N-Propargylamine protects SH-SY5Y cells from apoptosis induced by an endogenous neurotoxin, N-methyl(R)salsolinol, through stabilization of mitochondrial membrane and induction of anti-apoptotic Bcl-2

Neuroprotective Function of Rasagiline and Selegiline, Inhibitors of Type B Monoamine Oxidase, and Role of Monoamine Oxidases in Synucleinopathies

Synucleinopathies are a group of neurodegenerative disorders caused by the accumulation of toxic species of α-synuclein. The common clinical features are chronic progressive decline of motor, cognitive, behavioral, and autonomic functions. They include Parkinson’s disease, dementia with Lewy body, and multiple system atrophy. Their etiology has not been clarified and multiple pathogenic factors include oxidative stress, mitochondrial dysfunction, impaired protein degradation systems, and neuroinflammation. Current available therapy cannot prevent progressive neurodegeneration and “disease-modifying or neuroprotective” therapy has been proposed. This paper presents the molecular mechanisms of neuroprotection by the inhibitors of type B monoamine oxidase, rasagiline and selegiline. They prevent mitochondrial apoptosis, induce anti-apoptotic Bcl-2 protein family, and pro-survival brain- and glial cell line-derived neurotrophic factors. They also prevent toxic oligomerization and aggregation of α-synuclein. Monoamine oxidase is involved in neurodegeneration and neuroprotection, independently of the catalytic activity. Type A monoamine oxidases mediates rasagiline-activated signaling pathways to induce neuroprotective genes in neuronal cells. Multi-targeting propargylamine derivatives have been developed for therapy in various neurodegenerative diseases. Preclinical studies have presented neuroprotection of rasagiline and selegiline, but beneficial effects have been scarcely presented. Strategy to improve clinical trials is discussed to achieve disease-modification in synucleinopathies.

Riluzole-Rasagiline Hybrids: Toward the Development of Multi-Target-Directed Ligands for Amyotrophic Lateral Sclerosis

Polypharmacology is a new trend in amyotrophic lateral sclerosis (ALS) therapy and an effective way of addressing a multifactorial etiology involving excitotoxicity, mitochondrial dysfunction, oxidative stress, and microglial activation. Inspired by a reported clinical trial, we converted a riluzole (1)–rasagiline (2) combination into single-molecule multi-target-directed ligands. By a ligand-based approach, the highly structurally integrated hybrids 3–8 were designed and synthesized. Through a target- and phenotypic-based screening pipeline, we identified hit compound 6. It showed monoamine oxidase A (MAO-A) inhibitory activity (IC₅₀ = 6.9 μM) rationalized by in silico studies as well as in vitro brain permeability. By using neuronal and non-neuronal cell models, including ALS-patient-derived cells, we disclosed for 6 a neuroprotective/neuroinflammatory profile similar to that of the parent compounds and their combination. Furthermore, the unexpected MAO inhibitory activity of 1 (IC₅₀ = 8.7 μM) might add a piece to the puzzle of its anti-ALS molecular profile.

Preventative effects of 1-methyl-1,2,3,4-tetrahydroisoquinoline derivatives (N-functional group loading) on MPTP-induced parkinsonism in mice

1,2,3,4-Tetrahydroisoquinoline (TIQ) is endogenously present in human brain, and some of its derivatives are thought to contribute to the induction of Parkinson's disease (PD)-like signs in rodents and primates. In contrast, the endogenous TIQ derivative 1-methyl-TIQ (1-MeTIQ) is reported to be neuroprotective. In the present study, we compared the effects of artificially modified 1-MeTIQ derivatives (loading an N-propyl, N-propenyl, N-propargyl, or N-butynyl group) on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD-like signs in mice. In a behavioral study, MPTP-induced bradykinesia was significantly decreased by all compounds. However, only 1-Me-N-propargyl-TIQ showed an inhibitory effect by blocking the MPTP-induced reduction in striatal dopamine content and the number of nigral tyrosine hydroxylase-positive cells. Western blot analysis showed that 1-Me-N-propargyl-TIQ and 1-Me-N-butynyl-TIQ potently prevented the MPTP-induced decrease in dopamine transporter expression, whereas 1-MeTIQ and 1-Me-N-propyl-TIQ did not. These results suggest that although loading an N-propargyl group on 1-MeTIQ clearly enhanced neuroprotective effects, other N-functional groups showed distinct pharmacological properties characteristic of their functional groups. Thus, the number of bonds and length of the N-functional group may contribute to the observed differences in effect.

Can the interplay between autophagy and apoptosis be targeted as a novel therapy for Parkinson's disease?

Development of efficacious treatments for Parkinson's disease (PD) demands an improved understanding of mechanisms underlying neurodegeneration. Two cellular death pathways postulated to play key roles in PD are autophagy and apoptosis. Molecular overlap between these pathways was investigated through identifying studies that used therapeutic compounds to alter expression of specific molecular components of the pathways. Bcl-2 was identified as an important protein with the ability to suppress autophagy and apoptosis through inhibiting Beclin-1 and Bax, respectively. Involvement of c-Jun N-terminal kinases (JNK) and p38, was evident in the activation of apoptosis through increasing the Bax/Bcl-2 ratio. JNK-mediated phosphorylation also suppresses the inhibiting functions of Bcl-2, indicating an ability to induce not only apoptosis but also autophagy. Additionally, a p38-mediated increase in heme oxygenase-1 expression inhibits apoptosis. Moreover, besides inhibiting mammalian target of rapamycin, Akt is associated with decreased Bax expression, thereby acting as both an autophagy inducer and apoptosis inhibitor. Ultimately, manipulation of molecular components involved in autophagy and apoptosis regulation could be targeted as possible therapies for PD.

