FGF-receptor signalling controls neural cell diversity in the zebrafish hindbrain by regulating olig2 and sox9

Short-range Fgf signalling patterns hindbrain progenitors to induce the neurogenesis-to-oligodendrogenesis switch

In the vertebrate nervous system, neurogenesis generally precedes gliogenesis. The mechanisms driving the switch in cell type production and generation of the correct proportion of cell types remain unclear. Here, we show that Fgf20 signalling patterns progenitors to induce the switch from neurogenesis to oligodendrogenesis in the zebrafish hindbrain. Fgf20 emanating from earlier-born neurons signals at a short range to downregulate proneural gene expression in the segment centre with high spatial precision along both anterior-posterior and dorsal-ventral axes. This signal induces oligodendrocytes in the segment centre by upregulating olig2 and sox10 expression in pre-patterned competent progenitors. We show that the magnitude of proneural gene downregulation and the quantity of oligodendrocyte precursor cells specified is dependent on the extent of Fgf20 signalling. Overexpression of fgf20a induces precocious specification and differentiation of oligodendrocytes among olig2+ progenitors, resulting in an increase in oligodendrocytes at the expense of neurogenesis. Thus, Fgf20 signalling defines the proportion of each cell type produced. Taken together, Fgf20 signalling from earlier-born neurons patterns hindbrain segments spatially and temporally to induce the neurogenesis-to-oligodendrogenesis switch.

Fgf22 and Fgfr2b are required for neurogenesis and gliogenesis in the zebrafish forebrain

Fibroblast growth factors (Fgfs) play crucial roles in various developmental processes including brain development. We previously identified Fgf22 in zebrafish and found that fgf22 is involved in midbrain patterning during embryogenesis. Here, we investigated the role of Fgf22 in the formation of the zebrafish forebrain. We found that fgf22 was essential for determining the ventral properties of the telencephalon and diencephalon but not for cell proliferation. In addition, the knockdown of fgf22 inhibited the generation of glutamatergic neurons, γ-aminobutyric acid (GABA)ergic interneurons and astrocytes. Recently, Fgf signaling has received much attention because of its importance in the pathogenesis of multiple sclerosis, in which oligodendrocytes and myelin are destroyed. However, the effects of each Fgf on oligodendrocytes remain largely unknown. Therefore, we also investigated the role of Fgf22 in oligodendrocyte development and explored whether there is a difference between Fgf22 and other Fgfs. Knockdown of fgf22 promoted the generation of oligodendrocytes. Conversely, overexpression of fgf22 inhibited the generation of oligodendrocytes. Furthermore, the forebrain phenotypes of fgfr2b knockdown zebrafish were remarkably similar to those of fgf22 knockdown zebrafish. This establishes the Fgf22-Fgfr2b axis as a key ligand‒receptor partnership in neurogenesis and gliogenesis in the forebrain. Our results indicate that Fgf22 has a unique function in suppressing oligodendrocyte differentiation through Fgfr2b without affecting cell proliferation.

Hindbrain rhombomere centers harbor a heterogenous population of dividing progenitors which rely on Notch signaling

Tissue growth and morphogenesis are interrelated processes, whose tight coordination is essential for the production of different cell fates and the timely precise allocation of stem cell capacities. The zebrafish embryonic brainstem, the hindbrain, exemplifies such coupling between spatiotemporal cell diversity acquisition and tissue growth as the neurogenic commitment is differentially distributed over time. Here, we combined cell lineage and in vivo imaging approaches to reveal the emergence of specific cell population properties within the rhombomeres. We studied the molecular identity of hindbrain rhombomere centers and showed that they harbor different progenitor capacities that change over time. By clonal analysis, we revealed that cells within the center of rhombomeres decrease the proliferative capacity to remain mainly in the G1 phase. Proliferating progenitors give rise to neurons by asymmetric and symmetric neurogenic divisions while maintaining the pool of progenitors. The proliferative capacity of these cells differs from their neighbors, and they are delayed in the onset of Notch activity. Through functional studies, we demonstrated that they rely on Notch3 signaling to be maintained as non-committed progenitors. In this study, we show that cells in rhombomere centers, despite the neurogenic asynchrony, might share steps of a similar program with the rhombomere counterparts, to ensure proper tissue growth.

Mesenchymal Chondrosarcoma from Diagnosis to Clinical Trials

Mesenchymal chondrosarcoma (MCS) is a rare subtype of chondrosarcoma with a poor prognosis. Although these tumors are sensitive to radiotherapy/chemotherapy, the standard treatment for localized MCS is only surgical resection, and there are no established treatment guidelines for patients with advanced and metastatic MCS. Due to the low incidence of MCS, the pathology of these tumors is still unknown, and other therapeutic options are lacking. Some studies show the potential role of the PDGF/PPI3K/AKT, PKC/RAF/MEK/ERK, and pRB pathways, and BCL2 overexpression in the pathogenesis of MCS. These findings provide an opportunity to use protein kinases and BCL2 inhibitors as potential therapy in MCS. In this review, we summarize the current knowledge about MCS diagnosis and treatment options. We show the immunological and molecular biomarkers used in the diagnosis of MCS. In addition, we discuss the known prognostic and predictive factors in MCS. Finally, we present the novel trends, including targeted therapies and ongoing clinical trials using protein kinase inhibitors and the death receptor 5 (DR5) agonist, which may be the focus of future MCS treatment studies.

Zbtb16 mediates a switch between Fgf signalling regimes in the developing hindbrain

Developing tissues are sequentially patterned by extracellular signals that are turned on and off at specific times. In the zebrafish hindbrain, fibroblast growth factor (Fgf) signalling has different roles at different developmental stages: in the early hindbrain, transient Fgf3 and Fgf8 signalling from rhombomere 4 is required for correct segmentation, whereas later, neuronal Fgf20 expression confines neurogenesis to specific spatial domains within each rhombomere. How the switch between these two signalling regimes is coordinated is not known. We present evidence that the Zbtb16 transcription factor is required for this transition to happen in an orderly fashion. Zbtb16 expression is high in the early anterior hindbrain, then gradually upregulated posteriorly and confined to neural progenitors. In mutants lacking functional Zbtb16, fgf3 expression fails to be downregulated and persists until a late stage, resulting in excess and more widespread Fgf signalling during neurogenesis. Accordingly, the spatial pattern of neurogenesis is disrupted in Zbtb16 mutants. Our results reveal how the distinct stage-specific roles of Fgf signalling are coordinated in the zebrafish hindbrain.

Emerging concepts in oligodendrocyte and myelin formation, inputs from the zebrafish model

Oligodendrocytes (OLs) are the myelinating cells of the central nervous system (CNS), which are derived from OL precursor cells. Myelin insulates axons allowing the saltatory conduction of action potentials and also provides trophic and metabolic supports to axons. Interestingly, oligodendroglial cells have the capacity to sense neuronal activity, which regulates myelin sheath formation via the vesicular release of neurotransmitters. Neuronal activity-dependent regulation of myelination is mediated by specialized interaction between axons and oligodendroglia, involving both synaptic and extra-synaptic modes of communications. The zebrafish has provided key advantages for the study of the myelination process in the CNS. External development and transparent larval stages of this vertebrate specie combined with the existence of several transgenic reporter lines provided key advances in oligodendroglial cell biology, axo-glial interactions and CNS myelination. In this publication, we reviewed and discussed the most recent knowledge on OL development and myelin formation, with a focus on mechanisms regulating these fundamental biological processes in the zebrafish. Especially, we highlighted the critical function of axons and oligodendroglia modes of communications and calcium signaling in myelin sheath formation and growth. Finally, we reviewed the relevance of these knowledge's in demyelinating diseases and drug discovery of pharmacological compounds favoring myelin regeneration.

