Context-specific requirements for Fgfr1 signaling through Frs2 and Frs3 during mouse development

A novel mouse model for ataxia-telangiectasia with a N-terminal mutation displays a behavioral defect and a low incidence of lymphoma but no increased oxidative burden

Ataxia-telangiectasia (A-T) is a rare multi-system disorder caused by mutations in the ATM gene. Significant heterogeneity exists in the underlying genetic mutations and clinical phenotypes. A number of mouse models have been generated that harbor mutations in the distal region of the gene, and a recent study suggests the presence of residual ATM protein in the brain of one such model. These mice recapitulate many of the characteristics of A-T seen in humans, with the notable exception of neurodegeneration. In order to study how an N-terminal mutation affects the disease phenotype, we generated an inducible Atm mutant mouse model (Atm(tm1Mmpl/tm1Mmpl), referred to as A-T [M]) predicted to express only the first 62 amino acids of Atm. Cells derived from A-T [M] mutant mice exhibited reduced cellular proliferation and an altered DNA damage response, but surprisingly, showed no evidence of an oxidative imbalance. Examination of the A-T [M] animals revealed an altered immunophenotype consistent with A-T. In contrast to mice harboring C-terminal Atm mutations that disproportionately develop thymic lymphomas, A-T [M] mice developed lymphoma at a similar rate as human A-T patients. Morphological analyses of A-T [M] cerebella revealed no substantial cellular defects, similar to other models of A-T, although mice display behavioral defects consistent with cerebellar dysfunction. Overall, these results suggest that loss of Atm is not necessarily associated with an oxidized phenotype as has been previously proposed and that loss of ATM protein is not sufficient to induce cerebellar degeneration in mice.

Blimp1/Prdm1 Functions in Opposition to Irf1 to Maintain Neonatal Tolerance during Postnatal Intestinal Maturation

The neonatal intestine is a very complex and dynamic organ that must rapidly adapt and remodel in response to a barrage of environmental stimuli during the first few postnatal weeks. Recent studies demonstrate that the zinc finger transcriptional repressor Blimp1/Prdm1 plays an essential role governing postnatal reprogramming of intestinal enterocytes during this period. Functional loss results in global changes in gene expression patterns, particularly in genes associated with metabolic function. Here we engineered a knock-in allele expressing an eGFP-tagged fusion protein under control of the endogenous regulatory elements and performed genome wide ChIP-seq analysis to identify direct Blimp1 targets and further elucidate the function of Blimp1 in intestinal development. Comparison with published human and mouse datasets revealed a highly conserved core set of genes including interferon-inducible promoters. Here we show that the interferon-inducible transcriptional activator Irf1 is constitutively expressed throughout fetal and postnatal intestinal epithelium development. ChIP-seq demonstrates closely overlapping Blimp1 and Irf1 peaks at key components of the MHC class I pathway in fetal enterocytes. The onset of MHC class I expression coincides with down-regulated Blimp1 expression during the suckling to weaning transition. Collectively, these experiments strongly suggest that in addition to regulating the enterocyte metabolic switch, Blimp1 functions as a gatekeeper in opposition to Irf1 to prevent premature activation of the MHC class I pathway in villus epithelium to maintain tolerance in the neonatal intestine.

The transcriptional repressor Blimp1/Prdm1 plays a pivotal role in the metabolic switch that occurs in the small intestine during the suckling to weaning transition. Notably, expression profiling of perinatal Blimp1-deficient small intestine revealed premature activation of metabolic genes normally restricted to post-weaning enterocytes. To further elucidate the function of Blimp1 in intestinal development, we engineered a novel Blimp1-eGFP-fusion knock-in mouse strain to perform ChIP-seq analysis. In addition to identifying which metabolic genes are direct Blimp1 targets, ChIP-seq analysis revealed a highly conserved Blimp1/Irf-1 overlapping sites that function to control MHC class I antigen processing during acquisition of neonatal tolerance in the first weeks after birth during early colonization of the intestinal tract by commensal microorganisms. Moreover, immunohistochemical analysis of human fetal intestine suggests that a BLIMP1/IRF-1 axis may also function in human intestinal epithelium development.

Distinct sets of FGF receptors sculpt excitatory and inhibitory synaptogenesis

Neurons in the brain must establish a balanced network of excitatory and inhibitory synapses during development for the brain to function properly. An imbalance between these synapses underlies various neurological and psychiatric disorders. The formation of excitatory and inhibitory synapses requires precise molecular control. In the hippocampus, the structure crucial for learning and memory, fibroblast growth factor 22 (FGF22) and FGF7 specifically promote excitatory or inhibitory synapse formation, respectively. Knockout of either Fgf gene leads to excitatory-inhibitory imbalance in the mouse hippocampus and manifests in an altered susceptibility to epileptic seizures, underscoring the importance of FGF-dependent synapse formation. However, the receptors and signaling mechanisms by which FGF22 and FGF7 induce excitatory and inhibitory synapse differentiation are unknown. Here, we show that distinct sets of overlapping FGF receptors (FGFRs), FGFR2b and FGFR1b, mediate excitatory or inhibitory presynaptic differentiation in response to FGF22 and FGF7. Excitatory presynaptic differentiation is impaired in Fgfr2b and Fgfr1b mutant mice; however, inhibitory presynaptic defects are only found in Fgfr2b mutants. FGFR2b and FGFR1b are required for an excitatory presynaptic response to FGF22, whereas only FGFR2b is required for an inhibitory presynaptic response to FGF7. We further find that FGFRs are required in the presynaptic neuron to respond to FGF22, and that FRS2 and PI3K, but not PLCγ, mediate FGF22-dependent presynaptic differentiation. Our results reveal the specific receptors and signaling pathways that mediate FGF-dependent presynaptic differentiation, and thereby provide a mechanistic understanding of precise excitatory and inhibitory synapse formation in the mammalian brain.

Receptor tyrosine kinases modulate distinct transcriptional programs by differential usage of intracellular pathways

Receptor tyrosine kinases (RTKs) signal through shared intracellular pathways yet mediate distinct outcomes across many cell types. To investigate the mechanisms underlying RTK specificity in craniofacial development, we performed RNA-seq to delineate the transcriptional response to platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) signaling in mouse embryonic palatal mesenchyme cells. While the early gene expression profile induced by both growth factors is qualitatively similar, the late response is divergent. Comparing the effect of MEK (Mitogen/Extracellular signal-regulated kinase) and PI3K (phosphoinositide-3-kinase) inhibition, we find the FGF response is MEK dependent, while the PDGF response is PI3K dependent. Furthermore, FGF promotes proliferation but PDGF favors differentiation. Finally, we demonstrate overlapping domains of PDGF-PI3K signaling and osteoblast differentiation in the palate and increased osteogenesis in FGF mutants, indicating this differentiation circuit is conserved in vivo. Our results identify distinct responses to PDGF and FGF and provide insight into the mechanisms encoding RTK specificity.

DOI:http://dx.doi.org/10.7554/eLife.07186.001

Cells produce many different proteins that play a variety of important roles. For example, proteins called receptor tyrosine kinases can detect particular molecules and send signals to other parts of the cell to regulate the activity (or “expression”) of genes involved in cell division, movement, and other processes.

Humans have 58 receptor tyrosine kinases, and defects in these proteins have been linked to diseases such as cancer and diabetes. However, many different receptors regulate the activities of shared sets of genes, so it is not clear how an individual receptor can specifically control the genes involved in a particular process.

Two receptor tyrosine kinases called PDGFR and FGFR are crucial for the development of the face, palate, and head in humans and other animals. Vasudevan et al. used a technique called RNA-sequencing to find out which genes are regulated by these receptors in mouse palate cells. The experiments show that there is a common set of genes whose activities change quickly—within 1 hour—in response to the activation of either PDGFR or FGFR. However, several hours later, cells in which PDGFR is activated have different patterns of gene expression compared to those with active FGFR.

