Rom2-dependent phosphorylation of Elo2 controls the abundance of very long-chain fatty acids

Activation of the cell wall integrity pathway negatively regulates TORC2-Ypk1/2 signaling through blocking eisosome disassembly in Saccharomyces cerevisiae

The target of rapamycin complex 2 (TORC2) signaling is associated with plasma membrane (PM) integrity. In Saccharomyces cerevisiae, TORC2-Ypk1/2 signaling controls sphingolipid biosynthesis, and Ypk1/2 phosphorylation by TORC2 under PM stress conditions is increased in a Slm1/2-dependent manner, under which Slm1 is known to be released from an eisosome, a furrow-like invagination PM structure. However, it remains unsolved how the activation machinery of TORC2-Ypk1/2 signaling is regulated. Here we show that edelfosine, a synthetic lysophospholipid analog, inhibits the activation of TORC2-Ypk1/2 signaling, and the cell wall integrity (CWI) pathway is involved in this inhibitory effect. The activation of CWI pathway blocked the eisosome disassembly promoted by PM stress and the release of Slm1 from eisosomes. Constitutive activation of TORC2-Ypk1/2 signaling exhibited increased sensitivity to cell wall stress. We propose that the CWI pathway negatively regulates the TORC2-Ypk1/2 signaling, which is involved in the regulatory mechanism to ensure the proper stress response to cell wall damage.

Identification of distinct active pools of yeast serine palmitoyltransferase in sub-compartments of the ER

Sphingolipids (SPs) are one of the three major lipid classes in eukaryotic cells and serve as structural components of the plasma membrane. The rate-limiting step in SP biosynthesis is catalyzed by the serine palmitoyltransferase (SPT). In budding yeast (Saccharomyces cerevisiae), SPT is negatively regulated by the two proteins, Orm1 and Orm2. Regulating SPT activity enables cells to adapt SP metabolism to changing environmental conditions. Therefore, the Orm proteins are phosphorylated by two signaling pathways originating from either the plasma membrane or the lysosome (or vacuole in yeast). Moreover, uptake of exogenous serine is necessary for the regulation of SP biosynthesis, which suggests the existence of differentially regulated SPT pools based on their intracellular localization. However, measuring lipid metabolic enzyme activity in different cellular sub-compartments has been challenging. Combining a nanobody recruitment approach with SP flux analysis, we show that the nuclear endoplasmic reticulum (ER)-localized SPT and the peripheral ER localized SPT pools are differentially active. Thus, our data add another layer to the complex network of SPT regulation. Moreover, combining lipid metabolic enzyme re-localization with flux analysis serves as versatile tool to measure lipid metabolism with subcellular resolution.

Beyond membrane components: uncovering the intriguing world of fungal sphingolipid synthesis and regulation

Sphingolipids (SLs) are essential to fungal survival and represent a major class of structural and signaling lipids. Unique SL structures and their biosynthetic enzymes in filamentous fungi make them an ideal drug target. Several studies have contributed towards the functional characterization of specific SL metabolism genes, which have been complemented by advanced lipidomics methods which allow accurate identification and quantification of lipid structures and pathway mapping. These studies have provided a better understanding of SL biosynthesis, degradation and regulation networks in filamentous fungi, which are discussed and elaborated here.

Yeast Svf1 binds ceramides and contributes to sphingolipid metabolism at the ER cis-Golgi interface

Ceramides are essential precursors of complex sphingolipids and act as potent signaling molecules. Ceramides are synthesized in the endoplasmic reticulum (ER) and receive their head-groups in the Golgi apparatus, yielding complex sphingolipids (SPs). Transport of ceramides between the ER and the Golgi is executed by the essential ceramide transport protein (CERT) in mammalian cells. However, yeast cells lack a CERT homolog, and the mechanism of ER to Golgi ceramide transport remains largely elusive. Here, we identified a role for yeast Svf1 in ceramide transport between the ER and the Golgi. Svf1 is dynamically targeted to membranes via an N-terminal amphipathic helix (AH). Svf1 binds ceramide via a hydrophobic binding pocket that is located in between two lipocalin domains. We showed that Svf1 membrane-targeting is important to maintain flux of ceramides into complex SPs. Together, our results show that Svf1 is a ceramide binding protein that contributes to sphingolipid metabolism at Golgi compartments.

