Galectin-1 in secondary alveolar septae of neonatal mouse lung.

In mice, alveolarization occurs during postnatal days 4 through 12, when secondary alveolar septae create thin-walled alveoli in the distal lung. We hypothesized that genes predominantly expressed in newly forming secondary alveolar septae influence the process of alveolarization. To address this hypothesis, tips of secondary alveolar septae were isolated from sections of postnatal day 6 mouse lung tissue using laser capture microdissection. Total RNA was isolated and amplified from the dissected alveolar septal tips and from intact postnatal day 6 lung tissue. Gene expression in the samples was characterized using Affymetrix mouse U74AN2 GeneChips. Galectin-1 was an abundantly expressed transcript that was enriched in the alveolar septal tips compared with levels in the whole lung tissue. Galectins are beta-galactoside-binding proteins involved in the regulation of cell proliferation, differentiation, and apoptosis in fibroblasts, muscle cells and endothelial cells, cell types that are present in the alveolar wall. Immunostaining in postnatal day 6 lung tissue confirmed that galectin-1 protein is concentrated in the tips of secondary alveolar septae, predominantly in myofibroblasts. Fibroblasts isolated from day 6 neonatal mouse lung tissue contained galectin-1 protein. Real-time PCR demonstrated that galectin-1 mRNA levels in mouse lung tissue peak at postnatal day 6. Immunoblot analysis confirmed that peak levels of lung galectin-1 protein are found at postnatal days 6 to 12. The increased expression of galectin-1 at the site and time of ongoing alveolarization in the newborn mouse is suggestive that galectin-1 may play an important role in this critical aspect of lung development.

Alveolarization in retinoic acid receptor-beta-deficient mice.

Retinoids bind to nuclear receptors [retinoic acid receptors (RARs) and retinoid X receptors]. RARbeta, one of three isoforms of RARs (alpha, beta, and gamma), is expressed in the fetal and adult lung. We hypothesized that RARbeta plays a role in alveolarization. Using morphometric analysis, we determined that there was a significant increase in the volume density of airspace in the alveolar region of the lung at 28, 42, and 56 d postnatal age in RARbeta null mice when compared with wild-type controls. The mean cord length of the respiratory airspaces was increased in RARbeta null animals at 42 d postnatal age. Respiratory gas-exchange surface area per unit lung volume was significantly decreased in RARbeta null animals at 28, 42, and 56 d postnatal age. In addition, alveolar ducts tended to comprise a greater proportion of the lung airspaces in the RARbeta null mice. The RARbeta null mice also had impaired respiratory function when compared with wild-type control mice. There was no effect of RARbeta gene deletion on lung platelet-derived growth factor (PDGF) receptor alpha mRNA levels in postnatal lung tissue at several postnatal ages. However PDGF-A protein levels were significantly lower in the RARbeta null mice than in wild-type controls. Thus, deletion of the RARbeta gene impairs the formation of the distal airspaces during the postnatal phase of lung maturation in mice via a pathway that may involve PDGF-A.