Modulation of laminin integrin receptors in the postnatal and adult rat lung

Recent studies have shown that type II pneumocytes, at birth and day 3 postnatally, have a diffuse distribution and localize at alveolar 'corners' between 3 and 7 days. Since α3β₁ and α₆β₁ are laminin-binding receptors that are well expressed by rat type II alveolar epithelial cells, we postulated that they may play a role in the localization of the cells in the alveolus. To begin the evaluation of this hypothesis, we studied the temporal and spatial expression of the α₃, α₆, and β₁ integrin subunit protein and mRNA in whole rat lungs during postnatal development by immunofluorescence, confocal microscopy, and Northern blot analysis. The temporal expression of proteins analyzed by immunochemistry, with integrin subunit specific antibodies, increased during the 3- to 7-day postnatal period and in adult lungs. Densitometric values of the α₃, α₆, and β₁ mRNA expression, normalized to 28S rRNA, quadrupled from day 1 to day 3 postnatally. The mRNA expression of different integrin chains was elevated 1.5- to threefold from days 5 to 7 postnatally compared to day 1 levels. The α₃ and α₆ integrin subunit mRNA decreased to newborn levels in adult lungs, whereas the β₁ integrin mRNA in adult lungs was expressed at approximately 50% of its level in newborn lungs. We postulate that the increases in α₃, α₆, and β₁ integrin mRNA expression during the early neonatal period may be important for the spatial distribution of type II pneumocytes.