Protective Effect of Uridine on Structural and Functional Rearrangements in Heart Mitochondria after a High-Dose Isoprenaline Exposure Modelling Stress-Induced Cardiomyopathy in Rats.

The pyrimidine nucleoside uridine and its phosphorylated derivates have been shown to be involved in the systemic regulation of energy and redox balance and promote the regeneration of many tissues, including the myocardium, although the underlying mechanisms are not fully understood. Moreover, rearrangements in mitochondrial structure and function within cardiomyocytes are the predominant signs of myocardial injury. Accordingly, this study aimed to investigate whether uridine could alleviate acute myocardial injury induced by isoprenaline (ISO) exposure, a rat model of stress-induced cardiomyopathy, and to elucidate the mechanisms of its action related to mitochondrial dysfunction. For this purpose, a biochemical analysis of the relevant serum biomarkers and ECG monitoring were performed in combination with transmission electron microscopy and a comprehensive study of cardiac mitochondrial functions. The administration of ISO (150 mg/kg, twice with an interval of 24 h, s.c.) to rats caused myocardial degenerative changes, a sharp increase in the serum cardiospecific markers troponin I and the AST/ALT ratio, and a decline in the ATP level in the left ventricular myocardium. In parallel, alterations in the organization of sarcomeres with focal disorganization of myofibrils, and ultrastructural and morphological defects in mitochondria, including disturbances in the orientation and packing density of crista membranes, were detected. These malfunctions were improved by pretreatment with uridine (30 mg/kg, twice with an interval of 24 h, i.p.). Uridine also led to the normalization of the QT interval. Moreover, uridine effectively inhibited ISO-induced ROS overproduction and lipid peroxidation in rat heart mitochondria. The administration of uridine partially recovered the protein level of the respiratory chain complex V, along with the rates of ATP synthesis and mitochondrial potassium transport, suggesting the activation of the potassium cycle through the mitoKATP channel. Taken together, these results indicate that uridine ameliorates acute ISO-induced myocardial injury and mitochondrial malfunction, which may be due to the activation of mitochondrial potassium recycling and a mild uncoupling leading to decreased ROS generation and oxidative damage.

Brequinar inhibits African swine fever virus replication in vitro by activating ferroptosis.

BACKGROUND: African swine fever virus (ASFV) is one of the most fatal swine etiological agents and has a huge economic impact on the global pork industry. Given that no effective vaccines or anti-ASFV drugs are available, there remains a pressing need for novel anti-ASFV drugs. This study aimed to investigate the anti-African swine fever virus (ASFV) activity of brequinar, a DHODH inhibitor. METHODS: The anti-ASFV activity of brequinar was investigated using IFA, HAD, HAD50, qRT-PCR, and western blotting assays. The western blotting assay was used to investigate whether brequinar inhibits ASFV replication by killing ASFV particles directly or by acting on cell factors. The confocal microscopy and western blotting assays were used to investigate whether brequinar inhibits ASFV replication by activating ferroptosis. RESULTS: In this study, brequinar was found to effectively inhibit ASFV replication ex vivo in porcine alveolar macrophages (PAMs) in a dose-dependent manner. In kinetic studies, brequinar was found to maintain ASFV inhibition from 24 to 72 hpi. Mechanistically, the time-of-addition assay showed that brequinar exerted anti-ASFV activity in all treatment modes, including pre-, co-, and post-treatment rather than directly killing ASFV particles. Notably, FerroOrange, Mito-FerroGreen, and Liperfluo staining experiments showed that brequinar increased the accumulation of intracellular iron, mitochondrial iron, and lipid peroxides, respectively. Furthermore, we also found that ferroptosis agonist cisplatin treatment inhibited ASFV replication in a dose-dependent manner and the inhibitory effect of brequinar on ASFV was partially reversed by the ferroptosis inhibitor ferrostatin-1, suggesting that brequinar activates ferroptosis to inhibit ASFV replication. Interestingly, exogenous uridine supplementation attenuated the anti-ASFV activity of brequinar, indicating that brequinar inhibits ASFV replication by inhibiting DHODH activity and the depletion of intracellular pyrimidine pools; however, the induction of ferroptosis by brequinar treatment was not reversed by exogenous uridine supplementation, suggesting that brequinar activation of ferroptosis is not related to the metabolic function of pyrimidines. CONCLUSIONS: Our data confirm that brequinar displays potent antiviral activity against ASFV in vitro and reveal the mechanism by which brequinar inhibits ASFV replication by activating ferroptosis, independent of inhibiting pyrimidine synthesis, providing novel targets for the development of anti-ASFV drugs.

