An integrated cell manipulation platform is developed for use in conjunction with a single-probe mass spectrometry setup for the on-line analysis of individual suspension cells under ambient conditions.
Single cell mass spectrometry (SCMS) enables sensitive detection and accurate analysis of broad ranges of cellular species on the individual-cell level. The single-probe, a microscale sampling and ionization device, can be coupled with a mass spectrometer for on-line, rapid SCMS analysis of cellular constituents under ambient conditions. Previously, the single-probe SCMS technique was primarily used to measure cells immobilized onto a substrate, limiting the types of cells for studies. In the current study, the single-probe SCMS technology has been integrated with a cell manipulation system, typically used for in vitro fertilization. This integrated cell manipulation and analysis platform uses a cell-selection probe to capture identified individual floating cells and transfer the cells to the single-probe tip for microscale lysis, followed by immediate mass spectrometry analysis. This capture and transfer process removes the cells from the surrounding solution prior to analysis, minimizing the introduction of matrix molecules in the mass spectrometry analysis. This integrated setup is capable of SCMS analysis of targeted patient-isolated cells present in body fluids samples (e.g., urine, blood, saliva, etc.), allowing for potential applications of SCMS analysis to human medicine and disease biology.
Human biology, especially disease biology, is increasingly understood to be the result of activities on the level of individual cells, but the traditional analytical methods, such as liquid chromatography mass spectrometry (LCMS), are generally used to analyze samples prepared from populations of cells, whereas the acquired molecular information cannot accurately represent the chemical processes on the individual-cell level. These standard, traditional methods are unable to discern the effects of cellular heterogeneity on an analytical measurement, and the process of destroying and mixing the cells to prepare the lysate potentially leads to the alteration or loss of cellular components1,2. These limitations of traditional methods are especially important in the analysis of patient cells, in which the obtained samples can contain a complex mixture of many different cell types. To overcome these deficiencies, single cell molecular analysis methods, including single cell mass spectrometry (SCMS) methods, are increasingly being developed and applied to bioanalysis, especially of cellular metabolites and low molecular weight biomolecules3,4.
The first SCMS techniques developed used vacuum-based techniques to perform the analyses under non-ambient conditions2,5,6,7,8,9,10,11. Non-ambient SCMS techniques are capable of analyzing cellular lipids and metabolites, but require sample pretreatment under artificial conditions, and therefore are not suitable for real-time analysis. The sample preparation process for non-ambient analysis includes the addition of matrix components, and this preparation can alter cellular components from their natural environment12. Therefore, ambient mass spectrometry (MS) techniques, which do not require a vacuum for the sampling environment, are utilized to analyze cells in a near-native environment. Not having a vacuum environment allows for versatility in the experimental design; cameras can be added to monitor the cellular process and softer ionization techniques can be combined with separation techniques to receive better information from each single-cell experiment4,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42.
The single-probe SCMS method is an ambient technique that analyzes live, mammalian cancer cell lines in a near-native environment21,43,44,45,46. In addition, the single-probe device has been used for other mass spectrometry applications, including analysis of extracellular molecules in multicellular spheroids and MS imaging of tissues47,48,49,50,51,52. However, since cell immobilization on substrates is required for this method, suspension cells cannot be directly analyzed using this technique3,53. Therefore, the single-probe SCMS system could not be directly used to sample non-adherent single cells, such as non-adherent cell lines or suspension cells isolated from a patient’s blood or other bodily fluids54. In this work, an integrated cell manipulation platform (ICMP) is coupled with the single-probe SCMS technique to analyze live, suspension cells on-line with minimal sample preparation (Figure 1)46. The ICMP consists of an inverted microscope to monitor cell selection, a glass cell-selection probe, a microinjector to capture individual floating cells, a heated plate to maintain cellular temperature, two cell manipulation systems to control spatial movements of both the glass cell-selection probe and single-probe, and a digital microscope to observe cell transfer from the cell-selection probe tip to the single-probe tip. The fabrication of the single-probe is detailed in previous publications and will not be addressed here21,48. The ICMP/single-probe system is coupled to a high resolution mass spectrometer. This integrated setup allows for the sampling of identified single cells from complex biological samples with minimal effects from matrix molecules.
