Overview of the structure and function of the dopamine transporter and its protein interactions

PF-06447475 Molecule Attenuates the Neuropathology of Familial Alzheimer's and Coexistent Parkinson's Disease Markers in PSEN1 I416T Dopaminergic-like Neurons

Familial Alzheimer's disease (FAD) is a complex multifactorial disorder clinically characterized by cognitive impairment and memory loss. Pathologically, FAD is characterized by intracellular accumulation of the protein fragment Aβ42 (iAβ), hyperphosphorylated microtubule-associated protein TAU (p-TAU), and extensive degeneration of basal forebrain cholinergic neurons of the nucleus basalis of Meynert (NbM) and the medial septal nucleus (MSN), mainly caused by mutations in the amyloid precursor protein (APP), presenilin 1 (PSEN1), and PSEN2 gene. Since the dopaminergic system may contribute to FAD symptoms, alterations in the nigro-hippocampal pathway may be associated with cognitive impairment in FAD. Interestingly, p-α-synuclein (p-α-Syn), Aβ, and p-TAU have been found to coexist in vulnerable regions of postmortem AD brains. However, the mechanism by which Aβ, p-TAU, and α-Syn coexist in DAergic neurons in AD brains has not been determined. We generated PSEN1 I416T dopaminergic-like neurons (DALNs) from I416T menstrual stromal cells (MenSCs) in NeuroForsk 2.0 medium for 7 days and then cultured them in minimal culture medium (MCm) for another 4 days. On day 11, DALNs were analyzed for molecular and pathological markers by flow cytometry and fluorescence microscopy. We found that mutant DALNs showed increased accumulation of iAβ as well as increased phosphorylation of TAU at S202/T205 compared to WT DALNs. Thus, mutant DALNs exhibited typical pathological hallmarks of Alzheimer's disease. Furthermore, PSEN1 I416T DALNs showed concomitant signs of OS as evidenced by the appearance of oxidized sensor protein DJ-1 (i.e., DJ-1C106-SO3) and apoptotic markers TP53, pS63-c-JUN, PUMA, and cleavage caspase 3 (CC3). Notably, these DALNs exhibited PD-associated proteins such as intracellular accumulation of α-Syn (detected as aggregates of pS129-α-Syn) and phosphorylation of LRRK2 kinase at residue S935. In addition, mutant DALNs showed a 17.16- and 6.17-fold decrease in DA-induced Ca2+ flux, compared to WT DALNs. These observations suggest that iAβ and p-TAU, together with p-α-Syn, and p-LRRK2 kinase, may damage DAergic neurons and thereby contribute to the exacerbation of neuropathologic processes in FAD. Remarkably, the LRRK2 inhibitor PF-06447475 (PF-475) significantly reversed PSEN1 I416T-induced neuropathological markers in DAergic neurons. PF-465 inhibitor reduced iAβ, oxDJ-1C106-SO3, and p-TAU. In addition, this inhibitor reduced pS935-LRRK2, pS129-αSYN, pS63-c-JUN, and CC3. We conclude that the observed neuroprotective effects of PF-475 are due to direct inhibition of LRRK2 activity and that the LRRK2 protein is upstream of the molecular cascade of apoptosis and proteinopathy. Our results suggest that PF-475 is an effective neuroprotective agent against endogenous PSEN1 I416T-induced neurotoxicity in DALNs coexisting with Parkinson's disease markers. Therefore, PF-475 may be of great therapeutic value in FAD.

Synthesis and Preliminary Evaluation of an In Vivo Stable 131I-Labeled Deuterated Tropane for Mapping Dopamine Transporter

