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Method Article
Bulk gene expression measurements cloud individual cell differences in heterogeneous cell populations. Here, we describe a protocol for how single-cell gene expression analysis and index sorting by Florescence Activated Cell Sorting (FACS) can be combined to delineate heterogeneity and immunophenotypically characterize molecularly distinct cell populations.
Immunophenotypic characterization and molecular analysis have long been used to delineate heterogeneity and define distinct cell populations. FACS is inherently a single-cell assay, however prior to molecular analysis, the target cells are often prospectively isolated in bulk, thereby losing single-cell resolution. Single-cell gene expression analysis provides a means to understand molecular differences between individual cells in heterogeneous cell populations. In bulk cell analysis an overrepresentation of a distinct cell type results in biases and occlusions of signals from rare cells with biological importance. By utilizing FACS index sorting coupled to single-cell gene expression analysis, populations can be investigated without the loss of single-cell resolution while cells with intermediate cell surface marker expression are also captured, enabling evaluation of the relevance of continuous surface marker expression. Here, we describe an approach that combines single-cell reverse transcription quantitative PCR (RT-qPCR) and FACS index sorting to simultaneously characterize the molecular and immunophenotypic heterogeneity within cell populations.
In contrast to single-cell RNA sequencing methods, the use of qPCR with specific target amplification allows for robust measurements of low-abundance transcripts with fewer dropouts, while it is not confounded by issues related to cell-to-cell variations in read depth. Moreover, by directly index-sorting single-cells into lysis buffer this method, allows for cDNA synthesis and specific target pre-amplification to be performed in one step as well as for correlation of subsequently derived molecular signatures with cell surface marker expression. The described approach has been developed to investigate hematopoietic single-cells, but have also been used successfully on other cell types.
In conclusion, the approach described herein allows for sensitive measurement of mRNA expression for a panel of pre-selected genes with the possibility to develop protocols for subsequent prospective isolation of molecularly distinct subpopulations.
Each individual blood cell is believed to reside in a cellular hierarchy, where stem cells form the apex on top of a series of increasingly committed intermediate progenitors that eventually terminally differentiate into the final effector cells carrying specific biological functions1. Much of the knowledge about how stem cell systems are organized has been generated in the hematopoietic system, largely because of the ability to prospectively isolate distinct hematopoietic populations highly enriched for stem cells or various progenitors2 by FACS sorting. This has allowed for many of these populations to be analyzed functionally or molecularly, predominantly through gene expression profiling3,4. However when analyzing gene expression of bulk populations individual differences between cells are averaged out and lost5. Thus, incapacity to detect cell-to-cell variations within heterogeneous cell fractions may confound our understanding of critical biological processes if small subsets of cells account for the inferred biological function of that population6,7. Conversely, investigation of gene expression signatures at single-cell resolution offer a possibility to delineate heterogeneity and circumvent overshadowing influences from overrepresented subsets of cells8.
To date many protocols for single-cell gene expression analysis have been developed; with each approach having its own caveats. The earliest method was RNA Fluorescent in situ hybridization (RNA-FISH), which measures a limited number of transcripts at a time but is unique in that it allows for investigation of RNA localization9,11. Early methods using PCR and qPCR to detect a single or very few transcripts were also developed12. However, these have lately been replaced by microfluidics-based methods which can simultaneously analyze the expression of hundreds of transcripts per cell in hundreds of cells through qPCR and thus allow for high-dimensional heterogeneity analysis using pre-determined gene panels10,13. Recently RNA sequencing-based technologies have become widely used for single cell analysis, as these theoretically can measure the entire transcriptome of a cell and thereby add an exploratory dimension to heterogeneity analysis10,14. Multiplexed qPCR analysis and single-cell RNA sequencing have different features, thus the rationale for using either of the methods depends on the question asked as well as the number of cells in the target population. The high-throughput and low cost per cell together with unbiased, exploratory characteristics of single-cell RNA sequencing are desirable when unknown cell or large populations are investigated. However, single-cell RNA sequencing is also biased towards sequencing high abundant transcripts more frequently while transcripts with low abundance are prone to dropouts. This can lead to considerably complex data that puts high-demands on bioinformatic analysis to reveal important molecular signals that are often subtle or hidden in technical noise15. Thus, for well-characterized tissues, single-cell qPCR analysis using pre-determined primer panels selected for functionally important genes or molecular markers can serve as a sensitive, straightforward approach to determine the heterogeneity of a population. However, it should be noted that compared to single-cell RNA-seq, the cost per cell is generally higher for single-cell qPCR methods. Here, we describe an approach that combines single-cell RT-qPCR (modified from Teles J. et al.16), to FACS index sorting17 and bioinformatics analysis18 in order to simultaneously characterize the molecular and immunophenotypic heterogeneity within populations.
