Circulating Primitive Erythroblasts in the Murine Embryo Undergo Complex Proteomic and Metabolomic Changes during Terminal Maturation

Primitive erythropoiesis is a critical component of the fetal cardiovascular network and is essential for growth and survival of the mammalian embryo. The need to rapidly establish a functional cardiovascular system in the post-implantation murine embryo is met, in part, by the intravascular circulation of primitive erythroid precursors that mature as a single semi-synchronous cohort. To better understand the processes that regulate erythroid precursor maturation, we analyzed the proteome, metabolome and lipidome of primitive erythroblasts isolated from embryonic day (E)10.5 and E12.5 of mouse gestation, which represents their transition from basophilic erythroblast (BasoE) to orthochromatic erythroblast (OrthoE) stages of maturation. Previous transcriptional and biomechanical characterization of these precursors has highlighted a transition towards the expression of protein elements characteristic of mature red blood cell structure and function, including initial formation of a functional erythroid-specific cytoskeleton. Our analysis confirmed a loss of organelle-specific protein components - from mRNA maturation to translation, from metabolism to protein degradation - including ribosome, spliceosome, mitochondria and proteasomes. In parallel, we observed a metabolic rewiring towards the pentose phosphate pathway, glycolysis and the Rapoport-Luebering shunt, which is consistent with the progressively decreasing steady state levels of carboxylic acids, acylcarnitines and free fatty acids along with decreased mitochondrial content. In addition, activation of the pentose phosphate pathway in particular may stem from increases in the expression of mature red blood cell components including hemoglobin chains and band 3 that control oxygen-dependent metabolic modulation. Furthermore, this metabolic reprogamming may reflect increased control of oxidative stress in line with increased expression of several antioxidant enzymes, including catalase, peroxiredoxins and glutathione peroxidase 4. On the other hand, accumulation of oxylipins and cholesteryl esters in primitive OrthoE cells was paralleled by increased transcript levels of the p53-regulated cholesterol transporter (ABCA1) and decreased transcript levels of cholesterol synthetic enzymes including mevalonate kinase (MVK) and phosphomevalonate kinase (PVMK), highlighting transcriptional control of metabolic reprogramming during this developmental process. The present study characterizes the extensive metabolic rewiring that occurs in primary embryonic erythroid precursors as they prepare to enucleate and continue circulating without internal organelles.