Roles of phosphatidylserine and phospholipase C in the activation of TOR complex 2 signaling in Saccharomyces cerevisiae

The target of rapamycin (TOR) forms two distinct complexes, TORC1 and TORC2, to exert its functions essential for cellular growth and homeostasis. TORC1 signaling is regulated in response to nutrients such as amino acids and glucose; however, the mechanisms underlying the activation of TORC2 signaling are still poorly understood compared to TORC1 signaling. In the budding yeast Saccharomyces cerevisiae, TORC2 targets protein kinases Ypk1, Ypk2, and Pkc1 for phosphorylation. Plasma membrane stress is known to activate the TORC2-Ypk1/2 signaling. We have previously reported that methylglyoxal (MG), a metabolite derived from glycolysis, activates TORC2-Pkc1 signaling. In this study, we found that MG activates the TORC2-Ypk1/2 and TORC2-Pkc1 signaling, and that phosphatidylserine is involved in the activation of both signaling pathways. We also demonstrated that the Rho-family GTPase Cdc42 contributes to the plasma membrane stress-induced activation of TORC2-Ypk1/2 signaling. Furthermore, we revealed that phosphatidylinositol-specific phospholipase C, Plc1, contributes to the activation of both TORC2-Ypk1/2 and TORC2-Pkc1 signaling.

Membrane Contact Sites in Yeast: Control Hubs of Sphingolipid Homeostasis

Sphingolipids are the most diverse class of membrane lipids, in terms of their structure and function. Structurally simple sphingolipid precursors, such as ceramides, act as intracellular signaling molecules in various processes, including apoptosis, whereas mature and complex forms of sphingolipids are important structural components of the plasma membrane. Supplying complex sphingolipids to the plasma membrane, according to need, while keeping pro-apoptotic ceramides in check is an intricate task for the cell and requires mechanisms that tightly control sphingolipid synthesis, breakdown, and storage. As each of these processes takes place in different organelles, recent studies, using the budding yeast Saccharomyces cerevisiae, have investigated the role of membrane contact sites as hubs that integrate inter-organellar sphingolipid transport and regulation. In this review, we provide a detailed overview of the findings of these studies and put them into the context of established regulatory mechanisms of sphingolipid homeostasis. We have focused on the role of membrane contact sites in sphingolipid metabolism and ceramide transport, as well as the mechanisms that prevent toxic ceramide accumulation.

Dynamic regulation of membrane integrity to enhance l-malate stress tolerance in Candida glabrata

Microbial cell factories provide a sustainable and economical way to produce chemicals from renewable feedstocks. However, the accumulation of targeted chemicals can reduce the robustness of the industrial strains and affect the production performance. Here, the physiological functions of Mediator tail subunit CgMed16 at l-malate stress were investigated. Deletion of CgMed16 decreased the survival, biomass, and half-maximal inhibitory concentration (IC₅₀ ) by 40.4%, 34.0%, and 30.6%, respectively, at 25 g/L l-malate stress. Transcriptome analysis showed that this growth defect was attributable to changes in the expression of genes involved in lipid metabolism. In addition, tolerance transcription factors CgUSV1 and CgYAP3 were found to interact with CgMed16 to regulate sterol biosynthesis and glycerophospholipid metabolism, respectively, ultimately endowing strains with excellent membrane integrity to resist l-malate stress. Furthermore, a dynamic tolerance system (DTS) was constructed based on CgUSV1, CgYAP3, and an l-malate-driven promoter P_cgr-10 to improve the robustness and productive capacity of Candida glabrata. As a result, the biomass, survival, and membrane integrity of C. glabrata 012 (with DTS) increased by 22.6%, 31.3%, and 53.8%, respectively, compared with those of strain 011 (without DTS). Therefore, at shake-flask scale, strain 012 accumulated 35.5 g/L l-malate, and the titer and productivity of l-malate increased by 32.5% and 32.1%, respectively, compared with those of strain 011. This study provides a novel strategy for the rational design and construction of DTS for dynamically enhancing the robustness of industrial strains.

