Expression of FGFR1, FGFR2 and FGFR3 during early neural development in the chick embryo

Joint dimension reduction and clustering analysis of single-cell RNA-seq and spatial transcriptomics data

Dimension reduction and (spatial) clustering is usually performed sequentially; however, the low-dimensional embeddings estimated in the dimension-reduction step may not be relevant to the class labels inferred in the clustering step. We therefore developed a computation method, Dimension-Reduction Spatial-Clustering (DR-SC), that can simultaneously perform dimension reduction and (spatial) clustering within a unified framework. Joint analysis by DR-SC produces accurate (spatial) clustering results and ensures the effective extraction of biologically informative low-dimensional features. DR-SC is applicable to spatial clustering in spatial transcriptomics that characterizes the spatial organization of the tissue by segregating it into multiple tissue structures. Here, DR-SC relies on a latent hidden Markov random field model to encourage the spatial smoothness of the detected spatial cluster boundaries. Underlying DR-SC is an efficient expectation-maximization algorithm based on an iterative conditional mode. As such, DR-SC is scalable to large sample sizes and can optimize the spatial smoothness parameter in a data-driven manner. With comprehensive simulations and real data applications, we show that DR-SC outperforms existing clustering and spatial clustering methods: it extracts more biologically relevant features than conventional dimension reduction methods, improves clustering performance, and offers improved trajectory inference and visualization for downstream trajectory inference analyses.

Importance of clitellar tissue in the regeneration ability of earthworm Eudrilus eugeniae

Among the annelids, earthworms are renowned for their phenomenal ability to regenerate the lost segments. The adult earthworm Eudrilus eugeniae contains 120 segments and the body segments of the earthworm are divided into pre-clitellar, clitellar and post-clitellar segments. The present study denoted that clitellum plays vital role in the successful regeneration of the species. We have performed histological studies to identify among the three skin layers of the earthworm, which cellular layer supports the blastema formation and regeneration of the species. The histological evidences denoted that the proliferation of the longitudinal cell layer at the amputation site is crucial for the successful regeneration of the earthworm and it takes place only in the presence of an intact clitellum. Besides we have performed clitellar transcriptome analysis of the earthworm Eudrilus eugeniae to monitor the key differentially expressed genes and their associated functions and pathways controlling the clitellar tissue changes during both anterior and posterior regeneration of the earthworm. A total of 4707 differentially expressed genes (DEGs) were identified between the control clitellum and clitellum of anterior regenerated earthworms and 4343 DEGs were detected between the control clitellum and clitellum of posterior regenerated earthworms. The functional enrichment analysis confirmed the genes regulating the muscle mass shape and structure were significantly downregulated and the genes associated with response to starvation and anterior-posterior axis specification were significantly upregulated in the clitellar tissue during both anterior and posterior regeneration of the earthworm. The RNA sequencing data of clitellum and the comparative transcriptomic analysis were helpful to understand the complex regeneration process of the earthworm.

An Update on the Molecular Mechanism of the Vertebrate Isthmic Organizer Development in the Context of the Neuromeric Model

A crucial event during the development of the central nervous system (CNS) is the early subdivision of the neural tube along its anterior-to-posterior axis to form neuromeres, morphogenetic units separated by transversal constrictions and programed for particular genetic cascades. The narrower portions observed in the developing neural tube are responsible for relevant cellular and molecular processes, such as clonal restrictions, expression of specific regulatory genes, and differential fate specification, as well as inductive activities. In this developmental context, the gradual formation of the midbrain-hindbrain (MH) constriction has been an excellent model to study the specification of two major subdivisions of the CNS containing the mesencephalic and isthmo-cerebellar primordia. This MH boundary is coincident with the common Otx2-(midbrain)/Gbx2-(hindbrain) expressing border. The early interactions between these two pre-specified areas confer positional identities and induce the generation of specific diffusible morphogenes at this interface, in particular FGF8 and WNT1. These signaling pathways are responsible for the gradual histogenetic specifications and cellular identity acquisitions with in the MH domain. This review is focused on the cellular and molecular mechanisms involved in the specification of the midbrain/hindbrain territory and the formation of the isthmic organizer. Emphasis will be placed on the chick/quail chimeric experiments leading to the acquisition of the first fate mapping and experimental data to, in this way, better understand pioneering morphological studies and innovative gain/loss-of-function analysis.

Targeting fibroblast growth factor receptors to combat aggressive ependymoma

Ependymomas (EPN) are central nervous system tumors comprising both aggressive and more benign molecular subtypes. However, therapy of the high-risk subtypes posterior fossa group A (PF-A) and supratentorial RELA-fusion positive (ST-RELA) is limited to gross total resection and radiotherapy, as effective systemic treatment concepts are still lacking. We have recently described fibroblast growth factor receptors 1 and 3 (FGFR1/FGFR3) as oncogenic drivers of EPN. However, the underlying molecular mechanisms and their potential as therapeutic targets have not yet been investigated in detail. Making use of transcriptomic data across 467 EPN tissues, we found that FGFR1 and FGFR3 were both widely expressed across all molecular groups. FGFR3 mRNA levels were enriched in ST-RELA showing the highest expression among EPN as well as other brain tumors. We further identified high expression levels of fibroblast growth factor 1 and 2 (FGF1, FGF2) across all EPN subtypes while FGF9 was elevated in ST-EPN. Interrogation of our EPN single-cell RNA-sequencing data revealed that FGFR3 was further enriched in cycling and progenitor-like cell populations. Corroboratively, we found FGFR3 to be predominantly expressed in radial glia cells in both mouse embryonal and human brain datasets. Moreover, we detected alternative splicing of the FGFR1/3-IIIc variant, which is known to enhance ligand affinity and FGFR signaling. Dominant-negative interruption of FGFR1/3 activation in PF-A and ST-RELA cell models demonstrated inhibition of key oncogenic pathways leading to reduced cell growth and stem cell characteristics. To explore the feasibility of therapeutically targeting FGFR, we tested a panel of FGFR inhibitors in 12 patient-derived EPN cell models revealing sensitivity in the low-micromolar to nano-molar range. Finally, we gain the first clinical evidence for the activity of the FGFR inhibitor nintedanib in the treatment of a patient with recurrent ST-RELA. Together, these preclinical and clinical data suggest FGFR inhibition as a novel and feasible approach to combat aggressive EPN.

Increased expression of fibroblast growth factor 13 in cortical lesions of the focal cortical dysplasia

Focal cortical dysplasias (FCDs) are well recognized as important causes of medically intractable epilepsy in both children and adults. To explore the potential role of fibroblast growth factor 13 (FGF13) in intractable epilepsy caused by FCDs, we examined the expression of FGF13 in cortical lesions from 23 patients with FCD type Ia (FCDIa), 24 patients with FCD type IIa (FCDIIa), and 12 patients with FCD type IIb (FCDIIb), and we compared the results with the FGF13 expression levels in control cortex (CTX) brain tissues from 12 nonepileptic normal subjects. Both the mRNA levels and protein levels of FGF13 were significantly higher in the cortical lesions from patients with FCD than in the control cortices. The immunohistochemical results showed that strong FGF13 immunoreactivity was observed in misshapen cells, including neuronal microcolumns, hypertrophic neurons, dysmorphic neurons, and most balloon cells. Moreover, double-label immunofluorescence analyses confirmed that FGF13 was mainly localized in neurons and nearly absent in glia-like cells. Taken together, our results suggest that the overexpression of FGF13 in FCDs and the cell-specific distribution patterns of FGF13 in misshapen neurons in FCDs could potentially contribute to intractable epilepsy caused by FCDs.

