Characterization of monoamine oxidase-B (MAO-B) as a biomarker of reactive astrogliosis in Alzheimer's disease and related dementias

Reactive astrogliosis accompanies the two neuropathological hallmarks of Alzheimer's disease (AD)-Aβ plaques and neurofibrillary tangles-and parallels neurodegeneration in AD and AD-related dementias (ADRD). Thus, there is growing interest in developing imaging and fluid biomarkers of reactive astrogliosis for AD/ADRD diagnosis and prognostication. Monoamine oxidase-B (MAO-B) is emerging as a target for PET imaging radiotracers of reactive astrogliosis. However, a thorough characterization of MAO-B expression in postmortem control and AD/ADRD brains is lacking. We sought to: (1) identify the primary cell type(s) expressing MAO-B in control and AD brains; (2) quantify MAO-B immunoreactivity in multiple brain regions of control and AD donors as a proxy for PET radiotracer uptake; (3) correlate MAO-B level with local AD neuropathological changes, reactive glia, and cortical atrophy; (4) determine whether the MAOB rs1799836 SNP genotype impacts MAO-B expression level; (5) compare MAO-B immunoreactivity across AD/ADRD, including Lewy body diseases (LBD) and frontotemporal lobar degenerations with tau (FTLD-Tau) and TDP-43 (FTLD-TDP). We found that MAO-B is mainly expressed by subpial and perivascular cortical astrocytes as well as by fibrous white matter astrocytes in control brains, whereas in AD brains, MAO-B is significantly upregulated by both cortical reactive astrocytes and white matter astrocytes across temporal, frontal, and occipital lobes. By contrast, MAO-B expression level was unchanged and lowest in cerebellum. Cortical MAO-B expression was independently associated with cortical atrophy and local measures of reactive astrocytes and microglia, and significantly increased in reactive astrocytes surrounding Thioflavin-S+ dense-core Aβ plaques. MAO-B expression was not affected by the MAOB rs1799836 SNP genotype. MAO-B expression was also significantly increased in the frontal cortex and white matter of donors with corticobasal degeneration, Pick's disease, and FTLD-TDP, but not in LBD or progressive supranuclear palsy. These findings support ongoing efforts to develop MAO-B-based PET radiotracers to image reactive astrogliosis in AD/ADRD.

Identification of new small molecule monoamine oxidase-B inhibitors through pharmacophore-based virtual screening, molecular docking and molecular dynamics simulation studies

The discovery of a safe and efficacious drug is a complex, time-consuming, and expensive process. Computational methodologies driven by cheminformatics tools play a central role in the high-throughput lead discovery and optimization process especially when the structure of the biological target is known. Monoamine oxidases are the membrane-bound FAD-containing enzymes and the isoform monoamine oxidase-B (MAO-B) is an attractive target for treating diseases like Alzheimer's disease, Parkinson's disease, glioma, etc. In the current study, we have used a pharmacophore-based virtual screening technique for the identification of new small molecule MAO-B inhibitors. Safinamide was used for building a pharmacophore model and the developed model was used to probe the ZINC database for potential hits. The obtained hits were filtered against drug-likeness and PAINS. Out of the hit's library, two compounds ZINC02181408, ZINC08853942 (most active), and ZINC53327382 (least active) were further subjected to molecular docking and dynamics simulation studies to assess their virtual binding affinities and stability of the resultant protein-ligand complex. The docking studies revealed that active ligands were well accommodated within the active site of MAO-B and interacted with both substrate and entrance cavity residues. MD simulation studies unveiled additional hydrogen bond interactions with the substrate cavity residues, Tyr398 and Tyr435 that are crucial for the catalytic role of MAO-B. Moreover, the predicted ADMET parameters suggest that the compounds ZINC08853942 and ZINC02181408 are suitable for CNS penetration. Thus, the attempted computational campaign yielded two potential MAO-B inhibitors that merit further experimental investigation.Communicated by Ramaswamy H. Sarma.

