The spectrum of mutations in the PCFT gene, coding for an intestinal folate transporter, that are the basis for hereditary folate malabsorption

Impact of Drug-Mediated Inhibition of Intestinal Transporters on Nutrient and Endogenous Substrate Disposition…an Afterthought?

A large percentage (~60%) of prescription drugs and new molecular entities are designed for oral delivery, which requires passage through a semi-impervious membrane bilayer in the gastrointestinal wall. Passage through this bilayer can be dependent on membrane transporters that regulate the absorption of nutrients or endogenous substrates. Several investigations have provided links between nutrient, endogenous substrate, or drug absorption and the activity of certain membrane transporters. This knowledge has been key in the development of new therapeutics that can alleviate various symptoms of select diseases, such as cholestasis and diabetes. Despite this progress, recent studies revealed potential clinical dangers of unintended altered nutrient or endogenous substrate disposition due to the drug-mediated disruption of intestinal transport activity. This review outlines reports of glucose, folate, thiamine, lactate, and bile acid (re)absorption changes and consequent adverse events as examples. Finally, the need to comprehensively expand research on intestinal transporter-mediated drug interactions to avoid the unwanted disruption of homeostasis and diminish therapeutic adverse events is highlighted.

Low-dose daily folic acid (400 μg) supplementation does not affect regulation of folate transporters found present throughout the terminal ileum and colon of humans: a randomized clinical trial

BACKGROUND

Folic acid supplementation during the periconceptional period reduces the risk of neural tube defects in infants, but concern over chronic folic acid exposure remains. An improved understanding of folate absorption may clarify potential risks. Folate transporters have been characterized in the small intestine, but less so in the colon of healthy, free-living humans. The impact of folic acid fortification or supplementation on regulation of these transporters along the intestinal tract is unknown.

OBJECTIVE

The objective was to characterize expression of folate transporters/receptor (FT/R) and folate hydrolase, glutamate carboxypeptidase II (GCPII), from the terminal ileum and throughout the colon of adults and assess the impact of supplemental folic acid.

METHODS

In this 16-wk open-labeled randomized clinical trial, adults consumed a low folic acid-containing diet, a folate-free multivitamin, and either a 400 μg folic acid supplement or no folic acid supplement. Dietary intakes and blood were assessed at baseline, 8 wk, and 16 wk (time of colonoscopy). Messenger RNA (mRNA) expression and protein expression of FT/R and GCPII were assessed in the terminal ileum, cecum, and ascending and descending colon.

RESULTS

Among 24 randomly assigned subjects, no differences in dietary folate intake or blood folate were observed at baseline. Mean ± SD red blood cell folate at 16 wk was 1765 ± 426 and 911 ± 242 nmol/L in the 400 and 0 μg folic acid group, respectively (P < 0.0001). Reduced folate carrier, proton-coupled folate transporter, and folate-receptor alpha expression were detected in the terminal ileum and colon, as were efflux transporters of breast cancer resistance protein and multidrug resistance protein-3. Other than a higher mRNA expression of FR-alpha and GCPII in the 400 μg supplement group in the ascending colon, no treatment differences were observed (P < 0.02).

CONCLUSIONS

Folate transporters are present throughout the terminal ileum and colon; there is little evidence that a low dose of folic acid supplementation affects colonic absorption. This trial was registered at clinicaltrials.gov as NCT03421483.

Biology and therapeutic applications of the proton-coupled folate transporter

INTRODUCTION

The proton-coupled folate transporter (PCFT; SLC46A1) was discovered in 2006 as the principal mechanism by which folates are absorbed in the intestine and the causal basis for hereditary folate malabsorption (HFM). In 2011, it was found that PCFT is highly expressed in many tumors. This stimulated interest in using PCFT for cytotoxic drug targeting, taking advantage of the substantial levels of PCFT transport and acidic pH conditions commonly associated with tumors.

AREAS COVERED

We summarize the literature from 2006 to 2022 that explores the role of PCFT in the intestinal absorption of dietary folates and its role in HFM and as a transporter of folates and antifolates such as pemetrexed (Alimta) in relation to cancer. We provide the rationale for the discovery of a new generation of targeted pyrrolo[2,3-d]pyrimidine antifolates with selective PCFT transport and inhibitory activity toward de novo purine biosynthesis in solid tumors. We summarize the benefits of this approach to cancer therapy and exciting new developments in the structural biology of PCFT and its potential to foster refinement of active structures of PCFT-targeted anti-cancer drugs.

EXPERT OPINION

We summarize the promising future and potential challenges of implementing PCFT-targeted therapeutics for HFM and a variety of cancers.

B vitamin supply in plants and humans: the importance of vitamer homeostasis

B vitamins are a group of water-soluble micronutrients that are required in all life forms. With the lack of biosynthetic pathways, humans depend on dietary uptake of these compounds, either directly or indirectly, from plant sources. B vitamins are frequently given little consideration beyond their role as enzyme accessory factors and are assumed not to limit metabolism. However, it should be recognized that each individual B vitamin is a family of compounds (vitamers), the regulation of which has dedicated pathways. Moreover, it is becoming increasingly evident that individual family members have physiological relevance and should not be sidelined. Here, we elaborate on the known forms of vitamins B₁ , B₆ and B₉ , their distinct functions and importance to metabolism, in both human and plant health, and highlight the relevance of vitamer homeostasis. Research on B vitamin metabolism over the past several years indicates that not only the total level of vitamins but also the oft-neglected homeostasis of the various vitamers of each B vitamin is essential to human and plant health. We briefly discuss the potential of plant biology studies in supporting human health regarding these B vitamins as essential micronutrients. Based on the findings of the past few years we conclude that research should focus on the significance of vitamer homeostasis - at the organ, tissue and subcellular levels - which could improve the health of not only humans but also plants, benefiting from cross-disciplinary approaches and novel technologies.

An infantile case of hereditary folate malabsorption with sudden development of pulmonary hemorrhage: a case report

BACKGROUND

Hereditary folate malabsorption-a rare disorder caused by impairment of the folate transporter-can develop into severe folate deficiency manifesting as megaloblastic anemia and occasionally thrombocytopenia. Reportedly, megaloblastic anemia can manifest with hemorrhagic episodes, possibly due to ineffective platelet production and platelet dysfunction. However, life-threatening hemorrhage events in hereditary folate malabsorption have not been well investigated.

CASE PRESENTATION

A 3-month-old Japanese boy was transferred to our hospital due to thrombocytopenia and severe megaloblastic anemia. During a thorough examination of hematopoietic abnormalities, the patient suddenly went into cardiac arrest due to pulmonary hemorrhage. Although intravenous folate supplementation was started soon after the identification of folate deficiency, the patient died of circulatory defect and multiple organ failure. The cause of pulmonary hemorrhage, such as respiratory infection, could not be confirmed. Genetic investigation revealed a mutation in the SLC46A1 gene to be the cause of the hereditary folate malabsorption.

CONCLUSION

We report an infantile case of hereditary folate malabsorption that progressed to lethal pulmonary hemorrhage before folate deficiency was identified. Clinicians should consider that megaloblastic anemia could lead to severe bleeding without warning, and that nutrient supplementation should be initiated as soon as possible.

