Deficiency in the neural cell adhesion molecule 2 (NCAM2) reduces axonal levels of beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), affects axonal organization in the hippocampus, and leads to behavioral deficits

The neural cell adhesion molecule 2 (NCAM2) regulates axonal organization in the central nervous system via mechanisms that have remained poorly understood. We now show that NCAM2 increases axonal levels of beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), a protease that regulates axonal guidance. In brains of NCAM2-deficient mice, BACE1 levels are reduced in hippocampal mossy fiber projections, and the infrapyramidal bundle of these projections is shortened. This abnormal axonal organization correlates with impaired short-term spatial memory and cognitive flexibility in NCAM2-deficient male and female mice. Self-grooming, rearing, digging and olfactory acuity are increased in NCAM2-deficient male mice, when compared with littermate wild-type mice of the same sex. NCAM2-deficient female mice also show increased self-grooming, but are reduced in rearing, and do not differ from female wild-type mice in olfactory acuity and digging behavior. Our results indicate that errors in axonal guidance and organization caused by impaired BACE1 function can underlie the manifestation of neurodevelopmental disorders, including autism as found in humans with deletions of the NCAM2 gene.

The BACE1-generated C-terminal fragment of the neural cell adhesion molecule 2 (NCAM2) promotes BACE1 targeting to Rab11-positive endosomes

Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), also known as β-secretase, is an aspartic protease. The sorting of this enzyme into Rab11-positive recycling endosomes regulates the BACE1-mediated cleavage of its substrates, however, the mechanisms underlying this targeting remain poorly understood. The neural cell adhesion molecule 2 (NCAM2) is a substrate of BACE1. We show that BACE1 cleaves NCAM2 in cultured hippocampal neurons and NCAM2-transfected CHO cells. The C-terminal fragment of NCAM2 that comprises the intracellular domain and a small portion of NCAM2’s extracellular domain, associates with BACE1. This association is not affected in cells with inhibited endocytosis, indicating that the interaction of NCAM2 and BACE1 precedes the targeting of BACE1 from the cell surface to endosomes. In neurons and CHO cells, this fragment and BACE1 co-localize in Rab11-positive endosomes. Overexpression of full-length NCAM2 or a recombinant NCAM2 fragment containing the transmembrane and intracellular domains but lacking the extracellular domain leads to an increase in BACE1 levels in these organelles. In NCAM2-deficient neurons, the levels of BACE1 are increased at the cell surface and reduced in intracellular organelles. These effects are correlated with increased levels of the soluble extracellular domain of BACE1 in the brains of NCAM2-deficient mice, suggesting increased shedding of BACE1 from the cell surface. Of note, shedding of the extracellular domain of Sez6, a protein cleaved exclusively by BACE1, is reduced in NCAM2-deficient animals. These results indicate that the BACE1-generated fragment of NCAM2 regulates BACE1 activity by promoting the targeting of BACE1 to Rab11-positive endosomes.

Amyloid precursor protein (APP) and amyloid β (Aβ) interact with cell adhesion molecules: Implications in Alzheimer’s disease and normal physiology

Alzheimer’s disease (AD) is an incurable neurodegenerative disorder in which dysfunction and loss of synapses and neurons lead to cognitive impairment and death. Accumulation and aggregation of neurotoxic amyloid-β (Aβ) peptides generated via amyloidogenic processing of amyloid precursor protein (APP) is considered to play a central role in the disease etiology. APP interacts with cell adhesion molecules, which influence the normal physiological functions of APP, its amyloidogenic and non-amyloidogenic processing, and formation of Aβ aggregates. These cell surface glycoproteins also mediate attachment of Aβ to the neuronal cell surface and induce intracellular signaling contributing to Aβ toxicity. In this review, we discuss the current knowledge surrounding the interactions of cell adhesion molecules with APP and Aβ and analyze the evidence of the critical role these proteins play in regulating the processing and physiological function of APP as well as Aβ toxicity. This is a necessary piece of the complex AD puzzle, which we should understand in order to develop safe and effective therapeutic interventions for AD.

RNA polyadenylation patterns in the human transcriptome

The eukaryotic transcriptome undergoes various post-transcriptional modifications which assists gene expression. Polyadenylation is a molecular process occurring at the 3'-end of the RNA molecule which involves the poly(A) polymerase attaching adenine monophosphate molecules in a chain-like fashion to assemble a poly(A) tail. Multiple RNA isoforms are produced with differing 3'-UTR and exonic compositions through alternative polyadenylation (APA) which enhances the diversification of alternatively spliced mRNA transcripts. To study polyadenylation patterns, novel methods have been developed using short-read and long-read sequencing technologies to analyse the 3'-ends of the transcript. Recent studies have identified unique polyadenylation patterns in different cellular functions, including oncogenic activity, which could prove valuable in the understanding of medical genetics, particularly in the discovery of biomarkers in diseased states. We present a review of current literature reporting on polyadenylation and the biological relevance in the mammalian transcriptome, with a focus on the human transcriptome. Additionally, we have explored the various methods available to detect polyadenylation patterns using second and third generation sequencing technologies.

Neural glycomics: the sweet side of nervous system functions

The success of investigations on the structure and function of the genome (genomics) has been paralleled by an equally awesome progress in the analysis of protein structure and function (proteomics). We propose that the investigation of carbohydrate structures that go beyond a cell's metabolism is a rapidly developing frontier in our expanding knowledge on the structure and function of carbohydrates (glycomics). No other functional system appears to be suited as well as the nervous system to study the functions of glycans, which had been originally characterized outside the nervous system. In this review, we describe the multiple studies on the functions of LewisX, the human natural killer cell antigen-1 (HNK-1), as well as oligomannosidic and sialic (neuraminic) acids. We attempt to show the sophistication of these structures in ontogenetic development, synaptic function and plasticity, and recovery from trauma, with a view on neurodegeneration and possibilities to ameliorate deterioration. In view of clinical applications, we emphasize the need for glycomimetic small organic compounds which surpass the usefulness of natural glycans in that they are metabolically more stable, more parsimonious to synthesize or isolate, and more advantageous for therapy, since many of them pass the blood brain barrier and are drug-approved for treatments other than those in the nervous system, thus allowing a more ready access for application in neurological diseases. We describe the isolation of such mimetic compounds using not only Western NIH, but also traditional Chinese medical libraries. With this review, we hope to deepen the interests in this exciting field.

