Emerging pulmonary vasculature lacks fate specification

Therapeutic Potential of Endothelial Progenitor Cells in Pulmonary Diseases

Compromised alveolar development and pulmonary vascular remodeling are hallmarks of pediatric lung diseases such as bronchopulmonary dysplasia (BPD) and alveolar capillary dysplasia with misalignment of pulmonary veins (ACDMPV). Although advances in surfactant therapy, corticosteroids, and anti-inflammatory drugs have improved clinical management of preterm infants, still those who suffer with severe vascular complications lack viable treatment options. Paucity of the alveolar capillary network in ACDMPV causes respiratory distress and leads to mortality in a vast majority of ACDMPV infants. The discovery of endothelial progenitor cells (EPCs) in 1997 brought forth the paradigm of postnatal vasculogenesis and hope for promoting vascularization in fragile patient populations, such as those with BPD and ACDMPV. The identification of diverse EPC populations, both hematopoietic and nonhematopoietic in origin, provided a need to identify progenitor cell selective markers which are linked to progenitor properties needed to develop cell-based therapies. Focusing to the future potential of EPCs for regenerative medicine, this review will discuss various aspects of EPC biology, beginning with the identification of hematopoietic, nonhematopoietic, and tissue-resident EPC populations. We will review knowledge related to cell surface markers, signature gene expression, key transcriptional regulators, and will explore the translational potential of EPCs for cell-based therapy for BPD and ACDMPV. The ability to produce pulmonary EPCs from patient-derived induced pluripotent stem cells (iPSCs) in vitro, holds promise for restoring vascular growth and function in the lungs of patients with pediatric pulmonary disorders.

Characteristics of the pulmonary circulation in infants with complete atrioventricular septal defect

OBJECTIVE

Infants with complete atrioventricular septal defect occasionally accompany pulmonary hypertension; however, the pulmonary circulation can be altered by pulmonary vascular conditions as well as the left heart lesions. This study aimed to explore whether the left heart lesions were related to the pulmonary circulation among them.

METHODS

We performed echocardiography and cardiac catheterisation in 42 infants with complete atrioventricular septal defect and studied relationships between the pulmonary haemodynamic parameters and the left heart morphology.

RESULTS

Age and weight at preoperative evaluation were 65 days (47-114) (the median following interquartile range) and 5.5 kg (4.0-7.1), respectively. There were 27 individuals with Down syndrome. Gestational age was 38 weeks (37-39). Catheterisation showed mean pulmonary arterial pressure: 36 (29-46) mmHg, the ratio of pulmonary to systemic blood flow: 3.45 (2.79-4.98), pulmonary vascular resistance: 2.20 Wood units·m2 (1.53-3.65), and pulmonary arterial compliance: 2.78 (1.86-4.10) ml/Hg/m2. Echocardiography showed the Rastelli classification type A in 28 and type C in 14, moderate or severe left atrioventricular valve regurgitation in 19 patients (45%), atrioventricular valve index of 0.67 (0.56-0.79), left ventricular end-diastolic volume z score of 4.46 (1.96-7.78), and aortic valve diameter z score of -0.70 (-1.91 to 0.20). Multivariable regression analysis revealed that preoperative pulmonary vascular resistance was significantly correlated to gestational age (p = 0.002), and that preoperative pulmonary arterial compliance was significantly correlated to gestational age (p = 0.009) and Down syndrome (p = 0.036).

CONCLUSIONS

The pulmonary circulation does not depend upon the presence of left heart lesions but gestational age and Down syndrome in infants with complete atrioventricular septal defect.

Neurotrophic factors and their receptors in lung development and implications in lung diseases

Although lung innervation has been described by many studies in humans and rodents, the regulation of the respiratory system induced by neurotrophins is not fully understood. Here, we review current knowledge on the role of neurotrophins and the expression and function of their receptors in neurogenesis, vasculogenesis and during the embryonic development of the respiratory tree and highlight key implications relevant to respiratory diseases.

