Comprehensive stratum corneum ceramide profiling reveals reduced acylceramides in ichthyosis patient with CERS3 mutations

The stratum corneum (SC) of the epidermis acts as a skin permeability barrier, and abnormalities in SC formation lead to several skin disorders. Lipids, especially the epidermis-specific ceramide classes ω-O-acylceramides (acylceramides) and protein-bound ceramides, are essential for skin barrier formation. Ceramide synthase 3 (CERS3) is involved in the synthesis of acylceramides and protein-bound ceramides, and CERS3 mutations cause autosomal recessive congenital ichthyosis. In the present study, we measured ceramide synthase activity and performed comprehensive SC ceramide profiling in an ichthyosis patient with compound heterozygous CERS3 mutations: nonsense mutation p.Arg75* and missense mutation p.Arg229His. The activity of p.Arg75* and p.Arg229His mutant CERS3 proteins was reduced to 4% and 56%, respectively, of the wild-type protein. In the patient's SC, acylceramide levels were greatly reduced, but the levels of protein-bound ceramides remained almost unchanged. Non-acylated ceramide levels were also affected in the patient; in particular, the levels of ceramides composed of sphingosine and non-hydroxy or α-hydroxy fatty acid were substantially higher than in healthy controls. These results suggest that a reduction in acylceramide levels alone leads to ichthyosis. Although protein-bound ceramides are synthesized from acylceramides, levels of acylceramides and protein-bound ceramides are not necessarily correlated.

Production of branched-chain very-long-chain fatty acids by fatty acid elongases and their tissue distribution in mammals

Although most mammalian fatty acids (FAs) are straight-chain, there also exist branched-chain FAs such as iso- and anteiso-FAs, especially in the meibomian glands. Meibum lipids, which are secreted from the meibomian glands and are important for dry eye prevention, contain abundant branched-chain lipids, such as cholesteryl esters and wax esters with chain-lengths of ≥C21 (very-long-chain; VLC). However, the exact tissue distribution of branched-chain lipids or the enzymes involved in the production of branched-chain VLCFAs has remained poorly understood. Here, we revealed that FA elongases ELOVL1, ELOVL3, and ELOVL7, of the seven mammalian ELOVL isozymes, elongated saturated branched-chain acyl-CoAs. ELOVL3 was highly active toward iso-C17:0 and anteiso-C17:0 acyl-CoAs and elongated them up to iso-C23:0 and anteiso-C25:0 acyl-CoAs, respectively. ELOVL1 elongated both iso- and anteiso-C23:0 acyl-CoAs to C25:0 acyl-CoAs. By establishing a liquid chromatography-tandem mass spectrometry method capable of separating branched- and straight-chain lipids, we showed that essentially all of the cholesteryl esters and 88% of the wax esters in the mouse meibomian glands are branched. In Elovl1 mutant mice, the levels of ≥C24:0 branched-chain cholesteryl esters and ≥C25:0 branched-chain wax esters were decreased, indicating that ELOVL1 indeed elongates branched-chain VLC acyl-CoAs in vivo. In addition, substantial amounts of ceramides containing branched-chain FAs were present in the skin, meibomian glands, and liver. Our findings provide new insights into the molecular mechanisms that create FA and lipid diversity.

Comparative profiling and comprehensive quantification of stratum corneum ceramides in humans and mice by LC/MS/MS

Ceramides are the predominant lipids in the stratum corneum (SC) and are crucial components for normal skin barrier function. Although the composition of various ceramide classes in the human SC has been reported, that in mice is still unknown, despite mice being widely used as animal models of skin barrier function. Here, we performed LC/MS/MS analyses using recently available ceramide class standards to measure 25 classes of free ceramides and 5 classes of protein-bound ceramides from human and mouse SC. Phytosphingosine- and 6-hydroxy sphingosine-type ceramides, which both contain an additional hydroxyl group, were abundant in the human SC (35% and 45% of total ceramides, respectively). In contrast, in mice, phytosph-ingosine- and 6-hydroxy sphingosine-type ceramides were present at ∼1% and undetectable levels, respectively, and sphingosine-type ceramides accounted for ∼90%. In humans, ceramides containing α-hydroxy FA were abundant, whereas ceramides containing β-hydroxy or ω-hydroxy FA were abundant in mice. The hydroxylated β-carbon in β-hydroxy ceramides was in the (R) configuration. Genetic knockout of β-hydroxy acyl-CoA dehydratases in HAP1 cells increased β-hydroxy ceramide levels, suggesting that β-hydroxy acyl-CoA, an FA-elongation cycle intermediate in the ER, is a substrate for β-hydroxy ceramide synthesis. We anticipate that our methods and findings will help to elucidate the role of each ceramide class in skin barrier formation and in the pathogenesis of skin disorders.

