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

Tenascin-C: A Key Regulator in Angiogenesis during Wound Healing

(1) Background: Injury repair is a complex physiological process in which multiple cells and molecules are involved. Tenascin-C (TNC), an extracellular matrix (ECM) glycoprotein, is essential for angiogenesis during wound healing. This study aims to provide a comprehensive review of the dynamic changes and functions of TNC throughout tissue regeneration and to present an up-to-date synthesis of the body of knowledge pointing to multiple mechanisms of TNC at different restoration stages. (2) Methods: A review of the PubMed database was performed to include all studies describing the pathological processes of damage restoration and the role, structure, expression, and function of TNC in post-injury treatment; (3) Results: In this review, we first introduced the construction and expression signature of TNC. Then, the role of TNC during the process of damage restoration was introduced. We highlight the temporal heterogeneity of TNC levels at different restoration stages. Furthermore, we are surprised to find that post-injury angiogenesis is dynamically consistent with changes in TNC. Finally, we discuss the strategies for TNC in post-injury treatment. (4) Conclusions: The dynamic expression of TNC has a significant impact on angiogenesis and healing wounds and counters many negative aspects of poorly healing wounds, such as excessive inflammation, ischemia, scarring, and wound infection.

Matricellular protein tenascin C: Implications in glioma progression, gliomagenesis, and treatment

Matricellular proteins are nonstructural extracellular matrix components that are expressed at low levels in normal adult tissues and are upregulated during development or under pathological conditions. Tenascin C (TNC), a matricellular protein, is a hexameric and multimodular glycoprotein with different molecular forms that is produced by alternative splicing and post-translational modifications. Malignant gliomas are the most common and aggressive primary brain cancer of the central nervous system. Despite continued advances in multimodal therapy, the prognosis of gliomas remains poor. The main reasons for such poor outcomes are the heterogeneity and adaptability caused by the tumor microenvironment and glioma stem cells. It has been shown that TNC is present in the glioma microenvironment and glioma stem cell niches, and that it promotes malignant properties, such as neovascularization, proliferation, invasiveness, and immunomodulation. TNC is abundantly expressed in neural stem cell niches and plays a role in neurogenesis. Notably, there is increasing evidence showing that neural stem cells in the subventricular zone may be the cells of origin of gliomas. Here, we review the evidence regarding the role of TNC in glioma progression, propose a potential association between TNC and gliomagenesis, and summarize its clinical applications. Collectively, TNC is an appealing focus for advancing our understanding of gliomas.

Tenascin-C at a glance

Tenascin-C (TNC) is a hexameric, multimodular extracellular matrix protein with several molecular forms that are created through alternative splicing and protein modifications. It is highly conserved amongst vertebrates, and molecular phylogeny indicates that it evolved before fibronectin. Tenascin-C has many extracellular binding partners, including matrix components, soluble factors and pathogens; it also influences cell phenotype directly through interactions with cell surface receptors. Tenascin-C protein synthesis is tightly regulated, with widespread protein distribution in embryonic tissues, but restricted distribution of tenascin-C in adult tissues. Tenascin-C is also expressed de novo during wound healing or in pathological conditions, including chronic inflammation and cancer. First described as a modulator of cell adhesion, tenascin-C also directs a plethora of cell signaling and gene expression programs by shaping mechanical and biochemical cues within the cellular microenvironment. Exploitment of the pathological expression and function of tenascin-C is emerging as a promising strategy to develop new diagnostic, therapeutic and bioengineering tools. In this Cell Science at a Glance article and the accompanying poster we provide a succinct and comprehensive overview of the structural and functional features of tenascin-C and its potential roles in developing embryos and under pathological conditions.

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.

Tenascin-C: Form versus function

Tenascin-C is a large, multimodular, extracellular matrix glycoprotein that exhibits a very restricted pattern of expression but an enormously diverse range of functions. Here, we discuss the importance of deciphering the expression pattern of, and effects mediated by, different forms of this molecule in order to fully understand tenascin-C biology. We focus on both post transcriptional and post translational events such as splicing, glycosylation, assembly into a 3D matrix and proteolytic cleavage, highlighting how these modifications are key to defining tenascin-C function.

Embryology, classification, and surgical management of bony malformations of the craniovertebral junction

The embryology of the bony craniovertebral junction (CVJ) is reviewed with the purpose of explaining the genesis and unusual configurations of the numerous congenital malformations in this region. Functionally, the bony CVJ can be divided into a central pillar consisting of the basiocciput and dental pivot; and a two-tiered ring revolving round the central pivot, comprising the foramen magnum rim and occipital condyles above, and the atlantal ring below. Embryologically, the central pillar and the surrounding rings descend from different primordia, and accordingly, developmental anomalies at the CVJ can also be segregated into those affecting the central pillar and the surrounding rings, respectively. A logical classification of this seemingly unwieldy group of malformations is thus possible based on their ontogenetic lineage, morbid anatomy, and clinical relevance. Representative examples of the main constituents of this classification scheme are given, and their surgical treatments are selectively discussed.

HOX Gene Aberrant Expression in Skin Melanoma: A Review

The homeobox family and its subset of HOX gene products represent a family of transcription factors directing DNA-protein and protein-protein interactions. In the embryo, they are central regulators in cell differentiation during morphogenesis. A series of genes of the four HOX gene clusters A, B, C, and D were reported to show aberrant expressions in oncogenesis, particularly in cutaneous malignant melanoma (CMM). They are involved in cell proliferation and progression in the CMM metastatic path. We present relevant peer-reviewed literature findings about the aberrant expression of HOX genes in CMM. The number of CMM cell nuclei exhibiting aberrant HOX protein expression appears correlated with tumour progression.

