Spatial dependence of alveolar angiogenesis in post-pneumonectomy lung growth

Phenomena of Intussusceptive Angiogenesis and Intussusceptive Lymphangiogenesis in Blood and Lymphatic Vessel Tumors

Intussusceptive angiogenesis (IA) and intussusceptive lymphangiogenesis (IL) play a key role in the growth and morphogenesis of vessels. However, there are very few studies in this regard in vessel tumors (VTs). Our objective is to assess the presence, characteristics, and possible mechanisms of the formation of intussusceptive structures in a broad spectrum of VTs. For this purpose, examples of benign and malignant blood and lymphatic VTs were studied via conventional procedures, semithin sections, and immunochemistry and immunofluorescence microscopy. The results demonstrated intussusceptive structures (pillars, meshes, and folds) in benign (lobular capillary hemangioma or pyogenic granuloma, intravascular papillary endothelial hyperplasia or Masson tumor, sinusoidal hemangioma, cavernous hemangioma, glomeruloid hemangioma, angiolipoma, and lymphangiomas), low-grade malignancy (retiform hemangioendothelioma and Dabska tumor), and malignant (angiosarcoma and Kaposi sarcoma) VTs. Intussusceptive structures showed an endothelial cover and a core formed of connective tissue components and presented findings suggesting an origin through vessel loops, endothelialized thrombus, interendothelial bridges, and/or splitting and fusion, and conditioned VT morphology. In conclusion, the findings support the participation of IA and IL, in association with sprouting angiogenesis, in VTs, and therefore in their growth and morphogenesis, which is of pathophysiological interest and lays the groundwork for in-depth molecular studies with therapeutic purposes.

Vascularization of the adult mouse lung grafted onto the chick chorioallantoic membrane

In the later stages of angiogenesis, the vascular sprout transitions into a functional vessel by fusing with a target vessel. Although this process appears to routinely occur in embryonic tissue, the biologic rules for sprout fusion and lumenization in adult regenerating tissue are unknown. To investigate this process, we grafted portions of the regenerating post-pneumonectomy lung onto the chick chorioallantoic membrane (CAM). Grafts from all 4 lobes of the post-pneumonectomy right lung demonstrated peri-graft angiogenesis as reflected by fluorescent plasma markers; however, fluorescent microsphere perfusion primarily occurred in the lobe of the lung that is the dominant site of post-pneumonectomy angiogenesis-namely, the cardiac lobe. Vascularization of the cardiac lobe grafts was confirmed by active tissue growth (p < .05). Functional vascular connections between the cardiac lobe and the CAM vascular network were demonstrated by confocal fluorescence microscopy as well as corrosion casting and scanning electron microscopy (SEM). Bulk transcriptional profiling of the cardiac lobe demonstrated the enhanced expression of many genes relative to alveolar epithelial cell (CD11b-/CD31-) control cells, but only the upregulation of Ereg and Fgf6 compared to the less well-vascularized right upper lobe. The growth of actively regenerating non-neoplastic adult tissue on the CAM demonstrates that functional lumenization can occur between species (mouse and chick) and across the developmental spectrum (adult and embryo).

Transluminal Pillars-Their Origin and Role in the Remodelling of the Zebrafish Caudal Vein Plexus

Intussusceptive pillars, regarded as a hallmark of intussusceptive angiogenesis, have been described in developing vasculature of many organs and organisms. The aim of this study was to resolve the question about pillar formation and their further maturation employing zebrafish caudal vein plexus (CVP). The CVP development was monitored by in vivo confocal microscopy in high spatio-temporal resolution using the transgenic zebrafish model Fli1a:eGPF//Gata1:dsRed. We tracked back the formation of pillars (diameter ≤ 4 µm) and intercapillary meshes (diameter > 4 µm) and analysed their morphology and behaviour. Transluminal pillars in the CVP arose via a combination of sprouting, lumen expansion, and/or the creation of intraluminal folds, and those mechanisms were not associated directly with blood flow. The follow-up of pillars indicated that one-third of them disappeared between 28 and 48 h post fertilisation (hpf), and of the remaining ones, only 1/17 changed their cross-section area by >50%. The majority of the bigger meshes (39/62) increased their cross-section area by >50%. Plexus simplification and the establishment of hierarchy were dominated by the dynamics of intercapillary meshes, which formed mainly via sprouting angiogenesis. These meshes were observed to grow, reshape, and merge with each other. Our observations suggested an alternative view on intussusceptive angiogenesis in the CVP.

A pneumonectomy model to study flow-induced pulmonary hypertension and compensatory lung growth

In newborns, developmental disorders such as congenital diaphragmatic hernia (CDH) and specific types of congenital heart disease (CHD) can lead to defective alveolarization, pulmonary hypoplasia, and pulmonary arterial hypertension (PAH). Therapeutic options for these patients are limited, emphasizing the need for new animal models representative of disease conditions. In most adult mammals, compensatory lung growth (CLG) occurs after pneumonectomy; however, the underlying relationship between CLG and flow-induced pulmonary hypertension (PH) is not fully understood. We propose a murine model that involves the simultaneous removal of the left lung and right caval lobe (extended pneumonectomy), which results in reduced CLG and exacerbated reproducible PH. Extended pneumonectomy in mice is a promising animal model to study the cellular response and molecular mechanisms contributing to flow-induced PH, with the potential to identify new treatments for patients with CDH or PAH-CHD.

Intussusceptive angiogenesis facilitated by microthrombosis has an important example in angiolipoma. An ultrastructural and immunohistochemical study

The microvasculature of angiolipoma frequently presents thrombi. Our objectives are to assess whether intussusceptive angiogenesis (IA) participates in vasculature formation in non-infiltrating angiolipoma and, if so, to explore how thrombi are involved in the IA process. For this purpose, we studied angiolipoma specimens (n: 52), using immunohistochemistry, and confocal and electron microscopy. The results showed the presence of folds and pillars, hallmarks of IA, dividing the vessel lumen. Folds showed a cover formed by reoriented endothelial cells from the vessel wall, or from newly formed folds, and a core initially formed by thrombus fragments (clot components as transitional core), which was replaced by extracellular matrix and invaginating pericytes establishing numerous peg-and-socket junctions with endothelial cells (mature core). A condensed plasmatic electron-dense material surrounded and connected folds and pillars with each other and with the vascular wall, which suggests a clot role in fold/pillar arrangement. In conclusion, we contribute to IA participation in capillary network formation in angiolipoma and the immunohistochemical and ultrastructural events by which microthrombosis facilitates IA. Therefore, in addition to the histogenesis of angiolipoma, we provide an easily obtainable substrate for future studies on clot component action in IA, of clinical and therapeutic interest.

Comparison of the Behavior of Perivascular Cells (Pericytes and CD34+ Stromal Cell/Telocytes) in Sprouting and Intussusceptive Angiogenesis

Perivascular cells in the pericytic microvasculature, pericytes and CD34+ stromal cells/telocytes (CD34+SCs/TCs), have an important role in angiogenesis. We compare the behavior of these cells depending on whether the growth of endothelial cells (ECs) from the pre-existing microvasculature is toward the interstitium with vascular bud and neovessel formation (sprouting angiogenesis) or toward the vascular lumen with intravascular pillar development and vessel division (intussusceptive angiogenesis). Detachment from the vascular wall, mobilization, proliferation, recruitment, and differentiation of pericytes and CD34+SCs/TCs, as well as associated changes in vessel permeability and functionality, and modifications of the extracellular matrix are more intense, longer lasting over time, and with a greater energy cost in sprouting angiogenesis than in intussusceptive angiogenesis, in which some of the aforementioned events do not occur or are compensated for by others (e.g., sparse EC and pericyte proliferation by cell elongation and thinning). The governing mechanisms involve cell-cell contacts (e.g., peg-and-socket junctions between pericytes and ECs), multiple autocrine and paracrine signaling molecules and pathways (e.g., vascular endothelial growth factor, platelet-derived growth factor, angiopoietins, transforming growth factor B, ephrins, semaphorins, and metalloproteinases), and other factors (e.g., hypoxia, vascular patency, and blood flow). Pericytes participate in vessel development, stabilization, maturation and regression in sprouting angiogenesis, and in interstitial tissue structure formation of the pillar core in intussusceptive angiogenesis. In sprouting angiogenesis, proliferating perivascular CD34+SCs/TCs are an important source of stromal cells during repair through granulation tissue formation and of cancer-associated fibroblasts (CAFs) in tumors. Conversely, CD34+SCs/TCs have less participation as precursor cells in intussusceptive angiogenesis. The dysfunction of these mechanisms is involved in several diseases, including neoplasms, with therapeutic implications.