Vasudevan et al. also found that FGFR promotes cell division, while PDGFR promotes the changing of palate cells into different types with more specialized roles. These different outcomes arise because PDGFR and FGFR use different signaling pathways that involve distinct proteins. For example, a protein called PI3K is critical for changes in gene expression in response to PDGFR but not FGFR.

These results suggest that PGDRF and FGFR control different cellular processes in the palate by sending distinct signals into the cell. Understanding the receptor tyrosine kinases and the networks of genes they activate will help us to identify the signals that are important for other processes, such as the development of the face.

DOI:http://dx.doi.org/10.7554/eLife.07186.002

FGFR1 signaling in hypertrophic chondrocytes is attenuated by the Ras-GAP neurofibromin during endochondral bone formation

Aberrant fibroblast growth factor receptor 3 (FGFR3) signaling disrupts chondrocyte proliferation and growth plate size and architecture, leading to various chondrodysplasias or bone overgrowth. These observations suggest that the duration, intensity and cellular context of FGFR signaling during growth plate chondrocyte maturation require tight, regulated control for proper bone elongation. However, the machinery fine-tuning FGFR signaling in chondrocytes is incompletely defined. We report here that neurofibromin, a Ras-GAP encoded by Nf1, has an overlapping expression pattern with FGFR1 and FGFR3 in prehypertrophic chondrocytes, and with FGFR1 in hypertrophic chondrocytes during endochondral ossification. Based on previous evidence that neurofibromin inhibits Ras-ERK signaling in chondrocytes and phenotypic analogies between mice with constitutive FGFR1 activation and Nf1 deficiency in Col2a1-positive chondrocytes, we asked whether neurofibromin is required to control FGFR1-Ras-ERK signaling in maturing chondrocytes in vivo. Genetic Nf1 ablation in Fgfr1-deficient chondrocytes reactivated Ras-ERK1/2 signaling in hypertrophic chondrocytes and reversed the expansion of the hypertrophic zone observed in mice lacking Fgfr1 in Col2a1-positive chondrocytes. Histomorphometric and gene expression analyses suggested that neurofibromin, by inhibiting Rankl expression, attenuates pro-osteoclastogenic FGFR1 signaling in hypertrophic chondrocytes. We also provide evidence suggesting that neurofibromin in prehypertrophic chondrocytes, downstream of FGFRs and via an indirect mechanism, is required for normal extension and organization of proliferative columns. Collectively, this study indicates that FGFR signaling provides an important input into the Ras-Raf-MEK-ERK1/2 signaling axis in chondrocytes, and that this input is differentially regulated during chondrocyte maturation by a complex intracellular machinery, of which neurofibromin is a critical component.

Receptor tyrosine kinase signaling: regulating neural crest development one phosphate at a time

Receptor tyrosine kinases (RTKs) bind to a subset of growth factors on the surface of cells and elicit responses with broad roles in developmental and postnatal cellular processes. Receptors in this subclass consist of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular domain harboring a catalytic tyrosine kinase and regulatory sequences that are phosphorylated either by the receptor itself or by various interacting proteins. Once activated, RTKs bind signaling molecules and recruit effector proteins to mediate downstream cellular responses through various intracellular signaling pathways. In this chapter, we highlight the role of a subset of RTK families in regulating the activity of neural crest cells (NCCs) and the development of their derivatives in mammalian systems. NCCs are migratory, multipotent cells that can be subdivided into four axial populations, cranial, cardiac, vagal, and trunk. These cells migrate throughout the vertebrate embryo along defined pathways and give rise to unique cell types and structures. Interestingly, individual RTK families often have specific functions in a subpopulation of NCCs that contribute to the diversity of these cells and their derivatives in the mammalian embryo. We additionally discuss current methods used to investigate RTK signaling, including genetic, biochemical, large-scale proteomic, and biosensor approaches, which can be applied to study intracellular signaling pathways active downstream of this receptor subclass during NCC development.

Genetic Regulation of Puberty Timing in Humans

Understanding the regulation of puberty timing has relevance to developmental and human biology and to the pathogenesis of various diseases. Recent large-scale genome-wide association studies on puberty timing and adult height, body mass index (BMI) and central body shape provide evidence for shared biological mechanisms that regulate these traits. There is a substantial genetic overlap between age at menarche in women and BMI, with almost invariable directional consistency with the epidemiological associations between earlier menarche and higher BMI. By contrast, the genetic loci identified for age at menarche are largely distinct from those identified for central body shape, while alleles that confer earlier menarche can be associated with taller or shorter adult height. The findings of population-based studies on age at menarche show increasing relevance for other studies of rare monogenic disorders and enrich our understanding of the mechanisms that regulate the timing of puberty and reproductive function.

SRF regulates craniofacial development through selective recruitment of MRTF cofactors by PDGF signaling

Receptor tyrosine kinase signaling is critical for mammalian craniofacial development, but the key downstream transcriptional effectors remain unknown. We demonstrate that serum response factor (SRF) is induced by both platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) signaling in mouse embryonic palatal mesenchyme cells and that Srf neural crest conditional mutants exhibit facial clefting accompanied by proliferation and migration defects. Srf and Pdgfra mutants interact genetically in craniofacial development, but Srf and Fgfr1 mutants do not. This signal specificity is recapitulated at the level of cofactor activation: while both PDGF and FGF target gene promoters show enriched genome-wide overlap with SRF ChIP-seq peaks, PDGF selectively activates a network of MRTF-dependent cytoskeletal genes. Collectively, our results identify a role for SRF in proliferation and migration during craniofacial development and delineate a mechanism of receptor tyrosine kinase specificity mediated through differential cofactor usage, leading to a PDGF-responsive SRF-driven transcriptional program in the midface.

FGF signaling is required for anterior but not posterior specification of the murine liver bud

BACKGROUND

The definitive endoderm arises as a naive epithelial sheet that produces the entire gut tube and associated organs including the liver, pancreas and lungs. Murine explant studies demonstrate that fibroblast growth factor (FGF) signaling from adjacent tissues is required to induce hepatic gene expression from isolated foregut endoderm. The requirement of FGF signaling during liver development is examined by means of small molecule inhibition during whole embryo culture.

RESULTS

Loss of FGF signaling before hepatic induction results in morphological defects and gene expression changes that are confined to the anterior liver bud. In contrast the posterior portion of the liver bud remains relatively unaffected. Because FGF is thought to act as a morphogen during endoderm organogenesis, the ventral pancreas was also examined after FGF inhibition. Although the size of the ventral pancreas is not affected, loss of FGF signaling results in a significantly higher density of ventral pancreas cells.

CONCLUSIONS

The requirement for FGF-mediated induction of hepatic gene expression differs across the anterior/posterior axis of the developing liver bud. These results underscore the importance of studying tissue differentiation in the context of the whole embryo.

The E2 ubiquitin-conjugating enzyme UBE2J1 is required for spermiogenesis in mice

ER-resident proteins destined for degradation are dislocated into the cytosol by components of the ER quality control machinery for proteasomal degradation. Dislocation substrates are ubiquitylated in the cytosol by E2 ubiquitin-conjugating/E3 ligase complexes. UBE2J1 is one of the well-characterized E2 enzymes that participate in this process. However, the physiological function of Ube2j1 is poorly defined. We find that Ube2j1(-/-) mice have reduced viability and fail to thrive early after birth. Male Ube2j1(-/-) mice are sterile due to a defect in late spermatogenesis. Ultrastructural analysis shows that removal of the cytoplasm is incomplete in Ube2j1(-/-) elongating spermatids, compromising the release of mature elongate spermatids into the lumen of the seminiferous tubule. Our findings identify an essential function for the ubiquitin-proteasome-system in spermiogenesis and define a novel, non-redundant physiological function for the dislocation step of ER quality control.

