Myelin basic protein cleaves cell adhesion molecule L1 and promotes neuritogenesis and cell survival

Synaptic cell adhesion molecules contribute to the pathogenesis and progression of fragile X syndrome

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and a monogenic cause of autism spectrum disorders. Deficiencies in the fragile X messenger ribonucleoprotein, encoded by the FMR1 gene, lead to various anatomical and pathophysiological abnormalities and behavioral deficits, such as spine dysmorphogenesis and learning and memory impairments. Synaptic cell adhesion molecules (CAMs) play crucial roles in synapse formation and neural signal transmission by promoting the formation of new synaptic contacts, accurately organizing presynaptic and postsynaptic protein complexes, and ensuring the accuracy of signal transmission. Recent studies have implicated synaptic CAMs such as the immunoglobulin superfamily, N-cadherin, leucine-rich repeat proteins, and neuroligin-1 in the pathogenesis of FXS and found that they contribute to defects in dendritic spines and synaptic plasticity in FXS animal models. This review systematically summarizes the biological associations between nine representative synaptic CAMs and FMRP, as well as the functional consequences of the interaction, to provide new insights into the mechanisms of abnormal synaptic development in FXS.

Dual-layer conduit containing VEGF-A - Transfected Schwann cells promotes peripheral nerve regeneration via angiogenesis

Peripheral nerve injuries (PNIs) can cause neuropathies and significantly affect the patient's quality of life. Autograft transplantation is the gold standard for conventional treatment; however, its application is limited by nerve unavailability, size mismatch, and local tissue adhesion. Tissue engineering, such as nerve guidance conduits, is an alternative and promising strategy to guide nerve regeneration for peripheral nerve repair; however, only a few conduits could reach the high repair efficiency of autografts. The healing process of PNI is frequently accompanied by not only axonal and myelination regeneration but also angiogenesis, which initializes nerve regeneration through vascular endothelial growth factor A (VEGF-A). In this study, a composite nerve conduit with a poly (lactic-co-glycolic acid) (PLGA) hollow tube as the outer layer and gelatin methacryloyl (GelMA) encapsulated with VEGF-A transfected Schwann cells (SCs) as the inner layer was established to evaluate its promising ability for peripheral nerve repair. A rat model of peripheral nerve defect was used to examine the efficiency of PLGA/GelMA-SC (VA) conduits, whereas autograft, PLGA, PLGA/GelMA, and PLGA/GelMA-SC (NC) were used as controls. VEGF-A-transfected SCs can provide a stable source for VEGF-A secretion. Furthermore, encapsulation in GelMA cannot only promote proliferation and tube formation of human umbilical vein endothelial cells but also enhance dorsal root ganglia and neuronal cell extension. Previous animal studies have demonstrated that the regenerative effects of PLGA/GelMA-SC (VA) nerve conduit were similar to those of autografts and much better than those of other conduits. These findings indicate that combination of VEGF-A-overexpressing SCs and PLGA/GelMA conduit-guided peripheral nerve repair provides a promising method that enhances angiogenesis and regeneration during nerve repair. STATEMENT OF SIGNIFICANCE: Nerve guidance conduits shows promise for peripheral nerve repair, while achieving the repair efficiency of autografts remains a challenge. In this study, a composite nerve conduit with a PLGA hollow tube as the outer layer and gelatin methacryloyl (GelMA) encapsulated with vascular endothelial growth factor A (VEGF-A)-transfected Schwann cells (SCs) as the inner layer was established to evaluate its potential ability for peripheral nerve repair. This approach preserves growth factor bioactivity and enhances material properties. GelMA insertion promotes Schwann cell proliferation and morphology extension. Moreover, transfected SCs serve as a stable VEGF-A source and fostering angiogenesis. This study offers a method preserving growth factor efficacy and safeguarding SCs, providing a comprehensive solution for enhanced angiogenesis and nerve regeneration.

Mice Mutated in the First Fibronectin Domain of Adhesion Molecule L1 Show Brain Malformations and Behavioral Abnormalities

The X-chromosome-linked cell adhesion molecule L1 (L1CAM), a glycoprotein mainly expressed by neurons in the central and peripheral nervous systems, has been implicated in many neural processes, including neuronal migration and survival, neuritogenesis, synapse formation, synaptic plasticity and regeneration. L1 consists of extracellular, transmembrane and cytoplasmic domains. Proteolytic cleavage of L1's extracellular and transmembrane domains by different proteases generates several L1 fragments with different functions. We found that myelin basic protein (MBP) cleaves L1's extracellular domain, leading to enhanced neuritogenesis and neuronal survival in vitro. To investigate in vivo the importance of the MBP-generated 70 kDa fragment (L1-70), we generated mice with an arginine to alanine substitution at position 687 (L1/687), thereby disrupting L1's MBP cleavage site and obliterating L1-70. Young adult L1/687 males showed normal anxiety and circadian rhythm activities but enhanced locomotion, while females showed altered social interactions. Older L1/687 males were impaired in motor coordination. Furthermore, L1/687 male and female mice had a larger hippocampus, with more neurons in the dentate gyrus and more proliferating cells in the subgranular layer, while the thickness of the corpus callosum and the size of lateral ventricles were normal. In summary, subtle mutant morphological changes result in subtle behavioral changes.

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.

NPS-Crosslinked Fibrin Gels Load with EMSCs to Repair Peripheral Nerve Injury in Rats

Neural tissue engineering has been introduced as a novel therapeutic strategy for trauma-induced sciatic nerve defects. Here, a neuropeptide S (NPS)-crosslinked fibrin scaffolds (NPS@Fg) loaded with an ectomesenchymal stem cell (EMSC) system to bridge an 8-mm sciatic nerve defect in rats are reported. The Schwann cell-like and neural differentiation of the EMSCs on the engineered fibrin scaffolds are also assessed in vitro. These results show that the NPS@Fg promotes the differentiation of EMSCs into neuronal lineage cells, which may also contribute to the therapeutic outcome of the NPS@Fg+EMSCs strategy. After transplantation NPS@Fg+EMSCs into sciatic nerve defects in rats, nerve recovery is assessed up to 12 weeks postinjury. In vivo experiments show that the combination of NPS crosslinked fibrin scaffolds with EMSCs can significantly accelerate nerve healing and improve morphological repair. In the study, NPS@Fg+EMSCs may represent a new potential strategy for peripheral nerve reconstruction.

X-ray structure and function of fibronectin domains two and three of the neural cell adhesion molecule L1

The cell adhesion molecule L1 (L1CAM, L1 in short) plays crucial roles during neural development, regeneration after injury, synapse formation, synaptic plasticity and tumor cell migration. L1 belongs to the immunoglobulin superfamily and comprises in its extracellular part six immunoglobulin (Ig)-like domains and five fibronectin type III homologous repeats (FNs). The second Ig-like domain has been validated for self- (so-called homophilic) binding between cells. Antibodies against this domain inhibit neuronal migration in vitro and in vivo. The fibronectin type III homologous repeats FN2 and FN3 bind small molecule agonistic L1 mimetics and contribute to signal transduction. FN3 has a stretch of 25 amino acids that can be triggered with a monoclonal antibody, or the L1 mimetics, to enhance neurite outgrowth and neuronal cell migration in vitro and in vivo. To correlate the structural features of these FNs with function, we determined a high-resolution crystal structure of a FN2FN3 fragment, which is functionally active in cerebellar granule cells and binds several mimetics. The structure illustrates that both domains are connected by a short linker sequence allowing a flexible and largely independent organization of both domains. This becomes further evident by comparing the X-ray crystal structure with models derived from Small-Angle X-ray Scattering (SAXS) data for FN2FN3 in solution. Based on the X-ray crystal structure, we identified five glycosylation sites which we believe are crucial for folding and stability of these domains. Our study signifies an advance in the understanding of structure-functional relationships of L1.

