Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Sphingomyelinase, Acidic

  • Nadine Beckmann
  • Erich Gulbins
  • Katrin Anne Becker
  • Alexander Carpinteiro
Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101873


Historical Background

Acid sphingomyelinase (ASM, Enzyme Commission classification number is one of the hydrolases that catalyze the breakdown of sphingomyelin to phosphorylcholine and the signaling molecule ceramide. Its importance was first realized when ASM deficiency was identified as the cause of the lysosomal storage disorder Niemann-Pick disease types A and B. Originally, because of its acidic pH optimum in vitro (pH 4,5–5,0), ASM was assumed to be a purely lysosomal enzyme. Further studies have since revealed that ASM is not restricted to the lysosomes, but can also translocate to and function at the extracellular cell surface (Grassmé et al. 2001). This is important for ASM-mediated signaling, which typically occurs through the generation of ceramide-enriched plasma membrane platforms. This mechanism was first demonstrated for CD95-DISC formation (Grassmé et al. 2001). Since then, a large number of receptor-initiated and physical stimuli have been reported to signal via the ASM/ceramide axis, evidencing the importance of ASM-mediated signaling in a variety of contexts, including, i.e., infection, inflammation, irradiation, and oxidative stress.

Structure and Maturation of ASM

Acid sphingomyelinase is encoded by one conserved 5–6 kb gene on the p15.1-p15.4 region of chromosome 11. The human gene is referred to as SMPD1 and the murine as Smpd1. Both contain six exons and the resulting proteins (human ASM, murine Asm) have 82% amino acid similarity. So far, seven isoforms have been identified, but only acid sphingomyelinase-1 (GenBank Accession Number NM_000543.4) – containing all six exons – is catalytically active (Schuchman et al. 1991, Rhein et al. 2012).

The ASM protein encompasses a signal peptide, a saposin domain, a proline-rich linker, a catalytic metallophosphatase (phosphodiesterase) domain, and a carboxy-terminal region (Schuchman et al. 1991, Xiong et al. 2016, Gorelik et al. 2016). Recent crystallographic analysis of ASM shows that the catalytic domain adopts a calcineurin-like fold with two zinc ions, the linker wraps around the catalytic domain, and the saposin domain can either adopt a closed conformation by interacting with the C-terminal domain (inactive form of the enzyme) or establish an interface with the catalytic domain that is required for enzyme activity (Xiong et al. 2016, Gorelik et al. 2016).

The nascent polypeptide (pre-pro-ASM) is targeted to the endoplasmatic reticulum by the signal peptide, which is then cleaved. The pro-form is next subjected to glycosylation, which is required for later activity. Based on the glycosylation pattern and the resulting alternative trafficking, two forms of ASM can be distinguished: a lysosomal ASM (L-ASM) and a secretory ASM (S-ASM). L-ASM is targeted to the lysosomes by high-mannose oligosaccharides, whereas S-ASM does not receive mannose-phosphate residues. Instead S-ASM is N-glycosylated in a more complex manner and is secreted via the Golgi secretory pathway (Hurwitz et al. 1994, Schissel et al. 1998).

The alternative trafficking of the precursors also results in a different dependency on extracellular Zn2+ ions: Since ASM is a metalloenzyme, it contains several Zn2+-binding motifs and requires Zn2+ binding for activation (Schissel et al. 1998). L-ASM binds intracellular Zn2+ ions during trafficking to the lysosomes and thus requires no further addition of Zn2+. S-ASM, on the other hand, is sequestered from Zn2+ throughout the secretory pathway and is therefore dependent on extracellular sources of Zn2+.

ASM protein maturation also involves proteolytic cleavage (Hurwitz et al. 1994). The N-terminus of S-ASM begins at His62 (all amino acids are numbered according to reference sequence NM_000543.4), whereas L-ASM commences at Gly68, due to typical N-terminal processing of lysosomal proteins (Schissel et al. 1998). Similar to other lysosomal hydrolases, L-ASM is further processed at the C-terminus within the endolysosomal compartment. This step is essential for full activity, most likely by promoting the coordination of lysosomal Zn2+ ion. ASM has been suggested to be regulated by a mechanism essentially identical to the “cysteine switch” described for metalloproteinases: Involvement of the C-terminal Cys631 in the coordination of the active site Zn2+ ion keeps the enzyme in the low activity form, whereas the absence of Cys631 enhances activity. In mature L-ASM, Cys631 is absent due to C-terminal cleavage at either the canonical site or at a noncanonical site by caspase-7 (Edelmann et al. 2011). In S-ASM, Cys631 is still present and enables regulation of S-ASM: Oxidation of Cys631 has been reported to result in dimerization of ASM at this residue and subsequent full activation (Qiu et al. 2003) (Fig. 1).
Sphingomyelinase, Acidic, Fig. 1

