Genetic disorders of homocysteine (Hcy) or folate metabolism or high-methionine diets elevate plasma Hcy and its atherogenic metabolite Hcy-thiolactone. In humans, severe hyperhomocysteinemia due to genetic alterations in cystathionine beta-synthase (Cbs) or methylenetetrahydrofolate reductase (Mthfr) results in neurological abnormalities and premature death from vascular complications. In mouse models, dietary or genetic hyperhomocysteinemia results in liver or brain pathological changes and accelerates atherosclerosis. Hcy-thiolactone has the ability to form isopeptide bonds with protein lysine residues, which generates modified proteins (N-Hcy-protein) with autoimmunogenic and prothrombotic properties. Our aim was to determine how N-Hcy-protein levels are affected by genetic or nutritional disorders in Hcy or folate metabolism in mice. We found that plasma N-Hcy-protein was elevated 10-fold in mice fed a high-methionine diet compared with the animals fed a normal commercial diet. We also found that inactivation of Cbs, Mthfr, or the proton-coupled folate transporter (Pcft) gene resulted in a 10- to 30-fold increase in plasma or serum N-Hcy-protein levels. Liver N-Hcy-protein was elevated 3.4-fold in severely and 11-fold in extremely hyperhomocysteinemic Cbs-deficient mice, 3.6-fold in severely hyperhomocysteinemic Pcft mice, but was not elevated in mildly hyperhomocysteinemic Mthfr-deficient animals, suggesting that mice have a capacity to prevent accumulation of N-Hcy-protein in their organs. These findings provide evidence that N-Hcy-protein is an important metabolite associated with Hcy pathophysiology in the mouse.

IntroductionFolate is an essential cosubstrate of many biochemical reactions, such as the de novo synthesis of purines and pyrimidines, methionine, and deoxythymidylate monophosphate. 1,2 Mammals cannot synthesize folate; therefore, inadequate dietary supply or folate malabsorption results in defective DNA synthesis. One of the first manifestations of a folate deficiency is in the rapidly proliferating cells of the hematopoietic system, leading to pancytopenia and anemia of the megaloblastic type. Patients with megaloblastic anemia demonstrate ineffective erythropoiesis by harboring large immature red blood cell (RBC) precursors that fail to survive to the postmitotic, terminal stages, undergoing premature apoptosis. 2 Three mammalian folate transporter systems have been described to date in a variety of tissues: (1) the bidirectional reduced folate carrier 1 (RFC1), also known as SLC19A1, 3,4 (2) the glycosyl-phosphatidylinositol-anchored folate receptors (FOLR1, FOLR2, and FOLR4) and one secreted receptor in humans without a mouse homolog (FOLR3), 5 and (3) the human proton coupled folate transporter (PCFT). 6-8 The RFC1 transporter is expressed ubiquitously, including the brush-border membrane of epithelial cells in the small intestine. 9 Although RFC1 is necessary for folate transport in erythroid cells, its involvement in intestinal folate uptake has not been confirmed. 6 Inactivation of RFC1 in mice by homologous recombination led to either embryonic lethality or defective erythropoiesis in pups born to mothers who were supplemented with 1 mg daily subcutaneous doses of folic acid. 10,11 Because this transporter functions at a neutral pH optimum whereas the majority of intestinal folate transport occurs in an acidic luminal milieu, RFC1 is an unlikely candidate for an intestinal folate transporter. 7 The role of FOLR1 in folate absorption and transport has also been demonstrated, where a 60% to 70% reduction was observed in plasma folate of FOLR1 Ϫ/Ϫ mice fed low folate and normal chow. 12 FOLR1 also regulates folate homeostasis via endocytotic mechanisms during embryonic development, and mice rendered null for this receptor display severe morphogenetic abnormalities and die in utero unless provided supraphysiologic concentrations of either folinic acid or 5-methyltetrahydrofolate. 5,13 The PCFT transporter is highly expressed in tissues involved in folate and heme transport, including the duodenum and liver. 6,14 Initially, PCFT was identified as a low-affinity, pHindependent heme transporter 14 and then later described to function as a low pH-dependent folate transporter in intestinal cells. 6 The latter role of the transporter was confirmed by the identification of loss-of-function mutations in the human PCFT gene in persons diagnosed with hereditary folate malabsorption syndrome. 6 Studies have also indicated that PCFT facilitates folate transport during folate receptor-mediated endocytosis, where FOLR1 binds folate, and its export into the cytosol is driven by PCFT activity as the vesicle undergoes en...

