Binding of neural cell adhesion molecules (N-CAMs) to the cellular prion protein

To identify molecular interaction partners of the cellular prion protein (PrP(C)), we sought to apply an in situ crosslinking method that maintains the microenvironment of PrP(C). Mild formaldehyde crosslinking of mouse neuroblastoma cells (N2a) that are susceptible to prion infection revealed the presence of PrP(C) in high molecular mass (HMM) protein complexes of 200 to 225 kDa. LC/MS/MS analysis identified three murine splice-variants of the neural cell adhesion molecule (N-CAM) in the complexes, which isolate with caveolae-like domains (CLDs). Enzymatic removal of N-linked sugar moieties did not disrupt the complexes, arguing that the interaction of PrP with N-CAM occurs through amino acid side-chains. Additionally, similar levels of PrP/N-CAM complexes were found in N2a and prion-infected N2a (ScN2a) cells. With the use of an N-CAM-specific peptide library, the PrP-binding site was determined to comprise beta-strands C and C' within the two consecutive fibronectin type III (FNIII) modules found in proximity of the membrane-attachment site of N-CAM. As revealed by in situ crosslinking of PrP deletion mutants, the PrP face of the binding site is formed by the N terminus, helix A (residues 144-154) and the adjacent loop region of PrP. N-CAM-deficient (N-CAM(-/-)) mice that were intracerebrally challenged with scrapie prions succumbed to disease with a mean incubation period of 122 (+/-4.1, SEM) days, arguing that N-CAM is not involved in PrP(Sc) replication. Our findings raise the possibility that N-CAM may join with PrP(C) in carrying out some as yet unidentified physiologic cellular function.

The microbial receptor CEACAM3 is linked to the calprotectin complex in granulocytes

Engulfment of foreign pathogens is an evolutionary ancient host cell endocytic response. Signaling pathways effecting phagocytosis are divergent and largely depend on the structural features of the cell surface receptor utilized. CEACAM3, a member of the CD66 complex on human neutrophils, has been implicated as a cellular receptor promoting phagocytosis of microorganisms. The cytoplasmic domain of CEACAM3 (CEACAM3(cyt)) contains an immunoreceptor tyrosine-based activation motif. In this study we demonstrate that CEACAM3(cyt) is phosphorylated by protein kinase C, casein kinase I, and Src-kinase in vitro. To identify molecules binding to CEACAM3(cyt) in vivo, we used differentially phosphorylated recombinant expressed CEACAM cytoplasmic domains to isolate CEACAM3(cyt)-associated proteins from granulocyte extracts. Calprotectin, which modulates neutrophil integrin-mediated adhesion and leukocyte trafficking and displays antimicrobial activity, interacts specifically with CEACAM3(cyt). This interaction is calcium-modulated but independent of phosphorylation of CEACAM3(cyt). Although tyrosine-phosphorylated CEACAM3(cyt) binds and stimulates Src-kinases in vitro, no CEACAM3(cyt)-associated phosphokinase activity was copurified.

Immunoelectron microscopic localization of the neural recognition molecules L1, NCAM, and its isoform NCAM180, the NCAM-associated polysialic acid, beta1 integrin and the extracellular matrix molecule tenascin-R in synapses of the adult rat hippocampus

We have investigated the possibility that morphologically different excitatory glutamatergic synapses of the "trisynaptic circuit" in the adult rodent hippocampus, which display different types of long-term potentiation (LTP), may express the immunoglobulin superfamily recognition molecules L1 and NCAM, the extracellular matrix molecule tenascin-R, and the extracellular matrix receptor constituent beta1 integrin in a differential manner. The neural cell adhesion molecules L1, NCAM (all three major isoforms), NCAM180 (the largest major isoform with the longest cytoplasmic domain), beta1 integrin, polysialic acid (PSA) associated with NCAM, and tenascin-R were localized by pre-embedding immunostaining procedures in the CA3/CA4 region (mossy fiber synapses) and in the dentate gyrus (spine synapses) of the adult rat hippocampus. Synaptic membranes of mossy fiber synapses where LTP is expressed presynaptically did not show detectable levels of immunoreactivity for any of the molecules/epitopes studied. L1, NCAM, and PSA, but not NCAM180 or beta1 integrin, were detectable on axonal membranes of fasciculating mossy fibers. In contrast to mossy fiber synapses, spine synapses in the outer third of the molecular layer of the dentate gyrus, which display postsynaptic expression mechanisms of LTP, were both immunopositive and immunonegative for NCAM, NCAM180, beta1 integrin, and PSA. Those spine synapses postsynaptically immunoreactive for NCAM or PSA also showed immunoreactivity on their presynaptic membranes. NCAM180 was not detectable presynaptically in spine synapses. L1 could not be found in spine synapses either pre- or postsynaptically. Also, the extracellular matrix molecule tenascin-R was not detectable in synaptic clefts of all synapses tested, but was amply present between fasciculating axons, axon-astrocyte contact areas, and astrocytic gap junctions. Differences in expression of the membrane-bound adhesion molecules at both types of synapses may reflect the different mechanisms for induction and/or maintenance of synaptic plasticity.

