Xenopus laevis integrins. Structural conservation and evolutionary divergence of integrin beta subunits

Cell migration in the Xenopus gastrula

Xenopus gastrulation movements are in large part based on the rearrangement of cells by differential cell-on-cell migration within multilayered tissues. Different patterns of migration-based cell intercalation drive endoderm and mesoderm internalization and their positioning along their prospective body axes. C-cadherin, fibronectin, integrins, and focal contact components are expressed in all gastrula cells and play putative roles in cell-on-cell migration, but their actual functions in this respect are not yet understood. The gastrula can be subdivided into two motility domains, and in the vegetal, migratory domain, two modes of cell migration are discerned. Vegetal endoderm cells show ingression-type migration, a variant of amoeboid migration characterized by the lack of locomotory protrusions and by macropinocytosis as a mechanism of trailing edge resorption. Mesendoderm and prechordal mesoderm cells use lamellipodia in a mesenchymal mode of migration. Gastrula cell motility can be dissected into traits, such as cell polarity, adhesion, mobility, or protrusive activity, which are controlled separately yet in complex, combinatorial ways. Cells can instantaneously switch between different combinations of traits, showing plasticity as they respond to substratum properties. This article is categorized under: Early Embryonic Development > Gastrulation and Neurulation.

Early chordate origin of the vertebrate integrin αI domains

Half of the 18 human integrins α subunits have an inserted αI domain yet none have been observed in species that have diverged prior to the appearance of the urochordates (ascidians). The urochordate integrin αI domains are not human orthologues but paralogues, but orthologues of human αI domains extend throughout later-diverging vertebrates and are observed in the bony fish with duplicate isoforms. Here, we report evidence for orthologues of human integrins with αI domains in the agnathostomes (jawless vertebrates) and later diverging species. Sequence comparisons, phylogenetic analyses and molecular modeling show that one nearly full-length sequence from lamprey and two additional fragments include the entire integrin αI domain region, have the hallmarks of collagen-binding integrin αI domains, and we show that the corresponding recombinant proteins recognize the collagen GFOGER motifs in a metal dependent manner, unlike the α1I domain of the ascidian C. intestinalis. The presence of a functional collagen receptor integrin αI domain supports the origin of orthologues of the human integrins with αI domains prior to the earliest diverging extant vertebrates, a domain that has been conserved and diversified throughout the vertebrate lineage.

Evolution of integrin I domains

In humans, an ~200-residue "inserted" I domain, a von Willebrand factor A domain (vWFA), buds out from the β-propeller domain in 9 of 18 integrin α subunits. The vWFA domain is not unique to the α subunit as it is an integral part of all integrin β subunits and many other proteins. The βI domain has always been a component of integrins but the αI domain makes its appearance relatively late, in early chordates, since it is found in tunicates and later diverging species. The tunicate αI domains are distinct from the human collagen and leukocyte recognizing integrin α subunits, but fragments of integrins from agnathastomes suggest that the human-type αI domains arose in an ancestor of the very first vertebrate species. The rise of integrins with αI domains parallels the enormous changes in body plan and systemic development of the chordate line that began some 550 million or more years ago.

Integrins on eggs: focal adhesion kinase is activated at fertilization, forms a complex with integrins, and is necessary for cortex formation and cell cycle initiation

We investigate the proposal that integrins and focal adhesion kinase (FAK) form a complex that has structural and signaling functions in eggs. FAK protein is present in eggs and is phosphorylated at fertilization. pY(397)FAK localizes to the membrane 30 min after fertilization, which correlates with the expression of βC integrins and egg cortex development. The βC integrin and pY(397)FAK coimmunoprecipitate from egg cortex lysates. PF573 228 and Y11, inhibitors of FAK, interfere with pronuclear fusion and reduce the abundance of pY(397)FAK and cortical actin without affecting microvillar actin. Cyclin E normally accumulates in the nucleus 15 min after fertilization, then returns to background levels. PF573 228- or Y11-treated eggs accumulate cyclin E in the nucleus; however, levels remain high. In addition, PF573 228 interferes with the accumulation of pERK1/2 in the nucleus and in eggs initiating mitosis. Injection of eggs with a fusion protein consisting of the focal adhesion-targeting domain of FAK fused to green fluorescent protein interferes with cortex formation and produces abnormal nuclei. These data indicate that an integrin-FAK adhesion complex forms at the egg surface that functions in formation of actin arrays in the egg cortex and provides signaling inputs for cell cycle initiation.

Lineage of anuran epidermal basal cells and their differentiation potential in relation to metamorphic skin remodeling

The anuran remodels the larval epidermis into the adult one during metamorphosis. Larval and adult epidermal cells of the bullfrog were characterized by determining the presence of huge cytoplasmic keratin bundles and the expression profiles of specific marker genes, namely colalpha1 (collagen alpha1 (I)), rlk (larval keratin) and rak (adult keratin). We identified four types of epidermal basal cells: (i) basal skein cells that have keratin bundles and express colalpha1 and rlk; (ii) rak+-basal skein cells that have keratin bundles and express colalpha1, rlk, and rak; (iii) larval basal cells that express rlk and rak; and (iv) adult basal cells that express rak. These traits suggested that these basal cells are on the same lineage in which basal skein cells are the original progenitor cells that consecutively differentiate into rak+-basal skein cells into larval basal cells, and finally into adult basal cells. To directly verify the differentiation potential of larval basal cells into adult ones, the mono-layered epidermis composed of larval basal cells was cultured in the presence of aldosterone and thyroid hormone. In this culture, larval basal cells differentiated into adult basal cells that reconstituted the adult epidermis. Thus, it was concluded that larval basal cells are the direct progenitor cells of the adult epidermal stem cells.

