Lung vascular cell heterogeneity: endothelium, smooth muscle, and fibroblasts

Metformin-stimulated AMPK-α1 promotes microvascular repair in acute lung injury

Acute lung injury secondary to sepsis is a leading cause of mortality in sepsis-related death. Present therapies are not effective in reversing endothelial cell dysfunction, which plays a key role in increased vascular permeability and compromised lung function. AMP-activated protein kinase (AMPK) is a molecular sensor important for detection and mediation of cellular adaptations to vascular disruptive stimuli. In this study, we sought to determine the role of AMPK in resolving increased endothelial permeability in the sepsis-injured lung. AMPK function was determined in vivo using a rat model of endotoxin-induced lung injury, ex vivo using the isolated lung, and in vitro using cultured rat pulmonary microvascular endothelial cells (PMVECs). AMPK stimulation using N1-(α-d-ribofuranosyl)-5-aminoimidizole-4-carboxamide or metformin decreased the LPS-induced increase in permeability, as determined by filtration coefficient (Kf) measurements, and resolved edema as indicated by decreased wet-to-dry ratios. The role of AMPK in the endothelial response to LPS was determined by shRNA designed to decrease expression of the AMPK-α1 isoform in capillary endothelial cells. Permeability, wounding, and barrier resistance assays using PMVECs identified AMPK-α1 as the molecule responsible for the beneficial effects of AMPK in the lung. Our findings provide novel evidence for AMPK-α1 as a vascular repair mechanism important in the pulmonary response to sepsis and identify a role for metformin treatment in the management of capillary injury.

Genetic influence on the relation between exhaled nitric oxide and pulse wave reflection

Nitric oxide has an important role in the development of the structure and function of the airways and vessel walls. Fractional exhaled nitric oxide (FE(NO)) is inversely related to the markers and risk factors of atherosclerosis. We aimed to estimate the relative contribution of genes and shared and non-shared environmental influences to variations and covariation of FE(NO) levels and the marker of elasticity function of arteries. Adult Caucasian twin pairs (n = 117) were recruited in Hungary, Italy and in the United States (83 monozygotic and 34 dizygotic pairs; age: 48 ± 16 SD years). FE(NO) was measured by an electrochemical sensor-based device. Pulse wave reflection (aortic augmentation index, Aix(ao)) was determined by an oscillometric method (Arteriograph). A bivariate Cholesky decomposition model was applied to investigate whether the heritabilities of FE(NO) and Aix(ao) were linked. Genetic effects accounted for 58% (95% confidence interval (CI): 42%, 71%) of the variation in FE(NO) with the remaining 42% (95%CI: 29%, 58%) due to non-shared environmental influences. A modest negative correlation was observed between FE(NO) and Aix(ao) (r = -0.17; 95%CI:-0.32,-0.02). FE(NO) showed a significant negative genetic correlation with Aix(ao) (r(g) = -0.25; 95%CI:-0.46,-0.02). Thus in humans, variations in FE(NO) are explained both by genetic and non-shared environmental effects. Covariance between FE(NO) and Aix(ao) is explained entirely by shared genetic factors. This is consistent with an overlap among the sets of genes involved in the expression of these phenotypes and provides a basis for further genetic studies on cardiovascular and respiratory diseases.

Intrapulmonary shear stress enhancement: a new therapeutic approach in pulmonary arterial hypertension

OBJECTIVE

Pulmonary arterial hypertension (PAH) is a dysfunctional endothelium disease with increased pulmonary vascular resistance (PVR) and poor prognosis. Current therapies are still insufficient. Here we propose a new pulsatile device as a more effective tool for PAH management compared with traditional treatments.

MATERIALS AND METHODS

Twelve piglets (10.3 ± 3.8 kg) were given either intrapulmonary pulsatile [P (n = 6)] or nonpulsatile [NP (n = 6)] tadalafil treatment. After median sternotomy and heparin injection (250 IU/kg), both groups underwent aorto-pulmonary surgical shunt for 1 h. During a second 1 h period in group P, a catheter prototype, driven by a small ventilator, was introduced into the pulmonary trunk and pulsated intermittently at 110 bpm irrespective of heart rate (90.6 ± 10.74 bpm). In group NP, tadalafil was given orally (1 mg/kg).

