Source: Bolívar, B. E., et. al., Visualization of Inflammatory Caspases Induced Proximity in Human Monocyte-Derived Macrophages. J. Vis. Exp. (2022).
This video demonstrates the activation of caspase proteins in human macrophages using bimolecular fluorescence complementation reporter proteins. The pro-domains of caspase-1 are fused with non-fluorescent fragments of Venus proteins, which are recruited to inflammasome complexes. The proximity of inflammasomes induces the refolding and fluorescence of Venus fragments, confirming caspase activation in the macrophages.
All procedures involving human participants have been performed in compliance with the institutional, national, and international guidelines for human welfare and have been reviewed by the local institutional review board.
1. Isolation of human monocytes and differentiation into macrophages
2. Preparation of electroporation components
NOTE: This protocol is designed for a 10 µL-Neon tip (Table of Materials). For each transfection, use 1-2 x 105 cells. It is recommended to seed transfected cells on a 48-well plate or 8-well chambered dish (10 µL-transfected cells per well). 1x sterile DPBS (without Ca2+ and Mg2+) can be used in place of PBS.
3. Preparation of cells for electroporation
NOTE: The yield of MDM from a 10 cm dish at the end of the 7-day differentiation period is approximately 1.5 x 106 cells. 1x sterile DPBS (without Ca2+ and Mg2+) can be used in place of PBS. This protocol was optimized so that most macrophages are detached from the plate with the maintenance of cell viability and integrity. MDM is difficult to detach from cell culture plates. Therefore, it may be necessary to perform steps 3.2 and 3.3 twice to dissociate the cells. Ensure that each incubation time with trypsin-EDTA (0.25%) does not exceed 5 min.
4. Nucleofection of caspase BiFC components into human monocyte-derived macrophages
NOTE: This section of the protocol is performed using the Neon Transfection System (Table of Materials). This protocol outlines the steps to transfect 1-2 x 105 cells using a 10 µL Neon tip (Table of Materials). If using a 100 µL Neon tip, scale up accordingly. Avoid exposing cells to resuspension buffer R for more than 15 min, as this may decrease cell viability and transfection efficiency.
5. Treatment of transfected MDM and caspase BiFC data acquisition
NOTE: If planning to image the cells using an epifluorescence or confocal microscope, treatment with qVD-OPh (20 µM) for 1 h prior to treatment with the chosen stimulus is advised to prevent caspase-dependent cell death (predominantly apoptosis). This is used in imaging to prevent cells from lifting off due to apoptosis, making them very difficult to image as they move out of the focal plane. Note that caspase recruitment to the activation platform and the associated caspase BiFC is not dependent on the catalytic activity of the caspase, and consequently, caspase inhibition will not affect this step.
Figure 1: Schematic overview of the experimental workflow.
Figure 2: Differentiation of CD14-monocytes and transfection of human MDM. (A) Representative bright-field images of CD14+ monocytes from peripheral blood exposed to GM-CSF at days 1, 3, 4, and 7 of differentiation (Scale bar, 100 µm). (B-C) Human MDM was transfected with caspase-1 pro-BiFC pairs and the transfection reporter dsRedmito (50 ng, red). 24 h later, cells were primed with and without LPS (100 ng/mL) for 3 h and treated with nigericin (5 µM) in an imaging medium for 20 h. Representative confocal images of untreated and nigericin-treated cells are shown (Scale bar, 10 µm). The BiFC is shown in green. (D) Cells from (C) were assessed for the percentage of dsRedmito-positive cells (red, transfected cells) that were Venus-positive (green, caspase-1 BiFC complex) at 20 h. Error bars represent the SD of at least two independent counts per well.
