Biology subjects

Eskandari, R.

Publications and source records attributed to Eskandari, R..

3 recordsLinked to original sources

bioRxiv2026

The GNMT N-terminus Couples Folate Feedback to Methyl-donor Homeostasis

Maintenance of S-adenosylmethionine (SAM) homeostasis is essential for methylation of biomolecules, nucleotide and polyamine synthesis, and redox homeostasis. While all methyltransferases consume SAM, only a subset of highly tissue specific methyltransferases regulates SAM homeostasis. Among them, glycine N-methyltransferase (GNMT) is enriched in the liver and its dysregulated activity has been linked to compromised liver function. GNMT is inhibited by the methyl carrier 5-methyltetrahydrofolate (5mTHF), suggesting a negative-feedback mechanism regulating its activity. Here, we identify the GNMT N-terminal tail, and specifically phosphorylation at serine 9 (S9ph), as a regulatory modification linking folate-dependent feedback inhibition to SAM homeostasis. Structural and biochemical analyses and molecular dynamics simulations revealed that the N-terminal tail is required for catalytic turnover of SAM and for 5mTHF binding. Phosphoproteomic analysis showed that GNMT S9ph is abundant in mouse liver and further enriched in aged mice. Consistent with loss of folate-dependent negative feedback, both distal N-terminal truncation (residues 1-8) and a phosphomimetic substitution abolished 5mTHF binding while maintaining catalytic activity. In hepatocyte cell lines lacking endogenous GNMT, lentiviral overexpression of constitutively active GNMT mutants depleted SAM, increased SAH, disrupted protein methylation, impaired growth, and induced transcriptional responses consistent with methyl-donor stress. Together, these findings identify the GNMT N-terminus as a tunable phosphoregulatory domain that dynamically regulates GNMT activity and cellular methylation potential.

biochemistry↗

bioRxiv2025

EGO-like complex regulates TOR (Target of Rapamycin) activity and localization in Neurospora

The TOR (Target of Rapamycin) signalling pathway is found in all eukaryotes and integrates nutrient and stress signals to control cell growth. It is well-studied in yeast and mammals but is less well understood in filamentous fungi. We previously identified TOR pathway components VTA (homologous to the vacuole-bound EGO complex of yeast) and GTR2 (homologous to Rag GTPases) as essential to maintaining circadian rhythms in the filamentous fungus Neurospora crassa. Therefore we are interested in defining the TOR pathway and its regulation in N. crassa. In yeast and mammals, TOR kinase is activated by carbon sources and amino acids. We report here that on high glucose medium, TOR is insensitive to added amino acids. On low glucose, TOR is activated by added glucose and amino acids. VTA and GTR2 knockouts block the activation of TOR by amino acids but not by glucose, identifying their function in an amino acid-sensing pathway. Live cell microscopy of KOG1 (a component of TOR complex 1) and GTR2 localizes them to punctate bodies near the vacuole. This localization is lost and the proteins are largely cytoplasmic under starvation conditions, in the presence of TOR inhibitor Torin II, and in the VTA knockout. This indicates that VTA acts as the vacuolar anchor for activated TOR complex. Co-immunoprecipitation of KOG1-FLAG and GTR2-FLAG confirms their cytoplasmic localization in VTA knockout and identifies TORC1 complex components TCO89 and LST8. These results focus attention on amino acid sensing through VTA and GTR2 as potentially regulating circadian rhythmicity in N. crassa.

cell biology↗

bioRxiv2020

Limited Environmental Serine Confers Sensitivity to PHGDH Inhibition in Brain Metastasis

A hallmark of metastasis is the adaptation of tumor cells to new environments. Although it is well established that the metabolic milieu of the brain is severely deprived of nutrients, particularly the amino acids serine and its catabolite glycine, how brain metastases rewire their metabolism to survive in the nutrient-limited environment of the brain is poorly understood. Here we demonstrate that cell-intrinsic de novo serine synthesis is a major determinant of brain metastasis. Whole proteome comparison of triple-negative breast cancer (TNBC) cells that differ in their capacity to colonize the brain reveals that 3-phosphoglycerate dehydrogenase (PHGDH), which catalyzes the rate-limiting step of glucose-derived serine synthesis, is the most significantly upregulated protein in cells that efficiently metastasize to the brain. Genetic silencing or pharmacological inhibition of PHGDH attenuated brain metastasis and improved overall survival in mice, whereas expression of catalytically active PHGDH in a non-brain trophic cell line promoted brain metastasis. Collectively, these findings indicate that nutrient availability determines serine synthesis pathway dependence in brain metastasis, and suggest that PHGDH inhibitors may be useful in the treatment of patients with cancers that have spread to the brain. Statement of SignificanceOur study highlights how limited serine and glycine availability within the brain microenvironment potentiates tumor cell sensitivity to serine synthesis inhibition. This finding underscores the importance of studying cancer metabolism in physiologically-relevant contexts, and provides a rationale for using PHGDH inhibitors to treat brain metastasis.

cancer biology↗