Bcl-2 interaction with the inositol 1,4,5-trisphosphate receptor: role in Ca(2+) signaling and disease

Recent advances in canonical versus non-canonical Ca2+-signaling-related anti-apoptotic Bcl-2 functions and prospects for cancer treatment

Cell fate is tightly controlled by a continuous balance between cell survival and cell death inducing mechanisms. B-cell lymphoma 2 (Bcl-2)-family members, composed of effectors and regulators, not only control apoptosis at the level of the mitochondria but also by impacting the intracellular Ca2+ homeostasis and dynamics. On the one hand, anti-apoptotic protein Bcl-2, prevents mitochondrial outer membrane permeabilization (MOMP) by scaffolding and neutralizing proapoptotic Bcl-2-family members via its hydrophobic cleft (region composed of BH-domain 1-3). On the other hand, Bcl-2 suppress pro-apoptotic Ca2+ signals by binding and inhibiting IP3 receptors via its BH4 domain, which is structurally exiled from the hydrophobic cleft by a flexible loop region (FLR). As such, Bcl-2 prevents excessive Ca2+ transfer from ER to mitochondria. Whereas regulation of both pathways requires different functional regions of Bcl-2, both seem to be connected in cancers that overexpress Bcl-2 in a life-promoting dependent manner. Here we discuss the anti-apoptotic canonical and non-canonical role, via calcium signaling, of Bcl-2 in health and cancer and evolving from this the proposed anti-cancer therapies with their shortcomings. We also argue how some cancers, with the major focus on diffuse large B-cell lymphoma (DLBCL) are difficult to treat, although theoretically prime marked for Bcl-2-targeting therapeutics. Further work is needed to understand the non-canonical functions of Bcl-2 also at organelles beyond the mitochondria, the interaction partners outside the Bcl-2 family as well as their ability to target or exploit these functions as therapeutic strategies in diseases.

Structural and functional insight into a new emerging target IP3R in cancer

Calcium signaling has been identified as an important phenomenon in a plethora of cellular processes. Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ER-residing intracellular calcium (Ca2+) release channels responsible for cell bioenergetics by transferring calcium from the ER to the mitochondria. The recent availability of full-length IP3R channel structure has enabled the researchers to design the IP3 competitive ligands and reveal the channel gating mechanism by elucidating the conformational changes induced by ligands. However, limited knowledge is available for IP3R antagonists and the exact mechanism of action of these antagonists within a tumorigenic environment of a cell. Here in this review a summarized information about the role of IP3R in cell proliferation and apoptosis has been discussed. Moreover, structure and gating mechanism of IP3R in the presence of antagonists have been provided in this review. Additionally, compelling information about ligand-based studies (both agonists and antagonists) has been discussed. The shortcomings of these studies and the challenges toward the design of potent IP3R modulators have also been provided in this review. However, the conformational changes induced by antagonists for channel gating mechanism still display some major drawbacks that need to be addressed. However, the design, synthesis and availability of isoform-specific antagonists is a rather challenging one due to intra-structural similarity within the binding domain of each isoform. HighlightsThe intricate complexity of IP3R's in cellular processes declares them an important target whereby, the recently solved structure depicts the receptor's potential involvement in a complex network of processes spanning from cell proliferation to cell death.Pharmacological inhibition of IP3R attenuates the proliferation or invasiveness of cancers, thus inducing necrotic cell death.Despite significant advancements, there is a tremendous need to design new potential hits to target IP3R, based upon 3D structural features and pharmacophoric patterns.Communicated by Ramaswamy H. Sarma.

A cellular atlas of calcineurin signaling

Intracellular Ca2+ signals are temporally controlled and spatially restricted. Signaling occurs adjacent to sites of Ca2+ entry and/or release, where Ca2+-dependent effectors and their substrates co-localize to form signaling microdomains. Here we review signaling by calcineurin, the Ca2+/calmodulin regulated protein phosphatase and target of immunosuppressant drugs, Cyclosporin A and FK506. Although well known for its activation of the adaptive immune response via NFAT dephosphorylation, systematic mapping of human calcineurin substrates and regulators reveals unexpected roles for this versatile phosphatase throughout the cell. We discuss calcineurin function, with an emphasis on where signaling occurs and mechanisms that target calcineurin and its substrates to signaling microdomains, especially binding of cognate short linear peptide motifs (SLiMs). Calcineurin is ubiquitously expressed and regulates events at the plasma membrane, other intracellular membranes, mitochondria, the nuclear pore complex and centrosomes/cilia. Based on our expanding knowledge of localized CN actions, we describe a cellular atlas of Ca2+/calcineurin signaling.

Oxidative Stress-Induced Unscheduled CDK1-Cyclin B1 Activity Impairs ER-Mitochondria-Mediated Bioenergetic Metabolism

Targeting the activities of endoplasmic reticulum (ER)-mitochondrial-dependent metabolic reprogramming is considered one of the most promising strategies for cancer treatment. Here, we present biochemical subcellular fractionation, coimmunoprecipitation, gene manipulation, and pharmacologic evidence that induction of mitochondria-localized phospho (p)-cyclin dependent kinase 1 (CDK1) (Thr 161)-cyclin B1 complexes by apigenin in nasopharyngeal carcinoma (NPC) cells impairs the ER-mitochondrial bioenergetics and redox regulation of calcium (Ca⁺⁺) homeostasis through suppressing the B cell lymphoma 2 (BCL-2)/BCL-2/B-cell lymphoma-extra large (BCL-x_L)-modulated anti-apoptotic and metabolic functions. Using a specific inducer, inhibitor, or short hairpin RNA for acid sphingomyelinase (ASM) demonstrated that enhanced lipid raft-associated ASM activity confers alteration of the lipid composition of lipid raft membranes, which leads to perturbation of protein trafficking, and induces formation of p110α free p85α-unphosphorylated phosphatase and tensin homolog deleted from chromosome 10 complexes in the lipid raft membranes, causing disruption of phosphatidylinositol 3-kinase (PI3K)-protein kinase B (Akt)-GTP-ras-related C3 botulinum toxin substrate 1 (Rac1)-mediated signaling, thus triggering the p-CDK1 (Thr 161))-cyclin B1-mediated BCL-2 (Thr 69/Ser 87)/BCL-x_L (Ser 62) phosphorylation and accompanying impairment of ER-mitochondria-regulated bioenergetic, redox, and Ca⁺⁺ homeostasis. Inhibition of apigenin-induced reactive oxygen species (ROS) generation by a ROS scavenger N-acetyl-L-cysteine blocked the lipid raft membrane localization and activation of ASM and formation of ceramide-enriched lipid raft membranes, returned PI3K-Akt-GTP-Rac1-modulated CDK1-cyclin B1 activity, and subsequently restored the BCL-2/BCL-x_L-regulated ER-mitochondrial bioenergetic activity. Thus, this study reveals a novel molecular mechanism of the pro-apoptotic activity of ASM controlled by oxidative stress to modulate the ER-mitochondrial bioenergetic metabolism, as well as suggests the disruption of CDK1-cyclin B1-mediated BCL-2/BCL-x_L oncogenic activity by triggering oxidative stress-ASM-induced PI3K-Akt-GTP-Rac1 inactivation as a therapeutic approach for NPC.

