Electron microscopy of the plexiform lesion

A correlated light and electron microscopy approach to study the subcellular localization of phosphorylated vimentin in human lung tissue

Correlative microscopy is an important approach for bridging the resolution gap between fluorescence light and electron microscopy. Here, we describe a fast and simple method for correlative immunofluorescence and immunogold labeling on the same section to elucidate the localization of phosphorylated vimentin (P-Vim), a robust feature of pulmonary vascular remodeling in cells of human lung small arteries. The lung is a complex, soft and difficult tissue to prepare for transmission electron microscopy (TEM). Detailing the molecular composition of small pulmonary arteries (<500μm) would be of great significance for research and diagnostics. Using the classical methods of immunochemistry (either hydrophilic resin or thin cryosections), is difficult to locate small arteries for analysis by TEM. To address this problem and to observe the same structures by both light and electron microscopy, correlative microscopy is a reliable approach. Immunofluorescence enables us to know the distribution of P-Vim in cells but does not provide ultrastructural detail on its localization. Labeled structures selected by fluorescence microscope can be identified and further analyzed by TEM at high resolution. With our method, the morphology of the arteries is well preserved, enabling the localization of P-Vim inside pulmonary endothelial cells. By applying this approach, fluorescent signals can be directly correlated to the corresponding subcellular structures in areas of interest.

[Pulmonary Vascular Remodeling Characteristics of Pulmonary Arterial Hypertension Mouse Model Induced by Left Pneumonectomy and Jugular Vein Injection of Monocrotaline Pyrrole]

Objective

To compare and investigate the differences and characteristics of pulmonary vascular remodeling in three mouse models of pulmonary arterial hypertension (PAH) constructed by left pneumonectomy, jugular vein injection of monocrotaline pyrrole, and left pneumonectomy combined with jugular vein injection of monocrotaline pyrrole, to explore for a PAH animal model that approximates the clinical pathogenesis of PAH, and to create a model that will provide sound basis for thorough investigation into the pathogenesis of severe PAH.

Methods

59 male C57/BL mice (10-12 weeks, 24-30 g) were randomized into four groups, a control group ( n=9), a group that had left pneumonectomy (PE, n=15), a group that had jugular vein injection of monocrotaline pyrrole (MCTP, n=15), and the last group that had left pneumonectomy combined with jugular injection of monocrotaline pyrrole (P+M, n=20). To evaluate the effect of modeling and the characteristics of pulmonary vascular remodeling, hemodynamic and morphological parameters, including right ventricular systolic pressure (RVSP), right ventricle/(left ventricle plus septum) (RV/LV+S), percent of wall thickness in the pulmonary artery (WT%), muscularization of non-muscular arteries, neointima formation, and vascular obstruction score (VOS), were measured in each group.

Results

1) Compared with those of the control group, the RVSP, RV/LV+S, WT%, and the degree of small pulmonary arteries muscularization in the P+M group were significantly increased ( P<0.01). The MCTP group had just slightly higher findings for these indicators ( P<0.05), while no significant change in these indicators was observed in the PE group ( P>0.05). 2) Neointima formation in the acinus pulmonary arteries, which caused obvious stenosis of the lumen, was observed in the P+M group, the VOS being 1.25±0.80 points ( P<0.001). In contrast, neointima formation was not observed in the MCTP group or the PE groups, the VOS being 0 point ( P>0.05).

Conclusion

Left pneumonectomy combined with jugular intravenous injection of MCTP could induce severe PAH formation in mouse. The model provides a good simulation of neointima formation, the characteristic pathological change of clinical severe PAH.

From thrombosis to fibrosis in chronic thromboembolic pulmonary hypertension

The pathomechanisms underlying the development of thrombofibrotic pulmonary artery occlusions in Chronic Thromboembolic Pulmonary Hypertension (CTEPH) are largely unknown. The aim of this study was to allocate distinct cellular processes playing a role in thrombus resolution, such as inflammation, hypoxia, proliferation, apoptosis and angiogenesis, to different stages of thrombofibrotic remodelling. A total of 182 pulmonary endarterectomy (PEA) specimens were collected from 31 CTEPH patients. To facilitate co-localisation, Tissue MicroArrays were prepared and processed for (immuno)-histochemistry and confocal fluorescence microscopy. Murine venous thrombus formation and resolution was examined after inferior vena cava ligation. PEA tissues exhibited five morphologically distinct regions predominantly consisting of either fibrin-, erythrocyte- or extracellular matrix-rich thrombus, myofibroblasts, vessels or fibrotic tissue, and were found to resemble chronological stages of thrombus resolution in mice. Cellularity was highest in vessel-rich regions, and numerous cells were strongly positive for HIF1α or HIF2α as well as markers of activated VEGF signalling, including endothelial nitric oxide synthase. On the other hand, negative regulators of angiogenic growth factor signalling and reactive oxygen species were also highly expressed. Immune cells, primarily macrophages of the M2 subtype and CD117 haematopoietic progenitors were detected and highest in vascularised regions. Our findings demonstrate the simultaneous presence of different stages of thrombus organisation and suggest that hypoxia-induced endothelial, mesenchymal and immune cell activation may contribute to thrombofibrosis in CTEPH. This systematic histological characterisation of the material obstructing pulmonary vessels in CTEPH may provide a valuable basis for further studies aimed at determining causal factors underlying this disease.

Supplementary Material to this article is available online at www.thrombosis-online.com.

Endothelial hyperpermeability in severe pulmonary arterial hypertension: role of store-operated calcium entry

Here, we tested the hypothesis that animals with severe pulmonary arterial hypertension (PAH) display increased sensitivity to vascular permeability induced by activation of store-operated calcium entry. To test this hypothesis, wild-type and transient receptor potential channel 4 (TRPC4) knockout Fischer 344 rats were given a single injection of Semaxanib (SU5416; 20 mg/kg) followed by 3 wk of exposure to hypoxia (10% oxygen) and a return to normoxia (21% oxygen) for an additional 2-3 wk. This Semaxanib/hypoxia/normoxia (i.e., SU5416/hypoxia/normoxia) treatment caused PAH, as evidenced by development of right ventricular hypertrophy, pulmonary artery medial hypertrophy, and occlusive lesions within precapillary arterioles. Pulmonary artery pressure was increased fivefold in Semaxanib/hypoxia/normoxia-treated animals compared with untreated, Semaxanib-treated, and hypoxia-treated controls, determined by isolated perfused lung studies. Thapsigargin induced a dose-dependent increase in permeability that was dependent on TRPC4 in the normotensive perfused lung. This increase in permeability was accentuated in PAH lungs but not in Semaxanib- or hypoxia-treated lungs. Fluid accumulated in large perivascular cuffs, and although alveolar fluid accumulation was not seen in histological sections, Evans blue dye conjugated to albumin was present in bronchoalveolar lavage fluid of hypertensive but not normotensive lungs. Thus PAH is accompanied by a TRPC4-dependent increase in the sensitivity to edemagenic agents that activate store-operated calcium entry.