RESULTS

Hemodynamics and cardiac output (CO) were significantly (p < 0.05) improved in group P compared with group NP: CO was 0.56 ± 0.0.26 versus 0.54 ± 0.11 (L/min), respectively. Mean pulmonary artery pressure (PAP) was decreased in group P compared with group NP: PAP was 9.6 ± 2.97 versus 32.2 ± 5.07, respectively. Vascular resistances (dynes.s.cm(-5)/kg) were significantly lower in group P versus group NP: pulmonary resistance was 85 ± 42.12 versus 478 ± 192.91 and systemic resistance was 298.8 ± 172.85 versus 1301 ± 615.79, respectively. Using Western blot analysis, endogenous NO synthase expression in PA segments was nonsignificantly (p > 0.05) greater in group P (0.81 ± 0.78) versus (0.62 ± 0.35) group NP.

CONCLUSION

Induced with an appropriate device, intrapulmonary shear stress-mediated endothelial function enhancement provides a more effective nearly physiological therapy for PAH.

Pseudomonas aeruginosa exotoxin Y is a promiscuous cyclase that increases endothelial tau phosphorylation and permeability

Exotoxin Y (ExoY) is a type III secretion system effector found in ~ 90% of the Pseudomonas aeruginosa isolates. Although it is known that ExoY causes inter-endothelial gaps and vascular leak, the mechanisms by which this occurs are poorly understood. Using both a bacteria-delivered and a codon-optimized conditionally expressed ExoY, we report that this toxin is a dual soluble adenylyl and guanylyl cyclase that results in intracellular cAMP and cGMP accumulation. The enzymatic activity of ExoY caused phosphorylation of endothelial Tau serine 214, accumulation of insoluble Tau, inter-endothelial cell gap formation, and increased macromolecular permeability. To discern whether the cAMP or cGMP signal was responsible for Tau phosphorylation and barrier disruption, pulmonary microvascular endothelial cells were engineered for the conditional expression of either wild-type guanylyl cyclase, which synthesizes cGMP, or a mutated guanylyl cyclase, which synthesizes cAMP. Sodium nitroprusside stimulation of the cGMP-generating cyclase resulted in transient Tau serine 214 phosphorylation and gap formation, whereas stimulation of the cAMP-generating cyclase induced a robust increase in Tau serine 214 phosphorylation, gap formation, and macromolecular permeability. These results indicate that the cAMP signal is the dominant stimulus for Tau phosphorylation. Hence, ExoY is a promiscuous cyclase and edema factor that uses cAMP and, to some extent, cGMP to induce the hyperphosphorylation and insolubility of endothelial Tau. Because hyperphosphorylated and insoluble Tau are hallmarks in neurodegenerative tauopathies such as Alzheimer disease, acute Pseudomonas infections cause a pathophysiological sequela in endothelium previously recognized only in chronic neurodegenerative 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.

Studies on the cell biology of interendothelial cell gaps

Pain, redness, heat, and swelling are hallmarks of inflammation that were recognized as early as the first century AD. Despite these early observations, the mechanisms responsible for swelling, in particular, remained an enigma for nearly two millennia. Only in the past century have scientists and physicians gained an appreciation for the role that vascular endothelium plays in controlling the exudation that is responsible for swelling. One of these mechanisms is the formation of transient gaps between adjacent endothelial cell borders. Inflammatory mediators act on endothelium to reorganize the cytoskeleton, decrease the strength of proteins that connect cells together, and induce transient gaps between endothelial cells. These gaps form a paracellular route responsible for exudation. The discovery that interendothelial cell gaps are causally linked to exudation began in the 1960s and was accompanied by significant controversy. Today, the role of gap formation in tissue edema is accepted by many, and significant scientific effort is dedicated toward developing therapeutic strategies that will prevent or reverse the endothelial cell gaps that are present during the course of inflammatory illness. Given the importance of this field in endothelial cell biology and inflammatory disease, this focused review catalogs key historical advances that contributed to our modern-day understanding of the cell biology of interendothelial gap formation.

