Early development of the mesencephalic trigeminal nucleus

Early embryogenesis in CHDFIDD mouse model reveals facial clefts and altered cranial neurogenesis

CDK13-related disorder, also known as congenital heart defects, dysmorphic facial features and intellectual developmental disorder (CHDFIDD) is associated with mutations in the CDK13 gene encoding transcription-regulating cyclin-dependent kinase 13 (CDK13). Here, we focused on the development of craniofacial structures and analyzed early embryonic stages in CHDFIDD mouse models, with one model comprising a hypomorphic mutation in Cdk13 and exhibiting cleft lip/palate, and another model comprising knockout of Cdk13, featuring a stronger phenotype including midfacial cleft. Cdk13 was found to be physiologically expressed at high levels in the mouse embryonic craniofacial structures, namely in the forebrain, nasal epithelium and maxillary mesenchyme. We also uncovered that Cdk13 deficiency leads to development of hypoplastic branches of the trigeminal nerve including the maxillary branch. Additionally, we detected significant changes in the expression levels of genes involved in neurogenesis (Ache, Dcx, Mef2c, Neurog1, Ntn1, Pou4f1) within the developing palatal shelves. These results, together with changes in the expression pattern of other key face-specific genes (Fgf8, Foxd1, Msx1, Meis2 and Shh) at early stages in Cdk13 mutant embryos, demonstrate a key role of CDK13 in the regulation of craniofacial morphogenesis.

Olfactory placode generates a diverse population of neurons expressing GnRH, somatostatin mRNA, neuropeptide Y, or calbindin in the chick forebrain

The olfactory placode (OP) of vertebrates generates several classes of migrating cells, including hypothalamic gonadotropin-releasing hormone (GnRH)-producing neurons, which play essential roles in the reproduction system. Previous studies using OP cell labeling have demonstrated that OP-derived non-GnRH cells enter the developing forebrain; however, their final fates and phenotypes are less well understood. In chick embryos, a subpopulation of migratory cells from the OP that is distinct from GnRH neurons transiently expresses somatostatin (SS). We postulated that these cells are destined to develop into brain neurons. In this study, we examined the expression pattern of SS mRNA in the olfactory-forebrain region during development, as well as the destination of OP-derived migratory cells, including SS mRNA-expressing cells. Utilizing the Tol2 genomic integration system to induce long-term fluorescent protein expression in OP cells, we found that OP-derived migratory cells labeled at embryonic day (E) 3 resided in the olfactory nerve and medial forebrain at E17-19. A subpopulation of green fluorescent protein (GFP)-labeled GnRH neurons that remained in the olfactory nerve was considered to comprise terminal nerve neurons. In the forebrain, GFP-labeled cells showed a distribution pattern similar to that of GnRH neurons. A large proportion of GFP-labeled cells expressed the mature neuronal marker NeuN. Among the GFP-labeled cells, the percentage of GnRH neurons was low, while the remaining GnRH-negative neurons either expressed SS mRNA, neuropeptide Y, or calbindin D-28k or did not express any of them. These results indicate that a diverse population of OP-derived neuronal cells, other than GnRH neurons, integrates into the chick medial forebrain.

Early Cerebellar Development in Relation to the Trigeminal System

Despite the wealth of knowledge of adult cerebellar connectivity, little is known about the developmental mechanisms that underpin its development. Early connectivity is important because it is the foundation of the neural networks crucial for neuronal function and serves as a scaffold on which later tracts form. Conventionally, it is believed that afferents from the vestibular system are the first to invade the cerebellum, at embryonic days (E) 11-E12/13 in mice, where they target the new born Purkinje cells. However, we have demonstrated that pioneer axons that originate from the trigeminal ganglia are already present in the cerebellar primordium by E9, a stage at which afferents from the vestibular ganglia have not yet reached the brainstem, where they target neurons of the cerebellar nuclei. An early-born subset of cerebellar nuclei may be derived from the mesencephalon. These may be the target of the earliest pioneer axons. They form the early connectivity at the rostral end. This is consistent with the notion that the formation of the antero-posterior axis follows a rostro-caudal sequence. The finding that trigeminal ganglion-derived pioneer axons enter the cerebellar primordium before Purkinje cells are born and target the cerebellar nuclei, reveals a novel perspective on the development of early cerebellar connectivity.

