Reduced platelet-derived growth factor receptor expression is a primary feature of human bronchopulmonary dysplasia.

Animal studies have shown that platelet-derived growth factor (PDGF) signaling is required for normal alveolarization. Changes in PDGF receptor (PDGFR) expression in infants with bronchopulmonary dysplasia (BPD), a disease of hypoalveolarization, have not been examined. We hypothesized that PDGFR expression is reduced in neonatal lung mesenchymal stromal cells (MSCs) from infants who develop BPD. MSCs from tracheal aspirates of premature infants requiring mechanical ventilation in the first week of life were studied. MSC migration was assessed in a Boyden chamber. Human lung tissue was obtained from the University of Rochester Neonatal Lung Biorepository. Neonatal mice were exposed to air or 75% oxygen for 14 days. PDGFR expression was quantified by qPCR, immunoblotting, and stereology. MSCs were isolated from 25 neonates (mean gestational age 27.7 wk); 13 developed BPD and 12 did not. MSCs from infants who develop BPD showed lower PDGFR-α and PDGFR-ß mRNA and protein expression and decreased migration to PDGF isoforms. Lungs from infants dying with BPD show thickened alveolar walls and paucity of PDGFR-α-positive cells in the dysmorphic alveolar septa. Similarly, lungs from hyperoxia-exposed neonatal mice showed lower expression of PDGFR-α and PDGFR-ß, with significant reductions in the volume of PDGFR-α-positive alveolar tips. In conclusion, MSCs from infants who develop BPD hold stable alterations in PDGFR gene expression that favor hypoalveolarization. These data demonstrate that defective PDGFR signaling is a primary feature of human BPD.

Glycogen synthase kinase-3ß/ß-catenin signaling regulates neonatal lung mesenchymal stromal cell myofibroblastic differentiation.

In bronchopulmonary dysplasia (BPD), alveolar septa are thickened with collagen and α-smooth muscle actin-, transforming growth factor (TGF)-ß-positive myofibroblasts. We examined the biochemical mechanisms underlying myofibroblastic differentiation, focusing on the role of glycogen synthase kinase-3ß (GSK-3ß)/ß-catenin signaling pathway. In the cytoplasm, ß-catenin is phosphorylated on the NH(2) terminus by constitutively active GSK-3ß, favoring its degradation. Upon TGF-ß stimulation, GSK-3ß is phosphorylated and inactivated, allowing ß-catenin to translocate to the nucleus, where it activates transcription of genes involved in myofibroblastic differentiation. We examined the role of ß-catenin in TGF-ß1-induced myofibroblastic differentiation of neonatal lung mesenchymal stromal cells (MSCs) isolated from tracheal aspirates of premature infants with respiratory distress. TGF-ß1 increased ß-catenin expression and nuclear translocation. Transduction of cells with GSK-3ß S9A, a nonphosphorylatable, constitutively active mutant that favors ß-catenin degradation, blocked TGF-ß1-induced myofibroblastic differentiation. Furthermore, transduction of MSCs with ΔN-catenin, a truncation mutant that cannot be phosphorylated on the NH(2) terminus by GSK-3ß and is not degraded, was sufficient for myofibroblastic differentiation. In vivo, hyperoxic exposure of neonatal mice increases expression of ß-catenin in α-smooth muscle actin-positive myofibroblasts. Similar changes were found in lungs of infants with BPD. Finally, low-passage unstimulated MSCs from infants developing BPD showed higher phospho-GSK-3ß, ß-catenin, and α-actin content compared with MSCs from infants not developing this disease, and phospho-GSK-3ß and ß-catenin each correlated with α-actin content. We conclude that phospho-GSK-3ß/ß-catenin signaling regulates α-smooth muscle actin expression, a marker of myofibroblast differentiation, in vitro and in vivo. This pathway appears to be activated in lung mesenchymal cells from patients with BPD.