Alternative splicing of the cytoplasmic domain of neural cell adhesion molecule alters its ability to act as a substrate for neurite outgrowth

Alternative splicing switches: Important players in cell differentiation

Alternative splicing (AS) greatly expands the coding capacities of genomes by allowing the generation of multiple mature mRNAs from a limited number of genes. Although the massive switch in AS profiles that often accompanies variations in gene expression patterns occurring during cell differentiation has been characterized for a variety of models, their causes and mechanisms remain largely unknown. Here, we integrate foundational and recent studies indicating the AS switches that govern the processes of cell fate determination. We include some distinct AS events in pluripotent cells and somatic reprogramming and discuss new progresses on alternative isoform expression in adipogenesis, myogenic differentiation and stimulation of immune cells. Finally, we cover novel insights on AS mechanisms during neuronal differentiation, paying special attention to the role of chromatin structure.

Intragenic epigenetic changes modulate NCAM alternative splicing in neuronal differentiation

Alternative splicing contributes to cell type-specific transcriptomes. Here, we show that changes in intragenic chromatin marks affect NCAM (neural cell adhesion molecule) exon 18 (E18) alternative splicing during neuronal differentiation. An increase in the repressive marks H3K9me2 and H3K27me3 along the gene body correlated with inhibition of polymerase II elongation in the E18 region, but without significantly affecting total mRNA levels. Treatment with the general DNA methylation inhibitor 5-azacytidine and BIX 01294, a specific inhibitor of H3K9 dimethylation, inhibited the differentiation-induced E18 inclusion, pointing to a role for repressive marks in sustaining NCAM splicing patterns typical of mature neurons. We demonstrate that intragenic deployment of repressive chromatin marks, induced by intronic small interfering RNAs targeting NCAM intron 18, promotes E18 inclusion in undifferentiated N2a cells, confirming the chromatin changes observed upon differentiation to be sufficient to induce alternative splicing. Combined with previous evidence that neuronal depolarization causes H3K9 acetylation and subsequent E18 skipping, our results show how two alternative epigenetic marks regulate NCAM alternative splicing and E18 levels in different cellular contexts.

The diacylglycerol lipases: structure, regulation and roles in and beyond endocannabinoid signalling

The diacylglycerol lipases (DAGLs) hydrolyse diacylglycerol to generate 2-arachidonoylglycerol (2-AG), the most abundant ligand for the CB(1) and CB(2) cannabinoid receptors in the body. DAGL-dependent endocannabinoid signalling regulates axonal growth and guidance during development, and is required for the generation and migration of new neurons in the adult brain. At developed synapses, 2-AG released from postsynaptic terminals acts back on presynaptic CB(1) receptors to inhibit the secretion of both excitatory and inhibitory neurotransmitters, with this DAGL-dependent synaptic plasticity operating throughout the nervous system. Importantly, the DAGLs have functions that do not involve cannabinoid receptors. For example, 2-AG is the precursor of arachidonic acid in a pathway that maintains the level of this essential lipid in the brain and other organs. This pathway also drives the cyclooxygenase-dependent generation of inflammatory prostaglandins in the brain, which has recently been implicated in the degeneration of dopaminergic neurons in Parkinson's disease. Remarkably, we still know very little about the mechanisms that regulate DAGL activity-however, key insights can be gleaned by homology modelling against other α/β hydrolases and from a detailed examination of published proteomic studies and other databases. These identify a regulatory loop with a highly conserved signature motif, as well as phosphorylation and palmitoylation as post-translational mechanisms likely to regulate function.

Low-level lead exposure attenuates the expression of three major isoforms of neural cell adhesion molecule

Toxic lead (Pb) exposure poses serious risks to human health, especially to children at developmental stages, even at low exposure levels. Neural cell adhesion molecule (NCAM) is considered to be a potential early target in the neurotoxicity of Pb due to its role in cell adhesion, neuronal migration, synaptic plasticity, and learning and memory. However, the effect of low-level Pb exposure on the specific expression of NCAM isoforms has not been reported. In the present study, we found that Pb could concentration-dependently (1-100 nM) inhibit the expression of three major NCAM isoforms (NCAM-180, -140, and -120) in primary cultured hippocampal neurons. Furthermore, it was verified that levels of all three major isoforms of NCAM were reduced by Pb exposure in human embryonic kidney (HEK)-293 cells transiently transfected with NCAM-120, -140, or -180 isoform cDNA constructs. In addition, low-level Pb exposure delayed the neurite outgrowth and reduced the survival rate of cultured hippocampal neurons at different time-points. Together, our results demonstrate that developmental low-level Pb exposure can attenuate the expression of all three major NCAM isoforms, which may contribute to the observed Pb-mediated neurotoxicity.

Morphoregulation by acetylcholinesterase in fibroblasts and astrocytes

Acetylcholinesterase (AChE) terminates neurotransmission at cholinergic synapses by hydrolysing acetylcholine, but also has non-enzymatic morphoregulatory effects on neurons such as stimulation of neurite outgrowth. It is widely expressed outside the nervous system, but its function in non-neuronal cells is unclear. Here we have investigated the distribution and function of AChE in fibroblasts and astrocytes. We show that these cells express high levels of AChE protein that co-migrates with recombinant AChE but contains little catalytic activity. Fibroblasts express transcripts encoding the synaptic AChE-T isoform and its membrane anchoring peptide PRiMA-I. AChE is strikingly distributed in arcs, rings and patches at the leading edge of spreading and migrating fibroblasts and astrocytes, close to the cell-substratum interface, and in neuronal growth cones. During in vivo healing of mouse skin, AChE becomes highly expressed in re-epithelialising epidermal keratinocytes 1 day after wounding. AChE appears to be functionally important for polarised cell migration, since an AChE antibody reduces substratum adhesion of fibroblasts, and slows wound healing in vitro as effectively as a beta1-integrin antibody. Moreover, elevation of AChE expression increases fibroblast wound healing independently of catalytic activity. Interestingly, AChE surface patches precisely co-localise with amyloid precursor protein and the extracellular matrix protein perlecan, but not focal adhesions or alpha-dystroglycan, and contain a high concentration of tyrosine phosphorylated proteins in spreading cells. These findings suggest that cell surface AChE, possibly in a novel signalling complex containing APP and perlecan, contributes to a generalised mechanism for polarised membrane protrusion and migration in all adherent cells.