The development, patterning and evolution of neural crest cell differentiation into cartilage and bone

Neural crest cells are a vertebrate-specific migratory, multipotent cell population that give rise to a diverse array of cells and tissues during development. Cranial neural crest cells, in particular, generate cartilage, bone, tendons and connective tissue in the head and face as well as neurons, glia and melanocytes. In this review, we focus on the chondrogenic and osteogenic potential of cranial neural crest cells and discuss the roles of Sox9, Runx2 and Msx1/2 transcription factors and WNT, FGF and TGFβ signaling pathways in regulating neural crest cell differentiation into cartilage and bone. We also describe cranioskeletal defects and disorders arising from gain or loss-of-function of genes that are required for patterning and differentiation of cranial neural crest cells. Finally, we discuss the evolution of skeletogenic potential in neural crest cells and their function as a conduit for intraspecies and interspecies variation, and the evolution of craniofacial novelties.

Possible roles of parathyroid hormone, 1.25(OH)2D3, and fibroblast growth factor 23 on genes controlling calcium metabolism across different tissues of the laying hen

This study provides an integrative description of candidate gene expression across tissues involved in calcium (Ca) metabolism during the egg laying cycle, using the well-defined model of Ca supply as fine or coarse particles of calcium carbonate (CaCO₃). Plasma and tissue samples were collected from hens at the peak of laying at 0 to 1, 9 to 10, and 18 to 19 h postovulation (PO). After mRNA preparation from the parathyroid gland, medullary bone, liver, kidney, duodenum, and jejunum, gene expressions were quantified using RT-qPCR. The highest levels of parathyroid hormone (PTH) mRNA in the parathyroid gland (P < 0.05), and of the active form of vitamin D₃ 1.25(OH)₂D₃ in the plasma (P < 0.01) were observed at 18 to 19 h PO. During this active phase of eggshell formation, bone resorption was attested to high levels of plasma inorganic phosphorus (iP) and the receptor activation of nuclear factor-κB expression in the bone (P < 0.001 and P < 0.05, respectively). At this stage, 5 genes of the transcellular and the paracellular Ca absorption pathways in the intestine (P < 0.05) and the Ca channel transient receptor potential cation channel subfamily V member 5 (P < 0.05), involved in its reabsorption in the kidney, were overexpressed. At 0 to 1 h PO during the subsequent daylight period, 2 candidates of the transcellular and the paracellular Ca pathways (P < 0.05) remained at high levels in the intestine, while calbindin D 28K expression was the highest in the kidney (P < 0.05). As PTH mRNA and 1.25(OH)₂D₃ were low, bone accretion was likely active at this stage. The phosphaturic hormone fibroblast growth factor 23 (FGF23) was overexpressed at 18 to 19 h PO (P < 0.05) in the bone when plasma iP was high, which suggested a role in the subsequent reduction of P reabsorption in the kidney, as attested to the decreased expression of P cotransporters, leading to iP clearance from the plasma at 0 to 1 h PO (P < 0.05). The low levels of 1.25(OH)₂D₃ at this stage coincided with increased expression of the 24-hydroxylase gene in the kidney (P < 0.05). In hens fed fine particles of CaCO₃, higher plasma levels of 1,25(OH)₂D₃ and higher expression of several genes involved in bone turnover reflected a stronger challenge to Ca homeostasis. Altogether, these data support the hypothesis that FGF23 could drive vitamin D metabolism in the laying hen, as previously documented in other species and explain the tight link between P and Ca metabolisms.

Thalidomide and Its Analogs Differentially Target Fibroblast Growth Factor Receptors: Thalidomide Suppresses FGFR Gene Expression while Pomalidomide Dampens FGFR2 Activity

Thalidomide is an infamous teratogen and it is continuously being explored for its anticancer properties. Fibroblast growth factor receptors (FGFRs) are implicated in embryo development and cancer pathophysiology. With striking similarities observed between FGFR implicated conditions and thalidomide embryopathy, we hypothesized thalidomide targets FGFRs. We utilized three different cell lines and chicken embryo model to investigate the effects of thalidomide and analogs on FGFR expression. We performed molecular docking, KINOMEscan analysis, and kinase activity assays to study the drug-protein interactions. The expression of FGFR1 and FGFR2 was differentially regulated by all the three drugs in cells as well as in developing organs. Transcriptome analysis of thalidomide-treated chick embryo strongly suggests the modulation of FGFR signaling and key transcription factors. Corroboration with previous studies suggests that thalidomide might affect FGFR expression through the transcription factor, E2F1. At the protein level, molecular docking predicted all three analogs to interact with lysine residue at 517th and 508th positions of FGFR2 and FGFR3, respectively. This lysine coordinates the ATP binding site of FGFR, thus hinting at the possible perturbation of FGFR activity by thalidomide. Kinome analysis revealed that kinase activities of FGFR2 and FGFR3 (G697C) reduced by 31% and 65%, respectively, in the presence of 10 μM thalidomide. Further, we checked and confirmed that the analogs inhibited the FGFR2 kinase activity in a dose-dependent manner. This study suggests that FGFRs could be potential targets of thalidomide and the two analogs, and also endorses the link between the teratogenicity and antitumor activities of the drugs.

CD200 facilitates the isolation of corneal epithelial cells derived from human pluripotent stem cells

The in vitro induction of corneal epithelial cells (CECs) from human induced pluripotent stem cells (iPSCs) represents a new strategy for obtaining CE stem/progenitor cells for the surgical reconstruction of a diseased or injured ocular surface. The clinical promise of this strategy is considerable, but if the approaches’ potential is to be realised, robust methods for the purification of iPSC-derived CE lineage cells need to be developed to avoid contamination with other cells that may carry the risk of unwanted side effects, such as tumorigenesis. Experiments conducted here revealed that during CEC isolation, CD200-negative selection using a cell sorter considerably reduced the contamination of the cell population with various non-CECs compared with what could be achieved using TRA-1-60, a conventional negative marker for CECs. Furthermore, CD200-negative sorting did not affect the yield of CECs nor that of their stem/progenitor cells. Single-cell gene expression analysis for CEC sheets obtained using CD200-negative sorting showed that all analysed cells were CE-lineage cells, expressing PAX6, delta-N p63, and E-cadherin. Non-CECs, on the other hand, expressed non-CEC genes such as FGFR1 and RPE65. CD200, thus, represents a robust negative marker for purification of induced CE lineage cells, which is expressed by undifferentiated iPSCs and non-CECs, including iPSC-derived neural and retinal cells.

Intracranial Venous Hypertension in Craniosynostosis: Mechanistic Underpinnings and Therapeutic Implications

Patients with complex, multisutural, and syndromic craniosynostosis (CSO) frequently exhibit intracranial hypertension. The intracranial hypertension cannot be entirely attributed to the craniocephalic disproportion with calvarial restriction because cranial vault expansion has not consistently alleviated elevated intracranial pressure. Evidence has most strongly supported a multifactorial interaction, including venous hypertension along with other pathogenic processes. Patients with CSO exhibit marked venous anomalies, including stenosis of the jugular-sigmoid complex, transverse sinuses, and extensive transosseous venous collaterals. These abnormal intracranial-extracranial occipital venous collaterals might represent anomalous development, with persistence and subsequent enlargement of channels normally present in the fetus, either as a primary defect or as nonregression in response to failure of the development of the jugular-sigmoid complexes. It has been suggested by some investigators that venous hypertension in patients with CSO could be treated directly via jugular foraminoplasty, venous stenting, or jugular venous bypass, although these options are not in common clinical practice. Obstructive sleep apnea, occurring as a consequence of midface hypoplasia, can also contribute to intracranial hypertension in patients with syndromic CSO. Thus, correction of facial deformities, as well as posterior fossa decompression, could also play important roles in the treatment of intracranial hypertension. Determining the precise mechanistic underpinnings underlying intracranial hypertension in any given patient with CSO requires individualized evaluation and management.