The role of very long chain fatty acids in yeast physiology and human diseases

Fatty acids (FAs) are a highly diverse class of molecules that can have variable chain length, number of double bonds and hydroxylation sites. FAs with 22 or more carbon atoms are described as very long chain fatty acids (VLCFAs). VLCFAs are synthesized in the endoplasmic reticulum (ER) through a four-step elongation cycle by membrane embedded enzymes. VLCFAs are precursors for the synthesis of sphingolipids (SLs) and glycerophospholipids. Besides their role as lipid constituents, VLCFAs are also found as precursors of lipid mediators. Mis-regulation of VLCFA metabolism can result in a variety of inherited diseases ranging from ichthyosis, to myopathies and demyelination. The enzymes for VLCFA biosynthesis are evolutionary conserved and many of the pioneering studies were performed in the model organism Saccharomyces cerevisiae. A growing body of evidence suggests that VLCFA metabolism is intricately regulated to maintain lipid homeostasis. In this review we will describe the metabolism of VLCFAs, how they are synthesized, transported and degraded and how these processes are regulated, focusing on budding yeast. We will review how lipid metabolism and membrane properties are affected by VLCFAs and which impact mutations in the biosynthetic genes have on physiology. We will also briefly describe diseases caused by mis-regulation of VLCFAs in human cells.

Uptake of exogenous serine is important to maintain sphingolipid homeostasis in Saccharomyces cerevisiae

Sphingolipids are abundant and essential molecules in eukaryotes that have crucial functions as signaling molecules and as membrane components. Sphingolipid biosynthesis starts in the endoplasmic reticulum with the condensation of serine and palmitoyl-CoA. Sphingolipid biosynthesis is highly regulated to maintain sphingolipid homeostasis. Even though, serine is an essential component of the sphingolipid biosynthesis pathway, its role in maintaining sphingolipid homeostasis has not been precisely studied. Here we show that serine uptake is an important factor for the regulation of sphingolipid biosynthesis in Saccharomyces cerevisiae. Using genetic experiments, we find the broad-specificity amino acid permease Gnp1 to be important for serine uptake. We confirm these results with serine uptake assays in gnp1Δ cells. We further show that uptake of exogenous serine by Gnp1 is important to maintain cellular serine levels and observe a specific connection between serine uptake and the first step of sphingolipid biosynthesis. Using mass spectrometry-based flux analysis, we further observed imported serine as the main source for de novo sphingolipid biosynthesis. Our results demonstrate that yeast cells preferentially use the uptake of exogenous serine to regulate sphingolipid biosynthesis. Our study can also be a starting point to analyze the role of serine uptake in mammalian sphingolipid metabolism.

Sphingolipids (SPs) are membrane lipids globally required for eukaryotic life. In contrast to other lipid classes, SPs cannot be stored in the cell and therefore their levels have to be tightly regulated. Failure to maintain sphingolipid homeostasis can result in pathologies including neurodegeneration, childhood asthma and cancer. However, we are only starting to understand how SP biosynthesis is adjusted according to need. In this study, we use genetic and biochemical methods to show that the uptake of exogenous serine is necessary to maintain SP homeostasis in Saccharomyces cerevisiae. Serine is one of the precursors of long chain bases in cells, the first intermediate of SP metabolism. Our results suggest that the uptake of serine is directly coupled to SP biosynthesis at ER-plasma membrane contact sites. Overall, our study identifies serine uptake as a novel regulatory factor of SP homeostasis. While we use yeast as a discovery tool, these results also provide valuable insights into mammalian SP biology especially under pathological conditions.

