Graded levels of FGF protein span the midbrain and can instruct graded induction and repression of neural mapping labels

Roles of Fibroblast Growth Factors in the Axon Guidance

Fibroblast growth factors (FGFs) have been widely studied by virtue of their ability to regulate many essential cellular activities, including proliferation, survival, migration, differentiation and metabolism. Recently, these molecules have emerged as the key components in forming the intricate connections within the nervous system. FGF and FGF receptor (FGFR) signaling pathways play important roles in axon guidance as axons navigate toward their synaptic targets. This review offers a current account of axonal navigation functions performed by FGFs, which operate as chemoattractants and/or chemorepellents in different circumstances. Meanwhile, detailed mechanisms behind the axon guidance process are elaborated, which are related to intracellular signaling integration and cytoskeleton dynamics.

Coreceptor Functions of Cell Surface Heparan Sulfate Proteoglycans

Receptor-ligand interactions play an important role in many biological processes by triggering specific cellular responses. These interactions are frequently regulated by coreceptors that facilitate, alter, or inhibit signaling. Coreceptors work in parallel with other specific and accessory molecules to coordinate receptor-ligand interactions. Cell surface heparan sulfate proteoglycans (HSPGs) function as unique coreceptors because they can bind to many ligands and receptors through their HS and core protein motifs. Cell surface HSPGs are typically expressed in abundance of the signaling receptors and, thus, are capable of mediating the initial binding of ligands to the cell surface. HSPG coreceptors do not possess kinase domains or intrinsic enzyme activities and, for the most part, binding to cell surface HSPGs does not directly stimulate intracellular signaling. Because of these features, cell surface HSPGs primarily function as coreceptors for many receptor-ligand interactions. Given that cell surface HSPGs are widely conserved, they likely serve fundamental functions to preserve basic physiological processes. Indeed, cell surface HSPGs can support specific cellular interactions with growth factors, morphogens, chemokines, extracellular matrix (ECM) components, and microbial pathogens and their secreted virulence factors. Through these interactions, HSPG coreceptors regulate cell adhesion, proliferation, migration and differentiation, and impact the onset, progression, and outcome of pathophysiological processes, such as development, tissue repair, inflammation, infection, and tumorigenesis. This review seeks to provide an overview of the various mechanisms of how cell surface HSPGs function as coreceptors.

Crosstalk of Intercellular Signaling Pathways in the Generation of Midbrain Dopaminergic Neurons In Vivo and from Stem Cells

Dopamine-synthesizing neurons located in the mammalian ventral midbrain are at the center stage of biomedical research due to their involvement in severe human neuropsychiatric and neurodegenerative disorders, most prominently Parkinson’s Disease (PD). The induction of midbrain dopaminergic (mDA) neurons depends on two important signaling centers of the mammalian embryo: the ventral midline or floor plate (FP) of the neural tube, and the isthmic organizer (IsO) at the mid-/hindbrain boundary (MHB). Cells located within and close to the FP secrete sonic hedgehog (SHH), and members of the wingless-type MMTV integration site family (WNT1/5A), as well as bone morphogenetic protein (BMP) family. The IsO cells secrete WNT1 and the fibroblast growth factor 8 (FGF8). Accordingly, the FGF8, SHH, WNT, and BMP signaling pathways play crucial roles during the development of the mDA neurons in the mammalian embryo. Moreover, these morphogens are essential for the generation of stem cell-derived mDA neurons, which are critical for the modeling, drug screening, and cell replacement therapy of PD. This review summarizes our current knowledge about the functions and crosstalk of these signaling pathways in mammalian mDA neuron development in vivo and their applications in stem cell-based paradigms for the efficient derivation of these neurons in vitro.

Regulation of glycosaminoglycan biogenesis is critical for sensitive-period-dependent vocal ontogeny

In the brain, the extracellular matrix (ECM) plays a central role during neural development and thus modulates critical-period regulated behavioral ontogeny. The major components of the ECM are glycosaminoglycans (GAGs) including chondroitin sulfate (CS). However, the specific roles of GAGs in behavioral development are largely unknown. It has been shown that xylosides affect the biological functions of GAGs through modulating GAG biosynthesis. Particularly, xylosides affect GAG biosynthesis through priming of GAG chains (priming activity), competing with endogenous core proteins that carry GAG initiation sites (decoy activity), or both. Using birdsong as our model, we investigated, for the first time, how xyloside-mediated modulation of GAG biogenesis affects song development. Xylosides infused into motor cortex of juvenile birds alter song development by specifically affecting ontogeny of the stereotyped sequence rather than the acoustic structure of syllables. Further analyses reveal that observed changes can be attributed to the priming activity rather than the decoy activity of xylosides. Collectively, these results suggest that regulation of GAG biogenesis through chemical biology approaches may allow promising therapeutic interventions of critical-period-dependent central nervous system plasticity. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1401-1412, 2017.