Catalytic residues, substrate specificity, and role in carbon starvation of the 2-hydroxy FA dioxygenase Mpo1 in yeast

The yeast protein Mpo1 belongs to a protein family that is widely conserved in bacteria, fungi, protozoa, and plants, and is the only protein of this family whose function has so far been elucidated. Mpo1 is an Fe²⁺-dependent dioxygenase that catalyzes the α-oxidation reaction of 2-hydroxy (2-OH) long-chain FAs (LCFAs) produced in the degradation pathway of the long-chain base phytosphingosine. However, several biochemical characteristics of Mpo1, such as its catalytic residues, membrane topology, and substrate specificity, remain unclear. Here, we report that yeast Mpo1 contains two transmembrane domains and that both its N- and C-terminal regions are exposed to the cytosol. Mutational analyses revealed that three histidine residues conserved in the Mpo1 family are especially important for Mpo1 activity, suggesting that they may be responsible for the formation of coordinate bonds with Fe²⁺ We found that, in addition to activity toward 2-OH LCFAs, Mpo1 also exhibits activity toward 2-OH very-long-chain FAs derived from the FA moiety of sphingolipids. These results indicate that Mpo1 is involved in the metabolism of long-chain to very-long-chain 2-OH FAs produced in different pathways. We noted that the growth of mpo1Δ cells is delayed upon carbon deprivation, suggesting that the Mpo1-mediated conversion of 2-OH FAs to nonhydroxy FAs is important for utilizing 2-OH FAs as a carbon source under carbon starvation. Our findings help to elucidate the as yet unknown functions and activities of other Mpo1 family members.

Lipid polarity gradient formed by ω-hydroxy lipids in tear film prevents dry eye disease

Meibum lipids form a lipid layer on the outermost side of the tear film and function to prevent water evaporation and reduce surface tension. (O-Acyl)-ω-hydroxy fatty acids (OAHFAs), a subclass of these lipids, are thought to be involved in connecting the lipid and aqueous layers in tears, although their actual function and synthesis pathway have to date remained unclear. Here, we reveal that the fatty acid ω-hydroxylase Cyp4f39 is involved in OAHFA production. Cyp4f39-deficient mice exhibited damaged corneal epithelium and shortening of tear film break-up time, both indicative of dry eye disease. In addition, tears accumulated on the lower eyelid side, indicating increased tear surface tension. In Cyp4f39-deficient mice, the production of wax diesters (type 1ω and 2ω) and cholesteryl OAHFAs was also impaired. These OAHFA derivatives show intermediate polarity among meibum lipids, suggesting that OAHFAs and their derivatives contribute to lipid polarity gradient formation for tear film stabilization.

FTY720 Protects Against Ischemia-Reperfusion Injury by Preventing the Redistribution of Tight Junction Proteins and Decreases Inflammation in the Subacute Phase in an Experimental Stroke Model

Injury due to brain ischemia followed by reperfusion (I/R) may be an important therapeutic target in the era of thrombectomy. FTY720, a widely known sphingosine-1-phosphate receptor agonist, exerts various neuroprotective effects. The aim of this study was to examine the protective effect of FTY720 with respect to I/R injury, especially focusing on blood-brain barrier (BBB) protection and anti-inflammatory effects. Male rats were subjected to transient ischemia and administered vehicle or 0.5 or 1.5 mg/kg of FTY720 immediately before reperfusion. Positron emission tomography (PET) with [¹⁸F]DPA-714 was performed 2 and 9 days after the insult to serially monitor neuroinflammation. Bovine and rat brain microvascular endothelial cells (MVECs) were also subjected to oxygen-glucose deprivation (OGD) and reperfusion, and administered FTY720, phosphorylated-FTY720 (FTY720-P), or their inhibitor. FTY720 dose-dependently reduced cell death, the infarct size, cell death including apoptosis, and inflammation. It also ameliorated BBB disruption and neurological deficits compared to in the vehicle group. PET indicated that FTY720 significantly inhibited the worsening of inflammation in later stages. FTY720-P significantly prevented the intracellular redistribution of tight junction proteins but did not increase their mRNA expression. These results suggest that FTY720 can ameliorate I/R injury by protecting the BBB and regulating neuroinflammation.

