Axonal Growth on Solubilized and Reconstituted Matrix from the Embryonic Chicken Retina Inner Limiting Membrane

Basal laminae, thin sheets of extracellular matrix covering the basal side of all neuroepithelia, are strongly supportive for neurite outgrowth in vitro and may provide a permissive environment for growing neurites in vivo. To gain information about the biological activity and composition of in situ-derived basal laminae the inner limiting membranes from embryonic day (E) 7 to E11 chick and quail retinae were isolated. The basal laminae were solubilized with high-molar guanidine hydrochloride or urea, and the solubilized proteins reconstituted by dialysis. The matrix proteins were spotted or dried onto nitrocellulose or polylysine-coated dishes. When explants from retina or from dorsal root ganglia were incubated on the protein spots, neurite extension was very robust, at a level as high as on authentic basal lamina. Extracts from the pigment epithelial basement membrane did not support neurite extension. Western blot analysis showed that the explant from the retinal inner limiting membrane contained predominantly basal lamina-type proteins, such as laminin, collagen type IV and heparan sulphate proteoglycan, whereas the matrix extract from the pigment epithelium contained predominantly mesenchymal-type proteins, like collagen type I and tenascin. JG22, a beta1 integrin antibody that inhibited neurite extension on EHS tumour laminin substrate, had no effect on neurite outgrowth on retinal basal lamina matrix, indicating that embryonic basal laminae contain other or additional growth promoting substrate molecules.

Temporary disruption of the retinal basal lamina and its effect on retinal histogenesis

An experimental paradigm was devised to remove the retinal basal lamina for defined periods of development: the basal lamina was dissolved by injecting collagenase into the vitreous of embryonic chick eyes, and its regeneration was induced by a chase with mouse laminin-1 and alpha2-macroglobulin. The laminin-1 was essential in reconstituting a new basal lamina and could not be replaced by laminin-2 or collagen IV, whereas the macroglobulin served as a collagenase inhibitor that did not directly contribute to basal lamina regeneration. The regeneration occurred within 6 h after the laminin-1 chase by forming a morphologically complete basal lamina that included all known basal lamina proteins from chick embryos, such as laminin-1, nidogen-1, collagens IV and XVIII, perlecan, and agrin. The temporary absence of the basal lamina had dramatic effects on retinal histogenesis, such as an irreversible retraction of the endfeet of the neuroepithelial cells from the vitreal surface of the retina, the formation of a disorganized ganglion cell layer with an increase in ganglion cells by 30%, and the appearance of multiple retinal ectopias. Finally, basal lamina regeneration was associated with aberrant axons failing to correctly enter the optic nerve. The present data demonstrate that a transient disruption of the basal lamina leads to dramatic and probably irreversible aberrations in the histogenesis in the developing central nervous system.

Composition, synthesis, and assembly of the embryonic chick retinal basal lamina

To study the biology of basal laminae in the developing nervous system the protein composition of the embryonic retinal basal lamina was investigated, the site of synthesis of its proteins in the eye was determined, and basal lamina assembly was studied in vivo in two assay systems. Laminin, nidogen, agrin, collagen IV, and XVIII are major constituents of the retinal basal lamina. However, only agrin is synthesized by the retina, whereas the other matrix constituents originate from cells of the ciliary body, the lens, or the optic disc. The synthesis from extraretinal tissues infers that the retinal basal lamina proteins must be shed from their tissues of origin into the vitreous body and from there bind to receptor proteins provided by the retinal neuroepithelium. The fact that all proteins typical for the retinal basal lamina are abundant in the vitreous body and a new basal lamina is only formed when the vitreous body was directly adjacent to the retina is consistent with the contention of the vitreous body having a function in retinal basal lamina formation. Basal lamina assembly was also studied after disrupting the retinal basal lamina by intraocular injection of collagenase. The basal lamina regenerated after chasing the collagenase with Matrigel, which served as a collagenase inhibitor. The basal lamina was reconstituted within 6 h. However, the regenerated basal lamina was located deeper in the retina than normal by reconstituting along the retracted neuroepithelial endfeet demonstrating that these endfeet are the preferred site of basal lamina assembly.

Agrin binds to beta-amyloid (Abeta), accelerates abeta fibril formation, and is localized to Abeta deposits in Alzheimer's disease brain

Agrin is an extracellular matrix heparan sulfate proteoglycan (HSPG) well known for its role in modulation of the neuromuscular junction during development. Although agrin is one of the major HSPGs of the brain, its function there remains elusive. Here we provide evidence suggesting a possible function for agrin in Alzheimer's disease brain. Agrin protein binds the amyloidogenic peptide Abeta (1-40) in its fibrillar state via a mechanism that involves the heparan sulfate glycosaminoglycan chains of agrin. Furthermore, agrin is able to accelerate Abeta fibril formation and protect Abeta (1-40) from proteolysis, in vitro. Supporting a biological significance for these in vitro data, immunocytochemical studies demonstrate agrin's presence within senile plaques and cerebrovascular amyloid deposits, and agrin immunostained capillaries exhibit pathological alterations in AD brain. These data therefore suggest that agrin may be an important factor in the progression of Abeta peptide aggregation and/or its persistence in Alzheimer's disease brain.

Identification of extracellular matrix ligands for the heparan sulfate proteoglycan agrin

Agrin is a major brain heparan sulfate proteoglycan which is expressed in nearly all basal laminae and in early axonal pathways of the developing central nervous system. To further understand agrin's function during nervous system development, we have examined agrin's ability to interact with several heparin-binding extracellular matrix proteins. Our data show that agrin binds FGF-2 and thrombospondin by a heparan sulfate-dependent mechanism, merosin and laminin by both heparan sulfate-dependent and -independent mechanisms, and tenascin solely via agrin's protein core. Furthermore, agrin's heparan sulfate side chains encode a specificity in interactions with heparin-binding molecules since fibronectin and the cell adhesion molecule L1 do not bind agrin. Surface plasmon resonance studies (BIAcore) reveal a high affinity for agrin's interaction with FGF-2 and merosin (2.5 and 1.8 nM, respectively). Demonstrating a biological significance for these interactions, FGF-2, laminin, and tenascin copurify with immunopurified agrin and immunohistochemistry reveals a partial codistribution of agrin and its ECM ligands in the chick developing visual system. These studies and our previous studies, showing that merosin and NCAM also colocalize with agrin, provide evidence that agrin plays a crucial role in the function of the extracellular matrix and suggest a role for agrin in axon pathway development.

Collagen XVIII is a basement membrane heparan sulfate proteoglycan

The present study shows that collagen XVIII is, next to perlecan and agrin, the third basal lamina heparan sulfate proteoglycan (HSPG) and the first collagen/proteoglycan with heparan sulfate side chains. By using monoclonal antibodies to an unidentified HSPG in chick, 14 cDNA clones were isolated from a chick yolk sac library. All clones had a common nucleotide sequence that was homologous to the mRNA sequences of mouse and human collagen XVIII. The deduced amino acid sequence of the chick fragment shows an 83% overall homology with the human and mouse collagen XVIII. Similar to the human and mouse homologue, the chick collagen XVIII mRNA has a size of 4.5 kilobase pairs. In Western blots, collagen XVIII appeared as a smear with a molecular mass of 300 kDa. After treatment with heparitinase, the protein was reduced in molecular mass by 120 kDa to a protein core of 180 kDa. Collagen XVIII has typical features of a collagen, such as its existence, under non-denaturing conditions, as a non-covalently linked oligomer, and a sensitivity of the core protein to collagenase digestion. It also has characteristics of an HSPG, such as long heparitinase-sensitive carbohydrate chains and a highly negative net charge. Collagen XVIII is abundant in basal laminae of the retina, epidermis, pia, cardiac and striated muscle, kidney, blood vessels, and lung. In situ hybridization showed that the main expression of collagen XVIII HSPG in the chick embryo is in the kidney and the peripheral nervous system. As a substrate, collagen XVIII moderately promoted the adhesion of Schwann cells but had no such activity on peripheral nervous system neurons and axons.

