Old mice made too much glucagon, the hormone that raises blood sugar

Age is one of the strongest risk factors for type 2 diabetes, the form in which blood sugar stays too high because the body stops responding well to insulin. Research on why has mostly looked at beta cells, the cells in the pancreas that make insulin, the hormone that lowers blood sugar. A team from Miguel Hernández University in Spain looked at their neighbours instead and found that, in mice, age also changes the cells that make glucagon. In data from people, higher glucagon was linked to a higher risk of diabetes. Most of the evidence comes from mice, and the human findings are associations, not proof of cause.

Glucagon is insulin's counterpart. When blood sugar drops, it tells the liver to release more sugar; when blood sugar is high, its release should be switched off. It is made by alpha cells, which sit with beta cells in small clusters in the pancreas called islets. In people with type 2 diabetes, glucagon is often too high and is not switched off properly, which pushes blood sugar up further.

What they did

The team compared young male mice (3 months old) with old ones (20 months). Many old mice become insulin resistant: their tissues respond less to insulin. To separate age itself from insulin resistance, the researchers split the old mice into those whose insulin sensitivity was still like the young mice's and those with clearly lower sensitivity.

They measured blood glucagon after a six-hour fast and after an injection of arginine, an amino acid that makes the pancreas release hormones. They counted alpha cells in pancreas sections, tested isolated islets in a dish, recorded electrical currents in single alpha cells and looked inside them with an electron microscope. They also reanalysed a public collection of more than 300,000 single islet cells from nine earlier mouse studies.

Finally, they analysed stored blood data from 462 participants of a Spanish clinical trial, CORDIOPREV, who did not have diabetes at the start. Over a median of 60 months, 107 of them developed type 2 diabetes.

What they found

• More glucagon in old mice. Both groups of old mice had higher fasting glucagon than young mice, regardless of insulin resistance, and released more glucagon and insulin after arginine. Across all mice, lower insulin sensitivity went with higher glucagon.
• More alpha cells. The total mass of alpha cells grew with age and was clearly higher in the insulin-resistant old mice. Alpha cells were not bigger and divided less, so the team thinks old mice may have accumulated them over life.
• A weaker off switch. In islets from young mice, high sugar cut glucagon release by about 43%, and added insulin by about 54%. Islets from old mice with normal insulin sensitivity responded similarly (about 43% and 35%). In islets from insulin-resistant old mice, the drop (about 24% and 28%) was too small to count as a clear effect.
• Signs of stress. In both old groups, the endoplasmic reticulum, the part of the cell that folds new proteins, was enlarged in alpha cells, a typical sign of what biologists call ER stress. Stress genes were also more active in old alpha cells in the public dataset. Yet the cells' electrical activity and release machinery looked normal, and very few died (about 0.1% to 0.2%).
• Slightly blurred identity. Fewer glucagon cells carried a key alpha-cell marker protein, and insulin-resistant old mice had more cells containing both hormones at once. The team calls these changes moderate.
• In people. Older participants who later developed diabetes, and were insulin resistant, had higher fasting glucagon and a bigger glucagon response in a sugar-drink test than the other groups. People older than the median age (58) had a higher risk of developing diabetes, and those who also had above-median glucagon had the highest risk. The exact figures are in the paper's tables and figures.

What to keep in mind

• Small groups of male mice. Only male mice were used. The authors note that old mice with normal insulin sensitivity were a small part of the old group, so they were especially scarce: 5 to 13 per test. Some experiments used islets or cells from only three mice per group.
• Alpha cells are a small share of islet cells, which limited the molecular tests. The authors also say some electrical measurements that could explain the weaker off switch are still missing.
• Mice and people differ. The paper notes that in humans, alpha-cell mass stays fairly stable with age, unlike in these mice.
• The human data show links, not causes, from one trial population; the authors say other groups should be checked. "Older" was defined partly by telomere length (the protective caps on chromosomes), partly by age.
• Whether the cell stress, the identity changes and the extra glucagon cause one another is, in the authors' words, a question that "warrants further investigation".

Why it matters

Age-related diabetes may not be only a story of insulin. In old mice, alpha cells seemed to hold up better than beta cells, but they made more glucagon and were harder to switch off, especially with insulin resistance: the wrong direction for keeping blood sugar in check. The authors suggest exploring whether older adults could benefit from diabetes treatments that target glucagon, a question for future research, not a conclusion of this study.