extensive burden and complete penetrance and are histopathologically indistinguishable from their autochthonous counterparts. Altogether, this organoid platform is a powerful, versatile new model system to study LUAD.

Regulation of extracellular matrix composition by fibroblasts during perinatal cardiac maturation

BACKGROUND

Cardiac fibroblasts are the main non-myocyte population responsible for extracellular matrix (ECM) production. During perinatal development, fibroblast expansion coincides with the transition from hyperplastic to hypertrophic myocardial growth. Therefore, we investigated the consequences of fibroblast loss at the time of cardiomyocyte maturation by depleting fibroblasts in the perinatal mouse.

METHODS AND RESULTS

We evaluated the microenvironment of the perinatal heart in the absence of fibroblasts and the potential functional impact of fibroblast loss in regulation of cardiomyocyte cell cycle arrest and binucleation. Cre-mediated expression of diphtheria toxin A in PDGFRα expressing cells immediately after birth eliminated 70-80% of the cardiac fibroblasts. At postnatal day 5, hearts lacking fibroblasts appeared similar to controls with normal morphology and comparable numbers of endothelial and smooth muscle cells, despite a pronounced reduction in fibrillar collagen. Immunoblotting and proteomic analysis of control and fibroblast-deficient hearts identified differential abundance of several ECM proteins. In addition, fibroblast loss decreased tissue stiffness and resulted in increased cardiomyocyte mitotic index, DNA synthesis, and cytokinesis. Moreover, decellularized matrix from fibroblast-deficient hearts promoted cardiomyocyte DNA replication. While cardiac architecture was not overtly affected by fibroblast reduction, few pups survived past postnatal day 11, suggesting an overall requirement for PDGFRα expressing fibroblasts.

CONCLUSIONS

These studies demonstrate the key role of fibroblasts in matrix production and cardiomyocyte cross-talk during mouse perinatal heart maturation and revealed that fibroblast-derived ECM may modulate cardiomyocyte maturation in vivo. Neonatal depletion of fibroblasts demonstrated that although hearts can tolerate reduced ECM composition, fibroblast loss eventually leads to perinatal death as the approach simultaneously reduced fibroblast populations in other organs.

Ex situ-armus: experimental models for combating respiratory dysfunction

Ex situ experimental models have become a main stay in pulmonary research. Organoids and explant systems have uncovered novel stem cell subsets, served as disease models, delineated cell fate transitions, and aided high throughput pre-clinical drug screening. Integration of gene-editing and bioengineering approaches have further generated novel avenues for regenerative medicine and transplantation strategies. In this article, we highlight recent studies, aided by ex situ systems, which have contributed to significant advances in our understanding of the human lower respiratory tract. We present key observations from these studies to gain improved insights into human disease. We conclude this article with a summary of existing challenges and potential technological advances to successfully mirror human tissue physiology.

Immune determinants of chronic sequelae after respiratory viral infection

The acute effects of various respiratory viral infections have been well studied, with extensive characterization of the clinical presentation as well as viral pathogenesis and host responses. However, over the course of the recent COVID-19 pandemic, the incidence and prevalence of chronic sequelae after acute viral infections have become increasingly appreciated as a serious health concern. Post-acute sequelae of COVID-19, alternatively described as “long COVID-19,” are characterized by symptoms that persist for longer than 28 days after recovery from acute illness. Although there exists substantial heterogeneity in the nature of the observed sequelae, this phenomenon has also been observed in the context of other respiratory viral infections including influenza virus, respiratory syncytial virus, rhinovirus, severe acute respiratory syndrome coronavirus, and Middle Eastern respiratory syndrome coronavirus. In this Review, we discuss the various sequelae observed following important human respiratory viral pathogens and our current understanding of the immunological mechanisms underlying the failure of restoration of homeostasis in the lung.

Combinatorial Gli activity directs immune infiltration and tumor growth in pancreatic cancer

Proper Hedgehog (HH) signaling is essential for embryonic development, while aberrant HH signaling drives pediatric and adult cancers. HH signaling is frequently dysregulated in pancreatic cancer, yet its role remains controversial, with both tumor-promoting and tumor-restraining functions reported. Notably, the GLI family of HH transcription factors (GLI1, GLI2, GLI3), remain largely unexplored in pancreatic cancer. We therefore investigated the individual and combined contributions of GLI1-3 to pancreatic cancer progression. At pre-cancerous stages, fibroblast-specific Gli2/Gli3 deletion decreases immunosuppressive macrophage infiltration and promotes T cell infiltration. Strikingly, combined loss of Gli1/Gli2/Gli3 promotes macrophage infiltration, indicating that subtle changes in Gli expression differentially regulate immune infiltration. In invasive tumors, Gli2/Gli3 KO fibroblasts exclude immunosuppressive myeloid cells and suppress tumor growth by recruiting natural killer cells. Finally, we demonstrate that fibroblasts directly regulate macrophage and T cell migration through the expression of Gli-dependent cytokines. Thus, the coordinated activity of GLI1-3 directs the fibroinflammatory response throughout pancreatic cancer progression.

Impaired Alveolar Re-Epithelialization in Pulmonary Emphysema

Alveolar type II (ATII) cells are progenitors in alveoli and can repair the alveolar epithelium after injury. They are intertwined with the microenvironment for alveolar epithelial cell homeostasis and re-epithelialization. A variety of ATII cell niches, transcription factors, mediators, and signaling pathways constitute a specific environment to regulate ATII cell function. Particularly, WNT/β-catenin, YAP/TAZ, NOTCH, TGF-β, and P53 signaling pathways are dynamically involved in ATII cell proliferation and differentiation, although there are still plenty of unknowns regarding the mechanism. However, an imbalance of alveolar cell death and proliferation was observed in patients with pulmonary emphysema, contributing to alveolar wall destruction and impaired gas exchange. Cigarette smoking causes oxidative stress and is the primary cause of this disease development. Aberrant inflammatory and oxidative stress responses result in loss of cell homeostasis and ATII cell dysfunction in emphysema. Here, we discuss the current understanding of alveolar re-epithelialization and altered reparative responses in the pathophysiology of this disease. Current therapeutics and emerging treatments, including cell therapies in clinical trials, are addressed as well.