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.

DNA replication timing is maintained genome-wide in primary human myoblasts independent of D4Z4 contraction in FSH muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is linked to contraction of an array of tandem 3.3-kb repeats (D4Z4) at 4q35.2 from 11-100 copies to 1-10 copies. The extent to which D4Z4 contraction at 4q35.2 affects overall 4q35.2 chromatin organization remains unclear. Because DNA replication timing is highly predictive of long-range chromatin interactions, we generated genome-wide replication-timing profiles for FSHD and control myogenic precursor cells. We compared non-immortalized myoblasts from four FSHD patients and three control individuals to each other and to a variety of other human cell types. This study also represents the first genome-wide comparison of replication timing profiles in non-immortalized human cell cultures. Myoblasts from both control and FSHD individuals all shared a myoblast-specific replication profile. In contrast, male and female individuals were readily distinguished by monoallelic differences in replication timing at DXZ4 and other regions across the X chromosome affected by X inactivation. We conclude that replication timing is a robust cell-type specific feature that is unaffected by FSHD-related D4Z4 contraction.

A novel method for isolating individual cellular components from the adult human distal lung

A variety of lung diseases, such as pulmonary emphysema and idiopathic pulmonary fibrosis, develop in the lung alveoli. Multiple cell types are localized in the alveoli, including epithelial, mesenchymal, and endothelial cells. These resident cells participate in the pathogenesis of lung disease in various ways. To elaborate clearly on the mechanisms of these pathologic processes, cell type-specific analyses of lung disease are required. However, no method exists for individually isolating the different types of cells found in the alveoli. We report on the development of a FACS-based method for the direct isolation of individual cell types from the adult human distal lung. We obtained human lung tissue from lung resections, and prepared single-cell suspension. After depleting CD45-positive cells, a combination of antibodies against epithelial cell adhesion molecule (EpCAM), T1α, and vascular endothelial (VE)-cadherin as used to delineate alveolar cell types. Alveolar Type II cells were highly purified in the EpCAM(hi)/T1α(-) subset, whereas the EpCAM(+)/T1α(-/low) subset contained a mixed epithelial population consisting of alveolar Type I and bronchiolar epithelial cells. The EpCAM(-)/T1α(-) subset included both microvascular endothelial and mesenchymal cells, and these were separated by immunoreactivity to VE-cadherin. Lymphatic endothelial cells existed in the EpCAM(-)/T1α(hi) subset. Isolated cells were viable, and further cell culture studies could be performed. These results suggest that this novel method enables the isolation of different cellular components from normal and diseased lungs, and is capable of elucidating phenotypes specific to certain alveolar cell types indicative of lung disease.

TGFB1 disrupts the angiogenic potential of microvascular endothelial cells of the corpus luteum

Cyclical formation and regression of the ovarian corpus luteum is required for reproduction. During luteal regression, the microvasculature of the corpus luteum is extensively disrupted. Prostaglandin F2α, a primary signal for luteal regression, induces the expression of transforming growth factor β1 (TGFB1) in the corpus luteum. This study determined the actions of TGFB1 on microvascular endothelial cells isolated from the bovine corpus luteum (CLENDO cells). We hypothesized that TGFB1 participates in the disruption of the microvasculature during luteal regression. TGFB1 activated the canonical SMAD signaling pathway in CLENDO cells. TGFB1 (1 ng/ml) significantly reduced both basal and fetal-calf-serum-stimulated DNA synthesis, without reducing cell viability. TGFB1 also significantly reduced CLENDO cell transwell migration and disrupted the formation of capillary-like structures when CLENDO cells were plated on Matrigel. By contrast, CLENDO cells plated on fibrillar collagen I gels did not form capillary-like structures and TGFB1 induced cell death. Additionally, TGFB1 caused loss of VE-cadherin from cellular junctions and loss of cell-cell contacts, and increased the permeability of confluent CLENDO cell monolayers. These studies demonstrate that TGFB1 acts directly on CLENDO cells to limit endothelial cell function and suggest that TGFB1 might act in the disassembly of capillaries observed during luteal regression.