Low-flow intussusception and metastable VEGFR2 signaling launch angiogenesis in ischemic muscle

Efforts to promote sprouting angiogenesis in skeletal muscles of individuals with peripheral artery disease have not been clinically successful. We discovered that, contrary to the prevailing view, angiogenesis following ischemic muscle injury in mice was not driven by endothelial sprouting. Instead, real-time imaging revealed the emergence of wide-caliber, primordial conduits with ultralow flow that rapidly transformed into a hierarchical neocirculation by transluminal bridging and intussusception. This process was accelerated by inhibiting vascular endothelial growth factor receptor-2 (VEGFR2). We probed this response by developing the first live-cell model of transluminal endothelial bridging using microfluidics. Endothelial cells subjected to ultralow shear stress could reposition inside the flowing lumen as pillars. Moreover, the low-flow lumen proved to be a privileged location for endothelial cells with reduced VEGFR2 signaling capacity, as VEGFR2 mechanosignals were boosted. These findings redefine regenerative angiogenesis in muscle as an intussusceptive process and uncover a basis for its launch.

Alveologenesis: What Governs Secondary Septa Formation

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

Epigenetic Regulation of the Vascular Endothelium by Angiogenic LncRNAs

The functional properties of the vascular endothelium are diverse and heterogeneous between vascular beds. This is especially evident when new blood vessels develop from a pre-existing closed cardiovascular system, a process termed angiogenesis. Endothelial cells are key drivers of angiogenesis as they undergo a highly choreographed cascade of events that has both exogenous (e.g., hypoxia and VEGF) and endogenous regulatory inputs. Not surprisingly, angiogenesis is critical in health and disease. Diverse therapeutics target proteins involved in coordinating angiogenesis with varying degrees of efficacy. It is of great interest that recent work on non-coding RNAs, especially long non-coding RNAs (lncRNAs), indicates that they are also important regulators of the gene expression paradigms that underpin this cellular cascade. The protean effects of lncRNAs are dependent, in part, on their subcellular localization. For instance, lncRNAs enriched in the nucleus can act as epigenetic modifiers of gene expression in the vascular endothelium. Of great interest to genetic disease, they are undergoing rapid evolution and show extensive inter- and intra-species heterogeneity. In this review, we describe endothelial-enriched lncRNAs that have robust effects in angiogenesis.

Endothelial metabolism in pulmonary vascular homeostasis and acute respiratory distress syndrome

Capillary endothelial cells possess a specialized metabolism necessary to adapt to the unique alveolar-capillary environment. Here, we highlight how endothelial metabolism preserves the integrity of the pulmonary circulation by controlling vascular permeability, defending against oxidative stress, facilitating rapid migration and angiogenesis in response to injury, and regulating the epigenetic landscape of endothelial cells. Recent reports on single-cell RNA-sequencing reveal subpopulations of pulmonary capillary endothelial cells with distinctive reparative capacities, which potentially offer new insight into their metabolic signature. Lastly, we discuss broad implications of pulmonary vascular metabolism on acute respiratory distress syndrome, touching on emerging findings of endotheliitis in coronavirus disease 2019 (COVID-19) lungs.

Intussusceptive Angiogenesis and Peg-Socket Junctions between Endothelial Cells and Smooth Muscle Cells in Early Arterial Intimal Thickening

Angiogenesis in arterial intimal thickening (AIT) has been considered mainly in late AIT stages and only refers to sprouting angiogenesis. We assess angiogenesis during early AIT development and the occurrence of the intussusceptive type. For this purpose, we studied AIT development in (a) human arteries with vasculitis in gallbladders with acute cholecystitis and urgent (n = 25) or delayed (n = 20) cholecystectomy, using immunohistochemical techniques and (b) experimentally occluded arterial segments (n = 56), using semithin and ultrathin sections and electron microscopy. The results showed transitory angiogenic phenomena, with formation of an important microvasculature, followed by vessel regression. In addition to the sequential description of angiogenic and regressive findings, we mainly contribute (a) formation of intravascular pillars (hallmarks of intussusception) during angiogenesis and vessel regression and (b) morphological interrelation between endothelial cells (ECs) in the arterial wall and vascular smooth muscle cells (VSMCs), which adopt a pericytic arrangement and establish peg-and-socket junctions with ECs. In conclusion, angiogenesis and vessel regression play an important role in AIT development in the conditions studied, with participation of intussusceptive angiogenesis during the formation and regression of a provisional microvasculature and with morphologic interrelation between ECs and VSMCs.

Single-Cell Transcriptional Profiling of Cells Derived From Regenerating Alveolar Ducts

Lung regeneration occurs in a variety of adult mammals after surgical removal of one lung (pneumonectomy). Previous studies of murine post-pneumonectomy lung growth have identified regenerative “hotspots” in subpleural alveolar ducts; however, the cell-types participating in this process remain unclear. To identify the single cells participating in post-pneumonectomy lung growth, we used laser microdissection, enzymatic digestion and microfluidic isolation. Single-cell transcriptional analysis of the murine alveolar duct cells was performed using the C1 integrated fluidic circuit (Fluidigm) and a custom PCR panel designed for lung growth and repair genes. The multi-dimensional data set was analyzed using visualization software based on the tSNE algorithm. The analysis identified 6 cell clusters; 1 cell cluster was present only after pneumonectomy. This post-pneumonectomy cluster was significantly less transcriptionally active than 3 other clusters and may represent a transitional cell population. A provisional cluster identity for 4 of the 6 cell clusters was obtained by embedding bulk transcriptional data into the tSNE analysis. The transcriptional pattern of the 6 clusters was further analyzed for genes associated with lung repair, matrix production, and angiogenesis. The data demonstrated that multiple cell-types (clusters) transcribed genes linked to these basic functions. We conclude that the coordinated gene expression across multiple cell clusters is likely a response to a shared regenerative microenvironment within the subpleural alveolar ducts.

Intussusceptive angiogenesis and its counterpart intussusceptive lymphangiogenesis

Intussusceptive angiogenesis (IA) is currently considered an important alternative and complementary form of sprouting angiogenesis (SA). Conversely, intussusceptive lymphangiogenesis (IL) is in an initial phase of study. We compare their morphofunctional characteristics, since many can be shared by both processes. To that end, the following aspects are considered: A) The concept of IA and IL as the mechanism by which blood and lymphatic vessels split, expand and remodel through transluminal pillar formations (hallmarks of intussusception). B) Terminology and historical background, with particular reference to the group of Burri, including Djonov and Patan, who initiated and developed the vessel intussusceptive concept in blood vessels. C) Incidence in normal (e.g. in the sinuses of developing lymph nodes) and pathologic conditions, above all in vessel diseases, such as dilated veins in hemorrhoidal disease, intravascular papillary endothelial hyperplasia (IPEH), sinusoidal hemangioma, lobular capillary hemangioma, lymphangiomas/lymphatic malformations and vascular transformation of lymph nodes. D) Differences and complementarity between vessel sprouting and intussusception. E) Characteristics of the cover (endothelial cells) and core (connective tissue components) of pillars and requirements for pillar identification. F) Structures involved in pillar formation, including endothelial contacts of opposite vessel walls, interendothelial bridges, merged adjacent capillaries, vessel loops and spilt pillars. G) Structures resulting from pillars with intussusceptive microvascular growth, arborization, remodeling and segmentation (compartmentalization). H) Influence of intussusception in the morphogenesis of vessel tumors/ pseudotumors; and I) Hemodynamic and molecular control of vessel intussusception, including VEGF, PDGF BB, Hypoxia, Notch, Endoglobin and Nitric oxide.

