PDGF-Ralpha gene expression predicts proliferation, but PDGF-A suppresses transdifferentiation of neonatal mouse lung myofibroblasts

Identification of a myofibroblast differentiation program during neonatal lung development

Alveologenesis is the final stage of lung development in which the internal surface area of the lung is increased to facilitate efficient gas exchange in the mature organism. The first phase of alveologenesis involves the formation of septal ridges (secondary septae) and the second phase involves thinning of the alveolar septa. Within secondary septa, mesenchymal cells include a transient population of alveolar myofibroblasts (MyoFBs) and a stable but poorly described population of lipid-rich cells that have been referred to as lipofibroblasts or matrix fibroblasts (MatFBs). Using a unique Fgf18CreER lineage trace mouse line, cell sorting, single-cell RNA sequencing and primary cell culture, we have identified multiple subtypes of mesenchymal cells in the neonatal lung, including an immature progenitor cell that gives rise to mature MyoFB. We also show that the endogenous and targeted ROSA26 locus serves as a sensitive reporter for MyoFB maturation. These studies identify a MyoFB differentiation program that is distinct from other mesenchymal cell types and increases the known repertoire of mesenchymal cell types in the neonatal lung.

Dedifferentiated early postnatal lung myofibroblasts redifferentiate in adult disease

Alveolarization ensures sufficient lung surface area for gas exchange, and during bulk alveolarization in mice (postnatal day [P] 4.5-14.5), alpha-smooth muscle actin (SMA)+ myofibroblasts accumulate, secrete elastin, and lay down alveolar septum. Herein, we delineate the dynamics of the lineage of early postnatal SMA+ myofibroblasts during and after bulk alveolarization and in response to lung injury. SMA+ lung myofibroblasts first appear at ∼ P2.5 and proliferate robustly. Lineage tracing shows that, at P14.5 and over the next few days, the vast majority of SMA+ myofibroblasts downregulate smooth muscle cell markers and undergo apoptosis. Of note, ∼8% of these dedifferentiated cells and another ∼1% of SMA+ myofibroblasts persist to adulthood. Single cell RNA sequencing analysis of the persistent SMA- cells and SMA+ myofibroblasts in the adult lung reveals distinct gene expression profiles. For instance, dedifferentiated SMA- cells exhibit higher levels of tissue remodeling genes. Most interestingly, these dedifferentiated early postnatal myofibroblasts re-express SMA upon exposure of the adult lung to hypoxia or the pro-fibrotic drug bleomycin. However, unlike during alveolarization, these cells that re-express SMA do not proliferate with hypoxia. In sum, dedifferentiated early postnatal myofibroblasts are a previously undescribed cell type in the adult lung and redifferentiate in response to injury.

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

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

Prenatal FGFR2 Signaling via PI3K/AKT Specifies the PDGFRA+ Myofibroblast

It is well known that FGFR2 (fibroblast growth factor receptor 2) signaling is critical for proper lung development. Recent studies demonstrate that epithelial FGFR2 signaling during the saccular phase of lung development (sacculation) regulates alveolar type 1 (AT1) and AT2 cell differentiation. During sacculation, PDGFRA (platelet-derived growth factor receptor-α)-positive lung fibroblasts exist as three functional subtypes: contractile myofibroblasts, extracellular matrix-producing matrix fibroblasts, and lipofibroblasts. All three subtypes are required during alveolarization to establish a niche that supports AT2 epithelial cell self-renewal and AT1 epithelial cell differentiation. FGFR2 signaling directs myofibroblast differentiation in PDGFRA+ fibroblasts during alveolar reseptation after pneumonectomy. However, it remains unknown if FGFR2 signaling regulates PDGFRA+ myo-, matrix, or lipofibroblast differentiation during sacculation. In this study, FGFR2 signaling was inhibited by temporal expression of a secreted dominant-negative FGFR2b (dnFGFR2) by AT2 cells from embryonic day (E) 16.5 to E18.5. Fibroblast and epithelial differentiation were analyzed at E18.5 and postnatal days 7 and 21. At all time points, the number of myofibroblasts was reduced and the number of lipo-/matrix fibroblasts was increased. AT2 cells are increased and AT1 cells are reduced postnatally, but not at E18.5. Similarly, in organoids made with PDGFRA+ fibroblasts from dnFGFR2 lungs, increased AT2 cells and reduced AT1 cells were observed. In vitro treatment of primary wild-type E16.5 adherent saccular lung fibroblasts with recombinant dnFGFR2b/c resulted in reduced myofibroblast contraction. Treatment with the PI3K/AKT activator 740 Y-P rescued the lack of myofibroblast differentiation caused by dnFGFR2b/2c. Moreover, treatment with the PI3K/AKT activator 740 Y-P rescued myofibroblast differentiation in E18.5 fibroblasts isolated from dnFGFR2 lungs.

Identification of a myofibroblast differentiation program during neonatal lung development

Alveologenesis is the final stage of lung development in which the internal surface area of the lung is increased to facilitate efficient gas exchange in the mature organism. The first phase of alveologenesis involves the formation of septal ridges (secondary septae) and the second phase involves thinning of the alveolar septa. Within secondary septa, mesenchymal cells include a transient population of alveolar myofibroblasts (MyoFB) and a stable but poorly described population of lipid rich cells that have been referred to as lipofibroblasts or matrix fibroblasts (MatFB). Using a unique Fgf18CreER lineage trace mouse line, cell sorting, single cell RNA sequencing, and primary cell culture, we have identified multiple subtypes of mesenchymal cells in the neonatal lung, including an immature progenitor cell that gives rise to mature MyoFB. We also show that the endogenous and targeted ROSA26 locus serves as a sensitive reporter for MyoFB maturation. These studies identify a myofibroblast differentiation program that is distinct form other mesenchymal cells types and increases the known repertoire of mesenchymal cell types in the neonatal lung.

Impaired Myofibroblast Proliferation is a Central Feature of Pathologic Post-Natal Alveolar Simplification

Premature infants with bronchopulmonary dysplasia (BPD) have impaired alveolar gas exchange due to alveolar simplification and dysmorphic pulmonary vasculature. Advances in clinical care have improved survival for infants with BPD, but the overall incidence of BPD remains unchanged because we lack specific therapies to prevent this disease. Recent work has suggested a role for increased transforming growth factor-beta (TGFβ) signaling and myofibroblast populations in BPD pathogenesis, but the functional significance of each remains unclear. Here, we utilize multiple murine models of alveolar simplification and comparative single-cell RNA sequencing to identify shared mechanisms that could contribute to BPD pathogenesis. Single-cell RNA sequencing reveals a profound loss of myofibroblasts in two models of BPD and identifies gene expression signatures of increased TGFβ signaling, cell cycle arrest, and impaired proliferation in myofibroblasts. Using pharmacologic and genetic approaches, we find no evidence that increased TGFβ signaling in the lung mesenchyme contributes to alveolar simplification. In contrast, this is likely a failed compensatory response, since none of our approaches to inhibit TGFb signaling protect mice from alveolar simplification due to hyperoxia while several make simplification worse. In contrast, we find that impaired myofibroblast proliferation is a central feature in several murine models of BPD, and we show that inhibiting myofibroblast proliferation is sufficient to cause pathologic alveolar simplification. Our results underscore the importance of impaired myofibroblast proliferation as a central feature of alveolar simplification and suggest that efforts to reverse this process could have therapeutic value in BPD.

