Fasciclin 2


FAS2 is the homolog of the vertebrate neural cell adhesion molecule (N-CAM). The Drosophila Neuroglian, also an Ig superfamily member, is a homolog of vertebrate LI. The fact that LI and N-CAM have maintained their identity through evolution, indicates the selective pressure for their function. It also indicates that there has to be an even more primitive CAM, ancestral to both LI and N-CAM (Bieber, 1989). The homology for FAS2 to N-CAM is only 20%, although the domain structure is conserved (Grenningloh, 1991).

A particularly dramatic example of directed migration has been documented within the developing enteric nervous system (ENS) of the moth Manduca sexta. During the formation of the ENS, a population of approximately 300 post-mitotic cells (the EP cells) delaminates from a neurogenic placode to form a discrete packet of undifferentiated neurons at the foregut-midgut boundary. The completion of their development is delayed, however, until a new set of migratory pathways differentiates on the adjacent midgut, where a set of visceral muscle bands coalesce at eight specific locations around the midgut surface. Once these pathways have formed, subsets of EP cells then migrate rapidly onto each muscle band, traveling several hundred mm over the course of 7-10 hours. Only after this migratory phase is complete do the neurons form mature synaptic contacts and express transmitter phenotypes, a developmental sequence that is regulated in part by the migratory process (Wright, 1999).

The insect cell adhesion receptor fasciclin II is expressed by specific subsets of neural and non-neural cells during embryogenesis and has been shown to control growth cone motility and axonal fasciculation. Probes specific for Manduca fasciclin II (MFas II) show that while the EP cells express fasciclin II throughout embryogenesis, their muscle band pathways express fasciclin II only during the migratory period. Manipulations of fasciclin II in embryonic culture using blocking antibodies, recombinant fasciclin II fragments, and enzymatic removal of glycosyl phosphatidylinositol-linked fasciclin II produce concentration-dependent reductions in the extent of EP cell migration. These results support a novel role for fasciclin II, indicating that this homophilic adhesion molecule is required for the promotion or guidance of neuronal migration (Wright, 1999).

Still unresolved is the precise mechanism by which fasciclin II-mediated interactions affect neuronal motility. The simplest interpretation of the results presented in this paper is that the onset of MFas II expression within the newly formed muscle bands establishes a permissive substrate for the EP cells on the midgut, so that MFas II-mediated homophilic interactions between the neurons and the muscle bands lead to their migration along these pathways. This hypothesis is supported by observations that show that the muscle bands are both necessary and sufficient for neuronal migration, because the EP cells will not migrate onto the midgut musculature unless they are in direct contact with one of the muscle bands. Moreover, neither the application of anti-MFas II antibodies nor recombinant MFas II fragments results in a general dissociation of the premigratory packet of EP cells; rather, these treatments appear to interfere specifically with the formation of homophilic interactions between the neurons and the newly formed muscle bands. However, manipulations of MFas II in culture also perturb the coalescence of the muscle bands themselves, suggesting that fasciclin II may play a role in the differentiation of the bands from the visceral mesoderm. Although the EP cells will migrate in culture onto the muscle band cells even when the bands have not completely coalesced, it is possible that the experimental manipulations of MFas II affect migration indirectly by preventing the normal differentiation of their migratory pathways. Although the data support a role for MFas II in guiding the EP cells along the muscle bands once these pathways have formed, an additional process must stimulate the preferential release of fasciclin II-dependent adhesive interactions within the premigratory cluster in order for migration to occur. Following the migratory period, the fibers continue to extend axonal processes along the same muscle band pathways on the midgut. Intriguingly, this behavioral transition from migration to axon outgrowth coincides with the disappearance of MFas II expression from the muscle bands, although MFas II levels within the axons of the EP cells remain high. It is possible that the termination of fasciclin II expression in the muscle bands precludes further migration by the neurons, while other guidance cues associated with the muscle bands (such as neuroglian) continue to support axonal outgrowth (Wright, 1999).