Endoderm-specific deletion of Tbx1 reveals an FGF-independent role for Tbx1 in pharyngeal apparatus morphogenesis

BACKGROUND

The T-box transcription factor Tbx1, is essential for the normal development of multiple organ systems in the embryo. One of the most striking phenotypes in Tbx1-/- embryos is the failure of the caudal pharyngeal pouches to evaginate from the foregut endoderm. Despite considerable interest in the role of Tbx1 in development, the mechanisms whereby Tbx1 controls caudal pouch formation have remained elusive. In particular, the question as to how Tbx1 expression in the pharyngeal endoderm regulates pharyngeal pouch morphogenesis in the mouse embryo is not known.

RESULTS

To address this question, we produced mouse embryos in which Tbx1 was specifically deleted from the pharyngeal endoderm and, as expected, embryos failed to form caudal pharyngeal pouches. To determine the molecular mechanism, we examined expression of Fgf3 and Fgf8 ligands and downstream effectors. Although Fgf8 expression is greatly reduced in Tbx1-deficient endoderm, FGF signaling levels are unaffected. Furthermore, pouch morphogenesis is only partially perturbed by the loss of both Fgf3 and Fgf8 from the endoderm, indicating that neither are required for pouch formation.

CONCLUSIONS

Tbx1 deletion from the pharyngeal endoderm is sufficient to cause caudal pharyngeal arch segmentation defects by FGF-independent effectors that remain to be identified.

Role of FGF/FGFR signaling in skeletal development and homeostasis: learning from mouse models

Fibroblast growth factor (FGF)/fibroblast growth factor receptor (FGFR) signaling plays essential roles in bone development and diseases. Missense mutations in FGFs and FGFRs in humans can cause various congenital bone diseases, including chondrodysplasia syndromes, craniosynostosis syndromes and syndromes with dysregulated phosphate metabolism. FGF/FGFR signaling is also an important pathway involved in the maintenance of adult bone homeostasis. Multiple kinds of mouse models, mimicking human skeleton diseases caused by missense mutations in FGFs and FGFRs, have been established by knock-in/out and transgenic technologies. These genetically modified mice provide good models for studying the role of FGF/FGFR signaling in skeleton development and homeostasis. In this review, we summarize the mouse models of FGF signaling-related skeleton diseases and recent progresses regarding the molecular mechanisms, underlying the role of FGFs/FGFRs in the regulation of bone development and homeostasis. This review also provides a perspective view on future works to explore the roles of FGF signaling in skeletal development and homeostasis.

Pten loss induces autocrine FGF signaling to promote skin tumorigenesis

Inactivation of the Pten tumor suppressor negatively regulates the PI3K-mTOR pathway. In a model of cutaneous squamous cell carcinoma (SCC), we demonstrate that deletion of Pten strongly elevates Fgf10 protein levels without increasing Fgf10 transcription in vitro and in vivo. The translational activation of Fgf10 by Pten deletion is reversed by genetic disruption of the mTORC1 complex, which also prevents skin tumorigenesis in Pten mutants. We further show that ectopic expression of Fgf10 causes skin papillomas, whereas Pten deletion-induced skin tumors are inhibited by epidermal deletion of Fgfr2. Collectively, our data identify autocrine activation of FGF signaling as an essential mechanism in promoting Pten-deficient skin tumors.

FGFR1-Frs2/3 signalling maintains sensory progenitors during inner ear hair cell formation

Inner ear mechanosensory hair cells transduce sound and balance information. Auditory hair cells emerge from a Sox2-positive sensory patch in the inner ear epithelium, which is progressively restricted during development. This restriction depends on the action of signaling molecules. Fibroblast growth factor (FGF) signalling is important during sensory specification: attenuation of Fgfr1 disrupts cochlear hair cell formation; however, the underlying mechanisms remain unknown. Here we report that in the absence of FGFR1 signaling, the expression of Sox2 within the sensory patch is not maintained. Despite the down-regulation of the prosensory domain markers, p27^Kip1, Hey2, and Hes5, progenitors can still exit the cell cycle to form the zone of non-proliferating cells (ZNPC), however the number of cells that form sensory cells is reduced. Analysis of a mutant Fgfr1 allele, unable to bind to the adaptor protein, Frs2/3, indicates that Sox2 maintenance can be regulated by MAP kinase. We suggest that FGF signaling, through the activation of MAP kinase, is necessary for the maintenance of sensory progenitors and commits precursors to sensory cell differentiation in the mammalian cochlea.

The ability of our brain to perceive sound depends on its conversion into electrical impulses within the cochlea of the inner ear. The cochlea has dedicated specialized cells, called inner ear hair cells, which register sound energy. Environmental effects, genetic disorders or just the passage of time can damage these cells, and the damage impairs our ability to hear. If we could understand how these cells develop, we might be able to exploit this knowledge to generate new hair cells. In this study we address an old problem: how do signals from the fibroblast growth factor (FGF) family control hair cell number? We used mice in which one of the receptors for FGF (Fgfr1) is mutated and found that the expression of a stem cell protein, Sox2 is not maintained. Sox2 generally acts to keep precursors in the cochlea in a pre-hair cell state. However, in mutant mice Sox2 expression is transient, diminishing the ability of precursors to commit to a hair cell fate. These findings suggest that it may be possible to amplify the number of hair cell progenitors in culture by tuning FGF activity, providing a route to replace damaged inner ear hair cells.

Frs2α and Shp2 signal independently of Gab to mediate FGF signaling in lens development

Fibroblast growth factor (FGF) signaling requires a plethora of adaptor proteins to elicit downstream responses, but the functional significances of these docking proteins remain controversial. In this study, we used lens development as a model to investigate Frs2α and its structurally related scaffolding proteins, Gab1 and Gab2, in FGF signaling. We show that genetic ablation of Frs2α alone has a modest effect, but additional deletion of tyrosine phosphatase Shp2 causes a complete arrest of lens vesicle development. Biochemical evidence suggests that this Frs2α-Shp2 synergy reflects their epistatic relationship in the FGF signaling cascade, as opposed to compensatory or parallel functions of these two proteins. Genetic interaction experiments further demonstrate that direct binding of Shp2 to Frs2α is necessary for activation of ERK signaling, whereas constitutive activation of either Shp2 or Kras signaling can compensate for the absence of Frs2α in lens development. By contrast, knockout of Gab1 and Gab2 failed to disrupt FGF signaling in vitro and lens development in vivo. These results establish the Frs2α-Shp2 complex as the key mediator of FGF signaling in lens development.

Epicardial function of canonical Wnt-, Hedgehog-, Fgfr1/2-, and Pdgfra-signalling

AIMS

The embryonic epicardium is a source of smooth muscle cells and fibroblasts of the coronary vasculature and of the myocardium, but the signalling pathways that control mobilization and differentiation of epicardial cells are only partly known. We aimed to (re-)evaluate the relevance of canonical Wnt-, Hedgehog (Hh)-, Fibroblast growth factor receptor (Fgfr)1/2-, and platelet-derived growth factor receptor alpha (Pdgfra)-signalling in murine epicardial development.

METHODS AND RESULTS

We used a T-box 18 (Tbx18)(cre)-mediated conditional approach to delete and to stabilize, respectively, the downstream mediator of canonical Wnt-signalling, beta-catenin (Ctnnb1), to delete and activate the mediator of Hh-signalling, smoothened (Smo), and to delete Fgfr1/Fgfr2 and Pdgfra in murine epicardial development. We show that epicardial loss of Ctnnb1, Smo, or Fgfr1/Fgfr2 does not affect cardiac development, whereas the loss of Pdgfra prevents the differentiation of epicardium-derived cells into mature fibroblasts. Epicardial expression of a stabilized version of Ctnnb1 results in the formation of hyperproliferative epicardial cell clusters; epicardial expression of a constitutively active version of Smo leads to epicardial thickening and loss of epicardial mobilization.

