Senior Scholar Award in Aging
Douglas C. Wallace, Ph.D.
University of California - Irvine

Mitochondrial Aging in the Chimpanzee
Evidence continues to accumulate that aging is associated with a decline in mitochondrial function, suggesting that mitochondrial dysfunction maybe a major factor in the pathophysiology of aging and senescence. The mitochondria provide most of the cellular energy through oxidative phosphorylation (OXPHOS), and generate most of the endogenous oxygen radicals (reactive oxygen species, ROS) as a toxic by-product. The mitochondria are also the primary regulators of apoptosis, which is initiated by the opening of the mitochondrial permeability transition pore (mtPTP). This releases cytochrome c, procaspases, and the apoptosis initiating factor (AIF) from the mitochondrial inter-membrane space, causing degradation of cytosolic proteins and chromosomal DNA. The opening of the mtPTP is stimulated by a decline in mitochondrial energy and an increase in mitochondrial oxidative stress.

The mitochondria are assembled from the genes of both the nuclear DNA (nDNA) and the mitochondrial DNA (mtDNA). The mtDNA codes for 13 proteins essential for OXPHOS as well as the structural RNAs for mitochondrial protein synthesis. In humans, maternally-inherited mtDNA disease mutations cause many of the same symptoms as seen in aging, and mitochondrial OXPHOS declines with age in association with the accumulation of somatic mtDNA mutations. This suggests that aging is a mitochondrial disease.

To further explore the mitochondrial hypothesis of aging, we propose to study the chimpanzee, our closest relative. The chimp genome is 98.5% homologous to our own, but the chimp's life span is half of our own. Moreover, Emory's Yerkes Primate Center houses a colony of over 200 chimps, which are regulary monitored for health and available for detailed autopsy at death.

First, we will characterize the physiolgical and biochemical changes that occur in chimp mitochondria with age. Muscle physiology will be monitored using non-invasive Near Infra Red (NIR) Spectroscopy, and correlated with muscle biopsy mitochondrial OXPHOS enzymes, respiration, ROS production, and excitability of the mtPTP. These studies will be extended to the other major organs using fresh autopsy tissues.

Second, we will correlate the decline in mitochondrial function with the accumulation of somatic mtDNA mutations in muscle biopsy and autopsy tissues. Rearrangements in the mtDNA will be monitored by long extension-PCR (LX-PCR) and mtDNA point mutations will be analyzed by direct mtDNA sequencing and by Protein Nucleic Acid (PNA)-competitive PCR (PNA-PCR). The regional distribution of mtDNA mutations will be assessed by in situ PCR. We will then determine the functional significance of the somatic mtDNA mutations by recovering mutant mtDNAs from the brains of post-mortem animals by synaptosome fusions to human mtDNA-deficient (r°) cells. Synaptosome cybrids, harboring mutant mtDNAs, will then be characterized for the biochemical consequences of the mutation.

Finally, we will analyze the age-related changes in mitochondrial gene expression using our human mitochondrial gene DNA microarray (mitochondrial gene chip). Initially, we will profile the age-related changes in nDNA and mtDNA mitochondrial gene expression in muscle, and then compare these changes with those seen in the post-mortem tissues from older chimps. These mitochondrial gene aging profiles will then be compared to profiles that we obtain from the muscle biopsies and autopsy tissues of human patients harboring known pathogenic mtDNA mutations. If the chimpanzee aging profiles approximate the patient mitochondrial disease profiles, then this will provide strong evidence that aging is a mitochondrial disease.

Contact Dr. Wallace.