Alternative splicing of the neural cell adhesion molecule gene generates variant extracellular domain structure in skeletal muscle and brain

WITHDRAWN: NCAM and the FGF-Receptor

In this review, the structural biology of interaction between the neural cell adhesion molecule (NCAM) and the fibroblast growth factor (FGF) receptor is described and a possible mechanism of the FGF-receptor activation by NCAM is discussed. Most of the FGF-receptor molecules are thought to be constantly involved in a transient interaction with NCAM. However, the FGF-receptor becomes activated only when NCAM is involved the trans-homophilic binding (mediating cell-cell adhesion). The trans-homophilic binding between the NCAM molecules is believed to result in formation of either one- or two-dimensional 'zipper'-like arrays of the NCAM molecules, which leads to NCAM clustering and as a result to clustering of the FGF-receptor, which in turn may lead to its activation through a direct receptor-receptor dimerization (and thus activation) due to an increase in the local concentration of the receptor.

Structural biology of NCAM homophilic binding and activation of FGFR

In this review, we analyse the structural basis of the homophilic interactions of the neural cell adhesion molecule (NCAM) and the NCAM-mediated activation of the fibroblast growth factor receptor (FGFR). Recent structural evidence suggests that NCAM molecules form cis-dimers in the cell membrane through a high affinity interaction. These cis-dimers, in turn, mediate low affinity trans-interactions between cells via formation of either one- or two-dimensional 'zippers'. We provide evidence that FGFR is probably activated by NCAM very differently from the way by which it is activated by FGFs, reflecting the different conditions for NCAM-FGFR and FGF-FGFR interactions. The affinity of FGF for FGFR is approximately 10(6) times higher than that of NCAM for FGFR. Moreover, in the brain NCAM is constantly present on the cell surface in a concentration of about 50 microm, whereas FGFs only appear transiently in the extracellular environment and in concentrations in the nanomolar range. We discuss the structural basis for the regulation of NCAM-FGFR interactions by two molecular 'switches', polysialic acid (PSA) and adenosine triphosphate (ATP), which determine whether NCAM acts as a signalling or an adhesion molecule.

Accumulation of N-CAM 180 at Contact Sites Between Neuroblastoma Cells and Latex Beads Coated with Extracellular Matrix Molecules

Neuronal cells expressing neural cell adhesion molecule (N-CAM) accumulate the largest N-CAM component (N-CAM 180) at cell - cell contact sites. To test whether this accumulation is induced by interactions at the surface membrane, latex beads coated with several purified adhesion molecules or extracellular matrix (ECM) components were co-cultured with neuroblastoma cells. Beads coated with L1, N-CAM, the L2/HNK-1 carbohydrate epitope-carrying molecules from adult mouse brain or laminin from Engelbreth-Holm-Swarm (EHS) sarcoma did not induce an accumulation of N-CAM 180 or L1 at sites of contact suggesting that these molecules are not directly involved in N-CAM 180 accumulation or that their mobility is required for this process. Beads coated with ECM components of the PF-HR9 cell line induced accumulation of N-CAM 180 at sites of contact with neuroblastoma cells. Accumulation was seen at cell bodies of undifferentiated and differentiated neuroblastoma cells, as well as on neurites and growth cones of differentiated neuroblastoma cells. Accumulation of the neural adhesion molecule L1 was also seen, but less prominently and reproducibly. These observations suggest that molecules of the ECM can directly or indirectly, e.g. via molecules linked to N-CAM 180 on the cell surface, induce accumulation of N-CAM 180.

High Degree of NCAM Diversity Generated by Alternative RNA Splicing in Brain and Muscle

The neural cell adhesion molecules (NCAMs) are cell surface glycoproteins involved in vertebrate cell contact formation. Several NCAM mRNA types are generated from a single primary transcript by alternative splicing and differential polyadenylation. In this presentation we analyse sequence heterogeneities within NCAM transcripts detected in the junctions of exons 7/8, 12/13 and 13/14. The highest degree of evolutionary conservation is observed in the 30-nucleotide insertion (pi) between exons 7 and 8, coding for an identical peptide sequence in the mouse, rat and chicken. The most complex splicing pattern is found between exons 12 and 13, called splice site a. Three alternative exons of 15, 48 and 42 nucleotides can be inserted in various combinations, which may also contain the additional trinucleotide AAG. In mouse muscle cell lines, differential 'extra exon' and AAG usage in splice site a creates up to 16 NCAM diversity forms, some (if not all) of which are also expressed in mouse brain. Additional microdiversity is generated by the insertion of an alternative AAG trinucleotide in exon junction 13/14. If all combinations of splicing patterns identified so far were to occur and to be translated, there could be up to 192 different NCAM proteins.

The polysialic acid moiety of the neural cell adhesion molecule is involved in intraretinal guidance of retinal ganglion cell axons

We have characterized the antigen recognized by mab10, a monoclonal antibody that has been shown to modify outgrowth of thalamic and cortical axons in vitro, and investigated the influence of this antibody on axonal growth in the chicken retina in vivo. Immunopurification, peptide sequencing, and biochemical characterization proved the epitope recognized by mab10 to be polysialic acid (PSA), associated with the neural cell adhesion molecule (NCAM). Intravitreal injections of antibody-secreting hybridoma cells were combined with whole-mount studies using the fluorescent tracer 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (DiI). Pathfinding at the optic fissure was affected, resulting in a failure of axons to exit into the nerve. Misprojections also occurred in more peripheral areas of the retina; however, axons eventually oriented toward the center. Similar projection errors were observed after enzymatic removal of PSA by injecting endoneuraminidase N (endo N). Quantitative measurements of the optic nerve diameter as well as the width of the optic fiber layer confirmed that many axons failed to leave the retina and grew back in the optic fiber layer of the retina. Our findings suggest that NCAM-linked PSA is involved in guiding ganglion cell axons in the retina and at the optic fissure.