Dopamine transporter (DAT) is closely associated with neurodegenerative diseases such as Parkinson's disease (PD). To develop an in vivo stable radioligand targeting DAT, we synthesized a novel iodine-131-labeled tropane analog [131I]1 with deuteration on both the N-fluoropropyl and 2β-carbomethoxy positions of the tropane scaffold and then biologically compared with the previously reported analog [131I]FP-CIT-d6 with deuteration only on the N-fluoropropyl group. The radioligand [131I]1 was obtained via a radioiodine-destannylation reaction with a radiochemical yield of ~95%, a radiochemical purity of > 99% and a molar activity of 12.72 GBq/μmol. [131I]1 exhibited a high in vitro binding affinity for DAT with an IC50 value of 2.2 nM. Metabolic stability studies demonstrated that [131I]1 displayed superior in vivo stability compared with [131I]FP-CIT-d6. Biodistribution studies revealed that [131I]1 had better DAT affinity, specificity, and a higher target-to-background ratio than [131I]FP-CIT-d6. Ex vivo autoradiography and blocking experiments validated the selective and reversible binding of [131I]1 to DAT and superiority to [131I]FP-CIT-d6. These preliminary results suggest that deuterated radioligand [131I]1, with enhanced in vivo stability and higher target-to-background ratio, is a promising DAT probe, warranting the development and application of 123I-labeled counterpart ([123I]1) for SPECT imaging in DAT-related disorders.

Identification of novel amides and alkaloids as putative inhibitors of dopamine transporter for schizophrenia using computer-aided virtual screening

Schizophrenia is a complex psychiatric disorder marked by delusions, memory impairments, hallucinations, disorganized behavior, and severe cognitive deficits. Targeting the dopamine transporter (DAT) protein is promising for treating cognitive symptoms, especially in patients resistant to antipsychotic treatments. In this study, phytochemicals from six medicinal plants underwent virtual screening, and molecular dynamics simulation to identify potential agents targeting DAT. Key drug-like properties, safety, and biological activity were evaluated for identified hits. Pharmacokinetic simulation and pharmacophoric analysis were also performed. Among 990 screened phytochemicals, three alkaloids and six amides, predominantly from Piper retrofractum, and one diterpene were identified as potential antischizophrenic agents based on their stronger binding affinities and favorable docking scores compared to the standard (Lumateperone). Amides showed more potential for DAT than alkaloids. The dynamic behavior and stability of the top three amides, namely, Chenoalbicin, Dipiperamide G, and Lyciumamide C, were evaluated using various molecular dynamics analyses. RMSD (Root Mean Square Deviation), RMSF (Root Mean Square Fluctuation), Rg (Radius of Gyration), and SASA (Solvent Accessible Surface Area) analyses demonstrated favorable characteristics for all three ligands. However, binding free energy, cross-correlation, PCA (Principal Component Analysis) and FEL (Free Energy Landscape) analyses indicated that Lyciumamide C exhibited the highest stability and binding affinity in dynamic environments, Pharmacophoric features highlighted the distinct interacting components for the top three amides. Pharmacokinetic simulations revealed significant peak concentrations and sustained levels can be indicated as Lyciumamide C > Chenoalbicin > Dipiperamide G. The higher and more sustained brain concentrations of Lyciumamide C suggest its most promising pharmacokinetic profile for targeting DAT. Overall, our screened metabolites followed drug-ability criteria and require further experimental validation.

Is Cocaine Protonated When it Binds to the Dopamine Transporter?

There has been much controversy about whether the well-known alkaloid and tertiary amine base cocaine (pK a = 8.5) binds to the human dopamine transporter (DAT) in its protonated form. Most potent DAT inhibitors are also strong amines-yet there are some noteworthy examples where neutral cocaine analogues have high affinity, while the quaternary ammonium analog of cocaine, cocaine methiodide, is a comparatively poor inhibitor. In this paper, we show that a fluorescent cocaine analog, with a lower pK a than cocaine, becomes protonated in the DAT binding site and conclude that similar behavior must be expected from cocaine. By determining the pK a of the aspartate residue in DAT believed to interact with the amine of cocaine, we are able to explain the apparently contradictory structure-activity data of cocaine analogues.

