Decreased anxiety, altered place learning, and increased CA1 basal excitatory synaptic transmission in mice with conditional ablation of the neural cell adhesion molecule L1

Interaction of L1CAM with LC3 Is Required for L1-Dependent Neurite Outgrowth and Neuronal Survival

The neural cell adhesion molecule L1 (also called L1CAM or CD171) functions not only in cell migration, but also in cell survival, differentiation, myelination, neurite outgrowth, and signaling during nervous system development and in adults. The proteolytic cleavage of L1 in its extracellular domain generates soluble fragments which are shed into the extracellular space and transmembrane fragments that are internalized into the cell and transported to various organelles to regulate cellular functions. To identify novel intracellular interaction partners of L1, we searched for protein-protein interaction motifs and found two potential microtubule-associated protein 1 light-chain 3 (LC3)-interacting region (LIR) motifs within L1, one in its extracellular domain and one in its intracellular domain. By ELISA, immunoprecipitation, and proximity ligation assay using L1 mutant mice lacking the 70 kDa L1 fragment (L1-70), we showed that L1-70 interacts with LC3 via the extracellular LIR motif in the fourth fibronectin type III domain, but not by the motif in the intracellular domain. The disruption of the L1-LC3 interaction reduces L1-mediated neurite outgrowth and neuronal survival.

Mice Mutated in the Third Fibronectin Domain of L1 Show Enhanced Hippocampal Neuronal Cell Death, Astrogliosis and Alterations in Behavior

Adhesion molecules play major roles in cell proliferation, migration, survival, neurite outgrowth and synapse formation during nervous system development and in adulthood. The neural cell adhesion molecule L1 contributes to these functions during development and in synapse formation and synaptic plasticity after trauma in adulthood. Mutations of L1 in humans result in L1 syndrome, which is associated with mild-to-severe brain malformations and mental disabilities. Furthermore, mutations in the extracellular domain were shown to cause a severe phenotype more often than mutations in the intracellular domain. To explore the outcome of a mutation in the extracellular domain, we generated mice with disruption of the dibasic sequences RK and KR that localize to position 858RKHSKR863 in the third fibronectin type III domain of murine L1. These mice exhibit alterations in exploratory behavior and enhanced marble burying activity. Mutant mice display higher numbers of caspase 3-positive neurons, a reduced number of principle neurons in the hippocampus, and an enhanced number of glial cells. Experiments suggest that disruption of the dibasic sequence in L1 results in subtle impairments in brain structure and functions leading to obsessive-like behavior in males and reduced anxiety in females.

L1CAM promotes vasculogenic mimicry formation by miR-143-3p-induced expression of hexokinase 2 in glioma

In recent decades, antiangiogenic therapy, which blocks the supply of oxygen and nutrition to tumor cells, has become a promising clinical strategy for the treatment of patients with tumors. However, recent studies revealed that vasculogenic mimicry (VM), which is the process by which vascular morphological structures are formed by highly invasive tumor cells, has been considered a potential factor for the failure of antiangiogenic therapy in patients with tumors. Thus, inhibition of VM formation might be a potential target for improving the outcome of antiangiogenic strategies. However, the mechanism underlying VM formation is still incompletely elucidated. Herein, we report that L1CAM might be a critical regulator of VM formation in glioma, and might be associated with the resistance of glioma to antiangiogenic therapy. We found that the tumor-invasion and tube-formation capabilities of L1CAM-overexpressing cells were significantly enhanced in vitro and in vivo. In addition, the results indicated that miR-143-3p, which might directly target the 3'UTR of the hexokinase 2 (HK2) gene to regulate its protein expression, was subsequently involved in L1CAM-mediated VM formation by glioma cells. Further study revealed that the regulation of MMP2, MMP9, and VEGFA expression was involved in this process. Moreover, we identified that activation of the downstream PI3K/AKT signaling pathway of the L1CAM/HK2 cascade is critical for VM formation by glioma cells. Furthermore, we found that the combined treatment of anti-L1CAM neutralizing monoclonal antibody and bevacizumab increases efficacy beyond that of bevacizumab alone, and suppresses glioma growth in vivo, indicating that the inhibition of L1CAM-mediated VM formation might efficiently improve the effect of antiangiogenic treatment for glioma patients. Together, our findings demonstrated a critical role of L1CAM in regulating VM formation in glioma, and that L1CAM might be a potential target for ameliorating tumor resistance to antiangiogenic therapy in glioma patients.

The L1 cell adhesion molecule constrains dendritic spine density in pyramidal neurons of the mouse cerebral cortex

A novel function for the L1 cell adhesion molecule, which binds the actin adaptor protein Ankyrin was identified in constraining dendritic spine density on pyramidal neurons in the mouse neocortex. In an L1-null mouse mutant increased spine density was observed on apical but not basal dendrites of pyramidal neurons in diverse cortical areas (prefrontal cortex layer 2/3, motor cortex layer 5, visual cortex layer 4. The Ankyrin binding motif (FIGQY) in the L1 cytoplasmic domain was critical for spine regulation, as demonstrated by increased spine density and altered spine morphology in the prefrontal cortex of a mouse knock-in mutant (L1YH) harboring a tyrosine (Y) to histidine (H) mutation in the FIGQY motif, which disrupted L1-Ankyrin association. This mutation is a known variant in the human L1 syndrome of intellectual disability. L1 was localized by immunofluorescence staining to spine heads and dendrites of cortical pyramidal neurons. L1 coimmunoprecipitated with Ankyrin B (220 kDa isoform) from lysates of wild type but not L1YH forebrain. This study provides insight into the molecular mechanism of spine regulation and underscores the potential for this adhesion molecule to regulate cognitive and other L1-related functions that are abnormal in the L1 syndrome.