Disruption of the pial basal lamina during early avian embryonic development inhibits histogenesis and axonal pathfinding in the optic tectum

Bacterial collagenase was injected into the ventricular cavity of the optic tectum of chick and quail embryos. Histological examination up to 6 days after enzyme injection revealed that the collagenase disrupted the pial basal lamina, which was evident by the fragmented distribution of basal lamina proteins at the pial surface of the midbrain and the brainstem. Although the disrupted basal lamina was not reestablished at later stages of development, the pial basal lamina of the newly developing neuroepithelium in the caudal part of the tectum was continuous and intact. Western blot analysis showed that the collagenase digested collagens but spared noncollagenous proteins. The disruption of the pial basal lamina caused the neuroepithelial cells to retract their pial end feet and caused tectal axons to exit the brain tissue into the adjacent mesenchyme. The vertical migration of neuroblasts to the pial layers of the tectum was inhibited, leading to a disruption of the tectal histogenesis. In the developing optic pathways, retinal axons were misguided at the optic chiasma and terminated in the head mesenchyme instead of the tectum. None of the abnormalities in histogenesis and axonal pathways were observed when the basal lamina was disrupted at a later stage of embryonic development. The present experiments demonstrate that the pial basal lamina has an important function during brain morphogenesis in restricting axons to the brain, providing an anchoring of the neuroepithelial cells to the pial surface, and allowing the formation of a defined cytoarchitecture of the brain.

Disruption of the retinal basal lamina during early embryonic development leads to a retraction of vitreal end feet, an increased number of ganglion cells, and aberrant axonal outgrowth

Bacterial collagenase was injected into the vitreous of the eye of chick and quail embryos. Immunocytochemical and ultrastructural studies revealed that the collagenase dissolved the retinal basal lamina of the injected eye. The basal lamina disruption was first detectable 1 hour after enzyme injection and was complete within 3 hours. With further development, the retinal basal lamina was not reestablished; newly developing neuroepithelium in the peripheral retina, however, generated an intact basal lamina. Western blot analysis showed that Clostridial collagenase degraded various collagens but spared noncollagenous proteins. Basal lamina disruption of embryonic day 3 to 6 retinae led to the retraction of the end feet of the neuroepithelial cells, caused an increase in the number of Islet-1+ cells (most likely ganglion cells), an increase in the thickness of the optic fiber layer, and aberrant growth of optic axons on their way toward the optic disc. None of these changes were observed when retinal basal laminae were disrupted at later stages of development. The present data demonstrate that the retinal basal lamina, by anchoring the neuroepithelial cells to the pial surface of the retina, has an important function in the development of the normal cytoarchitecture of this structure. It is proposed that the altered extracellular environment in the vitreal part of the retina, resulting in the retraction of the neuroepithelial end feet, is responsible for the increased number of Islet-1+ cells and the aberrant axonal navigation.

A role of midkine in the development of the neuromuscular junction

Midkine (MK) is a member of a family of developmentally regulated neurotrophic and heparin-binding growth factors. It is expressed during the midgestation period in a retinoid-acid dependent manner during embryogenesis in the mouse. In vitro, it promotes neurite outgrowth from spinal cord neurons and cell migration. It expression is strongest in the central nervous system, thus suggesting a function for this protein in neural development. In this study, the role of MK in synaptogenesis was examined in the Xenopus system. A Xenopus MK cDNA was cloned from an embryonic library encompassing neurulation and synaptogenesis stages. By Northern blot analysis, MK mRNA was detected from the onset of neurulation and throughout the stages of synaptogenesis in the Xenopus embryo. This suggests that MK is also an important growth regulator in Xenopus embryogenesis. To study the function of MK in the development of the neuromuscular junction (NMJ), fusion proteins were made and their ability to induce the formation of acetylcholine receptor (AChR) clusters in cultured muscle cells was studied. Beads coated with MK strongly induce AChR clustering. When nerve-muscle cocultures were labeled with antibodies made against the MK fusion protein, MK immunoreactivity was detected at the NMJ. Unlike heparin-binding growth-associated molecule (HB-GAM), another member of this growth factor family, MK expression cannot be detected in the muscle but is present in spinal cord neurites. Consistent with these in vitro data is the observation that MK mRNA is only localized in the central nervous system but the protein is deposited at the intersomitic junction where the NMJ is located in vivo. Exogenously applied MK does bind to the heparan sulfate proteoglycan on the surface of Xenopus muscle cells. Agrin, a heparan-sulfate proteoglycan that induces the formation of AChR clusters in cultured muscle cells, binds strongly to MK. Bath application of MK in conjunction with agrin results in a change in the pattern of AChR clustering induced by agrin alone. These data suggest that MK is a neuron-derived factor that participates in the signal transduction process during NMJ development.

Chronic rejection causes early destruction of the intrinsic nervous system in rat intestinal transplants

Chronic rejection is the major cause of late intestinal allograft dysfunction. We have previously shown that chronic rejection alters the muscularis externa of the graft. This study determined structural and functional changes to the enteric nerves during chronic rejection. Chronic rejection was achieved in orthotopic intestinal transplants (ACI to Lewis) by limited immunosuppression. Syngeneic transplants (ACI to ACI) and unoperated ACI rats served as controls. Animals were clinically healthy and showed no significant alterations in the mucosal architecture on postoperative day 90. Staining for NADPH diaphorase activity (nitric oxide synthase-containing neurons) and with neurofilament antibody RT-97 revealed that chronic rejection decreased the number of jejunal myenteric ganglia by approximately 50%. Inhibitory junction potentials (IJPs) to circular muscle cells were determined by electrical field stimulation (EFS). In controls and syngeneic grafts, EFS caused a stimulus-dependent increase in IJP amplitude, with a maximal amplitude of 9 +/- 0.4 and 10 +/- 0.8 mV, respectively. Chronic rejection in allografts markedly increased the threshold for IJP initiation and decreased the maximal IJP amplitude (5 +/- 0.8 mV). Our data indicate that chronic rejection severely damages the muscularis and the enteric nervous system before mucosal changes become evident.

Distribution and substrate properties of agrin, a heparan sulfate proteoglycan of developing axonal pathways

The distribution and substrate properties of agrin, an extracellular matrix heparan sulfate proteoglycan (HSPG), was investigated in the developing chick nervous system by immunocytochemistry, Western blotting, and in neurite outgrowth assays. By comparing the distribution of agrin with that of laminin-1, merosin (laminin-2), neurofilament, and neural cell adhesion molecule (NCAM), it was found that throughout development, agrin is a constituent of all basal laminae. From embryonic day (E) 4 onwards, agrin is also abundant in axonal pathways of the central nervous system, such as the optic nerve, the tectobulbar pathway, the white matter of the spinal cord, and the marginal and the molecular layers of the forebrain and the cerebellum. The abundance of agrin in brain decreases from E13 onwards. In the peripheral nervous system, agrin is present throughout development as a constituent of the Schwann cell basal laminae. Western blots confirmed the immunocytochemical data, showing maximum expression of agrin occurs during the early to medium stages of brain development. Western blots also showed that in mouse and human brain, agrin exists as an HSPG. Purified agrin did not support neurite outgrowth, rather it inhibited retinal neurite extension on mixed agrin/merosin substrates. Despite the fact that agrin, when used as a substrate inhibited neurite outgrowth, its temporal and spatial overlap with growing axons suggests that agrin has a supportive role in the development of axonal pathways, possibly as a binding component for growth factors and cell adhesion proteins.

Expression pattern of collagen IX and potential role in the segmentation of the peripheral nervous system

Segmentation of the peripheral nervous system of vertebrates requires guidance cues located in the adjacent somitic mesoderm. Recent experiments suggest that inhibitory molecules in the posterior somite may influence segmentation by restricting the outgrowth of axons and the migration of neural crest cells to the anterior somite. A potential candidate for an inhibitory molecule is collagen IX, a chondroitin sulfate proteoglycan made by sclerotome cells of the somite and by the notochord. Immunohistochemical localization of collagen IX demonstrated that its expression in the posterior sclerotome of the somite correlates with axon outgrowth and neural crest cell migration through the anterior sclerotome. In vitro, sensory neurites on fibronectin, and motor neurites on basal lamina extract, avoid regions which contain substrate-bound collagen IX. This effect can be abolished by chondroitinase treatment, suggesting that the glycosaminoglycan component of the molecule is responsible for this activity. Further, collagen IX elicits a similar avoidance behavior by neural crest cells in vitro. These data suggest that collagen IX contributes to the segmentation of the peripheral nervous system in vivo.

Effects of hepatocellular mitogens on cytokine-induced nitric oxide synthesis in human hepatocytes

The synthesis of induced nitric oxide (NO) is regulated by several cytokines, including growth factors produced following hepatic injury and inflammation. However, little information is available on the role of growth factors in regulating the inducible NO synthase in human hepatocytes. The capacity of hepatocellular mitogens (HGF, EGF, and TGF-alpha) to regulate the inducible NO synthase (iNOS) was studied in human hepatocytes incubated with inflammatory cytokines and lipopolysaccharide (LPS). Furthermore, the effects of hepatic mitogens on NO-induced changes in DNA and protein synthesis was studied. It was found that NO-mediated decrease of protein and DNA synthesis were partially reversed by the mitogens. This was associated with a down-regulation in cytokine-mediated hepatocyte NO formation, iNOS mRNA expression, and NOS enzyme activity. Cytokine-induced NO formation or SNAP, an NO donor, added with cytokines increased hepatocyte chromatin condensation but no DNA fragmentation was observed. The increase in chromatin condensation was partially reversed by hepatic mitogens and corresponded with the inhibition of NO production. Thus, the hepatic mitogens, HGF, EGF, and TGF-alpha, all suppress iNOS expression and it is the suppression of iNOS that appears to be responsible for the mitogen-reduced preservation of DNA and protein synthesis and prevention of chromatin condensation.

