Vesicular monoamine transporter substrate/inhibitor activity of MPTP/MPP+ derivatives: a structure-activity study

Identification and optimization of nitrophenolic analogues as dopamine metabolic enzyme inhibitors for the treatment of Parkinson's disease

Progressive loss of dopaminergic neurons leads to the depletion of the striatal neurotransmitter dopamine, which is the main cause of Parkinson's disease (PD) motor symptoms. Simultaneous inhibition of the two key dopamine metabolic enzymes, catechol-O-methyltransferase (COMT) and monoamine oxidase B (MAO-B), could potentially be a breakthrough in achieving clinical efficacy. Representative compound C12 exhibits good COMT inhibitory activity (IC50 = 0.37 μM), metal chelation ability, and BBB permeability. Furthermore, results from in vivo biological activity evaluations indicate that C12 can improve dopamine levels and ameliorate MPTP-induced PD symptoms in mice. Preliminary in vivo and in vitro study results highlight the potential of compound C12 in PD treatment.

Inhibition of VMAT2 by β2-adrenergic agonists, antagonists, and the atypical antipsychotic ziprasidone

Vesicular monoamine transporter 2 (VMAT2) is responsible for packing monoamine neurotransmitters into synaptic vesicles for storage and subsequent neurotransmission. VMAT2 inhibitors are approved for symptomatic treatment of tardive dyskinesia and Huntington's chorea, but despite being much-studied inhibitors their exact binding site and mechanism behind binding and inhibition of monoamine transport are not known. Here we report the identification of several approved drugs, notably β2-adrenergic agonists salmeterol, vilanterol and formoterol, β2-adrenergic antagonist carvedilol and the atypical antipsychotic ziprasidone as inhibitors of rat VMAT2. Further, plausible binding modes of the established VMAT2 inhibitors reserpine and tetrabenazine and hit compounds salmeterol and ziprasidone were identified using molecular dynamics simulations and functional assays using VMAT2 wild-type and mutants. Our findings show VMAT2 as a potential off-target of treatments with several approved drugs in use today and can also provide important first steps in both drug repurposing and therapy development targeting VMAT2 function.

A Review on the Arylpiperazine Derivatives as Potential Therapeutics for the Treatment of Various Neurological Disorders

Abstract:

Neurological disorders are disease conditions related to the neurons and central nervous system (CNS). Any kind of structural, electrical, biochemical and functional abnormalities in neurons can lead to various types of disorders like Alzheimer’s disease (AD), depression, Parkinson’s disease (PD), epilepsy, stroke, etc. Currently available medicines are symptomatic and do not treat the disease state. Thus, novel CNS active agents with the potential of complete treatment of an illness are highly desired. A range of small organic molecules are being explored as potential drug candidates for the cure of different neurological disorders. In this context, arylpiperazine has been found to be a versatile scaffold and indispensable pharmacophore in many CNS active agents. A number of molecules with arylpiperazine nucleus have been developed as potent leads for the treatment of AD, PD, depression and other disorders. The arylpiperazine nucleus can be optionally substituted at different chemical structures and offer flexibility for the synthesis of large number of derivatives. In the current review article, we have explored the role of various arylpiperazine containing scaffolds against different neurological disorders, including AD, PD, and depression. The structure-activity relationship studies were conducted for recognizing potent lead compounds. This review article may provide important clues on the structural requirements for the design and synthesis of effective molecules as curative agents for different neurological disorders.

Caenorhabditis elegans Model Studies Show MPP+ Is a Simple Member of a Large Group of Related Potent Dopaminergic Toxins

Although the causes of Parkinson's disease (PD) are not fully understood, the consensus is that a combination of genetic and environmental factors plays a major role. The discovery that the synthetic chemical, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-derived N-methyl-4-phenylpyridinium (MPP⁺), recapitulates major pathophysiological characteristics of PD in humans and other mammals has provided the strongest support for this possibility; however, several key aspects of the mechanism remain unclear. In contrast to the widely accepted view that MPP⁺ is structurally unique and optimal for selective dopaminergic toxicity, previous in vitro studies have suggested that MPP⁺ is most likely a simple member of a large group of related dopaminergic toxins. Here we provide first in vivo evidence to support the above possibility using Caenorhabditis elegans PD models. We also provide in vivo evidence to show that the inherent predisposition of dopaminergic neurons to produce high oxidative stress and related downstream effects when exposed to MPP⁺ and related mitochondrial toxins is responsible for their selective vulnerability to these toxins. More significantly, present findings suggest that if this broad group of MPP⁺ related dopaminergic toxins are present in work places or in the environment, they could cause far-reaching public health consequences.

