Intra-neuronal vesicular uptake of catecholamines is decreased in patients with Lewy body diseases

Reduced vesicular storage of catecholamines causes progressive degeneration in the locus ceruleus

Parkinson's disease (PD) is the most common neurodegenerative motor disease. Pathologically, PD is characterized by Lewy body deposition and subsequent death of dopamine neurons in the substantia nigra pars compacta. PD also consistently features degeneration of the locus ceruleus, the main source of norepinephrine in the central nervous system. We have previously reported a mouse model of dopaminergic neurodegeneration based on reduced expression of the vesicular monoamine transporter (VMAT2 LO). To determine if reduced vesicular storage can also cause noradrenergic degeneration, we examined indices of damage to the catecholaminergic systems in brain and cardiac tissue of VMAT2 LO mice. At two months of age, neurochemical analyses revealed substantial reductions in striatal dopamine (94%), cortical dopamine (57%) and norepinephrine (54%), as well as cardiac norepinephrine (97%). These losses were accompanied by increased conversion of dopamine and norepinephrine to their deaminated metabolites. VMAT2 LO mice exhibited loss of noradrenergic innervation in the cortex, as determined by norepinephrine transporter immunoreactivity and (3)H-nisoxetine binding. Using unbiased stereological techniques, we observed progressive degeneration in the locus ceruleus that preceded degeneration of the substantia nigra pars compacta. In contrast, the ventral tegmental area, which is spared in human PD, remained unaffected. The coordinate loss of dopamine and norepinephrine neurons in VMAT2 LO mice parallels the pattern of neurodegeneration that occurs in human PD, and demonstrates that insufficient catecholamine storage can cause spontaneous degeneration in susceptible neurons, underscoring cytosolic catecholamine catabolism as a determinant of neuronal susceptibility in PD. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Vesicular integrity in Parkinson's disease

The defining motor characteristics of Parkinson's disease (PD) are mediated by the neurotransmitter dopamine (DA). Dopamine molecules spend most of their lifespan stored in intracellular vesicles awaiting release and very little time in the extracellular space or the cytosol. Without proper packaging of transmitter and trafficking of vesicles to the active zone, dopamine neurotransmission cannot occur. In the cytosol, dopamine is readily oxidized; excessive cytosolic dopamine oxidation may be pathogenic to nigral neurons in PD. Thus, factors that disrupt vesicular function may impair signaling and increase the vulnerability of dopamine neurons. This review outlines the many mechanisms by which disruption of vesicular function may contribute to the pathogenesis of PD. From direct inhibition of dopamine transport into vesicles by pharmacological or toxicological agents to alterations in vesicle trafficking by PD-related gene products, variations in the proper compartmentalization of dopamine can wreak havoc on a functional dopamine pathway. Findings from patient populations, imaging studies, transgenic models, and mechanistic studies will be presented to document the relationship between impaired vesicular function and vulnerability of the nigrostriatal dopamine system. Given the deleterious effects of impaired vesicular function, strategies aimed at enhancing vesicular function may be beneficial in the treatment of PD.

Biomarkers, mechanisms, and potential prevention of catecholamine neuron loss in Parkinson disease

This chapter is on biomarkers, mechanisms, and potential treatment of catecholamine neuron loss in Parkinson disease (PD). PD is characterized by a movement disorder from loss of nigrostriatal dopamine neurons. An intense search is going on for biomarkers of the disease process. Theoretically, cerebrospinal fluid (CSF) levels of the deaminated DA metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), should be superior to other neurochemical indices of loss of central dopamine. CSF DOPAC is low in PD-even in patients with recent onset of Parkinsonism. Cardiac norepinephrine depletion is as severe as the loss of putamen dopamine. PD importantly involves nonmotor manifestations, including anosmia, dementia, REM behavior disorder, and orthostatic hypotension, and all of these nonmotor features are associated with neuroimaging evidence for cardiac sympathetic denervation, which seems to occur independently of the movement disorder and striatal dopaminergic lesion. Analogy to a bank robber's getaway car conveys the catecholaldehyde hypothesis, according to which buildup of the dopamine metabolite 3,4-dihydroxyphenylacetaldehyde (DOPAL), the immediate product of the action of monoamine oxidase on cytosolic dopamine, causes or contributes to the death of dopamine neurons. Decreased vesicular uptake of dopamine and decreased DOPAL detoxification by aldehyde dehydrogenase (ALDH) determine this buildup. Vesicular uptake is also markedly decreased in the heart in PD. Multiple factors influence vesicular uptake and ALDH activity. Evidence is accruing for aging-related induction of positive feedback loops and an autotoxic final common pathway in the death of catecholamine neurons, mediated by metabolites produced continuously in neuronal life. The catecholaldehyde hypothesis also leads to testable experimental therapeutic ideas.

