Establishing the trochlear motor axon trajectory: role of the isthmic organiser and Fgf8

Fibroblast growth factor 8 promotes in vitro neurite outgrowth of placode-derived petrosal and nodose ganglia to varying degrees

Fibroblast growth factors (FGFs) are required for the specification and formation of the epibranchial placodes, which give rise to the distal part of the cranial sensory ganglia. However, it remains unclear whether FGFs play a role in regulating the neurite outgrowth of the epibranchial placode-derived ganglia during further development. Previous studies have shown that Fibroblast growth factor 8 (FGF8) promotes neurite outgrowth from the statoacoustic ganglion in vitro. However, these studies did not distinguish between the neural crest- and placode-derived components of the sensory ganglia. In this study, we focused on the petrosal and nodose ganglia as representatives of the epibranchial ganglia and investigated their axonal outgrowth under the influence of FGF8 signaling protein in vitro. To precisely isolate the placode-derived ganglion part, we labeled the placode and its derivatives with enhanced green fluorescent protein (EGFP) through electroporation. The isolated ganglia were then collected for qRT-PCR assay and cultured in a collagen gel with and without FGF8 protein. Our findings revealed that both placode-derived petrosal and nodose ganglia expressed FGFR1 and FGFR2. In culture, FGF8 exerted a neural trophic effect on the axon outgrowth of both ganglia. While the expression levels of FGFR1/2 were similar between the two ganglia, the petrosal ganglion exhibited greater sensitivity to FGF8 compared to the nodose ganglion. This indicates that the placode-derived ganglia have differential responsiveness to FGF8 signaling during axonal extension. Thus, FGF8 is not only required for the early development of the epibranchial placode, as shown in previous studies, but also promotes neurite outgrowth of placode-derived ganglia.

Evolution and development of extraocular motor neurons, nerves and muscles in vertebrates

The purpose of this review is to analyze the origin of ocular motor neurons, define the pattern of innervation of nerve fibers that project to the extraocular eye muscles (EOMs), describe congenital disorders that alter the development of ocular motor neurons, and provide an overview of vestibular pathway inputs to ocular motor nuclei. Six eye muscles are innervated by axons of three ocular motor neurons, the oculomotor (CNIII), trochlear (CNIV), and abducens (CNVI) neurons. Ocular motor neurons (CNIII) originate in the midbrain and innervate the ipsilateral orbit, except for the superior rectus and the levator palpebrae, which are contralaterally innervated. Trochlear motor neurons (CNIV) originate at the midbrain-hindbrain junction and innervate the contralateral superior oblique muscle. Abducens motor neurons (CNVI) originate variously in the hindbrain of rhombomeres r4-6 that innervate the posterior (or lateral) rectus muscle and innervate the retractor bulbi. Genes allow a distinction between special somatic (CNIII, IV) and somatic (CNVI) ocular motor neurons. Development of ocular motor neurons and their axonal projections to the EOMs may be derailed by various genetic causes, resulting in the congenital cranial dysinnervation disorders. The ocular motor neurons innervate EOMs while the vestibular nuclei connect with the midbrain-brainstem motor neurons.

Fibroblast growth factor signaling in axons: from development to disease

The fibroblast growth factor (FGF) family regulates various and important aspects of nervous system development, ranging from the well-established roles in neuronal patterning to more recent and exciting functions in axonal growth and synaptogenesis. In addition, FGFs play a critical role in axonal regeneration, particularly after spinal cord injury, confirming their versatile nature in the nervous system. Due to their widespread involvement in neural development, the FGF system also underlies several human neurological disorders. While particular attention has been given to FGFs in a whole-cell context, their effects at the axonal level are in most cases undervalued. Here we discuss the endeavor of the FGF system in axons, we delve into this neuronal subcompartment to provide an original view of this multipurpose family of growth factors in nervous system (dys)function. Video Abstract.

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.

Smoothened overexpression causes trochlear motoneurons to reroute and innervate ipsilateral eyes

The trochlear projection is unique among the cranial nerves in that it exits the midbrain dorsally to innervate the contralateral superior oblique muscle in all vertebrates. Trochlear as well as oculomotor motoneurons uniquely depend upon Phox2a and Wnt1, both of which are downstream of Lmx1b, though why trochlear motoneurons display such unusual projections is not fully known. We used Pax2-cre to drive expression of ectopically activated Smoothened (SmoM2) dorsally in the midbrain and anterior hindbrain. We documented the expansion of oculomotor and trochlear motoneurons using Phox2a as a specific marker at E9.5. We show that the initial expansion follows a demise of these neurons by E14.5. Furthermore, SmoM2 expression leads to a ventral exit and ipsilateral projection of trochlear motoneurons. We compare that data with Unc5c mutants that shows a variable ipsilateral number of trochlear fibers that exit dorsal. Our data suggest that Shh signaling is involved in trochlear motoneuron projections and that the deflected trochlear projections after SmoM2 expression is likely due to the dorsal expression of Gli1, which impedes the normal dorsal trajectory of these neurons.