The Etiology of Neuronal Development in Craniosynostosis: A Working Hypothesis

Craniosynostosis is one of the most common craniofacial conditions treated by neurologic and plastic surgeons. In addition to disfigurement, children with craniosynostosis experience significant cognitive dysfunction later in life. Surgery is performed in infancy to correct skull deformity; however, the field is at a crossroads regarding the best approach for correction. Since the cause of brain dysfunction in these patients has remained uncertain, the role and type of surgery might have in attenuating the later-observed cognitive deficits through impact on the brain has been unclear. Recently, however, advances in imaging such as event-related potentials, diffusion tensor imaging, and functional MRI, in conjunction with more robust clinical studies, are providing important insight into the potential etiologies of brain dysfunction in syndromic and nonsyndromic craniosynostosis patients. This review aims to outline the cause(s) of such brain dysfunction including the role extrinsic vault constriction might have on brain development and the current evidence for an intrinsic modular developmental error in brain development. Illuminating the cause of brain dysfunction will identify the role of surgery can play in improving observed functional deficits and thus direct optimal primary and adjuvant treatment.

Growth factors FGF8 and FGF2 and their receptor FGFR1, transcriptional factors Msx-1 and MSX-2, and apoptotic factors p19 and RIP5 participate in the early human limb development

The expression pattern of fibroblast growth factors FGF8 and FGF2 and their receptor FGFR1, transcription factors MSX-1 and MSX-2, as well as cell proliferation (Ki-67) and cell death associated caspase-3, p19 and RIP5 factors were analyzed in histological sections of eight 4th-9th-weeks developing human limbs by immunohistochemistry and semi-thin sectioning. Increasing expression of all analyzed factors (except FGF8) characterized both the multilayered human apical ectodermal ridge (AER), sub-ridge mesenchyme (progress zone) and chondrocytes in developing human limbs. While cytoplasmic co-expression of MSX-1 and MSX-2 was observed in both limb epithelium and mesenchyme, p19 displayed strong cytoplasmic expression in non-proliferating cells. Nuclear expression of Ki-67 proliferating cells, and partly of MSX-1 and MSX-2 was detected in the whole limb primordium. Strong expression of factors p19 and RIP5, both in the AER and mesenchyme of human developing limbs indicates their possible involvement in control of cell senescence and cell death. In contrast to animal studies, expression of FGFR1 in the surface ectoderm and p19 in the whole limb primordium might reflect interspecies differences in limb morphology. Expression of FGF2 and downstream RIP5 gene, and transcription factors Msx-1 and MSX-2 did not show human-specific changes in expression pattern. Based on their spatio-temporal expression during human limb development, our study indicates role of FGFs and Msx genes in stimulation of cell proliferation, limb outgrowth, digit elongation and separation, and additionally MSX-2 in control of vasculogenesis. The cascade of orchestrated gene expressions, including the analyzed developmental factors, jointly contribute to the complex human limb development.

Mouse embryonic dorsal root ganglia contain pluripotent stem cells that show features similar to embryonic stem cells and induced pluripotent stem cells

In the present study, we showed that the dorsal root ganglion (DRG) in the mouse embryo contains pluripotent stem cells (PSCs) that have developmental capacities equivalent to those of embryonic stem (ES) cells and induced pluripotent stem cells. Mouse embryonic DRG cells expressed pluripotency-related transcription factors [octamer-binding transcription factor 4, SRY (sex determining region Y)-box containing gene (Sox) 2, and Nanog] that play essential roles in maintaining the pluripotency of ES cells. Furthermore, the DRG cells differentiated into ectoderm-, mesoderm- and endoderm-derived cells. In addition, these cells produced primordial germ cell-like cells and embryoid body-like spheres. We also showed that the combination of leukemia inhibitor factor/bone morphogenetic protein 2/fibroblast growth factor 2 effectively promoted maintenance of the pluripotency of the PSCs present in DRGs, as well as that of neural crest-derived stem cells (NCSCs) in DRGs, which were previously shown to be present there. Furthermore, the expression of pluripotency-related transcription factors in the DRG cells was regulated by chromodomain helicase DNA-binding protein 7 and Sox10, which are indispensable for the formation of NCSCs, and vice versa. These findings support the possibility that PSCs in mouse embryonic DRGs are NCSCs.

Generation of thalamic neurons from mouse embryonic stem cells

The thalamus is a diencephalic structure that plays crucial roles in relaying and modulating sensory and motor information to the neocortex. The thalamus develops in the dorsal part of the neural tube at the level of the caudal forebrain. However, the molecular mechanisms that are essential for thalamic differentiation are still unknown. Here, we have succeeded in generating thalamic neurons from mouse embryonic stem cells (mESCs) by modifying the default method that induces the most-anterior neural type in self-organizing culture. A low concentration of the caudalizing factor insulin and a MAPK/ERK kinase inhibitor enhanced the expression of the caudal forebrain markers Otx2 and Pax6. BMP7 promoted an increase in thalamic precursors such as Tcf7l2⁺/Gbx2⁺ and Tcf7l2⁺/Olig3⁺ cells. mESC thalamic precursors began to express the glutamate transporter vGlut2 and the axon-specific marker VGF, similar to mature projection neurons. The mESC thalamic neurons extended their axons to cortical layers in both organotypic culture and subcortical transplantation. Thus, we have identified the minimum elements sufficient for in vitro generation of thalamic neurons. These findings expand our knowledge of thalamic development.

Analysis of the Fgfr2C342Y mouse model shows condensation defects due to misregulation of Sox9 expression in prechondrocytic mesenchyme

Syndromic craniosynostosis caused by mutations in FGFR2 is characterised by developmental pathology in both endochondral and membranous skeletogenesis. Detailed phenotypic characterisation of features in the membranous calvarium, the endochondral cranial base and other structures in the axial and appendicular skeleton has not been performed at embryonic stages. We investigated bone development in the Crouzon mouse model (Fgfr2^C342Y) at pre- and post-ossification stages to improve understanding of the underlying pathogenesis. Phenotypic analysis was performed by whole-mount skeletal staining (Alcian Blue/Alizarin Red) and histological staining of sections of CD1 wild-type (WT), Fgfr2^C342Y/+ heterozygous (HET) and Fgfr2^C342Y/C342Y homozygous (HOM) mouse embryos from embryonic day (E)12.5-E17.5 stages. Gene expression (Sox9, Shh, Fgf10 and Runx2) was studied by in situ hybridisation and protein expression (COL2A1) by immunohistochemistry. Our analysis has identified severely decreased osteogenesis in parts of the craniofacial skeleton together with increased chondrogenesis in parts of the endochondral and cartilaginous skeleton in HOM embryos. The Sox9 expression domain in tracheal and basi-cranial chondrocytic precursors at E13.5 in HOM embryos is increased and expanded, correlating with the phenotypic observations which suggest FGFR2 signalling regulates Sox9 expression. Combined with abnormal staining of type II collagen in pre-chondrocytic mesenchyme, this is indicative of a mesenchymal condensation defect. An expanded spectrum of phenotypic features observed in the Fgfr2^C342Y/C342Y mouse embryo paves the way towards better understanding the clinical attributes of human Crouzon-Pfeiffer syndrome. FGFR2 mutation results in impaired skeletogenesis; however, our findings suggest that many phenotypic aberrations stem from a primary failure of pre-chondrogenic/osteogenic mesenchymal condensation and link FGFR2 to SOX9, a principal regulator of skeletogenesis.

