Mitochondrial Oxidative Stress and Mitophagy Activation Contribute to TNF-Dependent Impairment of Myogenesis.

Many muscular pathologies are associated with oxidative stress and elevated levels of the tumor necrosis factor (TNF) that cause muscle protein catabolism and impair myogenesis. Myogenesis defects caused by TNF are mediated in part by reactive oxygen species (ROS), including those produced by mitochondria (mitoROS), but the mechanism of their pathological action is not fully understood. We hypothesized that mitoROS act by triggering and enhancing mitophagy, an important tool for remodelling the mitochondrial reticulum during myogenesis. We used three recently developed probes-MitoTracker Orange CM-H2TMRos, mito-QC, and MitoCLox-to study myogenesis in human myoblasts. Induction of myogenesis resulted in a significant increase in mitoROS generation and phospholipid peroxidation in the inner mitochondrial membrane, as well as mitophagy enhancement. Treatment of myoblasts with TNF 24 h before induction of myogenesis resulted in a significant decrease in the myoblast fusion index and myosin heavy chain (MYH2) synthesis. TNF increased the levels of mitoROS, phospholipid peroxidation in the inner mitochondrial membrane and mitophagy at an early stage of differentiation. Trolox and SkQ1 antioxidants partially restored TNF-impaired myogenesis. The general autophagy inducers rapamycin and AICAR, which also stimulate mitophagy, completely blocked myogenesis. The autophagy suppression by the ULK1 inhibitor SBI-0206965 partially restored myogenesis impaired by TNF. Thus, suppression of myogenesis by TNF is associated with a mitoROS-dependent increase in general autophagy and mitophagy.

Mitochondria-Targeted Antioxidant SkQ1 Prevents the Development of Experimental Colitis in Mice and Impairment of the Barrier Function of the Intestinal Epithelium.

Mitochondria-targeted antioxidants have become promising candidates for the therapy of various pathologies. The mitochondria-targeted antioxidant SkQ1, which is a derivative of plastoquinone, has been successfully used in preclinical studies for the treatment of cardiovascular and renal diseases, and has demonstrated anti-inflammatory activity in a number of inflammatory disease models. The present work aimed to investigate the therapeutic potential of SkQ1 and C12TPP, the analog of SkQ1 lacking the antioxidant quinone moiety, in the prevention of sodium dextran sulfate (DSS) experimental colitis and impairment of the barrier function of the intestinal epithelium in mice. DSS-treated animals exhibited weight loss, bloody stool, dysfunction of the intestinal epithelium barrier (which was observed using FITC-dextran permeability), reduced colon length, and histopathological changes in the colon mucosa. SkQ1 prevented the development of clinical and histological changes in DSS-treated mice. SkQ1 also reduced mRNA expression of pro-inflammatory molecules TNF, IL-6, IL-1ß, and ICAM-1 in the proximal colon compared with DSS-treated animals. SkQ1 prevented DSS-induced tight junction disassembly in Caco-2 cells. Pretreatment of mice by C12TPP did not protect against DSS-induced colitis. Furthermore, C12TPP did not prevent DSS-induced tight junction disassembly in Caco-2 cells. Our results suggest that SkQ1 may be a promising therapeutic agent for the treatment of inflammatory bowel diseases, in particular ulcerative colitis.

Comparison of chemiluminescent heterogeneous and homogeneous-heterogeneous assays coupled with isothermal circular strand-displacement polymerization reaction amplification for the quantification of miRNA-141.

Heterogeneous and homogeneous-heterogeneous assays for the quantitation of hsa-miR-141-3p (miRNA-141) were constructed. Both microplate assays were based on the use of the isothermal circular strand-displacement polymerization reaction (ICSDPR), which was carried out in heterogeneous and homogeneous media, respectively. In addition, a streptavidin-polyperoxidase conjugate and enhanced chemiluminescence were used to increase the assay's sensitivity. A comparison of the developed assays showed that the sensitivity of the heterogeneous assay was higher than that of the homogeneous-heterogeneous assay. The detection limit values of the heterogeneous and homogeneous-heterogeneous assays were 51 fM and 10 pM, respectively. The amplification index for the ICSDPR used in the heterogeneous assay of miRNA-141 was 100. Using miRNAs of the miRNA-200 family, the high specificity of the assay was demonstrated. MiRNA-141 in human cultured cells was determined by the heterogeneous ICSDPR-assisted assay with chemiluminescence detection. To assess the purification yield of miRNAs from cellular lysates, the heterogeneous assay of miRNA-39 developed on the same platform was used. The intracellular content of miRNA-141 in Caco-2, HepG2, MCF-7 and HeLa was shown to be 3400, 1400, 1300 and 470 copies per cell, respectively.

