Hematopoiesis is the process by which various types of mature blood cells with highly specialized functions are formed from HSCs1. HSCs are capable of self-renewal and differentiation into various multipotent and lineage-specific progenitor populations. These progenitors ultimately produce cells of lymphoid, myeloid, erythroid, and megakaryocyte lineages. To maintain their self-renewal capacity, HSCs remain largely quiescent and, like other tissue stem cells, are believed to rely on glycolysis rather than mitochondrial OxPHOS for ATP production2,3. Entry into the cell cycle leads to enhanced respiration and OxPHOS, resulting in elevated levels of reactive oxygen species (ROS), which are detrimental to HSC maintenance and function3. Repeated cell division thus may lead to reduced self-renewal capacity of HSCs and ultimately to their exhaustion.
In adult hematopoiesis, HSCs primarily undergo asymmetric cell division, during which one of the daughter cells retains HSC potential and continues to rely on glycolytic metabolism. The other daughter cell becomes a primitive progenitor cell that loses self-renewal capacity but proliferates and eventually gives rise to differentiated functional hematopoietic cells4. When HSCs differentiate to produce downstream progenitors, a switch from glycolysis to mitochondrial metabolism is thought to occur to supply the energy and building blocks needed to support this rapid transition5, as suggested by the observations that HSCs possess inactive mitochondrial mass6,7,8,9. In contrast, mitochondrial activity (indicated by linked ROS levels) is much higher in lineage-committed progenitors than in HSCs9,10,11. Metabolic changes that occur during the earliest step of hematopoiesis thus suggest a direct and crucial role of mitochondria in regulating HSC fate.
Dysfunctional mitochondria present under various stress conditions, such as aging, cancer, diabetes, and obesity12, can interfere with HSC self-renewal capacity, inducing an imbalance in HSC/progenitor differentiation by producing excessive amounts of ROS, impairing ATP production, and/or by altering other metabolic processes9,12,13. Perturbations in metabolic homeostasis in HSC/progenitor differentiation could significantly impact hematopoiesis, potentially contributing to the development of hematologic abnormalities13. Given the critical influences of glycolysis and mitochondrial OxPHOS on HSC stemness and differentiation, it is of interest to investigate both metabolic parameters under normal and stress conditions. Valuable insights into the glycolytic and mitochondrial function of HSCs and progenitor cells can be gained by assessing their ECAR and OCR, which are indicators of cellular glycolysis and mitochondrial respiration, respectively.
The Seahorse extracellular flux analyzer is a powerful apparatus equipped with two probes per well to simultaneously measure ECAR and OCR in live cells and thus can be used to assess cellular bioenergetics in real time, in response to various substrates or inhibitors. The assay cartridge used with the analyzer contains injection ports to hold up to four drugs for automated injection during the assay. A scheme of a typical glycolysis stress test is shown in Figure 1A. The assay starts with the measurement of ECAR of cells, incubated in glycolysis stress test medium containing glutamine but not glucose or pyruvate. This represents acidification occurring due to non-glycolytic activities of the cells and is reported as non-glycolytic acidification. This is followed by the injection of glucose at a saturating concentration. Via glycolysis, glucose in the cell is converted into pyruvate, which is further metabolized in the cytoplasm to produce lactate, or in mitochondria to produce CO2 and water.
Conversion of glucose to lactate causes net production and subsequent release of protons into the extracellular medium, resulting in a rapid increase in the ECAR14,15,16. This glucose-stimulated change in ECAR is reported as glycolysis under basal conditions. The second injection consists of oligomycin (an inhibitor of ATP synthase, a.k.a. complex V17), which inhibits mitochondrial ATP production. During oligomycin-mediated OxPHOS inhibition, cells maximally upregulate glycolysis to meet their energetic demands. This causes a further increase in ECAR, revealing the maximum glycolytic capacity of the cells. The difference between the maximum glycolytic capacity and basal glycolysis is referred to as glycolytic reserve. Finally, 2-DG is injected, which causes a significant drop in ECAR, usually close to non-glycolytic acidification levels. 2-DG is a glucose analog that competitively binds to hexokinase, resulting in inhibition of glycolysis18. Thus, the 2-DG-induced decrease in ECAR further confirms that glycolysis is indeed the source of ECAR observed after glucose and oligomycin injections.
Figure 1B displays the schematic for a typical mitochondrial stress test. The assay starts with baseline OCR measurement of the cells, incubated in mitochondrial stress test medium containing glucose, glutamine, and pyruvate. Following basal OCR measurements, oligomycin is injected in this assay, which inhibits complex V, thereby reducing electron flow through the electron transport chain (ETC)17. Consequently, OCR is reduced in response to oligomycin injection, and this decrease in OCR is linked to mitochondrial ATP production. The second injection consists of carbonyl cyanide-4 (trifluoromethoxy) phenylhydrazone (FCCP), a protonophore and an uncoupler of mitochondrial OxPHOS17. FCCP collapses the mitochondrial proton gradient by allowing the flow of protons across the mitochondrial inner membrane. Because of the FCCP injection, electron flow through the ETC is derepressed, and complex IV consumes oxygen at the maximal level. The difference between maximal OCR and basal OCR is referred to as the spare respiratory capacity, which is a measure of the cell's ability to respond to increased energy demand under stress conditions. Finally, a mixture of two ETC inhibitors (rotenone, a complex I inhibitor, and antimycin A, a complex III inhibitor17) is injected, which completely shuts down electron flow, and OCR decreases to a low level. OCR measured after rotenone and antimycin A injection corresponds to non-mitochondrial OCR driven by other processes inside the cells. Non-mitochondrial OCR enables the calculation of basal respiration, proton leak, and maximal respiration.
Basal respiration represents the difference between baseline OCR and non-mitochondrial OCR. Proton leak refers to the difference between OCR after oligomycin injection and non-mitochondrial OCR. Maximal respiration represents the difference between OCR after FCCP injection and non-mitochondrial OCR. Coupling efficiency is calculated as the percentage of ATP production rate to basal respiration rate. This method paper provides a detailed protocol to measure ECAR and OCR in lineage-negative HSPCs using the Seahorse XFe96 extracellular flux analyzer. This protocol describes approaches to isolate mouse lineage-negative HSPCs, explains the optimization of cell-seeding density and concentrations of various drugs used in extracellular flux assays, and describes drug treatment strategies.