Cracking the Code of Neuronal Cell Fate

Transcriptional regulation is fundamental to most biological processes and reverse-engineering programs can be used to decipher the underlying programs. In this review, we describe how genomics is offering a systems biology-based perspective of the intricate and temporally coordinated transcriptional programs that control neuronal apoptosis and survival. In addition to providing a new standpoint in human pathology focused on the regulatory program, cracking the code of neuronal cell fate may offer innovative therapeutic approaches focused on downstream targets and regulatory networks. Similar to computers, where faults often arise from a software bug, neuronal fate may critically depend on its transcription program. Thus, cracking the code of neuronal life or death may help finding a patch for neurodegeneration and cancer.

TEMPO enables sequential genetic labeling and manipulation of vertebrate cell lineages

During development, regulatory factors appear in a precise order to determine cell fates over time. Consequently, to investigate complex tissue development, it is necessary to visualize and manipulate cell lineages with temporal control. Current strategies for tracing vertebrate cell lineages lack genetic access to sequentially produced cells. Here, we present TEMPO (Temporal Encoding and Manipulation in a Predefined Order), an imaging-readable genetic tool allowing differential labeling and manipulation of consecutive cell generations in vertebrates. TEMPO is based on CRISPR and powered by a cascade of gRNAs that drive orderly activation and inactivation of reporters and/or effectors. Using TEMPO to visualize zebrafish and mouse neurogenesis, we recapitulated birth-order-dependent neuronal fates. Temporally manipulating cell-cycle regulators in mouse cortex progenitors altered the proportion and distribution of neurons and glia, revealing the effects of temporal gene perturbation on serial cell fates. Thus, TEMPO enables sequential manipulation of molecular factors, crucial to study cell-type specification.

Transcription Factor-Forced Astrocytic Differentiation Impairs Human Glioblastoma Growth In Vitro and In Vivo

Direct cellular reprogramming has recently gained attention of cancer researchers for the possibility to convert undifferentiated cancer cells into more differentiated, postmitotic cell types. While a few studies have attempted reprogramming of glioblastoma (GBM) cells toward a neuronal fate, this approach has not yet been used to induce differentiation into other lineages and in vivo data on reduction in tumorigenicity are limited. Here, we employ cellular reprogramming to induce astrocytic differentiation as a therapeutic approach in GBM. To this end, we overexpressed key transcriptional regulators of astroglial development in human GBM and GBM stem cell lines. Treated cells undergo a remarkable shift in structure, acquiring an astrocyte-like morphology with star-shaped bodies and radial branched processes. Differentiated cells express typical glial markers and show a marked decrease in their proliferative state. In addition, forced differentiation induces astrocytic functions such as induced calcium transients and ability to respond to inflammatory stimuli. Most importantly, forced differentiation substantially reduces tumorigenicity of GBM cells in an in vivo xenotransplantation model. The current study capitalizes on cellular plasticity with a novel application in cancer. We take advantage of the similarity between neural developmental processes and cancer hierarchy to mitigate, if not completely abolish, the malignant nature of tumor cells and pave the way for new intervention strategies.

Proper modulation of AHR signaling is necessary for establishing neural connectivity and oligodendrocyte precursor cell development in the embryonic zebrafish brain

2,3,7,8-tetrachlorodibenzo-[p]-dioxin (TCDD) is a persistent global pollutant that exhibits a high affinity for the aryl hydrocarbon receptor (AHR), a ligand activated transcription factor. Epidemiological studies have associated AHR agonist exposure with multiple human neuropathologies. Consistent with the human data, research studies using laboratory models have linked pollutant-induced AHR activation to disruptions in learning and memory as well as motor impairments. Our understanding of endogenous AHR functions in brain development is limited and, correspondingly, scientists are still determining which cell types and brain regions are sensitive to AHR modulation. To identify novel phenotypes resulting from pollutant-induced AHR activation and ahr2 loss of function, we utilized the optically transparent zebrafish model. Early embryonic TCDD exposure impaired embryonic brain morphogenesis, resulted in ventriculomegaly, and disrupted neural connectivity in the optic tectum, habenula, cerebellum, and olfactory bulb. Altered neural network formation was accompanied by reduced expression of synaptic vesicle 2. Loss of ahr2 function also impaired nascent network development, but did not affect gross brain or ventricular morphology. To determine whether neural AHR activation was sufficient to disrupt connectivity, we used the Gal4/UAS system to express a constitutively active AHR specifically in differentiated neurons and observed disruptions only in the cerebellum; thus, suggesting that the phenotypes resulting from global AHR activation likely involve multiple cell types. Consistent with this hypothesis, we found that TCDD exposure reduced the number of oligodendrocyte precursor cells and their derivatives. Together, our findings indicate that proper modulation of AHR signaling is necessary for the growth and maturation of the embryonic zebrafish brain.

Nucleolin loss of function leads to aberrant Fibroblast Growth Factor signaling and craniofacial anomalies

Ribosomal RNA (rRNA) transcription and ribosome biogenesis are global processes required for growth and proliferation of all cells, yet perturbation of these processes in vertebrates leads to tissue-specific defects termed ribosomopathies. Mutations in rRNA transcription and processing proteins often lead to craniofacial anomalies; however, the cellular and molecular reasons for these defects are poorly understood. Therefore, we examined the function of the most abundant nucleolar phosphoprotein, Nucleolin (Ncl), in vertebrate development. ncl mutant (ncl-/-) zebrafish present with craniofacial anomalies such as mandibulofacial hypoplasia. We observed that ncl-/- mutants exhibited decreased rRNA synthesis and p53-dependent apoptosis, consistent with a role in ribosome biogenesis. However, we found that Nucleolin also performs functions not associated with ribosome biogenesis. We discovered that the half-life of fgf8a mRNA was reduced in ncl-/- mutants, which perturbed Fgf signaling, resulting in misregulated Sox9a-mediated chondrogenesis and Runx2-mediated osteogenesis. Consistent with this model, exogenous FGF8 treatment significantly rescued the cranioskeletal phenotype in ncl-/- zebrafish, suggesting that Nucleolin regulates osteochondroprogenitor differentiation. Our work has therefore uncovered tissue-specific functions for Nucleolin in rRNA transcription and post-transcriptional regulation of growth factor signaling during embryonic craniofacial development.

From Cell States to Cell Fates: How Cell Proliferation and Neuronal Differentiation Are Coordinated During Embryonic Development

The central nervous system (CNS) exhibits an extraordinary diversity of neurons, with the right cell types and proportions at the appropriate sites. Thus, to produce brains with specific size and cell composition, the rates of proliferation and differentiation must be tightly coordinated and balanced during development. Early on, proliferation dominates; later on, the growth rate almost ceases as more cells differentiate and exit the cell cycle. Generation of cell diversity and morphogenesis takes place concomitantly. In the vertebrate brain, this results in dramatic changes in the position of progenitor cells and their neuronal derivatives, whereas in the spinal cord morphogenetic changes are not so important because the structure mainly grows by increasing its volume. Morphogenesis is under control of specific genetic programs that coordinately unfold over time; however, little is known about how they operate and impact in the pools of progenitor cells in the CNS. Thus, the spatiotemporal coordination of these processes is fundamental for generating functional neuronal networks. Some key aims in developmental neurobiology are to determine how cell diversity arises from pluripotent progenitor cells, and how the progenitor potential changes upon time. In this review, we will share our view on how the advance of new technologies provides novel data that challenge some of the current hypothesis. We will cover some of the latest studies on cell lineage tracing and clonal analyses addressing the role of distinct progenitor cell division modes in balancing the rate of proliferation and differentiation during brain morphogenesis. We will discuss different hypothesis proposed to explain how progenitor cell diversity is generated and how they challenged prevailing concepts and raised new questions.

