The Interplay between Myc and CTP Synthase in Drosophila

Architecture of CTPS filament networks revealed by cryo-electron tomography

The cytoophidium is a novel type of membraneless organelle, first observed in the ovaries of Drosophila using fluorescence microscopy. In vitro, purified Drosophila melanogaster CTPS (dmCTPS) can form metabolic filaments under the presence of either substrates or products, and their structures that have been analyzed using cryo-electron microscopy (cryo-EM). These dmCTPS filaments are considered the fundamental units of cytoophidia. However, due to the resolution gap between light and electron microscopy, the precise assembly pattern of cytoophidia remains unclear. In this study, we find that dmCTPS filaments can spontaneously assemble in vitro, forming network structures that reach micron-scale dimensions. Using cryo-electron tomography (cryo-ET), we reconstruct the network structures formed by dmCTPS filaments under substrate or product binding conditions and elucidate their assembly process. The dmCTPS filaments initially form structural bundles, which then further assemble into larger networks. By identifying, tracking, and statistically analyzing the filaments, we observed distinct characteristics of the structural bundles formed under different conditions. This study provides the first systematic analysis of dmCTPS filament networks, offering new insights into the relationship between cytoophidia and metabolic filaments.

The Impact of Developmental and Metabolic Cues on Cytoophidium Formation

The cytoophidium, composed mainly of CTP synthase (CTPS), is a newly discovered dynamic filamentous structure in various organisms such as archaea, bacteria, and humans. These filamentous structures represent a fascinating example of intracellular compartmentation and dynamic regulation of metabolic enzymes. Currently, cytoophidia have been proven to be tightly regulated and highly dynamic, responding rapidly to developmental and metabolic cues and playing a critical role in maintaining cellular homeostasis. In this review, we would like to discuss in detail the characteristics, mechanisms, functions, and potential applications of this conservative but promising organelle.

Dynamic Cytoophidia during Late-Stage Drosophila Oogenesis

CTP synthase (CTPS) catalyzes the final step of de novo synthesis of CTP. CTPS was first discovered to form filamentous structures termed cytoophidia in Drosophila ovarian cells. Subsequent studies have shown that cytoophidia are widely present in cells of three life domains. In the Drosophila ovary model, our previous studies mainly focused on the early and middle stages, with less involvement in the later stages. In this work, we focus on the later stages of female germline cells in Drosophila. We use live-cell imaging to capture the continuous dynamics of cytoophidia in Stages 10-12. We notice the heterogeneity of cytoophidia in the two types of germline cells (nurse cells and oocytes), manifested in significant differences in morphology, distribution, and dynamics. Surprisingly, we also find that neighboring nurse cells in the same egg chamber exhibit multiple dynamic patterns of cytoophidia over time. Although the described dynamics may be influenced by the in vitro incubation conditions, our observation provides an initial understanding of the dynamics of cytoophidia during late-stage Drosophila oogenesis.

Cytoophidia: a conserved yet promising mode of enzyme regulation in nucleotide metabolism

Nucleotide biosynthesis encompasses both de novo and salvage synthesis pathways, each characterized by significant material and procedural distinctions. Despite these differences, cells with elevated nucleotide demands exhibit a preference for the more intricate de novo synthesis pathway, intricately linked to modes of enzyme regulation. In this study, we primarily scrutinize the biological importance of a conserved yet promising mode of enzyme regulation in nucleotide metabolism-cytoophidia. Cytoophidia, comprising cytidine triphosphate synthase or inosine monophosphate dehydrogenase, is explored across diverse biological models, including yeasts, Drosophila, mice, and human cancer cell lines. Additionally, we delineate potential biomedical applications of cytoophidia. As our understanding of cytoophidia deepens, the roles of enzyme compartmentalization and polymerization in various biochemical processes will unveil, promising profound impacts on both research and the treatment of metabolism-related diseases.

Connecting Hippo Pathway and Cytoophidia in Drosophila Posterior Follicle Cells

CTP synthase (CTPS), the rate-limiting enzyme in the de novo synthesis of CTP, assembles into a filamentous structure termed the cytoophidium. The Hippo pathway regulates cell proliferation and apoptosis. The relationship of the nucleotide metabolism with the Hippo pathway is little known. Here, we study the impact of the Hippo pathway on the cytoophidium in Drosophila melanogaster posterior follicle cells (PFCs). We find that the inactivation of the Hippo pathway correlates with reduced cytoophidium length and number within PFCs. During the overexpression of CTPS, the presence of Hippo mutations also reduces the length of cytoophidia in PFCs. In addition, we observe that knocking down CTPS mitigates hpo (Hippo)-associated over-proliferation. In summary, our results suggest that there is a connection between the Hippo pathway and the nucleotide biosynthesis enzyme CTPS in PFCs.

Cytoophidia Influence Cell Cycle and Size in Schizosaccharomyces pombe

Cytidine triphosphate synthase (CTPS) forms cytoophidia in all three domains of life. Here we focus on the function of cytoophidia in cell proliferation using Schizosaccharomyces pombe as a model system. We find that converting His359 of CTPS into Ala359 leads to cytoophidium disassembly. By reducing the level of CTPS protein or specific mutation, the loss of cytoophidia prolongs the G2 phase and expands cell size. In addition, the loss-filament mutant of CTPS leads to a decrease in the expression of genes related to G2/M transition and cell growth, including histone chaperone slm9. The overexpression of slm9 alleviates the G2 phase elongation and cell size enlargement induced by CTPS loss-filament mutants. Overall, our results connect cytoophidia with cell cycle and cell size control in Schizosaccharomyces pombe.

Therapeutic potential of toosendanin: Novel applications of an old ascaris repellent as a drug candidate

Toosendanin (TSN), extracted from Melia. toosendan Sieb.et Zucc. and Melia. azedarach L., has been developed into an ascaris repellent in China. However, with the improvement of public health protection, the incidence of ascariasis has been reduced considerably, resulting in limited medical application of TSN. Therefore, it is questionable whether this old ascaris repellent can develop into a drug candidate. Modern studies have shown that TSN has strong pharmacological activities, including anti-tumor, anti-botulinum, anti-viral and anti-parasitic potentials. It also can regulate fat formation and improve inflammation. These researches indicate that TSN has great potential to be developed into a corresponding medical product. In order to better development and application of TSN, the availability, pharmacodynamics, pharmacokinetics and toxicology of TSN are summarized systematically. In addition, this review discusses shortcomings in the current researches and provides useful suggestions about how TSN developed into a drug candidate. Therefore, this paper illustrates the possibility of developing TSN as a medical product, aimed to provide directions for the clinical application and further research of TSN.

