Identification and characterization of the promoter for the cytotactin gene

Monogenic Causes of Low-Frequency Non-Syndromic Hearing Loss

BACKGROUND

Low-frequency non-syndromic hearing loss (LFNSHL) is a rare form of hearing loss (HL). It is defined as HL at low frequencies (≤2,000 Hz) resulting in a characteristic ascending audiogram. LFNSHL is usually diagnosed postlingually and is progressive, leading to HL affecting other frequencies as well. Sometimes it occurs with tinnitus. Around half of the diagnosed prelingual HL cases have a genetic cause and it is usually inherited in an autosomal recessive mode. Postlingual HL caused by genetic changes generally has an autosomal dominant pattern of inheritance and its incidence remains unknown.

SUMMARY

To date, only a handful of genes have been found as causing LFNSHL: well-established WFS1 and, reported in some cases, DIAPH1, MYO7A, TNC, and CCDC50 (respectively, responsible for DFNA6/14/38, DFNA1, DFNA11, DFNA56, and DFNA44). In this review, we set out audiological phenotypes, causative genetic changes, and molecular mechanisms leading to the development of LFNSHL.

KEY MESSAGES

LFNSHL is most commonly caused by pathogenic variants in the WFS1 gene, but it is also important to consider changes in other HL genes, which may result in similar audiological phenotype.

Transcriptional regulation of tenascin genes

Extracellular matrix proteins of the tenascin family resemble each other in their domain structure, and also share functions in modulating cell adhesion and cellular responses to growth factors. Despite these common features, the 4 vertebrate tenascins exhibit vastly different expression patterns. Tenascin-R is specific to the central nervous system. Tenascin-C is an “oncofetal” protein controlled by many stimuli (growth factors, cytokines, mechanical stress), but with restricted occurrence in space and time. In contrast, tenascin-X is a constituitive component of connective tissues, and its level is barely affected by external factors. Finally, the expression of tenascin-W is similar to that of tenascin-C but even more limited. In accordance with their highly regulated expression, the promoters of the tenascin-C and -W genes contain TATA boxes, whereas those of the other 2 tenascins do not. This article summarizes what is currently known about the complex transcriptional regulation of the 4 tenascin genes in development and disease.

Identification of specific protein/E-box-containing DNA complexes: lessons from the ubiquitously expressed USF transcription factors of the b-HLH-LZ super family

In order to determine how gene expression is regulated in response to environmental cues, it is necessary to identify the specific interaction between transcription factors and their cognate cis-regulatory DNA elements. Here we have out-lined electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) protocols to define in vitro and in vivo USFs specific interacting sequences. The proposed procedures have been optimized for the USFs transcription factor family, allowing the identification of USF-specific targets.

The role of cell adhesion molecules in ischemic epididymal injury

OBJECTIVE

The aim of this study was to investigate the role of adhesion molecules in epididymal injury induced by ischemia-reperfusion (I-R) in the rats.

STUDY DESIGN

About 20 male Sprague-Dawley rats were separated into two groups. A sham operation was performed in group 1 (control). In group 2 (I-R), following 6 h of unilateral spermatic cord torsion, 1-h detorsion of the testis was performed. Then, epididymides were removed to measure the tissue levels of malondialdehyde (MDA) and to make histological examination.

RESULTS

Malondialdehyde values increased in group 2. In group 2, the rats demonstrated significant disorganization of the epithelium and loss of microvilli in the epididymal tissue. No abnormal microscopic findings of the epididymis of the rats in the control group. The tenascin expression in the interstitial area of the epididymis was intense in group 2. Intercellular adhesion molecule-1 (ICAM-1) expression by intense brown staining was seen along the basement membrane in epididymal tissue from I-R group rats. The microvillus sites of the epithelia in I-R group were stained mildly by lectin.

CONCLUSION

The increased expression of adhesion molecules found in epididymal injury induced during of postischemic reperfusion may implicate importance of inflammatory infiltration.

The role of cell adhesion molecules in ischemic epididymal injury

OBJECTIVE

The aim of this study was to investigate the role of adhesion molecules in epididymal injury induced by I-R in the rats.

STUDY DESIGN

A total of 20 male Sprague-Dawley rats were separated into two groups. A sham operation was performed in group 1 (control). In group 2 (I-R), following 6 h of unilateral spermatic cord torsion, 1-h detorsion of the testis was performed. Then, epididymides were removed to measure the tissue levels of malondialdehyde (MDA) and to make histological examination.

RESULTS

MDA values increased in the group 2. In the group 2 rats demonstrated significant disorganization of the epithelium and loss of microvilli in the epididymal tissue. No abnormal microscopic findings of the epididymis of the rats in the control group. The tenascin expression in the interstitial area of the epididymis was intense in the group 2. ICAM-1 expression by intense brown staining was seen along the basement membrane in epididymal tissue from I to R group rats. The microvillus sites of the epithelia in I-R group were stained mildly by lectin.

CONCLUSION

The increased expression of adhesion molecules found in epididymal injury induced during postischemic reperfusion might implicate importance of inflammatory infiltration.

