Alveolar proliferation, retinoid synthesizing enzymes, and endogenous retinoids in the postnatal mouse lung. Different roles for Aldh-1 and Raldh-2

Global gene expression profiling in congenital diaphragmatic hernia (CDH) patients

Congenital diaphragmatic hernia (CDH) is an anomaly characterized by a defect in the diaphragm, leading to the passage of intra-abdominal organs into the thoracic cavity. Herein, the presented work analyzes the global gene expression profiles in nine CDH and one healthy newborn. All of the patients had left posterolateral (Bochdalek) diaphragmatic hernia, operated via an abdominal approach, and stomach and bowels in the thorax cavity. Some patients also had additional anomalies. A total of 560 differentially regulated genes were measured. Among them, 11 genes showed significant changes in expression associated with lung tissue, vascular structure development, and vitamin A metabolism, which are typical ontologies related to CDH etiology. Among them, SLC25A24 and RAB3IL1 are involved in angiogenesis, HIF1A and FOXC2-AS1 are related with the alveolus, MAGI2-AS3 is associated with the diaphragm, LHX4 and DHH are linked with the lung, and BRINP1, FZD9, WNT4, and BLOC1S1-RDH5 are involved in retinol. Besides, the expression levels of some previously claimed genes with CDH etiology also showed diverse expression patterns in different patients. All these indicated that CDH is a complex, multigenic anomaly, requiring holistic approaches for its elucidation.

Genetics of diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) is a life-threatening malformation characterised by failure of diaphragmatic development with lung hypoplasia and persistent pulmonary hypertension of the newborn (PPHN). The incidence is 1:2000 corresponding to 8% of all major congenital malformations. Morbidity and mortality in affected newborns are very high and at present, there is no precise prenatal or early postnatal prognostication parameter to predict clinical outcome in CDH patients. Most cases occur sporadically, however, genetic causes have long been discussed to explain a proportion of cases. These range from aneuploidy to complex chromosomal aberrations and specific mutations often causing a complex phenotype exhibiting multiple malformations along with CDH. This review summarises the genetic variations which have been observed in syndromic and isolated cases of congenital diaphragmatic hernia.

All Trans Retinoic Acid (ATRA) progresses alveolar epithelium regeneration by involving diverse signalling pathways in emphysematous rat

INTRODUCTION

Pulmonary emphysema is characterized by destruction of alveoli leading to inadequate oxygenation, disability and frequently death. This destruction was understood so far as irreversible. Published data has shown that ATRA (All Trans Retinoic Acid) reverses elastase-induced emphysema in rats. However, the molecular mechanisms governing regeneration process are so far unknown.

OBJECTIVE

To examine the therapeutic potential of ATRA on various molecular pathways and their coordination towards governance of alveolar epithelial regeneration in emphysematous rats.

METHODS

Emphysema was induced by elastase versus saline in Sprague-Dawley rats. On days 26-37, rats received daily intraperitoneal injections with ATRA (500 μg/kg b.w.) versus olive-oil. Lungs were removed at day 38 for histopathology and investigation of relative mRNA and protein expressions.

RESULTS

Histopathological analysis has shown that losses of alveoli were recovered in therapy (EA) group. Moreover, expressions of markers genes for alveolar cell proliferation, differentiation and EMT events at mRNA and protein levels were significantly increased in EA group than emphysema group (ES). Upon validation at genomics level, expressions of components of Notch, Hedgehog, Wnt, BMP and TGFβ pathways were significantly attenuated in EA group when compared with ES and were well comparable with the healthy group.

CONCLUSION

Therapeutic supplementation of ATRA rectifies the deregulated Notch, Hedgehog, Wnt, BMP and TGFβ pathways in emphysema condition, resulting in alveolar epithelium regeneration. Hence, ATRA may prove to be a potential drug in the treatment of emphysema. Nevertheless, elaborated studies are to be conducted.

Retinoic Acid: A Key Regulator of Lung Development

Retinoic acid (RA) is a key molecular player in embryogenesis and adult tissue homeostasis. In embryo development, RA plays a crucial role in the formation of different organ systems, namely, the respiratory system. During lung development, there is a spatiotemporal regulation of RA levels that assures the formation of a fully functional organ. RA signaling influences lung specification, branching morphogenesis, and alveolarization by regulating the expression of particular target genes. Moreover, cooperation with other developmental pathways is essential to shape lung organogenesis. This review focuses on the events regulated by retinoic acid during lung developmental phases and pulmonary vascular development; also, it aims to provide a snapshot of RA interplay with other well-known regulators of lung development.

Time-lapse Imaging of Alveologenesis in Mouse Precision-cut Lung Slices

Alveoli are the gas-exchange units of lung. The process of alveolar development, alveologenesis, is regulated by a complex network of signaling pathways that act on various cell types including alveolar type I and II epithelial cells, fibroblasts and the vascular endothelium. Dysregulated alveologenesis results in bronchopulmonary dysplasia in neonates and in adults, disrupted alveolar regeneration is associated with chronic lung diseases including COPD and pulmonary fibrosis. Therefore, visualizing alveologenesis is critical to understand lung homeostasis and for the development of effective therapies for incurable lung diseases. We have developed a technique to visualize alveologenesis in real-time using a combination of widefield microscopy and image deconvolution of precision-cut lung slices. Here, we describe this live imaging technique in step-by-step detail. This time-lapse imaging technique can be used to capture the dynamics of individual cells within tissue slices over a long time period (up to 16 h), with minimal loss of fluorescence or cell toxicity.