Transporters in drug development: International transporter consortium update on emerging transporters of clinical importance

During its 4^th transporter workshop in 2021, the International Transporter Consortium (ITC) provided updates on emerging clinically relevant transporters for drug development. Previously highlighted and new transporters were considered based on up-to-date clinical evidence of their importance in drug-drug interactions and potential for altered drug efficacy and safety, including drug-nutrient interactions leading to nutrient deficiencies. For the first time, folate transport pathways (PCFT, RFC, and FRα) were examined in-depth as a potential mechanism of drug-induced folate deficiency and related toxicities (e.g., neural tube defects, megaloblastic anemia). However, routine toxicology studies conducted in support of drug development appear sufficient to flag such folate deficiency toxicities, while prospective prediction from in vitro folate metabolism and transport inhibition is not well enough established to inform drug development. Previous suggestion of retrospective study of intestinal OATP2B1 inhibition to explain unexpected decreases in drug exposure were updated. Furthermore, when the absorption of a new molecular entity is more rapid and extensive than can be explained by passive permeability, evaluation of OATP2B1 transport may be considered. Emerging research on hepatic and renal OAT2 is summarized, but current understanding of the importance of OAT2 was deemed insufficient to justify specific consideration for drug development. Hepatic, renal, and intestinal MRPs (MRP2, MRP3, MRP4) were revisited. MRPs may be considered when they are suspected to be the major determinant of drug disposition (e.g., direct glucuronide conjugates); MRP2 inhibition as a mechanistic explanation for drug-induced hyperbilirubinemia remains justified. There were no major changes in recommendations from previous ITC whitepapers.

Luteolin Enhances Choroid Plexus 5-MTHF Brain Transport to Promote Hippocampal Neurogenesis in LOD Rats

Folates, provided by food, are commonly used antidepressant synergists in late-onset depression (LOD). However, increased intake of folic acid in the elderly population might lead to the accumulation of unmetabolized folic acid in the systemic circulation, leading to enhanced deterioration of the central nervous system function. In addition, folates cannot access the brain directly because of the blood-brain barrier. Choroid plexus (CP) 5-methyltetrahydrofolate (5-MTHF) brain transport plays a critical role in regulating the cerebrospinal fluid (CSF) 5-MTHF content. Luteolin is a natural flavonoid that has antidepressant effects and is involved in the anti-folate resistance pathway. It remains unclear whether the antidepressant effects of luteolin are associated with the CP 5-MTHF brain transport. In this study, 20-21-month-old Wistar rats were exposed to the chronic unpredictable mild stress (CUMS) protocol for 6 consecutive weeks to explore the long-term effects of luteolin on behavior, 5-MTHF levels, hippocampal neurogenesis, and folate brain transport of the CP. In vitro primary hippocampal neural stem cells (NSCs) cultured in media containing 10% CSF from each group of rats and choroid plexus epithelial cells (CPECs) cultured in media containing 20 μM luteolin were treated with 100 μM corticosterone and 40 mg/ml D-galactose. We found that aged rats exposed to CUMS showed a significantly reduced sucrose preference, decreased locomotion activity in the open field test and accuracy of the Morris water maze test, increased immobility time in the forced swimming test, accelerated dysfunctional neurogenesis and neuronal loss in the dentate gyrus of LOD rats, as well as decreased CSF and hippocampus 5-MTHF levels, and zona occludens protein 1 (ZO-1), proton-coupled folate transporter (PCFT), and reduced folate carrier (RFC) protein levels. In vitro assays showed media containing 10% aged CSF or LOD+ Luteolin-CSF significantly increased the viability of CORT + D-gal-injured NSCs and alleviated dysfunctional neurogenesis and neuronal loss compared with the CORT + D-gal medium. However, media containing 10% LOD-CSF had no such effect. In the meantime, induction of CORT + D-gal significantly decreased the ZO-1, PCFT, RFC, and folate receptor alpha (FR-α) protein levels and transepithelial electrical resistance in rat CPECs. As expected, luteolin treatment was effective in improving these abnormal changes. These findings suggested that luteolin could ameliorate CUMS-induced LOD-like behaviors by enhancing the folate brain transport.

The evolving biology of the proton‐coupled folate transporter: New insights into regulation, structure, and mechanism

The human proton‐coupled folate transporter (PCFT; SLC46A1) or hPCFT was identified in 2006 as the principal folate transporter involved in the intestinal absorption of dietary folates. A rare autosomal recessive hereditary folate malabsorption syndrome is attributable to human SLC46A1 variants. The recognition that hPCFT was highly expressed in many tumors stimulated substantial interest in its potential for cytotoxic drug targeting, taking advantage of its high‐level transport activity under acidic pH conditions that characterize many tumors and its modest expression in most normal tissues. To better understand the basis for variations in hPCFT levels between tissues including human tumors, studies have examined the transcriptional regulation of hPCFT including the roles of CpG hypermethylation and critical transcription factors and cis elements. Additional focus involved identifying key structural and functional determinants of hPCFT transport that, combined with homology models based on structural homologies to the bacterial transporters GlpT and LacY, have enabled new structural and mechanistic insights. Recently, cryo‐electron microscopy structures of chicken PCFT in a substrate‐free state and in complex with the antifolate pemetrexed were reported, providing further structural insights into determinants of (anti)folate recognition and the mechanism of pH‐regulated (anti)folate transport by PCFT. Like many major facilitator proteins, hPCFT exists as a homo‐oligomer, and evidence suggests that homo‐oligomerization of hPCFT monomeric proteins may be important for its intracellular trafficking and/or transport function. Better understanding of the structure, function and regulation of hPCFT should facilitate the rational development of new therapeutic strategies for conditions associated with folate deficiency, as well as cancer.

SLC gene mutations and pediatric neurological disorders: diverse clinical phenotypes in a Saudi Arabian population

The uptake and efflux of solutes across a plasma membrane is controlled by transporters. There are two main superfamilies of transporters, adenosine 5'-triphosphate (ATP) binding cassettes (ABCs) and solute carriers (SLCs). In the brain, SLC transporters are involved in transporting various solutes across the blood-brain barrier, blood-cerebrospinal fluid barrier, astrocytes, neurons, and other brain cell types including oligodendrocytes and microglial cells. SLCs play an important role in maintaining normal brain function. Hence, mutations in the genes that encode SLC transporters can cause a variety of neurological disorders. We identified the following SLC gene variants in 25 patients in our cohort: SLC1A2, SLC2A1, SLC5A1, SLC6A3, SLC6A5, SLC6A8, SLC9A6, SLC9A9, SLC12A6, SLC13A5, SLC16A1, SLC17A5, SLC19A3, SLC25A12, SLC25A15, SLC27A4, SLC45A1, SLC46A1, and SLC52A3. Eight patients harbored pathogenic or likely pathogenic mutations (SLC5A1, SLC9A6, SLC12A6, SLC16A1, SLC19A3, and SLC52A3), and 12 patients were found to have variants of unknown clinical significance (VOUS); these variants occurred in 11 genes (SLC1A2, SLC2A1, SLC6A3, SLC6A5, SLC6A8, SLC9A6, SLC9A9, SLC13A5, SLC25A12, SLC27A4, and SLC45A1). Five patients were excluded as they were carriers. In the remaining 20 patients with SLC gene variants, we identified 16 possible distinct neurological disorders. Based on the clinical presentation, we categorized them into genes causing intellectual delay (ID) or autism spectrum disorder (ASD), those causing epilepsy, those causing vitamin-related disorders, and those causing other neurological diseases. Several variants were detected that indicated possible personalized therapies: SLC2A1 led to dystonia or epilepsy, which can be treated with a ketogenic diet; SLC6A3 led to infantile parkinsonism-dystonia 1, which can be treated with levodopa; SLC6A5 led to hyperekplexia 3, for which unnecessary treatment with antiepileptic drugs should be avoided; SLC6A8 led to creatine deficiency syndrome type 1, which can be treated with creatine monohydrate; SLC16A1 led to monocarboxylate transporter 1 deficiency, which causes seizures that should not be treated with a ketogenic diet; SLC19A3 led to biotin-thiamine-responsive basal ganglia disease, which can be treated with biotin and thiamine; and SLC52A3 led to Brown-Vialetto-Van-Laere syndrome 1, which can be treated with riboflavin. The present study examines the prevalence of SLC gene mutations in our cohort of children with epilepsy and other neurological disorders. It highlights the diverse phenotypes associated with mutations in this large family of SLC transporter proteins, and an opportunity for personalized genomics and personalized therapeutics.

