Alpha-Synuclein Interacts Directly But Reversibly With Psychosine: Implications For Alpha-Synucleinopathies

Aggregation of alpha-synuclein, the hallmark of alpha-synucleinopathies such as Parkinson's disease, occurs in various glycosphingolipidoses. Although alpha-synuclein aggregation correlates with deficiencies in the lysosomal degradation of glycosphingolipids (GSL), the mechanism(s) involved in this aggregation remains unclear. We previously described the aggregation of alpha-synuclein in Krabbe's disease (KD), a neurodegenerative glycosphingolipidosis caused by lysosomal deficiency of galactosyl-ceramidase (GALC) and the accumulation of the GSL psychosine. Here, we used a multi-pronged approach including genetic, biophysical and biochemical techniques to determine the pathogenic contribution, reversibility, and molecular mechanism of aggregation of alpha-synuclein in KD. While genetic knock-out of alpha-synuclein reduces, but does not completely prevent, neurological signs in a mouse model of KD, genetic correction of GALC deficiency completely prevents alpha-synuclein aggregation. We show that psychosine forms hydrophilic clusters and binds the C-terminus of alpha-synuclein through its amino group and sugar moiety, suggesting that psychosine promotes an open/aggregation-prone conformation of alpha-synuclein. Dopamine and carbidopa reverse the structural changes of psychosine by mediating a closed/aggregation-resistant conformation of alpha-synuclein. Our results underscore the therapeutic potential of lysosomal correction and small molecules to reduce neuronal burden in alpha-synucleinopathies, and provide a mechanistic understanding of alpha-synuclein aggregation in glycosphingolipidoses.