Biological Regulatory Network (BRN) Analysis and Molecular Docking Simulations to Probe the Modulation of IP3R Mediated Ca2+ Signaling in Cancer

Inositol trisphosphate receptor (IP3R) mediated Ca+2 signaling is essential in determining the cell fate by regulating numerous cellular processes, including cell division and cell death. Despite extensive studies about the characterization of IP3R in cancer, the underlying molecular mechanism initiating the cell proliferation and apoptosis remained enigmatic. Moreover, in cancer, the modulation of IP3R in downstream signaling pathways, which control oncogenesis and cancer progression, is not well characterized. Here, we constructed a biological regulatory network (BRN), and describe the remodeling of IP3R mediated Ca2+ signaling as a central key that controls the cellular processes in cancer. Moreover, we summarize how the inhibition of IP3R affects the deregulated cell proliferation and cell death in cancer cells and results in the initiation of pro-survival responses in resistance of cell death in normal cells. Further, we also investigated the role of stereo-specificity of IP3 molecule and its analogs in binding with the IP3 receptor. Molecular docking simulations showed that the hydroxyl group at R6 position along with the phosphate group at R5 position in 'R' conformation is more favorable for IP3 interactions. Additionally, Arg-266 and Arg-510 showed π-π and hydrogen bond interactions and Ser-278 forms hydrogen bond interactions with the IP3 binding site. Thus, they are identified as crucial for the binding of antagonists.

VDAC1 at the Intersection of Cell Metabolism, Apoptosis, and Diseases

The voltage-dependent anion channel 1 (VDAC1) protein, is an important regulator of mitochondrial function, and serves as a mitochondrial gatekeeper, with responsibility for cellular fate. In addition to control over energy sources and metabolism, the protein also regulates epigenomic elements and apoptosis via mediating the release of apoptotic proteins from the mitochondria. Apoptotic and pathological conditions, as well as certain viruses, induce cell death by inducing VDAC1 overexpression leading to oligomerization, and the formation of a large channel within the VDAC1 homo-oligomer. This then permits the release of pro-apoptotic proteins from the mitochondria and subsequent apoptosis. Mitochondrial DNA can also be released through this channel, which triggers type-Ι interferon responses. VDAC1 also participates in endoplasmic reticulum (ER)-mitochondria cross-talk, and in the regulation of autophagy, and inflammation. Its location in the outer mitochondrial membrane, makes VDAC1 ideally placed to interact with over 100 proteins, and to orchestrate the interaction of mitochondrial and cellular activities through a number of signaling pathways. Here, we provide insights into the multiple functions of VDAC1 and describe its involvement in several diseases, which demonstrate the potential of this protein as a druggable target in a wide variety of pathologies, including cancer.

A robust TDP-43 knock-in mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal, adult-onset degenerative disorder of motor neurons. The diseased spinal cord motor neurons of more than 95% of amyotrophic lateral sclerosis (ALS) patients are characterized by the mis-metabolism of the RNA/DNA-binding protein TDP-43 (ALS-TDP), in particular, the presence of cytosolic aggregates of the protein. Most available mouse models for the basic or translational studies of ALS-TDP are based on transgenic overexpression of the TDP-43 protein. Here, we report the generation and characterization of mouse lines bearing homologous knock-in of fALS-associated mutation A315T and sALS-associated mutation N390D, respectively. Remarkably, the heterozygous TDP-43 (N390D/+) mice but not those heterozygous for the TDP-43 (A315T/+) mice develop a full spectrum of ALS-TDP-like pathologies at the molecular, cellular and behavioral levels. Comparative analysis of the mutant mice and spinal cord motor neurons (MN) derived from their embryonic stem (ES) cells demonstrates that different ALS-associated TDP-43 mutations possess critical ALS-causing capabilities and pathogenic pathways, likely modified by their genetic background and the environmental factors. Mechanistically, we identify aberrant RNA splicing of spinal cord Bcl-2 pre-mRNA and consequent increase of a negative regulator of autophagy, Bcl-2, which correlate with and are caused by a progressive increase of TDP-43, one of the early events associated with ALS-TDP pathogenesis, in the spinal cord of TDP-43 (N390D/+) mice and spinal cord MN derived from their ES cells. The TDP-43 (N390D/+) knock-in mice appear to be an ideal rodent model for basic as well as translational studies of ALS- TDP.

Creating a New Cancer Therapeutic Agent by Targeting the Interaction between Bcl-2 and IP3 Receptors

Bcl-2 is a member of a family of proteins that regulate cell survival. Expression of Bcl-2 is aberrantly elevated in many types of cancer. Within cells of the immune system, Bcl-2 has a physiological role in regulating immune responses. However, in cancers arising from cells of the immune system Bcl-2 promotes cell survival and proliferation. This review summarizes discoveries over the past 30 years that have elucidated Bcl-2's role in the normal immune system, including its actions in regulating calcium (Ca²⁺) signals necessary for the immune response, and for Ca²⁺-mediated apoptosis at the end of an immune response. How Bcl-2 modulates the release of Ca²⁺ from intracellular stores via inositol 1,4,5-trisphosphate receptors (IP₃R) is discussed, and in particular, the role of Bcl-2/IP₃R interactions in promoting the survival of cancer cells by preventing Ca²⁺-mediated cell death. The development and usage of a peptide, referred to as TAT-Pep8, or more recently, BIRD-2, that induces death of cancer cells by inhibiting Bcl-2's control over IP₃R-mediated Ca²⁺ elevation is discussed. Studies aimed at discovering a small molecule that mimics BIRD-2's anticancer mechanism of action are summarized, along with the prospect of such a compound becoming a novel therapeutic option for cancer.

Effects of vitamin D supplementation on depressive symptoms in type 2 diabetes mellitus patients: Randomized placebo-controlled double-blind clinical trial

AIM

Diabetes increases the odds of depression and depression is often associated with poor glycemic control and complications of diabetes. Vitamin D is also believed to improve glycemic control and ameliorate depressive symptoms. Therefore, we examined effects of vitamin D monotherapy (without antidepressant drugs) on depressive symptoms in Type 2 diabetic patients with mild to moderate depressive symptoms.

METHODS

We conducted 12 weeks, placebo-controlled, double-blind, randomized trial on 68 subjects with T2DM and mild to moderate depressive symptoms. Subjects received 100 μg (4000 IU) vitamin D (n = 32) or placebo (n = 34) daily. Beck Depression Inventory-II (BDI-II-PERSIAN) was applied for assessment of the severity of depression. Depression scores and metabolic profiles were measured at the beginning and end of trail.

RESULTS

after 3 months of vitamin D supplementation, mean values of 25(OH) D increased from 15.5 ± 8.8 to 32.2 ± 8.9 ng/ml (p-value <0.001) in the vitamin D group. Moreover, BDI-II scores decreased from 15.2 ± 9.6 to 9.8 ± 7.2 (p-value <0.001) in the vitamin D group and 15.5 ± 11.2 to 13.7 ± 11.5 (p-value = 0.03) in placebo group. This decrease in BDI-II scores were significant (27.6% vs 10.8%) compared with placebo (p-value = 0.02). In term of metabolic profiles, mean change in level of Hemoglobin A1c (HbA1c), insulin and triglycerides (TG) were significantly higher in response to the treatment with vitamin D compared to placebo (p-value <0.02).

CONCLUSIONS

In conclusion, supplementation of vitamin D in T2DM patients may protect these patients against the onset of major depressive disorder (MDD), with noticeable favorable effects on measures of metabolic profiles.

TRIAL REGISTRATION

NCT03008057.

Bok regulates mitochondrial fusion and morphology

Bok (Bcl-2-related ovarian killer) is a member of the Bcl-2 protein family that governs the intrinsic apoptosis pathway, but the cellular role that Bok plays is controversial. Remarkably, endogenous Bok is constitutively bound to inositol 1,4,5-trisphosphate receptors (IP₃Rs) and is stabilized by this interaction. Here we report that despite the strong association with IP₃Rs, deletion of Bok expression by CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/CRISPR-associated protein-9 nuclease)-mediated gene editing does not alter calcium mobilization via IP₃Rs or calcium influx into the mitochondria. Rather, Bok deletion significantly reduces mitochondrial fusion rate, resulting in mitochondrial fragmentation. This phenotype is reversed by exogenous wild-type Bok and by an IP₃R binding-deficient Bok mutant, and may result from a decrease in mitochondrial motility. Bok deletion also enhances mitochondrial spare respiratory capacity and membrane potential. Finally, Bok does not play a major role in apoptotic signaling, since Bok deletion does not alter responsiveness to various apoptotic stimuli. Overall, despite binding to IP₃Rs, Bok does not alter IP₃R-mediated Ca²⁺ signaling, but is required to maintain normal mitochondrial fusion, morphology, and bioenergetics.