Evolutionary Origins of the Oligodendrocyte Cell Type and Adaptive Myelination

Oligodendrocytes are multifunctional central nervous system (CNS) glia that are essential for neural function in gnathostomes. The evolutionary origins and specializations of the oligodendrocyte cell type are among the many remaining mysteries in glial biology and neuroscience. The role of oligodendrocytes as CNS myelinating glia is well established, but recent studies demonstrate that oligodendrocytes also participate in several myelin-independent aspects of CNS development, function, and maintenance. Furthermore, many recent studies have collectively advanced our understanding of myelin plasticity, and it is now clear that experience-dependent adaptations to myelination are an additional form of neural plasticity. These observations beg the questions of when and for which functions the ancestral oligodendrocyte cell type emerged, when primitive oligodendrocytes evolved new functionalities, and the genetic changes responsible for these evolutionary innovations. Here, I review recent findings and propose working models addressing the origins and evolution of the oligodendrocyte cell type and adaptive myelination. The core gene regulatory network (GRN) specifying the oligodendrocyte cell type is also reviewed as a means to probe the existence of oligodendrocytes in basal vertebrates and chordate invertebrates.

Peptide ligands targeting FGF receptors promote recovery from dorsal root crush injury via AKT/mTOR signaling

Background: Fibroblast growth factor receptors (FGFRs) are key targets for nerve regeneration and repair. The therapeutic effect of exogenous recombinant FGFs in vivo is limited due to their high molecular weight. Small peptides with low molecular weight, easy diffusion, low immunogenicity, and nontoxic metabolite formation are potential candidates. The present study aimed to develop a novel low-molecular-weight peptide agonist of FGFR to promote nerve injury repair.

Methods: Phage display technology was employed to screen peptide ligands targeting FGFR2. The peptide ligand affinity for FGFRs was detected by isothermal titration calorimetry. Structural biology-based computer virtual analysis was used to characterize the interaction between the peptide ligand and FGFR2. The peptide ligand effect on axon growth, regeneration, and behavioral recovery of sensory neurons was determined in the primary culture of sensory neurons and dorsal root ganglia (DRG) explants in vitro and a rat spinal dorsal root injury (DRI) model in vivo. The peptide ligand binding to other membrane receptors was characterized by surface plasmon resonance (SPR) and liquid chromatography-mass spectrometry (LC-MS)/MS. Intracellular signaling pathways primarily affected by the peptide ligand were characterized by phosphoproteomics, and related pathways were verified using specific inhibitors.

Results: We identified a novel FGFR-targeting small peptide, CH02, with seven amino acid residues. CH02 activated FGFR signaling through high-affinity binding with the extracellular segment of FGFRs and also had an affinity for several receptor tyrosine kinase (RTK) family members, including VEGFR2. In sensory neurons cultured in vitro, CH02 maintained the survival of neurons and promoted axon growth. Simultaneously, CH02 robustly enhanced nerve regeneration and sensory-motor behavioral recovery after DRI in rats. CH02-induced activation of FGFR signaling promoted nerve regeneration primarily via AKT and ERK signaling downstream of FGFRs. Activation of mTOR downstream of AKT signaling augmented axon growth potential in response to CH02.

Conclusion: Our study revealed the significant therapeutic effect of CH02 on strengthening nerve regeneration and suggested a strategy for treating peripheral and central nervous system injuries.

Segmentation and patterning of the vertebrate hindbrain

During early development, the hindbrain is sub-divided into rhombomeres that underlie the organisation of neurons and adjacent craniofacial tissues. A gene regulatory network of signals and transcription factors establish and pattern segments with a distinct anteroposterior identity. Initially, the borders of segmental gene expression are imprecise, but then become sharply defined, and specialised boundary cells form. In this Review, we summarise key aspects of the conserved regulatory cascade that underlies the formation of hindbrain segments. We describe how the pattern is sharpened and stabilised through the dynamic regulation of cell identity, acting in parallel with cell segregation. Finally, we discuss evidence that boundary cells have roles in local patterning, and act as a site of neurogenesis within the hindbrain.

Zebrafish Minichromosome Maintenance Protein 5 Gene Regulates the Development and Migration of Facial Motor Neurons via Fibroblast Growth Factor Signaling

Minichromosome maintenance protein 5 (MCM5), a member of the microchromosomal maintenance protein family, plays an important role in the initiation and extension of DNA replication. However, its role in neural development in zebrafish remains unclear. Here, we used morpholino (MO) and CRISPR/Cas9 to knock down mcm5 and investigated the developmental features of facial motor neurons (FMNs) in the hindbrain of zebrafish. We found that knockdown of mcm5 using mcm5 MO resulted in a small head, small eyes, and a blurred midbrain-hindbrain boundary, while MO injection of mcm5 led to decrease in FMNs and their migration disorder. However, the mutant of mcm5 only resulted in the migration defect of FMNs rather than quantity change. We further investigated the underlying mechanism of mcm5 in the development of hindbrain using in situ hybridization (ISH) and fgfr1a mRNA co-injected with mcm5 MO. Results from ISH showed that the fibroblast growth factor (FGF) signaling pathway was changed when the MCM5 function was lost, with the decrease in fgfr1a and the increase in fgf8, while that of pea3 had opposite trend. FMN development defects were rescued by fgfr1a mRNA co-injected with mcm5 MO. Our results demonstrated that FGF signaling pathway is required for FMN development in zebrafish. Specifically, mcm5 regulates FMN development during zebrafish growing.

OLIG2 Immunolabeling of Mesenchymal Chondrosarcoma: Report of 14 Cases

Mesenchymal chondrosarcoma (MC) is a rare aggressive mesenchymal sarcoma. Specific markers for the differential diagnosis of MCs remain to be developed. OLIG2 expression has been reported only in neuroepithelial tumors. Recently, OLIG2 expression was found to be involved in the development of NCOA2 fusion-positive alveolar rhabdomyosarcomas. Therefore, we investigated whether OLIG2 expression could be used as a diagnostic marker for MC. We report the clinical pathological and immunohistochemical features of 14 MCs. All tumors showed typical pathological features including biphasic patterns with sheets of primitive round mesenchymal cells and interspersed islands of cartilage. These tumors expressed BCL2, SOX9, and CD99. OLIG2 was robustly expressed in 12/14 of MCs. NCOA2 rearrangement was found in 12 cases. OLIG2 expression was not found in the NCOA2 rearrangement-negative MCs. Notably, OLIG2 expression was not detected in 52 neoplasms (8 Ewing sarcomas, 23 hemangiopericytomas, and 21 chondrosarcomas) that are frequently misdiagnosed as MC. Our findings provide convincing evidence that OLIG2 can serve as a reliable marker in the differential diagnosis of MC and may be a unique neurodevelopmental gene expression signature for the NCOA2 rearranged MCs.