ProNodal acts via FGFR3 to govern duration of Shh expression in the prechordal mesoderm

The secreted glycoprotein sonic hedgehog (Shh) is expressed in the prechordal mesoderm, where it plays a crucial role in induction and patterning of the ventral forebrain. Currently little is known about how Shh is regulated in prechordal tissue. Here we show that in the embryonic chick, Shh is expressed transiently in prechordal mesoderm, and is governed by unprocessed Nodal. Exposure of prechordal mesoderm microcultures to Nodal-conditioned medium, the Nodal inhibitor CerS, or to an ALK4/5/7 inhibitor reveals that Nodal is required to maintain both Shh and Gsc expression, but whereas Gsc is largely maintained through canonical signalling, Nodal signals through a non-canonical route to maintain Shh. Further, Shh expression can be maintained by a recombinant Nodal cleavage mutant, proNodal, but not by purified mature Nodal. A number of lines of evidence suggest that proNodal acts via FGFR3. ProNodal and FGFR3 co-immunoprecipitate and proNodal increases FGFR3 tyrosine phosphorylation. In microcultures, soluble FGFR3 abolishes Shh without affecting Gsc expression. Further, prechordal mesoderm cells in which Fgfr3 expression is reduced by Fgfr3 siRNA fail to bind to proNodal. Finally, targeted electroporation of Fgfr3 siRNA to prechordal mesoderm in vivo results in premature Shh downregulation without affecting Gsc. We report an inverse correlation between proNodal-FGFR3 signalling and pSmad1/5/8, and show that proNodal-FGFR3 signalling antagonises BMP-mediated pSmad1/5/8 signalling, which is poised to downregulate Shh. Our studies suggest that proNodal/FGFR3 signalling governs Shh duration by repressing canonical BMP signalling, and that local BMPs rapidly silence Shh once endogenous Nodal-FGFR3 signalling is downregulated.

Highlighted article: In the chick prechordal mesoderm, the Nodal precursor proNodal acts via a non-canonical route to inhibit BMP signalling and thus maintain Shh expression

The formation of the foramen magnum and its role in developing ventriculomegaly and Chiari I malformation in children with craniosynostosis syndromes

OBJECT

Craniosynostosis syndromes are characterized by prematurely fused skull sutures, however, less is known about skull base synchondroses. This study evaluates how foramen magnum (FM) size, and closure of its intra-occipital synchondroses (IOS) differ between patients with different craniosynostosis syndromes and control subjects; and whether this correlates to ventriculomegaly and/or Chiari malformation type I (CMI), intracranial disturbances often described in these patients.

METHODS

Surface area and anterior-posterior (A-P) diameter were measured in 175 3D-CT scans of 113 craniosynostosis patients, and in 53 controls (0-10 years old). Scans were aligned in a 3D multiplane-platform. The frontal and occipital horn ratio was used as an indicator of ventricular volume, and the occurrence of CMI was recorded. Synchondroses were studied in scans with a slice thickness ≤1.25 mm. A generalized linear mixed model and a repeated measures ordinal logistic regression model were used to study differences.

RESULTS

At birth, patients with craniosynostosis syndromes have a smaller FM than controls (p < 0.05). This is not related to the presence of CMI (p = 0.36). In Crouzon-Pfeiffer patients the anterior and posterior IOS fused prematurely (p < 0.01), and in Apert patients only the posterior IOS fused prematurely (p = 0.028).

CONCLUSION

The FM is smaller in patients with craniosynostosis syndromes than in controls, and is already smaller at birth. In addition to the timing of IOS closure, other factors may influence FM size.

Synergistic interaction between the fibroblast growth factor and bone morphogenetic protein signaling pathways in lens cells

Relatively little is known about how receptor tyrosine kinase ligands can positively cooperate with BMP signaling. Primary cultures of lens cells were used to reveal an unprecedented type of cross-talk between the canonical FGF and BMP signaling pathways that regulates lens cell differentiation and intercellular coupling.

Fibroblast growth factors (FGFs) play a central role in two processes essential for lens transparency—fiber cell differentiation and gap junction–mediated intercellular communication (GJIC). Using serum-free primary cultures of chick lens epithelial cells (DCDMLs), we investigated how the FGF and bone morphogenetic protein (BMP) signaling pathways positively cooperate to regulate lens development and function. We found that culturing DCDMLs for 6 d with the BMP blocker noggin inhibits the canonical FGF-to-ERK pathway upstream of FRS2 activation and also prevents FGF from stimulating FRS2- and ERK-independent gene expression, indicating that BMP signaling is required at the level of FGF receptors. Other experiments revealed a second type of BMP/FGF interaction by which FGF promotes expression of BMP target genes as well as of BMP4. Together these studies reveal a novel mode of cooperation between the FGF and BMP pathways in which BMP keeps lens cells in an optimally FGF-responsive state and, reciprocally, FGF enhances BMP-mediated gene expression. This interaction provides a mechanistic explanation for why disruption of either FGF or BMP signaling in the lens leads to defects in lens development and function.

Venous hypertension in syndromic and complex craniosynostosis: the abnormal anatomy of the jugular foramen and collaterals

Why craniosynostosis patients develop elevated intracranial pressure (ICP) is still a mystery. Our aim was to investigate jugular foramen size and its relation to venous hypertension and elevated ICP. Secondly, we evaluated whether occipital collateral veins develop as a compensatory mechanism for elevated ICP. We conducted a prospective study in 41 children with craniosynostosis who underwent a 3D-CT-angiography. We evaluated the anatomical course of the jugular vein, the diameter of the jugular foramen and the relation to the presence of papilledema. Additionally, we studied the anatomical variations of the cerebral venous drainage system. The diameter of the jugular foramen was significantly smaller in our patients. Abnormal venous collaterals were most often observed in patients with Apert, Crouzon-Pfeiffer and Saethre-Chotzen syndrome, even in children under two years of age. There was no significant difference in the number of collateral veins in patients with or without papilledema. Collaterals appear to reflect an inborn abnormality of the venous system, rather than a compensating mechanism for elevated ICP. This study confirms the presence of jugular foraminal narrowing in craniosynostosis patients and an abnormal venous system, which may predispose to elevated ICP.

LEVEL OF EVIDENCE

Diagnostic II.

Independently specified Atoh1 domains define novel developmental compartments in rhombomere 1

The rhombic lip gives rise to neuronal populations that contribute to cerebellar, proprioceptive and interoceptive networks. Cell production depends on the expression of the basic helix-loop-helix (bHLH) transcription factor Atoh1. In rhombomere 1, Atoh1-positive cells give rise to both cerebellar neurons and extra-cerebellar nuclei in ventral hindbrain. The origin of this cellular diversity has previously been attributed to temporal signals rather than spatial patterning. Here, we show that in both chick and mouse the cerebellar Atoh1 precursor pool is partitioned into initially cryptic spatial domains that reflect the activity of two different organisers: an isthmic Atoh1 domain, which gives rise to isthmic nuclei, and the rhombic lip, which generates deep cerebellar nuclei and granule cells. We use a combination of in vitro explant culture, genetic fate mapping and gene overexpression and knockdown to explore the role of isthmic signalling in patterning these domains. We show that an FGF-dependent isthmic Atoh1 domain is the origin of distinct populations of Lhx9-positive neurons in the extra-cerebellar isthmic nuclei. In the cerebellum, ectopic FGF induces proliferation while blockade reduces the length of the cerebellar rhombic lip. FGF signalling is not required for the specification of cerebellar cell types from the rhombic lip and its upregulation inhibits their production. This suggests that although the isthmus regulates the size of the cerebellar anlage, the downregulation of isthmic FGF signals is required for induction of rhombic lip-derived cerebellar neurons.

