Atomistic molecular dynamics simulations of typical and atypical antipsychotic drugs at the dopamine D2 receptor (D2R) elucidates their inhibition mechanism

Canonical and Non-Canonical Antipsychotics' Dopamine-Related Mechanisms of Present and Next Generation Molecules: A Systematic Review on Translational Highlights for Treatment Response and Treatment-Resistant Schizophrenia

Schizophrenia is a severe psychiatric illness affecting almost 25 million people worldwide and is conceptualized as a disorder of synaptic plasticity and brain connectivity. Antipsychotics are the primary pharmacological treatment after more than sixty years after their introduction in therapy. Two findings hold true for all presently available antipsychotics. First, all antipsychotics occupy the dopamine D2 receptor (D2R) as an antagonist or partial agonist, even if with different affinity; second, D2R occupancy is the necessary and probably the sufficient mechanism for antipsychotic effect despite the complexity of antipsychotics' receptor profile. D2R occupancy is followed by coincident or divergent intracellular mechanisms, implying the contribution of cAMP regulation, β-arrestin recruitment, and phospholipase A activation, to quote some of the mechanisms considered canonical. However, in recent years, novel mechanisms related to dopamine function beyond or together with D2R occupancy have emerged. Among these potentially non-canonical mechanisms, the role of Na²⁺ channels at the dopamine at the presynaptic site, dopamine transporter (DAT) involvement as the main regulator of dopamine concentration at synaptic clefts, and the putative role of antipsychotics as chaperones for intracellular D2R sequestration, should be included. These mechanisms expand the fundamental role of dopamine in schizophrenia therapy and may have relevance to considering putatively new strategies for treatment-resistant schizophrenia (TRS), an extremely severe condition epidemiologically relevant and affecting almost 30% of schizophrenia patients. Here, we performed a critical evaluation of the role of antipsychotics in synaptic plasticity, focusing on their canonical and non-canonical mechanisms of action relevant to the treatment of schizophrenia and their subsequent implication for the pathophysiology and potential therapy of TRS.

Efficiency of Homology Modeling Assisted Molecular Docking in G-protein Coupled Receptors

BACKGROUND

Molecular docking is in regular practice to assess ligand affinity on a target protein crystal structure. In the absence of protein crystal structure, the homology modeling or comparative modeling is the best alternative to elucidate the relationship details between a ligand and protein at the molecular level. The development of accurate homology modeling (HM) and its integration with molecular docking (MD) is essential for successful, rational drug discovery.

OBJECTIVE

The G-protein coupled receptors (GPCRs) are attractive therapeutic targets due to their immense role in human pharmacology. The GPCRs are membrane-bound proteins with the complex constitution, and the understanding of their activation and inactivation mechanisms is quite challenging. Over the past decade, there has been a rapid expansion in the number of solved G-protein-coupled receptor (GPCR) crystal structures; however, the majority of the GPCR structures remain unsolved. In this context, HM guided MD has been widely used for structure-based drug design (SBDD) of GPCRs.

METHODS

The focus of this review is on the recent (i) developments on HM supported GPCR drug discovery in the absence of GPCR crystal structures and (ii) application of HM in understanding the ligand interactions at the binding site, virtual screening, determining receptor subtype selectivity and receptor behaviour in comparison with GPCR crystal structures.

RESULTS

The HM in GPCRs has been extremely challenging due to the scarcity in template structures. In such a scenario, it is difficult to get accurate HM that can facilitate understanding of the ligand-receptor interactions. This problem has been alleviated to some extent by developing refined HM based on incorporating active /inactive ligand information and inducing protein flexibility. In some cases, HM proteins were found to outscore crystal structures.

CONCLUSION

The developments in HM have been highly operative to gain insights about the ligand interaction at the binding site and receptor functioning at the molecular level. Thus, HM guided molecular docking may be useful for rational drug discovery for the GPCRs mediated diseases.

A Complete Assessment of Dopamine Receptor- Ligand Interactions through Computational Methods

Background: Selectively targeting dopamine receptors (DRs) has been a persistent challenge in the last years for the development of new treatments to combat the large variety of diseases involving these receptors. Although, several drugs have been successfully brought to market, the subtype-specific binding mode on a molecular basis has not been fully elucidated. Methods: Homology modeling and molecular dynamics were applied to construct robust conformational models of all dopamine receptor subtypes (D₁-like and D₂-like). Fifteen structurally diverse ligands were docked. Contacts at the binding pocket were fully described in order to reveal new structural findings responsible for selective binding to DR subtypes. Results: Residues of the aromatic microdomain were shown to be responsible for the majority of ligand interactions established to all DRs. Hydrophobic contacts involved a huge network of conserved and non-conserved residues between three transmembrane domains (TMs), TM2-TM3-TM7. Hydrogen bonds were mostly mediated by the serine microdomain. TM1 and TM2 residues were main contributors for the coupling of large ligands. Some amino acid groups form electrostatic interactions of particular importance for D₁R-like selective ligands binding. Conclusions: This in silico approach was successful in showing known receptor-ligand interactions as well as in determining unique combinations of interactions, which will support mutagenesis studies to improve the design of subtype-specific ligands.