The Interactions of the 70 kDa Fragment of Cell Adhesion Molecule L1 with Topoisomerase 1, Peroxisome Proliferator-Activated Receptor γ and NADH Dehydrogenase (Ubiquinone) Flavoprotein 2 Are Involved in Gene Expression and Neuronal L1-Dependent Functions

The cell adhesion molecule L1 is essential not only for neural development, but also for synaptic functions and regeneration after trauma in adulthood. Abnormalities in L1 functions cause developmental and degenerative disorders. L1's functions critically depend on proteolysis which underlies dynamic cell interactions and signal transduction. We showed that a 70 kDa fragment (L1-70) supports mitochondrial functions and gene transcription. To gain further insights into L1-70's functions, we investigated several binding partners. Here we show that L1-70 interacts with topoisomerase 1 (TOP1), peroxisome proliferator-activated receptor γ (PPARγ) and NADH dehydrogenase (ubiquinone) flavoprotein 2 (NDUFV2). TOP1, PPARγ and NDUFV2 siRNAs reduced L1-dependent neurite outgrowth, and the topoisomerase inhibitors topotecan and irinotecan inhibited L1-dependent neurite outgrowth, neuronal survival and migration. In cultured neurons, L1 siRNA reduces the expression levels of the long autism genes neurexin-1 (Nrxn1) and neuroligin-1 (Nlgn1) and of the mitochondrially encoded gene NADH:ubiquinone oxidoreductase core subunit 2 (ND2). In mutant mice lacking L1-70, Nrxn1 and Nlgn1, but not ND2, mRNA levels are reduced. Since L1-70's interactions with TOP1, PPARγ and NDUFV2 contribute to the expression of two essential long autism genes and regulate important neuronal functions, we propose that L1 may not only ameliorate neurological problems, but also psychiatric dysfunctions.

Functional Diversity of Neuronal Cell Adhesion and Recognition Molecule L1CAM through Proteolytic Cleavage

The neuronal cell adhesion and recognition molecule L1 does not only 'keep cells together' by way of homophilic and heterophilic interactions, but can also promote cell motility when cleaved into fragments by several proteases. It has largely been thought that such fragments are signs of degradation. Now, it is clear that proteolysis contributes to the pronounced functional diversity of L1, which we have reviewed in this work. L1 fragments generated at the plasma membrane are released into the extracellular space, whereas other membrane-bound fragments are internalised and enter the nucleus, thus conveying extracellular signals to the cell interior. Post-translational modifications on L1 determine the sequence of cleavage by proteases and the subcellular localisation of the generated fragments. Inside the neuronal cells, L1 fragments interact with various binding partners to facilitate morphogenic events, as well as regenerative processes. The stimulation of L1 proteolysis via injection of L1 peptides or proteases active on L1 or L1 mimetics is a promising tool for therapy of injured nervous systems. The collective findings gathered over the years not only shed light on the great functional diversity of L1 and its fragments, but also provide novel mechanistic insights into the adhesion molecule proteolysis that is active in the developing and diseased nervous system.

The Cell Adhesion Molecule L1 Interacts with Methyl CpG Binding Protein 2 via Its Intracellular Domain

Cell adhesion molecule L1 regulates multiple cell functions, and L1 deficiency is linked to several neural diseases. Recently, we have identified methyl CpG binding protein 2 (MeCP2) as a potential binding partner of the intracellular L1 domain. By ELISA we show here that L1's intracellular domain binds directly to MeCP2 via the sequence motif KDET. Proximity ligation assay with cultured cerebellar and cortical neurons suggests a close association between L1 and MeCP2 in nuclei of neurons. Immunoprecipitation using MeCP2 antibodies and nuclear mouse brain extracts indicates that MeCP2 interacts with an L1 fragment of ~55 kDa (L1-55). Proximity ligation assay indicates that metalloproteases, β-site of amyloid precursor protein cleaving enzyme (BACE1) and ɣ-secretase, are involved in the generation of L1-55. Reduction in MeCP2 expression by siRNA decreases L1-dependent neurite outgrowth from cultured cortical neurons as well as the migration of L1-expressing HEK293 cells. Moreover, L1 siRNA, MeCP2 siRNA, or a cell-penetrating KDET-containing L1 peptide leads to reduced levels of myocyte enhancer factor 2C (Mef2c) mRNA and protein in cortical neurons, suggesting that the MeCP2/L1 interaction regulates Mef2c expression. Altogether, the present findings indicate that the interaction of the novel fragment L1-55 with MeCP2 affects L1-dependent functions, such as neurite outgrowth and neuronal migration.

A role for the Erk MAPK pathway in modulating SAX-7/L1CAM-dependent locomotion in Caenorhabditis elegans

L1CAMs are immunoglobulin cell adhesion molecules that function in nervous system development and function. Besides being associated with autism and schizophrenia spectrum disorders, impaired L1CAM function also underlies the X-linked L1 syndrome, which encompasses a group of neurological conditions, including spastic paraplegia and congenital hydrocephalus. Studies on vertebrate and invertebrate L1CAMs established conserved roles that include axon guidance, dendrite morphogenesis, synapse development, and maintenance of neural architecture. We previously identified a genetic interaction between the Caenorhabditis elegans L1CAM encoded by the sax-7 gene and RAB-3, a GTPase that functions in synaptic neurotransmission; rab-3; sax-7 mutant animals exhibit synthetic locomotion abnormalities and neuronal dysfunction. Here, we show that this synergism also occurs when loss of SAX-7 is combined with mutants of other genes encoding key players of the synaptic vesicle (SV) cycle. In contrast, sax-7 does not interact with genes that function in synaptogenesis. These findings suggest a postdevelopmental role for sax-7 in the regulation of synaptic activity. To assess this possibility, we conducted electrophysiological recordings and ultrastructural analyses at neuromuscular junctions; these analyses did not reveal obvious synaptic abnormalities. Lastly, based on a forward genetic screen for suppressors of the rab-3; sax-7 synthetic phenotypes, we determined that mutants in the ERK Mitogen-activated Protein Kinase (MAPK) pathway can suppress the rab-3; sax-7 locomotion defects. Moreover, we established that Erk signaling acts in a subset of cholinergic neurons in the head to promote coordinated locomotion. In combination, these results suggest a modulatory role for Erk MAPK in L1CAM-dependent locomotion in C. elegans.