Cell Adhesion Molecules and Protein Synthesis Regulation in Neurons

Cell adhesion molecules (CAMs) mediate interactions of neurons with the extracellular environment by forming adhesive bonds with CAMs on adjacent membranes or via binding to proteins of the extracellular matrix. Binding of CAMs to their extracellular ligands results in the activation of intracellular signaling cascades, leading to changes in neuronal structure and the molecular composition and function of neuronal contacts. Ultimately, many of these changes depend on the synthesis of new proteins. In this review, we summarize the evidence showing that CAMs regulate protein synthesis by modulating the activity of transcription factors, gene expression, protein translation, and the structure and distribution of organelles involved in protein synthesis and transport.

Trafficking and Activity of Glutamate and GABA Receptors: Regulation by Cell Adhesion Molecules

The efficient targeting of ionotropic receptors to postsynaptic sites is essential for the function of chemical excitatory and inhibitory synapses, constituting the majority of synapses in the brain. A growing body of evidence indicates that cell adhesion molecules (CAMs), which accumulate at synapses at the earliest stages of synaptogenesis, are critical for this process. A diverse variety of CAMs assemble into complexes with glutamate and GABA receptors and regulate the targeting of these receptors to the cell surface and synapses. Presynaptically localized CAMs provide an additional level of regulation, sending a trans-synaptic signal that can regulate synaptic strength at the level of receptor trafficking. Apart from controlling the numbers of receptors present at postsynaptic sites, CAMs can also influence synaptic strength by modulating the conductivity of single receptor channels. CAMs thus act to maintain basal synaptic transmission and are essential for many forms of activity dependent synaptic plasticity. These activities of CAMs may underlie the association between CAM gene mutations and synaptic pathology and represent fundamental mechanisms by which synaptic strength is dynamically tuned at both excitatory and inhibitory synapses.

Neural Cell Adhesion Molecule 2 (NCAM2)-Induced c-Src-Dependent Propagation of Submembrane Ca2+ Spikes Along Dendrites Inhibits Synapse Maturation

The neural cell adhesion molecule 2 (NCAM2) is encoded by a gene on chromosome 21 in humans. NCAM2 accumulates in synapses, but its role in regulation of synapse formation remains poorly understood. We demonstrate that an increase in NCAM2 levels results in increased instability of dendritic protrusions and reduced conversion of protrusions to dendritic spines in mouse cortical neurons. NCAM2 overexpression induces an increase in the frequency of submembrane Ca2+ spikes localized in individual dendritic protrusions and promotes propagation of submembrane Ca2+ spikes over segments of dendrites or the whole dendritic tree. NCAM2-dependent submembrane Ca2+ spikes are L-type voltage-gated Ca2+ channel-dependent, and their propagation but not initiation depends on the c-Src protein tyrosine kinase. Inhibition of initiation or propagation of NCAM2-dependent submembrane Ca2+ spikes reduces the NCAM2-dependent instability of dendritic protrusions. Synaptic boutons formed on dendrites of neurons with elevated NCAM2 expression are enriched in the protein marker of immature synapses GAP43, and the number of boutons with mature activity-dependent synaptic vesicle recycling is reduced. Our results indicate that synapse maturation is inhibited in NCAM2-overexpressing neurons and suggest that changes in NCAM2 levels and altered submembrane Ca2+ dynamics can cause defects in synapse maturation in Down syndrome and other brain disorders associated with abnormal NCAM2 expression.

NCAM regulates temporal specification of neural progenitor cells via profilin2 during corticogenesis

The development of cerebral cortex requires spatially and temporally orchestrated proliferation, migration, and differentiation of neural progenitor cells (NPCs). The molecular mechanisms underlying cortical development are, however, not fully understood. The neural cell adhesion molecule (NCAM) has been suggested to play a role in corticogenesis. Here we show that NCAM is dynamically expressed in the developing cortex. NCAM expression in NPCs is highest in the neurogenic period and declines during the gliogenic period. In mice bearing an NPC-specific NCAM deletion, proliferation of NPCs is reduced, and production of cortical neurons is delayed, while formation of cortical glia is advanced. Mechanistically, NCAM enhances actin polymerization in NPCs by interacting with actin-associated protein profilin2. NCAM-dependent regulation of NPCs is blocked by mutations in the profilin2 binding site. Thus, NCAM plays an essential role in NPC proliferation and fate decision during cortical development by regulating profilin2-dependent actin polymerization.

Early transcriptome changes in response to chemical long-term potentiation induced via activation of synaptic NMDA receptors in mouse hippocampal neurons

Long term potentiation (LTP) is a form of synaptic plasticity. In the present study LTP was induced via activation of synaptic NMDA receptors in primary hippocampal neuron cultures from neonate mice and RNA was isolated for RNA sequencing at 20 min following LTP induction. RNA sequencing and differential expression testing was performed to determine the identity and abundance of protein-coding and non-coding RNAs in control and LTP induced neuron cultures. We show that expression levels of a small group of transcripts encoding proteins involved in negative regulation of gene expression (Adcyap1, Id3), protein translation (Rpl22L1), extracellular structure organization (Bgn), intracellular signalling (Ppm1H, Ntsr2, Cldn10) and protein citrullination (PAD2) are downregulated in the stimulated neurons. Our results suggest that the early stages of LTP are accompanied by the remodelling of the biosynthetic machinery, interactions with the extracellular matrix and intracellular signalling pathways at the transcriptional level.