Characterization of human cleaved N-CAM and association with schizophrenia

The neural cell adhesion molecule (N-CAM) is a cell recognition molecule involved in cellular migration, synaptic plasticity, and CNS development. A 105- to 115-kDa isoform of N-CAM (cleaved N-CAM or cN-CAM) is increased in schizophrenia in hippocampus, prefrontal cortex, and CSF. We purified and partially characterized cN-CAM, a putative novel isoform, and confirmed that the first 9 amino acids were identical to exon 1 of N-CAM, without the signal sequence. Analysis of trypsin-digested cN-CAM fragments by matrix-assisted laser desorption ionization on a time-of-flight mass spectrometer (MALDI-TOF) yielded peptides that could be identified as being derived from the first 548 amino acid residues of the expected N-CAM amino acid sequence. Immunological identification with four specific N-CAM antisera directed toward cytoplasmic, secreted, variable alternative spliced exon, or GPI epitopes failed to indicate other known splice variants. Neuraminidase treatment of cN-CAM produced a minor alteration resulting in a faster migrating immunoreactive band, indicating partial glycosylation of cN-CAM. Membranous particles from cytosolic brain extract containing cN-CAM were obtained by ultracentrifugation; however, CSF contained few such particles. cN-CAM and synaptophysin were colocalized on these particles. Both cN-CAM and N-CAM 180 were present in synaptosomal preparations of human brain. Following incubation of synaptosomes or brain tissue without protease inhibitors, N-CAM 180 was degraded and cN-CAM was increased. A cN-CAM-like band was present in human fetal neuronal cultures, but not in fetal astrocyte cultures. Thus, cN-CAM represents a protease- and neuraminidase-susceptible fragment possibly derived by proteolytic cleavage of N-CAM 180. An enlargement in ventricular volume in a group of adult patients with schizophrenia over a 2-year interval was found to be correlated with CSF cN-CAM levels as measured at the time of the initial MRI scan (r = 0.53, P = 0.01). cN-CAM is associated with ventricular enlargement; thus, the release of N-CAM fragments may be part of the pathogenic mechanism of schizophrenia in vulnerable brain regions such as the hippocampus and prefrontal cortex. Alternatively, the increases in cN-CAM in schizophrenia may be a reflection of a more general abnormality in the regulation of proteolysis or of extracellular matrix stability.

Molecular properties of fibrin-based matrices for promotion of angiogenesis in vitro

The molecular properties of fibrin-based matrices, such as fibrillar structure and covalent modifications with adhesion domains, influence the angiogenic behavior of human umbilical vein endothelial cells (HUVECs) in vitro. The fibrillar structure of fibrin-based matrices was influenced by pH but not by covalent incorporation of exogenous adhesion domains. Native fibrin-based matrices polymerized at pH 10 formed organized and longitudinally oriented fibrin fibrils, which provided a good angiogenic substrate for endothelial cells. Furthermore, upon covalent incorporation of the model ligand L1Ig6, which binds to the integrin most prominently expressed on the surface of angiogenic endothelial cells, alpha(v)beta3, these matrices became angiogenesis-promoting when polymerized at physiological pH. The amount of incorporation of L1Ig6 into the matrices depended on the fibrinogen concentration on all three fibrin chains. Soluble forms of L1Ig6 diffused rapidly out of the matrix. Most important, L1Ig6-modified matrices were very specific in inducing the angiogenic phenotype of HUVECs, whereas control cells did not differentiate on these matrices. Our results indicate that artificial extracellular matrices can influence cell behavior in two ways. One way is based on the three-dimensional fibril structure of the matrix molecules themselves, and the other is due to providing specific binding sites for direct cell-matrix interactions that lead to the activation of second-messenger cascades and thus promoting angiogenic differentiation.