HEMCAM/CD146 downregulates cell surface expression of (β)1 integrins

HEMCAM/gicerin, an immunoglobulin superfamily protein, is involved in homophilic and heterophilic adhesion. It interacts with NOF (neurite outgrowth factor), a molecule of the laminin family. Alternative splicing leads to mRNAs coding for HEMCAM with a short (HEMCAM-s) or a long cytoplasmic tail (HEMCAM-l). To investigate the cellular function of these two variants, we stably transfected murine fibroblasts with either form of HEMCAM. Expression of each isoform of this protein in L cells delayed proliferation and modified their adhesion properties to purified extracellular matrix proteins. Expression of either HEMCAM-s or HEMCAM-l inhibited integrin-dependent adhesion and spreading of fibroblasts to laminin 1, showing that this phenomenon did not depend on the cytoplasmic region. By contrast, L-cell adhesion and spreading to fibronectin depended on the HEMCAM isoform expressed. Flow cytometry and immunoprecipitation studies revealed that the expression of HEMCAM downregulated expression of the laminin-binding integrins (α)3 (β)1, (α)6 (β)1 and (α)7 (β)1, and fibronectin receptor (α)5 (β)1 from the cell surface. Semi-quantitative PCR and northern blot experiments showed that the expression of (α)6 (β)1 integrin modified by HEMCAM occurred at a translation or maturation level. Thus, our data demonstrate that HEMCAM regulates fibroblast adhesion by controlling (β)1 integrin expression. http://www.biologists.com/JCS/movies/jcs1886.html

Molecular characterization and leukocyte distribution of a teleost beta1 integrin molecule

The beta1 integrin, in combination with the alpha subunit, is responsible for migration of leukocytes into areas of inflammation. Although identified in mammalian species; the beta1 or CD29 molecule has yet to be identified in fish. The present investigation has identified a full-length channel catfish, Ictalurus punctatus, cDNA beta1 molecule composed of 2786 bases and a deduced amino acid sequence of 797 amino acids. The catfish molecule has an amino acid identity ranging from 71.87 to 74.12% with bovine, feline, human, and Xenopus. The channel catfish molecule retains several characteristics of mammalian beta1 molecules, such as four cysteine-rich repeat regions, and eight potential N-linked glycosylation sites. Based on Western blotting the channel catfish beta1 molecule has a molecular mass of approximately 130kDa, essentially the same as that for mammalian species. These results confirm the existence and expression of a beta1 gene in channel catfish, a species phylogenetically distant from mammals.

Molecular cloning and characterization of the pig homologue to human CD29, the integrin beta1 subunit

BACKGROUND

CD29 is the beta1 subunit, a member of the integrin gene superfamily that function as receptor for cell adhesion molecules of the extracellular matrix. Porcine integrin beta1 subunit is involved in rejection of pig-to-human tissue xenografts as target of the natural antibodies present in the human serum. Moreover since CD29, as part of the beta1 integrins very late antigen 4 (VLA-4) and VLA-6, is involved in homing and differentiation of haematopoietic progenitor cells, its characterization in pig is critical to study the interaction of porcine adhesion molecules with human ligands in the induction of donor-specific tolerance toward porcine antigens, a process extremely desirable to prevent rejection of xenogeneic organs.

METHODS

The porcine CD29 cDNA has been isolated from a cDNA library and its structure determined. In addition, reverse transcription-polymerase chain reaction (RT-PCR) was performed to determine the expression of CD29 in different tissues.

RESULTS

The nucleotide sequence of the porcine cDNA includes an open reading frame encoding a polypeptide of 798 amino acids. Expression analysis showed that porcine CD29 is expressed in all lymphoid tissues tested and, in lower amounts, in nonlymphoid tissues. Pig CD29 deduced amino acid sequence displays extensive conservation compared with CD29 sequences from other species and a common structural feature with all the other CD29 molecules analyzed in mammals, including the 12 potential N-glycosilation sites. Punctual changes between human and swine CD29 molecule into the ligand binding domain, and/or into the regulatory domain, suggest potential differences between human and porcine CD29 relative to the human CD29 ligands.

CONCLUSIONS

Cloning of the swine CD29 gene offers a new tool for an alternative protocol of removing xenoreactive antibodies in the recipient. In addition, the determination of the differences between human and swine CD29 will help to understand the adhesion molecule-ligand interactions and their function across the swine-human barrier in xenotransplantation.

Neural crest-specific and general expression of distinct metalloprotease-disintegrins in early Xenopus laevis development

Metalloprotease-disintegrins are a family of membrane-anchored glycoproteins that have been implicated in diverse cellular processes, including fertilization and myoblast fusion, release of TNFalpha from the plasma membrane, and neurogenesis. Here we report the cloning of cDNAs encoding three full-length (xMDC9, xMDC11b, and xMDC13), and one partial (xMDC11a) metalloprotease-disintegrin from the amphibian Xenopus laevis, and the analysis of their expression during early X. laevis development and in adult tissues. The most notable finding was the highly localized and specific expression pattern of xmdc11a at the tailbud stage in the cranial neural crest and in a subset of neural tube cells in the trunk region. In contrast, expression of the closely related xmdc11b was not detectable during the early stages of X. laevis development, and remained low in the adult tissues examined here. Distinct expression patterns were also observed for two other highly related X. laevis genes, xmdc13 and adam13 (Alfandari et al., 1997). While adam13 is expressed in the somitic mesoderm and in neural crest cells, but not in adult testis, xmdc13 expression is low and ubiquitous in the developing embryo, but is clearly present in adult testis. Finally, xmdc9, the putative orthologue of human and mouse mdc9, was found at all stages of development, and in all tissues examined, suggesting a function that may be utilized by most or all cells. The noteworthy features of these four xmdc genes and the implications of their distinct spatial and temporal expression patterns are discussed.