Rasagiline and selegiline modulate mitochondrial homeostasis, intervene apoptosis system and mitigate α-synuclein cytotoxicity in disease-modifying therapy for Parkinson's disease

Parkinson's disease has been considered as a motor neuron disease with dopamine (DA) deficit caused by neuronal loss in the substantia nigra, but now proposed as a multi-system disorder associated with α-synuclein accumulation in neuronal and non-neuronal systems. Neuroprotection in Parkinson's disease has intended to halt or reverse cell death of nigro-striatal DA neurons and prevent the disease progression, but clinical studies have not presented enough beneficial results, except the trial of rasagiline by delayed start design at low dose of 1 mg/day only. Now strategy of disease-modifying therapy should be reconsidered taking consideration of accumulation and toxicity of α-synuclein preceding the manifest of motor symptoms. Hitherto neuroprotective therapy has been aimed to mitigate non-specific risk factors; oxidative stress, mitochondrial dysfunction, apoptosis, deficits of neurotrophic factors (NTFs), inflammation and accumulation of pathogenic protein. Future disease-modify therapy should target more specified pathogenic factors, including deregulated mitochondrial homeostasis, deficit of NTFs and α-synuclein toxicity. Selegiline and rasagiline, inhibitors of type B monoamine oxidase, have been proved to exhibit potent neuroprotective function: regulation of mitochondrial apoptosis system, maintenance of mitochondrial function, increased expression of genes coding antioxidant enzymes, anti-apoptotic Bcl-2 and pro-survival NTFs, and suppression of oligomerization and aggregation of α-synuclein and the toxicity in cellular and animal experiments. However, the present available pharmacological therapy starts too late to reverse disease progression, and future disease-modifying therapy should include also non-pharmacological complementary therapy during the prodromal stage.

Neurotrophic function of phytochemicals for neuroprotection in aging and neurodegenerative disorders: modulation of intracellular signaling and gene expression

Bioactive compounds in food and beverages have been reported to promote health and prevent age-associated decline in cognitive, motor and sensory activities, and emotional function. Phytochemicals, a ubiquitous class of plant secondary metabolites, protect neuronal cells by interaction with cellular activities, in addition to the antioxidant and anti-inflammatory function. In aging and age-associated neurodegenerative disorders, phytochemicals protect neuronal cells by neurotrophic factor-mimic activity, in addition to suppression of apoptosis signaling in mitochondria. This review presents the cellular mechanisms underlying anti-apoptotic function and neurotrophic function of phytochemicals in the brain. Phytochemicals bind to receptors of neurotrophic factors, and also receptors for γ-aminobutyric acid, acetylcholine, serotonin, and glutamate and estrogen, and activate downstream signal pathways. Phytochemicals also directly intervene intracellular signaling molecules to modify the brain function. Finally, phytochemicals enhance the endogenous biosynthesis of genes coding anti-apoptotic Bcl-2 and neurotrophic factors, such as brain-derived and glial cell line-derived neurotrophic factor. The gene induction may play a major role in the neuroprotective function of dietary compounds shown by epidemiological studies. Quantitative measurement of neurotrophic factors induced by phytochemicals in the serum, cerebrospinal fluid, and other clinical samples is proposed as a surrogate assay method to evaluate the neuroprotective potency. Development of novel neuroprotective compounds is expected among compounds chemically synthesized from the brain-permeable basic structure of phytochemicals.

Changes in neuronal dopamine homeostasis following 1-methyl-4-phenylpyridinium (MPP+) exposure

1-Methyl-4-phenylpyridinium (MPP(+)), the active metabolite of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, selectively kills dopaminergic neurons in vivo and in vitro via a variety of toxic mechanisms, including mitochondrial dysfunction, generation of peroxynitrite, induction of apoptosis, and oxidative stress due to disruption of vesicular dopamine (DA) storage. To investigate the effects of acute MPP(+) exposure on neuronal DA homeostasis, we measured stimulation-dependent DA release and non-exocytotic DA efflux from mouse striatal slices and extracellular, intracellular, and cytosolic DA (DAcyt) levels in cultured mouse ventral midbrain neurons. In acute striatal slices, MPP(+) exposure gradually decreased stimulation-dependent DA release, followed by massive DA efflux that was dependent on MPP(+) concentration, temperature, and DA uptake transporter activity. Similarly, in mouse midbrain neuronal cultures, MPP(+) depleted vesicular DA storage accompanied by an elevation of cytosolic and extracellular DA levels. In neuronal cell bodies, increased DAcyt was not due to transmitter leakage from synaptic vesicles but rather to competitive MPP(+)-dependent inhibition of monoamine oxidase activity. Accordingly, monoamine oxidase blockers pargyline and l-deprenyl had no effect on DAcyt levels in MPP(+)-treated cells and produced only a moderate effect on the survival of dopaminergic neurons treated with the toxin. In contrast, depletion of intracellular DA by blocking neurotransmitter synthesis resulted in ∼30% reduction of MPP(+)-mediated toxicity, whereas overexpression of VMAT2 completely rescued dopaminergic neurons. These results demonstrate the utility of comprehensive analysis of DA metabolism using various electrochemical methods and reveal the complexity of the effects of MPP(+) on neuronal DA homeostasis and neurotoxicity.

