Slav Angelov Semerdzhiev

Oligomers of Parkinson's Disease-Related α-Synuclein Mutants have Similar Structures but Distinctive Membrane Permeabilization Properties

Single-amino acid mutations in the human α-synuclein (αS) protein are related to early onset Parkinsons disease (PD). In addition to the well-known A30P, A53T, and E46K mutants, recently a number of new familial disease-related αS mutations have been discovered. How these mutations affect the putative physiological function of αS and the disease pathology is still unknown. Here we focus on the H50Q and G51D familial mutants and show that like wild-type αS, H50Q and G51D monomers bind to negatively charged membranes, form soluble partially folded oligomers with an aggregation number of ~30 monomers under specific conditions, and can aggregate into amyloid fibrils. We systematically studied the ability of these isolated oligomers to permeabilize membranes composed of anionic phospholipids (DOPG) and membranes mimicking the mitochondrial phospholipid composition (CL:POPE:POPC) using a calcein release assay. Small-angle X-ray scattering studies of isolated oligomers show that oligomers formed from wild-type αS and the A30P, E46K, H50Q, G51D, and A53T disease-related mutants are composed of a similar number of monomers. However, although the binding affinity of the monomeric protein and the aggregation number of the oligomers formed under our specific protocol are comparable for wild-type αS and H50Q and G51D αS, G51D oligomers cannot disrupt negatively charged and physiologically relevant model membranes. Replacement of the membrane-immersed glycine with a negatively charged aspartic acid at position 51 apparently abrogates membrane destabilization, whereas a mutation in the proximal but solvent-exposed part of the membrane-bound α-helix such as that found in the H50Q mutant has little effect on the bilayer disrupting properties of oligomers.

Self-assembly of protein fibrils into suprafibrillar aggregates: bridging the nano- and mesoscale

We report on in vitro self-assembly of nanometer-sized α-synuclein amyloid fibrils into well-defined micrometer-sized suprafibrillar aggregates with sheet-like or cylindrical morphology depending on the ionic strength of the solution. The cylindrical suprafibrillar structures are heavily hydrated, suggesting swollen gel-like particles. In contrast to higher order structures formed by other negatively charged biopolymers, multivalent ions are not required for the suprafibrillar aggregates to form. Their formation is induced by both mono- and divalent counterions. The self-assembly process is not mediated by protein-specific interactions but rather by the cooperative action of long-range electrostatic repulsion and short-range attraction. Understanding the mechanism driving the self-assembly might give us valuable insight into the pathological formation of fibrillar superstructures such as Lewy bodies and neurites-distinct signatures of Parkinsons disease-and will open the possibility to utilize the self-assembly process for the design of novel fibril-based smart nanostructured materials.

Non-uniform self-assembly: On the anisotropic architecture of α-synuclein supra-fibrillar aggregates

Although the function of biopolymer hydrogels in nature depends on structural anisotropy at mesoscopic length scales, the self-assembly of such anisotropic structures in vitro is challenging. Here we show that fibrils of the protein α-synuclein spontaneously self-assemble into structurally anisotropic hydrogel particles. While the fibrils in the interior of these supra-fibrillar aggregates (SFAs) are randomly oriented, the fibrils in the periphery prefer to cross neighboring fibrils at high angles. This difference in organization coincides with a significant difference in polarity of the environment in the central and peripheral parts of the SFA. We rationalize the structural anisotropy of SFAs in the light of the observation that αS fibrils bind a substantial amount of counterions. We propose that, with the progress of protein polymerization into fibrils, this binding of counterions changes the ionic environment which triggers a change in fibril organization resulting in anisotropy in the architecture of hydrogel particles.