Monoamine Oxidase Inhibitors Present in Tobacco Modulate Dopamine Balance Via the Dopamine Transporter

It has been reported that nicotine affects brain dopamine homeostasis. By binding to nicotinic acetylcholine receptors, including those expressed by dopaminergic neurons of the ventral tegmental area, nicotine stimulates dopamine release and signaling. Dopamine is taken up from the synaptic cleft by the dopamine transporter (DAT) into presynaptic neurons, where it is degraded by monoamine oxidase (MAO). Besides nicotine, other tobacco compounds play a role in dopamine modulation. To better understand the biological effects of nicotine and other tobacco compounds on dopamine regulation, we selected a group of tobacco compounds based on their potential affinity to bind human MAO-A and MAO-B enzymes using an in silico approach. Subsequently, we tested the putative compounds in an enzymatic assay to verify their ability to inhibit human MAO-A or MAO-B. The positive hits were harman, norharman, harmaline, and 1-ethyl-β-carboline. While harman and norharman have been extensively studied, both harmaline and 1-ethyl-β-carboline have not been described in the context of tobacco and MAO inhibition before. We investigated DAT activity in an overexpressing cell line and dopamine release and uptake in rat striatal synaptosomes. We clearly demonstrate that tested MAO-A inhibitors (MAO-AIs) significantly attenuated human DAT activity and consequent dopamine uptake, establishing a functional connection between MAOIs and dopamine uptake via DAT. Interestingly, the tested MAO-AIs elicited pronounced dopamine release in crude synaptosomal preparations. In summary, this in vitro study demonstrates that tested MAO-AIs found in cigarette smoke not only reduce MAO activity but also strongly impact dopamine homeostatic mechanisms via DAT. Further in vivo investigations would advance our understanding of the underlying mechanisms of dopamine regulation and homeostasis.

Moringa oleifera Lam. seed lectin (WSMoL) reduces chronic stress-induced anxiety and depression in mice by lessening inflammation and balancing brain chemicals

Phyto-based treatments for anxiety and depression are gaining attention. The efficacy of the water-soluble Moringa oleifera seed lectin (WSMoL) in reducing acute anxiolytic and depressive-like behaviors in mice has been previously demonstrated. In the present study, it was evaluated the effects of WSMoL on reducing anxiety and depressive-like symptoms in a mouse model of unpredictable chronic mild stress (UCMS). The animals were divided into groups and exposed to a four-week UCMS regimen. Following this, the mice received daily intraperitoneal injections of vehicle (non-stressed and UCMS control groups), WSMoL (2 or 4 mg/kg), or fluoxetine (10 mg/kg) for 21 days. Neurobehavioral tests included the open field test and elevated plus maze test to assess anxiety-like behavior, and the tail suspension test and sucrose preference test to evaluate depression-like behavior. Biochemical analyses measured serum corticosterone and cytokines as well brain levels of cytokines and monoamines. All tests indicated that WSMoL significantly (p < 0.05) reversed the anxiety and depression-like behaviors induced by UCMS. The stress protocol increased serum corticosterone levels and WSMoL treatment was not able to normalize corticosterone secretion. WSMoL treatment reduced serum and brain levels of IL-2, IL-6, and TNF-α, indicating reduced neuroinflammation, and increased brain levels of dopamine, serotonin, and noradrenaline. In summary, WSMoL mitigated UCMS-induced anxiety and depression-like behaviors by reducing neuroinflammation and modulating brain monoamine levels.

Monoamine transporter ubiquitination and inward-open conformation synergistically maximize transporter endocytosis

Monoamine transporters function in neuronal membranes to control extracellular concentrations of their substrates. Cell-surface expression of transporters is regulated by substrates and intracellular signaling, but the underlying mechanisms remain unclear. Here, we found that substrates of the dopamine transporter (DAT), amphetamine and dopamine, synergize with protein kinase C (PKC)-dependent DAT ubiquitination to markedly elevate clathrin-mediated endocytosis of DAT, which is accompanied by DAT movement out of plasma membrane protrusions with a negative curvature. Disruption of the outward-open (OO) DAT conformation or its stabilization in the inward-open (IO) conformation recapitulates substrate effects on DAT endocytosis. Amphetamine strongly increases PKC-dependent endocytosis of norepinephrine transporter (NET) but not of serotonin transporter (SERT), correlating with a substantially weaker ubiquitination of SERT compared to NET. We propose a "shape-transition" model whereby shifting from convex-shaped OO conformers to IO conformers minimizes retention of transporters in negatively curved membranes, which facilitates their PKC-dependent ubiquitination and recruitment to positively invaginated clathrin-coated membranes, driving robust transporter endocytosis.