Fungal plasma membrane domains

The plasma membrane (PM) performs a plethora of physiological processes, the coordination of which requires spatial and temporal organization into specialized domains of different sizes, stability, protein/lipid composition and overall architecture. Compartmentalization of the PM has been particularly well studied in the yeast Saccharomyces cerevisiae, where five non-overlapping domains have been described: The Membrane Compartments containing the arginine permease Can1 (MCC), the H+-ATPase Pma1 (MCP), the TORC2 kinase (MCT), the sterol transporters Ltc3/4 (MCL), and the cell wall stress mechanosensor Wsc1 (MCW). Additional cortical foci at the fungal PM are the sites where clathrin-dependent endocytosis occurs, the sites where the external pH sensing complex PAL/Rim localizes, and sterol-rich domains found in apically grown regions of fungal membranes. In this review, we summarize knowledge from several fungal species regarding the organization of the lateral PM segregation. We discuss the mechanisms of formation of these domains, and the mechanisms of partitioning of proteins there. Finally, we discuss the physiological roles of the best-known membrane compartments, including the regulation of membrane and cell wall homeostasis, apical growth of fungal cells and the newly emerging role of MCCs as starvation-protective membrane domains.

Myriocin-induced adaptive laboratory evolution of an industrial strain of Saccharomyces cerevisiae reveals its potential to remodel lipid composition and heat tolerance

The modification of lipid composition allows cells to adjust membrane biophysical properties in response to changes in environmental temperature. Here, we use adaptive laboratory evolution (ALE) in the presence of myriocin, a sphingolipid (SLs) biosynthesis inhibitor, to remodel the lipid profile of an industrial yeast strain (LH) of Saccharomyces cerevisiae. The approach enabled to obtain a heterogeneous population (LHev) of myriocin-tolerant evolved clones characterized by its growth capacity at high temperature. Myriocin exposure also caused tolerance to soraphen A, an inhibitor of the acetyl-CoA carboxylase Acc1, the rate-limiting enzyme in fatty acid de novo production, supporting a change in lipid metabolism during ALE. In line with this, characterization of two randomly selected clones, LH03 and LH09, showed the presence of lipids with increased saturation degree and reduced acyl length. In addition, the clone LH03, which displays the greater improvement in fitness at 40°C, exhibited higher SL content as compared with the parental strain. Analysis of the LH03 and LH09 genomes revealed a loss of chromosomes affecting genes that have a role in fatty acid synthesis and elongation. The link between ploidy level and growth at high temperature was further supported by the analysis of a fully isogenic set of yeast strains with ploidy between 1N and 4N which showed that the loss of genome content provides heat tolerance. Consistent with this, a thermotolerant evolved population (LH40°) generated from the parental LH strain by heat-driven ALE exhibited a reduction in the chromosome copy number. Thus, our results identify myriocin-driven evolution as a powerful approach to investigate the mechanisms of acquired thermotolerance and to generate improved strains.

Engineering microbial cell morphology and membrane homeostasis toward industrial applications

Microbial cell factory performance is significantly affected by the cell morphology and membrane homeostasis. It is important to ensure that cell factories are able to regulate cell morphology and maintain membrane homeostasis. Cell morphology can be controlled by regulating the formation of elongasomes and divisomes, which change the shapes of cells from rods to fibers, large spheres, or mini-cells. Membrane homeostasis can also be controlled by regulating the homeostasis of membrane lipids and proteins, thereby improving the robustness of microbes in toxic environments. In the present review, we discuss promising developments in cell morphology and membrane homeostasis engineering that have improved microbial cell factory performance.

