In healthy human midbrain neurons, α-Syn exists as both physiological monomers and HMW conformers. GSLs such as GluCer preferentially act on HMW conformers, and to a minor extent on monomers, to convert them into assembly-state oligomers (AOs) with molecular radius that is identical to HMW physiological conformers. HMW physiological species are non-toxic, loosely self-associated species and can be directly converted into AOs. AOs are cytotoxic, compact, and PK resistant, and can accelerate the formation of assembled amyloid fibrils from free monomers.