Etv1 and Ewsr1 cooperatively regulate limb mesenchymal Fgf10 expression in response to apical ectodermal ridge-derived fibroblast growth factor signal

Fibroblast growth factor 10

Fibroblast growth factor 10 (FGF10) is a major morphoregulatory factor that plays essential signaling roles during vertebrate multiorgan development and homeostasis. FGF10 is predominantly expressed in mesenchymal cells and signals though FGFR2b in adjacent epithelia to regulate branching morphogenesis, stem cell fate, tissue differentiation and proliferation, in addition to autocrine roles. Genetic loss of function analyses have revealed critical requirements for FGF10 signaling during limb, lung, digestive system, ectodermal, nervous system, craniofacial and cardiac development. Heterozygous FGF10 mutations have been identified in human genetic syndromes associated with craniofacial anomalies, including lacrimal and salivary gland aplasia. Elevated Fgf10 expression is associated with poor prognosis in a range of cancers. In addition to developmental and disease roles, FGF10 regulates homeostasis and repair of diverse adult tissues and has been identified as a target for regenerative medicine.

New developments in the biology of fibroblast growth factors

The fibroblast growth factor (FGF) family is composed of 18 secreted signaling proteins consisting of canonical FGFs and endocrine FGFs that activate four receptor tyrosine kinases (FGFRs 1-4) and four intracellular proteins (intracellular FGFs or iFGFs) that primarily function to regulate the activity of voltage-gated sodium channels and other molecules. The canonical FGFs, endocrine FGFs, and iFGFs have been reviewed extensively by us and others. In this review, we briefly summarize past reviews and then focus on new developments in the FGF field since our last review in 2015. Some of the highlights in the past 6 years include the use of optogenetic tools, viral vectors, and inducible transgenes to experimentally modulate FGF signaling, the clinical use of small molecule FGFR inhibitors, an expanded understanding of endocrine FGF signaling, functions for FGF signaling in stem cell pluripotency and differentiation, roles for FGF signaling in tissue homeostasis and regeneration, a continuing elaboration of mechanisms of FGF signaling in development, and an expanding appreciation of roles for FGF signaling in neuropsychiatric diseases. This article is categorized under: Cardiovascular Diseases > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology Congenital Diseases > Stem Cells and Development Cancer > Stem Cells and Development.

Androgen signaling connects short isoform production to breakpoint formation at Ewing sarcoma breakpoint region 1

Ewing sarcoma breakpoint region 1 (EWSR1) encodes a multifunctional protein that can cooperate with the transcription factor ERG to promote prostate cancer. The EWSR1 gene is also commonly involved in oncogenic gene rearrangements in Ewing sarcoma. Despite the cancer relevance of EWSR1, its regulation is poorly understood. Here we find that in prostate cancer, androgen signaling upregulates a 5′ EWSR1 isoform by promoting usage of an intronic polyadenylation site. This isoform encodes a cytoplasmic protein that can strongly promote cell migration and clonogenic growth. Deletion of an Androgen Receptor (AR) binding site near the 5′ EWSR1 polyadenylation site abolished androgen-dependent upregulation. This polyadenylation site is also near the Ewing sarcoma breakpoint hotspot, and androgen signaling promoted R-loop and breakpoint formation. RNase H overexpression reduced breakage and 5′ EWSR1 isoform expression suggesting an R-loop dependent mechanism. These data suggest that androgen signaling can promote R-loops internal to the EWSR1 gene leading to either early transcription termination, or breakpoint formation.

