Endoplasmic reticulum Ca(2+) handling in excitable cells in health and disease

Abstract

The endoplasmic reticulum (ER) is a morphologically and functionally diverse organelle capable of integrating multiple extracellular and internal signals and generating adaptive cellular responses. It plays fundamental roles in protein synthesis and folding and in cellular responses to metabolic and proteotoxic stress. In addition, the ER stores and releases Ca(2+) in sophisticated scenarios that regulate a range of processes in excitable cells throughout the body, including muscle contraction and relaxation, endocrine regulation of metabolism, learning and memory, and cell death. One or more Ca(2+) ATPases and two types of ER membrane Ca(2+) channels (inositol trisphosphate and ryanodine receptors) are the major proteins involved in ER Ca(2+) uptake and release, respectively. There are also direct and indirect interactions of ER Ca(2+) stores with plasma membrane and mitochondrial Ca(2+)-regulating systems. Pharmacological agents that selectively modify ER Ca(2+) release or uptake have enabled studies that revealed many different physiological roles for ER Ca(2+) signaling. Several inherited diseases are caused by mutations in ER Ca(2+)-regulating proteins, and perturbed ER Ca(2+) homeostasis is implicated in a range of acquired disorders. Preclinical investigations suggest a therapeutic potential for use of agents that target ER Ca(2+) handling systems of excitable cells in disorders ranging from cardiac arrhythmias and skeletal muscle myopathies to Alzheimer disease.

Fig. 1.

Structures of agents that activate or inhibit ryanodine receptors, IP₃ receptors, or the ER Ca²⁺-ATPase. Caffeine activates ryanodine receptors, ryanodine activates (low concentrations) or inhibits (high concentrations) ryanodine receptors, and dantrolene inhibits ryanodine receptors. Adenosine, inosine, uric acid, and xanthine have all been reported to modulate ryanodine-sensitive ER Ca²⁺ stores. Xestospongin C and low-molecular-weight heparin inhibit IP₃ receptor-mediated Ca²⁺ release. Thapsigargin selectively inhibits the ER Ca²⁺-ATPase.

Fig. 2.

Examples of pharmacological and genetic manipulations of ER Ca²⁺ dynamics. A, cultured neural cells were transfected with an empty control vector or with an expression vector containing the cDNA encoding the mitochondrial uncoupling protein 4 (UCP4). Intracellular Ca²⁺ concentrations were then monitored in the cells by ratiometric imaging of the Ca²⁺ indicator dye fura-2 at baseline and during exposures to the indicated experimental treatments. 0 Ca²⁺, culture medium lacking Ca²⁺; 2 mM Ca²⁺, culture medium containing 2 mM Ca²⁺; thapsigargin (1 μM); and A23187, the Ca²⁺ ionophore A23187 (calcimycin; 10 μM). Note that Ca²⁺-induced Ca²⁺ influx (in response to addition of extracellular Ca²⁺ in the presence of thapsigargin is attenuated in cells overexpressing UCP4. See Chan et al. (2006) for additional information. B, neural cells expressing a presenilin-1 mutation that causes Alzheimer disease exhibit an elevated ER pool of Ca²⁺. The indicated clones of PC12 cells were exposed to vehicle or 1 mM thapsigargin and the intracellular Ca²⁺ concentration was measured 30 min later. Con, untransfected control cells; vector, cells transfected with empty vector; wtPS1, cells overexpressing wild type presenilin-1; mutPS1, cells overexpressing the L286V presenilin-1 missense mutation. Note that thapsigargin-induced elevation of the intracellular Ca²⁺ level was greater in cells expressing mutant presenilin-1 compared with each of the other three cell clones. [Modified from Guo Q, Furukawa K, Sopher BL, Pham DG, Xie J, Robinson N, Martin GM, and Mattson MP (1996) Alzheimer's PS-1 mutation perturbs calcium homeostasis and sensitizes PC12 cells to death induced by amyloid beta-peptide. Neuroreport 8:379–383. Copyright © 1996 Lippincott Williams & Wilkins. Used with permission.]

Fig. 3.

The role of ER Ca²⁺ in synaptic plasticity. A, the ER can extend into both the pre- and postsynaptic compartments of a synapse. In presynaptic terminals, ER Ca²⁺ release can trigger spontaneous neurotransmitter release and can also integrate with voltage-gated Ca²⁺ entry elicited from action potential invasion to facilitate vesicle release and the repopulation of the ready-releasable pool of vesicles. NMDAR-mediated Ca²⁺ signals are amplified postsynaptically by RyR in dendritic spines and contribute to homosynaptic plasticity. At extrasynaptic sites, glutamate spillover triggers metabotropic glutamate (mGlu) receptor-mediated generation of IP₃ and activates a Ca²⁺ response outside of the synaptic contact point. Subsequent activation of IP₃Rs supports regenerative Ca²⁺ waves, which may be involved in heterosynaptic plasticity and gene expression. [Modified from Bardo S, Cavazzini MG, and Emptage N (2006) The role of endoplasmic reticulum Ca²⁺ store in the plasticity of central neurons. Trends Pharmacol Sci 27:78–84.). B, the Ca²⁺ generated by both plasma membrane Ca²⁺-permeable channels (e.g., NMDAR) and ER Ca²⁺ channels can subsequently trigger multiple Ca²⁺-dependent cascades that encode long-term plasticity. In the case of LTP, Ca²⁺ in dendritic spines locally activates effectors, including calmodulin, which in turn activates several kinase pathways such as adenylyl cyclase, CamKII, and PKC. These then trigger longer term cascades, such as the cAMP/phosphorylated cAMP response element-binding protein (pCREB) pathway, which results in protein translation and long-term structural and functional alterations to the neuron that support learning and memory encoding. AMPAR, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor.

Fig. 4.

The role of presenilin in ER Ca²⁺ signaling and dysregulation. The transmembrane-spanning presenilin protein largely functions as an aspartyl protease localized in the ER membrane. It cleaves several type 1 membrane proteins, including β-APP. As part of the γ-secretase complex, presenilin cleaves APP (scissors) to generate Aβ₄₀ and Aβ₄₂. Aβ₄₂ readily self-aggregates to form toxic oligomers that may damage neurons by inducing membrane-associated reactive oxygen species (ROS) that, in turn, impair the function of ion-motive ATPases, resulting in membrane depolarization and Ca²⁺ influx through glutamate and voltage-dependent channels. Ca²⁺ oligomers may also form Ca²⁺-conducting pores in the membrane. Presenilin-1 (PS1) mutations that cause Alzheimer disease result in increased levels of Aβ₄₂, rather than the more commonly produced and relatively inert Aβ₄₀ fragment generated by wild-type PS1. PS1 mutations also result in increased Ca²⁺ release from ER stores through a mechanism that probably involves IP₃R and RyR. This increased Ca²⁺ flux also accelerates Aβ formation, which in turn contributes to Ca²⁺ dyshomeostasis. PS1 may also be involved in ER Ca²⁺ homeostasis by serving as a leak channel; and PS1 mutations may impair this Ca²⁺ leak channel function, thereby leading to increased resting Ca²⁺ store levels and increased Ca²⁺ release upon activation of IP₃R or RyR.