Inhibition of Chaperone Activity Is a Shared Property of Several Cu,Zn-Superoxide Dismutase Mutants That Cause Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive motor neuron degeneration, paralysis, and death. Mutant Cu,Zn-superoxide dismutase (SOD1) causes a subset of ALS by an unidentified toxic property. Increasing evidence suggests that chaperone dysfunction plays a role in motor neuron degeneration in ALS. To investigate the relationship between mutant SOD1 expression and chaperone dysfunction, we measured chaperone function in central nervous system tissue lysates from normal mice and transgenic mice expressing human SOD1 variants. We observed a significant decrease in chaperone activity in tissues from mice expressing ALS-linked mutant SOD1 but not control mice expressing human wild type SOD1. This decrease was detected only in the spinal cord, became apparent by 60 days of age (before the onset of muscle weakness and significant motor neuron loss), and persisted throughout the late stages. In addition, this impairment of chaperone activity occurred only in cytosolic but not in mitochondrial and nuclear fractions. Furthermore, multiple recombinant human SOD1 mutants with differing biochemical and biophysical properties inhibited chaperone function in a cell-free extract of normal mouse spinal cords. Thus, mutant SOD1 proteins may impair chaperone function independent of gene expression in vivo, and this inhibition may be a shared property of ALS-linked mutant SOD1 proteins.

Mitochondrial electron transport chain complex dysfunction in a transgenic mouse model for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease that causes degeneration of motoneurons. Mutation of Cu,Zn superoxide dismutase (SOD1) is one cause for this disease. In mice, expression of mutant protein causes motoneuron degeneration and paralysis resembling the human disease. Morphological change, indicative of mitochondrial damage, occurs at early stages of the disease. To determine whether mitochondrial function changes during the course of disease progression, enzyme activities of mitochondrial electron transport chain in spinal cords from mice at different disease stages were measured using three different methods: spectrophotometric assay, in situ histochemical enzyme assay, and blue native gel electrophoresis combined with in-gel histochemical reaction. The enzyme activities were decreased in the spinal cord, particularly in the ventral horn, beginning at early disease stages. This decrease persisted throughout the course of disease progression. This decrease was not detected in the spinal cords of non-transgenic animals, of mice expressing the wild-type protein, and in cerebellum and dorsal horn of the spinal cords from mice expressing mutant protein. These results demonstrate a functional defect in mitochondria in the ventral horn region and support the view that mitochondrial damage plays a role in mutant SOD1-induced motoneuron degeneration pathway.

A quantitative histochemical assay for activities of mitochondrial electron transport chain complexes in mouse spinal cord sections

Mitochondrial dysfunction and degeneration are associated with neurodegenerative disorders. A dysfunctional mitochondrial electron transport chain (ETC) impairs ATP production and accelerates the generation of free radicals. To quantify ETC activity, solution-spectrophotometric assays and histochemical reactions on blue native polyacrylamide gel electrophoresis (BN-PAGE) gels have been used. These methods, however, do not provide information regarding mitochondrial ETC activities associated with specific regions in the central nervous system (CNS). Because neurodegenerative diseases often strike a specific subset of neurons within specific regions in the CNS, reliable methods for quantifying mitochondrial ETC activities in selected CNS regions are needed. We have studied the quantitative range of in situ histochemical assays for ETC complex I, II and IV and determined the optimal conditions for quantification of these ETC complex activities. We also demonstrate that these assays can detect a decrease in mitochondrial ETC activities in the ventral horn of spinal cords isolated from a transgenic mouse model for amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease.

Mutant Cu, Zn superoxide dismutase that causes motoneuron degeneration is present in mitochondria in the CNS

Mutations in Cu, Zn superoxide dismutase (SOD1) cause a fraction of amyotrophic lateral sclerosis (ALS), which involves motoneuron degeneration, paralysis, and death. An acquired activity by mutant SOD1 is responsible for the cellular toxicity, but how mutant SOD1 kills motoneurons is unclear. In transgenic mouse models of ALS, mitochondrial degeneration occurs early, before disease onset, raising the question of how mutant SOD1 damages mitochondria. Here we investigate the intracellular localization of SOD1 in the CNS to determine whether SOD1 is present in mitochondria, where it could directly damage this organelle. We show that endogenous mouse SOD1, wild-type human, and mutant human SOD1 (G93A), when expressed as transgenes, are colocalized with mitochondria in spinal cord by immunofluorescence confocal microscopy. By immunoelectron microscopy, we show that SOD1 is present within mitochondria at similar concentrations as in the cytoplasm. Thus SOD1, in addition to being a cytosolic enzyme, is present inside mitochondria in the CNS.