Molecular and topological reorganizations in mitochondrial architecture interplay during Bax-mediated steps of apoptosis

Abstract

During apoptosis, Bcl-2 proteins such as Bax and Bak mediate the release of pro-apoptotic proteins from the mitochondria by clustering on the outer mitochondrial membrane and thereby permeabilizing it. However, it remains unclear how outer membrane openings form. Here, we combined different correlative microscopy and electron cryo-tomography approaches to visualize the effects of Bax activity on mitochondria in human cells. Our data show that Bax clusters localize near outer membrane ruptures of highly variable size. Bax clusters contain structural elements suggesting a higher order organization of their components. Furthermore, unfolding of inner membrane cristae is coupled to changes in the supramolecular assembly of ATP synthases, particularly pronounced at membrane segments exposed to the cytosol by ruptures. Based on our results, we propose a comprehensive model in which molecular reorganizations of the inner membrane and sequestration of outer membrane components into Bax clusters interplay in the formation of outer membrane ruptures.Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Mitochondria as multifaceted regulators of cell death

“…Furthermore, compared with healthy mitochondria, the matrix of apoptotic mitochondria is more dilute 106 . Potentially, the extra pressure associated with the increased volume of a more dilute matrix may be an important driver of both macropore expansion and inner mitochondrial membrane extrusion and subsequent rupture.…”

“…This was unexpected because cGAS and STING reside outside the mitochondria, whereas mtDNA localises to the mitochondrial matrix and the inner mitochondrial membrane was thought to remain intact during apoptosis. Various studies employing different imaging approaches in murine embryonic fibroblasts as well as various cancer cell lines have addressed how mtDNA could be exposed to cGAS-STING [104][105][106] . Super-resolution imaging of cells undergoing mitochondrial apoptosis demonstrated that MOMP induction is followed, over time, by the formation of expanding pores on the mitochondrial outer membrane.…”

Mitochondrial Dysfunction, Oxidative Stress, and Neuroinflammation: Intertwined Roads to Neurodegeneration

“…In the case of programmed cell death, the activation of the Bax/Bak pathway may induce permeabilization of the outer mitochondrial membrane with the formation of pores that, under prolonged stress, become deeper until reaching the inner mitochondrial membrane. Via these structures, mtDNA nucleoids (i.e., mtDNA complexed with the mitochondrial transcription factor A) can be displaced into the cytosol [37,38,65]. Altered mitochondrial dynamics have also been associated with mtDNA-driven inflammation [34][35][36]39,40].…”

Tailoring cryo-electron microscopy grids by photo-micropatterning for in-cell structural studies

“…Despite a growing number of successfully reported in-cell cryo-ET studies 4,9,[15][16][17][18] , controlled on-grid positioning of adherent mammalian cells has remained largely unaddressed. Currently, optimization of grids for cellular cryo-ET is primarily carried out by adjusting the concentration of the cell suspension during seeding.…”