Morphomolecular motifs of pulmonary neoangiogenesis in interstitial lung diseases

The pathogenetic role of angiogenesis in interstitial lung diseases (ILDs) is controversial. This study represents the first investigation of the spatial complexity and molecular motifs of microvascular architecture in important subsets of human ILD. The aim of our study was to identify specific variants of neoangiogenesis in three common pulmonary injury patterns in human ILD.We performed comprehensive and compartment-specific analysis of 24 human lung explants with usual intersitial pneumonia (UIP), nonspecific interstitial pneumonia (NSIP) and alveolar fibroelastosis (AFE) using histopathology, microvascular corrosion casting, micro-comupted tomography based volumetry and gene expression analysis using Nanostring as well as immunohistochemistry to assess remodelling-associated angiogenesis.Morphometrical assessment of vessel diameters and intervascular distances showed significant differences in neoangiogenesis in characteristically remodelled areas of UIP, NSIP and AFE lungs. Likewise, gene expression analysis revealed distinct and specific angiogenic profiles in UIP, NSIP and AFE lungs.Whereas UIP lungs showed a higher density of upstream vascularity and lower density in perifocal blood vessels, NSIP and AFE lungs revealed densely packed alveolar septal blood vessels. Vascular remodelling in NSIP and AFE is characterised by a prominent intussusceptive neoangiogenesis, in contrast to UIP, in which sprouting of new vessels into the fibrotic areas is characteristic. The molecular analyses of the gene expression provide a foundation for understanding these fundamental differences between AFE and UIP and give insight into the cellular functions involved.

Endothelial MT1-MMP targeting limits intussusceptive angiogenesis and colitis via TSP1/nitric oxide axis

Pathological angiogenesis contributes to cancer progression and chronic inflammatory diseases. In inflammatory bowel disease, the microvasculature expands by intussusceptive angiogenesis (IA), a poorly characterized mechanism involving increased blood flow and splitting of pre-existing capillaries. In this report, mice lacking the protease MT1-MMP in endothelial cells (MT1iΔEC ) presented limited IA in the capillary plexus of the colon mucosa assessed by 3D imaging during 1% DSS-induced colitis. This resulted in better tissue perfusion, preserved intestinal morphology, and milder disease activity index. Combined in vivo intravital microscopy and lentiviral rescue experiments with in vitro cell culture demonstrated that MT1-MMP activity in endothelial cells is required for vasodilation and IA, as well as for nitric oxide production via binding of the C-terminal fragment of MT1-MMP substrate thrombospondin-1 (TSP1) to CD47/αvβ3 integrin. Moreover, TSP1 levels were significantly higher in serum from IBD patients and in vivo administration of an anti-MT1-MMP inhibitory antibody or a nonamer peptide spanning the αvβ3 integrin binding site in TSP1 reduced IA during mouse colitis. Our results identify MT1-MMP as a new actor in inflammatory IA and a promising therapeutic target for inflammatory bowel disease.

Endothelial YAP1 in Regenerative Lung Growth through the Angiopoietin-Tie2 Pathway

Angiogenesis, the formation of new blood capillaries, plays a key role in organ development and regeneration. Inhibition of lung angiogenesis through the blockade of angiogenic signaling pathways impairs compensatory and regenerative lung growth after unilateral pneumonectomy (PNX). The Hippo signaling transducer, Yes-associated protein (YAP) 1 binds to TEA domain transcription factor (TEAD) and controls organ size and regeneration. However, the role of endothelial YAP1 in lung vascular and alveolar morphogenesis remains unclear. In this report, we demonstrate that knockdown of YAP1 in endothelial cells (ECs) decreases angiogenic factor receptor Tie2 expression, and inhibits EC sprouting and epithelial cell budding in vitro and vascular and alveolar morphogenesis in the gel implanted on the mouse lung. The expression levels of YAP1, TEAD1, and Tie2 increase in ECs isolated from the remaining mouse lungs after unilateral PNX and vascular formation is stimulated in the post-PNX mouse lungs. Knockdown of endothelial YAP1 inhibits compensatory lung growth and vascular and alveolar morphogenesis after unilateral PNX. These findings suggest that endothelial YAP1 is required for lung vascular and alveolar regeneration and modulation of YAP1 in ECs may be novel interventions for the improvement of lung regeneration.

Vascular Niche in Lung Alveolar Development, Homeostasis, and Regeneration

Endothelial cells (ECs) constitute small capillary blood vessels and contribute to delivery of nutrients, oxygen and cellular components to the local tissues, as well as to removal of carbon dioxide and waste products from the tissues. Besides these fundamental functions, accumulating evidence indicates that capillary ECs form the vascular niche. In the vascular niche, ECs reciprocally crosstalk with resident cells such as epithelial cells, mesenchymal cells, and immune cells to regulate development, homeostasis, and regeneration in various organs. Capillary ECs supply paracrine factors, called angiocrine factors, to the adjacent cells in the niche and orchestrate these processes. Although the vascular niche is anatomically and functionally well-characterized in several organs such as bone marrow and neurons, the effects of endothelial signals on other resident cells and anatomy of the vascular niche in the lung have not been well-explored. This review discusses the role of alveolar capillary ECs in the vascular niche during development, homeostasis and regeneration.

Intussusceptive Lymphangiogenesis in Lymphatic Malformations/Lymphangiomas

Intussusception in lymphatic vessels has received less attention than in blood vessels. In tumors and pseudotumors of blood vessels with intravascular papillary structures, including sinusoidal hemangioma and intravascular papillary endothelial hyperplasia, we observed exuberant intussusceptive angiogenesis, as well as the similarity between papillae (term used by pathologists) and pillars/folds (hallmarks of intussusceptive angiogenesis). A similar response could be expected in lymphangiomas (lymphatic malformations and reactive processes rather than tumors) with papillae. The aim of this work is to assess whether papillae/pillars/folds and associated structures (vessel loops and septa) are present in lymphangiomas, and to establish the characteristics and formation of these structures. For this purpose, we selected lymphangiomas with intraluminal papillae (n = 18), including cystic, cavernous, circumscriptum, and progressive types, of which two cases of each type with a greater number of papillae were used for serial histologic sections and immunohistochemistry. The studies showed a) dilated lymphatic spaces giving rise to lymphatic-lymphatic vascular loops, which dissected and encircled perilymphatic structures (interstitial tissue structures/ITSs and pillars/posts), b) ITSs and pillars, surrounded by anti-podoplanin-positive endothelial cells, protruding into the lymphatic spaces (papillary aspect), and c) splitting, remodeling, linear arrangement, and fusion of papillae/pillars/folds, forming papillary networks and septa. In conclusion, as occurs in blood vessel diseases, the development of lymphatic vessel loops, papillae/pillars/folds, and septa (segmentation) supports intussusceptive lymphangiogenesis and suggests a piecemeal form of intussusception. This intussusceptive lymphangiogenesis in lymphatic diseases can provide a basis for further studies of lymphatic intussusception in other conditions, with clinical and therapeutic implications. Anat Rec, 302:2003-2013, 2019. © 2019 American Association for Anatomy.

