Molecular forms, binding functions, and developmental expression patterns of cytotactin and cytotactin-binding proteoglycan, an interactive pair of extracellular matrix molecules

Domains of neuronal heparan sulphate proteoglycans involved in neurite growth on laminin

A single neuronal cell assay of neurite growth was utilized to determine types and domains of neuronal proteoglycans involved in neurite growth on laminin. Perturbations of biosynthesis and processing, enzymatic digestion with specific lyases, and competition with glycosaminoglycan side chains produced complementary data consistent with a molecular model implicating glycosaminoglycan (GAG) residues of heparan sulphate proteoglycans (HSPGs) in neurite growth. The observations suggest that HSPGs promote neurite growth on laminin by bridging between binding domains for HSPGs on laminin and on the neuronal cell surface, and that the bridge is tethered at both ends by non-covalent interactions between the binding domains and GAG side chains. Sulphation of the GAGs of HSPGs appears to be critical to the tethering and/or neurite growth-promoting activity of neuronal HSPGs.

Isolation of chick tenascin variants and fragments. A C-terminal heparin-binding fragment produced by cleavage of the extra domain from the largest subunit splicing variant

The extracellular-matrix glycoprotein, tenascin, consists of disulfide-linked subunits of 190, 200 and 230 kDa (the three splicing variants reported in chicken) and usually exists as a six-armed structure under the electron microscope. We used monoclonal antibodies to isolate and characterize different splicing variants and proteolytic fragments obtained from the native protein. Purified monomeric tenascin has a native molecular mass of 216 kDa and is structured as single arms. Tenascin fragments obtained by pepsin digestion bind to monoclonal antibody (mAb) TnM1 which is directed against epidermal-growth-factor-like repeats in the N-terminal half of all subunits. These fragments represent the thin proximal part of the tenascin arms and they are still partially linked to dimers and trimers via disulfide bridges. Using mAb Tn68, that reacts with a fibronectin-type-III repeat towards the C-terminus, a tenascin fragment, generated by treatment with pronase, can be isolated. Ultrastructurally, this fragment looks like the thicker distal part of the tenascin arms. Only the 230-kDa variant of tenascin gives rise to this distal fragment after cleavage within the alternatively spliced fibronectin-type-III repeats. Native tenascin and all fragments containing the distal part of its arms bind to heparin-agarose, whereas the proximal fragments do not. Oligomeric and monomeric tenascin inhibit fibronectin-mediated fibroblast spreading with comparable efficiency when added to the culture medium, while the proximal fragment has no effect. The distal fragment as well as reduced and alkylated tenascin are active in this assay, but only at higher molar concentrations when compared to the native protein.

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.

On the possible role of endogenous lectins in early animal development

In this review I have tried to summarize the information available on the lectins of developing embryos. The emerging evidence indicates that during fertilization carbohydrate-binding proteins play a role in sperm adhesion and in the reorganization of the extracellular matrix of the fertilized egg. Results also indicate that in adult tissues lectins participate in cell recognition and adhesion, and that several galactose-binding lectins function as receptors for laminin and, in principle could also interact with polylactosamine groups of other extracellular matrix glycoproteins. Since in developing embryos lectins are located at the cell surface, and colocalize with extracellular matrix glycoproteins, they could play a role in transitory adhesive interactions and in the segregation of organ primordia. On the basis of experiments in cultured cell lines, it has been suggested that lectins are involved in lysosomal and nuclear glycoprotein transport. These carbohydrate-binding proteins could also regulate development by modulating these processes in the embryo. Since galactose-binding lectins are mitogenic, and are present in high concentration in the chick yolk sac, these proteins could be released into the embryonic circulation, bind to cells expressing appropriate receptors, and act as growth regulators, by modulating cell division of specific cell lineages.

Expression of adhesion molecules during the formation and differentiation of the avian endocardial cushion tissue

The expression of cytotactin, the cytotactin-binding (CTB) proteoglycan, and the neural cell adhesion molecule, N-CAM, was examined during the development of the avian endocardial cushion tissue (ECT). N-CAM was present in the cardiac mesoderm from its earliest time of development. At the time when endothelial cells converted to mesenchyme and began to migrate, they ceased their expression of N-CAM. Cytotactin and CTB proteoglycan were present in the cardiac jelly (into which the ECT cells migrate) in patterns that were correlated with cell migration. At early times of migration (stage 18), the region of the cardiac jelly near the endocardium contained cytotactin in the vicinity of the migrating cells. During later migration (stage 22), cytotactin remained associated with the leading zone of cell migration, but its expression began to decrease in areas where cells had accumulated. After ECT cell migration had ceased, cytotactin expression decreased, remaining high only in the peripheral portion of the aorticopulmonary septum and absent from its ridges. CTB proteoglycan was expressed during early migration at high levels in and adjacent to the myocardium. By stage 22, its distribution had become more uniform throughout the ECT regions and in the myocardium. The combined results of this study suggest that cytotactin, CTB proteoglycan, and N-CAM each play a distinct, critical role in pattern formation in the early heart.

Chondroitin sulfate proteoglycan surrounds a subset of human and rat CNS neurons

Chondroitin sulfate proteoglycan (CS-PG) bearing glycosaminoglycan (GAG) chains containing unsulfate (COS) and 6-sulfate (C6S) disaccharides was immunolocalized in rat and human CNS by using monoclonal antibodies (MAb) specific for the two disaccharides. The immunostaining with both MAb was restricted to the periphery of a neuronal subset in rat and human CNS. Double immunofluorescence showed codistribution of the antigens around the same neuronal population. The staining with anti-COS MAb was stronger than with anti-C6S MAb, suggesting that the proteoglycan (PG) contains mainly COS disaccharides. In different rat cortical areas, 40-60/mm2 positive interneurons were found, the visual cortex showing the highest value. In human cortex, positivity was also observed around the soma of some pyramidal cells. In the rat, positive neurons were also localized in deep cerebellar nuclei, reticular nucleus of the thalamus, and other structures of the midbrain and hindbrain. CA3 region of hippocampus and the external layer of pyriform cortex were characterized by positivity of the neuropil. Immunoelectronmicroscopy showed the antigens in the extracellular space around the neuronal soma, the synaptic elements and the cell processes of the neuropil. The neuronal surface of the soma and of the proximal dendrites were positive, but the pre- and postsynaptic membranes and clefts were negative.

