Stable Isotope Dilution Assays for Clinical Analyses of Folates and Other One-Carbon Metabolites: Application to Folate-Deficiency Studies

MR1-dependent immune surveillance of the skin contributes to pathogenesis and is a photobiological target of UV light therapy in a mouse model of atopic dermatitis

BACKGROUND

Mucosal-associated invariant T (MAIT) cells are unconventional T cells which recognize microbial metabolites presented by the major histocompatibility complex class I-related molecule MR1. Although MAIT cells have been shown to reside in human and murine skin, their contribution to atopic dermatitis (AD), an inflammatory skin disease associated with barrier dysfunction and microbial translocation, has not yet been determined.

METHODS

Genetic deletion of MR1 and topical treatment with inhibitory MR1 ligands, which result in the absence and functional inhibition of MAIT cells, respectively, were used to investigate the role of MR1-dependent immune surveillance in a MC903-driven murine model of AD.

RESULTS

The absence or inhibition of MR1 arrested AD disease progression through the blockade of both eosinophil activation and recruitment of IL-4- and IL-13-producing cells. In addition, the therapeutic efficacy of phototherapy against MC903-driven AD could be increased with prior application of folate, which photodegrades into the inhibitory MR1 ligand 6-formylpterin.

CONCLUSION

We identified MAIT cells as sentinels and mediators of cutaneous type 2 immunity. Their pathogenic activity can be inhibited by topical application or endogenous generation, via phototherapy, of inhibitory MR1 ligands.

Simultaneous quantification of intracellular concentrations of clinically important metabolites of folate-homocysteine cycle by LC-MS/MS

Inadequate folate status is detrimental to human development. Deficiency has been implicated in congenital birth defects and cancer, whereas excess has been linked to various negative neurocognitive development outcomes. We developed a method for translational studies involving lymphoblastoid cell models for studying role of folates in vital cell processes. We describe a simple, sensitive, and fast liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the simultaneous quantification of intracellular concentrations of clinically important metabolites of folate-homocysteine cycle; namely, folic acid (FA), 5-methyltetrahydrofolate (5-Me-THF), and homocysteine (Hcy). The method was validated for specificity, linearity, limits of quantification, repeatability, reproducibility, matrix effects, and stability. Method had a wide linear range between 0.341 and 71.053 ng Hcy/mg protein for Hcy, 0.004-0.526 ng FA/mg protein for FA and 0.003-0.526 ng 5-Me-THF/mg protein for 5-Me-THF. The method overcomes challenges associated with the quantification of endogenous molecules, poor stability, and extremely small amounts of the analytes. The method was successfully applied to evaluate the effects of FA and 5-Me-THF treatment of cells in vitro mimicking supplement therapy with various metabolically active species, and showed that 5-Me-THF is more effective than FA in increasing intracellular levels of the biologically active form of folate.

Quantitative analysis of glycine related metabolic pathways for one-carbon synthetic biology

Glycine is an essential one-carbon (C1) metabolite nested in a complex network of cellular metabolism. Glycine and its related metabolic pathways have important biochemical and biomedical implications and have thus been studied for a long time. However, quantitative and systems level knowledge about the interactions and regulations of the pathways are severely limited, especially for the purpose of reengineering the relevant pathways for C1-based biotechnological processes using synthetic biology and metabolic engineering approaches. In fact, quantitative analytic methods are missing for some of the key players of the glycine-related pathways, prominently the glycine cleavage system and folate cycle, particularly for intracellular processes under physiological conditions. Here, we pinpoint the existing gaps and highlight the need and challenges for future development.

Chemoenzymatic Assembly of Isotopically Labeled Folates

Pterin-containing natural products have diverse functions in life, but an efficient and easy scheme for their in vitro synthesis is not available. Here we report a chemoenzymatic 14-step, one-pot synthesis that can be used to generate ¹³C- and ¹⁵N-labeled dihydrofolates (H₂F) from glucose, guanine, and p-aminobenzoyl-l-glutamic acid. This synthesis stands out from previous approaches to produce H₂F in that the average yield of each step is >91% and it requires only a single purification step. The use of a one-pot reaction allowed us to overcome potential problems with individual steps during the synthesis. The availability of labeled dihydrofolates allowed the measurement of heavy-atom isotope effects for the reaction catalyzed by the drug target dihydrofolate reductase and established that protonation at N5 of H₂F and hydride transfer to C6 occur in a stepwise mechanism. This chemoenzymatic pterin synthesis can be applied to the efficient production of other folates and a range of other natural compounds with applications in nutritional, medical, and cell-biological research.

Characterization and Interrelations of One-Carbon Metabolites in Tissues, Erythrocytes, and Plasma in Mice with Dietary Induced Folate Deficiency

Studies on one-carbon metabolism for the assessment of folate deficiency have focused on either metabolites of folate metabolism or methionine cycle. To bridge the gap between deficiency markers in these pathways we designed a dietary induced folate deficiency study using male C57BL/6N mice. After weaning (3 weeks) mice were fed a defined control diet (1 week) before being fed a folate deficient diet (n = 6 mice) and the control diet (n = 6 mice) for 12 additional weeks. Thereafter, we determined total homocysteine in plasma and folate in erythrocytes as well as S-adenosylmethionine, S-adenosylhomocysteine, and six folate vitamers in tissues including 5-methyltetrahydrofolate, 5-formyltetrahydrofolate, 5,10-methenyltetrahydrofolate, tetrahydrofolate, 10-formylfolic acid, and folic acid by means of stable isotope dilution assays coupled with liquid chromatography tandem mass spectrometry. In all organs, except heart (mainly 5-mehtyltetrahydrofolate), tetrahydrofolate constitutes the main vitamer. Moreover, in liver tetrahydrofolate was most abundant followed by 5-methyltetrahydrofolate (heart: tetrahydrofolate), 5-formyltetrahydrofolate, and 5,10-methenyltetrahydrofolate. Because of the significant decrease (p < 0.05) of folate status and S-adenosylmethionine/S-adenosylhomocysteine ratio accompanied with increasing S-adenosylhomocysteine (p < 0.05), hepatocytes are most susceptible to folate deficiency. To the best of our knowledge, we herein present the first method for simultaneous quantitation of eight metabolites for both folate and methionine cycle in one tissue sample, tHcy in plasma, and erythrocyte folate to shed light on physiological interrelations of one-carbon metabolism.

Nutrients in Energy and One-Carbon Metabolism: Learning from Metformin Users

Metabolic vulnerability is associated with age-related diseases and concomitant co-morbidities, which include obesity, diabetes, atherosclerosis and cancer. Most of the health problems we face today come from excessive intake of nutrients and drugs mimicking dietary effects and dietary restriction are the most successful manipulations targeting age-related pathways. Phenotypic heterogeneity and individual response to metabolic stressors are closely related food intake. Understanding the complexity of the relationship between dietary provision and metabolic consequences in the long term might provide clinical strategies to improve healthspan. New aspects of metformin activity provide a link to many of the overlapping factors, especially the way in which organismal bioenergetics remodel one-carbon metabolism. Metformin not only inhibits mitochondrial complex 1, modulating the metabolic response to nutrient intake, but also alters one-carbon metabolic pathways. Here, we discuss findings on the mechanism(s) of action of metformin with the potential for therapeutic interpretations.