The Dopamine Transporter Is a New Target for Ischemic Stroke

AIMS

Dopamine transporter (DAT) can regulate DA homeostasis and has been implicated in many nervous system diseases. Whether DAT is involved in the protection against ischemic stroke is unclear.

METHODS

In vivo microdialysis measurements of DA were recorded in the ischemic penumbral area of mice with middle cerebral artery occlusion (MCAO). DAT coding gene, Slc6a3 mutation, and DAT overexpression animals were performed MCAO. Madopar (compound formulation of levodopa) and nomifensine (DA reuptake inhibitor) were administered in MCAO animals. Brain slices were prepared in Slc6a3 mutation or wild-type (WT) animals with MCAO to record miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs). The effects of DA and its dopamine-1 receptor (D1R) antagonists (SCH-23390) on mEPSCs, mIPSCs, and neurons protection were recorded.

RESULTS

MCAO caused a prominent increase in DA. Slc6a3 mutation significantly attenuated the ischemic injury, whereas DAT overexpression aggravated this injury. Both nomifensine and madopar protected against brain injury. Slc6a3 mutation and DA restored the disturbance of mEPSCs and mIPSC, and protected against neuron death, which was abolished by SCH-23390.

CONCLUSION

DAT inhibition might be explored as a strategy for ischemic stroke prevention. DA and D1R involve in the restoration of synaptic dysfunction and neuron protection.

Schild Analysis of the Interaction between Parthenolide and Cocaine Suggests an Allosteric Relationship for Their Effects on Planarian Motility

The freshwater planarian is an emerging animal model in neuroscience due to its centralized nervous system that closely parallels closely parallels the nervous system of vertebrates. Cocaine, an abused drug, is the 'founding member' of the local anesthetic family. Parthenolide, a sesquiterpene lactone, acts as a behavioral and physiological antagonist of cocaine in planarians and rats, respectively. Previous work from our laboratory showed that both parthenolide and cocaine reduced planarian motility and that parthenolide reversed the cocaine-induced motility decrease at concentrations where parthenolide does not affect the movement of the worms. However, the exact mechanism of the cocaine/parthenolide antagonism is unknown. Here, we report the results of a Schild analysis to explore the parthenolide/cocaine relationship in the planarian Girardia tigrina. The Schild slopes of a family of concentration-response curves of parthenolide ± a single concentration of cocaine and vice versa were -0.55 and -0.36, respectively. These slopes were not statistically different from each other. Interestingly, the slope corresponding to the parthenolide ± cocaine (but not the cocaine ± parthenolide) data set was statistically different from -1. Our data suggest an allosteric relationship between cocaine and parthenolide for their effect on planarian motility. To the best of our knowledge, this is the first study about the mechanism of action of the antagonism between cocaine and parthenolide. Further studies are needed to determine the specific nature of the parthenolide/cocaine target(s) in this organism.

Structural bioinformatics studies of serotonin, dopamine and norepinephrine transporters and their AlphaFold2 predicted water-soluble QTY variants and uncovering the natural mutations of L->Q, I->T, F->Y and Q->L, T->I and Y->F

Monoamine transporters including transporters for serotonin, dopamine, and norepinephrine play key roles in monoaminergic synaptic signaling, involving in the molecular etiology of a wide range of neurological and physiological disorders. Despite being crucial drug targets, the study of transmembrane proteins remains challenging due to their localization within the cell membrane. To address this, we present the structural bioinformatics studies of 7 monoamine transporters and their water-soluble variants designed using the QTY code, by systematically replacing the hydrophobic amino acids leucine (L), valine (V), isoleucine (I) and phenylalanine (F) with hydrophilic amino acids (glutamine (Q), threonine (T) and tyrosine (Y). The resulting QTY variants, despite significant protein transmembrane sequence differences (44.27%-51.85%), showed similar isoelectric points (pI) and molecular weights. While their hydrophobic surfaces significantly reduced, this change resulted in a minimal structural alteration. Quantitatively, Alphafold2 predicted QTY variant structures displayed remarkable similarity with RMSD 0.492Å-1.619Å. Accompanied by the structural similarities of substituted amino acids in the context of 1.5Å electron density maps, our study revealed multiple QTY and reverse QTY variations in genomic databases. We further analyzed their phenotypical and topological characteristics. By extending evolutionary game theory to the molecular foundations of biology, we provided insights into the evolutionary dynamics of chemically distinct alpha-helices, their usage in different chemotherapeutic applications, and open possibilities of diagnostic medicine. Our study rationalizes that QTY variants of monoamine transporters may not only become distinct tools for medical, structural, and evolutionary research, but these transporters may also emerge as contemporary therapeutic targets, providing a new approach to treatment for several conditions.

