Aberrant trajectory of thalamocortical axons associated with abnormal localization of neurocan immunoreactivity in the cerebral neocortex of reeler mutant mice

We examined the molecular mechanisms underlying the formation of the thalamocortical pathway in the cerebral neocortex of normal and reeler mutant mice. During normal development of the mouse neocortex, thalamic axons immunoreactive for the neural cell adhesion molecule L1 rarely invaded the cortical plate and ran centered in the subplate which is immunoreactive for neurocan, a brain-specific chondroitin sulfate proteoglycan. On the other hand, in homozygous reeler mutant mice, thalamic axons took an aberrant course to run obliquely through the cortical plate. Injection of bromodeoxyuridine at embryonic day 11 specifically labeled subplate neurons in normal mice, whilst in the reeler neocortex it labeled cells scattered in the cortical plate as well as in the superficial layer (superplate). Neurocan immunoreactivity was associated with the bromodeoxyuridine-positive cells in the superplate, as well as being present in oblique bands within the cortical plate, along which L1-bearing thalamic axons preferentially ran. The present results support our previous hypothesis proposed for normal rats that a heterophilic molecular interaction between L1 and neurocan is involved in determining the thalamocortical pathway within the neocortical anlage [T. Fukuda et al. (1997) Journal of Comparative Neurology, 382, 141-152].

L1-mediated branching is regulated by two ezrin-radixin-moesin (ERM)-binding sites, the RSLE region and a novel juxtamembrane ERM-binding region

We investigated how the neural cell adhesion molecule L1 mediates neurite outgrowth through L1-L1 homophilic interactions. Wild-type L1 and L1 with mutations in the cytoplasmic domain (CD) were introduced into L1 knock-out neurons, and transfected neurons were grown on an L1 substrate. Neurite length and branching were compared between wild-type L1 and L1CD mutations. Surprisingly, the L1CD is not required for L1-mediated neurite outgrowth but plays a critical role in neurite branching, through both the juxtamembrane region and the RSLE region. We demonstrate that both regions serve as ezrin-moesin-radixin-binding sites. A truncation mutant that deletes 110 of 114 amino acids of the L1CD still supports neurite outgrowth on an L1 substrate, suggesting that a coreceptor binds to L1 in cis and mediates neurite outgrowth and that L1-ankyrin interactions are not essential for neurite initiation or outgrowth. These data are consistent with a model in which L1 can influence L1-mediated neurite outgrowth and branching through both the L1CD and a coreceptor.

Polysialylated neural cell adhesion molecule promotes remodeling and formation of hippocampal synapses

Expression of the neural cell adhesion molecule (NCAM) has been shown to promote long-term potentiation (LTP) and stabilization of synapses during early synaptogenesis. Here, we searched for the mechanisms of synaptogenic activity of NCAM, focusing on the role of polysialic acid (PSA), an unusual carbohydrate preferentially associated with NCAM. We show that enzymatic removal of PSA with endoneuraminidase-N (endo-N) abolished preferential formation of synapses on NCAM-expressing cells in heterogenotypic cocultures of wild-type and NCAM-deficient hippocampal neurons. Transfection of NCAM-deficient neurons with either of three major NCAM isoforms (different in intracellular domains but identical in extracellular domains and carrying PSA) stimulated preferential synapse formation on NCAM isoform-expressing neurons. Enzymatic removal of heparan sulfates from cultured neurons and a mutation in the heparin-binding domain of NCAM diminished synaptogenic activity of neuronally expressed PSA-NCAM, suggesting that interaction of NCAM with heparan sulfate proteoglycans mediates this activity. PSA-NCAM-driven synaptogenesis was also blocked by antagonists to fibroblast growth factor receptor and NMDA subtype of glutamate receptors but not by blockers of non-NMDA glutamate receptors and voltage-dependent Na+ channels. Enzymatic removal of PSA and heparan sulfates also blocked the increase in the number of perforated spine synapses associated with NMDA receptor-dependent LTP in the CA1 region of organotypic hippocampal cultures. Thus, neuronal PSA-NCAM in complex with heparan sulfate proteoglycans promotes synaptogenesis and activity-dependent remodeling of synapses.