Fat body-specific reduction of CTPS alleviates HFD-induced obesity

Obesity induced by high-fat diet (HFD) is a multi-factorial disease including genetic, physiological, behavioral, and environmental components. Drosophila has emerged as an effective metabolic disease model. Cytidine 5'-triphosphate synthase (CTPS) is an important enzyme for the de novo synthesis of CTP, governing the cellular level of CTP and the rate of phospholipid synthesis. CTPS is known to form filamentous structures called cytoophidia, which are found in bacteria, archaea, and eukaryotes. Our study demonstrates that CTPS is crucial in regulating body weight and starvation resistance in Drosophila by functioning in the fat body. HFD-induced obesity leads to increased transcription of CTPS and elongates cytoophidia in larval adipocytes. Depleting CTPS in the fat body prevented HFD-induced obesity, including body weight gain, adipocyte expansion, and lipid accumulation, by inhibiting the PI3K-Akt-SREBP axis. Furthermore, a dominant-negative form of CTPS also prevented adipocyte expansion and downregulated lipogenic genes. These findings not only establish a functional link between CTPS and lipid homeostasis but also highlight the potential role of CTPS manipulation in the treatment of HFD-induced obesity.

STAMP: Spatio-Temporal Association Mapping of Proteins

We describe a tool, Spatio-Temporal Association Mapping of Proteins (STAMP), for identifying protein interactomes via proximity labeling. For a proof-of-principle study, we use cytidine 5'-triphosphate synthase (CTPS) as an example. CTPS, a metabolic enzyme, forms filamentous structures termed cytoophidia in various tissues. We apply STAMP to a variety of developmental stages and tissues in Drosophila including adult ovaries. Using a cell-specific GAL4 driver, we verify that TurboID can biotinylate the bait protein CTPS, making possible the identification of protein-protein interactions (PPIs) in individual cells. Using the wild-type and mutant CTPS as bait proteins, STAMP results in two distinct sets of proximate proteomes. Our results suggest that STAMP is a feasible tool to catch in vivo PPIs in situ at a defined spatiotemporal resolution.

Drosophila pVALIUM10 TRiP RNAi lines cause undesired silencing of Gateway-based transgenes

Post-transcriptional gene silencing using double-stranded RNA has revolutionized the field of functional genetics, allowing fast and easy disruption of gene function in various organisms. In Drosophila, many transgenic RNAi lines have been generated in large-scale efforts, including the Drosophila Transgenic RNAi Project (TRiP), to facilitate in vivo knockdown of virtually any Drosophila gene with spatial and temporal resolution. The available transgenic RNAi lines represent a fundamental resource for the fly community, providing an unprecedented opportunity to address a vast range of biological questions relevant to basic and biomedical research fields. However, caution should be applied regarding the efficiency and specificity of the RNAi approach. Here, we demonstrate that pVALIUM10-based RNAi lines, representing ∼13% of the total TRiP collection (1,808 of 13,410 pVALIUM TRiP-based RNAi lines), cause unintended off-target silencing of transgenes expressed from Gateway destination vectors. The silencing is mediated by targeting attB1 and attB2 sequences generated via site-specific recombination and included in the transcribed mRNA. Deleting these attB sites from the Gateway expression vector prevents silencing and restores expected transgene expression.

Cytoophidia Maintain the Integrity of Drosophila Follicle Epithelium

CTP synthase (CTPS) forms a filamentous structure termed the cytoophidium in all three domains of life. The female reproductive system of Drosophila is an excellent model for studying the physiological function of cytoophidia. Here, we use CTPS^H355A, a point mutation that destroys the cytoophidium-forming ability of CTPS, to explore the in vivo function of cytoophidia. In CTPS^H355A egg chambers, we observe the ingression and increased heterogeneity of follicle cells. In addition, we find that the cytoophidium-forming ability of CTPS, rather than the protein level, is the cause of the defects observed in CTPS^H355A mutants. To sum up, our data indicate that cytoophidia play an important role in maintaining the integrity of follicle epithelium.

A TRiP RNAi screen to identify molecules necessary for Drosophila photoreceptor differentiation

Drosophila rhabdomeric terminal photoreceptor differentiation is an extended process taking several days to complete. Following ommatidial patterning by the morphogenetic furrow, photoreceptors are sequentially recruited and specified, and terminal differentiation begins. Key events of terminal differentiation include the establishment of apical and basolateral domains, rhabdomere and stalk formation, inter-rhabdomeral space formation, and expression of phototransduction machinery. While many key regulators of these processes have been identified, the complete network of transcription factors to downstream effector molecules necessary for regulating each of these major events remains incomplete. Here, we report an RNAi screen to identify additional molecules and cellular pathways required for photoreceptor terminal differentiation. First, we tested several eye-specific GAL4 drivers for correct spatial and temporal specificity and identified Pph13-GAL4 as the most appropriate GAL4 line for our screen. We screened lines available through the Transgenic RNAi Project and isolated lines that when combined with Pph13-GAL4 resulted in the loss of the deep pseudopupil, as a readout for abnormal differentiation. In the end, we screened 6,189 lines, representing 3,971 genes, and have identified 64 genes, illuminating potential new regulatory molecules and cellular pathways for the differentiation and organization of Drosophila rhabdomeric photoreceptors.

Single-cell transcriptomics identifies Keap1-Nrf2 regulated collective invasion in a Drosophila tumor model

Apicobasal cell polarity loss is a founding event in epithelial-mesenchymal transition and epithelial tumorigenesis, yet how pathological polarity loss links to plasticity remains largely unknown. To understand the mechanisms and mediators regulating plasticity upon polarity loss, we performed single-cell RNA sequencing of Drosophila ovaries, where inducing polarity-gene l(2)gl-knockdown (Lgl-KD) causes invasive multilayering of the follicular epithelia. Analyzing the integrated Lgl-KD and wildtype transcriptomes, we discovered the cells specific to the various discernible phenotypes and characterized the underlying gene expression. A genetic requirement of Keap1-Nrf2 signaling in promoting multilayer formation of Lgl-KD cells was further identified. Ectopic expression of Keap1 increased the volume of delaminated follicle cells that showed enhanced invasive behavior with significant changes to the cytoskeleton. Overall, our findings describe the comprehensive transcriptome of cells within the follicle cell tumor model at the single-cell resolution and identify a previously unappreciated link between Keap1-Nrf2 signaling and cell plasticity at early tumorigenesis.

