Fibronectin: Molecular Structure, Fibrillar Structure and Mechanochemical Signaling

Abstract

The extracellular matrix (ECM) plays a key role as both structural scaffold and regulator of cell signal transduction in tissues. In times of ECM assembly and turnover, cells upregulate assembly of the ECM protein, fibronectin (FN). FN is assembled by cells into viscoelastic fibrils that can bind upward of 40 distinct growth factors and cytokines. These fibrils play a key role in assembling a provisional ECM during embryonic development and wound healing. Fibril assembly is also often upregulated during disease states, including cancer and fibrotic diseases. FN fibrils have unique mechanical properties, which allow them to alter mechanotransduction signals sensed and relayed by cells. Binding of soluble growth factors to FN fibrils alters signal transduction from these proteins, while binding of other ECM proteins, including collagens, elastins, and proteoglycans, to FN fibrils facilitates the maturation and tissue specificity of the ECM. In this review, we will discuss the assembly of FN fibrils from individual FN molecules; the composition, structure, and mechanics of FN fibrils; the interaction of FN fibrils with other ECM proteins and growth factors; the role of FN in transmitting mechanobiology signaling events; and approaches for studying the mechanics of FN fibrils.

Keywords: biomechanics; extracellular matrix; fibrillogenesis; fibronectin; mechanobiology.

Figure 1

A schematic of the domains of FN with relevant structural features, cleavage sites, nomenclature, and integrin binding. FN is an approximately 250 kDa protein that is secreted as a dimer. Individual domains are classified as Type I (rectangles), Type II (hexagons), Type III (ovals), or a variable region (diamond). Domains that spontaneously open are shown with a single red slash, while domains that are mechanically unfolded are shown with a red X. Domains that have exposed FN–FN binding sites are shown in gold, while FN domains that have been shown to exhibit cryptic FN–FN binding sites are shown in orange. Molecular weight of regions are directly below. Enzymes known to digest FN are shown at their specific sites with arrows and color coded appropriately. Regions and/or specific sequences that have been shown to bind other ECM constituents are labeled based on size and ligand, then integrins are listed below that, and common terminology for each FN fragment is listed below that. The dimerization of FN at its C-terminus is shown at bottom.

Figure 2

Assembly of FN fibrils from soluble FN. (A) FN exists in a soluble conformation, which binds to integrins on the cells surface. (B) Actomyosin force extends FN, facilitating (C) FN–FN binding. (D) As additional soluble FN molecules bind to the fibril, additional integrin binding drives progression and assembly of an insoluble fibril. (E) FN–FN interactions are detailed in the text, and involve interactions between homotypic Type I domain (black circle) interactions, and heterotypic Type I and Type III domain (gray oval) interactions.

Figure 3

Assembly of FN fibrils. FN assembly is facilitated by interactions between domains I^1–5 (yellow rectangles) and several Type III domains (various circles). FN is in an extended conformation in FN fibrils, and as such, most likely enables multiple FN–FN binding events within the cross section of the fibril. Since stretched III domains can bind via β-strand addition, it is possible that fibril maturation and insolubility is driven by these binding events. Computational models of FN fibrillogenesis have assumed a hexagonal packing organization [7,25], but actual molecular organization is not known. A cross section on the left demonstrates how different domains within the FN moelcule may interact with a range of domains from neighboring FN molecules within the fibril.