CONCLUSION

Canonical Wnt-, Hh-, and Fgfr1/Fgfr2-signalling are dispensable for epicardial development, but Pdgfra-signalling is crucial for the differentiation of cardiac fibroblasts from epicardium-derived cells.

Receptor tyrosine kinase (RTK) signalling in the control of neural stem and progenitor cell (NSPC) development

Important developmental responses are elicited in neural stem and progenitor cells (NSPC) by activation of the receptor tyrosine kinases (RTK), including the fibroblast growth factor receptors, epidermal growth factor receptor, platelet-derived growth factor receptors and insulin-like growth factor receptor (IGF1R). Signalling through these RTK is necessary and sufficient for driving a number of developmental processes in the central nervous system. Within each of the four RTK families discussed here, receptors are activated by sets of ligands that do not cross-activate receptors of the other three families, and therefore, their activation can be independently regulated by ligand availability. These RTK pathways converge on a conserved core of signalling molecules, but differences between the receptors in utilisation of signalling molecules and molecular adaptors for intracellular signal propagation become increasingly apparent. Intracellular inhibitors of RTK signalling are widely involved in the regulation of developmental signalling in NSPC and often determine developmental outcomes of RTK activation. In addition, cellular responses of NSPC to the activation of a given RTK may be significantly modulated by signal strength. Cellular propensity to respond also plays a role in developmental outcomes of RTK signalling. In combination, these mechanisms regulate the balance between NSPC maintenance and differentiation during development and in adulthood. Attribution of particular developmental responses of NSPC to specific pathways of RTK signalling becomes increasingly elucidated. Co-activation of several RTK in developing NSPC is common, and analysis of co-operation between their signalling pathways may advance knowledge of RTK role in NSPC development.

Stromal-epithelial crosstalk regulates kidney progenitor cell differentiation

Current models suggest that the fate of the kidney epithelial progenitors is solely regulated by signals from the adjacent ureteric bud. The bud provides signals that regulate the survival, renewal and differentiation of these cells. Recent data suggest that Wnt9b, a ureteric bud-derived factor, is sufficient for both progenitor cell renewal and differentiation. How the same molecule induces two seemingly contradictory processes is unknown. Here, we show that signals from the stromal fibroblasts cooperate with Wnt9b to promote differentiation of the progenitors. The atypical cadherin Fat4 encodes at least part of this stromal signal. Our data support a model whereby proper kidney size/function is regulated by balancing opposing signals from the ureteric bud and stroma to promote renewal and differentiation of the nehron progenitors.

Deficient FGF signaling causes optic nerve dysgenesis and ocular coloboma

FGF signaling plays a pivotal role in eye development. Previous studies using in vitro chick models and systemic zebrafish mutants have suggested that FGF signaling is required for the patterning and specification of the optic vesicle, but due to a lack of genetic models, its role in mammalian retinal development remains elusive. In this study, we show that specific deletion of Fgfr1 and Fgfr2 in the optic vesicle disrupts ERK signaling, which results in optic disc and nerve dysgenesis and, ultimately, ocular coloboma. Defective FGF signaling does not abrogate Shh or BMP signaling, nor does it affect axial patterning of the optic vesicle. Instead, FGF signaling regulates Mitf and Pax2 in coordinating the closure of the optic fissure and optic disc specification, which is necessary for the outgrowth of the optic nerve. Genetic evidence further supports that the formation of an Frs2α-Shp2 complex and its recruitment to FGF receptors are crucial for downstream ERK signaling in this process, whereas constitutively active Ras signaling can rescue ocular coloboma in the FGF signaling mutants. Our results thus reveal a previously unappreciated role of FGF-Frs2α-Shp2-Ras-ERK signaling axis in preventing ocular coloboma. These findings suggest that components of FGF signaling pathway may be novel targets in the diagnosis of and the therapeutic interventions for congenital ocular anomalies.

Tbx2 terminates shh/fgf signaling in the developing mouse limb bud by direct repression of gremlin1

Vertebrate limb outgrowth is driven by a positive feedback loop that involves Sonic hedgehog (Shh) and Gremlin1 (Grem1) in the posterior limb bud mesenchyme and Fibroblast growth factors (Fgfs) in the overlying epithelium. Proper spatio-temporal control of these signaling activities is required to avoid limb malformations such as polydactyly. Here we show that, in Tbx2-deficient hindlimbs, Shh/Fgf4 signaling is prolonged, resulting in increased limb bud size and duplication of digit 4. In turn, limb-specific Tbx2 overexpression leads to premature termination of this signaling loop with smaller limbs and reduced digit number as phenotypic manifestation. We show that Tbx2 directly represses Grem1 in distal regions of the posterior limb mesenchyme allowing Bone morphogenetic protein (Bmp) signaling to abrogate Fgf4/9/17 expression in the overlying epithelium. Since Tbx2 itself is a target of Bmp signaling, our data identify a growth-inhibiting positive feedback loop (Bmp/Tbx2/Grem1). We propose that proliferative expansion of Tbx2-expressing cells mediates self-termination of limb bud outgrowth due to their refractoriness to Grem1 induction.

Developmental defects of the limb skeleton, such as variations from the normal number of digits, can result from an abnormal size of the early limb bud. The mechanisms that restrict limb bud growth to avoid polydactyly, i.e. the formation of extra digits, are unclear. Gremlin 1 (Grem1) has been identified as a key regulator in this process via its role as secreted antagonist of Bone morphogenetic protein (Bmp) signaling. But it remains unknown how Grem1 expression is switched off appropriately to achieve normal limb bud size. Here we show in the mouse embryo that T-box transcription factor 2 (Tbx2) directly represses Grem1. We show that Tbx2-positive mesenchymal cells at the posterior margin of the limb bud create a Grem1-negative zone that expands concomitantly with limb bud growth. Progressive displacement of the source of Grem1 and its target region, the apical ectodermal ridge, eventually disrupts epithelial-mesenchymal signaling that is crucial for further proliferative expansion. Our data show how local control of signaling activities is translated into the architecture of the adult skeleton, i.e. the number or digits, which helps us to understand the molecular bases of human polydactyly.

Transcriptional profiling and miRNA-dependent regulatory network analysis of longissimus dorsi muscle during prenatal and adult stages in two distinct pig breeds

MicroRNAs (miRNAs) and mRNAs establish a complex regulatory network influencing diverse biological pathways including muscle development and growth. Elucidating miRNA-dependent regulatory networks involved in muscle development could provide additional insights into muscle traits largely predefined during prenatal development. The present study aimed to determine differentially expressed transcripts and functional miRNA-mRNA relationships associated with different stages of skeletal muscle development in two pig breeds, German Landrace and Pietrain, distinct in muscle characteristics. A comparative transcriptional profiling of longissimus dorsi muscle tissues from fetuses at 35, 63 and 91 days post-conception as well as adult pigs (180 days postnatum) was performed using the Affymetrix GeneChip porcine genome microarray. Differential expression patterns were identified to be associated with muscularly developmental stages and breed types. The integration of miRNA expression data and ingenuity pathways analysis (ipa) pathway analysis revealed several miRNA-dependent regulatory networks related to muscle growth and development. The present results provide insights into muscle biology for further improvement of porcine meat quality.