Decreased Pulmonary Arterial Compliance is a Predictor for Poor Outcomes in Infants with Isolated Atrial Septal Defect and Pulmonary Hypertension

This study aimed to investigate risk factors for poor outcomes in infants with isolated atrial septal defect (ASD) and pulmonary hypertension who had unexpectedly fatal course. We retrospectively reviewed 22 infants with isolated ASD and pulmonary hypertension, and analyzed the relationship between clinical outcomes and pulmonary hemodynamic parameters including pulmonary arterial resistance (Rp) and compliance (Cp) based on cardiac catheterization among them. Age and weight at cardiac catheterization were 5 (1-11) months and 4.9 (3.1-9.2) kg, respectively. There were 17 individuals with Down syndrome. Pulmonary hemodynamic parameters were shown as follows: the ratio of pulmonary to systemic blood flow: 2.0 (0.6-3.8), mean pulmonary arterial pressure: 41 (20-60) mmHg, the ratio of pulmonary to systemic mean pressure (Pp/Ps): 0.67 (0.46-1.13), Rp: 4.11 (0.68-15.80) Wood units m², and Cp: 1.80 (0.63-6.16) mL/mmHg m². There were 4 deaths during the follow-up period of 40 (7-241) months. Univariable logistic regression analysis revealed that Pp/Ps (odds ratio [OR]: 18,500, 95% confidential interval [CI] 1.48-230,659,690, P = 0.041) and Cp (OR: 0.03, 95% CI 0.001-0.73, P = 0.031) were significantly related to death. A Pp/Ps cutoff value more than 0.94 and a Cp cutoff value less than 0.97 mL/mmHg m² yielded as the predictors of death with sensitivity of 98% and 94%, specificity of 75% and 75%, respectively. Decreased Cp is a previously unrecognized predictor for poor outcome in infants with isolated ASD and pulmonary hypertension.

Early origins of lung disease: towards an interdisciplinary approach

The prenatal and perinatal environments can have profound effects on the development of chronic inflammatory diseases. However, mechanistic insight into how the early-life microenvironment can impact upon development of the lung and immune system and consequent initiation and progression of respiratory diseases is still emerging. Recent studies investigating the developmental origins of lung diseases have started to delineate the effects of early-life changes in the lung, environmental exposures and immune maturation on the development of childhood and adult lung diseases. While the influencing factors have been described and studied in mostly animal models, it remains challenging to pinpoint exactly which factors and at which time point are detrimental in lung development leading to respiratory disease later in life. To advance our understanding of early origins of chronic lung disease and to allow for proper dissemination and application of this knowledge, we propose four major focus areas: 1) policy and education; 2) clinical assessment; 3) basic and translational research; and 4) infrastructure and tools, and discuss future directions for advancement. This review is a follow-up of the discussions at the European Respiratory Society Research Seminar "Early origins of lung disease: towards an interdisciplinary approach" (Lisbon, Portugal, November 2019).

Building and Regenerating the Lung Cell by Cell

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

Unique aspects of the developing lung circulation: structural development and regulation of vasomotor tone

This review summarizes our current knowledge on lung vasculogenesis and angiogenesis during normal lung development and the regulation of fetal and postnatal pulmonary vascular tone. In comparison to that of the adult, the pulmonary circulation of the fetus and newborn displays many unique characteristics. Moreover, altered development of pulmonary vasculature plays a more prominent role in compromised pulmonary vasoreactivity than in the adult. Clinically, a better understanding of the developmental changes in pulmonary vasculature and vasomotor tone and the mechanisms that are disrupted in disease states can lead to the development of new therapies for lung diseases characterized by impaired alveolar structure and pulmonary hypertension.

Increased c-kit and stem cell factor expression in the pulmonary vasculature of nitrofen-induced congenital diaphragmatic hernia

PURPOSE

Persistent pulmonary hypertension(PPH) in congenital diaphragmatic hernia (CDH) is caused by increased vascular cell proliferation and endothelial cell (EC) dysfunction, thus leading to obstructive changes in the pulmonary vasculature. C-Kit and its ligand, stem cell factor(SCF), are expressed by ECs in the developing lung mesenchyme, suggesting an important role during lung vascular formation. Conversely, absence of c-Kit expression has been demonstrated in ECs of dysplastic alveolar capillaries. We hypothesized that c-Kit and SCF expression is increased in the pulmonary vasculature of nitrofen-induced CDH.