Neuorfascin, a member of the L1 family of ankyrin-binding cell adhesion molecules, is a substrate for protein tyrosine kinase(s) and phosphatase(s); it identifies the highly conserved FIGQY tyrosine in the cytoplasmic domain as the principal site of phosphorylation, and demonstrates that phosphorylation of the FIGQY tyrosine abolishes ankyrin-binding activity. Neurofascin expressed in neuroblastoma cells is subject to tyrosine phosphorylation after activation of tyrosine kinases by NGF or bFGF or inactivation of tyrosine phosphatases with vanadate or dephostatin. Furthermore, both neurofascin and the related molecule Nr-CAM are tyrosine phosphorylated in a developmentally regulated pattern in rat brain. The FIGQY sequence is present in the cytoplasmic domains of all members of the L1 family of neural cell adhesion molecules. Phosphorylation of the FIGQY tyrosine abolishes ankyrin binding, as determined by coimmunoprecipitation of endogenous ankyrin and in vitro ankyrin-binding assays. Measurements of fluorescence recovery after photobleaching demonstrate that phosphorylation of the FIGQY tyrosine also increases lateral mobility of neurofascin expressed in neuroblastoma cells to the same extent as removal of the cytoplasmic domain. Ankyrin binding, therefore, appears to regulate the dynamic behavior of neurofascin and is the target for regulation by tyrosine phosphorylation in response to external signals. These findings suggest that tyrosine phosphorylation at the FIGQY site represents a highly conserved mechanism, used by the entire class of L1-related cell adhesion molecules, for regulation of ankyrin-dependent connections to the spectrin skeleton (Garver, 1997).

During development of the primary olfactory projection, olfactory receptor axons must sort by odor specificity and seek particular sites in the brain in which to create odor-specific glomeruli. In the moth Manduca sexta, Fasciclin II, a cell adhesion molecule in the immunoglobulin superfamily, is expressed by the axons of a subset of olfactory receptor neurons during development and, in a specialized glia-rich 'sorting zone,' these axons segregate from non-Fasciclin II-expressing axons before entering the neuropil of the glomerular layer. The segregation into Fasciclin II-positive fascicles is dependent on the presence of the glial cells in the sorting zone. The expression patterns for different isoforms of Manduca Fasciclin II in the developing olfactory system has been explored. Olfactory receptor axons express transmembrane Fasciclin II during the period of axonal ingrowth and glomerulus development. Fascicles of TM-Fasciclin II+ axons target certain glomeruli and avoid others, such as the sexually dimorphic glomeruli. These results suggest that TM-Fasciclin II may play a role in the sorting and guidance of the axons. GPI-linked forms of Fasciclin II are expressed weakly by glial cells associated with the receptor axons before they reach the sorting zone, but not by sorting-zone glia. GPI-Fasciclin II may, therefore, be involved in axon-glia interactions related to stabilization of axons in the nerve, but probably not related to sorting (Higgins, 2002).

These results led to the hypothesis that the two isoforms of Fas II have distinct functions in formation of the olfactory pathway in Manduca. In the developing olfactory system, just as was seen in the developing embryonic enteric system, GPI-Fas II may serve an adhesive function between cells of the perineurial sheath. In the olfactory system, it may also play a role in axon-glia interactions in the antennal nerve. That role is unlikely to be essential to growth, since the glia that express GPI-linked Fas II migrate into the antennal nerve from the antenna after many olfactory receptor axons have already reached their target glomeruli; perhaps GPI-Fas II participates in some sort of glial stabilization of the axons in the nerve. TM-Fas II, found almost exclusively on olfactory axons that extend from antennal nerve rootlets to target glomeruli, probably serves a guidance function by allowing axons destined to terminate in a subset of 14-21 glomeruli to recognize and fasciculate with each other. The first axons to navigate a path to each incipient glomerulus site presumably would use other cues to find those sites. Subsequent axons would use Fas II and other adhesion molecules, in various combinations, to assort into molecularly specified fascicles specific for the 63 individual glomeruli that they form (Higgins, 2002).