Monoamine oxidase inhibitors: A concise review with special emphasis on structure activity relationship studies

Monoamine oxidase enzyme is necessary for the management of brain functions. It oxidatively metabolizes monoamines and produces ammonia, aldehyde and hydrogen peroxide as by-products. Excessive production of by-products of monoamine metabolism generates free radicals which cause cellular apoptosis and several neurodegenerative disorders for example Alzheimer's disease, Parkinson's disease, depression and autism. The inhibition of MAOs is an attractive target for the treatment of neurological disorders. Clinically approved MAO inhibitors for example selegiline, rasagiline, clorgyline, pargyline etc. are irreversible in nature and cause some adverse effects while recently studied reversible MAO inhibitors are devoid of harmful effects of old monoamine oxidase inhibitors. In this review article we have listed various synthesized molecules containing different moieties like coumarin, chalcone, thiazole, thiourea, caffeine, pyrazole, chromone etc. along with their activity, mode of action, structure activity relationship and molecular docking studies.

Synthesis and biological evaluation of 3-styrylchromone derivatives as selective monoamine oxidase B inhibitors

A series of 3-styrylchromone derivatives was synthesized and evaluated for monoamine oxidase (MAO) A and B inhibitory activities. Most of all derivatives inhibited MAO-B selectively, except compound 21. Compound 19, which had a methoxy group at R² on the chromone ring and chlorine at R⁴ on phenyl ring, potently inhibited MAO-B, with an IC₅₀ value of 2.2 nM. Compound 1 showed the highest MAO-B selectivity, with a selectivity index of >3700. Further analysis of these compounds indicated that compounds 1 and 19 were reversible and mixed-type MAO-B inhibitors, suggesting that their mode of action may be through tight-binding inhibition to MAO-B. Quantitative structure-activity relationship (QSAR) analyses of the 3-styrylchromone derivatives were conducted using their pIC₅₀ values, through Molecular Operating Environment (MOE) and Dragon. There were 1796 descriptors of MAO-B inhibitory activity, which showed significant correlations (P < 0.05). Further investigation of the 3-styrylchromone structures as useful scaffolds was performed through three-dimensional-QSAR studies using AutoGPA, which is based on the molecular field analysis algorithm using MOE. The MAO-B inhibitory activity model constructed using pIC₅₀ value index exhibited a determination coefficients (R²) of 0.972 and a Leave-One-Out cross-validated determination coefficients (Q²) of 0.914. These data suggest that the 3-styrylchromone derivatives assessed herein may be suitable for the design and development of novel MAO inhibitors.

Synthesis and evaluation of 2-pyrrolopyridinylquinoline derivatives as selective tau PET tracers for the diagnosis of Alzheimer's disease

INTRODUCTION

[¹⁸F]THK-5351 was originally developed as a positron emission tomography (PET) imaging tracer for the detection of accumulated tau proteins, the pathological hallmark of Alzheimer's disease (AD). However, clinical studies of [¹⁸F]THK-5351 revealed the existence of off-target binding to monoamine oxidase-B (MAO-B). To overcome this off-target binding, in this work, we synthesized and evaluated 2-pyrrolopyridinylquinoline (PPQ) derivatives as selective tau PET imaging tracers.

METHODS

The core structure of PPQ derivatives was synthesized mainly using the Buchwald-Hartwig amination coupling reaction. All derivatives were evaluated for binding affinity towards tau and MAO-B by in vitro competitive binding assay. Radiosynthesis of PPQ derivatives was performed by ¹⁸F-radiolabeling of their tosylate precursors with activated [¹⁸F]KF/Kryptofix222 complex in dimethylsulfoxide by heating at 110 °C for 10 min. The biological properties of these [¹⁸F]PPQ derivatives were characterized by in vitro autoradiography of postmortem AD brain sections and by assay of ex vivo biodistribution in mice.

RESULTS

The PPQ derivatives were synthesized, with yields of 49-84%. In vitro competitive binding assay revealed that two novel PPQ derivatives-PPQ8 and PPQ9-demonstrated high binding affinity for tau (IC₅₀ = 4.9 and 6.9 nM, respectively). The radiosynthesis of [¹⁸F]PPQ8 and [¹⁸F]PPQ9 yielded 1.4% and 50.1% isolated non-decay corrected radiochemical yield, respectively, with >99% radiochemical purity. The molar radioactivities of [¹⁸F]PPQ8 and [¹⁸F]PPQ9 were 16.9 and 64.8 GBq/μmol, respectively. The in vitro and ex vivo biological characterization of [¹⁸F]PPQ8 and [¹⁸F]PPQ9 revealed that these tracers were selective for tau in AD brain sections without off-target binding, and they furthermore demonstrated brain uptake in normal mice.

