Fgf9 signaling regulates inner ear morphogenesis through epithelial–mesenchymal interactions

Fgf9 promotes incisor dental epithelial stem cell survival and enamel formation

BACKGROUND

Understanding the role of cytokines in tooth development is critical for advancing dental tissue engineering. Fibroblast growth factor 9 (FGF9) is the only FGF consistently expressed throughout dental epithelial tissue, from the initiation of tooth bud formation to tooth maturation. However, mice lacking Fgf9 (Fgf9-/-) surprisingly show no obvious abnormalities in tooth development, suggesting potential compensation by other FGFs. Here we report findings from an Fgf9S99N mutation mouse model, a loss-of-function mutation with a dominant negative effect. Our study reveals that Fgf9 is crucial for dental epithelial stem cell (DESC) survival and enamel formation.

METHODS

To dissect the role of Fgf9 in tooth development, we performed the micro-CT, histomorphological analysis and gene expression assay in mice and embryos with S99N mutation. In addition, we assessed the effect of FGF9 on the DESC survival and dental epithelial differentiation by DESC sphere formation assay and tooth explant culture. Cell/tissue culture methods, gene expression analysis, specific inhibitors, and antibody blockage analysis were employed to explore how Fgf9 regulates enamel differentiation and DESC survival through both direct and indirect mechanisms.

RESULTS

The Fgf9S99N mutation in mice led to reduced ameloblasts, impaired enamel formation, and increased apoptosis in the cervical loop (CL). DESC sphere culture experiments revealed that FGF9 facilitated DESC survival via activating ERK/CREB signaling, without affecting cell proliferation. Furthermore, in vitro tissue culture experiments demonstrated that FGF9 promoted enamel formation in a manner dependent on the presence of mesenchyme. Interestingly, FGF9 stimulation inhibited enamel formation in isolated enamel epithelia and DESC spheres. Further investigation revealed that FGF9 supports DESC survival and promotes amelogenesis by stimulating the secretion of FGF3 and FGF10 in dental mesenchymal cells via the MAPK/ERK signaling pathway.

CONCLUSIONS

Our study demonstrates that Fgf9 is essential for DESC survival and enamel formation. Fgf9 performs as a dual-directional regulator of the dental enamel epithelium, not only inhibiting DESC differentiation into ameloblasts to preserve the stemness of DESC, but also promoting ameloblast differentiation through epithelial-mesenchymal interactions.

The fundamentals of fibroblast growth factor 9

Fibroblast growth factor 9 (FGF9) was first identified during a screen for factors acting on cells of the central nervous system (CNS). Research over the subsequent two decades has revealed this protein to be a critically important and elegantly regulated growth factor. A hallmark control feature is reciprocal compartmentalization, particularly during development, with epithelium as a dominant source and mesenchyme a prime target. This mesenchyme selectivity is accomplished by the high affinity of FGF9 to the IIIc isoforms of FGFR1, 2, and 3. FGF9 is expressed widely in the embryo, including the developing heart and lungs, and more selectively in the adult, including the CNS and kidneys. Global Fgf9-null mice die shortly after birth due to respiratory failure from hypoplastic lungs. As well, their hearts are dilated and poorly vascularized, the skeleton is small, the intestine is shortened, and male-to-female sex reversal can be found. Conditional Fgf9-null mice have revealed CNS phenotypes, including ataxia and epilepsy. In humans, FGF9 variants have been found to underlie multiple synostoses syndrome 3, a syndrome characterized by multiple joint fusions. Aberrant FGF9 signaling has also been implicated in differences of sex development and cancer, whereas vascular stabilizing effects of FGF9 could benefit chronic diseases. This primer reviews the attributes of this vital growth factor.

Chromatin profiling and state predictions reveal insights into epigenetic regulation during early porcine development

BACKGROUND

Given their physiological similarities to humans, pigs are increasingly used as model organisms in human-oriented biomedical studies. Additionally, their value to animal agriculture across the globe has led to the development of numerous studies to investigate how to improve livestock welfare and production efficiency. As such, pigs are uniquely poised as compelling models that can yield findings with potential implications in both human and animal contexts. Despite this, many gaps remain in our knowledge about the foundational mechanisms that govern gene expression in swine across different developmental stages, particularly in early development. To address some of these gaps, we profiled the histone marks H3K4me3, H3K27ac, and H3K27me3 and the SWI/SNF central ATPase BRG1 in two porcine cell lines representing discrete early developmental time points and used the resulting information to construct predicted chromatin state maps for these cells. We combined this approach with analysis of publicly available RNA-seq data to examine the relationship between epigenetic status and gene expression in these cell types.

RESULTS

In porcine fetal fibroblast (PFF) and trophectoderm cells (PTr2), we saw expected patterns of enrichment for each of the profiled epigenetic features relative to specific genomic regions. H3K4me3 was primarily enriched at and around global gene promoters, H3K27ac was enriched in promoter and intergenic regions, H3K27me3 had broad stretches of enrichment across the genome and narrower enrichment patterns in and around the promoter regions of some genes, and BRG1 primarily had detectable enrichment at and around promoter regions and in intergenic stretches, with many instances of H3K27ac co-enrichment. We used this information to perform genome-wide chromatin state predictions for 10 different states using ChromHMM. Using the predicted chromatin state maps, we identified a subset of genomic regions marked by broad H3K4me3 enrichment, and annotation of these regions revealed that they were highly associated with essential developmental processes and consisted largely of expressed genes. We then compared the identities of the genes marked by these regions to genes identified as cell-type-specific using transcriptome data and saw that a subset of broad H3K4me3-marked genes was also specifically expressed in either PFF or PTr2 cells.

CONCLUSIONS

These findings enhance our understanding of the epigenetic landscape present in early swine development and provide insight into how variabilities in chromatin state are linked to cell identity. Furthermore, this data captures foundational epigenetic details in two valuable porcine cell lines and contributes to the growing body of knowledge surrounding the epigenetic landscape in this species.

FGF9, a Potent Mitogen, Is a New Ligand for Integrin αvβ3, and the FGF9 Mutant Defective in Integrin Binding Acts as an Antagonist

FGF9 is a potent mitogen and survival factor, but FGF9 protein levels are generally low and restricted to a few adult organs. Aberrant expression of FGF9 usually results in cancer. However, the mechanism of FGF9 action has not been fully established. Previous studies showed that FGF1 and FGF2 directly bind to integrin αvβ3, and this interaction is critical for signaling functions (FGF-integrin crosstalk). FGF1 and FGF2 mutants defective in integrin binding were defective in signaling, whereas the mutants still bound to FGFR suppressed angiogenesis and tumor growth, indicating that they act as antagonists. We hypothesize that FGF9 requires direct integrin binding for signaling. Here, we show that docking simulation of the interaction between FGF9 and αvβ3 predicted that FGF9 binds to the classical ligand-binding site of αvβ3. We show that FGF9 bound to integrin αvβ3 and generated FGF9 mutants in the predicted integrin-binding interface. An FGF9 mutant (R108E) was defective in integrin binding, activating FRS2α and ERK1/2, inducing DNA synthesis, cancer cell migration, and invasion in vitro. R108E suppressed DNA synthesis and activation of FRS2α and ERK1/2 induced by WT FGF9 (dominant-negative effect). These findings indicate that FGF9 requires direct integrin binding for signaling and that R108E has potential as an antagonist to FGF9 signaling.

Shared features in ear and kidney development - implications for oto-renal syndromes

The association between ear and kidney anomalies has long been recognized. However, little is known about the underlying mechanisms. In the last two decades, embryonic development of the inner ear and kidney has been studied extensively. Here, we describe the developmental pathways shared between both organs with particular emphasis on the genes that regulate signalling cross talk and the specification of progenitor cells and specialised cell types. We relate this to the clinical features of oto-renal syndromes and explore links to developmental mechanisms.

