Three cell types differentiate in the early frog neural plate: neural crest at the lateral edges, floorplate at the midline and primary neurons in three bilateral stripes. Floorplate cells and ventral neurons are induced by Sonic hedgehog (Shh) and neural crest and dorsal neurons are induced by epidermal factors such as bone morphogenetic proteins (BMPs). Neurogenesis in a subset of cells within the stripes involves lateral inhibition. However, the process by which pools of precursors are defined in stereotypic domains in response to inductive signals is unknown. Here we show that frog Zic2 encodes a zinc-finger transcription factor of the Gli superfamily which is expressed in stripes that alternate with those in which primary neurons differentiate and overlap the domains of floorplate and neural crest progenitors. Zic2 inhibits neurogenesis and induces neural crest differentiation. Conversely, Gli proteins are widely expressed, induce neurogenesis and inhibit neural crest differentiation. Zic2 is therefore a vertebrate pre-pattern gene, encoding anti-neurogenic and crest-inducing functions that counteract the neurogenic but not the floorplate-inducing activity of Gli proteins. We propose that the combined function of Gli/Zic genes responds to inductive signals and induces patterned neural cell differentiation.

Through the direct analysis of cell behaviors, we address the mechanisms underlying anterior neural tube morphogenesis in the zebrafish and the role of the cell adhesion molecule N-cadherin (N-cad) in this process. We demonstrate that although the mode of neurulation differs at the morphological level between amphibians and teleosts, the underlying cellular mechanisms are conserved. Contrary to previous reports, the zebrafish neural plate is a multi-layered structure, composed of deep and superficial cells that converge medially while undergoing radial intercalation, to form a single cell-layered neural tube. Time-lapse recording of individual cell behaviors reveals that cells are polarized along the mediolateral axis and exhibit protrusive activity. In N-cad mutants, both convergence and intercalation are blocked. Moreover, although N-cad-depleted cells are not defective in their ability to form protrusions, they are unable to maintain them stably. Taken together, these studies uncover key cellular mechanisms underlying neural tube morphogenesis in teleosts, and reveal a role for cadherins in promoting the polarized cell behaviors that underlie cellular rearrangements and shape the vertebrate embryo.

SUMMARY Microtubules are essential regulators of cell polarity, architecture and motility. The organization of the microtubule network is context-specific. In non-polarized cells, microtubules are anchored to the centrosome and form radial arrays. In most epithelial cells, microtubules are noncentrosomal, align along the apico-basal axis and the centrosome templates a cilium. It follows that cells undergoing mesenchyme-to-epithelium transitions must reorganize their microtubule network extensively, yet little is understood about how this process is orchestrated. In particular, the pathways regulating the apical positioning of the centrosome are unknown, a central question given the role of cilia in fluid propulsion, sensation and signaling. In zebrafish, neural progenitors undergo progressive epithelialization during neurulation, and thus provide a convenient in vivo cellular context in which to address this question. We demonstrate here that the microtubule cytoskeleton gradually transitions from a radial to linear organization during neurulation and that microtubules function in conjunction with the polarity protein Pard3 to mediate centrosome positioning. Pard3 depletion results in hydrocephalus, a defect often associated with abnormal cerebrospinal fluid flow that has been linked to cilia defects. These findings thus bring to focus cellular events occurring during neurulation and reveal novel molecular mechanisms implicated in centrosome positioning.

SUMMARYThe central nervous system of vertebrate embryos originates from the neural tube (NT), a simple epithelium surrounding a central lumen. The mechanisms underlying the shaping of the NT, a process otherwise known as neurulation, have been the focus of numerous studies, using a variety of model systems. Yet, it remains unclear to what extent neurulation is conserved across vertebrates. This review provides a comparison between modes of neurulation, with a focus on cellular mechanisms. An emerging concept is that cell behaviors reveal similarities between modes of neurulation that cannot be predicted from morphological comparisons.Mol. Reprod. Dev. 76: 954-965, 2009. ß

The neurokinin-1 (NK-1, substance P) receptor belongs to the class of seven transmembrane domain (7-TM) receptors that interact with cellular effector systems via guanine nucleotide binding regulatory proteins (G-proteins). In this study, coupling mechanisms of functional NK-1 receptors endogenously expressed in a human astrocytoma cell line (U373MG) were analyzed. Stimulation with substance P (SP) resulted in 1) a rapid increase in inositol 1,4,5-trisphosphate (IP3) synthesis; 2) a rise in cytosolic free calcium concentration ([Ca2+]i); 3) induction of immediate early gene transcription as monitored by c-fos and c-jun expression; and 4) a significant increase in de novo DNA synthesis. Thus, the functional responses induced by stimulation of NK-1 receptors on U373MG strongly correlate with those observed after treatment of primary astrocytes with SP and make U373MG cells a useful in vitro model system for the analysis of NK-1 receptor function on astrocytes in vivo.