Intussusceptive lymphangiogenesis in vascular transformation of lymph node sinuses

Numerous lymphatic anastomosing channels in the lymph nodes are the most demonstrative finding of the rare lesion termed "vascular transformation of lymph node sinuses" (VTS). The mechanism of lymphatic vessel formation in VTS has not been studied. Vessel intussusception contributes to vascular expansion, and intraluminal pillars/posts, interstitial tissue structures or larger pillars (ITSs) and folds are the hallmarks of this process in blood vessels. The aim of this work is to assess whether these hallmarks of intussusception occur in VTS lymphatic vessels, indicating intussusceptive lymphangiogenesis. For this purpose, specimens of five cases of VTS were used for serial histological sections, immunohistochemistry and immunofluorescence in confocal microscopy, which enabled us to demonstrate the 3D image that defines the pillars. The studies showed a) meshworks of lymphatic vessels, which form complex loops, resembling sinuses of lymph nodes, b) presence of intralymphatic pillars, ITSs and folds, with a cover of lymphatic endothelial cells expressing podoplanin and a varying-sized connective core (e.g. collagen), and c) increase of vessel meshwork and linear arrangement, splitting and fusion of ITSs, pillars and folds, with remodelling and segmentation. In conclusion, the development of lymphatic vessel loops, ITSs, pillars and folds with segmentation in VTS supports intussusceptive lymphangiogenesis. This mechanism of intussusception is of interest because it participates in VTS histogenesis, contributes to general knowledge of intussusceptive lymphangiogenesis, which has received less attention than intussusception in blood vessels, and provides a basis for further studies in other lymphatic conditions.

SDF-1/CXCR4 signalling is involved in blood vessel growth and remodelling by intussusception

The precise mechanisms of SDF‐1 (CXCL12) in angiogenesis are not fully elucidated. Recently, we showed that Notch inhibition induces extensive intussusceptive angiogenesis by recruitment of mononuclear cells and it was associated with increased levels of SDF‐1 and CXCR4. In the current study, we demonstrated SDF‐1 expression in liver sinusoidal vessels of Notch1 knockout mice with regenerative hyperplasia by means of intussusception, but we did not detect any SDF‐1 expression in wild‐type mice with normal liver vessel structure. In addition, pharmacological inhibition of SDF‐1/CXCR4 signalling by AMD3100 perturbs intussusceptive vascular growth and abolishes mononuclear cell recruitment in the chicken area vasculosa. In contrast, treatment with recombinant SDF‐1 protein increased microvascular density by 34% through augmentation of pillar number compared to controls. The number of extravasating mononuclear cells was four times higher after SDF‐1 application and two times less after blocking this pathway. Bone marrow‐derived mononuclear cells (BMDC) were recruited to vessels in response to elevated expression of SDF‐1 in endothelial cells. They participated in formation and stabilization of pillars. The current study is the first report to implicate SDF‐1/CXCR4 signalling in intussusceptive angiogenesis and further highlights the stabilizing role of BMDC in the formation of pillars during vascular remodelling.

Lung development and emerging roles for type 2 immunity

Lung development is a complex process mediated through the interaction of multiple cell types, factors and mediators. In mice, it starts as early as embryonic day 9 and continues into early adulthood. The process can be separated into five different developmental stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar. Whilst lung bud formation and branching morphogenesis have been studied extensively, the mechanisms of alveolarisation are incompletely understood. Aberrant lung development can lead to deleterious consequences for respiratory health such as bronchopulmonary dysplasia (BPD), a disease primarily affecting preterm neonates, which is characterised by increased pulmonary inflammation and disturbed alveolarisation. While the deleterious effects of type 1-mediated inflammatory responses on lung development have been well established, the role of type 2 responses in postnatal lung development remains poorly understood. Recent studies indicate that type 2-associated immune cells, such as group 2 innate lymphoid cells and alveolar macrophages, are increased in number during postnatal alveolarisation. Here, we present the current state of understanding of the postnatal stages of lung development and the key cell types and mediators known to be involved. We also provide an overview of how stem cells are involved in lung development and regeneration, and the negative influences of respiratory infections. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

LRP5 in age-related changes in vascular and alveolar morphogenesis in the lung

Aging is associated with impaired angiogenesis and lung alveolar regeneration, which contributes to the increased susceptibility to chronic lung diseases (CLD). We have reported that the Wnt ligand co-receptor, low-density lipoprotein receptor-related protein 5 (LRP5), stimulates angiogenesis and lung alveolar regeneration. However, the role of LRP5 in age-related decline in vascular and alveolar morphogenesis remains unclear. In this report, we have demonstrated that vascular and alveolar structures are disrupted in the 24-month (24M) old mouse lungs. The expression of LRP5 and the major angiogenic factors, VEGFR2 and Tie2, is lower in endothelial cells (ECs) isolated from 24M old mouse lungs compared to those from 2M old mouse lungs. Vascular and alveolar formation is attenuated in the hydrogel implanted on the 24M old mouse lungs, while overexpression of LRP5, which restores angiogenic factor expression, reverses vascular and alveolar morphogenesis in the gel. Compensatory lung growth after unilateral pneumonectomy is inhibited in 24M old mice, which is reversed by overexpression of LRP5. These results suggest that LRP5 mediates age-related inhibition of angiogenesis and alveolar morphogenesis. Modulation of LRP5 may be a novel intervention to rejuvenate regenerative ability in aged lung and will lead to the development of efficient strategies for aging-associated CLD.

Structural and functional evidence for the scaffolding effect of alveolar blood vessels

A contribution of pulmonary blood distension to alveolar opening was first proposed more than 100 years ago. To investigate the contribution of blood distension to lung mechanics, we studied control mice (normal perfusion), mice after exsanguination (absent perfusion) and mice after varying degrees of parenchymal resection (supra-normal perfusion). On inflation, mean tracheal pressures were higher in the bloodless mouse (4.0 ± 2.5 cm H₂O); however, there was minimal difference between conditions on deflation (0.7 ± 0.9 cm H₂O). To separate the peripheral and central mechanical effects of blood volume, multi-frequency lung impedance data was fitted to the constant-phase model. The presence or absence of blood had no effect on central airway resistance (p > .05). In contrast, measures of tissue damping (G), tissue elastance (H) and hysteresivity (η) demonstrated a significant increase in bloodless mice relative to control mice (p < .001). After varying amount of surgical resection and associated supra-normal perfusion of the remaining lung, there was an increase in G and H. Although the absolute difference in G and H increased with the amount of parenchymal resection, the proportional contribution of blood was identical in all conditions. The presence of blood in the pulmonary vasculature resulted in a constant 64 ± 5% reduction in tissue damping (G) and a 55 ± 4% reduction in tissue elastance (H). This nearly-constant contribution of blood to lung hysteresivity was only reduced by positive end-expiratory pressure (PEEP). To identify a distinct structural subset of vessels in the lung potentially contributing to these observations, vascular casting and scanning electron microscopy of the lung demonstrated morphologically distinct vascular rings at the alveolar opening. Our results suggest that intravascular blood distension, likely attributable to a subset of vessels in the alveolar entrance ring, contributes a measurable scaffolding effect to the functional recruitment of the peripheral lung.

