Molecular Insights into Neuronal Dysfunction in GM2 Gangliosidoses

Glycosphingolipids (GSL) are important bioactive components of cellular membranes. Complex GSLs, containing one or more sialic acid residues are known as gangliosides and are highly abundant in the brain. Diseases of ganglioside metabolism often result in severe, early-onset neurodegeneration. The ganglioside GM2 is the substrate of the hydrolytic lysosomal β-hexosaminidase A (HexA) enzyme and when subunits of this enzyme are non-functional, GM2 lipid accumulates in cells leading to the GM2 gangliosidoses, Tay-Sachs and Sandhoff diseases. We have developed i3Neuron-based models of Tay-Sachs and Sandhoff diseases, which recapitulate cellular features of these diseases including endolysosomal storage of GM2 and formation of membrane whorls. Using proteomic approaches, we identify the molecular changes occurring within these neurons including accumulation of endolysosomal proteins consistent with disease phenotypes. Importantly, we demonstrate that in addition to lysosomal dysfunction, these diseases also result in significant changes in proteins associated with trafficking, as well as changes in the lipid and protein composition of the plasma membrane (PM). One of the mechanisms driving these changes is the exocytosis of lysosomal material resulting in the aberrant accumulation of lysosomal proteins and lipids on the cell surface. The PM analysis also revealed changes in the abundance of several key synaptic proteins and measurements of neuronal electrical activity reveals a reduced threshold for depolarisation, consistent with neuronal hyperactivity. This work provides deep molecular insights into the mechanisms driving neuronal dysfunction in the GM2 gangliosidoses with potential relevance more broadly to other lysosomal storage diseases and for late-onset diseases with sphingolipid dysregulation.