Loss of Hox5 function results in myofibroblast mislocalization and distal lung matrix defects during postnatal development

The Lung Elastin Matrix Undergoes Rapid Degradation Upon Adult Loss of Hox5 Function

Hox genes encode transcription factors that are critical for embryonic skeletal patterning and organogenesis. The Hoxa5, Hoxb5, and Hoxc5 paralogs are expressed in the lung mesenchyme and function redundantly during embryonic lung development. Conditional loss-of-function of these genes during postnatal stages leads to severe defects in alveologenesis, specifically in the generation of the elastin network, and animals display bronchopulmonary dysplasia (BPD) or BPD-like phenotype. Here we show the surprising results that mesenchyme-specific loss of Hox5 function at adult stages leads to rapid disruption of the mature elastin matrix, alveolar enlargement, and an emphysema-like phenotype. As the elastin matrix of the lung is considered highly stable, adult disruption of the matrix was not predicted. Just 2 weeks after deletion, adult Hox5 mutant animals show significant increases in alveolar space and changes in pulmonary function, including reduced elastance and increased compliance. Examination of the extracellular matrix (ECM) of adult Tbx4rtTA; TetOCre; Hox5a f a f bbcc lungs demonstrates a disruption of the elastin network although the underlying fibronectin, interstitial collagen and basement membrane appear unaffected. An influx of macrophages and increased matrix metalloproteinase 12 (MMP12) are observed in the distal lung 3 days after Hox5 deletion. In culture, fibroblasts from Hox5 mutant lungs exhibit reduced adhesion. These findings establish a novel role for Hox5 transcription factors as critical regulators of lung fibroblasts at adult homeostasis.

Recent advances in our understanding of the mechanisms of lung alveolarization and bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is the most common cause of morbidity and mortality in preterm infants. A key histopathological feature of BPD is stunted late lung development, where the process of alveolarization-the generation of alveolar gas exchange units-is impeded, through mechanisms that remain largely unclear. As such, there is interest in the clarification both of the pathomechanisms at play in affected lungs, and the mechanisms of de novo alveoli generation in healthy, developing lungs. A better understanding of normal and pathological alveolarization might reveal opportunities for improved medical management of affected infants. Furthermore, disturbances to the alveolar architecture are a key histopathological feature of several adult chronic lung diseases, including emphysema and fibrosis, and it is envisaged that knowledge about the mechanisms of alveologenesis might facilitate regeneration of healthy lung parenchyma in affected patients. To this end, recent efforts have interrogated clinical data, developed new-and refined existing-in vivo and in vitro models of BPD, have applied new microscopic and radiographic approaches, and have developed advanced cell-culture approaches, including organoid generation. Advances have also been made in the development of other methodologies, including single-cell analysis, metabolomics, lipidomics, and proteomics, as well as the generation and use of complex mouse genetics tools. The objective of this review is to present advances made in our understanding of the mechanisms of lung alveolarization and BPD over the period 1 January 2017-30 June 2019, a period that spans the 50th anniversary of the original clinical description of BPD in preterm infants.

Hox5 genes direct elastin network formation during alveologenesis by regulating myofibroblast adhesion

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