N-Methyl-4-phenylpyridinium Scaffold-Containing Lipophilic Compounds Are Potent Complex I Inhibitors and Selective Dopaminergic Toxins

Although the exact cause or causes of Parkinson's disease (PD) are not fully understood, it is believed that environmental factors play a major role. The discovery that a synthetic chemical, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-derived N-methyl-4-phenylpyridinium (MPP⁺), recapitulates major pathophysiological characteristics of PD in humans has provided the strongest support for this possibility. While the mechanism of the selective dopaminergic toxicity of MPP⁺ has been extensively studied and is, in most respects, well accepted, several key aspects of the mechanism are still debatable. In the present study, we use a series of structurally related, novel, and lipophilic MPP⁺ derivatives [ N-(2-phenyl-1-propene)-4-phenylpyridinium] to probe the mechanism of action of MPP⁺ using dopaminergic MN9D and non-neuronal HepG2 cells in vitro. Here we show that effective mitochondrial complex I inhibition is necessary and that the specific uptake through plasma membrane dopamine transporter is not essential for dopaminergic toxicity of MPP⁺ and related toxins. We also provide strong evidence to support our previous proposal that the selective vulnerability of dopaminergic cells to MPP⁺ and similar toxins is likely due to the high inherent propensity of these cells to produce excessive reactive oxygen species as a downstream effect of complex I inhibition. Based on the current and previous findings, we propose that MPP⁺ is the simplest of a larger group of unidentified environmental dopaminergic toxins, a possibility that may have major public health implications.

Characteristics of the mitochondrial and cellular uptake of MPP+, as probed by the fluorescent mimic, 4'I-MPP

The discovery that 1-methyl-4-phenylpyridinium (MPP⁺) selectively destroys dopaminergic neurons and causes Parkinson’s disease (PD) symptoms in mammals has strengthened the environmental hypothesis of PD. The current model for the dopaminergic toxicity of MPP⁺ is centered on its uptake into dopaminergic neurons, accumulation into the mitochondria, inhibition of the complex-I leading to ATP depletion, increased reactive oxygen species (ROS) production, and apoptotic cell death. However, some aspects of this mechanism and the details of the cellular and mitochondrial accumulation of MPP⁺ are still poorly understood. The aim of this study was to characterize a structural and functional MPP⁺ mimic which is suitable to study the cellular distribution and mitochondrial uptake of MPP⁺ in live cells and use it to identify the molecular details of these processes to advance the understanding of the mechanism of the selective dopaminergic toxicity of MPP⁺. Here we report the characterization of the fluorescent MPP⁺ derivative, 1-methyl-4-(4'-iodophenyl)pyridinium (4'I-MPP⁺), as a suitable candidate for this purpose. Using this novel probe, we show that cytosolic/mitochondrial Ca²⁺ play a critical role through the sodium-calcium exchanger (NCX) in the mitochondrial and cellular accumulation of MPP⁺ suggesting for the first time that MPP⁺ and related mitochondrial toxins may also exert their toxic effects through the perturbation of Ca²⁺ homeostasis in dopaminergic cells. We also found that the specific mitochondrial NCX (mNCX) inhibitors protect dopaminergic cells from the MPP⁺ and 4'I-MPP⁺ toxicity, most likely through the inhibition of the mitochondrial uptake, which could potentially be exploited for the development of pharmacological agents to protect the central nervous system (CNS) dopaminergic neurons from PD-causing environmental toxins.

Human dopamine transporter: the first implementation of a combined in silico/in vitro approach revealing the substrate and inhibitor specificities

Parkinson's disease (PD) is characterized by the loss of dopamine-generating neurons in the substantia nigra and corpus striatum. Current treatments alleviate PD symptoms rather than exerting neuroprotective effect on dopaminergic neurons. New drugs targeting the dopaminergic neurons by specific uptake through the human dopamine transporter (hDAT) could represent a viable strategy for establishing selective neuroprotection. Molecules able to increase the bioactive amount of extracellular dopamine, thereby enhancing and compensating a loss of dopaminergic neurotransmission, and to exert neuroprotective response because of their accumulation in the cytoplasm, are required. By means of homology modeling, molecular docking, and molecular dynamics simulations, we have generated 3D structure models of hDAT in complex with substrate and inhibitors. Our results clearly reveal differences in binding affinity of these compounds to the hDAT in the open and closed conformations, critical for future drug design. The established in silico approach allowed the identification of promising substrate compounds that were subsequently analyzed for their efficiency in inhibiting hDAT-dependent fluorescent substrate uptake, through in vitro live cell imaging experiments. Taken together, our work presents the first implementation of a combined in silico/in vitro approach enabling the selection of promising dopaminergic neuron-specific substrates.

