Transforming growth factor-beta-induced osteoblast elongation regulates osteoclastic bone resorption through a p38 mitogen-activated protein kinase- and matrix metalloproteinase-dependent pathway

Transforming growth factor-beta (TGF-beta) is a powerful modulator of bone metabolism, and both its anabolic and catabolic effects on bone have been described. Here we have tested the hypothesis that TGF-beta-induced changes in osteoblast shape promote bone resorption by increasing the surface area of bone that is accessible to osteoclasts. The addition of TGF-beta1 to MC3T3-E1 cells resulted in cytoskeletal reorganization, augmented expression of focal adhesion kinase, and cell elongation, accompanied by an increase in the area of cell-free substratum. TGF-beta1 also triggered activation of Erk1/2 and p38 mitogen-activated protein (MAP) kinase. The p38 MAP kinase inhibitor PD169316, but not an inhibitor of the Erk1/2 pathway, abrogated the effect of TGF-beta1 on cell shape. The matrix metalloproteinase inhibitor GM6001 also interfered with osteoblast elongation. Treatment of MC3T3-E1 cells seeded at confluence onto bone slices to mimic a bone lining cell layer with TGF-beta1 also induced cell elongation and increased pit formation by subsequently added osteoclasts. These effects were again blocked by PD169316 and GM6001. We propose that this novel pathway regulating osteoblast morphology plays an important role in the catabolic effects of TGF-beta on bone metabolism.

Proteinases in bone resorption: obvious and less obvious roles

Bone resorption is critical for the development and the maintenance of the skeleton, and improper regulation of bone resorption leads to pathological situations. Proteinases are necessary for this process. In this review, we show that this need of proteinases is not only because they are required for the solubilization of bone matrix, but also because they are key components of the mechanism that determines where and when bone resorption will be initiated. Moreover, there are indications that proteinases may also determine whether resorption will be followed by bone formation. Some of the proteinases involved in these different steps of the resorption processes were recently identified, as for instance cathepsin K, MMP-9 (gelatinase B), and interstitial collagenase. However, there is also increasing evidence showing that the critical proteinase(s) may vary depending on the bone type or on other factors.

Division of labor among the alpha6beta4 integrin, beta1 integrins, and an E3 laminin receptor to signal morphogenesis and beta-casein expression in mammary epithelial cells

Contact of cultured mammary epithelial cells with the basement membrane protein laminin induces multiple responses, including cell shape changes, growth arrest, and, in the presence of prolactin, transcription of the milk protein beta-casein. We sought to identify the specific laminin receptor(s) mediating the multiple cell responses to laminin. Using assays with clonal mammary epithelial cells, we reveal distinct functions for the alpha6beta4 integrin, beta1 integrins, and an E3 laminin receptor. Signals from laminin for beta-casein expression were inhibited in the presence of function-blocking antibodies against both the alpha6 and beta1 integrin subunits and by the laminin E3 fragment. The alpha6-blocking antibody perturbed signals mediated by the alpha6beta4 integrin, and the beta1-blocking antibody perturbed signals mediated by another integrin, the alpha subunit(s) of which remains to be determined. Neither alpha6- nor beta1-blocking antibodies perturbed the cell shape changes resulting from cell exposure to laminin. However, the E3 laminin fragment and heparin both inhibited cell shape changes induced by laminin, thereby implicating an E3 laminin receptor in this function. These results elucidate the multiplicity of cell-extracellular matrix interactions required to integrate cell structure and signaling and ultimately permit normal cell function.

The stromal proteinase MMP3/stromelysin-1 promotes mammary carcinogenesis

Matrix metalloproteinases (MMPs) are invariably upregulated in the stromal compartment of epithelial cancers and appear to promote invasion and metastasis. Here we report that phenotypically normal mammary epithelial cells with tetracycline-regulated expression of MMP3/stromelysin-1 (Str1) form epithelial glandular structures in vivo without Str1 but form invasive mesenchymal-like tumors with Str1. Once initiated, the tumors become independent of continued Str1 expression. Str1 also promotes spontaneous premalignant changes and malignant conversion in mammary glands of transgenic mice. These changes are blocked by coexpression of a TIMP1 transgene. The premalignant and malignant lesions have stereotyped genomic changes unlike those seen in other murine mammary cancer models. These data indicate that Str1 influences tumor initiation and alters neoplastic risk.

