enhanced plate reader assay for calcium mobilization in endothelial cells

Studying Protease-Activated Receptor Ligands in Endothelial Cells

Protease-activated receptors (PARs)1,2,3 are a subfamily of class A G protein-coupled receptors (GPCRs) that are expressed in a variety of cell types, including platelets and endothelial cells4,5,6,7. Unlike the majority of GPCRs, PARs have a unique intramolecular mode of activation. Most GPCRs are activated by soluble ligands interacting with a distinct binding pocket, but PARs are activated by the proteolytic cleavage of the N-terminus, which results in a new tethered ligand that can interact with the extracellular loop 2 domain on the surface of a cell6,8,9. This interaction activates the receptor and can initiate several signaling pathways, promoting effects such as inflammation and platelet activation4,10,11,12. Different proteases can activate PARs through cleavage at unique sites on the N-terminus, revealing different tethered ligands (TL) that stabilize receptor conformations, which initiate different signaling pathways9,13,14,15. For example, in the most well-studied member of the subfamily, PAR1, cleavage by thrombin is used to support numerous biological processes, including platelet activation and leukocyte recruitment to the endothelium, but can lead to deleterious effects when the receptor is overexpressed or overactivated4,16,17,18,19,20,21. Conversely, cleavage by activated protein C (aPC) can promote anti-inflammatory effects and maintenance of endothelial barriers15,22,23,24,25,26,27,28,29. PARs can also be activated by peptide analogs of the TLs in an intermolecular fashion13,30,31. These peptides are routinely used to measure PAR inhibition (modulation) in place of PAR-targeting proteases, and they are used in this protocol.
Numerous disorders are associated with pathological PAR1 signaling, including sepsis22,32, cardiovascular disease33,34,35,36,37,38, kidney disease39,40,41,42, sickle cell disease43, fibrosis44, osteoporosis and osteoarthritis45,46, neurodegeneration47,48,49,50,51, and cancer52,53,54,55,56,57,58,59. Antagonists of PAR1 have been studied since the 1990s as antiplatelet agents for cardiovascular disease, and the growing list of diseases associated with the receptor necessitates the identification of novel ligands for use as biological probes (tool compounds) or as potential therapeutics. Currently, there is only one FDA-approved PAR1 antagonist, vorapaxar, which is used to treat coronary artery disease in high-risk patients34,36,37,60. An alternative PAR1 antagonist, the pepducin PZ-128, completed a successful phase II study to prevent thrombosis in cardiac catheterization patients38. The Dockendorff group has focused on the medicinal chemistry and pharmacology of a separate class of small molecules, PAR1 ligands known as parmodulins61,62. Unlike reported PAR1 antagonists such as vorapaxar, parmodulins are allosteric, biased modulators of PAR1 that selectively block the G&#945;q pathway while promoting cytoprotective effects similar to aPC. Unlike potent orthosteric PAR1 antagonists such as vorapaxar, published parmodulins are also reversible63,64,65.
Initially, parmodulins were identified by Flaumenhaft and coworkers for&#160;their ability to inhibit P-selectin expression or granule secretion in platelets61,66. However, an alternative method was required to study the effects of parmodulins on endothelial cells. One common method to monitor GPCR-related signaling is to measure intracellular Ca2+ mobilization, an important secondary messenger that can be measured using a suitable intracellular calcium-binding dye67,68. Substantial evidence has been provided showing that calcium mobilization induced by PAR1 is through the activation of G&#945;q69,70. Once activated by its tethered ligand (or a suitable exogenous ligand), PAR1 undergoes a conformational change which causes guanosine diphosphate (GDP) bound to the G&#945;q subunit to be replaced by guanosine triphosphate (GTP)68. The G&#945;q subunit then activates phospholipase C&#946; (PLC-&#946;), which catalyzes the hydrolysis of phosphatidylinositol 4,5 bisphosphate (PIP2), forming 1,4,5-inositol triphosphate (IP3) and diacylglycerol (DAG). Finally, IP3 binds to IP3-sensitive Ca2+ channels in the membrane of the endoplasmic reticulum, allowing Ca2+ to be released into the cytoplasm, where it can bind to Ca2+-dependent fluorescent dyes, such as Fluo-4, that are added to the cells71.&#160;This process occurs within seconds and can increase the concentration of Ca2+ 100-fold, leading to a drastic change in the amount of calcium-bound dye and a robust fluorescence signal.
In 2018, the Dockendorff group disclosed a medium-throughput Ca2+ mobilization assay that could be used to identify antagonists of the G&#945;q pathway of PAR172. The assay used&#160;EA.hy92673, a hybrid human endothelial cell line, which can be used for multiple passages without a noticeable change in PAR1 expression, and is established for in vitro measurements of cytoprotective effects.&#160;
The original protocol used EA.hy926 cells in 96-well plates and loaded with Fluo-4/AM dye, which was chosen due to its intense fluorescence at 488 nm and high cell permeability. Once the dye was loaded into the cells, lengthy washing steps were performed with an automated 8-channel liquid handler (faster methods of liquid handling, such as a 96-channel washer, were inaccessible). The reproducibility of this assay was superior to that without the careful, automated robotic media changes. Antagonists were then incubated with the cells, PAR1 was activated through the sequential addition of a selective agonist (16 wells at a time), and changes in fluorescence resulting from calcium mobilization and dye binding were measured to determine activity.
While this protocol allows for the measurement of PAR1-mediated calcium mobilization, it is limited by the time required to assay each 96-well plate. Long experiment times are problematic not only because the number of compounds that can be screened each day is limited, but also because dye efflux occurs over time, narrowing the assay window by increasing the basal fluorescence. One contributor to the long experiment time is the use of an 8-channel liquid handler for plate washing, which adds over 30 min to each experiment. The required tips also became difficult to obtain due to supply chain problems. Here an updated protocol for the PAR-mediated calcium mobilization assay that does not require a liquid handler, and therefore can be run in higher throughput, is reported. This protocol should also be suitable for measuring signaling with other GPCRs that lead to intracellular calcium mobilization.&#160;This updated plate reader protocol&#160;is ideal for academic and small industrial labs that do not have the resources for expensive cell imaging instruments but have a need to rapidly screen numerous compounds. For an example of a calcium mobilization assay using a plate imager, see Caers et al.74.

