Chenodeoxycholic acid (CDCA) is an essential drug for patients with rare metabolic disease cerebrotendinous xanthomatosis (CTX). To ensure continuation of treatment, the Amsterdam UMC hospital pharmacy developed pharmacy compounded CDCA capsules when the authorized CDCA capsules were no longer available for Dutch patients. This study reports the safety of pharmacy compounded CDCA through pharmacovigilance monitoring and assesses its effectiveness by evaluating biochemical outcome measures, both in patients who were previously treated with authorized CDCA and subsequently switched to the compounded formulation and in new patients. Data were generated during routine patient care and collected retrospectively. Adverse events were reported by 45% of the patients; the most reported adverse events were diarrhea (16%), constipation (7%), and fatigue (7%). Biochemically, plasma cholestanol levels and urinary bile alcohol levels remained normalized before and after switching from the authorized product to pharmacy compounded capsules. It can be concluded that the pharmacy compounded CDCA capsules are well tolerated by patients with CTX and that the desired biochemical effect was maintained, supporting the use of a compounded formulation when the authorized product is unavailable.

A critical concern of newborn screening (NBS) for very-long chain acyl-CoA dehydrogenase deficiency (VLCADD) is the difficulty of predicting clinical outcomes. To address this, we investigated neonatal C18:2-carnitine concentrations as a possible predictor of VLCADD phenotype. To investigate the impact of sex, gestational age (GA) at birth, sampling day and birth weight on C18:2-carnitine, we analyzed NBS-dried blood spots (DBS) from Dutch newborns born between 2018 and 2020 (n = 209.785). After normalization for resulting confounders, C18:2-carnitine concentrations were investigated in NBS-DBS (n = 15) and neonatal plasma (n = 35) of Dutch VLCADD-patients, and in German NBS-DBS (n = 6) and correlated with clinical severity and diagnostic assays. Results showed that C18:2-carnitine concentrations were affected by GA, sampling day, birth weight and, to a lesser extent, by sex. High C18:2-carnitine, normalized for GA, sampling day and birth weight, reliably identified all VLCADD-patients with (expected) severe phenotypes. The differentiating C18:2-carnitine was identified as linoleylcarnitine. In conclusion, this study shows that neonatal C18:2-carnitine concentrations can serve to predict disease severity directly after positive NBS for VLCADD. Patients with high C18:2-carnitine concentrations can be considered "severe" and require strict dietary treatment and close monitoring. Patients with low C18:2-carnitine concentrations can be identified as "mild" and only need preventive dietary measures.

Exercise is fundamental to healthy aging, yet how it mitigates age-related molecular changes and how fitness level shapes exercise responses remain unclear. To address these questions, we performed transcriptomics, lipidomics and metabolomics on skeletal muscle of young and older adults with differing physical function, both before and after an acute bout of submaximal exercise. At baseline, older adults exhibited reduced expression of genes associated with cellular respiration and energy metabolism compared to young adults with comparable activity levels. Here we found that 50% of these age-related differences were absent in trained older adults, resulting in profiles resembling those of young adults. Although all participants displayed transcriptional immune and stress responses upon acute exercise, the magnitude of these responses in older adults was positively correlated with their physical fitness. Integrated multiomic analyses further revealed links among mitochondrial respiration, lipid metabolism, stress responses and NAD+ biology. These findings demonstrate that sustained physical training transforms age-related molecular profiles and provide a molecular atlas for study of fitness-dependent aging mechanisms.