A bipotential organoid model of respiratory epithelium recapitulates high infectivity of SARS-CoV-2 Omicron variant

The airways and alveoli of the human respiratory tract are lined by two distinct types of epithelium, which are the primary targets of respiratory viruses. We previously established long-term expanding human lung epithelial organoids from lung tissues and developed a ‘proximal’ differentiation protocol to generate mucociliary airway organoids. However, a respiratory organoid system with bipotential of the airway and alveolar differentiation remains elusive. Here we defined a ‘distal’ differentiation approach to generate alveolar organoids from the same source for the derivation of airway organoids. The alveolar organoids consisting of type I and type II alveolar epithelial cells (AT1 and AT2, respectively) functionally simulate the alveolar epithelium. AT2 cells maintained in lung organoids serve as progenitor cells from which alveolar organoids derive. Moreover, alveolar organoids sustain a productive SARS-CoV-2 infection, albeit a lower replicative fitness was observed compared to that in airway organoids. We further optimized 2-dimensional (2D) airway organoids. Upon differentiation under a slightly acidic pH, the 2D airway organoids exhibit enhanced viral replication, representing an optimal in vitro correlate of respiratory epithelium for modeling the high infectivity of SARS-CoV-2. Notably, the higher infectivity and replicative fitness of the Omicron variant than an ancestral strain were accurately recapitulated in these optimized airway organoids. In conclusion, we have established a bipotential organoid culture system able to reproducibly expand the entire human respiratory epithelium in vitro for modeling respiratory diseases, including COVID-19.

A Guide to Examining Intramuscular Fat Formation and its Cellular Origin in Skeletal Muscle

Fibro-adipogenic progenitors (FAPs) are mesenchymal stromal cells that play a crucial role during skeletal muscle homeostasis and regeneration. FAPs build and maintain the extracellular matrix that acts as a molecular myofiber scaffold. In addition, FAPs are indispensable for myofiber regeneration as they secrete a multitude of beneficial factors sensed by the muscle stem cells (MuSCs). In diseased states, however, FAPs are the cellular origin of intramuscular fat and fibrotic scar tissue. This fatty fibrosis is a hallmark of sarcopenia and neuromuscular diseases, such as Duchenne Muscular Dystrophy. One significant barrier in determining why and how FAPs differentiate into intramuscular fat is effective preservation and subsequent visualization of adipocytes, especially in frozen tissue sections. Conventional methods of skeletal muscle tissue processing, such as snap-freezing, do not properly preserve the morphology of individual adipocytes, thereby preventing accurate visualization and quantification. To overcome this hurdle, a rigorous protocol was developed that preserves adipocyte morphology in skeletal muscle sections allowing visualization, imaging, and quantification of intramuscular fat. The protocol also outlines how to process a portion of muscle tissue for RT-qPCR, enabling users to confirm observed changes in fat formation by viewing differences in the expression of adipogenic genes. Additionally, it can be adapted to visualize adipocytes by whole-mount immunofluorescence of muscle samples. Finally, this protocol outlines how to perform genetic lineage tracing of Pdgfrα-expressing FAPs to study the adipogenic conversion of FAPs. This protocol consistently yields high-resolution and morphologically accurate immunofluorescent images of adipocytes, along with confirmation by RT-qPCR, allowing for robust, rigorous, and reproducible visualization and quantification of intramuscular fat. Together, the analysis pipeline described here is the first step to improving our understanding of how FAPs differentiate into intramuscular fat, and provides a framework to validate novel interventions to prevent fat formation.

Function of epithelial stem cell in the repair of alveolar injury

Alveoli are the functional units of blood-gas exchange in the lung and thus are constantly exposed to outside environments and frequently encounter pathogens, particles and other harmful substances. For example, the alveolar epithelium is one of the primary targets of the SARS-CoV-2 virus that causes COVID-19 lung disease. Therefore, it is essential to understand the cellular and molecular mechanisms by which the integrity of alveoli epithelial barrier is maintained. Alveolar epithelium comprises two cell types: alveolar type I cells (AT1) and alveolar type II cells (AT2). AT2s have been shown to function as tissue stem cells that repair the injured alveoli epithelium. Recent studies indicate that AT1s and subgroups of proximal airway epithelial cells can also participate alveolar repair process through their intrinsic plasticity. This review discussed the potential mechanisms that drive the reparative behaviors of AT2, AT1 and some proximal cells in responses to injury and how an abnormal repair contributes to some pathological conditions.

The Patient-Derived Cancer Organoids: Promises and Challenges as Platforms for Cancer Discovery

The cancer burden is rapidly increasing in most countries, and thus, new anticancer drugs for effective cancer therapy must be developed. Cancer model systems that recapitulate the biological processes of human cancers are one of the cores of the drug development process. PDCO has emerged as a unique model that preserves the genetic, physiological, and histologic characteristics of original cancer, including inter- and intratumoral heterogeneities. Due to these advantages, the PCDO model is increasingly investigated for anticancer drug screening and efficacy testing, preclinical patient stratification, and precision medicine for selecting the most effective anticancer therapy for patients. Here, we review the prospects and limitations of PDCO compared to the conventional cancer models. With advances in culture success rates, co-culture systems with the tumor microenvironment, organoid-on-a-chip technology, and automation technology, PDCO will become the most promising model to develop anticancer drugs and precision medicine.

SARS-CoV-2 Infection and Lung Regeneration

The lung is the primary site of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-induced immunopathology whereby the virus enters the host cells by binding to angiotensin-converting enzyme 2 (ACE2). Sophisticated regeneration and repair programs exist in the lungs to replenish injured cell populations. However, known resident stem/progenitor cells have been demonstrated to express ACE2, raising a substantial concern regarding the long-term consequences of impaired lung regeneration after SARS-CoV-2 infection. Moreover, clinical treatments may also affect lung repair from antiviral drug candidates to mechanical ventilation. In this review, we highlight how SARS-CoV-2 disrupts a program that governs lung homeostasis. We also summarize the current efforts of targeted therapy and supportive treatments for COVID-19 patients. In addition, we discuss the pros and cons of cell therapy with mesenchymal stem cells or resident lung epithelial stem/progenitor cells in preventing post-acute sequelae of COVID-19. We propose that, in addition to symptomatic treatments being developed and applied in the clinic, targeting lung regeneration is also essential to restore lung homeostasis in COVID-19 patients.

