Shortening the thick filament by partial deletion of titin's C-zone alters cardiac function by reducing the operating sarcomere length range

Titin's C-zone is an inextensible segment in titin, comprised of 11 super-repeats and located in the cMyBP-C-containing region of the thick filament. Previously we showed that deletion of titin's super-repeats C1 and C2 (Ttn^ΔC1-2 model) results in shorter thick filaments and contractile dysfunction of the left ventricular (LV) chamber but that unexpectedly LV diastolic stiffness is normal. Here we studied the contraction-relaxation kinetics from the time-varying elastance of the LV and intact cardiomyocyte, cellular work loops of intact cardiomyocytes, Ca²⁺ transients, cross-bridge kinetics, and myofilament Ca²⁺ sensitivity. Intact cardiomyocytes of Ttn^ΔC1-2 mice exhibit systolic dysfunction and impaired relaxation. The time-varying elastance at both LV and single-cell levels showed that activation kinetics are normal in Ttn^ΔC1-2 mice, but that relaxation is slower. The slowed relaxation is, in part, attributable to an increased myofilament Ca²⁺ sensitivity and slower early Ca²⁺ reuptake. Cross-bridge dynamics showed that cross-bridge kinetics are normal but that the number of force-generating cross-bridges is reduced. In vivo sarcomere length (SL) measurements revealed that in Ttn^ΔC1-2 mice the operating SL range of the LV is shifted towards shorter lengths. This normalizes the apparent cell and LV diastolic stiffness but further reduces systolic force as systole occurs further down on the ascending limb of the force-SL relation. We propose that the reduced working SLs reflect titin's role in regulating diastolic stiffness by altering the number of sarcomeres in series. Overall, our study reveals that thick filament length regulation by titin's C-zone is critical for normal cardiac function.