Plasticity of mammary epithelia during normal development and neoplastic progression

The functional unit of the mammary gland is the epithelium. It consists of luminal epithelial cells and myoepithelial cells that are generated from self-renewing stem and progenitor cells. The latter two cell types are scattered throughout the mammary epithelium and are concentrated in specialized structures, the end buds. In transplantation studies the pluripotency of mammary stem cells has been confirmed by demonstrating that they can regenerate a complete mammary gland. The ability of mammary epithelial cells to produce an elaborate ductal system during puberty and to differentiate into milk-producing alveoli during pregnancy is not only influenced by their genetic make-up, but is also governed by local molecular signals. Recent studies suggest that the transdifferentiation of epithelial cells into tumor cells is under microenvironmental control, despite the prominence of genetic mutations in breast cancer. Consequently, disturbances of tissue homeostasis can alter mammary gland development or result in preneoplastic and neoplastic pathologies. The plasticity of mammary epithelia is not limited to the entry of cells into differentiation and transdifferentiation pathways, but extends to their ability to regain facets of their preceding stage of functionality. Deciphering the molecular cues that determine cell plasticity is prerequisite for establishing a unifying concept of mammary gland development and breast tumor progression.

alpha1 and alpha2 integrins mediate invasive activity of mouse mammary carcinoma cells through regulation of stromelysin-1 expression

Tumor cell invasion relies on cell migration and extracellular matrix proteolysis. We investigated the contribution of different integrins to the invasive activity of mouse mammary carcinoma cells. Antibodies against integrin subunits alpha6 and beta1, but not against alpha1 and alpha2, inhibited cell locomotion on a reconstituted basement membrane in two-dimensional cell migration assays, whereas antibodies against beta1, but not against alpha6 or alpha2, interfered with cell adhesion to basement membrane constituents. Blocking antibodies against alpha1 integrins impaired only cell adhesion to type IV collagen. Antibodies against alpha1, alpha2, alpha6, and beta1, but not alpha5, integrin subunits reduced invasion of a reconstituted basement membrane. Integrins alpha1 and alpha2, which contributed only marginally to motility and adhesion, regulated proteinase production. Antibodies against alpha1 and alpha2, but not alpha6 and beta1, integrin subunits inhibited both transcription and protein expression of the matrix metalloproteinase stromelysin-1. Inhibition of tumor cell invasion by antibodies against alpha1 and alpha2 was reversed by addition of recombinant stromelysin-1. In contrast, stromelysin-1 could not rescue invasion inhibited by anti-alpha6 antibodies. Our data indicate that alpha1 and alpha2 integrins confer invasive behavior by regulating stromelysin-1 expression, whereas alpha6 integrins regulate cell motility. These results provide new insights into the specific functions of integrins during tumor cell invasion.

An odyssey from breast to bone: multi-step control of mammary metastases and osteolysis by matrix metalloproteinases

Development of metastases distant to the primary site of solid tumors marks late stages of tumor progression. Almost all malignant mammary tumors are carcinomas arising from the breast epithelium, but the morphological and molecular alterations in the mammary stroma surrounding the premalignant and the growing tumor contribute to its conversion into neoplastic tissue. Two parameters are critical for initiation of the metastatic process and access of tumor cells to the circulation. These are the ability of tumor cells to invade the basement membrane and the stroma, and the neovascularization of breast tumor tissue. A major site for development of distant metastases is the skeleton. After colonizing the bone, tumor cells promote a cascade of events leading to recruitment of osteoclasts and subsequent osteolytic bone destruction. A ubiquitous theme of neoplastic progression of breast tumors is the overproduction of matrix metalloproteinases. In this review, we summarize the recent insights into the functional consequences of matrix metalloproteinase expression and activation during malignant conversion in the breast, and after bone colonization. The current literature supports the hypothesis that matrix metalloproteinases play a key role in the metastatic expansion of most, if not all, mammary tumors and in the ensuing bone loss.

The significance of matrix metalloproteinases during early stages of tumor progression

Matrix metalloproteinases (MMPs) orchestrate tissue remodeling and play diverse roles during organ development. They are produced excessively during the course of various pathological conditions, including solid tumors. An important function of MMPs during tumor progression is to provide the proteolytic activity that is necessary both for tumor cells to invade extracellular matrix (ECM) and for neovascularization of tumor tissue by endothelial cells. Recently, independent studies in transgenic animals suggest that MMPs may, in addition, promote very early stages of tumor progression. To investigate this possibility further, we have analyzed the consequences of MMP overexpression in functionally normal and nontumorigenic mouse mammary epithelial cells in culture. Our observations demonstrate that the MMP stromelysin-1 (SL-1) triggers an epigenetic molecular program in mammary epithelial cells that results in a number of phenotypic alterations that eventually culminate in the generation of a malignant tumor-cell phenotype.

