Extracellular glycine is necessary for optimal hemoglobinization of erythroid cells

Abstract

Vertebrate heme synthesis requires three substrates: succinyl-CoA, which regenerates in the tricarboxylic acid cycle, iron and glycine. For each heme molecule synthesized, one atom of iron and eight molecules of glycine are needed. Inadequate delivery of iron to immature erythroid cells leads to a decreased production of heme, but virtually nothing is known about the consequence of an insufficient supply of extracellular glycine on the process of hemoglobinization. To address this issue, we exploited mice in which the gene encoding glycine transporter 1 (GlyT1) was disrupted. Primary erythroid cells isolated from fetal livers of GlyT1 knockout (GlyT1 −/− ) and GlyT1-haplodeficient (GlyT1 +/− ) embryos had decreased cellular uptake of [2 −14 C]glycine and heme synthesis as revealed by a considerable decrease in [2- 14 C]glycine and 59 Fe incorporation into heme. Since GlyT1 −/− mice die during the first postnatal day, we analyzed blood parameters of newborn pups and found that GlyT1 −/− animals develop hypochromic microcytic anemia. Our finding that Glyt1-deficiency causes decreased heme synthesis in erythroblasts is unexpected, since glycine is a non-essential amino acid. It also suggests that GlyT1 represents a limiting step in heme and, consequently, hemoglobin production.

Enhanced phosphocholine metabolism is essential for terminal erythropoiesis

“…37 Although proteins such as glycine transporter 1 (GlyT1) and Band3 have known glycine-transporting properties, the mechanism by which de novo glycine synthesis contributes to the glycine supply is unclear. 38 Our results demonstrate that PHOSPHO1 and the glycolytic pathway both contribute to de novo glycine synthesis. Interestingly, 2-DG has also been reported to promote erythropoiesis by shunting glycolysis to the pentose phosphate pathway (PPP), 31 illustrating the flexibility of glycolysis and the Figure 5 (continued) Enzymes involved in the steps are labeled.…”

Glutamine via α-ketoglutarate dehydrogenase provides succinyl-CoA for heme synthesis during erythropoiesis

“…Fate of 13 C-labeled metabolic heme precursors during erythroid differentiation Protoheme IX has a total of 34 carbons with 26 of these derived from succinyl-CoA and 8 carbons derived from glycine ( Figure 1B-C). Assuming that glycine for heme synthesis is exogenously supplied, 22,23 then 76% of the heme carbons originate from a source that produces succinyl-CoA ( Figure 1A). During late erythropoiesis, elevation of heme synthesis creates a significantly increased demand for succinyl-CoA believed to come from the TCA cycle.…”

“…20 Glycine is abundant in plasma (;250 mM), and studies of glycine transporters are consistent with glycine being supplied from extracellular sources. [21][22][23] In contrast, our knowledge of how succinyl-CoA is provided for heme synthesis during erythroid differentiation is not well understood. The assumption that the tricarboxylic acid (TCA) cycle supplies succinyl-CoA for erythroid heme synthesis has been made, but there is a paucity of data to support this view.…”

Bitopertin, a selective oral GLYT1 inhibitor, improves anemia in a mouse model of β-thalassemia

“…In erythroblasts, glycine uptake via the glycine carrier system, GlyT1, is a key determinant of the rate of heme biosynthesis and bioavailability (18,19). Indeed, mice genetically lacking GlyT1 display a hypochromic microcytic anemia associated with a 25% reduced mean corpuscular hemoglobin (MCH) and mean corpuscular volume (MCV), supporting the importance of GlyT1 in providing glycine for heme biosynthesis during erythroid maturation (16).…”

“…Previous studies have shown that modulation of heme homeostasis might affect erythropoiesis (16) and beneficially affect β-thal erythroid maturation (17). In erythroblasts, glycine uptake via the glycine carrier system, GlyT1, is a key determinant of the rate of heme biosynthesis and bioavailability (18,19).…”