Heterophilic interactions between cell adhesion molecule L1 and alphavbeta3-integrin induce HUVEC process extension in vitro and angiogenesis in vivo

Cell adhesion molecule L1 was implicated in angiogenic processes, tumor formation and metastasis. Here, we provide evidence that the sixth Ig-like domain of L1 (L1Ig6) interacts with alpha(v)beta3 to induce process extension of human umbilical vein endothelial cells (HUVECs) in vitro and angiogenesis in vivo. HUVECs formed network-like structures on full-length L1 or L1Ig6 substrates comparable to structures found on matrigel. In the presence of mab alpha(v)beta3 or cyclic RGD, apoptosis was induced. In fibrin matrices where L1Ig6 was covalently incorporated, HUVECs formed multicellular and hollow processes through interactions between cell-surface alpha(v)beta3 and RGD-sites of matrix-immobilized L1Ig6. No such processes were induced by L1Ig6 having non-functional RDG-sites, or in the presence of mab alpha(v)beta3 or cyclic RGD. In those matrices, increased apoptosis was found. Co-immunoprecipitation of L1 or L1Ig6 with alpha(v)beta3 suggests close interactions. Furthermore, L1Ig6 stimulated HUVECs showed increased tyrosine phosphorylation of alpha(v)beta3 and phosphorylation of MAP kinases (ERK1 and ERK2) but not AKT indicating specific activation of alpha(v) and alpha(v)beta3 followed by activation of downstream kinases. Application of L1Ig6-modified fibrin matrices on CAMs induced 50-60% increased alpha(v) and alpha(v)beta3 protein expression and in vivo angiogenesis indicated by approximately 50% increased mean vascular length density. The results demonstrate angiogenic potential of L1Ig6 involving ligation and activation of alpha(v)beta3.

Impairment of sensorimotor gating in mice deficient in the cell adhesion molecule L1 or its close homologue, CHL1

Mutations in the gene encoding the cell adhesion molecule L1 or its close homologue, CHL1 (close homologue of L1), cause brain dysfunction in both humans and mice. Here we report that prepulse inhibition (PPI) of the acoustic startle response is impaired in mice deficient in either L1 or CHL1. This newly identified feature may provide a basis for using these mice as models for sensorimotor gating impairment found in some neuropsychiatric disorders such as schizophrenia.

Platelet homeostasis is regulated by platelet expression of CD47 under normal conditions and in passive immune thrombocytopenia

Interaction between target cell CD47 and the inhibitory macrophage receptor signal regulatory protein alpha (SIRPalpha) counteracts macrophage phagocytosis of CD47-expressing host cells. As platelets also express CD47, we asked whether inhibitory CD47/SIRPalpha signaling regulates normal platelet turnover and clearance of platelets in immune thrombocytopenic purpura (ITP). CD47(-/-) mice had a mild spontaneous thrombocytopenia, which was not due to a decreased platelet half-life as a result of increased expression of P-selectin, CD61, or phosphatidylserine. In contrast, CD47(-/-) platelets were rapidly cleared when transfused into CD47(+/+) recipients, whereas CD47(+/-) platelets had a nearly normal half-life in CD47(+/+) mice under nonautoimmune conditions. CD47(-/-) mice were more sensitive to ITP, as compared with CD47(+/+) mice. In vitro, macrophage phagocytosis of immunoglobulin G (IgG)-opsonized CD47(-/-) platelets was significantly higher than that for equally opsonized CD47(+/+) platelets. However, when SIRPalpha was blocked, phagocytosis of CD47(+/+) platelets increased to the level of CD47(-/-) platelets. Phagocytosis of opsonized CD47(+/-) platelets was higher than that for CD47(+/+) platelets, but lower than that for CD47(-/-) platelets, suggesting a gene-dose effect of CD47 in this system. In conclusion, we suggest that inhibitory CD47/SIRPalpha signaling is involved in regulating platelet phagocytosis in ITP, and that targeting SIRPalpha may be a new means of reducing platelet clearance in ITP.