Development of a system to analyze oral frailty associated with Alzheimer's disease using a mouse model

The rapid aging of the population makes the detection and prevention of frailty increasingly important. Oral frailty has been proposed as a novel frailty phenotype and is defined as a decrease in oral function coexisting with a decline in cognitive and physical functions. Oral frailty has received particular attention in relation to Alzheimer's disease (AD). However, the pathomechanisms of oral frailty related to AD remain unknown. It is assumed that the mesencephalic trigeminal nucleus (Vmes), which controls mastication, is affected by AD pathology, and as a result, masticatory function may be impaired. To investigate this possibility, we included male 3 × Tg-AD mice and their non-transgenic counterpart (NonTg) of 3–4 months of age in the present study. Immunohistochemistry revealed amyloid-β deposition and excessive tau phosphorylation in the Vmes of 3 × Tg-AD mice. Furthermore, vesicular glutamate transporter 1-immunopositive axon varicosities, which are derived from Vmes neurons, were significantly reduced in the trigeminal motor nucleus of 3 × Tg-AD mice. To investigate whether the AD pathology observed in the Vmes affects masticatory function, we analyzed electromyography of the masseter muscle during feeding. The 3 × Tg-AD mice showed a significant delay in masticatory rhythm compared to NonTg mice. Furthermore, we developed a system to simultaneously record bite force and electromyography of masseter, and devised a new method to estimate bite force during food chewing in mice. Since the muscle activity of the masseter showed a high correlation with bite force, it could be accurately estimated from the muscle activity. The estimated bite force of 3 × Tg-AD mice eating sunflower seeds was predominantly smaller than that of NonTg mice. However, there was no difference in masseter weight or muscle fiber cross-sectional area between the two groups, suggesting that the decreased bite force and delayed mastication rhythm observed in 3 × Tg-AD mice were not due to abnormality of the masseter. In conclusion, the decreased masticatory function observed in 3 × Tg-AD mice was most likely caused by AD pathology in the Vmes. Thus, novel quantitative analyses of masticatory function using the mouse model of AD enabled a comprehensive understanding of oral frailty pathogenesis.

Molecular Segmentation of the Spinal Trigeminal Nucleus in the Adult Mouse Brain

The trigeminal column is a hindbrain structure formed by second order sensory neurons that receive afferences from trigeminal primary (ganglionic) nerve fibers. Classical studies subdivide it into the principal sensory trigeminal nucleus located next to the pontine nerve root, and the spinal trigeminal nucleus which in turn consists of oral, interpolar and caudal subnuclei. On the other hand, according to the prosomeric model, this column would be subdivided into segmental units derived from respective rhombomeres. Experimental studies have mapped the principal sensory trigeminal nucleus to pontine rhombomeres (r) r2-r3 in the mouse. The spinal trigeminal nucleus emerges as a plurisegmental formation covering several rhombomeres (r4 to r11 in mice) across pontine, retropontine and medullary hindbrain regions. In the present work we reexamined the issue of rhombomeric vs. classical subdivisions of this column. To this end, we analyzed its subdivisions in an AZIN2-lacZ transgenic mouse, known as a reference model for hindbrain topography, together with transgenic reporter lines for trigeminal fibers. We screened as well for genes differentially expressed along the axial dimension of this structure in the adult and juvenile mouse brain. This analysis yielded genes from multiple functional families that display transverse domains fitting the mentioned rhombomeric map. The spinal trigeminal nucleus thus represents a plurisegmental structure with a series of distinct neuromeric units having unique combinatorial molecular profiles.

Why is the mesencephalic nucleus of the trigeminal nerve situated inside the brain?

Primary sensory neurons are usually situated in ganglia outside the brain, while the mesencephalic nucleus of the trigeminal nerve (Me5) is situated inside the brain. However, it remains unknown why only Me5 situated inside the brain is. The neurons of Me5 are the cell bodies of primary afferent fibers concerned with the muscles of mastication and periodontal receptors of both maxillary and mandibular teeth. Interestingly, there was no Me5 till the evolution level of the agnatha, vertebrates which lack jaws, while Me5 appeared with the evolution of jawed vertebrates, the gnathostomes. Thus, I speculate that the appearance of jaws necessitated the emergence of a novel sensory system including newly-made primary sensory neurons to co-ordinate jaw movement and this need was met by the appearance of Me5. Although primary sensory neurons are usually generated from the neural crest or the neurogenic placodes, primary sensory neurons in Me5 are derived from neuroepithelium of the dorsal midline of the midbrain. Taken together, I propose the following hypothesis; (1) Me5 did not exist till the evolution level of agnatha, which lacks jaw. (2) When jawed vertebrates evolved, a new sensory system including new primary sensory neurons for mastication was needed. (3) At that point, there was no capacity for the neural crest and neurogenic placodes to make primary sensory neurons. (4) However, there remained capacity only for the neuroepithelium of the midbrain to make primary sensory neurons. (5) Thus, Me5 was newly made inside the CNS.

