Mechanisms of Catecholaminergic Neurodegeneration

National Institute Of Neurological Disorders And Stroke

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Abstract

In this reporting period we have applied a mainly patient-oriented research approach to understand mechanisms of degeneration of nerve cells (neurons) that use particular body chemicals, catecholamines, as the chemical messengers (neurotransmitters). According to the catecholaldehyde hypothesis, neurons that produce the catecholamines dopamine and norepinephrine are susceptible in Lewy body diseases (LBDs), because 3,4-dihydroxyphenylacetaldehyde (DOPAL) formed within the neurons as part of normal metabolism is an autotoxin that poses a continuous challenge to neuronal integrity. Mutations that promote DOPAL formation, increase spontaneous oxidation of DOPAL, inhibit detoxification of DOPAL, or enhance DOPAL-induced misfolding of proteins may interfere with processes that keep neurotransmitter stores within healthy bounds (homeostasis). Harmful effects of DOPAL, especially via interactions with the protein alpha-synuclein (AS), could be cumulative and progress over years. Compensatory, adjustments that maintain neurotransmitter homeostasis eventually could give way, with rapid degeneration and death of the neurons and various manifestations resulting from catecholamine deficiency. (A) Computational modeling of the progression of catecholamine deficiency in LBDs: To investigate how catecholaminergic neurodegeneration progresses over years in LBDs, we constructed and evaluated a computational model that incorporates homeostasis and harmful DOPAL-AS interactions. Model predictions have been compared with empirical data about trends over years in cardiac 18F-dopamine- and putamen 18F-DOPA-derived radioactivity (respective indices of norepinephrine stores in the heart and of dopamine stores in the brain system most impacted in PD), post-mortem cardiac contents of AS and catecholamines, and effects of genetic mutations. Preliminarily, the model generates a tri-phasic curve for the loss of catecholamine stores in LBDs, with the stores maintained by compensatory homeostatic processes at first, but then rapid depletion of catecholamine stores when the homeostatic mechanisms are overwhelmed. Disease symptoms come on only relatively late in this second phase. In the third phase, further loss of catecholamine stores is slow because few living neurons are left. (B) Sympathetic noradrenergic denervation and decreased vesicular storage in pure autonomic failure: In patients with the rare LBD pure autonomic failure (PAF) we used 11C-methylreboxetine and 18F-dopamine positron emission tomographic (PET) scanning to evaluate cardiac noradrenergic innervation and intra-neuronal storage of catecholamines in vesicles in PAF. We obtained evidence that PAF entails a combination of a moderate amount of loss of noradrenergic nerves with reduced vesicular storage in the nerves that are left (PMID 32945121), an example of the sick-but-not-dead phenomenon (PMID 32372682, 32906170). (C) Pathophysiological significance of alpha-synuclein (AS) deposition in sympathetic nerves in skin, submandibular gland, and heart: We completed a post-mortem study about the pathophysiological significance of AS deposits in norepinephrine-producing (noradrenergic) nerve fibers in skin, submandibular gland, and heart tissues from patients with autopsy-proven PD. We have found preliminarily that there is increased deposition of AS in all three organs. In skin and submandibular gland, however, there is no loss of tissue norepinephrine content, whereas the same patients have profound norepinephrine deficiency in the heart. These findings suggest that the pathophysiological role of AS deposition depends on the organ that is involved and is prominent in the heart. (D) Cardioselective sympathetic noradrenergic deficiency in LBDs: The status of noradrenergic innervation in organs other than the heart in LBDs has been unclear. We reviewed 18F-dopamine positron emission tomographic (PET) scan images and post-mortem neurochemical data across several body organs of patients with the LBDs PD and PAF and a group of non-LB subjects. As expected, in vivo cardiac 18F-dopamine-derived radioactivity and post-mortem myocardial norepinephrine content were markedly decreased in the LBD group. In contrast, 18F-dopamine-derived radioactivity and tissue norepinephrine contents were normal in all the other organs examined. Therefore, by both in vivo sympathetic neuroimaging and post-mortem neurochemistry, in LBDs noradrenergic deficiency in peripheral organs is cardioselective (PMID 33216462). (E) Detection of the autotoxic catecholaldehyde DOPEGAL in pure autonomic failure (PAF): Bases for the association between catecholamine deficiency and AS deposition in LBDs are poorly understood. Previously we provided evidence for buildup of DOPAL, the catecholaldehyde of dopamine, in PD. Until now, no LBD case has been reported in which 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL), the catecholaldehyde of norepinephrine, has been detected. We have found preliminarily that in a PAF patient who had post-mortem tissues assayed for contents of AS and catecholaldehydes, submandibular gland tissue had increased co-localization of AS with tyrosine hydroxylase, indicating deposition of AS in noradrenergic nerves, and DOPEGAL detected by liquid chromatography with electrochemical detection (LCED). The results provide support for a pathogenetic link between intra-neuronal AS deposition and DOPEGAL in PAF. (F) Expression of COUP-TFII, an orphan nuclear receptor, is upregulated in PD patients and in mice accelerates phenotypic progression in a PD mouse model, MitoPark: In collaboration with investigators at Baylor we found that the MitoPark mouse model of PD involves dysfunctions of mitochondria (organelles that supply energy for cellular processes), decreased activity of aldehyde dehydrogenase (ALDH), which detoxifies DOPAL, and DOPAL buildup (PMID 32579581). These abnormalities may be linked, because mitochondrial Complex I generates NAD+, NAD+ is a required co-factor for ALDH, and decreased ALDH activity promotes DOPAL accumulation. (G) Tyrosine hydroxylase (TH)-over-expressing mice have increased DOPAL levels: In collaboration with investigators at the Univ. of Toronto we found that mice with over-expression of TH, the rate-limiting enzyme in dopamine biosynthesis, have elevated tissue contents of both dopamine and DOPAL in the putamen. There is also increased production of potentially toxic by-products of spontaneous oxidation of dopamine. Increased enzymatic and spontaneous oxidation of dopamine may contribute to aging-related catecholaminergic neurodegeneration in this model (PMID 24979780). (H) LC-MS/MS for catecholamine-related compounds: Biochemical assays of DOPAL and DOPEGAL in body fluids by LCED are limited technologically. For further development of means to elucidate mechanisms of catecholaminergic neurodegneration by measuring levels of DOPAL and DOPEGAL, liquid chromatography with tandem mass spectroscopy (LC-MS/MS) seems necessary. Under a technology transfer agreement we are assessing whether a commercially available LC-MS/MS system measures plasma DOPAL and DOPEGAL sensitively and specifically.