Deficient vesicular storage: A common theme in catecholaminergic neurodegeneration

Abstract

Several neurodegenerative diseases involve loss of catecholamine neurons--Parkinson's disease (PD) is a prototypical example. Catecholamine neurons are rare in the nervous system, and why they are lost has been mysterious. Accumulating evidence supports the concept of "autotoxicity"--inherent cytotoxicity caused by catecholamine metabolites. Since vesicular sequestration limits the buildup of toxic products of enzymatic and spontaneous oxidation of catecholamines, a vesicular storage defect could play a pathogenic role in the death of catecholaminergic neurons in a variety of neurodegenerative diseases. In putamen, deficient vesicular storage is revealed in vivo by accelerated loss of (18)F-DOPA-derived radioactivity and post-mortem by decreased tissue dopamine (DA):DOPA ratios; in myocardium in vivo by accelerated loss of (18)F-dopamine-derived radioactivity and post-mortem by increased 3,4-dihydroxyphenylglycol:norepinephrine (DHPG:NE) ratios; and in sympathetic noradrenergic nerves overall in vivo by increased plasma F-dihydroxyphenylacetic acid (F-DOPAC):DHPG ratios. We retrospectively analyzed data from 20 conditions with decreased or intact catecholaminergic innervation, involving different etiologies, pathogenetic mechanisms, and lesion locations. All conditions involving parkinsonism had accelerated loss of putamen (18)F-DOPA-derived radioactivity; in those with post-mortem data there were also decreased putamen DA:DOPA ratios. All conditions involving cardiac sympathetic denervation had accelerated loss of myocardial (18)F-dopamine-derived radioactivity; in those with post-mortem data there were increased myocardial DHPG:NE ratios. All conditions involving localized loss of catecholaminergic innervation had evidence of decreased vesicular storage specifically in the denervated regions. Thus, across neurodegenerative diseases, loss of catecholaminergic neurons seems to be associated with decreased vesicular storage in the residual neurons.

Keywords: Catecholamine; Dopamine; Neurodegeneration; Norepinephrine; Parkinson's disease.

Figure 1. Concept diagram depicting synthesis and intra-neuronal metabolic fate of dopamine (DA)

A vesicular storage defect would be expected to decrease vesicular DA content (DA) and the ratio of DA:DOPA. Decreased vesicular sequestration would shift the fate of cytoplasmic DA toward auto-oxidation to form quinones and toward enzymatic oxidative deamination catalyzed by monoamine oxidase-A (MAO-A) to form the catecholaldehyde, 3,4-dihydroxyphenylacetaldehyde (DOPAL). DA-quinone reacts with glutathione or cysteine to form 5-S-cysteinyl-DA and bonds covalently with cysteine residues in proteins. DOPAL auto-oxidizes to DOPAL-quinone, generates reactive oxygen species, bonds covalently with amine groups on lysine residues in proteins, and oligomerizes alpha-synuclein. Decreased aldehyde dehydrogenase (ALDH) attenuates metabolic conversion of DOPAL to 3,4-dihydroxyphenylacetic acid (DOPAC).

Figure 2. Diagrams depicting definitions of Fractional Loss 30′–120′ from ¹⁸F-DOPA PET data and 8′ Radioactivity and k8′–25′ from ¹⁸F-dopamine PET data

As a measure of loss of ¹⁸F-DOPA-derived radioactivity after vesicular loading, the fractional loss in radioactivity (Fractional Loss 30′–120′) was calculated from the radioactivity in the scan beginning at 30′ and the scan ending at 120′. Denervation alone (or blockade of the cell membrane DA transporter) would be expected to decrease peak ¹⁸F-DOPA-derived radioactivity (blue arrow). Denervation with decreased vesicular storage would be expected to both decrease peak PUT radioactivity and increase Fractional Loss 30′–120′. As a measure of neuronal uptake and therefore of innervation density in the myocardial interventricular septum, 8′ Radioactivity was measured at the midpoint of the 5-minute frame beginning at about 3.5 minutes after initiation of the 3-minute infusion of ¹⁸F-dopamine. As a measure of loss of ¹⁸F-dopamine-derived radioactivity, the slope of mono-exponential decline in radioactivity (k18′–25′) was calculated from radioactivity at 8′, 13′, 18′, and 25′. Previous work has shown that the decline is often bi-exponential, reflecting different rate constants for loss from 2 intra-neuronal vesicular pools. Denervation with decreased vesicular storage would be expected to decrease 8′ Radioactivity and increase k18′–25′.

