A mouse model of hereditary folate malabsorption: deletion of the PCFT gene leads to systemic folate deficiency

Abstract

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...

Choroid plexus transcytosis and exosome shuttling deliver folate into brain parenchyma

“…Further experimental studies are required to delineate the cooperative functioning of FRa together with the PCFT. PCFTnull mice 31 as well as FRa-deficient mice 32 that can both be rescued by prenatal and/or postnatal folate supplementation provide important tools to substantiate and elaborate the transmembrane transport and the intracellular routing of folates.…”

Mitochondrial heme: an exit strategy at last

“…Furthermore, in vivo investigation of transporter proteins identified in vitro or on the basis of their expression pattern has been misleading or has provided ambiguous results. For example, Hcp1, a putative apical intestinal heme importer, was eventually proven to be essential for folate transport in the intestine when it was found to be mutated in patients with congenital folate deficiency (8,9). Whether Hcp1 also contributes to the absorption of heme in the intestine is uncertain at this time.…”

Iron homeostasis: An anthropocentric perspective

“…The folate transporter proton-coupled folate transporter (PCFT) 2 (16) has been suggested as a candidate heme transporter (17), but its low affinity for heme and the poor absorption of heme in mice (3), despite abundant PCFT in the duodenum (18), have cast doubt on the role of PCFT in heme absorption.…”