Completion of neural crest cell production and emigration is regulated by retinoic-acid-dependent inhibition of BMP signaling

Abstract

Production and emigration of neural crest cells is a transient process followed by the emergence of the definitive roof plate. The mechanisms regulating the end of neural crest ontogeny are poorly understood. Whereas early crest development is stimulated by mesoderm-derived retinoic acid, we report that the end of the neural crest period is regulated by retinoic acid synthesized in the dorsal neural tube. Inhibition of retinoic acid signaling in the neural tube prevents the normal upregulation of BMP inhibitors in the nascent roof plate and prolongs the period of BMP responsiveness which otherwise ceases close to roof plate establishment. Consequently, neural crest production and emigration are extended well into the roof plate stage. In turn, extending the activity of neural crest-specific genes inhibits the onset of retinoic acid synthesis in roof plate suggesting a mutual repressive interaction between neural crest and roof plate traits. Although several roof plate-specific genes are normally expressed in the absence of retinoic acid signaling, roof plate and crest markers are co-expressed in single cells and this domain also contains dorsal interneurons. Hence, the cellular and molecular architecture of the roof plate is compromised. Collectively, our results demonstrate that neural tube-derived retinoic acid, via inhibition of BMP signaling, is an essential factor responsible for the end of neural crest generation and the proper segregation of dorsal neural lineages.

Recent advances in understanding cell type transitions during dorsal neural tube development

“…In a recent study, the loss of responsiveness to BMP was found to depend on RA newly produced in the dorsal NT 5 . Upon inhibition of RA signaling, expression of an NC-specific molecular signature, cell proliferation, lack of an organized apicobasal architecture, and (most importantly) cell emigration were all extended well into the RP stage 5 . These data show that the local synthesis of RA in the nascent RP, acting via BMP, is the switch that turns off the NC period of dorsal NT development.…”

“…In the previous section, we described that RA signaling emanating from the nascent RP itself inhibits BMP activity, which altogether accounts for the end of the NC stage 5 . A fundamental question is whether ending the production of NC cells is sufficient to induce RP formation or, alternatively, whether additional factors are required.…”

“…The expression of Raldh2, the enzyme responsible for RA synthesis, is upregulated in RP concomitant with the downregulation of foxd3 , snai2 , and sox9 . Furthermore, NC and RP genes stand in a mutually repressive interaction, suggesting that cross inhibition is a factor responsible for setting the timing of events in this domain 5 ( Figure 1 ).…”

“…A fundamental question is whether ending the production of NC cells is sufficient to induce RP formation or, alternatively, whether additional factors are required. Whereas inhibiting RA signaling was sufficient to prevent the timely expression of RP-specific genes that encode BMP inhibitors such as BAMBI , Grem1 , and hes4 , additional genes unique to RP, such as Rspo1 , draxin , and NDP , were normally expressed 5 . This suggests that RP formation takes place under these conditions and thus can be mechanistically distinguished from its NC predecessors in the dorsal NT.…”

“…Opposing morphogen gradients generate distinct identities along the dorso-ventral axis of the NT. Bone morphogenetic protein (BMP) and Wnt emanate from the dorsal NT and act across the dorsal third of the NT 5 , 6 , Sonic hedgehog produced in notochord and later floor plate acts primarily in the ventral region, and retinoic acid (RA) from the paraxial mesoderm acts at an early stage to pattern the dorso-ventral and rostro-caudal embryonic axes 7 – 10 . The integration of the above signals establishes a transcriptional gene regulatory network that first defines the premigratory neural crest (NC) 11 , 12 , an epithelial cell subset that transiently populates the NT.…”