Stress, cognitive impairment and cell adhesion molecules

Stress has profound effects on brain structure and function, but the underlying mechanisms are still poorly understood. Recent studies imply that neuronal cell adhesion molecules of the immunoglobulin superfamily--NCAM and L1--are important mediators of the effects of stress on the brain. Chronic stress regimes that lead to hippocampal atrophy and spatial-learning impairment in rodents simultaneously induce a pattern of changes in cell adhesion molecule expression that fits with a role for these molecules in stress-induced neuronal damage and neuroprotective mechanisms. These findings highlight cell adhesion molecules as potential therapeutic targets to treat stress-related cognitive disturbances.

Gene expression profiling reveals multiple novel intrinsic and extrinsic factors associated with axonal regeneration failure

In contrast to the regeneration-competent peripheral nervous system (PNS), lesions of nerve tracts within the central nervous system (CNS) lead to chronically impaired neuronal connections. We have analysed changes in gene expression patterns occurring as a consequence of postcommissural fornix transection at a time when spontaneous axonal growth has ceased at the lesion site. This was done in order to describe both extrinsic and intrinsic determinants of regeneration failure. Using a genomic approach we have identified a number of so far undetected factors such as bamacan and semaphorin 6B, which relate to chronic axonal growth arrest and therefore are promising candidates for lesion-induced axonal growth inhibitors. In addition, we observed that within the subiculum, where the fornix axons originate, neuronal Oct-6 was induced and NG2 was down-regulated, indicating that axotomized neurons as well as glial cells react at the level of gene expression to remote axotomy.

Neuronal differentiation and protection from nitric oxide-induced apoptosis require c-Jun-dependent expression of NCAM140

c-Jun, a crucial component of the dimeric transcription factor activating protein 1 (AP-1), can regulate apoptosis induced by oxidative stress and has been implicated in neuronal differentiation, but the mechanisms are largely unknown. We found that specific inhibition of transcription or stable transfection with cDNA encoding dominant-negative c-Jun sensitized SH-SY5Y neuroblastoma cells (TAM-67 cells) to apoptosis induced by the nitric oxide (NO) donor sodium nitroprusside or SIN-1. TAM-67 cells also became refractory to nerve growth factor (NGF)-induced neuronal differentiation. Dominant-negative c-Jun abolished expression of a 140-kDa neural cell adhesion molecule (NCAM140) and dramatically enhanced the expression of NCAM180 in TAM-67 cells. Inhibition of c-Jun in TAM-67 cells also resulted in a corresponding decrease in the amount of NCAM140 mRNA and an increase in the amount of NCAM180 mRNA. Reexpression of NCAM140 in TAM-67 cells restored NGF-induced neuronal differentiation and resistance to NO-induced apoptosis. Our results show that c-Jun/AP-1, through up-regulation of NCAM140, plays an important role in both NGF-induced neuronal differentiation and resistance to apoptosis induced by NO in neuroblastoma cells. As NCAM140 and NCAM180 are translated from differentially spliced mRNAs transcribed from the same gene, alternative splicing of NCAM pre-mRNA (and consequently the synthesis of the smaller NCAM140 species) appears to be regulated by c-Jun/AP-1.

NCAM regulates cell motility

Cell migration is required during development of the nervous system. The regulatory mechanisms for this process, however, are poorly elucidated. We show here that expression of or exposure to the neural cell adhesion molecule (NCAM) strongly affected the motile behaviour of glioma cells independently of homophilic NCAM interactions.

Expression of the transmembrane 140 kDa isoform of NCAM (NCAM-140) caused a significant reduction in cellular motility, probably through interference with factors regulating cellular attachment, as NCAM-140-expressing cells exhibited a decreased attachment to a fibronectin substratum compared with NCAM-negative cells. Ectopic expression of the cytoplasmic part of NCAM-140 also inhibited cell motility, presumably via the non-receptor tyrosine kinase p59fyn with which NCAM-140 interacts.

Furthermore, we showed that the extracellular part of NCAM acted as a paracrine inhibitor of NCAM-negative cell locomotion through a heterophilic interaction with a cell-surface receptor. As we showed that the two N-terminal immunoglobulin modules of NCAM, which are known to bind to heparin, were responsible for this inhibition, we presume that this receptor is a heparan sulfate proteoglycan. A model for the inhibitory effect of NCAM is proposed, which involves competition between NCAM and extracellular components for the binding to membrane-associated heparan sulfate proteoglycan.

Effects of chronic stress on contextual fear conditioning and the hippocampal expression of the neural cell adhesion molecule, its polysialylation, and L1

Chronic stress has been shown to induce time-dependent neurodegeneration in the hippocampus, ranging from a reversible damage to a permanent neuronal loss. This damage has been proposed to impair cognitive function in hippocampus-dependent learning tasks. In this study, we have used a 21-day restraint stress procedure in rats, previously reported to induce reversible atrophy of apical dendrites of CA3 pyramidal cells, to assess whether it may influence subsequent performance in the contextual fear conditioning task under experimental conditions involving high stress levels (1 mA shock intensity as the unconditioned stimulus). In addition, we were interested in the study of the possible cellular and molecular mechanisms involved in the reversible phase of neural damage. Cell adhesion molecules of the immunoglobulin superfamily, such as the neural cell adhesion molecule and L1, are cell-surface macromolecules that, through their recognition and adhesion properties, regulate cell-cell interactions and have been reported to play a key role in cognitive functioning. A second aim of this study was to evaluate whether chronic stress would modulate the expression of the neural cell adhesion molecule, its polysialylation, and L1 in the hippocampus. The results showed that chronic stress facilitated subsequent contextual fear conditioning. They also showed that chronically stressed rats displayed reduced hippocampal neural cell adhesion molecule, but increased polysialylated expression as well as a trend towards exhibiting increased L1 expression. In summary, these results support the view that a 21-day chronic stress regimen predisposes individuals to develop enhanced contextual fear conditioning responses. They also indicate that cell adhesion molecules might play a role in the structural remodelling that occurs in the hippocampus as a consequence of chronic stress exposure.