N-carbamoyl aspartate reduced body weight by stimulating the thermogenesis of iBAT.

Uridine has formerly been shown to alleviate obesity and hepatic lipid accumulation. N-carbamoyl aspartate (NCA) provides carbon atoms to uridine in de novo pyrimidine biosynthesis pathway. However, whether NCA is involved in the lipid metabolism remains elusive. Here we showed that NCA supplementation significantly decreased (P < 0.05) serum cholesterol (CHOL), high-density lipoprotein (HDL), lactate dehydrogenase (LDH), and alkaline phosphatase (ALP) levels of mice, and significantly increased (P < 0.05) relative mRNA expression of genes related to the synthesis of pyrimidine nucleotides and polyunsaturated fatty acids. Besides, supplemented with NCA significantly decreased body weight and area under the curve (AUC), and increased body temperature in the high-fat diet fed mice. For further, relative protein expression of uridine monophosphate synthase (UMPS), sterol regulatory element-binding protein 1(SREBP-1) and phosphorylated hormone-sensitive triglyceride lipase (P-HSL) in the liver, and uncoupling protein 1 (UCP-1) in interscapular brown adipose tissue (iBAT) also showed upregulated in the high-fat diet fed mice. Thus, NCA promoted de novo synthesis of pyrimidine and polyunsaturated fatty acid, and reduced body weight by stimulating high-fat diet-induced thermogenesis of iBAT.

Domain function and predicted structure of three heterodimeric endonuclease subunits of RNA editing catalytic complexes in Trypanosoma brucei.

Each of the three similar RNA Editing Catalytic Complexes (RECCs) that perform gRNA-directed uridine insertion and deletion during Trypanosoma brucei mitochondrial (mt) mRNA editing has a distinct endonuclease activity that requires two related RNase III proteins, with only one competent for catalysis. We identified multiple loss-of-function mutations in the RNase III and other motifs of the non-catalytic KREPB6, KREPB7, and KREPB8 components by random mutagenesis and screening. These mutations had various effects on growth, editing, and both the abundances and RECC associations of these RNase III protein pairs in bloodstream form (BF) and procyclic form (PF) cells. Protein structure modelling predicted that the Zinc Finger (ZnF) of each paired RNase III protein contacts RNA positioned at the heterodimeric active site which is flanked by helices of a novel RNase III-Associated Motif (RAM). The results indicate that the protein domains of the non-catalytic subunits function together in RECC integrity, substrate binding, and editing site recognition during the multistep RNA editing process. Additionally, several mutants display distinct functional consequences in different life cycle stages. These results highlight the complementary roles of protein pairs and three RECCs within the complicated T. brucei mRNA editing machinery that matures mt mRNAs differentially between developmental stages.

The Effect of Uridine on the State of Skeletal Muscles and the Functioning of Mitochondria in Duchenne Dystrophy.

Duchenne muscular dystrophy is caused by the loss of functional dystrophin that secondarily causes systemic metabolic impairment in skeletal muscles and cardiomyocytes. The nutraceutical approach is considered as a possible complementary therapy for this pathology. In this work, we have studied the effect of pyrimidine nucleoside uridine (30 mg/kg/day for 28 days, i.p.), which plays an important role in cellular metabolism, on the development of DMD in the skeletal muscles of dystrophin deficient mdx mice, as well as its effect on the mitochondrial dysfunction that accompanies this pathology. We found that chronic uridine administration reduced fibrosis in the skeletal muscles of mdx mice, but it had no effect on the intensity of degeneration/regeneration cycles and inflammation, pseudohypetrophy, and muscle strength of the animals. Analysis of TEM micrographs showed that uridine also had no effect on the impaired mitochondrial ultrastructure of mdx mouse skeletal muscle. The administration of uridine was found to lead to an increase in the expression of the Drp1 and Parkin genes, which may indicate an increase in the intensity of organelle fission and the normalization of mitophagy. Uridine had little effect on OXPHOS dysfunction in mdx mouse mitochondria, and moreover, it was suppressed in the mitochondria of wild type animals. At the same time, uridine restored the transport of potassium ions and reduced the production of reactive oxygen species; however, this had no effect on the impaired calcium retention capacity of mdx mouse mitochondria. The obtained results demonstrate that the used dose of uridine only partially prevents mitochondrial dysfunction in skeletal muscles during Duchenne dystrophy, though it mitigates the development of destructive processes in skeletal muscles.

