No abstract

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.

The regulatory elements of the Tie2/Tek promoter are commonly used in mouse models to direct transgene expression to endothelial cells. Tie2 is also expressed in hematopoietic cells, although this has not been fully characterized. We determine the lineages of adult hematopoietic cells derived from Tie2 expressing populations using Tie2-Cre;Rosa26R-EYFP mice. In Tie2-Cre;Rosa26R-EYFP mice, analysis of bone marrow cells showed Cre-mediated recombination in 85% of the population. In adult bone marrow and spleen, we analyzed sub-classes of early hematopoietic progenitors, T cells, monocytes, granulocytes, and B cells. We found that ~84% of each lineage was EYFP+, and nearly all cells that come from Tie2 expressing lineages are CD45+, confirming widespread contribution to definitive hematopoietic cells. In addition, more than 82% of blood cells within the embryonic yolk sac were of Tie2+ origin. Our findings of high levels of Tie2-Cre recombination in the hematopoietic lineage have implications for the use of the Tie2-Cre mouse as a lineage restricted driver strain.

SUMMARYRecent studies indicate that nephron progenitor cells of the embryonic kidney are arranged in a series of compartments of an increasing state of differentiation. The earliest progenitor compartment, distinguished by expression of CITED1, possesses greater capacity for renewal and differentiation than later compartments. Signaling events governing progression of nephron progenitor cells through stages of increasing differentiation are poorly understood, and their elucidation will provide key insights into normal and dysregulated nephrogenesis, as well as into regenerative processes that follow kidney injury. In this study, we found that the mouse CITED1 + progenitor compartment is maintained in response to receptor tyrosine kinase (RTK) ligands that activate both FGF and EGF receptors. This RTK signaling function is dependent on RAS and PI3K signaling but not ERK. In vivo, RAS inactivation by expression of sprouty 1 (Spry1) in CITED1 + nephron progenitors results in loss of characteristic molecular marker expression and in increased death of progenitor cells. Lineage tracing shows that surviving Spry1-expressing progenitor cells are impaired in their subsequent epithelial differentiation, infrequently contributing to epithelial structures. These findings demonstrate that the survival and developmental potential of cells in the earliest embryonic nephron progenitor cell compartment are dependent on FGF/EGF signaling through RAS.