New Insights Into the Intricacies of Proneural Gene Regulation in the Embryonic and Adult Cerebral Cortex

Historically, the mammalian brain was thought to lack stem cells as no new neurons were found to be made in adulthood. That dogma changed ∼25 years ago with the identification of neural stem cells (NSCs) in the adult rodent forebrain. However, unlike rapidly self-renewing mature tissues (e.g., blood, intestinal crypts, skin), the majority of adult NSCs are quiescent, and those that become 'activated' are restricted to a few neurogenic zones that repopulate specific brain regions. Conversely, embryonic NSCs are actively proliferating and neurogenic. Investigations into the molecular control of the quiescence-to-proliferation-to-differentiation continuum in the embryonic and adult brain have identified proneural genes encoding basic-helix-loop-helix (bHLH) transcription factors (TFs) as critical regulators. These bHLH TFs initiate genetic programs that remove NSCs from quiescence and drive daughter neural progenitor cells (NPCs) to differentiate into specific neural cell subtypes, thereby contributing to the enormous cellular diversity of the adult brain. However, new insights have revealed that proneural gene activities are context-dependent and tightly regulated. Here we review how proneural bHLH TFs are regulated, with a focus on the murine cerebral cortex, drawing parallels where appropriate to other organisms and neural tissues. We discuss upstream regulatory events, post-translational modifications (phosphorylation, ubiquitinylation), protein-protein interactions, epigenetic and metabolic mechanisms that govern bHLH TF expression, stability, localization, and consequent transactivation of downstream target genes. These tight regulatory controls help to explain paradoxical findings of changes to bHLH activity in different cellular contexts.

Gene regulatory networks controlling vertebrate retinal regeneration

Injury induces retinal Müller glia of certain cold-blooded vertebrates, but not those of mammals, to regenerate neurons. To identify gene regulatory networks that reprogram Müller glia into progenitor cells, we profiled changes in gene expression and chromatin accessibility in Müller glia from zebrafish, chick, and mice in response to different stimuli. We identified evolutionarily conserved and species-specific gene networks controlling glial quiescence, reactivity, and neurogenesis. In zebrafish and chick, the transition from quiescence to reactivity is essential for retinal regeneration, whereas in mice, a dedicated network suppresses neurogenic competence and restores quiescence. Disruption of nuclear factor I transcription factors, which maintain and restore quiescence, induces Müller glia to proliferate and generate neurons in adult mice after injury. These findings may aid in designing therapies to restore retinal neurons lost to degenerative diseases.

A single cell transcriptome atlas of the developing zebrafish hindbrain

Segmentation of the vertebrate hindbrain leads to the formation of rhombomeres, each with a distinct anteroposterior identity. Specialised boundary cells form at segment borders that act as a source or regulator of neuronal differentiation. In zebrafish, there is spatial patterning of neurogenesis in which non-neurogenic zones form at boundaries and segment centres, in part mediated by Fgf20 signalling. To further understand the control of neurogenesis, we have carried out single cell RNA sequencing of the zebrafish hindbrain at three different stages of patterning. Analyses of the data reveal known and novel markers of distinct hindbrain segments, of cell types along the dorsoventral axis, and of the transition of progenitors to neuronal differentiation. We find major shifts in the transcriptome of progenitors and of differentiating cells between the different stages analysed. Supervised clustering with markers of boundary cells and segment centres, together with RNA-seq analysis of Fgf-regulated genes, has revealed new candidate regulators of cell differentiation in the hindbrain. These data provide a valuable resource for functional investigations of the patterning of neurogenesis and the transition of progenitors to neuronal differentiation.

FGF Signaling Directs the Cell Fate Switch from Neurons to Astrocytes in the Developing Mouse Cerebral Cortex

During mammalian neocortical development, neural precursor cells generate neurons first and astrocytes later. The cell fate switch from neurons to astrocytes is a key process generating proper numbers of neurons and astrocytes. Although the intracellular mechanisms regulating this cell fate switch have been well characterized, extracellular regulators are still largely unknown. Here, we uncovered that fibroblast growth factor (FGF) regulates the cell fate switch from neurons to astrocytes in the developing cerebral cortex using mice of both sexes. We found that the FGF signaling pathway is activated in radial glial cells of the ventricular zone at time points corresponding to the switch in cell fate. Our loss- and gain-of-function studies using in utero electroporation indicate that activation of FGF signaling is necessary and sufficient to change cell fates from neurons to astrocytes. We further found that the FGF-induced neuron-astrocyte cell fate switch is mediated by the MAPK pathway. These results indicate that FGF is a critical extracellular regulator of the cell fate switch from neurons to astrocytes in the mammalian cerebral cortex.SIGNIFICANCE STATEMENT Although the intracellular mechanisms regulating the neuron-astrocyte cell fate switch in the mammalian cerebral cortex during development have been well studied, their upstream extracellular regulators remain unknown. By using in utero electroporation, our study provides in vivo data showing that activation of FGF signaling is necessary and sufficient for changing cell fates from neurons to astrocytes. Manipulation of FGF signaling activity led to drastic changes in the numbers of neurons and astrocytes. These results indicate that FGF is a key extracellular regulator determining the numbers of neurons and astrocytes in the mammalian cerebral cortex, and is indispensable for the establishment of appropriate neural circuitry.

FGF signaling controls Shh-dependent oligodendroglial fate specification in the ventral spinal cord

Background

Most oligodendrocytes of the spinal cord originate from ventral progenitor cells of the pMN domain, characterized by expression of the transcription factor Olig2. A minority of oligodendrocytes is also recognized to emerge from dorsal progenitors during fetal development. The prevailing view is that generation of ventral oligodendrocytes depends on Sonic hedgehog (Shh) while dorsal oligodendrocytes develop under the influence of Fibroblast Growth Factors (FGFs).

Results

Using the well-established model of the chicken embryo, we show that ventral spinal progenitor cells activate FGF signaling at the onset of oligodendrocyte precursor cell (OPC) generation. Inhibition of FGF receptors at that time appears sufficient to prevent generation of ventral OPCs, highlighting that, in addition to Shh, FGF signaling is required also for generation of ventral OPCs. We further reveal an unsuspected interplay between Shh and FGF signaling by showing that FGFs serve dual essential functions in ventral OPC specification. FGFs are responsible for timely induction of a secondary Shh signaling center, the lateral floor plate, a crucial step to create the burst of Shh required for OPC specification. At the same time, FGFs prevent down-regulation of Olig2 in pMN progenitor cells as these cells receive higher threshold of the Shh signal. Finally, we bring arguments favoring a key role of newly differentiated neurons acting as providers of the FGF signal required to trigger OPC generation in the ventral spinal cord.

Conclusion

Altogether our data reveal that the FGF signaling pathway is activated and required for OPC commitment in the ventral spinal cord. More generally, our data may prove important in defining strategies to produce large populations of determined oligodendrocyte precursor cells from undetermined neural progenitors, including stem cells. In the long run, these new data could be useful in attempts to stimulate the oligodendrocyte fate in residing neural stem cells.

Wnt/β-catenin signaling during early vertebrate neural development

The vertebrate central nervous system (CNS) is comprised of vast number of distinct cell types arranged in a highly organized manner. This high degree of complexity is achieved by cellular communication, including direct cell-cell contact, cell-matrix interactions, and cell-growth factor signaling. Among the several developmental signals controlling the development of the CNS, Wnt proteins have emerged as particularly critical and, hence, have captivated the attention of many researchers. With Wnts' evolutionarily conserved function as primordial symmetry breaking signals, these proteins and their downstream effects are responsible for simultaneously establishing cellular diversity and tissue organization. With their expansive repertoire of secreted agonists and antagonists, cell surface receptors, signaling cascades and downstream biological effects, Wnts are ideally suited to control the complex processes underlying vertebrate neural development. In this review, we will describe the mechanisms by which Wnts exert their potent effects on cells and tissues and highlight the many roles of Wnt signaling during neural development, starting from the initial induction of the neural plate, the subsequent patterning along the embryonic axes, to the intricately organized structure of the CNS. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1239-1259, 2017.

