1-Deoxysphingolipids

Syntheses of differentially fluorinated triazole-based 1-deoxysphingosine analogues en route to SphK inhibitors

This study focuses on the stereoselective syntheses of 1-deoxysphingosine analogues as potential inhibitors of sphingosine kinase (SphK), particularly targeting its isoforms SphK1 and SphK2, which are implicated in cancer progression and therapy resistance. The research builds on previous work by designing a series of analogues featuring systematic structural modifications like the incorporation of a triazole ring, varying degrees of fluorination, and different head groups (e.g., guanidino, N-methylamino, and N,N-dimethylamino). These modifications aimed to enhance polar and hydrophobic interactions especially with SphK2. The synthesized compounds were evaluated for their inhibitory activity, revealing that certain derivatives, particularly those with guanidino groups and heptafluoropropyl fragments at the lipidic tail, exhibited significant potency and selectivity towards SphK2. Docking studies supported these findings by showing favorable binding interactions within the SphK2 active site, which were less pronounced in SphK1, correlating with the observed selectivity. This work contributes to the development of novel 1-deoxysphingosine analogues targeting SphK inhibition, as well as to the knowledge of the differential topology of the active sites in SphK1 and SphK2.

The role of 1-Deoxysphingolipids and Polyamines in the pathogenesis of placental syndrome

BACKGROUND

Placental syndrome, mainly composed of preeclampsia and fetal growth restriction, has an impact on the health of mother and baby dyads. While impaired placentation is central to their pathophysiology, the underlying molecular mechanisms remain incompletely understood. This study investigates the association between placental syndrome and metabolic alterations in 1-deoxysphingolipids (1-deoxySLs) and polyamines, along with their regulatory enzymes.

METHODS

This prospective case-control study involved 26 healthy pregnant women and 17 with placental syndrome. Blood samples were collected from maternal, uterine venous, and umbilical cord veins. Levels of 1-deoxySL, spermine, and spermidine, as well as related enzymes of polyamine metabolism such as ornithine decarboxylase (ODC), spermidine/spermine N1-acetyltransferase (SSAT), polyamine oxidase (PAO), and spermine oxidase (SMO), were measured using the techniques of LC-MS and ELISA, respectively.

RESULTS

Women with placental syndrome had significantly higher levels of 1-deoxySL, spermine, and spermidine in all blood samples compared to the healthy pregnancy group. Additionally, ODC and SSAT levels were reduced significantly in the placental syndrome group, while PAO and SMO levels showed no significant differences. Strong positive correlations were found between the studied enzymes and biomolecules in healthy pregnancies, which were notably weaker in the placental syndrome group.

CONCLUSION

This study demonstrates significantly altered levels of 1-deoxySL and polyamines, with corresponding enzyme activity changes, in placental syndrome compared to healthy pregnancies. The disrupted correlations between these biomolecules suggest alterations in their metabolic pathways and potential utility as biomarkers. Further mechanistic studies are warranted to elucidate their role in placental syndrome pathophysiology.

Don't Be Surprised When These Surprise You: Some Infrequently Studied Sphingoid Bases, Metabolites, and Factors That Should Be Kept in Mind During Sphingolipidomic Studies

Sphingolipidomic mass spectrometry has provided valuable information-and surprises-about sphingolipid structures, metabolism, and functions in normal biological processes and disease. Nonetheless, many noteworthy compounds are not routinely determined, such as the following: most of the sphingoid bases that mammals biosynthesize de novo other than sphingosine (and sometimes sphinganine) or acquire from exogenous sources; infrequently considered metabolites of sphingoid bases, such as N-(methyl)n-derivatives; "ceramides" other than the most common N-acylsphingosines; and complex sphingolipids other than sphingomyelins and simple glycosphingolipids, including glucosyl- and galactosylceramides, which are usually reported as "monohexosylceramides". These and other subspecies are discussed, as well as some of the circumstances when they are likely to be seen (or present and missed) due to experimental conditions that can influence sphingolipid metabolism, uptake from the diet or from the microbiome, or as artifacts produced during extraction and analysis. If these compounds and factors are kept in mind during the design and interpretation of lipidomic studies, investigators are likely to be surprised by how often they appear and thereby advance knowledge about them.

Safe and Orally Bioavailable Inhibitor of Serine Palmitoyltransferase Improves Age-Related Sarcopenia

The accumulation of ceramides and related metabolites has emerged as a pivotal mechanism contributing to the onset of age-related diseases. However, small molecule inhibitors targeting the ceramide de novo synthesis pathway for clinical use are currently unavailable. We synthesized a safe and orally bioavailable inhibitor, termed ALT-007, targeting the rate-limiting enzyme of ceramide de novo synthesis, serine palmitoyltransferase (SPT). In a mouse model of age-related sarcopenia, ALT-007, administered through the diet, effectively restored muscle mass and function compromised by aging. Mechanistic studies revealed that ALT-007 enhances protein homeostasis in Caenorhabditis elegans and mouse models of aging and age-related diseases, such as sarcopenia and inclusion body myositis (IBM); this effect is mediated by a specific reduction in very-long chain 1-deoxy-sphingolipid species, which accumulate in both muscle and brain tissues of aged mice and in muscle cells from IBM patients. These findings unveil a promising therapeutic avenue for developing safe ceramide inhibitors to address age-related neuromuscular diseases.

Possible Metabolic Remodeling based on de novo Biosynthesis of L-serine in Se-Subtoxic or -Deficient Mammals

Current research studies point to an increased risk of diabetes with selenium (Se) intake beyond the physiological requirement used to prevent cancers. The existing hypothesis of "selenoprotein overexpression leads to intracellular redox imbalance" cannot clearly explain the U-shaped dose-effect relationship between Se intake and the risk of diabetes. In this review, it is speculated that metabolic remodeling based on the de novo biosynthesis of L-serine may occur in mammals at supranutritional or subtoxic levels of Se. It is also speculated that a large amount of L-serine is consumed by the body during insufficient Se intake, thus resulting in similar metabolic reprogramming. The increase in atypical ceramide and its derivatives due to the lack of L-serine may also play a role in the development of diabetes.

Long Chain Base Profiling with Multiple Reaction Monitoring Mass Spectrometry

Sphingolipids (SLs) are essential lipids with important functions in membrane formation and cell signaling. The presence of a long chain base (LCB) structure is common to all SLs. De novo SL synthesis is initiated by the enzyme serine-palmitoyltransferase (SPT), which forms an LCB by the conjugation from serine and fatty acyl-CoAs. SPT can metabolize a variety of acyl-CoA substrates, which form diverse LCB structures within and across species. The LCB then undergoes further metabolic modifications resulting in an extraordinarily diverse spectrum of sphingolipids formed. SL analysis, using liquid chromatography-mass spectrometry (LC-MS)-based methods, poses challenges due to the diverse range of frequently isobaric species. This complexity complicates the identification of underlying LCB structures using standard lipidomics approaches. Here, we describe a simplified method to analyze the LCB profile in cells, tissue, and blood. The procedure involves chemical hydrolysis to remove the conjugated headgroups and N-acyl chains, allowing to specifically resolve the underlying LCB structures by LC-MS. This method can also be combined with an isotope labeling approach to determine in vivo SPT activity and total SL de novo synthesis over time.

Copepod Lipidomics: Fatty Acid Substituents of Structural Lipids in Labidocerca aestiva, a Dominant Species in the Food Chain of the Apalachicola Estuary of the Gulf of Mexico

Zooplanktonic copepods represent a major biological mass in the marine food chain that can be affected by climate change. Monitoring the health of this critical biomass is essential for increasing our understanding of the impact of environmental changes on marine environments. Since the lipidomes of marine organisms are known to adapt to alterations in pH, temperature, and availability of metabolic precursors, lipidomics is one technology that can be used for monitoring copepod adaptations. Among the key lipid parameters that can be monitored are the fatty acid substituents of glycerolipids and glycerophospholipids. We utilized high-resolution tandem mass spectrometry (≤2 ppm mass error) to characterize the fatty acid substituents of triacylglycerols, glycerophosphocholines, ceramides, and sphingomyelins of Labidocerca aestiva. This included monitoring for furan fatty acid substituents, a family of fatty acids unique to marine organisms. These data will contribute to establishing a lipid database of the fatty acid substituents of essential structural lipids. The key findings were that polyunsaturated fatty acids (PUFAs) were only major substituents in glycerophosphocholines with 36 to 44 carbons. Triacylglycerols, ceramides, and sphingomyelins contained minimal PUFA substituents. Furan fatty acids were limited to mono- and di-acylglycerols. In summary, we have built a baseline database of the fatty acid substituents of key structural lipids in Labidocerca aestiva. With this database, we will next evaluate potential seasonal changes in these lipid substituents and long-term effects of climate change.