Isolation and Culture of Oculomotor, Trochlear, and Spinal Motor Neurons from Prenatal Islmn:GFP Transgenic Mice

Oculomotor neurons (CN3s) and trochlear neurons (CN4s) exhibit remarkable resistance to degenerative motor neuron diseases such as amyotrophic lateral sclerosis (ALS) when compared to spinal motor neurons (SMNs). The ability to isolate and culture primary mouse CN3s, CN4s, and SMNs would provide an approach to study mechanisms underlying this selective vulnerability. To date, most protocols use heterogeneous cell cultures, which can confound the interpretation of experimental outcomes. To minimize the problems associated with mixed-cell populations, pure cultures are indispensable. Here, the first protocol describes in detail how to efficiently purify and cultivate CN3s/CN4s alongside SMNs counterparts from the same embryos using embryonic day 11.5 (E11.5) IslMN:GFP transgenic mouse embryos. The protocol provides details on the tissue dissection and dissociation, FACS-based cell isolation, and in vitro cultivation of cells from CN3/CN4 and SMN nuclei. This protocol adds a novel in vitro CN3/CN4 culture system to existing protocols and simultaneously provides a pure species- and age-matched SMN culture for comparison. Analyses focusing on the morphological, cellular, molecular, and electrophysiological characteristics of motor neurons are feasible in this culture system. This protocol will enable research into the mechanisms that define motor neuron development, selective vulnerability, and disease.

The emergence of mesencephalic trigeminal neurons

Background

The cells of the mesencephalic trigeminal nucleus (MTN) are the proprioceptive sensory neurons that innervate the jaw closing muscles. These cells differentiate close to the two key signalling centres that influence the dorsal midbrain, the isthmus, which mediates its effects via FGF and WNT signalling and the roof plate, which is a major source of BMP signalling as well as WNT signalling.

Methods

In this study, we have set out to analyse the importance of FGF, WNT and BMP signalling for the development of the MTN. We have employed pharmacological inhibitors of these pathways in explant cultures as well as utilising the electroporation of inhibitory constructs in vivo in the chick embryo.

Results

We find that interfering with either FGF or WNT signalling has pronounced effects on MTN development whilst abrogation of BMP signalling has no effect. We show that treatment of explants with either FGF or WNT antagonists results in the generation of fewer MTN neurons and affects MTN axon extension and that inhibition of both these pathways has an additive effect. To complement these studies, we have used in vivo electroporation to inhibit BMP, FGF and WNT signalling within dorsal midbrain cells prior to, and during, their differentiation as MTN neurons. Again, we find that inhibition of BMP signalling has no effect on the development of MTN neurons. We additionally find that cells electroporated with inhibitory constructs for either FGF or WNT signalling can differentiate as MTN neurons suggesting that these pathways are not required cell intrinsically for the emergence of these neurons. Indeed, we also show that explants of dorsal mesencephalon lacking both the isthmus and roof plate can generate MTN neurons. However, we did find that inhibiting FGF or WNT signalling had consequences for MTN differentiation.

Conclusions

Our results suggest that the emergence of MTN neurons is an intrinsic property of the dorsal mesencephalon of gnathostomes, and that this population undergoes expansion, and maturation, along with the rest of the dorsal midbrain under the influence of FGF and WNT signalling.

The best-laid plans go oft awry: synaptogenic growth factor signaling in neuropsychiatric disease

Growth factors play important roles in synapse formation. Mouse models of neuropsychiatric diseases suggest that defects in synaptogenic growth factors, their receptors, and signaling pathways can lead to disordered neural development and various behavioral phenotypes, including anxiety, memory problems, and social deficits. Genetic association studies in humans have found evidence for similar relationships between growth factor signaling pathways and neuropsychiatric phenotypes. Accumulating data suggest that dysfunction in neuronal circuitry, caused by defects in growth factor-mediated synapse formation, contributes to the susceptibility to multiple neuropsychiatric diseases, including epilepsy, autism, and disorders of thought and mood (e.g., schizophrenia and bipolar disorder, respectively). In this review, we will focus on how specific synaptogenic growth factors and their downstream signaling pathways might be involved in the development of neuropsychiatric diseases.

Role of fibroblast growth factor 8 in neurite outgrowth from spiral ganglion neurons in vitro

Many neurons degenerate after injuries resulting from overstimulation, drugs, genetic mutations, and aging. Although several growth factors and neurotrophins delay degeneration and promote regrowth of neural processes, the role of fibroblast growth factor 8 (FGF8) in mammalian spiral ganglion neurons (SGN) neurite outgrowth has not been examined. This study develops and uses SGN cell cultures suitable for experimental analysis, it investigates whether FGF8a and FGF8b isoforms affect the neurite outgrowth from SGN cultured in vitro. We found that both FGF8a and FGF8b promoted the outgrowth of neurites from cultured SGN. This response is mediated by FGF receptors and involves the activation of IκBα-mediated NFκB signaling pathway. These findings suggest that, besides its morphogenetic role during development, FGF8 may have trophic functions in the adult which are relevant to regeneration.

Direct and indirect roles of Fgf3 and Fgf10 in innervation and vascularisation of the vertebrate hypothalamic neurohypophysis

The neurohypophysis is a crucial component of the hypothalamo-pituitary axis, serving as the site of release of hypothalamic neurohormones into a plexus of hypophyseal capillaries. The growth of hypothalamic axons and capillaries to the forming neurohypophysis in embryogenesis is therefore crucial to future adult homeostasis. Using ex vivo analyses in chick and in vivo analyses in mutant and transgenic zebrafish, we show that Fgf10 and Fgf3 secreted from the forming neurohypophysis exert direct guidance effects on hypothalamic neurosecretory axons. Simultaneously, they promote hypophyseal vascularisation, exerting early direct effects on endothelial cells that are subsequently complemented by indirect effects. Together, our studies suggest a model for the integrated neurohemal wiring of the hypothalamo-neurohypophyseal axis.

