Drastic reduction of sarcalumenin in Dp427 (dystrophin of 427 kDa)-deficient fibres indicates that abnormal calcium handling plays a key role in muscular dystrophy

Although the primary abnormality in dystrophin is the underlying cause for mdx (X-chromosome-linked muscular dystrophy), abnormal Ca²⁺ handling after sarcolemmal microrupturing appears to be the pathophysiological mechanism leading to muscle weakness. To develop novel pharmacological strategies for eliminating Ca²⁺-dependent proteolysis, it is crucial to determine the fate of Ca²⁺-handling proteins in dystrophin-deficient fibres. In the present study, we show that a key luminal Ca²⁺-binding protein SAR (sarcalumenin) is affected in mdx skeletal-muscle fibres. One- and two-dimensional immunoblot analyses revealed the relative expression of the 160 kDa SR (sarcoplasmic reticulum) protein to be approx. 70% lower in mdx fibres when compared with normal skeletal muscles. This drastic reduction in SAR was confirmed by immunofluorescence microscopy. Patchy internal labelling of SAR in dystrophic fibres suggests an abnormal formation of SAR domains. Differential co-immunoprecipitation experiments and chemical cross-linking demonstrated a tight linkage between SAR and the SERCA1 (sarcoplasmic/endoplasmic-reticulum Ca²⁺-ATPase 1) isoform of the SR Ca²⁺-ATPase. However, the relative expression of the fast Ca²⁺ pump was not decreased in dystrophic membrane preparations. This implies that the reduction in SAR and calsequestrin-like proteins plays a central role in the previously reported impairment of Ca²⁺ buffering in the dystrophic SR [Culligan, Banville, Dowling and Ohlendieck (2002) J. Appl. Physiol. 92, 435–445]. Impaired Ca²⁺ shuttling between the Ca²⁺-uptake SERCA units and calsequestrin clusters via SAR, as well as an overall decreased luminal ion-binding capacity, might indirectly amplify the Ca²⁺-leak-channel-induced increase in cytosolic Ca²⁺ levels. This confirms the idea that abnormal Ca²⁺ cycling is involved in Ca²⁺-induced myonecrosis. Hence, manipulating disturbed Ca²⁺ handling might represent new modes of abolishing proteolytic degradation in muscular dystrophy.