Metabolic role of the mitochondrial NAD-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase

Folate-dependent enzymes are compartmentalized between the cytoplasm and mitochondria of eukaryotic cells. The cytoplasm contains an NADP-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase-formyltetrahydrofolate synthetase that is involved in interconverting one-carbon substituted tetrahydrofolates that play essential roles in thymidylate and purine biosynthesis as well as in methionine regeneration. The extent of contribution of mitochondrial folate metabolism to cytoplasmic processes is an area of active investigation. A bifunctional NAD-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase (NMDMC) is located in mitochondria of certain mammalian cells. NMDMC expression in embryonic tissues and established cell lines but not in most adult tissues suggests that it plays a role in embryonic development. Examination of the expression pattern revealed that nmdmc is widely expressed in tissues of embryonic day (E)9.5 and E10.5 embryos. To determine its metabolic role, a knockout approach was used to disrupt the gene encoding NMDMC in mice. Heterozygous mice appear normal but nmdmc-/- embryos die in utero beyond E13.5. Nmdmc-/- fetal livers at E12.5 are smaller in size, lack the characteristic red coloring of a normal liver, but contain similar frequencies of hematopoietic progenitors as do the livers of wild type or heterozygous littermates. This indicates that the failure to establish definitive erythropoiesis in nmdmc-/- embryos is due to the inability of the hematopoietic progenitors to proliferate. Established fibroblasts were generated from wild type, heterozygous and null mutant embryos. Mitochondria of nmdmc-/- cells are structurally intact, retain a membrane potential and do not require NMDMC to support the initiation of protein synthesis. Null mutant fibroblasts are glycine auxotrophs which demonstrates that NMDMC is the only dehydrogenase/cyclohydrolase (synt