Researchers examine metabolism in defective cells

UAlberta researchers are taking a closer look at how two metabolic pathways interact to increase the lifespan of cells with mitochondrial defects. Magnus Friis (PhD ’10) is the lead author of the study, which was published online on April 10 and will be published in the April 24 issue of Cell Reports.

Mitochondria produce energy for cells through oxidative metabolism, but the process produces toxic byproducts that can accumulate and cause defects in the cell’s mitochondria. These defects, in turn, affect the cell’s ability to generate energy and can potentially lead to cell death and are associated with aging and various neurological diseases.

Friis, a postdoctoral fellow in Mike Schultz’s biochemistry lab, examined how dietary changes at the cell level can affect cell health. He exposed normal and defective yeast cells to two different energy sources: glucose, the preferred sugar of cells, and raffinose, a natural sugar found in vegetables and whole grains.

“[The dietary intervention] is a general shift in what we’re feeding the cells to get them to do something different with their whole nutrient metabolism,” Friis noted. “There are signaling pathways that allow a cell to sense its environment and co-ordinate events to allow the cell to adapt to what’s going on. In this case, [cells are responding to] which nutrients are available.”

Friis and Schultz examined two nutrient signaling pathways called the AMPK pathway and the retrograde response. AMPK responds to energy deficits in the cell by down-regulating energy consuming processes, which are often associated with cell growth, and up-regulating energy producing processes. The retrograde response pathway is specific to the yeast used in the study and supplies key amino acids to the cell by changing the metabolic process of the mitochondria.  

When activated individually, neither the AMPK pathway nor the retrograde response provided substantial benefits to cells with damaged mitochondria. When activated simultaneously, clear benefits became evident.

“We looked at the effect activating both pathways had on maintenance of cellular viability in what’s called a chronological aging experiment,” Friis said. “Even when they had defective mitochondria, the cells with the retrograde response and AMPK simultaneously activated during growth were able to live as long as cells with normal mitochondrial function.”

Working in collaboration with John Paul Glaves, a postdoctoral fellow in Bryan Sykes’ lab, and Tao Huan, a PhD student in Liang Li’s lab, Friis measured the molecules produced during the metabolic process. They found that the defective cells had higher levels of branched chain amino acids and trahelose, a carbohydrate found in yeast that can serve an energy source, similar to glycogen in human cells.

“By activating AMPK, we’ve removed certain blocks in metabolism. With the retrograde response, we’ve changed the amino acid metabolism in a way that allowed the cells to accumulate storage carbohydrates, which stabilize their function,” Friis said.

Activated AMPK and retrograde response pathways allow the cell to accumulate a storage carbohydrate, which can be metabolize normally despite mitochondrial defects that affect the cell’s metabolism. The additional energy stabilizes cell function and prevents premature cell death often caused by defects in mitochondria.

“No matter how many people are working on the problem in humans, mitochondrial disorders are too complicated to figure out the nuts and bolts without the work that Magnus is doing,” Schultz said. “This research opens the concept, a new concept on how to deal with these metabolic problems.”

Scientists find potential target for treating mitochondrial disorders

Mitochondria, long known as “cellular power plants” for their generation of the key energy source adenosine triphosphate (ATP), are essential for proper cellular functions. Mitochondrial defects are often observed in a variety of diseases, including cancer, Alzheimer’s disease, and Parkinson’s disease, and are the hallmarks of a number of genetic mitochondrial disorders whose manifestations range from muscle weakness to organ failure. Despite a fairly strong understanding of the pathology of such genetic mitochondrial disorders, efforts to treat them have been largely ineffective.

But now, graduate student Walter Chen and postdoctoral researcher Kivanc Birsoy, both part of Whitehead Institute Member David Sabatini’s lab, have unraveled how to rescue cells suffering from mitochondrial dysfunction, a finding that may lead to new therapies for this condition.

To find genetic mutations that would rescue the cells, Chen and Birsoy mimicked mitochondrial dysfunction in a haploid genetic system developed by former Whitehead Fellow Thijn Brummelkamp. After suppressing mitochondrial function using the drug antimycin, Chen and Birsoy saw that cells with mutations inactivating the gene ATPIF1 were protected against loss of mitochondrial function.

The protein ATPIF1 is part of a backup system to save starving cells. When cells are deprived of oxygen and sugars, a mitochondrial complex that usually produces ATP, called ATP synthase, switches to consuming it, a state that can be harmful to an already starving cell. ATPIF1 interacts with ATP synthase to shut it down and prevent it from consuming the mitochondrion’s dwindling ATP supply but, in the process, also worsens the mitochondrion’s membrane potential.

“In these diseases of mitochondrial dysfunction, in a sense, it’s a false starvation situation for the cell—there are plenty of nutrients, but because there’s a block in the mitochondria’s normal function, the mitochondria behave as if there’s not enough oxygen,” says Chen, who with Birsoy, authored a paper in the journal Cell Reports describing this work. “So in these situations, activation of ATPIF1 is not good, because there are still many nutrients around to provide ATP. Instead, blocking ATPIF1 is therapeutic because it allows for maintenance of the membrane potential.”

Liver cells are frequently affected in patients with severe mitochondrial disease, so Chen and Birsoy tested the effects of mitochondrial dysfunction in the liver cells of control mice and mice with ATPIF1 genetically knocked out. Again, the liver cells with suppressed ATPIF1 function dealt better with mitochondrial dysfunction than liver cells with normal ATPIF1 activity.

“It’s very simple—if you get rid of ATPIF1, you survive in the presence of mitochondrial dysfunction,” says Birsoy. “From what we see so far, there are no major side effects from blocking ATPIF1 in mice.”

For Chen and Birsoy, the next step in this line of research is to test the effects of ATPIF1 suppression in mouse models of mitochondrial dysfunction. Then they will try to identify therapeutics that effectively block ATPIF1 function.