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

Microexons: at the nexus of nervous system development, behaviour and autism spectrum disorder

The discovery and characterization of a network of highly conserved neuronal microexons has provided fundamental new insight into mechanisms underlying nervous system development and function, as well as an important basis for pathway convergence in autism spectrum disorder. In the past few years, considerable progress has been made in comprehensively determining the repertoires of factors that control neuronal microexons. These results have illuminated molecular mechanisms that activate the splicing of microexons, including those that control gene expression programs critical for neurogenesis, as well as synaptic protein translation and neuronal activity. Remarkably, individual disruption of specific microexons in these pathways results in autism-like phenotypes and cognitive impairment in mice. This review discusses these findings and their implications for delivering new therapeutic strategies for neurological disorders.

L1 increases adhesion-mediated proliferation and chemoresistance of retinoblastoma

Retinoblastoma is the most common intraocular cancer in children, affecting 1/20,000 live births. Currently, children with retinoblastoma were treated with chemotherapy using drugs such as carboplatin, vincristine, and etoposide. Unfortunately, if conventional treatment fails, the affected eyes should be removed to prevent extension into adjacent tissues and metastasis. This study is to investigate the roles of L1 in adhesion-mediated proliferation and chemoresistance of retinoblastoma. L1 was differentially expressed in 30 retinoblastoma tissues and 2 retinoblastoma cell lines. Furthermore, the proportions of L1-positive cells in retinoblastoma tumors were negatively linked with the number of Flexner-Wintersteiner rosettes, a characteristic of differentiated retinoblastoma tumors, in each tumor sample. Following in vitro experiments using L1-deleted and -overexpressing cells showed that L1 increased adhesion-mediated proliferation of retinoblastoma cells via regulation of cell cycle-associated proteins with modulation of Akt, extracellular signal-regulated kinase, and p38 pathways. In addition, L1 increased resistance against carboplatin, vincristine, and esoposide through up-regulation of apoptosis- and multidrug resistance-related genes. In vivo tumor formation and chemoresistance were also positively linked with the levels of L1 in an orthotopic transplantation model in mice. In this manner, L1 increases adhesion-mediated proliferation and chemoresistance of retinoblastoma. Targeted therapy to L1 might be effective in the treatment of retinoblastoma tumors, especially which rapidly proliferate and demonstrate resistance to conventional chemotherapeutic drugs.

Microexons: discovery, regulation, and function

The importance of RNA splicing in numerous cellular processes is well established. However, an underappreciated aspect is the ability of the spliceosome to recognize a set of very small (3-30 nucleotide, 1-10 amino acid) exons named microexons. Despite their small size, microexons and their regulation through alternative splicing have now been shown to play critical roles in protein and system function. Here we review the discovery of microexons over time and the mechanisms by which their splicing is regulated, including recent progress made through deep RNA sequencing. We also discuss the functional role of microexons in biology and disease. WIREs RNA 2017, 8:e1418. doi: 10.1002/wrna.1418 For further resources related to this article, please visit the WIREs website.

Splicing factor NSSR1 reduces neuronal injury after mouse transient global cerebral ischemia

