Ca(2+) -independent effects of BAPTA and EGTA on single-channel Cl(-) currents in brown adipocytes

Cellular effects of BAPTA: Are they only about Ca2+ chelation?

Intracellular Ca2+ signals play a vital role in a broad range of cell biological and physiological processes in all eukaryotic cell types. Dysregulation of Ca2+ signaling has been implicated in numerous human diseases. Over the past four decades, the understanding of how cells use Ca2+ as a messenger has flourished, largely because of the development of reporters that enable visualization of Ca2+ signals in different cellular compartments, and tools that can modulate cellular Ca2+ signaling. One such tool that is frequently used is BAPTA; a fast, high-affinity Ca2+-chelating molecule. By making use of a cell-permeable acetoxymethyl ester (AM) variant, BAPTA can be readily loaded into the cytosol of cells (referred to as BAPTAi), where it is trapped and able to buffer changes in cytosolic Ca2+. Due to the ease of loading of the AM version of BAPTA, this reagent has been used in hundreds of studies to probe the role of Ca2+ signaling in specific processes. As such, for decades, researchers have almost universally attributed changes in biological processes caused by BAPTAi to the involvement of Ca2+ signaling. However, BAPTAi has often been used without any form of control, and in many cases has neither been shown to be retained in cells for the duration of experiments nor to buffer any Ca2+ signals. Moreover, increasing evidence points to off-target cellular effects of BAPTA that are clearly not related to Ca2+ chelation. Here, we briefly introduce Ca2+ signaling and the history of Ca2+ chelators and fluorescent Ca2+ indicators. We highlight Ca2+-independent effects of BAPTAi on a broad range of molecular targets and describe some of BAPTAi's impacts on cell functions that occur independently of its Ca2+-chelating properties. Finally, we propose strategies for determining whether Ca2+ chelation, the binding of other metal ions, or off-target interactions with cell components are responsible for BAPTAi's effect on a particular process and suggest some future research directions.

Calcium chelation independent effects of BAPTA on endogenous ANO6 channels in HEK293T cells

Selective calcium chelator 1,2-Bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA) is a common tool to investigate calcium signaling. However, BAPTA expresses various effects on intracellular calcium signaling, which are not related to its ability to bind Ca2+. In patch clamp experiments, we investigated calcium chelation independent effects of BAPTA on endogenous calcium-activated chloride channels ANO6 (TMEM16F) in HEK293T cells. We have found that application of BAPTA to intracellular solution led to two distinct effects on channels properties. On the one hand, application of BAPTA acutely reduced amplitude of endogenous ANO6 channels induced by 10 μM Ca2+ in single channel recordings. On the other hand, BAPTA application by itself induced ANO6 channel activity in the absence of the intracellular calcium elevation. Open channel probability was enhanced by increasing the intracellular BAPTA concentration from 0.1 to 1 and 10 mM. Another calcium chelator EGTA did not demonstrate chelation independent effects on the ANO6 activity in the same conditions. Due to off-target effects BAPTA should be used with caution when studying calcium-activated ANO6 channels.

Intracellular BAPTA directly inhibits PFKFB3, thereby impeding mTORC1-driven Mcl-1 translation and killing MCL-1-addicted cancer cells

Intracellular Ca2+ signals control several physiological and pathophysiological processes. The main tool to chelate intracellular Ca2+ is intracellular BAPTA (BAPTAi), usually introduced into cells as a membrane-permeant acetoxymethyl ester (BAPTA-AM). Previously, we demonstrated that BAPTAi enhanced apoptosis induced by venetoclax, a BCL-2 antagonist, in diffuse large B-cell lymphoma (DLBCL). This finding implied a novel interplay between intracellular Ca2+ signaling and anti-apoptotic BCL-2 function. Hence, we set out to identify the underlying mechanisms by which BAPTAi enhances cell death in B-cell cancers. In this study, we discovered that BAPTAi alone induced apoptosis in hematological cancer cell lines that were highly sensitive to S63845, an MCL-1 antagonist. BAPTAi provoked a rapid decline in MCL-1-protein levels by inhibiting mTORC1-driven Mcl-1 translation. These events were not a consequence of cell death, as BAX/BAK-deficient cancer cells exhibited similar downregulation of mTORC1 activity and MCL-1-protein levels. Next, we investigated how BAPTAi diminished mTORC1 activity and identified its ability to impair glycolysis by directly inhibiting 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3) activity, a previously unknown effect of BAPTAi. Notably, these effects were also induced by a BAPTAi analog with low affinity for Ca2+. Consequently, our findings uncover PFKFB3 inhibition as an Ca2+-independent mechanism through which BAPTAi impairs cellular metabolism and ultimately compromises the survival of MCL-1-dependent cancer cells. These findings hold two important implications. Firstly, the direct inhibition of PFKFB3 emerges as a key regulator of mTORC1 activity and a promising target in MCL-1-dependent cancers. Secondly, cellular effects caused by BAPTAi are not necessarily related to Ca2+ signaling. Our data support the need for a reassessment of the role of Ca2+ in cellular processes when findings were based on the use of BAPTAi.

