Removal of C6 astrocytoma cell surface molecules with phosphatidylinositol phospholipase C: effect on regulation of neural cell adhesion molecule isoforms.

Different membrane-associated isoforms of the neural cell adhesion molecules have been described. One of them, N-CAM120, has been shown to be anchored to the membranes by a complex glycan-phosphatidylinositol group and to be releasable, under soluble form, by the bacterial enzyme phosphatidylinositol-phospholipase C. We used the C6 rat astrocytoma cell line expressing both N-CAM120 and the transmembrane isoform N-CAM140 as a model system. We investigated whether artificial depletion of cell membrane N-CAM120 influences the synthesis and the messenger RNA transcript levels of the isoforms of the neural cell adhesion molecules. Our results showed an increase in the rate of N-CAM120 protein synthesis, whereas the expression of N-CAM140 decreased. Additionally, perturbations in the levels of the 6.7-kb messenger RNA encoding for N-CAM140 were observed, whereas the 2.7-kb transcript encoding for N-CAM120 remained stable. Examination of the time course for the reexpression of N-CAM120 showed that control levels were recovered after 24 h. We provide evidence that N-CAM120 spontaneously released in the culture medium is not incorporated into the extracellular matrix; however, its concentration is important because, if the medium was changed, cells rapidly released a new pool.