Deletion of STAT5a/b in vascular smooth muscle abrogates the male bias in hypoxic pulmonary hypertension in mice: implications in the human disease

Chronic hypoxia typically elicits pulmonary hypertension (PH) in mice with a male-dominant phenotype. There is an opposite-sex bias in human PH, with a higher prevalence in women, but greater survival (the "estrogen paradox"). We investigated the involvement of the STAT5a/b species, previously established to mediate sexual dimorphism in other contexts, in the sex bias in PH. Mice with heterozygous or homozygous deletions of the STAT5a/b locus in vascular smooth muscle cells (SMCs) were generated in crosses between STAT5a/b(fl/fl) and transgelin (SM22α)-Cre(+/+) parents. Wild-type (wt) males subjected to chronic hypoxia showed significant PH and pulmonary arterial remodeling, with wt females showing minimal changes (a male-dominant phenotype). However, in conditional STAT5(+/-) or STAT5(-/-) mice, hypoxic females showed the severest manifestations of PH (a female-dominant phenotype). Immunofluorescence studies on human lung sections showed that obliterative pulmonary arterial lesions in patients with idiopathic pulmonary arterial hypertension (IPAH) or hereditary pulmonary arterial hypertension (HPAH), both male and female, overall had reduced STAT5a/b, reduced PY-STAT5 and reduced endoplasmic reticulum (ER) GTPase atlastin-3 (ATL3). Studies of SMCs and endothelial cell (EC) lines derived from vessels isolated from lungs of male and female IPAH patients and controls revealed instances of coordinate reductions in STAT5a, STAT5b and ATL3 in IPAH-derived cells, including SMCs and ECs from the same patient. Taken together, these data provide the first definitive evidence for a contribution of STAT5a/b to the sex bias in PH in the hypoxic mouse and implicate reduced STAT5 in the pathogenesis of the human disease.

Abnormal expression of vesicular transport proteins in pulmonary arterial hypertension in monocrotaline-treated rats

Intracellular vesicular transport is shown to be dysfunctional in pulmonary arterial hypertension (PAH). However, the expression of intracellular vesicular transport proteins in PAH remains unclear. To elucidate the possible role of these proteins in the development of PAH, the changes in the expressions of N-ethyl-maleimide-sensitive factor (NSF), α-soluble NSF attachment protein (α-SNAP), synaptosome-associated membrane protein 23 (SNAP23), type 2 bone morphogenetic receptor (BMPR2), caveolin-1 (cav-1), and endothelial nitric oxide synthase (eNOS) were examined in lung tissues of monocrotaline (MCT)-treated rats by real-time polymerase chain reaction and western blot analysis. In addition, caspase-3, also examined by western blot analysis, was used as an indicator of apoptosis. Our data showed that during the development of PAH, the expressions of NSF, α-SNAP, and SNAP23 were significantly increased before pulmonary arterial pressure started to increase and then significantly decreased after PAH was established. The expressions of BMPR2 and eNOS were similar to those of NSF, α-SNAP, and SNAP23; however, the expression of cav-1 was down-regulated after MCT treatment. Caspase-3 expression was increased after exposure to MCT. In conclusion, the expressions of NSF, α-SNAP, and SNPA23 changed greatly during the onset of PAH, which was accompanied by abnormal expressions of BMPR2, cav-1, and eNOS, as well as an increase in apoptosis. Thus, changes in NSF, α-SNAP, and SNAP23 expressions appear to be mechanistically associated with the development of PAH in MCT-treated rats.

Abnormal expression of NSF, α-SNAP and SNAP23 in pulmonary arterial hypertension in rats treated with monocrotaline

BACKGROUND

Recent researches have shown that dysfunctional intracellular vesicular trafficking exists in pulmonary arterial hypertension (PAH). However, the expression of proteins involved in intracellular vesicular trafficking in pulmonary vasculature in PAH remains unclear.

OBJECTIVE

To elucidate possible roles of proteins involved in intracellular vesicular trafficking in the development of PAH in rats treated with monocrotaline, changes in the expression of N-ethyl-maleimide-sensitive factor (NSF), α-soluble NSF attachment protein (α-SNAP) and synaptosome-associated membrane protein (SNAP) 23 were examined together with expression of caveolin-1 (cav-1), endogenous nitric oxide synthase (eNOS), type 2 bone morphogenetic receptor (BMPR2) and cellular apoptosis.

METHODS

The mRNA expression was investigated by real time-PCR and protein expression by immunoblot method in rat lung. Caspase-3 was used as an indicator of cellular apoptosis and examined by immunoblot method.

RESULTS

During the development of PAH, mRNA and protein expression of NSF, α-SNAP and SNAP23 all significantly increased before pulmonary arterial pressure started to increase, then all significantly decreased when PAH established. The expression of eNOS and BMPR2 changed similarly, while the mRNA and protein of cav-1 both downregulated after monocrotaline treatment. Caspase-3 was also increased after exposure to monocrotaline.

CONCLUSIONS

Since the expression of NSF, α-SNAP and SNAP23 changed greatly during the onset of PAH and accompanied with abnormal expression of eNOS, BMPR2 and cav-1 and with enhanced cellular apoptosis, NSF, α-SNAP and SNAP23 appear to be associated with the development of PAH in rats treated with monocrotaline.

Endothelial-to-mesenchymal transition in pulmonary hypertension

BACKGROUND

The vascular remodeling responsible for pulmonary arterial hypertension (PAH) involves predominantly the accumulation of α-smooth muscle actin-expressing mesenchymal-like cells in obstructive pulmonary vascular lesions. Endothelial-to-mesenchymal transition (EndoMT) may be a source of those α-smooth muscle actin-expressing cells.

