Coordinated Formation of IMPDH2 Cytoophidium in Mouse Oocytes and Granulosa Cells

The IMPDH cytoophidium couples metabolism and fetal development in mice

The cytoophidium is an evolutionarily conserved subcellular structure formed by filamentous polymers of metabolic enzymes. In vertebrates, inosine monophosphate dehydrogenase (IMPDH), which catalyses the rate-limiting step in guanosine triphosphate (GTP) biosynthesis, is one of the best-known cytoophidium-forming enzymes. Formation of the cytoophidium has been proposed to alleviate the inhibition of IMPDH, thereby facilitating GTP production to support the rapid proliferation of certain cell types such as lymphocytes, cancer cells and pluripotent stem cells (PSCs). However, past studies lacked appropriate models to elucidate the significance of IMPDH cytoophidium under normal physiological conditions. In this study, we demonstrate that the presence of IMPDH cytoophidium in mouse PSCs correlates with their metabolic status rather than pluripotency. By introducing IMPDH2 Y12C point mutation through genome editing, we established mouse embryonic stem cell (ESC) lines incapable of forming IMPDH polymers and the cytoophidium. Our data indicate an important role of IMPDH cytoophidium in sustaining a positive feedback loop that couples nucleotide biosynthesis with upstream metabolic pathways. Additionally, we find that IMPDH2 Y12C mutation leads to decreased cell proliferation and increased DNA damage in teratomas, as well as impaired embryo development following blastocoel injection. Further analysis shows that IMPDH cytoophidium assembly in mouse embryonic development begins after implantation and gradually increases throughout fetal development. These findings provide insights into the regulation of IMPDH polymerisation in embryogenesis and its significance in coordinating cell metabolism and development.

The molecular regulatory mechanisms of meiotic arrest and resumption in Oocyte development and maturation

In human female primordial germ cells, the transition from mitosis to meiosis begins from the fetal stage. In germ cells, meiosis is arrested at the diplotene stage of prophase in meiosis I (MI) after synapsis and recombination of homologous chromosomes, which cannot be segregated. Within the follicle, the maintenance of oocyte meiotic arrest is primarily attributed to high cytoplasmic concentrations of cyclic adenosine monophosphate (cAMP). Depending on the specific species, oocytes can remain arrested for extended periods of time, ranging from months to even years. During estrus phase in animals or the menstrual cycle in humans, the resumption of meiosis occurs in certain oocytes due to a surge of luteinizing hormone (LH) levels. Any factor interfering with this process may lead to impaired oocyte maturation, which in turn affects female reproductive function. Nevertheless, the precise molecular mechanisms underlying this phenomenon has not been systematically summarized yet. To provide a comprehensive understanding of the recently uncovered regulatory network involved in oocyte development and maturation, the progress of the cellular and molecular mechanisms of oocyte nuclear maturation including meiosis arrest and meiosis resumption is summarized. Additionally, the advancements in understanding the molecular cytoplasmic events occurring in oocytes, such as maternal mRNA degradation, posttranslational regulation, and organelle distribution associated with the quality of oocyte maturation, are reviewed. Therefore, understanding the pathways regulating oocyte meiotic arrest and resumption will provide detailed insight into female reproductive system and provide a theoretical basis for further research and potential approaches for novel disease treatments.

HIF-1α mediates osteoclast-induced disuse osteoporosis via cytoophidia in the femur of mice

Osteoporosis induced by disuse because of bed rest or the aerospace industry has become one of the most common skeletal disorders. However, mechanisms underlying the disuse osteoporosis remain largely unknown. We validated the tail-suspended model in mice and demonstrated that there is bone loss in the trabecular and cortical bones of the femur. Importantly, we showed that genetical deletion of hypoxia-inducible factor-1α (HIF-1α) in osteoclasts ameliorated osteoclastic bone resorption in the trabecular bone whereas pharmacological treatment with HIF-1α inhibitor protected the hindlimb-unloaded mice from disuse-induced osteoporosis in the trabecular and cortical bones. The HIF-1α knockout RAW264.7 cells and RNA-sequencing proved that HIF-1α is vital for osteoclastogenesis and bone resorption because it regulated the level of inosine monophosphate dehydrogenase (IMPDH) and cytidine triphosphate synthetase (CTPS) via cellular myelocytomatosis (c-Myc) oncogene. The IMPDH and CTPS are vital nucleotide metabolic enzymes which have an important functional role in cell metabolism, and they can assemble into intracellular linear or ring-shaped structures to cope with cell stress. Interestingly, both in vitro and in vivo, the IMPDH and CTPS cytoophidia were found in osteoclasts, and the level of HIF-1α correlated with osteoclastogenesis and bone-resorbing activity. Our data revealed that HIF-1α/c-Myc/cytoophidia signalling might be required for osteoclasts to mediate cell metabolism in disuse-induced osteoporosis. Overall, our results revealed a new role of HIF-1α/c-Myc/cytoophidia in supporting osteoclastogenesis and bone resorption and exposed evidence for its role in the pathogenesis of disuse osteoporosis, which might provide promising therapeutic targets.