UNC-52/perlecan isoform diversity and function in Caenorhabditis elegans

The unc-52 gene encodes the nematode homologue of mammalian perlecan, the major heparan sulphate proteoglycan of the extracellular matrix. This is a large complex protein with regions similar to low-density lipoprotein receptors, laminin and neural cell-adhesion molecules. Three major classes of UNC-52/perlecan isoforms are produced through alternative splicing, and these distinct proteins exhibit complex spatial and temporal expression patterns throughout development. The unc-52 gene plays an essential role in myofilament assembly in body-wall muscle during embryonic development.

Alternative use of a mini exon of the L1 gene affects L1 binding to neural ligands

Neural cell adhesion molecule L1 is a cell surface glycoprotein required for the correct development of the nervous system. L1 exists as two isoforms encoded by mRNA species that either collectively incorporate or exclude exons 2 and 27. Neurons utilize only the full-length isoform, whereas Schwann cells, kidney cells, and blood lymphocytes only express the short form of L1. Still other cells, oligodendrocytes, regulate L1 isoform expression in a maturation-dependent manner. The RSLE motif encoded by exon 27 is known to have a role in clathrin-mediated endocytosis of L1, but the function of the exon 2-encoded motif (YEGHHV) is unknown. Here we show that this motif is required for the optimal binding of L1 to several neural ligands and is likely to be important for nervous system development. Thus, alternative use of exon 2 is a mechanism for regulating ligand interactions with L1.

Regulation of cell adhesion by polysialic acid. Effects on cadherin, immunoglobulin cell adhesion molecule, and integrin function and independence from neural cell adhesion molecule binding or signaling activity

The polysialylation of neural cell adhesion molecule (NCAM) evolved in vertebrates to carry out biological functions related to changes in cell position and morphology. Many of these effects involve the attenuation of cell interactions that are not mediated through NCAM's own adhesion properties. A proposed mechanism for this global effect on cell interaction is the steric inhibition of membrane-membrane apposition based solely on polysialic acid (PSA) biophysical properties. However, it remains possible that the intrinsic binding or signaling properties of the NCAM polypeptide are also involved. To help resolve this issue, this study uses a quantitative cell detachment assay together with cells engineered to display different adhesion receptors together with a variety of polysialylated NCAM polypeptide isoforms and functional domain deletion mutations. The results obtained indicate that regulation by PSA occurs with adhesion receptors as diverse as an IgCAM, a cadherin and an integrin, and does not require NCAM functional domains other than those minimally required for polysialylation. These findings are most consistent with the cell apposition mechanism for PSA action, as this model predicts that the inhibitory effects of PSA-NCAM on cell adhesion should be independent of the nature of the adhesion system and of any intrinsic binding or signaling properties of the NCAM polypeptide itself.

Solution structure of the third immunoglobulin domain of the neural cell adhesion molecule N-CAM: can solution studies define the mechanism of homophilic binding?

Homophilic binding of the neural cell adhesion molecule (N-CAM) mediates the calcium-independent cell-cell adhesion that is involved in neuronal development. Two hypotheses have been advanced for the mechanism of homophilic binding. Cell-based experiments have implicated each of the five extracellular immunoglobulin (Ig) domains of N-CAM in the homophilic adhesion interaction, and have predicted that the third domain (Ig III) self-associates. The alternative hypothesis is based on solution observations, which implicate a specific antiparallel interaction between the first two Ig domains (Ig I and Ig II). In order to test these hypotheses, we have determined a high-resolution solution structure of recombinant Ig III (sequence derived from chicken N-CAM) and examined the aggregation behavior of isolated Ig domains in solution. The structure shows that Ig III adopts a canonical Ig fold, in which the beta strands ABED and A'GFCC' form two beta sheets that are linked by a disulfide bond. In contrast to the demonstrated aggregation of Ig III on solid supports, we were unable to demonstrate self-association of Ig III under any of a variety of solution conditions. The structure shows that the surface of Ig III is dominated by two large acidic patches, which may explain our failure to observe self-association in solution. To evaluate the involvement of the Ig I-Ig II interaction in cell-cell adhesion, we designed a point mutation in Ig I (F19S) that proved sufficient to abrogate the Ig I-Ig II interaction seen in solution. However, the introduction of this mutation into full-length N-CAM expressed in COS-7 cells failed to affect N-CAM-mediated cell-cell adhesion. The inability to observe Ig III self-association in solution, combined with the failure of the F19S mutation to affect N-CAM-mediated cell-cell adhesion, suggests that, although solution studies can give important insights into the structures of individual domains, the interactions observed in solution between the domains may not be representative of the interactions that occur on the cell surface.

