Triadin is a multiple proteins family, some isoforms being involved in muscle excitation-contraction coupling, and some having still unknown functions. To obtain clues on triadin functions, we engineered a triadin knock-out mouse line and characterized the physiological effect of triadin ablation on skeletal muscle function. These mice presented a reduced muscle strength, which seemed not to alter their survival and has been characterized in the present work. We first checked in these mice the expression level of the different proteins involved in calcium homeostasis and observed in fast muscles an increase in expression of dihydropyridine receptor, with a large reduction in calsequestrin expression. Electron microscopy analysis of KO muscles morphology demonstrated the presence of triads in abnormal orientation and a reduction in the sarcoplasmic reticulum terminal cisternae volume. Using calcium imaging on cultured myotubes, we observed a reduction in the total amount of calcium stored in the sarcoplasmic reticulum. Physiological studies have been performed to evaluate the influence of triadin deletion on skeletal muscle function. Muscle strength has been measured both on the whole animal model, using hang test or electrical stimulation combined with NMR analysis and strength measurement, or on isolated muscle using electrical stimulation. All the results obtained demonstrate an important reduction in muscle strength, indicating that triadin plays an essential role in skeletal muscle function and in skeletal muscle structure. These results indicate that triadin alteration leads to the development of a myopathy, which could be studied using this new animal model. Muscle contraction is activated by Ca 2ϩ release from the sarcoplasmic reticulum in response to plasma membrane depolarization. This process, called excitation-contraction coupling, takes place at the skeletal muscle triad junction, where T-tubules and the sarcoplasmic reticulum terminal cisternae are in close contact (1). Calcium release occurs via the calcium release complex, a macromolecular complex specifically localized in the skeletal muscle triad (2). The main components of this calcium release complex are the ryanodine receptor (RyR) 2 and the dihydropyridine receptor (DHPR) (3), both of which are calcium channels. It is now clear that a number of proteins are associated with these two calcium channels to form the calcium release complex, triadin being part of them. Triadin is an integral membrane protein of the sarcoplasmic reticulum, first identified in rabbit skeletal muscle as a 95-kDa glycoprotein specifically located in the triads (4, 5). Because of its co-localization with RyR in the triads, involvement of triadin in excitation-contraction coupling has been postulated (6, 7). Protein interaction studies have shown that the major molecular partners of triadin are RyR (8 -10), calsequestrin (CSQ), a protein that traps calcium inside the sarcoplasmic reticulum (11-13), and junctin (14). Functional studies have shown that triadin by itself r...

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