Methods for studying mitochondrial bioenergetics under physiologically relevant substrate concentrations in immune cells are limited. We provide a detailed protocol that uses high-resolution fluorespirometry to assess changes in the response of the mitochondrial membrane potential to energy demand in human T-cells, monocytes, and peripheral mononuclear cells.
Peripheral mononuclear cells (PBMCs) exhibit robust changes in mitochondrial respiratory capacity in response to health and disease. While these changes do not always reflect what occurs in other tissues, such as skeletal muscle, these cells are an accessible and valuable source of viable mitochondria from human subjects. PBMCs are exposed to systemic signals that impact their bioenergetic state. Thus, expanding our tools to interrogate mitochondrial metabolism in this population will elucidate mechanisms related to disease progression. Functional assays of mitochondria are often limited to using respiratory outputs following maximal substrate, inhibitor, and uncoupler concentrations to determine the full range of respiratory capacity, which may not be achievable in vivo. The conversion of adenosine diphosphate (ADP) to adenosine triphosphate (ATP) by ATP-synthase results in a decrease in mitochondrial membrane potential (mMP) and an increase in oxygen consumption. To provide a more integrated analysis of mitochondrial dynamics, this article describes the use of high-resolution fluorespirometry to measure the simultaneous response of oxygen consumption and mitochondrial membrane potential (mMP) to physiologically relevant concentrations of ADP. This technique uses tetramethylrhodamine methylester (TMRM) to measure mMP polarization in response to ADP titrations following maximal hyperpolarization with complex I and II substrates. This technique can be used to quantify how changes in health status, such as aging and metabolic disease, affect the sensitivity of mitochondrial response to energy demand in PBMCs, T-cells, and monocytes from human subjects.
A cell's ability to function and survive in a period of physiological stress is largely dependent on its ability to meet the energetic requirement to restore homeostasis1,2. Energy demand rises in response to a variety of stimuli. For instance, increased muscle contraction during exercise increases the utilization of ATP and glucose by skeletal muscle, and a rise in protein synthesis following infection increases the utilization of ATP by immune cells for cytokine production and proliferation3,4,5,6. A spike in energy demand triggers a series of bioenergetic processes to restore the ATP/ADP ratio. As ATP is consumed, ADP levels rise and stimulate F1F0 ATP-synthase (complex V), which requires a protonmotive force to drive its mechanical rotation and catalytic conversion of ADP to ATP within the mitochondrion7. The protonmotive force is an electrochemical gradient created by the pumping of protons during the transfer of electrons from substrates to oxygen through the electron transport system (ETS) within the inner mitochondrial membrane. The resulting difference in proton concentration (delta pH) and electrical potential (membrane potential) creates the protonmotive force that drives ATP synthesis and oxygen consumption in response to energy demand reducing the ATP/ADP ratio or raising ADP levels. The affinity of mitochondria to ADP can be determined by the calculation of the Km or EC50 of ADP-stimulated respiration of isolated mitochondria or permeabilized cells8,9. This method has shown that permeabilized muscle fibers from older humans require a greater concentration of ADP to stimulate 50% of their maximal oxidative phosphorylation capacity than those of younger subjects9. Similarly, aging mouse skeletal muscle requires more ADP to lower the production of mitochondrial reactive oxygen species (ROS)10,11. Additionally, ADP sensitivity is reduced in permeabilized muscle fibers of mice with diet-induced obesity relative to controls and is enhanced in the presence of insulin and following nitrate consumption12,13. Thus, the capacity of mitochondria to respond to energy demand varies under different physiological conditions, but this has not been previously explored in the context of immune cells.
