Development of oculomotor axon projections in the chick embryo

Oculomotor nerve guidance and terminal branching requires interactions with differentiating extraocular muscles

Muscle function is dependent on innervation by the correct motor nerves. Motor nerves are composed of motor axons which extend through peripheral tissues as a compact bundle, then diverge to create terminal nerve branches to specific muscle targets. As motor nerves approach their targets, they undergo a transition where the fasciculated nerve halts further growth then after a pause, the nerve later initiates branching to muscles. This transition point is potentially an intermediate target or guidepost to present specific cellular and molecular signals for navigation. Here we describe the navigation of the oculomotor nerve and its association with developing muscles in mouse embryos. We found that the oculomotor nerve initially grew to the eye three days prior to the appearance of any extraocular muscles. The oculomotor axons spread to form a plexus within a mass of cells, which included precursors of extraocular muscles and other orbital tissues and expressed the transcription factor Pitx2. The nerve growth paused in the plexus for more than two days, persisting during primary extraocular myogenesis, with a subsequent phase in which the nerve branched out to specific muscles. To test the functional significance of the nerve contact with Pitx2+ cells in the plexus, we used two strategies to genetically ablate Pitx2+ cells or muscle precursors early in nerve development. The first strategy used Myf5-Cre-mediated expression of diphtheria toxin A to ablate muscle precursors, leading to loss of extraocular muscles. The oculomotor axons navigated to the eye to form the main nerve, but subsequently largely failed to initiate terminal branches. The second strategy studied Pitx2 homozygous mutants, which have early apoptosis of Pitx2-expressing precursor cells, including precursors for extraocular muscles and other orbital tissues. Oculomotor nerve fibers also grew to the eye, but failed to stop to form the plexus, instead grew long ectopic projections. These results show that neither Pitx2 function nor Myf5-expressing cells are required for oculomotor nerve navigation to the eye. However, Pitx2 function is required for oculomotor axons to pause growth in the plexus, while Myf5-expressing cells are required for terminal branch initiation.

Coactosin Promotes F-Actin Protrusion in Growth Cones Under Cofilin-Related Signaling Pathway

During brain development, axon outgrowth and its subsequent pathfinding are reliant on a highly motile growth cone located at the tip of the axon. Actin polymerization that is regulated by actin-depolymerizing factors homology (ADF-H) domain-containing family drives the formation of lamellipodia and filopodia at the leading edge of growth cones for axon guidance. However, the precise localization and function of ADF-H domain-containing proteins involved in axon extension and retraction remain unclear. We have previously shown that transcripts and proteins of coactosin-like protein 1 (COTL1), an ADF-H domain-containing protein, are observed in neurites and axons in chick embryos. Coactosin overexpression analysis revealed that this protein was localized to axonal growth cones and involved in axon extension in the midbrain. We further examined the specific distribution of coactosin and cofilin within the growth cone using superresolution microscopy, structured illumination microscopy, which overcomes the optical diffraction limitation and is suitable to the analysis of cellular dynamic movements. We found that coactosin was tightly associated with F-actin bundles at the growth cones and that coactosin overexpression promoted the expansion of lamellipodia and extension of growth cones. Coactosin knockdown in oculomotor neurons resulted in an increase in the levels of the inactive, phosphorylated form of cofilin and dysregulation of actin polymerization and axonal elongation, which suggests that coactosin promoted axonal growth in a cofilin-dependent manner. Indeed, the application of a dominant-negative form of LIMK1, a downstream effector of GTPases, reversed the effect of coactosin knockdown on axonal growth by enhancing cofilin activity. Combined, our results indicate that coactosin functions promote the assembly of protrusive actin filament arrays at the leading edge for growth cone motility.

