Calcium-dependent protein folding in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a neurodegenerative disease characterized by a progressive loss of motor neurons. Although the etiology remains unclear, disturbances in Ca²⁺ homoeostasis and protein folding are essential features of neurodegeneration. The correct folding of proteins is managed by folding proteins, which are regulated by Ca²⁺ levels. Therefore, Ca²⁺-sensitive folding proteins represent an important link between disturbed Ca²⁺ handling and protein misfolding in amyotrophic lateral sclerosis. In the first part of this review, we focus on Ca²⁺ handling in the endoplasmic reticulum and mitochondria in terms of their roles in protein misfolding. In the second part, we draw attention to the main Ca²⁺-sensitive folding proteins that play a role in motor neuron degeneration such as calreticulin and calnexin, which are involved in the folding of glycosylated proteins. In addition, calmodulin and the Ca²⁺/calmodulin-dependent protein kinase are discussed as one correlation to oxidative stress. The heat shock protein endoplasmin is associated with the anti-apoptotic insulin-like growth factor pathway that is altered in amyotrophic lateral sclerosis. Grp78, which influences Ca²⁺ homeostasis in the intraluminal endoplasmic reticulum is upregulated in mice models and amyotrophic lateral sclerosis patients and constitutes a core component of the unfolded protein response. Lastly, the protein disulfide isomerase family is responsible for mediating oxidative protein folding in the endoplasmic reticulum.