Mitochondrial Membrane Organization in Regulation of Apoptosis

Abstract

Programmed cell death (apoptosis) is an essential mechanism in life. Key regulators of the intrinsic mitochondrial apoptotic pathway are pro- and anti-apoptotic members of the Bcl-2 family who meet at the mitochondrion's surface - as defined by its outer membrane system - where they arbitrate a life or death decision. Our main aim is to address this molecular regulation mechanism occurring at this mitochondrial outer membrane (MOM) level. For this purpose we use the anti-apoptotic Bcl-2 protein itself which is an integral membrane protein, and its counterpart, the pro-apoptotic Bax protein, to address specifically the importance of the MOM system in the recruitment of Bax.We could show that damaged mitochondrial outer membranes - generated upon oxidative stress - increase the affinity and therefore the translocation of Bax towards these membranes dramatically, and even promote partial penetration of full-length Bax [1]. This process was characterized by a combination of solid state NMR spectroscopy, differential scanning calorimetry and CD spectroscopy [1]. More recently, we found by applying fluorescence based leakage assays that Bax under these conditions is already able to induce MOM pore formation without additional mediator proteins (e.g. tBid); albeit creating a pore size not sufficient for cytochrome c release [2]. Surprisingly, the Bax-induced leakage increased with the amount of oxidized lipid present without any detectable threshold neither for oxidized phospholipids (OxPl) nor Bax. And the leakage rate correlated with i) Bax to lipid ratio and ii) the oxidized lipid content.This observation of an OxPls induced significant increase in Bax membrane affinity, challenged the traditional view of Bax activation. We therefore propose a model, where Bax in a dynamic equilibrium shuttles between cytosol and the membrane. Upon apoptotic stress this equilibrium shifts towards the membrane-bound state; a prerequisite for Bax induced permeabilization of the MOM and the final execution of cell death through the release of apoptotic factors from the mitochondrial intermembrane space.[1] M. Wallgren, M. Lidman, Q. Dat Pham, .. G. Grobner. The oxidized phospholipid PazePC modulates interactions between Bax and mitochondrial membranes. BBA Biomembranes 1818 (2012) 2718 - 2724.[2] M. Lidman, S. Pokorna, A.P.G. Dingeldein, T. Sparrman, .. G. Grobner. The oxidized phospholipid PazePC promotes the formation of Bax pores in mitochondrial membranes. BBA Biomembranes 1858 (2016) 1288 - 1297.