Regulation of self-renewing neural progenitors by FGF/ERK signaling controls formation of the inferior colliculus

The embryonic tectum displays an anteroposterior gradient in development and produces the superior colliculus and inferior colliculus. Studies suggest that partition of the tectum is controlled by different strengths and durations of FGF signals originated from the so-called isthmic organizer at the mid/hindbrain junction; however, the underlying mechanism is unclear. We show that deleting Ptpn11, which links FGF with the ERK pathway, prevents inferior colliculus formation by depleting a previously uncharacterized stem cell zone. The stem-zone loss is attributed to shortening of S phase and acceleration of cell cycle exit and neurogenesis. Expression of a constitutively active Mek1 (Mek1^DD), the known ERK activator, restores the tectal stem zone and the inferior colliculus without Ptpn11. By contrast, Mek1^DD expression fails to rescue the tectal stem zone and the inferior colliculus in the absence of Fgf8 and the isthmic organizer, indicating that FGF and Mek1^DD initiate qualitatively and/or quantitatively distinctive signaling. Together, our data show that the formation of the inferior colliculus relies on the provision of new cells from the tectal stem zone. Furthermore, distinctive ERK signaling mediates Fgf8 in the control of cell survival, tissue polarity and cytogenetic gradient during the development of the tectum.

Heparan sulfate proteoglycans: a sugar code for vertebrate development?

Heparan sulfate proteoglycans (HSPGs) have long been implicated in a wide range of cell-cell signaling and cell-matrix interactions, both in vitro and in vivo in invertebrate models. Although many of the genes that encode HSPG core proteins and the biosynthetic enzymes that generate and modify HSPG sugar chains have not yet been analyzed by genetics in vertebrates, recent studies have shown that HSPGs do indeed mediate a wide range of functions in early vertebrate development, for example during left-right patterning and in cardiovascular and neural development. Here, we provide a comprehensive overview of the various roles of HSPGs in these systems and explore the concept of an instructive heparan sulfate sugar code for modulating vertebrate development.

Summary: This Review article examines the role of heparan sulfate proteoglycans in vertebrate development and explores the concept of an instructive 'sugar code' for modulating development.

Mechanisms of FGF gradient formation during embryogenesis

Fibroblast growth factors (FGFs) have long been attributed to influence morphogenesis in embryonic development. Signaling by FGF morphogen encodes positional identity of tissues by creating a concentration gradient over the developing embryo. Various mechanisms that influence the development of such gradient have been elucidated in the recent past. These mechanisms of FGF gradient formation present either as an extracellular control over FGF ligand diffusion or as a subcellular control of FGF propagation and signaling. In this review, we describe our current understanding of FGF as a morphogen, the extracellular control of FGF gradient formation by heparan sulfate proteoglycans (HSPGs) and mechanisms of intracellular regulation of FGF signaling that influence gradient formation.

Engrailed homeoproteins in visual system development

Engrailed is a homeoprotein transcription factor. This family of transcription factors is characterized by their DNA-binding homeodomain and some members, including Engrailed, can transfer between cells and regulate protein translation in addition to gene transcription. Engrailed is intimately involved in the development of the vertebrate visual system. Early expression of Engrailed in dorsal mesencephalon contributes to the development and organization of a visual structure, the optic tectum/superior colliculus. This structure is an important target for retinal ganglion cell axons that carry visual information from the retina. Engrailed regulates the expression of Ephrin axon guidance cues in the tectum/superior colliculus. More recently it has been reported that Engrailed itself acts as an axon guidance cue in synergy with the Ephrin system and is proposed to enhance retinal topographic precision.

