Regulation of cellular and systemic sphingolipid homeostasis

Abstract

One hundred and fifty years ago, Johann Thudichum described sphingolipids as unusual "Sphinx-like" lipids from the brain. Today, we know that thousands of sphingolipid molecules mediate many essential functions in embryonic development and normal physiology. In addition, sphingolipid metabolism and signalling pathways are dysregulated in a wide range of pathologies, and therapeutic agents that target sphingolipids are now used to treat several human diseases. However, our understanding of sphingolipid regulation at cellular and organismal levels and their functions in developmental, physiological and pathological settings is rudimentary. In this Review, we discuss recent advances in sphingolipid pathways in different organelles, how secreted sphingolipid mediators modulate physiology and disease, progress in sphingolipid-targeted therapeutic and diagnostic research, and the trans-cellular sphingolipid metabolic networks between microbiota and mammals. Advances in sphingolipid biology have led to a deeper understanding of mammalian physiology and may lead to progress in the management of many diseases.

Fig. 1:. Overview of sphingolipid structures and metabolic pathways.

Sphingolipid metabolism is separated into several pathways that branch from the central metabolite ceramide. a, The de novo synthesis pathway is initiated by the action of the serine palmitoyltransferase (SPT) complex, which uses palmitoyl-CoA and serine to form 3-ketodihydrosphingosine. 3-Ketodihydrosphingosine reductase (KDSR) reduces 3-ketodihydrosphingosine to form dihydrosphingosine. Ceramide synthases (CERSs) further convert dihydrosphingosine into dihydroceramide by N-acylation. Finally, a Δ4 double bond is introduced by the dihydroceramide sphingolipid Δ4-desaturase DES1 (DEGS1), forming ceramide, the central hub of sphingolipid metabolism. b, Ceramide can be hydrolysed by ceramidase (CDase), releasing sphingosine and free fatty acids. In the salvage pathway, sphingosine can be hexadecenal is converted to palmitoyl-CoA, and ethanolamine phosphate is utilized to form phospholipids. d, Ceramide can be converted into sphingomyelin by sphingomyelin synthases (SMSs), in which a phosphocholine (PC) head group is added to the 1-hydroxyl group of ceramide. Degradation of sphingomyelin is mediated by various classes of sphingomyelinases (SMases), generating ceramide to be utilized in other pathways. e, Ceramide can be used to synthesize glycosphingolipids by the sequential addition of sugar groups to the 1-hydroxyl group of ceramide. As an example, glucosylceramide is synthesized by UDP-glucose ceramide glucosyltransferase (UGCG). Glucosylceramide provides the core structure for more complex glycosphingolipids containing additional sugar groups mediated by a family of glycosyltransferases. Sugar moieties of glycosphingolipids are mostly removed in the lysosome by various glycosidases, releasing ceramide. f, Ceramide can be phosphorylated by ceramide kinase (CERK) to form ceramide-1-phosphate, which acts as a second messenger and regulates many biological functions. The reverse reaction is catalysed by PLPPs to generate ceramide. g, Alternatively, O-acylation of ceramide can be catalysed by diacylglycerol O-acyltransferases (DGATs), forming 1-O-acylceramides, which are incorporated into lipid droplets.

Fig. 2:. Intracellular sphingolipid biosynthesis and distribution in organelles.