BMP receptor-activated Smads confer diverse functions during the development of the dorsal spinal cord

Bone Morphogenetic Proteins (BMPs) have multiple activities in the developing spinal cord: they specify the identity of the dorsal-most neuronal populations and then direct the trajectories of dorsal interneuron (dI) 1 commissural axons. How are these activities decoded by dorsal neurons to result in different cellular outcomes? Our previous studies have shown that the diverse functions of the BMPs are mediated by the canonical family of BMP receptors and then regulated by specific inhibitory (I) Smads, which block the activity of a complex of Smad second messengers. However, the extent to which this complex translates the different activities of the BMPs in the spinal cord has remained unresolved. Here, we demonstrate that the receptor-activated (R) Smads, Smad1 and Smad5 play distinct roles mediating the abilities of the BMPs to direct cell fate specification and axon outgrowth. Smad1 and Smad5 occupy spatially distinct compartments within the spinal cord, with Smad5 primarily associated with neural progenitors and Smad1 with differentiated neurons. Consistent with this expression profile, loss of function experiments in mouse embryos reveal that Smad5 is required for the acquisition of dorsal spinal neuron identities whereas Smad1 is critical for the regulation of dI1 axon outgrowth. Thus the R-Smads, like the I-Smads, have discrete roles mediating BMP-dependent cellular processes during spinal interneuron development.

Intrinsic properties guide proximal abducens and oculomotor nerve outgrowth in avian embryos

Proper movement of the vertebrate eye requires the formation of precisely patterned axonal connections linking cranial somatic motoneurons, located at defined positions in the ventral midbrain and hindbrain, with extraocular muscles. The aim of this research was to assess the relative contributions of intrinsic, population-specific properties and extrinsic, outgrowth site-specific cues during the early stages of abducens and oculomotor nerve development in avian embryos. This was accomplished by surgically transposing midbrain and caudal hindbrain segments, which had been pre-labeled by electroporation with an EGFP construct. Graft-derived EGFP+ oculomotor axons entering a hindbrain microenvironment often mimicked an abducens initial pathway and coursed cranially. Similarly, some EGFP+ abducens axons entering a midbrain microenvironment mimicked an oculomotor initial pathway and coursed ventrally. Many but not all of these axons subsequently projected to extraocular muscles that they would not normally innervate. Strikingly, EGFP+ axons also took initial paths atypical for their new location. Upon exiting from a hindbrain position, most EGFP+ oculomotor axons actually coursed ventrally and joined host branchiomotor nerves, whose neurons share molecular features with oculomotor neurons. Similarly, upon exiting from a midbrain position, some EGFP+ abducens axons turned caudally, elongated parallel to the brainstem, and contacted the lateral rectus muscle, their originally correct target. These data reveal an interplay between intrinsic properties that are unique to oculomotor and abducens populations and shared ability to recognize and respond to extrinsic directional cues. The former play a prominent role in initial pathway choices, whereas the latter appear more instructive during subsequent directional choices.

Analysis of Cdh22 expression and function in the developing mouse brain

Classical cadherins are important cell adhesion molecules specifying and separating brain nuclei and developmental compartments. Cadherin-22 (Cdh22) belongs to type II subfamily of classical cadherins, and is expressed at the midbrain-hindbrain boundary during early embryogenesis. In Fgfr1 mutant mouse embryos, which have a disturbed midbrain-hindbrain border, Cdh22 is down-regulated. Here, we studied expression of Cdh22 in developing mouse brain in more detail and compared it to expression of related family members. This revealed both complementary and overlapping patterns of Cdh22, Cdh11, Cdh8, and Cdh6 expression in distinct regions of the forebrain and midbrain. We used a mutated allele of Cdh22 to study its function in brain development. Loss of Cdh22 caused reduced postnatal viability. Despite strong Cdh22 expression in the developing brain, we did not observe defects in compartmentalization or abnormalities in the midbrain and forebrain nuclei in Cdh22 mutants. This may be explained by functional redundancy between type II cadherins.

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.

Crossing the border: molecular control of motor axon exit

Living organisms heavily rely on the function of motor circuits for their survival and for adapting to ever-changing environments. Unique among central nervous system (CNS) neurons, motor neurons (MNs) project their axons out of the CNS. Once in the periphery, motor axons navigate along highly stereotyped trajectories, often at considerable distances from their cell bodies, to innervate appropriate muscle targets. A key decision made by pathfinding motor axons is whether to exit the CNS through dorsal or ventral motor exit points (MEPs). In contrast to the major advances made in understanding the mechanisms that regulate the specification of MN subtypes and the innervation of limb muscles, remarkably little is known about how MN axons project out of the CNS. Nevertheless, a limited number of studies, mainly in Drosophila, have identified transcription factors, and in some cases candidate downstream effector molecules, that are required for motor axons to exit the spinal cord. Notably, specialized neural crest cell derivatives, referred to as Boundary Cap (BC) cells, pre-figure and demarcate MEPs in vertebrates. Surprisingly, however, BC cells are not required for MN axon exit, but rather restrict MN cell bodies from ectopically migrating along their axons out of the CNS. Here, we describe the small set of studies that have addressed motor axon exit in Drosophila and vertebrates, and discuss our fragmentary knowledge of the mechanisms, which guide motor axons out of the CNS.

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.