FGF9, a potent mitogen, is a new ligand for integrin αvβ3, and the FGF9 mutant defective in integrin binding acts as an antagonist

FGF9 is a potent mitogen and survival factor, but FGF9 protein level is generally low and restricted to a few adult organs. Aberrant expression of FGF9 usually results in cancer. However, the mechanism of FGF9 action has not been fully established. Previous studies showed that FGF1 and FGF2 directly bind to integrin αvβ3 and this interaction is critical for signaling functions (FGF-integrin crosstalk). FGF1 and FGF2 mutants defective in integrin binding were defective in signaling, whereas the mutants still bound to FGFR, and suppressed angiogenesis and tumor growth, indicating that they act as antagonists. We hypothesize that FGF9 requires direct integrin binding for signaling. Here we show that docking simulation of interaction between FGF9 and αvβ3 predicted that FGF9 binds to the classical ligand-binding site of αvβ3. We showed that FGF9 actually bound to integrin αvβ3, and generated an FGF9 mutants in the predicted integrin-binding interface. An FGF9 mutant (R108E) was defective in integrin binding, activating FRS2α and ERK1/2, inducing DNA synthesis, cancer cell migration, and invasion in vitro. R108E suppressed DNA synthesis induced by WT FGF9 and suppressed DNA synthesis and activation of FRS2α and ERK1/2 induced by WT FGF9 (dominant-negative effect). These findings indicate that FGF9 requires direct integrin binding for signaling and that R108E has potential as an antagonist to FGF9 signaling.

PRDM16 expression and function in mammalian cochlear development

BACKGROUND

PR domain containing 16 (PRDM16) is a key transcriptional regulator in the development of craniofacial, adipose, and neural tissues. Our lab identified PRDM16 expression in the epithelial cells of the Kölliker's organ (KO) that starts at ~E13.5 and is maintained until KO disappearance. A transgenic mouse model that carries a gene trap null allele of Prdm16 (Prdm16^cGT ) was used to characterize the impact of Prdm16 loss on cochlear development.

RESULTS

At P0 Prdm16^cGT null cochlea exhibited hypoplastic KO, shortened cochlear duct, increased density of hair cells (HCs) and supporting cells (SCs) in the apical turn as well as multiple isolated ectopic HCs within the KO domain. KO epithelial cells proliferation rate was reduced in the apical turn of the developing Prdm16^cGT null cochlea vs controls. Bulk RNA sequencing of cochlear duct cells at E14.5 followed by quantitative real time PCR and mRNA Fluorescence in-situ hybridization (FISH) validation identified differentially expressed genes in Prdm16^cGT null vs littermate control cochleae. Upregulated genes at E14.5 included Fgf20, as well as several Notch pathway genes (Lfng, Hes1, and Jag1).

CONCLUSIONS

This study characterizes Prdm16 expression during cochlear development and establishes its requirement for KO development.

Presence of chondroitin sulphate and requirement for heparan sulphate biosynthesis in the developing zebrafish inner ear

Epithelial morphogenesis to form the semicircular canal ducts of the zebrafish inner ear depends on the production of the large glycosaminoglycan hyaluronan, which is thought to contribute to the driving force that pushes projections of epithelium into the lumen of the otic vesicle. Proteoglycans are also implicated in otic morphogenesis: several of the genes coding for proteoglycan core proteins, together with enzymes that synthesise and modify their polysaccharide chains, are expressed in the developing zebrafish inner ear. In this study, we demonstrate the highly specific localisation of chondroitin sulphate to the sites of epithelial projection outgrowth in the ear, present before any morphological deformation of the epithelium. Staining for chondroitin sulphate is also present in the otolithic membrane, whereas the otoliths are strongly positive for keratan sulphate. We show that heparan sulphate biosynthesis is critical for normal epithelial projection outgrowth, otolith growth and tethering. In the ext2 mutant ear, which has reduced heparan sulphate levels, but continues to produce hyaluronan, epithelial projections are rudimentary, and do not grow sufficiently to meet and fuse to form the pillars of tissue that normally span the otic lumen. Staining for chondroitin sulphate and expression of versican b, a chondroitin sulphate proteoglycan core protein gene, persist abnormally at high levels in the unfused projections of the ext2 mutant ear. We propose a model for wild-type epithelial projection outgrowth in which hyaluronan and proteoglycans are linked to form a hydrated gel that fills the projection core, with both classes of molecule playing essential roles in zebrafish semicircular canal morphogenesis.

Hypermineralization of Hearing-Related Bones by a Specific Osteoblast Subtype

Auditory ossicles in the middle ear and bony labyrinth of the inner ear are highly mineralized in adult mammals. Cellular mechanisms underlying formation of dense bone during development are unknown. Here, we found that osteoblast-like cells synthesizing highly mineralized hearing-related bones produce both type I and type II collagens as the bone matrix, while conventional osteoblasts and chondrocytes primarily produce type I and type II collagens, respectively. Furthermore, these osteoblast-like cells were not labeled in a "conventional osteoblast"-specific green fluorescent protein (GFP) mouse line. Type II collagen-producing osteoblast-like cells were not chondrocytes as they express osteocalcin, localize along alizarin-labeled osteoid, and form osteocyte lacunae and canaliculi, as do conventional osteoblasts. Auditory ossicles and the bony labyrinth exhibit not only higher bone matrix mineralization but also a higher degree of apatite orientation than do long bones. Therefore, we conclude that these type II collagen-producing hypermineralizing osteoblasts (termed here auditory osteoblasts) represent a new osteoblast subtype. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

The Fibroblast Growth Factor 9 (Fgf9) Participates in Palatogenesis by Promoting Palatal Growth and Elevation

Cleft palate, a common global congenital malformation, occurs due to disturbances in palatal growth, elevation, contact, and fusion during palatogenesis. The Fibroblast growth factor 9 (FGF9) mutation has been discovered in humans with cleft lip and palate. Fgf9 is expressed in both the epithelium and mesenchyme, with temporospatial diversity during palatogenesis. However, the specific role of Fgf9 in palatogenesis has not been extensively discussed. Herein, we used Ddx4-Cre mice to generate an Fgf9^–/– mouse model (with an Fgf9 exon 2 deletion) that exhibited a craniofacial syndrome involving a cleft palate and deficient mandibular size with 100% penetrance. A smaller palatal shelf size, delayed palatal elevation, and contact failure were investigated to be the intrinsic causes for cleft palate. Hyaluronic acid accumulation in the extracellular matrix (ECM) sharply decreased, while the cell density correspondingly increased in Fgf9^–/– mice. Additionally, significant decreases in cell proliferation were discovered in not only the palatal epithelium and mesenchyme but also among cells in Meckel’s cartilage and around the mandibular bone in Fgf9^–/– mice. Serial sections of embryonic heads dissected at embryonic day 14.5 (E14.5) were subjected to craniofacial morphometric measurement. This highlighted the reduced oral volume owing to abnormal tongue size and descent, and insufficient mandibular size, which disturbed palatal elevation in Fgf9^–/– mice. These results indicate that Fgf9 facilitates palatal growth and timely elevation by regulating cell proliferation and hyaluronic acid accumulation. Moreover, Fgf9 ensures that the palatal elevation process has adequate space by influencing tongue descent, tongue morphology, and mandibular growth.