Why do we study sphingolipids?

Research on sphingolipids has proliferated exponentially over the past couple of decades, as exemplified in the findings reported at the International Leopoldina Symposium on Lipid Signaling held in Frankfurt in late 2023. Most researchers in the field study how sphingolipids function in regulating a variety of cellular processes and, in particular, how they are dysregulated in numerous human diseases; however, I now propose that we implement a more holistic research program in our study of sphingolipids, which embraces a sense of awe and wonder at the complexities and beauty of sphingolipids and of sphingolipid metabolism. I will outline the chemical complexity of sphingolipids, their modes of interaction within the lipid bilayer, and their biosynthetic pathways. I will then briefly touch upon the ability of current neo-Darwinian mechanisms to explain the emergence of both sphingolipids and of the complex pathways that generate them. Although such discussion is normally considered taboo in biological circles, I nevertheless submit that in-depth analysis of the minutiae of metabolic pathways, such as those of the sphingolipid biosynthetic pathway, raises challenges to current neo-Darwinian mechanisms that should not be shunned or ignored.

Development of a Multiplexed Sphingolipids Method for Diagnosis of Inborn Errors of Ceramide Metabolism

BACKGROUND

Sphingolipids play a crucial role in cellular functions and are essential components of cell membranes, signaling molecules, and lipid metabolism. In particular, ceramide is a key intermediate in sphingolipid metabolism and defects in ceramide metabolism can lead to various inborn errors of metabolism, making ceramides important targets for clinical screening and diagnosis. Detecting altered concentration patterns of sphingolipids is desirable for distinguishing related inborn errors of metabolism for diagnosis and treatment monitoring.

METHODS

We developed a liquid chromatography-tandem mass spectrometry method with a pathway-oriented approach to focus on sphingolipids involved in ceramide metabolism. A total of 47 sphingolipids bearing different head groups and side chains were targeted. Precision/reproducibility, linearity, and spike recovery extraction efficiency tests were performed on plasma and serum samples from confirmed cases of sphingolipidosis.

RESULTS

Linearity of the method showed the coefficient of determination (r2) for all standards to be >0.99 with a slope of 1.00 ± 0.01. Intra- and interday reproducibility of standards spiked into plasma and serum revealed a coefficient of variation <20%. Spike and recovery assessment showed recovery values of 80%-120% for all standards. Altered levels of sphingolipids from patients with hereditary sensory and autonomic neuropathy caused by pathogenic variants in SPTLC2 and hypomyelinating leukodystrophy related to variants in DEGS1 were detected, in agreement with trends reported in earlier studies confirming the utility of this pathway-centric method.

CONCLUSIONS

This method can serve as a useful tool to simultaneously monitor sphingolipids, enabling screening and diagnosis of inborn errors of ceramide metabolism.

Metabolic ripple effects - deciphering how lipid metabolism in cancer interfaces with the tumor microenvironment

Cancer cells require a constant supply of lipids. Lipids are a diverse class of hydrophobic molecules that are essential for cellular homeostasis, growth and survival, and energy production. How tumors acquire lipids is under intensive investigation, as these mechanisms could provide attractive therapeutic targets for cancer. Cellular lipid metabolism is tightly regulated and responsive to environmental stimuli. Thus, lipid metabolism in cancer is heavily influenced by the tumor microenvironment. In this Review, we outline the mechanisms by which the tumor microenvironment determines the metabolic pathways used by tumors to acquire lipids. We also discuss emerging literature that reveals that lipid availability in the tumor microenvironment influences many metabolic pathways in cancers, including those not traditionally associated with lipid biology. Thus, metabolic changes instigated by the tumor microenvironment have 'ripple' effects throughout the densely interconnected metabolic network of cancer cells. Given the interconnectedness of tumor metabolism, we also discuss new tools and approaches to identify the lipid metabolic requirements of cancer cells in the tumor microenvironment and characterize how these requirements influence other aspects of tumor metabolism.

Enzymes of sphingolipid metabolism as transducers of metabolic inputs

Sphingolipids (SLs) constitute a discrete subdomain of metabolism, and they display both structural and signaling functions. Accumulating evidence also points to intimate connections between intermediary metabolism and SL metabolism. Given that many SLs exhibit bioactive properties (i.e. transduce signals), these raise the possibility that an important function of SLs is to relay information on metabolic changes into specific cell responses. This could occur at various levels. Some metabolites are incorporated into SLs, whereas others may initiate regulatory or signaling events that, in turn, modulate SL metabolism. In this review, we elaborate on the former as it represents a poorly appreciated aspect of SL metabolism, and we develop the hypothesis that the SL network is highly sensitive to several specific metabolic changes, focusing on amino acids (serine and alanine), various fatty acids, choline (and ethanolamine), and glucose.

The Candida albicans quorum-sensing molecule farnesol alters sphingolipid metabolism in human monocyte-derived dendritic cells

Candida albicans, an opportunistic fungal pathogen, produces the quorum-sensing molecule farnesol, which we have shown alters the transcriptional response and phenotype of human monocyte-derived dendritic cells (DCs), including their cytokine secretion and ability to prime T cells. This is partially dependent on the nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR-γ), which has numerous ligands, including the sphingolipid metabolite sphingosine 1-phosphate. Sphingolipids are a vital component of membranes that affect membrane protein arrangement and phagocytosis of C. albicans by DCs. Thus, we quantified sphingolipid metabolites in monocytes differentiating into DCs by High-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Farnesol increased the activity of serine palmitoyltransferase, leading to increased levels of 3-keto-dihydrosphingosine, dihydrosphingosine, and dihydrosphingosine 1-phosphate and inhibited dihydroceramide desaturase by inducing oxidative stress, leading to increased levels of dihydroceramide and dihydrosphingomyelin species and reduced ceramide levels. Accumulation of dihydroceramides can inhibit mitochondrial function; accordingly, farnesol reduced mitochondrial respiration. Dihydroceramide desaturase inhibition increases lipid droplet formation, which we observed in farnesol-treated cells, coupled with an increase in intracellular triacylglycerol species. Furthermore, inhibition of dihydroceramide desaturase with either farnesol or specific inhibitors impaired the ability of DCs to prime interferon-γ-producing T cells. The effect of farnesol on sphingolipid metabolism, triacylglycerol synthesis, and mitochondrial respiration was not dependent on PPAR-γ. In summary, our data reveal novel effects of farnesol on sphingolipid metabolism, neutral lipid synthesis, and mitochondrial function in DCs that affect their instruction of T cell cytokine secretion, indicating that C. albicans can manipulate host cell metabolism via farnesol secretion.IMPORTANCECandida albicans is a common commensal yeast, but it is also an opportunistic pathogen which is one of the leading causes of potentially lethal hospital-acquired infections. There is growing evidence that its overgrowth in the gut can influence diseases as diverse as alcohol-associated liver disease and COVID-19. Previously, we found that its quorum-sensing molecule, farnesol, alters the phenotype of dendritic cells differentiating from monocytes, impairing their ability to drive protective T cell responses. Here, we demonstrate that farnesol alters the metabolism of sphingolipids, important structural components of the membrane that also act as signaling molecules. In monocytes differentiating to dendritic cells, farnesol inhibited dihydroceramide desaturase, resulting in the accumulation of dihydroceramides and a reduction in ceramide levels. Farnesol impaired mitochondrial respiration, known to occur with an accumulation of dihydroceramides, and induced the accumulation of triacylglycerol and oil bodies. Inhibition of dihydroceramide desaturase resulted in the impaired ability of DCs to induce interferon-γ production by T cells. Thus, farnesol production by C. albicans could manipulate the function of dendritic cells by altering the sphingolipidome.

