Influence of ER leak on resting cytoplasmic Ca2+ and receptor-mediated Ca2+ signalling in human macrophage

Insights into the mediation of Ca2+ signaling in the promoting effects of LETX-VI on the synthesis and release of dopamine

Latroeggtoxin-VI (LETX-VI) is an active protein and was previously demonstrated to have effects on the synthesis and release of dopamine. Hererin, the involvement of Ca2+ signaling in the effects of LETX-VI on dopamine was systematically investigated, using PC12 cells as a neuron model. LETX-VI was shown to promote dopamine release from PC12 cells both in the presence and absence of extracellular Ca2+; however the presence of extracellular Ca2+ was favorable for enhancing the promoting effects of LETX-VI on dopamine, because LETX-VI facilitated the influx of extracellular Ca2+ through the L-type calcium channels in plasma membrane (PM) to increase cytosolic Ca2+ concentration. LETX-VI was able to penetrate the PM of PC12 cells to act on the Ca2+ channel proteins IP3Rs and RyRs in the endoplasm reticulum (ER) membrane, opening the Ca2+ channels and promoting the release of ER Ca2+ to elevate cytosolic Ca2+ level. With the help of intracellular Ca2+ chelator BAPTA, the elevated cytosolic Ca2+ level was proven to play crucial role for the enhanced promoting effects of LETX-VI on dopamine. Taken together, LETX-VI is able to open the Ca2+ channels in both PM and ER membrane simultaneously to facilitate extracellular Ca2+ influx and ER Ca2+ release, and thus increases the cytosolic Ca2+ concentration to enhance the promoting effects on the synthesis and release of dopamine.

Sec61 complex/translocon: The role of an atypical ER Ca2+-leak channel in health and disease

The heterotrimeric Sec61 protein complex forms the functional core of the so-called translocon that forms an aqueous channel in the endoplasmic reticulum (ER). The primary role of the Sec61 complex is to allow protein import in the ER during translation. Surprisingly, a completely different function in intracellular Ca2+ homeostasis has emerged for the Sec61 complex, and the latter is now accepted as one of the major Ca2+-leak pathways of the ER. In this review, we first discuss the structure of the Sec61 complex and focus on the pharmacology and regulation of the Sec61 complex as a Ca2+-leak channel. Subsequently, we will pay particular attention to pathologies that are linked to Sec61 mutations, such as plasma cell deficiency and congenital neutropenia. Finally, we will explore the relevance of the Sec61 complex as a Ca2+-leak channel in various pathophysiological (ER stress, apoptosis, ischemia-reperfusion) and pathological (type 2 diabetes, cancer) settings.

Acute Induction of Translocon-Mediated Ca2+ Leak Protects Cardiomyocytes Against Ischemia/Reperfusion Injury

During myocardial infarction, dysregulation of Ca²⁺ homeostasis between the reticulum, mitochondria, and cytosol occurs in cardiomyocytes and leads to cell death. Ca²⁺ leak channels are thought to be key regulators of the reticular Ca²⁺ homeostasis and cell survival. The present study aimed to determine whether a particular reticular Ca²⁺ leak channel, the translocon, also known as translocation channel, could be a relevant target against ischemia/reperfusion-mediated heart injury. To achieve this objective, we first used an intramyocardial adenoviral strategy to express biosensors in order to assess Ca²⁺ variations in freshly isolated adult mouse cardiomyocytes to show that translocon is a functional reticular Ca²⁺ leak channel. Interestingly, translocon activation by puromycin mobilized a ryanodine receptor (RyR)-independent reticular Ca²⁺ pool and did not affect the excitation–concentration coupling. Second, puromycin pretreatment decreased mitochondrial Ca²⁺ content and slowed down the mitochondrial permeability transition pore (mPTP) opening and the rate of cytosolic Ca²⁺ increase during hypoxia. Finally, this translocon pre-activation also protected cardiomyocytes after in vitro hypoxia reoxygenation and reduced infarct size in mice submitted to in vivo ischemia-reperfusion. Altogether, our report emphasizes the role of translocon in cardioprotection and highlights a new paradigm in cardioprotection by functionally uncoupling the RyR-dependent Ca²⁺ stores and translocon-dependent Ca²⁺ stores.