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.

The KDET Motif in the Intracellular Domain of the Cell Adhesion Molecule L1 Interacts with Several Nuclear, Cytoplasmic, and Mitochondrial Proteins Essential for Neuronal Functions

Abnormal functions of the cell adhesion molecule L1 are linked to several neural diseases. Proteolytic L1 fragments were reported to interact with nuclear and mitochondrial proteins to regulate events in the developing and the adult nervous system. Recently, we identified a 55 kDa L1 fragment (L1-55) that interacts with methyl CpG binding protein 2 (MeCP2) and heterochromatin protein 1 (HP1) via the KDET motif. We now show that L1-55 also interacts with histone H1.4 (HistH1e) via this motif. Moreover, we show that this motif binds to NADH dehydrogenase ubiquinone flavoprotein 2 (NDUFV2), splicing factor proline/glutamine-rich (SFPQ), the non-POU domain containing octamer-binding protein (NonO), paraspeckle component 1 (PSPC1), WD-repeat protein 5 (WDR5), heat shock cognate protein 71 kDa (Hsc70), and synaptotagmin 1 (SYT1). Furthermore, applications of HistH1e, NDUFV2, SFPQ, NonO, PSPC1, WDR5, Hsc70, or SYT1 siRNAs or a cell-penetrating KDET-carrying peptide decrease L1-dependent neurite outgrowth and the survival of cultured neurons. These findings indicate that L1's KDET motif binds to an unexpectedly large number of molecules that are essential for nervous system-related functions, such as neurite outgrowth and neuronal survival. In summary, L1 interacts with cytoplasmic, nuclear and mitochondrial proteins to regulate development and, in adults, the formation, maintenance, and flexibility of neural functions.

Presentation and integration of multiple signals that modulate oligodendrocyte lineage progression and myelination

Myelination is critical for fast saltatory conduction of action potentials. Recent studies have revealed that myelin is not a static structure as previously considered but continues to be made and remodeled throughout adulthood in tune with the network requirement. Synthesis of new myelin requires turning on the switch in oligodendrocytes (OL) to initiate the myelination program that includes synthesis and transport of macromolecules needed for myelin production as well as the metabolic and other cellular functions needed to support this process. A significant amount of information is available regarding the individual intrinsic and extrinsic signals that promote OL commitment, expansion, terminal differentiation, and myelination. However, it is less clear how these signals are made available to OL lineage cells when needed, and how multiple signals are integrated to generate the correct amount of myelin that is needed in a given neural network state. Here we review the pleiotropic effects of some of the extracellular signals that affect myelination and discuss the cellular processes used by the source cells that contribute to the variation in the temporal and spatial availability of the signals, and how the recipient OL lineage cells might integrate the multiple signals presented to them in a manner dialed to the strength of the input.

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.

Cell adhesion molecule L1 interacts with the chromo shadow domain of heterochromatin protein 1 isoforms α, β, and ɣ via its intracellular domain

Cell adhesion molecule L1 regulates multiple cell functions and L1 deficiency is linked to several neural diseases. Proteolytic processing generates functionally decisive L1 fragments, which are imported into the nucleus. By computational analysis, we found at L1's C-terminal end the chromo shadow domain-binding motif PxVxL, which directs the binding of nuclear proteins to the heterochromatin protein 1 (HP1) isoforms α, β, and ɣ. By enzyme-linked immunosorbent assay, we show that the intracellular L1 domain binds to all HP1 isoforms. These interactions involve the HP1 chromo shadow domain and are mediated via the sequence ₁₁₅₈ KDET₁₁₆₁ in the intracellular domain of murine L1, but not by L1's C-terminal PxVxL motif. Immunoprecipitation using nuclear extracts from the brain and from cultured cerebellar and cortical neurons indicates that HP1 isoforms interact with a yet unknown nuclear L1 fragment of approximately 55 kDa (L1-55), which carries ubiquitin residues. Proximity ligation indicates a close association between L1-55 and the HP1 isoforms in neuronal nuclei. This association is reduced after the treatment of neurons with inhibitors of metalloproteases, β-site of amyloid precursor protein cleaving enzyme (BACE1), or ɣ-secretase, suggesting that cleavage of full-length L1 by these proteases generates L1-55. Reduction of HP1α, -β, or -ɣ expression by siRNA decreases L1-dependent neurite outgrowth from cultured cortical neurons and decreases the L1-dependent migration of L1-transfected HEK293 cells in a scratch assay. These findings indicate that the interaction of the novel fragment L1-55 with HP1 isoforms in nuclei affects L1-dependent functions, such as neurite outgrowth and neuronal migration.

Increased plasmin-mediated proteolysis of L1CAM in a mouse model of idiopathic normal pressure hydrocephalus

Idiopathic normal pressure hydrocephalus (iNPH) is the most common form of adult-onset hydrocephalus, but its etiology is poorly understood. Symptoms develop in previously normal individuals and include gait difficulty, incontinence, and dementia. We recently reported that 15% of iNPH patients harbor heterozygous loss-of-function deletions in CWH43, which encodes a protein that modifies other cell membrane proteins. Mice harboring CWH43 deletions develop hydrocephalus and gait dysfunction. Mutations affecting the L1CAM adhesion protein cause developmental brain abnormalities and hydrocephalus from birth. Here, we show that CWH43 deletion leads to L1CAM hypoglycosylation, decreased L1CAM association with lipid microdomains, increased plasmin-mediated L1CAM cleavage, and decreased L1CAM expression. Thus, decreased L1CAM expression appears to occur in adult-onset iNPH and congenital hydrocephalus.

Idiopathic normal pressure hydrocephalus (iNPH) is a common neurological disorder that is characterized by enlarged cerebral ventricles, gait difficulty, incontinence, and dementia. iNPH usually develops after the sixth decade of life in previously asymptomatic individuals. We recently reported that loss-of-function deletions in CWH43 lead to the development of iNPH in a subgroup of patients, but how this occurs is poorly understood. Here, we show that deletions in CWH43 decrease expression of the cell adhesion molecule, L1CAM, in the brains of CWH43 mutant mice and in human HeLa cells harboring a CWH43 deletion. Loss-of-function mutations in L1CAM are a common cause of severe neurodevelopmental defects that include congenital X-linked hydrocephalus. Mechanistically, we find that CWH43 deletion leads to decreased N-glycosylation of L1CAM, decreased association of L1CAM with cell membrane lipid microdomains, increased L1CAM cleavage by plasmin, and increased shedding of cleaved L1CAM in the cerebrospinal fluid. CWH43 deletion also decreased L1CAM nuclear translocation, suggesting decreased L1CAM intracellular signaling. Importantly, the increase in L1CAM cleavage occurred primarily in the ventricular and subventricular zones where brain CWH43 is most highly expressed. Thus, CWH43 deletions may contribute to adult-onset iNPH by selectively downregulating L1CAM in the ventricular and subventricular zone.

