Inositol 1, 4, 5-trisphosphate receptor is required for spindle assembly in Xenopus oocytes

The extent to which calcium signaling participates in specific events of animal cell meiosis or mitosis is a subject of enduring controversy. We have previously demonstrated that buffering intracellular calcium with 1,2-bis(2-aminophenoxy)ethane-N,N,N’,N’-tetraacetic acid (BAPTA, a fast calcium chelator), but not ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA, a slow calcium chelator), rapidly depolymerizes spindle microtubules in Xenopus oocytes, suggesting that spindle assembly and/or stability requires calcium nanodomains—calcium transients at extremely restricted spatial–temporal scales. In this study, we have investigated the function of inositol-1,4,5-trisphosphate receptor (IP(3)R), an endoplasmic reticulum (ER) calcium channel, in spindle assembly using Trim21-mediated depletion of IP(3)R. Oocytes depleted of IP(3)R underwent germinal vesicle breakdown but failed to emit the first polar body and failed to assemble proper meiotic spindles. Further, we developed a cell-free spindle assembly assay in which cytoplasm was aspirated from single oocytes. Spindles assembled in this cell-free system were encased in ER membranes, with IP(3)R enriched at the poles, while disruption of either ER organization or calcium signaling resulted in rapid spindle disassembly. As in intact oocytes, formation of spindles in cell-free oocyte extracts also required IP(3)R. We conclude that intracellular calcium signaling involving IP(3)R-mediated calcium release is required for meiotic spindle assembly in Xenopus oocytes.