Birthdate-dependent heterogeneity of oculomotor neurons is involved in transmedian migration in the developing mouse midbrain

During the formation of the oculomotor nucleus (nIII), a subset of cells undergoes transmedian migration, crossing the midline to join the contralateral nucleus. A recent study reported that the onset of transmedian migration of nIII neurons is regulated by Slit/Robo signaling. However, developmental programs that differentiate migratory subpopulations of the nIII remain elusive. Here, we identified cellular and molecular characteristics of nIII neurons that are correlated with their migratory behaviors. Birthdate analysis revealed that contralaterally migrating neurons in the caudal part of the nIII are generated at later stages than uncrossed neurons in the rostral part of the nIII. Furthermore, we found that Slit2 is expressed in the ventral midline of the midbrain and contralaterally migrating neurons. On the other hand, Robo2, a receptor of Sli2, is differentially expressed in subpopulations of rostral and caudal parts of the nIII: uncrossed neurons expressed Robo2 in the developing nIII. These results suggest that spatio-temporal regulation of developmental timings and the molecular signatures of oculomotor neurons are crucial for transmedian migration, which underlies appropriate positioning and stereotyped circuit formation of the nIII in the developing mouse midbrain.

The stepwise development of the lamprey visual system and its evolutionary implications

Lampreys, which represent the oldest group of living vertebrates (cyclostomes), show unique eye development. The lamprey larva has only eyespot-like immature eyes beneath a non-transparent skin, whereas after metamorphosis, the adult has well-developed image-forming camera eyes. To establish a functional visual system, well-organised visual centres as well as motor components (e.g. trunk muscles for locomotion) and interactions between them are needed. Here we review the available knowledge concerning the structure, function and development of the different parts of the lamprey visual system. The lamprey exhibits stepwise development of the visual system during its life cycle. In prolarvae and early larvae, the 'primary' retina does not have horizontal and amacrine cells, but does have photoreceptors, bipolar cells and ganglion cells. At this stage, the optic nerve projects mostly to the pretectum, where the dendrites of neurons in the nucleus of the medial longitudinal fasciculus (nMLF) appear to receive direct visual information and send motor outputs to the neck and trunk muscles. This simple neural circuit may generate negative phototaxis. Through the larval period, the lateral region of the retina grows again to form the 'secondary' retina and the topographic retinotectal projection of the optic nerve is formed, and at the same time, the extra-ocular muscles progressively develop. During metamorphosis, horizontal and amacrine cells differentiate for the first time, and the optic tectum expands and becomes laminated. The adult lamprey then has a sophisticated visual system for image-forming and visual decision-making. In the adult lamprey, the thalamic pathway (retina-thalamus-cortex/pallium) also transmits visual stimuli. Because the primary, simple light-detecting circuit in larval lamprey shares functional and developmental similarities with that of protochordates (amphioxus and tunicates), the visual development of the lamprey provides information regarding the evolutionary transition of the vertebrate visual system from the protochordate-type to the vertebrate-type.

Ocular congenital cranial dysinnervation disorders (CCDDs): insights into axon growth and guidance

Unraveling the genetics of the paralytic strabismus syndromes known as congenital cranial dysinnervation disorders (CCDDs) is both informing physicians and their patients and broadening our understanding of development of the ocular motor system. Genetic mutations underlying ocular CCDDs alter either motor neuron specification or motor nerve development, and highlight the importance of modulations of cell signaling, cytoskeletal transport, and microtubule dynamics for axon growth and guidance. Here we review recent advances in our understanding of two CCDDs, congenital fibrosis of the extraocular muscles (CFEOM) and Duane retraction syndrome (DRS), and discuss what they have taught us about mechanisms of axon guidance and selective vulnerability. CFEOM presents with congenital ptosis and restricted eye movements, and can be caused by heterozygous missense mutations in the kinesin motor protein KIF21A or in the β-tubulin isotypes TUBB3 or TUBB2B. CFEOM-causing mutations in these genes alter protein function and result in axon growth and guidance defects. DRS presents with inability to abduct one or both eyes. It can be caused by decreased function of several transcription factors critical for abducens motor neuron identity, including MAFB, or by heterozygous missense mutations in CHN1, which encodes α2-chimaerin, a Rac-GAP GTPase that affects cytoskeletal dynamics. Examination of the orbital innervation in mice lacking Mafb has established that the stereotypical misinnervation of the lateral rectus by fibers of the oculomotor nerve in DRS is secondary to absence of the abducens nerve. Studies of a CHN1 mouse model have begun to elucidate mechanisms of selective vulnerability in the nervous system.

Ocular Motor Nerve Development in the Presence and Absence of Extraocular Muscle

Purpose

To spatially and temporally define ocular motor nerve development in the presence and absence of extraocular muscles (EOMs).