Role of endothelial injury in disease mechanisms and contribution of progenitor cells in mediating endothelial repair

Recent research on the endothelium demonstrates complex interactions of endothelial cells with circulating immune cells, mediators such as cytokines, hormones and growth factors, and with the underlying parenchymal cells. These disparate interactions are involved in promotion of vascular development; maintenance of tissue homeostasis; and regulation of vascular repair. Injury to the endothelial monolayer is the sine qua non of organ dysfunction with endothelial repair the necessary first step needed for recovery. Thus, the capacity of the endothelium to regenerate itself is a key determinant of organ repair and survival after injury. Using the example of the lung, we will review the current state of knowledge regarding the importance of endothelium in the above mentioned processes with a focus on the role of stem cells, both endogenous (i.e., localized within the vessel wall) as well as exogenous (i.e., arriving in the vessel wall from distant sites such as the bone marrow) in promoting endothelial repair and regeneration. The subject of endothelial regeneration and the ways in which stem and progenitor cells contribute to this process has promise in treating vascular diseases. As we will highlight in this review, some questions have been addressed but many more remain and need to be addressed before cell-based therapies become a viable option.

Hypoxia-induced mitogenic factor (HIMF/FIZZ1/RELMalpha) increases lung inflammation and activates pulmonary microvascular endothelial cells via an IL-4-dependent mechanism

Hypoxia-induced mitogenic factor (HIMF), also known as found in inflammatory zone 1 and resistin-like molecule α, belongs to a novel class of cysteine-rich secreted proteins. It exhibits mitogenic and chemotactic properties during pulmonary hypertension-associated vascular remodeling, as well as fibrogenic properties during pulmonary fibrosis. HIMF expression in the lung was reported to be regulated by Th2 cytokines (IL-4 and IL-13) via the transcription factor STAT6 pathway in a bleomycin-induced pulmonary fibrosis model. However, in this study, we found that in the hypoxia-induced pulmonary hypertension model, lung HIMF expression is increased in IL-4 and STAT6 knockout (KO) mice to the same degree as in wild-type (WT) mice, suggesting that induction of HIMF expression does not require Th2 regulation in this model. We also found that HIMF-induced proliferative activity, hypertrophy, collagen, and extracellular matrix deposition in the pulmonary arteries are significantly less in IL-4 KO mice than in WT mice. In addition, HIMF-induced production of angiogenic factors/chemokines, such as vascular endothelial growth factor, MCP-1, and stromal-derived factor-1, in the lung resident cells, as well as macrophage infiltration, were significantly suppressed in the lungs of IL-4 KO mice. We also show that IL-4 was significantly increased in the lungs of HIMF-treated WT mice. Our in vitro studies using pulmonary microvascular endothelial cells revealed that HIMF stimulated cell proliferation, vascular endothelial growth factor expression, and MCP-1 production in a manner that is dependent on the IL-4/IL-4Rα system. These findings suggest that IL-4 signaling may play a significant role in HIMF-induced lung inflammation and vascular remodeling.

