Regulation of the ryanodine receptor by anti-apoptotic Bcl-2 is independent of its BH3-domain-binding properties

The BCL2 family: from apoptosis mechanisms to new advances in targeted therapy

The B cell lymphoma 2 (BCL2) protein family critically controls apoptosis by regulating the release of cytochrome c from mitochondria. In this cutting-edge review, we summarize the basic biology regulating the BCL2 family including canonical and non-canonical functions, and highlight milestones from basic research to clinical applications in cancer and other pathophysiological conditions. We review laboratory and clinical development of BH3-mimetics as well as more recent approaches including proteolysis targeting chimeras (PROTACs), antibody-drug conjugates (ADCs) and tools targeting the BH4 domain of BCL2. The first BCL2-selective BH3-mimetic, venetoclax, showed remarkable efficacy with manageable toxicities and has transformed the treatment of several hematologic malignancies. Following its success, several chemically similar BCL2 inhibitors such as sonrotoclax and lisaftoclax are currently under clinical evaluation, alone and in combination. Genetic analysis highlights the importance of BCL-XL and MCL1 across different cancer types and the possible utility of BH3-mimetics targeting these proteins. However, the development of BH3-mimetics targeting BCL-XL or MCL1 has been more challenging, with on-target toxicities including thrombocytopenia for BCL-XL and cardiac toxicities for MCL1 inhibitors precluding clinical development. Tumor-specific BCL-XL or MCL1 inhibition may be achieved by novel targeting approaches using PROTACs or selective drug delivery strategies and would be transformational in many subtypes of malignancy. Taken together, we envision that the targeting of BCL2 proteins, while already a success story of translational research, may in the foreseeable future have broader clinical applicability and improve the treatment of multiple diseases.

CISD2 counteracts the inhibition of ER-mitochondrial calcium transfer by anti-apoptotic BCL-2

CISD2, a 2Fe2S cluster domain-containing protein, is implicated in Wolfram syndrome type 2, longevity and cancer. CISD2 is part of a ternary complex with IP3 receptors (IP3Rs) and anti-apoptotic BCL-2 proteins and enhances BCL-2's anti-autophagic function. Here, we examined how CISD2 impacted the function of BCL-2 in apoptosis and in controlling IP3R-mediated Ca2+ signaling. Using purified proteins, we found a direct interaction between the cytosolic region of CISD2 and BCL-2's BH4 domain with a submicromolar affinity. At the functional level, the cytosolic region of CISD2, as a purified protein, did not affect the ability of BCL-2 to inhibit BAX-pore formation. In a cellular context, loss of CISD2 did not impede the suppression of apoptosis by BCL-2. Also, in Ca2+-signaling assays, absence of CISD2 did not affect the inhibition of IP3R-mediated Ca2+ release by BCL-2. Combined, these experiments indicate that CISD2 is not essential for BCL-2 function in apoptosis and cytosolic Ca2+ signaling. Instead, CISD2 overexpression enhanced BCL-2-mediated suppression of cytosolic IP3R-mediated Ca2+ release. However, consistent with the presence of CISD2 and BCL-2 at mitochondria-associated ER membranes (MAMs), the most striking effect was observed at the level of ER-mitochondrial Ca2+ transfer. While BCL-2 overexpression inhibited ER-mitochondrial Ca2+ transfer, overexpression of CISD2 together with BCL-2 abrogated the effect of BCL-2. The underlying mechanism is linked to ER-mitochondrial contact sites, since BCL-2 reduced ER-mitochondrial contact sites while co-expression of CISD2 together with BCL-2 abolished this effect. These findings reveal a unique interplay between BCL-2 and CISD2 at Ca2+-signaling nanodomains between ER and mitochondria.

Modulation of Ca2+ signaling by antiapoptotic Bcl-2 versus Bcl-xL: From molecular mechanisms to relevance for cancer cell survival

Members of the Bcl-2-protein family are key controllers of apoptotic cell death. The family is divided into antiapoptotic (including Bcl-2 itself, Bcl-xL, Mcl-1, etc.) and proapoptotic members (Bax, Bak, Bim, Bim, Puma, Noxa, Bad, etc.). These proteins are well known for their canonical role in the mitochondria, where they control mitochondrial outer membrane permeabilization and subsequent apoptosis. However, several proteins are recognized as modulators of intracellular Ca²⁺ signals that originate from the endoplasmic reticulum (ER), the major intracellular Ca²⁺-storage organelle. More than 25 years ago, Bcl-2, the founding member of the family, was reported to control apoptosis through Ca²⁺ signaling. Further work elucidated that Bcl-2 directly targets and inhibits inositol 1,4,5-trisphosphate receptors (IP₃Rs), thereby suppressing proapoptotic Ca²⁺ signaling. In addition to Bcl-2, Bcl-xL was also shown to impact cell survival by sensitizing IP₃R function, thereby promoting prosurvival oscillatory Ca²⁺ release. However, new work challenges this model and demonstrates that Bcl-2 and Bcl-xL can both function as inhibitors of IP₃Rs. This suggests that, depending on the cell context, Bcl-xL could support very distinct Ca²⁺ patterns. This not only raises several questions but also opens new possibilities for the treatment of Bcl-xL-dependent cancers. In this review, we will discuss the similarities and divergences between Bcl-2 and Bcl-xL regarding Ca²⁺ homeostasis and IP₃R modulation from both a molecular and a functional point of view, with particular emphasis on cancer cell death resistance mechanisms.