Ubiquitination regulates cytoophidium assembly in Schizosaccharomyces pombe

CTP synthase (CTPS), a metabolic enzyme responsible for the de novo synthesis of CTP, can form filamentous structures termed cytoophidia, which are evolutionarily conserved from bacteria to humans. Here we used Schizosaccharomyces pombe to study the cytoophidium assembly regulation by ubiquitination. We tested the CTP synthase's capacity to be post-translationally modified by ubiquitin or be affected by the ubiquitination state of the cell and showed that ubiquitination is important for the maintenance of the CTPS filamentous structure in fission yeast. We have identified proteins which are in complex with CTPS, including specific ubiquitination regulators which significantly affect CTPS filamentation, and mapped probable ubiquitination targets on CTPS. Furthermore, we discovered that a cohort of deubiquitinating enzymes is important for the regulation of cytoophidium's filamentous morphology. Our study provides a framework for the analysis of the effects that ubiquitination and deubiquitination have on the formation of cytoophidia.

Cytoophidia coupling adipose architecture and metabolism

Tissue architecture determines its unique physiology and function. How these properties are intertwined has remained unclear. Here we show that the metabolic enzyme CTP synthase (CTPS) form filamentous structures termed cytoophidia along the adipocyte cortex in Drosophila adipose tissue. Loss of cytoophidia, whether due to reduced CTPS expression or a point mutation that specifically abrogates its polymerization ability, causes impaired adipocyte adhesion and defective adipose tissue architecture. Moreover, CTPS influences integrin distribution and dot-like deposition of type IV collagen (Col IV). Col IV-integrin signaling reciprocally regulates the assembly of cytoophidia in adipocytes. Our results demonstrate that a positive feedback signaling loop containing both cytoophidia and integrin adhesion complex couple tissue architecture and metabolism in Drosophila adipose tissue.

MYC sensitises cells to apoptosis by driving energetic demand

The MYC oncogene is a potent driver of growth and proliferation but also sensitises cells to apoptosis, which limits its oncogenic potential. MYC induces several biosynthetic programmes and primary cells overexpressing MYC are highly sensitive to glutamine withdrawal suggesting that MYC-induced sensitisation to apoptosis may be due to imbalance of metabolic/energetic supply and demand. Here we show that MYC elevates global transcription and translation, even in the absence of glutamine, revealing metabolic demand without corresponding supply. Glutamine withdrawal from MRC-5 fibroblasts depletes key tricarboxylic acid (TCA) cycle metabolites and, in combination with MYC activation, leads to AMP accumulation and nucleotide catabolism indicative of energetic stress. Further analyses reveal that glutamine supports viability through TCA cycle energetics rather than asparagine biosynthesis and that TCA cycle inhibition confers tumour suppression on MYC-driven lymphoma in vivo. In summary, glutamine supports the viability of MYC-overexpressing cells through an energetic rather than a biosynthetic mechanism.

CTP synthase does not form cytoophidia in Drosophila interfollicular stalks

CTP synthase (CTPS) catalyzes the final step of de novo synthesis of the nucleotide CTP. In 2010, CTPS has been found to form filamentous structures termed cytoophidia in Drosophila follicle cells and germline cells. Subsequently, cytoophidia have been reported in many species across three domains of life: bacteria, eukaryotes and archaea. Forming cytoophidia appears to be a highly conserved and ancient property of CTPS. To our surprise, here we find that polar cells and stalk cells, two specialized types of cells composing Drosophila interfollicular stalks, do not possess obvious cytoophidia. We show that Myc level is low in these two types of cells. Treatment with a glutamine analog, 6-diazo-5-oxo-l-norleucine (DON), increases cytoophidium assembly in main follicle cells, but not in polar cells or stalk cells. Moreover, overexpressing Myc induces cytoophidium formation in stalk cells. When CTPS is overexpressed, cytoophidia can be observed both in stalk cells and polar cells. Our findings provide an interesting paradigm for the in vivo study of cytoophidium assembly and disassembly among different populations of follicle cells.

GTP-Dependent Regulation of CTP Synthase: Evolving Insights into Allosteric Activation and NH3 Translocation

Cytidine-5′-triphosphate (CTP) synthase (CTPS) is the class I glutamine-dependent amidotransferase (GAT) that catalyzes the last step in the de novo biosynthesis of CTP. Glutamine hydrolysis is catalyzed in the GAT domain and the liberated ammonia is transferred via an intramolecular tunnel to the synthase domain where the ATP-dependent amination of UTP occurs to form CTP. CTPS is unique among the glutamine-dependent amidotransferases, requiring an allosteric effector (GTP) to activate the GAT domain for efficient glutamine hydrolysis. Recently, the first cryo-electron microscopy structure of Drosophila CTPS was solved with bound ATP, UTP, and, notably, GTP, as well as the covalent adduct with 6-diazo-5-oxo-l-norleucine. This structural information, along with the numerous site-directed mutagenesis, kinetics, and structural studies conducted over the past 50 years, provide more detailed insights into the elaborate conformational changes that accompany GTP binding at the GAT domain and their contribution to catalysis. Interactions between GTP and the L2 loop, the L4 loop from an adjacent protomer, the L11 lid, and the L13 loop (or unique flexible “wing” region), induce conformational changes that promote the hydrolysis of glutamine at the GAT domain; however, direct experimental evidence on the specific mechanism by which these conformational changes facilitate catalysis at the GAT domain is still lacking. Significantly, the conformational changes induced by GTP binding also affect the assembly and maintenance of the NH₃ tunnel. Hence, in addition to promoting glutamine hydrolysis, the allosteric effector plays an important role in coordinating the reactions catalyzed by the GAT and synthase domains of CTPS.

Combined Inactivation of CTPS1 and ATR Is Synthetically Lethal to MYC-Overexpressing Cancer Cells

The "undruggable" oncogene MYC supports cancer cell proliferation and survival through parallel induction of multiple anabolic processes. Here we find that inhibiting CTP synthase (CTPS) selectively decreases cell viability and induces DNA replication stress in MYC-overexpressing cells. MYC-driven rRNA synthesis caused the selective DNA replication stress upon CTPS inhibition. Combined inhibition of CTPS and ataxia telangiectasia and Rad3-related protein (ATR) is synthetically lethal in MYC-overexpressing cells, promoting cell death in vitro and decreasing tumor growth in vivo. Unexpectedly, interfering with CTPS1 but not CTPS2 is required to induce replication stress in MYC-deregulated cancer cells and consequent cell death in the presence of an ATR inhibitor. These results highlight a specific and key role of CTPS1 in MYC-driven cancer, suggesting that selectively inhibiting CTPS1 in combination with ATR could be a promising strategy to combat disease progression.