Identification and Kinetics Analysis of a Novel Heparin-binding Site (KEDK) in Human Tenascin-C

The interaction between tenascin-C (TN-C), a multi-subunit extracellular matrix protein, and heparin was examined using a surface plasmon resonance-based technique on a Biacore system. The aims of the present study were to examine the affinity of fibronectin type III repeats of TN-C fragments (TNIII) for heparin, to investigate the role of the TNIII4 domains in the binding of TN-C to heparin, and to delineate a sequence of amino acids within the TNIII4 domain, which mediates cooperative heparin binding. At a physiological salt concentration, and pH 7.4, TNIII3-5 binds to heparin with high affinity (K(D) = 30 nm). However, a major heparin-binding site in TNIII5 produces a modest affinity binding at a K(D) near 4 microm, and a second site in TNIII4 enhances the binding by several orders of magnitude, although it was far too weak to produce an observable binding of TNIII4 by itself. Moreover, mutagenesis of the KEDK sequence in the TNIII4 domain resulted in the significant reduction of heparin-binding affinity. In addition, residues in the KEDK sequences are conserved in TN-C throughout mammalian evolution. Thus the structure-based sequence alignment, mutagenesis, and sequence conservation data together reveal a KEDK sequence in TNIII4 suggestive of a minor heparin-binding site. Finally, we demonstrate that TNIII4 contains binding sites for heparin sulfate proteoglycan and enhances the heparin sulfate proteoglycan-dependent human gingival fibroblast adhesion to TNIII5, thus providing the biological significance of heparin-binding site of TNIII4. These results suggest that the heparin-binding sites may traverse TNIII4-5 and thus require KEDK in TNIII4 for optimal heparin-binding.

Ets-1 regulates TNF-alpha-induced matrix metalloproteinase-9 and tenascin expression in primary bronchial fibroblasts

Increased subepithelial deposition of extracellular matrix proteins is a key feature in bronchial asthma. Matrix metalloproteinase-9 (MMP-9) is a proteolytic enzyme that degrades the extracellular matrix. Tenascin is an extracellular matrix glycoprotein that is abundant in thickened asthmatic subbasement membrane. The expression of MMP-9 and tenascin reflects disease activity in asthma and airway remodeling. The molecular mechanisms regulating the expression of these proteins remain unknown. Both MMP-9 and tenascin promoters contain an Ets binding site, suggesting control by Ets-1. Thus, we hypothesized that Ets-1 expression is increased in asthma and that it contributed to enhanced MMP-9 and tenascin expression. To test this hypothesis, we determined the expression of Ets-1 in bronchial biopsies obtained from asthmatic subjects and determined the expression of Ets-1, MMP-9, and tenascin by bronchial fibroblasts activated ex vivo. We observed that nuclear extracts from TNF-alpha-activated fibroblasts showed increased Ets-binding activity. In addition, TNF-alpha-activated fibroblasts had increased expression of Ets-1 mRNA and protein, which preceded an increase in MMP-9 and tenascin mRNA. Furthermore, treatment of fibroblasts with Ets-1 antisense oligonucleotides down-regulated TNF-alpha-induced Ets-1, MMP-9, and, to a lesser extent, tenascin protein expression or activity. Taken together, these data demonstrate that TNF-alpha increases MMP-9 and tenascin expression in bronchial fibroblasts via the transcription factor Ets-1, and suggest a role for Ets-1 in airway remodeling in asthma.

The role of ERG (ets related gene) in cartilage development

OBJECTIVE

Based on function and developmental fate, cartilage tissue can be broadly classified into two types: transient (embryonic or growth-plate) cartilage and permanent cartilage. Chondrocytes in transient cartilage undergo terminal differentiation into hypertrophic cells, induce cartilage-matrix mineralization, and eventually disappear and are replaced by bone. On the other hand, chondrocytes in permanent cartilage do not differentiate further, do not become hypertrophic, and persist throughout life at specific sites, including joints and tracheal rings. While many studies have described differences in structure, matrix composition and biological characteristics between permanent and transient cartilage, it is poorly understood how the fates of permanent and transient cartilage are determined. Previous studies demonstrated that chondrocytes isolated from permanent cartilage have the potential to express markers of the mature hypertrophic phenotype once grown in culture, suggesting that cell hypertrophy is an intrinsic property of all chondrocytes and must be actively silenced in permanent cartilage in vivo. These silencing mechanisms, however, are largely unknown. In this paper, we first review nature of chondrocytes in transient and permanent cartilages and then report the cloning and characterization of a novel variant of ets transcription factor chERG, hereafter called C-1-1, which might be involved in regulation of permanent cartilage development.

DESIGN

For cloning of a novel variant of chERG (C-1-1), we isolated RNA from the cartilaginous femur or tibiotarsus of Day 17 chick embryos and processed it for reverse transcription-polymerase chain reaction (RT-PCR) with the primers from sequences upstream and downstream of the 81 and 72 bp segments alternatively-spliced in mammals. For investigation of function of chERG and C-1-1, we over-expressed chERG or C-1-1 in cultured chick chondrocytes or the developing limb of chick embryo using a retrovirus (RCAS) system, and examined the phenotype changes in the infected chondrocytes or the infected limb elements.

RESULTS

C-1-1 is an alternative and novel variant lacking the 27 amino acids segment of chERG that has been reported previously. C-1-1 is preferentially expressed in developing articular cartilage, whereas chERG is preferentially expressed in growth plate cartilage. Growth of articular chondrocytes in culture was accompanied by decreasing C-1-1 expression after several passages, while expression of hypertrophic markers increased. Expression of C-1-1 in cultured chondrocytes inhibited cell hypertrophy, alkaline phosphatase activity, and cartilage matrix mineralization. In contrast, over-expression of chERG promoted chondrocyte maturation and mineralization.

