Investigations into tropomyosin function using mouse models

Abstract

Tropomyosin plays a key role in controlling calcium regulated sarcomeric contraction through its interactions with actin and the troponin complex. The focus of this review is on striated muscle tropomyosin isoforms and the in vivo approach we have taken to define the functional differences among these isoforms in regulating cardiac physiology. In addition, we address specific regions within tropomyosin that differ among the isoforms to impart differences in the physiological performance of muscle and the sarcomere itself. There is a high degree of amino acid identity among the three striated muscle α-, β-, and γ-tropomyosin isoforms; this identity ranges from 86% -91%. We employ transgenic mouse model systems that express the different tropomyosin isoforms or chimeric tropomyosin molecules specifically in the myocardium. Results show the three isoforms differentially regulate the rates of cardiac contraction and relaxation, along with conferring differences in myofilament calcium sensitivity and sarcomere tension development. We also found the putative troponin T binding regions of tropomyosin (amino acids 175-190 and 258-284) appear to a play significant role in imparting these physiological differences that are observed during cardiac and sarcomeric contraction/relaxation. In addition, we have successfully used chimeric tropomyosin molecules to rescue cardiomyopathic diseased mice by normalizing sarcomeric performance. These studies illustrate not only the importance of tropomyosin structure and function for understanding muscle physiology, but also demonstrate how this information can potentially be used for gene therapy.

Patterns of tropomyosin and troponin-T isoform expression in jaw-closing muscles of mammals and reptiles that express masticatory myosin

“…They reported that Tm- is associated with a slowing of relaxation and an increase in the Ca 2+ -sensitivity of force generation compared with control myocardium, which expresses predominantly fast-type Tm-a. Slowtype Tm-a increases isometric contraction and relaxation rates and decreases Ca 2+ -sensitivity in transgenic mouse hearts (Jagatheesan et al, 2010). Our results show that the jaw-closing muscles of members of Carnivora express little or no Tm-, which suggests that contractions could be faster and that the Ca 2+ -sensitivity of force generation would be lower.…”

“…At least four genes encode the Tm isoforms that are expressed in skeletal and cardiac muscles (reviewed in Perry, 2001;Gunning et al, 2005;Jagatheesan et al, 2010) and one gene codes for multiple fast-type TnT isoforms in vertebrate skeletal muscle (Medford et al, 1984;Wilkinson et al, 1984;Breitbart et al, 1985;Hastings et al, 1985). The expression of these proteins is regulated by extensive alternative RNA splicing, which has the potential to yield complex patterns between muscles of a given organism or even between individual fibers within a specific muscle (e.g.…”

“…Wieczorek and co-workers manipulated Tm isoform expression in mouse myocardium in a series of studies (reviewed in Jagatheesan et al, 2010) to examine the role of fast-type Tm-a, slow-type Tma (also referred to as Tm-g or TPM 3) and Tm- in the regulation of contractile properties of striated muscle. They reported that Tm- is associated with a slowing of relaxation and an increase in the Ca 2+ -sensitivity of force generation compared with control myocardium, which expresses predominantly fast-type Tm-a.…”

“…TnT and Tm are expressed as diverse families of isoforms through alternative RNA splicing (reviewed in Perry, 1998;Perry, 2001;Wolska and Wieczoreck, 2003;Gunning et al, 2005;Jagatheesan et al, 2010). Different isoforms of TnT and Tm are associated with variations in the Ca 2+ -sensitivity or cooperativity of force generation in muscle, as determined by shifts in the force- [Ca 2+ ] relationship in fibers in which the surface membrane is removed or rendered hyperpermeable (e.g.…”

Rescue of Familial Hypertrophic Cardiomyopathy by Altering Sarcomeric Exposure and Response to Calcium

“…To compensate for this increased calcium sensitivity, we generated a double-transgenic mouse expressing a chimeric Tm containing the α-Tm amino terminus and the carboxyl terminus of β-Tm, the fetal cardiac isoform (Chi 1) (30). Previous work demonstrated that myofilaments from these Chi 1 mice have a decreased sensitivity to calcium (33,34). The double-transgenic mice which were created by crossing the 2 lines displayed normalized myofilament calcium sensitivity.…”