Mitochondria amplify activation of caspases during apoptosis by releasing cytochrome c and other cofactors. This is accompanied by fragmentation of the organelle and remodeling of the cristae. Here we provide evidence that Optic Atrophy 1 (OPA1), a profusion dynamin-related protein of the inner mitochondrial membrane mutated in dominant optic atrophy, protects from apoptosis by preventing cytochrome c release independently from mitochondrial fusion. OPA1 does not interfere with activation of the mitochondrial "gatekeepers" BAX and BAK, but it controls the shape of mitochondrial cristae, keeping their junctions tight during apoptosis. Tightness of cristae junctions correlates with oligomerization of two forms of OPA1, a soluble, intermembrane space and an integral inner membrane one. The proapoptotic BCL-2 family member BID, which widens cristae junctions, also disrupts OPA1 oligomers. Thus, OPA1 has genetically and molecularly distinct functions in mitochondrial fusion and in cristae remodeling during apoptosis.

Programmed cell death is a distinct genetic and biochemical pathway essential to metazoans. An intact death pathway is required for successful embryonic development and the maintenance of normal tissue homeostasis. Apoptosis has proven to be tightly interwoven with other essential cell pathways. The identification of critical control points in the cell death pathway has yielded fundamental insights for basic biology, as well as provided rational targets for new therapeutics.

Mitochondrial permeability transition (PT) is a phenomenon induced by high levels of matrix calcium and is characterized by the opening of the PT pore (PTP). Activation of the PTP results in loss of mitochondrial membrane potential, expansion of the matrix, and rupture of the mitochondrial outer membrane. Consequently, PT has been implicated in both apoptotic and necrotic cell death. Cyclophilin D (CypD) appears to be a critical component of the PTP. To investigate the role of CypD in cell death, we created a CypD-deficient mouse. In vitro, CypD-deficient mitochondria showed an increased capacity to retain calcium and were no longer susceptible to PT induced by the addition of calcium. CypD-deficient primary mouse embryonic fibroblasts (MEFs) were as susceptible to classical apoptotic stimuli as the WT, suggesting that CypD is not a central component of cell death in response to these specific death stimuli. However, CypD-deficient MEFs were significantly less susceptible than their WT counterparts to cell death induced by hydrogen peroxide, implicating CypD in oxidative stress-induced cell death. Importantly, CypD-deficient mice displayed a dramatic reduction in brain infarct size after acute middle cerebral artery occlusion and reperfusion, strongly supporting an essential role for CypD in an ischemic injury model in which calcium overload and oxidative stress have been implicated. mitochondria ͉ oxidative stress ͉ calcium homeostasis N umerous toxic stimuli convert mitochondria from a lifesustaining organelle to an inducer of cell death (1-3). In response to cell death stimuli, prodeath proteins sequestered in the inner mitochondrial membrane space are released into the cytosol upon disruption of the mitochondrial outer membrane (MOM) (4-6). The proapoptotic BCL-2 family members BAX and BAK constitute a ''gateway'' to the apoptotic program by regulating events leading to disruption of the MOM (7). BCL-2 family members not only function at the MOM but also are located at the endoplasmic reticulum (ER), where they regulate calcium fluxes. By controlling steady-state calcium levels in the ER, BCL-2 family members regulate the amount of calcium released from the ER to transmit a death signal mediated by mitochondrial calcium uptake (8-11).Sequestration of high levels of calcium by mitochondria can also lead to disruption of the MOM, which, mechanistically, is thought to occur by means of the phenomenon of permeability transition (PT). PT is described as an abrupt increase of inner membrane permeability to solutes with molecular masses of Ͻ1,500 Da. These events are caused by the opening of a highly regulated channel [PT pore (PTP)], which leads to dissipation of the mitochondrial transmembrane potential and an influx of solutes, causing expansion of the matrix (reviewed in ref. 12). The latter event may result in sufficient swelling to rupture the MOM and cause cytochrome c release and subsequent caspase activation, resulting in apoptosis. However, dissipation of the membrane potential can also lead to a sudden d...

Glycolysis and apoptosis are considered major but independent pathways that are critical for cell survival. The activity of BAD, a pro-apoptotic BCL-2 family member, is regulated by phosphorylation in response to growth/survival factors. Here we undertook a proteomic analysis to assess whether BAD might also participate in mitochondrial physiology. In liver mitochondria, BAD resides in a functional holoenzyme complex together with protein kinase A and protein phosphatase 1 (PP1) catalytic units, Wiskott-Aldrich family member WAVE-1 as an A kinase anchoring protein, and glucokinase (hexokinase IV). BAD is required to assemble the complex in that Bad-deficient hepatocytes lack this complex, resulting in diminished mitochondria-based glucokinase activity and blunted mitochondrial respiration in response to glucose. Glucose deprivation results in dephosphorylation of BAD, and BAD-dependent cell death. Moreover, the phosphorylation status of BAD helps regulate glucokinase activity. Mice deficient for BAD or bearing a non-phosphorylatable BAD(3SA) mutant display abnormal glucose homeostasis including profound defects in glucose tolerance. This combination of proteomics, genetics and physiology indicates an unanticipated role for BAD in integrating pathways of glucose metabolism and apoptosis.