Strategic plan for lung vascular research: An NHLBI-ORDR Workshop Report

The Division of Lung Diseases of the National Heart, Lung, and Blood Institute, with the Office of Rare Diseases Research, held a workshop to identify priority areas and strategic goals to enhance and accelerate research that will result in improved understanding of the lung vasculature, translational research needs, and ultimately the care of patients with pulmonary vascular diseases. Multidisciplinary experts with diverse experience in laboratory, translational, and clinical studies identified seven priority areas and discussed limitations in our current knowledge, technologies, and approaches. The focus for future research efforts include the following: (1) better characterizing vascular genotype-phenotype relationships and incorporating systems biology approaches when appropriate; (2) advancing our understanding of pulmonary vascular metabolic regulatory signaling in health and disease; (3) expanding our knowledge of the biologic relationships between the lung circulation and circulating elements, systemic vascular function, and right heart function and disease; (4) improving translational research for identifying disease-modifying therapies for the pulmonary hypertensive diseases; (5) establishing an appropriate and effective platform for advancing translational findings into clinical studies testing; and (6) developing the specific technologies and tools that will be enabling for these goals, such as question-guided imaging techniques and lung vascular investigator training programs. Recommendations from this workshop will be used within the Lung Vascular Biology and Disease Extramural Research Program for planning and strategic implementation purposes.

Opposing effects of bim and bcl-2 on lung endothelial cell migration

Integration of cell adhesive, survival, and proliferative processes is essential for capillary morphogenesis of endothelial cells (EC) in vitro and vascular development and function in vivo. Unfortunately, the molecular and cellular mechanisms that impact these processes are poorly defined. Here we examined how lack of bim and/or bcl-2 expression impact lung EC function. The absence of bcl-2 or bim had a significant impact on EC adhesion and migration. Lack of bcl-2 expression decreased lung EC migration, whereas lack of bim expression increased migration compared with their wild-type counterparts. Decreased adhesion to fibronectin and vitronectin was observed in both bcl-2-/- and bim-/- lung EC, with bcl-2-/- EC having very little adhesion to either matrix protein. Capillary morphogenesis was greatly diminished in bcl-2-/- EC, which correlated with decreased lung alveolarization in vivo, an angiogenesis-dependent process. We also observed aberrant production of extracellular matrix proteins, eNOS expression, and nitric oxide production in bcl-2-/- lung EC, which could contribute to inability to undergo capillary morphogenesis. The changes in cell adhesion and migration noted in the absence of bim or bcl-2 were independent of their impact on apoptosis. We observed no significant affect on the steady-state rate of apoptosis of lung EC in the absence of bim or bcl-2. Thus, bcl-2 family members, bim and bcl-2, play a central role in modulation of EC proangiogenic properties, which goes beyond their role as simple mediators of mitochondrial homeostasis and apoptosis.

Autophagy in the lung

Autophagy is a cellular process for the disposal of damaged organelles or denatured proteins through a lysosomal degradation pathway. By reducing endogenous macromolecules to their basic components (i.e., amino acids, lipids), autophagy serves a homeostatic function by ensuring cell survival during starvation. Increased autophagy can be found in dying cells, although the relationships between autophagy and programmed cell death remain unclear. To date, few studies have examined the regulation and functional significance of autophagy in human lung disease. The lung, a complex organ that functions primarily in gas exchange, consists of diverse cell types (i.e., endothelial, epithelial, mesenchymal, inflammatory). In lung cells, autophagy may represent a general inducible adaptive response to injury resulting from exposure to stress agents, including hypoxia, oxidants, inflammation, ischemia-reperfusion, endoplasmic reticulum stress, pharmaceuticals, or inhaled xenobiotics (i.e., air pollution, cigarette smoke). In recent studies, we have observed increased autophagy in mouse lungs subjected to chronic cigarette smoke exposure, and in pulmonary epithelial cells exposed to cigarette smoke extract. Knockdown of autophagic proteins inhibited apoptosis in response to cigarette smoke exposure in vitro, suggesting that increased autophagy was associated with epithelial cell death. We have also observed increased morphological and biochemical markers of autophagy in human lung specimens from patients with chronic obstructive pulmonary disease (COPD). We hypothesize that increased autophagy contributes to COPD pathogenesis by promoting epithelial cell death. Further research will examine whether autophagy plays a homeostatic or maladaptive role in COPD and other human lung diseases.

