A time window for rescuing dying retinal ganglion cells

BACKGROUND

Retinal ganglion cell (RGC) degeneration and death cause vision loss in patients with glaucoma. Regulated cell death, once initiated, is generally considered to be an irreversible process. Recently, we showed that, by timely removing the cell death stimulus, stressed neuronal PC12 cells can recover from phosphatidylserine (PS) exposure, nuclear shrinkage, DNA damage, mitochondrial fragmentation, mitochondrial membrane potential loss, and retraction of neurites, all hallmarks of an activated cell death program. Whether the cell death process can be reversed in neurons of the central nervous system, like RGCs, is still unknown. Here, we studied reversibility of the activated cell death program in primary rat RGCs (prRGCs).

METHODS

prRGCs were exposed to ethanol (5%, vol/vol) to induce cell death. At different stages of the cell death process, ethanol was removed by washing and injured prRGCs were further cultured in fresh medium to see whether they recovered. The dynamics of single cells were monitored by high-resolution live-cell spinning disk microscopy. PS exposure, mitochondrial structure, membrane potential, and intracellular Ca2+ were revealed by annexin A5-FITC, Mito-tracker, TMRM, and Fluo 8-AM staining, respectively. The distribution of cytochrome c was investigated by immunofluorescence. The ultrastructure of mitochondria was studied by electron microscopy.

RESULTS

Analysis of temporal relationships between mitochondrial changes and PS exposure showed that fragmentation of the mitochondrial network and loss of mitochondrial membrane potential occurred before PS exposure. Mitochondrial changes proceeded caspase-independently, while PS exposure was caspase dependent. Interestingly, prRGCs recovered quickly from these mitochondrial changes but not from PS exposure at the plasma membrane. Correlative light and electron microscopy showed that stress-induced decrease in mitochondrial area, length and cristae number was reversible. Intracellular Ca2+ was elevated during this stage of reversible mitochondrial injury, but there was no sign of mitochondrial cytochrome c release.

CONCLUSIONS

Our study demonstrates that RGCs with impaired mitochondrial structure and function can fully recover if there is no mitochondrial cytochrome c release yet, and no PS is exposed at the plasma membrane. This finding indicates that there is a time window for rescuing dying or injured RGCs, by simply removing the cell death stimulus. Video Abstract.

Palmitoyl-CoA effect on cytochrome c release, a key process of apoptosis, from liver mitochondria of rat with sucrose diet-induced obesity

Cytochrome c (cyt-c) release from the mitochondria to the cytosol is a key process in the initiation of hepatocyte apoptosis involved in the progression of non-alcoholic fatty liver disease (NAFLD) to fibrosis, cirrhosis and hepatocellular carcinoma. Hepatocyte apoptosis may be related to lipotoxicity due to the accumulation of palmitic acid and palmitoyl-CoA (Pal-CoA). Therefore, the aim of this study is to examine whether Pal-CoA induces cyt-c release from liver mitochondria of sucrose-fed rat (SF). Pal-CoA-induced cyt-c release was sensitive to cyclosporine A indicating the involvement of the mitochondrial membrane permeability transition (mMPT). In addition, cyt-c release from SF mitochondria remains significantly lower than C mitochondria despite the increased rate of H2O2 generation in SF mitochondria. The decreased cyt-c release from SF may be also related to the increased proportion of the palmitic acid-enriched cardiolipin, due to the high availibilty of palmitic acid in SF liver. The enrichment of cardiolipin molecular species with palmitic acid makes cardiolipin more resistant to peroxidation, a mechanism involved in the dissociation of cyt-c from mitochondrial inner membrane. These results suggest that Pal-CoA may participate in the progression of NAFLD to more severe disease through mechanisms involving cyt-c release and mMPT, a key process of apoptosis.