Molecular cloning and developmental expression of the Xenopus homolog of integrin alpha 4

Integrin receptors containing an alpha 4 subunit mediate cell-cell adhesion by binding to VCAM and MadCAM-1 in addition to supporting cell-extracellular matrix (ECM) adhesion by binding to the alternatively spliced V-region of fibronectin (FN). Studies in chick and mouse embryos have implicated these integrins in neural crest migration, myotube formation, heart development, and placentation. Because integrin-FN adhesive interactions have been shown to play essential roles in mammalian development, studies were initiated of integrin alpha 4 in amphibian embryos, which are better suited to experimental analyses of the earliest stages of embryogenesis. Here, the cDNA cloning and pattern of expression of the Xenopus laevis homolog of integrin alpha 4 is reported. Xenopus alpha 4 is 55% identical at the amino-acid level to both its human and mouse counterparts, including conservation of an alpha 4-specific protease cleavage site, 11 potential N-linked glycosylation sites, and 24 cysteine residues. In situ hybridization analysis reveals that transcripts encoding alpha 4 are expressed in epidermis and the branchial arches. Although alpha 4 transcripts can be detected as early as gastrulation, the protein is observed only after tailbud stages of development and is spatially restricted to the epidermis and gills of tadpole stage embryos. From these data it is concluded that Xenopus integrin alpha 4 has structural features in common with other vertebrate alpha 4 homologs, but is detected in a more restricted tissue distribution during development than alpha 4 in other species.

Integrins, adhesion, and cardiac remodeling

Integrins are heterodimeric cell surface receptors that mediate a cell's ability to perceive its environment, respond to changed in its environment, and alter its environment. When activated, these receptors form focal adhesions, which are areas of close attachment of the cells to extracellular matrix proteins in which colocalization of cytoskeletal proteins, intracellular signaling molecules, and growth factor receptors occurs. In cardiac fibroblasts, integrins mediate cell growth and adhesion. Growth factors such as angiotensin II regulate DNA synthesis, protooncogene expression, extracellular matrix production, adhesion, and other actions of cardiac fibroblasts, many of which require integrin activation. In addition to controlling growth factor and hemodynamic effects, regulation ofintegrin activity may be useful to affect cardiac fibrosis and the remodeling process.

Cloning and characterization of cDNAs encoding the integrin alpha2 and alpha3 subunits from Xenopus laevis

Integrins containing the alpha2 and alpha3 subunits associate with the beta1 subunit to form distinct receptors with partially overlapping adhesive specificities. We report the cloning and sequence of cDNAs that encode the Xenopus orthologues of integrins alpha2 and alpha3 and the expression of these subunits during embryogenesis. Integrin alpha2 and alpha3 mRNAs are first expressed in the dorsal mesoderm and developing notochord at gastrulation. We also show that alpha3 mRNAs are expressed in the entire marginal zone of gastrulae dorsalized with LiCl but that this localization is lost in embryos ventralized by ultraviolet light. Immunoblots reveal that the alpha3 protein is expressed throughout early development, however, the alpha2 protein is not detected until late tailbud stages. Injection of full-length alpha3 transcripts into the animal poles of fertilized eggs results in embryonic defects in paraxial mesoderm attributed to the failure of somites to form segments. Injection of the alpha3 transcripts into the vegetal pole and overexpression of a 5'-truncated alpha3 control construct have no apparent affect on development or somite formation. These data suggest that normal position-specific expression of integrins is important in maintaining the proper organization of tissues during early amphibian morphogenesis.

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.

Genomic organization of the mouse beta 1 gene: conservation of the beta 1D but not of the beta 1B and beta 1C integrin splice variants

We have determined the genomic organization of the 3'-region of the murine beta 1 gene and cloned the murine beta 1D integrin splice variant. Overlapping genomic clones encompassing the region of the beta 1D-specific exons were isolated from a phage lambda FIXII library, mapped and partially sequenced. All of the exon-intron junctions identified in the murine beta 1 gene fit with the consensus splice donor and acceptor sequences and occur at the same positions as in their human counterparts. cDNA clones for the beta 1D integrin were isolated from a murine skeletal muscle library. The human and murine beta 1D sequences are conserved at the nucleotide (93%) and amino acid (100%) level, suggesting an important role of this muscle-specific variant throughout mammalian phylogenesis. In contrast, murine sequences for beta 1B are very different from human beta 1B at both the nucleotide as well as amino acid level. Moreover, no specific polyadenylation signal for the beta 1B variant could be identified in genomic clones, suggesting that this variant is not present in the mouse. Finally, we were not able to identify a murine beta 1C splice variant by sequencing analysis, Southern hybridization techniques or polymerase chain reaction of mRNA from platelets. These findings indicate that the beta 1B and beta 1C variants emerged relatively late in the phylogenesis of the beta 1 integrin family.

