Cranial and trunk neural crest cells use different mechanisms for attachment to extracellular matrices

Molecular and Cellular Insights: A Focus on Glycans and the HNK1 Epitope in Autism Spectrum Disorder

Autism Spectrum Disorder (ASD) is a synaptic disorder with a GABA/glutamate imbalance in the perineuronal nets and structural abnormalities such as increased dendritic spines and decreased long distance connections. Specific pregnancy disorders significantly increase the risk for an ASD phenotype such as preeclampsia, preterm birth, hypoxia phenomena, and spontaneous miscarriages. They are associated with defects in the glycosylation-immune placental processes implicated in neurogenesis. Some glycans epitopes expressed in the placenta, and specifically in the extra-villous trophoblast also have predominant functions in dendritic process and synapse function. Among these, the most important are CD57 or HNK1, CD22, CD24, CD33 and CD45. They modulate the innate immune cells at the maternal-fetal interface and they promote foeto-maternal tolerance. There are many glycan-based pathways of immunosuppression. N-glycosylation pathway dysregulation has been found to be associated with autoimmune-like phenotypes and maternal-autoantibody-related (MAR) autism have been found to be associated with central, systemic and peripheric autoimmune processes. Essential molecular pathways associated with the glycan-epitopes expression have been found to be specifically dysregulated in ASD, notably the Slit/Robo, Wnt, and mTOR/RAGE signaling pathways. These modifications have important effects on major transcriptional pathways with important genetic expression consequences. These modifications lead to defects in neuronal progenitors and in the nervous system's implementation specifically, with further molecular defects in the GABA/glutamate system. Glycosylation placental processes are crucial effectors for proper maternofetal immunity and endocrine/paracrine pathways formation. Glycans/ galectins expression regulate immunity and neurulation processes with a direct link with gene expression. These need to be clearly elucidated in ASD pathophysiology.

Time to go: neural crest cell epithelial-to-mesenchymal transition

Neural crest cells (NCCs) are a dynamic, multipotent, vertebrate-specific population of embryonic stem cells. These ectodermally-derived cells contribute to diverse tissue types in developing embryos including craniofacial bone and cartilage, the peripheral and enteric nervous systems and pigment cells, among a host of other cell types. Due to their contribution to a significant number of adult tissue types, the mechanisms that drive their formation, migration and differentiation are highly studied. NCCs have a unique ability to transition from tightly adherent epithelial cells to mesenchymal and migratory cells by altering their polarity, expression of cell-cell adhesion molecules and gaining invasive abilities. In this Review, we discuss classical and emerging factors driving NCC epithelial-to-mesenchymal transition and migration, highlighting the role of signaling and transcription factors, as well as novel modifying factors including chromatin remodelers, small RNAs and post-translational regulators, which control the availability and longevity of major NCC players.

Knockout of the gene encoding the extracellular matrix protein SNED1 results in early neonatal lethality and craniofacial malformations

BACKGROUND

The extracellular matrix (ECM) is a fundamental component of multicellular organisms that orchestrates developmental processes and controls cell and tissue organization. We previously identified the novel ECM protein SNED1 as a promoter of breast cancer metastasis and showed that its level of expression negatively correlated with breast cancer patient survival. Here, we sought to identify the roles of SNED1 during murine development.

RESULTS

We generated two novel Sned1 knockout mouse strains and showed that Sned1 is essential since homozygous ablation of the gene led to early neonatal lethality. Phenotypic analysis of the surviving knockout mice revealed a role for SNED1 in the development of craniofacial and skeletal structures since Sned1 knockout resulted in growth defects, nasal cavity occlusion, and craniofacial malformations. Sned1 is widely expressed in embryos, notably by cell populations undergoing epithelial-to-mesenchymal transition, such as the neural crest cells. We further show that mice with a neural-crest-cell-specific deletion of Sned1 survive, but display facial anomalies partly phenocopying the global knockout mice.

CONCLUSIONS

Our results demonstrate requisite roles for SNED1 during development and neonatal survival. Importantly, the deletion of 2q37.3 in humans, a region that includes the SNED1 locus, has been associated with facial dysmorphism and short stature.

Involvement of neural crest and paraxial mesoderm in oral mucosal development and healing

Tissue engineering therapies using adult stem cells derived from neural crest have sought accessible tissue sources of these cells because of their potential pluripotency. In this study, the gingiva and oral mucosa and their associated stem cells were investigated. Biopsies of these tissues produce neither scarring nor functional problems and are relatively painless, and fresh tissue can be obtained readily during different chairside dental procedures. However, the embryonic origin of these cells needs to be clarified, as does their evolution from the perinatal period to adulthood. In this study, the embryonic origin of gingival fibroblasts were determined, including gingival stem cells. To do this, transgenic mouse models were used to track neural crest derivatives as well as cells derived from paraxial mesoderm, spanning from embryogenesis to adulthood. These cells were compared with ones derived from abdominal dermis and facial dermis. Our results showed that gingival fibroblasts are derived from neural crest, and that paraxial mesoderm is involved in the vasculogenesis of oral tissues during development. Our in vitro studies revealed that the neuroectodermal origin of gingival fibroblasts (or gingival stem cells) endows them with multipotential properties as well as a specific migratory and contractile phenotype which may participate to the scar-free properties of the oral mucosa. Together, these results illustrate the high regenerative potential of neural crest-derived stem cells of the oral mucosa, including the gingiva, and strongly support their use in cell therapy to regenerate tissues with impaired healing.

Collective cell migration in development

During embryonic development, tissues undergo major rearrangements that lead to germ layer positioning, patterning, and organ morphogenesis. Often these morphogenetic movements are accomplished by the coordinated and cooperative migration of the constituent cells, referred to as collective cell migration. The molecular and biomechanical mechanisms underlying collective migration of developing tissues have been investigated in a variety of models, including border cell migration, tracheal branching, blood vessel sprouting, and the migration of the lateral line primordium, neural crest cells, or head mesendoderm. Here we review recent advances in understanding collective migration in these developmental models, focusing on the interaction between cells and guidance cues presented by the microenvironment and on the role of cell–cell adhesion in mechanical and behavioral coupling of cells within the collective.