CONCLUSION

Our data demonstrate for the first time that chERG and C-1-1 play distinct roles in skeletogenesis and may have crucial roles in the development and function of transient and permanent cartilages.

Tenascin-C in development and disease: gene regulation and cell function

Tenascin-C (TN-C) is a modular and multifunctional extracellular matrix (ECM) glycoprotein that is exquisitely regulated during embryonic development and in adult tissue remodeling. TN-C gene transcription is controlled by intracellular signals that are generated by multiple soluble factors, integrins and mechanical forces. These external cues are interpreted by particular DNA control elements that interact with different classes of transcription factors to activate or repress TN-C expression in a cell type- and differentiation-dependent fashion. Among the transcriptional regulators of the TN-C gene that have been identified, the homeobox family of proteins has emerged as a major player. Downstream from TN-C, intracellular signals that are relayed via specific cell surface receptors often impart contrary cellular functions, even within the same cell type. A key to understanding this behavior may lie in the dual ability of TN-C-enriched extracellular matrices to generate intracellular signals, and to define unique cellular morphologies that modulate these signal transduction pathways. Thus, despite the contention that TN-C null mice appear to develop and act normally, TN-C biology continues to provide a wealth of information regarding the complex nature of the ECM in development and disease.

Cellular signaling by neural cell adhesion molecules of the immunoglobulin superfamily

Neural cell adhesion molecules (CAMs) of the immunoglobulin superfamily nucleate and maintain groups of cells at key sites during early development and in the adult. In addition to their adhesive properties, binding of CAMs can affect intracellular signaling. Their ability to influence developmental events, including cell migration, proliferation, and differentiation can therefore result both from their adhesive as well as their signaling properties. This review focuses on the two CAMs for which the most information is known, the neural CAM, N-CAM, and L1. N-CAM was the first CAM to be characterized and, therefore, has been studied extensively. The binding of N-CAM to cells leads to a number of signaling events, some of which result in changes in gene expression. Interest in L1 derives from the fact that mutations in its gene lead to human genetic diseases including mental retardation. Much is known about modifications of the L1 cytoplasmic domain and its interaction with cytoskeletal molecules. The study of CAM signaling mechanisms has been assay-dependent rather than molecule-dependent, with particular emphasis on assays of neurite outgrowth and gene expression, an emphasis that is maintained throughout the review. The signals generated following CAM binding that lead to alterations in cell morphology and gene expression have been linked directly in only a few cases. We also review information on other CAMs, giving special consideration to those that are anchored in the membrane by a phospholipid anchor. These proteins, including a form of N-CAM, are presumed to be localized in lipid rafts, membrane substructures that include distinctive subsets of cytoplasmic signaling molecules such as members of the src-family of nonreceptor protein tyrosine kinases. In the end, these studies may reveal that what CAMs do after they bind cells together may have as profound consequences for the cells as the adhesive interactions themselves. This area will therefore remain a rich ground for future studies.

The tenascin family of ECM glycoproteins: structure, function, and regulation during embryonic development and tissue remodeling

The determination of animal form depends on the coordination of events that lead to the morphological patterning of cells. This epigenetic view of development suggests that embryonic structures arise as a consequence of environmental influences acting on the properties of cells, rather than an unfolding of a completely genetically specified and preexisting invisible pattern. Specialized cells of developing multicellular organisms are surrounded by a complex extracellular matrix (ECM), comprised largely of different collagens, proteoglycans, and glycoproteins. This ECM is a substrate for tissue morphogenesis, lends support and flexibility to mature tissues, and acts as an epigenetic informational entity in the sense that it transduces and integrates intracellular signals via distinct cell surface receptors. Consequently, ECM-receptor interactions have a profound influence on major cellular programs including growth, differentiation, migration, and survival. In contrast to many other ECM proteins, the tenascin (TN) family of glycoproteins (TN-C, TN-R, TN-W, TN-X, and TN-Y) display highly restricted and dynamic patterns of expression in the embryo, particularly during neural development, skeletogenesis, and vasculogenesis. These molecules are reexpressed in the adult during normal processes such as wound healing, nerve regeneration, and tissue involution, and in pathological states including vascular disease, tumorigenesis, and metastasis. In concert with a multitude of associated ECM proteins and cell surface receptors that include members of the integrin family, TN proteins impart contrary cellular functions, depending on their mode of presentation (i.e., soluble or substrate-bound) and the cell types and differentiation states of the target tissues. Expression of tenascins is regulated by a variety of growth factors, cytokines, vasoactive peptides, ECM proteins, and biomechanical factors. The signals generated by these factors converge on particular combinations of cis-regulatory elements within the recently identified TN gene promoters via specific transcriptional activators or repressors. Additional complexity in regulating TN gene expression is achieved through alternative splicing, resulting in variants of TN polypeptides that exhibit different combinations of functional protein domains. In this review, we discuss some of the recent advances in TN biology that provide insights into the complex way in which the ECM is regulated and how it functions to regulate tissue morphogenesis and gene expression.

Activation of neurospecific gene expression by antennapedia homeobox peptide

Antennapedia homeobox peptide has been reported to enhance neurite outgrowth and branching. Thus it is of interest to investigate whether antennapedia peptide is capable of modulating the expression of genes related to different events of neuronal development. In this paper we report the enhancement of a 68 KDa neurofilament subunit, choline acetyltransferase and acetylcholinesterase expression in spinal cord neurons, elicited by antennapedia peptide. Modulation of gene expression is different with respect to each gene product analyzed, suggesting a specific action of the peptide on diverse genes controlling different events of neuronal differentiation.

