Lysosomal enzymes in the macronucleus of Tetrahymena during its apoptosis-like degradation

Autophagy as a caretaker of nuclear integrity

Due to their essential functions, dysregulation of nuclear pore complexes (NPCs) is strongly associated with numerous human diseases, including neurodegeneration and cancer. On a cellular level, longevity of scaffold nucleoporins in postmitotic cells of both C. elegans and mammals renders them vulnerable to age-related damage, which is associated with an increase in pore leakiness and accumulation of intranuclear aggregates in rat brain cells. Thus, understanding the mechanisms which underpin the homeostasis of this complex, as well as other nuclear proteins, is essential. In this review, autophagy-mediated degradation pathways governing nuclear components in yeast will be discussed, with a particular focus on NPCs. Furthermore, the various nuclear degradation mechanisms identified thus far in diverse eukaryotes will also be highlighted.

Ran1 is essential for parental macronuclear import of apoptosis-inducing factor and programmed nuclear death in Tetrahymena thermophila

During programmed nuclear death (PND), apoptosis-inducing factor (AIF) translocates from mitochondria to the parental macronucleus (MAC) in Tetrahymena thermophila. In the degenerating parental MAC, AIF induces chromatin condensation and large-scale DNA fragmentation in a caspase-independent manner. However, the regulation of AIF nuclear translocation and molecular mechanism of PND are less clear. In this study, we demonstrated that the asymmetric distribution of nuclear GDP-bound Ran1-mimetic mutant Ran1T25N and cytoplasmic GTP-bound Ran1-mimetic mutant Ran1Q70L exists across the parental macronuclear-cytoplasmic barrier during PND. Knockdown of RAN1 led to defects in PND progression and failure of parental macronuclear accumulation of AIF. Moreover, AIF parental macronuclear import occurred in Ran1T25N mutants, while it was inhibited in Ran1Q70L mutants. Importantly, artificial accumulation of AIF in the parental MAC rescued PND progression defects in RAN1 knockdown mutants. These data suggest that Ran1 is essential for parental macronuclear import of AIF and PND in T. thermophila.

The programmed DNA elimination and formation of micronuclei in germ line cells of the natural hybridogenetic water frog Pelophylax esculentus

DNA elimination is a radical form of gene silencing and occurs both in somatic and germ cells. The programmed DNA elimination occurs during gametogenesis in interspecies hybrids that reproduce by hybridogenesis (stick insects, fishes, and amphibians) and concerns removal of whole genomes of one of the parental species and production of clonal gametes propagating the genome of the other species. The cellular mechanisms differ considerably in hybridogenetic insects and fishes but remains unknown in edible frogs Pelophylax esculentus, natural hybrids between Pelophylax lessonae and Pelophylax ridibundus. Here we report DNA elimination mechanism in early developing gonads of diploid and triploid hybrid frogs, studied by TEM, immunofluorescence, and cytochemistry. In gonocytes of both sexes (primary oogonia and prespermatogonia), micronuclei emerge as detached nuclear buds formed during interphase. We found depletion of nuclear pore complexes in micronuclear membrane and chromatin inactivation via heterochromatinization followed by degradation of micronuclei by autophagy. Micronuclei formation does not lead to apoptotic cell death showing that genome elimination is a physiological process. Chromatin elimination via micronuclei in P. esculentus is unique among hybridogenetic animals and contributes to broadening the knowledge about reproductive modes in animals.

Repair, Reuse, Recycle: The Expanding Role of Autophagy in Genome Maintenance

(Macro)Autophagy is a catabolic pathway that delivers excess, aggregated, or damaged proteins and organelles to lysosomes for degradation. Autophagy is activated in response to numerous cellular stressors such as increased levels of reactive oxygen species (ROS) and low levels of cellular nutrients as well as DNA damage. Although autophagy occurs in the cytoplasm, its inhibition leads to accumulation of DNA damage and genomic instability. In the past few years, our understanding of the interplay between autophagy and genomic stability has greatly increased. In this review we summarize these recent advances in understanding the molecular mechanisms linking autophagy to DNA repair.