Aldehyde dehydrogenase inhibition as a pathogenic mechanism in Parkinson disease

Parkinson disease (PD) is a neurodegenerative disorder particularly characterized by the loss of dopaminergic neurons in the substantia nigra. Pesticide exposure has been associated with PD occurrence, and we previously reported that the fungicide benomyl interferes with several cellular processes potentially relevant to PD pathogenesis. Here we propose that benomyl, via its bioactivated thiocarbamate sulfoxide metabolite, inhibits aldehyde dehydrogenase (ALDH), leading to accumulation of the reactive dopamine metabolite 3,4-dihydroxyphenylacetaldehyde (DOPAL), preferential degeneration of dopaminergic neurons, and development of PD. This hypothesis is supported by multiple lines of evidence. (i) We previously showed in mice the metabolism of benomyl to S-methyl N-butylthiocarbamate sulfoxide, which inhibits ALDH at nanomolar levels. We report here that benomyl exposure in primary mesencephalic neurons (ii) inhibits ALDH and (iii) alters dopamine homeostasis. It induces selective dopaminergic neuronal damage (iv) in vitro in primary mesencephalic cultures and (v) in vivo in a zebrafish system. (vi) In vitro cell loss was attenuated by reducing DOPAL formation. (vii) In our epidemiology study, higher exposure to benomyl was associated with increased PD risk. This ALDH model for PD etiology may help explain the selective vulnerability of dopaminergic neurons in PD and provide a potential mechanism through which environmental toxicants contribute to PD pathogenesis.

DJ-1 protects against dopamine toxicity: implications for Parkinson's disease and aging

Parkinson's disease (PD) is a common age-related neurodegenerative disorder. Dopamine neurotoxicity, mediated through oxidative stress, is implicated in disease pathogenesis. The vesicular monoamine transporter-2 (VMAT2) transfers dopamine into synaptic vesicles preparing it for exocytotic release and preventing its cytoplasmic oxidation. DJ-1 mutations cause early-onset familial PD. Here, we show that DJ-1 protects dopaminergic neurons and controls the vesicular sequestration of dopamine by upregulating VMAT2. Overexpression of DJ-1 protected cells against dopamine toxicity, reduced oxidative stress, and increased VMAT2 expression and function. Reduced DJ-1 levels resulted in opposite effects. Dopamine vesicular sequestration and its release upon depolarization were dependent on DJ-1 levels. Transcriptional regulation of VMAT2 expression by DJ-1 was confirmed by chromatin immunoprecipitation assay. The results were corroborated in vivo using 6-hydroxydopamine hemiparkinsonian mouse model and transgenic DJ-1 knockout mice. Our experimental data point to a novel potential protective function of DJ-1, which could be used as a therapeutic tool.

Cerebrospinal fluid biomarkers of central catecholamine deficiency in Parkinson's disease and other synucleinopathies

Central catecholamine deficiency characterizes α-synucleinopathies such as Parkinson's disease. We hypothesized that cerebrospinal fluid levels of neuronal metabolites of catecholamines provide neurochemical biomarkers of these disorders. To test this hypothesis we measured cerebrospinal fluid levels of catechols including dopamine, norepinephrine and their main respective neuronal metabolites dihydroxyphenylacetic acid and dihydroxyphenylglycol in Parkinson's disease and two other synucleinopathies, multiple system atrophy and pure autonomic failure. Cerebrospinal fluid catechols were assayed in 146 subjects-108 synucleinopathy patients (34 Parkinson's disease, 54 multiple system atrophy, 20 pure autonomic failure) and 38 controls. In 14 patients cerebrospinal fluid was obtained before or within 2 years after the onset of parkinsonism. The Parkinson's disease, multiple system atrophy and pure autonomic failure groups all had lower cerebrospinal fluid dihydroxyphenylacetic acid [0.86 ± 0.09 (SEM), 1.00 ± 0.09, 1.32 ± 0.12 nmol/l] than controls (2.15 ± 0.18 nmol/l; P < 0.0001; P < 0.0001; P = 0.0002). Dihydroxyphenylglycol was also lower in the three synucleinopathies (8.82 ± 0.44, 7.75 ± 0.42, 5.82 ± 0.65 nmol/l) than controls (11.0 ± 0.62 nmol/l; P = 0.009, P < 0.0001, P < 0.0001). Dihydroxyphenylacetic acid was lower and dihydroxyphenylglycol higher in Parkinson's disease than in pure autonomic failure. Dihydroxyphenylacetic acid was 100% sensitive at 89% specificity in separating patients with recent onset of parkinsonism from controls but was of no value in differentiating Parkinson's disease from multiple system atrophy. Synucleinopathies feature cerebrospinal fluid neurochemical evidence for central dopamine and norepinephrine deficiency. Parkinson's disease and pure autonomic failure involve differential dopaminergic versus noradrenergic lesions. Cerebrospinal fluid dihydroxyphenylacetic acid seems to provide a sensitive means to identify even early Parkinson's disease.

