Transforming growth factor-beta (TGF-?) is released from cells as part of a tripartite latent complex that includes, in addition to TGF-?, the latency associated protein (LAP) and latent TGF-? binding protein (LTBP), which is disulfide bonded to LAP. We have reversed the impaired terminal alveolar development phenotype observed in mice deficient in LTBP-4 by generating Ltbp4-/-;Tgfb2-/- mice and thereby lowering TGF-? levels. This result suggests that the defect in lung septation in Ltbp4-/- animals is related to increased TGF-?2 levels. We propose that LTBP-4 acts primarily as an organizer of elastic microfibrils, multi-protein assemblies, which contain fibrillins, fibulins, elastin, and LTBPs, and not as a binder of latent TGF-?. In our view, the TGF-?-mediated effects are secondary to abnormal matrix. We will test this hypothesis in two aims.
In Aim 1, we will generate mice in which the two cysteine residues in LTBP-4 that bind to LAP are mutated to serines so that Ltbp-4 cannot bind to TGF-?. These mice will produce Ltbp-4 and TGF-?, but no Ltbp-4-TGF-? complexes. If the lung alveolarization abnormality in Ltbp4-/- mice is due to the absence of the structural activity of LTBP-4, these new mutant animals should have a normal phenotype. Conversely, if the lung defect in Ltbp-4-/- mice relates to the loss of TGF-? bound to Ltbp-4, the mutant animals will display abnormal air sac septation. We will also validate our hypothesis in vitro using Ltbp4-/- cells and measuring matrix organization and active TGF-? levels under conditions in which either LTBP-4's structural function or TGF-? levels are normalized. We will normalize the LTBP-4 structural function by adding either cells that express WT LTBP-4 or purified LTBP-4 protein. TGF-? levels will be normalized by adding a pan-neutralizing antibody to TGF-?.
In Aim 2, we will examine the role and source of TGF- ? in the lung pathology. We will characterize the contribution of TGF-? to the lung defect by producing Ltbp4-/-;Tgfb1-/- mice and examining their phenotypes. The results of this experiment will establish whether normalization of the lung phenotype in Ltbp4-/-;Tgfb2-/- animals is due to a decrease in total TGF-?;i.e. the sum of TGF-?1 and TGF-?2, or is specific for TGF-?2. We will also identify the nature of the activator of latent TGF-? in cultured cells and/or animals deficient in LTBP-4 by using specific inhibitors of, or mice with null mutations for, latent TGF-? activators. Finally, we will determine whether the excess active TGF-? formed in the absence of LTBP- 4 derives from complexes of LTBP-1 or LTBP-3 with TGF-?, or from latent TGF-? not bound to an LTBP. These experiments will yield important insights as to how latent TGF-? is controlled in the lung and by cultured lung cells using novel genetic and cellular approaches. The results may suggest mechanisms for normalizing TGF-? in certain pathological states, such as lung fibrosis.
This research focuses on the mechanisms controlling the action of a potent signaling molecule in the extra cellular environment of the lung. Abnormalities in this mechanism result in pathologies of lung development. We will explore the functions of molecules that bind to the signaling protein and how those interactions affect function. Our long-term goal is to utilize this information in developing new therapeutic strategies.
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