Murine bubblegum orthologue is a microsomal very long-chain acyl-CoA synthetase

Role of ACSBG1 in brain lipid metabolism and X-linked adrenoleukodystrophy pathogenesis: Insights from a knockout mouse model

The "bubblegum" acyl-CoA synthetase (ACSBG1) is a pivotal player in lipid metabolism during the development of the mouse brain, facilitating the activation of long-chain fatty acids (LCFAs) and their integration into essential lipid species crucial for brain function. Through its enzymatic activity, ACSBG1 converts LCFAs into acyl-CoA derivatives, supporting vital processes like membrane formation, myelination, and energy production. Its regulatory role significantly influences neuronal growth, synaptic plasticity, and overall brain development, highlighting its importance in maintaining lipid homeostasis and proper brain function. Originally discovered in the fruit fly brain, ACSBG1 attracted attention for its potential implication in X-linked adrenoleukodystrophy (XALD) pathogenesis. Studies using Drosophila melanogaster lacking the ACSBG1 homolog, bubblegum, revealed adult neurodegeneration with elevated levels of very long-chain fatty acids (VLCFA). To explore ACSBG1's role in fatty acid (FA) metabolism and its relevance to XALD, we created an ACSBG1 knockout (Acsbg1-/-) mouse model and examined its impact on lipid metabolism during mouse brain development. Phenotypically, Acsbg1-/- mice resembled wild type (w.t.) mice. Despite its primary expression in tissues affected by XALD, brain, adrenal gland and testis, ACSBG1 depletion did not significantly reduce total ACS enzyme activity in these tissues when using LCFA or VLCFA as substrates. However, analysis unveiled intriguing developmental and compositional changes in FA levels associated with ACSBG1 deficiency. In the adult mouse brain, ACSBG1 expression peaked in the cerebellum, with lower levels observed in other brain regions. Developmentally, ACSBG1 expression in the cerebellum was initially low during the first week of life but increased dramatically thereafter. Cerebellar FA levels were assessed in both w.t. and Acsbg1-/- mouse brains throughout development, revealing notable differences. While saturated VLCFA levels were typically high in XALD tissues and in fruit flies lacking ACSBG1, cerebella from Acsbg1-/- mice displayed lower saturated VLCFA levels, especially after about 8 days of age. Additionally, monounsaturated ω9 FA levels exhibited a similar trend as saturated VLCFA, while ω3 polyunsaturated FA levels were elevated in Acsbg1-/- mice. Further analysis of specific FA levels provided additional insights into potential roles for ACSBG1. Notably, the decreased VLCFA levels in Acsbg1-/- mice primarily stemmed from changes in C24:0 and C26:0, while reduced ω9 FA levels were mainly observed in C18:1 and C24:1. ACSBG1 depletion had minimal effects on saturated long-chain FA or ω6 polyunsaturated FA levels but led to significant increases in specific ω3 FA, such as C20:5 and C22:5. Moreover, the impact of ACSBG1 deficiency on the developmental expression of several cerebellar FA metabolism enzymes, including those required for synthesis of ω3 polyunsaturated FA, was assessed; these FA can potentially be converted into bioactive signaling molecules like eicosanoids and docosanoids. In conclusion, despite compelling circumstantial evidence, it is unlikely that ACSBG1 directly contributes to the pathology of XALD. Instead, the effects of ACSBG1 knockout on processes regulated by eicosanoids and/or docosanoids should be further investigated.

PRSS37 deficiency leads to impaired energy metabolism in testis and sperm revealed by DIA-based quantitative proteomic analysis

Our previous studies have reported that a putative trypsin-like serine protease, PRSS37, is exclusively expressed in testicular germ cells during late spermatogenesis and essential for sperm migration from the uterus into the oviduct and sperm-egg recognition via mediating the interaction between PDILT and ADAM3. In the present study, the global proteome profiles of wild-type (wt) and Prss37^-/- mice in testis and sperm were compared employing data independent acquisition (DIA) technology. Overall, 2506 and 459 differentially expressed proteins (DEPs) were identified in Prss37-null testis and sperm, respectively, when compared to control groups. Bioinformatic analyses revealed that most of DEPs were related to energy metabolism. Of note, the DEPs associated with pathways for the catabolism such as glucose via glycolysis, fatty acids via β-oxidation, and amino acids via oxidative deamination were significantly down-regulated. Meanwhile, the DEPs involved in the tricarboxylic acid cycle (TCA cycle) and oxidative phosphorylation (OXPHOS) were remarkably decreased. The DIA data were further confirmed by a markedly reduction of intermediate metabolites (citrate and fumarate) in TCA cycle and terminal metabolite (ATP) in OXPHOS system after disruption of PRSS37. These outcomes not only provide a more comprehensive understanding of the male fertility of energy metabolism modulated by PRSS37 but also furnish a dynamic proteomic resource for further reproductive biology studies.