Author Spotlight: Developing Parmodulins to Target Protease-Activated Receptors for Inflammation Control

Characterizing Modulators of Protease-Activated Receptors with a Calcium Mobilization Assay Using a Plate Reader

Our lab is working to understand how the modulation of protease activated receptors can inhibit inflammation. We have developed a new class of small molecules called parmodulins that modulate the activity of PAR1. The goal is to discover parmodulins with improved properties for potential use as drug candidates, and also to understand their mode of action.In preclinical studies with our collaborators, we've determined that parmodulins may be particularly promising for the treatment of disorders driven by thromboinflammation, including kidney, liver, and cardiovascular disease. Their mode of action may also enable the development of compounds with a better safety profile than prior PAR1 targeting drug candidates. The receptor PAR1 is important for activating platelets, but it's signaling in endothelial cells is also important and relevant in numerous pathologies.Therefore, we wanted to develop a cheap and convenient protocol for testing the activity of parmodulins and cultured endothelial cells. We're here to share an updated protocol measuring calcium mobilization that uses a standard plate reader, and does not require an automated liquid handler. It's suitable for screening PAR1 ligands in endothelial cells, but in theory, it can be used to measure the activity of any compounds affecting calcium mobilization.

Enhanced Plate Reader Assay for Calcium Mobilization in Endothelial Cells

After growing endothelial cells in the 96 well plate, remove it from the incubator and confirm confluency using an inverted microscope. Flick the plate into a sink to remove the complete media, and blot the top of the plate with a paper towel. Add HBSS HEPES assay buffer plus probenecid solution to a 50 milliliter multi-channel pipette solvent reservoir.Using a multi-channel pipette, add 100 microliters of assay buffer to each well and allow the cells to sit for five minutes. Then, flick the plate to remove the assay buffer. Add downloading buffer to a separate 50 milliliter multi-channel pipette solvent reservoir.Using a multi-channel pipette, add 50 microliters of the dye-loading buffer to each well of the assay plate. Incubate the plate at 37 degrees Celsius and 5%carbon dioxide for 45 minutes. After incubation, observe the plate under a fluorescence microscope to confirm the dye uptake.Flick the plate to remove the dye solution. Wash the cells twice with 50 microliters of the HBSS HEPES assay buffer plus probenecid solution in each well. Flick the plate to remove the assay buffer.Add 92 microliters of HBSS HEPES assay buffer to each well, and again, view the cells with a fluorescence microscope to ensure excess dye has been fully removed and that cells remain adherent. Using a multi-channel pipette, add two microliters of the antagonist solutions and vehicle to appropriate wells. After turning on the plate reader, set the temperature to 37 degrees Celsius and incubate the plate for 15 minutes.Measure the background fluorescence in columns one and two, performing five scans per well. Eject the plate from the plate reader, then from an 8-tube PCR strip, quickly add six microliters of agonist solution using a multi-channel pipette to each well in columns one and two. Measure the change in fluorescence as calcium mobilization occurs in the cells.Perform 20 scans of each well. The calcium mobilization assay with endothelial cells demonstrated that positive control wells with no antagonist exhibited a rapid increase in fluorescence. Wells with high antagonist concentrations showed minimal change in fluorescence, similar to the negative control wells with no agonist.