Modulation of retinoid signaling: therapeutic opportunities in organ fibrosis and repair

The vitamin A metabolite, retinoic acid, is an important signaling molecule during embryonic development serving critical roles in morphogenesis, organ patterning and skeletal and neural development. Retinoic acid is also important in postnatal life in the maintenance of tissue homeostasis, while retinoid-based therapies have long been used in the treatment of a variety of cancers and skin disorders. As the number of people living with chronic disorders continues to increase, there is great interest in extending the use of retinoid therapies in promoting the maintenance and repair of adult tissues. However, there are still many conflicting results as we struggle to understand the role of retinoic acid in the multitude of processes that contribute to tissue injury and repair. This review will assess our current knowledge of the role retinoic acid signaling in the development of fibroblasts, and their transformation to myofibroblasts, and of the potential use of retinoid therapies in the treatment of organ fibrosis.

Mesothelium and Malignant Mesothelioma

The mesothelium is an epithelial structure derived from the embryonic mesoderm. It plays an important role in the development of a number of different organs, including the heart, lungs, and intestines. In this publication, we discuss aspects of the development of the mesothelium, where mesothelial structures can be found, and review molecular and cellular characteristics associated with the mesothelium. Furthermore, we discuss the involvement of the mesothelium in a number of disease conditions, in particular in the pathogenesis of mesotheliomas with an emphasis on malignant pleural mesothelioma (MPM)—a primary cancer developing in the pleural cavity.

Live imaging of alveologenesis in precision-cut lung slices reveals dynamic epithelial cell behaviour

Damage to alveoli, the gas-exchanging region of the lungs, is a component of many chronic and acute lung diseases. In addition, insufficient generation of alveoli results in bronchopulmonary dysplasia, a disease of prematurity. Therefore visualising the process of alveolar development (alveologenesis) is critical for our understanding of lung homeostasis and for the development of treatments to repair and regenerate lung tissue. Here we show live alveologenesis, using long-term, time-lapse imaging of precision-cut lung slices. We reveal that during this process, epithelial cells are highly mobile and we identify specific cell behaviours that contribute to alveologenesis: cell clustering, hollowing and cell extension. Using the cytoskeleton inhibitors blebbistatin and cytochalasin D, we show that cell migration is a key driver of alveologenesis. This study reveals important novel information about lung biology and provides a new system in which to manipulate alveologenesis genetically and pharmacologically.

The process of alveologenesis is incompletely understood, partly due to the lack of applicable real-time imaging methods. Here the authors describe the process of alveologenesis and the behaviour of epithelial cells in real-time, using widefield microscopy and image deconvolution in precision-cut lung slices, revealing the dominant role of epithelial cell migration.

Effect of Hyperoxia on Retinoid Metabolism and Retinoid Receptor Expression in the Lungs of Newborn Mice

BACKGROUND

Preterm newborns that receive oxygen therapy often develop bronchopulmonary dysplasia (BPD), which is abnormal lung development characterized by impaired alveologenesis. Oxygen-mediated injury is thought to disrupt normal lung growth and development. However, the mechanism of hyperoxia-induced BPD has not been extensively investigated. We established a neonatal mouse model to investigate the effects of normobaric hyperoxia on retinoid metabolism and retinoid receptor expression.

METHODS

Newborn mice were exposed to hyperoxic or normoxic conditions for 15 days. The concentration of retinol and retinyl palmitate in the lung was measured by HPLC to gauge retinoid metabolism. Retinoid receptor mRNA levels were assessed by real-time PCR. Proliferation and retinoid receptor expression in A549 cells were assessed in the presence and absence of exogenous vitamin A.

RESULTS

Hyperoxia significantly reduced the body and lung weight of neonatal mice. Hyperoxia also downregulated expression of RARα, RARγ, and RXRγ in the lungs of neonatal mice. In vitro, hyperoxia inhibited proliferation and expression of retinoid receptors in A549 cells.

CONCLUSION

Hyperoxia disrupted retinoid receptor expression in neonatal mice.

Retinoids and rexinoids in cancer prevention: from laboratory to clinic

Early in the age of modern medicine the consequences of vitamin A deficiency drew attention to the fundamental link between retinoid-dependent homeostatic regulation and malignant hyperproliferative diseases. The term "retinoid" includes a handful of endogenous and a large group of synthetic derivatives of vitamin A. These multifunctional lipid-soluble compounds directly regulate target genes of specific biological functions and critical signaling pathways to orchestrate complex functions from vision to development, metabolism, and inflammation. Many of the retinoid activities on the cellular level have been well characterized and translated to the regulation of processes like differentiation and cell death, which play critical roles in the outcome of malignant transformation of tissues. In fact, retinoid-based differentiation therapy of acute promyelocytic leukemia was one of the first successful examples of molecularly targeted treatment strategies. The selectivity, high receptor binding affinity and the ability of retinoids to directly modulate gene expression programs present a distinct pharmacological opportunity for cancer treatment and prevention. However, to fully exploit their potential, the adverse effects of retinoids must be averted. In this review we provide an overview of the biology of retinoid (activated by nuclear retinoic acid receptors [RARs]) and rexinoid (engaged by nuclear retinoid X receptors [RXRs]) action concluded from a long line of preclinical studies, in relation to normal and transformed states of cells. We will also discuss the past and current uses of retinoids in the treatment of malignancies, the potential of rexinoids in the cancer prevention setting, both as single agents and in combinations.

