IMP dehydrogenase rod/ring structures in acral melanomas

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.

Light-sensitive phosphorylation regulates retinal IMPDH1 activity and filament assembly

Inosine monophosphate dehydrogenase (IMPDH) is the rate-limiting enzyme in guanosine triphosphate (GTP) synthesis and assembles into filaments in cells, which desensitizes the enzyme to feedback inhibition and boosts nucleotide production. The vertebrate retina expresses two splice variants IMPDH1(546) and IMPDH1(595). In bovine retinas, residue S477 is preferentially phosphorylated in the dark, but the effects on IMPDH1 activity and regulation are unclear. Here, we generated phosphomimetic mutants to investigate structural and functional consequences of S477 phosphorylation. The S477D mutation resensitized both variants to GTP inhibition but only blocked assembly of IMPDH1(595) filaments. Cryo-EM structures of both variants showed that S477D specifically blocks assembly of a high-activity assembly interface, still allowing assembly of low-activity IMPDH1(546) filaments. Finally, we discovered that S477D exerts a dominant-negative effect in cells, preventing endogenous IMPDH filament assembly. By modulating the structure and higher-order assembly of IMPDH, S477 phosphorylation acts as a mechanism for downregulating retinal GTP synthesis in the dark when nucleotide turnover is decreased.

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.

The clinical value of indirect immunofluorescence for screening anti-rods and rings antibodies: A retrospective study of two centers in China

Objective

To investigate the distribution and clinical significance of the rods and rings (RR) pattern in various diseases.

Methods

A total of 169,891 patients in Peking Union Medical College Hospital (PUMCH) and 29,458 patients in Inner Mongolia People’s Hospital (IMPH) from January 2018 to December 2020 were included, and the results of ANA (antinuclear antibodies) and special antibodies were analyzed retrospectively.

Results

The positive rates of ANA and RR patterns were 34.84%, 0.16% in PUMCH, and 44.73%, 0.23% in IMPH. Anti-RR antibodies mainly appear in adults (≥ 41 years), mostly of low or medium fluorescence titers. Isolated RR patterns were mostly presented (60.30% and 69.12%, respectively), and the RR pattern mixed with the speckled pattern was most commonly observed among patients having two or more patterns. The RR pattern existed in a variety of diseases including hepatitis C, AIDs, pulmonary diseases, nephropathy diseases, and even healthy people. The highest prevalence of the RR pattern was observed in hepatic diseases, such as hepatic dysfunction (0.79%), hepatic cirrhosis (1.05%), PBC (0.85%), and AIH (0.65%), etc. The positive rate of specific antibodies in RR pattern cases was 31.25%, and anti-Ro52 (27, 20.61%) was the most common target antibody.

Conclusion

The RR pattern had a low prevalence in ANAs test samples and varied in different nationalities and regions. Except for hepatitis C, it could be observed in AIDs, pulmonary diseases, nephropathy, other hepatic diseases, and even healthy people, but the positive rate was slightly higher in hepatic diseases. Its mechanism of action and clinical relevance still need clarification.

Molecular crowding facilitates bundling of IMPDH polymers and cytoophidium formation

The cytoophidium is a unique type of membraneless compartment comprising of filamentous protein polymers. Inosine monophosphate dehydrogenase (IMPDH) catalyzes the rate-limiting step of de novo GTP biosynthesis and plays critical roles in active cell metabolism. However, the molecular regulation of cytoophidium formation is poorly understood. Here we show that human IMPDH2 polymers bundle up to form cytoophidium-like aggregates in vitro when macromolecular crowders are present. The self-association of IMPDH polymers is suggested to rely on electrostatic interactions. In cells, the increase of molecular crowding with hyperosmotic medium induces cytoophidia, while the decrease of that by the inhibition of RNA synthesis perturbs cytoophidium assembly. In addition to IMPDH, CTPS and PRPS cytoophidium could be also induced by hyperosmolality, suggesting a universal phenomenon of cytoophidium-forming proteins. Finally, our results indicate that the cytoophidium can prolong the half-life of IMPDH, which is proposed to be one of conserved functions of this subcellular compartment.

IMPDH forms the cytoophidium in zebrafish

Inosine monophosphate dehydrogenase (IMPDH) catalyzes the rate-limiting step in de novo guanine nucleotide biosynthesis. Its activity is negatively regulated by the binding of GTP. IMPDH can form a membraneless subcellular structure termed the cytoophidium in response to certain changes in the metabolic status of the cell. The polymeric form of IMPDH, which is the subunit of the cytoophidium, has been shown to be more resistant to the inhibition by GTP at physiological concentrations, implying a functional correlation between cytoophidium formation and the upregulation of GTP biosynthesis. Herein we demonstrate that zebrafish IMPDH1b and IMPDH2 isoforms can assemble abundant cytoophidium in most of cultured cells under stimuli, while zebrafish IMPDH1a shows distinctive properties of forming the cytoophidium in different cell types. Point mutations that disrupt cytoophidium structure in mammalian models also prevent the aggregation of zebrafish IMPDHs. In addition, we discover the presence of the IMPDH cytoophidium in various tissues of larval and adult fish under normal growth conditions. Our results reveal that polymerization and cytoophidium assembly of IMPDH can be a regulatory machinery conserved among vertebrates, and with specific physiological purposes.

The value of anti-rods and rings antibodies in patients with nonhepatitis virus infection: A single-center retrospective study from Southwest China

The aim of this study was to retrospectively investigate the clinical significance of anti-rods and rings (anti-RR) antibodies in nonhepatitis virus infection patients from Southwest China.Anti-RR antibodies were determined by indirect immunofluorescence assay in a group of 19,935 individuals with antinuclear antibodies test from January 2017 to December 2019. The laboratory and clinical data were collected. Finally, 66 samples with anti-RR antibodies (0.33%) were detected.In Wilcoxon rank sum test, gamma glutamyl transferase (Z = -3.364, P = .001), alpha-l-fucosidase (AFU) (Z = -2.312, P = .021), uric acid (Z = -1.634, P = .047) and red blood cell distribution width (Z = -2.285, P = .022) were higher in metabolic disease group than nonmetabolic disease group. In independent-samples t test, endogenous creatinine clearance was higher in metabolic disease group than nonmetabolic disease group (t = 2.061, P = .045). During the follow-up period of 37 patients with anti-RR antibodies for 1 to 60 months, the titers of anti-RR were significantly increased in the metabolic disease group (Z = -2.346, P = .019). In binary logistic regression analysis, triglycerides (odds ratio 3.679, 95% confidence interval 1.467-24.779, P = .048) was associated with elevated titers of anti-RR antibodies.In summary, anti-RR in non-hepatitis patients may be a manifestation of metabolic disorders, and has a certain correlation with routine laboratory indicators, which is worthy of the attention from clinicians.