New views on the neural crest epithelial-mesenchymal transition and neuroepithelial interkinetic nuclear migration

Cell cycle and cell fate in the developing nervous system: the role of CDC25B phosphatase

Deciphering the core machinery of the cell cycle and cell division has been primarily the focus of cell biologists, while developmental biologists have identified the signaling pathways and transcriptional programs controlling cell fate choices. As a result, until recently, the interplay between these two fundamental aspects of biology have remained largely unexplored. Increasing data show that the cell cycle and regulators of the core cell cycle machinery are important players in cell fate decisions during neurogenesis. Here, we summarize recent data describing how cell cycle dynamics affect the switch between proliferation and differentiation, with an emphasis on the roles played by the cell cycle regulators, the CDC25 phosphatases.

Sox2 acts as a rheostat of epithelial to mesenchymal transition during neural crest development

Precise control of self-renewal and differentiation of progenitor cells into the cranial neural crest (CNC) pool ensures proper head development, guided by signaling pathways such as BMPs, FGFs, Shh and Notch. Here, we show that murine Sox2 plays an essential role in controlling progenitor cell behavior during craniofacial development. A “Conditional by Inversion” Sox2 allele (Sox2^COIN) has been employed to generate an epiblast ablation of Sox2 function (Sox2^EpINV). Sox2^EpINV/+(H) haploinsufficient and conditional (Sox2^EpINV/mosaic) mutant embryos proceed beyond gastrulation and die around E11. These mutant embryos exhibit severe anterior malformations, with hydrocephaly and frontonasal truncations, which could be attributed to the deregulation of CNC progenitor cells during their epithelial to mesenchymal transition. This irregularity results in an exacerbated and aberrant migration of Sox10⁺ NCC in the branchial arches and frontonasal process of the Sox2 mutant embryos. These results suggest a novel role for Sox2 as a regulator of the epithelial to mesenchymal transitions (EMT) that are important for the cell flow in the developing head.

Neural crest-derived mesenchymal cells require Wnt signaling for their development and drive invagination of the telencephalic midline

Embryonic neural crest cells contribute to the development of the craniofacial mesenchyme, forebrain meninges and perivascular cells. In this study, we investigated the function of ß-catenin signaling in neural crest cells abutting the dorsal forebrain during development. In the absence of ß-catenin signaling, neural crest cells failed to expand in the interhemispheric region and produced ectopic smooth muscle cells instead of generating dermal and calvarial mesenchyme. In contrast, constitutive expression of stabilized ß-catenin in neural crest cells increased the number of mesenchymal lineage precursors suggesting that ß-catenin signaling is necessary for the expansion of neural crest-derived mesenchymal cells. Interestingly, the loss of neural crest-derived mesenchymal stem cells (MSCs) leads to failure of telencephalic midline invagination and causes ventricular system defects. This study shows that ß-catenin signaling is required for the switch of neural crest cells to MSCs and mediates the expansion of MSCs to drive the formation of mesenchymal structures of the head. Furthermore, loss of these structures causes striking defects in forebrain morphogenesis.

Cadherin dynamics during neural crest cell ontogeny

Cell membrane-associated junctional complexes mediate cell-cell adhesion, intercellular interactions, and other fundamental processes required for proper embryo morphogenesis. Cadherins are calcium-dependent transmembrane proteins at the core of adherens junctions and are expressed in distinct spatiotemporal patterns throughout the development of an important vertebrate cell type, the neural crest. Multipotent neural crest cells arise from the ectoderm as epithelial cells under the influence of inductive cues, undergo an epithelial-to-mesenchymal transition, migrate throughout the embryonic body, and then differentiate into multiple derivatives at predetermined destinations. Neural crest cells change their expressed cadherin repertoires as they undergo each new morphogenetic transition, providing insight into distinct functions of expressed cadherins that are essential for proper completion of each specific stage. Cadherins modulate neural crest cell morphology, segregation, migration, and tissue formation. This chapter reviews the knowledge base of cadherin regulation, expression, and function during the ontogeny of the neural crest.

Epithelial-mesenchymal transitions: insights from development

Epithelial-mesenchymal transition (EMT) is a crucial, evolutionarily conserved process that occurs during development and is essential for shaping embryos. Also implicated in cancer, this morphological transition is executed through multiple mechanisms in different contexts, and studies suggest that the molecular programs governing EMT, albeit still enigmatic, are embedded within developmental programs that regulate specification and differentiation. As we review here, knowledge garnered from studies of EMT during gastrulation, neural crest delamination and heart formation have furthered our understanding of tumor progression and metastasis.