In Vivo Characterization of an AHR-Dependent Long Noncoding RNA Required for Proper Sox9b Expression

Xenobiotic activation of the aryl hydrocarbon receptor (AHR) by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) prevents the proper formation of craniofacial cartilage and the heart in developing zebrafish. Downstream molecular targets responsible for AHR-dependent adverse effects remain largely unknown; however, in zebrafish sox9b has been identified as one of the most-reduced transcripts in several target organs and is hypothesized to have a causal role in TCDD-induced toxicity. The reduction of sox9b expression in TCDD-exposed zebrafish embryos has been shown to contribute to heart and jaw malformation phenotypes. The mechanisms by which AHR2 (functional ortholog of mammalian AHR) activation leads to reduced sox9b expression levels and subsequent target organ toxicity are unknown. We have identified a novel long noncoding RNA (slincR) that is upregulated by strong AHR ligands and is located adjacent to the sox9b gene. We hypothesize that slincR is regulated by AHR2 and transcriptionally represses sox9b. The slincR transcript functions as an RNA macromolecule, and slincR expression is AHR2 dependent. Antisense knockdown of slincR results in an increase in sox9b expression during both normal development and AHR2 activation, which suggests relief in repression. During development, slincR was expressed in tissues with sox9 essential functions, including the jaw/snout region, otic vesicle, eye, and brain. Reducing the levels of slincR resulted in altered neurologic and/or locomotor behavioral responses. Our results place slincR as an intermediate between AHR2 activation and the reduction of sox9b mRNA in the AHR2 signaling pathway.

Brorin is required for neurogenesis, gliogenesis, and commissural axon guidance in the zebrafish forebrain

Bmps regulate numerous neural functions with their regulators. We previously identified Brorin, a neural-specific secreted antagonist of Bmp signaling, in humans, mice, and zebrafish. Mouse Brorin has two cysteine-rich domains containing 10 cysteine residues in its core region, and these are located in similar positions to those in the cysteine-rich domains of Chordin family members, which are secreted Bmp antagonists. Zebrafish Brorin had two cysteine-rich domains with high similarity to those of mouse Brorin. We herein examined zebrafish brorin in order to elucidate its in vivo actions. Zebrafish brorin was predominantly expressed in developing neural tissues. The overexpression of brorin led to the inactivation of Bmp signaling. On the other hand, the knockdown of brorin resulted in the activation of Bmp signaling and brorin morphants exhibited defective development of the ventral domain in the forebrain. Furthermore, the knockdown of brorin inhibited the generation of γ–aminobutyric acid (GABA)ergic interneurons and oligodendrocytes and promoted the generation of astrocytes in the forebrain. In addition, brorin was required for axon guidance in the forebrain. The present results suggest that Brorin is a secreted Bmp antagonist predominantly expressed in developing neural tissues and that it plays multiple roles in the development of the zebrafish forebrain.

Retinoic Acid Signaling Mediates Hair Cell Regeneration by Repressing p27kip and sox2 in Supporting Cells

During development, otic sensory progenitors give rise to hair cells and supporting cells. In mammalian adults, differentiated and quiescent sensory cells are unable to generate new hair cells when these are lost due to various insults, leading to irreversible hearing loss. Retinoic acid (RA) has strong regenerative capacity in several organs, but its role in hair cell regeneration is unknown. Here, we use genetic and pharmacological inhibition to show that the RA pathway is required for hair cell regeneration in zebrafish. When regeneration is induced by laser ablation in the inner ear or by neomycin treatment in the lateral line, we observe rapid activation of several components of the RA pathway, with dynamics that position RA signaling upstream of other signaling pathways. We demonstrate that blockade of the RA pathway impairs cell proliferation of supporting cells in the inner ear and lateral line. Moreover, in neuromast, RA pathway regulates the transcription of p27(kip) and sox2 in supporting cells but not fgf3. Finally, genetic cell-lineage tracing using Kaede photoconversion demonstrates that de novo hair cells derive from FGF-active supporting cells. Our findings reveal that RA has a pivotal role in zebrafish hair cell regeneration by inducing supporting cell proliferation, and shed light on the underlying transcriptional mechanisms involved. This signaling pathway might be a promising approach for hearing recovery.

SIGNIFICANCE STATEMENT

Hair cells are the specialized mechanosensory cells of the inner ear that capture auditory and balance sensory input. Hair cells die after acoustic trauma, ototoxic drugs or aging diseases, leading to progressive hearing loss. Mammals, in contrast to zebrafish, lack the ability to regenerate hair cells. Here, we find that retinoic acid (RA) pathway is required for hair cell regeneration in vivo in the zebrafish inner ear and lateral line. RA pathway is activated very early upon hair cell loss, promotes cell proliferation of progenitor cells, and regulates two key genes, p27(kip) and sox2. Our results position RA as an essential signal for hair cell regeneration with relevance in future regenerative strategies in mammals.

Construction and characterization of a sox9b transgenic reporter line

The transcription factor SOX9 is a member of the SRY-related high-mobility-group box (SOX) superfamily of genes. In mammals, Sox9 plays important roles in many developmental processes including craniofacial, skeletal and heart morphogenesis, retinal and brain development, and gonad differentiation. Human mutations in SOX9 or the SOX9 promoter result in campomelic dysplasia, a severe genetic disorder, which disrupts skeletal, craniofacial, cardiac, neural and reproductive development. Due to the duplication of the teleost fish genome, zebrafish (Danio rerio) have two Sox9 genes: sox9a and sox9b. Loss of sox9b in zebrafish results in loss of function phenotypes that are similar to those observed in humans and mice. In order to generate a transgenic sox9b:EGFP reporter line, we cloned a 2450 bp fragment of the sox9b promoter and fused it to an EGFP reporter. Consistent with reported sox9b expression and function, we observed sox9b:EGFP in the developing heart, skeletal and craniofacial structures, brain, retina, and ovaries. Our resulting transgenic line is a useful tool for identifying and studying sox9b function in development and visualizing a number of zebrafish organs and tissues in which sox9b is normally expressed.

Fgf-signaling-dependent Sox9a and Atoh1a regulate otic neural development in zebrafish

Fibroblast growth factors (Fgfs) play important roles in developmental processes of the inner ear, including the ontogeny of the statoacoustic ganglia (SAG) and hair cells. However, the detailed genetic mechanism(s) underlying Fgf/Fgfr-dependent otic neural development remains elusive. Using conditional genetic approaches and inhibitory small molecules, we have revealed that Fgfr-PI3K/Akt signaling is mainly responsible for zebrafish SAG development and have determined that Sox9a and Atoh1a act downstream of Fgfr-Akt signaling to specify and/or maintain the otic neuron fate during the early segmentation stage. Sox9a and Atoh1a coregulate numerous downstream factors identified through our ChIP-seq analyses, including Tlx2 and Eya2. Fgfr-Erk1/2 signaling contributes to ultricular hair cell development during a critical period between 9 and 15 hours postfertilization. Our work reveals that a genetic network of the previously known sensory determinant Atoh1 and the neural crest determinant Sox9 plays critical roles in SAG development. These newly uncovered roles for Atoh1and Sox9 in zebrafish otic development may be relevant to study in other species.