Cardiovascular dysautonomia in Parkinson disease: from pathophysiology to pathogenesis

Signs or symptoms of impaired autonomic regulation of circulation often attend Parkinson disease (PD). This review covers biomarkers and mechanisms of autonomic cardiovascular abnormalities in PD and related alpha-synucleinopathies. The clearest clinical laboratory correlate of dysautonomia in PD is loss of myocardial noradrenergic innervation, detected by cardiac sympathetic neuroimaging. About 30-40% of PD patients have orthostatic hypotension (OH), defined as a persistent, consistent fall in systolic blood pressure of at least 20 mmHg or diastolic blood pressure of at least 10 mmHg within 3 min of change in position from supine to standing. Neuroimaging evidence of cardiac sympathetic denervation is universal in PD with OH (PD+OH). In PD without OH about half the patients have diffuse left ventricular myocardial sympathetic denervation, a substantial minority have partial denervation confined to the inferolateral or apical walls, and a small number have normal innervation. Among patients with partial denervation the neuronal loss invariably progresses over time, and in those with normal innervation at least some loss eventually becomes evident. Thus, cardiac sympathetic denervation in PD occurs independently of the movement disorder. PD+OH also entails extra-cardiac noradrenergic denervation, but this is not as severe as in pure autonomic failure. PD+OH patients have failure of both the parasympathetic and sympathetic components of the arterial baroreflex. OH in PD therefore seems to reflect a "triple whammy" of cardiac and extra-cardiac noradrenergic denervation and baroreflex failure. In contrast, most patients with multiple system atrophy, which can resemble PD+OH clinically, do not have evidence for cardiac or extra-cardiac noradrenergic denervation. Catecholamines in the neuronal cytoplasm are potentially toxic, via spontaneous and enzyme-catalyzed oxidation. Normally cytoplasmic catecholamines are efficiently taken up into vesicles via the vesicular monoamine transporter. The recent finding of decreased vesicular uptake in Lewy body diseases therefore suggests a pathogenetic mechanism for loss of catecholaminergic neurons in the periphery and brain. Parkinson disease (PD) is one of the most common chronic neurodegenerative diseases of the elderly, and it is likely that as populations age PD will become even more prevalent and more of a public health burden. Severe depletion of dopaminergic neurons of the nigrostriatal system characterizes and likely produces the movement disorder (rest tremor, slowness of movement, rigid muscle tone, and postural instability) in PD. Over the past two decades, compelling evidence has accrued that PD also involves loss of noradrenergic neurons in the heart. This finding supports the view that loss of catecholaminergic neurons, both in the nigrostriatal system and the heart, is fundamental in PD. By the time PD manifests clinically, most of the nigrostriatal dopaminergic neurons are already lost. Identifying laboratory measures-biomarkers-of the disease process is therefore crucial for advances in treatment and prevention. Deposition of the protein, alpha-synuclein, in the form of Lewy bodies in catecholaminergic neurons is a pathologic hallmark of PD. Alpha-synucleinopathy in autonomic neurons may occur early in the pathogenetic process. The timing of cardiac noradrenergic denervation in PD is therefore a key issue. This review updates the field of autonomic cardiovascular abnormalities in PD and related disorders, with emphasis on relationships among striatal dopamine depletion, sympathetic noradrenergic denervation, and alpha-synucleinopathy.