Gene expression profile in flow-associated pulmonary arterial hypertension with neointimal lesions

Pulmonary arterial hypertension (PAH) is a pulmonary angioproliferative disease with high morbidity and mortality, characterized by a typical pattern of pulmonary vascular remodeling including neointimal lesions. In congenital heart disease, increased pulmonary blood flow has appeared to be a key mediator in the development of these characteristic lesions, but the molecular mechanisms underlying the pulmonary vascular lesions are largely unknown. We employed a rat model of flow-associated PAH, which induced specific pulmonary neointimal lesions. We identified gene expression profiles in rats specifically related to the addition of increased pulmonary blood flow to monocrotaline and the associated occurrence of neointimal lesions. Increased pulmonary blood flow induced the expression of the transcription factors activating transcription factor-3 (ATF3) and early growth response factor-1 (EGR-1), for which presence was confirmed in neointimal lesions. Monocrotaline alone induced increased numbers of activated mast cells and their products. We further identified molecular pathways that may be involved in treatment with the prostacyclin analog iloprost, a vasoactive compound with clinically beneficial effects in patients with PAH, which were similar to pathways described in samples from patient studies. These pathways, associated with the development of angioproliferative lesions as well as with the response to therapy in PAH, may provide new therapeutic targets.

A microwell array device with integrated microfluidic components for enhanced single-cell analysis

Increasing awareness of the importance of cell heterogeneity in many biological and medical contexts is prompting increasing interest in systems that allow single-cell analysis rather than conventional bulk analysis (which provides average values for variables of interest from large numbers of cells). Recently, we presented a microwell chip for long-term, high-throughput single-cell analysis. The chip has proved to be useful for purposes such as screening individual cancer and stem cells for protein/gene markers. However, liquids in the wells can only be added or changed by manually rinsing the chip, or parts of it. This procedure has several well-known drawbacks--including risks of cross-contamination, large dead volumes and laboriousness--but there have been few previous attempts to integrate liquid rinsing/switching channels in "ready-to-use" systems for single-cell analysis. Here we present a microwell system designed (using flow simulations) for single-cell analysis with integrated microfluidic components (microchannels, magnetically driven micropumps and reservoirs) for supplying the cell culture wells with reagents, or rinsing, thus facilitating controlled, directed liquid handling. It can be used totally independently, since tubing is not essential. The practical utility of the integrated system has been demonstrated by culturing endothelial cells in the microwells, and successfully applying live-cell Calcein AM staining. Systems such as this combining high-density microwell chips with microfluidic components have great potential in numerous screening applications, such as exploring the important, but frequently undetected, heterogeneity in drug responses among individual cells.

Lung organogenesis

Developmental lung biology is a field that has the potential for significant human impact: lung disease at the extremes of age continues to cause major morbidity and mortality worldwide. Understanding how the lung develops holds the promise that investigators can use this knowledge to aid lung repair and regeneration. In the decade since the "molecular embryology" of the lung was first comprehensively reviewed, new challenges have emerged-and it is on these that we focus the current review. Firstly, there is a critical need to understand the progenitor cell biology of the lung in order to exploit the potential of stem cells for the treatment of lung disease. Secondly, the current familiar descriptions of lung morphogenesis governed by growth and transcription factors need to be elaborated upon with the reinclusion and reconsideration of other factors, such as mechanics, in lung growth. Thirdly, efforts to parse the finer detail of lung bud signaling may need to be combined with broader consideration of overarching mechanisms that may be therapeutically easier to target: in this arena, we advance the proposal that looking at the lung in general (and branching in particular) in terms of clocks may yield unexpected benefits.

National Heart, Lung, and Blood Institute perspective: lung progenitor and stem cells--gaps in knowledge and future opportunities

Because the lung stem cell field is so new, there remain many unanswered questions that are being addressed regarding the identification, location, and role of exogenous and endogenous stem and progenitor cell populations in growth, regeneration, and repair of the lung. Advancing lung stem cell biology will require multidisciplinary teams and a long term effort to unravel the biologic processes of stem cells in the lung. While no clinical research in lung stem cell therapies are currently funded by NHLBI, the knowledge gained by understanding the basic biology of the lung stem cell populations will be needed to translate to diagnostic and therapeutic strategies in the future.