Functional role of serine 318 of the proton-coupled folate transporter in methotrexate transport

This study revealed the importance of serine 318 (S318) residue for proton-coupled folate transporter (PCFT, SLC46A1) functioning. Substitution of S318 with arginine or lysine impaired transport of methotrexate (MTX), but substitution with alanine (has a simple side chain structure), or cysteine (structurally similar to serine), had no significant effect on MTX transport. The initial uptake rate of MTX by S318A and S318C mutant at pH 5.0, followed by Michaelis-Menten kinetics with a K_m value of approximately 2.3 μM (for S318A) and 2.9 μM (for S318C), was similar to that of the wild-type. The normalized V_max value of the S318A mutant, calculated by dividing the V_max value by the Western blot protein band's relative intensity, was approximately 2-fold greater than that of the wild-type. The normalized V_max value of the S318C mutant was approximately 0.8-fold smaller than the wild-type. Results obtained showed that the substitution of S318 with basic amino acid residues results in the loss of transport activity, even though PCFT mutants are expressed at the cell membrane. Alternatively, the substitution of S318 with neutral amino acids did not significantly affect the transport function of PCFT.

A proton-coupled folate transporter mutation causing hereditary folate malabsorption locks the protein in an inward-open conformation

The proton-coupled folate transporter (PCFT, SLC46A1) is required for folate intestinal absorption and transport across the choroid plexus. Recent work has identified a F392V mutation causing hereditary folate malabsorption. However, the residue properties responsible for this loss of function remains unknown. Using site-directed mutagenesis, we observed complete loss of function with charged (Lys, Asp, and Glu) and polar (Thr, Ser, and Gln) Phe-392 substitutions and minimal function with some neutral substitutions; however, F392M retained full function. Using the substituted-cysteine accessibility method (with N-biotinyl aminoethyl methanethiosulfonate labeling), Phe-392 mutations causing loss of function, although preserving membrane expression and trafficking, also resulted in loss of accessibility of the substituted cysteine in P314C-PCFT located within the aqueous translocation pathway. F392V function and accessibility of the P314C cysteine were restored by insertion of a G305L (suppressor) mutation. A S196L mutation localized in proximity to Gly-305 by homology modeling was inactive. However, when inserted into the inactive F392V scaffold, function was restored (mutually compensatory mutations), as was accessibility of the P314C cysteine residue. Reduced function, documented with F392H PCFT, was due to a 15-fold decrease in methotrexate influx V _max, accompanied by a decreased influx K_t (4.5-fold) and K_i (3-fold). The data indicate that Phe-392 is required for rapid oscillation of the carrier among its conformational states and suggest that this is achieved by dampening affinity of the protein for its folate substrates. F392V and other inactivating Phe-392 PCFT mutations lock the protein in its inward-open conformation. Reach (length) and hydrophobicity of Phe-392 appear to be features required for full activity.

Identification of the amino acid residue responsible for the myricetin sensitivity of human proton-coupled folate transporter

Human proton-coupled folate transporter (hPCFT/SLC46A1) has recently been found to be inhibited by myricetin by a sustained mechanism, raising a concern that the inhibition might lead to malabsorption of folates in the intestine, where hPCFT works for their epithelial uptake. However, rat PCFT (rPCFT) has more recently been found not to be inhibited by myricetin. Prompted by this finding, we attempted to determine the amino acid residue involved in that by analyses comparing between hPCFT and rPCFT. In the initial analysis, chimeric constructs prepared from hPCFT and rPCFT were examined for myricetin sensitivity to determine the hPCFT segment involved in the sensitivity. Focusing on the thereby determined segment from 83rd to 186th amino acid residue, hPCFT mutants having a designated amino acid residue replaced with its counterpart in rPCFT were prepared for the subsequent analysis. Among them, only G158N-substituted hPCFT was found to be transformed to be insensitive to myricetin and, accordingly, oppositely N158G-substituted rPCFT was transformed to be sensitive to myricetin. These results indicate the critical role of Gly¹⁵⁸ in the myricetin sensitivity of hPCFT. This finding would help advance the elucidation of the mechanism of the myricetin-induced inhibition of hPCFT and manage the potential risk arising from that.

Methotrexate pharmacogenetics in the treatment of rheumatoid arthritis

For many decades, methotrexate (MXT) has remained the drug of choice in the treatment of rheumatoid arthritis (RA). Unfortunately, a considerable number of patients do not achieve an appropriate therapeutic response. Pharmacogenetics studies do not give usable results regarding differences in MTX response among RA patients. The mechanism of MTX action in RA is not completely understood. We present and discuss data regarding the molecular basis of folate and adenosine pathways, the most obvious MTX targets, to explain possible causes of therapy failure. The molecular basis of the disease could also have an impact on therapy outcomes and in this review we explore this. Finally, we make a short review of available pharmacogenetics study results.

Discovery of amide-bridged pyrrolo[2,3-d]pyrimidines as tumor targeted classical antifolates with selective uptake by folate receptor α and inhibition of de novo purine nucleotide biosynthesis

We previously showed that classical 6-substituted pyrrolo[2,3-d]pyrimidine antifolates bind to folate receptor (FR) α and the target purine biosynthetic enzyme glycinamide ribonucleotide formyltransferase (GARFTase) with different cis and trans conformations. In this study, we designed novel analogs of this series with an amide moiety in the bridge region that can adopt both the cis and trans lowest energy conformations. This provides entropic benefit, by restricting the number of side-chain conformations of the unbound ligand to those most likely to promote binding to FRα and the target enzyme required for antitumor activity. NMR of the most active compound 7 showed both cis and trans amide bridge conformations in ~1:1 ratio. The bridge amide group in the best docked poses of 7 in the crystal structures of FRα and GARFTase adopted both cis and trans conformations, with the lowest energy conformations predicted by Maestro and evidenced by NMR within 1 kcal/mol. Compound 7 showed ~3-fold increased inhibition of FRα-expressing cells over its non-restricted parent analog 1 and was selectively internalized by FRα over the reduced folate carrier (RFC), resulting in significant in vitro antitumor activity toward FRα-expressing KB human tumor cells. Antitumor activity of 7 was abolished by treating cells with adenosine but was incompletely protected by 5-aminoimidazole-4-carboxamide (AICA) at higher drug concentrations, suggesting GARFTase and AICA ribonucleotide formyltransferase (AICARFTase) in de novo purine biosynthesis as the likely intracellular targets. GARFTase inhibition by compound 7 was confirmed by an in situ cell-based activity assay. Our results identify a "first-in-class" classical antifolate with a novel amide linkage between the scaffold and the side chain aryl L-glutamate that affords exclusive selectivity for transport via FRα over RFC and antitumor activity resulting from inhibition of GARFTase and likely AICARFTase. Compound 7 offers significant advantages over clinically used inhibitors of this class that are transported by the ubiquitous RFC, resulting in dose-limiting toxicities.

Human inborn errors of immunity to infection affecting cells other than leukocytes: from the immune system to the whole organism

Studies of vertebrate immunity have traditionally focused on professional cells, including circulating and tissue-resident leukocytes. Evidence that non-professional cells are also intrinsically essential (i.e. not via their effect on leukocytes) for protective immunity in natural conditions of infection has emerged from three lines of research in human genetics. First, studies of Mendelian resistance to infection have revealed an essential role of DARC-expressing erythrocytes in protection against Plasmodium vivax infection, and an essential role of FUT2-expressing intestinal epithelial cells for protection against norovirus and rotavirus infections. Second, studies of inborn errors of non-hematopoietic cell-extrinsic immunity have shown that APOL1 and complement cascade components secreted by hepatocytes are essential for protective immunity to trypanosome and pyogenic bacteria, respectively. Third, studies of inborn errors of non-hematopoietic cell-intrinsic immunity have suggested that keratinocytes, pulmonary epithelial cells, and cortical neurons are essential for tissue-specific protective immunity to human papillomaviruses, influenza virus, and herpes simplex virus, respectively. Various other types of genetic resistance or predisposition to infection in human populations are not readily explained by inborn variants of genes operating in leukocytes and may, therefore, involve defects in other cells. The probing of this unchartered territory by human genetics is reshaping immunology, by scaling immunity to infection up from the immune system to the whole organism.

