Titin-based stiffening of muscle fibers in Ehlers-Danlos Syndrome

Objective: tenascin-X (TNX) is an extracellular matrix glycoprotein whose absence leads to Ehlers-Danlos Syndrome (EDS). TNX-deficient EDS patients present with joint hypermobility and muscle weakness attributable to increased compliance of the extracellular matrix. We hypothesized that in response to the increased compliance of the extracellular matrix in TNX-deficient EDS patients, intracellular adaptations take place in the elastic properties of the giant muscle protein titin. Methods: we performed extensive single muscle fiber mechanical studies to determine active and passive properties in TNX-deficient EDS patients. Gel-electrophoresis, Western blotting, and microarray studies were used to evaluate titin expression and phosphorylation. X-ray diffraction was used to measure myofilament lattice spacing. Results: passive tension of muscle fibers from TNXdeficient EDS patients was markedly increased. Myofilament extraction experiments indicated that the increased passive tension is attributable to changes in the properties of the sarcomeric protein titin. Transcript and protein data indicated no changes in titin isoform expression. Instead, differences in posttranslational modifications within titin's elastic region were found. In patients, active tension was not different at maximal activation level, but at submaximal activation level it was augmented attributable to increased calcium sensitivity. This increased calcium sensitivity might be attributable to stiffer titin molecules. Conclusion: in response to the increased compliance of the extracellular matrix in muscle of TNX-deficient EDS patients, a marked intracellular stiffening occurs of the giant protein titin. The stiffening of titin partly compensates for the muscle weakness in these patients by augmenting submaximal active tension generation.