In multiple sclerosis (MS), the chronic, unresolved nature of neuroinflammation within the central nervous system (CNS) remains a major obstacle for effective therapeutic intervention. This challenge arises primarily due to an incomplete understanding of the dysregulated inflammatory and pro-resolving pathways underlying MS lesion progression. Bioactive lipid mediators (LMs), biosynthesized through the coordinated actions of specific enzymes like lipoxygenases (LOX) and cyclooxygenases (COX), are key regulators of both the initiation and resolution of an inflammatory response; however, their spatial organization and functional role during MS pathology have not been fully elucidated. Here, by using pneumatically assisted nanospray desorption electrospray ionization (PA nano-DESI) mass spectrometry imaging and immunohistochemistry, we reveal an increase in the LM leukotriene B4 (LTB4) in human MS white matter compared to controls, with further enrichment in MS lesions relative to perilesional areas, alongside elevated microglial 5-LOX activating protein (FLAP) expression. Pharmacological antagonism of FLAP suppresses LTB4 biosynthesis in human-induced pluripotent stem cell (iPSC)-derived microglia with only marginal effects on the microglia transcriptional phenotype as determined by RNA sequencing. Moreover, in vivo FLAP antagonism ameliorates disease severity and spinal cord inflammatory gene expression in the experimental autoimmune encephalomyelitis (EAE) model, an animal model of MS, in both a prophylactic and therapeutic settings. This coincided with reduced local LTB4 biosynthesis and reduced levels of inflammatory monocytes within the spinal cord during EAE. Together these findings establish the FLAP/LTB4 axis as a driver of neuroinflammation and a druggable therapeutic target for chronic inflammatory CNS disorders like MS.

Lysosomal function can be affected by components in cell culture. This in turn may influence cellular metabolism and, consequently, research and diagnostics outcomes. One such component is the commonly used pH buffer 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES). HEPES specifically impacts the trafficking of the lysosomal enzyme glucocerebrosidase, which is deficient in Gaucher disease (GD). Understanding how HEPES affects cellular models of GD is essential, since glucocerebrosidase is central to diagnostic testing and the investigation of GD pathophysiology. Therefore, we examined the broader effects of HEPES on cultured fibroblasts from individuals with GD and healthy controls. We cultured dermal fibroblasts of eight adults with GD and seven healthy age- and sex-matched controls. The cells were cultured in two culture media, Ham's F10 and DMEM, both with and without HEPES. We assessed glucocerebrosidase enzyme activity and sphingolipid concentrations using a quantitative UPLC-MS/MS method. Additionally, we conducted multi-omics analyses, consisting of lipidomics, metabolomics and proteomics, to explore the broader impact of HEPES in cell culture on fibroblasts. Glucocerebrosidase activity in cell lysates increased after HEPES exposure in both GD and control fibroblasts, to an extent that may influence diagnostic outcomes. In GD fibroblasts, substrate accumulation was absent and not altered by HEPES exposure. GD fibroblasts exhibited a multi-omics profile largely overlapping with healthy controls and lacking the typical pathological features associated with GD in other cell types, such as mitochondrial dysfunction, dysregulated autophagy, disruption of intracellular calcium homeostasis, ER stress and chronic oxidative stress. In addition, the multi-omics profile was altered by HEPES, however in a non-specific manner. In conclusion, HEPES influences fibroblasts in culture, both from healthy controls and from patients with GD. Furthermore, GD fibroblasts lack a specific disease-related profile. This renders cultured fibroblasts unsuitable for studying pathophysiological processes in GD. Culturing GD fibroblasts with HEPES may compromise the reliability of diagnostics.

OBJECTIVE:To assess the diagnostic accuracy of metabolites in cerebrospinal fluid (CSF) for central nervous system (CNS) infections. METHODS:Patients were derived from three prospective cohort studies in the Netherlands. All studies included adults suspected of a CNS infection who underwent a diagnostic lumbar puncture. Metabolomics was performed on CSF using ultra-high-performance liquid chromatography with tandem mass spectrometry on a discovery and validation cohort. Metabolite quantification was the index test; a microbiologically confirmed diagnosis was the reference standard. RESULTS:In total, 343 episodes were included, of whom 170 (50%) had a CNS infections and 173 (50%) episodes had other diagnoses. CNS infections included bacterial meningitis in 88 (26%), viral meningoencephalitis in 50 (15%), and other CNS infections in 32 (9%) episodes. Other diagnoses consisted of CNS autoimmune disorders in 21 (6%), other neurological diseases in 84 (24%), and systemic infections in 68 (20%) episodes. A distinct metabolomic profile was observed in CSF of CNS infections, particularly bacterial meningitis. Glucose, glycerate, 1.3-diphosphoglyceric acid, pyruvate, lactate, taurine, and alpha-ketoglutarate had the highest diagnostic accuracy (area under the curve 0.87 to 0.95). Combinations further improved diagnostic accuracy, resulting in models that outperformed both individual metabolites and CSF leukocytes. Episodes with CSF leukocytes between 5 and 1,000 cells per mm3 showed similar results. INTERPRETATION:CSF metabolites demonstrate high diagnostic accuracy for CNS infections, particularly bacterial meningitis. Combinations further improve the diagnostic performance, exceeding that of CSF leukocytes alone. These findings highlight the potential of cerebrospinal fluid metabolites to improve diagnostic accuracy in clinical practice. ANN NEUROL 2025;98:851-863.

