Hypoxia-Inducible Factor-1α in SM22α-Expressing Cells Modulates Alveolarization

Worldwide, the incidence of both preterm births and chronic lung disease of infancy, or bronchopulmonary dysplasia, remains high. Infants with bronchopulmonary dysplasia have larger and fewer alveoli, a lung pathology that can persist into adulthood. Although recent data point to a role for hypoxia-inducible factor-1 alpha (HIF-1 alpha) in mediating pulmonary angiogenesis and alveolarization, the cell-specific role of HIF-1 alpha remains incompletely understood. Thus, we hypothesized that HIF-1 alpha, in a distinct subset of mesenchymal cells, mediates postnatal alveolarization. To test the hypothesis, we generated mice with a cell-specific deletion of HIF-1 alpha by crossing SM22 alpha promoter-driven Cre mice with HIF-1 alpha(flox/flox) mice (SM22 alpha-HIF-1 alpha(-/-)), determined SM-22 alpha-expressing cell identity using single-cell RNA sequencing, and interrogated samples from preterm infants. Deletion of HIF-1 alpha in SM22 alpha-expressing cells had no effect on lung structure at day 3 of life. However, at 8 days, there were fewer and larger alveoli, a difference that persisted into adulthood. Microvascular density, elastin organization, and peripheral branching of the lung vasculature were decreased in SM22 alpha-HIF-1 alpha(-/-) mice, compared with control mice. Single-cell RNA sequencing demonstrated that three mesenchymal cell subtypes express SM22 alpha: myofibroblasts, airway smooth muscle cells, and vascular smooth muscle cells. Pulmonary vascular smooth muscle cells from SM22 alpha-HIF-1 alpha(-/-) mice had decreased angiopoietin-2 expression and, in coculture experiments, a diminished capacity to promote angiogenesis that was rescued by angiopoietin-2. Angiopoietin-2 expression in tracheal aspirates of preterm infants was inversely correlated with overall mechanical ventilation time, a marker of disease severity. We conclude that SM22 alpha-specific HIF-1 alpha expression drives peripheral angiogenesis and alveolarization in the lung, perhaps by promoting angiopoietin-2 expression.