Novel biallelic FA2H mutations in a Japanese boy with fatty acid hydroxylase-associated neurodegeneration

FA2H encodes fatty acid 2-hydroxylase, which plays a significant role in maintaining the neuronal myelin sheath. Previous reports have revealed that a FA2H mutation leads to spastic paraplegia, leukodystrophy, and neurodegeneration with brain iron accumulation, collectively referred to as fatty acid hydroxylase-associated neurodegeneration (FAHN). The disease severity of FAHN varies among individual patients and may be explained by the enzyme activity of FA2H mutant proteins. Here we report a 10-year-old Japanese boy with FAHN having novel heterozygous mutations in FA2H. The patient presented with a spastic gait since the age of 5 years and was unable to walk without a cane by the time he was 8 years old. Brain MRI demonstrated a partial thinning of the corpus callosum, slight reduction of cerebellar volume, and posterior dominant periventricular leukodystrophy. Whole exome sequencing revealed two novel missense mutations in FA2H with compound heterozygous inheritance (NM_024306, p.Val149Leu, and p.His260Gln mutations). The enzyme activities of the p.Val149Leu and p.His260Gln variants were 60%-80% and almost 0%, respectively. Our cell-based enzyme assay demonstrated partial functionality for one of the variants, indicating a milder phenotype. However, considered along with previous reports, there was no definite relationship between the disease severity and residual enzyme activity measured using a similar method. Further research is needed to precisely predict the phenotypic severity of this disorder.

N-glycosylation of Rim21 at an Unconventional Site Fine-tunes Its Behavior in the Plasma Membrane

The polytopic plasma membrane protein Rim21 senses both the elevation of ambient pH and alterations in plasma membrane lipid asymmetry in the Rim101 pathway in budding yeast. Rim21 is known to undergo N-glycosylation, but the site and function of the glycosylation modification is not known. Using a systematic mutation analysis, we found that Rim21 is N-glycosylated at an unconventional motif located in the N-terminal extracellular region. The Rim21 mutant protein that failed to receive N-glycosylation showed prolonged protein lifetime compared to that of WT Rim21 protein. Although both the WT and mutant Rim21 localized to the plasma membrane, they exhibited different biochemical fractionation profiles. The mutant Rim21, but not WT Rim21, was mainly fractionated into the heavy membrane fraction. Further, compared to WT Rim21, mutant Rim21 was more easily solubilized with digitonin but was conversely more resistant to solubilization with Triton X-100. Despite these different biochemical properties from WT Rim21, mutant Rim21 protein could still activate the Rim101 pathway in response to external alkalization. Collectively, N-glycosylation of Rim21 is not indispensable for its activity as a sensor protein, but modulates the residence of Rim21 protein to some microdomains within the plasma membrane with distinct lipid conditions, thereby affecting its turnover.Key words: plasma membrane, lipid asymmetry, N-linked glycosylation, microdomain, Saccharomyces cerevisiae.

Biosynthesis of the anti-lipid-microdomain sphingoid base 4,14-sphingadiene by the ceramide desaturase FADS3

Sphingolipids are multifunctional lipids. Among the sphingolipid-component sphingoid bases, 4,14-sphingadiene (SPD) is unique such that it has a cis double bond with a bent structure. Although SPD was discovered half a century ago, its tissue distribution, biosynthesis, and degradation remain poorly understood. Here, we established a specific and quantitative method for SPD measurement and found that SPD exists in a wide range of mammalian tissues. SPD was especially abundant in kidney, where the amount of SPD was ~2/3 of sphingosine, the most abundant sphingoid base in mammals. Although SPD is metabolized to ceramides and SPD 1-phosphate with almost the same efficiency as sphingosine, it is less susceptible to degradation by a cleavage reaction, at least in vitro. We identified the fatty acid desaturase family protein FADS3 as a ceramide desaturase that produces SPD ceramides by desaturating ceramides containing sphingosine. SPD sphingolipids were preferentially localized outside lipid microdomains, suggesting that SPD has different functions compared to other sphingoid bases in the formation of lipid microdomains. In summary, we revealed the biosynthesis and degradation pathways of SPD and its characteristic membrane localization. Our findings contribute to the elucidation of the molecular mechanism underlying the generation of sphingolipid diversity.

