Inhibitors and promoters of thalamic neuron adhesion and outgrowth in embryonic neocortex: functional association with chondroitin sulfate

Versican is upregulated in CNS injury and is a product of oligodendrocyte lineage cells

Chondroitin sulfate proteoglycan (CS-PG) expression is increased in response to CNS injury and limits the capacity for axonal regeneration. Previously we have shown that neurocan is one of the CS-PGs that is upregulated (Asher et al., 2000). Here we show that another member of the aggrecan family, versican, is also upregulated in response to CNS injury. Labeling of frozen sections 7 d after a unilateral knife lesion to the cerebral cortex revealed a clear increase in versican immunoreactivity around the lesion. Western blot analysis of extracts prepared from injured and uninjured tissue also revealed considerably more versican in the injured tissue extract. In vitro studies revealed versican to be a product of oligodendrocyte lineage cells (OLCs). Labeling was seen between the late A2B5-positive stage and the O1-positive pre-oligodendrocyte stage. Neither immature, bipolar A2B5-positive cells, nor differentiated, myelin-forming oligodendrocytes were labeled. The amount of versican in conditioned medium increased as these cells differentiated. Versican and tenascin-R colocalized in OLCs, and coimmunoprecipitation indicated that the two exist as a complex in oligodendrocyte-conditioned medium. Treatment of pre-oligodendrocytes with hyaluronidase led to the release of versican, indicating that its retention at the cell surface is dependent on hyaluronate (HA). In rat brain, approximately half of the versican is bound to hyaluronate. We also provide evidence of a role for CS-PGs in the axon growth-inhibitory properties of oligodendrocytes. Because large numbers of OLCs are recruited to CNS lesions, these results suggest that OLC-derived versican contributes to the inhospitable environment of the injured CNS.

Repellent guidance of regenerating optic axons by chondroitin sulfate glycosaminoglycans in zebrafish

We analyzed the role of chondroitin sulfate (CS) glycosaminoglycans, putative inhibitors of axonal regeneration in mammals, in the regenerating visual pathway of adult zebrafish. In the adult, CS immunoreactivity was not detectable before or after an optic nerve crush in the optic nerve and tract but was constitutively present in developing and adult nonretinorecipient pretectal brain nuclei, where CSs may form a boundary preventing regenerating optic fibers from growing into these inappropriate locations. Enzymatic removal of CSs by chondroitinase ABC after optic nerve crush significantly increased the number of animals showing erroneous growth of optic axons into the nonretinorecipient magnocellular superficial/posterior pretectal nucleus (83% vs 42% in controls). In vitro, a substrate border of CSs, but not heparan sulfates, strongly repelled regenerating retinal axons from adult zebrafish. We conclude that CSs contribute to repellent axon guidance during regeneration of the optic projection in zebrafish.

Roles of the telencephalic cells and their chondroitin sulfate proteoglycans in delimiting an anterior border of the retinal pathway

The axons of the retinal ganglion cells run on the diencephalotelencephalic boundary on their way to the tectum; however, they do not invade the telencephalon anteriorly. To investigate the mechanisms that prevent the retinal axons from entering the telencephalic territory, the effects of the telencephalic cells were examined on the outgrowth of the retinal axons in vitro; the retinal outgrowth was selectively inhibited by the cellular substrate derived from the telencephalon. The responsible factor for the selective inhibition was, furthermore, found in the telencephalic membranes and the fraction of peripheral membrane molecules from the telencephalon. Because the inhibitory effect was destroyed by chondroitinase ABC but not by heat, this inhibition was attributable to the carbohydrate chains of chondroitin sulfate proteoglycans (CSPGs) adhering to the membranes of the telencephalic cells. To understand the function of the telencephalic CSPGs on the retinal pathfinding in vivo, their carbohydrate chains [chondroitin sulfate glycosaminoglycan (CS-GAG)] were removed from the embryonic brains by intraventricular injection of chondroitinase ABC; the removal of CS-GAG resulted in an anterior enlargement of the optic tract. The results indicate that the telencephalic cells delimit the anterior border of the optic tract with their CSPGs and prevent the retinal axons from aberrantly entering the anterior territory.

Hyaluronan-associated adhesive cues control fiber segregation in the hippocampus

In various brain regions, particularly in the hippocampus, afferent fiber projections terminate in specific layers. Little is known about the molecular cues governing this laminar specificity. To this end we have recently shown that the innervation pattern of entorhinal fibers to the hippocampus is mimicked by the lamina-specific adhesion of entorhinal cells on living hippocampal slices, suggesting a role of adhesion molecules in the positioning of entorhinal fibers. Here, we have analyzed the role of extracellular matrix components in mediating this lamina-specific adhesion. We show that hyaluronidase treatment of hippocampal slices abolishes lamina-specific adhesion as well as layer-specific growth of entorhinal fibers to the dentate outer molecular layer in organotypic slice cultures. We conclude that hyaluronan-associated molecules play a crucial role in the formation of the lamina-specific entorhinal projection to the hippocampus.

Facilitation of learning and long-term ventral pallidal-cortical cholinergic activation by proteoglycan biglycan and chondroitin sulfate C

We have shown previously in the rat that biglycan, a recently discovered chondroitin sulfate proteoglycan, has neurotrophic effects which are mediated by its chondroitin/dermatan sulfate chains. Here we report that biglycan has neurochemical effects when injected into the nucleus basalis magnocellularis of the ventral pallidum, a site of dense cholinergic cell bodies. The effects on the cholinergic output in the frontal cortex are long lasting, indicating profound neuroactive function akin to that expected of a long-acting hormone. Injected into the same area of the brain, as well as into the ventricles in behaviorally impaired old animals, we found that biglycan can improve learning and memory in several behavioral paradigms. Furthermore, we show that both the neurochemical effectiveness as well as the promotion of learning is carried not by the proteoglycan per se, but rather by its chondroitin sulfate moiety, thus, demonstrating for the first time memory-promoting and neuroactive effects of a glycosaminoglycan, namely, chondroitin sulfate C. The results suggest that biglycan and other extracellular matrix molecules can have neurobehavioral and pharmacological functions for beyond those traditionally attributed to this class of molecules.

Inhibitory mechanism by polysialic acid for lamina-specific branch formation of thalamocortical axons

During development, thalamocortical axons form arbors primarily in layer 4 of the neocortex. This lamina-specific branch formation was studied in cultures of rat thalamic explants grown next to chemically fixed cortical slices. After a week in vitro, thalamic axons formed branches specifically in the target layer of fixed cortical slices, regardless of the orientation of the ingrowth. This in vitro system permits a direct assessment of contributions of membrane-associated molecules to thalamic axon branch formation. To this end, the present study uses three enzymatic perturbations: chondroitinase, phosphatidylinositol phospholipase C, or the polysialic acid (PSA)-specific endoneuraminidase (endo N). With endo N pretreatment of cortex, the number of branch points was increased significantly, whereas branch tip length was decreased. In addition, the localization of branch points to the target layer was weakened considerably. These features of branch formation were not altered by the other two enzymatic treatments, except that branch tips were shortened by chondroitinase treatment to the same extent as in endo N treatment. These results suggest that membrane-bound components are involved in lamina-specific branch formation of thalamocortical axons, and in particular that PSA moieties contribute to laminar specificity by inhibiting branch emergence in inappropriate layers.

