Ceramides in focus: Excess calorie intake can ultimately lead to a metabolic syndrome that interferes with fat or lipid metabolism. There are many different types of lipids, and it has been widely debated which are the true culprits underlying metabolic disorders. Chaurasia et al. report that ceramides are the major contributor to insulin resistance and fatty liver disease (see the Perspective by Kusminski and Scherer). This appears to be caused by the enzyme dihydroceramide desaturase 1 (DES1), which is normally involved in ceramide production by inserting a double bond into the backbone of the molecule. In mice fed a high-fat diet, deletion of DES1 improved glucose and lipid metabolism. Science , this issue p. 386 ; see also p. 319

Brown adipose tissue (BAT) is composed of thermogenic cells that convert chemical energy into heat to maintain a constant body temperature and counteract metabolic disease. The metabolic adaptations required for thermogenesis are not fully understood. Here, we explore how steady state levels of metabolic intermediates are altered in brown adipose tissue in response to cold exposure. Transcriptome and metabolome analysis revealed changes in pathways involved in amino acid, glucose, and TCA cycle metabolism. Using isotopic labeling experiments, we found that activated brown adipocytes increased labeling of pyruvate and TCA cycle intermediates from U¹³C-glucose. Although glucose oxidation has been implicated as being essential for thermogenesis, its requirement for efficient thermogenesis has not been directly tested. We show that mitochondrial pyruvate uptake is essential for optimal thermogenesis, as conditional deletion ofMpc1in brown adipocytes leads to impaired cold adaptation. Isotopic labeling experiments using U¹³C-glucose showed that loss of MPC1 led to impaired labeling of TCA cycle intermediates. Loss of MPC1 in BAT increased 3-hydroxybutyrate levels in blood and BAT in response to the cold, suggesting that ketogenesis provides an alternative fuel source to compensate. Collectively, these studies highlight that complete glucose oxidation is essential for optimal brown fat thermogenesis.

P2-type ATPase sodium-potassium pumps (Na⁺/K⁺-ATPases) are ion-transporting enzymes that use ATP to transport Na⁺ and K⁺ on opposite sides of the lipid bilayer against their electrochemical gradients to maintain ion concentration gradients across the membranes in all animal cells. Despite the available molecular architecture of the Na⁺/K⁺-ATPases, a complete molecular mechanism by which the Na⁺ and K⁺ ions access into and are released from the pump remains unknown. Here we report five cryo-electron microscopy (cryo-EM) structures of the human alpha3 Na⁺/K⁺-ATPase in its cytoplasmic side-open (E1), ATP-bound cytoplasmic side-open (E1•ATP), ADP-AlF₄⁻ trapped Na⁺-occluded (E1•P-ADP), BeF₃⁻ trapped exoplasmic side-open (E2P) and MgF₄²⁻ trapped K⁺-occluded (E2•P_i) states. Our work reveals the atomically resolved structural detail of the cytoplasmic gating mechanism of the Na⁺/K⁺-ATPase.

Sphingolipids, a major class of lipids in cell membranes, play diverse roles in biology. They are synthesized by a highly conserved biosynthetic pathway that leads to the production of ceramides, the major precursors of most complex sphingolipids. Almost all known stress stimuli including inflammatory agonists, chemotherapeutics, and saturated fatty acids induce the synthesis of ceramide and its metabolites. A panoply of recent studies has implicated ceramides in the development of the metabolic comorbidities of obesity such as diabetes and cardiovascular diseases. In particular, inhibition of ceramide biosynthesis in rodents ameliorates insulin resistance, diabetes, cardiomyopathy, atherosclerosis, and steatohepatitis. These data implicate ceramides as major contributors to the development of metabolic diseases. This review summarizes recent findings on this emerging class of bioactive lipids with an emphasis on studies using in vivo models to understand their role in metabolic disease.

Stable isotope-tracing assays track few metabolites, yet cells use many nutrients to sustain nitrogen metabolism. Here we create a platform for tracing 30 nitrogen isotope-labelled metabolites in parallel to enable a system-level understanding of cellular nitrogen metabolism. This platform reveals that while primitive cells engage both de novo and salvage pyrimidine synthesis pathways, differentiated cells nearly exclusively salvage uridine. This link between cell state and pyrimidine synthesis pathway preference persists in murine and human tissues. Mechanistically, we find that S1900 phosphorylation of CAD, the first enzyme of the de novo pathway, is induced by uridine deprivation in differentiated cells and constitutively enriched in primitive cells. Mimicking CAD S1900 phosphorylation in differentiated cells constitutively activates de novo pyrimidine synthesis, while blocking this modification impairs the cellular response to uridine starvation. Collectively, we establish a method for nitrogen metabolism profiling and define a mechanism of cell state-specific pyrimidine synthesis pathway choice.