Fgf9 Negatively Regulates Bone Mass by Inhibiting Osteogenesis and Promoting Osteoclastogenesis Via MAPK and PI3K/AKT Signaling

Fibroblast growth factor 9 (Fgf9) is a well-known factor that regulates bone development; however, its function in bone homeostasis is still unknown. Previously, we identified a point mutation in the FGF9 gene (p.Ser99Asn, S99N) and generated an isogeneic knock-in mouse model, which revealed that this loss-of-function mutation impaired early joint formation and was responsible for human multiple synostosis syndrome 3 (SYNS3). Moreover, newborn and adult S99N mutant mice exhibited significantly increased bone mass, suggesting that Fgf9 also participated in bone homeostasis. Histomorphology, tomography, and serological analysis of homozygous newborns and heterozygous adults showed that the Fgf9^S99N mutation immensely increased bone mass and bone formation in perinatal and adult bones and decreased osteoclastogenesis in adult bone. An in vitro differentiation assay further revealed that the S99N mutation enhanced bone formation by promoting osteogenesis and mineralization of bone marrow mesenchymal stem cells (BMSCs) and attenuating osteoclastogenesis of bone marrow monocytes (BMMs). Considering the loss-of-function effect of the S99N mutation, we hypothesized that Fgf9 itself inhibits osteogenesis and promotes osteoclastogenesis. An in vitro differentiation assay revealed that Fgf9 prominently inhibited BMSC osteogenic differentiation and mineralization and showed for the first time that Fgf9 promoted osteoclastogenesis by enhancing preosteoclast aggregation and cell-cell fusion. Furthermore, specific inhibitors and in vitro differentiation assays were used and showed that Fgf9 inhibited BMSC osteogenesis mainly via the MEK/ERK pathway and partially via the PI3K/AKT pathway. Fgf9 also promoted osteoclastogenesis as a potential costimulatory factor with macrophage colony-stimating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) by coactivating the MAPK and PI3K/AKT signaling pathways. Taken together, our study demonstrated that Fgf9 is a negative regulator of bone homeostasis by regulating osteogenesis and osteoclastogenesis and provides a potential therapeutic target for bone degenerative diseases. © 2020 American Society for Bone and Mineral Research (ASBMR).

Anti-Cancer Effect of Cordycepin on FGF9-Induced Testicular Tumorigenesis

Cordycepin, a bioactive constituent from the fungus Cordyceps sinensis, could inhibit cancer cell proliferation and promote cell death via induction of cell cycle arrest, apoptosis and autophagy. Our novel finding from microarray analysis of cordycepin-treated MA-10 mouse Leydig tumor cells is that cordycepin down-regulated the mRNA levels of FGF9, FGF18, FGFR2 and FGFR3 genes in MA-10 cells. Meanwhile, the IPA-MAP pathway prediction result showed that cordycepin inhibited MA-10 cell proliferation by suppressing FGFs/FGFRs pathways. The in vitro study further revealed that cordycepin decreased FGF9-induced MA-10 cell proliferation by inhibiting the expressions of p-ERK1/2, p-Rb and E2F1, and subsequently reducing the expressions of cyclins and CDKs. In addition, a mouse allograft model was performed by intratumoral injection of FGF9 and/or intraperitoneal injection of cordycepin to MA-10-tumor bearing C57BL/6J mice. Results showed that FGF9-induced tumor growth in cordycepin-treated mice was significantly smaller than that in a PBS-treated control group. Furthermore, cordycepin decreased FGF9-induced FGFR1-4 protein expressions in vitro and in vivo. In summary, cordycepin inhibited FGF9-induced testicular tumor growth by suppressing the ERK1/2, Rb/E2F1, cell cycle pathways, and the expressions of FGFR1-4 proteins, suggesting that cordycepin can be used as a novel anticancer drug for testicular cancers.

Genetic interactions support an inhibitory relationship between bone morphogenetic protein 2 and netrin 1 during semicircular canal formation

The semicircular canals of the mammalian inner ear are derived from epithelial pouches in which epithelial cells in the central region of each pouch undergo resorption, leaving behind the region at the rim to form a tube-shaped canal. Lack of proliferation at the rim and/or over-clearing of epithelial cells in the center of the pouch can obliterate canal formation. Otic-specific knockout of bone morphogenetic protein 2 (Bmp2) results in absence of all three semicircular canals; however, the common crus and ampullae housing the sensory tissue (crista) are intact. The lack of Bmp2 causes Ntn1 (which encodes netrin 1), which is required for canal resorption, to be ectopically expressed at the canal rim. Ectopic Ntn1 results in reduction of Dlx5 and Lmo4, which are required for rim formation. These phenotypes can be partially rescued by removing one allele of Ntn1 in the Bmp2 mutants, indicating that Bmp2 normally negatively regulates Ntn1 for canal formation. Additionally, non-resorption of the canal pouch in Ntn1^−/− mutants is partially rescued by removing one allele of Bmp2. Thus, reciprocal inhibition between Bmp2 and netrin 1 is involved in canal formation of the vestibule.

Summary:Bmp2-conditional mutant analyses support the hypothesis that presumptive crista induces canal genesis zone in the canal pouch to express Bmp2, which promotes canal formation by restricting Ntn1 expression to the resorption domain.

Spatial and temporal inhibition of FGFR2b ligands reveals continuous requirements and novel targets in mouse inner ear morphogenesis

Morphogenesis of the inner ear epithelium requires coordinated deployment of several signaling pathways, and disruptions cause abnormalities of hearing and/or balance. The FGFR2b ligands FGF3 and FGF10 are expressed throughout otic development and are required individually for normal morphogenesis, but their prior and redundant roles in otic placode induction complicates investigation of subsequent combinatorial functions in morphogenesis. To interrogate these roles and identify new effectors of FGF3 and FGF10 signaling at the earliest stages of otic morphogenesis, we used conditional gene ablation after otic placode induction, and temporal inhibition of signaling with a secreted, dominant-negative FGFR2b ectodomain. We show that both ligands are required continuously after otocyst formation for maintenance of otic neuroblasts and for patterning and proliferation of the epithelium, leading to normal morphogenesis of both the cochlear and vestibular domains. Furthermore, the first genome-wide identification of proximal targets of FGFR2b signaling in the early otocyst reveals novel candidate genes for inner ear development and function.

Sculpting the skull through neurosensory epithelial-mesenchymal signaling

The vertebrate skull is a complex structure housing the brain and specialized sensory organs, including the eye, the inner ear, and the olfactory system. The close association between bones of the skull and the sensory organs they encase has posed interesting developmental questions about how the tissues scale with one another. Mechanisms that regulate morphogenesis of the skull are hypothesized to originate in part from the encased neurosensory organs. Conversely, the developing skull is hypothesized to regulate the growth of neurosensory organs, through mechanical forces or molecular signaling. Here, we review studies of epithelial-mesenchymal interactions during inner ear and olfactory system development that may coordinate the growth of the two sensory organs with their surrounding bone. We highlight recent progress in the field and provide evidence that mechanical forces arising from bone growth may affect olfactory epithelium development. Developmental Dynamics 248:88-97, 2019. © 2018 Wiley Periodicals, Inc.

Fibroblast growth factor 9 subfamily and the heart

The fibroblast growth factor (FGF) 9 subfamily is a member of the FGF family, including FGF9, 16, and 20, potentially sharing similar biochemical functions due to their high degree of sequence homology. Unlike other secreted proteins which have a cleavable N-terminal secreted signal peptide, FGF9/16/20 have non-cleaved N-terminal signal peptides. As an intercellular signaling molecule, they are involved in a variety of complex responses in animal development. Cardiogenesis is controlled by many members of the transcription factor family. Evidence suggests that FGF signaling, including the FGF9 subfamily, has a pretty close association with these cardiac-specific genes. In addition, recent studies have shown that the FGF9 subfamily maintains functional adaptation and survival after myocardial infarction in adult myocardium. Since FGF9/16/20 are secreted proteins, their function characterization in cardiac regeneration can promote their potential to be developed for the treatment of cardioprotection and revascularization. Here, we conclude that the FGF9 subfamily roles in cardiac development and maintenance of postnatal cardiac homeostasis, especially cardiac function maturation and functional maintenance of the heart after injury.

