The Beta-Arrestin-Biased Dopamine D2 Receptor Ligand, UNC9994, Is a Partial Agonist at G-Protein-Mediated Potassium Channel Activation

A β-Arrestin 2-Biased Dopamine Receptor Type 2 (DRD2) Agonist Is More Efficacious Than Cabergoline in Reducing Cell Proliferation in PRL-Secreting but Not in Non-Functioning Pituitary Tumor Cells

The molecular events underlying the variable effectiveness of dopamine receptor type 2 (DRD2) agonists in pituitary neuroendocrine tumors (PitNETs) are not known. Besides the canonical pathway induced by DRD2 coupling with Gi proteins, the β-arrestin 2 pathway contributes to DRD2's antimitotic effects in PRL- and NF-PitNETs. A promising pharmacological strategy is the use of DRD2-biased agonists that selectively activate only one of these two pathways. The aim of the present study was to compare the effects of two biased DRD2 ligands, selectively activating the G protein (MLS1547) or β-arrestin 2 (UNC9994) pathway, with unbiased DRD2 agonist cabergoline in PRL- and NF-PitNET cells. In rat tumoral pituitary PRL-secreting MMQ cells, UNC9994 reduced cell proliferation with a greater efficacy compared to cabergoline (-40.2 ± 20.4% vs. -21 ± 10.9%, p < 0.05), whereas the G-protein-biased agonist induced only a slight reduction. β-arrestin 2 silencing, but not pertussis toxin treatment, reverted UNC9994 and cabergoline's antiproliferative effects. In a cabergoline-resistant PRL-PitNET primary culture, UNC9994 inhibited cell proliferation and PRL release. In contrast, in NF-PitNET primary cultures (n = 23), biased agonists did not show better antiproliferative effects than cabergoline. In conclusion, the preferential activation of the β-arrestin 2 pathway by UNC9994 improves DRD2-mediated antiproliferative effects in PRL-PitNETs, suggesting a new pharmacological approach for resistant or poorly responsive tumors.

Fluorescence-Based Measurements of Membrane-Bound Angiotensin Converting Enzyme 2 Activity Using Xenopus Laevis Oocytes

Functional investigations of enzymes involving cellular expression systems are important for pharmacological studies. The precise control of expression is challenging in transiently transfected mammalian cell lines. Here, we explored the ability of Xenopus laevis oocytes to express a membrane-bound enzyme for functional characterization using standard 96-well plates and a fluorescence-based plate reader assay. We microinjected oocytes with cRNA encoding the angiotensin converting enzyme 2 (ACE2) and measured the enzymatic activity in single oocytes using a commercial fluorescence-based assay. The injected oocytes showed up to a 50-fold increase in fluorescence compared to uninjected oocytes. This fluorescence intensity was dose-dependent on the amount of ACE2 cRNA. These results suggest that Xenopus oocytes can be used for the functional evaluation of membrane-bound enzymes, decreasing the experimental workload.

G protein-coupled receptor kinase-2 confers isoform-specific calcium sensitivity to dopamine D2 receptor desensitization

The dopamine D₂ receptor (D₂ R) functions as an autoreceptor on dopaminergic cell bodies and terminals and as a postsynaptic receptor on a variety of neurons in the central nervous system. As a result of alternative splicing, the D₂ R is expressed as two isoforms: long (D_2L R) and short (D_2S R) differing by a stretch of 29 residues in the third intracellular loop, with D_2S R being the predominant presynaptic isoform. Recent reports described a Ca²⁺ sensitivity of the desensitization time course of potassium currents elicited via D_2S R, but not via D_2L R, when either isoform was selectively expressed in dopaminergic neurons. Here, we aimed to study the mechanism behind this subtype-specific Ca²⁺ sensitivity. Thus, we measured the desensitization of potassium channel responses evoked by D_2L R and D_2S R using two-electrode voltage clamp in Xenopus oocytes in the absence and presence of different amounts of β-arrestin2 and G protein-coupled receptor kinase-2 (GRK2), both of which are known to play important roles in D₂ R desensitization in native cells. We found that co-expression of both GRK2 and β-arrestin2 was necessary for reconstitution of the Ca²⁺ sensitivity of D_2S R desensitization, while D_2L R did not display Ca²⁺ sensitivity under these conditions. The effect of Ca²⁺ chelation by BAPTA-AM to slow the rate of D_2S R desensitization was mimicked by the GRK2 inhibitor, Cmpd101, and by the kinase-inactivating GRK2 mutation, K220R, but not by the PKC inhibitor, Gö6976, nor by the calmodulin antagonist, KN-93. Thus, Ca²⁺ -sensitive desensitization of D_2S R appears to be mediated via a GRK2 phosphorylation-dependent mechanism.

