Fibulin-5 is an elastin-binding protein essential for elastic fibre development in vivo

Abstract

Extracellular elastic fibres provide mechanical elasticity to tissues and contribute towards the processes of organ remodelling by affecting cell-cell signalling. The formation of elastic fibres requires the assembly and crosslinking of tropoelastin monomers, and organization of the resulting insoluble elastin matrix into functional fibres. The molecules and mechanisms involved in this process are unknown. Fibulin-5 (also known as EVEC/DANCE) is an extracellular matrix protein abundantly expressed in great vessels and cardiac valves during embryogenesis, and in many adult tissues including the aorta, lung, uterus and skin, all of which contain abundant elastic fibres. Here we show that fibulin-5 is a calcium-dependent, elastin-binding protein that localizes to the surface of elastic fibres in vivo. fibulin-5-/- mice develop marked elastinopathy owing to the disorganization of elastic fibres, with resulting loose skin, vascular abnormalities and emphysematous lung. This phenotype, which resembles the cutis laxa syndrome in humans, reveals a critical function for fibulin-5 as a scaffold protein that organizes and links elastic fibres to cells. This function may be mediated by the RGD motif in fibulin-5, which binds to cell surface integrins, and the Ca2+-binding epidermal growth factor (EGF) repeats, which bind elastin.

New insights into elastic fiber assembly

“…Fibulin-4 has been localized to microfibrils (Kobayashi et al, 2007). Fibulin-5 colocalizes with elastic fibers in vitro (Nakamura et al, 2002) and can be seen at the elastin/microfibril interface in the aorta (Yanagisawa et al, 2002). Fibulin-5 binds tropoelastin and shows weak affinity for the C-and N-terminus of fibrillin-1 (Yanagisawa et al, 2002).…”

Elastic fiber homeostasis requires lysyl oxidase–like 1 protein

“…To obtain antibodies that were monospecific, we put affinity-purified antibodies to LOXL1C and to LOX through columns containing the respective homologous proteins immobilized on agarose beads (AminoLink Plus, Pierce). Fibulin-5 antibody (BSYN1923) made in rabbits was described previously 14 . The following antibodies to elastin, fibrillin-1 and fibrillin-2 were from Elastin Products Company: PR385 (rabbit antibody to mouse elastin, exons 6-17); PR387 (rabbit antibody to mouse elastin, exons 31-36); RT675 (goat antibody to rat elastin) used for double labeling of elastin with LOXL1, LOX or fibulin-5; PR210AP (rabbit antibody to fibrillin-1); PR225 (rabbit antibody to fibrillin-2).…”

“…Elastin binding protein (EBP) 13 , fibulin-5 (ref. 14) and an unspecified lysyl oxidase 15 were also reported to associate with elastic a). +/+, wild-type; +/-, heterozygous; -/-, homozygous Loxl1 -/-mutant.…”

“…Gene targeting studies show that fibulin-5 is required for elastic fiber development 14,16 . Disruption of the gene Lox leads to a reduction in collagen and elastin cross-links and perinatal lethality 17,18 , suggestive of a role in cross-linking both fibrillar collagens and elastin during development.…”

“…5g). Binding of fibulin-5 to both tropoelastin 14 and LOXL1 brings the enzyme and substrate into juxtaposition for efficient and spatially restricted polymer formation. LOXL1 converts tropoelastin into a lysyl-deaminated form, and the 'activated' tropoelastin associates with one another or deposits onto the existing polymer through coacervation 19 , followed by spontaneous covalent cross-linking.…”