Notch signalling stabilises boundary formation at the midbrain-hindbrain organiser

The midbrain-hindbrain interface gives rise to a boundary of particular importance in CNS development as it forms a local signalling centre, the proper functioning of which is essential for the formation of tectum and cerebellum. Positioning of the mid-hindbrain boundary (MHB) within the neuroepithelium is dependent on the interface of Otx2 and Gbx2 expression domains, yet in the absence of either or both of these genes, organiser genes are still expressed, suggesting that other, as yet unknown mechanisms are also involved in MHB establishment. Here, we present evidence for a role for Notch signalling in stabilising cell lineage restriction and regulating organiser gene expression at the MHB. Experimental interference with Notch signalling in the chick embryo disrupts MHB formation, including downregulation of the organiser signal Fgf8. Ectopic activation of Notch signalling in cells of the anterior hindbrain results in an exclusion of those cells from rhombomeres 1 and 2, and in a simultaneous clustering along the anterior and posterior boundaries of this area, suggesting that Notch signalling influences cell sorting. These cells ectopically express the boundary marker Fgf3. In agreement with a role for Notch signalling in cell sorting, anterior hindbrain cells with activated Notch signalling segregate from normal cells in an aggregation assay. Finally, misexpression of the Notch modulator Lfng or the Notch ligand Ser1 across the MHB leads to a shift in boundary position and loss of restriction of Fgf8 to the MHB. We propose that differential Notch signalling stabilises the MHB through regulating cell sorting and specifying boundary cell fate.

Inhibitory Smads differentially regulate cell fate specification and axon dynamics in the dorsal spinal cord

The roof plate resident BMPs have sequential functions in the developing spinal cord, establishing cell fate and orienting axonal trajectories. These activities are, however, restricted to the dI1-dI3 neurons in the most dorsal region of the spinal cord. What limits the extent of the action of the BMPs to these neurons? To address this question, we have examined both the distribution of the inhibitory Smads (I-Smads), Smad6 and Smad7 in the spinal cord and the consequence of ectopically expressing the I-Smads in chicken embryos. Our studies suggest that the I-Smads function in vivo to restrict the action of BMP signaling in the dorsal spinal cord. Moreover, the I-Smads have distinct roles in regulating the diverse activities of the BMPs. Thus, the ectopic expression of Smad7 suppresses the dI1 and dI3 neural fates and concomitantly increases the number of dI4-dI6 spinal neurons. In contrast, Smad6 most potently functions to block dI1 axon outgrowth. Taken together, these experiments suggest that the I-Smads have distinct roles in spatially limiting the response of cells to BMP signaling.

Lrrn1 is required for formation of the midbrain-hindbrain boundary and organiser through regulation of affinity differences between midbrain and hindbrain cells in chick

The midbrain–hindbrain boundary (MHB) acts as an organiser/signalling centre to pattern tectal and cerebellar compartments. Cells in adjacent compartments must be distinct from each other for boundary formation to occur at the interface. Here we have identified the leucine-rich repeat (LRR) neuronal 1 (Lrrn1) protein as a key regulator of this process in chick. The Lrrn family is orthologous to the Drosophila tartan/capricious (trn/caps) family. Differential expression of trn/caps promotes an affinity difference and boundary formation between adjacent compartments in a number of contexts; for example, in the wing, leg and eye imaginal discs. Here we show that Lrrn1 is expressed in midbrain cells but not in anterior hindbrain cells. Lrrn1 is down-regulated in the anterior hindbrain by the organiser signalling molecule FGF8, thereby creating a differential affinity between these two compartments. Lrrn1 is required for the formation of MHB — loss of function leads to a loss of the morphological constriction and loss of Fgf8. Cells overexpressing Lrrn1 violate the boundary and result in a loss of cell restriction between midbrain and hindbrain compartments. Lrrn1 also regulates the glycosyltransferase Lunatic Fringe, a modulator of Notch signalling, maintaining its expression in midbrain cells which is instrumental in MHB boundary formation. Thus, Lrrn1 provides a link between cell affinity/compartment segregation, and cell signalling to specify boundary cell fate.

The bone morphogenetic protein roof plate chemorepellent regulates the rate of commissural axonal growth

Commissural spinal axons extend away from the roof plate (RP) in response to a chemorepellent mediated by the bone morphogenetic proteins (BMPs). Previous studies have focused on the ability of commissural axons to translate a spatial gradient of BMPs into directional information in vitro. However, a notable feature of this system in vivo is that the gradient of BMPs is thought to act from behind the commissural cell bodies, making it possible for the BMPs to have a continued effect on commissural axons as they grow away from the RP. Here, we demonstrate that BMPs activate the cofilin regulator Lim domain kinase 1 (Limk1) to control the rate of commissural axon extension in the dorsal spinal cord. By modulating Limk1 activity in both rodent and chicken commissural neurons, the rate of axon growth can either be stalled or accelerated. Altering the activation state of Limk1 also influences subsequent guidance decisions: accelerated axons make rostrocaudal projection errors while navigating their intermediate target, the floor plate. These results suggest that guidance cues can specify information about the rate of growth, to ensure that axons reach subsequent signals either at particular times or speeds during development.