Replicable Expansion and Differentiation of Neural Precursors from Adult Canine Skin

Repopulation of brain circuits by neural precursors is a potential therapeutic strategy for neurodegenerative disorders; however, choice of cell is critical. Previously, we introduced a two-step culture system that generates a high yield of neural precursors from small samples of adult canine skin. Here, we probe their gene and protein expression profiles in comparison with dermal fibroblasts and brain-derived neural stem cells and characterize their neuronal potential. To date, we have produced >50 skin-derived neural precursor (SKN) lines. SKNs can be cultured in a highly replicable fashion and uniformly express a panel of identifying markers. Upon differentiation, they self-upregulate neural specification genes, generating neurons with basic electrophysiological functionality. This unique population of neural precursors, derived from mature skin, overcomes many of the practical issues that have limited clinical translation of alternative cell types. Easily accessible, neuronally committed, and patient specific, SKNs may have potential for the treatment of brain disorders.

Cell Adhesion Molecule Close Homolog of L1 (CHL1) Guides the Regrowth of Regenerating Motor Axons and Regulates Synaptic Coverage of Motor Neurons

The close homolog of L1 (CHL1) is a cell adhesion molecule involved in regulation of neuronal differentiation and survival, neurite outgrowth and axon guidance during development. In the mature nervous system, CHL1 regulates synaptic activity and plasticity. The aim of the present study was to evaluate the influence of CHL1 on peripheral nerve regeneration after trauma. Using the established model of mouse femoral nerve regeneration, CHL1 knock-out mice were investigated in comparison to the wild type littermates. First, non-injured mice of both genotypes were compared regarding the synaptic phenotypes in the corresponding spinal cord segment. While no differences in phenotypes were detectable in the femoral nerve, corresponding segments in the spinal cord were observed to differ in that inhibitory perisomatic innervation of motor neurons was increased in CHL1-deficient mice, and numbers of perisomatic cholinergic synapses on motor neuronal somata were reduced. Regarding the femoral nerve after injury, CHL1-deficient mice demonstrated preferential motor axon regrowth into the saphenous vs. quadriceps branch after nerve transection upstream of the nerve bifurcation by 8 weeks after transection, indicating decreased preferential motor re-innervation. Furthermore, in injured wild-type mice, enhanced CHL1 expression was observed in regenerating axons in the proximal nerve stump upstream of the bifurcation at days 1, 3, 5, 7 and 14, and in the distal stump at days 7 and 14 after injury, when compared to non-injured mice. Injury-related upregulation of CHL1 expression was more pronounced in axons than in Schwann cells. Despite a more pronounced capacity for preferential motor axon regrowth in wild-type vs. mutant mice, only a tendency for difference in recovery of motor functions was observed between genotypes, without statistical significance Taken together, these results indicate that CHL1 is involved in peripheral nerve regeneration, because it guides regrowing axons into the appropriate nerve branch and regulates synaptic coverage in the spinal cord.

Glycosylphosphatidylinositol-Anchored Immunoglobulin Superfamily Cell Adhesion Molecules and Their Role in Neuronal Development and Synapse Regulation

Immunoglobulin superfamily (IgSF) cell adhesion molecules (CAMs) are cell surface glycoproteins that not only mediate interactions between neurons but also between neurons and other cells in the nervous system. While typical IgSF CAMs are transmembrane molecules, this superfamily also includes CAMs, which do not possess transmembrane and intracellular domains and are instead attached to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. In this review, we focus on the role GPI-anchored IgSF CAMs have as signal transducers and ligands in neurons, and discuss their functions in regulation of neuronal development, synapse formation, synaptic plasticity, learning, and behavior. We also review the links between GPI-anchored IgSF CAMs and brain disorders.

Intracellular transport and cell surface delivery of the neural cell adhesion molecule (NCAM)

The neural cell adhesion molecule (NCAM) regulates differentiation and functioning of neurons by accumulating at the cell surface where it mediates the interactions of neurons with the extracellular environment. NCAM also induces a number of intracellular signaling cascades, which coordinate interactions at the cell surface with intracellular processes including changes in gene expression, transport and cytoskeleton remodeling. Since NCAM functions at the cell surface, its transport and delivery to the cell surface play a critical role. Here, we review recent advances in our understanding of the molecular mechanisms of the intracellular transport and cell surface delivery of NCAM. We also discuss the data suggesting a possibility of cross talk between activation of NCAM at the cell surface and the intracellular transport and cell surface delivery of NCAM.

KCa1.1, a calcium-activated potassium channel subunit alpha 1, is targeted by miR-17-5p and modulates cell migration in malignant pleural mesothelioma

Background

Malignant pleural mesothelioma (MPM) is an aggressive, locally invasive, cancer elicited by asbestos exposure and almost invariably a fatal diagnosis. To date, we are one of the leading laboratory that compared microRNA expression profiles in MPM and normal mesothelium samples in order to identify dysregulated microRNAs with functional roles in mesothelioma. We interrogated a significant collection of MPM tumors and normal pleural samples in our biobank in search for novel therapeutic targets.

Methods

Utilizing mRNA-microRNA correlations based on differential gene expression using Gene Set Enrichment Analysis (GSEA), we systematically combined publicly available gene expression datasets with our own MPM data in order to identify candidate targets for MPM therapy.

Results

We identified enrichment of target binding sites for the miR-17 and miR-30 families in both MPM tumors and cell lines. RT-qPCR revealed that members of both families were significantly downregulated in MPM tumors and cell lines. Interestingly, lower expression of miR-17-5p (P = 0.022) and miR-20a-5p (P = 0.026) was clearly associated with epithelioid histology. We interrogated the predicted targets of these differentially expressed microRNA families in MPM cell lines, and identified KCa1.1, a calcium-activated potassium channel subunit alpha 1 encoded by the KCNMA1 gene, as a target of miR-17-5p. KCa1.1 was overexpressed in MPM cells compared to the (normal) mesothelial line MeT-5A, and was also upregulated in patient tumor samples compared to normal mesothelium. Transfection of MPM cells with a miR-17-5p mimic or KCNMA1-specific siRNAs reduced mRNA expression of KCa1.1 and inhibited MPM cell migration. Similarly, treatment with paxilline, a small molecule inhibitor of KCa1.1, resulted in suppression of MPM cell migration.