Embryology and bony malformations of the craniovertebral junction

BACKGROUND

The embryology of the bony craniovertebral junction (CVJ) is reviewed with the purpose of explaining the genesis and unusual configurations of the numerous congenital malformations in this region. Functionally, the bony CVJ can be divided into a central pillar consisting of the basiocciput and dental pivot and a two-tiered ring revolving round the central pivot, comprising the foramen magnum rim and occipital condyles above and the atlantal ring below. Embryologically, the central pillar and the surrounding rings descend from different primordia, and accordingly, developmental anomalies at the CVJ can also be segregated into those affecting the central pillar and those affecting the surrounding rings, respectively.

DISCUSSION

A logical classification of this seemingly unwieldy group of malformations is thus possible based on their ontogenetic lineage, morbid anatomy, and clinical relevance. Representative examples of the main constituents of this classification scheme are given, and their surgical treatments are selectively discussed.

Neural Stem/Progenitor Cells Express 20 Tenascin C Isoforms That Are Differentially Regulated by Pax6

Tenascin C (Tnc) is an alternatively spliced, multimodular extracellular matrix glycoprotein present in the ventricular zone of the developing brain. Pax6-deficient small eye (sey) mouse mutants show an altered Tnc expression pattern. Here, we investigated the expression of Tnc isoforms in neural stem/progenitor cells and their regulation by the paired-box transcription factor Pax6. Neural stem/progenitor cells cultured as neurospheres strongly expressed Tnc on the protein level. The Tnc isoform expression in neural stem/progenitor cells was analyzed by reverse transcriptase-PCR and dot blot Southern hybridization. In total, 20 different Tnc isoforms were detected in neurospheres derived from embryonic fore-brain cell suspensions. The Tnc isoform containing the fibronectin type III domains A1A4BD is novel and might be neural stem/progenitor cell-specific. Transient overexpression of Pax6 in neurospheres of the medial ganglionic eminence did not alter the total Tnc mRNA expression level but showed a pronounced regulative effect on different Tnc isoforms. The larger Tnc isoforms containing four, five, and six additional alternatively spliced fibronectin type III domains were up-regulated, whereas the small Tnc isoforms without any or with one additional domain were down-regulated. Thus, Pax6 is a homeodomain protein that also modulates the splicing machinery. We conclude that the combinatorial code of Tnc isoform expression in the neural stem/progenitor cell is complex and regulated by Pax6. These findings suggest a functional significance for individual Tnc isoforms in neural stem/progenitor cells.

Hox genes and their candidate downstream targets in the developing central nervous system

1. Homeobox (Hox) genes were originally discovered in the fruit fly Drosophila, where they function through a conserved homeodomain as transcriptional regulators to control embryonic morphogenesis. Since then over 1000 homeodomain proteins have been identified in several species. In vertebrates, 39 Hox genes have been identified as homologs of the original Drosophila complex, and like their Drosophila counterparts they are organized within chromosomal clusters. Vertebrate Hox genes have also been shown to play a critical role in embryonic development as transcriptional regulators. 2. Both the Drosophila and vertebrate Hox genes have been shown to interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. These protein-protein interactions are believed to contribute to enhancing the specificity of target gene recognition in a cell-type or tissue- dependent manner. The regulatory activity of a particular Hox protein on a specific regulatory element is highly variable and dependent on its interacting partners within the transcriptional complex. 3. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing CNS, both along the anterioposterior and dorsoventral axis of the embryo. Their restricted gene expression is suggestive of a regulatory role in patterning of the CNS, as well as in cell specification. Determining the precise function of individual Hox genes in CNS morphogenesis through classical mutational analyses is complicated due to functional redundancy between Hox genes. 4. Understanding the precise mechanisms through which Hox genes mediate embryonic morphogenesis requires the identification of their downstream target genes. Although Hox genes have been implicated in the regulation of several pathways, few target genes have been shown to be under their direct regulatory control. Development of methodologies used for the isolation of target genes and for the analysis of putative targets will be beneficial in establishing the genetic pathways controlled by Hox factors. 5. Within the developing CNS various cell adhesion molecules and signaling molecules have been identified as candidate downstream target genes of Hox proteins. These targets play a role in processes such as cell migration and differentiation, and are implicated in contributing to neuronal processes such as plasticity and/or specification. Hence, Hox genes not only play a role in patterning of the CNS during early development, but may also contribute to cell specification and identity.

HOXA3 induces cell migration in endothelial and epithelial cells promoting angiogenesis and wound repair

Wound repair requires both the recruitment and coordination of numerous cell types including inflammatory cells, fibroblasts, endothelial and epithelial cells. Each cell type has a distinct set of cell behavior such as formation of granulation tissue and basement membrane, migration, proliferation and redifferentiation. These processes are dependent on cell-cell and cell-ECM signaling, intracellular signal transduction cascades, and ultimately, changes in gene transcription. We have investigated the role of the transcription factor HOXA3 in wound repair and angiogenesis. Here we show that HOXA3 increases endothelial cell migration, induces angiogenesis in vivo, and leads to increased expression of the matrix metalloproteinase-14 (MMP-14) and urokinase-type plasminogen activator receptor (uPAR) genes in endothelial cells in culture and in vivo in response to injury. We find that HOXA3 gene expression is upregulated during wound healing in angiogenic endothelial cells and keratinocytes, and that HOXA3 is not induced in genetically diabetic mice that have impaired angiogenesis and wound repair. We demonstrate that gene transfer of HOXA3 into diabetic mouse wounds leads to dramatic improvements in both angiogenesis and wound closure. In addition, we show that HOXA3 promotes migration of endothelial cells and keratinocytes in a uPAR-dependent manner. Together these findings illustrate how the morphoregulatory protein, HOXA3 can facilitate tissue remodeling via coordinated changes in both epithelial and endothelial cell gene expression and behavior in adult tissues during wound repair.