Polysialyltransferase ST8Sia II (STX) polysialylates all of the major isoforms of NCAM and facilitates neurite outgrowth

The neural cell adhesion molecule (NCAM) has different isoforms due to different sizes in its polypeptide and plays a significant role in neural development. In neural development, the function of NCAM is modified by polysialylation catalyzed by two polysialyltransferases, ST8Sia II and ST8Sia IV. Previously, it was reported by others that ST8Sia II polysialylates only transmembrane isoforms of the NCAM, such as NCAM-140 and NCAM-180, but not NCAM-120 and NCAM-125 anchored by a glycosylphosphotidylinositol. In the present study, we first discovered that ST8Sia II polysialylates all isoforms of the NCAM examined, and we demonstrated that polysialylation of NCAM expressed on 3T3 cells facilitates neurite outgrowth regardless of isoforms of NCAM, where polysialic acid is attached. We then show that neurite outgrowth is significantly facilitated only when polysialylated NCAM is present in cell membranes. Moreover, the soluble NCAM coated on plates did not have an effect on neurite outgrowth exerted by soluble L1 adhesion molecule coated on plates. These results, taken together, indicate that ST8Sia II plays critical roles in modulating the function of all major isoforms of NCAM. The results also support previous studies showing that a signal cascade initiated by NCAM differs from that initiated by L1 molecule.

Dysregulation of the neural cell adhesion molecule and neuropsychiatric disorders

Cell adhesion molecule proteins play a diverse role in neural development, signal transduction, structural linkages to extracellular and intracellular proteins, synaptic stabilization, neurogenesis, and learning. Three basic mRNA isoforms and potent posttranslational modifications differentially regulate these neurobiological properties of the neural cell adhesion molecule (N-CAM). Abnormal concentrations of N-CAM 105-115 kDa (cN-CAM), N-CAM variable alternative spliced exon (VASE), and N-CAM secreted exon (SEC) are related to schizophrenia and bipolar neuropsychiatric disorders. These N-CAM isoforms provide potential mechanisms for expression of multiple neurobiological alterations between controls and individuals with schizophrenia or bipolar illness. Multiple processes can trigger the dysregulation of N-CAM isoforms. Differences in neuropil volume, neuronal diameter, gray matter thickness, and ventricular size can be related to N-CAM neurobiological properties in neuropsychiatric disorders. Potential test of the N-CAM dysregulation hypothesis of neuropsychiatric disorder is whether ongoing dysregulation of N-CAM would cause cognitive impairments, increased lateral ventricle volume, and decreased hippocampal volume observed in schizophrenia and to a lesser extent in bipolar disorder. An indirect test of this theory conducted in animal experiments lend support to this N-CAM hypothesis. N-CAM dysregulation is consistent with a synaptic abnormality that could underlie the disconnection between brain regions consistent with neuroimaging reports. Synapse stability and plasticity may be part of the molecular neuropathology of these disorders.

Recovery of emotional behaviour in neural cell adhesion molecule (NCAM) null mutant mice through transgenic expression of NCAM180

In the present study we further investigate functions of the neural cell adhesion molecule (NCAM) in the mature central nervous system and its implications for animal behaviour. To this end we generated transgenic mice expressing the major NCAM isoform with the largest cytoplasmic domain, NCAM180, under control of a promoter for the small form neurofilament gene. Transgenic mice were also bred with mice deficient in endogenous NCAM (Ncam-/- mice) so that effects of NCAM180 could be analysed in the presence and absence of endogenous NCAM. While overexpression of transgenic NCAM180 was without apparent behavioural or morphological effect, its expression in Ncam-/- mice counteracted NCAM ablation-induced aggressive, anxiety-like and antidepressant-like behaviour. It furthermore prevented a hypersensitivity of Ncam-/- mice to the anxiolytic serotonin1A (5-HT1A) receptor agonist buspirone. Such recovery of emotional behaviour and behavioural 5-HT1A response occurred in spite of misdevelopment of the olfactory bulb and hippocampus that is characteristic of Ncam-/- mice, and without an apparent change in the expression of 5-HT1A binding sites in the brain. Hippocampus- and amygdala-dependent learning, though disturbed in Ncam-/- mice, remained unaffected by the transgenic NCAM180. We suggest an involvement of NCAM180-mediated cell recognition processes in the serotonergic modulation of emotional behaviour in adult mice.

Cellular signaling by neural cell adhesion molecules of the immunoglobulin superfamily

Neural cell adhesion molecules (CAMs) of the immunoglobulin superfamily nucleate and maintain groups of cells at key sites during early development and in the adult. In addition to their adhesive properties, binding of CAMs can affect intracellular signaling. Their ability to influence developmental events, including cell migration, proliferation, and differentiation can therefore result both from their adhesive as well as their signaling properties. This review focuses on the two CAMs for which the most information is known, the neural CAM, N-CAM, and L1. N-CAM was the first CAM to be characterized and, therefore, has been studied extensively. The binding of N-CAM to cells leads to a number of signaling events, some of which result in changes in gene expression. Interest in L1 derives from the fact that mutations in its gene lead to human genetic diseases including mental retardation. Much is known about modifications of the L1 cytoplasmic domain and its interaction with cytoskeletal molecules. The study of CAM signaling mechanisms has been assay-dependent rather than molecule-dependent, with particular emphasis on assays of neurite outgrowth and gene expression, an emphasis that is maintained throughout the review. The signals generated following CAM binding that lead to alterations in cell morphology and gene expression have been linked directly in only a few cases. We also review information on other CAMs, giving special consideration to those that are anchored in the membrane by a phospholipid anchor. These proteins, including a form of N-CAM, are presumed to be localized in lipid rafts, membrane substructures that include distinctive subsets of cytoplasmic signaling molecules such as members of the src-family of nonreceptor protein tyrosine kinases. In the end, these studies may reveal that what CAMs do after they bind cells together may have as profound consequences for the cells as the adhesive interactions themselves. This area will therefore remain a rich ground for future studies.