Methods

Myf5^cre mice, which in the homozygous state lack EOMs, were crossed to an Isl^MN:GFP reporter line to fluorescently label motor neuron cell bodies and axons. Embryonic day (E) 11.5 to E15.5 wild-type and Myf5^cre/cre:Isl^MN:GFP whole mount embryos and dissected orbits were imaged by confocal microscopy to visualize the developing oculomotor, trochlear, and abducens nerves in the presence and absence of EOMs. E11.5 and E18.5 brainstems were serially sectioned and stained for Islet1 to determine the fate of ocular motor neurons.

Results

At E11.5, all three ocular motor nerves in mutant embryos approached the orbit with a trajectory similar to that of wild-type. Subsequently, while wild-type nerves send terminal branches that contact target EOMs in a stereotypical pattern, the Myf5^cre/cre ocular motor nerves failed to form terminal branches, regressed, and by E18.5 two-thirds of their corresponding motor neurons died. Comparisons between mutant and wild-type embryos revealed novel aspects of trochlear and oculomotor nerve development.

Conclusions

We delineated mouse ocular motor nerve spatial and temporal development in unprecedented detail. Moreover, we found that EOMs are not necessary for initial outgrowth and guidance of ocular motor axons from the brainstem to the orbit but are required for their terminal branching and survival. These data suggest that intermediate targets in the mesenchyme provide cues necessary for appropriate targeting of ocular motor axons to the orbit, while EOM cues are responsible for terminal branching and motor neuron survival.

Contralateral migration of oculomotor neurons is regulated by Slit/Robo signaling

BACKGROUND

Oculomotor neurons develop initially like typical motor neurons, projecting axons out of the ventral midbrain to their ipsilateral targets, the extraocular muscles. However, in all vertebrates, after the oculomotor nerve (nIII) has reached the extraocular muscle primordia, the cell bodies that innervate the superior rectus migrate to join the contralateral nucleus. This motor neuron migration represents a unique strategy to form a contralateral motor projection. Whether migration is guided by diffusible cues remains unknown.

METHODS

We examined the role of Slit chemorepellent signals in contralateral oculomotor migration by analyzing mutant mouse embryos.

RESULTS

We found that the ventral midbrain expresses high levels of both Slit1 and 2, and that oculomotor neurons express the repellent Slit receptors Robo1 and Robo2. Therefore, Slit signals are in a position to influence the migration of oculomotor neurons. In Slit 1/2 or Robo1/2 double mutant embryos, motor neuron cell bodies migrated into the ventral midbrain on E10.5, three days prior to normal migration. These early migrating neurons had leading projections into and across the floor plate. In contrast to the double mutants, embryos which were mutant for single Slit or Robo genes did not have premature migration or outgrowth on E10.5, demonstrating a cooperative requirement of Slit1 and 2, as well as Robo1 and 2. To test how Slit/Robo midline repulsion is modulated, we found that the normal migration did not require the receptors Robo3 and CXCR4, or the chemoattractant, Netrin 1. The signal to initiate contralateral migration is likely autonomous to the midbrain because oculomotor neurons migrate in embryos that lack either nerve outgrowth or extraocular muscles, or in cultured midbrains that lacked peripheral tissue.

CONCLUSION

Overall, our results demonstrate that a migratory subset of motor neurons respond to floor plate-derived Slit repulsion to properly control the timing of contralateral migration.

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.

Dynamic expression of Notch-dependent neurogenic markers in the chick embryonic nervous system

The establishment of a functional nervous system requires a highly orchestrated process of neural proliferation and differentiation. The evolutionary conserved Notch signaling pathway is a key regulator of this process, regulating basic helix-loop-helix (bHLH) transcriptional repressors and proneural genes. However, little is known about downstream Notch targets and subsequently genes required for neuronal specification. In this report, the expression pattern of Transgelin 3 (Tagln3), Chromogranin A (Chga) and Contactin 2 (Cntn2) was described in detail during early chick embryogenesis. Expression of these genes was largely restricted to the nervous system including the early axon scaffold populations, cranial ganglia and spinal motor neurons. Their temporal and spatial expression were compared with the neuronal markers Nescient Helix-Loop-Helix 1 (Nhlh1), Stathmin 2 (Stmn2) and HuC/D. We show that Tagln3 is an early marker for post-mitotic neurons whereas Chga and Cntn2 are expressed in mature neurons. We demonstrate that inhibition of Notch signaling during spinal cord neurogenesis enhances expression of these markers. This data demonstrates that Tagln3, Chga and Cntn2 represent strong new candidates to contribute to the sequential progression of vertebrate neurogenesis.