Alveolar macrophages contribute to respiratory tolerance by inducing FoxP3 expression in naive T cells

Alveolar macrophages (AMs) from mice and humans have long been known to contribute to maintaining tolerance in the lung. Studies have shown that AMs can induce anergy in CD4(+) T cells. Nitric oxide, prostaglandins, and leukotrienes have been implicated in AM-mediated tolerance. However, it remains unclear what effect, if any, AMs exert on FoxP3 induction in CD4(+) T cells from mice and humans, and whether or not other immunomodulators might play a role. AMs were isolated from bronchoalveolar lavage (BAL) fluid from either mice or humans, and cocultured with enriched naive CD4(+)FoxP3(-) T cells. We show here for the first time that AMs and AM-conditioned media (AM-CM) from mice and humans induced FoxP3 expression in naive CD4(+) T cells in vitro, an outcome that was reversed in part either by inhibiting retinoic acid (RA) binding to its receptor (RAR), or by blocking transforming growth factor (TGF)-β₁ signaling. A nasal administration of the RAR antagonist reduced the frequencies of CD4(+)FoxP3(+) T cells in the lungs of mice after aerosol challenge with Bordetella pertussis. In addition, we found that the intranasal vaccination of mice with ovalbumin (OVA) protein in conjunction with an RAR inhibitor led to a significant increase in OVA-specific serum IgE. Our findings suggest that AMs can mediate tolerance in the lungs of mice and humans via RA and TGF-β₁. These data may have implications in the development of nasal vaccines in the future.

Mucin production during prenatal and postnatal murine lung development

Mucus is a protective gel that lines respiratory tract surfaces. To identify potential roles for secreted gel--forming mucins in lung development, we isolated murine lungs on embryonic days (E) 12.5-18.5, and postnatal days (PN) days 5, 14, and 28. We measured the mucin gene expression by quantitative RT-PCR, and localization by histochemical and immunohistochemical labeling. Alcian blue/periodic acid--Schiff--positive cells are present from E15.5 through PN28. Muc5b transcripts were abundant at all time points from E14.5 to PN28. By contrast, transcript levels of Muc5ac and Muc2 were approximately 300 and 85,000 times lower, respectively. These data are supported by immunohistochemical studies demonstrating the production and localization of Muc5ac and Muc5b protein. This study indicates that mucin production is prominent in developing murine lungs and that Muc5b is an early, abundant, and persistent marker of bronchial airway secretory cells, thereby implicating it as an intrinsic component of homeostatic mucosal defense in the lungs.

Is a regenerative approach viable for the treatment of COPD?

Degenerative lung diseases such as chronic obstructive pulmonary disease (COPD) are common with huge worldwide morbidity. Anti-inflammatory drug development strategies have proved disappointing and current treatment is aimed at symptomatic relief. Only lung transplantation with all its attendant difficulties offers hope of cure and the outlook for affected patients is bleak. Lung regeneration therapies aim to reverse the structural and functional deficits in COPD either by delivery of exogenous lung cells to replace lost tissue, delivery of exogenous stem cells to induce a local paracrine effect probably through an anti-inflammatory action or by the administration of small molecules to stimulate the endogenous regenerative ability of lung cells. In animal models of emphysema and disrupted alveolar development each of these strategies has shown some success but there are potential tumour-inducing dangers with a cellular approach. Small molecules such as all-trans retinoic acid have been successful in animal models although the mechanism is not completely understood. There are currently two Pharma-sponsored trials in progress concerning patients with COPD, one of a specific retinoic acid receptor gamma agonist and another using mesenchymal stem cells.

Altered gene expression profiles in the brain, kidney, and lung of one-month-old cloned pigs

Although numerous mammalian species have been successfully cloned by somatic cell nuclear transfer (SCNT), little is known about gene expression of cloned pigs by SCNT. In the present study, expression profiles of 1-month-old cloned pigs generated from fetal fibroblasts (n = 5) were compared to those of age-matched controls (n = 5) using a 13K oligonucleotide microarray. The brain, kidney, and lung were chosen for microarray analysis to represent tissues from endoderm, mesoderm, and ectoderm in origin. In clones, 179 and 154 genes were differentially expressed in the kidney and the lung, respectively (fold change >2, p < 0.05, false discovery rate = 0.05), whereas only seven genes were differentially expressed in the brain of clones. Functional analysis of the differentially expressed genes revealed that they were enriched in diabetic nephropathy in the kidney, delayed alveologenesis as well as downregulated MAPK signaling pathways in the lung, which was accompanied with collapsed alveoli in the histological examination of the lung. To evaluate whether the gene expression anomalies are associated with changes in DNA methylation, global concentration of the methylated cytosine was measured in lung DNA by HPLC. Clones were significantly hypermethylated (5.72%) compared to the controls (4.13%). Bisulfite-pyrosequencing analyses of the promoter regions of differentially expressed genes, MYC and Period 1 (PER1), however, did not show any differences in the degree of DNA methylation between controls and clones. Together, these findings demonstrate that cloned pigs have altered gene expression that may potentially cause organ dysfunction.

