Computational modeling reveals multiple abnormalities of myocardial noradrenergic function in Lewy body diseases

Abstract

Background: Lewy body diseases, a family of aging-related neurodegenerative disorders, entail loss of the catecholamine dopamine in the nigrostriatal system and equally severe deficiency of the closely related catecholamine norepinephrine in the heart. The myocardial noradrenergic lesion is associated with major non-motor symptoms and decreased survival. Numerous mechanisms determine norepinephrine stores, and which of these are altered in Lewy body diseases has not been examined in an integrated way. We used a computational modeling approach to assess comprehensively pathways of cardiac norepinephrine synthesis, storage, release, reuptake, and metabolism in Lewy body diseases. Application of a novel kinetic model identified a pattern of dysfunctional steps contributing to norepinephrine deficiency. We then tested predictions from the model in a new cohort of Parkinson disease patients.

Methods: Rate constants were calculated for 17 reactions determining intra-neuronal norepinephrine stores. Model predictions were tested by measuring post-mortem apical ventricular concentrations and concentration ratios of catechols in controls and patients with Parkinson disease.

Results: The model identified low rate constants for three types of processes in the Lewy body group-catecholamine biosynthesis via tyrosine hydroxylase and L-aromatic-amino-acid decarboxylase, vesicular storage of dopamine and norepinephrine, and neuronal norepinephrine reuptake via the cell membrane norepinephrine transporter. Post-mortem catechols and catechol ratios confirmed this triad of model-predicted functional abnormalities.

Conclusion: Denervation-independent impairments of neurotransmitter biosynthesis, vesicular sequestration, and norepinephrine recycling contribute to the myocardial norepinephrine deficiency attending Lewy body diseases. A proportion of cardiac sympathetic nerves are "sick but not dead," suggesting targeted disease-modification strategies might retard clinical progression.

Trial registration: This study was not a clinical trial.

Funding: The research reported here was supported by the Division of Intramural Research, NINDS.

Keywords: Cardiology; Cardiovascular disease; Neurodegeneration; Neurological disorders; Neuroscience.

Figure 1. Concept diagram depicting steps of catecholamine synthesis, storage, release, recycling, and metabolism in myocardial sympathetic nerves.

Reactions are in italics and amounts of reactants in plain text. Font sizes correspond roughly to amounts of reactants. Green arrows indicate dopamine (DA) synthesis and blue arrows norepinephrine (NE) vesicular uptake and leakage. After tyrosine (TYR) uptake, cytoplasmic TYR (TYRc) is converted to 3,4-dihydroxyphenylalanine (DOPA) via tyrosine hydroxylase (TH). Cytoplasmic DOPA (DOPAc) is converted to DA via aromatic l-amino acid decarboxylase (LAAAD) or undergoes spontaneous oxidation to form 5-S-cysteinyldopa (Cys-DOPA). Cytoplasmic DA (DAc) is converted to 3,4-dihydroxyphenylacetaldehyde (DOPAL) via monoamine oxidase (MAO), undergoes spontaneous oxidation to form 5-S-cysteinylDA, or is taken up into vesicles via the vesicular monoamine transporter (VMAT). DOPAL is metabolized by aldehyde dehydrogenase (ALDH) to form 3,4-dihydroxyphenylacetic acid (DOPAC) or by aldehyde/aldose reductase (AR) to form 3,4-dihydroxyphenylethanol. DA in vesicles is converted to NE via dopamine-β-hydroxylase (DBH) or leaks passively into the cytoplasm. NE in vesicles is released into the extracellular fluid or leaks passively into the cytoplasm. Cytoplasmic NE (NEc) is converted to 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL) via MAO, and DOPEGAL is metabolized to 3,4-dihydroxyphenylglycol (DHPG) via AR. NE in the extracellular fluid (NEe) is taken up into the nerve by Uptake-1 (U1), spills over into the cardiac venous drainage, or is removed by extraneuronal uptake (Uptake-2). Circulating epinephrine (EPI) can be taken up into the nerve by U1 and transported into vesicles via the VMAT.

Figure 2. Calculated rate constants for processes related to catecholamine synthesis and fate in cardiac sympathetic nerves.

Pink is the intraneuronal cytoplasm; blue and green are vesicles; and white is extracellular fluid. Numbers in gray are values in control subjects and in red are values in patients with a Lewy body disease. Green arrows highlight steps in DA synthesis; blue arrows highlight steps in vesicular norepinephrine (NEv) turnover. DAv, vesicular DA; DOPACc, cytoplasmic DOPAC. See Figure 1 for other abbreviations.

Figure 3. Model-generated curves relating amounts of intraneuronal reactants to fractional changes of rate constants.

(A) DOPAc and DOPACc versus kVMAT_DA. (B) DOPAc and DOPACc versus kLAAAD. (C) Vesicular NE and DA versus kU1. (D) DOPAc and DOPACc versus kTH. Vertical dashed lines indicate rate constants in Lewy body disease patients as a fraction of corresponding rate constants in controls. No single change in kVMAT_DA, kLAAAD, kU1, or kTH predicts the actual pattern of alterations in reactant amounts in the Lewy body compared with the control group.

Figure 4. Box-and-whisker plots for postmortem myocardial concentrations of catechols in controls and PD patients.

Highest, third quartile, median, second quartile, and lowest values are shown. Numbers in italics are P values for independent-means t tests conducted on log-transformed data comparing the control (shown in gray) and PD (shown in red) groups. NE, DA, and DHPG levels in PD are drastically decreased compared with controls, without a significant group difference in DOPA. NE: control, n = 11; PD, n = 11. DA: control, n = 11; PD, n = 7. DHPG: control, n = 11; PD, n = 7. DOPA: control, n = 11; PD, n = 7. DHPG, 3,4-dihydroxyphenylglycol.a

Figure 5. Box-and-whisker plots for postmortem indices of sympathetic intraneuronal functions in controls and PD patients.

Highest, third quartile, median, second quartile, and lowest values are shown. Numbers in italics are P values for independent-means t tests conducted on log-transformed data comparing the control and PD groups. DHPG/NE and DOPAC/NE ratios provided inverse indices of VMAT2 activity. The sum of Cys-DOPA and DOPA, divided by the sum of DA and its metabolites DOPAC and 3,4-dihydroxyphenylethanol, provided an inverse index of LAAAD activity. The sum of Cys-DOPA and DOPA, adjusted for LAAAD activity, provided an index of TH activity. The results indicate decreased VMAT2, LAAAD, and TH activities in PD (shown in red) compared with controls (shown in gray). DHPG/NE: control, n = 11; PD, n = 7. DOPAC/NE: control, n = 11; PD, n = 11. Inv, Index of LAAD: control, n = 11; PD, n = 11. Index of TH: control, n = 11; PD, n = 11. DOPAC, 3,4-dihydroxyphenylacetic acid; Inv., inverse.