IP3 Receptor Properties and Function at Membrane Contact Sites

The inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) is a ubiquitously expressed Ca²⁺-release channel localized in the endoplasmic reticulum (ER). The intracellular Ca²⁺ signals originating from the activation of the IP₃R regulate multiple cellular processes including the control of cell death versus cell survival via their action on apoptosis and autophagy. The exact role of the IP₃Rs in these two processes does not only depend on their activity, which is modulated by the cytosolic composition (Ca²⁺, ATP, redox status, …) and by various types of regulatory proteins, including kinases and phosphatases as well as by a number of oncogenes and tumor suppressors, but also on their intracellular localization, especially at the ER-mitochondrial and ER-lysosomal interfaces. At these interfaces, Ca²⁺ microdomains are formed, in which the Ca²⁺ concentration is finely regulated by the different ER, mitochondrial and lysosomal Ca²⁺-transport systems and also depends on the functional and structural interactions existing between them. In this review, we therefore discuss the most recent insights in the role of Ca²⁺ signaling in general, and of the IP₃R in particular, in the control of basal mitochondrial bioenergetics, apoptosis, and autophagy at the level of inter-organellar contact sites.

Bcl-2 regulates store-operated Ca2+ entry to modulate ER stress-induced apoptosis

Ca²⁺ plays a significant role in linking the induction of apoptosis. The key anti-apoptotic protein, Bcl-2, has been reported to regulate the movement of Ca²⁺ across the ER membrane, but the exact effect of Bcl-2 on Ca²⁺ levels remains controversial. Store-operated Ca²⁺ entry (SOCE), a major mode of Ca²⁺ uptake in non-excitable cells, is activated by depletion of Ca²⁺ in the ER. Depletion of Ca²⁺ in the ER causes translocation of the SOC channel activator, STIM1, to the plasma membrane. Thereafter, STIM1 binds to Orai1 or/and TRPC1 channels, forcing them to open and thereby allow Ca²⁺ entry. In addition, several anti-cancer drugs have been reported to induce apoptosis of cancer cells via the SOCE pathway. However, the detailed mechanism underlying the regulation of SOCE by Bcl-2 is not well understood. In this study, a three-amino acid mutation within the Bcl-2 BH1 domain was generated to verify the role of Bcl-2 in Ca²⁺ handling during ER stress. The subcellular localization of the Bcl-2 mutant (mt) is similar to that in the wild-type Bcl-2 (WT) in the ER and mitochondria. We found that mt enhanced thapsigargin and tunicamycin-induced apoptosis through ER stress-mediated apoptosis but not through the death receptor- and mitochondria-dependent apoptosis, while WT prevented thapsigargin- and tunicamycin-induced apoptosis. In addition, mt depleted Ca²⁺ in the ER lumen and also increased the expression of SOCE-related molecules. Therefore, a massive Ca²⁺ influx via SOCE contributed to caspase activation and apoptosis. Furthermore, inhibiting SOCE or chelating either extracellular or intracellular Ca²⁺ inhibited mt-mediated apoptosis. In brief, our results explored the critical role of Bcl-2 in Ca²⁺ homeostasis and the modulation of ER stress.

A double point mutation at residues Ile14 and Val15 of Bcl-2 uncovers a role for the BH4 domain in both protein stability and function

B-cell lymphoma 2 (Bcl-2) protein is the archetype apoptosis suppressor protein. The N-terminal Bcl-2-homology 4 (BH4) domain of Bcl-2 is required for the antiapoptotic function of this protein at the mitochondria and endoplasmic reticulum (ER). The involvement of the BH4 domain in Bcl-2's antiapoptotic functions has been proposed based on Gly-based substitutions of the Ile14/Val15 amino acids, two hydrophobic residues located in the center of Bcl-2's BH4 domain. Following this strategy, we recently showed that a BH4-domain-derived peptide in which Ile14 and Val15 have been replaced by Gly residues, was unable to dampen proapoptotic Ca²⁺ -release events from the ER. Here, we investigated the impact of these mutations on the overall structure, stability, and function of full-length Bcl-2 as a regulator of Ca²⁺ signaling and cell death. Our results indicate that full-length Bcl-2 Ile14Gly/Val15Gly, in contrast to wild-type Bcl-2, (a) displayed severely reduced structural stability and a shortened protein half-life; (b) failed to interact with Bcl-2-associated X protein (BAX), to inhibit the inositol 1,4,5-trisphosphate receptor (IP₃ R) and to protect against Ca²⁺ -mediated apoptosis. We conclude that the hydrophobic face of Bcl-2's BH4 domain (Ile14, Val15) is an important structural regulatory element by affecting protein stability and turnover, thereby likely reducing Bcl-2's ability to modulate the function of its targets, like IP₃ R and BAX. Therefore, Bcl-2 structure/function studies require pre-emptive and reliable determination of protein stability upon introduction of point mutations at the level of the BH4 domain.

Apoptosis as anticancer mechanism: function and dysfunction of its modulators and targeted therapeutic strategies

Apoptosis is a form of programmed cell death that results in the orderly and efficient removal of damaged cells, such as those resulting from DNA damage or during development. Apoptosis can be triggered by signals from within the cell, such as genotoxic stress, or by extrinsic signals, such as the binding of ligands to cell surface death receptors. Deregulation in apoptotic cell death machinery is an hallmark of cancer. Apoptosis alteration is responsible not only for tumor development and progression but also for tumor resistance to therapies. Most anticancer drugs currently used in clinical oncology exploit the intact apoptotic signaling pathways to trigger cancer cell death. Thus, defects in the death pathways may result in drug resistance so limiting the efficacy of therapies. Therefore, a better understanding of the apoptotic cell death signaling pathways may improve the efficacy of cancer therapy and bypass resistance. This review will highlight the role of the fundamental regulators of apoptosis and how their deregulation, including activation of anti-apoptotic factors (i.e., Bcl-2, Bcl-xL, etc) or inactivation of pro-apoptotic factors (i.e., p53 pathway) ends up in cancer cell resistance to therapies. In addition, therapeutic strategies aimed at modulating apoptotic activity are briefly discussed.

Vitamin D deficiency accelerates ageing and age-related diseases: a novel hypothesis

Ageing can occur at different rates, but what controls this variable rate is unknown. Here I have developed a hypothesis that vitamin D may act to control the rate of ageing. The basis of this hypothesis emerged from studyng the various cellular processes that control ageing. These processes such as autophagy, mitochondrial dysfunction, inflammation, oxidative stress, epigenetic changes, DNA disorders and alterations in Ca²⁺ and reactive oxygen species (ROS) signalling are all known to be regulated by vitamin D. The activity of these processes will be enhanced in individuals that are deficient in vitamin D. Not only will this increase the rate of ageing, but it will also increase the probability of developing age-related diseases such as Alzheimer's disease, Parkinson's disease, multiple sclerosis and cardiovascular disease. In individual with normal vitamin D levels, these ageing-related processes will occur at lower rates resulting in a reduced rate of ageing and enhanced protection against these age-related diseases.