A bi-modal function of Wnt signalling directs an FGF activity gradient to spatially regulate neuronal differentiation in the midbrain

FGFs and Wnts are important morphogens during midbrain development, but their importance and potential interactions during neurogenesis are poorly understood. We have employed a combination of genetic and pharmacological manipulations in zebrafish to show that during neurogenesis FGF activity occurs as a gradient along the anterior-posterior axis of the dorsal midbrain and directs spatially dynamic expression of the Hairy gene her5. As FGF activity diminishes during development, Her5 is lost and differentiation of neuronal progenitors occurs in an anterior-posterior manner. We generated mathematical models to explain how Wnt and FGFs direct the spatial differentiation of neurons in the midbrain through Wnt regulation of FGF signalling. These models suggested that a negative-feedback loop controlled by Wnt is crucial for regulating FGF activity. We tested Sprouty genes as mediators of this regulatory loop using conditional mouse knockouts and pharmacological manipulations in zebrafish. These reveal that Sprouty genes direct the positioning of early midbrain neurons and are Wnt responsive in the midbrain. We propose a model in which Wnt regulates FGF activity at the isthmus by driving both FGF and Sprouty gene expression. This controls a dynamic, posteriorly retracting expression of her5 that directs neuronal differentiation in a precise spatiotemporal manner in the midbrain.

Exploring mechanisms of FGF signalling through the lens of structural biology

Fibroblast growth factors (FGFs) mediate a broad range of functions in both the developing and adult organism. The accumulated wealth of structural information on the FGF signalling pathway has begun to unveil the underlying molecular mechanisms that modulate this system to generate a myriad of distinct biological outputs in development, tissue homeostasis and metabolism. At the ligand and receptor level, these mechanisms include alternative splicing of the ligand (FGF8 subfamily) and the receptor (FGFR1-FGFR3), ligand homodimerization (FGF9 subfamily), site-specific proteolytic cleavage of the ligand (FGF23), and interaction of the ligand and the receptor with heparan sulphate cofactor and Klotho co-receptor.

Regulation of ephrin-A expression in compressed retinocollicular maps

Retinotopic maps can undergo compression and expansion in response to changes in target size, but the mechanism underlying this compensatory process has remained a mystery. The discovery of ephrins as molecular mediators of Sperry's chemoaffinity process allows a mechanistic approach to this important issue. In Syrian hamsters, neonatal, partial (PT) ablation of posterior superior colliculus (SC) leads to compression of the retinotopic map, independent of neural activity. Graded, repulsive EphA receptor/ephrin-A ligand interactions direct the formation of the retinocollicular map, but whether ephrins might also be involved in map compression is unknown. To examine whether map compression might be directed by changes in the ephrin expression pattern, we compared ephrin-A2 and ephrin-A5 mRNA expression between normal SC and PT SC using in situ hybridization and quantitative real-time PCR. We found that ephrin-A ligand expression in the compressed maps was low anteriorly and high posteriorly, as in normal animals. Consistent with our hypothesis, the steepness of the ephrin gradient increased in the lesioned colliculi. Interestingly, overall levels of ephrin-A2 and -A5 expression declined immediately after neonatal target damage, perhaps promoting axon outgrowth. These data establish a correlation between changes in ephrin-A gradients and map compression, and suggest that ephrin-A expression gradients may be regulated by target size. This in turn could lead to compression of the retinocollicular map onto the reduced target. These findings have important implications for mechanisms of recovery from traumatic brain injury.

Fibroblast growth factor 8 organizes the neocortical area map and regulates sensory map topography

The concept of an "organizer" is basic to embryology. An organizer is a portion of the embryo producing signals that lead to the creation of a patterned mature structure from an embryonic primordium. Fibroblast growth factor 8 (FGF8) is a morphogen that disperses from a rostromedial source in the neocortical primordium (NP), forms a rostral-to-caudal (R/C) gradient, and regulates embryonic and neonatal R/C patterns of gene expression in neocortex. Whether FGF8 also has organizer activity that generates the postnatal neocortical area map is uncertain. To test this possibility, new sources of FGF8 were introduced into the mouse NP with in utero microelectroporation at embryonic day 10.5, close to the estimated peak of area patterning. Results differed depending on the position of ectopic FGF8. Ectopic FGF8 in the caudalmost NP could duplicate somatosensory cortex (S1) and primary visual cortex (V1). FGF8 delivered to the midlateral NP generated a sulcus separating rostral and caudal portions of the NP, in effect creating duplicate NPs. In the caudal NP, ectopic FGF8 induced a second, inclusive area map, containing frontal cortex, S1, V1, and primary auditory areas. Moreover, duplicate S1 showed plasticity to sensory deprivation, and duplicate V1 responded to visual stimuli. Our findings implicate FGF8 as an organizer signal, and its source in the rostromedial telencephalon as an organizer of the neocortical area map.