CONCLUSIONS

¹⁸F-labeled PPQ derivatives improved binding affinity and selectivity for tau aggregates in AD. Further structural optimization to improve pharmacokinetics for potent tau PET imaging tracers is required.

Rasagiline, a monoamine oxidase B inhibitor, reduces in vivo [18F]THK5351 uptake in progressive supranuclear palsy

BACKGROUND

[¹⁸F]THK5351 is a tau positron emission tomography tracer that has shown promise in quantifying tau distribution in tauopathies such as Alzheimer's disease (AD) and progressive supranuclear palsy (PSP). However, the interpretation of [¹⁸F]THK5351 uptake has been shown to be confounded by high monoamine oxidase B (MAO-B) availability across the brain in AD.

OBJECTIVES

To test the hypothesis that the MAO-B inhibitor, rasagiline reduces [¹⁸F]THK5351 uptake in PSP.

METHODS

Six individuals (4: PSP; 2: cognitively unimpaired, CU) underwent [¹⁸F]THK5351 and [¹⁸F]AZD4694 to quantify baseline tau and amyloid deposition, respectively. Following a 10-day course of 1 mg rasagiline, all participants received a post-challenge [¹⁸F]THK5351 scan. The baseline and post-rasagiline challenge standardized uptake value (SUV) were generated normalized for patient weight and injected radioactivity.

RESULTS

The post-rasagiline regional SUV was reduced on average by 69-89% in PSP, and 53-81% in CU. The distributions of post-rasagiline [¹⁸F]THK5351 SUV among PSP individuals were not consistent with the typical pattern of tau aggregates in PSP.

CONCLUSIONS

Similar to AD, the interpretation of [¹⁸F]THK5351 uptake in PSP is likely confounded by off-target binding to MAO-B binding sites. [¹⁸F]THK5351 is not sufficient in quantifying tau aggregates in PSP using the proposed rasagiline dosing regimen.

The pro-psychotic metabotropic glutamate receptor compounds fenobam and AZD9272 share binding sites with monoamine oxidase-B inhibitors in humans

The metabotropic glutamate receptor 5 (mGluR5) ligands fenobam and AZD9272 have been reported to induce psychosis-like adverse events and to bind at unknown, non-GluR5-related, sites. Based on similarities of the regional binding patterns for [¹¹C]AZD9272 and the monoamine oxidase-B (MAO-B) radioligand [¹¹C]L-deprenyl-D2 in PET studies of the human brain we tested the hypothesis that the unique binding of fenobam and AZD9272 may represent specific binding to the MAO-B. PET data previously acquired for subjects examined using [¹¹C]AZD9272 or [¹¹C]L-deprenyl-D2 were re-evaluated to assess the correlations between radioligand binding parameters in human brain. In addition, the pharmacology of AZD9272 binding sites was characterized using competition binding studies carried out in vivo in non-human primates (NHPs) and in vitro using autoradiography in selected human brain regions. The regional binding of [¹¹C]AZD9272 in human brain was closely correlated with that of [¹¹C]L-deprenyl-D2. In PET studies of NHP brain administration of the MAO-B ligand L-deprenyl inhibited binding of radiolabeled AZD9272 and administration of fenobam inhibited binding of [¹¹C]L-deprenyl-D2. Binding of radiolabeled AZD9272 in vitro was potently inhibited by fenobam or MAO-B compounds, and [¹¹C]L-deprenyl-D2 binding was inhibited by fenobam or AZD9272. The findings are consistent with the hypothesis that both fenobam and AZD9272 bind to the MAO-B, which may be of relevance for understanding the mechanism of the psychosis-like adverse events reported for these compounds. Such understanding may serve as a lead to generate new models for the pathophysiology of psychosis.

