Abstract

The endoplasmic reticulum (ER) is the largest organelle of the cell, composed of a continuous network of sheets and tubules, and is involved in protein, calcium (Ca²⁺) and lipid homeostasis. In neurons, the ER extends throughout the cell, both somal and axodendritic compartments, and is highly important for neuronal functions. A third of the proteome of a cell, secreted and membrane-bound proteins, are processed within the ER lumen and most of these proteins are vital for neuronal activity. The brain itself is high in lipid content and many structural lipids are produced, in part, by the ER. Cholesterol and steroid synthesis are strictly regulated in the ER of the blood-brain barrier protected brain cells. The high Ca²⁺ level in the ER lumen and low cytosolic concentration is needed for Ca²⁺-based intracellular signaling, also for synaptic signaling and Ca²⁺ waves, as well as preparing proteins for correct folding in the presence of high Ca²⁺ concentrations to cope with the high concentrations of extracellular milieu. Particularly, ER Ca²⁺ is controlled in axodendritic areas for proper neurito- and synaptogenesis and synaptic plasticity and remodeling. In this review, we cover the physiological functions of the neuronal ER and discuss it in context of common neurodegenerative diseases focusing on pharmacological regulation of ER Ca²⁺. Furthermore, we postulate that heterogeneity of the ER, its protein folding capacity and ensuring Ca²⁺ regulation is a crucial factor for the aging and selective vulnerability of neurons in various neurodegenerative diseases.

Significance Statement ER Ca²⁺ regulators are promising therapeutic targets for degenerative diseases for which efficacious drug therapies do not exist. The use of pharmacological probes targeting maintenance and restoration of ER Ca²⁺ can provide restoration of protein homeostasis, e.g. folding of complex plasma membrane signalling receptors and slow down the degeneration process of neurons.