Expression of truncated Sek-1 receptor tyrosine kinase disrupts the segmental restriction of gene expression in the Xenopus and zebrafish hindbrain

Interactions of Eph-related receptors and ligands confer rostrocaudal pattern to trunk neural crest migration

BACKGROUND

In the trunk of avian embryos, neural crest migration through the somites is segmental, with neural crest cells entering the rostral half of each somitic sclerotome but avoiding the caudal half. Little is known about the molecular nature of the cues-intrinsic to the somites-that are responsible for this segmental migration of neural crest cells.

RESULTS

We demonstrate that Eph-related receptor tyrosine kinases and their ligands are essential for the segmental migration of avian trunk neural crest cells through the somites. EphB3 localizes to the rostral half-sclerotome, including the neural crest, and the ligand ephrin-B1 has a complementary pattern of expression in the caudal half-sclerotome. To test the functional significance of this striking asymmetry, soluble ligand ephrin-B1 was added to interfere with receptor function in either whole trunk explants or neural crest cells cultured on alternating stripes of ephrin-B1 versus fibronection. Neural crest cells in vitro avoided migrating on lanes of immobilized ephrin-B1; the addition of soluble ephrin-B1 blocked this inhibition. Similarly, in whole trunk explants, the metameric pattern of neural crest migration was disrupted by addition of soluble ephrin-B1, allowing entry of neural crest cells into caudal portions of the sclerotome.

CONCLUSIONS

Both in vivo and in vitro, the addition of soluble ephrin-B1 results in a loss of the metameric migratory pattern and a disorganization of neural crest cell movement. These results demonstrate that Eph-family receptor tyrosine kinases and their transmembrane ligands are involved in interactions between neural crest and sclerotomal cells, mediating an inhibitory activity necessary to constrain neural precursors to specific territories in the developing nervous system.

Krox-20 is a key regulator of rhombomere-specific gene expression in the developing hindbrain

The morphogenesis of the vertebrate hindbrain involves a transient segmentation process leading to the formation of reiterated organisation units called rhombomeres (r). A number of regulatory genes expressed with a rhombomere-specific pattern have been identified, including the gene encoding the transcription factor Krox-20, which is restricted to r3 and r5. We have previously demonstrated that in r3 and r5 Krox-20 directly controls the transcription of Hoxa-2 and Hoxb-2. In the present study, we provide evidence that Krox-20 is required for the expression of another Hox gene, Hoxb-3, in r5 specifically. Furthermore, the regulatory role of Krox-20 is not restricted to the control of Hox gene expression, since it is also involved in the activation of a receptor tyrosine kinase gene, Sek-1, in r3 and r5 and in the repression of the follistatin gene in r3 but not in r5. In conclusion, at least five regulatory genes belonging to different families are under the direct or indirect control of Krox-20 in r3 and/or r5 and this transcription factor therefore appears as a key regulator of gene expression in the developing hindbrain.

Cell movements, neuronal organisation and gene expression in hindbrains lacking morphological boundaries

Rhombomeres are segmental units of the hindbrain that are separated from each other by a specialised zone of boundary cells. Retinoic acid application to a recently segmented hindbrain leads to disappearance of posterior rhombomere boundaries. Boundary loss is preceded by changes in segmental expression of Krox-20 and Cek-8 and followed by alterations in Hox gene expression. The characteristic morphology of boundary cells, their expression of follistatin and the periodic accumulation of axons normally associated with boundaries are all lost. In the absence of boundaries, we detect no change in anteroposterior dispersal of precursor cells and, in most cases, no substantial cell mixing between former rhombomeric units. This is consistent with the idea that lineage restriction can be maintained by processes other than a mechanical barrier composed of boundary cells. Much of the early organisation of the motor nuclei appears normal despite the loss of boundaries and altered Hox expression.

Novel Eph-family receptor tyrosine kinase is widely expressed in the developing zebrafish nervous system

In a search for novel tyrosine kinases involved in vertebrate development, we have isolated cDNAs corresponding to three distinct members of the Eph-family of receptor tyrosine kinases. Whole mount RNA in situ hybridization analysis showed all three genes were most abundantly expressed in the developing nervous system. zek1 (zebrafish Eph-like kinase1) encodes a 981 amino acid polypeptide closely related to the murine Sek1 and Bsk receptors. Cos-1 cells transfected with zek1 produce a 141 kilodalton tyrosine phosphorylated protein which is recognized by antibodies raised against two predicted Zek1 peptides. These antibodies also recognized a protein of the same apparent molecular weight in lysates from zebrafish embryos and adults. Widespread expression of zek1 in the developing brain and neural tube suggested a generalized function of the Zek1 receptor in neuronal cell ontogeny.