METHODS AND RESULTS

In situ evidence of EndoMT in human PAH was obtained by using confocal microscopy of multiple fluorescent stainings at the arterial level, and by using transmission electron microscopy and correlative light and electron microscopy at the ultrastructural level. Findings were confirmed by in vitro analyses of human PAH and control cultured pulmonary artery endothelial cells. In addition, the mRNA and protein signature of EndoMT was recognized at the arterial and lung level by quantitative real-time polymerase chain reaction and Western blot analyses. We confirmed our human observations in established animal models of pulmonary hypertension (monocrotaline and SuHx). After establishing the first genetically modified rat model linked to BMPR2 mutations (BMPR2(Δ140Ex1/+) rats), we demonstrated that EndoMT is linked to alterations in signaling of BMPR2, a gene that is mutated in 70% of cases of familial PAH and in 10% to 40% of cases of idiopathic PAH. We identified molecular actors of this pathological transition, including twist overexpression and vimentin phosphorylation. We demonstrated that rapamycin partially reversed the protein expression patterns of EndoMT, improved experimental PAH, and decreased the migration of human pulmonary artery endothelial cells, providing the proof of concept that EndoMT is druggable.

CONCLUSIONS

EndoMT is linked to alterations in BPMR2 signaling and is involved in the occlusive vas cular remodeling of PAH, findings that may have therapeutic implications.

How do we measure pathology in PAH (lung and RV) and what does it tell us about the disease

The current understanding of the pathology that underlies pulmonary vascular and right ventricular remodeling in pulmonary hypertension is discussed. Although recent studies underscored the importance of intima and media remodeling and, for the first time, the relevance of perivascular inflammation, much is needed to move the field forward. Reassessment of distribution and extension of the different vascular lesions requires state-of-the-art stereological tools, allied to three-dimensional casting and integration with data concerning cellular and molecular pathobiological processes. This integrated approach is ever more pressing in the right ventricle, because our understanding of key structural alterations of the failing right ventricle in pulmonary hypertension is lacking. This enterprise will enable better translation of pathogenetic processes to the human disease and provide key data to guide diagnostic and prognostic imaging approaches.

The metabolic basis of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a vascular remodeling disease of the lungs resulting in heart failure and premature death. Although, until recently, it was thought that PAH pathology is restricted to pulmonary arteries, several extrapulmonary organs are also affected. The realization that these tissues share a common metabolic abnormality (i.e., suppression of mitochondrial glucose oxidation and increased glycolysis) is important for our understanding of PAH, if not a paradigm shift. Here, we discuss an emerging metabolic theory, which proposes that PAH should be viewed as a syndrome involving many organs sharing a mitochondrial abnormality and explains many PAH features and provides novel biomarkers and therapeutic targets.

Subcellular mechanisms in pulmonary arterial hypertension: combinatorial modalities that inhibit anterograde trafficking and cause bone morphogenetic protein receptor type 2 mislocalization

Abstract The natural history of familial pulmonary arterial hypertension (PAH) typically involves mutations in and/or haploinsuffciency of BMPR2 (gene for bone morphogenetic protein receptor type 2) but with low penetrance (10%-15%), delayed onset (in the third or fourth decade), and a gender bias (two- to fourfold more prevalent in postpubertal women). Thus, investigators have sought an understanding of "second-hit" modalities that might affect BMPR2 anterograde trafficking and/or function. Indeed, vascular lung lesions in PAH have been reported to contain enlarged "vacuolated" endothelial and smooth muscle cells with dilated endoplasmic reticulum (ER) cisternae, increased ER structural protein reticulon 4 (also called Nogo-B), and enlarged and fragmented Golgi apparatus. We recently replicated this cellular phenotype in primary human pulmonary arterial endothelial cells and human pulmonary arterial smooth muscle cells in culture by acute knockdown of the estradiol 17β (E2)-responsive proteins signal transducer and activator of transcription 5a (STAT5a) and STAT5b using small interfering RNAs (siRNAs). We have now investigated whether functional haploinsufficiences of these molecules, alone or in combination with other modalities, might interfere with anterograde membrane trafficking using (a) the quantitative tsO45VSV-G-GFP trafficking assay and (b) assays for cell-surface localization of Flag-tagged BMPR2 molecules. The G glycoprotein of the vesicular stomatitis virus (VSV-G) trafficking assay was validated in EA.hy926 endothelial cells by showing that cells exposed to monocrotaline pyrrole displayed reduced anterograde trafficking. Thereafter, the combinatorial knockdowns of STAT5a, STAT5b, BMPR2, and/or endothelial nitric oxide synthase as well as exposure to E2 or 2-methoxyestradiol were observed to significantly inhibit VSV-G trafficking. These combinations also led to intracellular trapping of wild-type Flag-tagged BMPR2. Overexpression of the PAH disease-derived F14 and KDF mutants of BMPR2, which were trapped in the ER/Golgi, also inhibited VSV-G trafficking in trans. Moreover, probenecid, a chemical chaperone in clinical use today, partially restored cell-surface localization of the KDF but not the F14 mutant. These data identify several combinatorial modalities that inhibit VSV-G anterograde trafficking and cause mislocalization of BMPR2. These modalities merit consideration in defining aspects of the late-developing and gender-biased natural history of human PAH.

Pathogenesis of pulmonary hypertension: a case for caveolin-1 and cell membrane integrity

Pulmonary hypertension (PH) is a progressive disease with a high morbidity and mortality rate. Despite important advances in the field, the precise mechanisms leading to PH are not yet understood. Main features of PH are loss of vasodilatory response, the activation of proliferative and antiapoptotic pathways leading to pulmonary vascular remodeling and obstruction, elevated pressure and right ventricular hypertrophy, resulting in right ventricular failure and death. Experimental studies suggest that endothelial dysfunction may be the key underlying feature in PH. Caveolin-1, a major protein constituent of caveolae, interacts with several signaling molecules including the ones implicated in PH and modulates them. Disruption and progressive loss of endothelial caveolin-1 with reciprocal activation of proliferative pathways occur before the onset of PH, and the rescue of caveolin-1 inhibits proliferative pathways and attenuates PH. Extensive endothelial damage/loss occurs during the progression of the disease with subsequent enhanced expression of caveolin-1 in smooth muscle cells. This caveolin-1 in smooth muscle cells switches from being an antiproliferative factor to a proproliferative one and participates in cell proliferation and cell migration, possibly leading to irreversible PH. In contrast, the disruption of endothelial caveolin-1 is not observed in the hypoxia-induced PH, a reversible form of PH. However, proliferative pathways are activated in this model, indicating caveolin-1 dysfunction. Thus disruption or dysfunction of endothelial caveolin-1 leads to PH, and the status of caveolin-1 may determine the reversibility versus irreversibility of PH. This article reviews the role of caveolin-1 and cell membrane integrity in the pathogenesis and progression of PH.

