Study on Neutral Ceramidase involved in Sphingolipid Signaling

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Neutral ceramidases (CDases) are widely distributed from bacteria to humans. The bacteria and drosophila enzymes are exclusively secretory while the zebrafish and mammalian enzymes are typical type II integral membrane proteins which occasionally detach from cells after the processing of their NH2-terminal anchor. We found that the difference in the localization of the enzyme is entirely due to the presence of a mucin-like domain located downstream of the NH2-terminal signal/anchor sequence of the enzymes from vertebrates but not bacteria and invertebrates. Overexpression of mouse neutral CDase on the cell-surface or in the extracellular milieu but not in the ER significantly increased the intracellular concentration of sphingosine as well as sphingosine 1-phosphate (SIP) when the amount of cell-surface ceramide was increased by treatment of cells with bacterial sphingomyelinase. In contrast, knockdown of the CDase reduced the amount of both sphingolipid metabolites. The enzyme seems to participate in the generation of S1P in the vascular system in which cell-and lipoprotein-bound sphingomyelin are likely to be a source of ceramide. The knockdown of the neutral CDase impaired both S1P-mediated angiogenesis and the development of the heart during zebrafish embryogenesis, leading a lack of circulation of blood cells. Collectively, the enzyme is involved in the metabolism of ceramide at the plasma membranes and in the extracellular milieu, which could regulate the generation of SIP and S1P-mediated signaling through the production of sphingosine.