Attenuated Fgf Signaling Underlies the Forelimb Heterochrony in the Emu Dromaius novaehollandiae

Powered flight was fundamental to the establishment and radiation of birds. However, flight has been lost multiple times throughout avian evolution. Convergent losses of flight within the ratites (flightless paleognaths, including the emu and ostrich) often coincide with reduced wings. Although there is a wealth of anatomical knowledge for several ratites, the genetic mechanisms causing these changes remain debated. Here, we use a multidisciplinary approach employing embryological, genetic, and genomic techniques to interrogate the mechanisms underlying forelimb heterochrony in emu embryos. We show that the initiation of limb formation, an epithelial to mesenchymal transition (EMT) in the lateral plate mesoderm (LPM) and myoblast migration into the LPM, occur at equivalent stages in the emu and chick. However, the emu forelimb fails to subsequently proliferate. The unique emu forelimb expression of Nkx2.5, previously associated with diminished wing development, initiates after this stage (concomitant with myoblast migration into the LPM) and is therefore unlikely to cause this developmental delay. In contrast, RNA sequencing of limb tissue reveals significantly lower Fgf10 expression in the emu forelimb. Artificially increasing Fgf10 expression in the emu LPM induces ectodermal Fgf8 expression and a limb bud. Analyzing open chromatin reveals differentially active regulatory elements near Fgf10 and Sall-1 in the emu wing, and the Sall-1 enhancer activity is dependent on a likely Fgf-mediated Ets transcription factor-binding site. Taken together, our results suggest that regulatory changes result in lower expression of Fgf10 and a concomitant failure to express genes required for limb proliferation in the early emu wing bud.

Hoxa13 regulates expression of common Hox target genes involved in cartilage development to coordinate the expansion of the autopodal anlage

To elucidate the role of Hox genes in limb cartilage development, we identified the target genes of HOXA11 and HOXA13 by ChIP‐Seq. The ChIP DNA fragment contained evolutionarily conserved sequences and multiple highly conserved HOX binding sites. A substantial portion of the HOXA11 ChIP fragment overlapped with the HOXA13 ChIP fragment indicating that both factors share common targets. Deletion of the target regions neighboring Bmp2 or Tshz2 reduced their expression in the autopod suggesting that they function as the limb bud‐specific enhancers. We identified the Hox downstream genes as exhibiting expression changes in the Hoxa13 knock out (KO) and Hoxd11‐13 deletion double mutant (Hox13 dKO) autopod by Genechip analysis. The Hox downstream genes neighboring the ChIP fragment were defined as the direct targets of Hox. We analyzed the spatial expression pattern of the Hox target genes that encode two different categories of transcription factors during autopod development and Hox13dKO limb bud. (a) Bcl11a, encoding a repressor of cartilage differentiation, was expressed in the E11.5 autopod and was substantially reduced in the Hox13dKO. (b) The transcription factors Aff3, Bnc2, Nfib and Runx1t1 were expressed in the zeugopodal cartilage but not in the autopod due to the repressive or relatively weak transcriptional activity of Hox13 at E11.5. Interestingly, the expression of these genes was later observed in the autopodal cartilage at E12.5. These results indicate that Hox13 transiently suspends the cartilage differentiation in the autopodal anlage via multiple pathways until establishing the paddle‐shaped structure required to generate five digits.

Fibroblast Growth Factor 10 and Vertebrate Limb Development

Early limb development requires fibroblast growth factor (Fgf)-mediated coordination between growth and patterning to ensure the proper formation of a functional organ. The apical ectodermal ridge (AER) is a domain of thickened epithelium located at the distal edge of the limb bud that coordinates outgrowth along the proximodistal axis. Considerable amount of work has been done to elucidate the cellular and molecular mechanisms underlying induction, maintenance and regression of the AER. Fgf10, a paracrine Fgf that elicits its biological responses by activating the fibroblast growth factor receptor 2b (Fgfr2b), is crucial for governing proximal distal outgrowth as well as patterning and acts upstream of the known AER marker Fgf8. A transgenic mouse line allowing doxycycline-based inducible and ubiquitous expression of a soluble form of Fgfr2b has been extensively used to identify the role of Fgfr2b ligands at different time points during development. Overexpression of soluble Fgfr2b (sFgfr2b) post-AER induction leads to irreversible loss of cellular β-catenin organization and decreased Fgf8 expression in the AER. A similar approach has been carried out pre-AER induction. The observed limb phenotype is similar to the severe proximal truncations observed in human babies exposed to thalidomide, which has been proposed to block the Fgf10-AER-Fgf8 feedback loop. Novel insights on the role of Fgf10 signaling in limb formation pre- and post-AER induction are summarized in this review and will be integrated with possible future investigations on the role of Fgf10 throughout limb development.