Ets transcription factors cooperate with Sp1 to activate the human tenascin-C promoter

Tenascin-C (TN-C), an extracellular matrix glycoprotein is expressed during embryonic development, but is present only at low levels in normal adult tissues. TN-C is re-expressed during wound healing, fibrotic diseases and in cancer. To better understand the mechanisms that control TN-C gene expression, we examined the regulation of the human TN-C promoter in human fibroblasts. We demonstrate that a short segment of the TN-C promoter between bp -133 and -27 contains three evolutionarily conserved Ets binding sites (EBS). These three EBSs bind in vitro expressed Fli1 protein and mediate transactivation of the TN-C gene by Fli1. Furthermore, two proximal EBSs contribute significantly to basal activity of the TN-C promoter. GABP, which is present in human fibroblast nuclear extracts, interacts with the two proximal EBSs. In addition, several Sp1 and Sp3 binding sites have been located in close proximity to the EBSs within this promoter region. The studies performed in Drosophila cells demonstrate that either Fli1 or GABPalpha+beta1 functionally interact with Sp1 resulting in a synergistic stimulation of the TN-C promoter activity. In conclusion, this study shows for the first time that the TN-C gene is regulated by Ets proteins, which together with Sp1 act as potent activators of TN-C expression.

Domains of tenascin involved in glioma migration

Tenascin (TN) is an extracellular matrix protein found in areas of cell migration during development and expressed at high levels in migratory tumor cells. TN was previously shown to support the attachment and migration of glioma cells in culture. To determine the domains responsible for glioma migration and attachment, we produced recombinant fusion proteins that collectively span the majority of the molecule including its epidermal growth factor-like repeats, fibronectin type III repeats and fibrinogen domain. These domains were tested for their ability to support migration of C6 glioma cells in an aggregate migration assay. A recombinant fusion protein including fibronectin type III (FNIII) repeats 2–6 (TNfn2-6) was the only fragment found to promote migration of C6 glioma cells at levels similar to that promoted by intact TN. Evaluation of smaller segments and individual FNIII repeats revealed that TNfn3 promoted migration and attachment of glioma cells and TNfn6 promoted migration but not attachment. While TNfn3 and TNfn6 promoted migration individually, the presence of both TNfn3 and TNfn6 was required for migration on segments of the FNIII region that included TNfn5. TNfn5 inhibited migration in a dose dependent manner when mixed with TNfn3 and also promoted strong attachment and spreading of C6 glioma cells. TNfn3 and TNfn6 promote cell migration and may function cooperatively to overcome the inhibitory activity of TNfn5. Additional cell attachment studies suggested that both beta1 integrins and heparin may differentially influence the attachment of glioma cells to TN fragments. Together, these findings show that C6 glioma cells integrate their response upon binding to at least three domains within TN.

The genomic structure of the chicken ICSBP gene and its transcriptional regulation by chicken interferon

The chicken interferon consensus sequence binding protein (ChICSBP) gene spans over 9 kb of DNA and consists, as its murine homolog, of nine exons. The first untranslated exon was identified by 5'-RACE technology. The second exon contains the translation initiation codon. Canonical consensus splice sites are found on every exon/intron junction. The introns are generally smaller than their mammalian counterparts. The ChICSBP and ChIRF-1 genes have been mapped by fluorescence in situ hybridization to different microchromosomes. The transcription start site has been mapped by primer extension. Inspection of the DNA sequence of a genomic clone containing the first exon and the region 1700-bp upstream revealed several potential cisregulatory elements of transcription. The ChICSBP mRNA is induced by recombinant ChIFN type I and ChIFN-gamma. A palindromic IFN regulatory element (pIRE) with high sequence homology to gamma activation site (GAS) sequences was functionally required in transient transfection assays for the induction of transcription by ChIFN-gamma.

Multiple promoter elements differentially regulate the expression of the mouse tenascin gene

Tenascin (TN) is an extracellular matrix glycoprotein that is expressed in a characteristic spatiotemporal pattern during development and is up-regulated in the adult during tumorigenesis, wound healing, and nerve regeneration. In previous studies, we identified a promoter within the proximal 250 bp upstream of the mouse TN gene that contains several putative regulatory elements that are conserved among vertebrate TN genes. We have identified four different DNA elements within this promoter and show that they contribute in different ways to TN gene expression in NIH 3T3 fibroblasts, C6 glioma cells, and N2A neuroblastoma cells. These elements comprise a binding site for Krox proteins, one for nuclear factor 1, an octamer motif that binds POU-homeodomain proteins, and a novel TN control element. The nuclear factor 1 and TN control element had positive effects on TN promoter activity and formed similar DNA-protein complexes with nuclear extracts from all three cell lines. The Krox element had a negative effect on TN promoter activity in N2A cells, a positive effect in C6 cells, and no effect in NIH 3T3 cells. Two DNA binding complexes, one correlated with the negative and the other with the positive activities of the Krox element, were found to contain the protein Krox24. In cotransfection experiments, the octamer motif was required for induction of TN promoter activity by the POU-homeodomain protein Brn2 in N2A cells but was inactive in C6 cells. Consistent with these findings, N2A cells transfected with Brn2 formed octamer-binding complexes containing N-Oct3, the transcriptionally active form of Brn2, whereas complexes formed in C6 cells contained only N-Oct5A and N-Oct5B. Our results provide a striking example of the diversity of regulatory mechanisms that can be called forth by combining different promoter motifs with transcriptional activators or repressors.

