Cellular Polyamines Promote the Aggregation of α-Synuclein

Over the last two decades, significant strides has been made toward acquiring a better knowledge of both the etiology and pathogenesis of Parkinson's disease (PD). Experimental models are of paramount importance to obtain greater insights into the pathogenesis of the disease. Thus far, neurotoxin-based animal models have been the most popular tools employed to produce selective neuronal death in both in vitro and in vivo systems. These models have been commonly referred to as the pathogenic models. The current trend in modeling PD revolves around what can be called the disease gene-based models or etiologic models. The value of utilizing multiple models with a different mechanism of insult rests on the premise that dopamine-producing neurons die by stereotyped cascades that can be activated by a range of insults, from neurotoxins to downregulation and overexpression of disease-related genes. In this position article, we present the relevance of both pathogenic and etiologic models as well as the concept of clinically relevant designs that, we argue, should be utilized in the preclinical development phase of new neuroprotective therapies before embarking into clinical trials.

Animal models of Parkinson's disease: limits and relevance to neuroprotection studies.

The acetylenic selective monoamine oxidase (MAO) type B suicide inhibitor selegiline (previously called L-deprenyl) has proved to be a useful adjuvant to levodopa therapy and monotherapy of Parkinson's disease (PD). Selegiline is readily absorbed from the gastrointestinal tract and rapidly enters the brain and spinal cord following oral administration. The drug binds to brain regions with a high MAO-B content, such as the thalamus, the striatum, the cortex, and the brainstem. It is extensively metabolized in humans, mainly in the liver, to form desmethylselegiline and methamphetamine, which are further metabolized to amphetamine. Eighty-six percent of the 10-mg dose was recovered in the urine within 24 hours. These data suggest that accumulation of metabolites does not occur. Although not all features of its anti-PD action are known, studies using brain obtained at autopsy from patients who had been treated with 10 mg of selegiline showed that selective inhibition of MAO-B, with the concomitant increase of phenylethylamine and dopamine (DA) but not of serotonin or noradrenaline, in the basal ganglia may be regarded as its mode of action. The protective effects afforded by selegiline in PD, resulting in a delayed need for levodopa therapy, have been variously interpreted in terms of the involvement of an endogenous neurotoxin or an oxygen free radical mechanism (oxidative stress) in the development of PD. However, although many different hypotheses have been advanced and recent findings have emphasized the significance of oxidative stress in the pathogenesis of the disease, the cause of chronic nigral cell death and the underlying mechanisms remain, as yet, elusive. Therefore, there is no clear knowledge regarding an understanding of the reported effects of selegiline on the progression of PD. Nevertheless, selegiline might be expected to have some protective effects in reducing the production of potentially neurotoxic compounds resulting in the MAO-catalyzed oxidation of DA. In addition, some evidence suggests both an indirect (via induction of radical-scavenging enzymes) and a direct antioxidant function for selegiline. On the other hand, the reported protective effect of selegiline might also receive a contribution from the diminished potentiation of the N-methyl-D-aspartate receptor by the polyamine binding site. Finally, the effects of selegiline might also involve preventing, or perhaps to some extent reversing, the decline in resistance normally associated with cellular aging because of its neurotrophine-like action. However, even in the early clinical stage of PD, the sequence of events leading to nigral cell death may be too far advanced for selegiline to exhibit its maximum potential.

Pharmacology of selegiline

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) has been shown to induce parkinsonism in man and non-human primates. Monoamine-oxidase B (MAO-B) has been reported to be implicated in both MPTP-induced parkinsonism and Parkinson's disease, since selegiline (L-deprenyl), an irreversible MAO-B inhibitor, prevents MPTP-induced neurotoxicity in numerous species including mice, goldfish and drosophyla. However, one disadvantage of this substance relates to its metabolism to (-)-methamphetamine and (-)-amphetamine. Rasagiline (R-(+)-N-propyl-1-aminoindane) is a novel irrevesible MAO-B-inhibitor, which is not metabolized to metamphetamine and/or amphetamine. The present study compared the effects of high doses of selegiline and rasagiline (10 mg/kg body weight s.c.) on MPTP-induced dopaminergic neurotoxicity in a non-human primate (Callithrix jacchus) model of PD. Groups of four monkeys were assigned to the following six experimental groups: Group I: Saline, Group II: Selegiline/Saline, Group III: Rasagiline/Saline, Group IV: MPTP/Saline, Group V: Rasagiline/MPTP, Group VI: Selegiline/MPTP. Daily treatment with MAO-B-inhibitors (either rasagiline or selegiline, 10 mg/kg body weight s.c.) was initiated four days prior to MPTP-exposure (MPTP-HCl, 2 mg/kg body weight subcutaneously, separated by an interval of 24 hours for a total of four days) and was continued until the end of the experiment, i.e. 7 days after the cessation of the MPTP-injections, when animals were sacrificed. MPTP-treatment caused distinct behavioural, histological, and biochemical alterations: 1. significant reduction of motor activity assessed by clinical rating and by computerized locomotor activity measurements; 2. substantial loss (approx. 40%) of dopaminergic (tyrosine-hydroxylase-positive) cells in the substantia nigra, pars compacta; and 3. putaminal dopamine depletion of 98% and its metabolites DOPAC (88%) and HVA (96%). Treatment with either rasagiline or selegiline markedly attenuated the neurotoxic effects of MPTP at the behavioural, histological, and at the biochemical levels. There were no significant differences between rasagiline/MPTP and selegiline/MPTP-treated animals in respect to signs of motor impairment, the number of dopaminergic cells in the substantia nigra, and striatal dopamine levels. As expected, both inhibitors decreased the metabolism of dopamine, leading to reduced levels of HVA and DOPAC (by >95% and 45% respectively). In conclusion, rasagiline and selegiline at the dosages employed equally protect against MPTP-toxicity in the common marmoset, suggesting that selegiline-derived metabolites are not important for the neuroprotective effects of high dose selegiline in the non-human MPTP-primate model in the experimental design employed. However, unexpectedly, high dose treatment with both MAO-inhibitors caused a decrease of the cell sizes of nigral tyrosine hydroxylase positive neurons. It remains to be determined, if this histological observation represents potential adverse effects of high dose treatment with monoamine oxidase inhibitors.

