Fertilization stimulates long-lasting oscillations of CaMKII activity in mouse eggs

Abstract

Elucidation of the biochemical mechanisms by which specific proteins transduce the all important intracellular calcium (Ca2+) signal at fertilization into events of egg activation will increase our understanding of the regulation of the onset of development and the extent to which these signals can be experimentally modified. Previously, we reported data supporting the hypothesis that mouse eggs have the capability to generate oscillations of the activity of Ca2+ and calmodulin-dependent kinase II (CaMKII), regulating the cell cycle and secretion. This study directly demonstrates transient increases of enzyme activity in relatively close synchrony with Ca2+ oscillations for the first hour of fertilization in single mouse eggs monitored for both Ca2+ and CaMKII activity. The extent of the enzyme activity increase was correlated with the level of intracellular Ca2+. After a rise in activity, the decrease in activity did not appear to be due to negative feedback from elevated Ca2+ or CaMKII activity over time, since enzyme activity persisted after 8 min of elevated Ca2+ from 7% ethanol activation. The contribution of CaMKII from a single sperm to the rise in CaMKII activity at fertilization appeared to be negligible. Also, long-term cell cycle inhibition was observed in fertilized eggs with the CaMKII antagonist myrAIP (50 microM), which did not inhibit the first large Ca2+ transient or subsequent early oscillations but did reduce the percentage of eggs fertilized. Thus, mammalian eggs appear to drive many activation events over time to completion with repeated short bursts of Ca2+ oscillation-dependent CaMKII activity, rather than by a steady-state, continuously elevated level of CaMKII activity that is maintained by periodic Ca2+ oscillations.

The γ isoform of CaM kinase II controls mouse egg activation by regulating cell cycle resumption

“…Interestingly, CG exocytosis is abnormal in these eggs (28). Also, CaMKII activity oscillates in synchrony with each Ca 2+ transient (12). These and other reports led to the idea that CaMKII is a master integrator of several EEA (29).…”

“…CaMKII inhibitors inhibit cell cycle progression in both parthenogenetically activated and fertilized mouse eggs (12,25,26). Conversely, expression of a truncated, constitutively active form of CaMKIIα triggers cell cycle resumption (27), reduction in MAPK and MPF activities, PN formation, and maternal mRNA recruitment, all in the absence of Ca 2+ oscillations (28).…”

“…Multiple isoforms of CaMKII are encoded by four genes (α, β, γ, and δ) that have distinct but overlapping expression patterns (10). In mouse eggs, CaMKII activity was originally reported to increase after parthenogenetic activation (11) and to fluctuate in parallel with Ca 2+ oscillations (12). Additional investigations using pharmacological inhibitors or constitutively active mutant forms of the enzyme supported a role for CaMKII during egg activation [reviewed in (7)].…”

Transitioning from egg to embryo: Triggers and mechanisms of egg activation

“…Current data suggest that release from CSF arrest in mouse occurs in a similar manner to that in Xenopus. For instance, upon fertilization, elevated Ca 2ϩ levels activate CaMKII, whose activity is important for the activation of the APC (Nixon et al, 2002;Markoulaki et al, 2004;Madgwick et al, 2005). In addition, Emi2 is rapidly degraded in response to Ca 2ϩ .…”

Dynamics and unexpected localization of the plakin binding protein, kazrin, in mouse eggs and early embryos

“…Egg activation was initiated by incubation in 7% ethanol (EtOH) in FHM medium supplemented with 0.3% bovine serum albumin and cultured for 5 min at 35°C before being washed in EtOH-free medium (Markoulaki et al, 2004). Eggs were examined at 10,20,30,45,60,90, and 120 min.…”