Molecular cloning of ssd-form neural cell adhesion molecules (N-CAMs) as the major form in Xenopus heart

Different forms of neural cell adhesion molecule (N-CAM) are generated by alternative splicing of primary transcripts and considered to have distinct biological functions. We cloned cDNAs encoding a new form of N-CAMs from the Xenopus heart cDNA library. Comparison of the sequences with chicken and mouse N-CAMs revealed that these clones code for ssd-form N-CAM. We demonstrate by Northern blot analysis that the ssd form is the major form expressed in the Xenopus adult heart. We obtained two types of ssd-form N-CAM, which are transcripts from N-CAM 1 and N-CAM 2 genes. Both types contain muscle specific domain (MSD) but not pi domain. Northern blot analysis also indicated that this form is not expressed in adult brain, in which ld-form N-CAM is the main N-CAM expressed. It is possible that high levels of specific expression of ssd-form N-CAM are related with the differentiation of cardiac muscles.

Altered secondary myogenesis in transgenic animals expressing the neural cell adhesion molecule under the control of a skeletal muscle alpha-actin promoter

The majority of skeletal muscle fibers are generated through the process of secondary myogenesis. Cell adhesion molecules such as NCAM are thought to be intricately involved in the cell-cell interactions between developing secondary and primary myotubes. During secondary myogenesis, the expression of NCAM in skeletal muscle is under strict spatial and temporal control. To investigate the role of NCAM in the regulation of primary-secondary myotube interactions and muscle fusion in vivo, we have examined muscle development in transgenic mice expressing the 125-kD muscle-specific, glycosylphosphatidylinositol-anchored isoform of human NCAM, under the control of a human skeletal muscle alpha-actin promoter that is active from about embryonic day 15 onward. Analysis of developing muscle from transgenic animals revealed a significantly lower number of myofibers encased by basal lamina at postnatal day 1 compared with nontransgenic littermates, although the total number of developing myofibers was similar. An increase in muscle fiber size and decreased numbers of VCAM-1-positive secondary myoblasts at postnatal day 1 was also found, indicating enhanced secondary myoblast fusion in the transgenic animals. There was also a significant decrease in myofiber number but no increase in overall muscle size in adult transgenic animals; other measurements such as the number of nuclei per fiber and the size of individual muscle fibers were significantly increased, again suggesting increased secondary myoblast fusion. Thus the level of NCAM in the sarcolemma is a key regulator of cell-cell interactions occurring during secondary myogenesis in vivo and fulfills the prediction derived from transfection studies in vitro that the 125-kD NCAM isoform can enhance myoblast fusion.

The ability to re-express polysialylated NCAM in soleus muscle after denervation is reduced in aged rats compared to young adult rats

The neural cell-adhesion molecule, NCAM, contains an unusual homopolymer of sialic acid units, polysialic acid. This carbohydrate seems to be involved in neurite outgrowth, bundling and branching, processes which are important during reinnervation. In aged rats, reinnervation of denervated muscle fibres is incomplete. In this study, age-related changes in the degree of polysialylation of NCAM re-expressed after denervation were examined using a monoclonal antibody recognizing polysialic acid and a polyclonal antibody recognizing NCAM. The results show that, after denervation, the degree of polysialylation on NCAM was clearly reduced in rat soleus muscle of aged, compared to young, adult rats. This age-related change in expression of polysialic acid probably influences the reinnervation process in aged muscle.

Localization of NCAM on NCAM-B-expressing cells with inhibited migration in collagen

The extracellular matrix is a key element in neuronal development and tumour invasion, providing a substratum which sustains the adhesion and migration of cells. In order to study interactions between the neural cell adhesion molecule (NCAM) and collagen, we transfected mouse L cells with cDNA encoding the human transmembrane NCAM isoform of 140 kDa (NCAM-B). An L-cell/collagen type I system was used to study the influence of NCAM expression on in vitro invasion. We here report that migration of NCAM-expressing cells in collagen was inhibited compared to that of NCAM-negative cells transfected with the empty vector. Immunofluorescence confocal laser scanning microscopy (CLSM) and immunogold electron microscopy using anti-human NCAM antibodies demonstrated a heterogeneous distribution of NCAM on the plasma membrane of transfected L cells grown on collagen. NCAM was preferentially located at the surface of broad cytoplasmic protrusions and slender extensions, some of which were facing the collagen. This was in contrast to the homogeneous surface distribution of NCAM on cells grown on plastic. These data suggest that NCAM and collagen type I interact, and that this might lead to the migration inhibition of NCAM-expressing cells.

NCAM polypeptides in heart development: association with Z discs of forms that contain the muscle-specific domain

Previous studies of neural cell adhesion molecule (NCAM) cDNAs have revealed an alternatively spliced set of small exons (12A, 12B, 12C, and 12D) that encode a region in the extracellular portion of the molecule known as the muscle-specific domain (MSD). The entire MSD region can be expressed in skeletal muscle, heart, and skin; only exons 12A and 12D have been found in brain. These studies did not reveal which NCAM polypeptides contain the MSD region or the immunohistochemical distribution of these NCAM molecules. To address these questions, we prepared antibodies against the oligopeptides encoded by exons 12A and 12B and by exons 12C and 12D, and we used these antibodies to study the forms of NCAM containing the MSD region expressed during embryonic chicken heart development. These antibodies recognize certain forms of NCAM found in the heart, but they do not recognize brain NCAM. In the heart, each of the splice variants of NCAM (large cytoplasmic domain, small cytoplasmic domain, and small surface domain) that differ in their mode of attachment to the plasma membrane or in the size of their cytoplasmic domain is expressed in a form that contains and in a form that lacks the MSD region. No microheterogeneity is observed in the size of NCAM molecules containing the MSD region, even at the level of cyanogen bromide fragments, suggesting that exons 12A-D are expressed as a single unit. Depending on the site and the stage of development, the percent of NCAM molecules containing the MSD region can vary from nearly 0 to 100%. In general, this percentage increases during development. In immunohistochemical studies of hearts from stage 18 embryos, forms of NCAM containing the MSD region colocalized with Z discs. No other adhesion molecules were found in this distribution at this early stage of development. Studies on isolated cells in vitro demonstrate that the colocalization with Z discs of NCAM molecules containing the MSD region does not depend on cell-cell contact, and they raise the possibility that this form of NCAM is involved in cell-extracellular matrix interactions. The association of NCAM molecules containing the MSD region with Z discs suggests that this form of NCAM is involved in early myofibrillogenesis.