2-Styrylchromone derivatives as potent and selective monoamine oxidase B inhibitors

A series of eighteen 2-styrylchromone derivatives (see Chart 1) were synthesized and evaluated for their monoamine oxidase (MAO) A and B inhibitory activities. Many of the derivatives inhibited MAO-B comparable to pargyline (a positive control), and most of them inhibited MAO-B selectively. Of the eighteen derivatives, compound 9 having methoxy group at R¹ and chlorine at R⁴ showed both the best MAO-B inhibitory activity (IC₅₀ = 17 ± 2.4 nM) and the best MAO-B selectivity (IC₅₀ for MAO-A/IC₅₀ for MAO-B = 1500). The mode of inhibition of compound 9 against MAO-B was competitive and reversible. Quantitative structure-activity relationship (QSAR) analyses of the 2-styrylchromone derivatives were conducted using their pIC₅₀ values with the use of Molecular Operating Environment (MOE) and Dragon, demonstrating that the descriptors of MAO-B inhibitory activity and MAO-B selectivity were 1734 and 121, respectively, that showed significant correlations (P < 0.05). We then examined the 2-styrylchromone structures as useful scaffolds through three-dimensional-QSAR studies using AutoGPA, which is based on the molecular field analysis algorithm using MOE. The model using pIC₅₀ value indexes for MAO-B exhibited a determination coefficient (R²) of 0.873 as well as a Leave-One-Out cross-validated determination coefficient (Q²) of 0.675. These data suggested that the 2-styrylchromone structure might be a useful scaffold for the design and development of novel MAO-B inhibitors.

Probiotics mixture increases butyrate, and subsequently rescues the nigral dopaminergic neurons from MPTP and rotenone-induced neurotoxicity

Microbiota in the gut affect brain physiology via various pathways, and dysbiosis seems to play a role in the pathogenesis of Parkinson's disease (PD). Probiotics showed pleiotropic effects on functions of the central nervous system via microbiota-gut-brain axis. However, no studies displayed the neuroprotective effects of probiotics in the Parkinson's disease. This study aimed to test the neuroprotective effects of probiotics in two different models of PD. We evaluated neuroprotective effects of a probiotic cocktail containing Lactobacillus rhamnosus GG, Bifidobacterium animalis lactis, and Lactobacillus acidophilus in PD models induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or rotenone utilizing behavioral tests, immunohistochemistry and neurochemical analysis. To assure the neuroprotection came from increased production of butyrate, we further determined beneficial effects of butyrate in the MPTP-mediated PD model. The probiotic mixture overtly protected the dopaminergic neurons against MPTP neurotoxicity. However, the probiotics downregulated expression of monoamine oxidase (MAO) B in the striatum, which was accompanied by a lower level of 1-methyl-4-phenylpyridinium (MPP⁺), the main neurotoxic metabolite of MPTP. Thus, we extended the investigation into the rotenone-induced PD model. Rescuing effects of the probiotics were observed in the setup, which came with increased levels of neurotrophic factors and butyrate in the brain. Lactobacillus rhamnosus GG was identified to be a major contributor to the induction of neurotrophic factors and downregulation of MAO B. Finally, we demonstrated that sodium butyrate attenuated MPTP-induced neuronal loss in the nigrostriatal pathway. Probiotics could ameliorate neurodegeneration at least partially by increasing butyrate level. These data highlight the role of probiotics for brain health, and their potential as a preventive measure for neurodegenerative diseases such as PD.

Gender-related increase of tropomyosin-1 abundance in platelets of Alzheimer's disease and mild cognitive impairment patients