Progenitor potential of lung epithelial organoid cells in a transplantation model

Lung progenitor cells are crucial for regeneration following injury, yet it is unclear whether lung progenitor cells can be functionally engrafted after transplantation. We transplanted organoid cells derived from alveolar type II (AT2) cells enriched by SCA1-negative status (SNO) or multipotent SCA1-positive progenitor cells (SPO) into injured mouse lungs. Transplanted SNO cells are retained in the alveolar regions, whereas SPO cells incorporate into airway and alveolar regions. Single-cell transcriptomics demonstrate that transplanted SNO cells are comparable to native AT2 cells. Transplanted SPO cells exhibit transcriptional hallmarks of alveolar and airway cells, as well as transitional cell states identified in disease. Transplanted cells proliferate after re-injury of recipient mice and retain organoid-forming capacity. Thus, lung epithelial organoid cells exhibit progenitor cell functions after reintroduction to the lung. This study reveals methods to interrogate lung progenitor cell potential and model transitional cell states relevant to pathogenic features of lung disease in vivo.

Hedgehog-responsive PDGFRa(+) fibroblasts maintain a unique pool of alveolar epithelial progenitor cells during alveologenesis

The lung alveolus is lined with alveolar type 1 (AT1) and type 2 (AT2) epithelial cells. During alveologenesis, increasing demand associated with expanding alveolar numbers is met by proliferating progenitor AT2s (pAT2). Little information exists regarding the identity of this population and their niche microenvironment. We show that during alveologenesis, Hedgehog-responsive PDGFRa(+) progenitors (also known as SCMFs) are a source of secreted trophic molecules that maintain a unique pAT2 population. SCMFs are in turn maintained by TGFβ signaling. Compound inactivation of Alk5 TβR2 in SCMFs reduced their numbers and depleted the pAT2 pool without impacting differentiation of daughter cells. In lungs of preterm infants who died with bronchopulmonary dysplasia, PDGFRa is reduced and the number of proliferative AT2s is diminished, indicating that an evolutionarily conserved mechanism governs pAT2 behavior during alveologenesis. SCMFs are a transient cell population, active only during alveologenesis, making them a unique stage-specific niche mesodermal cell type in mammalian organs.

Extracellular vesicle-mediated cellular crosstalk in lung repair, remodelling and regeneration

The unperturbed lung is highly quiescent, with a remarkably low level of cell turnover. However, once damaged, the lung shows an extensive regenerative capacity, with resident progenitor cell populations re-entering the cell cycle and differentiating to promote repair. This quick and dramatic repair response requires interactions among more than 40 different cell lineages in the lung, and defects in any of these processes can lead to various lung pathologies. Understanding the mechanisms of interaction in lung injury, repair and regeneration thus has considerable practical and therapeutic implications. Moreover, therapeutic strategies for replacing lung progenitor cells and their progeny through cell therapy have gained increasing attention. In the last decade, extracellular vesicles (EVs), including exosomes, have been recognised as paracrine mediators through the transfer of biological cargo. Recent work has revealed that EVs are involved in lung homeostasis and diseases. In addition, EVs derived from specific cells or tissues have proven to be a promising cell-free modality for the treatment of lung diseases. This review highlights the EV-mediated cellular crosstalk that regulates lung homeostasis and discusses the potential of EV therapeutics for lung regenerative medicine.

FOXO1 Couples KGF and PI-3K/AKT Signaling to NKX2.1-Regulated Differentiation of Alveolar Epithelial Cells

NKX2.1 is a master regulator of lung morphogenesis and cell specification; however, interactions of NKX2.1 with various transcription factors to regulate cell-specific gene expression and cell fate in the distal lung remain incompletely understood. FOXO1 is a key regulator of stem/progenitor cell maintenance/differentiation in several tissues but its role in the regulation of lung alveolar epithelial progenitor homeostasis has not been evaluated. We identified a novel role for FOXO1 in alveolar epithelial cell (AEC) differentiation that results in the removal of NKX2.1 from surfactant gene promoters and the subsequent loss of surfactant expression in alveolar epithelial type I-like (AT1-like) cells. We found that the FOXO1 forkhead domain potentiates a loss of surfactant gene expression through an interaction with the NKX2.1 homeodomain, disrupting NKX2.1 binding to the SFTPC promoter. In addition, blocking PI-3K/AKT signaling reduces phosphorylated FOXO-1 (p-FOXO1), allowing accumulated nuclear FOXO1 to interact with NKX2.1 in differentiating AEC. Inhibiting AEC differentiation in vitro with keratinocyte growth factor (KGF) maintained an AT2 cell phenotype through increased PI3K/AKT-mediated FOXO1 phosphorylation, resulting in higher levels of surfactant expression. Together these results indicate that FOXO1 plays a central role in AEC differentiation by directly binding NKX2.1 and suggests an essential role for FOXO1 in mediating AEC homeostasis.

Prkg2 regulates alveolar type 2-mediated re-alveolarization

BACKGROUND

The cGMP-dependent type 2 protein kinase, encoded by the prkg2 gene, is highly expressed in alveolar type 2 epithelial (AT2) cells. It is unclear whether prkg2 regulates AT2 cell homeostasis and re-alveolarization of injured lungs. This study aimed to investigate the role of prkg2 in the regulation of the fate of AT2 in vitro.

METHODS

Primary AT2 cells of wild-type (wt) and prkg2^-/- mice were co-cultured with fibroblasts as three-dimensional organoids. The colony formation was analyzed between days 4 and 12 post-seeding. EdU assay was used to detect cells with active DNA synthesis. AT1 and AT2 cells in organoids were visualized with anti-podoplanin and anti-surfactant protein C antibodies, respectively.

RESULTS

Prkg2^-/- AT2 cells developed a greater number of organoids than wt controls. However, compared to wt organoids, a lower number of AT2 but a greater number of AT1 cells were visualized. In addition, a lower number of proliferated cells (EdU⁺) were observed in prkg2^-/- organoids compared to wt controls. The numbers of organoids and EdU⁺ cells were significantly reduced in protein kinase A (PKA) inhibitor H89-treated wt and prkg2^-/- cultures. Organoids and EdU⁺ cells were increased by lipopolysaccharides (LPS) in both wt and prkg2^-/- groups. The increase in the proportion of AT1 and AT2 cells in organoids was only seen in wt controls.

CONCLUSIONS

Prkg2 may regulate the lineage of AT2 cells, which is affected by endotoxins and the interactive PKA signaling pathway.

