title: Compensatory responses of protein import and transcription factor expression in mitochondrial DNA defects.
abstract: Defects in mitochondrial DNA (mtDNA) evoke distinctive responses in the nuclear genome, leading to altered mitochondrial biogenesis. We used C(2)C(12) cells depleted of mtDNA (rho(-) cells) and fibroblasts from a mitochondrial encephalopathy, lactic acidosis, and strokelike episodes (MELAS) patient to examine adaptations of the protein import machinery and transcription factors involved in mitochondrial biogenesis. In rho(-) cells, Tom20 and Tim23 protein levels were reduced by 25% and 59%, whereas mtHSP70 was induced by twofold relative to control cells. These changes were accompanied by a 21% increase in enhanced yellow fluorescent protein (EYFP) import into mitochondria in rho(-) cells (P < 0.05). In contrast, in MELAS cells mtHSP70 was elevated by 70%, whereas Tom20 and Tom34 protein levels were increased by 45% and 112% relative to control values. EYFP import was not altered in MELAS cells. In rho(-) cells, protein levels of the transcription factors nuclear respiratory factor-1 (NRF-1) and transcription factor A (Tfam) declined by 33% and 54%, whereas no change was observed for the coactivator peroxisome proliferator receptor-gamma coactivator-1alpha (PGC-1alpha). In contrast, Tfam was increased by 40% in MELAS cells. Rho(-) cells displayed reduced oxygen consumption (Vo(2)) and ATP levels, along with a twofold increase in lactate levels (P < 0.05). In electrically stimulated C(2)C(12) cells, 109%, 78%, 60%, and 67% increases were observed in mtDNA, Vo(2), cytochrome-c oxidase (COX) activity, and Tom34 levels, respectively (P < 0.05). Our findings suggest that compensatory adaptations occurred to maintain normal rates of protein import in response to mtDNA defects and support a role for contractile activity in reducing pathophysiology associated with mtDNA depletion. Because the expression of nuclear-encoded transcription factors and protein import machinery components was dependent on the type of mtDNA defect, these findings suggest involvement of distinct signaling cascades, each dependent on the type of mitochondrial defect, resulting in divergent changes in nuclear gene expression patterns.
authors.affiliation: Department of Biology, York University, Toronto, Canada M3J IP3.
authors.forename: Anna-Maria
authors.initials: AM
authors.lastname: Joseph
authors.forename: Arne A
authors.initials: AA
authors.lastname: Rungi
authors.forename: Brian H
authors.initials: BH
authors.lastname: Robinson
authors.forename: David A
authors.initials: DA
authors.lastname: Hood
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D004272:DNA, Mitochondrial / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000235:genetics; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism; D015870:Gene Expression; D006609:High Mobility Group Proteins; D017241:MELAS Syndrome / Q000378:metabolism* / Q000503:physiopathology*; D008564:Membrane Potentials / Q000502:physiology; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D024101:Mitochondrial Proteins; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000166:cytology / Q000502:physiology*; D032448:Myoblasts, Skeletal / Q000166:cytology / Q000378:metabolism; D051265:NF-E2-Related Factor 1; D009687:Nuclear Proteins; D051266:Nuclear Respiratory Factor 1; D051264:Nuclear Respiratory Factors; D011451:Prostaglandin-Endoperoxide Synthases / Q000378:metabolism; D021381:Protein Transport / Q000502:physiology; D011956:Receptors, Cell Surface / Q000235:genetics / Q000378:metabolism; D015534:Trans-Activators / Q000235:genetics; D014157:Transcription Factors / Q000235:genetics; D029867:Xenopus Proteins
chemical_list: D002352:Carrier Proteins; D004272:DNA, Mitochondrial; D004268:DNA-Binding Proteins; D006609:High Mobility Group Proteins; D026901:Membrane Transport Proteins; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D024101:Mitochondrial Proteins; D051265:NF-E2-Related Factor 1; C495634:Nrf1 protein, mouse; D009687:Nuclear Proteins; D051266:Nuclear Respiratory Factor 1; D051264:Nuclear Respiratory Factors; D011956:Receptors, Cell Surface; C084008:TOMM20 protein, human; C108485:TOMM34 protein, human; C081930:Tfam protein, mouse; C489997:Tomm20 protein, mouse; D015534:Trans-Activators; D014157:Transcription Factors; C097078:XL-MTTFA protein, Xenopus; D029867:Xenopus Proteins; C422562:mitochondrial transcription factor A; D011451:Prostaglandin-Endoperoxide Synthases