Development and migration of Purkinje cells in the mouse cerebellar primordium

The mode of Purkinje cell migration in the mouse cerebellar primordium was examined immunohistochemically, by marking Purkinje cells with anti-spot 35 antibody and labeling them with 5'-bromodeoxyuridine. The cells migrated radially from the neuroepithelium of the fourth ventricle towards the cortical surface between the 13th and 17th days (E13-E17) of gestation. Regional differences in the migratory process were evident: the final settlement of the Purkinje cells proceeded earlier in the lateral and posterior parts of the primordium, exhibiting latero-medial and posteroventral-anterodorsal diminishing sequences. To elucidate the factors involved in the migration, the arrangement of radial glial fibers, and expression of the cell adhesion molecule, tenascin, were examined immunohistochemically with the monoclonal antibody 1D11, a marker for both immature and mature astroglia, and an anti-tenascin antibody. At E14, 1D11-immunopositive fibers were seen to extend from the ventricle to the pial surface, and the cell bodies of immature glia migrated after E15 towards the cortex, shortening the radial processes whose end-feet were attached to the pia mater. Tenascin, which possesses a neuron-glial adhesiveness, was also expressed on the radial fibers during the migration of the Purkinje cells. The fibers were closely apposed to the migratory Purkinje cells, and their arrangement and orientation accorded with the migratory direction of the Purkinje cells. Further, changes in the molecular species of antigens detected by both the 1D11 and anti-tenascin antibodies were observed by immunoblotting analysis during the course of cerebellar development. These findings suggest that the arrangement of radial glia and expression of adhesion molecules may be involved in the control and guidance of Purkinje cell migration.

Purification and partial characterization of Xenopus laevis tenascin from the XTC cell line

We report here the purification of tenascin, an extracellular matrix molecule involved in the control of morphogenesis, from the conditioned medium of the Xenopus XTC cell line. Tenascin was purified by affinity chromatography on a column of the monoclonal antibody mAb TnM1; the molecule eluted from this column has a relative molecular mass of 210 kDa after reduction. Electrophoretic analysis under non-reducing conditions shows that the purified components are oligomeric disulfide-linked complexes which barely enter a 4% polyacrylamide gel. Upon rotary shadowing these molecules appear to possess a central globular domain to which pairs or triplets of arms are attached. Polyclonal antibodies have been raised against purified Xenopus tenascin. They recognise specifically the antigen on Western blots of XTC conditioned medium and adult brain, by immunofluorescence, these antibodies reveal large amounts of tenascin in the secretory vesicles as well as in the extracellular matrix of XTC cells. In the Xenopus tadpole, they stain the developing cartilage, the basal lamina of skin epidermis, myotendinous ligaments and restricted regions of the central nervous system.

Cholinesterases during development of the avian nervous system

1. Long before onset of synaptogenesis in the chicken neural tube, the closely related enzymes butyrylcholinesterase (BChE) and acetylcholinesterase (AChE) are expressed in a mutually exclusive manner. Accordingly, neuroblasts on the ventricular side of the neural tube transiently express BChE before they abruptly accumulate AChE while approaching the outer brain surface. 2. By exploiting AChE as a sensitive and early histochemical differentiation marker, we have demonstrated complex polycentric waves of differentiation spreading upon the cranial part of the chicken neural tube but a smooth rostrocaudal wave along the spinal cord. Shortly after expression of AChE, these cells extend long projecting neurites. In particular, segmented spinal motor axons originate from AChE-positive motoneurones; they navigate through a BChE-active zone within the rostral half of the sclerotomes before contacting BChE/AChE-positive myotome cells. At synaptogenetic stages, cholinesterases additionally are detectable in neurofibrillar laminae foreshadowing the establishment of cholinergic synapses. 3. In order to elucidate the functional significance of cholinesterases at early stages, we have investigated specific cholinesterase molecules and their mechanisms of action in vivo and in vitro. A developmental shift from the low molecular weight forms to the tetramers of both enzymes has been determined. In vitro, the addition of a selective BChE inhibitor leads to a reduction of AChE gene expression. Thus, in vivo and in vitro data suggest roles of cholinesterases in the regulation of cell proliferation and neurite growth. 4. Future research has to show whether neurogenetic functioning of cholinesterases can help to understand their reported alterations in neural tube defects, mental retardations, dementias and in some tumours.

Cells and cell-interactions that guide motor axons in the developing chick embryo

A considerable challenge confronts any developing neuron. Before it can establish a functional and specific connection, it must extend an axon over tens and sometimes hundreds of microns through a complex and mutable environment to reach one out of many possible destinations. The field of axonal guidance concerns the control of this navigation process. To satisfactorily identify the cell interactions and molecular mechanisms that mediate axonal guidance, it is essential to first identify the pertinent cell populations. Embryonic surgeries have provided solid information on which tissues are critical and which are irrelevant to the navigation of motor axons within the chick embryo. The gross anatomical nerve pattern is established as axons respond to both positive (path) and negative (barrier) tissue environments. Analysis of the interactions of motoneurons with these tissues reveals that several cellular interactions--chemotaxis, substratum preference, and perhaps contact paralysis--are important to the common patterns of motor axon advance. Axons simultaneously interact with population-specific cues that have begun to be identified on the tissue level.