Peripheral blood mononuclear cells (PBMCs) are commonly used to investigate cellular bioenergetics in human subjects14,15,16,17,18,19,20. This is largely due to cells being easily obtainable from uncoagulated blood samples in clinical studies, the responsiveness of cells to metabolic perturbations, and the methods developed by various groups to interrogate mitochondrial metabolism by using inhibitors and uncouplers to determine the maximal and minimal capacity of mitochondrial respiration21,22. These methods have led to an appreciation of the roles of bioenergetics in aging, metabolic disease, and immune function14,20,23,24. Mitochondrial respiratory capacity is often reduced in skeletal muscle and PBMCs under conditions of heart failure18,25. PBMC bioenergetics are also correlated with cardiometabolic risk factors in healthy adults17 and are responsive to treatments such as nicotinamide riboside18. PBMCs include neutrophils, lymphocytes (B-cells and T-cells), monocytes, natural killer cells, and dendritic cells, which all contribute to PBMC mitochondrial capacity26,27,28. In addition, cellular bioenergetics play a crucial role in immune cell activation, proliferation, and renewal23. However, a limitation of these methods is that the cells are not functioning under a physiological range of substrates. Additional methods are therefore required to interrogate mitochondrial function in substrate concentrations that are more relevant to what cells experience in vivo.
Mitochondrial membrane potential (mMP) is the major component of a protonmotive force and is essential for a variety of mitochondrial processes beyond ATP production, such as regulation of respiratory flux, reactive oxygen species production, protein and ion import, autophagy, and apoptosis. mMP can be assessed with electrochemical probes or fluorescent dyes sensitive to changes in membrane polarization like JC-1, Rhod123, DiOC6, tetramethyl rhodamine (TMRE) or methyl ester (TMRM), and safranin. The latter two are lipophilic cationic dyes that have been successfully used in high-resolution fluorespirometry of tissue homogenates, isolated mitochondria, and permeabilized tissue11,29,30,31,32,33. In this technique, TMRM is used in quench mode, where cells are exposed to a high concentration of TMRM that accumulates in the mitochondrial matrix when polarized (high mMP and protonmotive force), resulting in the quenching of cytosolic TMRM fluorescence. When mitochondria depolarize in response to ADP or uncouplers, the dye is released from the matrix, increasing the TMRM fluorescent signal34,35. The purpose of this method is to simultaneously measure changes in mitochondrial respiration and mMP in response to ADP titrations in human-derived PBMCs, circulating monocytes, and T-cells, and it can also be applied to mouse splenic T-cells.
The collection of blood samples for data and methods development presented herein was approved by the Internal Review Board of the University of Washington. Representative results also include data from male C57BL/6J mice (5-7 months old) purchased from Jackson Laboratories. All animal procedures were approved by the University of Washington Office of Animal Welfare. The protocol overview is pictured in Figure 1. Reagent preparation for this protocol can be found in Supplementary File 1.
Figure 1: Overview of the protocol. Workflow using high-resolution fluorespirometry to assess changes in mitochondrial membrane potential in isolated monocytes (CD14+) and T-cells (CD3+) from fresh human blood samples. Abbreviations: TMRM, tetramethylrhodamine methyl ester; SUIT, substrate-uncoupler-inhibitor titrations; ADP, adenosine diphosphate; Dig, digitonin; Mal, malate; Pyr, pyruvate; Glut, glutamate; D1-10, 10 consecutive ADP titrations. Please click here to view a larger version of this figure.
1. Separation of buffy coat from whole blood
NOTE: Cell isolation is modified from Kramer et al.27.
2. Magnetic separation of CD14+ and CD3+ cells
3. High-resolution fluorespirometry - Oxygen and TMRM fluorescence calibration
NOTE: This method was adapted from previous work done on permeabilized fibers by Pharaoh et al.11. A high, un-inhibitory concentration of TMRM is used for quench mode, where the relationship of mMP and TMRM concentration in the matrix is inverted. Thus, a decrease in mMP leads to the release of TMRM dye from the matrix and an increase in fluorescence32.
4. Substrate-uncoupler-inhibitor titration (SUIT) protocol
NOTE: Run blank experiments where 20 µL of Mir05 is injected into the chamber instead of 20 µL of cell suspension, as the TMRM signal will change in response to injections alone (discussed in representative results). Allow for the oxygen flux signal to stabilize (about 2-3 min) before the next injection for both blank and sample experiments. The following titration protocol and expected observations are in Table 1.