Construction and adhesive properties of a soluble MadCAM-1-Fc chimera expressed in a baculovirus system: phylogenetic conservation of receptor-ligand interaction

MAdCAM-1 is a high endothelial venule adhesion molecule composed of immunoglobulin and mucin-like domains which binds the leucocyte integrin LPAM-1 (alpha 4 beta 7), and is largely responsible for the selective homing of lymphocytes to mucosal tissues. A novel soluble form of mouse MAdCAM-1 which is normally membrane bound has been produced by joining the extracellular region of the receptor to the Fc domain of human IgG1. The MAdCAM-1-Fc cDNA was inserted into the genome of Autographa californica nuclear polyhedrosis virus (AcNPV). Spodoptera frugiperda insect cells infected with the recombinant virus produced MAdCAM-1-Fc as a disulfide-linked homodimer of 82 kDa polypeptides, which was secreted into the culture medium at > 1 microgram/ml. The product purified by Protein G-Sepharose was identified as authentic MAdCAM-1-Fc by the anti-MAdCAM-1 monoclonal antibody (MoAb) MECA-367 using Western blot and ELISA analysis. When immobilized on glass it was fully functional in supporting the binding of mouse alpha 4 beta 1+ alpha 4 beta 7+ mesenteric lymph node lymphocytes, and the alpha 4 beta 1- alpha 4 beta 7+ TK1 T cell lymphoma. Binding was enhanced by Mn(++)-induced integrin activation, and specifically blocked by anti-integrin alpha 4 subunit and anti-MAdCAM-1 MoAbs. Binding was blocked by pretreatment of cells with sodium azide, and EDTA, indicating that binding is an energy-dependent process which requires divalent cations. Thus the mouse MAdCAM-1-Fc chimera produced in insect cells retains certain functional properties that typify the native receptor, and should be valuable in analysing the role of MAdCAM-1 in lymphocyte recirculation and emigration. However it was not sialylated despite being post-translational modified with N- and O-linked carbohydrate moieties, suggesting that the ability of MAdCAM-1 to support cell adhesion under static conditions is sialylation-independent. A rabbit polyclonal antibody raised against the entire cytoplasmic domain of the human integrin beta 7 subunit recognized LPAM-1-like molecules in human, rat, and mouse cells, suggesting a high degree of conservation of the MAdCAM-1 receptor across species. In agreement with this notion MAdCAM-1-Fc immobilized on glass was fully functional in supporting the cation-dependent binding of peripheral blood or spleen cells from a range of other species including human, rat, and guinea pig; and for human myeloid HL60 cells, binding was mediated by alpha 4 integrins.

Adhesion and matrix in vertebrate development

The extracellular matrix supports the adhesion and migration of cells during morphogenesis and influences cell differentiation. Cell interactions with the extracellular matrix are mediated in large part by members of the integrin family of cell-surface receptors. Recent progress in this area has resulted in the identification of multiple integrins, many of which are expressed in position-specific patterns during vertebrate development. The contributions of these receptors to specific developmental events are now being investigated in a variety of systems using a combination of genetic, molecular and immunological approaches.

A beta 1-integrin associated alpha-chain is differentially expressed during Xenopus embryogenesis

The antigen of mAb 2F10 was identified as a Xenopus beta 1-integrin associated alpha-chain by the criteria (1) that it coprecipitates with anti beta 1-antibody, (2) that it changes molecular mass upon reduction in a way that is characteristic for integrin alpha-chains and (3) that it is present on cell membranes. This alpha-chain, termed alpha 2F10, is found in small amounts in the pregastrula stages of Xenopus development and accumulates thereafter in the embryo, alpha 2F10 can be detected by immunofluorescence first at stage 17 of embryogenesis on the cell membranes of the sensorial layer of the ectoderm, the notochord and the endoderm. This characteristic pattern of distribution is maintained throughout the following embryonic stages. Timed explanation experiments indicate that all cells of the pregastrula have the potency to express alpha 2F10. This potency becomes successively restricted during gastrulation to yield the ultimate pattern of expression.

The mouse VLA-2 homologue supports collagen and laminin adhesion but not virus binding

Human VLA-2 (alpha 2 beta 1) mediates cellular adhesion to collagen and laminin and cell attachment by the human pathogen echovirus 1. We report here the cloning, sequencing and functional expression of the mouse VLA-2 alpha subunit homologue. This integrin subunit is closely related to its human counterpart, with 84% amino acid identity between the human and murine proteins. Conserved structural features include an identical number of amino acids, the presence of an I domain, and identity in the number and position of N-linked glycosylation sites and putative divalent cation binding regions. Murine and human alpha 2 show 30% amino acid divergence within the cytoplasmic tail, a difference that can be detected with antisera directed against the C-terminal peptides. Functionally, mouse alpha 2 was capable of mediating cell attachment to collagen and laminin, and responded to both intra- and extracellular signals with changes in its ligand affinity. In contrast, unlike its human homologue, mouse alpha 2 did not promote binding of echovirus 1. Comparison of the primary structure of the homologues leads us to predict that echovirus 1 may bind in the region of the first two thirds of the human alpha 2 I domain, where the sequences are most divergent, whereas more conserved flanking regions, and the conserved terminal one third of the I domain, may be involved in adhesion to collagen and laminin.

