Regulation of necrosis and mitochondrial morphology: A novel role for Bax and Bak

Abstract

Cells die primarily by apoptosis or necrosis, and mitochondria play major roles in both processes. The defining event in apoptosis is mitochondrial outer membrane permeabilization (MOMP), which allows the release of apoptogens. In contrast, the triggering event in programmed necrosis is early opening of the inner membrane mitochondrial permeability transition pore (mPTP), which disrupts normal mitochondrial function and ATP synthesis.;The Bcl-2 proteins Bax and Bak are responsible for the initiation of MOMP and apoptosis. Unexpectedly, we find that Bax and Bak also critically modulate necrosis. Deletion of Bax and Bak in an in vivo model of myocardial infarction resulted in striking reductions in both apoptotic and necrotic injury. Triple knockout mice lacking Bax, Bak and cyclophilin D, a key regulator of mPTP opening, fail to show further reduction in infarct size over mice deficient in both Bax and Bak. Absence of Bax and Bak renders cells resistant to mPTP opening and necrosis, effects confirmed in isolated mitochondria. Reconstitution of Bax/Bak DKO cells or mitochondria with either wild-type Bax, or an oligomerization-deficient, apoptosis-incompetent mutant of Bax, restores mPTP opening and necrosis. These results demonstrate that Bax regulates both necrosis and apoptosis through mechanisms distinct from each other.;Bax/Bak-null cells exhibit highly fragmented mitochondrial morphology, similar to cells lacking Mitofusin 2. Both Bax/Bak null and Mitofusin-2 null cells are protected from mPTP opening, indicating that the fissile state protects cells from necrosis. Conversely, restoration of fused mitochondrial morphology through inhibition of fission potentiates mPTP opening in both Bax/Bak null and mitofusin-2 null cells, indicating that the fused state itself is a critical determinant for necrosis sensitivity. These data suggest that Bax promotes mitochondrial fusion, which subsequently lowers the threshold for mPTP opening and necrosis. Thus, Bax and Bak play wider roles in necrotic and apoptotic cell death than previously appreciated. As central regulators of both apoptosis and necrosis, Bax and Bak may be optimal therapeutic targets in conditions such as myocardial infarction and stroke, where necrosis and apoptosis are prominent.