The different effects on cranial and trunk neural crest cell behaviour following exposure to a low concentration of alcohol in vitro

Embryonic neural crest cells give rise to large regions of the face and peripheral nervous system. Exposure of these cells to high alcohol concentrations leads to cell death in the craniofacial region resulting in facial defects. However, the effects of low concentrations of alcohol on neural crest cells are not clear. In this study, cranial neural crest cells from Xenopus laevis were cultured in an ethanol concentration approximately equivalent to one drink. Techniques were developed to study various aspects of neural crest cell behaviour and a number of cellular parameters were quantified. In the presence of alcohol, a significant number of cranial neural crest cells emigrated from the explant on fibronectin but the liberation of individual cells was delayed. The cells also remained close to the explant and their morphology changed. Cranial neural crest cells did not grow on Type 1 collagen. For the purposes of comparison, the behaviour of trunk neural crest cells was also studied. The presence of alcohol correlated with increased retention of single cells on fibronectin but left other parameters unchanged. The behaviour of trunk neural crest cells growing on Type 1 collagen in the presence of alcohol did not differ from controls. Low concentrations of alcohol therefore significantly affected both cranial and trunk neural crest cells, with a wider variety of effects on cells from the cranial as opposed to the trunk region. The results suggest that low concentrations of alcohol may be more detrimental to early events in organ formation than currently suspected.

Slit/Robo1 signaling regulates neural tube development by balancing neuroepithelial cell proliferation and differentiation

Formation of the neural tube is the morphological hallmark for development of the embryonic central nervous system (CNS). Therefore, neural tube development is a crucial step in the neurulation process. Slit/Robo signaling was initially identified as a chemo-repellent that regulated axon growth cone elongation, but its role in controlling neural tube development is currently unknown. To address this issue, we investigated Slit/Robo1 signaling in the development of chick neCollege of Life Sciences Biocentre, University of Dundee, Dundee DD1 5EH, UKural tube and transgenic mice over-expressing Slit2. We disrupted Slit/Robo1 signaling by injecting R5 monoclonal antibodies into HH10 neural tubes to block the Robo1 receptor. This inhibited the normal development of the ventral body curvature and caused the spinal cord to curl up into a S-shape. Next, Slit/Robo1 signaling on one half-side of the chick embryo neural tube was disturbed by electroporation in ovo. We found that the morphology of the neural tube was dramatically abnormal after we interfered with Slit/Robo1 signaling. Furthermore, we established that silencing Robo1 inhibited cell proliferation while over-expressing Robo1 enhanced cell proliferation. We also investigated the effects of altering Slit/Robo1 expression on Sonic Hedgehog (Shh) and Pax7 expression in the developing neural tube. We demonstrated that over-expressing Robo1 down-regulated Shh expression in the ventral neural tube and resulted in the production of fewer HNK-1(+) migrating neural crest cells (NCCs). In addition, Robo1 over-expression enhanced Pax7 expression in the dorsal neural tube and increased the number of Slug(+) pre-migratory NCCs. Conversely, silencing Robo1 expression resulted in an enhanced Shh expression and more HNK-1(+) migrating NCCs but reduced Pax7 expression and fewer Slug(+) pre-migratory NCCs were observed. In conclusion, we propose that Slit/Robo1 signaling is involved in regulating neural tube development by tightly coordinating cell proliferation and differentiation during neurulation.

The developmental roles of the extracellular matrix: beyond structure to regulation

Cells in multicellular organisms are surrounded by a complex three-dimensional macromolecular extracellular matrix (ECM). This matrix, traditionally thought to serve a structural function providing support and strength to cells within tissues, is increasingly being recognized as having pleiotropic effects in development and growth. Elucidation of the role that the ECM plays in developmental processes has been significantly advanced by studying the phenotypic and developmental consequences of specific genetic alterations of ECM components in the mouse. These studies have revealed the enormous contribution of the ECM to the regulation of key processes in morphogenesis and organogenesis, such as cell adhesion, proliferation, specification, migration, survival, and differentiation. The ECM interacts with signaling molecules and morphogens thereby modulating their activities. This review considers these advances in our understanding of the function of ECM proteins during development, extending beyond their structural capacity, to embrace their new roles in intercellular signaling.

Fibronectin promotes differentiation of neural crest progenitors endowed with smooth muscle cell potential

The neural crest (NC) is a model system used to investigate multipotency during vertebrate development. Environmental factors control NC cell fate decisions. Despite the well-known influence of extracellular matrix molecules in NC cell migration, the issue of whether they also influence NC cell differentiation has not been addressed at the single cell level. By analyzing mass and clonal cultures of mouse cephalic and quail trunk NC cells, we show for the first time that fibronectin (FN) promotes differentiation into the smooth muscle cell phenotype without affecting differentiation into glia, neurons, and melanocytes. Time course analysis indicated that the FN-induced effect was not related to massive cell death or proliferation of smooth muscle cells. Finally, by comparing clonal cultures of quail trunk NC cells grown on FN and collagen type IV (CLIV), we found that FN strongly increased both NC cell survival and the proportion of unipotent and oligopotent NC progenitors endowed with smooth muscle potential. In contrast, melanocytic progenitors were prominent in clonogenic NC cells grown on CLIV. Taken together, these results show that FN promotes NC cell differentiation along the smooth muscle lineage, and therefore plays an important role in fate decisions of NC progenitor cells.

Neural crest motility on fibronectin is regulated by integrin activation

Cell migration is essential for proper development of numerous structures derived from embryonic neural crest cells (NCCs). Although recent work has shown that receptor recycling plays an important role in NCC motility on laminin, the molecular mechanisms regulating NCC motility on fibronectin remain unclear. One mechanism by which cells regulate motility is by modulating the affinity of integrin receptors. Here, we provide evidence that cranial and trunk NCCs rely on functional regulation of integrins to migrate efficiently on fibronectin (FN) in vitro. For NCCs cultured on fibronectin, velocity decreases after Mn2+ application (a treatment that activates all surface integrins) while velocity on laminin (LM) is not affected. The distribution of activated integrin beta 1 receptors on the surface of NCCs is also substratum-dependent. Integrin activation affects cranial and trunk NCCs differently when cultured on different concentrations of FN substrata; only cranial NCCs slow in a FN concentration-dependent manner. Furthermore, Mn2+ treatment alters the distribution and number of activated integrin beta 1 receptors on the surface of cranial and trunk NCCs in different ways. We provide a hypothesis whereby a combination of activated surface integrin levels and the degree to which those receptors are clustered determines NCC motility on fibronectin.

