Analysis of IP₃ dynamics during the intracellular Ca²⁺ oscillations in mammalian eggs

Abstract

At fertilization of mammalian eggs, the repetitive Ca²⁺ releases from intracellular stores through inositol 1,4,5-trisphosphate (IP₃) receptors are thought to be induced by the soluble factor originated from the sperm. Although phospholipase C zeta (PLCζ) is the possible candidate of such 'sperm factor', there are no evidence to show the elevation of intracellular IP₃ concentration at fertilization. We measured the changes in IP₃ concentration during Ca²⁺ oscillations in mouse eggs, using a novel FRET-based IP₃ probe, fretino. Eggs expressing fretino showed a transient decrease in FRET signal in response to the microinjection of IP₃, but not to inositol 1,3,4,5-tetrakisphosphate (IP₄). The signal decayed exponentially with a time constant of 100-180 s. When Ca²⁺ oscillations were induced in the eggs by insemination, the signal of fretino showed no detectable changes to indicate the increase in IP₃ concentration. On the other hand, eggs expressing a large amount of PLCζ showed significant decrease in the FRET signal from fretino. Furthermore, the FRET signal oscillated with Ca²⁺ in such eggs, suggesting the enhancement of PLCζ activity by cytoplasmic Ca²⁺. The magnitudes of Ca²⁺-induced IP₃ production in the fertilized eggs and in the eggs expressing PLCζ or PLCδ1, were also compared. [J Physiol Sci. 2006;56 Suppl:S109]