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MAP and cdc2 kinase activities at germinal vesicle breakdown in Chaetopterus
1Department of Biology, Howard University, Washington, DC 20059, USA.
Abstract:
In frog oocytes, activation of mitogen-activated protein kinase (MAPK, ERK) leads to activation of cdc2 and germinal vesicle breakdown (GVBD). By contrast, in starfish, MAPK is activated after GVBD. Here we have examined the relative involvements of MAPK and cdc2 in GVBD of Chaetopterus oocytes. MAPK was rapidly tyrosine-phosphorylated and activated (within 1-2 min) in response to exposure of the oocytes either to natural seawater (the normal trigger of GVBD in this organism) or to the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), which can also elicit GVBD. This response preceded the tyrosine dephosphorylation and activation of cdc2 by several minutes. MAPK phosphorylation and activation were transient, lasting only until GVBD occurred and the spindle migrated to the cortex. The enzyme was not phosphorylated again as a result of egg activation. These results are consistent with the hypothesis that the activation of MAPK has a role in GVBD. However, PD 98059, a potent and selective inhibitor of MEK, the protein kinase that phosphorylates and activates MAPK, blocked the phosphorylation of MAPK but did not block GVBD, the dephosphorylation and activation of cdc2, or spindle formation and migration. Oocytes that underwent GVBD in PD 98059 could be fertilized and cleaved normally. Ionophore A23187, although it caused germinal vesicles to disappear and caused transient phosphorylation of MAPK, did not cause dephosphorylation of cdc2, and therefore this disappearance is artifactual. These results suggest that MAPK activation is neither obligatory nor sufficient for either GVBD or meiotic metaphase arrest in Chaetopterus and that activation of MAPK and cdc2 occur on independent, parallel pathways.
Insights
In Chaetopterus oocytes, mitogen-activated protein kinase (MAPK) activation is not essential for germinal vesicle breakdown (GVBD) or meiotic arrest. MAPK and cdc2 activation occur independently, challenging previous models of oocyte maturation.
Area of Science:
- Cellular and Molecular Biology
- Developmental Biology
- Reproductive Biology
Background:
- Mitogen-activated protein kinase (MAPK) and cdc2 are key regulators of oocyte maturation.
- The precise roles and timing of MAPK and cdc2 activation in germinal vesicle breakdown (GVBD) vary across species.
- In frogs, MAPK activation precedes GVBD, while in starfish, it follows GVBD.
Purpose of the Study:
- To investigate the involvement of MAPK and cdc2 in the GVBD of Chaetopterus oocytes.
- To determine the sequential activation and interdependence of MAPK and cdc2 pathways during oocyte maturation.
Main Methods:
- Induction of GVBD using natural seawater and 12-O-tetradecanoylphorbol 13-acetate (TPA).
- Assessment of MAPK tyrosine phosphorylation and activation.
- Monitoring of cdc2 dephosphorylation and activation.
- Inhibition of MAPK activation using PD 98059, a MEK inhibitor.
- Evaluation of GVBD, spindle formation/migration, fertilization, and cleavage post-inhibition.
Main Results:
- MAPK was rapidly activated preceding cdc2 activation in response to seawater or TPA.
- MAPK activation was transient, ceasing upon GVBD and spindle migration.
- Inhibition of MAPK activation by PD 98059 did not prevent GVBD, cdc2 activation, or spindle dynamics.
- Oocytes treated with PD 98059 could be fertilized and cleaved normally.
- Ionophore A23187 induced transient MAPK phosphorylation but not cdc2 dephosphorylation, indicating artifactual GVBD.
Conclusions:
- MAPK activation is neither obligatory nor sufficient for GVBD or meiotic metaphase arrest in Chaetopterus oocytes.
- MAPK and cdc2 activation occur on independent, parallel pathways in Chaetopterus oocyte maturation.
- Findings challenge the conserved role of MAPK as a prerequisite for GVBD across different marine invertebrates.
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