Physiology and protein synthesis in programmed cell death. Early synthesis and DNA degradation

Protein Synthesis Inhibition and Activation of the c-Jun N-Terminal Kinase Are Potential Contributors to Cisplatin Ototoxicity

Cisplatin has been regarded as an effective and versatile chemotherapeutic agent for nearly 40 years. Though the associated dose-dependent ototoxicity is known, the cellular mechanisms by which cochleovestibular hair cell death occur are not well understood. We have previously shown that aminoglycoside ototoxicity is mediated in part by cytosolic protein synthesis inhibition. Despite a lack of molecular similarity, aminoglycosides were shown to elicit similar stress pathways to cisplatin. We therefore reasoned that there may be some role of protein synthesis inhibition in cisplatin ototoxicity. Employing a modification of the bioorthogonal noncanonical amino acid tagging (BONCAT) method, we evaluated the effects of cisplatin on cellular protein synthesis. We show that cisplatin inhibits cellular protein synthesis in organ of Corti explant cultures. Similar to what was found after gentamicin exposure, cisplatin activates both the c-Jun N-terminal kinase (JNK) and mammalian target of rapamycin (mTOR) pathways. In contrast to aminoglycosides, cisplatin also inhibits protein synthesis in all cochlear cell types. We further demonstrate that the multikinase inhibitor sorafenib completely prevents JNK activation, while providing only moderate hair cell protection. Simultaneous stimulation of cellular protein synthesis by insulin, however, significantly improved hair cell survival in culture. The presented data provides evidence for a potential role of protein synthesis inhibition in cisplatin-mediated ototoxicity.

A novel role of cytosolic protein synthesis inhibition in aminoglycoside ototoxicity

Ototoxicity is a main dose-limiting factor in the clinical application of aminoglycoside antibiotics. Despite longstanding research efforts, our understanding of the mechanisms underlying aminoglycoside ototoxicity remains limited. Here we report the discovery of a novel stress pathway that contributes to aminoglycoside-induced hair cell degeneration. Modifying the previously developed bioorthogonal noncanonical amino acid tagging method, we used click chemistry to study the role of protein synthesis activity in aminoglycoside-induced hair cell stress. We demonstrate that aminoglycosides inhibit protein synthesis in hair cells and activate a signaling pathway similar to ribotoxic stress response, contributing to hair cell degeneration. The ability of a particular aminoglycoside to inhibit protein synthesis and to activate the c-Jun N-terminal kinase (JNK) pathway correlated well with its ototoxic potential. Finally, we report that a Food and Drug Administration-approved drug known to inhibit ribotoxic stress response also prevents JNK activation and improves hair cell survival, opening up novel strategies to prevent and treat aminoglycoside ototoxicity.

Nuclear alterations associated to programmed cell death in larval salivary glands of Apis mellifera (Hymenoptera: Apidae)

The silk glands of bees are a good model for the study of cell death in insects. With the objective to detect the nuclear features during glandular regression stage, larvae at the last instar and pre-pupae were collected and their silk glands were dissected and processed for ultrastructural analysis and histologically for cytochemical and imunocytochemical analysis. The results showed that the cellular nuclei exhibited characteristics of death by atypical apoptosis as well as autophagic cell death. Among the apoptosis characteristic were: nuclear strangulation with bleb formation in some nuclei, DNA fragmentation in most of the nuclei and nucleolar fragmentation. Centripetal chromatin compaction was observed in many nuclei, forming a perichromatin halo differing from typical apoptotic nuclei. With regards to the characteristics of autophagic-programmed cell death, most relevant was the delay in the collapse of many nuclei.

Cell death and tendinopathy

Apoptosis and necrosis are presently recognized as the two major types of physiological and pathological cell death. Apoptosis is a tightly regulated cell deletion process that differs morphologically and biochemically from necrotic cell death. Tendinopathy is defined as a tendon injury that originates from intrinsic and extrinsic etiological factors. Excessive apoptosis has recently been described in degenerative tendon. The increased number of apoptotic tendon cells in degenerative tendon tissue could affect the rate of collagen synthesis and repair. Impaired or dysfunctional protein synthesis may lead to weaker tendon tissue and eventually increase the risk for tendon rupture. Clearly, there are many details to insert into this pathway, but there is hope that if the fine details of the pathway can be fleshed out, then strategies may be able to be developed to break the cycle at one or more points and prevent or treat tendinopathy more effectively.

