The BCL-2 family of proteins control a key checkpoint in apoptosis, that of mitochondrial outer membrane permeabilization or, simply, mitochondrial poration. The family consists of three subgroups: ...BH3-only initiators that respond to apoptotic stimuli; antiapoptotic guardians that protect against cell death; and the membrane permeabilizing effectors BAX, BAK, and BOK. On activation, effector proteins are converted from inert monomers into membrane permeabilizing oligomers. For many years, this process has been poorly understood at the molecular level, but a number of recent advances have provided important insights. We review the regulation of these effectors, their activation, subsequent conformational changes, and the ensuing oligomerization events that enable mitochondrial poration, which initiates apoptosis through release of key signaling factors such as cytochrome
We highlight the mysteries that remain in understanding these important proteins in an endeavor to provide a comprehensive picture of where the field currently sits and where it is moving toward.
The BCL-2 protein family determines the commitment of cells to apoptosis, an ancient cell suicide programme that is essential for development, tissue homeostasis and immunity. Too little apoptosis ...can promote cancer and autoimmune diseases; too much apoptosis can augment ischaemic conditions and drive neurodegeneration. We discuss the biochemical, structural and genetic studies that have clarified how the interplay between members of the BCL-2 family on mitochondria sets the apoptotic threshold. These mechanistic insights into the functions of the BCL-2 family are illuminating the physiological control of apoptosis, the pathological consequences of its dysregulation and the promising search for novel cancer therapies that target the BCL-2 family.
Apoptosis is a form of programmed cell death critical for the development and homeostasis of multicellular organisms. A key event within the mitochondrial pathway to apoptosis is the permeabilisation ...of the mitochondrial outer membrane (MOM), a point of no return in apoptotic progression. This event is governed by a complex interplay of interactions between BCL-2 family members. Here we discuss the roles of opposing factions within the family. We focus on the structural details of these interactions, how they promote or prevent apoptosis and recent developments towards understanding the conformational changes of BAK and BAX that lead to MOM permeabilisation. These interactions and structural insights are of particular interest for drug discovery, as highlighted by the development of therapeutics that target pro-survival family members and restore apoptosis in cancer cells.
The Structural Basis of Necroptotic Cell Death Signaling Petrie, Emma J.; Czabotar, Peter E.; Murphy, James M.
Trends in biochemical sciences (Amsterdam. Regular ed.),
January 2019, 2019-01-00, 20190101, Letnik:
44, Številka:
1
Journal Article
Recenzirano
The recent implication of the cell death pathway, necroptosis, in innate immunity and a range of human pathologies has led to intense interest in the underlying molecular mechanism. Unlike the ...better-understood apoptosis pathway, necroptosis is a caspase-independent pathway that leads to cell lysis and release of immunogens downstream of death receptor and Toll-like receptor (TLR) ligation. Here we review the role of recent structural studies of the core machinery of the pathway, the protein kinases receptor-interacting protein kinase (RIPK)1 and RIPK3, and the terminal effector, the pseudokinase mixed lineage kinase domain-like protein (MLKL), in shaping our mechanistic understanding of necroptotic signaling. Structural studies have played a key role in establishing models that describe MLKL’s transition from a dormant monomer to a killer oligomer and revealing important interspecies differences.
Structures of component signaling modules in all known necroptosis effectors – RIPK1, RIPK3, and MLKL – have been reported over the past 5 years.
Recent NMR studies have provided insights into how RIPK1 and RIPK3 form high-molecular-weight fibrils via their RHIM motifs.
Recent structural and biophysical data highlight interspecies differences in the terminal effector MLKL. MLKL assembles into higher-order complexes: mouse MLKL forms trimers while the human protein is tetrameric. Structural differences in the executioner 4HB domain and the regulatory pseudokinase domain between mouse and human MLKL may reflect mechanistic differences.
Several models have been proposed to describe the mechanism by which MLKL permeabilizes the plasma membrane, leading to debate about the nature of the membrane aperture. Recent studies suggest a role for necroptotic coeffectors in regulating the susceptibility of membranes to MLKL-mediated permeabilization.
