Grant: Project Grant

In current textbooks, muscle contraction is associated with the relative sliding of two filamentous proteins, actin and myosin and the cyclic interaction of myosin with the actin filament. This mechanism of muscle contraction is referred to as the "cross-bridge theory". The cross-bridge theory has been successful in explaining many experimental observations but has failed in explaining results obtained when a muscle is stretched, so-called eccentric contractions, which are important for everyday muscle function. In 2002, we discovered a new property of muscle, now referred to as "passive force enhancement". Passive force enhancement is the increase in force in a passive structure when a muscle is activated, compared to when muscle is relaxed. We identified that this "extra" force comes from a structural protein called titin and hypothesized that titin behaves like a spring whose stiffness increases when a muscle is active compared to when it is relaxed. After a decade of rejection, scientists now embrace this idea and are searching for how titin might change its stiffness upon muscle activation. The purpose of this study is to test the hypothesis that titin stiffness increases with muscle activation because titin is attaching to the rigid actin filament. We will test this idea by measuring the lengths of titin segments in active and relaxed conditions using fluorescent markers placed at defined locations on titin. We will further use high resolution cryo-electron tomography to visualize the interactions of titin with actin in active and relaxed conditions. Using these techniques, we hope to determine the molecular interactions between titin and actin, thereby solving the long-held mystery of how muscles contract. Since titin is implicated in many muscle and heart diseases, knowing its normal function might provide insight into the prevention of muscle and cardiac failure.