The inhibitory effect on neurite outgrowth of motoneurons exerted by the ligands ELF-1 and RAGS

Eph-related receptor tyrosine kinases and ligands are expressed at high levels in the developing nervous system, giving rise to the proposal that they are involved in neuronal connection. Cek8 was found to be predominantly expressed on a subset of motoneurons innervating limb but not body muscles during motoneuron axonal growth. Here we show that the ligands RAGS and ELF-1 were expressed in limb buds and that they activated Cek8 when presented in membrane-bound or clustered forms of Fc chimeric proteins but not in unclustered soluble forms. When chick embryonic motoneurons enriched by panning were cultured on clustered forms of RAGS-Fc and ELF-1-Fc, the neutrite growth of motoneurons expressing Cek8 was inhibited. Our results show a relationship between receptor phosphorylation and neurite growth inhibition and suggest that Eph-related kinases and ligands have a regulatory effect on the axon growth of motoneurons during development.

Segmental and neuronal architecture of the hindbrain of Krox-20 mouse mutants

The vertebrate hindbrain is transiently segmented during its early development with the formation of reiterated bulges, the rhombomeres (r). The Krox-20 gene, which encodes a zinc finger transcription factor, has been shown previously to be implicated in the maintenance of r3 and r5 (Schneider-Maunoury, S., Topilko, P., Seitanidou, T., Levi, G., Cohen-Tannoudji, M., Pournin, S., Babinet, C. and Charnay, P. (1993) Cell 75, 1199–1214; Swiatek, P. J. and Gridley, T. (1993) Genes Dev. 7, 2071–2084. However, it was not clear from these analyses how extensive the deletion of r3 and r5 was and whether the overall segmentation and internal architecture of the hindbrain was affected. We have now reinvestigated these issues by analysis of rhombomere boundaries, using both morphological and molecular markers, and of the fate of specific motor neuron populations, using retrograde and anterograde carbocyanine dye tracing. We conclude that r3 and r5 and their derivatives are completely eliminated in Krox-20(−/−) embryos while overall hindbrain segmentation is maintained. In addition, we show that the disappearance of these territories has important consequences for even-numbered rhombomeres as well, in particular on axonal navigation: (i) a population of r6 motoneurons, presumably normally fated to join the glossopharyngeal nerve, has its axons misrouted toward the facial exit point in r4; (ii) the trigeminal motor axons are also misrouted, presumably because of the proximity of the trigeminal and facial exit points. They fasciculate with facial axons outside the neural tube and enter the second branchial arch instead of the first arch. This navigational error could explain the disappearance, at around 17.5 dpc, of the trigeminal motor nucleus in Krox-20(−/−) embryos by inadequate supply of essential, possibly arch-specific survival factors.

Eph family transmembrane ligands can mediate repulsive guidance of trunk neural crest migration and motor axon outgrowth

In vertebrate embryos, neural crest cell migration and motor axon outgrowth are restricted to rostral somite halves by repulsive factors located in the caudal somite compartment. We show that two Eph family transmembrane ligands, Lerk2 and HtkL, are expressed in caudal somite halves, and that crest cells and motor axons express receptors for these ligands. In several independent in vitro assays, preclustered ligand-Fc fusion proteins can repulsively guide both crest migration and motor axon outgrowth. These repulsive activities depend on a graded or discontinuous presentation of the ligands when tested in the context of permissive substrates, such as laminin or fibronectin. These results identify Lerk2 and HtkL as potential determinants of segmental pattern in the peripheral nervous system.

Reciprocal expression of the Eph receptor Cek5 and its ligand(s) in the early retina

Recent evidence suggests that Eph receptor tyrosine kinases and their ligands provide positional information in the developing visual system. We previously found that the Eph receptor Cek5 is more highly expressed in the ventral than dorsal chicken embryonic retina. We now report the identification of a chicken ligand for Cek5 (cCek5-L) that is 75% identical to the ligand LERK2. In situ hybridization experiments do not reveal a dorsoventral gradient of cCek5-L transcripts in the optic tectum at Embryonic Day 8, suggesting that this ligand is not involved in guiding Cek5-expressing axons in the tectum. Surprisingly, it is in the retina that high levels of cCek5-L mRNA are present. In the early retina, cCek5-L is more highly expressed in the dorsal than the ventral aspect. Similarly, a Cek5 Ig chimera labels dorsal but not ventral retina, indicating that even if several Cek5 ligands are present, their overall distribution is complementary to that of Cek5. Hence, Cek5 and cCek5-L may both contribute to define anatomical compartments within the early retina. In contrast, in the 11-day embryonic retina the distributions of Cek5 and its ligand(s) show considerable overlap, suggesting changing functions as development progresses. In dissociated cultures of dorsal or ventral retinal cells seeded on plates coated with either receptor or ligand Ig chimeras, the interaction between Cek5 and its ligand(s) or cCek5-L and its receptor(s) is sufficient to mediate cell adhesion and allows neurite outgrowth.