Pho85 and PI(4,5)P2 regulate different lipid metabolic pathways in response to cold

Lipid homeostasis allows cells to adjust membrane biophysical properties in response to changes in environmental conditions. In the yeast Saccharomyces cerevisiae, a downward shift in temperature from an optimal reduces membrane fluidity, which triggers a lipid remodeling of the plasma membrane. How changes in membrane fluidity are perceived, and how the abundance and composition of different lipid classes is properly balanced, remain largely unknown. Here, we show that the levels of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2], the most abundant plasma membrane phosphoinositide, drop rapidly in response to a downward shift in temperature. This change triggers a signaling cascade transmitted to cytosolic diphosphoinositol phosphate derivatives, among them 5-PP-IP4 and 1-IP7, that exert regulatory functions on genes involved in the inositol and phospholipids (PLs) metabolism, and inhibit the activity of the protein kinase Pho85. Consistent with this, cold exposure triggers a specific program of neutral lipids and PLs changes. Furthermore, we identified Pho85 as playing a key role in controlling the synthesis of long-chain bases (LCBs) via the Ypk1-Orm2 regulatory circuit. We conclude that Pho85 orchestrates a coordinated response of lipid metabolic pathways that ensure yeast thermal adaptation.

Disruption of ergosterol and tryptophan biosynthesis, as well as cell wall integrity pathway and the intracellular pH homeostasis, lead to mono-(2-ethylhexyl)-phthalate toxicity in budding yeast

Endocrine disrupting chemicals (EDCs) are substances in the environment, food, and consumer products that interfere with hormone homeostasis, metabolism or reproduction in humans and animals. One such EDC, the plasticizer di-(2-ethylhexyl)-phthalate (DEHP), exerts its function through its principal bioactive metabolite, mono-(2-ethylhexyl)-phthalate (MEHP). To fully understand the effects of MEHP on cellular processes and metabolism as well as to assess the impact of genetic alteration on the susceptibility to MEHP-induced toxicity, we screened MEHP-sensitive mutations on a genome-scale in the eukaryotic model organism Saccharomyces cerevisiae. We identified a total of 96 chemical-genetic interactions between MEHP and gene mutations in this study. In response to MEHP treatment, most of these gene mutants accumulated higher intracellular MEHP content, which correlated with their MEHP sensitivity. Twenty-seven of these genes are involved in the metabolism, twenty-two of them play roles in protein sorting, and ten of them regulate ion homeostasis. Functional categorization of these genes indicated that the biosynthetic pathways of both ergosterol and tryptophan, as well as cell wall integrity and the intracellular pH homeostasis, were involved in the protective response of yeast cells to the MEHP toxicity. Our study demonstrated that a collection of yeast gene deletion mutants is useful for a functional toxicogenomic analysis of EDCs, which could provide important clues to the effects of EDCs on higher eukaryotic organisms.

Proteomic and phosphoproteomic analyses of yeast reveal the global cellular response to sphingolipid depletion

Sphingolipids are essential components of eukaryotic cells with important functions in membrane biology and cellular signaling. Their levels are tightly controlled and coordinated with the abundance of other membrane lipids. How sphingolipid homeostasis is achieved is not yet well understood. Studies performed primarily in yeast showed that the phosphorylation states of several enzymes and regulators of sphingolipid synthesis are important, although a global understanding for such regulation is lacking. Here, we used high-resolution MS-based proteomics and phosphoproteomics to analyze the cellular response to sphingolipid synthesis inhibition. Our dataset reveals that changes in protein phosphorylation, rather than protein abundance, dominate the response to blocking sphingolipid synthesis. We identified Ypk signaling as a pathway likely to be activated under these conditions, and we identified potential Ypk1 target proteins. Our data provide a rich resource for on-going mechanistic studies of key elements of the cellular response to the depletion of sphingolipid levels and the maintenance of sphingolipid homeostasis. All MS data have been deposited in the ProteomeXchange with identifier PXD003854 (http://proteomecentral.proteomexchange.org/dataset/PXD003854).