Vertebrate Sensory Ganglia: Common and Divergent Features of the Transcriptional Programs Generating Their Functional Specialization

Sensory fibers of the peripheral nervous system carry sensation from specific sense structures or use different tissues and organs as receptive fields, and convey this information to the central nervous system. In the head of vertebrates, each cranial sensory ganglia and associated nerves perform specific functions. Sensory ganglia are composed of different types of specialized neurons in which two broad categories can be distinguished, somatosensory neurons relaying all sensations that are felt and visceral sensory neurons sensing the internal milieu and controlling body homeostasis. While in the trunk somatosensory neurons composing the dorsal root ganglia are derived exclusively from neural crest cells, somato- and visceral sensory neurons of cranial sensory ganglia have a dual origin, with contributions from both neural crest and placodes. As most studies on sensory neurogenesis have focused on dorsal root ganglia, our understanding of the molecular mechanisms underlying the embryonic development of the different cranial sensory ganglia remains today rudimentary. However, using single-cell RNA sequencing, recent studies have made significant advances in the characterization of the neuronal diversity of most sensory ganglia. Here we summarize the general anatomy, function and neuronal diversity of cranial sensory ganglia. We then provide an overview of our current knowledge of the transcriptional networks controlling neurogenesis and neuronal diversification in the developing sensory system, focusing on cranial sensory ganglia, highlighting specific aspects of their development and comparing it to that of trunk sensory ganglia.

Loss of Axon Bifurcation in Mesencephalic Trigeminal Neurons Impairs the Maximal Biting Force in Npr2-Deficient Mice

Bifurcation of axons from dorsal root ganglion (DRG) and cranial sensory ganglion (CSG) neurons is mediated by a cGMP-dependent signaling pathway composed of the ligand C-type natriuretic peptide (CNP), the receptor guanylyl cyclase Npr2 and the cGMP-dependent protein kinase I (cGKI). Here, we demonstrate that mesencephalic trigeminal neurons (MTN) which are the only somatosensory neurons whose cell bodies are located within the CNS co-express Npr2 and cGKI. Afferents of MTNs form Y-shaped branches in rhombomere 2 where the ligand CNP is expressed. Analyzing mouse mutants deficient for CNP or Npr2 we found that in the absence of CNP-induced cGMP signaling MTN afferents no longer bifurcate and instead extend either into the trigeminal root or caudally in the hindbrain. Since MTNs provide sensory information from jaw closing muscles and periodontal ligaments we measured the bite force of conditional mouse mutants of Npr2 (Npr2^flox/flox;Engr1^Cre ) that lack bifurcation of MTN whereas the bifurcation of trigeminal afferents is normal. Our study revealed that the maximal biting force of both sexes is reduced in Npr2^flox/flox;Engr1^Cre mice as compared to their Npr2^flox/flox littermate controls. In conclusion sensory feedback mechanisms from jaw closing muscles or periodontal ligaments might be impaired in the absence of MTN axon bifurcation.

Molecular Analysis of Sensory Axon Branching Unraveled a cGMP-Dependent Signaling Cascade

Axonal branching is a key process in the establishment of circuit connectivity within the nervous system. Molecular-genetic studies have shown that a specific form of axonal branching—the bifurcation of sensory neurons at the transition zone between the peripheral and the central nervous system—is regulated by a cyclic guanosine monophosphate (cGMP)-dependent signaling cascade which is composed of C-type natriuretic peptide (CNP), the receptor guanylyl cyclase Npr2, and cGMP-dependent protein kinase Iα (cGKIα). In the absence of any one of these components, neurons in dorsal root ganglia (DRG) and cranial sensory ganglia no longer bifurcate, and instead turn in either an ascending or a descending direction. In contrast, collateral axonal branch formation which represents a second type of axonal branch formation is not affected by inactivation of CNP, Npr2, or cGKI. Whereas axon bifurcation was lost in mouse mutants deficient for components of CNP-induced cGMP formation; the absence of the cGMP-degrading enzyme phosphodiesterase 2A had no effect on axon bifurcation. Adult mice that lack sensory axon bifurcation due to the conditional inactivation of Npr2-mediated cGMP signaling in DRG neurons demonstrated an altered shape of sensory axon terminal fields in the spinal cord, indicating that elaborate compensatory mechanisms reorganize neuronal circuits in the absence of bifurcation. On a functional level, these mice showed impaired heat sensation and nociception induced by chemical irritants, whereas responses to cold sensation, mechanical stimulation, and motor coordination are normal. These data point to a critical role of axon bifurcation for the processing of acute pain perception.

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.

