Activation of the cytotactin promoter by the homeobox-containing gene Evx-1

Expression screening reveals an orphan receptor chick ovalbumin upstream promoter transcription factor I as a regulator of neurite/substrate-cell contacts and cell aggregation

A rat homologue of chick ovalbumin upstream promoter transcription factor I (COUP-TF I) was isolated using an expression cloning method developed to isolate neurite outgrowth inhibitors. Overexpression of COUP-TF I in 3T3 fibroblasts resulted in reduction of stable contact formation between neurites and transfected cells. Additionally, COUP-TF I enhanced retinoic acid response element-dependent reporter gene expression in 3T3 fibroblasts, indicating that COUP-TF I can modulate transcriptional activation in these cells. Our data suggest that COUP-TF transcription factors are involved in the regulation of cell surface molecules during neruogenesis.

Boundaries and inhibitory molecules in developing neural tissues

Numerous studies of the past decade have illuminated the importance of intercellular adhesion events for neural pattern formation. It has been documented that members of the Ig and cadherin gene superfamilies, that glycoproteins and, probably to some extent, proteoglycans of the extracellular matrix play a role in this context. Recent observations suggest that, in addition to adhesive interactions, repulsive and/or inhibitory phenoma are also of importance in regulating neural pattern formation. Several molecules are under study which are considered possible mediators of inhibitory interactions in the nervous system. The hypothesis has been advanced that some of these might be partially responsible for restrictive, boundary-like properties ascribed to glial cells in developing and regenerating tissues. The current review summarizes these studies and focusses on molecular aspects of boundary and compartmentation phenomena.

Structural and functional similarities between the promoters for mouse tenascin and chicken cytotactin

Cytotactin/tenascin is an extracellular matrix glycoprotein expressed in a restricted anteroposterior pattern during vertebrate development and is reexpressed in the adult during wound healing, tumorigenesis, and nerve regeneration. Previously, we have characterized the chicken cytotactin promoter and have shown its regulation by homeobox gene products in vitro. We have now isolated the promoter for the mouse tenascin gene in order to determine whether common or different DNA regulatory elements control the expression of this gene in these two species. Like the chicken cytotactin gene, the mouse tenascin gene has a single RNA start site that lies 27 bp downstream of a TATA box. A 4028-bp region of DNA upstream of the mouse tenascin gene was sequenced and examined for regulatory motifs in common with the upstream sequence from the chicken cytotactin promoter. Two hundred thirty base pairs of the proximal promoter regions from both genes had an extended sequence similarity and contained common regulatory motifs such as two tracts of homopolymeric dA.dT sequence, an octamer motif, an ATTA (TAAT) motif which is a common core sequence for binding of homeodomain transcription factors, and a TATA-box/cap-site region. Reporter gene constructs with various 5' deletions of the mouse tenascin upstream sequence were tested in transient transfections of mouse NIH 3T3 and chicken embryo fibroblasts. The conserved proximal promoter region of tenascin was responsible for most of the positive regulatory activity. In addition, an upstream region (-2478 to -247) repressed proximal promoter activity in mouse fibroblasts and also in chicken embryo fibroblasts. These data indicate that both the structure and function of the cytotactin/tenascin proximal promoters have remained conserved over 250 million years.

Human tenascin gene. Structure of the 5'-region, identification, and characterization of the transcription regulatory sequences

This report describes the genomic organization of the 5'-region of the human tenascin-C (TN) gene and the functional characterization of its promoter. Approximately 2300 base pairs of the TN gene 5'-flanking region have been cloned and sequenced. This genomic region contains several potential binding sites for transcription factors. By primer extension and S1 nuclease analysis we have localized the transcription start site. The first exon of the TN gene (179 base pairs long) is present in the two major TN transcripts, showing that the expression of these two mRNAs is regulated by a single promoter. The 220 bases upstream to the transcription start site are equally active in directing the expression of chloramphenicol acetyltransferase (CAT) reporter gene in TN producer and nonproducer cells. Using deletion fragments of the human 5'-flanking region we have shown the presence of putative "silencer" elements in the -220 to -2300 region active in both TN producer and nonproducer cell lines. Furthermore, we have demonstrated that the selective transcription in TN producing cells requires the presence of a 1.3-kilobase portion of the TN gene intron 1 in the CAT expression vectors. These findings indicate that complex mechanisms control the transcriptional regulation of TN gene.

