Proteolytic N-terminal Truncation of Cardiac Troponin I Enhances Ventricular Diastolic Function

John C Barbato,Qi-Quan Huang,M Moazzem Hossain,Meredith Bond,Jian-Ping Jin

doi:10.1074/jbc.m408525200

John C Barbato, Qi-Quan Huang + Show 3 more

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https://doi.org/10.1074/jbc.m408525200

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Abstract

Besides the core structure conserved in all troponin I isoforms, cardiac troponin I (cTnI) has an N-terminal extension that contains phosphorylation sites for protein kinase A under beta-adrenergic regulation. A restricted cleavage of this N-terminal regulatory domain occurs in normal cardiac muscle and is up-regulated during hemodynamic adaptation (Z.-B. Yu, L.-F. Zhang, and J.-P. Jin (2001) J. Biol. Chem. 276, 15753-15760). In the present study, we developed transgenic mice overexpressing the N-terminal truncated cTnI (cTnI-ND) in the heart to examine its biochemical and physiological significance. Ca(2+)-activated actomyosin ATPase activity showed that cTnI-ND myofibrils had lower affinity for Ca(2+) than controls, similar to the effect of isoproterenol treatment. In vivo and isolated working heart experiments revealed that cTnI-ND hearts had a significantly faster rate of relaxation and lower left ventricular end diastolic pressure compared with controls. The higher baseline relaxation rate of cTnI-ND hearts was at a level similar to that of wild type mouse hearts under beta-adrenergic stimulation. The decrease in cardiac output due to lowered preload was significantly smaller for cTnI-ND hearts compared with controls. These findings indicate that removal of the N-terminal extension of cTnI via restricted proteolysis enhances cardiac function by increasing the rate of myocardial relaxation and lowering left ventricular end diastolic pressure to facilitate ventricular filling, thus resulting in better utilization of the Frank-Starling mechanism.

Highlights

Besides the core structure conserved in all troponin I isoforms, cardiac troponin I has an N-terminal extension that contains phosphorylation sites for protein kinase A under ␤-adrenergic regulation
This paper is available on line at http://www.jbc.org cardiac function, we developed transgenic mice with postnatal overexpression of N terminus-truncated cardiac troponin I (cTnI) that lacks amino acids 1–28 for biochemical and physiological characterization. cTnI-ND cardiac myofibrils showed a lower affinity for Ca2ϩ than controls in actomyosin ATPase assay, similar to the effect of isoproterenol treatment. cTnI-ND hearts had a significantly faster rate of relaxation and lower left ventricular end diastolic pressure (LVEDP) compared with controls
Densitometry of Western blots showed that in wild type mouse hearts, cTnI-ND produced by endogenous proteolysis is within a range of up to 10% of the amount of total cTnI, similar to that found in the rat hearts [7]

Summary

Introduction

Besides the core structure conserved in all troponin I isoforms, cardiac troponin I (cTnI) has an N-terminal extension that contains phosphorylation sites for protein kinase A under ␤-adrenergic regulation. The decrease in cardiac output due to lowered preload was significantly smaller for cTnI-ND hearts compared with controls These findings indicate that removal of the N-terminal extension of cTnI via restricted proteolysis enhances cardiac function by increasing the rate of myocardial relaxation and lowering left ventricular end diastolic pressure to facilitate ventricular filling, resulting in better utilization of the Frank-Starling mechanism. In a rat tail-hanging model simulating the effect of weightlessness on the cardiovascular system, an N-terminal truncated cTnI was found to be up-regulated in the heart after 3– 4 weeks of simulated weightlessness [7]. The N terminus-truncated cTnI is present in normal hearts of all species examined [7], indicating that proteolytic removal of the N-terminal extension of cTnI, including the PKA substrate sites, may be a novel mechanism to regulate cardiac function under physiological and stress conditions. Understanding the functional effect of this structural modification of cTnI will increase our knowledge of mechanisms regulating myocardial contraction and may identify a molecular target for potential therapeutic enhancement of cardiac function

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