Hedgehog and Platelet-derived Growth Factor Signaling Intersect during Postnatal Lung Development

Normal lung development critically depends on HH (Hedgehog) and PDGF (platelet-derived growth factor) signaling, which coordinate mesenchymal differentiation and proliferation. PDGF signaling is required for postnatal alveolar septum formation by myofibroblasts. Recently, we demonstrated a requirement for HH in postnatal lung development involving alveolar myofibroblast differentiation. Given shared features of HH signaling and PDGF signaling and their impact on this key cell type, we sought to clarify their relationship during murine postnatal lung development. Timed experiments revealed that HH inhibition phenocopies the key lung myofibroblast phenotypes of Pdgfa (platelet-derived growth factor subunit A) and Pdgfra (platelet-derived growth factor receptor alpha) knockouts during secondary alveolar septation. Using a dual signaling reporter, Gli1lZ;PdgfraEGFP, we show that HH and PDGF pathway intermediates are concurrently expressed during alveolar septal myofibroblast accumulation, suggesting pathway convergence in the generation of lung myofibroblasts. Consistent with this hypothesis, HH inhibition reduces Pdgfra expression and diminishes the number of Pdgfra-positive and Pdgfra-lineage cells in postnatal lungs. Bulk RNA sequencing data of Pdgfra-expressing cells from Postnatal Day 8 (P8) lungs show that HH inhibition alters the expression not only of well-established HH targets but also of several putative PDGF target genes. This, together with the presence of Gli-binding sites in PDGF target genes, suggests HH input into PDGF signaling. We identified these HH/PDGF targets in several postnatal lung mesenchymal cell populations, including myofibroblasts, using single-cell transcriptomic analysis. Collectively, our data indicate that HH signaling and PDGF signaling intersect to support myofibroblast/fibroblast function during secondary alveolar septum formation. Moreover, they provide a molecular foundation relevant to perinatal lung diseases associated with impaired alveolarization.

Maladaptive functional changes in alveolar fibroblasts due to perinatal hyperoxia impair epithelial differentiation

Infants born prematurely worldwide have up to a 50% chance of developing bronchopulmonary dysplasia (BPD), a clinical morbidity characterized by dysregulated lung alveolarization and microvascular development. It is known that PDGFR alpha-positive (PDGFRA+) fibroblasts are critical for alveolarization and that PDGFRA+ fibroblasts are reduced in BPD. A better understanding of fibroblast heterogeneity and functional activation status during pathogenesis is required to develop mesenchymal population-targeted therapies for BPD. In this study, we utilized a neonatal hyperoxia mouse model (90% O2 postnatal days 0-7, PN0-PN7) and performed studies on sorted PDGFRA+ cells during injury and room air recovery. After hyperoxia injury, PDGFRA+ matrix and myofibroblasts decreased and PDGFRA+ lipofibroblasts increased by transcriptional signature and population size. PDGFRA+ matrix and myofibroblasts recovered during repair (PN10). After 7 days of in vivo hyperoxia, PDGFRA+ sorted fibroblasts had reduced contractility in vitro, reflecting loss of myofibroblast commitment. Organoids made with PN7 PDGFRA+ fibroblasts from hyperoxia in mice exhibited reduced alveolar type 1 cell differentiation, suggesting reduced alveolar niche-supporting PDGFRA+ matrix fibroblast function. Pathway analysis predicted reduced WNT signaling in hyperoxia fibroblasts. In alveolar organoids from hyperoxia-exposed fibroblasts, WNT activation by CHIR increased the size and number of alveolar organoids and enhanced alveolar type 2 cell differentiation.

Lysyl oxidase regulation and protein aldehydes in the injured newborn lung

During newborn lung injury, excessive activity of lysyl oxidases (LOXs) disrupts extracellular matrix (ECM) formation. Previous studies indicate that TGFβ activation in the O₂-injured mouse pup lung increases lysyl oxidase (LOX) expression. But how TGFβ regulates this, and whether the LOXs generate excess pulmonary aldehydes are unknown. First, we determined that O₂-mediated lung injury increases LOX protein expression in TGFβ-stimulated pup lung interstitial fibroblasts. This regulation appeared to be direct; this is because TGFβ treatment also increased LOX protein expression in isolated pup lung fibroblasts. Then using a fibroblast cell line, we determined that TGFβ stimulates LOX expression at a transcriptional level via Smad2/3-dependent signaling. LOX is translated as a pro-protein that requires secretion and extracellular cleavage before assuming amine oxidase activity and, in some cells, reuptake with nuclear localization. We found that pro-LOX is processed in the newborn mouse pup lung. Also, O₂-mediated injury was determined to increase pro-LOX secretion and nuclear LOX immunoreactivity particularly in areas populated with interstitial fibroblasts and exhibiting malformed ECM. Then, using molecular probes, we detected increased aldehyde levels in vivo in O₂-injured pup lungs, which mapped to areas of increased pro-LOX secretion in lung sections. Increased activity of LOXs plays a critical role in the aldehyde generation; an inhibitor of LOXs prevented the elevation of aldehydes in the O₂-injured pup lung. These results reveal new mechanisms of TGFβ and LOX in newborn lung disease and suggest that aldehyde-reactive probes might have utility in sensing the activation of LOXs in vivo during lung injury.

Pretreatment of aged mice with retinoic acid supports alveolar regeneration via upregulation of reciprocal PDGFA signalling

OBJECTIVES

Idiopathic pulmonary fibrosis (IPF) primarily affects the aged population and is characterised by failure of alveolar regeneration, leading to loss of alveolar type 1 (AT1) cells. Aged mouse models of lung repair have demonstrated that regeneration fails with increased age. Mouse and rat lung repair models have shown retinoic acid (RA) treatment can restore alveolar regeneration. Herein, we seek to determine the signalling mechanisms that become activated on RA treatment prior to injury, which support alveolar differentiation.

DESIGN

Partial pneumonectomy lung injury model and next-generation sequencing of sorted cell populations were used to uncover molecular targets regulating alveolar repair. In vitro organoids generated from epithelial cells of mouse or patient with IPF co-cultured with young, aged or RA-pretreated murine fibroblasts were used to test potential targets.

MAIN OUTCOME MEASUREMENTS

Known alveolar epithelial cell differentiation markers, including HOPX and AGER for AT1 cells, were used to assess outcome of treatments.

RESULTS

Gene expression analysis of sorted fibroblasts and epithelial cells isolated from lungs of young, aged and RA-pretreated aged mice predicted increased platelet-derived growth factor subunit A (PDGFA) signalling that coincided with regeneration and alveolar epithelial differentiation. Addition of PDGFA induced AT1 and AT2 differentiation in both mouse and human IPF lung organoids generated with aged fibroblasts, and PDGFA monoclonal antibody blocked AT1 cell differentiation in organoids generated with young murine fibroblasts.

CONCLUSIONS

Our data support the concept that RA indirectly induces reciprocal PDGFA signalling, which activates regenerative fibroblasts that support alveolar epithelial cell differentiation and repair, providing a potential therapeutic strategy to influence the pathogenesis of IPF.

