Eph signalling functions downstream of Val to regulate cell sorting and boundary formation in the caudal hindbrain

Rhombomeres are segmental units of the developing vertebrate hindbrain that underlie the reiterated organisation of cranial neural crest migration and neuronal differentiation. valentino (val), a zebrafish homologue of the mouse bzip transcription factor-encoding gene, kreisler, is required for segment boundary formation caudal to rhombomere 4 (r4). val is normally expressed in r5/6 and is required for cells to contribute to this region. In val(−) mutants, rX, a region one rhombomere in length and of mixed identity, lies between r4 and r7. While a number of genes involved in establishing rhombomeric identity are known, it is still largely unclear how segmental integrity is established and boundaries are formed. Members of the Eph family of receptor tyrosine kinases and their ligands, the ephrins, are candidates for functioning in rhombomere boundary formation. Indeed, expression of the receptor ephB4a coincides with val in r5/6, whilst ephrin-B2a, which encodes a ligand for EphB4a, is expressed in r4 and r7, complementary to the domain of val expression. Here we show that in val(−) embryos, ephB4a expression is downregulated and ephrin-B2a expression is upregulated between r4 and r7, indicating that Val is normally required to establish the mutually exclusive expression domains of these two genes. We show that juxtaposition of ephB4a-expressing cells and ephrin-B2a-expressing cells in the hindbrain leads to boundary formation. Loss of the normal spatial regulation of eph/ephrin expression in val mutants correlates not only with absence of boundaries but also with the inability of mutant cells to contribute to wild-type r5/6. Using a genetic mosaic approach, we show that spatially inappropriate Eph signalling underlies the repulsion of val(−) cells from r5/6. We propose that Val controls eph expression and that interactions between EphB4a and Ephrin-B2a mediate cell sorting and boundary formation in the segmenting caudal hindbrain.

Anteroposterior patterning is required within segments for somite boundary formation in developing zebrafish

Somite formation involves the establishment of a segmental prepattern in the presomitic mesoderm, anteroposterior patterning of each segmental primordium and formation of boundaries between adjacent segments. How these events are co-ordinated remains uncertain. In this study, analysis of expression of zebrafish mesp-a reveals that each segment acquires anteroposterior regionalisation when located in the anterior presomitic mesoderm. Thus anteroposterior patterning is occurring after the establishment of a segmental prepattern in the paraxial mesoderm and prior to somite boundary formation. Zebrafish fss(−), bea(−), des(−) and aei(−) embryos all fail to form somites, yet we demonstrate that a segmental prepattern is established in the presomitic mesoderm of all these mutants and hox gene expression shows that overall anteroposterior patterning of the mesoderm is also normal. However, analysis of various molecular markers reveals that anteroposterior regionalisation within each segment is disturbed in the mutants. In fss(−), there is a loss of anterior segment markers, such that all segments appear posteriorized, whereas in bea(−), des(−) and aei(−), anterior and posterior markers are expressed throughout each segment. Since somite formation is disrupted in these mutants, correct anteroposterior patterning within segments may be a prerequisite for somite boundary formation. In support of this hypothesis, we show that it is possible to rescue boundary formation in fss(−) through the ectopic expression of EphA4, an anterior segment marker, in the paraxial mesoderm. These observations indicate that a key consequence of the anteroposterior regionalisation of segments may be the induction of Eph and ephrin expression at segment interfaces and that Eph/ephrin signalling subsequently contributes to the formation of somite boundaries.

Eph signaling is required for segmentation and differentiation of the somites

Somitogenesis involves the segmentation of the paraxial mesoderm into units along the anteroposterior axis. Here we show a role for Eph and ephrin signaling in the patterning of presomitic mesoderm and formation of the somites. Ephrin-A-L1 and ephrin-B2 are expressed in an iterative manner in the developing somites and presomitic mesoderm, as is the Eph receptor EphA4. We have examined the role of these proteins by injection of RNA, encoding dominant negative forms of Eph receptors and ephrins. Interruption of Eph signaling leads to abnormal somite boundary formation and reduced or disturbed myoD expression in the myotome. Disruption of Eph family signaling delays the normal down-regulation of her1 and Delta D expression in the anterior presomitic mesoderm and disrupts myogenic differentiation. We suggest that Eph signaling has a key role in the translation of the patterning of presomitic mesoderm into somites.

