Rhombomere-specific origin of branchial and visceral motoneurons of the facial nerve in the rat embryo

Mammalian NREM and REM sleep: Why, when and how

This report proposes that fish use the spinal-rhombencephalic regions of their brain to support their activities while awake. Instead, the brainstem-diencephalic regions support the wakefulness in amphibians and reptiles. Lastly, mammals developed the telencephalic cortex to attain the highest degree of wakefulness, the cortical wakefulness. However, a paralyzed form of spinal-rhombencephalic wakefulness remains in mammals in the form of REMS, whose phasic signs are highly efficient in promoting maternal care to mammalian litter. Therefore, the phasic REMS is highly adaptive. However, their importance is low for singletons, in which it is a neutral trait, devoid of adaptive value for adults, and is mal-adaptive for marine mammals. Therefore, they lost it. The spinal-rhombencephalic and cortical wakeful states disregard the homeostasis: animals only attend their most immediate needs: foraging defense and reproduction. However, these activities generate allostatic loads that must be recovered during NREMS, that is a paralyzed form of the amphibian-reptilian subcortical wakefulness. Regarding the regulation of tonic REMS, it depends on a hypothalamic switch. Instead, the phasic REMS depends on an independent proportional pontine control.

Robo1 and 2 Repellent Receptors Cooperate to Guide Facial Neuron Cell Migration and Axon Projections in the Embryonic Mouse Hindbrain

The facial nerve is necessary for our ability to eat, speak, and make facial expressions. Both the axons and cell bodies of the facial nerve undergo a complex embryonic developmental pattern involving migration of the cell bodies caudally and tangentially through rhombomeres, and simultaneously the axons projecting to exit the hindbrain to form the facial nerve. Our goal in this study was to test the functions of the chemorepulsive receptors Robo1 and Robo2 in facial neuron migration and axon projection by analyzing genetically marked motor neurons in double-mutant mouse embryos through the migration time course, E10.0-E13.5. In Robo1/2 double mutants, axon projection and cell body migration errors were more severe than in single mutants. Most axons did not make it to their motor exit point, and instead projected into and longitudinally within the floor plate. Surprisingly, some facial neurons had multiple axons exiting and projecting into the floor plate. At the same time, a subset of mutant facial cell bodies failed to migrate caudally, and instead either streamed dorsally toward the exit point or shifted into the floor plate. We conclude that Robo1 and Robo2 have redundant functions to guide multiple aspects of the complex cell migration of the facial nucleus, as well as regulating axon trajectories and suppressing formation of ectopic axons.

Talking back: Development of the olivocochlear efferent system

Developing sensory systems must coordinate the growth of neural circuitry spanning from receptors in the peripheral nervous system (PNS) to multilayered networks within the central nervous system (CNS). This breadth presents particular challenges, as nascent processes must navigate across the CNS-PNS boundary and coalesce into a tightly intermingled wiring pattern, thereby enabling reliable integration from the PNS to the CNS and back. In the auditory system, feedforward spiral ganglion neurons (SGNs) from the periphery collect sound information via tonotopically organized connections in the cochlea and transmit this information to the brainstem for processing via the VIII cranial nerve. In turn, feedback olivocochlear neurons (OCNs) housed in the auditory brainstem send projections into the periphery, also through the VIII nerve. OCNs are motor neuron-like efferent cells that influence auditory processing within the cochlea and protect against noise damage in adult animals. These aligned feedforward and feedback systems develop in parallel, with SGN central axons reaching the developing auditory brainstem around the same time that the OCN axons extend out toward the developing inner ear. Recent findings have begun to unravel the genetic and molecular mechanisms that guide OCN development, from their origins in a generic pool of motor neuron precursors to their specialized roles as modulators of cochlear activity. One recurrent theme is the importance of efferent-afferent interactions, as afferent SGNs guide OCNs to their final locations within the sensory epithelium, and efferent OCNs shape the activity of the developing auditory system. This article is categorized under: Nervous System Development > Vertebrates: Regional Development.

Extrinsic mechanical forces mediate retrograde axon extension in a developing neuronal circuit

To form functional neural circuits, neurons migrate to their final destination and extend axons towards their targets. Whether and how these two processes are coordinated in vivo remains elusive. We use the zebrafish olfactory placode as a system to address the underlying mechanisms. Quantitative live imaging uncovers a choreography of directed cell movements that shapes the placode neuronal cluster: convergence of cells towards the centre of the placodal domain and lateral cell movements away from the brain. Axon formation is concomitant with lateral movements and occurs through an unexpected, retrograde mode of extension, where cell bodies move away from axon tips attached to the brain surface. Convergence movements are active, whereas cell body lateral displacements are of mainly passive nature, likely triggered by compression forces from converging neighbouring cells. These findings unravel a previously unknown mechanism of neuronal circuit formation, whereby extrinsic mechanical forces drive the retrograde extension of axons.How neuronal migration and axon growth coordinate during development is only partially understood. Here the authors use quantitative imaging to characterise the morphogenesis of the zebrafish olfactory placode and report an unexpected phenomenon, whereby axons extend through the passive movement of neuron cell bodies away from tethered axon tips.

Nuclear derivatives and axonal projections originating from rhombomere 4 in the mouse hindbrain

The r4-derived territory is located in the pontine region of the brainstem, forming a wedge-shaped slice that broadens from the choroidal roof to the ventral midline. R4-derived neuronal populations migrate radially inside and tangentially outside this rhombomere, forming nuclei of the sensorimotor auditory, vestibular, trigeminal and reticular systems. R4-derived fibre tracts contribute to the lateral lemniscus, the trigeminothalamic tracts, the medial tegmental tract and the medial forebrain bundle, which variously project to the midbrain, thalamus, hypothalamus and telencephalon. Other tracts such as the trigeminocerebellar and vestibulocerebellar tracts reach the cerebellum, while the medial and lateral vestibulospinal tracts, and the reticulospinal and trigeminal oro-spinal tracts extend into the spinal cord. Many r4-derived fibres are crossed; they decussate to the contralateral side traversing the midline through the cerebellar, collicular and intercollicular commissures, as well as the supraoptic decussation. Moreover, some fibres enter into the posterior and anterior commissures and some terminals reach the septum. Overall, this study provides an overview of all r4 neuronal populations and axonal tracts from their embryonic origin to the adult final location and target.

Cellular and Molecular Underpinnings of Neuronal Assembly in the Central Auditory System during Mouse Development

During development, the organization of the auditory system into distinct functional subcircuits depends on the spatially and temporally ordered sequence of neuronal specification, differentiation, migration and connectivity. Regional patterning along the antero-posterior axis and neuronal subtype specification along the dorso-ventral axis intersect to determine proper neuronal fate and assembly of rhombomere-specific auditory subcircuits. By taking advantage of the increasing number of transgenic mouse lines, recent studies have expanded the knowledge of developmental mechanisms involved in the formation and refinement of the auditory system. Here, we summarize several findings dealing with the molecular and cellular mechanisms that underlie the assembly of central auditory subcircuits during mouse development, focusing primarily on the rhombomeric and dorso-ventral origin of auditory nuclei and their associated molecular genetic pathways.