Increased expression of interferon-gamma in a rat model of chronic intestinal allograft rejection

Chronic rejection remains a major cause of late graft dysfunction. Although much research has focused on acute rejection, little is known about the mechanisms of chronic rejection. Our group has recently reported evidence of significant intestinal smooth muscle hypertrophy and hyperplasia associated with abnormal contractile and electrical activities in a rat model of chronic intestinal rejection. The changes in the smooth muscle layer are associated with a significant inflammatory infiltrate. In order to further delineate the immune mechanisms of chronic rejection, we sought to clarify the nature of this infiltrate. Orthotopic small bowel transplantation was performed using an allogeneic (ACI-Lewis) rat combination. The rats only received immunosuppression for the first 28 days posttransplantation (cyclosporine 15 mg/kg daily from postoperative day 0 to 6 and every other day from postoperative day 7 to 28). This led to chronic rejection of the graft by day 90, at which time the rats were sacrificed. Analysis by immunohistochemistry revealed NK and CD5+ leukocytes infiltrating the muscular layer. Examination of cytokine production by radiolabeled polymerase chain reaction showed high levels of steady state interferon-gamma mRNA in full thickness intestinal segments and within the isolated muscularis of chronically rejecting intestinal allografts as compared to syngeneic and control grafts. Interferon-gamma mRNA was localized to both the muscularis and mucosa. Interestingly, positively hybridized cells within the muscularis tended to preferentially localize to the myenteric and submucosal plexuses suggesting potential role for this cytokine in chronic intestinal ejection.

The behavior of optic axons on substrate gradients of retinal basal lamina proteins and merosin

To study the behavior of optic axons to continuously changing concentrations of their substrate, explants from embryonic retina were placed across gradients of retinal basal lamina proteins and merosin. The following growth patterns of axons in response to the substrate gradients were found: (1) Axons that grew up gradients, i.e., from low to high substrate concentrations, became longer and less fasciculated with increasing concentration of the substrate. On shallow basal lamina gradients, the axons also showed a directional response that resulted in guidance to higher substrate concentrations. (2) Axons that grew down gradients, i.e., from high to low substrate concentrations, became shorter and more fasciculated with decreasing concentrations of the substrate. On gradients of merosin, a significant alteration in the axonal growth direction toward higher substrate concentrations was detected. Axons heading down gradients never U turned to higher substrate concentrations. (3) Axons confronted with discontinuous substrates were confined to the borders of the substrate exclusively, whereas axons confronted with substrate gradients were able to cross into the territory beyond the substrate. (4) The growth patterns of axons on substrate gradients of basal lamina proteins and merosin were similar but not identical, indicating that axons may respond to substrate gradients dependent on its chemical composition. The present results show that substrate gradients can regulate length and fasciculation of neurites and have a limited capability to direct axons to higher substrate concentrations.

Intraretinal grafting reveals growth requirements and guidance cues for optic axons in the developing avian retina

To study environmental factors controlling the growth and navigation of optic axons in the eye, grafts of retinal, optic disc, optic tectum, and floor plate tissue were transplanted into organ-cultured embryonic chick or quail eyes. The growth of axons into and out of the graft was studied in cross sections of the cultured eyes and by DiI tracing in retinal whole mounts. Based on the location and trajectory of axons and based on the quantity of axons that entered and exited the grafts, several requirements for axonal navigation were established: (1) Axonal growth is restricted to an approximately 10-microm-thick layer at the vitreal surface of the retina. (2) The retinal neuroepithelium prior to axogenesis is nonpermissive for neurite outgrowth. This nonpermissive quality is transient and recedes peripherally as the differentiation of the retina progresses. (3) Embryonic axons are able to grow into neonatal and adult retinal grafts, demonstrating that older retina remains permissive for axonal growth. (4) The trajectory of axons into and from retinal grafts that had been rotated in their peripheral-central orientation showed that the retina has an inherent polarity that permits axon growth toward and away from the optic disc, but does not allow axon growth perpendicular to this direction. This centroperipheral cue operates locally rather than by long distance. (5) The optic disc provides an exit for the axons from the retina, but has no detectable neurotropic activity. Finally, optic axons from the host retina readily enter grafts of their target tissue, the optic tectum, but few axons are able to leave tectal transplants.

An ovomucin-like protein on the surface of migrating primordial germ cells of the chick and rat

A mucin was discovered on the surface of migratory primordial germ cells (PGCs) from chick and rat embryos by means of two monoclonal antibodies. The protein was found to be identical or closely related to ovomucin, a 600 X 10(3) relative molecular mass glycoprotein, and a major constituent of the vitelline membrane of the avian yolk. Based on its resemblance to ovomucin it is referred to as ovomucin-like protein (OLP). The OLP was expressed on PGCs from E3 to E7 female, and from E3 to E12 male chick embryos as the PGCs migrate and colonize the gonadal ridges. After the PGCs have settled in the gonads, they no longer express OLP. In tissue cultures of dissociated cells from E6 gonads, OLP was present only on cells that were positive for PAS staining, the standard histological method to identify PGCs in the chick embryo. Since unfixed PGCs were recognized by the antibodies, at least part of the OLP is localized on the cell surface. The anti-OLP antibodies also stained PGCs in the gonads of the rat embryo, showing that the expression of this antigen on PGCs is phylogenetically conserved. Ovomucin isolated from vitelline membrane prevented adhesion of fibroblasts but not PGCs when used a as a substratum in vitro. The anti-adhesive quality of the mucin resides in the sialic acid residues of the carbohydrate side chains. We propose that OLP has a similar anti-adhesive quality as the ovomucin from vitelline membrane, and that this anti-adhesive property is important to prevent precocious adhesion of migrating PGCs to blood vessel walls and to connective tissue in the mesentery as they migrate toward the gonadal ridges.

Agrin: an extracellular matrix heparan sulfate proteoglycan involved in cell interactions and synaptogenesis

Recent studies have documented important roles for heparan sulfate proteoglycans in the control of nervous system development. Agrin is an extracellular matrix protein identified and named based on its involvement in the aggregation of acetylcholine receptors (AChRs) during synaptogenesis at the neuromuscular junction. Recent studies have demonstrated that agrin is a large extracellular heparan sulfate proteoglycan, with a molecular mass in excess of 500 kDa and a protein core of 220 kDa. Emerging evidence indicates that agrin's function is not limited to its role in AChR aggregation during synaptogenesis, as the majority of agrin expression occurs in the developing central nervous system, especially in developing axonal tracts. This review examines recent studies suggesting a role for agrin in the regulation of cell-cell interactions, most notably by its ability to interact with the neural cell adhesion molecule. In addition, other potential roles for the heparan sulfate chains of agrin during nervous system development are explored.

Identification of a novel alternatively spliced agrin mRNA that is preferentially expressed in non-neuronal cells

A novel agrin isoform was identified based on the isolation of an agrin cDNA from E9 chick brain that lacked 21 base pairs (bp) in the NH2-terminal encoding region of the agrin mRNA. Reverse transcription-polymerase chain reaction (RT-PCR) of E9 chick brain mRNA confirmed the existence of this agrin isoform in brain, although the novel splice variant represents a minor fraction of agrin mRNA in brain. However, upon analysis of chick brain astrocyte mRNA, smooth muscle mRNA, and cardiac muscle mRNA by RT-PCR, we show that this novel agrin isoform is the predominant agrin isoform in these non-neuronal cell populations. We extended our analyses to examine the expression of this agrin mRNA isoform during chick development and show that the agrin mRNA lacking this 21-bp exon is up-regulated with brain development, consistent with the increase in glial number during brain development, while the agrin isoform that does not undergo splicing and thus contains the 21-bp exon is down-regulated in brain development. Because the 21-bp exon is inserted in the region of chick agrin which encodes the putative signal sequence of agrin, with the signal peptidase site immediately preceding the putative first amino acid of the mature protein being deleted as a result of splicing, these data raise the interesting possibility that the presence or absence of this alternatively spliced exon may differentially regulate processing of the agrin protein in neuronal and non-neuronal cells, respectively.

