Functional Importance of the Carboxyl-terminal Region of Striated Muscle Tropomyosin

Abstract

Striated muscle tropomyosin (TM) interacts with actin and the troponin complex to regulate calcium-mediated muscle contraction. Previous work by our laboratory established that ␣-and ␤-TM isoforms elicit physiological differences in sarcomeric performance. Heart myofilaments containing ␤-TM exhibit an increased sensitivity to calcium that is associated with a decrease in the rate of relaxation and a prolonged time of relaxation. To address whether the carboxyl-terminal, troponin T binding domain of ␤-TM is responsible for these physiological alterations, we exchanged the 27 terminal amino acids of ␣-TM (amino acids 258 -284) for the corresponding region in ␤-TM. Hearts of transgenic mice that express this chimeric TM protein exhibit significant decreases in their rates of contraction and relaxation when assessed by ex vivo work-performing cardiac analyses. There are increases in the time to peak pressure and a dramatic increase in end diastolic pressure. In myofilaments, this chimeric protein induces depression of maximum tension and ATPase rate, together with a significant decrease in sensitivity to calcium. Our data are the first to demonstrate that the TM isoformspecific carboxyl terminus is a critical determinant of sarcomere performance and calcium sensitivity in both the whole heart and in isolated myofilaments. Tropomyosin (TM),1 a ␣-helical coiled-coil dimer, plays an essential role in the regulation of contraction and relaxation of the sarcomere. TM regulates sarcomeric performance through its binding to actin and the troponin complex. During muscle relaxation when cytoplasmic levels of calcium are low, tropomyosin blocks the myosin-binding site on the filamentous striated muscle actin. Upon stimulation, calcium is released into the myofilament space from the sarcoplasmic reticulum, via the ryanodine receptor Ca 2ϩ channel, and from extracellular stores; this calcium binds to troponin C within the troponin complex, which causes a conformational change in the position of TM on actin, thereby exposing the myosin-binding site. Myosin binds to actin and triggers muscle contractile activity until the stimulation ceases, and calcium is re-sequestered into the sarcoplasmic reticulum.Recent studies (1-3) in our laboratory have demonstrated that there are functional 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 adult murine heart, this isoform constitutes ϳ98% of the total TM. The remaining 2% of TM is ␤-TM, which is 86% identical to ␣-TM. In the skeletal musculature, ␤-TM expression is much greater, with levels dependent upon the specific muscle. TPM 3 (TM 30sk, ␥-TM) is not expressed in the murine heart but is found in slow-twitch skeletal musculature, such as the soleus where it composes ϳ30% of all the TMs (4). The TPM 3 isoform is 93% identical to ␣-TM and is 86% identical to ␤-TM. Studies in our laboratory using transgenic mice that express ␤-TM in the heart demonstrate that ␤-TM increas...

Myofibrillar remodelling in cardiac hypertrophy, heart failure and cardiomyopathies

“…A markedly increased expression of β-TM in mouse hearts (α-TM to β-TM ratio of approximately 1:4) leads to the death of animals soon after birth, along with severe cardiac abnormalities in both contraction and relaxation (197). However, the overexpression of the wild type α-TM in the heart does not lead to any morphological or physiological alterations in cardiac performance (200). A potential mechanism for the difference in function between these two isoforms, which are 86% identical, is likely in their interaction with the neighbouring thin filament molecules.…”

“…A potential mechanism for the difference in function between these two isoforms, which are 86% identical, is likely in their interaction with the neighbouring thin filament molecules. Thus, TM isoform shift may account for weaker binding of fast skeletal β-TM to cTnT and may be a major determinant in modulating cardiac muscle performance and regulating Ca 2+ sensitivity of the myofilaments (196,200). Four mutations of the TM-encoding gene have been described in fHCM so far.…”

N Terminus Is Essential for Tropomyosin Functions

“…Most interesting, cells expressing the chimera retained normal microfilament architecture. More recent studies using transgenic expression of chimeras of ␣ and ␤ TMs have identified that the C terminal portion of TM is an important determinant of cardiac function (53).…”

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.…”