Differential expression of phosphacan/RPTPβ isoforms in the developing mouse visual system

The chondroitin sulfate proteoglycan DSD-1-PG/phosphacan represents one of four splice variants of receptor-protein-tyrosine-phosphatase-beta/zeta (RPTPbeta/zeta). This receptor is expressed by glial cells and occurs in two isoforms, RPTPbeta(long) and RPTPbeta(short). The secreted forms phosphacan and phosphacan short isoform (PSI) bind to extracellular matrix and adhesion molecules and might mediate astroglial effects on neuronal differentiation. Phosphacan and RPTPbeta(long) both carry the DSD-1 epitope, a glycosaminoglycan modification that is involved in stimulating neurite outgrowth of embryonic rat mesencephalic and hippocampal neurons in a polycationic environment. Additionally, phosphacan inhibits neurite outgrowth of embryonic DRG neurons in the presence of laminin. In the light of these functional properties we examined the expression patterns of the DSD-1 epitope and phosphacan isoforms in the developing mouse visual system. During retinal development the DSD-1 epitope appears around embryonic day (E)13, peaks around postnatal day (P)6, and is downregulated from P9 to adolescence. By comparison, the phosphacan core protein is first detectable at E12, reaches maximal levels around P14, and persists, although at lower levels, to adulthood. The DSD-1 epitope is restricted to the nerve fiber and the inner plexiform layers. In contrast, the phosphacan core protein immunoreactivity extends from the nerve fiber layer to the outer plexiform layer. The level of expression of the phosphacan/RPTPbeta gene was investigated by reverse-transcriptase polymerase chain reaction. These experiments suggest that there is a shift in the expression patterns of the different phosphacan/RPTPbeta isoforms during late embryonic and postnatal development. In situ hybridization experiments support the conclusion that at least one of the phosphacan/RPTPbeta isoforms in the retina is expressed by neurons.

The trunk neural crest and its early glial derivatives: a study of survival responses, developmental schedules and autocrine mechanisms

Regulation of survival during gliogenesis from the trunk neural crest is poorly understood. Using adapted survival assays, we directly compared crest cells and the crest-derived precursor populations that generate satellite cells and Schwann cells. A range of factors that supports Schwann cells and glial precursors does not rescue crest, with the major exception of neuregulin-1 that rescues crest cells provided they contact the extracellular matrix. Autocrine survival appears earlier in developing satellite cells than Schwann cells. Satellite cells also show early expression of S100beta, BFABP and fibronectin and early survival responses to IGF-1, NT-3 and PDGF-BB that in developing Schwann cells are not seen until the precursor/Schwann cell transition. These experiments define novel differences between crest cells and early glia and show that entry to the glial lineage is an important point for regulation of survival responses. They show that survival mechanisms among PNS glia differ early in development and that satellite cell development runs ahead of schedule compared to Schwann cells in several significant features.

Integrin alpha5beta1 supports the migration of Xenopus cranial neural crest on fibronectin

During early embryonic development, cranial neural crest cells emerge from the developing mid- and hindbrain. While numerous studies have focused on integrin involvement in trunk neural crest cell migration, comparatively little is known about mechanisms of cranial neural crest cell migration. We show that fibronectin, but not laminin, vitronectin, or type I collagen can support cranial neural crest cell migration and segmentation in vitro. These behaviors require both the RGD and "synergy" sites located within the central cell-binding domain of fibronectin. While these two sites are sufficient for cranial neural crest cell migration, we find that the second Heparin-binding domain of fibronectin can provide additional support for cranial neural crest cell migration in vitro. Finally, using a function blocking monoclonal antibody, we show that cranial neural crest cell migration on fibronectin requires the integrin alpha5beta1.

Neural crest motility and integrin regulation are distinct in cranial and trunk populations

The neural crest is a transient cell population that travels long distances through the embryo to form a wide range of derivatives. The extensive migration of the neural crest is highly unusual and incompletely understood. We examined the ability of neural crest cells (NCCs) to migrate under different conditions in vitro. Unlike most motile cell types, avian NCCs migrate efficiently on a wide range of fibronectin concentrations. Strikingly, the migration of NCCs on laminin depends on the axial level from which the crest is derived. On high concentrations of laminin, cranial NCCs migrate at approximately twice the rate of trunk NCCs and show greater persistence, a higher percentage of migratory cells, and a less organized cytoskeleton. The difference in migration between cranial and trunk neural crest is not due to transcriptional differences in integrin mRNA, but rather to differences in posttranslational regulation. Overexpression of a single integrin is sufficient to significantly slow the migration velocity of cranial neural crest cultured on high laminin densities. These results demonstrate that neural crest cells accommodate a wide range of ECM concentrations in vitro and suggest that differences in integrin regulation along the anterior-posterior axis may contribute to differences in neural crest migration and cell fate.

Trunk neural crest has skeletogenic potential

During early vertebrate development, neural crest cells emerge from the dorsal neural tube, migrate into the periphery, and form a wide range of derivatives. There is, however, a significant difference between the cranial and trunk neural crest with respect to the diversity of cell types that each normally produces. Thus, while crest cells from all axial levels form neurons, glia, and melanocytes, the cranial crest additionally generates skeletal derivatives such as bone and cartilage; trunk crest cells are generally thought to lack skeletogenic potential. Here, we show, however, that if avian trunk neural crest cells are cultured in appropriate media, they form both bone and cartilage cells, and if placed into the developing head, they contribute to cranial skeletal components. Thus, the neural crest from all axial levels can generate the full repertoire of crest derivatives. The skeletogenic potential of the trunk neural crest is significant, as it was likely realized in early vertebrates, which had extensive postcranial exoskeletal coverings.

The early steps of neural crest development

The neural crest is an intriguing cell population that gives rise to many derivatives which are all generated far from their final destinations. From its induction to the delamination of the cells, multiple signalling pathways converge to regulate the expression of effector genes, the products of which endow the cells with invasive and migratory properties reminiscent of those displayed by malignant cells in tumours. As such, the neural crest constitutes an excellent model to study cell migration.