Analysis of proteoglycan expression in developing chicken brain: characterization of a heparan sulfate proteoglycan that interacts with the neural cell adhesion molecule

In the present study we have characterized the major proteoglycans of chick brain, focusing on their pattern of expression in development and on identifying the heparan sulfate proteoglycan (HSPG) that binds to the neural cell adhesion molecule (NCAM). The major chondroitin sulfate proteoglycans (CSPG) are a heterogeneous group of molecules with an average MW of 450 kDa. Protein core analysis reveals multiple protein cores between 100 and 350 kDa. The HSPGs are somewhat smaller, with an average MW of 350 kDa, and the major brain HSPG possesses a 250 kDa protein core. During development the relative percentage of HSPG decreases from approximately 50% of total sulfate-labeled PG at E6 to 25% by E10. In order to begin to characterize the HSPG that interacts with NCAM, we initially used an antiserum produced against a HSPG which was previously shown to copurify with NCAM (Cole and Burg: Exp Cell Res 182:44-60, 1989). This antiserum immunoprecipitated a HSPG core protein of 250 kDa, corresponding to the major HSPG of chick brain. We also show that the major brain HSPG binds to a synthetic peptide that encodes the heparan sulfate-binding domain of NCAM, and that monoclonal antibodies to a recently identified chick retinal HSPG recognize this NCAM-binding HSPG. This HSPG was immunopurified from E10 chick brain using the 6D2 monoclonal antibody, and has been shown to bind an affinity column containing the heparan sulfate-binding peptide of NCAM. Consistent with its ability to bind NCAM, we show that the intact 6D2 HSPG inhibits cell adhesion to a HBD peptide substratum, and also binds chick brain cells when employed as a substratum.

Two chondroitin sulfate proteoglycans differentially expressed in the developing chick visual system

Two monoclonal antibodies, 2B9 and 9BA12, were used to identify and characterize two different chondroitin sulfate proteoglycans (CSPGs) associated with the embryonic chick visual system. Monoclonal antibody 2B9 recognizes a carbohydrate epitope of collagen type IX proteoglycan. Immunohistochemistry showed that collagen type IX proteoglycan was abundant in the vitreous body and meninges, but absent in brain and retina. In developing trunk regions, collagen type IX proteoglycan is segmentally distributed in the somites, appearing only in the posterior sclerotome. Monoclonal antibody 9BA12 recognizes collagen type IX proteoglycan from vitreous body and an unidentified chondroitin sulfate proteoglycan in retina and brain, herein referred to as 9BA12 CSPG. Immunohistochemistry showed that 9BA12 CSPG is present in the optic fiber layer of the retina, coinciding temporally and spatially with the onset and cessation of ganglion cell axon growth. In the trunk region, 9BA12 immunostaining appears in the developing spinal cord and throughout the sclerotome. In culture, neither the collagen type IX proteoglycan nor the brain-derived 9BA12 CSPG were able to support neurite outgrowth from retinal ganglion cell explants. In combination with basal lamina proteins, collagen type IX proteoglycan slightly inhibited neurite outgrowth and led to a stronger fasciculation of retinal axons. In contrast, 9BA12 CSPG had no inhibitory effect on the outgrowth of retinal axons and had no effect on their fasciculation. Our study demonstrates the existence of two chondroitin sulfate proteoglycans in the developing visual system of the chick. Based on the developmental expression and the results from neurite outgrowth experiments, it was concluded that the 9BA12 CSPG does not operate as a neurite outgrowth inhibitor for retinal axons.

Agrin is a heparan sulfate proteoglycan

In the present study we have identified the extracellular matrix protein agrin as a major heparan sulfate proteoglycan (HSPG) in embryonic chick brain. Using monoclonal antibodies and a polyclonal antiserum to the core protein of a previously identified HSPG from embryonic chick brain, our expression screened a random-primed E9 chick brain cDNA library. Twelve cDNAs were isolated that were shown to be identical to the chick extracellular matrix protein agrin. Western blot analysis and immunocytochemistry confirmed that agrin is a HSPG that is identical with the HSPG from embryonic chick brain. A polyclonal antiserum to recombinant agrin protein recognized agrin as a diffuse band of over 400 kDa in extracts from brain and vitreous humor. The agrin immunoreactivity on the blot was shifted to a defined band of approximately 250 kDa after treatment of the samples with heparitinase or nitrous acid, and this banding pattern was indistinguishable from immunoreactivity obtained with antibodies to the brain HSPG. We also show that agrin binds tightly to anion exchange beads, indicating that the molecule is highly negatively charged, which is a hallmark of all proteoglycans. Furthermore, the agrin antiserum recognizes the affinity purified HSPG from chick brain and vitreous humor. Immunocytochemistry demonstrated that agrin is expressed in developing brain, and is especially abundant in developing axonal tracts, in a distribution identical to the staining of the brain HSPG with monoclonal antibodies. We also show that the anti-HSPG antibodies stain the synaptic site of the neuromuscular junction, in agreement with agrin expression. Thus, our studies demonstrate that chick agrin is a HSPG that is prominent in the embryonic chick brain. Since previous studies from our laboratories have shown that this proteoglycan interacts with neural cell adhesion molecule, our studies raise the interesting possibility that neural cell adhesion molecule and agrin are interactive partners that may regulate a variety of cell adhesion processes during neural development, including synaptogenesis.

A new heparan sulfate proteoglycan in the extracellular matrix of the developing chick embryo

A new heparan sulfate proteoglycan was identified by two monoclonal antibodies. The antibodies (hybridoma clones 6C4 and 1B11) were generated from mice immunized with inner limiting membranes of the embryonic chick retina. The proteoglycan had an apparent molecular weight of 250 kDa with a core protein of 180 kDa. Antibodies to perlecan and to a recently identified brain-derived heparan sulfate proteoglycan did not cross-react with purified 6C4/1B11 antigen, confirming that the three proteoglycans are not related. The 6C4/1B11 proteoglycan was abundant in basal laminae, such as the inner limiting membrane of the retina, the lens capsule, the epidermal, the pial, and the muscle basal laminae, and the vitreous body. The distribution and developmental expression of the 6C4/1B11 proteoglycan was different than that of perlecan and the brain-derived heparan sulfate proteoglycan. When used as a substrate for embryonic retinal explants, the proteoglycan did not support axonal outgrowth in vitro. The data present a new heparan sulfate proteoglycan and demonstrate the existence of at least three different heparan sulfate proteoglycans in the developing chick embryo with partially overlapping distribution.

Axonin 1 is expressed primarily in subclasses of avian sensory neurons during outgrowth

A 120 kDa protein, which is expressed mainly on the surface of chick sensory neurons during outgrowth, was identified by monoclonal antibody 1A12. Crossreactivity studies showed that this protein was identical to axonin 1, a member of the immunoglobulin superfamily which promotes neurite outgrowth. Using the 1A12 antibody, we show that in the peripheral nervous system of the chick, axonin 1 is present on the cell bodies and processes of cutaneous and visceral neurons, but not on muscle afferents. In the central nervous system, axonin 1 is present in sensory pathways, such as fibers of the dorsal funiculi in the spinal cord and the optic pathway. However, axonin 1 is only expressed on growing nerve fibers. Late in embryonic development, it is present only on a small population of dorsal root ganglion cells, and is entirely absent on optic fibers. The disappearance of axonin 1 in the visual pathway coincides with the arrival of optic axons at the tectum, suggesting its expression is down regulated by axonal contact with its target. The localization of this protein on the surface of neuronal membranes was confirmed by EM immunohistochemistry and by labeling live nerve cells and their processes in tissue culture. The restricted spatio-temporal expression of axonin 1, together with its expression on the surface of neuronal membranes suggests that it is important for the development of sensory projections.

Tenascin protein and mRNA in the avian visual system: distribution and potential contribution to retinotectal development

The large glycoprotein tenascin is one of the extracellular matrix proteins that is abundant in the developing nervous system. To determine its distribution and possible role in the ontogenty of the avian retinotectal system, the distribution of the protein, the expression of its mRNA, and the effect of the protein on growing retinal neurites in vitro was investigated. Immunocytochemistry demonstrated that relatively little tenascin was present in the optic fiber layer of the retina, the optic nerve and tract. Tenascin, however, was abundant in the stratum opticum of the tectum, the target of retinal axons in the brain. Whereas tenascin protein is found only in discrete portions and layers of the brain, in situ hybridization studies showed that tenascin mRNA was expressed throughout development by radial glial cells at the ventricular surfaces of the brain, distant from the tissue localization of the protein. Injection of antitenascin antiserum into the tectal ventricle disturbed the distribution of the protein in the tectum by binding tenascin closer to its origin at the ventricular border. This suggests that the localization of tenascin in the brain is not restricted to its site of synthesis, but is mediated by tenascin-binding proteins that are expressed in defined areas and layers in the brain. In vitro studies showed that tenascin did not promote neurite outgrowth of retinal axons. On the contrary, the addition of tenascin to retinal explants growing on collagen or on L1 slowed the growth rate of optic axons by as much as 50%. The inhibitory function in vitro suggests that this protein has a modulatory role in axonal growth in vivo. The absence of tenascin from the optic fiber layer of retina, optic nerve, and optic tract and its abundance in the tectum suggest that tenascin may function to slow the rapidly growing optic nerve fibers once they arrive at the tectum. The slowing of optic fiber outgrowth may then facilitate terminal arborization and synapse formation within the tectum. The abundant tenascin in synaptic layers may also serve to stabilize synapses once they have formed.

