Latent TGF-\u03b2 binding protein 2 and 4 have essential overlapping functions in microfibril development

Yusuke Fujikawa,Hideyuki Yoshida,Toshiji Iwasaka,Kazuo Noda,Tomoyuki Nakamura,Tadashi Inoue,Tomoya O. Akama,Harald von Melchner,Tetsuya Ohbayashi,Ichiro Shiojima

doi:10.1038/srep43714

Abstract

Microfibrils are exracellular matrix components necessary for elastic fiber assembly and for suspending lenses. We previously reported that latent TGF-β binding protein 2 (LTBP-2), a microfibril-associated protein, is required for forming stable microfibril bundles in ciliary zonules. However, it was not understood why Ltbp2 null mice only showed an eye-specific phenotype, whereas LTBP-2 is abundantly expressed in other tissues containing microfibrils in wild type mice. Here, we show that LTBP-4, another microfibril-associated protein, compensates for the loss of LTBP-2 in microfibril formation. Ltbp2/4S double knockout (DKO) mice showed increased lethality due to emphysema, which was much more severe than that found in Ltbp4S null mice. Elastic fibers in the lungs of Ltbp2/4S DKO mice were severely disorganized and fragmented. Cultured mouse embryonic fibroblasts (MEFs) from Ltbp2/4S DKO embryos developed reduced microfibril meshwork in serum-free conditions, whereas the microfibril formation was restored by the addition of either recombinant LTBP-2 or -4. Finally, ectopic expression of LTBP-4 in the whole body restored ciliary zonule microfibril bundles in the eyes of Ltbp2 null mice. These data suggest that LTBP-2 and -4 have critical overlapping functions in forming the robust structure of microfibrils in vitro and in vivo.

Highlights

IntroductionBy tethering TGF-βpropeptide (latency-associated peptide or LAP) that binds to mature TGF-β, facilitating the secretion, storage in the extracellular matrix (ECM), and activation of TGF-β
By tethering TGF-βpropeptide that binds to mature TGF-β, facilitating the secretion, storage in the extracellular matrix (ECM), and activation of TGF-β
Assuming that other latent TGF-βbinding proteins (LTBPs) may compensate for the loss of LTBP-2 in stable microfibril formation in tissues other than the eyes in Ltbp[2] null mice, we analyzed the expression of Ltbp[1, 2, 3], and 4 mRNA in the lungs, aortae, and ciliary bodies of wild type and Ltbp[2] null mice using RT-PCR (Fig. 1)

Summary

Introduction

By tethering TGF-βpropeptide (latency-associated peptide or LAP) that binds to mature TGF-β, facilitating the secretion, storage in the ECM, and activation of TGF-β. Using cell culture and organ explant culture, we demonstrated that LTBP-2 is required for the formation of stable microfibril bundles in ciliary zonules. It is not understood why patients with LTBP2 mutations or Ltbp[2] null mice only show eye-related phenotypes, whereas LTBP-2 is abundantly expressed in other tissues such as lungs and arteries that are rich in elastic fibers in wild type mice[14,15,16]. Ectopic expression of LTBP-4 in Ltbp[2] null mouse eyes restored stable ciliary zonules This novel function of LTBP-4 in the development of microfibrils, at least in part, explains the absence of an extra-ocular phenotype in LTBP2-deficient patients

Methods

Results

Conclusion