A time-resolved transcriptome landscape of the developing mouse ovary

The ovarian structure is complex and diverse, including egg cells, granulosa cells and other cell types. Its organ function depends heavily on normal development. Numerous studies have focused on certain gene function changes during ovarian development, but systematic analyses of its molecular changes are extremely rare. Here, we present a comprehensive transcriptional profile of the mouse ovary from 11 time points across multiple developmental stages, which enables us to explore the dynamics of ovarian development. By performing coexpression analysis, we identified gene modules with similar expression trends and determined 159 functional gene interaction networks based on machine learning. Most of these gene interaction networks are related to biological processes involved in the development of the ovary, which provides functional predictions for some genes with unknown functions and a reference for subsequent functional research. In general, our study provides a resource for understanding ovarian development.

The Drosophila melanogaster as Genetic Model System to Dissect the Mechanisms of Disease that Lead to Neurodegeneration in Adrenoleukodystrophy

Drosophila melanogaster is the most successful genetic model organism to study different human disease with a recent increased popularity to study neurological disorders. Drosophila melanogaster has a complex yet well-defined brain with defined anatomical regions with specific functions. The neuronal network in the adult brain has a structural organization highly similar to human neurons, but in a brain that is much more amenable for complex analyses. The availability of sophisticated genetic tools to study neurons permits to examine neuronal functions at the single cell level in the whole brain by confocal imaging, which does not require sections. Thus, Drosophila has been used to successfully study many neurological disorders such as Parkinson's disease and has been recently adopted to understand the complex networks leading to neurological disorders with metabolic origins such as Leigh disease and X-linked adrenoleukodystrophy (X-ALD).In this review, we will describe the genetic tools available to study neuronal structures and functions and also illustrate some limitations of the system. Finally, we will report the experimental efforts that in the past 10 years have established Drosophila melanogaster as an excellent model organism to study neurodegenerative disorders focusing on X-ALD.

Identification of genes associated with resilience/vulnerability to sleep deprivation and starvation in Drosophila

BACKGROUND AND STUDY OBJECTIVES

Flies mutant for the canonical clock protein cycle (cyc(01)) exhibit a sleep rebound that is ∼10 times larger than wild-type flies and die after only 10 h of sleep deprivation. Surprisingly, when starved, cyc(01) mutants can remain awake for 28 h without demonstrating negative outcomes. Thus, we hypothesized that identifying transcripts that are differentially regulated between waking induced by sleep deprivation and waking induced by starvation would identify genes that underlie the deleterious effects of sleep deprivation and/or protect flies from the negative consequences of waking.

DESIGN

We used partial complementary DNA microarrays to identify transcripts that are differentially expressed between cyc(01) mutants that had been sleep deprived or starved for 7 h. We then used genetics to determine whether disrupting genes involved in lipid metabolism would exhibit alterations in their response to sleep deprivation.

SETTING

Laboratory.

PATIENTS OR PARTICIPANTS

Drosophila melanogaster.

INTERVENTIONS

Sleep deprivation and starvation.

MEASUREMENTS AND RESULTS

We identified 84 genes with transcript levels that were differentially modulated by 7 h of sleep deprivation and starvation in cyc(01) mutants and were confirmed in independent samples using quantitative polymerase chain reaction. Several of these genes were predicted to be lipid metabolism genes, including bubblegum, cueball, and CG4500, which based on our data we have renamed heimdall (hll). Using lipidomics we confirmed that knockdown of hll using RNA interference significantly decreased lipid stores. Importantly, genetically modifying bubblegum, cueball, or hll resulted in sleep rebound alterations following sleep deprivation compared to genetic background controls.

CONCLUSIONS

We have identified a set of genes that may confer resilience/vulnerability to sleep deprivation and demonstrate that genes involved in lipid metabolism modulate sleep homeostasis.

β-oxidation and rapid metabolism, but not uptake regulate brain eicosapentaenoic acid levels

The brain has a unique polyunsaturated fatty acid composition, with high levels of arachidonic and docosahexaenoic acids (DHA) while levels of eicosapentaenoic acid (EPA) are several orders of magnitude lower. As evidence accumulated that fatty acid entry into the brain was not selective and, in fact, that DHA and EPA enter the brain at similar rates, new mechanisms were required to explain their large concentration differences in the brain. Here we summarize recent research demonstrating that EPA is rapidly and extensively β-oxidized upon entry into the brain. Although the ATP generated from the β-oxidation of EPA is low compared to the use of glucose, fatty acid β-oxidation may serve to regulate brain fatty acid levels in the absence of selective transportation. Furthermore, when β-oxidation of EPA is blocked, desaturation of EPA increases and Land׳s recycling decreases to maintain low EPA levels.