Molecular Mechanisms of Alveolar Epithelial Stem Cell Senescence and Senescence-Associated Differentiation Disorders in Pulmonary Fibrosis

Pulmonary senescence is accelerated by unresolved DNA damage response, underpinning susceptibility to pulmonary fibrosis. Recently it was reported that the SARS-Cov-2 viral infection induces acute pulmonary epithelial senescence followed by fibrosis, although the mechanism remains unclear. Here, we examine roles of alveolar epithelial stem cell senescence and senescence-associated differentiation disorders in pulmonary fibrosis, exploring the mechanisms mediating and preventing pulmonary fibrogenic crisis. Notably, the TGF-β signalling pathway mediates alveolar epithelial stem cell senescence by mechanisms involving suppression of the telomerase reverse transcriptase gene in pulmonary fibrosis. Alternatively, telomere uncapping caused by stress-induced telomeric shelterin protein TPP1 degradation mediates DNA damage response, pulmonary senescence and fibrosis. However, targeted intervention of cellular senescence disrupts pulmonary remodelling and fibrosis by clearing senescent cells using senolytics or preventing senescence using telomere dysfunction inhibitor (TELODIN). Studies indicate that the development of senescence-associated differentiation disorders is reprogrammable and reversible by inhibiting stem cell replicative senescence in pulmonary fibrosis, providing a framework for targeted intervention of the molecular mechanisms of alveolar stem cell senescence and pulmonary fibrosis. Abbreviations: DPS, developmental programmed senescence; IPF, idiopathic pulmonary fibrosis; OIS, oncogene-induced replicative senescence; SADD, senescence-associated differentiation disorder; SALI, senescence-associated low-grade inflammation; SIPS, stress-induced premature senescence; TERC, telomerase RNA component; TERT, telomerase reverse transcriptase; TIFs, telomere dysfunction-induced foci; TIS, therapy-induced senescence; VIS, virus-induced senescence.

Short-term intermittent cigarette smoke exposure enhances alveolar type 2 cell stemness via fatty acid oxidation

Background

Cigarette smoke (CS) is associated with chronic obstructive pulmonary disease (COPD) and cancer. However, the underlying pathological mechanisms are not well understood. We recently reported that mice exposed to long-term intermittent CS for 3 months developed more severe emphysema and higher incidence of adenocarcinoma than mice exposed to long-term continuous CS for 3 months and long-term continuous CS exposure activated alveolar stem cell proliferation. However, the influence of variations in the CS exposure pattern in alveolar stem cell in unknown. Here, we exposed mice to 3 weeks of continuous or intermittent CS to identify whether different CS exposure patterns would result in differential effects on stem cells and the mechanisms underlying these potential differences.

Methods

Female mice expressing GFP in alveolar type 2 (AT2) cells, which are stem cells of the alveolar compartment, were exposed to mainstream CS via nasal inhalation. AT2 cells were collected based on their GFP expression by flow cytometry and co-cultured with fibroblasts in stem cell 3D organoid/colony-forming assays. We compared gene expression profiles of continuous and intermittent CS-exposed AT2 cells using microarray analysis and performed a functional assessment of a differentially expressed gene to confirm its involvement in the process using activator and inhibitor studies.

Results

AT2 cells sorted from intermittent CS-exposed mice formed significantly more colonies compared to those from continuous CS-exposed mice, and both CS-exposed groups formed significantly more colonies when compared to air-exposed cells. Comparative microarray analysis revealed the upregulation of genes related to fatty acid oxidation (FAO) pathways in AT2 cells from intermittent CS-exposed mice. Treatment of intermittent CS-exposed mice with etomoxir, an inhibitor of the FAO regulator Cpt1a, for 5 weeks resulted in a significant suppression of the efficiency of AT2 cell colony formation. In vitro treatment of naïve AT2 cells with a FAO activator and inhibitor further confirmed the relationship between FAO and AT2 stem cell function.

Conclusions

Alveolar stem cell function was more strongly activated by intermittent CS exposure than by continuous CS exposure. We provide evidence that AT2 stem cells respond to intermittent CS exposure by activating stem cell proliferation via the activation of FAO.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-022-01948-4.

Methionine deficiency and its hydroxy analogue influence chicken intestinal 3-dimensional organoid development

Methionine and its hydroxy analogue (MHA) have been shown to benefit mouse intestinal regeneration. The intestinal organoid is a good model that directly reflects the impact of certain nutrients or chemicals on intestinal development. Here, we aimed to establish a chicken intestinal organoid culture method first and then use the model to explore the influence of methionine deficiency and MHA on intestinal organoid development. The results showed that 125-μm cell strainer exhibited the highest efficiency for chicken embryo crypt harvesting. We found that transforming growth factor-β inhibitor (A8301) supplementation promoted enterocyte differentiation at the expense of the proliferation of intestinal stem cells (ISC). The mitogen-activated protein kinase p38 inhibitor (SB202190) promoted intestinal organoid formation and enterocyte differentiation but suppressed the differentiation of enteroendocrine cells, goblet cells and Paneth cells. However, the suppression of enteroendocrine cell and Paneth cell differentiation by SB202190 was alleviated at the presence of A8301. The glycogen synthase kinase 3 inhibitor (CHIR99021), valproic acid (VPA) alone and their combination promoted chicken intestinal organoid formation and enterocyte differentiation at the expense of the expression of Paneth cells and goblet cells. Chicken serum significantly improved organoid formation, especially in the presence of A8301, SB202190, CHIR99021, and VPA, but inhibited the differentiation of Paneth cells and enteroendocrine cells. Chicken serum at a concentration of 0.25% meets the requirement of chicken intestinal organoid development, and the beneficial effect of chicken serum on chicken intestinal organoid culture could not be replaced by fetal bovine serum and insulin-like growth factor-1. Moreover, commercial mouse organoid culture medium supplemented with A8301, SB202190, CHIR99021, VPA, and chicken serum promotes chicken organoid budding. Based on the chicken intestinal organoid model, we found that methionine deficiency mimicked by cycloleucine suppressed organoid formation and organoid size, and this effect was reinforced with increased cycloleucine concentrations. Methionine hydroxy analogue promoted regeneration of ISC but decreased cell differentiation compared with the results obtained with L-methionine. In conclusion, our results provide a potentially excellent guideline for chicken intestinal organoid culture and insights into methionine function in crypt development.

