CRISPR-Cas9 induced mutations along de novo purine synthesis in HeLa cells result in accumulation of individual enzyme substrates and affect purinosome formation

Enrichment-Sensing All-in-One Strategy Integrated in the La(OH)3-Au@AgNPs Substrate for Rapid Surface-Enhanced Raman Spectroscopy Analysis of Purine Components

Improving the speediness of complex sample analysis has attracted much research interest in analytical science. In this work, an enrichment-sensing all-in-one strategy was presented for rapid surface-enhanced Raman spectroscopy (SERS) analysis of purine components by using the La(OH)3-Au@AgNPs nanocomposite. Two-dimensional (2D) La(OH)3 nanosheets with nanothickness and accessible active sites not only acted as efficient media for the rapid enrichment of analytes but also provided flat planes for the intensive decoration of Au@AgNPs nanoparticles to amplify the SERS signals of adsorbed analytes. The nanocomposite could realize the rapid enrichment-sensing of purine components in 1 min, including mercaptopurine, thioguanine, adenine, and purine. Subsequently, the surface adsorption behaviors were explored by density functional theory and the enhancement mechanisms were simulated by the finite-difference time-domain method. Moreover, the nanocomposite also exhibited good SERS performances with relative standard deviations (RSDs) of uniformity less than 6.5% (n = 23), RSDs of batch-to-batch stability less than 7.3% (n = 9), and long-term stability over 9 weeks with RSDs within 6.6%. Finally, the enrichment-sensing strategy was applied for the rapid SERS analysis of two projects: mercaptopurine in tablets and adenine in beers with detection limits of 6.0 and 0.76 μg/L and spiked recoveries of 90.9-100 and 84.2-101%, respectively. Benefiting from the high-performance enrichment medium and closely packed plasmonic nanoparticles, the enrichment-sensing all-in-one strategy possesses great potential for rapid on-site detection in food safety and pharmaceutical analysis.

Improved diagnostics of purine and pyrimidine metabolism disorders using LC-MS/MS and its clinical application

OBJECTIVES

To develop a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to quantify 41 different purine and pyrimidine (PuPy) metabolites in human urine to allow detection of most known disorders in this metabolic pathway and to determine reference intervals.

METHODS

Urine samples were diluted with an aqueous buffer to minimize ion suppression. For detection and quantification, liquid chromatography was combined with electrospray ionization, tandem mass spectrometry and multiple reaction monitoring. Transitions and instrument settings were established to quantify 41 analytes and nine stable-isotope-labeled internal standards (IS).

RESULTS

The established method is precise (intra-day CV: 1.4-6.3%; inter-day CV: 1.3-15.2%), accurate (95.2% external quality control results within ±2 SD and 99.0% within ±3 SD; analyte recoveries: 61-121%), sensitive and has a broad dynamic range to quantify normal and pathological metabolite concentrations within one run. All analytes except aminoimidazole ribonucleoside (AIr) are stable before, during and after sample preparation. Moreover, analytes are not affected by five cycles of freeze-thawing (variation: -5.6 to 7.4%), are stable in thymol (variation: -8.4 to 12.9%) and the lithogenic metabolites also in HCl conserved urine. Age-dependent reference intervals from 3,368 urine samples were determined and used to diagnose 11 new patients within 7 years (total performed tests: 4,206).

CONCLUSIONS

The presented method and reference intervals enable the quantification of 41 metabolites and the potential diagnosis of up to 25 disorders of PuPy metabolism.

Metabolites of De Novo Purine Synthesis: Metabolic Regulators and Cytotoxic Compounds

