Dynamics of BMP and Hes1/Hairy1 signaling in the dorsal neural tube underlies the transition from neural crest to definitive roof plate

Abstract

BackgroundThe dorsal midline region of the neural tube that results from closure of the neural folds is generally termed the roof plate (RP). However, this domain is highly dynamic and complex, and is first transiently inhabited by prospective neural crest (NC) cells that sequentially emigrate from the neuroepithelium. It only later becomes the definitive RP, the dorsal midline cells of the spinal cord. We previously showed that at the trunk level of the axis, prospective RP progenitors originate ventral to the premigratory NC and progressively reach the dorsal midline following NC emigration. However, the molecular mechanisms underlying the end of NC production and formation of the definitive RP remain virtually unknown.ResultsBased on distinctive cellular and molecular traits, we have defined an initial NC and a subsequent RP stage, allowing us to investigate the mechanisms responsible for the transition between the two phases.We demonstrate that in spite of the constant production of BMP4 in the dorsal tube at both stages, RP progenitors only transiently respond to the ligand and lose competence shortly before they arrive at their final location. In addition, exposure of dorsal tube cells at the NC stage to high levels of BMP signaling induces premature RP traits, such as Hes1/Hairy1, while concomitantly inhibiting NC production. Reciprocally, early inhibition of BMP signaling prevents Hairy1 mRNA expression at the RP stage altogether, suggesting that BMP is both necessary and sufficient for the development of this RP-specific trait.Furthermore, when Hes1/Hairy1 is misexpressed at the NC stage, it inhibits BMP signaling and downregulates BMPR1A/Alk3 mRNA expression, transcription of BMP targets such as Foxd3, cell-cycle progression, and NC emigration. Reciprocally, Foxd3 inhibits Hairy1, suggesting that repressive cross-interactions at the level of, and downstream from, BMP ensure the temporal separation between both lineages.ConclusionsTogether, our data suggest that BMP signaling is important both for NC and RP formation. Given that these two structures develop sequentially, we speculate that the longer exposure of RP progenitors to BMP compared with that of premigratory NC cells may be translated into a higher signaling level in the former. This induces changes in responsiveness to BMP, most likely by downregulating the expression of Alk3 receptors and, consequently, of BMP-dependent downstream transcription factors, which exhibit spatial complementary expression patterns and mutually repress each other to generate alternative fates. This molecular dynamic is likely to account for the transition between the NC and definitive RP stages and thus be responsible for the segregation between central and peripheral lineages during neural development.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-016-0245-6) contains supplementary material, which is available to authorized users.

Zebrafish Rfx4 controls dorsal and ventral midline formation in the neural tube

“…The newly formed roof plate, in turn, becomes an important source of BMPs as well as Wnts, which organize DV pattern and are essential for dorsal interneuron specification (reviewed in (Chizhikov and Millen 2004). Earlier in development, the roof plate lineage shares a common progenitor with the neural crest lineage (Krispin et al 2010, Nitzan et al 2016). The Zic family of zinc-finger transcription factors is essential for the formation of both lineages during early neural development (Aruga et al 1996, Nakata et al 2000, Elms et al 2003, Elsen et al 2008, Milet et al 2013, Teslaa et al 2013, Plouhinec et al 2014, Sedykh et al 2017).…”

Evolution of vertebrate spinal cord patterning

“…References: Gnathostomes; Mouse, BMP2 , BMP6 , BMP7 , and Gdf7 are expressed in roof plate, BMP4 in the whole spinal cord E9.5 then restricted to dorsal ectoderm by E10.5 . Chick, BMP4 , BMP7 , and Gdf7 are in the roof plate, BMP6 expressed through out spinal cord except roof plate . Lizard, BMP4 in the roof plate .…”

Neural crest emigration: From start to stop

“…Following the completion of NC EMT, the dorsal neuroepithelium regains epithelial characteristics and generates the RP (Nitzan et al, ). The RP is a bona fide CNS cell type that constitutes the definitive dorsal midline of the spinal cord and behaves as a signaling center necessary for interneuron development (Chizhikov & Millen, ).…”

“…The early responsiveness of RP progenitors to BMP suggested that this factor may, nevertheless, be important for initial specification of these cells. In fact, exposure of dorsal tube cells at the NC stage to high levels of BMP signaling induced premature RP traits, such as the RP marker Hairy1 , while concomitantly inhibiting NC production (Nitzan et al, ). This is consistent with RP precursors being extensively exposed to BMP when compared to NC which “sense” the factor and undergo rapid EMT.…”

“…The former progenitors proliferate both before and after delamination (Burstyn-Cohen & Kalcheim, 2002). In contrast, RP cells adopt a typical morphology while becoming postmitotic, regaining apico-basal polarity, Ncadherin-mediated cell adhesion, and a continuous laminin-containing basement membrane (Kahane & Kalcheim, 1998;Nitzan et al, 2016) ( Figure 1b). At the molecular level, few genes are known to date to exhibit a differential expression between the NC and RP phases (Nitzan et al, 2016) and a thorough identification of additional RP-specific markers is needed both for localization and for functional studies.…”