Tropomyosin 3 expression leads to hypercontractility and attenuates myofilament length-dependent Ca²⁺activation

Abstract

Tropomyosin (TM), an integral component of the thin filament, is encoded by three striated muscle isoforms: alpha-TM, beta-TM, and TPM 3. Although the alpha-TM and beta-TM isoforms are well characterized, less is known about the function of the TPM 3 isoform, which is predominantly found in the slow-twitch musculature of mammals. To determine its functional significance, we ectopically expressed this isoform in the hearts of transgenic mice. We generated six transgenic mouse lines that produce varying levels of TPM 3 message with ectopic TPM 3 protein accounting for 40-60% of the total striated muscle tropomyosin. The transgenic mice have normal life spans and exhibit no morphological abnormalities in their sarcomeres or hearts. However, there are significant functional alterations in cardiac performance. Physiological assessment of these mice by using closed-chest analyses and a work-performing model reveals a hyperdynamic effect on systolic and diastolic function. Analysis of detergent-extracted fiber bundles demonstrates a decreased sensitivity to Ca(2+) in force generation and a decrease in length-dependent Ca(2+) activation with no detectable change in interfilament spacing as determined by using X-ray diffraction. Our data are the first to demonstrate that TM isoforms can affect sarcomeric performance by decreasing sensitivity to Ca(2+) and influencing the length-dependent Ca(2+) activation.

Comparison of gene expression of 2-mo denervated, 2-mo stimulated-denervated, and control rat skeletal muscles

“…There are three striated muscle-encoded isoforms: ␣-TPM, ␤-TPM, and TPM3. Switching from ␣-TPM to TPM3 is associated with a decrease in myofilament Ca 2ϩ sensitivity and an attenuation of length-dependent activation (76). TPM3 isoform is predominantly found in the slow-twitch muscles (75), and the upregulation of TPM3 mRNA expression correlates with the increase in the number of slow fibers in denervated EDL muscles seen in the present study.…”

“…Increased expression of ubiquitous actin-binding protein tropomyosin-3 (TPM3) in EDL muscles after denervation might be related to the stabilization of muscle structure. Tropomyosins are localized to the thin filament of the sarcomeres, and their function includes stabilization of thin filaments (76). There are three striated muscle-encoded isoforms: ␣-TPM, ␤-TPM, and TPM3.…”

The Insulin-like Growth Factor 1 Receptor Induces Physiological Heart Growth via the Phosphoinositide 3-Kinase(p110α) Pathway

“…In the current study, TPM3 was positively correlated with HW/BW ratio whereas TPM2 was inversely related to the hypertrophic phenotype. The striated muscle form of TPM3 is expressed in the adult human heart (64), and cardiac overexpression of TPM3 in transgenic mice leads to hypercontractility (65). By contrast, TPM2 is usually expressed at low levels during postnatal life (66), and transgenic mice with low overexpression of TPM2 displayed impairment of diastolic function, which was linked to impaired myocyte relaxation and decreased maximal tension in isolated preparations (67,68).…”

An internal domain of β-tropomyosin increases myofilament Ca2+sensitivity

“…Through its interactions with the troponin (Tn) complex, TM shifts its conformational position on actin in response to Ca 2ϩ binding to TnC; this repositioning of TM on actin exposes the myosin-binding site on actin and facilitates muscle contraction. Upon cessation of stimulation, there is a resequestration of Ca 2ϩ into the sarcoplasmic reticulum and extrusion of Ca 2ϩ from the cell, resulting in TM returning to its original position as the muscle fiber relaxes.Previous studies (18,24,25) in our laboratory have established there are physiological differences among the three highly conserved striated muscle TM isoforms. The ␣-TM isoform is the predominant isoform in both skeletal and cardiac musculature in the mouse, constituting ϳ98% of total TM protein in the heart.…”

“…Previous studies (18,24,25) in our laboratory have established there are physiological differences among the three highly conserved striated muscle TM isoforms. The ␣-TM isoform is the predominant isoform in both skeletal and cardiac musculature in the mouse, constituting ϳ98% of total TM protein in the heart.…”