Lower miRNA-155-5p expression in pancreatic β cells than in α cells is associated with higher susceptibility to innate immune pathway activators

Type 1 diabetes is a condition where the body's immune system mistakenly attacks and destroys the insulin-producing cells in the pancreas. These specific cells, known as beta cells, are essential for regulating blood sugar. While genetics play a major role in who develops the disease, the fact that identical twins do not always both get it suggests that something in the environment must trigger the attack. Scientists have long suspected that viruses, particularly a group called enteroviruses, are involved in this process. These viruses can infect the pancreas and confuse the body's internal alarm systems, which are designed to fight off infections. When these alarms go off, they can sometimes cause the very cells they are meant to protect to become damaged or die. However, the exact steps of how a viral infection turns into the destruction of beta cells have remained a mystery.

A new study from researchers at The Bartholin Institute and Roskilde University in Denmark has taken a closer look at this interaction, focusing on a specific type of molecule inside cells that helps control how they respond to stress and infection. The researchers wanted to understand why beta cells seem to be more vulnerable to these viral attacks than their neighbors, the alpha cells, which produce a different hormone called glucagon. To investigate this, they used laboratory-grown lines of mouse beta and alpha cells. They exposed these cells to substances that mimic the presence of a virus, specifically by activating the cell's internal sensors that usually detect viral genetic material. They also tested the cells under different sugar conditions, ranging from normal levels to very high levels, to see if high blood sugar made the cells more fragile.

The researchers found that when the cells were exposed to these viral mimics, the beta cells reacted differently and more severely than the alpha cells. One key sign of this stress was the production of nitric oxide, a molecule that can act as a warning signal but also damage cells if levels get too high. The beta cells produced significantly more of this stress signal than the alpha cells, especially when the sugar levels were high. Furthermore, when the researchers activated the specific sensor that detects viral RNA, the beta cells showed a sharp drop in their ability to release insulin, the hormone needed to lower blood sugar. In contrast, the alpha cells continued to function normally, releasing their hormone without issue. This suggests that the beta cells are uniquely sensitive to the combination of viral threats and high sugar levels.

Digging deeper into the cells' internal machinery, the team looked at a set of tiny regulatory molecules called microRNAs. These molecules act like dimmer switches for genes, turning the activity of specific genes up or down to help the cell adapt. The study revealed that one particular microRNA, known as miRNA-155-5p, was present at much lower levels in the beta cells compared to the alpha cells. This difference was consistent across all conditions. When the beta cells were exposed to high sugar or viral mimics, the levels of this protective microRNA dropped even further. The researchers also observed that another gene, ISG15, which is part of the body's antiviral defense, was turned on strongly in the beta cells but remained quiet in the alpha cells. This indicates that the beta cells are mounting a frantic, perhaps excessive, immune response that they cannot fully control, whereas the alpha cells remain more stable.

The study concludes that beta cells lack the same level of internal regulation that alpha cells possess when facing a viral threat. The low levels of miRNA-155-5p in beta cells appear to leave them unable to properly manage the stress signals triggered by a virus, especially when blood sugar is high. This inability to balance the immune response may be a key reason why beta cells are the primary targets in type 1 diabetes, while alpha cells survive the same environment. The findings suggest that the path to the disease involves a perfect storm where a viral infection and high blood sugar work together to overwhelm the beta cells' defenses. By identifying these specific molecular weaknesses, the research offers a clearer picture of why these cells are so easily damaged, pointing toward the need for therapies that could help strengthen their ability to withstand these dual attacks.