Neuronal post-developmentally acting SAX-7S/L1CAM can function as cleaved fragments to maintain neuronal architecture in C. elegans

Whereas remarkable advances have uncovered mechanisms that drive nervous system assembly, the processes responsible for the lifelong maintenance of nervous system architecture remain poorly understood. Subsequent to its establishment during embryogenesis, neuronal architecture is maintained throughout life in the face of the animal's growth, maturation processes, the addition of new neurons, body movements, and aging. The C. elegans protein SAX-7, homologous to the vertebrate L1 protein family of neural adhesion molecules, is required for maintaining the organization of neuronal ganglia and fascicles after their successful initial embryonic development. To dissect the function of sax-7 in neuronal maintenance, we generated a null allele and sax-7S-isoform-specific alleles. We find that the null sax-7(qv30) is, in some contexts, more severe than previously described mutant alleles, and that the loss of sax-7S largely phenocopies the null, consistent with sax-7S being the key isoform in neuronal maintenance. Using a sfGFP::SAX-7S knock-in, we observe sax-7S to be predominantly expressed across the nervous system, from embryogenesis to adulthood. Yet, its role in maintaining neuronal organization is ensured by post-developmentally acting SAX-7S, as larval transgenic sax-7S(+) expression alone is sufficient to profoundly rescue the null mutants' neuronal maintenance defects. Moreover, the majority of the protein SAX-7 appears to be cleaved, and we show that these cleaved SAX-7S fragments together, not individually, can fully support neuronal maintenance. These findings contribute to our understanding of the role of the conserved protein SAX-7/L1CAM in long-term neuronal maintenance, and may help decipher processes that go awry in some neurodegenerative conditions.

L1cam curbs the differentiation of adult-born hippocampal neurons

L1 is an immunoglobulin domain (Ig)-containing protein essential for a wide range of neurodevelopmental processes highly conserved across species from worms to humans. L1 can act as a cell adhesion molecule by binding to other Ig-containing proteins or as a ligand for certain tyrosine kinase receptors such as FGFRs and TRKs, which are required not only during neurodevelopment but also in hippocampal neurogenesis. Yet, the role of L1 itself in adult hippocampal neurogenesis remains unaddressed. Here, we used several Cre-driver lines in mice to conditionally delete a floxed allele of L1cam at different points along the differentiation lineage of new neurons and in surrounding neurons in the adult dentate gyrus of the hippocampus. We found that L1cam deletion in stem/progenitor cells increased: 1) the differentiation of progenitors into new neurons, 2) the complexity of dendritic arbors in immature neurons, and 3) anxiety-related behavior. In addition, deletion of L1cam in neurons leads to an earlier age-related decline in hippocampal neurogenesis. These data suggest that L1 is not only important for normal nervous system development, but also for maintaining certain neural processes in adulthood.

Negr1 controls adult hippocampal neurogenesis and affective behaviors

Recent genome-wide association studies on major depressive disorder have implicated neuronal growth regulator 1 (Negr1), a GPI-anchored cell adhesion molecule in the immunoglobulin LON family. Although Negr1 has been shown to regulate neurite outgrowth and synapse formation, the mechanism through which this protein affects mood disorders is still largely unknown. In this research, we characterized Negr1-deficient (negr1^-/-) mice to elucidate the function of Negr1 in anxiety and depression. We found that anxiety- and depression-like behaviors increased in negr1^-/- mice compared with wild-type mice. In addition, negr1^-/- mice had decreased adult hippocampal neurogenesis compared to wild-type mice. Concurrently, both LTP and mEPSC in the dentate gyrus (DG) region were severely compromised in negr1^-/- mice. In our effort to elucidate the underlying molecular mechanisms, we found that lipocalin-2 (Lcn2) expression was decreased in the hippocampus of negr1^-/- mice compared to wild-type mice. Heterologous Lcn2 expression in the hippocampal DG of negr1^-/- mice rescued anxiety- and depression-like behaviors and restored neurogenesis and mEPSC frequency to their normal levels in these mice. Furthermore, we discovered that Negr1 interacts with leukemia inhibitory factor receptor (LIFR) and modulates LIF-induced Lcn2 expression. Taken together, our data uncovered a novel mechanism of mood regulation by Negr1 involving an interaction between Negr1 and LIFR along with Lcn2 expression.

The cell adhesion protein CAR is a negative regulator of synaptic transmission

The Coxsackievirus and adenovirus receptor (CAR) is essential for normal electrical conductance in the heart, but its role in the postnatal brain is largely unknown. Using brain specific CAR knockout mice (KO), we discovered an unexpected role of CAR in neuronal communication. This includes increased basic synaptic transmission at hippocampal Schaffer collaterals, resistance to fatigue, and enhanced long-term potentiation. Spontaneous neurotransmitter release and speed of endocytosis are increased in KOs, accompanied by increased expression of the exocytosis associated calcium sensor synaptotagmin 2. Using proximity proteomics and binding studies, we link CAR to the exocytosis machinery as it associates with syntenin and synaptobrevin/VAMP2 at the synapse. Increased synaptic function does not cause adverse effects in KO mice, as behavior and learning are unaffected. Thus, unlike the connexin-dependent suppression of atrioventricular conduction in the cardiac knockout, communication in the CAR deficient brain is improved, suggesting a role for CAR in presynaptic processes.

Pericyte-like spreading by disseminated cancer cells activates YAP and MRTF for metastatic colonization

Metastatic seeding by disseminated cancer cells principally occurs in perivascular niches. Here, we show that mechanotransduction signalling triggered by the pericyte-like spreading of disseminated cancer cells on host tissue capillaries is critical for metastatic colonization. Disseminated cancer cells employ L1CAM (cell adhesion molecule L1) to spread on capillaries and activate the mechanotransduction effectors YAP (Yes-associated protein) and MRTF (myocardin-related transcription factor). This spreading is robust enough to displace resident pericytes, which also use L1CAM for perivascular spreading. L1CAM activates YAP by engaging β₁ integrin and ILK (integrin-linked kinase). L1CAM and YAP signalling enables the outgrowth of metastasis-initiating cells both immediately following their infiltration of target organs and after they exit from a period of latency. Our results identify an important step in the initiation of metastatic colonization, define its molecular constituents and provide an explanation for the widespread association of L1CAM with metastatic relapse in the clinic.