Nerve growth factor mediates monosialoganglioside-induced release of fibronectin and J1/tenascin from C6 glioma cells

C6 rat glioma cells incubated in serum-free medium with D-[14C]glucosamine secrete, on stimulation with nerve growth factor (NGF) or monosialogangliosides (MSGs), several glycoproteins (Gps), the most prominent of which are a 270-, 220-, and 69-kDa Gp. Several growth factors, hormones, phorbol ester, and disialo- and trisialogangliosides did not stimulate secretion. Western blot analysis of the conditioned medium from C6 cells stimulated with NGF or MSG identified one distinct band of approximately 220 kDa for fibronectin and J1/tenascin, which comigrated. Antiserum to NGF prevented NGF-stimulated release and also blocked MSG-evoked release. The 220-kDa band was labeled after pulse labeling with [35S]methionine in the presence of NGF, and by a 15-min chase period radioactively labeled J1/tenascin could be immunoprecipitated. Tunicamycin drastically inhibited almost completely release of the 220-kDa Gp labeled by D-[14C]glucosamine or [35S]methionine. These results extend the range of neurotrophic properties attributed to NGF to cells of glial origin and suggest that NGF regulates secretion of extracellular matrix proteins. MSG stimulation of fibronectin and J1/tenascin secretion may be mediated by NGF or an NGF-like molecule also secreted by the C6 glioma cells.

Tenascin expression in hyperproliferative skin diseases

The expression of tenascin, a recently discovered extracellular matrix glycoprotein, was studied by immunohistochemistry in normal human skin and in a number of skin diseases with epidermal hyperproliferation such as psoriasis, basal cell carcinoma, Bowen's disease and solar keratosis. Tenascin expression in the upper dermis of normal skin was found to vary from almost absent to patchy along the basal membrane. Staining was continuous and intense around blood vessels, hair follicles and eccrine sweat ducts. In basal cell carcinoma a marked expression of tenascin was found in the tumour stroma, especially adjacent to the basal membrane surrounding the tumour cell nests. In Bowen's disease and solar keratosis, tenascin expression was found in the dermis next to the keratinocytes. In psoriasis the dermal papillae of clinically involved skin were intensely stained and a continuous band of tenascin was present in the upper dermis along the basal membrane. The distribution of tenascin differed from other known extracellular matrix components.

Characterization of glycosaminoglycans produced by primary astrocytes in vitro

Quantitative biosynthetic studies with cultures highly enriched for glial fibrillary acidic protein (GFAP+) cells of neonatal mammalian brain demonstrated production of four proteoglycans: hyaluronate (HA), heparan sulphate (HS), chondroitin sulphate (CS), and dermatan sulphate (DS). The glycosaminoglycans were present in cell conditioned medium and in the cellular compartment. There were qualitative differences in the subcellular disposition of the various proteoglycans. The ratio of HS to CS/DS in cell extracts was 1:1, while in medium this ratio was 1:6. All of the glycosaminoglycans were associated with core proteins that were integral to the cell membrane and associated with the cell surface by non-covalent interactions involving glycosaminoglycans. Less than 20% of the HS was non-covalently associated with the astrocyte cell surface reflecting in part the proportionately smaller amounts of this proteoglycan released to astrocyte conditioned medium. HS released to medium was undersulphated relative to that associated with cells. The astrocyte can contribute proteoglycans to the extracellular milieu and displays cell surface proteoglycans that have the potential to provide appropriate substrates for neuron adhesion, process extension, and other cell-cell interactions.

Interaction of the NG2 chondroitin sulfate proteoglycan with type VI collagen

The NG2 chondroitin sulfate proteoglycan is a membrane-associated molecule of approximately 500 kD with a core glycoprotein of 300 kD. Both the complete proteoglycan and a smaller quantity of the 300-kD core are immunoprecipitable with polyclonal and monoclonal antibodies against purified NG2. From some cell lines, the antibodies coprecipitate NG2 and type VI collagen, the latter appearing on SDS-PAGE as components of 140 and 250 kD under reducing conditions. The immunoprecipitation of type VI collagen does not seem to be due to recognition of the collagen by the antibodies, but rather to binding of the collagen to NG2. Studies on the NG2-type VI collagen complex suggest that binding between the two molecules is mediated by protein-protein interactions rather than by ionic interactions involving the glycosaminoglycans. Immunofluorescence double labeling in frozen sections of embryonic rat shows that NG2 and type VI collagen are colocalized in structures such as the intervertebral discs and arteries of the spinal column. In vitro the two molecules are highly colocalized on the surface of several cell lines. Treatment of these cells resulting in a change in the distribution of NG2 on the cell surface also causes a parallel change in type VI collagen distribution. Our results suggest that cell surface NG2 may mediate cellular interactions with the extracellular matrix by binding to type VI collagen.

Chondroitin 4-sulfate proteoglycan forms an extracellular network in human and rat central nervous system

Chondroitin 4-sulfate proteoglycan (C4S-PG) was localized both in rat and human central nervous system (CNS) by monoclonal and polyclonal antisera recognizing the 4-sulfate disaccharide (C4S). In the rat the whole CNS was studied in serial coronal sections. A positive extracellular meshwork was observed both in white and grey matters. In the white matter (WM) C4S-PG formed a network around myelinated axons, sparing myelin sheaths and axoplasms. The neuropil of the grey matter (GM) showed a positive meshwork constituted by delicate intermingling filaments. The cytoplasms of neuronal, glial and endothelial cells were negative. Stronger straining than in the neuropil was observed around the soma and the proximal part of the cell processes of some neurons located in the cortex, in the deep cerebellar nuclei and in some other CNS nuclei. A similar pattern was also observed in human CNS, the only difference being a smaller amount of cortical neurons surrounded by a rim of C4S-PG. This study shows that a PG bearing C4S disaccharide is located extracellularly in the rodent and human CNS and that C4S disaccharides can be present in different types of CNS proteoglycans (PGs).

