<i>Hox5</i> genes direct elastin network formation during alveologenesis by regulating myofibroblast adhesion.

Abstract

<i>Hox5</i> genes (<i>Hoxa5</i>, <i>Hoxb5</i>, <i>Hoxc5</i>) are exclusively expressed in the lung mesenchyme during embryogenesis, and the most severe phenotypes result from constitutive loss of function of all three genes. Because <i>Hox5</i> triple null mutants exhibit perinatal lethality, the contribution of this paralogous group to postembryonic lung development is unknown. Intriguingly, expression of all three <i>Hox5</i> genes peaks during the first 2 weeks after birth, reaching levels far exceeding those measured at embryonic stages, and surviving <i>Hoxa5</i> single and <i>Hox5 AabbCc</i> compound mutants exhibit defects in the localization of alveolar myofibroblasts. To define the contribution of the entire <i>Hox5</i> paralogous group to this process, we generated an <i>Hoxa5</i> conditional allele to use with our existing null alleles for <i>Hoxb5</i> and <i>Hoxc5</i> Postnatally, mesenchymal deletion of <i>Hoxa5</i> in an <i>Hoxb5/Hoxc5</i> double-mutant background results in severe alveolar simplification. The elastin network required for alveolar formation is dramatically disrupted in <i>Hox5</i> triple 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 of <i>Hox5</i> triple 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 for <i>Hox5</i> genes in lung fibroblast adhesion necessary for proper elastin network formation during alveologenesis.