The cytoprotective effects of dantrolene: a ryanodine receptor antagonist

Abstract

Calcium, as a second messenger, has an important role in a variety of cellular functions. However, disruption of intracellular calcium homeostasis leads to cytotoxicity and cell death. Excessive calcium release from intracellular stores, via the calcium channel ryanodine receptor, contributes to cell damage. Dysfunction of calcium homeostasis is established in tissue culture and animal models of ischemia, hypoxia, seizure, trauma, anesthesia, and neurodegenerative diseases. Dantrolene, the primary drug to treat malignant hyperthermia, is a ryanodine receptor antagonist. Dantrolene inhibits abnormal calcium release from the sarco-endoplasmic reticulum, which is the primary intracellular calcium store. Dantrolene has been investigated widely for its possible cytoprotective effects against cell damage in different tissue culture or animal models of diseases involving cytotoxicity induced by disruption of intracellular calcium homeostasis in pathogenesis. In this review, we summarize the role of the disruption of intracellular calcium homeostasis on cell death, the pharmacologic and pharmacokinetic features of dantrolene, and the cytoprotective effects and potential application of dantrolene for the inhibition of cell damage in a wide variety of models of stress and disease.

Figure 1.

Chemical structures of dantrolene (A) and azumolene (B).

Figure 2.

The models of cytotoxicity induced by Ca²⁺ dysregulation in excitotoxicity (i.e., sepsis, seizure, trauma), neurodegenerative diseases, and general anesthesia. Under normal conditions, [Ca²⁺] in the extracellular space is 10,000 times higher than the cytosolic Ca²⁺ concentration ([Ca²⁺]_c). Upon electrical or receptor-mediated stimulation, [Ca²⁺]_c is increased by extracellular Ca²⁺ influx via specific ion channels on the plasma membrane including voltage-dependent calcium channels (VDCCs) and ligand-gated calcium channels or by Ca²⁺ release from intracellular stores. The main intracellular Ca²⁺ store in neurons is the endoplasmic reticulum (ER) and Ca²⁺ is released into the cytosol via activation of ryanodine receptors (RYRs) and inositol-1,4,5-triphosphate receptors (InsP₃Rs). Basal [Ca²⁺]_c is maintained through calcium binding and calcium buffering proteins or uptake into internal stores by the energy-dependent sarco-ER calcium pump (SERCA) at the ER membrane or by the mitochondrial uniporter. Disturbance in Ca²⁺ homeostasis leads to cytotoxicity. Abnormal increase in intracellular [Ca²⁺] is the result of either increased extracellular Ca²⁺ influx via Ca²⁺ channels or increased release from the ER by either calcium-induced calcium release (CICR) or overactivation of RYRs and InsP₃Rs. During excitotoxic conditions (i.e., ischemia, sepsis, seizure, trauma), glutamate release is increased. Glutamate stimulates N-methyl-d-aspartate (NMDA), kainate (KA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and metabotropic glutamate mGluR1/5 receptors. Amyloid-β peptide (Aβ) synthesis is increased in Alzheimer disease (AD) and forms oligomers. Both Aβ oligomers and α-synuclein, which aggregates in Parkinson disease (PD), can form Ca²⁺ permeable pores in the plasma membrane and facilitates the increase of [Ca²⁺]_c. Aβ oligomers also activate NMDA and KA receptors as well as VDCCs and cause Ca²⁺ influx. Activation of mGluR1/5 increases inositol-1,4,5-triphosphate (InsP₃), which activates the InsP₃R and causes Ca²⁺ release from the ER. Evidence suggests that the InsP₃R is sensitized to InsP₃ during Huntington disease (HD), spinothalamic cerebellar ataxies (SCAs), and general anesthesia by isoflurane. Dantrolene is cytoprotective via inhibiting RYRs and preventing excessive Ca²⁺ release from the ER, especially during CICR. Reduction in ER Ca²⁺ stores results in the misfolding of proteins, which stimulates the unfolding protein response as a cellular stress response. Influx of Ca²⁺ into mitochondria causes the formation of oxygen radicals and energy failure as a consequence of decreased adenosine triphosphate (ATP) production. When cytosolic Ca²⁺ binds to calmodulin (CaM), nitric oxide synthase (NOS) is activated and nitric oxide (NO) is produced. Oxygen radicals react with NO and forms peroxynitrite, which damages DNA and proteins. DNA cleavage activates the DNA-repair enzyme poly (ADP-ribose) polymerase (PARP), which requires energy for its activation. PARP-induced energy depletion worsens cellular stress. Increased [Ca²⁺] in mitochondria increases mitochondrial permeability via the mitochondrial permeability transition pore (MtPTP), which causes mitochondrial swelling, outer mitochondrial membrane rupture, and release of cytochrome c (CytC). CytC activates pro-apoptotic factors (caspases) by reacting with Ca²⁺-activated calpain and induces apoptosis via intrinsic pathways.