Reduced chain length in myelin sphingolipids and poorer motor coordination in mice deficient in the fatty acid elongase Elovl1

Very-long-chain fatty acids, with a chain length of >C20, are abundant in myelin sphingolipids. Recently, a de novo mutation in the ELOVL1 gene, which encodes fatty acid elongase, was identified in patients with neurocutaneous disorders involving skin ichthyosis and multiple neurological abnormalities, including hypomyelination, spastic paraplegia, and high-frequency deafness. However, the consequences of ELOVL1 deficiency for lipid composition and detailed pathological changes in the brain remain unclear. Here, we analyzed Elovl1 mutant mice as a model of human ELOVL1 deficiency. The mice exhibited a decreased postnatal survival rate, and some died of startle epilepsy. The acyl chain length of sphingolipids such as galactosylceramides, sulfatides, sphingomyelins, and ceramides in the brains of these mice was markedly shortened. Moreover, the mice exhibited reduced levels of galactosylceramides, which are important for myelin formation and stability. Electron microscope analysis of the corpus callosum in Elovl1 mutant mice revealed modest hypomyelination, especially in large-diameter axons. Furthermore, behavioral testing of the mice revealed deficits such as poorer motor coordination and reduced acoustic startle response to high-intensity stimulus. These findings provide clues to the molecular mechanism of the neurological symptoms of patients with the ELOVL1 mutation.

The very-long-chain fatty acid elongase Elo2 rescues lethal defects associated with loss of the nuclear barrier function in fission yeast cells

In eukaryotic cells, chromosomes are confined to the nucleus, which is compartmentalized by the nuclear membranes; these are continuous with the endoplasmic reticulum membranes. Maintaining the homeostasis of these membranes is an important cellular activity performed by lipid metabolic enzymes. However, how lipid metabolic enzymes affect nuclear membrane functions remains to be elucidated. We found that the very-long-chain fatty acid elongase Elo2 is located in the nuclear membrane and prevents lethal defects associated with nuclear membrane ruptures in mutants of the nuclear membrane proteins Lem2 and Bqt4 in the fission yeast Schizosaccharomyces pombe. Lipid composition analysis shows that t20:0/24:0 phytoceramide (a conjugate of C20:0 phytosphingosine and C24:0 fatty acid) is a major ceramide species in S. pombe The quantity of this ceramide is reduced in the absence of Lem2, and restored by increased expression of Elo2. Furthermore, loss of S. pombe Elo2 can be rescued by its human orthologs. These results suggest that the conserved very-long-chain fatty acid elongase producing the ceramide component is essential for nuclear membrane integrity and cell viability in eukaryotes.This article has an associated First Person interview with the first author of the paper.

Abstract WP547: FTY720 (Fingolimod) Ameliorates Ischemia Reperfusion Injury in Experimental Stroke Model

Background: Fingolimod (FTY720), a widely known sphingosine-1-phosphate (S1P) receptor agonist, was approved as a treatment for multiple sclerosis due to its strong anti-inflammatory effect.

Methods: In-vivo study, FTY720 was injected intraperitoneally just before reperfusion. Neurological score and infarct volume were evaluated at Day0, 1, 3, 5, and 7 after MCAO. Evans Blue analysis and immunohistochemistry were conducted at day1 and day7. [18F]DPA-714 PET was performed 2 and 9 days after MCAO. In vitro study, in order to perform OGD, Bovine Brain Microvascular Endothelial Cells (BBMVECs) were incubated in a glucose-free medium under 1% oxygen condition, and then reperfused for 4 hours. In vitro models of ischemia/reperfusion injury were exposed to FTY720, FTY720-P, S1P and SEW (100 nM) immediately after reperfusion. The effects of FTY720 on the blood brain barrier were examined by real-time PCR analysis and immunofluorescence staining.