Domains found in the ASM protein are schematically shown. Important posttranslational modifications are included. Modified amino acids are numbered according to the reference sequence (GenBank Accession Number NM_000543.4)

Activation and Translocation of ASM

A multitude of stimuli are known to activate ASM, including, for instance, pathogens, certain drugs, or soluble molecules (please refer to Table 1 for a more extensive list and references). At present the exact regulatory mechanisms underlying ASM activation are only partially understood. For most ASM activating stimuli, the activation of ASM has been reported to coincide with the formation of ceramide-enriched platforms in the plasma membrane. They serve as the origin of the downstream signaling (addressed in more detail in the next paragraph). Since these signaling platforms are located in the plasma membrane, whereas ASM is present in the endolysosomal compartment (L-ASM) or secretory vesicles (S-ASM), ASM activation has to coincide with the enzymes translocation to the plasma membrane in these cases. L-ASM is exposed to the extracellular leaflet of the cell membrane by the fusion of secretory lysosomes with the plasma membrane, a process that requires the t-SNARE protein syntaxin-4 (Perrotta et al. 2010). S-ASM is assumed to bind and act on the cell surface upon secretion.
Sphingomyelinase, Acidic, Table 1

ASM-activating stimuli and ceramide-interacting proteins

ASM-activating stimuli

Ceramide-interacting proteins



Cluster of differentiation molecules

Soluble molecules











•CD253 (TRAIL)

•IL-1 receptor

•Platelet-activating factor




Listeria monocytogenes

•Measles virus

Mycobacterium avium

Neisseria gonorrhoeae

Pseudomonas aeruginosa


Salmonella typhimurium

•Sindbis virus

•Staphylococcus aureus

•Cu2+ treatment

•Heat damage

•Ischemia-reperfusion injury

•Oxidative stress

•Oxygen radicals

•UV light


•Calcium release-activated calcium channels (CRAC)

•Cathepsin D

•Ceramide-activated protein serine-threonine phosphatases (CAPP)

•LC3B as a ceramide-interacting protein Kv1.3 potassium channel

•Phospholipase A2

•Protein kinase C isoforms

Table 1 provides an overview of stimuli that have been reported to signal via the ASM/ceramide axis. A similar compilation with references to the respective primary sources can be found in Beckmann et al. (2014)

ASM-Mediated Signaling Through Ceramide-Enriched Platforms

A common signaling principle upon ASM activation and translocation to the plasma membrane is the formation of specific membrane domains, termed ceramide-enriched membrane platforms. Biological membranes predominantly contain three classes of lipids: sterols, glycerolipids, and sphingolipids. Sphingolipids are lipids with a sphingoid base backbone (one of five unsaturated amino alcohols, e.g., sphingosine) linked to a fatty acid through an amide bond. Whereas the hydrophobic region is variable due to varying chain lengths, saturation status, and degree of hydroxylation of the fatty acid, the hydrophilic part is even more variable, as it can contain hydroxyl groups, phosphates, and/or sugar residues. Sphingolipids interact with each other and with cholesterol molecules by hydrophilic forces between the polar head groups and by hydrophobic van der Waal interactions between the hydrophobic tails. These interactions result in distinct membrane domains with a liquid-ordered status.

Ceramide, the product of the enzymatic reaction catalyzed by ASM, is the simplest sphingolipid: It is formed only by the sphingoid base backbone (sphingosine) and a fatty acid. Upon activation and translocation of ASM, ceramide is generated in the plasma membrane. Ceramide molecules self-associate by the abovementioned hydrophobic and hydrophilic forces and thereby separate themselves from other lipids in the membrane. This results in the formation of small, ceramide-enriched membrane domains. However, the changes to the membrane do not end with these domains, as they have a tendency to spontaneously fuse into even larger ceramide-enriched membrane domains, termed platforms. The formation of these platforms leads to a reorganization of receptors and signaling molecules within the membrane. Thus, the various stimuli that induce ceramide-enriched membrane platforms can have very different cellular effects. Therefore, signaling via ceramide-enriched membrane platforms is very distinct from the classical second messenger signaling, in which the respective messenger acts stoichiometrically on its specific target molecule. (For a more detailed review on the biology of ceramide-enriched membrane domains and the signaling through them, the reader is referred to Grassmé et al. 2007).

