Interaction of β-Sheet Folds with a Gold Surface

The adsorption of proteins on inorganic surfaces is of fundamental biological importance. Further, biomedical and nanotechnological applications increasingly use interfaces between inorganic material and polypeptides. Yet, the underlying adsorption mechanism of polypeptides on surfaces is not well understood and experimentally difficult to analyze. Therefore, we investigate here the interactions of polypeptides with a gold(111) surface using computational molecular dynamics (MD) simulations with a polarizable gold model in explicit water. Our focus in this paper is the investigation of the interaction of polypeptides with β-sheet folds. First, we concentrate on a β-sheet forming model peptide. Second, we investigate the interactions of two domains with high β-sheet content of the biologically important extracellular matrix protein fibronectin (FN). We find that adsorption occurs in a stepwise mechanism both for the model peptide and the protein. The positively charged amino acid Arg facilitates the initial contact formation between protein and gold surface. Our results suggest that an effective gold-binding surface patch is overall uncharged, but contains Arg for contact initiation. The polypeptides do not unfold on the gold surface within the simulation time. However, for the two FN domains, the relative domain-domain orientation changes. The observation of a very fast and strong adsorption indicates that in a biological matrix, no bare gold surfaces will be present. Hence, the bioactivity of gold surfaces (like bare gold nanoparticles) will critically depend on the history of particle administration and the proteins present during initial contact between gold and biological material. Further, gold particles may act as seeds for protein aggregation. Structural re-organization and protein aggregation are potentially of immunological importance

Plasma and cellular fibronectin: distinct and independent functions during tissue repair

Fibronectin (FN) is a ubiquitous extracellular matrix (ECM) glycoprotein that plays vital roles during tissue repair. The plasma form of FN circulates in the blood, and upon tissue injury, is incorporated into fibrin clots to exert effects on platelet function and to mediate hemostasis. Cellular FN is then synthesized and assembled by cells as they migrate into the clot to reconstitute damaged tissue. The assembly of FN into a complex three-dimensional matrix during physiological repair plays a key role not only as a structural scaffold, but also as a regulator of cell function during this stage of tissue repair. FN fibrillogenesis is a complex, stepwise process that is strictly regulated by a multitude of factors. During fibrosis, there is excessive deposition of ECM, of which FN is one of the major components. Aberrant FN-matrix assembly is a major contributing factor to the switch from normal tissue repair to misregulated fibrosis. Understanding the mechanisms involved in FN assembly and how these interplay with cellular, fibrotic and immune responses may reveal targets for the future development of therapies to regulate aberrant tissue-repair processes

Interdomain tilt angle determines integrin-dependent function of the ninth and tenth FIII domains of human fibronectin

Integrins are an important family of signaling receptors that mediate diverse cellular processes. The binding of the abundant extracellular matrix ligand fibronectin to integrins 51 and v3 is known to depend upon the Arg-Gly-Asp (RGD) motif on the tenth fibronectin FIII domain. The adjacent ninth FIII domain provides a synergistic effect on RGD-mediated integrin 51 binding and downstream function. The precise molecular basis of this synergy remains elusive. Here we have dissected further the function of FIII9 in integrin binding by analyzing the biological activity of the FIII9-10 interdomain interface variants and by determining their structural and dynamic properties in solution. We demonstrate that the contribution of FIII9 to both 51 and v3 binding and downstream function critically depends upon the interdomain tilt between the FIII9 and FIII10 domains. Our data suggest that modulation of integrin binding by FIII9 may arise in part from its steric properties that determine accessibility of the RGD motif. These findings have wider implications for mechanisms of integrin-ligand binding in the physiological context

Interdomain association in fibronectin: insight into cryptic sites and fibrillogenesis

The process by which fibronectin (FN), a soluble multidomain protein found in tissue fluids, forms insoluble fibrillar networks in the extracellular matrix is poorly understood. Cryptic sites found in FN type III domains have been hypothesized to function as nucleation points, thereby initiating fibrillogenesis. Exposure of these sites could occur upon tension-mediated mechanical rearrangement of type III domains. Here, we present the solution structures of the second type III domain of human FN (2FNIII), and that of an interaction complex between the first two type III domains (1−2FNIII). The two domains are connected through a long linker, flexible in solution. A weak but specific interdomain interaction maintains 1−2FNIII in a closed conformation that associates weakly with the FN N-terminal 30 kDa fragment (FN30 kDa). Disruption of the interdomain interaction by amino-acid substitutions dramatically enhances association with FN30 kDa. Truncation analysis of 1−2FNIII reveals that the interdomain linker is necessary for robust 1−2FNIII–FN30 kDa interaction. We speculate on the importance of this interaction for FN function and present a possible mechanism by which tension could initiate fibrillogenesis

Hydrogels with precisely controlled integrin activation dictate vascular patterning and permeability

Integrin binding to bioengineered hydrogel scaffolds is essential for tissue regrowth and regeneration, yet not all integrin binding can lead to tissue repair. Here, we show that through engineering hydrogel materials to promote α3/α5β1 integrin binding, we can promote the formation of a space-filling and mature vasculature compared with hydrogel materials that promote αvβ3 integrin binding. In vitro, α3/α5β1 scaffolds promoted endothelial cells to sprout and branch, forming organized extensive networks that eventually reached and anastomosed with neighbouring branches. In vivo, α3/α5β1 scaffolds delivering vascular endothelial growth factor (VEGF) promoted non-tortuous blood vessel formation and non-leaky blood vessels by 10 days post-stroke. In contrast, materials that promote αvβ3 integrin binding promoted endothelial sprout clumping in vitro and leaky vessels in vivo. This work shows that precisely controlled integrin activation from a biomaterial can be harnessed to direct therapeutic vessel regeneration and reduce VEGF-induced vascular permeability in vivo