An engineered monomer binding-protein forα-synuclein efficiently inhibits the proliferation of amyloid fibrils

Removing or preventing the formation of α -synuclein aggregates is a plausible strategy against Parkinson’s disease. To this end we have engineered the β -wrapin AS69 to bind monomeric α -synuclein with high affinity. In cultured cells, AS69 reduced the occurrence of α -synuclein oligomers and of visible α -synuclein aggregates. In flies, AS69 reduced α -synuclein aggregates and the locomotor deficit resulting from α -synuclein expression in neuronal cells. In a mouse model based on the intracerebral injection of pre-formed α -synuclein seed fibrills (PFFs), AS69 co-injection reduced the density of dystrophic neurites observed three months later. In biophysical experiments in vitro , AS69 highly sub-stoichiometrically inhibited auto-catalytic secondary nucleation processes, even in the presence of a large excess of monomer. We present evidence that the AS69- α -synuclein complex, rather than the free AS69, is the inhibitory species responsible for sub-stoichiometric inhibition. These results represent a new paradigm that high affinity monomer binders can be strongly sub-stoichiometric inhibitors of nucleation processes.