Mice that were engineered with a fat-burning pathway borrowed from bacteria (top) remained thin compared with normal mice (bottom) when both were fed a high-fat diet.
Credit: Jason Dean, University of California, Los Angeles
This is transgenic metabolism transfer. They take the metabolism of bacteria and plants and transferred [maybe re-activated dormant genes] it to mice. The changed genes were introduced into the liver and instead of converting fat to sugar, fat was converted to carbon dioxide. Cholesterol levels were also lower.
Someday, it may be possible to actually introduce these bacterial genes or proteins into humans, although Pei points out that such a feat poses many challenges, including a potential immune response to foreign genes. Another possibility would be to search for drugs that could mimic the effects of these enzymes. Furthermore, earlier studies reported glyoxylate shunt activity in chickens and rats, suggesting that higher organisms might retain the genes for this pathway but don’t use them; it might be possible to activate dormant genes.
Liao says that the study borrows strategies from synthetic biology, a field that has for the most part focused on engineering new functions into bacteria and other lower organisms. The study suggests that the same concepts could be applied to mammals: just as we create bacteria that produce biofuels, we could introduce new abilities into the bodies of humans and other animals.
Researchers at the University of California, Los Angeles (UCLA), who transplanted a fat-burning pathway used by bacteria and plants into mice. The genetic alterations enabled the animals to convert fat into carbon dioxide and remain lean while eating the equivalent of a fast-food diet.
Given the success in engineering synthetic phenotypes in microbes and mammalian cells, constructing non-native pathways in mammals has become increasingly attractive for understanding and identifying potential targets for treating metabolic disorders. Here, we introduced the glyoxylate shunt into mouse liver to investigate mammalian fatty acid metabolism. Mice expressing the shunt showed resistance to diet-induced obesity on a high-fat diet despite similar food consumption. This was accompanied by a decrease in total fat mass, circulating leptin levels, plasma triglyceride concentration, and a signaling metabolite in liver, malonyl-CoA, that inhibits fatty acid degradation. Contrary to plants and bacteria, in which the glyoxylate shunt prevents the complete oxidation of fatty acids, this pathway when introduced in mice increases fatty acid oxidation such that resistance to diet-induced obesity develops. This work suggests that using non-native pathways in higher organisms to explore and modulate metabolism may be a useful approach.
To investigate the effects of the glyoxylate shunt on fatty acid metabolism in mammals, Liao’s team cloned bacteria genes from Escherichia coli that would enable the shunt, then introduced the cloned E. coli genes into the mitochondria of liver cells in mice; mitochondria are where fatty acids are burned in cells.
The researchers found that the glyoxylate shunt cut the energy-generating pathway of the cell in half, allowing the cell to digest the fatty acid much faster than normal. They also found that by cutting through this pathway, they created an additional pathway for converting fatty acid into carbon dioxide. This new cycle allowed the cell to digest fatty acid more effectively.
“The significance of this is great. It is a unique approach to understanding metabolism. Perturbing metabolic pathways, such as introducing the glyoxylate shunt and seeing how it affects overall metabolism, is a novel way to understand the control of metabolism,” Dipple said.
The team also found that the new pathway decreased the regulatory signal malonyl-CoA. When malonyl-CoA levels are high, a signal is released that tells the body it is too full and that it needs to stop using fat and begin making it. Malonyl-CoA is high after eating a meal, blocking fatty acid metabolism. The new pathway, however, allowed for fat degradation even when the body was full.
Ultimately, the research team found that mice with the glyoxylate shunt that were fed the same high-fat diet — 60 percent of calories from fat — for six weeks remained skinny, compared with mice without the shunt.
“One exciting aspect of this study is that it provides a proof-of-principle for how engineering a specific metabolic pathway in the liver can affect the whole body adiposity and response to a high-fat diet,” said Karen Reue, a UCLA professor of human genetics and an author of the study. “This could have relevance in understanding, and potentially treating, human obesity and associated diseases, such as diabetes and heart disease
To create the fat-burning mice, the researchers focused on a metabolic strategy used by some bacteria and plants called the glyoxylate shunt. James Liao, a biomolecular-engineering professor at UCLA and a senior author of the study, says, “This pathway is essential for the cell to convert fat to sugar” and is used when sugar is not readily available or to convert the fat stored in plant seeds into usable energy. Liao also says that it’s not known why mammals lack this particular strategy, although it may be because our bodies are designed to store fat rather than burn it.
The glyoxylate shunt is composed of just two enzymes. The researchers first introduced genes for these enzymes from E. coli bacteria into cultured human cells and found that they increased the metabolism of fats in the cells. But surprisingly, rather than converting the fat into sugar as bacteria do, the cells burned the fat completely into carbon dioxide. The scientists analyzed gene expression in the cells and found that the new pathway promoted cellular responses that led the cells to metabolize fats rather than sugar.
The researchers then introduced the genes into the livers of mice. While normal mice gain weight when put on a high-fat diet, Liao says that the engineered mice “remained skinny despite the fact that they ate about the same and produced the same waste” and were as active as their normal counterparts. They also had lower fat levels in the liver and lower cholesterol levels. As in the cultured cells, the engineered mice did not convert the fat into sugar, which could have the dangerous side effect of promoting high blood sugar and diabetes. Instead, the scientists found a measured increase in their carbon dioxide output; the excess fat was literally released into thin air. The mice exhibited no visible side effects, although more detailed studies are necessary to verify that.