Dynamic expression of transcription factor Brn3b during mouse cranial nerve development

During development, transcription factor combinatorial codes define a large variety of morphologically and physiologically distinct neurons. Such a combinatorial code has been proposed for the differentiation of projection neurons of the somatic and visceral components of cranial nerves. It is possible that individual neuronal cell types are not specified by unique transcription factors but rather emerge through the intersection of their expression domains. Brn3a, Brn3b, and Brn3c, in combination with each other and/or transcription factors of other families, can define subgroups of retinal ganglion cells (RGC), spiral and vestibular ganglia, inner ear and vestibular hair cell neurons in the vestibuloacoustic system, and groups of somatosensory neurons in the dorsal root ganglia. The present study investigates the expression and potential role of the Brn3b transcription factor in cranial nerves and associated nuclei of the brainstem. We report the dynamic expression of Brn3b in the somatosensory component of cranial nerves II, V, VII, and VIII and visceromotor nuclei of nerves VII, IX, and X as well as other brainstem nuclei during different stages of development into adult stage. We find that genetically identified Brn3b(KO) RGC axons show correct but delayed pathfinding during the early stages of embryonic development. However, loss of Brn3b does not affect the anatomy of the other cranial nerves normally expressing this transcription factor.

Shox2 is required for the proper development of the facial motor nucleus and the establishment of the facial nerves

Background

Axons from the visceral motor neurons (vMNs) project from nuclei in the hindbrain to innervate autonomic ganglia and branchial arch-derived muscles. Although much is known about the events that govern specification of somatic motor neurons, the genetic pathways responsible for the development of vMNs are less well characterized. We know that vMNs, like all motor neurons, depend on sonic hedgehog signaling for their generation. Similarly, the paired-like homeobox 2b (Phox2b) gene, which is expressed in both proliferating progenitors and post-mitotic motor neurons, is essential for the development of vMNs. Given that our previous study identified a novel role for the short stature homeobox 2 (Shox2) gene in the hindbrain, and since SHOX2 has been shown to regulate transcription of islet 1 (Isl1), an important regulator of vMN development, we sought to determine whether Shox2 is required for the proper development of the facial motor nucleus.

Results

Using a Nestin-Cre driver, we show that elimination of Shox2 throughout the brain results in elevated cell death in the facial motor nucleus at embryonic day 12.5 (E12.5) and E14.5, which correlates with impaired axonal projection properties of vMNs. We also observed changes in the spatial expression of the vMN cell fate factors Isl1 and Phox2b, and concomitant defects in Shh and Ptch1 expression in Shox2 mutants. Furthermore, we demonstrate that elimination of Shox2 results in the loss of dorsomedial and ventromedial subnuclei by postnatal day 0 (P0), which may explain the changes in physical activity and impaired feeding/nursing behavior in Shox2 mutants.

Conclusions

Combined, our data show that Shox2 is required for development of the facial motor nucleus and its associated facial (VII) nerves, and serves as a new molecular tool to probe the genetic programs of this understudied hindbrain region.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-015-0176-0) contains supplementary material, which is available to authorized users.

De novo mutations in PLXND1 and REV3L cause Möbius syndrome

Möbius syndrome (MBS) is a neurological disorder that is characterized by paralysis of the facial nerves and variable other congenital anomalies. The aetiology of this syndrome has been enigmatic since the initial descriptions by von Graefe in 1880 and by Möbius in 1888, and it has been debated for decades whether MBS has a genetic or a non-genetic aetiology. Here, we report de novo mutations affecting two genes, PLXND1 and REV3L in MBS patients. PLXND1 and REV3L represent totally unrelated pathways involved in hindbrain development: neural migration and DNA translesion synthesis, essential for the replication of endogenously damaged DNA, respectively. Interestingly, analysis of Plxnd1 and Rev3l mutant mice shows that disruption of these separate pathways converge at the facial branchiomotor nucleus, affecting either motoneuron migration or proliferation. The finding that PLXND1 and REV3L mutations are responsible for a proportion of MBS patients suggests that de novo mutations in other genes might account for other MBS patients.

lt has been debated for decades if there is a genetic aetiology underlying Möbius syndrome, a neurological disorder characterized by facial paralysis. Here Tomas-Roca et al. use exome sequencing and identify de novo mutations in PLXND1 and REV3L, representing converging pathways in hindbrain development.

Motor neuron cell bodies are actively positioned by Slit/Robo repulsion and Netrin/DCC attraction

Motor neurons differentiate from a ventral column of progenitors and settle in static clusters, the motor nuclei, next to the floor plate. Within these cell clusters, motor neurons receive afferent input and project their axons out to muscle targets. The molecular mechanisms that position motor neurons in the neural tube remain poorly understood. The floor plate produces several types of guidance cues with well-known roles in attracting and repelling axons, including the Slit family of chemorepellents via their Robo receptors, and Netrin1 via its DCC attractive receptor. In the present study we found that Islet1(+) motor neuron cell bodies invaded the floor plate of Robo1/2 double mutant mouse embryos or Slit1/2/3 triple mutants. Misplaced neurons were born in their normal progenitor column, but then migrated tangentially into the ventral midline. Robo1 and 2 receptor expression in motor neurons was confirmed by reporter gene staining and anti-Robo antibody labeling. Mis-positioned motor neurons projected their axons longitudinally within the floor plate, and failed to reach their normal exit points. To test for potential counteracting ventral attractive signals, we examined Netrin-1 and DCC mutants, and found that motor neurons shifted dorsally in the hindbrain and spinal cord, suggesting that Netrin-1/DCC signaling normally attracts motor neurons closer to the floor plate. Our results show that motor neurons are actively migrating cells, and are normally trapped in a static position by Slit/Robo repulsion and Netrin-1/DCC attraction.

Neuronal organization: unsticking the cadherin code

Hindbrain cranial motor neurons are organized into discrete functional clusters. A new study demonstrates that coalescence of these nuclei is driven by the expression of distinct combinations of cadherin adhesion molecules by each motor neuron group.

Retinoic acid negatively regulates dact3b expression in the hindbrain of zebrafish embryos

Wnt signaling plays important roles in normal development as well as pathophysiological conditions. The Dapper antagonist of β-catenin (Dact) proteins are modulators of both canonical and non-canonical Wnt signaling via direct interactions with Dishevelled (Dvl) and Van Gogh like-2 (Vangl2). Here, we report the dynamic expression patterns of two zebrafish dact3 paralogs during early embryonic development. Our whole mount in situ hybridization (WISH) analysis indicates that specific dact3a expression starts by the tailbud stage in adaxial cells. Later, it is expressed in the anterior lateral plate mesoderm, somites, migrating cranial neural crest, and hindbrain neurons. By comparison, dact3b expression initiates on the dorsal side at the dome stage and soon after is expressed in the dorsal forerunner cells (DFCs) during gastrulation. At later stages, dact3b expression becomes restricted to the branchial neurons of the hindbrain and to the second pharyngeal arch. To investigate how zebrafish dact3 gene expression is regulated, we manipulated retinoic acid (RA) signaling during development and found that it negatively regulates dact3b in the hindbrain. Our study is the first to document the expression of the paralogous zebrafish dact3 genes during early development and demonstrate dact3b can be regulated by RA signaling. Therefore, our study opens up new avenues to study Dact3 function in the development of multiple tissues and suggests a previously unappreciated cross regulation of Wnt signaling by RA signaling in the developing vertebrate hindbrain.

