Overall Role of Contactins Expression in Neurodevelopmental Events and Contribution to Neurological Disorders

BACKGROUND

Neurodegenerative disorders may depend upon a misregulation of the pathways which sustain neurodevelopmental control. In this context, this review article focuses on Friedreich ataxia (FA), a neurodegenerative disorder resulting from mutations within the gene encoding the Frataxin protein, which is involved in the control of mitochondrial function and oxidative metabolism.

OBJECTIVE

The specific aim of the present study concerns the FA molecular and cellular substrates, for which available transgenic mice models are proposed, including mutants undergoing misexpression of adhesive/morphoregulatory proteins, in particular belonging to the Contactin subset of the immunoglobulin supergene family.

METHODS

In both mutant and control mice, neurogenesis was explored by morphological/morphometric analysis through the expression of cell type-specific markers, including -tubulin, the Contactin-1 axonal adhesive glycoprotein, as well as the Glial Fibrillary Acidic Protein (GFAP).

RESULTS

Specific consequences were found to arise from the chosen misexpression approach, consisting of a neuronal developmental delay associated with glial upregulation. Protective effects against the arising phenotype resulted from antioxidants (essentially epigallocatechin gallate (EGCG)) administration, which was demonstrated through the profiles of neuronal (-tubulin and Contactin 1) as well as glial (GFAP) markers, in turn indicating the concomitant activation of neurodegeneration and neuro repair processes. The latter also implied activation of the Notch-1 signaling.

CONCLUSION

Overall, this study supports the significance of changes in morphoregulatory proteins expression in the FA pathogenesis and of antioxidant administration in counteracting it, which, in turn, allows to devise potential therapeutic approaches.

The Cerebellar Dopaminergic System

In the central nervous system (CNS), dopamine (DA) is involved in motor and cognitive functions. Although the cerebellum is not been considered an elective dopaminergic region, studies attributed to it a critical role in dopamine deficit-related neurological and psychiatric disorders [e.g., Parkinson's disease (PD) and schizophrenia (SCZ)]. Data on the cerebellar dopaminergic neuronal system are still lacking. Nevertheless, biochemical studies detected in the mammalians cerebellum high dopamine levels, while chemical neuroanatomy studies revealed the presence of midbrain dopaminergic afferents to the cerebellum as well as wide distribution of the dopaminergic receptor subtypes (DRD1-DRD5). The present review summarizes the data on the cerebellar dopaminergic system including its involvement in associative and projective circuits. Furthermore, this study also briefly discusses the role of the cerebellar dopaminergic system in some neurologic and psychiatric disorders and suggests its potential involvement as a target in pharmacologic and non-pharmacologic treatments.

Molecular and Cellular Substrates for the Friedreich Ataxia. Significance of Contactin Expression and of Antioxidant Administration

In this study, the neural phenotype is explored in rodent models of the spinocerebellar disorder known as the Friedreich Ataxia (FA), which results from mutations within the gene encoding the Frataxin mitochondrial protein. For this, the M12 line, bearing a targeted mutation, which disrupts the Frataxin gene exon 4 was used, together with the M02 line, which, in addition, is hemizygous for the human Frataxin gene mutation (Pook transgene), implying the occurrence of 82-190 GAA repeats within its first intron. The mutant mice phenotype was compared to the one of wild type littermates in regions undergoing differential profiles of neurogenesis, including the cerebellar cortex and the spinal cord by using neuronal (β-tubulin) and glial (Glial Fibrillary Acidic Protein) markers as well as the Contactin 1 axonal glycoprotein, involved in neurite growth control. Morphological/morphometric analyses revealed that while in Frataxin mutant mice the neuronal phenotype was significantly counteracted, a glial upregulation occurred at the same time. Furthermore, Contactin 1 downregulation suggested that changes in the underlying gene contributed to the disorder pathogenesis. Therefore, the FA phenotype implies an alteration of the developmental profile of neuronal and glial precursors. Finally, epigallocatechin gallate polyphenol administration counteracted the disorder, indicating protective effects of antioxidant administration.

Modulation of Nerve Cell Differentiation: Role of Polyphenols and of Contactin Family Components

In this study the mechanisms are explored, which modulate expression and function of cell surface adhesive glycoproteins of the Immunoglobulin Supergene Family (IgSF), and in particular of its Contactin subset, during neuronal precursor developmental events. In this context, a specific topic concerns the significance of the expression profile of such molecules and their ability to modulate signaling pathways activated through nutraceuticals, in particular polyphenols, administration. Both in vitro and in vivo approaches are chosen. As for the former, by using as a model the human SH-SY5Y neuroblastoma line, the effects of grape seed polyphenols are evaluated on proliferation and commitment/differentiation events along the neuronal lineage. In SH-SY5Y cell cultures, polyphenols were found to counteract precursor proliferation while promoting their differentiation, as deduced by studying their developmental parameters through the expression of cell cycle and neuronal commitment/differentiation markers as well as by measuring neurite growth. In such cultures, Cyclin E expression and BrdU incorporation were downregulated, indicating reduced precursor proliferation while increased neuronal differentiation was inferred from upregulation of cell cycle exit (p27^–Kip) and neuronal commitment (NeuN) markers as well as by measuring neurite length through morphometric analysis. The polyphenol effects on developmental parameters were also explored in vivo, in cerebellar cortex, by using as a model the TAG/F3 transgenic line, which undergoes delayed neural development as a consequence of Contactin1 adhesive glycoprotein upregulation and premature expression under control of the Contactin2 gene (Cntn-2) promoter. In this transgenic line, a Notch pathway activation is known to occur and polyphenol treatment was found to counteract such an effect, demonstrated through downregulation of the Hes-1 transcription factor. Polyphenols also downregulated the expression of adhesive glycoproteins of the Contactin family themselves, demonstrated for both Contactin1 and Contactin2, indicating the involvement of changes in the expression of the underlying genes in the observed phenotype. These data support the hypothesis that the complex control exerted by polyphenols on neural development involves modulation of expression and function of the genes encoding cell adhesion molecules of the Contactin family and of the associated signaling pathways, indicating potential mechanisms whereby such compounds may control neurogenesis.

Performance assessment of a fully automated electro-chemiluminescence immunoassay system for serum S100B protein

The altered expression levels of S100 proteins can lead to four different categories of diseases: diseases of the heart and of the central nervous system, inflammatory disorders and cancer. Various studies have shown the lack of harmonization of the results obtained with different methods, mainly due to different performances and measurements of S100B. The purpose of this work was to compare quantitatively the fully automated Elecsys® immunoassay with the reference immunoenzimatic method CanAg® EIA for serum S100B protein. In the study serum samples were analyzed of 161 patients: 85 females (aged 22-83 years) and 76 males (aged 16-90 years), affected by oncological and non-oncological pathologies. Passing–Bablok regression was used to analyze the comparison between the assays; it showed a strong interassay correlation: r = 0.9350 (95% CI =0.9122 – 0.9520), with an intercept of 0.02063 (95% CI=-0.02850 – 0.01400) and a slope of 1.1125 (95% CI=1.0200 – 1.2417). Elecsys® S100 assay should be preferred to CanAg® S100 for better standardization, good reliability and precision but also with the aim to reduce costs and obtain results in a shorter time.