This study focuses on the function of NSSR1, a splicing factor, in neuronal injury in the ischemic mouse brain using the transient global cerebral ischemic mouse model and the cultured cells treated with oxygen-glucose deprivation (OGD). The results showed that the cerebral ischemia triggers the expression of NSSR1 in hippocampal astrocytes, predominantly the dephosphorylated NSSR1 proteins, and the Exon3 inclusive NCAM-L1 variant and the Exon4 inclusive CREB variant. While in the hippocampus of astrocyte-specific NSSR1 conditional knockdown (cKD) mice, where cerebral ischemia no longer triggers NSSR1 expression in astrocytes, the expression of Exon3 inclusive NCAM-L1 variant and Exon4 inclusive CREB variant were no longer triggered as well. In addition, the injury of hippocampal neurons was more severe in astrocyte-specific NSSR1 cKD mice compared with in wild-type mice after brain ischemia. Of note, the culture media harvested from the astrocytes with overexpression of NSSR1 or the Exon3 inclusive NCAM-L1 variant, or Exon4 inclusive CREB variant were all able to reduce the neuronal injury induced by OGD. The results provide the evidence demonstrating that: (1) Splicing factor NSSR1 is a new factor involved in reducing ischemic injury. (2) Ischemia induces NSSR1 expression in astrocytes, not in neurons. (3) NSSR1-mediated pathway in astrocytes is required for reducing ischemic neuronal injury. (4) NCAM-L1 and CREB are probably mediators in NSSR1-mediated pathway. In conclusion, our results suggest for the first time that NSSR1 may provide a novel mechanism for reducing neuronal injury after ischemia, probably through regulation on alternative splicing of NCAM-L1 and CREB in astrocytes.

Protein tyrosine phosphatases PTPδ, PTPσ, and LAR: presynaptic hubs for synapse organization

Synapse development requires differentiation of presynaptic neurotransmitter release sites and postsynaptic receptive apparatus coordinated by synapse organizing proteins. In addition to the well-characterized neurexins, recent studies identified presynaptic type IIa receptor-type protein tyrosine phosphatases (RPTPs) as mediators of presynaptic differentiation and triggers of postsynaptic differentiation, thus extending the roles of RPTPs from axon outgrowth and guidance. Similarly to neurexins, RPTPs exist in multiple isoforms generated by alternative splicing that interact in a splice-selective code with diverse postsynaptic partners. The parallel RPTP and neurexin hub design facilitates synapse self-assembly through cooperation, pairs presynaptic similarity with postsynaptic diversity, and balances excitation with inhibition. Upon mutation of individual genes in neuropsychiatric disorders, imbalance of this synaptic organizing network may contribute to impaired cognitive function.

Regulation of adhesion by flexible ectodomains of IgCAMs

To perform their diverse biological functions the adhesion activities of the cell adhesion molecules of the immunoglobulin superfamily (IgCAMs) might be regulated by local clustering, proteolytical shedding of their ectodomains or rapid recycling to and from the plasma membrane. Another form of regulation of adhesion might be obtained through flexible ectodomains of IgCAMs which adopt distinct conformations and which in turn modulate their adhesion activity. Here, we discuss variations in the conformation of the extracellular domains of CEACAM1 and CAR that might influence their binding and signaling activities. Furthermore, we concentrate on alternative splicing of single domains and short segments in the extracellular regions of L1 subfamily members that might affect the organization of the N-terminal located Ig-like domains. In particular, we discuss variations of the linker sequence between Ig-like domains 2 and 3 (D2 and D3) that is required for the horseshoe conformation.

A phylogenetic analysis of the L1 family of neural cell adhesion molecules

L1-type genes form one of several distinct gene families that encode adhesive proteins, which are predominantly expressed in developing and mature metazoan nervous systems. These proteins have a multitude of different important cellular functions in neuronal and glial cells. L1-type gene products are transmembrane proteins with a characteristic extracellular domain structure consisting of six immunoglobulin and three to five fibronectin type III protein folds. As reported here, L1-type proteins can be identified in most metazoan phyla with the notable exception of Porifera (sponges). This puts the origin of L1-type genes at a point in time when primitive cellular neural networks emerged, approximately 1,200 to 1,500 million years ago. Subsequently, several independent gene duplication events generated multiple paralogous L1-type genes in some phyla, allowing for a considerable diversification of L1 structures and the emergence of new functional features and molecular interactions. One such evolutionary newer feature is the appearance of RGD integrin-binding motifs in some vertebrate L1 family members.