The reorganization of receptors and signaling molecules is mediated by the ceramide-enriched domain acting as a hydrophobic platform or by direct attracting or repelling interactions of a given protein with ceramide moieties. For this, not only the protein’s composition is important but also its structure: For instance, ligand binding may alter the structure of the transmembrane domain, resulting in sorting of a receptor into the platforms only after ligand binding. Via these biophysical and energetic mechanisms, proteins can be specifically sorted, and receptors and their associated signaling molecules can be trapped and clustered in the ceramide-enriched membrane domains. The clustering of receptors in these domains enables the generation of a strong, localized signal. It also facilitates the transmission of this signal across the membrane and into the cell by resulting in a high receptor density, the spatial focusing of activated receptors with intracellular molecules, the exclusion of inhibitory molecules, and/or the transactivation of other enzymes. This signaling mechanism was first described for CD95 (Grassmé et al. 2001) and has since been reported for a number of receptor-initiated as well as physical signals (please refer to Table 1) (Fig. 2).
Sphingomyelinase, Acidic, Fig. 2

Sphingolipids are a major component of biological membranes. Sphingolipids interact with each other via hydrophobic and hydrophilic forces, resulting in the formation of distinct membrane microdomains. Sphingomyelin, the most abundant sphingolipid in biological membranes, is hydrolyzed to ceramide upon activation of ASM. The generation of ceramide changes the biophysical properties of the membrane, resulting in the formation of small ceramide-enriched membrane domains. These small domains can further fuse to large ceramide-enriched membrane platforms, which serve to reorganize receptors and signaling molecules. This facilitates the transmission of the respective receptor’s signal, e.g., by resulting in high receptor density, transactivation of other enzymes, and/or exclusion of inhibitory molecules

Next to the reorganization of the “signalosome,” ceramide can also have classical second messenger functions by direct interaction and regulation of different signaling proteins, i.e., cathepsin D, cytosolic phospholipase A2, kinase suppressor of Ras, ceramide-activated protein serine-threonine phosphatases, and protein kinase C isoforms (reviewed in Grassmé et al. 2007). Additionally, ceramide has been reported to regulate intracellular potassium and calcium levels by interfering with Kv1.3 and CRAC (for more details please also refer to Grassmé et al. 2007). Moreover, ceramides can form channels in the outer mitochondrial membrane themselves and may thereby contribute to apoptosis induction (Siskind and Colombini 2000).

Inhibition of ASM-Mediated Signaling

Several ASM inhibitory mechanisms have been described. For one, catalytically inactive alternative splice variants of ASM that lack the catalytic and/or carboxy-terminal domains (transcripts 5, 6, and 7) have been suggested to function as dominant-negative proteins (Rhein et al. 2012). Additionally, inositol phosphates have been reported to inhibit ASM activity, although the physiological significance of this is yet unknown (Kölzer et al. 2003).

The translocation of L-ASM to the plasma membrane can be inhibited by nitric oxide: Nitric oxide activates G-kinase, which phosphorylates syntaxin-4, thus inducing its proteasomal degradation. As syntaxin-4 is required for the fusion of the ASM-containing secretory lysosomes with the plasma membrane, L-ASM signaling is thus blocked (Perrotta et al. 2010).

Pharmacologically, a number of compounds have been identified as functional inhibitors of ASM (FIASMA) by inducing its detachment form the inner leaflet of the lysosomal membrane, resulting in inactivation and subsequent proteolytic degradation (Kornhuber et al. 2010). The fact that many of these compounds, i.e., amitriptyline, are clinically used as antidepressants has led to an investigation into the role of ASM in major depression. Since, it has been concluded that the ASM/ceramide system mediates the effects of these antidepressant drugs. Thus, a novel mechanism of action for tricyclic antidepressants has been put forward to replace the previously questioned hypothesis that these drugs function by inhibiting monoamine uptake (Gulbins et al. 2013).