The low-density lipoprotein (LDL) receptor-related protein 2 (LRP2) is a multifunctional cell-surface receptor expressed in the embryonic neuroepithelium. Loss of LRP2 in the developing murine central nervous system (CNS) causes impaired closure of the rostral neural tube at embryonic stage (E) 9.0. Similar neural tube defects (NTDs) have previously been attributed to impaired folate metabolism in mice. We therefore asked whether LRP2 might be required for the delivery of folate to neuroepithelial cells during neurulation. Uptake assays in whole-embryo cultures showed that LRP2-deficient neuroepithelial cells are unable to mediate the uptake of folate bound to soluble folate receptor 1 (sFOLR1). Consequently, folate concentrations are significantly reduced in Lrp2 2/2 embryos compared with control littermates. Moreover, the folic-aciddependent gene Alx3 is significantly downregulated in Lrp2 mutants. In conclusion, we show that LRP2 is essential for cellular folate uptake in the developing neural tube, a crucial step for proper neural tube closure.

Periconceptional folic acid can reduce the occurrence of neural tube defects (NTDs) by up to 70%, and autoantibodies for folate receptors (FRs) have been observed in serum from women with a pregnancy complicated by an NTD. This population-based cohort study has examined serum from pregnant mothers for autoantibodies to FRs, antibodies to bovine folate binding protein (FBP), and inhibition of folic acid binding to FR and FBP in association with NTD risk. The mid-gestational maternal serum specimens used for this study were collected during the 15th–18th week of pregnancy. Samples were obtained from the California Birth Defects Monitoring Program; 29 mothers had a pregnancy complicated by spina bifida and 76 mothers had unaffected children. The presence of IgG and IgM antibodies to human FR, bovine FBP, and inhibition of folic acid binding to FR and FBP was determined. Higher activity of IgM to FBP in cases verses controls was observed (P=.04). Higher activity of IgM and IgG autoantibodies to FR was observed (P<0.001 and P=.04, respectively). Risk estimates at two standard deviations above average control antibody concentrations were OR=2.07 (CI=1.02, 4.06) for anti-FBPIgM, OR=2.15 (CI=1.02, 4.69) for anti-FRIgG and OR=3.19 (CI=1.47, 6.92) for anti-FR IgM. These data support the hypothesis that high titers of antibodies and blocking of folic acid binding to FRs by maternal serum should be regarded as risk factors for NTDs.

Neural tube defects (NTDs) are serious malformations affecting approximately 1 per 1000 births, yet the mechanisms by which they arise are unknown. There have been consistent efforts in many fields of research to elucidate the etiology of this multifactorial condition. While no single gene has been identified as a major independent risk factor for NTDs, candidate genes have been proposed that may modify the effects of maternal and/or embryonic exposures. Folate supplementation effectively reduces the occurrence of NTDs and, consequently, has focused much research on metabolism of folate-related pathways during pregnancy and development. Further understanding of normal development and how teratogens can perturb these orchestrated processes also remains at the fore of modern scientific endeavors. The composite of these factors remains fragmented; the aim of this review is to provide the reader with a summary of sentinel and current works in the body of literature addressing NTD disease etiology.

The membrane receptor (TCblR/CD320) for transcobalamin (TC)-bound cobalamin (Cbl) facilitates the cellular uptake of Cbl. A genetically modified mouse model involving ablation of the CD320 gene was generated to study the effects on cobalamin homeostasis. The nonlethal nature of this knockout and the lack of systemic cobalamin deficiency point to other mechanisms for cellular Cbl uptake in the mouse. However, severe cobalamin depletion in the central nervous system (CNS) after birth (P<0.01) indicates that TCblR is the only receptor responsible for Cbl uptake in the CNS. Metabolic Cbl deficiency in the brain was evident from the increased methylmalonic acid (P<0.01-0.04), homocysteine (P<0.01), cystathionine (P<0.01), and the decreased S-adenosylmethionine/S-adenosyl homocysteine ratio (P<0.01). The CNS pathology of Cbl deficiency seen in humans may not manifest in this mouse model; however, it does provide a model with which to evaluate metabolic pathways and genes affected.

