Elsevier

Experimental Neurology

Volume 172, Issue 1, November 2001, Pages 29-46
Experimental Neurology

Regular Article
Characterization of Human Cleaved N-CAM and Association with Schizophrenia

https://doi.org/10.1006/exnr.2001.7790Get rights and content

Abstract

The neural cell adhesion molecule (N-CAM) is a cell recognition molecule involved in cellular migration, synaptic plasticity, and CNS development. A 105- 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 partially 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 signal sequence. Analysis of trypsin-digested cN-CAM fragments by matrix-assisted laser desorption ionization on a time-of-flight mass spectrometer (MALDI-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. Immunological identification with four specific N-CAM antisera directed toward 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 immunoreactive band, indicating partial glycosylation of cN-CAM. Membranous particles from cytosolic brain extract containing cN-CAM were obtained by ultracentrifugation; however, CSF contained few such particles. cN-CAM and synaptophysin were colocalized on these particles. Both cN-CAM and N-CAM 180 were present in synaptosomal preparations of human brain. Following incubation of synaptosomes or brain tissue without protease inhibitors, N-CAM 180 was degraded and cN-CAM was increased. A cN-CAM-like band was present in human fetal neuronal cultures, but not in fetal astrocyte cultures. Thus, cN-CAM represents a protease- and neuraminidase-susceptible fragment possibly derived by proteolytic cleavage of N-CAM 180. An enlargement in ventricular volume in a group of adult patients with schizophrenia over a 2-year interval was 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 ventricular enlargement; thus, the release of N-CAM fragments may be part of the pathogenic mechanism of schizophrenia in vulnerable brain regions such as the hippocampus and prefrontal cortex. Alternatively, the increases in cN-CAM in schizophrenia may be a reflection of a more general abnormality in the regulation of proteolysis or of extracellular matrix stability.

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      In addition, previous studies have reported the possible dysregulation of the NCAM system by examining cerebrospinal fluid (CSF), serum, and postmortem brain regions of schizophrenia patients. For example, early studies have found the increased NCAM in CSF (Lyons et al., 1988; Poltorak et al., 1995, 1997; Vawter et al., 2000, 2001; Vawter, Hemperly, Freed and Garver, 1998b) and postmortem brain tissues (Vawter et al., 1998a) of schizophrenia patients than healthy controls. Moreover, abnormal concentrations of CSF NCAM are associated with enlarged ventricular (Vawter et al., 2001) and clinical symptoms (Lyons et al., 1988; Vawter et al., 2000).

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      Schizophrenia is characterized by alterations to synaptic circuitry, where the spine density of cortical pyramidal neurons is markedly lower in schizophrenic patients relative to healthy controls (Garey et al., 1998; Glantz and Lewis, 2000; Glausier and Lewis, 2013). Interestingly, levels of cleaved NCAM are increased in the prefrontal cortex and hippocampus of human schizophrenia patients (Vawter et al., 2001). However, the effect or function of these cleaved NCAM fragments in the human brain is still unknown.

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