Expression of autoactivated stromelysin-1 in mammary glands of transgenic mice leads to a reactive stroma during early development

Extracellular matrix and extracellular matrix-degrading matrix metalloproteinases play a key role in interactions between the epithelium and the mesenchyme during mammary gland development and disease. In patients with breast cancer, the mammary mesenchyme undergoes a stromal reaction, the etiology of which is unknown. We previously showed that targeting of an autoactivating mutant of the matrix metalloproteinase stromelysin-1 to mammary epithelia of transgenic mice resulted in reduced mammary function during pregnancy and development of preneoplastic and neoplastic lesions. Here we examine the cascade of alterations before breast tumor formation in the mammary gland stroma once the expression of the stromelysin-1 transgene commences. Beginning in postpubertal virgin animals, low levels of transgene expression in mammary epithelia led to increased expression of endogenous stromelysin-1 in stromal fibroblasts and up-regulation of other matrix metalloproteinases, without basement membrane disruption. These changes were accompanied by the progressive development of a compensatory reactive stroma, characterized by increased collagen content and vascularization in glands from virgin mice. This remodeling of the gland affected epithelial-mesenchymal communication as indicated by inappropriate expression of tenascin-C starting by day 6 of pregnancy. This, together with increased transgene expression, led to basement membrane disruption starting by day 15 of pregnancy. We propose that the highly reactive stroma provides a prelude to breast epithelial tumors observed in these animals.

Tissue architecture: the ultimate regulator of epithelial function?

The architecture of a tissue is defined by the nature and the integrity of its cellular and extracellular compartments, and is based on proper adhesive cell-cell and cell-extracellular matrix interactions. Cadherins and integrins are major adhesion-mediators that assemble epithelial cells together laterally and attach them basally to a subepithelial basement membrane, respectively. Because cell adhesion complexes are linked to the cytoskeleton and to the cellular signalling pathways, they represent checkpoints for regulation of cell shape and gene expression and thus are instructive for cell behaviour and function. This organization allows a reciprocal flow of mechanical and biochemical information between the cell and its microenvironment, and necessitates that cells actively maintain a state of homeostasis within a given tissue context. The loss of the ability of tumour cells to establish correct adhesive interactions with their microenvironment results in disruption of tissue architecture with often fatal consequences for the host organism. This review discusses the role of cell adhesion in the maintenance of tissue structure and analyses how tissue structure regulates epithelial function.

Epimorphin functions as a key morphoregulator for mammary epithelial cells

Hepatocyte growth factor (HGF) and EGF have been reported to promote branching morphogenesis of mammary epithelial cells. We now show that it is epimorphin that is primarily responsible for this phenomenon. In vivo, epimorphin was detected in the stromal compartment but not in lumenal epithelial cells of the mammary gland; in culture, however, a subpopulation of mammary epithelial cells produced significant amounts of epimorphin. When epimorphin-expressing epithelial cell clones were cultured in collagen gels they displayed branching morphogenesis in the presence of HGF, EGF, keratinocyte growth factor, or fibroblast growth factor, a process that was inhibited by anti-epimorphin but not anti-HGF antibodies. The branch length, however, was roughly proportional to the ability of the factors to induce growth. Accordingly, epimorphin-negative epithelial cells simply grew in a cluster in response to the growth factors and failed to branch. When recombinant epimorphin was added to these collagen gels, epimorphin-negative cells underwent branching morphogenesis. The mode of action of epimorphin on morphogenesis of the gland, however, was dependent on how it was presented to the mammary cells. If epimorphin was overexpressed in epimorphin-negative epithelial cells under regulation of an inducible promoter or was allowed to coat the surface of each epithelial cell in a nonpolar fashion, the cells formed globular, alveoli-like structures with a large central lumen instead of branching ducts. This process was enhanced also by addition of HGF, EGF, or other growth factors and was inhibited by epimorphin antibodies. These results suggest that epimorphin is the primary morphogen in the mammary gland but that growth factors are necessary to achieve the appropriate cell numbers for the resulting morphogenesis to be visualized.