2-Selenouridine, a Modified Nucleoside of Bacterial tRNAs, Its Reactivity in the Presence of Oxidizing and Reducing Reagents.

The 5-substituted 2-selenouridines are natural components of the bacterial tRNA epitranscriptome. Because selenium-containing biomolecules are redox-active entities, the oxidation susceptibility of 2-selenouridine (Se2U) was studied in the presence of hydrogen peroxide under various conditions and compared with previously reported data for 2-thiouridine (S2U). It was found that Se2U is more susceptible to oxidation and converted in the first step to the corresponding diselenide (Se2U)2, an unstable intermediate that decomposes to uridine and selenium. The reversibility of the oxidized state of Se2U was demonstrated by the efficient reduction of (Se2U)2 to Se2U in the presence of common reducing agents. Thus, the 2-selenouridine component of tRNA may have antioxidant potential in cells because of its ability to react with both cellular ROS components and reducing agents. Interestingly, in the course of the reactions studied, we found that (Se2U)2 reacts with Se2U to form new 'oligomeric nucleosides' as linear and cyclic byproducts.

Peroxisomal-derived ether phospholipids link nucleotides to respirasome assembly.

The protein complexes of the mitochondrial electron transport chain exist in isolation and in higher order assemblies termed supercomplexes (SCs) or respirasomes (SC I+III2+IV). The association of complexes I, III and IV into the respirasome is regulated by unknown mechanisms. Here, we designed a nanoluciferase complementation reporter for complex III and IV proximity to determine in vivo respirasome levels. In a chemical screen, we found that inhibitors of the de novo pyrimidine synthesis enzyme dihydroorotate dehydrogenase (DHODH) potently increased respirasome assembly and activity. By-passing DHODH inhibition via uridine supplementation decreases SC assembly by altering mitochondrial phospholipid composition, specifically elevated peroxisomal-derived ether phospholipids. Cell growth rates upon DHODH inhibition depend on ether lipid synthesis and SC assembly. These data reveal that nucleotide pools signal to peroxisomes to modulate synthesis and transport of ether phospholipids to mitochondria for SC assembly, which are necessary for optimal cell growth in conditions of nucleotide limitation.

Cloning and high-level expression of monomeric human superoxide dismutase 1 (SOD1) and its interaction with pyrimidine analogs.

Superoxide dismutase 1 (SOD1) is known to be involved in the pathogenesis of Amyotrophic Lateral Sclerosis (ALS) and is therefore considered to be an important ALS drug target. Identifying potential drug leads that bind to SOD1 and characterizing their interactions by nuclear magnetic resonance (NMR) spectroscopy is complicated by the fact that SOD1 is a homodimer. Creating a monomeric version of SOD1 could alleviate these issues. A specially designed monomeric form of human superoxide dismutase (T2M4SOD1) was cloned into E. coli and its expression significantly enhanced using a number of novel DNA sequence, leader peptide and growth condition optimizations. Uniformly 15N-labeled T2M4SOD1 was prepared from minimal media using 15NH4Cl as the 15N source. The T2M4SOD1 monomer (both 15N labeled and unlabeled) was correctly folded as confirmed by 1H-NMR spectroscopy and active as confirmed by an in-gel enzymatic assay. To demonstrate the utility of this new SOD1 expression system for NMR-based drug screening, eight pyrimidine compounds were tested for binding to T2M4SOD1 by monitoring changes in their 1H NMR and/or 19F-NMR spectra. Weak binding to 5-fluorouridine (FUrd) was observed via line broadening, but very minimal spectral changes were seen with uridine, 5-bromouridine or trifluridine. On the other hand, 1H-NMR spectra of T2M4SOD1 with uracil or three halogenated derivatives of uracil changed dramatically suggesting that the pyrimidine moiety is the crucial binding component of FUrd. Interestingly, no change in tryptophan 32 (Trp32), the putative receptor for FUrd, was detected in the 15N-NMR spectra of 15N-T2M4SOD1 when mixed with these uracil analogs. Molecular docking and molecular dynamic (MD) studies indicate that interaction with Trp32 of SOD1 is predicted to be weak and that there was hydrogen bonding with the nearby aspartate (Asp96), potentiating the Trp32-uracil interaction. These studies demonstrate that monomeric T2M4SOD1 can be readily used to explore small molecule interactions via NMR.

