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.