ER Stress-Mediated Signaling: Action Potential and Ca(2+) as Key Players

Abstract

The proper functioning of the endoplasmic reticulum (ER) is crucial for multiple cellular activities and survival. Disturbances in the normal ER functions lead to the accumulation and aggregation of unfolded proteins, which initiates an adaptive response, the unfolded protein response (UPR), in order to regain normal ER functions. Failure to activate the adaptive response initiates the process of programmed cell death or apoptosis. Apoptosis plays an important role in cell elimination, which is essential for embryogenesis, development, and tissue homeostasis. Impaired apoptosis can lead to the development of various pathological conditions, such as neurodegenerative and autoimmune diseases, cancer, or acquired immune deficiency syndrome (AIDS). Calcium (Ca(2+)) is one of the key regulators of cell survival and it can induce ER stress-mediated apoptosis in response to various conditions. Ca(2+) regulates cell death both at the early and late stages of apoptosis. Severe Ca(2+) dysregulation can promote cell death through apoptosis. Action potential, an electrical signal transmitted along the neurons and muscle fibers, is important for conveying information to, from, and within the brain. Upon the initiation of the action potential, increased levels of cytosolic Ca(2+) (depolarization) lead to the activation of the ER stress response involved in the initiation of apoptosis. In this review, we discuss the involvement of Ca(2+) and action potential in ER stress-mediated apoptosis.

Keywords: action potential; apoptosis; calcium; endoplasmic reticulum stress; unfolded protein response.

Figure 1

Endoplasmic reticulum (ER) Stress and unfolded protein response (UPR). ER functions include protein synthesis, maturation, and the folding of proteins, ensuring cellular homeostasis. The disturbance of cellular adenosine triphosphate (ATP) levels, redox state, or Ca²⁺ concentration affects ER functioning, causing the accumulation and aggregation of unfolded proteins, and generating ER stress, which further triggers UPR. The UPR has three major roles: in adaptive response, feedback control, and cell fate. In the adaptive response, the UPR reduces ER stress and restores ER homeostasis. The UPR signaling is inhibited through a negative feedback mechanism. Depending on the severity of the ER stress, the UPR can regulate both cellular survival and death.

Figure 2

UPR mechanism. Upon the aggregation of the unfolded proteins, binding-immunoglobulin protein (BiP) dissociates from inositol requiring protein-1 (IRE1), protein kinase RNA-like ER kinase (PERK), and activating transcription factor-6 (ATF6), allowing their activation. Activated IRE1 splices X-box-binding protein 1 (XBP-1) mRNA, producing spliced XBP-1 (sXBP-1) that translocates to the nucleus and regulates the expression of C/EBP homologous protein (CHOP) transcription factor. IRE1 can recruit tumor necrosis factor receptor (TNFR)-associated factor-2 (TRAF2) and apoptosis-signaling-kinase 1 (ASK1), resulting in the downstream activation of c-Jun N-terminal protein kinase (JNK) and p38 mitogen-activation protein kinase (MAPK). Activated p38 MAPK phosphorylates and activates CHOP, whereas JNK translocates to the mitochondrial membrane, inhibiting B-cell lymphoma 2 (Bcl-2) and activating Bcl-2 interacting protein (Bim). IRE1 can activate Bcl-2 associated X protein (Bax) and Bcl-2 homologous antagonist killer protein (Bak) that induce inositol 1,4,5-triphosphate receptors (IP3Rs) to initiate the release of Ca²⁺ from the ER. Activated PERK phosphorylates eukaryotic initiation factor 2 (eIF2), which allows the translation of ATF4 through an eIF2-independent pathway, and ATF4 translocates to the nucleus and stimulates the transcription of proteins required to regain ER homeostasis. ATF6 is activated by the Golgi resident enzymes through a limited proteolysis, and it regulates the expression of CHOP. During the ER stress, all three UPR pathways result in the initiation of CHOP transcription.

Figure 3

Ionic basis of action potential. (A) A typical action potential. The membrane potential begins at −70 mV. When a stimulus is applied after 1 ms, the membrane potential raises above −40 mV (threshold potential). If a prolonged stimulation is applied, the membrane potential rapidly rises to the peak potential (+60 mV) at time = 2 ms. Afterward, the potential rapidly drops and overshoots to −90 mV at time = 4 ms, and finally the resting potential of −70 mV is reestablished at time = 5 ms; (B) the role of Ca²⁺ during an action potential. Depolarization occurs due to the influx of Na⁺ ions, which causes voltage-gated Ca²⁺ channels to open. This results in the change of membrane potential first from −70 mV to −40 mV (threshold level), and then to +60 mV. When the membrane potential reaches +60 mV, Ca²⁺ channels close and voltage-gated K⁺ channels open. The efflux of K⁺ results in the repolarization of cell membrane to −70 mV and then to −90 mV (hyperpolarization).

Figure 4

Action potential propagation induces ER stress-mediated apoptosis. During the action potential, extracellular Ca²⁺ enters into the cell through voltage-gated Ca²⁺ channels (VGCCs) and several ligand-gated calcium channels (LGCCs), while ER Ca²⁺ is released into the cytosol through IP3Rs or ryanodine receptors (RyRs). An increased level of intracellular Ca²⁺ leads to the membrane depolarization and the subsequent activation of ER stress response. Conformational changes of Bak and Bax in the ER membrane permit Ca²⁺ efflux, which activates m-calpain in the cytosol and subsequently cleaves and activates ER-resident procaspase-12, leading to the activation of the caspase cascade. Ca²⁺ is taken by mitochondria, leading to the depolarization of the inner membrane, the release of cytochrome c, and subsequent activation of Apaf-1/procaspase-9-regulated apoptosis. PERK and ATF6 can trigger pro-apoptotic signaling through the activation of downstream transcriptional target CHOP that inhibits the expression of Bcl-2 and thus promotes apoptosis. Activated IRE1 recruits TRAF2, which leads to the activation of ASK1/JNK and procaspase-12, subsequently activating caspase cascade.