Glypican-4 is an FGF2-binding heparan sulfate proteoglycan expressed in neural precursor cells

FGF2 is a crucial mitogen for neural precursor cells in the developing cerebral cortex. Heparan sulfate proteoglycans (HSPGs) are thought to play a role in cortical neurogenesis by regulating the action of FGF2 on neural precursor cells. In this article, we present data indicating that glypican-4 (K-glypican), a GPI-anchored cell surface HSPG, is involved in these processes. In the developing mouse brain, glypican-4 mRNA is expressed predominantly in the ventricular zone of the telencephalon. Neither the outer layers of the telencephalic wall nor the ventricular zone of other parts of the developing brain express significant levels of glypican-4, with the exception of the ventricular zone of the tectum. In cultures of E13 rat cortical precursor cells, glypican-4 is expressed in cells immunoreactive for nestin and the D1.1 antigen, markers of neural precursor cells. Glypican-4 expression was not detected in early postmitotic or fully differentiated neurons. Recombinant glypican-4 produced in immortalized neural precursor cells binds FGF2 through its heparan sulfate chains and suppressed the mitogenic effect of FGF2 on E13 cortical precursor cells. The spatiotemporal expression pattern of glypican-4 in the developing cerebral wall significantly overlaps with that of FGF2. These results suggest that glypican-4 plays a critical role in the regulation of FGF2 action during cortical neurogenesis.

Modulation of extracellular matrix adhesiveness by neurocan and identification of its molecular basis

Neurocan is one of the major chondroitin sulfate proteoglycans of perinatal rodent brain. HEK-293 cells producing neurocan recombinantly show changes in their behavior. The expression of full-length neurocan led to a detachment of the secreting cells and the formation of floating spheroids. This occurred in the continuous presence of 10% fetal bovine serum in the culture medium. Cells secreting fragments of neurocan-containing chondroitin sulfate chains and the C-terminal domain of the molecule showed a similar behavior, whereas cells expressing fragments of neurocan-containing chondroitin sulfate chains but lacking parts of the C-terminal domain did not show spheroid formation. Cells secreting the hyaluronan-binding N-terminal domain of neurocan showed an enhanced adhesiveness. When untransfected HEK-293 cells were plated on a surface conditioned by spheroid-forming cells, they also formed spheroids. This effect could be abolished by chondroitinase treatment of the conditioned surface. The observations indicate that the ability of the chondroitin sulfate proteoglycan neurocan to modulate the adhesive character of extracellular matrices is dependent on the structural integrity of the C-terminal domain of the core protein.

The polarity and magnitude of ambient charge influences three-dimensional neurite extension from DRGs

Sulfated proteoglycans have inhibitory effects on neurite extension, and the negative charge of the glycosaminoglycan side chains may be involved in the inhibitory process. The main goal of this study is to investigate the effects of charge on three-dimensional neurite extension. Various concentrations of dermatan sulfate (DS), a chondroitin sulfate glycosaminoglycan, and consequently, various degrees of negative charge were presented on three-dimensional agarose hydrogels and the effect of charge on neurite extension from primary neurons was investigated. Dose-response experiments were also performed with the polycationic (positively charged) polysaccharide chitosan covalently coupled to agarose. The amount of DS or chitosan coupled to the agarose gel was quantified via metachromatic dye or Fourier transform infrared spectroscopy methods, respectively. The length of embryonic day 9 (E9) chick dorsal root ganglia neurites extended through charged agarose gels is dependent on the polarity and quantity of ambient charge. The inhibitory effects of the sulfated DS and the enhancing effects of the polycationic chitosan on neurite extension decrease as the amount of DS or chitosan coupled to agarose is decreased. These findings indicate that primary neural process extension is influenced by the polarity of ambient charge in a dose-responsive manner.

MMP-2 and MMP-9 increase the neurite-promoting potential of schwann cell basal laminae and are upregulated in degenerated nerve

Compared to degenerated nerves, the ability of normal adult peripheral nerve to support axonal regeneration is poor and may be attributed to the inhibition of endoneurial laminin by chondroitin sulfate proteoglycan (CSPG). In cryoculture assays, neuritic growth of neonatal and adult peripheral neurons was increased on sections of normal nerve by pretreatment with CSPG-degrading enzymes, including the matrix metalloproteinases MMP-2 and MMP-9. Axonal regeneration is known to occur within the Schwann cell basal laminae of degenerated nerve. Similarly, deconvolution microscopy revealed that neuritic growth on nerve tissue sections occurred principally on the lumenal surface of enzymatically modified basal laminae. Compared to normal nerve, there was a marked increase in the neurite-promoting activity of the degenerated nerve, and this activity was not increased significantly by subsequent MMP treatment. Additionally, the expression and activation of MMP-2 and MMP-9 were elevated in degenerated nerve, suggesting that degradation of inhibitory CSPG by the MMPs contributes to the growth-promoting properties of degenerated nerve.

Dynamic spatiotemporal expression patterns of neurocan and phosphacan indicate diverse roles in the developing and adult mouse olfactory system

The chondroitin sulfate proteoglycans neurocan and phosphacan are believed to modulate neurite outgrowth by binding to cell adhesion molecules, tenascin, and the differentiation factors heparin-binding growth-associated molecule and amphoterin. To assess the role of these chondroitin sulfate proteoglycans in the olfactory system, we describe here their expression patterns during both embryonic and postnatal development in the mouse. Immunoreactivity for neurocan was first detected in primary olfactory neurons at embryonic day 11. 5 (E11.5). Neurocan was expressed by primary olfactory axons as they extended toward the rostral pole of the telencephalon as well as by their arbors in glomeruli after they contacted the olfactory bulb. The role of neurocan was examined by growing olfactory neurons on an extracellular matrix substrate containing neurocan or on extracellular matrix in the presence of soluble neurocan. In both cases, neurocan strongly promoted neurite outgrowth. These results suggest that neurocan supports the growth of primary olfactory axons through the extracellular matrix as they project to the olfactory bulb during development. Phosphacan, unlike neurocan, was present within the mesenchyme surrounding the E11.5 and E12.5 nasal cavity. This expression decreased at E13.5, concomitant with a transient appearance of phosphacan in nerve fascicles. Within the embryonic olfactory bulb, phosphacan was localised to the external and internal plexiform layers. However, during early postnatal development phosphacan was concentrated in the glomerular layer. These results suggest that phosphacan may play a role in delineating the pathway of growing olfactory axons as well as defining the laminar organization of the bulb. Together, the spatiotemporal expression patterns of neurocan and phosphacan indicate that these chondroitin sulfate proteoglycans have diverse in situ roles, which are dependent on context-specific interactions with extracellular and cell adhesion molecules within the developing olfactory nerve pathway.

Axon routing at the optic chiasm after enzymatic removal of chondroitin sulfate in mouse embryos

The effects of removing chondroitin sulfate from chondroitin sulfate proteoglycan molecules on guidance of retinal ganglion cell axons at the optic chiasm were investigated in a brain slice preparation of mouse embryos of embryonic day 13 to 15. Slices were grown for 5 hours and growth of dye-labeled axons was traced through the chiasm. After continuous enzymatic digestion of the chondroitin sulfate proteoglycans with chondroitinase ABC, which removes the glycosaminoglycan chains, navigation of retinal axons was disrupted. At embryonic day 13, before the uncrossed projection forms in normal development, many axons deviated from their normal course, crossing the midline at aberrant positions and invading the ventral diencephalon. In slices from embryonic day 14 embryos, axons that would normally form the uncrossed projection at this stage failed to turn into the ipsilateral optic tract. In embryonic day 15 slices, enzyme treatment caused a reduction of the uncrossed projection that develops at this stage. Growth cones in enzyme-treated slices showed a significant increase in the size both before and after they crossed the midline. This indicates that responses of retinal axons to guidance signals at the chiasm have changed after removal of the chondroitin sulfate epitope. We concluded that the chondroitin sulfate moieties of the proteoglycans are involved in patterning the early phase of axonal growth across the midline and at a later stage controlling the axon divergence at the chiasm.