METHODS

Timed-pregnant rats received nitrofen or vehicle on gestational day 9(D9). Fetuses were sacrificed on D15, D18, and D21, and divided into control and CDH group. Pulmonary gene expression levels of c-Kit and SCF were analyzed by qRT-PCR. Immunofluorescence double staining for c-Kit and SCF was combined with CD34 to evaluate protein expression in ECs of the pulmonary vasculature.

RESULTS

Relative mRNA levels of c-Kit and SCF were significantly increased in lungs of CDH fetuses on D15, D18, and D21 compared to controls. Confocal laser scanning microscopy confirmed markedly increased vascular c-Kit and SCF expression in mesenchymal ECs of CDH lungs on D15, D18, and D21 compared to controls.

CONCLUSION

Increased expression of c-Kit and SCF in the pulmonary vasculature of nitrofen-induced CDH lungs suggest that increased c-Kit signaling during lung vascular formation may contribute to vascular remodeling and thus to PPH.

Mesodermal lineages in the developing respiratory system

The life-sustaining air-blood interface of the respiratory system requires the exquisite integration of the epithelial lining with the mesenchymal capillary network, all supported by elastic smooth muscle and rigid cartilage keeping the expandable airways open. These intimate tissue interactions originate in the early embryo, where bidirectional paracrine signaling between the endoderm epithelium and adjacent mesoderm orchestrates lung and trachea development and controls the stereotypical branching morphogenesis. Although much attention has focused on how these interactions impact the differentiation of the respiratory epithelium, relatively less is known about the patterning and differentiation of the mesenchyme. Endothelial cells, smooth muscle cells, and chondrocytes together with other types of mesenchymal cells are essential components of a functional respiratory system, and malformation of these cells can lead to various congenital defects. In this review, we summarize the current understanding of mesenchymal development in the fetal trachea and lung, focusing on recent findings from animal models that have begun to shed light on the poorly understood respiratory mesenchyme lineages.

Pulmonary vascular development goes awry in congenital lung abnormalities

Pulmonary vascular diseases of the newborn comprise a wide range of pathological conditions with developmental abnormalities in the pulmonary vasculature. Clinically, pulmonary arterial hypertension (PH) is characterized by persistent increased resistance of the vasculature and abnormal vascular response. The classification of PH is primarily based on clinical parameters instead of morphology and distinguishes five groups of PH. Congenital lung anomalies, such as alveolar capillary dysplasia (ACD) and PH associated with congenital diaphragmatic hernia (CDH), but also bronchopulmonary dysplasia (BPD), are classified in group three. Clearly, tight and correct regulation of pulmonary vascular development is crucial for normal lung development. Human and animal model systems have increased our knowledge and make it possible to identify and characterize affected pathways and study pivotal genes. Understanding of the normal development of the pulmonary vasculature will give new insights in the origin of the spectrum of rare diseases, such as CDH, ACD, and BPD, which render a significant clinical problem in neonatal intensive care units around the world. In this review, we describe normal pulmonary vascular development, and focus on four diseases of the newborn in which abnormal pulmonary vascular development play a critical role in morbidity and mortality. In the future perspective, we indicate the lines of research that seem to be very promising for elucidating the molecular pathways involved in the origin of congenital pulmonary vascular disease.

Ephrin-B2 reverse signaling increases α5β1 integrin-mediated fibronectin deposition and reduces distal lung compliance

Alveolar growth abnormalities and severe respiratory dysfunction are often fatal. Identifying mechanisms that control epithelial proliferation and enlarged, poorly septated airspaces is essential in developing new therapies for lung disease. The membrane-bound ligand ephrin-B2 is strongly expressed in lung epithelium, and yet in contrast to its known requirement for arteriogenesis, considerably less is known regarding the function of this protein in the epithelium. We hypothesize that the vascular mediator ephrin-B2 governs alveolar growth and mechanics beyond the confines of the endothelium. We used the in vivo manipulation of ephrin-B2 reverse signaling to determine the role of this vascular mediator in the pulmonary epithelium and distal lung mechanics. We determined that the ephrin-B2 gene (EfnB2) is strongly expressed in alveolar Type 2 cells throughout development and into adulthood. The role of ephrin-B2 reverse signaling in the lung was assessed in Efnb2(LacZ/6YFΔV) mutants that coexpress the intracellular truncated ephrin-B2-β-galactosidase fusion and an intracellular point mutant ephrin-B2 protein that is unable to become tyrosine-phosphorylated or to interact with either the SH2 or PDZ domain-containing downstream signaling proteins. In these viable mice, we observed pulmonary hypoplasia and altered pulmonary mechanics, as evidenced by a marked reduction in lung compliance. Associated with the reduction in lung compliance was a significant increase in insoluble fibronectin (FN) basement membrane matrix assembly with FN deposition, and a corresponding increase in the α5 integrin receptor required for FN fibrillogenesis. These experiments indicate that ephrin-B2 reverse signaling mediates distal alveolar formation, fibrillogenesis, and pulmonary compliance.

