Polysialylated Neural Cell Adhesion Molecule Promotes Remodeling and Formation of Hippocampal Synapses

Abstract

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

L1 and NCAM adhesion molecules as signaling coreceptors in neuronal migration and process outgrowth

“…Premature removal of PSA from NCAM by endoglycosidase N treatment promotes synaptogenesis between GABAergic interneuron subpopulations and pyramidal cells in the visual cortex and regulates the onset of the critical period of visual plasticity in adolescence [32]. On the other hand, surface levels of PSA-NCAM are upregulated in adult hippocampal neurons undergoing activity-dependent plasticity [33]. Thus, PSA modification of NCAM at different places and times may regulate molecular interactions leading to either synaptogenesis or synaptic remodeling.…”

“…* of special interest: references 3,6,18,20,26,33,37 ** of exceptional interest as follows: ** 10. The authors used BAC transgenic reporter mice expressing green fluorescent protein in an interneuron subpopulation to show that a subcellular gradient of the Neurofascin 186 isoform along the dendritic-somal-axon initial segment axis of cerebellar Purkinje cells is formed by direct interaction with an ankyrin B isoform, and is responsible for axon targeting of basket interneurons to the axon initial segment.…”

Immobilized Pool of NCAM180 in the Postsynaptic Membrane Is Homeostatically Replenished by the Flux of NCAM180 from Extrasynaptic Regions

“…The neural cell adhesion molecule NCAM and, in particular, its largest NCAM180 isoform, is one of such components (14,25). Delivery of adhesion molecules to nascent synapses during development or to activated synapses during induction of long term potentiation was described as quantal, i.e.…”

“…Long lasting stability of synapses (months or years), however, suggests that synaptic pools of cell adhesion molecules (protein life time hours or days) have to be also homeostatically and continuously replenished over prolonged periods of time without compromising synapse stability and composition. Mechanisms of such replenishment remain poorly understood.Among synaptic adhesion molecules, the neural cell adhesion molecule (NCAM), 2 a member of the immunoglobulin superfamily of adhesion molecules, was shown to be involved in mechanical stabilization of interneuronal contacts and their transformation into synapses (12,14). In mice, deficiency in NCAM results in impaired long term potentiation, increased aggression and anxiety, deficits in spatial learning, and exploratory behavior (15-21).…”

“…Among synaptic adhesion molecules, the neural cell adhesion molecule (NCAM), 2 a member of the immunoglobulin superfamily of adhesion molecules, was shown to be involved in mechanical stabilization of interneuronal contacts and their transformation into synapses (12,14). In mice, deficiency in NCAM results in impaired long term potentiation, increased aggression and anxiety, deficits in spatial learning, and exploratory behavior (15)(16)(17)(18)(19)(20)(21).…”