One-Pot Phosphate-Mediated Synthesis of Novel 1,3,5-Trisubstituted Pyridinium Salts: A New Family of S. aureus Inhibitors

Polysubstituted pyridinium salts are valuable pharmacophores found in many biologically active molecules. Their synthesis typically involves the use of multistep procedures or harsh reaction conditions. Here, we report water-based phosphate mediated reaction conditions that promote the condensation of arylacetaldehydes with amines to give 1,3,5-pyridinium salts. The reaction, carried out at pH 6, provides conditions suitable for the use of less stable aldehydes and amines in this Chichibabin pyridine condensation. The evaluation of selected 1,3,5-trisubstituted pyridinium salts highlighted that they can inhibit the growth of S. aureus in the low μg/mL range. The synthetic accessibility of these compounds and preliminary growth inhibition data may pave the way towards the discovery of new anti-bacterials based on the 1,3,5-trisubstituted pyridinium scaffold.

Protection of cultured dopamine neurons from MPP(+) requires a combination of neurotrophic factors

Parkinson's disease is a progressive neurodegenerative disorder, caused in part by the loss of dopamine (DA) neurons in the substantia nigra (SN). Neurotrophic factors have been shown to increase the basal survival of DA neurons in vitro, as well as to protect the neurons from some toxins under certain in vitro conditions and in animal models. Although these factors have often been tested individually, they have rarely been studied in combinations. We therefore examined the effect of such combinations after acute exposure to the toxin 1-methyl-4-phenylpyridinium (MPP(+) ) using dissociated postnatal rat midbrain cultures isolated from SN and ventral tegmental area (VTA). We found that significant loss of DA neurons in the SN occurred with an LC50 of between 1 and 10 μm, whereas the LC50 of DA neurons from the VTA was approximately 1000-fold higher. We did not observe neuroprotection against MPP(+) by individual exposure to glial cell-line derived neurotrophic factor (GDNF), brain derived neurotrophic factor (BDNF), transforming growth factor beta (TGFβ), basic fibroblast growth factor (FGF-2) or growth/differentiation factor 5 (GDF5) at concentrations of 100 or 500 ng/mL. Combinations of two, three or four neurotrophic factors were also ineffective. However, when the SN cultures were exposed to a combination of all five neurotrophic factors, each at a concentration of 100 ng/mL, we observed a 30% increase in DA neuron survival in the presence of 10 and 500 μm MPP(+) . These results may be relevant to the use of neurotrophic factors as therapeutic treatments for Parkinson's disease.

Mechanisms of amphetamine action illuminated through optical monitoring of dopamine synaptic vesicles in Drosophila brain

Amphetamines elevate extracellular dopamine, but the underlying mechanisms remain uncertain. Here we show in rodents that acute pharmacological inhibition of the vesicular monoamine transporter (VMAT) blocks amphetamine-induced locomotion and self-administration without impacting cocaine-induced behaviours. To study VMAT's role in mediating amphetamine action in dopamine neurons, we have used novel genetic, pharmacological and optical approaches in Drosophila melanogaster. In an ex vivo whole-brain preparation, fluorescent reporters of vesicular cargo and of vesicular pH reveal that amphetamine redistributes vesicle contents and diminishes the vesicle pH-gradient responsible for dopamine uptake and retention. This amphetamine-induced deacidification requires VMAT function and results from net H⁺ antiport by VMAT out of the vesicle lumen coupled to inward amphetamine transport. Amphetamine-induced vesicle deacidification also requires functional dopamine transporter (DAT) at the plasma membrane. Thus, we find that at pharmacologically relevant concentrations, amphetamines must be actively transported by DAT and VMAT in tandem to produce psychostimulant effects.

Amphetamines are known to enhance extracellular dopamine levels, but the underlying mechanisms are unclear. Utilising a new pH biosensor for synaptic vesicles, the authors show that amphetamines diminish vesicle pH gradients, disrupting dopamine packaging and leading to increased neurotransmitter release.