A heparan sulfate proteoglycan in developing avian axonal tracts

A neuronal heparan sulfate proteoglycan was identified by a panel of four monoclonal antibodies. The antibodies were generated from mice immunized with embryonic chick retina basal lamina (clones 3A12, 3A3, and 9E10) and embryonic chick optic tract (clone 6D2). Cross-reactivity of all four antibodies with the purified proteoglycan confirmed that the antibodies were directed to the same antigen. Antibodies to heparan sulfate proteoglycan from embryonic chick muscle or EHS mouse tumor (perlecan) did not cross-react with the neuronal heparan sulfate proteoglycan, suggesting that the two proteoglycans are not related. In Western blots, the proteoglycan had a molecular weight of 600 kDa that dropped to 250 kDa when the samples were treated with heparitinase or nitric acid. Immunocytochemistry showed that in early stages of chick and quail development, the proteoglycan was exclusively localized in basal laminae and had a distribution similar to that of laminin. During further development, a strong labeling was also found in the extracellular environment of nerve tracts, such as the optic nerve and white matter areas of the brain and spinal cord. The labeling of axonal tracts declined from embryonic day 10 onward, while labeling in basal laminae persisted. Antibodies to muscle heparan sulfate proteoglycan or to perlecan did not label nerve fibers. The data show that embryonic neuronal tissue expresses a new type of heparan sulfate proteoglycan.

Tenascin in the developing chick visual system: distribution and potential role as a modulator of retinal axon growth

The distribution of the extracellular matrix protein tenascin was studied in the developing chick visual system to determine its possible regulatory role in retinotectal development. Little tenascin was present in the retinal optic fiber layer, and the optic nerve and tract, but was abundant in the stratum opticum of the tectum, the target of retinal axons in the brain. A high concentration of tenascin was found in areas bordering the developing visual pathway, such as the optic disc, the outer surface of the optic nerve, and the supraoptic commissure. In vitro studies showed that tenascin did not promote neurite outgrowth of retinal axons. When optic axons were confronted with a tenascin substrate in culture, they did not grow onto the tenascin suggesting that this protein inhibited optic axon outgrowth. Furthermore, the addition of tenascin to retinal explants in collagen gels slowed the growth rate of optic axons by as much as 50%. The distribution of tenascin in vivo and its inhibitory function in several in vitro systems suggest that this protein acts as a modulator of axonal growth in vivo. Tenascin may act as a barrier at specific sites along the visual pathway, and at the target, may slow the rate of axon outgrowth, and ultimately act as a stop molecule. The growth inhibitory activity of tenascin in retinal and tectal synaptic layers may also serve to stabilize synapses once appropriate connections have been made.

Effect of wound healing and tissue transplantation on the navigation of axons in organ-cultured embryonic chick eyes

Wound closure and repair of embryonic neuroepithelium were studied in organ-cultured embryonic retinae. Eyes from 3 to 4-day-old embryos were cultured after removing pieces of retinal tissue. During the subsequent 24 hours of incubation, the 150 to 200 microns wide holes in the retina closed completely. Histological studies showed that the wound closure was not accomplished by cell migration or cell proliferation, but by an approximation of the wound edges mediated by extracellular matrix fibrils of the vitreous body. The wound contraction facilitated the integration of transplants into the retinal neuroepithelium with a perfect alignment of the implants with the host at the vitreal surface. Within 24 hours, a continuous inner limiting membrane between transplant and host retina was established. The effect of wound healing and tissue transplantation on the navigation of optic axons in the retina was investigated. The wound contraction in the retina caused the optic axons near the lesion site to grow to the wound center, where the axons traversed the retina and formed a neuroma at the ventricular side, resembling the organization of axons at the optic disc. In the transplantation paradigm, axons from the host retina migrated into the transplant and vice versa. However, due to the wound contraction around the transplant, most axons grew into the interface between the transplant and host tissue.

Deposition of extracellular matrix along the pathways of migrating fibroblasts

Fibroblasts from rat, mouse and chick embryos cultured on poly-lysine/fibronectin- or poly-lysine/laminin-coated dishes were stained with antibodies directed to extracellular matrix molecules. The staining showed that cells had migrated during culture and deposited extracellular matrix components along their migration trails. Depending on the antigen, the staining of the matrix revealed fibrils, spots or a diffuse smear along the migration pathways. The major matrix components were fibronectin and heparan sulfate proteoglycan; however, laminin nidogen, tenascin, glia-derived nexin (GDN) and chondroitin-4-sulfate proteoglycan were also found. The migration trails were also detectable by scanning electron microscopy. Here, the fibrils were the prominent structures. The deposition of matrix was independent from the substratum: fibronectin was deposited on laminin, plain poly-lysine, basal lamina and even on fibronectin. Functional assays using anti-fibronectin or an antiserum to embryonic pigment epithelium basement membrane disturbed the formation of matrix fibrils, but did not inhibit cell attachment and translocation. Likewise, heparin in the culture medium only partially inhibited cell migration, despite the fact that it disturbed the formation of proper matrix fibrils. Our results suggest that the deposition of extracellular matrix by cells may not be mandatory for attachment and translocation. However, the deposition of matrix along defined trails might be important for the pathfinding of cells or nerve fibers that appear later in development.

Origin and distribution of enteric neurones in Xenopus

In Xenopus, we investigated the origin of enteric neurones and their distribution in relation to the extracellular matrix (ECM) components, fibronectin (FN) and tenascin (TN). Enteric neurone precursor cells originate from the anterior trunk neural crest (NC). They migrate along the ventromedial NC pathway (between somites and neural tube/notochord) into the primitive gut (via the dorsal mesentery/lateral plate mesoderm) where they differentiate into enteric neurones. NC cells were identified during their migration and in the gut using the X. laevis - X. borealis nuclear marker system. The neuronal character of NC cells in the gut could be demonstrated immunohistochemically with a monoclonal antibody against the HNK-1 epitope. This antibody is superior to N-CAM and neurofilament antibodies which proved insufficient in Xenopus. In early tadpoles (stage 45), enteric neurones occurred frequently in the mesenchymal lining of the oesophagus, either singly or in groups of two to three cells. In more distal portions of the digestive tract, enteric neurones were rarely found. In metamorphosing tadpoles (stage 62/63), enteric neurones were scattered singly beneath the mucosa, or formed small aggregates between the inner and outer muscle layer throughout the length of the digestive tract. The neurones occurred in positions corresponding to the myenteric and submucosal plexus of higher vertebrates. The distribution of enteric neurones was studied in relation to fibronectin (FN) and tenascin (TN), glycoproteins of the ECM, which support (FN) and inhibit (TN) amphibian NC cell migration. Using immunohistochemistry, FN was found during NC cell migration in ECM spaces along the ventromedial pathway, and in the gut between the mucosa and the muscle layers, where it would be able to support adhesion and migration of NC cells. TN, in contrast, appeared much later than FN, both in the dorsal trunk and also ventrally, in the gut. In older tadpoles, TN was present in the mesenchyme and muscle layers of the digestive tract, where it might have an inhibiting influence on the migration of enteric neurones within the gut wall.

Antisera to basal lamina and glial endfeet disturb the normal extension of axons on retina and pigment epithelium basal laminae

In order to determine the role of the extracellular matrix in regulating the directed growth of embryonic neurites, antisera to retina (a-RBL I and II), to pigment epithelium (a-PBL) and to glomerular (a-GBL) basal lamina were probed for an effect on the ordered extension of neurites. In the assays, retina explants from chick and quail were cultured on basal lamina from embryonic chick retina and pigment epithelium either in the presence of anti-basal lamina antisera or in the presence of the corresponding preimmune sera. In the presence of all anti-basal lamina antisera, normal extension of axons was greatly inhibited both on retina and on pigment epithelium basal lamina. The antisera affected the growth pattern and the morphology of the individual axons in two ways: in the presence of a-RBL I the short axons were less directed, developed more and longer side branches, and the lamellipodia of the growth cones were reduced in size compared to axons from control cultures. In the presence of a-RBL II and a-GBL, axons grew slowly out from the explants as very thick bundles, strikingly different from axons in control cultures. The antiserum to pigment epithelium basal lamina induced both strong fasciculation and disorganization of the linear fiber extension, being intermediate between the two types of effects observed after antiserum addition. The results suggest that adhesive matrix molecules in basal laminae have important functions in elongation, fasciculation and in the morphology of growing axons.

