Ceramides are bioactive sphingolipids, which are composed of sphingoid bases carrying acyl chains of various lengths. Ceramides are synthesized by a family of six ceramide synthases (CerS) in mammals, which produce ceramides with different N-linked acyl chains. Increased ceramide levels are known to contribute to the development of obesity and insulin resistance. Recently, it has been demonstrated that the ceramide acylation pattern is of particular importance for an organism to maintain energy homeostasis. However, which of the CerS family members are involved in this process is not yet completely known. Using newly developed CerS5 knock-out mice, we show here that CerS5 is essential to maintain cellular C 16:0 sphingolipid pools in lung, spleen, muscle, liver, and white adipose tissue. Glycerophospholipid levels in CerS5-deficient mice were not altered. We found a strong impact of CerS5-dependent ceramide synthesis in white adipose tissue after high fat diet feeding. In skeletal muscle, liver, and spleen, C 16:0 -ceramide levels were altered independent of feeding conditions. The loss of CerS5 is associated with reduced weight gain and improved systemic health, including maintenance of glucose homeostasis and reduced white adipose tissue inflammation after high fat diet challenge. Our findings indicate that reduction of endogenous C 16:0 -ceramide by genetic inhibition of CerS5 is sufficient to ameliorate obesity and its comorbidities.Ceramide synthases are sphingosine N-acyltransferases and represent an important metabolic hub in the ceramide synthesis pathway (Fig. 1A). They acylate sphingoid bases with fatty acid acyl chains of different length and saturation ( Fig. 1B) (1-6), thereby creating ceramides with diverse biological properties (7-9). The ceramide synthase enzyme family contains six members (CerS1-6) 6 in mammals (1, 9, 10). The individual enzymes differ in their substrate specificity and show different expression patterns (Fig. 1A) (1, 11). CerS1 is specifically expressed in muscle and neurons and has strong substrate specificity toward C 18:0 -CoA (1, 4, 11), whereas CerS2 and CerS4 are broadly expressed with specificity toward very long chain C 20: 0 -26:0 CoAs and C 18:0 -C 22:0 CoAs, respectively (1, 11). CerS3 is highly expressed in the epidermis and testis showing a substrate specificity for ultra-long chain CoAs (2, 10). CerS6 is expressed in most tissues at low levels and shows substrate specificity toward long chain C 14:0 -16:0 -CoAs (Fig. 1B) (1, 11, 12). CerS5 expression has also been studied at the mRNA level and is expressed in most tissues at low levels (11) and has a specificity toward the long chain CoAs C 14:0 -18:0 (1).Most murine CerS family members have also been characterized using knock-out (KO) mouse models. CerS1-deficient mice show behavioral abnormalities and Purkinje cell loss (4, 13), whereas CerS2 knock-out mice develop hepatocarcinomas and show myelination defects (3,5,14). CerS3 KO mice are lethal shortly after birth due to skin barrier disruption (2), and it was s...

Choroideremia (CHM) is an X- linked retinal degeneration that is symptomatic in the 1st or 2nd decade of life causing nyctalopia and loss of peripheral vision. The disease progresses through mid-life, when most patients become blind. CHM is a favorable target for gene augmentation therapy, as the disease is due to loss of function of a protein necessary for retinal cell health, Rab Escort Protein 1 (REP1).The CHM cDNA can be packaged in recombinant adeno-associated virus (rAAV), which has an established track record in human gene therapy studies, and, in addition, there are sensitive and quantitative assays to document REP1 activity. An animal model that accurately reflects the human condition is not available. In this study, we tested the ability to restore REP1 function in personalized in vitro models of CHM: lymphoblasts and induced pluripotent stems cells (iPSCs) from human patients. The initial step of evaluating safety of the treatment was carried out by evaluating for acute retinal histopathologic effects in normal-sighted mice and no obvious toxicity was identified. Delivery of the CHM cDNA to affected cells restores REP1 enzymatic activity and also restores proper protein trafficking. The gene transfer is efficient and the preliminary safety data are encouraging. These studies pave the way for a human clinical trial of gene therapy for CHM.

Protein prenylation is a widespread post-translational modification in eukaryotes that plays a crucial role in membrane targeting and signal transduction. RabGTPases is the largest group of post-translationally C-terminally geranylgeranylated. All Rabs are processed by Rab geranylgeranyl-transferase and Rab escort protein (REP). Human genetic defects resulting in the loss one of two REP isoforms REP-1, lead to underprenylation of RabGTPases that manifests in retinal degradation and blindness known as choroideremia. In this study we used a combination of microinjections and chemo-enzymatic tagging to establish whether Rab GTPases are prenylated and delivered to their target cellular membranes with the same rate. We demonstrate that although all tested Rab GTPases display the same rate of membrane delivery, the extent of Rab prenylation in 5 hour time window vary by more than an order of magnitude. We found that Rab27a, Rab27b, Rab38 and Rab42 display the slowest prenylation in vivo and in the cell. Our work points to possible contribution of Rab38 to the emergence of choroideremia in addition to Rab27a and Rab27b.

Protein prenylation is required for membrane anchorage of small GTPases. Correct membrane targeting is essential for their biological activity. Signal output of the prenylated proto-oncogene Ras in addition critically depends on its organization into nanoscale proteolipid assemblies of the plasma membrane, so called nanoclusters. While protein prenylation is an established drug target, only a handful of nanoclustering inhibitors are known, partially due to the lack of appropriate assays to screen for such compounds. Here, we describe three cell-based high-throughput screening amenable Förster resonance energy transfer NANOclustering and Prenylation Sensors (NANOPS) that are specific for Ras, Rho, and Rab proteins. Rab-NANOPS provides the first evidence for nanoclustering of Rab proteins. Using NANOPS in a cell-based chemical screen, we now identify macrotetrolides, known ionophoric antibiotics, as submicromolar disruptors of Ras nanoclustering and MAPK signaling.

Hundreds of eukaryotic signaling proteins require myristoylation to functionally associate with intracellular membranes. N-myristoyl transferases (NMT) responsible for this modification are established drug targets in cancer and infectious diseases. Here we describe NANOMS (NANOclustering and Myristoylation Sensors), biosensors that exploit the FRET resulting from plasma membrane nanoclustering of myristoylated membrane targeting sequences of Gαi2, Yes- or Src-kinases fused to fluorescent proteins. When expressed in mammalian cells, NANOMS report on loss of membrane anchorage due to chemical or genetic inhibition of myristoylation e.g. by blocking NMT and methionine-aminopeptidase (Met-AP). We used Yes-NANOMS to assess inhibitors of NMT and a cherry-picked compound library of putative Met-AP inhibitors. Thus we successfully confirmed the activity of DDD85646 and fumagillin in our cellular assay. The developed assay is unique in its ability to identify modulators of signaling protein nanoclustering, and is amenable to high throughput screening for chemical or genetic inhibitors of functional membrane anchorage of myristoylated proteins in mammalian cells.