BEN/SC1/DM-GRASP expression during neuromuscular development: a cell adhesion molecule regulated by innervation

Frizzled 3 acts upstream of Alcam during embryonic eye development

Formation of a functional eye during vertebrate embryogenesis requires different processes such as cell differentiation, cell migration, cell-cell interactions as well as intracellular signalling processes. It was previously shown that the non-canonical Wnt receptor Frizzled 3 (Fzd3) is required for proper eye formation, however, the underlying mechanism is poorly understood. Here we demonstrate that loss of Fzd3 induces severe malformations of the developing eye and that this defect is phenocopied by loss of the activated leukocyte cell adhesion molecule (Alcam). Promoter analysis revealed the presence of a Fzd3 responsive element within the alcam promoter, which is responsible for alcam expression during anterior neural development. In-depth analysis identified the jun N-terminal protein kinase 1 (JNK1) and the transcription factor paired box 2 (Pax2) to be important for the activation of alcam expression. Altogether our study reveals that alcam is activated through non-canonical Wnt signalling during embryonic eye development in Xenopus laevis and shows that this pathway plays a similar role in different tissues.

Internalization and recycling of ALCAM/CD166 detected by a fully human single-chain recombinant antibody

Activated leukocyte cell adhesion molecule (ALCAM/CD166), a member of the immunoglobulin superfamily with five extracellular immunoglobulin-like domains, promotes heterophilic (ALCAM-CD6) and homophilic (ALCAM-ALCAM) cell-cell interactions. Here we describe a fully human single-chain antibody fragment (scFv) directed to ALCAM/CD166. We selected the I/F8 scFv from a phage display library of human V-gene segments by cell panning and phage internalization into IGROV-I human ovary carcinoma cells. The I/F8 specificity was identified as ALCAM/CD166 by matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) peptide mass fingerprinting of the I/F8-immunoprecipitated protein. The I/F8 scFv reacts with the human, monkey and murine ALCAM/CD166 molecule, indicating that the recognized epitope is highly conserved. The I/F8 scFv completely abolished binding of both ALCAM/Fc and CD6/Fc soluble ligands, whereas it did not compete with the anti-ALCAM/CD166 murine monoclonal antibodies J4-81 and 3A6 and therefore recognizes a different epitope. Engagement through I/F8 scFv, 3A6 monoclonal antibody or CD6/Fc ligand induced ALCAM/CD166 internalization, with a kinetics slower than that of transferrin in the same cells. Newly internalized I/F8-ALCAM complexes colocalized with clathrin but not with caveolin and we demonstrated, using surface biotinylation and recycling assays, that endocytosed ALCAM/CD166 recycles back to the cell surface. Such an endocytic pathway allows the efficient delivery of an I/F8 scFv-saporin immunotoxin into tumor cells, as the conjugates are able to selectively kill cell lines expressing ALCAM/CD166. Altogether these data provide evidence of the suitability of the I/F8 scFv for further functional analysis of ALCAM/CD166 and intracellular delivery of effector moieties.

Quantitative proteomic analysis of sokotrasterol sulfate-stimulated primary human endothelial cells

The endothelium forms a continuous monolayer at the interface between blood and tissue and contributes significantly to the sensing and transducing of signals between blood and tissue. New blood vessel formation, or angiogenesis, is initiated by the activation of endothelial cells and is an important process required for various pathological and physiological situations. This study used cleavable isotope-coded affinity tag reagents combined with mass spectrometry to investigate the molecular basis of a recently discovered angiogenesis-promoting steroid, sokotrasterol sulfate. Changes in the relative abundances of over 1000 proteins within human endothelial cells treated with sokotrasterol sulfate and vehicle-treated cells were identified and quantitated using this technique. A method that examines the entire ensemble of quantitative measurements was developed to identify proteins that showed a statistically significant change in relative abundance resulting from treatment with sokotrasterol sulfate. A total of 93 proteins was significantly up-regulated, and 37 were down-regulated in response to sokotrasterol sulfate stimulation of endothelial cells. Among the up-regulated proteins, several were identified that are novel to endothelial cells and are likely involved in cell communication and morphogenesis. These findings are consistent with a role for sokotrasterol sulfate in endothelial sprouting.

