The Department of Biomedicine

BBB seminar: Werner J. H. Koopman

Quantitative live cell imaging of mitochondrial morphology and function

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Werner J. H. Koopman
Department of Biochemistry, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands

Mitochondria are critically involved in cell cycle regulation, apoptosis, calcium signaling, organismal development, immune responses and dynamic modulation of respiratory capacity. In this way, the relevance of mitochondria for proper cell function by far exceeds their role as ATP generators. This places mitochondrial (dys)function not only at the heart of normal cell function but also implicates this organelle in the etiology of human disorders including diabetes, genetic oxidative phosphorylation defects, and neurodegenerative disorders such as certain forms of Parkinson and Alzheimer's disease.

Mitochondrial function is directly related to mitochondrial shape and position within the cell. Our research aims to understand the interconnection between mitochondrial dynamics and function at the molecular and live cell level. As a model system we use dermal fibroblasts and cancer cell lines in which a deficiency in mitochondrial complex I (CI) is chemically induced as well as cells of children (patients) with inherited CI deficiency. During the last years we combined confocal, video and fluorescence correlation microscopy of chemical and genetically-encoded reporter molecules with image analysis and native electrophoresis to establish the quantitative relationship between CI assembly/activity, mitochondrial dynamics, reactive oxygen species (ROS), mitochondrial membrane potential (Δψ), redox environment, NADH levels, and
Ca 2+ /ATP homeostasis.

It was found that cells with CI deficiency displayed aberrant cytosolic and mitochondrial Ca 2+ /ATP homeostasis. At the morphological level, cells from patients with a very low residual CI activity (class I) contained fragmented mitochondria, whereas in those cells with a moderate reduction in CI activity, mitochondrial shape was normal (class II). Western blot analysis revealed that these distinct morphological phenotypes were associated with altered expression of mitochondrial fission and fusion proteins. Although all cells from patients displayed a reduced amount/activity of CI and increased ROS and NADH levels, these changes were significantly more pronounced in class I cells.

We conclude that increased ROS production, when not counterbalanced by the cells antioxidant defense systems, induces mitochondrial fragmentation. In line with this hypothesis we observed that application of exogenous antioxidants reversed the fragmented phenotype and improved mitochondrial function in cells from patients.

Selected references:

Koopman, W.J.H., Verkaart, S., Visch, H.J., van Emst-de Vries, S.E., Nijtmans, L.G.J., Smeitink, J.A.M., Willems, P.H.G.M. ( 2007 ) Human NADH:ubiquinone oxidoreductase deficiency: radical changes in mitochondrial morphology? Am. J. Physiol. Cell Physiol. 293, C22-C29.

Willems, P.H.G.M., Valsecchi, F., Distelmaier, F., Verkaart, S., Visch, H.J, Smeitink, J.A.M., Koopman, W.J.H. (2008) Mitochondrial Ca2+ homeostasis in human NADH:ubiquinone oxidoreductase deficiency. Cell Calcium. 2008 Jul;44(1):123-33.

Host: Karl Johan Tronstad <karl.tronstad@biomed.uib.no>, Department of Biomedicine