SUMMARY Molecular signatures have identified several subsets of Diffuse Large B-Cell Lymphoma (DLBCL) and rational targets within the B-cell receptor (BCR) signaling axis. The OxPhos-DLBCL subset, which harbors the signature of genes involved in mitochondrial metabolism, is insensitive to inhibition of BCR survival signaling, but is functionally undefined. We show that compared with BCR-DLBCLs, OxPhos-DLBCLs display enhanced mitochondrial energy transduction, greater incorporation of nutrient-derived carbons into the TCA cycle and increased glutathione levels. Importantly, perturbation of the fatty acid oxidation program and glutathione synthesis proved selectively toxic to this tumor subset. Our analysis provides evidence for distinct metabolic fingerprints and associated survival mechanisms in DLBCL and may have therapeutic implications.

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Summary A hallmark of type 2 diabetes mellitus (T2DM) is the development of pancreatic β cell failure, resulting in insulinopenia and hyperglycemia. We show that the adipokine adipsin has a beneficial role in maintaining β cell function. Animals genetically lacking adipsin have glucose intolerance due to insulinopenia; isolated islets from these mice have reduced glucose-stimulated insulin secretion. Replenishment of adipsin to diabetic mice treated hyperglycemia by boosting insulin secretion. We identify C3a, a peptide generated by adipsin, as a potent insulin secretagogue and show that the C3a receptor is required for these beneficial effects of adipsin. C3a acts on islets by augmenting ATP levels, respiration and cytosolic free Ca2+. Finally, we demonstrate that T2DM patients with β cell failure are deficient in adipsin. These findings indicate that the adipsin/C3a pathway connects adipocyte function to β cell physiology and manipulation of this molecular switch may serve as a novel therapy in T2DM.

Apoptosis is a morphologically distinct form of programmed cell death essential for normal development and tissue homeostasis. Aberrant regulation of this pathway is linked to multiple human diseases, including cancer, autoimmunity, neurodegenerative disorders, and diabetes. The BCL-2 family of proteins constitutes a critical control point in apoptosis residing immediately upstream of irreversible cellular damage, where family members control the release of apoptogenic factors from mitochondria.The cardinal member of this family, BCL-2, was originally discovered as the defining oncogene in follicular lymphomas, located at one reciprocal breakpoint of the t(14;18) (q32;q21) chromosomal translocation. Since this original discovery, remarkable efforts marshaled by many investigators around the world have advanced our knowledge of the basic biology, molecular mechanisms, and therapeutic targets in the apoptotic pathway. This review highlights findings from many laboratories that have helped uncover some of the critical control points in apoptosis. The emerging picture is that of an intricate cellular machinery orchestrated by tightly regulated molecular interactions and conformational changes within BCL-2 family proteins that ultimately govern the cellular commitment to apoptotic death.Apoptosis is a conserved genetic and biochemical pathway whose basic tenets are present in all metazoans (1, 2). In mammals, the execution of this pathway is governed by two molecular programs which ultimately lead to the activation of select members of the caspase (cysteinyl aspartate -specific protease) family. Subsequently, cleavage of key cellular substrates ensues, leading to cell demise. The two molecular programs are known as the extrinsic pathway operating downstream of death receptors, such as Fas and the tumor necrosis factor receptor family, and the intrinsic pathway, which is activated by a diverse array of stress signals. The identification of cytochrome c as an apoptogenic factor released from mitochondria marked a pivotal breakthrough in uncovering the importance of this organelle in the intrinsic pathway of apoptosis (3). The ''point of no return'' in this pathway is defined by mitochondrial outer membrane permeabilization (MOMP), which leads to the release of cytochrome c (4). BCL-2 family proteins regulate MOMP and thereby determine the cellular commitment to apoptosis. This review is limited in scope to the intrinsic pathway and its regulation by BCL-2 family of proteins. In particular, recent advances in understanding the interplay between distinct members of the BCL-2 family and the molecular mechanisms underlying their regulation of MOMP are described. Broader overview of the BCL-2 family can be found in reviews published elsewhere (1, 2). Likewise, the role of the intrinsic pathway and apoptosis in the larger context of mechanisms of cell death are the subject of another review in this issue of CCR Focus (5). Execution of the Apoptotic ProgramCaspases are present as inactive zymogens that are activated ...