SNT1/FRS2 mediates germinal vesicle breakdown induced by an activated FGF receptor1 in Xenopus oocytes

Follicular cells protect Xenopus oocyte from abnormal maturation via integrin signaling downregulation and O-GlcNAcylation control

Xenopus oocytes are encompassed by a layer of follicular cells that contribute to oocyte growth and meiosis in relation to oocyte maturation. However, the effects of the interaction between follicular cells and the oocyte surface on meiotic processes are unclear. Here, we investigated Xenopus follicular cell function using oocyte signaling and heterologous-expressing capabilities. We found that oocytes deprotected from their surrounding layer of follicular cells and expressing the epidermal growth factor (EGF) receptor (EGFR) and the Grb7 adaptor undergo accelerated prophase I to metaphase II meiosis progression upon stimulation by EGF. This unusual maturation unravels atypical spindle formation but is rescued by inhibiting integrin β1 or Grb7 binding to the EGFR. In addition, we determined that oocytes surrounded by their follicular cells expressing EGFR-Grb7 exhibit normal meiotic resumption. These oocytes are protected from abnormal meiotic spindle formation through the recruitment of O-GlcNAcylated Grb7, and OGT (O-GlcNAc transferase), the enzyme responsible for O-GlcNAcylation processes, in the integrin β1-EGFR complex. Folliculated oocytes can be forced to adopt an abnormal phenotype and exclusive Grb7 Y338 and Y188 phosphorylation instead of O-GlcNAcylation under integrin activation. Furthermore, an O-GlcNAcylation increase (by inhibition of O-GlcNAcase), the glycosidase that removes O-GlcNAc moieties, or decrease (by inhibition of OGT) amplifies oocyte spindle defects when follicular cells are absent highlighting a control of the meiotic spindle by the OGT-O-GlcNAcase duo. In summary, our study provides further insight into the role of the follicular cell layer in oocyte meiosis progression.

Fibroblast growth factor (FGF) signaling regulates transforming growth factor beta (TGFβ)-dependent smooth muscle cell phenotype modulation

Smooth muscle cells (SMCs) in normal blood vessels exist in a highly differentiate state characterized by expression of SMC-specific contractile proteins ("contractile phenotype"). Following blood vessel injury in vivo or when cultured in vitro in the presence of multiple growth factors, SMC undergo a phenotype switch characterized by the loss of contractile markers and appearance of expression of non-muscle proteins ("proliferative phenotype"). While a number of factors have been reported to modulate this process, its regulation remains uncertain. Here we show that induction of SMC FGF signaling inhibits TGFβ signaling and converts contractile SMCs to the proliferative phenotype. Conversely, inhibition of SMC FGF signaling induces TGFβ signaling converting proliferating SMCs to the contractile phenotype, even in the presence of various growth factors in vitro or vascular injury in vivo. The importance of this signaling cross-talk is supported by in vivo data that show that an SMC deletion of a pan-FGF receptor adaptor Frs2α (fibroblast growth factor receptor substrate 2 alpha) in mice profoundly reduces neointima formation and vascular remodelling following carotid artery ligation. These results demonstrate that FGF-TGFβ signaling antagonism is the primary regulator of the SMC phenotype switch. Manipulation of this cross-talk may be an effective strategy for treatment of SMC-proliferation related diseases.

Fibroblast growth factor receptor 1 is a key inhibitor of TGFβ signaling in the endothelium

Abnormal vascular homeostasis can lead to increased proliferation of smooth muscle cells and deposition of extracellular matrix, resulting in neointima formation, which contributes to vascular lumen narrowing, a pathology that underlies diseases including transplant vasculopathy, the recurrence of stenosis, and atherosclerosis. Growth of neointima is in part due to endothelial-to-mesenchymal transition (EndMT), a transforming growth factor-β (TGFβ)-driven process, which leads to increased numbers of smooth muscle cells and fibroblasts and deposition of extracellular matrix. We reported that endothelial cell-specific knockout of fibroblast growth factor receptor 1 (FGFR1) led to activation of TGFβ signaling and development of EndMT in vitro and in vivo. Furthermore, EndMT in human diseased vasculature correlated with decreased abundance of FGFR1. These findings identify FGFR1 as the key regulator of TGFβ signaling and EndMT development.