Extreme Diversity of the Human Vascular Mesenchymal Cell Landscape

Background Human mesenchymal cells are culprit factors in vascular (patho)physiology and are hallmarked by phenotypic and functional heterogeneity. At present, they are subdivided by classic umbrella terms, such as "fibroblasts," "myofibroblasts," "smooth muscle cells," "fibrocytes," "mesangial cells," and "pericytes." However, a discriminative marker-based subclassification has to date not been established. Methods and Results As a first effort toward a classification scheme, a systematic literature search was performed to identify the most commonly used phenotypical and functional protein markers for characterizing and classifying vascular mesenchymal cell subpopulation(s). We next applied immunohistochemistry and immunofluorescence to inventory the expression pattern of identified markers on human aorta specimens representing early, intermediate, and end stages of human atherosclerotic disease. Included markers comprise markers for mesenchymal lineage (vimentin, FSP-1 [fibroblast-specific protein-1]/S100A4, cluster of differentiation (CD) 90/thymocyte differentiation antigen 1, and FAP [fibroblast activation protein]), contractile/non-contractile phenotype (α-smooth muscle actin, smooth muscle myosin heavy chain, and nonmuscle myosin heavy chain), and auxiliary contractile markers (h1-Calponin, h-Caldesmon, Desmin, SM22α [smooth muscle protein 22α], non-muscle myosin heavy chain, smooth muscle myosin heavy chain, Smoothelin-B, α-Tropomyosin, and Telokin) or adhesion proteins (Paxillin and Vinculin). Vimentin classified as the most inclusive lineage marker. Subset markers did not separate along classic lines of smooth muscle cell, myofibroblast, or fibroblast, but showed clear temporal and spatial diversity. Strong indications were found for presence of stem cells/Endothelial-to-Mesenchymal cell Transition and fibrocytes in specific aspects of the human atherosclerotic process. Conclusions This systematic evaluation shows a highly diverse and dynamic landscape for the human vascular mesenchymal cell population that is not captured by the classic nomenclature. Our observations stress the need for a consensus multiparameter subclass designation along the lines of the cluster of differentiation classification for leucocytes.

The elephant in the lung: Integrating lineage-tracing, molecular markers, and single cell sequencing data to identify distinct fibroblast populations during lung development and regeneration

During lung development, the mesenchyme and epithelium are dependent on each other for instructive morphogenic cues that direct proliferation, cellular differentiation and organogenesis. Specification of epithelial and mesenchymal cell lineages occurs in parallel, forming cellular subtypes that guide the formation of both transitional developmental structures and the permanent architecture of the adult lung. While epithelial cell types and lineages have been relatively well-defined in recent years, the definition of mesenchymal cell types and lineage relationships has been more challenging. Transgenic mouse lines with permanent and inducible lineage tracers have been instrumental in identifying lineage relationships among epithelial progenitor cells and their differentiation into distinct airway and alveolar epithelial cells. Lineage tracing experiments with reporter mice used to identify fibroblast progenitors and their lineage trajectories have been limited by the number of cell specific genes and the developmental timepoint when the lineage trace was activated. In this review, we discuss major developmental mesenchymal lineages, focusing on time of origin, major cell type, and other lineage derivatives, as well as the transgenic tools used to find and define them. We describe lung fibroblasts using function, location, and molecular markers in order to compare and contrast cells with similar functions. The temporal and cell-type specific expression of fourteen "fibroblast lineage" genes were identified in single-cell RNA-sequencing data from LungMAP in the LGEA database. Using these lineage signature genes as guides, we clustered murine lung fibroblast populations from embryonic day 16.5 to postnatal day 28 (E16.5-PN28) and generated heatmaps to illustrate expression of transcription factors, signaling receptors and ligands in a temporal and population specific manner.

Single-Cell Deconvolution of Fibroblast Heterogeneity in Mouse Pulmonary Fibrosis

Fibroblast heterogeneity has long been recognized in mouse and human lungs, homeostasis, and disease states. However, there is no common consensus on fibroblast subtypes, lineages, biological properties, signaling, and plasticity, which severely hampers our understanding of the mechanisms of fibrosis. To comprehensively classify fibroblast populations in the lung using an unbiased approach, single-cell RNA sequencing was performed with mesenchymal preparations from either uninjured or bleomycin-treated mouse lungs. Single-cell transcriptome analyses classified and defined six mesenchymal cell types in normal lung and seven in fibrotic lung. Furthermore, delineation of their differentiation trajectory was achieved by a machine learning method. This collection of single-cell transcriptomes and the distinct classification of fibroblast subsets provide a new resource for understanding the fibroblast landscape and the roles of fibroblasts in fibrotic diseases.

Secondary crest myofibroblast PDGFRα controls the elastogenesis pathway via a secondary tier of signaling networks during alveologenesis

Postnatal alveolar formation is the most important and the least understood phase of lung development. Alveolar pathologies are prominent in neonatal and adult lung diseases. The mechanisms of alveologenesis remain largely unknown. We inactivated Pdgfra postnatally in secondary crest myofibroblasts (SCMF), a subpopulation of lung mesenchymal cells. Lack of Pdgfra arrested alveologenesis akin to bronchopulmonary dysplasia (BPD), a neonatal chronic lung disease. The transcriptome of mutant SCMF revealed 1808 altered genes encoding transcription factors, signaling and extracellular matrix molecules. Elastin mRNA was reduced, and its distribution was abnormal. Absence of Pdgfra disrupted expression of elastogenic genes, including members of the Lox, Fbn and Fbln families. Expression of EGF family members increased when Tgfb1 was repressed in mouse. Similar, but not identical, results were found in human BPD lung samples. In vitro, blocking PDGF signaling decreased elastogenic gene expression associated with increased Egf and decreased Tgfb family mRNAs. The effect was reversible by inhibiting EGF or activating TGFβ signaling. These observations demonstrate the previously unappreciated postnatal role of PDGFA/PDGFRα in controlling elastogenic gene expression via a secondary tier of signaling networks composed of EGF and TGFβ.

Bronchiolitis Obliterans and Pulmonary Fibrosis after Sulfur Mustard Inhalation in Rats

Inhalation of powerful chemical agents, such as sulfur mustard (SM), can have debilitating pulmonary consequences, such as bronchiolitis obliterans (BO) and parenchymal fibrosis (PF). The underlying pathogenesis of disorders after SM inhalation is not clearly understood, resulting in a paucity of effective therapies. In this study, we evaluated the role of profibrotic pathways involving transforming growth factor-β (TGF-β) and platelet-derived growth factor (PDGF) in the development of BO and PF after SM inhalation injury using a rat model. Adult Sprague-Dawley rats were intubated and exposed to SM (1.0 mg/kg), then monitored daily for respiratory distress, oxygen saturation changes, and weight loss. Rats were killed at 7, 14, 21, or 28 days, and markers of injury were determined by histopathology; pulmonary function testing; and assessment of TGF-β, PDGF, and PAI-1 concentrations. Respiratory distress developed over time after SM inhalation, with progressive hypoxemia, respiratory distress, and weight loss. Histopathology confirmed the presence of both BO and PF, and both gradually worsened with time. Pulmonary function testing demonstrated a time-dependent increase in lung resistance, as well as a decrease in lung compliance. Concentrations of TGF-β, PDGF, and PAI-1 were elevated at 28 days in lung, BAL fluid, and/or plasma. Time-dependent development of BO and PF occurs in lungs of rats exposed to SM inhalation, and the elevated concentrations of TGF-β, PDGF, and PAI-1 suggest involvement of these profibrotic pathways in the aberrant remodeling after injury.