α2-Chimaerin regulates a key axon guidance transition during development of the oculomotor projection

The ocular motor system consists of three nerves which innervate six muscles to control eye movements. In humans, defective development of this system leads to eye movement disorders, such as Duane Retraction Syndrome, which can result from mutations in the α2-chimaerin signaling molecule. We have used the zebrafish to model the role of α2-chimaerin during development of the ocular motor system. We first mapped ocular motor spatiotemporal development, which occurs between 24 and 72 h postfertilization (hpf), with the oculomotor nerve following an invariant sequence of growth and branching to its muscle targets. We identified 52 hpf as a key axon guidance "transition," when oculomotor axons reach the orbit and select their muscle targets. Live imaging and quantitation showed that, at 52 hpf, axons undergo a switch in behavior, with striking changes in the dynamics of filopodia. We tested the role of α2-chimaerin in this guidance process and found that axons expressing gain-of-function α2-chimaerin isoforms failed to undergo the 52 hpf transition in filopodial dynamics, leading to axon stalling. α2-chimaerin loss of function led to ecotopic and misguided branching and hypoplasia of oculomotor axons; embryos had defective eye movements as measured by the optokinetic reflex. Manipulation of chimaerin signaling in oculomotor neurons in vitro led to changes in microtubule stability. These findings demonstrate that a correct level of α2-chimaerin signaling is required for key oculomotor axon guidance decisions, and provide a zebrafish model for Duane Retraction Syndrome.

Development of the rat larynx: a histological study

OBJECTIVES/HYPOTHESIS

To evaluate and describe the cartilaginous and muscular development of the rat larynx.

STUDY DESIGN

Histologic evaluation.

METHODS

The larynges of Sprague Dawley rats of embryonic day (E) 13, 15, 17, 19, 21, postnatal day 0, 14, and adult of 250 gm were collected. Four larynges of each age were harvested, cut into 15-μm serial sections, stained with hematoxylin and eosin, and evaluated under light microscopy. Representative digital images were recorded and evaluated at the preglottic (supraglottic in humans), glottic, and postglottic (subglottic in humans) levels.

RESULTS

Brachial arches were observed at E13. At E17, immature structures of the larynx, including skeletal muscle, cartilage, and the lumen were identifiable. Chondrification and muscle formation were clearly seen by E19. The muscular and cartilagenous components of the larynx were well established by E21. During the span between birth and adult maturation, the size of the larynx increased from a height of 1.10 mm to 2.90 mm, and from a width of 1.80 mm to 5.40 mm, and from a length of 1.38 mm to 4.77 mm in the stained section. Although developed at E21, the laryngeal structures continued to grow by approximately 30%.

CONCLUSION

Rat laryngeal development parallels that in mice and humans. In the rat, at E17 immature structures of the larynx are identifiable, they are well developed at birth and grow by approximately 30% into adulthood. Understanding the chronology and morphology of the embryogenesis of the rat laryngeal musculature is essential and will allow for further evaluation of the embryologic innervation of these muscles.

Axon guidance in the developing ocular motor system and Duane retraction syndrome depends on Semaphorin signaling via alpha2-chimaerin

