Connecting Events in Aging Process
The lab of Daniel Gottschling, Ph.D., an investigator at the Fred Hutchinson Cancer Research Center at the University of Washington, uses the simplest eukaryotes to study complex processes using systems biology. They work with budding yeast, which divide asymmetrically into a mother cell and a daughter cell.
Mother cells continue to divide until at some point no more divisions occur. This finite number of divisions is called the replicative lifespan, which Dr. Gottschling studies as a model for the cellular events that govern the aging process. However, their research had been hampered by the difficulty in isolating mother cells.
“The way this has been done historically is by single cell micromanipulation, and you just can’t do a lot of experiments,” said Dr. Gottschling. “But with our new Mother Enrichment Program, we can now.”
This protocol enables them to isolate large numbers of mother cells more easily. “We’re doing large genetic screens, and we’ve been able to do biology on every organelle, and watch as the cells get older,” he continued. “It has sort of opened up a door.”
Dr. Gottschling began using yeast to study cellular events of aging after observing changes in genome instability as a function of the mother cell’s replicative lifespan.
“We discovered that the mitochondria were messed up just before we saw the nuclear genome instability,” he explained. “The mitochondria, when they became dysfunctional, no longer produced something called iron sulfur clusters,” which are co-factors for many types of proteins, including at least four proteins responsible for genome integrity.
This caused double-stranded breaks, nucleotide excision repair, and base excision repair, to all be compromised in the older cells, according to Dr. Gottschling.
Their recent development of the Mother Enrichment Program has jumpstarted their research, screening for genes involved in aging. They’ve overexpressed each of 250 genes known to be important in mitochondrial dysfunction in yeast, to see whether any could delay the onset of aging.
Using the Mother Enrichment Program to age the cells, and labeled mitochondria for detection, they looked at the effects of each individual transformation on the health of the mitochondria.
They discovered that the yeast vacuole (the equivalent of lysosomes in other eukaryotes) was becoming defective before the mitochondria, and the resulting change in pH of the cytoplasm then caused the mitochondria to become defective.
The lysosome is important for storing amino acids, ions, and small molecules, and requires an acidic pH that is maintained by transporters in their membranes. According to Dr. Gottchling’s hypothesis, when the transporters don’t function properly, a subset of amino acids accumulates in the cytoplasm, destroying the membrane potential of the mitochondria that they require to function, so they become defective.
“In this idea of systems biology, we are actually beginning to dissect causal network interactions,” he pointed out. “One part of the network starts to break down, that is the vacuole, and then because of the interconnectedness to the mitochondria, that then starts to pull it down the hole, and then after that, when the mitochondria start to become defective, the nucleus starts to become defective.”
Dr. Gottschling is excited about the possibility that his research could shed some light on human diseases as well, possibly even explaining previously inexplicable observations of diseased tissue.
“We see fragmentation of mitochondria as they get older,” he noted. “And that’s seen in all these human neuronal degenerative diseases like Alzheimer’s. There are also lysosome defects, but they’ve just seen them as correlates, they’ve never understood why. And so we’re kind of excited that this might explain it. That it might be more than correlative."