V(+)-fibronectin expression and localization prior to gastrulation in Xenopus laevis embryos

The V-region represents one of three alternatively spliced segments in Xenopus fibronectin. Here, we identify this V-region as binding epitope of the monoclonal antibody (MAb 6D9) that we generated against Xenopus plasma fibronectin. By the use of this antibody we obtained new results that change the present view of the fibronectin expression pattern before gastrulation: (1) the V(+)-fibronectin is the major isoform expressed during early development since only a single fibronectin band is found in Western blots up to tadpole stages. (2) In contrast to previously published data we demonstrate that fibronectin expression is induced by progesterone during oocyte maturation. (3) During cleavage stages the protein is stored in the cytoplasm where it is predominantly associated with plasma membranes. Immunoelectronmicroscopy reveals that V(+)-fibronectin is present at the surface of animal pole blastomeres and secreted into intercellular spaces. This extracellular localization of fibronectin is predominantly observed in the marginal zone, surrounding single cells of the outer cell layer baso-laterally. In the vegetal hemisphere V(+)-fibronectin is restricted to the cytoplasm and accumulated at plasma membranes. With the onset of gastrulation the intracellular and membrane associated fibronectin disappears and fibronectin becomes detectable at the blastocoel roof. Since reaggregation of dissociated blastula cells was not blocked by addition of GRGDS peptide or antibodies against fibronectin, we assume that the early expression and secretion of fibronectin serves as store to allow a rapid matrix assembly with onset of mesodermal cell migration.

A novel, tissue-specific integrin subunit, beta nu, expressed in the midgut of Drosophila melanogaster

The integrins are a family of cell surface receptors for extracellular matrix proteins and counter-receptors on other cells. We have used the polymerase chain reaction to identify a novel integrin receptor beta subunit in Drosophila melanogaster. The deduced amino acid sequence of this subunit, which we have termed beta v (beta-neu), indicates that it has several unusual properties. The beta v subunit is roughly 33% identical with each of the previously sequenced vertebrate and Drosophila beta subunits and is lacking four of the 56 cysteine residues characteristic of most members of this protein family. The expression of the beta v gene is strikingly restricted. It is temporally regulated, with maximal expression occurring at 12–15 hours of embryonic development. In situ hybridization analyses and antibody localization on whole-mount embryos reveal that beta v expression is tissue-specific and confined to the developing midgut endoderm and its precursors during embryogenesis. Tissue specificity of expression is maintained through later stages of development as beta v transcripts are found exclusively in the larval midgut. Within this structure, beta v transcripts are especially concentrated in the cells of the midgut imaginal islands which give rise to the adult midgut.

Characterization of Xenopus laevis gamma-crystallin-encoding genes

In order to gain insight into crystallin (Cry)-encoding gene (cry) evolution and developmental function, we have determined the gene structure and sequence of several Xenopus laevis gamma-cry. These encode the most abundant Cry in the embryonic lens. Four of the X. laevis gamma-cry, which are part of a multigene family, were isolated from a X. laevis genomic library and demonstrated to have the same gene structure as gamma-cry from other vertebrates, thereby providing further evidence that the split between beta and gamma members of the beta gamma cry family occurred relatively early in evolution. Sequence comparisons indicate that these X. laevis genes share 88-90% nucleotide sequence identity in the protein coding regions, which is slightly higher than the identity observed between gamma-cry of other species. The 5' upstream regions of X. laevis gamma-cry contain a few short stretches of homology and one putative promoter element conserved among all cry genes but lack other regions common to gamma-cry promoters from other organisms. The deduced amino acid sequences of all four genes and one cDNA suggest that the structure of X. laevis gamma-Cry is highly conserved with that of other vertebrate gamma-Cry, as deduced from the known three-dimensional structure of bovine gamma B Cry.

The evolution of the thrombospondin gene family

Thrombospondin-1 is an adhesive glycoprotein that is involved in cellular attachment, spreading, migration, and proliferation. To date, four genes have been identified that encode for the members of the thrombospondin gene family. These four genes are homologous to each other in the EGF-like (type 2) repeats, the calcium-binding (type 3) motifs, and the COOH-terminal. The latter has been reported to be a cell-binding domain in thrombospondin-1. Phylogenetic trees have been constructed from the multisequence alignment of thrombospondin sequences from human, mouse, chicken, and frog. Two different algorithms generate comparable results in terms of the topology and the branch lengths. The analysis indicates that an early form of the thrombospondin gene duplicated about 925 million years ago. The gene duplication that produced the thrombospondin-1 and -2 branches of the family is predicted to have occurred 583 million years ago, whereas the gene duplication that produced the thrombospondin-3 and -4 branches of the family is predicted to have occurred 644 million years ago. These results indicate that the members of the thrombospondin gene family have existed throughout the evolution of the animal kingdom and thus probably participate in functions that are common to most of its members.

Integrin alpha subunit mRNAs are differentially expressed in early Xenopus embryos