Congenital diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) is a congenital anomaly consisting of a posterolateral defect in the diaphragm also known as a Bochdalek hernia. It occurs in 1 in 2000 to 3000 newborns and is associated with a variable degree of pulmonary hypoplasia (PH) and persistent pulmonary hypertension (PPH). Despite remarkable advances in neonatal resuscitation and intensive care and the new postnatal treatment strategies, many newborns with CDH continue to have high rates of mortality and morbidity as the result of severe respiratory failure secondary to PH and PPH. The pathogenesis of CDH and associated PH and PPH is poorly understood. Herein, we aim to review diaphragm and pulmonary development and correlate this to the abnormalities found in CDH.

Structural features of the NAD-dependent in situ retinoic acid supply system in esophageal mucosa

BACKGROUND

We previously reported that an NAD-dependent in situ retinoic acid supply system, which comprises some isoforms of alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) and provides retinoic acid from retinol via a 2-step oxidation process, exists in the rat esophagus. Herein, their isoforms responsible for the pathway and its localization in the rat esophagus was examined.

METHODS

The expressions of mRNAs of various isoforms of ADH and ALDH were examined in the fraction mainly comprising mucosal layer of the rat esophagus by RT-PCR. Expression levels of Class IV ADH and ALDH 1A1 were compared between the fractions and that mainly comprising muscle layer of the rat esophagus by quantitative PCR. The catalytic activities producing retinoic acid from retinal were compared between the 2 fractions and its optimum pH was also determined.

RESULTS

Classes I, III, and IV ADHs and ALDHs 1A1 and 3A1 were predominant isoforms in the rat esophageal mucosa. The expression levels of mRNA of Class IV ADH and ALDH 3A1 were significantly higher in the mucosal than in the muscle layer. Consistently, the catalytic activities producing retinoic acid from retinal were significantly higher in the former than the latter. The optimum pH of the process was 9.0.

CONCLUSIONS

Considering the affinities for retinol and retinal of ADHs and ALDHs expressed in the rat esophagus, the NAD-dependent in situ retinoic acid supply system in the rat esophagus is thought to comprise Class IV ADH and ALDH 1A1. In the rat esophagus, the system exists predominantly in the mucosal layer.

Congenital diaphragmatic hernia and retinoids: searching for an etiology

Congenital diaphragmatic hernia (CDH) is a major life-threatening cause of respiratory failure in the newborn. Recent data reveal the role of a retinoid-signaling pathway disruption in the pathogenesis of CDH. We describe the epidemiology and pathophysiology of human CDH, the metabolism of retinoids and the implications of retinoids in the development of the diaphragm and lung. Finally, we describe the existing evidence of a disruption of the retinoid-signaling pathway in CDH.

Retinoic acid induced alveolar regeneration: critical differences in strain sensitivity

In emphysema, the lung cannot spontaneously regenerate lost alveolar tissue. Treatment with retinoic acid (RA) in rodent models of emphysema induces alveolar regeneration. However, some animal studies have failed to show regeneration when using different species and strains. We have previously shown that dexamethasone (Dex) treatment of newborn TO outbred strain mice permanently disrupts alveolar development. Later RA treatment restores alveolar architecture to normal. To determine whether this model of alveolar regeneration is strain specific, our protocol was repeated with two new outbred mouse strains. ICR and NIHS mice received Dex from Postnatal Days 4 to P15 (P4- P15). From P46 to P57, mice received RA (2 mg/kg) or vehicle. An additional ICR group received 5x RA (10 mg/kg) from P46 to P57. Control groups received vehicle at both treatment points. All mice were killed at P90 and lung morphology analyzed. Dex-treated ICR and NIHS mice showed increased mean alveolar chord length (Lm) and reduced alveolar surface area (SA) and SA/lung volume (SA/LV) compared with controls. RA-treated NIHS mice showed return of Lm, SA, and SA/LV toward control values, indicating alveolar regeneration. ICR RA group mice did not regenerate, but 5x RA mice showed Lm, SA, and SA/LV values consistent with alveolar regeneration. In conclusion, the Dex-treated mouse model of emphysema is robust and repeatable in different strains. RA-induced alveolar regeneration is not a strain-specific phenomenon. RA dose threshold for inducing alveolar regeneration is higher in ICR mice, suggesting a difference in retinoid pharmacokinetics between strains. These results provide a possible explanation for previous failed studies of RA-induced alveolar regeneration.

Gene expression profiling in lung fibroblasts reveals new players in alveolarization

Little is known about the molecular basis of lung alveolarization. We used a microarray profiling strategy to identify novel genes that may regulate the secondary septation process. Rat lung fibroblasts were extemporaneously isolated on postnatal days 2, 7, and 21, i.e., before, during, and after septation, respectively. Total RNA was extracted, and cRNAs were hybridized to Affymetrix rat genome 230 2.0 microarrays. Expression levels of a selection of genes were confirmed by real-time PCR. In addition to genes already known to be upregulated during alveolarization including drebrin, midkine, Fgfr3, and Fgfr4, the study allowed us to identify two remarkable groups of genes with opposite profiles, i.e., gathering genes either transiently up- or downregulated on day 7. The former group includes the transcription factors retinoic acid receptor (RXR)-gamma and homeobox (Hox) a2, a4, and a5 and genes involved in Wnt signaling (Wnt5a, Fzd1, and Ndp); the latter group includes the extracellular matrix components Comp and Opn and the signal molecule Slfn4. Profiling in whole lung from fetal life to adulthood confirmed that changes were specific for alveolarization. Two treatments that arrest septation, hyperoxia and dexamethasone, inhibited the expression of genes that are upregulated during alveolarization and conversely enhanced that of genes weakly expressed during alveolarization and upregulated thereafter. The possible roles of these genes in secondary septation are discussed. Gene expression profiling analysis on freshly isolated cells represents a powerful approach to provide new information about differential regulation of genes during alveolarization and pathways potentially involved in the pathogenesis of bronchopulmonary dysplasia.