Stromal cell-derived factor-1 and hepatocyte growth factor guide axon projections to the extraocular muscles

Vertebrate eye movements depend on the co-ordinated function of six extraocular muscles that are innervated by the oculomotor, trochlear, and abducens nerves. Here, we show that the diffusible factors, stromal cell-derived factor-1 (SDF-1) and hepatocyte growth factor (HGF), guide the development of these axon projections. SDF-1 is expressed in the mesenchyme around the oculomotor nerve exit point, and oculomotor axons fail to exit the neuroepithelium in mice mutant for the SDF-1 receptor CXCR4. Both SDF-1 and HGF are expressed in or around the ventral and dorsal oblique muscles, which are distal targets for the oculomotor and trochlear nerves, respectively, as well as in the muscles which are later targets for oculomotor axon branches. We find that in vitro SDF-1 and HGF promote the growth of oculomotor and trochlear axons, whereas SDF-1 also chemoattracts oculomotor axons. Oculomotor neurons show increased branching in the presence of SDF-1 and HGF singly or together. HGF promotes the growth of trochlear axons more than that of oculomotor axons. Taken together, these data point to a role for both SDF-1 and HGF in extraocular nerve projections and indicate that SDF-1 functions specifically in the development of the oculomotor nerve, including oculomotor axon branch formation to secondary muscle targets. HGF shows some specificity in preferentially enhancing development of the trochlear nerve.

Guidance of postural motoneurons requires MAPK/ERK signaling downstream of fibroblast growth factor receptor 1

Identification of intracellular signaling pathways necessary for appropriate axon guidance is challenging because many CNS populations used to study these events contain multiple cell types. Here, we resolve this issue by using mouse embryonic stem (ES) cells that were directed to differentiate into a population of motoneurons that exclusively innervate epaxial muscles [medial median motor column (MMCm) motoneurons]. These ES cell-derived MMCm motoneurons, like their endogenous counterparts, express fibroblast growth factor receptor 1 (FGFR1) and selectively extend axons toward the epaxial trophin FGF8. Unlike wild-type MMCm motoneurons, FGFR1(-/-) MMCm motoneurons show guidance defects when transplanted into the neural tube of chick embryos. Furthermore, activation of FGFR1 selectively signals through mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) for appropriate guidance in vitro, whereas overexpression of constitutively active MAPK/ERK in transplanted, or endogenous chick, MMCm cells causes guidance defects in vivo. These results indicate that MAPK/ERK activation downstream of FGFR1 is necessary for MMCm motor axon guidance and that ES cell-derived neurons provide an important tool for dissecting intracellular pathways required for axon guidance.

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.

FGF8 signaling regulates growth of midbrain dopaminergic axons by inducing semaphorin 3F

Accumulating evidence indicates that signaling centers controlling the dorsoventral (DV) polarization of the neural tube, the roof plate and the floor plate, play crucial roles in axon guidance along the DV axis. However, the role of signaling centers regulating the rostrocaudal (RC) polarization of the neural tube in axon guidance along the RC axis remains unknown. Here, we show that a signaling center located at the midbrain-hindbrain boundary (MHB) regulates the rostrally directed growth of axons from midbrain dopaminergic neurons (mDANs). We found that beads soaked with fibroblast growth factor 8 (FGF8), a signaling molecule that mediates patterning activities of the MHB, repelled mDAN axons that extended through the diencephalon. This repulsion may be mediated by semaphorin 3F (sema3F) because (1) FGF8-soaked beads induced an increase in expression of sema3F, (2) sema3F expression in the midbrain was essentially abolished by the application of an FGF receptor tyrosine kinase inhibitor, and (3) mDAN axonal growth was also inhibited by sema3F. Furthermore, mDAN axons expressed a sema3F receptor, neuropilin-2 (nrp2), and the removal of nrp-2 by gene targeting caused caudal growth of mDAN axons. These results indicate that the MHB signaling center regulates the growth polarity of mDAN axons along the RC axis by inducing sema3F.

Weaving the neuronal net with target-derived fibroblast growth factors

Fibroblast growth factors (FGFs) are a large family of secreted growth factors that are involved in the development, regeneration and repair of various tissues. In the nervous system, FGFs have been implicated in early developmental processes, such as neural induction, proliferation and patterning. Accumulating data indicate that FGFs are also important for the formation of functional neural networks. The role of FGFs in axon guidance, target recognition and synaptic differentiation as target-derived factors, and how they cooperate with cell adhesion molecules that are also involved in the wiring of the nervous system are the focus of this review.

BMP type I receptor complexes have distinct activities mediating cell fate and axon guidance decisions

The finding that morphogens, signalling molecules that specify cell identity, also act as axon guidance molecules has raised the possibility that the mechanisms that establish neural cell fate are also used to assemble neuronal circuits. It remains unresolved, however, how cells differentially transduce the cell fate specification and guidance activities of morphogens. To address this question, we have examined the mechanism by which the Bone morphogenetic proteins (BMPs) guide commissural axons in the developing spinal cord. In contrast to studies that have suggested that morphogens direct axon guidance decisions using non-canonical signal transduction factors, our results indicate that canonical components of the BMP signalling pathway, the type I BMP receptors (BMPRs), are both necessary and sufficient to specify the fate of commissural neurons and guide their axonal projections. However,whereas the induction of cell fate is a shared property of both type I BMPRs,axon guidance is chiefly mediated by only one of the type I BMPRs, BMPRIB. Taken together, these results indicate that the diverse activities of BMP morphogens can be accounted for by the differential use of distinct components of the canonical BMPR complex.

Initiation to end point: the multiple roles of fibroblast growth factors in neural development

From a wealth of experimental findings, derived from both in vitro and in vivo experiments, it is becoming clear that fibroblast growth factors regulate processes that are central to all aspects of nervous system development. Some of these functions are well known, whereas others, such as the roles of these proteins in axon guidance and synaptogenesis, have been established only recently. The emergent picture is one of remarkable economy, in which this family of ligands is deployed and redeployed at successive developmental stages to sculpt the nervous system.

