Plakoglobin supports length-dependent force generation and cardiac resistance to fasting

Muscles are not just passive bundles of fibers that move our limbs; they are dynamic tissues that constantly sense their environment and adjust their size based on how hard they work. When we exercise, muscles grow stronger and larger, but when we stop moving or face severe stress like starvation, they shrink. This shrinking process, known as atrophy, is a major health concern because losing muscle mass reduces our ability to move, recover from illness, and maintain overall health. For decades, scientists have understood that physical activity protects muscles from wasting away, but the exact mechanism by which a muscle cell translates the physical force of a contraction into a chemical signal to stay healthy has remained a mystery. The question is simple yet profound: how does a muscle cell know it is being stretched or pulled, and how does that physical sensation tell the cell to build more protein rather than break it down?

A team of researchers at the Technion Institute of Technology in Israel and the University of Chicago has uncovered a crucial piece of this puzzle, focusing on a specific protein called plakoglobin. This protein acts like a structural anchor, sitting at the boundary where the muscle cell's outer membrane meets its internal skeleton. In healthy, active muscles, plakoglobin helps organize a complex network of proteins that transmit signals telling the cell to grow. The researchers discovered that when skeletal muscles are starved of food, this anchor protein disappears from the cell's surface, the signaling network collapses, and the muscle begins to waste away. However, the heart, which never stops beating, behaves very differently. Even during starvation, the heart keeps its plakoglobin anchors firmly in place, maintains its signaling networks, and refuses to shrink. By studying how these two types of muscle respond to stress, the team found that plakoglobin is essential for muscles to generate maximum force when they are stretched, and that the heart's ability to hold onto this protein is a key reason it survives starvation while other muscles do not.

To understand how this works, the researchers first looked at what happens inside a muscle cell when an animal is fasted. They used advanced microscopy to take incredibly sharp, high-resolution images of mouse leg muscles. In well-fed mice, they saw plakoglobin as distinct, bright dots scattered along the edge of the muscle fibers, tightly connected to the internal scaffolding protein called desmin. But after just two days without food, these dots vanished from the edge of the cells in the leg muscles. The protein had retreated from the surface, taking the vital signaling machinery with it. This loss coincided with the activation of genes that tell the muscle to break itself down. The researchers confirmed that without plakoglobin at the surface, the muscle loses its ability to receive growth signals from insulin, a hormone that normally tells cells to build mass.

The story changes completely when looking at the heart. The researchers compared the leg muscles of fasted mice with their heart muscles. While the leg muscles shrank significantly and lost their plakoglobin anchors, the heart muscle maintained its mass and kept its plakoglobin complexes intact. Even though the heart was in the same starving animal with low levels of insulin circulating in the blood, its internal signaling remained active. The heart continued to produce the proteins needed to prevent breakdown. This suggested that the heart has a special defense mechanism that skeletal muscles lack, one that allows it to hold onto its structural anchors and stay strong even when the body is under extreme stress.

To prove that plakoglobin was the key to this difference, the scientists performed a direct test. They used a harmless virus to deliver a genetic instruction that specifically turned down the production of plakoglobin in the mice. In the leg muscles, this worked exactly as expected: the plakoglobin levels dropped, the muscle fibers lost their structural integrity, and the muscles began to waste away. However, when they looked at the hearts of these same mice, something remarkable happened. Despite the virus successfully entering the heart cells and delivering the instruction, the heart managed to keep its plakoglobin levels high. The heart seemed to have a built-in resistance to losing this protein, protecting itself from the genetic attack that crippled the skeletal muscles. This finding confirmed that the heart possesses a unique ability to preserve these critical anchors, which is likely why it does not waste away during starvation.

The researchers then investigated why this matters for how muscles actually work. They isolated single muscle fibers from mice and stretched them to different lengths to measure how much force they could generate. They found that when a muscle fiber is at its normal, resting length, removing plakoglobin did not change how much force it could produce. However, when they stretched the fiber to a longer length, mimicking the state of a muscle being pulled during movement, the fibers without plakoglobin failed to generate the extra force that healthy fibers produced. In other words, plakoglobin is not needed for basic muscle contraction, but it is essential for the muscle to respond to being stretched. Without it, the muscle cannot translate the physical stretch into the increased power needed to handle the load.

Finally, the team sought to understand how the heart manages to keep its signaling active during starvation. They analyzed the proteins that travel with the insulin receptor in the heart and found a cluster of molecules involved in a specific chemical pathway known as the MEK-ERK pathway. This pathway is known to respond to mechanical forces. When the researchers blocked this pathway in fasted mice using a drug, the heart lost its protection. The heart muscle began to shrink, the signaling for growth stopped, and the genes for muscle breakdown turned on. This proved that the heart relies on this mechanical signaling pathway to maintain its mass when food is scarce. The heart's continuous beating likely provides the constant mechanical stimulation needed to keep this pathway active, which in turn keeps the plakoglobin anchors in place and the muscle healthy.

The study reveals a clear and elegant distinction between how our skeletal muscles and our heart handle stress. Skeletal muscles are designed to be flexible; when resources are low, they let go of their structural anchors and shrink to conserve energy. The heart, however, is built to endure. It holds onto its plakoglobin anchors and its mechanical signaling pathways, ensuring it never loses the strength needed to keep pumping. This research highlights that the physical force of a muscle's movement is not just a result of its contraction but a vital signal that tells the cell how to survive. By understanding how plakoglobin links the physical stretch of a muscle to its chemical survival signals, scientists have taken a significant step toward understanding how to protect muscles from wasting away in disease and aging.