Neural Cell Adhesion Molecules of the Immunoglobulin Superfamily Regulate Synapse Formation, Maintenance, and Function

Immunoglobulin superfamily adhesion molecules are among the most abundant proteins in vertebrate and invertebrate nervous systems. Prominent family members are the neural cell adhesion molecules NCAM and L1, which were the first to be shown to be essential not only in development but also in synaptic function and as key regulators of synapse formation, synaptic activity, plasticity, and synaptic vesicle recycling at distinct developmental and activity stages. In addition to interacting with each other, adhesion molecules interact with ion channels and cytokine and neurotransmitter receptors. Mutations in their genes are linked to neurological disorders associated with abnormal development and synaptic functioning. This review presents an overview of recent studies on these molecules and their crucial impact on neurological disorders.

Down-regulation of neuronal L1 cell adhesion molecule expression alleviates inflammatory neuronal injury

In multiple sclerosis (MS), the immune cell attack leads to axonal injury as a major cause for neurological disability. Here, we report a novel role of the cell adhesion molecule L1 in the crosstalk between the immune and nervous systems. L1 was found to be expressed by CNS axons of MS patients and human T cells. In MOG_35-55-induced murine experimental neuroinflammation, CD4⁺ T cells were associated with degenerating axons in the spinal cord, both expressing L1. However, neuronal L1 expression in the spinal cord was reduced, while levels of the transcriptional repressor REST (RE1-Silencing Transcription Factor) were up-regulated. In PLP_139-151-induced relapsing-remitting neuroinflammation, L1 expression was low at the peak stage of disease, reached almost normal levels in the remission stage, but decreased again during disease relapse indicating adaptive expression regulation of L1. In vitro, activated CD4⁺ T cells caused contact-dependent down-regulation of L1, up-regulation of its repressor REST and axonal injury in co-cultured neurons. T cell adhesion to neurons and axonal injury were prevented by an antibody blocking L1 suggesting that down-regulation of L1 ameliorates neuroinflammation. In support of this hypothesis, antibody-mediated blocking of L1 in C57BL/6 mice as well as neuron-specific depletion of L1 in synapsin^Cre × L1^fl/fl mice reduces disease severity and axonal pathology despite unchanged immune cell infiltration of the CNS. Our data suggest that down-regulation of neuronal L1 expression is an adaptive process of neuronal self-defense in response to pro-inflammatory T cells, thereby alleviating immune-mediated axonal injury.

Functions of the Alzheimer's Disease Protease BACE1 at the Synapse in the Central Nervous System

Inhibition of the protease β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) is a promising treatment strategy for Alzheimer's disease, and a number of BACE inhibitors are currently progressing through clinical trials. The strategy aims to decrease production of amyloid-β (Aβ) peptide from the amyloid precursor protein (APP), thus reducing or preventing Aβ toxicity. Over the last decade, it has become clear that BACE1 proteolytically cleaves a number of substrates in addition to APP. These substrates are not known to be involved in the pathogenesis of Alzheimer's disease but have other roles in the developing and/or mature central nervous system. Consequently, BACE inhibition and knockout in mice results in synaptic and other neuronal dysfunctions and the key substrates responsible for these deficits are still being elucidated. Of the BACE1 substrates that have been validated to date, a number may contribute to the synaptic deficits seen with BACE blockade, including neuregulin 1, close homologue of L1 and seizure-related gene 6. It is important to understand the impact that BACE blockade may have on these substrates and other proteins detected in substrate screens and, if necessary, develop substrate-selective BACE inhibitors.

Cell adhesion molecule L1 contributes to neuronal excitability regulating the function of voltage-gated Na+ channels

L1 (also known as L1CAM) is a trans-membrane glycoprotein mediating neuron-neuron adhesion through homophilic and heterophilic interactions. Although experimental evidence has implicated L1 in axonal outgrowth, fasciculation and pathfinding, its contribution to voltage-gated Na(+) channel function and membrane excitability has remained unknown. Here, we show that firing rate, single cell spiking frequency and Na(+) current density are all reduced in hippocampal excitatory neurons from L1-deficient mice both in culture and in slices owing to an overall reduced membrane expression of Na(+) channels. Remarkably, normal firing activity was restored when L1 was reintroduced into L1-deficient excitatory neurons, indicating that abnormal firing patterns are not related to developmental abnormalities, but are a direct consequence of L1 deletion. Moreover, L1 deficiency leads to impairment of action potential initiation, most likely due to the loss of the interaction of L1 with ankyrin G that produces the delocalization of Na(+) channels at the axonal initial segment. We conclude that L1 contributes to functional expression and localization of Na(+) channels to the neuronal plasma membrane, ensuring correct initiation of action potential and normal firing activity.

Conditional deletion of L1CAM in human neurons impairs both axonal and dendritic arborization and action potential generation

Patzke et al. create human embryonic stem cell–derived neurons that enable the generation of conditional loss-of-function mutations of L1CAM. Deletion of L1CAM impairs axonal elongation, dendritic arborization, and action potential generation.

Hundreds of L1CAM gene mutations have been shown to be associated with congenital hydrocephalus, severe intellectual disability, aphasia, and motor symptoms. How such mutations impair neuronal function, however, remains unclear. Here, we generated human embryonic stem (ES) cells carrying a conditional L1CAM loss-of-function mutation and produced precisely matching control and L1CAM-deficient neurons from these ES cells. In analyzing two independent conditionally mutant ES cell clones, we found that deletion of L1CAM dramatically impaired axonal elongation and, to a lesser extent, dendritic arborization. Unexpectedly, we also detected an ∼20–50% and ∼20–30% decrease, respectively, in the levels of ankyrinG and ankyrinB protein, and observed that the size and intensity of ankyrinG staining in the axon initial segment was significantly reduced. Overexpression of wild-type L1CAM, but not of the L1CAM point mutants R1166X and S1224L, rescued the decrease in ankyrin levels. Importantly, we found that the L1CAM mutation selectively decreased activity-dependent Na⁺-currents, altered neuronal excitability, and caused impairments in action potential (AP) generation. Thus, our results suggest that the clinical presentations of L1CAM mutations in human patients could be accounted for, at least in part, by cell-autonomous changes in the functional development of neurons, such that neurons are unable to develop normal axons and dendrites and to generate normal APs.