Sphingolipid metabolism is compartmentalized in cells, where each organelle has a unique sphingolipid signature due to the presence of specific enzymes and regulatory proteins. Details for individual organelles are found in the text. a, The de novo biosynthesis pathway occurs in the endoplasmic reticulum (ER), forming ceramide as the central hub for other sphingolipid pathways. ORM1-like proteins (ORMDLs) are sensors of ceramide levels and negatively regulate the activity of the serine palmitoyltransferase (SPT) complex, which catalyses the first step of ceramide synthesis using serine and palmitoyl-CoA as substrates (see Fig. 1 and Box 1 for details). b, Ceramide is rapidly transported to the Golgi through both transporter-dependent and vesicular-dependent mechanisms. Ceramide transfer protein (CERT) transports ceramide to the trans-Golgi complex. Here, sphingomyelin synthase 1 (SMS1) transfers the phosphocholine group from phosphatidylcholines to ceramide, generating sphingomyelin and diacylglycerol (DAG). DAG, in turn, activates protein kinase D (PKD), which serves as a negative feedback loop by inhibiting CERT activity. This mechanism ensures proper regulation of sphingolipid flux from the ER to the Golgi. Ceramide at the ER–cis-Golgi contact site is shuttled via vesicular transport and converted to glucosylceramide by UDP-glucose ceramide glucosyltransferase (UGCG) in the cytosolic side of the Golgi. It is then transported to the trans-Golgi by transporter protein phosphoinositol 4-phosphate adapter protein 2 (FAPP2; also known as PLEKHA8), for synthesis of lactosylceramide by β-1,4-galactosyltransferase 5 (B4GALT5) or B4GALT6, allowing subsequent formation of various glycosphingolipids such as gangliosides and globosides. c, Sphingomyelin and glycosphingolipids reach the plasma membrane through vesicular transport and become enriched in the outer leaflet of the plasma membrane and clustered with other lipids such as cholesterol. Sphingomyelin in the plasma membrane can be degraded by neutral sphingomyelinase (nSMase) to generate ceramide, which is subsequently used to generate sphingosine-1-phosphate (S1P). d, Sphingomyelin and glycosphingolipids are transported to lysosomes by endocytic and/or pinocytic pathways and are metabolized into ceramide by the action of acid sphingomyelinase (aSMase) and glycosidases, respectively. Ceramide is further degraded by acid ceramidase (aCDase), releasing sphingosine (Sph) and free fatty acid. Sph can exit the lysosome via transporters, such as NPC intracellular cholesterol transporter 1 (NPC1), StAR-related lipid transfer protein 3 (STARD3) and protein spinster homologue 1 (SPNS1), at ER–lysosome contact sites. Lysosomal Sph can also flip to the outer leaflet of the lysosome and be phosphorylated by sphingosine kinase 1 (SPHK1) to form S1P. S1P transfer from the lysosome to the ER may take place at contact sites between these two organelles. At the ER, Sph can be recycled to form ceramide (catalysed by ceramide synthases (CERSs)) or converted to S1P by SPHKs. S1P can then be degraded by S1P lyase 1 to hexadecenal and ethanolamine phosphate in the degradation pathway or dephosphorylated into Sph by S1P phosphatase 1 and 2 (SGPP1 and SGPP2). e, Some sphingolipid metabolic enzymes have been identified in association with mitochondria, including SMase, CDase, CERS and SPHK2. Although sphingolipid metabolic enzymes involved in the de novo synthesis pathway, including SPT complex and dihydroceramide sphingolipid Δ4-desaturase DES1 (DEGS1), are also identified at ER–mitochondria contact sites, it remains unclear whether de novo synthesis of sphingolipid occurs in mitochondria itself or only at mitochondria–ER contact sites, supplying sphingolipids to the mitochondria. f, At ER–lipid droplet contact sites, ceramide can be O-acylated by diacylglycerol O-acyltransferases (DGATs) in coordination with long-chain fatty acid CoA ligase 5 (ACSL5) for fatty acid synthesis, forming acylceramide, which is stored in lipid droplets.

Fig. 3:. Systemic regulation of sphingolipids.

