Mechanisms by which Bak and Bax permeabilise mitochondria during apoptosis

Apoptosis-induced changes in mitochondrial lipids

Apoptosis is an active and tightly regulated form of cell death, which can also be considered a stress-induced process of cellular communication. Recent studies reveal that the lipid network within cells is involved in the regulation and propagation of death signalling. Despite the vast growth of our current knowledge on apoptosis, little is known of the specific role played by lipid molecules in the central event of apoptosis-the piercing of mitochondrial membranes. Here we review the information regarding changes in mitochondrial lipids that are associated with apoptosis and discuss whether they may be involved in the permeabilization of mitochondria to release their apoptogenic factors, or just lie downstream of this permeabilization leading to the amplification of caspase activation. We focus on the earliest changes that physiological apoptosis induces in mitochondrial membranes, which may derive from an upstream alteration of phospholipid metabolism that reverberates on the mitochondrial re-modelling of their characteristic lipid, cardiolipin. Hopefully, this review will lead to an increased understanding of the role of mitochondrial lipids in apoptosis and also help revealing new stress sensing mechanisms in cells. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Still embedded together binding to membranes regulates Bcl-2 protein interactions

The dysregulation of apoptosis is a key step in developing tumours, and mediates resistance to cancer therapy. Many different signals for cell death converge on permeabilization of the outer mitochondrial membrane, which is controlled by the Bcl-2 family of proteins. The importance of this step is becoming increasingly relevant as the first generation of small molecules that inhibit the interaction of Bcl-2 family proteins enters clinical trials as anticancer agents. The Bcl-2 family can be divided into three classes: BH3-only proteins that are activated by various forms of cellular stress, Bax and Bak proteins that mediate mitochondrial membrane permeabilization, and inhibitory proteins such as Bcl-2 and Bcl-XL. The recently proposed embedded together model emphasizes the fact that many of the regulatory interactions between different classes of Bcl-2 family members occur at intracellular membranes, and binding to membranes causes conformational changes in the proteins that dictate functions in a dynamic manner. Within this context, recent results indicate that Bcl-XL functions as a dominant-negative Bax, a concept that resolves the paradox of similar structures but opposite functions of Bcl-XL and Bax. We have also shown that the conformational change that allows Bax to insert into the outer mitochondrial membrane is the rate-limiting step in the multistep process of Bax activation. Nevertheless, investigating the structure of activated Bax or Bak as monomers and as components of the oligomeric structures that mediate membrane permeabilization is the focus of ongoing research (and controversy) at many laboratories worldwide.

Neuroprotective effect of Bax-inhibiting peptide on neonatal brain injury

BACKGROUND AND PURPOSE

Mitochondria play a critical role in mediating cell death in both the adult and immature brain. The cyclophilin D mitochondrial membrane permeability transition pore is critical in adult ischemia, whereas in neonatal hypoxic-ischemic (HI) brain injury, mitochondrial permeabilization appears to be primarily Bax-dependent. The aim of this study was to evaluate the neuroprotective effect of a cell-penetrating Bax-inhibiting peptide (BIP) on neonatal mouse HI brain injury.

METHODS

BIP (5 microL, 5 mg/mL) or a BIP-negative control (5 microL, 5 mg/mL) was injected intracerebroventricularly immediately before HI in postnatal day 9 mice. Mice were euthanized at different time points after HI for evaluation of brain injury, Bax activation, release of proapoptotic proteins, and caspase activation. The trace fear conditioning and cylinder tests were performed for evaluation of the functional recovery after BIP treatment.

RESULTS

At 5 days after HI, there was a 41.2% reduction of brain injury in BIP-treated mice compared with BIP-negative control treated animals. Myelin basic protein and neurofilament quantification revealed that BIP reduced white matter injury. BIP treatment conferred improvement in both sensorimotor and memory functions at 7 weeks after HI. BIP protection was associated with a reduction of Bax activation, mitochondrial permeabilization, and downstream caspase activation.

CONCLUSIONS

Bax inhibition provides neuroprotection and functional improvement in a neonatal mouse model of HI.

Conformational changes in BAK, a pore-forming proapoptotic Bcl-2 family member, upon membrane insertion and direct evidence for the existence of BH3-BH3 contact interface in BAK homo-oligomers

During apoptosis, the pro-apoptotic Bcl-2 family proteins BAK and BAX form large oligomeric pores in the mitochondrial outer membrane. Apoptotic factors, including cytochrome c, are released through these pores from the mitochondrial intermembrane space into the cytoplasm where they initiate the cascade of events leading to cell death. To better understand this pivotal step toward apoptosis, a method was developed to induce membrane permeabilization by BAK in the membrane without using the full-length protein. Using a soluble form of BAK with a hexahistidine tag at the C terminus and a liposomal system containing the Ni(2+)-nitrilotriacetic acid lipid analog that can bind hexahistidine-tagged proteins, BAK oligomers were formed in the presence of the activator protein p7/p15Bid. In this system, we determined the conformational changes in BAK upon membrane insertion by applying the site-directed spin labeling method of EPR to 13 different amino acid locations. Upon membrane insertion, the BH3 domains were reorganized, and the alpha5-alpha6 helical hairpin structure was partially exposed to the membrane environment. The monomer-monomer interface in the oligomeric structure was also mapped by measuring the distance-dependent spin-spin interactions for each residue location. Spin labels attached in the BH3 domain were juxtaposed within 5-10 A distance in the oligomeric form in the membrane. These results are consistent with the current hypothesis that BAK or BAX forms homodimers, and these homodimers assemble into a higher order oligomeric pore. Detailed analyses of the data provide new insights into the structure of the BAX or BAK homodimer.