5. Calculation of mitochondrial membrane potential and analysis
Figure 2: Calculating mitochondrial membrane potential (mMP) and ADP sensitivity from TMRM fluorescence. Steps for calculating mitochondrial membrane potential (mMP) and ADP sensitivity from measurements of TMRM fluorescence by high-resolution fluorespirometry of one sample of T-cells (n = 1). Step 1: TMRM fluorescence is measured in blank samples as done in the biological sample. Step 2: Determine the ratio in the TMRM signal with each titration relative to the signal prior to the sample for each blank experiment. Calculate the average for each titration of all blank experiments. Step 3: Calculate the background for each sample experiment by multiplying the "pre-sample" fluorescence by the average background ratio for each titration. Step 4: Calculate the difference between background and sample TMRM fluorescence for each titration to express data as mMP or mitochondrial TMRM uptake. Step 5: Correct mMP so that full uncoupling with FCCP reflects zero mMP. Step 6: Perform non-linear regression to graph changes in mMP with increasing ADP concentrations. Measurements were performed in 0.5 mL chambers, one containing 5 million T-cells from a healthy volunteer. Averaged data are expressed as mean ± SEM. Single data points of a single replicate are expressed without error bars. Please click here to view a larger version of this figure.
To illustrate the differences in optimal cell concentration for the assay, 5 million T-cells were loaded into one 0.5 mL chamber (10 million cells/mL), and 1.25 million cells were loaded into another chamber (2.5 million cells/mL) containing 1 µM TMRM (Figure 3A-G). Three blank experiments were also included to calculate the TMRM background. We found that a higher concentration of T-cells resulted in a more distinguishable change in TMRM fluorescence relative to the background (Figure 3B,D). In addition, a higher cell concentration allowed us to detect the expected increase in oxygen consumption and simultaneous depletion of the mMP in response to the addition of FCCP (Figure 3E,F). Using a low concentration of cells yielded a weak change in fluorescence that paralleled the background. Since the calculation of mMP subtracts the background from the signal, a low cell concentration does not allow for the determination of changes in mMP in response to substrates and uncouplers. In addition to using the higher concentrations of cells in this assay, we recommend keeping the cell concentration constant for each cell type between experiments.
To validate the influence of ATP-synthase in the dissipation of mMP with ADP titrations, we ran parallel experiments on PBMCs and T-cells where one chamber received oligomycin before ADP titration (Figure 4). We found no dissipation of mMP in response to ADP in cells treated with oligomycin, suggesting that the gradual decrease in mMP with ADP is a result of proton flux through ATP-synthase (Figure 4A-F). We also compared ADP sensitivity between T-cells and PBMCs of the same participant and found ADP sensitivity to be lower (higher EC50) in the T-cell fraction (Figure 4G,H).
We conducted a series of blank experiments to determine the influence of time or the SUIT protocol on TMRM fluorescence. We found that the TMRM signal in blank experiments is mostly influenced by SUIT titrations (Figure 5A) as opposed to the timing of the titrations (Figure 5B).
We compared ADP-driven changes in oxygen consumption rates (OCR) and in mMP in T-cells and monocytes from 11 healthy, community-dwelling volunteers (Figure 6A-H). Similar to the results of previously published experiments using extracellular flux and enzymatic assays, monocytes exhibited a greater mitochondrial respiratory capacity than lymphocytes26,27 (Figure 6A,H). However, we did not detect a typical dose-response increase in OCR with ADP in either cell type (Figure 6C,D), contrary to what this method shows when using highly metabolic tissues like mouse liver (Figure 7A-H). On the other hand, the use of TMRM allowed us to detect a gradual decline in mMP with ADP in human immune cells (Figure 6E-G) and in splenic T-cells from mice (Figure 7E-H). While we did not directly compare human and mouse T-cells using the same titration protocol, we did find that the IC50 of mouse T cells was lower by a factor of 10 compared with that of circulating T-cells from human subjects.