Role of CD47 in erythroid cells and in autoimmunity

The cell surface glycoprotein CD47 (Integrin-associated protein/IAP) was originally identified as a regulator of integrin-dependent responses to extracellular matrix proteins. However, CD47 is ubiquitously expressed, also by cells that do not express integrins. Thus, during the last few years, it has been shown that CD47 has several important functions besides assisting integrin activation. This review will focus on the role of CD47 in erythrocytes. In these cells, CD47 was found to be an important link in the interaction between the band 3 complex and the Rh complex in the maintenance of erythrocyte membrane integrity. CD47 can also function as a marker of self on erythrocytes, and likely also on other cells, by binding to the inhibitory receptor SIRPalpha. In this way, SIRPalpha-expressing cells, like macrophages and dendritic cells, are less likely to phagocytose an autoimmune sensitized cell with CD47 on its surface than a CD47-deficient cell where this inhibitory mechanism will not be engaged. The interaction between CD47 and SIRPalpha seems to be important to limit destruction of host cells in autoimmune diseases like autoimmune hemolytic anemia (AIHA), where macrophages destroy antibody or complement opsonized cells.

Transcriptional Regulation of Signal Regulatory Protein α1 Inhibitory Receptors by Epidermal Growth Factor Receptor Signaling

Signal regulatory protein (SIRP) alpha1 is a membrane glycoprotein and a member of the SIRP receptor family. These transmembrane receptors have been shown to exert negative effects on signal transduction by receptor tyrosine kinases via immunoreceptor tyrosine-based inhibitory motifs in the carboxyl domain. Previous work has demonstrated that SIRPs negatively regulate many signaling pathways leading to reduction in tumor migration, survival, and cell transformation. Thus, modulation of SIRP expression levels or activity could be of great significance in the field of cancer therapy. The aim of the present study was to determine the factors that regulate levels of SIRPalpha1 in human glioblastoma cells that frequently overexpress the epidermal growth factor receptor (EGFR) because SIRPs have been shown to negatively regulate EGFR signaling. Northern blot analysis and immunoprecipitation assays showed variable expression levels of endogenous SIRPalpha transcripts in nine well-characterized glioblastoma cell lines. We examined SIRPalpha1 regulation in U87MG and U373MG cells in comparison with clonal derivatives that express a truncated form of erbB2, which negatively regulates EGFR signaling by inducing the formation of nonfunctional heterodimeric complexes. Mutant erbB2-expressing cells contained more SIRPalpha1 mRNA when compared with the parental cells in presence or absence of serum. Similarly, immunoprecipitation assays showed increased SIRPalpha1 protein levels in erbB-inhibited cells when compared with parental cells. Messenger RNA stability assays revealed that the increased mRNA levels in EGFR-inhibited cells were due to an induction of transcription. Consistent with this finding, expression of the erbB2 mutant receptor up-regulated SIRPalpha1 promoter activity in all cell lines tested. Interestingly, pharmacological inhibition of the kinase activities of EGFR, erbB2, and src and activation of mitogen-activated protein kinase, but not phosphatidylinositol 3'-kinase, significantly up-regulated SIRPalpha1 promoter activity. Based on these observations, we hypothesize that down-modulation of EGFR signaling leads to transcriptional up-regulation of the inhibitory SIRPalpha1 gene. These data may be important in the application of erbB-inhibitory strategies and for design of therapies for the treatment of glial tumors and other epithelial malignancies.