Changes in glycosaminoglycans during the neuritogenesis in PC12 pheochromocytoma cells induced by nerve growth factor

Previously, we had suggested that heparan sulfate (HS) makes some contribution to a flat-shaped morphology of PC12D cells. Therefore, we carried out quantitative and qualitative analyses of glycosaminoglycans (GAGs), the polysaccharide moiety of proteoglycans, during neuritogenesis in PC12 cells that is induced by nerve growth factor (NGF). (a) In PC12 cells, NGF induced a flat-shaped morphology with a few short processes after 3 days of culture, and then it elicited short and long neurites after 6 (in approximately 30% of cells) and 9 (in 60-70%) days of culture, respectively. (b) HS and chondroitin sulfate (CS) were detected in the cell layer at all times. Only CS was found in the medium at 3 and 6 days, whereas a low level of HS, in addition to CS, was detectable on day 9. (c) In the NGF-treated cultures, the amounts of cell-associated HS per cell were two to three times as high as those in the respective nontreated cultures at all times, whereas the amount based on phospholipid was about twofold higher after 3 days of culture. (d) The levels of HS labeled with [35S]sulfate during the last 48 h of the culture were 1.5- to twofold higher in the NGF-treated cultures than in the respective controls at any time. (e) The amount of cell-associated CS per cell (or per unit of phospholipid), but not of labeled CS per cell, was transiently enhanced at 3 days in culture with or without NGF.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and biochemical characterization of a neural proteoglycan expressing the L5 carbohydrate epitope

The monoclonal L5 antibody reacts with an N-glycosidically linked carbohydrate structure which is present on the neural cell adhesion molecule L1, neural chondroitin sulfate proteoglycans, and other not yet identified glycosylated proteins. Using this antibody, we isolated and characterized proteoglycans from adult mouse brain and cultured astrocytes biosynthetically labeled with Na2 35SO4 and a 3H-amino acid mixture. Our data suggest that the L5 proteoglycans of both sources are identical in their biochemical properties. The apparent molecular mass of the L5 proteoglycan is approximately 500 kDa. Digestion of the iodinated L5 proteoglycan from mouse brain and of the [35S]methionine-labeled L5 proteoglycan from cultured astrocytes with proteinase-free chondroitinases ABC and AC revealed three major core proteins with apparent molecular masses of approximately 380, 360, and 260 kDa. These represent molecularly distinct protein cores.

Cell adhesion molecules (CAMs) in adrenal medulla in situ and in vitro: enhancement of chromaffin cell L1/Ng-CAM expression by NGF

We have studied the expression of the cell adhesion molecules (CAMs) L1/Ng-CAM, N-CAM, J1/tenascin, and myelin-associated glycoprotein and their common carbohydrate L2/HNK-1 epitope in normal rat adrenal gland sections as well as in adrenal medulla cell culture with and without NGF stimulation. In situ L1/Ng-CAM was observed on the surface of some but not all chromaffin cell clusters, including their closely associated extracellular matrix (ECM). N-CAM immunoreactivity was present on all chromaffin cells and ECM. The ECM of whole medullas also expressed J1/tenascin molecules. In long-term cultures, nerve growth factor (NGF) stimulation enhanced L1/Ng-CAM, N-CAM, and Thy 1.1 immunolabeling on chromaffin cells and their processes. Process outgrowth was greater from chromaffin cell clusters containing S-100 positive Schwann cells as compared to dispersed single chromaffin cells. When long bundles of chromaffin cell fibers were present, S-100, L1/Ng-CAM, and N-CAM positive Schwann cells were always found and were grouped in distinct clusters in the intervals between the chromaffin cells. In some areas, however, after NGF stimulation some chromaffin cell process development occurred despite an apparent lack of close contact with Schwann cells. NGF-activated chromaffin cells also demonstrated neurofilament- and vimentin-like-immunoreactive filaments within cell bodies and their processes. Chromaffin cells were usually found on a layer of N-CAM and fibronectin positive fibroblasts, and often were associated with laminin-immunoreactive material. These data suggest a possible role of N-CAM and L1/Ng-CAM as well as ECM laminin in process outgrowth from chromaffin cells.

Cell adhesion molecules in adrenal medulla grafts: enhancement of chromaffin cell L1/Ng-CAM expression and reorganization of extracellular matrix following transplantation

Intracerebral adrenal medulla grafts have been used in human patients as an experimental treatment for Parkinson's disease, based on studies in animal models of this disorder. However, alterations in chromaffin cell properties after transplantation and the factors controlling graft survival are poorly understood. Since cell adhesion molecules (CAMs) are involved in regeneration and development of neural tissue in vivo and in vitro, the present study was undertaken to determine the expression of CAMs in adrenal medulla isografts. Fragments of rat adrenal medulla were implanted into the right lateral ventricle. The majority of grafts survived quite well, for up to 2 months (the longest studied period). The implanted chromaffin cells did not develop extensive processes. The cells retained tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) immunoreactivity, while phenylethanolamine N-methyltransferase (PNMT) expression was decreased. Surviving transplanted chromaffin cells showed enhancement and spreading of surface L1/Ng-CAM expression as compared to normal chromaffin cells in adrenal medulla. The implanted chromaffin cells demonstrated only partial conversion to neuronal phenotypes. These chromaffin cells did not develop extensive processes, but showed an enhancement of L1/Ng-CAM expression. Surviving chromaffin cells were accompanied by reorganization of their closely associated extracellular matrix (ECM). As compared to normal in situ adrenal medulla, graft ECM demonstrated a substantial increase of L1/Ng-CAM and laminin immunoreactivities and a distinct decrease in J1/tenascin expression. Some adrenal medulla grafts degenerated, particularly when misplaced within the host brain parenchyma. In these cases the grafts showed fragmentation of ECM and gradual disappearance of CAMs. These results suggest that surviving adrenal medulla grafts exhibit increased synthesis of certain CAMs by chromaffin cells, which may be involved in interactions between chromaffin cells and the surrounding ECM. It is speculated that both surviving and degenerating adrenal medulla grafts could provide CAMs and ECM components including laminin to host brain and this way contribute to functional effects of grafts.