Adhesion of cells to extracellular matrix proteins is mediated, in large part, by transmembrane receptors of the integrin family. The identification of specific integrins expressed in early embryos is an important first step to understanding the roles of these receptors in developmental processes. We have used polymerase chain reaction methods and degenerate oligodeoxynucleotide primers to identify and clone Xenopus integrin alpha subunits from neurula-stage (stage 17) cDNA. Partial cDNAs encoding integrin subunits alpha 2, alpha 3, alpha 4, alpha 5, alpha 6 and an alpha IIb-related subunit were cloned and used to investigate integrin mRNA expression in early embryos by RNase protection assay and whole-mount in situ hybridization methods. Considerable integrin diversity is apparent early in development with integrins alpha 2, alpha 3, alpha 4, alpha 5 and alpha 6 each expressed by the end of gastrulation. Both alpha 3 and alpha 5 are expressed as maternal mRNAs. Zygotic expression of alpha 2, alpha 3, alpha 4 and alpha 6 transcripts begins during gastrulation. Integrin alpha 5 is expressed at relatively high levels during cleavage, blastula and gastrula stages suggesting that it may represent the major integrin expressed in the early embryo. We demonstrated previously that integrin beta 1 protein synthesis remains constant following induction of stage 8 animal cap cells with activin (Smith, J. C., Symes, K., Hynes, R. O. and DeSimone, D. W. (1990) Development 108, 289–298.). Here we report that integrin alpha 3, alpha 4 and alpha 6 mRNA levels increase following induction with 10 U/ml activin-A whereas alpha 5, beta 1 and beta 3 mRNA levels remain unchanged. Whole-mount in situ hybridization reveals that alpha 3 mRNAs are expressed by cells of the involuting mesoderm in the dorsal lip region of early gastrulae. As gastrulation proceeds, alpha 3 expression is localized to a stripe of presumptive notochordal cells along the dorsal midline. In neurulae, alpha 3 mRNA is highly expressed in the notochord but becomes progressively more restricted to the caudalmost portion of this tissue as development proceeds from tailbud to tadpole stages. In addition, alpha 3 is expressed in the forebrain region of later stage embryos. These data suggest that integrin-mediated adhesion may be involved in the process of mesoderm involution at gastrulation and the organization of tissues during embryogenesis.

Early opsin expression in Xenopus embryos precedes photoreceptor differentiation

The visual pigment which serves as the first step in the phototransduction cycle in vertebrate rod cells consists of a retinal chromophore which is linked to the transmembrane protein, opsin. Opsin genes have been isolated from a number of different organisms and studies have shown opsin to be developmentally regulated with both mRNA and protein expression associated with the morphological differentiation of photoreceptor cells. Due to its potential utility as a marker for rod photoreceptor determination in studies of retinal tissue interactions, and because no amphibian opsin genes have as yet been cloned, we isolated cDNA clones of the Xenopus laevis opsin gene. Sequence analysis shows that within the coding region Xenopus opsin shares a high degree of identity with other rod opsin genes, except at the C-terminal where it more closely resembles the mammalian color opsins. A developmental analysis, on the other hand, reveals that Xenopus opsin transcripts are detectable in a retina-specific fashion early in retinal development. Using in situ hybridization we find that Xenopus opsin mRNA is initially restricted to a few isolated cells in the presumptive photoreceptor layer which express the gene at relatively high levels. This suggests that rod photoreceptor determination occurs in single cells, and that the mechanisms controlling opsin expression in Xenopus are initiated well before any evidence of morphological differentiation.

Familial genetic defect in a case of leukocyte adhesion deficiency

Leukocyte adhesion deficiency (LAD) is an inherited immunodeficiency disorder caused by CD18 subunit abnormality dependent defective expression of beta 2 integrins on the surface of leukocytes. On analysis of the CD18 molecular defect in a female Japanese patient with a severe deficiency LAD phenotype, neither CD11a nor CD18 molecules could be detected on the patient's EBV-transformed B lymphoblastoid cell line. The mRNA of the patient's B cells was normal in size, but was diminished in quantity, to approximately half normal levels. Sequencing of the CD18 cDNA of the patient revealed a C605 to T transition, resulting in a Pro178-->Leu substitution. This was heterozygous in the genomic DNA, and shown to be of maternal origin by family study. Only a few transcripts from the other allele without the Pro178-->Leu mutation were detectable. Northern blot analysis revealed reduced CD18 mRNA levels, not only in the patient, but also in the father and brother. These results indicate that our case is a compound heterozygote with two different mutant alleles: one causing a single amino acid substitution and the other causing defective expression of mRNA.

Further characterisation of the talin-binding site in the cytoskeletal protein vinculin

The cytoskeletal protein vinculin is a component of adherens-type junctions where it is one of a number of interacting proteins thought to link the cytoplasmic domain of adhesion receptors to F-actin. Vinculin has been shown to bind to at least three other cytoskeletal proteins, talin, paxillin and alpha-actinin. In this study, we further characterise the talin-binding domain in vinculin using a series of chick vinculin polypeptides expressed as glutathione-S-transferase fusion proteins in Escherichia coli. Thus 125I-talin bound to a fusion protein spanning residues 1–398, but not to those spanning residues 399–881 or 881–1066 in an SDS-PAGE gel-blot assay. We have previously characterised two chick vinculin cDNAs (2.89 kb cDNA and cVin5) which are identical in the region of overlap except that cVin5 lacks coding sequence for residues 167–207. Interestingly, a fusion protein spanning residues 1–398, but lacking residues 167–207, was unable to bind talin. However, further analysis showed that residues 167–207 are insufficient to support binding, and deletion of as few as 31 N-terminal residues abolished binding activity. The results of the gel-blot assay were essentially confirmed using purified fusion proteins adsorbed to glutathione-agarose beads. The smallest vinculin fusion protein able to bind talin contained residues 1–258. This fusion protein was as effective as whole vinculin in inhibiting the binding of 125I-vinculin to talin-coated microtitre wells. Interestingly, mutations which altered the charge characteristics of the highly conserved residues 178 and 181 abolished binding, whereas conservative substitutions were without effect. However, such mutations did not abolish the ability of mutant polypeptides spanning residues 1–398 to target to cell-matrix junctions in Cos cells. We have investigated the possible origin of the cDNA clone cVin5 by defining the structure of a 5′ portion of the chicken vinculin gene, and by analysing vinculin transcripts in a variety of adult tissues and embryonic fibroblasts using reverse transcriptase and polymerase chain reaction. Although residues 167–207 are encoded on a separate exon, we have been unable to identify a tissue where this exon is alternatively spliced.