Florid Intussusceptive-like Microvascular Dysangiogenesis in a Preterm Lung

The cellular mechanisms underlying the microvascular dysangiogenesis of bronchopulmonary dysplasia (chronic lung disease of the newborn) remain largely undetermined. We report unusual pulmonary vascular findings in a 27-week-gestation male newborn who died on the second day of life from intractable respiratory failure, following a pregnancy complicated by prolonged membrane rupture and persistent severe oligohydramnios. As expected, postmortem examination revealed pulmonary hypoplasia (lung/body weight ratio: 2.23%; 10th percentile for 27 weeks: 2.59%). In addition, lung microscopy revealed complex networks of non-sprouting, tortuous, and bulbously dilated capillaries, randomly distributed in widened airspace septa. Anti-smooth muscle actin immunohistochemistry demonstrated immunoreactive central densities within capillary lumina, suggestive of intravascular pillar formation. The plexus-forming, non-sprouting type of angiogenesis and apparent transluminal pillar formation are consistent with intussusceptive ("longitudinal splitting") angiogenesis. In concordance with previous observations made in human fetal lung xenografts, these findings support the notion that human postcanalicular lungs have the capacity to switch from sprouting to non-sprouting, intussusceptive-like angiogenesis, possibly representing an adaptive response activated by hemodynamic flow alterations and/or hypoxia. The possible relationship between the intussusceptive-like vascular changes observed in this case and the microvascular dysangiogenesis characteristic of bronchopulmonary dysplasia remains to be determined.

Compensatory lung growth after bilobectomy in emphysematous rats

Lung volume reduction surgery (LVRS) is an option for emphysematous patients who are awaiting lung transplantation. LVRS reduces nonfunctional portions of lung tissues and favors the compensatory lung growth (CLG) of the remaining lobes. This phenomenon diminishes dyspnea and improves both the respiratory mechanics and quality of life for the patients. An animal model of elastase-induced pulmonary emphysema was used to investigate the structural and functional lung response after LVRS. Bilobectomy was performed six weeks after elastase instillation. Two weeks after bilobectomy, CLG effects were evaluated by lung mechanics and histomorphometric analysis. After bilobectomy, the emphysematous animals presented decreased mean linear intercepts, increased elastic fiber proportion, and increased alveolar surface density, total volumes of airspace, tissue and respiratory region and absolute surface area. We conclude that bilobectomy promoted CLG in emphysematous animals, resulting in alveolar architecture repair.

Alveolar septal patterning during compensatory lung growth: Part II the effect of parenchymal pressure gradients

In most mammals, compensatory lung growth occurs after the removal of one lung (pneumonectomy). Although the mechanism of alveolar growth is unknown, the patterning of complex alveolar geometry over organ-sized length scales is a central question in regenerative lung biology. Because shear forces appear capable of signaling the differentiation of important cells involved in neoalveolarization (fibroblasts and myofibroblasts), interstitial fluid mechanics provide a potential mechanism for the patterning of alveolar growth. The movement of interstitial fluid is created by two basic mechanisms: 1) the non-uniform motion of the boundary walls, and 2) parenchymal pressure gradients external to the interstitial fluid. In a previous study (Haber et al., Journal of Theoretical Biology 400: 118-128, 2016), we investigated the effects of non-uniform stretching of the primary septum (associated with its heterogeneous mechanical properties) during breathing on generating non-uniform Stokes flow in the interstitial space. In the present study, we analyzed the effect of parenchymal pressure gradients on interstitial flow. Dependent upon lung microarchitecture and physiologic conditions, parenchymal pressure gradients had a significant effect on the shear stress distribution in the interstitial space of primary septa. A dimensionless parameter δ described the ratio between the effects of a pressure gradient and the influence of non-uniform primary septal wall motion. Assuming that secondary septa are formed where shear stresses were the largest, it is shown that the geometry of the newly generated secondary septa was governed by the value of δ. For δ smaller than 0.26, the alveolus size was halved while for higher values its original size was unaltered. We conclude that the movement of interstitial fluid, governed by parenchymal pressure gradients and non-uniform primary septa wall motion, provides a plausible mechanism for the patterning of alveolar growth.

Evidence for pleural epithelial-mesenchymal transition in murine compensatory lung growth

In many mammals, including rodents and humans, removal of one lung results in the compensatory growth of the remaining lung; however, the mechanism of compensatory lung growth is unknown. Here, we investigated the changes in morphology and phenotype of pleural cells after pneumonectomy. Between days 1 and 3 after pneumonectomy, cells expressing α-smooth muscle actin (SMA), a cytoplasmic marker of myofibroblasts, were significantly increased in the pleura compared to surgical controls (p < .01). Scanning electron microscopy of the pleural surface 3 days post-pneumonectomy demonstrated regions of the pleura with morphologic features consistent with epithelial-mesenchymal transition (EMT); namely, cells with disrupted intercellular junctions and an acquired mesenchymal (rounded and fusiform) morphotype. To detect the migration of the transitional pleural cells into the lung, a biotin tracer was used to label the pleural mesothelial cells at the time of surgery. By post-operative day 3, image cytometry of post-pneumonectomy subpleural alveoli demonstrated a 40-fold increase in biotin⁺ cells relative to pneumonectomy-plus-plombage controls (p < .01). Suggesting a similar origin in space and time, the distribution of cells expressing biotin, SMA, or vimentin demonstrated a strong spatial autocorrelation in the subpleural lung (p < .001). We conclude that post-pneumonectomy compensatory lung growth involves EMT with the migration of transitional mesothelial cells into subpleural alveoli.

Recruited Monocytes and Type 2 Immunity Promote Lung Regeneration following Pneumonectomy

To investigate the role of immune cells in lung regeneration, we used a unilateral pneumonectomy model that promotes the formation of new alveoli in the remaining lobes. Immunofluorescence and single-cell RNA sequencing found CD115+ and CCR2+ monocytes and M2-like macrophages accumulating in the lung during the peak of type 2 alveolar epithelial stem cell (AEC2) proliferation. Genetic loss of function in mice and adoptive transfer studies revealed that bone marrow-derived macrophages (BMDMs) traffic to the lung through a CCL2-CCR2 chemokine axis and are required for optimal lung regeneration, along with Il4ra-expressing leukocytes. Our data suggest that these cells modulate AEC2 proliferation and differentiation. Finally, we provide evidence that group 2 innate lymphoid cells are a source of IL-13, which promotes lung regeneration. Together, our data highlight the potential for immunomodulatory therapies to stimulate alveologenesis in adults.

Piecemeal Mechanism Combining Sprouting and Intussusceptive Angiogenesis in Intravenous Papillary Formation Induced by PGE2 and Glycerol

Recently, we demonstrated that in human intravascular papillary endothelial hyperplasia (IPEH), vein wall vascularization occurs in association with myriad papillae, a large part of which formed in the vascularized vein wall. Previously, using an animal model, we observed that PGE2 and glycerol administration around the femoral vein originates intense vascularization of the vein wall from its intimal endothelial cells (ECs). This vascularization is similar to that in IPEH. The aim of this study is to assess the mechanism of papillary formation, using this model after demonstrating papillary development in neo-vascularized femoral vein walls. In semithin and ultrathin sections, the sequential vascular and papillary development was as follows: (a) activation of vein intimal ECs, (b) sprouting of intimal ECs towards the vein media layer and microvessel development, (c) interconnection between neighboring microvessels originated elementary loops, which encircled vein wall components and formed papillae. The encircling ECs formed the papillary cover, and the encircled component formed the core. The papillae showed a similar structure to that of folds and pillars in intussusceptive angiogenesis, and (d) origin of secondary and complex loop systems by interconnection of neighboring elementary loops and by splitting of papillae by new loops, with abundant papillary development. In conclusion, the results support a piecemeal angiogenic mechanism in papillary formation, with association of sprouting and intussusceptive types of angiogenesis. Further studies are needed to assess whether the intravascular papillae described in several pathologic processes, including vessel tumors, such as Dabska's tumor, retiform hemangioendothelioma, and angiosarcoma, follow a similar mechanism. Anat Rec, 2017. © 2017 Wiley Periodicals, Inc. Anat Rec, 300:1781-1792, 2017. © 2017 Wiley Periodicals, Inc.