Hox genes and region-specific sensorimotor circuit formation in the hindbrain and spinal cord

Homeobox (Hox) genes were originally discovered in the fruit fly Drosophila, where they function through a conserved homeodomain as transcriptional regulators to control embryonic morphogenesis. In vertebrates, 39 Hox genes have been identified and like their Drosophila counterparts they are organized within chromosomal clusters. Hox genes interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing hindbrain and spinal cord, and are considered crucial determinants of segmental identity and cell specification along the anterioposterior and dorsoventral axes of the embryo. Here, we review their later roles in the assembly of neuronal circuitry, in stereotypic neuronal migration, axon pathfinding, and topographic connectivity. Importantly, we will put some emphasis on how their early-segmented expression patterns can influence the formation of complex vital hindbrain and spinal cord circuitries.

Assembly of the auditory circuitry by a Hox genetic network in the mouse brainstem

Rhombomeres (r) contribute to brainstem auditory nuclei during development. Hox genes are determinants of rhombomere-derived fate and neuronal connectivity. Little is known about the contribution of individual rhombomeres and their associated Hox codes to auditory sensorimotor circuitry. Here, we show that r4 contributes to functionally linked sensory and motor components, including the ventral nucleus of lateral lemniscus, posterior ventral cochlear nuclei (VCN), and motor olivocochlear neurons. Assembly of the r4-derived auditory components is involved in sound perception and depends on regulatory interactions between Hoxb1 and Hoxb2. Indeed, in Hoxb1 and Hoxb2 mutant mice the transmission of low-level auditory stimuli is lost, resulting in hearing impairments. On the other hand, Hoxa2 regulates the Rig1 axon guidance receptor and controls contralateral projections from the anterior VCN to the medial nucleus of the trapezoid body, a circuit involved in sound localization. Thus, individual rhombomeres and their associated Hox codes control the assembly of distinct functionally segregated sub-circuits in the developing auditory brainstem.

Sound perception and sound localization are controlled by two distinct circuits in the central nervous system. However, the cellular and molecular determinants underlying their development are poorly understood. Here, we show that a spatially restricted region of the brainstem, the rhombomere 4, and two members of the Hox gene family, Hoxb1 and Hoxb2, are directly implicated in the development of the circuit leading to sound perception and sound amplification. In the absence of Hoxb1 and Hoxb2 function, we found severe morphological defects in the hair cell population implicated in transducing the acoustic signal, leading ultimately to severe hearing impairments in adult mutant mice. In contrast, the expression in the cochlear nucleus of another Hox member, Hoxa2, regulates the guidance receptor Rig1 and contralateral connectivity in the sound localization circuit. Some of the auditory dysfunctions described in our mouse models resemble pathological hearing conditions in humans, in which patients have an elevated hearing threshold sensitivity, as recorded in audiograms. Thus, this study provides mechanistic insight into the genetic and functional regulation of Hox genes during development and assembly of the auditory system.

olig2-Expressing hindbrain cells are required for migrating facial motor neurons

The complicated trajectory of facial motor neuron migration requires coordination of intrinsic signals and cues from the surrounding environment. Migration begins in rhombomere (r) 4 where facial motor neurons are born and proceeds in a caudal direction. Once facial motor neurons reach their target rhombomeres, they migrate laterally and radially from the ventral neural tube. In zebrafish, as facial motor neurons migrate through r5/r6, they pass near cells that express olig2, which encodes a bHLH transcription factor. In this study, we found that olig2 function is required for facial motor neurons to complete their caudal migration into r6 and r7 and form stereotypical clusters. Additionally, embryos that lack mafba function, in which facial motor neurons also fail to complete caudal migration, lack olig2 expression in r5 and r6. Our data raise the possibility that cells expressing olig2 are intermediate targets that help guide facial motor neuron migration.

Ongoing roles of Phox2 homeodomain transcription factors during neuronal differentiation

Transcriptional determinants of neuronal identity often stay expressed after their downstream genetic program is launched. Whether this maintenance of expression plays a role is for the most part unknown. Here, we address this question for the paralogous paired-like homeobox genes Phox2a and Phox2b, which specify several classes of visceral neurons at the progenitor stage in the central and peripheral nervous systems. By temporally controlled inactivation of Phox2b, we find that the gene, which is required in ventral neural progenitors of the hindbrain for the production of branchio-visceral motoneuronal precursors, is also required in these post-mitotic precursors to maintain their molecular signature - including downstream transcription factors - and allow their tangential migration and the histogenesis of the corresponding nuclei. Similarly, maintenance of noradrenergic differentiation during embryogenesis requires ongoing expression of Phox2b in sympathetic ganglia, and of Phox2a in the main noradrenergic center, the locus coeruleus. These data illustrate cases where the neuronal differentiation program does not unfold as a transcriptional `cascade' whereby downstream events are irreversibly triggered by an upstream regulator, but instead require continuous transcriptional input from it.

Wnt activity guides facial branchiomotor neuron migration, and involves the PCP pathway and JNK and ROCK kinases

Background

Wnt proteins play roles in many biological processes, including axon guidance and cell migration. In the mammalian hindbrain, facial branchiomotor (FBM) neurons undergo a striking rostral to caudal migration, yet little is known of the underlying molecular mechanisms. In this study, we investigated a possible role of Wnts and the planar cell polarity (PCP) pathway in this process.

Results

Here we demonstrate a novel role for Wnt proteins in guiding FBM neurons during their rostral to caudal migration in the hindbrain. We found that Wnt5a is expressed in a caudal^high to rostral^low gradient in the hindbrain. Wnt-coated beads chemoattracted FBM neurons to ectopic positions in an explant migration assay. The rostrocaudal FBM migration was moderately perturbed in Wnt5a mutant embryos and severely disrupted in Frizzled3 mutant mouse embryos, and was aberrant following inhibition of Wnt function by secreted Frizzled-related proteins. We also show the involvement of the Wnt/PCP pathway in mammalian FBM neuron migration. Thus, mutations in two PCP genes, Vangl2 and Scribble, caused severe defects in FBM migration. Inhibition of JNK and ROCK kinases strongly and specifically reduced the FBM migration, as well as blocked the chemoattractant effects of ectopic Wnt proteins.

