Ephrin-mediated restriction of ERK1/2 activity delimits the number of pigment cells in the Ciona CNS

Abstract

Recent evidence suggests that ascidian pigment cells are related to neural crest-derived melanocytes of vertebrates. Using live-imaging, we determine a revised cell lineage of the pigment cells in Ciona intestinalis embryos. The neural precursors undergo successive rounds of anterior-posterior (A-P) oriented cell divisions, starting at the blastula 64-cell stage. A previously unrecognized fourth A-P oriented cell division in the pigment cell lineage leads to the generation of the post-mitotic pigment cell precursors. We provide evidence that MEK/ERK signals are required for pigment cell specification until approximately 30 minutes after the final cell division has taken place. Following each of the four A-P oriented cell divisions, ERK1/2 is differentially activated in the posterior sister cells, into which the pigment cell lineage segregates. Eph/ephrin signals are critical during the third A-P oriented cell division to spatially restrict ERK1/2 activation to the posterior daughter cell. Targeted inhibition of Eph/ephrin signals results in, at neurula stages, anterior expansion of both ERK1/2 activation and a pigment cell lineage marker and subsequently, at larval stages, supernumerary pigment cells. We discuss the implications of these findings with respect to the evolution of the vertebrate neural crest.

T-type Calcium Channel Regulation of Neural Tube Closure and EphrinA/EPHA Expression

“…In contrast to the results for control embryos, three of these embryos genotyped as homozygous for the bug allele (Figure 7B), indicating that they were rescued. Note also that the example rescued bug embryo (yellow arrow) has extra pigment cells, a previously described phenotype of dnEPH3 expression (Haupaix et al, 2014). Finally, we observed a number of embryos that appeared to be partially rescued by dnEPH3 (Figure 7C).…”

“…Because of the role of EphrinA in mouse neural tube closure and the similar expression patterns of ephrinA-d and CAV3 (Imai et al, 2009), the C. savignyi ephrinA-d upregulation was of particular interest. In C. intestinalis , ephrinA-d has been shown to be necessary for the proper patterning of the neural plate, although a later role in neural tube closure has not been reported (Haupaix et al, 2014, Stolfi et al, 2011). We examined the normal developmental time-course of ephrinA-d expression by qRT-PCR.…”

Diverse ETS transcription factors mediate FGF signaling in the Ciona anterior neural plate

“…1C and D), including the a9.49-derived pigmented cells. It has recently been shown that the a9.49 cells undergo two additional rounds of cell division before becoming post-mitotic, with pigmented cell fates segregating to the posterior granddaughters of a9.49 (Haupaix et al, 2014; Racioppi et al, 2014). While these studies and others have shown that specification of the pigmented cells depends on sequential FGF signaling from mid-gastrula through neurula stages (Squarzoni et al, 2011; Haupaix et al, 2014; Racioppi et al, 2014), the role of FGF in patterning the medial cells of row III (a9.33 and a9.37 pairs) has not been thoroughly investigated.…”

“…1B). At this stage, FGF expression is restricted to row II, and the MAP kinase (MAPK) pathway component ERK1/2 is activated in the neighboring rows I and III (Hudson et al, 2007; Haupaix et al, 2014). Several recent studies show that FGF signaling is required for the specification of the pigmented cells of the otolith and ocellus, which arise from the lateral a9.49 cells of row III (Squarzoni et al, 2011; Haupaix et al, 2014; Racioppi et al, 2014).…”

Patterning of brain precursors in ascidian embryos

“…Like the A-lineage-derived neural plate, differential FGF/ERK signalling also patterns the a-lineage-derived neural plate along its anteriorposterior axis. Specifically, FGF/ERK signalling is required to promote row III over row IV cell identities (Haupaix et al, 2014;Racioppi et al, 2014). Similarly, as in the A-lineage neural plate, Nodal signalling is implicated in specification of the lateral part of the a-lineage neural plate, as lateral gene expression is lost in the alineage cells when Nodal signalling is inhibited (Hudson and Yasuo, 2005;Imai et al, 2006;Ohtsuka et al, 2014).…”