FRS2 via fibroblast growth factor receptor 1 is required for platelet-derived growth factor receptor beta-mediated regulation of vascular smooth muscle marker gene expression

Vascular smooth muscle cells (VSMC) exhibit phenotypic plasticity and change from a quiescent contractile phenotype to a proliferative synthetic phenotype during physiological arteriogenesis and pathological conditions such as atherosclerosis and restenosis. Platelet-derived growth factor (PDGF)-BB is a potent inducer of the VSMC synthetic phenotype; however, much less is known about the role of fibroblast growth factor-2 (FGF2) in this process. Here, we show using signal transduction mutants of FGF receptor 1 (FGFR1) expressed in rat VSMC that the adaptor protein FRS2 is essential for FGFR1-mediated phenotypic modulation and down-regulation of VSMC smooth muscle alpha-actin (SMA) gene expression. In addition, we show that PDGF-BB and FGF2 act synergistically to induce cell proliferation and down-regulate SMA and SM22alpha in VSMC. Furthermore, we show that PDGF-BB induces tyrosine phosphorylation of FGFR1 and that this phosphorylation is mediated by PDGF receptor-beta (PDGFRbeta), but not c-Src. We demonstrate that FRS2 co-immunoprecipitates with PDGFRbeta in a complex that requires FGFR1 and that both the extracellular and the intracellular domains of FGFR1 are required for association with PDGFRbeta, whereas the cytoplasmic domain of FGFR1 is required for FRS2 association with the FGFR1-PDGFRbeta complex. Knockdown of FRS2 in VSMC by RNA interference inhibited PDGF-BB-mediated down-regulation of SMA and SM22alpha without affecting PDGF-BB mediated cell proliferation or ERK activation. Together, these data support the notion that PDGFRbeta down-regulates SMA and SM22alpha through formation of a complex that requires FGFR1 and FRS2 and prove novel insight into VSMC phenotypic plasticity.

FGFR1 forms an FRS2-dependent complex with mTOR to regulate smooth muscle marker gene expression

Vascular smooth muscle cells (VSMCs) switch from a contractile to a synthetic phenotype in human cardiovascular disease such as atherosclerosis and restenosis after angioplasty. VSMCs show reduced expression of contractile proteins and are capable of responding to mitogens by increasing expression of growth factor receptors. Fibroblast growth factor receptor-1 (FGFR1) signaling is one of several signaling pathways involved in this VSMC phenotypic switching. The aim of this study was to examine the signaling pathway downstream of FGFR1 in the regulation of SM marker gene expression. We found that FGFR1 activated Akt/mTOR pathway and that the mTOR inhibitor rapamycin partially reversed FGFR1-mediated downregulation of SM marker gene expression. Furthermore, we showed that mTOR forms a multi-protein complex with FGFR1 in VSMCs. These findings provide novel information for future development of therapeutic strategies for the treatment of human cardiovascular disease.

Mechanisms Regulating Oocyte Meiotic Resumption: Roles of Mitogen-Activated Protein Kinase

Oocyte meiotic maturation is one of the important physiological requirements for species survival. However, little is known about the detailed events occurring during this process. A number of studies have demonstrated that MAPK plays a pivotal role in the regulation of meiotic cell cycle progression in oocytes, but controversial findings have been reported in both lower vertebrates and mammals. In this review, we summarized the roles of MAPK cascade and related signal pathways in oocyte meiotic reinitiation in both lower vertebrates and mammals. We also tried to reconcile the paradoxical results and highlight the new findings concerning the function of MAPK in both oocytes and the surrounding follicular somatic cells. The unresolved questions and future research directions regarding the role of MAPK in meiotic resumption are addressed.

The adaptor-like protein ROG-1 is required for activation of the Ras-MAP kinase pathway and meiotic cell cycle progression in Caenorhabditis elegans