Deformation-induced transitional myofibroblasts contribute to compensatory lung growth

In many mammals, including humans, removal of one lung (pneumonectomy) results in the compensatory growth of the remaining lung. Compensatory growth involves not only an increase in lung size, but also an increase in the number of alveoli in the peripheral lung; however, the process of compensatory neoalveolarization remains poorly understood. Here, we show that the expression of α-smooth muscle actin (SMA)-a cytoplasmic protein characteristic of myofibroblasts-is induced in the pleura following pneumonectomy. SMA induction appears to be dependent on pleural deformation (stretch) as induction is prevented by plombage or phrenic nerve transection (P < 0.001). Within 3 days of pneumonectomy, the frequency of SMA+ cells in subpleural alveolar ducts was significantly increased (P < 0.01). To determine the functional activity of these SMA+ cells, we isolated regenerating alveolar ducts by laser microdissection and analyzed individual cells using microfluidic single-cell quantitative PCR. Single cells expressing the SMA (Acta2) gene demonstrated significantly greater transcriptional activity than endothelial cells or other discrete cell populations in the alveolar duct (P < 0.05). The transcriptional activity of the Acta2+ cells, including expression of TGF signaling as well as repair-related genes, suggests that these myofibroblast-like cells contribute to compensatory lung growth.

Regulation of Injury-Induced Ovarian Regeneration by Activation of Oogonial Stem Cells

Some animals have the ability to generate large numbers of oocytes throughout life. This raises the question whether persistent adult germline stem cell populations drive continuous oogenesis and whether they are capable of mounting a regenerative response after injury. Here we demonstrate the presence of adult oogonial stem cells (OSCs) in the adult axolotl salamander ovary and show that ovarian injury induces OSC activation and functional regeneration of the ovaries to reproductive capability. Cells that have morphological similarities to germ cells were identified in the developing and adult ovaries via histological analysis. Genes involved in germ cell maintenance including Vasa, Oct4, Sox2, Nanog, Bmp15, Piwil1, Piwil2, Dazl, and Lhx8 were expressed in the presumptive OSCs. Colocalization of Vasa protein with H3 mitotic marker showed that both oogonial and spermatogonial adult stem cells were mitotically active. Providing evidence of stemness and viability of adult OSCs, enhanced green fluorescent protein (EGFP) adult OSCs grafted into white juvenile host gonads gave rise to EGFP OSCs, and oocytes. Last, the axolotl ovaries completely regenerated after partial ovariectomy injury. During regeneration, OSC activation resulted in rapid differentiation into new oocytes, which was demonstrated by Vasa⁺ /BrdU⁺ coexpression. Furthermore, follicle cell proliferation promoted follicle maturation during ovarian regeneration. Overall, these results show that adult oogenesis occurs via proliferation of endogenous OSCs in a tetrapod and mediates ovarian regeneration. This study lays the foundations to elucidate mechanisms of ovarian regeneration that will assist regenerative medicine in treating premature ovarian failure and reduced fertility. Stem Cells 2017;35:236-247.

Intussusceptive-like angiogenesis in human fetal lung xenografts: Link with bronchopulmonary dysplasia-associated microvascular dysangiogenesis?

BACKGROUND

Human fetal lung xenografts display an unusual pattern of non-sprouting, plexus-forming angiogenesis that is reminiscent of the dysmorphic angioarchitecture described in bronchopulmonary dysplasia (BPD). The aim of this study was to determine the clinicopathological correlates, growth characteristics and molecular regulation of this aberrant form of graft angiogenesis.

METHODS

Fetal lung xenografts, derived from 12 previable fetuses (15 to 22 weeks' gestation) and engrafted in the renal subcapsular space of SCID-beige mice, were analyzed 4 weeks posttransplantation for morphology, vascularization, proliferative activity and gene expression.

RESULTS

Focal plexus-forming angiogenesis (PFA) was observed in 60/230 (26%) of xenografts. PFA was characterized by a complex network of tortuous nonsprouting vascular structures with low endothelial proliferative activity, suggestive of intussusceptive-type angiogenesis. There was no correlation between the occurrence of PFA and gestational age or time interval between delivery and engraftment. PFA was preferentially localized in the relatively hypoxic central subcapsular area. Microarray analysis suggested altered expression of 15 genes in graft regions with PFA, of which 7 are known angiogenic/lymphangiogenic regulators and 5 are known hypoxia-inducible genes. qRT-PCR analysis confirmed significant upregulation of SULF2, IGF2, and HMOX1 in graft regions with PFA.

CONCLUSION

These observations in human fetal lungs ex vivo suggest that postcanalicular lungs can switch from sprouting angiogenesis to an aberrant intussusceptive-type of angiogenesis that is highly reminiscent of BPD-associated dysangiogenesis. While circumstantial evidence suggests hypoxia may be implicated, the exact triggering mechanisms, molecular regulation and clinical implications of this angiogenic switch in preterm lungs in vivo remain to be determined.

Bridging Lung Development with Chronic Obstructive Pulmonary Disease. Relevance of Developmental Pathways in Chronic Obstructive Pulmonary Disease Pathogenesis

Chronic obstructive pulmonary disease (COPD) is characterized by chronic airflow limitation. This generic term encompasses emphysema and chronic bronchitis, two common conditions, each having distinct but also overlapping features. Recent epidemiological and experimental studies have challenged the traditional view that COPD is exclusively an adult disease occurring after years of inhalational insults to the lungs, pinpointing abnormalities or disruption of the pathways that control lung development as an important susceptibility factor for adult COPD. In addition, there is growing evidence that emphysema is not solely a destructive process because it is also characterized by a failure in cell and molecular maintenance programs necessary for proper lung development. This leads to the concept that tissue regeneration required stimulation of signaling pathways that normally operate during development. We undertook a review of the literature to outline the contribution of developmental insults and genes in the occurrence and pathogenesis of COPD, respectively.

Integration of CD45-positive leukocytes into newly forming lymphatics of adult mice

The embryonic origin of lymphatic endothelial cells (LECs) has been a matter of controversy since more than a century. However, recent studies in mice have supported the concept that embryonic lymphangiogenesis is a complex process consisting of growth of lymphatics from specific venous segments as well as the integration of lymphangioblasts into the lymphatic networks. Similarly, the mechanisms of adult lymphangiogenesis are poorly understood and have rarely been studied. We have recently shown that endothelial progenitor cells isolated from the lung of adult mice have the capacity to form both blood vessels and lymphatics when grafted with Matrigel plugs into the skin of syngeneic mice. Here, we followed up on these experiments and studied the behavior of host leukocytes during lymphangiogenesis in the Matrigel plugs. We observed a striking co-localization of CD45(+) leukocytes with the developing lymphatics. Numerous CD45(+) cells expressed the LEC marker podoplanin and were obviously integrated into the lining of lymphatic capillaries. This indicates that, similar to inflammation-induced lymphangiogenesis in man, circulating CD45(+) cells of adult mice are capable of initiating lymphangiogenesis and of adopting a lymphvasculogenic cellular differentiation program. The data are discussed in the context of embryonic and inflammation-induced lymphangiogenesis.

Acceleration of Lung Regeneration by Platelet-Rich Plasma Extract through the Low-Density Lipoprotein Receptor-Related Protein 5-Tie2 Pathway

Angiogenesis, the growth of new blood vessels, plays a key role in organ development, homeostasis, and regeneration. The cooperation of multiple angiogenic factors, rather than a single factor, is required for physiological angiogenesis. Recently, we have reported that soluble platelet-rich plasma (PRP) extract, which contains abundant angiopoietin-1 and multiple other angiogenic factors, stimulates angiogenesis and maintains vascular integrity in vitro and in vivo. In this report, we have demonstrated that mouse PRP extract increases phosphorylation levels of the Wnt coreceptor low-density lipoprotein receptor-related protein 5 (LRP5) and thereby activates angiogenic factor receptor Tie2 in endothelial cells (ECs) and accelerates EC sprouting and lung epithelial cell budding in vitro. PRP extract also increases phosphorylation levels of Tie2 in the mouse lungs and accelerates compensatory lung growth and recovery of exercise capacity after unilateral pneumonectomy in mice, whereas soluble Tie2 receptor or Lrp5 knockdown attenuates the effects of PRP extract. Because human PRP extract is generated from autologous peripheral blood and can be stored at -80°C, our findings may lead to the development of novel therapeutic interventions for various angiogenesis-related lung diseases and to the improvement of strategies for lung regeneration.