Dual REST-dependence of L1CAM: from gene expression to alternative splicing governed by Nova2 in neural cells

L1 cell adhesion molecule (L1CAM), an adhesion/signaling protein encoded by a gene target of the transcription repressor RE-1-Silencing Transcription factor (REST), is expressed in two alternatively spliced isoforms. The full-length isoform, typical of low-REST neural cells, plays key roles in survival/migration, outgrowth/fasciculation/regeneration of axons, synaptic plasticity; the isoform missing two mini-exons, abundant in a few high-REST non-neural cells, maintains some effect on migration and proliferation. To investigate whether and how L1CAM alternative splicing depends on REST we used neural cell models expressing low or high levels of REST (PC12, SH-SY5Y, differentiated NT2/D1 and primary neurons transduced or not with REST). The short isoform was found to rise when the low-REST levels of neural cells were experimentally increased, while the full-length isoform increased in high-REST cells when the repressor tone was attenuated. These results were due to Nova2, a neural cell-specific splicing factor shown here to be repressed by REST. REST control of L1CAM occurs therefore by two mechanisms, transcription and alternative splicing. The splicing mechanism, affecting not only L1CAM but all Nova2 targets (∼7% of brain-specific splicing, including the mRNAs of other adhesion and synaptic proteins) is expected to be critical during development and important also for the structure and function of mature neural cells.

Isoform diversity and its importance for axon regeneration

Axon regeneration is a fundamental problem facing neuroscientists and clinicians. Failure of axon regeneration is caused by both extrinsic and intrinsic mechanisms. New techniques to examine gene expression such as Next Generation Sequencing of the Transcriptome (RNA-Seq) drastically increase our knowledge of both gene expression complexity (RNA isoforms) and gene expression regulation. By utilizing RNA-Seq, gene expression can now be defined at the level of isoforms, an essential step for understanding the mechanisms governing cell identity, growth and ultimately cellular responses to injury and disease.

Human L1CAM carrying the missense mutations of the fibronectin-like type III domains is localized in the endoplasmic reticulum and degraded by polyubiquitylation

Any mutations in the human neural cell adhesion molecule L1 (hL1CAM) gene might cause various types of serious neurological syndromes in humans, characterized by increased mortality, mental retardation, and various malformations of the nervous system. Such missense mutations often cause severe abnormalities or even fatalities, and the reason for this may be a disruption of the adhesive function of L1CAM resulting from a misdirection of the degradative pathway. Transfection studies using neuroblastoma N2a cells demonstrated that hL1CAM carrying the missense mutations in the fibronectin-like type III (FnIII) domains most likely is located within the endoplasmic reticulum (ER), but it is less well expressed on the cell surface. One mutant, L935P, in the fourth FnIII domain, was chosen from six mutants (K655 and G698 at Fn1, L935P and P941 at Fn4, W1036 and Y1070 at Fn5) in the FnIII domains to study in detail the functions of hL1CAM(200 kDa) , such as the intracellular traffic and degradation, because only a single band at 200 kDa was detected in the hL1CAM(L935P) -transfected cells. hL1CAM(200 kDa) is expressed predominantly in the ER but not on the cell surface. In addition, this missense mutated hL1CAM(200 kDa) is polyubiquitylated at some sites in the extracellular domain and thus becomes degraded by proteasomes via the ER-associated degradation pathway. These observations demonstrate that the missense mutations of hL1CAM in the FnIII domain may cause the resultant pathogenesis because of a loss of expression on the cell surface resulting from misrouting to the degradative pathway.

Full-length L1CAM and not its Δ2Δ27 splice variant promotes metastasis through induction of gelatinase expression