Prolyl isomerase Pin1-mediated conformational change and subnuclear focal accumulation of Runx2 are crucial for fibroblast growth factor 2 (FGF2)-induced osteoblast differentiation

Fibroblast growth factor 2 (FGF2) signaling plays a pivotal role in bone growth/differentiation through the activation of osteogenic master transcription factor Runx2, which is mediated by the ERK/MAPK-dependent phosphorylation and the p300-dependent acetylation of Runx2. In this study, we found that Pin1-dependent isomerization of Runx2 is the critical step for FGF2-induced Runx2 transactivation function. We identified four serine or threonine residues in the C-terminal domain of Runx2 that are responsible for Pin1 binding and structural modification. Confocal imaging studies indicated that FGF2 treatment strongly stimulated the focal accumulation of Pin1 in the subnuclear area, which recruited Runx2. In addition, active forms of RNA polymerase-II also colocalized in the same subnuclear compartment. Dipentamethylene thiuram monosulfide, a Pin1 inhibitor, strongly attenuated their focal accumulation as well as Runx2 transactivation activity. The Pin1-mediated structural modification of Runx2 is an indispensable step connecting phosphorylation and acetylation and, consequently, transcriptional activation of Runx2 by FGF signaling. Thus, the modulation of Pin1 activity may be a target for the regulation of bone formation.

FGF2 delays tectal neurogenesis, increases tectal cell numbers, and alters tectal lamination in embryonic chicks

Intraventricular injections of the fibroblast growth factor 2 (FGF2) are known to increase the size of the optic tectum in embryonic chicks. Here we show that this increase in tectum size is due to a delay in tectal neurogenesis, which by definition extends the proliferation of tectal progenitors. Specifically, we use cumulative labeling with the thymidine analog EdU to demonstrate that FGF2 treatment on embryonic day 4 (ED4) reduces the proportion and absolute number of unlabeled cells in the rostroventral tectum when EdU infusions are begun on ED5, as one would expect if FGF2 retards tectal neurogenesis. We also examined FGF2′s effect on neurogenesis in the caudodorsal tectum, which is born 2-3 days after the rostroventral tectum, by combining FGF2 treatment on ED4 with EDU infusions beginning on ED8. Again, FGF2 treatment reduced the proportion and number of EdU-negative (i.e., unlabeled) cells, consistent with a delay in neurogenesis. Collectively, these data indicate FGF2 in embryonic chicks delays neurogenesis throughout much of the tectum and continues to do so for several days after the FGF2 injection. One effect of this delay in neurogenesis is that tectal cell numbers more than double. In addition, tectal laminae that are born early in development become abnormally thin and cell-sparse after FGF2 treatment, whereas late-born layers remain unaffected. Combined with the results of prior work, these data indicate that FGF2 delays tectal neurogenesis and, thereby, triggers a cascade of changes in tectum size and morphology.

PLZF regulates fibroblast growth factor responsiveness and maintenance of neural progenitors

A transcription factor called Promyelocytic Leukemia Zinc Finger (PLZF) calibrates the balance between spinal cord progenitor maintenance and differentiation by enhancing their sensitivity to mitogens that are present in developing embryos.

Distinct classes of neurons and glial cells in the developing spinal cord arise at specific times and in specific quantities from spatially discrete neural progenitor domains. Thus, adjacent domains can exhibit marked differences in their proliferative potential and timing of differentiation. However, remarkably little is known about the mechanisms that account for this regional control. Here, we show that the transcription factor Promyelocytic Leukemia Zinc Finger (PLZF) plays a critical role shaping patterns of neuronal differentiation by gating the expression of Fibroblast Growth Factor (FGF) Receptor 3 and responsiveness of progenitors to FGFs. PLZF elevation increases FGFR3 expression and STAT3 pathway activity, suppresses neurogenesis, and biases progenitors towards glial cell production. In contrast, PLZF loss reduces FGFR3 levels, leading to premature neuronal differentiation. Together, these findings reveal a novel transcriptional strategy for spatially tuning the responsiveness of distinct neural progenitor groups to broadly distributed mitogenic signals in the embryonic environment.

The embryonic spinal cord is organized into an array of discrete neural progenitor domains along the dorsoventral axis. Most of these domains undergo two periods of differentiation, first producing specific classes of neurons and then generating distinct populations of glial cells at later times. In addition, each of these progenitors pools exhibit marked differences in their proliferative capacities and propensity to differentiate to produce the appropriate numbers and diversity of neurons and glia needed to form functional neural circuits. The mechanisms behind this regional control of neural progenitor behavior, however, remain unclear. In this study, we identify the transcription factor Promyelocytic Leukemia Zinc Finger (PLZF) as a critical regulator of this process in the chick spinal cord. We show that PLZF is initially expressed by all spinal cord progenitors and then becomes restricted to a central domain, where it helps to limit the rate of neuronal differentiation and to preserve the progenitor pool for subsequent glial production. We also demonstrate that PLZF acts by promoting the expression of Fibroblast Growth Factor (FGF) Receptor 3, thereby enhancing the proliferative response of neural progenitors to FGFs present in developing embryos. Together, these findings reveal a novel developmental strategy for spatially controlling neural progenitor behavior by tuning their responsiveness to broadly distributed growth-promoting signals in the embryonic environment.

Cubilin, a high affinity receptor for fibroblast growth factor 8, is required for cell survival in the developing vertebrate head

Cubilin (Cubn) is a multiligand endocytic receptor critical for the intestinal absorption of vitamin B12 and renal protein reabsorption. During mouse development, Cubn is expressed in both embryonic and extra-embryonic tissues, and Cubn gene inactivation results in early embryo lethality most likely due to the impairment of the function of extra-embryonic Cubn. Here, we focus on the developmental role of Cubn expressed in the embryonic head. We report that Cubn is a novel, interspecies-conserved Fgf receptor. Epiblast-specific inactivation of Cubn in the mouse embryo as well as Cubn silencing in the anterior head of frog or the cephalic neural crest of chick embryos show that Cubn is required during early somite stages to convey survival signals in the developing vertebrate head. Surface plasmon resonance analysis reveals that fibroblast growth factor 8 (Fgf8), a key mediator of cell survival, migration, proliferation, and patterning in the developing head, is a high affinity ligand for Cubn. Cell uptake studies show that binding to Cubn is necessary for the phosphorylation of the Fgf signaling mediators MAPK and Smad1. Although Cubn may not form stable ternary complexes with Fgf receptors (FgfRs), it acts together with and/or is necessary for optimal FgfR activity. We propose that plasma membrane binding of Fgf8, and most likely of the Fgf8 family members Fgf17 and Fgf18, to Cubn improves Fgf ligand endocytosis and availability to FgfRs, thus modulating Fgf signaling activity.

Signals and switches in Mammalian neural crest cell differentiation

Neural crest cells (NCCs) comprise a multipotent, migratory cell population that generates a diverse array of cell and tissue types during vertebrate development. These include cartilage and bone, tendons, and connective tissue, as well as neurons, glia, melanocytes, and endocrine and adipose cells; this remarkable lineage potential persists into adult life. Taken together with a limited capacity for self-renewal, neural crest cells bear the hallmarks of stem and progenitor cells and are considered to be synonymous with vertebrate evolution. The neural crest has provided a system for exploring the mechanisms that govern developmental processes such as morphogenetic induction, cell migration, and fate determination. Today, much of the focus on neural crest cells revolves around their stem cell-like characteristics and potential for use in regenerative medicine. A thorough understanding of the signals and switches that govern mammalian neural crest patterning is central to potential therapeutic application of these cells and better appreciation of the role that neural crest cells play in vertebrate evolution, development, and disease.

Apert syndrome with fused thalami

Apert syndrome (Acrocephalosyndactyly type I; AS) is a rare but well-known autosomal dominant disorder characterized by craniosynostosis, midface hypoplasia, bony/cutaneous syndactyly of fingers and toes as well as a variety of associated congenital anomalies involving the brain, heart, limbs and other organ systems. We report the case of a fetus with molecularly confirmed Apert syndrome and additional fusion of the thalamic nuclei. Various central nervous system anomalies, have been reported in patients with AS. However, as far as we know cases of fused thalami in Apert syndrome have never been reported so far.

