Treating Parkinson’s disease : a matter of density?

Abstract

Dopamine agonists constitute the main first-line therapeutic option for Parkinson’s disease (PD). These compounds exert their beneficial effect on locomotion by activating the D2 receptor and thereby compensating for the declining dopaminergic transmission in the striatum. Brain imaging studies have highlighted that the density of dopamine D2 receptors markedly fluctuates across the stages of PD and its pharmacological treatment. Since receptor density constitutes a molecular determinant for the signalling profile of D2 receptors ligands, we hypothesized that variations in receptor expression could influence the response to antiparkinsonian drugs mediated by D2 receptors, most notably relying on Gi1 and β-arrestin2 proteins. The recruitment bias of dopamine, pramipexole, ropinirole, and rotigotine was examined using a live-cell nanoluciferase-based biosensor capable of monitoring the interactions between the D2L receptor and either Gi1 or β-arrestin2. The characterization of the functional selectivity of these D2 agonists was performed at two distinct D2L receptor densities by taking advantage of an engineered cellular model enabling the overexpression of the D2L receptor when exposed to doxycycline. A high receptor density oriented the signalling profile of dopamine towards a preferential recruitment of Gi1. It also moderated the marked Gi1 and β-arrestin2 biases of pramipexole and rotigotine, respectively. The Gi1 bias of ropinirole appeared as not being influenced by D2L receptor density. Given that D2 receptor density was also demonstrated to be significantly elevated in schizophrenic patients, and being exacerbated by antipsychotic treatments, we wondered whether the observed influence of receptor density on the pharmacological profile of antiparkinsonian agents could be also observed for dopamine partial agonists commonly prescribed as antipsychotics. Hence, we extended our experimental paradigm to measure the recruitment bias of aripiprazole, brexpiprazole and cariprazine at both low and high receptor densities. Increasing the dopamine D2 receptor density reoriented aripiprazole’s preferential recruitment from Gi1 to β-arrestin2. Brexpiprazole showed inverse agonism for β-arrestin2 recruitment at the lower receptor density tested, and as an inverse agonism for Gi1 recruitment when tested at a high receptor expression level. At variance, cariprazine evoked a potent partial agonism for β-arrestin2 recruitment only, in all tested conditions. Observing that receptor density was capable of influencing D2 receptor pharmacology with a considerable breadth prompted us to investigate a putative biochemical mechanism underlying our results. Since D2L receptors are known to assemble in homodimers, and that this biochemical feature was demonstrated to be influenced by receptor density and ligand binding, we explored the influence that the D2 receptor ligands exert on D2L homodimeric assembly at distinct receptor densities. By applying a D2L homodimerization assay relying upon a nanoluciferase-based biosensor to a cell model enabling the tetracycline-inducible expression of D2L receptors, we were able to show that increasing receptor density promoted constitutive dopamine D2L receptor homodimerization. Receptor full agonists promoted homodimerization, while antagonists and partial agonists disrupted dopamine D2L receptor homodimers. High receptor densities enhanced this inhibitory effect only for receptor antagonists. Taken together, these observations highlight receptor density as a key driver of both signal transducer recruitment and homodimerization evoked by dopamine agonists, either employed as antiparkinsonian or antipsychotic agents. Importantly, the effect of receptor density on both biochemical phenomena appeared as ligand specific, without any strict correlation with ligands’ intrinsic activity. Finally, this study provides molecular insights positioning rotigotine as the putative most beneficial D2 receptor agonist for the treatment of early and late Parkinson’s disease, while depicting brexpiprazole as the most beneficial option for the management of antipsychotic switches.(BIFA - Sciences biomédicales et pharmaceutiques) -- UCL, 202