Evolutionary Conservation of the Early Axon Scaffold in the Vertebrate Brain

The early axon scaffold is the first axonal structure to appear in the rostral brain of vertebrates, paving the way for later, more complex connections. Several early axon scaffold components are conserved between all vertebrates; most notably two main ventral longitudinal tracts, the tract of the postoptic commissure and the medial longitudinal fascicle. While the overall structure is remarkably similar, differences both in the organization and the development of the early tracts are apparent. This review will bring together extensive data from the last 25 years in different vertebrates and for the first time, the timing and anatomy of these early tracts have been directly compared. Representatives of major vertebrate clades, including cat shark, Xenopus, chick, and mouse embryos, will be compared using immunohistochemistry staining based on previous results. There is still confusion over the nomenclature and homology of these tracts which this review will aim to address. The discussion here is relevant both for understanding the evolution of the early axon scaffold and for future studies into the molecular regulation of its formation.

Specification of sensory neurons occurs through diverse developmental programs functioning in the brain and spinal cord

BACKGROUND

Vertebrates possess two populations of sensory neurons located within the central nervous system: Rohon-Beard (RB) and mesencephalic trigeminal nucleus (MTN) neurons. RB neurons are transient spinal cord neurons whilst MTN neurons are the proprioceptive cells that innervate the jaw muscles. It has been suggested that MTN and RB neurons share similarities and may have a common developmental program, but it is unclear how similar or different their development is.

RESULTS

We have dissected RB and MTN neuronal specification in zebrafish. We find that RB and MTN neurons express a core set of genes indicative of sensory neurons, but find these are also expressed by adjacent diencephalic neurons. Unlike RB neurons, our evidence argues against a role for the neural crest during MTN development. We additionally find that neurogenin1 function is dispensable for MTN differentiation, unlike RB cells and all other sensory neurons. Finally, we demonstrate that, although Notch signalling is involved in RB development, it is not involved in the generation of MTN cells.

CONCLUSIONS

Our work reveals fundamental differences between the development of MTN and RB neurons and suggests that these populations are non-homologous and thus have distinct developmental and, probably, evolutionary origins.

The effects of bilateral lesions of the mesencephalic trigeminal sensory nucleus on nocturnal feeding and related behaviors in mice

AIMS

The mesencephalic trigeminal sensory nucleus (Me5), which receives signals originating from oral proprioceptors and projects its fibers to the hypothalamus, regulates mastication and modulates satiation. Because the Me5 neurons display circadian rhythms in circadian mPer1 gene expression and bilateral Me5 lesions change feeding and exploratory behavior profiles, we speculated that Me5 may influence the daily timing of feeding. Therefore, we explored the effects of bilateral caudal Me5 lesions on the circadian profiles of feeding and its related behaviors.

MAIN METHODS

We measured the activities of feeding, drinking, and locomotion for 24h using an automated feeding behavior measurement apparatus and analyzed the mRNA expression levels of hypothalamic orexigenic and anorexigenic signaling molecules in both Me5-lesioned and sham-operated mice.

KEY FINDINGS

Food and water intake and locomotor activity decreased significantly in Me5-lesioned mice during the dark phase without affecting these total indexes when measured over the entire day. Analysis of the mRNA expression levels of hypothalamic orexigenic and anorexigenic signaling molecules showed that prepro-orexin (orexin) mRNA in the perifornical area was significantly decreased during the dark phase only in Me5-lesioned mice.

SIGNIFICANCE

Bilateral caudal Me5 lesions alter the nocturnal properties of food and water intake and locomotor activity in mice and decrease the mRNA expression level of orexin in the perifornical area during the dark phase. These results suggest that Me5 activity may influence the nocturnal properties of feeding and its related behaviors by adjusting the activity of orexin neurons in the perifornical area.

Onecut factors control development of the Locus Coeruleus and of the mesencephalic trigeminal nucleus

The Locus Coeruleus (LC), the main noradrenergic nucleus in the vertebrate CNS, contributes to the regulation of several processes including arousal, sleep, adaptative behaviors and stress. Regulators controlling the formation of the LC have been identified but factors involved in its maintenance remain unknown. Here, we show that members of the Onecut (OC) family of transcription factors, namely HNF-6, OC-2 and OC-3, are required for maintenance of the LC phenotype. Indeed, in embryos lacking any OC proteins, LC neurons properly differentiate but abnormally migrate and eventually lose their noradrenergic characteristics. Surprisingly, the expression of Oc genes in these neurons is restricted to the earliest differentiation stages, suggesting that OC factors may regulate maintenance of the LC in a non cell-autonomous manner. Accordingly, the OC factors are present throughout development in a population directly adjacent to the LC, the rhombencephalic portion of the mesencephalic trigeminal nucleus (MTN). In the absence of OC factors, rhombencephalic MTN neurons fail to be generated, suggesting that OC proteins cell-autonomously control their production. Hence, we propose that OC factors are required at early developmental stages for differentiation of the MTN neurons that are in turn necessary for maintenance of the LC.