Cytotoxicity of de novo purine synthesis (DNPS) metabolites is critical to the pathogenesis of three known and one putative autosomal recessive disorder affecting DNPS. These rare disorders are caused by biallelic mutations in the DNPS genes phosphoribosylformylglycineamidine synthase (PFAS), phosphoribosylaminoimidazolecarboxylase/phosphoribosylaminoimidazolesuccinocarboxamide synthase (PAICS), adenylosuccinate lyase (ADSL), and aminoimidazole carboxamide ribonucleotide transformylase/inosine monophosphate cyclohydrolase (ATIC) and are clinically characterized by developmental abnormalities, psychomotor retardation, and nonspecific neurological impairment. At a biochemical level, loss of function of specific mutated enzymes results in elevated levels of DNPS ribosides in body fluids. The main pathogenic effect is attributed to the accumulation of DNPS ribosides, which are postulated to be toxic to the organism. Therefore, we decided to characterize the uptake and flux of several DNPS metabolites in HeLa cells and the impact of DNPS metabolites to viability of cancer cell lines and primary skin fibroblasts. We treated cells with DNPS metabolites and followed their flux in purine synthesis and degradation. In this study, we show for the first time the transport of formylglycinamide ribotide (FGAR), aminoimidazole ribotide (AIR), succinylaminoimidazolecarboxamide ribotide (SAICAR), and aminoimidazolecarboxamide ribotide (AICAR) into cells and their flux in DNPS and the degradation pathway. We found diminished cell viability mostly in the presence of FGAR and AIR. Our results suggest that direct cellular toxicity of DNPS metabolites may not be the primary pathogenetic mechanism in these disorders.

Disorders of purine biosynthesis metabolism

Purines are essential molecules that are components of vital biomolecules, such as nucleic acids, coenzymes, signaling molecules, as well as energy transfer molecules. The de novo biosynthesis pathway starts from phosphoribosylpyrophosphate (PRPP) and eventually leads to the synthesis of inosine monophosphate (IMP) by means of 10 sequential steps catalyzed by six different enzymes, three of which are bi-or tri-functional in nature. IMP is then converted into guanosine monophosphate (GMP) or adenosine monophosphate (AMP), which are further phosphorylated into nucleoside di- or tri-phosphates, such as GDP, GTP, ADP and ATP. This review provides an overview of inborn errors of metabolism pertaining to purine synthesis in humans, including either phosphoribosylpyrophosphate synthetase (PRS) overactivity or deficiency, as well as adenylosuccinate lyase (ADSL), 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase (ATIC), phosphoribosylaminoimidazole succinocarboxamide synthetase (PAICS), and adenylosuccinate synthetase (ADSS) deficiencies. ITPase deficiency is being described as well. The clinical spectrum of these disorders is broad, including neurological impairment, such as psychomotor retardation, epilepsy, hypotonia, or microcephaly; sensory involvement, such as deafness and visual disturbances; multiple malformations, as well as muscle presentations or consequences of hyperuricemia, such as gouty arthritis or kidney stones. Clinical signs are often nonspecific and, thus, overlooked. It is to be hoped that this is likely to be gradually overcome by using sensitive biochemical investigations and next-generation sequencing technologies.

The Human Ntn-Hydrolase Superfamily: Structure, Functions and Perspectives

N-terminal nucleophile (Ntn)-hydrolases catalyze the cleavage of amide bonds in a variety of macromolecules, including the peptide bond in proteins, the amide bond in N-linked protein glycosylation, and the amide bond linking a fatty acid to sphingosine in complex sphingolipids. Ntn-hydrolases are all sharing two common hallmarks: Firstly, the enzymes are synthesized as inactive precursors that undergo auto-proteolytic self-activation, which, as a consequence, reveals the active site nucleophile at the newly formed N-terminus. Secondly, all Ntn-hydrolases share a structural consistent αββα-fold, notwithstanding the total lack of amino acid sequence homology. In humans, five subclasses of the Ntn-superfamily have been identified so far, comprising relevant members such as the catalytic active subunits of the proteasome or a number of lysosomal hydrolases, which are often associated with lysosomal storage diseases. This review gives an updated overview on the structural, functional, and (patho-)physiological characteristics of human Ntn-hydrolases, in particular.

Purine biosynthetic enzymes assemble into liquid-like condensates dependent on the activity of chaperone protein HSP90