Binding partners L1 cell adhesion molecule and the ezrin-radixin-moesin (ERM) proteins are involved in development and the regenerative response to injury of hippocampal and cortical neurons

Regeneration of the adult central nervous system may require recapitulation of developmental events and therefore involve the re-expression of developmentally significant proteins. We have investigated whether the L1 cell adhesion molecule, and its binding partner, the ezrin-radixin-moesin (ERM) proteins are involved in the neuronal regenerative response to injury. Hippocampal and cortical neurons were cultured in vitro on either an L1 substrate or poly-L-lysine, and ERM and other neuronal proteins were localized immunocytochemically both developmentally and following neurite transection of neurons maintained in long-term culture. Activated ERM was localized to growth cones up to 7 days in vitro but relatively mature cultures (21 days in vitro) were devoid of active ERM proteins. However, ERM proteins were localized to the growth cones of sprouting neuronal processes that formed several hours after neurite transection. In addition, the L1 substrate, relative to poly-L-lysine, resulted in significantly longer regenerative neurites, as well as larger growth cones with more filopodia. Furthermore, neurons derived from the cortex formed significantly longer post-injury neurite sprouts at 6 h post-injury than hippocampal derived neurons grown on both substrates. We have demonstrated that L1 and the ERM proteins are involved in the neuronal response to injury, and that neurons derived from the hippocampus and cortex may have different post-injury regenerative neurite sprouting abilities.

Axon behaviour at Schwann cell - astrocyte boundaries: manipulation of axon signalling pathways and the neural adhesion molecule L1 can enable axons to cross

Axon regeneration in vivo is blocked at boundaries between Schwann cells and astrocytes, such as occur at the dorsal root entry zone and around peripheral nerve or Schwann cell grafts. We have created a tissue culture model of these boundaries in Schwann cell - astrocyte monolayer co-cultures. Axon behaviour resembles that in vivo, with axons showing a strong preference for Schwann cells over astrocytes. At boundaries between the two cell types, axons growing on astrocytes cross readily onto Schwann cells, but only 15% of axons growing on Schwann cells are able to cross onto astrocytes. Treatment with chondroitinase or chlorate to reduce inhibition by proteoglycans did not change this behaviour. The neural adhesion molecule L1 is present on Schwann cells and not astrocytes, and manipulation of L1 by application of an antibody, L1-Fc in solution, or adenoviral transduction of L1 into astrocytes increased the proportion of axons able to cross onto astrocytes to 40-50%. Elevating cAMP levels increased crossing from Schwann cells onto astrocytes in live and fixed cultures, and had a co-operative effect with NT-3 but not with NGF. Inactivation of Rho with a cell-permeant form of C3 exoenzyme also increased crossing from Schwann cells to astrocytes. Our experiments indicate that the preference of axons for Schwann cells is largely mediated by the presence of L1 on Schwann cells but not astrocytes, and that manipulation of growth cone signalling pathways can allow axons to disregard boundaries between the two cell types.

Negative regulation of hepatocellular carcinoma cell growth by signal regulatory protein alpha1

Signal regulatory protein (SIRP) alpha1 is a member of the SIRP family that undergoes tyrosine phosphorylation and binds SHP-2 tyrosine phosphatase in response to various mitogens. The expression levels of SIRPalpha1 were decreased in HCC tissues, compared with the matched normal tissues. Exogenous expression of wild type SIRPalpha1, but not of a mutant SIRPalpha1 lacking the tyrosine phosphorylation sites, in SIRPalpha1-negative Huh7 human HCC cells resulted in suppression of tumor cell growth both in vitro and in vivo. Treatment of Huh7 transfectants with EGF or HGF induced tyrosine phosphorylation of SIRPalpha1 and its association with SHP-2, which were accompanied by reduced ERK1 activation. Expression of SIRPalpha1 significantly suppressed activation of NF-kappaB and also sensitized Huh7 cells to TNFalpha or cisplatin-induced cell death. In addition, SIRPalpha1-transfected Huh7 cells displayed reduced cell migration and cell spreading in a fashion that was dependent on SIRPalpha1/SHP-2 complex formation. In conclusion, a negative regulatory effect of SIRPalpha1 on hepatocarcinogenesis is exerted, at least in part, through inhibition of ERK and NF-kappaB pathways.