The incidence of Alzheimer's disease (AD) is higher in elderly women than in men. The molecular background of this gender-related risk, however, is largely unknown. In a previous proteomics study, we identified significantly elevated levels of monoamine oxidase-B and tropomyosin-1 in AD patients, together with significant changes of the genetic AD risk factors apolipoprotein E4 (APOE4) and glutathione S-transferase omega 1 (GSTO1), in platelets - a promising source for AD blood biomarkers. The present study aimed to investigate the gender-specificity as well as the disease-stage dependency of these biomarkers in AD patients and those with mild cognitive impairment (MCI). Tropomyosin-1 and monoamine oxidase-B protein levels were quantified by 2-D DIGE and 1-D Western blotting. Here, for the first time, we revealed a significant increase of 38&39kDa tropomyosin-1 protein levels in female but not male AD (+56%; p=0.008) and MCI patients (+46%; p=0.041) measured by 1-D WB. In contrast, levels of monoamine oxidase-B were, independently of gender, elevated in AD patients (+52%; p=0.009) but unaltered in MCI compared to control subjects. Moreover, we confirmed that APOE4-positive females are at a higher risk (OR=18.7; p=9.7E-09) of developing AD compared to APOE4-positive males (OR=6.5; p=5.9E-04). No gender-related effects were observed for GSTO1.

SIGNIFICANCE

Platelet tropomyosin-1 constitutes a gender-related and stage-dependent protein in cognitive impairment. In contrast, platelet monoamine oxidase-B, frequently described to be increased in platelets and brains of AD patients, shows a gender-independent but stage-related increase since it is unaltered in MCI subjects. A blood biomarker test for this preceding stage of AD that considers gender-specificity is not yet available. The newly described AD-related platelet protein profiles might refine and facilitate routine diagnosis and enable early as well as tailored interventions.

Substantial protection against MPTP-associated Parkinson's neurotoxicity in vitro and in vivo by anti-cancer agent SU4312 via activation of MEF2D and inhibition of MAO-B

We have previously demonstrated the unexpected neuroprotection of the anti-cancer agent SU4312 in cellular models associated with Parkinson's disease (PD). However, the precise mechanisms underlying its neuroprotection are still unknown, and the effects of SU4312 on rodent models of PD have not been characterized. In the current study, we found that the protection of SU4312 against 1-methyl-4-phenylpyridinium ion (MPP⁺)-induced neurotoxicity in PC12 cells was achieved through the activation of transcription factor myocyte enhancer factor 2D (MEF2D), as evidenced by the fact that SU4312 stimulated myocyte enhancer factor 2 (MEF2) transcriptional activity and prevented the inhibition of MEF2D protein expression caused by MPP⁺, and that short hairpin RNA (ShRNA)-mediated knockdown of MEF2D significantly abolished the neuroprotection of SU4312. Additionally, Western blotting analysis revealed that SU4312 potentiated pro-survival PI3-K/Akt pathway to down-regulate MEF2D inhibitor glycogen synthase kinase-3beta (GSK3β). Furthermore, using the in vivo PD model of C57BL/6 mice insulted with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), we found that intragastrical administration of SU4312 (0.2 and 1 mg/kg) greatly ameliorated Parkinsonian motor defects, and restored protein levels of MEF2D, phosphorylated-Ser473-Akt and phosphorylated-Ser9-GSK3β. Meanwhile, SU4312 effectively reversed the decrease in protein expression of tyrosine hydroxylase in substantia nigra pars compacta dopaminergic neurons, inhibited oxidative stress, maintained mitochondrial biogenesis and partially prevented the depletion of dopamine and its metabolites. Very encouragingly, SU4312 was able to selectively inhibit monoamine oxidase-B (MAO-B) activity both in vitro and in vivo, with an IC₅₀ value of 0.2 μM. These findings suggest that SU4312 provides therapeutic benefits in cellular and animal models of PD, possibly through multiple mechanisms including enhancement of MEF2D through the activation of PI3-K/Akt pathway, maintenance of mitochondrial biogenesis and inhibition of MAO-B activity. SU4312 thus may be an effective drug candidate for the prevention or even modification of the pathological processes of PD.

Monoamine oxidase B inhibitor, selegiline, reduces 18F-THK5351 uptake in the human brain

Background

¹⁸F-THK5351 is a quinoline-derived tau imaging agent with high affinity to paired helical filaments (PHF). However, high levels of ¹⁸F-THK5351 retention in brain regions thought to contain negligible concentrations of PHF raise questions about the interpretation of the positron emission tomography (PET) signals, particularly given previously described interactions between quinolone derivatives and monoamine oxidase B (MAO-B). Here, we tested the effects of MAO-B inhibition on ¹⁸F-THK5351 brain uptake using PET and autoradiography.