ESRP1 Mutations Cause Hearing Loss due to Defects in Alternative Splicing that Disrupt Cochlear Development

Alternative splicing contributes to gene expression dynamics in many tissues, yet its role in auditory development remains unclear. We performed whole-exome sequencing in individuals with sensorineural hearing loss (SNHL) and identified pathogenic mutations in Epithelial Splicing-Regulatory Protein 1 (ESRP1). Patient-derived induced pluripotent stem cells showed alternative splicing defects that were restored upon repair of an ESRP1 mutant allele. To determine how ESRP1 mutations cause hearing loss, we evaluated Esrp1-/- mouse embryos and uncovered alterations in cochlear morphogenesis, auditory hair cell differentiation, and cell fate specification. Transcriptome analysis revealed impaired expression and splicing of genes with essential roles in cochlea development and auditory function. Aberrant splicing of Fgfr2 blocked stria vascularis formation due to erroneous ligand usage, which was corrected by reducing Fgf9 gene dosage. These findings implicate mutations in ESRP1 as a cause of SNHL and demonstrate the complex interplay between alternative splicing, inner ear development, and auditory function.

FGF signaling: diverse roles during cochlear development

Mammalian inner ear comprises of six sensory organs; cochlea, utricle, saccule, and three semicircular canals. The cochlea contains sensory epithelium known as the organ of Corti which senses sound through mechanosensory hair cells. Mammalian inner ear undergoes series of morphogenesis during development beginning thickening of ectoderm nearby hindbrain. These events require tight regulation of multiple signaling cascades including FGF, Wnt, Notch and Bmp signaling. In this review, we will discuss the role of newly emerging signaling, FGF signaling, for its roles required for cochlear development. [BMB Reports 2017; 50(10): 487-495].

Regulation of Receptor Binding Specificity of FGF9 by an Autoinhibitory Homodimerization

The epithelial fibroblast growth factor 9 (FGF9) subfamily specifically binds and activates the mesenchymal "c" splice isoform of FGF receptors 1-3 (FGFR1-3) to regulate organogenesis and tissue homeostasis. The unique N and C termini of FGF9 subfamily ligands mediate a reversible homodimerization that occludes major receptor binding sites within the ligand core region. Here we provide compelling X-ray crystallographic, biophysical, and biochemical data showing that homodimerization controls receptor binding specificity of the FGF9 subfamily by keeping the concentration of active FGF9 monomers at a level, which is sufficient for a normal FGFR "c" isoform binding/signaling, but is insufficient for an illegitimate FGFR "b" isoform binding/signaling. We show that deletion of the N terminus or alanine substitutions in the C terminus of FGF9 skews the delicate ligand equilibrium toward active FGF9 monomers causing off-target binding and activation of FGFR b isoforms. Our study is the first to implicate ligand homodimerization in the regulation of ligand-receptor specificity.

Characterization of the Transcriptomes of Lgr5+ Hair Cell Progenitors and Lgr5- Supporting Cells in the Mouse Cochlea

Cochlear supporting cells (SCs) have been shown to be a promising resource for hair cell (HC) regeneration in the neonatal mouse cochlea. Previous studies have reported that Lgr5+ SCs can regenerate HCs both in vitro and in vivo and thus are considered to be inner ear progenitor cells. Lgr5+ progenitors are able to regenerate more HCs than Lgr5- SCs, and it is important to understand the mechanism behind the proliferation and HC regeneration of these progenitors. Here, we isolated Lgr5+ progenitors and Lgr5- SCs from Lgr5-EGFP-CreERT2/Sox2-CreERT2/Rosa26-tdTomato mice via flow cytometry. As expected, we found that Lgr5+ progenitors had significantly higher proliferation and HC regeneration ability than Lgr5- SCs. Next, we performed RNA-Seq to determine the gene expression profiles of Lgr5+ progenitors and Lgr5- SCs. We analyzed the genes that were enriched and differentially expressed in Lgr5+ progenitors and Lgr5- SCs, and we found 8 cell cycle genes, 9 transcription factors, and 24 cell signaling pathway genes that were uniquely expressed in one population but not the other. Last, we made a protein–protein interaction network to further analyze the role of these differentially expressed genes. In conclusion, we present a set of genes that might regulate the proliferation and HC regeneration ability of Lgr5+ progenitors, and these might serve as potential new therapeutic targets for HC regeneration.

The expression profiles of fibroblast growth factor 9 and its receptors in developing mice testes

An expressional lack of fibroblast growth factor 9 (FGF9) would cause male-to-female sex reversal in the mouse, implying the essential role of FGF9 in testicular organogenesis and maturation. However, the temporal expression of FGF9 and its receptors during testicular development remains elusive. In this study, immunohistochemistry was used to identify the localization of FGF9 and its receptors at different embryonic and postnatal stages in mice testes. Results showed that FGF9 continuously expressed in the testis during development. FGF9 had highest expression in the interstitial region at 17-18 d post coitum (dpc) and in the spermatocytes, spermatids and Leydig cell on postnatal days (pnd) 35-65. Regarding receptor expression, FGFR1 and FGFR4 were evenly expressed in the whole testis during the embryonic and postnatal stages. However, FGFR2 and FGFR3 were widely expressed during the embryonic testis development with higher FGFR2 expression in seminiferous tubules at 16-18 dpc and higher FGFR3 expression in interstitial region at 17-18 dpc. In postnatal stage, FGFR2 extensively expressed with higher expression at spermatids and Leydig cells on 35-65 pnd and FGFR3 widely expressed in the whole testis. Taken together, these results strongly suggest that FGF9 is correlated with the temporal expression profiles of FGFR2 and FGFR3 and possibly associated with testis development.

Identification of molecular signatures specific for distinct cranial sensory ganglia in the developing chick

Background

The cranial sensory ganglia represent populations of neurons with distinct functions, or sensory modalities. The production of individual ganglia from distinct neurogenic placodes with different developmental pathways provides a powerful model to investigate the acquisition of specific sensory modalities. To date there is a limited range of gene markers available to examine the molecular pathways underlying this process.

Results

Transcriptional profiles were generated for populations of differentiated neurons purified from distinct cranial sensory ganglia using microdissection in embryonic chicken followed by FAC-sorting and RNAseq. Whole transcriptome analysis confirmed the division into somato- versus viscerosensory neurons, with additional evidence for subdivision of the somatic class into general and special somatosensory neurons. Cross-comparison of distinct ganglia transcriptomes identified a total of 134 markers, 113 of which are novel, which can be used to distinguish trigeminal, vestibulo-acoustic and epibranchial neuronal populations. In situ hybridisation analysis provided validation for 20/26 tested markers, and showed related expression in the target region of the hindbrain in many cases.

Conclusions

One hundred thirty-four high-confidence markers have been identified for placode-derived cranial sensory ganglia which can now be used to address the acquisition of specific cranial sensory modalities.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-016-0057-y) contains supplementary material, which is available to authorized users.

Fibroblast Growth Factor-9 Activates c-Kit Progenitor Cells and Enhances Angiogenesis in the Infarcted Diabetic Heart

We hypothesized that fibroblast growth factor-9 (FGF-9) would enhance angiogenesis via activating c-kit positive stem cells in the infarcted nondiabetic and diabetic heart. In brief, animals were divided into three groups: Sham, MI, and MI+FGF-9. Two weeks following MI or sham surgery, our data suggest that treatment with FGF-9 significantly diminished vascular apoptosis compared to the MI group in both C57BL/6 and db/db mice (p < 0.05). Additionally, the number of c-kit^+ve/SM α-actin^+ve cells and c-kit^+ve/CD31^+ve cells were greatly enhanced in the MI+FGF-9 groups relative to the MI suggesting FGF-9 enhances c-Kit cell activation and their differentiation into vascular smooth muscle cells and endothelial cells, respectively (p < 0.05). Histology shows that the total number of vessels were quantified for all groups and our data suggest that the FGF-9 treated groups had significantly more vessels than their MI counterparts (p < 0.05). Finally, echocardiographic data suggests a significant improvement in left ventricular output, as indicated by fractional shortening and ejection fraction in both nondiabetic and diabetic animals treated with FGF-9 (p < 0.05). Overall, our data suggests FGF-9 has the potential to attenuate vascular cell apoptosis, activate c-Kit progenitor cells, and enhance angiogenesis and neovascularization in C57BL/6 and db/db mice leading to improved cardiac function.

