The invention provides blood-brain barrier shuttles comprising monoclonal antibody 5C1 and related antibodies linked to a monovalent binding entity for a blood-brain barrier receptor. The 5C1 antibody binds to an epitope within residues 118-126 of alpha-synuclein. The antibodies are useful, for example, for treating and/or diagnosing synucleinopathies including Lewy body diseases, such as Parkinson's disease, diffuse Lewy body disease (DLBD), Lewy body variant of Alzheimer's disease (LBV), Alzheimer's and Parkinson's disease comorbidity, and pure autonomic failure, as well as multiple system atrophy (MSA).