Results: In this study, we observed that FTY720 reduced infarction size and improved neurological score after MCAO. [18F]DPA-714 PET showed a high standardized uptake value (SUV) around the ischemic area 2 days after MCAO. Although SUV was increased further 9 days after MCAO in both treatment groups, the increase was significantly inhibited in 1.5mg/kg group. In addition, FTY720 recovered the integrity of blood brain barrier which has been disrupted by ischemia/reperfusion injury. However, we observed that FTY720 can improve the integrity of blood brain barrier by inducing the translocation of tight junction and adherins junction, but FTY720 cannot ameliorate the expression of tight junction and adherins junction.

Conclusions: The present results suggest that FTY720 improves neurological score, reduces infarction size, inflammatory activation and neuronal apoptosis. Especially ameliorates the integrity of Blood-Brain Barrier by inducing the translocation of tight junction and adherins junction after transient MCAO.

De novo mutation in ELOVL1 causes ichthyosis, acanthosis nigricans, hypomyelination, spastic paraplegia, high frequency deafness and optic atrophy

BACKGROUND

Very long-chain fatty acids (VLCFAs) are essential for functioning of biological membranes. ELOVL fatty acid elongase 1 catalyses elongation of saturated and monounsaturated C22-C26-VLCFAs. We studied two patients with a dominant ELOVL1 mutation. Independently, Kutkowska-Kaźmierczak et al. had investigated the same patients and found the same mutation. We extended our study towards additional biochemical, functional, and therapeutic aspects.

METHODS

We did mutation screening by whole exome sequencing. RNA-sequencing was performed in patient and control fibroblasts. Ceramide and sphingomyelin levels were measured by LC-MS/MS. ELOVL1 activity was determined by a stable isotope-labelled [¹³C]malonyl-CoA elongation assay. ELOVL1 expression patterns were investigated by immunofluorescence, in situ hybridisation and RT-qPCR. As treatment option, we investigated VLCFA loading of fibroblasts.

RESULTS

Both patients carried an identical heterozygous de novo ELOVL1 mutation (c.494C>T, NM_001256399; p.S165F) not deriving from a founder allele. Patients suffered from epidermal hyperproliferation and increased keratinisation (ichthyosis). Hypomyelination of the central white matter explained spastic paraplegia and central nystagmus, while optic atrophy was causative for reduction of peripheral vision and visual acuity. The mutation abrogated ELOVL1 enzymatic activity and reduced ≥C24 ceramides and sphingomyelins in patient cells. Fibroblast loading with C22:0-VLCFAs increased C24:0-ceramides and sphingomyelins. We found competitive inhibition for ceramide and sphingomyelin synthesis between saturated and monounsaturated VLCFAs. Transcriptome analysis revealed upregulation of modules involved in epidermal development and keratinisation, and downregulation of genes for neurodevelopment, myelination, and synaptogenesis. Many regulated genes carried consensus proliferator-activated receptor (PPAR)α and PPARγ binding motifs in their 5'-regions.

CONCLUSION

A dominant ELOVL1 mutation causes a neuro-ichthyotic disorder possibly amenable to treatment with PPAR-modulating drugs.

Molecular mechanism of the ichthyosis pathology of Chanarin-Dorfman syndrome: Stimulation of PNPLA1-catalyzed ω-O-acylceramide production by ABHD5

BACKGROUND

ABHD5 mutations cause Chanarin-Dorfman syndrome accompanied by ichthyosis. ω-O-Acylceramide (acylceramide) is essential for skin permeability barrier formation. Acylceramide production is impaired in Abhd5 knockout mice. The transacylase PNPLA1 catalyzes the final step of acylceramide production: transfer of linoleic acid in triglyceride to ω-hydroxyceramide.

OBJECTIVE

We aimed to elucidate the role of ABHD5 in acylceramide production and the molecular mechanism of the ichthyosis symptoms of Chanarin-Dorfman syndrome.

METHODS

We investigated how ABHD5 influences acylceramide production using an acylceramide-producing cell system. The effects of ABHD5 and PNPLA1 expression on the morphology of lipid droplets were examined by indirect immunofluorescent microscopy and immunoelectron microscopy.

RESULTS

When ABHD5 was expressed in the acylceramide-producing cell system, acylceramide synthesis by PNPLA1 was enhanced. Dispersed localization of PNPLA1 was observed by immunofluorescent microscopy in HeLa cells under lipid droplet-forming conditions. Co-expression with ABHD5 caused PNPLA1 to localize on the lipid droplet membranes or their periphery. This staining pattern was observed in cells where PNPLA1 and ABHD5 were expressed at low levels. In contrast, lipid droplets disappeared in cells where PNPLA1 and ABHD5 were highly expressed. Immunoelectron microscopic analyses suggested that lipid droplets underwent morphological changes, transforming into vesicles or becoming incorporated into the endoplasmic reticulum. ABHD5 mutations found in Chanarin-Dorfman syndrome patients reduced ABHD5's ability to promote PNPLA1-dependent acylceramide production.