Nuclear Receptors and the Hidden Language of the Metabolome

Nuclear hormone receptors (NHRs) are a family of ligand-regulated transcription factors that control key aspects of development and physiology. The regulation of NHRs by ligands derived from metabolism or diet makes them excellent pharmacological targets, and the mechanistic understanding of how NHRs interact with their ligands to regulate downstream gene networks, along with the identification of ligands for orphan NHRs, could enable innovative approaches for cellular engineering, disease modeling and regenerative medicine. We review recent discoveries in the identification of physiologic ligands for NHRs. We propose new models of ligand-receptor co-evolution, the emergence of hormonal function and models of regulation of NHR specificity and activity via one-ligand and two-ligand models as well as feedback loops. Lastly, we discuss limitations on the processes for the identification of physiologic NHR ligands and emerging new methodologies that could be used to identify the natural ligands for the remaining 17 orphan NHRs in the human genome.

Ceramides are fuel gauges on the drive to cardiometabolic disease

Ceramides are signals of fatty acid excess that accumulate when a cell's energetic needs have been met and its nutrient storage has reached capacity. As these sphingolipids accrue, they alter the metabolism and survival of cells throughout the body including in the heart, liver, blood vessels, skeletal muscle, brain, and kidney. These ceramide actions elicit the tissue dysfunction that underlies cardiometabolic diseases such as diabetes, coronary artery disease, metabolic-associated steatohepatitis, and heart failure. Here, we review the biosynthesis and degradation pathways that maintain ceramide levels in normal physiology and discuss how the loss of ceramide homeostasis drives cardiometabolic pathologies. We highlight signaling nodes that sense small changes in ceramides and in turn reprogram cellular metabolism and stimulate apoptosis. Finally, we evaluate the emerging therapeutic utility of these unique lipids as biomarkers that forecast disease risk and as targets of ceramide-lowering interventions that ameliorate disease.

Unraveling the multifaceted insights into amyotrophic lateral sclerosis: Genetic underpinnings, pathogenesis, and therapeutic horizons

Amyotrophic Lateral Sclerosis (ALS), a progressive neurodegenerative disease, primarily impairs upper and lower motor neurons, leading to debilitating motor dysfunction and eventually respiratory failure, widely known as Lou Gehrig's disease. ALS presents with diverse symptomatology, including dysarthria, dysphagia, muscle atrophy, and hyperreflexia. The prevalence of ALS varies globally, with incidence rates ranging from 1.5 to 3.8 per 100,000 individuals, significantly affecting populations aged 45-80. A complex interplay of genetic and environmental factors underpins ALS pathogenesis. Key genetic contributors include mutations in chromosome 9 open reading frame 72 (C9ORF72), superoxide dismutase type 1 (SOD1), Fusedin sarcoma (FUS), and TAR DNA-binding protein (TARDBP) genes, accounting for a considerable fraction of both familial (fALS) and sporadic (sALS) cases. The disease mechanism encompasses aberrant protein folding, mitochondrial dysfunction, oxidative stress, excitotoxicity, and neuroinflammation, contributing to neuronal death. This review consolidates current insights into ALS's multifaceted etiology, highlighting the roles of environmental exposures (e.g., toxins, heavy metals) and their interaction with genetic predispositions. We emphasize the polygenic nature of ALS, where multiple genetic variations cumulatively influence disease susceptibility and progression. This aspect underscores the challenges in ALS diagnosis, which currently lacks specific biomarkers and relies on symptomatology and familial history. Therapeutic strategies for ALS, still in nascent stages, involve symptomatic management and experimental approaches targeting molecular pathways implicated in ALS pathology. Gene therapy, focusing on specific ALS mutations, and stem cell therapy emerge as promising avenues. However, effective treatments remain elusive, necessitating a deeper understanding of ALS's genetic architecture and the development of targeted therapies based on personalized medicine principles. This review aims to provide a comprehensive understanding of ALS, encouraging further research into its complex genetic underpinnings and the development of innovative, effective treatment modalities.

Therapeutic implications for sphingolipid metabolism in metabolic dysfunction-associated steatohepatitis

The prevalence of metabolic dysfunction-associated steatotic liver disease (MASLD), a leading cause of chronic liver disease, has increased worldwide along with the epidemics of obesity and related dysmetabolic conditions characterized by impaired glucose metabolism and insulin signaling, such as type 2 diabetes mellitus (T2D). MASLD can be defined as an excessive accumulation of lipid droplets in hepatocytes that occurs when the hepatic lipid metabolism is totally surpassed. This metabolic lipid inflexibility constitutes a central node in the pathogenesis of MASLD and is frequently linked to the overproduction of lipotoxic species, increased cellular stress, and mitochondrial dysfunction. A compelling body of evidence suggests that the accumulation of lipid species derived from sphingolipid metabolism, such as ceramides, contributes significantly to the structural and functional tissue damage observed in more severe grades of MASLD by triggering inflammatory and fibrogenic mechanisms. In this context, MASLD can further progress to metabolic dysfunction-associated steatohepatitis (MASH), which represents the advanced form of MASLD, and hepatic fibrosis. In this review, we discuss the role of sphingolipid species as drivers of MASH and the mechanisms involved in the disease. In addition, given the absence of approved therapies and the limited options for treating MASH, we discuss the feasibility of therapeutic strategies to protect against MASH and other severe manifestations by modulating sphingolipid metabolism.

Ceramides and mitochondrial homeostasis

Lipotoxicity arises from the accumulation of lipid intermediates in non-adipose tissue, precipitating cellular dysfunction and death. Ceramide, a toxic byproduct of excessive free fatty acids, has been widely recognized as a primary contributor to lipotoxicity, mediating various cellular processes such as apoptosis, differentiation, senescence, migration, and adhesion. As the hub of lipid metabolism, the excessive accumulation of ceramides inevitably imposes stress on the mitochondria, leading to the disruption of mitochondrial homeostasis, which is typified by adequate ATP production, regulated oxidative stress, an optimal quantity of mitochondria, and controlled mitochondrial quality. Consequently, this review aims to collate current knowledge and facts regarding the involvement of ceramides in mitochondrial energy metabolism and quality control, thereby providing insights for future research.

Sphingolipid-Induced Bone Regulation and Its Emerging Role in Dysfunction Due to Disease and Infection

The human skeleton is a metabolically active system that is constantly regenerating via the tightly regulated and highly coordinated processes of bone resorption and formation. Emerging evidence reveals fascinating new insights into the role of sphingolipids, including sphingomyelin, sphingosine, ceramide, and sphingosine-1-phosphate, in bone homeostasis. Sphingolipids are a major class of highly bioactive lipids able to activate distinct protein targets including, lipases, phosphatases, and kinases, thereby conferring distinct cellular functions beyond energy metabolism. Lipids are known to contribute to the progression of chronic inflammation, and notably, an increase in bone marrow adiposity parallel to elevated bone loss is observed in most pathological bone conditions, including aging, rheumatoid arthritis, osteoarthritis, and osteomyelitis. Of the numerous classes of lipids that form, sphingolipids are considered among the most deleterious. This review highlights the important primary role of sphingolipids in bone homeostasis and how dysregulation of these bioactive metabolites appears central to many chronic bone-related diseases. Further, their contribution to the invasion, virulence, and colonization of both viral and bacterial host cell infections is also discussed. Many unmet clinical needs remain, and data to date suggest the future use of sphingolipid-targeted therapy to regulate bone dysfunction due to a variety of diseases or infection are highly promising. However, deciphering the biochemical and molecular mechanisms of this diverse and extremely complex sphingolipidome, both in terms of bone health and disease, is considered the next frontier in the field.