All-trans retinoic acid increases expression of aquaporin-5 and plasma membrane water permeability via transactivation of Sp1 in mouse lung epithelial cells

Aquaporin-5 (AQP5) is a water-selective channel protein that is expressed in lacrimal glands, salivary glands, and distal lung. Several studies using AQP5 knockout mice have revealed that AQP5 plays an important role in maintaining water homeostasis in the lung. We report here that all-trans retinoic acid (atRA) increases plasma membrane water permeability, AQP5 mRNA and protein expression, and AQP5 promoter activity in MLE-12 cells. The promoter activation induced by atRA was diminished by mutation at the Sp1/Sp3 binding element (SBE), suggesting that the SBE mediates the effects of atRA. In addition, atRA increased the binding of Sp1 to the SBE without changing the levels of Sp1 in the nucleus. Taken together, our data indicate that atRA increases AQP5 expression through transactivation of Sp1, leading to an increase in plasma membrane water permeability.

Localization of retinaldehyde dehydrogenases and retinoid binding proteins to sustentacular cells, glia, Bowman's gland cells, and stroma: potential sites of retinoic acid synthesis in the postnatal rat olfactory organ

Work from our laboratory suggests that retinoic acid (RA) influences neuron development in the postnatal olfactory epithelium (OE). The studies reported here were carried out to identify and localize retinaldehyde dehydrogenase (RALDH) expression in postnatal rat OE to gain a better understanding of potential in vivo RA synthesis sites in this continuously regenerating tissue. RALDH 1, 2, and 3 mRNAs were detected in postnatal rat olfactory tissue by RT-PCR analysis, but RALDH 1 and 2 transcripts were predominant. RALDH 1 immunoreactivity was localized to sustentacular cells in the OE and to Bowman's gland cells, and GFAP(+)/p75(-) olfactory ensheathing cells (OECs) in the underlying lamina propria (LP). RALDH 2 did not colocalize with RALDH 1, but appeared to be expressed in GFAP(-)/RALDH 1(-) OECs as well as in unidentified structures in the LP. Cellular RA binding protein (CRABP II) colocalized with RALDH 1. Cellular retinol/retinaldehyde binding protein (CRBP I) was localized to RALDH 1(+) sites in the OE and LP and RALDH 2(+) sites, primarily surrounding nerve fiber bundles in the LP. Vitamin A deficiency altered RALDH 1, but not RALDH 2 protein expression. The isozymes and binding proteins exhibited random variability in levels and areas of expression both within and between animals. These findings support the hypothesis that RA is synthesized in the postnatal OE (catalyzed by RALDH 1) and underlying LP (differentially catalyzed by RALDH 1 and RALDH 2) at sites that could influence the development, maturation, targeting, and/or turnover of olfactory receptor neurons throughout the olfactory organ.

Retinoids have differing efficacies on alveolar regeneration in a dexamethasone-treated mouse

We have investigated the relative efficacy of a range of natural and synthetic retinoids on the induction of alveolar regeneration in a dexamethasone-treated mouse model. The aim was to explore the roles of the different retinoic acid receptors using receptor-selective agonists and to determine whether other natural retinoids in addition to all-trans-retinoic acid (tRA) were effective. Dexamethasone treatment of newborn pups led to a reduced lung surface area and increased mean chord length. Subsequently, tRA induced alveolar repair, improved mean chord length, and improved the lung surface area to volume ratio. We found that 4-oxo-RA and a retinoic acid receptor (RAR) alpha-selective compound were as effective as tRA at inducing alveolar regeneration, with neither showing a significantly better efficacy. An RARbeta-selective compound was also effective, whereas a RARgamma-selective compound was not. Other retinoids, such as 9-cis-RA, 13-cis-RA, retinol, and a pan retinoid X receptor (RXR) agonist, do not induce significant responses. Neither did granulocyte colony-stimulating factor. We also showed that an RARbeta-null mutant mouse line responded to dexamethasone by failing to develop alveoli appropriately and that tRA induced alveolar regeneration, suggesting that RARbeta was not required for the regenerative response.

Upregulated genes in sporadic, idiopathic pulmonary arterial hypertension

Background

To elucidate further the pathogenesis of sporadic, idiopathic pulmonary arterial hypertension (IPAH) and identify potential therapeutic avenues, differential gene expression in IPAH was examined by suppression subtractive hybridisation (SSH).