An evolutionary balance: conservation vs innovation in ciliate membrane trafficking

As most of eukaryotic diversity lies in single-celled protists, they represent unique opportunities to ask questions about the balance of conservation and innovation in cell biological features. Among free-living protists the ciliates offer ease of culturing, a rich array of experimental approaches, and versatile molecular tools, particularly in Tetrahymena thermophila and Paramecium tetraurelia. These attributes have been exploited by researchers to analyze a wealth of cellular structures in these large and complex cells. This mini-review focuses on 3 aspects of ciliate membrane dynamics, all linked with endolysosomal trafficking. First is nutrition based on phagocytosis and maturation of food vacuoles. Secondly, we discuss regulated exocytosis from vesicles that have features of both dense core secretory granules but also lysosome-related organelles. The third topic is the targeting, breakdown and resorption of parental nuclei in mating partners. For all 3 phenomena, it is clear that elements of the canonical membrane-trafficking system have been retained and in some cases repurposed. In addition, there is evidence that recently evolved, lineage-specific proteins provide determinants in these pathways.

Signalling in ciliates: long- and short-range signals and molecular determinants for cellular dynamics

In ciliates, unicellular representatives of the bikont branch of evolution, inter- and intracellular signalling pathways have been analysed mainly in Paramecium tetraurelia, Paramecium multimicronucleatum and Tetrahymena thermophila and in part also in Euplotes raikovi. Electrophysiology of ciliary activity in Paramecium spp. is a most successful example. Established signalling mechanisms include plasmalemmal ion channels, recently established intracellular Ca²⁺ -release channels, as well as signalling by cyclic nucleotides and Ca²⁺ . Ca²⁺ -binding proteins (calmodulin, centrin) and Ca²⁺ -activated enzymes (kinases, phosphatases) are involved. Many organelles are endowed with specific molecules cooperating in signalling for intracellular transport and targeted delivery. Among them are recently specified soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), monomeric GTPases, H⁺ -ATPase/pump, actin, etc. Little specification is available for some key signal transducers including mechanosensitive Ca²⁺ -channels, exocyst complexes and Ca²⁺ -sensor proteins for vesicle-vesicle/membrane interactions. The existence of heterotrimeric G-proteins and of G-protein-coupled receptors is still under considerable debate. Serine/threonine kinases dominate by far over tyrosine kinases (some predicted by phosphoproteomic analyses). Besides short-range signalling, long-range signalling also exists, e.g. as firmly installed microtubular transport rails within epigenetically determined patterns, thus facilitating targeted vesicle delivery. By envisaging widely different phenomena of signalling and subcellular dynamics, it will be shown (i) that important pathways of signalling and cellular dynamics are established already in ciliates, (ii) that some mechanisms diverge from higher eukaryotes and (iii) that considerable uncertainties still exist about some essential aspects of signalling.

Colchicine-induced degeneration of the micronucleus during conjugation in Tetrahymena

One of the most dramatic examples of nuclear morphogenesis occurs during conjugation in Tetrahymena when the micronucleus elongates to a size longer than the cell itself. After contraction to a spherical shape, the nucleus moves directly to chromosome separation in the first meiotic division. Here we investigate the consequences of interrupting the elongation process. Colchicine, a microtubule inhibitor, caused retraction of elongated structures. With time, cells began to lose their micronuclei, and by five hours more than half of the paired cells had at least one cell missing a micronucleus. After reversing the colchicine block, existing micronuclei did not undergo elongation again, nor did meiosis occur. These observations indicate that micronuclear elongation is critical to subsequent meiotic division. Further, nuclear elimination occurs, which could be due to meiotic failure or possibly a problem downstream from meiosis. An analysis of the process of colchicine-induced micronuclear degeneration indicated that it was regulated by a caspase-dependent mechanism, characteristic of apoptosis, and then resorbed by a lysosome-dependent autophagic mechanism. Amicronucleate cells failed to grow when returned to nutrient medium, likely because of a lesion in the post-conjugation reconstruction of a functioning oral apparatus. The ease by which a large number of nuclei are induced to "self-destruct" may make this system useful in investigating the link between colchicine treatment and nuclear death in Tetrahymena, and in investigating how nuclear death could be regulated in living cells more generally. Finally, we note that this phenomenon might relate to the evolution of amicronucleate species of Tetrahymena.

Beyond transcriptional silencing: is methylcytosine a widely conserved eukaryotic DNA elimination mechanism?