Congenital NAD+ deficiency disorders (CNDD) represent a novel category of hereditary diseases that affect NAD+ biosynthesis from tryptophan. CNDD include kynureninase (KYNU) deficiency with only ten documented patients to date. Here, we report two new cases. In addition to the previously described clinical phenotype of congenital heart defects, skeletal abnormalities, hearing loss and airway malacia, our patients also had hypothyroidism, recurrent infections, and necrotizing enterocolitis. The first patient's exome analysis identified compound heterozygosity for two novel KYNU variants, pathogenicity being supported by extensive metabolic profiling. The second patient was homozygous for a known KYNU missense variant. For the first time we describe a potential treatment of KYNU deficiency as both patients were treated with oral nicotinamide (a B3 vitamer and NAD+ precursor) and pyridoxine (a vitamin B6, cofactor of KYNU) from young age, which improved biochemistry without evident short-term adverse effects. Furthermore, we show that maternal supplementation with nicotinamide riboside (another B3 vitamer and NAD+ precursor) during pregnancy appears to be safe. In conclusion, KYNU deficiency is an extremely rare disorder associated with severe congenital defects, demonstrating the indispensable role of de novo NAD+ biosynthesis in foetal development. Metabolomic profiling is a valuable tool in the analysis of new genetic variants. NAD+ precursor supplementation in the form of nicotinamide, combined with pyridoxine, may be a novel therapeutic approach.

Complex lipid metabolism plays a crucial role in regulating aging. We recently discovered that the phospholipid bis(monoacylglycero)phosphate (BMP) increases in aged human muscles and many mouse tissues. The phospholipase PLA2G15 is reportedly involved in BMP synthesis, however, its specific role in aging remains unknown. To elucidate the role of PLA2G15 in aging, we used Caenorhabditis elegans as a model. When silencing plag-15, the predicted worm orthologue of PLA2G15, we observed improved healthspan and lifespan extension. Semi-targeted lipidomics highlighted that instead of changes related to BMP, plag-15 RNAi led to lower levels of lysophosphatidic acid, lysophosphatidylcholine, and lysophosphatidylethanolamine. Transcriptome-guided epistasis experiments identified that the lifespan extension of plag-15 RNAi worms is regulated by transcription factors hlh-30 and elt-3, and lysosomal vitamin B12 transporter pmp-5 (human TFEB, GATA, and ABCD4 respectively). Overall, we conclude that targeting phospholipid remodeling through plag-15 could be a promising strategy to promote healthy aging.

