Carbohydrate-carrying molecules in the nervous system have important roles during development, regeneration and synaptic plasticity. Carbohydrates mediate interactions between recognition molecules, thereby contributing to the formation of a complex molecular meshwork at the cell surface and in the extracellular matrix. The tremendous structural diversity of glycan chains allows for immense combinatorial possibilities that might underlie the fine-tuning of cell-cell and cell-matrix interactions.

Extracellular matrix molecules--including chondroitin sulfate proteoglycans, hyaluronan, and tenascin-R--are enriched in perineuronal nets (PNs) associated with subsets of neurons in the brain and spinal cord. In the present study, we show that similar cell type-dependent extracellular matrix aggregates are formed in dissociated cell cultures prepared from early postnatal mouse hippocampus. Starting from the 5th day in culture, accumulations of lattice-like extracellular structures labeled with Wisteria floribunda agglutinin were detected at the cell surface of parvalbumin-expressing interneurons, which developed after 2-3 weeks into conspicuous PNs localized around synaptic contacts at somata and proximal dendrites, as well as around axon initial segments. Physiological recording and intracellular labeling of PN-expressing neurons revealed that these are large fast-spiking interneurons with morphological characteristics of basket cells. To study mechanisms of activity-dependent formation of PNs, we performed pharmacological analysis and found that blockade of action potentials, transmitter release, Ca2+ permeable AMPA subtype of glutamate receptors or L-type Ca2+ voltage-gated channels strongly decreased the extracellular accumulation of PN components in cultured neurons. Thus, we suggest that Ca2+ influx via AMPA receptors and L-type channels is necessary for activity-dependent formation of PNs. To study functions of chondroitin sulfate-rich PNs, we treated cultures with chondroitinase ABC that resulted in a prominent reduction of several major PN components. Removal of PNs did not affect the number and distribution of perisomatic GABAergic contacts but increased the excitability of interneurons in cultures, implicating the extracellular matrix of PNs in regulation of interneuronal activity.

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Background:Various glycan microarrays are currently widely used, but systematic cross-comparisons are lacking. Results: We compare and contrast two sialoglycan microarrays using a variety of sialic acid-binding proteins. Conclusion: Diverse array formats can strengthen the quality of information, but differences between arrays may be observed. Significance: Glycan arrays with similar glycan structures cannot be simply assumed to give similar results.

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Background:The neural cell adhesion molecule L1 is important in the developing and adult nervous system. Results: L1 stimulation leads to sumoylation and proteolytic processing of L1 and translocation of a sumoylated transmembrane fragment to the nucleus. Conclusion: Sumoylation and nuclear localization of the L1 fragment are required for L1-dependent functions. Significance: Unraveling the molecular mechanisms underlying L1-activated cellular responses helps understanding L1-linked disorders.

Among the recognition molecules that determine a neuron's interaction with other cells, L1 and CD24 have been suggested to cooperate with each other in neurite outgrowth and signal transduction. Here we report that binding of CD24 to L1 depends on ␣2,3-sialic acid on CD24, which determines the CD24 induced and cell typespecific promotion or inhibition of neurite outgrowth. Using knockout mutants, we could show that the CD24-induced effects on neurite outgrowth are mediated via L1, and not GPI-linked CD24, by trans-interaction of L1 with sialylated CD24. This glycoform is excluded together with L1 from raft microdomains, suggesting that molecular compartmentation in the surface membrane could play a role in signal transduction.Path-finding of growth cones and neurite outgrowth toward targets are important events in the developing and regenerating nervous system and in synaptic remodeling during learning and memory. Axonal guidance depends on molecules at the cell surface and in the extracellular matrix. The different and often changing combinations of molecularly associated recognition molecules at the cell surface are important determinants of the ways by which the cell surface communicates with the cell interior, where cell surface signals are integrated to influence cell behavior.Two recognition molecules, L1 of the immunoglobulin superfamily and CD24, a highly glycosylated mucin type glycoprotein, interact with each other functionally (1, 2). L1 is a 200-kDa homophilic and heterophilic adhesion molecule expressed by many postmitotic neurons in the central nervous system (for reviews, see Refs. 3 and 4). It is one of the most potent promoters of neurite outgrowth in vitro known so far. Mutants of L1 in mice and men strongly underscore its importance during embryonic development in vivo (3,5,6).CD24 is linked to the surface membrane by a glycosyl phosphatidylinositol anchor and is, therefore, unable to directly interact with cytoplasmic proteins. It is also known as heatstable antigen or nectadrin with a peptide core of only 30 amino acids (for references, see Ref. 1). Similar to L1, it is highly expressed by neurons (7,8). The apparent molecular weight of CD24 varies considerably among cell types and also within each cell type, depending on its developmental stage due to differences in glycosylation pattern (for references, see Refs. 1 and 7). These observations suggest that post-translational modifications of CD24 play an important functional role. CD24 acts as a co-stimulator for various physiological functions. In the nervous system, CD24 has been reported to interact with L1 to stimulate cell adhesion and to increase intracellular Ca 2ϩ levels (1, 2). Interestingly, CD24 has been shown to inhibit neurite outgrowth of neonatal retinal ganglion cells and dorsal root ganglion neurons in culture (8) by yet unknown signal transduction mechanisms.Based on these observations on the functional interplay and molecular association between L1 and CD24, we decided to further study their functional interdependence. Her...