The gene of chicken axonin-1. Complete structure and analysis of the promoter

We have isolated and characterised the gene encoding the chicken axonal cell adhesion molecule axonin-1. This gene comprises 23 exons distributed over approximately 40 kb. Each of the six immunoglobulin-like domains and the four fibronectin-type-III-like domains of axonin-1 is encoded by two exons. The introns between two domains are exclusively phase I. Their exon/intron borders correspond to the domain borders of the protein, suggesting that the gene of axonin-1 had been generated by exon shuffling. Three transcripts with a length of 4.3 kb, 5 kb, and 8 kb are found, and we provide evidence that they result from alternative use of polyadenylation signals. In situ hybridization revealed co-localisation of these transcripts in time and space in the developing chicken retina. Several identical transcription initiation sites were found in retina, brain, and cerebellum by RNase protection assay and anchored polymerase chain reaction. By transfection of HeLa cells, rat PC-12 phaeochromocytoma cells, and chicken embryonic fibroblasts with serially truncated segments of the 5'-flanking region linked to a luciferase reporter gene, we have found that the sequence from -91 to +56 relative to the transcription initiation site is sufficient to promote efficient gene expression. Tissue-specific expression of the axonin-1 gene seems to be regulated in part by sequences more than 1 kb upstream of the transcription initiation site. As revealed by computer analysis, the sequence immediately upstream of exon 1 contains an AP-2 binding site, a tumor phorbol-ester-responsive element, and a homeodomain protein binding site, but no canonical TATA box. A second AP-2 binding site and a homeodomain protein binding site are located within exon 1.

Tenascin-C expression by fibroblasts is elevated in stressed collagen gels

Chick embryo fibroblasts cultured on a collagen matrix exert tractional forces leading to the contraction of unrestrained, floating collagen gels and to the development of tension in attached, restrained gels. On a restrained, attached collagen gel the fibroblasts synthesize large quantities of tenascin-C, whereas in a floating, contracting gel tenascin-C synthesis is decreased. This regulation of tenascin-C synthesis can be observed by the secretion of metabolically labeled tenascin-C into the conditioned medium, as well as by the deposition of tenascin-C into the collagen matrix as judged by immunofluorescence. Regulation appears to occur at the transcriptional level, because when cells on attached or floating collagen gels are transfected with promoter constructs of the tenascin-C gene, luciferase expression driven by the tenascin-C promoter parallels the effects measured for endogenous tenascin-C synthesis, whereas luciferase expression under the control of the SV40 promoter does not depend on the state of the collagen gel. The promoter region responsible for tenascin-C induction on attached collagen gels is distinct from the region important for the induction of tenascin-C by serum, and may define a novel kind of response element. By joining this tenascin-C sequence to the SV40 promoter of a reporter plasmid, its activity can be transferred to the heterologous promoter. We propose that the tenascin-C promoter is directly or indirectly activated in fibroblasts generating and experiencing mechanical stress within a restrained collagen matrix. This may be an important aspect of the regulation of tenascin-C expression during embryogenesis as well as during wound healing and other regenerative and morphogenetic processes.

Ipsi- and contralateral commissural growth cones react differently to the cellular environment of the ventral zebrafish spinal cord

Early commissural axons in the zebrafish spinal cord extend along a pathway consisting of a ventrally directed ipsilateral, a contralateral diagonal, and a contralateral longitudinal segment. The midline floor plate cell is one important cue at the transition from the ipsilateral to the contralateral pathway segments. In order to identify additional guidance cues, the interactions between commissural growth cones and their substrates were examined at the electron microscopic level in the different pathway segments. The growth cones extended near the superficial margin of the spinal cord, within filopodial reach of three bilateral longitudinal axon pathways that were ignored irrespective of whether other axons were already present. Ultimately the commissural growth cones pioneered an additional independent longitudinal pathway in the dorsolateral spinal cord. Neuroepithelial cells were extensively contacted in the lateral marginal zone of the dorsal spinal cord and are thus in a position to contribute to the establishment of the longitudinal commissural pathway segment. The extent of contact with neuroepithelial cells in the ventral spinal cord was dependent on whether commissural growth cones had already crossed the ventral midline: ipsilateral, but not contralateral, growth cones showed extensive contacts with neuroepithelial processes and minor contacts with the basal lamina. In marked contrast, commissural growth cones that had already crossed the ventral midline and entered the diagonal pathway segment showed major appositions to the basal lamina. Extensive contact with the basal lamina was first established in the ventral midline region, where crossing growth cones always inserted between the basal lamina and the base of the midline floor plate cells. This indicates that a change occurs in the response characteristics of commissural growth cones as they cross the ventral midline of the spinal cord. Such a change could help to explain why the growth cones extend first toward but then away from the ventral midline.