Role of the adhesion molecule F3/Contactin in synaptic plasticity and memory

Cell adhesion molecules (CAMs) have a pivotal role in building and maintaining synaptic structures during brain development participating in axonal elongation and pathfinding, glial guidance of neuronal migration, as well as myelination. CAMs expression persists in the adult brain particularly in structures undergoing postnatal neurogenesis and involved in synaptic plasticity and memory as the hippocampus. Among the neural CAMs, we have recently focused on F3/Contactin, a glycosylphosphatidyl inositol-anchored glycoprotein belonging to the immunoglobulin superfamily, involved in neuronal development, synaptic maintenance and organization of neuronal networks. Here, we discuss our recent data suggesting that F3/Contactin exerts a role in hippocampal synaptic plasticity and memory in adult and aged mice. In particular, we have studied long-term potentiation (LTP), spatial and object recognition memory, and phosphorylation of the transcription factor cAMP-Responsive-Element Binding protein (CREB) in a transgenic mouse model of F3/Contactin overexpression. We also investigated whether F3/Contactin might influence neuronal apoptosis and the production of amyloid-beta peptide (Aβ), known to be one of the main pathogenetic hallmarks of Alzheimer's disease (AD). In conclusion, a further understanding of F3/Contactin role in synaptic plasticity and memory might have interesting clinical outcomes in cognitive disorders, such as aging and AD, offering innovative therapeutic opportunities.

The role of Gpi-anchored axonal glycoproteins in neural development and neurological disorders

This review article focuses on the Contactin (CNTN) subset of the Immunoglobulin supergene family (IgC2/FNIII molecules), whose components share structural properties (the association of Immunoglobulin type C2 with Fibronectin type III domains), as well as a general role in cell contact formation and axonal growth control. IgC2/FNIII molecules include 6 highly related components (CNTN 1-6), associated with the cell membrane via a Glycosyl Phosphatidyl Inositol (GPI)-containing lipid tail. Contactin 1 and Contactin 2 share ~50 (49.38)% identity at the aminoacid level. They are components of the cell surface, from which they may be released in soluble forms. They bind heterophilically to multiple partners in cis and in trans, including members of the related L1CAM family and of the Neurexin family Contactin-associated proteins (CNTNAPs or Casprs). Such interactions are important for organising the neuronal membrane, as well as for modulating the growth and pathfinding of axon tracts. In addition, they also mediate the functional maturation of axons by promoting their interactions with myelinating cells at the nodal, paranodal and juxtaparanodal regions. Such interactions also mediate differential ionic channels (both Na⁺ and K⁺) distribution, which is of critical relevance in the generation of the peak-shaped action potential. Indeed, thanks to their interactions with Ankyrin G, Na⁺ channels map within the nodal regions, where they drive axonal depolarization. However, no ionic channels are found in the flanking Contactin1-containing paranodal regions, where CNTN1 interactions with Caspr1 and with the Ig superfamily component Neurofascin 155 in cis and in trans, respectively, build a molecular barrier between the node and the juxtaparanode. In this region K⁺ channels are clustered, depending upon molecular interactions with Contactin 2 and with Caspr2. In addition to these functions, the Contactins appear to have also a role in degenerative and inflammatory disorders: indeed Contactin 2 is involved in neurodegenerative disorders with a special reference to the Alzheimer disease, given its ability to work as a ligand of the Alzheimer Precursor Protein (APP), which results in increased Alzheimer Intracellular Domain (AICD) release in a γ-secretase-dependent manner. On the other hand Contactin 1 drives Notch signalling activation via the Hes pathway, which could be consistent with its ability to modulate neuroinflammation events, and with the possibility that Contactin 1-dependent interactions may participate to the pathogenesis of the Multiple Sclerosis and of other inflammatory disorders.

Role of F3/contactin expression profile in synaptic plasticity and memory in aged mice

We have recently shown that overexpression of the F3/contactin adhesive glycoprotein (also known as Contactin-1) promotes neurogenesis in adult hippocampus, which correlates with improved synaptic plasticity and memory. Because F3/contactin levels physiologically decrease with age, here, we aim at investigating whether its overexpression might counteract the cognitive decline in aged animals. For this we use 20- to 24-month-old TAG/F3 transgenic mice in which F3/contactin overexpression is driven by regulatory sequences from the gene encoding the transient axonal glycoprotein TAG-1 throughout development. We show that aged TAG/F3 mice display improved hippocampal long-term potentiation and memory compared with wild-type littermates. The same mice undergo a decrease of neuronal apoptosis at the hippocampal level, which correlated to a decrease of active caspase-3; by contrast, procaspase-3 and Bax as well as the anti-apoptotic and plasticity-related pathway BDNF/CREB/Bcl-2 were rather increased. Interestingly, amyloid-precursor protein processing was shifted toward sAPPα generation, with a decrease of sAPPβ and amyloid-beta levels. Our data confirm that F3/contactin plays a role in hippocampal synaptic plasticity and memory also in aged mice, suggesting that it acts on molecular pathways related to apoptosis and amyloid-beta production.

Neurobehavioral studies, in transgenic F3/CONTACTIN (C57BL/6J × CBA) mice, on cognitive and anxiety aspects during late-adolescential period

OBJECTIVE

Besides than in the control of developmental events, axonal adhesive glycoproteins may be also involved in functions requiring fine organization and connectivity of the nervous tissue. We previously demonstrated morphological alterations and functional cerebellar deficits in transgenic mice (TAG/F3 mice) ectopically expressing the F3/Contactin axonal glycoprotein under the control of a selected regulatory region from the Transient Axonal Glycoprotein (TAG-1) gene. In the present study, the hippocampal function was explored by evaluating the ability of TAG/F3 mice to encode spatial and non-spatial relationships between discrete stimuli and to analyze an anxiety-related behavior.

MATERIALS AND METHODS

To the first end, mice were placed in an "open-Field" containing five objects and, after three sessions of habituation (S2-S4), their reactivity to objects displacement (S5-S4) and object substitution (S7-S6) was examined.To the second end, mice were placed in the "elevated zero maze", a standard test to explore the anxiety-related behavior, in order to study, in transgenic mice, the effects of F3 misexpression on emotional reactivity by measuring the avoidance of the unsheltered open sectors.

RESULTS

Statistical evaluations of reactivity to object novelty, TAG-F3 mice showed a lower DO exploration with respect to wild-type mice and, regarding DOs, TAG/F3 mice interacted less than wild-type mice, showing an impaired spatial change response. Furthermore, the number of HDIPS in transgenic TAG/F3 mice resulted significantly lower with respect to the controls (wild type).

CONCLUSIONS

These results indicate that the coordinated expression of axonal adhesive glycoproteins may be relevant for the functional maturation of the hippocampus.

F3/Contactin promotes hippocampal neurogenesis, synaptic plasticity, and memory in adult mice

F3/contactin, a cell-adhesion molecule belonging to the immunoglobulin supergene family, is involved in several aspects of neural development including synapse building, maintenance and functioning. Here, we examine F3/contactin function in adult hippocampal neurogenesis, synaptic plasticity, and memory, using as a model TAG/F3 transgenic mice, where F3/contactin overexpression was induced under control of regulatory sequences from the human TAG-1 (TAX-1) gene. Transgenic mice aged 5 (M5) and 12 (M12) months exhibited an increase in hippocampal size, which correlated with positive effects on precursor proliferation and NeuN expression, these data suggesting a possible role for F3/contactin in promoting adult hippocampal neurogenesis. On the functional level, TAG/F3 mice exhibited increased CA1 long-term potentiation and improved spatial and object recognition memory, notably at 12 months of age. Interestingly, these mice showed an increased expression of the phosphorylated transcription factor CREB, which may represent the main molecular correlate of the observed morphological and functional effects. Altogether, these findings indicate for the first time that F3/contactin plays a role in promoting adult hippocampal neurogenesis and that this effect correlates with improved synaptic function and memory.