Isothermal chemiluminescent assay based on circular stand-displacement polymerization reaction amplification for cel-miRNA-39-3p determination in cell extracts.

A sensitive and specific heterogeneous assay for quantitation of cel-miRNA-39-3p (miRNA-39) was constructed. To improve the assay sensitivity an amplification strategy based on the use of isothermal circular strand-displacement polymerization reaction (ICSDPR), polyperoxidase conjugated with streptavidin and enhanced chemiluminescence was used. The detection limit of the proposed assay was 4 × 10-13 M. The coefficient of variation (CV) for quantitation of miRNA-39 within the working range was below 8%. The study of cross-reactivity of different miRNAs including miRNA-39 demonstrated high specificity of the proposed assay. Comparison of the calibration curves of miRNA-39 dissolved in the buffer and the lysate of MCF-7 cells (prepared by lysis of the cells with phenol/guanidine thiocyanate mixture and purified using silica membrane spin column) has demonstrated a negligible matrix effect. The proposed assay makes it possible to estimate the yield of purification of miRNAs from cells, which is necessary for the quantitative calculation of the intracellular content of miRNAs measured with the isothermal assay coupled with ICSDPR.

Novel Fluorescent Mitochondria-Targeted Probe MitoCLox Reports Lipid Peroxidation in Response to Oxidative Stress In Vivo.

A new mitochondria-targeted probe MitoCLox was designed as a starting compound for a series of probes sensitive to cardiolipin (CL) peroxidation. Fluorescence microscopy reported selective accumulation of MitoCLox in mitochondria of diverse living cell cultures and its oxidation under stress conditions, particularly those known to cause a selective cardiolipin oxidation. Ratiometric fluorescence measurements using flow cytometry showed a remarkable dependence of the MitoCLox dynamic range on the oxidation of the sample. Specifically, MitoCLox oxidation was induced by low doses of hydrogen peroxide or organic hydroperoxide. The mitochondria-targeted antioxidant 10-(6'-plastoquinonyl)decyltriphenyl-phosphonium (SkQ1), which was shown earlier to selectively protect cardiolipin from oxidation, prevented hydrogen peroxide-induced MitoCLox oxidation in the cells. Concurrent tracing of MitoCLox oxidation and membrane potential changes in response to hydrogen peroxide addition showed that the oxidation of MitoCLox started without a delay and was complete during the first hour, whereas the membrane potential started to decay after 40 minutes of incubation. Hence, MitoCLox could be used for splitting the cell response to oxidative stress into separate steps. Application of MitoCLox revealed heterogeneity of the mitochondrial population; in living endothelial cells, a fraction of small, rounded mitochondria with an increased level of lipid peroxidation were detected near the nucleus. In addition, the MitoCLox staining revealed a specific fraction of cells with an increased level of oxidized lipids also in the culture of human myoblasts. The fraction of such cells increased in high-density cultures. These specific conditions correspond to the initiation of spontaneous myogenesis in vitro, which indicates that oxidation may precede the onset of myogenic differentiation. These data point to a possible participation of oxidized CL in cell signalling and differentiation.

Mitochondria-Targeted Antioxidant SkQ1 Improves Dermal Wound Healing in Genetically Diabetic Mice.

