Research
In humans, as in most animal cells, genetic information is housed not only in the nucleus, but also in mitochondria (see Figure 1). Mitochondrial DNA (mtDNA) encodes thirteen essential proteins of the oxidative phosphorylation complexes as well as 22 tRNAs and 2 rRNAs required to translate these thirteen mRNAs in the mitochondrial matrix. Mutations in mtDNA cause maternally inherited neuromuscular disorders due to declines in cellular energy metabolism. In addition, mtDNA mutations accumulate in normal aging tissues, certain tumors, and have been implicated in late-onset diseases such a Alzheimer's, Parkinson's, and diabetes, indicating that the pathology of dysfunctional mitochondria is only beginning to be unraveled. The research in my laboratory is directed toward understanding the mechanism of gene expression in human mitochondria and its impact on human aging and disease. The ultimate goal is to understand the full impact of dysfunctional mitochondrial gene expression on human health and use this information to design specific interventions to treat mitochondria-based disease and age-related pathology.
Specifically, we focus on nucleus-encoded factors that are imported into the organelle to regulate transcription, translation, replication, and maintenance of mtDNA. We are also concerned with signaling pathways that connect the nuclear and mitochondrial genomes to coordinate gene expession patterns in both compartments. We use multiple approaches to this problem including the employment of mouse and yeast (S. cerevisiae) genetic model systems, biochemical characterization of mitochondrial transcription events and interactions, and in vivo approaches in cultured mammalian cells.
Major Ongoing Projects:
1. Nuclear Control of Mitochondrial Gene Expression. This project involves 1) the characterization of human mitochondrial transcription factors and complexes in vivo and in vitro. This involves the study of two dual-function transcription factors/rRNA methyltransferases (h-mtTFB1 and mtTFB2) that play critical roles in mtDNA transcription and replication, mitochondrial ribosome biogenesis, and overall mitochondrial function and biogenesis; 2) charcterization of the functional significance of the direct binding of mitochondrial ribosomal protein L12 to the mitochondrial RNA polymerase (POLRMT); 3) the role of mitochondrial 12S rRNA methylation in maternally inherited deafness (due to the A1555G mtDNA mutation); and 4) generation of mouse gene knock-outs to probe in vivo functions and tissue-specific pathology associated with loss of mtDNA regulation.
2. Mitochondrial Dysfunction and Oxidative Stress in Ataxia-Telangiectasia. We recently discovery that the Ataxia-Telangiectasia Mutated (ATM) checkpoint signaling pathway and its key downstream target ribonucleotide reductase (RNR) are involved in maintaining proper mtDNA copy number and stability in mammalian cells and tissues. Mutations in the ATM kinase cause the multi-faceted disease Ataxia-Telangiectasia (A-T), a key feature of which is oxidative stress in affected tissues. Our preliminary results show that disrupted ATM signaling results in aberrant expression of RNR subunits, improperly regulated mtDNA copy number, increased mtDNA mutagenesis, and ROS accumulation, leading us to hypothesize that mitochondrial dysfunction contributes to the oxidative stress-associated pathology of A-T. This project will dissect mechanistically how disruptions in this pathway affect mitochondrial function in cultured cell and mouse models of the disease, with the goal of identifying potential new therapeutic strategies for A-T pathology.
3. The Role of the Target of Rapamycin (TOR) Pathway in Regulating Mitochondrial Gene Expression, Respiration, and Life Span. Here we are continuing to exploit the budding yeast genetic model system to follow up mechanistically our recent discovery that reduced TOR signaling results in increased mitochondrial translation and respiration, and extension of chronological lifespan.
4. Epigenetic Regulation of the SDHD Gene, Encoding a Subunit of Mitochondrial OXPHOS Complex II. Mutations in the SDHD gene cause hereditary paraganglioma (PGL), a disease characterized by vascularized tumors in the head and neck (commonly in the carotid body, an organ that senses blood oxygen levels). Inheritance of this disease is only observed when mutant SDHD alleles are present on the paternal chromosome, suggesting a unique allele- and tissue-specific form of gene silencing is at play. Defining this imprinting mechanism is the goal of this project.
5. Other Projects. Other projects include
1) the role of the mammalian Pif1 helicase in mtDNA replication and stability;
2) the contribution of mtDNA stability and mitochondrial ROS to intestinal
tumorigenesis, and 3) development of methods to transform mammalian mitochondrial
with exogenous mtDNA to generate mouse models of mtDNA-based disease pathology.
-top-
People
Left to right- Taylor LaFlam, Maria Lebedeva, Justin Cotney, Anthony
D'Souza, Gerald Shadel, Marc Chatenay-Lapointe, Linda Alila, Sharen McKay,
Yong Pan, Tim Shutt
not pictured: Tom Gilliland, Bora Baysal, Lauren Dunn,
Czestochowa Francois
Shadel Lab Party April 2008
-top-
Publications
Recent Publications:
Bonawitz ND, Chatenay-Lapointe M, Pan Y, Shadel GS. (2007) Reduced TOR Signaling Extends Chronological Life Span via Increased Respiration and Upregulation of Mitochondrial Gene Expression. Cell Metabolism 5:265-77. [see additional commentary by Bonawitz ND and Shadel GS. (2007) Rethinking the mitochondrial theory of aging: the role of mitochondrial gene expression in lifespan determination. Cell Cycle 6:1574-1578].
Eaton, JS et al. (2007) Ataxia-Telangiectasia Mutated kinase regulates ribonucleotide reductase and mitochondrial homeostasis. Journal of Clinical Investigation. 117:2723-2734.
Cotney J, Wang Z, Shadel GS. (2007) Relative abundance of the human mitochondrial transcription system and distinct roles for h-mtTFB1 and h-mtTFB2 in mitochondrial biogenesis and gene expression. Nucleic Acids Res. 2007;35(12):4042-4054.
Wang Z, Cotney J, Shadel GS. (2007) Human mitochondrial ribosomal protein MRPL12 interacts directly with mitochondrial RNA polymerase to modulate mitochondrial gene expression. Journal of Bioliological Chemistry 282(17):12610-12618.
Bonawitz ND, Rodeheffer MS, Shadel GS. (2006) Defective mitochondrial gene expression results in reactive oxygen species-mediated inhibition of respiration and reduction of yeast life span. Molecular & Cellular Biology 26:4818-29.
McCulloch V & Shadel GS. (2003) Human mitochondrial transcription factor B interacts with the C-terminal activation region of h-mtTFA and stimulates transcription independently of its RNA methyltransferase activity. Molecular & Cellular Biology 23:5816-5824.
Seidel-Rogol BL, McCulloch V, Shadel GS (2003) Human mitochondrial transcription factor B1 methylates ribosomal RNA at a conserved stem-loop. Nature Genetics 33: 23-4.
Reviews and Commentaries: Bonawitz ND, Clayton DA, Shadel (2006) Initiation and beyond: multiple functions of the human mitochondrial transcription machinery. Molecular Cell 24:813-25. Shadel, GS (2005) "Mitochondrial DNA, Aconitase wraps it up" Trends in Biochemical Sciences 30:294-296. Shadel GS (2004) "A dual-function mitochondrial transcription factor tunes out deafness" Molecular Genetics and Metabolism. 82:1-3. Shadel GS. (2004) Coupling the mitochondrial transcription machinery to human disease. Trends Genet. 2004 Oct;20(10):513-9. Shadel GS and Clayton DA. (1997) "Mitochondrial DNA maintenance in vertebrates" Annual Review of Biochemistry 66:409-35.
Search PubMed Database for Gerald Shadel
-top-