Mesenchymal-epithelial crosstalk shapes intestinal regionalisation via Wnt and Shh signalling

Organs are anatomically compartmentalised to cater for specialised functions. In the small intestine (SI), regionalisation enables sequential processing of food and nutrient absorption. While several studies indicate the critical importance of non-epithelial cells during development and homeostasis, the extent to which these cells contribute to regionalisation during morphogenesis remains unexplored. Here, we identify a mesenchymal-epithelial crosstalk that shapes the developing SI during late morphogenesis. We find that subepithelial mesenchymal cells are characterised by gradients of factors supporting Wnt signalling and stimulate epithelial growth in vitro. Such a gradient impacts epithelial gene expression and regional villus formation along the anterior-posterior axis of the SI. Notably, we further provide evidence that Wnt signalling directly regulates epithelial expression of Sonic Hedgehog (SHH), which, in turn, acts on mesenchymal cells to drive villi formation. Taken together our results uncover a mechanistic link between Wnt and Hedgehog signalling across different cellular compartments that is central for anterior-posterior regionalisation and correct formation of the SI.

The small intestine forms via crosstalk between epithelial and mesenchymal cell compartments. Here, the authors show that a gradient of Wnt signalling along the anterior-posterior axis regulates Sonic Hedgehog which is required for correct formation and regionalization of the small intestine.

Insights into the mechanisms of alveolarization - Implications for lung regeneration and cell therapies

Although the lung has extensive regenerative capacity, some diseases affecting the distal lung result in irreversible loss of pulmonary alveoli. Hitherto, treatments are supportive and do not specifically target tissue repair. Regenerative medicine offers prospects to promote lung repair and regeneration. The neonatal lung may be particularly receptive, because of its growth potential, compared to the adult lung. Based on our current understanding of neonatal lung injury, the ideal therapeutic approach includes mitigation of inflammation and fibrosis, and induction of regenerative signals. Cell-based therapies have shown potential to prevent and reverse impaired lung development. Their mechanisms of action suggest effects on both, mitigating the pathophysiological processes and promoting lung growth. Here, we review our current understanding of normal and impaired alveolarization, provide some rationale for the use of cell-based therapies and summarize current evidence for the therapeutic potential of cell-based therapies for pulmonary regeneration in preterm infants.

Multiple roles of mothers against decapentaplegic homolog 4 in tumorigenesis, stem cells, drug resistance, and cancer therapy

The transforming growth factor (TGF)-β signaling pathway controls many cellular processes, including proliferation, differentiation, and apoptosis. Abnormalities in the TGF-β signaling pathway and its components are closely related to the occurrence of many human diseases, including cancer. Mothers against decapentaplegic homolog 4 (Smad4), also known as deleted in pancreatic cancer locus 4, is a typical tumor suppressor candidate gene locating at q21.1 of human chromosome 18 and the common mediator of the TGF-β/Smad and bone morphogenetic protein/Smad signaling pathways. It is believed that Smad4 inactivation correlates with the development of tumors and stem cell fate decisions. Smad4 also interacts with cytokines, miRNAs, and other signaling pathways, jointly regulating cell behavior. However, the regulatory function of Smad4 in tumorigenesis, stem cells, and drug resistance is currently controversial. In addition, Smad4 represents an attractive therapeutic target for cancer. Elucidating the specific role of Smad4 is important for understanding the mechanism of tumorigenesis and cancer treatment. Here, we review the identification and characterization of Smad4, the canonical TGF-β/Smad pathway, as well as the multiple roles of Smad4 in tumorigenesis, stem cells, and drug resistance. Furthermore, we provide novel insights into the prospects of Smad4-targeted cancer therapy and the challenges that it will face in the future.

A census of the lung: CellCards from LungMAP

The human lung plays vital roles in respiration, host defense, and basic physiology. Recent technological advancements such as single-cell RNA sequencing and genetic lineage tracing have revealed novel cell types and enriched functional properties of existing cell types in lung. The time has come to take a new census. Initiated by members of the NHLBI-funded LungMAP Consortium and aided by experts in the lung biology community, we synthesized current data into a comprehensive and practical cellular census of the lung. Identities of cell types in the normal lung are captured in individual cell cards with delineation of function, markers, developmental lineages, heterogeneity, regenerative potential, disease links, and key experimental tools. This publication will serve as the starting point of a live, up-to-date guide for lung research at https://www.lungmap.net/cell-cards/. We hope that Lung CellCards will promote the community-wide effort to establish, maintain, and restore respiratory health.

Single-cell transcriptomics reveals lasting changes in the lung cellular landscape into adulthood after neonatal hyperoxic exposure

Ventilatory support, such as supplemental oxygen, used to save premature infants impairs the growth of the pulmonary microvasculature and distal alveoli, leading to bronchopulmonary dysplasia (BPD). Although lung cellular composition changes with exposure to hyperoxia in neonatal mice, most human BPD survivors are weaned off oxygen within the first weeks to months of life, yet they may have persistent lung injury and pulmonary dysfunction as adults. We hypothesized that early-life hyperoxia alters the cellular landscape in later life and predicts long-term lung injury. Using single-cell RNA sequencing, we mapped lung cell subpopulations at postnatal day (pnd)7 and pnd60 in mice exposed to hyperoxia (95% O2) for 3 days as neonates. We interrogated over 10,000 cells and identified a total of 45 clusters within 32 cell states. Neonatal hyperoxia caused persistent compositional changes in later life (pnd60) in all five type II cell states with unique signatures and function. Premature infants requiring mechanical ventilation with different durations also showed similar alterations in these unique signatures of type II cell states. Pathologically, neonatal hyperoxic exposure caused alveolar simplification in adult mice. We conclude that neonatal hyperoxia alters the lung cellular landscape in later life, uncovering neonatal programing of adult lung dysfunction.

Alveologenesis: What Governs Secondary Septa Formation

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

It takes all kinds: heterogeneity among satellite cells and fibro-adipogenic progenitors during skeletal muscle regeneration

Vertebrate skeletal muscle is composed of multinucleate myofibers that are surrounded by muscle connective tissue. Following injury, muscle is able to robustly regenerate because of tissue-resident muscle stem cells, called satellite cells. In addition, efficient and complete regeneration depends on other cells resident in muscle - including fibro-adipogenic progenitors (FAPs). Increasing evidence from single-cell analyses and genetic and transplantation experiments suggests that satellite cells and FAPs are heterogeneous cell populations. Here, we review our current understanding of the heterogeneity of satellite cells, their myogenic derivatives and FAPs in terms of gene expression, anatomical location, age and timing during the regenerative process - each of which have potentially important functional consequences.