The Ras-MAP kinase pathway regulates varieties of fundamental cellular events. In Caenorhabditis elegans, this pathway is required for oocyte development; however, the nature of its up-stream regulators has remained elusive. Here, we identified a C. elegans gene, rog-1, which encodes the only protein having the IRS-type phosphotyrosine-binding (PTB) domain in the worms. ROG-1 has no obvious domain structure aside from the PTB domain, suggesting that it could serve as an adaptor down-stream of protein-tyrosine kinases (PTKs). RNA interference (RNAi)-mediated down-regulation of rog-1 mRNA significantly decreased brood size. rog-1(tm1031) truncation mutants showed a severe disruption in progression of developing oocytes from pachytene to diakinesis, as was seen in worms carrying a loss-of-function mutation in the let-60 Ras or mpk-1 MAP kinase gene. Furthermore, let-60 Ras-regulated activation of MPK-1 in the gonad is undetectable in rog-1(tm1031) mutants. Conversely, a gain-of-function mutation in the let-60 Ras gene rescues the brood size reduction and germ cell abnormality in rog-1(tm1031) worms. Consistently, rog-1 is preferentially expressed in the germ cells and its expression in the gonad is essential for oocyte development. Thus, ROG-1 is a key positive regulator of the Ras-MAP kinase pathway that permits germ cells to exit from pachytene.

A role for extracellular and transmembrane domains of Sef in Sef-mediated inhibition of FGF signaling

Sef (similar expression to fgf genes) is a member of the fibroblast growth factor (FGF) synexpression group that negatively regulates FGF receptor (FGFR) signaling in zebrafish during early embryonic development and in mammalian cell culture systems. The mechanism by which Sef exerts its inhibitory effect remains controversial. It has been reported that Sef functions either through binding to and inhibiting FGFR1 activation or by acting downstream of FGF receptors at the level of MEK/ERK kinases. In both cases, the intracellular domain of Sef was found to play a role in the inhibitory function of Sef. Here we demonstrated that both extracellular and transmembrane domains of Sef contributed to Sef-mediated negative regulation of FGF signaling. In fact, over-expression studies in NIH3T3 cells showed that a truncated mutant of Sef, which lacks the intracellular domain (SefECTM), exerted the inhibitory activity on FGF signaling by inhibiting FGFR1 tyrosine phosphorylation and subsequent activation of the Raf/MEK/ERK signaling cascade. We also showed that SefECTM associated with FGFR1, and inhibited FGF-induced ERK activation in HEK293T cells. Furthermore, we demonstrated that the over-expression of SefECTM was able to inhibit the function of a constitutively activated form of FGFR1, FGFR1-C289R, but not FGFR1-K562E. Finally, we found that SefECTM reduced cell viability when over-expressed in human umbilical vein endothelial cells (HUVEC). These data provide additional insight into the structure-activity relationship in the mechanism of inhibitory action of Sef on FGFR1-mediated signaling.

Gab1 is required for cell cycle transition, cell proliferation, and transformation induced by an oncogenic met receptor

We have shown previously that either Grb2- or Shc-mediated signaling from the oncogenic Met receptor Tpr-Met is sufficient to trigger cell cycle progression in Xenopus oocytes. However, direct binding of these adaptors to Tpr-Met is dispensable, implying that another Met binding partner mediates these responses. In this study, we show that overexpression of Grb2-associated binder 1 (Gab1) promotes cell cycle progression when Tpr-Met is expressed at suboptimal levels. This response requires that Gab1 possess an intact Met-binding motif, the pleckstrin homology domain, and the binding sites for phosphatidylinositol 3-kinase and tyrosine phosphatase SHP-2, but not the Grb2 and CrkII/phospholipase Cgamma binding sites. Importantly, we establish that Gab1-mediated signals are critical for cell cycle transition promoted by the oncogenic Met and fibroblast growth factor receptors, but not by progesterone, the natural inducer of cell cycle transition in Xenopus oocytes. Moreover, Gab1 is essential for Tpr-Met-mediated morphological transformation and proliferation of fibroblasts. This study provides the first evidence that Gab1 is a key binding partner of the Met receptor for induction of cell cycle progression, proliferation, and oncogenic morphological transformation. This study identifies Gab1 and its associated signaling partners as potential therapeutic targets to impair proliferation or transformation of cancer cells in human malignancies harboring a deregulated Met receptor.