Matrix metalloproteinase stromelysin-1 triggers a cascade of molecular alterations that leads to stable epithelial-to-mesenchymal conversion and a premalignant phenotype in mammary epithelial cells

Matrix metalloproteinases (MMPs) regulate ductal morphogenesis, apoptosis, and neoplastic progression in mammary epithelial cells. To elucidate the direct effects of MMPs on mammary epithelium, we generated functionally normal cells expressing an inducible autoactivating stromelysin-1 (SL-1) transgene. Induction of SL-1 expression resulted in cleavage of E-cadherin, and triggered progressive phenotypic conversion characterized by disappearance of E-cadherin and catenins from cell-cell contacts, downregulation of cytokeratins, upregulation of vimentin, induction of keratinocyte growth factor expression and activation, and upregulation of endogenous MMPs. Cells expressing SL-1 were unable to undergo lactogenic differentiation and became invasive. Once initiated, this phenotypic conversion was essentially stable, and progressed even in the absence of continued SL-1 expression. These observations demonstrate that inappropriate expression of SL-1 initiates a cascade of events that may represent a coordinated program leading to loss of the differentiated epithelial phenotype and gain of some characteristics of tumor cells. Our data provide novel insights into how MMPs function in development and neoplastic conversion.

Inhibitors of protein kinases abolish ECM-mediated promotion of neuronal polarity

Different extracellular matrix (ECM) molecules, when presented to hippocampal neurons in culture in a substrate-bound form, exert strikingly similar effects on the establishment of neuronal polarity, i.e., the growth of axon-like major neurites is favored, whereas extension of dendrite-like minor neurites is inhibited. To gain insight into the underlying signal transduction processes, we have investigated the effects of modulators of protein kinase activity on the morphology of neurons cultured on tenascin-R, tenascin-C, and laminin-entactin substrates. We found differential effects of broad-spectrum protein kinase inhibitors: H-7 promoted the growth of minor neurites, whereas H-8 reduced the growth of major neurites on ECM but not control substrates. In contrast, chelerythrine, a specific inhibitor of protein kinase C, selectively affected growth of both minor and major neurites on control, but not on ECM substrates. Finally, reagents which elevate intracellular cAMP levels facilitated growth of minor neurites and inhibited growth of major neurites and thus interfered with the establishment of a polarized phenotype on both ECM and control substrates. Our results suggest that protein kinases mediate the effects of ECM molecules on neuronal polarity and that different kinases control extension of axons and dendrites.

Misregulation of stromelysin-1 expression in mouse mammary tumor cells accompanies acquisition of stromelysin-1-dependent invasive properties

Stromelysin-1 is a member of the metalloproteinase family of extracellular matrix-degrading enzymes that regulates tissue remodeling. We previously established a transgenic mouse model in which rat stromelysin-1 targeted to the mammary gland augmented expression of endogenous stromelysin-1, disrupted functional differentiation, and induced mammary tumors. A cell line generated from an adenocarcinoma in one of these animals and a previously described mammary tumor cell line generated in culture readily invaded both a reconstituted basement membrane and type I collagen gels, whereas a nonmalignant, functionally normal epithelial cell line did not. Invasion of Matrigel by tumor cells was largely abolished by metalloproteinase inhibitors, but not by inhibitors of other proteinase families. Inhibition experiments with antisense oligodeoxynucleotides revealed that Matrigel invasion of both cell lines was critically dependent on stromelysin-1 expression. Invasion of collagen, on the other hand, was reduced by only 40-50%. Stromelysin-1 was expressed in both malignant and nonmalignant cells grown on plastic substrata. Its expression was completely inhibited in nonmalignant cells, but up-regulated in tumor cells, in response to Matrigel. Thus misregulation of stromelysin-1 expression appears to be an important aspect of mammary tumor cell progression to an invasive phenotype.

Distinct effects of recombinant tenascin-C domains on neuronal cell adhesion, growth cone guidance, and neuronal polarity

Using a set of recombinantly expressed proteins, distinct domains of the mouse extracellular matrix glycoprotein tenascin-C, hereafter called tenascin, have been identified to confer adhesion, anti-adhesion, and changes in morphology of neuronal cells. In short-term adhesion assays (1 hr), cerebellar and hippocampal neurons adhered to several domains, encompassing the fibronectin type III-like (FN III) repeats 1-2 and 6-8, as well as to the alternatively spliced FN III repeats and to tenascin itself. Although no short-term adhesion to the EGF repeats containing fragment could be detected under the conditions used, it was anti-adhesive for neuronal cell bodies and repellent for growth cone advance and neuritogenesis. FN III repeats 3-5 were repellent only for growth cones but not for neuronal cell bodies. Neurite outgrowth promoting activities at early stages and induction of a polarized neuronal morphology at later stages of differentiation were associated with the EGF repeats and the FN III repeats 6-8. These observations suggest differential effects of particular domains of the tenascin molecule on distinct cellular compartments, i.e., cell body, axon and dendrite, and existence of multiple neuronal receptors with distinct intracellular signaling features.