The attachment of nematocytes from the primitive invertebrate Hydra to fibronectin is specific and RGD-dependent

The transient attachment of cells to components of the extracellular matrix is an important step in the complex molecular mechanisms involved in amoeboid cell locomotion. We have analyzed the attachment of nematocytes from the freshwater cnidarian Hydra to fibronectin which is a constituent of the mesoglea, the extracellular matrix, of the polyps. The percentage of attaching cells increased gradually in a concentration-dependent manner and reached a plateau value at a fibronectin concentration of 50 micrograms/ml. Attachment was inhibited by exposure of the fibronectin-coated surfaces to antibodies against the cell binding domain of fibronectin or by incubating the cells with peptides containing the recognition sequence Arg-Gly-Asp (RGD) known from vertebrate cells. This, together with data obtained by affinity chromatography, indicates that RGD-dependent binding to fibronectin, mediated by a receptor which possibly belongs to the integrin family, already occurs in Hydra, a member of an evolutionary low invertebrate phylum.

Are beta 1 integrins involved in Xenopus gastrulation?

The molecular basis of vertebrate gastrulation is poorly understood. Work on urodele amphibians has implicated beta 1-containing integrins, but the limited information available for Xenopus indicates otherwise: peptides containing the RGD sequence do not inhibit gastrulation and induction of cell spreading in presumptive ectodermal cells by activin is not accompanied by an increase in synthesis of integrin beta 1. Here we report that beta 1-containing integrins are, nevertheless, the principal fibronectin receptors in the Xenopus gastrula, although their cell surface levels are low. Antibodies recognizing the external domain of the molecule can, unlike peptides containing the RGD site, block gastrulation when introduced into the blastocoel. These results allow us to propose a model to explain the role of integrin beta 1 in Xenopus gastrulation.

Identification and characterization of alternatively spliced fibronectin mRNAs expressed in early Xenopus embryos

Sequence analysis of cDNA clones encoding fibronectin (FN) from Xenopus laevis reveals extensive amino acid identities with other vertebrate FNs, including the presence of the Arg-Gly-Asp (RGD) cell attachment site in type III-10 and of a second, cell-binding site (EILDV) in the alternative spliced V region of the protein. These cDNAs have been used to study the expression of FN mRNAs during early development. Overall, levels of maternal FN mRNA remain constant until the mid- to late-gastrula stage when the accumulation of new FN transcripts is first apparent. RNase protection analyses reveal that the pattern of FN alternative splicing is similar to that reported for other species and does not change with the shift from maternal to zygotic mRNA expression. The cellular forms of the FN protein predominate in the early embryo with the EIIIA and EIIIB exons included in most mRNAs at this time. A comparison of V-region alternative splicing between embryonic and adult liver RNAs indicates a segment of 345 nucleotides that can be either completely excluded or included in mature FN transcripts but there is no evidence for additional V-region variants. Maternal mRNAs encoding alternatively spliced forms of FN can be specifically eliminated from Xenopus oocytes following the injection of antisense oligodeoxynucleotides into the cytoplasm, thereby making it possible to analyze the structure, composition, and function of FN mRNAs in early embryos.

Molecular cloning of the human alpha 6 integrin subunit. Alternative splicing of alpha 6 mRNA and chromosomal localization of the alpha 6 and beta 4 genes

We have isolated cDNAs encoding the alpha 6 subunit from a lambda gt11 expression library from human keratinocytes by combined screening with a rabbit polyclonal anti-alpha 6 antibody and the polymerase chain reaction. The alpha 6 subunit encoded by this cDNA consists of 1050 amino acids with a 991-amino-acid extracellular, a 23-amino-acid transmembrane and a 36-amino-acid cytoplasmic domain. The extracellular domain contains three putative divalent cation-binding sites and nine potential N-linked glycosylation sites. From a cDNA library from normal human mammary gland cells two different cDNAs for alpha 6 were isolated, one of which is identical to the above cDNA. The two alpha 6 subunits, called alpha 6A and alpha 6B, encoded by the two cDNAs each have a unique cytoplasmic domain, that of alpha 6B being 18 amino acids longer than that of alpha 6A. Different carcinoma cell lines contain transcripts for both alpha 6 subunits. K562 leukemic cells have little alpha 6A or alpha 6B mRNAs. The overall level of expression varies in the carcinoma cell lines, but reflects alpha 6 cell surface expression. In A375 melanoma cells, however, cell surface expression of alpha 6 was low in spite of a high level of mRNA. This suggest that other mechanisms may be involved in regulating the expression of alpha 6 on the surface of these cells. The mRNA for both alpha 6 subunits is around 6 kb. The alpha 6 subunits are similar to other alpha subunits (26-31% identity with cleaved alpha subunits) of the integrin family but they are more similar to the alpha 3 subunit (40% identity). This high degree of similarity may be the basis for their functional resemblance since both alpha 3 and alpha 6 subunits, when associated with beta 1, function as laminin receptors and bind to the long arm of laminin. The genes for alpha 6 and beta 4, the alternative beta subunit with which alpha 6 combines on certain epithelial cells, were mapped to chromosome 2 and 17q11-qter, respectively.

