Molecular basis of cell migration in the fish lateral line: role of the chemokine receptor CXCR4 and of its ligand, SDF1.

Cell migration plays an essential role in many morphogenetic processes, and its deregulation has many dramatic consequences. Yet how migration is controlled during normal development is still a largely unresolved question. We examined this process in the case of the posterior lateral line (PLL), a mechanosensory system present in fish and amphibians. In zebrafish, the embryonic PLL comprises seven to eight sense organs (neuromasts) aligned from head to tail along the flank of the animal and is formed by a primordium that originates from a cephalic placode. This primordium migrates along a stereotyped pathway toward the tip of the tail and deposits in its wake discrete groups of cells, each of which will become a neuromast. We show that a trail of SDF1-like chemokine is present along the pathway of the primordium and that a CXCR4-like chemokine receptor is expressed by the migrating cells. The inactivation of either the ligand or its receptor blocks migration, whereas in mutants in which the normal SDF1 trail is absent, the primordium path is redirected to the next, more ventral sdf1 expression domain. In all cases, the sensory axons remain associated to the primordium, indicating that the extension of the neurites to form the PLL nerve depends on the movement of the primordium. We conclude that both the formation and the innervation of this system depend on the SDF1-CXCR4 system, which has also been implicated in several migration events in humans, including metastasis formation and lymphocyte homing.

Requirement for endoderm and FGF3 in ventral head skeleton formation.

The vertebrate head skeleton is derived in part from neural crest cells, which physically interact with head ectoderm, mesoderm and endoderm to shape the pharyngeal arches. The cellular and molecular nature of these interactions is poorly understood, and we explore here the function of endoderm in this process. By genetic ablation and reintroduction of endoderm in zebrafish, we show that it is required for the development of chondrogenic neural crest cells, including their identity, survival and differentiation into arch cartilages. Using a genetic interference approach, we further identify Fgf3 as a critical component of endodermal function that allows the development of posterior arch cartilages. Together, our results reveal for the first time that the endoderm provides differential cues along the anteroposterior axis to control ventral head skeleton development and demonstrate that this function is mediated in part by Fgf3.

Molecular integration of casanova in the Nodal signalling pathway controlling endoderm formation.

Endoderm originates from a large endomesodermal field requiring Nodal signalling. The mechanisms that ensure segregation of endoderm from mesoderm are not fully understood. We first show that the timing and dose of Nodal activation are crucial for endoderm formation and the endoderm versus mesoderm fate choice, because sustained Nodal signalling is required to ensure endoderm formation but transient signalling is sufficient for mesoderm formation. In zebrafish, downstream of Nodal signals, three genes encoding transcription factors (faust, bonnie and clyde and the recently identified gene casanova) are required for endoderm formation and differentiation. However their positions within the pathway are not completely established. In the present work, we show that casanova is the earliest specification marker for endodermal cells and that its expression requires bonnie and clyde. Furthermore, we have analysed the molecular activities of casanova on endoderm formation and found that it can induce endodermal markers and repress mesodermal markers during gastrulation, as well as change the fate of marginal blastomeres to endoderm. Overexpression of casanova also restores endoderm markers in the absence of Nodal signalling. In addition, casanova efficiently restores later endodermal differentiation in these mutants, but this process requires, in addition, a partial activation of Nodal signalling.

Regulation of nodal signalling and mesendoderm formation by TARAM-A, a TGFbeta-related type I receptor.

Nodal signalling is essential for many developmental events during vertebrate development, including the establishment of left-right asymmetry, of dorsoventral axis of the central nervous system, and endoderm and mesoderm formation. The zebrafish TGFbeta-related type I receptor, TARAM-A (Tar), is expressed in the prospective mesendodermal territory and, when activated, can transfate early blastomeres into endoderm, suggesting that Nodal and Tar may represent similar signalling pathways. We have analysed the functional relationships between those two pathways in zebrafish. We first demonstrate that tar and the zebrafish nodal genes cyc and sqt functionally interact. We also show that a dominant-negative isoform of Tar, TarMR, interferes specifically with the function of Cyc and Sqt in vitro, but does not interfere with the function of BMP2, another TGFbeta-related molecule. TarMR interferes also with Nodal signalling in vivo since it enhances the phenotype of embryos with weakened Nodal signalling. Overexpression of tarMR in wild-type embryos interfered with the formation of endoderm-derived structures. Conversely, overexpression of tar enlarged the presumptive mesendodermal region at the onset of gastrulation. Together, our results point to Tar as an essential factor for endoderm formation and an important modulator of Nodal signalling, potentially representing one of the Nodal receptors. (c)2001 Elsevier Science.