Significance of F3/Contactin gene expression in cerebral cortex and nigrostriatal development

F3/Contactin is a neuronal surface glycoprotein, which plays a general role in neural development and, in particular, in neuronal and oligodendrocyte differentiation. In a previous study using the F3/EGFP transgenic mice, which express an EGFP reporter under control of the regulatory region from the mouse F3/Contactin gene, the activation of the F3/Contactin promoter was found to correlate with granule and Purkinje neuron differentiation in developing cerebellar cortex. Here we report that in developing cerebral cortex and basal ganglia the F3/Contactin gene is mostly activated during early commitment of neuronal precursors, thus indicating a region-specific profile of its developmental activation. We also report that, in the same structures of F3/EGFP mice, a downregulation of the endogenous F3/Contactin gene occurs, which correlates with upregulation of the dopaminergic phenotype and with locomotor pattern abnormalities. Therefore, F3/EGFP transgenic mice exhibit morphological and functional phenotypes recapitulating those arising from imbalance of the striatal dopaminergic pathway. As for the underlying mechanisms, we postulate that in F3/EGFP mice F3/Contactin downregulation results from the ability of transgene promoter sequences to interfere with the activation of the endogenous gene, thus realizing an F3/Contactin knockdown model, while dopaminergic upregulation is consistent with a general F3/Contactin inhibitory effect on the neuronal phenotype.

F3/contactin and TAG1 play antagonistic roles in the regulation of sonic hedgehog-induced cerebellar granule neuron progenitor proliferation

Modulation of the sonic hedgehog (SHH) pathway is a crucial factor in cerebellar morphogenesis. Stimulation of granule neuron progenitor (GNP) proliferation is a central function of SHH signalling, but how this is controlled locally is not understood. We show that two sequentially expressed members of the contactin (CNTN) family of adhesion molecules, TAG1 and F3, act antagonistically to control SHH-induced proliferation: F3 suppresses SHH-induced GNP proliferation and induces differentiation, whereas TAG1 antagonises F3. Production of GNPs in TAG1-null mice is delayed and reduced. F3 and TAG1 colocalise on GNPs with the related L1-like adhesion molecule NrCAM, and F3 fails to suppress the SHH-induced proliferation of NrCAM-deficient GNPs. We show that F3 and SHH both primarily affect a group of intermediate GNPs (IPs), which, though actively dividing, also express molecules associated with differentiation, including β-tubulin III (TuJ1) and TAG1. In vivo, intermediate progenitors form a discrete layer in the middle of the external germinal layer (mEGL), while F3 becomes expressed on the axons of postmitotic granule neurons as they leave the inner EGL (iEGL). We propose, therefore, that F3 acts as a localised signal in the iEGL that induces SHH-stimulated cells in the overlying mEGL to exit cell cycle and differentiate. By contrast, expression of TAG1 on GNPs antagonises this signal in the mEGL, preventing premature differentiation and sustaining GNP expansion in a paracrine fashion. Together, these findings indicate that CNTN and L1-like proteins play a significant role in modulating SHH-induced neuronal precursor proliferation.

Characterization and role of Helix contactin-related proteins in cultured Helix pomatia neurons

We report on the structural and functional properties of the Helix contactin-related proteins (HCRPs), a family of closely related glycoproteins previously identified in the nervous system of the land snail Helix pomatia through antibodies against the mouse F3/contactin glycoprotein. We focus on HCRP1 and HCRP2, soluble FNIII domains-containing proteins of 90 and 45 kD bearing consensus motifs for both N- and O-glycosylation. Using the anti-HCRPs serum, we find secreted HCRPs in Helix nervous tissue isotonic extracts and in culture medium conditioned by Helix ganglia. In addition, we demonstrate expression of HCRPs on neuronal soma and on neurite extensions. Functionally, in Helix neurons, the antisense HCRP2 mRNA counteracts neurite elongation, and the recombinant HCRP2 protein exerts a strong positive effect on neurite growth when used as substrate. These data point to HCRPs as novel neurite growth-promoting molecules expressed in invertebrate nervous tissue.

The mouse F3/contactin glycoprotein: structural features, functional properties and developmental significance of its regulated expression

F3/Contactin is an immunoglobulin superfamily component expressed in the nervous tissue of several species. Here we focus on the structural and functional properties of its mouse relative, on the mechanisms driving its regulated expression and on its developmental role. F3/Contactin is differentially expressed in distinct populations of central and peripheral neurons and in some non-neuronal cells. Accordingly, the regulatory region of the underlying gene includes promoter elements undergoing differential activation, associated with an intricate splicing profile, indicating that transcriptional and posttranscriptional mechanisms contribute to its expression. Transgenic models allowed to follow F3/Contactin promoter activation in vivo and to modify F3/Contactin gene expression under a heterologous promoter, which resulted in morphological and functional phenotypes. Besides axonal growth and pathfinding, these concerned earlier events, including precursor proliferation and commitment. This wide role in neural ontogenesis is consistent with the recognized interaction of F3/Contactin with developmental control genes belonging to the Notch pathway.

F3/contactin-related proteins in Helix pomatia nervous tissue (HCRPs): distribution and function in neurite growth and neurotransmitter release

By using antibodies against mouse F3/contactin, we found immunologically related glycoproteins expressed in the nervous tissue of the snail Helix pomatia. Helix contactin-related proteins (HCRPs) include different molecules ranging in size from 90 to 240 kD. Clones isolated from a cDNA expression library allowed us to demonstrate that these proteins are translated from a unique 6.3-kb mRNA, suggesting that their heterogeneity depends on posttranslational processing. This is supported by the results of endoglycosidase F treatment, which indicate that the high-molecular-weight components are glycosylation variants of the 90-kD chain. In vivo and in cultures, HCRPs antibodies label neuronal soma and neurite extensions, giving the appearance of both cytoplasmic and cell surface immunostaining. On the other hand, no expression is found on nonneural tissues. Functionally, HCRPs are involved in neurite growth control and appear to modulate neurotransmitter release, as indicated by the inhibiting effects of specific antibodies on both functions. These data allow the definition of HCRPs glycoproteins as growth-promoting molecules, suggesting that they play a role in neurite development and presynaptic terminal maturation in the invertebrate nervous system.

Activation profile of the F3/Contactin gene in the developing mouse cerebellum

In this study, we address the activation profile of the gene encoding the mouse axonal glycoprotein F3/Contactin. Promoter sequences previously characterized in vitro are used to drive an Enhanced Green Fluorescent Protein reporter in transgenic mice. In developing cerebellum, differential transgene expression occurs within distinct cell populations. At P0 the transgene is activated in postmitotic granule neurons undergoing radial migration, a sharp upregulation occurring at P6-P8, with a gradual decline from this stage onward. In Purkinje cells, promoter activation, first detected at P3, peaks at around P6 and is fully downregulated by P16. The transgene is also expressed in Ng2- and O4-positive cells, mostly at the end of the first postnatal week, suggesting correlation with early oligodendrocyte differentiation. These data indicate that the complex organization of the regulatory region of the F3/Contactin gene is necessary for directing its articulated expression in different neural cells types and for its developmental function.