Amelioration of the abnormal phenotype of a new L1 syndrome mouse mutation with L1 mimetics

L1 syndrome is a rare developmental disorder characterized by hydrocephalus of varying severity, intellectual deficits, spasticity of the legs, and adducted thumbs. Therapy is limited to symptomatic relief. Numerous gene mutations in the L1 cell adhesion molecule (L1CAM, hereafter abbreviated L1) were identified in L1 syndrome patients, and those affecting the extracellular domain of this transmembrane type 1 glycoprotein show the most severe phenotypes. Previously analyzed rodent models of the L1 syndrome focused on L1-deficient animals or mouse mutants with abrogated cell surface expression of L1, making it difficult to test L1 function-triggering mimetic compounds with potential therapeutic value. To overcome this impasse, we generated a novel L1 syndrome mouse with a mutation of aspartic acid at position 201 in the extracellular part of L1 (p.D201N, hereafter termed L1-201) that displays a cell surface-exposed L1 accessible to the L1 mimetics. Behavioral assessment revealed an increased neurological deficit score and increased locomotor activity in male L1-201 mice carrying the mutation on the X-chromosome. Histological analyses of L1-201 mice showed features of the L1 syndrome, including enlarged ventricles and reduced size of the corpus callosum. Expression levels of L1-201 protein as well as extent of cell surface biotinylation and immunofluorescence labelling of cultured cerebellar neurons were normal. Importantly, treatment of these cultures with the L1 mimetic compounds duloxetine, crotamiton, and trimebutine rescued impaired cell migration and survival as well as neuritogenesis. Altogether, the novel L1 syndrome mouse model provides a first experimental proof-of-principle for the potential therapeutic value of L1 mimetic compounds.

Myelin basic protein enhances axonal regeneration from neural progenitor cells

Introduction

Stem cell therapy using neural progenitor cells (NPCs) shows promise in mitigating the debilitating effects of spinal cord injury (SCI). Notably, myelin stimulates axonal regeneration from mammalian NPCs. This led us to hypothesize that myelin-associated proteins may contribute to axonal regeneration from NPCs.

Methods

We conducted an R-based bioinformatics analysis to identify key gene(s) that may participate in myelin-associated axonal regeneration from murine NPCs, which identified the serine protease myelin basic protein (Mbp). We employed E12 murine NPCs, E14 rat NPCs, and human iPSC-derived Day 1 NPCs (D1 hNPCs) with or without CRISPR/Cas9-mediated Mbp knockout in combination with rescue L1-70 overexpression, constitutively-active VP16-PPARγ2, or the PPARγ agonist ciglitazone. A murine dorsal column crush model of SCI utilizing porous collagen-based scaffolding (PCS)-seeded murine NPCs with or without stable Mbp overexpression was used to assess locomotive recovery and axonal regeneration in vivo.

Results

Myelin promotes axonal outgrowth from NPCs in an Mbp-dependent manner and that Mbp’s stimulatory effects on NPC neurite outgrowth are mediated by Mbp’s production of L1-70. Furthermore, we determined that Mbp/L1-70’s stimulatory effects on NPC neurite outgrowth are mediated by PPARγ-based repression of neuron differentiation-associated gene expression and PPARγ-based Erk1/2 activation. In vivo, PCS-seeded murine NPCs stably overexpressing Mbp significantly enhanced locomotive recovery and axonal regeneration in post-SCI mice.

Conclusions

We discovered that Mbp supports axonal regeneration from mammalian NPCs through the novel Mbp/L1cam/Pparγ signaling pathway. This study suggests that bioengineered, NPC-based interventions can promote axonal regeneration and functional recovery post-SCI.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00584-7.

The 5-HT4 receptor interacts with adhesion molecule L1 to modulate morphogenic signaling in neurons

Morphological remodeling of dendritic spines is critically involved in memory formation and depends on adhesion molecules. Serotonin receptors are also implicated in this remodeling, though the underlying mechanisms remain enigmatic. Here, we uncovered a signaling pathway involving the adhesion molecule L1CAM (L1) and serotonin receptor 5-HT₄ (5-HT₄R, encoded by HTR4). Using Förster resonance energy transfer (FRET) imaging, we demonstrated a physical interaction between 5-HT₄R and L1, and found that 5-HT₄R-L1 heterodimerization facilitates mitogen-activated protein kinase activation in a G_s-dependent manner. We also found that 5-HT₄R-L1-mediated signaling is involved in G₁₃-dependent modulation of cofilin-1 activity. In hippocampal neurons in vitro, the 5-HT₄R-L1 pathway triggers maturation of dendritic spines. Thus, the 5-HT₄R-L1 signaling module represents a previously unknown molecular pathway regulating synaptic remodeling.

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.

Revisiting the proteolytic processing of cell adhesion molecule L1

The important functions of cell adhesion molecule L1 in the nervous system depend on diverse proteolytic enzymes which generate different L1 fragments. It has been reported that cleavage in the third fibronectin type III (FNIII) homologous domain generates the fragments L1-80 and L1-140, while cleavage in the first FNIII domain yields the fragments L1-70 and L1-135. These results raised questions concerning the L1 cleavage sites. We thus generated gene-edited mice expressing L1 with mutations of the cleavage sites either in the first or third FNIII domain. By immunoprecipitations and immunoblot analyses using brain homogenates and different L1 antibodies, we show that L1-70 and L1-135 are generated in wild-type mice, but not or only to a low extent in L1 mutant mice. L1-80 and L1-140 were not detected in wild-type or mutant mice. Mass spectrometry confirmed the results from immunoprecipitations and immunoblot analyses. Based on these observations, we propose that L1-70 and L1-135 are the predominant fragments in the mouse nervous system and that the third FNIII domain is decisive for generating these fragments. Treatment of cultured cerebellar neurons with trypsin or plasmin, which were both proposed to generate L1-80 and L1-140 by cleaving in the third FNIII domain, showed by immunoprecipitations and immunoblot analyses that both proteases lead to the generation of L1-70 and L1-135, but not L1-80 and L1-140. We discuss previous observations on the basis of our new results and propose a novel view on the molecular features that render previous and present observations compatible.

The FAM171A2 gene is a key regulator of progranulin expression and modifies the risk of multiple neurodegenerative diseases

Progranulin (PGRN) is a secreted pleiotropic glycoprotein associated with the development of common neurodegenerative diseases. Understanding the pathophysiological role of PGRN may help uncover biological underpinnings. We performed a genome-wide association study to determine the genetic regulators of cerebrospinal fluid (CSF) PGRN levels. Common variants in region of FAM171A2 were associated with lower CSF PGRN levels (rs708384, P = 3.95 × 10-12). This was replicated in another independent cohort. The rs708384 was associated with increased risk of Alzheimer's disease, Parkinson's disease, and frontotemporal dementia and could modify the expression of the FAM171A2 gene. FAM171A2 was considerably expressed in the vascular endothelium and microglia, which are rich in PGRN. The in vitro study further confirmed that the rs708384 mutation up-regulated the expression of FAM171A2, which caused a decrease in the PGRN level. Collectively, genetic, molecular, and bioinformatic findings suggested that FAM171A2 is a key player in regulating PGRN production.