Intercellular communication in the cumulus-oocyte complex during folliculogenesis: A review

During folliculogenesis, the oocyte and surrounding cumulus cells form an ensemble called the cumulus-oocyte complex (COC). Due to their interdependence, research on the COC has been a hot issue in the past few decades. A growing body of literature has revealed that intercellular communication is critical in determining oocyte quality and ovulation. This review provides an update on the current knowledge of COC intercellular communication, morphology, and functions. Transzonal projections (TZPs) and gap junctions are the most described structures of the COC. They provide basic metabolic and nutrient support, and abundant molecules for signaling pathways and regulations. Oocyte-secreted factors (OSFs) such as growth differentiation factor 9 and bone morphogenetic protein 15 have been linked with follicular homeostasis, suggesting that the communications are bidirectional. Using advanced techniques, new evidence has highlighted the existence of other structures that participate in intercellular communication. Extracellular vesicles can carry transcripts and signaling molecules. Microvilli on the oocyte can induce the formation of TZPs and secrete OSFs. Cell membrane fusion between the oocyte and cumulus cells can lead to sharing of cytoplasm, in a way making the COC a true whole. These findings give us new insights into related reproductive diseases like polycystic ovary syndrome and primary ovarian insufficiency and how to improve the outcomes of assisted reproduction.

Biochemical alterations in the follicular fluid of bovine peri-ovulatory follicles and association with final oocyte maturation

Follicular fluid (FF), a product of vascular transudate and granulosa and thecal cell secretions, is the milieu that has evolved to support oocyte growth and maturation which plays a central role in oocyte quality determination. Therefore, a suboptimal FF composition may be reflected in compromised oocyte progression through maturation, fertilization or embryo development. To date, the composition of bovine FF remains understudied. To address this, we comprehensively characterized the metabolomic constituency of bovine FF in the period during which the oocyte undergoes meiotic maturation. More specifically, FF from pre (-24 h) and peri (-2 h) -ovulatory follicles was profiled by high-throughput untargeted ultra-high-performance liquid chromatography tandem mass spectroscopy. A total of 634 metabolites were identified, comprising: lipids (37.1%), amino acids (30.0%), xenobiotics (11.5%), nucleotides (6.8%), carbohydrates (4.4%), cofactors and vitamins (4.4%), peptides (3.6%) and energy substrates (2.1%). The concentrations of 67 metabolites were significantly affected by stage of follicle development, 33.3% (n=21) were reduced (P≤0.05) by a mean of 9.0-fold, whereas 46 were elevated (P≤0.05) by a mean of 1.7-fold in peri vs. pre -ovulatory FF. The most pronounced individual metabolite concentration decreases were hypoxanthine (98.9-fold), xanthine (65.7-fold), 17β-oestradiol (12.4-fold), and inosine (4.6-fold). In contrast, the greatest increases were in retinal (4.9-fold), 1-methyl-5-imidazoleacetate (2.7-fold), and isovalerylcarnitine (2.7-fold). This global metabolomic analysis of bovine FF temporal dynamics provides new information for understanding the environment supporting oocyte maturation and facilitating ovulation, that has the potential for improving oocyte quality both in vivo and in vitro.

Asymmetric inheritance of cytoophidia could contribute to determine cell fate and plasticity: The onset of alternative differentiation patterns in daughter cells may rely on the acquisition of either CTPS or IMPDH cytoophidia: The onset of alternative differentiation patterns in daughter cells may rely on the acquisition of either CTPS or IMPDH cytoophidia

Two enzymes involved in the synthesis of pyrimidine and purine nucleotides, CTP synthase (CTPS) and IMP dehydrogenase (IMPDH), can assemble into a single or very few large filaments called rods and rings (RR) or cytoophidia. Most recently, asymmetric cytoplasmic distribution of organelles during cell division has been described as a decisive event in hematopoietic stem cell fate. We propose that cytoophidia, which could be considered as membrane-less organelles, may also be distributed asymmetrically during mammalian cell division as previously described for Schizosaccharomyces pombe. Furthermore, because each type of nucleotide intervenes in distinct processes (e.g., membrane synthesis, glycosylation, and G protein-signaling), alterations in the rate of synthesis of specific nucleotide types could influence cell differentiation in multiple ways. Therefore, we hypothesize that whether a daughter cell inherits or not CTPS or IMPDH filaments determines its fate and that this asymmetric inheritance, together with the dynamic nature of these structures enables plasticity in a cell population.

Metabolic control of oocyte development

Well balanced and timed metabolism is essential for oocyte development. The effects of extrinsic nutrients on oocyte maturation have been widely reported. In contrast, intrinsic control of oogenesis by intracellular metabolites and metabolic enzymes has received little attention. The comprehensive characterization of metabolic patterns could lead to more complete understanding of regulatory mechanisms underlying oocyte development. A cell's metabolic state is integrated with epigenetic regulation. Epigenetic modifications in germ cells are therefore sensitive to parental environmental exposures. Nevertheless, direct genetic evidence for metabolites involvement in epigenetic establishment during oocyte development is still lacking. Moreover, metabolic disorder-induced epigenetic perturbations during oogenesis might mediate the inter/transgenerational effects of environmental insults. The molecular mechanisms responsible for this deserve further investigation. Here, we summarize the findings on metabolic regulation in oocyte maturation, and how it contributes to oocyte epigenetic modification. Finally, we propose a mouse model that metabolic disorder in oocyte serves as a potential factor mediating the maternal environment effects on offspring health.