Neural cell adhesion molecules in activity-dependent development and synaptic plasticity

Cell adhesion molecules (CAMs) have a vital role in forming connections between neurons during embryonic development. Increasing evidence suggests that CAMs also participate in activity-dependent plasticity during development and synaptic plasticity in adults. Neural impulses of appropriate patterns can regulate expression of specific CAMs in mouse neurons from dorsal-root ganglia, alter cell-cell adhesion and produce structural reorganization of axon terminals in culture. Synaptic plasticity in Aplysia, learning in chick and long-term potentiation in rat hippocampus are accompanied by changes in CAM expression. Long-term potentiation can be blocked by disrupting CAM function in rat hippocampus, and learning deficits result from antibody blockade of CAMs in chicks and in transgenic mice lacking specific CAMs. Cell adhesion molecules might produce these effects by controlling several cellular processes, including cell adhesion, cytoskeletal structure and intracellular signaling.

Tenascin-X expression in tumor cells and fibroblasts: glucocorticoids as negative regulators in fibroblasts

Tenascin-X has recently been shown to be a novel member of the tenascin family and its distribution is often reciprocal to that of tenascin-C in the developing mouse embryo. We have investigated the expression of tenascin-X in fibroblasts and carcinoma cells in culture. Tenascin-X protein was secreted in vitro in the conditioned media at an apparent molecular mass of approximately 450 kDa. In addition fibroblasts contained a major tenascin-X isoform of 220 kDa. On northern blots, a single major transcript with a size of approximately 13 kb was detected. No overexpression of tenascin-X protein was found in primary fibroblasts of the tenascin-C-gene knockout mice. Steroid hormone glucocorticoids, were found to downregulate tenascin-X mRNA levels and protein synthesis in fibroblasts but not carcinoma cells at physiological concentrations. None of the growth factors or cytokines examined affected the expression level of tenascin-X. As in vivo study, carcinoma cells were transplanted into nude mice. In contrast to the ubiquitous presence of tenascin-X in adult skin, expression of tenascin-X protein during tumorigenesis was found to be down-regulated considerably not only in tumor cells themselves but also in tumor stroma. These findings provide evidence that the expression of tenascin-X can be influenced by stromal-epithelial interactions. We have identified glucocorticoids as physiological inhibitors of tenascin-X and suggest that glucocorticoids may in part participate in the downregulation of tenascin-X in fibroblasts in vivo.

Kainic acid activates transient expression of tenascin-C in the adult rat hippocampus

Kainic acid-induced limbic seizures enhance expression of tenascin-C (TN) in the hippocampus of adult rats. TN mRNA was detectable by in situ hybridization in many granule cells in the dentate gyrus 4.5 hr after kainic acid injection but not in saline-injected animals (controls) or in animals killed 2 or 24 hr after injection. Thirty days after kainic acid injection, TN mRNA was detectable only in pyramidal cells of CA3 and CA1. At the protein level, TN was detectable by immunocytochemistry in control animals in the strata oriens and lacunosum moleculare of CA1, in the molecular layer, and within a narrow area at the inner surface of the granule cell layer in the dentate gyrus. Twenty-four hours after kainic acid injection, TN immunoreactivity was enhanced in these areas and throughout the granule cell layer. Thirty days after kainic acid injection, TN immunoreactivity was downregulated in these areas, while it was prominent in the stratum oriens and in clusters of immunoreactivity in the stratum lucidum of CA3. Western blot analysis of the hippocampus showed a peak of TN expression 24 hr after kainic acid injection. These observations show that TN expression is upregulated in predominantly neuronal cells already by 4.5 hr after kainic acid injection, coincident with activation of granule cells and sprouting of axon terminals, whereas the remaining TN expression 30 days after injection relates to pyramidal cells in CA1 and CA3, coincident with an astroglial response, as marked by a strong expression of glial fibrillary acidic protein.

Engrailed, retinotectal targeting, and axonal patterning in the midbrain during Xenopus development: an antisense study

Axonal tracts in the vertebrate brain seem to respect domains of homeobox gene expression. To test the role of engrailed in tract formation in the midbrain, we inhibited its expression using antisense (AS) oligonucleotides. Phosphorothioate-modified AS oligos caused navigational errors in both the optic projection (OP) and the intertectal commissure (ITC). These oligos, however, also inhibited bFGF binding to the brain. To determine whether these tract phenotypes were due to inhibition of bFGF function or engrailed expression, we used partially phosphorothioate-modified (pp) oligos, which inhibit engrailed expression but do not affect bFGF binding. These ppAS oligos caused the ITC phenotype but had no effect on the OP. Thus, interference with bFGF function correlates with the OP phenotype, while the ITC phenotype is directly related to engrailed expression.

Different human tenascin-C variants in the extracellular matrix of cultured human fibroblasts

Using an immunoadsorbent prepared with a monoclonal antibody specific for the high molecular mass isoform of human tenascin-C, we purified tenascin-C molecules containing at least one large subunit from the extracellular matrix of cultured normal human fibroblasts. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis and immunoblotting analyses have shown that both high and low molecular mass subunits are present in these tenascin-C preparations. Because the monoclonal antibody used is able to bind only the high molecular mass isoform, the present data show that part of the tenascin-C present in the fibroblast extracellular matrix is made up of heterohexameric molecules.