L1 homologs in C. elegans

The L1 family of cell adhesion molecules is predominantly expressed in the nervous system. Mutations in human L1 cause neuronal diseases such as HSAS, MASA, and SPG1. sax-7 gene encodes an L1 homologue in Caenorhabditis elegans. In sax-7 mutants, the organization of ganglia and positioning of neurons are abnormal in the adult stage, but these abnormalities are not observed in early larval stage. Misplacement of neurons in sax-7 mutants is triggered by mechanical force linked to body movement. Short and long forms of SAX-7 exhibited strong and weak homophilic adhesion activities in in vitro aggregation assay, respectively, which correlates with their different activities in vivo. SAX-7 is localized on plasma membranes of neurons in vivo. Expression of SAX-7 only in a single neuron in sax-7 mutants cell-autonomously restores the neuron's normal neuronal position. Expression of SAX-7 in two different head neurons in sax-7 mutants leads to the forced attachment of these neurons. It is proposed that both homophilic and heterophilic interactions of SAX-7 are essential for maintenance of neuronal positions in organized ganglia (Sasakura, 2005 ).

L1 Acts as a Ligand for Integrins

A single immunoglobulin-like domain of the human neural cell adhesion molecule L1 supports adhesion by multiple vascular and platelet integrins (see Drosophila Myospheroid). The L1 has been shown to function as a homophilic ligand in a variety of dynamic neurological processes. The sixth immunoglobulin-like domain of human L1 (L1-Ig6) can function as a heterophilic ligand for multiple members of the integrin superfamily, including alphavbeta3, alphavbeta1, alpha5beta1, and alphaIIbbeta3. The interaction between L1-Ig6 and alphaIIbbeta3 is found to support the rapid attachment of activated human platelets, whereas a corresponding interaction with alphavbeta3 and alphavbeta1 supports the adhesion of umbilical vein endothelial cells. Mutation of the single Arg-Gly-Asp (RGD) motif in human L1-Ig6 effectively abrogates binding by the aforementioned integrins. An L1 peptide containing this RGD motif and corresponding flanking amino acids (PSITWRGDGRDLQEL) effectively blocks L1 integrin interactions and, as an immobilized ligand, supports adhesion via alphavbeta3, alphavbeta1, alpha5beta1, and alphaIIbbeta3. Whereas beta3 integrin binding to L1-Ig6 is evident in the presence of either Ca2+, Mg2+, or Mn2+, a corresponding interaction with the beta1 integrins is only observed in the presence of Mn2+. Furthermore, such Mn2+-dependent binding by alpha5beta1 and alphavbeta1 is significantly inhibited by exogenous Ca2+. These findings suggest that physiological levels of calcium will impose a hierarchy of integrin binding to L1 such that alphavbeta3 or active alphaIIbbeta3 > alphavbeta1 > alpha5beta1. Given that L1 can interact with multiple vascular or platelet integrins it is significant that de novo L1 expression on blood vessels is associated with certain neoplastic or inflammatory diseases. Together these findings suggest an expanded and novel role for L1 in vascular and thrombogenic processes (Felding-Habermann, 1997).

L1 associates with neuropilin-1 to form a receptor for Sema3A

Mutations in the L1 gene induce a spectrum of human neurological disorders due to abnormal development of several brain structures and fiber tracts. Among its binding partners, L1 immunoglobulin superfamily adhesion molecule (Ig CAM) associates with neuropilin-1 (NP-1) to form a semaphorin3A (Sema3A) receptor and soluble L1 converts Sema3A-induced axonal repulsion into attraction. Using L1 constructs containing missense pathological mutations, it has been shown that this reversion is initiated by a specific trans binding of L1 to NP-1, but not to L1 or other Ig CAMs. This binding leads to activation of the NO/cGMP pathway. The L1-NP-1-binding site has been identified in a restricted sequence of L1 Ig domain 1, since a peptide derived from this region could reverse Sema3A repulsive effects. A pathological L1 missense mutation located in this sequence specifically disrupts both L1-NP-1 complex formation and Sema3A reversion, suggesting that the cross-talk between L1 and Sema3A might participate in human brain development (Castellani, 2002).