1. Glass cell-selection probe fabrication
2. Integrated cell manipulation platform assembly
3. Create an extended ion transfer tube for the mass spectrometer inlet
4. Couple the ICMP with a single-probe setup
5. Suspended cell sample preparation
6. Perform SCMS measurements using the ICMP/single-probe setup
First, untreated K562 cells are used to establish the experimental method. In a typical SCMS experiment, obvious changes of mass spectra can be observed from transferring a cell, during the detection of cellular contents, and after finishing the measurement (Figure S1). Three common cellular lipid peaks (phosphatidylcholine, PC), including PC(34:4) (m/z 754.536), PC(36:4) (m/z 782.567), and PC(38:5) (m/z 808.583), are monitored to ensure the cell is successfully transferred and cellular contents are detected (Figure S2)21,43,46,55,56. If lipid peaks are not seen within 5 s, the mineral oil level in the microinjector is altered to reduce the suction holding the cell at the cell-selection probe tip; caution needs to be taken so that no mineral oil is pushed out from the cell-selection probe. The identity of many PC’s in the mass range of m/z 750-850 are confirmed using MS/MS on untreated cell lysate samples (Figure 3, Figure S2, Table 1)46.
K562 cells are also subjected to treatment with various drug compounds to expand the versatility of the method. K562 cells are incubated with gemcitabine (1 µM) and taxol (1 µM) for 1 h and OSW-1 (100 nM, 1 µM) for 4 h and 2 h, respectively. Cells are then washed with PBS to minimize the detection of drug compounds from extracellular content. The contribution of matrix (e.g., ions from cell culture medium, PBS, and solvent) to mass spectra of cellular contents can be eliminated through data subtraction, due to their significantly different ion signals (Figure S3). All three drug compounds are detected using the ICMP/single-probe MS setup (Figure S4)46. These results suggest this method can be used to study intracellular lipids, drugs, and metabolites on the single-cell level from cells in solution in a near-native environment.
Figure 1. Experimental setup for single suspension cell MS experiments. (A) The integrated cell manipulation platform (ICMP) coupled with a mass spectrometer. (B) Schematic for analysis of suspended cells. (C) Experimental view of K562 cells to be selected using the cell-selection probe. Reprinted with permission from Standke et al.46. Copyright 2019 American Chemical Society. Please click here to view a larger version of this figure.
Figure 2. Photos of a modified single-probe and a cell-selection probe utilized for single suspension cell MS experiments. Please click here to view a larger version of this figure.
Figure 3. Zoomed-in mass spectrum from a single cell showing the representative species (m/z 750-850). Chemical structures are confirmed using MS/MS analysis (Figure S1). Reprinted with permission from Standke et al46. Copyright 2019 American Chemical Society. Please click here to view a larger version of this figure.
Drug Molecule* | m/z | Mass Error (ppm) |
[Gemcitabine + H]+ | 264.076 | 11.32 |
[Taxol + Na]+ | 876.318 | 2.74 |
[OSW-1 + Na]+ | 895.445 | 0.89 |
Cellular Lipids | ||
[PC(34:4) + H]+ | 754.535 | 3.71 |
[PC(34:3) + H]+ | 756.551 | 3.44 |
[PC(34:2) + H]+ | 758.569 | 0.66 |
[PC(36:5) + H]+ | 780.551 | 3.07 |
[PC(36:4) + H]+ | 782.568 | 2.17 |
[PC(36:3) + H]+ | 784.585 | 0.64 |
[PC(38:7) + H]+ | 804.551 | 4.1 |
[PC(38:6) + H]+ | 806.567 | 2.48 |
[PC(38:5) + H]+ | 808.583 | 2.72 |
[PC(38:4) + H]+ | 810.601 | 0 |
[PC(40:7) + H]+ | 832.583 | 3.12 |
Table 1. Identified cellular components using the ICMP/Single-probe setup. The detection of all drug compounds were confirmed by comparing the MS/MS results with standard compound.
The integrated cell manipulation and analysis platform is constructed to expand the versatility of the single-probe MS method, allowing for on-line, rapid analysis of non-adherent cells in a near-native environment. A major advantage of the technique is that minimal sample preparation is required, so the cells are analyzed in conditions that mimic their standard state. Particularly, individual cells of interest can be visually identified and selected, minimizing the influence of matrix effect on MS ionization efficiency while maintaining cells in their natural environment, so the results are more representative cells’ native status (Figure S3). This technique can be potentially used to study patient cells suspended in biofluids in future studies. Another advantage of this technique is the flexible selection of the sampling solvent. It is important to include acetonitrile as the main sampling solvent so that microscale lysis can occur rapidly. Potentially, internal standards (e.g., isotopically-labeled drug compounds) can be added into the sampling solvent for quantification of molecules of interest (e.g., drug molecules) from individual cells, including those can play a key role in revolutionizing personalizing drug treatments in the future54.