Expression and endocytosis of integrin VLA receptors for collagen, fibronectin and laminin by normal human keratinocytes

The Very Late Activation (VLA) antigen family is involved in cell-extracellular matrix interactions and consists of six heterodimeric cell surface receptors with a common beta 1 and a variable alpha subunit. Using a panel of specific antibodies, we showed that human epidermal basal cells expressed VLA-2, VLA-3 and VLA-6 but failed to express VLA-4. Their functional roles were investigated and VLA-2 appeared as a specific receptor for type IV collagen and also as a laminin receptor. VLA-3 appeared as a receptor for fibronectin and laminin and to a lesser extent as a type I collagen receptor. VLA-6 appeared as a specific receptor for laminin. It also appeared that the VLA-alpha subunit specifically mediates the recognition of ligand but the beta 1 subunit is also involved in adhesion and that both subunits have a synergistic influence. Immunoprecipitation analyses confirmed that VLA-2, VLA-3 and VLA-6 were expressed by basal keratinocytes. Endocytosis of VLA-2 and VLA-3 was observed involving coated vesicles and endosomes that are structures characteristic of a receptor-mediated pathway. These findings provide first evidence that normal human basal keratinocytes are able of endocytosis mediated by receptors. Taken together, these results indicate that multiple VLA receptors function in combination to mediate epidermal basal cell adhesion to extracellular matrix.

Extracellular matrix molecules and their receptors: an overview with special emphasis on periodontal tissues

Knowledge of extracellular matrix molecules and their cell receptors has increased exponentially during the last 2 decades. It is now known that the structure and function of each tissue is based on specific combinations of matrix molecules. The major constituents of the extracellular matrix are collagens, proteoglycans, and adhesive glycoproteins. The rapid development of biochemical, molecular biological, and immunological research has revealed a lot of interesting details pertaining to these molecules. Several new collagen types have been discovered. In addition to being responsible for the strength and form of tissues, each collagen type has specific sequences providing them with special features such as flexibility and the ability to interact with other matrix molecules and cells. Proteoglycans are another large group of matrix molecules with a variety of functions. Proteoglycans play an important role in tissue resilience and filtering. Some proteoglycans have a capacity to specifically bind other matrix molecules and growth factors, while others act as matrix receptors on the cell surface. An important part of regulation of the cell behavior is played by adhesive glycoproteins belonging to the fibronectin and laminin families. Several isoforms of fibronectin and laminin that result from alternative RNA splicing serve specific functions such as controlling the attachment, migration, and synthetic activity of cells. A major group of cell receptors for cell-matrix and cell-cell interactions is termed integrins. The integrins are cell surface proteins composed of two polypeptides whose structure dictates the specificity of each receptor. The cytoplasmic domain of the integrins interacts with cytoskeletal elements within the cell, and thereby relays the information from the extracellular space into the protein synthesis machinery. The expression of the integrins is controlled by the extracellular matrix and growth factors, most notably TGF beta. During periodontal diseases several aspects of the cell-matrix interactions may be disturbed. Therefore, an understanding of the special features of the extracellular matrix and their receptors in periodontal tissues is a prerequisite for developing new approaches to the prevention and treatment of periodontal diseases.

Identity between the novel integrin beta 7 subunit and an antigen found highly expressed on intraepithelial lymphocytes in the small intestine

A cDNA clone encoding the N-terminal sequence of the murine integrin beta 7 subunit, a novel member of the leukocyte cell adhesion molecule subset (Leu-CAM), has been isolated. An N-terminal region of 13 contiguous amino acids deduced from the cDNA shows complete identity with the N-terminus of the 120 kDa subunit of the M290 antigen, a surface molecule found highly expressed on mouse intestinal intraepithelial lymphocytes (IEL). This unexpected result focuses two previously unconnected areas of research and suggests that integrins may have a special role to play in the defence of the gut mucosa.

Diversity amongst the microglia in growing and regenerating fish CNS: immunohistochemical characterization using FL.1, an anti-macrophage monoclonal antibody

We have immunohistochemically characterized the forms and distribution of microglia--the macrophages of the CNS--in fish, using a new monoclonal antibody (mAb), FL.1. This mAb specifically reacts with resident macrophages throughout the body in Oreochromine fish, including Kuppfer cells, gut-associated myeloid cells, and peritoneal macrophages, as well as with microglia, but circulating monocytes are not labelled with FL.1. The FL.1-epitope, which is lost following treatment with reducing agents, has an extracellular location and is associated with three integral membrane glycoprotein variants. FL.1-staining shows that microglia are extremely abundant throughout the fish CNS. For example, they comprise a third of the glia in the optic nerve, and 30% of all cells, including neurons, in the spinal cord, i.e., fish have about tenfold more microglia than mammals. Two forms of FL.1-positive microglia are predominant in fish, one resembling their mammalian counterparts, but less ramified, and the other comprising smaller rounded cells with very little cytoplasm, which are most numerous in the ependymal region of the optic tectum. Apart from the conventional microglia, the optic nerves also contain large lipid-laden macrophages which comprise a third form of FL.1-positive cell in the CNS. Fish optic nerves contain astrocytes of a distinct type which form reticular networks, but lack connections to capillaries (Maggs and Scholes, J. Neurosci. 1990;10:1600-1614). The co-distribution of foamy macrophages may have a metabolic role that is performed by ordinary astrocytes elsewhere in the CNS. An antiserum against the beta 2 subunit of the human leukocyte integrins (Kishimoto et al., Cell 1987a; 50:193-202) was found selectively to recognize the foamy macrophages in Oreochromis. Following lesion to the optic nerve, FL.1-labelling shows that microglia proliferate throughout the visual pathway. In the optic tectum, the additional FL.1-positive cells are concentrated in the vicinity of degenerating retinal axons and their terminals. Most of the microglia in the injured optic nerve have amoeboid morphologies, and the foamy macrophages become depleted.