BACKGROUND:GRIN2B-neurodevelopmental disorder (GRIN2B-NDD) is a rare genetic disorder caused by pathogenic variants in GRIN2B, leading to impaired N-methyl d-aspartate receptor (NMDAR) function. l-serine, a precursor to d-serine that modulates NMDAR activity, has shown therapeutic potential for GRIN2B loss-of-function (LoF) variants. METHODS:The efficacy of oral l-serine supplementation in 4 children with GRIN2B LoF variants were evaluated in the first double-blind, randomized, placebo-controlled, one-year n-of-1 trials. The trial consisted of 2 cycles of 6 months. RESULTS:The Perceive, Recall, Plan, and Perform Assessment (PRPP-A) showed a significant improvement in Performance Mastery at 1.5 months (p = 0.0373), while 11 of 14 other PRPP-A measures showed mean differences that were numerically in the same direction toward a positive l-serine effect (not significant). Secondary outcomes varied across patients, for those with statistical group analysis, no significant difference were observed. Individual improvements were noted in information processing/adaptive function (n = 3/4), quality of life (n = 3/4), sleep (n = 1/2), irritability (n = 2/4), and language (n = 1/3), based on objective assessments and anecdotal parent reports. CONCLUSION:These pioneering n-of-1 trials provide insights into l-serine's potential for GRIN2B-NDD, with improvements in two of four patients, though no clear distinguishing responder-characteristics were identified. Future trials should focus on refining patient selection, the use of multiple baseline designs, establishing a core outcome set and pooling treatment data to better understand patient-specific responses.

STUDY QUESTION:What is the composition of currently available commercial human embryo culture media provided by seven suppliers, for each stage of human preimplantation embryo development? SUMMARY ANSWER:While common trends existed across brands, distinct differences in composition underlined the absence of a clear standard for human embryo culture medium formulation. WHAT IS KNOWN ALREADY:The reluctance of manufacturers to fully disclose the composition of their human embryo culture media generates uncertainty regarding the culture conditions that are used for human preimplantation embryo culture. The critical role of the embryo culture environment is well-recognized, with proven effects on IVF success rates and child outcomes, such as birth weight. The lack of comprehensive composition details restricts research efforts crucial for enhancing our understanding of its impacts on these outcomes. The ongoing demand for greater transparency remains unmet, highlighting a significant barrier in embryo culture medium optimization. STUDY DESIGN, SIZE, DURATION:For this study, 47 different human embryo culture media and protein supplements were purchased between December 2019 and June 2020; they comprise complete media (n = 23), unsupplemented media (n = 14), and supplements (n = 10). Unsupplemented media were supplemented with each available supplement from the same brand (n = 33 combinations). All samples were directly frozen in liquid nitrogen and stored at -80°C until composition analysis. PARTICIPANTS/MATERIALS, SETTING, METHODS:We determined the concentrations of 40 components in all samples collected (n = 80). Seven electrolytes (calcium, chloride, iron, magnesium, phosphate, potassium, sodium), glucose, immunoglobulins A, G, and M (IgA, IgG, IgM), uric acid, alanine aminotransferase (ALAT), aspartate aminotransferase (ASAT), and albumin, as well as the total protein concentration, were determined in each sample using a Cobas 8000 Analyser (Roche Diagnostics). Analysis of pyruvate, lactate, carnitine, and 21 amino acids was achieved with Ultra-High Performance Liquid Chromatography-Mass Spectrometry (UPLC-MS/MS). MAIN RESULTS AND THE ROLE OF CHANCE:Our analysis showed that generally, the concentrations of components of ready-to-use human embryo culture media align with established assumptions about the changing needs of an embryo during early development. For instance, glucose concentrations displayed a high-low-high pattern in sequential media systems from all brands: 2.5-3 mM in most fertilization media, 0.5 mM or below in all cleavage stage media, and 2.5-3.3 mM in most blastocyst stage media. Continuous media generally resembled glucose concentrations of cleavage stage media. However, for other components, such as lactate, glycine, and potassium, we observed clear differences in medium composition across different brands. No two embryo culture media compositions were the same. Remarkably, even embryo culture media from brands that belong to the same parent company differed in composition. Additionally, the scientific backing for the specific concentrations used and the differences in the composition of sequential media is quite limited and often based on minimal in vivo studies of limited sample size or studies using animal models. LARGE SCALE DATA:N/A. LIMITATIONS, REASONS FOR CAUTION:We used a targeted approach and performed a selection of tests which limit the composition analysis to this set of analytes. WIDER IMPLICATIONS OF THE FINDINGS:Comprehensive disclosure and complete transparency concerning the composition of human embryo culture media, including the exact concentration of each component, are crucial for evidence-based improvements of culture media for human preimplantation embryos. STUDY FUNDING/COMPETING INTEREST(S):This research was supported by ZonMw (https://www.zonmw.nl/en), Programme Translational Research 2 (project number 446002003). M.G. declares an unrestricted research grant from Ferring not related to the presented work, paid to the institution VU Medical Center. The remaining authors have no conflicts of interest to declare. TRIAL REGISTRATION NUMBER:N/A.