Axon fasciculation defects and retinal dysplasias in mice lacking the immunoglobulin superfamily adhesion molecule BEN/ALCAM/SC1

The immunoglobulin superfamily adhesion molecule BEN (other names include ALCAM, SC1, DM-GRASP, neurolin, and CD166) has been implicated in the control of numerous developmental and pathological processes, including the guidance of retinal and motor axons to their targets. To test hypotheses about BEN function, we disrupted its gene via homologous recombination and analyzed the resulting mutant mice. Mice lacking BEN are viable and fertile, and display no external morphological defects. Despite grossly normal trajectories, both motor and retinal ganglion cell axons fasciculated poorly and were occasionally misdirected. In addition, BEN mutant retinae exhibited evaginated or invaginated regions with photoreceptor ectopias that resembled the "retinal folds" observed in some human retinopathies. Together, these results demonstrate that BEN promotes fasciculation of multiple axonal populations and uncover an unexpected function for BEN in retinal histogenesis.

Towards a molecular pathway for myoblast fusion in Drosophila

Intercellular fusion among myoblasts is required for the generation of multinucleated muscle fibers during skeletal muscle development. Recent studies in Drosophila have shed light on the molecular mechanisms that underlie this process, and a signaling pathway that relays fusion signals from the cell membrane to the cytoskeleton has emerged. In this article, we review these recent advances and discuss how Drosophila offers a powerful model system to study myoblast fusion in vivo.

Ectopic Myf5 or MyoD prevents the neuronal differentiation program in addition to inducing skeletal muscle differentiation, in the chick neural tube

Forced expression of the bHLH myogenic factors, Myf5 and MyoD, in various mammalian cell lines induces the full program of myogenic differentiation. However, this property has not been extensively explored in vivo. We have taken advantage of the chick model to investigate the effect of electroporation of the mouse Myf5 and MyoD genes in the embryonic neural tube. We found that misexpression of either mouse Myf5 or MyoD in the chick neural tube leads to ectopic skeletal muscle differentiation, assayed by the expression of the myosin heavy chains in the neural tube and neural crest derivatives. We also showed that the endogenous neuronal differentiation program is inhibited under the influence of either ectopic mouse Myf5 or MyoD. We used this new system to analyse, in vivo, the transcriptional regulation between the myogenic factors. We found that MyoD and Myogenin expression can be activated by ectopic mouse Myf5 or MyoD, while Myf5 expression cannot be activated either by mouse MyoD or by itself. We also analysed the transcriptional regulation between the myogenic factors and the different genes involved in myogenesis, such as Mef2c, Pax3, Paraxis, Six1, Mox1, Mox2 and FgfR4. We established the existence of an unexpected regulatory loop between MyoD and FgfR4. The consequences for myogenesis are discussed.

BEN/DM-GRASP/SC1 expression during mouse facial development: differential expression and regulation in molars and incisors

The cell adhesion molecule BEN/DM-GRASP/SC1 is expressed in a variety of tissues during embryogenesis. Here, we studied the expression pattern of BEN/DM-GRASP/SC1 in different organs involved in facial mouse development, especially in the developing teeth. BEN/DM-GRASP/SC1 was expressed in nose, whisker, gland, and tongue epithelia, as well as in myogenic mesenchyme. In molars, BEN/DM-GRASP/SC1 was firstly expressed in the condensed mesenchyme and thereafter expression was confined to mesenchymal cells of the dental follicle. In contrast, in incisors, transient BEN/DM-GRASP/SC1 expression was restricted to epithelium. In tissue recombination experiments, BEN/DM-GRASP/SC1 expression in mesenchyme was activated by molar, but not incisor epithelium.

