The proton-coupled folate transporter (PCFT) is the mechanism by which folates are absorbed across the apical brush-border membrane of the small intestine. The transporter is also expressed in the choroid plexus and is required for transport of folates into the cerebrospinal fluid. Loss of PCFT function, as occurs in the autosomal recessive disorder "hereditary folate malabsorption" (HFM), results in a syndrome characterized by severe systemic and cerebral folate deficiency. Folate-receptor alpha (FRα) is expressed in the choroid plexus, and loss of function of this protein, as also occurs in an autosomal recessive disorder, results solely in "cerebral folate deficiency" (CFD), the designation for this disorder. This paper reviews the current understanding of the functional and structural properties and regulation of PCFT, an electrogenic proton symporter, and contrasts PCFT properties with those of the reduced folate carrier (RFC), an organic anion antiporter, that is the major route of folate transport to systemic tissues. The clinical characteristics of HFM and its treatment, based upon the thirty-seven known cases with the clinical syndrome, of which thirty have been verified by genotype, are presented. The ways in which PCFT and FRα might interact at the level of the choroid plexus such that each is required for folate transport from blood to cerebrospinal fluid are considered along with a basis for the different clinical presentations of HFM and CFD.

Even though pancreatic ductal adenocarcinoma (PDAC) is associated with fibrotic stroma, the molecular pathways regulating the formation of cancer associated fibroblasts (CAFs) are not well elucidated. An epigenomic analysis of patient-derived and de-novo generated CAFs demonstrated widespread loss of cytosine methylation that was associated with overexpression of various inflammatory transcripts including CXCR4. Co-culture of neoplastic cells with CAFs led to increased invasiveness that was abrogated by inhibition of CXCR4. Metabolite tracing revealed that lactate produced by neoplastic cells leads to increased production of alpha-ketoglutarate (aKG) within mesenchymal stem cells (MSCs). In turn, aKG mediated activation of the demethylase TET enzyme led to decreased cytosine methylation and increased hydroxymethylation during de novo differentiation of MSCs to CAF. Co-injection of neoplastic cells with TET-deficient MSCs inhibited tumor growth in vivo. Thus, in PDAC, a tumor-mediated lactate flux is associated with widespread epigenomic reprogramming that is seen during CAF formation.

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bIn cells, N 10 -formyltetrahydrofolate (N 10 -fTHF) is required for formylation of eubacterial/organellar initiator tRNA and purine nucleotide biosynthesis. Biosynthesis of N 10 -fTHF is catalyzed by 5,10-methylene-tetrahydrofolate dehydrogenase/cyclohydrolase (FolD) and/or 10-formyltetrahydrofolate synthetase (Fhs). All eubacteria possess FolD, but some possess both FolD and Fhs. However, the reasons for possessing Fhs in addition to FolD have remained unclear. We used Escherichia coli, which naturally lacks fhs, as our model. We show that in E. coli, the essential function of folD could be replaced by Clostridium perfringens fhs when it was provided on a medium-copy-number plasmid or integrated as a single-copy gene in the chromosome. The fhs-supported folD deletion (⌬folD) strains grow well in a complex medium. However, these strains require purines and glycine as supplements for growth in M9 minimal medium. The in vivo levels of N 10 -fTHF in the ⌬folD strain (supported by plasmid-borne fhs) were limiting despite the high capacity of the available Fhs to synthesize N 10 -fTHF in vitro. Auxotrophy for purines could be alleviated by supplementing formate to the medium, and that for glycine was alleviated by engineering THF import into the cells. The ⌬folD strain (harboring fhs on the chromosome) showed a high NADP ؉ -to-NADPH ratio and hypersensitivity to trimethoprim. The presence of fhs in E. coli was disadvantageous for its aerobic growth. However, under hypoxia, E. coli strains harboring fhs outcompeted those lacking it. The computational analysis revealed a predominant natural occurrence of fhs in anaerobic and facultative anaerobic bacteria.T he pathway of one-carbon metabolism is central to the synthesis of purine nucleotides, thymidylate, glycine, and methionine (Fig. 1). The enzymes that catalyze interconversions of the pathway intermediates are highly conserved across the three domains of life (1-6). Serine hydroxymethyltransferase (GlyA) catalyzes the reversible reaction of conversion of serine and tetrahydrofolate (THF) to glycine and 5,10-methylene-tetrahydrofolate (5,10-CH 2 -THF) (7). FolD, a bifunctional enzyme, carries out sequential steps of reversible conversions of 5,10-CH 2 -THF to 5,10-methenyltetrahydrofolate (5,10-CH ϩ -THF), followed by the conversion of the latter to N 10 -formyltetrahydrofolate (N 10 -fTHF) by its dehydrogenase and cyclohydrolase activities, respectively (8). Availability of N 10 -fTHF is crucial for the de novo pathway of purine nucleotide biosynthesis and formylation of the initiator tRNA (tRNA fMet ) to initiate protein synthesis in eubacteria and eukaryotic organelles (9). N 10 -fTHF can also be synthesized by formyltetrahydrofolate synthetase, Fhs (also known as formate-tetrahydrofolate ligase), by utilizing THF, formate, and ATP (Fig. 1). The dual scheme of N 10 -fTHF synthesis is conserved in eukaryotes and some archaea (6). Many eukaryotic organisms possess FolD with trifunctional activities of dehydrogenase-cyclohydrolase-synthetase (10, 11). Among eubacter...