Mitochondrial fragmentation in neurodegeneration

Mitochondria are remarkably dynamic organelles that migrate, divide and fuse. Cycles of mitochondrial fission and fusion ensure metabolite and mitochondrial DNA mixing and dictate organelle shape, number and bioenergetic functionality. There is mounting evidence that mitochondrial dysfunction is an early and causal event in neurodegeneration. Mutations in the mitochondrial fusion GTPases mitofusin 2 and optic atrophy 1, neurotoxins and oxidative stress all disrupt the cable-like morphology of functional mitochondria. This results in impaired bioenergetics and mitochondrial migration, and can trigger neurodegeneration. These findings suggest potential new treatment avenues for neurodegenerative diseases.     

DNA fragmentation in apoptosis

Cleavage of chromosomal DNA into oligonucleosomal size fragments is an integral part of apoptosis.Elegant biochemical work identified the DNA fragmentation factor(DFF) as a major apoptotic endonuclease for DNA fragmentation in vitro Genetic studies in mice support the importence of DFF in DNA fragmentation and possibly in apoptosis in vivo.Recent work also suggests the existence of additional endonucleases for DNA degradation.Understanding the roles of individual endonucleases in apoptosis,and how they might coordinate to degrade DNA in different tissues during normal development and homeostasis,as well as in various diseased states,will be a major research focus in the near future.     

Mitochondrial fragmentation in excitotoxicity requires ROCK activation

Mitochondria morphology constantly changes through fission and fusion processes that regulate mitochondrial function, and it therefore plays a prominent role in cellular homeostasis. Cell death progression is associated with mitochondrial fission. Fission is mediated by the mainly cytoplasmic Drp1, which is activated by different post-translational modifications and recruited to mitochondria to perform its function. Our research and other studies have shown that in the early moments of excitotoxic insult Drp1 must be nitrosylated to mediate mitochondrial fragmentation in neurons. Nonetheless, mitochondrial fission is a multistep process in which filamentous actin assembly/disassembly and myosin-mediated mitochondrial constriction play prominent roles. Here we establish that in addition to nitric oxide production, excitotoxicity-induced mitochondrial fragmentation also requires activation of the actomyosin regulator ROCK. Although ROCK1 has been shown to phosphorylate and activate Drp1, experiments using phosphor-mutant forms of Drp1 in primary cortical neurons indicate that in excitotoxic conditions, ROCK does not act directly on Drp1 to mediate fission, but may act on the actomyosin complex. Thus, these data indicate that a wider range of signaling pathways than those that target Drp1 are amenable to be inhibited to prevent mitochondrial fragmentation as therapeutic option.     

Prothymosin alpha fragmentation in apoptosis

We observed fragmentation of an essential proliferation-related human nuclear protein prothymosin alpha in the course of apoptosis induced by various stimuli. Prothymosin alpha cleavage occurred at the DDVD(99) motif. In vitro, prothymosin alpha could be cleaved at D(99) by caspase-3 and -7. Caspase hydrolysis disrupted the nuclear localization signal of prothymosin alpha and abrogated the ability of the truncated protein to accumulate inside the nucleus. Prothymosin alpha fragmentation may therefore be proposed to disable intranuclear proliferation-related function of prothymosin alpha in two ways: by cleaving off a short peptide containing important determinants, and by preventing active nuclear uptake of the truncated protein.     

Mitochondrial fission in apoptosis

Mitochondria fuse and divide continuously within cells to form a dynamic network. One of the steps in apoptosis is the fragmentation of mitochondria, and recent evidence indicates that the mitochondrial fission machinery actively participates in the process of programmed cell death.     