Molecular dynamics simulation of biased agonists at the dopamine D2 receptor suggests the mechanism of receptor functional selectivity

The dopamine D2 receptor (D2R) is the primary target for antipsychotic drugs. Besides schizophrenia, this receptor is linked to dementia, Parkinson's disease, and depression. Recent studies have shown that β-arrestin biased agonists at this receptor treat schizophrenia with less side effects. Although the high resolution structure of this receptor exists, the mechanism of biased agonism at the receptor is unknown. In this study, dopamine, the endogenous unbiased G-protein agonist, MLS1547, a G-protein biased agonist, and UNC9975, a G-protein antagonist and a β-arrestin biased agonist, were docked to a homology model of the whole D2R including all flexible loops, and molecular dynamics simulations were conducted to study the potential mechanisms of biased agonism. Our thorough analysis on the protein-ligand interaction, secondary structure, tertiary structure, structure dynamics, and molecular switches of all three systems indicates that ligand binding to transmembrane 3 might be essential for G-protein recruitment, while ligand binding to transmembrane 6 might be essential for β-arrestin recruitment. Our analysis also suggests changes in both the secondary and the tertiary structures of TM5 and TM7, molecular switches and ICL3 flexibility are important in biased signaling. Communicated by Ramaswamy H. Sarma.

Molecular Mechanism and Dynamics of S-Deoxyephedrine Moving through Molecular Channels within D3R

In this article, the trajectories of S-deoxyephedrine (SBD) along molecular channels within the complex protein structure of third dopamine receptor (D₃R) are analyzed via molecular dynamic techniques, including potential mean force calculations of umbrella samplings from the 4.5 version of the GROMACS program. Changes in free energy due to the movement of SBD within D₃R are determined, and the molecular dynamic mechanisms of SBD transmitting along molecular channels are probed. Molecular simulated results show that the change in free energy is calculated as 171.7 kJ·mol^-1 for the transmission of SBD toward the outside of the cell along the y+ axis functional molecular channel and is 275.0 kJ·mol^-1 for movement toward the intracellular structure along the y- axis. Within the internal structure of D₃R, the changes in free energy are determined to be 103.6, 242.1, 459.7, and 127.8 kJ·mol^-1 for transmission of SBD along the x+, x-, z+, and z- axes, respectively, toward the cell bilayer membrane, which indicates that SBD leaves much more easily along the x+ axis through the gap between the TM5 (the fifth transmembrane helix) and TM6 (the sixth transmembrane helix) from the internal structure of D₃R. The values of free-energy changes indicate that SBD molecules can clear the protective channel within D₃R, which helps dopamine molecules to leave the D₃R internal structure along the x+ axis and to prevent them for exerting excessive neurotransmitter function. Therefore, our results suggest that SBD is effective for development as a drug for treating schizophrenia and its pharmacology is closely related to its dynamics and mechanisms within the molecular pathway of dopamine receptors.

Analysis of the Glutamate Agonist LY404,039 Binding to Nonstatic Dopamine Receptor D2 Dimer Structures and Consensus Docking

Dopamine receptor D2 (D2R) plays an important role in the human central nervous system and is a focal target of antipsychotic agents. The D2^HighR and D2^LowR dimeric models previously developed by our group are used to investigate the prediction of binding affinity of the LY404,039 ligand and its binding mechanism within the catalytic domain. The computational data obtained using molecular dynamics simulations fit well with the experimental results. The calculated binding affinities of LY404,039 using MM/PBSA for the D2^HighR and D2^LowR targets were -12.04 and -9.11 kcal/mol, respectively. The experimental results suggest that LY404,039 binds to D2^HighR and D2^LowR with binding affinities (K_i) of 8.2 and 1640 nM, respectively. The high binding affinity of LY404,039 in terms of binding to [³H]domperidone was inhibited by the presence of a guanine nucleotide, indicating an agonist action of the drug at D2^HighR. The interaction analysis demonstrated that while Asp114 was among the most critical amino acids for D2^HighR binding, residues Ser193 and Ser197 were significantly more important within the binding cavity of D2^LowR. Molecular modeling analyses are extended to ensemble docking as well as structure-based pharmacophore model (E-pharmacophore) development using the bioactive conformation of LY404,039 at the binding pocket as a template and screening of small-molecule databases with derived pharmacophore models.