Decreased expression of folate transport proteins in oral cancer

OBJECTIVES

The purpose of this study was to assess the expression of the 3 major folate transporters-folate receptors (FRs), reduced folate carrier (RFC), and proton-coupled folate transporter (PCFT)-in oral squamous cell carcinoma (OSCC). We hypothesized that patterns of expression of folate transporters would be different in OSCC compared with normal oral epithelium.

STUDY DESIGN

We used immunohistochemistry to examine the expression of FR, RFC, and PCFT in 15 primary specimens collected from patients with OSCC, 2 human cadaveric samples of OSCC, and 12 normal human cadaveric oral tissues from a medical gross anatomy laboratory. Possible correlations between the expression of each folate transporter and patients' clinical data were determined.

RESULTS

All 3 folate transporters were highly expressed in normal oral epithelium. In contrast, OSCC samples generally demonstrated low expression of FR, RFC, and PCFT, with wide distribution in the invading cancer cells. There were no differences in folate transporter expression between OSCC samples collected from patients and from human cadavers. The lowest expression of FR and PCFT characterized less-differentiated tumors, and the lowest expression of RFC correlated with higher lymph node involvement.

CONCLUSIONS

Human oral cancer samples expressed decreased amounts of all 3 major folate transport proteins compared with controls from normal cadaveric oral tissues.

Substitutions that lock and unlock the proton-coupled folate transporter (PCFT-SLC46A1) in an inward-open conformation

The proton-coupled folate transporter (PCFT) mediates intestinal absorption of folates and their transport from blood to cerebrospinal fluid across the choroid plexus. Substitutions at Asp-109 in the first intracellular loop between the first and second transmembrane domains (TMDs) abolish PCFT function, but protein expression and trafficking to the cell membrane are retained. Here, we used site-directed mutagenesis, the substituted-cysteine accessibility method, functional analyses, and homology modeling to determine whether the D109A substitution locks PCFT in one of its conformational states. Cys-substituted residues lining the PCFT aqueous translocation pathway and accessible in WT PCFT to the membrane-impermeable cysteine-biotinylation reagent, MTSEA-biotin, lost accessibility when introduced into the D109A scaffold. Substitutions at Gly-305 located exofacially within the eighth TMD, particularly with bulky residues, when introduced into the D109A scaffold largely restored function and MTSEA-biotin accessibility to Cys-substituted residues within the pathway. Likewise, Ser-196 substitution in the fifth TMD, predicted by homology modeling to be in proximity to Gly-305, also partially restored function found in solute transporters, is critical to oscillation of the carrier among its conformational states. Substitutions at Asp-109 and Gly-112 lock PCFT in an inward-open conformation, resulting in the loss of function. However, the integrity of the locked protein is preserved, indicated by the restoration of function after insertion of a second "unlocking" mutation. and accessibility. Similarly, the inactivating G112K substitution within the first intracellular loop was partially reactivated by introducing the G305L substitution. These data indicate that the first intracellular loop, with a sequence identical to "motif A" (GXXXDXXGR(R/K)).

Toward a better understanding of folate metabolism in health and disease

Folate metabolism is crucial for many biochemical processes, including purine and thymidine monophosphate (dTMP) biosynthesis, mitochondrial protein translation, and methionine regeneration. These biochemical processes in turn support critical cellular functions such as cell proliferation, mitochondrial respiration, and epigenetic regulation. Not surprisingly, abnormal folate metabolism has been causally linked with a myriad of diseases. In this review, we provide a historical perspective, delve into folate chemistry that is often overlooked, and point out various missing links and underdeveloped areas in folate metabolism for future exploration.

Fluorine-Substituted Pyrrolo[2,3- d]Pyrimidine Analogues with Tumor Targeting via Cellular Uptake by Folate Receptor α and the Proton-Coupled Folate Transporter and Inhibition of de Novo Purine Nucleotide Biosynthesis

Novel fluorinated 2-amino-4-oxo-6-substituted pyrrolo[2,3- d]pyrimidine analogues 7-12 were synthesized and tested for selective cellular uptake by folate receptors (FRs) α and β or the proton-coupled folate transporter (PCFT) and for antitumor efficacy. Compounds 8, 9, 11, and 12 showed increased in vitro antiproliferative activities (∼11-fold) over the nonfluorinated analogues 2, 3, 5, and 6 toward engineered Chinese hamster ovary and HeLa cells expressing FRs or PCFT. Compounds 8, 9, 11, and 12 also inhibited proliferation of IGROV1 and A2780 epithelial ovarian cancer cells; in IGROV1 cells with knockdown of FRα, 9, 11, and 12 showed sustained inhibition associated with uptake by PCFT. All compounds inhibited glycinamide ribonucleotide formyltransferase, a key enzyme in the de novo purine biosynthesis pathway. Molecular modeling studies validated in vitro cell-based results. NMR evidence supports the presence of an intramolecular fluorine-hydrogen bond. Potent in vivo efficacy of 11 was established with IGROV1 xenografts in severe compromised immunodeficient mice.

Folate promotes S-adenosyl methionine reactions and the microbial methylation cycle and boosts ruminants production and reproduction

Folate has gained significant attention due to its vital role in biological methylation and epigenetic machinery. Folate, or vitamin (B₉), is only produced through a de novo mechanism by plants and micro-organisms in the rumen of mature animals. Although limited research has been conducted on folate in ruminants, it has been noted that ruminal synthesis could not maintain folate levels in high yielding dairy animals. Folate has an essential role in one-carbon metabolism and is a strong antiproliferative agent. Folate increases DNA stability, being crucial for DNA synthesis and repair, the methylation cycle, and preventing oxidation of DNA by free radicals. Folate is also critical for cell division, metabolism of proteins, synthesis of purine and pyrimidine, and increasing the de novo delivery of methyl groups and S-adenosylmethionine. However, in ruminants, metabolism of B₁₂ and B₉ vitamins are closely connected and utilization of folate by cells is significantly affected by B₁₂ vitamin concentration. Supplementation of folate through diet, particularly in early lactation, enhanced metabolic efficiency, lactational performance, and nutritional quality of milk. Impaired absorption, oxidative degradation, or deficient supply of folate in ruminants affects DNA stability, cell division, homocysteine remethylation to methionine, de novo synthesis of S-adenosylmethionine, and increases DNA hypomethylation, uracil misincorporation into DNA, chromosomal damage, abnormal cell growth, oxidative species, premature birth, low calf weight, placental tube defects, and decreases production and reproduction of ruminant animals. However, more studies are needed to overcome these problems and reduce enormous dietary supplement waste and impaired absorption of folate in ruminants. This review was aimed to highlight the vital role of folic acid in ruminants performance.

The promise and challenges of exploiting the proton-coupled folate transporter for selective therapeutic targeting of cancer

This review considers the "promise" of exploiting the proton-coupled folate transporter (PCFT) for selective therapeutic targeting of cancer. PCFT was discovered in 2006 and was identified as the principal folate transporter involved in the intestinal absorption of dietary folates. The recognition that PCFT was highly expressed in many tumors stimulated substantial interest in using PCFT for cytotoxic drug targeting, taking advantage of its high level transport activity under the acidic pH conditions that characterize many tumors. For pemetrexed, among the best PCFT substrates, transport by PCFT establishes its importance as a clinically important transporter in malignant pleural mesothelioma and non-small cell lung cancer. In recent years, the notion of PCFT-targeting has been extended to a new generation of tumor-targeted 6-substituted pyrrolo[2,3-d]pyrimidine compounds that are structurally and functionally distinct from pemetrexed, and that exhibit near exclusive transport by PCFT and potent inhibition of de novo purine nucleotide biosynthesis. Based on compelling preclinical evidence in a wide range of human tumor models, it is now time to advance the most optimized PCFT-targeted agents with the best balance of PCFT transport specificity and potent antitumor efficacy to the clinic to validate this novel paradigm of highly selective tumor targeting.