Conclusion

These functional data implicating KCa1.1 in MPM cell migration support our integrative approach using MPM gene expression datasets to identify novel and potentially druggable targets.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-016-0529-z) contains supplementary material, which is available to authorized users.

Reciprocal Interactions between Cell Adhesion Molecules of the Immunoglobulin Superfamily and the Cytoskeleton in Neurons

Cell adhesion molecules of the immunoglobulin superfamily (IgSF) including the neural cell adhesion molecule (NCAM) and members of the L1 family of neuronal cell adhesion molecules play important functions in the developing nervous system by regulating formation, growth and branching of neurites, and establishment of the synaptic contacts between neurons. In the mature brain, members of IgSF regulate synapse composition, function, and plasticity required for learning and memory. The intracellular domains of IgSF cell adhesion molecules interact with the components of the cytoskeleton including the submembrane actin-spectrin meshwork, actin microfilaments, and microtubules. In this review, we summarize current data indicating that interactions between IgSF cell adhesion molecules and the cytoskeleton are reciprocal, and that while IgSF cell adhesion molecules regulate the assembly of the cytoskeleton, the cytoskeleton plays an important role in regulation of the functions of IgSF cell adhesion molecules. Reciprocal interactions between NCAM and L1 family members and the cytoskeleton and their role in neuronal differentiation and synapse formation are discussed in detail.

Aβ-dependent reduction of NCAM2-mediated synaptic adhesion contributes to synapse loss in Alzheimer's disease

Alzheimer's disease (AD) is characterized by synapse loss due to mechanisms that remain poorly understood. We show that the neural cell adhesion molecule 2 (NCAM2) is enriched in synapses in the human hippocampus. This enrichment is abolished in the hippocampus of AD patients and in brains of mice overexpressing the human amyloid-β (Aβ) precursor protein carrying the pathogenic Swedish mutation. Aβ binds to NCAM2 at the cell surface of cultured hippocampal neurons and induces removal of NCAM2 from synapses. In AD hippocampus, cleavage of the membrane proximal external region of NCAM2 is increased and soluble extracellular fragments of NCAM2 (NCAM2-ED) accumulate. Knockdown of NCAM2 expression or incubation with NCAM2-ED induces disassembly of GluR1-containing glutamatergic synapses in cultured hippocampal neurons. Aβ-dependent disassembly of GluR1-containing synapses is inhibited in neurons overexpressing a cleavage-resistant mutant of NCAM2. Our data indicate that Aβ-dependent disruption of NCAM2 functions in AD hippocampus contributes to synapse loss.

Understanding how ß-amyloid contributes to synapse loss and dysfunction is a central goal of Alzheimer's disease research. Here, Leshchyns'ka et al. identify a novel mechanism by which Aß disassembles hippocampal glutamatergic synapses via cleavage of a neural cell adhesion molecule 2 (NCAM2).

Age-dependent loss of parvalbumin-expressing hippocampal interneurons in mice deficient in CHL1, a mental retardation and schizophrenia susceptibility gene

In humans, deletions/mutations in the CHL1/CALL gene are associated with mental retardation and schizophrenia. Juvenile CHL1-deficient (CHL1(-/-) ) mice have been shown to display abnormally high numbers of parvalbumin-expressing (PV(+) ) hippocampal interneurons and, as adults, display behavioral traits observed in neuropsychiatric disorders. Here, we addressed the question whether inhibitory interneurons and synaptic plasticity in the CHL1(-/-) mouse are affected during brain maturation and in adulthood. We found that hippocampal, but not neocortical, PV(+) interneurons were reduced with age in CHL1(-/-) mice, from a surplus of +27% at 1 month to a deficit of -20% in adulthood compared with wild-type littermates. This loss occurred during brain maturation, correlating with microgliosis and enhanced interleukin-6 expression. In parallel with the loss of PV(+) interneurons, the inhibitory input to adult CA1 pyramidal cells was reduced and a deficit in short- and long-term potentiation developed at CA3-CA1 excitatory synapses between 2 and 9 months of age in CHL1(-/-) mice. This deficit could be abrogated by a GABAA receptor agonist. We propose that region-specific aberrant GABAergic synaptic connectivity resulting from the mutation and a subsequently enhanced synaptic elimination during brain maturation lead to microgliosis, increase in pro-inflammatory cytokine levels, loss of interneurons, and impaired synaptic plasticity. Close homolog of L1-deficient (CHL1(-/-) ) mice have abnormally high numbers of parvalbumin (PV)-expressing hippocampal interneurons in juvenile animals, but in adult animals a loss of these cells is observed. This loss correlates with an increased density of microglia (M), enhanced interleukin-6 (IL6) production and a deficit in short- and long-term potentiation at CA3-CA1 excitatory synapses. Furthermore, adult CHL1(-/-) mice display behavioral traits similar to those observed in neuropsychiatric disorders of humans.

Kinesin-1 promotes post-Golgi trafficking of NCAM140 and NCAM180 to the cell surface

The neural cell adhesion molecule (NCAM, also known as NCAM1) is important during neural development, because it contributes to neurite outgrowth in response to its ligands at the cell surface. In the adult brain, NCAM is involved in regulating synaptic plasticity. The molecular mechanisms underlying delivery of NCAM to the neuronal cell surface remain poorly understood. We used a protein macroarray and identified the kinesin light chain 1 (KLC1), a component of the kinesin-1 motor protein, as a binding partner of the intracellular domains of the two transmembrane isoforms of NCAM, NCAM140 and NCAM180. KLC1 binds to amino acids CGKAGPGA within the intracellular domain of NCAM and colocalizes with kinesin-1 in the Golgi compartment. Delivery of NCAM180 to the cell surface is increased in CHO cells and neurons co-transfected with kinesin-1. We further demonstrate that the p21-activated kinase 1 (PAK1) competes with KLC1 for binding to the intracellular domain of NCAM and contributes to the regulation of the membrane insertion of NCAM. Our results indicate that NCAM is delivered to the cell surface through a kinesin-1-mediated transport mechanism in a PAK1-dependent manner.