Nervous system proteoglycans as modulators of neurite outgrowth

The proteoglycans are multifunctional macromolecules composed of a core polypeptide and a variable number of glycosaminoglycan chains. The structural diversity and complexities of proteoglycan expression in the developing and adult Nervous System underlies the variety of biological functions that these molecules fulfill. Thus, in the Nervous System, proteoglycans regulate the structural organisation of the extracellular matrix, modulate growth factor activities and cellular adhesive and motility events, such as cell migration and axon outgrowth. This review summarises the evidences indicating that proteoglycans have an important role as modulators of neurite outgrowth and neuronal polarity. Special emphasis will be placed on those studies that have shown that proteoglycans of certain subtypes inhibit neurite extension either during the development and/or the regeneration of the vertebrate Central Nervous System.

Effects of lipid-derivatized glycosaminoglycans (GAGs), a novel probe for functional analyses of GAGs, on cell-to-substratum adhesion and neurite elongation in primary cultures of fetal rat hippocampal neurons

The effects of glycosaminoglycans (GAG) on cell-to-substratum adhesion and neurite elongation were examined in primary cultures of fetal rat hippocampal neurons using tissue culture dishes coated with GAGs coupled to dipalmitoylphosphatidylethanolamine (PE), a novel probe for biological functions of GAGs. Both chondroitin sulfate conjugate to PE (CS-PE) and hyaluronic acid conjugate to PE (HA-PE) promoted neurite elongation from neurons in a dose-dependent manner when immobilized onto polylysine-coated dishes at various concentrations up to 1.0 microg/ml. The coating of CS-PE or HA-PE at a concentration higher than 1.0 microg/ml resulted in failure of neurite extension and adhesion of neurons to the substrata. In contrast, heparin conjugate to PE (HP-PE) did not exert any effects on neurite elongation or on cell attachment at these concentrations. These findings suggest that GAGs serve as a modulator for neurite elongation during neuronal network formation in the developing central nervous system.

Neurocan is upregulated in injured brain and in cytokine-treated astrocytes

Injury to the CNS results in the formation of the glial scar, a primarily astrocytic structure that represents an obstacle to regrowing axons. Chondroitin sulfate proteoglycans (CSPG) are greatly upregulated in the glial scar, and a large body of evidence suggests that these molecules are inhibitory to axon regeneration. We show that the CSPG neurocan, which is expressed in the CNS, exerts a repulsive effect on growing cerebellar axons. Expression of neurocan was examined in the normal and damaged CNS. Frozen sections labeled with anti-neurocan monoclonal antibodies 7 d after a unilateral knife lesion to the cerebral cortex revealed an upregulation of neurocan around the lesion. Western blot analysis of extracts prepared from injured and uninjured tissue also revealed substantially more neurocan in the injured CNS. Western blot analysis revealed neurocan and the processed forms neurocan-C and neurocan-130 to be present in the conditioned medium of highly purified rat astrocytes. The amount detected was increased by transforming growth factor beta and to a greater extent by epidermal growth factor and was decreased by platelet-derived growth factor and, to a lesser extent, by interferon gamma. O-2A lineage cells were also capable of synthesizing and processing neurocan. Immunocytochemistry revealed neurocan to be deposited on the substrate around and under astrocytes but not on the cells. Astrocytes therefore lack the means to retain neurocan at the cell surface. These findings raise the possibility that neurocan interferes with axonal regeneration after CNS injury.

Chondroitin sulfates affect the formation of the segmental motor nerves in zebrafish embryos

Chondroitin sulfates have been implicated in the promotion and in the inhibition of axon growth. In the zebrafish embryo, chondroitin sulfates are present at the interface of the somites and the notochord where spinal motor axons extend ventrally to establish the midsegmental ventral motor nerves. Injection of chondroitinase ABC prior to motor axon outgrowth effectively removed all chondroitin sulfate immunoreactivity and induced abnormal axonal outgrowth in many (39%) of the ventral motor nerves. The most common abnormality was the formation of side branches, approximately half of which extended posteriorly, the others anteriorly. The effect was specific to the removal of chondroitin sulfates, since injections of vehicle solution or of heparinase III did not affect the ventral motor nerves. Electron microscopic examination demonstrated that the injections caused no damage to spinal cord, somite, and notochord. This suggests that chondroitin sulfates normally constrain the outgrowth of the ventral motor nerves. Consistent with this hypothesis, injections of soluble chondroitin sulfates, either as a mixture or individually, led to truncated or missing ventral motor nerves. Truncations were most frequent after injection of chondroitin sulfate-B (up to 23%) while chondroitin sulfate-A had a lesser, and chondroitin sulfate-C no apparent, effect.

Contribution of heparan sulfate to the non-permissive role of the midline glia to the growth of midbrain neurites

Radial glial cells and astrocytes are heterogeneous with respect to morphology, cytoskeletal- and membrane-associated molecules and intercellular interactions. Astrocytes derived from lateral (L) and medial (M) midbrain sectors differ in their abilities to support neuritic growth of midbrain neurons in coculture (Garcia-Abreu et al. J Neurosci Res 40:471, 1995). There is a correlation between these abilities and the differential patterns of laminin (LN) organization that is fibrillar in growth-permissive L astrocytes and punctate in the non-permissive M astroglia (Garcia-Abreu et al. NeuroReport 6:761, 1995). There are also differences in the production of glycosaminoglycans (GAGs) by L and M midbrain astrocytes (Garcia-Abreu et al. Glia 17:339, 1996). We show that the relative amounts of the glycoproteins laminin LN, fibronectin (FN) and tenascin (TN) are virtually identical in L and M glia, thus, confirming that an abundant content of LN is not sufficient to promote neurite growth. To further analyze the role of GAGs in the properties of M and L glia, we employed enzymatic degradation of the GAGs chondroitin sulfate (CS) and heparan sulfate (HS). Treatment with chondroitinase has little effect on the non-permissive properties of M glia but reduces the growth-supporting ability of L glia. By contrast, heparitinase I produces no significant changes on L glia but leads to neurite growth promotion by M glia. Taken together, these results suggest that glial CS helps to promote neurite growth and, more importantly, they indicate that a HS proteoglycan is, at least, partially responsible for the non-permissive role of the midline glia to the growth of midbrain neurites.

The chondroitin sulfate proteoglycans neurocan and phosphacan are expressed by reactive astrocytes in the chronic CNS glial scar

Chondroitin sulfate proteoglycans (CS-PGs) expressed by reactive astrocytes may contribute to the axon growth-inhibitory environment of the injured CNS. The specific potentially inhibitory CS-PGs present in areas of reactive gliosis, however, have yet to be thoroughly examined. In this study, we used immunohistochemistry, combined immunohistochemistry-in situ hybridization, immunoblot analysis, and reverse transcription-PCR to examine the expression of specific CS-PGs by reactive astrocytes in an in vivo model of reactive gliosis: that is, the glial scar, after cortical injury. Neurocan and phosphacan can be localized to reactive astrocytes 30 d after CNS injury, whereas brevican and versican are not expressed in the chronic glial scar. Neurocan is also expressed by astrocytes in primary cell culture. Relative to the amount present in cultured astrocytes or uninjured cortex, neurocan expression increases significantly in the glial scar resulting from cortical injury, including the re-expression of the neonatal isoform of neurocan. In contrast, phosphacan protein levels are decreased in the glial scar compared with the uninjured brain. Because these CS-PGs are capable of inhibiting neurite outgrowth in vitro, our data suggest that phosphacan and neurocan in areas of reactive gliosis may contribute to axonal regenerative failure after CNS injury.