Conclusion

These results provide in vivo evidence that Wnts chemoattract mammalian FBM neurons and that Wnt5a is a candidate to mediate this process. Molecules of the PCP pathway and the JNK and ROCK kinases also play a role in the FBM migration and are likely mediators of Wnt signalling.

Hox genes in neural patterning and circuit formation in the mouse hindbrain

The mammalian hindbrain is the seat of regulation of several vital functions that involve many of the organ systems of the body. Such functions are controlled through the activity of intricate arrays of neuronal circuits and connections. The establishment of ordered patterns of neuronal specification, migration, and axonal topographic connectivity during development is crucial to build such a complex network of circuits and functional connectivity in the mature hindbrain. The early development of the vertebrate hindbrain proceeds according to a fundamental metameric partitioning along the anteroposterior axis into cellular compartments known as rhombomeres. Such an organization has been highly conserved in vertebrate evolution and has a fundamental impact on the hindbrain adult structure, nuclear organization, and connectivity. Here, we review the cellular and molecular mechanisms underlying hindbrain neuronal circuitry in the mouse, with a specific focus on the role of the homeodomain transcription factors of the Hox gene family. The Hox genes are crucial determinants of rhombomere segmental identity and anteroposterior patterning. However, recent findings suggest that, in addition to their well-known roles at early embryonic stages, the Hox genes may play important roles also in later aspect of neuronal circuit development, including stereotypic neuronal migration, axon pathfinding, and topographic mapping of connectivity.

Plexin A3 and plexin A4 convey semaphorin signals during facial nerve development

In vertebrates, class 3 semaphorins (SEMA3) control axon behaviour by binding to neuronal cell surface receptors composed of a ligand binding subunit termed neuropilin (NRP) and a signal transduction subunit of the A-type plexin family (PLXNA). We have determined the requirement for SEMA3/NRP/PLXN signalling in the development of the facial nerve, which contains axons from two motor neuron populations, branchiomotor and visceromotor neurons. Loss of either SEMA3A/NRP1 or SEMA3F/NRP2 caused defasciculation and ectopic projection of facial branchiomotor axons. In contrast, facial visceromotor axons selectively required SEMA3A/NRP1. Thus, the greater superficial petrosal nerve was defasciculated, formed ectopic projections and failed to branch in its target area when either SEMA3A or NRP1 were lost. To examine which A-type plexin conveyed SEMA3/neuropilin signals during facial nerve development, we combined an expression analysis with loss of function studies. Even though all four A-type plexins were expressed in embryonic motor neurons, PLXNA1 and PLXNA2 were not essential for facial nerve development. In contrast, loss of PLXNA4 phenocopied the defects of SEMA3A and NRP1 mutants, and loss of PLXNA3 phenocopied the defects of SEMA3F and NRP2 mutants. The combined loss of PLXNA3 and PLXNA4 impaired facial branchiomotor axon guidance more severely than loss of either plexin alone, suggesting that SEMA3A and SEMA3F signals, even though both essential, are partially redundant.

Cdk5 selectively affects the migration of different populations of neurons in the developing spinal cord

It has been shown that cyclin-dependent kinase 5 (Cdk5) is crucial for neuronal migration and survival in the brain. However, the role of Cdk5 in neuronal migration in the spinal cord has never been investigated. The present study is the first to show that Cdk5 affects the migration of different populations of neurons in the developing spinal cord. In the absence of Cdk5, at least four neuronal populations failed to migrate to their final destinations: sympathetic and parasympathetic preganglionic neurons, as well as dorsally originating and ventrally originating (U-shaped group) diaphorase-positive dorsal horn interneurons. In contrast, the migration of somatic motor neurons and various types of ventral and dorsal interneurons was unaffected by the absence of Cdk5. Moreover, our results suggest that Cdk5-dependent migration in the developing spinal cord is axon- or glial fiber-mediated. Finally, our results show that sympathetic preganglionic neurons and somatic motor neurons in Cdk5-deficient mice continue to extend processes and project toward their normal target areas, suggesting that Cdk5 has no obvious effects on axonal outgrowth and guidance mechanisms of these two neuronal populations in spinal cord development.

Ambiguus motoneurons innervating laryngeal and esophageal muscles are malpositioned in the Reelin-deficient mutant rat, Shaking Rat Kawasaki

CONCLUSIONS

The present study confirmed that ambiguus motoneurons innervating intrinsic laryngeal and esophageal muscles are radially malpositioned in the brainstem of the Shaking Rat Kawasaki (SRK), a reelin-deficient mutant rat.

OBJECTIVES

Ambiguus motoneurons innervating the striated muscles of the larynx and esophagus take a long migration from their original birth plate in the floor of the fourth ventricle to their final settlement in the ventral margin of the medulla oblongata. To examine whether the migration of ambiguus nucleus neurons is affected in SRK, we studied localization of ambiguus motoneurons of postnatal day 21 (P21) normal and SRK rats.

MATERIALS AND METHODS

To label ambiguus motoneurons retrogradely, horseradish peroxidase (HRP) was injected into some laryngeal muscles including cricothyroid, thyroarytenoid and posterior cricoarytenoid muscles, and the cervical and abdominal esophageal muscles of the SRK and normal controls 2 days before sacrifice.

RESULTS

In the P21 normal rat, HRP-positive laryngeal and esophageal motoneurons were found in the nucleus ambiguus, whereas in the P21 SRK, they were scattered from the base of the fourth ventricle to the ventro-lateral margin of the medulla, suggesting that radial migration of ambiguus motoneurons from their birthplace to their final settlement is guided by Reelin protein.

The development of mastication in rodents: from neurons to behaviors

The development of suckling behavior is a fundamental characteristic of mammalian development. The occurrence of this behavior across mammals allows us to extrapolate information from animal models to better understand normal and abnormal masticatory development in infants. This review focuses on prenatal cell, molecular, and morphological changes in rat and/or mouse masticatory muscles, trigeminal motoneurons (Mo5) and mesencephalic trigeminal neurons (Me5) that accompany the development of suckling behavior. A special emphasis is placed on N-methyl-d-aspartate (NMDA) receptor subunit expression because of the important role that NMDA receptors play in the production of rhythmical jaw movements and neuronal development. Prenatally the timing of NMDA subunit changes follows neuromuscular junction formation in masticatory muscles, and is coincident with the emergence of rhythmical jaw movements and in vitro rhythmical trigeminal activity. Our data suggest that NMDA receptor subunit changes in Mo5 and Me5 are synchronized with the emergence of rhythmical jaw movements and trigeminal motor activity.