Acyl-CoA metabolism and partitioning

Long-chain fatty acyl-coenzyme As (CoAs) are critical regulatory molecules and metabolic intermediates. The initial step in their synthesis is the activation of fatty acids by one of 13 long-chain acyl-CoA synthetase isoforms. These isoforms are regulated independently and have different tissue expression patterns and subcellular locations. Their acyl-CoA products regulate metabolic enzymes and signaling pathways, become oxidized to provide cellular energy, and are incorporated into acylated proteins and complex lipids such as triacylglycerol, phospholipids, and cholesterol esters. Their differing metabolic fates are determined by a network of proteins that channel the acyl-CoAs toward or away from specific metabolic pathways and serve as the basis for partitioning. This review evaluates the evidence for acyl-CoA partitioning by reviewing experimental data on proteins that are believed to contribute to acyl-CoA channeling, the metabolic consequences of loss of these proteins, and the potential role of maladaptive acyl-CoA partitioning in the pathogenesis of metabolic disease and carcinogenesis.

Very long-chain acyl-CoA synthetase 3: overexpression and growth dependence in lung cancer

Lung cancer is the leading cause of cancer deaths worldwide. In the United States, only one in six lung cancer patients survives five years after diagnosis. These statistics may improve if new therapeutic targets are identified. We previously reported that an enzyme of fatty acid metabolism, very long-chain acyl-CoA synthetase 3 (ACSVL3), is overexpressed in malignant glioma, and that depleting glioblastoma cells of ACSVL3 diminishes their malignant properties. To determine whether ACSVL3 expression was also increased in lung cancer, we studied tumor histologic sections and lung cancer cell lines. Immunohistochemical analysis of normal human lung showed moderate ACSVL3 expression only in bronchial epithelial cells. In contrast, all of 69 different lung tumors tested, including adeno-, squamous cell, large cell, and small cell carcinomas, had robustly elevated ACSVL3 levels. Western blot analysis of lung cancer cell lines derived from these tumor types also had significantly increased ACSVL3 protein compared to normal bronchial epithelial cells. Decreasing the growth rate of lung cancer cell lines did not change ACSVL3 expression. However, knocking down ACSVL3 expression by RNA interference reduced cell growth rates in culture by 65–76%, and the ability of tumor cells to form colonies in soft agar suspension by 65–80%. We also conducted studies to gain a better understanding of the biochemical properties of human ACSVL3. ACSVL3 mRNA was detected in many human tissues, but the expression pattern differed somewhat from that of the mouse. The enzyme activated long- and very long-chain saturated fatty acid substrates, as well as long-chain mono- and polyunsaturated fatty acids to their respective coenzyme A derivatives. Endogenous human ACSVL3 protein was found in a punctate subcellular compartment that partially colocalized with mitochondria as determined by immunofluorescence microscopy and subcellular fractionation. From these studies, we conclude that ACSVL3 is a promising new therapeutic target in lung cancer.

Lessons from the gonadotropin-regulated long chain acyl-CoA synthetase (GR-LACS) null mouse model: a role in steroidogenesis, but not result in X-ALD phenotype

Gonadotropin-regulated long chain fatty acid Acyl-CoA synthetase (GR-LACS), is a member of the LACS family that is regulated by gonadotropin in the rat Leydig cell (LC). Its mouse/human homologs, lipidosin/bubblegum, have been suggested to participate in X-linked adrenoleukodystrophy (X-ALD), an adreno/neurodegenerative disorder with accumulation of very long chain fatty acids (VLCFA) in tissues and plasma. To further gain insights into its regulatory function, a GR-LACS/lipidosin null mouse was generated. No apparent phenotypic abnormalities were observed in the X-ALD target tissues (brain, testis, adrenal). Nuclear inclusions seen in mice >15 month-old, were present in LC of 9 month-old GR-LACS(-/-) mice. LC of the null mice showed refractoriness to the gonadotropin-induced desensitization of testosterone production that is observed in adult animals. LCFAs were moderately increased in the testis, ovary and brain, but not in the adrenal gland of GR-LACS(-/-) mice, with no major changes in VLCFA. No change in LACS activity was observed in these tissues, suggesting a compensatory mechanism exhibited by other LACS members. The GR-LACS(-/-) model did not support its association with X-ALD. These studies revealed a role of GR-LACS in reducing the aging process of the LC, and its participation in gonadotropin-induced testicular desensitization of testosterone production.