SIGNIFICANCE

Inhibition of CTPS in MYC-overexpressing cells blocks pyrimidine synthesis while maintaining ribosome synthesis activity to create an anabolic imbalance that induces replication stress, providing a new approach to selectively target MYC-driven cancer. See related commentary by Chabanon and Postel-Vinay, p. 969.

CTP synthase: the hissing of the cellular serpent

CTP biosynthesis is carried out by two pathways: salvage and de novo. CTPsyn catalyzes the latter. The study of CTPsyn activity in mammalian cells began in the 1970s, and various fascinating discoveries were made regarding the role of CTPsyn in cancer and development. However, its ability to fit into a cellular serpent-like structure, termed 'cytoophidia,' was only discovered a decade ago by three independent groups of scientists. Although the self-assembly of CTPsyn into a filamentous structure is evolutionarily conserved, the enzyme activity upon this self-assembly varies in different species. CTPsyn is required for cellular development and homeostasis. Changes in the expression of CTPsyn cause developmental changes in Drosophila melanogaster. A high level of CTPsyn activity and formation of cytoophidia are often observed in rapidly proliferating cells such as in stem and cancer cells. Meanwhile, the deficiency of CTPsyn causes severe immunodeficiency leading to immunocompromised diseases caused by bacteria, viruses, and parasites, making CTPsyn an attractive therapeutic target. Here, we provide an overview of the role of CTPsyn in cellular and disease perspectives along with its potential as a drug target.

Drosophila intestinal homeostasis requires CTP synthase

CTP synthase (CTPS) senses all four nucleotides and forms filamentous structures termed cytoophidia in all three domains of life. How CTPS and cytoophidia function in a developmental context, however, remains underexplored. We report that CTPS forms cytoophidia in a subset of cells in the Drosophila midgut. We found that cytoophidia exist in intestinal stem cells (ISC) and enteroblasts in similar proportions. Both refeeding after starvation and feeding with dextran sulfate sodium (DSS) induce ISC proliferation and elongate cytoophidia. Knockdown of CTPS inhibits ISC proliferation. Remarkably, disruption of CTPS cytoophidia inhibits DSS-induced ISC proliferation. Taken together, these data suggest that both the expression level and the filament-form property of CTPS are crucial for intestinal homeostasis in Drosophila.

Polyploidy in development and tumor models in Drosophila

Polyploidy, a cell status defined as more than two sets of genomic DNA, is a conserved strategy across species that can increase cell size and biosynthetic production, but the functional aspects of polyploidy are nuanced and vary across cell types. Throughout Drosophila developmental stages (embryo, larva, pupa and adult), polyploid cells are present in numerous organs and help orchestrate development while contributing to normal growth, well-being and homeostasis of the organism. Conversely, increasing evidence has shown that polyploid cells are prevalent in Drosophila tumors and play important roles in tumor growth and invasiveness. Here, we summarize the genes and pathways involved in polyploidy during normal and tumorigenic development, the mechanisms underlying polyploidization, and the functional aspects of polyploidy in development, homeostasis and tumorigenesis in the Drosophila model.

Highly effective proximate labeling in Drosophila

The protein-protein interaction (PPI) is a basic strategy for life to operate. The analysis of PPIs in multicellular organisms is very important but extremely challenging because PPIs are particularly dynamic and variable among different development stages, tissues, cells, and even organelles. Therefore, understanding PPI needs a good resolution of time and space. More importantly, understanding in vivo PPI needs to be realized in situ. Proximity-based biotinylation combined with mass spectrometry (MS) has emerged as a powerful approach to study PPI networks and protein subcellular compartmentation. TurboID, the newly engineered promiscuous ligase, has been reported to label proximate proteins effectively in various species. In Drosophila, we systematically apply TurboID-mediated biotinylation in a wide range of developmental stages and tissues, and demonstrate the feasibility of TurboID-mediated labeling system in desired cell types. For a proof-of-principle, we use the TurboID-mediated biotinylation coupled with MS to distinguish CTP synthase with or without the ability to form filamentous cytoophidia, retrieving two distinct sets of proximate proteomes. Therefore, this makes it possible to map PPIs in vivo and in situ at a defined spatiotemporal resolution, and demonstrates a referable resource for cytoophidium proteome in Drosophila.

ASNS disruption shortens CTPS cytoophidia in Saccharomyces cerevisiae

Asparagine synthetase (ASNS) and CTP synthase (CTPS) are two metabolic enzymes that catalyze the biosynthesis of asparagine and CTP, respectively. Both CTPS and ASNS have been identified to form cytoophidia in Saccharomyces cerevisiae. Glutamine is a common substrate for both these enzymes, and they play an important role in glutamine homeostasis. Here, we find that the ASNS cytoophidia are shorter than the CTPS cytoophidia, and that disruption of ASNS shortens the length of CTPS cytoophidia. However, the deletion of CTPS has no effect on the formation and length of ASNS cytoophidia, or on the ASNS protein level. We also find that Asn1 overexpression induces the formation of a multi-dot structure in diauxic phase which suggests that the increased protein level may trigger cytoophidia formation. Collectively, our results reveal a connection between ASNS cytoophidia and CTPS cytoophidia.

CTPS forms the cytoophidium in zebrafish

Cytidine triphosphate synthase (CTPS) catalyzes the rate-limiting step of de novo CTP biosynthesis. An intracellular structure of CTPS, the cytoophidium, has been found in many organisms including prokaryotes and eukaryotes. Formation of the cytoophidium has been suggested to regulate the activity and stability of CTPS and may participate in certain physiological events. Herein, we demonstrate that both CTPS1a and CTPS1b in zebrafish are able to form the cytoophidium in cultured cells. A point mutation, H355A, abrogates cytoophidium assembly of zebrafish CTPS1a and CTPS1b. In addition, we show the presence of CTPS cytoophidia in multiple tissues of larval and adult fish under normal conditions, while treatment with a CTPS inhibitor 6-diazo-5-oxo-l-norleucine (DON) can induce more cytoophidia in some tissues. Our findings reveal that forming the CTPS cytoophidium is a natural phenomenon of zebrafish and provide valuable information for future research on the physiological importance of this intracellular structure in vertebrates.

Formation of autotriploid Carassius auratus and its fertility-related genes analysis

Background

Formation of triploid organism is useful in genetics and breeding. In this study, autotriploid Carassius auratus (3nRR, 3n = 150) was generated from Carassius auratus red var. (RCC, 2n = 100) (♀) and autotetraploid Carassius auratus (4nRR, 4n = 200) (♂). The female 3nRR produced haploid, diploid and triploid eggs, whereas the male 3nRR was infertile. The aim of the present study was to explore fertility of potential candidate genes of 3nRR.

