Development of a Sensitive and HTS-Compatible Reporter Gene Assay for Functional Analysis of Human Adenosine A2a Receptors in CHO-K1 Cells

The establishment of mouse embryonic stem cell cultures on 96-well plates for high-throughput screening

Embryonic stem (ES) cells can be valuable for monitoring differentiation processes and for improving applications in basic developmental biology. The application of ES cells can be a useful tool for drug discovery and toxicology. Therefore, we suggest the high-throughput screening (HTS) system based on ES cells in this study. Firstly, we optimized the feeder-free condition and seeding cell number which can maintained for at least 7 days without over-confluency. We analyzed the system by cell viability, proliferation activity, RT-PCR and morphologic/immunohistochemical evaluations. The optimal cell seeding number was 30/well that was maintained the typical colonial morphology over 9 d with 1,000 U/ml LIF in the limited space. The cell in optimized condition expressed ALP, SSEA-1, Oct 4 and Nanog and the genetic expressions showed similar to protein expressions. The cell lineage marker expressions showed faint or none. The cell viability and proliferation activity were increased in time-dependent manner in our optimized HTS system. In conclusion, the novel HTS system using ES cells can by useful for developing models for drug discovery as well as toxicological screening in the near future.

Development of a cyclic adenosine monophosphate assay for Gi-coupled G protein-coupled receptors by utilizing the endogenous calcitonin activity in Chinese hamster ovary cells

Activation of G(i)-coupled G protein-coupled receptor (GPCRs) by their ligands leads to inhibition of adenylyl cyclase (AC) and reduction of cyclic adenosine monophosphate (cAMP) levels in cells. The traditional cAMP assay for G(i)-coupled GPCRs commonly uses forskolin, a nonspecific AC activator, to increase the basal cAMP level in cells to create an assay window for ligand detection. However, there is still a need to develop a nonforskolin-based cAMP assay because of the challenges inherent in titrating the concentration of forskolin to achieve a reliable assay window, along with issues related to the cAMP-independent effects of forskolin. Herein, we describe such an assay by utilizing the endogenous activity of the calcitonin receptor in Chinese hamster ovary (CHO) cells. The calcitonin receptor is a G(s)-coupled GPCR that, when activated by calcitonin, leads to the stimulation of AC and increases cAMP in cells. Thus, we use calcitonin, instead of forskolin, to increase the basal cAMP level in CHO cells to achieve an assay window. We demonstrated that calcitonin peptides robustly increased cAMP accumulation in several CHO cell lines stably expressing well-known G(i)-coupled GPCRs, such as the Dopamine D2 receptor, the Opioid μ receptor, or the Cannabinoid receptor-1. Agonists of these G(i)-coupled GPCRs attenuated calcitonin-induced cAMP production in their receptor stable cell lines. On the other hand, antagonists and/or inverse agonists blocked the effects of their agonists on calcitonin-induced cAMP production. This calcitonin-based cAMP assay has been demonstrated to be sensitive and robust and exhibited acceptable assay windows (signal/noise ratio) and, thus, can be applied to screen for agonists and antagonists/inverse agonists of G(i)-coupled GPCRs in high-throughput screening formats.

Indexing TNF-alpha gene expression using a gene-targeted reporter cell line

Background

Current cell-based drug screening technologies utilize randomly integrated reporter genes to index transcriptional activity of an endogenous gene of interest. In this context, reporter expression is controlled by known genetic elements that may only partially capture gene regulation and by unknown features of chromatin specific to the integration site. As an alternative technology, we applied highly efficient gene-targeting with recombinant adeno-associated virus to precisely integrate a luciferase reporter gene into exon 1 of the HeLa cell tumor necrosis factor-alpha (TNF-α) gene. Drugs known to induce TNF-α expression were then used to compare the authenticity of gene-targeted and randomly integrated transcriptional reporters.

Results

TNF-α-targeted reporter activity reflected endogenous TNF-α mRNA expression, whereas randomly integrated TNF-α reporter lines gave variable expression in response to transcriptional and epigenetic regulators. 5,6-Dimethylxanthenone-4-acetic acid (DMXAA), currently used in cancer clinical trials to induce TNF-α gene transcription, was only effective at inducing reporter expression from TNF-α gene-targeted cells.

Conclusion

We conclude that gene-targeted reporter cell lines provide predictive indexing of gene transcription for drug discovery.

Functional coupling of the Galpha(olf) variant XLGalpha(olf) with the human adenosine A2A receptor

A recently identified novel Galphaolf variant, XLGalphaolf, is shown to functionally couple to the human adenosine A2A receptor (A2AR). In Sf9 cells expressing A2AR, beta1, and gamma2, co-expression of XLGalphaolf increased NECA-induced [35S]GTPgammaS binding from approximately 130% to 300% of basal levels. Pharmacological characteristics of A2AR ligands on these cells were evaluated by using [3H]ZM241385- and [35S]GTPgammaS- binding assays. The rank order of the equilibrium binding constants (Kd or Ki) of adenosine receptor ligands were [3H]ZM241385 approximately CGS15943 < MRS1220 < < CV1808 approximately NECA < CGS21680 approximately adenosine < IBMECA < HEMADO approximately CPA approximately CCPA. The rank order of EC50 values for agonists were CV1808 approximately NECA < adenosine approximately CGS26180 < IBMECA < HEMADO approximately CPA approximately CCPA. This pharmacology is consistent with the literature for A2AR and suggests that Sf9 cells co-expressing A2AR, beta1, gamma2, and XLGalphaolf could serve as a heterologous expression system for A2AR drug screening.

Distinctive and synergistic signaling of human adenosine A2a and dopamine D2L receptors in CHO cells

Adenosine A(2a) receptor (A(2a)R) colocalizes with dopamine D(2) receptor (D(2)R) in the basal ganglia and modulates D(2)R-mediated dopaminergic activities. A(2a)R and D(2)R couple to stimulatory and inhibitory G proteins, respectively. Their opposing roles in regulating neuronal activities, such as locomotion and alcohol consumption, are mediated by their opposite actions on adenylate cyclase, which often serves as "co-incidence detector" of various activators. On the other hand, the neural actions of A(2a)R and D(2)R are also, at least partially, independent of each other, as indicated by studies using D(2)R and A(2a)R knock-out mice. Here we co-expressed human A(2a)R and human D(2L)R in CHO cells and examined their signaling characteristics. Human A(2a)R desensitized rapidly upon agonist stimulation. A(2a)R activity (80%) was diminished after 2 hr of pretreatment with its agonist CGS21680. In contrast, human D(2L)R activity was sustained even after 2 hr and 18 hr pretreatment with its agonist quinpirole. Long-term (18 hr) stimulation of human D(2L)R also increased basal cAMP levels in CHO cells, whereas long-term (18 hr) activation of human A(2a)R did not affect basal cAMP levels. Furthermore, long-term (18 hr) activation of D(2L)R dramatically sensitized A(2a)R-induced stimulation of adenylate cyclase in a pertussis toxin-sensitive way. Forskolin-induced cAMP accumulation was significantly increased after short-term (2 hr) human D(2L)R stimulation and further elevated after long-term (18 hr) D(2L)R activation. However, neither short-term (2 hr) nor long-term (18 hr) stimulation of A(2a)R affected the inhibitory effects of D(2L)R on adenylate cyclase. Co-stimulation of A(2a)R and D(2L)R could not induce desensitization or sensitization of D(2L)R either. In summary, signaling through A(2a)R and D(2L)R is distinctive and synergistic, supporting their unique and yet integrative roles in regulating neuronal functions when both receptors are present.