Med15B Regulates Acid Stress Response and Tolerance in Candida glabrata by Altering Membrane Lipid Composition

Candida glabrata is a promising producer of organic acids. To elucidate the physiological function of the Mediator tail subunit Med15B in the response to low-pH stress, we constructed a deletion strain, C. glabratamed15BΔ, and an overexpression strain, C. glabrata HTUΔ/CgMED15B Deletion of MED15B caused biomass production, glucose consumption rate, and cell viability to decrease by 28.3%, 31.7%, and 26.5%, respectively, compared with those of the parent (HTUΔ) strain at pH 2.0. Expression of lipid metabolism-related genes was significantly downregulated in the med15BΔ strain, whereas key genes of ergosterol biosynthesis showed abnormal upregulation. This caused the proportion of C_18:1 fatty acids, the ratio of unsaturated to saturated fatty acids (UFA/SFA), and the total phospholipid content to decrease by 11.6%, 27.4%, and 37.6%, respectively. Cells failed to synthesize fecosterol and ergosterol, leading to the accumulation and a 60.3-fold increase in the concentration of zymosterol. Additionally, cells showed reductions of 69.2%, 11.6%, and 21.8% in membrane integrity, fluidity, and H⁺-ATPase activity, respectively. In contrast, overexpression of Med15B increased the C_18:1 levels, total phospholipids, ergosterol content, and UFA/SFA by 18.6%, 143.5%, 94.5%, and 18.7%, respectively. Membrane integrity, fluidity, and H⁺-ATPase activity also increased by 30.2%, 6.9%, and 51.8%, respectively. Furthermore, in the absence of pH buffering, dry weight of cells and pyruvate concentrations were 29.3% and 61.2% higher, respectively, than those of the parent strain. These results indicated that in C. glabrata, Med15B regulates tolerance toward low pH via transcriptional regulation of acid stress response genes and alteration in lipid composition.IMPORTANCE This study explored the role of the Mediator tail subunit Med15B in the metabolism of Candida glabrata under acidic conditions. Overexpression of MED15B enhanced yeast tolerance to low pH and improved biomass production, cell viability, and pyruvate yield. Membrane lipid composition data indicated that Med15B might play a critical role in membrane integrity, fluidity, and H⁺-ATPase activity homeostasis at low pH. Thus, controlling membrane composition may serve to increase C. glabrata productivity at low pH.

A conserved signaling network monitors delivery of sphingolipids to the plasma membrane in budding yeast

In budding yeast, signals generated in response to membrane growth are required for cell cycle progression. A mass spectrometry screen for signals triggered by an arrest of membrane growth identified sphingolipid signaling pathways. Delivery of sphingolipids to the plasma membrane could generate signals that control cell growth and the cell cycle.

In budding yeast, cell cycle progression and ribosome biogenesis are dependent on plasma membrane growth, which ensures that events of cell growth are coordinated with each other and with the cell cycle. However, the signals that link the cell cycle and ribosome biogenesis to membrane growth are poorly understood. Here we used proteome-wide mass spectrometry to systematically discover signals associated with membrane growth. The results suggest that membrane trafficking events required for membrane growth generate sphingolipid-dependent signals. A conserved signaling network appears to play an essential role in signaling by responding to delivery of sphingolipids to the plasma membrane. In addition, sphingolipid-dependent signals control phosphorylation of protein kinase C (Pkc1), which plays an essential role in the pathways that link the cell cycle and ribosome biogenesis to membrane growth. Together these discoveries provide new clues as to how growth-­dependent signals control cell growth and the cell cycle.

Flux control through protein phosphorylation in yeast

Protein phosphorylation is one of the most important mechanisms regulating metabolism as it can directly modify metabolic enzymes by the addition of phosphate groups. Attributed to such a rapid and reversible mechanism, cells can adjust metabolism rapidly in response to temporal changes. The yeast Saccharomyces cerevisiae, a widely used cell factory and model organism, is reported to show frequent phosphorylation events in metabolism. Studying protein phosphorylation in S. cerevisiae allows for gaining new insight into the function of regulatory networks, which may enable improved metabolic engineering as well as identify mechanisms underlying human metabolic diseases. Here we collect functional phosphorylation events of 41 enzymes involved in yeast metabolism and demonstrate functional mechanisms and the application of this information in metabolic engineering. From a systems biology perspective, we describe the development of phosphoproteomics in yeast as well as approaches to analysing the phosphoproteomics data. Finally, we focus on integrated analyses with other omics data sets and genome-scale metabolic models. Despite the advances, future studies improving both experimental technologies and computational approaches are imperative to expand the current knowledge of protein phosphorylation in S. cerevisiae.