Results

Gonadal transcriptome profiling of four groups (3 females RCC (FRCC), 3 males 4nRR (M4nRR), 3 males 3nRR (M3nRR) and 3 females 3nRR (F3nRR)) was performed using RNA-SEq. A total of 78.90 Gb of clean short reads and 24,262 differentially expressed transcripts (DETs), including 20,155 in F3nRR vs. FRCC and 4,107 in M3nRR vs. M4nRR were identified. A total of 106 enriched pathways were identified through KEGG enrichment analysis. Out of the enriched pathways, 44 and 62 signalling pathways were identified in F3nRR vs. FRCC and M3nRR vs. M4nRR, respectively. A total of 80 and 25 potential candidate genes for fertility-related in F3nRR and M3nRR were identified, respectively, through GO, KEGG analyses and the published literature. Moreover, protein-protein interaction (PPI) network construction of these fertility-associated genes were performed. Analysis of the PPI networks showed that 6 hub genes (MYC, SOX2, BMP4, GATA4, PTEN and BMP2) were involved in female fertility of F3nRR, and 2 hub genes (TP53 and FGF2) were involved in male sterility of M3nRR.

Conclusions

Establishment of autotriploid fish offers an ideal model to study reproductive traits of triploid fish. RNA-Seq data revealed 6 genes, namely, MYC, SOX2, BMP4, GATA4, PTEN and BMP2, involved in the female fertility of the F3nRR. Moreover, 2 genes, namely, TP53 and FGF2, were related to the male sterility of the M3nRR. These findings provide information on reproduction and breeding in triploid fish.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07753-5.

Freedom of assembly: metabolic enzymes come together

Many different enzymes in intermediate metabolism dynamically assemble filamentous polymers in cells, often in response to changes in physiological conditions. Most of the enzyme filaments known to date have only been observed in cells, but in a handful of cases structural and biochemical studies have revealed the mechanisms and consequences of assembly. In general, enzyme polymerization functions as a mechanism to allosterically tune enzyme kinetics, and it may play a physiological role in integrating metabolic signaling. Here, we highlight some principles of metabolic filaments by focusing on two well-studied examples in nucleotide biosynthesis pathways—inosine-5’-monophosphate (IMP) dehydrogenase and cytosine triphosphate (CTP) synthase.

CTPS and IMPDH form cytoophidia in developmental thymocytes

The cytoophidium, a filamentous structure formed by metabolic enzymes, has emerged as a novel regulatory machinery for certain proteins. The rate-limiting enzymes of de novo CTP and GTP synthesis, cytidine triphosphate synthase (CTPS) and inosine monophosphate dehydrogenase (IMPDH), are the most characterized cytoophidium-forming enzymes in mammalian models. Although the assembly of CTPS cytoophidia has been demonstrated in various organisms including multiple human cancers, a systemic survey for the presence of CTPS cytoophidia in mammalian tissues in normal physiological conditions has not yet been reported. Herein, we examine major organs of adult mouse and observe that CTPS cytoophidia are displayed by a specific thymocyte population ranging between DN3 to early DP stages. Most of these cytoophidium-presenting cells have both CTPS and IMPDH cytoophidia and undergo rapid cell proliferation. In addition, we show that cytoophidium formation is associated with active glycolytic metabolism as the cytoophidium-presenting cells exhibit higher levels of c-Myc, phospho-Akt and PFK. Inhibition of glycolysis with 2DG, however, disrupts most of cytoophidium structures and impairs cell proliferation. Our findings not only indicate that the regulation of CTPS and IMPDH cytoophidia are correlated with the metabolic switch triggered by pre-TCR signaling, but also suggest physiological roles of the cytoophidium in thymocyte development.

Gm14230 controls Tbc1d24 cytoophidia and neuronal cellular juvenescence

It is not fully understood how enzymes are regulated in the tiny reaction field of a cell. Several enzymatic proteins form cytoophidia, a cellular macrostructure to titrate enzymatic activities. Here, we show that the epileptic encephalopathy-associated protein Tbc1d24 forms cytoophidia in neuronal cells both in vitro and in vivo. The Tbc1d24 cytoophidia are distinct from previously reported cytoophidia consisting of inosine monophosphate dehydrogenase (Impdh) or cytidine-5'-triphosphate synthase (Ctps). Tbc1d24 cytoophidia is induced by loss of cellular juvenescence caused by depletion of Gm14230, a juvenility-associated lncRNA (JALNC) and zeocin treatment. Cytoophidia formation is associated with impaired enzymatic activity of Tbc1d24. Thus, our findings reveal the property of Tbc1d24 to form cytoophidia to maintain neuronal cellular juvenescence.

CTP Synthase 2 From Arabidopsis thaliana Is Required for Complete Embryo Development

Pyrimidine de novo synthesis is an essential pathway in all organisms. The final and rate-limiting step in the synthesis of the nucleotide cytidine triphosphate (CTP) is catalyzed by CTP synthase (CTPS), and Arabidopsis harbors five isoforms. Single mutant lines defective in each one of the four isoforms do not show apparent phenotypical alterations in comparison to wild-type plants. However, Arabidopsis lines that contain T-DNA insertions in the CTPS2 gene were unable to produce homozygous offspring. Here, we show that CTPS2 exhibits a distinct expression pattern throughout embryo development, and loss-of-function mutants are embryo lethal, as siliques from +/ctps2 plants contained nearly 25% aborted seeds. This phenotype was rescued by complementation with CTPS2 under control of its endogenous promoter. CTPS2::GFP lines revealed expression only in the tip of columella cells in embryo root tips of the heart and later stages. Furthermore, CTPS2 expression in mature roots, most pronounced in the columella cells, shoots, and vasculature tissue of young seedlings, was observed. Filial generations of +/ctps2 plants did not germinate properly, even under external cytidine supply. During embryo development, the CTPS2 expression pattern resembled the established auxin reporter DR5::GFP. Indeed, the cloned promoter region we used in this study possesses a repeat of an auxin response element, and auxin supply increased CTPS2 expression in a cell-type-specific manner. Thus, we conclude that CTPS2 is essential for CTP supply in developing embryos, and loss-of-function mutants in CTPS2 are embryo lethal.