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

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

c-Kit immunoexpression delineates a putative endothelial progenitor cell population in developing human lungs

Expression of c-Kit and its ligand, stem cell factor (SCF), in developing human lung tissue was investigated by immunohistochemistry. Twenty-eight human fetal lungs [age range 13 to 38 gestational wk (GW)] and 12 postnatal lungs (age range 1-79 yr) were evaluated. We identified c-Kit(+) cells in the lung mesenchyme as early as 13 GW. These mesenchymal c-Kit(+) cells in the lung did not express mast cell tryptase or α-smooth muscle actin. However, these cells did express CD34, VEGFR2, and Tie-2, indicating their endothelial lineage. Three-dimensional reconstructions of confocal laser scanning images revealed that c-Kit(+) cells displayed a closed-end tube formation that did not contain hematopoietic cells. From the pseudoglandular phase to the canalicular phase, c-Kit(+) cells appeared to continuously proliferate, to connect with central pulmonary vessels, and finally, to develop the lung capillary plexus. The spatial distribution of c-Kit- and SCF-positive cells was also demonstrated, and these cells were shown to be in close association. Our results suggest that c-Kit expression in early fetal lungs marks a progenitor population that is restricted to endothelial lineage. This study also suggests the potential involvement of c-Kit signaling in lung vascular development.

Physiological and pathological angiogenesis in the adult pulmonary circulation

Angiogenesis occurs during growth and physiological adaptation in many systemic organs, for example, exercise-induced skeletal and cardiac muscle hypertrophy, ovulation, and tissue repair. Disordered angiogenesis contributes to chronic inflammatory disease processes and to tumor growth and metastasis. Although it was previously thought that the adult pulmonary circulation was incapable of supporting new vessel growth, over that past 10 years new data have shown that angiogenesis within this circulation occurs both during physiological adaptive processes and as part of the pathogenic mechanisms of lung diseases. Here we review the expression of vascular growth factors in the adult lung, their essential role in pulmonary vascular homeostasis and the changes in their expression that occur in response to physiological challenges and in disease. We consider the evidence for adaptive neovascularization in the pulmonary circulation in response to alveolar hypoxia and during lung growth following pneumonectomy in the adult lung. In addition, we review the role of disordered angiogenesis in specific lung diseases including idiopathic pulmonary fibrosis, acute adult distress syndrome and both primary and metastatic tumors of the lung. Finally, we examine recent experimental data showing that therapeutic enhancement of pulmonary angiogenesis has the potential to treat lung diseases characterized by vessel loss.

Caveolins and lung function

The primary function of the mammalian lung is to facilitate diffusion of oxygen to venous blood and to ventilate carbon dioxide produced by catabolic reactions within cells. However, it is also responsible for a variety of other important functions, including host defense and production of vasoactive agents to regulate not only systemic blood pressure, but also water, electrolyte and acid-base balance. Caveolin-1 is highly expressed in the majority of cell types in the lung, including epithelial, endothelial, smooth muscle, connective tissue cells, and alveolar macrophages. Deletion of caveolin-1 in these cells results in major functional aberrations, suggesting that caveolin-1 may be crucial to lung homeostasis and development. Furthermore, generation of mutant mice that under-express caveolin-1 results in severe functional distortion with phenotypes covering practically the entire spectrum of known lung diseases, including pulmonary hypertension, fibrosis, increased endothelial permeability, and immune defects. In this Chapter, we outline the current state of knowledge regarding caveolin-1-dependent regulation of pulmonary cell functions and discuss recent research findings on the role of caveolin-1 in various pulmonary disease states, including obstructive and fibrotic pulmonary vascular and inflammatory diseases.