enhanced-plate-reader-assay-for-calcium-mobilization-endothelial

Research

JoVE Journal

Biochemistry

Changes in calcium concentration in cells are rapidly monitored in a high-throughput fashion with the use of intracellular, fluorescent, calcium-binding dyes and imaging instruments that can measure fluorescent emissions from up to 1,536 wells simultaneously. However, these instruments are much more expensive and can be challenging to maintain relative to widely available plate readers that scan wells individually. Described here is an optimized plate reader assay for use with an endothelial cell line (EA.hy926) to measure the protease-activated receptor (PAR)-driven activation of G&#945;q signaling and subsequent calcium mobilization using the calcium-binding dye Fluo-4. This assay has been used to characterize a range of PAR ligands, including the allosteric PAR1-targeting anti-inflammatory "parmodulin" ligands identified in the Dockendorff lab. This protocol obviates the need for an automated liquid handler and permits the medium-throughput screening of PAR ligands in 96-well plates and should be applicable to the study of other receptors that initiate calcium mobilization.

An improved protocol for a calcium mobilization assay with endothelial cells, used to identify ligands of protease-activated receptors (PARs), has been developed. The new protocol reduces total assay time by 90-120 min and yields reproducible concentration-response curves.

Changes in calcium concentration in cells are rapidly monitored in a high-throughput fashion with the use of intracellular, fluorescent, calcium-binding ...

Culturing the Endothelial Cell Line in 96-Well Plates for the Calcium Mobilization Assay

To begin, arrange the required experimental materials on the working platform. Add 100 microliters of sterile 0.4%gelatin solution to the wells of a black walled 96 well clear bottom plate. Incubate the plate at 37 degrees Celsius and 5%carbon dioxide for 30 minutes.Under an inverted microscope, confirm that endothelial cells in a T75 flask are 80 to 100%confluent. Remove the DMEM complete medium from the flask via aspiration. To wash the cells, add 10 milliliters of PBS and gently shake the flask.Replace the PBS with five milliliters of cell dissociation solution. Incubate the flask at 37 degrees Celsius and 5%carbon dioxide for approximately 12 minutes. Tap the flask after six minutes.To help facilitate dissociation. Add five milliliters of the prewarmed DMEM complete medium to the flask, and mix the cell suspension. Using a 10 milliliter serological pipette, transfer the cell suspension into a sterile 50 milliliter tube and centrifuge at 150 G for five minutes.In the meantime, using a pastor pipette connected to a vacuum, aspirate the gelatin solution from the 96 well plate. After centrifugation of the cells, remove the supernatin from the tube without disturbing the pellet. Using a serological pipette, add eight milliliters of fresh DMEM complete medium to the tube, and gently pipette the suspension to break up the cell pellet.Count the cells using trippen blue on a hemocytometer. Add DMEM complete medium to make a density of 600, 000 cells per milliliter suspension. Mix the cell suspension by gently inverting the tube.Add the cell suspension to a 50 milliliter multi-channel pipette reservoir. Every 30 seconds, gently rock the reservoir side to side to mix the suspension and use a multi-channel pipette to add 100 microliters to each well of the 96 well plate coated with gelatin. Replace the transparent plate cover and gently shake the plate by sliding it on the surface of the sterile hood from north to south and east to west.Incubate the plate at 37 degrees Celsius with 5%carbon dioxide for 16 to 24 hours.