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.

Cochlear progenitor number is controlled through mesenchymal FGF receptor signaling

The sensory and supporting cells (SCs) of the organ of Corti are derived from a limited number of progenitors. The mechanisms that regulate the number of sensory progenitors are not known. Here, we show that Fibroblast Growth Factors (FGF) 9 and 20, which are expressed in the non-sensory (Fgf9) and sensory (Fgf20) epithelium during otic development, regulate the number of cochlear progenitors. We further demonstrate that Fgf receptor (Fgfr) 1 signaling within the developing sensory epithelium is required for the differentiation of outer hair cells and SCs, while mesenchymal FGFRs regulate the size of the sensory progenitor population and the overall cochlear length. In addition, ectopic FGFR activation in mesenchyme was sufficient to increase sensory progenitor proliferation and cochlear length. These data define a feedback mechanism, originating from epithelial FGF ligands and mediated through periotic mesenchyme that controls the number of sensory progenitors and the length of the cochlea.

DOI:http://dx.doi.org/10.7554/eLife.05921.001

Mammalian ears contain several structures that are involved in hearing. Within the inner ear is a spiral-shaped structure called the cochlea. This contains an array of cells called sensory hair cells that convert sound vibrations into electrical signals, which are then conveyed to the brain. Sounds of differing pitch are detected at different points along the cochlea, so its overall length helps to determine the range of sounds that an individual can hear.

In the embryo, sensory hair cells and their associated supporting cells develop from ‘cochlear progenitor’ cells. The final length of the cochlea is determined by the numbers of progenitor cells that commit to becoming either sensory hair cells or supporting cells. Two proteins called FGF9 and FGF20 are involved in the formation of the cochlea. FGF20 promotes the formation of the hair cells and supporting cells, but the precise roles of both proteins are not clear.

Here, Huh et al. studied FGF9 and FGF20 in the inner ear of mice at an early stage of development. The experiments show that these proteins work together to control the number of progenitor cells and the length of the cochlea. FGF20 is produced by the same tissue structure (called an ‘epithelium’) that gives rise to the hair cells and supporting cells. In contrast, FGF9 is produced in another epithelium tissue that produces the cells that line the fluid-filled tubes of the inner ear.

The experiments also show that both FGF9 and FGF20 act as signals to cells in an adjacent tissue called the mesenchyme, where they activate other proteins known as FGF receptors. These receptors, in turn, regulate an unknown molecule in the mesenchyme that influences the growth of progenitor cells and the length of the cochlea.

Sensory hair cells can be injured or lost by excessive sound exposure, some medications and as part of normal aging. These cells are not replaced, and so their loss is a major cause of permanent hearing loss. Understanding the signals that produce the progenitor cells will take us one step closer to being able to grow these cells in the laboratory for use in therapies to replace or repair damaged sensory hair cells.

DOI:http://dx.doi.org/10.7554/eLife.05921.002

Fibroblast growth factor-9 enhances M2 macrophage differentiation and attenuates adverse cardiac remodeling in the infarcted diabetic heart

Inflammation has been implicated as a perpetrator of diabetes and its associated complications. Monocytes, key mediators of inflammation, differentiate into pro-inflammatory M1 macrophages and anti-inflammatory M2 macrophages upon infiltration of damaged tissue. However, the inflammatory cell types, which propagate diabetes progression and consequential adverse disorders, remain unclear. The current study was undertaken to assess monocyte infiltration and the role of fibroblast growth factor-9 (FGF-9) on monocyte to macrophage differentiation and cardioprotection in the diabetic infarcted heart. Db/db diabetic mice were assigned to sham, myocardial infarction (MI), and MI+FGF-9 groups. MI was induced by permanent coronary artery ligation and animals were subjected to 2D transthoracic echocardiography two weeks post-surgery. Immunohistochemical and immunoassay results from heart samples collected suggest significantly increased infiltration of monocytes (Mean ± SEM; MI: 2.02% ± 0.23% vs. Sham 0.75% ± 0.07%; p<0.05) and associated pro-inflammatory cytokines (TNF-α, MCP-1, and IL-6), adverse cardiac remodeling (Mean ± SEM; MI: 33% ± 3.04% vs. Sham 2.2% ± 0.33%; p<0.05), and left ventricular dysfunction (Mean ± SEM; MI: 35.4% ± 1.25% vs. Sham 49.19% ± 1.07%; p<0.05) in the MI group. Importantly, treatment of diabetic infarcted myocardium with FGF-9 resulted in significantly decreased monocyte infiltration (Mean ± SEM; MI+FGF-9: 1.39% ± 0.1% vs. MI: 2.02% ± 0.23%; p<0.05), increased M2 macrophage differentiation (Mean ± SEM; MI+FGF-9: 4.82% ± 0.86% vs. MI: 0.85% ± 0.3%; p<0.05) and associated anti-inflammatory cytokines (IL-10 and IL-1RA), reduced adverse remodeling (Mean ± SEM; MI+FGF-9: 11.59% ± 1.2% vs. MI: 33% ± 3.04%; p<0.05), and improved cardiac function (Fractional shortening, Mean ± SEM; MI+FGF-9: 41.51% ± 1.68% vs. MI: 35.4% ± 1.25%; p<0.05). In conclusion, our data suggest FGF-9 possesses novel therapeutic potential in its ability to mediate monocyte to M2 differentiation and confer cardiac protection in the post-MI diabetic heart.

Fgf10 is required for specification of non-sensory regions of the cochlear epithelium

The vertebrate inner ear is a morphologically complex sensory organ comprised of two compartments, the dorsal vestibular apparatus and the ventral cochlear duct, required for motion and sound detection, respectively. Fgf10, in addition to Fgf3, is necessary for the earliest stage of otic placode induction, but continued expression of Fgf10 in the developing otic epithelium, including the prosensory domain and later in Kolliker׳s organ, suggests additional roles for this gene during morphogenesis of the labyrinth. While loss of Fgf10 was implicated previously in semicircular canal agenesis, we show that Fgf10(-/+) embryos also exhibit a reduction or absence of the posterior semicircular canal, revealing a dosage-sensitive requirement for FGF10 in vestibular development. In addition, we show that Fgf10(-/-) embryos have previously unappreciated defects of cochlear morphogenesis, including a somewhat shortened duct, and, surprisingly, a substantially narrower duct. The mutant cochlear epithelium lacks Reissner׳s membrane and a large portion of the outer sulcus-two non-contiguous, non-sensory domains. Marker gene analyses revealed effects on Reissner׳s membrane as early as E12.5-E13.5 and on the outer sulcus by E15.5, stages when Fgf10 is expressed in close proximity to Fgfr2b, but these effects were not accompanied by changes in epithelial cell proliferation or death. These data indicate a dual role for Fgf10 in cochlear development: to regulate outgrowth of the duct and subsequently as a bidirectional signal that sequentially specifies Reissner׳s membrane and outer sulcus non-sensory domains. These findings may help to explain the hearing loss sometimes observed in LADD syndrome subjects with FGF10 mutations.