Recurrent de novo SPTLC2 variant causes childhood-onset amyotrophic lateral sclerosis (ALS) by excess sphingolipid synthesis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease of the upper and lower motor neurons with varying ages of onset, progression and pathomechanisms. Monogenic childhood-onset ALS, although rare, forms an important subgroup of ALS. We recently reported specific SPTLC1 variants resulting in sphingolipid overproduction as a cause for juvenile ALS. Here, we report six patients from six independent families with a recurrent, de novo, heterozygous variant in SPTLC2 c.778G>A [p.Glu260Lys] manifesting with juvenile ALS.

METHODS

Clinical examination of the patients along with ancillary and genetic testing, followed by biochemical investigation of patients' blood and fibroblasts, was performed.

RESULTS

All patients presented with early-childhood-onset progressive weakness, with signs and symptoms of upper and lower motor neuron degeneration in multiple myotomes, without sensory neuropathy. These findings were supported on ancillary testing including nerve conduction studies and electromyography, muscle biopsies and muscle ultrasound studies. Biochemical investigations in plasma and fibroblasts showed elevated levels of ceramides and unrestrained de novo sphingolipid synthesis. Our studies indicate that SPTLC2 variant [c.778G>A, p.Glu260Lys] acts distinctly from hereditary sensory and autonomic neuropathy (HSAN)-causing SPTLC2 variants by causing excess canonical sphingolipid biosynthesis, similar to the recently reported SPTLC1 ALS associated pathogenic variants. Our studies also indicate that serine supplementation, which is a therapeutic in SPTLC1 and SPTCL2-associated HSAN, is expected to exacerbate the excess sphingolipid synthesis in serine palmitoyltransferase (SPT)-associated ALS.

CONCLUSIONS

SPTLC2 is the second SPT-associated gene that underlies monogenic, juvenile ALS and further establishes alterations of sphingolipid metabolism in motor neuron disease pathogenesis. Our findings also have important therapeutic implications: serine supplementation must be avoided in SPT-associated ALS, as it is expected to drive pathogenesis further.

Circadian organization of lipid landscape is perturbed in type 2 diabetic patients

Lipid homeostasis in humans follows a diurnal pattern in muscle and pancreatic islets, altered upon metabolic dysregulation. We employ tandem and liquid-chromatography mass spectrometry to investigate daily regulation of lipid metabolism in subcutaneous white adipose tissue (SAT) and serum of type 2 diabetic (T2D) and non-diabetic (ND) human volunteers (n = 12). Around 8% of ≈440 lipid metabolites exhibit diurnal rhythmicity in serum and SAT from ND and T2D subjects. The spectrum of rhythmic lipids differs between ND and T2D individuals, with the most substantial changes observed early morning, as confirmed by lipidomics in an independent cohort of ND and T2D subjects (n = 32) conducted at a single morning time point. Strikingly, metabolites identified as daily rhythmic in both serum and SAT from T2D subjects exhibit phase differences. Our study reveals massive temporal and tissue-specific alterations of human lipid homeostasis in T2D, providing essential clues for the development of lipid biomarkers in a temporal manner.

Pilot Lipidomics Study of Copepods: Investigation of Potential Lipid-Based Biomarkers for the Early Detection and Quantification of the Biological Effects of Climate Change on the Oceanic Food Chain

Maintenance of the health of our oceans is critical for the survival of the oceanic food chain upon which humanity is dependent. Zooplanktonic copepods are among the most numerous multicellular organisms on earth. As the base of the primary consumer food web, they constitute a major biomass in oceans, being an important food source for fish and functioning in the carbon cycle. The potential impact of climate change on copepod populations is an area of intense study. Omics technologies offer the potential to detect early metabolic alterations induced by the stresses of climate change. One such omics approach is lipidomics, which can accurately quantify changes in lipid pools serving structural, signal transduction, and energy roles. We utilized high-resolution mass spectrometry (≤2 ppm mass error) to characterize the lipidome of three different species of copepods in an effort to identify lipid-based biomarkers of copepod health and viability which are more sensitive than observational tools. With the establishment of such a lipid database, we will have an analytical platform useful for prospectively monitoring the lipidome of copepods in a planned long-term five-year ecological study of climate change on this oceanic sentinel species. The copepods examined in this pilot study included a North Atlantic species (Calanus finmarchicus) and two species from the Gulf of Mexico, one a filter feeder (Acartia tonsa) and one a hunter (Labidocerca aestiva). Our findings clearly indicate that the lipidomes of copepod species can vary greatly, supporting the need to obtain a broad snapshot of each unique lipidome in a long-term multigeneration prospective study of climate change. This is critical, since there may well be species-specific responses to the stressors of climate change and co-stressors such as pollution. While lipid nomenclature and biochemistry are extremely complex, it is not essential for all readers interested in climate change to understand all of the various lipid classes presented in this study. The clear message from this research is that we can monitor key copepod lipid families with high accuracy, and therefore potentially monitor lipid families that respond to environmental perturbations evoked by climate change.

The atypical sphingolipid SPB 18:1(14Z);O2 is a biomarker for DEGS1 related hypomyelinating leukodystrophy

Sphingolipids (SL) represent a structurally diverse class of lipids that are central to cellular physiology and neuronal development and function. Defects in the sphingolipid metabolism are typically associated with nervous system disorders. The C4-dihydroceramide desaturase (DEGS1) catalyzes the conversion of dihydroceramide to ceramide, the final step in the SL de-novo synthesis. Loss of function mutations in DEGS1 cause a hypomyelinating leukodystrophy, which is associated with increased plasma dihydrosphingolipids (dhSL) and with the formation of an atypical SPB 18:1(14Z);O2 metabolite. Here, we characterize two novel DEGS1 variants of unknown significance (VUS), provide a structural model with a predicted substrate binding site, and propose a regulatory link between DEGS1 and fatty acid desaturase 3 (FADS3). Both VUS involve single amino acid substitutions near the C-terminus within conserved regions of the enzyme. Patient 1 (p.R311K variant) shows severe progressive tetraspasticity, intellectual disability, and epilepsy in combination with brain magnetic resonance imaging (MRI) findings, typical for DEGS1-related leukodystrophy. Patient 2 (p.G270E variant) presents with delayed psychomotor development, oculomotor apraxia, and a normal brain MRI. Plasma from the p.R311K carrier showed a significantly elevated dhSL species and the presence of SPB 18:1(14Z);O2, while the plasma SL profile for the p.G270E variant was not altered. This suggests the p.R331K variant is pathogenic, while the p.G270E appears benign. As an increase in dihydroSL species is also seen in other pathological disorders of the SL metabolism, the SPB 18:1(14Z);O2 seems to be a more specific biomarker to discriminate between pathogenic and benign DEGS1 variants.

Metabolic bias: Lipid structures as determinants of their metabolic fates

Cellular lipids have an enormous diversity in their chemical structures, which affect the physicochemical properties of lipids and membranes, as well as their regulatory roles on protein functions. Here, I review additional roles of lipid structures. Multiple studies show that structural differences affect how lipids, even from the same class, are metabolically converted via distinct pathways. I propose the name "structure-guided metabolic bias" for this phenomenon, and discuss its biological relevance. This metabolic bias seems implicated in the buildup of basic cellular lipid compositions, as well as genetic predisposition to diseases. Thus, guiding metabolic biases is an important function of lipid structures, while having the characteristic of being difficult to study by in vitro biochemical reconstitutions.

Unraveling the mysteries of macular telangiectasia 2: the intersection of philanthropy, multimodal imaging and molecular genetics. The 2022 founders lecture of the pan American vitreoretinal society

PURPOSE

Offer a personal perspective on the scientific advances on macular telangiectasia type 2 (MacTel2) since the launch of the MacTel Project in 2005.

DESIGN

Literature review and personal perspective.

METHODS

Critical review of the peer-reviewed literature and personal perspective.