Epigenetic regulation of the neural transcriptome and alcohol interference during development

Alcohol intoxicated cells broadly alter their metabolites – among them methyl and acetic acid can alter the DNA and histone epigenetic codes. Together with the promiscuous effect of alcohol on enzyme activities (including DNA methyltransferases) and the downstream effect on microRNA and transposable elements, alcohol is well placed to affect intrinsic transcriptional programs of developing cells. Considering that the developmental consequences of early alcohol exposure so profoundly affect neural systems, it is not unfounded to reason that alcohol exploits transcriptional regulators to challenge canonical gene expression and in effect, intrinsic developmental pathways to achieve widespread damage in the developing nervous system. To fully evaluate the role of epigenetic regulation in alcohol-related developmental disease, it is important to first gather the targets of epigenetic players in neurodevelopmental models. Here, we attempt to review the cellular and genomic windows of opportunity for alcohol to act on intrinsic neurodevelopmental programs. We also discuss some established targets of fetal alcohol exposure and propose pathways for future study. Overall, this review hopes to illustrate the known epigenetic program and its alterations in normal neural stem cell development and further, aims to depict how alcohol, through neuroepigenetics, may lead to neurodevelopmental deficits observed in fetal alcohol spectrum disorders.

Integration of signals along orthogonal axes of the vertebrate neural tube controls progenitor competence and increases cell diversity

FGF gates competence to generate Floor Plate and Neural Crest in response to Shh and BMP signals by controlling expression of the transcription factor Nkx1.2.

A relatively small number of signals are responsible for the variety and pattern of cell types generated in developing embryos. In part this is achieved by exploiting differences in the concentration or duration of signaling to increase cellular diversity. In addition, however, changes in cellular competence—temporal shifts in the response of cells to a signal—contribute to the array of cell types generated. Here we investigate how these two mechanisms are combined in the vertebrate neural tube to increase the range of cell types and deliver spatial control over their location. We provide evidence that FGF signaling emanating from the posterior of the embryo controls a change in competence of neural progenitors to Shh and BMP, the two morphogens that are responsible for patterning the ventral and dorsal regions of the neural tube, respectively. Newly generated neural progenitors are exposed to FGF signaling, and this maintains the expression of the Nk1-class transcription factor Nkx1.2. Ventrally, this acts in combination with the Shh-induced transcription factor FoxA2 to specify floor plate cells and dorsally in combination with BMP signaling to induce neural crest cells. As development progresses, the intersection of FGF with BMP and Shh signals is interrupted by axis elongation, resulting in the loss of Nkx1.2 expression and allowing the induction of ventral and dorsal interneuron progenitors by Shh and BMP signaling to supervene. Hence a similar mechanism increases cell type diversity at both dorsal and ventral poles of the neural tube. Together these data reveal that tissue morphogenesis produces changes in the coincidence of signals acting along orthogonal axes of the neural tube and this is used to define spatial and temporal transitions in the competence of cells to interpret morphogen signaling.

During embryonic development different cell types arise at different times and places. This diversity is produced by a relatively small number of signals and depends, at least in part, on changes in the way cells respond to each signal. One example of this so-called change in “competence” is found in the vertebrate spinal cord where a signal, Sonic Hedgehog (Shh), induces a glial cell type known as floor plate (FP) at early developmental times, while the same signal later induces specific types of neurons. Here, we dissected the molecular mechanism underlying the change in competence, and found that another signal, FGF, is involved through its control of the transcription factor Nkx1.2. In embryos, Shh and FGF are produced perpendicular to one another and FP is induced where the two signals intersect. The position of this intersection changes as the embryo elongates and this determines the place and time FP is produced. A similar strategy also appears to apply to another cell type, neural crest. In this case, the intersection of FGF with BMP signal is crucial. Together the data provide new insight into the spatiotemporal control of cell type specification during development of the vertebrate spinal cord.

Endocytosis of Fgf8 is a double-stage process and regulates spreading and signaling

Tightly controlled concentration gradients of morphogens provide positional information and thus regulate tissue differentiation and morphogenesis in multicellular organisms. However, how such morphogenetic fields are formed and maintained remains debated. Here we show that fibroblast growth factor 8 (Fgf8) morphogen gradients in zebrafish embryos are established and maintained by two essential mechanisms. Firstly, Fgf8 is taken up into the cell by clathrin-mediated endocytosis. The speed of the uptake rate defines the range of the morphogenetic gradient of Fgf8. Secondly, our data demonstrate that after endocytosis the routing of Fgf8 from the early endosome to the late endosome shuts down signaling. Therefore, intracellular endocytic transport regulates the intensity and duration of Fgf8 signaling. We show that internalization of Fgf8 into the early endosome and subsequent transport towards the late endosome are two independent processes. Therefore, we hypothesize that Fgf8 receiving cells control both, the propagation width and the signal strength of the morphogen.

Multi-study integration of brain cancer transcriptomes reveals organ-level molecular signatures

We utilized abundant transcriptomic data for the primary classes of brain cancers to study the feasibility of separating all of these diseases simultaneously based on molecular data alone. These signatures were based on a new method reported herein – Identification of Structured Signatures and Classifiers (ISSAC) – that resulted in a brain cancer marker panel of 44 unique genes. Many of these genes have established relevance to the brain cancers examined herein, with others having known roles in cancer biology. Analyses on large-scale data from multiple sources must deal with significant challenges associated with heterogeneity between different published studies, for it was observed that the variation among individual studies often had a larger effect on the transcriptome than did phenotype differences, as is typical. For this reason, we restricted ourselves to studying only cases where we had at least two independent studies performed for each phenotype, and also reprocessed all the raw data from the studies using a unified pre-processing pipeline. We found that learning signatures across multiple datasets greatly enhanced reproducibility and accuracy in predictive performance on truly independent validation sets, even when keeping the size of the training set the same. This was most likely due to the meta-signature encompassing more of the heterogeneity across different sources and conditions, while amplifying signal from the repeated global characteristics of the phenotype. When molecular signatures of brain cancers were constructed from all currently available microarray data, 90% phenotype prediction accuracy, or the accuracy of identifying a particular brain cancer from the background of all phenotypes, was found. Looking forward, we discuss our approach in the context of the eventual development of organ-specific molecular signatures from peripheral fluids such as the blood.

From a multi-study, integrated transcriptomic dataset, we identified a marker panel for differentiating major human brain cancers at the gene-expression level. The ISSAC molecular signatures for brain cancers, composed of 44 unique genes, are based on comparing expression levels of pairs of genes, and phenotype prediction follows a diagnostic hierarchy. We found that sufficient dataset integration across multiple studies greatly enhanced diagnostic performance on truly independent validation sets, whereas signatures learned from only one dataset typically led to high error rate. Molecular signatures of brain cancers, when obtained using all currently available gene-expression data, achieved 90% phenotype prediction accuracy. Thus, our integrative approach holds significant promise for developing organ-level, comprehensive, molecular signatures of disease.

Fibroblast growth factor receptor 2c signaling is required for intestinal cell differentiation in zebrafish

Background

There are four cell lineages derived from intestinal stem cells that are located at the crypt and villus in the mammalian intestine the non-secretory absorptive enterocytes, and the secretory cells, which include mucous-secreting goblet cells, regulatory peptide-secreting enteroendocrine cells and antimicrobial peptide-secreting Paneth cells. Although fibroblast growth factor (Fgf) signaling is important for cell proliferation and differentiation in various tissues, its role in intestinal differentiation is less well understood.

Methodology/Principal Findings

We used a loss of function approach to investigate the importance of Fgf signaling in intestinal cell differentiation in zebrafish; abnormal differentiation of goblet cells was observed when Fgf signaling was inhibited using SU5402 or in the Tg(hsp70ldnfgfr1-EGFP) transgenic line. We identified Fgfr2c as an important receptor for cell differentiation. The number of goblet cells and enteroendocrine cells was reduced in fgfr2c morphants. In addition to secretory cells, enterocyte differentiation was also disrupted in fgfr2c morphants. Furthermore, proliferating cells were increased in the morphants. Interestingly, the loss of fgfr2c expression repressed secretory cell differentiation and increased cell proliferation in the mib^ta52b mutant that had defective Notch signaling.