The HNK-1 carbohydrate epitope in the eye: basic science and functional implications

The HNK-1 carbohydrate epitope is part of many cell membrane and extracellular matrix molecules. It has been implicated in cell to cell and cell to extracellular matrix adhesion, and antibodies to the HNK-1 epitope are emerging as a versatile tool in eye research. They have been used to identify a novel cell type in the human eye, the subepithelial matrix cells that reside in the inner connective tissue layer (ICTL) of the ciliary body. Although these cells resemble fibroblasts in ultrastructure, they form a distinct cell population that differs in its antigenic profile from fibroblasts of other tissues. These cells are associated with the elastic fiber system of the ICTL. Other structures in the human eye that harbor the HNK-1 epitope in a nonrandom pattern are the ciliary and iris epithelia, the zonular lamella, the lens capsule, the retina, glial cells of the optic and ciliary nerves, and scleral fibroblasts. The HNK-1 epitope in the eye appears early during embryonic development and is phylogenetically conserved, but many interspecies differences exist in its distribution. The role of the HNK-1 epitope may be to structurally stabilize the ciliary body and the retina, and to participate in zonular attachments. The HNK-1 epitope has been linked with many common eye diseases. The subepithelial matrix cells seem to be susceptible to undergo irreversible damage as a result of glaucoma, thermal injury, and tissue compression. This epitope has proved to be useful in identifying intraocular deposits of exfoliation syndrome. It can explain the adhesiveness of exfoliation material. Intraocular exfoliation material differs in HNK-1 immunoreactivity from the extraocular fibrillopathy of exfoliation syndrome and its presence in fellow eyes also argues against the concept of unilateral exfoliation syndrome. The HNK-1 epitope is found in the extracellular matrix of secondary cataract and anterior subcapsular cataract, and it may contribute to their pathogenesis. Finally, the HNK-1 epitope can be used to trace neuroepithelial derivatives of the optic vesicle in developmental anomalies and in tumors of the eye. Eventual identification of molecules that bear the HNK-1 epitope in the eye will likely shed light on many aspects of ocular physiology and pathobiology

Adhesion and migration of avian neural crest cells on fibronectin require the cooperating activities of multiple integrins of the (beta)1 and (beta)3 families

Based on genetic, functional and histological studies, the extracellular matrix molecule fibronectin has been proposed to play a key role in the migration of neural crest cells in the vertebrate embryo. In the present study, we have analyzed in vitro the repertoire and function of integrin receptors involved in the adhesive and locomotory responses of avian truncal neural crest cells to fibronectin. Immunoprecipitation experiments showed that neural crest cells express multiple integrins, namely (alpha)3(beta)1, (alpha)4(beta)1, (alpha)5(beta)1, (alpha)8(beta)1, (alpha)v(beta)1, (alpha)v(beta)3 and a (beta)8 integrin, as potential fibronectin receptors, and flow cytometry analyses revealed no major heterogeneity among the cell population for expression of integrin subunits. In addition, the integrin repertoire expressed by neural crest cells was found not to change dramatically during migration. At the cellular level, only (alpha)v(beta)1 and (alpha)v(beta)3 were concentrated in focal adhesion sites in connection with the actin microfilaments, whereas the other integrins were predominantly diffuse over the cell surface. In inhibition assays with function-perturbing antibodies, it appeared that complete abolition of cell spreading and migration could be achieved only by blocking multiple integrins of the (beta)1 and (beta)3 families, suggesting possible functional compensations between different integrins. In addition, these studies provided evidence for functional partitioning of integrins in cell adhesion and migration. While spreading was essentially mediated by (alpha)v(beta)1 and (alpha)8(beta)1, migration involved primarily (alpha)4(beta)1, (alpha)v(beta)3 and (alpha)8(beta)1 and, more indirectly, (alpha)3(beta)1. (alpha)5(beta)1 and the (beta)8 integrin were not found to play any major role in either adhesion or migration. Finally, consistent with the results of inhibition experiments, recruitment of (alpha)4(beta)1 and (alpha)v(beta)3, individually or in combination using antibodies or recombinant VCAM-1 and PECAM-1 molecules as a substratum, was required for migration but was not sufficient to produce migration of the cell population as efficiently as with fibronectin. In conclusion, our study indicates that neural crest cells express a multiplicity of fibronectin-binding integrins and suggests that dispersion of the cell population requires cooperation between distinct integrins regulating different events of cell adhesion, locomotion and, possibly, proliferation and survival.

Laminin and beta1 integrins are crucial for normal mammary gland development in the mouse

We have examined the role of integrin-extracellular matrix interactions in the morphogenesis of ductal structures in vivo using the developing mouse mammary gland as a model. At puberty, ductal growth from terminal end buds results in an arborescent network that eventually fills the gland, whereupon the buds shrink in size and become mitotically inactive. End buds are surrounded by a basement membrane, which we show contains laminin-1 and collagen IV. To address the role of cell-matrix interactions in gland development, pellets containing function-perturbing anti-beta1 integrin, anti-alpha6 integrin, and anti-laminin antibodies respectively were implanted into mammary glands at puberty. Blocking beta1 integrins dramatically reduced both the number of end buds per gland and the extent of the mammary ductal network, compared with controls. These effects were specific to the end buds since the rest of the gland architecture remained intact. Reduced development was still apparent after 6 days, but end buds subsequently reappeared, indicating that the inhibition of beta1 integrins was reversible. Similar results were obtained with anti-laminin antibodies. In contrast, no effect on morphogenesis in vivo was seen with anti-alpha6 integrin antibody, suggesting that alpha6 is not the important partner for beta1 in this system. The studies with beta1 integrin were confirmed in a culture model of ductal morphogenesis, where we show that hepatocyte growth factor (HGF)-induced tubulogenesis is dependent on functional beta1 integrins. Thus integrins and HGF cooperate to regulate ductal morphogenesis. We propose that both laminin and beta1 integrins are required to permit cellular traction through the stromal matrix and are therefore essential for maintaining end bud structure and function in normal pubertal mammary gland development.

Vaccinia virus induces Ca2+-independent cell-matrix adhesion during the motile phase of infection

Vaccinia virus (VV) induces two forms of cell motility: cell migration, which is dependent on the expression of early genes, and the formation of cellular projections, which requires the expression of late genes. The need for viral gene expression prior to cell motility suggests that VV proteins may affect how infected cells interact with the extracellular matrix. To address this, we have analyzed changes in cell-matrix adhesion after infection of BS-C-1 cells with VV. Whereas uninfected cells round up and detach from the culture flask in the presence of EGTA, infected cells remain attached to the culture flask with a stellate morphology. Ca2+-independent cell-matrix adhesion was evident by 10 h postinfection, after the onset of cell motility but before the formation of virus-induced cellular projections. Progression to Ca2+-independent adhesion required the expression of late viral genes but not the formation of intracellular enveloped virus particles or intracellular actin tails. Analyses of specific matrix proteins identified vitronectin and fibronectin as optimal ligands for Ca2+-independent adhesion and the formation of cellular projections. Adhesion to fibronectin was mediated via RGD motifs alone and was not inhibited by 500 micrograms of heparin/ml. Kistrin, a disintegrin which binds preferentially to the alphav beta3 (vitronectin/fibronectin) receptor inhibited the formation of cellular projections without disrupting preformed matrix interactions. Finally, we show that Ca2+-independent cell-matrix adhesion is a dynamic process which mediates changes in the morphology of VV-infected cells and uninfected cells which exhibit a transformed phenotype.