CONCLUSION

ABHD5 enhances PNPLA1-catalyzed acylceramide production. We speculate that ABHD5 retains triglycerides in the endoplasmic reticulum, and presents them to PNPLA1 to promote substrate recognition.

Sphingosine 1-phosphate receptor modulator ONO-4641 stimulates CD11b+Gr-1+ cell expansion and inhibits lymphocyte infiltration in the lungs to ameliorate murine pulmonary emphysema

Sphingolipids play a pivotal role in the pathogenesis of chronic obstructive pulmonary disease (COPD). However, little is known about the precise roles of sphingosine-1-phosphate (S1P), a bioactive sphingolipid metabolite, and its receptor modulation in COPD. In this study, we demonstrated that the S1P receptor modulator ONO-4641 induced the expansion of lung CD11b⁺Gr-1⁺ cells and lymphocytopenia in naive mice. ONO-4641-expanded CD11b⁺Gr-1⁺ cells showed higher arginase-1 activity, decreased T cell proliferation, and lower IFN-γ production in CD3⁺ T cells, similar to the features of myeloid-derived suppressor cells. ONO-4641 treatment decreased airspace enlargement in elastase-induced and cigarette smoke-induced emphysema models and attenuated emphysema exacerbation induced by post-elastase pneumococcal infection, which was also associated with an increased number of lung CD11b⁺Gr-1⁺ cells. Adoptive transfer of ONO-4641-expanded CD11b⁺Gr-1⁺ cells protected against elastase-induced emphysema. Lymphocytopenia observed in these models likely contributed to beneficial ONO-4641 effects. Thus, ONO-4641 attenuated murine pulmonary emphysema by expanding lung CD11b⁺Gr-1⁺ cell populations and inducing lymphocytopenia. The S1P receptor might be a promising target for strategies aimed at ameliorating pulmonary emphysema progression.

The role of PNPLA1 in ω-O-acylceramide synthesis and skin barrier function

The human genome encodes nine enzymes belonging to the patatin-like phospholipase domain-containing lipase (PNPLA)/Ca²⁺-independent phospholipase A₂ (iPLA₂) family. Although most PNPLA/iPLA₂ enzymes are widely distributed and act on phospholipids or neutral lipids as (phospho)lipases to play homeostatic roles in lipid metabolism, the function of PNPLA1 remained a mystery until a few years ago. However, the recent finding that mutations in the human PNPLA1 gene are linked to autosomal recessive congenital ichthyosis (ARCI), as well as evidence obtained from biochemical and gene knockout studies, has shed light on the function of this enzyme in skin-specific sphingolipid metabolism rather than glycerophospholipid metabolism. PNPLA1 is specifically expressed in differentiated keratinocytes and plays a crucial role in the biosynthesis of ω-O-acylceramide, a particular class of sphingolipids that is essential for skin barrier function. PNPLA1 acts as a unique transacylase that specifically transfers linoleic acid from triglyceride to ω-hydroxy fatty acid in ceramide, thus giving rise to ω-O-acylceramide. In this review, we overview the biosynthetic route and biological role of epidermal ω-O-acylceramide, highlight the function of PNPLA1 as a bona fide acylceramide synthase required for proper skin barrier function and keratinocyte differentiation, and summarize the mutations of PNPLA1 currently identified in ARCI patients. This article is part of a Special Issue entitled Novel functions of phospholipase A₂ Guest Editors: Makoto Murakami and Gerard Lambeau.