Solid-phase binding analysis of N-CAM interactions with brain fodrin

The large cytoplasmic domain form of the neural cell adhesion molecule N-CAM has been reported to interact specifically with fodrin, a submembranous cytoskeletal protein. We tested the abilities of fodrins from bovine brain and embryonic chicken brain to bind to N-CAM that had been isolated from differentiated or undifferentiated mouse N2A neuroblastoma cells or from the brains of embryonic day 11 or day 14 chickens. Labeled fodrin samples bound with immobilized fodrin at a minimum soluble fodrin concentration of 2.5 x 10(-8) M, but the labeled fodrin did not bind to the immobilized N-CAM when incubated at 20-fold higher fodrin concentrations.

Hot spots of antigenicity in the neural cell adhesion molecule NCAM

Monoclonal antibodies (MAbs) ranked together as small-cell-lung-cancer (SCLC) Cluster I MAbs are directed against the neural cell adhesion molecule NCAM (CD 56) and have been shown to be useful reagents in SCLC diagnosis and therapy. We analyzed the epitopes recognized by 5 SCLC cluster-I MAbs (123C3, 123A8, ERIC-I, MB2, and Leu 19) and a closely related anti CD 56 MAb (T199). Our results show that within the NCAM molecule Ig-like domain 3 and the segment of about 200 amino acids comprised by exons 11-13 are immunodominant regions. The MAbs investigated in this study can be combined into 2 groups. Group 1 consists of MAbs MB2, Leu 19 and T199, which are directed against epitopes located in the 3rd Ig-like domain. These MAbs recognize closely related but distinctive conformational epitopes. MAbs ERIC-1, 123C3 and 123A8 form Group 2 and are directed against a membrane-proximal region of the NCAM molecule. The data presented suggest that the 3 Group-2 MAbs bind to closely related or identical epitopes.

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.

Intact transmembrane isoforms of the neural cell adhesion molecule are released from the plasma membrane

Three soluble neural cell adhesion molecule (NCAM) polypeptide classes of M(r) values 190,000 (NCAM-s1), 135,000 (NCAM-s2) and 115,000-110,000 (NCAM-s3) have been demonstrated in rat brain and cerebrospinal fluid [Krog, Olsen, Dalseg, Roth and Bock (1992) J. Neurochem. 59, 838-847]. NCAM-s3 is known to arise from released glycosylphosphatidylinositol (GPI)-linked NCAM [He, Finne and Goridis (1987) J. Cell. Biol. 105, 2489-2500] as well as from extracellularly cleaved transmembrane NCAM isoforms [Nybroe, Linnemann and Bock (1989) J. Neurochem. 53, 1372-1378]. In this study the origin of NCAM-s1 and NCAM-s2 and the function of soluble NCAM forms were investigated. It was shown that all three soluble forms could be released from brain membranes with M(r) values identical to the three major membrane-associated forms: the large transmembrane 190,000-M(r) form (NCAM-A), the smaller transmembrane 135,000-M(r) form (NCAM-B) and the GPI-anchored 115,000-110,000-M(r) form (NCAM-C). A polyclonal antibody, directed against transmembrane and cytoplasmic epitopes common to NCAM-A and NCAM-B, was shown to react with NCAM-s1 and NCAM-s2. Furthermore, NCAM-B was shown to be shed in a presumably intact soluble form from membranes of cells transfected with this isoform. Thus, NCAM-s1 and NCAM-s2 probably represent intact released transmembrane NCAM-A and NCAM-B. The soluble transmembrane forms are likely to exist in vivo, as NCAM-s1 and NCAM-s2 were readily demonstrated in cerebrospinal fluid. By density-gradient centrifugation it was shown that shed transmembrane NCAM-B was present in fractions of high, as well as low, density, indicating that a fraction of the shed NCAM is associated with minor plasma membrane fragments. Finally, it was shown that isolated soluble NCAM inhibited cell binding to an immobilized NCAM substratum, attributing a pivotal role to soluble NCAM in vivo as a modulator of NCAM-mediated cell behaviour.

Cystic fibrosis transmembrane conductance regulator splice variants are not conserved and fail to produce chloride channels

In the human CFTR only the rare exon 4- splice variant is conserved in mice. We have discovered two novel murine variants, exon 5- and exon 11b+. The exon 5- variant represents up to 40% of mRNA in all CFTR-expressing tissues and leaves the reading frame intact. The exon 11b+ variant inserts a novel exon between exons 11 and 12 with expression restricted to the testis. Two variants of 11b have been found and both introduce premature stop codons. When we expressed human CFTR variants lacking either exon 5 or exon 9 in HeLa cells, they failed to generate cAMP-mediated chloride transport, due to defective intracellular processing. The lack of conservation of splice variants between species and the inability of the more abundant splice variants to generate protein that is correctly processed argue against a physiological role and may simply represent aberrant splicing that is tolerated by the cell and organism.

Age-related changes in expression of the neural cell adhesion molecule in skeletal muscle: a comparative study of newborn, adult and aged rats

