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SNT1/FRS2 mediates germinal vesicle breakdown induced by an activated FGF receptor1 in Xenopus oocytes
Kathleen Mood1, Robert Friesel, Ira O Daar
1Regulation of Cell Growth Laboratory, NCI-Frederick, National Institutes of Health, Frederick, Maryland 21702, USA.
Abstract:
The docking protein SNT1/FRS2 (fibroblast growth factor receptor substrate 2) is implicated in the transmission of extracellular signals from the fibroblast growth factor receptor (FGFR), which plays vital roles during embryogenesis. Activating FGFR mutations cause several craniosynostoses and dwarfism syndromes in humans. Here we show that the Xenopus homolog of mammalian FRS-2 (XFRS2) is essential for the induction of oocyte maturation by an XFGFR1 harboring an activating mutation (XFGFR1act). Using a dominant-negative form of kinase suppressor of Ras, we show the Mek activity is required for germinal vesicle breakdown (GVBD) induced by co-expression of XFGFR1act and XFRS2, but this activity is not required for progesterone-induced GVBD. Furthermore, Mek/MAPK activity is critical for the induction and/or maintenance of H1 kinase activity at metaphase of meiosis II in progesterone-treated oocytes. An activated XFGFR1 containing a mutation in the phospholipase Cgamma binding site (XFGFR1actY672F) displayed a reduced ability to induce cell-cycle progression in oocytes, suggesting phospholipase Cgamma may not be necessary but that it augments XFGFR signaling in this system. Oocytes co-expressing XFGFR1act and XFRS2 showed substantial H1 kinase activity, but this activity was blocked when the oocytes were treated with the phosphatidylinositol 3-kinase inhibitor LY294002. Although phosphatidylinositol 3-kinase activity is essential for XFGFR1act/XFRS2-induced oocyte maturation, this activity is not required for maturation induced by progesterone. Finally, ectopic expression of Xspry2, a negative regulator of XFGFR signaling, greatly reduced MAPK activation and GVBD induced by the expression of either XFGFR1act plus XFRS2 or activated Ras (H-RasV12). In contrast, Xspry2 did not prevent GVBD induced by an activated form of Raf1, suggesting that Xspry2 exerts its inhibitory function upstream or parallel to Raf and downstream of Ras.
Insights
Fibroblast growth factor receptor substrate 2 (FRS2) is crucial for oocyte maturation signaling in Xenopus. Mek/MAPK and phosphatidylinositol 3-kinase pathways are essential for FRS2-mediated maturation but not progesterone-induced maturation.
Area of Science:
- Cellular signaling pathways
- Developmental biology
- Molecular endocrinology
Background:
- Fibroblast growth factor receptor (FGFR) signaling is vital for embryogenesis.
- Mutations in FGFR are linked to human craniosynostoses and dwarfism syndromes.
- The docking protein SNT1/FRS2 mediates signal transmission from FGFR.
Purpose of the Study:
- To investigate the role of Xenopus FRS2 (XFRS2) in oocyte maturation.
- To elucidate the downstream signaling pathways involved in XFGFR1-induced oocyte maturation.
- To compare XFGFR1 signaling with progesterone-induced oocyte maturation.
Main Methods:
- Expression of wild-type and mutated XFGFR1 and XFRS2 in Xenopus oocytes.
- Use of dominant-negative kinase suppressor of Ras to inhibit Mek activity.
- Treatment with specific inhibitors (LY294002 for PI3K) and ectopic expression of Xspry2.
Main Results:
- XFRS2 is essential for XFGFR1act-induced oocyte maturation.
- Mek/MAPK and phosphatidylinositol 3-kinase activities are required for XFGFR1act/XFRS2-induced maturation but not progesterone-induced maturation.
- Xspry2 inhibits MAPK activation downstream of Ras but upstream or parallel to Raf1.
Conclusions:
- XFRS2 acts as an essential mediator for XFGFR1 signaling in Xenopus oocyte maturation.
- Distinct signaling pathways (Mek/MAPK, PI3K) are activated by XFGFR1 compared to progesterone.
- Xspry2 functions as a negative regulator upstream or parallel to Raf1 in the XFGFR signaling cascade.