The immunoglobulin superfamily protein SYG-1 determines the location of specific synapses in C. elegans

During nervous system development, neurons form reproducible synapses onto specific targets. Here, we analyze the development of stereotyped synapses of the C. elegans HSNL neuron in vivo. Postsynaptic neurons and muscles were not required for accurate synaptic vesicle clustering in HSNL. Instead, vulval epithelial cells that contact HSNL act as synaptic guidepost cells that direct HSNL presynaptic vesicles to adjacent regions. The mutant syg-1(ky652) has defects in synapse formation that resemble those in animals that lack vulval epithelial cells: HSNL synaptic vesicles fail to accumulate at normal synaptic locations and form ectopic anterior clusters. syg-1 encodes an immunoglobulin superfamily protein that acts in the presynaptic HSNL axon. SYG-1 protein is localized to the site of future synapses, where it initiates synapse formation and localizes synaptic connections in response to the epithelial signal. SYG-1 is related to Drosophila IrreC and vertebrate NEPH1 proteins, which mediate cell-cell recognition in diverse developmental contexts.

Activated leukocyte cell adhesion molecule (CD166/ALCAM): developmental and mechanistic aspects of cell clustering and cell migration

Activated leukocyte cell adhesion molecule (ALCAM/CD166) is a member of the immunoglobulin superfamily and belongs to a recent subgroup with five extracellular immunoglobulin-like domains (VVC2C2C2). ALCAM mediates both heterophilic (ALCAM-CD6) and homophilic (ALCAM-ALCAM) cell-cell interactions. While expressed in a wide variety of tissues, ALCAM is usually restricted to subsets of cells involved in dynamic growth and/or migration, including neural development, branching organ development, hematopoiesis, immune response and tumor progression. Recent structure-function analyses of ALCAM hint at how its cytoskeletal anchoring and the integrity of the extracellular immunoglobulin-like domains may regulate complex cellular properties in regard to cell adhesion, growth and migration. Accumulating evidence suggests that ALCAM expression may reflect the onset of a cellular program for homeostatic control of growth saturation, which induces either growth arrest or cell migration when the upper limits are exceeded.

Identification of CD146 as a component of the endothelial junction involved in the control of cell-cell cohesion

CD146 is a cell-surface molecule belonging to the immunoglobulin superfamily and expressed in all types of human endothelial cells. Confocal and electron microscopic analysis of confluent human umbilical vein endothelial cells (HUVECs) were used to demonstrate that CD146 is a component of the endothelial junction. Double immunolabeling with vascular endothelial cadherin showed that CD146 is localized outside the adherens junction. Moreover, CD146 expression is not restricted to the junction, since part of the labeling was detectable at the apical side of the HUVECs. Interestingly, cell-surface expression of CD146 increased when HUVECs reached confluence. In addition, the paracellular permeability of CD146-transfected fibroblast cells was decreased compared with that of control cells. Finally, CD146 colocalized with actin, was partly resistant to Triton X-100 extraction, and had its expression altered by actin-disrupting agents, indicating that CD146 is associated with the actin cytoskeleton. These results show the regulated expression of CD146 at areas of cell-cell junction and strongly suggest involvement of CD146 as a mediator of cell-cell interaction.

The cell adhesion molecule BEN defines a prosensory patch in the developing avian otocyst

The distribution of the cell adhesion molecule BEN in the developing chick inner ear is described. BEN is first detected in the otic placode at stage 11. As the placode begins to invaginate, BEN becomes concentrated in a ventromedial region extending from the anterior to the posterior end of the otic pit. BEN expression levels increase in this region as the pit closes to form the otocyst, and distinct boundaries become defined along the dorsal and ventral edges of the ventromedial band of BEN expression. BEN expression also becomes concentrated dorsally within the otic epithelium as the pit closes and is observed in the condensing otic ganglion. By stage 22, the ventromedial band of BEN expression splits into two distinct regions, a small caudal patch within which the posterior crista will develop, and a larger anterior patch. By stage 26, this larger anterior patch of cells expressing BEN becomes subdivided into five separate areas corresponding to the regions within which the anterior crista, the lateral crista, the utricle, the saccule, and both the basilar papilla and lagenar macula form. Hair cells only develop within these regions defined by BEN distribution. The data suggest that the ventromedial patch of BEN expression observed from stage 11 onwards defines a single sensory competent zone from which all sensory organs of the inner ear develop. BEN immunoreactivity in the inner ear declines after stage 38. In response to noise exposure, upregulation of BEN expression is mainly detected in regions of the posthatch papilla where the damage is severe and regenerating hair cells are not observed. The regenerating hair and supporting cells do not express BEN, highlighting a molecular difference between the processes of development and regeneration.