Mitochondria as pharmacological targets

In the last decade, mitochondria have provided a vast area of research for the pharmacologist, with a wealth of potential targets for drug action. Correct target identification and subsequent pharmacological manipulation might greatly help in the prevention and/or treatment of a number of the most prevalent diseases of our time including cancer, neurodegenerative disease and myocardial infarction. This is a commentary to accompany the publication of three papers in this issue of the BJP by Kurz et al., Pravdic et al. and Puerta et al. on different aspects of pharmacology involving mitochondria.

Mitochondrial fragmentation and neuronal cell death in response to the Bcl-2/Bcl-x(L)/Bcl-w antagonist ABT-737

Inhibition of pro-survival Bcl-2 family proteins by BH3-only proteins is a key initial step leading to apoptotic cell death. In neurons, investigating cell death pathways is often hampered by the multi-factorial nature of the stress stimuli employed. Here we investigate the action of ABT-737, a small molecule inhibitor which specifically targets the BH3-protein binding domain of pro-survival Bcl-2, Bcl-X(L) and Bcl-w. ABT-737 produced a time- and concentration-dependent neuronal cell death which displayed the classical hallmarks of apoptosis. Cell death was maximal by around 4 h ABT-737 treatment, and the effect of ABT-737 could be delayed by the broad spectrum caspase inhibitor zVADfmk. Examining, using real-time confocal microscopy, the molecular basis for the onset of response demonstrated recruitment of pro-apoptotic Bax to specific mitochondrial foci, followed by mitochondrial fragmentation. Treatment of neurons with ABT-737 also produced cleavage of Bid, a BH3-only protein known to be a caspase substrate. Interestingly, cleaved Bid translocated to mitochondria but did not colocalise with Bax foci. zVADfmk inhibited Bid cleavage and slowed the rate of fragmentation, suggesting a role for cleaved Bid in the amplification of the apoptotic response. siRNA-mediated knockdown of Bax significantly inhibited ABT-737 induced cell death, whereas knockdown of the BH3-only proteins Bid or Bim had no effect. ABT-737 therefore appears to be a useful tool with which to examine neuronal apoptotic pathways. Our data suggests that caspase-dependent cleavage of Bid may be a downstream amplification event which enhances the rate of mitochondrial fragmentation.

Bax forms an oligomer via separate, yet interdependent, surfaces

Interactions of Bcl-2 family proteins regulate permeability of the mitochondrial outer membrane and apoptosis. In particular, Bax forms an oligomer that permeabilizes the membrane. To map the interface of the Bax oligomer we used Triton X-100 as a membrane surrogate and performed site-specific photocross-linking. Bax-specific adducts were formed through photo-reactive probes at multiple sites that can be grouped into two surfaces. The first surface overlaps with the BH1-3 groove formed by Bcl-2 Homology motif 1, 2, and 3; the second surface is a rear pocket located on the opposite side of the protein from the BH1-3 groove. Further cross-linking experiments using Bax BH3 peptides and mutants demonstrated that the two surfaces interact with their counterparts in neighboring proteins to form two separated interfaces and that interaction at the BH1-3 groove primes the rear pocket for further interaction. Therefore, Bax oligomerization proceeds through a series of interactions that occur at separate, yet allosterically, coupled interfaces.

Bak activation for apoptosis involves oligomerization of dimers via their alpha6 helices

A pivotal step toward apoptosis is oligomerization of the Bcl-2 relative Bak. We recently reported that its oligomerization initiates by insertion of an exposed BH3 domain into the groove of another Bak monomer. We now report that the resulting BH3:groove dimers can be converted to the larger oligomers that permeabilize mitochondria by an interface between alpha6 helices. Cysteine residues placed in alpha6 could be crosslinked only after apoptotic signaling. Cysteines placed at both interfaces established that the BH3:groove dimer is symmetric and that the alpha6:alpha6 interface can link these dimers into homo-oligomers containing at least 18 Bak molecules. A putative zinc-binding site in alpha6 was not required to form the alpha6:alpha6 interface, and its mutation in full-length Bak did not affect Bak conformation, oligomerization, or function. We conclude that alpha6:alpha6 interaction occurs during Bak oligomerization and proapoptotic function, but we find no evidence that zinc binding to that interface regulates apoptosis.