A transcriptome database for astrocytes, neurons, and oligodendrocytes: a new resource for understanding brain development and function

Understanding the cell-cell interactions that control CNS development and function has long been limited by the lack of methods to cleanly separate neural cell types. Here we describe methods for the prospective isolation and purification of astrocytes, neurons, and oligodendrocytes from developing and mature mouse forebrain. We used FACS (fluorescent-activated cell sorting) to isolate astrocytes from transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of an S100beta promoter. Using Affymetrix GeneChip Arrays, we then created a transcriptome database of the expression levels of >20,000 genes by gene profiling these three main CNS neural cell types at various postnatal ages between postnatal day 1 (P1) and P30. This database provides a detailed global characterization and comparison of the genes expressed by acutely isolated astrocytes, neurons, and oligodendrocytes. We found that Aldh1L1 is a highly specific antigenic marker for astrocytes with a substantially broader pattern of astrocyte expression than the traditional astrocyte marker GFAP. Astrocytes were enriched in specific metabolic and lipid synthetic pathways, as well as the draper/Megf10 and Mertk/integrin alpha(v)beta5 phagocytic pathways suggesting that astrocytes are professional phagocytes. Our findings call into question the concept of a "glial" cell class as the gene profiles of astrocytes and oligodendrocytes are as dissimilar to each other as they are to neurons. This transcriptome database of acutely isolated purified astrocytes, neurons, and oligodendrocytes provides a resource to the neuroscience community by providing improved cell-type-specific markers and for better understanding of neural development, function, and disease.

Long-chain acyl-CoA synthetases and fatty acid channeling

Thirteen homologous proteins comprise the long-chain acyl-CoA synthetase (ACSL), fatty acid transport protein (FATP), and bubblegum (ACSBG) subfamilies that activate long-chain and very-long-chain fatty acids to form acyl-CoAs. Gain- and loss-of-function studies show marked differences in the ability of these enzymes to channel fatty acids into different pathways of complex lipid synthesis. Further, the ability of the ACSLs and FATPs to enhance cellular FA uptake does not always require these proteins to be present on the plasma membrane; instead, FA uptake can be increased by enhancing its conversion to acyl-CoA and its metabolism in downstream pathways. Since altered fatty acid metabolism is a hallmark of numerous metabolic diseases and pathological conditions, the ACSL, FATP and ACSBG isoforms are likely to play important roles in disease etiology.

Brain neutral lipids mass is increased in alpha-synuclein gene-ablated mice

Because alpha-synuclein (Snca) has a role in brain lipid metabolism, we determined the impact that Snca deletion had on whole brain lipid composition. We analysed masses of individual phospholipid (PL) classes and neutral lipid mass as well as PL acyl chain composition in brains from wild-type and Snca-/- mice. Although total brain PL mass was not altered, cardiolipin and phosphatidylglycerol mass decreased 16% and 27%, respectively, in Snca-/- mice. In addition, no changes were observed in plasmalogen or polyphosphoinositide mass. In ethanolamine glycerophospholipids and phosphatidylserine, docosahexaenoic acid (22 : 6n-3) was decreased 7%, while 16 : 0 was increased 1.1-fold and 1.4-fold, respectively. Surprisingly, brain cholesterol, cholesteryl ester, and triacylglycerol mass were increased 1.1-fold, 1.6-fold, and 1.4-fold, respectively in Snca-/- mice. In isolated myelin, cholesterol mass was also increased 1.3-fold, but because there was also a net increase in myelin PL mass, the cholesterol to PL ratio was unaltered. No changes in the expression of cholesterogenic enzymes were observed, suggesting these did not account for the observed changes in cholesterol. These data extend our previous results in astrocytes and kinetic studies in vivo demonstrating a role for Snca in brain lipid metabolism and demonstrate a clear impact on brain neutral lipid metabolism.

Expression of an acyl-CoA synthetase, lipidosin, in astrocytes of the murine brain and its up-regulation during remyelination following cuprizone-induced demyelination

Lipidosin is an 80-kDa protein with long-chain acyl-CoA synthetase activity expressed in the brain, adrenal gland, testis, and ovary, which are selectively damaged in X-linked adrenoleukodystrophy (X-ALD). Western blot analysis of the cerebrum and cerebellum revealed a gradual increase in the expression of lipidosin postnatally. Light microscopic immunohistochemistry using a panel of specific monoclonal antibodies showed that the lipidosin-immunopositive cells were ubiquitously distributed in the brain and were denser in the gray matter than in the white matter. Lipidosin immunoreactivity was colocalized with GFAP immunoreactivity but not with ubiquitin C-terminal hydrolase 1 (= PGP9.5) immunoreactivity, a neuronal marker, and lipidosin-producing cells detected by an antisense probe specific for lipidosin mRNA were also GFAP immunopositive. These data together with Western blot analysis of primary cultured astrocytes indicate that lipidosin is expressed in astrocytes. Immunoelectron microscopic analysis revealed that lipidosin immunoreactivity was widely distributed from perivascular endfeet to perisynaptic processes without being limited to peroxisomes. Lipidosin immunoreactivity was greatly increased in astrocytes in the area of remyelination following experimental demyelination induced by the administration of cuprizone to mice. These data suggest that lipidosin was involved in fatty acid metabolism during reconstruction of the myelin sheath.

