Role of the Sympathetic Nervous System in Amyotrophic Lateral Sclerosis (ALS)

Abstract

Amyotrophic lateral sclerosis (ALS) is a progressive and lethal neuromuscular degenerative disease that affects 0.01% (~10/100,000) of the population worldwide. Most ALS patients (familial or sporadic) display a progressive motor disorder after the disease onset; therefore, characterizing it as a degenerative syndrome triggered by a major disruption of the neuromuscular axis. However, ~50% of ALS patients also have non-motor symptoms, raising the question other systems (extrinsic to the neuromuscular axis) contribute to the onset and progression of ALS. The autonomic system is among the systems extrinsically to the neuromuscular axis that might affect ALS pathophysiology. Both patients and experimental models of ALS have an autonomic imbalance triggered by hyperactivation of the sympathetic nervous system. These clinically relevant but still associative data suggest that increased sympathetic nervous system activity might be involved in the pathophysiology of ALS, as seen in other degenerative diseases including Parkinson's, Alzheimer's and heart failure. Based on this information, we hypothesized that sympathetic nervous hyperactivation contributes to the onset and progression of ALS. To test this hypothesis, we established three aims in the present proposal: 1. To characterize the activation profile of the sympathetic nervous system in the progression of ALS (pre-symptomatic and symptomatic states) using genetically modified male mice that develop ALS as a result of overexpression of the mutant enzyme human SOD1-G93A (ELA-SOD1-G93A); 2- To test the effect of isoproterenol (an agonist of beta-adrenergic receptors) on the onset and progression of ALS in ELA-SOD1-G93A mice; and 3- To evaluate the influence of chronic sympathetic hyperactivity induced by the deletion of ±2a and ±2c-adrenergic receptors on the establishment and progression of ALS in triple transgenic mice (ARKO-ELA-SOD1-G93A). For this, we will divide the project into three stages. In the first step, we will perform the phenotypic characterization of genetically modified male mice (ELA-SOD1-G93A) that develop ALS as a result of the hyperexpression of the human SOD1-G93A enzyme. This characterization will be performed in the pre-symptomatic and symptomatic phases of ALS. In the second stage, we will administer, still in the acute phase of ALS, a single intraperitoneal injection of isoproterenol (200 mg/kg) in the ELA-SOD1-G93A animals, mimicking a transient hyperactivation of the sympathetic nervous system, and we will evaluate the ALS signs. This acute protocol with isoproterenol induces transient morphological and functional changes in target tissues of the sympathetic nervous system, which will certainly impact the pathophysiology of ALS. In the third and final step, we will cross male C57BL/6J mice that overexpress the human SOD1-G93A enzyme (ELA-SOD1-G93A) with female C57BL6/J double knockout for ±2a and ±2c-adrenergic receptors (ARKO). ARKO animals present as a phenotype the intrinsic and chronic hyperactivation of the sympathetic nervous system, as previously demonstrated by our group. Therefore, we will generate triple transgenic animals that will develop ALS and present lifelong sympathetic nervous system hyperactivation (ARKO-ELA-SOD1-G93A). At all stages of the project, we will carry out a series of behavioral and functional analyzes in the pre-symptomatic and symptomatic phases of ALS. Furthermore, we will perform biochemical analyzes on ALS target tissues (motor cortex, spinal cord and skeletal muscle) and target tissues of sympathetic nervous hyperactivation (heart and vessels) throughout the three stages. These measurements will be of great value to dissect the direct (neuromuscular axis) and indirect contribution of sympathetic nervous system activation in the pathophysiology of ALS.