Monoamine oxidase-inhibition and MPTP-induced neurotoxicity in the non-human primate: comparison of rasagiline (TVP 1012) with selegiline.

Schizophrenia is a common and etiologically heterogeneous disorder. Although inheritance of schizophrenic syndromes is complex with genetic and environmental factors contributing to the clinical phenotype, periodic catatonia, a familial subtype of catatonic schizophrenia, appears to be transmitted in an autosomal dominant manner. We report here that a Leu309Met mutation in WKL1, a positional candidate gene on chromosome 22q13.33 encoding a putative non-selective cation channel expressed exclusively in brain, co-segregates with periodic catatonia in an extended pedigree. Structural analyses revealed that this missense mutation results in conformational changes of the mutant protein. Our results not only underscore the importance of genetic mechanisms in the etiology of schizophrenic syndromes, but also provide a better understanding of the pathogenesis and incapacitating course of catatonic schizophrenia and related disorders.

A missense mutation in a novel gene encoding a putative cation channel is associated with catatonic schizophrenia in a large pedigree

There have been many claims that the selective monoamine oxidase type B (MAO-B) inhibitor selegiline may have distinct properties in slowing the progression of Parkinson's disease (PD). Degeneration of nigro-striatal dopaminergic neurons is the primary histopathological feature of PD. Although many different hypotheses have been advanced, the cause of chronic nigral cell death and the underlying mechanisms remain elusive as yet. Therefore, there is no clear knowledge regarding an understanding of the reported effects of selegiline on the progression of PD. However, there is a considerable body of indirect evidence that oxidative stress may play a role in the pathogenesis of this illness. Oxidative stress refers to cytotoxic consequences of hydrogen peroxide and oxygen-derived free radicals such as the hydroxyl radical (.OH), the superoxide anion (.O2), and nitric oxide (NO), which are generated as byproducts of normal and aberrant metabolic processes that utilize molecular oxygen. On the other hand, an increasing body of experimental data has implicated excitotoxicity as a mechanism of cell death in both acute and chronic neurological disease. One of the receptor which is particularly involved in the toxic effects of excitatory amino acids is the NMDA (N-methyl-D-aspartate) receptor. Excessive stimulation of this type of receptor by glutamic acid or NMDA agonists leads to a massive influx of calcium ions into the neuron followed by activation of a variety of calcium-dependent enzymes, impaired mitochondrial function, and the generation of free radicals. This article will consider the concept that excitotoxicity is linked with the generation of free radicals. In view of this idea it will be further discussed how selegiline might exert its neuroprotective effects via indirect actions on the polyamine binding site of the NMDA receptor. Under treatment with the MAO-B inhibitor selegiline, the degradation of putrescine via MAO, a key factor in regulating the polyamine metabolism, might be diminished in the Parkinsonian brain, which in turn would suppress the polyamine synthesis. Hence, the reported neuroprotective effect of selegiline might also receive a contribution from the diminished potentiation of the NMDA receptor by the polyamine binding site. On the other hand, since N1-acetylated spermine and spermidine are also good substrates of MAO-B, it is likely that these compounds will be present in the brain in increased concentrations. It therefore seems possible that they will exert a neuroprotective effect via an antagonistic modulation of the polyamine binding site of the NMDA receptor.

H. Desser

Papers

New horizons in molecular mechanisms underlying Parkinson’s disease and in our understanding of the neuroprotective effects of selegiline