Gamma-butyrobetaine hydroxylase (BBOX1) catalyses the last step of carnitine biosynthesis, converting γ-butyrobetaine (γ-BB) into L-carnitine. Here we show, for the first time, that biallelic variants in BBOX1 are associated with decreased levels of L-carnitine and increased plasma levels of γ-BB in three patients from two unrelated families presenting with myopathic, neurodevelopmental, and late-onset psychiatric manifestations. Using a knockout C. elegans model of BBOX1 homolog, gbh-1, and strains harboring patient-derived variants (gbh-1(D72G) for p.Asp59Gly, gbh-1(G283R) for p.Gly263Arg, and gbh-1(G247Vfs6) for p.Gly227Valfs*6), we show very low L-carnitine levels and significantly elevated γ-BB in c.675delA and c.787G>A mutants, and moderately elevated γ-BB in c.176A>G. Furthermore, we observed a lethal embryonic phenotype for the gbh-1 loss-of-function strains, which was rescued upon L-carnitine supplementation. Our study provides novel insights into the clinical and biochemical consequences of BBOX1-related L-carnitine biosynthesis deficiency and establishes C. elegans as a model to study the effects of BBOX1 deficiency.

Purpose: The Amsterdam UMC pharmacy has been compounding chenodeoxycholic acid (CDCA) capsules for Dutch cerebrotendinous xanthomatosis patients since 2018. However, limited data are available on the pharmacokinetics and bioequivalence of therapeutic CDCA formulations. Methods: An open-label, single-center, randomized, two-period, two-sequence, cross-over study was conducted in 12 healthy volunteers to compare the pharmacokinetic profile of pharmacy-compounded CDCA capsules to that of the authorized CDCA product. Results: Both formulations reached peak plasma concentrations (tmax) at approximately 1 h post-dose. The mean AUC(0–6h) values were 262.4 (±69.4) µmol∙min/L for the compounded capsules and 248.0 (±78.1) µmol∙min/L for the authorized capsules, with a 90% confidence interval (CI) for the AUC(0–6h) ratio of 0.89–1.30, exceeding the accepted bioequivalence range of 0.80–1.25. The mean Cmax for the compounded formulation (2.96 ± 0.91 µmol/L) was significantly lower than that of the comparator product (4.42 ± 1.36 µmol/L; p = 0.0040), with a 90% CI for the Cmax ratio of 0.57–0.80, also outside the bioequivalence range. Conclusions: Overall, the pharmacy-compounded and authorized capsules demonstrate a comparable AUC(0–6h) and tmax. Bioequivalence could not be demonstrated, primarily due to high variation, a significantly lower Cmax, and an AUC(0–6h) ratio outside the accepted limits. These findings indicate that the compounded formulation results in reduced systemic peak exposure compared with the authorized product. However, given the high variation, a larger sample size would be needed to further investigate bioequivalence in future studies.

Barth syndrome (BTHS) is a rare X-linked recessively inherited disorder caused by variants in the TAFAZZIN gene, leading to impaired conversion of monolysocardiolipin (MLCL) into mature cardiolipin (CL). Accumulation of MLCL and CL deficiency are diagnostic markers for BTHS. Clinically, BTHS includes cardiomyopathy, skeletal myopathy, neutropenia, and growth delays. Severely affected patients may require early cardiac transplants due to unpredictable cardiac phenotypes. The pathophysiological mechanisms of BTHS are poorly understood, and treatments remain symptomatic. This study analyzed heart samples from five pediatric male BTHS patients (5 months-15 years) and compared them to tissues from 24 non-failing donors (19-71 years) using an integrated omics method combining metabolomics, lipidomics, and proteomics. The analysis confirmed changes in diagnostic markers (CL and MLCL), severe mitochondrial alterations, metabolic shifts, and elevated heart-failure markers. It also revealed significant interindividual differences among BTHS patients. This study describes a powerful analytical tool for the in-depth analysis of metabolic disorders and a solid foundation for the understanding of BTHS disease phenotypes in cardiac tissues.