Figure 3: High-resolution fluorespirometry experiments. (A-D) Trace of high-resolution fluorespirometry experiments using T-cell concentrations of 10 million cells/mL and 2.5 million cells/mL in 0.5 mL chambers. (A) 10 million cells/mL in 0.5 mL chambers. (C) 2.5 million cells/mL in 0.5 mL chambers. Oxygen flux (pmol/s/mL) is shown in the top panel (red), and the calibrated TMRM signal is shown in the bottom panel (black). Changes in TMRM throughout the SUIT for the sample and its calculated background were plotted for the chambers containing (B) 10 million cells/mL and (D) 2.5 million cells/mL. (E) For each cell concentration, oxygen flux (pmol/s/million cells) and (F) mitochondrial membrane potential were calculated. (G) ADP sensitivity curve was plotted and fit to a non-linear regression model (solid lines). Abbreviations: mMP, mitochondrial membrane potential; TMRM, tetramethylrhodamine methyl ester; SUIT, substrate-uncoupler-inhibitor titrations; ADP, adenosine diphosphate; Dig, digitonin; Mal, malate; Pyr, pyruvate; Glut, glutamate; D1-11, 11 consecutive ADP titrations; U, uncoupler FCCP of 0.5 and 1.0 μM; AMA, antimycin A. Please click here to view a larger version of this figure.
Figure 4: ATP-synthase drives ADP-driven decrease in membrane potential in T-cells and PBMCs. (A-H) The protocol described here was tested in PBMCs and T-cells. Two O2K chambers were injected with PBMCs, and two chambers of an additional O2K were injected with T-cells from the same participant. After injecting substrates malate, pyruvate, and glutamate in all chambers, one chamber of PBMCs and T-cells received oligomycin. Oligomycin prevented any ADP-driven rise in respiration in (A) PBMCs and (D) T-cells or decline in mitochondrial membrane potential in (B,C) PBMCs and (E,F) T-cells. (G,H) ADP sensitivity was greater in PBMCs compared to T-cells. Please click here to view a larger version of this figure.
Figure 5: Blank experiments show the change in TMRM fluorescence in response to time and titrations of substrates, uncouplers, and inhibitors (SUIT). (A) Change in TMRM fluorescence in response to titration. (B) Change in TMRM fluorescence in response to time. Experiments were conducted in 0.5 mL chambers filled with Mir05 containing 1 μM TMRM. One chamber did not receive any SUIT titrations (no injection); two chambers in two different instruments received a standard suit protocol (standard injection); one chamber received the same SUIT titrations but with a delay between each injection (delayed injection). Please click here to view a larger version of this figure.
Figure 6: Differences in ADP sensitivity between T-cells and monocytes using OCR and mMP. (A) Trace of high-resolution fluorespirometry experiment from a subject's monocyte and T-cell sample. (B) Oxygen consumption in monocytes (n= 11) and T-cells (n= 13) from the blood of healthy volunteers. (C,D) Non-linear regression fitting of the plotted rise in respiration with ADP titrations to calculate an EC50. (E) Simultaneous measurement of mitochondrial membrane potential. (F,G) Non-linear regression fitting of the plotted decline in mitochondrial membrane potential with ADP titrations to calculate an IC50. (H) Parameters of respiratory capacity of monocytes and T-cells. Data are expressed as mean ± SEM for line graphs and mean ± SD for bar graphs. Statistically significant differences following t-tests are expressed as *p < 0.05. **p < 0.01, and ****p < for 0.0001. Please click here to view a larger version of this figure.