Non-genomic STAT5-dependent effects at the endoplasmic reticulum and Golgi apparatus and STAT6-GFP in mitochondria

STAT protein species are well-known as transcription factors that regulate nuclear gene expression. Recent novel lines of research suggest new non-genomic functions of STAT5A/B and STAT6. It was discovered in human pulmonary arterial endothelial cells that STAT5A, including STAT5A-GFP, constitutively associated with the Golgi apparatus, and both STAT5A and B with the endoplasmic reticulum. Acute siRNA-mediated knockdown of STAT5A/B led to the rapid development of a dramatic cystic change in the endoplasmic reticulum (ER) characterized by deposition of the ER structural protein reticulon-4 (RTN4; also called Nogo-B) and the ER-resident GTPase atlastin-3 (ATL3) along cyst membranes and cyst-zone boundaries, accompanied by Golgi fragmentation. Functional consequences included reduced anterograde trafficking, an ER stress response (increased GRP78/BiP) and eventual mitochondrial fragmentation. This phenotype was "non-genomic" in that it was elicited in enucleated cytoplasts. In cross-immunopanning assays STAT5A and B species associated with ATL3, and the ER-lumen spacer CLIMP63 (also called cytoskeleton-associated protein 4, CKAP4) but not RTN4. From a disease significance perspective we posit that STAT5, which is known to be affected by estradiol-17β and prolactin, represents the gender-sensitive determinant in the pathogenesis of idiopathic pulmonary hypertension (IPAH), a disease which includes ER/Golgi dysfunctions but with a 2- to 4-fold higher prevalence in postpubertal women. A separate line of recent research produced evidence for the association of STAT6-GFP, but not STAT3-GFP, STAT3-DsRed, or STAT3-Flag, with mitochondria in live-cell, immunofluorescence, and immunoelectron microscopy. An N-terminal truncation of STAT6-GFP (1-459), which lacked the SH2 domain and Tyr-phosphorylation site, constitutively associated with mitochondria. Thus, the emergent new of biology STAT proteins includes non-genomic roles-structurally and functionally-in the three closely related membrane organelles consisting of the endoplasmic reticulum, Golgi apparatus, and mitochondria.

Definitive evidence using enucleated cytoplasts for a nongenomic basis for the cystic change in endoplasmic reticulum structure caused by STAT5a/b siRNAs

STAT5a/b species are well known as transcription factors that regulate nuclear gene expression. In a novel line of research in human pulmonary arterial endothelial cells (HPAECs), we previously observed that STAT5a associated with the Golgi apparatus and that siRNA-mediated knockdown of STAT5a/b led to the rapid development of a dramatic cystic change in the endoplasmic reticulum (ER) characterized by deposition along cyst membranes and tubule-to-cyst boundaries of the proteins reticulon-4 (RTN4; also called Nogo-B) and the ER-resident GTPase atlastin-3 (ATL3) and Golgi fragmentation. We now report that STAT5a can be observed in ER sheets in digitonin-permeabilized HPAECs and that anti-STAT5a cross- immunopanned ATL3 but not RTN4. Moreover, there was marked accumulation of the 63-kDa cytoskeleton-linking membrane protein and ER-spacer CLIMP63 (also called cytoskeleton-associated protein 4, CKAP4) and KDEL-mCherry within the cysts. That the STAT5a/b-siRNA-induced cystic ER phenotype developed in the presence of the transcription inhibitor 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole (DRB) had suggested that the mechanism was independent of the transcription factor functions of STAT5a/b, i.e., was "nongenomic." We have now definitively tested the requirement for the nucleus in eliciting the STAT5a/b-siRNA-induced cystic ER phenotype. Enucleated HPAEC cytoplasts were prepared using adherent 35-mm cultures using the cytochalasin B-centrifugation method (typically yielding 65-75% enucleation). STAT5a/b siRNAs readily elicited the cystic ER phenotype including the marked luminal accumulation of CLIMP63 and Golgi fragmentation in the recovered HPAEC cytoplasts demonstrably lacking a nucleus. These studies provide unequivocal evidence using enucleated cytoplasts for a nongenomic mechanism(s) underlying the cystic change in ER structure elicited by STAT5a/b knockdown.

Attenuating Endoplasmic Reticulum Stress as a Novel Therapeutic Strategy in Pulmonary Hypertension

Background—

Evidence suggestive of endoplasmic reticulum (ER) stress in the pulmonary arteries of patients with pulmonary arterial hypertension has been described for decades but has never been therapeutically targeted. ER stress is a feature of many conditions associated with pulmonary arterial hypertension like hypoxia, inflammation, or loss-of-function mutations. ER stress signaling in the pulmonary circulation involves the activation of activating transcription factor 6, which, via induction of the reticulin protein Nogo, can lead to the disruption of the functional ER-mitochondria unit and the increasingly recognized cancer-like metabolic shift in pulmonary arterial hypertension that promotes proliferation and apoptosis resistance in the pulmonary artery wall. We hypothesized that chemical chaperones known to suppress ER stress signaling, like 4-phenylbutyrate (PBA) or tauroursodeoxycholic acid, will inhibit the disruption of the ER-mitochondrial unit and prevent/reverse pulmonary arterial hypertension.

Methods and Results—

PBA in the drinking water both prevented and reversed chronic hypoxia–induced pulmonary hypertension in mice, decreasing pulmonary vascular resistance, pulmonary artery remodeling, and right ventricular hypertrophy and improving functional capacity without affecting systemic hemodynamics. These results were replicated in the monocrotaline rat model. PBA and tauroursodeoxycholic acid improved ER stress indexes in vivo and in vitro, decreased activating transcription factor 6 activation (cleavage, nuclear localization, luciferase, and downstream target expression), and inhibited the hypoxia-induced decrease in mitochondrial calcium and mitochondrial function. In addition, these chemical chaperones suppressed proliferation and induced apoptosis in pulmonary artery smooth muscle cells in vitro and in vivo.

Conclusions—

Attenuating ER stress with clinically used chemical chaperones may be a novel therapeutic strategy in pulmonary hypertension with high translational potential.

Mitochondria in vascular health and disease

The eukaryote's mitochondrial network is perhaps the cell's most sophisticated and dynamic responsive sensing system. Integrating metabolic, oxygen, or danger signals with inputs from other organelles, as well as local and systemic signals, mitochondria have a profound impact on vascular function in both health and disease. This review highlights recently discovered aspects of mitochondrial function (oxygen sensing, inflammation, autophagy, and apoptosis) and discusses their role in diseases of both systemic and pulmonary vessels. We also emphasize the role of mitochondria as therapeutic targets for vascular disease. We highlight the intriguing similarities of mitochondria-driven molecular mechanisms in terms of both pathogenesis and therapies in very diverse diseases, such as atherosclerosis, pulmonary hypertension, and cancer, to support the foundation of a new field in medicine: mitochondrial medicine.