Nuphar alkaloids induce very rapid apoptosis through a novel caspase-dependent but BAX/BAK-independent pathway

Nuphar alkaloids, originally isolated from water lilies, induce apoptosis in mammalian cells in less than 1 h, making them possibly the fastest known inducers. However, the mechanism by which this rapid apoptosis occurs remains unknown. We have investigated canonical aspects of apoptosis to determine how the nuphar alkaloid, (+)-6-hydroxythiobinupharidine (6HTBN), induces apoptosis. 6HTBN induced rapid apoptosis in various leukemia, lymphoma, and carcinoma cell lines, suggesting that its mechanism is cell-type independent. It also circumvented resistance of patient-derived chronic lymphocytic leukemia cells generated by co-culture on survival-promoting stroma. Intriguingly, 6HTBN failed to induce apoptosis in platelets. The mechanism of apoptosis involves activation of caspase 9 and caspase 3, but not caspase 8 as previously reported. The release of cytochrome c from mitochondria occurred even in the absence of BAX/BAK and in cells that retained mitochondrial membrane potential. These results suggest a novel mechanism of apoptosis that has previously not been reported. The molecular target of the nuphar alkaloids remains to be determined.

Methods to Probe Conformational Activation and Mitochondrial Activity of Proapoptotic BAK

Mitochondrial outer membrane permeabilization (MOMP) is a crucial initiating event in apoptosis that activates the caspase cascade to execute cell demise. The effector B-cell lymphoma 2 (BCL-2) antagonist killer (BAK) forms mitochondrial apoptotic pores to mediate MOMP. In healthy cells, BAK resides at the outer mitochondrial membrane as a dormant monomer. Upon direct interactions with the BCL-2 homology 3 (BH3)-only proapoptotic proteins during apoptosis, BAK undergoes conformational changes to form the active species associated with apoptotic pores. We describe methods to purify mitochondria for MOMP assays and to detect conformational changes in native BAK associated with MOMP by using limited proteolysis and cross-linking analyses.

Toxicity of Manganese Titanate on Rat Vital Organ Mitochondria

The TiO2, which is a main material in the field of photocatalytic reactions, includes rutile and anatase phase. Titanium dioxide has possessed notice due to its promising applications in the environmental photocatalytic degradation of pollutants of organic compound in waste water and utilization of solar energy. The nanosized manganese titanate (pyrophanite) MnTiO3 was collected by oxidation of Mn(OH)2 with TiO2 powder in cetyltrimethylammonium bromide (CTAB) micelle solutions and the calcinations of the produced powders. Therefore, it was decided to determine the Mechanistic mitochondria toxicity of nanoparticles towards liver, kidney, heart, and brain via new and reliable methods. Our results showed that nanoparticles induced mitochondria dysfunction via an increase in ROS production and membrane potential collapse, correlated to cytochrome c release. Also, increased disturbance in oxidative phosphorylation was also shown by the decrease in ATP. Recent studies have suggested that nanoparticles leading to cytosolic release of lysosomal content, and ultimately apoptosis. This study suggests that mitochondrial oxidative stress and impairment of oxidative phosphorylation in vital organ Mitochondria may play a key role in manganese titanate toxicity.

DRP1-dependent apoptotic mitochondrial fission occurs independently of BAX, BAK and APAF1 to amplify cell death by BID and oxidative stress

During apoptosis mitochondria undergo cristae remodeling and fragmentation, but how the latter relates to outer membrane permeabilization and downstream caspase activation is unclear. Here we show that the mitochondrial fission protein Dynamin Related Protein (Drp) 1 participates in cytochrome c release by selected intrinsic death stimuli. While Bax, Bak double deficient (DKO) and Apaf1(-/-) mouse embryonic fibroblasts (MEFs) were less susceptible to apoptosis by Bcl-2 family member BID, H(2)O(2), staurosporine and thapsigargin, Drp1(-/-) MEFs were protected only from BID and H(2)O(2). Resistance to cell death of Drp1(-/-) and DKO MEFs correlated with blunted cytochrome c release, whereas mitochondrial fragmentation occurred in all cell lines in response to all tested stimuli, indicating that other mechanisms accounted for the reduced cytochrome c release. Indeed, cristae remodeling was reduced in Drp1(-/-) cells, potentially explaining their resistance to apoptosis. Our results indicate that caspase-independent mitochondrial fission and Drp1-dependent cristae remodeling amplify apoptosis. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

