Structural Comparison of the Human G93A Mutant SOD1 to the Wild-type SOD1 Filaments

Area of Science:

• Neuroscience
• Biochemistry
• Structural Biology

Background:

• Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease.
• Protein misfolding and aggregation, particularly of Cu, Zn-superoxide dismutase (SOD1), are key pathological hallmarks.
• The G93A mutation in SOD1 is strongly associated with familial ALS and is studied in mouse models.

Purpose of the Study:

• To investigate the filament structure of G93A mutant SOD1.
• To compare filament formation pathways between G93A mutant SOD1 and wild-type (WT) SOD1.
• To elucidate molecular mechanisms contributing to ALS pathogenesis.

Main Methods:

• Cryo-electron microscopy to determine fibril structure.
• Self- and cross-seeding experiments to assess filament formation.
• Proteolysis susceptibility assays and mass spectrometry to analyze protein structure and intermediates.

Main Results:

• G93A mutant SOD1 fibrils formed single protofilaments with a left-handed helical twist, similar to WT SOD1.
• Seeding experiments enhanced filament formation for both WT and G93A mutant SOD1.
• G93A mutant SOD1 showed increased proteolysis susceptibility; mass spectrometry identified a disordered electrostatic loop as a common intermediate.

Conclusions:

• Shared filament formation pathways exist for both WT and G93A mutant SOD1.
• These findings provide novel insights into the molecular basis of ALS.
• The structurally disordered electrostatic loop is a critical common intermediate in SOD1 aggregation.