Tumour-specific splicing is known to contribute to cancer progression. In the case of the L1 cell adhesion molecule (L1CAM), which is expressed in many human tumours and often linked to bad prognosis, alternative splicing results in a full-length form (FL-L1CAM) and a splice variant lacking exons 2 and 27 (SV-L1CAM). It has not been elucidated so far whether SV-L1CAM, classically considered as tumour-associated, or whether FL-L1CAM is the metastasis-promoting isoform. Here, we show that both variants were expressed in human ovarian carcinoma and that exposure of tumour cells to pro-metastatic factors led to an exclusive increase of FL-L1CAM expression. Selective overexpression of one isoform in different tumour cells revealed that only FL-L1CAM promoted experimental lung and/or liver metastasis in mice. In addition, metastasis formation upon up-regulation of FL-L1CAM correlated with increased invasive potential and elevated Matrix metalloproteinase (MMP)-2 and -9 expression and activity in vitro as well as enhanced gelatinolytic activity in vivo. In conclusion, we identified FL-L1CAM as the metastasis-promoting isoform, thereby exemplifying that high expression of a so-called tumour-associated variant, here SV-L1CAM, is not per se equivalent to a decisive role of this isoform in tumour progression.

L1CAM malfunction in the nervous system and human carcinomas

Research over the last 25 years on the cell adhesion molecule L1 has revealed its pivotal role in nervous system function. Mutations of the human L1CAM gene have been shown to cause neurodevelopmental disorders such as X-linked hydrocephalus, spastic paraplegia and mental retardation. Impaired L1 function has been also implicated in the aetiology of fetal alcohol spectrum disorders, defective enteric nervous system development and malformations of the renal system. Importantly, aberrant expression of L1 has emerged as a critical factor in the development of human carcinomas, where it enhances cell proliferation, motility and chemoresistance. This discovery promoted collaborative work between tumour biologists and neurobiologists, which has led to a substantial expansion of the basic knowledge about L1 function and regulation. Here we provide an overview of the pathological conditions caused by L1 malfunction. We further discuss how the available data on gene regulation, molecular interactions and posttranslational processing of L1 may contribute to a better understanding of associated neurological and cancerous diseases.

Inside-out regulation of L1 conformation, integrin binding, proteolysis, and concomitant cell migration

The ectodomain structure and function of the neural cell adhesion molecule L1 is shown to be regulated by the intracellular phosphorylation of a novel threonine, T1172. In pancreatic cancer cells, T1172 exhibits steady-state saturated phosphorylation, an event regulated by CKII and PKC, and which further regulates cell migration.

Previous reports on the expression of the cell adhesion molecule L1 in pancreatic ductal adenocarcinoma (PDAC) cells range from absent to high. Our data demonstrate that L1 is expressed in poorly differentiated PDAC cells in situ and that threonine-1172 (T1172) in the L1 cytoplasmic domain exhibits steady-state saturated phosphorylation in PDAC cells in vitro and in situ. In vitro studies support roles for casein kinase II and PKC in this modification, consistent with our prior studies using recombinant proteins. Importantly, T1172 phosphorylation drives, or is associated with, a change in the extracellular structure of L1, consistent with a potential role in regulating the shift between the closed conformation and the open, multimerized conformation of L1. We further demonstrate that these distinct conformations exhibit differential binding to integrins αvβ3 and αvβ5 and that T1172 regulates cell migration in a matrix-specific manner and is required for a disintegrin and metalloproteinase-mediated shedding of the L1 ectodomain that has been shown to regulate cell migration. These data define a specific role for T1172 of L1 in regulating aspects of pancreatic adenocarcinoma cell phenotype and suggest the need for further studies to elucidate the specific ramifications of L1 expression and T1172 phosphorylation in the pathobiology of pancreatic cancer.

Kinetic analysis of L1 homophilic interaction: role of the first four immunoglobulin domains and implications on binding mechanism