Methylation of cytosine DNA residues is a well-studied epigenetic modification with important roles in formation of heterochromatic regions of the genome, and also in tissue-specific repression of transcription. However, we recently found that the ciliate Oxytricha uses methylcytosine in a novel DNA elimination pathway important for programmed genome restructuring. Remarkably, mounting evidence suggests that methylcytosine can play a dual role in ciliates, repressing gene expression during some life-stages and directing DNA elimination in others. In this essay, I describe these recent advances in the DNA methylation field and discuss whether this unexpected novel role for methylcytosine in DNA elimination might be more widely conserved in eukaryotic biology, particularly in apoptotic pathways.

Role of class III phosphatidylinositol 3-kinase during programmed nuclear death of Tetrahymena thermophila

Programmed nuclear death (PND) in the ciliate protozoan Tetrahymena thermophila is a novel type of autophagy that occurs during conjugation, in which only the parental somatic macronucleus is destined to die and is then eliminated from the progeny cytoplasm. Other coexisting nuclei, however, such as new micro- and macronuclei are unaffected. PND starts with condensation in the nucleus followed by apoptotic DNA fragmentation, lysosomal acidification, and final resorption. Because of the peculiarity in the process and the absence of some ATG genes in this organism, the mechanism of PND has remained unclear. In this study, we focus on the role of class III phosphatidylinositol 3-kinase (PtdIns3K, corresponding to yeast Vps34) in order to identify central regulators of PND. We identified the sole Tetrahymena thermophila ortholog (TtVPS34) to yeast Vps34 and human PIK3C3 (the catalytic subunit of PtdIns3K), through phylogenetic analysis, and generated the gene knockdown mutant for functional analysis. Loss of TtVPS34 activity prevents autophagosome formation on the parental macronucleus, and this nucleus escapes from the lysosomal pathway. In turn, DNA fragmentation and final resorption of the nucleus are drastically impaired. These phenotypes are similar to the situation in the ATG8Δ mutants of Tetrahymena, implying an inextricable link between TtVPS34 and TtATG8s in controlling PND as well as general macroautophagy. On the other hand, TtVPS34 does not appear responsible for the nuclear condensation and does not affect the progeny nuclear development. These results demonstrate that TtVPS34 is critically involved in the nuclear degradation events of PND in autophagosome formation rather than with an involvement in commitment to the death program.

Three steps to the immortality of cancer cells: senescence, polyploidy and self-renewal

Metastatic cancer is rarely cured by current DNA damaging treatments, apparently due to the development of resistance. However, recent data indicates that tumour cells can elicit the opposing processes of senescence and stemness in response to these treatments, the biological significance and molecular regulation of which is currently poorly understood. Although cellular senescence is typically considered a terminal cell fate, it was recently shown to be reversible in a small population of polyploid cancer cells induced after DNA damage. Overcoming genotoxic insults is associated with reversible polyploidy, which itself is associated with the induction of a stemness phenotype, thereby providing a framework linking these separate phenomena. In keeping with this suggestion, senescence and autophagy are clearly intimately involved in the emergence of self-renewal potential in the surviving cells that result from de-polyploidisation. Moreover, subsequent analysis indicates that senescence may paradoxically be actually required to rejuvenate cancer cells after genotoxic treatments. We propose that genotoxic resistance is thereby afforded through a programmed life-cycle-like process which intimately unites senescence, polyploidy and stemness.

Nucleophagy at a glance

Under certain circumstances, the removal of damaged or non-essential parts of the nucleus, or even an entire nucleus, is crucial in order to promote cell longevity and enable proper function. A selective form of autophagy, known as nucleophagy, can be used to accomplish the degradation of nucleus-derived material. In this Cell Science at a Glance article and the accompanying poster, we summarize the similarities and differences between the divergent modes of nucleophagy that have been described to date, emphasizing, where possible, the molecular mechanism, the membrane interactions and rearrangements, and the nature of the nucleus-derived material that is degraded. In turn, we will consider nucleophagy processes in the lower eukaryotes, the budding yeast Saccharomyces cerevisiae, filamentous fungi Aspergillus and Magnaporthe oryzae and the ciliated protozoan Tetrahymena thermophila, and finally in mammalian cells. We will also briefly discuss the emerging links between nucleophagy and human disease.

Macroautophagy-aided elimination of chromatin: sorting of waste, sorting of fate?

