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 Ca2+ 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 Ca2+ dynamics. Four cell lines with varying expression of NCX and GM1 in the NE were transfected with cameleon-fluorescent Ca2+ indicators genetically targeted to NE/endoplasmic reticulum (ER) and nucleoplasm to monitor [Ca2+](ne/er) and [Ca2+](n) respectively. Cytosolic Ca2+ ([Ca2+](cyt)) was indicated with fura-2. Thapsigargin caused progressive loss of [Ca2+](ne/er), which was rapidly replaced on addition of extrinsic Ca2+ to those cells containing fully functional NCX/GM1: differentiated NG 108-15 and C6 cells. Reduced elevation of [Ca2+](ne/er) following thapsigargin depletion occurred in cells containing little or no GM1 in the NE: undifferentiated NG 108-15 and NG-CR72 cells. No change in [Ca2+](ne/er) due to applied Ca2+ was seen in Jurkat cells, which entirely lack NCX. Ca2+ entry to NE/ER was also blocked by KB-R7943, inhibitor of NCX. [Ca2+](n) and [Ca2+](cyt) were elevated independent of [Ca2+](ne/er) and remained in approximate equilibrium with each other. Ca2+ 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 Ca2+ 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 Ca2+.