Neural cell adhesion molecule (NCAM) is expressed by muscle and involved in muscle-neuron and muscle-muscle cell interactions. The expression in muscle is regulated during myogenesis and by the state of innervation. In aged muscle, both neurogenic and myogenic degenerative processes occur. We here report quantitative and qualitative changes in NCAM protein and mRNA forms during aging in normal rat skeletal muscle. Determination of the amount of NCAM by e.l.i.s.a. showed that the level decreased from perinatal to adult age, followed by a considerable increase in 24-month-old rat muscle. Thus NCAM concentration in aged muscle was sixfold higher than in young adult muscle. In contrast with previous reports, NCAM polypeptides of 200, 145, 125 and 120 kDa were observed by immunoblotting throughout postnatal development and aging, the relative proportions of the individual NCAM polypeptides remaining virtually unchanged at all ages examined. However, changes in the extent of sialylation of NCAM were demonstrated. Even though the relative amounts of the various NCAM polypeptides were unchanged during aging, distinct changes in NCAM mRNA classes were observed. Three NCAM mRNA classes of 6.7, 5.2 and 2.9 kb were present in perinatal and young adult skeletal muscle, whereas only the 5.2 and 2.9 kb mRNA classes could be demonstrated in aged muscle. This indicates that metabolism of the various NCAM polypeptides is individually regulated during aging. Alternative splicing of NCAM mRNA in skeletal muscle was studied by Northern blotting using DNA oligonucleotide probes specifically hybridizing to selected exons or exon combinations. Exon VASE, which has previously been shown to be present in both brain and heart NCAM mRNA, was virtually absent from skeletal muscle at all ages studied. In contrast, the majority of NCAM mRNA in postnatal skeletal muscle was shown to contain extra exons inserted between exons 12 and 13. Of the various possible exon combinations at this splice site, the combinations 12-a-AAG-13 and 12-a-b seemed to be prevalent in postnatal skeletal muscle. No significant change in the relative proportion of these two exon combinations occurred during aging. The observed upregulation of NCAM protein in aged muscle supports the assumption that an increasing proportion of muscle fibres are denervated in aged muscle. Selective upregulation of the 5.2 and 2.9 kb mRNA forms have previously been demonstrated in muscle cell lines and in primary cultures of muscle cells during formation of myotubes in vitro, and this switch in NCAM mRNA classes has been suggested to correlate with myogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of soluble neural cell adhesion molecule in rat brain, CSF, and plasma

The polypeptide composition and glycosylation of soluble isoforms of neural cell adhesion molecule (NCAM) in developing rat brain, CSF, and plasma were characterized. Soluble NCAM in rat brain consisted of several glycosylated isoforms. The degree of glycosylation was developmentally regulated. After desialylation, four polypeptides of M(r) values of approximately 190,000 (s1), 135,000 (s2), 115,000 (s3), and 110,000 (s4) were observed. Polypeptides s1, s2, and s3 were also present in CSF, whereas only s3 and s4 were observed in plasma. Treatment of soluble brain NCAM with N-glycosidase F, which removes N-linked carbohydrates, produced polypeptides of M(r) values of approximately 190,000, 125,000, and 108,000-97,000. The monoclonal antibody OB11, which recognizes an epitope on the cytoplasmic part of transmembrane forms of NCAM, did not react with any of the soluble isoforms. Purified soluble NCAM, consisting mainly of s3, contained an N-terminal sequence identical to that of membrane-associated NCAM. Gel filtration of s3 indicated that it was present as a dimer under the chosen conditions. NCAM-expressing glioma cells adhered specifically to immobilized soluble NCAM. This implies that functionally significant soluble forms of NCAM are present in the extracellular fluid.

Expression of NCAM isoforms during skeletal myogenesis in the mouse embryo

We have examined the developmental patterns of neural cell adhesion molecule (NCAM) gene expression in embryonic mouse skeletal muscle cells by in situ hybridization. Moreover, by utilising exon-specific cRNA probes, we have examined tissue specific splicing of the NCAM gene. We show that there is a distinct sequence of NCAM isoform expression during skeletal muscle development. Since NCAMs are also expressed in other cell types, particularly neurons, NCAM mRNAs have been colocalised with acetylcholine receptor alpha (AChR alpha) gene transcripts to identify muscle-specific expression. NCAM is first detected in somites as they first form, prior to their differentiation into muscle and nonmuscle compartments. Myotomes, the first skeletal muscle masses to form in the embryo, express mRNAs for the transmembrane 180 and 140 kDa isoforms of NCAM. Both of these transcripts are also detected in the neural tube, and their spatial pattern of expression changes with development. Transcripts containing the muscle-specific domain (MSD) of the NCAM gene are not detected prior to 11 days postcoitum (p.c.), at a time when rostral somites already contain well-developed myotomes. As the level of MSD mRNAs increases at 12 days p.c., the 140 and 180 kDa transcript levels decrease in skeletal muscle masses. The level of all NCAM isoform transcripts declines between 13 and 15 days p.c. in muscle. However, the 180 and 140 kDa NCAM isoforms are expressed at a high level in neural tissue and in other locations in the developing embryo such as in smooth muscle, around vibrissae follicles, and in the perichondrial zone of digits.

NCAM: structural diversity, function and regulation of expression

NCAM is a large family of structurally closely related proteins with cell-cell adhesive properties and a temporo-spatially regulated expression throughout development. This review covers recent work on NCAM with an emphasis on the still open questions of the full extent of structural diversity and the mechanism whereby it arises, the chemistry and functional consequences of the binding event and the intricacies of the developmental regulation of NCAM, all of which have ramifications in its likely role as an effector of morphogenesis.

Alternative splicing of neural-cell-adhesion molecule mRNA in human small-cell lung-cancer cell line H69

The neural-cell-adhesion molecule (NCAM) is expressed in all small-cell lung cancers (SCLC) and in approximately 20% of non-small-cell lung tumors (non-SCLC). These NCAM-positive lung tumors have a poor prognosis compared with NCAM-negative tumors. Multiple NCAM protein isoforms are expressed from a single-copy gene as a result of alternative splicing and/or post-translational modifications. Therefore, we studied the NCAM isoforms expressed in a human small-cell lung-cancer cell line, H69. NCAM mRNA transcripts of 7.2, 6.7, 4.3 and 4.0 were detected in these cells on Northern blots. Since the various NCAM isoforms may have different biological properties, we performed a more precise examination of NCAM mRNAs, using polymerase chain reactions (PCR) with primers flanking the various NCAM exon boundaries. The shortest alternatively spliced sequence that we found was the trinucleotide AAG located between exon 12 and 13 in the so-called hinge region of the NCAM protein. This AAG trinucleotide was present in the majority of the NCAM mRNAs. A second alternatively spliced 30 nt-exon VASE (immunoglobulin-variable domain-like alternatively spliced exon) was present in all NCAM transcript isoforms at the exon 7/exon 8 junction. VASE resulted in the insertion of 10 amino acids into the 4th immunoglobulin-like loop of the NCAM protein. Within the limits of the PCR methodology, no evidence for the presence of mRNA containing exon 15, encoding the glycosyl-phosphatidylinositol-linked (GPI-linked) NCAM isoform in H69 cells was obtained. Considering that H69 cells express 2 major NCAM protein classes (NCAM-180 and NCAM-140), and that the VASE and AAG alternative mRNA splice variants result in minor differences in protein sizes, at least 8 polypeptide isoforms of NCAM might be expressed in H69 cells that contribute to the binding interactions of NCAM.