FGF signalling pathways in development of the midbrain and anterior hindbrain

Development of the central nervous system is coordinated by intercellular signalling centres established within the neural tube. The isthmic organizer (IsO), located between the midbrain and anterior hindbrain, is one such centre. Important signal molecules secreted by the IsO include members of the fibroblast growth factor and Wnt families. These signals are integrated with dorsally and ventrally derived signals to regulate development of the midbrain and rhombomere 1 of the hindbrain. The IsO is operational for a remarkably long period of time. Depending on the developmental stage, it controls a variety of processes such as cell survival, cell identity, neural precursor proliferation, neuronal differentiation and axon guidance. This review focuses on the fibroblast growth factor signalling, its novel molecular regulatory mechanisms and how this pathway regulates multiple aspects of cell behaviour in the developing midbrain and anterior hindbrain.

Motor axon guidance of the mammalian trochlear and phrenic nerves: dependence on the netrin receptor Unc5c and modifier loci

Netrin signaling is important to guide migrating neurons and axons in many systems. Experiments with vertebrate CNS explants suggested netrin is bifunctional, attracting some axons and repelling others. Netrin1-expressing cells attracted spinal commissural axons and repelled trochlear cranial nerve axons in these experiments. Subsequent genetic studies demonstrated that multiple axon types, including those of the spinal commissural neurons, are attracted to netrin in vivo; however, an in vivo role for netrin signaling in trochlear nerve repulsion has not been observed. Here, we demonstrate that mice with a null mutation in the netrin receptor Unc5c on the inbred C57BL/6J (B6) genetic background have ventral/ipsilateral trochlear nerve misprojections. These misprojections are attenuated on a hybrid B6 x SJL background. In addition, B6.Unc5c(-/-) mice die as neonates of apparent respiratory distress and have incomplete phrenic nerve innervation of the diaphragm muscle. Neither the trochlear nerve misprojections nor the phrenic nerve phenotype was observed in B6 embryos lacking the netrin receptors DCC or Neogenin1, or the ligand netrin1, indicating these signaling molecules are dispensable for guidance of these axons. Like the trochlear nerve, the phrenic nerve phenotype is modified in a B6 x SJL hybrid background. To identify these modifier loci, we performed genome scans of the hybrid Unc5c(-/-) mice and found a major SJL-derived suppressor locus on Chromosome 17. Our results provide the first evidence that genes involved in netrin signaling are necessary for proper mammalian spinal motor axon development and trochlear axon guidance. In addition, they demonstrate the importance of modifier genes in vertebrate axonal guidance.

FGF as a Target-Derived Chemoattractant for Developing Motor Axons Genetically Programmed by the LIM Code

LIM transcription factors confer developing axons with specific navigational properties, but the downstream guidance receptors and ligands are not well defined. The dermomyotome, a transient structure from which axial muscles arise, is the source of a secreted long-range chemoattractant specific for medial-class spinal motor neuron axons (MMCm axons). We show that fibroblast growth factors (FGFs) produced by the dermomyotome selectively attract MMCm axons in vitro. FGF receptor 1 (FGFR1) expression is restricted to MMCm neurons, and conditional deletion of FGFR1 causes motor axon guidance defects. Furthermore, reprogramming the identity of limb-innervating motor neurons to that of dermomyotome-innervating MMCm cells using the LIM factor Lhx3 induces FGFR1 expression and shifts an increased number of motor axons to an FGF-responsive state. These results point to a role for FGF signaling in axon guidance and further unravel how downstream effectors of LIM codes direct wiring of the developing nervous system.

Molecular control of spinal accessory motor neuron/axon development in the mouse spinal cord

Within the developing vertebrate spinal cord, motor neuron subtypes are distinguished by the settling positions of their cell bodies, patterns of gene expression, and the paths their axons follow to exit the CNS. The inclusive set of cues required to guide a given motor axon subtype from cell body to target has yet to be identified, in any species. This is attributable, in part, to the unavailability of markers that demarcate the complete trajectory followed by a specific class of spinal motor axons. Most spinal motor neurons extend axons out of the CNS through ventral exit points. In contrast, spinal accessory motor neurons (SACMNs) project dorsally directed axons through lateral exit points (LEPs), and these axons assemble into the spinal accessory nerve (SAN). Here we show that an antibody against BEN/ALCAM/SC1/DM-GRASP/MuSC selectively labels mouse SACMNs and can be used to trace the pathfinding of SACMN axons. We use this marker, together with a battery of transcription factor-deficient or guidance cue/receptor-deficient mice to identify molecules required for distinct stages of SACMN development. Specifically, we find that Gli2 is required for the initial extension of axons from SACMN cell bodies, and that netrin-1 and its receptor Dcc are required for the proper dorsal migration of these cells and the dorsally directed extension of SACMN axons toward the LEPs. Furthermore, in the absence of the transcription factor Nkx2.9, SACMN axons fail to exit the CNS. Together, these findings suggest molecular mechanisms that are likely to regulate key steps in SACMN development.