OsSalT gene cloned from rice provides evidence of its role in salinity and drought stress tolerance

Abiotic stresses impose a huge threat to agricultural productivity and global food security. To counter this challenge, the precise identification of the right candidate gene (s) for conferring abiotic stress tolerance without compromising the growth and yield is crucial. OsSalT is identified as a salt stress responsive gene located on SalTol QTL of chromosome 1 of rice, however, there is no genetic evidence of its function and probable pathway of its regulation. To get better insights into its functioning, earlier we elucidated the structure of SALT protein at atomic scale {PDB ID (5GVY)} and solution state that provided key clues on the probable mode of its action. Herein, we report the modulation of OsSalT gene in response to various factors and its functional characterization. Results indicate that OsSalT operates through both abscisic acid and gibberellic acid-dependent pathways and is linked to the adaptive stress mechanisms of plants. Its overexpression in a model plant resulted in improved salinity and drought stress tolerance. The OsSalT transformed plants also showed vigorous root growth, early flowering, and better seed germination. The triggering of multiple responses by OsSalT suggested that modulation of such mannose-binding lectin could be a potential game-changer for the improvement of many crops in future.

L-type Voltage-Gated Calcium Channel Modulators Inhibit Glutamate-Induced Morphology Changes in U118-MG Astrocytoma Cells

The excitatory neurotransmitter glutamate evokes physiological responses within the astrocytic network that lead to fine morphological changes. However, the mechanism by which astrocytes couple glutamate sensing with cellular calcium rise remains unclear. We tested a possible connection between L-type voltage-gated calcium channels (Ca_v) and glutamate-induced response in U118-MG astrocytoma cells. While astrocytoma cells differ from primary astrocytes, they demonstrate the same response to glutamate. In this study, the extension of U118-MG processes upon glutamate exposure was shown to depend on extracellular calcium entry via L-type Ca_v's. Drugs known to bind to the pore-forming subunit of Ca_v's decreased the astrocytic filopodia extension caused by glutamate, and ligands of the α2δ auxiliary subunit inhibited all process growth (e.g., gabapentinoids). The observed phenotypic responses suggest that α2δ is a main contributor to the role of Ca_vs in glutamate-dependent filopodiagenesis, thereby opening new avenues of research on the role of α2δ in astrocytic neurochemical signaling.

Pharmacological Strategies for Manipulating Plant Ca2+ Signalling

Calcium is one of the most pleiotropic second messengers in all living organisms. However, signalling specificity is encoded via spatio-temporally regulated signatures that act with surgical precision to elicit highly specific cellular responses. How this is brought about remains a big challenge in the plant field, in part due to a lack of specific tools to manipulate/interrogate the plant Ca2+ toolkit. In many cases, researchers resort to tools that were optimized in animal cells. However, the obviously large evolutionary distance between plants and animals implies that there is a good chance observed effects may not be specific to the intended plant target. Here, we provide an overview of pharmacological strategies that are commonly used to activate or inhibit plant Ca2+ signalling. We focus on highlighting modes of action where possible, and warn for potential pitfalls. Together, this review aims at guiding plant researchers through the Ca2+ pharmacology swamp.

ATP release during cell swelling activates a Ca2+-dependent Cl- current by autocrine mechanism in mouse hippocampal microglia

Microglia cells, resident immune cells of the brain, survey brain parenchyma by dynamically extending and retracting their processes. Cl⁻ channels, activated in the cellular response to stretch/swelling, take part in several functions deeply connected with microglia physiology, including cell shape changes, proliferation, differentiation and migration. However, the molecular identity and functional properties of these Cl⁻ channels are largely unknown. We investigated the properties of swelling-activated currents in microglial from acute hippocampal slices of Cx3cr1^+/GFP mice by whole-cell patch-clamp and imaging techniques. The exposure of cells to a mild hypotonic medium, caused an outward rectifying current, developing in 5–10 minutes and reverting upon stimulus washout. This current, required for microglia ability to extend processes towards a damage signal, was carried mainly by Cl⁻ ions and dependent on intracellular Ca²⁺. Moreover, it involved swelling-induced ATP release. We identified a purine-dependent mechanism, likely constituting an amplification pathway of current activation: under hypotonic conditions, ATP release triggered the Ca²⁺-dependent activation of anionic channels by autocrine purine receptors stimulation. Our study on native microglia describes for the first time the functional properties of stretch/swelling-activated currents, representing a key element in microglia ability to monitor the brain parenchyma.

Hysteresis of gating underlines sensitization of TRPV3 channels

Vanilloid receptors of the transient receptor potential family have functions in thermal sensation and nociception. Among them, transient receptor potential vanilloid (TRPV)3 displays a unique property by which the repeated stimulation causes successive increases in its activity. The property has been known as sensitization and is observed in both native cells and cells heterologously expressing TRPV3. Transient increases in intracellular calcium levels have been implicated to play a key role in this process by mediating interaction of calmodulin with the channel. In support of the mechanism, BAPTA, a fast calcium chelator, accelerates the sensitization, whereas the slow chelator EGTA is ineffectual. Here, we show that the sensitization of TRPV3 also occurred independently of Ca²⁺. It was observed in both inside-out and outside-out membrane patches. BAPTA, but not EGTA, has a direct potentiation effect on channel activation. Analogues of BAPTA lacking Ca²⁺-buffering capability were similarly effective. The stimulation-induced sensitization and the potentiation by BAPTA are distinguishable in reversibility. We conclude that the sensitization of TRPV3 is intrinsic to the channel itself and occurs as a result of hysteresis of channel gating. BAPTA accelerates the sensitization process by potentiating the gating of the channel.