br Conclusions br Transparency document br Acknowledgements
Acknowledgements This work was supported by a grant from the European Commission FP6 “Neuroprion” – Network of Excellence and Royal Veterinary College Bioveterinary science research project funding. We thank Professor Alun Williams (Cambridge University) for supplying Alzheimer's brain samples and Professor E. Koo (National University of Singapore) for 7PA2 cells.
Introduction Alzheimer's disease (AD) is one of the most populated chronic neurodegenerative disorders, which has caused worldwide health problem and huge economic loss in the word [, , ]. It has been recognized that senile plaques in the brain parenchyma are the hallmark of AD . Moreover, the accumulation of amyloid β-protein (Aβ) is responsible for senile plaques, which disrupts membrane functions and causes primary neuronal dysfunction . Many studies have proved that soluble Aβ oligomers  and protofibrils  are the most toxic species, responsible for neuronal dysfunction and death. Aβ is produced by the sequential proteolytic cleavages of the transmembrane amyloid precursor protein (APP) by β- and γ-secretases [7,8]. Thus, decreasing Aβ production, preventing Aβ aggregation, neutralizing or removing cytotoxic species are suggested for the treatment of AD . By considering the essential physiological functions of the secretases and APP, however, inhibition of Aβ aggregation is considered as a promising strategy for the treatment or delaying the onset of AD . Recently, many inhibitors, such as small molecules [, , ], 7545 [, , ], peptides and peptide mimetics [, , ], proteins  and nanoparticles [21,22], have been studied to prevent Aβ aggregation and reduce its associated cytotoxicity. Among them, peptide inhibitors have been acquired extensive interesting due to their easy synthesis and broad bioavailability [23,24]. Previous experiments have demonstrated that Aβ fragments, such as Aβ17–21 [25,26], Aβ30–34 , Aβ32–37 [28,29], Aβ38–42 , display a protective effect against Aβ-mediated neurotoxicity. Based on the sequence of Aβ16–21, a heptapeptide (Ac-LVFFARK-NH2, LK7)  was designed. Biochemical and biophysical experiments demonstrated that LK7 was more effective in inhibiting Aβ fibrillogenesis than other peptide analogous derived from the same hydrophobic core (Aβ16–21). However, the shortcomings of the peptide, such as poor solubility, propensity to self-assembly and the strong cytotoxicity of its aggregates, limited its utilization. In order to improve the anti-aggregation effects of peptide-based inhibitors or to obtain additional functions on the therapy of AD, nanoparticles [19,31] and dendritic macromolecules  have been utilized to modify peptides. However, most of the nanoparticle inhibitors are too large to traverse the blood–brain barrier to arrive the focus location. Cyclodextrins (CyDs), cyclic oligosaccharide compounds, have been used successfully in the treatment of various types of neurodegenerative disorders to gain higher therapeutic effect with lower dosage and to reduce side effects . Numerous researches demonstrated that CyDs were useful in improving the effectiveness of drugs, mainly to the compounds of poor solubility . More importantly, as a functional host, CyDs were proved to be nontoxic and to possess high water solubility . All these features make them suitable for functional modifications. Previously, βCyD was attempted to reduce Aβ fibrillogenesis , but the binding affinity between βCyD and Aβ was too weak to make it an effective inhibitor.
Materials and methods
Results and discussion
Conclusions In this work, βCyD was conjugated to our previously designed heptapeptide inhibitor of Aβ aggregation, Ac-LVFFARK-NH2 (LK7), which had poor solubility and aggregation-prone under the physiological condition. The LK7 conjugation to βCyD resulted in a significant improvement of its solubility and greatly suppressed its self-assembly propensity. In addition, the conjugation changed the secondary structure of the heptapeptide as demonstrated by the CD spectra. These made LK7-βCyD conjugate present significantly higher potency on inhibiting Aβ40 fibrillization than LK7. βCyD showed no effect on Aβ40 aggregation at the experimental concentrations (25–250 μM) because it did not interact with the protein. This proved that the superiority of LK7-βCyD on inhibiting Aβ40 aggregation was attributed to the improved inhibitory properties of the conjugated peptide. ITC results indicated that the conjugation of LK7 and βCyD distinctly increased the hydrophobic interactions between the LK7 moiety and Aβ40, leading to an enhanced binding affinity for Aβ40. The binding of LK7-βCyD stabilized the secondary structure of Aβ40, inhibited the nucleation of the amyloid protein, and induced the formation of innocuous amorphous aggregates, thus made LK7-βCyD present a promising protective effect on cultured cells. By comparison, LK7 could not inhibit the amyloid cytotoxicity, but only showed moderate inhibition on Aβ40 aggregation. The results suggest that this modification, in addition to potentiating prototype's inhibitory efficacy, would open a new way to create more potent agents against amyloid proteins via functionalization of inhibitors of high hydrophobicity and self-aggregation tendency (e.g., LK7).