Extracellular matrix and neuronal movement

During brain development, both neuronal migration and axon guidance are influenced by extracellular matrix molecules present in the environment of the migrating neuronal cell bodies and nerve fibers. Glial laminin is an extracellular matrix protein which these early brain cells preferentially attach to. Extracellular glycosaminoglycans are suggested to function in restricting neuronal cell bodies and axons from certain brain areas. Since laminin is deposited along the radial glial fibers and along the developing nerve pathways in punctate form, the punctate assemblies may be one of the key factors in routing the developing neurons in vivo. This review discusses the role of laminin in neuronal movement given the present concept of the extracellular matrix molecules and their proposed interactions.

Cholinesterases preceding major tracts in vertebrate neurogenesis

The role of acetylcholinesterase (AChE) in neurotransmission is well known. But long before synapses are formed in vertebrates, AChE is expressed in young postmitotic neuroblasts that are about to extend the first long tracts. AChE histochemistry can thus be used to map primary steps of brain differentiation. Preceding and possibly inducing AChE in avian brains, the closely related butyrylcholinesterase (BChE) spatially foreshadows AChE-positive cell areas and the course of their axons. In particular, before spinal motor axons grow, their corresponding rostral sclerotomes and myotomes express BChE, and both their neuronal source and myotomal target cells express AChE. Since axon growth has been found inhibited by acetylcholine, it is postulated that both cholinesterases can attract neurite growth cones by neutralizing the inhibitor. Thus, the early expression of both cholinesterases that is at least partially independent from classical cholinergic synaptogenesis, sheds new light on the developmental and medical significance of these enzymes.

Identification and characterization of the promoter for the cytotactin gene

The extracellular glycoprotein cytotactin is expressed in a characteristic and complex spatiotemporal sequence during development of the chicken embryo. To identify the various control elements underlying its expression, the promoter region of the cytotactin gene has been isolated and characterized. Clones were isolated from genomic libraries by using a fragment near the 5' end of the cDNA sequence. The sequence of this cDNA fragment was found to be distributed over two exons separated by a large first intron. The site of transcription initiation was determined by S1 nuclease and primer-extension mapping. Sequencing of a 4.3-kilobase (kb) genomic DNA clone that contains 3986 base pairs (bp) upstream of the RNA start site, the first exon, and part of the first intron revealed a number of sequence motifs implicated in the regulation and expression of eukaryotic genes. These included CCAAT boxes, phorbol ester-responsive elements, enhancer elements, and a consensus TATA sequence located 24 bp upstream of the major RNA cap site. The flanking sequence also contained a number of regions of dyad symmetry and direct repeats unique to cytotactin, as well as an array of A + T-rich sequences that resemble engrailed elements. Constructs containing fragments of the upstream region of the cytotactin gene fused to a promoterless gene for chloramphenicol acetyltransferase were transiently transfected into chicken embryo fibroblasts to define functional promoter sequences. Although sequences from -721 to +121 exhibited minimal promoter activity, the entire region between -3986 to +374 was required to yield maximal expression in chicken embryo fibroblasts. Transfection of the -3986/+374 chloramphenicol acetyltransferase plasmid into the human U251MG astrocytoma cells but not HT1080 fibrosarcoma cells resulted in chloramphenicol acetyltransferase expression, consistent with the observed synthesis of cytotactin protein only by the U251MG cell line. These data indicate that the chicken cytotactin promoter can control expression in a cell type-specific fashion within cells of another species. These studies provide a basis for the dissection of cis elements and trans factors that govern the developmental expression of the cytotactin gene.

Astrocyte topography and tenascin cytotactin expression: correlation with the ability to support neuritic outgrowth

We have observed a heterogeneity in the ability of a monolayer of cultured rat astrocytes to support the attachment and growth of dissociated embryonic hypothalamic neurons in culture. Areas of the monolayer which have an uneven surface ('rocky' astrocytes) are poor substrates for neuronal attachment and neuritic outgrowth, while surrounding areas of the glial monolayer ('flat' astrocytes) support extensive neuronal growth. Astrocytes obtained from both neonatal cerebral cortex or hypothalamus displayed 'rocky' morphology. We utilized immunocytochemical techniques with antibodies directed against putative adhesion molecules to investigate the source of this heterogeneity. Antibodies against tenascin/cytotacin, fibronectin, laminin, N-CAM, thrombospondin, heparan sulfate proteoglycan, and the p185 protein product of the neu oncogene were employed in indirect-immunofluorescence experiments. We found that the difference in the surface properties of astrocytes appears to be correlated with the expression of the extracellular matrix molecule tenascin/cytotacin, but not with any of the other molecules we tested. Our data suggest that tenascin/cytotactin is inhibitory to neuronal attachment and process outgrowth in the developing nervous system.

Localization during development of alternatively spliced forms of cytotactin mRNA by in situ hybridization

Cytotactin, an extracellular glycoprotein found in neural and nonneural tissues, influences a variety of cellular phenomena, particularly cell adhesion and cell migration. Northern and Western blot analysis and in situ hybridization were used to determine localization of alternatively spliced forms of cytotactin in neural and nonneural tissues using a probe (CT) that detected all forms of cytotactin mRNA, and one (VbVc) that detected two of the differentially spliced repeats homologous to the type III repeats of fibronectin. In the brain, the levels of mRNA and protein increased from E8 through E15 and then gradually decreased until they were barely detectable by P3. Among the three cytotactin mRNAs (7.2, 6.6, and 6.4 kb) detected in the brain, the VbVc probe hybridized only to the 7.2-kb message. In isolated cerebella, the 220-kD polypeptide and 7.2-kb mRNA were the only cytotactin species present at hatching, indicating that the 220-kD polypeptide is encoded by the 7.2-kb message that contains the VbVc alternatively spliced insert. In situ hybridization showed cytotactin mRNA in glia and glial precursors in the ventricular zone throughout the central nervous system. In all regions of the nervous system, cytotactin mRNAs were more transient and more localized than the polypeptides. For example, in the radial glia, cytotactin mRNA was observed in the soma whereas the protein was present externally along the glial fibers. In the telencephalon, cytotactin mRNAs were found in a narrow band at the edge of a larger region in which the protein was wide-spread. Hybridization with the VbVc probe generally overlapped that of the CT probe in the spinal cord and cerebellum, consistent with the results of Northern blot analysis. In contrast, in the outermost tectal layers, differential hybridization was observed with the two probes. In nonneural tissues, hybridization with the CT probe, but not the VbVc probe, was detected in chondroblasts, tendinous tissues, and certain mesenchymal cells in the lung. In contrast, hybridization with both probes was observed in smooth muscle and lung epithelium. Both epithelium and mesenchyme expressed cytotactin mRNA in varying combinations: in the choroid plexus, only epithelial cells expressed cytotactin mRNA; in kidney, only mesenchymal cells; and in the lung, both of these cell types contained cytotactin mRNA. These spatiotemporal changes during development suggest that the synthesis of the various alternatively spliced cytotactin mRNAs is responsive to tissue-specific local signals and prompt a search for functional differences in the various molecular forms of the protein.