Investigation into a role for the primitive streak in development of the murine allantois

Despite its importance as the source of one of three major vascular systems in the mammalian conceptus, little is known about the murine allantois, which will become the umbilical cord of the chorio-allantoic placenta. During gastrulation, the allantois grows into the exocoelomic cavity as a mesodermal extension of the posterior primitive streak. On the basis of morphology, gene expression and/or function, three cell types have been identified in the allantois: an outer layer of mesothelial cells, whose distal portion will become transformed into chorio-adhesive cells, and endothelial cells within the core. Formation of endothelium and chorio-adhesive cells begins in the distal region of the allantois, farthest from the streak. Over time, endothelium spreads to the proximal allantoic region, whilst the distal outer layer of presumptive mesothelium gradually acquires vascular cell adhesion molecule (VCAM1) and mediates chorio-allantoic union. Intriguingly, the VCAM1 domain does not extend into the proximal allantoic region. How these three allantoic cell types are established is not known, although contact with the chorion has been discounted. In this study, we have investigated how the allantois differentiates, with the goal of discriminating between extrinsic mechanisms involving the primitive streak and an intrinsic role for the allantois itself. Exploiting previous observations that the streak contributes mesoderm to the allantois throughout the latter's early development, microsurgery was used to remove allantoises at ten developmental stages. Subsequent whole embryo culture of operated conceptuses resulted in the formation of regenerated allantoises at all time points. Aside from being generally shorter than normal, none of the regenerates exhibited abnormal differentiation or inappropriate cell relationships. Rather, all of them resembled intact allantoises by morphological, molecular and functional criteria. Moreover, fate mapping adjacent yolk sac and amniotic mesoderm revealed that these tissues and their associated bone morphogenetic protein 4 (BMP4) did not contribute to restoration of allantoic outgrowth and differentiation during allantoic regeneration. Thus, on the basis of these observations, we conclude that specification of allantoic endothelium, mesothelium and chorio-adhesive cells does not occur by a streak-related mechanism during the time that proximal epiblast travels through it and is transformed into allantoic mesoderm. Rather, all three cell-types are established by mechanisms intrinsic to the allantois, and possibly include roles for cell age and cell position. However, although chorio-adhesive cells were not specified within the streak, we discovered that the streak nonetheless plays a role in establishing VCAM1's expression domain, which typically began and was thereafter maintained at a defined distance from the primitive streak. When allantoises were removed from contact with the streak, normally VCAM1-negative proximal allantoic regions acquired VCAM1. These results suggested that the streak suppresses formation of chorio-adhesive cells in allantoic mesoderm closest to it. Together with previous results, findings presented here suggest a model of differentiation of allantoic mesoderm that invokes intrinsic and extrinsic mechanisms, all of which appear to be activated once the allantoic bud has formed.

Mechanisms of pattern formation in development and evolution

We present a classification of developmental mechanisms that have been shown experimentally to generate pattern and form in metazoan organisms. We propose that all such mechanisms can be organized into three basic categories and that two of these may act as composite mechanisms in two different ways. The simple categories are cell autonomous mechanisms in which cells enter into specific arrangements ('patterns') without interacting, inductive mechanisms in which cell communication leads to changes in pattern by reciprocal or hierarchical alteration of cell phenotypes ('states') and morphogenetic mechanisms in which pattern changes by means of cell interactions that do not change cell states. The latter two types of mechanism can be combined either morphostatically, in which case inductive mechanisms act first, followed by the morphogenetic mechanism, or morphodynamically, in which case both types of mechanisms interact continuously to modify each other's dynamics. We propose that this previously unexplored distinction in the operation of composite developmental mechanisms provides insight into the dynamics of many developmental processes. In particular, morphostatic and morphodynamic mechanisms respond to small changes in their genetic and microenvironmental components in dramatically different ways. We suggest that these differences in 'variational properties' lead to morphostatic and morphodynamic mechanisms being represented to different extents in early and late stages of development and to their contributing in distinct ways to morphological transitions in evolution.

The repressor activity of Even-skipped is highly conserved, and is sufficient to activate engrailed and to regulate both the spacing and stability of parasegment boundaries

During segmentation of the Drosophila embryo, even skipped is required to activate engrailed stripes and to organize odd-numbered parasegments. A 16 kb transgene containing the even skipped coding region can rescue normal engrailed expression, as well as all other aspects of segmentation, in even skipped null mutants. To better understand its mechanism of action, we functionally dissected the Even-skipped protein in the context of this transgene. We found that Even-skipped utilizes two repressor domains to carry out its function. Each of these domains can function autonomously in embryos when fused with the Gal4 DNA-binding domain. A chimeric protein consisting only of the Engrailed repressor domain and the Even-skipped homeodomain, but not the homeodomain alone, was able to restore function, indicating that the repression of target genes is sufficient for even skipped function at the blastoderm stage, while the homeodomain is sufficient to recognize those target genes. When Drosophila Even skipped was replaced by its homologs from other species, including a mouse homolog, they could provide substantial function, indicating that these proteins can recognize similar target sites and also provide repressor activity. Using this rescue system, we show that broad, early even skipped stripes are sufficient for activation of both odd- and even-numbered engrailed stripes. Furthermore, these ‘unrefined’ stripes organize odd-numbered parasegments in a dose-dependent manner, while the refined, late stripes, which coincide cell-for-cell with parasegment boundaries, are required to ensure the stability of the boundaries.

