Tropomyosin in the groove? Molecular insights into an inherited myopathy

Inherited myopathies in skeletal and cardiac muscle can result from mutations in troponin or tropomyosin (Michele et al. 2002; Ahmad et al. 2005; Robinson et al. 2007) – components of the thin filament that confer Ca²⁺ regulation to striated muscle contraction (Gordon et al. 2000). Mechanisms connecting mutation to disease are elusive. The first physiological study on a novel tropomyosin mutation in distal arthrogryposis (DA) provides new and unexpected molecular insights into contractile dysfunction (Ochala et al. 2007).

Troponin (Tn) is a complex of three distinct polypeptides (TnT, TnI and TnC), while tropomyosin (Tm) is a dimer of similar or identical polypeptides that form an α-helical coiled-coil. Tm molecules join end-to-end (head-to-tail), with two such strands extending the length of a thin filament (Perry, 2001). Ca²⁺ binding to Tn triggers contraction by dynamically displacing Tm on actin, exposing sites for myosin cross-bridge formation (Gordon et al. 2000).

Tn and Tm mutations often alter Ca²⁺ sensitivity of contractility. In the heart, severity of familial hypertrophic cardiomyopathy is related to the degree to which Ca²⁺ sensitivity is increased by mutations (Gomes & Potter, 2004). Adding to this small but growing list of disease-related mutations in thin filament proteins, Tajsharghi et al. (2007) identified a point mutation in the β-Tm gene (TPM2) that correlated with DA type 2B; sequence analyses of genes for troponin subunits I (TNNI2) and T (TNNT3) were normal, and unaffected relatives did not have the TPM2 mutation. The two patients (mother and daughter) exhibited contractures of the distal joints at birth, and weakness as adults in proximal and distal muscles most prominent in the hands and feet (Tajsharghi et al. 2007) (additional details about DA type IIB are available in the Online Mendelian Inheritance in Man (OMIM) database entry 601680). Missense mutation R133W is near the middle of the Tm molecule, in the region coded for by exon 4 that is common to all alternatively spliced variants of β-Tm; splice variants of β-Tm transcript are found not only in skeletal muscle along with α-Tm (TPM1), but also in smooth muscle and fibroblasts (Perry, 2001). The R133W mutation removes a charge predicted to stabilize Tm's α-helical coiled-coil by its location at the g position of the seven-residue structural repeat (Tajsharghi et al. 2007). Based on this predicted destabilization of structure and the clinical observation of muscle hypercontraction at birth, β-Tm R133W would be expected to increase Ca²⁺ sensitivity of contractility and, perhaps, permit actomyosin interactions at nominally ‘relaxing’ Ca²⁺ levels; such results have been obtained from in vitro assays with four other DA mutations in Tn or β-Tm (Robinson et al. 2007).

In this issue of The Journal of Physiology, Ochala et al. (2007) report that, surprisingly, there is no change in Ca²⁺ regulation of isometric force in slow type I muscle fibres – the fibre type with the highest proportion of β-Tm – biopsied from the two DA patients reported by Tajsharghi et al. (2007) compared with six unrelated control subjects. Both type I and type IIa fibres from the patients generally expressed less β-Tm (and more α-Tm), although this difference did not achieve significance. In type I fibres at saturating Ca²⁺, there were significant changes in force per cross-sectional area (decrease), velocity of unloaded shortening (V_U; increase), and rate of tension redevelopment (k_TR; decrease). These measurements point toward the R133W mutation in β-Tm increasing actomyosin cross-bridge detachment rate while decreasing attachment rate; motility assays with unregulated actin demonstrated that kinetic changes could not be explained by a modification of myosin function alone. Rather than altering Ca²⁺ sensitivity, this Tm mutation affects cross-bridge kinetics. Additional observations support the idea that Tn and Tm can modulate actomyosin function both in isometric and unloaded biomechanical assays (Gordon et al. 1997; Schoffstall et al. 2006; Moreno-Gonzalez et al. 2007). Muscle weakness experienced by the adult DA patients can be explained by a kinetically determined reduction in the proportion of strong binding cross-bridges.

The elegant experiments of Ochala et al. (2007) raise new questions. Does the β-Tm R133W mutation in fact cause changes in Ca²⁺ sensitivity that are masked by other factors in the affected adults? What causes the progression from hypercontractility at birth to muscle weakness in adulthood; are normal developmental changes responsible, or are there adaptive changes within the muscle fibres? And what is the mechanism by which the β-Tm mutation affects actomyosin kinetics, particular the faster V_U at saturating Ca²⁺? Perhaps destabilization of Tm structure alters its flexural rigidity. While it is tempting to speculate that such a change in Tm mechanics could alter its interactions with cross-bridges (Gordon et al. 2000), and possibly superactivation of actomyosin function (Gordon et al. 1997), these are certainly not the only possibilities. Now that we know the effects of β-Tm R133W in muscle fibres from patients, answers to these and other questions await in vitro experiments where the proportions of α- versus β-Tm, and homo- versus hetero-dimers, along with other factors, can be experimentally controlled.