Paclitaxel Reduces Axonal Bclw to Initiate IP3R1-Dependent Axon Degeneration

Chemotherapy-induced peripheral neuropathy (CIPN) is a debilitating side effect of many cancer treatments. The hallmark of CIPN is degeneration of long axons required for transmission of sensory information; axonal degeneration causes impaired tactile sensation and persistent pain. Currently the molecular mechanisms of CIPN are not understood, and there are no available treatments. Here we show that the chemotherapeutic agent paclitaxel triggers CIPN by altering IP₃ receptor phosphorylation and intracellular calcium flux, and activating calcium-dependent calpain proteases. Concomitantly paclitaxel impairs axonal trafficking of RNA-granules and reduces synthesis of Bclw (bcl2l2), a Bcl2 family member that binds IP₃R1 and restrains axon degeneration. Surprisingly, Bclw or a stapled peptide corresponding to the Bclw BH4 domain interact with axonal IP₃R1 and prevent paclitaxel-induced degeneration, while Bcl2 and Bclx_L cannot do so. Together these data identify a Bclw-IP₃R1-dependent cascade that causes axon degeneration and suggest that Bclw-mimetics could provide effective therapy to prevent CIPN.

Non-genetic mechanisms of diabetic nephropathy

Diabetic nephropathy (DN) is one of the most common microvascular complications in diabetes mellitus patients and is characterized by thickened glomerular basement membrane, increased extracellular matrix formation, and podocyte loss. These phenomena lead to proteinuria and altered glomerular filtration rate, that is, the rate initially increases but progressively decreases. DN has become the leading cause of end-stage renal disease. Its prevalence shows a rapid growth trend and causes heavy social and economic burden in many countries. However, this disease is multifactorial, and its mechanism is poorly understood due to the complex pathogenesis of DN. In this review, we highlight the new molecular insights about the pathogenesis of DN from the aspects of immune inflammation response, epithelial-mesenchymal transition, apoptosis and mitochondrial damage, epigenetics, and podocyte-endothelial communication. This work offers groundwork for understanding the initiation and progression of DN, as well as provides ideas for developing new prevention and treatment measures.

Modulation of Ca2+ Signaling by Anti-apoptotic B-Cell Lymphoma 2 Proteins at the Endoplasmic Reticulum-Mitochondrial Interface

Mitochondria are important regulators of cell death and cell survival. Mitochondrial Ca²⁺ levels are critically involved in both of these processes. On the one hand, excessive mitochondrial Ca²⁺ leads to Ca²⁺-induced mitochondrial outer membrane permeabilization and thus apoptosis. On the other hand, mitochondria need Ca²⁺ in order to efficiently fuel the tricarboxylic acid cycle and maintain adequate mitochondrial bioenergetics. For obtaining this Ca²⁺, the mitochondria are largely dependent on close contact sites with the endoplasmic reticulum (ER), the so-called mitochondria-associated ER membranes. There, the inositol 1,4,5-trisphosphate receptors are responsible for the Ca²⁺ release from the ER. It comes as no surprise that this Ca²⁺ release from the ER and the subsequent Ca²⁺ uptake at the mitochondria are finely regulated. Cancer cells often modulate ER-Ca²⁺ transfer to the mitochondria in order to promote cell survival and to inhibit cell death. Important regulators of these Ca²⁺ signals and the onset of cancer are the B-cell lymphoma 2 (Bcl-2) family of proteins. An increasing number of reports highlight the ability of these Bcl-2-protein family members to finely regulate Ca²⁺ transfer from ER to mitochondria both in healthy cells and in cancer. In this review, we focus on recent insights into the dynamic regulation of ER-mitochondrial Ca²⁺ fluxes by Bcl-2-family members and how this impacts cell survival, cell death and mitochondrial energy production.

Vitamin D deficiency and diabetes

Vitamin D deficiency has been linked to the onset of diabetes. This review summarizes the role of Vitamin D in maintaining the normal release of insulin by the pancreatic beta cells (β-cells). Diabetes is initiated by the onset of insulin resistance. The β-cells can overcome this resistance by releasing more insulin, thus preventing hyperglycaemia. However, as this hyperactivity increases, the β-cells experience excessive Ca2+ and reactive oxygen species (ROS) signalling that results in cell death and the onset of diabetes. Vitamin D deficiency contributes to both the initial insulin resistance and the subsequent onset of diabetes caused by β-cell death. Vitamin D acts to reduce inflammation, which is a major process in inducing insulin resistance. Vitamin D maintains the normal resting levels of both Ca2+ and ROS that are elevated in the β-cells during diabetes. Vitamin D also has a very significant role in maintaining the epigenome. Epigenetic alterations are a feature of diabetes by which many diabetes-related genes are inactivated by hypermethylation. Vitamin D acts to prevent such hypermethylation by increasing the expression of the DNA demethylases that prevent hypermethylation of multiple gene promoter regions of many diabetes-related genes. What is remarkable is just how many cellular processes are maintained by Vitamin D. When Vitamin D is deficient, many of these processes begin to decline and this sets the stage for the onset of diseases such as diabetes.

Potential Roles of Mitochondria-Associated ER Membranes (MAMs) in Traumatic Brain Injury

The endoplasmic reticulum (ER) and mitochondria have both been shown to be critical in cellular homeostasis. The functions of the ER and mitochondria are independent but interrelated. These two organelles could form physical interactions, known as MAMs, to regulate physiological functions between ER and mitochondria to maintain Ca²⁺, lipid, and metabolite exchange. Several proteins are located in MAMs, including RNA-dependent protein kinase (PKR)-like ER kinase, inositol 1,4,5-trisphosphate receptors, phosphofurin acidic cluster sorting protein-2 and sigma-1 receptor to ensure regulation. Recent studies indicated that MAMs participate in inflammation and apoptosis in various conditions. All of these functions are crucial in determining cell fate following traumatic brain injury (TBI). We hypothesized that MAMs may associate with TBI and could contribute to mitochondrial dysfunction, ER stress, autophagy dysregulation, dysregulation of Ca²⁺ homeostasis, and oxidative stress. In this review, we summarize the latest understanding of MAM formation and their potential regulatory role in TBI pathophysiology.

The Inositol Trisphosphate/Calcium Signaling Pathway in Health and Disease

Many cellular functions are regulated by calcium (Ca2+) signals that are generated by different signaling pathways. One of these is the inositol 1,4,5-trisphosphate/calcium (InsP3/Ca2+) signaling pathway that operates through either primary or modulatory mechanisms. In its primary role, it generates the Ca2+ that acts directly to control processes such as metabolism, secretion, fertilization, proliferation, and smooth muscle contraction. Its modulatory role occurs in excitable cells where it modulates the primary Ca2+ signal generated by the entry of Ca2+ through voltage-operated channels that releases Ca2+ from ryanodine receptors (RYRs) on the internal stores. In carrying out this modulatory role, the InsP3/Ca2+ signaling pathway induces subtle changes in the generation and function of the voltage-dependent primary Ca2+ signal. Changes in the nature of both the primary and modulatory roles of InsP3/Ca2+ signaling are a contributory factor responsible for the onset of a large number human diseases.

Intracellular Ca(2+) signaling and Ca(2+) microdomains in the control of cell survival, apoptosis and autophagy

The endoplasmic reticulum (ER), mitochondria and lysosomes are physically and/or functionally linked, establishing close contact sites between these organelles. As a consequence, Ca(2+) release events from the ER, the major intracellular Ca(2+)-storage organelle, have an immediate effect on the physiological function of mitochondria and lysosomes. Also, the lysosomes can act as a Ca(2+) source for Ca(2+) release into the cytosol, thereby influencing ER-based Ca(2+) signaling. Given the important role for mitochondria and lysosomes in cell survival, cell death and cell adaptation processes, it has become increasingly clear that Ca(2+) signals from or towards these organelles impact these processes. In this review, we discuss the most recent insights in the emerging role of Ca(2+) signaling in cellular survival by controlling basal mitochondrial bioenergetics and by regulating apoptosis, a mitochondrial process, and autophagy, a lysosomal process, in response to cell damage and cell stress.