In this approach, the cell population of interest is stained, and single-cells are sorted by FACS directly into lysis buffer in 96-well PCR plates. Simultaneously, the expression levels of an additional set of cell-surface markers are recorded for each single cell during FACS-sorting, a method that is referred to as index-sorting. The lysed cell material is subsequently amplified and the gene expression of a selected set of genes analyzed with RT-qPCR, using a microfluidic platform. This strategy enables molecular analysis of the sorted single-cell as well as simultaneous characterization of each individual cell's cell-surface marker expression. By directly mapping molecularly distinct subsets of cells to the expression of the indexed sorted markers, the subpopulations can be linked to a specific immunophenotype that can be used for their prospective isolation. The method is outlined step by step in Figure 1. A pre-determined gene panel further contributes to a higher resolution of the targeted gene expression, since it circumvents measurement of irrelevant abundant genes that can otherwise occlude subtle gene expression signals. Moreover, the specific target amplification, one step reverse transcription, and amplification allows for robust measurement of low expressed transcripts, like transcription factors or non-poly-adenylated RNAs. Importantly, qPCR methods allow for measurement of mRNA from fusion proteins, which are important when investigating certain malignant diseases19. Finally, the focused number of genes investigated, low drop-out rates, and limited technical differences between cells make this method easily analyzed compared to higher dimensional methods, such as single-cell RNA-seq. By following the protocol, an entire experiment can be performed, from sorting cells to analyzed results, within three days, making this an uncomplicated and quick method for sensitive, high-throughput single-cell gene expression analysis.
1. Preparation of Lysis Plates
2. Preparation of Cells for Cell Sorting
3. Cell Sorting
4. Reverse Transcription and Specific Target Amplification
5. Preparation of Sample and Assay Plates for Multiplex Microfluidic Gene Expression Analysis
6. Loading of Microfluidic Chip
7. Running Chip on Multiplex Microfluidic Gene Expression Platform
8. Preliminary Analysis of Chip Run
9. Single-cell Analysis Using SCExV
NOTE: An introductory film is present20 to introduce the tool. Here, a short recommendation of how to do analysis using the controls introduced in the protocol is presented.
10. Index-sorting Analysis
The protocol described is quick, easily performed and highly reliable. An overview of the experimental set-up is presented in Figure 1. The entire protocol, from sorting of single-cells, to specific target amplification, gene expression measurements and preliminary analysis can be performed in three days. An example of analyzed results in the form of a heat map that represents preliminary analyzed data from single-cell gene expression analysis using 96 primer...
In recent years, single-cell gene expression analysis has become a valuable addition to define the heterogeneity of various cell populations23. The advent of RNA sequencing technologies theoretically provides a possibility to measure the entire transcriptome of a cell, however these methods are complicated by variations in cell-to-cell sequencing depths and drop-outs. Single-cell qPCR offers a sensitive and robust analysis of the expression of hundreds of critical genes where all cells are treated...
The authors have nothing to disclose.
This work is supported by grants from the Swedish Cancer Society, The Swedish Research Council, The Swedish Society for Medical Research, The Swedish Childhood Cancer Foundation, The Ragnar Söderberg Foundation, and The Knut and Alice Wallenberg Foundation