DNA fragmentation in apoptosis and homeostasis

Apoptosis is a highly regulated physiological process critical in development and tissue homeostasis. Abnormal apoptosis can lead to disease conditions including neurodegeneration, autoimmunity and cancer. DNA fragmentation is an integral part of apoptosis and has long been suspected to be of critical importance in cleaning up potentially antigenic DNA and genetic material capable of inducing neoplasmic transformation in neighboring cells. Direct evidence for this function of DNA fragmentation however, is still lacking. The identification of a heterodimeric DNA fragmentation factor 45 and 40 (DFF45 and DFF40, also called ICAD for Inhibitor of Caspase Activated DNase and CAD for Caspase Activated DNase respectively) as well as     

Mitochondrial disruption in Drosophila apoptosis

Mitochondrial disruption is a conserved aspect of apoptosis, seen in many species from mammals to nematodes. Despite significant conservation of other elements of the apoptotic pathway in Drosophila, a broad role for mitochondrial changes in apoptosis in flies remains unconfirmed. Here, we show that Drosophila mitochondria become permeable in response to the expression of Reaper and Hid, endogenous regulators of developmental apoptosis. Caspase activation in the absence of Reaper and Hid is not sufficient to permeabilize mitochondria, but caspases play a role in Reaper- and Hid-induced mitochondrial changes. Reaper and Hid rapidly localize to mitochondria, resulting in changes in mitochondrial ultrastructure. The dynamin-related protein, Drp1, is important for Reaper- and DNA-damage-induced mitochondrial disruption. Significantly, we show that inhibition of Reaper or Hid mitochondrial localization or inhibition of Drp1 significantly inhibits apoptosis, indicating a role for mitochondrial disruption in fly apoptosis.     

线粒体在细胞凋亡中的介导作用

线粒体是细胞内产生能量的重要细胞器,被认为是细胞生存与死亡的调节中心。Bcl-2家族蛋白、内质网和溶酶体能引起线粒体膜通透性的改变,造成线粒体功能损伤,诱导细胞凋亡。本文主要综述线粒体在Bcl-2家族蛋白、内质网和溶酶体诱导细胞凋亡中作用的研究进展。     

Mitochondrial fragmentation in human macrophages attenuates palmitate-induced inflammatory responses

Macrophages in adipose tissue contribute to inflammation and the development of insulin resistance in obesity. Exposure of macrophages to saturated fatty acids alters cell metabolism and activates pro-inflammatory signaling. How fatty acids influence macrophage mitochondrial dynamics is unclear. We investigated the mechanism of palmitate-induced mitochondrial fragmentation and its impact on inflammatory responses in primary human macrophages. Fatty acids, such as palmitate, caused mitochondrial fragmentation in human macrophages. Increased mitochondrial fragmentation was also observed in peritoneal macrophages from hyperlipidemic apolipoprotein E knockout mice. Fatty acid-induced mitochondrial fragmentation was independent of the fatty acid chain saturation and required dynamin-related protein 1 (DRP1). Mechanistically, mitochondrial fragmentation was regulated by incorporation of palmitate into mitochondrial phospholipids and their precursors. Palmitate-induced endoplasmic reticulum stress and loss of mitochondrial membrane potential did not contribute to mitochondrial fragmentation. Macrophages treated with palmitate maintained intact mitochondrial respiration and ATP levels. Pharmacological or genetic inhibition of DRP1 enhanced palmitate-induced mitochondrial ROS production, c-Jun phosphorylation, and inflammatory cytokine expression. Our results indicate that mitochondrial fragmentation is a protective mechanism attenuating inflammatory responses induced by palmitate in human macrophages.     

Mitochondrial dynamics during apoptosis

Mitochondria exist as dynamic networks that often change shape and subcellular distribution. The morphology of mitochondria within a cell is controlled by precisely regulated rates of organelle fusion and fission. Several reports have described dramatic alterations in mitochondrial morphology during the early stages of apoptosis: a fragmentation of the network and the cristae remodeling. However, whether this mitochondrial fragmentation is a required step for apoptosis is highly debated. In this review the recent progress in understanding the mechanisms governing mitochondrial morphology during apoptosis and the latest advances connecting the regulation of mitochondrial morphology with apoptosis are discussed.     

The role of DNA fragmentation in apoptosis

The formation of distinct DNA fragments of oligonucleosomal size (180-200 bp lengths) is a biochemical hallmark of apoptosis in many cells. Recent observations also suggest large DNA fragments and even single-strand cleavage events occur during cell death. These observations have raised many questions. What are the types of DNA cleavage observed during apoptosis? What are the nucleases involved? And what is the role of these nucleolytic events in apoptosis?     