Neuroimaging and neurological findings in patients with hypochondroplasia and FGFR3 N540K mutation

Hypochondroplasia (HCH), an autosomal dominant skeletal dysplasia caused by mutations in the FGFR3 gene, has not been commonly associated with neurological problems. Temporal lobe dysgenesis associated with epilepsy was recently described in single patients. In this retrospective study, we assessed neurological and neuroimaging aspects of 13 FGFR3 (N540K) mutation verified HCH patients in Finland. Eight patients had neurocognitive difficulties, ranging from specific learning disorder (2/13) to mild intellectual disability (5/13) or global developmental delay (1/13). Six of 13 patients had a history of seizures or epilepsy. Eight patients had undergone MRI. They all had structural abnormalities consistent with temporal lobe dysgenesis. Six patients had peritrigonal white matter reduction, and 4 had abnormally shaped lateral ventricles. We recommend a close follow-up of development in patients with HCH and a low threshold for neuroimaging.

Fasciculation and guidance of spinal motor axons in the absence of FGFR2 signaling

During development, fibroblast growth factors (FGF) are essential for early patterning events along the anterior-posterior axis, conferring positional identity to spinal motor neurons by activation of different Hox codes. In the periphery, signaling through one of four fibroblast growth factor receptors supports the development of the skeleton, as well as induction and maintenance of extremities. In previous studies, FGF receptor 2 (FGFR2) was found to interact with axon bound molecules involved in axon fasciculation and extension, thus rendering this receptor an interesting candidate for the promotion of proper peripheral innervation. However, while the involvement of FGFR2 in limb bud induction has been extensively studied, its role during axon elongation and formation of distinct nervous projections has not been addressed so far. We show here that motor neurons in the spinal cord express FGFR2 and other family members during the establishment of motor connections to the forelimb and axial musculature. Employing a conditional genetic approach to selectively ablate FGFR2 from motor neurons we found that the patterning of motor columns and the expression patterns of other FGF receptors and Sema3A in the motor columns of mutant embryos are not altered. In the absence of FGFR2 signaling, pathfinding of motor axons is intact, and also fasciculation, distal advancement of motor nerves and gross morphology and positioning of axonal projections are not altered. Our findings therefore show that FGFR2 is not required cell-autonomously in motor neurons during the formation of initial motor projections towards limb and axial musculature.

Primary cultures of embryonic chick lens cells as a model system to study lens gap junctions and fiber cell differentiation

A major limitation in lens gap junction research has been the lack of experimentally tractable ex vivo systems to study the formation and regulation of fiber-type gap junctions. Although immortalized lens-derived cell lines are amenable to both gene transfection and siRNA-mediated knockdown, to our knowledge none are capable of undergoing appreciable epithelial-to-fiber differentiation. Lens central epithelial explants have the converse limitation. A key advance in the field was the development of a primary embryonic chick lens cell culture system by Drs. Sue Menko and Ross Johnson. Unlike central epithelial explants, these cultures also include cells from the peripheral (preequatorial and equatorial) epithelium, which is the most physiologically relevant population for the study of fiber-type gap junction formation. We have modified the Menko/Johnson system and refer to our cultures as dissociated cell-derived monolayer cultures (DCDMLs). We culture DCDMLs without serum to mimic the avascular lens environment and on laminin, the major matrix component of the lens capsule. Here, I review the features of the DCDML system and how we have used it to study lens gap junctions and fiber cell differentiation. Our results demonstrate the power of DCDMLs to generate new findings germane to the mammalian lens and how these cultures can be exploited to conduct experiments that would be impossible, prohibitively expensive and/or difficult to interpret using transgenic animals in vivo.

Expansion, folding, and abnormal lamination of the chick optic tectum after intraventricular injections of FGF2

Comparative research has shown that evolutionary increases in brain region volumes often involve delays in neurogenesis. However, little is known about the influence of such changes on subsequent development. To get at this question, we injected FGF2--which delays cell cycle exit in mammalian neocortex--into the cerebral ventricles of chicks at embryonic day (ED) 4. This manipulation alters the development of the optic tectum dramatically. By ED7, the tectum of FGF2-treated birds is abnormally thin and has a reduced postmitotic layer, consistent with a delay in neurogenesis. FGF2 treatment also increases tectal volume and ventricular surface area, disturbs tectal lamination, and creates small discontinuities in the pia mater overlying the tectum. On ED12, the tectum is still larger in FGF2-treated embryos than in controls. However, lateral portions of the FGF2-treated tectum now exhibit volcano-like laminar disturbances that coincide with holes in the pia, and the caudomedial tectum exhibits prominent folds. To explain these observations, we propose that the tangential expansion of the ventricular surface in FGF2-treated tecta outpaces the expansion of the pial surface, creating abnormal mechanical stresses. Two alternative means of alleviating these stresses are tectal foliation and the formation of pial holes. The latter probably alter signaling gradients required for normal cell migration and may generate abnormal patterns of cerebrospinal fluid flow; both abnormalities would generate disturbances in tectal lamination. Overall, our findings suggest that evolutionary expansion of sheet-like, laminated brain regions requires a concomitant expansion of the pia mater.

Expression of hindbrain boundary markers is regulated by FGF3

Compartment boundaries act as organizing centers that segregate adjacent areas into domains of gene expression and regulation, and control their distinct fates via the secretion of signalling factors. During hindbrain development, a specialized cell-population forms boundaries between rhombomeres. These boundary cells demonstrate unique morphological properties and express multiple genes that differs them from intra-rhombomeric cells. Yet, little is known regarding the mechanisms that controls the expression or function of these boundary markers.Multiple components of the FGF signaling system, including ligands, receptors, downstream effectors as well as proteoglycans are shown to localize to boundary cells in the chick hindbrain. These patterns raise the possibility that FGF signaling plays a role in regulating boundary properties. We provide evidence to the role of FGF signaling, particularly the boundary-derived FGF3, in regulating the expression of multiple markers at hindbrain boundaries. These findings enable further characterization of the unique boundary-cell population, and expose a new function for FGFs as regulators of boundary-gene expression in the chick hindbrain.

Detection of isoform-specific fibroblast growth factor receptors by whole-mount in situ hybridization in early chick embryos

We have developed "b" and "c" isoform-specific chicken fibroblast growth factor (FGF) receptor 1-3 probes for in situ hybridization. We rigorously demonstrate the specificity of these probes by using both dot blot hybridization and whole-mount in situ hybridization during neurulation and early postneurulation stages, and we compare expression patterns of each of the three isoform-specific probes to one another and to generic probes to each of the three (non-isoform-specific) FGF receptors. We show that the expression pattern of each receptor is represented by the collective expression of each of its two isoforms, with the expression of each FGF receptor being most similar to that of its "c" isoform at two of the three stages studied, and that tissue and stage differences exist in the patterns of expression of the six isoforms. We demonstrate the usefulness of these probes for defining the differential tissue expression of FGF receptor 1-3 isoforms.

FGF signalling through RAS/MAPK and PI3K pathways regulates cell movement and gene expression in the chicken primitive streak without affecting E-cadherin expression

Background

FGF signalling regulates numerous aspects of early embryo development. During gastrulation in amniotes, epiblast cells undergo an epithelial to mesenchymal transition (EMT) in the primitive streak to form the mesoderm and endoderm. In mice lacking FGFR1, epiblast cells in the primitive streak fail to downregulate E-cadherin and undergo EMT, and cell migration is inhibited. This study investigated how FGF signalling regulates cell movement and gene expression in the primitive streak of chicken embryos.