NCAM is a receptor for GDNF family ligands

Intercellular communication involves either direct cell-cell contact or release and uptake of diffusible signals, two strategies mediated by distinct and largely nonoverlapping sets of molecules. The neural cell adhesion molecule NCAM can function as a signaling receptor for members of the glial cell line-derived neurotrophic factor (GDNF) ligand family. Association of NCAM with GFRalph1, a GPI-anchored receptor for GDNF, downregulates NCAM-mediated cell adhesion and promotes high-affinity binding of GDNF to p140NCAM, resulting in rapid activation of cytoplasmic protein tyrosine kinases Fyn and FAK in cells lacking RET, a known GDNF signaling receptor. GDNF stimulates Schwann cell migration and axonal growth in hippocampal and cortical neurons via binding to NCAM and activation of Fyn, but independently of RET. These results uncover an unexpected intersection between short- and long-range mechanisms of intercellular communication and reveal a pathway for GDNF signaling that does not require the RET receptor (Paratcha, 2003).

Signalling by Mammalian N-CAM and L1 from the membrane

The neural cell adhesion molecule (N-CAM) is a homophilic cell adhesion protein that influences neurite outgrowth. A model for N-CAM homophilic binding has been proposed in which the Ig domains bind in a pairwise antiparallel manner such that Ig I binds Ig V, Ig II binds Ig IV, and Ig III binds Ig III. Astrocyte proliferation induced by growth factors such as basic fibroblast growth factor (bFGF) is inhibited by N-CAM. This inhibition is partially reversed by the glucocorticoid antagonist RU-486, suggesting that N-CAM signaling might activate the glucocorticoid receptor. Signaling after N-CAM binding in neurons has been examined in neurite outgrowth assays. It has been proposed that N-CAM signaling occurs through the cis interaction of N-CAM with the FGF receptor and intracellular pathways stimulated by the FGF receptor. It is therefore important to assess whether signaling after N-CAM binding involves both the glucocorticoid receptor and FGF receptor pathways or whether there may be different pathways involved, depending on cell type or specific cellular events (Krushel,1998).

N-CAM inhibits astrocyte proliferation in vitro and in vivo, and this effect is partially reversed by the glucocorticoid antagonist RU-486. The present studies have tested the hypothesis that N-CAM-mediated inhibition of astrocyte proliferation is caused by homophilic binding and involves the activation of glucocorticoid receptors. It was observed that all N-CAM Ig domains inhibit astrocyte proliferation in parallel with their ability to influence N-CAM binding. At an intermediate concentration of the Ig domains there is a distinct difference in the ability of Ig domain fragments to inhibit proliferation. The proliferation of other N-CAM-expressing cells also is inhibited by the addition of affinity chromatography purified N-CAM. In contrast, the proliferation of astrocytes from knockout mice lacking N-CAM is not inhibited by added N-CAM. These findings support the hypothesis that it is binding of soluble N-CAM to N-CAM on the astrocyte surface that leads to decreased proliferation. Signaling pathways stimulated by growth factors include activation of mitogen-activated protein (MAP) kinase. Addition of N-CAM inhibits MAP kinase activity induced by basic fibroblast growth factor in astrocytes. In accord with previous findings, that RU-486 can partially prevent the proliferative effects of N-CAM, inhibition of MAP kinase activity by N-CAM is reversed by RU-486. MAP kinase activity in astrocytes is increased over 4-fold after bFGF treatment. However, when N-CAM is added simultaneously with bFGF, MAP kinase activity is reduced to 46% of the value stimulated by bFGF alone. This inhibitory effect on MAP kinase activity is completely reversed if the glucocorticoid antagonist RU486 is included with bFGF and N-CAM. The addition of N-CAM alone produces a small, but reproducible, decrease in basal MAP kinase activity, whereas the addition of RU486 alone has little or no effect. These results suggest that soluble N-CAM inhibits growth factor-induced MAP kinase activity and that this inhibition requires activation of the glucocorticoid receptor (Krushel, 1998).