Enzymes within the de novo purine biosynthetic pathway spatially organize into dynamic intracellular assemblies called purinosomes. The formation of purinosomes has been correlated with growth conditions resulting in high purine demand, and therefore, the cellular advantage of complexation has been hypothesized to enhance metabolite flux through the pathway. However, the properties of this cellular structure are unclear. Here, we define the purinosome in a transient expression system as a biomolecular condensate using fluorescence microscopy. We show that purinosomes, as denoted by formylglycinamidine ribonucleotide synthase granules in purine-depleted HeLa cells, are spherical and appear to coalesce when two come into contact, all liquid-like characteristics that are consistent with previously reported condensates. We further explored the biophysical and biochemical means that drive the liquid-liquid phase separation of these structures. We found that the process of enzyme condensation into purinosomes is likely driven by the oligomeric state of the pathway enzymes and not a result of intrinsic disorder, the presence of low-complexity domains, the assistance of RNA scaffolds, or changes in intracellular pH. Finally, we demonstrate that the heat shock protein 90 KDa helps to regulate the physical properties of the condensate and maintain their liquid-like state inside HeLa cells. We show that disruption of heat shock protein 90 KDa activity induced the transformation of formylglycinamidine ribonucleotide synthase clusters into more irregularly shaped condensates, suggesting that its chaperone activity is essential for purinosomes to retain their liquid-like properties. This refined view of the purinosome offers new insight into how metabolic enzymes spatially organize into dynamic condensates within human cells.

Multienzyme interactions of the de novo purine biosynthetic protein PAICS facilitate purinosome formation and metabolic channeling

There is growing evidence that mammalian cells deploy a mitochondria-associated metabolon called the purinosome to perform channeled de novo purine biosynthesis (DNPB). However, the molecular mechanisms of this substrate-channeling pathway are not well defined. Here, we present molecular evidence of protein-protein interactions (PPIs) between the human bifunctional phosphoribosylaminoimidazole carboxylase/succinocarboxamide synthetase (PAICS) and other known DNPB enzymes. We employed two orthogonal approaches: bimolecular fluorescence complementation, to probe PPIs inside live, intact cells, and co-immunoprecipitation using StrepTag-labeled PAICS that was reintegrated into the genome of PAICS-knockout HeLa cells (crPAICS). With the exception of amidophosphoribosyltransferase, the first enzyme of the DNPB pathway, we discovered PAICS interacts with all other known DNPB enzymes and with MTHFD1, an enzyme which supplies the 10-formyltetrahydrofolate cofactor essential for DNPB. We show these interactions are present in cells grown in both purine-depleted and purine-rich conditions, suggesting at least a partial assembly of these enzymes may be present regardless of the activity of the DNPB pathway. We also demonstrate that tagging of PAICS on its C terminus disrupts these interactions and that this disruption is correlated with disturbed DNPB activity. Finally, we show that crPAICS cells with reintegrated N-terminally tagged PAICS regained effective DNPB with metabolic signatures of channeled synthesis, whereas crPAICS cells that reintegrated C-terminally tagged PAICS exhibit reduced DNPB intermediate pools and a perturbed partitioning of inosine monophosphate into AMP and GMP. Our results provide molecular evidence in support of purinosomes and suggest perturbing PPIs between DNPB enzymes negatively impact metabolite flux through this important pathway.

Combined Targeted and Untargeted Profiling of HeLa Cells Deficient in Purine De Novo Synthesis

Three genetically determined enzyme defects of purine de novo synthesis (PDNS) have been identified so far in humans: adenylosuccinate lyase (ADSL) deficiency, 5-amino-4-imidazole carboxamide-ribosiduria (AICA-ribosiduria), and deficiency in bifunctional enzyme phosphoribosylaminoimidazole carboxylase and phosphoribosylaminoimidazolesuccinocarboxamide synthase (PAICS). Clinical signs of these defects are mainly neurological, such as seizures, psychomotor retardation, epilepsy, autistic features, etc. This work aims to describe the metabolic changes of CRISPR-Cas9 genome-edited HeLa cells deficient in the individual steps of PDNS to better understand known and potential defects of the pathway in humans. High-performance liquid chromatography coupled with mass spectrometry was used for both targeted and untargeted metabolomic analyses. The statistically significant features from the untargeted study were identified by fragmentation analysis. Data from the targeted analysis were processed in Cytoscape software to visualize the most affected metabolic pathways. Statistical significance of PDNS intermediates preceding deficient enzymes was the highest (p-values 10 × 10−7–10 × 10−15) in comparison with the metabolites from other pathways (p-values of up to 10 × 10−7). Disturbed PDNS resulted in an altered pool of adenine and guanine nucleotides. However, the adenylate energy charge was not different from controls. Different profiles of acylcarnitines observed among deficient cell lines might be associated with a specific enzyme deficiency rather than global changes related to the PDNS pathway. Changes detected in one-carbon metabolism might reduce the methylation activity of the deficient cells, thus affecting the modification state of DNA, RNA, and proteins.