Pdgfra marks a cellular lineage with distinct contributions to myofibroblasts in lung maturation and injury response

Pdgfra-expressing (Pdgfra+) cells have been implicated as progenitors in many mesenchymal tissues. To determine lineage potential, we generated Pdgfra^rtTA knockin mice using CRISPR/Cas9. During lung maturation, counter to a prior study reporting that Pdgfra+ cells give rise equally to myofibroblasts and lipofibroblasts, lineage tracing using Pdgfra^rtTA;tetO-cre mice indicated that ~95% of the lineaged cells are myofibroblasts. Genetic ablation of Pdgfra⁺cells using Pdgfra^rtTA-driven diphtheria toxin (DTA) led to alveolar simplification, demonstrating that these cells are essential for building the gas exchange surface area. In the adult bleomycin model of lung fibrosis, lineaged cells increased to contribute to pathological myofibroblasts. In contrast, in a neonatal hyperoxia model of bronchopulmonary dysplasia (BPD), lineaged cells decreased and do not substantially contribute to pathological myofibroblasts. Our findings revealed complexity in the behavior of the Pdgfra-lineaged cells as exemplified by their distinct contributions to myofibroblasts in normal maturation, BPD and adult fibrosis.

[Regulatory role of Shh signaling pathway in lung development in fetal mice]

OBJECTIVE

To investigate the regulatory role of classical Shh signaling pathway in the development of the epithelium and mesenchyme (bronchial cartilage and smooth muscles) during lung development in fetal mice.

METHODS

Immunohistochemical technique was used to detect the expression of Shh signaling pathway receptor Smo and Pdgfr-α in murine fetal lungs to explore the spatial and temporal characteristics of their expression. Based on the interstitial specificity of Pdgfr-α expression, we constructed a Pdgfr-α-cre to establish a E12.5 - E16.5 transgenic mice with specific knockout of the key Shh signaling molecule Smo in the pulmonary interstitium with tamoxifen induction. Immunofluorescence technique was used to observe the epithelium and mesenchyme (bronchial cartilage and smooth muscle) during fetal lung development in the transgenic mice to assess the role of Shh signaling pathway in the epithelial-to-mesenchymal (EMT) transition during the lung development.

RESULTS

Smo was highly expressed in the epithelial and stromal lung tissues in the pseudoglandular stage and was gradually lowered over time with its distribution mainly in the interstitial tissues. Pdgfr-α was enriched in the distal lung epithelial and mesenchy tissues in early embryonic lungs and gradually migrated to the proximal stroma until becoming concentrated around the main bronchial proximal stroma. We successfully specific established mouse models of specific mesenchymal Smo knockout. Compared with the control group, the transgenic mice during E12.5-E16.5 showed significantly reduced lung the volume and bronchial branching with also decreased expression of the proximal epithelial P63 (P<0.05). The transgenic mice exhibited alterations in the expression of α-smooth muscle actin with delayed bronchial cartilage development and decreased expression of mucoprotein.

CONCLUSION

The temporospatial specific expression of Shh signaling pathway plays an important role in developmental regulation of mouse embryonic lung epithelium and mesenchyme (bronchial cartilage and smooth muscle).

Neuropilin-1 and platelet-derived growth factor receptors cooperatively regulate intermediate filaments and mesenchymal cell migration during alveolar septation

Generation of secondary alveolar septa occurs primarily after birth in humans and is complete in mice postnatally, when mechanical stresses vary as air space pressure oscillates. Alveolar mesenchymal cells deposit elastic fibers, which limit cell strain; although when the elastic fiber network is incomplete, this function is also served by the intracellular cytoskeleton. Intermediate filament proteins support deformation during cell division and migration, which occur during septal elongation. Because platelet-derived growth factor receptor-α (PDGFRα) signaling is essential for alveolar septation, we hypothesized that neuropilin-1 (NRP1) may link PDGFRα to cytoskeletal deformation. During cell migration, NRP1 links receptor tyrosine kinase signaling to cytoskeletal and focal adhesion remodeling. Therefore, we examined the consequences of nrp1 gene deletion in alveolar mesenchymal cells (myofibroblasts and pericytes). NRP1 depletion reduced the proportion of mesenchymal cells that contain nestin and desmin within the subpopulation that lacked PDGFRα but contained PDGFRβ. Desmin was reduced at alveolar entry rings, air spaces were enlarged, and surface area was reduced after NRP1 depletion. PDGFRα and NRP1 colocalized to membrane lipid rafts, which are known to contain Src kinase. NRP1 depletion reduced alveolar mesenchymal cell migration and PDGF-A-mediated activation of Src kinase, which may limit accumulation of desmin at septal tips (alveolar entry rings). Cooperation between NRP1 and PDGF signaling is required for secondary septation, and manipulation of NRP1 could promote alveolar regeneration without producing fibrosis.

Use of multitarget tyrosine kinase inhibitors to attenuate platelet-derived growth factor signalling in lung disease

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) play a fundamental role in the embryonic development of the lung. Aberrant PDGF signalling has been documented convincingly in a large variety of pulmonary diseases, including idiopathic pulmonary arterial hypertension, lung cancer and lung fibrosis. Targeting PDGF signalling has been proven to be effective in these diseases. In clinical practice, the most effective way to block PDGF signalling is to inhibit the activity of the intracellular PDGFR kinases. Although the mechanism of action of such drugs is not specific for PDGF signalling, the medications have a broad therapeutic index that allows clinical use. The safety profile and therapeutic opportunities of these and future medications that target PDGFs and PDGFRs are reviewed.

An increasing role for PDGF signalling inhibitors in clinical trials for the treatment of various pulmonary diseaseshttp://ow.ly/buaI30f9HcN

Platelet-derived growth factor receptor-α and Ras-related C3 botulinum toxin substrate-1 regulate mechano-responsiveness of lung fibroblasts

Platelet-derived growth factor (PDGF)-A, which only signals through PDGF-receptor-α (PDGFR-α), is required for secondary alveolar septal formation. Although PDGFR-α distinguishes mesenchymal progenitor cells during the saccular stage, PDGFR-α-expressing alveolar cells persist through adulthood. PDGF-A sustains proliferation, limits apoptosis, and maintains α-smooth muscle actin (α-SMA)-containing alveolar cells, which congregate at the alveolar entry ring at postnatal day (P)12. PDGFR-α-expressing, α-SMA-containing alveolar cells redistribute in the elongating septum, suggesting that they migrate to the alveolar entry rings, where mechanical tension is higher. We hypothesized that PDGFR-α and Ras-related C3 botulinum toxin substrate 1(Rac1) are required for mechanosensitive myofibroblast migration. Spreading of PDGFR-α-deficient lung fibroblasts was insensitive to increased rigidity, and their migration was not reduced by Rac1-guanine exchange factor (GEF)-inhibition. PDGFR-α-expressing fibroblasts migrated toward stiffer regions within two-dimensional substrates by increasing migrational persistence (durotaxis). Using a Förster resonance energy transfer (FRET) biosensor for Rac1-GTP, we observed that PDGFR-α was required for fibroblast Rac1 responsiveness to stiffness within a three-dimensional collagen substrate, which by itself increased Rac1-FRET. Rho-GTPase stabilized, whereas Rac1-GTPase increased the turnover of focal adhesions. Under conditions that increased Rac1-GTP, PDGFR-α signaled through both phosphoinositide-3-kinase (PIK) or Src to engage the Rac1 GEF dedicator of cytokinesis-1 (Dock180) and p21-activated-kinase interacting exchange factor-β (βPIX). In cooperation with collagen fibers, these signaling pathways may guide fibroblasts toward the more rigid alveolar entry ring during secondary septation. Because emphysema and interstitial fibrosis disrupt the parenchymal mechanical continuum, understanding how mechanical factors regulate fibroblast migration could elicit strategies for alveolar repair and regeneration.