Organoid models: assessing lung cell fate decisions and disease responses

Organoids can be derived from various cell types in the lung, and they provide a reproducible and tractable model for understanding the complex signals driving cell fate decisions in a regenerative context. In this review, we provide a retrospective account of organoid methodologies and outline new opportunities for optimizing these methods to further explore emerging concepts in lung biology. Moreover, we examine the benefits of integrating organoid assays with in vivo modeling to explore how the various niches and compartments in the respiratory system respond to both acute and chronic lung disease. The strategic implementation and improvement of organoid techniques will provide exciting new opportunities to understand and identify new therapeutic approaches to ameliorate lung disease states.

Organ Specificity and Heterogeneity of Cancer-Associated Fibroblasts in Colorectal Cancer

Fibroblasts constitute a ubiquitous mesenchymal cell type and produce the extracellular matrix (ECM) of connective tissue, thereby providing the structural basis of various organs. Fibroblasts display differential transcriptional patterns unique to the organ of their origin and they can be activated by common stimuli such as transforming growth factor-β (TGF-β) and platelet-derived growth factor (PDGF) signaling. Cancer-associated fibroblasts (CAFs) reside in the cancer tissue and contribute to cancer progression by influencing cancer cell growth, invasion, angiogenesis and tumor immunity. CAFs impact on the tumor microenvironment by remodeling the ECM and secreting soluble factors such as chemokines and growth factors. Differential expression patterns of molecular markers suggest heterogeneous features of CAFs in terms of their function, pathogenic role and cellular origin. Recent studies elucidated the bimodal action of CAFs on cancer progression and suggest a subgroup of CAFs with tumor-suppressive effects. This review attempts to describe cellular features of colorectal CAFs with an emphasis on their heterogeneity and functional diversity.

Co-culture of type I and type II pneumocytes as a model of alveolar epithelium

The epithelial tissues of the distal lung are continuously exposed to inhaled air, and are of research interest in studying respiratory exposure to both hazardous and therapeutic materials. Pharmaco-toxicological research depends on the development of sophisticated models of the alveolar epithelium, which better represent the different cell types present in the native lung and interactions between them. We developed an air-liquid interface (ALI) model of the alveolar epithelium which incorporates cell lines which bear features of type I (hAELVi) and type II (NCI-H441) epithelial cells. We compared morphology of single cells and the structure of cell layers of the two lines using light and electron microscopy. Working both in monotypic cultures and cocultures, we measured barrier function by trans-epithelial electrical resistance (TEER), and demonstrated that barrier properties can be maintained for 30 days. We created a mathematical model of TEER development over time based on these data in order to make inferences about the interactions occurring in these culture systems. We assessed expression of a panel of relevant genes that play important roles in barrier function and differentiation. The coculture model was observed to form a stable barrier akin to that seen in hAELVi, while expressing surfactant protein C, and having a profile of expression of claudins and aquaporins appropriate for the distal lung. We described cavities which arise within stratified cell layers in NCI-H441 and cocultured cells, and present evidence that these cavities represent an aberrant apical surface. In summary, our results support the coculture of these two cell lines to produce a model which better represents the breadth of functions seen in native alveolar epithelium.

Parallels between the extracellular matrix roles in developmental biology and cancer biology

Interaction of a tumor with its microenvironment is an emerging field of investigation, and the crosstalk between cancer cells and the extracellular matrix is of particular interest, since cancer patients with abundant and stiff extracellular matrices display a poorer prognosis. At the post-juvenile stage, the extracellular matrix plays predominantly a structural role by providing support to cells and tissues; however, during development, matrix proteins exert a plethora of diverse signals to guide the movement and determine the fate of pluripotent cells. Taking a closer look at the communication between the extracellular matrix and cells of a developing body may bring new insights into cancer biology and identify cancer weaknesses. This review discusses parallels between the extracellular matrix roles during development and tumor growth.

A systematic summary of survival and death signalling during the life of hair follicle stem cells

Hair follicle stem cells (HFSCs) are among the most widely available resources and most frequently approved model systems used for studying adult stem cells. HFSCs are particularly useful because of their self-renewal and differentiation properties. Additionally, the cyclic growth of hair follicles is driven by HFSCs. There are high expectations for the use of HFSCs as favourable systems for studying the molecular mechanisms that contribute to HFSC identification and can be applied to hair loss therapy, such as the activation or regeneration of hair follicles, and to the generation of hair using a tissue-engineering strategy. A variety of molecules are involved in the networks that critically regulate the fate of HFSCs, such as factors in hair follicle growth and development (in the Wnt pathway, Sonic hedgehog pathway, Notch pathway, and BMP pathway), and that suppress apoptotic cues (the apoptosis pathway). Here, we review the life cycle, biomarkers and functions of HFSCs, concluding with a summary of the signalling pathways involved in HFSC fate for promoting better understanding of the pathophysiological changes in the HFSC niche. Importantly, we highlight the potential mechanisms underlying the therapeutic targets involved in pathways associated with the treatment of hair loss and other disorders of skin and hair, including alopecia, skin cancer, skin inflammation, and skin wound healing.

Fibroblasts: Origins, definitions, and functions in health and disease

Fibroblasts are diverse mesenchymal cells that participate in tissue homeostasis and disease by producing complex extracellular matrix and creating signaling niches through biophysical and biochemical cues. Transcriptionally and functionally heterogeneous across and within organs, fibroblasts encode regional positional information and maintain distinct cellular progeny. We summarize their development, lineages, functions, and contributions to fibrosis in four fibroblast-rich organs: skin, lung, skeletal muscle, and heart. We propose that fibroblasts are uniquely poised for tissue repair by easily reentering the cell cycle and exhibiting a reversible plasticity in phenotype and cell fate. These properties, when activated aberrantly, drive fibrotic disorders in humans.

