Assembly of fibrillin microfibrils governs extracellular deposition of latent TGFβ

Abstract

SummaryControl of the bioavailability of the growth factor TGF is essential for tissue formation and homeostasis, yet precisely how latent TGF is incorporated into the extracellular matrix is unknown. Here, we show that deposition of a large latent TGF complex (LLC), which contains latent TGF-binding protein 1 (LTBP-1), is directly dependent on the pericellular assembly of fibrillin microfibrils, which interact with fibronectin during higher-order fibrillogenesis. LTBP-1 formed pericellular arrays that colocalized with microfibrils, whereas fibrillin knockdown inhibited fibrillar LTBP-1 and/or LLC deposition. Blocking 51 integrin or supplementing cultures with heparin, which both inhibited microfibril assembly, disrupted LTBP-1 deposition and enhanced Smad2 phosphorylation. Full-length LTBP-1 bound only weakly to N-terminal pro-fibrillin-1, but this association was strongly enhanced by heparin. The microfibrilassociated glycoprotein MAGP-1 (MFAP-2) inhibited LTBP-1 binding to fibrillin-1 and stimulated Smad2 phosphorylation. By contrast, fibulin-4, which interacted strongly with full-length LTBP-1, did not induce Smad2 phosphorylation. Thus, LTBP-1 and/or LLC deposition is dependent on pericellular microfibril assembly and is governed by complex interactions between LTBP-1, heparan sulfate, fibrillin-1 and microfibril-associated molecules. In this way, microfibrils control TGF bioavailability.

Fibulin-4 E57K Knock-in Mice Recapitulate Cutaneous, Vascular and Skeletal Defects of Recessive Cutis Laxa 1B with both Elastic Fiber and Collagen Fibril Abnormalities

“…Biochemical analysis showed that fibulin-4 and LTBP1 compete for fibrillin-1 binding (43). Fibulin-4 also binds strongly to LTBP1; fibrillin-1 and LTBP1 can bind fibulin-4 simultaneously, forming a ternary complex (44). Indeed, there is significant overlap between clinical features of ARCL 1B and Marfan syndrome caused by fibrillin-1 mutations, such as thoracic aortic aneurysm/dilatation, emphysematous lung, joint laxity, pectus deformity, arachnodactyly, scoliosis, and kyphosis.…”

EMILIN-3, Peculiar Member of Elastin Microfibril Interface-located Protein (EMILIN) Family, Has Distinct Expression Pattern, Forms Oligomeric Assemblies, and Serves as Transforming Growth Factor β (TGF-β) Antagonist

“…Although this activity largely relies upon the EMI domain, the precise molecular mechanisms involved in the in vivo regulation of TGF-␤ activity by EMILIN proteins remain to be understood. In this respect, it will be interesting to investigate whether EMILIN proteins contribute to the regulation of the bioavailability of mature TGF-␤ ligands by also interacting with the large ECM functional complex composed by latent TGF-␤-binding proteins, fibrillin-1 and -2, and other microfibril-associated proteins, such as MAGP-1 and fibulin-4 (40,41). Moreover, the finding that EMILIN-3 binds heparin suggests that this protein may participate in the modulation in the extracellular space of the availability and distribution of other secreted factors known to be regulated by heparan sulfate proteoglycans, such as Wnt, Hedgehog, or bone morphogenetic protein ligands (42).…”

Genetics of Thoracic Aortic Aneurysm

“…FBLN4 and LTBP1 bind with high affinity, which suggests a key role for FBLN4 in the association of LTBP1 with microfibrils and the latent TGF-β cytokine. 89 Indeed, upregulated TGF-β signaling was found both in fibroblasts of cutis laxa patients with FBLN4 mutations and in the aortic wall of Fbln-4 hypomorphic mice (Figure). 88 Interestingly, FBLN4 is also involved in recruiting lysyl oxidase to the elastic fiber, and lysyl oxidase is known to inhibit TGF-β enzymatically.…”