Temporal, spatial, and phenotypical changes of PDGFRα expressing fibroblasts during late lung development

Many studies have investigated the source and role of epithelial progenitors during lung development; such information is limited for fibroblast populations and their complex role in the developing lung. In this study, we characterized the spatial location, mRNA expression and Immunophenotyping of PDGFRα⁺ fibroblasts during sacculation and alveolarization. Confocal microscopy identified spatial association of PDGFRα expressing fibroblasts with proximal epithelial cells of the branching bronchioles and the dilating acinar tubules at E16.5; with distal terminal saccules at E18.5; and with alveolar epithelial cells at PN7 and PN28. Immunohistochemistry for alpha smooth muscle actin revealed that PDGFRα⁺ fibroblasts contribute to proximal peribronchiolar smooth muscle at E16.5 and to transient distal alveolar myofibroblasts at PN7. Time series RNA-Seq analyses of PDGFRα⁺ fibroblasts identified differentially expressed genes that, based on gene expression similarity were clustered into 7 major gene expression profile patterns. The presence of myofibroblast and smooth muscle precursors at E16.5 and PN7 was reflected by a two-peak gene expression profile on these days and gene ontology enrichment in muscle contraction. Additional molecular and functional differences between peribronchiolar smooth muscle cells at E16.5 and transient intraseptal myofibroblasts at PN7 were suggested by a single peak in gene expression at PN7 with functional enrichment in cell projection and muscle cell differentiation. Immunophenotyping of subsets of PDGFRα⁺ fibroblasts by flow cytometry confirmed the predicted increase in proliferation at E16.5 and PN7, and identified subsets of CD29⁺ myofibroblasts and CD34⁺ lipofibroblasts. These data can be further mined to develop novel hypotheses and valuable understanding of the molecular and cellular basis of alveolarization.

Understanding the developmental pathways pulmonary fibroblasts may follow during alveolar regeneration

Although pulmonary alveolar interstitial fibroblasts are less specialized than their epithelial and endothelial neighbors, they play essential roles during development and in response to lung injury. At birth, they must adapt to the sudden mechanical changes imposed by the onset of respiration and to a higher ambient oxygen concentration. In diseases such as bronchopulmonary dysplasia and interstitial fibrosis, their adaptive responses are overwhelmed leading to compromised gas-exchange function. Thus, although fibroblasts do not directly participate in gas-exchange, they are essential for creating and maintaining an optimal environment at the alveolar epithelial-endothelial interface. This review summarizes new information and concepts about the ontogeny differentiation, and function of alveolar fibroblasts. Alveolar development will be emphasized, because the development of strategies to evoke alveolar repair and regeneration hinges on thoroughly understanding the way that resident fibroblasts populate specific locations in which extracellular matrix must be produced and remodeled. Other recent reviews have described the disruption that diseases cause to the fibroblast niche and so my objective is to illustrate how the unique developmental origins and differentiation pathways could be harnessed favorably to augment certain fibroblast subpopulations and to optimize the conditions for alveolar regeneration.

Genetic tools for identifying and manipulating fibroblasts in the mouse

The use of mouse genetic tools to track and manipulate fibroblasts has provided invaluable in vivo information regarding the activities of these cells. Recently, many new mouse strains have been described for the specific purpose of studying fibroblast behavior. Colorimetric reporter mice and lines expressing Cre are available for the study of fibroblasts in the organs prone to fibrosis, including heart, kidney, liver, lung, and skeletal muscle. In this review we summarize the current state of the models that have been used to define tissue resident fibroblast populations. While these complex genetic reagents provide unique insights into the process of fibrosis, they also require a thorough understanding of the caveats and limitations. Here, we discuss the specificity and efficiency of the available genetic models and briefly describe how they have been used to document the mechanisms of fibrosis.

Diversity of Interstitial Lung Fibroblasts Is Regulated by Platelet-Derived Growth Factor Receptor α Kinase Activity

Epithelial-mesenchymal cell interactions and factors that control normal lung development are key players in lung injury, repair, and fibrosis. A number of studies have investigated the roles and sources of epithelial progenitors during lung regeneration; such information, however, is limited in lung fibroblasts. Thus, understanding the origin, phenotype, and roles of fibroblast progenitors in lung development, repair, and regeneration helps address these limitations. Using a combination of platelet-derived growth factor receptor α-green fluorescent protein (PDGFRα-GFP) reporter mice, microarray, real-time polymerase chain reaction, flow cytometry, and immunofluorescence, we characterized two distinct interstitial resident fibroblasts, myo- and matrix fibroblasts, and identified a role for PDGFRα kinase activity in regulating their activation during lung regeneration. Transcriptional profiling of the two populations revealed a myo- and matrix fibroblast gene signature. Differences in proliferation, smooth muscle actin induction, and lipid content in the two subpopulations of PDGFRα-expressing fibroblasts during alveolar regeneration were observed. Although CD140α(+)CD29(+) cells behaved as myofibroblasts, CD140α(+)CD34(+) appeared as matrix and/or lipofibroblasts. Gain or loss of PDGFRα kinase activity using the inhibitor nilotinib and a dominant-active PDGFRα-D842V mutation revealed that PDGFRα was important for matrix fibroblast differentiation. We demonstrated that PDGFRα signaling promotes alveolar septation by regulating fibroblast activation and matrix fibroblast differentiation, whereas myofibroblast differentiation was largely PDGFRα independent. These studies provide evidence for the phenotypic and functional diversity as well as the extent of specificity of interstitial resident fibroblasts differentiation during regeneration after partial pneumonectomy.

Type IV collagen drives alveolar epithelial-endothelial association and the morphogenetic movements of septation

BACKGROUND

Type IV collagen is the main component of the basement membrane that gives strength to the blood-gas barrier (BGB). In mammals, the formation of a mature BGB occurs primarily after birth during alveologenesis and requires the formation of septa from the walls of the saccule. In contrast, in avians, the formation of the BGB occurs rapidly and prior to hatching. Mutation in basement membrane components results in an abnormal alveolar phenotype; however, the specific role of type IV collagen in regulating alveologenesis remains unknown.

RESULTS

We have performed a microarray expression analysis in late chick lung development and found that COL4A1 and COL4A2 were among the most significantly upregulated genes during the formation of the avian BGB. Using mouse models, we discovered that mutations in murine Col4a1 and Col4a2 genes affected the balance between lung epithelial progenitors and differentiated cells. Mutations in Col4a1 derived from the vascular component were sufficient to cause defects in vascular development and the BGB. We also show that Col4a1 and Col4a2 mutants displayed disrupted myofibroblast proliferation, differentiation and migration. Lastly, we revealed that addition of type IV collagen protein induced myofibroblast proliferation and migration in monolayer culture and increased the formation of mesenchymal-epithelial septal-like structures in co-culture.

