Molecular mechanisms of dysfunction of muscle fibres associated with Glu139 deletion in TPM2 gene

Yurii S Borovikov,Adam Piers,Nikita A Rysev,Stanislava V Avrova,Olga E Karpicheva,Charles S Redwood,Vladimir V Sirenko

doi:10.1038/s41598-017-17076-9

Abstract

Deletion of Glu139 in β-tropomyosin caused by a point mutation in TPM2 gene is associated with cap myopathy characterized by high myofilament Ca2+-sensitivity and muscle weakness. To reveal the mechanism of these disorders at molecular level, mobility and spatial rearrangements of actin, tropomyosin and the myosin heads at different stages of actomyosin cycle in reconstituted single ghost fibres were investigated by polarized fluorescence microscopy. The mutation did not alter tropomyosin’s affinity for actin but increased strongly the flexibility of tropomyosin and kept its strands near the inner domain of actin. The ability of troponin to switch actin monomers “on” and “off” at high and low Ca2+, respectively, was increased, and the movement of tropomyosin towards the blocked position at low Ca2+ was inhibited, presumably causing higher Ca2+-sensitivity. The mutation decreased also the amount of the myosin heads which bound strongly to actin at high Ca2+ and increased the number of these heads at relaxation; this may contribute to contractures and muscle weakness.

Highlights

The muscle thin filament is a cooperative-allosteric system composed of a backbone of actin monomers with the troponin-tropomyosin (TN-Tpm) complex located lengthwise along the F-actin[1]
We showed that the ΔE139 mutation increases flexibility of Tpm strands and “freezes” Tpm in a position close to the inner domains of actin throughout the ATPase cycle, and postulate that this may be one of the reasons for the higher Ca2+- sensitivity and induction of contractures and muscle fibre weakness
The filaments were assembled with the wild-type tropomyosin (WTTpm) or ΔE139Tpm (Fig. 1) and used in measurements of actin-activated myosin S1 ATPase activity at increasing Ca2+ concentrations

Summary

Introduction

The muscle thin filament is a cooperative-allosteric system composed of a backbone of actin monomers with the troponin-tropomyosin (TN-Tpm) complex located lengthwise along the F-actin[1]. At low levels of Ca2+, TN interacts with actin to change the conformation of actin monomers and to switch them “off ”3,4, and constrains Tpm in a position close to the outer domain of actin (“blocked position”)[5] This is the so-called “OFF” state of the thin filament in which Tpm and actin[3,5] inhibit the strong binding of myosin cross-bridges to actin and, the actin-activated myosin ATPase and muscle contraction[1]. When the myosin heads strongly attach to the actin filament, Tpm takes a position over the inner domain of actin (“open position”)[5] and the majority of actin monomers are “switched on”[3], the thin filament transits to the so-called “ON” state In this state Tpm fully exposes the myosin binding sites on F-actin[1,5] and, activates the actin-activated myosin ATPase and initiates muscle contraction. We showed that the ΔE139 mutation increases flexibility of Tpm strands and “freezes” Tpm in a position close to the inner domains of actin throughout the ATPase cycle, and postulate that this may be one of the reasons for the higher Ca2+- sensitivity and induction of contractures and muscle fibre weakness

Objectives

Methods

Results

Conclusion