Preclinical MRGPRX2, Mrgprb2, Mast Cell Activation, and Evans Blue
Assays
Calcium mobilization in heterologous cells - Clonal HEK293 cells
expressing human MRGPRX2 and Galpha15 were plated in wells of a 96-well
plate, loaded with Fluo-4 acetoxymethyl ester (Fluo-4 AM) for 45minutes
at 37°C and allowed to rest for 30minutes before use (previously
described; McNeil et al., 2015). 2X elamipretide was added by manual
pipetting at designated time points, and fluorescence intensity before,
during, and after elamipretide incubation was measured with a confocal
microscope using the FITC filter. A similar protocol was used for
MRGPRX2-transfected Chem-1 cells and their parental cell line, Chem-1.
Cells were cultured in Dulbecco’s Modified Eagle Medium (DMEM) high
glucose medium (4.5g/L) with 10% fetal bovine serum, non-essential
amino acids, and HEPES. Geneticin (G418) was used to maintain receptor
expression in the MRGPRX2 cell line. These cells were seeded to 96-well
plates, loaded with Fluo-4 AM for 30 minutes at 37°C, washed, and
allowed to rest for 30 minutes before use. Cells were resuspended in
50µL of PBS with calcium and magnesium and 50µL of 2X elamipretide or
ionomycin were added to stimulate the cells. A no-drug vehicle control
and ionomycin positive control were also included. Plates were read
every six seconds for two minutes using a Biotek plate reader.
EC50 calculation - Clonal HEK293 cells expressing human
MRGPRX2 and Galpha15 were plated in a 96-well plate, loaded with Fluo-4
AM for 45 minutes at 37°C, and allowed to rest for 30 minutes before
use. Assay was performed using a fluorescent plate reader and baseline
fluorescence was calculated as the average of a 30-second read. 2X
elamipretide was added manually after baseline recordings, and response
was defined as the maximum signal within 90 seconds after addition of
elamipretide minus the baseline fluorescence signal. Concentrations were
tested in duplicate, the assay was performed six times, and the curve
was calculated as a four-parameter non-linear fit with variable slope.
Peritoneal mast cell activation assay - Primary peritoneal mast cells
were isolated from wild type and Mrgprb2 knockout mice (McNeil 2015),
selected because Mrgprb2 is the mouse ortholog of MRGPRX2. Mast cells
were incubated in DMEM and supplemented with 10% fetal bovine serum and
100ng/mL human stem cell factor for 2 hours in a 96-well plate. Cells
were then loaded with Fluo-4 AM for 30 minutes at room temperature,
washed, and allowed to rest for 30 minutes before use. Free
intracellular calcium levels in each mast cell were measured
(fluorescence microscope). If the Fluo-4 signal rose by at least 50%
for ≥10 seconds, cells were identified as responding.
Evans Blue assay - Anesthetized wild type and Mrgprb2 knockout mice up
to 8 months of age were injected intravenously with 50µL of 12.5mg/mL
Evans Blue in saline, followed by injection of 5µL of 0.5mg/mL
elamipretide in one paw and saline in the other paw. Mice were
sacrificed 15 minutes after elamipretide injection and paw tissue was
collected, dried for 24 hours at 50°C, and weighed. Evans Blue was
extracted by a 24-hour incubation in formamide at 50°C and the optical
density was read at 600nm and 740nm using a spectrophotometer. The value
at 740nm was subtracted from the value at 600nm to attain the final
readout.