We have optimized a protocol to isolate and purify murine cardiac pericytes for basic research and investigation of their biology and therapeutic potential.
Pericytes, perivascular cells of microvessels and capillaries, are known to play a part in angiogenesis, vessel stabilization, and endothelial barrier integrity. However, their tissue-specific functions in the heart are not well understood. Moreover, there is currently no protocol utilizing readily accessible materials to isolate and purify pericytes of cardiac origin. Our protocol focuses on using the widely used mammalian model, the mouse, as our source of cells. Using the enzymatic digestion and mechanical dissociation of heart tissue, we obtained a crude cell mixture that was further purified by fluorescence activating cell sorting (FACS) by a plethora of markers. Because there is no single unequivocal marker for pericytes, we gated for cells that were CD31-CD34-CD45-CD140b+NG2+CD146+. Following purification, these primary cells were cultured and passaged multiple times without any changes in morphology and marker expression. With the ability to regularly obtain primary murine cardiac pericytes using our protocol, we hope to further understand the role of pericytes in cardiovascular physiology and their therapeutic potential.
Perivascular cells known as pericytes surround the microvessels and capillaries of the vascular tree1,2. Physiologically, they are known to promote and play a part in angiogenesis, increase barrier integrity due to their close relationship with endothelial cells as well as stabilize and mature vessels1,2. Moreover, the dysfunction and/or loss of these cells have been implicated in diseases such as Alzheimer's disease2,3 and various cardiovascular diseases4. These cells are found throughout the entire body, but the cell numbers are tissue-dependent. Pericytes have most notably been studied in the brain due to high vascularization of the blood-brain barrier1,2. However, in the heart, the biology of pericytes is understudied.
Recently, there are increased interests in the field for cardiac pericytes, but there is currently no streamlined protocol available for their isolation from one of the most used tools in biology — the mouse. There are protocols in the literature on isolating pericytes from the brain5, retina6, placenta7, and skeletal muscle8,9; however, few protocols are on isolating pericytes from the heart. There are several groups that have isolated cardiac pericytes. Nees et al. were able to isolate an abundant amount of cardiac pericytes from multiple species including the mouse; however, their methods used specific in-house built equipment which decreases reproducibility10. Avolio et al.11, Chen et al.12, and Baily et al.13 also successfully isolated cardiac pericytes from human heart tissue, but human tissues are not always available and hard to obtain for some investigators. Here, we have developed an isolation method to obtain cardiac pericytes from mouse models for investigators to further study their biology with readily available materials.
Using enzymatic digestion and fluorescence activated cell sorting (FACS) with known key pericyte markers14, our protocol allows us to isolate and purify a population of pericytes that are characterized by CD31-CD34-CD45-CD140b+NG2+CD146+. Our panel of markers contain both inclusion and exclusion markers. CD45 is used as a marker to exclude hematopoietic cells. CD31 is used as a marker to exclude endothelial cells. CD34 is used as a marker to exclude both hematopoietic and endothelial progenitor cells. CD146 is a marker for perivascular cells. Lastly, NG2 and CD140b (also known as platelet derived growth factor receptor beta — PDGFRβ) are both accepted markers for pericytes14.The primary culture obtained can be cultured and passaged multiple times with no changes in morphology or marker expression. Furthermore, these cells can be co-cultured with endothelial cells to study their interactions and crosstalk with each other. This cell isolation method will allow investigators to study the biology and pathophysiology of cardiac pericytes from wild type, disease, and genetically variant mouse models.
All animals were housed and used in an Association for Assessment and Accreditation of Laboratory Animal Care International (AAALAC) accredited facility and all animal work was conducted under appropriate veterinary oversight and under the Institutional Animal Care and Use Committee (IACUC) approved protocol of Amgen Inc.
1. Preparation of Tools and Culture Media
2. Preparation of Animal and Procurement of Cardiac Tissue
3. Dissociation of Heart Tissue
4. Purification of Pericytes from Crude Cell Mixture Using FACS
5. Culturing of Pericytes
6. Characterization of Pericytes
After enzymatic digestion and dissociation of the whole heart and before FACS purification of the cells, cells are a crude mixture that contains many different cell types from the heart (Figure 1A). After FACS purification and culturing, cells are homogenous. They are single nucleated, quite flat, and have the typical pericyte rhomboid morphology (Figure 1B).