48-well tissue culture2:34 plates | Genesee Scientific | 25-108 | |
10 cm Tissue Culture Dishes | VWR | 25382-166 | |
2 Mercaptoethanol 1000x | Thermo Fisher Scientific | 21985023 | |
8-well chambered coverglass with 1.5 HP coverglass | Cellvis | c8-1.5H-N | |
AutoMACS columns | Miltenyi (Biotec) | 130-021-101 | For automated separation using AutoMACS Pro Separator only |
AutoMACS Pro Separator | Miltenyi (Biotec) | 130-092-545 | For automated separation using AutoMACS Pro Separator only |
AutoMACS Pro Washing Solution | Miltenyi (Biotec) | 130-092-987 | For automated separation using AutoMACS Pro Separator only |
AutoMACS Rinsing Solution | Miltenyi (Biotec) | 130-091-222 | For automated separation using AutoMACS Pro Separator only |
AutoMacs running buffer | Miltenyi (Biotec) | 130-091-221 | For manual or automated separation using QuadroMACS or AutoMACS pro Separator |
Axio Observer Z1 motorized inverted microscope equipped with a CSU-X1A 5000 spinning disk unit | Zeiss | Any confocal microscope equipped with a laser module fitted with laser lines of 568 nm (RFP) and 488 or 512 nm (GFP or YFP) wavelengths can be used | |
AxioObserver A1, Research-Grade Inverted Microscope | Zeiss | Any epifluorescence microscope with fluorescence filters capable of exciting 568 nm (RFP) and 488 or 512 nm (GFP or YFP) wavelengths can be used | |
CD14+ MICROBEADS | Miltenyi (Biotec) | 130-050-201 | For manual or automated separation using QuadroMACS or AutoMACS pro Separator |
DPBS without calcium chloride and magnesium chloride | Sigma | D8537-6x500ML | |
DsRed mito plasmid | Clontech | 632421 | Similar plasmids that can be used as fluorescent reporters can be found on Addgene |
Fetal Bovine Serum | Thermo Fisher Scientific | 10437028 | |
Ficoll-Paque PLUS 6 x 100 mL | Sigma | GE17-1440-02 | |
GlutaMAX Supplement (100x) | Thermo Fisher Scientific | 35050079 | |
GM-CSF | Thermo Fisher Scientific | PHC2011 | |
Hemin BioXtra, from Porcine, INSERT CHARACTR96.0% (HPLC) | Sigma | 51280-1G | |
HEPES | Thermo Fisher Scientific | 15630106 | |
Inflammatory caspase BiFC plasmids | Available by request from LBH lab | ||
LPS-EB Ultrapure | Invivogen | TLRL-3PELPS | |
LS Columns | Miltenyi (Biotec) | 130-042-401 | For manual separation using QuadroMACS Separator only |
MACS 15 mL Tube Rack | Miltenyi (Biotec) | 130-091-052 | For manual separation using QuadroMACS Separator only |
MACS MultiStand | Miltenyi (Biotec) | 130-042-303 | For manual separation using QuadroMACS Separator only |
mCherry plasmid | Yungpeng Wang Lab | Similar plasmids that can be used as fluorescent reporters can be found on Addgene | |
Neon Transfection System | Thermo Fisher Scientific | MPK5000 | Includes Neon electroporation device, pipette and pipette station |
Neon Transfection System 10 µL Kit | Thermo Fisher Scientific | MPK1096 | Includes resuspension buffer R, resuspension buffer T, electrolytic buffer E, 96 x 10 µL Neon tips and Neon electroporation tubes |
Nigericin sodium salt, ready made solution | Sigma | SML1779-1ML | |
Penicillin-Streptomycin (10,000 U/mL) | Thermo Fisher Scientific | 15140122 | |
Poly-D-Lysine Hydrobromide | Sigma | P7280-5mg | |
QuadroMACS Separator | Miltenyi (Biotec) | 130-090-976 | For manual separation using QuadroMACS Separator only |
qVD-OPh | Fisher (ApexBio) | 50-101-3172 | |
RPMI 1640 Medium | Thermo Fisher Scientific | 11875119 | |
Trypsin-EDTA (0.25%), phenol red | Thermo Fisher Scientific | 25200072 | |
UltraPure 0.5 M EDTA, pH 8.0 | Thermo Fisher Scientific | 15575020 |