The alpha4 subunit of integrin is important for neural crest cell migration

We identify the alpha4 subunit of integrin as a predominant integrin expressed by neural crest cells in both avian and murine embryos. Using degenerate primers, we obtained a PCR fragment of the chick integrin alpha4 subunit that was subsequently used to clone the full-length subunit with a predicted amino acid sequence 60% identical to human and mouse alpha4 subunits. In situ hybridization demonstrates that chick integrin alpha4 mRNA is expressed at high levels by migrating neural crest cells and neural crest-derived ganglia at both cranial and trunk levels. An antibody against the murine alpha4 subunit revealed similar distribution patterns in mouse to chick. In addition to neural crest cells, the integrin alpha4 subunit was later observed on the muscle masses of the limb, the apical ectodermal ridge, and the developing liver. To examine the functional role of the integrin alpha4 subunit in neural crest cell migration, we used an explant preparation that allows visualization of neural crest cells in their normal environment with or without perturbing reagents. In the presence of a blocking antibody against the mouse integrin alpha4 subunit, there was a profound abrogation of neural crest cell migration at trunk and hindbrain levels. Both the numbers of migrating neural crest cells and the total distance traversed were markedly reduced. Similarly, avian embryos injected with synthetic peptides that contain the integrin alpha4 binding site in fibronectin displayed abnormal neural crest cell migration. Our results suggest that the integrin alpha4 subunit is important for normal neural crest cell migration and may be one of the primary alpha subunits used for neural crest cell migration in vivo. Furthermore, the integrin alpha4 subunit represents a useful neural crest marker in the mouse.

Presence of polysialic acid and HNK-1 carbohydrate on brain glycoproteins from beta-1,4-galactosyltransferase-knockout mice

Polysialic acid and HNK-1 carbohydrate are expressed on Gal beta 1-->4GlcNAc outer chains of N-linked sugar chain of neural cell recognition molecules at certain developmental stages and involved in neural tissue formation. Targeted inactivation of the mouse beta-1,4-galactosyltransferase (beta-1,4-GalT) gene resulted in short life of the mice which supposedly do not have such carbohydrate antigens but have no defects in neural tissue formation. Analysis of the mutant mouse brain glycoproteins revealed that polysialic acid and HNK-1 carbohydrate are normally expressed in an age-dependent manner. In support of this, protein bands reacted with Ricinus communis agglutinin-I, which interacts with oligosaccharides terminated with the Gal beta 1-->4GlcNAc group, and beta-1,4-GalT activity toward GlcNAc beta-S-pNP were detected in the mutant mouse brain, indicating that brain contains another functional beta-1,4-GalT important for the expression of the carbohydrate antigens.

Fibronectins Are Essential for Heart and Blood Vessel Morphogenesis But Are Dispensable for Initial Specification of Precursor Cells

The underlying mechanisms of lethal cardiovascular defects associated with the fibronectin-null (FN.null) mutation in mouse embryos were investigated by lineage analysis of myocardial, endocardial, and endothelial cells. A wide variation in phenotype was observed on two genetic backgrounds. In the less severe class (C57/BL6 background), FN.null embryos display a defective heart. Myocardial cells express the specific marker MF-20 and are correctly localized in the anterior trunk region, but myocardial organization is disrupted, resulting in a bulbous heart tube. Endocardial cells express the specific marker platelet-endothelial cell adhesion molecule-1 (PECAM-1) and are localized within the myocardium, but the endocardium appears collapsed. Endothelial cells of two vascular beds are specified, but the aortae are distended and lack contact with the surrounding mesenchyme, while no vessels form in the yolk sac. Defects in the more severe class suggest that FNs are essential earlier in development on the 129/Sv background. Myocardial and endocardial cells are specified, but morphogenesis of the myocardium and endocardium does not occur. Aortic endothelial cells are specified and localized normally, but remain scattered. Yolk sac endothelial cells resemble those of the less severe class. We conclude that FNs are essential for organization of heart and blood vessels, but are dispensable for cellular specification in the appropriate regions within the embryo.

Maturation of the mammalian dorsal root entry zone--from entry to no entry

Interfaces between glial cell precursors of the PNS and CNS are established early in development and form the sites where sensory axons enter and motor axons exit the developing CNS. The molecular and cellular interactions that lead to the formation of these glial interfaces are only now becoming apparent. New in-vitro techniques are providing clues as to how the maturation of PNS-CNS glial interfaces generates barriers to regenerating axons.

HNK-1 carbohydrate-mediated cell adhesion to laminin-1 is different from heparin-mediated and sulfatide-mediated cell adhesion

The sulfated HNK-1 carbohydrate present on glycolipids and on several neural recognition molecules has been shown to mediate the adhesion of murine small cerebellar neurons and astrocytes to the extracellular matrix molecule laminin-1. In this study, we characterized the binding of the HNK-1 carbohydrate to laminin-1 extracted from the Engelbreth-Holm-Swarm (EHS) sarcoma and distinguished it unequivocally from binding sites for other sulfated carbohydrates. Electron microscopic analysis of rotary shadowed complexes of laminin-1 and a HNK-1 neoglycoprotein revealed a major binding site on the G domain that comprises the C-terminal globule of the laminin alpha1 chain. The HNK-1 carbohydrate also interacted with placental laminin isoforms containing an alpha chain variant. It bound to the proteolytic laminin-1 fragment E8 comprising the domains G1-G3, but not to fragment E3 that carries the major heparin-binding site on domains G4-G5. No binding was observed to the short arm containing fragments E1XNd or P1. Binding studies with native or denatured laminin E8 fragments and proteolytic or recombinant fragments of the G domain localized the HNK-1 carbohydrate binding site to domain G2. The binding could be clearly distinguished from binding sites for other sulfated carbohydrates such as heparin and sulfatides. Further, the binding could not be abolished by reduction and alkylation or by urea treatment of laminin-1 and was independent of the native conformation of laminin-1 and of Ca2+. The G2 domain is also involved in the adhesion of HNK-1 carbohydrate expressing early postnatal cerebellar neurons and is different from heparin- and sulfatide-mediated cell adhesion to laminin-1. HNK-1 carbohydrate-mediated cell adhesion appears, however, to be dependent on the native conformation of laminin-1 indicating a more complex cellular recognition process.