The L1 cell adhesion molecule affects protein kinase D1 activity in the cerebral cortex in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by deposition of β-amyloid protein (Aβ), neurofibrillary tangles and cognitive deficits resulting from neuronal cell death. In search for the molecular underpinnings of the disease, we were interested in the relationship between Aβ, L1 cell adhesion molecule and protein kinase D1 (PKD1), which are not only implicated in neural development and functional maintenance in the adult, but are also neuroprotective under pathological conditions. Based on our observations that L1 and phosphorylated, i.e. activated, protein kinase PKD1 (pPKD1) co-localize in cultured neurons, we investigated the functional relationship between L1 and pPKD1 in the frontal lobe of an AD human cortical tissue microarray, and found increased and positively correlating levels of both molecules when compared to a non-affected human brain. Also in the APP_SWE mouse model of AD, L1 and pPKD1 levels were increased in the frontal lobe. To investigate whether L1 influences PKD1-based functions in AD, cultured cortical neurons were stressed with either H₂O₂ or oligomeric Aβ_1-42, in the presence or absence of recombinant L1 extracellular domain, and PKD1 phosphorylation was measured. As indicated by the cell viability assay, L1 maintained neuronal survival under oxidative stress and under application of oligomeric Aβ_1-42, when PKD1 activity was inhibited, suggesting that L1 ameliorates some aspects of Aβ_1-42 pathology in parallel with reducing PKD1 function.

The cell adhesion molecule L1 interacts with nuclear proteins via its intracellular domain

Proteolytic cleavage of the cell adhesion molecule L1 (L1) in brain tissue and in cultured cerebellar neurons results in the generation and nuclear import of a 30 kDa fragment comprising most of L1's C-terminal, intracellular domain. In search of molecules that interact with this domain, we performed affinity chromatography with the recombinant intracellular L1 domain and a nuclear extract from mouse brains, and identified potential nuclear L1 binding partners involved in transcriptional regulation, RNA processing and transport, DNA repair, chromatin remodeling, and nucleocytoplasmic transport. By co-immunoprecipitation and enzyme-linked immunosorbent assay using recombinant proteins, we verified the direct interaction between L1 and the nuclear binding partners non-POU domain containing octamer-binding protein and splicing factor proline/glutamine-rich. The proximity ligation assay confirmed this close interaction in cultures of cerebellar granule cells. Our findings suggest that L1 fragments regulate multiple nuclear functions in the nervous system. We discuss possible physiological and pathological roles of these interactions in regulation of chromatin structure, gene expression, RNA processing, and DNA repair.

Differentiation of NSC-34 cells is characterized by expression of NGF receptor p75, glutaminase and NCAM L1, activation of mitochondria, and sensitivity to fatty acid intervention

Motor neuronal damage due to diseases, traumatic insults or de-afferentation of the spinal cord is often incurable because of poor intrinsic regenerative capacity. Hence, medical basic research has to provide a better understanding of development-/regeneration-related cellular processes as only way to develop new and successful therapeutic strategies. Here, we investigated the neuronal differentiation of the NSC-34 hybrid cell line, which is an accepted model for spinal cord motor neurons. Their differentiation was stimulated by switching from normal to differentiation medium and by supplementation with palmitic and oleic acid. To characterize neuro-differentiation of NSC-34 cells, expression of nicotinic acetylcholine receptor alpha 4, NGF p75 receptor, IGF I alpha receptor, glutaminase, NCAM L1, ADAM10 and myelin basic protein as well as activation of mitochondria were analyzed. Both switch from normal to differentiation medium and fatty acid application stimulated NSC-34 differentiation. Differentiation was characterized by diminishing expression of the nicotinic acetylcholine receptor alpha 4 and enhancing expression of the NGF receptor p75, of glutaminase, of NCAM L1 and it's partially transformation from the cell surface into the cell. Fatty acid intervention stabilized the expression of the nicotinic acetylcholine receptor alpha 4, diminished the expression of the NGF receptor p75, consolidated the expression profile of NCAM L1, and intensified the expression of the relevant for NCAM L1 cleavage ADAM10. However, NCAM L1 cleavage itself was unaffected by fatty acid intervention, as was the differentiation-relevant activation of mitochondria and their transformation into neuronal filopodia.

Transcriptomic Landscape of von Economo Neurons in Human Anterior Cingulate Cortex Revealed by Microdissected-Cell RNA Sequencing

The von Economo neurons (VENs) are specialized large bipolar projection neurons with restricted distribution in the human brain, and they are far more abundant in humans than in non-human primates. However, VEN functions remain elusive due to the difficulty of isolating VENs and dissecting their connections in the brain. Here, we combined laser-capture-microdissection with RNA sequencing to describe the transcriptomic profile of VENs from human anterior cingulate cortex (ACC). Using pyramidal neurons as reference cells, we identified 344 genes with VEN-associated expression differences, including 215 higher and 129 lower expression genes. Functional enrichment and protein–protein interaction network analyses showed that these genes with VEN-associated expression differences are involved in VEN morphogenesis and functions, such as dendrite branching and axon myelination, and many of them are associated with human social-emotional disorders. With the use of in situ hybridization and immunohistochemistry assays, we validated four novel VEN markers (VAT1L, CHST8, LYPD1, and SULF2). Collectively, we generated a full-spectrum expression profile of VENs from human ACC, greatly enlarging the pool of genes with VEN-associated expression differences that can help researchers to understand the role of VENs in normal and disordered human brains.

Combination Therapy With Hyperbaric Oxygen and Erythropoietin Inhibits Neuronal Apoptosis and Improves Recovery in Rats With Spinal Cord Injury

BACKGROUND

Apoptosis plays an important role in various diseases, including spinal cord injury (SCI). Hyperbaric oxygen (HBO) and erythropoietin (EPO) promote the recovery from SCI, but the relationship between apoptosis and the combination therapeutic effect is not completely clear.

OBJECTIVE

The purpose of this study was to investigate the effects of HBO and EPO on SCI and the mechanisms that underlie their therapeutic benefits.

DESIGN

The study was designed to explore the effects of HBO and EPO on SCI through a randomized controlled trial.

METHODS

Sixty young developing female Sprague-Dawley rats were randomly divided into groups of 12 rats receiving sham, SCI, HBO, EPO, or HBO plus EPO. The SCI model was modified with the Allen method to better control consistency. HBO was performed for 1 hour per day for a total of 21 days, and EPO was given once per week for a total of 3 weeks. Both methods were performed 2 hours after SCI. Locomotor function was evaluated with the 21-point Basso-Beattie-Bresnahan Locomotor Rating Scale, an inclined-plane test, and a footprint analysis. All genes were detected by Western blotting and immunohistochemistry. The level of cell apoptosis was determined by Hoechst staining.

RESULTS

The results showed that HBO and EPO promoted the recovery of locomotor function in the hind limbs of rats by inhibiting the apoptosis of neurons. During this period, the expression of B-cell lymphoma/leukemia 2 protein (Bcl-2) increased significantly, whereas the expression of Bcl-2-associated X protein (Bax) and cleaved caspase 3 decreased significantly, indicating the inhibition of apoptosis. Meanwhile, the expression of G protein-coupled receptor 17 decreased, and that of myelin basic protein increased, suggesting that there may be a potential connection between demyelination and neuronal apoptosis.

LIMITATIONS

The limitations of the study include deviations in the preparation of SCI models; lack of reverse validation of molecular mechanisms; absence of in vitro cell experiments; and only one time point after SCI was studied.