ASM in Disease

Next to its pathophysiological role in major depression, the ASM/ceramide system has also been reported to be involved in cardiovascular, metabolic, hepatic, inflammatory, and infectious diseases, as well as in cancer and tumor metastasis (please see Table 2 for examples, and refer to Beckmann et al. 2014 or Kornhuber et al. 2015 for more extensive reviews). The signaling concepts involved differ depending on the context. For instance, in infectious diseases, ASM-mediated formation of ceramide-enriched platforms has been shown to be critical for cellular invasion by pathogens (Grassmé et al. 1997), whereas ceramide mediates inflammation, cell death, and infection susceptibility in cystic fibrosis (Teichgräber et al. 2008). The extensive studies on the role of ASM/ceramide in cystic fibrosis have resulted in an ongoing phase III clinical study to confirm the effectiveness of ASM inhibition by amitriptyline. Another emerging concept is the ASM-mediated clustering of adhesion molecules in ceramide-enriched plasma membrane domains. This was first reported to be crucial for tumor cell adhesion and hematogenous metastasis (Carpinteiro et al. 2015), but is now also studied in lymphocyte adhesion and transmigration and may thus be important in inflammatory and autoimmune diseases as well.
Sphingomyelinase, Acidic, Table 2

ASM/ceramide system in disease


Infectious Diseases

Metabolic Diseases

 Kawasaki disease

 Bacterial infections


 Multiple sclerosis

  L. monocytogenes

 Diabetic retinopathy

 Systemic sclerosis

  M. avium

 Obesity-induced kidney damage


  N. gonorrhoeae



  P. aeruginosa

Neurological Disorders

 Irradiation and radiotherapy

  S. aureus

 Alzheimer disease


  S. typhimurium

 Major depression

Cardiovascular diseases

 Endotoxic shock syndrome

 Parkinson’s disease



Skin conditions

 Cardiomyocyte apoptosis (cardioplegia/reperfusion)

 Virus infections

 Atopic dermatitis

 Thrombus formation

  Measles virus


Genetic disorders


Respiratory diseases

 Sickle-cell disease

  Sindbis virus

 Acute lung injury

 Niemann-Pick disease A and B

Inflammatory diseases

Aspiration Pneumonia

 Wilson disease/liver cirrhosis


 Cystic fibrosis

 Hemophagocytic lymphohistiocytosis

 Hypoxemic respiratory failure

 Hepatic fibrosis

 Lung fibrosis

 Inflammatory bowel disease


 Mast cell function/allergies

Table 2 provides an overview of diseases in which ASM/ceramide signaling have been implicated. For more details about the pathological roles of ASM/ceramide signaling, the reader is referred to Beckmann et al. (2014) and Kornhuber et al. (2015)


ASM is a hydrolase that presents in a lysosomal and secreted form. Protein maturation requires glycosylation, proteolytic processing, and Zn2+ binding. Activity is induced by a variety of receptor-initiated and physical stimuli and is typically accompanied by translocation of the enzyme to the plasma membrane. The ASM-catalyzed hydrolysis of sphingomyelin leads to the generation of ceramide. Ceramide can act directly as a second messenger, and/or ceramide generation alters the membrane microenvironment, resulting in the formation of ceramide-enriched membrane platforms. These platforms reorganize signaling molecules within the membrane and thus enable, facilitate, and/or enhance their respective signal. Despite solid evidence that this signaling mechanism plays an important role in cell death and senescence, stress signaling, infection, and inflammation, the exact molecular mechanisms underlying ASM activation and –translocation and ceramide-mediated signalosome–reorganization must still be determined. Another future perspective is the expansion of the clinical targeting of ASM/ceramide signaling, as a growing number of in vivo and in vitro studies report dysregulated ASM/ceramide signaling in disease.


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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  • Nadine Beckmann
    • 1
  • Erich Gulbins
    • 1
    • 2
  • Katrin Anne Becker
    • 1
  • Alexander Carpinteiro
    • 1
  1. 1.Department of Molecular BiologyUniversity of Duisburg-EssenEssenGermany
  2. 2.Department of SurgeryUniversity of CincinnatiCincinnatiUSA