Three-dimensional imaging reveals endo(sarco)plasmic reticulum-containing invaginations within the nucleoplasm of...

The mammalian nucleus has invaginations from the cytoplasm, termed nucleoplasmic reticulum (NR). With increased resolution of cellular imaging, progress has been made in understanding the formation and function of NR. In fact, nucleoplasmic Ca²⁺ homeostasis has been implicated in the regulation of gene expression, DNA repair, and cell death. However, the majority of studies focus on cross-sectional or single-plane analyses of NR invaginations, providing an incomplete assessment of its distribution and content. Here, we provided advanced imaging and three-dimensional reconstructive analyses characterizing the molecular constituents of nuclear invaginations in the nucleoplasm in HEK293 cells, murine C₂C₁₂ muscle cells, and cardiac myocytes. We demonstrated the presence of critical Ca²⁺ regulatory channels, including sarco(endo)plasmic reticulum Ca²⁺-ATPase 2a (SERCA2a), stromal interaction molecule 1 (STIM1), and Ca²⁺ release-activated Ca²⁺ channel protein 1 (ORAI1), in the nucleoplasm in isolated primary mouse cardiomyocytes. We have shown for the first time the presence of STIM1 and ORAI1 in the nucleoplasm, suggesting the presence of store-operated calcium entry (SOCE) mechanism in nucleoplasmic Ca²⁺ regulation. These results show that nucleoplasmic invaginations contain continuous endoplasmic reticulum components, mitochondria, and intact nuclear membranes, highlighting the extremely detailed and complex nature of this organellar structure.