2, 2'- and 4, 4'-Cyanines are transporter-independent in vitro dopaminergic toxins with the specificity and mechanism of toxicity similar to MPP⁺

Specific uptake through dopamine transporter followed by the inhibition of the mitochondrial complex-I have been accepted as the cause of the specific dopaminergic toxicity of 1-methyl-4-phenylpyridinium (MPP(+) ). However, MPP(+) is taken up into many cell types through other transporters, suggesting that, in addition to the efficient uptake, intrinsic vulnerability of dopaminergic cells may also contribute to their high sensitivity to MPP(+) and similar toxins. To test this possibility, two simple cyanines were employed in a comparative study based on their unique characteristics and structural similarity to MPP(+) . Here, we show that they freely accumulate in dopaminergic (MN9D and SH-SY5Y) as well as in liver (HepG2) cells, but are specifically and highly toxic to dopaminergic cells with IC50s in the range of 50-100 nM, demonstrating that they are about 1000-fold more toxic than MPP(+) under similar experimental conditions. They cause mitochondrial depolarization non-specifically, but increase the reactive oxygen species specifically in dopaminergic cells leading to the apoptotic cell death parallel to MPP(+) . These and other findings suggest that the specific dopaminergic toxicity of these cyanines is due to the inherent vulnerability of dopaminergic cells toward mitochondrial toxins that lead to the excessive production of reactive oxygen species. Therefore, the specific dopaminergic toxicity of MPP(+) must also be, at least partly, due to the specific vulnerability of dopaminergic neurons. Thus, these cyanines could be stronger in vivo dopaminergic toxins than MPP(+) and their in vivo toxicities must be evaluated. Here, we show that cationic lipophilic cyanines with structural similarity to 1-methyl-4-phenylpyridinium (MPP(+) ) freely accumulate non-specifically, but only toxic to dopaminergic cells. They are 1000-fold more toxic than MPP(+) under similar conditions. They cause mitochondrial depolarization non-specifically, but increase the ROS specifically in dopaminergic cells leading to the apoptotic cell death parallel to MPP(+) . Thus, the specific dopaminergic toxicity of MPP(+) and related toxins could be due to the intrinsic vulnerability of dopaminergic cells toward mitochondrial oxidative stress.

Binding-induced fluorescence of serotonin transporter ligands: A spectroscopic and structural study of 4-(4-(dimethylamino)phenyl)-1-methylpyridinium (APP(+)) and APP(+) analogues

The binding-induced fluorescence of 4-(4-(dimethylamino)-phenyl)-1-methylpyridinium (APP(+)) and two new serotonin transporter (SERT)-binding fluorescent analogues, 1-butyl-4-[4-(1-dimethylamino)phenyl]-pyridinium bromide (BPP(+)) and 1-methyl-4-[4-(1-piperidinyl)phenyl]-pyridinium (PPP(+)), has been investigated. Optical spectroscopy reveals that these probes are highly sensitive to their chemical microenvironment, responding to variations in polarity with changes in transition energies and responding to changes in viscosity or rotational freedom with emission enhancements. Molecular docking calculations reveal that the probes are able to access the nonpolar and conformationally restrictive binding pocket of SERT. As a result, the probes exhibit previously not identified binding-induced turn-on emission that is spectroscopically distinct from dyes that have accumulated intracellularly. Thus, binding and transport dynamics of SERT ligands can be resolved both spatially and spectroscopically.

Enhancing a CH-π Interaction to Increase the Affinity for 5-HT1A Receptors

An electrostatic interaction related to a favorable position of the distal phenyl ring and a phenylalanine residue in the binding pocket would explain the higher 5-HT1A affinity of a 4-phenyl-1,2,3,6-tetrahydropyridine (THP) analogue compared to the corresponding 4-phenylpiperazine analogue. To explore a possible reinforcement of this interaction to increase the affinity for 5-HT1A receptors, different 4-substituted-phenyl analogues were synthesized and tested. The most important increase of affinity is obtained with two electron-donating methyl groups in positions 3 and 5.