Caveolins and lung function

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

Caveolins and lung function

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

Nongenomic STAT5-dependent effects on Golgi apparatus and endoplasmic reticulum structure and function

We report unexpected nongenomic functions of signal transducer and activator of transcription (STAT) 5 species in the cytoplasm aimed at preserving the structure and function of the Golgi apparatus and rough endoplasmic reticulum (ER) in vascular cells. Immunoimaging and green fluorescent protein-tagged-STAT5a protein localization studies showed the constitutive association of nonphosphorylated STAT5a, and to a lesser extent STAT5b, with the Golgi apparatus and of STAT5a with centrosomes in human pulmonary arterial endothelial and smooth muscle cells. Acute knockdown of STAT5a/b species using small interfering RNAs (siRNAs), including in the presence of an mRNA synthesis inhibitor (5,6-dichloro-1-β-d-ribofuranosylbenzimidazole), produced a dramatic phenotype within 1 day, consisting of dilatation and fragmentation of Golgi cisternae, a marked tubule-to-cyst change in the ER, increased accumulation of reticulon-4 (RTN4)/Nogo-B and atlastin-3 (ATL3) at cyst-zone boundaries, cystic separation of the outer and inner nuclear membranes, accompanied by scalloped/lunate distortion of the nucleus, with accumulation of RTN4 on convex sides of distorted nuclei. These cells showed inhibition of vesicular stomatitis virus G protein glycoprotein trafficking, mitochondrial fragmentation, and reduced mitochondrial function. STAT5a/b(-/-) mouse embryo fibroblasts also showed altered ER/Golgi dynamics. RTN4 knockdown using siRNA did not affect development of the cystic phenotype; ATL3 siRNA led to effacement of cyst-zone boundaries. In magnetic-bead cross-immunopanning assays, ATL3 bound both STAT5a and STAT5b. Remarkably, this novel cystic ER/lunate nucleus phenotype was characteristic of vascular cells in arterial lesions of idiopathic pulmonary hypertension, an unrelentingly fatal human disease. These data provide evidence of a STAT-family protein regulating the structure of a cytoplasmic organelle and implicate this mechanism in the pathogenesis of a human disease.

Protein trafficking dysfunctions: Role in the pathogenesis of pulmonary arterial hypertension

Earlier electron microscopic data had shown that a hallmark of the vascular remodeling in pulmonary arterial hypertension (PAH) in man and experimental models includes enlarged vacuolated endothelial and smooth muscle cells with increased endoplasmic reticulum and Golgi stacks in pulmonary arterial lesions. In cell culture and in vivo experiments in the monocrotaline model, we observed disruption of Golgi function and intracellular trafficking with trapping of diverse vesicle tethers, SNAREs and SNAPs in the Golgi membranes of enlarged pulmonary arterial endothelial cells (PAECs) and pulmonary arterial smooth muscle cells (PASMCs). Consequences included the loss of cell surface caveolin-1, hyperactivation of STAT3, mislocalization of eNOS with reduced cell surface/caveolar NO and hypo-S-nitrosylation of trafficking-relevant proteins. Similar Golgi tether, SNARE and SNAP dysfunctions were also observed in hypoxic PAECs in culture and in PAECs subjected to NO scavenging. Strikingly, a hypo-NO state promoted PAEC mitosis and cell proliferation. Golgi dysfunction was also observed in pulmonary vascular cells in idiopathic PAH (IPAH) in terms of a marked cytoplasmic dispersal and increased cellular content of the Golgi tethers, giantin and p115, in cells in the proliferative, obliterative and plexiform lesions in IPAH. The question of whether there might be a causal relationship between trafficking dysfunction and vasculopathies of PAH was approached by genetic means using HIV-nef, a protein that disrupts endocytic and trans-Golgi trafficking. Macaques infected with a chimeric simian immunodeficiency virus (SIV) containing the HIV-nef gene (SHIV-nef), but not the non-chimeric SIV virus containing the endogenous SIV-nef gene, displayed pulmonary arterial vasculopathies similar to those in human IPAH. Only macaques infected with chimeric SHIV-nef showed pulmonary vascular lesions containing cells with dramatic cytoplasmic dispersal and increase in giantin and p115. Specifically, it was the HIV-nef–positive cells that showed increased giantin. Elucidating how each of these changes fits into the multifactorial context of hypoxia, reduced NO bioavailability, mutations in BMPR II, modulation of disease penetrance and gender effects in disease occurrence in the pathogenesis of PAH is part of the road ahead.

The role of Nogo and the mitochondria-endoplasmic reticulum unit in pulmonary hypertension

Pulmonary arterial hypertension (PAH) is caused by excessive proliferation of vascular cells, which occlude the lumen of pulmonary arteries (PAs) and lead to right ventricular failure. The cause of the vascular remodeling in PAH remains unknown, and the prognosis of PAH remains poor. Abnormal mitochondria in PAH PA smooth muscle cells (SMCs) suppress mitochondria-dependent apoptosis and contribute to the vascular remodeling. We hypothesized that early endoplasmic reticulum (ER) stress, which is associated with clinical triggers of PAH including hypoxia, bone morphogenetic protein receptor II mutations, and HIV/herpes simplex virus infections, explains the mitochondrial abnormalities and has a causal role in PAH. We showed in SMCs from mice that Nogo-B, a regulator of ER structure, was induced by hypoxia in SMCs of the PAs but not the systemic vasculature through activation of the ER stress-sensitive transcription factor ATF6. Nogo-B induction increased the distance between the ER and mitochondria and decreased ER-to-mitochondria phospholipid transfer and intramitochondrial calcium. In addition, we noted inhibition of calcium-sensitive mitochondrial enzymes, increased mitochondrial membrane potential, decreased mitochondrial reactive oxygen species, and decreased mitochondria-dependent apoptosis. Lack of Nogo-B in PASMCs from Nogo-A/B-/- mice prevented these hypoxia-induced changes in vitro and in vivo, resulting in complete resistance to PAH. Nogo-B in the serum and PAs of PAH patients was also increased. Therefore, triggers of PAH may induce Nogo-B, which disrupts the ER-mitochondria unit and suppresses apoptosis. This could rescue PASMCs from death during ER stress but enable the development of PAH through overproliferation. The disruption of the ER-mitochondria unit may be relevant to other diseases in which Nogo is implicated, such as cancer or neurodegeneration.