Results

We find that pharmacological inhibition of FGFR activity blocks migration of cells through the primitive streak of chicken embryos without apparent alterations in the level or intracellular localization of E-cadherin. E-cadherin protein is localized to the periphery of epiblast, primitive streak and some mesodermal cells. FGFR inhibition leads to downregulation of a large number of regulatory genes in the preingression epiblast adjacent to the primitive streak, the primitive streak and the newly formed mesoderm. This includes members of the FGF, NOTCH, EPH, PDGF, and canonical and non-canonical WNT pathways, negative modulators of these pathways, and a large number of transcriptional regulatory genes. SNAI2 expression in the primitive streak and mesoderm is not altered by FGFR inhibition, but is downregulated only in the preingression epiblast region with no significant effect on E-cadherin. Furthermore, over expression of SNAIL has no discernable effect on E-cadherin protein levels or localization in epiblast, primitive streak or mesodermal cells. FGFR activity modulates distinct downstream pathways including RAS/MAPK and PI3K/AKT. Pharmacological inhibition of MEK or AKT indicate that these downstream effectors control discrete and overlapping groups of genes during gastrulation. FGFR activity regulates components of several pathways known to be required for cell migration through the streak or in the mesoderm, including RHOA, the non-canonical WNT pathway, PDGF signalling and the cell adhesion protein N-cadherin.

Conclusions

In chicken embryos, FGF signalling regulates cell movement through the primitive streak by mechanisms that appear to be independent of changes in E-cadherin expression or protein localization. The positive and negative effects on large groups of genes by pharmacological inhibition of FGF signalling, including major signalling pathways and transcription factor families, indicates that the FGF pathway is a focal point of regulation during gastrulation in chicken.

Analysis of expression and function of FGF-MAPK signaling components in the hindbrain reveals a central role for FGF3 in the regulation of Krox20, mediated by Pea3

The development of the vertebrate hindbrain requires multiple coordinated signals which act via several pathways. One such signal is Fibroblast Growth Factor (FGF), which is necessary for the patterning of a major transcription factor in the hindbrain, Krox20. However, in the chick, it is still not known which specific FGF ligand is responsible for the regulation of Krox20 and how the signal is dispatched. The most characterized signaling pathway which FGF acts through in the nervous system is the MAPK/Erk1/2 pathway. Nevertheless, a detailed analysis of the hindbrain distribution of various components of this pathway has not been fully described. In this study we present a comprehensive atlas of the FGF ligands, receptors and members of the MAPK/Erk1/2 signaling components in subsequent stages of avian hindbrain development. Moreover, we show that FGF is a major signaling pathway that contributes to the activation of ERK1/2 and expression of the downstream targets Pea3 and Erm. Central to this study, we provide multiple evidence that FGF3 is required for the upregulation of Pea3 that in turn is necessary for Krox20 distribution in rhombomeres 3 and 5. These results show for the first time that Pea3 mediates the FGF3 signal to regulate the hindbrain expression of Krox20.

Modeling gastrulation in the chick embryo: formation of the primitive streak

The body plan of all higher organisms develops during gastrulation. Gastrulation results from the integration of cell proliferation, differentiation and migration of thousands of cells. In the chick embryo gastrulation starts with the formation of the primitive streak, the site of invagination of mesoderm and endoderm cells, from cells overlaying Koller's Sickle. Streak formation is associated with large-scale cell flows that carry the mesoderm cells overlying Koller's sickle into the central midline region of the embryo. We use multi-cell computer simulations to investigate possible mechanisms underlying the formation of the primitive streak in the chick embryo. Our simulations suggest that the formation of the primitive streak employs chemotactic movement of a subpopulation of streak cells, as well as differential adhesion between the mesoderm cells and the other cells in the epiblast. Both chemo-attraction and chemo-repulsion between various combinations of cell types can create a streak. However, only one combination successfully reproduces experimental observations of the manner in which two streaks in the same embryo interact. This finding supports a mechanism in which streak tip cells produce a diffusible morphogen which repels cells in the surrounding epiblast. On the other hand, chemotactic interaction alone does not reproduce the experimental observation that the large-scale vortical cell flows develop simultaneously with streak initiation. In our model the formation of large scale cell flows requires an additional mechanism that coordinates and aligns the motion of neighboring cells.

FGFR3 expression in Xenopus laevis

We studied the expression of FGF receptor 3 (FGFR3) mRNA throughout early development of Xenopus laevis by RT-PCR and in situ hybridization. RT-PCR shows that FGFR3 mRNA is localized within the gastrula; regionalized staining is detected by the neural plate stage and continues throughout embryonic development. Strong expression is seen in developing neural structures, especially in the forebrain and hindbrain, including the developing eyes, and in lateral mesoderm. Comparison of these data with previous reports of FGF expression in this species suggests possible FGF-FGFR3 interactions. The pattern of FGFR3 expression appears to be strongly conserved among vertebrate embryos.

Early embryonic brain development in rats requires the trophic influence of cerebrospinal fluid

Cerebrospinal fluid has shown itself to be an essential brain component during development. This is particularly evident at the earliest stages of development where a lot of research, performed mainly in chick embryos, supports the evidence that cerebrospinal fluid is involved in different mechanisms controlling brain growth and morphogenesis, by exerting a trophic effect on neuroepithelial precursor cells (NPC) involved in controlling the behaviour of these cells. Despite it being known that cerebrospinal fluid in mammals is directly involved in corticogenesis at fetal stages, the influence of cerebrospinal fluid on the activity of NPC at the earliest stages of brain development has not been demonstrated. Here, using "in vitro" organotypic cultures of rat embryo brain neuroepithelium in order to expose NPC to or deprive them of cerebrospinal fluid, we show that the neuroepithelium needs the trophic influence of cerebrospinal fluid to undergo normal rates of cell survival, replication and neurogenesis, suggesting that NPC are not self-sufficient to induce their normal activity. This data shows that cerebrospinal fluid is an essential component in chick and rat early brain development, suggesting that its influence could be constant in higher vertebrates.

Boundary cells regulate a switch in the expression of FGF3 in hindbrain rhombomeres

Background

During formation of the vertebrate central nervous system, the hindbrain is organized into segmental units, called rhombomeres (r). These cell-lineage restricted segments are separated by a subpopulation of cells known as boundary cells. Boundary cells display distinct molecular and cellular properties such as an elongated shape, enriched extracellular matrix components and a reduced proliferation rate compared to intra-rhombomeric cells. However, little is known regarding their functions and the mechanisms that regulate their formation.

Results

Hindbrain boundary cells express several signaling molecules, such as FGF3, which at earlier developmental stages is transiently expressed in specific rhombomeres. We show that chick embryos that lack boundary cells due to overexpression of truncated EphA4 receptor in the hindbrain have continued segmental expression of FGF3 at stages when it is normally restricted to hindbrain boundaries. Furthermore, surgical ablation of the boundary between r3 and r4, or blocking of the contact of r4 with boundary cells, results in sustained FGF3 expression in this segment.

Conclusion

These findings suggest that boundary cells are required for the downregulation of segmental FGF3, presumably mediated by a soluble factor(s) that emanates from boundaries. We propose that this new function of boundary cells enables a switch in gene expression that may be required for stage-specific functions of FGF3 in the developing hindbrain.

Why the embryo still matters: CSF and the neuroepithelium as interdependent regulators of embryonic brain growth, morphogenesis and histiogenesis

The key focus of this review is that both the neuroepithelium and embryonic cerebrospinal fluid (CSF) work in an integrated way to promote embryonic brain growth, morphogenesis and histiogenesis. The CSF generates pressure and also contains many biologically powerful trophic factors; both play key roles in early brain development. Accumulation of fluid via an osmotic gradient creates pressure that promotes rapid expansion of the early brain in a developmental regulated way, since the rates of growth differ between the vesicles and for different species. The neuroepithelium and ventricles both contribute to this growth but by different and coordinated mechanisms. The neuroepithelium grows primarily by cell proliferation and at the same time the ventricle expands via hydrostatic pressure generated by active transport of Na(+) and transport or secretion of proteins and proteoglycans that create an osmotic gradient which contribute to the accumulation of fluid inside the sealed brain cavity. Recent evidence shows that the CSF regulates relevant aspects of neuroepithelial behavior such as cell survival, replication and neurogenesis by means of growth factors and morphogens. Here we try to highlight that early brain development requires the coordinated interplay of the CSF contained in the brain cavity with the surrounding neuroepithelium. The information presented is essential in order to understand the earliest phases of brain development and also how neuronal precursor behavior is regulated.

