ABSTRACT
Although carbohydrates have been implicated in cell interactions in the nervous system, the molecular bases of their functions have remained largely obscure. Here, we show that promotion or inhibition of neurite outgrowth of cerebellar or dorsal root ganglion neurons, respectively, induced by the mucin-type adhesion molecule CD24 depends on alpha2,3-linked sialic acid and Lewis(x) present on glia-specific CD24 glycoforms. Alpha2,3-sialyl residues of CD24 bind to a structural motif in the first fibronectin type III domain of the adhesion molecule L1. Following the observation that the adhesion molecules TAG-1 and Contactin show sequence homologies with fucose-specific lectins, we obtained evidence that TAG-1 and Contactin mediate Lewis(x)-dependent CD24-induced effects on neurite outgrowth. Thus, L1, TAG-1, and Contactin function as lectin-like neuronal receptors. Their cis interactions with neighboring adhesion molecules, e.g., Caspr1 and Caspr2, and with their triggered signal transduction pathways elicit cell type-specific promotion or inhibition of neurite outgrowth induced by glial CD24 in a glycan-dependent trans interaction.
Subject(s)
CD24 Antigen/metabolism , Cell Adhesion Molecules, Neuronal/physiology , Lewis X Antigen/pharmacology , Neurites/drug effects , Neurons/cytology , Sialic Acids/pharmacology , Animals , Animals, Newborn , Binding Sites/drug effects , CD24 Antigen/genetics , Cell Adhesion Molecules, Neuronal/deficiency , Cells, Cultured , Cerebellum/cytology , Contactin 2 , Contactins , Cricetinae , Cricetulus , Dose-Response Relationship, Drug , Enzyme-Linked Immunosorbent Assay/methods , Ganglia, Spinal/cytology , Glycosylation/drug effects , Immunoprecipitation/methods , Leukocyte L1 Antigen Complex/metabolism , Locomotion/genetics , Mice , Mice, Inbred C57BL , Mice, Knockout , Neurites/physiology , Neurons/classification , Neurons/drug effects , Neurons/physiology , Peptides/pharmacology , Protein Binding/drug effects , Recovery of Function/genetics , Spinal Cord Injuries/genetics , Spinal Cord Injuries/pathology , Spinal Cord Injuries/physiopathology , Transfection/methodsABSTRACT
The cell adhesion molecule CD24 is a highly glycosylated glycoprotein that plays important roles in the central nervous system, the immune system and in tumor biology. Since CD24 comprises only a short protein core of approximately 30 amino acids and low conservation among species, it has been proposed that the functions of CD24 are mediated by its glycosylation pattern. Our present study provides evidence that interaction of CD24 with the cell adhesion molecule L1 is mediated by O-linked glycans carrying alpha2,3-linked sialic acid. Furthermore, de-N-glycosylated CD24 was shown to promote or inhibit neurite outgrowth of cerebellar neurons or dorsal root ganglion neurons, respectively, to the same extent as untreated CD24. Therefore, this study is focused on the structural elucidation of the chemically released, permethylated CD24 O-glycans by electrospray ionization ion trap mass spectrometry. Our analyses revealed the occurrence of a diverse mixture of mucin-type and O-mannosyl glycans carrying, in part, functionally relevant epitopes, such as 3-linked sialic acid, disialyl motifs, Le(X), sialyl-Le(X) or HNK-1 units. Hence, our data provide the basis for further studies on the contribution of carbohydrate determinants to CD24-mediated biological activities.