The perinotochordal mesenchyme acts as a barrier to axon advance in the chick embryo: implications for a general mechanism of axonal guidance

To test the hypothesis that the perinotochordal mesenchyme (the sclerotome ventral to the spinal nerve pathway) is a barrier to axonal advance in the chick embryo, we determined whether axons directly confronted with perinotochordal mesenchyme would turn to avoid it. The initial direction of motor axon outgrowth was altered by rotating the right half of the neural tube after deleting the left half. Perinotochordal mesenchyme was identified histologically or by peanut agglutinin (PNA) binding. We found that axons turned to avoid the perinotochordal mesenchyme and traversed only the dorsal-anterior sclerotome at all stages of outgrowth. When the ventral root was positioned at the midline, axons projected around the perinotochordal mesenchyme and formed spinal nerves on both sides of the embryo. Furthermore, neural crest cells and sensory axons did not penetrate perinotochordal mesenchyme, even in the absence of motor axons. In contrast, perinotochordal mesenchyme did not exhibit inhibitory function and did not differentially bind PNA when the notochord was deleted; axons ramified widely within it. We conclude that the dorsal-anterior sclerotome is permissive and that the perinotochordal mesenchyme is relatively inhibitory for the advance of axons and neural crest cells. Two additional pairs of tissues provide similar permissive/inhibitory contrasts in the embryo, the anterior/posterior sclerotome and the plexus/pelvic girdle mesenchyme. We hypothesize that guidance by all three pairs is mediated by the same set of cellular interactions and has a common molecular basis. We further propose that the transient expression of substances characteristic of these contrasting tissue pairs could serve to guide axons elsewhere, in both the peripheral and the central nervous systems.

Expression of the novel extracellular matrix component tenascin in normal and diseased human liver. An immunohistochemical study

The novel extracellular matrix glycoprotein tenascin was studied immunohistochemically in normal and fibrotic human liver. Its localization was compared to that of laminin, fibronectin and collagen type IV. In the normal liver, a weak staining for tenascin was detected along sinusoids, while portal tracts were negative. In both alcoholic and cholestatic liver disease and acute and chronic hepatitis, sinusoidal immunoreactivity for tenascin was variably increased as compared to the normal liver. Most striking, however, was the preferential accumulation of tenascin at connective tissue-parenchymal interfaces between proliferating ductules and in areas of piecemeal necrosis. As compared to laminin, fibronectin and collagen type IV, tenascin has the most restricted distribution. Our findings indicate that tenascin is a component of the extracellular matrix of the human liver. Its preferential expression at connective tissue-parenchymal interfaces in fibrosing areas in contrast to its absence from mature fibrous septa suggest a transient role in early matrix organization.

Expression of adhesion molecules and the establishment of boundaries during embryonic and neural development

Evidence is accumulating that molecules involved in cell-cell and cell-substratum interactions are important in the establishment and maintenance of borders between cell groups during development. In this report, we review evidence supporting this conclusion, particularly in regard to the role of adhesion molecules in the formation of cell collectives and in the modulation of cell and neurite movements.

A diverse set of developmentally regulated proteoglycans is expressed in the rat central nervous system

Cellular interactions in neural development are influenced by various extracellular proteins, many of which bind glycosaminoglycans or proteoglycans. Precise functions of nervous system proteoglycans remain unknown, in part because neural proteoglycan composition is poorly understood. In this study, 25 putative proteoglycan core proteins were identified in subcellular fractions of rat brain. Levels of many of these varied considerably during development. Membrane-associated proteoglycans included two heparan sulfate proteoglycans (cores of 50 and 59 kd) that are covalently linked to glycosyl-phosphatidylinositol lipid, as well as several that appear to aggregate either with themselves or with copurifying proteins. These data indicate that brain proteoglycans exhibit the abundance, structural diversity, and developmental regulation that would be anticipated for molecules with diverse developmental functions.

Purification of hexabrachion (tenascin) from cell culture conditioned medium, and separation from a cell adhesion factor

We describe a protocol for purifying hexabrachion from conditioned medium of cell cultures, using gel filtration chromatography on Sephacryl 500, followed by anion-exchange chromatography on a Mono Q column, followed optionally by a second gel filtration or zone sedimentation on glycerol gradients. The protocol has several advantages over previous procedures based on affinity chromatography on monoclonal antibodies. Perhaps foremost, the protein is never exposed to the denaturing solvents that are required for elution from the antibody column. The Mono Q column also separated hexabrachion from a prominent cell adhesion activity that eluted with the hexabrachion on the first gel filtration, and co-sedimented with hexabrachions on glycerol gradients. The cell adhesion fractions showed several bands between 190 and 400 kDa. A single band at 220 kDa stained prominently with a polyclonal antibody against mouse EHS laminin, and a band at 190 kDa stained with a monoclonal antibody against s-laminin. The purification protocol gave hexabrachion at high concentration and with no detectable contamination by fibronectin or laminin. The highest yield of hexabrachion (1-4 mg from 400 ml of conditioned medium) was from human glioblastoma cell cultures, but the same procedure allowed us to purify and characterize the rat hexabrachion. Protein purified from primary cultures of rat embryo fibroblasts showed approximately equal amounts of three subunit sizes: 280, 230, and 220 kDa. These different subunits, presumably derived from alternative RNA splicing, appeared to be segregated into large and small hexabrachions, which could be separated on glycerol gradients.