Characterization of factors regulating lamina-specific growth of thalamocortical axons

During development, most thalamocortical axons extend through the deep layers to terminate in layer 4 of neocortex. To elucidate the molecular mechanisms that underlie the formation of layer-specific thalamocortical projections, axon outgrowth from embryonic rat thalamus onto postnatal neocortical slices which had been fixed chemically was used as an experimental model system. When the thalamic explant was juxtaposed to the lateral edge of fixed cortical slice, thalamic axons extended farther in the deep layers than the upper layers. Correspondingly, thalamic axons entering from the ventricular side extended farther than those from the pial side. In contrast, axons from cortical explants cultured next to fixed cortical slices tended to grow nearly as well in the upper as in the deep layers. Biochemical aspects of lamina-specific thalamic axon growth were studied by applying several enzymatic treatments to the cortical slices prior to culturing. Phosphatidylinositol phospholipase C treatment increased elongation of thalamic axons in the upper layers without influencing growth in the deep layers. Neither chondroitinase, heparitinase, nor neuraminidase treatment influenced the overall projection pattern, although neuraminidase slightly decreased axonal elongation in the deep layers. These findings suggest that glycosylphosphatidylinositol-linked molecules in the cortex may contribute to the laminar specificity of thalamocortical projections by suppressing thalamic axon growth in the upper cortical layers.

Embryonic neurons adapt to the inhibitory proteoglycan aggrecan by increasing integrin expression

The primary mediators of cell migration during development, wound healing and metastasis, are receptors of the integrin family. In the developing and regenerating nervous system, chondroitin sulfate proteoglycans (CSPGs) inhibit the integrin-dependent migration of neuronal growth cones. Here we report that embryonic sensory neurons cultured on the growth-promoting molecule laminin in combination with the inhibitory CSPG aggrecan rapidly adapt to inhibition. Adaptation is associated with a two- to threefold increase in the levels of RNA and surface protein for two laminin receptors, integrin alpha6beta1 and alpha3beta1, indicating that integrin expression is regulated by aggrecan. Increased integrin expression is associated both with increases in neuronal cell adhesion/outgrowth and with decreases in the ability of aggrecan to inhibit cell adhesion. Directly increasing integrin expression by adenoviral infection is sufficient to eliminate the inhibitory effects of aggrecan, indicating that upregulation of integrin receptors may promote neuronal regeneration in the presence of inhibitory matrix components.

Chondroitin sulphate-binding molecules may pattern central projections of sensory axons within the cranial mesenchyme of the developing mouse

During mammalian hindbrain development, sensory axons grow along highly stereotyped routes within the cranial mesenchyme to reach their appropriate entry points into the neuroepithelium. Thus, trigeminal ganglion axons always project to rhombomere (r)2, whilst facial/acoustic ganglia axons always project to r4. Axons are never observed to enter the mesenchyme adjacent to r3, raising the possibility that r3 mesenchyme contains an axon growth-inhibitory activity. Conversely, in mice which lack the erbB4 receptor (normally expressed in r3), trigeminal and facial/acoustic ganglia axons misproject into r3 mesenchyme, suggesting that the putative axon barrier is absent. To investigate this hypothesis, we have developed an in vitro model in which dissociated wild-type embryonic trigeminal ganglion neurons are cultured on longitudinal cryosections of embryonic mouse head. We observed that on wild-type embryonic day 10 (E10) cryosections, neurites generally failed to grow into r3 mesenchyme from the adjacent r2 or r4 mesenchyme. This barrier was removed if cryosections were pretreated with chondroitinase or were washed with excess chondroitin 6-sulphate or hypertonic saline. By contrast, when trigeminal neurons were seeded onto cryosections of E10 erbB4 -/- embryo heads their neurites readily entered mutant r3 mesenchyme. Immunohistochemical analysis demonstrated chondroitin-sulphated proteoglycans throughout the cranial mesenchyme in both wild-type and erbB4 -/- embryos. We propose that trigeminal axons are excluded from wild-type r3 mesenchyme by a growth-inhibitory activity which associates with chondroitin-sulphated proteoglycans and that the synthesis of this activity may rely on signals transduced by erbB receptors.

Anatomical and functional reconstruction of the nigrostriatal system in vitro: selective innervation of the striatum by dopaminergic neurons

To study development of the nigrostriatal pathway in an in vitro model system, organotypic slices obtained from rat pups (P4) and containing the striatum and the cortex were grown together with apposed embryonic (E13.5) mesencephalic blocks according to the static slice culture method of Stoppini et al. (1991; J. Neurosci. Methods 37:173-182). Under these conditions, mesencephalic dopaminergic (DA) fibers rapidly grow through the slice, preferentially its striatal portion. This innervation provides a true synaptic innervation to the striatum, as shown by the presence of DA terminals on striatal neurons. DA fibers are able to exert a functional influence, as seen by their ability to modulate c-Fos expression in striatal neurons in the same way as in vivo. Thus, blockade, under basal conditions, of the effect of spontaneously released dopamine by the D2 receptor antagonist haloperidol leads to the activation of c-Fos expression in the striatum. Furthermore, stimulation of DA release by amphetamine induces striatal c-Fos expression in a D1 receptor-dependent manner. Next, the mechanisms of the selective striatal innervation were examined. Indeed, DA fibers innervated specifically the striatum, avoiding the cortical portion of the slice. This selectivity seems to be specific for DA neurons; no selectivity could be observed when noradrenergic neurons were substituted for DA neurons. Short-term cocultures in a collagen gel of mesencephalic blocks with striatal blocks failed to reveal any oriented outgrowth of DA fibers from the mesencephalon, suggesting that the selective innervation observed in the organotypic slices results from some contact-dependent, presumably adhesive interactions rather than from the presence of some diffusible substance orienting the growth of DA fibers towards the striatum. On the other hand, DA neurons seeded onto striatal slices did not attach selectively onto the striatal portion of the slice, indicating that the putative specific adhesive interactions governing the selective striatal innervation are not the same as those determining the adhesion of the DA neurons. These results show that cocultures of cortex-striatum and mesencephalic slices result in a system that displays a number of the morphological and functional traits of the normal nigrostriatal system and that can be relied on as a good in vitro model of in vivo development.

Neurite outgrowth inhibition by chondroitin sulfate proteoglycan: stalling/stopping exceeds turning in human neuroblastoma growth cones

Chondroitin sulfate proteoglycan (CSPG) inhibits outgrowth from embryonic chick and rodent neurons in vivo and in vitro and is upregulated during development and following injury. The role of CSPG in outgrowth from human neurons has been largely untested, but is critical for our understanding of regeneration in humans following nervous system injury. Here we determined the effects of CSPG on platelet-derived growth factor (PDGF)-stimulated neurite outgrowth from SH-SY5Y human neuroblastoma cells, a well-accepted model of neuronal differentiation. Cells were plated on glass coverslips adsorbed with laminin (LN), CSPG, or a patterned substratum consisting of alternating stripes of the two molecules. Similar to other studies using chick or rodent neurons, SH-SY5Y cells extend neurites on LN, displaying a 15.2% increase in the total neurite length/cell as compared to cells plated on glass. Cells plated on CSPG alone exhibited reduced neurite outgrowth compared to cells plated on glass or LN. Interestingly, SH-SY5Y growth cones extending on LN and then encountering a CSPG border display more stopping/stalling (62.3%) than turning (27.9%) behaviors. Soluble CSPG inhibits neurite initiation from SH-SY5Y cells plated on glass, but not on LN. These data demonstrate that several CSPG-elicited responses of human neuron-like cells are similar to those from nonhuman neurons. However, approximately 70% of SH-SY5Y growth cones stop or stall at a CSPG border while over 80% of chick sensory neurons turn at a CSPG border. The experimental difference between these models may well indicate a functional difference between animal and human neuronal regeneration.

