A dynamic code of dorsal neural tube genes regulates the segregation between neurogenic and melanogenic neural crest cells

Abstract

SUMMARYUnderstanding when and how multipotent progenitors segregate into diverse fates is a key question during embryonic development. The neural crest (NC) is an exemplary model system with which to investigate the dynamics of progenitor cell specification, as it generates a multitude of derivatives. Based on 'in ovo' lineage analysis, we previously suggested an early fate restriction of premigratory trunk NC to generate neural versus melanogenic fates, yet the timing of fate segregation and the underlying mechanisms remained unknown. Analysis of progenitors expressing a Foxd3 reporter reveals that prospective melanoblasts downregulate Foxd3 and have already segregated from neural lineages before emigration. When this downregulation is prevented, late-emigrating avian precursors fail to upregulate the melanogenic markers Mitf and MC/1 and the guidance receptor Ednrb2, generating instead glial cells that express P0 and Fabp. In this context, Foxd3 lies downstream of Snail2 and Sox9, constituting a minimal network upstream of Mitf and Ednrb2 to link melanogenic specification with migration. Consistent with the gain-of-function data in avians, loss of Foxd3 function in mouse NC results in ectopic melanogenesis in the dorsal tube and sensory ganglia. Altogether, Foxd3 is part of a dynamically expressed gene network that is necessary and sufficient to regulate fate decisions in premigratory NC. Their timely downregulation in the dorsal neural tube is thus necessary for the switch between neural and melanocytic phases of NC development.

Neural crest and Schwann cell progenitor-derived melanocytes are two spatially segregated populations similarly regulated by Foxd3

“…1 D-D"). Consistent with the precedent results, the majority of Ednrb2 and MC/1 + cells (5,8) were apparent in the epaxial domain of the body at flank regions of embryonic day (E)5 embryos with very few pigment cells in the abdominal area at this stage (Fig. 1 E, G, and I).…”

“…2F. This approach enabled stable lineage tracing of the enhancer expressing cells (8,24,25). Six hours after ventral to dorsal electroporation of NTs, GFP + cells were restricted to the dorsal NT, where cells expressing endogenous Foxd3 are located (8).…”

“…Next we examined whether the nerve-derived SCP-melanocyte switch is regulated by Foxd3 as previously observed for melanocytes directly issued from the NC (8,13,14). Separate lineage tracing of each population revealed that NC and SCP-derived melanocytes are differentially restricted to the epaxial and hypaxial body domains, respectively, respecting the boundaries of the somite vs. lateral plate mesoderm-derived dermis through which they migrate.…”

“…Furthermore, although both populations are initially part of the Foxd3 lineage, epaxial melanocytes segregate from Foxd3-positive neural progenitors already in the dorsal NT, whereas hypaxial melanocytes lose Foxd3 at late stages upon separation from the nerve. Gainand loss-of-function experiments in avians and mice, respectively, reveal that Foxd3 is both sufficient and necessary for regulating the balance between melanocyte and SC development, similar to its role in epaxial melanocytes (8). Foxd3 is thus a negative regulator of both melanocyte populations.…”

“…This dynamic behavior of dorsal NT progenitors is also reflected by the differential expression of at least three transcription factors, Snail2, Sox9, and Foxd3, in early neural progenitors and their down-regulation in the late subset of prospective melanoblasts, hence suggesting an early segregation between the precedent lineages (4,5). Along this line, gain of Foxd3 activity in vivo induced the development of glial cells while inhibiting melanocytes, and conversely, genetic ablation of Foxd3 function enhanced ectopic melanogenesis in the NT and later in DRG at the expense of both neurons and glia (8).…”