Methods

Eight participants (five mild cognitive impairment, two Alzheimer’s disease, and one progressive supranuclear palsy) had baseline ¹⁸F-AZD4694 and ¹⁸F-THK5351 scans in order to quantify brain amyloid and PHF load, respectively. A second ¹⁸F-THK5351 scan was conducted 1 week later, 1 h after a 10-mg oral dose of selegiline. Three out of eight patients also had a third ¹⁸F-THK5351 scan 9–28 days after the selegiline administration. The primary outcome measure was standardized uptake value (SUV), calculated using tissue radioactivity concentration from 50 to 70 min after ¹⁸F-THK5351 injection, normalizing for body weight and injected radioactivity. The SUV ratio (SUVR) was determined using the cerebellar cortex as the reference region. ¹⁸F-THK5351 competition autoradiography studies in postmortem tissue were conducted using 150 and 500 nM selegiline.

Results

At baseline, ¹⁸F-THK5351 SUVs were highest in the basal ganglia (0.64 ± 0.11) and thalamus (0.62 ± 0.14). In the post-selegiline scans, the regional SUVs were reduced on average by 36.7% to 51.8%, with the greatest reduction noted in the thalamus (51.8%) and basal ganglia (51.4%). MAO-B inhibition also reduced ¹⁸F-THK5351 SUVs in the cerebellar cortex (41.6%). The SUVs remained reduced in the three patients imaged at 9–28 days. Tissue autoradiography confirmed the effects of MAO-B inhibition on ¹⁸F-THK5351 uptake.

Conclusions

These results indicate that the interpretation of ¹⁸F-THK5351 PET images, with respect to tau, is confounded by the high MAO-B availability across the entire brain. In addition, the heterogeneous MAO-B availability across the cortex may limit the interpretation of ¹⁸F-THK5351 scans using reference region methods.

Detrimental effects of oxidative losses in parkin activity in a model of sporadic Parkinson's disease are attenuated by restoration of PGC1alpha

Loss of parkin E3 ligase activity as a result of parkin gene mutation in rare familial forms of Parkinson's disease (PD) has been shown to be detrimental to mitochondrial function and to contribute to ensuing neurodegeneration. This has been shown by ourselves and others to be in part due to reductions in parkin-mediated ubiquitination of the transcriptional repressor PARIS, limiting the protein's subsequent degradation by the proteasome. Subsequent elevations in PARIS protein levels result in reduced expression of the master mitochondrial regulator PGC-1α, impacting in turn on mitochondrial function. Here, we report that oxidatively-mediated reductions in parkin solubility and function in a mouse model of age-related sporadic PD coincides with increased PARIS levels and reduced PGC-1α signaling. Furthermore, restoration of PGC-1α expression was found to abrogate losses in mitochondrial function and degeneration of dopaminergic (DAergic) neurons within the substantia nigra pars compacta (SNpc) associated with this particular model. These findings suggest that the PGC-1α signaling pathway constitutes a viable therapeutic target for the treatment of not only familial PD, but also more common sporadic forms of the disorder.

Monoamine oxidase B layer-by-layer film fabrication and characterization toward dopamine detection

In this work nanostructured film composites of the monoamine oxidase B (MAO-B) enzyme, free or encapsulated in liposomes, were fabricated by the layer-by-layer (LbL) self-assembly technique, employing polyethylene imine (PEI) as polycation. Initially, the MAO-B enzyme was incorporated into liposomes in order to preserve its enzymatic structure ensuring their activity and catalytic stability. The LbL film growth was monitored by surface plasmon resonance (SPR) by gold resonance angle shift analysis after each bilayer deposition. Subsequently, the films were applied as amperometric biosensors for dopamine detection using Prussian Blue (PB) as the electron mediator. The biosensor fabricated by MAO-B incorporated into liposomes composed of DPPG:POPG in the ratio (1:4) (w/w) showed the best performance with a sensitivity of 0.86 (μA cm(-2))/(mmol L(-1)) and a detection limit of 0.33 mmol L(-1).