Name | Company | Catalog Number | Comments |
CD14 PECY5 | eBioscience | 15-0149-42 | Clone: 61D3 |
CD16 PECY5 | Biolegend | 302010 | Clone: 3G8 |
CD56 PECY5 | Biolegend | 304608 | Clone: MEM-188 |
CD19 PECY5 | Biolegend | 302210 | Clone: HIB19 |
CD2 PECY5 | Biolegend | 300210 | Clone: RPA-2.10 |
CD3 PECY5 | Biolegend | 300310 | Clone: HIT3a |
CD123 PECY5 | Biolegend | 306008 | Clone: 6H6 |
CD235A PECY5 | BD Pharma | 559944 | Clone GAR2 |
CD34 FITC | Biolegend | 343604 | Clone: 561 |
CD38 APC | Biolegend | 303510 | Clone: Hit2 |
CD90 PE | Biolegend | 328110 | Clone: 5E10 |
CD45RA BV421 | BD bioscience | 560362 | Clone: HI100 |
CD49f Pecy7 | eBioscience | 25-0495-82 | Clone: eBioGOH3 |
FBS | HyClone | SV30160.3 | |
PBS | HyClone | SH30028.02 | |
96-well u-bottom Plate | VWR | 10861-564 | |
SFEM | Stem cell technologies | 9650 | |
Penicillin streptomycin | HyClone | SV30010 | |
TPO | Peprotech | 300-18 | |
SCF | Peprotech | 300-07 | |
FLT3L | Peprotech | 300-19 | |
Falcon Tube 15 mL | Sarstedt | 62.554.502 | |
Eppendorph tube | Sarstedt | 72.690.001 | |
CST beads | BD | 642412 | |
Accudrop Beads | BD | 345249 | 6 µm particles |
Adhesive film Clear | Thermo scientific | AB-1170 | |
Adhesive film Foil | Thermo scientific | AB-0626 | |
96 well PCR plate | Axygen | PCR-96M2-HS-C | |
PCR 1.5 mL tube | Axygen | MCT-150-L-C | |
T100 PCR cycler | BioRad | 186-1096 | |
10% NP40 | Thermo scientific | 85124 | |
10 mM dNTP | Takara | 4030 | |
0.1 M DTT | Invitrogen | P2325 | |
RNAsout | Invitrogen | 10777-019 | RNAse inhibitor |
CellsDirect One-Step qRT-PCR Kit | Invitrogen | 11753-100 | |
Neuclease free water | Invitrogen | 11753-100 | from CellsDirect kit |
2x Reaction Mix | Invitrogen | 11753-100 | from CellsDirect kit |
SuperScript III RT/Platinum Taq Mix | Invitrogen | 11753-100 | from CellsDirect kit |
Platinum Taq DNA Polymerase | Invitrogen | 10966026 | |
TaqMan Cells-to-CT Control Kit | Invitrogen | 4386995 | |
Xeno RNA Control | Invitrogen | 4386995 | From TaqMan Cells-to-CT Control Kit |
20x Xeno RNA Control Taqman Gene Expression Assay | Invitrogen | 4386995 | From TaqMan Cells-to-CT Control Kit |
96.96 Sample/Loading Kit—10 IFCs | Fluidigm | BMK-M10-96.96 | |
2x Assay Loading Reagent | Fluidigm | From 96.96 Sample/Loading Kit | |
20x GE Sample Loading Reagent | Fluidigm | From 96.96 Sample/Loading Kit | |
Control line fluid | Fluidigm | From 96.96 Sample/Loading Kit | |
TaqMan Gene Expression Master Mix | Applied Biosystems | 4369016 | |
BioMark HD | Fluidigm | BMKHD-BMKHD | |
96.96 Dynamic Array IFC | Fluidigm | BMK-M10-96.96GT | |
Excel | Microsoft | Microsoft | |
FlowJo V10 | TreeStar | TreeStar | |
Fluidigm real time PCR analysis | Fluidigm | Fluidigm | |
CD179a.VPREB1 | Thermofisher scientific | Hs00356766_g1 | |
ACE | Thermofisher scientific | Hs00174179_m1 | |
AHR | Thermofisher scientific | Hs00169233_m1 | |
BCR_ABL.52 | Thermofisher scientific | Hs03043652_ft | |
BCR_ABL41 | Thermofisher scientific | Hs03024541_ft | |
BMI1 | Thermofisher scientific | Hs00995536_m1 | |
CCNA2 | Thermofisher scientific | Hs00996788_m1 | |
CCNB1 | Thermofisher scientific | Hs01030099_m1 | |
CCNB2 | Thermofisher scientific | Hs01084593_g1 | |
CCNC | Thermofisher scientific | Hs01029304_m1 | |
CCNE1 | Thermofisher scientific | Hs01026535_g1 | |
CCNF | Thermofisher scientific | Hs00171049_m1 | |
CCR9 | Thermofisher scientific | Hs01890924_s1 | |
CD10.MME | Thermofisher scientific | Hs00153510_m1 | |
CD11a | Thermofisher scientific | Hs00158218_m1 | |
CD11c.ITAX | Thermofisher scientific | Hs00174217_m1 | |
CD123.IL3RA | Thermofisher scientific | Hs00608141_m1 | |
CD133.PROM1 | Thermofisher scientific | Hs01009250_m1 | |
CD151 | Thermofisher scientific | Hs00911635_g1 | |
CD220.INSR | Thermofisher scientific | Hs00961554_m1 | |
CD24.HSA | Thermofisher scientific | Hs03044178_g1 | |