Hearing loss in syndromic craniosynostoses: introduction and consideration of mechanisms

PURPOSE

There are a number of craniosynostosis syndromes with hearing loss-including Muenke, Apert, Pfeiffer, Crouzon, Beare-Stevenson, Crouzon with acanthosis nigricans, and Jackson-Weiss syndromes-that result from mutations in the fibroblast growth factor receptor (FGFR) genes. Studies of FGFRs and their ligands, fibroblast growth factors (FGFs), have revealed clues to the precise contribution of aberrant FGFR signaling to inner ear morphogenesis and the hearing loss encountered in craniosynostoses. The purpose of this article is to review basic studies of FGFRs with emphasis on their function and expression in the inner ear and surrounding structures.

METHOD

A Medline search was performed to find basic science articles regarding FGFR, their ligands, and their expression and relevant mouse models. Additional items searched included clinical descriptions and studies of individuals with FGFR-related craniosynostosis syndromes.

RESULTS

The FGF signaling pathway is essential for the morphogensis and proper function of the inner ear and auditory sensory epithelium.

CONCLUSION

The variable auditory phenotypes seen in individuals with Muenke syndrome may have a genetic basis, likely due to multiple interacting factors in the genetic environment or modifying factors. Further analysis and studies of mouse models of Muenke syndrome, in particular, may provide clues to the specific effects of the defining mutation in FGFR3 in the inner ear not only at birth but also into adulthood. In particular, investigations into these models may give insight into the variable expression and incomplete penetrance of this phenotype.

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.

Canonical Wnt signaling regulates the proliferative expansion and differentiation of fibrocytes in the murine inner ear

Otic fibrocytes tether the cochlear duct to the surrounding otic capsule but are also critically involved in maintenance of ion homeostasis in the cochlea, thus, perception of sound. The molecular pathways that regulate the development of this heterogenous group of cells from mesenchymal precursors are poorly understood. Here, we identified epithelial Wnt7a and Wnt7b as possible ligands of Fzd-mediated β-catenin (Ctnnb1)-dependent (canonical) Wnt signaling in the adjacent undifferentiated periotic mesenchyme (POM). Mice with a conditional deletion of Ctnnb1 in the POM exhibited a complete failure of fibrocyte differentiation, a severe reduction of mesenchymal cells surrounding the cochlear duct, loss of pericochlear spaces, a thickening and partial loss of the bony capsule and a secondary disturbance of cochlear duct coiling shortly before birth. Analysis at earlier stages revealed that radial patterning of the POM in two domains with highly condensed cartilaginous precursors and more loosely arranged inner mesenchymal cells occurred normally but that proliferation in the inner domain was reduced and cytodifferentiation failed. Cells with mis/overexpression of a stabilized form of Ctnnb1 in the entire POM mesenchyme sorted to the inner mesenchymal compartment and exhibited increased proliferation. Our analysis suggests that Wnt signals from the cochlear duct epithelium are crucial to induce differentiation and expansion of fibrocyte precursor cells. Our findings emphasize the importance of epithelial-mesenchymal signaling in inner ear development.

Expression of signaling components in embryonic eyelid epithelium

Closure of an epithelium opening is a critical morphogenetic event for development. An excellent example for this process is the transient closure of embryonic eyelid. Eyelid closure requires shape change and migration of epithelial cells at the tip of the developing eyelids, and is dictated by numerous signaling pathways. Here we evaluated gene expression in epithelial cells isolated from the tip (leading edge, LE) and inner surface epithelium (IE) of the eyelid from E15.5 mouse fetuses by laser capture microdissection (LCM). We showed that the LE and IE cells are different at E15.5, such that IE had higher expression of muscle specific genes, while LE acquired epithelium identities. Despite their distinct destinies, these cells were overall similar in expression of signaling components for the “eyelid closure pathways”. However, while the LE cells had more abundant expression of Fgfr2, Erbb2, Shh, Ptch1 and 2, Smo and Gli2, and Jag1 and Notch1, the IE cells had more abundant expression of Bmp5 and Bmpr1a. In addition, the LE cells had more abundant expression of adenomatosis polyposis coli down-regulated 1 (Apcdd1), but the IE cells had high expression of Dkk2. Our results suggest that the functionally distinct LE and IE cells have also differential expression of signaling molecules that may contribute to the cell-specific responses to morphogenetic signals. The expression pattern suggests that the EGF, Shh and NOTCH pathways are preferentially active in LE cells, the BMP pathways are effective in IE cells, and the Wnt pathway may be repressed in LE and IE cells via different mechanisms.

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.

Evolution and development of the tetrapod auditory system: an organ of Corti-centric perspective

The tetrapod auditory system transmits sound through the outer and middle ear to the organ of Corti or other sound pressure receivers of the inner ear where specialized hair cells translate vibrations of the basilar membrane into electrical potential changes that are conducted by the spiral ganglion neurons to the auditory nuclei. In other systems, notably the vertebrate limb, a detailed connection between the evolutionary variations in adaptive morphology and the underlying alterations in the genetic basis of development has been partially elucidated. In this review, we attempt to correlate evolutionary and partially characterized molecular data into a cohesive perspective of the evolution of the mammalian organ of Corti out of the tetrapod basilar papilla. We propose a stepwise, molecularly partially characterized transformation of the ancestral, vestibular developmental program of the vertebrate ear. This review provides a framework to decipher both discrete steps in development and the evolution of unique functional adaptations of the auditory system. The combined analysis of evolution and development establishes a powerful cross-correlation where conclusions derived from either approach become more meaningful in a larger context which is not possible through exclusively evolution or development centered perspectives. Selection may explain the survival of the fittest auditory system, but only developmental genetics can explain the arrival of the fittest auditory system. [Modified after (Wagner 2011)].

Continued expression of GATA3 is necessary for cochlear neurosensory development

Hair cells of the developing mammalian inner ear are progressively defined through cell fate restriction. This process culminates in the expression of the bHLH transcription factor Atoh1, which is necessary for differentiation of hair cells, but not for their specification. Loss of several genes will disrupt ear morphogenesis or arrest of neurosensory epithelia development. We previously showed in null mutants that the loss of the transcription factor, Gata3, results specifically in the loss of all cochlear neurosensory development. Temporal expression of Gata3 is broad from the otic placode stage through the postnatal ear. It therefore remains unclear at which stage in development Gata3 exerts its effect. To better understand the stage specific effects of Gata3, we investigated the role of Gata3 in cochlear neurosensory specification and differentiation utilizing a LoxP targeted Gata3 line and two Cre lines. Foxg1^Cre∶Gata3^f/f mice show recombination of Gata3 around E8.5 but continue to develop a cochlear duct without differentiated hair cells and spiral ganglion neurons. qRT-PCR data show that Atoh1 was down-regulated but not absent in the duct whereas other hair cell specific genes such as Pou4f3 were completely absent. In addition, while Sox2 levels were lower in the Foxg1^Cre:Gata3^f/f cochlea, Eya1 levels remained normal. We conclude that Eya1 is unable to fully upregulate Atoh1 or Pou4f3, and drive differentiation of hair cells without Gata3. Pax2-Cre∶Gata3^f/f mice show a delayed recombination of Gata3 in the ear relative to Foxg1^Cre:Gata3^f/f. These mice exhibited a cochlear duct containing patches of partially differentiated hair cells and developed only few and incorrectly projecting spiral ganglion neurons. Our conditional deletion studies reveal a major role of Gata3 in the signaling of prosensory genes and in the differentiation of cochlear neurosenory cells. We suggest that Gata3 may act in combination with Eya1, Six1, and Sox2 in cochlear prosensory gene signaling.