Cloning muscle isoforms of neural cell adhesion molecule using an episomal shuttle vector

Three distinct transcripts encoding two phosphatidylinositol (PI) linked isoforms of the neural cell adhesion molecule (NCAM) are induced during the differentiation of C2C12 myoblasts into myotubes. Corresponding NCAM clones were isolated from a mouse muscle cDNA library made in an Epstein-Barr virus shuttle vector that replicates extrachromosomally in human cells. Following transfection with the library, human cells expressing mouse NCAM were enriched using the fluorescence-activated cell sorter. Episomal NCAM clones recovered from sorted cells contain an 18-bp insert between exons 12 and 13. Two other NCAM cDNAs encode identical polypeptides containing a 108-bp insert homologous to the complete MSD1 domain, but differ in their 3' untranslated regions. Induction of MSD1-containing transcripts in advance of myotube formation suggests that muscle-specific NCAMs contribute to myogenesis from the earliest stages of differentiation. Moreover, our studies demonstrate the feasibility of cloning tissue-specific molecules by transfection and expression of cDNA libraries in episomal vectors.

Structural variants of the neural cell adhesion molecule (N-CAM) in developing feathers

The neural cell adhesion molecule (N-CAM) is expressed in a specific spatiotemporal pattern during feather development, suggesting that adhesion mediated by this molecule is involved in feather morphogenesis. To begin to investigate N-CAM's function in developing feathers, we determined what forms of N-CAM polypeptide are present and the distribution of polysialic acid (PSA), a carbohydrate moiety that decreases N-CAM-mediated cellular adhesion. N-CAM in skin appears as a Mr 145-kDa polypeptide compared to the 140-kDa brain N-CAM polypeptide, and is encoded by a 6.4-kb mRNA, compared to the 6.1-kb mRNA in brain. Polymerase chain reaction analysis of the exon splicing pattern of skin N-CAM shows that the 6.4-kb mRNA band represents two transcripts, with and without a 93-bp insert between exons 12 and 13. Thus, two N-CAM polypeptides are expressed in skin, but the 93-bp insert does not account for the larger size of the skin mRNAs and polypeptides. We show that the size difference of the polypeptides is instead due to N-linked oligosaccharides attached to the skin N-CAM proteins. The larger size of the skin mRNAs may be due to use of a different transcriptional start site. Staining of skin sections and wholemounts confirms previous descriptions of N-CAM in developing feathers, but reveals that N-CAM is also present at low levels on epidermal cells as early as stage 29 (E6). We find that PSA is expressed only on a subset of the cells that express N-CAM, in particular on dermal cells in the feather rudiments from stage 35-36 (E9-10) and on smooth muscle cells at the base of the filaments from stage 37 (E11) until the latest stage examined (stage 44, E18). The known effects on cell-cell adhesion of amount of N-CAM and PSA suggest that the variations we observe in skin may regulate cell-cell interactions that are important in feather development.

Clusters of neural cell adhesion molecule at sites of cell-cell contact

I have examined the distribution of neural cell adhesion molecule (N-CAM) in cultured C2 myogenic cells and other cell lines to determine if N-CAM accumulates at sites of cell-cell contact. C2 cells growing in log phase display large clusters of neural cell adhesion molecule where they contact each other. These clusters are remarkably stable, do not form at cell-substrate contacts, and appear not to be enriched in a number of other cytoskeletal, membrane, or extracellular proteins. Thus, N-CAM clusters form preferentially in response to cell-cell contact and are specifically enriched in N-CAM. As C2 cultures mature and differentiate, clusters persist at contacts between aligning myoblasts and between myotubes, consistent with a role in myogenesis. N-CAM is also enriched at cell-cell contacts in cultures of PC12, NRK, and CHO cells. These cells have significant amounts of N-CAM as detected on immunoblots. Clusters are not seen in L929 cells, which do not have detectable amounts of N-CAM. Coculture of these cells with C2 cells results in the clustering of N-CAM at heterologous contacts between C2 cells and NRK, CHO, or PC12 cells, but not between C2 cells and L929 cells. These results suggest that N-CAM specifically accumulates where N-CAM-bearing cells contact one another. Clustering of N-CAM may be an important step in strengthening intercellular adhesion.

Developmental regulation of alternative splicing in the mRNA encoding Xenopus laevis neural cell adhesion molecule (NCAM)

The neural cell adhesion molecule (NCAM) is thought to play a role in the formation of the vertebrate nervous system. In mammals and chicken, it is known that more than 100 different forms of the NCAM protein can be generated by alternative splicing of one primary transcript and it is possible that these different forms have distinct biological functions. A large part of the diversity is generated by alternative mRNA splicing in two regions, called the pi and the muscle specific domain (MSD), that encode portions of the extracellular domain of the NCAM protein. In this report, we describe the tissue and developmental expression of the pi and MSD sequences in the amphibian, Xenopus laevis. Our experiments show that NCAM transcripts are present in all tissues examined including muscle, heart, liver, kidney, and brain. We have identified a 30-base exon, similar to the pi domain observed in mammals, that is not present in maternal NCAM RNA but appears in a subset of the NCAM mRNA population shortly after neural induction. At the predicted location of the MSD we have detected only two alternatively spliced exons, 3 bases and 15 bases in length. In no X. laevis tissue examined did we detect the two additional alternatively spliced exons which are present in the MSD region of mammalian and chicken NCAM RNAs. Finally, the analysis has revealed a dynamic and complex pattern of expression of alternatively spliced NCAM mRNAs during embryogenesis. High levels of expression of specific forms of NCAM RNA correlate with major morphogenic events such as neural tube formation and metamorphosis.