Mechanistic insights into dopaminergic and serotonergic neurotransmission - concerted interactions with helices 5 and 6 drive the functional outcome

Brain functions rely on neurotransmitters that mediate communication between billions of neurons. Disruption of this communication can result in a plethora of psychiatric and neurological disorders. In this work, we combine molecular dynamics simulations, live-cell biosensor and electrophysiological assays to investigate the action of the neurotransmitter dopamine at the dopaminergic D2 receptor (D2R). The study of dopamine and closely related chemical probes reveals how neurotransmitter binding translates into the activation of distinct subsets of D2R effectors (i.e.: Gi2, GoB, Gz and β-arrestin 2). Ligand interactions with key residues in TM5 (S5.42) and TM6 (H6.55) in the D2R binding pocket yield a dopamine-like coupling signature, whereas exclusive TM5 interaction is typically linked to preferential G protein coupling (in particular GoB) over β-arrestin. Further experiments for serotonin receptors indicate that the reported molecular mechanism is shared by other monoaminergic neurotransmitter receptors. Ultimately, our study highlights how sequence variation in position 6.55 is used by nature to fine-tune β-arrestin recruitment and in turn receptor signaling and internalization of neurotransmitter receptors.

Exploiting D2 receptor β-arrestin2-biased signalling to suppress tumour growth of pituitary adenomas

BACKGROUND AND PURPOSE

Dopamine agonists targeting D₂ receptor have been used for decades in treating pituitary adenomas. There has been little clear evidence implicating the canonical G protein signalling as the mechanism by which D₂ receptor suppresses the growth of pituitary tumours. We hypothesize that β-arrestin2-dependent signalling is the molecular mechanism dictating D₂ receptor inhibitory effects on pituitary tumour growth.

EXPERIMENTAL APPROACH

The involvement of G protein and β-arrestin2 in bromocriptine-mediated growth suppression in rat MMQ and GH3 tumour cells was assessed. The anti-growth effect of a β-arrestin2-biased agonist, UNC9994, was tested in cultured cells, tumour-bearing nude mice and primary cultured human pituitary adenomas. The effect of G protein signalling on tumour growth was also analysed by using a G protein-biased agonist, MLS1547, and a Gβγ inhibitor, gallein, in vitro.

KEY RESULTS

β-arrestin2 signalling but not G protein pathways mediated the suppressive effect of bromocriptine on pituitary tumour growth. UNC9994 inhibited pituitary tumour cell growth in vitro and in vivo. The suppressive function of UNC9994 was obtained by inducing intracellular reactive oxygen species generation through downregulating mitochondrial complex I subunit NDUFA1. The effects of Gαi/o signalling and Gβγ signalling via D₂ receptor on pituitary tumour growth were cell-type-dependent.

CONCLUSION AND IMPLICATIONS

Given the very low expression of Gαi/o proteins in pituitary tumours and the complexity of the responses of pituitary tumours to G protein signalling pathways, our study reveals D₂ receptor β-arrestin2-biased ligand may be a more promising choice to treat pituitary tumours with improved therapeutic selectivity.