Tenascin-C induction by the diffusible factor epidermal growth factor in stromal-epithelial interactions

Tenascin-C, a six-armed extracellular matrix glycoprotein, is expressed in a temporally and spatially restricted pattern during carcinogenesis and invasion or metastasis of carcinoma cells in association with stromal-epithelial interactions. The human epidermoid carcinoma-derived cell lines, A431 and HEp-2, which do not express tenascin-C by themselves in vitro, do express tenascin-C after transplantation into nude mice, and transforming growth factor beta 1 (TGF-beta 1) induces them to express tenascin-C in vitro. Epidermal growth factor (EGF) induced tenascin-C in these cells more effectively (about 3.5-fold greater) than did TGF-beta 1. Hepatocyte growth factor (HGF) and platelet-derived growth factor (PDGF) had little effect on the induction of tenascin-C. EGF also induced other extracellular matrix components, fibronectin and laminin. Tenascin-C was also induced when the carcinoma cells were co-cultured with embryonic fibroblasts from mice which were homozygous for a null mutation in the tenascin-C gene, or when the conditioned medium from these cells was added. The induction of tenascin-C in the co-culture was reduced by treating the cells with antibodies against EGF or its receptor. The addition of EGF caused both cell types to disrupt their cytoskeleton and focal contacts as evidenced by the loss of stress fibers and vinculin plaques. EGF did neither induce tenascin-C nor affect the morphology in tenascin-C-nonproducing A549 carcinoma cells, which did not produce tenascin-C after transplantation. Thus, EGF induces tenascin-C in tenascin-C-nonproducing human carcinoma cells through EGF receptors. Furthermore, in stromalepithelial interactions, the diffusible factor EGF participates in the induction of human tenascin-C in these cells through EGF receptors.

The complexity in regulating the expression of tenascins

The tenascins are a growing family of extracellular matrix proteins of typical multidomain structure. The prototype to be discovered was tenascin-C. It shows a highly regulated expression pattern during embryonic development and is often transiently associated with morphogenetic tissue interactions during organogenesis. In the adult organism reexpression of tenascin-C occurs in tumors and many other pathological conditions. Tenascin-C expression can be regulated by many different growth factors and hormones. Furthermore, mechanical strain exerted by fibroblasts seems to induce the expression of tenascin-C. This could represent a mechanism of translating mechanical forces into protein patterns, a step of potential relevance in the organization of embryogenesis. Tenascin-C as well as tenascin-R are believed to counteract the cell adhesion and spreading activity of fibronectin, thereby facilitating cell movement.

Regulation of mesenchymal extracellular matrix protein synthesis by transforming growth factor-beta and glucocorticoids in tumor stroma

We have here studied the composition and regulation of stromal extracellular matrix components in an experimental tumor model. Nude mice were inoculated with WCCS-1 cells, a human Wilms' tumor cell line. In the formed tumors the stroma was found to contain mesenchymal extracellular matrix proteins such as tenascin-C, fibulins-1 and 2 and fibronectin, but no nidogen. Nidogen was confined to basement membranes of tumor blood vessels. Since glucocorticoids have been shown to downregulate tenascin-C expression in vitro, we tested whether dexamethasone can influence biosynthesis of extracellular matrix components during tumor formation in vivo. A downregulation of tenascin-C mRNA and an upregulation of fibronectin mRNA expression by dexamethasone was noted. Transforming growth factor-beta 1 mRNA levels were unaffected by the dexamethasone treatment. Glucocorticoids can thus downregulate tenascin-C synthesis although local stimulatory growth factors are present. The competition between a negative and a positive extrinsic factor on synthesis of stromal extracellular matrix components was studied in a fibroblast/preadipocyte cell line. Transforming growth factor-beta 1 stimulated tenascin-C synthesis but did not affect fibronectin or fibulin-2 synthesis. Dexamethasone at high concentrations could completely suppress the effect of transforming growth factor-beta 1 on tenascin-C mRNA expression. Transforming growth factor-beta 1 could in turn overcome the downregulation of tenascin-C mRNA expression caused by a lower concentration of dexamethasone. We therefore suggest that the limited expression of tenascin-C in part is due to a continuous suppression by physiological levels of glucocorticoids, which can be overcome by local stimulatory growth factors when present in sufficient amounts.

Different susceptibility of small and large human tenascin-C isoforms to degradation by matrix metalloproteinases

Two major tenascin-C (TN-C) isoforms are generated by the alternative splicing of the pre-mRNA. The large isoform contains seven extra type three repeats that, by contrast, are omitted in the small TN-C isoform. The large TN-C isoform is mainly expressed at the onset of cellular processes that entail active cell migration, proliferation, or tissue remodeling such as occur in neoplasia, wound healing, and during development. Thus, the large TN-C isoform seems to be a specific component of the provisional extracellular matrix. Here we have studied the degradation of the large and small TN-C isoforms by matrix metalloproteinases (MMPs) 2, 3, 7, and 9. Among these proteolytic enzymes only MMP-7 can degrade the small TN-C isoform removing the NH2-terminal knob. The large TN-C isoform shows the same MMP-7-sensitive site adjacent to the NH2-terminal sequence, but is further degraded in the splicing area where three fibronectin-like type III repeats are completely digested. Moreover, the large TN-C isoform is degraded by MMP-2 and MMP-3 which completely digest a single type III repeat inside the splicing area. By contrast, the large TN-C isoform is resistant to MMP-9 digestion. The results show that the presence of the spliced sequence introduces new protease-sensitive sites in the large TN-C isoform.