RESULTS

Generous financial support from the Lowy Medical Research Institute laid the foundations of the MacTel Project. MacTel Project investigators used state of the art multimodal retinal imaging and advanced modern biological methods to unravel many of the mysteries surrounding MacTel2. Major accomplishments includes elucidation of the pathogenic role that low serine levels, elevated 1-deoxysphingolipids and other mechanisms induce mitochondrial dysfunction which lead to Müller cell and photoreceptor degeneration; the use of objective measures of retinal structures such as the area of ellipsoid zone disruption as an outcome measure in clinical trials; the demonstration that the ciliary neurotrophic factor slows down retinal degeneration and the development of a new severity scale classification based on multimodal imaging findings.

CONCLUSIONS

MacTel2 is a predominantly metabolic disease characterized by defects in energy metabolism. Despite relatively good visual acuities, MacTel2 patients experience significant visual disability. The Mac Tel Project has been instrumental in advancing MacTel2 knowledge in the past two decades.

Hepatocyte ABCA1 deficiency is associated with reduced HDL sphingolipids

ATP binding cassette transporter A1 (ABCA1) limits the formation of high density lipoproteins (HDL) as genetic loss of ABCA1 function causes virtual HDL deficiency in patients with Tangier disease. Mice with a hepatocyte-specific ABCA1 knockout (Abca1 HSKO) have 20% of wild type (WT) plasma HDL-cholesterol levels, suggesting a major contribution of hepatic ABCA1 to the HDL phenotype. Whether plasma sphingolipids are reduced in Tangier disease and to what extent hepatic ABCA1 contributes to plasma sphingolipid (SL) levels is unknown. Here, we report a drastic reduction of total SL levels in plasma of a Tangier patient with compound heterozygosity for mutations in ABCA1. Compared to mutation-free controls, heterozygous mutations in ABCA1 had no significant effect on total SLs in plasma; however, apoB-depleted plasma showed a reduction in total SL also in het carriers. Similarly, liver specific Abca1 KO mice (Abca1 HSKO) showed reduced total sphingolipids in plasma and liver. In parallel, apoM and sphingosine-1-phosphate (S1P) levels were reduced in plasma of Abca1 HSKO mice. Primary hepatocytes from Abca1 HSKO mice showed a modest, but significant reduction in total SLs concentration compared to WT hepatocytes, although SL de novo synthesis and secretion were slightly increased in Abca1 HSKO hepatocytes. We conclude that hepatic ABCA1 is a signficant contributor to maintaining total plasma pool of HDL sphingolipids, including sphingomyelins and S1P.

Imbalanced unfolded protein response signaling contributes to 1-deoxysphingolipid retinal toxicity

The accumulation of atypical, cytotoxic 1-deoxysphingolipids (1-dSLs) has been linked to retinal diseases such as diabetic retinopathy and Macular Telangiectasia Type 2. However, the molecular mechanisms by which 1-dSLs induce toxicity in retinal cells remain poorly understood. Here, we integrate bulk and single-nucleus RNA-sequencing to define biological pathways that modulate 1-dSL toxicity in human retinal organoids. Our results demonstrate that 1-dSLs differentially activate signaling arms of the unfolded protein response (UPR) in photoreceptor cells and Müller glia. Using a combination of pharmacologic activators and inhibitors, we show that sustained PERK signaling through the integrated stress response (ISR) and deficiencies in signaling through the protective ATF6 arm of the UPR are implicated in 1-dSL-induced photoreceptor toxicity. Further, we demonstrate that pharmacologic activation of ATF6 mitigates 1-dSL toxicity without impacting PERK/ISR signaling. Collectively, our results identify new opportunities to intervene in 1-dSL linked diseases through targeting different arms of the UPR.

Impact of sphingolipids on protein membrane trafficking

Membrane trafficking is essential to maintain the spatiotemporal control of protein and lipid distribution within membrane systems of eukaryotic cells. To achieve their functional destination proteins are sorted and transported into lipid carriers that construct the secretory and endocytic pathways. It is an emerging theme that lipid diversity might exist in part to ensure the homeostasis of these pathways. Sphingolipids, a chemical diverse type of lipids with special physicochemical characteristics have been implicated in the selective transport of proteins. In this review, we will discuss current knowledge about how sphingolipids modulate protein trafficking through the endomembrane systems to guarantee that proteins reach their functional destination and the proposed underlying mechanisms.

Insulin-regulated serine and lipid metabolism drive peripheral neuropathy

Diabetes represents a spectrum of disease in which metabolic dysfunction damages multiple organ systems including liver, kidneys and peripheral nerves1,2. Although the onset and progression of these co-morbidities are linked with insulin resistance, hyperglycaemia and dyslipidaemia3-7, aberrant non-essential amino acid (NEAA) metabolism also contributes to the pathogenesis of diabetes8-10. Serine and glycine are closely related NEAAs whose levels are consistently reduced in patients with metabolic syndrome10-14, but the mechanistic drivers and downstream consequences of this metabotype remain unclear. Low systemic serine and glycine are also emerging as a hallmark of macular and peripheral nerve disorders, correlating with impaired visual acuity and peripheral neuropathy15,16. Here we demonstrate that aberrant serine homeostasis drives serine and glycine deficiencies in diabetic mice, which can be diagnosed with a serine tolerance test that quantifies serine uptake and disposal. Mimicking these metabolic alterations in young mice by dietary serine or glycine restriction together with high fat intake markedly accelerates the onset of small fibre neuropathy while reducing adiposity. Normalization of serine by dietary supplementation and mitigation of dyslipidaemia with myriocin both alleviate neuropathy in diabetic mice, linking serine-associated peripheral neuropathy to sphingolipid metabolism. These findings identify systemic serine deficiency and dyslipidaemia as novel risk factors for peripheral neuropathy that may be exploited therapeutically.

Long-term effects of l-serine supplementation upon a mouse model of diabetic neuropathy

Deoxysphingolipids (1-deoxySLs) are neurotoxic sphingolipids associated with obesity and diabetic neuropathy (DN) and have been linked to severity of functional peripheral neuropathies. While l-serine supplementation can reduce 1-deoxySL accumulation and improve insulin sensitivity and sensory nerve velocity, long-term outcomes have not yet been examined. To assess this, we treated 2 month old db/db mice, a model of DN, with 5-20 % oral l-serine for 6 months and longitudinally quantified the extent of functional neuropathy progression. We examined putative biomarkers of neuropathy in blood and tissue and quantified levels of small fiber neuropathy, looking for associations between lowered 1-deoxySL and phenotypes. Toxic 1-deoxySLs were suppressed long-term in plasma and various tissue including the sciatic nerve, which is particularly targeted in DN. Functional neuropathy and sensory modalities were significantly improved in the treatment group well into advanced stages of disease. However, structural assessments revealed prominent axonal degeneration, apoptosis and Schwann cell pathology, suggesting that neuropathy was ongoing. Hyperglycemia and dyslipidemia persisted during our study, and high levels of glutathione were seen in the spinal cord. Our results demonstrate that despite significant functional improvements, l-serine does not prevent chronic degenerative changes specifically at the structural level, pointing to other processes such as oxidative damage and hyperglycemia, that persist despite 1-deoxySL reduction.

Sphingolipids accumulate in aged muscle, and their reduction counteracts sarcopenia

Age-related muscle dysfunction and sarcopenia are major causes of physical incapacitation in older adults and currently lack viable treatment strategies. Here we find that sphingolipids accumulate in mouse skeletal muscle upon aging and that both genetic and pharmacological inhibition of sphingolipid synthesis prevent age-related decline in muscle mass while enhancing strength and exercise capacity. Inhibition of sphingolipid synthesis confers increased myogenic potential and promotes protein synthesis. Within the sphingolipid pathway, we show that accumulation of dihydroceramides is the culprit disturbing myofibrillar homeostasis. The relevance of sphingolipid pathways in human aging is demonstrated in two cohorts, the UK Biobank and Helsinki Birth Cohort Study in which gene expression-reducing variants of SPTLC1 and DEGS1 are associated with improved and reduced fitness of older individuals, respectively. These findings identify sphingolipid synthesis inhibition as an attractive therapeutic strategy for age-related sarcopenia and co-occurring pathologies.