Cell adhesion and intracellular calcium signaling in neurons

Cell adhesion molecules (CAMs) play indispensable roles in the developing and mature brain by regulating neuronal migration and differentiation, neurite outgrowth, axonal fasciculation, synapse formation and synaptic plasticity. CAM-mediated changes in neuronal behavior depend on a number of intracellular signaling cascades including changes in various second messengers, among which CAM-dependent changes in intracellular Ca²⁺ levels play a prominent role. Ca²⁺ is an essential secondary intracellular signaling molecule that regulates fundamental cellular functions in various cell types, including neurons. We present a systematic review of the studies reporting changes in intracellular Ca²⁺ levels in response to activation of the immunoglobulin superfamily CAMs, cadherins and integrins in neurons. We also analyze current experimental evidence on the Ca²⁺ sources and channels involved in intracellular Ca²⁺ increases mediated by CAMs of these families, and systematically review the role of the voltage-dependent Ca²⁺ channels (VDCCs) in neurite outgrowth induced by activation of these CAMs. Molecular mechanisms linking CAMs to VDCCs and intracellular Ca²⁺ stores in neurons are discussed.

The neural cell adhesion molecule promotes maturation of the presynaptic endocytotic machinery by switching synaptic vesicle recycling from adaptor protein 3 (AP-3)- to AP-2-dependent mechanisms

Newly formed synapses undergo maturation during ontogenetic development via mechanisms that remain poorly understood. We show that maturation of the presynaptic endocytotic machinery in CNS neurons requires substitution of the adaptor protein 3 (AP-3) with AP-2 at the presynaptic plasma membrane. In mature synapses, AP-2 associates with the intracellular domain of the neural cell adhesion molecule (NCAM). NCAM promotes binding of AP-2 over binding of AP-3 to presynaptic membranes, thus favoring the substitution of AP-3 for AP-2 during formation of mature synapses. The presynaptic endocytotic machinery remains immature in adult NCAM-deficient (NCAM-/-) mice accumulating AP-3 instead of AP-2 and its partner protein AP180 in synaptic membranes and vesicles. NCAM deficiency or disruption of the NCAM/AP-2 complex in wild-type (NCAM+/+) neurons by overexpression of AP-2 binding-defective mutant NCAM interferes with efficient retrieval of the synaptic vesicle v-SNARE synaptobrevin 2. Abnormalities in synaptic vesicle endocytosis and recycling may thus contribute to neurological disorders associated with mutations in NCAM.

Lipid raft-dependent endocytosis of close homolog of adhesion molecule L1 (CHL1) promotes neuritogenesis

CHL1 plays a dual role by either promoting or inhibiting neuritogenesis. We report here that neuritogenesis-promoting ligand-dependent cell surface clustering of CHL1 induces palmitoylation and lipid raft-dependent endocytosis of CHL1. We identify βII spectrin as a binding partner of CHL1, and we show that partial disruption of the complex between CHL1 and βII spectrin accompanies CHL1 endocytosis. Inhibition of the association of CHL1 with lipid rafts by pharmacological disruption of lipid rafts or by mutation of cysteine 1102 within the intracellular domain of CHL1 reduces endocytosis of CHL1. Endocytosis of CHL1 is also reduced by nifedipine, an inhibitor of the L-type voltage-dependent Ca(2+) channels. CHL1-dependent neurite outgrowth is reduced by inhibitors of lipid raft assembly, inhibitors of voltage-dependent Ca(2+) channels, and overexpression of CHL1 with mutated cysteine Cys-1102. Our results suggest that ligand-induced and lipid raft-dependent regulation of CHL1 adhesion via Ca(2+)-dependent remodeling of the CHL1-βII spectrin complex and CHL1 endocytosis are required for CHL1-dependent neurite outgrowth.

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.

Immobilized pool of NCAM180 in the postsynaptic membrane is homeostatically replenished by the flux of NCAM180 from extrasynaptic regions

Homeostatic mechanisms maintaining high levels of adhesion molecules in synapses over prolonged periods of time remain incompletely understood. We used fluorescence recovery after photobleaching experiments to analyze the steady state turnover of the immobile pool of green fluorescent protein-labeled NCAM180, the largest postsynaptically accumulating isoform of the neural cell adhesion molecule (NCAM). We show that there is a continuous flux of NCAM180 to the postsynaptic membrane from nonsynaptic regions of dendrites by diffusion. In the postsynaptic membrane, the newly delivered NCAM180 slowly intermixes with the immobilized pool of NCAM180. Preferential immobilization and accumulation of NCAM180 in the postsynaptic membrane is reduced after disruption of the association of NCAM180 with the spectrin cytoskeleton and in the absence of the homophilic interactions of NCAM180 in synapses. Our observations indicate that the homophilic interactions and binding to the cytoskeleton promote immobilization of NCAM180 and its accumulation in the postsynaptic membrane. Flux of NCAM180 from extrasynaptic regions and its slow intermixture with the immobile pool of NCAM180 in the postsynaptic membrane may be important for the continuous homeostatic replenishment of NCAM180 protein at synaptic contacts without compromising the long term synaptic contact stability.