Human Lymphocytes Interact Directly with CD47 through a Novel Member of the Signal Regulatory Protein (SIRP) Family

Two closely related proteins, signal regulatory protein alpha (SIRPalpha; SHPS-1/CD172) and SIRPbeta, have been described in humans. The existence of a third SIRP protein has been suggested by cDNA sequence only. We show that this third SIRP is a separate gene that is expressed as a protein with unique characteristics from both alpha and beta genes and suggest that this gene should be termed SIRPgamma. We have expressed the extracellular region of SIRPgamma as a soluble protein and have shown that, like SIRPalpha, it binds CD47, but with a lower affinity (K(d), approximately 23 microM) compared with SIRPalpha (K(d), approximately 2 microM). mAbs specific to SIRPgamma show that it was not expressed on myeloid cells, in contrast to SIRPalpha and -beta, being expressed instead on the majority of T cells and a proportion of B cells. The short cytoplasmic tail of SIRPgamma does not contain any known signaling motifs, nor does it contain a characteristic lysine, as with SIRPbeta, that is required for DAP12 interaction. DAP12 coexpression is a requirement for SIRPbeta surface expression, whereas SIRPgamma is expressed in its absence. The SIRPgamma-CD47 interaction may therefore not be capable of bidirectional signaling as with the SIRPalpha-CD47, but, instead, use unidirectional signaling via CD47 only.

Src homology 2 domain-containing protein tyrosine phosphatase substrate 1 regulates the migration of Langerhans cells from the epidermis to draining lymph nodes

Src homology 2 domain-containing protein tyrosine phosphatase substrate 1 (SHPS-1) is a member of the signal regulatory protein family in which the extracellular region interacts with its ligand, CD47. Recent studies have demonstrated that SHPS-1 plays an important role in cell migration and cell adhesion. We demonstrate in this study, using immunohistochemical and flow cytometric analyses, that murine Langerhans cells (LCs) express SHPS-1. Treatment of mice ears with 2,4-dinitro-1-fluorobenzene significantly reduced the number of epidermal LCs, and that reduction could be reversed by pretreatment with mAb to SHPS-1 or the CD47-Fc fusion protein. Treatment with the SHPS-1 mAb in vivo reduced the number of FITC-bearing cells in the lesional lymph nodes after the application of FITC to the skin. The SHPS-1 mAb inhibited the in vivo TNF-alpha-induced migration of LCs. The emigration of dendritic cells expressing I-A(b+) from skin explants to the medium was also reduced by the SHPS-1 mAb. We further demonstrate that the chemotaxis of a murine dendritic cell line, XS52, by macrophage inflammatory protein-3beta was significantly inhibited by treatment with the SHPS-1 mAb or CD47-Fc recombinant protein. Finally, we show that migration of LCs was attenuated in mutant mice that lack the intracellular domain of SHPS-1. These observations show that the ligation of SHPS-1 with the SHPS-1 mAb or with CD47-Fc abrogates the migration of LCs in vivo and in vitro, which suggests that the SHPS-1-CD47 interaction may negatively regulate LC migration.

Cell adhesion molecule L1 promotes neurite outgrowth of septal neurons

To establish if the cell adhesion molecule L1 could promote neurite outgrowth of septal neurons, L1-positive substrates were prepared by genetically modifying 3T3 fibroblasts with a retroviral vector encoding human L1 under the control of a negative tetracycline-regulatory system. In several clones of L1-transfected fibroblasts, L1 expression at the cell surface was prominent and efficiently regulated by doxycycline, a tetracycline analogue. In co-culture of septal neurons and fibroblasts, a two-dimensional fractionator probe provided systematic random sampling of the neurites to be measured. Septal neurons, isolated at embryonic Day 17, were found to express L1 in vitro and to extend significantly longer neurites when plated on L1-expressing fibroblasts compared to control fibroblasts. The neurite outgrowth-promoting effect of L1 was inhibited after a doxycycline treatment, which specifically suppressed L1 expression from the modified fibroblasts. The findings that septal neurons at embryonic Day 17 in vitro express L1 and respond to L1-modulation suggest that this molecule is involved in development of the septohippocampal pathway.

