Abstract

Several lines of evidence indicate that increases in nuclear Ca2+ have specific biological effects that differ from those of cytosolic Ca2+, suggesting that they occur independently. The mechanisms involved in controlling nuclear Ca2+ signaling are both controversial and still poorly understood. Using hypotonic shock combined with mechanical disruption, we obtained and characterized a fraction of purified nuclei from cultured rat skeletal myotubes. Both immunoblot studies and radiolabeled inositol 1,4,5-trisphosphate [IP3] binding revealed an important concentration of IP3 receptors in the nuclear fraction. Immunofluorescence and immunoelectron microscopy studies localized type-1 and type-3 IP3 receptors in the nucleus with type-1 receptors preferentially localized in the inner nuclear membrane. Type-2 IP3 receptor was confined to the sarcoplasmic reticulum. Isolated nuclei responded to IP3 with rapid and transient Ca2+ concentration elevations, which were inhibited by known blockers of IP3 signals. Similar results were obtained with isolated nuclei from the 1B5 cell line, which does not express ryanodine receptors but releases nuclear Ca2+ in an IP3-dependent manner. Nuclear Ca2+ increases triggered by IP3 evoked phosphorylation of CAMP response element binding protein with kinetics compatible with sequential activation. These results support the idea that Ca2+ signals, mediated by nuclear IP3 receptors in muscle cells, are part of a distinct Ca2+ release component that originates in the nucleus and probably participates in gene regulation mediated by cAMP response element binding protein.