Intact transmembrane isoforms of the neural cell adhesion molecule are released from the plasma membrane

Abstract

Three soluble neural cell adhesion molecule (NCAM) polypeptide classes of M(r) values 190,000 (NCAM-s1), 135,000 (NCAM-s2) and 115,000-110,000 (NCAM-s3) have been demonstrated in rat brain and cerebrospinal fluid [Krog, Olsen, Dalseg, Roth and Bock (1992) J. Neurochem. 59, 838-847]. NCAM-s3 is known to arise from released glycosylphosphatidylinositol (GPI)-linked NCAM [He, Finne and Goridis (1987) J. Cell. Biol. 105, 2489-2500] as well as from extracellularly cleaved transmembrane NCAM isoforms [Nybroe, Linnemann and Bock (1989) J. Neurochem. 53, 1372-1378]. In this study the origin of NCAM-s1 and NCAM-s2 and the function of soluble NCAM forms were investigated. It was shown that all three soluble forms could be released from brain membranes with M(r) values identical to the three major membrane-associated forms: the large transmembrane 190,000-M(r) form (NCAM-A), the smaller transmembrane 135,000-M(r) form (NCAM-B) and the GPI-anchored 115,000-110,000-M(r) form (NCAM-C). A polyclonal antibody, directed against transmembrane and cytoplasmic epitopes common to NCAM-A and NCAM-B, was shown to react with NCAM-s1 and NCAM-s2. Furthermore, NCAM-B was shown to be shed in a presumably intact soluble form from membranes of cells transfected with this isoform. Thus, NCAM-s1 and NCAM-s2 probably represent intact released transmembrane NCAM-A and NCAM-B. The soluble transmembrane forms are likely to exist in vivo, as NCAM-s1 and NCAM-s2 were readily demonstrated in cerebrospinal fluid. By density-gradient centrifugation it was shown that shed transmembrane NCAM-B was present in fractions of high, as well as low, density, indicating that a fraction of the shed NCAM is associated with minor plasma membrane fragments. Finally, it was shown that isolated soluble NCAM inhibited cell binding to an immobilized NCAM substratum, attributing a pivotal role to soluble NCAM in vivo as a modulator of NCAM-mediated cell behaviour.

Cholinergic agonists stimulate secretion of soluble full-length amyloid precursor protein in neuroendocrine cells.

“…In contrast, we were able to detect solAPPcyt in human cerebrospinal fluid (CSF) without prior binding to heparin (Fig. 5B) (30), showed that functional forms of the neural cell adhesion molecule (N-CAM) with intact transmembrane sequence, are present in CSF and plasma (31). Similarly, a form of the insulin receptor, a type I transmembrane protein, was secreted with its transmembrane sequence from cultured cells and was also detected in human plasma (32).…”

Detection, Purification and Identification of An Endogenous Inhibitor of l-Dopa Decarboxylase Activity from Human Placenta

“…These results indicate the existence of a possible enzymatically-based mechanism, which leads to the release of the hydrophobic inhibitor molecule into the cytoplasm. Such a mechanism has been demonstrated for the enzymatic release from the membranes of various type I transmembrane proteins, such as, the precursor of the epidermic growth factor [35], neural cell adhesion molecule [36], insulin receptor [37] and the amyloid precursor [38], all of which have been detected intact in soluble forms.…”

Expression of the neural cell adhesion molecule in astrocytic tumors

“…7,8 The same group also found that transfection of 140kD isoform, but not GPI-anchored 120kD isoform, a polypeptide cleaved from transmembrane 180 and/or 140kD isoforms, etc., 23,24 and some diversity in form, down-regulated secretion of matrix metalloproteinases 1 and 9. 28 Regarding proliferative activity, it the positions of the bands could be attributed to such a minor difference in the polypeptide chain and/or has been reported that the addition of purified rat NCAM inhibited astrocyte proliferation in primary posttranslational modifications, such as the addition of asparagine-linked saccharides.…”