Elevated concentration of N-CAM VASE isoforms in schizophrenia

Neural cell adhesion molecule (N-CAM) is a cell recognition molecule, four major isoforms (180, 140, 120, and 105-115 kDa) of which are present in brain. N-CAM has several roles in cellular organization and CNS development. Previously we have found an elevation in CSF N-CAM 120 kDa in the CSF of patients with schizophrenia, bipolar disorder, and depression. We now report an increase in the variable alternative spliced exon (VASE), a 10 amino acid sequence inserted into the fourth N-CAM domain, in the CSF of patients with schizophrenia, but not in bipolar disorder or depression. VASE-immunoreactive (VASE-ir) bands were measured in CSF from patients with schizophrenia (n = 14), bipolar disorder I (n = 7), bipolar disorder II (n = 9), unipolar depression (n = 17) and matched controls (n = 37) by Western immunoblotting. Three VASE-ir bands were distinguished in lumbar CSF corresponding to heavy (165 kDa), medium (155 kDa) and low (140 kDa) MW. A logarithmic transformation was applied to the VASE protein units and analyzed with a MANOVA. There was a 51% and 45% increase in VASE heavy (p = 0.0008) and medium (p = 0.04) MW protein, respectively, in patients with schizophrenia as compared with normal controls. Current neuroleptic treatment in patients with schizophrenia had no effect on CSF VASE concentrations. VASE concentration correlated significantly with behavioral ratings in patients with schizophrenia but not affective disorders. Thus, VASE immunoreactivity is increased in schizophrenia but not in affective disorders. These results provide further evidence of an abnormality of N-CAM protein in chronic schizophrenia and suggest differences between schizophrenia and affective disorders in regulation of N-CAM.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

Structural mosaicism on the submicron scale in the plasma membrane

The lateral mobility of the neural cell adhesion molecule (NCAM) was examined using single particle tracking (SPT). Various isoforms of human NCAM, differing in their ectodomain, their membrane anchorage mode, or the size of their cytoplasmic domain, were expressed in National Institutes of Health 3T3 cells and C2C12 muscle cells. On a 6.6-s time scale, SPT measurements on both transmembrane and glycosylphosphatidylinositol (GPI) anchored isoforms of NCAM expressed in 3T3 cells could be classified into mobile (Brownian diffusion), slow diffusion, corralled diffusion, and immobile subpopulations. On a 90-s time scale, SPT studies in C2C12 cells revealed that 40-60% of transfected NCAM was mobile, whereas a smaller fraction (approximately 10-30%) experienced much slower diffusion. In addition, a fraction of approximately 30% of both transfected GPI and transmembrane isoforms and endogenous NCAM isoforms in C2C12 cells experienced transient confinement for approximately 8 s within regions of approximately 300-nm diameter. Diffusion within both these and the slow diffusion regions was anomalous, consistent with movements through a dense field of obstacles, whereas diffusion outside these regions was normal. Thus the membrane appears as a mosaic containing regions that permit free diffusion as well as regions in which NCAM is transiently confined to small or more extended domains. These results, including a large, freely diffusing fraction, similar confinement of transmembrane and GPI isoforms, a significant slowly diffusing fraction, and relatively large interdomain distances, are at some variance with the membrane skeleton fence model (Kusumi and Sako, 1996). Possible revisions to the model that incorporate these data are discussed.

Cellular determinants of the lateral mobility of neural cell adhesion molecules

The lateral mobility of the neural cell adhesion molecule (NCAM) was examined using fluorescence recovery after photobleaching (FRAP). Various isoforms of human NCAM, differing in their ectodomain, their membrane anchorage mode or in the size of their cytoplasmic domain, were expressed in NIH 3T3 cells and C2C12 muscle cells. When the various isoforms were compared in 3T3 cells, FRAP studies showed both GPI-anchored and transmembrane isoforms diffused rapidly and only small differences in either the diffusion coefficients (D) or the mobile fractions (mf) were measured, suggesting the importance of the ectodomain in regulating lateral diffusion. However, the mobility of all NCAM isoforms was greatly reduced in regions of cell-cell contact, presumably due to homophilic trans interactions between NCAMs on adjacent cells. NCAM isoforms transfected into C2C12 cells which express NCAM naturally usually displayed a significantly lower D compared to the same isoforms transfected into 3T3 cells. Thus, NCAM lateral mobility is modulated in regions where cells interact and by the structure of the host cell membrane.

Effects of tenascin-C in cultured hippocampal astrocytes: NCAM and fibronectin immunoreactivity changes

Tenascin-C is an extracellular matrix glycoprotein with trophic and repulsive properties on neuronal cells, involved in migratory processes of immature neurons. Previous reports demonstrated that this molecule is produced and secreted by astrocytes, in vitro after activation by bFGF or in vivo after CNS lesion. In injured brain the expression of tenascin-C has been correlated with the glial reaction since it was observed in regions suffering a dramatic glial proliferation and hypertrophy. In this report we show that the treatment of cultured hippocampal astrocytes with tenascin-C results in an increased fibronectin and NCAM immunoreactivities. In addition, treated astrocytes form longer extensions than control ones. The number of cells as well as the levels of GFAP mRNA and protein immunoreactivity are not modified after tenascin-C treatment. The present changes may, therefore, be related to the modification of the adhesive properties of astrocytes to the substrate. These observations are compatible with the hypothesis that tenascin-C may contribute to the glial scarring process.