How tumor cells process damaged or unwanted DNA is a matter of much interest. Recently, Rello-Varona et al. (Cell Cycle 2012; 11:170–76) reported the involvement of macroautophagy (hereon autophagy) in the elimination of micronuclei (MN) from osteosarcoma cells. Prior to that, diminution of whole nuclei from multinucleated TP53-mutant tumor cells was described. Here, we discuss these two kinds of chromatin autophagy evoked after genotoxic stress in the context of the various biological processes involved: (1) endopolyploidy and the ploidy cycle; (2) the timing of DNA synthesis; (3) DNA repair; (4) chromatin:nuclear envelope interactions; and (5) cytoplasmic autophagy. We suggest that whereas some MN can be reunited with the main nucleus (through interactions with envelope-limited chromatin sheets) and participate in DNA repair, failure of repair serves as a signal for the chromatin autophagy of MN. In turn, autophagy of whole sub-nuclei in multi-nucleated cells appears to favor de-polyploidization, mitigation of aneuploidy with its adverse effects, thereby promoting the survival fitness of descendents and treatment resistance. Thus, both kinds of chromatin autophagy provide tumor cells with the opportunity to repair DNA, sort and resort chromatin, reduce DNA content, and enhance survival.

The selectivity and specificity of autophagy in Drosophila

Autophagy is a process of cellular self-degradation and is a major pathway for elimination of cytoplasmic material by the lysosomes. Autophagy is responsible for the degradation of damaged organelles and protein aggregates and therefore plays a significant role in cellular homeostasis. Despite the initial belief that autophagy is a nonselective bulk process, there is growing evidence during the last years that sequestration and degradation of cellular material by autophagy can be accomplished in a selective and specific manner. Given the role of autophagy and selective autophagy in several disease related processes such as tumorigenesis, neurodegeneration and infections, it is very important to dissect the molecular mechanisms of selective autophagy, in the context of the system and the organism. An excellent genetically tractable model organism to study autophagy is Drosophila, which appears to have a highly conserved autophagic machinery compared with mammals. However, the mechanisms of selective autophagy in Drosophila have been largely unexplored. The aim of this review is to summarize recent discoveries about the selectivity of autophagy in Drosophila.

Role of ATG8 and autophagy in programmed nuclear degradation in Tetrahymena thermophila

Autophagy is an evolutionarily conserved mechanism for the degradation of cellular components, but its role in enucleation during differentiation has not been established. Tetrahymena thermophila is a unicellular eukaryote with two functionally distinct nuclei, the somatic (macro-) and the germ line (micro-) nuclei. These nuclei are produced during sexual reproduction (conjugation), which involves differentiation and selective degradation of several specific nuclei. To examine the role of autophagy in nuclear degradation, we studied the function of two ATG8 genes in Tetrahymena. Through fluorescent protein tagging, we found that both proteins are targeted to degrading nuclei at specific stages, with some enrichment on the nuclear periphery, suggesting the formation of autophagosomes surrounding these nuclei. In addition, ATG8 knockout mutant cells showed a pronounced delay in nuclear degradation without apparently preventing the completion of other developmental events. This evidence provided direct support for a critical role for autophagy in programmed nuclear degradation. The results also showed differential roles for two ATG8 genes, with ATG8-65 playing a more significant role in starvation than ATG8-2, although both are important in nuclear degradation.

Sirtuin-mediated nuclear differentiation and programmed degradation in Tetrahymena

BACKGROUND

The NAD(+)-dependent histone deacetylases, known as "sirtuins", participate in a variety of processes critical for single- and multi-cellular life. Recent studies have elucidated the importance of sirtuin activity in development, aging, and disease; yet, underlying mechanistic pathways are not well understood. Specific sirtuins influence chromatin structure and gene expression, but differences in their pathways as they relate to distinct chromatin functions are just beginning to emerge. To further define the range of global chromatin changes dependent on sirtuins, unique biological features of the ciliated protozoan Tetrahymena thermophila can be exploited. This system offers clear spatial and temporal separation of multiple whole genome restructuring events critical for the life cycle.

RESULTS

Inhibition with nicotinamide revealed that sirtuin deacetylase activity in Tetrahymena cells promotes chromatin condensation during meiotic prophase, differentiation of heterochromatin from euchromatin during development, and chromatin condensation/degradation during programmed nuclear death. We identified a class I sirtuin, called Thd14, that resides in mitochondria and nucleoli during vegetative growth, and forms a large sub-nuclear aggregate in response to prolonged cell starvation that may be peripherally associated with nucleoli. During sexual conjugation and development Thd14 selectively concentrates in the parental nucleus prior to its apoptotic-like degradation.

