Oocyte-cumulus cell interactions regulate free intracellular zinc in mouse oocytes

Zinc increases in the oocyte during maturation and is required for progression and completion of meiosis. The objective of this study was to determine whether cumulus cells regulate the levels of free intracellular zinc in the oocyte during maturation. In the cumulus-oocyte complex (COC) the relative level of free intracellular zinc was almost fourfold higher in cumulus cells compared with the resident germinal vesicle-stage oocyte. Removal of cumulus cells caused a fourfold increase in intracellular zinc in the oocyte by 1 h after cumulus cell removal, but subsequent coculture of denuded oocytes with COC decreased free intracellular zinc in the oocyte by 65%. Thus, cumulus cells suppress free intracellular zinc in the oocyte. The mRNA transcripts for the zinc transporter proteins Slc39a6, Slc39a8, Slc39a9, Slc39a10, Slc39a12, Slc30a2, Slc30a4, Slc30a5 and Slc30a8 mRNAs were higher in oocytes, while Slc39a1, Slc39a7, Slc39a13, Slc39a14, Slc30a6, Slc30a7 and Slc30a9 mRNAs were higher in cumulus cells. Thus a complex zinc transport network is present in the COC. Pretreatment with epidermal growth factor for 4 h abolished the ability of COCs to restrict free intracellular zinc in denuded oocytes. Coculture of denuded metaphase II oocytes with COC lowers free intracellular zinc in mature oocytes. Oocytes matured in vivo or oocytes from older mice had lower levels of free intracellular zinc than oocytes matured in vitro or from younger mice. Thus, a precise mechanism for regulating oocyte zinc homeostasis has been uncovered in the COC that is disrupted with increasing age or by removal of cumulus cells.