Self-Assembled Curcumin-Poly(carboxybetaine methacrylate) Conjugates: Potent Nano-Inhibitors against Amyloid β-Protein Fibrillogenesis and Cytotoxicity

Fibrillogenesis of amyloid beta-protein (A beta) is a pathological hallmark of Alzheimer's disease, so inhibition of A beta aggregation is considered as an important strategy for the precaution and treatment of AD. Curcumin (Cur) has been recognized as an effective inhibitor of A beta fibrillogenesis, but its potential application is limited by its poor bioavailability. Herein, we proposed to conjugate Cur to a zwitterionic polymer, poly(carboxybetaine methacrylate) (pCB), and synthesized three Cur@pCB conjugates of different degrees of substitution (DS, 1.9-2.9). Cur@pCB conjugates self-assembled into nanogels of 120-190 nm. The inhibition effects of Cur@pCB conjugates on the fibrillation and cytotoxicity of A beta(42) was investigated by extensive biophysical and biological analyses. Thioflavin T fluorescence assays and atomic force microscopic observations revealed that the Cur@pCB conjugates were much more efficient than molecular curcumin on inhibiting A beta(42) fibrillation, and cytotoxicity assays also indicated the same tendency. Of the three conjugates, Curl@pCB of the lowest DS (1.97) exhibited the best performance; 5 mu M Curl@pCB functioned similarly with 25 mu M free curcumin. Moreover, 5 mu M Curl@pCB increased the cell viability by 43% but free curcumin at the same concentration showed little effect. It is considered that the highly hydrated state of the zwitterionic polymers resulted in the superiority of Cur@pCB over free curcumin. Namely, the dense hydration layer on the conjugates strongly stabilized the bound A beta on curcumin anchored on the polymer, suppressing the conformational transition of the protein to beta-sheet-rich structures. This was demonstrated by circular dichroism spectroscopy, in which Curl@pCB was proven to be the strongest in the three conjugates. The research has thus revealed a new function of zwitterionic polymer pCBMA and provided new insights into the development of more potent nanoinhibitors for suppressing A beta fibrillogenesis and cytotoxicity.