Ectodomain Shedding of SHPS-1 and Its Role in Regulation of Cell Migration

SHPS-1 is a transmembrane protein whose cytoplasmic region undergoes tyrosine phosphorylation and then binds the protein-tyrosine phosphatase SHP-2. Formation of the SHPS-1-SHP-2 complex is implicated in regulation of cell migration. In addition, SHPS-1 and its ligand CD47 constitute an intercellular recognition system that contributes to inhibition of cell migration by cell-cell contact. The ectodomain of SHPS-1 has now been shown to be shed from cells in a reaction likely mediated by a metalloproteinase. This process was promoted by activation of protein kinase C or of Ras, and the released ectodomain exhibited minimal CD47-binding activity. Metalloproteinases catalyzed the cleavage of a recombinant SHPS-1-Fc fusion protein in vitro, and the primary cleavage site was localized to the juxtamembrane region of SHPS-1. Forced expression of an SHPS-1 mutant resistant to ectodomain shedding impaired cell migration, cell spreading, and reorganization of the actin cytoskeleton. It also increased the tyrosine phosphorylation of paxillin and FAK triggered by cell adhesion. These results suggest that shedding of the ectodomain of SHPS-1 plays an important role in regulation of cell migration and spreading by this protein.

Neural cell adhesion molecule (NCAM) null mice do not show a deficit in odour discrimination learning

Polysialylated neural cell adhesion molecule (PSA-NCAM) is predominantly expressed during development where it regulates biological functions including axon targeting and neuronal precursor cell migration. Although dramatically down regulated after birth in most regions of the nervous system, PSA-NCAM remains highly expressed into adulthood in areas that have ongoing regeneration and plasticity such as in the olfactory bulb and hippocampus. Consequently, lack of PSA-NCAM in NCAM null mice results in distinct morphological changes to these areas. The functional correlates of these changes are not well defined although there have been reports that learning is impaired in NCAM null mice. In the present study, we assessed the ability of old and young NCAM null mice to learn an odour discrimination task. We tested male and female experimental and control animals of two different ages: 30-60 days and 12-15 months. During 4 days of training, NCAM null and C57BL/6J received trials where one odour (CS+) was paired with sugar while another odour (CS-) was not. In a subsequent preference test, conducted in the absence of sugar, all animals, regardless of strain or age, spent significantly more time digging in the CS+ odour than in the CS- odour. In addition, there was no significant difference in digging behaviour in the CS+ between the NCAM null and the control animals. These data indicate that deletion of the NCAM gene may change the morphology of the olfactory bulb but does not interfere with the ability to learn an odour discrimination task.

Promotion of neurite and filopodium formation by CD47: roles of integrins, Rac, and Cdc42

Axon extension during development is guided by many factors, but the signaling mechanisms responsible for its regulation remain largely unknown. We have now investigated the role of the transmembrane protein CD47 in this process in N1E-115 neuroblastoma cells. Forced expression of CD47 induced the formation of neurites and filopodia. Furthermore, an Fc fusion protein containing the extracellular region of the CD47 ligand SHPS-1 induced filopodium formation, and this effect was enhanced by CD47 overexpression. SHPS-1-Fc also promoted neurite and filopodium formation triggered by serum deprivation. Inhibition of Rac or Cdc42 preferentially blocked CD47-induced formation of neurites and filopodia, respectively. Overexpression of CD47 resulted in the activation of both Rac and Cdc42. The extracellular region of CD47 was sufficient for the induction of neurite formation by forced expression, but the entire structure of CD47 was required for enhancement of filopodium formation by SHPS-1-Fc. Neurite formation induced by CD47 was also inhibited by a mAb to the integrin beta3 subunit. These results indicate that the interaction of SHPS-1 with CD47 promotes neurite and filopodium formation through the activation of Rac and Cdc42, and that integrins containing the beta3 subunit participate in the effect of CD47 on neurite formation.

