Determinants of Myofilament Length Dependent Activation

Myofilament length-dependent activation (LDA) is a universal property of striated muscle, yet the molecular mechanisms that underlie it are incompletely understood. LDA is characterized by an increase in both maximal Ca2+-activated force (Fmax) and calcium sensitivity upon an increase in sarcomere length (SL). My first aim was to determine the dynamic rate by which a change in SL is sensed and then transduced to the myofilaments. We employed the rapid solution-switch single myofibril technique which allows for the study of contraction activation/relaxation dynamics in the virtual absence of diffusion delays. We compared submaximal contraction kinetics at steady-state SL with contractions following rapid SL ramps to that same SL just prior to activation. Neither the activation/relaxation kinetics nor final submaximal force development was significantly different between the two contraction modes for SL ramps as fast as 5ms. We concluded that the transduction of the length signal by the cardiac sarcomere to modulate thin filament activation levels occurs virtually instantaneously. The giant protein titin spans the entire sarcomere, transducing strain between the myofilaments. Additionally, the magnitude of LDA exhibited in striated muscle has been shown to vary with titin isoform length. Recently, a rat that harbors a homozygous mutation (HM) causing preferential expression of a longer titin isoform was discovered. Accordingly, my second aim was to investigate the role of titin isoform length on the magnitude of LDA exhibited in striated muscle. We measured myofilament force development and cross-bridge cycling kinetics as a function of SL in tibialis anterior and trabeculae isolated from wild-type (WT) and HM rats. SL-dependent changes in passive tension, Fmax, calcium sensitivity, and ATP consumption were attenuated in HM skeletal muscles. HM trabeculae exhibited a reduction in SL-dependent changes in Fmax and calcium sensitivity compared to WT. Additionally, activation kinetics were reduced in skeletal HM myofibrils compared to WT, while relaxation kinetics were not affected. We established that the presence of a long titin isoform is associated with reduced myofilament force development and cross-bridge cycling kinetics, and a blunting of LDA. In conclusion, LDA develops virtually instantaneously, possibly from titin-strain mediated geometric rearrangement of thick and thin filament contractile proteins.