Genetic deletion of RALA and RALB small GTPases reveals redundant functions in development and tumorigenesis

RAL small GTPases, encoded by the Rala and Ralb genes, are members of the RAS superfamily of small GTPases and can act as downstream effectors of RAS [1]. Although highly similar, distinct functions have been identified for RALA and RALB: RALA has been implicated in epithelial cell polarity [2], insulin secretion [3], GLUT4 translocation [4, 5], neurite branching, and neuronal polarity [6, 7], and RALB in tumor cell survival [8], migration/invasion [9-12], TBK1 activation [13], and autophagy [14]. To investigate RAL GTPases in vivo, we generated null and conditional knockout mice. Ralb null mice are viable with no overt phenotype; the Rala null leads to exencephaly and embryonic lethality. The exencephaly phenotype is exacerbated in Rala(-/-);Ralb(+/-) embryos; embryos null for Rala and Ralb do not live past gastrulation. Using a Kras-driven non-small cell lung carcinoma mouse model, we found that either RALA or RALB is sufficient for tumor growth. However, deletion of both Ral genes blocks tumor formation. Either RALA or RALB is sufficient for cell proliferation, but cells lacking both fail to proliferate. These studies demonstrate functions of RAL proteins in development, tumorigenesis, and cell proliferation and show that RALA and RALB act in a redundant fashion.

Evolutionarily conserved requirement of Cdx for post-occipital tissue emergence

Mouse Cdx genes are involved in axial patterning and partial Cdx mutants exhibit posterior embryonic defects. We found that mouse embryos in which all three Cdx genes are inactivated fail to generate any axial tissue beyond the cephalic and occipital primordia. Anterior axial tissues are laid down and well patterned in Cdx null embryos, and a 3' Hox gene is initially transcribed and expressed in the hindbrain normally. Axial elongation stops abruptly at the post-occipital level in the absence of Cdx, as the posterior growth zone loses its progenitor activity. Exogenous Fgf8 rescues the posterior truncation of Cdx mutants, and the spectrum of defects of Cdx null embryos matches that resulting from loss of posterior Fgfr1 signaling. Our data argue for a main function of Cdx in enforcing trunk emergence beyond the Cdx-independent cephalo-occipital region, and for a downstream role of Fgfr1 signaling in this function. Cdx requirement for the post-head section of the axis is ancestral as it takes place in arthropods as well.

Ureteric morphogenesis requires Fgfr1 and Fgfr2/Frs2α signaling in the metanephric mesenchyme

Conditional deletion of fibroblast growth factor receptors (Fgfrs) 1 and 2 in the metanephric mesenchyme (MM) of mice leads to a virtual absence of MM and unbranched ureteric buds that are occasionally duplex. Deletion of Fgfr2 in the MM leads to kidneys with cranially displaced ureteric buds along the Wolffian duct or duplex ureters. Mice with point mutations in Fgfr2's binding site for the docking protein Frs2α (Fgfr2(LR/LR)), however, have normal kidneys; the roles of the Fgfr2/Frs2α signaling axis in MM development and regulating the ureteric bud induction site are incompletely understood. Here, we generated mice with both Fgfr1 deleted in the MM and Fgfr2(LR/LR) point mutations (Fgfr1(Mes-/-)Fgfrf2(LR/LR)). Unlike mice lacking both Fgfr1 and Fgfr2 in the MM, these mice had no obvious MM defects but had cranially displaced or duplex ureteric buds, probably as a result of decreased Bmp4 expression. Fgfr1(Mes-/-)Fgfr2(LR/LR) mice also had subsequent defects in ureteric morphogenesis, including dilated, hyperproliferative tips and decreased branching. Ultimately, they developed progressive renal cystic dysplasia associated with abnormally oriented cell division. Furthermore, mutants had increased and ectopic expression of Ret and its downstream targets in ureteric trunks, and exhibited upregulation of Ret/Etv4/5 signaling effectors, including Met, Myb, Cxcr4, and Crlf1. These defects were associated with reduced expression of Bmp4 in mesenchymal cells near mutant ureteric bud tips. Taken together, these results demonstrate that Fgfr2/Frs2α signaling in the MM promotes Bmp4 expression, which represses Ret levels and signaling in the ureteric bud to ensure normal ureteric morphogenesis.

FGFs: Neurodevelopment's Jack-of-all-Trades - How Do They Do it?

From neurulation to postnatal processes, the requirements for FGF signaling in many aspects of neural precursor cell biology have been well documented. However, identifying a requirement for FGFs in a particular neurogenic process provides only an initial and superficial understanding of what FGF signaling is doing. How FGFs specify cell types in one instance, yet promote cell survival, proliferation, migration, or differentiation in other instances remains largely unknown and is key to understanding how they function. This review describes what we have learned primarily from in vivo vertebrate studies about the roles of FGF signaling in neurulation, anterior–posterior patterning of the neural plate, brain patterning from local signaling centers, and finally neocortex development as an example of continued roles for FGFs within the same brain area. The potential explanations for the diverse functions of FGFs through differential interactions with cell intrinsic and extrinsic factors is then discussed with an emphasis on how little we know about the modulation of FGF signaling in vivo. A clearer picture of the mechanisms involved is nevertheless essential to understand the behavior of neural precursor cells and to potentially guide their fates for therapeutic purposes.

Regulation of insulin-like growth factor signaling by Yap governs cardiomyocyte proliferation and embryonic heart size

The Hippo signaling pathway regulates growth of the heart and other tissues. Hippo pathway kinases influence the activity of various targets, including the transcriptional coactivator Yap, but the specific role of Yap in heart growth has not been investigated. We show that Yap is necessary and sufficient for embryonic cardiac growth in mice. Deletion of Yap in the embryonic mouse heart impeded cardiomyocyte proliferation, causing myocardial hypoplasia and lethality at embryonic stage 10.5. Conversely, forced expression of a constitutively active form of Yap in the embryonic heart increased cardiomyocyte number and heart size. Yap activated the insulin-like growth factor (IGF) signaling pathway in cardiomyocytes, resulting in inactivation of glycogen synthase kinase 3β, which led to increased abundance of β-catenin, a positive regulator of cardiac growth. Our results point to Yap as a critical downstream effector of the Hippo pathway in the control of cardiomyocyte proliferation and a nexus for coupling the IGF, Wnt, and Hippo signaling pathways with the developmental program for heart growth.

The master negative regulator REST/NRSF controls adult neurogenesis by restraining the neurogenic program in quiescent stem cells

Transcriptional regulation is a critical mechanism in the birth, specification, and differentiation of granule neurons in the adult hippocampus. One of the first negative-acting transcriptional regulators implicated in vertebrate development is repressor element 1-silencing transcription/neuron-restrictive silencer factor (REST/NRSF)--thought to regulate hundreds of neuron-specific genes--yet its function in the adult brain remains elusive. Here we report that REST/NRSF is required to maintain the adult neural stem cell (NSC) pool and orchestrate stage-specific differentiation. REST/NRSF recruits CoREST and mSin3A corepressors to stem cell chromatin for the regulation of pro-neuronal target genes to prevent precocious neuronal differentiation in cultured adult NSCs. Moreover, mice lacking REST/NRSF specifically in NSCs display a transient increase in adult neurogenesis that leads to a loss in the neurogenic capacity of NSCs and eventually diminished granule neurons. Our work identifies REST/NRSF as a master negative regulator of adult NSC differentiation and offers a potential molecular target for neuroregenerative approaches.