The Purinosome: A Case Study for a Mammalian Metabolon

Over the past fifteen years, we have unveiled a new mechanism by which cells achieve greater efficiency in de novo purine biosynthesis. This mechanism relies on the compartmentalization of de novo purine biosynthetic enzymes into a dynamic complex called the purinosome. In this review, we highlight our current understanding of the purinosome with emphasis on its biophysical properties and function and on the cellular mechanisms that regulate its assembly. We propose a model for functional purinosomes in which they consist of at least ten enzymes that localize near mitochondria and carry out de novo purine biosynthesis by metabolic channeling. We conclude by discussing challenges and opportunities associated with studying the purinosome and analogous metabolons. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Pathway specific effects of ADSL deficiency on neurodevelopment

Adenylosuccinate lyase (ADSL) functions in de novo purine synthesis (DNPS) and the purine nucleotide cycle. ADSL deficiency (ADSLD) causes numerous neurodevelopmental pathologies, including microcephaly and autism spectrum disorder. ADSLD patients have normal serum purine nucleotide levels but exhibit accumulation of dephosphorylated ADSL substrates, S-Ado, and SAICAr, the latter being implicated in neurotoxic effects through unknown mechanisms. We examined the phenotypic effects of ADSL depletion in human cells and their relation to phenotypic outcomes. Using specific interventions to compensate for reduced purine levels or modulate SAICAr accumulation, we found that diminished AMP levels resulted in increased DNA damage signaling and cell cycle delays, while primary ciliogenesis was impaired specifically by loss of ADSL or administration of SAICAr. ADSL-deficient chicken and zebrafish embryos displayed impaired neurogenesis and microcephaly. Neuroprogenitor attrition in zebrafish embryos was rescued by pharmacological inhibition of DNPS, but not increased nucleotide concentration. Zebrafish also displayed phenotypes commonly linked to ciliopathies. Our results suggest that both reduced purine levels and impaired DNPS contribute to neurodevelopmental pathology in ADSLD and that defective ciliogenesis may influence the ADSLD phenotypic spectrum.

Glutamine Antagonist GA-607 Causes a Dramatic Accumulation of FGAR which can be used to Monitor Target Engagement

BACKGROUND

Metabolomic analyses from our group and others have shown that tumors treated with glutamine antagonists (GA) exhibit robust accumulation of formylglycinamide ribonucleotide (FGAR), an intermediate in the de novo purine synthesis pathway. The increase in FGAR is attributed to the inhibition of the enzyme FGAR amidotransferase (FGAR-AT) that catalyzes the ATP-dependent amidation of FGAR to formylglycinamidine ribonucleotide (FGAM). While perturbation of this pathway resulting from GA therapy has long been recognized, no study has reported systematic quantitation and analyses of FGAR in plasma and tumors.

OBJECTIVE

Herein, we aimed to evaluate the efficacy of our recently discovered tumor-targeted GA prodrug, GA-607 (isopropyl 2-(6-acetamido-2-(adamantane-1-carboxamido)hexanamido)-6-diazo-5-oxohexanoate), and demonstrate its target engagement by quantification of FGAR in plasma and tumors.

METHODS

Efficacy and pharmacokinetics of GA-607 were evaluated in a murine EL4 lymphoma model followed by global tumor metabolomic analysis. Liquid chromatography-mass spectrometry (LC-MS) based methods employing the ion-pair chromatography approach were developed and utilized for quantitative FGAR analyses in plasma and tumors.

RESULTS

GA-607 showed preferential tumor distribution and robust single-agent efficacy in a murine EL4 lymphoma model. While several metabolic pathways were perturbed by GA-607 treatment, FGAR showed the highest increase qualitatively. Using our newly developed sensitive and selective LC-MS method, we showed a robust >80- and >10- fold increase in tumor and plasma FGAR levels, respectively, with GA-607 treatment.

CONCLUSION

These studies describe the importance of FGAR quantification following GA therapy in cancer and underscore its importance as a valuable pharmacodynamic marker in the preclinical and clinical development of GA therapies.