Refinement of the locus for hereditary congenital facial palsy on chromosome 3q21 in two unrelated families and screening of positional candidate genes

Hereditary congenital facial palsy (HCFP) is an autosomal-dominant disorder consisting of paresis or paralysis of the VIIth (facial) cranial nerve. Genetic heterogeneity for this disorder has been suggested based on linkage analysis in two large Dutch families. Two loci have been identified, one on chromosome 3q21.2-q22.1 (HCFP1) and another on chromosome 10q21.3-q22.1 (HCFP2). Here, we report linkage analysis in a large Pakistani family with dominant congenital facial palsy. A region cosegregating with the disorder was identified on the long arm of chromosome 3, which overlaps with the previously identified HCFP1 locus on chromosome 3q21-q22, thus confirming the involvement of this locus in HCFP. The critical region could be reduced from 5.7 to 3.0 cM between the markers D3S3607 and GDB ID:11524500. In addition, mutation analysis on seven candidate genes: KLF15, FLJ40083, PODXL2, TMCC1, PLEXIN-A1, PLEXIN-D1, and GATA-2, was performed. All genes are located within the critical interval of the Dutch HCFP1 family. The genes PODXL2, PLEXIN-D1, GATA-2, and TMCC1 are also located within the smaller critical interval of the Pakistani HCFP family. Based on the results obtained, all seven genes could be excluded as causative genes in HCFP.

Mash1 and Math3 are required for development of branchiomotor neurons and maintenance of neural progenitors

Basic helix-loop-helix (bHLH) transcription factors are known to play important roles in neuronal determination and differentiation. However, their exact roles in neural development still remain to be determined because of the functional redundancy. Here, we examined the roles of neural bHLH genes Mash1 and Math3 in the development of trigeminal and facial branchiomotor neurons, which derive from rhombomeres 2-4. In Math3-null mutant mice, facial branchiomotor neurons are misspecified, and both trigeminal and facial branchiomotor neurons adopt abnormal migratory pathways. In Mash1;Math3 double-mutant mice, trigeminal and facial branchiomotor neurons are severely reduced in number partly because of increased apoptosis. In addition, neurons with migratory defects are intermingled over the midline from either side of the neural tube. Furthermore, oligodendrocyte progenitors of rhombomere 4 are reduced in number. In the absence of Mash1 and Math3, expression of Notch signaling components is severely downregulated in rhombomere 4 and neural progenitors are not properly maintained, which may lead to intermingling of neurons and a decrease in oligodendrocyte progenitors. These results indicate that Mash1 and Math3 not only promote branchiomotor neuron development but also regulate the subsequent oligodendrocyte development and the cytoarchitecture by maintaining neural progenitors through Notch signaling.

Preservation of segmental hindbrain organization in adult frogs

To test for possible retention of early segmental patterning throughout development, the cranial nerve efferent nuclei in adult ranid frogs were quantitatively mapped and compared with the segmental organization of these nuclei in larvae. Cranial nerve roots IV-X were labeled in larvae with fluorescent dextran amines. Each cranial nerve efferent nucleus resided in a characteristic segmental position within the clearly visible larval hindbrain rhombomeres (r). Trochlear motoneurons were located in r0, trigeminal motoneurons in r2-r3, facial branchiomotor and vestibuloacoustic efferent neurons in r4, abducens and facial parasympathetic neurons in r5, glossopharyngeal motoneurons in r6, and vagal efferent neurons in r7-r8 and rostral spinal cord. In adult frogs, biocytin labeling of cranial nerve roots IV-XII and spinal ventral root 2 in various combinations on both sides of the brain revealed precisely the same rostrocaudal sequence of efferent nuclei relative to each other as observed in larvae. This indicates that no longitudinal migratory rearrangement of hindbrain efferent neurons occurs. Although rhombomeres are not visible in adults, a segmental map of adult cranial nerve efferent nuclei can be inferred from the strict retention of the larval hindbrain pattern. Precise measurements of the borders of adjacent efferent nuclei within a coordinate system based on external landmarks were used to create a quantitative adult segmental map that mirrors the organization of the larval rhombomeric framework. Plotting morphologically and physiologically identified hindbrain neurons onto this map allows the physiological properties of adult hindbrain neurons to be linked with the underlying genetically specified segmental framework.

Reciprocal gene replacements reveal unique functions for Phox2 genes during neural differentiation

The paralogous paired-like homeobox genes Phox2a and Phox2b are involved in the development of specific neural subtypes in the central and peripheral nervous systems. The different phenotypes of Phox2 knockout mutants, together with their asynchronous onset of expression, prompted us to generate two knock-in mutant mice, in which Phox2a is replaced by the Phox2b coding sequence, and vice versa. Our results indicate that Phox2a and Phox2b are not functionally equivalent, as only Phox2b can fulfill the role of Phox2a in the structures that depend on both genes. Furthermore, we demonstrate unique roles of Phox2 genes in the differentiation of specific motor neurons. Whereas the oculomotor and the trochlear neurons require Phox2a for their proper development, the migration of the facial branchiomotor neurons depends on Phox2b. Therefore, our analysis strongly indicates that biochemical differences between the proteins rather than temporal regulation of their expression account for the specific function of each paralogue.

Slit-mediated repulsion is a key regulator of motor axon pathfinding in the hindbrain

The floor plate is known to be a source of repellent signals for cranial motor axons, preventing them from crossing the midline of the hindbrain. However, it is unknown which molecules mediate this effect in vivo. We show that Slit and Robo proteins are candidate motor axon guidance molecules, as Robo proteins are expressed by cranial motoneurons, and Slit proteins are expressed by the tissues that delimit motor axon trajectories, i.e. the floor plate and the rhombic lip. We present in vitro evidence showing that Slit1 and Slit2 proteins are selective inhibitors and repellents for dorsally projecting, but not for ventrally projecting, cranial motor axons. Analysis of mice deficient in Slit and Robo function shows that cranial motor axons aberrantly enter the midline, while ectopic expression of Slit1 in chick embryos leads to specific motor axon projection errors. Expression of dominant-negative Robo receptors within cranial motoneurons in chick embryos strikingly perturbs their projections, causing some motor axons to enter the midline, and preventing dorsally projecting motor axons from exiting the hindbrain. These data suggest that Slit proteins play a key role in guiding dorsally projecting cranial motoneurons and in facilitating their neural tube exit.

Nucleotide variation analysis does not support a causal role for plexin-A1 in hereditary congenital facial paresis

Hereditary congenital facial paresis is a rare autosomal dominantly inherited disorder, in which pathological changes in the brainstem affect the paired facial nuclei and nerves. Previously, the neuropilin-1 protein has been shown to control axon guidance and cell body position of facial motor neurons, and mice with a targeted disruption of neuropilin-1 present with developmental defects of the facial nerve nuclei. Plexin-A1 can function as a signal transducing subunit for the neuronal neuropilin receptor, and its gene is located in the linkage interval for hereditary congenital facial paresis at chromosome 3q21-q22 (MIM601471), making it an excellent candidate gene for this disorder. During mouse embryogenesis, the murine ortholog of plexin-A1 gene showed restricted spatial and temporal expression in the hindbrain, consistent with a role in cell body movement, or axonal guidance during facial nerve development. Sequence analysis of the plexin-A1 gene in patients from the 3q21-q22-linked hereditary congenital facial paresis family revealed several nucleotide changes. However, none of the nucleotide changes led to an amino acid substitution, and reverse transcriptase polymerase chain reaction analysis did not detect aberrant RNA processing. We therefore conclude that it is highly unlikely that Plexin-A1 is involved in the pathogenicity of hereditary congenital facial paresis.