3-O-(2'E,4'Z-decadienoyl)-20-O-acetylingenol induces apoptosis in intestinal epithelial cells of rats via mitochondrial pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Euphorbia kansui is a traditional Chinese medicine widely used for the treatment of edema, ascite and asthma in China for centuries. However, its serious gastrointestinal toxicity restricted its safe clinical application. 3-O-(2'E,4'Z-decadienoyl)-20-O-acetylingenol (3EZ,20Ac-ingenol), a diterpenoid compound derived from kansui, has obvious gastrointestinal cytotoxicity in cells. Until now, its gastrointestinal cytotoxic mechanism is mostly unknown. This study focused on elucidating the cytotoxic mechanism of 3EZ,20Ac-ingenol in intestinal epithelial cells of rats (IEC-6 cells) to guide safer application of this herb in clinic.

MATERIALS AND METHODS

3EZ,20Ac-ingenol was isolated from the EtOAc extract of kansui. Cell morphology was detected by inverted phase contrast microscope and transmission electron microscope (TEM). Cell apoptosis was examined by Annexin V-FITC/PI dual-staining or Hoechst staining. ROS generation was detected with DCFH-DA staining by laser scanning confocal microscope. MMP change was examined with JC-1 staining by high content screening (HCS). Further, the release of cytochrome c, the expressions of Bax, Bcl-2, AIF and Apaf-1 were analyzed by western blot and the activities of caspase-3, 8, 9 were determined by ELISA. Additionally, cell cycle analysis was performed to detect the effects of 3EZ,20Ac-ingenol on cell cycle in IEC-6 cells by flow cytometry.

RESULTS

The study showed that 3EZ,20Ac-ingenol significantly reduced IEC-6 cells viability in a dose-dependent manner and the IC50 value was 5.74 μg/mL. Consistently, 3EZ,20Ac-ingenol could elevate reactive oxygen species (ROS), disrupt mitochondrial membrane potential (MMP), induce the release of cytochrome c from mitochondria to cytosol, enhance the expressions of Bax, AIF and Apaf-1, suppress the expression of Bcl-2, then activate caspase-3, 8, 9 cascade, and subsequently result in apoptosis. Additionally, 3EZ,20Ac-ingenol also could cause G2/M phase arrest in IEC-6 cells.

CONCLUSIONS

The results indicated that 3EZ,20Ac-ingenol induced the cytotoxicity of IEC-6 cells depends on induction of cell apoptosis via mitochondrial pathway and cell cycle arrest.

Very long chain ceramides interfere with C16-ceramide-induced channel formation: A plausible mechanism for regulating the initiation of intrinsic apoptosis

Mitochondria mediate both cell survival and death. The intrinsic apoptotic pathway is initiated by the permeabilization of the mitochondrial outer membrane to pro-apoptotic inter-membrane space (IMS) proteins. Many pathways cause the egress of IMS proteins. Of particular interest is the ability of ceramide to self-assemble into dynamic water-filled channels. The formation of ceramide channels is regulated extensively by Bcl-2 family proteins and dihydroceramide. Here, we show that the chain length of biologically active ceramides serves as an important regulatory factor. Ceramides are synthesized by a family of six mammalian ceramide synthases (CerS) each of which produces a subset of ceramides that differ in their fatty acyl chain length. Various ceramides permeabilize mitochondria differentially. Interestingly, the presence of very long chain ceramides reduces the potency of C16-mediated mitochondrial permeabilization indicating that the intercalation of the lipids in the dynamic channel has a destabilizing effect, reminiscent of dihydroceramide inhibition of ceramide channel formation (Stiban et al., 2006). Moreover, mitochondria isolated from cells overexpressing the ceramide synthase responsible for the production of C16-ceramide (CerS5) are permeabilized faster upon the exogenous addition of C16-ceramide whereas they are resistant to permeabilization with added C24-ceramide. On the other hand mitochondria isolated from CerS2-overexpressing cells show the opposite pattern, indicating that the product of CerS2 inhibits C16-channel formation ex vivo and vice versa. This interplay between different ceramide metabolic enzymes and their products adds a new dimension to the complexity of mitochondrial-mediated apoptosis, and emphasizes its role as a key regulatory step that commits cells to life or death.