Eye movements depend on correct patterns of connectivity between cranial motor axons and the extraocular muscles. Despite the clinical importance of the ocular motor system, little is known of the molecular mechanisms underlying its development. We have recently shown that mutations in the Chimaerin-1 gene encoding the signaling protein α2-chimaerin (α2-chn) perturb axon guidance in the ocular motor system and lead to the human eye movement disorder, Duane retraction syndrome (DRS). The axon guidance cues that lie upstream of α2-chn are unknown; here we identify candidates to be the Semaphorins (Sema) 3A and 3C, acting via the PlexinA receptors. Sema3A/C are expressed in and around the developing extraocular muscles and cause growth cone collapse of oculomotor neurons in vitro. Furthermore, RNAi knockdown of α2-chn or PlexinAs in oculomotor neurons abrogates Sema3A/C-dependent growth cone collapse. In vivo knockdown of endogenous PlexinAs or α2-chn function results in stereotypical oculomotor axon guidance defects, which are reminiscent of DRS, whereas expression of α2-chn gain-of-function constructs can rescue PlexinA loss of function. These data suggest that α2-chn mediates Sema3-PlexinA repellent signaling. We further show that α2-chn is required for oculomotor neurons to respond to CXCL12 and hepatocyte growth factor (HGF), which are growth promoting and chemoattractant during oculomotor axon guidance. α2-chn is therefore a potential integrator of different types of guidance information to orchestrate ocular motor pathfinding. DRS phenotypes can result from incorrect regulation of this signaling pathway.

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

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

Drebrin controls neuronal migration through the formation and alignment of the leading process

Formation of a functional nervous system requires neurons to migrate to the correct place within the developing brain. Tangentially migrating neurons are guided by a leading process which extends towards the target and is followed by the cell body. How environmental cues are coupled to specific cytoskeletal changes to produce and guide leading process growth is unknown. One such cytoskeletal modulator is drebrin, an actin-binding protein known to induce protrusions in many cell types and be important for regulating neuronal morphology.

Using the migration of oculomotor neurons as a model, we have shown that drebrin is necessary for the generation and guidance of the leading process. In the absence of drebrin, leading processes are not formed and cells fail to migrate although axon growth and pathfinding appear grossly unaffected. Conversely, when levels of drebrin are elevated the leading processes turn away from their target and as a result the motor neuron cell bodies move along abnormal paths within the brain. The aberrant trajectories were highly reproducible suggesting that drebrin is required to interpret specific guidance cues. The axons and growth cones of these neurons display morphological changes, particularly increased branching and filopodial number but despite this they extend along normal developmental pathways.

Collectively these results show that drebrin is initially necessary for the formation of a leading process and subsequently for this to respond to navigational signals and grow in the correct direction. Furthermore, we have shown that the actions of drebrin can be segregated within individual motor neurons to direct their migration independently of axon guidance.

Cranial nerve fasciculation and Schwann cell migration are impaired after loss of Npn-1

Interaction of the axon guidance receptor Neuropilin-1 (Npn-1) with its repulsive ligand Semaphorin 3A (Sema3A) is crucial for guidance decisions, fasciculation, timing of growth and axon-axon interactions of sensory and motor projections in the embryonic limb. At cranial levels, Npn-1 is expressed in motor neurons and sensory ganglia and loss of Sema3A-Npn-1 signaling leads to defasciculation of the superficial projections to the head and neck. The molecular mechanisms that govern the initial fasciculation and growth of the purely motor projections of the hypoglossal and abducens nerves in general, and the role of Npn-1 during these events in particular are, however, not well understood. We show here that selective removal of Npn-1 from somatic motor neurons impairs initial fasciculation and assembly of hypoglossal rootlets and leads to reduced numbers of abducens and hypoglossal fibers. Ablation of Npn-1 specifically from cranial neural crest and placodally derived sensory tissues recapitulates the distal defasciculation of mixed sensory-motor nerves of trigeminal, facial, glossopharyngeal and vagal projections, which was observed in Npn-1(-/-) and Npn-1(Sema-) mutants. Surprisingly, the assembly and fasciculation of the purely motor hypoglossal nerve are also impaired and the number of Schwann cells migrating along the defasciculated axonal projections is reduced. These findings are corroborated by partial genetic elimination of cranial neural crest and embryonic placodes, where loss of Schwann cell precursors leads to aberrant growth patterns of the hypoglossal nerve. Interestingly, rostral turning of hypoglossal axons is not perturbed in any of the investigated genotypes. Thus, initial hypoglossal nerve assembly and fasciculation, but not later guidance decisions depend on Npn-1 expression and axon-Schwann cell interactions.