New phosphosite-specific antibodies to unravel the role of GRK phosphorylation in dopamine D2 receptor regulation and signaling

The dopamine D₂ receptor (D₂R) is the target of drugs used to treat the symptoms of Parkinson’s disease and schizophrenia. The D₂R is regulated through its interaction with and phosphorylation by G protein receptor kinases (GRKs) and interaction with arrestins. More recently, D₂R arrestin-mediated signaling has been shown to have distinct physiological functions to those of G protein signalling. Relatively little is known regarding the patterns of D₂R phosphorylation that might control these processes. We aimed to generate antibodies specific for intracellular D₂R phosphorylation sites to facilitate the investigation of these mechanisms. We synthesised double phosphorylated peptides corresponding to regions within intracellular loop 3 of the hD₂R and used them to raise phosphosite-specific antibodies to capture a broad screen of GRK-mediated phosphorylation. We identify an antibody specific to a GRK2/3 phosphorylation site in intracellular loop 3 of the D₂R. We compared measurements of D₂R phosphorylation with other measurements of D₂R signalling to profile selected D₂R agonists including previously described biased agonists. These studies demonstrate the utility of novel phosphosite-specific antibodies to investigate D₂R regulation and signalling.

Dopamine D2 Receptor Agonist Binding Kinetics-Role of a Conserved Serine Residue

The forward (k_on) and reverse (k_off) rate constants of drug–target interactions have important implications for therapeutic efficacy. Hence, time-resolved assays capable of measuring these binding rate constants may be informative to drug discovery efforts. Here, we used an ion channel activation assay to estimate the k_ons and k_offs of four dopamine D₂ receptor (D₂R) agonists; dopamine (DA), p-tyramine, (R)- and (S)-5-OH-dipropylaminotetralin (DPAT). We further probed the role of the conserved serine S193^5.42 by mutagenesis, taking advantage of the preferential interaction of (S)-, but not (R)-5-OH-DPAT with this residue. Results suggested similar k_offs for the two 5-OH-DPAT enantiomers at wild-type (WT) D₂R, both being slower than the k_offs of DA and p-tyramine. Conversely, the k_on of (S)-5-OH-DPAT was estimated to be higher than that of (R)-5-OH-DPAT, in agreement with the higher potency of the (S)-enantiomer. Furthermore, S193^5.42A mutation lowered the k_on of (S)-5-OH-DPAT and reduced the potency difference between the two 5-OH-DPAT enantiomers. Kinetic K_ds derived from the k_off and k_on estimates correlated well with EC₅₀ values for all four compounds across four orders of magnitude, strengthening the notion that our assay captured meaningful information about binding kinetics. The approach presented here may thus prove valuable for characterizing D₂R agonist candidate drugs.

Targeting β-Arrestins in the Treatment of Psychiatric and Neurological Disorders

Therapies for psychiatric and neurological disorders have been in the development and refinement process for the past 5 decades. Yet, most of these therapies lack optimal therapeutic efficacy and have multiple debilitating side effects. Recent advances in understanding the pathophysiological processes of psychiatric and neurological disorders have revealed an important role for β-arrestins, which are important regulators of G-protein-coupled receptor (GPCR) function, including desensitization and intracellular signaling. These findings have pushed β-arrestins to the forefront as potential therapeutic targets. Here, we highlight current knowledge on β-arrestin functions in certain psychiatric and neurological disorders (schizophrenia, Parkinson's disease, and substance abuse disorders), and how this has been leveraged to develop new therapeutic strategies. Furthermore, we discuss the obstacles impacting the field of β-arrestin-based therapeutic development and future approaches that might help advance strategies to develop optimal β-arrestin-based therapies.

