Mitochondrial Cristae Shape Determines Respiratory Chain Supercomplexes Assembly and Respiratory Efficiency

摘要

class="head no_bottom_margin" id="sec1title">IntroductionMitochondria are key organelles in intermediate cellular metabolism, energy conversion, and calcium homeostasis (). They also integrate and amplify apoptosis induced by intrinsic stimuli, releasing cytochrome c and other proapoptotic factors required for the activation of caspases (). Cytochrome c release is regulated by proteins of the BCL-2 family that control the permeabilization of the outer membrane (OMM) ().Energy conversion occurs at the inner mitochondrial membrane (IMM) that can be further divided into two subcompartments: the so-called “boundary membrane” and the cristae, separated from the former by narrow tubular junctions (). The cristae shape is dynamic: upon activation of mitochondrial respiration, “orthodox” mitochondria become “condensed,” with an expanded cristae space (). During apoptosis, the curvature of the cristae membrane is inverted in a remodeling process required for the complete release of cytochrome c, normally confined in the cristae (). Cristae remodeling occurs in response to proapoptotic BH3-only BCL-2 family members, such as BID, BIM-S, and BNIP3, and independently of the outer membrane multidomain BCL-2 family members BAX and BAK (). Whether changes in morphology of the cristae, where respiratory chain complexes (RCCs) mainly localize (), affect oxidative phosphorylation efficiency, as originally predicted (), is unclear. This issue is further complicated by the assembly of RCC in supercomplexes (RCS) (), quaternary supramolecular structures that, by channeling electrons among individual RCCs, allow the selective use of RCC subsets for nicotine adenine dinucleotide (NADH)- or flavin adenine dinucleotide-derived electrons (). Such a supramolecular organization is common in cristae: also, the mitochondrial ATP synthase is assembled into dimers with greater adenosine triphosphatase (ATPase) activity (). Interestingly, cristae shape and ATPase dimers are linked: in yeast mutants where the ATPase cannot dimerize, cristae are disorganized (), whereas in mammalian cells, increased cristae density favors ATPase dimerization during autophagy (). On the contrary, despite their importance in mitochondrial bioenergetics, the relationship between RCS and cristae shape remains unclear.Mitochondrial morphology and ultrastructure depends on “mitochondria-shaping” proteins that regulate organellar fusion and fission (href="#bib23" rid="bib23" class=" bibr popnode">Griparic and van der Bliek, 2001). Mitofusins (MFN) 1 and 2, highly homologous dynamin-related proteins of the OMM, orchestrate fusion (href="#bib33" rid="bib33 bib27 bib6 bib34" class=" bibr popnode">Santel and Fuller, 2001; Legros et al., 2002; Chen et al., 2003; Santel et al., 2003). MFN1 primarily participates in fusion, cooperating with the IMM dynamin-related protein optic atrophy 1 (OPA1) (href="#bib10" rid="bib10" class=" bibr popnode">Cipolat et al., 2004), whereas MFN2 also tethers mitochondria to the endoplasmic reticulum (href="#bib15" rid="bib15" class=" bibr popnode">de Brito and Scorrano, 2008). Mitochondrial fission is regulated by the cytoplasmic dynamin-related protein 1 that, upon calcineurin-dependent dephosphorylation, translocates to mitochondria (href="#bib46" rid="bib46 bib37 bib5" class=" bibr popnode">Yoon et al., 2001; Smirnova et al., 2001; Cereghetti et al., 2008). Genetic depletion of OPA1 leads to disorganization of the cristae (href="#bib19" rid="bib19" class=" bibr popnode">Frezza et al., 2006), and oligomers that contain a soluble and a membrane-bound form of OPA1 keep the cristae junctions tight, independently from OPA1 role in fusion (href="#bib19" rid="bib19 bib11" class=" bibr popnode">Frezza et al., 2006; Cipolat et al., 2006). During apoptosis, these oligomers are early targets of BID, BIM-S, and BNIP3, as well as of intrinsic death stimuli (href="#bib19" rid="bib19 bib45 bib25 bib12" class=" bibr popnode">Frezza et al., 2006; Yamaguchi et al., 2008; Landes et al., 2010; Costa et al., 2010). Whereas our knowledge of the molecular determinants of cristae shape and their role in apoptosis is increasing, the relationship between cristae morphology and mitochondrial function remains unexplored. We therefore set out to genetically dissect whether and how cristae shape regulates mitochondrial respiration. We show that cristae morphology determines assembly and stability of RCS and hence optimal mitochondrial respiratory function during life and death of the cell.