Using FACS, cells are purified to homogeny. The unstained control cell sample is used to show the gating strategy (Figure 2). First, debris and doublets were gated out based on forward and side scatter distributions. Then dead cells were gated out due to their amine reaction with the dye which produces a signal greater and more intense than live cells. Of the live cells, hematopoietic cells were gated out by being CD45+. To further remove hematopoietic and endothelial cells, CD34+ and CD31+ cells were gated out. Finally, NG2+Â and CD140b+/CD146+ cells were selected for being perivascular cells with expression of typical pericyte markers (Figure 3). The marker panel was also tested on mouse coronary endothelial cells as a control (Supplemental Figure 1). Only about 1% of crude cell mixture consisted of pericytes after sorting.
To validate that the cells were indeed pericytes, we passaged the cells for further characterization. Cells grew rapidly once they reached P3 in the T-75 flasks without changes in viability as they became older (Supplemental Figure 2). When compared with human brain pericytes, the cells had a similar morphology (Figure 4A). When compared with mouse and human smooth muscle cells, the cells had a different morphology (Figure 4A). There were also no observed changes in morphology or marker expression at P7 when immunostained or by flow cytometry analysis after passaging (Figure 4B,C).
Figure 1: Crude cells versus purified cells. (A) The brightfield image of crude cell mixture post whole heart enzymatic digestion and dissociation which has been cultured in a T25 flask for 14 days. (B) The brightfield image of a homogenous population of cardiac pericytes post-sorting and culturing after 14 days. Scale bar = 100 µm. Please click here to view a larger version of this figure.
Figure 2: Representative images of FACS analysis of unstained cells. Schematic representation of the gating strategy used to purify crude cell mixture. Gate for cells that are single, live, CD45-, CD31-, CD34-, NG2+, CD146+, and CD140b+. Please click here to view a larger version of this figure.
Figure 3: Representative images of FACS analysis of crude cells. Schematic representation of the sorting used to obtain a homogenous population of cardiac pericytes. Roughly 1% of crude cells are CD31-CD34-CD45-CD140b+NG2+CD146+. Please click here to view a larger version of this figure.
Figure 4: Characterization of primary isolated cardiac pericytes (A) Brightfield images of cultured cells from human brain (hPC) and mouse hearts (mPC) show similar pericyte cell morphology but different morphology from human smooth muscle cells hSMC) and mouse smooth muscle cells (mSMC). Scale bar = 100 µm. (B) Phenotypic characterization of cells at P7 by immunocytochemistry for pericyte markers. Scale bar = 100 µm. (C) Analysis by flow cytometry of the pericytes at P7 where they were gated for negative markers CD31, CD34, CD45 and positive markers NG2, CD140b, and CD146. Population remains homogenous. Please click here to view a larger version of this figure.
Supplemental Figure 1: Representative images of flow cytometry analysis of endothelial cells using marker panel. A mouse coronary endothelial cell line was used as control for the binding specificity for the markers. Using the same gating strategy that was used in the sort except for a positive gate for CD31 instead of a negative gate, the endothelial cells were negative for CD45, CD34, NG2, CD140b, and CD146 but positive for CD31 as expected. Please click here to download this figure.
Supplemental Figure 2: Representative images of flow cytometry analysis of different passages of mPC. Primary isolated cardiac pericytes were cultured and passaged up to passage 12. Cells were stained with propidium iodide and analyzed on a flow cytometer. Control population is a mixture of dead cells and live cells. There were no significant differences in number of viable cells between passages. Please click here to download this figure.
As studies on cardiac pericytes are relatively new, the role of pericytes in cardiovascular physiology and pathophysiology have yet to be defined. In other organs, they have been shown to play key roles in vessel homeostasis and perfusion1,2. Compared to the literature of pericytes from other organs such as the brain, there are significantly fewer publications on cardiac pericytes. The isolation of cardiac pericytes is critical to the understanding of their functional characteristics and signaling mechanisms. Therefore, this protocol will provide investigators with an easier way to access cardiac pericytes from a more readily available tissue source and promote studies on their biology. It will help answer questions on how cardiac pericytes contribute to cardiac homeostasis and pathophysiology as well as investigate their therapeutic potential.
The pericyte population isolated from murine heart and characterized by CD31-CD34-CD45-CD140b+NG2+CD146+ has been passaged multiple times (up to P12 and was still going strong), which does not decrease in viability and propagates quickly (Supplementary Figure 2). The cells have also been cryofrozen and recovered with at least 95% viability. However, we prefer to use cells P7 or younger for our experiments. Comparing brightfield images of our pericytes with human brain pericytes, the two cell lines have comparable cell morphology (Figure 4A) while they differ in morphology from smooth muscle cells (Figure 4A). Our P7 cells were characterized by immunocytochemistry for pericyte markers, some from our FACS panel (NG2 and CD140b), and a few not in the panel (vimentin, desmin, αSMA) and we found that the cells expressed pericyte markers homogenously (Figure 4B). Additionally, our P7 cells were analyzed by flow cytometry again with the same marker panel to assess for changes in marker expression due to passaging and we found that there were no changes (Figure 4C). Therefore, both phenotypically and morphologically, our cells are pericytes.