Acyl-CoA synthetase activity links wild-type but not mutant alpha-synuclein to brain arachidonate metabolism

Because alpha-synuclein (Snca) has a role in brain lipid metabolism, we determined the impact that the loss of alpha-synuclein had on brain arachidonic acid (20:4n-6) metabolism in vivo using Snca-/- mice. We measured [1-(14)C]20:4n-6 incorporation and turnover kinetics in brain phospholipids using an established steady-state kinetic model. Liver was used as a negative control, and no changes were observed between groups. In Snca-/- brains, there was a marked reduction in 20:4n-6-CoA mass and in microsomal acyl-CoA synthetase (Acsl) activity toward 20:4n-6. Microsomal Acsl activity was completely restored after the addition of exogenous wild-type mouse or human alpha-synuclein, but not by A30P, E46K, and A53T forms of alpha-synuclein. Acsl and acyl-CoA hydrolase expression was not different between groups. The incorporation and turnover of 20:4n-6 into brain phospholipid pools were markedly reduced. The dilution coefficient lambda, which indicates 20:4n-6 recycling between the acyl-CoA pool and brain phospholipids, was increased 3.3-fold, indicating more 20:4n-6 was entering the 20:4n-6-CoA pool from the plasma relative to that being recycled from the phospholipids. This is consistent with the reduction in Acsl activity observed in the Snca-/- mice. Using titration microcalorimetry, we determined that alpha-synuclein bound free 20:4n-6 (Kd = 3.7 microM) but did not bind 20:4n-6-CoA. These data suggest alpha-synuclein is involved in substrate presentation to Acsl rather than product removal. In summary, our data demonstrate that alpha-synuclein has a major role in brain 20:4n-6 metabolism through its modulation of endoplasmic reticulum-localized acyl-CoA synthetase activity, although mutant forms of alpha-synuclein fail to restore this activity.

X-linked adrenoleukodystrophy: clinical, biochemical and pathogenetic aspects

X-linked adrenoleukodystrophy (X-ALD) is a clinically heterogeneous disorder ranging from the severe childhood cerebral form to asymptomatic persons. The overall incidence is 1:16,800 including hemizygotes as well as heterozygotes. The principal molecular defect is due to inborn mutations in the ABCD1 gene encoding the adrenoleukodystrophy protein (ALDP), a transporter in the peroxisome membrane. ALDP is involved in the transport of substrates from the cytoplasm into the peroxisomal lumen. ALDP defects lead to characteristic accumulation of saturated very long-chain fatty acids, the diagnostic disease marker. The pathogenesis is unclear. Different molecular mechanisms seem to induce inflammatory demyelination, neurodegeneration and adrenocortical insufficiency involving the primary ABCD1 defect, environmental factors and modifier genes. Important information has been derived from the X-ALD mouse models; species differences however complicate the interpretation of results. So far, bone marrow transplantation is the only effective long-term treatment for childhood cerebral X-ALD, however, only when performed at an early-stage of disease. Urgently needed novel therapeutic strategies are under consideration ranging from dietary approaches to gene therapy.

Uptake and metabolism of plasma-derived erucic acid by rat brain

We examined the ability of erucic acid (22:1n-9) to cross the blood-brain barrier (BBB) by infusing [14-14C]22:1n-9 (170 microCi/kg, iv and icv) into awake, male rats. [1-14C]arachidonic acid (20:4n-6) [intravenous (i.v.)] was the positive control. After i.v. infusion, 0.011% of the plasma [14-14C]22:1n-9 was extracted by the brain, compared with 0.055% of the plasma [1-14C]20:4n-6. The [14-14C]22:1n-9 was extensively beta-oxidized (60%), compared with 30% for [1-14C]20:4n-6. Although 20:4n-6 was targeted primarily to phospholipid pools, 22:1n-9 was targeted to cholesteryl esters, triglycerides, and phospholipids. When [14-14C]22:1n-9 was infused directly into the fourth ventricle of the brain [intracerebroventricular (i.c.v.)] for 7 days, 60% of the tracer entered the phospholipid pools, similar to the distribution observed for [1-14C]20:4n-6. This demonstrates plasticity in the ability of the brain to esterify 22:1n-9 in an exposure-dependent manner. In i.v. and i.c.v. infused rats, a significant amount of tracer found in the phospholipid pools underwent sequential rounds of chain shortening and was found as [12-14C]20:1n-9 and [10-14C]oleic acid. These results demonstrate for the first time that intact 22:1n-9 crosses the BBB, is incorporated into specific lipid pools, and is chain-shortened.