Detection of DNA fragmentation and endonucleases in apoptosis

DNA degradation during apoptosis is endonuclease mediated and proceeds through an ordered series of stages commencing with the production of large DNA pieces of 300 kb which are then degraded to fragments of 50 kb. The 50-kb fragments are further degraded, in some but not all cells, to smaller pieces (10-40 kb) releasing the small oligonucleosome fragments that are detected as a characteristic DNA ladder on conventional agarose gels. Methodology is presented for the detection of both DNA ladders and the initial stages of DNA fragmentation using pulsed-field gel electrophoresis. We have developed electrophoresis conditions that resolve large fragments of DNA and also retain the smaller fragments on the same gel. Methods for the detection of endonuclease activities responsible for the cleavage of DNA during apoptosis are also presented.     

Mitochondrial outer-membrane permeabilization and remodelling in apoptosis

Many human pathologies are associated with defects in mitochondria such as diabetes, neurodegenerative diseases or cancer. This tiny organelle is involved in a plethora of processes in mammalian cells, including energy production, lipid metabolism and cell death. In the so-called intrinsic apoptotic pathway, the outer mitochondrial membrane (MOM) is premeabilized by the pro-apoptotic Bcl-2 members Bax and Bak, allowing the release of apoptogenic factors such as cytochrome c from the inter-membrane space into the cytosol. At the same time, mitochondria fragment in response to Drp-1 activation suggesting that mitochondrial fission could play a role in mitochondrial outer-membrane permeabilization (MOMP). In this review, we will discuss the link that could exist between mitochondrial fission and fusion machinery, Bcl-2 family members and MOMP.     

Mitochondrial intermembrane space proteins in apoptosis process

Mitochondria, despite their function in cellular energy metabolism, play an important role in the apoptotic signaling pathways. These organelles in response to the death signal undergo changes resulting in the release of proteins which are essential to conduct apoptosis via mitochondrial pathway. This article is focused on the properties and functions of apoptogenic proteins released from the mitochondrial intermembrane space, i.e., caspases, cytochrome c, Smac/DIABLO, serine protease Omi/HtrA2, AIF and endonuclease G.     

Mitochondrial permeability transition in apoptosis and necrosis

Apoptosis has classically been viewed as a process not involving mitochondria, whereas the implication of mitochondrial dysfunction in necrosis has been recognized for several decades. Recently, it has become clear that apoptosis implies a disruption of mitochondrial membrane intregrity that is decisive for the cell death process. Cytofluorometric methods assessing the mitochondrial membrane function and structure can be employed to demonstrate that, at least in most models of apoptosis, mitochondrial changes precede caspase and nuclease activation. Moreover, pharmacological and genetic experiments suggest that the loss of mitochondrial membrane integrity is a critical event of the apoptotic process, beyond or at the point of no return of programmed cell death. Inhibitors of the mitochondrial megachannel (= permeability transition pore) can prevent both the mitochondrial and the post-mitochondrial manifestations of apoptosis.     

Mitochondrial mechanisms of neural cell apoptosis

The importance of calcium overload, mitochondrial dysfunction, and free radical generation to neuropathological processes has been recognized for many years. Only more recently has evidence accumulated that the programmed cell death process of apoptosis plays an integral role not only in the development of the nervous system, but in the loss of cells following acute neurological insults and chronic disease. In 1996 came the landmark discovery that cytochrome c, an evolutionary old and essential component of the respiratory chain, has a second and deadly function when it escapes the mitochondrion: triggering the cell death cascade. A flurry of activity has since ensued in an effort to understand the mechanistic events associated with mitochondrial permeabilization during apoptosis and regulation by an enigmatic family of proteins characterized by homology to the proto-oncogene Bcl-2. This review discusses the evidence for various release mechanisms of apoptotic proteins (e.g. cytochrome c) from neural cell mitochondria, focusing particularly on roles for calcium, Bax, p53, and oxidative stress. The need for new drugs that act at the level of the mitochondrion to prevent apoptosis is also highlighted.