Serotonin receptor 6 mediates defective brain development in monoamine oxidase A-deficient mouse embryos

Monoamine oxidases A and B (MAO-A and MAO-B) are enzymes of the outer mitochondrial membrane that metabolize biogenic amines. In the adult central nervous system, MAOs have important functions for neurotransmitter homeostasis. Expression of MAO isoforms has been detected in the developing embryo. However, suppression of MAO-B does not induce developmental alterations. In contrast, targeted inhibition and knockdown of MAO-A expression (E7.5-E10.5) caused structural abnormalities in the brain. Here we explored the molecular mechanisms underlying defective brain development induced by MAO-A knockdown during in vitro embryogenesis. The developmental alterations were paralleled by diminished apoptotic activity in the affected neuronal structures. Moreover, dysfunctional MAO-A expression led to elevated levels of embryonic serotonin (5-hydroxytryptamine (5-HT)), and we found that knockdown of serotonin receptor-6 (5-Htr6) expression or pharmacologic inhibition of 5-Htr6 activity rescued the MAO-A knockdown phenotype and restored apoptotic activity in the developing brain. Our data suggest that excessive 5-Htr6 activation reduces activation of caspase-3 and -9 of the intrinsic apoptotic pathway and enhances expression of antiapoptotic proteins Bcl-2 and Bcl-XL. Moreover, we found that elevated 5-HT levels in MAO-A knockdown embryos coincided with an enhanced activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and a reduction of proliferating cell numbers. In summary, our findings suggest that excessive 5-HT in MAO-A-deficient mouse embryos triggers cellular signaling cascades via 5-Htr6, which suppresses developmental apoptosis in the brain and thus induces developmental retardations.

Revelation in the neuroprotective functions of rasagiline and selegiline: the induction of distinct genes by different mechanisms

In Parkinson's disease, cell death of dopamine neurons in the substantia nigra progresses and neuroprotective therapy is required to halt neuronal loss. In cellular and animal models, selegiline [(-)deprenyl] and rasagiline, inhibitors of type B monoamine oxidase (MAO)-B, protect neuronal cells from programmed cell death. In this paper, the authors review their recent results on the molecular mechanisms by which MAO inhibitors prevent the cell death through the induction of antiapoptotic, prosurvival genes. MAO-A mediates the induction of antiapoptotic bcl-2 and mao-a itself by rasagiline, whereas a different mechanism is associated with selegiline. Rasagiline and selegiline preferentially increase GDNF and BDNF in nonhuman primates and Parkinsonian patients, respectively. Enhanced neurotrophic factors might be applicable to monitor the neurorescuing activity of neuroprotection.

Formation of a salsolinol-like compound, the neurotoxin, 1-acetyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, in a cellular model of hyperglycemia and a rat model of diabetes

There are statistical data indicating that diabetes is a risk factor for Parkinson's disease (PD). Methylglyoxal (MG), a biologically reactive byproduct of glucose metabolism, the levels of which have been shown to be increase in diabetes, reacts with dopamine to form 1-acetyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (ADTIQ); this formation may provide further insight into the connection between PD and diabetes. In this study, we investigated the role of ADTIQ in these two diseases to determine in an aim to enhance our understanding of the link between PD and diabetes. To this end, a cell model of hyperglycemia and a rat model of diabetes were established. In the cell model of hyperglycemia, compared with the control group, the elevated glucose levels promoted free hydroxyl radical formation (p<0.01). An ADTIQ assay was successfully developed and ADTIQ levels were detected and quantified. The levels of its precursors, MG and dopamine (DA), were determined in both the cell model of hyperglycemia and the rat model of diabetes. The proteins related to glucose metabolism were also assayed. Compared with the control group, ADTIQ and MG levels were significantly elevated not only in the cell model of hyperglycemia, but also in the brains of rats with diabetes (p<0.01). Seven key enzymes from the glycolytic pathway were found to be significantly more abundant in the brains of rats with diabetes. Moreover, it was found that adenosine triphosphate (ATP) synthase and superoxide dismutase (SOD) expression levels were markedly decreased in the rats with diabetes compared with the control group. Therefore, ADTIQ expression levels were found to be elevated under hyperglycemic conditions. The results reported herein demonstrate that ADTIQ, which is derived from MG, the levels of which are increased in diabetes, may serve as a neurotoxin to dopaminergic neurons, eventually leading to PD.