Expression of a Dominant Negative FGF Receptor in Developing GNRH1 Neurons Disrupts Axon Outgrowth and Targeting to the Median Eminence1

During development, neurons that synthesize and release gonadotropin-releasing hormone (GNRH1) extend their axons to the median eminence (ME) to establish neurosecretory contacts necessary for hormone secretion. Signals that coordinate this process are not known, but could involve the activation of fibroblast growth factor receptors (FGFRs) expressed on developing GNRH1 neurons. Using both whole-animal and cell culture approaches, this study examines the direct role of FGFR signaling in the extension and guidance of GNRH1 axons to the ME. In vivo retrograde labeling with fluorogold (FG) first showed a significant reduction in the projections of GNRH1 axons to the circumventricular organs (including the ME) in transgenic mice expressing a dominant negative FGF receptor (dnFGFR) in GNRH1 neurons. Using a primary GNRH1 neuronal culture system, we examined if compromised axon extension and directional growth led to the reduced axon targeting efficiency seen in vivo. Primary cultures of GNRH1 neurons were established from Embryonic Day 15.5 embryos, an age when GNRH1 neurons are actively targeting the ME. Cultured GNRH1 neurons expressing dnFGFR (dnFGFR neurons) exhibited attenuated activation of signaling pathways and reduced neurite outgrowth in response to FGF2. Further, dnFGFR neurons failed to preferentially target neurites toward cocultured ME explant and FGF2-coated beads, suggesting a defect in axon pathfinding. Together, these findings describe a direct role of FGFR signaling in the elongation and guidance of GNRH1 axons to the ME.

Longitudinal axon guidance

Our knowledge about molecular mechanisms underlying axon guidance along the antero-posterior axis in contrast to the dorso-ventral axis of the developing nervous system is very limited. During the past two years in vitro and in vivo studies have indicated that morphogens have a role in longitudinal axon guidance. Morphogens are secreted proteins that act in a concentration-dependent manner on susceptible groups of precursor cells and induce their differentiation to a specific cell fate. Thus, gradients of morphogens are responsible for the appropriate patterning of the nervous system during early phases of neural development. Therefore, it was surprising to find that gradients of two of these morphogens, Wnt4 and Shh, can be re-used for longitudinal axon guidance during later stages of nervous system development.

The differentiation and morphogenesis of craniofacial muscles

Unraveling the complex tissue interactions necessary to generate the structural and functional diversity present among craniofacial muscles is challenging. These muscles initiate their development within a mesenchymal population bounded by the brain, pharyngeal endoderm, surface ectoderm, and neural crest cells. This set of spatial relations, and in particular the segmental properties of these adjacent tissues, are unique to the head. Additionally, the lack of early epithelialization in head mesoderm necessitates strategies for generating discrete myogenic foci that may differ from those operating in the trunk. Molecular data indeed indicate dissimilar methods of regulation, yet transplantation studies suggest that some head and trunk myogenic populations are interchangeable. The first goal of this review is to present key features of these diversities, identifying and comparing tissue and molecular interactions regulating myogenesis in the head and trunk. Our second focus is on the diverse morphogenetic movements exhibited by craniofacial muscles. Precursors of tongue muscles partly mimic migrations of appendicular myoblasts, whereas myoblasts destined to form extraocular muscles condense within paraxial mesoderm, then as large cohorts they cross the mesoderm:neural crest interface en route to periocular regions. Branchial muscle precursors exhibit yet another strategy, establishing contacts with neural crest populations before branchial arch formation and maintaining these relations through subsequent stages of morphogenesis. With many of the prerequisite stepping-stones in our knowledge of craniofacial myogenesis now in place, discovering the cellular and molecular interactions necessary to initiate and sustain the differentiation and morphogenesis of these neglected craniofacial muscles is now an attainable goal.

Development of midline cell types and commissural axon tracts requires Fgfr1 in the cerebrum

The adult cerebral hemispheres are connected to each other by specialized midline cell types and by three axonal tracts: the corpus callosum, the hippocampal commissure, and the anterior commissure. Many steps are required for these tracts to form, including early patterning and later axon pathfinding steps. Here, the requirement for FGF signaling in forming midline cell types and commissural axon tracts of the cerebral hemispheres is examined. Fgfr1, but not Fgfr3, is found to be essential for establishing all three commissural tracts. In an Fgfr1 mutant, commissural neurons are present and initially project their axons, but these fail to cross the midline that separates the hemispheres. Moreover, midline patterning defects are observed in the mutant. These defects include the loss of the septum and three specialized glial cell types, the indusium griseum glia, midline zipper glia, and glial wedge. Our findings demonstrate that FGF signaling is required for generating telencephalic midline structures, in particular septal and glial cell types and all three cerebral commissures. In addition, analysis of the Fgfr1 heterozygous mutant, in which midline patterning is normal but commissural defects still occur, suggests that at least two distinct FGF-dependent mechanisms underlie the formation of the cerebral commissures.

Morphogens as growth cone signalling molecules

Morphogen signalling among cells is one of the most important mechanisms underlying the progressive patterning of embryos. Members of the hedgehog (Hh), wingless (Wnt), transforming growth factor-beta (TGFbeta), and fibroblast growth factor (Fgf) families of extracellular signalling molecules act as morphogens. Recent studies have demonstrated that members of these four families of proteins, secreted by well-characterised organiser centres in the central nervous system (CNS) as floor plate or midbrain-hindbrain boundary, are reused at later developmental stages to control axon growth. Here, we have summarised the evidence for this novel idea with a particular emphasis on those related to Shh and Wnt signalling-the object of some works in our laboratory.