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

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

PHOX2A regulation of oculomotor complex nucleogenesis

Brain nuclei are spatially organized collections of neurons that share functional properties. Despite being central to vertebrate brain circuitry, little is known about how nuclei are generated during development. We have chosen the chick midbrain oculomotor complex (OMC) as a model with which to study the developmental mechanisms of nucleogenesis. The chick OMC comprises two distinct cell groups: a dorsal Edinger-Westphal nucleus of visceral oculomotor neurons and a ventral nucleus of somatic oculomotor neurons. Genetic studies in mice and humans have established that the homeobox transcription factor gene PHOX2A is required for midbrain motoneuron development. We probed, in forced expression experiments, the capacity of PHOX2A to generate a spatially organized midbrain OMC. We found that exogenous Phox2a delivery to embryonic chick midbrain can drive a complete OMC molecular program, including the production of visceral and somatic motoneurons. Phox2a overexpression was also able to generate ectopic motor nerves. The exit points of such auxiliary nerves were invested with ectopic boundary cap cells and, in four examples, the ectopic nerves were seen to innervate extraocular muscle directly. Finally, Phox2a delivery was able to direct ectopic visceral and somatic motoneurons to their correct native spatial positions, with visceral motoneurons settling close to the ventricular surface and somatic motoneurons migrating deeper into the midbrain. These findings establish that in midbrain, a single transcription factor can both specify motoneuron cell fates and orchestrate the construction of a spatially organized motoneuron nuclear complex.

The specific targeting of guidance receptors within neurons: who directs the directors?

Guidance molecules present in both axonal and dendritic growth cones mediate neuronal responses to extracellular cues thereby ensuring correct neurite pathfinding and development of the nervous system. Little is known though about the mechanisms employed by neurons to deliver these receptors, specifically and efficiently, to the extending growth cone. A deeper understanding of this process is crucial if guidance receptors are to be manipulated to promote nervous system repair. Studies in other polarised cells, notably epithelial, have elucidated fundamental routes to the intracellular segregation of molecules mediated by endosomal pathways. Due to their extreme complexity and specialisation, neurons appear to have built upon these generic systems to evolve sophisticated trafficking networks. A striking feature is the axon initial segment which acts like a valve to tightly regulate the flux of molecules both entering and leaving the axon. Once in the growth cone, further controls operate to enhance the retention or rejection, as appropriate, of membrane receptors. We discuss the current state of knowledge regarding the intracellular trafficking of axon guidance receptors and how this relates to their developmental roles. We highlight the various facets still to be properly elucidated and by building on existing data regarding neuronal polarity and intracellular sorting mechanisms suggest ways to fill these gaps.

Human CHN1 mutations hyperactivate alpha2-chimaerin and cause Duane's retraction syndrome

Duane's retraction syndrome (DRS) is a complex congenital eye movement disorder caused by aberrant innervation of the extraocular muscles by axons of brainstem motor neurons. Studying families with a variant form of the disorder (DURS2-DRS), we have identified causative heterozygous missense mutations in CHN1, a gene on chromosome 2q31 that encodes alpha2-chimaerin, a Rac guanosine triphosphatase-activating protein (RacGAP) signaling protein previously implicated in the pathfinding of corticospinal axons in mice. We found that these are gain-of-function mutations that increase alpha2-chimaerin RacGAP activity in vitro. Several of the mutations appeared to enhance alpha2-chimaerin translocation to the cell membrane or enhance its ability to self-associate. Expression of mutant alpha2-chimaerin constructs in chick embryos resulted in failure of oculomotor axons to innervate their target extraocular muscles. We conclude that alpha2-chimaerin has a critical developmental function in ocular motor axon pathfinding.

Neural tube derived signals and Fgf8 act antagonistically to specify eye versus mandibular arch muscles

Recent knockout experiments in the mouse generated amazing craniofacial skeletal muscle phenotypes. Yet none of the genes could be placed into a molecular network, because the programme to control the development of muscles in the head is not known. Here we show that antagonistic signals from the neural tube and the branchial arches specify extraocular versus branchiomeric muscles. Moreover, we identified Fgf8 as the branchial arch derived signal. However, this molecule has an additional function in supporting the proliferative state of myoblasts, suppressing their differentiation, while a further branchial arch derived signal, namely Bmp7, is an overall negative regulator of head myogenesis.