Pentyl-4-yn-valproic acid enhances both spatial and avoidance learning, and attenuates age-related NCAM-mediated neuroplastic decline within the rat medial temporal lobe

2-N-Pentyl-4-pentynoic acid [pentyl-4-yn-valproic acid (VPA)] is an analogue of valproic acid that induces neuritogenesis and increases neural cell adhesion molecule (NCAM) prevalence in cultured neural cells. As memory consolidation involves synapse growth, aided by cell adhesion molecule function, we determined whether or not pentyl-4-yn-VPA had cognition-enhancing properties. Pentyl-4-yn-VPA (16-85 mg/kg) significantly improved water maze learning and task retention when given prior to each training session. Acute administration of pentyl-4-yn-VPA also influenced memory consolidation processes as, when given at 3 h post-passive avoidance training, the amnesia induced by scopolamine given 6 h post-training was prevented in a dose-dependent manner. Chronic administration of pentyl-4-yn-VPA (16.8 or 50.4 mg/kg) also significantly reduced escape latencies in the water maze task, 24 h following the last drug administration. This improved spatial learning was accompanied by enhanced neuroplasticity as the expression of NCAM polysialylated neurons in the infragranular zone of the dentate gyrus and in layer II of the perirhinal and piriform cortex was increased significantly following chronic drug treatment. The cognition-enhancing qualities of pentyl-4-yn-VPA, combined with its ability to attenuate the age-related loss of the NCAM polysialylation state, suggest that it may effectively slow the onset of cognitive decline.

Developmental Profile of Neural Cell Adhesion Molecule Glycoforms with a Varying Degree of Polymerization of Polysialic Acid Chains

More precise information on the degree of polymerization (DP) of polysialic acid (polySia) chains expressed on neural cell adhesion molecule (NCAM) and its developmental stage-dependent variation are considered important in understanding the mechanism of regulated polysialylation and fine-tuning of NCAM-mediated cell adhesion by polySia. In this paper, first we performed a kinetic study of acid-catalyzed hydrolysis of polySia and report our findings that (a) in (-->8Neu5Ac alpha 2-->)(n)-->8Neu5Ac alpha 2-->3Gal beta 1-->R, the proximal Neu5Ac residue alpha 2-->3 linked to Gal is cleaved about 2.5-4 times faster than the alpha 2-->8 linkages and (b) in contrary to general belief that alpha 2-->8 linkages in polySia are extremely labile, the kinetic consideration showed that they are not so unstable, and every ketosidic bond is hydrolyzed at the same rate. These findings are the basis of our strategy for DP analysis of polySia on NCAM. Second, using the recently developed method that provides base-line resolution of oligo/polySia from DP 2 to >80 with detection thresholds of 1.4 fmol per resolved peak, we have determined the DP of polySia chains expressed in embryonic chicken brains at different developmental stages. Our results support the presence of numerous NCAM glycoforms differing in DPs of oligo/polySia chains and a delicate change in their distribution during development.