Transcriptome and metabolome analysis of crGART, a novel cell model of de novo purine synthesis deficiency: Alterations in CD36 expression and activity

In humans, GART [phosphoribosylglycinamide formyltransferase (EC 2.1.2.2) / phosphoribosylglycinamide synthetase (EC 6.3.4.13) / phosphoribosylaminoimidazole synthetase (EC 6.3.3.1)] is a trifunctional protein which catalyzes the second, third, and fifth reactions of the ten step de novo purine synthesis (DNPS) pathway. The second step of DNPS is conversion of phosphoribosylamine (5-PRA) to glycineamide ribonucleotide (GAR). 5-PRA is extremely unstable under physiological conditions and is unlikely to accumulate in the absence of GART activity. Recently, a HeLa cell line null mutant for GART was constructed via CRISPR-Cas9 mutagenesis. This cell line, crGART, is an important cellular model of DNPS inactivation that does not accumulate DNPS pathway intermediates. In the current study, we characterized the crGART versus HeLa transcriptomes in purine-supplemented and purine-depleted growth conditions. We observed multiple transcriptome changes and discuss pathways and ontologies particularly relevant to Alzheimer disease and Down syndrome. We selected the Cluster of Differentiation (CD36) gene for initial analysis based on its elevated expression in crGART versus HeLa as well as its high basal expression, high log2 value, and minimal P-value.

Targeting Future Pandemics, a Case for De Novo Purine Synthesis and Basic Research

We are currently experiencing a deadly novel viral pandemic with no efficacious, readily available anti-viral therapies to SARS-CoV-2. Viruses will hijack host cellular machinery, including metabolic processes. Here, I provide theory and evidence for targeting the host de novo purine synthetic pathway for broad spectrum anti-viral drug development as well as the pursuit of basic science to mitigate the risks of future novel viral outbreaks.

Human de novo purine biosynthesis

The focus of this review is the human de novo purine biosynthetic pathway. The pathway enzymes are enumerated, as well as the reactions they catalyze and their physical properties. Early literature evidence suggested that they might assemble into a multi-enzyme complex called a metabolon. The finding that fluorescently-tagged chimeras of the pathway enzymes form discrete puncta, now called purinosomes, is further elaborated in this review to include: a discussion of their assembly; the role of ancillary proteins; their locus at the microtubule/mitochondria interface; the elucidation that at endogenous levels, purinosomes function to channel intermediates from phosphoribosyl pyrophosphate to AMP and GMP; and the evidence for the purinosomes to exist as a protein condensate. The review concludes with a consideration of probable signaling pathways that might promote the assembly and disassembly of the purinosome, in particular the identification of candidate kinases given the extensive phosphorylation of the enzymes. These collective findings substantiate our current view of the de novo purine biosynthetic metabolon whose properties will be representative of how other metabolic pathways might be organized for their function.

Surveying purine biosynthesis across the domains of life unveils promising drug targets in pathogens

Purines play an integral role in cellular processes such as energy metabolism, cell signaling and encoding the genetic makeup of all living organisms-ensuring that the purine metabolic pathway is maintained across all domains of life. To gain a deeper understanding of purine biosynthesis via the de novo biosynthetic pathway, the genes encoding purine metabolic enzymes from 35 archaean, 69 bacterial and 99 eukaryotic species were investigated. While the classic elements of the canonical purine metabolic pathway were utilized in all domains, a subset of familiar biochemical roles was found to be performed by unrelated proteins in some members of the Archaea and Bacteria. In the Bacteria, a major differentiating feature of de novo purine biosynthesis is the increasing prevalence of gene fusions, where two or more purine biosynthesis enzymes that perform consecutive biochemical functions in the pathway are encoded by a single gene. All species in the Eukaryota exhibited the most common fusions seen in the Bacteria, in addition to new gene fusions to potentially increase metabolic flux. This complexity is taken further in humans, where a reversible biomolecular assembly of enzymes known as the purinosome has been identified, allowing short-term regulation in response to metabolic cues while expanding on the benefits that can come from gene fusion. By surveying purine metabolism across all domains of life, we have identified important features of the purine biosynthetic pathway that can potentially be exploited as prospective drug targets.

