Disruption of Exocytosis in Sympathoadrenal Chromaffin Cells from Mouse Models of Neurodegenerative Diseases

Abstract

Synaptic disruption and altered neurotransmitter release occurs in the brains of patients and in murine models of neurodegenerative diseases (NDDs). During the last few years, evidence has accumulated suggesting that the sympathoadrenal axis is also affected as disease progresses. Here, we review a few studies done in adrenal medullary chromaffin cells (CCs), that are considered as the amplifying arm of the sympathetic nervous system; the sudden fast exocytotic release of their catecholamines-stored in noradrenergic and adrenergic cells-plays a fundamental role in the stress fight-or-flight response. Bulk exocytosis and the fine kinetics of single-vesicle exocytotic events have been studied in mouse models carrying a mutation linked to NDDs. For instance, in R6/1 mouse models of Huntington's disease (HD), mutated huntingtin is overexpressed in CCs; this causes decreased quantal secretion, smaller quantal size and faster kinetics of the exocytotic fusion pore, pore expansion, and closure. This was accompanied by decreased sodium current, decreased acetylcholine-evoked action potentials, and attenuated [Ca²⁺]c transients with faster Ca²⁺ clearance. In the SOD1^G93A mouse model of amyotrophic lateral sclerosis (ALS), CCs exhibited secretory single-vesicle spikes with a slower release rate but higher exocytosis. Finally, in the APP/PS1 mouse model of Alzheimer's disease (AD), the stabilization, expansion, and closure of the fusion pore was faster, but the secretion was attenuated. Additionally, α-synuclein that is associated with Parkinson's disease (PD) decreases exocytosis and promotes fusion pore dilation in adrenal CCs. Furthermore, Huntington-associated protein 1 (HAP1) interacts with the huntingtin that, when mutated, causes Huntington's disease (HD); HAP1 reduces full fusion exocytosis by affecting vesicle docking and controlling fusion pore stabilization. The alterations described here are consistent with the hypothesis that central alterations undergone in various NDDs are also manifested at the peripheral sympathoadrenal axis to impair the stress fight-or-flight response in patients suffering from those diseases. Such alterations may occur: (i) primarily by the expression of mutated disease proteins in CCs; (ii) secondarily to stress adaptation imposed by disease progression and the limitations of patient autonomy.

Keywords: chromaffin cell; exocytosis; neurodegenerative diseases; sympathoadrenal axis.

Figure 1

Detection of single-vesicle catecholamine release with a carbon-fiber microelectrode (panel A). In this technique, the process of exocytosis is recorded by placing a polarized microelectrode close to the cell membrane, which oxidizes the released catecholamines. At the right of the cell scheme, the sequential time course of the generation of the exocytotic event is shown: 1, vesicle docking to the plasma membrane; 2, pore formation, with a small release of catecholamine causing a mild baseline elevation, the so-called spike-foot; 3, pore expansion and full secretion of vesicle contents (fast rise of the spike); 4, pore closure and spike relaxation to baseline. The different kinetic parameters of the spike are shown in panel (B). In several neurodegenerative diseases (NDDs), mouse models spike kinetic changes in a characteristic manner. (Adapted from de Diego and García, 2018, [27]).

Figure 2

Schematic representation of altered exocytosis in chromaffin cells (CCs) from wildtype mice (WT) and mice carrying a mutation of Huntington’s disease (HD, R6/1), amyotrophic lateral sclerosis (ALS, SOD1^G93A), and Alzheimer’s disease. CCs were stimulated with 1-min pulses of acetylcholine (ACh); bulk secretion (summatory areas of all spikes recorded in 1-min ACh stimulation) is represented in the middle traces (Qamp versus time) and the averaged modelled single-spike kinetics are represented in the right traces. (A), typical time-course secretory curve and a representation of a spike with foot and the parameters measured in each individual spike. (B), bulk secretion and spike kinetics in WT and R6/1 CCs [28]; (C), bulk secretion and spike kinetics in SOD1^G93A CCs [29]; (D), cumulative bulk secretion was not monitored in this study; overlapping WT and APP/PS1 spikes are represented on the right part of this panel [33]. See text for further explanation.