Screening the Human Genome for New Mitochondrial and Longevity Regulators

Abstract

Over the past two decades, genes and pathways have been discovered that can prolong lifespan and delay the onset of age-related diseases. The vast majority of these longevity genes were discovered in screens conducted in lower organisms such as yeast and worms. Given that similar large-scale genetic screens in mammals would be cost prohibitive, it is likely that many mammalian longevity genes remain to be discovered. One way to circumvent this issue is to conduct the screen in mammalian cells and instead use cellular phenotypes that correlate with lifespan extension as a read-out. Mitochondrial dysfunction has been observed in aging and implicated in many age-related diseases. Conversely, calorie restriction, a dietary regimen found to extend lifespan and delay the onset of age-related diseases, is associated with maintained or increased mitochondrial function. Thus, increased mitochondrial content and function is a candidate phenotype for a cell-based screen for mammalian lifespan-regulating genes.

We have performed a genetic screen of 15,483 human genes for open reading frames (ORFs) that modulate mitochondrial function. A total of 76 ORFs were found and validated to be positive regulators of mitochondrial mass while 18 were negative regulators. The gene products of these ORFs include secreted factors, transcription factors and a variety of predicted polypeptides with unknown function.

One of the hits identified from the screen was GLTSCR2 (glioma tumor suppressor candidate region gene 2), a nucleolar protein that has been found to be involved in apoptosis and the cell cycle. GLTSCR2 was discovered as a positive regulator of mitochondrial mass and its overexpression was found to increase cellular respiration. This upregulation of respiration was found to be mediated by Myc, connecting role of GLTSCR2 in oxidative metabolism discovered in this study to its previously reported role in the cell cycle.

Another hit identified from the screen was Cpne2 (copine 2), a member of the Copine family of proteins. Members of the Copine family are calcium-dependent phospholipid binding proteins and although discovered over a decade ago, the cellular role of Cpne2 remains unknown. Cpne2 was found as a negative regulator of mitochondria and its depletion was found to increase mitochondrial mass and function. This increase in mitochondria was not due to an increase in the transcriptional program of mitochondrial biogenesis or a defect in depolarization-induced mitophagy, suggesting the involvement of another independent pathway.

Collectively, this study describes the discovery and characterization of novel mitochondrial regulators, some of which may be longevity genes and may provide a better understanding of the aging process.