The Stability and Expression Level of Bok Are Governed by Binding to Inositol 1,4,5-Trisphosphate Receptors

Bok is a member of the Bcl-2 protein family that governs the intrinsic apoptosis pathway, although the role that Bok plays in this pathway is unclear. We have shown previously in cultured cell lines that Bok interacts strongly with inositol 1,4,5-trisphosphate receptors (IP3Rs), suggesting that it may contribute to the structural integrity or stability of IP3R tetramers. Here we report that Bok is similarly IP3R-assocated in mouse tissues, that essentially all cellular Bok is IP3R bound, that it is the helical nature of the Bok BH4 domain, rather than specific amino acids, that mediates binding to IP3Rs, that Bok is dramatically stabilized by binding to IP3Rs, that unbound Bok is ubiquitinated and degraded by the proteasome, and that binding to IP3Rs limits the pro-apoptotic effect of overexpressed Bok. Agents that stimulate IP3R activity, apoptosis, phosphorylation, and endoplasmic reticulum stress did not trigger the dissociation of mature Bok from IP3Rs or Bok degradation, indicating that the role of proteasome-mediated Bok degradation is to destroy newly synthesized Bok that is not IP3R associated. The existence of this unexpected proteolytic mechanism that is geared toward restricting Bok to that which is bound to IP3Rs, implies that unbound Bok is deleterious to cell viability and helps explain the current uncertainty regarding the cellular role of Bok.

PACS-2 mediates the ATM and NF-κB-dependent induction of anti-apoptotic Bcl-xL in response to DNA damage

Nuclear factor kappa B (NF-κB) promotes cell survival in response to genotoxic stress by inducing the expression of anti-apoptotic proteins including Bcl-xL, which protects mitochondria from stress-induced mitochondrial outer membrane permeabilization (MOMP). Here we show that the multifunctional sorting protein Pacs-2 (phosphofurin acidic cluster sorting protein-2) is required for Bcl-xL induction following DNA damage in primary mouse thymocytes. Consequently, in response to DNA damage, Pacs-2(-/-) thymocytes exhibit a blunted induction of Bcl-xL, increased MOMP and accelerated apoptosis. Biochemical studies show that cytoplasmic PACS-2 promotes this DNA damage-induced anti-apoptotic pathway by interacting with ataxia telangiectasia mutated (ATM) to drive NF-κB activation and induction of Bcl-xL. However, Pacs-2 was not required for tumor necrosis factor-α-induced NF-κB activation, suggesting a role for PACS-2 selectively in NF-κB activation in response to DNA damage. These findings identify PACS-2 as an in vivo mediator of the ATM and NF-κB-dependent induction of Bcl-xL that promotes cell survival in response to DNA damage.

How Shigella Utilizes Ca(2+) Jagged Edge Signals during Invasion of Epithelial Cells

Shigella, the causative agent of bacillary dysentery invades intestinal epithelial cells using a type III secretion system (T3SS). Through the injection of type III effectors, Shigella manipulates the actin cytoskeleton to induce its internalization in epithelial cells. At early invasion stages, Shigella induces atypical Ca(2+) responses confined at entry sites allowing local cytoskeletal remodeling for bacteria engulfment. Global Ca(2+) increase in the cell triggers the opening of connexin hemichannels at the plasma membrane that releases ATP in the extracellular milieu, favoring Shigella invasion and spreading through purinergic receptor signaling. During intracellular replication, Shigella regulates inflammatory and death pathways to disseminate within the epithelium. At later stages of infection, Shigella downregulates hemichannel opening and the release of extracellular ATP to dampen inflammatory signals. To avoid premature cell death, Shigella activates cell survival by upregulating the PI3K/Akt pathway and downregulating the levels of p53. Furthermore, Shigella interferes with pro-apoptotic caspases, and orients infected cells toward a slow necrotic cell death linked to mitochondrial Ca(2+) overload. In this review, we will focus on the role of Ca(2+) responses and their regulation by Shigella during the different stages of bacterial infection.

Protective effect of trans-δ-viniferin against high glucose-induced oxidative stress in human umbilical vein endothelial cells through the SIRT1 pathway

Oxidative stress plays a critical role in the pathogenesis of diabetic vascular complications. Trans-δ-viniferin (TVN), a polyphenolic compound, has recently attracted much attention as an antioxidant exhibiting a hypoglycemic potential. In the present study, we aimed at investigating the protective effect of TVN against high glucose-induced oxidative stress in human umbilical vein endothelial cells (HUVECs) and the potential mechanism involved. We found that TVN attenuated reactive oxygen species (ROS) production, increased catalase (CAT) activity and decreased malondialdehyde (MDA) levels to ameliorate cell survival induced by 35 mM glucose. Meanwhile, it inhibited high glucose-induced apoptosis by maintaining Ca(2+) and preserving mitochondrial membrane potential (MMP) levels. The immunoblot analysis indicated that TVN efficiently regulated the cleavage of caspase family, p53, Bax and Bcl-2, all mediated by SIRT1. Furthermore, the increased level of SIRT1 induced by TVN was inhibited by nicotinamide and siRNA-medicated SIRT1 silencing (si-SIRT1), thereby confirming the significant role of SIRT1 in these events. In conclusion, our results indicated that TVN efficiently reduced oxidative stress and maintained mitochondrial function related with activating SIRT1 in high glucose-treated HUVECs. It suggested that TVN is pharmacologically promising for treating diabetic cardiovascular complications.

Beneficial effects of dantrolene on sepsis-induced diaphragmatic dysfunction are associated with downregulation of high-mobility group box 1 and calpain-caspase-3 proteolytic pathway

BACKGROUND

Intracellular calcium overload is a major contributing factor to diaphragmatic dysfunction triggered by sepsis. In this study, the possible role of dantrolene, a ryanodine receptor inhibitor, in preventing the release of calcium from the sarcoplasmic reticulum in diaphragmatic dysfunction and weakness was explored.

METHODS

A middle-grade severity sepsis rat model was established for the effects of treatment with dantrolene, on diaphragm harvested 24 h after cecal ligation and puncture (CLP), and analyzed using functional, histologic, and biomarker assays.

RESULTS

It was found that in septic rats, treatment with dantrolene significantly improved the contractility, relaxation, and fatigue index of the diaphragm in a dose-dependent manner. The benefits are associated with improvement in ultrastructural changes of Z band integrity and myofilament arrangements along with increases both in the ratio of slow-twitch type composition. Moreover, dantrolene effectively inhibits the overexpression of high-mobility group box 1 and reduces the calpain-1-caspase-3 proteolytic activity.

CONCLUSIONS

Dantrolene can effectively attenuate the dysfunction of diaphragm in septic rats; Furthermore, the beneficial effects were associated with downregulation of high-mobility group box 1 and calpain-1-caspase-3 proteolytic activity.

Vertebrate Reproduction

Vertebrate reproduction requires a myriad of precisely orchestrated events-in particular, the maternal production of oocytes, the paternal production of sperm, successful fertilization, and initiation of early embryonic cell divisions. These processes are governed by a host of signaling pathways. Protein kinase and phosphatase signaling pathways involving Mos, CDK1, RSK, and PP2A regulate meiosis during maturation of the oocyte. Steroid signals-specifically testosterone-regulate spermatogenesis, as does signaling by G-protein-coupled hormone receptors. Finally, calcium signaling is essential for both sperm motility and fertilization. Altogether, this signaling symphony ensures the production of viable offspring, offering a chance of genetic immortality.