Immunocytochemical Localization of Glia-Derived Nexin, Laminin and Fibronectin on the Surface or Extracellular Matrix of C6 Rat Glioma Cells, Astrocytes and Fibroblasts

The expression and cellular distribution of glia-derived nexin (GDN), laminin and fibronectin on C6 rat glioma cells, rat brain astrocytes and rat fibroblasts were investigated by immunoblotting and immunocytochemistry. Western blot analysis of C6 cell homogenates confirmed the specificities of the antibodies. Immunocytochemical staining of C6 cells, astrocytes and fibroblasts showed that laminin, fibronectin and GDN were abundant on the surface of glioma cells and astrocytes whereas on fibroblasts fibronectin was abundant though only traces of GDN and laminin could be detected. The light microscopy data were confirmed by ultrastructural studies showing that each antigen was present on the surface of the C6 rat glioma cells as numerous spots with slightly different distribution patterns for each of the antigens. In fibroblast cultures, the antigens were also localized in the extracellular matrix in the vicinity of the cells. Migrating fibroblasts but not migrating glioma cells or astrocytes deposit the matrix-proteins onto the substratum leaving behind a track of GDN, laminin and fibronectin. When the cells were treated with heparin prior to antibody incubation, the GDN immunoreactivity completely disappeared, whereas the distribution and abundance of laminin and fibronectin was not affected. Our data show that GDN binds, possibly by a heparin-like molecule, to the outer surface of cells or to the extracellular matrix and may protect cells and matrix proteins against proteolytic degradation.

The effect of tenascin and embryonic basal lamina on the behavior and morphology of neural crest cells in vitro

We have investigated the morphology and migratory behavior of quail neural crest cells on isolated embryonic basal laminae or substrata coated with fibronectin or tenascin. Each of these substrata have been implicated in directing neural crest cell migration in situ. We also observed the altered behavior of cells in response to the addition of tenascin to the culture medium independent of its effect as a migratory substratum. On tenascin-coated substrata, the rate of neural crest cell migration from neural tube explants was significantly greater than on uncoated tissue culture plastic, on fibronectin-coated plastic, or on basal lamina isolated from embryonic chick retinae. Neural crest cells on tenascin were rounded and lacked lamellipodia, in contrast to the flattened cells seen on basal lamina and fibronectin-coated plastic. In contrast, when tenascin was added to the culture medium of neural crest cells migrating on isolated basal lamina, a significant reduction in the rate of cell migration was observed. To study the nature of this effect, we used human melanoma cells, which have a number of characteristics in common with quail neural crest cells though they would be expected to have a distinct family of integrin receptors. A dose-dependent reduction in the rate of translocation was observed when tenascin was added to the culture medium of the human melanoma cell line plated on isolated basal laminae, indicating that the inhibitory effect of tenascin bound to the quail neural crest surface is probably not solely the result of competitive inhibition by tenascin for the integrin receptor. Our results show that tenascin can be used as a migratory substratum by avian neural crest cells and that tenascin as a substratum can stimulate neural crest cell migration, probably by permitting rapid detachment. Tenascin in the medium, on the other hand, inhibits both the migration rates and spreading of motile cells on basal lamina because it binds only the cell surface and not the underlying basal lamina. Cell surface-bound tenascin may decrease cell-substratum interactions and thus weaken the tractional forces generated by migrating cells. This is in contrast to the action of fibronectin, which when added to the medium stimulates cell migration by binding both to neural crest cells and the basal lamina, thus providing a bridge between the motile cells and the substratum.

Induction of tenascin in healing wounds

The distribution of the extracellular matrix glycoprotein, tenascin, in normal skin and healing skin wounds in rats, has been investigated by immunohistochemistry. In normal skin, tenascin was sparsely distributed, predominantly in association with basement membranes. In wounds, there was a marked increase in the expression of tenascin at the wound edge in all levels of the skin. There was also particularly strong tenascin staining at the dermal-epidermal junction beneath migrating, proliferating epidermis. Tenascin was present throughout the matrix of the granulation tissue, which filled full-thickness wounds, but was not detectable in the scar after wound contraction was complete. The distribution of tenascin was spatially and temporally different from that of fibronectin, and tenascin appeared before laminin beneath migrating epidermis. Tenascin was not entirely codistributed with myofibroblasts, the contractile wound fibroblasts. In EM studies of wounds, tenascin was localized in the basal lamina at the dermal-epidermal junction, as well as in the extracellular matrix of the adjacent dermal stroma, where it was either distributed homogeneously or bound to the surface of collagen fibers. In cultured skin explants, in which epidermis migrated over the cut edge of the dermis, tenascin, but not fibronectin, appeared in the dermis underlying the migrating epithelium. This demonstrates that migrating, proliferating epidermis induces the production of tenascin. The results presented here suggest that tenascin is important in wound healing and is subject to quite different regulatory mechanisms than is fibronectin.

Migratory behavior of cells on embryonic retina basal lamina

In order to study cell translocation in vitro on a physiological substrate a novel cell migration assay was developed using the inner limiting membrane of the avian embryonic retina. The matrix sheet consists of a laminin-rich basal lamina covered by a dense layer of neuroepithelial endfeet. The retina basal lamina does not contain fibronectin. Cells translocating on this substrate displace the neuroepithelial endfeet, leaving behind tracks in the endfeet monolayer. Motility of cells and the relative forward to lateral migration can be quantitated by measuring lengths, widths, and areas of the tracks. Using this assay system, the conditions and patterns of cell migration for a variety of cells have been examined. In the absence of serum all cell types show only minor migratory activity and addition of serum to the culture medium always enhances the rate of cell migration in a saturable, dose-response manner. The serum cannot be replaced by fibronectin or vitronectin (serum spreading factor). For maximum cell migration, serum has to be constantly present in the medium; however, 58% cell migration is obtained in serum-free medium when the matrix is preincubated with serum. According to the area and linearity of the tracks, the migratory behavior of the different cells can be classified into three groups: (i) fibroblasts and the nonpigmented Bowes melanoma cells form straight and long tracks; (ii) glioma, sarcoma, and carcinoma cells from straight but short tracks, and (iii) neuronal tumor cells, epithelial cells, and pigmented B16 melanoma cells form wide and short tracks. Comparative studies with low and high metastatic clones of tumorgenic cell lines show that migratory activity and metastatic potential of cells do not necessarily correlate. Finally, we show that fibroblasts deposit fibronectin fibrils on their paths as they migrate on the basal lamina. Fibronectin trails are also seen when fibroblasts are cultured on plain basal laminae that are pretreated with detergent to remove the endfeet monolayer. Likewise, when fibroblasts are cultured in the presence of antifibronectin antibodies, the fibronectin secreted by cells is detectable. Due to antibody treatment the cellular fibronectin is precipitated and its normal fibril formation is inhibited; however, the translocation of fibroblasts is not impaired.

The distribution of fibronectin and tenascin along migratory pathways of the neural crest in the trunk of amphibian embryos

It is generally assumed that in amphibian embryos neural crest cells migrate dorsally, where they form the mesenchyme of the dorsal fin, laterally (between somites and epidermis), where they give rise to pigment cells, and ventromedially (between somites and neural tube), where they form the elements of the peripheral nervous system. While there is agreement about the crest migratory routes in the axolotl (Ambystoma mexicanum), different opinions exist about the lateral pathway in Xenopus. We investigated neural crest cell migration in Xenopus (stages 23, 32, 35/36 and 41) using the X. laevis-X. borealis nuclear marker system and could not find evidence for cells migrating laterally. We have also used immunohistochemistry to study the distribution of the extracellular matrix (ECM) glycoproteins fibronectin (FN) and tenascin (TN), which have been implicated in directing neural crest cells during their migrations in avian and mammalian embryos, in the neural crest migratory pathways of Xenopus and the axolotl. In premigratory stages of the crest, both in Xenopus (stage 22) and the axolotl (stage 25), FN was found subepidermally and in extracellular spaces around the neural tube, notochord and somites. The staining was particularly intense in the dorsal part of the embryo, but it was also present along the visceral and parietal layers of the lateral plate mesoderm. TN, in contrast, was found only in the anterior trunk mesoderm in Xenopus; in the axolotl, it was absent. During neural crest cell migration in Xenopus (stages 25–33) and the axolotl (stages 28–35), anti-FN stained the ECM throughout the embryo, whereas anti-TN staining was limited to dorsal regions. There it was particularly intense medially, i.e. in the dorsal fin, around the neural tube, notochord, dorsal aorta and at the medial surface of the somites (stage 35 in both species). During postmigratory stages in Xenopus (stage 40), anti-FN staining was less intense than anti-TN staining. In culture, axolotl neural crest cells spread differently on FN- and TN-coated substrata. On TN, the onset of cellular outgrowth was delayed for about 1 day, but after 3 days the extent of outgrowth was indistinguishable from cultures grown on FN. However, neural crest cells in 3-day-old cultures were much more flattened on FN than on TN. We conclude that both FN and TN are present in the ECM that lines the neural crest migratory pathways of amphibian embryos at the time when the neural crest cells are actively migrating. FN is present in the embryonic ECM before the onset of neural crest migration.(ABSTRACT TRUNCATED AT 400 WORDS)