Histone transcription regulator Slm9 is required for cytoophidium biogenesis

The cytoophidium, a subcellular structure composed of CTP synthase, can be observed during the division of Schizosaccharomyces pombe. Cytoophidium formation changes periodically with the cell cycle of yeast cells. Here, we find that histone chaperone Slm9 is required for the integrity of cytoophidia in fission yeast. When the slm9 gene is knocked out, we observe that morphological characteristics, the abundance of cytoophidia and the division of the yeast cells are significantly affected. Fragmented cytoophidia occur in slm9 mutant cells, a phenomenon rarely observed in wild-type cells. Our study reveals a potential link between a chromosomal regulatory factor and cytoophidium biogenesis.

Polarised maintenance of cytoophidia in Drosophila follicle epithelia

The metabolic enzyme CTP synthase (CTPS) can form filamentous structures named cytoophidia in numerous types of cells, including follicle cells. However, the regulation of cytoophidium assembly remains elusive. The apicobasal polarity, a defining characteristic of Drosophila follicle epithelium, is established and regulated by a variety of membrane domains. Here we show that CTPS can form cytoophidia in Drosophila epithelial follicle cells. Cytoophidia localise to the basolateral side of follicle cells. If apical polarity regulators are knocked down, cytoophidia become unstable and distribute abnormally. Knockdown of basolateral polarity regulators has no significant effect on cytoophidia, even though the polarity is disturbed. Our results indicate that cytoophidia are maintained via polarised distribution on the basolateral side of Drosophila follicle epithelia, which is primarily achieved through the apical polarity regulators.

Energy deficiency caused by CTPS downregulation in decidua may contribute to pre-eclampsia by impairing decidualization

Pre-eclampsia (PE) is a pregnancy-related disorder that occurs after 20 weeks of gestation. It seriously affects the health of maternity and the fetus. However, the pathogenesis of PE is still unknown. Decidualization deficiency is considered a contributing factor to the development of PE. CTP synthetase (CTPS) which is the rate-limiting enzyme in the CTP de novo biosynthesis, is essential for nucleic acid synthesis and cellular energy metabolism, and often appears as cytoophidium in many cell types. Here, we found that the expression of CTPS was significantly downregulated in decidual tissues of patients with severe PE compared with healthy pregnant women. During in vitro decidualization, changes in CTPS were accompanied by opposite fluctuation of the AMPK signaling pathway. Moreover, the downregulation of CTPS by glutamine analogs or CTPS small interfering RNA inhibited the decidualization process and the AMPK signaling pathway. Investigating the underlying mechanism of action by co-immunoprecipitation coupled with mass spectrometry showed that CTPS interacted with ATP synthase (ATPS) and maintained the content of ATP on Day 3 of decidualization. However, when combined with mitochondrial stress protein STRESS-70 instead of ATPS, the concentration of ATP on Day 6 of induction was reduced. Corresponding to this, CTPS was mainly distributes in the cytoplasm on Day 3 of induction, while it appeared both in the cytoplasm and the nucleus on Day 6 in decidualized cells, which was similar to that in cells before induction. In summary, we believe that CTPS plays an important role in decidualization by participating in energy metabolism. Abnormal expression of CTPS in decidualization would lead to abnormal decidualization and consequently result in the occurrence of PE.

CTP synthase polymerization in germline cells of the developing Drosophila egg supports egg production

Polymerization of metabolic enzymes into micron-scale assemblies is an emerging mechanism for regulating their activity. CTP synthase (CTPS) is an essential enzyme in the biosynthesis of the nucleotide CTP and undergoes regulated and reversible assembly into large filamentous structures in organisms from bacteria to humans. The purpose of these assemblies is unclear. A major challenge to addressing this question has been the inability to abolish assembly without eliminating CTPS protein. Here we demonstrate that a recently reported point mutant in CTPS, Histidine 355A (H355A), prevents CTPS filament assembly in vivo and dominantly inhibits the assembly of endogenous wild-type CTPS in the Drosophila ovary. Expressing this mutant in ovarian germline cells, we show that disruption of CTPS assembly in early stage egg chambers reduces egg production. This effect is exacerbated in flies fed the glutamine antagonist 6-diazo-5-oxo-L-norleucine, which inhibits de novo CTP synthesis. These findings introduce a general approach to blocking the assembly of polymerizing enzymes without eliminating their catalytic activity and demonstrate a role for CTPS assembly in supporting egg production, particularly under conditions of limited glutamine metabolism.This article has an associated First Person interview with the first author of the paper.

The atlas of cytoophidia in Drosophila larvae

In 2010, cytidine 5'-triphosphate synthase (CTPS) was reported to form the filamentous or serpentine structure in Drosophila, which we termed the cytoophidium. In the last decade, CTPS filaments/cytoophidia have been found in bacteria, budding yeast, human cells, mice, fission yeast, plants, and archaea, indicating that this mechanism is highly conserved in evolution. In addition to CTPS, other metabolic enzymes have been identified to have the characteristics of forming cytoophidia or similar advanced structures, demonstrating that this is a basic strategy of cells. Nevertheless, our understanding of the physiological function of the cytoophidium remains incomplete and elusive. Here, we took the larva of Drosophila melanogaster as a model to systematically describe the localization and distribution of cytoophidia in different tissues during larval development. We found that the distribution pattern of CTPS cytoophidia is dynamic and heterogenic in larval tissues. Our study provides a road map for further understanding of the function and regulatory mechanism of cytoophidia.

SNAP29 mediates the assembly of histidine-induced CTP synthase filaments in proximity to the cytokeratin network

Under metabolic stress, cellular components can assemble into distinct membraneless organelles for adaptation. One such example is cytidine 5'-triphosphate synthase (CTPS, for which there are CTPS1 and CTPS2 forms in mammals), which forms filamentous structures under glutamine deprivation. We have previously demonstrated that histidine (His)-mediated methylation regulates the formation of CTPS filaments to suppress enzymatic activity and preserve the CTPS protein under glutamine deprivation, which promotes cancer cell growth after stress alleviation. However, it remains unclear where and how these enigmatic structures are assembled. Using CTPS-APEX2-mediated in vivo proximity labeling, we found that synaptosome-associated protein 29 (SNAP29) regulates the spatiotemporal filament assembly of CTPS along the cytokeratin network in a keratin 8 (KRT8)-dependent manner. Knockdown of SNAP29 interfered with assembly and relaxed the filament-induced suppression of CTPS enzymatic activity. Furthermore, APEX2 proximity labeling of keratin 18 (KRT18) revealed a spatiotemporal association of SNAP29 with cytokeratin in response to stress. Super-resolution imaging suggests that during CTPS filament formation, SNAP29 interacts with CTPS along the cytokeratin network. This study links the cytokeratin network to the regulation of metabolism by compartmentalization of metabolic enzymes during nutrient deprivation.