A missense mutation in Fgfr1 causes ear and skull defects in hush puppy mice

The hush puppy mouse mutant has been shown previously to have skull and outer, middle, and inner ear defects, and an increase in hearing threshold. The fibroblast growth factor receptor 1 (Fgfr1) gene is located in the region of chromosome 8 containing the mutation. Sequencing of the gene in hush puppy heterozygotes revealed a missense mutation in the kinase domain of the protein (W691R). Homozygotes were found to die during development, at approximately embryonic day 8.5, and displayed a phenotype similar to null mutants. Reverse transcription PCR indicated a decrease in Fgfr1 transcript in heterozygotes and homozygotes. Generation of a construct containing the mutation allowed the function of the mutated receptor to be studied. Immunocytochemistry showed that the mutant receptor protein was present at the cell membrane, suggesting normal expression and trafficking. Measurements of changes in intracellular calcium concentration showed that the mutated receptor could not activate the IP₃ pathway, in contrast to the wild-type receptor, nor could it initiate activation of the Ras/MAP kinase pathway. Thus, the hush puppy mutation in fibroblast growth factor receptor 1 appears to cause a loss of receptor function. The mutant protein appears to have a dominant negative effect, which could be due to it dimerising with the wild-type protein and inhibiting its activity, thus further reducing the levels of functional protein. A dominant modifier, Mhspy, which reduces the effect of the hush puppy mutation on pinna and stapes development, has been mapped to the distal end of chromosome 7 and may show imprinting.

Dissecting the role of Fgf signaling during gastrulation and left-right axis formation in mouse embryos using chemical inhibitors

Fgf signaling plays pivotal roles in mouse gastrulation and left-right axis formation. However, although genetic analyses have revealed important aspects of Fgf signaling in these processes, the temporal resolution of genetic studies is low. Here, we combined whole-embryo culture with application of chemical compounds to inhibit Fgf signaling at specific time points. We found that sodium chlorate and PD173074 are potent inhibitors of Fgf signaling in early mouse embryos. Fgf signaling is required for the epithelial-to-mesenchymal transition of the primitive streak before the onset of gastrulation. Once gastrulation begins, Fgf signaling specifies mesodermal fates via the Ras/MAPK downstream cascade. Finally, Fgf signaling on the posterior side of the embryo during gastrulation induces Nodal expression in the node via Tbx6-Dll1, the initial event required for Nodal expression in the left lateral plate mesoderm.

Mammalian Fbh1 is important to restore normal mitotic progression following decatenation stress

We have addressed the role of the F-box helicase 1 (Fbh1) protein during genome maintenance in mammalian cells. For this, we generated two mouse embryonic stem cell lines deficient for Fbh1: one with a homozygous deletion of the N-terminal F-box domain (Fbh1(f/f)), and the other with a homozygous disruption (Fbh1(-/-)). Consistent with previous reports of Fbh1-deficiency in vertebrate cells, we found that Fbh1(-/-) cells show a moderate increase in Rad51 localization to DNA damage, but no clear defect in chromosome break repair. In contrast, we found that Fbh1(f/f) cells show a decrease in Rad51 localization to DNA damage and increased cytoplasmic localization of Rad51. However, these Fbh1(f/f) cells show no clear defects in chromosome break repair. Since some Rad51 partners and F-box-associated proteins (Skp1-Cul1) have been implicated in progression through mitosis, we considered whether Fbh1 might play a role in this process. To test this hypothesis, we disrupted mitosis using catalytic topoisomerase II inhibitors (bisdioxopiperazines), which inhibit chromosome decatenation. We found that both Fbh1(f/f) and Fbh1(-/-) cells show hypersensitivity to topoisomerase II catalytic inhibitors, even though the degree of decatenation stress was not affected. Furthermore, following topoisomerase II catalytic inhibition, both Fbh1-deficient cell lines show substantial defects in anaphase separation of chromosomes. These results indicate that Fbh1 is important for restoration of normal mitotic progression following decatenation stress.

Midline signaling regulates kidney positioning but not nephrogenesis through Shh

The role of axial structures, especially the notochord, in metanephric kidney development has not been directly examined. Here, we showed that disruption of the notochord and floor plate by diphtheria toxin (DTA)-mediated cell ablation did not disrupt nephrogenesis, but resulted in kidney fusions, resembling horseshoe kidneys in humans. Axial disruptions led to more medially positioned metanephric mesenchyme (MM) in midgestation. However, neither axial disruption nor the ensuing positional shift of the MM affected the formation of nephrons and other structures within the kidney. Response to Shh signaling was greatly reduced in midline cell populations in the mutants. To further ascertain the molecular mechanism underlying these abnormalities, we specifically inactivated Shh in the notochord and floor plate. We found that depleting the axial source of Shh was sufficient to cause kidney fusion, even in the presence of the notochord. These results suggested that the notochord is dispensable for nephrogenesis but required for the correct positioning of the metanephric kidney. Axial Shh signal appears to be critical in conferring the effects of axial structures on kidney positioning along the mediolateral axis. These studies also provide insights into the pathogenesis of horseshoe kidneys and how congenital kidney defects can be caused by signals outside the renal primordia.

Posterior malformations in Dact1 mutant mice arise through misregulated Vangl2 at the primitive streak

Mice homozygous for mutations in Dact1 (Dpr/Frodo) phenocopy human malformations involving the spine, genitourinary system, and distal digestive tract. We trace this phenotype to disrupted germ layer morphogenesis at the primitive streak (PS). Remarkably, heterozygous mutation of Vangl2, a transmembrane component of the Planar Cell Polarity (PCP) pathway, rescues recessive Dact1 phenotypes, whereas loss of Dact1 reciprocally rescues semidominant Vangl2 phenotypes. We show that Dact1, an intracellular protein, forms a complex with Vangl2. In Dact1 mutants, Vangl2 is increased at the PS where cells ordinarily undergo an epithelial-mesenchymal transition. This is associated with abnormal E-cadherin distribution and changes in biochemical measures of the PCP pathway. We conclude that Dact1 contributes to morphogenesis at the PS by regulating Vangl2 upstream of cell adhesion and the PCP pathway.

The FRS2 family of docking/scaffolding adaptor proteins as therapeutic targets of cancer treatment

BACKGROUND

There are two members--FRS2alpha and FRS2beta--in the fibroblast growth factor receptor substrate 2 (FRS2) family of docking/scaffolding adaptor proteins. These proteins function downstream of certain kinds of receptor tyrosine kinases (RTKs) that are important for tumorigenesis. FRS2alpha acts as a control centre for fibroblast growth factor receptor signalling and encourages tumorigenesis, while FRS2beta regulates EGFR signalling negatively, and might have a tumour suppressive role. Therefore, both proteins could be good therapeutic targets for the treatment of cancer.

OBJECTIVE

To examine the physiological and pathological roles of FRS2, especially in cancer, and describe their potential value as therapeutic targets.

METHODS

A review of relevant literature.

RESULTS/CONCLUSIONS

Although it is still difficult to develop small compounds to modify functions of FRS2 adaptor proteins, such compounds may be useful as the next generation of molecular targeting drugs. Combination therapy with RTK-targeting drugs and FRS2-targeting drugs may be useful for cancer treatment in the near future.

FGF-receptor substrate 2 functions as a molecular sensor integrating external regulatory signals into the FGF pathway

Fibroblast growth factor (FGF) receptor substrate 2alpha (FRS2alpha) is the main mediator of signaling in the FGF pathway. Recent studies have shown that mitogen-activated protein kinase (MAPK) phosphorylates serine and threonine residues in FRS2, negatively affecting FGF-induced tyrosine phosphorylation (PY) of FRS2. Several kinds of stimuli can induce serine/threonine phosphorylation (PS/T) of FRS2, indicating that FRS2 may be useful for studying crosstalk between growth factor signaling pathways. Here, we report that FGF-induced PY of FRS2 can be attenuated by EGF co-stimulation in PC12 cells; this inhibitory effect could be completely reversed by U0126, an inhibitor of MEK. We further identified the ERK1/2-binding motif in FRS2 and generated FRS2-3KL, a mutant lacking MAPK binding and PT upon FGF and/or EGF stimulation. Unlike wild-type (WT) FRS2, FGF-induced PY of FRS2-3KL could not be inhibited by EGF co-stimulation, and FRS2-3KL-expressing PC12 cells exhibited more differentiating potential than FRS2-WT-expressing cells in response to FGF treatment. These results suggest that PS/T of FRS2 mediated by the FRS2-MAPK negative regulatory loop may function as a molecular switch integrating negative regulatory signals from other pathways into FGFR-generated signal transduction.