The GPR120 Agonist TUG-891 Inhibits the Motility and Phagocytosis of Mouse Alveolar Macrophages

Movement and phagocytosis characterize the fundamental actions of macrophages. Although it is known that the free fatty acid receptor GPR120 is expressed in macrophages and regulates cytokine expression to exert anti-inflammatory activities, the effects of GPR120 activation on the motility and phagocytosis of macrophages are not clear. In this study, mouse alveolar macrophages (AM) were stimulated with the GPR120 agonist TUG-891, and the changes in cell motility, intracellular Ca²⁺ concentration ([Ca²⁺]i), and the ability of phagocytosis were measured. Mouse AM in controls exhibited active movement in vitro, and TUG-891 significantly restrained AM movement. Meanwhile, TUG-891 stimulated a quick increase in [Ca²⁺]i in AM, which was blocked separately by the Gq protein inhibitor YM-254890, the phospholipase C (PLC) inhibitor U73122, or depletion of endoplasmic reticulum (ER) Ca²⁺ store by thapsigargin. The inhibition of AM movement by TUG-891 was eliminated by YM-254890, U73122, thapsigargin, and chelation of cytosolic Ca²⁺ by BAPTA. Moreover, TUG-891 inhibited AM phagocytosis of fluorescent microspheres, which was also blocked by YM-254890, U73122, thapsigargin, and BAPTA. In conclusion, GPR120 activation in mouse AM increases [Ca²⁺]i but inhibits the motility and phagocytosis via Gq protein/PLC-mediated Ca²⁺ release from ER Ca²⁺ store.

A Current Overview of the Biological and Cellular Effects of Nanosilver

Nanosilver plays an important role in nanoscience and nanotechnology, and is becoming increasingly used for applications in nanomedicine. Nanosilver ranges from 1 to 100 nanometers in diameter. Smaller particles more readily enter cells and interact with the cellular components. The exposure dose, particle size, coating, and aggregation state of the nanosilver, as well as the cell type or organism on which it is tested, are all large determining factors on the effect and potential toxicity of nanosilver. A high exposure dose to nanosilver alters the cellular stress responses and initiates cascades of signalling that can eventually trigger organelle autophagy and apoptosis. This review summarizes the current knowledge of the effects of nanosilver on cellular metabolic function and response to stress. Both the causative effects of nanosilver on oxidative stress, endoplasmic reticulum stress, and hypoxic stress—as well as the effects of nanosilver on the responses to such stresses—are outlined. The interactions and effects of nanosilver on cellular uptake, oxidative stress (reactive oxygen species), inflammation, hypoxic response, mitochondrial function, endoplasmic reticulum (ER) function and the unfolded protein response, autophagy and apoptosis, angiogenesis, epigenetics, genotoxicity, and cancer development and tumorigenesis—as well as other pathway alterations—are examined in this review.

Silver nanoparticles induce SH-SY5Y cell apoptosis via endoplasmic reticulum- and mitochondrial pathways that lengthen endoplasmic reticulum-mitochondria contact sites and alter inositol-3-phosphate receptor function

Silver nanoparticles (AgNPs) have many medical and commercial applications, but their effects on human health are poorly understood. The aim of this study was to assess the effect of AgNPs on the human neuroblastoma cell line SH-SY5Y and to explore their potential mechanisms of action. We found that AgNPs decreased SH-SY5Y cell viability in a dose- and time-dependent manner. Exposure to AgNPs activated endoplasmic reticulum (ER) stress, as reflected by upregulated expression of glucose-regulated protein 78 (GRP78), phosphorylated PKR-like endoplasmic reticulum kinase (p-PERK), phosphorylated eukaryotic translation initiation factor 2α (p-eIF2α), C/EBP homology protein (CHOP), spliced X-box binding protein-1 (XBP1), and phosphorylated inositol-requiring enzyme (p-IRE), all of which are involved in the cellular unfolded protein response. Prolonged exposure of cells to AgNPs damaged calcium (Ca²⁺) homeostasis, increased the length of contact sites between the ER and mitochondria, altered IP₃R function by the increased levels of phosphatase and tensin homolog deleted on chromosome ten (PTEN) in the ER and enhanced mitochondrial Ca²⁺ uptake. Finally, Ca²⁺ overload and disrupted homeostasis in the mitochondria triggered apoptotic cell death. Our results suggest that caution should be exercised in the use of AgNPs in humans.