Tracing the evolutionary history of Ca2+-signaling modulation by human Bcl-2: Insights from the Capsaspora owczarzaki IP3 receptor ortholog

Recently, a functional IP₃R ortholog (CO.IP₃R-A) capable of IP₃-induced Ca²⁺ release has been discovered in Capsaspora owczarzaki, a close unicellular relative to Metazoa. In contrast to mammalian IP₃Rs, CO.IP₃R-A is not modulated by Ca²⁺, ATP or PKA. Protein-sequence analysis revealed that CO.IP₃R-A contained a putative binding site for anti-apoptotic Bcl-2, although Bcl-2 was not detected in Capsaspora owczarzaki and only appeared in Metazoa. Here, we examined whether human Bcl-2 could form a complex with CO.IP₃R-A channels and modulate their Ca²⁺-flux properties using ectopic expression approaches in a HEK293 cell model in which all three IP₃R isoforms were knocked out. We demonstrate that human Bcl-2 via its BH4 domain could functionally interact with CO.IP₃R-A, thereby suppressing Ca²⁺ flux through CO.IP₃R-A channels. The BH4 domain of Bcl-2 was sufficient for interaction with CO.IP₃R-A channels. Moreover, mutating the Lys17 of Bcl-2's BH4 domain, the residue critical for Bcl-2-dependent modulation of mammalian IP₃Rs, abrogated Bcl-2's ability to bind and inhibit CO.IP₃R-A channels. Hence, this raises the possibility that a unicellular ancestor of animals already had an IP₃R that harbored a Bcl-2-binding site. Bcl-2 proteins may have evolved as controllers of IP₃R function by exploiting this pre-existing site, thereby counteracting Ca²⁺-dependent apoptosis.

Bcl-2-Protein Family as Modulators of IP3 Receptors and Other Organellar Ca2+ Channels

The pro- and antiapoptotic proteins belonging to the B-cell lymphoma-2 (Bcl-2) family exert a critical control over cell-death processes by enabling or counteracting mitochondrial outer membrane permeabilization. Beyond this mitochondrial function, several Bcl-2 family members have emerged as critical modulators of intracellular Ca²⁺ homeostasis and dynamics, showing proapoptotic and antiapoptotic functions. Bcl-2 family proteins specifically target several intracellular Ca²⁺-transport systems, including organellar Ca²⁺ channels: inositol 1,4,5-trisphosphate receptors (IP₃Rs) and ryanodine receptors (RyRs), Ca²⁺-release channels mediating Ca²⁺ flux from the endoplasmic reticulum, as well as voltage-dependent anion channels (VDACs), which mediate Ca²⁺ flux across the mitochondrial outer membrane into the mitochondria. Although the formation of protein complexes between Bcl-2 proteins and these channels has been extensively studied, a major advance during recent years has been elucidating the complex interaction of Bcl-2 proteins with IP₃Rs. Distinct interaction sites for different Bcl-2 family members were identified in the primary structure of IP₃Rs. The unique molecular profiles of these Bcl-2 proteins may account for their distinct functional outcomes when bound to IP₃Rs. Furthermore, Bcl-2 inhibitors used in cancer therapy may affect IP₃R function as part of their proapoptotic effect and/or as an adverse effect in healthy cells.

Bcl-2 and IP3 compete for the ligand-binding domain of IP3Rs modulating Ca2+ signaling output

Bcl-2 proteins have emerged as critical regulators of intracellular Ca2+ dynamics by directly targeting and inhibiting the IP3 receptor (IP3R), a major intracellular Ca2+-release channel. Here, we demonstrate that such inhibition occurs under conditions of basal, but not high IP3R activity, since overexpressed and purified Bcl-2 (or its BH4 domain) can inhibit IP3R function provoked by low concentration of agonist or IP3, while fails to attenuate against high concentration of agonist or IP3. Surprisingly, Bcl-2 remained capable of inhibiting IP3R1 channels lacking the residues encompassing the previously identified Bcl-2-binding site (a.a. 1380-1408) located in the ARM2 domain, part of the modulatory region. Using a plethora of computational, biochemical and biophysical methods, we demonstrate that Bcl-2 and more particularly its BH4 domain bind to the ligand-binding domain (LBD) of IP3R1. In line with this finding, the interaction between the LBD and Bcl-2 (or its BH4 domain) was sensitive to IP3 and adenophostin A, ligands of the IP3R. Vice versa, the BH4 domain of Bcl-2 counteracted the binding of IP3 to the LBD. Collectively, our work reveals a novel mechanism by which Bcl-2 influences IP3R activity at the level of the LBD. This allows for exquisite modulation of Bcl-2's inhibitory properties on IP3Rs that is tunable to the level of IP3 signaling in cells.

