A retrograde signal from calsequestrin for the regulation of store-operated Ca2+ entry in skeletal muscle

Calsequestrin (CSQ) is a high capacity Ca2+-binding protein present in the lumen of sarcoplasmic reticulum (SR) in striated muscle cells and has been shown to regulate the ryanodine receptor Ca2+ release channel activity through interaction with other proteins present in the SR. Here we show that overexpression of wildtype CSQ or a CSQ mutant lacking the junction binding region (amino acids 86-191; Deltajunc-CSQ) in mouse skeletal C2Cl2 myotube enhanced caffeine- and voltage-induced Ca2+ release by increasing the Ca2+ load in SR, whereas overexpression of a mutant CSQ lacking a Ca2+ binding, aspartate-rich domain (amino acids 352-367; Deltaasp-CSQ) showed the opposite effects. Depletion of SR Ca2+ by thapsigargin initiated store-operated Ca2+ entry (SOCE) in C2Cl2 myotubes. A large component of SOCE was inhibited by overexpression of wild-type CSQ or Deltajunc-CSQ, whereas myotubes transfected with Deltaasp-CSQ exhibited normal function of SOCE. These results indicate that the aspartate-rich segment of CSQ, under conditions of overexpression, can sustain structural interactions that interfere with the SOCE mechanism. Such retrograde activation mechanisms are possibly taking place at the junctional site of the SR.