010. Coordinating the transition from egg to embryo in mammals

Abstract

At fertilization of mammalian oocytes, the sperm induces a series of increases in the concentration of intracellular Ca²⁺. These Ca²⁺ oscillations trigger all the events of egg activation, including cortical granule exocytosis, completion of meiosis and entry into the first mitotic division. Thus, intracellular Ca²⁺ plays a pivotal role in coordinating the transition from egg to embryo. Our work is focussed on understanding how the oocyte prepares for fertilisation, how the Ca²⁺ oscillations are controlled and how Ca²⁺ stimulates signalling pathways that lead to optimal early embryonic development. In this lecture I will focus on the downstream pathways of Ca²⁺ signalling at fertilisation. Conventional Protein Kinase C (cPKC) is the major downstream target of Ca²⁺ in many cell functions. Using PKC-GFP fusion proteins we have found that cPKC is recruited to the membrane in a manner that is dependent on the frequency and amplitude of the Ca²⁺ oscillations. Recruitment of cPKC appears to promote the Ca²⁺ influx necessary to sustain the generation of long lasting Ca²⁺ oscillations. In other cell types cytosolic Ca²⁺ increases are known to stimulate mitochondrial respiration. We have found that maintenance of resting Ca²⁺ levels and sperm-induced Ca²⁺ oscillations are critically dependent on mitochondrial ATP production: a feature not shared by many cell types. Since Ca²⁺ release increases ATP consumption we investigated whether the Ca²⁺ transients increase mitochondrial activity so as to meet this increase in demand. Monitoring autofluorescence from NADH and flavoproteins reveals that Ca²⁺ transients stimulate a change in redox state of mitochondria, presumably by activating Ca²⁺-sensitive dehydrogenases of the TCA cycle. Thus, through activation of downstream pathways, including PKC, cyclin B degradation and mitochondrial activity, intracellular Ca²⁺ provides a signal that orchestrates the activation of early mammalian development.