Figure 7: Comparing ADP response in respiration and mitochondrial membrane potential (mMP) in permeabilized mouse splenic T-cells and liver. (A-D) Response in respiration in permeabilized mouse splenic T-cells and liver. (E-H) Response in mMP in permeabilized mouse splenic T-cells and liver. Fresh liver and spleen were dissected from three mice following cervical dislocation. Splenic Pan T-cells were isolated using antibody-conjugated magnetic bead separation. Both samples underwent the same SUIT protocol in the presence of 1 μM TMRM. (I,J) Comparison of EC50 calculated from the increase in oxygen consumption (OCR) and IC50 from the decrease in mMP in response to ADP. N = 3 per group. Data are expressed as mean ± SEM. Please click here to view a larger version of this figure.
Table 1: Example SUIT protocol to assess mitochondrial membrane potential in freshly isolated T-cells and monocytes using the 0.5 mL chambers. Please click here to download this Table.
Table 2: Recommended ADP titration for 0.5 mL chamber. Please click here to download this Table.
Table 3: Calculating the average background ratio using five independent blank experiments. Please click here to download this Table.
Table 4: Calculating mitochondrial membrane potential (mMP) from sample experiment. Please click here to download this Table.
Supplementary Figure 1: Effect of Mir05 and DMSO on mitochondrial respiration and membrane potential. Please click here to download this File.
Supplementary File 1: Reagent preparation and protocol for isolating T-cells from mouse spleen. Please click here to download this File.
This protocol uses high-resolution fluorespirometry to measure the sensitivity of the mitochondrial response to energy demand by measuring the dissipation of mMP in response to increasing levels of ADP in PBMCs, monocytes, and T-cells. This is done by adding complex I and II substrates to maximize the mitochondrial membrane potential and titrating ADP to gradually stimulate ATP-synthase to use the proton gradient for ATP generation.
Critical steps in the protocol include setting the gain and intensity of the fluorophore to 1000 and making sure a TMRM fluorescent signal is acquired during the TMRM titration. Because TMRM fluorescence declines following each titration (a limitation of this method), it is imperative to run background experiments using blank samples. We have also found that DMSO has an inhibitory effect on mitochondrial respiration and membrane potential and, therefore, recommend diluting the working solution of TMRM in Mir05 (Supplementary Figure 1).
Some modifications that may be used when trying this protocol are adjusting cell concentrations and using the standard 2 mL chamber. However, the 0.5 mL chamber is preferred for T-cells and monocytes because of the high concentration of cells needed for optimal response in membrane potential and oxygen flux. A lower concentration of cells may be optimal when testing cells with greater respiratory capacity, like macrophages.
Additional limitations of the method presented here include the requirement for at least 5 million T-cells and 2.5 million monocytes. We can often obtain enough cells from ~20 mL of blood from healthy participants, but these numbers can vary by health status, age, and sex26. In addition, as in most methods assessing mitochondrial capacity, the cells need to be freshly isolated. However, this method could be tried in cryopreserved cells in the future. In comparison with the yield from human blood, the T-cell yield from spleens of healthy mice is high enough to conduct this assay.
Circulating T-cells, particularly long-lived memory (TM) and regulatory (Treg) cells, rely on oxidative phosphorylation for energy37. While their energy demand and oxygen consumption are low (e.g., compared with that of resting muscle), their survival is essential for an effective immune response to reinfection and cancer38,39,40. A reduction in T-cell oxidative phosphorylation results in impaired proliferative capacity and promotes T-cell exhaustion and senescence5,41. Additionally, mitochondrial hyperpolarization promotes a sustained production of cytokines (IL-4 and IL-21) by effector CD4 T-cells during activation42. Upon infection, the energy requirement for activation and proliferation of immune cells can be as high as 25%-30% of the basal metabolic rate43. Therefore, immune cells function in a wide and extreme range of energy demands, and this protocol can test mitochondrial responses within that range.
Chronic inflammation is a common feature of obesity, diabetes, and aging. Dysregulated levels of circulating hormones, lipids, and glucose have systemic impacts and can thus affect how mitochondria respond to an energetic challenge. Here, we have presented a method to assess mitochondrial ADP sensitivity in circulating PBMCs. Further studies are needed to determine how ADP sensitivity may be modulated in metabolic disease and how it impacts health status.