Intercellular protein-protein interactions at synapses

Chemical synapses are asymmetric intercellular junctions through which neurons send nerve impulses to communicate with other neurons or excitable cells. The appropriate formation of synapses, both spatially and temporally, is essential for brain function and depends on the intercellular protein-protein interactions of cell adhesion molecules (CAMs) at synaptic clefts. The CAM proteins link pre- and post-synaptic sites, and play essential roles in promoting synapse formation and maturation, maintaining synapse number and type, accumulating neurotransmitter receptors and ion channels, controlling neuronal differentiation, and even regulating synaptic plasticity directly. Alteration of the interactions of CAMs leads to structural and functional impairments, which results in many neurological disorders, such as autism, Alzheimer's disease and schizophrenia. Therefore, it is crucial to understand the functions of CAMs during development and in the mature neural system, as well as in the pathogenesis of some neurological disorders. Here, we review the function of the major classes of CAMs, and how dysfunction of CAMs relates to several neurological disorders.

Cognitive assessment of mice strains heterozygous for cell-adhesion genes reveals strain-specific alterations in timing

We used a fully automated system for the behavioural measurement of physiologically meaningful properties of basic mechanisms of cognition to test two strains of heterozygous mutant mice, Bfc (batface) and L1, and their wild-type littermate controls. Both of the target genes are involved in the establishment and maintenance of synapses. We find that the Bfc heterozygotes show reduced precision in their representation of interval duration, whereas the L1 heterozygotes show increased precision. These effects are functionally specific, because many other measures made on the same mice are unaffected, namely: the accuracy of matching temporal investment ratios to income ratios in a matching protocol, the rate of instrumental and classical conditioning, the latency to initiate a cued instrumental response, the trials on task and the impulsivity in a switch paradigm, the accuracy with which mice adjust timed switches to changes in the temporal constraints, the days to acquisition, and mean onset time and onset variability in the circadian anticipation of food availability.

Role of stress system disturbance and enhanced novelty response in spatial learning of NCAM-deficient mice

The neural cell adhesion molecule (NCAM) plays a crucial role in stress-related brain function, emotional behavior and memory formation. In this study, we investigated the functions of the glucocorticoid and serotonergic systems in mice constitutively deficient for NCAM (NCAM-/- mice). Our data provide evidence for a hyperfunction of the hypothalamic-pituitary-adrenal axis, with enlarged adrenal glands and increased stress-induced corticosterone release, but reduced hippocampal glucocorticoid receptor expression in NCAM-/- mice when compared to NCAM+/+ mice. We also obtained evidence for a hypofunction of 5-HT1A autoreceptors as indicated by increased 8-0H-DPAT-induced hypothermia. These findings suggest a disturbance of both humoral and neural stress systems in NCAM-/- mice. Accordingly, we not only confirmed previously observed hyperarousal of NCAM-/- mice in various anxiety tests, but also observed an increased response to novelty exposure in these animals. Spatial learning deficits of the NCAM-/- mice in a Morris Water maze persisted, even when mice were pretrained to prevent effects of novelty or stress. We suggest that NCAM-mediated processes are involved in both novelty/stress-related emotional behavior and in cognitive function during spatial learning.

The neurobiology of X-linked intellectual disability

X-linked intellectual disability (XLID) affects 1% to 3% of the population. XLID subsumes several heterogeneous conditions, all of which are marked by cognitive impairment and reduced adaptive skills. XLID arises from mutations on the X chromosome; to date, 102 XLID genes have been identified. The proteins encoded by XLID genes are involved in higher brain functions, such as cognition, learning and memory, and their molecular role is the subject of intense investigation. Here, we review recent findings concerning a representative group of XLID proteins: the fragile X mental retardation protein; methyl-CpG-binding protein 2 and cyclin-dependent kinase-like 5 proteins, which are involved in Rett syndrome; the intracellular signaling molecules of the Rho guanosine triphosphatases family; and the class of cell adhesion molecules. We discuss how XLID gene mutations affect the structure and function of synapses.

Spatial memory tasks in rodents: what do they model?

The analysis of spatial learning and memory in rodents is commonly used to investigate the mechanisms underlying certain forms of human cognition and to model their dysfunction in neuropsychiatric and neurodegenerative diseases. Proper interpretation of rodent behavior in terms of spatial memory and as a model of human cognitive functions is only possible if various navigation strategies and factors controlling the performance of the animal in a spatial task are taken into consideration. The aim of this review is to describe the experimental approaches that are being used for the study of spatial memory in rats and mice and the way that they can be interpreted in terms of general memory functions. After an introduction to the classification of memory into various categories and respective underlying neuroanatomical substrates, I explain the concept of spatial memory and its measurement in rats and mice by analysis of their navigation strategies. Subsequently, I describe the most common paradigms for spatial memory assessment with specific focus on methodological issues relevant for the correct interpretation of the results in terms of cognitive function. Finally, I present recent advances in the use of spatial memory tasks to investigate episodic-like memory in mice.

Neuregulin 1 enhances cell adhesion molecule l1 expression in human glioma cells and promotes their migration as a function of malignancy

Similar functions of L1, a cell adhesion molecule, and the cytokine neuregulin 1 (Nrg1) have been suggested in tumorigenesis and the promotion of metastasis. We studied the relationships of Nrg1 and L1 expression in human gliomas. Using immunofluorescence staining on a human glioma tissue microarray, we found a positive correlation between levels of L1 and Nrg1α or Nrg1β expression; expression tended to increase with increasing WHO (World Health Organization) tumor grade. L1 was also found to colocalize with either Nrg1 isoform. In cultures of U87-MG human glioblastoma and human U251 and SHG-44 glioma cells, the base levels of full-length L1 expression were increased by the 2 Nrg1 molecules in the nanomolar range, and Nrg1 siRNA downregulated full-length L1 expression in these tumor cell lines. U87-MG cells treated with either Nrg1 isoform also showed enhanced migration when compared with that treated with vehicle control. In addition, administration of either lapatinib (a dual inhibitor of both the epidermal growth factor receptor and ErbB-2) or erlotinib (an inhibitor of the epidermal growth factor receptor) in combination with either Nrg1α or Nrg1β inhibited the L1 expression elicited by these cytokines in U87-MG cells. Together, our data suggest that Nrg1 regulates L1 expression in gliomas, and that Nrg1 may contribute to malignancy by upregulating the L1 expression in glioblastoma cells, thereby enhancing their migration.