Phospholipase D2 functions as a downstream signaling molecule of MAP kinase pathway in L1-stimulated neurite outgrowth of cerebellar granule neurons

Stimulation of the neuronal cell adhesion molecule L1 in cerebellar granule neurons (CGNs) enhances neurite outgrowth and this response is inhibited by the primary alcohol ethanol. Because primary alcohols suppress the formation of the signaling lipid phosphatidic acid (PA) by phospholipase D (PLD), this observation prompted us to investigate whether PLD plays a role in the L1-mediated neurite outgrowth in CGNs. In the cerebellum of postnatal day 8 mice, PLD2 protein was abundantly expressed, while PLD1 expression was not detected. The L1-stimulated neurite outgrowth was inhibited by primary alcohols and by overexpression of lipase-deficient PLD2. Increases in cellular PA levels by direct PA application or overexpression of wild-type PLD2 mimicked the L1-dependent stimulation of neurite outgrowth. Furthermore, it was found that L1 stimulation in CGNs increased PLD activity concomitantly with phosphorylation of extracellular signal-regulated kinase (ERK), both of which were inhibited by the MAP kinase-ERK kinase (MEK) inhibitor. These results provide evidence that PLD2 functions as a downstream signaling molecule of ERK to mediate the L1-dependent neurite outgrowth of CGNs, a mechanism that may be related to alcohol-related neurodevelopmental disorders.

The role of L1 in axon pathfinding and fasciculation

The neural cell adhesion molecule L1 has been found to play important roles in axon growth and fasciculation. Our main objective was to determine the role of L1 during the development of connections between thalamus and cortex. We find that thalamocortical and corticothalamic axons in mice lacking L1 are hyperfasciculated, a subset of thalamocortical axons make pathfinding errors and thalamocortical axon growth cones are abnormally long in the subplate. These defects occur despite formation of six cortical layers and formation of topographically appropriate thalamocortical connections. The loss of L1 is accompanied by loss of expression of ankyrin-B, an intracellular L1 binding partner, suggesting that L1 is involved in the regulation of Ank2 stability. We postulate that the pathfinding errors, growth cone abnormalities and hyperfasciculation of axons following loss of L1 reflect both a shift in binding partners among axons and different substrates and a loss of appropriate interactions with the cytoskeleton.

Involvement of highly polysialylated neural cell adhesion molecule (PSA-NCAM)-positive granule cells in the amygdaloid-kindling-induced sprouting of a hippocampal mossy fiber trajectory

The mossy fiber system in the hippocampus of amygdaloid-kindled rats was examined by using highly polysialylated neural cell adhesion molecule (PSA-NCAM) as a marker for immunohistochemical detection of immature dentate granule cells and mossy fibers in combination with bromodeoxyuridine (BrdU) labeling of newly generated granule cells. Statistically significant increases in BrdU-labeled cells and PSA-NCAM-positive cells occurred in the dentate gyrus following kindling. The increase in PSA-NCAM-immunoreactive neurites was confined to the entire stratum lucidum of CA3. Immunoelectron-microscopic examination also revealed that PSA-NCAM-positive immature synaptic terminals of the sprouting mossy fibers increased in the stratum lucidum of CA3 in the kindled rats. The increase in the numbers of PSA-NCAM-positive granule cells correlated well with the increase in the immunopositive neurites and synaptic terminals on the mossy fiber trajectory. The increase in these PSA-NCAM-immunopositive structures is thought to reflect the enhancement of sprouting and synaptogenesis of mossy fibers by a subset of granule cells newly generated during amygdaloid-kindling and suggests that the reorganization of the mossy fiber system on the normal trajectory at least in part contributes to the acquisition and maintenance of an epileptogenic state.