Neural cell adhesion molecules in activity-dependent development and synaptic plasticity

Cell adhesion molecules (CAMs) have a vital role in forming connections between neurons during embryonic development. Increasing evidence suggests that CAMs also participate in activity-dependent plasticity during development and synaptic plasticity in adults. Neural impulses of appropriate patterns can regulate expression of specific CAMs in mouse neurons from dorsal-root ganglia, alter cell-cell adhesion and produce structural reorganization of axon terminals in culture. Synaptic plasticity in Aplysia, learning in chick and long-term potentiation in rat hippocampus are accompanied by changes in CAM expression. Long-term potentiation can be blocked by disrupting CAM function in rat hippocampus, and learning deficits result from antibody blockade of CAMs in chicks and in transgenic mice lacking specific CAMs. Cell adhesion molecules might produce these effects by controlling several cellular processes, including cell adhesion, cytoskeletal structure and intracellular signaling.

Sprouting adult CNS cholinergic axons express NILE and associate with astrocytic surfaces expressing neural cell adhesion molecule

To assess the cellular and molecular substrates for cholinergic axon growth in the adult central nervous system (CNS), we implanted grafts of control and nerve growth factor (NGF)-producing genetically modified fibroblasts within the striatum of rats. Sprouting cholinergic axonal processes that grew into grafts of NGF-producing fibroblasts were fasciculated and followed the surface of astrocytic processes for long distances within the grafts. The close and long distance anatomical relationship between the sprouted axons and the astrocytes supported previous ultrastructural evidence that astrocytes may serve as a cellular substrate for sprouting cholinergic axons in vivo. The sprouted axon processes were associated with the expression of nerve growth factor-inducible large external (NILE) glycoprotein on their surfaces. NILE expression was not seen in control grafts where there was an absence of cholinergic ingrowth. NILE has been demonstrated to play a role in axon fasciculation in a number of other neural systems. The astrocytic processes in both control and NGF-producing fibroblast grafts expressed neural cell adhesion molecule (NCAM), suggesting that NCAM-mediated adhesion may be responsible for the close relationship between the axons and astrocytes within the grafts. NGF-induced heterotypic interactions between neuronal NILE and astroglial NCAM may also be required for adult cholinergic axonal sprouting.

Review: a role for the FGF receptor in the axonal growth response stimulated by cell adhesion molecules?

Cell adhesion molecules (CAMs) have been shown to stimulate axonal growth. The molecular basis of this response has been extensively studied and a range of agents that promote or inhibit CAM stimulated axonal growth have now been identified. These studies have led to the suggestion that following homophilic and/or heterophilic interactions CAM specific signal transduction pathways are activated which are directly responsible for promotion of axonal growth. In this review we will suggest that the axonal growth response stimulated by three CAMs (NCAM, N-cadherin and L1) can be operationally divided into a number of phases. During the first phase homophilic and/or heterophilic binding between the CAMs expressed on the axonal growth cone and cellular substrate take place. This is followed by an interaction of the neuronal CAMs with the fibroblast growth factor receptor (FGFR), leading to receptor activation by autophosphorylation. This results in the recruitment and activation of additional effector molecules via interactions of their SH2 domains with the activated receptor. In this context the key event in terms of neurite outgrowth appears to be the activation of phospholipase C gamma (PLC gamma) which sets into motion a second messenger cascade that ultimately leads to a modification, most likely by phosphorylation, of cytoskeletal elements that are involved in growth cone motility.

Expression and functional analysis of a novel isoform of gicerin, an immunoglobulin superfamily cell adhesion molecule

We have cloned a novel cDNA of gicerin, a cell adhesion molecule belonging to the immunoglobulin superfamily. Both gicerin isoforms share the same extracellular domain, which has five immunoglobulin-like loop structures and a transmembrane domain as s-gicerin, but differ in the cytoplasmic tail domain. As the newly identified form has a larger cytoplasmic domain than the previously reported form, we refer to them as l-gicerin and s-gicerin, respectively. l-gicerin is transcribed from a distinct mRNA containing an inserted sequence not found in s-gicerin mRNA which caused a frameshift for the coding region for a cytoplasmic domain. Previous studies demonstrated that gicerin showed a doublet band of 82 and 90 kDa in chicken gizzard smooth muscle. We report that the 82-kDa protein corresponds to s-gicerin and the 90-kDa protein to l-gicerin. We also found that the two gicerin isoforms are expressed differentially in the developing nervous system. Functional analysis of these gicerin isoforms in stable transfectants revealed that they had differ in their homophilic adhesion properties, as well as in heterophilic cell adhesion assayed with neurite outgrowth factor. In addition, these isoforms have neurite-promoting activity by their homophilic adhesion, but differ in their ability to promote neurite outgrowth.

The cytoplasmic domain of the Drosophila cell adhesion molecule neuroglian is not essential for its homophilic adhesive properties in S2 cells

Drosophila neuroglian is a transmembrane glycoprotein that has strong structural and sequence homology to the vertebrate L1 gene family of cell adhesion molecules (Bieber, A.J., Snow, P.M., Hortsch, M., Patel, N.H., Jacobs, J.R., Traquina, Z.R., Schilling, J., and Goodman, C.S. (1989) Cell 59, 447-460. Two different neuroglian protein forms that are generated by a differential splicing process are expressed in a tissue-specific fashion by embryonic and larval cells (Hortsch, M., Bieber, A.J., Patel, N.H., and Goodman, C.S. (1990) Neuron 4, 697-709). The two neuroglial polypeptides differ only in their cytoplasmic domains. Both of these neuroglian species, when transfected into the expressed in Drosophila S2 cells, induce the calcium-independent, homophilic aggregation of transformed cells. A third artificial neuroglian protein form was constructed by substituting the neuroglian transmembrane segment and cytoplasmic domains with the glycosyl phosphatidylinositol attachment signal of the Drosophila fasciclin I protein. This cDNA construct generates a glycosyl phosphatidylinositol-anchored form of neuroglian, which retains the ability to induce homophilic cell aggregation when expressed in S2 cells, and was able to interact with both of the two naturally occurring neuroglian polypeptides. These results demonstrate that neuroglian mediates a calcium-independent, homophilic cell adhesion activity and that neither cytoplasmic neuroglian domains nor a direct interaction with cytoskeletal elements is essential for this property.