Pulmonary hypertension associated with HIV infection: pulmonary vascular disease: the global perspective

The success of antiretroviral therapies in improving the survival of patients infected with HIV and reducing HIV-associated opportunistic infections is undisputed. Nevertheless, long-term outcomes such as noninfectious cardiovascular complications, including cardiomegaly, pericarditis, myocarditis, and pulmonary arterial hypertension, are now serious concerns. The lung is a frequent target organ for disorders associated with HIV infection. HIV-related pulmonary arterial hypertension (HRPAH) affects more individuals who are infected with HIV than individuals who are uninfected. Moreover, the long-standing estimated prevalence of HRPAH in developed countries (calculated at 0.5%) is increasing as more clinician-scientists unify their efforts to screen patients who are pulmonary asymptomatic for pulmonary arterial hypertension. In order to decrease mortality, efforts are directed at early detection, diagnosis, and therapeutic interventions before the disease compromises patients' quality of life. This article reviews the logistics of screening approaches for HRPAH and discusses the substantial disease burden currently faced by developing countries, where the prevalence of HIV infection is higher and complicated by hyperendemic risk factors, limited access to antiretrovirals, and lack of screening tools. We also present mechanistic insights into HRPAH, including the role of HIV proteins and their potential use as screening tools, and, finally, areas that still need intense research.

Animal models of pulmonary arterial hypertension: the hope for etiological discovery and pharmacological cure

At present, six groups of chronic pulmonary hypertension (PH) are described. Among these, group 1 (and 1′) comprises a group of diverse diseases termed pulmonary arterial hypertension (PAH) that have several pathophysiological, histological, and prognostic features in common. PAH is a particularly severe and progressive form of PH that frequently leads to right heart failure and premature death. The diagnosis of PAH must include a series of defined clinical parameters, which extend beyond mere elevations in pulmonary arterial pressures and include precapillary PH, pulmonary hypertensive arteriopathy (usually with plexiform lesions), slow clinical onset (months or years), and a chronic time course (years) characterized by progressive deterioration. What appears to distinguish PAH from other forms of PH is the severity of the arteriopathy observed, the defining characteristic of which is “plexogenic arteriopathy.” The pathogenesis of this arteriopathy remains unclear despite intense investigation in a variety of animal model systems. The most commonly used animal models (“classic” models) are rodents exposed to either hypoxia or monocrotaline. Newer models, which involve modification of classic approaches, have been developed that exhibit more severe PH and vascular lesions, which include neointimal proliferation and occlusion of small vessels. In addition, genetically manipulated mice have been generated that have provided insight into the role of specific molecules in the pulmonary hypertensive process. Unfortunately, at present, there is no perfect preclinical model that completely recapitulates human PAH. All models, however, have provided and will continue to provide invaluable insight into the numerous pathways that contribute to the development and maintenance of PH. Use of both classic and newly developed animal models will allow continued rigorous testing of new hypotheses regarding pathogenesis and treatment. This review highlights progress that has been made in animal modeling of this important human condition.

Undifferentiated Perivascular Cells in Myxoid Mesenchymal Tumors: An Ultrastructural Study

The current WHO classification of soft tissue tumors is based on the lineage of differentiation of the proliferating cells. Since mature mesenchymal cells have a broad phenotypic plasticity it has been considered unnecessary to recur to a hypothetical stem cell to explain the origin of these neoplasms. In spite of this assumption, the target cell of the oncogenic mutations in mesenchymal tumors is still a controversial item. Myxoid mesenchymal tumors constitute a heterogeneous group of neoplasms sharing in an ample mucinous matrix that separates neoplastic cells and facilitates their single submicroscopic study under electron microscopy examination. The authors have studied, by electron microscopy, 74 myxoid mesenchymal tumors, including a large variety of nosologic entities, to assess their madurational gradient. In 43 of 74 cases, a common element has been found: medium-sized cells, with high nucleo-cytoplasmic ratio, lacking lineage specific features, which were arranged around the capillary vessels. In some cases, the authors were able to demonstrate gradual differentiation in these cells, as they moved away from the vessels. These features support the hypothesis that at least some mesenchymal tumors originate from perivascular undifferentiated cells. In addition, the findings might contribute to define both topographic and morphologic characteristics of adult stem mesenchymal cells.

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

The pulmonary circulation represents a unique vascular bed, receiving 100% of the cardiac output while maintaining low blood pressure. Multiple different cell types, including endothelium, smooth muscle, and fibroblasts, contribute to normal vascular function, and to the vascular response to injury. Our understanding of the basic cell biology of these various cell types, and the roles they play in vascular homeostasis and disease, remains quite limited despite several decades of study. Recent advances in approaches that enable the mapping of cell origin and the study of the molecular basis of structure and function have resulted in a rapid accumulation of new information that is essential to vascular biology. A recent National Institutes of Health workshop was held to discuss emerging concepts in lung vascular biology. The findings of this workshop are summarized in this article.

Age-related changes among 25 patients with congenital cardiac left-to-right shunts and irreversible plexogenic pulmonary arteriopathy

BACKGROUND

Some patients with congenital heart disease and irreversible plexogenic pulmonary arteriopathy survive into adulthood. The purpose of this study was to compare histopathological and antigen expression patterns in adults and children with congenital heart disease and plexogenic pulmonary arteriopathy.

METHODS

Autopsy/explant lung tissues from 25 patients with congenital heart disease and plexogenic pulmonary arteriopathy were reviewed for 24 histopathological parameters associated with plexogenic pulmonary arteriopathy, including the prevalence and character of plexiform lesions. Immunohistochemistry using antibodies against CD31, C-kit, smooth muscle actin, CD68, and Fli-1 was performed to evaluate plexiform lesions.

RESULTS

Group 1 consisted of 14 patients aged <20 years, and Group 2 included 11 patients aged >or=20 years (range, 20-69 years). Cellular plexiform lesions trended toward greater prevalence in Group 1 than in Group 2 (P=.081), whereas involuted plexiform lesions more frequently affected Group 2 than Group 1 (P=.0037). In addition, recent platelet-fibrin thrombi within plexiform lesions occurred more often in Group 1 than in Group 2 (P=.0419). Intimal proliferation/fibrosis of elastic arteries and pulmonary vein dilation were more common in Group 2 (P=.0172 and P=.0048, respectively). CD31 and C-kit staining in non-lumen-lining cells of plexiform lesions was more frequent in Group 2 than in Group 1 (P=.0858 and P=.0173, respectively). No statistically significant differences in expression patterns between cellular and involuted plexiform lesions existed.

CONCLUSIONS

Among patients with congenital heart disease and plexogenic pulmonary arteriopathy, histopathological differences were observed between those surviving into adulthood and those dying in childhood/adolescence. Plexiform lesions in adults undergo remodeling from a cellular morphology to an involuted morphology. It is unclear whether this simply represents the natural progression of plexiform lesions or also confers a survival advantage.