Dietary sphingolipids, mostly enriched in sphingomyelin (SM), are broken down into the website functhis may affect how tions. Please view our privacy policy for further details on ceramide in the gut by alkhow we aline sphingprocess omyoyur inelinasformae (tioAlkn. Di-SMassmisse). Subsequently, ceramide is metabolized by neutral ceramidase (ASAH2) at the brush border of enterocytes, releasing sphingosine (Sph) and free fatty acids (FAs). Extracellular Sph and ceramide in the intestine can impact microbiome function in the gut. Conversely, microbiota-derived sphingolipids can regulate host gut health by regulating enterocytes and immune cells in the gut. Sph and free FAs can also be absorbed by enterocytes. The majority of Sph is converted to sphingosine-1-phosphate (S1P) in the endoplasmic reticulum (ER) and enters the degradation pathway, generating palmitic acid (PA), which can be incorporated into triglycerides (TGs) along with other FAs in the ER. Only a small portion of Sph is converted to ceramide. Both ceramide and TGs can then be transferred onto nascent apolipoprotein B48 (APOB48) by microsomal transfer protein (MTP) during pre-chylomicron formation. Pre-chylomicrons are transported via pre-chylomicron transport vesicles (PCTV) and reach the Golgi complex, where ceramide-derived sphingomyelin can be further incorporated. Subsequently, sphingolipids are secreted along with chylomicrons from the enterocytes via exocytosis of secretory vesicles. In the circulation, sphingolipids are present within various lipoproteins, including chylomicron, very-low-density lipoprotein (VLDL), low-density lipoprotein (LDL) and high-density lipoprotein (HDL). Whereas sphingomyelins are present in all lipoproteins, ceramide content is high in VLDL and LDL, and S1P is enriched in HDL. Sphingolipid transfer between lipoproteins can occur through spontaneous exchange and phospholipid transporter (PLTP)-dependent mechanisms. In addition to lipoproteins, many cells generate extracellular vesicles (EVs), which are enriched with sphingolipids such as sphingomyelin and glycosphingolipids. Sphingolipids in EVs can regulate EV biosynthesis and are involved in intercellular communication events that regulate pathophysiological conditions. In many disease conditions, such as cardiovascular, metabolic and neurodegenerative diseases, changes in circulatory sphingolipid profiles can serve as biomarkers or may be targeted for therapeutic purposes. CERS, ceramide synthase; SGPL1, S1P lyase 1; SPHKs, sphingosine kinases.

Fig. 4:. S1P metabolism and its therapeutic strategies.

a, Circulatory sphingosine-1-phosphate (S1P) is mostly provided by erythrocytes, endothelial cells and platelets. In the endothelium, sphingomyelin (SM) in the plasma membrane can be converted by neutral sphingomyelinase (nSMase), generating ceramide (Cer). Ceramide can be converted back to sphingomyelin by sphingomyelin synthase 2 (SMS2) or be degraded by ceramidase (CDase) into sphingosine (Sph). Sph can be converted back to Cer by ceramide synthases (CERSs) or flip to the inner leaflet of the plasma membrane. By the action of sphingosine kinase 1 (SPHK1), S1P can be formed and released extracellularly by the S1P transporter protein spinster homologue 2 (SPNS2). Erythrocytes and platelets utilize exogenous Sph to form S1P by sphingosine kinase 1 (SPHK1) or SPHK2 and export it through the S1P transporter major facilitator superfamily domain-containing protein 2B (MFSD2B). b, Once released, S1P is bound to chaperones such as apolipoprotein M (APOM) and albumin, whereby the affinity to APOM on high-density lipoprotein (HDL) is higher than the affinity to albumin. The S1P chaperone system stabilizes S1P in the circulation to facilitate its action in autocrine and paracrine manners. Our group developed engineered APOM conjugated with the Fc domain of IgG (APOM-Fc) to restore depleted S1P in circulation, which is seen in diabetes and cardiovascular diseases. This approach may be useful in these pathological conditions. c, S1P binds to its Gprotein-coupled receptors S1P receptors 1–5 (S1PR1–S1PR5) on the same cells or other cells, activating various downstream signalling pathways and regulating many biological processes, including cell migration, survival, cell fate and gene expression. d, Extracellular S1P can be dephosphorylated by phospholipid phosphatase 3 (PLPP3) into Sph, which can flip into the inner leaflet of the plasma membrane. Sph is re-phosphorylated by SPHK1 into S1P and reaches the endoplasmic reticulum (ER) for imidazole (THI), 4-deoxypyridoxine (DOP), LX2931, LX2932, LX3305 and A6770 . An inhibitor for SPNS2, 16d, has been developed recently. The development of inhibitors or activators targeting MFSD2B is still under investigation.