NCAM/spectrin complex disassembly results in PSD perforation and postsynaptic endocytic zone formation

Mechanisms inducing perforation of the postsynaptic density (PSD) are poorly understood. We show that neural cell adhesion molecule- deficient (NCAM-/-) hippocampal neurons have an abnormally high percentage of synapses with perforated PSDs. The percentage of synapses with perforated PSDs is also increased in wild-type (NCAM+/+) neurons after the disruption of the NCAM/spectrin complex indicating that the NCAM-assembled spectrin cytoskeleton maintains the structural integrity of PSDs. We demonstrate that PSD perforations contain endocytic zones involved in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) internalization. Induction of long-term potentiation in NCAM+/+ neurons accompanied by insertion of AMPAR into the neuronal cell surface is subsequently followed by formation of perforated synapses and AMPAR endocytosis suggesting that perforation of PSDs is important for membrane homeostasis in activated synapses. In NCAM-/- or NCAM+/+ neurons with dissociated spectrin meshwork, AMPAR endocytosis is enhanced under conditions of basal activity. An abnormally high rate of postsynaptic membrane endocytosis may thus contribute to brain pathologies associated with mutations in NCAM or spectrin.

Clustering of the neural cell adhesion molecule (NCAM) at the neuronal cell surface induces caspase-8- and -3-dependent changes of the spectrin meshwork required for NCAM-mediated neurite outgrowth

Changes in neuronal morphology underlying neuronal differentiation depend on rapid and sustained cytoskeleton rearrangements in the growing neurites. Whereas cell adhesion molecules are well established as regulators of neuronal differentiation, less is known about the signaling mechanisms by which they influence the cytoskeleton. Here we show that the neural cell adhesion molecule (NCAM) associates with the active form of caspase-8 and that clustering of NCAM at the neuronal cell surface leads to activation of caspase-8 and -3 followed by the cleavage of the sub-membranous brain spectrin meshwork, but not of the actin or tubulin cytoskeleton. Inhibitors of caspase-8 and -3 specifically block the NCAM-dependent spectrin cleavage and abolish NCAM-dependent neurite outgrowth. NCAM-dependent rearrangements of the membrane associated spectrin meshwork via caspase-8 dependent caspase-3 activation are thus indispensable for NCAM-mediated neurite outgrowth.

CHL1 is a selective organizer of the presynaptic machinery chaperoning the SNARE complex

Proteins constituting the presynaptic machinery of vesicle release undergo substantial conformational changes during the process of exocytosis. While changes in the conformation make proteins vulnerable to aggregation and degradation, little is known about synaptic chaperones which counteract these processes. We show that the cell adhesion molecule CHL1 directly interacts with and regulates the activity of the synaptic chaperones Hsc70, CSP and αSGT. CHL1, Hsc70, CSP and αSGT form predominantly CHL1/Hsc70/αSGT and CHL1/CSP complexes in synapses. Among the various complexes formed by CHL1, Hsc70, CSP and αSGT, SNAP25 and VAMP2 induce chaperone activity only in CHL1/Hsc70/αSGT and CHL1/CSP complexes, respectively, indicating a remarkable selectivity of a presynaptic chaperone activity for proteins of the exocytotic machinery. In mice with genetic ablation of CHL1, chaperone activity in synapses is reduced and the machinery for synaptic vesicle exocytosis and, in particular, the SNARE complex is unable to sustain prolonged synaptic activity. Thus, we reveal a novel role for a cell adhesion molecule in selective activation of the presynaptic chaperone machinery.

NCAM induces CaMKIIalpha-mediated RPTPalpha phosphorylation to enhance its catalytic activity and neurite outgrowth

Receptor protein tyrosine phosphatase α (RPTPα) phosphatase activity is required for intracellular signaling cascades that are activated in motile cells and growing neurites. Little is known, however, about mechanisms that coordinate RPTPα activity with cell behavior. We show that clustering of neural cell adhesion molecule (NCAM) at the cell surface is coupled to an increase in serine phosphorylation and phosphatase activity of RPTPα. NCAM associates with T- and L-type voltage-dependent Ca²⁺ channels, and NCAM clustering at the cell surface results in Ca²⁺ influx via these channels and activation of NCAM-associated calmodulin-dependent protein kinase IIα (CaMKIIα). Clustering of NCAM promotes its redistribution to lipid rafts and the formation of a NCAM–RPTPα–CaMKIIα complex, resulting in serine phosphorylation of RPTPα by CaMKIIα. Overexpression of RPTPα with mutated Ser180 and Ser204 interferes with NCAM-induced neurite outgrowth, which indicates that neurite extension depends on NCAM-induced up-regulation of RPTPα activity. Thus, we reveal a novel function for a cell adhesion molecule in coordination of cell behavior with intracellular phosphatase activity.

Glial scar expression of CHL1, the close homolog of the adhesion molecule L1, limits recovery after spinal cord injury

The Ig superfamily adhesion molecule CHL1, the close homolog of the adhesion molecule L1, promotes neurite outgrowth, neuronal migration, and survival in vitro. We tested whether CHL1, similar to its close homolog L1, has a beneficial impact on recovery from spinal cord injury using adult CHL1-deficient (CHL1-/-) mice and wild-type (CHL1+/+) littermates. In contrast to our hypothesis, we found that functional recovery, assessed by locomotor rating and video-based motion analyses, was improved in CHL1-/- mice compared with wild-type mice at 3-6 weeks after compression of the thoracic spinal cord. Better function was associated with enhanced monoaminergic reinnervation of the lumbar spinal cord and altered pattern of posttraumatic synaptic rearrangements around motoneurons. Restricted recovery of wild-type mice was likely related to early and persistent (3-56 d after lesion) upregulation of CHL1 in GFAP-positive astrocytes at the lesion core. In both the intact spinal cord and cultured astrocytes, enhanced expression of CHL1 and GFAP was induced by application of basic fibroblast growth factor, a cytokine involved in the pathophysiology of spinal cord injury. This upregulation was abolished by inhibitors of FGF receptor-dependent extracellular signal-regulated kinase, calcium/calmodulin-dependent kinase, and phosphoinositide-3 kinase signaling pathways. In homogenotypic and heterogenotypic cocultures of neurons and astrocytes, reduced neurite outgrowth was observed only if CHL1 was simultaneously present on both cell types. These findings and novel in vitro evidence for a homophilic CHL1-CHL1 interaction indicate that CHL1 is a glial scar component that restricts posttraumatic axonal growth and remodeling of spinal circuits by homophilic binding mechanisms.