Oncogenic ETS Factors in Prostate Cancer

Prostate cancer is unique among carcinomas in that a fusion gene created by a chromosomal rearrangement is a common driver of the disease. The TMPRSS2/ERG rearrangement drives aberrant expression of the ETS family transcription factor ERG in 50% of prostate tumors. Similar rearrangements promote aberrant expression of the ETS family transcription factors ETV1 and ETV4 in another 10% of cases. Together, these three ETS factors are thought to promote tumorigenesis in the majority of prostate cancers. A goal of precision medicine is to be able to apply targeted therapeutics that are specific to disease subtypes. ETS gene rearrangement positive tumors represent the largest molecular subtype of prostate cancer, but to date there is no treatment specific to this marker. In this chapter we will review the latest findings regarding the molecular mechanisms of ETS factor function in the prostate. These molecular details may provide a path towards new therapeutic targets for this subtype of prostate cancer. Further, we will describe efforts to target the oncogenic functions of ETS family transcription factors directly as well as indirectly.

Characterization of cis-regulatory elements for Fgf10 expression in the chick embryo

BACKGROUND

Fgf10 is expressed in various tissues and organs, such as the limb bud, heart, inner ear, and head mesenchyme. Previous studies identified Fgf10 enhancers for the inner ear and heart. However, Fgf10 enhancers for other tissues have not been identified.

RESULTS

By using primary culture chick embryo lateral plate mesoderm cells, we compared activities of deletion constructs of the Fgf10 promoter region, cloned into a promoter-less luciferase reporter vector. We identified a 0.34-kb proximal promoter that can activate luciferase expression. Then, we cloned 11 evolutionarily conserved sequences located within or outside of the Fgf10 gene into the 0.34-kb promoter-luciferase vector, and tested their activities in vitro using primary cultured cells. Two sequences showed the highest activities. By using the Tol2 system and electroporation into chick embryos, activities of the 0.34-kb promoter with and without the two sequences were tested in vivo. No activities were detected in limb buds. However, the 0.34-kb promoter exhibited activities in the dorsal midline of the brain, while Fgf10 is detected in broader region in the brain. The two noncoding sequences negatively acted on the 0.34-kb promoter in the brain.

CONCLUSIONS

The proximal 0.34-kb promoter has activities to drive expression in restricted areas of the brain. Developmental Dynamics 247:1253-1263, 2018. © 2018 Wiley Periodicals, Inc.

Regulation of FGF10 Signaling in Development and Disease

Fibroblast Growth Factor 10 (FGF10) is a multifunctional mesenchymal-epithelial signaling growth factor, which is essential for multi-organ development and tissue homeostasis in adults. Furthermore, FGF10 deregulation has been associated with human genetic disorders and certain forms of cancer. Upon binding to FGF receptors with heparan sulfate as co-factor, FGF10 activates several intracellular signaling cascades, resulting in cell proliferation, differentiation, and invasion. FGF10 activity is modulated not only by heparan sulfate proteoglycans in the extracellular matrix, but also by hormones and other soluble factors. Despite more than 20 years of research on FGF10 functions, context-dependent regulation of FGF10 signaling specificity remains poorly understood. Emerging modes of FGF10 signaling regulation will be described, focusing on the role of FGF10 trafficking and sub-cellular localization, heparan sulfate proteoglycans, and miRNAs. Systems biology approaches based on quantitative proteomics will be considered for globally investigating FGF10 signaling specificity. Finally, current gaps in our understanding of FGF10 functions, such as the relative contribution of receptor isoforms to signaling activation, will be discussed in the context of genetic disorders and tumorigenesis.