The ability of N-CAM to promote neurite outgrowth in neurons has been postulated to occur through cis binding of N-CAM to a segment of the FGF receptor called the CAM homology domain. This conclusion was based on the observation that the presence of a 20-aa synthetic peptide corresponding to this domain could inhibit N-CAM-dependent neurite outgrowth. Proliferation stimulated by bFGF in astrocytes is reduced in the presence of N-CAM. The effects of the FGF receptor peptide was tested on the ability of N-CAM to affect astrocyte proliferation. Inclusion of N-CAM with either the FGF receptor peptide or a control peptide with the same amino acids in a random order reduces the amount of [3H]thymidine incorporation to levels equivalent to those of N-CAM alone, although addition of either peptide alone results in a slight decrease in proliferation. These results suggest that the ability of N-CAM to inhibit astrocyte proliferation is not likely to occur through direct interactions of N-CAM with the FGF receptor. Together, these findings indicate that homophilic N-CAM binding leads to inhibition of astrocyte proliferation via a pathway involving the glucocorticoid receptor and that the ability of N-CAM to influence astrocyte proliferation and neurite outgrowth involves different signal pathways (Krushel, 1998).

To examine the neural function of Csk (C-terminal Src kinase), a membrane-targeted form of Csk (Src/Csk) and its kinase-defective variant (DK-Src/Csk) were both expressed in the embryonic carcinoma cell line P19. Expression of Src/Csk, but not DK-Src/Csk, causes reduction of the specific activities of Src and Fyn in the differentiated P19 cells. During neural differentiation, the specific activity of Src is elevated in the control P19 cells, whereas the activation is completely eliminated in the Src/Csk transfectant. In normally differentiated P19 cells, cross-linking of a cell adhesion molecule, L1, induces a short-term activation of Src and Fyn. In the Src/Csk transfectant, L1 stimulation induces delayed activation of Src and Fyn peaking at much lower levels than in the control cells. Src/Csk transfectants develop normally in the initial stages of neural differentiation, but exhibit an apparent defect in cell-to-cell interaction (neurite fasciculation and aggregation of cell bodies) in the later stages. These findings imply that Csk is involved in the regulation of Src family kinases that play roles in cell-to-cell interaction mediated by cell adhesion molecules (Takayama, 1997).

Axonal growth cones respond to adhesion molecules and to extracellular matrix components by rapid morphological changes and growth rate modification. Neurite outgrowth mediated by the neural cell adhesion molecule (NCAM) requires the src family tyrosine kinase p59(fyn) in nerve growth cones, but the molecular basis for this interaction has not been defined. The NCAM140 isoform, which is found in migrating growth cones, selectively co-immunoprecipitates with p59(fyn) from nonionic detergent extracts of early postnatal mouse cerebellum and transfected rat B35 neuroblastoma and COS-7 cells. Whereas p59(fyn) is constitutively associated with NCAM140, the focal adhesion kinase p125(fak), a nonreceptor tyrosine kinase known to mediate integrin-dependent signaling, becomes recruited to the NCAM140-p59(fyn) complex when cells are reacted with antibodies against the extracellular region of NCAM. Treatment of cells with a soluble NCAM fusion protein or with NCAM antibodies causes a rapid and transient increase in tyrosine phosphorylation of p125(fak) and p59(fyn). These results suggest that NCAM140 binding interactions at the cell surface induce the assembly of a molecular complex of NCAM140, p125(fak), and p59(fyn) and activate the catalytic function of these tyrosine kinases, initiating a signaling cascade that may modulate growth cone migration (Beggs, 1997).

The cell adhesion molecules (CAMs) NCAM, N-cadherin (see Drosophila Cadherin-N), and L1 are homophilic binding molecules that stimulate axonal growth. It has been postulated that the above CAMs can stimulate axonal growth by activating the fibroblast growth factor receptor (FGFR) in neurons (See Drosophila Breathless). Activation of NCAM and L1 can lead to phosphorylation of the FGFR. Both this and the neurite outgrowth response stimulated by all three of the above CAMs are lost when a kinase-deleted, dominant negative form of FGFR1 is expressed in PC12 cells. Transgenic mice have been generated that express the dominant negative FGFR under control of the neuron-specific enolase (NSE) promoter. Cerebellar neurons isolated from these mice have also lost their ability to respond to NCAM, N-cadherin, and L1. A peptide inhibitor of phospholipase C gamma (PLCgamma) that inhibits neurite outgrowth stimulated by FGF also inhibits neurite outgrowth stimulated by the CAMs. It is concluded that activation of the FGFR is both necessary and sufficient to account for the ability of the above CAMs to stimulate axonal growth, and that PLCgamma is a key downstream effector of this response (Saffell, 1997).

