Costamere proteins and their involvement in myopathic processes.

Muscle fibres are very specialised cells with a complex structure that requires a high level of organisation of the constituent proteins. For muscle contraction to function properly, there is a need for not only sarcomeres, the contractile structures of the muscle fibre, but also costameres. These are supramolecular structures associated with the sarcolemma that allow muscle adhesion to the extracellular matrix. They are composed of protein complexes that interact and whose functions include maintaining cell structure and signal transduction mediated by their constituent proteins. It is important to improve our understanding of these structures, as mutations in various genes that code for costamere proteins cause many types of muscular dystrophy. In this review, we provide a description of costameres detailing each of their constituent proteins, such as dystrophin, dystrobrevin, syntrophin, sarcoglycans, dystroglycans, vinculin, talin, integrins, desmin, plectin, etc. We describe as well the diseases associated with deficiency thereof, providing a general overview of their importance.

Substrate stiffness affects sarcomere and costamere structure and electrophysiological function of isolated adult cardiomyocytes.

INTRODUCTION: The mechanical environment is a key regulator of function in cardiomyocytes. We studied the role of substrate stiffness on the organization of sarcomeres and costameres in adult rat cardiomyocytes and further examined the resulting changes in cell shortening and calcium dynamics. METHODS: Cardiomyocytes isolated from adult rats were plated on laminin-coated polydimethylsiloxane substrates of defined stiffness (255 kPa, 117 kPa, 27 kPa, and 7 kPa) for 48 h. Levels of α-actinin and ß1 integrins were determined by immunofluoresence imaging and immunoblotting, both in the absence and presence of the phosphatase inhibitor calyculin A. Quantitative reverse transcriptase polymerase chain reaction was used to measure message levels of key structural proteins (α-actinin, α7 integrin, ß1 integrin, vinculin). Sarcomere shortening and calcium dynamics were measured at 2, 24, and 48 h. RESULTS: Overall cardiomyocyte morphology was similar on all substrates. However, well organized sarcomere structures were observed on only the stiffest (255 kPa) and the most compliant (7 kPa) substrates. Levels of α-actinin in cells were the same on all substrates, while message levels of structural proteins were up-regulated on substrates of intermediate stiffness. Inhibition of phosphatase activity blocked the degradation of contractile structures, but altered overall cardiomyocyte morphology. Shortening and calcium dynamics also were dependent on substrate stiffness; however, there was no clear causative relationship between the phenomena. CONCLUSIONS: Extracellular matrix stiffness can affect structural remodeling by adult cardiomyocytes, and the resulting contractile activity. These findings illuminate changes in cardiomyocyte function in cardiac fibrosis, and may suggest cardiac-specific phosphatases as a target for therapeutic intervention.

Desmoplakin and talin2 are novel mRNA targets of fragile X-related protein-1 in cardiac muscle.

RATIONALE: The proper function of cardiac muscle requires the precise assembly and interactions of numerous cytoskeletal and regulatory proteins into specialized structures that orchestrate contraction and force transmission. Evidence suggests that posttranscriptional regulation is critical for muscle function, but the mechanisms involved remain understudied. OBJECTIVE: To investigate the molecular mechanisms and targets of the muscle-specific fragile X mental retardation, autosomal homolog 1 (FXR1), an RNA binding protein whose loss leads to perinatal lethality in mice and cardiomyopathy in zebrafish. METHODS AND RESULTS: Using RNA immunoprecipitation approaches we found that desmoplakin and talin2 mRNAs associate with FXR1 in a complex. In vitro assays indicate that FXR1 binds these mRNA targets directly and represses their translation. Fxr1 KO hearts exhibit an up-regulation of desmoplakin and talin2 proteins, which is accompanied by severe disruption of desmosome as well as costamere architecture and composition in the heart, as determined by electron microscopy and deconvolution immunofluorescence analysis. CONCLUSIONS: Our findings reveal the first direct mRNA targets of FXR1 in striated muscle and support translational repression as a novel mechanism for regulating heart muscle development and function, in particular the assembly of specialized cytoskeletal structures.