Apoptosis is a form of programmed cell death that is regulated by the balance between prosurvival and proapoptotic BCL-2 protein family members. Evasion of apoptosis is a hallmark of cancer that ...arises when this balance is tipped in favour of survival. One form of anticancer therapeutic, termed 'BH3-mimetic drugs', has been developed to directly activate the apoptosis machinery in malignant cells. These drugs bind to and inhibit specific prosurvival BCL-2 family proteins, thereby mimicking their interaction with the BH3 domains of proapoptotic BCL-2 family proteins. The BCL-2-specific inhibitor venetoclax is approved by the US Food and Drug Administration and many regulatory authorities worldwide for the treatment of chronic lymphocytic leukaemia and acute myeloid leukaemia. BH3-mimetic drugs targeting other BCL-2 prosurvival proteins have been tested in preclinical models of cancer, and drugs targeting MCL-1 or BCL-X
have advanced into phase I clinical trials for certain cancers. As with all therapeutics, efficacy and tolerability need to be carefully balanced to achieve a therapeutic window whereby there is significant anticancer activity with an acceptable safety profile. In this Review, we outline the current state of BH3-mimetic drugs targeting various prosurvival BCL-2 family proteins and discuss emerging data regarding primary and acquired resistance to these agents and approaches that may overcome this. We highlight issues that need to be addressed to further advance the clinical application of BH3-mimetic drugs, both alone and in combination with additional anticancer agents (for example, standard chemotherapeutic drugs or inhibitors of oncogenic kinases), for improved responses in patients with cancer.
In stressed cells, apoptosis ensues when Bcl-2 family members Bax or Bak oligomerize and permeabilize the mitochondrial outer membrane. Certain BH3-only relatives can directly activate them to ...mediate this pivotal, poorly understood step. To clarify the conformational changes that induce Bax oligomerization, we determined crystal structures of BaxΔC21 treated with detergents and BH3 peptides. The peptides bound the Bax canonical surface groove but, unlike their complexes with prosurvival relatives, dissociated Bax into two domains. The structures define the sequence signature of activator BH3 domains and reveal how they can activate Bax via its groove by favoring release of its BH3 domain. Furthermore, Bax helices α2–α5 alone adopted a symmetric homodimer structure, supporting the proposal that two Bax molecules insert their BH3 domain into each other’s surface groove to nucleate oligomerization. A planar lipophilic surface on this homodimer may engage the membrane. Our results thus define critical Bax transitions toward apoptosis.
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► Activator BH3 domains bind the Bax canonical groove and help free the Bax BH3 ► Bax:BH3 peptide complexes reveal sequence characteristics of activator BH3 domains ► Bax activation separates its “core” domain (helices α1–α5) and “latch” (α6–α8) ► The freed core forms a symmetric BH3-in-groove dimer with a lipophilic surface
The structural transitions that set proapoptotic Bax on the pathway to forming a pore in the outer mitochondrial membrane are revealed in crystal structures of Bax bound to activator BH3 peptides.
Bax and Bak are two nuclear-encoded proteins present in higher eukaryotes that are able to pierce the mitochondrial outer membrane to mediate cell death by apoptosis. Thus, organelles recruited by ...nucleated cells to supply energy can be recruited by Bax and Bak to kill cells. The two proteins lie in wait in healthy cells where they adopt a globular α-helical structure, seemingly as monomers. Following a variety of stress signals, they convert into pore-forming proteins by changing conformation and assembling into oligomeric complexes in the mitochondrial outer membrane. Proteins from the mitochondrial intermembrane space then empty into the cytosol to activate proteases that dismantle the cell. The arrangement of Bax and Bak in membrane-bound complexes, and how the complexes porate the membrane, is far from being understood. However, recent data indicate that they first form symmetric BH3:groove dimers which can be linked via an interface between the α6-helices to form high order oligomers. Here, we review how Bax and Bak change conformation and oligomerize, as well as how oligomers might form a pore. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.