Retinoic acid influences the development of the inferior olivary nucleus in the rodent

All-trans retinoic acid (atRA) is an endogenous morphogen that regulates gene transcription. Maternal exposure to atRA results in severe developmental abnormalities by disrupting normal patterns of atRA distribution. Previously, we have shown that the pontine nucleus, which originates from the rhombic lip, is severely atrophied in the mouse on exposure to atRA at gestational days 9 and 10. In this study, we show that this same period of atRA exposure has the contrary effect on the inferior olive and this rhombic lip derivative is expanded in volume and probably contains an increased number of cells. The posterior region of the inferior olive maintains a relatively normal shape but is significantly expanded in size. In contrast, the organization of the anterior inferior olive is severely disrupted. Because endogenous atRA levels are known to be higher in the region of the posterior inferior olive at the time of birth of inferior olivary neurons, these results suggest that endogenous atRA may promote the generation, or select the fate, of posterior neurons of the inferior olive. In support of this concept, a reduction in atRA resulting from vitamin A deficiency results in loss of cells of the posterior inferior olive.

Reelin signaling is necessary for a specific step in the migration of hindbrain efferent neurons

The cytoarchitecture of the hindbrain results from precise and co-ordinated sequences of neuronal migrations. Here, we show that reelin, an extracellular matrix protein involved in neuronal migration during CNS development, is necessary for an early, specific step in the migration of several hindbrain nuclei. We identified two cell populations not previously known to be affected in reeler mutants that show a common migratory defect: the olivocochlear efferent neurons and the facial visceral motor nucleus. In control embryos, these cells migrate first toward a lateral position within the neural tube, and then parallel to the glial cell processes, to a ventral position where they settle close to the pial surface. In reelermutants, the first migration is not affected, but the neurons are unable to reach the pial surface and remain in an ectopic position. Indeed, this is the first evidence that the migration of specific hindbrain nuclei can be divided into two parts: a reelin-independent and a reelin-dependent migration. We also show that reelin is expressed at high levels at the final destination of the migratory process, while the reelin intracellular effector Dab1 was expressed by cell groups that included the two populations affected. Mice mutant at the Dab1 locus, called scrambler, exhibit the same phenotype, a failure of final migration. However, examination of mice lacking both reelin receptors, ApoER2 and VLDLR, did not reveal the same phenotype, suggesting involvement of an additional reelin-binding receptor. In the hindbrain, reelin signaling might alter the adhesive properties of efferent neurons and their ability to respond to directional cues, as has been suggested for the migration of olfactory bulb precursors.

Pontine influences on breathing: an overview

Historical and contemporary views of the functional organization of the lateral pontine regions influencing breathing are reviewed. In vertebrates, the rhombencephalon generates a breathing rhythm and detailed motor pattern that persist throughout life. Key to this process is an essentially continuous column of neurons extending from the spino-medullary border through the ventrolateral medulla, continuing through the ventral pons and arcing into the dorsolateral medulla. Comparative neuroanatomy and physiology indicate this is a richly interconnected network divided into serial, functionally distinct compartments. Serial compartmentalization of pontomedullary structures related to breathing also reflects the developmental segmentation of the rhombencephalon. However, with migration of cell groups such as the facial nucleus from the pons to the medulla during ontogeny, the boundaries of the adult pons are sometimes difficult to precisely define. Accordingly, a working definition of rostral and caudal pontine boundaries for adult mammals is depicted.

Role of SDF1 chemokine in the development of lateral line efferent and facial motor neurons

Most sensory systems are innervated by efferent neurons as well as by afferent neurons. The efferent innervation modulates the sensitivity of the receptor cells or of the sensory terminals. In the posterior lateral line system of the zebrafish, two efferent nuclei have been described in the hindbrain. Here we examine the development of the efferent neurons. We show that their axons are guided toward the target organ along the lateral line nerve while their cell bodies migrate posteriorward across rhombomeres to achieve their final position in rhombomeres 6/7. This migration depends on the SDF1 chemokine. We show that the migration of motor neurons of the facial nucleus from rhombomere 4 to 6 is also affected in sdf1a morphants (embryos injected with morpholine-conjugated antisense oligonucleotides). We propose that SDF1/CXCR4-mediated cell migration is preferentially associated with movement along the anteroposterior axis of the animal.

Vascular endothelial growth factor controls neuronal migration and cooperates with Sema3A to pattern distinct compartments of the facial nerve

Developing neurons accurately position their somata within the neural tube to make contact with appropriate neighbors and project axons to their preferred targets. Taking advantage of a collection of genetically engineered mouse mutants, we now demonstrate that the behavior of somata and axons of the facial nerve is regulated independently by two secreted ligands for the transmembrane receptor neuropilin 1 (Nrp1), the semaphorin Sema3A and the VEGF164 isoform of Vascular Endothelial Growth Factor. Although Sema3A is known to control the guidance of facial nerve axons, we now show that it is not required for the pathfinding of their somata. Vice versa, we find that VEGF164 is not required for axon guidance of facial motor neurons, but is essential for the correct migration of their somata. These observations demonstrate, for the first time, that VEGF contributes to neuronal patterning in vivo, and that different compartments of one cell can be co-ordinately patterned by structurally distinct ligands for a shared receptor.

Cytoarchitecture and musculotopic organization of the facial motor nucleus in Cebus apella monkey

The architecture and musculotopic organization of the facial motor nucleus in the Cebus apella monkey (a New World primate) were investigated using histological techniques and a multiple labelling strategy, in which horseradish peroxidase-conjugated neuroanatomical tracers (CTB-HRP and WGA-HRP) and fluorescent tracers were injected into individual facial muscles. The facial motor nucleus was formed by multipolar motoneurons and had an ovoid shape, with its rostrocaudal axis measuring on average 1875 micro m. We divided the nucleus into four different subnuclei: medial, intermediate, dorsal and lateral. Retrograde labelling patterns revealed that individual muscles were innervated by longitudinal functional columns of motoneurons. The columns of the orbicularis oculi, zygomaticus, orbicularis oris, auricularis superior, buccinator and platysma muscles were located in the dorsal, intermediate, lateral, medial, lateral and intermediate subnuclei, respectively. However, the motoneuron columns of the levator labii superioris alaeque nasi muscle and frontalis muscle could not be associated with a specific subnucleus. The present results confirm previous studies regarding the musculotopic organization of the facial motor nucleus. However, we observed some particularities in terms of the relative size of each column in C. apella, which might be related to the functional and behavioral importance of each muscle in the particular context of this primate.