Development of the early axon scaffold in the rostral brain of the chick embryo

The arrangement of the early nerve connections in the embryonic vertebrate brain follows a well-conserved pattern, forming the early axon scaffold. The early axon tracts have been described in a number of anamniote species and in mouse, but a detailed analysis in chick is lacking. We have used immunostaining, axon tracing and in situ hybridisation to analyse the development of the early axon scaffold in the embryonic chick brain in relation to the neuromeric organisation of the brain. The first tract to be formed is the medial longitudinal fascicle (MLF), shortly followed by the tract of the postoptic commissure to pioneer the ventral longitudinal tract system. The MLF was found to originate from three different populations of neurones located in the diencephalon. Neurones close to the dorsal midline of the mesencephalon establish the descending tract of the mesencephalic nucleus of the trigeminus. Their axons pioneer the lateral longitudinal tract. At later stages, the tract of the posterior commissure emerges in the caudal pretectum as the first transversal tract. It is formed by dorsally projecting axons from neurones located in the ventral pretectum, and by ventrally projecting axons from neurones located in the dorsal pretectum. The organisation of neurones and axons in the chick brain is similar to that described in the mouse, though tracts form in a different order and appear more clearly distinguished than in the mammalian model.

The formation of the superior and jugular ganglia: insights into the generation of sensory neurons by the neural crest

The superior and jugular ganglia (S/JG) are the proximal ganglia of the IXth and Xth cranial nerves and the sensory neurons of these ganglia are neural crest derived. However, it has been unclear the extent to which their differentiation resembles that of the Dorsal Root Ganglia (DRGs). In the DRGs, neural crest cells undergo neuronal differentiation just after the onset of migration and there is evidence suggesting that these cells are pre-specified towards a sensory fate. We have analysed sensory neuronal differentiation in the S/JG. We show, in keeping with previous studies, that neuronal differentiation initiates long after the cessation of neural crest migration. We also find no evidence for the existence of migratory neural crest cells pre-specified towards a sensory phenotype prior to ganglion formation. Rather our results suggest that sensory neuronal differentiation in the S/JG is the result of localised spatiotemporal cues.

Krox-20 gene expression: influencing hindbrain-craniofacial developmental interactions

Krox-20 is a C(2)H(2)-type zinc-finger transcription factor that plays an essential role in hindbrain development. The Krox-20 null mutation results in hindbrain anomalies that result in neonatal death due to respiratory and feeding deficits. Here we review our studies of how the Krox- 20 null mutation impacts the development of motor and sensory systems critical for the production of consummatory behaviors (suckling/chewing). First, we demonstrated that Krox-20 null mutants suffer a selective loss of primary jaw-opening muscles during prenatal development. In vivo and in vitro studies are reviewed that highlight intrinsic defects in mutant jaw-opener muscles that contribute to muscle degeneration. Next we focus on the impact of the mutation on proprioceptive neurons activated during consummatory behaviors. Mesencephalic trigeminal (Me5) neurons are primary sensory neurons that relay jaw proprioception to the central nervous system. These cells are unique because their cell bodies are located in the central as opposed to the peripheral nervous system. Data are reviewed that demonstrate the impact of the mutation on Me5 neurons, a cell group traditionally thought to emerge from the mesencephalon. We show that Krox-20 null mutants have twice as many Me5 neurons relative to wildtypes at E15, but by birth have half the number of Me5 cells as wildtypes. TUNEL assays performed in each set of studies reveal that Krox-20 expression acts to protect both muscle and mesencephalic trigeminal neurons against apoptosis, suggesting that Krox-20, in addition to its role in hindbrain patterning, has a broader, long-lasting role in development.

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.

The derivatives of the Wnt3a lineage in the central nervous system

The dorsal midline of the vertebrate neural tube is a source of signals that direct cell fate specification and proliferation. Using genetic fate mapping in the mouse and a previously generated Wnt3aCre line, we report here that genetically labeled cells of the Wnt3a lineage migrate widely from the dorsal midline into the dorsal half of the adult brain and spinal cord, contributing to diverse structures in the diencephalon, midbrain, and brainstem and extensively populating the rostral spinal cord. Conspicuously, many of these structures are linked in specific functional networks. Wnt3a lineage cells populate nuclei of the central auditory system from the medulla to thalamus, and the trigeminal sensory system from the cervical spinal cord to the midbrain. Our findings reveal the rich contributions of the Wnt3a lineage to a variety of brain structures and show that functionally integrated nuclei can share a molecular identity, provided by transient gene expression early in their development.