Hoxa 11 is upstream of Integrin alpha8 expression in the developing kidney

Mutation of the functionally redundant Hoxa 11/Hoxd 11 genes gives absent or rudimentary kidneys resulting from a dramatic reduction of the growth and branching of the ureteric bud. To understand better the molecular mechanisms of Hoxa 11/Hoxd 11 function in kidney development, it is necessary to identify the downstream target genes regulated by their encoded transcription factors. To this end, we conducted a screen for Hoxa 11-responsive genes in two kidney cell lines. HEK293 cells, which usually do not express Hoxa 11, were modified to allow inducible Hoxa 11 expression. The mK10 cells, derived specifically for this study from Hoxa 11/Hoxd 11 double-mutant mice, were also modified to give cell populations with and without Hoxa 11 expression. Differential display, Gene Discovery Arrays, and Affymetrix genechip probe arrays were used to screen for genes up- or down-regulated by Hoxa 11. Nine genes, PDGF A, Cathepsin L, annexin A1, Mm.112139, Est2 repressor factor, NrCAM, ZNF192, integrin-associated protein, and GCM1, showed reproducible 3-fold or smaller changes in gene expression in response to Hoxa 11. One gene, the Integrin alpha8, was up-regulated approximately 20-fold after Hoxa 11 expression. The Integrin alpha8 gene is expressed together with Hoxa 11 in metanephric mesenchyme cells, and mutation of Integrin alpha8 gives a bud-branching morphogenesis defect very similar to that observed in Hoxa 11/Hoxd 11 mutant mice. In situ hybridizations showed a dramatic regional reduction in Integrin alpha8 expression in the developing kidneys of Hoxa 11/Hoxd 11 mutant mice. This work suggests that the Integrin alpha8 gene may be a major effector of Hoxa 11/Hoxd 11 function in the developing kidney.

Tenascin-C in developing mouse teeth: expression of splice variants and stimulation by TGFbeta and FGF

Tenascin-C is a protein of the extracellular matrix which has been suggested to regulate organogenesis. We have analysed the expression of tenascin-C mRNA during mouse tooth development. We show that it is transiently expressed during epithelial budding in the condensed dental mesenchyme, and that it reappears later in the dental papilla mesenchyme where it persists in the dental pulp but is downregulated in odontoblasts. Probes corresponding to the domains A4, B, and D of the differentially spliced and domain 7 of the constant region of the FNIII-like domain show similar patterns of hybridization. Dental epithelium has been shown to induce tenascin-C in early dental mesenchyme, and we show that growth factors in the transforming growth factor beta (TGFbeta) and fibroblast growth factor (FGF) families can mimic this effect. FGF-4, -8 and TGFbeta-1 proteins were applied locally by beads on dissected dental mesenchyme, and tenascin-C expression was analysed after 24 h culture by reverse transcriptase-polymerase chain reaction (RT-PCR) in situ hybridization, and immunohistochemistry. FGF-4 and TGFbeta-1 stimulated tenascin-C expression in E12 dental mesenchymes. RT-PCR showed induction of several tenascin-C isoforms by both TGFbeta-1 and FGFs. We conclude that several splice forms are expressed during mouse tooth development, and that TGFbeta- and FGF-family growth factors may act as epithelial signals inducing tenascin expression in the dental mesenchyme.

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.

Transcriptional regulation of laminin gene expression

Laminins are the most abundant structural non-collagenous glycoproteins ubiquitously present in basement membranes. They are multidomain molecules consisting of of alpha, beta, and gamma chains. Although the precise functional differences between the laminin variants are not well understood, the diversity of laminin isoforms may reflect the formation of distinct basement membranes. The laminins display a remarkable restricted expression profile, suggesting a fine regulation of their genes. In this review, we focus on the most recent developments of laminin biology, centering on transcriptional and posttranscriptional controls. We discuss only those laminin chains whose gene organization and promoter elements have been characterized and proved to be functional. When possible, we correlate the effects of growth factors, cytokines, retinoids, and transcription factors on laminin gene expression with the identity of cis-acting elements in their genomic control regions.

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.

Mechanical modulation of tenascin-C and collagen-XII expression during avian synovial joint formation

The objective of this study was to investigate how temporal and spatial patterns of characteristic extracellular matrix molecules are altered in the absence of normal functional skeletal muscle contractions during avian synovial joint development. By using in situ detection of protein and mRNA expression in developing avian feet and femorotibial joints from a wide range of developmental stages, we demonstrate that the morphological abnormalities that result from embryonic immobilization are associated with altered patterns of tenascin-C and collagen-XII expression within developing joint structures. As the joints fuse in immobilized embryos, the cells of the presumptive articular surface differentiate from flattened fibroblasts to more rounded chondrocytes and collagens XII and I are no longer detected at sites of complete joint fusion. Although the expression of collagen XII persists at normal levels elsewhere within the immobilized joint, tenascin-C expression is diminished within the chondroepiphysis, synovium, and tendons, as well as within the remains of the fibrous articular surface. This effect is most notable for the shortest tenascin variant (Tn190) within the chondroepiphysis and the largest variant (Tn230) within tendons, synovium, and the fibrous surface layer of the joint. This study thus provides in vivo support of previous in vitro work that suggests that tenascin expression is sensitive to external changes in mechanical loading environment. However, these data do not support a similar conclusion for collagen XII during early development.