Neural symptoms in a gene knockout mouse model of Sjögren-Larsson syndrome are associated with a decrease in 2-hydroxygalactosylceramide

Insulation by myelin lipids is essential to fast action potential conductivity: changes in their quality or amount can cause several neurologic disorders. Sjögren-Larsson syndrome (SLS) is one such disorder, which is caused by mutations in the fatty aldehyde dehydrogenase ALDH3A2. To date, the molecular mechanism underlying SLS pathology has remained unknown. In this study, we found that Aldh3a2 is expressed in oligodendrocytes and neurons in the mouse brain, and neurons of Aldh3a2 knockout (KO) mice exhibited impaired metabolism of the long-chain base, a component of sphingolipids. Aldh3a2 KO mice showed several abnormalities corresponding to SLS symptoms in behavioral tests, including increased paw slips on a balance beam and light-induced anxiety. In their brain tissue, 2-hydroxygalactosylceramide, an important lipid for myelin function and maintenance, was reduced by the inactivation of fatty acid 2-hydroxylase. Our findings provide important new insights into the molecular mechanisms responsible for neural pathogenesis caused by lipid metabolism abnormalities.-Kanetake, T., Sassa, T., Nojiri, K., Sawai, M., Hattori, S., Miyakawa, T., Kitamura, T., Kihara, A. Neural symptoms in a gene knockout mouse model of Sjögren-Larsson syndrome are associated with a decrease in 2-hydroxygalactosylceramide.

Sphingolipids activate the endoplasmic reticulum stress surveillance pathway

In yeast, the ER stress surveillance (ERSU) pathway ensures that daughter cells inherit a functional ER during ER stress. Piña et al. show that phytosphingosine (PHS) levels increase during ER stress and PHS alone is sufficient to activate the hallmarks of ERSU activation: cER inheritance block, septin mislocalization, and Slt2 phosphorylation.

Proper inheritance of functional organelles is vital to cell survival. In the budding yeast, Saccharomyces cerevisiae, the endoplasmic reticulum (ER) stress surveillance (ERSU) pathway ensures that daughter cells inherit a functional ER. Here, we show that the ERSU pathway is activated by phytosphingosine (PHS), an early biosynthetic sphingolipid. Multiple lines of evidence support this: (1) Reducing PHS levels with myriocin diminishes the ability of cells to induce ERSU phenotypes. (2) Aureobasidin A treatment, which blocks conversion of early intermediates to downstream complex sphingolipids, induces ERSU. (3) orm1Δorm2Δ cells, which up-regulate PHS, show an ERSU response even in the absence of ER stress. (4) Lipid analyses confirm that PHS levels are indeed elevated in ER-stressed cells. (5) Lastly, the addition of exogenous PHS is sufficient to induce all ERSU phenotypes. We propose that ER stress elevates PHS, which in turn activates the ERSU pathway to ensure future daughter-cell viability.

Structure-inspired design of a sphingolipid mimic sphingosine-1-phosphate receptor agonist from a naturally occurring sphingomyelin synthase inhibitor

A monophosphate derivative of ginkgolic acid binds to sphingosine 1-phosphate (S1P) receptors to perform similar functions to the lipid mediator S1P.

Decreased Skin Barrier Lipid Acylceramide and Differentiation-Dependent Gene Expression in Ichthyosis Gene Nipal4-Knockout Mice

NIPAL4 is one of the causative genes for autosomal recessive congenital ichthyosis. However, the role of NIPAL4 in skin barrier formation and the molecular mechanism of ichthyosis pathology caused by NIPAL4 mutations, have not yet been determined. Here, we found that Nipal4-knockout (KO) mice exhibited neonatal lethality due to skin barrier defects. Histological analyses showed several morphological abnormalities in the Nipal4-KO epidermis, including impairment of lipid multilayer structure formation, hyperkeratosis, immature keratohyalin granules, and developed heterochromatin structures. The levels of the skin barrier lipid acylceramide were decreased in Nipal4-KO mice. Expression of genes involved in skin barrier formation normally increases during keratinocyte differentiation, in which chromatin remodeling is involved. However, the induction of Krt1, Lor, Flg, Elovl1, and Dgat2 was impaired in Nipal4-KO mice. NIPAL4 is a putative Mg²⁺ transporter, and Mg²⁺ concentration in differentiated keratinocytes of Nipal4-KO mice was indeed lower than that of wild-type mice. Our results suggest that low Mg²⁺ concentration causes aberration in the proper chromatin remodeling process, which in turn leads to failure of differentiation-dependent gene induction in keratinocytes. Our findings provide insights into Mg²⁺-dependent regulation of gene expression and skin barrier formation during keratinocyte differentiation.