Dynamic expression of RGS4 in the developing nervous system and regulation by the neural type-specific transcription factor Phox2b

Previous studies have shown that members of the family of regulators of G-protein signaling (RGS), including RGS4, have a discrete expression pattern in the adult brain (Gold et al., 1997). Here, we describe for RGS4 a distinct, mostly transient phase of neuronal expression, during embryonic development: transcription of RGS4 occurs in a highly dynamic manner in a small set of peripheral and central neuronal precursors. This expression pattern overlaps extensively with that of the paired-like homeodomain protein Phox2b, a determinant of neuronal identity. In embryos deficient for Phox2b, RGS4 expression is downregulated in the locus coeruleus, sympathetic ganglia, and cranial motor and sensory neurons. Moreover, Phox2b cooperates with the basic helix-loop-helix protein Mash1 to transiently switch on RGS4 after ectopic expression in the chicken spinal cord. Intriguingly, we also identify a heterotrimeric G-protein alpha-subunit, gustducin, as coexpressed with RGS4 in developing facial motor neurons, also under the control of Phox2b. Altogether, these data identify components of the heterotrimeric G-protein signaling pathway as part of the type-specific program of neuronal differentiation.

Nkx6.1 controls migration and axon pathfinding of cranial branchio-motoneurons

As many studies have focused on the mechanisms of motoneuron specification,little is known about the factors that control the subsequent development of postmitotic motoneurons. Previously, we showed that the transcription factor Nkx6.1 is required for the early specification of somatic motoneuron progenitors in the spinal cord. Our present analysis of hindbrain motoneuron development in Nkx6.1-deficient mouse embryos reveals that the early specification of branchio-motoneurons is independent of Nkx6.1 function, but that it is required for their subsequent development. In Nkx6.1mutant mice, we observed defects in the migration, as well as in the axon projections of branchio-motoneurons. A detailed analysis of the migratory defect in facial branchio-motoneurons reveals ectopic expression of the cell surface receptors Ret and Unc5h3 in premigratory neurons,but no changes in the rhombomeric environment. Taken together, our findings demonstrate a requirement for Nkx6.1 in the development of postmitotic motoneurons, and suggest a cell-autonomous function in the control of branchio-motoneuron migration.

Neuronal defects in the hindbrain of Hoxa1, Hoxb1 and Hoxb2 mutants reflect regulatory interactions among these Hox genes

Hox genes are instrumental in assigning segmental identity in the developing hindbrain. Auto-, cross- and para-regulatory interactions help establish and maintain their expression. To understand to what extent such regulatory interactions shape neuronal patterning in the hindbrain, we analysed neurogenesis, neuronal differentiation and motoneuron migration in Hoxa1, Hoxb1 and Hoxb2 mutant mice. This comparison revealed that neurogenesis and differentiation of specific neuronal subpopulations in r4 was impaired in a similar fashion in all three mutants, but with different degrees of severity. In the Hoxb1 mutants, neurons derived from the presumptive r4 territory were re-specified towards an r2-like identity. Motoneurons derived from that territory resembled trigeminal motoneurons in both their migration patterns and the expression of molecular markers. Both migrating motoneurons and the resident territory underwent changes consistent with a switch from an r4 to r2 identity. Abnormally migrating motoneurons initially formed ectopic nuclei that were subsequently cleared. Their survival could be prolonged through the introduction of a block in the apoptotic pathway. The Hoxa1 mutant phenotype is consistent with a partial misspecification of the presumptive r4 territory that results from partial Hoxb1 activation. The Hoxb2 mutant phenotype is a hypomorph of the Hoxb1 mutant phenotype, consistent with the overlapping roles of these genes in facial motoneuron specification. Therefore, we have delineated the functional requirements in hindbrain neuronal patterning that follow the establishment of the genetic regulatory hierarchy between Hoxa1, Hoxb1 and Hoxb2.

Role of Phox2b and Mash1 in the generation of the vestibular efferent nucleus

The inner ear (vestibular and cochlear) efferent neurons are a group of atypical motor-like hindbrain neurons which innervate inner ear hair cells and their sensory afferents. They are born in the fourth rhombomere, in close association with facial branchial motor neurons, from which they subsequently part through a specific migration route. Here, we demonstrate that the inner ear efferents depend on Phox2b for their differentiation, behaving in that respect like hindbrain visceral and branchial motor neurons. We also show that the vestibular efferent nucleus is no longer present at its usual site in mice inactivated for the bHLH transcription factor Mash 1. The concomitant appearance of an ectopic branchial-like nucleus at the location where both inner ear efferents and facial branchial motor neurons are born suggests that Mash1 is required for the migration of a subpopulation of rhombomere 4-derived efferents.

Autonomous and nonautonomous functions for Hox/Pbx in branchiomotor neuron development

The vertebrate branchiomotor neurons are organized in a pattern that corresponds with the segments, or rhombomeres, of the developing hindbrain and have identities and behaviors associated with their position along the anterior/posterior axis. These neurons undergo characteristic migrations in the hindbrain and project from stereotyped exit points. We show that lazarus/pbx4, which encodes an essential Hox DNA-binding partner in zebrafish, is required for facial (VIIth cranial nerve) motor neuron migration and for axon pathfinding of trigeminal (Vth cranial nerve) motor axons. We show that lzr/pbx4 is required for Hox paralog group 1 and 2 function, suggesting that Pbx interacts with these proteins. Consistent with this, lzr/pbx4 interacts genetically with hoxb1a to control facial motor neuron migration. Using genetic mosaic analysis, we show that lzr/pbx4 and hoxb1a are primarily required cell-autonomously within the facial motor neurons; however, analysis of a subtle non-cell-autonomous effect indicates that facial motor neuron migration is promoted by interactions amongst the migrating neurons. At the same time, lzr/pbx4 is required non-cell-autonomously to control the pathfinding of trigeminal motor axons. Thus, Pbx/Hox can function both cell-autonomously and non-cell-autonomously to direct different aspects of hindbrain motor neuron behavior.

Morphology of the decrementing expiratory neurons in the brainstem of the rat

In anesthetized and artificially-ventilated rats, the morphological properties of decrementing expiratory (E-DEC) neurons were studied using intracellular recording and labeling with Neurobiotin. Sixteen E-DEC neurons were successfully labeled; ten of which were cranial motoneurons located in the facial (FN) and ambiguus (NA) nuclei. Two interneurons were labeled in the Bötzinger complex (BOT) and the ventral respiratory group (VRG) rostral to the obex, and the remaining four in the VRG caudal to the obex. All the interneurons had extensive intramedullary collaterals within the ventrolateral medulla. Terminal-like boutons were distributed ventral to the NA at the level of the BOT, both ventral to and within the NA at the level rostral to the obex and largely within the cell column tentatively designed as the ambiguous-retroambiguus complex (NA/NRA) caudal to the obex. The four interneurons in the NA/NRA had axons projecting to the spinal cord as well. The extensive intramedullary projections suggest that these E-DEC interneurons of the BOT and the VRG play a significant role in respiration. The simultaneous projections from the caudal E-DEC neurons to both the spinal cord and the NA suggest that these neurons also play integrative roles in non-respiratory behaviors including vocalization, swallowing and defecation.

