Most cells rely on aspartic acid synthesized autonomously by cells. Aspartic acid synthesis and depletion is controlled by two reversible reactions – its synthesis is achieved in the mitochondria by glutamic acid and oxaloacetic acid via glutamic acid-oxaloacetic acid transaminase 2 (GOT2), which after entering the cytoplasm is consumed by glutamate-oxaloacetic acid aminotransferase 1 (GOT1). Electron transport chain can promote the synthesis of aspartic acid by promoting the production of oxaloacetic acid in the tricarboxylic acid (TCA) cycle, and it has been found that in the case of the lack of function of the electron transport chain, the addition of exogenous aspartic acid can save the proliferation of cancer cells, and supplementation of nucleosides or asparagine to cells can also partially eliminate this restriction [1-3].
Studies have shown that one of the most important functions of electron transport chains is to promote aspartic acid synthesis, which can be used by cells to synthesize proteins, other amino acids (arginine, asparagine, glutamate and N-acetyltrate) and nucleotides (purines and pyrimidine), in addition, aspartic acid can also promote TCA. Aspartic acid synthesis is not only important for the proliferation of cancer cells, but also for some cells with normal proliferation, including T cells and lung endothelial cells. Aspartic acid-glutamate vector 1 (transporting aspartic acid from the mitochondria to the cytoplasm) is required for oligodendrocyte progenitor proliferation. In addition, studies have found that the function of stem cells (including hematopoietic stem cell HSCs) usually requires the activity of electron transport chains, but this is not always the case, and mitochondrial function seems to have complex effects in different stem cells. Some stem cells express the glutamate/aspartate transporter GLAST (Slc1a3), including neural stem cells, which enable them to absorb extracellular aspartic acid. Knockdown of malate dehydrogenase 1 (a component of malate-aspartate shuttle enzyme) reduces the recombinant capacity of Irradiated fetal liver HSCs in irradiated mice. So we have to ask: Do aspartic acid levels limit the function of normal stem cells?
On August 26, 2021, Sean J. Morrison's team from the University of Texas Southwestern Medical Center published an article titled Aspartate availability limits hematopoietic stem cell function during hematopoietic regeneration at Cell Stem Cell. It was demonstrated that hematopoietic stem cells (HSCs) were completely dependent on aspartic acid synthesized autonomously by cells, and that the function of HSCs was limited by the availability of aspartic acid. During regeneration, HSCs can increase the synthesis of aspartic acid, thereby promoting the function of HSCs by increasing the synthesis of asparagine and purines.
The researchers found that, contrary to the endothelial protein C receptor (Epcr, also known as PRORC) expressed in HSCs and pluripotent progenitor cells (MPP), the expression of aspartic acid transporters [Slc1a1, Slc1a2, Slc1a3 (Glast), Slc1a6, and Slc1a7] was almost undetectable in HSCs or other bone marrow hematopoietic progenitor cells (HPCs). Therefore, the researchers first constructed a mouse model of GLAST overexpression (GLAST-OE), culturing wild-type or GLAST-OE HSCs for 7 days under conditions that promote HSC self-renewal, and supplementing physiological levels with 13C-labeled aspartic acid to observe the absorption of aspartic acid by hematopoietic cells. It was found that wild-type HSCs/HPCs were almost unable to absorb aspartic acid, but the overexpression of GLAST conferred on them this ability, and the overexpression of GLIST in hematopoietic cells increased the level of aspartic acid in HSC/MPP and the amount of HSC in the bone marrow, while increasing the function of HSCs in the process of hematopoietic regeneration. At the same time, the researchers constructed a mouse model of hematopoietic cell conditional knockout got1, and the results showed that although there were no significant differences in the number of hematopoietic stem cells, MPP, HPC, restrictive progenitor cells, differentiated cells, and colony-forming progenitor cells, the lack of Got1 increased aspartic acid levels and hematopoietic stem cell function during hematopoietic regeneration. In addition, the study also found that aspartic acid synthesis increased when HSC was activated, and aspartic acid synthesis increased in rapidly dividing myeloid progenitor cells compared with resting HSC, suggesting that the rate of aspartic acid synthesis in HSC/HPC varied with anabolic needs. Subsequently, the researchers constructed a mouse model of conditioned knockout got2 and found that, contrary to GLAD overexpression and Got1 deletion, the lack of Got2 reduced aspartic acid levels and hematopoietic stem cell function during hematopoietic regeneration. This shows that during hematopoietic regeneration, aspartic acid levels limit the function of hematopoietic stem cells.
Using GLIST overexpressed HSCs, isotope tracking experiments have found that in resting or proliferating HSCs, aspartic acid levels have no effect on protein synthesis, but exogenous aspartic acids can promote TCA cycling or be used to synthesize nucleotides and asparagine. At the same time, the deficiency of Got1 significantly reduced the enrichment of the aspartate fraction in the intermediate product of the TCA cycle, suggesting that the deficiency of Got1 reduced the carbon entering the TCA cycle from aspartic acid. However, the researchers found that the effect of aspartic acid on TCA cycling did not increase the number or function of HSCs. Similarly, the experimental results show that GRASS does not promote HSC function by increasing the synthesis of pyrimidine or N-acetyl aspartic acid (NAA). So how exactly does aspartic acid affect HSC function? By constructing a mouse model of asparagine synthase deletion, the researchers found that after transplantation, the deletion of asparagine synthase significantly reduced the level of donor bone marrow cells, B cells and T cells and stem cells/progenitor cells in the blood of recipient mice, and also reduced the remodeling of GRASS overexpressed cells during secondary transplantation, thus indicating that the synthesis of asparagine helped to improve the function of GLAST overexpressed hematopoietic stem cells. During hematopoietic regeneration, the level of asparagine limits the function of HSCs, and the synthesis or uptake of exogenous asparagine can enhance it. Similarly, irradiated mice transplanted with control or GLAST overexpressed bone marrow cells were given 6-mercaptopurine (6MP, an inhibitor of purine synthesis), which significantly reduced the frequency of donor cells produced by GLAST overexpression bone marrow cells in the recipient's blood and bone marrow compared with the control, suggesting that the synthesis of purines also participated in the effect of GLIST overexpression on HSC function. Moreover, in the process of hematopoietic regeneration, in addition to the overexpressed HSCs of GRASS, purine synthesis can also promote the function of wild-type HSCs.
In summary, this study found that after HSC activation, aspartic acid synthesis increased, and in the process of hematopoietic regeneration, aspartic acid levels limited the function of HSC, while HSC can increase the synthesis of aspartic acid, asparagine and purine in the hematopoietic regeneration process to meet increased anabolic demand.
In situ link:
https://doi.org/10.1016/j.stem.2021.07.011
bibliography
1. Birsoy, K., Wang, T., Chen, W.W., et al. (2015). An Essential Role of the Mitochondrial Electron Transport Chain in Cell Proliferation Is to Enable Aspartate Synthesis. Cell 162, 540–551.
2. Sullivan, L.B., Luengo, A., Danai, L.V., et al. (2018). Aspartate is an endogenous metabolic limitation for tumour growth. Nat. Cell Biol. 20, 782–788.
3. Krall, A.S., Mullen, P.J., Surjono, F., et al. (2021). Asparagine couples mitochondrial respiration to ATF4 activity and tumor growth. Cell Metab. 33, 1013–1026.e6.