CONCLUSIONS

HBO and EPO treatments are beneficial for SCI, especially when the 2 therapies are combined.

GDP-l-fucose synthase is a CD4+ T cell-specific autoantigen in DRB3*02:02 patients with multiple sclerosis

Multiple sclerosis is an immune-mediated autoimmune disease of the central nervous system that develops in genetically susceptible individuals and likely requires environmental triggers. The autoantigens and molecular mimics triggering the autoimmune response in multiple sclerosis remain incompletely understood. By using a brain-infiltrating CD4⁺ T cell clone that is clonally expanded in multiple sclerosis brain lesions and a systematic approach for the identification of its target antigens, positional scanning peptide libraries in combination with biometrical analysis, we have identified guanosine diphosphate (GDP)-l-fucose synthase as an autoantigen that is recognized by cerebrospinal fluid-infiltrating CD4⁺ T cells from HLA-DRB3*-positive patients. Significant associations were found between reactivity to GDP-l-fucose synthase peptides and DRB3*02:02 expression, along with reactivity against an immunodominant myelin basic protein peptide. These results, coupled with the cross-recognition of homologous peptides from gut microbiota, suggest a possible role of this antigen as an inducer or driver of pathogenic autoimmune responses in multiple sclerosis.

Glioma malignancy is linked to interdependent and inverse AMOG and L1 adhesion molecule expression

BACKGROUND

Gliomas account for the majority of primary human brain tumors and remain a challenging neoplasm for cure due to limited therapeutic options. Cell adhesion molecules play pivotal roles in the growth and progression of glial tumors. Roles of the adhesion molecules on glia (AMOG) and L1CAM (L1) in glioma cells have been shown to correlate with tumorigenesis: Increased expression of L1 and decreased expression of AMOG correlate with degree of malignancy.

METHODS

We evaluated the interdependence in expression of these molecules by investigating the role of AMOG in vitro via modulation of L1 expression and analyzing apoptosis and cell senescence of glioma cells.

RESULTS

Immunohistochemical staining of normal human cortical and glioma tissue microarrays demonstrated that AMOG expression was lower in human gliomas compared to normal tissue and is inversely correlated with the degree of malignancy. Moreover, reduction of AMOG expression in human glioblastoma cells elevated L1 expression, which is accompanied by decreased cell apoptosis as well as senescence.

CONCLUSION

AMOG and L1 interdependently regulate their expression levels not only in U-87 MG cells but also in U251 and SHG44 human glioma cell lines. The capacity of AMOG to reduce L1 expression suggests that methods for increasing AMOG expression may provide a therapeutic choice for the management of glial tumors with high expression of L1.

L1cam-mediated developmental processes of the nervous system are differentially regulated by proteolytic processing

Normal brain development depends on tight temporal and spatial regulation of connections between cells. Mutations in L1cam, a member of the immunoglobulin (Ig) superfamily that mediate cell-cell contacts through homo- and heterophilic interactions, are associated with several developmental abnormalities of the nervous system, including mental retardation, limb spasticity, hydrocephalus, and corpus callosum aplasia. L1cam has been reported to be shed from the cell surface, but the significance of this during different phases of brain development is unknown. We here show that ADAM10-mediated shedding of L1cam is regulated by its fibronectin type III (FNIII) domains. Specifically, the third FNIII domain is important for maintaining a conformation where access to a membrane proximal cleavage site is restricted. To define the role of ADAM10/17/BACE1-mediated shedding of L1cam during brain development, we used a zebrafish model system. Knockdown of the zebrafish, l1camb, caused hydrocephalus, defects in axonal outgrowth, and myelination abnormalities. Rescue experiments with proteinase-resistant and soluble L1cam variants showed that proteolytic cleavage is not required for normal axonal outgrowth and development of the ventricular system. In contrast, metalloproteinase-mediated shedding is required for efficient myelination, and only specific fragments are able to mediate this stimulatory function of the shedded L1cam.

High N-glycan multiplicity is critical for neuronal adhesion and sensitizes the developing cerebellum to N-glycosylation defect

Proper brain development relies highly on protein N-glycosylation to sustain neuronal migration, axon guidance and synaptic physiology. Impairing the N-glycosylation pathway at early steps produces broad neurological symptoms identified in congenital disorders of glycosylation. However, little is known about the molecular mechanisms underlying these defects. We generated a cerebellum specific knockout mouse for Srd5a3, a gene involved in the initiation of N-glycosylation. In addition to motor coordination defects and abnormal granule cell development, Srd5a3 deletion causes mild N-glycosylation impairment without significantly altering ER homeostasis. Using proteomic approaches, we identified that Srd5a3 loss affects a subset of glycoproteins with high N-glycans multiplicity per protein and decreased protein abundance or N-glycosylation level. As IgSF-CAM adhesion proteins are critical for neuron adhesion and highly N-glycosylated, we observed impaired IgSF-CAM-mediated neurite outgrowth and axon guidance in Srd5a3 mutant cerebellum. Our results link high N-glycan multiplicity to fine-tuned neural cell adhesion during mammalian brain development.

A Fragment of Adhesion Molecule L1 Binds to Nuclear Receptors to Regulate Synaptic Plasticity and Motor Coordination

Proteolytic cleavage of the neuronal isoform of the murine cell adhesion molecule L1, triggered by stimulation of the cognate L1-dependent signaling pathways, results in the generation and nuclear import of an L1 fragment that contains the intracellular domain, the transmembrane domain, and part of the extracellular domain. Here, we show that the LXXLL and FXXLF motifs in the extracellular and transmembrane domain of this L1 fragment mediate the interaction with the nuclear estrogen receptors α (ERα) and β (ERβ), peroxisome proliferator-activated receptor γ (PPARγ), and retinoid X receptor β (RXRβ). Mutations of the LXXLL motif in the transmembrane domain and of the FXXLF motif in the extracellular domain disturb the interaction of the L1 fragment with these nuclear receptors and, when introduced by viral transduction into mouse embryos in utero, result in impaired motor coordination, learning and memory, as well as synaptic connectivity in the cerebellum, in adulthood. These impairments are similar to those observed in the L1-deficient mouse. Our findings suggest that the interplay of nuclear L1 and distinct nuclear receptors is associated with synaptic contact formation and plasticity.

A fragment of adhesion molecule L1 is imported into mitochondria, and regulates mitochondrial metabolism and trafficking

The cell adhesion molecule L1 (also known as L1CAM) plays important roles in the mammalian nervous system under physiological and pathological conditions. We have previously reported that proteolytic cleavage of L1 by myelin basic protein leads to the generation of a 70 kDa transmembrane L1 fragment (L1-70) that promotes neuronal migration and neuritogenesis. Here, we provide evidence that L1-70 is imported from the cytoplasm into mitochondria. Genetic ablation of L1, inhibition of mitochondrial import of L1-70 or prevention of myelin basic protein-mediated generation of L1-70 all lead to reduced mitochondrial complex I activity, and impaired mitochondrial membrane potential, fusion, fission and motility, as well as increased retrograde transport. We identified NADH dehydrogenase ubiquinone flavoprotein 2 as a binding partner for L1, suggesting that L1-70 interacts with this complex I subunit to regulate complex I activity. The results of our study provide insights into novel functions of L1 in mitochondrial metabolism and cellular dynamics. These functions are likely to ameliorate the consequences of acute nervous system injuries and chronic neurodegenerative diseases.