The studies by Nees et al.10, Avolio et al.11, Chen et al.12, and Baily et al.13 have shown successful cardiac pericyte isolations. However, the use of an in-house custom built equipment to detach the pericytes from the microvessels by Nees et al.10 involved two chambers with pumps that perfused protease solution back and forth through a mesh net stack, which was hard to replicate as they did not provide a schematic and/or picture of the apparatus and how it was built. Although Nees et al.10Â successfully isolated cardiac pericytes from many species, we were never able to reproduce their method. Our pericyte detachment step in our protocol simply uses an orbital shaker (to dissociate all cells) which is available in most, if not all laboratories, with the tissue and enzyme solution in a conical tube followed by a mechanical dissociation step. There is no custom apparatus required. Secondly, the remaining protocols involve the use of human tissues and thus the procurement of human tissue is limiting to investigators. Our protocol is a modification and optimization of current protocols9,12 using mouse models (wild type, genetically modified, diseased) and materials that are readily available to all investigators.
Because perivascular cells in general are sensitive, viability of the cells is critical to obtain a good yield. During procurement of cardiac tissue and staining of cells, the tissue/cells need to be kept ice cold. Secondly, the enzymatic digestion of the tissue may require optimization on an individual basis. Depending on the units of activity on one's vials of enzymes, concentration and digestion time may need to be optimized. Make sure that the enzymatic solution is prepared fresh each time otherwise yield will decrease. Thirdly, the crude mixture contains a lot of cells, some dead and/or dying, it is best to lower the concentration of FBS in the staining buffer from 5% to 2%. If you are having trouble with cells clogging the nozzle during sort, enrich the cells first by using a dead cell removal kit. You can also add EDTA/HEPES buffer or DNase treatment to the cell pre-sort to prevent cell clumping. Lastly, because our panel of antibodies is rather large and uses many fluorophores, be sure your FMO controls and compensation controls are done correctly.
One limitation to this method is the amount of cardiac pericytes that can be obtained per heart. In our case, only 1.1% of our crude mixture from one mouse heart were pericytes which is comparable to the percent in the human heart isolations, but the number of cells is significantly less due to the amount of heart tissue a mouse provides. Because the starting number of cells is so low after FACS, it would be better to isolate from multiple hearts at once. However, the problem with that is the sheer number of cells that you need to sort through in one day. If you have more than 30 million cells, it will be difficult to get through the sort without affecting the viability of the cells. If the investigator had multiple cell sorters, isolating from multiple hearts in a day would be doable. Another limitation is that because we do not know if there are subpopulations of pericytes in the heart like there is skeletal muscle15,16, we do not know if we are eliminating a subtype in our gating strategy. We are in the process of characterizing our cardiac pericytes and thus far in our unpublished data, they are functionally like other pericytes in the literature.
Our protocol will enable investigators to answer questions on cardiac pericyte properties, characteristics, functionality, and other aspects that will help define their contribution to cardiac homeostasis and hemodynamics. These cells could have therapeutic potential to cardiovascular disease once their biology is better understood.
The authors would like to thank the Amgen Flow Cytometry Core for their help with fluorophore panel design, troubleshooting, and cell sorting.