Lung Multipotent Stem Cells of Mesenchymal Nature: Cellular Basis, Clinical Relevance, and Implications for Stem Cell Therapy

Significance: Tissue-resident stem cells are essential for normal organ homeostasis as well as for functional tissue regeneration after severe injury. Herein, mesenchymal stem cells, also designated as mesenchymal stromal cells (MSCs), contribute to the maintenance of organ integrity by their ability to replace dysfunctional cells or secrete cytokines locally and thus support the repair and healing processes of affected tissues. Recent Advances: Besides epithelial stem and progenitor cells, substantial evidence exists that tissue-resident multipotent stem cells of mesenchymal nature also exist in adult human lungs. These lung MSCs may function to regulate pulmonary tissue repair and/or regeneration, inflammation, fibrosis, and tumor formation. Critical Issues: Although therapeutically applied MSCs turned out to be a valuable therapeutic option for the prevention of lung diseases and/or the regeneration of diseased lung tissue, the true function of tissue-resident MSCs within the lung, and identification of their niche, which presumably dictates function, remain elusive. Future Directions: A detailed understanding of lung MSC localization (in the potential vascular stem cell niche) as well as of the signaling pathways controlling stem cell fate is prerequisite to unravel how (i) endogenous MSCs contribute to lung diseases, (ii) exogenous MSCs affect the proliferation of endogenous stem cells to repair damaged tissue, and (iii) a potential on-site manipulation of these cells directly within their endogenous niche could be used for therapeutic benefits. This review focuses on the central role of lung-resident MSCs, which are closely associated with the pulmonary vasculature, in a variety of chronic and acute lung diseases. Antioxid. Redox Signal. 35, 204-216.

Effects of bone morphogenetic proteins on epithelial repair

Epithelial tissue has important functions such as protection, secretion, and sensation. Epithelial damage is involved in various pathological processes. Bone morphogenetic proteins (BMPs) are a class of growth factors with multiple functions. They play important roles in epithelial cells, including in differentiation, proliferation, and migration during the repair of the epithelium. This article reviews the functions and mechanisms of the most profoundly studied BMPs in the process of epithelial damage repair and their clinical significance.

Tempo-spatial regulation of the Wnt pathway by FAM13A modulates the stemness of alveolar epithelial progenitors

BACKGROUND

Family with Sequence Similarity 13, Member A (FAM13A) gene has been consistently associated with COPD by Genome-wide association studies (GWAS). Our previous study demonstrated that FAM13A was mainly expressed in the lung epithelial progenitors including Club cells and alveolar type II epithelial (ATII) cells. Fam13a^-/- mice were resistant to cigarette smoke (CS)-induced emphysema through promoting β-catenin/Wnt activation. Given the important roles of β-catenin/Wnt activation in alveolar regeneration during injury, it is unclear when and where FAM13A regulates the Wnt pathway, the requisite pathway for alveolar epithelial repair, in vivo during CS exposure in lung epithelial progenitors.

METHODS

Fam13a^+/+ or Fam13a^-/- mice were crossed with TCF/Lef:H2B-GFP Wnt-signaling reporter mouse line to indicate β-catenin/Wnt-activated cells labeled with GFP followed by acute (1 month) or chronic (7 months) CS exposure. Fluorescence-activated flow cytometry analysis, immunofluorescence and organoid culture system were performed to identify the β-catenin/Wnt-activated cells in Fam13a^+/+ or Fam13a^-/- mice exposed to CS. Fam13a;SftpcCreERT2;Rosa26RmTmG mouse line, where GFP labels ATII cells, was generated for alveolar organoid culture followed by analyses of organoid number, immunofluorescence and gene expression. Single cell RNA-seq data from COPD ever smokers and nonsmoker control lungs were further analyzed.

FINDINGS

We found that FAM13A-deficiency significantly increased Wnt activation mainly in lung epithelial cells. Consistently, after long-term CS exposure in vivo, FAM13A deficiency bestows alveolar epithelial progenitor cells with enhanced proliferation and differentiation in the ex vivo organoid model. Importantly, expression of FAM13A is significantly increased in human COPD-derived ATII cells compared to healthy ATII cells as suggested by single cell RNA-sequencing data.

INTERPRETATION

Our findings suggest that FAM13A-deficiency promotes the Wnt pathway-mediated ATII cell repair/regeneration, and thereby possibly mitigating CS-induced alveolar destruction. FUND: This project is funded by the National Institutes of Health of United States of America (NIH) grants R01HL127200, R01HL137927, R01HL148667 and R01HL147148 (XZ).

State of the art on lung organoids in mammals

The number and severity of diseases affecting lung development and adult respiratory function have stimulated great interest in developing new in vitro models to study lung in different species. Recent breakthroughs in 3-dimensional (3D) organoid cultures have led to new physiological in vitro models that better mimic the lung than conventional 2D cultures. Lung organoids simulate multiple aspects of the real organ, making them promising and useful models for studying organ development, function and disease (infection, cancer, genetic disease). Due to their dynamics in culture, they can serve as a sustainable source of functional cells (biobanking) and be manipulated genetically. Given the differences between species regarding developmental kinetics, the maturation of the lung at birth, the distribution of the different cell populations along the respiratory tract and species barriers for infectious diseases, there is a need for species-specific lung models capable of mimicking mammal lungs as they are of great interest for animal health and production, following the One Health approach. This paper reviews the latest developments in the growing field of lung organoids.

Mesenchymal growth hormone receptor deficiency leads to failure of alveolar progenitor cell function and severe pulmonary fibrosis

Recent studies have identified impaired type 2 alveolar epithelial cell (ATII) renewal in idiopathic pulmonary fibrosis (IPF) human organoids and severe fibrosis when ATII is defective in mice. ATIIs function as progenitor cells and require supportive signals from the surrounding mesenchymal cells. The mechanisms by which mesenchymal cells promote ATII progenitor functions in lung fibrosis are incompletely understood. We identified growth hormone receptor (GHR) is mainly expressed in mesenchymal cells, and its expression is substantially decreased in IPF lungs. Higher levels of GHR expression correlated with better lung function in patients with IPF. Profibrotic mesenchymal cells retarded ATII growth and were associated with suppressed vesicular GHR expression. Vesicles enriched with Ghr promote ATII proliferation and diminished pulmonary fibrosis in mesenchymal Ghr-deficient mice. Our findings demonstrate a previously unidentified mesenchymal paracrine signaling coordinated by GHR that is capable of supporting ATII progenitor cell renewal and limiting the severity of lung fibrosis.