α7 nicotinic acetylcholine receptor upregulation by anti-apoptotic Bcl-2 proteins

Nicotinic acetylcholine receptors (nAChRs) mediate and modulate synaptic transmission throughout the brain, and contribute to learning, memory, and behavior. Dysregulation of α7-type nAChRs in neuropsychiatric as well as immunological and oncological diseases makes them attractive targets for pharmaceutical development. Recently, we identified NACHO as an essential chaperone for α7 nAChRs. Leveraging the robust recombinant expression of α7 nAChRs with NACHO, we utilized genome-wide cDNA library screening and discovered that several anti-apoptotic Bcl-2 family proteins further upregulate receptor assembly and cell surface expression. These effects are mediated by an intracellular motif on α7 that resembles the BH3 binding domain of pro-apoptotic Bcl-2 proteins, and can be blocked by BH3 mimetic Bcl-2 inhibitors. Overexpression of Bcl-2 member Mcl-1 in neurons enhanced surface expression of endogenous α7 nAChRs, while a combination of chemotherapeutic Bcl2-inhibitors suppressed neuronal α7 receptor assembly. These results demonstrate that Bcl-2 proteins link α7 nAChR assembly to cell survival pathways.

The α7 nicotinic acetylcholine receptor (nAChR) plays a major role in shaping the activity of neuronal circuits and contributes to the pathophysiology of several neurological disorders. Following cDNA library screening, the authors identify anti-apoptotic, Bcl-2 family proteins as enhancers of α7 nAChR assembly, acting through an intracellular BH3-like domain during receptor biogenesis in the endoplasmic reticulum.

ABT-199 (Venetoclax), a BH3-mimetic Bcl-2 inhibitor, does not cause Ca2+ -signalling dysregulation or toxicity in pancreatic acinar cells

BACKGROUND AND PURPOSE

Many cancer cells depend on anti-apoptotic B-cell lymphoma 2 (Bcl-2) proteins for their survival. Bcl-2 antagonism through Bcl-2 homology 3 (BH3) mimetics has emerged as a novel anti-cancer therapy. ABT-199 (Venetoclax), a recently developed BH3 mimetic that selectively inhibits Bcl-2, was introduced into the clinic for treatment of relapsed chronic lymphocytic leukaemia. Early generations of Bcl-2 inhibitors evoked sustained Ca²⁺ responses in pancreatic acinar cells (PACs) inducing cell death. Therefore, BH3 mimetics could potentially be toxic for the pancreas when used to treat cancer. Although ABT-199 was shown to kill Bcl-2-dependent cancer cells without affecting intracellular Ca²⁺ signalling, its effects on PACs have not yet been determined. Hence, it is essential and timely to assess whether this recently approved anti-leukaemic drug might potentially have pancreatotoxic effects.

EXPERIMENTAL APPROACH

Single-cell Ca²⁺ measurements and cell death analysis were performed on isolated mouse PACs.

KEY RESULTS

Inhibition of Bcl-2 via ABT-199 did not elicit intracellular Ca²⁺ signalling on its own or potentiate Ca²⁺ signalling induced by physiological/pathophysiological stimuli in PACs. Although ABT-199 did not affect cell death in PACs, under conditions that killed ABT-199-sensitive cancer cells, cytosolic Ca²⁺ extrusion was slightly enhanced in the presence of ABT-199. In contrast, inhibition of Bcl-xL potentiated pathophysiological Ca²⁺ responses in PACs, without exacerbating cell death.

CONCLUSION AND IMPLICATIONS

Our results demonstrate that apart from having a modest effect on cytosolic Ca²⁺ extrusion, ABT-199 does not substantially alter intracellular Ca²⁺ homeostasis in normal PACs and should be safe for the pancreas during cancer treatment.

LINKED ARTICLES

This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc.

Ryanodine Receptors in Autophagy: Implications for Neurodegenerative Diseases?