CONCLUSIONS

Sirtuin activity is important for several functionally distinct events requiring global chromatin condensation. Our findings suggest a novel role for sirtuins in promoting programmed pycnosis by acting on chromatin destined for degradation. The sirtuin Thd14, which displays physiological-dependent differential localization within the nucleus, is a candidate for a chromatin condensation enzyme that is coupled to nuclear degradation.

Macroautophagy-mediated degradation of whole nuclei in the filamentous fungus Aspergillus oryzae

Filamentous fungi consist of continuum of multinucleate cells called hyphae, and proliferate by means of hyphal tip growth. Accordingly, research interest has been focusing on hyphal tip cells, but little is known about basal cells in colony interior that do not directly contribute to proliferation. Here, we show that autophagy mediates degradation of basal cell components in the filamentous fungus Aspergillus oryzae. In basal cells, enhanced green fluorescent protein (EGFP)-labeled peroxisomes, mitochondria, and even nuclei were taken up into vacuoles in an autophagy-dependent manner. During this process, crescents of autophagosome precursors matured into ring-like autophagosomes to encircle apparently whole nuclei. The ring-like autophagosomes then disappeared, followed by dispersal of the nuclear material throughout the vacuoles, suggesting the autophagy-mediated degradation of whole nuclei. We also demonstrated that colony growth in a nutrient-depleted medium was significantly inhibited in the absence of functional autophagy. This is a first report describing autophagy-mediated degradation of whole nuclei, as well as suggesting a novel strategy of filamentous fungi to degrade components of existing hyphae for use as nutrients to support mycelial growth in order to counteract starvation.

Gigantic macroautophagy in programmed nuclear death of Tetrahymena thermophila

Programmed nuclear death (PND) in Tetrahymena is a unique process during conjugation, in which only the parental macronucleus is degraded and then eliminated from the progeny cytoplasm, but other co-existing nuclei such as new micro- and macronuclei are unaffected. PND through autophagic elimination is expected to be strictly controlled, considering the significant roles in ciliates such as turnover of disused organelles and production of the next generation. Here we demonstrate that PND in Tetrahymena involves peculiar aspects of autophagy, which differ from mammalian or yeast macroautophagy. Drastic change of the parental macronucleus occurs when differentiation of new macronuclei is initiated. Combined use of monodansylcadaverine and a lysosome indicator LysoTracker Red showed that prior to nuclear condensation, the envelope of the parental macronucleus changed its nature as if it is an autophagic membrane, without the accumulation of a pre-autophagosomal structure from the cytoplasm. Subsequently, lysosomes approached only to the parental macronucleus and localized at the envelope until a final resorption stage. In addition, we found that the parental macronucleus exhibits certain sugars and phosphatidylserine on the envelope, which are possible "attack me" signals, that are not found on other types of nuclei. These findings suggest that PND is a highly elaborated process, different from the typical macroautophagy seen in other systems, and is executed through interaction between specific molecular signals on the parental macronuclear envelope and autophagic/lysosomal machineries.

Role of apoptosis-inducing factor (AIF) in programmed nuclear death during conjugation in Tetrahymena thermophila

Background

Programmed nuclear death (PND), which is also referred to as nuclear apoptosis, is a remarkable process that occurs in ciliates during sexual reproduction (conjugation). In Tetrahymena thermophila, when the new macronucleus differentiates, the parental macronucleus is selectively eliminated from the cytoplasm of the progeny, concomitant with apoptotic nuclear events. However, the molecular mechanisms underlying these events are not well understood. The parental macronucleus is engulfed by a large autophagosome, which contains numerous mitochondria that have lost their membrane potential. In animals, mitochondrial depolarization precedes apoptotic cell death, which involves DNA fragmentation and subsequent nuclear degradation.

Results

We focused on the role of mitochondrial apoptosis-inducing factor (AIF) during PND in Tetrahymena. The disruption of AIF delays the normal progression of PND, specifically, nuclear condensation and kilobase-size DNA fragmentation. AIF is localized in Tetrahymena mitochondria and is released into the macronucleus prior to nuclear condensation. In addition, AIF associates and co-operates with the mitochondrial DNase to facilitate the degradation of kilobase-size DNA, which is followed by oligonucleosome-size DNA laddering.