Caveolins and lung function

The primary function of the mammalian lung is to facilitate diffusion of oxygen to venous blood and to ventilate carbon dioxide produced by catabolic reactions within cells. However, it is also responsible for a variety of other important functions, including host defense and production of vasoactive agents to regulate not only systemic blood pressure, but also water, electrolyte and acid-base balance. Caveolin-1 is highly expressed in the majority of cell types in the lung, including epithelial, endothelial, smooth muscle, connective tissue cells, and alveolar macrophages. Deletion of caveolin-1 in these cells results in major functional aberrations, suggesting that caveolin-1 may be crucial to lung homeostasis and development. Furthermore, generation of mutant mice that under-express caveolin-1 results in severe functional distortion with phenotypes covering practically the entire spectrum of known lung diseases, including pulmonary hypertension, fibrosis, increased endothelial permeability, and immune defects. In this Chapter, we outline the current state of knowledge regarding caveolin-1-dependent regulation of pulmonary cell functions and discuss recent research findings on the role of caveolin-1 in various pulmonary disease states, including obstructive and fibrotic pulmonary vascular and inflammatory diseases.

Critical role of the axonal guidance cue EphrinB2 in lung growth, angiogenesis, and repair

RATIONALE

Lung diseases characterized by alveolar damage currently lack efficient treatments. The mechanisms contributing to normal and impaired alveolar growth and repair are incompletely understood. Axonal guidance cues (AGC) are molecules that guide the outgrowth of axons to their targets. Among these AGCs, members of the Ephrin family also promote angiogenesis, cell migration, and organogenesis outside the nervous system. The role of Ephrins during alveolar growth and repair is unknown.

OBJECTIVES

We hypothesized that EphrinB2 promotes alveolar development and repair.

METHODS

We used in vitro and in vivo manipulation of EphrinB2 signaling to assess the role of this AGC during normal and impaired lung development.

MEASUREMENTS AND MAIN RESULTS

In vivo EphrinB2 knockdown using intranasal siRNA during the postnatal stage of alveolar development in rats arrested alveolar and vascular growth. In a model of O(2)-induced arrested alveolar growth in newborn rats, air space enlargement, loss of lung capillaries, and pulmonary hypertension were associated with decreased lung EphrinB2 and receptor EphB4 expression. In vitro, EphrinB2 preserved alveolar epithelial cell viability in O(2), decreased O(2)-induced alveolar epithelial cell apoptosis, and accelerated alveolar epithelial cell wound healing, maintained lung microvascular endothelial cell viability, and proliferation and vascular network formation. In vivo, treatment with intranasal EphrinB2 decreased alveolar epithelial and endothelial cell apoptosis, preserved alveolar and vascular growth in hyperoxic rats, and attenuated pulmonary hypertension.

CONCLUSION

The AGC EphrinB2 may be a new therapeutic target for lung repair and pulmonary hypertension.

Vascular abnormalities in human newborns with pulmonary hypertension

Pulmonary vascular disease embodies all congenital or acquired pathologies that affect the pulmonary vasculature. One of them is pulmonary hypertension of the newborn (PHN), which is clinically characterized by a persistent high pulmonary vascular resistance postnatally and an abnormal vascular response. Morphologically, the vascular walls of the small pulmonary arteries become thickened, leading to increased resistance of these vessels and thus a worsening of gas exchange. PHN occurs as a primary disease or in association with abnormal lung development, for example as in congenital diaphragmatic hernia, and is a critical determinant of morbidity and mortality. Here we review the current knowledge about vascular abnormalities in PHN and discuss the vascular abnormalities in different conditions associated with pulmonary hypertension in human newborns in relation to recent findings from molecular biology.

The building blocks of mammalian lung development

Progress has recently been made in identifying progenitor cell populations in the embryonic lung. Some progenitor cell types have been definitively identified by lineage-tracing studies. However, others are not as well characterized and their existence is inferred on the basis of lung morphology, or mutant phenotypes. Here, I focus on lung development after the specification of the initial lung primordium. The evidence for various lung embryonic progenitor cell types is discussed and future experiments are suggested. The regulation of progenitor proliferation in the embryonic lung, and its coordinate control with morphogenesis, is also discussed. In addition, the relationship between embryonic and adult lung progenitors is considered.