Figure 3. Mean (±SEM) values for (left) the fractional loss of ¹⁸F-DOPA-derived radioactivity between 30 and 120 minutes (Fractional Loss 30′–120′) and (right) mean CSF DOPAC as a function of Fractional Loss 30′–120′ in parkinsonian and non-parkinsonian conditions

Patients with parkinsonism had elevated values for the fractional loss of radioactivity, whereas patients without parkinsonism had normal values; CSF DOPAC was low in conditions involving parkinsonism.

Figure 4. Mean (±SEM) values for putamen ¹⁸F-DOPA-derived radioactivity as a function of time after i.v. administration of ¹⁸F-DOPA in patient groups with (red arrows) or without (green arrows) parkinsonism

Gray circles indicate normal mean values. Parkinsonian patients had accelerated loss of putamen ¹⁸F-DOPA-derived radioactivity; non-parkinsonian patients did not.

Figure 5. Dopamine content and an index of vesicular storage in putamen of patients with parkinsonism and controls

(A) Mean (±SEM) values for putamen tissue DA in MSA (blue), Gaucher/PD (yellow), sporadic PD (red), and controls (gray). Numbers in italics are p values for comparisons with control subjects, based on post-hoc testing after ANOVA. (B) Mean (±SEM) values for DA:DOPA. (C) Individual data for DA:DOPA as a function of DA content. Patient groups with different disease etiologies (sporadic PD, Gaucher PD, MSA) had in common DA depletion and decreased vesicular storage. Across individuals, DA:DOPA was positively related with DA.

Figure 6. Myocardial interventricular septal ¹⁸F-DA-derived radioactivity (means ± SEM) as a function of time in patient groups with (red arrows) and without (green arrows) sympathetic denervation

Abbreviations: PARK1=familial Parkinson’s disease (PD) from A53T mutation of the gene encoding alpha-synuclein; PARK4=familial PD from alpha-synuclein gene triplication; Amyloid=acquired amyloidosis; PAF=pure autonomic failure; PD+OH=PD with orthostatic hypotension; FD=familial dysautonomia; MSA=multiple system atrophy; AAG=autoimmune autonomic ganglionopathy. Also shown are data for normal volunteers treated with 125 mg of oral desipramine (DMI) before ¹⁸F-dopamine administration. Conditions involving sympathetic denervation are associated with accelerated declines of septal ¹⁸F-DA-derived radioactivity.

Figure 7. Scatter plot relating mean values for k to mean values for 8′ ¹⁸F-DA-derived radioactivity in various patient groups

Pink rectangle indicates groups with diffuse cardiac sympathetic denervation, green partial cardiac sympathetic denervation, and blue intact cardiac sympathetic innervation. All patient groups with septal sympathetic denervation as indicated by low 8′ ¹⁸F-DA-derived radioactivity had accelerated loss of radioactivity. Additional abbreviation: AMYL=amyloidosis; MSA-P=parkinsonian form of MSA; MSA-C=cerebellar form of MSA; MSA+D=MSA with cardiac sympathetic denervation, defined by 8′ radioactivity more than 2 standard deviations below the normal mean; NL=normal; POTS=postural tachycardia syndrome; PSP=progressive supranuclear palsy;.

Figure 8. Scatter plot relating individual values for the ratio of DHPG:NE as a function of NE content in apical myocardial tissue

Additional abbreviation: DLB=dementia with Lewy bodies. Note that regardless of diagnosis, DHPG:NE, an index of a vesicular sequestration-to-oxidative deamination shift in the fate of cytoplasmic NE, is associated with myocardial NE depletion.

Figure 9. Mean values (±SEM) for peak arterial plasma FDOPAC:DHPG in various patient groups

Additional abbreviations: PD No OH=PD without orthostatic hypotension. Note elevated peak FDOPAC:DHPG in PD+OH, which is associated with generalized noradrenergic denervation, and normal peak FDOPAC:DHPG in PD No OH, which is associated with relatively selective cardiac noradrenergic denervation.