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.

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.

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.

Induction of human tenascin (neuronectin) by growth factors and cytokines: cell type-specific signals and signalling pathways

The extracellular matrix protein tenascin (TN) is expressed with precise temporo-spatial patterns during embryonic and fetal development and is induced in healing wounds, inflammatory lesions and solid tumors. These tissue patterns suggest that TN synthesis may be modulated by soluble factors present in developing tissues or released from injured, inflammatory or neoplastic cells. To characterize the extrinsic control of human TN we examined the effects of several signalling molecules on cultured neural, melanocytic and fibroblastic cells. Results obtained with alpha TN antibodies in enzyme-linked immunosorbent and immunoprecipitation assays indicate that TN expression is tightly regulated in a cell type-specific manner: (1) Primitive neuroectodermal tumor (PNET) cells grown in chemically defined, serum-free media show up to > 100-fold TN induction in response to fibroblast growth factors (aFGF, bFGF, K-FGF) and phorbol ester, independent of changes in cell proliferation or total protein synthesis; no induction is seen in PNET cultures stimulated with serum or other growth and differentiation factors. (2) Normal melanocytes, which require FGF and phorbol ester for survival in vitro, fail to express TN; however, they produce TN following oncogenic transformation. (3) Fibroblasts derived from disparate tissues differ up to 100-fold in basal TN production; for example, fetal lung fibroblasts are TNhigh, but conjunctival fibroblasts derived from the same donors and fetal leptomeningeal cells are TNlow. (4) TNlow fibroblasts treated with interleukin-1, tumor necrosis factor-alpha, and interleukin-4 show up to > 100-fold increased TN secretion and TN incorporation into their extracellular matrix. Transforming growth factor-beta, which acts as an inducer of fibronectin, collagen, and integrin-type matrix receptors, has variable effects on fibroblast TN, ranging from increased deposition in the extracellular matrix of fetal conjunctival fibroblasts to reduced secretion in newborn foreskin fibroblasts. In contrast, FGFs (which are potent fibroblast mitogens), phorbol ester, bone morphogenetic proteins, and several other factors tested produced no discernible effects on fibroblast TN expression. These findings suggest that discrete sets of extrinsic signals modify TN expression in specific cell types, with the effects of a given ligand/receptor system determined by cell type-specific signalling pathways that may be linked to unique cis-regulatory elements of the TN gene. As a result, a limited set of regulatory peptides may produce highly diversified TN distribution patterns in developing and lesional tissues.

Identification of serum-inducible genes: different patterns of gene regulation during G0-->S and G1-->S progression

We have identified, by differential cDNA library screening, 15 serum inducible genes in the human diploid fibroblast cell line WI-38. The genes fall into two classes that are distinguished by their dependence on protein synthesis for the induction by serum, i.e., primary and secondary genes. While 11 of these genes encode known proteins, 4 other genes have not been described to date. The former genes encode proteins of diverse functions, including the monocyte-derived neutrophil chemotactic factor (MONAP), calmodulin, tropomyosin, tenascin, collagenase, plasminogen activator inhibitor-2a, the ‘sperm-specific’ cleavage signal-1 protein, metallothionein IIa and the mitochondrial chaperonin hsp-60. Interestingly, one of the unknown genes contains a large open reading frame for a polypeptide that is highly homologous to a previously unidentified long open reading frame in the opposite strand of the gene coding for the transcription factor HTF-4. We also studied the regulation of these serum-induced genes during cell cycle progression in normally cycling WI-38 and HL-60 cells separated by counterflow elutriation as well as in serum-stimulated HL-60 cells. Our results clearly show that, in contrast to the prevailing opinion, the expression of most genes induced after mitogen stimulation is not subject to a significant regulation in normally proliferating cells. This supports the hypothesis that the progression into S from either G0 or G1 are distinct processes with specific patterns of gene expression.

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.

Human carcinoma cells synthesize and secrete tenascin in vitro

Tenascin is an extracellular matrix glycoprotein produced in response to epithelial-mesenchymal interactions that initiate fetal organogenesis, and it is also found in the stroma of benign and malignant neoplasms. Thirty-five human cell lines representing a variety of cancers were examined by immunoprecipitation and polyacrylamide gel electrophoresis of radiolabeled tenascin proteins from conditioned media. Two forms of tenascin with relative molecular masses of 190,000 and 250,000 were identified. Eight cell lines produced both forms. With the exception of myeloid and lymphoid leukemias and Burkitt's lymphoma, all of the mesodermal and neuroectodermal tumor lines were found to synthesize tenascin. Unexpectedly, tenascin was secreted by several mammary and colonic adenocarcinomas as well as by a line derived from normal mammary epithelial cells, and in some cases increased production was induced by transforming growth factor beta in serum-free medium. Cells producing fibronectin but not tenascin attached and spread on plastic culture dishes, while those producing tenascin alone remained suspended in the medium or were rarely attached. Tenascin also inhibited fibronectin-mediated adhesion of MCF7 breast carcinoma cells in vitro. The results suggest that tenascins synthesized and secreted by some cancer cells, especially those of epithelial origin, may have specific roles in determining tumor cell adhesion and ultimately the ability to form invasive outgrowths.