Signal transduction by cell adhesion receptors

Over the last few years, it has become clear that cell adhesion receptors function in signal transduction processes leading to the regulation of cell growth and differentiation. Signal transduction by both integrins and CAMs has been shown to involve activation of tyrosine kinases, while CAM signaling in neural cells involves G proteins as well. In the case of integrins, some of the downstream signaling events intersect with the Ras pathway, particularly the activation of MAP kinases. In fibroblasts, integrin mediated anchorage to the substratum regulates cell cycle traverse, while in epithelial cells, loss of anchorage can trigger programmed cell death. In many cell types, but particularly monocytic cells, integrin ligation has a profound impact on gene expression. Preliminary evidence also implicates CAMs and selectins in gene regulation. A consistent theme in signal transduction mediated by adhesion receptors concerns the role of the cytoskeleton. Integrin mediated signaling processes are interrupted by cytoskeletal disassembly. Identification of the APC and neurofibromatosis type 2 tumor suppressors suggest that cytoskeletal complexes also play a key role in signaling by cadherins and CD44, respectively. Thus, signaling by cell adhesion receptors may involve aspects that impinge on previously known signaling pathways including the RTK/Ras pathway and serpentine receptor/G protein pathways. However, novel aspects of signal transduction involving cytoskeletal assemblies may also be critical.

Neural cell adhesion molecule (NCAM) as a quantitative marker in synaptic remodeling

The neural cell adhesion molecule (NCAM) participates in adhesion and neuritic outgrowth during nervous system development. In the adult brain, NCAM is considered to be involved in neuronal sprouting and synaptic remodeling. The NCAM concentration of brain tissue has proved to be a useful marker of these processes, especially when viewed in comparison with the concentration of a marker of mature synapses, e.g. D3-protein (SNAP-25) or synaptophysin. The present review focusses on studies of adult brain in which NCAM concentration estimates and NCAM/D3 ratios have been used to evaluate the rate of synaptic remodeling in brain damage and degenerative diseases.

The neural cell adhesion molecule and synaptic plasticity

Highly stereotyped patterns of neuronal connections are laid down during the development of the nervous system via a range of activity independent and activity dependent mechanisms. Whereas the coarse hard-wiring of the nervous system appears to rely on molecular recognition events between the neuron, its pathway, and its target, the establishment of precisely patterned functional circuits is thought to be driven by neuronal activity. In this review we discuss the role that the neuronal cell adhesion molecule (NCAM) plays in morphological plasticity. Recent studies on NCAM and its probable species homologue in Aplysia (apCAM) suggests that an individual CAM can function to both promote synaptic plasticity and maintain the structure of the synapse. In the adult brain, changes between stability and plasticity are likely to underlie dynamic morphological changes in synaptic structures associated with learning and memory. In this review we use NCAM as an example to illustrate mechanisms that can change the function of an individual CAM from a molecule that promotes plasticity to one that does not. We also discuss evidence that NCAM promotes plasticity by activating a conventional signal transduction cascade, rather than by modulating adhesion per se. Finally, we consider the evidence that supports a role for NCAM in learning and memory.

Expression of four immunoglobulin superfamily adhesion molecules (L1, Nr-CAM/Bravo, neurofascin/ABGP, and N-CAM) in the developing mouse spinal cord

To identify cell adhesion molecules (CAMs) expressed by mammalian motoneurons, we applied the polymerase chain reaction to a murine motor neuron-like cell line, NSC-34. Using primers derived from a group of L1-related CAMs, we cloned two alternatively spliced forms of mouse L1, which differ by a 12-base-pair insert, plus putative murine orthologs of the chicken cell adhesion molecules Nr-CAM/Bravo and neurofascin. All four mRNAs are expressed in NSC-34 cells, but only neurofascin and the insert-minus form of L1 are expressed in its neuroblastoma parent, N18TG2. Analysis of RNA in neonatal tissues reveals expression largely restricted to the brain and spinal cord. In situ hybridization histochemistry of spinal cord shows that motoneurons express L1, Nr-CAM, and neurofascin as well as N-CAM. L1 and N-CAM RNAs are detected throughout the period studied (from embryonic day [E]11 to postnatal day [P]28), whereas Nr-CAM is expressed only at early ages (< E15) and neurofascin is predominantly expressed postnatally. Moreover, each CAM is expressed by distinct subsets of neighboring cells and at distinct times. For example, Nr-CAM mRNA is present in floor plate cells of embryonic spinal cord, whereas neurofascin is expressed by a subset of glia postnatally. Finally, we show that each CAM has a distinct spatiotemporal pattern of expression in dorsal root ganglia.

Transient upregulation of NCAM mRNA in astrocytes in response to entorhinal cortex lesions and ischemia

Axonal sprouting and synaptic reorganization play an important role in the adaptation of the CNS to injury. However, the molecular mechanisms underlying this neuronal plasticity are poorly understood. In the present study we used in situ hybridization to examine the expression of NCAM mRNA in normal hippocampus, and in response to entorhinal cortex (EC) lesions and transient global ischemia. Both neurons and astrocytes were labeled by digoxygenin-tagged cRNA probes which recognize all three major NCAM isoforms of the adult CNS. In contrast, NCAM180-specific probes labeled only neurons in the hippocampus. After unilateral EC lesion, a transient and anatomically restricted upregulation of NCAM120/140 mRNA in reactive astrocytes in the denervated molecular layer of the dentate gyrus was observed. This increase was only present 2-4 days after the lesion whereas the GFAP mRNA increase was present up to 30 days postlesion. Following global ischemia a similar, transient increase of NCAM120/140 mRNA labeling of reactive astrocytes was observed; this increase was anatomically restricted to CA1, where neuronal loss occurred. Results suggest that the transient upregulation of NCAM120/140 mRNA in reactive astrocytes shortly after injury might be an important molecular mechanism in the cascade of events underlying neuronal plasticity in the adult CNS.

