Toxic Role of K⁺Channel Oxidation in Mammalian Brain

Abstract

Potassium (K ϩ ) channels are essential to neuronal signaling and survival. Here we show that these proteins are targets of reactive oxygen species in mammalian brain and that their oxidation contributes to neuropathy. Thus, the KCNB1 (Kv2.1) channel, which is abundantly expressed in cortex and hippocampus, formed oligomers upon exposure to oxidizing agents. These oligomers were ϳ10-fold more abundant in the brain of old than young mice. Oxidant-induced oligomerization of wild-type KCNB1 enhanced apoptosis in neuronal cells subject to oxidative insults. Consequently, a KCNB1 variant resistant to oxidation, obtained by mutating a conserved cysteine to alanine, (C73A), was neuroprotective. The fact that oxidation of KCNB1 is toxic, argues that this mechanism may contribute to neuropathy in conditions characterized by high levels of oxidative stress, such as Alzheimer's disease (AD). Accordingly, oxidation of KCNB1 channels was exacerbated in the brain of a triple transgenic mouse model of AD (3xTg-AD). The C73A variant protected neuronal cells from apoptosis induced by incubation with ␤-amyloid peptide (A␤ 1-42 ). In an invertebrate model (Caenorhabditis elegans) that mimics aspects of AD, a C73A-KCNB1 homolog (C113S-KVS-1) protected specific neurons from apoptotic death induced by ectopic expression of human A␤ 1-42 . Together, these data underscore a novel mechanism of toxicity in neurodegenerative disease. IntroductionPotassium (K ϩ ) channels are a diverse and ubiquitous family of ion-channels that operate in nonexcitable and excitable cells. K ϩ channels are essential for the function of the nervous system where they modulate the shape and frequency of action potentials and consequently, neurotransmitter release. As such, conditions that result in the impairment of K ϩ channels have the potential to cause neuronal dysfunction by affecting the excitability of the cell, which is intrinsically dependent on these proteins. For example, mutations in KCNQ2, KCNQ3, KCNMA1, KCNA1 and KCNC3 genes which lead to the synthesis of mutants with defective properties have been identified in patients affected by epilepsy, ataxia and episodic ataxia type 1 (for review, see Kullmann, 2002). In addition to genetic predisposition, acquired impairment of K ϩ channel's function can lead to neurological diseases, such as, acquired neuromyotonia, limbic encephalitis, and Morvan's syndrome (for review, see Vernino, 2007).Oxygen metabolism leads to the synthesis of reactive and thus potentially toxic molecules known as reactive oxygen species (ROS). The increase in unbuffered ROS levels in a cell, a phenomenon commonly referred to as oxidative stress, is thought to cause significant cellular damage and to contribute to cellular aging (Harman, 1956). Moreover, oxidative stress is elevated in neuropathies such as Alzheimer's disease (AD) (for review, see Lin and Beal, 2006). Oxidative damage occurs early in AD, before the onset of significant plaque formation (Nunomura et al., 2001;Praticò et al., 2001;Reddy et al., 2004), and consequent...

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