Homeobox B3 promotes capillary morphogenesis and angiogenesis

Endothelial cells (EC) express several members of the Homeobox (Hox) gene family, suggesting a role for these morphoregulatory mediators during angiogenesis. We have previously established that Hox D3 is required for expression of integrin alphavbeta3 and urokinase plasminogen activator (uPA), which contribute to EC adhesion, invasion, and migration during angiogenesis. We now report that the paralogous gene, Hox B3, influences angiogenic behavior in a manner that is distinct from Hox D3. Antisense against Hox B3 impaired capillary morphogenesis of dermal microvascular EC cultured on basement membrane extracellular matrices. Although levels of Hox D3-dependent genes were maintained in these cells, levels of the ephrin A1 ligand were markedly attenuated. Capillary morphogenesis could be restored, however, by addition of recombinant ephrin A1/Fc fusion proteins. To test the impact of Hox B3 on angiogenesis in vivo, we constitutively expressed Hox B3 in the chick chorioallantoic membrane using avian retroviruses that resulted in an increase in vascular density and angiogenesis. Thus, while Hox D3 promotes the invasive or migratory behavior of EC, Hox B3 is required for the subsequent capillary morphogenesis of these new vascular sprouts and, together, these results support the hypothesis that paralogous Hox genes perform complementary functions within a particular tissue type.

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.

Induction of vascular smooth muscle cell tenascin-C gene expression by denatured type I collagen is dependent upon a beta3 integrin-mediated mitogen-activated protein kinase pathway and a 122-base pair promoter element

Tenascin-C is an extracellular matrix glycoprotein, the expression of which is upregulated in remodeling arteries. In previous studies we showed that the presence of tenascin-C alters vascular smooth muscle cell shape and amplifies their proliferative response by promoting growth factor receptor clustering and phosphorylation. Moreover, we demonstrated that denatured type I collagen induces smooth muscle cell tenascin-C protein production via beta3 integrins. In the present study, we examine the pathway by which beta3 integrins stimulate expression of tenascin-C, and define a promoter sequence that is critical for its induction. On native collagen, A10 smooth muscle cells adopt a stellate morphology and produce low levels of tenascin-C mRNA and protein, whereas on denatured collagen they spread extensively and produce high levels of tenascin-C mRNA and protein, which is incorporated into an elaborate extracellular matrix. Increased tenascin-C synthesis on denatured collagen is associated with elevated protein tyrosine phosphorylation, including activation of extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2). beta3 integrin function-blocking antibodies attenuate ERK1/2 activation and tenascin-C protein synthesis. Consistent with these findings, treatment with the specific MEK inhibitor, PD 98059, results in suppression of tenascin-C protein synthesis. To investigate whether beta3 integrin-dependent activation of ERK1/2 regulates the tenascin-C promoter, we transfected A10 cells with a full-length (approx. 4 kb) mouse tenascin-C gene promoter-chloramphenicol acetyltransferse reporter construct and showed that, relative to native collagen, its activity is increased on denatured collagen. Next, to identify regions of the promoter involved, we examined a series of tenascin-C promoter constructs with 5′ deletions and showed that denatured collagen-dependent promoter activity was retained by a 122-base pair element, located -43 to -165 bp upstream of the RNA start site. Activation of this element was suppressed either by blocking beta3 integrins, or by preventing ERK1/2 activation. These observations demonstrate that smooth muscle cell binding to beta3 integrins activates the mitogen activated protein kinase pathway, which is required for the induction of tenascin-C gene expression via a potential extracellular matrix response element in the tenascin-C gene promoter. Our data suggest a mechanism by which remodeling of type I collagen modulates tenascin-C gene expression via a beta3 integrin-mediated signaling pathway, and as such represents a paradigm for vascular development and disease whereby smooth muscle cells respond to perturbations in extracellular matrix composition by altering their phenotype and patterns of gene expression.

Cellular and molecular bases of axonal pathfinding during embryogenesis of the fish central nervous system

The accessibility of the zebrafish embryo offers unique possibilities to study the mechanisms that guide growing axons in the developing vertebrate central nervous system. This review examines the current understanding of the pathfinding decisions by the growing axons, their substrates, and the recognition molecules that mediate axon-substrate interactions. The detailed analysis of pathfinding at the level of individual axons demonstrates that growing axons chose their paths unerringly. To do so, they rely on cues presented by their environment, in particular by neuroepithelial cells. Our understanding of the molecular bases of axon-substrate interactions is increasing. Members of most classes of recognition molecules have been identified in fish. Experimental evidence for the functions of these molecules in the zebrafish nervous system is accumulating. In the future, this analysis is expected to profit greatly from genetic screens that have recently been initiated.

Expression of Msx genes in regenerating and developing limbs of axolotl

Msx genes, homeobox-containing genes, have been isolated as homologues of the Drosophila msh gene and are thought to play important roles in the development of chick or mouse limb buds. We isolated two Msx genes, Msx1 and Msx2, from regenerating blastemas of axolotl limbs and examined their expression patterns using Northern blot and whole mount in situ hybridization during regeneration and development. Northern blot analysis revealed that the expression level of both Msx genes increased during limb regeneration. The Msx2 expression level increased in the blastema at the early bud stage, and Msx1 expression level increased at the late bud stage. Whole mount in situ hybridization revealed that Msx2 was expressed in the distal mesenchyme and Msx1 in the entire mesenchyme of the blastema at the late bud stage. In the developing limb bud, Msx1 was expressed in the entire mesenchyme, while Msx2 was expressed in the distal and peripheral mesenchyme. The expression patterns of Msx genes in the blastemas and limb buds of the axolotl were different from those reported for chick or mouse limb buds. These expression patterns of axolotl Msx genes are discussed in relation to the blastema or limb bud morphology and their possible roles in limb patterning.

