Synaptic Mitochondria Are More Susceptible to Ca2+Overload than Nonsynaptic Mitochondria

Abstract

Mitochondria in nerve terminals are subjected to extensive Ca 2؉ fluxes and high energy demands, but the extent to which the synaptic mitochondria buffer Ca 2؉ is unclear. In this study, we identified a difference in the Ca 2؉ clearance ability of nonsynaptic versus synaptic mitochondrial populations enriched from rat cerebral cortex. Mitochondria were isolated using Percoll discontinuous gradients in combination with high pressure nitrogen cell disruption. Mitochondria in the nonsynaptic fraction originate from neurons and other cell types including glia, whereas mitochondria enriched from a synaptosomal fraction are predominantly neuronal and presynaptic in origin. There were no differences in respiration or initial Ca 2؉ loads between nonsynaptic and synaptic mitochondrial populations. Following both bolus and infusion Ca 2؉ addition, nonsynaptic mitochondria were able to accumulate significantly more exogenously added Ca 2؉ than the synaptic mitochondria before undergoing mitochondrial permeability transition, observed as a loss in mitochondrial membrane potential and decreased Ca 2؉ uptake. The limited ability of synaptic mitochondria to accumulate Ca 2؉ could result from several factors including a primary function of ATP production to support the high energy demand of presynaptic terminals, their relative isolation in comparison with the threads or clusters of mitochondria found in the soma of neurons and glia, or the older age and increased exposure to oxidative damage of synaptic versus nonsynaptic mitochondria. By more readily undergoing permeability transition, synaptic mitochondria may initiate neuron death in response to insults that elevate synaptic levels of intracellular Ca 2؉ , consistent with the early degeneration of distal axon segments in neurodegenerative disorders.Mitochondria are important regulators of cellular Ca 2ϩ homeostasis, producers of ATP via oxidative phosphorylation, and regulators of cell death pathways (for reviews see Refs. 1 and 2). Mitochondria assist in maintaining Ca 2ϩ homeostasis by sequestering and releasing Ca 2ϩ (2-4). Normal Ca 2ϩ cycling occurs by the movement of Ca 2ϩ into mitochondria via the Ca 2ϩ uniporter and slow efflux via the Na ϩ /Ca 2ϩ antiporter or by Na ϩ -independent mechanisms (1, 3). Isolated mitochondria in the presence of phosphate take up Ca 2ϩ to a fixed capacity, in a membrane potential (⌬⌿ m )-dependent fashion (5-7). When the mitochondria become overloaded with Ca 2ϩ , they undergo the cataclysmic mitochondrial permeability transition (mPT) 3 via formation of a nonselective pore that allows solutes of 1500 daltons or smaller to pass through the usually impermeable inner mitochondrial membrane with a resultant rupture of the outer mitochondrial membrane caused by osmotic swelling (2, 8 -12).Previous studies have demonstrated substantial mitochondrial heterogeneity that exists among organs and within the CNS. Nonsynaptic brain mitochondria are more resistant to Ca 2ϩ -induced opening of mPT, assessed by mitochondrial swelling, when compared w...

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