Early coevolution of adhesive but not antiadhesive tenascin-R ligand-receptor pairs in vertebrates: A phylogenetic study

Axon growth inhibitory CNS matrix proteins, such as tenascin-R (TN-R), have been supposed to contribute to the poor regenerative capacity of adult mammalian CNS. With regard to TN-R function in low vertebrates capable of CNS regeneration, questions of particular interest concern the (co)evolution of ligand-receptor pairs and cellular response mechanisms associated with axon growth inhibition and oligodendrocyte differentiation. We address here these questions in a series of comparative in vivo and in vitro analyses using TN-R proteins purified from different vertebrates (from fish to human). Our studies provide strong evidence that unlike TN-R of higher vertebrates, fish TN-R proteins are not repellent for fish and less repellent for mammalian neurons and do not interfere with F3/contactin- and fibronectin-mediated mammalian cell adhesion and axon growth. However, axonal repulsion is induced in fish neurons by mammalian TN-R proteins, suggesting that the intracellular inhibitory machinery induced by TN-R-F3 interactions is already present during early vertebrate evolution. In contrast to TN-R-F3, TN-R-sulfatide interactions, mediating oligodendrocyte adhesion and differentiation, are highly conserved during vertebrate evolution. Our findings thus indicate the necessity of being cautious about extrapolations of the function of ligand-receptor pairs beyond a species border and, therefore, about the phylogenetic conservation of a molecular function at the cellular/tissue level.

A key role for the HLH transcription factor EBF2COE2,O/E-3 in Purkinje neuron migration and cerebellar cortical topography

Early B-cell factor 2 (EBF2) is one of four mammalian members of an atypical helix-loop-helix transcription factor family (COE). COE proteins have been implicated in various aspects of nervous and immune system development. We and others have generated and described mice carrying a null mutation of Ebf2, a gene previously characterized in the context of Xenopus laevis primary neurogenesis and neuronal differentiation. In addition to deficits in neuroendocrine and olfactory development, and peripheral nerve maturation, Ebf2 null mice feature an ataxic gait and obvious motor deficits associated with clear-cut abnormalities of cerebellar development. The number of Purkinje cells (PCs) in the Ebf2 null is markedly decreased, resulting in a small cerebellum with notable foliation defects,particularly in the anterior vermis. We show that this stems from the defective migration of a molecularly defined PC subset that subsequently dies by apoptosis. Part of the striped cerebellar topography is disrupted due to cell death and, in addition, many of the surviving PCs, that would normally adopt a zebrin II-negative phenotype, transdifferentiate to Zebrin II-positive, an unprecedented finding suggesting that Ebf2 is required for the establishment of a proper cerebellar cortical map.

Cross-talk between F3/contactin and Notch at axoglial interface: a role in oligodendrocyte development

Increasing evidence has shown that the Notch signalling pathway regulates oligodendrogliogenesis. Upon binding to classical Delta/Serrate/Lag-2 ligands, Notch signalling promotes generation of oligodendrocyte precursor cells while inhibiting their further differentiation into myelinating oligodendrocytes. In our recent studies, we have found that two neural cell adhesion molecules, F3/contactin and NB-3 interact with Notch receptors and promote oligodendrocyte development. Remarkably, all these F3 and NB-3/Notch cascade-related events required Deltex1 as the intermediate element. Experiments using several animal models further imply the function of F3/Notch signalling in vivo, which designates Notch signalling as a ligand-dependent, multipotential cascade involved in oligodendrocyte development.

Transgenic models for studying expression and function of axonal adhesive glycoproteins

In this study, by using two transgenic models, we address the general topic of the significance of axonal glycoproteins regulated expression in nervous tissue maturation. The immunoglobulin superfamily components F3/Contactin (F3) and TAG-1 are used as the molecular models in this respect. First, a minigene including the relevant regulatory sequences of the F3 gene, deduced by a previous in vitro study, has been fused to an EGFP (Enhanced Green Fluorescent Protein) reporter and expressed in transgenic mice, which provided information about the profile of F3 gene developmental activation. In a complementary model, transgenic mice have been generated which express the F3 cDNA under control of a selected regulatory region from the TAG-1 gene. While leading to ectopic expression of F3, this perturbed neuronal precursor proliferation and differentiation. The arising effects were even stronger than those coming from the overall suppression of the F3 or, respectively, TAG-1 genes, thus supporting the hypothesis that the mechanisms underlying axonal glycoprotein regulated expression are themselves endowed with a key significance in neural development.

The neuronal growth and regeneration associated Cntn1 (F3/F11/Contactin) gene is duplicated in fish: expression during development and retinal axon regeneration

The Cntn1 (Contactin/F3/F11) cell adhesion molecule is involved in axon growth and guidance, fasciculation, synapse formation, and myelination in birds and mammals. We identified Cntn1 genes in goldfish, zebrafish, and fugu, and provide evidence for a fish-specific duplication leading to Cntn1a and Cntn1b. Our analyses suggest a subfunctionalization for the Cntn1 paralogs in zebrafish compared to other vertebrates which have a single Cntn1 gene. Similar to Cntn1a, Cntn1b transcripts are found in subsets of sensory and motor neurons. However, Cntn1b is detected later and more restricted than Cntn1a. This spatio-temporal expression pattern of the two zebrafish Cntn1 paralogs suggests functions related to those of mammalian Cntn1. In adult goldfish, Cntn1b is expressed in oligodendrocytes and is upregulated in retinal ganglion cells after optic nerve transection, which is consistent with an additional role during regeneration.

Transgenic mice expressing F3/contactin from the transient axonal glycoprotein promoter undergo developmentally regulated deficits of the cerebellar function

We have shown that transgenic transient axonal glycoprotein (TAG)/F3 mice, in which the mouse axonal glycoprotein F3/contactin was misexpressed from a regulatory region of the gene encoding the transient axonal glycoprotein TAG-1, exhibit a transient disruption of cerebellar granule and Purkinje cell development [Development 130 (2003) 29]. In the present study we explore the neurobehavioural consequences of this mutation. We report on assays of reproductive parameters (gestation length, litter size and offspring viability) and on somatic and neurobehavioural end-points (sensorimotor development, homing performance, motor activity, motor coordination and motor learning). Compared with wild-type littermates, TAG/F3 mice display delayed sensorimotor development, reduced exploratory activity and impaired motor activity, motor coordination and motor learning. The latter parameters, in particular, were affected also in adult mice, despite the apparent recovery of cerebellar morphology, suggesting that subtle changes of neuronal circuitry persist in these animals after development is complete. These behavioural deficits indicate that the finely coordinated expression of immunoglobulin-like cell adhesion molecules such as TAG-1 and F3/contactin is of key relevance to the functional, as well as morphological maturation of the cerebellum.

Transgenic mice expressing F3/contactin from the TAG-1 promoter exhibit developmentally regulated changes in the differentiation of cerebellar neurons

F3/contactin (CNTN1) and TAG-1 (CNTN2) are closely related axonal glycoproteins that are differentially regulated during development. In the cerebellar cortex TAG-1 is expressed first as granule cell progenitors differentiate in the premigratory zone of the external germinal layer. However, as these cells begin radial migration, TAG-1 is replaced by F3/contactin. To address the significance of this differential regulation, we have generated transgenic mice in which F3/contactin expression is driven by TAG-1 gene regulatory sequences, which results in premature expression of F3/contactin in granule cells. These animals (TAG/F3 mice) display a developmentally regulated cerebellar phenotype in which the size of the cerebellum is markedly reduced during the first two postnatal weeks but subsequently recovers. This is due in part to a reduction in the number of granule cells, most evident in the external germinal layer at postnatal day 3 and in the inner granular layer between postnatal days 8 and 11. The reduction in granule cell number is accompanied by a decrease in precursor granule cell proliferation at postnatal day 3, followed by an increase in the number of cycling cells at postnatal day 8. In the same developmental window the size of the molecular layer is markedly reduced and Purkinje cell dendrites fail to elaborate normally. These data are consistent with a model in which deployment of F3/contactin on granule cells affects proliferation and differentiation of these neurons as well as the differentiation of their synaptic partners, the Purkinje cells. Together, these findings indicate that precise spatio-temporal regulation of TAG-1 and F3/contactin expression is critical for normal cerebellar morphogenesis.

