Energy landscapes reveal the myopathic effects of tropomyosin mutations

Abstract

Striated muscle contraction is regulated by an interaction network connecting the effects of troponin, Ca2+, and myosin-heads to the azimuthal positioning of tropomyosin along thin filaments. Many missense mutations, located at the actin-tropomyosin interface, however, reset the regulatory switching mechanism either by weakening or strengthening residue-specific interactions, leading to hyper- or hypo-contractile pathologies. Here, we compute energy landscapes for the actin-tropomyosin interface and quantify contributions of single amino acid residues to actin-tropomyosin binding. The method is a useful tool to assess effects of actin and tropomyosin mutations, potentially relating initial stages of myopathy to alterations in thin filament stability and regulation. Landscapes for mutant filaments linked to hyper-contractility provide a simple picture that describes a decrease in actin-tropomyosin interaction energy. Destabilizing the blocked (relaxed)-state parallels previously noted enhanced Ca2+-sensitivity conferred by these mutants. Energy landscapes also identify post-translational modifications that can rescue regulatory imbalances. For example, cardiomyopathy-associated E62Q tropomyosin mutation weakens actin-tropomyosin interaction, but phosphorylation of neighboring S61 rescues the binding-deficit, results confirmed experimentally by in vitro motility assays. Unlike results on hyper-contractility-related mutants, landscapes for tropomyosin mutants tied to hypo-contractility do not present a straightforward picture. These mutations may affect other components of the regulatory network, e.g., troponin-tropomyosin signaling.

Investigating the effects of tropomyosin mutations on its flexibility and interactions with filamentous actin using molecular dynamics simulation

“…Previous MD simulation studies have provided rich structural and dynamic information for the shape and flexibility of Tpm Li et al 2010aLi et al , b, c, 2011Li et al , 2014Zheng et al 2013), for the effects of Tpm mutations Moore et al 2011), and for actin dynamics (Zheng et al 2007). In complement with the MD approach, an energy landscape method was recently used to probe how disease mutations perturb the interactions between Tpm and F-actin as rigid bodies (Marston et al 2013;Orzechowski et al 2014). …”

Predicting Effects of Tropomyosin Mutations on Cardiac Muscle Contraction through Myofilament Modeling

“…Having obtained a parameter set for skinned cardiac muscle, we perturbed each of the model parameters that could potentially be affected by tropomyosin mutations (Li et al, 2012; Bai et al, 2013; Orzechowski et al, 2014a) in order to understand their separate effects on the steady state force-pCa relationship (Figure 3). The same relative changes in tropomyosin stiffness γ, BC equilibrium constant K BC , and myosin duty cycle δ were used to generate force-pCa curves (Figures 3A–C) with their properties quantified as maximum force, fraction of maximum force present at diastolic (low) calcium, calcium sensitivity, and Hill coefficient.…”

“…This suggests that mutation effects are not confined to stiffness alone and that other molecular mechanisms should be considered. Indeed, tropomyosin mutants have previously been determined to alter the interactions between tropomyosin and the actin surface (Orzechowski et al, 2014a; Zheng et al, 2016), which may in turn affect both the blocked-to-closed and closed-to-open (myosin-induced) transitions of tropomyosin across actin. We therefore entertained the possibility that introducing changes to the BC equilibrium constant and duty cycle in addition to the assumed stiffness changes could produce a reasonable fit to mutant data.…”

“…Instead, we must rely on indirect evidence to corroborate our estimates. For instance, in previous work, the electrostatic interactions was computed between tropomyosin and actin residues as tropomyosin is moved axially and azimuthally over the actin surface (Orzechowski et al, 2014a). This allows construction of energy landscapes for wild-type and mutant tropomyosins.…”

“…Changes in tropomyosin flexibility are widely believed to affect thin filament cooperativity (Loong et al, 2012a; Moore et al, 2016). However, recent work has shown that point mutations in tropomyosin can also affect the energy landscape of tropomyosin interactions with actin, providing another potential route for mutations to affect thin filament calcium regulation (Orzechowski et al, 2014a,b). Altered actin binding by mutant tropomyosin is also supported by numerous experimental and computational studies (Boussouf et al, 2007; Janco et al, 2012; Zheng et al, 2016).…”