Patterns of paired-related homeobox genes PRX1 and PRX2 suggest involvement in matrix modulation in the developing chick vascular system

PRX1 (MHox) and PRX2 (S8) were previously shown to be expressed throughout embryogenesis in complex, mostly mesenchyme-specific patterns. In the developing cardiovascular system both genes were highly expressed in prospective connective tissues, that is, endocardial cushions and valves, the epicardium, and the wall of the great arteries and veins. We further scrutinised expression of PRX1 and PRX2 in the developing vascular system of the chicken embryo and compared patterns with those of established vascular differentiation markers (muscle-actin, procollagen I, and fibrillin-2). PRX1 and PRX2 expression were associated with the primary vessel wall from early stages onward and became increasingly restricted to the adventitial and outer medial cell layers. PRX1 eventually colocalised strikingly with procollagen I and fibrillin-2 expression and generally excluded high smooth muscle actin expression. Furthermore, PRX1 expression preceded the segregation of very distinct nonmuscular cells and smooth muscle cells in the media of the great arteries. PRX2 patterns deviated at later stages from those of PRX1 and showed specific and high transcript levels in the ductus arteriosus from embryonic day 6 onward. Results suggest that PRX genes are not essential in smooth muscle contractile differentiation, but may be involved in matrix modulation in the vascular system and possibly in defining the noncontractile cellular phenotype and in media-adventitia definition.

Anterior structural defects by misexpression of Xgbx-2 in early Xenopus embryos are associated with altered expression of cell adhesion molecules

The RNA of the noncluster homeobox gene, Xgbx-2, is localized during neurulation to a narrow band of tissue at the midbrain hindbrain boundary (anterior hindbrain). The localized expression of Xgbx-2 within the nervous system prompted us to assess its function during early development by injection of synthetic Xgbx-2 RNA into the animal pole region of both dorsal blastomeres at the four-cell stage. Injection of Xgbx-2 RNA leads to dose-dependent alterations in anterior dorsal structures. These defects include abnormal eye development including reduced and missing eyes, reduced or missing cement glands, and abnormal brain development. Additionally, coinjection with lineage label (either beta-galactosidase or green fluorescent protein) shows there is a dose-dependent misplacement of cells. These misplaced cells can be found in such locations as the blastocoele, gastrocoele, or ventricles in the brain. In some spawnings, misplaced cells are expelled from the embryo into the periviteline space. In general, the phenotype of Xgbx-2 RNA-injected embryos is strikingly similar to the phenotypes observed when dominant-negative RNA constructs of Ca2+-dependent cell-adhesion molecules are injected into similar regions of early embryos. Xgbx-2 misexpression enhanced the dissociation of animal hemisphere cells, and inhibited Ca2+-dependent cell adhesion in dissociated animal hemisphere cells in vitro. Additionally, when the expression of various calcium-dependent cadherins was tested, it was shown that misexpression of Xgbx-2 prevents N-cadherin expression during early neurulation. These observations suggest that the transcription factor, Xgbx-2, functions normally in the regionalization of the neural tube (specifically the anterior hindbrain) by regulating differential cell adhesion and subsequently cell identity.

Expression of Hoxa-1 and Hoxb-7 is regulated by extracellular matrix-dependent signals in mammary epithelial cells

Homeobox-containing genes encode transcriptional regulators involved in cell fate and pattern formation during embryogenesis. Recently, it has become clear that their expression in continuously developing adult tissues, as well as in tumorigenesis, may be of equal importance. In the mouse mammary gland, expression patterns of several homeobox genes suggest a role in epithelial-stromal interactions. Because the stroma and the extracellular matrix (ECM) are known to influence both functional and morphological development of the mammary gland, we asked whether these genes would be expressed postnatally in the gland and also in cell lines in culture and whether they could be modulated by ECM. Using a polymerase chain reaction-base strategy five members of the Hox gene clusters a and b were shown to be expressed in cultured mouse mammary cells. Hoxa-1 and Hoxb-7 were chosen for further analysis. Hoxb-7 was chosen because it had not been described previously in the mammary gland and was modulated at different stages of gland development. Hoxa-1 was chosen because it was reported previously to be expressed only in mammary tumors, and not in normal glands. We showed that culturing the mammary epithelial cell lines SCp2 and CID-9 on a basement membrane (BM) that was previously shown to induce a lactational phenotype was necessary to turn off Hoxb-7, but a change in cell shape, brought about by culturing the cells on an inert substratum such as polyHEMA, was sufficient to downregulate Hoxa-1. This is the first report of modulation of homeobox genes by ECM. The results provide a rationale for the differential pattern of expression in vivo of Hoxa-1 and Hoxb-7 during different stages of development. The culture model should permit further in-depth analysis of the molecular mechanisms involved in how ECM signaling and homeobox genes may interact to bring about tissue organization.

HOX homeobox genes exhibit spatial and temporal changes in expression during human skin development

The spatial and temporal deployment of HOX homeobox genes along the spinal axis and in limb buds during fetal development is a key program in embryonic pattern formation. Although we have previously reported that several of the HOX homeobox genes are expressed during murine skin development, there is no information about developmental expression of HOX genes in human skin. We have now used reverse transcriptase polymerase chain reaction, in conjunction with a set of degenerate oligonucleotide primers, to identify a subset of HOX genes that are expressed during human fetal skin development. In situ hybridization analyses demonstrated that there were temporal and spatial shifts in expression of these genes. Strong HOXA4 expression was detected in the basal cell layers of 10 wk fetal epidermis and throughout the epidermis and dermis of 17 wk skin, whereas weak signal was present in the granular layer of newborn and adult skin. The expression patterns of HOXA5 and HOXA7 were similar, but their expression was weaker. In situ hybridization analysis also revealed strong HOXC4 and weaker HOXB7 expression throughout fetal development, whereas HOXB4 was expressed at barely detectable levels. Differential HOX gene expression was also observed in developing hair follicles, and sebaceous and sweat glands. None of the HOX genes examined were detected in the adult dermis.

