Isolation, characterization, and substrate properties of the external limiting membrane from the avian embryonic optic tectum

Integrins mediate adhesion of medulloblastoma cells to tenascin and activate pathways associated with survival and proliferation

Medulloblastoma spreads by leptomeningeal dissemination rather than by infiltration that characterizes other CNS tumors, eg, gliomas. This study represents an initial attempt to identify both the molecules that mediate medulloblastoma adhesion to leptomeninges and the pathways that are key to survival and proliferation of tumor following adhesion. As a first step in molecule identification, we produced adhesion of D283 medulloblastoma cells to the extracellular matrix (ECM) of H4 glioma cells in vitro. Within this context, D283 cells preferentially expressed the alpha9 and beta1 integrin subunits; antibody and disintegrin blockade of alpha9 and beta1 binding eliminated the adhesion. The H4 ECM was enriched in tenascin, a binding partner for the alpha9beta1 integrin heterodimer. Purified tenascin-C supported D283 cell adhesion. The adhesion was blocked by antibodies to alpha9 and beta1 integrin. In vivo data were similar; immunohistochemistry of primary human medulloblastomas with leptomeningeal extension demonstrated increased expression of alpha9 and beta1 integrins as well as tenascin at the interface of brain and leptomeningeal tumor. These data suggest that tumor-cell expressions of alpha9 and beta1 integrins in combination with extracellular tenascin are necessary for medulloblastoma adhesion to the leptomeninges. As a first step in the identification of pathways that mediate survival and proliferation of tumor following adhesion, we demonstrated that adhesion to H4 ECM was associated with survival and proliferation of D283 cells as well as activation of the MAPK pathway in a growth factor deficient environment. Antibody blockade of alpha9 and beta1 integrin binding that eliminated adhesion also eliminated the in vitro survival benefit. These data suggest that adhesion of medulloblastoma to the meninges is necessary for the survival and proliferation of these tumor cells at the secondary site.

Clustering transmembrane-agrin induces filopodia-like processes on axons and dendrites

The transmembrane form of agrin (TM-agrin) is primarily expressed in the CNS, particularly on neurites. To analyze its function, we clustered TM-agrin on neurons using anti-agrin antibodies. On axons from the chick CNS and PNS as well as on axons and dendrites from mouse hippocampal neurons anti-agrin antibodies induced the dose- and time-dependent formation of numerous filopodia-like processes. The processes appeared within minutes after antibody addition and contained a complex cytoskeleton. Formation of processes required calcium, could be inhibited by cytochalasine D, but was not influenced by staurosporine, heparin or pervanadate. Time-lapse video microscopy revealed that the processes were dynamic and extended laterally along the entire length of the neuron. The lateral processes had growth cones at their tips that initially adhered to the substrate, but subsequently collapsed and were retracted. These data provide the first evidence for a specific role of TM-agrin in shaping the cytoskeleton of neurites in the developing nervous system.

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.

Tissue-specific neuro-glia interactions determine neurite differentiation in ganglion cells

Guided formation and extension of axons versus dendrites is considered crucial for structuring the nervous system. In the chick visual system, retinal ganglion cells (RGCs) extend their axons into the tectum opticum, but not into glial somata containing retina layers. We addressed the question whether the different glia of retina and tectum opticum differentially affect axon growth. Glial cells were purified from retina and tectum opticum by complement-mediated cytolysis of non-glial cells. RGCs were purified by enzymatic delayering from flat mounted retina. RGCs were seeded onto retinal versus tectal glia monolayers. Subsequent neuritic differentiation was analysed by immunofluorescence microscopy and scanning electron microscopy. Qualitative and quantitative evaluation revealed that retinal glia somata inhibited axons. Time-lapse video recording indicated that axonal inhibition was based on the collapse of lamellipodia- and filopodia-rich growth cones of axons. In contrast to retinal glia, tectal glia supported axonal extension. Notably, retinal glia were not inhibitory for neurons in general, because in control experiments axon extension of dorsal root ganglia was not hampered. Therefore, the axon inhibition by retinal glia was neuron type-specific. In summary, the data demonstrate that homotopic (retinal) glia somata inhibit axonal outgrowth of RGCs, whereas heterotopic (tectal) glia of the synaptic target area support RGC axon extension. The data underscore the pivotal role of glia in structuring the developing nervous system.