A novel mammalian bubblegum-related acyl-CoA synthetase restricted to testes and possibly involved in spermatogenesis

We have characterized a new, membrane-associated acyl-CoA synthetase (ACS), termed bubblegum-related protein (BGR), which upon functional analysis demonstrated ACS activity capable of activating long- and very long-chain fatty acids. By multiple tissue RNA array and Northern blot analyses, human BGR mRNA was exclusively detected in testes. Murine Bgr mRNA was specifically expressed in pubertal and adult testes and was further demonstrated to be enriched in germ cells and Sertoli cells while present at a lower level in Leydig cells both by in situ hybridization and cell type fractionation. The complex 5'-end of the BGR mRNA appears to underlie translational control leading to differential utilization of alternative translation start sites. Thus, the BGR gene expands the bubblegum ACS family with a testes-specific, developmentally regulated member that may play a role in spermatogenesis.

Tissue-cell- and species-specific expression of gonadotropin-regulated long chain acyl-CoA synthetase (GR-LACS) in gonads, adrenal and brain. Identification of novel forms in the brain

Gonadotropin-regulated long chain acyl-CoA synthetase (GR-LACS) is a novel hormonally regulated fatty acyl-CoA synthetase (FACS) with activity for long-chain fatty acids. The presence of this enzyme in the Leydig cells of the mature rat testis and its mode of regulation suggest that it participates in testicular steroidogenesis. This study demonstrates that GR-LACS expression is tissue, cell and species-specific. The 79 kDa GR-LACS protein is expressed in rodent gonads and brain, and only in the mouse in the adrenal cortex. In the ovary of both species it is associated with follicles undergoing atresia. It is present in the newborn and immature testis tubules and after puberty only in the Leydig cells. A distinct GR-LACS protein species of 64 kDa that was more abundant than the 79 kDa long form was found in the rat brain. Also, a minor 73 kDa form was observed in the rat brain and mouse ovary. Two novel species resulting from alternatively splicing of the GR-LACS gene were identified in a rat brain cDNA library: a short form 1 (S1) lacking exon 8 and short form 2 (S2) lacking exons 6-8. Expression studies revealed that the sizes of the S1/S2 proteins are comparable to those of the endogenous variant species. Neither S form contains FACSs activity, suggesting that exon 8 is essential for the enzymatic function. GR-LACS variants exhibit small but significant dominant negative effects on the FACS activity of the long form. GR-LACS variants may regulate the long form's activity in the brain.

The second member of the human and murine bubblegum family is a testis- and brainstem-specific acyl-CoA synthetase

Acyl-CoA synthetases that activate fatty acids to their CoA derivatives play a central role in fatty acid metabolism. ACSBG1, an acyl-CoA synthetase originally identified in the fruit fly mutant bubblegum, was hypothesized to contribute to the biochemical pathology of X-linked adrenoleukodystrophy. We looked for homologous proteins and identified ACSBG2 in humans, mice, and rats. Human ACSBG1 and ACSBG2 amino acid sequences are 50% identical. ACSBG2 expression was confined to the testis and brainstem. Immunohistochemistry and in situ hybridization studies further localized ACSBG2 expression to testicular Sertoli cells and large motoneurons in the medulla oblongata and cervical spinal cord. Full-length cDNA encoding human and mouse ACSBG2 was cloned. In transfected COS-1 cells, both human and murine ACSBG2 were detected as 75- to 80-kDa proteins by Western blot. Cells overexpressing ACSBG2 had increased ability to activate oleic acid (C18:1omega9) and linoleic acid (C18:2omega6) but not other fatty acid substrates tested. Within a highly conserved motif known to be important for catalysis, human ACSBG2 contains a histidine residue where all other known acyl-CoA synthetases, including mouse and rat ACSBG2, contain an arginine. This substitution resulted in a shift of the human ACSBG2 pH optimum to a more acidic pH. Mutation of this histidine to arginine improved catalytic function at neutral pH by shifting the pH profile without affecting substrate specificity. Although the role of ACSBG2 in testicular and neuronal lipid metabolism remains unclear, the limited tissue expression pattern and limited substrate specificity rule out a likely role for this enzyme in X-linked adrenoleukodystrophy pathology.