Chondroitin sulfate E promotes neurite outgrowth of rat embryonic day 18 hippocampal neurons

In light of controversial reports concerning the effects of chondroitin sulfates on neurite outgrowth, several glycosaminoglycans belonging to this structural class were compared with regard to their influence on axon formation by embryonic day 18 hippocampal neurons. In these studies, chondroitin sulfate A (CS-A), CS-B and CS-C proved weak or inefficient in the neurite outgrowth promotion assay. As expected, CS-D stimulated both the fraction of neurite bearing neurons and the length of their processes. This effect could be neutralized by the monoclonal antibody (mAb) 473HD. In contrast, CS-E enacted a dramatic promotion of neurite outgrowth. This effect persisted in the presence of mAb 473HD, consistent with the observation that this antibody did not react with CS-E in glycosaminoglycan transfer and blotting techniques. We conclude that CSE contains a novel glycosaminoglycan based neurite outgrowth promoting motif, which is distinct from other known activities.

DSD-1-proteoglycan is the mouse homolog of phosphacan and displays opposing effects on neurite outgrowth dependent on neuronal lineage

DSD-1-PG is a chondroitin sulfate proteoglycan (CSPG) expressed by glial cells that can promote neurite outgrowth from rat embryonic mesencephalic (E14) and hippocampal (E18) neurons, an activity that is associated with the CS glycosaminoglycans (GAGs). Further characterization of DSD-1-PG has included sequencing of peptides from the core protein and the cloning of the corresponding cDNA using polyclonal antisera against DSD-1-PG to screen phage expression libraries. On the basis of these studies we have identified DSD-1-PG as the mouse homolog of phosphacan, a neural rat CSPG. Monoclonal antibodies 3H1 and 3F8 against carbohydrate residues on rat phosphacan recognize these epitopes on DSD-1-PG. The epitopes of the antibodies, L2/HNK-1 and L5/Lewis-X, which have been implicated in functional interactions, are also found on DSD-1-PG. Although DSD-1-PG has previously been shown to promote neurite outgrowth, its upregulation after stab wounding of the CNS and its localization in regions that are considered boundaries to axonal extension suggested that it may also have inhibitory functions. Neonatal dorsal root ganglion (DRG) explants grown on a rich supportive substrate (laminin) with and without DSD-1-PG were strikingly inhibited by the proteoglycan. The inhibitory effects of DSD-1-PG on the DRG explants were not relieved by removal of the CS GAGs, indicating that this activity is associated with the core glycoprotein. The neurite outgrowth from embryonic hippocampal neurons on laminin was not affected by the addition of DSD-1-PG. This indicates that DSD-1-PG/mouse phosphacan can have opposing effects on the process of neurite outgrowth dependent on neuronal lineage.

Defects in thalamocortical axon pathfinding correlate with altered cell domains in Mash-1-deficient mice

We have analyzed the pathfinding of thalamocortical axons (TCAs) from dorsal thalamus to neocortex in relation to specific cell domains in the forebrain of wild-type and Mash-1-deficient mice. In wild-type mice, we identified four cell domains that constitute the proximal part of the TCA pathway. These domains are distinguished by patterns of gene expression and by the presence of neurons retrogradely labeled from dorsal thalamus. Since the cells that form these domains are generated in forebrain proliferative zones that express high levels of Mash-1, we studied Mash-1 mutant mice to assess the potential roles of these domains in TCA pathfinding. In null mutants, each of the domains is altered: the two Pax-6 domains, one in ventral thalamus and one in hypothalamus, are expanded in size; a complementary RPTP(delta) domain in ventral thalamus is correspondingly reduced and the normally graded expression of RPTP(delta) in that domain is no longer apparent. In ventral telencephalon, a domain characterized in the wild type by Netrin-1 and Nkx-2.1 expression and by retrogradely labeled neurons is absent in the mutant. Defects in TCA pathfinding are localized to the borders of each of these altered domains. Many TCAs fail to enter the expanded, ventral thalamic Pax-6 domain that constitutes the most proximal part of the TCA pathway, and form a dense whorl at the border between dorsal and ventral thalamus. A proportion of TCAs do extend further distally into ventral thalamus, but many of these stall at an aberrant, abrupt border of high RPTP(delta) expression. A small proportion of TCAs extend around the RPTP(delta) domain and reach the ventral thalamic-hypothalamic border, but few of these axons turn at that border to extend into the ventral telencephalon. These findings demonstrate that Mash-1 is required for the normal development of cell domains that in turn are required for normal TCA pathfinding. In addition, these findings support the hypothesis that ventral telencephalic neurons and their axons guide TCAs through ventral thalamus and into ventral telencephalon.

Extracellular matrix molecules play diverse roles in the growth and guidance of central nervous system axons

Axon growth and guidance represent complex biological processes in which probably intervene diverse sets of molecular cues that allow for the appropriate wiring of the central nervous system (CNS). The extracellular matrix (ECM) represents a major contributor of molecular signals either diffusible or membrane-bound that may regulate different stages of neural development. Some of the brain ECM molecules form tridimensional structures (tunnels and boundaries) that appear during time- and space-regulated events, possibly playing relevant roles in the control of axon elongation and pathfinding. This short review focuses mainly on the recognized roles played by proteoglycans, laminin, fibronectin and tenascin in axonal development during ontogenesis.

Development of signals influencing the growth and termination of thalamocortical axons in organotypic culture

Explants of embryonic or postnatal rat cortex, organotypically cultured in serum-free medium, maintain their structural integrity and their upper layers continue to mature. Coculture of portions of embryonic thalamus with cortical slices taken at different ages reveals a temporal cascade of cortical signals. (1) Slices of occipital cortex taken at E19 or earlier stimulate axonal outgrowth from explants of embryonic lateral geniculate nucleus but do not allow the fibers to invade. (2) In cortical slices taken after E19 but before P2, thalamic axons enter the slice, from any direction, and extend radially across the entire depth of the cortical plate without branching or terminating. (3) In slices taken after P2, fibers slow down, arborize, and terminate in the maturing layer 4 of the cortex. If the thalamic explant is placed against the pial surface of the cortical slice, axons still enter and branch in the same layer. These findings imply that the developing cortex expresses a diffusible growth-promoting factor and then itself becomes growth permissive, and finally the maturing layer 4 expresses a "stop signal." In triple cocultures of one thalamic explant with a "choice" of two neighboring slices, thalamic axons will not invade slices of cerebellum but behave indistinguishably in response to slices from any region of the hemisphere. Thus the initial tangential distribution of the thalamic projection in vivo (which is achieved by about E16) is unlikely to be controlled by regional variation in signals produced by the cortex. When cortical slices were precultured alone for 7-14 days before the addition of an explant of embryonic thalamus for 4 further days of coculture, the pattern of innervation was more appropriate to the chronological age of the slice than the age at which it was first taken. Thus the timing of the cascade of cortical properties is at least partly intrinsically determined. This sequence of expression of these signals suggests that they play a part in vivo in controlling the outgrowth of thalamic fibers, their accumulation under the cortical plate, their invasion of the plate, and their arborization in layer 4.