Development of the central nervous system in the larvacean Oikopleura dioica and the evolution of the chordate brain

In non-vertebrate chordates, central nervous system (CNS) development has been studied in only two taxa, the Cephalochordata and a single Class (Ascidiacea) of the morphologically diverse Urochordata. To understand development and molecular regionalization of the brain in a different deeply diverging chordate clade, we isolated and determined the expression patterns of orthologs of vertebrate CNS markers (otxa, otxb, otxc, pax6, pax2/5/8a, pax2/5/8b, engrailed, and hox1) in Oikopleura dioica (Subphylum Urochordata, Class Larvacea). The three Oikopleura otx genes are expressed similarly to vertebrate Otx paralogs, demonstrating that trans-homologs converged on similar evolutionary outcomes by independent neo- or subfunctionalization processes during the evolution of the two taxa. This work revealed that the Oikopleura CNS possesses homologs of the vertebrate forebrain, hindbrain, and spinal cord, but not the midbrain. Comparing larvacean gene expression patterns to published results in ascidians disclosed important developmental differences and similarities that suggest mechanisms of development likely present in their last common ancestor. In contrast to ascidians, the lack of a radical reorganization of the CNS as larvaceans become adults allows us to relate embryonic gene expression patterns to three subdivisions of the adult anterior brain. Our study of the Oikopleura brain provides new insights into chordate CNS evolution: first, the absence of midbrain is a urochordate synapomorphy and not a peculiarity of ascidians, perhaps resulting from their drastic CNS metamorphosis; second, there is no convincing evidence for a homolog of a midbrain-hindbrain boundary (MHB) organizer in urochordates; and third, the expression pattern of "MHB-genes" in the urochordate hindbrain suggests that they function in the development of specific neurons rather than in an MHB organizer.

Patterning molecules; multitasking in the nervous system

Classical patterning molecules that were previously implicated in controlling cell fate choices in the nervous system have recently been shown to play additional roles in axon guidance and synaptogenesis. Bone morphogenetic proteins (BMPs), Sonic hedgehog (Shh), Wnts, and fibroblast growth factors (FGFs) all participate in multiple acts of controlling neuronal circuit assembly. Depending on the cellular context, they can provide instructive signals at the growth cone or synapse or alternatively can elicit responses in the nucleus initiating transcriptional changes. Differences in signal transduction pathways may contribute to the diversity of the functional repertoire of these versatile molecules.

Morphogen signaling at the vertebrate growth cone: A few cases or a general strategy?

Axon navigation relies on the competence of growth cones to sense and interpret attractive and repulsive guidance cues present along their trajectory. For most neurons, this process is mediated by a limited number of conserved families of ligand-receptor signaling systems, including Ephrin/Eph, Netrins/DCC-Unc5, Slits/Robo, and Semaphorins/Plexin-Neuropilin. Recent studies have demonstrated that some neurons respond also to well-known secreted signaling molecules, best known for their roles as morphogens, such as BMP7, SHH, FGF8, and Wnt. Thus, retina ganglion cell axon navigation is influenced by FGF, SHH, and possibly BMP signaling. Similarly, commissural neurons in the spinal cord respond sequentially to the activity of BMP, SHH, and Wnt to extend toward and away from their intermediate target, the floor plate. The data that support this conclusion will be summarized and how morphogens may signal at the growth cone will be discussed.

Evolutionary Patterns of Cranial Nerve Efferent Nuclei in Vertebrates

All vertebrates have a similar series of rhombomeric hindbrain segments within which cranial nerve efferent nuclei are distributed in a similar rostrocaudal sequence. The registration between these two morphological patterns is reviewed here to highlight the conserved vs. variable aspects of hindbrain organization contributing to diversification of efferent sub-nuclei. Recent studies of segmental origins and migrations of branchiomotor, visceromotor and octavolateral efferent neurons revealed more segmental similarities than differences among vertebrates. Nonetheless, discrete variations exist in the origins of trigeminal, abducens and glossopharyngeal efferent nuclei. Segmental variation of the abducens nucleus remains the sole example of efferent neuronal homeosis during vertebrate hindbrain evolution. Comparison of cranial efferent segmental variations with surrounding intrinsic neurons will distinguish evolutionary changes in segment identity from lesser transformations in expression of unique neuronal types. The diversification of motoneuronal subgroups serving new muscles and functions appears to occur primarily by elaboration within and migration from already established segmental efferent pools rather than by de novo specification in different segmental locations. Identifying subtle variations in segment-specific neuronal phenotypes requires studies of cranial efferent organization within highly diverse groups such as teleosts and mammals.

Divide et Impera – the midbrain–hindbrain boundary and its organizer

The midbrain-hindbrain organizer (MHO) is a signalling centre that orchestrates development of the mesencephalic and anterior metencephalic primordia. In recent years, details have been revealed about the molecular nature of these signals, their transmission and reception, and the regulatory processes associated with MHO function. This article reviews recent progress in understanding the genetic and molecular components of the MHO, and how they synergize to control brain development.

Local inhibition guides the trajectory of early longitudinal tracts in the developing chick brain

During development of the chick central nervous system, the trajectories of the descending medial and lateral longitudinal fascicles (MLF and LLF) are pioneered by axons originating from the interstitial nucleus of Cajal (INC) and the mesencephalic trigeminal nucleus (MTN), respectively. Both tracts cross rhombomere 1 at two specific locations in the basal plate. In this study, we have investigated the molecular properties of these crossing points and find that they are permissive regions situated in an otherwise inhibitory boundary region. We show that the dorsal part of rhombomere 1 is inhibitory for the growth of both MTN and INC axons. Ventrally, MLF and LLF axons are repelled from the midline by Slit proteins. Our results reveal the existence of a new repulsive/inhibitory mechanism for axons in the alar plate in addition to the ventral repulsion by Slit proteins. This suggests a model where MLF and LLF axons are channeled longitudinally within the neural tube by both dorsal and ventral constraints.