Molecular Mechanisms of Muscle Weakness Associated with E173A Mutation in Tpm3.12. Troponin Ca2+ Sensitivity Inhibitor W7 Can Reduce the Damaging Effect of This Mutation

Abstract

Substitution of Ala for Glu residue in position 173 of γ-tropomyosin (Tpm3.12) is associated with muscle weakness. Here we observe that this mutation increases myofilament Ca2+-sensitivity and inhibits in vitro actin-activated ATPase activity of myosin subfragment-1 at high Ca2+. In order to determine the critical conformational changes in myosin, actin and tropomyosin caused by the mutation, we used the technique of polarized fluorimetry. It was found that this mutation changes the spatial arrangement of actin monomers and myosin heads, and the position of the mutant tropomyosin on the thin filaments in muscle fibres at various mimicked stages of the ATPase cycle. At low Ca2+ the E173A mutant tropomyosin shifts towards the inner domains of actin at all stages of the cycle, and this is accompanied by an increase in the number of switched-on actin monomers and myosin heads strongly bound to F-actin even at relaxation. Contrarily, at high Ca2+ the amount of the strongly bound myosin heads slightly decreases. These changes in the balance of the strongly bound myosin heads in the ATPase cycle may underlie the occurrence of muscle weakness. W7, an inhibitor of troponin Ca2+-sensitivity, restores the increase in the number of myosin heads strongly bound to F-actin at high Ca2+ and stops their strong binding at relaxation, suggesting the possibility of using Ca2+-desensitizers to reduce the damaging effect of the E173A mutation on muscle fibre contractility.

Looking for Targets to Restore the Contractile Function in Congenital Myopathy Caused by Gln147Pro Tropomyosin

“…In control, the activation of thin filaments by troponin-high Ca 2+ and the myosin heads is accompanied by the increase in the Φ E value for actin-FITC, and by the decrease in this value for Tpm-AF and S1-AEDANS. This pattern of changes has also been observed in our previous studies [ 3 , 26 , 28 ]. The changes in the value of Φ E for Tpm-AF relative to actin are interpreted here according to a widespread theory of the Tpm shifting over actin surface from the outer to the inner actin domains [ 4 ].…”

“…The value of ε for Tpm varies ambiguously, but as a rule, it changes in the opposite direction as compared with actin ( Figure 2 b). As was proposed previously [ 7 , 28 ], the changes in bending stiffness of the filaments mean the changes in their persistence length. Usually, an increase in the persistence length of Tpm strands correlates with a decrease in the persistence length of actin filaments and vice versa.…”

“…Usually, an increase in the persistence length of Tpm strands correlates with a decrease in the persistence length of actin filaments and vice versa. These changes easily explain the spatial rearrangement of Tpm relative to actin in the process of regulation of actin-myosin interaction [ 28 ].…”

“…Since the decrease in the value of Φ E for Tpm-AF is observed in the control under activation conditions, then such changes are considered by us as a shift of Tpm strands towards the inner actin domains [ 3 , 26 , 29 ]. The change in a position of the Tpm strands relative to the inner domain of actin may be associated with a disparity in the alterations in the persistence length of Tpm strands and actin filaments that presumably cause azimuthal shifting of the Tpm strands [ 6 , 26 , 28 , 29 ]. For example, if the Tpm strands at transition from the ON to the OFF state undergo a greater elongation than does F-actin, it may cause an azimuthal shift of the Tpm strands towards the outer domain of actin [ 6 , 26 , 28 , 29 , 51 ].…”

“…The change in a position of the Tpm strands relative to the inner domain of actin may be associated with a disparity in the alterations in the persistence length of Tpm strands and actin filaments that presumably cause azimuthal shifting of the Tpm strands [ 6 , 26 , 28 , 29 ]. For example, if the Tpm strands at transition from the ON to the OFF state undergo a greater elongation than does F-actin, it may cause an azimuthal shift of the Tpm strands towards the outer domain of actin [ 6 , 26 , 28 , 29 , 51 ]. Conversely, a lower compared to F-actin shortening of the Tpm strands move them to the inner domain of actin [ 6 , 26 , 28 , 29 , 51 ].…”