Latent TGFß complexes are transglutaminase cross-linked to fibrillin to facilitate TGFß activation.

TGFß superfamily members are potent growth factors in the extracellular matrix with essential roles in all aspects of cellular behaviour. Latent TGFß binding proteins (LTBPs) are co-expressed with TGFß, essential for correct folding and secretion of the growth factor, to form large latent complexes. These large latent complexes bind extracellular proteins such as fibrillin for sequestration of TGFß in the matrix, essential for normal tissue function, and dysregulated TGFß signalling is a hallmark of many fibrillinopathies. Transglutaminase-2 (TG2) cross-linking of LTBPs is known to play a role in TGFß activation but the underlying molecular mechanisms are not resolved. Here we show that fibrillin is a matrix substrate for TG2 and that TG2 cross-linked complexes can be formed between fibrillin and LTBP-1 and -3, and their latent TGFß complexes. The structure of the fibrillin-LTBP1 complex shows that the two elongated proteins interact in a perpendicular arrangement which would allow them to form distal interactions between the matrix and the cell surface. Formation of the cross-link with fibrillin does not change the interaction between latent TGFß and integrin αVß6 but does increase TGFß activation in cell-based assays. The activating effect may be due to direction of the latent complexes to the cell surface by fibrillin, as competition with heparan sulphate can ameliorate the activating effect. Together, these data support that TGFß activation can be enhanced by covalent tethering of LTBPs to the matrix via fibrillin.

Independent multimerization of Latent TGFß Binding Protein-1 stabilized by cross-linking and enhanced by heparan sulfate.

TGFß plays key roles in fibrosis and cancer progression, and latency is conferred by covalent linkage to latent TGFß binding proteins (LTBPs). LTBP1 is essential for TGFß folding, secretion, matrix localization and activation but little is known about its structure due to its inherent size and flexibility. Here we show that LTBP1 adopts an extended conformation with stable matrix-binding N-terminus, extended central array of 11 calcium-binding EGF domains and flexible TGFß-binding C-terminus. Moreover we demonstrate that LTBP1 forms short filament-like structures independent of other matrix components. The termini bind to each other to facilitate linear extension of the filament, while the N-terminal region can serve as a branch-point. Multimerization is enhanced in the presence of heparin and stabilized by the matrix cross-linking enzyme transglutaminase-2. These assemblies will extend the span of LTBP1 to potentially allow simultaneous N-terminal matrix and C-terminal fibrillin interactions providing tethering for TGFß activation by mechanical force.

Mutations in LTBP3 cause acromicric dysplasia and geleophysic dysplasia.

BACKGROUND: Acromelic dysplasias are a group of disorders characterised by short stature, brachydactyly, limited joint extension and thickened skin and comprises acromicric dysplasia (AD), geleophysic dysplasia (GD), Myhre syndrome and Weill-Marchesani syndrome. Mutations in several genes have been identified for these disorders (including latent transforming growth factor ß (TGF-ß)-binding protein-2 (LTBP2), ADAMTS10, ADAMSTS17 and fibrillin-1 (FBN1) for Weill-Marchesani syndrome, ADAMTSL2 for recessive GD and FBN1 for AD and dominant GD), encoding proteins involved in the microfibrillar network. However, not all cases have mutations in these genes. METHODS: Individuals negative for mutations in known acromelic dysplasia genes underwent whole exome sequencing. RESULTS: A heterozygous missense mutation (exon 14: c.2087C>G: p.Ser696Cys) in latent transforming growth factor ß (TGF-ß)-binding protein-3 (LTBP3) was identified in a dominant AD family. Two distinct de novo heterozygous LTPB3 mutations were also identified in two unrelated GD individuals who had died in early childhood from respiratory failure-a donor splice site mutation (exon 12 c.1846+5G>A) and a stop-loss mutation (exon 28: c.3912A>T: p.1304*Cysext*12). CONCLUSIONS: The constellation of features in these AD and GD cases, including postnatal growth retardation of long bones and lung involvement, is reminiscent of the null ltbp3 mice phenotype. We conclude that LTBP3 is a novel component of the microfibrillar network involved in the acromelic dysplasia spectrum.

Elastic fibres in health and disease.

Elastic fibres are insoluble components of the extracellular matrix of dynamic connective tissues such as skin, arteries, lungs and ligaments. They are laid down during development, and comprise a cross-linked elastin core within a template of fibrillin-based microfibrils. Their function is to endow tissues with the property of elastic recoil, and they also regulate the bioavailability of transforming growth factor ß. Severe heritable elastic fibre diseases are caused by mutations in elastic fibre components; for example, mutations in elastin cause supravalvular aortic stenosis and autosomal dominant cutis laxa, mutations in fibrillin-1 cause Marfan syndrome and Weill-Marchesani syndrome, and mutations in fibulins-4 and -5 cause autosomal recessive cutis laxa. Acquired elastic fibre defects include dermal elastosis, whereas inflammatory damage to fibres contributes to pathologies such as pulmonary emphysema and vascular disease. This review outlines the latest understanding of the composition and assembly of elastic fibres, and describes elastic fibre diseases and current therapeutic approaches.