Ectopic expression of NCAM in mouse fibroblasts stimulates self-aggregation, and promotes integration into primary cerebellum cell aggregates

We have undertaken aggregation experiments using mouse LMTK(-)-fibroblasts transfected with various isotypes of the neural cell adhesion molecule, NCAM. We found that self-aggregation of NCAM-positive fibroblasts is enhanced compared to control-transfected cells. The aggregation properties are partly dependent on the expressed NCAM isotype. Fibroblasts expressing a NCAM 140 isotype with exons a3 and pi were further tested in primary cerebellum cell re-aggregation experiments. While control-transfected fibroblasts could not be found in forming aggregates, fibroblasts ectopically expressing NCAM were integrated into neural cell aggregates. Time-lapse photography indicated that the nascent primary cell aggregates actively participated in the integration process by migration and attachment to nearby NCAM-positive fibroblasts.

Expression of NCAM containing VASE in neurons can account for a developmental loss in their neurite outgrowth response to NCAM in a cellular substratum

Binding of the neural cell adhesion molecule (NCAM) in neurons to NCAM on non-neuronal cells can stimulate axonal growth. A developmentally regulated loss of this response is associated with the insertion of 10 amino acids (called VASE) into the fourth Ig domain in up to 50% of the NCAM receptors in neurons. In the present study we have transfected PC12 cells with the major neuronal isoforms of human NCAM and tested cells expressing these isoforms for their ability to respond to NCAM in a cellular substratum. Whereas both the 140- and 180-kD isoforms of NCAM can act as functional receptors for neurite outgrowth, the presence of the VASE sequence in a minority of the receptors specifically inhibited this response. A synthetic peptide containing the VASE sequence inhibits neurite outgrowth from PC12 cells and primary neurons stimulated by NCAM. The same peptide has no effect on integrin dependent neurite outgrowth or neurite outgrowth stimulated by N-cadherin or L1. We discuss the possibility that the VASE peptide inhibits the NCAM response by preventing NCAM from binding to the FGF receptor in the plasma membrane.

Differential effects of over-expressed neural cell adhesion molecule isoforms on myoblast fusion

We have used a transfection based approach to analyze the role of neural cell adhesion molecule (NCAM) in myogenesis at the stage of myoblast fusion to form multinucleate myotubes. Stable cell lines of myogenic C2 cells were isolated that express the transmembrane 140- or 180-kD NCAM isoforms or the glycosylphosphatidylinositol (GPI) linked isoforms of 120 or 125 kD. We found that expression of the 140-kD transmembrane isoform led to a potent enhancement of myoblast fusion. The 125-kD GPI-linked NCAM also enhanced the rate of fusion but less so when a direct comparison of cell surface levels of the 140-kD transmembrane form was carried out. While the 180-kD transmembrane NCAM isoform was effective in promoting C2 cell fusion similar to the 140-kD isoform, the 120-kD isoform did not have an effect on fusion parameters. It is possible that these alterations in cell fusion are associated with cis NCAM interactions in the plane of the membrane. While all of the transfected human NCAMs (the transmembrane 140- and 180-kD isoforms and the 125- and 120-kD GPI isoforms) could be clustered in the plane of the plasma membrane by species-specific antibodies there was a concomitant clustering of the endogenous mouse NCAM protein in all cases except with the 120-kD human isoform. These studies show that different isoforms of NCAM can undergo specific interactions in the plasma membrane which are likely to be important in fusion. While the transmembrane and the 125-kD GPI-anchored NCAMs are capable of enhancing fusion the 120-kD GPI NCAM is not. Thus it is likely that interactions associated with NCAM intracellular domains and also the muscle specific domain (MSD) region in the extracellular domain of the GPI-linked 125-kD NCAM are important. In particular this is the first role ascribed to the O-linked carbohydrate containing MSD region which is specifically expressed in skeletal muscle.

Genetic deletion of a neural cell adhesion molecule variant (N-CAM-180) produces distinct defects in the central nervous system

N-CAM is abundantly expressed in the nervous system in the form of numerous structural variants with characteristic distribution patterns and functional properties. N-CAM-180, the variant having the largest cytoplasmic domain, is expressed by all neurons. The N-CAM-180-specific exon 18 has been deleted to generate homozygous mice unable to express this N-CAM form. The most conspicuous mutant phenotype was in the olfactory bulb, where granule cells were both reduced in number and disorganized. In addition, precursors of these cells were found to be accumulated at their origin in the subependymal zone at the lateral ventricle. Analysis of the mutant in this region suggests that the mutant phenotype involves a defect in cell migration, possibly through specific loss of the polysialylated form of N-CAM-180, which is expressed in the migration pathway. Subtle but distinct abnormalities also were observed in other regions of the brain.

Adhesion molecules of the nervous system

The structural and functional characterization of neural cell-cell adhesion molecules continues to progress at a rapid rate, exposing the complex nature of these large multidomain receptors and their intricate relationship with the cell. Over the past year, significant progress has been made in the description of multiple binding activities, signaling potentials, and cytoplasmic associations of cell-cell adhesion molecules. In addition, new modes for developmental regulation of adhesion molecule function have been identified.

Neurite outgrowth of spinal neurons on tissue sections of embryonic muscle is largely integrin dependent

Embryonic chick spinal neurons have been cultured over sections of human foetal muscle to determine which cell adhesion molecules present in embryonic muscle are important in promoting neurite outgrowth. Using blocking antibodies against the major cell adhesion molecules, neural cell adhesion molecule (NCAM), N-cadherin and the beta 1 subunit of the integrins, neurite outgrowth was significantly blocked only by anti-integrin antibodies. In addition other agents that block neurite outgrowth stimulated by NCAM, N-cadherin and L1, such as the calcium channel antagonists verapamil and omega-conotoxin and pertussis toxin which inactivates G-proteins also had no effect. This suggests that in this culture system integrins are able to promote neurite outgrowth whereas NCAM and N-cadherin are not.