Exploring the structural basis of the selective inhibition of monoamine oxidase A by dicarbonitrile aminoheterocycles: role of Asn181 and Ile335 validated by spectroscopic and computational studies

Since cyanide potentiates the inhibitory activity of several monoamine oxidase (MAO) inhibitors, a series of carbonitrile-containing aminoheterocycles was examined to explore the role of nitriles in determining the inhibitory activity against MAO. Dicarbonitrile aminofurans were found to be potent, selective inhibitors against MAO A. The origin of the MAO A selectivity was identified by combining spectroscopic and computational methods. Spectroscopic changes induced in MAO A by mono- and dicarbonitrile inhibitors were different, providing experimental evidence for distinct binding modes to the enzyme. Similar differences were also found between the binding of dicarbonitrile compounds to MAO A and to MAO B. Stabilization of the flavin anionic semiquinone by monocarbonitrile compounds, but destabilization by dicarbonitriles, provided further support to the distinct binding modes of these compounds and their interaction with the flavin ring. Molecular modeling studies supported the role played by the nitrile and amino groups in anchoring the inhibitor to the binding cavity. In particular, the results highlight the role of Asn181 and Ile335 in assisting the interaction of the nitrile-containing aminofuran ring. The network of interactions afforded by the specific attachment of these functional groups provides useful guidelines for the design of selective, reversible MAO A inhibitors.

Monoamine oxidase inhibitors and neuroprotection: a review

Monoamine oxidase inhibitors have been available for more than 50 years, initially developed as antidepressants but currently used in a variety of psychiatric and neurological conditions. There has been a recent surge of interest in monoamine oxidase inhibitors because of their reported neuroprotective and/or neurorescue properties. Interestingly, it seems that often these properties are independent of their ability to inhibit monoamine oxidase. This review article presents an overview of the neuroprotective/neurorescue properties of these multifaceted drugs and focuses on phenelzine, (-)-deprenyl, rasagiline, ladostigil, tranylcypromine, moclobemide, and clorgyline and their possible neuroprotective mechanisms.

Increased vulnerability of parkin knock down PC12 cells to hydrogen peroxide toxicity: the role of salsolinol and NM-salsolinol

Dopamine-derived neurotoxins, 1-methyl-4-phenyl-1,2,3,4-tetrahydroisoquinoline (salsolinol) and 1(R),2(N)-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (NM-salsolinol) are the two most possible 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-like endogenous neurotoxin candidates that involved in the pathogenesis of Parkinson's disease (PD). The levels of endogenously synthesized salsolinol and NM-salsolinol are increased in the cerebrospinal fluid (CSF) of PD patients. Both of them lead to neurotoxicity in dopaminergic cells by inhibiting mitochondrial electron transport chain. To study the role of salsolinol and NM-salsolinol in Parkin deficiency-induced dopaminergic cell damage, we determined the cellular level of oxidative stress, the formation of salsolinol and NM-salsolinol, the level of mitochondrial damage and cell viability with/without the presence of exogenous H₂O₂ using differentiated dopaminergic PC12 cells. Our data show that parkin knock down elevates cellular oxidative stress, salsolinol and NM-salsolinol levels, which are responsible for the higher cell mortality in Parkin-deficient cells upon exposure to exogenous H₂O₂. The level of mitochondrial membrane potential loss, cristae disruption and the release of cytochrome c increased significantly along with the increased level of salsolinol and NM-salsolinol, whereas compared to parkin knock down cells in the presence of H₂O₂, the mitochondrial damage and higher cell mortality were both diminished when the levels of salsolinol and NM-salsolinol was reduced. The results not only indicate the elevated level of salsolinol and NM-salsolinol, but also reveal the potential role of salsolinol and NM-salsolinol in parkin knock down-induced cell vulnerability. We assume that parkin deficiency is the trigger of excessive oxidative stress, elevated endogenous neurotoxin levels and mitochondrial damage, which eventually results in cell death of dopaminergic cells.

Type A monoamine oxidase is associated with induction of neuroprotective Bcl-2 by rasagiline, an inhibitor of type B monoamine oxidase

Rasagiline and (-)deprenyl (selegiline), irreversible type B monoamine oxidase (MAO-B) inhibitors, protect neuronal cells through gene induction of pro-survival Bcl-2 and neurotrophic factors in the cellular models of neurodegenerative disorders. In this paper, the role of MAO in the up-regulation of neuroprotective Bcl-2 gene by these inhibitors was studied using type A MAO (MAO-A) expressing wild SH-SY5Y cells and the transfection-enforced MAO-B overexpressed cells. Rasagiline and (-)deprenyl, and also befloxatone, a reversible MAO-A inhibitor, increased Bcl-2 mRNA and protein in SH-SY5Y cells. Silencing MAO-A expression with short interfering (si) RNA suppressed Bcl-2 induction by rasagiline, but not by (-)deprenyl. MAO-B overexpression inhibited Bcl-2 induction by rasagiline and befloxatone, but did not affect that by (-)deprenyl, suggesting the different mechanisms behind Bcl-2 gene induction by these MAO-B inhibitors. The novel role of MAO-A in Bcl-2 induction by rasagiline is discussed with regard to the molecular mechanism underlying neuroprotection by the MAO inhibitors.