L1 is a cell adhesion molecule of the immunoglobulin (Ig) superfamily, critical for central nervous system development, and involved in several neuronal biological events. It is a type I membrane glycoprotein. The L1 ectodomain, composed of six Ig-like and five fibronectin (Fn) type-III domains, is involved in homophilic binding. Here, co-immunoprecipitation studies between recombinant truncated forms of human L1 expressed and purified from insect Spodoptera frugiperda Sf9 cells, and endogenous full-length L1 from human NT2N neurons, showed that the L1 ectodomain (L1/ECD) and L1/Ig1-4 interacted homophilically in trans, contrary to mutants L1/Ig1-3 and L1/Ig2-Fn5. All mutants were correctly folded as evaluated by combination of far-UV CD and fluorescence spectroscopy. Surface plasmon resonance analysis showed comparable dissociation constants of 116 +/- 2 and 130 +/- 6 nm for L1/ECD-L1/ECD and L1/ECD-L1/Ig1-4, respectively, whereas deletion mutants for Ig1 or Ig4 did not interact. Accordingly, in vivo, Sf9 cells stably expressing L1 were found to adhere only to L1/ECD- and L1/Ig1-4-coated surfaces. Furthermore, only these mutants bound to HEK293 cells overexpressing L1 at the cell surface. Enhancement of neurite outgrowth, which is the consequence of signaling events caused by L1 homophilic binding, was comparable between L1/ECD and L1/Ig1-4. Altogether, these results showed that domains Ig1 to Ig4 are necessary and sufficient for L1 homophilic binding in trans, and that the rest of the molecule does not contribute to the affinity under the conditions of the current study. Furthermore, they are compatible with a cooperative interaction between modules Ig1-Ig4 in a horseshoe conformation.

An alcohol binding site on the neural cell adhesion molecule L1

Prenatal ethanol exposure causes fetal alcohol spectrum disorders (FASD) in part by disrupting the neural cell adhesion molecule L1. L1 gene mutations cause neuropathological abnormalities similar to those of FASD. Ethanol and 1-butanol inhibit L1-mediated cell-cell adhesion (L1 adhesion), whereas 1-octanol antagonizes this action. To test the hypothesis that there are alcohol binding sites on L1, we used 3-azibutanol and 3-azioctanol, the photoactivatable analogs of 1-butanol and 1-octanol, to photolabel the purified Ig1-4 domain of human L1 (hL1 Ig1-4). 3-Azibutanol (11 mM), like ethanol, inhibited L1 adhesion in NIH/3T3 cells stably transfected with hL1, whereas subanesthetic concentrations of 3-azioctanol (14 microM) antagonized ethanol inhibition of L1 adhesion. 3-Azibutanol (100-1,000 microM) and 3-azioctanol (10-100 microM) photoincorporated into Tyr-418 on Ig4 and into two adjacent regions in the N terminus, Glu-33 and Glu-24 to Glu-27. A homology model of hL1 Ig1-4 (residues 33-422), based on the structure of the Ig1-4 domains of axonin-1, suggests that Glu-33 and Tyr-418 hydrogen-bond at the interface of Ig1 and Ig4 to stabilize a horseshoe conformation of L1 that favors homophilic binding. Furthermore, this alcohol binding pocket lies within 7 A of Leu-120 and Gly-121, residues in which missense mutations cause neurological disorders similar to FASD. These data suggest that ethanol or selected mutations produce neuropathological abnormalities by disrupting the domain interface between Ig1 and Ig4. Characterization of alcohol agonist and antagonist binding sites on L1 will aid in understanding the molecular basis for FASD and might accelerate the development of ethanol antagonists.

Cell adhesion molecule L1 disrupts E-cadherin-containing adherens junctions and increases scattering and motility of MCF7 breast carcinoma cells

The first steps of invasion and metastasis include the dissociation of adherens junctions and the induction of migratory phenotype, through a program that resembles epithelial-mesenchymal transition (EMT). The L1 cell adhesion molecule, which is normally found primarily in the brain, was recently shown to be expressed in different types of cancer and to have tumor-promoting activity. We now find that L1 mediates EMT-like events in MCF7 breast carcinoma cells. MCF7 predominantly expresses the nonneuronal isoform of L1, as do 16 of 17 other cell lines derived from different types of cancer. L1 protein expression in MCF7 cells, which form E-cadherin-containing adherens junctions, is inversely related to cell density. Analysis of MCF7 cells with overexpression or knockdown of nonneuronal L1 isoform revealed that L1 expression leads to the disruption of adherens junctions and increases beta-catenin transcriptional activity. As a result, L1 expression promotes the scattering of epithelial cells from compact colonies. Expression of the full-length L1 protein, but not of its soluble extracellular moiety, increases the motility of the MCF7 epithelial monolayer in a wound-healing assay, in which L1 expression is preferentially observed and required in cells leading the movement of the monolayer. Based on these results, we propose a model for the role of L1 as a trigger of EMT-like events in transformed epithelial cells.

