Pathological forms of alpha-synuclein (α-synuclein), a presynaptic neuronal protein. In its native state, α-synuclein is soluble and functional. However, in synucleinopathies like MSA and PD, it misfolds and aggregates into toxic oligomers and insoluble fibrils, which form intraneuronal inclusions known as Lewy bodies (in PD) and glial cytoplasmic inclusions (in MSA). These aggregates are believed to propagate through the brain in a prion-like manner, driving neurodegeneration and disease progression.