CONCLUSIONS

Our study showed that type IV collagen and, therefore the basement membrane, play fundamental roles in coordinating alveolar morphogenesis. In addition to its role in the formation of epithelium and vasculature, type IV collagen appears to be key for alveolar myofibroblast development by inducing their proliferation, differentiation and migration throughout the developing septum.

A three-dimensional study of alveologenesis in mouse lung

Alveologenesis is the final step of lung maturation, which subdivides the alveolar region of the lung into smaller units called alveoli. Each of the nascent dividers serves as a new gas-exchange surface, and collectively they drastically increase the surface area for breathing. Disruption of alveologenesis results in simplification of alveoli, as is seen in premature infants diagnosed with bronchopulmonary dysplasia (BPD), a prevalent lung disease that is often associated with lifelong breathing deficiencies. To date, a majority of studies of alveologenesis rely on two-dimensional (2D) analysis of tissue sections. Given that an overarching theme of alveologenesis is thinning and extension of the epithelium and mesenchyme to facilitate gas exchange, often only a small portion of a cell or a cellular structure is represented in a single 2D plane. Here, we use a three-dimensional (3D) approach to examine the structural architecture and cellular composition of myofibroblasts, alveolar type 2 cells, elastin and lipid droplets in normal as well as BPD-like mouse lung. We found that 2D finger-like septal crests, commonly used to depict growing alveolar septae, are often artifacts of sectioning through fully established alveolar walls. Instead, a more accurate representation of growing septae are 3D ridges that are lined by platelet-derived growth factor receptor alpha (PDGFRA) and alpha smooth muscle actin (α-SMA)-expressing myofibroblasts, as well as the elastin fibers that they produce. Accordingly in 3D, both α-SMA and elastin were each found in connected networks underlying the 3D septal ridges rather than as isolated dots at the tip of 2D septal crests. Analysis through representative stages of alveologenesis revealed unappreciated dynamic changes in these patterns. PDGFRA-expressing cells are only α-SMA-positive during the first phase of alveologenesis, but not in the second phase, suggesting that the two phases of septae formation may be driven by distinct mechanisms. Thin elastin fibers are already present in the alveolar region prior to alveologenesis, suggesting that during alveologenesis, there is not only new elastin deposition, but also extensive remodeling to transform thin and uniformly distributed fibers into thick cables that rim the nascent septae. Analysis of several genetic as well as hyperoxia-induced models of BPD revealed that the myofibroblast organization is perturbed in all, regardless of whether the origin of defect is epithelial, mesenchymal, endothelial or environmental. Finally, analysis of relative position of PDGFRA-positive cells and alveolar type 2 cells reveal that during alveologenesis, these two cell types are not always adjacent to one another. This result suggests that the niche and progenitor relationship afforded by their close juxtaposition in the adult lung may be a later acquired property. These insights revealed by 3D reconstruction of the septae set the foundation for future investigations of the mechanisms driving normal alveologenesis, as well as causes of alveolar simplification in BPD.

Divergent fibroblast growth factor signaling pathways in lung fibroblast subsets: where do we go from here?

Lung fibroblasts play a key role in postnatal lung development, namely, the formation of the alveolar gas exchange units, through the process of secondary septation. Although evidence initially highlighted roles for fibroblasts in the production and remodeling of the lung extracellular matrix, more recent studies have described the presence of different fibroblast subsets in the developing lung. These subsets include myofibroblasts and lipofibroblasts and their precursors. These cells are believed to play different roles in alveologenesis and are localized to different regions of the developing septa. The precise roles played by these different fibroblast subsets remain unclear. Understanding the signaling pathways that control the discrete functions of these fibroblast subsets would help to clarify the roles and the regulation of lung fibroblasts during lung development. Here, we critically evaluate a recent report that described divergent fibroblast growth factor (FGF) signaling pathways in two different subsets of lung fibroblasts that express different levels of green fluorescent protein (GFP) driven by the platelet-derived growth factor receptor-α promoter. The GFP expression was used as a surrogate for lipofibroblasts (GFP(low)) and myofibroblasts (GFP(high)). It was suggested that Fgf10/Fgf1 and Fgf18/Fgfr3 autocrine pathways may be operative in GFP(low) and GFP(high) cells, respectively, and that these pathways might regulate the proliferation and migration of different fibroblast subsets during alveologenesis. These observations lay important groundwork for the further exploration of FGF function during normal lung development, as well as in aberrant lung development associated with bronchopulmonary dysplasia.

Characterization of the platelet-derived growth factor receptor-α-positive cell lineage during murine late lung development

A reduced number of alveoli is the structural hallmark of diseases of the neonatal and adult lung, where alveoli either fail to develop (as in bronchopulmonary dysplasia), or are progressively destroyed (as in chronic obstructive pulmonary disease). To correct the loss of alveolar septa through therapeutic regeneration, the mechanisms of septa formation must first be understood. The present study characterized platelet-derived growth factor receptor-α-positive (PDGFRα(+)) cell populations during late lung development in mice. PDGFRα(+) cells (detected using a PDGFRα(GFP) reporter line) were noted around the proximal airways during the pseudoglandular stage. In the canalicular stage, PDGFRα(+) cells appeared in the more distal mesenchyme, and labeled α-smooth muscle actin-positive tip cells in the secondary crests and lipofibroblasts in the primary septa during alveolarization. Some PDGFRα(+) cells appeared in the mesenchyme of the adult lung. Over the course of late lung development, PDGFRα(+) cells consistently expressed collagen I, and transiently expressed markers of mesenchymal stem cells. With the use of both, a constitutive and a conditional PDGFRα(Cre) line, it was observed that PDGFRα(+) cells generated alveolar myofibroblasts including tip cells of the secondary crests, and lipofibroblasts. These lineages were committed before secondary septation. The present study provides new insights into the time-dependent commitment of the PDGFRα(+) cell lineage to lipofibroblasts and myofibroblasts during late lung development that is needed to better understand the cellular contribution to the process of alveolarization.

Fibroblast growth factor signaling in myofibroblasts differs from lipofibroblasts during alveolar septation in mice

Pulmonary alveolar fibroblasts produce extracellular matrix in a temporally and spatially regulated pattern to yield a durable yet pliable gas-exchange surface. Proliferation ensures a sufficient complement of cells, but they must differentiate into functionally distinct subtypes: contractile myofibroblasts (MF), which generate elastin and regulate air-flow at the alveolar ducts, and, in mice and rats, lipofibroblasts (LF), which store neutral lipids. PDGF-A is required but acts in conjunction with other differentiation factors arising from adjacent epithelia or within fibroblasts. We hypothesized that FGF receptor (FGFR) expression and function vary for MF and LF and contributes to their divergent differentiation. Whereas approximately half of the FGFR3 was extracellular in MF, FGFR2 and FGFR4 were primarily intracellular. Intracellular FGFR3 localized to the multivesicular body, and its abundance may be modified by Sprouty and interaction with heat shock protein-90. FGF18 mRNA is more abundant in MF, whereas FGF10 mRNA predominated in LF, which also express FGFR1 IIIb, a receptor for FGF10. FGF18 diminished fibroblast proliferation and was chemotactic for cultured fibroblasts. Although PDGF receptor-α (PDGFR-α) primarily signals through phosphoinositide 3-kinase and Akt, p42/p44 MAP kinase (Erk1/2), a major signaling pathway for FGFRs, influenced the abundance of cell-surface PDGFR-α. Observing different FGFR and ligand profiles in MF and LF is consistent with their divergent differentiation although both subpopulations express PDGFR-α. These studies also emphasize the importance of particular cellular locations of FGFR3 and PDGFR-α, which may modify their effects during alveolar development or repair.