In Vitro Antibacterial Activity of 4-Phenyl-1-(2-phenyl-allyl)pyridinium bromide: A Novel Class of Pyridinium Based Antibacterial Compounds

The continuing increase in the incidence of multi drug resistant pathogenic bacteria and shortage of new antimicrobial agents are the prime driver in efforts to identify the novel antimicrobial classes. In vitro antibacterial activity of 4-phenyl-1-(2-phenylallyl) pyridinium bromide was tested against Gram positive Staphylococcus aureus, Streptococcus species, Bacillus subtilis, and Gram negative Klebsiella aerogenes and Escherichia coli using disk diffusion method. Among them S. aureus showed strong antibacterial activity (21.99 ± 0.03 mm) while E. coli showed very little activity (8.97 ± 0.06 mm) towards the compound. The MIC of 4-phenyl-1-(2-phenyl-allyl)-pyridinium bromide for 90% S. aureus was ≤20 μg/ml and was compared with phenoxymethylpenicillin, cloxacillin, erythromycin and vancomycin. When 4-phenyl-1-(2-phenyl-allyl)pyridinium bromide showed MIC at ≤20 μg/ml, all others showed MIC at ≤100 μg/ml. Strong antibacterial activity of 4-phenyl-1-(2-phenyl-allyl)pyridinium bromide against S. aureus indicates that there is a possibility to use it as an effective antibacterial agent.

Vesicular monoamine transporters: structure-function, pharmacology, and medicinal chemistry

Vesicular monoamine transporters (VMAT) are responsible for the uptake of cytosolic monoamines into synaptic vesicles in monoaminergic neurons. Two closely related VMATs with distinct pharmacological properties and tissue distributions have been characterized. VMAT1 is preferentially expressed in neuroendocrine cells and VMAT2 is primarily expressed in the CNS. The neurotoxicity and addictive properties of various psychostimulants have been attributed, at least partly, to their interference with VMAT2 functions. The quantitative assessment of the VMAT2 density by PET scanning has been clinically useful for early diagnosis and monitoring of the progression of Parkinson's and Alzheimer's diseases and drug addiction. The classical VMAT2 inhibitor, tetrabenazine, has long been used for the treatment of chorea associated with Huntington's disease in the United Kingdom, Canada, and Australia, and recently approved in the United States. The VMAT2 imaging may also be useful for exploiting the onset of diabetes mellitus, as VMAT2 is also expressed in the β-cells of the pancreas. VMAT1 gene SLC18A1 is a locus with strong evidence of linkage with schizophrenia and, thus, the polymorphic forms of the VMAT1 gene may confer susceptibility to schizophrenia. This review summarizes the current understanding of the structure-function relationships of VMAT2, and the role of VMAT2 on addiction and psychostimulant-induced neurotoxicity, and the therapeutic and diagnostic applications of specific VMAT2 ligands. The evidence for the linkage of VMAT1 gene with schizophrenia and bipolar disorder I is also discussed.

The biochemical and cellular basis for nutraceutical strategies to attenuate neurodegeneration in Parkinson's disease

Future therapeutic intervention that could effectively decelerate the rate of degeneration within the substantia nigra pars compacta (SNc) could add years of mobility and reduce morbidity associated with Parkinson’s disease (PD). Neurodegenerative decline associated with PD is distinguished by extensive damage to SNc dopaminergic (DAergic) neurons and decay of the striatal tract. While genetic mutations or environmental toxins can precipitate pathology, progressive degenerative succession involves a gradual decline in DA neurotransmission/synaptic uptake, impaired oxidative glucose consumption, a rise in striatal lactate and chronic inflammation. Nutraceuticals play a fundamental role in energy metabolism and signaling transduction pathways that control neurotransmission and inflammation. However, the use of nutritional supplements to slow the progression of PD has met with considerable challenge and has thus far proven unsuccessful. This review re-examines precipitating factors and insults involved in PD and how nutraceuticals can affect each of these biological targets. Discussed are disease dynamics (Sections 1 and 2) and natural substances, vitamins and minerals that could impact disease processes (Section 3). Topics include nutritional influences on α-synuclein aggregation, ubiquitin proteasome function, mTOR signaling/lysosomal-autophagy, energy failure, faulty catecholamine trafficking, DA oxidation, synthesis of toxic DA-quinones, o-semiquinones, benzothiazolines, hyperhomocyseinemia, methylation, inflammation and irreversible oxidation of neuromelanin. In summary, it is clear that future research will be required to consider the multi-faceted nature of this disease and re-examine how and why the use of nutritional multi-vitamin-mineral and plant-based combinations could be used to slow the progression of PD, if possible.