Cyclin-dependent kinase 5/p35 contributes synergistically with Reelin/Dab1 to the positioning of facial branchiomotor and inferior olive neurons in the developing mouse hindbrain

Cyclin-dependent kinase 5 (Cdk5)/p35 is a serine/threonine kinase, and its activity is detected primarily in postmitotic neurons. Mice lacking Cdk5/p35 display migration defects of the cortical neurons in the cerebrum and cerebellum. In this study, we demonstrate that although most brainstem nuclei are found in their proper positions, the motor nucleus of the facial nerve is ectopically located and neurons of the inferior olive fail to position correctly, resulting in the lack of their characteristic structures in the hindbrain of Cdk5-/- mice. Despite the defective migration of these neurons, axonal exits of the facial nerve from brainstem and projections of the inferior cerebellar axons appear unchanged in Cdk5-/- mice. Defective neuronal migration in Cdk5-/- hindbrain was rescued by the neuron-specific expression of Cdk5 transgene. Because developmental defects of these structures have been reported in reeler and Dab1 mutant mice, we analyzed the double-null mutants of p35 and Dab1 and found more extensive ectopia of VII motor nuclei in these mice. These results indicate that Cdk5/p35 and Reelin signaling regulates the selective mode of neuronal migration in the developing mouse hindbrain.

Branchiogenic motoneurons innervating facial, masticatory, and esophageal muscles show aberrant distribution in the reeler-phenotype mutant rat, Shaking Rat Kawasaki

Shaking Rat Kawasaki (SRK) is an autosomal recessive mutant rat that is characterized by cerebellar ataxia. Although previous studies indicated many points of similarity between this mutant rat and the reeler mouse, nonlaminated structures such as the facial nucleus have not been studied in this mutant rat. Nissl-stained sections through the brainstem showed that the cytoarchitecture of the facial, motor trigeminal, and ambiguus nuclei was abnormal in SRK, especially in the lateral cell group of the facial nucleus and the compact formation of the ambiguus nucleus. To examine whether orofacial motoneurons are also malpositioned in the SRK rat, horseradish peroxidase (HRP) was injected into the facial, masticatory, and abdominal esophageal muscles of the SRK rats and normal controls to label facial, trigeminal, and ambiguus motoneurons, respectively. HRP-labeled facial, trigeminal, and ambiguus motoneurons of the SRK rat were distributed more widely than those of their normal counterparts, as in the case of the reeler mouse, with the one exception that labeled facial motoneurons innervating the nasolabial muscle were distributed more widely in the ventrolateral-to-dorsomedial direction in comparison with those of the reeler mutant. These data demonstrate that nonlaminated structures in the brainstem of the SRK rat are affected severely, as is the case in the reeler mutant mouse.

Initiation of facial motoneurone migration is dependent on rhombomeres 5 and 6

In mammals, facial branchiomotor (FBM) neurones are born in ventral rhombomere (r) 4 and migrate through r5 to dorsal r6 where they form the facial motor nucleus. This pattern of migration gives rise to the distinctive appearance of the internal genu of the facial nerve, which is lacking in birds. To distinguish between extrinsic cues and intrinsic factors in the caudal migration of FBM neurones, this study takes advantage of the evolutionary migratory difference between mouse and chick in generating mouse-chick chimaeras in ovo. After the homotopic transplantation of mouse r5 and/or r6 into a chick embryo, chick ventral r4 neurones redirected their cell bodies towards the ectopic mouse source and followed a caudal migratory path, reminiscent of mouse FBM neurones. In a second series of grafting experiments, when mouse r4 was transplanted in place of chick r4, mouse r4 neurones were unable to migrate into chick r5, although mouse and chick cells were able to mix freely within r4. Thus, these data suggest that local environmental cues embedded in mouse r5 and r6 are directly involved in initiating caudal migration of FBM neurones. In addition, they demonstrate that chick FBM neurones are competent to recapitulate a migratory behaviour that has been lost during avian phylogeny.

The COE--Collier/Olf1/EBF--transcription factors: structural conservation and diversity of developmental functions

One major conclusion of studies in Developmental Biology during the last two decades is that, despite profound anatomical differences, the building of vertebrate and arthropod bodies relies on the same fundamental molecular networks, including conserved cell signalling and transcription-regulatory cascades. Rodent Early B-Cell Factor/Olfactory-1 and Drosophila Collier belong to a recently defined, novel family of transcription factors, the Collier/Olf1/EBF (COE) proteins which have a unique DNA-binding domain. Early investigations revealed that, despite their high degree of sequence identity, the different vertebrate and invertebrate COE proteins play a variety of developmental roles. We review here the current evidence for this diversity of COE functions, including in the specification and differentiation of various neuronal populations. We also discuss the existence of an evolutionarily conserved pathway linking Notch signalling and COE regulatory functions in various developmental decisions.

Rhombomere interactions control the segmental differentiation of hindbrain neurons

The embryonic hindbrain is subdivided into a series of metameric units termed rhombomeres, which display features that strongly suggest they are autonomous developmental units. However, some aspects of their phenotype develop nonautonomously. Here we have analyzed the possibility that interrhombomere interactions generate the pattern of segmental neuronal differentiation. The differentiation of both projection interneurons and motor neurons in the hindbrain is retarded in rhombomeres 3 and 5. We demonstrate here that if either rhombomere 3 or 5 is isolated from the influence of their neighbours, either in vitro or in vivo, then these segments no longer display delayed neuronal diufferentiation. We further show that the retardation of motor neurons differentiation in rhombomeres 3 and 5 is, at least in part, mediated by Bmp-4. If this molecule is inhibited, by grafting cells expressing chordin, then the motor neurons of these rhombomeres develop ahead of their normal schedule.

Correlated patterns of neuron differentiation and Hox gene expression in the hindbrain: a comparative analysis

Hindbrain neurons are organized into coherent subpopulations with characteristic projection patterns and functions. Many of these serve vital functions that have been conserved throughout the vertebrate radiation, but diversification to modified or highly specialized functions has also occurred. The differentiation of identifiable neuron groups in specific spatial domains must involve the regional expression of determinants within the hindbrain neuroepithelium. The Hox genes are involved in longitudinal regionalization of the neural tube, and their expression patterns in the hindbrain are closely related to the rhombomeres which partition the hindbrain into morphogenetic units. Hox gene expression also exhibits conserved patterning as well as phylogenetic variation. One plausible mechanism that may have contributed to evolutionary diversification in hindbrain neuron populations is therefore the emergence of species-specific differences in Hox gene expression. This article presents a comparative overview of the regional patterning of selected Hox genes and hindbrain neuron populations in several embryologically important species. Although tantalizing correlations exist, the relationship between Hox genes and neuronal patterning is complex, and complicated by dynamic features in each. Much more comparative and developmental data must be obtained before the link between Hox gene expression and hindbrain neuron patterning can be elucidated satisfactorily in an evolutionary context.