Function of latent TGFβ binding protein 4 and fibulin 5 in elastogenesis and lung development

Abstract

Mice deficient in Latent TGFβ Binding Protein 4 (Ltbp4) display a defect in lung septation and elastogenesis. The lung septation defect is normalized by genetically decreasing TGFβ2 levels. However, the elastic fiber assembly is not improved in Tgfb2-/-;Ltbp4S-/- compared to Ltbp4S-/- lungs. We found that decreased levels of TGFβ1 or TGFβ3 did not improve lung septation indicating that the TGFβ isoform elevated in Ltbp4S-/- lungs is TGFβ2. Expression of a form of Ltbp4 that could not bind latent TGFβ did not affect lung phenotype indicating that normal lung development does not require the formation of LTBP4-latent TGFβ complexes. Therefore, the change in TGFβ level in the lungs is not directly related to Ltbp4 deficiency but probably is a consequence of changes in the extracellular matrix. Interestingly, combination of the Ltbp4S-/- mutation with a fibulin-5 null mutant in Fbln5-/-;Ltbp4S-/- mice improves the lung septation compared to Ltbp4S-/- lungs. Large globular elastin aggregates characteristic for Ltbp4S-/- lungs do not form in Fbln5-/-;Ltbp4S-/- lungs and EM studies showed that elastic fibers in Fbln5-/-;Ltbp4S-/- lungs resemble those found in Fbln5-/- mice. These results are consistent with a role for TGFβ2 in lung septation and for Ltbp4 in regulating fibulin-5 dependent elastic fiber assembly.