Dissection of astrocyte-mediated cues in neuronal guidance and process extension

Neurites are believed to be guided by astrocyte boundaries during development. We have previously shown that in vitro astrocyte boundaries can be generated by combining two different astrocyte cell lines, one which is inhibitory to neurite outgrowth (Neu7) with one that is permissive (A7). The extracellular matrix molecules tenascin-C, chondroitin sulfate proteoglycans (CSPG) and keratan sulfate proteoglycans (KSPG) were implicated in boundary formation. We have now further addressed the roles of these molecules using additional astrocyte cell lines that differ in their potential to permit neurite extension and in their expression of extracellular matrix molecules. T34-2 and 27A1 cells are permissive to neurite extension. T34-2 cells express high amounts of tenascin-C, but very low levels of proteoglycans, while 27A1 cells express CSPG and KSPG, but very little tenascin-C. T34-2 cells formed boundaries to neurites, and these boundaries are greatly reduced in the presence of blocking antitenascin-C antiserum. The addition of the antiserum did not affect neurite extension. 27A1 cells also formed boundaries without affecting neurite extension. Chondroitinase ABC, but not keratanase, treatment reduced the boundary, suggesting that CSPG is a major boundary component. These results demonstrate that astrocyte tenascin-C and proteoglycans are distinct components of astrocyte boundaries. More importantly, these results suggest that growing neurites can be directed to their targets by astrocyte-derived guidance molecules independent of effects on process extension.

Heparan sulphate and HB-GAM (heparin-binding growth-associated molecule) in the development of the thalamocortical pathway of rat brain

Extracellular matrix (ECM) molecules, such as laminin, tenascin, chondroitin sulphate proteoglycans and heparan sulphate proteoglycans have been suggested to have 'signpost' and directing roles in the formation of axonal projections in cortical development. We show here that the expression of the neurite outgrowth-promoting protein heparin-binding growth-associated molecule (HB-GAM) and N-syndecan, a transmembrane heparan sulphate proteoglycan previously isolated as a receptor for HB-GAM, is spatiotemporally associated with the developing thalamocortical pathway in the rat brain. Using in situ hybridization, thalamic neurons were shown to express mRNA for N-syndecan, and in vitro, thalamic neurons grew more neurites on HB-GAM than on laminin. The HB-GAM-induced neurite outgrowth in thalamic neurons was inhibited by heparitinase, heparin, soluble N-syndecan and by an excess of soluble HB-GAM in the culture medium. In a pathway assay, thalamic neurons selectively preferred attaching and growing neurites on matrices containing HB-GAM than on those containing fibronectin or laminin alone, suggesting that HB-GAM may modulate the effect of other ECM proteins. On an unfixed brain slice preparation, thalamic neurons repeatedly showed a typical neurite outgrowth and attachment pattern resembling the expression pattern of HB-GAM. On the brain slices, the neurite outgrowth was significantly inhibited by heparitinase, heparin and soluble HB-GAM, thus displaying features of neurite outgrowth on matrix-bound HB-GAM. Our results suggest that HB-GAM is important for the neurite outgrowth of thalamic neurons and it may function as an ECM-bound guidance cue for thalamic neurons that possess HB-GAM-binding heparan sulphates on their cell membrane.

Formation of lamina-specific synaptic connections

In many parts of the vertebrate central nervous system, inputs of distinct types confine their synapses to individual laminae. Such laminar specificity is a major determinant of synaptic specificity. Recent studies of several laminated structures have begun to identify some of the cells (such as guidepost neurons in hippocampus), molecules (such as N-cadherin in optic tectum, semaphorin/collapsin in spinal cord, and ephrins in cerebral cortex), and mechanisms (such as activity-dependent refinement in lateral geniculate) that combine to generate laminar specificity.

Anterior-posterior subdivision of the somite in embryonic zebrafish: implications for motor axon guidance

The ventral branches of the segmental peripheral nerves in the zebrafish embryo are pioneered by the caudal primary (CaP) motor axons, which extend midsegmentally at the interface of the somite and the notochord. The signals that define the CaP pathway are not well understood. To gain insight into the nature of the guidance cues, we have examined the environment of the CaP motor axons by using electron microscopy and histochemistry. Specifically, we have mapped the distribution of the transcription factor engrailed, of a chondrotin sulfate epitope, and of the recognition molecules zebrafish semaphorin z1b and zebrafish tenascin C. Ultrastructural examination of dye-labeled CaP motor axons revealed a close association with the medial surface of the somite but not with the notochord. The CaP axons were always accompanied by cells that appeared to migrate at the interface of somite and notochord. These cells were confined to the posterior half of the somite. Some of the cells may be neural crest derived, many others are probably of sclerotomal origin. The putative migratory cells expressed a chondroitin sulfate epitope that is a marker of sclerotome in the chick. The pathway of the CaP axon and the distribution of the putative neural crest and sclerotome cells correlated with a subdivision of the myotome into an anterior and posterior components, which were evident at the histological level and by the expression of the markers engrailed, semaphorin z1b, chondroitin sulfate, and tenascin C. We suggest that both the pathway choice of the CaP axon and the route of migratory cells reflect this anterior-posterior bipartition of the myotome.

Chondroitin sulfates modulate axon guidance in embryonic Xenopus brain

Chondroitin sulfate proteoglycans display both inhibitory and stimulatory effects on cell adhesion and neurite outgrowth in vitro. The functional activity of these proteoglycans appears to be context specific and dependent on the presence of different chondroitin sulfate-binding molecules. Little is known about the role of chondroitin sulfate proteoglycans in the growth and guidance of axons in vivo. To address this question, we examined the effects of exogenous soluble chondroitin sulfates on the growth and guidance of axons arising from a subpopulation of neurons in the vertebrate brain which express NOC-2, a novel glycoform of the neural cell adhesion molecule N-CAM. Intact brains of stage 28 Xenopus embryos were unilaterally exposed to medium containing soluble exogenous chondroitin sulfates. When exposed to chondroitin sulfate, NOC-2(+) axons within the tract of the postoptic commissure failed to follow their normal trajectory across the ventral midline via the ventral commissure in the midbrain. Instead, these axons either stalled or grew into the dorsal midbrain or continued growing longitudinally within the ventral longitudinal tract. These findings suggest that chondroitin sulfate proteoglycans indirectly modulate the growth and guidance of a subpopulation of forebrain axons by regulating either matrix-bound or cell surface cues at specific choice points within the developing vertebrate brain.

Expression cloning and characterization of NSIST, a novel sulfotransferase expressed by a subset of neurons and postsynaptic targets

Synapses are distinguished by localized concentrations of specific proteins, many of which bear the marks of posttranslational processing such as glycosylation and sulfation. One strategy to elucidate this posttranslational tailoring is to identify the enzymes that create these modifications. Monoclonal antibody 3B3 recognizes a carbohydrate-containing epitope expressed on dystroglycan and other constituents of Torpedo electric organ synaptic membranes. We used mAb 3B3 in an immunofluorescence-based expression-cloning method and isolated a cDNA clone conferring mAb-3B3 immunoreactivity to transfected COS cells. The deduced polypeptide has a predicted molecular weight of 51 kDa, a type II transmembrane topology, and four potential N-linked glycosylation sites. The polypeptide, which we term NSIST (nervous system involved sulfotransferase), shows extensive, although not complete, homology to a chondroitin-6-sulfotransferase and limited homology to other sulfotransferases. In NSIST-transfected COS cells, 35SO4 incorporation and chondroitin-sulfate-like immunoreactivity are increased. In vivo, NSIST occurs as a single 2.4 kb transcript abundant in Torpedo electric organ, moderately expressed in spinal cord and electric lobe, and undetectable in non-neural tissues. Immunohistochemistry shows that NSIST is expressed in a punctate distribution in the innervated portion of electrocytes. In the CNS, NSIST-like immunoreactivity is localized within the somas of motor neurons and neurons of the electromotor nucleus, whereas mAb-3B3 immunostaining is associated with cell surfaces and neuropil. Neuronal NSIST is therefore likely to exert its effects extracellularly; although NSIST is synthesized by neurons, its product, the 3B3 epitope, is found outside neuronal cell bodies. Our evidence indicates that NSIST participates in nervous system specific posttranslational modifications, perhaps including those at synapses.