Computational Chemistry in Structure-Based Solute Carrier Transporter Drug Design: Recent Advances and Future Perspectives

Solute carrier transporters (SLCs) are a class of important transmembrane proteins that are involved in the transportation of diverse solute ions and small molecules into cells. There are approximately 450 SLCs within the human body, and more than a quarter of them are emerging as attractive therapeutic targets for multiple complex diseases, e.g., depression, cancer, and diabetes. However, only 44 unique transporters (∼9.8% of the SLC superfamily) with 3D structures and specific binding sites have been reported. To design innovative and effective drugs targeting diverse SLCs, there are a number of obstacles that need to be overcome. However, computational chemistry, including physics-based molecular modeling and machine learning- and deep learning-based artificial intelligence (AI), provides an alternative and complementary way to the classical drug discovery approach. Here, we present a comprehensive overview on recent advances and existing challenges of the computational techniques in structure-based drug design of SLCs from three main aspects: (i) characterizing multiple conformations of the proteins during the functional process of transportation, (ii) identifying druggability sites especially the cryptic allosteric ones on the transporters for substrates and drugs binding, and (iii) discovering diverse small molecules or synthetic protein binders targeting the binding sites. This work is expected to provide guidelines for a deep understanding of the structure and function of the SLC superfamily to facilitate rational design of novel modulators of the transporters with the aid of state-of-the-art computational chemistry technologies including artificial intelligence.

Unlocking the Potential of High-Quality Dopamine Transporter Pharmacological Data: Advancing Robust Machine Learning-Based QSAR Modeling

The dopamine transporter (DAT) plays a critical role in the central nervous system and has been implicated in numerous psychiatric disorders. The ligand-based approaches are instrumental to decipher the structure-activity relationship (SAR) of DAT ligands, especially the quantitative SAR (QSAR) modeling. By gathering and analyzing data from literature and databases, we systematically assemble a diverse range of ligands binding to DAT, aiming to discern the general features of DAT ligands and uncover the chemical space for potential novel DAT ligand scaffolds. The aggregation of DAT pharmacological activity data, particularly from databases like ChEMBL, provides a foundation for constructing robust QSAR models. The compilation and meticulous filtering of these data, establishing high-quality training datasets with specific divisions of pharmacological assays and data types, along with the application of QSAR modeling, prove to be a promising strategy for navigating the pertinent chemical space. Through a systematic comparison of DAT QSAR models using training datasets from various ChEMBL releases, we underscore the positive impact of enhanced data set quality and increased data set size on the predictive power of DAT QSAR models.

Leaky lessons learned: Efflux prone dopamine transporter variant reveals sex and circuit specific contributions of D2 receptor signalling to neuropsychiatric disease

Aberrant dopamine (DA) signalling has been implicated in various neuropsychiatric disorders, including attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder (ASD), schizophrenia, bipolar disorder (BPD) and addiction. The availability of extracellular DA is sculpted by the exocytotic release of vesicular DA and subsequent transporter-mediated clearance, rendering the presynaptic DA transporter (DAT) a crucial regulator of DA neurotransmission. D2-type DA autoreceptors (D2ARs) regulate multiple aspects of DA homeostasis, including (i) DA synthesis, (ii) vesicular release, (iii) DA neuron firing and (iv) the surface expression of DAT and DAT-mediated DA clearance. The DAT Val559 variant, identified in boys with ADHD or ASD, as well as in a girl with BPD, supports anomalous DA efflux (ADE), which we have shown drives tonic activation of D2ARs. Through ex vivo and in vivo studies of the DAT Val559 variant using transgenic knock-in mice, we have uncovered a circuit and sex-specific capacity of D2ARs to regulate DAT, which consequently disrupts DA signalling and behaviour differently in males and females. Our studies reveal the ability of the construct-valid DAT Val559 model to elucidate endogenous mechanisms that support DA signalling, findings that may be of translational and/or therapeutic importance.

