Elevated type IV collagen in bronchoalveolar lavage fluid from infants with bronchopulmonary dysplasia

SM22α cell-specific HIF stabilization mitigates hyperoxia-induced neonatal lung injury

Though survival rates for preterm infants are improving, the incidence of chronic lung disease of infancy, or bronchopulmonary dysplasia (BPD), remains high. Histologically, BPD is characterized by larger and fewer alveoli. Hypoxia-inducible factors (HIFs) may be protective in the context of hyperoxia-induced lung injury, but the cell-specific effects of HIF expression in neonatal lung injury remain unknown. Thus, we sought to determine whether HIF stabilization in SM22α-expressing cells can limit hyperoxia-induced neonatal lung injury. We generated SM22α-specific HIF-1α-stabilized mice (SM22α-PHD1/2^-/- mice) by cross-breeding SM22α-promotor-driven Cre recombinase mice with prolyl hydroxylase PHD1^flox/flox and PHD2^flox/flox mice. Neonatal mice were randomized to 21% O₂ (normoxia) or 80% O₂ (hyperoxia) exposure for 14 days. For the hyperoxia recovery studies, neonatal mice were recovered from normoxia for an additional 10 wk. SM22α-specific HIF-1α stabilization mitigated hyperoxia-induced lung injury and preserved microvessel density compared with control mice for both neonates and adults. In SM22α-PHD1/2^-/- mice, pulmonary artery endothelial cells (PAECs) were more proliferative and pulmonary arteries expressed more collagen IV compared with control mice, even under hyperoxic conditions. Angiopoietin-2 (Ang2) mRNA expression in pulmonary artery smooth muscle cells (PASMC) was greater in SM22α-PHD1/2^-/- compared with control mice in both normoxia and hyperoxia. Pulmonary endothelial cells (PECs) cocultured with PASMC isolated from SM22α-PHD1/2^-/- mice formed more tubes and branches with greater tube length compared with PEC cocultured with PASMC isolated from SM22α-PHD1/2^+/+ mice. Addition of Ang2 recombinant protein further augmented tube formation for both PHD1/2^+/+ and PHD1/2^-/- PASMC. Cell-specific deletion of PHD1 and 2 selectively increases HIF-1α expression in SM22α-expressing cells and protects neonatal lung development despite prolonged hyperoxia exposure. HIF stabilization in SM22α-expressing cells preserved endothelial cell proliferation, microvascular density, increased angiopoietin-2 expression, and lung structure, suggesting a role for cell-specific HIF-1α stabilization to prevent neonatal lung injury.

The critical role of collagen VI in lung development and chronic lung disease

Type VI collagen (collagen VI) is an obligate extracellular matrix component found mainly in the basement membrane region of many mammalian tissues and organs, including skeletal muscle and throughout the respiratory system. Collagen VI is probably most recognized in medicine as the genetic cause of a spectrum of muscular dystrophies, including Ullrich Congenital Myopathy and Bethlem Myopathy. Collagen VI is thought to contribute to myopathy, at least in part, by mediating muscle fiber integrity by anchoring myoblasts to the muscle basement membrane. Interestingly, collagen VI myopathies present with restrictive respiratory insufficiency, thought to be due primarily to thoracic muscular weakening. Although it was recently recognized as one of the (if not the) most abundant collagens in the mammalian lung, there is a substantive knowledge gap concerning its role in respiratory system development and function. A few studies have suggested that collagen VI insufficiency is associated with airway epithelial cell survival and altered lung function. Our recent work suggested collagen VI may be a genomic risk factor for chronic lung disease in premature infants. Using this as motivation, we thoroughly assessed the role of collagen VI in lung development and in lung epithelial cell biology. Here, we describe the state-of-the-art for collagen VI cell and developmental biology within the respiratory system, and reveal its essential roles in normal developmental processes and airway epithelial cell phenotype and intracellular signaling.