Distribution and possible roles of the highly polysialylated neural cell adhesion molecule (NCAM-H) in the developing and adult central nervous system

The neural cell adhesion molecule (NCAM) is a cell surface glycoprotein which is thought to mediate cell adhesion and recognition. During developmental stages, NCAM is highly polysialylated (NCAM-H) by a unique alpha-2,8-linked polysialic acid chain (PSA), and this PSA portion of NCAM-H has been found to be closely associated with various developmental processes of the nervous system. Further, recent immunohistochemical investigations have revealed that even in the adult nervous system, a persistent PSA expression has been found confined to several regions: the olfactory bulb, the piriform cortex, the hippocampal dentate gyrus, the hypothalamus, some nuclei of the medulla and the dorsal horn of the spinal cord, which are related directly or indirectly to sensory systems. Moreover, in the dentate gyrus and olfactory bulb the expression is connected with adult neurogenesis that may add new neuronal circuits to the adult neural tissue. Therefore, the possible role of NCAM-H in the central nervous system may be associated not only with neural development, but also with adult functions, such as the processing system of sensory information and neuronal plasticity.

Axon growth is enhanced by NCAM lacking the VASE exon when expressed in either the growth substrate or the growing axon

The neural cell adhesion molecule NCAM exists as several related peptides formed by alternative splicing of the single NCAM gene. Here the ability of NCAM containing and lacking the alternatively spliced VASE exon to act as a permissive growth substrate was tested by examining retinal axon outgrowth on normal L cell fibroblasts and L cells expressing stably transfected 140 kD NCAM +/- VASE. L cells expressing either NCAM form were a more permissive substrate than control L cells. At higher substrate cell densities, greater axon outgrowth occurred on substrate cells expressing NCAM - VASE than on those expressing NCAM + VASE. Similar experiments tested retinal axon growth on neuronal substrates by utilizing clonal B35 cells, C3 cells that are NCAM lacking variants of B35, and C3 cells into which 140 kD NCAM +/- VASE has been restored by transfection. Axon growth on C3 cells transfected with NCAM - VASE was greater than that on all other substrates including cells transfected with NCAM + VASE. In these experiments C3 cells and transfected C3 expressing NCAM + VASE cell promoted similar outgrowth. The influence on neurite growth of the NCAM isoform of the neurite itself was tested by examining neurite formation using combinations of C3 cells and C3 NCAM transfectants both in the growth monolayer and as responding cells. C3 cells were able to extend neurites, indicating NCAM is not required for neurite growth. However, C3 derivatives transfected with NCAM +/- VASE had greater neurite outgrowth. The most extensive neurite growth was found when NCAM - VASE was expressed by both substrate cells and the responding neurite growing cells. Thus NCAM enhances axon or neurite outgrowth when present either in the growth substrate or on the growing axon. NCAM - VASE has a significantly greater growth promoting capability than NCAM + VASE. The expression of NCAM + VASE by more mature neural cells could thus be a significant factor in the reduced axonation capabilities of mature neurons.

Calcium influx into neurons can solely account for cell contact-dependent neurite outgrowth stimulated by transfected L1

We have used monolayers of control 3T3 cells and 3T3 cells expressing transfected human L1 as a culture substrate for rat PC12 cells and rat cerebellar neurons. PC12 cells and cerebellar neurons extended longer neurites on human L1 expressing cells. Neurons isolated from the cerebellum at postnatal day 9 responded equally as well as those isolated at postnatal day 1-4, and this contrasts with the failure of these older neurons to respond to the transfected human neural cell adhesion molecule (NCAM). Human L1-dependent neurite outgrowth could be blocked by antibodies that bound to rat L1 and, additionally, the response could be fully inhibited by pertussis toxin and substantially inhibited by antagonists of L- and N-type calcium channels. Calcium influx into neurons induced by K+ depolarization fully mimics the L1 response. Furthermore, we show that L1- and K+(-)dependent neurite outgrowth can be specifically inhibited by a reduction in extracellular calcium to 0.25 microM, and by pretreatment of cerebellar neurons with the intracellular calcium chelator BAPTA/AM. In contrast, the response was not inhibited by heparin or by removal of polysialic acid from neuronal NCAM both of which substantially inhibit NCAM-dependent neurite outgrowth. These data demonstrate that whereas NCAM and L1 promote neurite outgrowth via activation of a common CAM-specific second messenger pathway in neurons, neuronal responsiveness to NCAM and L1 is not coordinately regulated via posttranslational processing of NCAM. The fact that NCAM- and L1-dependent neurite outgrowth, but not adhesion, are calcium dependent provides further evidence that adhesion per se does not directly contribute to neurite outgrowth.

Use of the neural cell adhesion molecule VASE exon by neurons is associated with a specific down-regulation of neural cell adhesion molecule-dependent neurite outgrowth in the developing cerebellum and hippocampus

The development of the CNS is associated with an increasing use of the 30-bp variable alternative, spliced exon (VASE) in neural cell adhesion molecule (NCAM). We have assessed the relative usage of VASE by reverse transcriptase-linked polymerase chain reaction in the developing cerebellum and hippocampus at times when neurons isolated from these tissues can respond to substrate-associated NCAM by increased axonal growth and also at later developmental stages, when they are no longer responsive to substrate-associated NCAM. Neurons isolated from the developing cerebellum at postnatal day 6 respond to NCAM with increased neurite growth. NCAM transcripts from these cells were found to have negligible levels of VASE usage. In contrast, neurons that are isolated at later stages of development (postnatal days 8, 10, and 11) and do not respond to NCAM were found to synthesise a much higher proportion of NCAM transcripts containing VASE. In the hippocampus, embryonic day 18 neurons, which are responsive to NCAM, express low levels of VASE, whereas postnatal days 4 and 5 neurons, which are not responsive to NCAM, have a greater proportion of transcripts containing VASE. Thus, the level of NCAM VASE exon usage by neurons appears to be a good indicator of the ability of these cells to respond to non-VASE-containing NCAM (expressed in a cellular substratum) by increased neurite outgrowth.

Cell adhesion molecules, second messengers and axonal growth

Recent studies on NCAM-related molecules suggest that individual cell adhesion molecules might function to both promote axonal growth during development and maintain synaptic structure in the adult. Evidence that differential alternative splicing contributes to this apparent bifunctionality of cell adhesion molecules is discussed.