Voltage-Dependent Dopamine Potency at D1-Like Dopamine Receptors

In recent years, transmembrane voltage has been found to modify agonist potencies at several G protein-coupled receptors (GPCRs). Whereas the voltage sensitivities of the Gα_i/o-coupled dopamine D₂-like receptors (D₂R, D₃R, D₄R) have previously been investigated, the putative impact of transmembrane voltage on agonist potency at the mainly Gα_s/olf-coupled dopamine D₁-like receptors (D₁R, D₅R) has hitherto not been reported. Here, we assayed the potency of dopamine in activating G protein-coupled inward rectifier potassium (GIRK) channels co-expressed with D₁R and D₅R in Xenopus oocytes, at -80 mV and at 0 mV. Furthermore, GIRK response deactivation rates upon dopamine washout were measured to estimate dopamine dissociation rate (k_off) constants. Depolarization from -80 to 0 mV was found to reduce dopamine potency by about 7-fold at both D₁R and D₅R. This potency reduction was accompanied by an increase in estimated dopamine k_offs at both receptors. While the GIRK response elicited via D₁R was insensitive to pertussis toxin (PTX), the response evoked via D₅R was reduced by 64% (-80 mV) and 71% (0 mV) in the presence of PTX. Injection of oocytes with Gα_s antisense oligonucleotide inhibited the D₁R-mediated response by 62% (-80 mV) and 76% (0 mV) and abolished the D₅R response when combined with PTX. Our results suggest that depolarization decreases dopamine affinity at D₁R and D₅R. The voltage-dependent affinities of dopamine at D₁R and D₅R may be relevant to the functions of these receptors in learning and memory.

Ligand with Two Modes of Interaction with the Dopamine D2 Receptor-An Induced-Fit Mechanism of Insurmountable Antagonism

A solid understanding of the mechanisms governing ligand binding is crucial for rational design of therapeutics targeting the dopamine D₂ receptor (D₂R). Here, we use G protein-coupled inward rectifier potassium (GIRK) channel activation in Xenopus oocytes to measure the kinetics of D₂R antagonism by a series of aripiprazole analogues, as well as the recovery of dopamine (DA) responsivity upon washout. The aripiprazole analogues comprise an orthosteric and a secondary pharmacophore and differ by the length of the saturated carbon linker joining these two pharmacophores. Two compounds containing 3- and 5-carbon linkers allowed for a similar extent of recovery from antagonism in the presence of 1 or 100 μM DA (>25 and >90% of control, respectively), whereas recovery was less prominent (∼20%) upon washout of the 4-carbon linker compound, SV-III-130, both with 1 and 100 μM DA. Prolonging the coincubation time with SV-III-130 further diminished recovery. Curve-shift experiments were consistent with competition between SV-III-130 and DA. Two mutations in the secondary binding pocket (V91A and E95A) of D₂R decreased antagonistic potency and increased recovery from SV-III-130 antagonism, whereas a third mutation (L94A) only increased recovery. Our results suggest that the secondary binding pocket influences recovery from inhibition by the studied aripiprazole analogues. We propose a mechanism, supported by in silico modeling, whereby SV-III-130 initially binds reversibly to the D₂R, after which the drug-receptor complex undergoes a slow transition to a second ligand-bound state, which is dependent on secondary binding pocket integrity and irreversible during the time frame of our experiments.

Evaluation of Functional Selectivity of Haloperidol, Clozapine, and LASSBio-579, an Experimental Compound With Antipsychotic-Like Actions in Rodents, at G Protein and Arrestin Signaling Downstream of the Dopamine D2 Receptor

LASSBio-579, an N-phenylpiperazine antipsychotic lead compound, has been previously reported as a D₂ receptor (D₂R) ligand with antipsychotic-like activities in rodent models of schizophrenia. In order to better understand the molecular mechanism of action of LASSBio-579 and of its main metabolite, LQFM 037, we decided to address the hypothesis of functional selectivity at the D₂R. HEK-293T cells transiently coexpressing the human long isoform of D₂ receptor (D₂LR) and bioluminescence resonance energy transfer (BRET)-based biosensors were used. The antagonist activity was evaluated using different concentrations of the compounds in the presence of a submaximal concentration of dopamine (DA), after 5 and 20 min. For both signaling pathways, haloperidol, clozapine, and our compounds act as DA antagonists in a concentration-dependent manner, with haloperidol being by far the most potent, consistent with its nanomolar D₂R affinity measured in binding assays. In our experimental conditions, only haloperidol presented a robust functional selectivity, being four- to fivefold more efficient for inhibiting translocation of β-arrestin-2 (β-arr2) than for antagonizing Gi activation. Present data are the first report on the effects of LASSBio-579 and LQFM 037 on the β-arr2 signaling pathway and further illustrate that the functional activity could vary depending on the assay conditions and approaches used.