The muscle specific domain of mouse N-CAM: structure and alternative splicing patterns

The neural cell adhesion molecule (N-CAM) is an important mediator of calcium independent cell-cell interactions. Variations in the primary structure of the protein are due to alternative splicing of pre-mRNA in the region encoding the extracellular, trans-membrane and cytoplasmic domains. In order to identify the patterns of exon usage during development of skeletal muscle and brain of the mouse, a coupled reverse-transcriptase/polymerase chain reaction was used to identify the murine homologues of the muscle-specific domain (MSD), located between exons 12 and 13 in human N-CAM mRNA. The cDNAs produced have been cloned and sequenced, or analysed directly. The amplification reactions were shown to maintain the concentration ratios of the initial cDNAs. The results indicate that the mouse homologue to exon MSD1a is under tissue and developmental regulation that is independent of exons MSD1b and MSD1c. The inclusion of the triplet exon AAG is also regulated in a cell- and stage-specific manner, which is independent of the other alternatively spliced exons of this domain.

The formation of axonal pathways in developing cranial nerves

The roles of a variety of molecules including cell adhesion molecules and growth factors in the development of cranial nerves have begun to be understood in detail. In the course of embryonic development, cranial nerves are differentiated in concordance with the development of the metameric facial structure called 'ectomeres'. Each ectomere parallels the segmentation of the hindbrain called the 'rhombomere', in which pairs of metameric units cooperate to generate the repeating sequence of cranial branchiomotor nerves. A number of genes, including homeobox genes, are expressed in a rhombomere-specific pattern. For the formation of the olfactory nerve, it is suggested that several carbohydrate residues play important roles in receptor-target specificity. In the optic nerve, a combination of multiple cell adhesion molecules contributes to neurite growth in a developmental stage-specific manner. The development of the trigeminal nerve is under the control of both cell adhesion molecules and several growth factors. There is evidence that some of the adhesion molecules are expressed in a modality-specific way. There are also several molecules, such as 11p15 or TAG1/SNAP which are expressed only in selected cranial nerves. The growth rate of neurites also varies according to the individual nerves. Thus each cranial nerve has its own intrinsic properties and their outgrowth is the outcome of these properties and their interactions with surrounding non-neuronal tissues.

Cell adhesion molecules and the regulation of development

Cell adhesion molecules, in conjunction with the other morphoregulatory molecules, substrate adhesion molecules and cell junctional molecules, are dynamically expressed in coordinate patterns throughout development. Their activities are linked to a variety of cellular processes, and their ability to influence mechanochemical processes allows them to influence a variety of other fundamental developmental events. The clinical significance of these molecules remains to be determined, but they are clearly involved in a number of pathologic conditions and could become the focus of a wide range of diagnostic techniques and eventually even therapeutic designs.

At least 27 alternatively spliced forms of the neural cell adhesion molecule mRNA are expressed during rat heart development

The major membrane-associated or transmembrane isoforms of the neural cell adhesion molecule (NCAM) are generated by alternative splicing at the 3' end of the mRNA. Further diversity in NCAM structure is observed in the extracellular region of the polypeptide, where the insertion of additional amino acid residues can result from alternative splicing events occurring at the exon 7-exon 8 and exon 12-exon 13 junctions. Here we report the characterization of tissue-specific patterns of alternative splicing at the exon 12-exon 13 junction by using the polymerase chain reaction. Nine alternatively spliced sequences in rat heart between exon 12 and exon 13 were identified. Each sequence consisted of different combinations of the three small exons (15, 48, and 42 bp in length) and the AAG triplet that make up MSD1, the 108-bp muscle-specific sequence found in human skeletal muscle NCAM (G. Dickson, H.J. Gower, C. H. Barton, H. M. Prentice, V. L. Elsom, S. E. Moore, R. D. Cox, C. Quinn, W. Putt, and F. S. Walsh, Cell 50:1119-1130, 1987). Although the rat equivalent of MSD1 (designated 15+ 48+ 42+ 3+) was detected in all ages of heart examined, it was only one of four or five major splice combinations at any given age. The only alternatively spliced sequence found in the exon 7-exon 8 junction of heart NCAM mRNA was the 30-bp variable alternatively spliced exon previously identified in rat brain. Twenty-seven NCAM forms with distinct sequences were found by analysis of individual NCAM transcripts from postnatal day 1 heart tissue for alternative splicing at the exon 7-exon 8 junction, the exon 12-exon 13 junction and the 3' end. Several combinations of splicing patterns in these three different regions of the gene appeared to be preferentially expressed. The observation that the expression of alternatively spliced forms of NCAM is developmentally regulated suggests a role for NCAM diversity in cardiac development.

At least 27 alternatively spliced forms of the neural cell adhesion molecule mRNA are expressed during rat heart development

The major membrane-associated or transmembrane isoforms of the neural cell adhesion molecule (NCAM) are generated by alternative splicing at the 3' end of the mRNA. Further diversity in NCAM structure is observed in the extracellular region of the polypeptide, where the insertion of additional amino acid residues can result from alternative splicing events occurring at the exon 7-exon 8 and exon 12-exon 13 junctions. Here we report the characterization of tissue-specific patterns of alternative splicing at the exon 12-exon 13 junction by using the polymerase chain reaction. Nine alternatively spliced sequences in rat heart between exon 12 and exon 13 were identified. Each sequence consisted of different combinations of the three small exons (15, 48, and 42 bp in length) and the AAG triplet that make up MSD1, the 108-bp muscle-specific sequence found in human skeletal muscle NCAM (G. Dickson, H.J. Gower, C. H. Barton, H. M. Prentice, V. L. Elsom, S. E. Moore, R. D. Cox, C. Quinn, W. Putt, and F. S. Walsh, Cell 50:1119-1130, 1987). Although the rat equivalent of MSD1 (designated 15+ 48+ 42+ 3+) was detected in all ages of heart examined, it was only one of four or five major splice combinations at any given age. The only alternatively spliced sequence found in the exon 7-exon 8 junction of heart NCAM mRNA was the 30-bp variable alternatively spliced exon previously identified in rat brain. Twenty-seven NCAM forms with distinct sequences were found by analysis of individual NCAM transcripts from postnatal day 1 heart tissue for alternative splicing at the exon 7-exon 8 junction, the exon 12-exon 13 junction and the 3' end. Several combinations of splicing patterns in these three different regions of the gene appeared to be preferentially expressed. The observation that the expression of alternatively spliced forms of NCAM is developmentally regulated suggests a role for NCAM diversity in cardiac development.