Cross-species collapse activity of polarized radial glia on retinal ganglion cell axons

Inhibition of incorrect axonal outgrowth has been shown to be a crucial guidance mechanism during the development of the nervous system. Within the visual system of chick and rat, extension of retinal ganglion cell axons is essentially restricted to distinct layers of the retina and distinct brain regions such as the tectum opticum. In addition, populations of ganglion cells from defined retina locations project topographically to defined tectal areas, their growth possibly being inhibited by radial glia in incorrect tectal regions. In the current study, we aimed to analyse potential inhibitory activity of retinal glia during outgrowth of ganglion cell axons of embryonic chick and rat. The response of ganglion cell axons originating from different retina locations when exposed to purified retinal radial glia cell membranes were monitored in collapse assays by time lapse video recording. The interaction of axons growing on purified glial somata or glial endfeet was analysed in outgrowth assays. Our results indicate that (1) nasal and temporal chick growth cones are equally induced to collapse by cell membranes from retinal radial glia: 75% nasal and 72% temporal. (2) The collapse inducing component of radial glia can be inactivated by defined heat treatment, reducing collapsing activity to 6% nasal and 5% temporal. (3) Rat growth cones respond in a similar way to chick radial glia. (4) Rat axons grow perfectly on endfeet but not on somata of radial glia of the chick. In summary, the data suggest that radial glia are functionally polarized with permissive endfeet and inhibitory somata based on heat-labile proteins. Glia polarization is likely to inhibit aberrant growth of ganglion cell axons into outer retina layers. However, retinal radial glia are unlikely to participate in preordering axons within the retina and therefore do not affect the topographic projection. Finally, the inhibitory function of radial glia is conserved between birds and mammals and represents possibly a fundamental mechanism for structuring the central nervous system.

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.

Biochemical and functional characterization of basal lamina-bound agrin in the chick central nervous system

Agrin is a high-molecular weight extracellular matrix molecule, initially purified from the electric organ of the marine ray Torpedo californica, which induces on the surface of cultured myotubes the formation of postsynaptic specializations similar to those found at the neuromuscular junction. Agrin immunoreactivity is highly concentrated in the basal lamina of the synaptic cleft but is also found in a number of other tissues where its function is not known. We characterized agrin associated with two basal laminae from the central nervous system, the inner limiting membrane of the retina and the mesencephalic external limiting membrane. A major broad band with an apparent molecular weight of > 300 kDa was identified in immunoblots of isolated basal laminae from retina, mesencephalon, kidney and muscle, showing that basal lamina-bound agrin from the central nervous system and that from non-neural tissues have similar molecular sizes. Agrin is stably but not covalently bound to the inner limiting membrane and could be completely removed only with strong detergents. Agrin could be partially extracted with buffers that are also able to partially release acetylcholine receptor aggregation activity from the neuromuscular junction or from the electric organ. Despite these immunological and biochemical similarities, agrin from both central nervous system-derived basal laminae was not able to induce acetylcholine receptor aggregation on cultured myotubes. This shows that functionally different agrin isoforms are associated with basal laminae in the central nervous system compared to the neuromuscular junction or the electric organ.

Expression of chondroitin sulfate and keratan sulfate proteoglycans in the path of growing retinal axons in the developing chick

Previous investigations have identified proteoglycans in the central nervous system during development and have implicated some proteoglycans as axon guidance molecules that act by inhibiting axon extension. The present study investigated the pattern of immunoreactivity for several glycosaminoglycans common to certain proteoglycans relative to growing retinal axons in the developing chick visual system and in retinal explant cultures. Immunostaining for chondroitin-6-sulfate, chondroitin-4-sulfate, and keratan sulfate was observed to colocalize with retinal axons throughout the retinofugal pathway during the entire period of retinal axon growth. The proteoglycan form of collagen IX, however, was only observed in the retina, primarily peripheral to the areas with actively growing axons. The pattern of immunostaining for chondroitin sulfate in tissue sections suggested that the retinal axons might be a source for some of the chondroitin sulfate immunostaining in the developing visual pathway. This was confirmed in that chondroitin sulfate immunostaining was also observed on neurites emanating from cultured retinal explants. These findings indicate that retinal axons grow in the presence of chondroitin sulfate and keratan sulfate proteoglycans and that these proteoglycans in the developing chick visual pathway have functions other than to inhibit axon growth.