Regulation of fibroblast lipid storage and myofibroblast phenotypes during alveolar septation in mice

Signaling through platelet-derived growth factor receptor-α (PDGFRα) is required for alveolar septation and participates in alveolar regeneration after pneumonectomy. In both adipose tissue and skeletal muscle, bipotent pdgfrα-expressing progenitors expressing delta-like ligand-1 or sex-determining region Y box 9 (Sox9) may differentiate into either lipid storage cells or myofibroblasts. We analyzed markers of mesenchymal progenitors and differentiation in lung fibroblasts (LF) with different levels (absent, low, or high) of pdgfrα gene expression. A larger proportion of pdgfrα-expressing than nonexpressing LF contained Sox9. Neutral lipids, CD166, and Tcf21 were more abundant in LF with a lower compared with a higher level of pdgfrα gene expression. PDGF-A increased Sox9 in primary LF cultures, suggesting that active signaling through PDGFRα is required to maintain Sox9. As alveolar septation progresses from postnatal day (P) 8 to P12, fewer pdgfrα-expressing LF contain Sox9, whereas more of these LF contain myocardin-like transcription factor-A, showing that Sox9 diminishes as LF become myofibroblasts. At P8, neutral lipid droplets predominate in LF with the lower level of pdgfrα gene expression, whereas transgelin (tagln) was predominantly expressed in LF with higher pdgfrα gene expression. Targeted deletion of pdgfrα in LF, which expressed tagln, reduced Sox9 in α-actin (α-SMA, ACTA2)-containing LF, whereas it increased the abundance of cell surface delta-like protein-1 (as well as peroxisome proliferator-activated receptor-γ and tcf21 mRNA in LF, which also expressed stem cell antigen-1). Thus pdgfrα deletion differentially alters delta-like protein-1 and Sox9, suggesting that targeting different downstream pathways in PDGF-A-responsive LF could identify strategies that promote lung regeneration without initiating fibrosis.

DA-Raf-dependent inhibition of the Ras-ERK signaling pathway in type 2 alveolar epithelial cells controls alveolar formation

Alveolar formation is coupled to the spatiotemporally regulated differentiation of alveolar myofibroblasts (AMYFs), which contribute to the morphological changes of interalveolar walls. Although the Ras-ERK signaling pathway is one of the key regulators for alveolar formation in developing lungs, the intrinsic molecular and cellular mechanisms underlying its role remain largely unknown. By analyzing the Ras-ERK signaling pathway during postnatal development of lungs, we have identified a critical role of DA-Raf1 (DA-Raf)-a dominant-negative antagonist for the Ras-ERK signaling pathway-in alveolar formation. DA-Raf-deficient mice displayed alveolar dysgenesis as a result of the blockade of AMYF differentiation. DA-Raf is predominantly expressed in type 2 alveolar epithelial cells (AEC2s) in developing lungs, and DA-Raf-dependent MEK1/2 inhibition in AEC2s suppresses expression of tissue inhibitor of matalloprotienase 4 (TIMP4), which prevents a subsequent proteolytic cascade matrix metalloproteinase (MMP)14-MMP2. Furthermore, MMP14-MMP2 proteolytic cascade regulates AMYF differentiation and alveolar formation. Therefore, DA-Raf-dependent inhibition of the Ras-ERK signaling pathway in AEC2s is required for alveolar formation via triggering MMP2 activation followed by AMYF differentiation. These findings reveal a pivotal role of the Ras-ERK signaling pathway in the dynamic regulation of alveolar development.

Paracrine cellular and extracellular matrix interactions with mesenchymal progenitors during pulmonary alveolar septation

Alveolar development in humans primarily occurs postnatally and requires a carefully orchestrated expansion of distal epithelial and mesenchymal progenitor populations and coordinated differentiation, to create a highly segmented gas-exchange surface. The regulation of alveolarization normally assimilates cues from paracrine cell-cell, cell-extracellular matrix, and mechanical interactions which are superimposed on cells and the extracellular matrix through phasic respiratory movement. In bronchopulmonary dysplasia, the entire process is precociously initiated when cellular and extracellular components are adapted to the saccular stage where movement and circulation are much more limited. This review focuses on mesenchymal cells (fibroblasts, endothelial cells, and pericytes), and epithelial cells are primarily discussed as sources of growth factor ligands or recipients of ligands produced by mesenchymal cells. Some interstitial fibroblasts differentiate to contractile myofibroblasts, containing a smooth muscle-actin rich cytoskeleton, which connects with tensile and elastic elements in the extracellular matrix, and together comprise a load-bearing network that diffuses mechanical forces during respiration. Other interstitial fibroblasts assimilate neutral lipid droplets, which regulate the differentiation of distal epithelial progenitors and surfactant production by alveolar type 2 cells. Pericytes organize and reinforce the capillary network as it expands to match the coverage of type 1 epithelial cells. Hyperoxia and the mechanical load imposed by positive pressure mechanical ventilation disrupt these paracrine interactions, leaving thickened alveolar walls, airways and arterioles, thereby diminishing gas-exchange surface area. Better understanding of these mechanisms of alveolar septation will lead to more effective treatments to preserve and perhaps augment the surface usual sequence of events that drive alveolarization.

Soluble guanylate cyclase modulates alveolarization in the newborn lung

Nitric oxide (NO) regulates lung development through incompletely understood mechanisms. NO controls pulmonary vascular smooth muscle cell (SMC) differentiation largely through stimulating soluble guanylate cyclase (sGC) to produce cGMP and increase cGMP-mediated signaling. To examine the role of sGC in regulating pulmonary development, we tested whether decreased sGC activity reduces alveolarization in the normal and injured newborn lung. For these studies, mouse pups with gene-targeted sGC-α1 subunit truncation were used because we determined that they have decreased pulmonary sGC enzyme activity. sGC-α1 knockout (KO) mouse pups were observed to have decreased numbers of small airway structures and lung volume compared with wild-type (WT) mice although lung septation and body weights were not different. However, following mild lung injury caused by breathing 70% O2, the sGC-α1 KO mouse pups had pronounced inhibition of alveolarization, as evidenced by an increase in airway mean linear intercept, reduction in terminal airway units, and decrease in lung septation and alveolar openings, as well as reduced somatic growth. Because cGMP regulates SMC phenotype, we also tested whether decreased sGC activity reduces lung myofibroblast differentiation. Cellular markers revealed that vascular SMC differentiation decreased, whereas myofibroblast activation increased in the hyperoxic sGC-α1 KO pup lung. These results indicate that lung development, particularly during hyperoxic injury, is impaired in mouse pups with diminished sGC activity. These studies support the investigation of sGC-targeting agents as therapies directed at improving development in the newborn lung exposed to injury.

Platelet-derived growth factor-A regulates lung fibroblast S-phase entry through p27(kip1) and FoxO3a

BACKGROUND

Secondary pulmonary alveolar septal formation requires platelet derived growth factor (PDGF-A) and platelet derived growth factor receptor-alpha (PDGFRα), and their regulation influences alveolar septal areal density and thickness. Insufficient PDGFRα expression in lung fibroblasts (LF) results in failed septation.