Detection of glia-derived nexin in the olfactory system of the rat

Glia-derived nexin (GDN) is a 43 kd cell-secreted protease inhibitor with neurite promoting activity. We have raised specific polyclonal antisera to rat GDN. These antibodies stain a single band at 43 kd on immunoblots of concentrated C6 glioma-conditioned medium and have been used to demonstrate that GDN is present in the olfactory system of the rat. One band at 43 kd is recognized by the GDN antibodies on immunoblots of olfactory bulb homogenate. Immunohistochemistry shows that GDN occurs predominantly in the olfactory nerve layer of the olfactory bulb and in the olfactory submucosa. Comparative studies with antibodies against vimentin, GFAP, and fibronectin suggest that anti-GDN recognizes cells associated with the olfactory system, but not exclusively the olfactory neurons themselves. Data from the immunohistochemical studies were confirmed by RNA blots and GDN mRNA expression throughout development of the olfactory bulb. The high levels of GDN in the rat olfactory system may be related to the continuous degeneration and regeneration phenomena taking place in these structures.

Aberrant optic axons in the retinal pigment epithelium during chick and quail visual pathway development

Examination of a large number of retinal pigment epithelia revealed that, in a small proportion, optic axons in chick and quail eyes aberrantly entered the pigment cell layer between embryonic day (E) 7 to E14. The aberrant retinal axons originated from the main stream of retinal fibers in the optic nerve and invaded the pigment layer from various positions of the optic nerve head or fissure by growing along the basal side of the pigment epithelium. The axon bundles grew several millimeters into the epithelial sheet and arborized at the margin of the eye. As shown by electron microscopy the nerve fibers occurred as bundles of three to several hundred axons. They always were located at the basal side of the epithelium, and were enveloped by processes of epithelial cells. Very large bundles of axons, however, displaced the epithelial cells from the basal matrix. These retinal axons contacted the pigment epithelial basal lamina. The basal extracellular matrix from the retinal pigment epithelium was isolated and used as substratum for in vitro cultures of various types of neural explants. The matrix preparations consisted of a sheet of a 50 nm thick basal lamina with a central lamina densa, two laminae rarae, and a 15 micron thick stroma. Axons from avian retina explants, as well as sensory ganglia, grew on the basal lamina side of the pigment cell matrix with the same growth rate and with the same fiber density as on similarly prepared basal laminae from the neural retina. These experiments show that the matrix from the pigment epithelium of the avian eye does not have negative effects on axonal growth and indicate that a basal lamina from a normally non-innervated tissue can provide a favorable matrix for axonal growth.

The distribution of tenascin coincides with pathways of neural crest cell migration

The distribution of the extracellular matrix (ECM) glycoprotein, tenascin, has been compared with that of fibronectin in neural crest migration pathways of Xenopus laevis, quail and rat embryos. In all species studied, the distribution of tenascin, examined by immunohistochemistry, was more closely correlated with pathways of migration than that of fibronectin, which is known to be important for neural crest migration. In Xenopus laevis embryos, anti-tenascin stained the dorsal fin matrix and ECM along the ventral route of migration, but not the ECM found laterally between the ectoderma and somites where neural crest cells do not migrate. In quail embryos, the appearance of tenascin in neural crest pathways was well correlated with the anterior-to-posterior wave of migration. The distribution of tenascin within somites was compared with that of the neural crest marker, HNK-1, in quail embryos. In the dorsal halves of quail somites which contained migrating neural crest cells, the predominant tenascin staining was in the anterior halves of the somites, codistributed with the migrating cells. In rat embryos, tenascin was detectable in the somites only in the anterior halves. Tenascin was not detectable in the matrix of cultured quail neural crest cells, but was in the matrix surrounding somite and notochord cells in vitro. Neural crest cells cultured on a substratum of tenascin did not spread and were rounded. We propose that tenascin is an important factor controlling neural crest morphogenesis, perhaps by modifying the interaction of neural crest cells with fibronectin.

Nondirected axonal growth on basal lamina from avian embryonic neural retina

The vitreous surface of the embryonic avian retinal neuroepithelium was isolated by mechanical disruption of the retina mounted between 2 adhesive substrata. The 200-micron-thick sheath covered an area of up to 1 cm2 and consisted of the vitreal basal lamina with a lamina densa, 2 laminae rarae, and a carpet of ventricular cell endfeet on top of the lamina. The vitreal endfeet were removed by detergent treatment and an extracellular basal lamina was obtained. The laminae were further characterized by immunohistochemistry and immunoblotting. A 190 kDa laminin protein was detected in laminae with and without vitreal endfeet, whereas the membrane-bound neural cell adhesion molecule (N-CAM) was detectable only on the endfeet of the ventricular cells and was absent in the detergent-treated basal laminae. Neither immunoblotting nor immunostaining revealed fibronectin in these preparations. Explants of retina, sensory ganglia, and cerebellum from chick, quail, and mouse were cultured on the basal lamina as a substratum. In all cases axonal outgrowth was excellent, with a growth rate similar to that in situ. Outgrowing axons from sensory ganglia and cerebellar explants were accompanied by migratory cells, which, in the case of sensory ganglia, were flat cells and, in the case of cerebellar explants, resembled granular neurons. Optic axons grew on the laminae in an asymmetric, explant-inherent pattern specific for the position of origin of the explant. On detergent-treated basal laminae, as well as on laminin, the retinal axons grew in a clockwise orientation. This axonal growth pattern was specific for retinal tissue and was not observed with axons from other neural explants. In spite of the excellent substrate properties provided by the substratum, cues for growing axons (toward or away from the optic disk) were not detectable in the basal lamina preparations.

Immunohistochemical localization of laminin, neural cell adhesion molecule, collagen type IV and T-61 antigen in the embryonic retina of the Japanese quail by in vivo injection of antibodies

Antibodies against laminin (LN), fibronectin (FN), collagen type IV (Col IV), neural cell adhesion molecule (N-CAM), T-61 antigen, actin, tubulin and neurofilament protein were injected into the eyes of quail embryos (Coturnix coturnix japonica) of different ages. Twenty h after injection, the heads of the embryos were fixed and the antibodies visualized in sections with the use of fluorescein-isothiocyanate (FITC) or peroxidase-labeled second antibodies by light- and electron microscopy. Antibodies against cell surface molecules, such as N-CAM, LN, Col IV and T 61, labeled matrix and membrane components of the retinal cells in different antigen-specific patterns. Antibodies against intracellular antigens, such as actin, tubulin and neurofilament protein labeled nonspecifically the vitreous body and the inner basal lamina of the retina, but resulted in only a very weak and diffuse labeling of retinal cells. N-CAM was detected in high concentration in the optic fiber layer on the surface of axons and on the membranes of all retinal cells. Col IV, LN and T 61 antigen were found predominantly in the optic fiber layer. LN and Col IV were located on the surface of axons and the endfeet of ventricular (neuroepithelial) cells in a patchy distribution. The T-61 antigen was found in early stages in the cell-free space of the optic fiber layer, on the surface of ventricular cells and axons, and at later stages also in high-density patches between nerve fibers. The distribution of LN and T-61 antigen together with data from in vitro experiments suggests a crucial role of these proteins in axon extension in the avian retina during early development of the optic fiber layer.