Hair follicle stem cells as a skin-organizing signaling center during adult homeostasis

Stem cells are the essential source of building blocks for tissue homeostasis and regeneration. Their behavior is dictated by both cell-intrinsic cues and extrinsic cues from the microenvironment, known as the stem cell niche. Interestingly, recent work began to demonstrate that hair follicle stem cells (HFSCs) are not only passive recipients of signals from the surroundings, but also actively send out signals to modulate the organization and function of their own niches. Here, we discuss recent findings, and briefly refer to the old, on the interaction of HFSCs and their niches with the emphasis on the outwards signals from HFSCs toward their niches. We also highlight recent technology advancements that further promote our understanding of HFSC niches. Taken together, the HFSCs emerge as a skin-organizing center rich in signaling output for niche remodeling during various stages of adult skin homeostasis. The intricate crosstalk between HFSCs and their niches adds important insight to skin biology that will inform clinical and bioengineering fields aiming to build complete and functional 3D organotypic cultures for skin replacement therapies.

Understanding Lung Carcinogenesis from a Morphostatic Perspective: Prevention and Therapeutic Potential of Phytochemicals for Targeting Cancer Stem Cells

Lung cancer is still one of the deadliest cancers, with over two million incidences annually. Prevention is regarded as the most efficient way to reduce both the incidence and death figures. Nevertheless, treatment should still be improved, particularly in addressing therapeutic resistance due to cancer stem cells—the assumed drivers of tumor initiation and progression. Phytochemicals in plant-based diets are thought to contribute substantially to lung cancer prevention and may be efficacious for targeting lung cancer stem cells. In this review, we collect recent literature on lung homeostasis, carcinogenesis, and phytochemicals studied in lung cancers. We provide a comprehensive overview of how normal lung tissue operates and relate it with lung carcinogenesis to redefine better targets for lung cancer stem cells. Nine well-studied phytochemical compounds, namely curcumin, resveratrol, quercetin, epigallocatechin-3-gallate, luteolin, sulforaphane, berberine, genistein, and capsaicin, are discussed in terms of their chemopreventive and anticancer mechanisms in lung cancer and potential use in the clinic. How the use of phytochemicals can be improved by structural manipulations, targeted delivery, concentration adjustments, and combinatorial treatments is also highlighted. We propose that lung carcinomas should be treated differently based on their respective cellular origins. Targeting quiescence-inducing, inflammation-dampening, or reactive oxygen species-balancing pathways appears particularly interesting.

Circulating Nestin-GFP+ Cells Participate in the Pathogenesis of Paracoccidioides brasiliensis in the Lungs

Multiple infectious diseases lead to impaired lung function. Revealing the cellular mechanisms involved in this impairment is crucial for the understanding of how the lungs shift from a physiologic to a pathologic state in each specific condition. In this context, we explored the pathogenesis of Paracoccidioidomycosis, which affects pulmonary functioning. The presence of cells expressing Nestin-GFP has been reported in different tissues, and their roles as tissue-specific progenitors have been stablished in particular organs. Here, we explored how Nestin-GFP⁺ cells are affected after lung infection by Paracoccidioides brasiliensis, a model of lung granulomatous inflammation with fibrotic outcome. We used Nestin-GFP transgenic mice, parabiosis surgery, confocal microscopy and flow cytometry to investigate the participation of Nestin-GFP⁺ cells in Paracoccidioides brasiliensis pathogenesis. We revealed that these cells increase in the lungs post-Paracoccidioides brasiliensis infection, accumulating around granulomas. This increase was due mainly to Nestin-GPF⁺ cells derived from the blood circulation, not associated to blood vessels, that co-express markers suggestive of hematopoietic cells (Sca-1, CD45 and CXCR4). Therefore, our findings suggest that circulating Nestin-GFP⁺ cells participate in the Paracoccidioides brasiliensis pathogenesis in the lungs.

The diversity of adult lung epithelial stem cells and their niche in homeostasis and regeneration

The adult lung, a workhorse for gas exchange, is continually subjected to a barrage of assaults from the inhaled particles and pathogens. Hence, homeostatic maintenance is of paramount importance. Epithelial stem cells interact with their particular niche in the adult lung to orchestrate both natural tissue rejuvenation and robust post-injury regeneration. Advances in single-cell sequencing, lineage tracing, and living tissue imaging have deepened our understanding about stem cell heterogeneities, transition states, and specific cell lineage markers. In this review, we provided an overview of the known stem/progenitor cells and their subpopulations in different regions of the adult lung, and explored the regulatory networks in stem cells and their respective niche which collectively coordinated stem cell quiescence and regeneration states. We finally discussed relationships between dysregulated stem cell function and lung disease.

The Cellular and Physiological Basis for Lung Repair and Regeneration: Past, Present, and Future

The respiratory system, which includes the trachea, airways, and distal alveoli, is a complex multi-cellular organ that intimately links with the cardiovascular system to accomplish gas exchange. In this review and as members of the NIH/NHLBI-supported Progenitor Cell Translational Consortium, we discuss key aspects of lung repair and regeneration. We focus on the cellular compositions within functional niches, cell-cell signaling in homeostatic health, the responses to injury, and new methods to study lung repair and regeneration. We also provide future directions for an improved understanding of the cell biology of the respiratory system, as well as new therapeutic avenues.

Resident interstitial lung fibroblasts and their role in alveolar stem cell niche development, homeostasis, injury, and regeneration

Developing, regenerating, and repairing a lung all require interstitial resident fibroblasts (iReFs) to direct the behavior of the epithelial stem cell niche. During lung development, distal lung fibroblasts, in the form of matrix-, myo-, and lipofibroblasts, form the extra cellular matrix (ECM), create tensile strength, and support distal epithelial differentiation, respectively. During de novo septation in a murine pneumonectomy lung regeneration model, developmental processes are reactivated within the iReFs, indicating progenitor function well into adulthood. In contrast to the regenerative activation of fibroblasts upon acute injury, chronic injury results in fibrotic activation. In murine lung fibrosis models, fibroblasts can pathologically differentiate into lineages beyond their normal commitment during homeostasis. In lung injury, recently defined alveolar niche cells support the expansion of alveolar epithelial progenitors to regenerate the epithelium. In human fibrotic lung diseases like bronchopulmonary dysplasia (BPD), idiopathic pulmonary fibrosis (IPF), and chronic obstructive pulmonary disease (COPD), dynamic changes in matrix-, myo-, lipofibroblasts, and alveolar niche cells suggest differential requirements for injury pathogenesis and repair. In this review, we summarize the role of alveolar fibroblasts and their activation stage in alveolar septation and regeneration and incorporate them into the context of human lung disease, discussing fibroblast activation stages and how they contribute to BPD, IPF, and COPD.