Biological organization of the extraocular muscles

Extraocular muscle is fundamentally distinct from other skeletal muscles. Here, we review the biological organization of the extraocular muscles with the intent of understanding this novel muscle group in the context of oculomotor system function. The specific objectives of this review are threefold. The first objective is to understand the anatomic arrangement of the extraocular muscles and their compartmental or layered organization in the context of a new concept of orbital mechanics, the active pulley hypothesis. The second objective is to present an integrated view of the morphologic, cellular, and molecular differences between extraocular and the more traditional skeletal muscles. The third objective is to relate recent data from functional and molecular biology studies to the established extraocular muscle fiber types. Developmental mechanisms that may be responsible for the divergence of the eye muscles from a skeletal muscle prototype also are considered. Taken together, a multidisciplinary understanding of extraocular muscle biology in health and disease provides insights into oculomotor system function and malfunction. Moreover, because the eye muscles are selectively involved or spared in a variety of neuromuscular diseases, knowledge of their biology may improve current pathogenic models of and treatments for devastating systemic diseases.

Development of the neuromuscular junction in extraocular muscles of white Leghorn chicks

Relatively little is known about the development of the neuromuscular junction of extraocular muscles (EOMs). In recent years, chicks have been increasingly used as a developmental model in ophthalmological research. To utilize this model system for understanding the development and plasticity of the extraocular motor system, we investigated the structural changes that occur at the developing neuromuscular junction in the chick between embryonic day 14 (E14) and posthatch day 2 (P2). Axons and nerve terminals were visualized with fluorescent neurofilament antibodies and motor endplates with rhodamine-conjugated alpha-bungarotoxin. Nerve fibers and endplates were colabeled within the same tissue samples. Motor endplates (density, length, width, and area) were measured and numbers of axons per neuromuscular junction were counted using confocal and conventional microscopy. In P2 chicks, densities of motor endplates were significantly greater in the superior oblique muscle when compared with the superior rectus and lateral gastrocnemius muscle. EOMs showed a two- to threefold larger area of motor endplate size as compared to gastrocnemius muscle. Motor endplate size also differed among EOMs with the superior oblique muscle having endplates with a larger area than those of the superior rectus muscle. The period of synapse elimination was similar between EOM and gastrocnemius muscle. Synapse elimination began at about E18 and was completed by P2. By describing the normal morphological changes in developing EOMs, this study provides a baseline for future work to elucidate underlying molecular mechanisms that regulate EOM innervation and strength.

Identification and distribution of mouse carboxypeptidase A-6

Carboxypeptidase A-6 (CPA6) was recently discovered in the human genome. To gain information regarding the potential function of this novel protein, the mouse homolog of CPA6 was identified using a combination of bioinformatics and reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, homologs in rat, chicken, and frog were identified using a bioinformatics approach. The distribution of CPA6 mRNA in mouse tissues was examined using RT-PCR and in situ hybridization. A strong RT-PCR signal is detectable in olfactory bulb, and much lower levels are present in other regions such as the cerebral cortex, hippocampus, hypothalamus, striatum, and medulla. In peripheral tissues, a moderate RT-PCR signal is present in epididymis, and low levels are detectable in colon and spleen. The high level of CPA6 in adult mouse brain olfactory bulb was confirmed by in situ hybridization. Lower levels of CPA6 mRNA were found to be present in the cingulate cortex, lateral septum, pontine nucleus, and inferior olivary nucleus of the hindbrain. Within the olfactory bulb, CPA6 mRNA is enriched in the mitral and granular layer. A lower level of CPA6 mRNA is present in the internal and external plexiform layers, and no signal is detectable in the olfactory nerve layer. The distribution was also examined in whole embryos at embryonic day 14.5 and CPA6 mRNA was found to be enriched in eye, ear, osteoblasts, stomach, skin, dorsal root ganglia, and throughout the CNS. The presence of CPA6 mRNA in the rectus muscle layer of the eye at embryonic day 14.5 is consistent with the observation that the CPA6 gene is disrupted in a patient with Duane syndrome, a congenital eye defect. Taken together, the distribution of CPA6 suggests a specific role in a limited number of tissues, and it is possible that this role involves an aspect of cell migration.