Soluble forms of NCAM and F3 neuronal cell adhesion molecules promote Schwann cell migration: identification of protein tyrosine phosphatases zeta/beta as the putative F3 receptors on Schwann cells

Neural cell adhesion molecule (NCAM) and F3 are both axonal adhesion molecules which display homophilic (NCAM) or heterophilic (NCAM, F3) binding activities and participate in bidirectional exchange of information between neurones and glial cells. Engineered Fc chimeric molecules are fusion proteins that contain the extracellular part of NCAM or F3 and the Fc region of human IgG1. Here, we investigated the effect of NCAM-Fc and F3-Fc chimeras on Schwann cell (SC) migration. Binding sites were identified at the surface of cultured SCs by chimera coated fluorospheres. The functional effect of NCAM-Fc and F3-Fc binding was studied in two different SC migration models. In the first, migration is monitored at specific time intervals inside a 1-mm gap produced in a monolayer culture of SCs. In the second, SCs from a dorsal root ganglion explant migrate on a sciatic nerve cryosection. In both systems addition of the chimeras significantly increased the extent of SC migration and this effect could be prevented by the corresponding anti-NCAM or anti-F3 blocking antibodies. Furthermore, antiproteoglycan-type protein tyrosine phosphatase zeta/beta (RPTPzeta/beta) antibodies identified the presence of RPTPzeta/beta on SCs and prevented the enhancing effect of soluble F3 on SC motility by 95%. The F3-Fc coated Sepharose beads precipitated RPTPzeta/beta from SC lysates. Altogether these data point to RPTPzeta/beta is the putative F3 receptor on SCs. These results identify F3 and NCAM receptors on SC as potential mediators of signalling occurring between axons and glial cells during peripheral nerve development and regeneration.

Molecular diversity in zebrafish NCAM family: three members with different VASE usage and distinct localization

NCAM in vertebrates and its related molecules, apCAM in Aplysia, fasciclin II in Drosophila, and OCAM in mammals, play key roles in various aspects of brain development and functions. In this study, we have identified and characterized three members of the NCAM gene family in zebrafish, designated as zNCAM, zOCAM, and zPCAM. Three molecules exhibit similar domain organization: an amino-terminal signal peptide, five immunoglobulin-like domains, two fibronectin type III-like domains, a transmembrane segment, and a carboxy-terminal cytoplasmic region. A novel molecule zPCAM is most closely related to zNCAM with 66% amino acid identity. Diversity in the extracellular region of zPCAM is generated by insertion of two different types of variable alternatively spliced exons. In situ hybridization analysis revealed that three molecules were specifically expressed by the central and peripheral nervous systems from early developmental stages in region-specific and cell-type-specific manners. For example, zPCAM showed a neuromere-specific segmental expression pattern, while zOCAM first appeared in specific clusters of secondary neurons in the forebrain. These results suggest that each member of the NCAM gene family plays distinct roles in the formation and maintenance of functional neuronal networks in the zebrafish nervous system.

SHP-2 complex formation with the SHP-2 substrate-1 during C2C12 myogenesis

Myogenesis is a highly ordered process that involves the expression of muscle-specific genes, cell-cell recognition and multinucleated myotube formation. Although protein tyrosine kinases have figured prominently in myogenesis, the involvement of tyrosine phosphatases in this process is unknown. SHP-2 is an SH2 domain-containing tyrosine phosphatase, which positively regulates growth and differentiation. We show that in C2C12 myoblasts, SHP-2 becomes upregulated early on during myogenesis and associates with a 120 kDa tyrosyl-phosphorylated complex. We have identified that the 120 kDa complex consists of the SHP-2 substrate-1 (SHPS-1) and the Grb2-associated binder-1 (Gab-1). SHPS-1, but not Gab-1, undergoes tyrosyl phosphorylation and association with SHP-2 during myogenesis, the kinetics of which correlate with the expression of MyoD. Either constitutive expression or inducible activation of MyoD in 10T½ fibroblasts promotes SHPS-1 tyrosyl phosphorylation and its association with SHP-2. It has been shown that p38 mitogen-activated protein kinase (MAPK) activity is required for the expression/activation of MyoD and MyoD-responsive genes. Inhibition of p38 MAPK by SB203580 in differentiating C2C12 myoblasts blocks MyoD expression, SHPS-1 tyrosyl phosphorylation and the association of SHPS-1 with SHP-2. These data suggest that SHPS-1/SHP-2 complex formation is an integral signaling component of skeletal muscle differentiation.