Characterization of Pax-6 and Hoxa-1 binding to the promoter region of the neural cell adhesion molecule L1

The neural cell adhesion molecule L1, a member of the immunoglobulin superfamily, mediates cell interactions in the developing and regenerating nervous system of mammals and is also detectable in the immune system and in the epithelia of intestine, skin, lung, and kidney. This diverse pattern of expression begs the question as to the regulatory mechanisms underlying transcription of the L1 gene. We demonstrate here that the paired domain and homeodomain containing Pax-6 protein binds to three different sites in the promoter region of the L1 gene. The promoter proximal binding site is also recognized by Hoxa-1 and lies approximately 60 bp upstream from the transcription start site only few base pairs upstream of a putative binding site for the TFII-I transcription initiation factor. On the basis of this sequence, we have characterized the binding of Pax-6 and explored two modes of its DNA binding activities.

Regulation in vitro of an L-CAM enhancer by homeobox genes HoxD9 and HNF-1

Previous studies have shown that in vitro expression of the neural cell adhesion molecule (N-CAM) can be regulated by the products of homeobox genes HoxB9, -B8, and -C6. N-CAM is a Ca(2+)-independent immunoglobulin-related CAM that plays an important role in neural development. In the present study, we investigated whether the liver cell adhesion molecule (L-CAM) a member of the Ca(2+)-dependent CAM family (cadherins) is also regulated by homeobox-containing genes. In transient cotransfection experiments of NIH 3T3 cells, we observed that both HoxD9 and liver-enriched POU-homeodomain transcription factor, HNF-1, activated chloramphenicol acetyltransferase gene reporter constructs containing the L-CAM promoter and an enhancer present in the second intron of the chicken L-CAM gene. Using electrophoretic mobility-shift assays, we found that components of cell extracts from NIH 3T3 cells transfected with HoxD9 bound to a small region of the L-CAM enhancer having a consensus sequence that is a putative binding site for HNF-1. Components of extracts from the chicken hepatoma cell line LMH that had been transfected with an HNF-1 expression vector also bound to this same site. In nuclear run-on experiments with nuclei from LMH cells that were transfected with expression vectors for HoxD9 or HNF-1, L-CAM RNA levels were increased 33-fold and 4-fold respectively. Using the same run-on procedure, it was confirmed that nuclei prepared from normal embryonic chicken liver cells expressed the RNAs for HoxD9, HNF-1, and L-CAM. Taken together with previous observations, these data raise the possibility that homeobox-containing genes will have a widespread role in the place-dependent expression of CAMs belonging both to immunoglobulin-related and to cadherin families.

Targeted disruption of the even-skipped gene, evx1, causes early postimplantation lethality of the mouse conceptus

Implantation within the mammalian uterus elicits dramatic changes in the growth, differentiation, and morphogenesis of the conceptus. This process is interrupted in mice carrying a targeted disruption of the murine evx1 gene, a homolog of the Drosophila even-skipped (eve) gene. Upon implantation, presumptive evx1- homozygotes elicit a decidual response, invade the uterine epithelium, and attach to the basement membrane between uterine stroma and epithelium, but fail to differentiate extraembryonic tissues or to form egg cylinders prior to resorption. Retrograde analysis of embryo genotypes demonstrates that homozygotes could be isolated as free-floating blastocysts but not as gastrulating egg cylinders. Homozygous mutant blastocysts appeared normal and, when grown in vitro, attach, proliferate, and form trophoblastic giant cells surrounding a growing inner cell mass before rapidly degenerating. In situ hybridization analysis demonstrates evx1 gene expression within the visceral endoderm after implantation and prior to gastrulation, at a time in which the mutant phenotype is first detected.

Role of epidermal-dermal tissue interactions in regulating tenascin expression during development of the chick scutate scale

During normal chicken development tenascin begins to accumulate in the dermis of anterior metatarsal skin at the time of scutate scale ridge formation, and is localized in a distinct pattern along the outer scale surface. Anterior metatarsal skin from scaleless (sc/sc) embryos, which do not form scutate scales, begins to accumulate tenascin 4 days later than normal skin. This study shows that normal and scaleless anterior metatarsal dermis accumulate the same tenascin isoforms and undergo the same isoform changes in the post-hatch period, but there is less tenascin accumulated in scaleless dermis and there is no pattern to its distribution. In both normal and scaleless anterior metatarsal skin, tenascin mRNA is localized in the dermis and is distributed in the same way as the protein. Thus, scaleless skin is defective in the ability to accumulate appropriate amounts of tenascin and to maintain the tenascin in the patterned manner of normal. Recombinant skin cultures show that epidermal-dermal interactions are required for tenascin accumulation. The dermis specifies the way that tenascin is organized, but interaction with epidermis is required to maintain this organization. The epidermal role appears to be permissive because in heterotypic recombinants, neither scaleless anterior metatarsal epidermis nor normal footpad epidermis changes the way that tenascin appears in the normal anterior metatarsal dermis; and in reciprocal recombinants, normal anterior metatarsal epidermis does not change the way tenascin is accumulated in either scaleless anterior metatarsal dermis or normal footpad dermis.

