View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector FEBS Letters 584 (2010) 1895–1900 journal homepage: www.FEBSLetters.org Review Acid sphingomyelinase, cell membranes and human disease: Lessons from Niemann–Pick disease Edward H. Schuchman * Department of Genetics and Genomic Sciences, Mount Sinai School of Medicine, Icahn Medical Institute, Floor 14 Room 14-20A, 1425 Madison Avenue, New York, NY 10029, USA article info abstract Article history: Acid sphingomyelinase (ASM) plays an important role in normal membrane turnover through the Received 29 October 2009 hydrolysis of sphingomyelin, and is one of the key enzymes responsible for the production of cera- Accepted 24 November 2009 mide. ASM activity is deficient in the genetic disorder Types A and B Niemann–Pick disease (NPD). Available online 26 November 2009 ASM knockout (ASMKO) mice were originally constructed to study this disorder, and numerous defects in ceramide-related signaling have been shown. Studies in these mice have further sug- Edited by Sandro Sonnino gested that ASM may be involved in the pathogenesis of several common diseases through the reor- ganization of membrane microdomains. This review will focus on the role of ASM in membrane Keywords: biology, with a specific emphasis on what a rare genetic disorder (NPD) has taught us about more Sphingomyelin Ceramide common events. Lysosome Ó 2009 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved. Enzyme Membrane microdomain 1. Introduction Numerous reviews are available on the function of ASM in cell signaling, as well as on its involvement in specific human diseases The fluid mosaic model of the cell membrane, first proposed in (e.g. [5–7]). The purpose of this review is to summarize informa- the early 1970s, suggested that membranes exist in a disorder sta- tion regarding the role of ASM in membrane biology. In order to tus without significant selectivity [1]. This concept rapidly estab- provide a biological context, a systems approach will be used. lished itself as dogma, although in recent years a body of Much of this information comes from studies using ASMKO mice, literature has shown that the cell membrane is, in fact, composed which were originally created as a model of the human genetic dis- of small ‘‘microdomains” that exist in a liquid-ordered phase [2]. order, Types A and B Niemann–Pick disease (NPD) [8]. A brief back- These domains are static within the membrane, and can coalesce ground on ASM and NPD is provided below, followed by a and reorganize in response to various stimuli. Several laboratories summary of studies using the ASMKO mice that reveal the function also have shown that sphingolipids and cholesterol associate with of ASM on cell membranes. Despite the fact that NPD was de- these microdomains, and that these associations are integral to scribed nearly a century ago and ASM was identified over 40 years membrane function. The sphingolipid and cholesterol-enriched ago, this literature has mostly emerged during the past decade. It is membrane microdomains have been referred to as lipid ‘‘rafts” therefore an evolving field, but one that has already integrated di- [3,4]. Despite a growing literature, the concept of membrane verse scientific disciplines, ranging from physicians, biophysicists, microdomains has remained controversial, principally because lipid biochemists and signal transduction biologists, and identified data demonstrating the existence of these domains in vivo is lim- ASM as a target for numerous, common diseases. ited. As summarized below, studies of one sphingolipid hydrolase, acid sphingomyelinase (ASM), specifically those using ASM knock- 1.1. Acid sphingomyelinase: historical perspective out mice (ASMKO), have provided some of the strongest evidence to date supporting the concept of membrane microdomains in Acid sphingomyelinase (ASM; EC 3.1.4.12) is one member of a vivo. They also have highlighted the important role of this enzyme family of enzymes that catalyzes the breakdown of sphingomyelin in normal cell function and the pathogenesis of many common by cleavage of the phosphorylcholine linkage, thereby producing diseases. ceramide. The existence of such a ‘‘sphingomyelin cleaving en- zyme” was first demonstrated in 1938 by the pioneering work of Thannhauser, Reichel and colleagues [9]. During the ensuing 25 * Fax: +1 212 849 2447. years, several similar enzymatic activities were identified that dif- E-mail address: [email protected] fered mostly in their tissue distribution and pH optimum. The first 0014-5793/$36.00 Ó 2009 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.febslet.2009.11.083 1896 E.H. Schuchman / FEBS Letters 584 (2010) 1895–1900 clear description of a sphingomyelin hydrolase that worked opti- totic effects of diverse stimuli, including but not limited to Fas/ mally at acidic pH (i.e., ASM) was made by Gatt and colleagues in CD95 [36], ischemia [37], radiation [35,38], chemotherapy [39], 1963 [10]. In addition to ASM, at