L1 is a neural cell adhesion molecule that has been shown to help guide nascent axons to their targets. This guidance is based on specific interactions of L1 with its binding partners and is likely to involve signaling cascades that alter cytoskeletal elements in response to these binding events. The phosphorylation of L1 and the role it may have in L1-directed neurite outgrowth has been examined. The phosphorylation site has been determined to be Ser1152. The L1 kinase activity from PC12 cells that phosphorylates this site co-elutes with the S6 kinase, p90(rsk). Moreover, S6 kinase activity and p90(rsk) immunoreactivity co-immunoprecipitate with L1 from brain. The phosphorylation site is located in a region of high conservation between mammalian L1 sequences as well as L1-related molecules in vertebrates from fish to birds. Neurons were loaded with peptides that encompass the phosphorylation site, as well as the flanking regions, and their effects on neurite outgrowth were observed. The peptides, which include Ser1152, inhibit neurite outgrowth on L1 but not on a control substrate, laminin. These data demonstrate that the membrane-proximal 15 amino acids of the cytoplasmic domain of L1 are important for neurite outgrowth on L1, and the interactions it mediates may be regulated by phosphorylation of Ser1152 (Wong, 1996a).

Casein kinase II (CKII), a ubiquitous serine/threonine kinase enriched in brain is able to phosphorylate recombinant L1 cytoplasmic domain (L1CD), which consists of residues 1,144-1,257. CKII is able to phosphorylate a peptide encompassing amino acids 1,173-1,185, as well as a related peptide representing an alternatively spliced nonneuronal L1 isoform that lacks amino acids 1,177-1,180. Serine to alanine substitutions in these peptides indicate that the CKII phosphorylation site is at Ser1,181. When L1 immunoprecipitates are used to phosphorylate L1CD, one of the residues phosphorylated is the same residue phosphorylated by CKII. Ser1,181 is found to be phosphorylated in newborn rat brain. These data show that CKII is associated with and able to phosphorylate L1. This phosphorylation may be important in regulating certain aspects of L1 function, such as adhesivity or signal transduction (Wong, 1996b).

Cell-cell interactions mediated via cell adhesion molecules (CAMs) are dynamically regulated during nervous system development. One mechanism to control the amount of cell surface CAMs is to regulate their recycling from the plasma membrane. The L1 subfamily of CAMs has a highly conserved cytoplasmic domain that contains a tyrosine, followed by the alternatively spliced RSLE (Arg-Ser-Leu-Glu) sequence. The resulting sequence of YRSL conforms to a tyrosine-based sorting signal that mediates the clathrin-dependent endocytosis of signal-bearing proteins. L1 associates in rat brain with AP-2, a clathrin adaptor that captures plasma membrane proteins that contain tyrosine-containing motifs, thus aiding endocytosis of these proteins by coated pits. In vitro assays demonstrate that this interaction occurs via the YRSL sequence of L1 and the mu 2 chain of AP-2. In L1-transfected 3T3 cells, L1 endocytosis is blocked by dominant-negative dynamin that specifically disrupts clathrin-mediated internalization. Furthermore, endocytosed L1 colocalizes with the transferrin receptor (TfR), a marker for clathrin-mediated internalization. Mutant forms of L1 that lack the YRSL do not colocalize with TfR, indicating that the YRSL mediates endocytosis of L1. In neurons, L1 is endocytosed preferentially at the rear of axonal growth cones, colocalizing with Eps15, another marker for the clathrin endocytic pathway. These results establish a mechanism by which L1 can be internalized from the cell surface and suggest that an active region of L1 endocytosis at the rear of growth cones is important in L1-dependent axon growth (Kamiguchi, 1998b).