Oxidative stress is widely recognized as an important factor in the delayed wound healing in diabetes. However, the role of mitochondrial reactive oxygen species in this process is unknown. It was assumed that mitochondrial reactive oxygen species are involved in many wound-healing processes in both diabetic humans and animals. We have applied the mitochondria-targeted antioxidant 10-(6'-plastoquinonyl)decyltriphenylphosphonium (SkQ1) to explore the role of mitochondrial reactive oxygen species in the wound healing of genetically diabetic mice. Healing of full-thickness excisional dermal wounds in diabetic C57BL/KsJ-db-/db- mice was significantly enhanced after long-term (12 weeks) administration of SkQ1. SkQ1 accelerated wound closure and stimulated epithelization, granulation tissue formation, and vascularization. On the 7th day after wounding, SkQ1 treatment increased the number of α-smooth muscle actin-positive cells (myofibroblasts), reduced the number of neutrophils, and increased macrophage infiltration. SkQ1 lowered lipid peroxidation level but did not change the level of the circulatory IL-6 and TNF. SkQ1 pretreatment also stimulated cell migration in a scratch-wound assay in vitro under hyperglycemic condition. Thus, a mitochondria-targeted antioxidant normalized both inflammatory and regenerative phases of wound healing in diabetic mice. Our results pointed to nearly all the major steps of wound healing as the target of excessive mitochondrial reactive oxygen species production in type II diabetes.

Low concentration of uncouplers of oxidative phosphorylation decreases the TNF-induced endothelial permeability and lethality in mice.

Mitochondrial dysfunctions occur in many diseases linked to the systemic inflammatory response syndrome (SIRS). Mild uncoupling of oxidative phosphorylation is known to rescue model animals from pathologies related to mitochondrial dysfunctions and overproduction of reactive oxygen species (ROS). To study the potential of SIRS therapy by uncoupling, we tested protonophore dinitrophenol (DNP) and a free fatty acid (FFA) anion carrier, lipophilic cation dodecyltriphenylphosphonium (C12TPP) in mice and in vitro models of SIRS. DNP and C12TPP prevented the body temperature drop and lethality in mice injected with high doses of a SIRS inducer, tumor necrosis factor (TNF). The mitochondria-targeted antioxidant plastoquinonyl decyltriphenylphosphonium (SkQ1) which also catalyzes FFA-dependent uncoupling revealed similar protective effects and downregulated expression of the NFκB-regulated genes (VCAM1, ICAM1, MCP1, and IL-6) involved in the inflammatory response of endothelium in aortas of the TNF-treated mice. In vitro mild uncoupling rescued from TNF-induced endothelial permeability, disassembly of cell contacts and VE-cadherin cleavage by the matrix metalloprotease 9 (ММР9). The uncouplers prevented TNF-induced expression of MMP9 via inhibition of NFκB signaling. Water-soluble antioxidant Trolox also prevented TNF-induced activation and permeability of endothelium in vitro via inhibition of NFκB signaling, suggesting that the protective action of the uncouplers is linked to their antioxidant potential.

Mitochondria-Targeted Antioxidants and Uncouplers of Oxidative Phosphorylation in Treatment of the Systemic Inflammatory Response Syndrome (SIRS).

Systemic inflammatory response syndrome (SIRS) development is accompanied by mitochondrial dysfunction and excessive ROS production. Mitochondrial dysfunctions also occur in many SIRS-related diseases and may be critical for their pathogenesis; therefore, a use of mitochondria-targeted drugs is a promising trend in SIRS research and therapy. Here, we review recent studies concerning the application of the mitochondria-targeted antioxidants and uncouplers of oxidative phosphorylation in animal models of SIRS and related diseases. We propose that a new class of uncouplers of oxidative phosphorylation, lipophilic cations could be a base for a new generation of drugs for SIRS treatment. J. Cell. Physiol. 232: 904-912, 2017. © 2016 Wiley Periodicals, Inc.

Mitochondria-targeted antioxidant SkQ1 improves impaired dermal wound healing in old mice.

The process of skin wound healing is delayed or impaired in aging animals. To investigate the possible role of mitochondrial reactive oxygen species (mtROS) in cutaneous wound healing of aged mice, we have applied the mitochondria-targeted antioxidant SkQ1. The SkQ1 treatment resulted in accelerated resolution of the inflammatory phase, formation of granulation tissue, vascularization and epithelization of the wounds. The wounds of SkQ1-treated mice contained increased amount of myofibroblasts which produce extracellular matrix proteins and growth factors mediating granulation tissue formation. This effect resembled SkQ1-induced differentiation of fibroblasts to myofibroblast, observed earlierin vitro. The Transforming Growth Factor beta (TGFb) produced by SkQ1-treated fibroblasts was found to stimulated motility of endothelial cells in vitro, an effect which may underlie pro-angiogenic action of SkQ1 in the wounds. In vitro experiments showed that SkQ1 prevented decomposition of VE-cadherin containing contacts and following increase in permeability of endothelial cells monolayer, induced by pro-inflammatory cytokine TNF. Prevention of excessive reaction of endothelium to the pro-inflammatory cytokine(s) might account for anti-inflammatory effect of SkQ1. Our findings point to an important role of mtROS in pathogenesis of age-related chronic wounds.