Hereditary folate malabsorption with a novel mutation on SLC46A1: A case report

RATIONALE

Hereditary folate malabsorption (HFM) is characterized by folate deficiency with impaired intestinal folate absorption and impaired folate transport into the central nervous system. Its manifestations mainly include macrocytic anemia, recurrent infections, and neurological deficits. The neurological manifestations include progressive psychomotor retardation, behavioral disorders, and early-onset seizures.

PATIENT CONCERNS

From early infancy, a Chinese boy had experienced macrocytic anemia, leukopenia, thrombocytopenia, recurrent pneumonia, diarrhea, and mouth ulcers. He also presented with progressive neurological symptoms.

DIAGNOSIS

A novel mutation in the SLC46A1 gene was identified, and HFM was diagnosed at 18 months of age.

INTERVENTIONS

After the HFM diagnosis, the boy was treated with folinic acid.

LESSONS

Folinic acid supplementation is effective and may offer life-changing therapy for patients with HFM.

Residues in the eighth transmembrane domain of the proton-coupled folate transporter (SLC46A1) play an important role in defining the aqueous translocation pathway and in folate substrate binding

The proton-coupled folate transporter (PCFT-SLC46A1) is required for intestinal folate absorption and folate transport across the choroid plexus. This report addresses the structure/function of the 8th transmembrane helix. Based upon biotinylation of cysteine-substituted residues by MTSEA-biotin, 14 contiguous exofacial residues to Leu316 were accessible to the extracellular compartment of the 23 residues in this helix (Leu303-Leu325). Pemetrexed blocked biotinylation of six Cys-substituted residues deep within the helix implicating an important role for this region in folate binding. Accessibility decreased at 4°C vs RT. The influx K_t, K_i and V_max were markedly increased for the P314C mutant, similar to what was observed for Y315A and Y315P mutants. However, the K_t, alone, was increased for the P314Y mutant. To correlate these observations with PCFT structural changes during the transport cycle, homology models were built for PCFT based upon the recently reported structures of bovine and rodent GLUT5 fructose transporters in the inward-open and outward- open conformations, respectively. The models predict substantial structural alterations in the exofacial region of the eighth transmembrane helix as it cycles between its conformational states that can account for the extended and contiguous aqueous accessibility of this region of the helix. Further, a helix break in one of the two conformations can account for the critical roles Pro314 and Tyr315, located in this region, play in PCFT function. The data indicates that the 8th transmembrane helix of PCFT plays an important role in defining the aqueous channel and the folate binding pocket.

Role of Intramuscular Levofolinate Administration in the Treatment of Hereditary Folate Malabsorption: Report of Three Cases

Hereditary folate malabsorption is a rare autosomal recessive disorder caused by impaired active folate transport across membranes and into the central nervous system due to loss-of-function mutations in proton-coupled folate transporter (PCFT). Newborns with this condition have initially normal folate stores, but as they are unable to absorb dietary folate and use rapidly their stores because of their growth demands, symptoms appear in the early infancy. Significant neurological morbidity usually follows the initial non-specific clinical presentation and delayed initiation of treatment. High dose oral and parenteral folinic acid treatment have been previously reported in literature to improve the clinical outcome without achieving optimal cerebrospinal fluid (CSF) folate levels though. The active isomer of 5-formyltetrahydrofolate, also known as levofolinic acid, is available for administration. We report our experience in achieving normal (age dependent) CSF 5-Methyltetrahydrofolate (5-MTHF) levels following daily intramuscular administration of levofolinic acid in three patients with HFM. Follow-up assessment with repeated lumbar punctures has shown a stabilization of 5-MTHF levels within normal range. Clinical features and brain MRI findings had as well either improvement or stabilization. To the best of our knowledge, we provide as well for the first time data in regard to the im levofolinate treatment dosage.

Maternal folic acid consumption during gestation and its long-term effects on offspring's liver: a systematic review

Abstract Objectives: describing the effects of maternal supplementation with folic acid (FA) exclusively during gestation on offspring's liver at later stages in life. Supplementation with FA during gestation has been recommended by the medical society worldwide. The liver has a central role on the substances of metabolism and homeostasis and some studies have shown that a high intake of FA at other periods in life may cause hepatic damage. Methods: a systematic review through which the following databases were consulted: Medline, through platforms of Pubmed, Lilacs and Scielo. The research was performed by keywords such as: "Folic acid", "Gestation", "Rat", "Offspring" and "Liver". Articles which evaluate the effect of FA consumption during both gestation and lactation were excluded. Results: FA consumption avoids disorders on expression of peroxisome proliferator-activated receptor alpha (PPARα) and glucocorticoid receptor (GccR), its lack did not change enzyme activity of the male offspring's liver in adulthood. Supplementation with FA during gestation did not change iron hepatic levels or lipid composition, but had an antioxidant effect on it. Conclusions: supplementation with FA at recommended doses did not cause toxic effects and is very likely to avoid deleterious effects in the liver of the offspring regarding the epigenetic level.

Impact of posttranslational modifications of engineered cysteines on the substituted cysteine accessibility method: evidence for glutathionylation

The substituted cysteine accessibility method (SCAM) is widely used to study the structure and function of channels, receptors and transporters. In its usual application, a cysteine residue is introduced into a protein which lacks native cysteines following which the accessibility of the residue to the aqueous compartment is assessed. Implicit, and generally assumed, is that if the cysteine-substituted residue is not available to react with sulfhydryl reagents it is not exposed to the extracellular compartment or within the aqueous translocation pathway. We demonstrate here, in a Hela-derived cell line, that some cysteine-substituted residues of the proton-coupled folate transporter (PCFT, SLC46A1) that are inaccessible to 2-((biotinoyl)amino)ethyl methanethiosulfonate are glutathionylated by biotinylated glutathione ethyl ester in the absence of an oxidizing agent. Intramolecular disulfide formation involving cysteine-substituted residues was also identified in some instances. These posttranslational modifications limit the accessibility of the cysteine residues to sulfhydryl-reactive reagents and can have a profound impact on the interpretation of SCAM but may not alter function. When a posttranslationally modified residue is used as a reference extracellular control, the high level of exposure required for detection on Western blot results in erroneous detection of otherwise inaccessible intracellular cysteine-substituted residues. The data indicate that in the application of SCAM, when a cysteine-substituted residue does not appear to be accessible to sulfhydryl-reactive reagents, the possibility of a posttranslational modification should be excluded. The data explain the discrepancies in the assessment, and confirm the localization, of the first intracellular loop of PCFT.

The proton-coupled folate transporter (PCFT-SLC46A1) and the syndrome of systemic and cerebral folate deficiency of infancy: Hereditary folate malabsorption

The proton-coupled folate transporter (PCFT-SLC46A1) is the mechanism by which folates are absorbed across the 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 the different clinical presentations of HFM and CFD.