Disrupted Schwann cell-axon interactions in peripheral nerves of mice with altered L1-integrin interactions

The cell adhesion molecule L1 is important for peripheral nerve development. Mice lacking the 6th Ig domain of L1 (L1-6D mice) lose L1 homophilic binding and RGD dependent LI-integrin binding [Itoh,K., Cheng, L., Kamei, Y., Fushiki, S., Kamiguchi, H., Gutwein, P.,Stoeck, A., Arnold, B., Altevogt, P., Lemmon, V., 2004. Brain development in mice lacking Li-L homophilic adhesion. J. Cell Biol.165, 145-154]. We examined the ultrastructure of sciatic nerves from L1-6D at postnatal day 7 and 8 weeks. Unmyelinated axons frequently detached at the edge of Schwann cells, and naked axons were observed. Myelin was thinner in L1-6D and abnormal, multiple axons wrapped in a single myelin sheath were routinely observed. Previous work has shown that L1 on axons interacts with a heterophilic binding partner on Schwann cells to facilitate normal peripheral nerve formation. Taken together, it is likely that L1 on axons binds integrins on Schwann cells, resulting in interactions between axons and Schwann cells that are essential for ensheathment and myelination.

Cell adhesion and neurite outgrowth are promoted by neurofascin NF155 and inhibited by NF186

Neurofascin (NF) is a neural cell adhesion molecule in the L1-family containing six Ig domains and multiple fibronectin type III (FnIII) repeats in its extracellular region. NF has many splicing variants and two of these are exemplars that have different cellular patterns of expression during development. NF186, which is expressed on neurons, contains an unusual mucin-like region and NF155, which is expressed on glia, contains a unique FnIII repeat with an RGD motif. Analysis of Fc fusion proteins representing different extracellular regions of NF indicate that NF186 inhibits cell adhesion and neurite outgrowth, and the inhibition is associated with the region containing the mucin-like domain. NF155 promotes neural cell adhesion and neurite outgrowth, and the RGD motif in its third FnIII repeat is critical for cell spreading and neurite outgrowth. The results suggest that different splicing variants of NF expressed on neurons and glia play distinct roles during neural development.

Pathological missense mutations of neural cell adhesion molecule L1 affect neurite outgrowth and branching on an L1 substrate

A number of pathological missense mutations of L1CAM have been shown to disrupt L1-L1 homophilic binding and/or affect surface expression. To investigate whether these mutations disrupt L1-mediated neurite outgrowth, cerebellar neurons from L1 knockout mice are transfected with WT human L1 or L1 mutant constructs, and grown on an L1 substrate. Various parameters of neurite growth are quantified. Most L1 mutations do not affect neurite length significantly but several mutations cause a significant decrease in branching. Comparison of these data with data on L1 expression levels and homophilic binding strength show that changes in neurite growth cannot be simply explained by reductions in either of these parameters. Our results suggest that a coreceptor is involved in L1-mediated neurite outgrowth. Some pathological mutations have little effect on L1 mediated neurite growth, so it is unlikely that a failure of L1-mediated neurite outgrowth is the principle cause of brain defects in patients with L1 mutations.