Elastin Cables Define the Axial Connective Tissue System in the Murine Lung

The axial connective tissue system is a fiber continuum of the lung that maintains alveolar surface area during changes in lung volume. Although the molecular anatomy of the axial system remains undefined, the fiber continuum of the lung is central to contemporary models of lung micromechanics and alveolar regeneration. To provide a detailed molecular structure of the axial connective tissue system, we examined the extracellular matrix of murine lungs. The lungs were decellularized using a 24 hr detergent treatment protocol. Systematic evaluation of the decellularized lungs demonstrated no residual cellular debris; morphometry demonstrated a mean 39 ± 7% reduction in lung dimensions. Scanning electron microscopy (SEM) demonstrated an intact structural hierarchy within the decellularized lung. Light, fluorescence, and SEM of precision-cut lung slices demonstrated that alveolar duct structure was defined by a cable line element encased in basement membrane. The cable line element arose in the distal airways, passed through septal tips and inserted into neighboring blood vessels and visceral pleura. The ropelike appearance, collagenase resistance and anti-elastin immunostaining indicated that the cable was an elastin macromolecule. Our results indicate that the helical line element of the axial connective tissue system is composed of an elastin cable that not only defines the structure of the alveolar duct, but also integrates the axial connective tissue system into visceral pleura and peripheral blood vessels.

Restoration of cerebral and systemic microvascular architecture in APP/PS1 transgenic mice following treatment with Liraglutide™

OBJECTIVE

Cerebral microvascular impairments occurring in AD may reduce Aβ peptide clearance and impact upon circulatory ultrastructure and function. We hypothesized that microvascular pathologies occur in organs responsible for systemic Aβ peptide clearance in a model of AD and that Liraglutide (Victoza(®)) improves vessel architecture.

METHODS

Seven-month-old APP/PS1 and age-matched wild-type mice received once-daily intraperitoneal injections of either Liraglutide or saline (n = 4 per group) for eight weeks. Casts of cerebral, splenic, hepatic, and renal microanatomy were analyzed using SEM.

RESULTS

Casts from wild-type mice showed regularly spaced microvasculature with smooth lumenal profiles, whereas APP/PS1 mice revealed evidence of microangiopathies including cerebral microanuerysms, intracerebral microvascular leakage, extravasation from renal glomerular microvessels, and significant reductions in both splenic sinus density (p = 0.0286) and intussusceptive microvascular pillars (p = 0.0412). Quantification of hepatic vascular ultrastructure in APP/PS1 mice revealed that vessel parameters (width, length, branching points, intussusceptive pillars and microaneurysms) were not significantly different from wild-type mice. Systemic administration of Liraglutide reduced the incidence of cerebral microanuerysms and leakage, restored renal microvascular architecture and significantly increased both splenic venous sinus number (p = 0.0286) and intussusceptive pillar formation (p = 0.0129).

CONCLUSION

Liraglutide restores cerebral, splenic, and renal architecture in APP/PS1 mice.

Keeping it together: Pulmonary alveoli are maintained by a hierarchy of cellular programs

The application of in vivo genetic lineage tracing has advanced our understanding of cellular mechanisms for tissue renewal in organs with slow turnover, like the lung. These studies have identified an adult stem cell with very different properties than classically understood ones that maintain continuously cycling tissues such as the intestine. A portrait has emerged of an ensemble of cellular programs that replenish the cells that line the gas exchange (alveolar) surface, enabling a response tailored to the extent of cell loss. A capacity for differentiated cells to undergo direct lineage transitions allows for local restoration of proper cell balance at sites of injury. We present these recent findings as a paradigm for how a relatively quiescent tissue compartment can maintain homeostasis throughout a lifetime punctuated by injuries ranging from mild to life-threatening, and discuss how dysfunction or insufficiency of alveolar repair programs produce serious health consequences like cancer and fibrosis.

Remodeling of alveolar septa after murine pneumonectomy

In most mammals, removing one lung (pneumonectomy) results in the compensatory growth of the remaining lung. In mice, stereological observations have demonstrated an increase in the number of mature alveoli; however, anatomic evidence of the early phases of alveolar growth has remained elusive. To identify changes in the lung microstructure associated with neoalveolarization, we used tissue histology, electron microscopy, and synchrotron imaging to examine the configuration of the alveolar duct after murine pneumonectomy. Systematic histological examination of the cardiac lobe demonstrated no change in the relative frequency of dihedral angle components (Ends, Bends, and Junctions) (P > 0.05), but a significant decrease in the length of a subset of septal ends ("E"). Septal retraction, observed in 20-30% of the alveolar ducts, was maximal on day 3 after pneumonectomy (P < 0.01) and returned to baseline levels within 3 wk. Consistent with septal retraction, the postpneumonectomy alveolar duct diameter ratio (Dout:Din) was significantly lower 3 days after pneumonectomy compared to all controls except for the detergent-treated lung (P < 0.001). To identify clumped capillaries predicted by septal retraction, vascular casting, analyzed by both scanning electron microscopy and synchrotron imaging, demonstrated matted capillaries that were most prominent 3 days after pneumonectomy. Numerical simulations suggested that septal retraction could reflect increased surface tension within the alveolar duct, resulting in a new equilibrium at a higher total energy and lower surface area. The spatial and temporal association of these microstructural changes with postpneumonectomy lung growth suggests that these changes represent an early phase of alveolar duct remodeling.

Mouse pneumonectomy model of compensatory lung growth

In humans, disrupted repair and remodeling of injured lung contributes to a host of acute and chronic lung disorders which may ultimately lead to disability or death. Injury-based animal models of lung repair and regeneration are limited by injury-specific responses making it difficult to differentiate changes related to the injury response and injury resolution from changes related to lung repair and lung regeneration. However, use of animal models to identify these repair and regeneration signaling pathways is critical to the development of new therapies aimed at improving pulmonary function following lung injury. The mouse pneumonectomy model utilizes compensatory lung growth to isolate those repair and regeneration signals in order to more clearly define mechanisms of alveolar re-septation. Here, we describe our technique for performing mouse pneumonectomy and sham pneumonectomy. This technique may be utilized in conjunction with lineage tracing or other transgenic mouse models to define molecular and cellular mechanism of lung repair and regeneration.

Laser microdissection of the alveolar duct enables single-cell genomic analysis

Complex tissues such as the lung are composed of structural hierarchies such as alveoli, alveolar ducts, and lobules. Some structural units, such as the alveolar duct, appear to participate in tissue repair as well as the development of bronchioalveolar carcinoma. Here, we demonstrate an approach to conduct laser microdissection of the lung alveolar duct for single-cell PCR analysis. Our approach involved three steps. (1) The initial preparation used mechanical sectioning of the lung tissue with sufficient thickness to encompass the structure of interest. In the case of the alveolar duct, the precision-cut lung slices were 200 μm thick; the slices were processed using near-physiologic conditions to preserve the state of viable cells. (2) The lung slices were examined by transmission light microscopy to target the alveolar duct. The air-filled lung was sufficiently accessible by light microscopy that counterstains or fluorescent labels were unnecessary to identify the alveolar duct. (3) The enzymatic and microfluidic isolation of single cells allowed for the harvest of as few as several thousand cells for PCR analysis. Microfluidics based arrays were used to measure the expression of selected marker genes in individual cells to characterize different cell populations. Preliminary work suggests the unique value of this approach to understand the intra- and intercellular interactions within the regenerating alveolar duct.