The proline synthesis enzyme P5CS forms cytoophidia in Drosophila

Compartmentation of enzymes via filamentation has arisen as a mechanism for the regulation of metabolism. In 2010, three groups independently reported that CTP synthase (CTPS) can assemble into a filamentous structure termed the cytoophidium. In searching for CTPS-interacting proteins, here we perform a yeast two-hybrid screening of Drosophila proteins and identify a putative CTPS-interacting protein, △¹-pyrroline-5-carboxylate synthase (P5CS). Using the Drosophila follicle cell as the in vivo model, we confirm that P5CS forms cytoophidia, which are associated with CTPS cytoophidia. Overexpression of P5CS increases the length of CTPS cytoophidia. Conversely, filamentation of CTPS affects the morphology of P5CS cytoophidia. Finally, in vitro analyses confirm the filament-forming property of P5CS. Our work links CTPS with P5CS, two enzymes involved in the rate-limiting steps in pyrimidine and proline biosynthesis, respectively.

Histidine-Dependent Protein Methylation Is Required for Compartmentalization of CTP Synthase

CTP synthase (CTPS) forms compartmentalized filaments in response to substrate availability and environmental nutrient status. However, the physiological role of filaments and mechanisms for filament assembly are not well understood. Here, we provide evidence that CTPS forms filaments in response to histidine influx during glutamine starvation. Tetramer conformation-based filament formation restricts CTPS enzymatic activity during nutrient deprivation. CTPS protein levels remain stable in the presence of histidine during nutrient deprivation, followed by rapid cell growth after stress relief. We demonstrate that filament formation is controlled by methylation and that histidine promotes re-methylation of homocysteine by donating one-carbon intermediates to the cytosolic folate cycle. Furthermore, we find that starvation stress and glutamine deficiency activate the GCN2/ATF4/MTHFD2 axis, which coordinates CTPS filament formation. CTPS filament formation induced by histidine-mediated methylation may be a strategy used by cancer cells to maintain homeostasis and ensure a growth advantage in adverse environments.

MicroRNA regulation of CTP synthase and cytoophidium in Drosophila melanogaster

CTPsyn is a crucial metabolic enzyme which synthesizes CTP nucleotides. It has the extraordinary ability to compartmentalize into filaments termed cytoophidia. Though the structure is evolutionarily conserved across kingdoms, the mechanisms behind their formation remain unknown. MicroRNAs (miRNAs) are short single-stranded RNA capable of directing mRNA silencing and degradation. D. melanogaster has a high total gene count to miRNA gene number ratio, alluding to the possibility that CTPsyn too may come under their regulation. A thorough miRNA overexpression involving 123 miRNAs was conducted, followed by CTPsyn-specific staining upon cytoophidia-rich egg chambers. This revealed a small group of candidates which confer either a lengthening or truncating effect on cytoophidia, suggesting they may play a role in regulating CTPsyn. MiR-975 and miR-1014 are both cytoophidia-elongating, whereas miR-190 and miR-932 are cytoophidia-shortening. Though target prediction shows that miR-975 and miR-932 do indeed have binding sites on CTPsyn mRNA, in vitro assays instead revealed a low probability of this being true, instead indicating that the effects asserted by overexpressed miRNAs indirectly reach CTPsyn and its cytoophidia through the actions of middling elements. In silico target prediction and qPCR quantification indicated that, at least for miR-932 and miR-1014, these undetermined elements may be players in fat metabolism. This is the first study to thoroughly investigate miRNAs in connection to CTPsyn expression and activity in any species. The findings presented could serve as a basis for further queries into not only the fundamental aspects of the enzyme's regulation, but may uncover new facets of closely related pathways as well.

The TOR pathway modulates cytoophidium formation in Schizosaccharomyces pombe

CTP synthase (CTPS) has been demonstrated to form evolutionarily-conserved filamentous structures termed cytoophidia whose exact cellular functions remain unclear, but they may play a role in intracellular compartmentalization. We have previously shown that the mammalian target of rapamycin complex 1 (mTORC1)-S6K1 pathway mediates cytoophidium assembly in mammalian cells. Here, using the fission yeast Schizosaccharomyces pombe as a model of a unicellular eukaryote, we demonstrate that the target of rapamycin (TOR)-signaling pathway regulates cytoophidium formation (from the S. pombe CTPS ortholog Cts1) also in S. pombe Conducting a systematic analysis of all viable single TOR subunit-knockout mutants and of several major downstream effector proteins, we found that Cts1 cytoophidia are significantly shortened and often dissociate when TOR is defective. We also found that the activities of the downstream effector kinases of the TORC1 pathway, Sck1, Sck2, and Psk1 S6, as well as of the S6K/AGC kinase Gad8, the major downstream effector kinase of the TORC2 pathway, are necessary for proper cytoophidium filament formation. Interestingly, we observed that the Crf1 transcriptional corepressor for ribosomal genes is a strong effector of Cts1 filamentation. Our findings connect TOR signaling, a major pathway required for cell growth, with the compartmentalization of the essential nucleotide synthesis enzyme CTPS, and we uncover differences in the regulation of its filamentation among higher multicellular and unicellular eukaryotic systems.

Targeting snoRNAs as an emerging method of therapeutic development for cancer

The relevance of the dysregulation of snoRNAs in human cancer has been widely investigated and has challenged the view that snoRNAs merely function as house-keeping genes for the posttranscriptional modification of rRNAs. Accumulating evidence has shown the intimate connection between snoRNAs and proliferation, apoptosis, invasion and migration of tumor cells via manual intervention patterns of snoRNA expression. In this review, we focused on how snoRNAs are dysregulated and its regulation of the formation and development of cancer. We summarized the non-classical functions of snoRNAs in the context of their regulations of the signaling pathways involving PI3K-AKT and K-Ras and p53-dependant manner. Under these novel functions and characteristics, snoRNAs can act as potential and feasible biomarkers for diagnosis. Simultaneously, these promising therapeutic strategies should be considered to counteract the perturbations of snoRNAs.