Combinatorial ligand-directed lung targeting

Phage display of random peptide libraries is a powerful, unbiased method frequently used to discover ligands for virtually any protein of interest and to reveal functional protein-protein interaction partners. Moreover, in vivo phage display permits selection of peptides that bind specifically to different vascular beds without any previous knowledge pertaining to the nature of their corresponding receptors. Vascular targeting exploits molecular differences inherent in blood vessels within given organs and tissues, as well as diversity between normal and angiogenic blood vessels. Over the years, our group has identified phage capable of homing to lung blood vessels based on screenings using immortalized lung endothelial cells combined with in vivo selections after intravenous administration of combinatorial libraries. Peptides targeting lung vasculature have been extensively characterized and a lead homing peptide has shown interesting biological properties, bringing novel insights as to the implications of lung endothelial cell apoptosis in the pathogenesis of emphysema. We have also designed and developed targeted nanoparticles with imaging capabilities and/or drug delivery functions by combining phage display technology and elemental gold (Au) nanoparticles, constituting a promising platform for the development of therapeutic agents for imaging and treatment of lung disorders. Given the important role of the endothelium in the pathogenesis and progression of several diseases associated with the airways, ligand-directed discovery of lung vascular markers is an important milestone toward the development of future targeted therapies.

Pulmonary vascular heterogeneity and the Starling hypothesis

It has generally been assumed that movement of fluid between the pulmonary microvasculature and surrounding tissues is governed by a "Starling" balance of hydrostatic and protein osmotic forces similar to that which prevails in the extremities. However, both recent and older observations suggest that the lungs are more resistant to edema formation than most other organs. Several structural aspects of the lung may account for protection of the airspaces from edema formation. The pulmonary microvasculature, which comprises >70% of the pulmonary circulatory bed, appears to be less permeable to fluid and electrolytes than the endothelium of the pulmonary arteries and veins and other microvascular exchange areas. This arrangement may help explain why early edema is confined to the perivascular and peribronchial regions and why lymphatics do not reach the alveoli. Unlike the peripheral vasculature, which is compressed by edema formation, the extra-alveolar vessels remain tethered open by airway distention, even when interstitial pressures rise above those in the vessels. This may also facilitate return of proteins to the circulation. Ultrafiltration of plasma may lower local protein concentrations in the interstitium, thereby slowing further edema formation. Transendothelial reabsorption of fluid may also be altered by vesicular transport.

Long-term effects of prenatal hypoxia on endothelium-dependent relaxation responses in pulmonary arteries of adult sheep

Chronic hypoxia during the course of pregnancy is a common insult to the fetus. However, its long-term effect on the pulmonary vasculature in adulthood has not been described. In this study, the vasorelaxation responses of conduit pulmonary arteries in adult female sheep that were chronically hypoxic as fetuses and raised postnatally at sea level were investigated. Vessel tension studies revealed that endothelium-dependent relaxation responses were attenuated in pulmonary arteries from adult sheep that experienced prenatal hypoxia. Endothelial nitric oxide synthase (eNOS) protein expression was unchanged, but eNOS activity was significantly decreased in pulmonary arteries from prenatally hypoxic sheep. Protein expression of eNOS partners, caveolin-1, calmodulin, and heat shock protein 90 (Hsp90) did not change following prenatal hypoxia. However, the association between eNOS and caveolin-1, its inhibitory binding partner, was significantly increased, whereas association between eNOS and its stimulatory partners calmodulin and Hsp90 was greatly decreased. Furthermore, phosphorylation of Ser(1177) in eNOS decreased, whereas phosphorylation of Thr(495) increased, in the prenatally hypoxic pulmonary arteries, events that are related to eNOS activity. These data demonstrate that prenatal hypoxia results in persistent abnormalities in endothelium-dependent relaxation responses of pulmonary arteries in adult sheep due to decreased eNOS activity resulting from altered posttranslational regulation.