Methods

Peripheral lung samples were obtained immediately after removal from patients undergoing lung transplant for IPAH without familial disease, and control tissues consisted of similarly sampled pieces of donor lungs not utilised during transplantation. Pools of lung mRNA from IPAH cases containing plexiform lesions and normal donor lungs were used to generate the tester and driver cDNA libraries, respectively. A subtracted IPAH cDNA library was made by SSH. Clones isolated from this subtracted library were examined for up regulated expression in IPAH using dot blot arrays of positive colony PCR products using both pooled cDNA libraries as probes. Clones verified as being upregulated were sequenced. For two genes the increase in expression was verified by northern blotting and data analysed using Student's unpaired two-tailed t-test.

Results

We present preliminary findings concerning candidate genes upregulated in IPAH. Twenty-seven upregulated genes were identified out of 192 clones examined. Upregulation in individual cases of IPAH was shown by northern blot for tissue inhibitor of metalloproteinase-3 and decorin (P < 0.01) compared with the housekeeping gene glyceraldehydes-3-phosphate dehydrogenase.

Conclusion

Four of the up regulated genes, magic roundabout, hevin, thrombomodulin and sucrose non-fermenting protein-related kinase-1 are expressed specifically by endothelial cells and one, muscleblind-1, by muscle cells, suggesting that they may be associated with plexiform lesions and hypertrophic arterial wall remodelling, respectively.

Congenital diaphragmatic hernia: a retinoid-signaling pathway disruption during lung development?

Congenital diaphragmatic hernia (CDH) usually occurs sporadically. The prognosis remains poor, with a 50% perinatal mortality rate. Most deaths result from hypoxemia due to lung hypoplasia and abnormal development of pulmonary vasculature that results in persistent pulmonary hypertension. Our current understanding of the pathogenesis of CDH is based on an assumption linking herniation of abdominal viscera into the thorax with compression of the developing lung. Pulmonary hypoplasia, however, can also result from reduced distension of the developing lung secondary to impaired fetal breathing movements. Moreover, a nitrofen-induced CDH model shows that lung hypoplasia precedes the diaphragmatic defect, leading to a "dual-hit hypothesis." Recent data reveal the role of a retinoid-signaling pathway disruption in the pathogenesis of CDH. We describe the clinical and epidemiological aspects of human CDH, the metabolic and molecular aspects of the retinoid-signaling pathway, and the implications of retinoids in the development of the diaphragm and the lung. Finally, we highlight the existing links between CDH and disruption of the retinoid-signaling pathway, which may suggest an eventual use of retinoids in the treatment of CDH.

Developmental Expression of Aldehyde Dehydrogenase in Rat: a Comparison of Liver and Lung Development

Metabolism is one of the major determinants for age-related changes in susceptibility to chemicals. Aldehydes are highly reactive molecules present in the environment that also can be produced during biotransformation of xenobiotics and endogenous metabolism. Although the lung is a major target for aldehyde toxicity, early development of aldehyde dehydrogenases (ALDHs) in lung has been poorly studied. The expression of ALDH in liver and lung across ages (postnatal day 1, 8, 22, and 60) was investigated in Wistar-Han rats. In adult, the majority of hepatic ALDH activity was found in mitochondria, while cytosolic ALDH activity was the highest contributor in lung. Total aldehyde oxidation capability in liver increases with age, but stays constant in lung. These overall developmental profiles of ALDH expression in a tissue appear to be determined by the different composition of ALDH isoforms within the tissue and their independent temporal and tissue-specific development. ALDH2 showed the most notable tissue-specific development. Hepatic ALDH2 was increased with age, while the pulmonary form did not. ALDH1 was at its maximum value at postnatal day 1 (PND1) and decreased thereafter both in liver and lung. ALDH3 increased with age in liver and lung, although ALDH3A1 was only detectible in lung. Collectively, the present study indicates that, in the case of aldehyde exposure, the in vivo responses would be tissue and age dependent.

Immunolocalization of retinoic acid biosynthesis systems in selected sites in rat

Vitamin A deficiency leads to focal metaplasia of numerous epithelial tissues with altered differentiation from columnar (in general) to stratified squamous cells. This process can be reversed with vitamin A repletion. Previously, we described a system of retinoic acid (RA) synthesis in the cycling rat uterus consisting of cellular retinol binding protein (Crbp), epithelial retinol dehydrogenase (eRoldh), retinal dehydrogenase 2 (Aldh1a2), and cellular retinoic acid binding protein type II (Crabp2). Western blot analysis, RT-PCR, and immunohistochemistry were performed to test whether this retinoic acid synthesis system was also present in other vitamin A sensitive tissues. We found that combinations of Crbp, eRoldh, Aldh1a2 or Aldh1a3, and Crabp2 were present in all vitamin A sensitive tissues examined. In the ureter, while eRoldh was present, another short chain alcohol dehydrogenase reductase (possibly Roldh 1, 2, or 3) was in higher concentration in the transitional epithelia. In several tissues, Crbp, Aldh1a2, and/or Aldh1a3 localized to mesenchyme and/or epithelial cells, while eRoldh and Crabp2 were expressed only in epithelial cells. This suggests that mesenchymal-epithelial interactions may be as important in the adult as they are during development and that local synthesis of RA is important in maintenance of these tissues.

