Neural cell adhesion molecule in human serum. Increased levels in dementia of the Alzheimer type

Abstract

Memory impairment is a process associated with alterations in neuronal plasticity, synapses formation, and stabilization. As the neural cell adhesion molecule (NCAM) plays a key role in synaptic bond stabilization, we analyzed the usefulness of soluble NCAM isoforms in the diagnosis of patients with dementia of the Alzheimer type (DAT).

NCAM was measured in the sera of 70 control subjects and 43 DAT patients (with different severity of cognitive impairment, GDS), employing Western blot and densitometric quantification. LMW-NCAM bands (100–130 kDa) decreased significantly with age independently of sex. DAT patients presented values of LMW-NCAM and HMW-NCAM significantly higher than healthy controls of similar age (higher than 130 kDa). Only LMW-NCAM was associated with GDS. Our results suggest that NCAM could be involved in the pathogenesis of DAT disorder and that serum NCAM levels could be useful as differential diagnostic markers of the disease.

Introduction

Alzheimer's disease is the most prevalent cause of dementia worldwide and affects 5–10% of the population aged over 65 years old. Still, the diagnosis of the pathology is only defined after autopsy or brain biopsy, by observation of histological manifestations, found predominantly in the hippocampus amygdalus and the cerebral cortex, such as neurofibrillar degeneration, intraneuronal fibrillary tangles, extracellular senile plaques, and vascular deposits of amyloid (Merched et al., 1977). Because of the difficulty of its diagnosis and to avoid implicit assumptions about the pathophysiology of this heterogeneous syndrome, the disease is called dementia of the Alzheimer type (DAT) (Schellenberg et al., 1992). Therefore, the ante mortem diagnosis of the disease is based on clinical history, physical and neurological examination, together with the exclusion of other causes of dementia by neuroimaging. Biochemical markers to assist diagnosis and help the treatment would be clinically helpful but currently are unavailable.

Glycoproteins involved in the adhesion of neural cells are believed to regulate the stabilization of synaptic junctions, neurite outgrowth, wound repair, functional plasticity, and control release of neurotransmitter biosynthetic enzymes. These molecules belong to one of three major structural adhesion families: the immunoglobulin superfamily, cadherins, and integrins, which are expressed at high levels on neurons and non-neuronal cells (Hampel et al., 1996). Recent evidence suggests that members of the immunoglobulin superfamily are involved in functional neuronal plasticity in the adult animal, such as that associated with learning Fields and Itoh, 1996, Rose, 1995, and that both short- and long-term memory requires some adhesion molecules to allow synapses formation (Ranheim et al., 1996). One of such options is the glycoprotein NCAM, which mediates homophilic binding between neighboring cells such as neurons, astrocytes, and muscle cells and heterophilic interactions between cells and extracellular matrix components (Rønn et al., 2000).

A single gene encodes NCAM, but alternative splicing of the pre-mRNA produces at least three major isoforms, NCAM 120, NCAM 140, and NCAM 180 Ricard et al., 2000, Rønn et al., 2000. While NCAM 120 lacks a cytoplasmatic domain and is attached to the membrane by a covalent-bound phosphatidylinositol anchor, NCAM 140 and 180 present transmembrane and intracytoplasmatic domains. In the vertebrate nervous system, NCAM is the dominant carrier of an unusual carbohydrate consisting of long homopolymers of sialic acid (PSA), as isoforms of 200–250 kDa (PSA-NCAM). PSA-NCAM is abundantly expressed in the central nervous system during development and in early postnatal brain Hampel et al., 1996, Ni Dhuill et al., 1999, Seki and Arai, 1993. In adult brain, PSA-NCAM is down-regulated but confined to a few restricted areas, such as the olfactory system and the mossy fiber system in the hippocampal formation, characterized by a high level of structural remodeling. It was proposed that the down-regulation of PSA-NCAM accompanies a change in NCAM function from a plasticity-promoting to a stability-promoting molecule. According to this hypothesis, the presence of long PSA chains on NCAM inhibits cell adhesion therefore allowing structural remodeling to occur (Rønn et al., 2000).

Besides, it was found that NCAM of high molecular weight is up-regulated in astrocytes exposed to elevated hydrostatic pressure in vitro, in neurodegenerative disease and after neural trauma Cotman et al., 1998, Ricard et al., 2000, Roche et al., 1997.

Nearly all biochemical analysis on NCAM has been performed on membrane-bound forms of the molecule. However, soluble forms of NCAM have been observed in conditioned media from cultured neural or muscle cells Bock et al., 1987, Thomaidou et al., 2001 and in different body fluids Lynch et al., 1997, Poltorak et al., 1995, although its role in different pathological process is unknown.

In the present work, we study the pattern of NCAM isoforms found in human serum. We demonstrate that circulating NCAM is significantly increased in DAT patients, suggesting that the measurement of serum NCAM could be useful to improve the diagnosis of neurological diseases with cognitive deficit.