Alternative promoters drive the expression of the gene encoding the mouse axonal glycoprotein F3/contactin

F3/Contactin is a neuronal glycoprotein which mediates axonal growth control via complex interactions with a number of cell surface or matrix components. As part of this developmental role, its expression undergoes differential regulation during the maturation of definite neuronal populations within the central and peripheral nervous tissue. To elucidate the underlying molecular mechanisms we study here the organization of the regulatory region of the mouse F3/Contactin gene. We show that this region displays peculiar features in that it spans more than 80 kb, bears very large introns and includes four untranslated exons which undergo complex splicing events leading to 11 potential arrangements of the F3/Contactin mRNA 5' end. Within this region we identify three alternative neurospecific promoters which, as deduced from the developmental profile of the associated 5' exons (A1,C1,0), drive two different patterns of F3/Contactin gene expression. The activity of the A1 exon-associated promoter displays only minor developmental changes and is likely to contribute to the basal level of the F3/Contactin gene expression; by contrast, the activities of the exon C1- and exon 0-associated promoters are significantly upregulated at the end of the first postnatal week. The data indicate that differential regulation of the F3/Contactin expression during development may depend upon alternative utilization of distinct promoter elements and may involve complex splicing events of the 5' untranslated exons. Several consensuses for homeogene transcription factors are scattered within the identified regulatory region, in agreement with the general assumption of homeotic gene regulation of neural morphoregulatory molecules.

Regional distribution and cell type-specific expression of the mouse F3 axonal glycoprotein: a developmental study

The expression of the mouse axonal adhesive glycoprotein F3 and of its mRNA was studied on sections of mouse cerebellar cortex, cerebral cortex, hippocampus, and olfactory bulb from postnatal days 0 (P0) to 30 (P30). In cerebellar cortex, a differential expression of F3 in granule versus Purkinje neurons was observed. F3 was highly expressed during migration of and initial axonal growth from cerebellar granule cells. The molecule was then downregulated on cell bodies and remained expressed, although at low levels, on their axonal extensions. On Purkinje cells, F3 was strongly expressed on cell bodies and processes at the beginning of the second postnatal week; by P16 it was restricted to neurites of Purkinje cells subpopulations. In the cerebral cortex, the molecule was highly expressed on migrating neurons at P0; by P16, it was found essentially within the neuropil with a diffuse pattern. In the hippocampal formation, where F3 was expressed on both pyramidal and granule neurons, a clear shift from the cell bodies to neurite extensions was observed on P3. In the olfactory pathway, F3 was expressed mainly on olfactory nerve fibers, mitral cells, and the synaptic glomeruli from P0 to P3, with a sharp decline from P11 to P16. As a whole, the data show that F3 protein expression is regulated at the regional, cellular, and subcellular levels and suggest that, in different regions, it can be proposed as a reliable neuronal differentiation marker.

Regulated expression of the cell adhesion glycoprotein F3 in adult hypothalamic magnocellular neurons

F3, a glycoprotein of the immunoglobulin superfamily implicated in axonal growth, occurs in oxytocin (OT)-secreting and vasopressin (AVP)-secreting neurons of the adult hypothalamo-neurohypophysial system (HNS) whose axons undergo morphological changes in response to stimulation. Immunocytochemistry and immunoblot analysis showed that during basal conditions of HNS secretion, there are higher levels of this glycosylphosphatidyl inositol-anchored protein in the neurohypophysis, where their axons terminate, than in the hypothalamic nuclei containing their somata. Physiological stimulation (lactation, osmotic challenge) reversed this pattern and resulted in upregulation of F3 expression, paralleling that of OT and AVP under these conditions. In situ hybridization revealed that F3 expression in the hypothalamus is restricted to its magnocellular neurons and demonstrated a more than threefold increase in F3 mRNA levels in response to stimulation. Confocal and electron microscopy localized F3 in secretory granules in all neuronal compartments, a localization confirmed by detection of F3 immunoreactivity in granule-enriched fractions obtained by sucrose density gradient fractionation of rat neurohypophyses. F3 was not visible on any cell surface in the magnocellular nuclei. In contrast, in the neurohypophysis, it was present not only in secretory granules but also on the surface of axon terminals and glia and in extracellular spaces. Taken together, our observations reveal that the cell adhesion glycoprotein F3 is colocalized with neurohypophysial peptides in secretory granules. It follows, therefore, the regulated pathway of secretion in HNS neurons to be released by exocytosis at their axon terminals in the neurohypophysis, where it may intervene in activity-dependent structural axonal plasticity.

Functional organization of the promoter region of the mouse F3 axonal glycoprotein gene

F3 is a developmentally regulated adhesive glycoprotein expressed by subpopulations of central and peripheral neurons which mediates neurite growth and fasciculation via cis- and trans-interactions with cell-surface or matrix components. We previously reported on the characterization of the F3 gene 5' flanking region in which we identified promoter and enhancer elements. Here, we report on the functional organization of the F3 gene regulatory regions. We show that the F3 promoter is built of linearly arranged positive and negative elements scattered through the 5' flanking region of the F3 gene and the 1st exon (exon 0). Neural- and cell type-specific expression of F3 appears to be governed by elements located in the most proximal promoter region which includes a neural-specific enhancer. In retardation assays, all these cis-acting elements bind nuclear proteins, three of which interact with the identified enhancer element while a single species interacts with sequences located within exon 0. Some of these proteins are also specifically expressed within the brain, indicating that they could correspond to neural-specific trans-acting factors. Elements located immediately upstream of the cell type-specific enhancer and within exon 0 are responsible for regulation of F3 expression by cAMP and retinoic acid.

Expression of the immunoglobulin superfamily cell adhesion molecule F3 by oligodendrocyte-lineage cells

We have analysed the expression of glycosylphosphatidylinositol (GPI)-anchored proteins by oligodendrocyte-lineage cells. Biosynthetic labeling of mouse oligodendroglial primary cultures and an oligodendroglial precursor cell line demonstrated that these cells synthesise a variety of different GPI-anchored proteins. GPI-anchored proteins were isolated as a bulk preparation from the precursor cell line, and the individual proteins separated by 2D gel electrophoresis and analysed by microsequencing after tryptic digestion of the separated components. One of the most prominent GPI-anchored proteins synthesised by the cell line was identified as the cell adhesion molecule F3, previously thought to be exclusively expressed by neurons. Western blotting and immunoprecipitation with several polyclonal sera confirmed the expression of F3 by oligodendrocyte-lineage cells and demonstrated the presence of F3 in myelin. Double staining with a panel of oligodendrocyte-specific antibodies and anti-F3 antibodies of cerebellar cultures, as well as oligodendrocytes isolated by panning, showed a colocalization of F3 with oligodendrocyte markers. Oligodendrocyte F3 is shown to be susceptible to phosphatidylinositol-phospholipase C (PI-PLC) cleavage, similar to neuronal F3. Northern blots demonstrated that the oligodendroglial F3 mRNA is the same size as the neuronal message; however, no F3 mRNA could be detected in cortical astrocytes and an astrocytic cell line. Thus, in addition to the expression by neurons, the cell-type specificity of F3 expression must be extended to oligodendroglial cells, underscoring the importance of this Ig superfamily member in the nervous system.