Control mechanisms of lung alveolar development and their disorders in bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a chronic lung disease that occurs in very premature infants and is characterized by impaired alveologenesis. This ultimate phase of lung development is mostly postnatal and allows growth of gas-exchange surface area to meet the needs of the organism. Alveologenesis is a highly integrated process that implies cooperative interactions between interstitial, epithelial, and vascular compartments of the lung. Understanding of its underlying mechanisms has considerably progressed recently with identification of structural, signaling, or remodeling molecules that are crucial in the process. Thus, the pivotal role of elastin deposition in lung walls has been demonstrated, and many key control-molecules have been identified, including various transcription factors, growth factors such as platelet-derived growth factor, fibroblast growth factors, and vascular endothelial growth factor, matrix-remodeling enzymes, and retinoids. BPD-associated changes in lung expression/content have been evidenced for most of these molecules, especially for signaling pathways, through both clinical investigations in premature infants and the use of animal models, including the premature baboon or lamb, neonatal exposure to hyperoxia in rodents, and maternal-fetal infection. These findings open therapeutic perspectives to correct imbalanced signaling. Unraveling the intimate molecular mechanisms of alveolar building appears as a prerequisite to define new strategies for the prevention and care of BPD.

Induction of compensatory lung growth in pulmonary emphysema improves surgical outcomes in rats

RATIONALE AND OBJECTIVES

Although lung volume reduction surgery (LVRS) has been widely used as a therapeutic strategy for pulmonary emphysema, the procedure carries significant disadvantages, including significant operative mortality and a limited duration of effective response. Pulmonary resection is known to elicit compensatory growth in remnant lung tissues; however, it remains unclear whether and how compensatory growth occurs and contributes to clinical outcomes after LVRS. The goal of the present study was to characterize the role of hepatocyte growth factor (HGF) in compensatory lung growth after LVRS in a rat model of elastase-induced emphysema, since HGF is a potent pulmotrophic factor responsible for the regeneration of lung parenchyma in damaged lungs, including after a pulmonary resection.

METHODS AND MAIN RESULTS

Unexpectedly, LVRS did not cause apparent increases in the endogenous HGF profiles of emphysematous lungs. Further, the lowered HGF production reflected a histologically inferior regenerative capacity in remnant lungs and was linked with impaired pulmonary functional recoveries after LVRS. When HGF was exogenously supplemented by gene transfection into emphysematous lungs simultaneously with LVRS, compensatory lung growth (as evidenced by increased lobe weight and alveolar regeneration and angiogenesis) was significantly enhanced as compared with rats that underwent LVRS alone. Consequently, pulmonary function and gas exchange were also significantly improved.

CONCLUSIONS

We concluded that the induction of compensatory growth by growth factors after LVRS may be a new strategy to further improve clinical outcomes of LVRS in patients with pulmonary emphysema.

FGF-18 is upregulated in the postnatal rat lung and enhances elastogenesis in myofibroblasts

The fibroblast growth factors (FGFs) are key players in fetal lung development, but little is known about their status in postnatal lung. Here, we investigated the expression pattern of FGF-18 transcripts through the perinatal period and evidenced a sevenfold increase after birth that paralleled changes in elastin expression. In vitro, recombinant human (rh)FGF-18 had a mitogenic activity on day 21 fetal rat lung fibroblasts and stimulated its own expression in the latter, whereas FGF-2 inhibited it. At 50 or 100 ng/ml, rhFGF-18 increased the expression of α-smooth muscle actin (α-SMA; 2.5-fold), a characteristic marker of myofibroblasts, of tropoelastin (6.5-fold), of lysyl oxidase (2-fold), and of fibulins 1 and 5 (8- and 2.2-fold) in confluent fibroblasts isolated from fetal day 21 lung; similar results were obtained with fibroblasts from day 3 postnatal lungs. Elastin protein expression was also slightly increased in fetal fibroblasts. Lung analysis on day 4 in rat pups that had received rhFGF-18 (3 μg) on days 0 and 1 showed a 1.7-fold increase of tropoelastin transcripts, whereas α-SMA transcripts were unchanged. In contrast, rhFGF-2 markedly decreased expression of elastin in vitro and in vivo and of fibulin 5 in vitro. In addition, vitamin A, which is known to enhance alveolar development, elevated FGF-18 and elastin expressions in day 2 lungs, thus advancing the biological increase. We postulate that FGF-18 is involved in postnatal lung development through stimulating myofibroblast proliferation and differentiation.

Retinoic acid in alveolar development, maintenance and regeneration

Recent data suggest that exogenous retinoic acid (RA), the biologically active derivative of vitamin A, can induce alveolar regeneration in a rat model of experimental emphysema. Here, we describe a mouse model of disrupted alveolar development using dexamethasone administered postnatally. We show that the effects of dexamethasone are concentration dependent, dose dependent, long lasting and result in a severe loss of alveolar surface area. When RA is administered to these animals as adults, lung architecture and the surface area per unit of body weight are completely restored to normal. This remarkable effect may be because RA is required during normal alveolar development and administering RA re-awakens gene cascades used during development. We provide evidence that RA is required during alveologenesis in the mouse by showing that the levels of the retinoid binding proteins, the RA receptors and two RA synthesizing enzymes peak postnatally. Furthermore, an inhibitor of RA synthesis, disulphiram, disrupts alveologenesis. We also show that RA is required throughout life for the maintenance of lung alveoli because when rats are deprived of dietary retinol they lose alveoli and show the features of emphysema. Alveolar regeneration with RA may therefore be an important novel therapeutic approach to the treatment of respiratory diseases characterized by a reduced gas-exchanging surface area such as bronchopulmonary dysplasia and emphysema for which there are currently no treatments.