Oncogenic Met receptor induces cell-cycle progression in Xenopus oocytes independent of direct Grb2 and Shc binding or Mos synthesis, but requires phosphatidylinositol 3-kinase and Raf signaling

Biological responses of hepatocyte growth factor (HGF) are mediated by the Met receptor tyrosine kinase. Although HGF is a potent mitogen for a variety of cells, the signals required for cell-cycle progression by the Met/HGF receptor are poorly defined. In this study, we have used the Xenopus oocyte system to define the role of various Met proximal-binding partners and downstream signaling pathways in cell-cycle regulation. We show that cell-cycle progression and activation of MAPK and JNK mediated by the oncogenic Met receptor, Tpr-Met, are dependent on its kinase activity and the presence of the twin phosphotyrosine (Y482 & Y489) residues in its C-terminus, but that the recruitment of Grb2 and Shc adaptor proteins is dispensable, implicating other signaling molecules. However, using Met receptor oncoproteins engineered to recruit specific signaling proteins, we demonstrate that recruitment of Grb2 or Shc adaptor proteins is sufficient to induce cell-cycle progression and activation of MAPK and JNK, while the binding of phospholipase-Cgamma or phosphatidylinositol 3-kinase alone fails to elicit these responses. Using various means to block phosphatidylinositol 3-kinase, phospholipase-Cgamma, MEK, JNK, Mos, and Raf1 activity, we show that unlike the fibroblast growth factor receptor, MEK-dependent and independent signaling contribute to Met receptor-mediated cell-cycle progression, but phospholipase-Cgamma or JNK activity and Mos synthesis are not critical. Notably, we demonstrate that Raf1 and phosphatidylinositol 3-kinase signaling are required for cell-cycle progression initiated by the Met receptor, a protein frequently deregulated in human tumors.

Tyr-298 in ephrinB1 is critical for an interaction with the Grb4 adaptor protein

The Eph family of receptor tyrosine kinases and their membrane-bound ligands, the ephrins, are thought to play a role in the regulation of cell adhesion and migration during development by mediating cell-to-cell signalling events. The transmembrane ephrinB protein is a bidirectional signalling molecule that sends a forward signal through the activation of its cognate receptor tyrosine kinase residing on another cell. The reverse signal is transduced into the ephrinB-expressing cell via tyrosine phosphorylation of its conserved C-terminal cytoplasmic domain. Previous work from our laboratory has implicated the activated FGFR1 (fibroblast growth factor receptor 1) as a regulator of a de-adhesion signal that results from overexpression of ephrinB1. In the present study, we report the isolation of Xenopus Grb4 (growth-factor-receptor-bound protein 4), an ephrinB1-interacting protein, and we show that when expressed in Xenopus oocytes, ephrinB1 interacts with Grb4 in the presence of an activated FGFR1. Amino acid substitutions were generated in Grb4, and the resulting mutants were expressed along with ephrinB1 and an activated FGFR in Xenopus oocytes. Co-immunoprecipitation analysis shows that the FLVR motif within the Src homology 2 domain of Xenopus Grb4 is vital for this phosphorylation-dependent interaction with ephrinB1. More importantly, using deletion and substitution analysis we identify the tyrosine residue at position 298 of ephrinB1 as being required for the physical interaction with Grb4, whereas Tyr-305 and Tyr-310 are dispensable. Moreover, we show that the region between amino acids 301 and 304 of ephrinB1 is also required for this critical tyrosine-phosphorylation-dependent event.

Sef inhibits fibroblast growth factor signaling by inhibiting FGFR1 tyrosine phosphorylation and subsequent ERK activation

Signaling through fibroblast growth factor receptors (FGFRs) is essential for many cellular processes including proliferation and migration as well as differentiation events such as angiogenesis, osteogenesis, and chondrogenesis. Recently, genetic screens in Drosophila and gene expression screens in zebrafish have resulted in the identification of several feedback inhibitors of FGF signaling. One of these, Sef (similar expression to fgf genes), encodes a transmembrane protein that belongs to the FGF synexpression group. Here we show that like zebrafish Sef (zSef), mouse Sef (mSef) interacts with FGFR1 and that the cytoplasmic domain of mSef mediates this interaction. Overexpression of mSef in NIH3T3 cells results in a decrease in FGF-induced cell proliferation associated with a decrease in Tyr phosphorylation of FGFR1 and FRS2. As a consequence, there is a reduction in the phosphorylation of Raf-1 at Ser(338), MEK1/2 at Ser(217) and Ser(221), and ERK1/2 at Thr(202) and Tyr(204). Furthermore, mSef inhibits ERK activation mediated by a constitutively activated FGFR1 but not by a constitutively active Ras and decreases FGF but not PDGF-mediated activation of Akt. These results indicate that Sef exerts its inhibitory effects at the level of FGFR and upstream of Ras providing an additional level of negative regulation of FGF signaling.