Glycosylation of CAMs

To identify neuronal cell surface glycoproteins in the Drosophila embryo, antisera against horseradish peroxidase (HRP) was used to recognize a carbohydrate epitope that is selectively expressed in the insect nervous system. A large number of neuronal glycoproteins (denoted "HRP proteins") apparently bear the HRP carbohydrate epitope. Polyclonal anti-HRP antibodies were used to purify these proteins from Drosophila embryos. Three major HRP proteins are Neurotactin, Fasciclin I, and an R-PTP, DPTP69D. Western blotting data suggest that Fasciclin II, Neuroglian, DPTP10D, and DPTP99A are also HRP proteins (Desai, 1994).

LeechCAM is an NCAM/FasII/ApCAM homolog, whereas Tractin, another Ig superfamily member, is a cleaved protein with several unique features, including a PG/YG repeat domain that may be part of or interact with the extracellular matrix. Tractin and LeechCAM are widely expressed neural proteins that are differentially glycosylated in sets and subsets of peripheral sensory neurons. These neurons form specific fascicles in the central nervous system. In vivo antibody perturbation of the Lan3-2 glycoepitope demonstrates that it can selectively regulate extension of neurites and filopodia. It is thought that local interactions involving the Lan3-2 epitope may be regulating defasciculation of sensillar neurons and facilitating the segregation of the axons into specific tracts. Thus, these experiments provide evidence that differential glycosylation can confer functional diversity and specificity to widely expressed neural proteins. The interaction between surface oligosaccharides and carbohydrate binding proteins could mediate pathfinding aspects of axonogenesis (Huang, 1997).

The mossy fiber axons of both the developing and adult dentate gyrus express the highly polysialylated form of neural cell adhesion molecule (NCAM) as they innervate the proximal apical dendrites of pyramidal cells in the CA3 region of the hippocampus. The present study used polysialic acid (PSA)-deficient and NCAM mutant mice to evaluate the role of PSA in mossy fiber development. The results indicate that removal of PSA by either specific enzymatic degradation or mutation of the NCAM-180 isoform that carries PSA in the brain causes an aberrant and persistent innervation of the pyramidal cell layer by mossy fibers, including excessive collateral sprouting and/or defasciculation of these processes, as well as formation of ectopic mossy fiber synaptic boutons. These results are considered in terms of two possible effects of PSA removal: an increase in the number of mossy fibers that can grow into the pyramidal cell layer and an inhibition of process retraction by formation of stable junctions including synapses. Because these defects on granule cells in the adult animal and PSA-positive granule cells continue to be produced in the mature brain, the present findings may be relevant to previous studies suggesting that PSA-NCAM function is required for long-term potentiation, long-term depression, and learning behaviors associated with hippocampus (Seki, 1998).

Sensory afferents in the leech are labeled with both constitutive and developmentally regulated glycosylations (markers) of their cell adhesion molecules (CAMs). A constitutive mannose marker borne by these cells, recognized by Lan3-2 monoclonal antibody (mAb), mediates the formation of diffuse central arbors in these neurons. At the ultrastructural level, the arbors consist of large, loosely organized axons rich with filopodia and synaptic vesicles. Perturbing the mannose-specific adhesion of this first targeting step leads to a gain in cell-cell contact but a loss of filopodia and synaptic vesicles. During the second targeting step, galactose markers divide afferents into different subsets. A subset is labeled by the marker recognized by Laz2-369 mAb. Initially, the galactose marker appears where afferents contact central neurons. Subsequently it spreads proximally and distally, covering the entire afferent surface. Afferents now gain cell-cell contact, with central neurons and self-similar afferents, but lose filopodia and synaptic vesicles. Extant synaptic vesicles prevail where afferents are apposed to central neurons. These neurons develop postsynaptic densities and en passant synapses are formed. Perturbing the galactose-specific adhesion of this second targeting step causes a loss of cell-cell contact but a gain in filopodia and synaptic vesicles, essentially returning afferents to the first targeting step. The transformation of afferent growth, progressing from mannose- to galactose-specific adhesion, is consistent with a change from cell-matrix to cell-cell adhesion. By performing opposing functions in a temporal sequence, constitutive and developmentally regulated glycosylations of CAMs collaborate in the synaptogenesis of afferents and the consolidation of self-similar afferents (Tai, 1999).

Functional coupling of L1 and Ankyrin

Fasciclin 2 Evolutionary homologs: Continued part 2/3 | part 3/3

Fasciclin 2: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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