Uridine natural products: Challenging targets and inspiration for novel small molecule inhibitors.

Nucleoside derivatives, in particular those featuring uridine, are familiar components of the nucleoside family of bioactive natural products. The structural complexity and biological activities of these compounds have inspired research from organic chemistry and chemical biology communities seeking to develop novel approaches to assemble the challenging molecular targets, to gain inspiration for enzyme inhibitor development and to fuel antibiotic discovery efforts. This review will present recent case studies describing the total synthesis and biosynthesis of uridine natural products, and de novo synthetic efforts exploiting features of the natural products to produce simplified scaffolds. This research has culminated in the development of complementary strategies that can lead to effective uridine-based inhibitors and antibiotics. The strengths and challenges of the juxtaposing methods will be illustrated by examining select uridine natural products. Moreover, structure-activity relationships (SAR) for each natural product-inspired scaffold will be discussed, highlighting the impact on inhibitor development, with the aim of future uridine-based small molecule expansion.

Maternal dietary uridine supplementation reduces diarrhea incidence in piglets by regulating the intestinal mucosal barrier and cytokine profiles.

BACKGROUND: Nucleotides play an important role in the regulation of cellular energy and protein homeostasis, which facilitate the repair, recovery and repletion of tissue function. This study tested the effects of maternal uridine (UR) supplementation during late pregnancy and lactation of sows on the immune function of the small intestine in neonatal and suckling piglets. RESULTS: Results showed that compared to the control group, maternal dietary UR supplementation significantly decreased incidence of diarrhea in suckling piglets (P < 0.01); and increased both duodenal and ileal average villus height (P < 0.01) as well as villus height/crypt depth in ileum (P = 0.017) in neonatal piglets. RT-qPCR results showed that maternal UR supplementation decreased mRNA expression of claudin-1 in jejunum and ileum of neonatal piglets (P < 0.05), while significantly increased mRNA expression of claudin-1 in duodenum and jejunum of suckling piglets. Furthermore, in suckling piglets, maternal dietary UR supplementation increased mRNA expression of IL-6, IL-8 and IL-1ß in duodenum, jejunum and ileum (P < 0.05), increased IL-10 expression in both jejunal and ileal mucosa (P < 0.05) and increased mRNA expression of IKB and TLR4 in ileal mucosa (P < 0.05). CONCLUSIONS: These results suggest that maternal dietary supplementation with UR contributed to reducing incidence of diarrhea by regulating cytokine secretion and intestinal mucosal barrier function in suckling piglets. © 2020 Society of Chemical Industry.

Dietary supplement with nucleotides in the form of uridine monophosphate or uridine stimulate intestinal development and promote nucleotide transport in weaned piglets.