We tested the hypothesis that in utero exposure to heavy metals increases autism-like behavioral phenotypes in adult animals and induces epigenetic changes in genes that have roles in the etiology of autism. Mouse dams were treated with cadmium, lead, arsenate, manganese, and mercury via drinking water from gestational days (E) 1–10. Valproic acid (VPA) injected intraperitoneally once on (E) 8.5 served as a positive control. Young male offspring were tested for behavioral deficits using four standardized behavioral assays. In this study, in utero exposure to heavy metals resulted in multiple behavioral abnormalities that persisted into adulthood. VPA and manganese induced changes in perseverative/impulsive behavior and social dominance behavior, arsenic caused changes only in perseverative/impulsive behavior, and lead induced abnormalities in social interaction in comparison to the control animals. Brain samples from Mn, Pb, and VPA treated and control animals were evaluated for changes in CpG island methylation in promoter regions and associated changes in gene expression. The Chd7 gene, essential for neural crest cell migration and patterning, was found to be hypomethylated in each experimental animal tested compared to water-treated controls. Furthermore, distinct patterns of CpG island methylation yielded novel candidate genes for further investigation.

Human immunodeficiency virus (HIV) integrase inhibitors are increasingly being used for antiretroviral therapy (ART), and dolutegravir (DTG/Tivicay) has emerged as a leading core agent. In 2018, the Tsepamo study reported a 6-to 9-fold increase for neural tube defect (NTD) risk among the offspring of mothers receiving DTG during early gestation. Maternal folate (vitamin B9) status is the largest known modifier of NTD risk, so we evaluated folate-related mechanisms of action and the critical period for DTG developmental toxicity. Folate receptor (FOLR1) binding studies indicate DTG is a non-competitive FOLR1 antagonist at therapeutic concentrations. In vitro testing indicates calcium (2mM) increases FOLR1-folate interactions and alters DTG-FOLR1-folate interactions and cytotoxicity. DTG does not inhibit downstream folate metabolism by dihydrofolate reductase (DHFR). Early embryonic exposure toDTG is developmentally toxic in zebrafish, and supplemental folic acid can mitigate DTG developmental toxicity. The results from these studies are expected to inform and guide future animal models and clinical studies of DTG-based ART in women of childbearing age. The human immunodeficiency virus and acquired immune deficiency syndrome (HIV/AIDS) epidemic remains a prominent public health challenge. Globally, there are approximately 36.7 million people living with HIV, with 2.1 million new infections occurring annually, resulting in 1.1 million AIDS-related deaths per year 1 . Current HIV antiretroviral therapy (ART) includes C-C chemokine receptor type 5 (CCR5) antagonists, fusion inhibitors, integrase inhibitors, nucleoside and non-nucleoside reverse transcriptase inhibitors (NRTIs and NNRTIs), and protease inhibitors. Dolutegravir (DTG) is the clinically preferred integrase inhibitor used alone (Tivicay, ViiV Healthcare), or in combination (e.g. Triumeq, ViiV Healthcare) as ART for HIV infection in adults and children. A teratogenic risk for DTG was recently reported after neural tube defects (NTDs) were observed in four infants from mothers who had been taking DTG at the time of conception in Botswana 2 . The World Health Organization (WHO) subsequentlyprovided guidelines that DTG use be avoided by women of childbearing potential unless they used adequate contraception methods 3 . Based on this cohort study, the current guidelines are warranted and prudent, but more evidence is needed before scientific and medical communities can support a causal relationship between DTG and developmental toxicity, including NTDs. Developmental Toxicity of DTG:There is increasing usage of the HIV integrase inhibitors are for ART, and DTG has emerged as the leading compound in this class of medications. The DTG/Tivicay manufacturer reports (US FDA approved Tivicay Prescribing Information, 09/2018) animal reproduction studies showed no evidence of adverse developmental outcomes. Specifically, DTG was administered orally at up to 1,000 mg per kg daily, in rats and rabbits, during the period of organogenesis, days 6-17 and 6-18, respectively...