The role of fibroblast growth factors in early Xenopus development

In recent years we and others have been attempting to identify the molecular nature of the inducing signals in early Xenopus development. We have found that most members of the fibroblast growth factor (FGF) family are biologically active as mesoderm-inducing factors when applied to ectoderm from blastulae. In addition to this, they will support continued expression of the pan-mesodermal transcription factor Xbra in the mesoderm of gastrula stage embryos. We have studied the expression pattern of four types of FGF in early embryos. Two types (FGF-2 and FGF-9) are expressed maternally and are thus present at the time of natural mesoderm induction. The expression of two other types (FGF-3 and FGF-4) is activated in the newly formed mesoderm of the gastrula. If the activity of the FGF family is inhibited by overexpression of a dominant-negative FGF receptor, there is a reduction in mesoderm formation, there are abnormalities arising from an inhibition of normal gastrulation movements and there is a defect in formation of the posterior parts. We believe that the mesoderm formation and cell movement effects are attributable to loss of Xbra expression, and the posterior defects to lack of posterior HOX gene activity. Overexpression of eFGF gives rise to a posteriorized phenotype, in which posterior HOX genes are expressed in a more anterior position. We conclude that the FGF system has multiple functions in early development, including mesoderm formation, gastrulation movements and anteroposterior patterning.

Studies of cytochrome b-245 translocation in the PMA stimulation of the human neutrophil NADPH-oxidase

The human neutrophil respiratory burst, activated by phorbol 12-myristate 13-acetate (PMA), results from specific receptor-ligand binding and activation of the NADPH-oxidase in the plasma membrane. The role of granule membrane constituents has been elucidated with neutrophils disrupted by nitrogen cavitation and then fractionated in Percoll gradients to resolve four postnuclear fractions: cytoplasm, light membranes or gamma fraction (site of the NADPH-oxidase), a light granule (beta) fraction containing putative constituents of the NADPH-oxidase (cytochrome b-245 and an associated flavoprotein), and a fraction of heavy granules. Cytochrome b-245 is localized to two pools of specific granules within the beta fraction as assessed by differing sedimentation in narrow Percoll gradients and translocates upon PMA-stimulation from one of these specific granule sub-pools to the plasma membrane where it exhibits no change in its midpoint redox potential. Translocation of cytochrome b-245 parallels O2-production initiated by PMA stimulation as assessed in the time course of each activity. The finding of increased amounts of the b cytochrome in cytoplast membranes relative to plasma membranes of unstimulated cells suggests that the cytoplasts, devoid of granules yet capable of O2-generation upon PMA-stimulation, are useful in assessing post-translocation events in the activation pathway of the NADPH-oxidase. These data support the hypothesis that translocation of NADPH-oxidase components from an intracellular granular pool contributes to respiratory burst expression.

Studies of cytochrome b-245 translocation in the PMA stimulation of the human neutrophil NADPH-oxidase

The human neutrophil respiratory burst, activated by phorbol 12-myristate 13-acetate (PMA), results from specific receptor-ligand binding and activation of the NADPH-oxidase in the plasma membrane. The role of granule membrane constituents has been elucidated with neutrophils disrupted by nitrogen cavitation and then fractionated in Percoll gradients to resolve four postnuclear fractions: cytoplasm, light membranes or gamma fraction (site of the NADPH-oxidase), a light granule (beta) fraction containing putative constituents of the NADPH-oxidase (cytochrome b-245 and an associated flavoprotein), and a fraction of heavy granules. Cytochrome b-245 is localized to two pools of specific granules within the beta fraction as assessed by differing sedimentation in narrow Percoll gradients and translocates upon PMA-stimulation from one of these specific granule sub-pools to the plasma membrane where it exhibits no change in its midpoint redox potential. Translocation of cytochrome b-245 parallels O2-production initiated by PMA stimulation as assessed in the time course of each activity. The finding of increased amounts of the b cytochrome in cytoplast membranes relative to plasma membranes of unstimulated cells suggests that the cytoplasts, devoid of granules yet capable of O2-generation upon PMA-stimulation, are useful in assessing post-translocation events in the activation pathway of the NADPH-oxidase. These data support the hypothesis that translocation of NADPH-oxidase components from an intracellular granular pool contributes to respiratory burst expression.