The membrane disordering effect of ethanol on neural crest cells in vitro and the protective role of GM1 ganglioside

The teratogenic effect of ethanol appears to be related to excessive cell death in selected cell populations including craniofacial neural crest. Because there is a large body of evidence suggesting that a primary site of action of ethanol is at the membrane level, the current study was designed to examine and attempt to ameliorate ethanol-induced neural crest cell membrane changes that proceed cell death. To this end, neural crest cells were grown as primary cultures from mouse cranial neural tube be explants. In these cultured cells, the relationships between changes in membrane lipid lateral mobility (a measure of membrane fluidity) as determined using the technique of fluorescence recovery after photobleaching (FRAP), ethanol-induced cell death, and the protective role of GM1 ganglioside were examined. A dose-response study showed that treatment with 50, 100, 150, or 200 mM ethanol respectively, for 24 h was positively correlated with membrane lipid lateral mobility and negatively correlated with cell viability. Pre- or co-treatment of the cells with GM1 ganglioside diminished the ethanol-induced increases in membrane fluidity and decreases in cell viability. The results of this study suggest that change in membrane fluidity can account, in part, for ethanol-induced neural crest cell death and that the protection conferred by GM1 ganglioside may result from membrane stabilization and subsequent preservation of the biophysical properties and biological function of the ethanol-exposed cell membranes.

Free radicals and ethanol-induced cytotoxicity in neural crest cells

Associations between ethanol-induced cranial neural crest cell (NCC) damage in mammalian embryos and subsequent malformations as observed in human fetal alcohol syndrome have previously been illustrated. The vulnerability of NCCs to this teratogen may result, at least in part, from their sensitivity to free radical damage. To examine relationships between free radical generation and NCC cytotoxicity, primary culture of mouse NCCs was used. NCC viability was determined in both dose- and time-response studies involving ethanol exposure. After 48 hr of culture, cell viability was significantly diminished at all doses tested (i.e., 50, 100, 150, and 200 mM ethanol). At 100 mM ethanol (a dosage that is teratogenic in vivo and in whole embryo culture), cell viability decreased to approximately 50% of control values over the first 12 hr of culture, and decreased further, to approximately 20% by 48 hr. Using nitroblue tetrazolium as a probe, it was observed that exposure of NCCs to ethanol stimulated the production of superoxide anion radicals. Co-treatment of the ethanol-exposed NCCs with free radical scavengers including 300 units/ml of superoxide dismutase, catalase (500 units/ml), or alpha-tocopherol (300 microM) significantly improved NCC viability. These results suggest that the ethanol-induced NCC injury is mediated, at least in part, through the generation of free radicals. To test this hypothesis further, NCCs were exposed in culture to xanthine/xanthine oxidase. Exogenous free radicals generated by the xanthine/xanthine oxidase system resulted in reduced NCC viability, the severity of which increased in a time and enzyme concentration-related manner. Superoxide dismutase (300 units/ml) and catalase (500 units/ml) significantly reduced the effects of the xanthine/xanthine oxidase-generated free radicals on NCC viability. The similarity between the susceptibility of NCCs to ethanol and their susceptibility to exogenous free radicals in concert with the free radical scavenger-mediated amelioration of ethanol and exogenous free radical-induced NCC death strongly suggest that free radicals play a significant role in ethanol-induced NCC death.

Integrin alpha 6 expression is required for early nervous system development in Xenopus laevis

The integrin alpha 6 subunit pairs with both the beta 1 and beta 4 subunits to form a subfamily of laminin receptors. Here we report the cDNA cloning and primary sequence for the Xenopus homologue of the mammalian integrin alpha 6 subunit. We present data demonstrating the spatial and temporal expression of alpha 6 mRNA and protein during early development. Initially, alpha 6 transcripts are expressed in the dorsal ectoderm and future neural plate at the end of gastrulation. Later in development, alpha 6 mRNAs are expressed in a variety of neural derivatives, including the developing sensory placodes (otic and olfactory) and commissural neurons within the neural tube. Integrin alpha 6 is also expressed in the elongating pronephric duct as well as a subset of the rhombencephalic neural crest, which will form the Schwann cells lining several cranial nerves (VII, VIII and X). In vivo expression of an alpha 6 antisense transcript in the animal hemisphere leads to a reduction in alpha 6 protein expression, a loss of adhesion to laminin, and severe defects in normal development. In 35% of cases, reduced levels of alpha 6 expression result in embryos that complete gastrulation normally but arrest at neurulation prior to the formation of the neural plate. In an additional 22% of cases, embryos develop with severe axial defects, including complete loss of head or tail structures. In contrast, overexpression of the alpha 6 subunit by injection of full-length mRNA has no apparent effect on embryonic development. Co-injection of antisense and sense plasmid constructs results in a partial rescue of the antisense-generated phenotypes. These data indicate that the integrin alpha 6 subunit is critical for the early development of the nervous system in amphibians.

Differential neural crest cell attachment and migration on avian laminin isoforms

A number of laminin isoforms have recently been identified and proposed to exert different functions during embryonic development. In the present study, we describe the purification and partial characterization of several isoforms isolated from chick heart and gizzard, and provide data on the molecular mechanisms underlying the interaction of avian neural crest cells with these molecules in vitro. Laminins extracted from heart and gizzard tissues were separated by gel filtration and purified to homogeneity by sequential lectin and immunoaffinity chromatography by utilizing monoclonal antibodies directed against the avian alpha 2, beta 2 and gamma 1 laminin chains. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) banding pattern of the polypeptide complexes obtained and immunoblotting with polyclonal antisera allowed the identification of Laminin-2 (alpha 2 beta 1 gamma 1), Laminin-4 (alpha 2 beta 2 gamma 1), and laminins comprising the beta 1, beta 2 and gamma 1 chains associated with a shorter alpha chain which, in SDS-PAGE, co-migrate with the beta/gamma complex in the 200 kDa region. These latter laminins, which are here arbitrarily denoted Laminin-alpha x (heart tissue) and Laminin-G (gizzard tissue), are somewhat distinct in their apparent molecular weight, are differentially associated with nidogen, and appear as "T"-shaped particles similar to Laminin-6 and Laminin-7 when analyzed by transmission electron microscopy following rotary shadowing. In contrast, the avian Laminin-2 and Laminin-4 isoforms exhibit the characteristic cruciform shape described previously for their mammalian counterparts. Isolated neural crest cells differentially attached and migrated on these laminin isoforms, showing a clear preference for Laminin-G. Similarly to the EHS Laminin-1, neural crest cells recognized all avian isoforms through their alpha 1 beta 1 integrin, shown previously to be the primary laminin-binding receptor on these cells. Neural crest cell interaction with the avian laminins was dependent upon maintenance of the secondary and tertiary structure of the molecules, as shown by the marked reduction in cell attachment and migration upon disruption of the alpha-helical coiled-coil structure of their constituent chains. The results demonstrate that different laminin isoforms may be differentially involved in the regulation of neural crest cell migration and suggest that this regulation operates through interaction of the cells with a structurally conserved cell binding site recognized by the alpha 1 beta 1 integrin.