Genetic analysis of an overlapping functional requirement for L1- and NCAM-type proteins during sensory axon guidance in Drosophila

L1- and NCAM-type cell adhesion molecules represent distinct protein families that function as specific receptors for different axon guidance cues. However, both L1 and NCAM proteins promote axonal growth by inducing neuronal tyrosine kinase activity and are coexpressed in subsets of axon tracts in arthropods and vertebrates. We have studied the functional requirements for the Drosophila L1- and NCAM-type proteins, Neuroglian (Nrg) and Fasciclin II (FasII), during postembryonic sensory axon guidance. The rescue of the Neuroglian loss-of-function (LOF) phenotype by transgenically expressed L1- and NCAM-type proteins demonstrates a functional interchangeability between these proteins in Drosophila photoreceptor pioneer axons, where both proteins are normally coexpressed. In contrast, the ectopic expression of Fasciclin II in mechanosensory neurons causes a strong enhancement of the axonal misguidance phenotype. Moreover, our findings demonstrate that this functionally redundant specificity to mediate axon guidance has been conserved in their vertebrate homologs, L1-CAM and NCAM.

Function of alternative splicing

Alternative splicing is one of the most important mechanisms to generate a large number of mRNA and protein isoforms from the surprisingly low number of human genes. Unlike promoter activity, which primarily regulates the amount of transcripts, alternative splicing changes the structure of transcripts and their encoded proteins. Together with nonsense-mediated decay (NMD), at least 25% of all alternative exons are predicted to regulate transcript abundance. Molecular analyses during the last decade demonstrate that alternative splicing determines the binding properties, intracellular localization, enzymatic activity, protein stability and posttranslational modifications of a large number of proteins. The magnitude of the effects range from a complete loss of function or acquisition of a new function to very subtle modulations, which are observed in the majority of cases reported. Alternative splicing factors regulate multiple pre-mRNAs and recent identification of physiological targets shows that a specific splicing factor regulates pre-mRNAs with coherent biological functions. Therefore, evidence is now accumulating that alternative splicing coordinates physiologically meaningful changes in protein isoform expression and is a key mechanism to generate the complex proteome of multicellular organisms.

Characterization of the genomic structure of the mouse limbic system-associated membrane protein (Lsamp) gene

The Lsamp gene encodes the limbic system-associated membrane protein (LAMP) an immunoglobulin (Ig) superfamily member with three Ig domains and a glycosylphosphatidylinositol anchor. LAMP is expressed by neurons composing the limbic system, is highly conserved between rodents and human, and has structural and functional properties that substantiate its role in the formation of limbic circuits. We report here the genomic organization of the Lsamp gene. The Lsamp gene is composed of 11 exons distributed over 2.2 megabases (Mb). Two exons 1 are separated by approximately 1.6 Mb and contribute to the unusual large size of the gene. Alternative spliced Lsamp mRNAs are generated from distinct promoter regions associated with the two exons 1 that encode distinct signal peptides and thus generate identical native mature polypetides. Additional diversity is created by the use of two small exons to include an insertion of 23 amino acids within the polypeptide C-terminal region of the mature protein. The genomic features of the Lsamp gene described here indicate an intricate mechanism of gene expression regulation that may be relevant in the context of human neuropsychiatric and neurological disorders, where LAMP expression may be altered.

NSSR1 promotes neuronal differentiation of mouse embryonic carcinoma P19 cells

We generated the small interference RNAs to specifically silence the expression of neural salient serine/arginine rich protein 1 (NSSR1) and showed that the inhibition of NSSR1 expression in mouse embryonic carcinoma cells (P19) reduces neuronal differentiation. By contrast, its over-expression promotes the differentiation. Neither inhibition nor over-expression shows distinct effect on cell proliferation. The over-expression increases the inclusion of NCAM L1 exon2 while the inhibition reduces the inclusion. The splicing of kinase insert free isoform of TrkC (TrkC-K1) is increased by the over-expression. The results demonstrate that NSSR1 promotes neuronal differentiation and the splicing of NCAML1 exon2 and TrkC-K1.