Bcl-2 associated with severity of manic symptoms in bipolar patients in a manic phase

B cell lymphoma protein-2 (Bcl-2) may contribute to the pathophysiology of bipolar disorder, and may be involved in the therapeutic action of anti-manic drugs. The aim of this study was to investigate serum levels of Bcl-2 in bipolar patients in a manic phase, and evaluate the Bcl-2 changes after treatment. We consecutively enrolled 23 bipolar inpatients in a manic phase and 40 healthy subjects; 20 bipolar patients were followed up with treatment. Serum Bcl-2 levels were measured with assay kits. All 20 patients were evaluated by examining the correlation between Bcl-2 levels and Young Mania Rating Scale (YMRS) scores, using Spearman׳s correlation coefficients. The serum Bcl-2 levels in bipolar patients in a manic phase were higher than in healthy subjects, but without a significant difference. The YMRS scores were significantly negatively associated with serum Bcl-2 levels (p=0.042). Bcl-2 levels of the 20 bipolar patients were measured at the end of treatment. Using the Wilcoxon Signed Rank test, we found no significant difference in the Bcl-2 levels of bipolar patients after treatment. Our results suggest that Bcl-2 levels might be an indicator of severity of manic symptoms in bipolar patients in a manic phase.

The BH4 domain of anti-apoptotic Bcl-XL, but not that of the related Bcl-2, limits the voltage-dependent anion channel 1 (VDAC1)-mediated transfer of pro-apoptotic Ca2+ signals to mitochondria

Excessive Ca(2+) fluxes from the endoplasmic reticulum to the mitochondria result in apoptotic cell death. Bcl-2 and Bcl-XL proteins exert part of their anti-apoptotic function by directly targeting Ca(2+)-transport systems, like the endoplasmic reticulum-localized inositol 1,4,5-trisphosphate receptors (IP3Rs) and the voltage-dependent anion channel 1 (VDAC1) at the outer mitochondrial membranes. We previously demonstrated that the Bcl-2 homology 4 (BH4) domain of Bcl-2 protects against Ca(2+)-dependent apoptosis by binding and inhibiting IP3Rs, although the BH4 domain of Bcl-XL was protective independently of binding IP3Rs. Here, we report that in contrast to the BH4 domain of Bcl-2, the BH4 domain of Bcl-XL binds and inhibits VDAC1. In intact cells, delivery of the BH4-Bcl-XL peptide via electroporation limits agonist-induced mitochondrial Ca(2+) uptake and protects against staurosporine-induced apoptosis, in line with the results obtained with VDAC1(-/-) cells. Moreover, the delivery of the N-terminal domain of VDAC1 as a synthetic peptide (VDAC1-NP) abolishes the ability of BH4-Bcl-XL to suppress mitochondrial Ca(2+) uptake and to protect against apoptosis. Importantly, VDAC1-NP did not affect the ability of BH4-Bcl-2 to suppress agonist-induced Ca(2+) release in the cytosol or to prevent apoptosis, as done instead by an IP3R-derived peptide. In conclusion, our data indicate that the BH4 domain of Bcl-XL, but not that of Bcl-2, selectively targets VDAC1 and inhibits apoptosis by decreasing VDAC1-mediated Ca(2+) uptake into the mitochondria.

The mystery of BCL2 family: Bcl-2 proteins and apoptosis: an update

Apoptosis is a critically important biological process that plays an essential role in cell fate and homeostasis. An important component of the apoptotic pathway is the family of proteins commonly known as the B cell lymphoma-2 (Bcl-2). The primary role of Bcl-2 family members is the regulation of apoptosis. Although the structure of Bcl-2 family of proteins was reported nearly 10 years ago, however, it still surprises us with its structural and functional complexity and diversity. A number of studies have demonstrated that Bcl-2 family influences many other cellular processes beyond apoptosis which are generally independent of the regulation of apoptosis, suggesting additional roles for Bcl-2. The disruption of the regulation of apoptosis is a causative event in many diseases. Since the Bcl-2 family of proteins is the key regulator of apoptosis, the abnormalities in its function have been implicated in many diseases including cancer, neurodegenerative disorders, ischemia and autoimmune diseases. In the past few years, our understanding of the mechanism of action of Bcl-2 family of proteins and its implications in various pathological conditions has enhanced significantly. The focus of this review is to summarize the current knowledge on the structure and function of Bcl-2 family of proteins in apoptotic cellular processes. A number of drugs have been developed in the past few years that target different Bcl-2 members. The role of Bcl-2 proteins in the pathogenesis of various diseases and their pharmacological significance as effective molecular therapeutic targets is also discussed.

The type 2 inositol 1,4,5-trisphosphate receptor, emerging functions for an intriguing Ca²⁺-release channel

The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) type 2 (IP3R2) is an intracellular Ca²⁺-release channel located on the endoplasmic reticulum (ER). IP3R2 is characterized by a high sensitivity to both IP3 and ATP and is biphasically regulated by Ca²⁺. Furthermore, IP3R2 is modulated by various protein kinases. In addition to its regulation by protein kinase A, IP3R2 forms a complex with adenylate cyclase 6 and is directly regulated by cAMP. Finally, in the ER, IP3R2 is less mobile than the other IP3R isoforms, while its functional properties appear dominant in heterotetramers. These properties make the IP3R2 a Ca²⁺ channel with exquisite properties for setting up intracellular Ca²⁺ signals with unique characteristics. IP3R2 plays a crucial role in the function of secretory cell types (e.g. pancreatic acinar cells, hepatocytes, salivary gland, eccrine sweat gland). In cardiac myocytes, the role of IP3R2 appears more complex, because, together with IP3R1, it is needed for normal cardiogenesis, while its aberrant activity is implicated in cardiac hypertrophy and arrhythmias. Most importantly, its high sensitivity to IP3 makes IP3R2 a target for anti-apoptotic proteins (e.g. Bcl-2) in B-cell cancers. Disrupting IP3R/Bcl-2 interaction therefore leads in those cells to increased Ca²⁺ release and apoptosis. Intriguingly, IP3R2 is not only implicated in apoptosis but also in the induction of senescence, another tumour-suppressive mechanism. These results were the first to unravel the physiological and pathophysiological role of IP3R2 and we anticipate that further progress will soon be made in understanding the function of IP3R2 in various tissues and organs.

The role of intrinsic apoptotic signaling in hemorrhagic shock-induced microvascular endothelial cell barrier dysfunction

Hemorrhagic shock leads to endothelial cell barrier dysfunction resulting in microvascular hyperpermeability. Hemorrhagic shock-induced microvascular hyperpermeability is associated with worse clinical outcomes in patients with traumatic injuries. The results from our laboratory have illustrated a possible pathophysiological mechanism showing involvement of mitochondria-mediated "intrinsic" apoptotic signaling in regulating hemorrhagic shock-induced microvascular hyperpermeability. Hemorrhagic shock results in overexpression of Bcl-2 family of pro-apoptotic protein, BAK, in the microvascular endothelial cells. The increase in BAK initiates "intrinsic" apoptotic signaling cascade with the release of mitochondrial cytochrome c in the cytoplasm and activation of downstream effector caspase-3, leading to loss of endothelial cell barrier integrity. Thus, this review article offers a brief overview of important findings from our past and present research work along with new leads for future research. The summary of our research work will provide information leading to different avenues in developing novel strategies against microvascular hyperpermeability following hemorrhagic shock.

Apoptosis in differentiating C2C12 muscle cells selectively targets Bcl-2-deficient myotubes

Muscle cell apoptosis accompanies normal muscle development and regeneration, as well as degenerative diseases and aging. C2C12 murine myoblast cells represent a common model to study muscle differentiation. Though it was already shown that myogenic differentiation of C2C12 cells is accompanied by enhanced apoptosis in a fraction of cells, either the cell population sensitive to apoptosis or regulatory mechanisms for the apoptotic response are unclear so far. In the current study we characterize apoptotic phenotypes of different types of C2C12 cells at all stages of differentiation, and report here that myotubes of differentiated C2C12 cells with low levels of anti-apoptotic Bcl-2 expression are particularly vulnerable to apoptosis even though they are displaying low levels of pro-apoptotic proteins Bax, Bak and Bad. In contrast, reserve cells exhibit higher levels of Bcl-2 and high resistance to apoptosis. The transfection of proliferating myoblasts with Bcl-2 prior to differentiation did not protect against spontaneous apoptosis accompanying differentiation of C2C12 cells but led to Bcl-2 overexpression in myotubes and to significant protection from apoptotic cell loss caused by exposure to hydrogen peroxide. Overall, our data advocate for a Bcl-2-dependent mechanism of apoptosis in differentiated muscle cells. However, downstream processes for spontaneous and hydrogen peroxide induced apoptosis are not completely similar. Apoptosis in differentiating myoblasts and myotubes is regulated not through interaction of Bcl-2 with pro-apoptotic Bcl-2 family proteins such as Bax, Bak, and Bad.