BACKGROUND: Nucleotides are key constituents of milk, where they are utilized in cell replication, although there are limited studies for weaned piglets. This study evaluated the effects of uridine monophosphate (UMP) with uridine (UR) feed supplementation on the intestinal development and nucleotide transport in weaned piglets. RESULTS: Supplementation with UMP significantly increased (P < 0.05) plasma glucose, and UR supplementation significantly reduced (0.05 < P < 0.10) the plasma total cholesterol (TC) of piglets when compared with that of the control group, although non-significant difference (P > 0.05) in growth performance was observed among three groups. Piglets fed supplementary UR exhibited greater (P < 0.05) crypt depth in the duodenum and ileum when compared with those in the supplementary UMP and control groups. Real-time quantitative polymerase chain reaction (RT-qPCR) results revealed that UR supplementation increased (P < 0.05) the relative mRNA levels of genes encoding the transmembrane proteins ZO-1 and occludin in the duodenum mucosa, and ZO-1 in the jejunum mucosa (P < 0.05). Similarly, UR supplementation increased (P < 0.05) expression of solute carriers SLC28A1 and SLC29A1 in the duodenum mucosa. Conversely, claudin-1 expression in the duodenum mucosa was inhibited (P < 0.05) by dietary supplementation with UMP or UR. CONCLUSION: Collectively, our data indicated that dietary supplementation with UMP or UR was conducive to stimulating intestinal development and promoting nucleotide transport in weaned piglets. © 2019 Society of Chemical Industry.

Uridine/UMP metabolism and their function on the gut in segregated early weaned piglets.

Uridine monophosphate (UMP) is a major nucleotide analogue in mammalian milk and uridine (UR) is its gastro-intestinal metabolite in vivo. This study aims to investigate the functional effects of UMP and UR on the gut in vitro and in vivo. Twenty-one piglets were randomly allotted into three groups, the control group, UMP group and UR group, and orally administered UMP or UR for 10 days. Results showed that UMP and UR supplements improved the ADG of piglets, and decreased the diarrhea rate. UR increased the jejunum villus length/crypt depth ratio, Claudin-3 and E-cadherin expression, and the pyrimidine nucleotide metabolic enzymes including CMPK1, RRM2, UPRT, CTPS1 and CTPS2 in the duodenal mucosa. Both the UMP and UR decreased the expression of CAD and RRM2 at the jejunal mucosa. Moreover, UMP and UR increased the apoptosis ratio of intestinal epithelial cells in in vivo and in vitro experiments. Taken together, oral administration of UR and UMP could improve the small intestinal morphology, promote epithelial cell apoptosis and renewal of intestinal villus tips, and benefit intestinal development and health thus improving the growth performance and reducing the risk of diarrhea in early-weaned piglets.

Metabolomic analysis of mouse prefrontal cortex reveals upregulated analytes during wakefulness compared to sleep.

By identifying endogenous molecules in brain extracellular fluid metabolomics can provide insight into the regulatory mechanisms and functions of sleep. Here we studied how the cortical metabolome changes during sleep, sleep deprivation and spontaneous wakefulness. Mice were implanted with electrodes for chronic sleep/wake recording and with microdialysis probes targeting prefrontal and primary motor cortex. Metabolites were measured using ultra performance liquid chromatography-high resolution mass spectrometry. Sleep/wake changes in metabolites were evaluated using partial least squares discriminant analysis, linear mixed effects model analysis of variance, and machine-learning algorithms. More than 30 known metabolites were reliably detected in most samples. When used by a logistic regression classifier, the profile of these metabolites across sleep, spontaneous wake, and enforced wake was sufficient to assign mice to their correct experimental group (pair-wise) in 80-100% of cases. Eleven of these metabolites showed significantly higher levels in awake than in sleeping mice. Some changes extend previous findings (glutamate, homovanillic acid, lactate, pyruvate, tryptophan, uridine), while others are novel (D-gluconate, N-acetyl-beta-alanine, N-acetylglutamine, orotate, succinate/methylmalonate). The upregulation of the de novo pyrimidine pathway, gluconate shunt and aerobic glycolysis may reflect a wake-dependent need to promote the synthesis of many essential components, from nucleic acids to synaptic membranes.

Escherichia coli tRNA 2-selenouridine synthase (SelU) converts S2U-RNA to Se2U-RNA via S-geranylated-intermediate.

To date the only tRNAs containing nucleosides modified with a selenium (5-carboxymethylaminomethyl-2-selenouridine and 5-methylaminomethyl-2-selenouridine) have been found in bacteria. By using tRNA anticodon-stem-loop fragments containing S2U, Se2U, or geS2U, we found that in vitro tRNA 2-selenouridine synthase (SelU) converts S2U-RNA to Se2U-RNA in a two-step process involving S2U-RNA geranylation (with ppGe) and subsequent selenation of the resulting geS2U-RNA (with SePO33- ). No 'direct' S2U-RNA→Se2U-RNA replacement is observed in the presence of SelU/SePO33- only (without ppGe). These results suggest that the in vivo S2U→Se2U and S2U→geS2U transformations in tRNA, so far claimed to be the elementary reactions occurring independently in the same domain of the SelU enzyme, should be considered a combination of two consecutive events - geranylation (S2U→geS2U) and selenation (geS2U→Se2U).