Specification and segmentation of the paraxial mesoderm

Somite formation in the mouse embryo begins with the recruitment of mesenchymal cells into the paraxial mesoderm. Cells destined for the paraxial mesoderm are recruited from a progenitor population found first in the embryonic ectoderm and later in the primitive streak and the tail bud. Experimental evidence suggests that the allocation of precursor cells to different mesodermal lineages may be related to the site at which the cells ingress through the primitive streak. An increasing number of genes, such as those encoding growth factor and transcription factors, are now known to be expressed in the primitive streak. It is not known whether the specification of mesodermal cell fate has any relationship with the activity of genes that are expressed in the restricted cell populations of the primitive streak. Somitomeres, which are spherical clusters of mesenchymal cells in the presomitic mesoderm, presage the segmentation of somites in the paraxial mesoderm. The somitomeric organization denotes a pre-pattern of segmentation that defines the physical boundary and the bilateral symmetry of the mesodermal segments in the body axis. The establishment of new somitomeres seems to require the interaction of a resident cell population in the presomitic mesoderm and the incoming primitive streak cells. Cell mixing, which occurs in the somitomeres prior to somite segmentation, poses problems in understanding the developmental role of the somitomere and the real significance of the partitioning of the node-derived and primitive streak-derived cells in the mesodermal segments. In the presomitic mesoderm, the expression of some genes that encode transcription factors, growth factors or tyrosine kinase receptor, and the localization of certain cell adhesion molecules are closely associated with distinct morphogenetic events, such as cell clustering in the presomitic mesoderm and the formation of epithelial somites. There is, however, very little direct relationship between the spatial pattern of gene expression and the somitomeric organization in the presomitic mesoderm. Results of somite transplantation experiments suggest that both the segmental address and the morphogenetic characteristics of the somite may be determined during somite segmentation. Regional identity of the paraxial mesodermal segment is conferred by the expression of a combination of Hox genes in the sclerotome and probably other lineage-specific genes that are subject to imprinting. Superimposed on the global metameric pattern, two orthogonal polarities of cell differentiation are endowed in each mesodermal segment. The rostro-caudal polarity is established prior to somite segmentation. This polarity is later manifested by the subdivision of the sclerotome and the alliance of the neural crest cells and motor axons with the rostral half-somite.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell adhesion alters gene transcription in chicken embryo brain cells and mouse embryonal carcinoma cells

To determine whether changes in gene expression occur in embryonic cells as a consequence of changes in cellular aggregation, chicken embryo brain (CEB) cells isolated from 8-day embryos were allowed to aggregate or prevented from aggregating by treatment with anti-neural cell adhesion molecule (N-CAM) Fab' fragments. A subtractive hybridization cloning strategy was employed to identify genes that might show different levels of expression in the two populations of cells. In addition, the transcription rates of a number of genes specifying CAMs and transcription factors were directly estimated by using nuclear run-off transcription assays. The transcription rates of several genes, including those encoding N-CAM, Ng-CAM, alpha-N-catenin, HoxA4 (Hox1.4), a fatty acid-binding protein, and a subunit of the mitochondrially encoded cytochrome-c oxidase enzyme decreased upon CEB cell aggregation. The transcription rates of several previously unidentified genes either increased or decreased upon aggregation, while the transcription of other genes remained unchanged. The transcription rate of the N-CAM gene was 3.3-fold higher in dissociated than in aggregated CEB cells. This rate of transcription also increased when the brain tissue was dissociated into single cells and the increased rate was maintained by keeping the cells dissociated in the presence of Fab' fragments of antibodies to N-CAM. Decreased transcription rates of the N-CAM gene were also observed upon aggregation of P19 cells, a mouse embryonal carcinoma cell line. Primary chicken embryo liver cells, which aggregate primarily by calcium-dependent adhesion mechanisms, did not show changes in the N-CAM gene or in the other genes whose transcription rates changed in CEB cells and P19 cells. These observations suggest that the types of genes regulated by cell aggregation include those for CAMs themselves as well as for transcription factors that may control the expression of CAMs and other molecules significant for morphogenesis.