The Extracellular Matrix in Bronchopulmonary Dysplasia: Target and Source

Bronchopulmonary dysplasia (BPD) is a common complication of preterm birth that contributes significantly to morbidity and mortality in neonatal intensive care units. BPD results from life-saving interventions, such as mechanical ventilation and oxygen supplementation used to manage preterm infants with acute respiratory failure, which may be complicated by pulmonary infection. The pathogenic pathways driving BPD are not well-delineated but include disturbances to the coordinated action of gene expression, cell-cell communication, physical forces, and cell interactions with the extracellular matrix (ECM), which together guide normal lung development. Efforts to further delineate these pathways have been assisted by the use of animal models of BPD, which rely on infection, injurious mechanical ventilation, or oxygen supplementation, where histopathological features of BPD can be mimicked. Notable among these are perturbations to ECM structures, namely, the organization of the elastin and collagen networks in the developing lung. Dysregulated collagen deposition and disturbed elastin fiber organization are pathological hallmarks of clinical and experimental BPD. Strides have been made in understanding the disturbances to ECM production in the developing lung, but much still remains to be discovered about how ECM maturation and turnover are dysregulated in aberrantly developing lungs. This review aims to inform the reader about the state-of-the-art concerning the ECM in BPD, to highlight the gaps in our knowledge and current controversies, and to suggest directions for future work in this exciting and complex area of lung development (patho)biology.

MMP-9 and TIMP-1 in the cord blood of premature infants developing BPD

We investigated matrix metalloproteinase-9 (MMP-9) and tissue inhibitor of metalloproteinase 1 (TIMP-1) levels in the cord blood of 29 premature infants who were <30 weeks gestation. One, 8, and 14 infants developed severe, moderate and mild bronchopulmonary dysplasia (BPD), respectively, and 6 did not. MMP-9 and TIMP-1 levels in the cord blood were determined by ELISA. MMP-9/TIMP-1 ratios in the cord blood of infants who developed severe or moderate BPD (n = 9) were significantly higher than those who developed mild BPD or did not develop BPD (n = 20; P = 0.015). Multivariate linear regressions demonstrated that MMP-9 levels and MMP-9/TIMP-1 ratios in the cord blood of the premature infants correlated with the oxygen supplementation period (r = 0.58, P = 0.003 and r = 0.41, P = 0.030, respectively). The MMP-9 levels and MMP-9/TIMP-1 ratios correlated with the severity of maternal chorioamnionitis (both trend P = 0.006). The MMP-9 levels and MMP-9/TIMP-1 ratios in the cord blood may be related to the pathogenesis and severity of BPD and maternal chorioamnionitis.

Modeling and Remodeling of the Lung in Neonatal Chronic Lung Disease

Neonatal chronic lung disease (CLD) is the major long-term pulmonary complication of preterm birth affecting about 20% of infants who need mechanical ventilation. CLD is the result of abnormal repair processes following inflammatory lung injury that lead to remodeling of the lung. Inflammation may be initiated by a variety of stimuli including mechanical ventilation, oxygen toxicity and infection. The resultant neutrophil chemotaxis and degranulation leads to the release of enzymes such as matrix metalloproteinases that can cause proteolysis of the lung extracellular matrix. Abnormal healing with remodeling leads to poorly compliant lungs with reduced capacity for gas exchange. Drugs can influence the normal process of lung modeling or remodeling. Fetal lung development can be influenced by glucocorticosteroids and inflammation. Both can cause abnormal lung modeling with fewer, larger alveoli and accelerated lung maturation, which confers benefits in terms of reduced morbidity and mortality from respiratory distress syndrome but potentially increases the risk of subsequent lung injury. Antioxidants, such as retinol (vitamin A), administered post-natally may reduce the effects of oxidative stress leading to a modest reduction in CLD but they require repeated intramuscular injections. Postnatal glucocorticosteroid therapy can modify the lung inflammatory response and reduce CLD but it can also have detrimental effects on the developing brain and lung, thereby creating a clinical dilemma for neonatologists. Proteinase inhibitors may be a rational therapy but more research is needed before they can be accepted as a treatment for preterm neonates.'Modeling' is defined as planning or forming that follows a set pattern. The term is used to describe the normal process of lung growth and development that culminates in mature branching alveolar air spaces surrounded by a network of capillaries. Normal lung modeling occurs under a variety of genetic and hormonal influences that can be altered, leading to abnormal patterns of growth. 'Remodeling' is defined as altering the structure of or re-making and, in the case of the lung, is used to describe the abnormal patterns of lung growth that occur after lung injury. Modeling and remodeling of the lungs occur to an extent throughout life but never more rapidly than during the fetal and early neonatal periods, and factors that influence this process may lead to development of neonatal CLD. Some of the factors involved in normal and abnormal lung modeling and inflammation and glucocorticosteroid-induced remodeling in the perinatal period, in the context of neonatal CLD, are reviewed with considerations of how various drugs may influence these processes.