Conclusions

Our results suggest that Tetrahymena AIF plays an important role in the degradation of DNA at an early stage of PND, which supports the notion that the mitochondrion-initiated apoptotic DNA degradation pathway is widely conserved among eukaryotes.

MY01, a class XIV myosin, affects developmentally-regulated elimination of the macronucleus during conjugation of Tetrahymena thermophila

BACKGROUND INFORMATION

Nuclear dimorphism is characteristic of ciliated protozoa. A transcriptionally-active macronucleus co-exists with a transcriptionally-silent micronucleus, which is activated only at conjugation. During conjugation, each conjugant develops two new genetically matched macronuclei and micronuclei, and the pre-existing macronucleus is eliminated. Elimination of the pre-existing macronucleus during conjugation is an apoptotic-like process. The macronucleus becomes highly condensed, DNA laddering occurs, caspase activity increases, acidic enzymes accumulate within the nucleoplasm, and the nucleus shrinks in size. The current study focused on the involvement of actin and myosin in nuclear events of conjugation. A myosin knockout strain was mated with wild-type, and the nuclear events were monitored with confocal microscopy.

RESULTS

Early nuclear events, including development of new macronuclei and micronuclei, appeared qualitatively normal in knockout conjugants. Completion of nuclear condensation and acidification in the pre-existing macronucleus was blocked in 44% of knockout conjugants. Knockout conjugants that failed to fully achieve nuclear condensation and acidification did not eliminate the pre-existing macronucleus. In control experiments, blockage of chromatin condensation, nuclear acidification, and macronuclear elimination was never observed in wild-type conjugants.

CONCLUSIONS

Perturbation of either DNA fragmentation, chromatin condensation or nuclear acidification can lead to blockage of apoptotic-like elimination of the macronucleus in MYO1-knockout conjugants. Consistent with the known motor function of myosins and the involvement of Myo1 in vesicle trafficking in Tetrahymena, we argue that Myo1 could specifically affect condensation of chromatin and acidification of the nucleus through direct interaction with chromatin and through Myo1-dependent vesicle trafficking to the nucleus.

Programmed cell death in protists

Programmed cell death in protists does not seem to make sense at first sight. However, apoptotic markers in unicellular organisms have been observed in all but one of the six/eight major groups of eukaryotes suggesting an ancient evolutionary origin of this regulated process. This review summarizes the available data on apoptotic markers in non-opisthokonts and elucidates potential functions and evolution of programmed cell death. A newly discovered family of caspase-like proteases, the metacaspases, is considered to exert the function of caspases in unicellular organisms. Important results on metacaspases, however, showed that they cannot be always correlated to the measured proteolytic activity during protist cell death. Thus, a major challenge for apoptosis research in a variety of protists remains the identification of the molecular cell death machinery.

Myo1 localizes to phagosomes, some of which traffic to the nucleus in a Myo1-dependent manner in Tetrahymena thermophila

Myo1 is one of 13 myosins in Tetrahymena thermophila. Initially, twelve of the myosins in Tetrahymena were assigned to Class XX in the myosin superfamily but recently re-assigned to a subclass within Class XIV. In a previous study, we reported that genomic knockout of MYO1 affected phagocytosis and macronuclear amitosis. These two phenotypes have appeared disparate because a possible mechanism linking phagocytosis and amitosis was unknown. In the present study, Myo1 localization was investigated in order to further link machinery for phagocytosis and amitosis. Antibodies directed against the Myo1 motor domain detected an immunospecific polypeptide at 175-180 kDa on immunoblots of wild-type proteins. The 175-180 kDa polypeptide was not detected on immunoblots of proteins from the knockout strain. For immunofluorescence microscopy, cells were allowed to internalize fluorescent beads as markers for phagosomes. In wild-type cells, anti-Myo1 and anti-actin antibodies co-localized to the periphery of phagosomes and the macronucleus. In the MYO1-knockout strain only background fluorescence was observed with anti-Myo1 antibody. Confocal x-z series through macronuclei revealed fluorescent beads within the nucleoplasm. Statistical analysis showed a significant difference between the mean distributions of fluorescent beads in the nucleoplasm of wild-type and MYO1-knockout cells. A fluorescent dye was used to label plasma membrane in living cells. Dye-labeled vacuoles trafficked to the macronucleus. Trafficking of phagosomes to the macronucleus in a myosin-dependent manner is a novel finding and a possible mechanism for targeting myosin and actin to the nucleus.

