Downsizing the Giant Titin Reveals its Dominant Roles in Skeletal Muscle Passive Stiffness and Longitudinal Hypertrophy

Abstract

Titin, the largest protein known, forms an elastic myofilament in the muscle sarcomere. To establish titin's contribution to skeletal muscle passive stiffness, relative to that of the extracellular matrix, a mouse model was created in which titin's molecular spring region was shortened by deleting 46 exons, the TtnΔ112-158 model. RNA sequencing and super-resolution microscopy predict a much stiffer titin molecule. Mechanical studies with this novel mouse model show that titin is the main determinant of skeletal muscle passive stiffness. Unexpectedly, the in vivo sarcomere length working range was shifted to a shorter length in TtnΔ112-158 mice, due to a ∼30% increase in the number of sarcomeres in series (longitudinal hypertrophy). The effect of this shift on active force generation was minimized through a shortening of thin filaments that was discovered in TtnΔ112-158 mice. Thus, titin is the dominant determinant of the physiological passive stiffness of skeletal muscle and drives longitudinal hypertrophy.