Fat mice lead researchers to new feeding control pathway

Published May 4 2016

Researchers injected littermates with either a control virus or a virus that knocked out OGT in a part of the brain. The mouse missing OGT (left) ate twice as much as its normal sibling. The photo was taken about five weeks after the virus injection. OLOF LAGERLOF

When their lab mice unexpectedly packed on weight, Richard Huganir at the Johns Hopkins School of Medicine and his colleagues had to figure out why the mice suddenly turned obese. In a paper in the March 11 issue of the journal Science, the investigators describe their discovery of a protein-modification pathway in the brain that plays a surprising role in feeding control and satiety.

“This was a serendipitous discovery,” says Huganir, a neuroscientist and member of the American Society for Biochemistry and Molecular Biology. “We had to learn a whole new area of biology — feeding control, metabolism and obesity. Luckily, we had great collaborators at Hopkins who had the expertise to help us figure out what was going on. We eventually found out that the mice had impaired satiety and ate larger meals.”

The investigators originally were working on deciphering the role of an enzyme called O-GlcNac transferase, known as OGT, in regulating synaptic transmission and plasticity in the brain as well as its potential role in learning and memory. OGT catalyzes the attachment of a short sugar molecule to proteins; the sugar molecule then influences the function of the proteins.

As part of their project, Huganir and colleagues genetically modified the brains of mice so that the researchers could turn off the expression of OGT in the forebrain and hippocampus. These two regions of the brain are important for learning and memory.

“Much to our surprise, a couple of weeks after we knocked out OGT, the mice got very, very fat,” says Huganir. “We stopped studying learning and started studying feeding control.”

The parts of the brain the investigators had targeted in their mice usually are not associated with feeding control. But the hypothalamus is.

When the investigators looked at the hypothalamus, they discovered that they inadvertently had removed OGT in specific cells in a region of the hypothalamus called the paraventricular nucleus.

To make sure that OGT in the paraventricular nucleus cells was what was influencing the feeding and satiety of the mice, Huganir and colleagues created another set of genetically modified mice. These mice had OGT missing only in the paraventricular nucleus cells. “Knocking out OGT in only these cells inhibited their activity and produced the same overeating phenotype,” says Huganir.

The investigators now know that OGT plays an important role in the paraventricular nucleus cells in feeding control, but the molecular details are still unknown. For one, the investigators don’t know what substrates OGT acts on in the paraventricular nucleus cells to regulate their activity.

And, as with any work done on mice, the implications for humans have to be worked out. “This work in mice does suggest similar mechanisms are important in human satiety,” says Huganir. “However, much more work is needed to identify potential therapeutic targets to modify this pathway in humans to regulate food intake.”

Rajendrani Mukhopadhyay Rajendrani Mukhopadhyay is the chief science correspondent for the American Society for Biochemistry and Molecular Biology. Follow her on Twitter.