Biphosphinic palladacycle complex mediates lysosomal-membrane permeabilization and cell death in K562 leukaemia cells

The cell death mechanism of cytotoxicity induced by the Biphosphinic Palladacycle Complex (BPC) was studied using a K562 leukaemia cell line. The IC50 values obtained for K562 cells post-72 h of BPC were less than 5.0 microM by using 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) and trypan blue assays. Using the Acridine Orange vital staining combining fluorescence microscopy it was observed that the complex triggers apoptosis in K562 cells, inducing DNA fragmentation, as analysed through electrophoresis. Lysosomal-membrane permeabilization was also observed in K562 cells post-5 h of BPC, which suggests intralysosomal accumulation by proton-trapping, since its pKa value ranged from 5.1 to 6.5. Caspase-3, and -6 activity induced by BPC in K562 cells was prevented by the cathepsin-B inhibitor [N-(L-3-trans-propylcarbamoyl-oxirane-2-carbonyl)-L-isoleucyl-L-proline] (CA074). These events occurred in the presence of endogenous bcl-2 and bax expression. Acute toxicological studies demonstrated that BPC produces no lesions for liver and kidney fourteen-days after drug administration (100 mg/kg--i.p.). White and red blood cells of BPC-treated mice presented normal morphological characteristics. Taken together, these data suggest a novel lysosomal pathway for BPC-induced apoptosis, in which lysosomes are the primary target and cathepsin B acts as death mediator.

Apoptosis in Ichthyophthirius multifiliis is associated with expression of the Fas receptor of theronts

The expression of type I membrane Fas receptors on the surface of Ichthyophthirius multifiliis (Ich) theronts and the possible association between Fas expression and theront apoptosis induced by the immune antibody was examined. Fas receptors were detected on the theront surface using fluorescein isothiocyanate-conjugated mouse monoclonal antibody against Fas. Fas-positive theronts significantly increased with time during in vitro incubation and with increasing theront concentration. Furthermore, the immune cutaneous antibody induced theront apoptosis; however, Fas ligand did not. A highly significant correlation was noted between theront Fas expression and immune cutaneous antibody-induced theront apoptosis. Numbers of apoptotic theronts increased with increasing number of Fas-positive theronts. The data indicated that theront apoptosis induced by immune cutaneous antibody appears to be positively correlated with the expression of Fas on the surface of Ich theronts.

Differential gene expression during stationary phase between amicronucleates and micronucleates of the ciliated protist, Pseudourostyla cristata

Ciliates are unicellular eukaryotic organisms with two types of nuclei, the 'germline' micronucleus (MIC) and the 'somatic' macronucleus (MAC). We previously reported that when the MIC of Pseudourostyla cristata was eliminated by amputation, the resultant amicronucleate organisms exhibited a lower viability and abnormal oral structures. To gain insight into the genetic reorganization involved in or induced by removal of the MIC and the mechanism by which nuclear dimorphism was established, we investigated gene expression differences between amicronucleates and micronucleates, using suppression subtractive hybridization (SSH) techniques. Approximately 250 clones from each library were screened by cDNA array dot blotting. Altogether, 22 unique genes from the forward-subtractive library (micronucleates as tester, amicronucleates as driver) and 23 unique genes from the reverse-subtractive library (micronucleates as driver and amicronucleates as tester) were shown to be differentially expressed. These 45 differentially expressed genes were found to be homologs of genes involved in various cellular processes including signal transduction, transcription, cell cycle accomplishment and general metabolism, cell structure, and stress response. We highlighted 14 genes, 7 that were unique from both the forward-subtractive and the reverse-subtractive libraries, using real time semi-quantitative RT-PCR. The characterization of these cDNAs represents a starting point in understanding the molecular mechanisms of amicronucleates disruption.