Functional dissection of a transcriptionally active, target-specific Hox-Pbx complex

Hox genes control cell fates and specify regional identities in vertebrate development. Hox proteins show a relaxed DNA-binding selectivity in vitro, suggesting that functional specificity is achieved in vivo through the action of transcriptional co-factors. Pbx proteins are good candidates for such a role, on the basis of both genetic and biochemical evidence. We report that the human Pbx1 and HOXB1 proteins can cooperatively activate transcription through a genetically characterized Hox target, i.e. an autoregulatory element directing spatially restricted expression of the murine Hoxb-1 gene (b1-ARE) in the developing hindbrain. On the b1-ARE, only a restricted subset of HOX proteins (HOXA1, HOXB1, HOXA2) are able to bind cooperatively with Pbx1 and activate transcription. Selective recognition of the b1-ARE is mediated by the N-terminal region of the HOX homeodomain. The DNA-binding and protein-protein interaction functions of HOXB1 and Pbx1 are all necessary for the assembly of a transcriptionally active complex on the b1-ARE. Functional dissection of the complex allowed the localization of the main activation domain in the HOXB1 N-terminal region, and of an additional one in the C-terminal region of Pbx1 contained in the Pbx1a but not in the alternatively spliced Pbx1b isoform. Our results indicate that Pbx1 acts as a transcriptional co-factor of Hox proteins, allowing selective recognition and cooperative activation of regulatory target sequences.

The human homeodomain protein OTX2 binds to the human tenascin-C promoter and trans-represses its activity in transfected cells

Homeodomain-containing proteins mediate many transcriptional processes in eukaryotes during development. Recently, mammalian homeodomain proteins involved in the anterior head formation have been discovered, but their effect on gene transcription has never been investigated. Here we report on the ability of the human homeodomain protein OTX2 to bind with high affinity to a target sequence present in the promoter of the gene encoding the human extracellular matrix protein tenascin-C and to repress its transcriptional activity in transiently transfected cells.

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.

Engrailed and retinotectal topography

We examine the role of the Engrailed homeobox gene in establishment of local tectal topography. In the mesencephalon, a gradient of Engrailed appears early and defines the rostrocaudal axis of the tectum. Various experiments that cause ectopic Engrailed expression cause predictable readjustments of the retinotectal map. The newly discovered 'realisators' of the retinotopic map, such as receptor tyrosine kinase ligands ELF-1 and RAGS could be controlled directly by Engrailed. Indeed, recent results show that Engrailed regulates the expression of these ligands. The Engrailed gradient itself appears to be set up by signals including FGF8 and WNTI, allowing us to begin to trace the molecular cascade that is responsible for the correct wiring of the visual projection back into the early embryo.

HOXB7 constitutively activates basic fibroblast growth factor in melanomas

Homeobox (HOX) genes control axial specification during mammalian development and also regulate skin morphogenesis. Although selected HOX genes are variably expressed in leukemias and kidney and colon cancer cell lines, their relationship with the neoplastic phenotype remains unclear. In both normal development and neoplastic transformation, HOX target genes are largely unknown. We investigated the expression and function of HOXB cluster genes in human melanoma. The HOXB7 gene was constitutively expressed in all 25 melanoma cell lines and analyzed under both normal and serum-starved conditions, as well as in in vivo primary and metastatic melanoma cells; conversely, HOXB7 was expressed in proliferating but not quiescent normal melanocytes. Treatment of melanoma cell lines with antisense oligomers targeting HOXB7 mRNA markedly inhibited cell proliferation and specifically abolished expression of basic fibroblast growth factor (bFGF) mRNA. Band shift and cotransfection experiments showed that HOXB7 directly transactivates the hFGF gene through one out of five putative homeodomain binding sites present in its promoter. These novel findings indicate a key role for constitutive HOXB7 expression in melanoma cell proliferation via bFGF. The results also raise the possibility that growth factor genes are critical HOX target genes in other developmental and/or neoplastic cell systems.

Embryonic expression patterns of the neural cell adhesion molecule gene are regulated by homeodomain binding sites

During development of the vertebrate nervous system, the neural cell adhesion molecule (N-CAM) is expressed in a defined spatiotemporal pattern. We have proposed that the expression of N-CAM is controlled, in part, by proteins encoded by homeobox genes. This hypothesis has been supported by previous in vitro experiments showing that products of homeobox genes can both bind to and transactivate the N-CAM promoter via two homeodomain binding sites, HBS-I and HBS-II. We have now tested the hypothesis that the N-CAM gene is a target of homeodomain proteins in vivo by using transgenic mice containing native and mutated N-CAM promoter constructs linked to a beta-galactosidase reporter gene. Segments of the 5' flanking region of the mouse N-CAM gene were sufficient to direct expression of the reporter gene in the central nervous system in a pattern consistent with that of the endogenous N-CAM gene. For example, at embryonic day (E) 11, beta-galactosidase staining was found in postmitotic neurons in dorsolateral and ventrolateral regions of the spinal cord; at E14.5, staining was seen in these neurons throughout the spinal cord. In contrast, mice carrying an N-CAM promoter-reporter construct with mutations in both homeodomain binding sites (HBS-I and HBS-II) showed altered expression patterns in the spinal cord. At E11, beta-galactosidase expression was seen in the ventrolateral spinal cord, but was absent in the dorsolateral areas, and at E 14.5, beta-galactosidase expression was no longer detected in any cells of the cord. Homeodomain binding sites found in the N-CAM promoter thus appear to be important in determining specific expression patterns of N-CAM along the dorsoventral axis in the developing spinal cord. These experiments suggest that the N-CAM gene is an in vivo target of homeobox gene products in vertebrates.