The Adhesion Molecule CHL1 Regulates Uncoating of Clathrin-Coated Synaptic Vesicles

In searching for binding partners of the intracellular domain of the immunoglobulin superfamily adhesion molecule CHL1, we identified the clathrin-uncoating ATPase Hsc70. CHL1 gene ablation resulted in reduced targeting of Hsc70 to the synaptic plasma membrane and synaptic vesicles, suggesting CHL1 as a synapse-targeting cue for Hsc70. CHL1 accumulates in presynaptic membranes and, in response to synapse activation, is targeted to synaptic vesicles by endocytosis. CHL1 deficiency or disruption of the CHL1/Hsc70 complex results in accumulation of abnormally high levels of clathrin-coated synaptic vesicles with a reduced ability to release clathrin. Generation of new clathrin-coated synaptic vesicles in an activity-dependent manner is inhibited when the CHL1/Hsc70 complex is disrupted, resulting in impaired uptake and release of FM dyes in synaptic boutons. Abnormalities in clathrin-dependent synaptic vesicle recycling may thus underlie brain malfunctions in humans and mice that carry mutations in the CHL1 gene.

NCAM promotes assembly and activity-dependent remodeling of the postsynaptic signaling complex

The neural cell adhesion molecule (NCAM) regulates synapse formation and synaptic strength via mechanisms that have remained unknown. We show that NCAM associates with the postsynaptic spectrin-based scaffold, cross-linking NCAM with the N-methyl-d-aspartate (NMDA) receptor and Ca2+/calmodulin-dependent protein kinase II alpha (CaMKIIalpha) in a manner not firmly or directly linked to PSD95 and alpha-actinin. Clustering of NCAM promotes formation of detergent-insoluble complexes enriched in postsynaptic proteins and resembling postsynaptic densities. Disruption of the NCAM-spectrin complex decreases the size of postsynaptic densities and reduces synaptic targeting of NCAM-spectrin-associated postsynaptic proteins, including spectrin, NMDA receptors, and CaMKIIalpha. Degeneration of the spectrin scaffold in NCAM-deficient neurons results in an inability to recruit CaMKIIalpha to synapses after NMDA receptor activation, which is a critical process in NMDA receptor-dependent long-term potentiation. The combined observations indicate that NCAM promotes assembly of the spectrin-based postsynaptic signaling complex, which is required for activity-associated, long-lasting changes in synaptic strength. Its abnormal function may contribute to the etiology of neuropsychiatric disorders associated with mutations in or abnormal expression of NCAM.

Electroporation-based gene transfer for efficient transfection of neural precursor cells

Transplantation of neural precursor cells (NPCs) is a potential tool to replace dysfunctional or degenerated neuronal or glial cell types in the central nervous system. Furthermore, transplantation of genetically engineered neural precursor cells might provide a strategy to target therapeutic gene products to the diseased nervous system. Here, we describe a novel and highly efficient electroporation-based transfection protocol for mitogen-expanded mouse NPCs. Transfection of NPCs with the reporter gene enhanced green fluorescent protein (EGFP) or the neural adhesion molecule L1 revealed transfection efficacies of more than 70% as estimated by the number of EGFP-positive or L1-immunoreactive cells 1 day after transfection in vitro. The percentage of EGFP- or L1-positive cells decreased with increasing time in culture. Positive cells were detectable for up to 3 weeks after transfection. When EGFP- or L1-transfected NPCs were grafted into the retina of adult wild-type or L1-deficient mice, they differentiated into glial cells some of which expressed EGFP and L1 for up to 2 and 3 weeks, respectively, the longest post-transplantation periods investigated.

Prion protein recruits its neuronal receptor NCAM to lipid rafts to activate p59fyn and to enhance neurite outgrowth

In spite of advances in understanding the role of the cellular prion protein (PrP) in neural cell interactions, the mechanisms of PrP function remain poorly characterized. We show that PrP interacts directly with the neural cell adhesion molecule (NCAM) and associates with NCAM at the neuronal cell surface. Both cis and trans interactions between NCAM at the neuronal surface and PrP promote recruitment of NCAM to lipid rafts and thereby regulate activation of fyn kinase, an enzyme involved in NCAM-mediated signaling. Cis and trans interactions between NCAM and PrP promote neurite outgrowth. When these interactions are disrupted in NCAM-deficient and PrP-deficient neurons or by PrP antibodies, NCAM/PrP-dependent neurite outgrowth is arrested, indicating that PrP is involved in nervous system development cooperating with NCAM as a signaling receptor.

Polysialylated neural cell adhesion molecule promotes remodeling and formation of hippocampal synapses

Expression of the neural cell adhesion molecule (NCAM) has been shown to promote long-term potentiation (LTP) and stabilization of synapses during early synaptogenesis. Here, we searched for the mechanisms of synaptogenic activity of NCAM, focusing on the role of polysialic acid (PSA), an unusual carbohydrate preferentially associated with NCAM. We show that enzymatic removal of PSA with endoneuraminidase-N (endo-N) abolished preferential formation of synapses on NCAM-expressing cells in heterogenotypic cocultures of wild-type and NCAM-deficient hippocampal neurons. Transfection of NCAM-deficient neurons with either of three major NCAM isoforms (different in intracellular domains but identical in extracellular domains and carrying PSA) stimulated preferential synapse formation on NCAM isoform-expressing neurons. Enzymatic removal of heparan sulfates from cultured neurons and a mutation in the heparin-binding domain of NCAM diminished synaptogenic activity of neuronally expressed PSA-NCAM, suggesting that interaction of NCAM with heparan sulfate proteoglycans mediates this activity. PSA-NCAM-driven synaptogenesis was also blocked by antagonists to fibroblast growth factor receptor and NMDA subtype of glutamate receptors but not by blockers of non-NMDA glutamate receptors and voltage-dependent Na+ channels. Enzymatic removal of PSA and heparan sulfates also blocked the increase in the number of perforated spine synapses associated with NMDA receptor-dependent LTP in the CA1 region of organotypic hippocampal cultures. Thus, neuronal PSA-NCAM in complex with heparan sulfate proteoglycans promotes synaptogenesis and activity-dependent remodeling of synapses.

