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J. Biol. Chem., Vol. 282, Issue 36, 26235-26244, September 7, 2007

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Mitochondrial Dysfunction and Dendritic Beading during Neuronal Toxicity^*

From the Neurosciences Institute, Ninewells Medical School, University of Dundee, Dundee DD19SY, Scotland, United Kingdom

Mitochondrial dysfunction (depolarization and structural collapse), cytosolic ATP depletion, and neuritic beading are early hallmarks of neuronal toxicity induced in a variety of pathological conditions. We show that, following global exposure to glutamate, mitochondrial changes are spatially and temporally coincident with dendritic bead formation. During oxygen-glucose deprivation, mitochondrial depolarization precedes mitochondrial collapse, which in turn is followed by dendritic beading. These events travel as a wave of activity from distal dendrites toward the neuronal cell body. Despite the spatiotemporal relationship between dysfunctional mitochondria and dendritic beads, mitochondrial depolarization and cytoplasmic ATP depletion do not trigger these events. However, mitochondrial dysfunction increases neuronal vulnerability to these morphological changes during normal physiological activity. Our findings support a mechanism whereby, during glutamate excitotoxicity, Ca²⁺ influx leads to mitochondrial depolarization, whereas Na⁺ influx leads to an unsustainable increase in ATP demand (Na⁺,K⁺-ATPase activity). This leads to a drop in ATP levels, an accumulation of intracellular Na⁺ ions, and the subsequent influx of water, leading to microtubule depolymerization, mitochondrial collapse, and dendritic beading. Following the removal of a glutamate challenge, dendritic recovery is dependent upon the integrity of the mitochondrial membrane potential, but not on a resumption of ATP synthesis or Na⁺,K⁺-ATPase activity. Thus, dendritic recovery is not a passive reversal of the events that induce dendritic beading. These findings suggest that the degree of calcium influx and mitochondrial depolarization inflicted by a neurotoxic challenge, determines the ability of the neuron to recover its normal morphology.

Received for publication, May 31, 2007 , and in revised form, July 5, 2007.

^* This work was supported by Biotechnology and Biological Sciences Research Council Grant 94/C17336, Tenovus Scotland, and the Anonymous Trust (to C. N. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S3 and Movies 1–3.

¹ To whom correspondence should be addressed. Tel.: 44-1382-632527; Fax: 44-1382-667120; E-mail: c.n.connolly{at}dundee.ac.uk.

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