Cell adhesion molecule L1 in folded (horseshoe) and extended conformations

We have investigated the structure of the cell adhesion molecule L1 by electron microscopy. We were particularly interested in the conformation of the four N-terminal immunoglobulin domains, because x-ray diffraction showed that these domains are bent into a horseshoe shape in the related molecules hemolin and axonin-1. Surprisingly, rotary-shadowed specimens showed the molecules to be elongated, with no indication of the horseshoe shape. However, sedimentation data suggested that these domains of L1 were folded into a compact shape in solution; therefore, this prompted us to look at the molecules by an alternative technique, negative stain. The negative stain images showed a compact shape consistent with the expected horseshoe conformation. We speculate that in rotary shadowing the contact with the mica caused a distortion of the protein, weakening the bonds forming the horseshoe and permitting the molecule to extend. We have thus confirmed that the L1 molecule is primarily in the horseshoe conformation in solution, and we have visualized for the first time its opening into an extended conformation. Our study resolves conflicting interpretations from previous electron microscopy studies of L1.

Plasmin-mediated release of the guidance molecule F-spondin from the extracellular matrix

Serine proteases are implicated in a variety of processes during neurogenesis, including cell migration, axon outgrowth, and synapse elimination. Tissue-type plasminogen activator and urokinase-type activator are expressed in the floor plate during embryonic development. F-spondin, a gene also expressed in the floor plate, encodes a secreted, extracellular matrix-attached protein that promotes outgrowth of commissural axons and inhibits outgrowth of motor axons. F-spondin is processed in vivo to yield an amino half protein that contains regions of homology to reelin and mindin, and a carboxyl half protein that contains either six or four thrombospondin type I repeats (TSRs). We have tested F-spondin to see whether it is subjected to processing by plasmin and to determine whether the processing modulates its biological activity. Plasmin cleaves F-spondin at its carboxyl terminus. By using nested deletion proteins and mutating potential plasmin cleavage sites, we have identified two cleavage sites, the first between the fifth and sixth TSRs, and the second at the fifth TSR. Analysis of the extracellular matrix (ECM) attachment properties of the TSRs revealed that the fifth and sixth TSRs bind to the ECM, but repeats 1-4 do not. Structural functional experiments revealed that two basic motives are required to elicit binding of TSR module to the ECM. We demonstrate further that plasmin releases the ECM-bound F-spondin protein.

Characterization of N-glycans from mouse brain neural cell adhesion molecule

The N-glycosylation pattern of the neural cell adhesion molecule (NCAM), isolated from brains of newborn mice, has been analyzed. Following digestion with trypsin, generated glycopeptides were fractionated by serial immunoaffinity chromatography using immobilized monoclonal antibodies specifically recognizing polysialic acid (PSA) units or the HNK1-carbohydrate epitope. Subsequent analyses of the resulting (glyco)peptides by Edman degradation and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) revealed polysialylated glycans to be exclusively linked to glycosylation sites 5 (Asn(431)) and 6 (Asn(460)), whereas glycans carrying the HNK1-epitope could be assigned to sites 2 (Asn(297)), 5, 6, and, to a lesser extent, site 3 (Asn(329)). PSA-, HNK1-, and non-PSA/HNK1-glycan fractions were characterized by carbohydrate constituent and methylation analyses as well as MALDI-TOF-MS in conjunction with chromatographic fractionation techniques. The results revealed that the core structures of PSA-glycans represented predominantly fucosylated, partially sulfated 2,6-branched isomers of triantennary as well as tetraantennary complex-type glycans, whereas carbohydrate chains bearing the HNK1-epitope were dominated by diantennary species carrying in part bisecting GlcNAc residues. Non-PSA/HNK1-glycans exhibited a highly heterogeneous pattern of partially truncated, mostly diantennary structures being characterized by the presence of additional fucose, bisecting GlcNAc and/or sulfate residues. In conclusion, our results revealed that the glycosylation pattern of murine NCAM displays high structural and regional selectivity, which might play an important role in controlling the biological activities of this molecule.

Molecular cloning of a novel human gene (SIRP-B2) which encodes a new member of the SIRP/SHPS-1 protein family

A full-length cDNA encoding a novel protein was isolated and sequenced from a human placental cDNA library. This cDNA consists of 1,735 base pairs and has a predicted open reading frame (ORF) encoding 354 amino acids. It possesses a putative signal sequence, a long extracellular domain, a transmembrane region, a short intracellular domain, and no catalytic domain, which is highly homologous to signal-regulatory protein (SIRP)-beta suggesting that it seems to be a new member of the SIRP family. Polymerase chain reaction (PCR)-based mapping with both a monochromosomal hybrid panel and radiation hybrid cell panels placed the gene to human chromosome 20p13 near the marker D20S906.

