The Supramolecular Organization of Fibrillin-Rich Microfibrils

Abstract

We propose a new model for the alignment of fibrillin molecules within fibrillin microfibrils. Automated electron tomography was used to generate three-dimensional microfibril reconstructions to 18.6-Å resolution, which revealed many new organizational details of untensioned microfibrils, including heart-shaped beads from which two arms emerge, and interbead diameter variation. Antibody epitope mapping of untensioned microfibrils revealed the juxtaposition of epitopes at the COOH terminus and near the proline-rich region, and of two internal epitopes that would be 42-nm apart in unfolded molecules, which infers intramolecular folding. Colloidal gold binds microfibrils in the absence of antibody. Comparison of colloidal gold and antibody binding sites in untensioned microfibrils and those extended in vitro, and immunofluorescence studies of fibrillin deposition in cell layers, indicate conformation changes and intramolecular folding. Mass mapping shows that, in solution, microfibrils with periodicities of <70 and >140 nm are stable, but periodicities of ∼100 nm are rare. Microfibrils comprise two in-register filaments with a longitudinal symmetry axis, with eight fibrillin molecules in cross section. We present a model of fibrillin alignment that fits all the data and indicates that microfibril extensibility follows conformation-dependent maturation from an initial head-to-tail alignment to a stable approximately one-third staggered arrangement.

Subectodermal microfibrillar bundles are organized into a distinct parallel array in the developing chick limb bud

“…Moreover, a widespread distribution of microfibrils (or FB1-immunoreactivity in this study) in the embryonic mesenchyme may imply their fundamental role to constitute a basic framework for the extracellular space of the early embryo. Considering that microfibrils have inherent, reversible extensibility (Baldock et al, 2001), a microfibrillar framework would endow a fragile embryonic tissue with both stiffness and flexibility. This framework may play a critical role in positioning cells and tissues into a specific compartments or regions during early development.…”

Fibrillin Assembly Requires Fibronectin

“…Although many publications in recent years have addressed the spatial organization of fibrillins in microfibrils Downing et al, 1996;Liu et al, 1996;Reinhardt et al, 1996;Qian and Glanville, 1997;Baldock et al, 2001;Lee et al, 2004;Baldock et al, 2006;Kuo et al, 2007), relatively little information is available about mechanisms and components involved in microfibril formation. Revealing these mechanisms is ultimately critical for the understanding of the pathobiology of microfibrillopathies associated with mutations in fibrillins.…”

“…Extracted microfibrils from cell culture or tissues display a typical bead-on-a-string ultrastructure, having a 50 -55-nm periodicity when analyzed by electron microscopy after rotary shadowing Kielty et al, 1991). Although many publications over recent years have addressed the spatial organization of fibrillins as it relates to the bead-on-a-string microfibrillar structure Reinhardt et al, 1996;Downing et al, 1996;Liu et al, 1996;Qian and Glanville, 1997;Baldock et al, 2001Baldock et al, , 2006Lee et al, 2004;Kuo et al, 2007), little information is available about the dynamic processes and components involved in microfibril formation. The pathogenetic relevance of this is highlighted by the fact that microfibril assembly is frequently disturbed in patients with fibrillinopathies (Tiedemann et al, 2004).…”

Elastic fibres in health and disease

“…Assembled microfibrils comprise fibrillin monomers aligned in a head-to-tail arrangement and laterally associated (probably eight molecules in cross-section) ( Refs 73,74,75). …”