Impaired cardiomyocyte relaxation and diastolic function in transgenic mice expressing slow skeletal troponin I in the heart

Abstract

To assess the specific functions of the cardiac isoform of troponin I (cTnI), we produced transgenic mice that expressed slow skeletal troponin I (ssTnI) specifically in cardiomyocytes. Cardiomyocytes from these mice displayed quantitative replacement of cTnI with transgene‐encoded ssTnI. The ssTnI transgenic mice were viable and fertile and did not display increased mortality or detectable cardiovascular histopathology. They exhibited normal ventricular weights and heart rates. Permeabilized transgenic cardiomyocytes demonstrated an increased Ca2+ sensitivity of tension and a lack of contractile responsiveness to cAMP‐dependent protein kinase (PKA). Isolated cardiomyocytes from transgenic mice had normal velocities of unloaded shortening but unlike wild‐type controls exhibited no enhancement of the velocity of shortening in response to treatment with isoprenaline. Transgenic cardiomyocytes exhibited greater extents of shortening than non‐transgenic cardiomyocytes at baseline and after treatment with isoprenaline. The rates of rise of intracellular [Ca2+] and the peak amplitudes of the intracellular [Ca2+] transients were similar in transgenic and wild‐type myocytes. However, the half‐time of intracellular [Ca2+] decay was significantly greater in the transgenic myocytes. This change in decay of intracellular [Ca2+] was correlated with an increase in the re‐lengthening time of the transgenic cells. These changes in cardiomyocyte function in vitro were manifested in vivo as impaired diastolic function both at baseline and after stimulation with isoprenaline. Thus, cTnI has important roles in regulating the Ca2+ sensitivity of cardiac myofibrils and controlling cardiomyocyte relaxation and cardiac diastolic function. cTnI is also required for the normal responsiveness of cardiomyocytes to β‐adrenergic receptor stimulation.

Effect of Cardiac Myosin Binding Protein-C on Mechanoenergetics in Mouse Myocardium

“…27,34 The strikingly similar calcium sensitivities in the t/t PTU and ϩ/ϩ PTU we report here may therefore indicate that cMyBP-C does not significantly affect calcium sensitivity of isometric tension, just as cMyBP-C phosphorylation also does not affect calcium sensitivity. 17,18 A reduction in isometric tension with increasing [P i ], as observed in the myofilaments lacking cMyBP-C, can be explained by a higher probability of reversal of the forceproducing power stroke of the acto-myosin crossbridge, which would occur with a higher probability of a backreaction of the phosphate-release step. 35,36 Our finding that a lack of cMyBP-C furthermore increases the characteristic frequency and the phosphate-dependence of oscillatory work production is intriguing.…”

“…[12][13][14] The possible effects of cMyBP-C and of its phosphorylation on actomyosin crossbridge kinetics are not yet clear, 15,16 although its phosphorylation does not apparently affect the tension-pCa relationship. 17,18 The effect of cMyBP-C itself on the tensionpCa relationship has so far proved equivocal: the chemical extraction of cMyBP-C from rat cardiomyocytes enhances calcium activation, 16 whereas the absence of cMyBP-C in mouse myocardium inhibits, 19 does not affect, 11 or enhances 9 calcium activation. Some of the discrepancies in these observations may be caused by differing experimental preparations and methods and/or the variable shifts of sarcomeric protein isoforms, such as myosin heavy chain (MHC) isoform from ␣-MHC to ␤-MHC, that occur in mouse models deficient in cMyBP-C. 19,20 Curiously, in both mouse models so far demonstrated to lack sarcomeric cMyBP-C, the LV is dilated, systolic elastance is dramatically reduced compared with normal, and the duration of ejection phase is similarly abbreviated.…”

The unique functions of cardiac troponin I in the control of cardiac muscle contraction and relaxation

“…We chose the slow skeletal isoform for these studies inasmuch as it is naturally expressed in the embryonic/neonatal period. Hearts of TG-ssTnI mice had slower relaxation kinetics, depressed effects of β-adrenergic stimulation [8], and protection against acidosis [9,10] and ischemia/reperfusion injury [11]. All of these properties are properties of preparations from neonatal rat hearts, and indicated that the special cardiac function in the neonatal period is importantly controlled by the isoform of TnI.…”

“…Although it had been reported that phosphorylation of this region depressed sarcomeric response to Ca 2+ and increased cross-bridge kinetics [2,3], studies with transgenic models have provided the strongest evidence for a critical role of Tn, especially cTnI, in cardiac function. The first series of studies were carried out with a transgenic mouse model expressing ssTnI (TG-ssTnI) to the exclusion of cTnI in the cardiac compartment [8]. We chose the slow skeletal isoform for these studies inasmuch as it is naturally expressed in the embryonic/neonatal period.…”

The Role of Troponins in Muscle Contraction

“…Permeabilized cardiomyocytes from transgenic mice overexpressing STnI in the heart displayed an increased Ca 2+ sensitivity of force and a lack of responsiveness to PKA phosphorylation (48). It has been demonstrated that different TnI isoforms contribute to different responses of cardiac vs. fast skeletal vs. slow skeletal muscles to acidic pH, with the CTnI isoform contributing the most to the susceptibility of the muscle to acidosis-dependent deactivation (49).…”