Expression of the immunoglobulin superfamily cell adhesion molecule F3 by oligodendrocyte-lineage cells

We have analysed the expression of glycosylphosphatidylinositol (GPI)-anchored proteins by oligodendrocyte-lineage cells. Biosynthetic labeling of mouse oligodendroglial primary cultures and an oligodendroglial precursor cell line demonstrated that these cells synthesise a variety of different GPI-anchored proteins. GPI-anchored proteins were isolated as a bulk preparation from the precursor cell line, and the individual proteins separated by 2D gel electrophoresis and analysed by microsequencing after tryptic digestion of the separated components. One of the most prominent GPI-anchored proteins synthesised by the cell line was identified as the cell adhesion molecule F3, previously thought to be exclusively expressed by neurons. Western blotting and immunoprecipitation with several polyclonal sera confirmed the expression of F3 by oligodendrocyte-lineage cells and demonstrated the presence of F3 in myelin. Double staining with a panel of oligodendrocyte-specific antibodies and anti-F3 antibodies of cerebellar cultures, as well as oligodendrocytes isolated by panning, showed a colocalization of F3 with oligodendrocyte markers. Oligodendrocyte F3 is shown to be susceptible to phosphatidylinositol-phospholipase C (PI-PLC) cleavage, similar to neuronal F3. Northern blots demonstrated that the oligodendroglial F3 mRNA is the same size as the neuronal message; however, no F3 mRNA could be detected in cortical astrocytes and an astrocytic cell line. Thus, in addition to the expression by neurons, the cell-type specificity of F3 expression must be extended to oligodendroglial cells, underscoring the importance of this Ig superfamily member in the nervous system.

Use of chimeric F3-NCAM molecules to explore the properties of VASE exon in modulating polysialylation and neurite outgrowth

Differential splicing of VASE exon in the fourth immunoglobulin (Ig) domain and attachment to the fifth Ig domain of alpha 2-8 linked sialic acid (PSA) both dramatically change, in opposite manner, Neural Cell Adhesion Molecule (NCAM) functional properties. Reciprocal patterns of VASE and PSA expression suggest that they might be mutually exclusive. Here, we tested whether informations conferring polysialylation reside in NCAM-Ig domains 4 and 5 and the influence of the VASE exon encoded sequence on this process. We also examined if the VASE sequence was still able to inhibit neurite outgrowth when presented out of its normal NCAM context. Constructs have been prepared encoding NCAM-Ig domains 4 (with or without the VASE exon) and 5 fused to the F3 molecule. Stable clones expressing the chimeric molecules or wild type F3 were then obtained in the AtT-20 cell line. Although the chimeric molecules were expressed on the cell surface none of them was bearing PSA. Thus, polysialylation cannot be conferred to proteins by addition of the NCAM-Ig domains 4 and 5 modular motif and in this molecular context, the VASE sequence is not influencing the process. These chimeric molecules, either expressed at the surface of RIN or COS cells or presented as soluble forms, were examined for their effect on neurite outgrowth. In all cases, the length of neurites of sensory neurons was significantly reduced when grown in presence of the VASE containing chimera by comparison with the chimera without VASE or wild type F3. When neurons from NCAM knock-out mice were used for the assay, the VASE inhibition could not be detected. Thus VASE is able to act as a modular motif and NCAM expressed on neurons participates in transducing its effect.

Characterization of the 5' and promoter regions of the gene encoding the mouse neuronal cell adhesion molecule F3

F3 is a 135 kDa neuronal cell surface adhesive glycoprotein belonging to the immunoglobulin supergene family (IgSF) which mediates heterophilic contact formation among neural cells and is involved in the control of neurite growth. F3 expression is regulated, during critical developmental periods, on neuronal subpopulations thus suggesting that control of F3 gene expression could be of morphogenetic relevance. To shed light on the mechanism involved in the control of F3 gene expression we isolated clones covering about 50 kilobases of the F3 gene which also included the promoter region. The study of F3 gene exon/intron organization revealed that, like other neural IgSF molecules, each of the first two F3 C2 domains is encoded by two exons while the N-terminus, the signal peptide and the 5' untranslated region are each encoded by distinct exons. A single transcription start site was identified, surrounded by a short 114 bp sequence able to direct reporter gene expression in both F3-expressing and -non-expressing cells. In addition, a cell type-specific enhancer, only active in F3-expressing cells, was found immediately upstream to it. Structural analysis of the promoter region revealed consensus sequences for binding transcription factors involved in cell type-specific and/or developmental regulations. Most of them are homeobox containing transcription factors thus suggesting that regulation of F3 gene expression could be part of a large developmental program.

Different domains of the F3 neuronal adhesion molecule are involved in adhesion and neurite outgrowth promotion

The mouse F3 cell surface protein is preferentially expressed on axons of subpopulations of neurons and is anchored to the membrane by a glycosyl-phosphatidylinositol group. It consists of six immunoglobulin-like domains and four fibronectin type III homologous repeats, and can be found both in membrane-anchored and soluble forms. We have previously established that F3 fulfills the operational criteria of a cell adhesion molecule when anchored to the plasma membrane and that its soluble form stimulates neurite initiation and neurite outgrowth. To further characterize F3-mediated adhesion and to investigate whether adhesion and neurite outgrowth promoting activities are displayed by different parts of the molecule, we (i) selected F3 transfected CHO cells expressing increasing levels of F3 at their surface and (ii) prepared transfectants expressing an F3 molecule with its fibronectin type III repeats deleted. We show that the F3 molecule mediates divalent-cation-independent, temperature-dependent binding. The levels of aggregation of F3 transfectants are proportional to the level of F3 expression. Transfectants expressing F3 deleted of the fibronectin type III repeats lose their adhesive properties; conversely, cells expressing wild-type F3 and treated with collagenase, specifically removing the immunoglobulin-like domains, are still able to aggregate. Therefore, in this model adhesion site(s) mapped to the fibronectin type III repeats. By contrast, transfectants expressing deleted F3, as well as the soluble forms of this F3 deleted molecule, were able to stimulate neurite outgrowth of sensory neurons similarly to wild-type F3. Our data indicate that F3 is a multifunctional molecule and that adhesion and neurite outgrowth promoting properties are expressed by distinct and independent domains.

Functional studies and cellular distribution of the F3 GPI-anchored adhesion molecule

Many adhesion molecules of the immunoglobulin superfamily expressed in the nervous system are attached to the neuronal membrane by a glycan-phosphatidylinositol. Using neuronal glycoprotein F3 as a model we will discuss how this lipid modification might confer on molecules specific properties which may be particularly well suited to a role in modulating neuronal interactions. In particular, the following data dealing with the question of how the glycosylphosphatidylinositol (GPI) anchor influences the function, transport and localization of this molecule will be presented. 1) When anchored to the plasma membrane, F3 fulfills the operational criteria of an adhesion molecule while its soluble form is able to stimulate neurite outgrowth of sensory neurons in culture. 2) In the hypothalamo-hypophyseal system, immunoblot analysis indicates that there is more F3 in the neurohypophysis where secretory axons terminate than in the hypothalamic nuclei where the molecule is synthesized. In addition, GPI-linked forms predominate in the nuclei while there are mainly soluble forms in the neurohypophysis, suggesting that there is conversion of the GPI-bearing form to the soluble form during axonal transport. 3) In the cerebellum, F3 is polarized to the tips of the axons of granule cells, the major neuronal population in this system, as an indication that indeed GPI might be a signal for targeting molecules to axons. However, some neurons such as Golgi cells express F3 over all their surface.