A Drosophila homologue of human Sp1 is a head-specific segmentation gene

Segmentation in Drosophila is based on a cascade of hierarchical gene interactions initiated by maternally deposited morphogens that define the spatially restricted domains of gap gene expression at blastoderm (reviewed in ref. 1). Although segmentation of the embryonic head is morphologically obscured, the repeated patterns of expression of the segment polarity genes reflect the formation of seven head segments; two of these depend on the segmentation and homeotic genes used in the trunk, whereas the others form as a result of the activity of the head-specific genes orthodenticle (otd), empty spiracles (ems) and buttonhead (btd). The genes ems and otd encode homeodomain proteins, suggesting that they may function as transcription factors. They are expressed in overlapping stripes in the early embryonic head of Drosophila, and their vertebrate homologues, otx and emx, are expressed in overlapping domains in the anterior central nervous system of the mouse embryo. We show here that btd is expressed in a stripe covering the head analgen of the segments affected in btd lack-of-function mutants and that btd encodes a zinc-finger-type transcription factor with sequence and functional similarity to the prototype mammalian transcription factor Sp1 (ref. 9). When expressed in the spatial pattern of btd, a transgene providing Sp1 activity can support development of the mandibular segment in the head of btd mutant embryos. A ubiquitous transcription factor from humans can therefore replace an essential component of the genetic circuitry required to specify the development of a particular head segment in the fly.

Neurotrophic activity of the Antennapedia homeodomain depends on its specific DNA-binding properties

In previous reports we have demonstrated that the 60-aa peptide corresponding to the homeodomain of the Drosophila protein Antennapedia (pAntp) translocates through the membrane of neurons in culture, accumulates in neuronal nuclei, and promotes neurite growth. To analyze the importance of specific pAntp DNA-binding properties in this phenomenon we have constructed three mutant versions of pAntp that differ in their ability to translocate through the membrane and to bind specifically the cognate sequence for homeodomains present in the promoter of HoxA5. We demonstrate that removing two hydrophobic residues of the third helix inhibits pAntp internalization and suppresses its neurotrophic activity. We also show that pAntp neurotrophic activity is lost when mutations are introduced in positions preserving its penetration and nuclear accumulation but abolishing its capacity to bind specifically the cognate DNA-binding motif for homeoproteins. Our results strongly suggest that pAntp neurotrophicity requires both its internalization and its specific binding to homeobox cognate sequences. We propose that homeoproteins might regulate important events in the morphological differentiation of the postmitotic neuron.

Position, guidance, and mapping in the developing visual system

Positional identity in the visual system affects the topographic projection of the retina onto its central targets. In this review we discuss gradients and positional information in the retina, when and how they arise, and their functional significance in development. When the axons of retinal ganglion cells leave the eye, they navigate through territory in the central nervous system that is rich in positional information. We review studies that explore the navigational cues that the growth cones of retinal axons use to orient towards their target and organize themselves as they make this journey. Finally, these axons arrive at their central targets and make a precise topographic map of visual space that is crucial for adaptive visual behavior. In the last section of this review, we examine the topographic cues in the tectum, what they are, when, and how they arise, and how retinal axons respond to them. We also touch on the role of neural activity in the refinement of this topography.

Ectopic neural expression of a floor plate marker in frog embryos injected with the midline transcription factor Pintallavis

The floor plate, a cell group that develops at the midline of the neural plate in response to inductive signals from the notochord, has been implicated in the control of dorsoventral neural pattern. The frog Pintallavis gene, encoding a member of the HNF-3/fork head transcription factor family, is expressed in the notochord and in midline neural plate cells that give rise to the floor plate. To examine whether Pintallavis might be involved in regulating the differentiation of the floor plate, we ectopically expressed Pintallavis by injection of synthetic mRNA into two-cell frog embryos. Injection of Pintallavis mRNA resulted in the ectopic expression of F-spondin, a gene encoding a floor plate-specific adhesion molecule, at the dorsal midline of the neural tube. The expression of Pintallavis in midline cells may therefore contribute to the establishment of the floor plate fate.

Pair-rule gene expression in the somitic stage chick embryo: association with somite segmentation and border formation

In vertebrates, metameric organization is high-lighted by the formation of somites from mesenchymal cells of the segmental plate which then differentiate into dermamyotomal and sclerotomal tissues. The resegmentation of the sclerotome into rostral and caudal halves follows, coincident with the production of specific extracellular matrix molecules at the abutment of these two cell types. Ultimately, cells from the caudal sclerotome migrate ventrally and contribute to the chondrogenic prevertebrae. The objective of this work is to investigate the molecular steps regulating these events. Our study is focused on the paired-box containing genes, which have been implicated in delineating boundaries early in development. A chick embryo system, which is readily accessible to manipulation and observation during early development, is used in this study. We have identified the existence of the paired-box motif in the chicken genome by polymerase chain reaction and hybridization with the mouse Pax 1 paired-box sequence. Expression of paired-box genes occurs early in development as shown by Northern analysis, and is localized by in situ hybridization to the edge of each somite, a patch at the central core of each somite, and the periphery of the neural tube. This specific spatial pattern of expression is consistent with the hypothesis that the pair-rule genes function as effecters of border formation in the early embryo. Moreover, the patch of positive cells at the center of a resegmenting somite appear to migrate ventrally, and may contribute to structures of the prevertebrae. These findings are relevant to our understanding of the mechanism of somite resegmentation and implicate the involvement of pair-rule genes in the process.