The CRISPR-Cas9 crATIC HeLa transcriptome: Characterization of a novel cellular model of ATIC deficiency and ZMP accumulation

In de novo purine biosynthesis (DNPS), 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase (EC 2.1.2.3)/inosine monophosphate cyclohydrolase (EC 3.5.4.10) (ATIC) catalyzes the last two reactions of the pathway: conversion of 5-aminoimidazole-4-carboxamide ribonucleotide [aka Z-nucleotide monophosphate (ZMP)] to 5-formamido-4-imidazolecarboxamide ribonucleotide (FAICAR) then to inosine monophosphate (IMP). Mutations in ATIC cause an untreatable and devastating inborn error of metabolism in humans. ZMP is an adenosine monophosphate (AMP) mimetic and a known activator of AMP-activated protein kinase (AMPK). Recently, a HeLa cell line null mutant for ATIC was constructed via CRISPR-Cas9 mutagenesis. This mutant, crATIC, accumulates ZMP during purine starvation. Given that the mutant can accumulate ZMP in the absence of treatment with exogenous compounds, crATIC is likely an important cellular model of DNPS inactivation and ZMP accumulation. In the current study, we characterize the crATIC transcriptome versus the HeLa transcriptome in purine-supplemented and purine-depleted growth conditions. We report and discuss transcriptome changes with particular relevance to Alzheimer's disease and in genes relevant to lipid and fatty acid synthesis, neurodevelopment, embryogenesis, cell cycle maintenance and progression, extracellular matrix, immune function, TGFβ and other cellular processes.

Hypoxia drives the assembly of the multienzyme purinosome complex

The purinosome is a dynamic metabolic complex composed of enzymes responsible for de novo purine biosynthesis, whose formation has been associated with elevated purine demand. However, the physiological conditions that govern purinosome formation in cells remain unknown. Here, we report that purinosome formation is up-regulated in cells in response to a low-oxygen microenvironment (hypoxia). We demonstrate that increased purinosome assembly in hypoxic human cells requires the activation of hypoxia inducible factor 1 (HIF-1) and not HIF-2. Hypoxia-driven purinosome assembly was inhibited in cells lacking 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase (ATIC), a single enzyme in de novo purine biosynthesis, and in cells treated with a small molecule inhibitor of ATIC homodimerization. However, despite the increase in purinosome assembly in hypoxia, we observed no associated increase in de novo purine biosynthesis in cells. Our results indicate that this was likely due to a reduction in mitochondrial one-carbon metabolism, resulting in reduced mitochondrion-derived one-carbon units needed for de novo purine biosynthesis. The findings of our study further clarify and deepen our understanding of purinosome formation by revealing that this process does not solely depend on cellular purine demand.

PAICS deficiency, a new defect of de novo purine synthesis resulting in multiple congenital anomalies and fatal outcome

We report for the first time an autosomal recessive inborn error of de novo purine synthesis (DNPS)-PAICS deficiency. We investigated two siblings from the Faroe Islands born with multiple malformations resulting in early neonatal death. Genetic analysis of affected individuals revealed a homozygous missense mutation in PAICS (c.158A>G; p.Lys53Arg) that affects the structure of the catalytic site of the bifunctional enzyme phosphoribosylaminoimidazole carboxylase (AIRC, EC 4.1.1.21)/phosphoribosylaminoimidazole succinocarboxamide synthetase (SAICARS, EC 6.3.2.6) (PAICS). The mutation reduced the catalytic activity of PAICS in heterozygous carrier and patient skin fibroblasts to approximately 50 and 10% of control levels, respectively. The catalytic activity of the corresponding recombinant enzyme protein carrying the mutation p.Lys53Arg expressed and purified from E. coli was reduced to approximately 25% of the wild-type enzyme. Similar to other two known DNPS defects-adenylosuccinate lyase deficiency and AICA-ribosiduria-the PAICS mutation prevented purinosome formation in the patient's skin fibroblasts, and this phenotype was corrected by transfection with the wild-type but not the mutated PAICS. Although aminoimidazole ribotide (AIR) and aminoimidazole riboside (AIr), the enzyme substrates that are predicted to accumulate in PAICS deficiency, were not detected in patient's fibroblasts, the cytotoxic effect of AIr on various cell lines was demonstrated. PAICS deficiency is a newly described disease that enhances our understanding of the DNPS pathway and should be considered in the diagnosis of families with recurrent spontaneous abortion or early neonatal death.

