Huang, Xupei

Troponin I is a contractile protein and plays an important role in cardiac function.
We have generated cTnl knockout and cTnI(R192H) transgenic mouse models. All of
cTnl knockout homozygous mice die at 17-18 days after birth. Some of cTnI(R192H)
transgenic mice die at early life stages, some mice develop heart failure at late stages.
High-resolution ultrasound imaging and Doppler echocardiography have been used to
evaluate cardiac function on cTnl deficient mice and cTnl(R192H) transgenic mice.
cTnI mice have damaged relaxation with gradually decreased E/A ratio(E/A<1). FS
and cardiac output dramatically decrease on 17-day-o1d cTnI mice indicating severe
cardiac dysfunction. We find that the damaged heart function is correspondent with the
Tnl expression level decline. 6-8 weeks transgenic mice have shown that the dimension
of left and right atria increase. In 15-month-old transgenic mice, the E/A ratio shows a
pseudonormal pattern indicating a diastolic dysfunction. This study demonstrate that
damaged heart function is tightly associated with Tnl levels in the heart.

Cardiomyopathy is a disorder which affects the heart muscle and causes varied physiological dysfunctions. Restrictive cardiomyopathy RCM is a cardiac muscle disorder in which the left ventricle becomes stiff, due to relaxation impairment. Mutations of the sarcomeric protein cardiac troponin I cTnI gene is found to cause idiopathic RCM. These mutations of cTnI are located in the C-terminus and affect cardiac relaxation. Transgenic mouse models presenting the pathology observed in clinical patients with RCM have been generated by expressing the mutant cTnI in the heart. RCM-linked mutations increase cardiac myofilament Ca2 sensitivity and promote diastolic dysfunction in the heart. In our laboratory, we have generated double transgenic mice cTnI R193H/ND/KO by crossing the cTnI R193H mice with transgenic cTnI-N terminal truncated mice cTnI-ND. Previous studies using the double transgenic mice showed that ventricular relaxation is enhanced in these mice. This study’s aim is to investigate whether or not the cardiac troponin I N-terminal deletion cTnI-ND corrects the debilitating effects caused by the cTnI R193H high expression in the heart. In doing so, we have first confirmed the genetypes of each experimental group. Echocardiography measurements have been performed on the animals at age 19-21 days. Our data indicate that cTnI R193H/KO showed a significant diastolic dysfunction, whereas the cardiac function in double transgenic mice did not show any difference compared to that in the wild type group, suggesting a functional correction by cTnI-ND in RCM phenotype.
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Restrictive cardiomyopathy (RCM) is represented in part by left ventricular stiffness
and diastolic dysfunction. Missense mutations of the cardiac troponin I (cTnI) gene cause
idiopathic RCM. These mutations are located in the C-terminus of cTnI and affect cardiac
relaxation. Transgenic mouse models presenting the pathology observed in clinical
patients with RCM have been generated previously and express the mutant cTnI in their
hearts. RCM-linked mutations increase cardiac myofilament Ca2+ sensitivity and promote
diastolic dysfunction in the heart. Previous studies using double transgenic mice
(cTnI/R193H/ND) showed that ventricular relaxation is enhanced in the cTnI/R193H
transgenic mice. In this study, another double transgenic mouse model,
(cTnI/R193H/ND/KO), provides an avenue to investigate its rescuing effects on RCMlinked
mutations in the cTnI /R193H/KO mouse. Use of molecular biological techniques,
transgenic animal developments and murine echocardiography in this study has
culminated into a greater understanding of RCM and diastolic dysfunction.

Troponin I (TnI) plays an important role in cardiac muscle contraction. Two TnI genes (cardiac and slow skeletal TnI) are predominantly expressed in the heart. In cTnI knockout mice, myocardial TnI deficiency results in a diastolic dysfunction and a sudden death in homozygous mutants. In the present studies, energy metabolism has been analyzed in myocardial cells from cTnI null hearts. Our results have demonstrated that damaged relaxation and increased Ca2+-independent force production in cTnI null hearts stimulated myofibril MgATPase activities accompanied by the increase of mitochondria quantity and ATPase activities. In addition, an increase of ssTnI phosphorylation level has been observed in cTnI null hearts. The results indicate that TnI deficiency can cause the disturbance of energy metabolism and some protein overphosphorylation.