Characterization of human cleaved N-CAM and association with schizophrenia

The neural cell adhesion molecule (N-CAM) is a cell recognition molecule in volved in cellular migration, synaptic plasticity, and CNS development. A 1 05- to 115-kDa isoform. of N-CAM (cleaved N-CAM or cN-CAM) is increased in schizophrenia in hippocampus, prefrontal cortex, and CSF. We purified and p artially characterized cN-CAM, a putative novel isoform, and confirmed that the first 9 amino acids were identical to exon 1 of N-CAM, without the sig nal sequence. Analysis of trypsin-digested cN-CAM fragments by matrix-assis ted laser desorption ionization on a time-of-flight mass spectrometer (MALD I-TOF) yielded peptides that could be identified as being derived from the first 548 amino acid residues of the expected N-CAM amino acid sequence. Im munological identification with four specific N-CAM antisera directed towar d cytoplasmic, secreted, variable alternative spliced exon, or GPI epitopes failed to indicate other known splice variants. Neuraminidase treatment of cN-CAM produced a minor alteration resulting in a faster migrating immunor eactive band, indicating partial glycosylation of cN-CAM. Membranous partic les from cytosolic brain extract containing cN-CAM were obtained by ultrace ntrifugation; however, CSF contained few such particles. cN-CAM and synapto physin were colocalized on these particles. Both cN-CAM and N-CAM 180 were present in synaptosomal preparations of human brain. Following incubation o f synaptosomes or brain tissue without protease inhibitors, N-CAM 180 was d egraded and cN-CAM was increased. A cN-CAM-like band was present in human f etal neuronal cultures, but not in fetal astrocyte cultures. Thus, cN-CAM r epresents a protease- and neuraminidase-susceptible fragment possibly deriv ed by proteolytic cleavage of N-CAM 180. An enlargement in ventricular volu me in a group of adult patients with schizophrenia over a 2-year interval w as found to be correlated with CSF cN-CAM levels as measured at the time of the initial MRI scan (r=0.53, P=0.01). cN-CAM is associated with ventricul ar enlargement; thus, the release of N-CAM fragments may be part of the pat hogenic mechanism of schizophrenia in vulnerable brain regions such as the hippocampus and prefrontal cortex. Alternatively, the increases in cN-CAM i n schizophrenia may be a reflection of a more general abnormality in the re gulation of proteolysis or of extracellular matrix stability. (C) 2001 Acad emic Press.