Explaining ADAGIO: a critical review of the biological basis for the clinical effects of rasagiline

The ADAGIO study demonstrated a symptomatic benefit for rasagiline in early Parkinson's disease (PD) and suggested a disease-modifying effect. Evidence indicates that mitochondrial dysfunction plays a role in the pathogenesis of PD and that this may be the site of effect for rasagiline. In this systematic review, evidence for the role of mitochondria in the pathogenesis of PD are reviewed in light of other proposed mechanisms of neuronal degeneration and the actions of rasagiline and its component parts, namely propargylamine and the metabolite, aminoindan. Evidence for the role of mitochondria in the pathogenesis and treatment of PD are reviewed in light of other proposed mechanisms of neuronal degeneration and clinical actions of rasagiline. Monoamine oxidase B (MAO-B) located in the outer mitochondrial membrane controls dopamine metabolism in early PD, and this is the likely location for the symptomatic action of rasagiline. Accumulating evidence indicates that mitochondrial impairment contributes to dopaminergic neuronal loss in PD, either directly or through other mechanisms such as oxidative stress or protein misfolding. Further rasagiline affects numerous mitochondrial mechanisms that prevent apoptotic cell death including prevention of opening of the mitochondrial transition pore, decreased release of cytochrome C, alterations in pro-antiapoptotic genes and proteins, and the nuclear translocation of glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Thus, the functional neuroprotective actions of rasagiline may not be dependent on MAO-B inhibition, but rather may involve actions of the propargylamine moiety and the aminoindan metabolite. An accumulating body of literature indicates a mitochondrial site of action for rasagiline and highlights the neuroprotective action of the drug, providing strong biological plausibility for disease-modifying effects of the drug such as those observed in ADAGIO.

Role and treatment of mitochondrial DNA-related mitochondrial dysfunction in sporadic neurodegenerative diseases

Several sporadic neurodegenerative diseases display phenomena that directly or indirectly relate to mitochondrial function. Data suggesting altered mitochondrial function in these diseases could arise from mitochondrial DNA (mtDNA) are reviewed. Approaches for manipulating mitochondrial function and minimizing the downstream consequences of mitochondrial dysfunction are discussed.

Novel MAO-B inhibitors: potential therapeutic use of the selective MAO-B inhibitor PF9601N in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease that is characterized by preferential loss of dopaminergic neurons in the substantia nigra pars compacta, leading to declining levels of dopamine in the striatum. In our search for compounds able not only to extend the effects of dopamine by preventing its degradation but also to halt or slow the neurodegenerative process, we designed, synthesized, and biologically tested a series of propargylamines for their potential use as therapeutic agents for PD. Among them, PF9601N, [N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine], showed high potency and selectivity as a MAO-BI (monoamine oxidase type B inhibitor) and also demonstrated remarkable neuroprotective properties in several in vivo and cellular models of PD. In this chapter, we describe the preclinical evidence revealing the novel MAO-BI PF9601N as an interesting candidate for the treatment of PD.

Type A monoamine oxidase regulates life and death of neurons in neurodegeneration and neuroprotection

In Parkinson's disease, type B monoamine oxidase (MAO-B) is proposed to play an important role in the pathogenesis through production of reactive oxygen species and neurotoxins from protoxicants, such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. In addition, inhibitors of MAO-B protect neurons in the cellular and animal models of Parkinson's and Alzheimer's diseases. However, the role of type A MAO (MAO-A) in neuronal death and neuroprotection by MAO-B inhibitors has been scarcely elucidated. This chapter presents our recent results on the involvement of MAO-A in the activation of mitochondrial death signal pathway and in the induction of prosurvival genes to prevent cell death with MAO-B inhibitors. The roles of MAO-A in the regulation of neuronal survival and death are discussed in concern to find a novel strategy to protect neurons in age-associated neurodegenerative disorders and depression.

The role of rasagiline in the treatment of Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder, affecting 1% to 2% of people older than 60 years. Treatment of PD consists of symptomatic therapies while neuroprotective strategies have remained elusive. Rasagiline is a novel, potent, and irreversible monoamine oxidase type B (MAO-B) inhibitor which has been approved for treatment of PD. Rasagiline inhibits MAO-B more potently than selegiline and has the advantage of once-daily dosing. In several large, randomized, placebo-controlled trials, rasagiline has demonstrated efficacy as monotherapy in early PD and as adjunctive therapy in advanced PD. In addition, rasagiline has been shown to have neuroprotective effects in in vitro and in vivo studies. The recently completed delayed-start ADAGIO (Attenuation of Disease Progression with Azilect Given Once-daily) trial suggests a potential disease-modifying effect for rasagiline 1 mg/day, though the clinical import of this finding has yet to be established.

The neuroprotective mechanism of 1-(R)-aminoindan, the major metabolite of the anti-parkinsonian drug rasagiline

The anti-parkinsonian drug, rasagiline [N-propargyl-1-(R)-aminoindan; Azilect(R)], is a secondary cyclic benzylamine and indane derivative, which provides irreversible, potent monoamine oxidase-B (MAO-B) inhibition and possesses neuroprotective and neurorestorative activities. A prospective clinical trial has shown that rasagiline confers significant symptomatic improvement and demonstrated alterations in Parkinson's disease progression. Rasagiline is primarily metabolized by hepatic cytochrome P-450 to form its major metabolite, 1-(R)-aminoindan, a non-amphetamine, weak reversible MAO-B inhibitor compound. Recent studies indicated the potential neuroprotective effect of 1-(R)-aminoindan, suggesting that it may contribute to the overall neuroprotective and antiapoptotic effects of its parent compound, rasagiline. This review article briefly highlights the molecular mechanisms underlying the neuroprotective properties of the active metabolite of rasagiline, 1-(R)-aminoindan, supporting the valuable potential of rasagiline for disease modification.