The BCL2L1 and PGAM5 axis defines hypoxia-induced receptor-mediated mitophagy

Receptor-mediated mitophagy is one of the major mechanisms of mitochondrial quality control essential for cell survival. We previously have identified FUNDC1 as a mitophagy receptor for selectively removing damaged mitochondria in mammalian systems. A critical unanswered question is how receptor-mediated mitophagy is regulated in response to cellular and environmental cues. Here, we report the striking finding that BCL2L1/Bcl-xL, but not BCL2, suppresses mitophagy mediated by FUNDC1 through its BH3 domain. Mechanistically, we demonstrate that BCL2L1, but not BCL2, interacts with and inhibits PGAM5, a mitochondrially localized phosphatase, to prevent the dephosphorylation of FUNDC1 at serine 13 (Ser13), which activates hypoxia-induced mitophagy. Our results showed that the BCL2L1-PGAM5-FUNDC1 axis is critical for receptor-mediated mitophagy in response to hypoxia and that BCL2L1 possesses unique functions distinct from BCL2.

Calcium in health and disease

Evolution has exploited the chemical properties of Ca(2+), which facilitate its reversible binding to the sites of irregular geometry offered by biological macromolecules, to select it as a carrier of cellular signals. A number of proteins bind Ca(2+) to specific sites: those intrinsic to membranes play the most important role in the spatial and temporal regulation of the concentration and movements of Ca(2+) inside cells. Those which are soluble, or organized in non-membranous structures, also decode the Ca(2+) message to be then transmitted to the targets of its regulation. Since Ca(2+) controls the most important processes in the life of cells, it must be very carefully controlled within the cytoplasm, where most of the targets of its signaling function reside. Membrane channels (in the plasma membrane and in the organelles) mediate the entrance of Ca(2+) into the cytoplasm, ATPases, exchangers, and the mitochondrial Ca(2+) uptake system remove Ca(2+) from it. The concentration of Ca(2+) in the external spaces, which is controlled essentially by its dynamic exchanges in the bone system, is much higher than inside cells, and can, under conditions of pathology, generate a situation of dangerous internal Ca(2+) overload. When massive and persistent, the Ca(2+) overload culminates in the death of the cell. Subtle conditions of cellular Ca(2+) dyshomeostasis that affect individual systems that control Ca(2+), generate cell disease phenotypes that are particularly severe in tissues in which the signaling function of Ca(2+) has special importance, e.g., the nervous system.

Structural basis for a pH-sensitive calcium leak across membranes

Calcium homeostasis balances passive calcium leak and active calcium uptake. Human Bax inhibitor-1 (hBI-1) is an antiapoptotic protein that mediates a calcium leak and is representative of a highly conserved and widely distributed family, the transmembrane Bax inhibitor motif (TMBIM) proteins. Here, we present crystal structures of a bacterial homolog and characterize its calcium leak activity. The structure has a seven-transmembrane-helix fold that features two triple-helix sandwiches wrapped around a central C-terminal helix. Structures obtained in closed and open conformations are reversibly interconvertible by change of pH. A hydrogen-bonded, pKa (where Ka is the acid dissociation constant)-perturbed pair of conserved aspartate residues explains the pH dependence of this transition, and biochemical studies show that pH regulates calcium influx in proteoliposomes. Homology models for hBI-1 provide insights into TMBIM-mediated calcium leak and cytoprotective activity.

Modulating Ca2+ release by the IP3R/Ca2+ channel as a potential therapeutic treatment for neurological diseases

Recent research into neurodegenerative disorders found that their pathogeneses have a link to the inositol 1,4,5-trisphosphate receptors (IP3R). This is encouraging, because despite extensive efforts, researchers have not fully understood the pathophysiologies of those disorders, and have yet to find the cure. The IP3R provides a possible point of convergence that new therapeutic drugs can target. This review highlights patents that manipulate activities of the IP3R. They generally involve the use of peptides designed from the amino acid sequences of IP3R-binding proteins, and of buffers that limit the availability of its ligand, IP3. Additionally, one of them details the use of a chromophore-conjugated small synthetic molecule to directly inhibit the IP3R in a highly spatiotemporally specific manner. Although many of them have only been tested in vitro or are in the early stages of in vivo application, more research-effective therapies for neurodegenerative diseases can hopefully be developed.

A dual role for the anti-apoptotic Bcl-2 protein in cancer: mitochondria versus endoplasmic reticulum

Anti-apoptotic Bcl-2 contributes to cancer formation and progression by promoting the survival of altered cells. Hence, it is a prime target for novel specific anti-cancer therapeutics. In addition to its canonical anti-apoptotic role, Bcl-2 has an inhibitory effect on cell-cycle progression. Bcl-2 acts at two different intracellular compartments, the mitochondria and the endoplasmic reticulum (ER). At the mitochondria, Bcl-2 via its hydrophobic cleft scaffolds the Bcl-2-homology (BH) domain 3 (BH3) of pro-apoptotic Bcl-2-family members. Small molecules (like BH3 mimetics) can disrupt this interaction, resulting in apoptotic cell death in cancer cells. At the ER, Bcl-2 modulates Ca(2+) signaling, thereby promoting proliferation while increasing resistance to apoptosis. Bcl-2 at the ER acts via its N-terminal BH4 domain, which directly binds and inhibits the inositol 1,4,5-trisphosphate receptor (IP3R), the main intracellular Ca(2+)-release channel. Tools targeting the BH4 domain of Bcl-2 reverse Bcl-2's inhibitory action on IP3Rs and trigger pro-apoptotic Ca(2+) signaling in cancer B-cells, including chronic lymphocytic leukemia (CLL) cells and diffuse large B-cell lymphoma (DLBCL) cells. The sensitivity of DLBCL cells to BH4-domain targeting tools strongly correlated with the expression levels of the IP3R2 channel, the IP3R isoform with the highest affinity for IP3. Interestingly, bio-informatic analysis of a database of primary CLL patient cells also revealed a transcriptional upregulation of IP3R2. Finally, this review proposes a model, in which cancer cell survival depends on Bcl-2 at the mitochondria and/or the ER. This dependence likely will have an impact on their responses to BH3-mimetic drugs and BH4-domain targeting tools. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Ca(2+)-mediated regulation of VDAC1 expression levels is associated with cell death induction

VDAC1, an outer mitochondrial membrane (OMM) protein, is crucial for regulating mitochondrial metabolic and energetic functions and acts as a convergence point for various cell survival and death signals. VDAC1 is also a key player in apoptosis, involved in cytochrome c (Cyto c) release and interactions with anti-apoptotic proteins. Recently, we demonstrated that various pro-apoptotic agents induce VDAC1 oligomerization and proposed that a channel formed by VDAC1 oligomers mediates cytochrome c release. As VDAC1 transports Ca(2+) across the OMM and because Ca(2+) has been implicated in apoptosis induction, we addressed the relationship between cytosolic Ca(2+) levels ([Ca(2)(+)]i), VDAC1 oligomerization and apoptosis induction. We demonstrate that different apoptosis inducers elevate cytosolic Ca(2+) and induce VDAC1 over-expression. Direct elevation of [Ca(2+)]i by the Ca(2+)-mobilizing agents A23187, ionomycin and thapsigargin also resulted in VDAC1 over-expression, VDAC1 oligomerization and apoptosis. In contrast, decreasing [Ca(2+)]i using the cell-permeable Ca(2+)-chelating reagent BAPTA-AM inhibited VDAC1 over-expression, VDAC1 oligomerization and apoptosis. Correlation between the increase in VDAC1 levels and oligomerization, [Ca(2+)]i levels and apoptosis induction, as induced by H2O2 or As2O3, was also obtained. On the other hand, cells transfected to overexpress VDAC1 presented Ca(2+)-independent VDAC1 oligomerization, cytochrome c release and apoptosis, suggesting that [Ca(2+)]i elevation is not a pre-requisite for apoptosis induction when VDAC1 is over-expressed. The results suggest that Ca(2+) promotes VDAC1 over-expression by an as yet unknown signaling pathway, leading to VDAC1 oligomerization, ultimately resulting in apoptosis. These findings provide a new insight into the mechanism of action of existing anti-cancer drugs involving induction of VDAC1 over-expression as a mechanism for inducing apoptosis. This article is part of a Special Issue entitled: Calcium Signaling in Health and Disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Inositol 1,4,5-trisphosphate receptor-isoform diversity in cell death and survival