Assessment of the relationship between the dopaminergic pathway and severe acute respiratory syndrome coronavirus 2 infection, with related neuropathological features, and potential therapeutic approaches in COVID-19 infection

Dopamine is a known catecholamine neurotransmitter involved in several physiological processes, including motor control, motivation, reward, cognition, and immune function. Dopamine receptors are widely distributed throughout the nervous system and in immune cells. Several viruses, including human immunodeficiency virus and Japanese encephalitis virus, can use dopaminergic receptors to replicate in the nervous system and are involved in viral neuropathogenesis. In addition, studies suggest that dopaminergic receptors may play a role in the progression and pathogenesis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. When SARS-CoV-2 binds to angiotensin-converting enzyme 2 receptors on the surface of neuronal cells, the spike protein of the virus can bind to dopaminergic receptors on neighbouring cells to accelerate its life cycle and exacerbate neurological symptoms. In addition, recent research has shown that dopamine is an important regulator of the immune-neuroendocrine system. Most immune cells express dopamine receptors and other dopamine-related proteins, indicating the importance of dopaminergic immune regulation. The increase in dopamine concentration during SARS-CoV2 infection may reduce immunity (innate and adaptive) that promotes viral spread, which could lead to neuronal damage. In addition, dopaminergic signalling in the nervous system may be affected by SARS-CoV-2 infection. COVID -19 can cause various neurological symptoms as it interacts with the immune system. One possible treatment strategy for COVID -19 patients could be the use of dopamine antagonists. To fully understand how to protect the neurological system and immune cells from the virus, we need to study the pathophysiology of the dopamine system in SARS-CoV-2 infection.

Targeting dopamine transporter to ameliorate cognitive deficits in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the pathologic deposition of amyloid and neurofibrillary tangles in the brain, leading to neuronal damage and defective synapses. These changes manifest as abnormalities in cognition and behavior. The functional deficits are also attributed to abnormalities in multiple neurotransmitter systems contributing to neuronal dysfunction. One such important system is the dopaminergic system. It plays a crucial role in modulating movement, cognition, and behavior while connecting various brain areas and influencing other neurotransmitter systems, making it relevant in neurodegenerative disorders like AD and Parkinson's disease (PD). Considering its significance, the dopaminergic system has emerged as a promising target for alleviating movement and cognitive deficits in PD and AD, respectively. Extensive research has been conducted on dopaminergic neurons, receptors, and dopamine levels as critical factors in cognition and memory in AD. However, the exact nature of movement abnormalities and other features of extrapyramidal symptoms are not fully understood yet in AD. Recently, a previously overlooked element of the dopaminergic system, the dopamine transporter, has shown significant promise as a more effective target for enhancing cognition while addressing dopaminergic system dysfunction in AD.

Refining S-acylation: Structure, regulation, dynamics, and therapeutic implications

With a limited number of genes, cells achieve remarkable diversity. This is to a large extent achieved by chemical posttranslational modifications of proteins. Amongst these are the lipid modifications that have the unique ability to confer hydrophobicity. The last decade has revealed that lipid modifications of proteins are extremely frequent and affect a great variety of cellular pathways and physiological processes. This is particularly true for S-acylation, the only reversible lipid modification. The enzymes involved in S-acylation and deacylation are only starting to be understood, and the list of proteins that undergo this modification is ever-increasing. We will describe the state of knowledge on the enzymes that regulate S-acylation, from their structure to their regulation, how S-acylation influences target proteins, and finally will offer a perspective on how alterations in the balance between S-acylation and deacylation may contribute to disease.