Why should we use multifunctional neuroprotective and neurorestorative drugs for Parkinson's disease?

Parkinson's disease (PD) is a severe neurodegenerative disorder, with no available drugs able to prevent the neuronal cell loss characteristic in brains of patients suffering from PD. Due to the complex cascade of molecular events involved in the etiology of PD, an innovative approach towards neuroprotection or neurorescue may entail the use of multifunctional pharmaceuticals that target an array of pathological pathways, each of which is believed to contribute to events that ultimately lead to neuronal cell death. Here we discuss examples of novel multifunctional ligands that may have potential as neuroprotective and neurorestorative therapeutics in PD. The compounds discussed originate from synthetic chemistry as well as from natural sources where various moieties, identified in research to possess neuroprotective and neurorestorative properties, have been introduced into the structures of several monomodal drugs, some of which are used in the clinic.

TVP1022 and propargylamine protect neonatal rat ventricular myocytes against doxorubicin-induced and serum starvation-induced cardiotoxicity

We recently reported that propargylamine derivatives such as rasagiline (Azilect) and its S-isomer TVP1022 are neuroprotective. The aim of this study was to test the hypothesis that the neuroprotective agents TVP1022 and propargylamine (the active moiety of propargylamine derivatives) are also cardioprotective. We specifically investigated the protective efficacy of TVP1022 and propargylamine in neonatal rat ventricular myocytes (NRVM) against apoptosis induced by the anthracycline chemotherapeutic agent doxorubicin and by serum starvation. We demonstrated that pretreatment of NRVM cultures with TVP1022 or propargylamine attenuated doxorubicin-induced and serum starvation-induced apoptosis, inhibited the increase in cleaved caspase 3 levels, and reversed the decline in Bcl-2/Bax ratio. These cytoprotective effects were shown to reside in the propargylamine moiety. Finally, we showed that TVP1022 neither caused proliferation of the human cancer cell lines HeLa and MDA-231 nor interfered with the anti-cancer efficacy of doxorubicin. These results suggest that TVP1022 should be considered as a novel cardioprotective agent against ischemic insults and against anthracycline cardiotoxicity and can be coadministered with doxorubicin in the treatment of human malignancies.

The novel cholinesterase-monoamine oxidase inhibitor and antioxidant, ladostigil, confers neuroprotection in neuroblastoma cells and aged rats

The current therapeutic advance in which future drugs are designed to possess varied pharmacological properties and act on multiple targets has stimulated the development of the multimodal drug, ladostigil (TV3326; (N-propargyl-(3R) aminoindan-5yl)-ethyl methyl carbamate). Ladostigil combines neuroprotective effects with monoamine oxidase (MAO)-A and MAO-B and cholinesterase (ChE) inhibitory activities in a single molecule, as a potential treatment for Alzheimer's disease (AD) and Lewy body disease. In the present study, we demonstrate that ladostigil (10(-6)-10 muM) dose-dependently increased cell viability, associated with increased activity of catalase and glutathione reductase and decrease of intracellular reactive oxygen species production in a cytotoxic model of human SH-SY5Y neuroblastoma cells exposed to hydrogen peroxide (H(2)O(2)). In addition, ladostigil significantly upregulated mRNA levels of several antioxidant enzymes (catalase, NAD(P)H quinone oxidoreductase 1 and peroxiredoxin 1) in both H(2)O(2)-treated SH-SY5Y cells, as well as in the high-density human SK-N-SH neuroblastoma cultured apoptotic models. In vivo chronic treatment with ladostigil (1 mg/kg per os per day for 30 days) markedly upregulated mRNA expression levels of various enzymes involved in metabolism and oxidation processes in aged rat hippocampus. In addition to its unique combination of ChE and MAO enzyme inhibition, these results indicate that ladostigil displays neuroprotective activity against oxidative stress-induced cell apoptosis, which might be valuable for aging and age-associated neurodegenerative diseases.

The neuroprotective effect of ladostigil against hydrogen peroxide-mediated cytotoxicity

The multifunctional, anti-Alzheimer drug, ladostigil (TV3326) [(N-propargyl-(3R) aminoindan-5yl)-ethyl methyl carbamate] combines the neuroprotective effects of the anti-Parkinson drug, rasagiline, a selective monoamine oxidase (MAO)-B inhibitor, with the cholinesterase (ChE) inhibitory activity of rivastigmine in a single molecule. Ladostigil has been shown to possess potent antiapoptotic and neuroprotective activities in various oxidative insults in vitro and in vivo, such as prevention of the fall in mitochondrial membrane potential and regulation of Bcl-2 family proteins. In the present study, we demonstrate that ladostigil (1 microM) increased cell viability, associated with the increase of catalase activity and decrease of intracellular reactive oxygen species (ROS) production in human SH-SY5Y neuroblastoma cells exposed to (hydrogen peroxide) H(2)O(2). Furthermore, ladostigil significantly elevated mRNA levels of the antioxidants enzymes, catalase, NAD(P)H quinone oxidoreductase 1 (NQO1) and peroxiredoxin 1 (Prx 1) in H(2)O(2)-treated SH-SY5Y cells. Chronic treatment with ladostigil (1 mg/kg gavage per day for 30 days) markedly up-regulated mRNA expression levels of various antioxidant enzymes in aged rat hippocampus (e.g. glutathione peroxidase precursor (GSHPX-P), glutathione S-transferase (GST) and glucose-6-phosphate dehydrogenase (G6PD)). These findings indicate that in addition to its multiple neuroprotective characteristics, ladostigil also possesses antioxidant properties, which might be beneficial for the treatment of oxidative stress (OS) in aging and age-associated neurodegenerative diseases.