Glycosaminoglycan-dependent restriction of FGF diffusion is necessary for lacrimal gland development

Glycosaminoglycans (GAGs) play a central role in embryonic development by regulating the movement and signaling of morphogens. We have previously demonstrated that GAGs are the co-receptors for Fgf10 signaling in the lacrimal gland epithelium, but their function in the Fgf10-producing periocular mesenchyme is still poorly understood. In this study, we have generated a mesenchymal ablation of UDP-glucose dehydrogenase (Ugdh), an essential biosynthetic enzyme for GAGs. Although Fgf10 RNA is expressed normally in the periocular mesenchyme, Ugdh mutation leads to excessive dispersion of Fgf10 protein, which fails to elicit an FGF signaling response or budding morphogenesis in the presumptive lacrimal gland epithelium. This is supported by genetic rescue experiments in which the Ugdh lacrimal gland defect is ameliorated by constitutive Ras activation in the epithelium but not in the mesenchyme. We further show that lacrimal gland development requires the mesenchymal expression of the heparan sulfate N-sulfation genes Ndst1 and Ndst2 but not the 6-O and 2-O-sulfation genes Hs6st1, Hs6st2 and Hs2st. Taken together, these results demonstrate that mesenchymal GAG controls lacrimal gland induction by restricting the diffusion of Fgf10.

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

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

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.

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.

FGF8 signaling is chemotactic for cardiac neural crest cells

Cardiac neural crest cells migrate into the pharyngeal arches where they support development of the pharyngeal arch arteries. The pharyngeal endoderm and ectoderm both express high levels of FGF8. We hypothesized that FGF8 is chemotactic for cardiac crest cells. To begin testing this hypothesis, cardiac crest was explanted for migration assays under various conditions. Cardiac neural crest cells migrated more in response to FGF8. Single cell tracing indicated that this was not due to proliferation and subsequent transwell assays showed that the cells migrate toward an FGF8 source. The migratory response was mediated by FGF receptors (FGFR) 1 and 3 and MAPK/ERK intracellular signaling. To test whether FGF8 is chemokinetic and/or chemotactic in vivo, dominant negative FGFR1 was electroporated into the premigratory cardiac neural crest. Cells expressing the dominant negative receptor migrated slower than normal cardiac neural crest cells and were prone to remain in the vicinity of the neural tube and die. Treating with the FGFR1 inhibitor, SU5402 or an FGFR3 function-blocking antibody also slowed neural crest migration. FGF8 over-signaling enhanced neural crest migration. Neural crest cells migrated to an FGF8-soaked bead placed dorsal to the pharynx. Finally, an FGF8 producing plasmid was electroporated into an ectopic site in the ventral pharyngeal endoderm. The FGF8 producing cells attracted a thick layer of mesenchymal cells. DiI labeling of the neural crest as well as quail-to-chick neural crest chimeras showed that neural crest cells migrated to and around the ectopic site of FGF8 expression. These results showing that FGF8 is chemotactic and chemokinetic for cardiac neural crest adds another dimension to understanding the relationship of FGF8 and cardiac neural crest in cardiovascular defects.

Gbx2 and Fgf8 are sequentially required for formation of the midbrain-hindbrain compartment boundary

In vertebrates, the common expression border of two homeobox genes, Otx2 and Gbx2, demarcates the prospective midbrain-hindbrain border (MHB) in the neural plate at the end of gastrulation. The presence of a compartment boundary at the MHB has been demonstrated, but the mechanism and timing of its formation remain unclear. We show by genetic inducible fate mapping using a Gbx2(CreER) knock-in mouse line that descendants of Gbx2(+) cells as early as embryonic day (E) 7.5 do not cross the MHB. Without Gbx2, hindbrain-born cells abnormally populate the entire midbrain, demonstrating that Gbx2 is essential for specifying hindbrain fate. Gbx2(+) and Otx2(+) cells segregate from each other, suggesting that mutually exclusive expression of Otx2 and Gbx2 in midbrain and hindbrain progenitors is responsible for cell sorting in establishing the MHB. The MHB organizer gene Fgf8, which is expressed as a sharp transverse band immediately posterior to the lineage boundary at the MHB, is crucial in maintaining the lineage-restricted boundary after E7.5. Partial deletion of Fgf8 disrupts MHB lineage separation. Activation of FGF pathways has a cell-autonomous effect on cell sorting in midbrain progenitors. Therefore, Fgf8 from the MHB may signal the nearby mesencephalic cells to impart distinct cell surface characteristics or induce local cell-cell signaling, which consequently prevents cell movements across the MHB. Our findings reveal the distinct function of Gbx2 and Fgf8 in a stepwise process in the development of the compartment boundary at the MHB and that Fgf8, in addition to its organizer function, plays a crucial role in maintaining the lineage boundary at the MHB by restricting cell movement.