Emergence of the sensory nervous system as defined by Foxs1 expression

Peripheral sensory neurons are derived from two distinct structures, the ectodermal placodes and the neural crest. Here, we establish the forkhead family transcription factor Foxs1 as an early sensory neuronal marker. Early embryonic Foxs1 expression was present in all the sensory nervous system regardless of cellular origin, but was not found in other placode and neural crest-derived cell types. Foxs1 expression was turned on in the sensory neuron precursors of the trunk. Consistently, expression of Sox10, that is present in undifferentiated multipotent neural crest cells (NCCs), was mutually exclusive to Foxs1. Acquirement of Foxs1 expression was used to study the emergence of the dorsal root ganglion (DRG) sensory neurons. Migrating pioneering Foxs1 expressing NCCs were found at the anterior dorsal somitic lip at the 18-somite stage. These cells showed limited proliferation and migrated to form a cluster in the ventral aspect of the coalescing ganglion, surrounded by Foxs1(-)/Sox10(+) migrating NCCs retaining a high rate of proliferation. Sensory neurogenesis of the Foxs1(-)/Sox10(+) precursors occurred within the condensed DRG starting with neurogenin-1 (Ngn1) and Brn3a expression. These data define a sequential emergence of neuronal precursors of the sensory nervous system with different molecular characteristics, starting during migration and continuing well after DRG condensation.

POU-domain factor Brn3a regulates both distinct and common programs of gene expression in the spinal and trigeminal sensory ganglia

Background

General somatic sensation is conveyed to the central nervous system at cranial levels by the trigeminal ganglion (TG), and at spinal levels by the dorsal root ganglia (DRG). Although these ganglia have similar functions, they have distinct embryological origins, in that both contain neurons originating from the neural crest, while only the TG includes cells derived from the placodal ectoderm.

Results

Here we use microarray analysis of E13.5 embryos to demonstrate that the developing DRG and TG have very similar overall patterns of gene expression. In mice lacking the POU-domain transcription factor Brn3a, the DRG and TG exhibit many common changes in gene expression, but a subset of Brn3a target genes show increased expression only in the TG. In the wild-type TG these Brn3a-repressed genes are silent, yet their promoter regions exhibit histone H3-acetylation levels similar to constitutively transcribed gene loci. This increased H3-acetylation is not observed in the DRG, suggesting that chromatin modifications play a role in cell-specific target gene regulation by Brn3a.

Conclusion

These results demonstrate that one developmental role of Brn3a is to repress potential differences in gene expression between sensory neurons generated at different axial levels, and to regulate a convergent program of developmental gene expression, in which functionally similar populations of neurons are generated from different embryological substrates.

Migratory routes and fates of cells transcribing the Wnt-1 gene in the murine hindbrain

To investigate the origins, migrations, and fates of Wnt-1-expressing cells in the murine hindbrain, mice carrying a Wnt-1 enhancer/lacZ transgene were observed from embryonic day (E) 8 through postnatal day 18. The transgene-stained ventricular layer waxed and waned prior to and following migrations from it. Stained cells migrated first external to the hindbrain as neural crest and then within it to form typical populations of the rhombic lip, as well as others not recognized as lip derivatives. Migrations originated in a temporally defined sequence, many from discrete rhombomeres. All moved first radially, then rostrally and/or ventrally, ipsi-, or contralaterally, in the mantle or marginal layers. These movements ultimately formed elements of several nuclei, aligned in four longitudinal bands: dorsal (including the gracile, cuneate, cochlear, and vestibular nuclei, plus cerebellar granular cells), dorsal intermediate (including trigeminal sensory, parvicellular reticular, and deep cerebellar nuclei), ventral intermediate (including lateral and intermediate reticular nuclei), and ventral (including the raphe obscurus and pontine nuclei). Transgene staining often persisted long enough to identify stained cells in their definitive, adult nuclei. However, staining was transient. The strength of the staining, however, was in its ability to reveal origins and migrations in both whole-mounts and sections, in single cell detail. The present results will permit analyses of the effects of genetic manipulations on Wnt-1 lineage cells.

Tyrosine kinase receptor immunoreactivity in trigeminal mesencephalic and motor neurons following transection of masseteric nerve of the rat