The gonadotropin-regulated long-chain acyl CoA synthetase gene: A novel downstream Sp1/Sp3 binding element critical for transcriptional promoter activity

The 79 kD gonadotropin-regulated testicular long chain acyl-CoA synthetase gene (GR-LACS) is a hormone-regulated member of the acyl-CoA synthetase family that is expressed abundantly in Leydig cells and to a lesser extent in germinal cells of the adult testis. GR-LACS possesses an ATP/AMP binding domain and the fatty acyl-CoA synthetase (FACS) signature motif. To gain insights into the transcriptional regulation of GR-LACS in gonadal cells, we determined the genomic organization of the gene, including the upstream flanking sequences. The mouse GR-LACS gene spans over at least 45 kb and the coding region is encoded by exons 1-14. All exon-intron junction sites correspond to the consensus splice sequence GT-AG. Exon 7 and 11 comprise the conserved ATP/AMP binding domain and the FACS signature motif, respectively. Primer extension and S1 nuclease analyses demonstrated four transcriptional start sites located at -266/-216 bp 5' to the ATG codon. The minimal promoter domain resides within -254/-217 bp 5' to ATG codon, and upstream sequences to -404 bp (-1035/-405 bp) contribute to the inhibition of transcription in the expressing mouse Leydig tumor cells. Removal of -217/-1 bp, containing a 23 nt GC rich sequence (-112/-90) with an Sp1/Sp3 binding element, within the 1st exon of this TATA-less promoter, significantly reduced GR-LACS gene transcription. Transcriptional activity was abolished by a 2 nt mutation of this element. Thus, functional analyses of this promoter domain indicate that transcription of GR-LACS gene requires an Sp1/Sp3 binding element downstream of the transcriptional start sites which is essential for basal promoter activity.

Decreased expression of ABCD4 and BG1 genes early in the pathogenesis of X-linked adrenoleukodystrophy

Childhood cerebral adrenoleukodystrophy (CCER), adrenomyeloneuropathy (AMN) and AMN with cerebral demyelination (AMN-C) are the main phenotypic variants of X-linked adrenoleukodystrophy (ALD). It is caused by mutations in the ABCD1 gene encoding a half-size peroxisomal transporter that has to dimerize to become functional. The biochemical hallmark of ALD is the accumulation of very-long-chain fatty acids (VLCFA) in plasma and tissues. However, there is no correlation between the ALD phenotype and the ABCD1 gene mutations or the accumulation of VLCFA in plasma and fibroblast from ALD patients. The absence of genotype-phenotype correlation suggests the existence of modifier genes. To elucidate the mechanisms underlying the phenotypic variability of ALD, we studied the expression of ABCD1, three other peroxisomal transporter genes of the same family (ABCD2, ABCD3 and ABCD4) and two VLCFA synthetase genes (VLCS and BG1) involved in VLCFA metabolism, as well as the VLCFA concentrations in the normal white matter (WM) from ALD patients with CCER, AMN-C and AMN phenotypes. This study shows that: (1) ABCD1 gene mutations leading to truncated ALD protein are unlikely to cause variation in the ALD phenotype; (2) accumulation of saturated VLCFA in normal-appearing WM correlates with ALD phenotype and (3) expression of the ABCD4 and BG1, but not of the ABCD2, ABCD3 and VLCS genes, tends to be correlated with the severity of the disease, acting early in the pathogenesis of ALD.

Mouse very long-chain Acyl-CoA synthetase 3/fatty acid transport protein 3 catalyzes fatty acid activation but not fatty acid transport in MA-10 cells

The family of proteins that includes very long-chain acyl-CoA synthetases (ACSVL) consists of six members. These enzymes have also been designated fatty acid transport proteins. We cloned full-length mouse Acsvl3 cDNA and characterized its protein product ACSVL3/fatty acid transport protein 3. The predicted amino acid sequence contains two highly conserved motifs characteristic of acyl-CoA synthetases. Northern blot analysis revealed that the mouse Acsvl3 mRNA is highly expressed in adrenal gland, testis, and ovary, with lower expression in the brain of adult mice. A developmental Northern blot revealed that Acsvl3 mRNA levels were significantly higher in embryonic mouse brain (embryonic days 12-14) than in newborn or adult mice, suggesting a possible role in nervous system development. Immunohistochemistry revealed high ACSVL3 expression in adrenal cortical cells, spermatocytes and interstitial cells of the testis, theca cells of the ovary, cerebral cortical neurons, and cerebellar Purkinje cells. Endogenous ACSVL3 was found primarily in mitochondria of MA-10 and Neuro2a cells by both Western blot analysis of subcellular fractions and immunofluorescence analysis. In MA-10 cells, loss-of-function studies using RNA interference confirmed that endogenous ACSVL3 is an acyl-CoA synthetase capable of activating both long-chain (C16:0) and very long-chain (C24:0) fatty acids. However, despite decreased acyl-CoA synthetase activity, initial rates of fatty acid uptake were unaffected by knockdown of Acsvl3 expression in MA-10 cells. These studies cast doubt on the designation of ACSVL3 as a fatty acid transport protein.