Systematic analysis of Ca2+ homeostasis in Saccharomyces cerevisiae based on chemical-genetic interaction profiles

We investigated the global landscape of Ca2+ homeostasis in budding yeast based on high-dimensional chemical-genetic interaction profiles. The morphological responses of 62 Ca2+-sensitive (cls) mutants were quantitatively analyzed with the image processing program CalMorph after exposure to a high concentration of Ca2+ After a generalized linear model was applied, an analysis of covariance model was used to detect significant Ca2+-cls interactions. We found that high-dimensional, morphological Ca2+-cls interactions were mixed with positive (86%) and negative (14%) chemical-genetic interactions, whereas one-dimensional fitness Ca2+-cls interactions were all negative in principle. Clustering analysis with the interaction profiles revealed nine distinct gene groups, six of which were functionally associated. In addition, characterization of Ca2+-cls interactions revealed that morphology-based negative interactions are unique signatures of sensitized cellular processes and pathways. Principal component analysis was used to discriminate between suppression and enhancement of the Ca2+-sensitive phenotypes triggered by inactivation of calcineurin, a Ca2+-dependent phosphatase. Finally, similarity of the interaction profiles was used to reveal a connected network among the Ca2+ homeostasis units acting in different cellular compartments. Our analyses of high-dimensional chemical-genetic interaction profiles provide novel insights into the intracellular network of yeast Ca2+ homeostasis.

The 3-hydroxyacyl-CoA dehydratases HACD1 and HACD2 exhibit functional redundancy and are active in a wide range of fatty acid elongation pathways

Differences among fatty acids (FAs) in chain length and number of double bonds create lipid diversity. FA elongation proceeds via a four-step reaction cycle, in which the 3-hydroxyacyl-CoA dehydratases (HACDs) HACD1-4 catalyze the third step. However, the contribution of each HACD to 3-hydroxyacyl-CoA dehydratase activity in certain tissues or in different FA elongation pathways remains unclear. HACD1 is specifically expressed in muscles and is a myopathy-causative gene. Here, we generated Hacd1 KO mice and observed that these mice had reduced body and skeletal muscle weights. In skeletal muscle, HACD1 mRNA expression was by far the highest among the HACDs However, we observed only an ∼40% reduction in HACD activity and no changes in membrane lipid composition in Hacd1-KO skeletal muscle, suggesting that some HACD activities are redundant. Moreover, when expressed in yeast, both HACD1 and HACD2 participated in saturated and monounsaturated FA elongation pathways. Disruption of HACD2 in the haploid human cell line HAP1 significantly reduced FA elongation activities toward both saturated and unsaturated FAs, and HACD1 HACD2 double disruption resulted in a further reduction. Overexpressed HACD3 exhibited weak activity in saturated and monounsaturated FA elongation pathways, and no activity was detected for HACD4. We therefore conclude that HACD1 and HACD2 exhibit redundant activities in a wide range of FA elongation pathways, including those for saturated to polyunsaturated FAs, with HACD2 being the major 3-hydroxyacyl-CoA dehydratase. Our findings are important for furthering the understanding of the molecular mechanisms in FA elongation and diversity.

Biallelic Mutations in KDSR Disrupt Ceramide Synthesis and Result in a Spectrum of Keratinization Disorders Associated with Thrombocytopenia

Mutations in ceramide biosynthesis pathways have been implicated in a few Mendelian disorders of keratinization, although ceramides are known to have key roles in several biological processes in skin and other tissues. Using whole-exome sequencing in four probands with undiagnosed skin hyperkeratosis/ichthyosis, we identified compound heterozygosity for mutations in KDSR, encoding an enzyme in the de novo synthesis pathway of ceramides. Two individuals had hyperkeratosis confined to palms, soles, and anogenital skin, whereas the other two had more severe, generalized harlequin ichthyosis-like skin. Thrombocytopenia was present in all patients. The mutations in KDSR were associated with reduced ceramide levels in skin and impaired platelet function. KDSR enzymatic activity was variably reduced in all patients, resulting in defective acylceramide synthesis. Mutations in KDSR have recently been reported in inherited recessive forms of progressive symmetric erythrokeratoderma, but our study shows that biallelic mutations in KDSR are implicated in an extended spectrum of disorders of keratinization in which thrombocytopenia is also part of the phenotype. Mutations in KDSR cause defective ceramide biosynthesis, underscoring the importance of ceramide and sphingosine synthesis pathways in skin and platelet biology.