The F3/11 cell adhesion molecule mediates the repulsion of neurons by the extracellular matrix glycoprotein J1-160/180

The oligodendrocyte-derived extracellular matrix protein J1-160/180 displays repellent substrate properties toward neurons. In a search for neuronal ligands mediating the response to J1-160/180, we have identified the F3/11 cell surface protein, a glyco-phosphatidylinositol-anchored member of the immunoglobulin superfamily. F3/11 mediates the initial recognition between a J1-160/180 substrate and cerebellar neurons or F3-transfected CHO cells. In cerebellar neurons, the F3/11-J1-160/180 interaction induces a repulsion consisting of the loss of substrate adhesion with time in culture and inhibition of neurite outgrowth. Antibody blocking experiments show that the avoidance response of neurites at J1-160/180 substrate borders is also mediated by F3/11. Active cell-cell and cell-substrate repulsion is considered a major mechanism governing the extent and directionality of axonal growth, but the ligand-receptor interactions involved have remained unknown. Our results show that F3/11 mediates the neuronal response to the repellent molecule J1-160/180 and may thus be involved in signal transduction leading to cell repulsion.

A soluble form of the F3 neuronal cell adhesion molecule promotes neurite outgrowth

The F3 molecule is a member of the immunoglobulin superfamily anchored to membranes by a glycane-phosphatidylinositol, and is predominantly expressed on subsets of axons of the central and peripheral nervous system. In a previous paper (Gennarini, G., P. Durbec, A. Boned, G. Rougon, and C. Goridis. 1991. Neuron. 6:595-606), we have established that F3 fulfills the operational definition of a cell adhesion molecule and that it stimulates neurite outgrowth when presented to sensory neurons as a surface component of transfected CHO cells. In the present study the question as to whether soluble forms of F3 would be functionally active was addressed in vitro on cultures of mouse dorsal root ganglion neurons. We observed that preparations enriched in soluble F3 had no effect on neuron attachment but enhanced neurite initiation and neurite outgrowth in a dose-dependent manner. By contrast, soluble NCAM-120 does not have any measurable effect on these phenomena. Addition of anti-F3 monovalent antibodies reduced the number of process-bearing neurons and the neuritic output per neuron to control values. Addition of cerebrospinal fluid, a natural source of soluble F3, also stimulated neurite extension, and this effect was partially blocked by anti-F3 antibodies. Our results suggest that the soluble forms of adhesive proteins with neurite outgrowth-promoting properties could act at a distance from their site of release in a way reminiscent of growth and trophic factors.

F3/F11 cell surface molecule expression in the developing mouse cerebellum is polarized at synaptic sites and within granule cells

The distribution of the F3/F11 neuronal cell surface molecule was investigated in the developing and adult mouse cerebellum by immunocytochemistry at the light and electron microscopic levels. F3/F11 was confined to subsets of neuronal types, since the Purkinje cell body and dendritic arborization as well as the stellate cells were not immunoreactive. In the young developing cerebellum, the granule cell axons strongly express F3/F11 as soon as they begin to grow, consistent with a functional role in promoting directional outgrowth of neuronal processes. In 10-d-old and adult cerebella, the granule cell bodies and dendrites were not immunoreactive whereas the parallel fibers, which are the granule cell axons, were labeled including in their presynaptic varicosities. By contrast, dendrites, cell bodies, and axons of Golgi cells were labeled by anti-F3 antibodies. Hence, F3/F11 can either be expressed throughout the cell or be polarized to the axons. This raises the question of how segregation of the glypiated F3/F11 molecule between different subcellular compartments depending on the type of neuron is achieved. F3/F11 was found to be present at three types of synaptic sites, suggesting that it might play a role in the formation and maintenance of synapses. However, in each type of synpase, F3/F11 was present at only the pre- or postsynaptic site, never at both: the parallel fiber varicosities contained F3/F11 whereas the postsynaptic compartment in contact, that is, the Purkinje cell dendritic spines, did not. The granule cell dendrites were unlabeled while the mossy fiber terminals contacting them were immunoreactive, and finally, the Golgi cell dendrites and dendritic spines were labeled while the presynaptic compartment contacting them was not. If F3/F11 functions as an adhesion molecule in vivo as indicated by in vitro assays, F3/F11-mediated adhesion is likely to be heterophilic.

Expression and release of phosphatidylinositol anchored cell surface molecules by a cell line derived from sensory neurons

Early postnatal mouse dorsal root ganglion neurons were found to express several glycosylphosphatidylinositol-anchored (GPI) molecules from the immunoglobulin superfamily (neural cell adhesion molecule 120 kD isoform, F3, Thy1) whose expression is developmentally regulated. A hybrid cell line (ND26), made by fusing postmitotic rat dorsal root ganglion (DRG) neurons with the mouse neuroblastoma N18Tg2, could be induced to differentiate by manipulating the composition of the culture medium and expressed similar GPI molecules to DRG neurons. We used this model system to investigate the metabolism of GPI-anchored molecules. We found that neural cell adhesion molecule 120 Kd isoform expression decreased upon differentiation, whereas the level of F3 and Thy1 increased, suggesting a role in neurite outgrowth processes. The ratio of molecules cleavable by exogenous phosphatidylinositol phospholipase C (PI-PLC) was similar for all the GPI-anchored molecules, which could mean that cell-specific modifications of the basic anchoring structure determine the level of potentially releasable molecules. Measurements of spontaneous release indicated that this reflected the overall level of expression of these molecules by the ND26 cell line. Finally, we observed an effect of dibutyryl cAMP on the level of expression of F3 and Thy1 but not of N-CAM. However, we could not detect any significant effect of nerve growth factor (NGF) either on the level of expression or on the amount of spontaneously released molecules.

Transfected F3/F11 neuronal cell surface protein mediates intercellular adhesion and promotes neurite outgrowth

The mouse neuronal F3 glycoprotein and its chicken homolog F11 belong to a subclass of proteins of the immunoglobulin superfamily with preferential localization on axons and neurites. We have transfected F3 cDNA into CHO cells. Biochemical analysis establishes that the cDNA we have cloned codes for a 130 kd phosphatidylinositol-anchored polypeptide. F3-expressing transfectants exhibited enhanced self-adhesive properties, aggregating with faster kinetics and forming larger aggregates than F3-negative control cells. When used as a culture substrate for sensory neurons, F3-transfected cells showed a markedly enhanced ability to promote neurite outgrowth compared with nontransfected cells. The results support the idea that F3/F11 and other closely similar proteins function as cell adhesion molecules that play a role in axonal growth and guidance.