The attenuating effect of memantine on staurosporine-, salsolinol- and doxorubicin-induced apoptosis in human neuroblastoma SH-SY5Y cells

Memantine, a clinically used N-methyl-D-aspartate (NMDA)-receptor antagonist, has been shown to prevent apoptotic neuronal damage connected with the over-activity of NMDA receptors. In the present study, we examined the effect of memantine on staurosporine-, salsolinol- and doxorubicin-induced apoptosis in the SH-SY5Y cell line which does not possess functional NMDA receptors. Electrophysiological recordings and toxicity studies showed no response to NMDA-evoked currents in this cell line, irrespective of the stage of its neuronal differentiation. Memantine (0.1-2 microM) attenuated staurosporine-induced apoptosis as evidenced by reversal of the changes in mitochondrial membrane potential (DeltaPsi(m)) and decreased caspase-3 activity, lactate dehydrogenase (LDH) release and DNA fragmentation. Wortmannin (10 nM) and LY 294002 (10 microM) (inhibitors of phosphatidylinositol-3-kinase, PI3-K) reversed the inhibitory effect of memantine on the staurosporine-induced LDH release, suggesting that the PI3-K/Akt prosurvival pathway is a possible target for antiapoptotic action of memantine. Memantine at low micromolar concentrations also attenuated salsolinol- and doxorubicin-induced LDH release and DNA fragmentation, but only in the case of salsolinol was this effect accompanied by a decrease in caspase-3 activity. The present data indicate that memantine attenuates the toxic effects of various proapoptotic agents and the cytoprotective effect of memantine does not seem to be connected with its action on NMDA receptor but rather with its influence on intracellular pathways engaged in cellular survival/apoptotic processes.

Rasagiline Promotes Regeneration of Substantia Nigra Dopaminergic Neurons in Post-MPTP-induced Parkinsonism via Activation of Tyrosine Kinase Receptor Signaling Pathway

The anti-Parkinson drug rasagiline (Azilect), an irreversible and selective monoamine oxidase (MAO)-B inhibitor, was shown to possess neuroprotective activities, involving multiple survival pathways among them the up-regulation of protein kinase C (PKC)alpha, PKCepsilon, the anti-apoptotic Bcl-2, Bcl-xL, and Bcl-w and the induction of brain-derived- and glial cell line-derived neurotrophic factors (BDNF, GDNF). More recently, employing conventional neurochemical techniques, as well as transcriptomic and proteomic screening tools, combined with a biology-based clustering method, it was shown that rasagiline also possesses neurorescue/neurogenesis activity in mice midbrain dopaminergic neurons when given chronically, post-MPTP (N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). This action was attributed to the activation of cell signaling mediators associated with neurotrophic factors responsive-tyrosine kinase receptor (Trk) pathway, including ShcC, SOS, AF6, Rin1, and Ras and the increase in the Trk-downstream effecter phosphatidylinositol 3 kinase (PI3K) protein and its substrate, Akt/PKB. It is interesting to determine whether a similar effect is seen in Parkinsonian patients after long-term treatment with rasagiline, which may have implications as a possible disease modifying agent.

Multifunctional neuroprotective–neurorescue drugs for Parkinson’s disease

Parkinson’s disease (PD) is a severe neurodegenerative disorder, with no drugs currently approved to prevent the neuronal cell loss characteristic of brains of patients suffering from PD. Owing to the complex etiology of PD, an innovative approach towards neuroprotection or neurorescue may be the use of multifunctional pharmaceuticals that target an array of pathological pathways, each of which is believed to contribute to the cascade that ultimately leads to neuronal cell death. In this review, we discuss examples of novel multifunctional ligands that may have potential as neuroprotective–neurorescue therapeutics in PD. The compounds discussed originate from synthetic chemistry as well as from natural sources.

A link between monoamine oxidase-A and apoptosis in serum deprived human SH-SY5Y neuroblastoma cells

Increased monoamine oxidase (MAO) activity was recently shown to accompany apoptotic cell death of various neuronal cells following growth factor deprivation. Here we show that in serum deprived SH-SY5Y cells, MAO-A mRNA levels and catalytic activities are increased, linked with activation of the apoptotic executioner caspase-3. Importantly, specific inhibition of MAO-A activity resulted in loss of apoptotic cell morphology. Our study indicates that MAO catalytic activity is involved in apoptotic signalling in response to serum withdrawal in neuronal cells.