Heterozygosity for the mutated X-chromosome-linked L1 cell adhesion molecule gene leads to increased numbers of neurons and enhanced metabolism in the forebrain of female carrier mice

Mutations in the X-chromosomal L1CAM gene lead to severe neurological deficits. In this study, we analyzed brains of female mice heterozygous for L1 (L1+/-) to gain insights into the brain structure of human females carrying one mutated L1 allele. From postnatal day 7 onward into adulthood, L1+/- female mice show an increased density of neurons in the neocortex and basal ganglia in comparison to wild-type (L1+/+) mice, correlating with enhanced metabolic parameters as measured in vivo. The densities of astrocytes and parvalbumin immunoreactive interneurons were not altered. No significant differences between L1+/- and L1+/+ mice were seen for cell proliferation in the cortex during embryonic days 11.5-15.5. Neuronal differentiation as estimated by analysis of doublecortin-immunoreactive cortical cells of embryonic brains was similar in L1+/- and L1+/+ mice. Interestingly, at postnatal days 3 and 5, apoptosis was reduced in L1+/- compared to L1+/+ mice. We suggest that reduced apoptosis leads to increased neuronal density in adult L1+/- mice. In conclusion, L1+/- mice display an unexpected phenotype that is not an intermediate between L1+/+ mice and mice deficient in L1 (L1-/y), but a novel phenotype which is challenging to understand regarding its underlying molecular and cellular mechanisms.

Comparison of L1 expression and secretion in glioblastoma and neuroblastoma cells

The expression of cell adhesion molecule L1 has been identified in a vast spectrum of tumors; however, its expression pattern with regard to tumor type is rarely discussed. In the present study, we studied L1 levels in human glioblastomas and neuroblastomas, and compared the expression and secretion of L1 in human glioblastoma U87-MG and neuroblastoma SK-N-SH cells. Immunofluorescence staining revealed different grades of L1 staining in human glioblastoma and neuroblastoma samples. In U87-MG cells, full-length L1 was weakly detected in cell lysates (CLs), while greater levels of abundant soluble L1 were confined in conditioned culture medium (CCM). In contrast, higher levels of full-length L1 were confined in SK-N-SH CLs, while almost no soluble forms of L1 were detected in CCM. Our data indicates various expression patterns of L1 in U87-MG and SK-N-SH cells, which may underlie the different malignancies of the two neural tumor types and further stress the importance of soluble L1-mediated signaling pathways in cell malignancy.

Comparison in the biological characteristics between primary cultured sensory and motor Schwann cells

Schwann cells (SCs) express distinct sensory and motor phenotypes, which are associated with modality-specific promotion of axon growth. Here we compared cell proliferation and migration of primary cultured sensory and motor SCs and determined the mRNA expression of several genes, nap1l1, dok4, lpp, mmp-9 and l1cam, in two phenotypes of SCs. The results showed that the rate of cell proliferation or migration was higher in sensory SCs than in motor SCs, and the five proliferation or migration-related genes also had higher expression in sensory SCs than in motor SCs. These findings may provide a basis for deeply studying the biological differences between sensory and motor SCs.

L1CAM increases MAP2 expression via the MAPK pathway to promote neurite outgrowth

The neural cell adhesion molecule L1 (L1CAM) promotes neurite outgrowth via mechanisms that are not completely understood, but are known to involve the cytoskeleton. Here, we show that L1 binds directly to the microtubule associated protein 2c (MAP2c). This isoform of MAP2 is predominantly expressed in developing neurons. We found that the mRNA and protein levels of MAP2c, but not of MAP2a/b, are reduced in brains of young adult L1-deficient transgenic mice. We show via ELISA, that MAP2c, but not MAP2a/b, binds directly to the intracellular domain of L1. Remarkably, all these MAP2 isoforms co-immunoprecipitate with L1, suggesting that MAP2a/b associates with L1 via intermediate binding partners. The expression levels of MAP2a/b/c correlate with those of L1 in different brain regions of early postnatal mice, while expression levels of heat shock cognate protein 70 (Hsc70) or actin do not. L1 enhances the expression of MAP2a/b/c in cultured hippocampal neurons depending on activation of the mitogen-activated protein kinase (MAPK) pathway. Deficiency in both L1 and MAP2a/b/c expression results in reduced neurite outgrowth in vitro. We propose that the L1-triggered increase in MAP2a/b/c expression is required to promote neurite outgrowth.

Generation and nuclear translocation of sumoylated transmembrane fragment of cell adhesion molecule L1

The functions of the cell adhesion molecule L1 in the developing and adult nervous system are triggered by homophilic and heterophilic interactions that stimulate signal transductions that activate cellular responses. Here, we show that stimulation of signaling by function-triggering L1 antibodies or L1-Fc leads to serine protease-dependent cleavage of full-length L1 at the plasma membrane and generation of a sumoylated transmembrane 70-kDa fragment comprising the intracellular and transmembrane domains and part of the extracellular domain. The 70-kDa transmembrane fragment is transported from the plasma membrane to a late endosomal compartment, released from endosomal membranes into the cytoplasm, and transferred from there into the nucleus by a pathway that depends on importin and chromatin-modifying protein 1. Mutation of the sumoylation site at Lys(1172) or of the nuclear localization signal at Lys(1147) abolished L1-stimulated generation or nuclear import of the 70-kDa fragment, respectively. Nuclear import of the 70-kDa fragment may activate cellular responses in parallel or in association with phosphorylation-dependent signaling pathways. Alterations in the levels of the 70-kDa fragment during development and in the adult after spinal cord injury or in a mouse model of Alzheimer disease suggest that this fragment is functionally implicated in development, regeneration, neurodegeneration, tumorigenesis, and possibly synaptic plasticity in the mature nervous system.

