Tissue-specific differences in mitochondrial activity and biogenesis Erika Fernández-Vizarra a,1 , José A. Enríquez a,b , Acisclo Pérez-Martos a , Julio Montoya a,c , Patricio Fernández-Silva a, ⁎ a Departamento de Bioquímica y Biología Molecular y Celular, Universidad de Zaragoza, Pedro Cerbuna, 12. 50009 Zaragoza, Spain b Centro Nacional de Investigaciones Cardiovasculares Carlos III, Melchor Fernández Almagro, 3. 28029 Madrid, Spain c CIBER de Enfermedades Raras (CIBERER), Spain abstract article info Article history: Received 5 May 2010 Received in revised form 3 August 2010 Accepted 28 September 2010 Available online 7 October 2010 Keywords: OXPHOS capacity mtDNA Tissue-specificity Copy number Each cell type develops and maintains a specific oxidative phosphorylation (OXPHOS) capacity to satisfy its metabolic and energetic demands. This implies that there are differences between tissues in mitochondrial number, function, protein composition and morphology. The OXPHOS system biogenesis requires the coordinated expression of both mitochondrial and nuclear genomes. Mitochondrial DNA (mtDNA) expression can be regulated at different levels (replication, transcription, translation and post-translational levels) to contribute to the final observed OXPHOS activities. By analyzing five mammalian tissues, we evaluated the differences in the cellular amount of mtDNA and its correlation with the final observed mitochondrial activity. © 2010 Elsevier B.V. and Mitochondria Research Society. All rights reserved. 1. Introduction Most of the ATP synthesized in eukaryotic cells relying on aerobic metabolism is produced in the mitochondria by the process called oxidative phosphorylation (OXPHOS). Each animal organ or tissue has a different metabolic profile and variable energetic demands due to the inherently different functions or, in a same tissue, to changes in ATP demand due to physiological or pathological conditions (Kunz, 2003; Leary et al., 1998; Leverve and Fontaine, 2001; Pfeiffer et al., 2001). Thus, it is reasonable to expect that their mitochondrial respiratory capacity would also be different and related to variations between tissues in mitochondrial function, protein composition and morphology (Benard et al., 2006; Johnson et al., 2007a,b; Mootha et al., 2003; Pagliarini et al., 2008). These differences are probably reflecting the different metabolic roles each tissue plays in the organism and the contribution of OXPHOS to the main metabolic pathways, namely, cardiomyocyte mitochondria are specialized in ATP production mostly by fatty-acid oxidation, in liver organelles several aspects of the urea cycle are enhanced, kidney seems to rely on the metabolism of some amino acids and brain is specialized in neurotransmitter metabolism and reactive oxygen species (ROS) detoxification (Johnson et al., 2007a). It is important to define the tissue-specific characteristics of the OXPHOS capacity as a contribu- tion to the understanding of why some tissues are more affected than others in mitochondrial disorders due to a primary malfunction of the OXPHOS system (Kunz, 2003; Rossignol et al., 1999). The aforemen- tioned system is unique in the cell as its biogenesis requires the coordinated expression of both mitochondrial and nuclear genomes. Different nuclear-encoded factors are required for the maintenance and expression of the mitochondrial DNA (mtDNA) (Falkenberg et al., 2007; Spremulli et al., 2004) so the regulation of their expression will in turn influence that of the mtDNA. The regulation at the transcriptional level of some nuclear factors needed for mitochondrial biogenesis has been well described (Scarpulla, 2008). Moreover, mtDNA expression is regulated at different levels to significantly contribute to the finally observed OXPHOS activities (Fernandez- Vizarra et al., 2008). While it is generally accepted that there are differences between tissues in respiratory capacity and mtDNA content in mammalian tissues, the extent and entity of these differences is not clear, as the published data are confusing due to the heterogeneous measuring methods and normalization criteria (Gadaleta et al., 1992; Gagnon et al., 1991; Van den Bogert et al., 1992, 1993; Veltri et al., 1990; Wiesner et al., 1992). Therefore, the aim of this study was to evaluate the importance and entity of the differences in OXPHOS capacity and mtDNA levels, between five mammalian tissues with different functions and metabolic profiles in a reliable and adequate manner. We have used a very straightforward procedure intended to avoid loss of activities and to allow an easy comparison among the samples. To establish the comparisons, first, cytochrome c oxidase (COX) and citrate synthase (CS) activities were measured in brain, heart, liver, skeletal muscle and kidney total homogenates from male and female Mitochondrion 11 (2011) 207–213 ⁎ Corresponding author. Tel.: + 34 976761285; fax: + 34 976762123. E-mail address: pfsilva@unizar.es (P. Fernández-Silva). 1 Present address: Unidad de Investigación Traslacional, Instituto Aragonés de Ciencias de la Salud, Hospital Universitario Miguel Servet, Pº Isabel la Católica, 1-3. 50009 Zaragoza, Spain. 1567-7249/$ – see front matter © 2010 Elsevier B.V. and Mitochondria Research Society. All rights reserved. doi:10.1016/j.mito.2010.09.011 Contents lists available at ScienceDirect Mitochondrion journal homepage: www.elsevier.com/locate/mito