METHODS

Mice in which the endogenous PDGFRα-gene regulates expression of the green fluorescent protein were used to temporally and spatially track PDGFRα-signaling. Transition from the G₁/G₀ to the S-phase of the cell cycle was compared in PDGFRα-expressing and non-expressing LF using flow cytometry. Laser scanning confocal microscopy was used to quantify p27(kip1) and forkhead box "other" 3a (FoxO3a) in the nuclei of alveolar cells from mice bearing the PDGFRα-GFP knock-in, and p27(kip1) in mice with a conditional deletion of PDGFRα-gene function. The effects of PDGF-A on the phosphorylation and the intracellular location of FoxO3a were examined using Western immuoblotting and immunocytochemistry.

RESULTS

In neonatal mouse lungs, entry of the PDGFRα-expressing LF subpopulation into the S-phase of the cell cycle diminished sooner than in their non-expressing LF counterparts. This preferential diminution was influenced by PDGFRα-mediated signaling, which phosphorylates and promotes cytoplasmic localization of FoxO3a. Comparative observations of LF at different ages during secondary septation and in mice that lack PDGFRα in alveolar LF demonstrated that nuclear localization of the G₁ cyclin-dependent kinase inhibitor p27(kip1) correlated with reduced LF entry into S-phase.

CONCLUSIONS

Nuclear localization of FoxO3a, an important regulator of p27(kip1) gene-expression, correlates with diminished proliferation of the PDGFRα-expressing LF subpopulation. These mechanisms for diminishing the effects of PDGFRα-mediated signaling likely regulate secondary septal formation and their derangement may contribute to imbalanced fibroblast cell kinetics in parenchymal lung diseases.

Platelet-derived growth factor-A and sonic hedgehog signaling direct lung fibroblast precursors during alveolar septal formation

Alveolar septal formation is required to support the respiration of growing mammals; in humans effacement of the alveolar surface and impaired gas exchange are critical features of emphysema and pulmonary fibrosis. Platelet-derived growth factor-A (PDGF-A) and its receptor PDGF-receptor-α (PDGFRα) are required for secondary septal elongation in mice during postnatal days 4 through 12 and they regulate the proliferation and septal location of interstitial fibroblasts. We examined lung fibroblasts (LF) to learn whether PDGFRα expression distinguished a population of precursor cells, with enhanced proliferative and migratory capabilities. We identified a subpopulation of LF that expresses sonic hedgehog (Shh) and stem cell antigen-1 (Sca1). PDGF-A and Shh both increased cytokinesis and chemotaxis in vitro, but through different mechanisms. In primary LF cultures, Shh signaled exclusively through a noncanonical pathway involving generation of Rac1-GTP, whereas both the canonical and noncanonical pathways were used by the Mlg neonatal mouse LF cell line. LF preferentially oriented their primary cilia toward their anterior pole during migration. Furthermore, a larger proportion of PDGFRα-expressing LF, which are more abundant at the septal tips, bore primary cilia compared with other alveolar cells. In pulmonary emphysema, destroyed alveolar septa do not regenerate, in part because cells fail to assume a configuration that allows efficient gas exchange. Better understanding how LF are positioned during alveolar development could identify signaling pathways, which promote alveolar septal regeneration.

Correlation between PDGFs and their receptor and liver fibrosis

Liver fibrosis occurs as a result of various injurious processes and is the common pathologic basis for all chronic hepatic diseases. Platelet-derived growth factors (PDGFs) are homodimers or heterodimers consisting of two polypeptide chains. Previous studies have proved that PDGFs are closely related to the occurrence and development of liver fibrosis. In this article, we review the advances in research on the role of PDGFs and their receptor in pathogenesis and therapy of liver fibrosis.

Dynamic regulation of platelet-derived growth factor receptor α expression in alveolar fibroblasts during realveolarization

Although the importance of platelet-derived growth factor receptor (PDGFR)-α signaling during normal alveogenesis is known, it is unclear whether this signaling pathway can regulate realveolarization in the adult lung. During alveolar development, PDGFR-α-expressing cells induce α smooth muscle actin (α-SMA) and differentiate to interstitial myofibroblasts. Fibroblast growth factor (FGF) signaling regulates myofibroblast differentiation during alveolarization, whereas peroxisome proliferator-activated receptor (PPAR)-γ activation antagonizes myofibroblast differentiation in lung fibrosis. Using left lung pneumonectomy, the roles of FGF and PPAR-γ signaling in differentiation of myofibroblasts from PDGFR-α-positive precursors during compensatory lung growth were assessed. FGF receptor (FGFR) signaling was inhibited by conditionally activating a soluble dominant-negative FGFR2 transgene. PPAR-γ signaling was activated by administration of rosiglitazone. Changes in α-SMA and PDGFR-α protein expression were assessed in PDGFR-α-green fluorescent protein (GFP) reporter mice using immunohistochemistry, flow cytometry, and real-time PCR. Immunohistochemistry and flow cytometry demonstrated that the cell ratio and expression levels of PDGFR-α-GFP changed dynamically during alveolar regeneration and that α-SMA expression was induced in a subset of PDGFR-α-GFP cells. Expression of a dominant-negative FGFR2 and administration of rosiglitazone inhibited induction of α-SMA in PDGFR-α-positive fibroblasts and formation of new septae. Changes in gene expression of epithelial and mesenchymal signaling molecules were assessed after left lobe pneumonectomy, and results demonstrated that inhibition of FGFR2 signaling and increase in PPAR-γ signaling altered the expression of Shh, FGF, Wnt, and Bmp4, genes that are also important for epithelial-mesenchymal crosstalk during early lung development. Our data demonstrate for the first time that a comparable epithelial-mesenchymal crosstalk regulates fibroblast phenotypes during alveolar septation.

Fibroblasts expressing PDGF-receptor-alpha diminish during alveolar septal thinning in mice

In mice, secondary alveolar septal formation primarily occurs during a brief postnatal period and is accompanied by transient expansion of the interstitial lung fibroblast (LF) population. PDGF-A, which solely signals through PDGF-receptor-alpha (PDGF-Rα), is required for expansion, but the receptor's relevant downstream targets remain incompletely defined. We have evaluated the proliferation, apoptosis, and differential response to the selective protein tyrosine kinase inhibitor, imatinib, by pdgfrα-expressing LF (pdgfrα-LF) and compared them with their nonexpressing LF counterparts. Our objective was to determine whether diminished signaling through PDGF-Rα-mediated pathways regulates the decline in myofibroblasts, which accompanies septal thinning and ensures more efficient alveolar gas exchange. Using quantitative stereology and flow cytometry at postnatal d 12 and 14, we observed that imatinib caused a selective suppression of proliferation and an increase in apoptosis. The number of the alpha smooth muscle actin (αSMA) producing pdgfrα-LF was also reduced. Using cultures of neonatal mouse LF, we showed that imatinib did not suppress PDGF-Rα gene expression but reduced PDGF-A-mediated Akt phosphorylation, potentially explaining the increase in apoptosis. Our findings are relevant to bronchopulmonary dysplasia in which positive pressure ventilation interferes with myofibroblast depletion, septal thinning, and capillary maturation.