► During apoptosis, Bax and Bak puncture the mitochondrial outer membrane. ► Bax and Bak activity is regulated by other members of the Bcl-2 family. ► Activation exposes the Bax and Bak BH3 domains to allow oligomerization. ► Bax and Bak oligomers form pores in the outer membrane via unknown mechanisms. ► Structures of oligomeric Bax or Bak may reveal novel drug targets.
BCL-2 family antagonists for cancer therapy Lessene, Guillaume; Czabotar, Peter E; Colman, Peter M
Nature reviews. Drug discovery,
12/2008, Letnik:
7, Številka:
12
Journal Article
Recenzirano
Overexpression of members of the BCL-2 family of pro-survival proteins is commonly associated with unfavourable pathogenesis in cancer. The convergence of cytotoxic stress signals on the extended ...BCL-2 protein family provides the biological rationale for directly targeting this family to induce apoptotic cell death. Recently, several compounds have been described that inhibit the interaction between BCL-2 family members and their natural ligand, a helical peptide sequence known as the BH3 domain. Here, we review preclinical and clinical data on these compounds, and recommend four criteria that define antagonists of the BCL-2 protein family.
Apoptotic stimuli activate and oligomerize the proapoptotic proteins Bak and Bax, resulting in mitochondrial outer-membrane permeabilization and subsequent cell death. This activation can occur when ...certain BH3-only proteins interact directly with Bak and Bax. Recently published crystal structures reveal that Bax separates into core and latch domains in response to BH3 peptides. The distinguishing characteristics of BH3 peptides capable of directly activating Bax were also elucidated. Here we identify specific BH3 peptides capable of “unlatching” Bak and describe structural insights into Bak activation and oligomerization. Crystal structures and crosslinking experiments demonstrate that Bak undergoes a conformational change similar to that of Bax upon activation. A structure of the Bak core domain dimer provides a high-resolution image of this key intermediate in the pore-forming oligomer. Our results confirm an analogous mechanism for activation and dimerization of Bak and Bax in response to certain BH3 peptides.
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•Bak separates into core and latch subdomains upon activation•Core/latch separation is required for Bak-mediated mitochondrial permeabilization•The freed core subdomain forms BH3:groove symmetric dimers•BH3:groove dimers possess a lipophilic surface that may engage membranes
BH3-only proteins activate the proapoptotic protein Bak, resulting in its oligomerization and disruption of the mitochondrial outer membrane. Here, Brouwer et al. show that Bak separates into core and latch domains upon activation. Released core domains form BH3:groove dimers, the likely building block for the larger Bak oligomer.
Intrinsic apoptosis, reliant on BAX and BAK, has been postulated to be fundamental for morphogenesis, but its precise contribution to this process has not been fully explored in mammals. Our ...structural analysis of BOK suggests close resemblance to BAX and BAK structures. Notably, Bok−/−Bax−/−Bak−/− animals exhibited more severe defects and died earlier than Bax−/−Bak−/− mice, implying that BOK has overlapping roles with BAX and BAK during developmental cell death. By analyzing Bok−/−Bax−/−Bak−/− triple-knockout mice whose cells are incapable of undergoing intrinsic apoptosis, we identified tissues that formed well without this process. We provide evidence that necroptosis, pyroptosis, or autophagy does not substantially substitute for the loss of apoptosis. Albeit very rare, unexpected attainment of adult Bok−/−Bax−/−Bak−/− mice suggests that morphogenesis can proceed entirely without apoptosis mediated by these proteins and possibly without cell death in general.
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•BOK, BAX, and BAK have similar structures and overlapping roles during development•BOK/BAX/BAK triple-null mice have more severe defects than BAX/BAK double knockouts•Intrinsic apoptosis is critical for midline fusion and aortic arch formation•A small number of BOK/BAX/BAK triple-null mice develop entirely without apoptosis
Although apoptosis has been observed in many organs and tissues during embryonic development, it is not strictly required for the development of most organs.
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