Postnatal expression pattern of calcium-binding proteins in organotypic thalamic cultures and in the dorsal thalamus in vivo

The present study describes the postnatal expression of calbindin, calretinin and parvalbumin and glutamic acid decarboxylase (GAD) and microtubule-associated protein 2 (MAP2) in organotypic monocultures of rat dorsal thalamus compared to the thalamus in vivo. Cultures were maintained for up to 7 weeks. Cortex-conditioned medium improved the survival of thalamic cultures. MAP2-immunoreactive material was present in somata and dendrites of small and large-sized neurons throughout the cultures. Parvalbumin immunoreactivity was present in larger multipolar or bitufted neurons along the edge of a culture. These neurons also displayed strong parvalbumin mRNA and GAD mRNA expression, and GABA immunoreactivity. They likely corresponded to cells of the nucleus reticularis thalami. Parvalbumin mRNA, but neither parvalbumin protein nor GAD mRNA, was expressed in neurons with large somata within the explant. They likely represented relay cells. GAD mRNA, but not parvalbumin mRNA, was expressed in small neurons within the explants. Small neurons also displayed calbindin- and calretinin-immunoreactivity. The small neurons likely represented local circuit neurons. The time course of expression of the calcium-binding proteins revealed that all were present at birth with the predicted molecular weights. A low, but constant parvalbumin expression was observed in vitro without the developmental increase seen in vivo, which most likely represented parvalbumin from afferent sources. In contrast, the explantation transiently downregulated the calretinin and calbindin expression, but the neurons recovered the expression after 14 and 21 days, respectively. In conclusion, thalamic monocultures older than three weeks represent a stable neuronal network containing well differentiated neurons of the nucleus reticularis thalami, relay cells and local circuit neurons.

Lamina-specific cell adhesion on living slices of hippocampus

Laminar distribution of fiber systems is a characteristic feature of hippocampal organization. Ingrowing afferents, e.g. the fibers from the entorhinal cortex, terminate in specific layers, which implies the existence of laminar recognition cues. To identify cues that are involved in the laminar segregation of fiber systems in the hippocampus, we used an in vitro assay to study the adhesion of dissociated entorhinal cells on living hippocampal slices. Here we demonstrate that dissociated entorhinal cells adhere to living hippocampal slices with a lamina-specific distribution that reflects the innervation pattern of the entorhino-hippocampal projection. In contrast, laminae which are not invaded by entorhinal fibers are a poor substrate for cell adhesion. Lamina-specific cell adhesion does not require the neural cell adhesion molecule or the extracellular matrix glycoprotein reelin, as revealed in studies with mutants. However, the pattern of adhesive cues in the reeler mouse hippocampus mimics characteristic alterations of the entorhinal projection in this mutant, suggesting a role of layer-specific adhesive cues in the pathfinding of entorhinal fibers. Lamina-specific cell adhesion is independent of divalent cations, is abolished after cryofixation or paraformaldehyde fixation and is recognized across species. By using a novel membrane adhesion assay, we show that lamina-specific cell adhesion can be mimicked by membrane-coated fluorescent microspheres. Recognition of the adhesive properties of different hippocampal laminae by growing axons, as either a growth permissive or a non-permissive substrate, may provide a developmental mechanism underlying the segregation of lamina-specific fiber projections.

Patterns of chondroitin sulfate immunoreactivity in the developing tectum reflect regional differences in glycosaminoglycan biosynthesis

The glycosaminoglycan chondroitin sulfate (CS) is expressed in many parts of the developing brain, both in regions where axons preferentially grow and in areas that axons distinctly avoid. Some in vitro studies suggest that CS and proteoglycans (PGs) that carry CS enhance axon growth, whereas others suggest that CS and CSPGs inhibit it. In the developing hamster, there is evidence that midbrain raphe cells act as a barrier to prevent growth of optic axons across the tectal midline. Here we show that in the newborn hamster, CS immunoreactivity is substantially higher in midline than in lateral tectum, raising the possibility that CSPGs play a role in the unilateral containment of optic axons. However, analysis of tectal PGs by anion exchange chromatography and denaturing gel electrophoresis failed to detect substantial differences between midline and lateral tectum in either the types or relative amounts of CSPG and heparan sulfate PG protein cores. In contrast, metabolic labeling of tectal slices in vitro documented that incorporation of 35S-sulfate into macromolecules is significantly increased at the tectal midline, in a pattern resembling chondroitin sulfate immunoreactivity. This difference was evident whether slices were labeled for 1 hr or overnight and was not paralleled by a difference in overall protein synthesis, suggesting that the rate of synthesis of sulfated macromolecules is specifically elevated in midline tectum. We propose that the concentration of CS at the midline of the developing tectum is a reflection of a higher rate of synthesis or sulfation of glycosaminoglycans by midline cells, rather than a higher level of production of any particular CSPG. These results suggest that the distribution of some axon guidance signals in development may be controlled by differential regulation of glycosaminoglycan biosynthetic enzymes.

Mechanisms underlying the early establishment of thalamocortical connections in the rat

We labeled axonal projections using carbocyanine dyes in the developing rat brain to study cellular interactions that might underlie the establishment of thalamocortical connectivity. By embryonic day 14 (E14), groups of neurons in the ventral diencephalon and the primitive internal capsule have established projections to the dorsal thalamus, and thalamic fibers pass in topographic order among them. Simultaneously, axons from the early-born cells in both subplate and marginal zone (i.e., the original cortical preplate) establish an ordered array that fills the intermediate zone. Thalamic axons and preplate fibers meet in the lateral part of the internal capsule (at E15 for occipital cortex and dorsolateral thalamus). Subsequently, selective labeling of corresponding thalamic and early corticofugal projections reveals thalamic fibers growing in association with early corticofugal axons, right up to the cortical subplate. A small carbocyanine crystal implanted at any point in the cortex shortly after the arrival of thalamic axons (E16 for the occipital cortex) labels a single, tight bundle containing both descending and ascending fibers, rather than two separate tracts, providing further evidence for intimate topographic association of the two axon systems. Crystals placed in a row, parasagittally or coronally along the hemisphere, reveal separate, topographically distributed, discrete fiber bundles throughout the pathway, leading to spatially ordered groups of back-labeled thalamic cells. These results indicate that the topography of thalamic axons is maintained throughout the pathway and that they reach the cortex by associating with the projections of a number of preexisting cells, including the preplate scaffold.

Developmental regulation of decorin expression in postnatal rat brain

Here, we report on the expression of the small chondroitin/dermatan sulfate proteoglycan decorin in the developing postnatal rat brain. Northern analysis of brain RNA demonstrated decorin transcripts with peak expression on postnatal day 3 followed by a slow decline to the lower adult level. In situ hybridization and immunohistochemistry revealed postnatal decorin expression in the grey matter of neocortex, hippocampus and thalamus, in myelinated fibre tracts and in several mesenchymal tissues (blood vessels, pia mater and the choroid plexus). In the neocortex, decorin is expressed in a specific laminar pattern with intense staining of the cortical plate and its derivatives, which differs remarkably from the distributions observed for other proteoglycans [B. Miller, A.M. Sheppard, A.R. Bicknese, A.L. Pearlman, Chondroitin sulfate proteoglycans in the developing cerebral cortex: the distribution of neurocan distinguishes forming afferent and efferent axonal pathways, J. Comp. Neurol. 355 (1995) 615-28]. Thus, decorin seems to serve yet unknown functions in the developing rat brain parenchyma in addition to its well-established role as a constituent of the mesenchymal extracellular matrix.