Bronchopulmonary dysplasia: new insights

The pathology of BPD has changed over time, with the old BPD characterized by airway injury, inflammation, and parenchymal fibrosis giving way to the new BPD manifesting less fibrosis but with decreased alveolar and vascular development. The pathogenesis of BPD involves factors affecting the severity and management of RDS, alterations in lung development and maturation, alveolar-vascular interactions, and extracellular matrix remodeling. These factors in pathogenesis are mediated and modulated by hyperoxic lung injury, antioxidants, NO, the pulmonary neuroendocrine system and peptide growth factors, the immune system, and various genetic polymorphisms and predispositions. Future therapeutic interventions are likely to target one or more of these abnormalities in lung development, maturation, and response to injury.

The role of matrix metalloproteinases -9 and -2 in development of neonatal chronic lung disease

AIM

Matrix metalloproteinases (MMPs) -9 and -2 degrade type-IV collagen, a major constituent of lung basement membrane, and may have a role in the pathogenesis of neonatal chronic lung disease (CLD). We determined factors influencing MMP levels in neonatal bronchoalveolar lavage (BAL) fluid to establish whether an imbalance between MMP and its inhibitor could be implicated in CLD.

METHODS

We measured MMP-9 and -2 and tissue inhibitor of metalloproteinase-1 (TIMP-1) levels in 316 BAL fluid samples from 121 babies of gestational ages 23 to 42 wk over the first 14 d of life to determine effects of gestation and postnatal age. Median MMP-9, -2, TIMP-1 and MMP-9/TIMP-1 ratio in BAL were further studied in a subgroup of 85 babies <33 wk gestation to determine their ability to predict CLD and to establish effects of antenatal corticosteroid therapy (ANCS).

RESULTS

MMP-9, -2 and TIMP levels did not vary with postnatal age over the first week. Median MMP-9 levels and MMP-9/TIMP-1 ratio increased with decreasing gestation in preterm babies. The MMP-9/TIMP-1 ratio was higher in babies who developed CLD, implying a proteinase/antiproteinase imbalance, but this association disappeared when controlled for gestational age. ANCS had no effect on BAL fluid MMP or TIMP levels.

CONCLUSION

MMPs may have a role in the development of lung injury and fibrosis, but estimating their levels in the first week of life does not help with prediction of CLD.

Chorioamnionitis and increased neonatal lung lavage fluid matrix metalloproteinase-9 levels: implications for antenatal origins of chronic lung disease

OBJECTIVE

Matrix metalloproteinase-9 (MMP-9) degrades type IV collagen, the major constituent of lung basement membrane. We studied the effects of chorioamnionitis and antenatal corticosteroids on bronchoalveolar lavage (BAL) fluid levels of MMP-9, and its inhibitor, TIMP-1 in preterm infants.

STUDY DESIGN

A prospective study was performed on serial BAL samples from 79 ventilated preterm infants at less than 33 weeks' gestation, 18 of whom were from pregnancies complicated by chorioamnionitis. MMP-9 levels were measured by gelatin zymography and TIMP-1 by enzyme-linked immunosorbent assay, and the median value for each infant was calculated. The presence and severity of chorioamnionitis were defined histologically.

RESULTS

BAL fluid MMP-9 levels were higher in preterm infants in the chorioamnionitis group (86 [29-518] vs 13 [3-43] ng/mL, P =.001), and levels increased stepwise with the increasing severity of chorioamnionitis. Antenatal corticosteroids had no effect on median MMP-9 levels. Infants in the chorioamnionitis group were more likely to have chronic lung disease (CLD) develop (55% vs 28%, P <.05). TIMP-1 levels were no different between groups.

CONCLUSION

Chorioamnionitis is associated with increased lung type IV collagenase levels in the ventilated preterm infant. Antenatal lung inflammation with up-regulation of MMP-9 may be important in the pathogenesis of CLD.

Collagen scaffolding during development and its deformation with chronic lung disease

OBJECTIVE

Infants with chronic lung disease (CLD) have an arrest of primary and secondary septation. We hypothesized that this may be related to damage or abnormal development of lung collagen secondary to positive pressure ventilation. Our aims were to identify the sites and quantity of collagen in control infants 22 to 72 weeks' postconceptional age and compare these with infants with various degrees of severity of CLD.

