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Contents
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FGF8 Takes Center Stage During Kidney Development
Vertebrates are segmented, as demonstrated by somite formation (Figure 1, part A)—blocks of mesoderm lining the anterior-posterior embryonic axis and giving rise to muscle, dermis, and vertebrae. Manipulation of the chick embryo suggested that FGF8 regulates somitogenesis by keeping the presomitic mesoderm unsegmented until the appropriate cue induces the next somite in the embryo’s tail end. However, this idea could not be tested genetically for lack of an appropriate Cre mouse that would inactivate Fgf8 expression in forming somites and yet allow normal gastrulation. In our recent publication cited above, we characterize such a mouse line, called T-Cre, because Cre is controlled by regulatory elements of the T (or Brachyury) gene and hence is expressed prior to somitogenesis in the early mesoderm as it forms during gastrulation. T-Cre–mediated Fgf8 inactivation yielded embryos that gastrulated normally and generated presomitic mesoderm devoid of Fgf8 gene product (Figure 1, part A). These embryos gave us a surprising result: FGF8 is not required for somitogenesis because the somites and their derivatives were normal (Figure 1, part A). Our current unpublished work addresses this conundrum by demonstrating that the role of FGF8 in this process is partially redundant with a subset of five other FGF genes coexpressed in this region. Although somitogenesis was unaffected in these mutants, neonates died because they lacked functional kidneys (Figure 1, part B). A central event during kidney development is a reciprocal induction between two lineages: the ureteric bud (UB) and metanephric mesenchyme (MM). As a result, the MM condenses and converts to an epithelium that undergoes a series of morphogenetic changes to form the structures of the nephron. In turn, the UB branches outward toward the periphery of the growing kidney where this mutual induction event repeats as the kidney grows. The end result is a functioning kidney consisting of a large number of nephrons connected by the UB-derived collecting ducts. We determined that Fgf8 was expressed in the condensing mesenchyme and that mutants suffered aberrant apoptosis in the MM of the kidney cortex, preventing new nephron formation (Figure 1, part C). Besides this role as a survival factor for this progenitor population, we also found that FGF8 regulates the expression of specific genes crucial for normal kidney development. Microarray analysis of microdissected kidneys at 12.5 days gestation, when mutant and control kidneys cannot be distinguished grossly, revealed a number of genes misregulated in the mutant tissue. Follow-up work led us to focus on two of these genes that proved to be pivotal to understanding the Fgf8 kidney phenotype. One of these genes encodes the secreted signaling molecule WNT4 and the other the transcription factor LIM1. MM-specific loss of either gene causes an arrest in kidney development that resembles the T-Cre–mediated inactivation of Fgf8. By determining the expression pattern of each gene in mouse mutants lacking one of the other two genes, the epistatic relationship was determined. This information, along with in vitro explant experiments indicated that both FGF8 and WNT sources are required in parallel for normal development, resulting in the model shown in Figure 1, part D. Figure 1. A) Somites (arrows) of mutant embryos at 9.5 days of gestation display no Fgf8 expression. B) Kidneys of mutant neonates are hypoplastic and nonfunctional. C) A ring of aberrant cell death (red) occurs in mutant kidneys at 14.5 days of gestation, where nephrogenesis would normally be taking place. D) Data from mutant analysis and in vitro explant experiments were used to generate a model wherein FGF8 induces Wnt4 gene expression and then both FGF8 and WNT4 are required for Lim1 gene expression and nephrogenesis. ad, adrenal gland; bl, bladder; ki, kidney; ov, ovary; ur, ureter; ut, uterus. It is intriguing that the FGF/WNT nexus we uncovered occurs in such processes as brain and limb development as well as during oncogenesis. Therefore, the task before us is to determine how the molecular interactions of these signaling pathways regulate normal development and how they cause disease when they go awry.
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he family of human and mouse fibroblast growth factors (FGFs) is large, numbering 22. Originally named for their effect on cultured cells, they regulate a wide variety of cellular and morphogenetic processes. Arguably, FGF8 is the busiest family member. It was isolated from mammary tumor cells and has since been implicated in the oncogenesis of sex hormone–related cancers of the breast and prostate. Most of our knowledge of how FGF8 controls morphogenesis comes from studying its various roles during mouse development. FGF8 is required for normal gastrulation, the embryonic stage when the three germ layers—mesoderm, ectoderm, and endoderm—are formed. Thanks to techniques of conditional mutagenesis, we also know that FGF8 is required for left/right asymmetry and regulates the development of different brain regions, the eyes, heart, limbs, and face. Equally diverse are the cellular processes that FGF8 regulates; depending on the embryonic stage, it controls cell growth, apoptosis, migration, and gene expression.