RPTPalpha is essential for NCAM-mediated p59fyn activation and neurite elongation

The neural cell adhesion molecule (NCAM) forms a complex with p59^fyn kinase and activates it via a mechanism that has remained unknown. We show that the NCAM140 isoform directly interacts with the intracellular domain of the receptor-like protein tyrosine phosphatase RPTPα, a known activator of p59^fyn. Whereas this direct interaction is Ca²⁺ independent, formation of the complex is enhanced by Ca²⁺-dependent spectrin cytoskeleton–mediated cross-linking of NCAM and RPTPα in response to NCAM activation and is accompanied by redistribution of the complex to lipid rafts. Association between NCAM and p59^fyn is lost in RPTPα-deficient brains and is disrupted by dominant-negative RPTPα mutants, demonstrating that RPTPα is a link between NCAM and p59^fyn. NCAM-mediated p59^fyn activation is abolished in RPTPα-deficient neurons, and disruption of the NCAM–p59^fyn complex in RPTPα-deficient neurons or with dominant-negative RPTPα mutants blocks NCAM-dependent neurite outgrowth, implicating RPTPα as a major phosphatase involved in NCAM-mediated signaling.

Trans-Golgi network delivery of synaptic proteins in synaptogenesis

Synapse formation, stabilization and maintenance comprise several remarkably precise and rapid stages. The initial steps involve delivery to the site of initial contact between axon and dendrite of transport carriers containing several sets of synaptic proteins necessary for proper synaptic function. This occurs both pre- and postsynaptically and is mediated by apparently distinct vesicular carriers that fuse with the synaptic plasma membrane to deliver receptors for neurotransmitters, ion channels, transporters and pumps. The presynaptic carriers in the developing axon give rise to synaptic vesicles. On the postsynaptic side, the so-called spine apparatus may represent a tubular reservoir that gives rise to the postsynaptic players in synaptic function. Recent evidence indicates that recognition molecules, particularly neural cell adhesion molecule (NCAM), are associated with trans-Golgi-network-derived structures and thus can provide a signal for accumulation of these transport carriers at nascent synapses.

Neural cell adhesion molecule (NCAM) association with PKCbeta2 via betaI spectrin is implicated in NCAM-mediated neurite outgrowth

In hippocampal neurons and transfected CHO cells, neural cell adhesion molecule (NCAM) 120, NCAM140, and NCAM180 form Triton X-100-insoluble complexes with betaI spectrin. Heteromeric spectrin (alphaIbetaI) binds to the intracellular domain of NCAM180, and isolated spectrin subunits bind to both NCAM180 and NCAM140, as does the betaI spectrin fragment encompassing second and third spectrin repeats (betaI2-3). In NCAM120-transfected cells, betaI spectrin is detectable predominantly in lipid rafts. Treatment of cells with methyl-beta-cyclodextrin disrupts the NCAM120-spectrin complex, implicating lipid rafts as a platform linking NCAM120 and spectrin. NCAM140/NCAM180-betaI spectrin complexes do not depend on raft integrity and are located both in rafts and raft-free membrane domains. PKCbeta2 forms detergent-insoluble complexes with NCAM140/NCAM180 and spectrin. Activation of NCAM enhances the formation of NCAM140/NCAM180-spectrin-PKCbeta2 complexes and results in their redistribution to lipid rafts. The complex is disrupted by the expression of dominant-negative betaI2-3, which impairs binding of spectrin to NCAM, implicating spectrin as the bridge between PKCbeta2 and NCAM140 or NCAM180. Redistribution of PKCbeta2 to NCAM-spectrin complexes is also blocked by a specific fibroblast growth factor receptor inhibitor. Furthermore, transfection with betaI2-3 inhibits NCAM-induced neurite outgrowth, showing that formation of the NCAM-spectrin-PKCbeta2 complex is necessary for NCAM-mediated neurite outgrowth.

Neural cell adhesion molecule promotes accumulation of TGN organelles at sites of neuron-to-neuron contacts

Transformation of a contact between axon and dendrite into a synapse is accompanied by accumulation of the synaptic machinery at this site, being delivered in intracellular organelles mainly of TGN origin. Here, we report that in cultured hippocampal neurons, TGN organelles are linked via spectrin to clusters of the neural cell adhesion molecule (NCAM) in the plasma membrane. These complexes are translocated along neurites and trapped at sites of initial neurite-to-neurite contacts within several minutes after initial contact formation. The accumulation of TGN organelles at contacts with NCAM-deficient neurons is reduced when compared with wild-type cells, suggesting that NCAM mediates the anchoring of intracellular organelles in nascent synapses.

Cosignaling of NCAM via lipid rafts and the FGF receptor is required for neuritogenesis

The neural cell adhesion molecule (NCAM) has been reported to stimulate neuritogenesis either via nonreceptor tyrosine kinases or fibroblast growth factor (FGF) receptor. Here we show that lipid raft association of NCAM is crucial for activation of the nonreceptor tyrosine kinase pathway and induction of neurite outgrowth. Transfection of hippocampal neurons of NCAM-deficient mice revealed that of the three major NCAM isoforms only NCAM140 can act as a homophilic receptor that induces neurite outgrowth. Disruption of NCAM140 raft association either by mutation of NCAM140 palmitoylation sites or by lipid raft destruction attenuates activation of the tyrosine focal adhesion kinase and extracellular signal-regulated kinase 1/2, completely blocking neurite outgrowth. Likewise, NCAM-triggered neurite outgrowth is also completely blocked by a specific FGF receptor inhibitor, indicating that cosignaling via raft-associated kinases and FGF receptor is essential for neuritogenesis.