Cardiolipin and mitochondrial cristae organization

Abstract

A fundamental question in cell biology, under investigation for over six decades, is the structural organization of mitochondrial cristae. Long known to harbor electron transport chain proteins, crista membrane integrity is key to establishment of the proton gradient that drives oxidative phosphorylation. Visualization of cristae morphology by electron microscopy/tomography has provided evidence that cristae are tube-like extensions of the mitochondrial inner membrane (IM) that project into the matrix space. Reconciling ultrastructural data with the lipid composition of the IM provides support for a continuously curved cylindrical bilayer capped by a dome-shaped tip. Strain imposed by the degree of curvature is relieved by an asymmetric distribution of phospholipids in monolayer leaflets that comprise cristae membranes. The signature mitochondrial lipid, cardiolipin (~18% of IM phospholipid mass), and phosphatidylethanolamine (34%) segregate to the negatively curved monolayer leaflet facing the crista lumen while the opposing, positively curved, matrix-facing monolayer leaflet contains predominantly phosphatidylcholine. Associated with cristae are numerous proteins that function in distinctive ways to establish and/or maintain their lipid repertoire and structural integrity. By combining unique lipid components with a set of protein modulators, crista membranes adopt and maintain their characteristic morphological and functional properties. Once established, cristae ultrastructure has a direct impact on oxidative phosphorylation, apoptosis, fusion/fission as well as diseases of compromised energy metabolism.

Keywords: Cardiolipin; Cristae; Electron transport chain; Membrane curvature; Mitochondria; Non-bilayer lipid; Phospholipid.

Figure 1. Ultrastructure and morphology of mitochondria

A) Image of cell interior depicting mitochondria (gold) of different shapes and sizes. Cristae are seen as digitiform extensions from the periphery into the matrix space. The cell nucleus is shown in red (from [74] with permission). B) Transmission electron micrograph (longitudinal section) of a bat pancreas mitochondrion. Cristae fine structure is depicted, revealing closely apposed, roughly parallel, membrane extensions (see black arrow; image by Keith R. Porter). C) Electron tomography image of cristae from a chick cerebellum mitochondrion. This computer-derived model was generated from segmented 3D tomograms. Cristae are depicted in yellow and the IBM in blue (from [6] with permission).

Figure 2. Diagram of mitochondrial energy metabolism and cristae organization

Fatty acid β-oxidation, glycolysis and amino acid metabolism generate acetyl CoA that fuels the TCA cycle, producing NADH and FADH₂. These reduced cofactors are oxidized by Complexes I and II, respectively, of the ETC. The transmembrane ETC complexes are depicted as various shaped and colored objects within each crista. The outermost cristae in the diagram shows the exterior, matrix facing membrane surface while the two cristae toward the center of the diagram show a cut-away view, revealing their interior lumen space. As NADH and FADH₂ are oxidized to NAD⁺ and FAD⁺, free energy from electron transfer is used to pump H⁺ into the crista lumen, generating an electrochemical gradient across the crista membrane. This gradient is the driving force for ATP synthesis via Complex V (ATP synthase; depicted as yellow objects).

Figure 3. Phospholipid asymmetry in cristae membranes

To stabilize a bilayer membrane with an ∼15 nm radius of curvature, phosphatidylchoine (PC), phosphatidylinositol (PI) and PS segregate into the positively curved leaflet while cardiolipin (CL) and PE partition into the apposing negatively curved leaflet. Phospholipid segregation in this manner decreases torsional strain that would otherwise exist in a highly curved bilayer. Not shown are the abundant protein components of cristae membranes, including ETC complexes. The phospholipid composition of the IM of rat liver mitochondria [29] is presented.

Figure 4. PHB ring orientation in a crista membrane

A) Electron micrograph of a mitochondrion. A portion of a single crista stalk region (yellow box) has been schematically expanded (B) to indicate a continuously curved, cylindrical bilayer membrane that envelops the crista lumen space. In (C) an assembled, membrane-associated, PHB ring circumscribes the interior of the crista membrane, providing a structural scaffold and/or diffusion barrier (adapted from Osman et al [68] with permission).