Cell-death and -survival decisions are critically controlled by intracellular Ca(2+) homeostasis and dynamics at the level of the endoplasmic reticulum (ER). Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) play a pivotal role in these processes by mediating Ca(2+) flux from the ER into the cytosol and mitochondria. Hence, it is clear that many pro-survival and pro-death signaling pathways and proteins affect Ca(2+) signaling by directly targeting IP3R channels, which can happen in an IP3R-isoform-dependent manner. In this review, we will focus on how the different IP3R isoforms (IP3R1, IP3R2 and IP3R3) control cell death and survival. First, we will present an overview of the isoform-specific regulation of IP3Rs by cellular factors like IP3, Ca(2+), Ca(2+)-binding proteins, adenosine triphosphate (ATP), thiol modification, phosphorylation and interacting proteins, and of IP3R-isoform specific expression patterns. Second, we will discuss the role of the ER as a Ca(2+) store in cell death and survival and how IP3Rs and pro-survival/pro-death proteins can modulate the basal ER Ca(2+) leak. Third, we will review the regulation of the Ca(2+)-flux properties of the IP3R isoforms by the ER-resident and by the cytoplasmic proteins involved in cell death and survival as well as by redox regulation. Hence, we aim to highlight the specific roles of the various IP3R isoforms in cell-death and -survival signaling. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Feedback regulation mediated by Bcl-2 and DARPP-32 regulates inositol 1,4,5-trisphosphate receptor phosphorylation and promotes cell survival

Bcl-2 interacts with the inositol 1,4,5-trisphosphate receptor (InsP3R) and thus prevents InsP3-induced Ca(2+) elevation that induces apoptosis. Here we report that Bcl-2 binds dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), a protein kinase A (PKA)-activated and calcineurin (CaN)-deactivated inhibitor of protein phosphatase 1 (PP1). Bcl-2 docks DARPP-32 and CaN in a complex on the InsP3R, creating a negative feedback loop that prevents exaggerated Ca(2+) release by decreasing PKA-mediated InsP3R phosphorylation. T-cell activation increases PKA activity, phosphorylating both the InsP3R and DARPP-32. Phosphorylated DARPP-32 inhibits PP1, enhancing InsP3R phosphorylation and Ca(2+) release. Elevated Ca(2+) activates CaN, which dephosphorylates DARPP-32 to dampen Ca(2+) release by eliminating PP1 inhibition to enable it to dephosphorylate the InsP3R. Knocking down either Bcl-2 or DARPP-32 abrogates this feedback mechanism, resulting in increased Ca(2+) elevation and apoptosis. This feedback mechanism appears to be exploited by high levels of Bcl-2 in chronic lymphocytic leukemia cells, repressing B-cell receptor-induced Ca(2+) elevation and apoptosis.

Regulation of tumor necrosis factor-α-induced microvascular endothelial cell hyperpermeability by recombinant B-cell lymphoma-extra large

BACKGROUND

Tumor necrosis factor-α (TNF-α), a cytotoxic cytokine, induces endothelial cell barrier dysfunction and microvascular hyperpermeability, leading to tissue edema, a hallmark of traumatic injuries. The objective of the present study was to determine whether B-cell lymphoma-extra large (Bcl-xL), an antiapoptotic protein, would regulate and protect against TNF-α-mediated endothelial cell barrier dysfunction and microvascular hyperpermeability.

METHODS

Rat lung microvascular endothelial cells were grown as monolayers on Transwell membranes, and fluorescein isothiocyanate-bovine albumin flux (5 mg/mL) across the monolayer was measured fluorometrically to indicate changes in monolayer permeability. The rat lung microvascular endothelial cell adherens junctional integrity and actin cytoskeleton was studied using β-catenin immunofluorescence and rhodamine phalloidin dye, respectively. Pretreatment of caspase-8 inhibitor (Z-IETD-FMK, 100 μM) for 1 hour and transfection of Bcl-2-homology domain 3-interacting domain death agonist small interfering RNA (10 μM) for 48 hours were performed to study their respective effects on TNF-α-induced (10 ng/mL; 1-hour treatment) monolayer permeability. Recombinant Bcl-xL protein (2.5 μg/ml) was transfected in rat lung microvascular endothelial cells for 1 hour, and its effect on permeability was demonstrated using a permeability assay. Caspase-3 activity was assayed fluorometrically.

RESULTS

Z-IETD-FMK pretreatment protected the adherens junctions and decreased TNF-α-induced monolayer hyperpermeability. Bcl-2-homology domain 3-interacting domain death agonist small interfering RNA transfection attenuated the TNF-α-induced increase in monolayer permeability. Recombinant Bcl-xL protein showed protection against TNF-α-induced actin stress fiber formation, an increase in caspase-3 activity, and monolayer hyperpermeability.

CONCLUSIONS

Our results have demonstrated the protective effects of recombinant Bcl-xL protein against TNF-α-induced endothelial cell adherens junction damage and microvascular endothelial cell hyperpermeability. These findings support the potential for Bcl-xL-based drug development against microvascular hyperpermeability and tissue edema.

The Bcl-2 protein family member Bok binds to the coupling domain of inositol 1,4,5-trisphosphate receptors and protects them from proteolytic cleavage

Bok is a member of the Bcl-2 protein family that controls intrinsic apoptosis. Bok is most closely related to the pro-apoptotic proteins Bak and Bax, but in contrast to Bak and Bax, very little is known about its cellular role. Here we report that Bok binds strongly and constitutively to inositol 1,4,5-trisphosphate receptors (IP3Rs), proteins that form tetrameric calcium channels in the endoplasmic reticulum (ER) membrane and govern the release of ER calcium stores. Bok binds most strongly to IP3R1 and IP3R2, and barely to IP3R3, and essentially all cellular Bok is IP3R bound in cells that express substantial amounts of IP3Rs. Binding to IP3Rs appears to be mediated by the putative BH4 domain of Bok and the docking site localizes to a small region within the coupling domain of IP3Rs (amino acids 1895-1903 of IP3R1) that is adjacent to numerous regulatory sites, including sites for proteolysis. With regard to the possible role of Bok-IP3R binding, the following was observed: (i) Bok does not appear to control the ability of IP3Rs to release ER calcium stores, (ii) Bok regulates IP3R expression, (iii) persistent activation of inositol 1,4,5-trisphosphate-dependent cell signaling causes Bok degradation by the ubiquitin-proteasome pathway, in a manner that parallels IP3R degradation, and (iv) Bok protects IP3Rs from proteolysis, either by chymotrypsin in vitro or by caspase-3 in vivo during apoptosis. Overall, these data show that Bok binds strongly and constitutively to IP3Rs and that the most significant consequence of this binding appears to be protection of IP3Rs from proteolysis. Thus, Bok may govern IP3R cleavage and activity during apoptosis.