Mutant cell lines resistant to azetidine-2-carboxylic acid: alterations in the synthesis of proline from glutamic acid

L-Proline Prevents Endoplasmic Reticulum Stress in Microglial Cells Exposed to L-azetidine-2-carboxylic Acid

L-Azetidine-2-carboxylic acid (AZE) is a non-protein amino acid that shares structural similarities with its proteogenic L-proline amino acid counterpart. For this reason, AZE can be misincorporated in place of L-proline, contributing to AZE toxicity. In previous work, we have shown that AZE induces both polarization and apoptosis in BV2 microglial cells. However, it is still unknown if these detrimental effects involve endoplasmic reticulum (ER) stress and whether L-proline co-administration prevents AZE-induced damage to microglia. Here, we investigated the gene expression of ER stress markers in BV2 microglial cells treated with AZE alone (1000 µM), or co-treated with L-proline (50 µM), for 6 or 24 h. AZE reduced cell viability, nitric oxide (NO) secretion and caused a robust activation of the unfolded protein response (UPR) genes (ATF4, ATF6, ERN1, PERK, XBP1, DDIT3, GADD34). These results were confirmed by immunofluorescence in BV2 and primary microglial cultures. AZE also altered the expression of microglial M1 phenotypic markers (increased IL-6, decreased CD206 and TREM2 expression). These effects were almost completely prevented upon L-proline co-administration. Finally, triple/quadrupole mass spectrometry demonstrated a robust increase in AZE-bound proteins after AZE treatment, which was reduced by 84% upon L-proline co-supplementation. This study identified ER stress as a pathogenic mechanism for AZE-induced microglial activation and death, which is reversed by co-administration of L-proline.

A bifunctional enzyme (delta 1-pyrroline-5-carboxylate synthetase) catalyzes the first two steps in proline biosynthesis in plants

Many plants synthesize and accumulate proline in response to osmotic stress. Despite the importance of this pathway, however, the exact metabolic route and enzymes involved in the synthesis of proline in plants have not been unequivocally identified. We report here the isolation of a mothbean (Vigna aconitifolia) cDNA clone encoding a bifunctional enzyme, delta 1-pyrroline-5-carboxylate synthetase (P5CS), with both gamma-glutamyl kinase and glutamic-gamma-semialdehyde dehydrogenase activities that catalyzes the first two steps in proline biosynthesis. The two enzymatic domains of P5CS correspond to the ProB and ProA proteins of Escherichia coli and contain a leucine zipper in each domain, which may facilitate inter- or intramolecular interaction of this protein. The Vigna P5CS enzyme activity is feedback regulated by proline but is less sensitive to end-product inhibition than is the E. coli gamma-glutamyl kinase. The P5CS gene is expressed at high levels in Vigna leaves and is inducible in roots subjected to salt stress, suggesting that P5CS plays a key role in proline biosynthesis, leading to osmoregulation in plants.

Synthesis of the proline analogue [2,3-3H]azetidine-2-carboxylic acid. Uptake and incorporation in Arabidopsis thaliana and Escherichia coli

Azetidine-2-carboxylic acid, the 4-membered ring noranalogue of proline, is regularly used in the study of proline metabolism as well as the study of protein conformation. We prepared D,L-[2,3-3H]azetidine-2-carboxylic acid with an optimized 10% yield from commercially available 4-amino-[2,3-3H]butyric acid. Purification was performed by fast-protein liquid chromatography. The biological activity was checked in both Arabidopsis thaliana and Escherichia coli. The obtained specific activity of 10 mCi/mmol was sufficient for most uptake and incorporation studies.

Characterization of a gamma-glutamyl kinase from Escherichia coli that confers proline overproduction and osmotic tolerance

Mutation(s) in the proBA operon of Escherichia coli confers proline overproduction and enhanced osmotic tolerance in enteric bacteria (L. N. Csonka, Mol. Gen. Genet. 182:82-86, 1981; M. J. Mahan and L. N. Csonka, J. Bacteriol. 156:1249-1262, 1983). A glutamate-dependent ATPase assay was developed and used to determine proB-encoded gamma-glutamyl kinase activity in the absence of glutamate-gamma-semialdehyde dehydrogenase. This assay indicated that the feedback insensitivity of mutant gamma-glutamyl kinase was independent of glutamate-gamma-semialdehyde dehydrogenase. However, the capacity of glutamate-gamma-semialdehyde dehydrogenase from the osmotolerant mutant to interact with the kinase was altered in thermal stability, suggesting that mutations in both proB and proA may be required for osmotolerance.