Towards understanding skeletal muscle regeneration

Factors which effect proliferation and fusion of muscle precursor cells have been studied extensively in tissue culture, although little is known about these events in vivo. This review assesses the tissue culture derived data with a view to understanding factors which may control the regeneration of mature skeletal muscle in vivo. The following topics are discussed in the light of recent developments in cell and molecular biology: 1) Injury and necrosis of mature skeletal muscle fibres 2) Phagocytosis of myofibre debris 3) Revascularisation of injured muscle 4) Activation and proliferation of muscle precursor cells (mpc) in vivo Identification of mpcs; Satellite cell relationships; Extracellular matrix; Growth factors; Hormones; Replication. 5) Differentiation and fusion of muscle precursor cells in vivo Differentiation; Fusion; Extracellular matrix; Cell surface molecules: Growth factors and prostaglandins 6) Myotubes and innervation.

Expression of the unique NCAM VASE exon is independently regulated in distinct tissues during development

During development of the rat central nervous system, neural cell adhesion molecule (NCAM) mRNAs containing in the extracellular domain a 30-bp alternative exon, here named VASE, replace RNAs that lack this exon. The presence of this alternative exon between previously described exons 7 and 8 changes the predicted loop structure of the derived polypeptide from one resembling an immunoglobulin constant region domain to one resembling an immunoglobulin variable domain. This change could have significant effects on NCAM polypeptide function and cell-cell interaction. In this report we test multiple rat tissues for the presence of additional alternative exons at this position and also examine the regulation of splicing of the previously described exon. To sensitively examine alternative splicing, polymerase chain reactions (PCRs) with primers flanking the exon 7/exon 8 alternative splicing site were performed. Four categories of RNA samples were tested for new exons: whole brain from embryonic day 11 to adult, specific brain regions dissected from adult brain, clonal lines of neural cells in vitro, and muscle cells and tissues cultured in vitro and obtained by dissection. Within the limits of the PCR methodology, no evidence for any alternative exon other than the previously identified VASE was obtained. The regulation of expression of this exon was found to be complex and tissue specific. Expression of the 30-bp exon in the heart and nervous system was found to be regulated independently; a significant proportion of embryonic day 15 heart NCAM mRNAs contain VASE while only a very small amount of day 15 nervous system mRNAs contain VASE. Some adult central nervous system regions, notably the olfactory bulb and the peripheral nervous system structures adrenal gland and dorsal root ganglia, express NCAM which contains very little VASE. VASE is undetectable in NCAM PCR products from the olfactory epithelium. Other nervous system regions express significant quantities of NCAM both with and without VASE. Clonal cell lines in culture generally expressed very little VASE. These results indicate that a single alternative exon, VASE, is found in NCAM immunoglobulin-like loop 4 and that distinct tissues and nervous system regions regulate expression of VASE independently both during development and in adult animals.

Myocardial localization and isoforms of neural cell adhesion molecule (N-CAM) in the developing and transplanted human heart

Neural cell adhesion molecule (N-CAM) has been implicated in cellular interactions involved in cardiac morphogenesis and innervation. Immunohistochemical techniques and Western blot analysis were used to determine the localization and isoforms of N-CAM in the developing and extrinsically denervated human heart. Myocardial and conducting cells in the fetal heart (7-24 wk gestation) exhibited sarcolemmal immunoreactivity, the major desialo N-CAM isoforms being 150, 145, 120, 115, and 110 kD. N-CAM expression appeared to be downregulated in the myocardium during adult life, with relatively little sarcolemmal immunoreactivity being detected in normal donor tissues. In contrast to the temporal changes observed in the myocardium, both the developing and mature cardiac innervation displayed N-CAM immunofluorescence staining, localized to neuronal cell bodies, nerve fascicles and fibres. Extrinsically denervated cardiac allografts, obtained 2 d to 91 mo after transplantation, showed extensive sarcolemmal and intercalated disc immunostaining and expression of 125-, 120-, and 115-kD isoforms. Tissues from explanted recipient hearts and atrial appendage samples obtained during coronary bypass graft operations were also examined and displayed varying amounts of N-CAM immunoreactivity. We conclude that the expression of N-CAM immunoreactivity and isoforms in the human heart is developmentally regulated and may be modulated by factors such as cardiac innervation and myocardial hypertrophy.

Neurite outgrowth in response to transfected N-CAM changes during development and is modulated by polysialic acid

We have used monolayers of control 3T3 cells and 3T3 cells transfected with a cDNA encoding human N-CAM as a culture substrate for embryonic chick retinal ganglion cells (RGCs). At embryonic day 6 (E6), but not at E11, RGCs extended longer neurites on monolayers of N-CAM-transfected cells. This loss of RGC responsiveness was not associated with substantial changes in the level of N-CAM expression on RGC growth cones. The neurite outgrowth response from E6 RGCs could be inhibited by removal of N-CAM from the monolayer, by removal of alpha 2-8-linked polysialic acid from neuronal N-CAM, or by antibodies that bind exclusively to chick (neuronal) N-CAM. In contrast, the response was not dependent on neuronal beta 1 integrin function. These data provide substantive evidence for a homophilic binding mechanism directly mediating N-CAM-dependent neurite outgrowth, and suggest that changes in polysialic acid expression on neuronal N-CAM may modulate N-CAM-dependent axonal growth during development.