Distribution of heparan sulfate proteoglycans in embryonic chicken neural retina and isolated inner limiting membrane

Quantitative distribution of proteoglycans was studied in retinal neural epithelium and its basement membrane (inner limiting membrane). Heparan sulfate proteoglycans (HSPGs) were primarily associated with both inner and outer plexiform (synaptic) layers, and inner limiting membrane (ILM), as determined by autoradiographs of lyase-digested cryosections. Based on distribution of 35S-sulfate-labeled proteoglycans, the isolated ILM contained on average approximately three fourths of its proteoglycans as HSPGs and one fourth as chondroitin sulfate/dermatan sulfate proteoglycans (CS/DSPGs), whereas the remaining retina contained approximately equal amounts of the two proteoglycans (PGs). Immunohistochemical staining indicates that the core proteins of the HSPGs in the ILM are distinct from those of the plexiform layers. The photoreceptor layer, which other studies have shown to contain much of the extracellular CS/DSPGs, was not examined. Enrichment of distinct HSPGs in the ILM and plexiform layers support the conclusion that the HSPGs may be intimately involved in the different developmental events characterizing the two regions: development and extension of ganglion cell axons in the former, synaptogenesis and neuronal function in the latter.

Neuritogenesis on collagen substrates. Involvement of integrin-like matrix receptors in retinal fibre outgrowth on collagen

Extracellular matrix molecules such as laminin, fibronectin and collagen promote neurite outgrowth in vitro. We have investigated the capacity of hydrated gels of collagen types I-III and monomeric collagen types I-VI on plastic surfaces to support neuritogenesis. The attachment and survival of explants from the day 6 chick embryo were studied and neurite outgrowth measured as mean elongation rate and maximal neurite length. Collagen types I and III, both as three-dimensional gels or as native monomers supported neuritogenesis equal to or better than laminin. Collagen type V also supported neurite out-growth although less effectively. Collagen types II, IV and VI, as well as denatured collagens of all types tested, did not support outgrowth. The monoclonal anti-beta 1 integrin antibody (CSAT), as well as rabbit polyclonal antibodies directed to the integrin beta 1-chain, effectively inhibited neurite outgrowth on permissive collagenous substrata, indicating that collagen-binding integrins were involved in the neuritogenesis. These beta 1-integrins were independent of Arg-Gly-Asp (RGD) since neurite formation proceeded in the presence of synthetic RGD-containing peptides. Fluorescence immunohistochemistry revealed the presence of the integrin beta 1-chain on the outgrowing neurites. The results suggest a possible function of collagen and collagen-binding integrins in the development of the visual system.

Molecular analysis of the ependymin gene and functional test of its promoter region by transient expression in Brachydanio rerio

Ependymins are secretory products of meningeal cells and represent the predominant glycoproteins in the cerebrospinal fluid from various orders of teleost fish. In the zebrafish, their expression starts between 48 and 72 h post-fertilization. Generally, they share characteristics with proteins involved in cell-contact phenomena. Here, we characterize the ependymin gene from Brachydanio rerio and its flanking regions. The sequence was obtained from clones generated using the polymerase chain reaction (PCR), including a variation of an "anchored" PCR. Also, clones from a conventional phage library were analyzed. We found that the transcribed portion is arranged in six exons. Transient expression of an ependymin-promoter-lacZ gene fusion in zebrafish embryos revealed that the 2.0-kb upstream regulatory region used is sufficient to direct the ependymin-specific correct temporal and spatial expression pattern of the lacZ reporter gene.

Neuronal interactions with the extracellular matrix

The interactions of neurons with extracellular cues are important in directing the formation of precise neuronal networks during the development of the nervous system. This review will focus on recent progress towards the understanding of the molecular machinery involved in the interactions of neurons with the extracellular matrix.

Molecular mechanisms separating two axonal pathways during embryonic development of the avian optic tectum

During embryonic development of the avian optic tectum, retinal and tectobulbar axons form an orthogonal array of nerve processes. Growing axons of both tracts are transiently very closely apposed to each other. Despite this spatial proximity, axons from the two pathways do not intermix, but instead restrict their growth to defined areas, thus forming two separate plexiform layers, the stratum opticum and the stratum album centrale. In this study we present experimental evidence indicating that the following three mechanisms might play a role in segregating both axonal populations: Retinal and tectobulbar axons differ in their ability to use the extracellular matrix protein laminin as a substrate for axonal elongation; the environment in the optic tectum is generally permissive for retinal axons, but is specifically nonpermissive for tectobulbar axons, resulting in a strong fasciculation of the latter; and growth cones of temporal retinal axons are reversibly inhibited in their motility by direct contact with the tectobulbar axon's membrane.