FRS2alpha is required for the separation, migration, and survival of pharyngeal-endoderm derived organs including thyroid, ultimobranchial body, parathyroid, and thymus

The docking protein FRS2alpha plays an important role in fibroblast growth factor (FGF)-induced intracellular signal transduction by linking FGF receptors (FGFRs) to a variety of intracellular signaling pathways. In FRS2alpha(2F/2F) mutant mice at embryonic day (E)18.5, in which the Shp2-binding sites of FRS2alpha were disrupted, the thyroid glands were aplastic or hypoplastic. C cells were absent or present in low numbers and rarely formed a compact mass of cells. Parathyroid glands were mostly connected to thymus tissues. At E10.5, the formations of pharyngeal pouches and thyroid primordium were normally initiated in the mutant mice. At E11.5 to E12.5, the thyroid primordium of wild-type embryos was located close to the aortic sac, and the epithelial buds of pharyngeal-derived organs, including the parathyroid gland, thymus and ultimobranchial body, were separated from the epithelium and began to migrate to their final destinations. In the FRS2alpha(2F/2F) mutants, however, the thyroid primordium became hypoplastic and the pharyngeal-derived organ primordia remained affiliated with the pharyngeal epithelium. At these stages, organ-specific differentiation markers (i.e., Nkx2-1/TTF1 for the thyroid lobe and ultimobranchial body; Pax8 for the thyroid lobe; parathormone (PTH), chromogranin A, P75(NTR), and S100 protein for the parathyroid gland; and p63 for the thymus) were normally expressed in the mutant tissues. Thus, the separation, migration, and survival of the pharyngeal organs were impaired in the FRS2alpha(2F/2F) mutants.

The PDGF signaling pathway controls multiple steroid-producing lineages

The platelet-derived growth factor (PDGF) signaling pathway regulates numerous lineages of mesenchymal cell origin during development and in the adult. The transcriptional targets of this pathway have been shown to be required in several PDGF-dependent processes, but the roles of these targets in specific tissues is just beginning to be identified. In this study, we show that five different PDGF target genes are essential for male and/or female fertility. Mutations in each of these five different genes lead to defects in the steroid-producing cells in the testis and/or ovary and altered hormone production, suggesting that the PDGF pathway controls steroidogenesis through these genes in both sexes. Furthermore, conditional mutations of both PDGF receptors revealed a requirement in steroid-producing cells in multiple organs, including the testis, ovary, and adrenal cortex. Therefore, PDGF signaling may constitute a common mechanism in the control of multiple steroidogenic lineages.

Frs2alpha-deficiency in cardiac progenitors disrupts a subset of FGF signals required for outflow tract morphogenesis

The cardiac outflow tract (OFT) is a developmentally complex structure derived from multiple lineages and is often defective in human congenital anomalies. Although emerging evidence shows that fibroblast growth factor (FGF) is essential for OFT development, the downstream pathways mediating FGF signaling in cardiac progenitors remain poorly understood. Here, we report that FRS2alpha (FRS2), an adaptor protein that links FGF receptor kinases to multiple signaling pathways, mediates crucial aspects of FGF-dependent OFT development in mouse. Ablation of Frs2alpha in mesodermal OFT progenitor cells that originate in the second heart field (SHF) affects their expansion into the OFT myocardium, resulting in OFT misalignment and hypoplasia. Moreover, Frs2alpha mutants have defective endothelial-to-mesenchymal transition and neural crest cell recruitment into the OFT cushions, resulting in OFT septation defects. These results provide new insight into the signaling molecules downstream of FGF receptor tyrosine kinases in cardiac progenitors.

The t(1;9)(p34;q34) and t(8;12)(p11;q15) fuse pre-mRNA processing proteins SFPQ (PSF) and CPSF6 to ABL and FGFR1

We have investigated two patients with acquired chromosomal rearrangements, a male presenting with a t(1;9)(p34;q34) and B cell progenitor acute lymphoid leukemia and a female presenting with a t(8;12)(p11;q15) and the 8p11 myeloproliferative syndrome. We determined that the t(1;9) fused ABL to SFPQ (also known as PSF), a gene mapping to 1p34 that encodes a polypyrimidine tract-binding protein-associated splicing factor. The t(8;12) fused CPSF6, a cleavage and polyadenylation specificity factor, to FGFR1. The fusions were confirmed by amplification of the genomic breakpoints and RT-PCR. The predicted oncogenic products of these fusions, SFPQ-ABL and CPSF6-FGFR1, are in-frame and encode the N-terminal domain of the partner protein and the entire tyrosine kinase domain and C-terminal sequences of ABL and FGFR1. SFPQ interacts with two FGFR1 fusion partners, ZNF198 and CPSF6, that are functionally related to the recurrent PDGFRalpha partner FIP1L1. Our findings thus identify a group of proteins that are important for pre-mRNA processing as fusion partners for tyrosine kinases in hematological malignancies.

Regulation of growth factor signaling by FRS2 family docking/scaffold adaptor proteins

The FRS2 family of adaptor/scaffold proteins has two members, FRS2alpha and FRS2beta. Both proteins contain N-terminal myristylation sites for localization on the plasma membrane and a PTB domain for binding to limited species of receptor tyrosine kinases (RTKs), including the FGF receptor, the neurotophin receptor, RET, and ALK. Activation of these RTKs allows FRS2 proteins to become phosphorylated of tyrosine residues and then bind to Grb2 and Shp2, a SH2 domain-containing adaptor and a tyrosine phosphatase, respectively. Subsequently, Shp2 activates a Ras/ERK pathway and Grb2 activates a Ras/ERK, phosphatidyl inositol (PI)-3 kinase and ubiquitination/degradation pathways by binding to SOS, Gab1, and Cbl via the SH3 domains of Grb2. FRS2alpha acts as 'a conning center' in FGF signaling mainly because it induces sustained levels of activation of ERK via Shp2-binding sites and Grb2-binding sites, though the contribution of the former is greater. Indeed, FRS2alpha knockout mice and mice with mutated Shp2-binding sites exhibit a variety of phenotypes due to defects in FGF signaling in vivo. Although FRS2beta binds to the EGF receptor, it does not induce tyrosine phosphorylation on the receptor. Instead, it inhibits EGF signaling, resulting in inhibition of EGF-induced cell proliferation and cell transformation. Based on these findings, the involvement of FRS2 proteins in tumorigenesis should be studied extensively to be validated as candidate biomarkers for the effectiveness of treatments targeting RTKs such as the FGF receptor and EGF receptor.