Determination of the Main Nucleosides and Nucleobases in Natural and Cultured Ophiocordyceps xuefengensis.

Ophiocordyceps xuefengensis, a recently described species of Ophiocordycepsthat is associated with the larvae of Phassusnodus (Hepialidae) in the living root or trunk of the medicinal plant Clerodendrumcyrtophyllum, isthe largest known Cordycepsspecies and is recognized as a desirable alternative for natural Ophiocordycepssinensis. This study investigated the main nucleosides and nucleobases in natural and cultured Ophiocordycepsxuefengensis. The contents of the nucleosides and nucleobases in the natural and cultured samples were determined by reverse phase HPLC. The highest concentration of adenosine was found in the natural fruit body and the cultured stroma, with almost no adenosine in the cadaver of Phassusnodus. The contents of adenine, guanosine, uridine and uracil in the cultured mycelium were significantly higher than those in the natural sample. Inosine was only detected in the natural samples. Thymidine and 2-deoxyadenosine were only found in the cadaver of Phassusnodus. Cordycepin was not detected in the five samples examined. These results suggested that the cultured mycelium and cultured stroma of Ophiocordycepsxuefengensis might be a promising substitute for natural O. xuefengensis.

Comparison of the redox chemistry of sulfur- and selenium-containing analogs of uracil.

Selenium is present in proteins in the form of selenocysteine, where this amino acid serves catalytic oxidoreductase functions. The use of selenocysteine in nature is strongly associated with redox catalysis. However, selenium is also found in a 2-selenouridine moiety at the wobble position of tRNAGlu, tRNAGln and tRNALys. It is thought that the modifications of the wobble position of the tRNA improves the selectivity of the codon-anticodon pair as a result of the physico-chemical changes that result from substitution of sulfur and selenium for oxygen. Both selenocysteine and 2-selenouridine have widespread analogs, cysteine and thiouridine, where sulfur is used instead. To examine the role of selenium in 2-selenouridine, we comparatively analyzed the oxidation reactions of sulfur-containing 2-thiouracil-5-carboxylic acid (s2c5Ura) and its selenium analog 2-selenouracil-5-carboxylic acid (se2c5Ura) using 1H-NMR spectroscopy, 77Se-NMR spectroscopy, and liquid chromatography-mass spectrometry. Treatment of s2c5Ura with hydrogen peroxide led to oxidized intermediates, followed by irreversible desulfurization to form uracil-5-carboxylic acid (c5Ura). In contrast, se2c5Ura oxidation resulted in a diselenide intermediate, followed by conversion to the seleninic acid, both of which could be readily reduced by ascorbate and glutathione. Glutathione and ascorbate only minimally prevented desulfurization of s2c5Ura, whereas very little deselenization of se2c5Ura occurred in the presence of the same antioxidants. In addition, se2c5Ura but not s2c5Ura showed glutathione peroxidase activity, further suggesting that oxidation of se2c5Ura is readily reversible, while oxidation of s2c5Ura is not. The results of the study of these model nucleobases suggest that the use of 2-selenouridine is related to resistance to oxidative inactivation that otherwise characterizes 2-thiouridine. As the use of selenocysteine in proteins also confers resistance to oxidation, our findings suggest a common mechanism for the use of selenium in biology.

Exploiting Substrate Diversity of NRPS Led to the Generation of New Sansanmycin Analogs.

Further exploration of substrate diversity of the sansaninycin biosynthetic pathway using available halogen- and methyl-phenylalanines led to the generation of diverse sansanmycin derivatives, either at the single C- or N-terminus alone or at both C- and N-termini. The structures of all of these derivatives were determined by MS/MS spectra, and amongst them, the structures of [2-Cl-Phe]-sansanmycin H (1) and [2-Cl-Phe]-sansanmycin A (2) were further identified by NMR. Both the C-terminal derivative I and the N-terminal derivative 2 were assayed for their antibacterial activitiesi and compound 1 exhibited moderate activity against P. aeruginosa and ΔtolC mutant E. coli.