Conclusions/Significance

In conclusion, we found that Fgfr2c signaling derived from mesenchymal cells is important for regulating the differentiation of zebrafish intestine epithelial cells by promoting cell cycle exit. The results of Fgfr2c knockdown in mib^ta52b mutants indicated that Fgfr2c signaling is required for intestinal cell differentiation. These findings provide new evidences that Fgf signaling is required for the differentiation of intestinal cells in the zebrafish developing gut.

Development of hypothalamic serotoninergic neurons requires Fgf signalling via the ETS-domain transcription factor Etv5b

Serotonin is a monoamine neurotransmitter that is involved in numerous physiological functions and its dysregulation is implicated in various psychiatric diseases. In all non-placental vertebrates, serotoninergic (5-HT) neurons are present in several regions of the brain, including the hypothalamus. In placental mammals, however, 5-HT neurons are located in the raphe nuclei only. In all species, though, 5-HT neurons constitute a functionally and molecularly heterogeneous population. How the non-raphe 5-HT populations are developmentally encoded is unknown. Using the zebrafish model we show that, in contrast to the raphe populations, hypothalamic 5-HT neurons are generated independently of the ETS-domain transcription factor Pet1 (Fev). By applying a combination of pharmacological tools and gene knockdown and/or overexpression experiments, we demonstrate that Fgf signalling acts via another ETS-domain transcription factor, Etv5b (Erm), to induce hypothalamic 5-HT neurons. We provide evidence that Etv5b exerts its effects by regulating cell cycle parameters in 5-HT progenitors. Our results highlight a novel role for Etv5b in neuronal development and provide support for the existence of a developmental heterogeneity among 5-HT neurons in their requirement for ETS-domain transcription factors.

Protein tyrosine phosphatase receptor type O (Ptpro) regulates cerebellar formation during zebrafish development through modulating Fgf signaling

Protein activities controlled by receptor protein tyrosine phosphatases (RPTPs) play comparably important roles in transducing cell surface signals into the cytoplasm by protein tyrosine kinases. Previous studies showed that several RPTPs are involved in neuronal generation, migration, and axon guidance in Drosophila, and the vertebrate hippocampus, retina, and developing limbs. However, whether the protein tyrosine phosphatase type O (ptpro), one kind of RPTP, participates in regulating vertebrate brain development is largely unknown. We isolated the zebrafish ptpro gene and found that its transcripts are primarily expressed in the embryonic and adult central nervous system. Depletion of zebrafish embryonic Ptpro by antisense morpholino oligonucleotide knockdown resulted in prominent defects in the forebrain and cerebellum, and the injected larvae died on the 4th day post-fertilization (dpf). We further investigated the function of ptpro in cerebellar development and found that the expression of ephrin-A5b (efnA5b), a Fgf signaling induced cerebellum patterning factor, was decreased while the expression of dusp6, a negative-feedback gene of Fgf signaling in the midbrain-hindbrain boundary region, was notably induced in ptpro morphants. Further analyses demonstrated that cerebellar defects of ptpro morphants were partially rescued by inhibiting Fgf signaling. Moreover, Ptpro physically interacted with the Fgf receptor 1a (Fgfr1a) and dephosphorylated Fgfr1a in a dose-dependant manner. Therefore, our findings demonstrate that Ptpro activity is required for patterning the zebrafish embryonic brain. Specifically, Ptpro regulates cerebellar formation during zebrafish development through modulating Fgf signaling.

Divergent requirements for fibroblast growth factor signaling in zebrafish maxillary barbel and caudal fin regeneration

The zebrafish maxillary barbel is an integumentary organ containing skin, glands, pigment cells, taste buds, nerves, and endothelial vessels. The maxillary barbel can regenerate (LeClair & Topczewski 2010); however, little is known about its molecular regulation. We have studied fibroblast growth factor (FGF) pathway molecules during barbel regeneration, comparing this system to a well-known regenerating appendage, the zebrafish caudal fin. Multiple FGF ligands (fgf20a, fgf24), receptors (fgfr1-4) and downstream targets (pea3, il17d) are expressed in normal and regenerating barbel tissue, confirming FGF activation. To test if specific FGF pathways were required for barbel regeneration, we performed simultaneous barbel and caudal fin amputations in two temperature-dependent zebrafish lines. Zebrafish homozygous for a point mutation in fgf20a, a factor essential for caudal fin blastema formation, regrew maxillary barbels normally, indicating that the requirement for this ligand is appendage-specific. Global overexpression of a dominant negative FGF receptor, Tg(hsp70l:dn-fgfr1:EGFP)(pd1) completely blocked fin outgrowth but only partially inhibited barbel outgrowth, suggesting reduced requirements for FGFs in barbel tissue. Maxillary barbels expressing dn-fgfr1 regenerated peripheral nerves, dermal connective tissue, endothelial tubes, and a glandular epithelium; in contrast to a recent report in which dn-fgfr1 overexpression blocks pharyngeal taste bud formation in zebrafish larvae (Kapsimali et al. 2011), we observed robust formation of calretinin-positive tastebuds. These are the first experiments to explore the molecular mechanisms of maxillary barbel regeneration. Our results suggest heterogeneous requirements for FGF signaling in the regeneration of different zebrafish appendages (caudal fin versus maxillary barbel) and taste buds of different embryonic origin (pharyngeal endoderm versus barbel ectoderm).

A Sox9/Fgf feed-forward loop maintains pancreatic organ identity

All mature pancreatic cell types arise from organ-specific multipotent progenitor cells. Although previous studies have identified cell-intrinsic and -extrinsic cues for progenitor cell expansion, it is unclear how these cues are integrated within the niche of the developing organ. Here, we present genetic evidence in mice that the transcription factor Sox9 forms the centerpiece of a gene regulatory network that is crucial for proper organ growth and maintenance of organ identity. We show that pancreatic progenitor-specific ablation of Sox9 during early pancreas development causes pancreas-to-liver cell fate conversion. Sox9 deficiency results in cell-autonomous loss of the fibroblast growth factor receptor (Fgfr) 2b, which is required for transducing mesenchymal Fgf10 signals. Likewise, Fgf10 is required to maintain expression of Sox9 and Fgfr2 in epithelial progenitors, showing that Sox9, Fgfr2 and Fgf10 form a feed-forward expression loop in the early pancreatic organ niche. Mirroring Sox9 deficiency, perturbation of Fgfr signaling in pancreatic explants or genetic inactivation of Fgf10 also result in hepatic cell fate conversion. Combined with previous findings that Fgfr2b or Fgf10 are necessary for pancreatic progenitor cell proliferation, our results demonstrate that organ fate commitment and progenitor cell expansion are coordinately controlled by the activity of a Sox9/Fgf10/Fgfr2b feed-forward loop in the pancreatic niche. This self-promoting Sox9/Fgf10/Fgfr2b loop may regulate cell identity and organ size in a broad spectrum of developmental and regenerative contexts.

miR-9 controls the timing of neurogenesis through the direct inhibition of antagonistic factors

The timing of commitment and cell-cycle exit within progenitor populations during neurogenesis is a fundamental decision that impacts both the number and identity of neurons produced during development. We show here that microRNA-9 plays a key role in this process through the direct inhibition of targets with antagonistic functions. Across the ventricular zone of the developing zebrafish hindbrain, miR-9 expression occurs at a range of commitment stages. Abrogating miR-9 function transiently delays cell-cycle exit, leading to the increased generation of late-born neuronal populations. Target protection analyses in vivo identify the progenitor-promoting genes her6 and zic5 and the cell-cycle exit-promoting gene elavl3/HuC as sequential targets of miR-9 as neurogenesis proceeds. We propose that miR-9 activity generates an ambivalent progenitor state poised to respond to both progenitor maintenance and commitment cues, which may be necessary to adjust neuronal production to local extrinsic signals during late embryogenesis.

