The effect of mitochondrial toxins on cytochrome c oxidase subunit IV isoforms and its consequences on mitochondrial function and neural cell survival
Year: 2010
Type: dissertation
Abstract: Mitochondria play a crucial role in neurodegenerative diseases. Different respiratory chain defects have been identified for Parkinson's disease (PD: complex I), Huntington's disease (HD: complex II) or Alzheimer’s disease (AD: complex IV). Additionally, studies with toxins that mimic neurodegenerative disorders when applied in vivo clearly support the pivotal role of mitochondrial enzyme complexes in neurodegeneration. Thus, I examined the effects of two structurally unrelated mitochondrial toxins on mitochondrial regulation, function and cell survival: 6-hydroxydopamine (6-OHDA) which impairs NADH dehydrogenase (ND; complex I) and 3-nitropropionic acid (NPA) which inhibits succinate dehydrogenase (SDH; complex II). Data shown here indicate that the terminal enzyme of respiratory chain, cytochrome c oxidase (COX), is also involved in the in vitro effects exerted by these two neurotoxins. With respect to the potential role of COX in toxin-induced neural cell death, I found that COX subunit IV isoforms play an essential brain region-specific role for mitochondrial energy metabolism, oxidative stress, and consequently survival of astrocytes under toxic conditions. NPA-treated astrocytes from striatum showed a higher vulnerability than those from cortex and mesencephalon. The lowest ATP level together with the highest reactive oxygen species (ROS) production in striatal astrocytes could explain the highest level of necrotic cell death rates in astrocyte cultures from striatum. The changes in ATP and ROS levels were caused by increased COX IV-2/COX IV-1 ratio due to up-regulated COX isoform IV-2 and down-regulated COX isoform IV-1 expression which is most pronounced in striatal astrocytes. Thus, COX plays a brain region-specific role in cell viability under toxic conditions. The underlying mechanism of this regulatory process involves the binding of ATP to COX subunit VI-1 in the presence of a high ATP/ADP ratio, thereby causing an allosteric inhibition of COX. This mechanism is abolished by the expression of COX isoform IV-2 (Singh et al., 2009). Overall, my in vitro observations support the concept of a selective vulnerability of striatal astrocytes in HD, which is, at least in part, based on increased COX IV-2 transcription accompanied by elevated ROS levels and increased necrosis rates. In the second part, I analyzed the possible role of nucleus-encoded COX subunit IV isoforms and mitochondria-encoded catalytic subunits of MRC complexes for the survival of mesencephalic female and male neurons during 6-OHDA intoxication. I found that female neurons are protected better than male cells from apoptotic and necrotic cell death. This protection may be explained by a less severe down-regulation of catalytic subunits of complex I and V accompanied by higher ATP level and lower ROS production in female when compared to male neurons. I show that estrogen (E) counteracts 6-OHDA-induced increase of ROS levels thereby decreasing cell death rates of mesencephalic neurons. The data presented herein support the hypothesis that mitochondrial toxins lead to increased ROS production through changes of MRC function, thereby contributing to mitochondrial toxin-induced cell death. Further exploration of the postulated regulatory role of COX subunit IV and its isoforms during pathological conditions can provide development of new therapeutical strategies that may target basic mitochondrial processes such as energy production and ROS generation. Numerous studies demonstrate the direct or indirect actions of mutated proteins which are present in neurodegenerative diseases such as mutated huntingtin (Htt) or alpha-synuclein on mitochondrial structure and function. Focus of future research on the influence of pathogenic proteins on MRC enzymes can provide additional information on the processes occurring during neurodegeneration. Therefore, we think, that based on our in vitro studies, the role of COX in the regulation of mitochondria during neurodegeneration should be verified in animal models with particular emphasis on brain region- and gender-specificity.
Source: RWTH Publications (RWTH Aachen)
Authors: Magdalena Misiak, Susanne Arnold
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2588616702