Sphingolipid homeostasis in the endoplasmic reticulum and beyond

Abstract

Sphingolipids are a diverse group of lipids that have essential cellular roles as structural components of membranes and as potent signaling molecules. In recent years, a detailed picture has emerged of the basic biochemistry of sphingolipids-from their initial synthesis in the endoplasmic reticulum (ER), to their elaboration into complex glycosphingolipids, to their turnover and degradation. However, our understanding of how sphingolipid metabolism is regulated in response to metabolic demand and physiologic cues remains incomplete. Here I discuss new insights into the mechanisms that ensure sphingolipid homeostasis, with an emphasis on the ER as a critical regulatory site in sphingolipid metabolism. In particular, Orm family proteins have recently emerged as key ER-localized mediators of sphingolipid homeostasis. A detailed understanding of how cells sense and control sphingolipid production promises to provide key insights into membrane function in health and disease.

Figure 1.

Structures of sphingolipids and other cellular lipids. (A–C) Representative structures of (A) sphingolipids, (B) glycerolipids, and (C) sterols. (D) Formation of sphingolipids from key building blocks, long chain bases (LCBs), and coenzyme A-linked fatty acids (FA-CoAs) that often have a very long acyl chain (VLCFA-CoA). Serine palmitoyltransferase (SPT) produces the LCB intermediate 3-keto-dihydrosphingosine, which is then reduced to yield LCBs that are used by ceramide synthase (CerS) to form ceramides. Sphingolipid structural diversity arises from (a) headgroup modifications including phosphorylation, glycosylation, or phosphocholine addition, (b) LCB hydroxylation, (c) LCB desaturation, (d) variability in the length of the N-linked acyl chain, and (e) desaturation of the N-linked acyl chain.

Figure 2.

Overview of sphingolipid metabolism. Sphingolipid synthesis begins in the ER with the condensation of serine and coenzyme A-linked fatty acids (FA-CoAs) to form long chain bases (LCBs). LCBs may be phosphorylated to produce LCB-Ps or N-acylated to form ceramides (Cer). Ceramides are then transported to the Golgi by vesicular and nonvesicular means (e.g., the transport protein CERT). In mammalian cells (right), headgroup modifications in the Golgi yield sphingomyelin (SM) and glycosphingolipids (GSL) such as glucosylceramide (GlcCer). In yeast (left), ceramide is progressively modified to form inositolphosphorylceramide (IPC), mannosyl-inositolphosphorylceramide (MIPC), and mannosyl-diinositolphosphorylceramide (M[IP]₂C). Sphingolipids then traffic to the plasma membrane, where they are most abundant. Most biosynthetic reactions are reversible, which enables turnover of mature sphingolipids and production of signaling molecules such as sphingosine (Sph) and sphingosine-1-phosphate (S1P). The lysosome is a key site of sphingolipid catabolism, generating breakdown products that can be recycled for sphingolipid synthesis. Alternatively, conversion to LCB-Ps enables terminal degradation of sphingolipids via a lyase enzyme that generates acyl aldehydes and ethanolamine phosphate. Filled arrows indicate biosynthetic steps; dashed arrows indicate degradative and recycling steps.

Figure 3.

Sphingolipid homeostasis by Orm family proteins. The SPOTS complex contains serine palmitoyltransferase (formed in yeast by Lcb1, Lcb2, and the Tsc3 accessory subunit), Orm proteins, and the phosphoinositide phosphatase Sac1. Serine palmitoyltransferase carries out the first and rate-limiting step in sphingolipid synthesis: condensation of coenzyme A-linked fatty acids (FA-CoAs) with serine to yield long chain bases (LCBs). When the supply of sphingolipids is adequate (left), SPT activity is inhibited by Orm proteins, and the SPOTS complex shows an increased degree of dimerization/oligomerization. When sphingolipid levels become too low (right), Orm proteins are inactivated by phosphorylation, relieving their inhibition of SPT and enabling a compensatory increase in de novo sphingolipid synthesis. Phosphorylation also triggers a shift to monomeric SPOTS complex organization. Note that the membrane topology of SPT and Orm proteins is not definitely known; one potential topology is shown for simplicity.

Figure 4.

Mechanisms for sensing sphingolipids. (A) Sphingolipid sensing by direct recognition of a specific sphingolipid metabolite by a soluble or transmembrane sensor protein. (B) Sphingolipid sensing by detection of changes in membrane physical properties such as propensity to form sphingolipid-dependent membrane domains. (C) Sensing of non-sphingolipid metabolites that are functionally or metabolically coupled to sphingolipid biosynthesis. (D) Multiple sphingolipid-sensing mechanisms may control Orm-mediated sphingolipid homeostasis. Orm proteins are phosphorylated by Ypk1, and Ypk1 is activated by sphingolipid depletion via the Torc2 kinase complex. Torc2-mediated activation of Ypk1 is in turn regulated by Slm proteins. Slm proteins may be sphingolipid sensors, as their movement between two membrane domains—sphingolipid-containing eisosomes and a compartment containing Torc2—is regulated by sphingolipid levels. Disruption of sphingolipid synthesis causes relocalization of Slm proteins from eisosomes to Torc2 domains, leading to Ypk1 activation. In addition to Slm-mediated sensing of complex sphingolipids in the plasma membrane, upstream metabolites such as LCBs or ceramide may also regulate Orm1/2 phosphorylation (dotted lines).