The mouse neuronal cell surface protein F3: a phosphatidylinositol-anchored member of the immunoglobulin superfamily related to chicken contactin

Several members of the Ig superfamily are expressed on neural cells where they participate in surface interactions between cell bodies and processes. Their Ig domains are more closely related to each other than to Ig variable and constant domains and have been grouped into the C2 set. Here, we report the cloning and characterization of another member of this group, the mouse neuronal cell surface antigen F3. The F3 cDNA sequence contains an open reading frame that could encode a 1,020-amino acid protein consisting of a signal sequence, six Ig-like domains of the C2 type, a long premembrane region containing two segments that exhibit sequence similarity to fibronectin type III repeats and a moderately hydrophobic COOH-terminal sequence. The protein does not contain a typical transmembrane segment but appears to be attached to the membrane by a phosphatidylinositol anchor. Antibodies against the F3 protein recognize a prominent 135-kD protein in mouse brain. In fetal brain cultures, they stain the neuronal cell surface and, in cultures maintained in chemically defined medium, most prominently neurites and neurite bundles. The mouse f3 gene maps to band F of chromosome 15. The gene transcripts detected in the brain by F3 cDNA probes are developmentally regulated, the highest amounts being expressed between 1 and 2 wk after birth. The F3 nucleotide and deduced amino acid sequence show striking similarity to the recently published sequence of the chicken neuronal cell surface protein contactin. However, there are important differences between the two molecules. In contrast to F3, contactin has a transmembrane and a cytoplasmic domain. Whereas contactin is insoluble in nonionic detergent and is tightly associated with the cytoskeleton, about equal amounts of F3 distribute between buffer-soluble, nonionic detergent-soluble, and detergent-insoluble fractions. Among other neural cell surface proteins, F3 most resembles the neuronal cell adhesion protein L1, with 25% amino acid identity between their extracellular domains. Based on its structural similarity with known cell adhesion proteins of nervous tissue and with L1 in particular, we propose that F3 mediates cell surface interactions during nervous system development.

Identification and cDNA cloning of a new member of the L2/HNK-1 family of neural surface glycoproteins

Rabbit antibodies raised against a 135- to 140-kD glycoprotein isolated from the culture medium of mouse forebrain explants were used for the identification and cloning of a complex of mouse neural cell surface glycoproteins. The antibodies recognized a 135-kD surface protein which shared the L2/HNK-1 epitope with several neural cell adhesion molecules. Three homologous complementary deoxyribonucleic acid (cDNA) clones were isolated from a mouse brain cDNA library prepared in the expression vector lambda gt11, one of which was sequenced and found to lack sequence homologies with known proteins. In Northern blots, this clone hybridized with a single 6.3 kb messenger ribonucleic acid (mRNA). In immunoblots of mouse brain extracts, antibodies raised in rabbits against the fusion protein encoded by it stained two glycoproteins of 135 and 90 kD, which we designated F3.135 and F3.90. In the developing mouse cerebellum, F3 antigenic sites were found predominantly on parallel fibers and on postmitotic neurons. In fetal brain cell cultures, F3 antigen was detected at the surface of cells with neuronal morphology, but the antibodies also stained some non-neuronal cells in a pattern characteristic of matrix components. Because all proteins carrying the L2/HNK-1 epitope identified so far have a role in cell adhesion, it can be anticipated that the F3 surface proteins also are involved in cell-interaction phenomena.

Differential expression of mouse neural cell-adhesion molecule (N-CAM) mRNA species during brain development and in neural cell lines

The cell-adhesion molecules N-CAM (neural cell-adhesion molecule) are ligands in the formation of cell-cell bonds and have been shown to play important roles during neuro-ontogenesis. They exist in several molecular forms which differ at the protein and carbohydrate levels. The regulation of the expression of these different forms is an important issue that bears on such questions as to how adhesive interactions between cells are modulated during morphogenesis. In the present study we have used N-CAM cDNA clones to investigate the expression of the cognate mRNAs in the mouse and rat brain and in 2 neural cell lines. The results were compared with the levels of the different N-CAM proteins. We made the following observations. A complex set of 5 size classes of mRNAs--which show developmental, regional, and cell-type-dependent variations in their expression--hybridize to 1 of our cDNA probes. While embryonic brain contains N-CAM gene transcripts 7.4, 6.7, and 4.3 kilobases (kb) in length, 2 additional mRNAs of 5.2 and 2.9 kb appear postnatally. Transformed brain cells of an astrocytic character express predominantly mRNAs of 6.7, 4.3, and 2.9 kb and a neuroblastoma line those of 7.4, 6.7, 4.3, and 2.9 kb. There are important quantitative changes in the amount of N-CAM message expressed during brain development, with a peak around birth, suggesting that N-CAM synthesis is controlled at the transcriptional level. A comparison of N-CAM protein and mRNA levels reveals a striking correlation between the relative concentrations of the Mr 120,000 N-CAM protein (N-CAM120) and the 5.2 kb transcript.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of mouse N-CAM-related cDNA: detection and cloning using monoclonal antibodies

Clones coding for the mouse neural cell adhesion molecule (N-CAM) were isolated from a cDNA library prepared in the expression vector lambda gt 11 from mRNA extracted from a mouse neuroblastoma cell line. This library was screened with two anti-N-CAM monoclonal antibodies directed against different sites on the molecule and with rabbit anti-N-CAM serum. Two clones were identified with the first monoclonal antibody, three with the second one, none reacted with both. The relevance of these cDNA clones to N-CAM was confirmed by several observations. First, cDNA sequences detected with one monoclonal antibody cross-hybridized with those identified by the other antibody. Second, the different fusion proteins all bound the rabbit serum in addition to one monoclonal antibody. Finally, the probes hybridized to discrete mRNA species of sufficient lengths to code for the very large N-CAM polypeptides in RNA preparations from N-CAM-expressing, but not from N-CAM-negative cells. An additional mRNA species not seen in embryonic brain was expressed in adult mouse brain. Genomic blot experiments indicated that sequences corresponding to one of our probes are present only a few times in the mouse genome.

Studies on the transmembrane disposition of the neural cell adhesion molecule N-CAM. The use of liposome-inserted radioiodinated N-CAM to study its transbilayer orientation

The transmembrane orientation of the polypeptide chains present in preparations of adult and neonatal mouse N-CAM was studied using, as a model system, liposome-inserted purified N-CAM preparations. N-CAM purified from adult or neonatal mouse brain was 125I-labeled and reconstituted into artificial lipid vesicles. After trypsin digestion, the peptides that remained associated with the liposomes were isolated by floatation of the vesicles on sucrose gradients. In control experiments the liposomes were lysed before trypsin treatment. Large, overlapping peptides were obtained after this treatment, several of which were protected by the liposome membrane. Sialic-acid-bearing peptides were revealed by their sensitivity to neuraminidase. To distinguish between peptides corresponding to intracellular or extracellular domains use was made of the P61 and H28.123 monoclonal antibodies, which recognize determinants located on the cytoplasmic and the extracellular part of the molecules respectively. There was no indication that the N-CAM chains were inserted in an inside-out configuration. Peptides protected from trypsin attack by the liposomes and recognized only by P61 had Mr values of 92 000, 42 000 and 35 000. The H28.123 determinant could be mapped to a 32 000-Mr peptide located close to the membrane at the vesicle's exterior. The bulk of the sialic acid seemed to be carried by a rather short sequence distal to the H28.123-reactive peptide but at some distance from the N terminus. Fragments of very similar Mr were generated from young and adult material. However, a 45 000-Mr peptide from neonatal N-CAM appeared to migrate in the higher-Mr region of sodium dodecyl sulfate/polyacrylamide gels in its fully sialylated form. It is concluded that (a) identical polypeptide chains are present in young and adult preparation, (b) the 180 000-Mr, 140 000-Mr and 120 000-Mr chains differ by the length of their cytoplasmic extensions and (c) the largest cytoplasmic sequences have a Mr close to 90 000. A tentative linear model of the transmembrane topography of the N-CAM polypeptides is presented.