Chromosomal localization of the human hexabrachion (tenascin) gene and evidence for recent reduplication within the gene

Using analysis of rodent-human somatic cell hybrids as well as in situ hybridization of hexabrachion cDNA probes to normal human metaphase chromosomes, we have localized the human hexabrachion gene to chromosome 9, bands q32-q34. We also put forward the hypothesis that there has been a recent reduplication of a small segment of the human hexabrachion gene. We support this hypothesis by comparison of codon usage in this segment of the gene to codon usage in the remainder of the gene. This hypothesis is also supported by comparison of the sequence of human hexabrachion to that of the chicken hexabrachion. In addition, the latter comparison shows that the reduplication most likely occurred after the divergence of mammalian and avian species.

Extracellular matrix of cultured glial cells: selective expression of chondroitin 4-sulfate by type-2 astrocytes and their progenitors

We have studied the extracellular matrix composition of cultured glial cells by immunocytochemistry with different monoclonal and polyclonal antibodies. Double immunofluorescence experiments and metabolic labeling with [3H]glucosamine performed in different types of cerebellar and cortical cultures showed that bipotential progenitors for type-2 astrocytes and for oligodendrocytes (recognized by the monoclonal antibody LB1 at early stages of their development) synthesize chondroitin sulfate (CS) and deposit this proteoglycan in their extracellular matrix. The distribution of the various [3H]glucosamine-labeled glycosaminoglycans between the intracellular and the extracellular space was different. CS was present both within the cells and in the culture medium, although in different amounts. Bi-potential progenitors became also O4-positive during their development in vitro. At the stage of O4-positivity they were still stained with antibodies against CS. However, when the progenitor cells were maintained in serum-free medium and differentiated into Gal-C-positive oligodendrocytes, they became CS-negative. In the presence of fetal calf serum in the culture medium, the bipotential progenitors differentiated into GFAP-positive type-2 astrocytes. These cells still expressed CS: their Golgi area and their surface were stained with anti-CS antibodies. Staining with monoclonal antibodies specific for different types of CS (4-sulfate, 6-sulfate, and unsulfated) revealed that both bipotential progenitors and type-2 astrocytes synthesized only chondroitin 4-sulfate. Type-1 astrocytes were negative for both the polyclonal and the monoclonal anti-CS antibodies. Finally, type-2 astrocytes and their progenitors were weakly stained with anti-laminin antibodies and unstained with anti-fibronectin. Type-1 astrocytes were positive for both anti-laminin and anti-fibronectin antibodies and appeared to secrete fibronectin in the extracellular space.

Non-specific interaction of proteoglycans with surfaces and matrices

Evidence is presented that reversible non-specific adsorption of proteoglycans (PGs) to surfaces and matrices is an inherent property of the PGs. This adsorption is dependent on the intact PG structure as the glycosaminoglycans (GAGs), which are isolated after papain digestion of the PG show no such non-specific adsorption. The interaction of the PG with surfaces and matrices is also highly dependent on the internal milieu and can be both inhibited and enhanced by such factors as the ionic composition and concentration, pH, detergents and chaotropic reagents such as guanidine hydrochloride (Gdn-HC1). It is suggested that this inherent stickiness of the PGs allows them to function like a reversible fluid adhesant in the connective tissues. This weak binding force thus not only aids in maintaining the integrity of the connective tissues, but its reversible nature may provide for easy movement of other materials through the connective tissue matrix.

Exogenous tenascin inhibits mesodermal cell migration during amphibian gastrulation

We have used amphibian gastrulation as a model system to study the action of the extracellular matrix (ECM) glycoprotein tenascin on mesodermal cell migration. Tenascin function was assayed in vitro during spreading of isolated cells from the dorsal marginal zone (DMZ) and during cell migration from DMZ explants. Plastic coated with bovine fibronectin or gastrula ECM was used as a substratum. In both cases, tenascin added to the medium inhibited spreading and migration of mesodermal cells. In addition, a substratum coated with a mixture of fibronectin and tenascin was found to prevent mesodermal cell migration. Tenascin was also microinjected into the blastocoel cavity of living embryos at the late blastula stage. This led to a complete arrest of gastrulation in more than 80% of the cases. Scanning electron microscopy of fractures from arrested gastrulae showed that mesodermal cell migration was blocked. Similar injection experiments carried out at the middle gastrula stage demonstrated that tenascin is able to inhibit cell migration after cells have already contacted the ECM. Mesodermal cell migration in the presence of tenascin could be restored in vitro and in vivo by the monoclonal antibody mAb Tn68 which is known to mask a cell binding site of the molecule. Finally, tenascin microinjected into the blastocoel of blastula or gastrula stage embryos bound within 15 min to the ECM fibrils at all the stages studied. Our results show that exogenous tenascin can be incorporated into embryonic ECM and interferes in vivo with the interactions of cells with a fibronectin-rich matrix.

Characterization of cell-associated proteoglycans synthesized by embryonic neural retinal cells

To begin to understand the properties of the neuronal heparan sulfate proteoglycan (HSPG) that interacts with the neural cell adhesion molecule (NCAM), we have analyzed proteoglycans synthesized by dissociated embryonic retinal cells in culture. Because NCAM plays an integral role in cell-cell interactions, we have focused on cell-associated HSPGs. Proteoglycans were isolated from embryonic Day 10 retinal cell cultures labeled with 35SO4 and separated into fractions that either flowed through or were retained on phenyl-Sepharose. Molecules binding phenyl-Sepharose have been proposed to be capable of insertion into the plasma membrane, and thus may be involved in binding to NCAM. Proteoglycans binding to phenyl-Sepharose had an estimated molecular mass of 400-500 kDa, and contained 60% HSPG and 40% chondroitin sulfate proteoglycan (CSPG). The putative membrane-associated HSPGs, with an average molecular weight of 360 kDa, were shown to contain heparan sulfate chains of 40 and 20 kDa, and multiple core proteins with the major core protein having a molecular weight of approximately 130 kDa. The membrane-associated CSPGs also exhibited multiple core proteins, with sizes ranging from 120 to 220 kDa. These data suggest that multiple membrane-associated HSPGs and CSPGs are synthesized by embryonic neural retina cells, which may explain the diversity in function of this class of proteoglycans.