Fibroblast growth factor receptor 1 (FGFR1) tyrosine phosphorylation regulates binding of FGFR substrate 2alpha (FRS2alpha) but not FRS2 to the receptor

Binding of the fibroblast growth factor (FGF) to the FGF receptor (FGFR) tyrosine kinase leads to receptor tyrosine autophosphorylation as well as phosphorylation of multiple downstream signaling molecules that are recruited to the receptor either by direct binding or through adaptor proteins. The FGFR substrate 2 (FRS2) family consists of two members, FRS2alpha and FRS2beta, and has been shown to recruit multiple signaling molecules, including Grb2 and Shp2, to FGFR1. To better understand how FRS2 interacted with FGFR1, in vivo binding assays with coexpressed FGFR1 and FRS2 recombinant proteins in mammalian cells were carried out. The results showed that the interaction of full-length FRS2alpha, but not FRS2beta, with FGFR1 was enhanced by activation of the receptor kinase. The truncated FRS2alpha mutant that was comprised only of the phosphotyrosine-binding domain (PTB) bound FGFR1 constitutively, suggesting that the C-terminal sequence downstream the PTB domain inhibited the PTB-FGFR1 binding. Inactivation of the FGFR1 kinase and substitutions of tyrosine phosphorylation sites of FGFR1, but not FRS2alpha, reduced binding of FGFR1 with FRS2alpha. The results suggest that although the tyrosine autophosphorylation sites of FGFR1 did not constitute the binding sites for FRS2alpha, phosphorylation of these residues was essential for optimal interaction with FRS2alpha. In addition, it was demonstrated that the Grb2-binding sites of FRS2alpha are essential for mediating signals of FGFR1 to activate the FiRE enhancer of the mouse syndecan 1 gene. The results, for the first time, demonstrate the specific signals mediated by the Grb2-binding sites and further our understanding of FGF signal transmission at the adaptor level.

Genetic analysis of molecular oscillators in mammalian somitogenesis: Clues for studies of human vertebral disorders

The repeating pattern of the human vertebral column is shaped early in development, by a process called somitogenesis. In this embryonic process, pairs of mesodermal segments called somites are serially laid down along the developing neural tube. Somitogenesis is an iterative process, repeating at regular time intervals until the last somite is formed. This process lays down the vertebrate body axis from head to tail, making for a progression of developmental steps along the rostral-caudal axis. In this review, the roles of the Notch, Wnt, fibroblast growth factor, retinoic acid and other pathways are described during the following key steps in somitogenesis: formation of the presomitic mesoderm (PSM) and establishment of molecular gradients; prepatterning of the PSM by molecular oscillators; patterning of rostral-caudal polarity within the somite; formation of somite borders; and maturation and resegmentation of somites to form musculoskeletal tissues. Disruption of somitogenesis can lead to severe vertebral birth defects such as spondylocostal dysostosis (SCD). Genetic studies in the mouse have been instrumental in finding mutations in this disorder, and ongoing mouse studies should provide functional insights and additional candidate genes to help in efforts to identify genes causing human spinal birth defects.

PDGF signaling specificity is mediated through multiple immediate early genes

Growth factor signaling leads to the induction or repression of immediate early genes, but how these genes act collectively as effectors of downstream processes remains unresolved. We have used gene trap-coupled microarray analysis to identify and mutate multiple platelet-derived growth factor (PDGF) intermediate early genes in mice. Mutations in these genes lead to a high frequency of phenotypes that affect the same cell types and processes as those controlled by the PDGF pathway. We conclude that these genes form a network that controls specific processes downstream of PDGF signaling.

Attenuation of signaling pathways stimulated by pathologically activated FGF-receptor 2 mutants prevents craniosynostosis

Craniosynostosis, the fusion of one or more of the sutures of the skull vault before the brain completes its growth, is a common (1 in 2,500 births) craniofacial abnormality, approximately 20% of which occurrences are caused by gain-of-function mutations in FGF receptors (FGFRs). We describe a genetic and pharmacological approach for the treatment of a murine model system of Crouzon-like craniosynostosis induced by a dominant mutation in Fgfr2c. Using genetically modified mice, we demonstrate that premature fusion of sutures mediated by Crouzon-like activated Fgfr2c mutant is prevented by attenuation of signaling pathways by selective uncoupling between the docking protein Frs2alpha and activated Fgfr2c, resulting in normal skull development. We also demonstrate that attenuation of Fgfr signaling in a calvaria organ culture with an Fgfr inhibitor prevents premature fusion of sutures without adversely affecting calvaria development. These experiments show that attenuation of FGFR signaling by pharmacological intervention could be applied for the treatment of craniosynostosis or other severe bone disorders caused by mutations in FGFRs that currently have no treatment.

Fgfr2 and Fgfr3 are not required for patterning and maintenance of the midbrain and anterior hindbrain

The mid-/hindbrain organizer (MHO) is characterized by the expression of a network of genes, which controls the patterning and development of the prospective midbrain and anterior hindbrain. One key molecule acting at the MHO is the fibroblast growth factor (Fgf) 8. Ectopic expression of Fgf8 induces genes that are normally expressed at the mid-/hindbrain boundary followed by the induction of midbrain and anterior hindbrain structures. Inactivation of the Fgf receptor (Fgfr) 1 gene, which was thought to be the primary transducer of the Fgf8 signal at the MHO, in the mid-/hindbrain region, leads to a deletion of dorsal structures of the mid-/hindbrain region, whereas ventral tissues are less severely affected. This suggests that other Fgfrs might be responsible for ventral mid-/hindbrain region development. Here we report the analysis of Fgfr2 conditional knockout mice, lacking the Fgfr2 in the mid-/hindbrain region and of Fgfr3 knockout mice with respect to the mid-/hindbrain region. In both homozygous mouse mutants, patterning of the mid-/hindbrain region is not altered, neuronal populations develop normal and are maintained into adulthood. This analysis shows that the Fgfr2 and the Fgfr3 on their own are dispensable for the development of the mid-/hindbrain region. We suggest functional redundancy of Fgf receptors in the mid-/hindbrain region.

Retroviral lineage analysis of fibroblast growth factor receptor signaling in FGF2 inhibition of oligodendrocyte progenitor differentiation

Fibroblast growth factor 2 (FGF2) inhibits oligodendrocyte progenitor cell (OPC) differentiation during development and limits remyelination following chronic demyelination. The current study examines the mechanism underlying this effect of FGF2 expression on OPC differentiation. Retroviral lineage tracing demonstrates a direct in vivo effect of FGF receptor (FGFR) signaling on OPC differentiation. Retrovirus expressing a dominant negative FGFR construct (FGFRdn) and green fluorescent protein (GFP) was injected into the dorsal columns of postnatal day 7 (P7) mice followed by perfusion at P28. Among the GFP-labeled cells, FGFRdn retrovirus generated a higher proportion of oligodendrocytes than did control infections. This result from FGFRdn expression in OPCs was similar to the result obtained in our previous study using control retrovirus in FGF2 null mice. Further, in vitro retroviral siRNA expression distinguishes the function of specific FGFR isoforms in OPC responses to FGF2. FGF2 inhibition of OPC differentiation was effectively blocked by siRNA targeted to FGFR1, but not FGFR2 or FGFR3. We propose a model of direct FGF2 activation of FGFR1 leading to inhibition of OPC differentiation. This signaling pathway may be an important regulator of oligodendrocyte generation during myelination in development and may perturb OPC generation of remyelinating oligodendrocytes in demyelinating disease.

Chemotactic cell movement during Dictyostelium development and gastrulation

Many developmental processes involve chemotactic cell movement up or down dynamic chemical gradients. Studies of the molecular mechanisms of chemotactic movement of Dictyostelium amoebae up cAMP gradients highlight the importance of PIP3 signaling in the control of cAMP-dependent actin polymerization, which drives the protrusion of lamellipodia and filopodia at the leading edge of the cell, but also emphasize the need for myosin thick filament assembly and motor activation for the contraction of the back of the cell. These process become even more important during the multicellular stages of development, when propagating waves of cAMP coordinate the chemotactic movement of tens of thousands of cells, resulting in multicellular morphogenesis. Recent experiments show that chemotaxis, especially in response to members of the FGF, PDGF and VEGF families of growth factors, plays a key role in the guidance of mesoderm cells during gastrulation in chick, mouse and frog embryos. The molecular mechanisms of signal detection and signaling to the actin-myosin cytoskeleton remain to be elucidated.