Electroacupuncture Restores Locomotor Functions After Mouse Spinal Cord Injury in Correlation With Reduction of PTEN and p53 Expression

Background: We previously showed that electroacupuncture (EA) at Jiaji points promotes expression of adhesion molecule L1 in spinal cord tissue after mouse spinal cord injury (SCI) and contributes to recovery of neural functions. Objective: We investigated the effects of EA on downstream signaling molecules of L1 and molecules relevant to apoptosis with the aim to understand the underlying molecular mechanisms. Methods: Female C57BL/6 mice were divided into a sham group, injury group, injury+acupuncture (AP) group and injury+EA group. We investigated the changes in cognate L1-triggered signaling molecules after SCI by immunofluorescence staining and immunoblot analysis. Results: Protein levels of phosphatase and tensin homolog (PTEN) and p53 were decreased by EA at different time points after injury, whereas the levels of phosphorylated mammalian target of rapamycin (pmTOR), p-Akt and phosphorylated extracellular signal-regulatedkinase (p-Erk) were increased. Also, levels of myelin basic protein (MBP) were increased by EA. AP alone showed less pronounced changes in expression of the investigated molecules, when compared to EA. Conclusion: We propose that EA contributes to neuroprotection by inhibiting PTEN and p53 expression and by increasing the levels of pmTOR/Akt/Erk and of MBP after SCI. These observations allow novel insights into the beneficial effects of EA via L1-triggered signaling molecules after injury.

A Small Organic Compound Mimicking the L1 Cell Adhesion Molecule Promotes Functional Recovery after Spinal Cord Injury in Zebrafish

Tacrine is a small organic compound that was discovered to mimic the functions of the neural cell adhesion molecule L1 by promoting the cognate functions of L1 in vitro, such as neuronal survival, neuronal migration, neurite outgrowth, and myelination. Based on studies indicating that L1 enhances functional recovery in different central and peripheral nervous system disease paradigms of rodents, it deemed interesting to investigate the beneficial role of tacrine in the attractive zebrafish animal model, by evaluating functional recovery after spinal cord injury. To this aim, larval and adult zebrafish were exposed to tacrine treatment after spinal cord injury and monitored for locomotor recovery and axonal regrowth. Tacrine promoted the rapid recovery of locomotor activities in both larval and adult zebrafish, enhanced regrowth of severed axons and myelination, and reduced astrogliosis in the spinal cords. Tacrine treatment upregulated the expression of L1.1 (a homolog of the mammalian recognition molecule L1) and enhanced the L1.1-mediated intracellular signaling cascades in the injured spinal cords. These observations lead to the hope that, in combination with other therapeutic approaches, this old drug may become a useful reagent to ameliorate the deficits resulting from acute and chronic injuries of the mammalian nervous system.

Phenelzine, a small organic compound mimicking the functions of cell adhesion molecule L1, promotes functional recovery after mouse spinal cord injury

BACKGROUND

Neural cell adhesion molecule L1 contributes to nervous system development and maintenance by promoting neuronal survival, neuritogenesis, axonal regrowth/sprouting, myelination, and synapse formation and plasticity. L1 also enhances recovery after spinal cord injury and ameliorates neurodegenerative processes in experimental rodent models. Aiming for clinical translation of L1 into therapy we screened for and functionally characterized in vitro the small organic molecule phenelzine, which mimics characteristic L1 functions.

OBJECTIVE

The present study was designed to evaluate the potential of this compound in vivo in a mouse model of spinal cord injury.

METHODS AND RESULTS

In mice, intraperitoneal injection of phenelzine immediately after severe thoracic compression, and thereafter once daily for 6 weeks, improved hind limb function, reduced astrogliosis and promoted axonal regrowth/sprouting at 4 and 5 weeks after spinal cord injury compared to vehicle control-treated mice. Phenelzine application upregulated L1 expression in the spinal cord and stimulated the cognate L1-mediated intracellular signaling cascades in the spinal cord tissue. Phenelzine-treated mice showed decreased levels of pro-inflammatory cytokines, such as interleukin-1β, interleukin-6, and tumor necrosis factor-α in the injured spinal cord during the acute phase of inflammation.

CONCLUSIONS

This study provides new insights into the role of phenelzine in L1-mediated neural functions and modulation of inflammation. The combined results raise hopes that phenelzine may develop into a therapeutic agent for nervous system injuries.

Proteolytic cleavage of transmembrane cell adhesion molecule L1 by extracellular matrix molecule Reelin is important for mouse brain development

The cell adhesion molecule L1 and the extracellular matrix protein Reelin play crucial roles in the developing nervous system. Reelin is known to activate signalling cascades regulating neuronal migration by binding to lipoprotein receptors. However, the interaction of Reelin with adhesion molecules, such as L1, has remained poorly explored. Here, we report that full-length Reelin and its N-terminal fragments N-R2 and N-R6 bind to L1 and that full-length Reelin and its N-terminal fragment N-R6 proteolytically cleave L1 to generate an L1 fragment with a molecular mass of 80 kDa (L1-80). Expression of N-R6 and generation of L1-80 coincide in time at early developmental stages of the cerebral cortex. Reelin-mediated generation of L1-80 is involved in neurite outgrowth and in stimulation of migration of cultured cortical and cerebellar neurons. Morphological abnormalities in layer formation of the cerebral cortex of L1-deficient mice partially overlap with those of Reelin-deficient reeler mice. In utero electroporation of L1-80 into reeler embryos normalised the migration of cortical neurons in reeler embryos. The combined results indicate that the direct interaction between L1 and Reelin as well as the Reelin-mediated generation of L1-80 contribute to brain development at early developmental stages.

Trimebutine, a small molecule mimetic agonist of adhesion molecule L1, contributes to functional recovery after spinal cord injury in mice

Curing spinal cord injury (SCI) in mammals is a daunting task because of the lack of permissive mechanisms and strong inhibitory responses at and around the lesion. The neural cell adhesion molecule L1CAM (L1) has been shown to favor axonal regrowth and enhance neuronal survival and synaptic plasticity but delivery of full-length L1 or its extracellular domain could encounter difficulties in translation to therapy in humans. We have, therefore, identified several small organic compounds that bind to L1 and stimulate neuronal survival, neuronal migration and neurite outgrowth in an L1-dependent manner. Here, we assessed the functions of two L1 mimetics, trimebutine and honokiol, in regeneration following SCI in young adult mice. Using the Basso Mouse Scale (BMS) score, we found that ground locomotion in trimebutine-treated mice recovered better than honokiol-treated or vehicle-receiving mice. Enhanced hindlimb locomotor functions in the trimebutine group were observed at 6 weeks after SCI. Immunohistology of the spinal cords rostral and caudal to the lesion site showed reduced areas and intensities of glial fibrillary acidic protein immunoreactivity in both trimebutine and honokiol groups, whereas increased regrowth of axons was observed only in the trimebutine-treated group. Both L1- and L1 mimetic-mediated intracellular signaling cascades in the spinal cord lesion sites were activated by trimebutine and honokiol, with trimebutine being more effective than honokiol. These observations suggest that trimebutine and, to a lesser extent under the present experimental conditions, honokiol have a potential for therapy in regeneration of mammalian spinal cord injuries.

