A(a)LS: Ammonia-induced amyotrophic lateral sclerosis

Abstract

Amyotrophic lateral sclerosis (ALS) is a dreadful, devastating and incurable motor neuron disease. Aetiologically, it is a multigenic, multifactorial and multiorgan disease. Despite intense research, ALS pathology remains unexplained. After the literature review, this paper posits a new integrative explanation. This framework proposes that ammonia neurotoxicity is a main player in ALS pathogenesis. According to this explanation, a combination of impaired ammonia removal— mainly because of impaired hepatic urea cycle dysfunction—and increased ammoniagenesis— mainly because of impaired glycolytic metabolism in fast twitch skeletal muscle—causes chronic hyperammonia in ALS. In the absence of neuroprotective calcium binding proteins (calbindin, calreticulin and parvalbumin), elevated ammonia—a neurotoxin—damages motor neurons. Ammonia-induced motor neuron damage occurs through multiple mechanisms such as macroautophagy-endolysosomal impairment, ER-stress, CDK5 activation, oxidative/nitrosative stress and neuroinflammation. Furthermore, the regional pattern of calcium binding proteins’ loss, owing to either ER stress and/or impaired oxidative metabolism, determines clinical viabilities of ALS. Most importantly, this new framework can be generalised to explain other neurodegenerative disorders such as Huntington’s disease.