Role of mitochondrial reactive oxygen species in age-related inflammatory activation of endothelium.

Vascular aging is accompanied by increases in circulatory proinflammatory cytokines leading to inflammatory endothelial response implicated in early atherogenesis. To study the possible role of mitochondria-derived reactive oxygen species (ROS) in this phenomenon, we applied the effective mitochondria-targeted antioxidant SkQ1, the conjugate of plastoquinone with dodecyltriphenylphosphonium. Eight months treatment of (CBAxC57BL/6) F1 mice with SkQ1 did not prevent age-related elevation of the major proinflammatory cytokines TNF and IL-6 in serum, but completely abrogated the increase in adhesion molecule ICAM1 expression in aortas of 24-month-old animals. In endothelial cell culture, SkQ1 also attenuated TNF-induced increase in ICAM1, VCAM, and E-selectin expression and secretion of IL-6 and IL-8, and prevented neutrophil adhesion to the endothelial monolayer. Using specific inhibitors to transcription factor NF-κB and stress-kinases p38 and JNK, we demonstrated that TNF-induced ICAM1 expression depends mainly on NF-κB activity and, to a lesser extent, on p38. SkQ1 had no effect on p38 phosphorylation (activation) but significantly reduced NF-κB activation by inhibiting phosphorylation and proteolytic cleavage of the inhibitory subunit IκBα. The data indicate an important role of mitochondrial reactive oxygen species in regulation of the NF-κB pathway and corresponding age-related inflammatory activation of endothelium.

Prevention of cardiolipin oxidation and fatty acid cycling as two antioxidant mechanisms of cationic derivatives of plastoquinone (SkQs).

The present state of the art in studies on the mechanisms of antioxidant activities of mitochondria-targeted cationic plastoquinone derivatives (SkQs) is reviewed. Our experiments showed that these compounds can operate as antioxidants in two quite different ways, i.e. (i) by preventing peroxidation of cardiolipin [Antonenko et al., Biochemistry (Moscow) 73 (2008) 1273-1287] and (ii) by fatty acid cycling resulting in mild uncoupling that inhibits the formation of reactive oxygen species (ROS) in mitochondrial State 4 [Severin et al. Proc. Natl. Acad. Sci. USA 107 (2009), 663-668]. The quinol and cationic moieties of SkQ are involved in cases (i) and (ii), respectively. In case (i) SkQH2 interrupts propagation of chain reactions involved in peroxidation of unsaturated fatty acid residues in cardiolipin, the formed SkQ- being reduced back to SkQH2 by heme bH of complex III in an antimycin-sensitive way. Molecular dynamics simulation showed that there are two stable conformations of SkQ1 with the quinol residue localized near peroxyl radicals at C9 or C13 of the linoleate residue in cardiolipin. In mechanism (ii), fatty acid cycling mediated by the cationic SkQ moiety is involved. It consists of (a) transmembrane movement of the fatty acid anion/SkQ cation pair and (b) back flows of free SkQ cation and protonated fatty acid. The cycling results in a protonophorous effect that was demonstrated in planar phospholipid membranes and liposomes. In mitochondria, the cycling gives rise to mild uncoupling, thereby decreasing membrane potential and ROS generation coupled to reverse electron transport in the respiratory chain. In yeast cells, dodecyltriphenylphosphonium (capital ES, Cyrillic12TPP), the cationic part of SkQ1, induces uncoupling that is mitochondria-targeted since capital ES, Cyrillic12TPP is specifically accumulated in mitochondria and increases the H+ conductance of their inner membrane. The conductance of the outer cell membrane is not affected by capital ES, Cyrillic12TPP.

Novel mechanism of elimination of malfunctioning mitochondria (mitoptosis): formation of mitoptotic bodies and extrusion of mitochondrial material from the cell.