Functional and mechanistic roles of the human proton-coupled folate transporter transmembrane domain 6-7 linker

The proton-coupled folate transporter (PCFT; SLC46A1) is a folate-proton symporter expressed in solid tumors and is used for tumor-targeted delivery of cytotoxic antifolates. Topology modeling suggests that the PCFT secondary structure includes 12 transmembrane domains (TMDs) with TMDs 6 and 7 linked by an intracellular loop (positions 236-265) including His247, implicated as functionally important. Single-cysteine (Cys) mutants were inserted from positions 241 to 251 in Cys-less PCFT and mutant proteins were expressed in PCFT-null (R1-11) HeLa cells; none were reactive with 2-aminoethyl methanethiosulfonate biotin, suggesting that the TMD6-7 loop is intracellular. Twenty-nine single alanine mutants spanning the entire TMD6-7 loop were expressed in R1-11 cells; activity was generally preserved, with the exception of the 247, 250, and 251 mutants, partly due to decreased surface expression. Coexpression of PCFT TMD1-6 and TMD7-12 half-molecules in R1-11 cells partially restored transport activity, although removal of residues 252-265 from TMD7-12 abolished transport. Chimeric proteins, including a nonhomologous sequence from a thiamine transporter (ThTr1) inserted into the PCFT TMD6-7 loop (positions 236-250 or 251-265), were active, although replacement of the entire loop with the ThTr1 sequence resulted in substantial loss of activity. Amino acid replacements (Ala, Arg, His, Gln, and Glu) or deletions at position 247 in wild-type and PCFT-ThTr1 chimeras resulted in differential effects on transport. Collectively, our findings suggest that the PCFT TMD6-7 connecting loop confers protein stability and may serve a unique functional role that depends on secondary structure rather than particular sequence elements.

Role of the tryptophan residues in proton-coupled folate transporter (PCFT-SLC46A1) function

The proton-coupled folate transporter (PCFT) mediates folate absorption across the brush-border membrane of the proximal small intestine and is required for folate transport across the choroid plexus into the cerebrospinal fluid. In this study, the functional role and accessibility of the seven PCFT Trp residues were assessed by the substituted-cysteine accessibility method. Six Trp residues at a lipid-aqueous interface tolerated Cys substitution in terms of protein stability and function. W85C, W202C, and W213C were accessible to N-biotinyl aminoethylmethanethiosulfonate; W48C and W299C were accessible only after treatment with dithiotreitol (DTT), consistent with modification of these residues by an endogenous thiol-reacting molecule and their extracellular location. Neither W107C nor W333C was accessible (even after DTT) consistent with their cytoplasmic orientation. Biotinylation was blocked by pemetrexed only for the W48C (after DTT), W85C, W202C residues. Function was impaired only for the W299C PCFT mutant located in the 4th external loop between the 7th and 8th transmembrane helices. Despite its aqueous location, function could only be fully preserved with Phe and, to a lesser extent, Ala substitutions. There was a 6.5-fold decrease in the pemetrexed influx Vmax and a 3.5- and 6-fold decrease in the influx Kt and Ki, respectively, for the W299S PCFT. The data indicate that the hydrophobicity of the W299 residue is important for function suggesting that during the transport cycle this residue interacts with the lipid membrane thereby impacting on the oscillation of the carrier and, indirectly, on the folate binding pocket.

Experimentally optimized threading structures of the proton-coupled folate transporter

The proton‐coupled folate transporter (PCFT, SLC46A1) transports folic acid across the plasma membrane, together with an excess of protons such that the net charge translocation is positive. We developed 3D structural models of PCFT threaded onto the X‐ray structures of major facilitator superfamily (MFS) members that were identified as close structural homologues. The model of PCFT threaded onto the glycerol‐3‐phosphate transporter (GlpT) structure is consistent with detailed accessibility studies in the absence of extracellular substrate and at pH 7.4 presented here, and additionally with a multitude of other mutagenesis and functional studies. Characteristic MFS structural features are preserved in this PCFT model, such as 12 transmembrane helices divided into two pseudosymmetric bundles, and a high density of positive charges on the periphery of the cytoplasmic site that allow interactions with negatively charged lipid head‐groups. Under the experimental conditions, PCFT predominantly samples the resting state, which in this case is inward‐open. Several positions lining the substrate cavity have been identified. Motif A, a helix‐turn‐helix motif that is a hallmark of MFS transporters between transmembrane segments II and III is oriented appropriately to interact with residues from transmembrane segments IV as well as XI upon conformational transition to the outward‐open state. A charge‐relay system between three charged residues as well as apposing glycines in two α‐helices, both contributed to by motif A, become engaged when PCFT is modeled on the outward‐open state of a putative proton‐driven transporter (YajR).

Identification of an Extracellular Gate for the Proton-coupled Folate Transporter (PCFT-SLC46A1) by Cysteine Cross-linking

The proton-coupled folate transporter (PCFT, SLC46A1) is required for intestinal folate absorption and folate homeostasis in humans. A homology model of PCFT, based upon theEscherichia coliglycerol 3-phosphate transporter structure, predicted that PCFT transmembrane domains (TMDs) 1, 2, 7, and 11 form an extracellular gate in the inward-open conformation. To assess this model, five residues (Gln(45)-TMD1, Asn(90)-TMD2, Leu(290)-TMD7, Ser(407)-TMD11 and Asn(411)-TMD11) in the predicted gate were substituted with Cys to generate single and nine double mutants. Transport function of the mutants was assayed in transient transfectants by measurement of [(3)H]substrate influx as was accessibility of the Cys residues to biotinylation. Pairs of Cys residues were assessed for spontaneous formation of a disulfide bond, induction of a disulfide bond by oxidization with dichloro(1,10-phenanthroline)copper (II) (CuPh), or the formation of a Cd(2+)complex. The data were consistent with the formation of a spontaneous disulfide bond between the N90C/S407C pair and a CuPh- and Cd(2+)-induced disulfide bond and complex, respectively, for the Q45C/L290C and L290C/N411C pairs. The decrease in activity induced by cross-linkage of the Cys residue pairs was due to a decrease in the influxVmaxconsistent with restriction in the mobility of the transporter. The presence of folate substrate decreased the CuPh-induced inhibition of transport. Hence, the data support the glycerol 3-phosphate transporter-based homology model of PCFT and the presence of an extracellular gate formed by TMDs 1, 2, 7, and 11.

Evaluation of proton-coupled folate transporter (SLC46A1) polymorphisms as risk factors for neural tube defects and oral clefts

Many folate-related genes have been investigated for possible causal roles in neural tube defects (NTDs) and oral clefts. However, no previous reports have examined the major gene responsible for folate uptake, the proton-coupled folate transporter (SLC46A1). We tested for association between these birth defects and single nucleotide polymorphisms in the SLC46A1 gene. The NTD study population included 549 complete and incomplete case-family triads, and 999 controls from Ireland. The oral clefts study population comprised a sample from Utah (495 complete and incomplete case-family triads and 551 controls) and 221 Filipino multiplex cleft families. There was suggestive evidence of increased NTD case risk with the rs17719944 minor allele (odds ratio (OR): 1.29; 95% confidence intervals (CI): [1.00-1.67]), and decreased maternal risk of an NTD pregnancy with the rs4795436 minor allele (OR: 0.62; [0.39-0.99]). In the Utah sample, the rs739439 minor allele was associated with decreased case risk for cleft lip with cleft palate (genotype relative risk (GRR): 0.56 [0.32-0.98]). Additionally, the rs2239907 minor allele was associated with decreased case risk for cleft lip with cleft palate in several models, and with cleft palate only in a recessive model (OR: 0.41; [0.20-0.85]). These associations did not remain statistically significant after correcting for multiple hypothesis testing. Nominal associations between SLC46A1 polymorphisms and both Irish NTDs and oral clefts in the Utah population suggest some role in the etiology of these birth defects, but further investigation in other populations is needed.

New insights into the metabolic and nutritional determinants of severe combined immunodeficiency

Human mutations in MTHFD1 have recently been identified in patients with severe combined immunodeficiency (SCID). SCID results from inborn errors of metabolism that cause impaired T- and B-cell proliferation and function. One of the most common causes of SCID is adenosine deaminase (ADA) deficiency, which ultimately inhibits DNA synthesis and cell division. MTHFD1 has been shown to translocate to the nucleus during S-phase of the cell cycle; this localization is critical for synthesis of thymidyate (dTMP or the “T” base in DNA) and subsequent progression through the cell cycle and cell proliferation. Identification of MTHFD1 mutations that are associated with SCID highlights the potential importance of adequate dTMP synthesis in the etiology of SCID.