Characterization of cDNA clones defining variant forms of human neural cell adhesion molecule N-CAM

The neural cell adhesion molecule N-CAM has been identified in a number of species and comprises at least three major cell surface polypeptides of different molecular structures and tissue distributions. We report here the isolation and characterization of cDNA clones encoding two of the three major forms of N-CAM from a human neuroblastoma cDNA library. One of the clones, NII-6, provides the first complete sequence of a small cytoplasmic domain (140 kDa) form of the molecule in humans and differs in a number of respects from cDNA clones derived from human muscle. These differences include the presence of a 30-bp insert in the fourth immunoglobulin-like domain of N-CAM, a 3-bp insert in the extracellular portion of the molecule, and an additional 6 bp in the middle of the membrane-spanning segment. Based on the analysis of a genomic DNA clone spanning these regions of N-CAM, the first two differences arise by alternate splicing of RNA and occur in some, but not all clones; the additional 6 bp may reflect a genetic polymorphism. A second cDNA clone, NI-10, encodes the complete sequence of a segment that is specific to the large cytoplasmic domain (180 kDa) polypeptide of human N-CAM and is very similar to corresponding segments of mouse, chicken, and rat N-CAM. This sequence also arises by alternative splicing of RNA. In addition, we have identified a genomic DNA segment encoding sequences specific to the third, small surface domain (120 kDa) polypeptide of N-CAM. The data presented here and previously define the DNA sequences of the membrane-bound forms and known variants of human N-CAM. From these sequences, a wide variety of probes can be generated for investigating the expression of particular N-CAM polypeptides in normal and pathological tissues.

Identification of positive and negative regulatory elements governing cell-type-specific expression of the neural cell adhesion molecule gene

The neural cell adhesion molecule (NCAM) is one of the most prevalent cell adhesion molecules in vertebrates. Its expression is subject to complex cell-type- and developmental-stage-dependent regulation. To study this regulation at the level of transcription, we analyzed the promoter region of the mouse NCAM gene. The NCAM promoter did not contain a typical TATA box. Transcription started at several sites that were used indiscriminately by different cell types, implying that the different NCAM isoforms are expressed from a single promoter. Sequences responsible for both promotion and inhibition of transcription resided within 840 base pairs upstream of the main transcriptional start site. The sequence from positions -645 to -37 relative to the translation initiation site directed high levels of expression in NCAM-expressing N2A cells. The same fragment was six times less active but still significantly active in L cells, but this activity was repressed by inclusion of an additional upstream segment. We mapped eight domains of interactions with nuclear proteins within the 840-base-pair region. The segment with maximum promoter activity contained two adjacent footprints, the occupation of which appeared to be mutually exclusive. One of them corresponded to an Sp1-factor-binding consensus site, the other one bound a factor with nuclear factor I activity. The single protected domain in the fragment harboring a repressor activity consisted of a GGA repeat resembling negative regulatory elements in other promoters. Three adjacent binding sites occupied an A + T-rich segment and contained ATTA motifs also found in the recognition elements of homeodomain proteins. These results show that negative and positive elements interact to regulate the tissue-specific patterns of expression of the NCAM gene and indicate that a factor related to nuclear factor I is involved in its transcriptional control.

Characterization of the human N-CAM promoter

In contrast with the complex series of splicing choices that generate the various membrane-associated isoforms of the neural cell-adhesion molecule alternative splicing of 5' exons does not contribute to additional molecular diversity. A single regulatory unit in genomic DNA, mapping to a 5 kb restriction-endonuclease-HindIII fragment, controls the expression of all major RNA size classes. DNA sequence analysis of a 2 kb fragment spanning the two major identified transcriptional initiation sites (194 and 188 bp from the ATG codon) and translation start codon indicates that the regulatory unit does not possess classical TATA or CCAAT motifs. The region of the putative promoter exhibits a GC-rich content and a high frequency of the dinucleotide CpG, both characteristics of a HTF(HpaII tiny fragments)-island. Introduction of deletion-mutant chimaeric-gene constructs into human and rodent N-CAM-expressing cell lines defines an active promoter region of 467 bp (-144 to -611 bp from the ATG codon). This region of genomic DNA contains consensus sites for the interaction of known transcriptional factors.

Identification of positive and negative regulatory elements governing cell-type-specific expression of the neural cell adhesion molecule gene

The neural cell adhesion molecule (NCAM) is one of the most prevalent cell adhesion molecules in vertebrates. Its expression is subject to complex cell-type- and developmental-stage-dependent regulation. To study this regulation at the level of transcription, we analyzed the promoter region of the mouse NCAM gene. The NCAM promoter did not contain a typical TATA box. Transcription started at several sites that were used indiscriminately by different cell types, implying that the different NCAM isoforms are expressed from a single promoter. Sequences responsible for both promotion and inhibition of transcription resided within 840 base pairs upstream of the main transcriptional start site. The sequence from positions -645 to -37 relative to the translation initiation site directed high levels of expression in NCAM-expressing N2A cells. The same fragment was six times less active but still significantly active in L cells, but this activity was repressed by inclusion of an additional upstream segment. We mapped eight domains of interactions with nuclear proteins within the 840-base-pair region. The segment with maximum promoter activity contained two adjacent footprints, the occupation of which appeared to be mutually exclusive. One of them corresponded to an Sp1-factor-binding consensus site, the other one bound a factor with nuclear factor I activity. The single protected domain in the fragment harboring a repressor activity consisted of a GGA repeat resembling negative regulatory elements in other promoters. Three adjacent binding sites occupied an A + T-rich segment and contained ATTA motifs also found in the recognition elements of homeodomain proteins. These results show that negative and positive elements interact to regulate the tissue-specific patterns of expression of the NCAM gene and indicate that a factor related to nuclear factor I is involved in its transcriptional control.

Differential splicing generates a nervous system-specific form of Drosophila neuroglian

We recently described the characterization and cloning of Drosophila neuroglian, a member of the immunoglobulin superfamily. Neuroglian contains six immunoglobulin-like domains and five fibronectin type III domains and shows strong sequence homology to the mouse neural cell adhesion molecule L1. Here we show that the neuroglian gene generates at least two different protein products by tissue-specific alternative splicing. The two protein forms differ in their cytoplasmic domains. The long form is restricted to the surface of neurons in the CNS and neurons and some support cells in the PNS; in contrast, the short form is expressed on a wide range of other cells and tissues. Thus, whereas the mouse L1 gene appears to encode only one protein that functions largely as a neural cell adhesion molecule, its Drosophila homolog, the neuroglian gene, encodes at least two protein forms that may play two different roles, one as a neural cell adhesion molecule and the other as a more general cell adhesion molecule involved in other tissues and imaginal disc morphogenesis.