Electro-Acupuncture Modulates L1 Adhesion Molecule Expression After Mouse Spinal Cord Injury

Spinal cord injury is a devastating neurological disease in desperate need of a cure. We have previously shown that overexpression of the adhesion molecule L1 contributes to locomotor recovery after injury and were therefore interested in how electro-acupuncture would influence the expression of this molecule. Here, we investigated the effects of electro-acupuncture at "Jiaji" points (EX-B2), newly established by us, in young adult mice to determine whether improved recovery via electro-acupuncture could be due to enhanced L1 expression. Locomotor function, as evaluated by the Basso Mouse Scale score and by catwalk gait parameters, was improved by electro-acupuncture at different time points after injury in parallel with enhanced levels of L1 expression. Interestingly, the levels of the astrocytic marker glial fibrillary acidic protein (GFAP) were also increased, but only in the early phase after injury, being reduced at later stages during recovery. Acupuncture alone showed less pronounced changes in expression of these molecules. We propose that electro-acupuncture improves regeneration in part by promoting the L1 expression and beneficial activation of stem cells, and by differentially modulating the expression of GFAP by promoting regeneration-conductive astrocytic responses at initial stages and reducing regeneration-adversive activation in the secondary stages. Expression of the stem cell marker nestin was upregulated by electro-acupuncture in the acute stage. The combined observations show for the first time in mice the beneficial functions of electro-acupuncture at Jiaji points in the spinal cord injury mouse model and provide novel insights into some molecular mechanisms underlying electro-acupuncture in spinal cord injury.

Small Molecule Agonists of Cell Adhesion Molecule L1 Mimic L1 Functions In Vivo

Lack of permissive mechanisms and abundance of inhibitory molecules in the lesioned central nervous system of adult mammals contribute to the failure of functional recovery after injury, leading to severe disabilities in motor functions and pain. Peripheral nerve injury impairs motor, sensory, and autonomic functions, particularly in cases where nerve gaps are large and chronic nerve injury ensues. Previous studies have indicated that the neural cell adhesion molecule L1 constitutes a viable target to promote regeneration after acute injury. We screened libraries of known drugs for small molecule agonists of L1 and evaluated the effect of hit compounds in cell-based assays in vitro and in mice after femoral nerve and spinal cord injuries in vivo. We identified eight small molecule L1 agonists and showed in cell-based assays that they stimulate neuronal survival, neuronal migration, and neurite outgrowth and enhance Schwann cell proliferation and migration and myelination of neurons in an L1-dependent manner. In a femoral nerve injury mouse model, enhanced functional regeneration and remyelination after application of the L1 agonists were observed. In a spinal cord injury mouse model, L1 agonists improved recovery of motor functions, being paralleled by enhanced remyelination, neuronal survival, and monoaminergic innervation, reduced astrogliosis, and activation of microglia. Together, these findings suggest that application of small organic compounds that bind to L1 and stimulate the beneficial homophilic L1 functions may prove to be a valuable addition to treatments of nervous system injuries.

Myelin Basic Protein Cleaves Cell Adhesion Molecule L1 and Improves Regeneration After Injury

Myelin basic protein (MBP) is a serine protease that cleaves neural cell adhesion molecule L1 and generates a transmembrane L1 fragment which facilitates L1-dependent functions in vitro, such as neurite outgrowth, neuronal cell migration and survival, myelination by Schwann cells as well as Schwann cell proliferation, migration, and process formation. Ablation and blocking of MBP or disruption of its proteolytic activity by mutation of a proteolytically active serine residue abolish L1-dependent cellular responses. In utero injection of adeno-associated virus encoding proteolytically active MBP into MBP-deficient shiverer mice normalizes differentiation, myelination, and synaptogenesis in the developing postnatal spinal cord, in contrast to proteolytically inactive MBP. Application of active MBP to the injured wild-type spinal cord and femoral nerve augments levels of a transmembrane L1 fragment, promotes remyelination, and improves functional recovery after injury. Application of MBP antibody impairs recovery. Virus-mediated expression of active MBP in the lesion site after spinal cord injury results in improved functional recovery, whereas injection of virus encoding proteolytically inactive MBP fails to do so. The present study provides evidence for a novel L1-mediated function of MBP in the developing spinal cord and in the injured adult mammalian nervous system that leads to enhanced recovery after acute trauma.

Gender differences in white matter pathology and mitochondrial dysfunction in Alzheimer's disease with cerebrovascular disease

BACKGROUND

Dementia risk in women is higher than in men, but the molecular neuropathology of this gender difference remains poorly defined. In this study, we used unbiased, discovery-driven quantitative proteomics to assess the molecular basis of gender influences on risk of Alzheimer's disease with cerebrovascular disease (AD + CVD).

RESULTS

We detected modulation of several redox proteins in the temporal lobe of AD + CVD subjects, and we observed sex-specific alterations in the white matter (WM) and mitochondria proteomes of female patients. Functional proteomic analysis of AD + CVD brain tissues revealed increased citrullination of arginine and deamidation of glutamine residues of myelin basic protein (MBP) in female which impaired degradation of degenerated MBP and resulted in accumulation of non-functional MBP in WM. Female patients also displayed down-regulation of ATP sub-units and cytochromes, suggesting increased severity of mitochondria impairment in women.

CONCLUSIONS

Our study demonstrates that gender-linked modulation of white matter and mitochondria proteomes influences neuropathology of the temporal lobe in AD + CVD.

Cell Adhesion Molecules and Ubiquitination-Functions and Significance

Cell adhesion molecules of the immunoglobulin (Ig) superfamily represent the biggest group of cell adhesion molecules. They have been analyzed since approximately 40 years ago and most of them have been shown to play a role in tumor progression and in the nervous system. All members of the Ig superfamily are intensively posttranslationally modified. However, many aspects of their cellular functions are not yet known. Since a few years ago it is known that some of the Ig superfamily members are modified by ubiquitin. Ubiquitination has classically been described as a proteasomal degradation signal but during the last years it became obvious that it can regulate many other processes including internalization of cell surface molecules and lysosomal sorting. The purpose of this review is to summarize the current knowledge about the ubiquitination of cell adhesion molecules of the Ig superfamily and to discuss its potential physiological roles in tumorigenesis and in the nervous system.

Temporal lobe in human aging: A quantitative protein profiling study of samples from Chinese Human Brain Bank

The temporal lobe is a portion of the cerebral cortex with critical functionality. The age-related protein profile changes in the human temporal lobe have not been previously studied. This 4-plex tandem mass tag labeled proteomic study was performed on samples of temporal lobe from Chinese donors. Tissue samples were assigned to four age groups: Group A (the young, age: 34±13 years); Group B (the elderly, 62±5 years); Group C (the aged, 84±4 years) and Group D (the old, 95±1 years). Pooled samples from the different groups were subjected to proteomics and bioinformatics analysis to identify age-related changes in protein expression and associated pathways. We isolated 5072 proteins, and found that 67 proteins were downregulated and 109 proteins were upregulated in one or more groups during the aging process. Western blotting assays were performed to verify the proteomic results. Bioinformatic analysis identified proteins involved in neuronal degeneration, including proteins involved in neuronal firing, myelin sheath damage, and cell structure stability. We also observed the accumulation of extracellular matrix and lysosomal proteins which imply the occurrence of fibrosis and autophagy. Our results suggest a series of changes across a wide range of proteins in the human temporal lobe that may relate to aging and age-related neurodegenerative disorders.

