Functional role of the polysialylated neural cell adhesion molecule in fear conditioning of mice (Mus musculus L., 1758)

Considerable progress has been made in uncovering the numerous functions of the neural cell adhesion molecule (NCAM) and its associated carbohydrate, polysialic acid (PSA), in ontogenetic development and in synaptic plasticity, learning and memory in the adult. Perturbation of NCAM functions by ablation of NCAM or by disruption of NCAM-mediated interactions in vitro or in vivo impaired synaptic plasticity in the hippocampus, fear conditioning, induced amnesia in a passive avoidance task, and caused spatial memory deficits. The mechanisms underlying action of NCAM and PSA at different phases of learning and memory consolidation are, however, unknown. To investigate the contributions of PSA versus the extracellular domain of the NCAM glycoprotein backbone on learning we acutely injected NCAM, PSA-NCAM and PSA into the dorsal hippocampus of wild-type mice at different phases of auditory fear conditioning. As an internal control that injections into the hippocampus would not have unspecific effects on hippocampus-independent forms of learning, we used a fear conditioning paradigm in which an animal was simultaneously subjected to hippocampus-independent learning of tone and hippocampusdependent learning of context. Only PSA, either attached to NCAM or alone, but not NCAM, injected before training, impaired formation of hippocampus-dependent contextual memory, as measured by the freezing response. Consolidation of contextual memory was affected only by PSA-NCAM when injected during its late, but not its early phases. None of the tested compounds disturbed extrahippocampal cued memory. Mice lacking the polysialyltransferase (ST8SialV/PST) responsible for attachment of PSA to NCAM in adulthood showed a mild deficit only in hippocampal contextual learning, when compared to NCAM deficient mice that were disturbed in both contextual and cued memories. Formation of contextual memory in NCAM deficient mice could be restored by pre-training injection of PSA-NCAM into the hippocampus, suggesting that a mechanism for PSA-NCAM function in learning is not mediated by modulation of NCAM-NCAM homophilic interactions but rather through its heterophilic partners. These in vivo results are supported by in vitro data showing the inhibitory effects of PSA on long-term potentiation (LTP) in the CA1 region of the hippocampus in wild-type mice and restorative effects of PSA on Oleg Senkov PhD thesis 8 Abstract impaired CA1 LTP in NCAM deficient hippocampal slices. In summary, our data support the view that polysialylated NCAM is involved in both formation and late consolidation of contextual memory, and supports synaptic plasticity. By using tamoxifen-inducible Cre-recombinase LoxP system which allowed us to induce temporally controlled ablation of the NCAM gene in the brain of adult mice – we show that NCAM is required for formation and/or retention of hippocampus-dependent contextual memory, whereas amygdala-contingent cued memory is less dependent on inducible ablation of NCAM. These results are intriguing since constitutive prenatal NCAM ablation causes both hippocampaland amygdala-dependent deficits in fear conditioning, and thus raise the question whether NCAM is critical for development of the amygdala, or the levels of NCAM expression in the amygdala after inducible ablation remain sufficiently high to mediate its functions. Strikingly, severe cognitive deficits in learning and memory of contextual and cued information in NCAM deficient mice could be rescued by facilitation of activity of the NMDA subtype glutamate receptors via systemic administration of D-cycloserine, as an agonist of the glycine site of these receptors. These results correspond to data from ongoing in vitro studies demonstrating a rescue of CA1 LTP in NCAM or PSAdeficient hippocampal slices by D-cycloserine. Thus, we suggest that a loss of NCAM and/or PSA leads to a deficit in NMDA receptor function during induction of synaptic plasticity and fear learning. Since lower concentrations of D-cycloserine were sufficient to restore levels of hippocampus-dependent contextual memory as compared to amygdala-based cued fear memory, there may be interesting differences in regulation of NMDA receptors by PSA-NCAM in the hippocampus and amygdala. The significance of these findings is underscored by studies uncovering a genetic link between schizophrenia and mutations in the polysialyltransferase ST8SiaII/STX, and a deficit in PSA and overproduction of soluble NCAM in schizophrenic brains.