Neuroprotection by propargylamines in Parkinson's disease: intracellular mechanism underlying the anti-apoptotic function and search for clinical markers

In Parkinson's and other neurodegenerative diseases, a therapeutic strategy has been proposed to halt progressive cell death. Propargylamine derivatives, rasagiline and (-)deprenyl (selegiline), have been confirmed to protect neurons against cell death induced by various insults in cellular and animal models of neurodegenerative disorders. In this paper, the mechanism and the markers of the neuroprotection are reviewed. Propargylamines prevent the mitochondrial permeabilization, membrane potential decline, cytochrome c release, caspase activation and nuclear translocation of glyceraldehyde 3-phosphate dehydrogenase. At the same time, rasagiline induces anti-apoptotic pro-survival proteins, Bcl-2 and glial cell-line derived neurotrophic factor, which is mediated by activated ERK-NF-kappaB signal pathway. DNA array studies indicate that rasagiline increases the expression of the genes coding mitochondrial energy synthesis, inhibitors of apoptosis, transcription factors, kinases and ubiquitin-proteasome system, sequentially in a time-dependent way. Products of cell survival-related gene induced by propargylamines may be applied as markers of neuroprotection in clinical samples.

Involvement of type A monoamine oxidase in neurodegeneration: regulation of mitochondrial signaling leading to cell death or neuroprotection

In neurodegenerative diseases, including Parkinson's and Alzheimer's diseases, apoptosis is a common type of cell death, and mitochondria emerge as the major organelle to initiate death cascade. Monoamine oxidase (MAO) in the mitochondrial outer membrane produces hydrogen peroxide by oxidation of monoamine substrates, and induces oxidative stress resulting in neuronal degeneration. On the other hand, a series of inhibitors of type B MAO (MAO-B) protect neurons from cell death. These results suggest that MAO may be involved in the cell death process initiated in mitochondria. However, the direct involvement of MAO in the apoptotic signaling has been scarcely reported. In this paper, we present our recent results on the role of MAO in activating and regulating cell death processing in mitochondria. Type A MAO (MAO-A) was found to bind an endogenous dopaminergic neurotoxin, N-methyl(R)salsolinol, and induce apoptosis in dopaminergic SH-SY5Y cells containing only MAO-A. To examine the intervention of MAO-B in apoptotic process, human MAO-B cDNA was transfected to SH-SY5Y cells, but the sensitivity to N-methyl(R)salsolinol was not affected, even though the activity and protein of MAO-B were expressed markedly. MAO-B oxidized dopamine with production of hydrogen peroxide, whereas in control cells expressing only MAO-A, dopamine autoxidation produced superoxide and dopamine-quinone, and induced mitochondrial permeability transition and apoptosis. Rasagiline and other MAO-B inhibitors prevent the activation of apoptotic cascade and induce prosurvival genes, such as bcl-2 and glial cell line-derived neurotrophic factor, in MAO-A-containing cells. These results demonstrate a novel function of MAO-A in the induction and regulation of apoptosis. Future studies will clarify more detailed mechanism behind regulation of mitochondrial death signaling by MAO-A, and bring out new strategies to cure or ameliorate the decline of neurons in neurodegenerative disorders.

Activation of tyrosine kinase receptor signaling pathway by rasagiline facilitates neurorescue and restoration of nigrostriatal dopamine neurons in post-MPTP-induced parkinsonism

The anti-Parkinson monoamine oxidase (MAO)-B inhibitor rasagiline (Azilect) was shown to possess neuroprotective activities, involving the induction of brain-derived- and glial cell line-derived neurotrophic factors (BDNF, GDNF). Employing conventional neurochemical techniques, transcriptomics and proteomic screening tools combined with a biology-based clustering method, we show that rasagiline, given chronically post-MPTP (N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), exerts neurorescue/neurotrophic activity in mice midbrain dopamine neurons. Rasagiline induced the activation of cell signaling mediators associated with neurotrophic factors responsive-tyrosine kinase receptor (Trk) pathway including ShcC, SOS, AF6, Rin1 and Ras and the increase in the Trk-downstream effector phosphatidylinositol 3 kinase (PI3K) protein. Confirmatory Western and immunohistochemical analyses indicated activation of the substrate of PI3K, Akt and phosphorylative inactivation of glycogen synthase kinase-3beta and Raf1. Thus, the activation of Ras-PI3K-Akt survival pathway may contribute to rasagiline-mediated neurorescue effect. It is interesting to determine whether a similar effect is seen in parkinsonian patients after long-term treatment with rasagiline.

Deprenyl: from chemical synthesis to neuroprotection

During the last decades (-)-deprenyl has become the golden standard of MAO-B inhibitors. It possesses dopamine potentiating and antioxidant properties; however, its effects cannot be explained solely by the enzyme inhibitory action. (-)-Deprenyl prevents the toxicity of certain selective neurotoxins and recently it was demonstrated to increase cell-cell adhesion as well. The complexity of its pharmacological effects reflects the action of both the parent compound and the active metabolites. (-)-Deprenyl and related propargylamines (DRPs) show neuroprotective features in a variety of in vitro and in vivo models that is dependent on the propargyl moiety. The main presumptive targets to date include glyceraldehyde-3-phosphate dehydrogenase, poly(ADP-ribose) polymerase, some kinase cascades, as well as pro- and antiapoptotic proteins, beside the inhibition of MAO-B. The antiapoptotic activity of DRPs converges upon the maintenance of mitochondrial integrity, due to the initiation of a complex transcriptional program, the details of which are yet to be elucidated.