least three other sphingomyelin- and TNFalpha [40]. This effect of ASM has been attributed to local ases have been described in mammalian cells that vary in their pH changes in sphingomyelin, ceramide and cholesterol content, and optimum and co-factor dependence [11–13]. While these enzymes ultimately reorganization of membrane microdomains. and an existing de novo synthetic pathway are alternative mecha- Another significant advancement in ASM research was the nisms for ceramide generation, each with their own implications large-scale purification of the recombinant enzyme from the media on cell signaling and disease, this review will specifically focus of genetically engineered Chinese hamster ovary cells [41]. With on the contributions of ASM. the availability of recombinant ASM, good antibodies against the By the late 1960s, researchers reported that the deficiency of enzyme also soon became available, stimulating new avenues of ASM was responsible for the rare, recessively inherited lysosomal research. Importantly, several investigators showed by immunocy- storage disorder, Niemann–Pick disease (Types A and B NPD; see tochemistry that when cells were exposed to various forms of below), stimulating intensive efforts to purify and characterize it stress, the location of this protein changed from primarily lyso- [14–18]. Early investigations identified ASM as a glycoprotein, somal/endosomal to the cell surface (e.g. Ref. [42]). This observa- and because the pH optimum of the enzyme in vitro was between tion marked a watershed in ASM biology, as it is at the outer 4.5 and 5.0, coupled with the fact that the majority of storage leaflet of the cell membrane where ASM initiates cell signaling, material in NPD patients was found in lysosomes and/or late endo- and thus exerts its impact on the pathogenesis of a diverse group somes, the enzyme was classified as a lysosomal protein [19]. The of diseases. cDNA and gene encoding ASM (designated SMPD1) were cloned in The precise mechanism by which ASM, which normally resides 1989 and 1992, respectively [20,21]. They predicted a 629 amino within lysosomes, is translocated to the cell surface remains un- acid polypeptide that included a 46 amino acid signal peptide re- known. Recently, Zeidan et al. showed that phosphorylation of a gion and two in-frame ATG initiation sites. Mutation analysis in specific serine residue on ASM (S508) by PKCdelta is required for NPD patients showed that both ATG initiation sites were functional its activation in response to UV irradiation and movement to the in vivo [22]. membrane [43,44]. These investigators suggested that the phos- SDS–PAGE analysis of ASM purified from various sources re- phorylation occurs within the lysosomes, although it is also possi- vealed an estimated molecular weight of 72 kDa; enzymatic de- ble that a cytosolic pool of ASM serves as the substrate for glycosylation reduced the molecular weight to 60 kDa [14,23]. PKCdelta, or that phosphorylation occurs within some other com- Processing studies performed in COS-1 cells further showed that partment at or near the cell surface. Deciphering the precise traf- ASM was synthesized as a 75-kDa ‘‘prepro” protein that was traf- ficking and processing of ASM during cell signaling is likely to be ficked to lysosomes. Interestingly, in these same studies, a 57 kDa a fruitful area for future research. secreted form of ASM also was identified [24]. Subsequent charac- terization studies revealed that 5 of 6 N-glycosylation sites of the 1.2. Types A and B NPD: brief overview enzyme were occupied [25], and that the oligosaccharide side chains contained mannose-6-phosphate residues, typical of lyso- As noted above, until recently, interest into the biology of ASM somal proteins [26]. In addition, the disulfide bond structure of was due to its role in the genetic disorder, Types A and B NPD. Both ASM was characterized [27], and it was found the terminal cys- forms of this disorder are caused by recessive mutations in the teine at amino acid residue 629 was the only cysteine not involved SMPD1 gene encoding ASM. Type A NPD is the infantile form of in intra-molecular disulfide linkages. In fact, this terminal cysteine ASM deficiency, characterized by a rapidly progressive neurode- residue must be removed to obtain full ASM activity [28], and it is generative course that leads to death by age 2–3. In contrast, Type thought that the retention of this residue in the mature protein B NPD is the later-onset form in which patients exhibit little or no may lead to the formation of inactive, higher molecular weight neurological symptoms, but may have severe and progressive vis- aggregates. ceral organ abnormalities, including hepatosplenomegaly, pul- Around this time, Tabas and co-workers identified a zinc- monary insufficiency and cardiovascular disease [45]. The dependant, secreted form of ASM that also was encoded by the different clinical presentations of Types A and B NPD are likely SMPD1 gene [29].