Glycans and the modulation of neural-recognition molecule function

Neural-recognition molecules are carbohydrate-bearing glycoproteins, glycolipids or proteoglycans that are found at the cell surface or in the extracellular matrix that regulate cell interactions during development, modification of synaptic activity and regeneration of nerve connections after damage in the adult. The expression of the carbohydrates appears to be fine tuned to these functions. Among the identified carbohydrates are polysialic acid, a 3'-sulfated glucuronic acid, and oligomannosidic residues. They act not only between apposing partner cell surfaces (trans-interaction) but also between recognition molecules within the surface membrane of one cell (cis-interaction), thereby forming complexes that influence transduction of signals to the cell interior.

The L2/HNK-1 carbohydrate is carried by the myelin associated glycoprotein and sulphated glucuronyl glycolipids in muscle but not cutaneous nerves of adult mice

We have previously shown that myelinating Schwann cells associated with motor, but not sensory, axons in peripheral nerves of adult mice express the L2/HNK-1 carbohydrate epitope. This carbohydrate structure carried by glycolipids and neural cell adhesion molecules has been suggested to specifically foster regrowth of motor as opposed to sensory axons after infliction of a lesion. To determine which molecular components may be the carriers of the L2 carbohydrate in motor axon-associated myelinating Schwann cells, we have isolated the purely sensory, cutaneous branch and the mixed sensory and motor muscle branch of the femoral nerve of adult mice, isolated the myelin fraction thereof and analysed the molecules expressing the L2 carbohydrate by several immunochemical methods. L2 immunoreactivity in myelin of the muscle branch was four to five times higher than that of the cutaneous branch. The 110 kDa L2-immunoreactive glycoprotein in myelin of the muscle branch, which is not L2-immunoreactive in the cutaneous branch, was identified as the myelin-associated glycoprotein by a combination of immunoprecipitation and Western blot analysis. Myelin extraction with organic solvents additionally revealed the two L2-carrying glycolipids, which amounted to approximately 40 ng glycolipid/mg dry weight in myelin of the muscle branch, whereas no significant amounts of the L2 glycolipids were found in myelin of the cutaneous branch. These observations suggest an astonishing degree of differential regulation of carbohydratesynthesizing activities in myelinating Schwann cells.

Specific roles of the alpha V beta 1, alpha V beta 3 and alpha V beta 5 integrins in avian neural crest cell adhesion and migration on vitronectin

To identify potentially important extracellular matrix adhesive molecules in neural crest cell migration, the possible role of vitronectin and its corresponding integrin receptors was examined in the adhesion and migration of avian neural crest cells in vitro. Adhesion and migration on vitronectin were comparable to those found on fibronectin and could be almost entirely abolished by antibodies against vitronectin and by RGD peptides. Immunoprecipitation and immunocytochemistry analyses revealed that neural crest cells expressed primarily the alpha V beta 1, alpha V beta 3 and alpha V beta 5 integrins as possible vitronectin receptors. Inhibition assays of cellular adhesion and migration with function-perturbing antibodies demonstrated that adhesion of neural crest cells to vitronectin was mediated essentially by one or more of the different alpha V integrins, with a possible preeminence of alpha V beta 1, whereas cell migration involved mostly the alpha V beta 3 and alpha V beta 5 integrins. Immunofluorescence labeling of cultured motile neural crest cells revealed that the alpha V integrins are differentially distributed on the cell surface. The beta 1 and alpha V subunits were both diffuse on the surface of cells and in focal adhesion sites in association with vinculin, talin and alpha-actinin, whereas the alpha V beta 3 and alpha V beta 5 integrins were essentially diffuse on the cell surface. Finally, vitronectin could be detected by immunoblotting and immunohistochemistry in the early embryo during the ontogeny of the neural crest. It was in particular closely associated with the surface of migrating neural crest cells. In conclusion, our study indicates that neural crest cells can adhere to and migrate on vitronectin in vitro by an RGD-dependent mechanism involving at least the alpha V beta 1, alpha V beta 3 and alpha V beta 5 integrins and that these integrins may have specific roles in the control of cell adhesion and migration.

Rostrocaudal differences in the expression of extracellular matrix proteins by avian neural crest cells in vitro

Cranial and trunk neural crest cells developing in vitro differed in their patterns of expression of two major extracellular matrix proteins, fibronectin and collagen I. Cranial neural crest cells showed two distinct phases of fibronectin expression: the first occurred during the initial migration of cells from explants onto the culture dish; the second was associated with the differentiative period of in vitro development. Fibronectin-immunoreactive cells eventually represented one of the most abundant cell types in cranial cultures. Large numbers of procollagen I-immunoreactive cells also developed in cranial cultures, and procollagen I was colocalized with fibronectin in individual cranial neural crest cells. Neither fibronectin nor procollagen I immunoreactivities were seen in either neurons or melanocytes, consistent with the idea that the fibronectin-immunoreactive cells in cranial neural crest cultures are committed to the mesenchymal lineage. In contrast to cranial neural crest, trunk neural crest produced very few fibronectin-immunoreactive cells at any time in vitro. Trunk neutral crest also produced smaller proportions of procollagen I-immunoreactive cells than did cranial explants. Mitotic labelling experiments showed that the differing proportions of fibronectin- and procollagen I-immunoreactive cells in cranial versus trunk cultures did not result from differences in rates of cell proliferation.