Integrin-associated protein (CD47) and its ligands

Integrin-associated protein (IAP or CD47) is a receptor for thrombospondin family members, a ligand for the transmembrane signaling protein SIRP alpha and a component of a supramolecular complex containing specific integrins, heterotrimeric G proteins and cholesterol. Peptides containing a VVM motif in the C-terminal domain of thrombospondins are agonists for CD47, initiating heterotrimeric Gi protein signaling that augments the functions of integrins of the beta 1, beta 2 and beta 3 families, thus modulating a range of cell activities including platelet activation, cell motility and adhesion, and leukocyte adhesion, migration and phagocytosis.

The highly sialylated isoform of the neural cell adhesion molecule is required for estradiol-induced morphological synaptic plasticity in the adult arcuate nucleus

The large quantities of polysialic acid (PSA) characterizing highly sialylated isoform of the neural cell adhesion molecule (PSA-NCAM), greatly reduce cell adhesion and render this particular cell surface adhesion molecule a likely candidate to intervene in dynamic neuronal phenomena, such as synaptic plasticity. The hypothalamic arcuate nucleus expresses high levels of PSA-NCAM and maintains a high capacity for neuroplastic changes in the adult. Thus, in the arcuate nucleus of female rats, varying circulating levels of estrogen give rise to a reversible reduction in the number of axo-somatic GABA synapses, together with a changing ensheathing of neuronal somata by astrocytes. To examine the role of PSA in such changes, we perturbed its expression, either by blockade with antibodies raised against this carbohydrate moiety (delivered intracerebroventricularly), or by its enzymatic cleavage after microinjection of endoneuraminidase N over the arcuate nucleus. Either procedure was performed in ovariectomized adult rats that received concurrent treatment with 17 beta-estradiol. Morphological synaptic plasticity was analysed using the unbiased disector method to assess synaptic densities in ultrathin sections of the arcuate nucleus immunogold-labelled for GABA. As expected, 17 beta-estradiol induced a significant reduction in the number of GABAergic axo-somatic synapses, a reduction which did not occur after infusion of anti-PSA antibodies or in vivo enzymatic removal of PSA from NCAM. Taken together, our results provide strong evidence that the presence of large quantities of the PSA moiety on NCAM is a necessary prerequisite for estrogen-induced phasic remodelling of synapses in the adult female arcuate nucleus.

Human papillomavirus infection requires cell surface heparan sulfate

Using pseudoinfection of cell lines, we demonstrate that cell surface heparan sulfate is required for infection by human papillomavirus type 16 (HPV-16) and HPV-33 pseudovirions. Pseudoinfection was inhibited by heparin but not dermatan or chondroitin sulfate, reduced by reducing the level of surface sulfation, and abolished by heparinase treatment. Carboxy-terminally deleted HPV-33 virus-like particles still bound efficiently to heparin. The kinetics of postattachment neutralization by antiserum or heparin indicated that pseudovirions were shifted on the cell surface from a heparin-sensitive into a heparin-resistant mode of binding, possibly involving a secondary receptor. Alpha-6 integrin is not a receptor for HPV-33 pseudoinfection.

NF-kappaB activation by N-CAM and cytokines in astrocytes is regulated by multiple protein kinases and redox modulation

Interaction of the neural cell adhesion molecule (N-CAM) with astrocytes activates a transcription factor, NF-kappaB, that mediates inflammatory responses after neural injury. Here we describe intracellular signaling events that link N-CAM binding to NF-kappaB-mediated transcription. Addition of the third immunoglobulin domain of N-CAM (Ig III), which mimics the activity of intact N-CAM, or of cytokines (interleukin-1beta or tumor necrosis factor-alpha), increased transcription from an NF-kappaB-responsive luciferase reporter gene construct that had been transiently transfected into neonatal rat forebrain astrocytes. NF-kappaB activity induced by Ig III or cytokines was decreased by inhibition of nonreceptor protein tyrosine kinases (PTKs), phospholipase C, protein kinase C (PKC), calcium/calmodulin-dependent protein kinase II (CaMKII), or oxidative stress. Inhibition of PKC blocked nuclear translocation of NF-kappaB protein while binding of NF-kappaB to DNA was decreased by modulation of redox homeostasis. In contrast, inhibition of CaMKII and nonreceptor PTKs altered neither nuclear translocation nor DNA binding, suggesting that these kinases affect NF-kappaB transactivation. A number of agents that inhibit NF-kappaB activation in other cell types did not affect activation in astrocytes. These findings suggest that activation of NF-kappaB by N-CAM and cytokines in astrocytes involves multiple signals that differentially affect NF-kappaB nuclear translocation, DNA binding, and transactivation.