A possible role of mitochondria in the apoptotic-like programmed nuclear death of Tetrahymena thermophila

The ciliated protozoan Tetrahymena has a unique apoptosis-like process, which is called programmed nuclear death (PND). During conjugation, the new germinal micro- and somatic macro-nuclei differentiate from a zygotic fertilized nucleus, whereas the old parental macronucleus degenerates, ensuring that only the new macronucleus is responsible for expression of the progeny genotype. As is the case with apoptosis, this process encompasses chromatin cleavage into high-molecular mass DNA, oligonucleosomal DNA laddering, and complete degradation of the nuclear DNA, with the ultimate outcome of nuclear resorption. Caspase-8- and caspase-9-like activities are involved in the final resorption process of PND. In this report, we show evidence for mitochondrial association with PND. Mitochondria and the degenerating macronucleus were colocalized in autophagosome using two dyes for the detection of mitochondria. In addition, an endonuclease with similarities to mammalian endonuclease G was detected in the isolated mitochondria. When the macronuclei were incubated with isolated mitochondria in a cell-free system, DNA fragments of 150-400 bp were generated, but no DNA ladder appeared. Taking account of the present observations and the timing of autophagosome formation, we conclude that mitochondria might be involved in Tetrahymena PND, probably with the process of oligonucleosomal laddering.

Cutaneous antibodies from channel catfish, Ictalurus punctatus (Rafinesque), immune to Ichthyophthirius multifiliis (Ich) may induce apoptosis of Ich theronts

This study explored the existence of apoptosis (programmed cell death) in Ichthyophthirius multifiliis Fouquet (Ich) theronts and determined the effect of cutaneous antibodies in skin culture fluid from fish immune to Ich on theront apoptosis. Apoptosis was detected in theronts and was clearly distinguished by fluorescent microscopy after staining with acridine orange and propidium iodide. The apoptotic theronts showed characteristic chromatin condensation and nuclear fragments containing chromatin pieces. The externalization of phosphatidylserine on the plasma membrane of apoptotic theronts was detected with fluorescein isothiocyanate-conjugated annexin using flow cytometry. Theront apoptosis was induced using the skin culture fluid from fish immune to Ich, which contained cutaneous antibodies against Ich. The highest apoptosis appeared in theronts exposed to immune skin culture fluid at a 1:10 dilution, compared with those at 1:20 and 1:40 dilutions. A direct correlation was noted between the percentage of apoptotic theronts and exposure duration to immune skin culture fluid. The study indicated that antibody reaction with theronts (immobilization) played an important role in theront apoptosis, but it could not be excluded that other components released from the excised skin had effects on theronts.

Caspase-like Activity is Required for Programmed Nuclear Elimination during Conjugation in Tetrahymena

During conjugation in the binucleate ciliate, Tetrahymena thermophila, the old macronucleus is eliminated as new macronuclei and micronuclei are ontogenetically derived from the zygote nucleus. The mechanism of programmed nuclear elimination in ciliates may be related to the mechanism of apoptosis in higher organisms since its chromatin undergoes major condensation, its DNA is digested into nucleosome-sized fragments, and it stains positively for TUNEL. The present study explores whether caspases are involved in programmed macronuclear degradation in Tetrahymena. We show here that caspase-like activity is detectable using two specific colorimetric substrates, and that the activity is reduced with specific caspase inhibitors. In addition, using the fluorigenic substrate PhiPhiLux, active caspase-like activity is detected in living cells, localized to cytoplasmic vesicles; activity is not detected in pre- or post-condensed macronuclei. Finally, three different inhibitors of caspase activity cause a block to macronuclear chromatin condensation and elimination. Therefore, a caspase-like enzyme activity is necessary for regulating macronuclear elimination in Tetrahymena. These data support the possibility that macronuclear elimination is related, evolutionarily, to regulated cell death in multicellular organisms.

Caspase-like activity in programmed nuclear death during conjugation of Tetrahymena thermophila

Apoptosis, or programmed cell death, is common in a variety of eucaryotes, from unicellular protozoa to vertebrates. The ciliated protozoan Tetrahymena thermophila has a unique apoptosis-like nuclear death during conjugation, called programmed nuclear death. This death program involves nuclear condensation (pyknosis) and oligonucleosomal DNA fragmentation in the parental macronucleus. Subsequently, the condensed nucleus is entirely resorbed in the autophagosome. Here we demonstrate that caspase-8- and -9-like activity was detected, but no caspase-3-like activity, by in vitro assay during the nuclear resorption process, suggesting that caspase-like activity is associated with both programmed cell death and apoptosis-like nuclear death in Tetrahymena. The use of indicator dye to detect the loss of mitochondrial membrane potential suggested the uptake of mitochondria and the degenerating macronucleus by the autophagosome. An involvement of mitochondria in the programmed nuclear death is discussed.