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

Mutations in the human L1CAM gene cause X‐linked hydrocephalus and MASA (Mental retardation, Aphasia, Shuffling gait, Adducted thumbs) syndrome. In vitro studies have shown that the L1 cytoplasmic domain (L1CD) is involved in L1 trafficking, neurite branching, signaling, and interactions with the cytoskeleton. L1cam knockout (L1^KO) mice have hydrocephalus, a small cerebellum, hyperfasciculation of corticothalamic tracts, and abnormal peripheral nerves. To explore the function of the L1CD, we made three new mice lines in which different parts of the L1CD have been altered. In all mutant lines L1 protein is expressed and transported into the axon. Interestingly, these new L1CD mutant lines display normal brain morphology. However, the expression of L1 protein in the adult is dramatically reduced in the two L1CD mutant lines that lack the ankyrin‐binding region and they show defects in motor function. Therefore, the L1CD is not responsible for the major defects observed in L1^KO mice, yet it is required for continued L1 protein expression and motor function in the adult. J. Comp. Neurol. 518:1113–1132, 2010. © 2009 Wiley‐Liss, Inc.     

Water maze learning and forebrain mRNA expression of the neural cell adhesion molecule L1

L1 and NCAM, two cell adhesion molecules of the immunoglobulin superfamily, have been implicated in the formation of neural circuits, synaptic plasticity, and cognitive function. In this study, we sought to investigate whether differences in the steady-state levels of L1 and NCAM expression in specific brain regions could account for individual differences in learning abilities. Using adult male Wistar rats, we evaluated mRNA levels of L1, NCAM, and the NCAM180 isoform in different brain regions (hippocampus, thalamus, striatum, prefrontal and frontal cortices) immediately after submitting rats to a massed training protocol in the water maze. The results showed that untrained and trained rats exhibited similar levels of mRNA for these molecules, which supports the view that training did not influence their immediate level of expression. However, in most of the brain regions we investigated (with the exception of prefrontal and frontal cortices), L1 mRNA levels were positively correlated with the latency to find the hidden platform in the water maze task and with posttraining plasma corticosterone levels. However, no correlations were observed for total NCAM or NCAM180 mRNA in the brain regions examined in this study. Given that animals with a slower spatial acquisition curve exhibited more anxiety-like responses, including thigmotactic behavior in the water maze and increased corticosterone levels, and that recent genetic studies indicate a role for L1 in anxiety, the current findings suggest a relationship among L1, anxiety, and cognitive processes.     

L2/HNK-1 carbohydrate and protein-protein interactions mediate the homophilic binding of the neural adhesion molecule P0.

The neural adhesion molecule P0, the most abundant glycoprotein in peripheral myelin of mammals, is a member of the immunoglobulin superfamily and expresses the L2/HNK-1 and L3 oligosaccharides at a single N-glycosylation site. It acts in both homophilic and heterophilic binding mechanisms. To investigate the molecular requirements for homophilic interaction, we have used P0 from human sciatic nerve and the extracellular domain of P0 expressed in bacteria to determine binding of P0 to P0 in solid phase and bead aggregation assays. The binding of P0 to P0 could be partially inhibited in both assays by antibodies to the L2/HNK-1 epitope and by the L2/HNK-1 carbohydrate, but not by L3 antibodies or other carbohydrates. Inhibition of binding was also seen with polyclonal antibodies reacting with the protein backbone of P0. These observations indicate that both carbohydrate and protein structures are involved in the binding of P0 to P0 and that P0 acts as a presenter of and a receptor for a functionally important carbohydrate.     

Binding partners L1 cell adhesion molecule and the ezrin-radixin-moesin (ERM) proteins are involved in development and the regenerative response to injury of hippocampal and cortical neurons

Regeneration of the adult central nervous system may require recapitulation of developmental events and therefore involve the re-expression of developmentally significant proteins. We have investigated whether the L1 cell adhesion molecule, and its binding partner, the ezrin-radixin-moesin (ERM) proteins are involved in the neuronal regenerative response to injury. Hippocampal and cortical neurons were cultured in vitro on either an L1 substrate or poly-L-lysine, and ERM and other neuronal proteins were localized immunocytochemically both developmentally and following neurite transection of neurons maintained in long-term culture. Activated ERM was localized to growth cones up to 7 days in vitro but relatively mature cultures (21 days in vitro) were devoid of active ERM proteins. However, ERM proteins were localized to the growth cones of sprouting neuronal processes that formed several hours after neurite transection. In addition, the L1 substrate, relative to poly-L-lysine, resulted in significantly longer regenerative neurites, as well as larger growth cones with more filopodia. Furthermore, neurons derived from the cortex formed significantly longer post-injury neurite sprouts at 6 h post-injury than hippocampal derived neurons grown on both substrates. We have demonstrated that L1 and the ERM proteins are involved in the neuronal response to injury, and that neurons derived from the hippocampus and cortex may have different post-injury regenerative neurite sprouting abilities.     

