Disuse atrophy is the loss of skeletal muscle mass due to inactivity or lower than 'normal' use. It is not only a furtive component of the 'modern' sedentary lifestyle but also a part of numerous pathologies, where muscle loss is linked to disease specific and/or other toxicity factors, eventually leading to wasting (cachexia). Whether disuse-or-disease induced, muscle loss leads to weakness and metabolic comorbidities with a high societal and financial cost. This review discusses the intricate network of interacting signalling pathways including Atrogin-1/MAFbx, IGF1-Akt, myostatin, glucocorticoids, NF-kB, MAPKs and caspases that seem to regulate disuse atrophy but also share common activation patterns in other states of muscle loss such as sarcopenia or cachexia. Reactive oxygen species are also important regulators of cell signalling pathways that can accelerate proteolysis and depress protein synthesis. Exercise is an effective countermeasure and antioxidants may show some benefit. We discuss how the experimental model used can crucially affect the outcome and hence our understanding of atrophy. Timing of sampling is crucial as some signalling mechanisms reach their peak early during the atrophy process to rapidly decline thereafter, while other present high levels even weeks and months after study initiation. The importance of such differences lays in future consideration of appropriate treatment targets. Apart from attempting to correct defective genes or negate their effects, technological advances in new rational ways should aim to regulate specific gene expression at precise time points for the treatment of muscle atrophy in therapeutic protocols depending on the origin of atrophy induction.

α-Tropomyosin (Tm) carrying hypertrophic cardiomyopathy mutation D175N or E180G was expressed in Escherichia coli. We have assembled dimers of two polypeptide chains in vitro that carry one (αα*) or two (α*α*) copies of the mutation. We found that the presence of the mutation has little effect on dimer assembly, thereby predicting that individuals heterozygous for the Tm mutations are likely to express both αα* and α*α* Tm. Depending on the expression level, the heterodimer may be the predominant form in individuals carrying the mutation. Thus, it is important to define differences in the properties of Tm molecules carrying one or two copies of the mutation. We examined the Tm homo- and heterodimer properties: actin affinity, thermal stability, calcium regulation of myosin subfragment 1 binding, and calcium regulation of myofibril force. We report that the properties of the heterodimer may be similar to those of the wild-type homodimer (actin affinity, thermal stability, D175N αα*), similar to those of the mutant homodimer (calcium sensitivity, D175N αα*), intermediate between the two (actin affinity, E180G αα*), or different from both (thermal stability, E180G αα*). Thus, the properties of the homodimer are not a completely reliable guide to the properties of the heterodimer.

Tropomyosin (Tm) is a dimer made of two alpha helical chains associated into a parallel coiled-coil. In mammalian skeletal and cardiac muscle, the Tm is expressed from two separate genes to give the α- and β-Tm isoforms. These associate in vivo to form homo- (α(2)) and heterodimers (α·β) with little β(2) normally observed. The proportion of α(2) vs α·β varies across species and across muscle types from almost 100% α(2)- to 50% α·β-Tm. The ratio can also vary during development and in disease. The functional significance of the presence of these two isoforms has not been defined because it is difficult to isolate or purify the α·β dimer for functional studies. Here we report an effective method for purifying bacterially expressed Tm as α·β dimers using a cleavable N-terminal tag on one of the two chains. The same method can be used to isolate Tm dimers in which one chain carries a mutation. We go on to show that the α·β dimers differ in key properties (actin affinity, thermal stability) from either the α(2)- or β(2)-Tm. However, the ability to regulate myosin binding when combined with cardiac troponin appears unaffected.

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Cardiac muscle contraction occurs through an interaction of the myosin head with the actin filaments, a process which is regulated by the troponin complex together with tropomyosin and is Ca(2+) dependent. Mutations in genes encoding sarcomeric proteins are a common cause of familial hypertrophic and dilated cardiomyopathies. The scope of this review is to gather information from studies regarding the in vitro characterisation of six HCM and six DCM mutations on the cardiac TnC gene and to suggest, if possible, how they may lead to dysfunction. Since TnC is the subunit responsible for Ca(2+) binding, mutations in the TnC could possibly have a strong impact on Ca(2+) binding affinities. Furthermore, the interactions of mutant TnCs with their binding partners could be altered. From the characterisation studies available to date, we can conclude that the HCM mutations on TnC increase significantly the Ca(2+) sensitivity of force development or of ATPase activity, producing large pCa shifts in comparison to WT TnC. In contrast, the DCM mutations on TnC have a tendency to decrease the Ca(2+) sensitivity of force development or of ATPase activity in comparison to WT TnC. Furthermore, the DCM mutants of TnC are not responsive to the TnI phosphorylation signal resulting in filaments that preserve their Ca(2+) sensitivity in contrast to WT filaments that experience a decrease in Ca(2+) sensitivity upon TnI phosphorylation.