Prickle 1 regulates cell movements during gastrulation and neuronal migration in zebrafish

Interplay of mechanical deformation and patterned gene expression in developing embryos

The shaping of the early embryo requires pattern formation as well as geometric and topological morphogenesis of the developing tissues. The morphogenetic movements that lead to geometric shape changes are controlled by patterned gene expression. How particular movements are related to patterning genes, and which underlying molecular and cellular mechanisms lead to coordinated macroscopic movements that induce morphogenesis, remain the challenging questions of embryonic development. How morphogenetic movements could modulate the expression of developmental genes is an emerging question, potentially opening new horizons in developmental biology. This question instigates the task of characterizing the molecular and cellular mechanisms underlying these mechano-transcription events.

Adhesion remodeling underlying tissue morphogenesis

Cell-adhesion molecules localized at adherens junctions (AJs) maintain the polarized architecture of epithelial cells but limit their movements. The morphogenesis of a developing epithelium is associated with the control of both cell shape and cell contacts. Epithelial cells remodel their contacts, and intercellular adhesion controlled by cadherin molecules is spatially and temporally regulated. Cell shape depends, in part, on the regulation of cell adhesion between different groups of cells. Patterned epithelial cell movements such as those that occur during cell intercalation--a universal process whereby cells exchange neighbors--rely on the polarized remodeling of AJs. Recent studies show that the understanding of adhesion will benefit from studies of developing organisms in which adhesion is regulated.

Identification of chick and mouse Daam1 and Daam2 genes and their expression patterns in the central nervous system

Genes controlling the planar cell polarity (PCP) pathway have recently been described in Drosophila. Although a number of PCP-related genes were identified, it remains unknown whether the same genetic programs found in invertebrate embryos operate in vertebrate embryos, especially with regard to central nervous system development. To gain insights into the roles played by vertebrate PCP-related genes, we cloned and examined the expression patterns of chick and mouse homologues of the Xenopus Daam gene, an essential component of the PCP pathway and involved in the convergent extension movement of cells. The observed expression patterns in developing central nervous tissues suggested that vertebrate Daam genes were involved in pivotal steps in neuronal cell differentiation and movement.

Independent mutations in mouse Vangl2 that cause neural tube defects in looptail mice impair interaction with members of the Dishevelled family

Mammalian Vangl1 and Vangl2 are highly conserved membrane proteins that have evolved from a single ancestral protein Strabismus/Van Gogh found in Drosophila. Mutations in the Vangl2 gene cause a neural tube defect (craniorachischisis) characteristic of the looptail (Lp) mouse. Studies in model organisms indicate that Vangl proteins play a key developmental role in establishing planar cell polarity (PCP) and in regulating convergent extension (CE) movements during embryogenesis. The role of Vangl1 in these processes is virtually unknown, and the molecular function of Vangl1 and Vangl2 in PCP and CE is poorly understood. Using a yeast two-hybrid system, glutathione S-transferase pull-down and co-immunoprecipitation assays, we show that both mouse Vangl1 and Vangl2 physically interact with the three members of the cytoplasmic Dishevelled (Dvl) protein family. This interaction is shown to require both the predicted cytoplasmic C-terminal half of Vangl1/2 and a portion of the Dvl protein containing PDZ and DIX domains. In addition, we show that the two known Vangl2 loss-of-function mutations identified in two independent Lp alleles associated with neural tube defects impair binding to Dvl1, Dvl2, and Dvl3. These findings suggest a molecular mechanism for the neural tube defect seen in Lp mice. Our observations indicate that Vangl1 biochemical properties parallel those of Vangl2 and that Vangl1 might, therefore, participate in PCP and CE either in concert with Vangl2 or independently of Vangl2 in discrete cell types.

Models of morphogenesis: the mechanisms and mechanics of cell rearrangement

The directional rearrangement of cells is a key mechanism for reshaping embryos. Despite substantial recent progress in understanding the basic signal transduction pathways that allow cells to orient themselves in space, the extrinsic cues that activate these pathways are just beginning to be understood. Even less-well understood are the physical mechanisms cells use to change position, especially when those cells are epithelial, and how mechanical forces within the embryo affect those movements. Recent studies are providing clues regarding how this fundamental process occurs with such remarkable reliability.

Identification and developmental expression pattern of van gogh-like 1, a second zebrafish strabismus homologue

Cell movement plays a central role in both normal embryogenesis and the development of diseases such as cancer. Therefore, identification and analysis of proteins controlling cell movement is of special importance. The zebrafish trilobite locus encodes a Van Gogh/Strabismus homologue, which regulates diverse cell migratory behaviors during embryogenesis. Trilobite is most similar to human Van Gogh-like 2 (VANGL2)/Strabismus 1 and mouse Loop-tail associated protein/Lpp1. Both human and mouse genomes encode a second Strabismus homologue referred to as VANGL1/Strabismus 2 and Lpp2, respectively. This prompted us to ask whether another van gogh/strabismus gene, one more closely related to human VANGL1, exists in the zebrafish genome. This paper describes the identification of zebrafish vangl1 and provides the first spatiotemporal expression and functional analysis of a vertebrate vangl1 homologue. Our data indicate that vangl1 and trilobite/vangl2 are expressed in largely non-overlapping domains during embryogenesis. Injection of synthetic vangl1 RNA partially suppressed the gastrulation defect in trilobite mutant embryos, suggesting that Vangl1 and Trilobite/Vangl2 have similar biochemical activities.

T-box genes in early embryogenesis

The T-box gene family, encoding related DNA-binding transcriptional regulators, plays an essential role in controlling many aspects of embryogenesis in a wide variety of organisms. The T-box genes exhibit diverse patterns of spatial and temporal expression in the developing embryo, and both genetic and molecular embryological studies have demonstrated their importance in regulating cell fate decisions that establish the early body plan, and in later processes underlying organogenesis. Despite these studies, little is known of either the regulation of the T-box genes or the identities of their transcriptional targets. The aim of this review is to examine the diverse yet conserved roles of several T-box genes in regulating early patterning in chordates and to discuss possible mechanisms through which this functional diversity might arise. Developmental Dynamics 229:201-218, 2004.

Turning heads: development of vertebrate branchiomotor neurons

The cranial motor neurons innervate muscles that control eye, jaw, and facial movements of the vertebrate head and parasympathetic neurons that innervate certain glands and organs. These efferent neurons develop at characteristic locations in the brainstem, and their axons exit the neural tube in well-defined trajectories to innervate target tissues. This review is focused on a subset of cranial motor neurons called the branchiomotor neurons, which innervate muscles derived from the branchial (pharyngeal) arches. First, the organization of the branchiomotor pathways in zebrafish, chick, and mouse embryos will be compared, and the underlying axon guidance mechanisms will be addressed. Next, the molecular mechanisms that generate branchiomotor neurons and specify their identities will be discussed. Finally, the caudally directed or tangential migration of facial branchiomotor neurons will be examined. Given the advances in the characterization and analysis of vertebrate genomes, we can expect rapid progress in elucidating the cellular and molecular mechanisms underlying the development of these vital neuronal networks. Developmental Dynamics 229:143-161, 2004.

A second canon. Functions and mechanisms of beta-catenin-independent Wnt signaling

More is becoming known about so-called noncanonical Wnt pathways that signal independently of beta-catenin. Here we review recent developments in both the functions and mechanisms of noncanonical Wnt signaling. We also discuss some unresolved and vexing questions. How many noncanonical Wnt pathways are there? How extensive are the parallels between Drosophila planar polarization and vertebrate convergence and extension? Last, we will outline some challenges and difficulties we foresee for this exciting but still very young field.