AVS2010 Session BI-WeA: Proteins & Peptides on Surfaces

Alzheimer’s disease (AD) is a major cause of dementia in elderly people, affecting 5 million people in USA alone. AD is caused by the abnormal accumulation of aggregated amyloid beta peptides (39 to 43 amino acid residues) in the brain. Amyloid beta peptides are proteolytic products of the amyloid precursor protein of unknown function. Unfortunately, there are no cures available for this disease. However, there are several mechanisms purposed to cause and cure AD. The lipid membrane has been shown to mediate the fibrillogenesis and toxicity of Alzheimer's disease amyloid beta (Aβ) peptide. Several reports have linked the insertion of Aβ peptide into membranes as a possible mechanism of neurotoxicity. We hypothesized that small molecules capable of preventing the insertion of Aβ into membranes may ameroliate Aβ toxicity. Therefore, we investigated the effect of curcumin, a naturally occurring anti-inflammatory and antioxidant compound that suppresses oxidative damage, inflammation, cognitive deficits, and amyloid accumulation, on Aβ40 induced toxicity and in Aβ40 insertion into 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) monolayer using surface pressure insertion isotherms and fluorescence microscopic techniques. We found that curcumin attenuates Aβ40 induced neuronal toxicity by inhibiting the insertion of Aβ into membranes possibly by interacting with membranes. Our data also demonstrated that neuroprotective action of curcumin in Aβ induced toxicity does not exclusively come through oligomerization inhibition, indicating that c urcumin-membrane interaction but not curcumin-Aβ is associated in curcumin mediated neuroprotection. Altogether, our study suggests that curcumin-like small molecules inhibitors of Aβ insertions into membranes could be potential target to cure AD.

Protein adsorption and orientation plays a critical role in many biomedical applications. Fibronectin (FN) is an extracellular matrix protein that is involved in many cell processes such as adhesion, migration and growth. The orientation and conformation of FN adsorbed onto surfaces can therefore play a critical role on cell-surface interactions. In this study, the adsorbed orientation and conformation of the 7-10 fragment of FNIII was studied on three different model surfaces (self-assembled monolayers (SAMs) of C₁₁ alkanethiols on Au with -CH₃, -NH₂, and -COOH functional groups). X-ray photoelectron spectroscopy (XPS) was used to quantify the amount of protein adsorbed on the different surfaces and time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to characterize their orientation and conformation. A trehalose coating was also used to inhibit the conformation changes due to the dehydration of the sample. With the help of principal component analysis (PCA), the peaks which are responsible for the variance observed between the spectra relative to the protein adsorbed on the different surfaces could be identified. Because the surface sensitivity of the ToF-SIMS technique is lower than average protein size, these changes in the spectra reflect differences in the conformation and the orientation of the FN fragment. Comparison of trehalose protected and unprotected samples show a significant difference in the ratio between hydrophilic and hydrophobic amino acids. The results suggest that the more hydrophilic amino acids stay on the outside of the trehalose protected protein while the more hydrophobic ones get exposed to the protein air interface upon drying. Comparison of the trehalose protected fragment on -CH₃ and ‑COOH terminated SAMs show more intense signals of Arg and Asp on the -COOH surface and more intense Val, Pro and Leu signals on the -CH₃ SAMs. The detection of these different amino acids for the protein on the different SAMs suggests that the fragment might partly denature upon adsorption to the hydrophobic surface.

von Willebrand Factor (vWF) is a blood-soluble clotting protein responsible for binding platelets through the glycoprotein 1b (GP1b) receptor on the platelet surface. vWF can become activated and bind platelets when bound to exposed collagen in blood vessels or when vWF experiences increased shear.

vWF can also bind platelets when adsorbed to synthetic surfaces and participate in clot formation, which is not desirable for blood-contacting biomaterials. There is evidence that surface properties can influence vWF adsorption. Previous studies showed differences in protein topography (1) and conformation (2) when vWF was adsorbed on mica (1), octadecyltrichlorosilane modified glass (1,2), and collagen VI (2). However, studies were not performed to relate adsorption differences to vWF function.

To more fully characterize the adsorption properties of vWF, we adsorbed the platelet binding domain of vWF (A1 domain) to three surfaces: polystyrene, tissue culture polystyrene, and glass. Protein structure was investigated using x-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). Protein function was tested by measuring platelet binding in a physiologically relevant flow assay.

Using nitrogen as a marker of protein, XPS showed similar amounts of vWF A1 adsorbed to the three surfaces. However, the flow assay showed significantly different platelet binding to vWF A1 on each surface, as measured by platelet rolling velocity. Rolling velocity was highest on glass, indicating lowest platelet binding. The slowest rolling velocity was observed on polystyrene, indicating the highest level of platelet binding. ToF-SIMS data was analyzed using principle component analysis (PCA). PCA showed separation of the three surfaces with adsorbed vWF A1, indicating conformational differences between the proteins on each surface.

These studies show that surface properties influence structure and function of adsorbed vWF domains. Although there was a similar amount of protein on each surface, protein function was different. Polystyrene, the most hydrophobic of the surfaces, appeared to have the strongest activating effect on vWF. ToF-SIMS studies showed conformational differences, suggesting that conformational differences contribute to the observed functional differences.

Understanding the structure and function of adsorbed vWF gives insight into how vWF behaves on biomaterial surfaces and how this might affect platelet binding. Characterizing vWF adsorption also allows in vitro behavior to be more accurately related to in vivo thrombosis events.

1. Raghavachari. Colloids Surf B (2000) 19:315.
2. Kang. Thromb Res (2007) 119: 731.

The adsorption behavior of model GG-X-GG and X_n oligopeptides on Au and native Si oxide substrates was investigated to elucidate the contributions of different amino acids (AAs) to peptide-surface interactions. The manner in which peptides and proteins interact with surfaces is of critical importance in many biological and technological systems. The mechanisms underlying surface adsorption of proteins, however, are poorly understood, largely due to the inherent complexity of natural proteins. Accordingly, in this work simple model peptides were chosen to systematically examine the interactions between natural AAs and inorganic surfaces. Surface interactions of a series of AAs were probed by incubating inorganic substrates in aqueous solutions of model GG-X-GG pentapeptides, in which an AA of interest was flanked with Gly. The effects of cooperative adsorption were also examined using model X_n oligopeptides (n = 5, 10). The amount of peptides that irreversibly adsorbed on each substrate was quantified by X-ray photoelectron spectroscopy (XPS), the resulting systematic data revealed several trends in surface adsorption of oligopeptides as a function of their composition and length. On the negatively-charged, hydrophilic native SiO_x layer of a Si wafer, only peptides containing positively-charged residues (Lys and Arg) and polar residues (Ser and Thr) adsorbed at significant levels. Peptides adsorbed more readily on Au-coated Si wafers, on which the maximum surface coverage was ca. 3 times greater than that on the native SiO_x. For a particular AA (X), adsorption tended to increase, sometimes dramatically, with increasing units of X (GG-X-GG < X₅ < X₁₀). In pairs of AAs having side chains that only vary by alkyl chain length (L and V, Q and N, R and K, E and D), the AA with the longer alkyl chain adsorbed more readily, although this trend diminished with the increasing number of X residues.