Cooperative DNA binding of the human HoxB5 (Hox-2.1) protein is under redox regulation in vitro

The human HoxB5 (Hox-2.1) gene product is a sequence-specific DNA binding protein. Cooperative interactions stabilize in vitro DNA binding of the HoxB5 protein to tandem binding sites by at least 100-fold relative to binding to a single site. The HoxB5 homeodomain is sufficient for sequence-specific DNA binding but not for cooperative DNA binding. Here we report that the additional protein sequence required for cooperativity is a small domain adjacent to the homeodomain on the amino-terminal side. We further show that cooperative DNA binding is under redox regulation. The HoxB5 protein binds to DNA in vitro both when oxidized or reduced but binds cooperatively only when oxidized. Mutational analysis has revealed that the cysteine residue in the turn between homeodomain helices 2 and 3 is necessary for cooperative binding and redox regulation. The enhanced DNA binding of oxidized HoxB5 protein is the opposite of the redox regulation reported for other mammalian transcription factors such as Fos, Jun, USF, NF-kappa B, c-Myb, and v-Rel, in which oxidation of cysteine residues inhibits DNA binding. Thus, specific oxidation of nuclear proteins is a potential regulatory mechanism that can act to either decrease or increase their DNA binding activity.

A six-armed, tenascin-like protein extracted from the Porifera Oscarella tuberculata (Homosclerophorida)

A six-armed complex could be extracted from the marine sponge Oscarella tuberculata by a two-step incubation, first in Tris-buffered saline containing EDTA, then in Tris-buffered saline containing urea. The crude extracts contained, in addition, collagen fibrils with surface filaments, individual filaments resembling collagen molecules, and laminin/nidogen-like complexes. The extracts were subsequently purified by gel-filtration chromatography and low-pressure ion-exchange chromatography on DEAE-cellulose, then analyzed by SDS/PAGE and immunoblotting methods. A glycoprotein of high molecular mass was isolated, and reduced to subunits of 230 kDa. After transfer to nitrocellulose, both the complex and its subunits were faintly stained by antibodies against amphibian tenascin. Electron microscopy of the purified extracts demonstrated the presence of a large population of tenascin-like molecules and complexes of several molecules interacting with each other by their central globule.

The making of a feather: homeoproteins, retinoids and adhesion molecules

We have been using feather development as a model for understanding the molecular basis of pattern formation and to explore the roles of homeoproteins, retinoids and adhesion molecules in this process. Two kinds of homeobox (Hox) protein gradients in the skin have been identified: a 'microgradient' within a single feather bud and a 'macrogradient' across the feather tract. The asynchronous alignment of different Hox macrogradients establishes a unique repertoire of Hox expression patterns in skin appendages within the integument, designated here as the 'Hox codes of skin appendages'. It is hypothesized that these Hox codes contribute to the phenotypic determination of skin appendages. High doses of retinoic acid cause a morphological transformation between feather and scale, while low doses of retinoic acid cause an alteration of the axial orientation of skin appendages. We have tested the ability of molecules directly involved in the feather formation process to mediate the action of the Hox codes, and surmise that adhesion molecules are potential candidates. Using specific Fabs to suppress the activity of adhesion molecules, we have found that L-CAM is involved in the formation of the hexagonal pattern, N-CAM is involved in mediating dermal condensations, tenascin is involved in feather bud growth and elongation, and integrin beta-1 is essential for epithelial-mesenchymal interactions. More work is in progress to fully understand the molecular pathways regulating the feather formation process.

Cooperative DNA binding of the human HoxB5 (Hox-2.1) protein is under redox regulation in vitro

The human HoxB5 (Hox-2.1) gene product is a sequence-specific DNA binding protein. Cooperative interactions stabilize in vitro DNA binding of the HoxB5 protein to tandem binding sites by at least 100-fold relative to binding to a single site. The HoxB5 homeodomain is sufficient for sequence-specific DNA binding but not for cooperative DNA binding. Here we report that the additional protein sequence required for cooperativity is a small domain adjacent to the homeodomain on the amino-terminal side. We further show that cooperative DNA binding is under redox regulation. The HoxB5 protein binds to DNA in vitro both when oxidized or reduced but binds cooperatively only when oxidized. Mutational analysis has revealed that the cysteine residue in the turn between homeodomain helices 2 and 3 is necessary for cooperative binding and redox regulation. The enhanced DNA binding of oxidized HoxB5 protein is the opposite of the redox regulation reported for other mammalian transcription factors such as Fos, Jun, USF, NF-kappa B, c-Myb, and v-Rel, in which oxidation of cysteine residues inhibits DNA binding. Thus, specific oxidation of nuclear proteins is a potential regulatory mechanism that can act to either decrease or increase their DNA binding activity.