Control of neuronal morphology in vitro: interplay between adhesive substrate forces and molecular instruction

Among the factors which influence neuronal morphology, the degree of substrate adhesivity has been suggested to play an important role in the growth and guidance of neurites. The present study was undertaken to investigate apparently contradictory results relating substrate adhesivity to the extent of neurite outgrowth. By using substrates coated with different concentrations of polyornithine to vary adhesivity, we could show that intermediate levels of neuron-to-substrate adhesive strength favored neurite outgrowth more than substrates of high or low adhesivity. However, when neurons were plated on substrates derived from the extracellular matrix, the strength of neuron-to-substrate adhesion was important for the growth of dendrite-like minor neurites, but not for the extension of axon-like major neurites, which grew independently of adhesive forces. On substrates of the cell adhesion molecule L1, growth of both major and minor neurites was adhesion-independent. Finally, in the presence of tenascin added to the culture medium, neurite growth was inhibited irrespective of the adhesivity of the substrate and the presence of substrate-bound extracellular matrix molecules or L1. These observations suggest that intermediate forces of adhesivity favor neurite growth in general, but that purely adhesive forces can be dominated by specific molecular instructions which differentially affect growth of major and minor neurites in positive and negative ways.

Involvement of extracellular matrix constituents in breast cancer

It has recently been established that the extracellular matrix is required for normal functional differentiation of mammary epithelia not only in culture, but also in vivo. The mechanisms by which extracellular matrix affects differentiation, as well as the nature of extracellular matrix constituents which have major impacts on mammary gland function, have only now begun to be dissected. The intricate variety of extracellular matrix-mediated events and the remarkable degree of plasticity of extracellular matrix structure and composition at virtually all times during ontogeny, make such studies difficult. Similarly, during carcinogenesis, the extracellular matrix undergoes gross alterations, the consequences of which are not yet precisely understood. Nevertheless, an increasing amount of data suggests that the extracellular matrix and extracellular matrix-receptors might participate in the control of most, if not all, of the successive stages of breast tumors, from appearance to progression and metastasis.

Isolation of a neural chondroitin sulfate proteoglycan with neurite outgrowth promoting properties

Proteoglycans are expressed in various tissues on cell surfaces and in the extracellular matrix and display substantial heterogeneity of both protein and carbohydrate constituents. The functions of individual proteoglycans of the nervous system are not well characterized, partly because specific reagents which would permit their isolation are missing. We report here that the monoclonal antibody 473HD, which binds to the surface of early differentiation stages of murine astrocytes and oligodendrocytes, reacts with the chondroitin sulfate/dermatan sulfate hybrid epitope DSD-1 expressed on a central nervous system chondroitin sulfate proteoglycan designated DSD-1-PG. When purified from detergent-free postnatal days 7 to 14 mouse brain extracts, DSD-1-PG displays an apparent molecular mass between 800-1,000 kD with a prominent core glycoprotein of 350-400 kD. Polyclonal anti-DSD-1-PG antibodies and monoclonal antibody 473HD react with the same molecular species as shown by immunocytochemistry and sequential immunoprecipitation performed on postnatal mouse cerebellar cultures, suggesting that the DSD-1 epitope is restricted to one proteoglycan. DSD-1-PG promotes neurite outgrowth of embryonic day 14 mesencephalic and embryonic day 18 hippocampal neurons from rat, a process which can be blocked by monoclonal antibody 473HD and by enzymatic removal of the DSD-1-epitope. These results show that the hybrid glycosaminoglycan structure DSD-1 supports the morphological differentiation of central nervous system neurons.