Taming the sphinx: Mechanisms of cellular sphingolipid homeostasis

Sphingolipids are important structural membrane components of eukaryotic cells, and potent signaling molecules. As such, their levels must be maintained to optimize cellular functions in different cellular membranes. Here, we review the current knowledge of homeostatic sphingolipid regulation. We describe recent studies in Saccharomyces cerevisiae that have provided insights into how cells sense changes in sphingolipid levels in the plasma membrane and acutely regulate sphingolipid biosynthesis by altering signaling pathways. We also discuss how cellular trafficking has emerged as an important determinant of sphingolipid homeostasis. Finally, we highlight areas where work is still needed to elucidate the mechanisms of sphingolipid regulation and the physiological functions of such regulatory networks, especially in mammalian cells. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

The GARP complex is required for cellular sphingolipid homeostasis

Sphingolipids are abundant membrane components and important signaling molecules in eukaryotic cells. Their levels and localization are tightly regulated. However, the mechanisms underlying this regulation remain largely unknown. In this study, we identify the Golgi-associated retrograde protein (GARP) complex, which functions in endosome-to-Golgi retrograde vesicular transport, as a critical player in sphingolipid homeostasis. GARP deficiency leads to accumulation of sphingolipid synthesis intermediates, changes in sterol distribution, and lysosomal dysfunction. A GARP complex mutation analogous to a VPS53 allele causing progressive cerebello-cerebral atrophy type 2 (PCCA2) in humans exhibits similar, albeit weaker, phenotypes in yeast, providing mechanistic insights into disease pathogenesis. Inhibition of the first step of de novo sphingolipid synthesis is sufficient to mitigate many of the phenotypes of GARP-deficient yeast or mammalian cells. Together, these data show that GARP is essential for cellular sphingolipid homeostasis and suggest a therapeutic strategy for the treatment of PCCA2.

DOI:http://dx.doi.org/10.7554/eLife.08712.001

Every cell is enveloped by a membrane that forms a barrier between the cell and its environment. This membrane contains fat molecules called ‘sphingolipids’, which help to maintain the structure of the membrane and enable it to work correctly. These molecules are also used as signals to send information around the interior of the cell and are required for the cell to grow and divide normally. The levels of sphingolipids in the membrane have to be tightly controlled because any imbalance can cause stress to the cell and can lead to serious diseases.

Sphingolipids are made inside the cell and are then sent to a compartment called the Golgi before being delivered to the membrane. To regulate the amount of sphingolipids in the membrane, these molecules are routinely returned to the interior of the cell in small structures called endosomes. From here, they can either be broken down or recycled back to the membrane via the Golgi.

A group of proteins known as the Golgi-associated retrograde protein complex (or GARP) is involved in the movement of endosomes from the membrane to the Golgi. People that have a mutation in the gene that encodes GARP suffer from a severe neurodegenerative disease known as ‘progressive cerebello-cerebral atrophy type 2’ (PCCA2) in which brain cells die prematurely. Researchers have assumed that the most important role of GARP is to sort proteins, and that the missorting of proteins leads to PCCA2.

Here, Frohlich et al. used a combination of genetic analysis and biochemical techniques to study GARP in yeast cells. The experiments show that GARP is critical for sphingolipid recycling, and that a lack of GARP leads to more sphingolipids being degraded, which results in a build-up of toxic molecules. Frohlich et al. generated yeast cells that have the same mutations in the gene that encodes GARP as those in human patients with PCCA2. These cells grew much slower than normal yeast and were less able to transport sphingolipids from the endosome to the Golgi.

Like the yeast cells, human cells in which the gene that encodes GARP was less active also accumulated toxic molecules. Together, these findings suggest that a build-up of toxic fat molecules may be responsible for the symptoms observed in PCCA2 patients. A future challenge is to find out whether this also applies to patients with Alzheimer's disease and other conditions that also affect endosomes.

DOI:http://dx.doi.org/10.7554/eLife.08712.002