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. 2011 May 27;18(5):563-8.
doi: 10.1016/j.chembiol.2011.02.016.

NIR-mbc94, a Fluorescent Ligand That Binds to Endogenous CB(2) Receptors and Is Amenable to High-Throughput Screening

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NIR-mbc94, a Fluorescent Ligand That Binds to Endogenous CB(2) Receptors and Is Amenable to High-Throughput Screening

Michelle Sexton et al. Chem Biol. .
Free PMC article

Abstract

High-throughput screening (HTS) of chemical libraries is often used for the unbiased identification of compounds interacting with G protein-coupled receptors (GPCRs), the largest family of therapeutic targets. However, current HTS methods require removing GPCRs from their native environment, which modifies their pharmacodynamic properties and biases the screen toward false positive hits. Here, we developed and validated a molecular imaging (MI) agent, NIR-mbc94, which emits near infrared (NIR) light and selectively binds to endogenously expressed cannabinoid CB(2) receptors, a recognized target for treating autoimmune diseases, chronic pain and cancer. The precision and ease of this assay allows for the HTS of compounds interacting with CB(2) receptors expressed in their native environment.

Figures

Figure 1
Figure 1. NIR-mbc94, a SR144528 Analog, Binds to CB2 Receptors in Homogenates
(A) Chemical structure of SR144528. Shown are the three modification sites to which alkyl amino linkers were conjugated. Note that NIR-mbc94 was synthesized by conjugating the R3 position a carbon linker and IRDye 800CW, a NIR emitting fluorophore. For NIR-mbc94 structure, see Figure S1A. (B) Radioligand binding competition curve of SR144528, mbc94 and NIR-mbc94. Experiments were performed using [3H]-CP-55,940 (~3 nM) and homogenates prepared from CB2 mid DBT cells. Specific binding is shown on y axis. Each data point represensts the mean ± SEM from three experiments, each performed in triplicate (using homogenates prepared from independent cell cultures).
Figure 2
Figure 2. NIR-mbc94 Binds to CB2 Receptors Expressed by Intact Cells: Heterologous Expression
(A) Saturation and competition of NIR-mbc94 binding to CB2 receptors. Fluorescent signal emitted by increasing concentrations of NIR-mbc94 bound to CB2 mid DBT cells was measured with a Li-Cor Odyssey scanner. Specific binding data are expressed as fluorescence values or relative functional units (RFU on the y axis) as a function of MI concentration (on the x axis). Data points for specific binding were obtained by subtracting the amount of fluorescence emitted by NIR-mbc94 incubated with CB2 mid DBT cells minus fluorescence emitted by NIR-mbc94 incubated with untransfected DBT cells, using the values for corresponding concentrations. Inset shows these two curves. Data represent the mean ± SEM from three experiments, each performed in triplicate and on independent cell cultures. (B) Image of NIR-mbc94 bound to CB2 receptors. Shown is a representative image of NIR-mbc94 (1 μM) bound to CB2 mid DBT cells, gated to the fluorescent signal emitted by NIR-mbc94 bound to untransfected DBT cells image (i.e. non-specific binding) (See Figure S2 for detailed images). (C and D) Concentration-dependent competition of NIR-mbc94 bound to CB2 receptors. CB2 mid DBT cells were preincubated for 15 min with increasing concentrations of SR144528 (C) or WIN55212-2 (D), and then incubated for 30 min with NIR-mbc94 (200 nM). Fluorescence signal was measured with a Li-Cor Odyssey scanner. Data represent the mean ± SEM from at least three experiments, each performed in triplicate and on independent cell cultures. See also Table S1.
Figure 3
Figure 3. NIR-mbc94 Binds to CB2 Receptors Expressed by Intact Cells: Endogenous Expression
(A and B) NIR-mbc94 binds to CB2 receptors expressed by (A) BV-2 cells and (B) mouse microglia in primary culture. Cells were preincubated for 15 min with either (A) increasing concentrations WIN55212-2 or (B), microglial cells from wild-type or CB2−/− mouse pups were pretreated with or without IL-4 (10 ng/ml, 18 hr). Data represent the mean ± SEM of three experiments, each performed in triplicate and on independent cell cultures. **p < 0.01 and ***p < 0.001 significantly different from wild-type basal (one-way ANOVA); # p < 0.05 significantly different from wild-type basal (Students t test, two-tailed). See also Figure S3.

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