X-linked adrenoleukodystrophy: role of very long-chain acyl-CoA synthetases

The principal biochemical abnormality in the neurodegenerative disorder X-linked adrenoleukodystrophy (X-ALD) is elevated plasma and tissue levels of very long-chain fatty acids (VLCFA). Enzymes with very long-chain acyl-CoA synthetase (VLACS) activity are required for VLCFA metabolism, including degradation by peroxisomal beta-oxidation or incorporation into complex lipids, and may also participate in VLCFA synthesis. Two enzymes with VLACS activity, ACSVL1 and BG1, were investigated for their potential role in X-ALD biochemical pathology. Skin fibroblast mRNA levels for ACSVL1, an enzyme previously shown to be in peroxisomes and to participate in VLCFA beta-oxidation, were not significantly different between normal controls, patients with childhood cerebral X-ALD, and patients with adrenomyeloneuropathy. Similar results were obtained with mRNA for BG1, a non-peroxisomal enzyme that is highly expressed in nervous system, adrenal gland, and testis, the principal tissues pathologically affected in X-ALD. No significant differences in the immunohistochemical staining patterns of tissues expressing either ACSVL1 or BG1 were observed when wild-type and X-ALD mice were compared. Western blot analysis of BG1 protein levels showed no differences between fibroblasts from controls, cerebral X-ALD, or adrenomyeloneuropathy patients. BG1 protein levels were similar in wild-type and X-ALD mouse brain, spinal cord, testis, and adrenal gland. We hypothesized that one function of BG1 was to direct VLCFA into the cholesterol ester synthesis pathway. However, BG1 depletion in Neuro2a cells using RNA interference did not decrease incorporation of labeled VLCFA into cholesterol esters. We conclude that the role, if any, of ACSVL1 and BG1 in X-ALD biochemical pathology is indirect.

Progress in X-linked adrenoleukodystrophy

PURPOSE OF REVIEW

The purpose of this article is to review and evaluate the new information about X-linked adrenoleukodystrophy that has been reported in 2002 and 2003.

RECENT FINDINGS

X-linked adrenoleukodystrophy has two distinct neurological phenotypes: adrenomyeloneuropathy, a non-inflammatory axonopathy mostly in adults, and an intensely inflammatory cerebral myelinopathy mostly in children. The two forms often co-occur in the same family. Heterozygous women and the X-linked adrenoleukodystrophy mouse model often have the adrenomyeloneuropathy phenotype. More than 500 distinct mutations in the defective gene (ABCD1) have been identified, and except in one unique family, do not correlate with the phenotype. Bone marrow transplantation is beneficial in patients with early cerebral involvement. A panel of brain neuroimaging studies aids the selection of patients for bone marrow transplantation. Lorenzo's oil administered to neurologically asymptomatic boys who are less than 6 years old and have a normal magnetic resonance imaging scan appears to reduce the probability of developing neurological abnormalities later in life.

SUMMARY

Progress has been achieved in the delineation of the phenotypes, pathogenesis, diagnosis and prevention of X-linked adrenoleukodystrophy, and therapies are emerging.

Structural basis of the substrate-specific two-step catalysis of long chain fatty acyl-CoA synthetase dimer

Long chain fatty acyl-CoA synthetases are responsible for fatty acid degradation as well as physiological regulation of cellular functions via the production of long chain fatty acyl-CoA esters. We report the first crystal structures of long chain fatty acyl-CoA synthetase homodimer (LC-FACS) from Thermus thermophilus HB8 (ttLC-FACS), including complexes with the ATP analogue adenosine 5'-(beta,gamma-imido) triphosphate (AMP-PNP) and myristoyl-AMP. ttLC-FACS is a member of the adenylate forming enzyme superfamily that catalyzes the ATP-dependent acylation of fatty acid in a two-step reaction. The first reaction step was shown to propagate in AMP-PNP complex crystals soaked with myristate solution. Myristoyl-AMP was identified as the intermediate. The AMP-PNP and the myristoyl-AMP complex structures show an identical closed conformation of the small C-terminal domains, whereas the uncomplexed form shows a variety of open conformations. Upon ATP binding, the fatty acid-binding tunnel gated by an aromatic residue opens to the ATP-binding site. The gated fatty acid-binding tunnel appears only to allow one-way movement of the fatty acid during overall catalysis. The protein incorporates a hydrophobic branch from the fatty acid-binding tunnel that is responsible for substrate specificity. Based on these high resolution crystal structures, we propose a unidirectional Bi Uni Uni Bi Ping-Pong mechanism for the two-step acylation by ttLC-FACS.