Neurotrophins are known to promote survival after neural injury. To determine the relative importance of tyrosine kinase receptors on the survival of axotomized trigeminal nuclear neurons, we examined the temporal expression profile of tyrosine kinase A, tyrosine kinase B and tyrosine kinase C receptors in the mesencephalic trigeminal nucleus and the motor trigeminal nucleus following transection of the masseteric nerve in rats. Axotomized neurons in these nuclei were retrogradely identified with FluoroGold. We found increase in tyrosine kinase A-immunoreactive mesencephalic trigeminal nucleus neurons in the second week after axotomy but no change in the number of tyrosine kinase A-immunoreactive motor trigeminal nucleus neurons. There was no change in the number of tyrosine kinase B-immunoreactive mesencephalic trigeminal nucleus neurons but the significant increase of tyrosine kinase B-immunoreactive motor trigeminal nucleus neurons throughout the period of observation (3 weeks) peaked at approximately 1 week after axotomy. There was no alteration in the number of tyrosine kinase C-immunoreactive mesencephalic trigeminal nucleus neurons but significant increase in tyrosine kinase C-immunoreactive motor trigeminal nucleus neurons observable by 4 days post-axotomy was followed by decline to levels lower than the control in 2 weeks. Temporal changes in the expression of individual tyrosine kinase receptors in mesencephalic trigeminal nucleus and motor trigeminal nucleus neurons following transection of the masseteric nerve suggest differential contribution of tyrosine kinase-specific neurotrophins to the survival of these neurons after axotomy.

A role forfoxd3andsox10in the differentiation of gonadotropin-releasing hormone (GnRH) cells in the zebrafishDanio rerio

Gonadotropin-releasing hormone (GnRH) is found in a wide range of vertebrate tissues, including the nervous system. In general, GnRH has two functions: endocrine, acting as a releasing hormone; and neuromodulatory,affecting neural activity in the peripheral and central nervous system. The best understood population of GnRH cells is that of the hypothalamus, which is essential for reproduction. Less well understood are the populations of GnRH cells found in the terminal nerve and midbrain, which appear to be neuromodulatory in function. The GnRH-containing cells of the midbrain are proposed to arise from the mesencephalic region of the neural tube. Previously, we showed that neuromodulatory GnRH cells of the terminal nerve arise from cranial neural crest. To test the hypothesis that neuromodulatory GnRH cells of the midbrain also arise from neural crest, we used gene knockdown experiments in zebrafish to disrupt neural crest development. We demonstrate that decrement of the function of foxd3 and/or sox10, two genes important for the development and specification of neural crest, resulted in a reduction and/or loss of GnRH cells of the midbrain, as well as a reduction in the number of terminal nerve GnRH cells. Therefore, our data support a neural crest origin for midbrain GnRH cells. Additionally, we demonstrate that knockdown of kallmann gene function resulted in the loss of endocrine GnRH cells of the hypothalamus, but not of neuromodulatory GnRH cells of the midbrain and terminal nerve, thus providing additional evidence for separate pathways controlling the development of neuromodulatory and endocrine GnRH cells.

Brn-3a is required for the generation of proprioceptors in the mesencephalic trigeminal tract nucleus

The distribution of motor and proprioceptive neurons was investigated in the trigeminal nervous system of wild-type and Brn-3a knockout mice at embryonic day 18.5 and postnatal day 0. We found that the trigeminal motor nucleus (Mo5) contained abundant motoneurons in wild-type (mean number +/- SD per section = 128 +/- 22, range = 93-167) and knockout (mean number +/- SD per section = 121 +/- 23, range = 75-158) mice and that the cell size of Mo5 neurons was similar between these mice (wild-type, mean +/- SD = 165 +/- 59 microm2, range = 65-326 microm2; knockout, mean +/- SD = 167 +/- 59 microm2, range = 71-327 microm2). Mo5 neurons were immunoreactive for calcitonin gene-related peptide and such immunoreactive neurons were abundant in both wild-type and mutant mice. In the mesencephalic tract nucleus (Mes5) of wild-type mice, many proprioceptors (mean number +/- SD per section = 56 +/- 19, range = 27-85) that contained parvalbumin immunoreactivity were also observed. In knockout mice, however, Mes5 neurons could not be detected. The area of brainstems which normally contained the Mes5 was devoid of parvalbumin-immunoreactive proprioceptors. The present study suggests that Brn-3a is required for the development of proprioceptors but not motoneurons in the trigeminal nervous system.

Specification of dorsoventral polarity in the embryonic chick mesencephalon and its presumptive role in midbrain morphogenesis

The chick midbrain is subdivided into functionally distinct ventral and dorsal domains, tegmentum and optic tectum. In the mature tectum, neurons are organized in layers, while they form discrete nuclei in the tegmentum. Dorsoventral (DV) specification of the early midbrain should thus play a crucial role for the organization of the neuronal circuitry in optic tectum and tegmentum. To investigate regional commitment and establishment of cellular differences along the midbrain DV axis, we examined the commitment of gene expression patterns in isolated ventral and dorsal tissue in vivo and in vitro, and studied their cell mixing properties. Use of explant cultures, and grafting of dorsal midbrain into a ventral environment or vice versa, revealed a gradual increase in the autonomy of region-specific gene regulation between stages 12 and 18 (embryonic day 2 to 3). This process becomes independent of the activity of midline organizers, such as floor and roof plate, by stage 16. Once the DV axis polarity is fixed, cells from dorsal and ventral midbrain adopt differential adhesive properties. Thus between stages 18 to 23 (embryonic day 3 and 4), cells of dorsal and ventral origin start to separate from each other, at a time-point when the majority of midbrain cells is not yet differentiated. Hence, our results suggest that progressive specification of the midbrain DV axis is accompanied by progressively reduced cell mixing between dorsal and ventral precursors, leading to a partial regionalization of midbrain tissue into autonomous units of precursor cell populations.