NCOR1 | Thermofisher scientific | Hs01094540_m1 | |
CD26.DPP4 | Thermofisher scientific | Hs00175210_m1 | |
CD274 | Thermofisher scientific | Hs01125301_m1 | |
CD276 | Thermofisher scientific | Hs00987207_m1 | |
CD32.FCGR2B | Thermofisher scientific | Hs01634996_s1 | |
CD33 | Thermofisher scientific | Hs01076281_m1 | |
CD34 | Thermofisher scientific | Hs00990732_m1 | |
CD344.FZD4 | Thermofisher scientific | Hs00201853_m1 | |
CD352.SLAMF6 | Thermofisher scientific | Hs01559920_m1 | |
CD38 | Thermofisher scientific | Hs01120071_m1 | |
CD4 | Thermofisher scientific | Hs01058407_m1 | |
CD41.ITGA2B | Thermofisher scientific | Hs01116228_m1 | |
CD49f.ITGA6 | Thermofisher scientific | Hs01041011_m1 | |
CD56.NCAM1 | Thermofisher scientific | Hs00941830_m1 | |
CD9 | Thermofisher scientific | Hs00233521_m1 | |
CD97 | Thermofisher scientific | Hs00173542_m1 | |
CD99 | Thermofisher scientific | Hs00908458_m1 | |
CDK6 | Thermofisher scientific | Hs01026371_m1 | |
CDKN1A | Thermofisher scientific | Hs00355782_m1 | |
CDKN1B | Thermofisher scientific | Hs01597588_m1 | |
CDKN1C | Thermofisher scientific | Hs00175938_m1 | |
CEBPa | Thermofisher scientific | Hs00269972_s1 | |
CSF1r | Thermofisher scientific | Hs00911250_m1 | |
CSF2RA | Thermofisher scientific | Hs00531296_g1 | |
CSF3RA | Thermofisher scientific | Hs01114427_m1 | |
E2A.TCF3 | Thermofisher scientific | Hs00413032_m1 | |
EBF1 | Thermofisher scientific | Hs01092694_m1 | |
ENG | Thermofisher scientific | Hs00923996_m1 | |
EPOR | Thermofisher scientific | Hs00959427_m1 | |
ERG | Thermofisher scientific | Hs01554629_m1 | |
FLI1 | Thermofisher scientific | Hs00956711_m1 | |
FLT3 | Thermofisher scientific | Hs00174690_m1 | |
FOXO1 | Thermofisher scientific | Hs01054576_m1 | |
GAPDH | Thermofisher scientific | Hs02758991_g1 | |
GATA1 | Thermofisher scientific | Hs00231112_m1 | |
GATA2 | Thermofisher scientific | Hs00231119_m1 | |
GATA3 | Thermofisher scientific | Hs00231122_m1 | |
GFI1 | Thermofisher scientific | Hs00382207_m1 | |
HES1 | Thermofisher scientific | Hs01118947_g1 | |
HLF | Thermofisher scientific | Hs00171406_m1 | |
HMGA2 | Thermofisher scientific | Hs00171569_m1 | |
HOXA5 | Thermofisher scientific | Hs00430330_m1 | |
HOXB4 | Thermofisher scientific | Hs00256884_m1 | |
ID2 | Thermofisher scientific | Hs04187239_m1 | |
IGF2BP1 | Thermofisher scientific | Hs00198023_m1 | |
IGF2BP2 | Thermofisher scientific | Hs01118009_m1 | |
IKZF1 | Thermofisher scientific | Hs00172991_m1 | |
IL1RAP | Thermofisher scientific | Hs00895050_m1 | |
IL2RG | Thermofisher scientific | Hs00953624_m1 | |
IRF8 | Thermofisher scientific | Hs00175238_m1 | |
ITGB7 | Thermofisher scientific | Hs01565750_m1 | |
KIT | Thermofisher scientific | Hs00174029_m1 | |
Lin28B | Thermofisher scientific | Hs01013729_m1 | |
LMO2 | Thermofisher scientific | Hs00153473_m1 | |
LYL1 | Thermofisher scientific | Hs01089802_g1 | |
Meis1 | Thermofisher scientific | Hs01017441_m1 | |
mKi67 | Thermofisher scientific | Hs01032443_m1 | |
MPL | Thermofisher scientific | Hs00180489_m1 | |
MPO | Thermofisher scientific | Hs00924296_m1 | |
NFIB | Thermofisher scientific | Hs01029175_m1 | |
Notch1 | Thermofisher scientific | Hs01062011_m1 | |
Pten | Thermofisher scientific | Hs02621230_s1 | |
RAG2 | Thermofisher scientific | Hs01851142_s1 | |
RPS18 | Thermofisher scientific | Hs01375212_g1 | |
RUNX1 | Thermofisher scientific | Hs00231079_m1 | |
Shisa2 | Thermofisher scientific | Hs01590823_m1 | |
Spi1 | Thermofisher scientific | Hs02786711_m1 | |
Sterile.IgH | Thermofisher scientific | Hs00378435_m1 | |
TAL1 | Thermofisher scientific | Hs01097987_m1 | |
THY1 | Thermofisher scientific | Hs00264235_s1 | |
Tim.3.HAVCR2 | Thermofisher scientific | Hs00958618_m1 | |
VWF | Thermofisher scientific | Hs00169795_m1 |
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