Precision mouse models of Yars/dominant intermediate Charcot-Marie-Tooth disease type C and Sptlc1/hereditary sensory and autonomic neuropathy type 1

Animal models of neurodegenerative diseases such as inherited peripheral neuropathies sometimes accurately recreate the pathophysiology of the human disease, and sometimes accurately recreate the genetic perturbations found in patients. Ideally, models achieve both, but this is not always possible; nonetheless, such models are informative. Here we describe two animal models of inherited peripheral neuropathy: mice with a mutation in tyrosyl tRNA-synthetase, YarsE196K , modeling dominant intermediate Charcot-Marie-Tooth disease type C (diCMTC), and mice with a mutation in serine palmitoyltransferase long chain 1, Sptlc1C133W , modeling hereditary sensory and autonomic neuropathy type 1 (HSAN1). YarsE196K mice develop disease-relevant phenotypes including reduced motor performance and reduced nerve conduction velocities by 4 months of age. Peripheral motor axons are reduced in size, but there is no reduction in axon number and plasma neurofilament light chain levels are not increased. Unlike the dominant human mutations, the YarsE196K mice only show these phenotypes as homozygotes, or as compound heterozygotes with a null allele, and no phenotype is observed in E196K or null heterozygotes. The Sptlc1C133W mice carry a knockin allele and show the anticipated increase in 1-deoxysphingolipids in circulation and in a variety of tissues. They also have mild behavioral defects consistent with HSAN1, but do not show neurophysiological defects or axon loss in peripheral nerves or in the epidermis of the hind paw or tail. Thus, despite the biochemical phenotype, the Sptlc1C133W mice do not show a strong neuropathy phenotype. Surprisingly, these mice were lethal as homozygotes, but the heterozygous genotype studied corresponds to the dominant genetics seen in humans. Thus, YarsE196K homozygous mice have a relevant phenotype, but imprecisely reproduce the human genetics, whereas the Sptlc1C133W mice precisely reproduce the human genetics, but do not recreate the disease phenotype. Despite these shortcomings, both models are informative and will be useful for future research.

SPTLC1 variants associated with ALS produce distinct sphingolipid signatures through impaired interaction with ORMDL proteins

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that affects motor neurons. Mutations in the SPTLC1 subunit of serine palmitoyltransferase (SPT), which catalyzes the first step in the de novo synthesis of sphingolipids (SLs), cause childhood-onset ALS. SPTLC1-ALS variants map to a transmembrane domain that interacts with ORMDL proteins, negative regulators of SPT activity. We show that ORMDL binding to the holoenzyme complex is impaired in cells expressing pathogenic SPTLC1-ALS alleles, resulting in increased SL synthesis and a distinct lipid signature. C-terminal SPTLC1 variants cause peripheral hereditary sensory and autonomic neuropathy type 1 (HSAN1) due to the synthesis of 1-deoxysphingolipids (1-deoxySLs) that form when SPT metabolizes L-alanine instead of L-serine. Limiting L-serine availability in SPTLC1-ALS-expressing cells increased 1-deoxySL and shifted the SL profile from an ALS to an HSAN1-like signature. This effect was corroborated in an SPTLC1-ALS pedigree in which the index patient uniquely presented with an HSAN1 phenotype, increased 1-deoxySL levels, and an L-serine deficiency. These data demonstrate how pathogenic variants in different domains of SPTLC1 give rise to distinct clinical presentations that are nonetheless modifiable by substrate availability.

1-deoxysphingolipid synthesis compromises anchorage-independent growth and plasma membrane endocytosis in cancer cells

Serine palmitoyltransferase (SPT) predominantly incorporates serine and fatty acyl-CoAs into diverse sphingolipids that serve as structural components of membranes and signaling molecules within or amongst cells. However, SPT also uses alanine as a substrate in the contexts of low serine availability, alanine accumulation, or disease-causing mutations in hereditary sensory neuropathy type I (HSAN1), resulting in the synthesis and accumulation of 1-deoxysphingolipids. These species promote cytotoxicity in neurons and impact diverse cellular phenotypes, including suppression of anchorage-independent cancer cell growth. While altered serine and alanine levels can promote 1-deoxysphingolipid synthesis, they impact numerous other metabolic pathways important for cancer cells. Here we combined isotope tracing, quantitative metabolomics, and functional studies to better understand the mechanistic drivers of 1-deoxysphingolipid toxicity in cancer cells. We determined that both alanine treatment and SPTLC1^C133W expression induce 1-deoxy(dihydro)ceramide synthesis and accumulation but fail to broadly impact intermediary metabolism, abundances of other lipids, or growth of adherent cells. However, we found spheroid culture and soft agar colony formation were compromised when endogenous 1-deoxysphingolipid synthesis was induced via SPTLC1^C133W expression. Consistent with these impacts on anchorage-independent cell growth, we observed that 1-deoxysphingolipid synthesis reduced plasma membrane endocytosis. These results highlight a potential role for SPT promiscuity in linking altered amino acid metabolism to plasma membrane endocytosis.

Sphingolipids and their role in health and disease in the central nervous system

Sphingolipids (SLs) are lipids derived from sphingosine, and their metabolism involves a broad and complex network of reactions. Although SLs are widely distributed in the body, it is well known that they are present in high concentrations within the central nervous system (CNS). Under physiological conditions, their abundance and distribution in the CNS depend on brain development and cell type. Consequently, SLs metabolism impairment may have a significant impact on the normal CNS function, and has been associated with several disorders, including sphingolipidoses, Parkinson's, and Alzheimer's. This review summarizes the main SLs characteristics and current knowledge about synthesis, catabolism, regulatory pathways, and their role in physiological and pathological scenarios in the CNS.

Description of a patient cohort with Hereditary Sensory Neuropathy Type 1 without retinal disease Macular Telangiectasia type 2 - implications for retinal screening in HSN1

BACKGROUND AND AIMS

Pathogenic variants in the genes encoding serine palmitoyl transferase (SPTLC1 or SPTLC2) are the most common causes of the rare peripheral nerve disorder Hereditary Sensory Neuropathy Type 1 (HSN1). Macular telangiectasia type 2 (MacTel), a retinal disorder associated with disordered serine-glycine metabolism and has been described in some patients with HSN1. This study aims to further investigate this association in a cohort of people with HSN1.

METHODS

Fourteen patients with a clinically and genetically confirmed diagnosis of HSN1 from the National Hospital for Neurology and Neurosurgery (NHNN, University College London Hospitals NHS Foundation Trust, London, United Kingdom) were recruited to the MacTel Registry, between July 2018 and April 2019. Two additional patients were identified from the dataset of the international clinical registry study (www.lmri.net). Ocular examination included fundus autofluorescence, blue light and infrared reflectance, macular pigment optical density mapping, and optical coherence tomography.

RESULTS

Twelve patients had a pathogenic variant in the SPTLC1 gene, with p.Cys133Trp in eleven cases (92%) and p.Cys133Tyr in one case (8%). Four patients had a variant in the SPTLC2 gene. None of the patients showed clinical evidence of MacTel.

INTERPRETATION

The link between HSN1 and MacTel seems more complex than can solely be explained by the genetic variants. An extension of the spectrum of SPTLC1/2-related disease with phenotypic pleiotropy is proposed. HSN1 patients should be screened for visual symptoms and referred for specialist retinal screening, but the association of the two diseases is likely to be variable and remains unexplained. This article is protected by copyright. All rights reserved.

Circadian rhythm of lipid metabolism

Lipids comprise a diverse group of metabolites that are indispensable as energy storage molecules, cellular membrane components and mediators of inter- and intra-cellular signaling processes. Lipid homeostasis plays a crucial role in maintaining metabolic health in mammals including human beings. A growing body of evidence suggests that the circadian clock system ensures temporal orchestration of lipid homeostasis, and that perturbation of such diurnal regulation leads to the development of metabolic disorders comprising obesity and type 2 diabetes. In view of the emerging role of circadian regulation in maintaining lipid homeostasis, in this review, we summarize the current knowledge on lipid metabolic pathways controlled by the mammalian circadian system. Furthermore, we review the emerging connection between the development of human metabolic diseases and changes in lipid metabolites that belong to major classes of lipids. Finally, we highlight the mechanisms underlying circadian organization of lipid metabolic rhythms upon the physiological situation, and the consequences of circadian clock dysfunction for dysregulation of lipid metabolism.