Retinoic acid fails to reverse emphysema in adult mouse models

BACKGROUND

Previous work has shown that all-trans-retinoic acid reverses elastase induced emphysema in rats. Since there is currently no effective treatment for pulmonary emphysema, the effect of retinoic acid should be further investigated in other adult species. A study was undertaken using two murine models of emphysema to evaluate the effect of retinoic acid.

METHODS

The models used were an elastase induced emphysema model for acute alveolar destruction and a tumour necrosis factor (TNF)-alpha transgenic mouse which exhibits chronic air space enlargement, loss of elastic recoil, increased lung volume, and pulmonary hypertension comparable to human pulmonary emphysema. All-trans-retinoic acid (2 mg/kg) was injected for 12 successive days after the establishment of emphysema. The effects of treatment were evaluated using physiological and morphometric analyses.

RESULTS

In contrast to the rat, administration of all-trans-retinoic acid in these murine models did not improve the emphysema. Moreover, worsening of emphysema was observed in TNF-alpha transgenic mice treated with all-trans-retinoic acid. The level of keratinocyte chemoattractant (KC), a CXC chemokine, in bronchoalveolar lavage fluid was increased in TNF-alpha transgenic mice following retinoic acid treatment. These data raise the possibility that retinoic acid causes deterioration of emphysema by promoting inflammation in this model.

CONCLUSIONS

In these models, retinoic acid did not show positive effects on emphysema. The effect of retinoic acid in the treatment of pulmonary emphysema remains controversial, and further studies are required to determine its physiological effects under a variety of experimental conditions.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Retinoic acid, a regeneration-inducing molecule

Retinoic acid (RA) is the biologically active metabolite of vitamin A. It is a low molecular weight, lipophilic molecule that acts on the nucleus to induce gene transcription. In amphibians and mammals, it induces the regeneration of several tissues and organs and these examples are reviewed here. RA induces the "super-regeneration" of organs that can already regenerate such as the urodele amphibian limb by respecifying positional information in the limb. In organs that cannot normally regenerate such as the adult mammalian lung, RA induces the complete regeneration of alveoli that have been destroyed by various noxious treatments. In the mammalian central nervous system (CNS), which is another tissue that cannot regenerate, RA does not induce neurite outgrowth as it does in the embryonic CNS, because one of the retinoic acid receptors, RAR beta 2, is not up-regulated. When RAR beta 2 is transfected into the adult spinal cord in vitro, then neurite outgrowth is stimulated. In all these cases, RA is required for the development of the organ, in the first place suggesting that the same gene pathways are likely to be used for both development and regeneration. This suggestion, therefore, might serve as a strategy for identifying potential tissue or organ targets that have the capacity to be stimulated to regenerate.

Functional specialization of maize mitochondrial aldehyde dehydrogenases

The maize (Zea mays) rf2a and rf2b genes both encode homotetrameric aldehyde dehydrogenases (ALDHs). The RF2A protein was shown previously to accumulate in the mitochondria. In vitro import experiments and ALDH assays on mitochondrial extracts from rf2a mutant plants established that the RF2B protein also accumulates in the mitochondria. RNA gel-blot analyses and immunohistolocation experiments revealed that these two proteins have only partially redundant expression patterns in organs and cell types. For example, RF2A, but not RF2B, accumulates to high levels in the tapetal cells of anthers. Kinetic analyses established that RF2A and RF2B have quite different substrate specificities; although RF2A can oxidize a broad range of aldehydes, including aliphatic aldehydes and aromatic aldehydes, RF2B can oxidize only short-chain aliphatic aldehydes. These two enzymes also have different pH optima and responses to changes in substrate concentration. In addition, RF2A, but not RF2B or any other natural ALDHs, exhibits positive cooperativity. These functional specializations may explain why many species have two mitochondrial ALDHs. This study provides data that serve as a basis for identifying the physiological pathway by which the rf2a gene participates in normal anther development and the restoration of Texas cytoplasm-based male sterility. For example, the observations that Texas cytoplasm anthers do not accumulate elevated levels of reactive oxygen species or lipid peroxidation and the kinetic features of RF2A make it unlikely that rf2a restores fertility by preventing premature programmed cell death.

Temporal/spatial expression of retinoid binding proteins and RAR isoforms in the postnatal lung

Endogenous retinoids have been implicated in alveologenesis in both the rat and the mouse, and exogenous retinoic acid (RA) can reverse or partially reverse experimental emphysema in adult rat and mouse models by an unknown mechanism. In this study, we examine the cellular and molecular biology of retinoid signaling during alveologenesis in the mouse. We describe the temporal and spatial expression of the retinoid binding proteins CRBP-I, CRBP-II, and CRABP-I using RT-PCR and immunohistochemistry. We identify the retinoic acid receptor isoforms RAR-alpha 1, RAR-beta 2, RAR-beta 4, and RAR-gamma 2 and describe their temporal and spatial expression using RT-PCR and in situ hybridization. We demonstrate that both retinoid binding proteins and RAR isoforms are temporally regulated and found within the alveolar septal regions during alveologenesis. These data support a role of dynamic endogenous RA signaling during alveolar formation.