Brain development in mice lacking L1-L1 homophilic adhesion

A new mouse line has been produced in which the sixth Ig domain of the L1 cell adhesion molecule has been deleted. Despite the rather large deletion, L1 expression is preserved at normal levels. In vitro experiments showed that L1–L1 homophilic binding was lost, along with L1-α5β1 integrin binding. However, L1–neurocan and L1–neuropilin binding were preserved and sema3a responses were intact. Surprisingly, many of the axon guidance defects present in the L1 knockout mice, such as abnormal corticospinal tract and corpus callosum, were not observed. Nonetheless, when backcrossed on the C57BL/6 strain, a severe hydrocephalus was observed and after several generations, became an embryonic lethal. These results imply that L1 binding to L1, TAG-1, or F3, and L1-α5β1 integrin binding are not essential for normal development of a variety of axon pathways, and suggest that L1–L1 homophilic binding is important in the production of X-linked hydrocephalus.

Ankyrin binding mediates L1CAM interactions with static components of the cytoskeleton and inhibits retrograde movement of L1CAM on the cell surface

The function of adhesion receptors in both cell adhesion and migration depends critically on interactions with the cytoskeleton. During cell adhesion, cytoskeletal interactions stabilize receptors to strengthen adhesive contacts. In contrast, during cell migration, adhesion proteins are believed to interact with dynamic components of the cytoskeleton, permitting the transmission of traction forces through the receptor to the extracellular environment. The L1 cell adhesion molecule (L1CAM), a member of the Ig superfamily, plays a crucial role in both the migration of neuronal growth cones and the static adhesion between neighboring axons. To understand the basis of L1CAM function in adhesion and migration, we quantified directly the diffusion characteristics of L1CAM on the upper surface of ND-7 neuroblastoma hybrid cells as an indication of receptor-cytoskeleton interactions. We find that cell surface L1CAM engages in diffusion, retrograde movement, and stationary behavior, consistent with interactions between L1CAM and two populations of cytoskeleton proteins. We provide evidence that the cytoskeletal adaptor protein ankyrin mediates stationary behavior while inhibiting the actin-dependent retrograde movement of L1CAM. Moreover, inhibitors of L1CAM-ankyrin interactions promote L1CAM-mediated axon growth. Together, these results suggest that ankyrin binding plays a crucial role in the anti-coordinate regulation of L1CAM-mediated adhesion and migration.

L1 mediated homophilic binding and neurite outgrowth are modulated by alternative splicing of exon 2

The neural cell adhesion molecule (CAM) L1 is a member of the immunoglobulin superfamily that has been implicated in neuronal adhesion, neurite outgrowth, and axon guidance. The clinical importance of L1 is illustrated by pathological mutations that lead to hydrocephalus, mental retardation, motor defects, and early mortality. The L1 gene is composed of 28 exons, including exons 2 and 27 that are spliced alternatively, and mutations in exon 2 are associated with severe neurological abnormalities in humans. To elucidate the role of L1 exon 2, a recombinant Fc fusion protein called Delta2L1 was constructed lacking the second exon in the extracellular domain of L1. When bound to fluorescent beads, L1 exhibited homophilic binding while Delta2L1 did not. However, L1 beads coaggregated with the Delta2L1 beads. Similarly, in cell binding studies, L1 bound to L1 and Delta2L1 did not bind to Delta2L1 but it bound moderately to L1. Given the reduced binding of Delta2L1, we tested its effect on neurons. By comparison to L1, a lower percentage of dissociated neurons extended neurites on Delta2L1, and there was a modest decrease in the length of the neurites that grew. Neurite outgrowth from reaggregated neurons was much less robust on Delta2L1 than on L1. The combined results indicate that Delta2L1 does not bind homophilically but it can interact with L1 containing exon 2. The reduced binding and neurite promoting activity of Delta2L1 provides an explanation for certain pathological mutations in L1 that lead to clinically apparent disease in the absence of the normal form of L1 in the nervous system.