Although this integrated system can be conveniently used to analyze broad ranges of cells, a limitation of the method is that neither the single-probe nor cell-selection probe is commercially-available; dictating the need for optimization of many parameters (e.g., flow rate, voltage, length between the nano-ESI emitter and ion transfer tubing, etc.) prior to each experiment. In addition, due to the smallness of the Single-probe and cell-selection probe, environmental perturbation (e.g., air flow) may result in difficulties establishing a junction between the two probes. A short-term solution is the bending of the cell-selection probe close to the end to minimize the length of tapering. Future work includes the development of a housing to enclose the critical parts of the setup to minimize environmental effects. Due to the limited amount of cellular contents and short acquisition time (~2-3 s) from a cell, MS/MS analysis can be only conducted for relatively abundant species. Other factors influencing the detection sensitivity include the suppressed ionization efficiency due to the introduction of matrix along with the cell and potential ion loss through the extended ion transfer tubing.
The authors have nothing to disclose.
The authors thank Naga Rama Kothapalli for her work in developing sample preparation for both suspension cells and cell lysate experiments. Additionally, the authors thank the NIH (R01GM116116 and R21CA204706) for funding.
Acetontrile | Millipore Co. | AX0145-1 | Sampling solvent |
CellTram Vario | Eppendorf | 6221 | ICMP |
Copper wire | stores.ebay.com/jewelerheaven | Dead soft, round, 20 guage, 25 ft | Conductive union setup |
Digital stereomicroscope | Shenzhen D&F Co. | Supereyes T004 | Analysis |
Disposable micropipette, 1-5 µL | Rochester Scientific | 5065 | Cell-selection probe fabrication |
Dual bore quartz tubing, 1.120"x0.005"x12" | Friedrich & Dimmock, Inc. | MBT-005-020-2Q | Single-probe fabrication |
Epoxy resin | Devcon | Part No. 20945 | Single-probe fabrication |
Eppendorf cell manipulation system | Eppendorf | Transferman NK517800397-U.R. | ICMP |
External nut | VALCO*CHEMINERT | EN1 | Ion transfer tube fabrication |
Formic acid | Sigma-Aldrich | 399388-500ML | Sampling solvent |
Fused silica capillary, ID: 40 µm, OD: 100 µm | Polymicro Technologies | TSP040105 | Single-probe fabrication, conductive union setup |
Fused silica capillary, ID: 50 µm, OD: 150 µm | Polymicro Technologies | 1068150015 | Conductive union setup |
HyClone Synthetic fetal bovine serum (FBS) | Fischer Sci | SH3006603 | Cell culture |
Inline MicroFilter | IDEX Health & Science LLC | M-520 | Conductive union setup |
Laser puller | Sutter Instrument Co. | Model P-2000 | Single-probe fabrication |
LED UV lamp | Foshan Liang Ya Dental Equipment | LY-C240 | Single-probe fabrication |
LTQ Orbitrap mass spectrometer | Thermo Scientific | LTQ Orbitrap XL | Analysis |
Microforge | Narishige, Co. | MF-9 | Cell-selection probe fabrication |
Microunion | IDEX Health & Science LLC | M-539 | Conductive union |
PEEK tubing, 1/32×0.005x 5ft | IDEX Health & Science LLC | 1576 | Conductive union setup |
PEEK tubing, 1/32×0.007x 5ft | IDEX Health & Science LLC | 1577 | Conductive union setup |
Penicillin/Streptomycin | Gibco/Life Technologies | 15140-122 | Cell culture |
Petri dish, 35×10 mm | VWR | 25382-334 | Sample preparation |
Phosphate Buffered Saline (PBS) | VWR | 0780-50L | Cell culture |
Platinum wire | Narishige, Co. | Model PT-A | Microforge |
Power supply | Nikon | PSM-2120 | ICMP |
RPMI, 1X with Corning glutagro | Corning | 10-104-CV | Cell culture |
Single-bore tubes | Boralex | 5065 | Cell-selection probe fabrication |
Stainless steel ferrules, for 1/16" OD | IDEX Health & Science LLC | VHP-200-01x | Ion transfer tube fabrication |
Stainless steel tubing, 1/32x 205 µm x30 cm | IDEX Health & Science LLC | U-1128 | Ion transfer tube fabrication |
Syringe, 250 µL | Hamilton | 1725LTN250UL | Sampling syringe |
T25 flask | CellStar | 690160 | Cell culture |
Thermo LTQ XL ion source interface flange | New Objective | PB5500 | Analysis |
ThermoPlate | TokaiHit | 55R30N | ICMP |
TrypLE Express | Gibco | 12605-010 | Cell culture |
Tube cutter, for 1/16" stainless steel | SUPELCO | 58692-U | Ion transfer tube fabrication |
USB digital photography microscope | dx.com | SO2 25~500X | Analysis |
UV curing resin | Prime Dental | Item No. 006.030 | Single-probe fabrication |
Vertical pipette puller | David Kopf Instruments | Model 720 | Cell-selection probe fabrication |
Voltage housing | PicoChip | PCH-A00120 | ICMP/MS interface |
Wire cutter | Craftsman | 4 1/2 in end nipper | Conductive union setup |