Signalling from hindbrain boundaries regulates neuronal clustering that patterns neurogenesis

During central nervous system development, neural progenitors are patterned to form discrete neurogenic and non-neurogenic zones. In the zebrafish hindbrain, neurogenesis is organised by Fgf20a emanating from neurons located at each segment centre that inhibits neuronal differentiation in adjacent progenitors. Here, we have identified a molecular mechanism that clusters fgf20a-expressing neurons in segment centres and uncovered a requirement for this positioning in the regulation of neurogenesis. Disruption of hindbrain boundary cell formation alters the organisation of fgf20a-expressing neurons, consistent with a role of chemorepulsion from boundaries. The semaphorins Sema3fb and Sema3gb, which are expressed by boundary cells, and their receptor Nrp2a are required for clustering of fgf20a-expressing neurons at segment centres. The dispersal of fgf20a-expressing neurons that occurs following the disruption of boundaries or of Sema3fb/Sema3gb signalling leads to reduced FGF target gene expression in progenitors and an increased number of differentiating neurons. Sema3 signalling from boundaries thus links hindbrain segmentation to the positioning of fgf20a-expressing neurons that regulates neurogenesis.

From cradle to grave: the multiple roles of fibroblast growth factors in neural development

The generation of a functional nervous system involves a multitude of steps that are controlled by just a few families of extracellular signaling molecules. Among these, the fibroblast growth factor (FGF) family is particularly prominent for the remarkable diversity of its functions. FGFs are best known for their roles in the early steps of patterning of the neural primordium and proliferation of neural progenitors. However, other equally important functions have emerged more recently, including in the later steps of neuronal migration, axon navigation, and synaptogenesis. We review here these diverse functions and discuss the mechanisms that account for this unusual range of activities. FGFs are essential components of most protocols devised to generate therapeutically important neuronal populations in vitro or to stimulate neuronal repair in vivo. How FGFs promote the development of the nervous system and maintain its integrity will thus remain an important focus of research in the future.

swap70 promotes neural precursor cell cycle exit and oligodendrocyte formation

Multipotent neural precursors produce oligodendrocyte lineage cells, which then migrate throughout the central nervous system and extend multiple, long membrane processes to wrap and myelinate axons. These dynamic cellular behaviors imply dynamic regulation of the cytoskeleton. In a previous microarray screen for new oligodendrocyte genes we identified swap70, which encodes a protein with domains that predict numerous signaling activities. Because mouse Swap70 can promote cell motility by functioning as a guanine nucleotide exchange factor for Rac1, we hypothesized that zebrafish Swap70 promotes oligodendrocyte progenitor cell (OPC) motility and axon wrapping. To test this we investigated Swap70 localization in OPCs and differentiating oligodendrocytes and we performed a series of gain and loss of function experiments. Our tests of gene function did not provide evidence that Swap70 regulates oligodendrocyte lineage cell behavior. Instead, we found that swap70 deficient larvae had excess neural precursors and a deficit of OPCs. Cells associated with neural proliferative zones express swap70. Therefore, our data reveal a potential new role for Swap70 in regulating transition of dividing neural precursors to specified OPCs.

Use of different morphological techniques to analyze the cellular composition of the adult zebrafish optic tectum

Cellular composition of the adult zebrafish (Danio rerio) optic tectal cortex was examined in this study. Morphological techniques such as 1 μm thick serial plastic sections stained with osmium tetroxide and toluidine blue, modified rapid Golgi silver impregnation, GFAP immunohistochemistry, confocal microscopy, as well as scanning and transmission electron microscopy were used. Neuronal and glial components are described and the layers of the cortex are revisited. Specific neuronal arrangements as well as unique glial/ependymal cells are described. A three dimensional rendering of the astrocytic fiber arrangement in the marginal zone is presented and a composite drawing summarizes the cellular composition of the optic tectum.

Fibroblast growth factor signaling is required for the generation of oligodendrocyte progenitors from the embryonic forebrain

Fibroblast growth factors (FGFs) comprise a family of developmental regulators implicated in a wide variety of neurological functions. FGF receptors 1, 2, and 3 (Fgfrs) are expressed in the embryonic forebrain, including regions overlapping with ventral sites of oligodendrocyte progenitor (OLP) generation. Although FGF signaling is known to influence the proliferation of OLPs in vitro, functions of different Fgfrs in vivo are lacking. Here, we examined single and double mutants with conditional disruption of Fgfrs, specifically in the embryonic forebrain, to investigate the effect of FGFs on the generation and proliferation of OLPs in vivo. FGF signaling, through cooperation between Fgfr1 and Fgfr2 but not Fgfr3, is required for the initial generation of OLPs in the mouse ventral forebrain, with Fgfr1 being a stronger inducer than Fgfr2. In cultures derived from embryonic mutant forebrains or from normal forebrains grown in the presence of Fgfr inhibitor, a strong attenuation of OLP generation was observed, supporting the role of FGF signaling in vivo. Contrary to in vitro findings, Fgfr1 and Fgfr2 signaling is not required for the proliferation of OLPs in vivo. Finally, failure of OLP generation in the Fgfr mutants occurred without loss of sonic hedgehog (Shh) signaling; and pharmacological inhibition of either Fgfr or hedgehog signaling in parallel cultures strongly inhibited OLP generation, suggesting that Fgfrs cooperate with Shh to generate OLPs. Overall, our results reveal for the first time an essential role of FGF signaling in vivo, where the three Fgfrs differentially control the normal generation of OLPs from the embryonic ventral forebrain.

Developmental genetics of vertebrate glial-cell specification

Oligodendrocytes and astrocytes are macroglial cells of the vertebrate central nervous system. These cells have diverse roles in the maintenance of neurological function. In the embryo, the genetic mechanisms that underlie the specification of macroglial precursors in vivo appear strikingly similar to those that regulate the development of the diverse neuron types. The switch from producing neuronal to glial subtype-specific precursors can be modelled as an interplay between region-restricted components and temporal regulators that determine neurogenic or gliogenic phases of development, contributing to glial diversity. Gaining insight into the developmental genetics of macroglia has great potential to improve our understanding of a variety of neurological disorders in humans.

Zebrafish: an integrative system for neurogenomics and neurosciences

Rapid technological advances over the past decade have moved us closer to a high throughput molecular approach to neurobiology, where we see the merging of neurogenetics, genomics, physiology, imaging and pharmacology. This is the case more in zebrafish than in any other model organism commonly used. Recent improvements in the generation of transgenic zebrafish now allow genetic manipulation and live imaging of neuronal development and function in early embryonic, larval, and adult animals. The sequenced zebrafish genome and comparative genomics give unprecedented insights into genome evolution and its relation to genome structure and function. There is now information on embryonic and larval expression of over 12,000 genes and just under 1000 mutant phenotypes. We review the remarkable similarity of the zebrafish genetic blueprint for the nervous system to that of mammals and assess recent technological advances that make the zebrafish a model of choice for elucidating the development and function of neuronal circuitry, transgene-based neuroanatomy, and small molecule neuropharmacology.