Sodium-calcium exchanger complexed with GM1 ganglioside in nuclear membrane transfers calcium from nucleoplasm to endoplasmic reticulum.

The inner membrane of the nuclear envelope (NE) was previously shown to contain a Na/Ca exchanger (NCX) tightly linked to GM1 ganglioside that mediates transfer of nucleoplasmic Ca(2+) to the NE lumen and constitutes a cytoprotective mechanism. This transfer was initially observed with isolated nuclei and is now demonstrated in living cells in relation to subcellular Ca(2+) dynamics. Four cell lines with varying expression of NCX and GM1 in the NE were transfected with cameleon-fluorescent Ca(2+) indicators genetically targeted to NE/endoplasmic reticulum (ER) and nucleoplasm to monitor Ca(2+) and Ca(2+) respectively. Cytosolic Ca(2+) (Ca(2+)) was indicated with fura-2. Thapsigargin caused progressive loss of Ca(2+), which was rapidly replaced on addition of extrinsic Ca(2+) to those cells containing fully functional NCX/GM1: differentiated NG108-15 and C6 cells. Reduced elevation of Ca(2+) following thapsigargin depletion occurred in cells containing little or no GM1 in the NE: undifferentiated NG108-15 and NG-CR72 cells. No change in Ca(2+) due to applied Ca(2+) was seen in Jurkat cells, which entirely lack NCX. Ca(2+) entry to NE/ER was also blocked by KB-R7943, inhibitor of NCX. Ca(2+) and Ca(2+) were elevated independent of Ca(2+) and remained in approximate equilibrium with each other. Ca(2+) rise in the ER originated in the NE region and extended to the entire ER network. These results indicate the nuclear NCX/GM1 complex acts to gate Ca(2+) transfer from cytosol to ER, an alternate route to the sarcoplasmic/endoplasmic reticulum calcium ATPase pump. They also suggest a possible contributory mechanism for independent regulation of nuclear Ca(2+).