NG2-expressing cells support capillary-rich skeletal muscle homeostasis, and their repeated depletion is associated with increased mortality
Repeated depletion of NG2-expressing perivascular cells, which are essential for maintaining capillary-rich skeletal muscle homeostasis, leads to severe systemic deterioration and increased mortality due to the loss of muscle mass and function in posture- and swallowing-related muscles.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Muscles are often thought of as simple engines that power movement, but they are living tissues that constantly renew themselves. Even when the body is at rest, muscle fibers break down old parts and replace them with new ones, a process that requires a steady supply of fresh building blocks. For decades, scientists believed that a specific group of stem cells, known as satellite cells, were the primary workers responsible for this maintenance. However, recent research has suggested that the environment surrounding these muscle fibers, particularly the tiny blood vessels that feed them, plays an equally critical role. These vessels are wrapped in a layer of support cells that help keep the blood supply stable and may also contribute to muscle repair. Understanding how these different parts work together is vital because the loss of muscle mass and strength, a condition known as sarcopenia, is a major cause of frailty and death in aging populations.
A team of researchers at Asahikawa Medical University in Japan has now uncovered a deeper layer of this biological story. They focused on a specific type of support cell that expresses a protein called NG2. These cells are found wrapped around the smallest blood vessels in the body, known as capillaries, and they are also present near the junctions where nerves connect to muscles. The scientists wanted to know what happens if these NG2 cells are removed from the body over a long period. To find out, they used a genetic technique to selectively eliminate these cells in mice. Crucially, they did not just remove them once; they repeated the process every two weeks for several months. This approach was designed to prevent the body from simply replacing the lost cells, allowing the researchers to see the true long-term consequences of their absence.
The results were dramatic and revealed a vulnerability that had not been fully appreciated before. While the mice appeared healthy for the first two months, a sudden and severe decline began after three months of repeated cell removal. The animals lost significant body weight, developed a hunched posture, and began to die at a high rate. When the researchers examined the lungs of the mice that died, they found signs of inflammation and foreign material, such as hair, lodged in the airways. This pattern strongly suggested that the mice were suffering from aspiration pneumonia, a condition where food or liquid is accidentally inhaled into the lungs. The cause appeared to be a failure of the muscles involved in swallowing and posture. The muscles along the spine, which keep the back straight, and the muscles in the throat and jaw, which control swallowing, had shrunk and weakened. Without these muscles functioning correctly, the mice could no longer hold their heads up or swallow safely, leading to fatal lung infections.
What made this discovery particularly surprising was that the muscle loss did not follow the rules scientists had previously expected. It was long believed that slow-twitch muscles, which are used for endurance and posture, relied more heavily on these support cells than fast-twitch muscles, which are used for quick bursts of speed. However, the study showed that the NG2 cells were essential for muscle groups regardless of whether they were fast or slow. The muscles that failed were not defined by their fiber type, but by how richly they were supplied with blood vessels. The muscles that atrophied, including the fast-twitch muscles of the jaw and throat, were packed with tiny capillaries and the NG2 cells that surrounded them. In contrast, other fast-twitch muscles with fewer blood vessels remained relatively healthy. This indicates that the need for these support cells is dictated by the local blood supply rather than the type of muscle fiber itself.
Further investigation revealed that the loss of NG2 cells also damaged the connection between nerves and muscles. These connections, called neuromuscular junctions, are the communication hubs where nerve signals tell muscles to contract. In the mice with depleted NG2 cells, these junctions became fragmented and unstable. The researchers found that the NG2 cells are not just passive supporters of blood vessels; they are also active participants in maintaining the health of the nerve-muscle connection. When these cells vanished, the nerve signals became unreliable, and the muscle fibers began to waste away. The genetic analysis of the affected muscles showed that the cells were struggling to maintain their structure and were reverting to an immature state, unable to complete the repair process.
The study suggests that the body has a limited capacity to compensate for the loss of these support cells. Initially, other cells may step in to fill the gap, but this backup system eventually fails under the stress of repeated depletion. Once the threshold is crossed, the microvascular network collapses, the nerve connections break down, and the muscles essential for life-sustaining functions like breathing and swallowing begin to fail. This finding challenges the simple view that muscle health depends only on the muscle cells themselves or a single type of stem cell. Instead, it highlights a complex, interdependent system where the tiny blood vessels and their surrounding support cells are just as critical as the muscle fibers they nourish. The research provides a new framework for understanding why certain muscles fail in aging and disease, pointing toward the microvascular environment as a key target for preserving muscle function and preventing the severe decline that leads to mortality.
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