The transmembrane and multidomain neural adhesion molecule L1 plays important functional roles in the developing and adult nervous system. L1 is proteolytically processed at two distinct sites within the extracellular domain, leading to the generation of different fragments. In this report, we present evidence that the proprotein convertase PC5A is the protease that cleaves L1 in the third fibronectin type III domain, whereas the proprotein convertases furin, PC1, PC2, PACE4, and PC7 are not effective in cleaving L1. Analysis of mutations revealed Arg 845 to be the site of cleavage generating the N-terminal 140-kDa fragment. This fragment was present in the hippocampus, which expresses PC5A, but was not detectable in the cerebellum, which does not express PC5A. The 140-kDa L1 fragment was found to be tightly associated with the full-length 200-kDa L1 molecule. The complex dissociated from the membrane upon cleavage by a protease acting at a more membrane-proximal site of full-length L1. This proteolytic cleavage was inhibited by the metalloprotease inhibitor GM 6001 and enhanced by a calmodulin inhibitor. L1-dependent neurite outgrowth of cerebellar neurons was inhibited by GM 6001, suggesting that proteolytic processing of L1 by a metalloprotease is involved in neurite outgrowth.Proteolytic processing of cell-surface proteins is of prime importance for regulating the functional properties of these proteins (for reviews, see Refs. 1-5). Cleavage of recognition molecules at the cell surface has been implicated in neuronal migration, neurite outgrowth, and synaptic plasticity (6 -13). Among the neural adhesion molecules, L1 has been shown to undergo proteolytic cleavage, which has been suggested to be involved in several functions of this molecule.L1 is a member of the immunoglobulin superfamily consisting of immunoglobulin-like domains and fibronectin type III repeats (for reviews, see Refs. 14 and 15). In the central nervous system, L1 is expressed only by post-mitotic neurons and mainly on non-myelinated axons, whereas in the peripheral nervous system, it is expressed by neurons as well as by non-myelinating Schwann cells. L1 is also expressed by nonneural cells, including normal and transformed cells of hematopoietic and epithelial origin. L1 is involved in neuronal migration, neurite outgrowth, and myelination (for review, see Ref. 14) as well as axon guidance, fasciculation, and regeneration (16,17). Furthermore, it enhances cell survival (18) and synaptic plasticity (19). The importance of L1 in nervous system development is underscored by the abnormal phenotypes of L1 mutations in humans and mice (for review, see Ref. 20). L1 engages in homophilic and heterophilic cell interactions (for reviews, see Refs. 14 and 15) Heterophilic binding partners are the RGD-binding integrins and TAG-1/ axonin-1, F3/F11/contactin, NCAM, CD9, CD24, and phosphacan (Ref. 21 and references therein). These interactions are likely to depend on the presentation of the L1 molecule either as a membrane-bound form or as a proteolytic...