MyelStones: the executive roles of myelin basic protein in myelin assembly and destabilization in multiple sclerosis

The classic isoforms of myelin basic protein (MBP, 14–21.5 kDa) are essential to formation of the multilamellar myelin sheath of the mammalian central nervous system (CNS). The predominant 18.5-kDa isoform links together the cytosolic surfaces of oligodendrocytes, but additionally participates in cytoskeletal turnover and membrane extension, Fyn-mediated signalling pathways, sequestration of phosphoinositides and maintenance of calcium homoeostasis. All MBP isoforms are intrinsically disordered proteins (IDPs) that interact via molecular recognition fragments (MoRFs), which thereby undergo local disorder-to-order transitions. Their conformations and associations are modulated by environment and by a dynamic barcode of post-translational modifications, particularly phosphorylation by mitogen-activated and other protein kinases and deimination [a hallmark of demyelination in multiple sclerosis (MS)]. The MBPs are thus to myelin what basic histones are to chromatin. Originally thought to be merely structural proteins forming an inert spool, histones are now known to be dynamic entities involved in epigenetic regulation and diseases such as cancer. Analogously, the MBPs are not mere adhesives of compact myelin, but active participants in oligodendrocyte proliferation and in membrane process extension and stabilization during myelinogenesis. A central segment of these proteins is pivotal in membrane-anchoring and SH3 domain (Src homology 3) interaction. We discuss in the present review advances in our understanding of conformational conversions of this classic basic protein upon membrane association, including new thermodynamic analyses of transitions into different structural ensembles and how a shift in the pattern of its post-translational modifications is associated with the pathogenesis and potentially onset of demyelination in MS.

Association between brain metastasis from lung cancer and the serum level of myelin basic protein

The aim of the present study was to determine the association between the expression of myelin basic protein in the serum and the metastasis of lung cancer to the brain. A total of 68 lung cancer patients, treated in the Department of Respiratory Medicine of the People’s Hospital of Rizhao (Rizhao, China), were divided into two groups, those with brain metastasis (32 cases) and those without brain metastasis (36 cases). The expression levels of myelin basic protein were measured for all the patients. The results indicated that the expression levels of myelin basic protein in the brain metastasis group were significantly higher when compared with those in the group without metastasis (P<0.05). However, there was no statistically significant correlation between the size of the brain metastasis and the expression levels of myelin basic protein (P>0.05). Furthermore, no statistically significant difference was found in the average level of myelin basic protein between the two subgroups of patients with brain tumor diameters of >1.5 cm and <1.5 cm (P>0.05). Therefore, the results demonstrated a statistically significant correlation between the expression of myelin basic protein in the serum and the metastasis of lung cancer to the brain. Myelin basic protein may thus prove useful in the early diagnosis of brain metastases in lung cancer patients.

A fibrin matrix promotes the differentiation of EMSCs isolated from nasal respiratory mucosa to myelinating phenotypical Schwann-like cells

Because Schwann cells perform the triple tasks of myelination, axon guidance and neurotrophin synthesis, they are candidates for cell transplantation that might cure some types of nervous-system degenerative diseases or injuries. However, Schwann cells are difficult to obtain. As another option, ectomesenchymal stem cells (EMSCs) can be easily harvested from the nasal respiratory mucosa. Whether fibrin, an important transplantation vehicle, can improve the differentiation of EMSCs into Schwann-like cells (SLCs) deserves further research. EMSCs were isolated from rat nasal respiratory mucosa and were purified using anti-CD133 magnetic cell sorting. The purified cells strongly expressed HNK-1, nestin, p75(NTR), S-100, and vimentin. Using nuclear staining, the MTT assay and Western blotting analysis of the expression of cell-cycle markers, the proliferation rate of EMSCs on a fibrin matrix was found to be significantly higher than that of cells grown on a plastic surface but insignificantly lower than that of cells grown on fibronectin. Additionally, the EMSCs grown on the fibrin matrix expressed myelination-related molecules, including myelin basic protein (MBP), 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and galactocerebrosides (GalCer), more strongly than did those grown on fibronectin or a plastic surface. Furthermore, the EMSCs grown on the fibrin matrix synthesized more neurotrophins compared with those grown on fibronectin or a plastic surface. The expression level of integrin in EMSCs grown on fibrin was similar to that of cells grown on fibronectin but was higher than that of cells grown on a plastic surface. These results demonstrated that fibrin not only promoted EMSC proliferation but also the differentiation of EMSCs into the SLCs. Our findings suggested that fibrin has great promise as a cell transplantation vehicle for the treatment of some types of nervous system diseases or injuries.

Transcriptional upregulation of myelin components in spontaneous myelin basic protein-deficient mice

Myelin is essential for efficient signal transduction in the nervous system comprising of multiple proteins. The intricacies of the regulation of the formation of myelin, and its components, are not fully understood. Here, we describe the characterization of a novel myelin basic protein (Mbp) mutant mouse, mbp(jive), which spontaneously occurred in our mouse colony. These mice displayed the onset of a shaking gait before 3 weeks of age and seizure onset before 2 months of age. Due to a progressive increase of seizure intensity, mbp(jive) mice experienced premature lethality at around 3 months of age. Mbp mRNA transcript or protein was undetectable and, accordingly, genetic analysis demonstrated a homozygous loss of exons 3 to 6 of Mbp. Peripheral nerve conductance was mostly unimpaired. Additionally, we observed grave structural changes in white matter predominant structures were detected by T1, T2 and diffusion weighted magnetic resonance imaging. We additionally observed that Mbp-deficiency results in an upregulation of Qkl, Mag and Cnp, suggestive of a regulatory feedback mechanism whereby compensatory increases in Qkl have downstream effects on Mag and Cnp. Further research will clarify the role and specifications of this myelin feedback loop, as well as determine its potential role in therapeutic strategies for demyelinating disorders.

Function-triggering antibodies to the adhesion molecule L1 enhance recovery after injury of the adult mouse femoral nerve

L1 is among the few adhesion molecules that favors repair after trauma in the adult central nervous system of vertebrates by promoting neuritogenesis and neuronal survival, among other beneficial features. In the peripheral nervous system, L1 is up-regulated in Schwann cells and regrowing axons after nerve damage, but the functional consequences of this expression remain unclear. Our previous study of L1-deficient mice in a femoral nerve injury model showed an unexpected improved functional recovery, attenuated motoneuronal cell death, and enhanced Schwann cell proliferation, being attributed to the persistent synthesis of neurotrophic factors. On the other hand, transgenic mice over-expressing L1 in neurons led to improved remyelination, but not improved functional recovery. The present study was undertaken to investigate whether the monoclonal L1 antibody 557 that triggers beneficial L1 functions in vitro would trigger these also in femoral nerve repair. We analyzed femoral nerve regeneration in C57BL/6J mice that received this antibody in a hydrogel filled conduit connecting the cut and sutured nerve before its bifurcation, leading to short-term release of antibody by diffusion. Video-based quantitative analysis of motor functions showed improved recovery when compared to mice treated with conduits containing PBS in the hydrogel scaffold, as a vehicle control. This improved recovery was associated with attenuated motoneuron loss, remyelination and improved precision of preferential motor reinnervation. We suggest that function-triggering L1 antibodies applied to the lesion site at the time of injury over a limited time period will not only be beneficial in peripheral, but also central nervous system regeneration.