In 2018 the Amsterdam University Medical Centre decided to prepare chenodeoxycholic acid (CDCA) capsules (also known as pharmacy compounding) for patients with the genetic metabolic disease cerebrotendinous xanthomatosis (CTX) when the product with a marketing authorization was commercially unavailable for patients. However, after reanalysis, unknown impurities were identified in the CDCA active pharmaceutical ingredient (API) using thin-layer chromatography from the European Pharmacopoeia (Ph.Eur.) monograph. Therefore, the API did not comply with the Ph.Eur. specifications for related substances and as a result, pharmacy compounding was halted and an investigation was initiated to identify and quantify the unknown impurities. Meanwhile, a second CDCA API was sourced from another manufacturer. However, this API also appeared to contain an unknown impurity. This impurity could be identified as a dimer of CDCA using reversed phase liquid chromatography mass spectrometry. Since the Ph.Eur. at the time did not describe a suitable analytical method for the quantification of this new impurity, a high pressure liquid chromatography with differential refractometer (HPLC-RI) method was developed to quantify the dimer. Subsequently, in 2019, a new draft version of the CDCA Ph.Eur. monograph was published, including the dimer as a new impurity together with a HPLC-RI method for its identification and quantification. The CDCA-dimer is classified as non-toxic and permitted in the CDCA API up to a maximum of 0.5 %. Because the API complied with the updated Ph.Eur. specifications, pharmacy compounding of CDCA capsules could be resumed.

Metachromatic leukodystrophy (MLD) is a rare autosomal recessive lysosomal storage disorder caused by disease-causing variants in the gene coding for arylsulfatase A, leading to deficient enzyme activity and subsequent accumulation of sulfatides. MLD is characterized by demyelination and neurodegeneration of the central and peripheral nervous system, manifesting as progressive motor and cognitive defects in affected individuals. This review provides a comprehensive overview of the significant progress made in MLD research in the past decade, regarding natural history, disease and treatment mechanisms, and newborn screening (NBS). Traditionally, MLD has been classified according to age at onset (late-infantile, early-juvenile and late-juvenile, and adult MLD), with earlier forms leading to more rapid neurologic decline. New data show that the type of presenting symptoms further influences the dynamic of disease progression. Patients with a cognitive presentation have a much slower or even no motor decline than patients with a mixed motor and cognitive presentation. Research advancements have enabled improved understanding of the effects of allogeneic hematopoietic stem cell transplantation and the development of novel therapeutic approaches, including hematopoietic stem cell gene therapy, which is now authorized in the EU, United Kingdom, and United States as treatment for selected patients with early-onset forms of MLD. Both hematopoietic stem cell transplantation and hematopoietic stem cell gene therapy are most effective when administered before disease onset. To identify presymptomatic patients, NBS for MLD is becoming available in several countries, resulting in new challenges. Decisions regarding patient eligibility for these treatments in already symptomatic individuals, as well as the timing of treatment for patients identified through NBS, require thorough understanding of disease progression. Biomarkers may be helpful for disease staging and prediction of disease evolution. Moreover, apart from timing, challenges remain regarding optimal treatment strategies across MLD subtypes, especially late-onset MLD, and management of the clinical heterogeneity and course of the disease. Another important issue is ensuring therapy accessibility, which forms a substantial barrier for equitable care. Continued research and international collaboration are essential to address these challenges, with the goal of improving care and outcomes for patients with MLD and their families.

