Signaling from the [Ca2+]i rise to egg activation
The signaling downstream the [Ca2+]i rise leading to egg activation is another essential subject. An increase in [Ca2+]i induces cortical granule exocytosis in various species (Fig. 4), as is generally the case in secretory cells. This event causes formation of the fertilization membrane and establishment of the block to polyspermy in sea urchin eggs. In mammals, the loss of cortical granules due to exocytotic secretion advances depending on the number of Ca2+ spikes (32). The released substance modifies proteins in the zona pellucida that surrounds the egg (zona reaction), resulting in polyspermy block (33).
[Ca2+]I rise triggers resumption of meiotic cell division. The second polar body is formed and subsequently the male and female pronuclei are formed (33) (Fig. 4). One of the central molecules in egg activation is maturation- (or metaphase-) promoting factor (MPF: Cdk1/cyclin B1 complex), a ubiquitous cell cycle regulator (34) (Fig. 4). Vertebrate mature eggs are arrested at the metaphase of the second meiosis (M II) due to sustained activity of MPF by the aid of cytostatic factor (CSF). Eggs are released from the M II arrest by fertilization as a consequence of the degradation of cyclin B1 by ubiqutin/proteasome-mediated proteolysis (Fig. 4). The Ca2+ response at fertilization is linked to activation of ubiqutin /proteasome. Upon [Ca2+]I rise at fertilization, Ca2+ binds to calmodulin and thereby activates calmodulin-dependent kinase II (CaMK II) (35). In fertilized mouse eggs, activation of CaMK II occurs at each Ca2+ peak (36). To be subjected to proteolysis, cyclin B1 is poly-ubiquitinated by anaphase promoting complex or cyclosome (APC/C), an E3 ubiquitin ligase,. This process is prevented by CSF (34, 37) (Fig. 4). In mammals, CaMK II causes inhibition of CSF, plausibly acting on the putative CSF component Emi 1, Mad2, or Bub1 (the most downstream component of the c-mos-MAPK (mitogen-activated protein kinase) pathway) (37). Taken together, the current view for the possible signaling pathway in mammals can be illustrated as in Fig. 4.
Repetitive [Ca2+]i rises at fertilization is necessary to accomplish degradation of MPF. If the number of Ca2+ spikes is insufficient, fertilized eggs are arrested at the M III stage after formation of the second polar body and fail to form the PN (32). Long-lasting Ca2+ oscillations in mouse eggs are responsible for pronucleus formation. Sufficient number of Ca2+ spikes causes reduction of MAPK activity and thereby leads to pronucleus formation (32) (Fig. 4). It has also been reported that the frequency and amplitude of Ca2+ oscillations affect the processes occurring much later during embryonic development in rabbits such as compaction, blastocyst formation, and the rate of successful transplantation of 4-cell embryo to host mothers (13).
The mechanism of fertilization has not been fully revealed, despite the long research history over a century. The sperm factor /Ca2+ increase in the egg /egg activation described here is the central point of cell signaling at fertilization, and research on this point is the most advanced in mammals at present. Further understanding of the mechanism involved in fertilization and early embryonic development depends on future studies.
This work was supported by Grant-in-Aid for General Scientific Research (B) to S.M. from the Japan Ministry of Education, Culture, Sports, Science, and Technology.