One fundamental problem in development is to determine how cells decide to remain multipotent or to differentiate. This problem lies at the heart of how pattern is generated in the embryo and, conversely, how pattern is disrupted when these processes are damaged as in tumor formation. The developing limb is an ideal system to address these problems because a wealth of data exists concerning limb development due to studies in genetics and experimental embryology. Previously, we have shown an essential role for Fibroblast Growth Factor (FGF) signaling during limb development by examining mice lacking genes that encode FGF ligands (Lewandoski et al. 2000 <I>Nature Genetics</I> 28:167, Sun et al 2000 <I>Nature Genetics</I>25: 6 ). However, the complexity caused by <I>Fgf</I> gene redundancy has led us to consider approaching the problem by examining FGF receptor mutants leading to the insight that FGF signaling controls limb size (Verheyden et al 2005<I>Development</I>132:4235). Signaling through the pathway governed by Bone Morphogenetic Proteins (BMPs) is thought to play a role in all aspects of limb outgrowth: early patterning in all three axes, programmed cell death and bone formation. The task before us is to understand how BMP and FGF signaling pathways interact during limb development. One process that BMP are thought to play a direct role in is as effectors of normal programmed cell death that occurs in mesenchymal interdigit cells, thus removing them and sculpting the final digit pattern in animals that are born without webbed limbs. However, we have challenged this paradigm. By simultaneously inactivating the Bmp receptor gene, <I>Bmpr1a</I>as well as <I>Fgf8</I>and <I>Fgf4</I>specifically in the limb bud ectoderm, we have produced genetic evidence for a novel model in which the surface ectoderm must receive a BMP signal, resulting in down regulation of <I>Fgfs</I>which in turn induces apoptosis of the underlying mesenchyme (Pajni-Underwood S. et al 2007 <I>Development</I>134: 2359). Thus we demonstrated that BMPs control programmed cell death indirectly, by regulating FGF signaling. We have extended these studies by studying the role of BMP and FGF signaling in various aspects of limb development using mouse lines that express Cre in specific region of the developing limb. For example the only way to test the hypothesis that BMPs act as direct effectors of cell death is to inactivate BMPs receptors only in the lineage that undergoes cells death, without affecting FGF expression in nearby cells. We have achieved this using new Cre lines that allow Cre-mediated gene inactivation in these lineages. With these lines are asking: are BMPs are direct effectors of normal programmed cell death? If not, how is programmed cell death controlled? If so, how do BMPs achieve this endpont? In another study, we have uncovered an important node of signaling between FGFs and BMP that is essential for normal development of the limb skeleton. Our previous work, cited above, demonstrates that specific FGFs, secreted from a distal structure in the limb bud, regulate the normal outgrowth and patterning of the limb. In current work, we are generating genetic evidence that BMP signaling to the progenitor population of the skeletal elements regulates this FGF signal by controlling the expression of an FGF antagonist. This linking of the two signaling pathways is not only a unique insight into how the limb is patterned but may provide a model for how the two pathways interact in other developmental contexts or during cancer.
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