The extracellular matrix molecule janusin regulates neuronal morphology in a substrate- and culture time-dependent manner

Janusin is an extracellular matrix glycoprotein with structural homology to tenascin. In search of extracellular matrix components which govern the differentiation of neurons in the central nervous system, we have investigated the influence of janusin on the differentiation of hippocampal neurons in vitro. Janusin coated onto nitrocellulose was a good substrate for attachment of cell bodies and neurite outgrowth after 21 h of culture. Most cells exhibited a polarized morphology with one long major neurite and one or two short minor neurites. When janusin was coated onto a polyornithine-conditioned plastic surface, it increased the polarity of neurons in that the length of major neurites was increased and the length and number of minor neurites were decreased when compared with the control polyornithine-conditioned plastic without janusin. As we have shown before for tenascin, laminin and fibronectin, polarization was preceded by an increase in the number and length of all neurites during the first hours after cell plating. This study therefore adds janusin to the increasing number of extracellular matrix glycoproteins which promote axonal but not dendritic growth.

Tenascin and extracellular matrix glycoproteins: from promotion to polarization of neurite growth in vitro

The extracellular matrix molecules tenascin, laminin, and fibronectin, the cell adhesion molecule L1, and the lectin concanavalin A (ConA) were tested for their effects on neuritogenesis in cultures of hippocampal neurons. We analyzed neurite outgrowth between 3 and 21 hr after plating and found that, on polyornithine as control substrate, lengths of axon-like major neurites and dendrite-like minor neurites increased continuously with time in culture. Moreover, growth of minor neurites was faster than growth of major neurites. When the extracellular matrix molecules tenascin, laminin, and fibronectin were coated on polyornithine substrates, growth of all neurites was faster than on control substrates during the first hours of culture. After this initial phase of enhanced neurite outgrowth, elongation of major neurites continued at a higher rate than on the control substrate and growth of minor neurites ceased after 12 hr. Correspondingly, neuronal polarity was strongly increased on the extracellular matrix substrates during later phases of culture. In contrast, lengths of both major and minor neurites were increased over control values on L1 and ConA substrates at all time points investigated. Thus, neuronal polarity was similar for control, L1, and ConA substrates. Spreading of neuronal cell bodies was reduced by about 50% on tenascin, laminin, and fibronectin and by less than 20% on L1 and ConA substrates after 21 hr of culture, when compared to the control substrate. Neuron-to-substrate adhesion was reduced on all three extracellular matrix substrates but not affected on L1 or ConA substrates, after 3 and 21 hr of culture. These observations indicate that induction of neuronal polarity is not a general feature of neurite outgrowth-promoting molecules, such as L1 or ConA, but a distinctive property of the three extracellular matrix glycoproteins studied, and may suggest that enhancement of polarity is correlated with decreased strength of adhesion.

J1/tenascin in substrate-bound and soluble form displays contrary effects on neurite outgrowth

The influence of J1/tenascin adsorbed to polyornithine-conditioned plastic (substrate-bound J1/tenascin) and J1/tenascin present in the culture medium (soluble J1/tenascin) on neurite outgrowth was studied with cultured single cells from hippocampus and mesencephalon of embryonic rats. Neurons at low density grew well on J1/tenascin substrates and extended neurites that were approximately 40% longer than on the polyornithine control substrate after 24 h in vitro. The neurite outgrowth promoting effect of substrate bound J1/tenascin was largely abolished in the presence of mAb J1/tn2, but not by mAb J1/tn1. In contrast to the neurite growth-promoting effects of substrate bound J1/tenascin, neurite outgrowth on polyornithine, laminin, fibronectin, or J1/tenascin as substrates was inhibited by addition of soluble J1/tenascin to the cultures. Neither of the two mAbs neutralized the neurite outgrowth-inhibitory properties of soluble J1/tenascin. In contrast to their opposite effects on neurite outgrowth, both substrate-bound and soluble J1/tenascin reduced spreading of the neuronal cell bodies, suggesting that the neurite outgrowth-promoting and antispreading effects are mediated by two different sites on the molecule. This was further supported by the inability of the mAb J1/tn2 to neutralize the antispreading effect. The J1/tn2 epitope localizes to a fibronectin type III homology domain that is presumably distinct from the putative Tn68 cell-binding domain of chicken tenascin for fibroblasts, as shown by electronmicroscopic localization of antibody binding sites. We infer from these experiments that J1/tenascin contains a neurite outgrowth promoting domain that is distinguishable from the cell-binding site and presumably not involved in the inhibition of neurite outgrowth or cell spreading. Our observations support the notion that J1/tenascin is a multifunctional extracellular matrix molecule.