Expression of tenascin mRNA in mesoderm during Xenopus laevis embryogenesis: the potential role of mesoderm patterning in tenascin regionalization

In Xenopus embryos, the extracellular matrix (ECM) protein tenascin (TN) is expressed dorsally in a very restricted pattern. We have studied the spatial and temporal expression of TN mRNA in tailbud-stage embryos by RNAase protection and in situ hybridization using a cDNA probe for Xenopus TN obtained by PCR amplification. We report that TN transcripts are principally expressed in cells dispersed around the neural tube and notochord as well as in myotome and sclerotome cells. No TN mRNA could be detected in lateral plate mesoderm, but expression was detectable beneath tail fin epidermis. In a second series of experiments, we studied the expression of TN mRNA and protein in combinations between animal and vegetal stage-6 blastomeres and in stage-8 blastula animal caps treated with activin A or basic fibroblastic growth factor (b-FGF). Isolated animal cap tissue cultured alone differentiates into epidermis, which expresses neither TN protein nor TN mRNA. TN expression is, however, elicited in response to isolated dorsal vegetal blastomeres and in response to high concentrations of activin, both of which treatments lead to formation of muscle and/or notochord. Low concentrations of activin, and ventral vegetal blastomeres, treatments that induce mesoderm of ventral character, are poor inducers of TN. However, b-FGF, which also induces ventral mesoderm, elicits strong expression. These results indicate that TN regionalization is a complex process, dependent both on the pattern of differentiation of mesodermal tissues and on the agent with which they are induced. The data further show that “ventral mesoderm” induced by low concentrations of activin is distinct from that induced by b-FGF, and imply that activin induces ventral mesoderm of the trunk while b-FGF induces posterior mesoderm of the tailbud.

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

Cytotactin is a morphoregulatory molecule of the extracellular matrix affecting cell shape, division, and migration that appears in a characteristic and complex site-restricted pattern during embryogenesis. The promoter region of the gene that encodes chicken cytotactin contains a variety of potential regulatory sequences. These include putative binding sites for homeodomain proteins and a phorbol 12-O-tetradecanoate 13-acetate response element (TRE)/AP-1 element, a potential target for transcription factors thought to be involved in growth-factor signal transduction. To determine the effects of homeobox-containing genes on cytotactin promoter activity, we conducted a series of cotransfection experiments on NIH 3T3 cells using cytotactin promoter-chloramphenicol acetyltransferase (CAT) reporter gene constructs and plasmids driving the expression of mouse homeobox genes Evx-1 and Hox-1.3. cotransfection with Evx-1 stimulated cytotactin promoter activity whereas cotransfection in control experiments with Hox-1.3 had no effect. To localize the sequences required for Evx-1 activation, we tested a series of deletions in the cytotactin promoter. An 89-base-pair region containing a consensus TRE/AP-1 element was found to be required for activation. An oligonucleotide segment containing this TRE/AP-1 site was found to confer Evx-1 inducibility on a simian virus 40 minimal promoter; mutation of the TRE/AP-1 site abolished this activity. To explore the potential role of growth factors in cytotactin promoter activation, chicken embryo fibroblasts, which are known to synthesize cytotactin, were first transfected with cytotactin promoter constructs and cultured under minimal conditions in 1% fetal bovine serum. Although the cells exhibited only low levels of CAT activity under these conditions, cells exposed for 12 h to 10% (vol/vol) fetal bovine serum showed a marked increase in CAT activity. Cotransfection with Evx-1 and cytotactin promoter constructs of cells cultured in 1% fetal bovine serum was sufficient, however, to produce high levels of CAT activity. These findings are consistent with the hypothesis that Evx-1, a homeobox-containing gene, may activate the cytotactin promoter by a mechanism involving a growth-factor signal transduction pathway. More generally, the results support the hypothesis that the place-dependent expression of morphoregulatory molecules may depend upon local cues provided by homeobox genes and their encoded proteins.

Cell adhesion molecules as targets for Hox genes: neural cell adhesion molecule promoter activity is modulated by cotransfection with Hox-2.5 and -2.4

In an effort to determine whether homeobox genes modulate the activity of the promoter of the mouse neural cell adhesion molecule (N-CAM) gene, we have carried out a series of cotransfection experiments using NIH 3T3 cells. Plasmids were constructed containing Xenopus laevis Hox-2.5 and -2.4 coding sequences linked to a human cytomegalovirus promoter (CMV-Hox-2.5 and CMV-Hox-2.4). A 4.9-kilobase DNA fragment containing 5' flanking and first exon sequences of the mouse N-CAM gene was linked to a chloramphenicol acetyltransferase (CAT) reporter gene (N-CAM-Pro-CAT). Cotransfection with CMV-Hox-2.5 and N-CAM-Pro-CAT resulted in a strong induction of CAT activity. The N-CAM promoter contained two potential homeodomain binding sites (sites I and II) within a 47-base-pair segment (512-559 base pairs upstream of the ATG codon in the first exon of the N-CAM gene). This segment was linked to a minimal promoter (simian virus 40 early) and a downstream CAT gene. Although this construct was transcriptionally active at a low level in NIH 3T3 cells, cotransfection of CMV-Hox-2.5 resulted in CAT activity that was greatly elevated. Mutational studies revealed that it was the homeodomain binding site II sequence that was required for this regulation. In contrast, cotransfection with CMV-Hox-2.4 eliminated the CAT activity that was driven by the CMV-Hox-2.5 construct. Thus, the products of two related Hox genes, which are located adjacent to each other in the Hox-2 complex, can differentially modulate transcription from the promoter of a cell adhesion molecule gene. The results suggest that the N-CAM gene is likely to be a target for regulation by Hox gene products.