To Phosphorylate or Not to Phosphorylate: The Role of Tropomyosin Phosphorylation in Cardiac Function and Disease

Tropomyosin (Tm) is an α -helical coiled-coil protein crucial in the calcium dependent regulation of the thin filament of the sarcomere. α-Tm is phosphorylated solely at serine 283 and phosphorylation levels are tightly regulated. Approximately 70% of total cardiac Tm phosphorylated during fetal development, decreasing to 30% during adulthood. Total Tm phosphorylation is altered in multiple mouse models of cardiac disease, including hypertrophic cardiomyopathy, dilated cardiomyopathy and myocardial infarction, indicating that Tm phosphorylation may play a role in the initiation, progression or modulation of cardiac disease. To determine the effect of loss of α-Tm phosphorylation, a transgenic (TG) mouse model was generated in which cardiac specific &alspha;-Tm expresses an alanine at amino acid 283 rather than a serine (α-Tm S283A). Counter to previous studies on Tm phosphorylation performed <i>in vitro</i>, at basal levels, significantly decreased α-Tm phosphorylation has no effect. TG animals exhibit normal cardiac function, efficient contractility and relaxation under basal conditions and under α-adrenergic stimulation. When α-Tm S283A TG animals are subjected to transaortic constriction (TAC), the TG TAC animals fail more quickly than the non-transgenic (NTG) TAC animals. In TG hearts, there is an increase in SERCA2a expression and an increase in PLN phosphorylation at Ser16. This increase in energetic demand placed on the heart both by the increase in SERCA2a activity coupled with the increased energetic demand that occurs during the onset of cardiac disease may be the basis of the more rapid cardiac failure in the TG TAC mice. After examining the effect of decreased α-Tm phosphorylation in the context of an acute, extrinsic cardiac stressor, a model of decreased α-Tm phosphorylation was made in the context of a chronic, intrinsic stressor. These α-Tm 180-S283A double mutant TG (DMTG) animals were surprising in that the α-Tm 180 familial hypertrophic cardiomyopathy phenotype was completely rescued in a TG line that exhibited Tm phosphorylation levels similar those seen in the α-Tm S283A TG hearts. This rescue occurred at morphological and physiological levels, including echocardiographic analysis. In addition, examination of detergent extracted skinned fiber bundles indicate that Ca²⁺ sensitivity and cooperativity (<i>n_H</i>) is rescued to NTG levels, indicating that the rescue occurs primarily at the level of the sarcomere. DMTG mutant hearts show a significant decrease in PLN phosphorylation at both Ser16 and Thr17 compared to NTG and α-Tm S283A TG hearts. However, DMTG levels of PLN phosphorylation at both sites are significantly increased compared to α-Tm 180 levels, which may account for the hypercontractile phenotype seen in DMTG mice when evaluated via echocardiography. These data seem to indicate that dephosphorylating Tm in the context of hypertrophy might be of benefit to the overall function of the heart. However, it is important to examine the effect of increased Tm phosphorylation in the context of other cardiac disease, such as dilated cardiomyopathy or myocardial infarction. The studies presented here, in addition to the proposed studies, would increase understanding of the functional consequences of Tm phosphorylation and may lead to possible therapeutic interventions.

Name thesis:degree_name

PhD

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Medicine: Molecular Genetics, Biochemistry, and Microbiology

Grantor dc:publisher

University of Cincinnati

Year dc:date

2012

Author dc:creator

• Schulz, Emily M.

Contributors dc:contributor

• Wieczorek, David

• Molecular Biology
• tropomyosin
• phosphorylation
• cardiomyopathy
• rescue
• cardiac

Statement dc:rights

• unrestricted
• This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.

Language dc:language

English

Repository record dc:identifier

http://rave.ohiolink.edu/etdc/view?acc_num=ucin1355156757

OAI identifier oai:identifier

oai:etd.ohiolink.edu:ucin1355156757