The interplay between BAX and BAK tunes apoptotic pore growth to control mitochondrial-DNA-mediated inflammation.

BAX and BAK are key apoptosis regulators that mediate the decisive step of mitochondrial outer membrane permeabilization. However, the mechanism by which they assemble the apoptotic pore remains obscure. Here, we report that BAX and BAK present distinct oligomerization properties, with BAK organizing into smaller structures with faster kinetics than BAX. BAK recruits and accelerates BAX assembly into oligomers that continue to grow during apoptosis. As a result, BAX and BAK regulate each other as they co-assemble into the same apoptotic pores, which we visualize. The relative availability of BAX and BAK molecules thereby determines the growth rate of the apoptotic pore and the relative kinetics by which mitochondrial contents, most notably mtDNA, are released. This feature of BAX and BAK results in distinct activation kinetics of the cGAS/STING pathway with implications for mtDNA-mediated paracrine inflammatory signaling.

BCL-2-family protein tBID can act as a BAX-like effector of apoptosis.

During apoptosis, the BCL-2-family protein tBID promotes mitochondrial permeabilization by activating BAX and BAK and by blocking anti-apoptotic BCL-2 members. Here, we report that tBID can also mediate mitochondrial permeabilization by itself, resulting in release of cytochrome c and mitochondrial DNA, caspase activation and apoptosis even in absence of BAX and BAK. This previously unrecognized activity of tBID depends on helix 6, homologous to the pore-forming regions of BAX and BAK, and can be blocked by pro-survival BCL-2 proteins. Importantly, tBID-mediated mitochondrial permeabilization independent of BAX and BAK is physiologically relevant for SMAC release in the immune response against Shigella infection. Furthermore, it can be exploited to kill leukaemia cells with acquired venetoclax resistance due to lack of active BAX and BAK. Our findings define tBID as an effector of mitochondrial permeabilization in apoptosis and provide a new paradigm for BCL-2 proteins, with implications for anti-bacterial immunity and cancer therapy.

Biallelic NFATC1 mutations cause an inborn error of immunity with impaired CD8+ T-cell function and perturbed glycolysis.

The nuclear factor of activated T cells (NFAT) family of transcription factors plays central roles in adaptive immunity in murine models; however, their contribution to human immune homeostasis remains poorly defined. In a multigenerational pedigree, we identified 3 patients who carry germ line biallelic missense variants in NFATC1, presenting with recurrent infections, hypogammaglobulinemia, and decreased antibody responses. The compound heterozygous NFATC1 variants identified in these patients caused decreased stability and reduced the binding of DNA and interacting proteins. We observed defects in early activation and proliferation of T and B cells from these patients, amenable to rescue upon genetic reconstitution. Stimulation induced early T-cell activation and proliferation responses were delayed but not lost, reaching that of healthy controls at day 7, indicative of an adaptive capacity of the cells. Assessment of the metabolic capacity of patient T cells revealed that NFATc1 dysfunction rendered T cells unable to engage in glycolysis after stimulation, although oxidative metabolic processes were intact. We hypothesized that NFATc1-mutant T cells could compensate for the energy deficit due to defective glycolysis by using enhanced lipid metabolism as an adaptation, leading to a delayed, but not lost, activation responses. Indeed, we observed increased 13C-labeled palmitate incorporation into citrate, indicating higher fatty acid oxidation, and we demonstrated that metformin and rosiglitazone improved patient T-cell effector functions. Collectively, enabled by our molecular dissection of the consequences of loss-of-function NFATC1 mutations and extending the role of NFATc1 in human immunity beyond receptor signaling, we provide evidence of metabolic plasticity in the context of impaired glycolysis observed in patient T cells, alleviating delayed effector responses.

Visualization of BOK pores independent of BAX and BAK reveals a similar mechanism with differing regulation.

BOK is a poorly understood member of the BCL-2 family of proteins that has been proposed to function as a pro-apoptotic, BAX-like effector. However, the molecular mechanism and structural properties of BOK pores remain enigmatic. Here, we show that the thermal stability and pore activity of BOK depends on the presence of its C-terminus as well as on the mitochondrial lipid cardiolipin. We directly visualized BOK pores in liposomes by electron microscopy, which appeared similar to those induced by BAX, in line with comparable oligomerization properties quantified by single molecule imaging. In addition, super-resolution STED imaging revealed that BOK organized into dots and ring-shaped assemblies in apoptotic mitochondria, also reminiscent of those found for BAX and BAK. Yet, unlike BAX and BAK, the apoptotic activity of BOK was limited by partial mitochondrial localization and was independent of and unaffected by other BCL-2 proteins. These results suggest that, while BOK activity is kept in check by subcellular localization instead of interaction with BCL-2 family members, the resulting pores are structurally similar to those of BAX and BAK.

MERLIN: a novel BRET-based proximity biosensor for studying mitochondria-ER contact sites.

The contacts between the ER and mitochondria play a key role in cellular functions such as the exchange of lipids and calcium between both organelles, as well as in apoptosis and autophagy signaling. The molecular architecture and spatiotemporal regulation of these distinct contact regions remain obscure and there is a need for new tools that enable tackling these questions. Here, we present a new bioluminescence resonance energy transfer-based biosensor for the quantitative analysis of distances between the ER and mitochondria that we call MERLIN (Mitochondria-ER Length Indicator Nanosensor). The main advantages of MERLIN compared with available alternatives are that it does not rely on the formation of artificial physical links between the two organelles, which could lead to artifacts, and that it allows to study contact site reversibility and dynamics. We show the applicability of MERLIN by characterizing the role of the mitochondrial dynamics machinery on the contacts of this organelle with the ER.