Signal regulation by family conspiracy

The signal regulating proteins (SIRPs) are a family of ubiquitously expressed transmembrane glycoproteins composed of two subgroups: SIRP alpha and SIRP beta, containing more than ten members. SIRP alpha has been shown to inhibit signalling through a variety of receptors including receptor tyrosine kinases and cytokine receptors. This function involves protein tyrosine kinases and is dependent on immunoreceptor tyrosine-based inhibition motifs which recruit key protein tyrosine phosphatases to the membrane. Negative regulation by SIRP alpha may also involve its ligand, CD47, in a bi-directional signalling mechanism. The SIRP beta subtype has no cytoplasmic domain but instead associates with at least one other transmembrane protein (DAP-12, or KARAP). DAP-12 possesses immunoreceptor tyrosine-based activation motifs within its cytoplasmic domain that are thought to link SIRP beta to activating machinery. SIRP alpha and SIRP beta thus have complementary roles in signal regulation and may conspire to tune the response to a stimulus.

SHPS-1 regulates integrin-mediated cytoskeletal reorganization and cell motility

The transmembrane glycoprotein SHPS-1 binds the protein tyrosine phosphatase SHP-2 and serves as its substrate. Although SHPS-1 has been implicated in growth factor- and cell adhesion-induced signaling, its biological role has remained unknown. Fibroblasts homozygous for expression of an SHPS-1 mutant lacking most of the cytoplasmic region of this protein exhibited increased formation of actin stress fibers and focal adhesions. They spread more quickly on fibronectin than did wild-type cells, but they were defective in subsequent polarized extension and migration. The extent of adhesion-induced activation of Rho, but not that of Rac, was also markedly reduced in the mutant cells. Activation of the Ras-extracellular signal-regulated kinase signaling pathway and of c-Jun N-terminal kinases by growth factors was either unaffected or enhanced in the mutant fibroblasts. These results demonstrate that SHPS-1 plays crucial roles in integrin-mediated cytoskeletal reorganization, cell motility and the regulation of Rho, and that it also negatively modulates growth factor-induced activation of mitogen-activated protein kinases.

Morphological and histochemical changes in the regenerating vomeronasal epithelium

Receptor cell degeneration and regeneration within the vomeronasal organ (VNO) of the rat was studied using both electron microscopy and histochemical methods. Electron microscopy was employed to examine the morphological changes along the surface of the sensory epithelium, and histochemical markers were used to monitor the changes in the epithelial cell layers. Transection of the vomeronasal nerves induced selective degeneration of the receptor cells, and within six days, a significant decrease in the number of receptor cells was observed. During the subsequent stage of receptor cell regeneration, cilia and bud-like structures characteristic of a developing sensory epithelium were seen. By day 15, thin microvilli covering the surface of the receptor cells reappeared in the sensory epithelium. The neural cell adhesion molecule (NCAM) and two vomeronasal system-specific lectins; 1) Bandeiraea simplicifolia lectin (BSL-I) and 2) Vicia villosa agglutinin (VVA) were used as the histochemical markers. NCAM immunoreactivity on the surface of the epithelium was observed to be decreased significantly six days after nerve transection, and was restored during receptor cell regeneration (day 15). The reactivity of the two lectins, BSL-I and VVA, was decreased slightly during degeneration, but was still detectable at the time of maximum receptor cell degeneration (day 6). Lectin reactivity was restored to control levels by day 15. These findings suggest that (1) NCAM is a useful marker for vomeronasal receptor cells and that the vomeronasal system-specific lectins may bind to both receptor and supporting cells and (2) degeneration of vomeronasal receptor cells occurs during the first week (day 6) following nerve transection and the receptor cell population begins to recover within 15 days. The morphological changes observed during receptor cell regeneration suggest that the stages of VNO receptor cell regeneration are similar to those observed during development.