Name | Company | Catalog Number | Comments |
0.25% Trypsin-EDTA | Corning | 25-053-Cl | dilute with 1x DPBS to get 0.1% |
100 μM Cell strainer | FisherSci | 22363549 | |
15 mL Falcon conical tubes | BD | 352096 | |
24-well plate | Corning | CLS3527 | |
25 gauge butterfly needle | FisherSci | 22-253-146 | |
31 gauge needle syringe | FisherSci | B328446 | |
50 mL Falcon conical tubes | BD | 352098 | |
6-well plate | Corning | CLS3516 | |
anti-alpha smooth muscle actin rabbit mAb | abcam | ab32575 | Antibody used in ICC 1:100 dilution |
anti-CD140b rabbit mAb | Cell Signaling | 28E1 | Antibody used in ICC 1:100 dilution |
anti-CD31 rabbit pAb | abcam | ab28364 | Antibody used in ICC 1:100 dilution |
anti-desmin rabbit pAb | abcam | ab8592 | Antibody used in ICC 1:100 dilution |
anti-NG2 conjugated to AF488 | Millipore | MAB5384A4 | Antibody used in ICC 1:100 dilution |
anti-vimentin rabbit mAb | abcam | ab92547 | Antibody used in ICC 1:100 dilution |
ArC Amine Reactive Compensation bead kit | Invitrogen | A10346 | compensation beads for Live/Dead Near IR dye |
Brightfield Microscope | camera attached | ||
CD140b-PE (clone APB5) | eBioscience | 12-1402-81 | Antibody used in FACS 1:100 dilution |
CD146-BV605 (clone ME-9F1) | BD | 740434 | Antibody used in FACS 1:100 dilution |
CD31-APC (clone MEC 13.3) | BD | 551262 | Antibody used in FACS 1:100 dilution |
CD34-BV421 (clone RAM 34) | BD | 56268 | Antibody used in FACS 1:100 dilution |
CD45-PE-Cy7 (clone 30-F11) | BD | 552848 | Antibody used in FACS 1:100 dilution |
Centrifuge | eppendorf | ||
Collagenase B | Roche | 11088815001 | 0.226 U/mg lyo. |
Confocal Microscope | |||
DAPI | ThermoFisher | D1306 | nuclear stain |
DMEM with 4.5 g/L glucose, L-glutamine & sodium pyruvate | Corning | 10-013-CV | 500 mL |
Dowell scissors | FST | 15040-11 | |
Dulbecco's Phosphate-Buffered Saline (DPBS) | Corning | 21-030-CV | 500 mL |
Dulbecco's Phosphate-Buffered Saline without Ca and Mg (CMF-DPBS) | Corning | 21-031-CV | 500 mL |
Dumont #5 Fine Forceps | FST | 11254-20 | |
FACSAria cell sorter | BD | Lasers: 405 nm 50 mW, 488 nm 100 mW, 561 nm 50mW, 633 nm 11 mW | |
FACSAria software | BD | ||
Falcon tube round-bottom polypropylene, 5 mL | BD | 38057 | |
Falcon tube with cell strainer cap, 5 mL | BD | 08-771-23 | |
Fetal Bovine Serum | Corning | 35-015-CV | 500 mL |
Fine scissors | FST | 14060-09 | |
FlowJo software | FlowJo LLC | ||
Fortessa LSR flow cytometer | BD | Lasers: 405 nm 50 mW, 488 nm 100 mW, 561 nm 50mW, 633 nm 11 mW | |
Gelatin-based coating | Cell Biologics | 6950 | |
Goat anti-rabbit IgG (H+L) Cross-Absorbed Secondary antibody, Alexa Fluor 488 | Invitrogen | A-11008 | Antibody used in ICC 1:1000 dilution |
Graefe Forceps | FST | 11049-10 | |
Heparin sodium solution | Hospira | NDC 0409-2720-02 | 10,000 USP units/10 mL; from porcine intestines |
Incubator | set at 37 °C, 5% CO2, 95% O2 | ||
Live/Dead-Near IR | Life Technologies | L10119 | |
Microscope slides | FisherSci | 12-550-343 | |
NG2-FITC | Millipore | AB5320A4 | Antibody used in FACS 1:100 dilution |
Oribital shaker | VWR | Inside 37 °C incubator or room | |
Paraformaldehyde | FisherSci | 50-980-487 | dilute with 1x DPBS to get 4% |
Penicillin-Streptomycin | Corning | 30-002-CI | |
 Petri dish | FisherSci | FB0875714 | |
Pipette and tips | |||
ProLong Diamond | ThermoFisher | P36965 | mounting media |
Propidum Iodide | ThermoFisher | cell viability dye for supplemental figure 2 | |
Rabbit IgG FITC | eBiosciences | 11-4614-80 | Isotype control antibody - FITC |
Rat IgG2a APC | Biolegend | 400512 | Isotype control antibody - APC |
Rat IgG2a BV421 | Biolegend | 400536 | Isotype control antibody - BV421 |
Rat IgG2a BV605 | BD | 563144 | Isotype control antibody - BV605 |
Rat IgG2a PE | Biolegend | 400308 | Isotype control antibody - PE |
Rat IgG2b PE-Cy7 | Biolegend | 400617 | Isotype control antibody - PE-Cy7 |
SuperBlock | ThermoFisher | 37515 | blocking buffer |
T75 | ThermoFisher | 156499 | |
Triton X-100 | Sigma | X100 | detergent, dilute with x DPBS to get 0.1% |
UltraComp beads | Invitrogen | 01-2222-42 | compensation beads |
Variable-Flow Peristaltic Pump | FisherSci | 13-876-1 | |
ViCell Cell counter | Beckman | ||
Wash buffer | 1:10 dilution of Superblock in 1x DPBS |
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