Cardiolipin or MTCH2 can serve as tBID receptors during apoptosis

Abstract

During apoptosis, proapoptotic BAX and BAK trigger mitochondrial outer membrane (MOM) permeabilization by a mechanism that is not yet fully understood. BH3-only proteins such as tBID, together with lipids of the MOM, are thought to play a key role in BAX and BAK activation. In particular, cardiolipin (CL) has been shown to stimulate tBID-induced BAX activation in vitro. However, it is still unclear whether this process also relies on CL in the cell, or whether it is more dependent on MTCH2, a proposed receptor for tBID present in the MOM. To address this issue, we deleted both alleles of cardiolipin synthase in human HCT116 cells by homologous recombination, which resulted in a complete absence of CL. The CL-deficient cells were fully viable in glucose but displayed impaired oxidative phosphorylation and an inability to grow in galactose. Using these cells, we found that CL was not required for either tBID-induced BAX activation, or for apoptosis in response to treatment with TRAIL. Downregulation of MTCH2 in HCT116 cells also failed to prevent recruitment of tBID to mitochondria in apoptotic conditions. However, when both CL and MTCH2 were depleted, a significant reduction in tBID recruitment was observed, suggesting that in HCT116 cells, CL and MTCH2 can have redundant functions in this process.

Inactivation of cardiolipin synthase triggers changes in mitochondrial morphology

“…4C,D), indicating that mitochondrial elongation is not caused by ectopic expression of mito-RFP. A previous study reported that human HCT116 cells lacking the CRLS1 gene have a normal mitochondrial morphology [29], which is inconsistent with our findings. Notably, the number of cells with elongated mitochondria gradually diminished over 48 h, whereas cells with an abnormal mitochondrial morphology appeared after 24-h incubation.…”

MOAP-1 Mediates Fas-Induced Apoptosis in Liver by Facilitating tBid Recruitment to Mitochondria

“…Furthermore, in contrast to the MOAP-1 KO MEFs ( Figure 1G), MTCH2 KO MEFs were not protected against Bax-induced apoptosis (Zaltsman et al, 2010), providing support to the argument that MOAP-1 may regulate the Fas apoptotic signaling through both upstream (mitochondrial targeting of tBid) and downstream (oligomerization of Bax) signaling events. Martinou and colleagues (Raemy et al, 2016) have recently confirmed that MTCH2 is critical for tBid recruitment in HeLa cells, but in HCT116 cells, MTCH2 and cardiolipin appear to play a redundant role in facilitating mitochondrial recruitment of tBid. However, our data demonstrated that depletion of MOAP-1 is sufficient to abolish mitochondrial accumulation of tBid and inhibit apoptosis in several type II cells including hepatocytes and HCT116 cells ( Figures 3A-3C, S1B, S2B, and S2C).…”

Physiological and Pharmacological Control of BAK, BAX, and Beyond

“…In addition to BAX, the activated form of BID (cleaved by caspase-8; cBID) is also regulated by cardiolipin. Although cBID does not require cardiolipin for the initial binding to a membrane, cardiolipin may promote conformational changes in cBID that enhance BAX activation; Mtch2, a carrier protein localized to the OMM, may facilitate a similar and/or redundant mechanism [32, 33]. …”