Embryonic morphogenesis is driven by a suite of cell behaviours, including coordinated shape changes, cellular rearrangements and individual cell migrations, whose molecular determinants are largely unknown. In the zebrafish, Dani rerio, trilobite mutant embryos have defects in gastrulation movements and posterior migration of hindbrain neurons. Here, we have used positional cloning to demonstrate that trilobite mutations disrupt the transmembrane protein Strabismus (Stbm)/Van Gogh (Vang), previously associated with planar cell polarity (PCP) in Drosophila melanogaster, and PCP and canonical Wnt/beta-catenin signalling in vertebrates. Our genetic and molecular analyses argue that during gastrulation, trilobite interacts with the PCP pathway without affecting canonical Wnt signalling. Furthermore, trilobite may regulate neuronal migration independently of PCP molecules. We show that trilobite mediates polarization of distinct movement behaviours. During gastrulation convergence and extension movements, trilobite regulates mediolateral cell polarity underlying effective intercalation and directed dorsal migration at increasing velocities. In the hindbrain, trilobite controls effective migration of branchiomotor neurons towards posterior rhombomeres. Mosaic analyses show trilobite functions cell-autonomously and non-autonomously in gastrulae and the hindbrain. We propose Trilobite/Stbm mediates cellular interactions that confer directionality on distinct movements during vertebrate embryogenesis.

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.

During hindbrain development, facial branchiomotor neurons (FBM neurons) migrate from medial rhombomere (r) 4 to lateral r6. In zebrafish, mutations in planar cell polarity genes celsr2 and frizzled3a block caudal migration of FBM neurons. Here, we investigated the role of cadherins Celsr1-3, and Fzd3 in FBM neuron migration in mice. In Celsr1 mutants (knock-out and Crash alleles), caudal migration was compromised and neurons often migrated rostrally into r2 and r3, as well as laterally. These phenotypes were not caused by defects in hindbrain patterning or neuronal specification. Celsr1 is expressed in FBM neuron precursors and the floor plate, but not in FBM neurons. Consistent with this, conditional inactivation showed that the function of Celsr1 in FBM neuron migration was non-cell autonomous. In Celsr2 mutants, FBM neurons initiated caudal migration but moved prematurely into lateral r4 and r5. This phenotype was enhanced by inactivation of Celsr3 in FBM neurons and mimicked by inactivation of Fzd3. Furthermore, Celsr2 was epistatic to Celsr1. These data indicate that Celsr1-3 differentially regulate FBM neuron migration. Celsr1 helps to specify the direction of FBM neuron migration, whereas Celsr2 and 3 control its ability to migrate.

During development, facial branchiomotor (FBM) neurons, which innervate muscles in the vertebrate head, migrate caudally and radially within the brainstem to form a motor nucleus at the pial surface. Several components of the Wnt/planar cell polarity (PCP) pathway, including the transmembrane protein Vangl2, regulate caudal migration of FBM neurons in zebrafish, but their roles in neuronal migration in mouse have not been investigated in detail. Therefore, we analyzed FBM neuron migration in mouse looptail (Lp) mutants, in which Vangl2 is inactivated. In Vangl2 Lp/+ and Vangl2 Lp/Lp embryos, FBM neurons failed to migrate caudally from rhombomere (r) 4 into r6. Although caudal migration was largely blocked, many FBM neurons underwent normal radial migration to the pial surface of the neural tube. In addition, hindbrain patterning and FBM progenitor specification were intact, and FBM neurons did not transfate into other non-migratory neuron types, indicating a specific effect on caudal migration. Since loss-of-function in some zebrafish Wnt/PCP genes does not affect caudal migration of FBM neurons, we tested whether this was also the case in mouse. Embryos null for Ptk7, a regulator of PCP signaling, had severe defects in caudal migration of FBM neurons. However, FBM neurons migrated normally in Dishevelled (Dvl) 1/2 double mutants, and in zebrafish embryos with disrupted Dvl signaling, suggesting that Dvl function is essentially dispensable for FBM neuron caudal migration. Consistent with this, loss of Dvl2 function in Vangl2 Lp/+ embryos did not exacerbate the Vangl2 Lp/+ neuronal migration phenotype. These data indicate that caudal migration of FBM neurons is regulated by multiple components of the Wnt/PCP pathway, but, importantly, may not require Dishevelled function. Interestingly, genetic-interaction experiments suggest that rostral FBM neuron migration, which is normally suppressed, depends upon Dvl function.

Candida albicans WO-1 switches spontaneously, frequently, and reversibly between a hemispherical white and a flat gray (opaque) colony-forming phenotype. This transition affects a number of morphological and physiological parameters and involves the activation and deactivation of phase-specific genes. The WH11 gene is transcribed in the white but not the opaque phase. A chimeric WH11-firefly luciferase gene containing the 5 upstream region of WH11 was demonstrated to be under phase regulation regardless of the site of integration, and a series of promoter deletion constructs was used to delineate two white-phase-specific transcription activation domains. Gel retardation experiments with the individual distal or proximal domain and white-phase or opaque-phase protein extract demonstrated the formation of one distal white-phasespecific complex and two proximal white-phase-specific complexes. Specific subfragments were tested for their ability to compete with the entire domain in the formation of complexes with white-phase protein extract in order to map the proximal domain sequence involved in white-phase-specific complex formation. Our results indicate that white-phase-specific transcription of WH11 is positively regulated by trans-acting factors interacting with two cis-acting activation sequences in the WH11 promoter.As is the case for a number of microbial pathogens (4,11,12,36), Candida albicans switches spontaneously, reversibly, and at high frequencies between a number of general phenotypes, distinguishable by colony morphology (21,24,26). However, switching in C. albicans differs from switching in other microbial pathogens because of its pleiotropic consequences (26). In the white-opaque transition in strain WO-1 (25), a switch between a hemispherical white and a flat grey (opaque) colony morphology affects cell size, cell mass, wall morphology, budding pattern, sugar assimilation pattern, adhesion, drug susceptibility, sensitivity to oxidants, and patterns of tissue invasion in systemic mouse models (reviewed in references 26 and 28). The pleiotropy of this transition suggested that it involved the coordinate regulation of phase-specific genes, and differences in the in vitro translation products of white-phase and opaque-phase RNAs supported this hypothesis (27). The hypothesis was verified by the isolation and characterization of white-phase-specific and opaque-phase-specific genes. In the opaque phase, cells differentially express the opaque-phasespecific genes PEP1 (19), which encodes a secreted aspartyl proteinase (13) and is also referred to as SAP1 (40); SAP3, which encodes a second secreted aspartyl proteinase (40); and Op4 (18), which encodes a unique protein which is selectively expressed not only in the opaque phase of strain WO-1 but also in the variant phenotypes of strain 3153A (17). In the white phase, cells differentially express the white-phase-specific gene WH11 (33), which encodes a protein homologous to the glucose-lipid-regulated GLP1 protein of Saccharomyces cerevisiae (34). Since th...