Differential regulation of sphingomyelin synthesis and catabolism in oligodendrocytes and neurons

Abstract

Neurons (both primary cultures of 3‐day rat hippocampal neurons and embryonic chick neurons) rapidly converted exogenous NBD‐sphingomyelin (SM) to NBD‐Cer but only slowly converted NBD‐Cer to NBD‐SM. This was confirmed by demonstrating low in vitro sphingomyelin synthase (SMS) and high sphingomyelinase (SMase) activity in neurons. Similar results were observed in a human neuroblastoma cell line (LA‐N‐5). In contrast, primary cultures of 3‐day‐old rat oligodendrocytes only slowly converted NBD‐SM to NBD‐Cer but rapidly converted NBD‐Cer to NBD‐SM. This difference was confirmed by high in vitro SMS and low SMase activity in neonatal rat oligodendrocytes. Similar results were observed in a human oligodendroglioma cell line. Mass‐Spectrometric analyses confirmed that neurons had a low SM/Cer ratio of (1.5 : 1) whereas oligodendroglia had a high SM/Cer ratio (9 : 1). Differences were also confirmed by [³H]palmitate‐labeling of ceramide, which was higher in neurons compared with oligodendrocytes. Stable transfection of human oligodendroglioma cells with neutral SMase, which enhanced the conversion of NBD‐SM to NBD‐Cer and increased cell death, whereas transfection with SMS1 or SMS2 enhanced conversion of NBD‐Cer to NBD‐SM and was somewhat protective against cell death. Thus, SMS rather than SMases may be more important for sphingolipid homeostasis in oligodendrocytes, whereas the reverse may be true for neurons.