The cell adhesion molecule L1 regulates the expression of choline acetyltransferase and the development of septal cholinergic neurons

Mutations in the L1 gene cause severe brain malformations and mental retardation. We investigated the potential roles of L1 in the regulation of choline acetyltransferase (ChAT) and in the development of septal cholinergic neurons, which are known to project to the hippocampus and play key roles in cognitive functions. Using stereological approaches, we detected significantly fewer ChAT-positive cholinergic neurons in the medial septum and vertical limb of the diagonal band of Broca (MS/VDB) of 2-week-old L1-deficient mice compared to wild-type littermates (1644 ± 137 vs. 2051 ± 165, P = 0.038). ChAT protein levels in the septum were 53% lower in 2-week-old L1-deficient mice compared to wild-type littermates. ChAT activity in the septum was significantly reduced in L1-deficient mice compared to wild-type littermates at 1 (34%) and 2 (40%) weeks of age. In vitro, increasing doses of L1-Fc induced ChAT activity in septal neurons with a significant linear trend (*P = 0.0065). At 4 weeks of age in the septum and at all time points investigated in the caudate-putamen (CPu), the number of ChAT-positive neurons and the levels of ChAT activity were not statistically different between L1-deficient mice and wild-type littermates. The total number of cells positive for the neuronal nuclear antigen (NeuN) in the MS/VDB and CPu was not statistically different in L1-deficient mice compared to wild-type littermates, and comparable expression of the cell cycle marker Ki67 was observed. Our results indicate that L1 is required for the timely maturation of septal cholinergic neurons and that L1 promotes the expression and activity of ChAT in septal neurons.

L1 syndrome mutations impair neuronal L1 function at different levels by divergent mechanisms

Mutations in the human L1CAM gene cause neurodevelopmental disorders collectively referred to as L1 syndrome. Here, we investigated cellular pathomechanisms underlying two L1 syndrome mutations, R184Q and W1036L. We demonstrate that these mutations cause partial endoplasmic reticulum (ER) retention of L1, reduce L1 cell surface expression, but do not induce ER stress in neuronal NSC-34 cells. We provide evidence that surface trafficking of mutated L1 is affected by defective sorting to ER exit sites and attenuated ER export. However, in differentiated neuronal cultures and long-term cultured hippocampal slices, the L1-R184Q protein is restricted to cell bodies, whereas L1-W1036L also aberrantly localizes to dendrites. These trafficking defects preclude axonal targeting of L1, thereby affecting L1-mediated axon growth and arborization. Our results indicate that L1 syndrome mutations impair neuronal L1 function at different levels, firstly by attenuating ER export and secondly by interfering with polarized neuronal trafficking.

Role of the cytoplasmic domain of the L1 cell adhesion molecule in brain development

Mutations in the human L1CAM gene cause X-linked hydrocephalus and MASA (Mental retardation, Aphasia, Shuffling gait, Adducted thumbs) syndrome. In vitro studies have shown that the L1 cytoplasmic domain (L1CD) is involved in L1 trafficking, neurite branching, signaling, and interactions with the cytoskeleton. L1cam knockout (L1(KO)) mice have hydrocephalus, a small cerebellum, hyperfasciculation of corticothalamic tracts, and abnormal peripheral nerves. To explore the function of the L1CD, we made three new mice lines in which different parts of the L1CD have been altered. In all mutant lines L1 protein is expressed and transported into the axon. Interestingly, these new L1CD mutant lines display normal brain morphology. However, the expression of L1 protein in the adult is dramatically reduced in the two L1CD mutant lines that lack the ankyrin-binding region and they show defects in motor function. Therefore, the L1CD is not responsible for the major defects observed in L1(KO) mice, yet it is required for continued L1 protein expression and motor function in the adult.

Altered gene expression of histone deacetylases in mood disorder patients

Chromatin remodeling such as changes in histone acetylation has been suggested to play an important role in the pathophysiology and treatment of mood disorders. In the present study, we investigated whether the expression of histone deacetylase (HDAC) genes are altered in mood disorder patients. We used quantitative real-time PCR to measure the mRNA levels of 11 HDACs (HDAC1-11) in peripheral white blood cells of major depressive disorder (MDD) and bipolar disorder (BPD) patients during depressive and remissive episodes and in the first-degree relatives of BPD patients. In addition, we investigated the effect of antidepressants and mood stabilizers on the mRNA levels of HDACs using mice. In MDD, the expression of HDAC2 and -5 mRNA was increased in a depressive state, but not in a remissive state, compared to controls. In BPD, the expression of HDAC4 mRNA was increased only in a depressive state, and the expression of HDAC6 and -8 was decreased in both depressive and remissive states compared to controls, whereas the first-degree relatives did not show any significant alteration in expression levels. Animal study showed that the expression of HDAC2 and -5 or HDAC4, -6 and -8 mRNAs in the mouse leukocytes were not affected by chronic treatment with antidepressants or mood stabilizers. Our data suggest that aberrant transcriptional regulation caused by the altered expression of HDACs is associated with the pathophysiology of mood disorders.

Modulation of synaptic transmission and plasticity by cell adhesion and repulsion molecules

Adhesive and repellent molecular cues guide migrating cells and growing neurites during development. They also contribute to synaptic function, learning and memory in adulthood. Here, we review the roles of cell adhesion molecules of the immunoglobulin superfamily (Ig-CAMs) and semaphorins (some of which also contain Ig-like domains) in regulation of synaptic transmission and plasticity. Interestingly, among the seven studied Ig-CAMs, the neuronal cell adhesion molecule proved to be important for all tested forms of hippocampal plasticity, while its associated unusual glycan polysialic acid is necessary and sufficient part for synaptic plasticity only at CA3-CA1 synapses. In contrast, Thy-1 and L1 specifically regulate long-term potentiation (LTP) at synapses formed by entorhinal axons in the dentate gyrus and cornu ammonis, respectively. Contactin-1 is important for long-term depression but not for LTP at CA3-CA1 synapses. Analysis of CHL1-deficient mice illustrates that at intermediate stages of development a deficit in a cell adhesion molecule is compensated but appears as impaired LTP during early and late postnatal development. The emerging mechanisms by which adhesive Ig-CAMs contribute to synaptic plasticity involve regulation of activities of NMDA receptors and L-type Ca2+ channels, signaling via mitogen-activated protein kinase p38, changes in GABAergic inhibition and motility of synaptic elements. Regarding repellent molecules, available data for semaphorins demonstrate their activity-dependent regulation in normal and pathological conditions, synaptic localization of their receptors and their potential to elevate or inhibit synaptic transmission either directly or indirectly.

