Motor Neuron Pathology Leads to Respiratory Deficiency in Optn−/− Mice

Abstract

Amyotrophic Lateral Sclerosis (ALS) is a devastating and fatal neurodegenerative disease with no current cure. Respiratory failure is the leading cause of death in ALS. Death occurs3–5 years after diagnosis when patients with ALS ultimately succumb to inadequate ventilation, hypoxia, and respiratory failure. Several genes are associated with ALS. One of these genes encodes optineurin (OPTN) which is associated with neurodegeneration in both ALS and glaucoma. Optineurin has multiple roles in various biochemical pathways such as regulation of inflammation and autophagy. However, the exact mechanism by which loss of OPTN results in progressive neural degeneration and respiratory failure in ALS is still unclear.In order to understand the impact of OPTN deficiency on respiratory function, our goal was to study the respiratory pathophysiology in the Optn−/− mouse. The hypothesis driving this work is that the Optn−/− mouse has respiratory insufficiency due to degeneration of the respiratory motor unit – motor neuron, nerve, neuromuscular junction and muscle. Physiological, histological and molecular outcome measures were used to assess the impact of OPTN on the respiratory system. We used whole body plethysmography (WBP) to assess breathing at baseline and during a respiratory challenge with hypercapnic and hypoxic conditions (FiCO2: 0.07, FiO2: 0.10; nitrogen balance). During the respiratory challenge, compared to the WT mice, Optn−/− mice had significantly lower tidal volume, minute ventilation, peak inspiratory flow, and peak expiratory flow starting at 6 months of age indicating weakened muscle strength. Furthermore, throughout the challenge period Optn−/− mice spent a greater amount of time in apnea indicating pathology in the respiratory control centers. The weakness and pathology in control of breathing during the respiratory challenge progresses as the mice age. At 1 year of age, Optn−/− mice have stunted growth accompanied by reduced diaphragm size compared to WT mice. Postmortem immunohistochemical studies of the medulla reveal Optn−/− mice have fewer hypoglossal motor neurons. Finally, the hypoglossal (XII) nerves of Optn−/− mice had significantly reduced g‐ratio which indicates pathology and decompaction of myelin sheaths. Within the XII nerve, there are an increase in both the number of mitochondria and area of mitochondria. These along with elevated levels of LC3‐II to LC3‐I indicate reduced mitophagy. In conclusion, the Optn−/− ALS mouse model displays motor neuron pathology, increased nerve demyelination and aberrant control of breathing leading to respiratory insufficiency.