Spinal nerve segmentation in the chick embryo: analysis of distinct axon-repulsive systems

In higher vertebrates, the segmental organization of peripheral spinal nerves is established by a repulsive mechanism whereby sensory and motor axons are excluded from the posterior half-somite. A number of candidate axon repellents have been suggested to mediate this barrier to axon growth, including Sema3A, Ephrin-B, and peanut agglutinin (PNA)-binding proteins. We have tested the candidacy of these factors in vitro by examining their contribution to the growth cone collapse-inducing activity of somite-derived protein extracts on sensory, motor, and retinal axons. We find that Sema3A is unlikely to play a role in the segmentation of sensory or motor axons and that Ephrin-B may contribute to motor but not sensory axon segmentation. We also provide evidence that the only candidate molecule(s) that induces the growth cone collapse of both sensory and motor axons binds to PNA and is not Sema3A or Ephrin-B. By grafting primary sensory, motor, and quail retinal neurons into the chick trunk in vivo, we provide further evidence that the posterior half-somite represents a universal barrier to growing axons. Taken together, these results suggest that the mechanisms of peripheral nerve segmentation should be considered in terms of repellent molecules in addition to the identified molecules.

Expression of DM-GRASP/BEN in the developing mouse spinal cord and various epithelia

The expression pattern of the immunoglobulin DM-GRASP/BEN gene was studied in the mouse embryo using in situ hybridization. DM-GRASP/BEN is expressed in the spinal cord in a subset of motoneurons expressing Islet-1, and non homogeneously in the dorsal root ganglia (DRG). In contrast, it's expression is homogeneous in the vestibulo-cochlear and trigemminal ganglia. DM-GRASP/BEN is also expressed in various epithelia of ectodermal or endodermal origin like the nasal, buccopharyngal and lung epithelia. In upper lip, DM-GRASP/BEN transcripts are present in the epidermal cells of the developing hair vibrissa follicles. First detected in the hair placode, DM-GRASP/BEN expression is localized in the central cells of the epithelial hair peg and then in a thin layer of cells crushed against the outer root sheath by the outgrowth of the hair shaft.

DM-GRASP is necessary for nonradial cell migration during chick diencephalic development

Cell migration is fundamental to normal CNS development. Radial migration, along radial glial fibers, has been the principal pathway studied, however, nonradial or tangential cell migration has increasingly been identified at all levels of the CNS. Receptors, cell adhesion molecules, and extracellular matrix molecules have all been shown to participate in radial cell migration. In contrast, the molecular basis of nonradial cell migration has only recently begun to be elucidated. Using replication defective retroviral vectors we have determined the location and time when nonradial cell migration begins in the developing chick diencephalon. We have identified three molecules that are expressed in spatially and temporally restricted domains that are consistent with them playing a role in nonradial cell migration. One of these molecules, DM-GRASP, a transmembrane protein with five extracellular Ig domains, is expressed on the nonradially migrating cells in addition to axons. To test the hypothesis that DM-GRASP participates in guiding nonradial cell migration, we injected a replication-defective retroviral vector used for lineage tracing followed by a DM-GRASP blocking antibody. Embryos injected with the blocking antibody showed a near complete block in nonradial cell migration specifically where DM-GRASP is expressed. Furthermore, morphological analyses revealed disruption of the normal architecture of the diencephalon indicating nonradial cell migration is necessary for normal morphological development of the brain. Our data indicate that DM-GRASP is necessary for nonradial cell migration in the chick diencephalon and have provided a system to further explore the function of nonradial cell migration during CNS development.