Functional analysis of acid and neutral sphingomyelinases in vitro and in vivo

The molecular cloning and the elucidation of the gene structures of the aci d (aSMase) and a neutral sphingomyelinases (nSMase) of mouse and human faci litated the structural and functional analysis of these enzymes responsible for the catabolism of sphingomyelin present ubiquitously in the membrane l ipid bilayer of mammalian cells. The protein and enzymic properties of the glycoprotein aSMase and of a non-glycosylated nSMase residing in the membra nes of the endoplasmic reticulum have been analysed in the native as well a s in the recombinant shingomyelinases. Important insight was gained from ge ne targeting experiments in which an aSMase deficient mouse line was genera ted which mimics the neurovisceral form of the human Niemann-Pick disease. The availability of the cloned aSMase and nSMases discovered so far led to a genetic approach to the verification of the concept that these enzymes in the 'sphingomelin cycle' are responsible for the generation of ceramide re garded as a lipophilic second messenger in the intracellular signal cascade s activated by e.g. TNF-alpha, Fas ligand or cellular stress. All the avail able evidence derived from the aSMase deficient mouse line and several cell lines overexpressing aSMase and nSMase questions a role of ceramide releas ed by the mammalian sphingomyelinases known so far in intracellular signal transduction. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.