Identification of putative SNPs in progressive retinal atrophy affected Canis lupus familiaris using exome sequencing

Progressive retinal atrophy (PRA) is one of the major causes of retinal photoreceptor cell degeneration in canines. The inheritance pattern of PRA is autosomal recessive and genetically heterogeneous. Here, using targeted sequencing technology, we have performed exome sequencing of 10 PRA-affected (Spitz=7, Cocker Spaniel=1, Lhasa Aphso=1 and Spitz-Labrador cross breed=1) and 6 normal (Spitz=5, Cocker Spaniel=1) dogs. The high-throughput sequencing using 454-Roche Titanium sequencer generated about 2.16 Giga bases of raw data. Initially, we have successfully identified 25,619 single nucleotide polymorphisms (SNPs) that passed the stringent SNP calling parameters. Further, we performed association study on the cohort, and the highly significant (0.001) associations were short-listed and investigated in-depth. Out of the 171 significant SNPs, 113 were previously unreported. Interestingly, six among them were non-synonymous coding (NSC) SNPs, which includes CPPED1 A>G (p.M307V), PITRM1 T>G (p.S715A), APP G>A (p.T266M), RNF213 A>G (p.V1482A), C>A (p.V1456L), and SLC46A3 G>A (p.R168Q). On the other hand, 35 out of 113 unreported SNPs were falling in regulatory regions such as 3'-UTR, 5'-UTR, etc. In-depth bioinformatics analysis revealed that majority of NSC SNPs have damaging effect and alter protein stability. This study highlighted the genetic markers associated with PRA, which will help to develop genetic assay-based screening in effective breeding.

Determinants of the activities of antifolates delivered into cells by folate-receptor-mediated endocytosis

PURPOSE

Elements in the endocytic process that are determinants of the activities of antifolates delivered by folate-receptor alpha (FRα) were explored.

METHODS

Antifolate growth inhibition was assessed with a 1- or 5-day exposure in reduced folate carrier-null HeLa cell lines that express a high level of FRα in the presence or absence of the proton-coupled folate transporter (PCFT). pH-dependent rates of dissociation from FRα were also determined.

RESULTS

With a 1-day drug exposure which is relevant to the pulse clinical administration of these drugs, FRα expression enhanced raltitrexed activity and modestly enhanced ZD9331 activity, but did not significantly augment the activity of pemetrexed or lomotrexol. With a 5-day drug exposure, FRα-mediated growth inhibition was increased for raltitrexed and ZD9331 and emerged for lomotrexol. While the FRα-augmented activity of lomotrexol and raltitrexed did not require PCFT, augmentation of ZD9331 activity required the co-expression of PCFT with both 1- and 5-day exposures. In contrast, there was no augmentation of pemetrexed activity by FRα under any condition. The activities of these agents correlated with their rate of dissociation from the receptor at acidic pH: raltitrexed > ZD9331 > lomotrexol > pemetrexed consistent with insufficient pemetrexed release from FRα for export from the endosomes.

CONCLUSIONS

FRα is unlikely to contribute to the pharmacological activity of antifolates, such as pemetrexed, that bind tightly to, and dissociate slowly from, the receptor particularly when the exposure time is brief. While PCFT was required for FRα-mediated ZD9931 activity, the activities of the other antifolates was independent of PCFT.

Novel insights in folate receptors and transporters: implications for disease and treatment of immune diseases and cancer

Folate receptors and transporters as well as folate enzymes play an essential role in human disease and form important targets for the treatment of immune diseases and cancer. To discuss new developments in this area, every 2 years a multidisciplinary meeting is held, which aims to be an informal forum for fundamental scientists and clinicians. During this meeting, the regulation of folate transporters and folate enzymes is discussed at the level of expression, transcription, translation, post-translational modification, and splicing and enzyme regulation. Importantly, this knowledge is applied and translated into exciting clinical applications by clinicians with various backgrounds, such as surgeons, nephrologists, rheumatologists and oncologists. Moreover, the meeting provides an excellent forum for a scientific interaction between academia and industry.

CSF 5-Methyltetrahydrofolate Serial Monitoring to Guide Treatment of Congenital Folate Malabsorption Due to Proton-Coupled Folate Transporter (PCFT) Deficiency

Hereditary folate malabsorption is characterized by folate deficiency with impaired folate transport into the central nervous system (CNS). This disease is characterized by megaloblastic anemia of early appearance, combined immunodeficiency, seizures, and cognitive impairment. The anemia and immunologic disease are responsive but neurological signs are refractory to folic-acid treatment. We report a 7-year-old girl who has congenital folate deficiency and SLC46A1 gene mutation who is unable to transport folate from her gut to the circulatory system and consequently from the blood to the cerebrospinal fluid (CSF). As a result she developed undetectable 5-methyltetrahydrofolate levels in her plasma and CSF and became immunocompromised and quite ill. Intramuscular treatment with 5-formyltetrahydrofolate (folinic acid) was therapeutic at her presentation and has been successful preventing other signs and symptoms of hereditary folate malabsorption even at relatively low CSF levels. Although difficult, early detection and diagnosis of cerebral folate deficiency are important because folinic acid at a pharmacologic dose may normalize outcome in PCFT gene defects, as well as bypass autoantibody-blocked folate receptors and enter the cerebrospinal fluid by way of the reduced folate carrier. This route elevates the 5-methyltetrahydrofolate level within the central nervous system and can prevent the neuropsychiatric disorder. CSF levels of 5-methyltetrahydrofolate between 18 and 46 nmol/L may be sufficient to eradicate CNS disease.

Structural determinants of human proton-coupled folate transporter oligomerization: role of GXXXG motifs and identification of oligomeric interfaces at transmembrane domains 3 and 6

The human proton-coupled folate transporter (hPCFT) is expressed in solid tumours and is active at pHs characterizing the tumour microenvironment. Recent attention focused on exploiting hPCFT for targeting solid tumours with novel cytotoxic anti-folates. hPCFT has 12 transmembrane domains (TMDs) and forms homo-oligomers with functional significance. The hPCFT primary sequence includes GXXXG motifs in TMD2 (G(93)XXXG(97)) and TMD4 (G(155)XXXG(159)). To investigate roles of these motifs in hPCFT function, stability and surface expression, we mutated glycine to leucine to generate single or multiple substitution mutants. Only the G93L and G159L mutants preserved substantial [(3)H]methotrexate (Mtx) transport when expressed in hPCFT-null (R1-11) HeLa cells. Transport activity of the glycine-to-leucine mutants correlated with surface hPCFT by surface biotinylation and confocal microscopy with ECFP*-tagged hPCFTs, suggesting a role for GXXXG in hPCFT stability and intracellular trafficking. When co-expressed in R1-11 cells, haemagglutinin-tagged glycine-to-leucine mutants and His10-tagged wild-type (WT) hPCFT co-associated on nickel affinity columns, suggesting that the GXXXG motifs are not directly involved in hPCFT oligomerization. This was substantiated by in situ FRET experiments with co-expressed ECFP*- and YFP-tagged hPCFT. Molecular modelling of dimeric hPCFT structures showed juxtaposed TMDs 2, 3, 4 and 6 as potential structural interfaces between monomers. hPCFT cysteine insertion mutants in TMD3 (Q136C and L137C) and TMD6 (W213C, L214C, L224C, A227C, F228C, F230C and G231C) were expressed in R1-11 cells and cross-linked with 1,6-hexanediyl bismethanethiosulfonate, confirming TMD juxtapositions. Altogether, our results imply that TMDs 3 and 6 provide critical interfaces for formation of hPCFT oligomers, which might be facilitated by the GXXXG motifs in TMD2 and TMD4.

