Molecular and Physiological Effects of Overexpressing Striated Muscle β-Tropomyosin in the Adult Murine Heart

doi:10.1074/jbc.270.51.30593

Molecular and Physiological Effects of Overexpressing Striated Muscle β-Tropomyosin in the Adult Murine Heart (*)

From the (1)Department of Molecular Genetics, Biochemistry, and Microbiology, the
(2)Department of Pharmacology and Cell Biophysics, the
(3)Department of Physiology and Biophysics and Medicine, and the
(4)Department of Pathology and Laboratory Medicine, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267

^¶Recipient of a grant from the Muscular Dystrophy Association and an Established Investigator of the American Heart Association, with funds contributed in part by the American Heart Association, Ohio Affiliate. To whom correspondence should be addressed: Tel.: 513-558-0058; Fax: 513-558-8474.

↵^§ Present Address: Dept. of Veterinary Pathobiology, Texas A & M University, College Station, TX 77843.

Abstract

Tropomyosins comprise a family of actin-binding proteins that are central to the control of calcium-regulated striated muscle contraction. To understand the functional role of tropomyosin isoform differences in cardiac muscle, we generated transgenic mice that overexpress striated muscle-specific β-tropomyosin in the adult heart. Nine transgenic lines show a 150-fold increase in β-tropomyosin mRNA expression in the heart, along with a 34-fold increase in the associated protein. This increase in β-tropomyosin message and protein causes a concomitant decrease in the level of α-tropomyosin transcripts and their associated protein. There is a preferential formation of the αβ-heterodimer in the transgenic mouse myofibrils, and there are no detectable alterations in the expression of other contractile protein genes, including the endogenous β-tropomyosin isoform. When expression from the β-tropomyosin transgene is terminated, α-tropomyosin expression returns to normal levels. No structural changes were observed in these transgenic hearts nor in the associated sarcomeres. Interestingly, physiological analyses of these hearts using a work-performing model reveal a significant effect on diastolic function. As such, this study demonstrates that a coordinate regulatory mechanism exists between α- and β-tropomyosin gene expression in the murine heart, which results in a functional correlation between α- and β-tropomyosin isoform content and cardiac performance.

Footnotes

↵* This work was partially supported by National Institutes of Health Grant HL 46826 (to D. F. W.) and by American Heart Association, Ohio Affiliate, Grant SW-93-36-F (to M. M.). Additional support for transgenic animal production was received from Marion Merrell Dow. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
↵¹ The abbreviations used are:

TM

tropomyosin

TMstr

striated tropomyosin

Tn

troponin

MHC

myosin heavy chain

TG

transgenic

NTG

nontransgenic

UTR

untranslated region

kb

kilobase(s)

PCR

polymerase chain reaction

bp

base pair(s)

PTU

5-propyl-2-thiouracil
↵²J. Robbins, personal communication.
↵³L. Leinwand, personal communication.
↵⁴J. Solaro, personal communication.
- Received July 11, 1995.
- Revision received September 5, 1995.