Behavioral and neurobiological changes in mice deficient in synaptic cell adhesion molecules NEGR1 and NCAM2

Interactions between neurons are mediated by cell adhesion molecules (CAMs). NCAM2 and NEGR1 are two CAMs, which accumulate in synapses. Abnormal expression of these synaptic CAMs is associated with behavioral abnormalities and neurodevelopmental disorders in humans. The effects caused by deficiency in these CAMs in the brain remain, however, unclear. To better understand human conditions associated with the loss of NCAM2 and NEGR1, in this thesis, we analyzed the behavior of NCAM2 and NEGR1 deficient mice and performed immunohistochemical analyses of the brain to study the underlying mechanisms of these effects. We found that NCAM2 deficiency in mice leads to the impairment in short-term memory, mild cognitive deficit, and causes an increase in olfactory acuity and repetitive and self-grooming behaviors. Our immunohistochemical studies showed shortening of the infrapyramidal bundle of the mossy fibers in the hippocampus of the NCAM2 knockout mice suggesting axonal guidance defects. Similar axon guidance defects and changes in behavior were also observed in mice deficient in BACE1, an enzyme involved in processing of NCAM2. Further analysis showed changes in distribution and a reduction in levels of BACE1 protein in the hippocampus in NCAM2 knockout mice. Additionally, cell culture studies showed that NCAM2 knockout neurons were characterized by altered organization of the Golgi body organelles and lower levels of the microtubule-associated proteins tau and CRMP2, which are required for transporting protein cargoes and maintaining the cytoskeleton. We observed spatial and short-term memory impairment, hyposmia, and a reduction in motivation towards highly palatable food rewards in NEGR1 knockout mice. These mice had lower densities of synapses in the hippocampus, particularly of those formed by mossy fibers. The overall size of the suprapyramidal bundle of mossy fibers was reduced in NEGR1 knockout mice. These effects were accompanied by the deposition of alpha-synuclein aggregates in the hippocampus. Similarly, the density of inhibitory synapses was reduced in the CA3 pyramidal cell layer of the hippocampus and arcuate nucleus of hypothalamus, which is involved in regulating the feeding behavior. In conclusion, we found that NCAM2 and NEGR1 deficiency causes synaptic changes in the brain, which are accompanied by abnormalities in learning, suggesting that both proteins are involved in regulating neuronal development and plasticity. Our data indicate that NCAM2 and NEGR1 deficiency can directly contribute to neurodevelopmental and other types of disorders in people with variations in the genes coding for NCAM2 and NEGR1