Identification of a translocation deficiency in cortical granule secretionin preovulatory mouse oocytes

Preovulatory, germinal vesicle (GV)-stage mouse oocytes are unable to under go normal cortical granule (CG) secretion. Full secretory competence is obs erved by metaphase II (MII) of meiosis and involves the development of calc ium response mechanisms. To identify the deficient or inhibited step in CG secretion, preovulatory GV-stage oocytes were stimulated and tested for the ir ability to undergo translocation, docking, and/or fusion. The mean CG di stance to the plasma membrane was not reduced in fertilized or sperm fracti on-injected, GV-stage oocytes relative to that in control GV-stage oocytes. In addition, analysis of individual CG distances to the plasma membrane in dicated no subpopulation of CGs competent to translocate. Further analysis demonstrated that secretory incompetence likely is not due to a lack of pro ximity of CGs to the egg's primary calcium store, the endoplasmic reticulum . Calcium/calmodulin-dependent protein kinase II (CaMKII), which is reporte dly involved in secretory granule translocation and secretion in many cells , including eggs, was investigated. A 60-kDa CaMKII isoform detected by Wes tern blot analysis increased 150% during oocyte maturation. The CaMKII acti vity assays indicated that MII-stage eggs correspondingly have 110% more ma ximal activity than GV-stage oocytes. These data demonstrate that the prima ry secretory deficiency is due to a failure of CG translocation, and that a maturation-associated increase in CaMKII correlates with the acquisition o f secretory competence and the ability of the egg to undergo normal activat ion.