Intracellular Ca²⁺ signaling is important in the regulation of several cellular processes including autophagy. The endoplasmic reticulum (ER) is the main and largest intracellular Ca²⁺ store. At the ER two protein families of Ca²⁺ release channels, inositol 1,4,5-trisphosphate receptors (IP₃Rs) and ryanodine receptors (RyRs), are expressed. Several studies have reported roles in the regulation of autophagy for the ubiquitously expressed IP₃R. For instance, IP₃R-mediated Ca²⁺ release supresses basal autophagic flux by promoting mitochondrial metabolism, while also promoting the rapid initial increase in autophagic flux in response to nutrient starvation. Insights into the contribution of RyRs in autophagy have been lagging significantly compared to the advances made for IP₃Rs. This is rather surprising considering that RyRs are predominantly expressed in long-lived cells with specialized metabolic needs, such as neurons and muscle cells, in which autophagy plays important roles. In this review article, recent studies revealing roles for RyRs in the regulation of autophagy will be discussed. Several RyR-interacting proteins that have been established to modulate both RyR function and autophagy will also be highlighted. Finally, the involvement of RyRs in neurodegenerative diseases will be addressed. Inhibition of RyR channels has not only been shown to be beneficial for treating several of these diseases but also regulates autophagy.

The trans-membrane domain of Bcl-2α, but not its hydrophobic cleft, is a critical determinant for efficient IP3 receptor inhibition

The anti-apoptotic Bcl-2 protein is emerging as an efficient inhibitor of IP3R function, contributing to its oncogenic properties. Yet, the underlying molecular mechanisms remain not fully understood. Using mutations or pharmacological inhibition to antagonize Bcl-2's hydrophobic cleft, we excluded this functional domain as responsible for Bcl-2-mediated IP3Rs inhibition. In contrast, the deletion of the C-terminus, containing the trans-membrane domain, which is only present in Bcl-2α, but not in Bcl-2β, led to impaired inhibition of IP3R-mediated Ca2+ release and staurosporine-induced apoptosis. Strikingly, the trans-membrane domain was sufficient for IP3R binding and inhibition. We therefore propose a novel model, in which the Bcl-2's C-terminus serves as a functional anchor, which beyond mere ER-membrane targeting, underlies efficient IP3R inhibition by (i) positioning the BH4 domain in the close proximity of its binding site on IP3R, thus facilitating their interaction; (ii) inhibiting IP3R-channel openings through a direct interaction with the C-terminal region of the channel downstream of the channel-pore. Finally, since the hydrophobic cleft of Bcl-2 was not involved in IP3R suppression, our findings indicate that ABT-199 does not interfere with IP3R regulation by Bcl-2 and its mechanism of action as a cell-death therapeutic in cancer cells likely does not involve Ca2+ signaling.

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.

Bcl-2 family proteins: master regulators of cell survival

The most prominent function of proteins of the Bcl-2 family is regulation of the initiation of intrinsic (mitochondrial) pathways of apoptosis. However, recent research has revealed that in addition to regulation of mitochondrial apoptosis, proteins of the Bcl-2 family play important roles in regulating other cellular pathways with a strong impact on cell survival like autophagy, endoplasmic reticulum (ER) stress response, intracellular calcium dynamics, cell cycle progression, mitochondrial dynamics and energy metabolism. This review summarizes the recent knowledge about functions of Bcl-2 family proteins that are related to cell survival.

ER functions of oncogenes and tumor suppressors: Modulators of intracellular Ca(2+) signaling

Intracellular Ca(2+) signals that arise from the endoplasmic reticulum (ER), the major intracellular Ca(2+)-storage organelle, impact several mitochondrial functions and dictate cell survival and cell death processes. Furthermore, alterations in Ca(2+) signaling in cancer cells promote survival and establish a high tolerance towards cell stress and damage, so that the on-going oncogenic stress does not result in the activation of cell death. Over the last years, the mechanisms underlying these oncogenic alterations in Ca(2+) signaling have started to emerge. An important aspect of this is the identification of several major oncogenes, including Bcl-2, Bcl-XL, Mcl-1, PKB/Akt, and Ras, and tumor suppressors, such as p53, PTEN, PML, BRCA1, and Beclin 1, as direct and critical regulators of Ca(2+)-transport systems located at the ER membranes, including IP3 receptors and SERCA Ca(2+) pumps. In this way, these proteins execute part of their function by controlling the ER-mitochondrial Ca(2+) fluxes, favoring either survival (oncogenes) or cell death (tumor suppressors). Oncogenic mutations, gene deletions or amplifications alter the expression and/or function of these proteins, thereby changing the delicate balance between oncogenes and tumor suppressors, impacting oncogenesis and favoring malignant cell function and behavior. In this review, we provided an integrated overview of the impact of the major oncogenes and tumor suppressors, often altered in cancer cells, on Ca(2+) signaling from the ER Ca(2+) stores. This article is part of a Special Issue entitled: Calcium and Cell Fate. Guest Editors: Jacques Haiech, Claus Heizmann, Joachim Krebs, Thierry Capiod and Olivier Mignen.