Stereoselective recognition of amethopterin enantiomers by the rat proton-coupled folate transporter

The stereoselective transport of methotrexate (L-amethopterin, L-MTX) and its enantiomer (D-amethopterin, D-MTX) by the rat proton-coupled folate transporter (rPCFT) were examined using rPCFT-expressing HEK293 cells. The initial rate of uptake of [3H]-L-MTX by the rPCFT followed Michaelis-Menten kinetics, with a Km value of 2.1 µM. Dixon plots revealed that the uptake of L-MTX by the rPCFT was inhibited in a competitive manner by unlabeled L-MTX and D-MTX, with Ki values of approximately 1.3 and 150 µM, respectively. The initial rate of uptake of D-MTX by the rPCFT also followed Michaelis-Menten kinetics, with a Km value of 190 µM. The results of the current study demonstrate that the different enantiomers of MTX are transported in a highly stereoselective manner by the rPCFT, with the uptake clearance of L-MTX being approximately 46-fold greater than that of D-MTX. The observed stereoselectivity of the rPCFT was found to be comparable with that of the human PCFT.

Identification of Tyr residues that enhance folate substrate binding and constrain oscillation of the proton-coupled folate transporter (PCFT-SLC46A1)

The proton-coupled folate transporter (PCFT) mediates intestinal folate absorption and transport of folates across the choroid plexus. This study focuses on the role of Tyr residues in PCFT function. The substituted Cys-accessibility method identified four Tyr residues (Y291, Y362, Y315, and Y414) that are accessible to the extracellular compartment; three of these (Y291, Y362, and Y315) are located within or near the folate binding pocket. When the Tyr residues were replaced with Cys or Ala, these mutants showed similar (up to 6-fold) increases in influx Vmax and Kt/Ki for [(3)H]methotrexate and [(3)H]pemetrexed. When the Tyr residues were replaced with Phe, these changes were moderated or absent. When Y315A PCFT was used as representative of the mutants and [(3)H]pemetrexed as the transport substrate, this substitution did not increase the efflux rate constant. Furthermore, neither influx nor efflux mediated by Y315A PCFT was transstimulated by the presence of substrate in the opposite compartment; however, substantial bidirectional transstimulation of transport was mediated by wild-type PCFT. This resulted in a threefold greater efflux rate constant for cells that express wild-type PCFT than for cells that express Y315 PCFT under exchange conditions. These data suggest that these Tyr residues, possibly through their rigid side chains, secure the carrier in a high-affinity state for its folate substrates. However, this may be achieved at the expense of constraining the carrier's mobility, thereby decreasing the rate at which the protein oscillates between its conformational states. The Vmax generated by these Tyr mutants may be so rapid that further augmentation during transstimulation may not be possible.

The first Chinese case report of hereditary folate malabsorption with a novel mutation on SLC46A1

BACKGROUND

Hereditary folate malabsorption is a rare, autosomal recessive disorder of proton-coupled folate transporter deficiency resulting in folate deficiency. Left untreated, the condition can cause severe brain damage and megaloblastic anemia, leading to progressive psychomotor retardation, seizures and other neurological problems. Early diagnosis and treatment are crucial. No case has been documented yet in Mainland China until now.

METHODS

A Chinese girl affected by hereditary folate malabsorption was studied. The girl presented with recurrent megaloblastic anemia from the age of 7 months. Paroxysmal limbs trembling and seizures were presented from the age of three years. Intracranial calcification was noted by CT. At her age of 5 years, mental regression, lower-extremity weakness and sleeping problems were observed. Her plasma folate decreased to 4.49 nmol/L (normal control>6.8nmol/L). Plasma total homocysteine elevated to 28.11 μmol/L (normal control<15 μmol/L). Folate and 5-methylterahydrofolate in cerebrospinal fluid were significantly decreased to undetectable level.

RESULTS

On SLC46A1 gene, a novel mutation, c.1A>T (M1L), and a reported mutation c.194-195 insG (p.Cys66LeufsX99) were identified, supported the diagnosis of hereditary folate malabsorption. Each parent carries one of two mutations. Folinic calcium supplement resulted in rapid clinical improvement. She is currently 6 years old with normal development and routine blood features.

CONCLUSION

Hereditary folate malabsorption is one of the few easily-treatable inherited metabolic diseases. Measurements of folate and 5-methyltetrahydrofolate in cerebrospinal fluid are keys for the diagnosis of the patients.

The intestinal absorption of folates

The properties of intestinal folate absorption were documented decades ago. However, it was only recently that the proton-coupled folate transporter (PCFT) was identified and its critical role in folate transport across the apical brush-border membrane of the proximal small intestine established by the loss-of-function mutations identified in the PCFT gene in subjects with hereditary folate malabsorption and, more recently, by the Pcft-null mouse. This article reviews the current understanding of the properties of PCFT-mediated transport and how they differ from those of the reduced folate carrier. Other processes that contribute to the transport of folates across the enterocyte, along with the contribution of the enterohepatic circulation, are considered. Important unresolved issues are addressed, including the mechanism of intestinal folate absorption in the absence of PCFT and regulation of PCFT gene expression. The impact of a variety of ions, organic molecules, and drugs on PCFT-mediated folate transport is described.

Inborn errors of metabolism underlying primary immunodeficiencies

A number of inborn errors of metabolism (IEM) have been shown to result in predominantly immunologic phenotypes, manifesting in part as inborn errors of immunity. These phenotypes are mostly caused by defects that affect the (i) quality or quantity of essential structural building blocks (e.g., nucleic acids, and amino acids), (ii) cellular energy economy (e.g., glucose metabolism), (iii) post-translational protein modification (e.g., glycosylation) or (iv) mitochondrial function. Presenting as multisystemic defects, they also affect innate or adaptive immunity, or both, and display various types of immune dysregulation. Specific and potentially curative therapies are available for some of these diseases, whereas targeted treatments capable of inducing clinical remission are available for others. We will herein review the pathogenesis, diagnosis, and treatment of primary immunodeficiencies (PIDs) due to underlying metabolic disorders.

Substituted cysteine accessibility reveals a novel transmembrane 2-3 reentrant loop and functional role for transmembrane domain 2 in the human proton-coupled folate transporter

The proton-coupled folate transporter (PCFT) is a folate-proton symporter highly expressed in solid tumors that can selectively target cytotoxic antifolates to tumors under acidic microenvironment conditions. Predicted topology models for PCFT suggest that the loop domain between transmembrane domains (TMDs) 2 and 3 resides in the cytosol. Mutations involving Asp-109 or Arg-113 in the TMD2-3 loop result in loss of activity. By structural homology to other solute carriers, TMD2 may form part of the PCFT substrate binding domain. In this study we mutated the seven cysteine (Cys) residues of human PCFT to serine, creating Cys-less PCFT. Thirty-three single-Cys mutants spanning TMD2 and the TMD2-3 loop in a Cys-less PCFT background were transfected into PCFT-null HeLa cells. All 33 mutants were detected by Western blotting, and 28 were active for [(3)H]methotrexate uptake at pH 5.5. For the active residues, we performed pulldown assays with membrane-impermeable 2-aminoethyl methanethiosulfonate-biotin and streptavidin beads to determine their aqueous-accessibilities. Multiple residues in TMD2 and the TMD2-3 loop domain reacted with 2-aminoethyl methanethiosulfonate-biotin, establishing aqueous accessibilities. Pemetrexed pretreatment inhibited biotinylation of TMD2 mutants G93C and F94C, and biotinylation of these residues inhibited methotrexate transport activity. Our results suggest that the TMD 2-3 loop domain is aqueous-accessible and forms a novel reentrant loop structure. Residues in TMD2 form an aqueous transmembrane pathway for folate substrates, and Gly-93 and Phe-94 may contribute to a substrate binding domain. Characterization of PCFT structure is essential to understanding the transport mechanism including the critical determinants of substrate binding.