An Interaction with Ewing's Sarcoma Breakpoint Protein EWS Defines a Specific Oncogenic Mechanism of ETS Factors Rearranged in Prostate Cancer

More than 50% of prostate tumors have a chromosomal rearrangement resulting in aberrant expression of an oncogenic ETS family transcription factor. However, mechanisms that differentiate the function of oncogenic ETS factors expressed in prostate tumors from non-oncogenic ETS factors expressed in normal prostate are unknown. Here, we find that four oncogenic ETS (ERG, ETV1, ETV4, and ETV5), and no other ETS, interact with the Ewing's sarcoma breakpoint protein, EWS. This EWS interaction was necessary and sufficient for oncogenic ETS functions including gene activation, cell migration, clonogenic survival, and transformation. Significantly, the EWS interacting region of ERG has no homology with that of ETV1, ETV4, and ETV5. Therefore, this finding may explain how divergent ETS factors have a common oncogenic function. Strikingly, EWS is fused to various ETS factors by the chromosome translocations that cause Ewing's sarcoma. Therefore, these findings link oncogenic ETS function in both prostate cancer and Ewing's sarcoma.

FGF10: A multifunctional mesenchymal-epithelial signaling growth factor in development, health, and disease

The FGF family comprises 22 members with diverse functions in development and health. FGF10 specifically activates FGFR2b in a paracrine manner with heparan sulfate as a co-factor. FGF10and FGFR2b are preferentially expressed in the mesenchyme and epithelium, respectively. FGF10 is a mesenchymal signaling molecule in the epithelium. FGF10 knockout mice die shortly after birth due to the complete absence of lungs as well as fore- and hindlimbs. FGF10 is also essential for the development of multiple organs. The phenotypes of Fgf10 knockout mice are very similar to those of FGFR2b knockout mice, indicating that FGF10 acts as a ligand that is specific to FGFR2b in mouse multi-organ development. FGF10 also plays roles in epithelial-mesenchymal transition, the repair of tissue injury, and embryonic stem cell differentiation. In humans, FGF10 loss-of-function mutations result in inherited diseases including aplasia of lacrimal and salivary gland, lacrimo-auriculo-dento-digital syndrome, and chronic obstructive pulmonary disease. FGF10 is also involved in the oncogenicity of pancreatic and breast cancers. Single nucleotide polymorphisms in FGF10 are also potential risk factors for limb deficiencies, cleft lip and palate, and extreme myopia. These findings indicate that FGF10 is a crucial paracrine signal from the mesenchyme to epithelium for development, health, and disease.

The Fibroblast Growth Factor signaling pathway

The signaling component of the mammalian Fibroblast Growth Factor (FGF) family is comprised of eighteen secreted proteins that interact with four signaling tyrosine kinase FGF receptors (FGFRs). Interaction of FGF ligands with their signaling receptors is regulated by protein or proteoglycan cofactors and by extracellular binding proteins. Activated FGFRs phosphorylate specific tyrosine residues that mediate interaction with cytosolic adaptor proteins and the RAS-MAPK, PI3K-AKT, PLCγ, and STAT intracellular signaling pathways. Four structurally related intracellular non-signaling FGFs interact with and regulate the family of voltage gated sodium channels. Members of the FGF family function in the earliest stages of embryonic development and during organogenesis to maintain progenitor cells and mediate their growth, differentiation, survival, and patterning. FGFs also have roles in adult tissues where they mediate metabolic functions, tissue repair, and regeneration, often by reactivating developmental signaling pathways. Consistent with the presence of FGFs in almost all tissues and organs, aberrant activity of the pathway is associated with developmental defects that disrupt organogenesis, impair the response to injury, and result in metabolic disorders, and cancer. For further resources related to this article, please visit the WIREs website.