Specific regulation of immediate early genes by patterned neuronal activity

Electrical activity shapes development of the nervous system, presumably in part by regulating gene expression. A set of regulatory genes, immediate early genes (IEGs), which are responsive to a number of extrinsic cellular stimuli have been proposed to play a role in coupling such activity to gene expression. Using a semiquantitative polymerase chain reaction assay, we show that in dissociated mouse dorsal root ganglion neurons the expression of two IEGs, c-fos and nur/77, is differentially sensitive to patterns of electrical stimulation. Differences in c-fos activation did not correlate with the peak intracellular calcium [Ca++]i produced by the different stimulation patterns or with residual [Ca++]i following stimulation. However, the net increase in [Ca++]i (calcium time integral) was greater for the pulsed stimulus that activated c-fos (6 impulses/min), compared to the ineffective stimulus (12 impulses/2 min). This system of genes seems suited to mediating the coupling between electrical activity and other functional genes.

Binding and transcriptional activation of the promoter for the neural cell adhesion molecule by HoxC6 (Hox-3.3)

Scores of homeobox gene-encoded transcription factors are expressed in a definite spatiotemporal pattern during embryogenesis and regulate a series of as yet unidentified target genes to help coordinate the morphogenetic process. We have suggested that homeobox gene products modulate the expression of adhesion molecule genes and have shown in cotransfection experiments that the promoters for the neural cell adhesion molecule (N-CAM) and cytotactin/tenascin genes respond to cues from different homeobox-containing genes. In this study, we show that the HoxC6 (Hox-3.3)-encoded homeoprotein binds to a DNA sequence in the N-CAM promoter CCTAATTATTAA, designated homeodomain binding site I (HBS-I). To test whether HoxC6 regulated N-CAM promoter activity, we cotransfected the Long and Short reading frame variants of Xenopus HoxC6 (CMV-HoxC6-L and CMV-HoxC6-S) driven by the human cytomegalovirus (CMV) promoter together with a chloramphenicol acetyltransferase (CAT) reporter gene driven by the mouse N-CAM promoter (N-CAM-Pro-CAT). Cotransfection of NIH 3T3 cells with either of the CMV-HoxC6 expression vectors stimulated N-CAM promoter-driven CAT expression. A 47-bp region from the N-CAM promoter that included HBS-I and an adjacent potential HBS, HBS-II, conferred HoxC6 regulation on a simian virus 40 minimal promoter. HBS-I was sufficient for transactivation of the minimal promoter by CMV-HoxC6-S. However, transcriptional activation by CMV-HoxC6-L required both HBS-I and HBS-II, inasmuch as mutation of either HBS-I, HBS-II, or both motifs abolished the response. These studies suggest that HBS-I is a target site for binding and transcriptional control of the N-CAM promoter by homeoproteins, although accessory DNA sequences (such as HBS-II) may also be required. Together with previous studies, these results support the notion that N-CAM gene expression may be controlled by different combinations of homeoproteins that appear in a place-dependent manner during embryogenesis.

Later embryogenesis: regulatory circuitry in morphogenetic fields

The subject of this review is the nature of regulatory processes underlying the spatial subdivision of morphogenetic regions in later embryogenesis. I have applied a non-classical definition of morphogenetic field, the progenitor field, which is a region of an embryo composed of cells whose progeny will constitute a given morphological structure. An important feature of such fields is that they have sharp spatial boundaries, across which lie cells whose progeny will express different fates. Two examples of the embryonic specification and development of such fields are considered. These are the formation of the archenteron in the sea urchin embryo and the formation of dorsal axial mesoderm in the Xenopus embryo. From these and a number of additional examples, from vertebrate, Drosophila, Caenorhabditis elegans and sea urchin embryos, it is concluded that the initial formation of the boundaries of morphogenetic progenitor fields depends on both positive and negative transcription control functions. Specification of morphogenetic progenitor fields, organization of the boundaries and their subsequent regionalization or subdivision are mediated by intercellular signaling. Genes encoding regionally expressed transcription factors that are activated in response to intercell signaling, and that in turn mediate signaling changes downstream, appear as fundamental regulatory circuit elements. Such [signal-->transcription factor gene-->signal] circuit elements appear to be utilized, often repetitively, in many different morphogenetic processes.

Homeobox genes and skin development: a review

Homeobox (HOX) genes are a gene family that encode information critical for the normal embryologic development of many different organisms, including vertebrates. HOX genes encode transcriptional regulatory factors that bind to multiple different genes and thereby determine the developmental fate of a cell. The role of HOX genes in the development of skin is undetermined but, based on information from other organisms and recent experimental data from skin models, it is likely that this class of genes is important for the normal development of skin adnexae, pigmentary system, and stratified epidermis during embryogenesis. The purpose of this review is to briefly summarize what is known about HOX genes and to familiarize the reader with recent insights into how HOX genes may function in skin development.