The Role of Sphingolipid Signaling in Oxidative Lung Injury and Pathogenesis of Bronchopulmonary Dysplasia

Premature infants are born with developing lungs burdened by surfactant deficiency and a dearth of antioxidant defense systems. Survival rate of such infants has significantly improved due to advances in care involving mechanical ventilation and oxygen supplementation. However, a significant subset of such survivors develops the chronic lung disease, Bronchopulmonary dysplasia (BPD), characterized by enlarged, simplified alveoli and deformed airways. Among a host of factors contributing to the pathogenesis is oxidative damage induced by exposure of the developing lungs to hyperoxia. Recent data indicate that hyperoxia induces aberrant sphingolipid signaling, leading to mitochondrial dysfunction and abnormal reactive oxygen species (ROS) formation (ROS). The role of sphingolipids such as ceramides and sphingosine 1-phosphate (S1P), in the development of BPD emerged in the last decade. Both ceramide and S1P are elevated in tracheal aspirates of premature infants of <32 weeks gestational age developing BPD. This was faithfully reflected in the murine models of hyperoxia and BPD, where there is an increased expression of sphingolipid metabolites both in lung tissue and bronchoalveolar lavage. Treatment of neonatal pups with a sphingosine kinase1 specific inhibitor, PF543, resulted in protection against BPD as neonates, accompanied by improved lung function and reduced airway remodeling as adults. This was accompanied by reduced mitochondrial ROS formation. S1P receptor1 induced by hyperoxia also aggravates BPD, revealing another potential druggable target in this pathway for BPD. In this review we aim to provide a detailed description on the role played by sphingolipid signaling in hyperoxia induced lung injury and BPD.

Progressive alterations in amino acid and lipid metabolism correlate with peripheral neuropathy in PolgD257A mice

Mitochondria are central to metabolic homeostasis, and progressive mitochondrial defects have diverse metabolic consequences that could drive distinct pathophysiological states. Here, we comprehensively characterized metabolic alterations in PolgD257A mice. Plasma alanine increased markedly with time, with other organic acids accumulating to a lesser extent. These changes were reflective of increased Cori and Cahill cycling in PolgD257A mice and subsequent hypoglycemia, which did not occur during normal mouse aging. Tracing with [15N]ammonium further supported this shift in amino acid metabolism with mild impairment of the urea cycle. We also measured alterations in the lipidome, observing a reduction in canonical lipids and accumulation of 1-deoxysphingolipids, which are synthesized from alanine via promiscuous serine palmitoyltransferase activity and correlate with peripheral neuropathy. Consistent with this metabolic link, PolgD257A mice exhibited thermal hypoalgesia. These results highlight the longitudinal changes that occur in intermediary metabolism upon mitochondrial impairment and identify a contributing mechanism to mitochondria-associated neuropathy.

Metabolism of HSAN1- and T2DM-associated 1-deoxy-sphingolipids inhibits the migration of fibroblasts

Hereditary sensory neuropathy type 1 (HSAN1) is a rare axonopathy, characterized by a progressive loss of sensation (pain, temperature, and vibration), neuropathic pain, and wound healing defects. HSAN1 is caused by several missense mutations in the serine palmitoyltransferase long-chain base subunit 1 and serine palmitoyltransferase long-chain base subunit 2 of the enzyme serine palmitoyltransferase-the key enzyme for the synthesis of sphingolipids. The mutations change the substrate specificity of serine palmitoyltransferase, which then forms an atypical class of 1-deoxy-sphinglipids (1-deoxySLs). Similarly, patients with type 2 diabetes mellitus also present with elevated 1-deoxySLs and a comparable clinical phenotype. The effect of 1-deoxySLs on neuronal cells was investigated in detail, but their impact on other cell types remains elusive. Here, we investigated the consequences of externally added 1-deoxySLs on the migration of fibroblasts in a scratch assay as a simplified cellular wound-healing model. We showed that 1-deoxy-sphinganine (1-deoxySA) inhibits the migration of NIH-3T3 fibroblasts in a dose- and time-dependent manner. This was not seen for a non-native, L-threo stereoisomer. Supplemented 1-deoxySA was metabolized to 1-deoxy-(dihydro)ceramide and downstream to 1-deoxy-sphingosine. Inhibiting downstream metabolism by blocking N-acylation rescued the migration phenotype. In contrast, adding 1-deoxy-sphingosine had a lesser effect on cell migration but caused the massive formation of intracellular vacuoles. Further experiments showed that the effect on cell migration was primarily mediated by 1-deoxy-dihydroceramides rather than by the free base or 1-deoxyceramides. Based on these findings, we suggest that limiting the N-acylation of 1-deoxySA could be a therapeutic approach to improve cell migration and wound healing in patients with HSAN1 and type 2 diabetes mellitus.

1-Deoxysphingolipids cause autophagosome and lysosome accumulation and trigger NLRP3 inflammasome activation

1-Deoxysphingolipids (deoxySLs) are atypical sphingolipids of clinical relevance as they are elevated in plasma of patients suffering from hereditary sensory and autonomic neuropathy (HSAN1) or type 2 diabetes. Their neurotoxicity is described best but they inflict damage to various cell types by an uncertain pathomechanism. Using mouse embryonic fibroblasts and an alkyne analog of 1-deoxysphinganine (doxSA), the metabolic precursor of all deoxySLs, we here study the impact of deoxySLs on macroautophagy/autophagy, the regulated degradation of dysfunctional or expendable cellular components. We find that deoxySLs induce autophagosome and lysosome accumulation indicative of an increase in autophagic flux. The autophagosomal machinery targets damaged mitochondria that have accumulated N-acylated doxSA metabolites, presumably deoxyceramide and deoxydihydroceramide, and show aberrant swelling and tubule formation. Autophagosomes and lysosomes also interact with cellular lipid aggregates and crystals that occur upon cellular uptake and N-acylation of monomeric doxSA. As crystals entering the lysophagosomal apparatus in phagocytes are known to trigger the NLRP3 inflammasome, we also treated macrophages with doxSA. We demonstrate the activation of the NLRP3 inflammasome by doxSLs, prompting the release of IL1B from primary macrophages. Taken together, our data establish an impact of doxSLs on autophagy and link doxSL pathophysiology to inflammation and the innate immune system.Abbreviations: alkyne-doxSA: (2S,3R)-2-aminooctadec-17yn-3-ol; alkyne-SA: (2S,3R)-2- aminooctadec-17yn-1,3-diol; aSA: alkyne-sphinganine; ASTM-BODIPY: azido-sulfo-tetramethyl-BODIPY; CerS: ceramide synthase; CMR: clonal macrophage reporter; deoxySLs: 1-deoxysphingolipids; dox(DH)Cer: 1-deoxydihydroceramide; doxCer: 1-deoxyceramide; doxSA: 1-deoxysphinganine; FB1: fumonisin B1; HSAN1: hereditary sensory and autonomic neuropathy type 1; LC3: MAP1LC3A and MAP1LC3B; LPS: lipopolysaccharide; MEF: mouse embryonal fibroblasts; MS: mass spectrometry; N3635P: azido-STAR635P; N3Cy3: azido-cyanine 3; N3picCy3: azido-picolylcyanine 3; NLRP3: NOD-like receptor pyrin domain containing protein 3; P4HB: prolyl 4-hydroxylase subunit beta; PINK1: PTEN induced putative kinase 1; PYCARD/ASC: PYD and CARD domain containing; SPTLC1: serine palmitoyltransferase long chain base subunit 1; SQSTM1: sequestosome 1; TLC: thin layer chromatography.