Isolation from chick somites of a glycoprotein fraction that causes collapse of dorsal root ganglion growth cones

The segmented pattern of peripheral spinal nerves in higher vertebrates is generated by interactions between nerve cells and somites. Neural crest cells, motor axons, and sensory axons grow exclusively through anterior-half sclerotome. In chick embryos, posterior cells bind the lectins peanut agglutinin (PNA) and Jacalin. When liposomes containing somite extracts are applied to cultures of chick sensory neurons, growth cones collapse abruptly, recovering within 4 hr of liposome removal. Collapse activity is eliminated by immobilized PNA, and SDS-PAGE demonstrates two major components (48K and 55K), which are absent from anterior-half sclerotome. Rabbit polyclonal antibodies against these components recognize only posterior cells and may also be used to eliminate collapse activity. We suggest that spinal nerve segmentation is produced by inhibitory interactions between these components and growth cones.

Molecular cloning of SC1: a putative brain extracellular matrix glycoprotein showing partial similarity to osteonectin/BM40/SPARC

We describe the cloning of SC1, a novel cDNA that was selected from a rat brain expression library using a mixed polyclonal antibody directed against synaptic junction glycoproteins. SC1 detects a 3.2 kb mRNA expressed throughout postnatal development of the brain and present at high levels in the adult. In situ hybridization reveals that the SC1 mRNA is expressed widely in the brain and is present in many types of neurons. DNA sequence data suggest that the SC1 product is a secreted, calcium binding glycoprotein. Strikingly, the carboxy-terminal region of the SC1 protein shows substantial similarity to the extracellular matrix glycoprotein osteonectin/BM40/SPARC. These data are consistent with the hypothesis that SC1 is an extracellular matrix glycoprotein in the brain.

Cytotactin and its proteoglycan ligand mark structural and functional boundaries in somatosensory cortex of the early postnatal mouse

The expression of the extracellular matrix molecules cytotactin, which is synthesized by glia, and cytotactin-binding (CTB) proteoglycan, which is synthesized by neurons, was examined in the developing brain of the mouse, specifically in the cortical barrel field, using highly specific polyclonal antibodies to the purified molecules. Both molecules appeared early in the development of the cortex but were excluded from the centers of the developing barrels at the time of entry and arborization of thalamocortical axons. Of the two major forms of cytotactin (220 and 200 kDa), the larger form predominated during development of the mouse brain and also predominated in mixed neuron-glia cultures but not in pure glial cultures. Both cytotactin and CTB proteoglycan were recognized by various lectins that have been shown in other studies to demarcate the barrel field: both molecules were recognized by lentil lectin and concanavalin A and CTB proteoglycan was also recognized by peanut and wheat germ agglutinins. The HNK-1 carbohydrate antigen, present on cytotactin, CTB proteoglycan, and other adhesion molecules, was also found in the barrel walls and diminished in the barrel hollows. Cytotactin and CTB proteoglycan were preferentially expressed in barrel walls through P12. After this time, their expression became uniform even though the histological pattern of barrel walls and hollows was maintained. The fusion of a row of barrels which results from peripheral damage to a row of whiskers was accompanied by the loss of patterned expression of both molecules following electrocauterization of a row of whisker follicles at P1.5. We conclude that activity from the periphery is important not only to development of anatomical pattern but also of the molecular pattern and that the expression of both glial and neuronal proteins can respond to such activity. The results are consistent with previous studies showing that incoming thalamocortical axons play a primary role in barrel field formation. They also suggest that both the migration of cortical neurons on glia and the refinement of the mapping between the peripheral whisker field and its cortical representation may depend upon the distribution of substrate adhesion molecules.

Glycosaminoglycan-related epitopes surrounding different subsets of mammalian central neurons

Among a panel of monoclonal antibodies generated against monkey brain tissue, a class of antibodies was found to produce perineuronal staining of small subsets of mammalian central neurons. Three antibodies (MAbs 473, 376, 528) we report here define two different, though partially overlapping, neuronal subsets in the monkey neocortex. All 3 antibodies stain in addition certain chondrocytes. The neural immunoreactivities were lost, and the chondral immunoreactivities either lost or enhanced, after treatment of the sections with chondroitinase ABC. Independently, 3 other antibodies (MAbs 1B5, 9A2, 3B3) with established specificity to glycosaminoglycan epitopes also produced perineuronal staining of a related subset of central neurons. Immunoblot experiments with two of the antibodies revealed bands of high molecular weight. These findings indicate that certain glycosaminoglycans occur surrounding mammalian central neurons, and suggest that different neuronal subsets are associated with different combinations of proteoglycan epitopes.

Two contrary functions of tenascin: dissection of the active sites by recombinant tenascin fragments

A structural and functional model of tenascin was elaborated using recombinant parts of three alternatively spliced tenascin variants and anti-tenascin monoclonal antibodies. The fusion proteins were compared with intact tenascin for their functions and by electron microscopy. A strong cell binding site was localized within 104 amino acids. This fragment also contains the epitope of the monoclonal antibody anti-Tn68, which inhibits cell attachment to tenascin and binds near the tips of the six arms of tenascin. In contrast, constructs containing the 13 1/2 EGF-like repeats of tenascin showed an antiadhesive effect. The coexistence of the two contrary signals on the same molecule might be responsible for the versatile features of tenascin.