Divergent roles for Wnt/β-catenin signaling in epithelial maintenance and breakdown during semicircular canal formation

The morphogenetic program that shapes the three semicircular canals (SSCs) must be executed with extreme precision to satisfy their complex vestibular function. The SSCs emerge from epithelial outgrowths of the dorsal otocyst, the central regions of which fuse and resorb to leave three fluid-filled canals. The Wnt/β-catenin signaling pathway is active at multiple stages of otic development, including during vestibular morphogenesis. How Wnt/β-catenin functionally integrates with other signaling pathways to sculpt the SSCs and their sensory patches is unknown. We used a genetic strategy to spatiotemporally modulate canonical Wnt signaling activity during SSC development in mice. Our findings demonstrate that Wnt/β-catenin signaling functions in a multifaceted manner during SSC formation. In the early phase, Wnt/β-catenin signaling is required to preserve the epithelial integrity of the vertical canal pouch perimeter (presumptive anterior and posterior SSCs) by establishing a sensory-dependent signaling relay that maintains expression of Dlx5 and opposes expression of the fusion plate marker netrin 1. Without this Wnt signaling activity the sensory to non-sensory signaling cascade fails to be activated, resulting in loss of vestibular hair and support cells and the anterior and posterior SSCs. In the later phase, Wnt/β-catenin signaling becomes restricted to the fusion plate where it facilitates the timely resorption of this tissue. Mosaic recombination of β-catenin in small clusters of canal pouch cells prevents their resorption, causing instead the formation of ectopic SSCs. Together, these disparate functions of the Wnt/β-catenin pathway in epithelial maintenance and resorption help regulate the size, shape and number of SSCs.

Mutanlallemand (mtl) and Belly Spot and Deafness (bsd) are two new mutations of Lmx1a causing severe cochlear and vestibular defects

Mutanlallemand (mtl) and Belly Spot and Deafness (bsd) are two new spontaneous alleles of the Lmx1a gene in mice. Homozygous mutants show head tossing and circling behaviour, indicative of vestibular defects, and they have short tails and white belly patches of variable size. The analysis of auditory brainstem responses (ABR) showed that mtl and bsd homozygotes are deaf, whereas heterozygous and wildtype littermates have normal hearing. Paint-filled inner ears at E16.5 revealed that mtl and bsd homozygotes lack endolymphatic ducts and semicircular canals and have short cochlear ducts. These new alleles show similarities with dreher (Lmx1a) mutants. Complementation tests between mtl and dreher and between mtl and bsd suggest that mtl and bsd are new mutant alleles of the Lmx1a gene. To determine the Lmx1a mutation in mtl and bsd mutant mice we performed PCR followed by sequencing of genomic DNA and cDNA. The mtl mutation is a single point mutation in the 3′ splice site of exon 4 leading to an exon extension and the activation of a cryptic splice site 44 base pairs downstream, whereas the bsd mutation is a genomic deletion that includes exon 3. Both mutations lead to a truncated LMX1A protein affecting the homeodomain (mtl) or LIM2-domain (bsd), which is critical for LMX1A protein function. Moreover, the levels of Lmx1a transcript in mtl and bsd mutants are significantly down-regulated. Hmx2/3 and Pax2 expression are also down-regulated in mtl and bsd mutants, suggesting a role of Lmx1a upstream of these transcription factors in early inner ear morphogenesis. We have found that these mutants develop sensory patches although they are misshapen. The characterization of these two new Lmx1a alleles highlights the critical role of this gene in the development of the cochlea and vestibular system.

Notch prosensory effects in the Mammalian cochlea are partially mediated by Fgf20

Hearing loss is becoming an increasingly prevalent problem affecting more than 250 million people worldwide. During development, fibroblast growth factors (FGFs) are required for inner ear development as well as hair cell formation in the mammalian cochlea and thus make attractive therapeutic candidates for the regeneration of sensory cells. Previous findings showed that Fgfr1 conditional knock out mice exhibited hair cell and support cell formation defects. Immunoblocking with Fgf20 antibody in vitro produced a similar phenotype. While hair cell differentiation in mice starts at embryonic day (E)14.5, beginning with the inner hair cells, Fgf20 expression precedes hair cell differentiation at E13.5 in the cochlea. This suggests a potential role for Fgf20 in priming the sensory epithelium for hair cell formation. Treatment of explants with a gamma-secretase inhibitor, DAPT, decreased Fgf20 mRNA, suggesting that Notch is upstream of Fgf20. Notch signaling also plays an early role in prosensory formation during cochlear development. In this report we show that during development, Notch-mediated regulation of prosensory formation in the cochlea occurs via Fgf20. Addition of exogenous FGF20 compensated for the block in Notch signaling and rescued Sox2, a prosensory marker, and Gfi1, an early hair cell marker in explant cultures. We hypothesized that Fgf20 plays a role in specification, amplification, or maintenance of Sox2 expression in prosensory progenitors of the developing mammalian cochlea.

Molecular mechanisms of inner ear development

The inner ear is a structurally complex vertebrate organ built to encode sound, motion, and orientation in space. Given its complexity, it is not surprising that inner ear dysfunction is a relatively common consequence of human genetic mutation. Studies in model organisms suggest that many genes currently known to be associated with human hearing impairment are active during embryogenesis. Hence, the study of inner ear development provides a rich context for understanding the functions of genes implicated in hearing loss. This chapter focuses on molecular mechanisms of inner ear development derived from studies of model organisms.

The effect of co-culturing costal chondrocytes and dental pulp stem cells combined with exogenous FGF9 protein on chondrogenesis and ossification in engineered cartilage

Dental pulp stem cells (DPSCs), which arise from cranial neural crest cells, are multipotent, making them a candidate for use in tissue engineering that may be especially useful for craniofacial tissues. Costal chondrocytes (CCs) can be easily obtained and demonstrate higher initial cell yields and expansion than articular chondrocytes. CCs have been found to retain chondrogenic capacity that can effectively repair articular defects. In this study, human CCs were co-cultured with human DPSCs, and the results showed that the CCs were able to supply a chondro-inductive niche that promoted the DPSCs to undergo chondrogenic differentiation and to enhance the formation of cartilage. Although CCs alone could not prevent the mineralization of chondro-differentiated DPSCs, CCs combined with exogenous FGF9 were able to simultaneously promote the chondrogenesis of DPSCs and partially inhibit their mineralization. Furthermore, FGF9 may activate this inhibition by binding to FGFR3 and enhancing the phosphorylation of ERK1/2 in DPSCs. Our results strongly suggest that the co-culture of CCs and DPSCs combined with exogenous FGF9 can simultaneously enhance chondrogenesis and partially inhibit ossification in engineered cartilage.

Fgf signaling regulates development and transdifferentiation of hair cells and supporting cells in the basilar papilla

The avian basilar papilla (BP) is a likely homolog of the auditory sensory epithelium of the mammalian cochlea, the organ of Corti. During mammalian development Fibroblast growth factor receptor-3 (Fgfr3) is known to regulate the differentiation of auditory mechanosensory hair cells (HCs) and supporting cells (SCs), both of which are required for sound detection. Fgfr3 is expressed in developing progenitor cells (PCs) and SCs of both the BP and the organ of Corti; however its role in BP development is unknown. Here we utilized an in vitro whole organ embryonic culture system to examine the role of Fgf signaling in the developing avian cochlea. SU5402 (an antagonist of Fgf signaling) was applied to developing BP cultures at different stages to assay the role of Fgf signaling during HC formation. Similar to the observed effects of inhibition of Fgfr3 in the mammalian cochlea, Fgfr inhibition in the developing BP increased the number of HCs that formed. This increase was not associated with increased proliferation, suggesting that inhibition of the Fgf pathway leads to the direct conversion of PCs or supporting cells into HCs, a process known as transdifferentiation. This also implies that Fgf signaling is required to prevent the conversion of PCs and SCs into HCs. The ability of Fgf signaling to inhibit transdifferentiation suggests that its down-regulation may be essential for the initial steps of HC formation, as well as for the maintenance of SC phenotypes.