In lipid metabolism, the fatty acid (FA) elongation system synthesises a wide array of FAs, crucial for various biological functions. The role of this system is to lengthen FA carbon chains to produce FAs with ≥C16, and notably, very long-chain FAs (VLCFAs, C24-C26) and ultra long-chain FAs (ULCFAs, C28 to ≥C36). Elongation occurs in the endoplasmic reticulum (ER) through the actions of a complex of four ER-embedded enzymes, which includes the ELOVL proteins. Together with desaturases that introduce double bonds, these processes significantly increase the variety of FAs. VLCFAs and ULCFAs are required for the biosynthesis of complex lipids, notably glycero(phospho)lipids, ether(phospho)lipids and sphingolipids. The FA elongation system is therefore fundamental for membrane biogenesis and lipid homeostasis, and also for signalling pathways associated with inflammation and cell proliferation. This review focuses on the elongase enzymes, encoded by the ELOVL genes, which catalyze the first and rate-limiting step of the FA elongation cycle. We summarize the physiological roles of the elongase system, with emphasis on the less-characterized ULCFAs, their biological functions, and the functional tools, biomarkers and lipidomic studies used to study them. Additionally, we discuss how ELOVL enzyme defects contribute to disorders at the intersection of metabolic and neurodegenerative conditions, driven by disrupted lipid metabolism and misfolded enzymes in the ER and Golgi.

Patients deficient in the peroxisomal membrane protein ACBD5 regularly exhibit a dystrophy of the retina along with decline in visual acuity. Despite the prevalent retinal phenotype, information on the pathogenesis of the retinodystrophy is limited. To gain insight into the cellular, subcellular and molecular alterations occurring in the retina, we analyzed an ACBD5-deficient mouse model by immunofluorescence microscopy, electron microscopy, full-field electroretinography (ffERG) and as well as analytical and spatial mass spectrometry (MS)-based lipidomics techniques. Histological results implied that ACBD5-deficient mice exhibit a moderate degeneration of photoreceptor, bipolar, ganglion and retinal pigment epithelial cells accompanied, however, by a prominent activation of astroglia and microglia. Reduced a- and b-wave amplitudes from ffERG point to a severe functional dysregulation of retinal signal transduction with a focus at the level of the information-processing cell of the inner retina. At the lipidome level, very long-chain polyunsaturated fatty acids (VLC-PUFA) accumulated in phosphatidylcholines from retina homogenates, most likely disrupted by a decline in peroxisome functions. Remarkably, as revealed by MALDI MS imaging, these lipidome changes affected neither the whole retina nor the photoreceptor outer segments (POS), where VLC-PUFAs display the highest concentration in phospholipids of POS membrane discs. In contrast, VLC-PUFAs in ACBD5-deficient mice consistently accumulated in the inner retinal region from the outer (OPL) to inner plexiform layer (IPL). In line with VLC-PUFA-accumulations, photoreceptor ribbon synapses in the OPL showed morphological signs of degeneration on the ultrastructural level. Hence, peroxisomal dysfunction appears to affect cell type-specific lipid homeostasis, thereby disrupting local retinal membrane physiology leading to a severe neuroinflammation of the ACBD5-deficient mouse retina.

Cardiolipin (CL) is the signature phospholipid of the inner mitochondrial membrane, where it stabilizes electron transport chain protein complexes1. The final step in CL biosynthesis relates to its remodelling: the exchange of nascent acyl chains with longer, unsaturated chains1. However, the enzyme responsible for cleaving nascent CL (nCL) has remained elusive. Here, we describe ABHD18 as a candidate deacylase in the CL biosynthesis pathway. Accordingly, ABHD18 converts CL into monolysocardiolipin (MLCL) in vitro, and its inactivation in cells and mice results in a shift to nCL in serum and tissues. Notably, ABHD18 deactivation rescues the mitochondrial defects in cells and the morbidity and mortality in mice associated with Barth syndrome. This rare genetic disease is characterized by the build-up of MLCL resulting from inactivating mutations in TAFAZZIN (TAZ), which encodes the final enzyme in the CL-remodelling cascade1. We also identified a selective, covalent, small-molecule inhibitor of ABHD18 that rescues TAZ mutant phenotypes in fibroblasts from human patients and in fish embryos. This study highlights a striking example of genetic suppression of a monogenic disease revealing a canonical enzyme in the CL biosynthesis pathway.