Enhancing Mitochondrial Calcium Buffering Capacity Reduces Aggregation of Misfolded SOD1 and Motor Neuron Cell Death without Extending Survival in Mouse Models of Inherited Amyotrophic Lateral Sclerosis

Abstract

Mitochondria have been proposed as targets for toxicity in amyotrophic lateral sclerosis (ALS), a progressive, fatal adult-onset neurodegenerative disorder characterized by the selective loss of motor neurons. A decrease in the capacity of spinal cord mitochondria to buffer calcium (Ca2+) has been observed in mice expressing ALS linked mutants of SOD1 that develop motor neuron disease with many of the key pathological hallmarks seen in ALS patients. In mice expressing three different ALS-causing SOD1 mutants, we now test the contribution of the loss of mitochondrial Ca2+ buffering capacity to disease mechanism(s) by eliminating ubiquitous expression of cyclophilin D, a critical regulator of Ca2+ mediated opening of the mitochondrial permeability transition pore (mPTP) that determines mitochondrial calcium content. A chronic increase in mitochondrial buffering of Ca2+ in the absence of cyclophilin D was maintained throughout disease course and was associated with improved mitochondrial ATP synthesis, reduced mitochondrial swelling, and retention of normal morphology. This was accompanied by an attenuation of glial activation, reduction in levels of misfolded SOD1 aggregates in the spinal cord, and a significant suppression of motor neuron death throughout disease. Despite this, muscle denervation, motor axon degeneration, and disease progression and survival were unaffected, thereby eliminating mutant SOD1-mediated loss of mitochondrial Ca2+ buffering capacity, altered mitochondrial morphology, motor neuron death, and misfolded SOD1 aggregates, as primary contributors to disease mechanism for fatal paralysis in these models of familial ALS.

Translational profiling identifies a cascade of damage initiated in motor neurons and spreading to glia in mutant SOD1-mediated ALS

“…Approximately 10% of all of the gene changes are in this category, consistent with synapse dysfunction accompanying the known denervation that occurs as one of the earliest detected changes in this mouse line (34). Up-regulated gene changes were enriched in genes involved in metabolism, with notable perturbation in the alanine, aspartate, and glutamate metabolism pathways (marked with red stars and red shade in SI Appendix, Fig.…”

Endogenous macrophage migration inhibitory factor reduces the accumulation and toxicity of misfolded SOD1 in a mouse model of ALS

“…Of note, a previous study has shown that reducing the levels of aggregated misfolded SOD1 by deleting cyclophilin D does not ameliorate the pathogenesis of ALS in mutant SOD1 mouse models (32). Therefore, SOD1 toxicity in vivo appears to derive from the soluble form of the misfolded SOD1, rather than from its highly aggregated form.…”

F-ATPase of Drosophila melanogaster Forms 53-Picosiemen (53-pS) Channels Responsible for Mitochondrial Ca2+-induced Ca2+ Release

“…Prolonged openings cause mitochondrial depolarization, osmotic swelling, outer mitochondrial membrane rupture, and release of apoptogenic proteins such as cytochrome c; transient openings, on the other hand, may be involved in physiological Ca 2ϩ homeostasis and may protect mitochondria from Ca 2ϩ overload (10,11), as indicated by studies in isolated cardiomyocytes, CyPD knock-out mice, adult cortical neurons, and spinal chord mitochondria (51)(52)(53)(54). It is tempting to speculate that the different PTP conductances observed in mammals, Drosophila, and yeast underscore different physiological functions.…”