Regulation of the Pulmonary Circulation in the Fetus and Newborn

During the development of the pulmonary vasculature in the fetus, many structural and functional changes occur to prepare the lung for the transition to air breathing. The development of the pulmonary circulation is genetically controlled by an array of mitogenic factors in a temporo-spatial order. With advancing gestation, pulmonary vessels acquire increased vasoreactivity. The fetal pulmonary vasculature is exposed to a low oxygen tension environment that promotes high intrinsic myogenic tone and high vasocontractility. At birth, a dramatic reduction in pulmonary arterial pressure and resistance occurs with an increase in oxygen tension and blood flow. The striking hemodynamic differences in the pulmonary circulation of the fetus and newborn are regulated by various factors and vasoactive agents. Among them, nitric oxide, endothelin-1, and prostaglandin I2 are mainly derived from endothelial cells and exert their effects via cGMP, cAMP, and Rho kinase signaling pathways. Alterations in these signaling pathways may lead to vascular remodeling, high vasocontractility, and persistent pulmonary hypertension of the newborn.

Preparing for the first breath: genetic and cellular mechanisms in lung development

The mammalian respiratory system--the trachea and the lungs--arises from the anterior foregut through a sequence of morphogenetic events involving reciprocal endodermal-mesodermal interactions. The lung itself consists of two highly branched, tree-like systems--the airways and the vasculature--that develop in a coordinated way from the primary bud stage to the generation of millions of alveolar gas exchange units. We are beginning to understand some of the molecular and cellular mechanisms that underlie critical processes such as branching morphogenesis, vascular development, and the differentiation of multipotent progenitor populations. Nevertheless, many gaps remain in our knowledge, the filling of which is essential for understanding respiratory disorders, congenital defects in human neonates, and how the disruption of morphogenetic programs early in lung development can lead to deficiencies that persist throughout life.

Chorioamnionitis stimulates angiogenesis in saccular stage fetal lungs via CC chemokines

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

Endothelial cell activation in a VEGF-A gradient: relevance to cell fate decisions

Distribution of vascular endothelial cell growth factor A (VEGF-A) as a gradient determines microvascular endothelial cell (EC) fate during organogenesis. While much is understood about mechanisms of differential distribution, less is known about how EC perceive and interpret a graded VEGF-A signal to generate positional target gene activation. Using microvascular EC, we analyzed the effect of time and graded VEGF-A input on VEGFR2 autophosphorylation, signal kinase activation and induction of immediate-early genes. The threshold and time to peak activation of VEGFR2 were dependent on signal strength over a 50-fold range in concentration with 3-fold concentration differences readily distinguished. Longer duration of exposure did not compensate for low concentration of VEGF-A, suggesting intensity and duration of signal were not interpreted equivalently. With the same conditions, graded and time-sensitive information was transduced through the PLCgamma/p44/p42MAPK signal pathway but not the parallel AKT pathway. Analysis of MAPK-induced angiogenic immediate-early genes determined that EGR-1, EGR-3, and NR4A1 were dependent on graded input while NR4A2 and DSCR1 were independent with 'switch-like' induction. These data demonstrate rapid, linear integration of VEGF-A levels but independent interpretation of duration of signal and identify potential nodes for segregation of gradient-dependent and -independent responses. These results describe how microvascular EC fate decisions can be determined by comparatively moderate changes in VEGF signal strength, resulting in combinatorial changes in the repertoire of immediate-early genes for transcription effectors.

Guidance molecules in lung cancer

Guidance molecules were first described in the nervous system to control axon outgrowth direction. They are also widely expressed outside the nervous system where they control cell migration, tissue development and establishment of the vascular network. In addition, they are involved in cancer development, tumor angiogenesis and metastasis. This review is primarily focused on their functions in lung cancer and their involvement in lung development is also presented. Five guidance molecule families and their corresponding receptors are described, including the semaphorins/neuropilins/plexins, ephrins and Eph receptors, netrin/DCC/UNC5, Slit/Robo and Notch/Delta. In addition, the possibility to target these molecules as a therapeutic approach in cancer is discussed.