Unraveling the mechanism of elastic fiber assembly: The roles of short fibulins

Abstract

Evolution of elastic fibers is associated with establishment of the closed circulation system. Primary roles of elastic fibers are to provide elasticity and recoiling to tissues and organs and to maintain the structural integrity against mechanical strain over a lifetime. Elastic fibers are comprised of an insoluble elastin core and surrounding mantle of microfibrils. Elastic fibers are formed in a regulated, stepwise manner, which includes the formation of a microfibrillar scaffold, deposition and integration of tropoelastin monomers into the scaffold, and cross-linking of the monomers to form an insoluble, functional polymer. In recent years, an increasing number of glycoproteins have been identified and shown to be located on or surrounding elastic fibers. Among them, the short fibulins-3, -4 and -5 particularly drew attention because of their potent elastogenic activity. Fibulins-3, -4 and -5 are characterized by tandem repeats of calcium binding EGF-like motifs and a C-terminal fibulin module, which is conserved throughout fibulin family members. Initial biochemical characterization and gene expression studies predicted that fibulins might be involved in structural support and/or matrix-cell interactions. Recent analyses of short fibulin knockout mice have revealed their critical roles in elastic fiber development in vivo. We review recent findings on the elastogenic functions of short fibulins and discuss the molecular mechanism underlying their activity in vitro and in vivo.

Figure 1. Human short fibulins

(A) Schematic representation of short fibulins-3, -4, -5 and -7 are shown. Identity and homology (parenthesis) between the C-terminal fibulin modules are indicated. (B) Phylogenetic map of the human fibulin family proteins. GenBank accession numbers used to obtain primary sequences are fibulin-1A (CAQ0836), fibulin-2 (AAH51690), fibulin-3 (NP_001034438), fibulin-4 (CAA10791), fibulin-5 (CAB38568), fibulin-6 (CAC37630) and fibulin-7 (AAH35784).

Figure 2. Electron micrographs of dermal elastic fibers in wild-type and Fbln5-null mouse skin

(A) Elastic fibers in wild-type dermis show a core of elastin (e) integrated within a bundle of microfibrils (mf). (B) In contrast, the elastic fibers in Fbln5-null dermis are composed of large elastin aggregates (e) located outside extensive bundles of microfibrils (mf). Scale bar = 0.25 μm, coll = collagen. Adapted and used with permission from Choi et al. Matrix Biol. 28:211-220, 2009.

Figure 3

A working model of elastic fiber formation. (Left) The normal elastogenesis process, including coacervation, cross-linking and deposition, and organization of elastic fibers is shown. 1) Following secretion of tropoelastin (TE) from elastogenic cells, coacervation takes place near the cells. Fibulins-4 and -5 (green) regulate this process. Inactive Lox proteins (orange) are present in the coacervate. 2) Fibronectin fibrils (yellow) are tethered to cells and provide a scaffold for microfibril assembly (blue). Microfibrils, in turn, provide a scaffold for deposition of tropoelastin. Fibulins bind to Lox proteins and compartmentalize them to the microfibrils and/or potentiate proteolytic activation of Lox proteins. 3) Lox-mediated cross-linking and polymerization of tropoelastin takes place. Fibulins-4 and -5 most likely remain on the periphery of assembled elastic fibers and/or microfibrils. (Right) Hypothetical condition without fibulins-4 and -5 is shown. 1) In the absence of fibulins-4 and -5, coacervation is decreased and/or maturation is dysregulated, leading to inefficient formation of tropoelastin coacervates and a compromised condition for subsequent cross-linking. 2) Lox/Loxl1-tropoelastin complexes cannot be incorporated onto the microfibril bundle and Lox proteins may not be properly cleaved to become active. The association between tropoelastin and Lox proteins may be decreased without fibulins-4 and -5. 3) As a consequence, aggregated tropoelastin globules are formed and a significant reduction of mature elastic fibers is observed.