Circulating angiogenic precursors in idiopathic pulmonary arterial hypertension

Vascular remodeling in idiopathic pulmonary arterial hypertension (IPAH) involves hyperproliferative and apoptosis-resistant pulmonary artery endothelial cells. In this study, we evaluated the relative contribution of bone marrow-derived proangiogenic precursors and tissue-resident endothelial progenitors to vascular remodeling in IPAH. Levels of circulating CD34+ CD133+ bone marrow-derived proangiogenic precursors were higher in peripheral blood from IPAH patients than in healthy controls and correlated with pulmonary artery pressure, whereas levels of resident endothelial progenitors in IPAH pulmonary arteries were comparable to those of healthy controls. Colony-forming units of endothelial-like cells (CFU-ECs) derived from CD34+ CD133+ bone marrow precursors of IPAH patients secreted high levels of matrix metalloproteinase-2, had greater affinity for angiogenic tubes, and spontaneously formed disorganized cell clusters that increased in size in the presence of transforming growth factor-beta or bone morphogenetic protein-2. Subcutaneous injection of NOD SCID mice with IPAH CFU-ECs within Matrigel plugs, but not with control CFU-ECs, produced cell clusters in the Matrigel and proliferative lesions in surrounding murine tissues. Thus, mobilization of high levels of proliferative bone marrow-derived proangiogenic precursors is a characteristic of IPAH and may participate in the pulmonary vascular remodeling process.

Pulmonary arterial hypertension: a disease of tethers, SNAREs and SNAPs?

Histological and electron microscopic studies over the past four decades have highlighted "plump," "enlarged" endothelial, smooth muscle, and fibroblastic cellular elements with increased endoplasmic reticulum, Golgi stacks, and vacuolation in pulmonary arterial lesions in human and in experimental (hypoxia and monocrotaline) pulmonary arterial hypertension. However, the contribution of disrupted intracellular membrane trafficking in the pathobiology of this disease has received insufficient attention. Recent studies suggest a pathogenetic role of the disruption of intracellular trafficking of vasorelevant proteins and cell-surface receptors in the development of this disease. The purpose of this essay is to highlight the molecular regulation of vesicular trafficking by membrane tethers, SNAREs and SNAPs, and to suggest how their dysfunction, directly and/or indirectly, might contribute to development of pulmonary arterial hypertension in experimental models and in humans, including that due to mutations in bone morphogenetic receptor type 2.

Dysfunctional intracellular trafficking in the pathobiology of pulmonary arterial hypertension

Discussions of the initiation of pulmonary arterial hypertension (PAH) in man and in experimental models have centered around intimal and medial proliferation in medium-sized pulmonary arteries. The histologic events are thought to include disordered proliferation of enlarged, vacuolated endothelial cells, neo-muscularization of the affected blood vessels, and vascular pruning. The discovery of the association of familial and sporadic PAH with mutations in BMPR2 has generated intense interest in cytokine receptor trafficking and function in the endothelial cell and how this might be disrupted to yield an enlarged proliferative cell phenotype. Nevertheless, considerations of the subcellular machinery of membrane trafficking in the endothelial cell and consequences of the disruption of this outward and inward membrane trafficking are largely absent from discussions of the pathobiology of PAH. Long-standing electron microscopy data in the PAH field has demonstrated marked disruptions of intracellular membrane trafficking in human and experimental PAH. Further, a role of the membrane-trafficking regulator Nef in simian HIV-induced PAH in macaques and in HIV-induced PAH in man is now evident. Additionally, monocrotaline and hypoxia are known to disrupt the function of Golgi tethers, SNAREs, SNAPs, and N-ethylmaleimide-sensitive factor ("the Golgi blockade hypothesis"). These results, along with recent reports demonstrating the trapping of PAH-associated human BMPR2 mutants in the Golgi, highlight the implications of disrupted intracellular membrane trafficking in the pathobiology of PAH. The purpose of this review is to present a brief overview of the molecular basis of intracellular trafficking and relate these considerations to the pathobiology of PAH.

Angiogenesis and pulmonary hypertension: a unique process in a unique disease

Severe pulmonary hypertension is a fatal condition associated with marked alterations of the cellular components of pulmonary arteries. In this review, we discuss the component of endothelial cell proliferation present in pulmonary arteries in patients with severe pulmonary hypertension. Because these proliferated endothelial cells exhibit markers of angiogenesis, we have named this process as "disordered or misguided angiogenesis." We also discuss the recent evidence that germline or somatic inactivating mutations in tumor suppressor or proapoptotic genes may play a significant role in the abnormal proliferation of pulmonary endothelial cells. The unraveling of the pathobiology of severe pulmonary hypertension may lead us to novel therapies and approaches to better diagnose the disease.

Vascular smooth muscle cell phenotypes in primary pulmonary hypertension

Primary pulmonary hypertension (PPH) is associated with specific structural alterations, including cellular intimal thickening, intimal fibrosis, and plexiform lesions. To determine the phenotypes of smooth muscle cells (SMCs) in such lesions, the authors conducted an immunohistochemical analysis of lung tissues from two patients with PPH, using two antimuscle actin antibodies, HHF35 and CGA7, and two anti-SMC myosin heavy chain markers, anti-SM1 and anti-SM2 antibodies and related antibodies. Cells that stained positive (+) with HHF35, CGA7, anti-SM1, and anti-SM2 were considered to be SMCs of a mature state. Conversely, those that stained positive with HHF35 and anti-SM1, but weakly positive (+/-) or negative (-) with CGA7 and anti-SM2, were considered to be SMCs exhibiting an immature state. Cellular intimal thickening was composed of SMCs of an immature phenotype (HHF35+, CGA7+/-, SM1+, SM2+/-), accompanied by the expression of fibronectin and the presence of macrophages; intimal fibrosis contained mature SMCs (HHF35+, CGA7+, SM1+, SM2+); and plexiform lesion consisted of proliferative endothelial cells (von Willebrand factor-positive cells, proliferating cell nuclear antigen-positive cells) and underlying immature SMCs (HHF35+, CGA7-, SM1+, SM2-) associated with fibronectin expression and macrophage infiltration. These findings suggest that smooth muscle cells with specific phenotypes may contribute to the development of specific vascular lesions in primary pulmonary hypertension.