Anterograde tracing of retinal axons in the avian embryo with low molecular weight derivatives of biotin

Several reactive biotin esters were injected into the eyes of chick and quail embryos at various stages of development. Four of the biotin esters reacted with molecules of the eye tissue and were detected with light and electron microscopy in fluorescein isothiocyanate and peroxidase-avidin incubated sections and whole mounts. Intra and extracellular components of the lens, the vitreous body, and the retina were labeled to different degrees. Three of the biotin esters (biotin-N-hydroxysuccinimidester, biotin-epsilon-aminocaproic acid-N-hydroxysuccinimidester, and desthiobiotin-N-hydroxysuccinimidester) prominently marked the optic fiber layer in the retina and the biotin labels were transported along the optic pathway. The tracers were detected up to the growth cone of axons 24 to 36 hr after injection. Explants from biotin marked retinas were cultured on collagen or basal laminae. During culturing axons grew out from these explants into the substratum showing that labeled tissue and nerve fibers were viable. The development of the optic pathway at the chiasma of quail embryos was studied using the biotin/avidin tracing. The bulk of fibers emerging from the retina crossed as shown by double labeling of both optic nerves in a complex pattern of segregated and interdigitizing axon bundles at the chiasma toward the contralateral side of the brain. From stage 25 onward a minor ipsilateral projection was found. At the same developmental stage a few fibers traveled into the contralateral optic nerve and grew retrogradely toward the contralateral eye. The percentage of specimens having this retino-retinal projection increased during development from 53% (stage 24 to 27; E3.5-E5.5) to 89% (stage 29 to 35; E6-E8) and declined to 40% at late embryogenesis (stage 37 to 41; E9-E12). The fact that all retinal axons were found within predictable pathways with some of them running in the wrong direction suggests that nerve fiber pathways provide accurate positional information, but at best weak directional information for growing nerve fibers.

Inhibition of cell proliferation by cytosine-arabinoside and its interference with spatial and temporal differentiation patterns in the chick retina

Incubation of chick embryos with 200 nmoles/egg of cytosine arabinoside (AraC) completely inhibits cell proliferation in the embryo. At an age older than embryonic day 4 (E4) more than 90% of the embryos survive this treatment. The drug induces various malformations; in particular the retina is heavily affected. This simple method offers the possibility to study the effect of a more or less decreased cell production on processes of further differentiation and histogenesis of retinal tissue. The following results are obtained: In spite of the inhibition of cell proliferation in the retina by AraC an abnormally thick basal lamina is found and the expansion of the eye still proceeds, indicating that eye growth is not only dependent on retinal cell numbers. Stereotyped malformations of retinal histogenesis are induced and categorized into three groups: in addition to areas of normal structure cells are found arranged in rosettes and in half-rosettes sometimes linked by areas of undefined transient cell arrangements. The results point to a strong tendency of a severely diminished cell population to form regularly laminated retinal-like structures as long as a minimal ratio of cell types is given. The spatio-temporal appearance of the type of retinal malformation in a given retinal area is dependent on the time of AraC exposure and thus on the degree of differentiation reached at a "spatio-temporal spot": Full rosettes develop at earlier, and half-rosettes at later stages of AraC interference. Furthermore, deformities first appear on temporal and ventral sides. Thus, the establishment of these malformations follows and reflects the normal sequence of differentiation within the retina. Cells within rosettes organize in specific layers and start to differentiate normally. This shows that earlier formed cells are not dependent on the influence of factors derived from cells that are formed later for their proper differentiation.

Axonal pathfinding in organ-cultured embryonic avian retinae

Eye cups from stage 14-28 (E2 to E5) chick and quail embryos consisting of neural retina, lens, and vitreous body were cultured for 1 or 2 days. These eyes expanded by proliferation of the retinal cells and the surface areas of the retinae increased several-fold. The area covered by ganglion cells and axons also expanded in vitro. [3H]Thymidine labeling showed extensive proliferation of the neuroepithelial cells including the formation of new ganglion cells. Culturing eyes from embryos before stage 17 results, as in vivo, in the generation of the first ganglion cells of the retina, but unlike in the in vivo situation, the outgrowing axons always formed a random fiber net in the central portion of the retina. A defined axonal pattern identical to the in vivo developed only in specimens from embryos of stage 17 and older. Some aberrant axons, however, were also observed at the retinal periphery in specimens from embryos of more advanced stages (20-24), but only during the second day of culturing. Axons in retinae from embryos of stages 23 to 26 heading toward the optic fissure often crossed the fissure and, in contrast to the situation in vivo, invaded the opposite retinal side. These axons of wrong polarity followed the pathways of axons growing centripetally but in reverse direction. This suggests that the polarity of growing nerve fibers and their course are determined by different factors. Culturing the eyes of embryos from stages 20 to 25 in the presence of antibodies showed that the antibodies penetrated the entire retina with 6 hr. Neither anti-N-CAM nor the T-61 antibody--both recognizing membrane proteins of retinal cells--affected the growth of the eyes in vitro. The development of the axonal pattern in vitro was not affected by incubation with N-CAM-antibodies at concentrations up to 500 micron/ml, whereas the T-61 antibody which is known to block neurite extention in vitro (S. Henke-Fahle, W. Reckhaus, and R. Babiel (l984). "Developmental Neuroscience: Physiological, Pharmacological, and Clinical Aspects," pp. 393-398. Elsevier, Amsterdam/New York) showed inhibition of axonal growth in retina cultures at 50 micron/ml. These results indicate that the eye cultures can be used as a test system for antibodies against antigens which could be involved in axon extension and neurite pathfinding in situ.

The formation of the axonal pattern in the embryonic avian retina

Both the polarity of the axonal growth and the formation of the optic fiber pattern early in retinal morphogenesis were studied in silver stained whole mounts of embryonic chick, quail, and pigeon retinae. The surface area of the retina and of the optic fiber layer increases in size exponentially, the optic fiber layer expanding faster than the retina. The optic fiber layer covers the retinal surface at E5 in quail and at E6 in chick and pigeon. In all species studied, the retinal fiber layer does not expand homogeneously with the optic nerve head as the center. Instead, the retinal fiber layer enlarges with polarities in the dorsal to ventral and nasal to temporal direction. The very first axon bearing ganglion cells appear at stage 16 in the dorsal and central portion of the retina and grow ventrally to merge at the optic disk. From stage 23 on, the optic fiber layer expands faster in the temporal than in the nasal side. Measurements on the initial polarization of young axonal processes show that the axonal growth is directed toward the optic fissure and the optic nerve head. This growth polarization is found at the onset of growth cone formation and in axons far from the nearest ganglion cells or ganglion cell axons. Therefore axon-axon interaction cannot be involved in the initial axon orientation early in retinal morphogenesis.

Axon growth in embryonic chick and quail retinal whole mounts in vitro

Whole retinae of 4- to 10-day-old chick and quail embryos were spread on membrane filters and kept in culture for up to 4 days. Axon growth during culture was demonstrated by silver staining, anterograde labeling of fibers with RITC, time-lapse recording, and SEM. Fiber growth was observed in specimens from chick embryos up to 7 days old, with a growth maximum at E6 and from quail embryos up to E6 with the maximum at E5. Newly growing axons followed the optic fiber pattern already existing and, like axons in vivo, grew predominantly toward the optic fissure. Directional and orientational adaptation of newly growing axons to the preexisting fibers increased with the donor age. Retinae from donors up to E5 in chick and up to E4 in quail showed a high proportion of axons which crossed the optic fissure during the culture period and invaded the opposite retinal fiber layer. These fibers showed a correct radial orientation while growing in the opposite direction to normal. Likewise, in cultures from these young donors some fibers grew out initially in the diametrically opposite direction to normal toward the tissue periphery. Since all of the wrongly directed axons grew at the same rate as normal and adapted correctly to the already formed axon pattern, this suggests independent signals for the direction and orientation of growing fibers. Treatment of mounted retinae with collagenase or trypsin removed the vitreal retinal surface, leaving the existing axon pattern intact. Subsequently, new axons grew profusely in culture, but lost both their orientational and directional characteristics.

Oriented axon outgrowth from avian embryonic retinae in culture

The neural retina of avian embryos was spread on a membrane filter and cut in any desired orientation. Strips cut across the retina of 4- to 7-day chick or 3- to 6-day quail embryos were explanted onto collagen gels. Vigorous neurite outgrowth was seen for about 3 days, by which time many neurites were 3 mm long. Horseradish peroxidase (HRP) labeling showed that the cells producing the neurites were large and formed a layer near the inner limiting membrane, indicating that the neurites in vitro were axons of retinal ganglion cells. The size of the neurite population and the regions from which neurites emerged varied with the donor age, while most neurites sprouted from the side of the explant formerly closest to the optic fissure. This pattern closely resembled that of axon growth in the normal retina, as revealed by SEM, silver staining, and HRP labeling. Mitotic inhibitors (Ara-C and FUdR) did not alter the neurite outgrowth. Pretreatment of retinae with trypsin or collagenase did not disorganize axons at the time of explanation, but tended to equalize neurite emergence on each side of the retinal strips. We suggest that microenvironmental factors, especially the enzyme-labile inner limiting membrane, are important for axon guidance in the retina.