Two Eph receptor tyrosine kinase ligands control axon growth and may be involved in the creation of the retinotectal map in the zebrafish

The isolation and characterisation of two zebrafish Eph receptor ligand cDNAs which we have called zfEphL3 and zfEphL4 is described. These genes are expressed in the presumptive midbrain of developing embryos from 6 somites. By 24 hours L3 is expressed throughout the midbrain including the region of the presumptive tectum whereas L4 is strongly expressed in the midbrain caudal to the presumptive tectum. At later stages of development L3 is expressed in a graded fashion throughout the tectum and L4 is maintained at its posterior margin. Growth cone collapse and pathway selection assays demonstrate that both these proteins have a collapse activity for retinal ganglion cells. When faced with a choice of substrate on which to grow, temporal axons from chick retinal ganglion cells selectively avoided membranes from Cos cells transfected with L3, whereas nasal axons did not. Both temporal and nasal axons avoided membranes from Cos cells transfected with L4. The expression patterns together with the functional data suggest that although both ligands may be able to guide retinal ganglion cells axons in vitro, they have different roles in the guidance of retinotectal projections in vivo. The expression of L3 is consistent with a role in the guidance of retinal ganglion cells to their targets on the tectum whereas that of L4 suggests a role in delineating the posterior boundary of the optic tectum.

Patterning the vertebrate neuraxis

Neuraxial patterning is a continuous process that extends over a protracted period of development. During gastrulation a crude anteroposterior pattern, detectable by molecular markers, is conferred on the neuroectoderm by signals from the endomesoderm that are largely inseparable from those of neural induction itself. This coarse-grained pattern is subsequently reinforced and refined by diverse, locally acting mechanisms. Segmentation and long-range signaling from organizing centers are prominent among the emerging principles governing regional pattern.

Bidirectional signalling through the EPH-family receptor Nuk and its transmembrane ligands

Receptor tyrosine kinases of the EPH class have been implicated in the control of axon guidance and fasciculation, in regulating cell migration, and in defining compartments in the developing embryo. Efficient activation of EPH receptors generally requires that their ligands be anchored to the cell surface, either through a transmembrane (TM) region or a glycosyl phosphatidylinositol (GPI) group. These observations have suggested that EPH receptors can transduce signals initiated by direct cell-cell interaction. Genetic analysis of Nuk, a murine EPH receptor that binds TM ligands, has raised the possibility that these ligands might themselves have a signalling function. Consistent with this, the three known TM ligands have a highly conserved cytoplasmic region, with multiple potential sites for tyrosine phosphorylation. Here we show that challenging cells that express the TM ligands Elk-L or Htk-L with the clustered ectodomain of Nuk induces phosphorylation of the ligands on tyrosine, a process that can be mimicked both in vitro and in vivo by an activated Src tyrosine kinase. Co-culture of cells expressing a TM ligand with cells expressing Nuk leads to tyrosine phosphorylation of both the ligand and Nuk. These results suggest that the TM ligands are associated with a tyrosine kinase, and are inducibly phosphorylated upon binding Nuk, in a fashion reminiscent of cytokine receptors. Furthermore, we show that TM ligands, as well as Nuk, are phosphorylated on tyrosine in mouse embryos, indicating that this is a physiological process. EPH receptors and their TM ligands therefore mediate bidirectional cell signalling.

Regionalized expression of Nkx5-1, Nkx5-2, Pax2 and sek genes during mouse inner ear development

Nkx5-1 and Nkx5-2 are two highly related homeobox genes which are expressed during mouse development in the inner ear. Here, we present the detailed expression of both genes within the developing ear and a comparison to the expression of other potential control genes in this organ. Both genes are active between E13.5 and birth in non-sensory epithelium of the semicircular canals, utricle and saccule. Nkx5-1 and Nkx5-2 are also expressed in the cochlea, where the expression is restricted to the stria vascularis. The endolymphatic duct is devoid of any Nkx5 transcripts. Pax2 is expressed in epithelial cells of the ventral part of the membranous labyrinth where it overlaps with the Nkx5 expression domain. sek shows a complementary pattern to Nkx5 in the vestibular epithelium. In the cochlea sek is expressed throughout the mesenchyme and epithelium but not in the stria vascularis. In the vestibulum Pax2 and sek is limited to the ventral part whereas Nkx5 genes are active throughout. These data suggest that Nkx5 genes, Pax2 and sek play different roles in the patterning of inner ear structures.

