Research Interest: Developmental Neurobiology: Cellular and molecular mechanisms that control the specification, delamination and migration of neural progenitors.  The neural crest (NC) is a transient group of vertebrate progenitors. Its  component cells yield an extensive variety of derivatives such as melanocytes, peripheral nervous system neurons of many kinds, glial, ectomesenchymal and endocrine cells. The questions we investigate in the lab concern the mechanisms responsible for the generation of the above cell identities from initial multipotent progenitors; the onset of cellular movement and their coalescence into organized and functional ganglia. A recent project deals with the end of neural crest production and the formation of the definitive roof plate of the CNS; e.g., the separation between PNS and CNS during nervous system development.  Initially, NC cells are epithelial and then undergo a time and axial level-specific conversion into mesenchyme that generates cell migration. NC cells then advance through stereotyped pathways, reach their homing sites and differentiate (Le Douarin and Kalcheim, 1999). The molecular network underlying the generation of cellular movement remains incompletely understood (Kalcheim and Burstyn-Cohen, 2005). This process involves an epithelial-to-mesenchymal transition (EMT) of the premigratory NC cells residing in the dorsal neural tube (NT) followed by delamination. Work from our lab has shown that a balance between BMP and its inhibitor noggin underlies the emigration of trunk-level NC independently of earlier cell specification (Sela-Donenfeld and Kalcheim, 1999). A decreasing rostrocaudal gradient of BMP4 activity is established along the NT by a reciprocal gradient of noggin. Noggin downregulation  is in turn triggered by the developing somites which  thus determine the timing of NC emigration (Sela-Donenfeld and Kalcheim, 1999; Sela-Donenfeld and Kalcheim, 2000; Sela-Donenfeld and Kalcheim, 2002). BMP then induces EMT of NC by triggering Wnt1 transcription. The latter promotes G1/S transition which is a necessary step for delamination of trunk NC (Burstyn-Cohen and Kalcheim, 2002; Burstyn-Cohen et al., 2004).   Acting downstream of BMP is the activity of effector genes that act on re-organisation of the actin cytoskeleton, alterations in adhesive properties and consequent loss of epithelial polarity. In this context, N-cadherin was found to be a component of the BMP-dependent network leading to NC EMT. N-cadherin inhibits the onset of NC delamination both by a cell adhesion-dependent mechanism as well as  by repressing canonical Wnt signaling. Relief from N-cadherin-mediated inhibition is attained in the dorsal NT during the onset of cell emigration. This is accounted for by an ADAM10-dependent  cleavage of the full-length molecule into a soluble domain with pro-delamination properties, a process triggered by BMP (Shoval et al., 2007). Rho GTPases act as molecular switches to control a variety of signal transduction pathways. We reported that RhoA and RhoB, through Rock, act downstream of BMP and of G1/S transition to maintain NC cells in an epithelial conformation. They exert these effects both by stabilizing assembly of the actin cytoskeleton and by enhancing N-cadherin-mediated intercellular adhesion (Groysman et al, 2008). During the course of the above studies, we observed that in the trunk, NC cells continuously delaminate from the NT for over two days, raising the fundamental question of the source and mechanisms accounting for the production of successive waves of NC progenitors. We found that the first NC to delaminate reside in the dorsal midline of the NT and generate sympathetic ganglia, and successive waves translocate ventrodorsally in the NT to replenish the dorsal midline and then delaminate. Hence, the dorsal midline is a dynamic region traversed sequentially by progenitors that colonize NC derivatives in a ventral to dorsal order (chromaffin cells, sympathetic ganglia, then Schwann cells, DRG and finally melanocytes). Based on our data invoking a dynamic behavior of premigratory NC cells, we hypothesize the existence  of a dynamic spatiotemporal fate map of derivatives already within the NT. Preliminary data suggest the existence of such a map as well as of a fate restriction of premigratory cells to generate discrete derivatives (Krispin et al, 2010). We are currently investigating the identity and mechanism of action of the molecular code underlying specification to various fates. We also examine the relationship between cell specification and migration by asking whether restriction to a certain fate conveys the cell with knowledge of the migratory pathway to follow and consequently, of its homing site, or alternatively, whether these are separable and distinctly regulated events. The regulation of epithelial-mesenchymal conversion in association with cell fate are among the most puzzling problems raised by morphogenesis, which go beyond developmental biology to  concern the stability of the histiotypic state and its disruption in disease.