The structural biology of the FGF19 subfamily

The ability of the Fibroblast Growth Factor (FGF) 19 subfamily to signal in an endocrine fashion sets this subfamily apart from the remaining five FGF subfamilies known for their paracrine functions during embryonic development. Compared to the members of paracrine FGF subfamiles, the three members of the FGF19 subfamily, namely FGF19, FGF21 and FGF23, have poor affinity for heparan sulfate (HS) and therefore can diffuse freely in the HS-rich extracellular matrix to enter into the bloodstream. In further contrast to paracrine FGFs, FGF19 subfamily members have unusually poor affinity for their cognate FGF receptors (FGFRs) and therefore cannot bind and activate them in a solely HS-dependent fashion. As a result, the FGF19 subfamily requires α/βklotho coreceptor proteins in order to bind, dimerize and activate their cognate FGFRs. This klotho-dependency also determines the tissue specificity of endocrine FGFs. Recent structural and biochemical studies have begun to shed light onto the molecular basis for the klotho-dependent endocrine mode of action of the FGF19 subfamily. Crystal structures of FGF19 and FGF23 show that the topology of the HS binding site (HBS) of FGF19 subfamily members deviates drastically from the common topology adopted by the paracrine FGFs. The distinct topologies of the HBS of FGF19 and FGF23 prevent HS from direct hydrogen bonding with the backbone atoms of the HBS of these ligands and accordingly decrease the HS binding affinity of this subfamily. Recent biochemical data reveal that the ?klotho ectodomain binds avidly to the ectodomain of FGFR1c, the main cognate FGFR of FGF23, creating a de novo high affinity binding site for the C-terminal tail of FGF23. The isolated FGF23 C-terminus can be used to effectively inhibit the formation of the FGF23-FGFR1c-αklotho complex and alleviate hypophosphatemia in renal phosphate disorders due to elevated levels of FGF23.

Restriction of Wnt signaling in the dorsal otocyst determines semicircular canal formation in the mouse embryo

The mouse inner ear develops from a simple epithelial pouch, the otocyst, with the dorsal and ventral portions giving rise to the vestibule and cochlea, respectively. The otocyst undergoes a morphological change to generate flattened saclike structures, known as outpocketings, in the dorsal and lateral regions. The semicircular canals of the vestibule form from the periphery of the outpocketings, with the central region (the fusion plate) undergoing de-epithelialization and disappearing. However, little is known of the mechanism that orchestrates formation of the semicircular canals. We now show that the area of canonical Wnt signaling changes dynamically in the dorsal otocyst during its morphogenesis. The genes for several Wnt ligands were found to be expressed in the dorsal otocyst according to specific patterns, whereas those for secreted inhibitors of Wnt ligands were expressed exclusively in the ventral otocyst. With the use of whole-embryo culture in combination with potent modulators of canonical Wnt signaling, we found that forced persistence of such signaling resulted in impaired formation both of the lateral outpocketing and of the fusion plates of the dorsal outpocketing. Canonical Wnt signaling was found to suppress Netrin1 expression and to preserve the integrity of the outpocketing epithelium. In addition, inhibition of canonical Wnt signaling reduced the size of the otocyst, likely through suppression of cell proliferation and promotion of apoptosis. Our stage-specific functional analysis suggests that strict regulation of canonical Wnt signaling in the dorsal otocyst orchestrates the process of semicircular canal formation.

Defects in sensory organ morphogenesis and generation of cochlear hair cells in Gata3-deficient mouse embryos

The development of the inner ear sensory epithelia involves a complex network of transcription factors and signaling pathways and the whole process is not yet entirely understood. GATA3 is a DNA-binding factor that is necessary for otic morphogenesis and without GATA3 variable defects have been observed already at early stages in mouse embryos. In the less severe phenotypes, one small oval shaped vesicle is formed whereas in the more severe cases, the otic epithelium becomes disrupted and the endolymphatic domain becomes separated from the rest of the otic epithelium. Despite these defects, the early sensory fate specification occurs in Gata3-/- otic epithelium. However, due to the early lethality of Gata3-deficient embryos, the later morphogenesis and sensory development have remained unclear. To gain information of these later processes we produced drug-rescued Gata3-/- embryos that survived up to late gestation. In these older Gata3-/- embryos, a similar variability was observed as earlier. In the more severely affected ears, the development of the separate endolymphatic domain arrested completely whereas the remaining vesicle formed an empty cavity with variable forms, but without any distinguishable otic compartments or morphologically distinct sensory organs. However, the dorsal part of this vesicle was able to adopt a sensory fate and to produce some hair cells. In the less severe cases of Gata3-/- ears, distinct utricular, saccular and cochlear compartments were present and hair cells could be detected in the vestibular sensory epithelia. Although clear cristae and maculae formed, the morphology and size of these sensory areas were abnormal and they remained often un-separated. In contrast to the vestibule, the cochlear sensory compartment remained more immature and no hair or supporting cells could be detected. Our results suggest that GATA3 is critical for normal vestibular and cochlear morphogenesis and that it is especially important for cochlear sensory differentiation.

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 molecular basis of organ of Corti development: Where are we now?

This review summarizes recent progress in our understanding of the molecular basis of cochlear duct growth, specification of the organ of Corti, and differentiation of the different types of hair cells. Studies of multiple mutations suggest that developing hair cells are involved in stretching the organ of Corti through convergent extension movements. However, Atoh1 null mutants have only undifferentiated and dying organ of Corti precursors but show a near normal extension of the cochlear duct, implying that organ of Corti precursor cells can equally drive this process. Some factors influence cochlear duct growth by regulating the cell cycle and proliferation. Shortened cell cycle and premature cell cycle exit can lead to a shorter organ of Corti with multiple rows of hair cells (e.g., Foxg1 null mice). Other genes affect the initial formation of a cochlear duct with or without affecting the organ of Corti. Such observations are consistent with evolutionary data that suggest some developmental uncoupling of cochlear duct from organ of Corti formation. Positioning the organ of Corti requires multiple genes expressed in the organ of Corti and the flanking region. Several candidate factors have emerged but how they cooperate to specify the organ of Corti and the topology of hair cells remains unclear. Atoh1 is required for differentiation of all hair cells, but regulation of inner versus outer hair cell differentiation is still unidentified. In summary, the emerging molecular complexity of organ of Corti development demands further study before a rational approach towards regeneration of unique types of hair cells in specific positions is possible.

Pax2/8 proteins coordinate sequential induction of otic and epibranchial placodes through differential regulation of foxi1, sox3 and fgf24

Vertebrate cranial placodes contribute vitally to development of sensory structures of the head. Amongst posterior placodes, the otic placode forms the inner ear whereas nearby epibranchial placodes produce sensory ganglia within branchial clefts. Though diverse in fate, these placodes show striking similarities in their early regulation. In zebrafish, both are initiated by localized Fgf signaling plus the ubiquitous competence factor Foxi1, and both express pax8 and sox3 in response. It has been suggested that Fgf initially induces a common otic/epibranchial field, which later subdivides in response to other signals. However, we find that otic and epibranchial placodes form at different times and by distinct mechanisms. Initially, Fgf from surrounding tissues induces otic expression of pax8 and sox3, which cooperate synergistically to establish otic fate. Subsequently, pax8 works with related genes pax2a/pax2b to downregulate otic expression of foxi1, a necessary step for further otic development. Additionally, pax2/8 activate otic expression of fgf24, which induces epibranchial expression of sox3. Knockdown of fgf24 or sox3 causes severe epibranchial deficiencies but has little effect on otic development. These findings clarify the roles of pax8 and sox3 and support a model whereby the otic placode forms first and induces epibranchial placodes through an Fgf-relay.