Restricted expression of homeobox genes distinguishes fetal from adult human smooth muscle cells

Smooth muscle cell plasticity is considered a prerequisite for atherosclerosis and restenosis following angioplasty and bypass surgery. Identification of transcription factors that specify one smooth muscle cell phenotype over another therefore may be of major importance in understanding the molecular basis of these vascular disorders. Homeobox genes exemplify one class of transcription factors that could govern smooth muscle cell phenotypic diversity. Accordingly, we screened adult and fetal human smooth muscle cell cDNA libraries with a degenerate oligonucleotide corresponding to a highly conserved region of the homeodomain with the idea that homeobox genes, if present, would display a smooth muscle cell phenotype-dependent pattern of expression. No homeobox genes were detected in the adult human smooth muscle cell library; however, five nonparalogous homeobox genes were uncovered from the fetal library (HoxA5, HoxA11, HoxB1, HoxB7, and HoxC9). Northern blotting of adult and fetal tissues revealed low and restricted expression of all five homeobox genes. No significant differences in transcripts of HoxA5, HoxA11, and HoxB1 were detected between adult or fetal human smooth muscle cells in culture. HoxB7 and HoxC9, however, showed preferential mRNA expression in fetal human smooth muscle cells that appeared to correlate with the age of the donor. This phenotype-dependent expression of homeobox genes was also noted in rat pup versus adult smooth muscle cells. While similar differences in gene expression have been reported between subsets of smooth muscle cells from rat vessels of different-aged animals or clones of rat smooth muscle, our findings represent a demonstration of a transcription factor distinguishing two human smooth muscle cell phenotypes.

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.

Transcriptional repression by the human homeobox protein EVX1 in transfected mammalian cells

The human homeobox protein EVX1 (EVX1) is thought to play an important role during embryogenesis. In this study, the effect of EVX1 on gene transcription has been investigated in transfected mammalian cells. EVX1 expression represses transcription of a reporter gene directed by either cell-specific or viral promoter/enhancer sequences in a variety of mammalian cell lines and in a concentration-dependent manner. Transcriptional repression is independent of the presence of DNA-binding sites for EVX1 in all the promoters we tested. Furthermore, repression by EVX1 is evident also using a TATA-less minimal promoter in the reporter construct. A carboxyl-terminal proline/alanine-rich region of EVX1 seems to be responsible for the transcriptional repression activity, as suggested by transfection of EVX1 mutants. We speculate that the repressor function of EVX1 contributes to its proposed role in embryogenesis.

Tenascin-C mRNA is expressed in cranial neural crest cells, in some placodal derivatives, and in discrete domains of the embryonic zebrafish brain

A partial zebrafish tenascin-C cDNA clone was isolated from an embryonic zebrafish cDNA library on the basis of homology to mouse tenascin-C. The expression pattern in the head of embryonic zebrafish was analyzed by in situ hybridization. Tenascin-C mRNA was detected in neural crest cells during the period of their migration and differentiation. Expression also occurred in differentiating placodal tissues and in mesodermal cells. In the developing brain, tenascin-C mRNA was expressed in specific domains. In the hindbrain the pattern of the domains was dynamic. At 18 to 22 h postfertilization, expression was widespread in rhombomeres 3, 5, and 6, confined to periventricular cells in rhombomere 2, and not detectable in rhombomere 4. At 32 h postfertilization, tenascin-C was expressed at the rhombomere boundaries. In contrast to the hindbrain, the pattern in the forebrain and midbrain did not show any major changes between 22 and 32 h postfertilization. Domains expressing tenascin-C alternated with regions devoid of it. The most anterior domain of expression was observed at the telencephalic-diencephalic border, surrounding the optic recess. A second domain, at the border between the diencephalon and the midbrain, and a third domain, in the caudal midbrain tegmentum, appeared restricted to the basal plate. Additionally, expression of tenascin-C mRNA was detected in the hypothalamus and in the developing epiphysis. These expression patterns suggest that tenascin-C may play a role in neural crest cell migration and during the differentiation of neural crest, placodal, and mesodermal derivatives. In the developing brain, tenascin-C may be involved in the consolidation of different regional identities.

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.

A hierarchy of ECM-mediated signalling regulates tissue-specific gene expression

A dynamic and reciprocal flow of information between cells and the extracellular matrix contributes significantly to the regulation of form and function in developing systems. Signals generated by the extracellular matrix do not act in isolation. Instead, they are processed within the context of global signalling hierarchies whose constituent inputs and outputs are constantly modulated by all the factors present in the cell's surrounding microenvironment. This is particularly evident in the mammary gland, where the construction and subsequent destruction of such a hierarchy regulates changes in tissue-specific gene expression, morphogenesis and apoptosis during each developmental cycle of pregnancy, lactation and involution.

Extracellular matrix components in intestinal development

Intestinal morphogenesis and differentiation are dependent on heterotypic cell interactions between embryonic epithelial cells (endoderm) and stromal cells (mesenchyme). Extracellular matrix molecules represent attractive candidates for regulators of these interactions. The structural and functional diversity of the extracellular matrix as intestinal development proceeds is demonstrated by 1) spatio-temporal specific expression of the classically described constituents, 2) the finding of laminin and collagen IV variants, 3) changes in the ratio of individual constituent chains, and 4) a stage-specific regulation of basement membrane molecule production, in particular by glucocorticoids. The orientation/assembly of these extracellular matrix molecules could direct precise cellular functions through interactions via integrin molecules. The involvement of extracellular matrix, and in particular basement membrane molecules in heterotypic cell interactions leading to epithelial cell differentiation, has been highlighted by the use of experimental models such as cocultures, hybrid intestines and antisense approaches. These models allowed us to conclude that a correct elaboration and assembly of the basement membrane, following close contacts between epithelial and fibroblastic cells, is necessary for the expression of differentiation markers such as digestive enzymes.