Pathogenesis and evolution of plexiform lesions in pulmonary hypertension associated with scleroderma and human immunodeficiency virus infection

Patients with primary pulmonary hypertension develop vascular lesions characterized by proliferated blood channels, the so-called plexiform lesions. These lesions are often associated with concentric intimal obliteration of pulmonary vessels. We report that the lungs of three patients with scleroderma-associated pulmonary hypertension showed a predominance of obliterative-concentric lesions, with relatively few plexiform or combined lesions. In contrast, plexiform lesions predominated in the lungs obtained from three patients with human immunodeficiency virus (HIV)-associated pulmonary hypertension; pure obliterative-concentric lesions were infrequent. Both plexiform and concentric obliterative lesions stained strongly positive for the endothelial cell marker factor VIII-related antigen. Muscle-specific actin immunostaining highlighted the smooth muscle cells of the tunica media of plexiform vessels, but not the luminal layers of the concentric-obliterative lesions. Proliferating cells, as determined by immunostaining with the MIB-1 antibody, were only detected in the plexiform vascular lesions. We postulate that concentric-obliterative lesions and plexiform lesions are temporally and etiologically related. A scaffolding of proliferating endothelial cells could be the common denominator of both lesions. Our hypothesis that there exists a chronological continuum, proceeding from early, proliferative plexiform lesions to late, nonproliferative concentric-obliterative lesions in primary and secondary pulmonary hypertension, may lead to better targeted treatment strategies and disease classification.

Exploration of the pulmonary circulation. Festschrift to Professor Donald Heath

Images

Plexogenic arteriopathy

Images

Pathology of pulmonary hypertension: a human and experimental study

To clarify the histopathological characteristics of pulmonary hypertension (PH) in Japan, and to clarify the role of serotonin and endothelin in monocrotaline induced PH, human histopathological studies and experimental studies were carried out. An epidemiological study based on the Annual of the Pathological Autopsy Cases in Japan, and a morphological study on autopsy cases of congenital heart disease and idiopathic PH were performed. Plasma levels of serotonin and endothelin, vascular responsiveness to serotonin, and the effects of a selective serotonin antagonist, DV-7028, were investigated after monocrotaline injection. Plexogenic pulmonary arteriopathy was prevalent, and recurrent pulmonary thromboembolism and pulmonary veno-occlusive disease extremely rare among primary pulmonary hypertension in Japan. In secondary PH, systemic lupus erythematosus and mixed connective tissue disease were frequent and showed particularly severe intimal and medial thickening. After an injection of monocrotaline, plasma serotonin and endothelin levels were raised, and pulmonary arteries showed hyperreactivity to serotonin. DV-7028 (5-HT2 receptor antagonist) attenuated the rise in pulmonary artery pressure and the various effects of monocrotaline. There may be some genetic difference between PH in Japan and other countries. Roles for serotonin and endothelin in the initiation and progression of monocrotaline induced PH are suggested.

Exuberant endothelial cell growth and elements of inflammation are present in plexiform lesions of pulmonary hypertension

The plexiform lesion in primary pulmonary hypertension is a glomeruloid structure forming channels in branches of the pulmonary artery. These lesions have been considered an abnormal growth of modified smooth muscle cells. We present immunohistochemical evidence in 10 cases of plexogenic pulmonary hypertension that the plexiform channels and the concentric obliterative arteriopathy associated with these channels represent abnormal growth of factor VIII-related antigen-positive endothelial cells. In addition, these cells strongly expressed vimentin, a growth- and differentiation-related intermediate filament. Morphologically and immunohistochemically, the lesions resembled the neovascularization associated with the brain tumor glioblastoma multiform. Furthermore, we noted an exclusively perivascular inflammatory cell infiltrate (but no vasculitis) in seven of the 10 cases with plexogenic arteriopathy composed of T cells, B cells, and macrophages. Our findings indicate that the plexiform lesion may result from a deregulated growth of endothelial cells. The presence of perivascular inflammatory cells suggested that cytokines and growth factors may further influence the development of the plexiform lesion.

Active collagen synthesis by pulmonary arteries in human primary pulmonary hypertension

Immunohistochemistry was performed on lung tissue obtained from patients with severe unexplained pulmonary hypertension using an antibody to the amino-terminal end of the procollagen type I propeptide. This antibody identifies newly synthesized alpha I(I) procollagen before cleavage of the amino-terminal propeptide following secretion and, therefore, can identify sites of active collagen deposition. Procollagen was detected in the media, media and neointima, or neointima alone of a large number of small muscular arteries from hypertensive lungs. Normal adult lungs were negative. Neointimal cells in remodeled small muscular arteries stained positively for alpha-smooth muscle actin and desmin consistent with a smooth muscle lineage. These data suggest smooth muscle-like cells in small muscular arteries are actively synthesizing collagen in patients with severe unexplained pulmonary hypertension.

The ultrastructure of plexogenic pulmonary arteriopathy

The lungs from 16 cases of plexogenic pulmonary arteriopathy obtained at heart-lung transplantation, half of which had primary pulmonary hypertension, were examined by electron microscopy. From these the probable pathogenesis of pulmonary arterial intimal fibrosis in plexogenic pulmonary arteriopathy was deduced. The earliest detectable change was migration of smooth muscle cells from the media, through the internal elastic lamina into the intima. These cells collected beneath the endothelium and lost many of their myofilaments to become myofibroblasts. They were associated with ground substance but scanty collagen fibrils. As the quantity of interstitial collagen increased, the myofibroblasts reverted to a muscular structure, became elongated, and assumed a regular, circumferential orientation. This later stage coincided with the development of plexiform lesions. At both early and later stages, the muscular pulmonary arteries were contracted but not markedly so, and muscular evaginations were not seen. On the other hand, the cellular intimal proliferations developed early and were occlusive. This suggests that occlusion of small pulmonary arterial vessels by myofibroblasts may be at least as important as vasoconstriction in the early elevation of the pulmonary vascular resistance in primary pulmonary hypertension.

Plexiform lesion in bronchopulmonary anastomosis

A cellular lesion developed in the bronchial component of a bronchopulmonary anastomosis in a girl of 6 years with primary pulmonary hypertension. Its histological features were consistent with those of a plexiform lesion.

Ultrastructure of early plexogenic pulmonary arteriopathy

A lung biopsy specimen from a young woman with the clinical features of primary pulmonary hypertension showed grade 2 plexogenic pulmonary arteriopathy. Electron microscopy revealed 'dark', electron-dense smooth muscle cells in the inner part of the media of muscular pulmonary arteries. Many of these transformed myocytes had migrated into the lumens of pulmonary arteries and arterioles which they occluded. This migration of smooth muscle cells was associated with a substantial increase in the number of pulmonary endocrine cells in the bronchioles containing bombesin and calcitonin.