Energy catastrophe, when mitochondria hydrolyze glycolytic ATP instead of producing respiratory ATP, has been modeled. In highly glycolyzing HeLa cells, 30-50% of the population survived after inhibition of respiration and uncoupling of oxidative phosphorylation for 2-4 days. The survival was accompanied by selective elimination of mitochondria. This type of mitoptosis includes (i) fission of mitochondrial filaments, (ii) clustering of the resulting roundish mitochondria in the perinuclear area, (iii) occlusion of mitochondrial clusters by a membrane (formation of a "mitoptotic body"), (iv) decomposition of mitochondria inside this body to small membrane vesicles, (v) protrusion of the body from the cell, and (vi) disruption of the body boundary membrane. Autophagy was not involved in this mitoptotic program. Increased production of reactive oxygen species (ROS) was necessary for execution of the program, since antioxidants prevent mitoptosis and kill the cells treated with the mitochondrial poisons as if a ROS-linked mitoptosis serves for protection of the cells under conditions of severe mitochondrial stress. It is suggested that exocytosis of mitoptotic bodies may be involved in maturation of reticulocytes and lens fiber cells.

Sulfatides inhibit leukotriene synthesis in human polymorphonuclear granulocytes by a mechanism involving lipid rearrangement in intracellular membranes.

Sulfatides - sulfated derivatives of galactocerebroside - are endogenous ligands for P- and L-selectins and are able to induce intracellular signaling in neutrophils through a L-selectin dependent pathway. Sulfatides are implicated in a variety of physiological functions and have been found to suppress the synthesis of 5-lipoxygenase (5-LO) metabolites and impede 5-LO translocation to the nuclear envelope in adherent human polymorphonuclear leukocytes (PMNs) [Sud'ina, G. F., Brock, T. G., Pushkareva, M. A., Galkina, S. I., Turutin, D. V., Peters-Golden, M., et al. (2001). Sulphatides trigger polymorphonuclear granulocyte spreading on collagen-coated surfaces and inhibit subsequent activation of 5-lipoxygenase. The Biochemical Journal, 359, 621-629]. In this study we investigated the mechanism of the leukotriene (LT) synthesis inhibition by sulfatides. Sulfatides neither attenuated the ionophore-induced rise in [Ca(2+)](i) nor promoted PKA activation. We demonstrated that sulfatides directly inhibited 5-LO enzyme activity in a cell-free assay. BODIPY-labeled sulfatides were able to rapidly penetrate into the cells. Sulfatides induced rearrangement and redistribution of cytoskeletal components in adherent PMNs. The lipid incorporation as well as sulfatide-induced inhibition of LT synthesis were abolished by cytochalasin D, an inhibitor of actin polymerization and endocytosis. Importantly, sulfatides caused a prominent intracellular cholesterol redistribution, increasing its abundance at the uropod region. On the basis of these data, we suggest that increased cholesterol accumulation in cell compartments represents a novel mechanism by which sulfatides abrogate 5-LO translocation and activation.

Purification and characterization of windmill palm tree (Trachycarpus fortunei) peroxidase.

High peroxidase activity was demonstrated to be present in the leaf of several species of cold-resistant palms. Histochemical studies of the leaf of windmill palm tree (Trachycarpus fortunei) showed the peroxidase activity to be localized in hypoderma, epidermis, cell walls, and conducting bundles. However, chlorophyll-containing mesophyll cells had no peroxidase at all. The leaf windmill palm tree peroxidase (WPTP) was purified to homogeneity and had a specific activity of 6230 units/mg, RZ = 3.0, a molecular mass of 50 kDa, and an isoelectric point of pI 3.5. The electronic spectrum of WPTP with a Soret band at 403 nm was typical of plant peroxidases. The N-terminal amino acid sequence of WPTP was determined. The substrate specificity of WPTP was distinct from that of other palm peroxidases, and the best substrate for WPTP was 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonic acid). The palm peroxidase showed an unusually high stability at elevated temperatures and high concentrations of guanidine.

"Wages of fear": transient threefold decrease in intracellular ATP level imposes apoptosis.