The close homologue of the neural adhesion molecule L1 (CHL1): patterns of expression and promotion of neurite outgrowth by heterophilic interactions

The close homologue of L1 (CHL1), a member of the L1 family of neural adhesion molecules, is first expressed at times of neurite outgrowth during brain development, and is detectable in subpopulations of neurons, astrocytes, oligodendrocyte precursors and Schwann cells of the mouse and rat. Aggregation assays with CHL1-transfected cells show that CHL1 does not promote homophilic adhesion or does it mediate heterophilic adhesion with L1. CHL1 promotes neurite outgrowth by hippocampal and small cerebellar neurons in substrate-bound and soluble form. The observation that CHL1 and L1 show overlapping, but also distinct patterns of synthesis in neurons and glia, suggests differential effects of L1-like molecules on neurite outgrowth.     

“Juvenile stress” alters maturation‐related changes in expression of the neural cell adhesion molecule L1 in the limbic system: Relevance for stress‐related psychopathologies

L1 is critically involved in neural development and maturation, activity‐dependent synaptic plasticity, and learning processes. Among adult rats, chronic stress protocols that affect L1 functioning also induce impaired cognitive and neural functioning and heightened anxiety reminiscent of stress‐induced mood and anxiety disorders. Epidemiological studies indicate that childhood trauma is related predominantly to higher rates of both mood and anxiety disorders in adulthood and is associated with altered limbic system functioning. Exposing rats to stress during the juvenile period (“juvenile stress”) has comparable effects and was suggested as a model of induced predisposition for these disorders. This study examined the effects of juvenile stress on rats aversive learning and on L1 expression soon after exposure and in adulthood, both following additional exposure to acute stress and in its absence. Adult juvenile‐stressed rats exhibited enhanced cued fear conditioning, reduced novel‐setting exploration, and impaired avoidance learning. Furthermore, juvenile stress increased L1 expression in the BLA, CA1, DG, and EC both soon after the stressful experience and during adulthood. It appears that juvenile stress affects the normative maturational decrease in L1 expression. The results support previous indications that juvenile stress alters the maturation of the limbic system and further support a role for L1 regulation in the mechanisms that underlie the predisposition to exhibit mood and/or anxiety disorders in adulthood. Furthermore, the findings support the “network hypothesis,” which postulates that information‐processing problems within relevant neural networks might underlie stress‐induced mood and anxiety disorders. © 2009 Wiley‐Liss, Inc.     

CD9 of mouse brain is implicated in neurite outgrowth and cell migration in vitro and is associated with the α6/β1 integrin and the neural adhesion molecule L1

We describe here a novel monoclonal antibody (mab INTRODUCTION H6) which recognizes CD9, an integral cell surface constituent previously described in cells of the hematopoietic lineage and involved in the aggregation of platelets. Mab H6 was raised against membranes of immature mouse astrocytes and reacted with a protein of 25–27 kD in detergent extracts of adult mouse brain membranes. Sequence analysis of the N-terminal amino acids revealed an identity of 96% with CD9 from mouse kidney. CD9 was localized in the central and peripheral mouse nervous systems: in the spinal cord of 11-day-old mouse embryos, CD9 was strongly expressed in the floor and roof plates. In the adult mouse sciatic nerve, myelin sheaths were highly CD9immunoreactive. Mab H6 reacted with the cell surfaces of both glial cells and neurons in culture and inhibited migration of neuronal cell bodies, neurite fasciculation and outgrowth of astrocytic processes from cerebellar microexplants. Neurite outgrowth from isolated small cerebellar neurons was increased in the presence of mab H6 on substrate-coated laminin, but not on substrate-coated poly-L-lysine. Addition of mab H6 elicited an increase in intracellular Ca²⁺ concentration in these cells on substrate-coated laminin. Immunoprecipitates of CD9 from cultured mouse neuroblastoma N2A cells contained the α6/β1 integrin. Moreover, preparations of CD9 immunoaffinity-purified from adult mouse brain using a mab H6 column contained the neural adhesion molecule L1, but not other neural adhesion molecules. CD9 bound to L1, but not to NCAM or MAG. Both the α6/β1 integrin and L1 could be induced to coredistribute with CD9 on the surface of cultured neuroblastoma N2A cells. The combined observations suggest that CD9 can associate with L1 and α6/β1 integrin to influence neural cell interactions in vitro. © 1996 Wiley-Liss, Inc.