Metabolomics and mass spectrometry imaging reveal channeled de novo purine synthesis in cells

Metabolons, multiprotein complexes consisting of sequential enzymes of a metabolic pathway, are proposed to be biosynthetic "hotspots" within the cell. However, experimental demonstration of their presence and functions has remained challenging. We used metabolomics and in situ three-dimensional submicrometer chemical imaging of single cells by gas cluster ion beam secondary ion mass spectrometry (GCIB-SIMS) to directly visualize de novo purine biosynthesis by a multienzyme complex, the purinosome. We found that purinosomes comprise nine enzymes that act synergistically, channeling the pathway intermediates to synthesize purine nucleotides, increasing the pathway flux, and influencing the adenosine monophosphate/guanosine monophosphate ratio. Our work also highlights the application of high-resolution GCIB-SIMS for multiplexed biomolecular analysis at the level of single cells.

The CRISPR-Cas9 crADSL HeLa transcriptome: A first step in establishing a model for ADSL deficiency and SAICAR accumulation

Adenylosuccinate lyase (ADSL) catalyzes two steps in de novo purine synthesis (DNPS). Mutations in ADSL can result in inborn errors of metabolism characterized by developmental delay and disorder phenotypes, with no effective treatment options. Recently, SAICAR, a metabolic substrate of ADSL, has been found to have alternative roles in the cell, complicating the role of ADSL. crADSL, a CRISPR KO of ADSL in HeLa cells, was constructed to investigate DNPS and ADSL in a human cell line. Here we employ this cell line in an RNA-seq analysis to initially investigate the effect of DNPS and ADSL deficiency on the transcriptome as a first step in establishing a cellular model of ADSL deficiency. We report transcriptome changes in genes relevant to development, vascular development, muscle, and cancer biology, which provide interesting avenues for future research.

Mass spectrometric analysis of purine de novo biosynthesis intermediates

Purines are essential molecules for all forms of life. In addition to constituting a backbone of DNA and RNA, purines play roles in many metabolic pathways, such as energy utilization, regulation of enzyme activity, and cell signaling. The supply of purines is provided by two pathways: the salvage pathway and de novo synthesis. Although purine de novo synthesis (PDNS) activity varies during the cell cycle, this pathway represents an important source of purines, especially for rapidly dividing cells. A method for the detailed study of PDNS is lacking for analytical reasons (sensitivity) and because of the commercial unavailability of the compounds. The aim was to fully describe the mass spectrometric fragmentation behavior of newly synthesized PDNS-related metabolites and develop an analytical method. Except for four initial ribotide PDNS intermediates that preferentially lost water or phosphate or cleaved the forming base of the purine ring, all the other metabolites studied cleaved the glycosidic bond in the first fragmentation stage. Fragmentation was possible in the third to sixth stages. A liquid chromatography-high-resolution mass spectrometric method was developed and applied in the analysis of CRISPR-Cas9 genome-edited HeLa cells deficient in the individual enzymatic steps of PDNS and the salvage pathway. The identities of the newly synthesized intermediates of PDNS were confirmed by comparing the fragmentation patterns of the synthesized metabolites with those produced by cells (formed under pathological conditions of known and theoretically possible defects of PDNS). The use of stable isotope incorporation allowed the confirmation of fragmentation mechanisms and provided data for future fluxomic experiments. This method may find uses in the diagnosis of PDNS disorders, the investigation of purinosome formation, cancer research, enzyme inhibition studies, and other applications.

Potential Mechanisms Connecting Purine Metabolism and Cancer Therapy

Unrestricted cell proliferation is a hallmark of cancer. Purines are basic components of nucleotides in cell proliferation, thus impaired purine metabolism is associated with the progression of cancer. The de novo biosynthesis of purine depends on six enzymes to catalyze the conversion of phosphoribosylpyrophosphate to inosine 5'-monophosphate. These enzymes cluster around mitochondria and microtubules to form purinosome, which is a multi-enzyme complex involved in de novo purine biosynthesis and purine nucleotides requirement. In this review, we highlighted the purine metabolism and purinosome biology with emphasis on the therapeutic potential of manipulating of purine metabolism or purinosome in cancers. We also reviewed current advances in our understanding of mammalian target of rapamycin for regulating purinosome formation or purine metabolism in cancers and discussed the future prospects for targeting purinosome to treat cancers.