FGF signaling gradient maintains symmetrical proliferative divisions of midbrain neuronal progenitors

For the correct development of the central nervous system, the balance between self-renewing and differentiating divisions of the neuronal progenitors must be tightly regulated. To maintain their self-renewing identity, the progenitors need to retain both apical and basal interfaces. However, the identities of fate-determining signals which cells receive via these connections, and the exact mechanism of their action, are poorly understood. The conditional inactivation of Fibroblast growth factor (FGF) receptors 1 and 2 in the embryonic mouse midbrain-hindbrain area results in premature neuronal differentiation. Here, we aim to elucidate the connection between FGF signaling and neuronal progenitor maintenance. Our results reveal that the loss of FGF signaling leads to downregulation of Hes1 and upregulation of Ngn2, Dll1, and p57 in the ventricular zone (VZ) cells, and that this increased neurogenesis occurs cell-autonomously. Yet the cell cycle progression, apico-basal-polarity, cell-cell connections, and the positioning of mitotic spindle in the mutant VZ appear unaltered. Interestingly, FGF8-protein is highly concentrated in the basal lamina. Thus, FGFs may act through basal processes of neuronal progenitors to maintain their progenitor status. Indeed, midbrain neuronal progenitors deprived in vitro of FGFs switched from symmetrical proliferative towards symmetrical neurogenic divisions. We suggest that FGF signaling in the midbrain VZ is cell-autonomously required for the maintenance of symmetrical proliferative divisions via Hes1-mediated repression of neurogenic genes.

Wiring the brain: the biology of neuronal guidance

The mammalian brain is the most complex organ in the body. It controls all aspects of our bodily functions and interprets the world around us through our senses. It defines us as human beings through our memories and our ability to plan for the future. Crucial to all these functions is how the brain is wired in order to perform these tasks. The basic map of brain wiring occurs during embryonic and postnatal development through a series of precisely orchestrated developmental events regulated by specific molecular mechanisms. Below we review the most important features of mammalian brain wiring derived from work in both mammals and in nonmammalian species. These mechanisms are highly conserved throughout evolution, simply becoming more complex in the mammalian brain. This fascinating area of biology is uncovering the essence of what makes the mammalian brain able to perform the everyday tasks we take for granted, as well as those which give us the ability for extraordinary achievement.

Roles of heparan sulfate in mammalian brain development current views based on the findings from Ext1 conditional knockout studies

Development of the mammalian central nervous system proceeds roughly in four major steps, namely the patterning of the neural tube, generation of neurons from neural stem cells and their migration to genetically predetermined destinations, extension of axons and dendrites toward target neurons to form neural circuits, and formation of synaptic contacts. Earlier studies on spatiotemporal expression patterns and in vitro function of heparan sulfate (HS) suggested that HS is functionally involved in various aspects of neural development. Recent studies using knockout of genes involved in HS biosynthesis have provided more physiologically relevant information as to the role of HS in mammalian neural development. This chapter reviews the current understanding of the in vivo function of HS deduced from the phenotypes of conditional Ext1 knockout mice.

Differential interactions of FGFs with heparan sulfate control gradient formation and branching morphogenesis

The developmental activities of morphogens depend on the gradients that they form in the extracellular matrix. Here, we show that differences in the binding of fibroblast growth factor 7 (FGF7) and FGF10 to heparan sulfate (HS) underlie the formation of different gradients that dictate distinct activities during branching morphogenesis. Reducing the binding affinity of FGF10 for HS by mutating a single residue in its HS-binding pocket converted FGF10 into a functional mimic of FGF7 with respect to gradient formation and regulation of branching morphogenesis. In particular, the mutant form of FGF10 caused lacrimal and salivary gland epithelium buds to branch rather than to elongate. In contrast, mutations that reduced the affinity of the FGF10 for its receptor affected the extent, but not the nature, of the response. Our data may provide a general model for understanding how binding to HS regulates other morphogenetic gradients.