Hox5genes direct elastin network formation during alveologenesis by regulating myofibroblast adhesion

Abstract

Hox5genes (Hoxa5,Hoxb5,Hoxc5) are exclusively expressed in the lung mesenchyme during embryogenesis, and the most severe phenotypes result from constitutive loss of function of all three genes. BecauseHox5triple null mutants exhibit perinatal lethality, the contribution of this paralogous group to postembryonic lung development is unknown. Intriguingly, expression of all threeHox5genes peaks during the first 2 weeks after birth, reaching levels far exceeding those measured at embryonic stages, and survivingHoxa5single andHox5 AabbCccompound mutants exhibit defects in the localization of alveolar myofibroblasts. To define the contribution of the entireHox5paralogous group to this process, we generated anHoxa5conditional allele to use with our existing null alleles forHoxb5andHoxc5. Postnatally, mesenchymal deletion ofHoxa5in anHoxb5/Hoxc5double-mutant background results in severe alveolar simplification. The elastin network required for alveolar formation is dramatically disrupted inHox5triple mutants, while the basal lamina, interstitial matrix, and fibronectin are normal. Alveolar myofibroblasts remain Pdgfrα+/SMA+ double positive and present in normal numbers, indicating that the irregular elastin network is not due to fibroblast differentiation defects. Rather, we observe that SMA+ myofibroblasts ofHox5triple mutants are morphologically abnormal both in vivo and in vitro with highly reduced adherence to fibronectin. This loss of adhesion is a result of loss of the integrin heterodimer Itga5b1 in mutant fibroblasts. Collectively, these data show an important role forHox5genes in lung fibroblast adhesion necessary for proper elastin network formation during alveologenesis.