Sarcomere length fluctuations and flow in capillary endothelial cells

Actomyosin stress fibers are known to generate tensile force in non-muscle cells. However, how sarcomere numbers and lengths change dynamically remains unclear. Prior research has shown that stress fibers contain contractile units called sarcomeres. It was already known that these structures are involved in cell tension and adhesion. No prior work had resolved the mechanisms of sarcomere length fluctuations or their incorporation into fibers. This gap motivated the use of live cell imaging to observe sarcomere behavior in real time. The static appearance of stress fibers contrasts with their dynamic internal structure. Understanding these dynamics could clarify how cells maintain tension under various conditions.

The aim of this work is to investigate how sarcomere lengths and numbers change within stress fibers. The specific problem is the lack of understanding about sarcomere dynamics despite their role in cell tension. Researchers sought to determine if sarcomere length fluctuations are tension-dependent. They also aimed to identify where new sarcomeres form and how they integrate into fibers. The motivation comes from the need to clarify the mechanisms behind stress fiber plasticity. Observing these processes in live cells allows direct measurement of sarcomere behavior. The study focuses on capillary endothelial cells as a model system. The findings may help explain how cells regulate contractile forces dynamically.

Live cell imaging was used to track labeled sarcomeres in capillary endothelial cells. Researchers observed sarcomere length fluctuations over time intervals. The fluctuation relaxation time was measured as approximately 20 minutes. New sarcomeres were identified forming at focal adhesions using fluorescent labeling. The incorporation speed of these sarcomeres into fibers was quantified. The speed of sarcomere convection was found to be independent of focal adhesion size. Disappearance points, or 'sinks,' of sarcomeres were also tracked along stress fibers. The data were analyzed to assess whether tension modulates sarcomere length or speed.

Sarcomere lengths were found to fluctuate continuously with a relaxation time of about 20 minutes. New sarcomeres formed at focal adhesions and moved into fibers at a uniform speed. The convection speed was independent of focal adhesion size or tension levels. Sarcomeres disappeared at specific points along stress fibers termed 'sinks.' The incorporation speed remained consistent across different regions of the cell. These findings suggest that sarcomere length is not modulated by tension. Instead, the speed of contraction or expansion may be tension-dependent. The results support a model where sarcomere dynamics are not directly controlled by tension.

The authors propose that sarcomere length fluctuations are not directly controlled by tension. Instead, the speed of sarcomere contraction or expansion may be tension-dependent. Stress fibers appear static but are highly dynamic structures. New sarcomeres form at focal adhesions and integrate into fibers at a uniform rate. The disappearance of sarcomeres at specific 'sinks' suggests a regulated process. The fluctuation relaxation time of 20 minutes indicates a time-dependent mechanism. These findings challenge assumptions about how tension is modulated in stress fibers. The model proposed aligns with the observed independence of sarcomere speed from tension.