De novo hem- and lymphangiogenesis by endothelial progenitor and mesenchymal stem cells in immunocompetent mice

Cellular pro-angiogenic therapies may be applicable for the treatment of peripheral vascular diseases. Interactions between mesenchymal stem cells (MSCs) and endothelial progenitor cells (EPCs) may provide such a treatment option. With the exception of some studies in man, experiments have only been performed in immunodeficient mice and rats. We studied an immunocompetent syngeneic mouse model. We isolated MSCs from bone marrow and EPCs from the lung of adult C57/Bl.6 mice and co-injected them in Matrigel subcutaneously in adult C57/Bl.6 mice. We demonstrate development of both blood vessels and lymphatics. Grafted EPCs integrated into the lining of the two vessel types, whereas MSCs usually did not incorporate into the vessel wall. Injections of each separate cell type did not, or hardly, reveal de novo angiogenesis. The release of VEGF-A by MSCs has been shown before, but its inhibitors, e.g., soluble VEGF receptors, have not been studied. We performed qualitative and quantitative studies of the proteins released by EPCs, MSCs, and cocultures of the cells. Despite the secretion of VEGF inhibitors (sVEGFR-1, sVEGFR-2) by EPCs, VEGF-A was secreted by MSCs at bioavailable amounts (350 pg/ml). We confirm the secretion of PlGF, FGF-1, MCP-1, and PDGFs by EPCs/MSCs and suggest functions for VEGF-B, amphiregulin, fractalkine, CXCL10, and CXCL16 during MSC-induced hem- and lymphangiogenesis. We assume that lymphangiogenesis is induced indirectly by growth factors from immigrating leukocytes, which we found in close association with the lymphatic networks. Inflammatory responses to the cellular markers GFP and cell-tracker red (CMPTX) used for tracing of EPCs or MSCs were not observed. Our studies demonstrate the feasibility of pro-angiogenic/lymphangiogenic therapies in immunocompetent animals and indicate new MSC/EPC-derived angiogenic factors.

Lung regeneration and translational implications of the postpneumonectomy model

Lung regeneration research is yielding data with increasing translational value. The classical models of lung development, postnatal alveolarization, and postpneumonectomy alveolarization have contributed to a broader understanding of the cellular participants including stem-progenitor cells, cell-cell signaling pathways, and the roles of mechanical deformation and other physiologic factors that have the potential to be modulated in human and animal patients. Although recent information is available describing the lineage fate of lung fibroblasts, genetic fate mapping, and clonal studies are lacking in the study of lung regeneration and deserve further examination. In addition to increasing knowledge concerning classical alveolarization (postnatal, postpneumonectomy), there is increasing evidence for remodeling of the adult lung after partial pneumonectomy. Though limited in scope, compelling data have emerged describing restoration of lung tissue mass in the adult human and in large animal models. The basis for this long-term adaptation to pneumonectomy is poorly understood, but investigations into mechanisms of lung regeneration in older animals that have lost their capacity for rapid re-alveolarization are warranted, as there would be great translational value in modulating these mechanisms. In addition, quantitative morphometric analysis has progressed in conjunction with developments in advanced imaging, which allow for longitudinal and nonterminal evaluation of pulmonary regenerative responses in animals and humans. This review focuses on the cellular and molecular events that have been observed in animals and humans after pneumonectomy because this model is closest to classical regeneration in other mammalian systems and has revealed several new fronts of translational research that deserve consideration.

Sprouting and intussusceptive angiogenesis in postpneumonectomy lung growth: mechanisms of alveolar neovascularization

In most rodents and some other mammals, the removal of one lung results in compensatory growth associated with dramatic angiogenesis and complete restoration of lung capacity. One pivotal mechanism in neoalveolarization is neovascularization, because without angiogenesis new alveoli can not be formed. The aim of this study is to image and analyze three-dimensionally the different patterns of neovascularization seen following pneumonectomy in mice on a sub-micron-scale. C57/BL6 mice underwent a left-sided pneumonectomy. Lungs were harvested at various timepoints after pneumonectomy. Volume analysis by microCT revealed a striking increase of 143 percent in the cardiac lobe 14 days after pneumonectomy. Analysis of microvascular corrosion casting demonstrated spatially heterogenous vascular densitities which were in line with the perivascular and subpleural compensatory growth pattern observed in anti-PCNA-stained lung sections. Within these regions an expansion of the vascular plexus with increased pillar formations and sprouting angiogenesis, originating both from pre-existing bronchial and pulmonary vessels was observed. Also, type II pneumocytes and alveolar macrophages were seen to participate actively in alveolar neo-angiogenesis after pneumonectomy. 3D-visualizations obtained by high-resolution synchrotron radiation X-ray tomographic microscopy showed the appearance of double-layered vessels and bud-like alveolar baskets as have already been described in normal lung development. Scanning electron microscopy data of microvascular architecture also revealed a replication of perialveolar vessel networks through septum formation as already seen in developmental alveolarization. In addition, the appearance of pillar formations and duplications on alveolar entrance ring vessels in mature alveoli are indicative of vascular remodeling. These findings indicate that sprouting and intussusceptive angiogenesis are pivotal mechanisms in adult lung alveolarization after pneumonectomy. Various forms of developmental neoalveolarization may also be considered to contribute in compensatory lung regeneration.

Effect of unilateral diaphragmatic paralysis on postpneumonectomy lung growth

Respiratory muscle-associated stretch has been implicated in normal lung development (fetal breathing movements) and postpneumonectomy lung growth. To test the hypothesis that mechanical stretch from diaphragmatic contraction contributes to lung growth, we performed left phrenic nerve transections (PNT) in mice with and without ipsilateral pneumonectomy. PNT was demonstrated by asymmetric costal margin excursion and confirmed at autopsy. In mice with two lungs, PNT was associated with a decrease in ipsilateral lung volume (P<0.05) and lung weight (P<0.05). After pneumonectomy, PNT was not associated with a change in activity level, measureable hypoxemia, or altered minute ventilation; however, microCT scanning demonstrated altered displacement and underinflation of the cardiac lobe within the first week after pneumonectomy. Coincident with the altered structural realignment, lung impedance measurements, fitted to the constant-phase model, demonstrated elevated airway resistance (P<0.05), but normal peripheral tissue resistance (P>0.05). Most important, PNT appeared to abrogate compensatory lung growth after pneumonectomy; the weight of the lobes of the right lung was significantly less than pneumonectomy alone (P<0.001) and indistinguishable from nonsurgical controls (P>0.05). We conclude that the cyclic stretch associated with diaphragmatic muscle contraction is a controlling factor in postpneumonectomy compensatory lung growth.

Mapping cyclic stretch in the postpneumonectomy murine lung

In many mammalian species, the removal of one lung [pneumonectomy (PNX)] is associated with the compensatory growth of the remaining lung. To investigate the hypothesis that parenchymal deformation may trigger lung regeneration, we used respiratory-gated micro-computed tomography scanning to create three-dimensional finite-element geometric models of the murine cardiac lobe with cyclic breathing. Models were constructed of respiratory-gated micro-computed tomography scans pre-PNX and 24 h post-PNX. The computational models demonstrated that the maximum stretch ratio map was patchy and heterogeneous, particularly in subpleural, juxta-diaphragmatic, and cephalad regions of the lobe. In these parenchymal regions, the material line segments at peak inspiration were frequently two- to fourfold greater after PNX; some regions of the post-PNX cardiac lobe demonstrated parenchymal compression at peak inspiration. Similarly, analyses of parenchymal maximum shear strain demonstrated heterogeneous regions of mechanical stress with focal regions demonstrating a threefold increase in shear strain after PNX. Consistent with previously identified growth patterns, these subpleural regions of enhanced stretch and shear strain are compatible with a mechanical signal, likely involving cyclic parenchymal stretch, triggering lung growth.