Metformin Targets Brain Rap1 To Lower Blood Sugar In Mice

A diabetes workhorse has been hiding a brain secret, and scientists finally caught it in action.

In new research from Baylor College of Medicine and collaborators, metformin lowered blood sugar by acting on a neural pathway in the ventromedial hypothalamus, specifically through Rap1 signaling. Published in Science Advances, the mouse study shows that low, clinically relevant doses of metformin require brain Rap1 to work, reframing how this common diabetes drug helps control glucose.

Why The Brain Matters For A Diabetes Pill

For decades, most explanations centered on metformin’s effects in the liver and gut. The new data add a third organ to the story, the brain. The team focused on Rap1, a small GTPase, inside neurons of the ventromedial hypothalamus, a control hub for whole body glucose metabolism.

“It’s been widely accepted that metformin lowers blood glucose primarily by reducing glucose output in the liver. Other studies have found that it acts through the gut,” said corresponding author Dr. Makoto Fukuda of Baylor College of Medicine. “We investigated whether the brain would also play a role, as it is widely recognized as a key regulator of whole-body glucose metabolism.”

What The Experiments Showed

• Rap1 is required at low doses: Mice lacking Rap1 in forebrain neurons did not respond to low doses of metformin, even though they still responded to insulin, GLP-1 agonists, SGLT2 inhibitors, and other drugs.
• Tiny brain doses were potent: Injecting metformin directly into the brain lowered blood glucose with microgram doses, thousands of times smaller than typical peripheral doses.
• VMH neurons lit up: Metformin increased activity in VMH SF1 neurons. In brain slices, metformin depolarized most SF1 neurons, but only when Rap1 was present.
• Turning Rap1 on blocked the drug: Forcing Rap1 to stay active in the VMH blunted metformin’s glucose lowering effects.

Key Numbers At A Glance

Therapeutically relevant serum metformin concentrations are about 10 to 40 micromolar in humans. In mice, intraperitoneal 150 mg per kg yielded about 23 micromolar in serum at four hours. By contrast, microgram level brain injections lowered blood sugar, suggesting the brain pathway is highly sensitive.

“This discovery changes how we think about metformin,” Fukuda said. “It’s not just working in the liver or the gut, it’s also acting in the brain. We found that while the liver and intestines need high concentrations of the drug to respond, the brain reacts to much lower levels.”

Inside The VMH, A Switch For Blood Sugar

The ventromedial hypothalamus houses SF1 neurons that help coordinate glucose balance. Metformin activated these neurons and required Rap1 to do so. “We also investigated which cells in the VMH were involved in mediating metformin’s effects,” Fukuda said. “We found that SF1 neurons are activated when metformin is introduced into the brain, suggesting they’re directly involved in the drug’s action.”

What Does This Mean For Patients

Most people who take metformin will not receive brain injections, of course. The takeaway is mechanistic. If low-dose metformin depends on brain Rap1, future therapies might target this neural pathway more directly. Could a brain directed strategy help patients who do not respond well to standard dosing

Clinically Relevant Questions To Watch

• Can drugs that modulate Rap1 in the VMH improve glucose control without higher systemic metformin exposure
• Do individual differences in central Rap1 signaling predict variable metformin response
• Could the same pathway explain reported effects of metformin on brain aging and cognition

Limits And Next Steps

The research was done in mice, and the team notes that very high metformin doses can bypass the brain mechanism by engaging classic hepatic targets. The next step is tracing how a signal from VMH neurons drives changes in liver and muscle that lower glucose.

“These findings open the door to developing new diabetes treatments that directly target this pathway in the brain,” Fukuda said. “In addition, metformin is known for other health benefits, such as slowing brain aging. We plan to investigate whether this same brain Rap1 signaling is responsible for other well-documented effects of the drug on the brain.”

Study Details

Title: Low-dose metformin requires brain Rap1 for its antidiabetic action

Lead authors: Hsiao-Yun Lin, Weisheng Lu, Yanlin He, Yukiko Fu, Kentaro Kaneko, Peimeng Huang, Ana B. De la Puente-Gomez, Chunmei Wang, Yongjie Yang, Feng Li, and Makoto Fukuda

Institutions: Baylor College of Medicine, Louisiana State University, Nagoya University, Meiji University

Citation

Science Advances, July 30, 2025. DOI: 10.1126/sciadv.adu3700