Impaired regulation of mitochondrial dynamics, which shifts the balance towards fission, is associated with neuronal death in age-related neurodegenerative diseases, such as Alzheimer's disease or Parkinson's disease. A role for mitochondrial dynamics in acute brain injury, however, has not been elucidated to date. Here, we investigated the role of dynamin-related protein 1 (Drp1), one of the key regulators of mitochondrial fission, in neuronal cell death induced by glutamate toxicity or oxygen-glucose deprivation (OGD) in vitro, and after ischemic brain damage in vivo. Drp1 siRNA and small molecule inhibitors of Drp1 prevented mitochondrial fission, loss of mitochondrial membrane potential (MMP), and cell death induced by glutamate or tBid overexpression in immortalized hippocampal HT-22 neuronal cells. Further, Drp1 inhibitors protected primary neurons against glutamate excitotoxicity and OGD, and reduced the infarct volume in a mouse model of transient focal ischemia. Our data indicate that Drp1 translocation and associated mitochondrial fission are key features preceding the loss of MMP and neuronal cell death. Thus, inhibition of Drp1 is proposed as an efficient strategy of neuroprotection against glutamate toxicity and OGD in vitro and ischemic brain damage in vivo. Mitochondria play crucial roles in energy metabolism, regulation of free radical formation and calcium storage, thereby determining essential metabolic functions and cell survival. 1 Further, mitochondria are highly dynamic organelles that undergo constant fission and fusion and these morphological changes are required for efficient ATP production, calcium buffering, regulation of signal transduction and apoptosis. 2 In neurons, mitochondrial fission is also essential for axonal transport of the organelles into areas of high metabolic demand, 3 whereas mitochondrial fusion supports substitution and regeneration of mitochondrial proteins, mtDNA repair and functional recovery. 2,4 Consistent with the critical roles of mitochondrial dynamics in neurons, defects in mitochondrial fission and fusion proteins are associated with a wide array of inherited or acquired neurodegenerative diseases such as Charcot-Marie-Tooth disease or Alzheimer's disease, respectively. 2 Fission and fusion defects may limit mitochondrial motility, decrease energy production, promote oxidative stress and lead to accumulating of mtDNA defects, thereby promoting neuronal dysfunction and cell death. 1 Recently, enhanced mitochondrial fragmentation was associated with induction of neuronal death triggered by oxidative stress. 5 These data imply that during neuronal cell death, the tubular mitochondrial network is fragmented into smaller and functionally impaired organelles. 5 It is, however, a matter of ongoing controversy, whether mitochondrial fragmentation is cause or consequence in programmed cell death.Current knowledge of the mechanisms regulating mitochondrial dynamics indicates that fission and fusion of mitochondria are under control of highly conserved dynamin-relate...

Glutamate toxicity involves increases in intracellular calcium levels and enhanced formation of reactive oxygen species (ROS) causing neuronal dysfunction and death in acute and chronic neurodegenerative disorders. The molecular mechanisms mediating glutamate-induced ROS formation are, however, still poorly defined. Using a model system that lacks glutamateoperated calcium channels, we demonstrate that glutamate-induced acceleration of ROS levels occurs in two steps and is initiated by lipoxygenases (LOXs) and then significantly accelerated through Bid-dependent mitochondrial damage. The Bid-mediated secondary boost of ROS formation downstream of LOX activity further involves mitochondrial fragmentation and release of mitochondrial apoptosis-inducing factor (AIF) to the nucleus. These data imply that the activation of Bid is an essential step in amplifying glutamate-induced formation of lipid peroxides to irreversible mitochondrial damage associated with further enhanced free radical formation and AIF-dependent execution of cell death. Cell Death and Differentiation (2011) 18, 282-292; doi:10.1038/cdd.2010.92; published online 6 August 2010Glutamate toxicity is a well-established cause for neuronal dysfunction and cell death in many acute and chronic neurological diseases. For example, increases in extracellular glutamate levels after acute brain damage by ischemic stroke, epilepsy or brain trauma may reach millimolar concentrations and induce massive Ca 2 þ influx and excitotoxic damage through activation of glutamate receptors such as N-methyl-D-aspartic acid (NMDA) receptors or a-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/kainate receptors. 1,2 The initial increase in intracellular Ca 2 þ levels after stimulation of these glutamate receptors is rather short and the following molecular mechanisms of glutamate excitotoxicity in neurons are poorly defined. While inhibition of the glutamate-induced Ca 2 þ influx by NMDA-receptor antagonists or antagonists of AMPA/kainate receptors protected neurons from glutamate excitotoxicity in experimental settings, the therapeutic time window of such neuroprotective effects is limited in vivo, and (post-)treatment strategies with glutamate receptor antagonists have failed in clinical studies to date. 3,4 Therefore, understanding the mechanisms of glutamate toxicity beyond the initial stimulation of Ca 2 þ influx is of utmost importance to provide efficient strategies of neuroprotection by targeting sustained mechanisms of glutamate-induced neuronal cell death. Such mechanisms include, for example, increased formation of reactive oxygen species (ROS), the activation of apoptosis-related death signaling, mitochondrial damage and DNA degradation.In particular, oxidative stress has been considered to cause neuronal dysfunction and cell death triggered by glutamate after acute brain injury and in age-related chronic neurodegenerative diseases. Therefore, recent research has focused on better understanding ROS formation and dissecting ROS-triggered neuronal death s...

No abstract

Background: SK2 channels modulate NMDA-dependent neuronal excitability and provide neuroprotection against excitotoxicity. Results: We identify mito SK2/K Ca 2.2 channels in neuronal mitochondria and demonstrate their protective function in cells lacking NMDAR. Conclusion: SK2 channels prevent mitochondrial dysfunction and completely restore cell viability independently of NMDAR modulation. Significance: Understanding how mitochondrial SK2 channels operate is crucial to develop novel therapeutic strategies for diseases caused by mitochondrial demise.

No abstract