Biology subjects

Seim, G. L.

Publications and source records attributed to Seim, G. L..

3 recordsLinked to original sources

bioRxiv2026

Discovery of metabolites produced by reactions between central carbon metabolites and cysteine that mark inflammatory macrophages

Identifying metabolites and metabolic reactions specific to a cellular state, such as inflammatory state in immune cells, is of great interest, as it can provide important biomarkers and point to compounds and reactions of specific biological functions. However, many cell state-specific metabolites remain in the unannotated part of metabolome. Here we identified a series of sulfur-containing metabolites that are actively produced in macrophages upon classical activation, but not in resting state or alternative activation state. Isotopic tracing, in vitro assays and genetic perturbations further revealed that they are formed from reactions between free cysteine and several important intermediates in glycolysis and TCA cycle. Upon classical activation, macrophages specifically upregulate the import of cystine via Slc7a11, supporting the production of these adducts. Their production dynamically responds to changes in central metabolism, environmental nutrient levels, and is regulated by nitric oxide. Finally, we confirmed these newly identified compounds also present in human samples, and most of them are significantly elevated in inflammatory granuloma annulare lesions. This work elucidated a previously uncharted part of metabolic network that is associated with inflammation and metabolic stress condition, which has important implications and set foundation for many future discoveries.

biochemistry↗

bioRxiv2023

Macrophages undergo functionally significant reprograming of nucleotide metabolism upon classical activation

During an immune response, macrophages specifically rewire their metabolism to support functional changes. Using a multi-omics approach, we identified nucleotide metabolism as one of the most significantly rewired pathways across the metabolic network in classically activated macrophages. Further isotopic tracing studies revealed the substantial changes in nucleotide de novo synthesis, degradation, and salvage fluxes in stimulated macrophages, as well as the key reactions where metabolic regulation occurs: 1) de novo synthesis of purine nucleotides is shut down and particularly blocked at the last step of IMP synthesis catalyzed by ATIC; 2) de novo synthesis of pyrimidines is maintained up to UMP, but further synthesis of CTP (catalyzed by CTPS) and dTMP (catalyzed by TYMS) is greatly reduced; 3) Nucleotide degradation to nitrogenous bases is increased, but further oxidation of purine bases (catalyzed by XOR) is inhibited, causing a great accumulation of nucleosides and bases; and 4) cells switch to salvaging the nucleosides and bases as the primary means to maintain purine nucleotides. Mechanistically, we found these changes are driven by a combination of transcriptional regulation and enzyme inhibition. Nitric oxide (NO) was identified as a major regulator, driving the strong inhibition of ATIC and XOR, and the transcriptional downregulation of Tyms. To understand the functional impact of the activation-induced switch from purine de novo synthesis to salvage, we knocked out the purine salvage enzyme Hprt. Hprt knockout significantly alters functional gene expression in activated macrophages, suppresses macrophage migration, and increases pyroptosis. Furthermore, knocking out Hprt or Xor increases the proliferation of the intracellular parasite Toxoplasma gondii in macrophages. Together, these results comprehensively uncovered the dynamic rewiring of nucleotide metabolism in classically activated macrophages, elucidated the key regulatory mechanisms, and identified the functional significance of such rewiring.

cell biology↗

bioRxiv2022

Modifications of lipoic arm by reactive nitrogen species regulate α-ketoacid dehydrogenases

Mitochondrial -ketoacid dehydrogenases, including the pyruvate dehydrogenase complex (PDHC) and the oxoglutarate dehydrogenase complex (OGDC), are a family of multi-subunit enzyme complexes that use a lipoic arm to transfer an acyl group to coenzyme A (CoA). The regulation of -ketoacid dehydrogenases plays crucial roles in mitochondrial metabolism and cellular energy homeostasis. We previously found that PDHC and OGDC become profoundly inhibited in macrophages upon classical activation, causing substantial remodeling of the TCA cycle. This inhibition was driven by the loss of the catalytically active lipoic moiety; however, the molecular mechanism causing this loss was not clear. Here we show that reactive nitrogen species (RNS), which are produced by activated macrophages, can cause a series of thiol-modifications to the lipoic arm that inactivate PDHC and OGDC. CoA-SNO, the non-enzymatic product between RNS and the E2 subunits natural substrate CoA, plays a key role in efficiently delivering RNS mediated modifications onto the lipoic arm. This work reveals a new biochemical mechanism capable of substantially regulating mitochondrial -ketoacid dehydrogenases, which has potential relevance for a range of physiological and pathological conditions.

biochemistry↗