Bronchopulmonary Dysplasia: Endothelial Cells in the Driver’s Seat

In this issue of the Journal, the detailed work by Appuhn and colleagues (pp. 81–91) highlights the importance of focusing on the structural elements of the lung, particularly the alveolar unit, in the hyperoxia mouse model of bronchopulmonary dysplasia (BPD). Their studies build on the knowledge of how the postnatal mouse lung matures (notably in the first 3 weeks of life) (1), on stereological data of the maturing and adult mouse lung (2), and on development of novel methods to assess the structure of the alveolar–capillary bed (3). All these advances followed the groundbreaking work by Weibel and Gomez aimed at defining the structure–function determinants of the human lung (4), which positioned stereology as the gold standard for assessing lung structure as it relates to function.

In the first 2 weeks of life, the mouse lung exhibits thickened alveolar ducts/sacs and primary septa with double capillary circulation, which progressively thins out and acquires a single capillary network, as there is an exponential expansion of fully functional alveoli. The postnatal maturation of the mouse lung in the first 2 or 3 weeks of life broadly resembles the transition from the canalicular to alveolar stages occurring in humans between Week 28 and Week 32 in utero. However, there are significant differences involving stereological parameters of the mouse and human lungs. An adult mouse lung has a volume of ∼0.74 ml versus 7 L in a 74 kg human (4-log difference), the mouse alveolar surface area (SA) is 0.068 m² versus the human 82 m² (3-log), the alveolar–capillary SA of 0.059 m² versus 70 mm² versus (3-log), the capillary volume is 0.084 ml versus 160 ml (more than 2-log), and the maximal lung O₂ diffusion of 0.147 versus 169 ml/mm Hg (∼2.5-log). Importantly, a newborn mouse has ∼2 × 10⁶ alveoli (2), whereas a newborn human has ∼10 times as many alveoli (5)

One of the major consequences of a disrupted early perinatal lung structure is its impact on gas exchange (i.e., the key characteristic of “what makes a good lung”), as reviewed by Weibel (6). He estimated that the adult human lung has a maximal capacity to diffuse oxygen (Dl _O2) of ∼160 ml/min/mm Hg, which was derived from the integration of physical properties of diffusion and solubility of oxygen in lung tissue and plasma and morphometric data of the total number of alveoli, epithelial and capillary SAs, alveolar airspace thickness, and oxygen interaction with red cells in alveolar capillaries. Weibel elegantly summarized that, among numerous animal species, Dl _O2 correlates with overall body weight, lung size, and, importantly, basal oxygen consumption (5).

As mentioned, gas exchange occurs through an SA equivalent to a tennis court in adult humans. In a mature alveolus, the epithelial side of the SA is slightly larger than the capillary component by ∼10% (in the immature lung, the capillary SA is larger than the epithelial SA, probably because of the double capillary circulation). The studies by Appuhn and colleagues indicate that on Days 7, 14, and 21 after hyperoxia exposure of mouse neonates, the disruption of the alveolar–capillary network precedes a reduction of the overall alveolar epithelial cell lining (1); this is documented by the reduction of the stereological ratio of capillary/epithelial surface area for gas exchange. These are allied to abnormalities of endothelial cell size and capillary lumen and remodeling (i.e., thickening of the alveolar interstitium). The aggregate of these structural changes will likely result in a decrease in the gas exchange area, with overly enlarged and simplified airspaces. Although it is not known whether these changes worsen over time in humans as patients with BPD reach early adulthood, a decreased Dl _O2 will likely impair physical activity and growth and limit overall survival.

The functional implications of these structural studies lend support to the so-called vascular hypothesis of BPD, in which abnormalities of lung capillaries, endothelial cells, and their molecular signaling drive the disease (7). There is growing evidence that endothelial cells of lung microvasculature provide critical developmental and reparative cues, which constitute the concept of angiocrine signaling (8). Neonatal mice in which the VEGFR (vascular endothelial growth factor receptor) signaling is blocked in the early postnatal period display impaired alveolarization similar to that in BPD (9). VEGFR-2 has been implicated in alveolar regeneration beyond the neonatal period. In a unilateral pneumectomy model, Rafii and colleagues (10) demonstrated that pulmonary capillary endothelial cells induce MMP (matrix metallopeptidase)14, which, by promoting the expansion of alveolar progenitor cells, stimulates alveologenesis, thus supporting the hypothesis of a lung vascular–epithelial niche. Conversely, mice with endothelial-specific deletion of VEGFR-2 or fibroblast growth factor receptor-1 displayed decreased production of MMP14, resulting in impaired alveolarization. Furthermore, adult rats treated with VEGFR-blocker SU5416 showed increased emphysematous alveolar enlargement due to alveolar septal cell apoptosis (11). These shared roles of VEGF in BPD and emphysema underscore the overall resemblance of alveolar simplification and capillary dropout in both BPD and chronic obstructive pulmonary disease (12).

The findings of Appuhn and colleagues not only support the vascular hypothesis but also suggest that potential therapeutic interventions aimed at BPD should target the wellbeing of the capillary circulation (1). Intratracheally instilled mesenchymal stem cells improved alveolarization in neonatal rats with hyperoxia-induced BPD (13), possibly by reconstituting growth factors essential for lung development in fetal lung tissue. Furthermore, Appuhn and colleagues outline the potential reparative effects of supplementary hepatocyte growth factor and VEGF (1).

These interventions may be considered preventive, as they are aimed at stopping or reversing the vascular abnormalities already present in the stage equivalent to P7 in the mouse (endothelial cell injury and an increase in alveolar diameter). They could curb oxidative stress and endothelial cell injury (apoptosis or other forms of cell death), which, if unabated, will contribute to a decrease in alveolar epithelial–endothelial cell cross-talk and endothelial cell SA. They can also minimize the increase in septal connective tissue apparent at postnatal Day 21. However, curative therapies face more significant challenges. It is unclear whether the growth of normal alveoli after neonatal injury given can occur in a BPD-damaged lung; most of the lung growth in humans occurs in the first 2 years of life, extending through adolescence (14). Whether this growth occurs in the human BPD lungs is unclear. The approach taken by Appuhn and colleagues should be extended to human BPD lungs, with emphasis on how the structural characteristic of the lung changes throughout postnatal life.