Ontogeny of substantia nigra dopamine neurons

Understanding the ontogeny of A9 dopamine (DA) neurons is critical not only to determining basic developmental events that facilitate the emergence of the substantia nigra pars compacta (SNc) but also to the extraction and de novo generation of DA neurons as a potential cell therapy for Parkinson's disease. Recent research has identified a precise window for DA cell birth (differentiation) in the ventral mesencephalon (VM) as well as a number of factors that may facilitate this process. However, application of these factors in vitro has had limited success in specifying a dopaminergic cell fate from undifferentiated cells, suggesting that other cell/molecular signals may as yet remain undiscovered. To resolve this, current work seeks to identify particularly potent and novel DA neuron differentiation factors within the developing VM specifically at the moment of ontogeny. Through such (past and present) studies, a catalog of proteins that play a pivotal role in the generation of nigral DA neurons during normal CNS development has begun to emerge. In the future, it will be crucial to continue to evaluate the critical developmental window where DA neuron ontogeny occurs, not only to facilitate our potential to protect these cells from degeneration in the adult brain but also to mimic the developmental environment in a way that enhances our ability to generate these cells anew either in vitro or in vivo. Here we review our present understanding of factors that are thought to be involved in the emergence of the A9 dopamine neuron group from the ventral mesencephalon.

Neural tube derived Wnt signals cooperate with FGF signaling in the formation and differentiation of the trigeminal placodes

BACKGROUND

Neurogenic placodes are focal thickenings of the embryonic ectoderm that form in the vertebrate head. It is within these structures that the precursors of the majority of the sensory neurons of the cranial ganglia are specified. The trigeminal placodes, the ophthalmic and maxillomandibular, form close to the midbrain-hindbrain boundary and many lines of evidence have shown that signals emanating from this level of the neuraxis are important for the development of the ophthalmic placode.

RESULTS

Here, we provide the first evidence that both the ophthalmic and maxillomandibular placodes form under the influence of isthmic Wnt and FGF signals. Activated Wnt signals direct development of the Pax3 expressing ophthalmic placodal field and induce premature differentiation of both the ophthalmic and the maxillomandibular placodes. Similarly, overexpression of Fgf8 directs premature differentiation of the trigeminal placodes. Wnt signals require FGF receptor activity to initiate Pax3 expression and, subsequently, the expression of neural markers, such as Brn3a, within the cranial ectoderm. Furthermore, fibroblast growth factor signaling via the mitogen activated protein kinase pathway is required to maintain early neuronal differentiation within the trigeminal placodes.

CONCLUSION

We demonstrate the identity of inductive signals that are necessary for trigeminal ganglion formation. This is the first report that describes how isthmic derived Wnt signals act in concert with fibroblast growth factor signaling. Together, both are necessary and sufficient for the establishment and differentiation of the ophthalmic and maxillomandibular placodes and, consequently, the trigeminal ganglion.

Inhibition of BMPs by follistatin is required for FGF3 expression and segmental patterning of the hindbrain

A network of molecular interactions is required in the developing vertebrate hindbrain for the formation and anterior-posterior patterning of the rhombomeres. FGF signaling is required in this network to upregulate the expression of the Krox20 and Kreisler segmentation genes, but little is known of how FGF gene expression is regulated in the hindbrain. We show that the dynamic expression of FGF3 in chick hindbrain segments and boundaries is similar to that of the BMP antagonist, follistatin. Consistent with a regulatory relationship between BMP signaling and FGF3 expression, we find that an increase in BMP activity due to blocking of follistatin translation by morpholino antisense oligonucleotides or overexpression of BMP results in strong inhibition of FGF3 expression. Conversely, addition of follistatin leads to an increase in the level of FGF3 expression. Furthermore, the segmental inhibition of BMP activity by follistatin is required for the expression of Krox20, Hoxb1 and EphA4 in the hindbrain. In addition, we show that the maintenance of FGF3 gene expression requires FGF activity, suggestive of an autoregulatory loop. These results reveal an antagonistic relationship between BMP activity and FGF3 expression that is required for correct segmental gene expression in the chick hindbrain, in which follistatin enables FGF3 expression by inhibiting BMP activity.

The emergence of ectomesenchyme

In the head, neural crest cells generate ectomesenchymal derivatives: cartilage, bone, and connective tissue. Indeed, these cells generate much of the cranial skeleton. There have, however, been few studies of how this lineage is established. Here, we show that neural crest cells stop expressing early neural crest markers upon entering the pharyngeal arches and switch to become ectomesenchymal. By contrast, those neural crest cells that do not enter the arches persist in their expression of early neural crest markers. We further show that fibroblast growth factor (FGF) signaling is involved in directing neural crest cells to become ectomesenchymal. If neural crest cells are rendered insensitive to FGFs, they persist in their expression of early neural crest markers, even after entering the pharyngeal arches. However, our results further suggest that, although FGF signaling is required for the realization of the ectomesenchymal lineages, other cues from the pharyngeal epithelia are also likely to be involved.

The mechanisms underlying primitive streak formation in the chick embryo

Formation of the primitive streak is one of the key events in the early development of amniote embryos. The streak is the site where during gastrulation the mesendoderm cells ingress to take up their correct topographical positions in the embryo. The process of streak formation can be conveniently observed in the chick embryo, where the streak forms as an accumulation of cells in the epiblast in the posterior pole of the embryo and extends subsequently in anterior direction until it covers 80% of the epiblast. A prerequisite for streak formation is the differentiation of mesoderm, which is induced in the epiblast at the interface between the posterior Area Opaca and Area Pellucida in a sickle shaped domain overlying Koller's sickle. Current views on the molecular mechanisms of mesoderm induction by inducing signals from the Area Opaca and inhibitory signals from the hypoblast are briefly discussed. During streak formation the sickle of mesoderm cells transforms into an elongated structure in the central midline of the embryo. We discuss possible cellular mechanisms underlying this process, such as oriented cell division, cell-cell intercalation, chemotactic cell movement in response to attractive and repulsive signals and a combination of chemotaxis and contact following. We review current experimental evidence in favor and against these different hypotheses and outline some the outstanding questions. Since many of the interactions between cells signaling and moving are dynamic and nonlinear in nature they will require detailed modeling and computer simulations to be understood in detail.

Fibroblast growth factor receptors cooperate to regulate neural progenitor properties in the developing midbrain and hindbrain

Fibroblast growth factors (FGFs) secreted from the midbrain-rhombomere 1 (r1) boundary instruct cell behavior in the surrounding neuroectoderm. For example, a combination of FGF and sonic hedgehog (SHH) can induce the development of the midbrain dopaminergic neurons, but the mechanisms behind the action and integration of these signals are unclear. We studied how FGF receptors (FGFRs) regulate cellular responses by analyzing midbrain-r1 development in mouse embryos, which carry different combinations of mutant Fgfr1, Fgfr2, and Fgfr3 alleles. Our results show that the FGFRs act redundantly to support cell survival in the dorsal neuroectoderm, promote r1 tissue identity, and regulate the production of ventral neuronal populations, including midbrain dopaminergic neurons. The compound Fgfr mutants have apparently normal WNT/SHH signaling and neurogenic gene expression in the ventral midbrain, but the number of proliferative neural progenitors is reduced as a result of precocious neuronal differentiation. Our results suggest a SoxB1 family member, Sox3, as a potential FGF-induced transcription factor promoting progenitor renewal. We propose a model for regulation of progenitor cell self-renewal and neuronal differentiation by combinatorial intercellular signals in the ventral midbrain.