Stereoselective Synthesis of Novel Sphingoid Bases Utilized for Exploring the Secrets of Sphinx

Sphingolipids are ubiquitous in eukaryotic plasma membranes and play major roles in human and animal physiology and disease. This class of lipids is usually defined as being derivatives of sphingosine, a long-chain 1,3-dihydroxy-2-amino alcohol. Various pathological conditions such as diabetes or neuropathy have been associated with changes in the sphingolipidome and an increased biosynthesis of structurally altered non-canonical sphingolipid derivatives. These unusual or non-canonical sphingolipids hold great promise as potential diagnostic markers. However, due to their low concentrations and the unavailability of suitable standards, the research to explore the secret of this class of 'Sphinx' lipids is ultimately hampered. Therefore, the development of efficient and facile syntheses of standard compounds is a key endeavor. Here, we present various chemical approaches for stereoselective synthesis and in-depth chemical characterization of a set of novel sphingoid bases which were recently utilized as valuable tools to explore the metabolism and biophysical properties of sphingolipids, but also to develop efficient analytical methods for their detection and quantification.

Lipidomics of Environmental Microbial Communities. II: Characterization Using Molecular Networking and Information Theory

Structurally diverse, specialized lipids are crucial components of microbial membranes and other organelles and play essential roles in ecological functioning. The detection of such lipids in the environment can reveal not only the occurrence of specific microbes but also the physicochemical conditions to which they are adapted to. Traditionally, liquid chromatography coupled with mass spectrometry allowed for the detection of lipids based on chromatographic separation and individual peak identification, resulting in a limited data acquisition and targeting of certain lipid groups. Here, we explored a comprehensive profiling of microbial lipids throughout the water column of a marine euxinic basin (Black Sea) using ultra high-pressure liquid chromatography coupled with high-resolution tandem mass spectrometry (UHPLC-HRMS/MS). An information theory framework combined with molecular networking based on the similarity of the mass spectra of lipids enabled us to capture lipidomic diversity and specificity in the environment, identify novel lipids, differentiate microbial sources within a lipid group, and discover potential biomarkers for biogeochemical processes. The workflow presented here allows microbial ecologists and biogeochemists to process quickly and efficiently vast amounts of lipidome data to understand microbial lipids characteristics in ecosystems.

Sphingolipids in metabolic disease: The good, the bad, and the unknown

The bioactive sphingolipid metabolites ceramide and sphingosine-1-phosphate (S1P) are a recent addition to the lipids accumulated in obesity and have emerged as important molecular players in metabolic diseases. Here we summarize evidence that dysregulation of sphingolipid metabolism correlates with pathogenesis of metabolic diseases in humans. This review discusses the current understanding of how ceramide regulates signaling and metabolic pathways to exacerbate metabolic diseases and the Janus faces for its further metabolite S1P, the kinases that produce it, and the multifaceted and at times opposing actions of S1P receptors in various tissues. Gaps and limitations in current knowledge are highlighted together with the need to further decipher the full array of their actions in tissue dysfunction underlying metabolic pathologies, pointing out prospects to move this young field of research toward the development of effective therapeutics.

The long chain base unsaturation has a stronger impact on 1-deoxy(methyl)-sphingolipids biophysical properties than the structure of its C1 functional group

1-deoxy-sphingolipids, also known as atypical sphingolipids, are directly implicated in the development and progression of hereditary sensory and autonomic neuropathy type 1 and diabetes type 2. The mechanisms underlying their patho-physiological actions are yet to be elucidated. Accumulating evidence suggests that the biological actions of canonical sphingolipids are triggered by changes promoted on membrane organization and biophysical properties. However, little is known regarding the biophysical implications of atypical sphingolipids. In this study, we performed a comprehensive characterization of the effects of the naturally occurring 1-deoxy-dihydroceramide, 1-deoxy-ceramide^Δ14Z and 1-deoxymethyl-ceramide^Δ3E in the properties of a fluid membrane. In addition, to better define which structural features determine sphingolipid ability to form ordered domains, the synthetic 1-O-methyl-ceramide^Δ4E and 1-deoxy-ceramide^Δ4E were also studied. Our results show that natural and synthetic 1-deoxy(methyl)-sphingolipids fail to laterally segregate into ordered domains as efficiently as the canonical C16-ceramide. The impaired ability of atypical sphingolipids to form ordered domains was more dependent on the presence, position, and configuration of the sphingoid base double bond than on the structure of its C1 functional group, due to packing constraints introduced by an unsaturated backbone. Nonetheless, absence of a hydrogen bond donor and acceptor group at the C1 position strongly reduced the capacity of atypical sphingolipids to form gel domains. Altogether, the results showed that 1-deoxy(methyl)-sphingolipids induce unique changes on the biophysical properties of the membranes, suggesting that these alterations might, in part, trigger the patho-biological actions of these lipids.

Discovery of deoxyceramide analogs as highly selective ACER3 inhibitors in live cells

Acid (AC), neutral (NC) and alkaline ceramidase 3 (ACER3) are the most ubiquitous ceramidases and their therapeutic interest as targets in cancer diseases has been well sustained. This supports the importance of discovering potent and specific inhibitors for further use in combination therapies. Although several ceramidase inhibitors have been reported, most of them target AC and a few focus on NC. In contrast, well characterized ACER3 inhibitors are lacking. Here we report on the synthesis and screening of two series of 1-deoxy(dihydro)ceramide analogs on the three enzymes. Activity was determined using fluorogenic substrates in recombinant human NC (rhNC) and both lysates and intact cells enriched in each enzyme. None of the molecules elicited a remarkable AC inhibitory activity in either experimental setup, while using rhNC, several compounds of both series were active as non-competitive inhibitors with K_i values between 1 and 5 μM. However, a dramatic loss of potency occurred in NC-enriched cell lysates and no activity was elicited in intact cells. Interestingly, several compounds of Series 2 inhibited ACER3 dose-dependently in both cell lysates and intact cells with IC₅₀'s around 20 μM. In agreement with their activity in live cells, they provoked a significant increase in the amounts of ceramides. Overall, this study identifies highly selective ACER3 activity blockers in intact cells, opening the door to further medicinal chemistry efforts aimed at developing more potent and specific compounds.

Oxidative stress increases 1-deoxysphingolipid levels in chronic kidney disease

Chronic kidney disease (CKD) leads to deep changes in lipid metabolism and obvious dyslipidemia. The dysregulation of lipid metabolism in turn results in CKD progression and the complications of cardiovascular diseases. To obtain a profound insight into the associated dyslipidemia in CKD, we performed lipidomic analysis to measure lipid metabolites in the serum from a rat 5/6 nephrectomy (5/6 Nx) model of CKD as well as in the serum from CKD patients. HK-2 cells were also used to examine oxidative stress-induced sphingolipid changes. Totally 182 lipid species were identified in 5/6 Nx rats. We found glycerolipids, total free fatty acids, and sphingolipids levels were significantly upregulated in 5/6 Nx rats. The atypical sphingolipids, 1-deoxysphingolipids, were significantly altered in both CKD animals and human CKD patients. The levels of 1-deoxysphingolipids directly relevant to the level of oxidative stress in vivo and in vitro. These results demonstrate that 1-deoxysphingolipid levels are increased in CKD and this increase directly correlates with increased kidney oxidative stress.

Ceramide synthases: Reflections on the impact of Dr. Lina M. Obeid

Sphingolipids are a family of lipids that are critical to cell function and survival. Much of the recent work done on sphingolipids has been performed by a closely-knit family of sphingolipid researchers, which including our colleague, Dr. Lina Obeid, who recently passed away. We now briefly review where the sphingolipid field stands today, focusing in particular on areas of sphingolipid research to which Dr. Obeid made valued contributions. These include the 'many-worlds' view of ceramides and the role of a key enzyme in the sphingolipid biosynthetic pathway, namely the ceramide synthases (CerS). The CerS contain a number of functional domains and also interact with a number of other proteins in lipid metabolic pathways, fulfilling Dr. Obeid's prophecy that ceramides, and the enzymes that generate ceramides, form the critical hub of the sphingolipid metabolic pathway.