METHODS

The controls were 22 to 42 weeks' gestation (n = 30), received minimal ventilator care, and died within 48 hours of birth, plus 5 term infants who died at 43 to 72 weeks' postconceptional age from nonpulmonary causes. Infants who were 23 to 30 weeks' gestation, were at risk for CLD, and lived 5 to 94 days (n = 33) were separated into 3 groups on the basis of respiratory score (score group; the integrated area under the curve of the average daily fraction of inspired oxygen x mean airway pressure [cm H2O] over the number of days lived). The score groups, <20, 20 to 69, and 70 to 500, related clinically to mild to moderate and severe lung disease. The lungs were tracheally perfused and formalin fixed. Total lung volume was determined by water displacement. The paraffin-embedded lung blocks were sectioned 5 micro m thick, stained with Gomori's reticulum stain, hematoxylin and eosin, and immunohistochemically for collagen IV. The parenchyma was point-counted, and the volume density of collagen was measured. The chord diameter of the peripheral airway saccules and alveoli was measured. Descriptive collagen data were assessed on en face 40- micro m-thick sections through the alveolar or saccular walls on all infants at risk for CLD and in selected controls.

RESULTS

In the controls, the volume density of collagen decreased from a maximum of 9% at 22 weeks to 5% at term and 72 weeks. With Scores < or =69, the fraction of collagen was similar to controls, but in infants with scores 70 to 500, it was increased relative to controls. However, when collagen was expressed as the volume density of interstitial tissue, ie, excluding parenchymal air space, it increased from a low of 5% at 22 weeks to 25% at 72 weeks. In infants with scores 70 to 500, 79% of infants had collagens greater than controls. Saccular and alveolar diameter increased from 40 micro m at 23 weeks to 100 microm at 72 weeks. Most infants with severe CLD (scores > or =70) had diameters more than twice that of controls at the same age. The total lung parenchymal collagen had a similar pattern as the volume density of collagen in interstitial tissue, increasing from 0.4 cm3 at 23 weeks to 9.7 cm3 at 72 weeks in the controls. Eighty-five percent of infants with scores 70 to 500 had total parenchymal collagen greater than the 95% confidence interval of the controls. With en face sections, a fine collagen mesh was seen at 23 weeks, which progressively increased in fiber size and quantity until 72 weeks. With severe CLD, the secondary collagen fibers in the saccular wall were thickened, tortuous, and disorganized relative to same-aged controls. Under 30 weeks, in the controls, the interstitium contained a wide, delicate network of interconnected collagen fibers. After positive pressure ventilation, some saccules markedly increased their diameter, which compressed and obliterated the interstitial network. In contrast with severe CLD, the interstitium was wide, with coarse wavy collagen fibers.

CONCLUSIONS

Parenchymal collagen increases throughout development. Before 30 weeks, there is a delicate complex interstitial collagen network, which may be important for primary septation and subsequent normal development. Positive pressure ventilation, if excessive, and depending on lung maturity and disease state, over a short time can severely compress the interstitium and damage this collagen network and prevent normal primary septation and arrest or distort future lung development. With severe CLD, distal air space diameter increases. There is a failure of primary and secondary septation, arrested lung development and remodeling, with thickened cnt and remodeling, with thickened collagenous saccular walls, and a wide interstitium with increased quantity and size of collagen fibers that can affect the mechanics of ventilation. We conclude that normal lung development is dependent on a normal interstitium and, perhaps, collagen architecture and that origins of CLD begin early in the course of positive pressure ventilation.

Fibroblast mitogenic activity of lung lavage fluid from infants with chronic lung disease of prematurity

BACKGROUND

Lung fibrosis is thought to be important in chronic lung disease of prematurity (CLD).

METHODS

Fibroblast proliferative activity was assessed in 207 bronchoalveolar lavage fluid (BALF) samples from 43 infants. Sixteen developed CLD (birth weight 765 g (630-1230), gestation 26.5 weeks (23-29)), 18 developed respiratory distress syndrome (RDS) (birth weight 1415 g (430-4160), gestation 31 weeks (23-39)), and nine control infants (birth weight 2110 g (900-3720), gestation 32 weeks (26-41)) received mechanical ventilation for non-pulmonary reasons.

RESULTS

The fibroblast proliferative activity relative to 10% fetal calf serum was 64-75% in infants with CLD, 55-86% in the RDS group, and 42-68% in control infants during the first 5 weeks of life. Only at day 3 was there a difference between the groups (CLD 72% v control 42%, p < 0.01; RDS 63% v control 42%, p < 0.05). With the use of neutralising antibodies, platelet derived growth factor BB (PDGF-BB) and epidermal growth factor were undetectable, and insulin-like growth factor I (IGF-I) accounted for 14% (p < 0.05) and 11% (p < 0.005) of BALF mitogenic activity from the RDS and CLD groups respectively.

CONCLUSIONS

The mitogenic activity of BALF was similar in the three groups studied and was only partially accounted for by IGF-I. Growth factors other than PDGF-BB and IGF-I contribute significantly to this process.