Characterization of the Xenopus Hox 2.4 gene and identification of control elements in its intron

We report on the Xenopus homolog of the Hox 2.4 gene. This gene occupies the next to 5'-most position in the Xenopus Hox2 complex. Hox 2.4 RNA is first detected at the early neurula stage, reaching a peak at the early tailbud stage, and is localized in the middle and posterior portions of the embryos. Antibodies raised against a fusion protein show expression of Hox 2.4 protein in Xenopus embryos in a band located in the mid spinal cord. Thus, the protein is expressed in a narrower domain than that of Hox 2.4 mRNA. The Xenopus Hox 2.4 antibody cross-reacts readily with mouse embryonic tissue, where the protein is detected in migrating neural crest cells, the dorsal portion of the spinal cord, somites, lateral plate mesoderm, and in the forelimb bud. The Xenopus Hox 2.4 intron shares considerable sequence identity with the intron in the mouse homolog. A reporter gene containing an element from this intron which can bind homeodomain proteins is activated following microinjection into Xenopus embryos. The short distance between the end of the Hox 2.4 cDNA and the start site of the neighboring gene in the complex raises the possibility that this transcriptional element might be shared by two Hox genes.

Specific binding of cytotactin to sulfated glycolipids

The binding of the glial glycoprotein, cytotactin, to a variety of purified glycolipids was examined. Clear-cut evidence was found for binding of radiolabeled cytotactin to sulfatides purified from bovine brain, but the molecule did not bind to gangliosides or cerebrosides. The sulfatide binding was sensitive to pH and ionic strength and was dependent on the presence of divalent cations. Binding was inhibited by purified unlabeled cytotactin, by polyclonal antibodies to cytotactin, and by several monosaccharides and polysaccharides. It was not inhibited by fibronectin, a chondroitin sulfate proteoglycan, or the HNK-1 monoclonal antibody, all of which are known to bind to cytotactin. These findings raise the possibilities that sulfated glycolipids may function as cellular receptors for cytotactin and that binding by sulfatides may modulate the varied effects of cytotactin on cellular processes.

Characterization of multiple adhesive and counteradhesive domains in the extracellular matrix protein cytotactin

The extracellular matrix molecule cytotactin is a multidomain protein that plays a role in cell migration, proliferation, and differentiation during development. To analyze the structure-function relationships of the different domains of this glycoprotein, we have prepared a series of fusion constructs in bacterial expression vectors. Results obtained using a number of adhesion assays suggest that at least four independent cell binding regions are distributed among the various cytotactin domains. Two of these are adhesive; two others appear to be counteradhesive in that they inhibit cell attachment to otherwise favorable substrates. The adhesive regions were mapped to the fibronectin type III repeats II-VI and the fibrinogen domain. The morphology of the cells plated onto these adhesive fragments differed; the cells spread on the fibronectin type III repeats as they do on fibronectin, but remained round on the fibrinogen domain. The counteradhesive properties of the molecule were mapped to the EGF-like repeats and the last two fibronectin type III repeats, VII-VIII. The latter region also contained a cell attachment activity that was observed only after proteolysis of the cells. Several cell types were used in these analyses, including fibroblasts, neurons, and glia, all of which are known to bind to cytotactin. The different domains exert their effects in a concentration-dependent manner and can be inhibited by an excess of the soluble molecule, consistent with the hypothesis that the observed properties are mediated by specific receptors. Moreover, it appears that some of these receptors are restricted to particular cell types. For example, glial cells bound better than neurons to the fibrinogen domain and fibroblasts bound better than glia and neurons to the EGF fragment. These results provide a basis for understanding the multiple activities of cytotactin and a framework for isolating different receptors that mediate the various cellular responses to this molecule.

Mice develop normally without tenascin

Tenascin, an extracellular matrix protein, is expressed in an unusually restricted pattern during embryogenesis and has been implicated in a variety of morphogenetic phenomena. To directly assess the function of tenascin in vivo, we generated mutant mice in which the tenascin gene was nully disrupted by replacing it with the lacZ gene. In mutant mice, lacZ was expressed in place of tenascin, and no tenascin product was detected. Homozygous mutant mice were, however, obtained in accordance with Mendelian laws, and both females and males produced offspring normally. No anatomical or histological abnormalities were detected in any tissues, and no major changes were observed in distribution of fibronectin, laminin, collagen, and proteoglycan. The existence of these mutant mice, lacking tenascin yet phenotypically normal, casts doubt on the theory that tenascin plays and essential role in normal development.