Biochemical changes induced by grape seed extract and low level laser therapy administration during intraoral wound healing in rat liver: an experimental and in silico study

In the present study, the changes that occur in rat liver tissue as a result of the use of grape seed extract (GSE) and low level laser therapy (LLLT) in intraoral wound (IW) healing are analyzed using biochemical parameters. Diode laser application groups received 8 J/cm² dose LLLT once a day for 4 days (810 nm wavelength, continuous mode, 0.25 W, 9 s). As a result of the biological parameter analysis, it was determined that the oxidative damage caused by the IWs and recovery period on 7th and 14th days could be substantially removed with GSE applications that have antioxidant capacity especially in rat liver tissue. In addition, the active compound of grape seed, catechin is studied in the active site of glycogen synthase kinase 3 (GSK3) target using molecular modeling approaches. Post-processing molecular dynamics (MD) results for catechin is compared with a standard GSK3 inhibitor. MD simulations assisted for better understanding of inhibition mechanism and the crucial amino acids contributing in the ligand binding. These results along with a through free energy analysis of ligands using sophisticated simulations methods are quite striking and it suggests a greater future role for simulation in deciphering complex patterns of molecular mechanism in combination with methods for understanding drug-receptor interactions.

The First Negative Allosteric Modulator for Dopamine D2 and D3 Receptors, SB269652 May Lead to a New Generation of Antipsychotic Drugs

D₂ and D₃ dopamine receptors belong to the largest family of cell surface proteins in eukaryotes, the G protein-coupled receptors (GPCRs). Considering their crucial physiologic functions and their relatively accessible cellular locations, GPCRs represent one of the most important classes of therapeutic targets. Until recently, the only strategy to develop drugs regulating GPCR activity was through the identification of compounds that directly acted on the orthosteric sites for endogenous ligands. However, many efforts have recently been made to identify small molecules that are able to interact with allosteric sites. These sites are less well-conserved, therefore allosteric ligands have greater selectivity on the specific receptor. Strikingly, the use of allosteric modulators can provide specific advantages, such as an increased selectivity for GPCR subunits and the ability to introduce specific beneficial therapeutic effects without disrupting the integrity of complex physiologically regulated networks. In 2010, our group unexpectedly found that N-[(1r,4r)-4-[2-(7-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-1H-indole-2-carboxamide (SB269652), a compound supposed to interact with the orthosteric binding site of dopamine receptors, was actually a negative allosteric modulator of D₂- and D₃-receptor dimers, thus identifying the first allosteric small molecule acting on these important therapeutic targets. This review addresses the progress in understanding the molecular mechanisms of interaction between the negative modulator SB269652 and D₂ and D₃ dopamine receptor monomers and dimers, and surveys the prospects for developing new dopamine receptor allosteric drugs with SB269652 as the leading compound.

The signaling pathway of dopamine D2 receptor (D2R) activation using normal mode analysis (NMA) and the construction of pharmacophore models for D2R ligands

G-protein-coupled receptors (GPCRs) are targets of more than 30% of marketed drugs. Investigation on the GPCRs may shed light on upcoming drug design studies. In the present study, we performed a combination of receptor- and ligand-based analysis targeting the dopamine D2 receptor (D2R). The signaling pathway of D2R activation and the construction of universal pharmacophore models for D2R ligands were also studied. The key amino acids, which contributed to the regular activation of the D2R, were in detail investigated by means of normal mode analysis (NMA). A derived cross-correlation matrix provided us an understanding of the degree of pair residue correlations. Although negative correlations were not observed in the case of the inactive D2R state, a high degree of correlation appeared between the residues in the active state. NMA results showed that the cytoplasmic side of the TM5 plays a significant role in promoting of residue-residue correlations in the active state of D2R. Tracing motions of the amino acids Arg219, Arg220, Val223, Asn224, Lys226, and Ser228 in the position of the TM5 are found to be critical in signal transduction. Complementing the receptor-based modeling, ligand-based modeling was also performed using known D2R ligands. The top-scored pharmacophore models were found as 5-sited (AADPR.671, AADRR.1398, AAPRR.3900, and ADHRR.2864) hypotheses from PHASE modeling from a pool consisting of more than 100 initial candidates. The constructed models using 38 D2R ligands (in the training set) were validated with 15 additional test set compounds. The resulting model correctly predicted the pIC₅₀ values of an additional test set compounds as true unknowns.