Target selection: invasion, mapping and cell choice

Recent research has shown that changes in the concentration of particular molecules lead axons to invade their target, and that concentration changes in other molecules at the borders of the target prevent axons from leaving the target area. After invasion, topographic and lamina-specific cues guide axons to the correct location within the target field. At the level of a single cell or part of a cell, the evidence raises the possibility that axon targeting might be a combinatorial affair whereby specific axons compare the relative concentrations of several molecules on the surface of postsynaptic cells in order to choose a particular target. Both proteins and carbohydrates of various classes play major roles in these processes.

N-syndecan and HB-GAM (heparin-binding growth-associated molecule) associate with early axonal tracts in the rat brain

Heparin-Binding Growth-Associated Molecule (HB-GAM)/pleiotrophin is an 18 kDa extracellular matrix- and cell-surface-associated protein shown to enhance neurite outgrowth of perinatal forebrain neurones in vitro. The heparan sulphate proteoglycan N-syndecan (Raulo et al., 1994) has been isolated as a receptor/coreceptor for the HB-GAM. We have investigated, whether HB-GAM and N-syndecan could have a similar role in neurite outgrowth and axon guidance in early axonal tracts of brain. In the present study N-syndecan was found to be spatiotemporally associated with the developing axonal tracts already on embryonic day 9 in rat, as revealed by coexpression with class III beta-tubulin, which is one of the earliest neuronal markers (Easter et al., 1993; Brittis et al., 1995). Later, N-syndecan and HB-GAM were detected in the first afferent serotonergic projections arising from the pontine raphe nuclei. The expression pattern of HB-GAM peaked in the developing rhombencephalon at embryonic stage (E) 13-14. At the same time, N-syndecan was expressed in the developing raphe neurones growing neurites towards the diencephalon along HB-GAM immunoreactive pathways. When rhombencephalic neurones were cultured on decreasing concentrations of substrate-bound HB-GAM, E13 neurones showed a significantly better neurite outgrowth response than E11, E16 or E18 neurones. The neurite outgrowth of raphe neurones in vitro was inhibited by adding soluble heparin or N-syndecan into the culture medium, whereas addition of chondroitin sulphate had no effect. In a simple pathway assay, E13 raphe neurones selectively preferred attaching and growing neurites on pathways containing HB-GAM as compared with regions containing either laminin or fibronectin alone. Our results suggest that HB-GAM may function as a developmentally regulated cue for rhombencephalic neurones that possess N-syndecan on their cell membrane.

Activated macrophages and the blood-brain barrier: inflammation after CNS injury leads to increases in putative inhibitory molecules

The cellular responses to spinal cord or brain injury include the production of molecules that modulate wound healing. This study examined the upregulation of chondroitin sulfate proteoglycans, a family of molecules present in the wound healing matrix that may inhibit axon regeneration in the central nervous system (CNS) after trauma. We have demonstrated increases in these putative inhibitory molecules in brain and spinal cord injury models, and we observed a close correlation between the tissue distribution of their upregulation and the presence of inflammation and a compromised blood-brain barrier. We determined that the presence of degenerating and dying axons injured by direct trauma does not provide a sufficient signal to induce the increases in proteoglycans observed after injury. Activated macrophages, their products, or other serum components that cross a compromised blood-brain barrier may provide a stimulus for changes in extracellular matrix molecules after CNS injury. While gliosis is associated with increased levels of proteoglycans, not all reactive astrocytes are associated with augmented amounts of these extracellular matrix molecules, which suggests a heterogeneity among glial cells that exhibit a reactive phenotype. Chondroitin sulfate also demarcates developing cavities of secondary necrosis, implicating these types of boundary molecules in the protective response of the CNS to trauma.

Proteoglycans provide neurite guidance at an astrocyte boundary

Astrocytes in the developing brain direct neurites through their synthesis of cell surface and extracellular matrix molecules. We introduce a novel culture system to identify and examine the guidance properties of astrocyte-derived molecules. The permissive A7 and nonpermissive Neu7 cell lines were co-cultured to form an A7/Neu7 monolayer. Neurites extended on A7 cells but avoided Neu7 cells and instead stopped or turned at the A7/Neu7 Interface. Enzymatic treatment with trypsin and hyaluronic acid increased neurite extension, but neither altered the boundary. Only, removal of keratan and chondroitin sulfate residues reduced the guidance capacity of the A7/Neu7 boundary. Since no treatment individually abolished the boundary, neurite guidance appears to be due to a combination of factors. The A7/Neu7 astrocyte substrate demonstrates the functional role for KSPGs and CSPGs, but more interestingly, suggests that simply increasing the capacity of a substrate to permit neurite outgrowth does not necessarily eliminate or even reduce its guidance properties.

Cellular delivery of neurotrophin-3 promotes corticospinal axonal growth and partial functional recovery after spinal cord injury

The injured adult mammalian spinal cord shows little spontaneous recovery after injury. In the present study, the contribution of projections in the dorsal half of the spinal cord to functional loss after adult spinal cord injury was examined, together with the effects of transgenic cellular delivery of neurotrophin-3 (NT-3) on morphological and functional disturbances. Adult rats underwent bilateral dorsal column spinal cord lesions that remove the dorsal corticospinal projections or underwent more extensive resections of the entire dorsal spinal cord bilaterally that remove corticospinal, rubrospinal, and cerulospinal projections. Long-lasting functional deficits were observed on a motor grid task requiring detailed integration of sensorimotor skills, but only in animals with dorsal hemisection lesions as opposed to dorsal column lesions. Syngenic primary rat fibroblasts genetically modified to produce NT-3 were then grafted to acute spinal cord dorsal hemisection lesion cavities. Up to 3 months later, significant partial functional recovery occurred in NT-3-grafted animals together with a significant increase in corticospinal axon growth at and distal to the injury site. These findings indicate that (1) several spinal pathways contribute to loss of motor function after spinal cord injury, (2) NT-3 is a neurotrophic factor for the injured corticospinal projection, and (3) functional deficits are partially ameliorated by local cellular delivery of NT-3. Lesions of the corticospinal projection may be necessary, but insufficient in isolation, to cause sensorimotor dysfunction after spinal cord injury in the rat.

Essential role of heparan sulfates in axon navigation and targeting in the developing visual system

Heparan sulfate (HS) is abundant in the developing brain and is a required co-factor for many types of fibroblast growth factor (FGF) signaling in vitro. We report that some HSs, when added exogenously to the developing Xenopus optic pathway, severely disrupt target recognition causing axons from the retina to bypass their primary target, the optic tectum. Significantly, HS sidechains from a neuroepithelial perlecan variant that preferentially bind FGF-2, HS(FGF-2), cause aberrant targeting, whereas those that preferentially bind FGF-1 do not. Charge-matched fragments of HS(FGF-2) show that the mistargeting activity associates with the FGF-binding fragments. Heparitinase removal of native HSs at the beginning of optic tract formation retards retinal axon elongation; addition of FGF-2 restores axon extension but axons lose directionality. Late HS removal, after axons have extended through the tract, elicits a tectal bypass phenotype indicating a growth promoting and guidance function for native HSs. Our results demonstrate that different HS sidechains from the same core protein differentially affect axon growth in vivo, possibly due to their distinct FGF-binding preferences, and suggest that growth factors and HSs are important partners in regulating axon growth and guidance in the developing visual system.