Effect of in ovo retinoic acid exposure on forebrain neural crest: in vitro analysis reveals up-regulation of N-CAM and loss of mesenchymal phenotype

In a prior study of in ovo exogenous retinoic acid (RA) exposure, we observed a prolonged expression of cell surface N-CAM in cranial neural crest (NC) cells exhibiting migratory failure. In the present studies, we employed an experimental strategy in which embryos were first exposed to exogenous RA in ovo and incubated for 45-60 hr; this was followed by extirpation and in vitro culturing of these same RA-exposed cranial neural tubes. NC cell outgrowth from the explant was assayed, as was the immunohistochemical localization of HNK-1 and N-CAM antigens. In RA-exposed explants, the size of the NC cell outgrowths were comparable to controls. However, almost all NC cells lost their mesenchymal phenotype and were arranged in an "epithelioid" pattern of tightly packed polygonal cells that expressed N-CAM at adjacent cell boundaries. By contrast, control NC cells were flattened and multipolar in shape and expressed HNK-1, rarely co-expressing N-CAM. These observations indicate that RA modulates NC cell N-CAM expression and microanatomical phenotype, a finding consistent with prior in ovo studies of RA-exposure. Several possible explanations are considered.

Adhesion molecules in neural crest development

Peripheral nerve cells, various endocrine and pigment cells and cranial connective tissue cells of vertebrates stem mainly from the embryonic neural crest. This originates with the central nervous system, but the crest cells detach from this tissue, via a decrease of cell-cell adhesion involving, particularly, a reduction of the adherens junction cell adhesive molecule A-CAM. This epithelio-mesenchymal transformation allows crest cells to migrate along pathways that are defined partly by the distribution of substrate adhesion molecules, the archetype being fibronectin, an extracellular matrix molecule recognized by integrin receptors on crest cells. Many other molecules, however, may act in the same way. In contrast, some molecules may define migration pathways by reducing adhesion; chondroitin sulfate proteoglycan is a candidate for this role. Pathway selection is most likely achieved by balanced combinations of molecules that promote and reduce adhesion. Cessation of migration, in the case of the nervous ganglia, correlated with re-expression of cell-cell adhesion molecules like A-CAM and others, consistent with an adhesive basis, although functional tests have not yet been performed. The development of the neural crest system provides a useful model that emphasizes the role of adhesion in morphogenesis.

Molecular mechanisms of neural crest cell attachment and migration on types I and IV collagen

We have examined the mechanisms involved in the interaction of avian neural crest cells with collagen types I and IV (Col I and IV) during their adhesion and migration in vitro. For this purpose native Col IV was purified from chicken tissues, characterized biochemically and ultrastructurally. Purified chicken Col I and Col IV, and various proteolytic fragments of the collagens, were used in quantitative cell attachment and migration assays in conjunction with domain-specific collagen antibodies and antibodies to avian integrin subunits. Neural crest cells do not distinguish between different macromolecular arrangements of Col I during their initial attachment, but do so during their migration, showing a clear preference for polymeric Col I. Interaction with Col I is mediated by the alpha 1 beta 1 integrin, through binding to a segment of the alpha 1(I) chain composed of fragment CNBr3. Neural crest cell attachment and migration on Col IV involves recognition of conformation-dependent sites within the triple-helical region and the noncollagenous, carboxyl-terminal NC1 domain. This recognition requires integrity of inter- and intrachain disulfide linkages and correct folding of the molecule. Moreover, there also is evidence that interaction sites within the NC1 domain may be cryptic, being exposed during migration of the cells in the intact collagen as a result of the prolonged cell-substratum contact. In contrast to Col I, neural crest cell interaction with Col IV is mediated by beta 1-class integrins other than alpha 1 beta 1.

Plasticity in neural crest cell differentiation

The neural crest is a pluripotent population of cells that are endowed with migratory capacities. It has long been known that the differentiation pathway taken by cells derived from the neural crest is largely controlled by the microenvironment to which they home after their migration phase, indicating a high degree of plasticity in their developmental fate. Recent progress has been made concerning the factors which influence survival, growth and differentiation of selected sets of precursors in each embryonic site colonised by derivatives of the neural crest.

Functional properties of alternatively spliced forms of the Drosophila PS2 integrin alpha subunit

The Drosophila alpha PS2 protein is encoded by two alternatively spliced transcripts. The respective alpha PS2 proteins differ by the presence of 25 amino acids in the alpha PS2(C) protein, not found in the alpha PS2(m8) subunit, in a region thought to be important for ligand binding. We examined the functional properties of Drosophila S2 cells transformed with genes expressing either of these proteins, in association with a beta PS subunit. Both PS2 integrins support cell spreading on vertebrate vitronectin or, to a lesser extent, on fibronectin. Interestingly, the PS2(C) form promotes spreading more efficiently on vitronectin than does the PS2(m8) form, with an opposite relative efficiency seen for fibronectin. Also, the two forms of PS2 show different requirements for divalent cations in order to mediate efficient cell spreading. These divalent cations are not required to maintain the association of alpha and beta subunits. Spreading of both cell types is similarly RGD sensitive, and both PS2 integrins appear to associate with the actin cytoskeleton. To our knowledge, this represents the first demonstration of functional differences in integrin subunits resulting from splicing variation to generate different extracellular, ligand binding domains.

5-Bromo-2-deoxyuridine regulates invasiveness and expression of integrins and matrix-degrading proteinases in a differentiated hamster melanoma cell

Cell interactions with the extracellular matrix play a critical role in regulating complex processes such as terminal differentiation and tumor progression. In these studies we describe a melanoma cell system that should be useful in addressing the regulation of cell-matrix interactions and the roles they play in regulating differentiation and cell invasiveness. CS (suspension)-1 melanoma cells are relatively well differentiated: they are melanotic, responsive to melanocyte-stimulating hormone, and express TA99, a melanosome membrane differentiation marker. Their repertoire of integrin receptors for extracellular matrix ligands is limited; in particular, they lack receptors for vitronectin, accounting for the observation that they are nonadherent when cultured in the presence of serum. CS-1 cells are noninvasive as well, and express low levels of both metalloproteinases and activated plasminogen activators. Treatment of these cells with melanocyte-stimulating hormone causes them to increase melanin production and assume an arborized phenotype, suggesting that it promotes their further differentiation. In contrast, treatment of CS-1 with the thymidine analog 5-bromodeoxyuridine, converts them to a highly invasive cell population (termed BCS-1) that loses its differentiated properties and responsiveness to melanocyte-stimulating hormone, acquires a broad integrin repertoire (including vitronectin receptors), and expresses elevated levels of metalloproteinases and activated urokinase. From these observations and findings of others on BrdU treatment of other developmental lineages, we hypothesize that BrdU both suppresses differentiation and promotes invasiveness of CS-1 cells. The demonstrated manipulability of CS-1 cells should make them extremely useful for studying the regulation of both terminal differentiation and tumor progression in the melanocyte lineage.