Blood feeding activates the vitellogenic stage of oogenesis in the mosquito Aedes aegypti through inhibition of glycogen synthase kinase 3 by the insulin and TOR pathways

Most mosquitoes, including Aedes aegypti, only produce eggs after blood feeding on a vertebrate host. Oogenesis in A. aegypti consists of a pre-vitellogenic stage before blood feeding and a vitellogenic stage after blood feeding. Primary egg chambers remain developmentally arrested during the pre-vitellogenic stage but complete oogenesis to form mature eggs during the vitellogenic stage. In contrast, the signaling factors that maintain primary egg chambers in pre-vitellogenic arrest or that activate vitellogenic growth are largely unclear. Prior studies showed that A. aegypti females release insulin-like peptide 3 (ILP3) and ovary ecdysteroidogenic hormone (OEH) from brain neurosecretory cells after blood feeding. Here, we report that primary egg chambers exit pre-vitellogenic arrest by 8 h post-blood meal as evidenced by proliferation of follicle cells, endoreplication of nurse cells, and formation of cytoophidia. Ex vivo assays showed that ILP3 and OEH stimulate primary egg chambers to exit pre-vitellogenic arrest in the presence of nutrients but not in their absence. Characterization of associated pathways indicated that activation of insulin/insulin growth factor signaling (IIS) by ILP3 or OEH inactivated glycogen synthase kinase 3 (GSK3) via phosphorylation by phosphorylated Akt. GSK3 inactivation correlated with accumulation of the basic helix-loop-helix transcription factor Max and primary egg chambers exiting pre-vitellogenic arrest. Direct inhibition of GSK3 by CHIR-99021 also stimulated Myc/Max accumulation and primary egg chambers exiting pre-vitellogenic arrest. Collectively, our results identify GSK3 as a key factor in regulating the pre- and vitellogenic stages of oogenesis in A. aegypti.

Ectopic miR-975 induces CTP synthase directed cell proliferation and differentiation in Drosophila melanogaster

CTP synthase (CTPSyn) is an essential metabolic enzyme, synthesizing precursors required for nucleotides and phospholipids production. Previous studies have also shown that CTPSyn is elevated in various cancers. In many organisms, CTPSyn compartmentalizes into filaments called cytoophidia. In Drosophila melanogaster, only its isoform C (CTPSynIsoC) forms cytoophidia. In the fruit fly's testis, cytoophidia are normally seen in the transit amplification regions close to its apical tip, where the stem-cell niche is located, and development is at its most rapid. Here, we report that CTPSynIsoC overexpression causes the lengthening of cytoophidia throughout the entirety of the testicular body. A bulging apical tip is found in approximately 34% of males overexpressing CTPSynIsoC. Immunostaining shows that this bulged phenotype is most likely due to increased numbers of both germline cells and spermatocytes. Through a microRNA (miRNA) overexpression screen, we found that ectopic miR-975 concurrently increases both the expression levels of CTPSyn and the length of its cytoophidia. The bulging testes phenotype was also recovered at a penetration of approximately 20%. However, qPCR assays reveal that CTPSynIsoC and miR-975 overexpression each provokes a differential response in expression of a number of cancer-related genes, indicating that the shared CTPSyn upregulation seen in either case is likely the cause of observed testicular overgrowth. This study presents the first instance of consequences of miRNA-asserted regulation upon CTPSyn in D. melanogaster, and further reaffirms the enzyme's close ties to germline cells overgrowth.

mTOR-S6K1 pathway mediates cytoophidium assembly

CTP synthase (CTPS), the rate-limiting enzyme in de novo CTP biosynthesis, has been demonstrated to assemble into evolutionarily conserved filamentous structures, termed cytoophidia, in Drosophila, bacteria, yeast and mammalian cells. However, the regulation and function of the cytoophidium remain elusive. Here, we provide evidence that the mechanistic target of rapamycin (mTOR) pathway controls cytoophidium assembly in mammalian and Drosophila cells. In mammalian cells, we find that inhibition of mTOR pathway attenuates cytoophidium formation. Moreover, CTPS cytoophidium assembly appears to be dependent on the mTOR complex 1 (mTORC1) mainly. In addition, knockdown of the mTORC1 downstream target S6K1 can inhibit cytoophidium formation, while overexpression of the constitutively active S6K1 reverses mTOR knockdown-induced cytoophidium disassembly. Finally, reducing mTOR protein expression results in a decrease of the length of cytoophidium in Drosophila follicle cells. Therefore, our study connects CTPS cytoophidium formation with the mTOR signaling pathway.

"Pinching" the ammonia tunnel of CTP synthase unveils coordinated catalytic and allosteric-dependent control of ammonia passage

Molecular gates within enzymes often play important roles in synchronizing catalytic events. We explored the role of a gate in cytidine-5'-triphosphate synthase (CTPS) from Escherichia coli. This glutamine amidotransferase catalyzes the biosynthesis of CTP from UTP using either l-glutamine or exogenous NH₃ as a substrate. Glutamine is hydrolyzed in the glutaminase domain, with GTP acting as a positive allosteric effector, and the nascent NH₃ passes through a gate located at the end of a ~25-Å tunnel before entering the synthase domain where CTP is generated. Substitution of the gate residue Val 60 by Ala, Cys, Asp, Trp, or Phe using site-directed mutagenesis and subsequent kinetic analyses revealed that V60-substitution impacts glutaminase activity, nucleotide binding, salt-dependent inhibition, and inter-domain NH₃ transport. Surprisingly, the increase in steric bulk present in V60F perturbed the local structure consistent with "pinching" the tunnel, thereby revealing processes that synchronize the transfer of NH₃ from the glutaminase domain to the synthase domain. V60F had a slightly reduced coupling efficiency at maximal glutaminase activity that was ameliorated by slowing down the glutamine hydrolysis reaction, consistent with a "bottleneck" effect. The inability of V60F to use exogenous NH₃ was overcome in the presence of GTP, and more so if CTPS was covalently modified by 6-diazo-5-oxo-l-norleucine. Use of NH₂OH by V60F as an alternative bulkier substrate occurred most efficiently when it was concomitant with the glutaminase reaction. Thus, the glutaminase activity and GTP-dependent activation act in concert to open the NH₃ gate of CTPS to mediate inter-domain NH₃ transport.

CTP synthase forms the cytoophidium in human hepatocellular carcinoma

CTP synthase (CTPS) can aggregate into an intracellular macrostructure, the cytoophidium, in various organisms including human cells. Previous studies have shown that assembly of human CTPS cytoophidia may be correlated with the cellular metabolic status, and is able to promote the activity of CTPS. A correlation between the cytoophidium and cancer metabolism has been proposed but not yet been revealed. In the current study we provide clear evidence of the presence of CTPS cytoophidia in various human cancers and some non-cancerous tissues. Moreover, among 203 tissue samples of hepatocellular carcinoma, 56 (28%) samples exhibited many cytoophidia, whereas no cytoophidia were detected in adjacent non-cancerous hepatocytes for all samples. Our findings suggest that the CTPS cytoophidium may participate in the adaptive metabolism of human hepatocellular carcinoma.