We would like to thank the kind volunteers who donated blood for this project. We also extend our sincere appreciation to Dr. Ellen Schur and her team for providing us with additional samples from their study. We would also like to thank Andrew Kirsh for reviewing the manuscript and editing it for legibility. This work was supported by the following funding sources: P01AG001751, R01AG078279, P30AR074990, P30DK035816, P30DK017047, R01DK089036, K01HL154761, T32AG066574.
Name | Company | Catalog Number | Comments |
Adenosine Diphosphate | Sigma-Aldrich | A5285 | Fluorespirometry |
Antimycin A | Sigma-Aldrich | A8674 | Fluorespirometry |
Bovine Serum Albumin (BSA) | Sigma-Aldrich | A6003 | Mir05 buffer |
Bovine Serum Albumin | Sigma-Aldrich | A6003 | Cell isolation |
Carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone | Sigma-Aldrich | C2920 | Fluorespirometry |
Cell strainers | Fisher Scientific | 22-363-548 | Isolation of T-cells from mouse spleen protocol |
CD14 Microbeads, human | Miltenyi Biotec | 130-050-201 | Cell isolation |
CD3 Microbeads, human | Miltenyi Biotec | 130-050-101 | Cell isolation |
DatLab | Oroboros | Version 8 | |
Digitonin | Sigma-Aldrich | D141 | Fluorespirometry |
D-Sucrose | Sigma-Aldrich | 84097 | Mir05 buffer |
Ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA) | Sigma-Aldrich | E4378 | Mir05 buffer |
Filter Set AmR | Oroboros | 44321-01 | |
HBSS (10x) | Gibco | 12060-040 | |
HEPES sodium salt | Sigma-Aldrich | H7523 | Mir05 buffer |
Histopaque 1077 | Sigma-Aldrich | 10771 | Cell isolation |
K2EDTA blood collection tubes | BD Vacutainer | 366643 | Cell isolation |
Lactobionic acid | Sigma-Aldrich | 153516 | Mir05 buffer |
L-Glutamic acid | Sigma-Aldrich | G1626 | Fluorespirometry |
L-Malic Acid | Sigma-Aldrich | M1000 | Fluorespirometry |
LS Columns | Miltenyi Biotec | 130-042-401 | Cell isolation |
Magnesium Chloride (MgCl2) | Sigma-Aldrich | M9272 | Mir05 buffer |
Multi-MACS stand and MidiMACS Separator | Miltenyi Biotec | 130-042-301 | Cell isolation |
O2k-Fluo Smart-Module | Oroboros | 12100-03 | |
O2k-FluoRespirometer series J | Oroboros | 10201-03 | |
O2k-sV-Module (0.5 chamber) | Oroboros | 11200-01 | |
Oligomycin | Sigma-Aldrich | 04876 | Fluorespirometry |
Pan T Cell Isolation Kit II, mouse | Miltenyi | 130095130 | Isolation of T-cells from mouse spleen protocol |
Potassium dihydrogen phosphate (KH2PO4) | Sigma-Aldrich | P0662 | Mir05 buffer |
Potassium Hydroxide (KOH) | Sigma-Aldrich | 221473 | Mir05 buffer |
Prism | GraphPad | Version 10 | |
Rotenone | Sigma-Aldrich | R8875 | Fluorespirometry |
RPMI Buffer | Corning | 17-105-CV | Cell isolation |
Sodium Pyruvate | Sigma-Aldrich | P2256 | Fluorespirometry |
Succinate disodium salt | Sigma-Aldrich | S2378 | Fluorespirometry |
Taurine | Sigma-Aldrich | T0625 | Mir05 buffer |
Tetramethyrhodamine methyl ester perchlorite | Sigma-Aldrich | T5428 | Fluorespirometry |
This article has been published
Video Coming Soon
SOBRE A JoVE
Copyright © 2024 MyJoVE Corporation. Todos os direitos reservados