The initial uridine of primary piRNAs does not create the tenth adenine that Is the hallmark of secondary piRNAs.

PIWI-interacting RNAs (piRNAs) silence transposons in animal germ cells. PIWI proteins bind and amplify piRNAs via the "Ping-Pong" pathway. Because PIWI proteins cleave RNAs between target nucleotides t10 and t11-the nucleotides paired to piRNA guide positions g10 and g11-the first ten nucleotides of piRNAs participating in the Ping-Pong amplification cycle are complementary. Drosophila piRNAs bound to the PIWI protein Aubergine typically begin with uridine (1U), while piRNAs bound to Argonaute3, which are produced by Ping-Pong amplification, often have adenine at position 10 (10A). The Ping-Pong model proposes that the 10A is a consequence of 1U. We find that 10A is not caused by 1U. Instead, fly Aubergine as well as its homologs, Siwi in silkmoth and MILI in mice, have an intrinsic preference for adenine at the t1 position of their target RNAs; during Ping-Pong amplification, this t1A subsequently becomes the g10A of a piRNA bound to Argonaute3.

Trypanosoma brucei (UMP synthase null mutants) are avirulent in mice, but recover virulence upon prolonged culture in vitro while retaining pyrimidine auxotrophy.

African trypanosomes are capable of both de novo synthesis and salvage of pyrimidines. The last two steps in de novo synthesis are catalysed by UMP synthase (UMPS) - a bifunctional enzyme comprising orotate phosphoribosyl transferase (OPRT) and orotidine monophosphate decarboxylase (OMPDC). To investigate the essentiality of pyrimidine biosynthesis in Trypanosoma brucei, we generated a umps double knockout (DKO) line by gene replacement. The DKO was unable to grow in pyrimidine-depleted medium in vitro, unless supplemented with uracil, uridine, deoxyuridine or UMP. DKO parasites were completely resistant to 5-fluoroorotate and hypersensitive to 5-fluorouracil, consistent with loss of UMPS, but remained sensitive to pyrazofurin indicating that, unlike mammalian cells, the primary target of pyrazofurin is not OMPDC. The null mutant was unable to infect mice indicating that salvage of host pyrimidines is insufficient to support growth. However, following prolonged culture in vitro, parasites regained virulence in mice despite retaining pyrimidine auxotrophy. Unlike the wild-type, both pyrimidine auxotrophs secreted substantial quantities of orotate, significantly higher in the virulent DKO line. We propose that this may be responsible for the recovery of virulence in mice, due to host metabolism converting orotate to uridine, thereby bypassing the loss of UMPS in the parasite.

Disruption of uridine homeostasis links liver pyrimidine metabolism to lipid accumulation.

We report in this study an intrinsic link between pyrimidine metabolism and liver lipid accumulation utilizing a uridine phosphorylase 1 transgenic mouse model UPase1-TG. Hepatic microvesicular steatosis is induced by disruption of uridine homeostasis through transgenic overexpression of UPase1, an enzyme of the pyrimidine catabolism and salvage pathway. Microvesicular steatosis is also induced by the inhibition of dihydroorotate dehydrogenase (DHODH), an enzyme of the de novo pyrimidine biosynthesis pathway. Interestingly, uridine supplementation completely suppresses microvesicular steatosis in both scenarios. The effective concentration (EC(50)) for uridine to suppress microvesicular steatosis is approximately 20 µM in primary hepatocytes of UPase1-TG mice. We find that uridine does not have any effect on in vitro DHODH enzymatic activity. On the other hand, uridine supplementation alters the liver NAD(+)/NADH and NADP(+)/NADPH ratios and the acetylation profile of metabolic, oxidation-reduction, and antioxidation enzymes. Protein acetylation is emerging as a key regulatory mechanism for cellular metabolism. Therefore, we propose that uridine suppresses fatty liver by modulating the liver protein acetylation profile. Our findings reveal a novel link between uridine homeostasis, pyrimidine metabolism, and liver lipid metabolism.