Study of purinosome assembly in cell-based model systems with de novo purine synthesis and salvage pathway deficiencies

BACKGROUND

The enzymes involved in de novo purine synthesis (DNPS), one of the basic processes in eukaryotic cells, transiently and reversibly form a dynamic multienzyme complex called the purinosome in the cytoplasm. The purinosome has been observed in a broad spectrum of cells, but some studies claim that it is an artefact of the constructs used for visualization or stress granules resulting from the exposure of cells to nutrient-reduced growth media. Both may be true depending on the method of observation. To clarify this point, we combined two previously used methods, transfection and immunofluorescence, to detect purinosomes in purinosome-free cells deficient in particular DNPS steps (CR-DNPS cells) and in cells deficient in the salvage pathway, which resulted in construction of the purinosome regardless of purine level (CR-HGPRT cells).

METHODS AND FINDINGS

To restore or disrupt purinosome formation, we transiently transfected CR-DNPS and CR-HGPRT cells with vectors encoding BFP-labelled wild-type (wt) proteins and observed the normalization of purinosome formation. The cells also ceased to accumulate the substrate(s) of the defective enzyme. The CR-DNPS cell line transfected with a DNA plasmid encoding an enzyme with zero activity served as a negative control for purinosome formation. No purinosome formation was observed in these cells regardless of the purine level in the growth medium.

CONCLUSION

In conclusion, both methods are useful for the detection of purinosomes in HeLa cells. Moreover, the cell-based models prepared represent a unique system for the study of purinosome assembly with deficiencies in DNPS or in the salvage pathway as well as for the study of purinosome formation under the action of DNPS inhibitors. This approach is a promising step toward the treatment of purine disorders and can also provide targets for anticancer therapy.

Life and Work of Stress Granules and Processing Bodies: New Insights into Their Formation and Function

The dynamic formation of stress granules (SGs), processing bodies (PBs), and related RNA organelles regulates diverse cellular processes, including the coordination of functionally connected messengers, the translational regulation at the synapse, and the control of viruses and retrotransposons. Recent studies have shown that pyruvate kinase and other enzymes localize in SGs and PBs, where they become protected from stress insults. These observations may have implications for enzyme regulation and metabolic control exerted by RNA-based organelles. The formation of these cellular bodies is governed by liquid-liquid phase separation (LLPS) processes, and it needs to be strictly controlled to prevent pathogenic aggregation. The intracellular concentration of key metabolites, such as ATP and sterol derivatives, may influence protein solubility, thus affecting the dynamics of liquid organelles. LLPS in vitro depends on the thermal diffusion of macromolecules, which is limited inside cells, where the condensation and dissolution of membrane-less organelles are helped by energy-driven processes. The active transport by the retrograde motor dynein helps SG assembly, whereas the anterograde motor kinesin mediates SG dissolution; a tug of war between these two molecular motors allows transient SG formation. There is evidence that the efficiency of dynein-mediated transport increases with the number of motor molecules associated with the cargo. The dynein-dependent transport may be influenced by cargo size as larger cargos can load a larger number of motors. We propose a model based on this emergent property of dynein motors, which would be collectively stronger during SG condensation and weaker during SG breakdown, thus allowing kinesin-mediated dispersion.

Spatial Organization of Metabolic Enzyme Complexes in Cells

The organization of metabolic multienzyme complexes has been hypothesized to benefit metabolic processes and provide a coordinated way for the cell to regulate metabolism. Historically, their existence has been supported by various in vitro techniques. However, it is only recently that the existence of metabolic complexes inside living cells has come to light to corroborate this long-standing hypothesis. Indeed, subcellular compartmentalization of metabolic enzymes appears to be widespread and highly regulated. On the other hand, it is still challenging to demonstrate the functional significance of these enzyme complexes in the context of the cellular milieu. In this review, we discuss the current understanding of metabolic enzyme complexes by primarily focusing on central carbon metabolism and closely associated metabolic pathways in a variety of organisms, as well as their regulation and functional contributions to cells.

A New View into the Regulation of Purine Metabolism: The Purinosome

Other than serving as building blocks for DNA and RNA, purine metabolites provide a cell with the necessary energy and cofactors to promote cell survival and proliferation. A renewed interest in how purine metabolism may fuel cancer progression has uncovered a new perspective into how a cell regulates purine need. Under cellular conditions of high purine demand, the de novo purine biosynthetic enzymes cluster near mitochondria and microtubules to form dynamic multienzyme complexes referred to as 'purinosomes'. In this review, we highlight the purinosome as a novel level of metabolic organization of enzymes in cells, its consequences for regulation of purine metabolism, and the extent that purine metabolism is being targeted for the treatment of cancers.