Calcium Ions Promote Superoxide Dismutase 1 (SOD1) Aggregation into Non-fibrillar Amyloid: A LINK TO TOXIC EFFECTS OF CALCIUM OVERLOAD IN AMYOTROPHIC LATERAL SCLEROSIS (ALS)?* |
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Authors: | Sónia S Leal Isabel Cardoso Joan S Valentine Cláudio M Gomes |
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Institution: | From the ‡Instituto Tecnologia Química e Biológica, Universidade Nova de Lisboa, Av. República 127, 2780-756 Oeiras, Portugal.;the §Molecular Neurobiology Unit, Instituto Biologia Molecular e Celular, 4150-180 Porto, Portugal, and ;the ¶Department of Chemistry and Biochemistry, UCLA, Los Angeles, California 90095 |
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Abstract: | Imbalance in metal ion homeostasis is a hallmark in neurodegenerative conditions involving protein deposition, and amyotrophic lateral sclerosis (ALS) is no exception. In particular, Ca2+ dysregulation has been shown to correlate with superoxide dismutase-1 (SOD1) aggregation in a cellular model of ALS. Here we present evidence that SOD1 aggregation is enhanced and modulated by Ca2+. We show that at physiological pH, Ca2+ induces conformational changes that increase SOD1 β-sheet content, as probed by far UV CD and attenuated total reflectance-FTIR, and enhances SOD1 hydrophobicity, as probed by ANS fluorescence emission. Moreover, dynamic light scattering analysis showed that Ca2+ boosts the onset of SOD1 aggregation. In agreement, Ca2+ decreases SOD1 critical concentration and nucleation time during aggregation kinetics, as evidenced by thioflavin T fluorescence emission. Attenuated total reflectance FTIR analysis showed that Ca2+ induced aggregates consisting preferentially of antiparallel β-sheets, thus suggesting a modulation effect on the aggregation pathway. Transmission electron microscopy and analysis with conformational anti-fibril and anti-oligomer antibodies showed that oligomers and amyloidogenic aggregates constitute the prevalent morphology of Ca2+-induced aggregates, thus indicating that Ca2+ diverts SOD1 aggregation from fibrils toward amorphous aggregates. Interestingly, the same heterogeneity of conformations is found in ALS-derived protein inclusions. We thus hypothesize that transient variations and dysregulation of cellular Ca2+ levels contribute to the formation of SOD1 aggregates in ALS patients. In this scenario, Ca2+ may be considered as a pathogenic effector in the formation of ALS proteinaceous inclusions. |
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Keywords: | Amyloid Amyotrophic Lateral Sclerosis (Lou Gehrig''s Disease) Biophysics Calcium Circular Dichroism (CD) Electron Microscopy (EM) Infrared Spectroscopy Neurodegenerative Diseases Protein Aggregation Superoxide Dismutase (SOD) |
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