Age matters

The age of an experimental animal can be a critical variable, yet age matters are often overlooked within neuroscience. Many studies make use of young animals, without considering possible differences between immature and mature subjects. This is especially problematic when attempting to model traits or diseases that do not emerge until adulthood. In this commentary we discuss the reasons for this apparent bias in age of experimental animals, and illustrate the problem with a systematic review of published articles on long-term potentiation. Additionally, we review the developmental stages of a rat and discuss the difficulty of using the weight of an animal as a predictor of its age. Finally, we provide original data from our laboratory and review published data to emphasize that development is an ongoing process that does not end with puberty. Developmental changes can be quantitative in nature, involving gradual changes, rapid switches, or inverted U-shaped curves. Changes can also be qualitative. Thus, phenomena that appear to be unitary may be governed by different mechanisms at different ages. We conclude that selection of the age of the animals may be critically important in the design and interpretation of neurobiological studies.

The adhesion molecule L1 regulates transendothelial migration and trafficking of dendritic cells

The adhesion molecule L1, which is extensively characterized in the nervous system, is also expressed in dendritic cells (DCs), but its function there has remained elusive. To address this issue, we ablated L1 expression in DCs of conditional knockout mice. L1-deficient DCs were impaired in adhesion to and transmigration through monolayers of either lymphatic or blood vessel endothelial cells, implicating L1 in transendothelial migration of DCs. In agreement with these findings, L1 was expressed in cutaneous DCs that migrated to draining lymph nodes, and its ablation reduced DC trafficking in vivo. Within the skin, L1 was found in Langerhans cells but not in dermal DCs, and L1 deficiency impaired Langerhans cell migration. Under inflammatory conditions, L1 also became expressed in vascular endothelium and enhanced transmigration of DCs, likely through L1 homophilic interactions. Our results implicate L1 in the regulation of DC trafficking and shed light on novel mechanisms underlying transendothelial migration of DCs. These observations might offer novel therapeutic perspectives for the treatment of certain immunological disorders.

The molecular and cellular biology of enhanced cognition

Most molecular and cellular studies of cognitive function have focused on either normal or pathological states, but recent research with transgenic mice has started to address the mechanisms of enhanced cognition. These results point to key synaptic and nuclear signalling events that can be manipulated to facilitate the induction or increase the stability of synaptic plasticity, and therefore enhance the acquisition or retention of information. Here, we review these surprising findings and explore their implications to both mechanisms of learning and memory and to ongoing efforts to develop treatments for cognitive disorders. These findings represent the beginning of a fundamental new approach in the study of enhanced cognition.

Enriched environment experience overcomes learning deficits and depressive-like behavior induced by juvenile stress

Mood disorders affect the lives and functioning of millions each year. Epidemiological studies indicate that childhood trauma is predominantly associated with higher rates of both mood and anxiety disorders. Exposure of rats to stress during juvenility (JS) (27–29 days of age) has comparable effects and was suggested as a model of induced predisposition for these disorders. The importance of the environment in the regulation of brain, behavior and physiology has long been recognized in biological, social and medical sciences. Here, we studied the effects of JS on emotional and cognitive aspects of depressive-like behavior in adulthood, on Hypothalamic-Pituitary-Adrenal (HPA) axis reactivity and on the expression of cell adhesion molecule L1 (L1-CAM). Furthermore, we combined it with the examination of potential reversibility by enriched environment (EE) of JS – induced disturbances of emotional and cognitive aspects of behavior in adulthood. Three groups were tested: Juvenile Stress –subjected to Juvenile stress; Enriched Environment – subjected to Juvenile stress and then, from day 30 on to EE; and Naïves. In adulthood, coping and stress responses were examined using the elevated plus-maze, open field, novel setting exploration and two way shuttle avoidance learning. We found that, JS rats showed anxiety- and depressive-like behaviors in adulthood, altered HPA axis activity and altered L1-CAM expression. Increased expression of L1-CAM was evident among JS rats in the basolateral amygdala (BLA) and Thalamus (TL). Furthermore, we found that EE could reverse most of the effects of Juvenile stress, both at the behavioral, endocrine and at the biochemical levels. The interaction between JS and EE resulted in an increased expression of L1-CAM in dorsal cornu ammonis (CA) area 1 (dCA1).

Looking beyond development: maintaining nervous system architecture

Neuronal circuitries established in development must persist throughout life. This poses a serious challenge to the structural integrity of an embryonically patterned nervous system as an animal dramatically increases its size postnatally, remodels parts of its anatomy, and incorporates new neurons. In addition, body movements, injury, and ageing generate physical stress on the nervous system. Specific molecular pathways maintain intrinsic properties of neurons in the mature nervous system. Other factors ensure that the overall organization of entire neuronal ensembles into ganglia and fascicles is appropriately maintained upon external challenges. Here, we discuss different molecules underlying these neuronal maintenance mechanisms, with a focus on lessons learned from the nematode Caenorhabditis elegans.

L1 cell adhesion molecule is essential for the maintenance of hyperalgesia after spinal cord injury

Spinal cord injury (SCI) results in a loss of normal motor and sensory function, leading to severe disability and reduced quality of life. A large proportion of individuals with SCI also suffer from neuropathic pain symptoms. The causes of abnormal pain sensations are not well understood, but can include aberrant sprouting and reorganization of injured or spared sensory afferent fibers. L1 is a cell adhesion molecule that contributes to axonal outgrowth, guidance and fasciculation in development as well as synapse formation and plasticity throughout life. In the present study, we used L1 knockout (KO) mice to determine whether this adhesion molecule contributes to sensory dysfunction after SCI. Both wild-type (WT) and KO mice developed heat hyperalgesia following contusion injury, but the KO mice recovered normal response latencies beginning at 4 weeks post-injury. Histological analyses confirmed increased sprouting of sensory fibers containing calcitonin-gene related peptide (CGRP) in the deep dorsal horn of the lumbar spinal cord and increased numbers of interneurons expressing protein kinase C gamma (PKCgamma) in WT mice 6 weeks after injury. In contrast, L1 KO mice had less CGRP(+) fiber sprouting, but even greater numbers of PKCgamma(+) interneurons at the 6 week time point. These data demonstrate that L1 plays a role in maintenance of thermal hyperalgesia after SCI in mice, and implicate CGRP(+) fiber sprouting and the upregulation of PKCgamma expression as potential contributors to this response.