Patterning axonal guidance molecules using a novel strategy for microcontact printing

We present here a two-step strategy for micropatterning proteins on a substrate to control neurite growth in culture. First, conventional microcontact printing is used to prepare a micropattern of protein A, which binds the Fc fragment of immunoglobulins. Then, a chimeric protein, consisting of the extracellular domain of a guidance protein recombinantly linked to the Fc fragment of IgG (prepared using conventional molecular techniques), is applied from solution. The chimeric protein binds to the patterned protein A, taking on its geometric pattern. Using this method, we have micropatterned the extracellular domain of the cell adhesion molecule, L1 (as an L1-Fc chimera) and demonstrated that it retains its ability to selectively guide axonal growth. L1-Fc micropatterned on a background of poly-L-lysine resulted in selective growth of the axons on the micropattern, whereas the somata and dendrites were unresponsive. Substrates bearing simultaneous micropatterns of L1-Fc and poly-L-lysine on a background of untreated glass were also created. Using this approach, cell body position was controlled by manipulating the dimensions of the poly-L-lysine pattern, and the dendrites were constrained to the poly-L-lysine pattern, while the axons grew preferentially on L1-Fc. The two-step microcontact printing method allows preparation of substrates that contain guidance proteins in geometric patterns with resolution of approximately 1 microm. This method should be broadly applicable to many classes of proteins.

Plasticity following Injury to the Adult Central Nervous System: Is Recapitulation of a Developmental State Worth Promoting?

The adult central nervous system (CNS) appears to initiate a transient increase in plasticity following injury, including increases in growth-related proteins and generation of new cells. Recent evidence is reviewed that the injured adult CNS exhibits events and patterns of gene expression that are also observed during development and during regeneration following damage to the mature peripheral nervous system (PNS). The growth of neurons during development or regeneration is correlated, in part, with a coordinated expression of growth-related proteins, such as growth-associated-protein-43 (GAP-43), microtubule-associated-protein-1B (MAP1B), and polysialylated-neural-cell-adhesion-molecule (PSA-NCAM). For each of these proteins, evidence is discussed regarding its specific role in neuronal development, signals that modify its expression, and reappearance following injury. The rate of adult hippocampal neurogenesis is also affected by numerous endogenous and exogenous factors including injury. The continuing study of developmental neurobiology will likely provide further gene and protein targets for increasing plasticity and regeneration in the mature adult CNS.

Untangling the functional potential of PSA-NCAM-expressing cells in CNS development and brain repair strategies

Central nervous system (CNS) neural stem cells (NSCs), which are mostly defined by their ability to self-renew and to generate the three main cell lineages of the CNS, were isolated from discrete regions of the adult mammalian CNS including the subventricular zone (SVZ) of the lateral ventricle and the dentate gyrus in the hippocampus. At early stages of CNS cell fate determination, NSCs give rise to progenitors that express the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). PSA-NCAM(+) cells persist in adult brain regions where neuronal plasticity and sustained formation of new neurons occur. PSA-NCAM has been shown to be involved in the regulation of CNS myelination as well as in changes of cell morphology that are necessary for motility, axonal guidance, synapse formation, and functional plasticity in the CNS. Although being preferentially committed to a restricted either glial or neuronal fate, cultured PSA-NCAM(plus) progenitors do preserve a relative degree of multipotentiality. Considering that PSA-NCAM(+) cells can be neatly used for brain repair purposes, there is much interest for studying signaling factors regulating their development. With this regard, it is noteworthy that neurotransmitters, which belong to the micro-environment of neural cells in vivo, regulate morphogenetic events preceding synaptogenesis such as cell proliferation, migration, differentiation and death. Consistently, several ionotropic but also G-protein-coupled neurotransmitter receptors were found to be expressed in CNS embryonic and postnatal progenitors. In the present review, we outlined the ins and outs of PSA-NCAM(plus) cells addressing to what extent our understanding of extrinsic and in particular neurotransmitter-mediated signaling in these CNS precursor cells might represent a new leading track to develop alternative strategies to stimulate brain repair.