Topographically specific effects of ELF-1 on retinal axon guidance in vitro and retinal axon mapping in vivo

Topographic maps, which maintain the spatial order of neurons in the order of their axonal connections, are found throughout the nervous system. In the visual retinotectal projection, ELF-1, a ligand in the tectum, and its receptors in the retina show complementary gradients in expression and binding, indicating they may be positional labels for map development. Here we show that ELF-1 acts as a repellent axon guidance factor in vitro. In vivo, when the tectal ELF-1 pattern is modified by retroviral overexpression, retinal axons avoid ectopic ELF-1 patches and map to abnormally anterior positions. All these effects were seen on axons from temporal but not nasal retina, indicating that ELF-1 could determine nasal versus temporal retinotectal specificity, and providing a direct demonstration of a cell recognition molecule with topographically specific effects on neural map development.

Nuk controls pathfinding of commissural axons in the mammalian central nervous system

Eph family receptor tyrosine kinases have been proposed to control axon guidance and fasciculation. To address the biological functions of the Eph family member Nuk, two mutations in the mouse germline have been generated: a protein null allele (Nuk1) and an allele that encodes a Nuk-beta gal fusion receptor lacking the tyrosine kinase and C-terminal domains (Nuk(lacZ)). In Nuk1 homozygous brains, the majority of axons forming the posterior tract of the anterior commissure migrate aberrantly to the floor of the brain, resulting in a failure of cortical neurons to link the two temporal lobes. These results indicate that Nuk, a receptor that binds transmembrane ligands, plays a critical and unique role in the pathfinding of specific axons in the mammalian central nervous system.

Eph receptors and ligands comprise two major specificity subclasses and are reciprocally compartmentalized during embryogenesis

We report that the many Eph-related receptor tyrosine kinases, and their numerous membrane-bound ligands, can each be grouped into only two major specificity subclasses. Receptors in a given subclass bind most members of a corresponding ligand subclass. The physiological relevance of these groupings is suggested by viewing the collective distributions of all members of a subclass. These composite distributions, in contrast with less informative patterns seen with individual members of the family, reveal that the developing embryo is subdivided into domains defined by reciprocal and apparently mutually exclusive expression of a receptor subclass and its corresponding ligands. Receptors seem to encounter their ligands only at the interface between these domains. This reciprocal compartmentalization implicates the Eph family in the formation of spatial boundaries that may help to organize the developing body plan.

Embryonic expression of eph signalling factors in Xenopus

Cellular communication in the developing embryo is mediated by receptor-ligand interactions at the cell surface. Receptor protein tyrosine kinases (RTKs) have been shown to play a critical role in the development of the vertebrate embryo. The eph receptors are a large subclass of RTKs for which a corresponding ligand family has only recently been described. The restricted expression patterns of several eph receptors imply roles for these molecules in early vertebrate development. We have isolated both a ligand of the eph ligand family (ELF), that we have named XELF-a, and an eph-related receptor, XE10, the likely homolog of the murine eck/Sek-2 receptor. At least two forms of the XELF-a transcript are present in the developing embryo. A truncated form of the XELF-a ligand, XELF-á, is the first ELF ligand isolated that lacks both the membrane-spanning and membrane-anchoring motifs conserved among this family, suggesting that ELF ligands can function as fully soluble molecules in vivo. XELF-a and XE10 are expressed maternally and throughout early embryogenesis, while XELF-á is only expressed zygotically. The dynamic expression patterns of these signalling molecules, in both mesoderm and neurectoderm, suggest that they may play a role in the patterning of the early vertebrate embryo.

Function of the Eph-related kinase rtk1 in patterning of the zebrafish forebrain

Early during its development, the vertebrate brain is subdivided into regions that have distinct fates and correlate with the expression domains of regulatory genes, but little is known about the cell-cell interactions that establish this spatial pattern. Candidates for regulating such interactions are the Eph-related receptor tyrosine kinases (RTKs) which have spatially restricted expression in the developing brain. These RTKs may mediate cell-contact-dependent signalling by interacting with membrane-bound ligands, and have been implicated in axon repulsion and the segmental restriction of gene expression in the hindbrain, but nothing is known regarding their function in the rostral neural epithelium. Here we use a dominant-negative approach in the zebrafish embryo to interfere with the function of Rtk1, an Eph-related RTK expressed in the developing diencephalon. We find that expression of a truncated receptor leads to expansion of the eye field into diencephalic territory and loss of diencephalic structures, indicating a role for Rtk1 in patterning the developing forebrain.

Eph receptor tyrosine kinases and their ligands in neural development

Receptor tyrosine kinases play important roles in many developmental phenomena, including the regulation of cell proliferation, differentiation, and survival. The largest subfamily of receptor tyrosine kinases are the Eph receptors, which are widely expressed in the nervous system. With the recent identification of several Eph ligands, it has become evident that Eph receptors and their ligands are involved in the guidance of retinal axons and in the process of hindbrain segmentation.