Fetal airway branching morhogenesis and airway repair following injury requires coordinated epithelial cell migration, proliferation, and differentiation. During airway branching, the extracellular matrix (ECM) provides structural support for branching airways. Airway epithelial cells communicate with the ECM through transmembrane integrin receptors, connecting the ECM with intracellular actin-based stress fibers. The Β1 integrin subunit is the major Β integrin expressed in the lung, associating with multiple a subunits to form the receptors for the ECM components collagen, laminin, and fibronectin. The role Β1 integrin receptors play during lung formation and repair has not been characterized. We hypothesize that epithelial Β1 integrin is required for normal lung development and maintenance of adult airway structures.
We specifically deleted Β1 integrin in lung epithelium using an SP-C-Cre transgenic mouse strain. We performed timed matings using Β1 flox/flox and SP-C-Cre: Β1 flox/+ mice. Embryos expressing SP-C-Cre and homozygous for the Β1 flox/flox allele (referred to as SP-C:Β1KO) lack Β1 integrin expression in the airway epithelia from E10.5. Fetal and adult lung morphology was measured by H&E and immunostaining. Proliferation was measured by calculation of epithelial cell mitotic index in SP-C:Β1KO mice and littermate controls.
SP-C:Β1KO mice were born in the expected Mendelian ratio (21%, N=34). Newborn SP-C:Β1KO mice survived without cyanosis, respiratory distress, major structural anomalies, or disrupted organogenesis. The SP-C:Β1KO lungs appeared to be normal size with the appropriate number and configuration of lobes. However, epithelial deletion of Β1 integrin altered airway morphology in the fetal lung. When studied at E15, SP-C:Β1KO lungs contained larger airways with simplified branching patterns. Deletion of Β1 integrin from the developing epithelium also led to aberrant epithelial localization, not seen in control littermates. Immunostaining E15 lung sections with the epithelial cell marker E-cadherin identified individual epithelial cells within the lung interstitium, separated from the epithelial monolayer lining the airways. Immunostaining suggested a higher number of mitotic figures in SP-C:Β1KO lungs. We quantified the increase in epithelial cell division in SP-C:Β1KO lungs by calculation of the mitotic index within the airway epithelia. SP-C:Β1KO lungs had approximately twice the number of phospho-histone H3-positive epithelial cells as control littermates, .039 vs .021, respectively (P<0.05). As adults, the SP-C:Β1KO mice have a shortened lifespan, surviving less than 4 months. The adult lungs appeared emphysematous, with markedly dilated and expanded distal airspaces.
These findings demonstrate that epithelial Β1 integrin is required for normal embryonal lung morphogenesis and formation or maintenance of adult lung structures. The role of Β1 integrin in growth factor signaling during airway branching morphogenesis is not yet understood. Whether the adult structural abnormalities result from persistence of airway dilation similar as a continuation of the embryonic phenotype or destruction and loss of mature alveoli is currently under investigation.