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.

Cannabinoid receptor, CB1, expression follows neuronal differentiation in the early chick embryo

The role of the CB1 cannabinoid receptor and endocannabinoid signalling has been widely studied in the adult nervous system. However, an emerging body of evidence suggests that the CB1 receptor may also play a role during development. Here we have scrutinized the expression profile of the CB1 receptor from the onset of neurogenesis in the chick embryo. We find that this gene exhibits a dynamic expression pattern that spatially and temporally follows neuronal differentiation in the early embryo.

Fgf signalling is required for formation of cartilage in the head

Characterisation of human craniofacial syndromes and studies in transgenic mice have demonstrated the requirement for Fgf signalling during morphogenesis of membrane bone of the cranium. Here, we report that Fgf activity is also required for development of the oro-pharyngeal skeleton, which develops first as cartilage with some elements subsequently becoming ossified. We show that inhibition of FGF receptor activity in the zebrafish embryo following neural crest emigration from the neural tube results in complete absence of neurocranial and pharyngeal cartilages. Moreover, this Fgf signal is required during a 6-h period soon after initiation of neural crest migration. The spatial and temporal expression of Fgf3 and Fgf8 in pharyngeal endoderm and ventral forebrain and its correlation with patterns of Fgf signalling activity in migrating neural crest makes them candidate regulators of cartilage development. Inhibition of Fgf3 results in the complete absence of cartilage elements that normally form in the third, fourth, fifth, and sixth pharyngeal arches, while those of the first, second, and seventh arches are largely unaffected. Inhibition of Fgf8 alone has variable, but mild, effects. However, inhibition of both Fgf3 and Fgf8 together causes a complete absence of pharyngeal cartilages and the near-complete loss of the neurocranial cartilage. These data implicate Fgf3 and Fgf8 as key regulators of cartilage formation in the vertebrate head.

Sonic hedgehog regulates the position of the trigeminal ganglia

The trigeminal ganglia differentiate in part from specialized ectodermal structures in the embryonic head termed the trigeminal placodes. However, the signals which govern the migration of trigeminal precursors and the final morphology of the ganglia are poorly defined. Here, we show that notochord or floor plate tissue can induce the formation of ectopic sensory ganglia adjacent to the developing dorsal mesencephalon. Neurons within these ganglia coexpress the transcription factors Brn3a and Islet, which together characterize primary sensory neurons throughout the developing embryo. The ectopic ganglia originate from Pax3-expressing regions of the surface ectoderm that normally contribute to the ophthalmic trigeminal (op5), and can only be induced at developmental stages during which op5 precursors are present in the mesencephalic region. The migration of trigeminal precursors is also blocked by a local source of recombinant Shh, while in mouse embryos lacking Shh, these cells continue to migrate until they fuse into a single ganglion at the ventral midline. Together, these results suggest that Shh acts to arrest the migration of sensory precursors rather than to induce sensory neurons de novo. Consistent with this hypothesis, Shh induces the expression of the proteoglycan PG-M/versican in the cranial mesoderm, which has been previously implicated in the regulation of the movement of sensory neural precursors.

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

Formation of the trochlear nerve within the anterior hindbrain provides a model system to study a simple axonal projection within the vertebrate central nervous system. We show that trochlear motor neurons are born within the isthmic organiser and also immediately posterior to it in anterior rhombomere 1. Axons of the most anterior cells follow a dorsal projection, which circumnavigates the isthmus, while those of more posterior trochlear neurons project anterodorsally to enter the isthmus. Once within the isthmus, axons form large fascicles that extend to a dorsal exit point. We investigated the possibility that the projection of trochlear axons towards the isthmus and their subsequent growth within that tissue might depend upon chemoattraction. We demonstrate that both isthmic tissue and Fgf8 protein are attractants for trochlear axons in vitro, while ectopic Fgf8 causes turning of these axons away from their normal routes in vivo. Both inhibition of FGF receptor activation and inhibition of Fgf8 function in vitro affect formation of the trochlear projection within explants in a manner consistent with a guidance function of Fgf8 during trochlear axon navigation.