In HeLa cells, complete inhibition of oxidative phosphorylation by oligomycin, myxothiazol or FCCP combined with partial inhibition of glycolysis by DOG resulted in a steady threefold decrease in the intracellular ATP level. The ATP level recovers when the DOG-containing medium was replaced by that with high glucose. In 48 h after a transient (3 h) [ATP] lowering followed by recovery of the ATP level, the majority of the cells commits suicide by means of apoptosis. The cell death does not occur if DOG or an oxidative phosphorylation inhibitor was added separately, treatments resulting in 10-35% lowering of [ATP]. Apoptosis is accompanied by Bax translocation to mitochondria, cytochrome c release into cytosol, caspase activation, reactive oxygen species (ROS) generation, and reorganization and decomposition of chromatin. Apoptosis appears to be sensitive to oncoprotein Bcl-2 and a pancaspase inhibitor zVADfmk. In the latter case, necrosis is shown to develop instead of apoptosis. The cell suicide is resistant to cyclosporine A, a phospholipase inhibitor trifluoroperazine, the JNK and p38 kinase inhibitors, oligomycin, N-acetyl cysteine and mitoQ, differing in these respects from the tumor necrosis factor (TNF)- and H(2)O(2)-induced apoptoses. It is suggested that the ATP concentration in the cell is monitored by intracellular "ATP-meter(s)" generating a cell suicide signal when ATP decreases, even temporarily, below some critical level (around 1 mM).

Inhibition of mitochondrial bioenergetics: the effects on structure of mitochondria in the cell and on apoptosis.

The effects of specific inhibitors of respiratory chain, F(o)F(1)ATP synthase and uncouplers of oxidative phosphorylation on survival of carcinoma HeLa cells and on the structure of mitochondria in the cells were studied. The inhibitors of respiration (piericidin, antimycin, myxothiazol), the F(1)-component of ATP synthase (aurovertin) and uncouplers (DNP, FCCP) did not affect viability of HeLa cells, apoptosis induced by TNF or staurosporin and the anti-apoptotic action of Bcl-2. Apoptosis was induced by combined action of respiratory inhibitors and uncouplers indicating possible pro-apoptotic action of reactive oxygen species (ROS) generated by mitochondria. Short-term incubation of HeLa cells with the mitochondrial inhibitors and 2-deoxyglucose followed by 24-48 h recovery resulted in massive apoptosis. Apoptosis correlated to transient (3-4 h) and limited (60-70%) depletion of ATP. More prolonged or more complete transient ATP depletion induced pronounced necrosis. The inhibitors of respiration and uncouplers caused fragmentation of tubular mitochondria and formation of small round bodies followed by swelling. These transitions were not accompanied with release of cytochrome c into the cytosol and were fully reversible. The combined effect of respiratory inhibitors and uncouplers developed more rapidly indicating possible involvement of ROS generated by mitochondria. More prolonged (48-72 h) incubation with this combination of inhibitors caused clustering and degradation of mitochondria.

Thread-grain transition of mitochondrial reticulum as a step of mitoptosis and apoptosis.

Association of mitochondrial population to a mitochondrial reticulum is typical of many types of the healthy cells. This allows the cell to organize a united intracellular power-transmitting system. However, such an association can create some difficulties for the cell when a part of the reticulum is damaged or when mitochondria should migrate from one cell region to another. It is shown that in these cases decomposition of extended mitochondria to small roundish organelles takes place (the thread-grain transition). As an intermediate step of this process, formation of beads-like mitochondria occurs when several swollen parts of the mitochondrial filament are interconnected with thin thread-like mitochondrial structures. A hypothesis is put forward that the thread-grain transition is used as a mechanism to isolate a damaged part of the mitochondrial system from its intact parts. If the injury is not repaired, spherical mitochondrion originated from the damaged part of the reticulum is assumed to convert to a small ultracondensed and presumably dead mitochondrion (this process is called 'mitoptosis'). Then the dead mitochondrion is engulfed by an autophagosome. Sometimes, an ultracondensed mitoplast co-exists with a normal mitoplast, both of them being surrounded by a common outer mitochondrial membrane. During apoptosis, massive thread-grain transition is observed which, according to Youle et al. (S. Frank et al., Dev Cell 1: 515, 2002), is mediated by a dynamin-related protein and represents an obligatory step of the mitochondria-mediated apoptosis. We found that there is a lag phase between addition of an apoptogenic agent and the thread-grain transition. When started, the transition occurs very fast. It is also found that this event precedes complete de-energization of mitochondria and cytochrome c release to cytosol. When formed, small mitochondria migrate to (and in certain rare cases even into) the nucleus. It is suggested that small mitochondria may serve as a transportable form of organelles ('cargo boats' transporting some apoptotic proteins to their nuclear targets).