Many ceramides

Abstract

Intensive research over the past 2 decades has implicated ceramide in the regulation of several cell responses. However, emerging evidence points to dramatic complexities in ceramide metabolism and structure that defy the prevailing unifying hypothesis on ceramide function that is based on the understanding of ceramide as a single entity. Here, we develop the concept that "ceramide" constitutes a family of closely related molecules, subject to metabolism by >28 enzymes and with >200 structurally distinct mammalian ceramides distinguished by specific structural modifications. These ceramides are synthesized in a combinatorial fashion with distinct enzymes responsible for the specific modifications. These multiple pathways of ceramide generation led to the hypothesis that individual ceramide molecular species are regulated by specific biochemical pathways in distinct subcellular compartments and execute distinct functions. In this minireview, we describe the "many ceramides" paradigm, along with the rationale, supporting evidence, and implications for our understanding of bioactive sphingolipids and approaches for unraveling these pathways.

FIGURE 1.

Basic blueprint of sphingolipid metabolism. Shown are the de novo pathway of ceramide formation, the production of complex sphingolipids from ceramide, the degradation of ceramide to sphingosine, the formation of S1P from sphingosine, and the clearance of S1P through the lyase reaction. CERK, ceramide kinase; GCS, glucosylceramide synthase; GBA, acid glucocerebrosidase; SK, sphingosine kinase; SMS, sphingomyelin synthase; SPP, S1P phosphatase.

FIGURE 2.

Compartmentalization of sphingolipid metabolism. Ceramide (Cer) is synthesized de novo in the ER and then is transported either via ceramide transfer protein (CERT) to the Golgi, where it serves as a substrate for the synthesis of sphingomyelin (SM), or is transported by vesicular traffic for the synthesis of glucosylceramide (gluCer) (97). Sphingomyelin and glycosphingolipids (GlycoSL) are, in turn, transported to the plasma membrane through vesicular trafficking, and they also undergo vesicular trafficking in the endosomal system and clearance through lysosomal degradation. Ceramide can also be transformed to galactosylceramide (GalCer) in the ER, a process enriched in neural tissues. SLs, sphingolipids; SMS, sphingomyelin synthase; glySL, glycosphingolipids; dhSph, dihydrosphingosine; aCDase, acid ceramidase; ma-nSMase, mitochondrial associated SMase; aSMase, acid SMase; SK, sphingosine kinase; Sph, sphingosine; CDase, ceramidase; dhCer, dihydroceramide; Mito, mitochondria; Nuc, nucleus.

FIGURE 3.

Many ceramides. A, complexity of ceramide structure. Ceramide is a family of closely related molecules. Distinct enzymes control the introduction of OH on the acyl chain (1), resulting in two variants, an OH on the sphingoid base or a double bond in the sphingoid base (2; total of three variants); the chain length and desaturation of the acyl chain (3; at least 10 variants); the length of the sphingoid base (4; at least three major variants); and the OH at the 1-position (5; two variants). Because these are independent modifications (at each of the sites), one can calculate the upper limit of possible ceramides as the product of these modifications (i.e. 360), but not all these ceramides necessarily exist (e.g. some modifications may preclude others because of enzyme specificities). On the other hand, any new discovery of additional variations would enlarge this number. B, partial representation of the metabolic domains of distinct ceramides. Each box represents a structurally distinct ceramide and the sphingolipids (SLS) derived from that particular ceramide. Box 1 illustrates the formation of C₁₆-dihydroceramide with a C₁₈-sphingoid backbone (18C₁₆dhCer). Likewise, each of the other boxes illustrates the combinatorial action of unique enzymes/subunits to effect the formation of unique ceramides and subsequent sphingolipids. Given the structural uniqueness of each ceramide, we recommend the shorthand designations shown, where the initial prefix designates the length of the sphingoid base and the number of double bonds in it (e.g. 18:1), followed by C_x indicating the length of the acyl chain, followed by an indication of acyl chain modifications (e.g. 2′-OH for hydroxylation at the 2-position of the fatty acid). These nine boxes are representative of all individual ceramide species, of which >100–150 can be detected using current LC-MS/MS technology. SPTLC, subunits of SPT; FA2H, fatty acid 2-hydroxylase; Pal, palmitoyl-CoA; Myr, myristoyl-CoA; Sph, sphingosine; C1P, ceramide 1-phosphate.

FIGURE 4.

Defining a lipid species. Three levels of “definition” must be considered as shown in the figure.