VDAC2 is required for truncated BID‐induced mitochondrial apoptosis by recruiting BAK to the mitochondria

Abstract

Truncated BID (tBID), a proapoptotic BCL2 family protein, induces BAK/BAX‐dependent release of cytochrome c and other mitochondrial intermembrane proteins to the cytosol to induce apoptosis. The voltage‐dependent anion channels (VDACs) are the primary gates for solutes across the outer mitochondrial membrane (OMM); however, their role in apoptotic OMM permeabilization remains controversial. Here, we report that VDAC2−/− (V2−/−) mouse embryonic fibroblasts (MEFs) are virtually insensitive to tBID‐induced OMM permeabilization and apoptosis, whereas VDAC1−/−, VDAC3−/− and VDAC1−/−/VDAC3−/− MEFs respond normally to tBID. V2−/− MEFs regain tBID sensitivity after VDAC2 expression. Furthermore, V2−/− MEFs are deficient in mitochondrial BAK despite normal tBID–mitochondrial binding and BAX/BAK expression. tBID sensitivity of BAK−/− MEFs is also reduced, although not to the same extent as V2−/− MEFs, which might result from their strong overexpression of BAX. Indeed, addition of recombinant BAX also sensitized V2−/− MEFs to tBID. Thus, VDAC2 acts as a crucial component in mitochondrial apoptosis by allowing the mitochondrial recruitment of BAK, thereby controlling tBID‐induced OMM permeabilization and cell death.

Bax targets mitochondria by distinct mechanisms before or during apoptotic cell death: a requirement for VDAC2 or Bak for efficient Bax apoptotic function

“…Thus, our findings reconcile the observation that Vdac2 À / À cells are sensitized to Bakmediated cell death, 15,20 whilst constitutive recruitment of Bak to the MOM is reduced in the absence of VDAC2. 16,39 Deletion of Bak rescues the apoptosis defect of Vdac2 13,19 In summary, VDAC2 facilitates the mitochondrial association and apoptotic function of Bax. Thus, elucidating the molecular mechanism involved in the Bax-VDAC2 association may reveal a novel target to perturb Bax-mediated apoptosis upstream of mitochondrial damage.…”

“…Studies have also implicated an additional role for the VDACs in the regulation of Bak or Bax apoptotic function or potentially even constituting a component of the Bak/Bax apoptotic pore. [15][16][17][18] However, these studies have provided contrasting findings relating to whether VDACs might positively or negatively regulate Bak/Bax apoptotic function. We used blue native-PAGE (BN-PAGE) to investigate how Bax oligomerizes in the MOM during apoptosis.…”

“…21 Our studies support VDAC2 as an important determinant of the constitutive subcellular distribution of Bax and Bak in healthy cells. 16,32 This role for VDAC2 is seemingly specific for Bak and Bax as the mitochondrial targeting of other Bcl-2 family members including Bcl-x L , Mcl-1 and tBid was not affected by the absence of VDAC2. Recently, Bcl-x L was shown to interact with VDAC1, 33 but not VDAC2, 34 suggesting that other VDAC isoforms may play a role in mitochondrial targeting of these Bcl-2 family proteins.…”

Bak apoptotic pores involve a flexible C-terminal region and juxtaposition of the C-terminal transmembrane domains

“…5,30,31 The hydrophobic stretch is important, as substituting polar or charged residues decreased targeting of Bak and Bax. 10,32 Mitochondrial targeting is also controlled by basic residues at the far C termini, [32][33][34] and by interaction with VDAC2 35,36 via the Bak and Bax C termini. 37,38 Retrotranslocation of Bak and Bax was also altered by swapping the C termini.…”

VDAC1 selectively transfers apoptotic Ca2+ signals to mitochondria

“…Indeed, VDACs have been shown to interact with cytoskeletal elements such as actin and tubulin, 4,5 metabolic enzymes, 6 Bcl2-family members including Bak, 7 Bad, 8 tBid 9 and Bcl-xL 10 or other channels such as ANT, 11 or the IP 3 R. 12,13 This scenario is further complicated by evidence showing that VDAC contribution to cell death can be isoform and stimulus dependent: VDAC1 acts predominantly as a pro-apoptotic protein [14][15][16][17] whereas VDAC2 protects from a number of apoptosis inducers, 18 but concurrently appears to be necessary for tBid-mediated cell-death. 9 However, these different effects are not supported by the apparent redundancy in the electrophysiological properties of the isoforms. 19,20 Mitochondrial [Ca 2 þ ] is commonly regarded as an important determinant in cell sensitivity to apoptotic stimuli.…”