Evidence for a role of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase in thapsigargin and Bcl-2 induced changes in Xenopus laevis oocyte maturation

Thapsigargin (Tg), a selective inhibitor of sarcoplasmic/ endoplasmic retic ulum Ca2+-ATPase (SERCA), causes depiction of intracellular Ca2+ stores, he nce activation of capacitative Ca2+ entry (CCE). Incubation of Xenopus laev is oocytes with Tg resulted in an increased rate of progesterone-induced me iotic maturation. Non-mitochondrial Ca-45(2+) uptake by SERCA-containing mi crosomes prepared from control wild-type oocytes microinjected with sterile water was inhibited essentially 100% by Tg. However, overexpression of Bcl -2, an oncogene known to protect against Tg-induced apoptosis in certain ce ll types, resulted in only 40% inhibition of microsomal Ca-45(2+) uptake by Tg while non-inhibited Ca-45(2+) uptake remained unchanged, Moreover Bcl-2 overexpression also protected against inhibition of CCE. I-Cl(Ca) was simi lar in Bcl-2-overexpressing and control oocytes when intracellular Ca2+ sto re depletion was induced by microinjection of inositol 1,4,5-trisphosphate (InsP(3)) and other means and when CCE was induced by means independent of SERCA inhibition. Our data indicate that Bcl-2 affects neither the InsP(3) receptor nor Ca2+ entry itself. At the end of a 24-h period after progester one addition to the medium, only 25% of Bcl-2-overexpressing oocytes had ma tured compared to 85% of control oocytes. Our data suggest that SERCA parti cipates in Xenopus oocyte maturation by controlling cytosolic Ca2+ and/or i ntracellular Ca2+ stores, hence CCE. An observed progesterone-dependent pro tein kinase-catalysed phosphorylation of SERCA is further indication of its role in oocyte maturation.