Quantification of axotomized ganglion cell death by explant culture of the rat retina

We first demonstrated a temporal profile of retinal ganglion cell (RGC) death after axotomy in situ using a newly developed retinal explant culture system. 1,1'- dioctadecyl- 3,3,3',3'-tetramethylindocarbocyanine perchlorate, a fluorescent tracer, was administered to the superior colliculi of 2 day old Wistar rats to label RGCs retrogradely. Small pieces of retinas were dissected and maintained at the interface between a 5% CO(2) atmosphere and culture media, and temporally observed by fluorescent microscopy. The number of surviving RGCs, identified as fluorescent spots, gradually decreased during the course of experiments for up to 10 days in vitro. We identified apoptotic RGCs by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling. Administration of cycloheximide, actinomycin D, or a caspase-3 inhibitor to media significantly decreased RGC death. This system provides a method of quantifying axotomized RGC death in relation to time-dependent changes in an identical retinal slip.

Dependence of vital cell function on endoplasmic reticulum calcium levels: implications for the mechanisms underlying neuronal cell injury in different pathological states

The endoplasmic reticulum (ER) is a subcellular compartment playing a pivotal role in the control of vital calcium-related cell functions, including calcium storage and signalling. In addition, newly synthesized membrane and secretory proteins are folded and processed in the ER, reactions which are strictly calcium dependent. The ER calcium activity is therefore high, being several orders of magnitude above that of the cytoplasm. Depletion of ER calcium stores causes an accumulation of unfolded proteins in the ER lumen, a pathological situation which induces the activation of two highly conserved stress responses, the ER overload response (EOR) and the unfolded protein response (UPR). EOR triggers activation of the transcription factor NF kappa B, which, in turn, activates the expression of target genes. UPR triggers two downstream processes: it activates the expression of genes coding for ER-resident stress proteins, and it causes a suppression of the initiation of protein synthesis. A similar stress response is activated in pathological states of the brain including cerebral ischaemia, implying common underlying mechanisms. Depending on the extent and duration of the disturbance, an isolated impairment of ER function is sufficient to induce cell injury. In this review, evidence is presented that ER function is indeed disturbed in various diseases of the brain, including acute pathological states (e.g. cerebral ischaemia) and degenerative diseases (e.g. Alzheimer's disease). A body of evidence suggests that disturbances of ER function could be a global pathomechanism underlying neuronal cell injury in various acute and chronic disorders of the central nervous system. If that is true, restoration of ER function or attenuation of secondary disturbances induced by ER dysfunction could present a highly promising new avenue for pharmacological intervention to minimize neuronal cell injury in different pathological states of the brain.

Protein synthesis, DNA degradation, and morphological changes during programmed cell death in labial glands of Manduca sexta

Labial glands of the tobacco hornworm Manduca sexta (Lepidoptera: Sphingiidae, homologues of Drosophila salivary glands, undergo programmed cell death (PCD) in a 4-day period during larva-to-pupa metamorphosis. The programmed death of the labial gland was examined by electron microscopy and measurement of protein synthesis as well as measurement of DNA synthesis, end-labeling of single strand breaks, and pulsed-field gel electrophoresis. One of the earliest changes observed is a sharp drop in synthesis of most proteins, coupled with synthesis of a glycine-rich protein, reminiscent of silk-like proteins. From a morphological standpoint, during the earliest phases the most prominent changes are the formation of small autophagic vacuoles containing ribosomes and an apparent focal dissolution of the membranes of the endoplasmic reticulum, whereas later changes include differing destruction at the lumenal and basal surfaces of the cell and erosion of the basement membrane. By the fourth day of metamorphosis, individual cells become rapidly vacuolated in a cell-independent manner. In the vacuolated cells on day 3, chromatin begins to coalesce. It is at this period that unequivocal nucleosomal ladders are seen and end-labeling in situ or electrophoretic techniques document single on double-strand breaks, respectively. DNA synthesis ceases shortly after the molt to the fifth instar, as detected by incorporation of tritiated thymidine and weak TUNEL labeling. Large size fragments of DNA are seen shortly after DNA synthesis ceases and thence throughout the instor, raising the possibility of potential limitations built into the cells before their final collapse.

Pharmacological intervention in age-associated brain disorders by Flupirtine: Alzheimer's and prion diseases

Alzheimer's disease, a major form of dementia in the elderly has become an increasingly important health problem in developed countries. In vitro studies on primary neurons demonstrate that Flupirtine (Katadolon) at a concentration of 1 microg/ml, significantly reduces the neurotoxic (apoptotic) effect displayed by A beta25-35, a segment of the amyloid beta-protein precursor the etiologic agent of Alzheimer's disease. Flupirtine, which has been in clinical use since 10 years ago, prevents the toxic effect of PrP, the presumed etiologic agent of the Creutzfeldt-Jakob disease as well as the excitatory amino acid glutamate on cortical neurons. Flupirtine displays a bimodal activity. Its strongest cytoprotective effect against glutamate-induced neurotoxicity was measured if administered at least 120 min prior to the addition of the glutamate. A likewise potent anti-apoptotic activity was measured if cells were simultaneously incubated with Flupirtine and the apoptotic inducers. Administration of Flupirtine during postincubation time in the experiments with glutamate did not result in neuroprotection. In parallel with the determination of the effect of Flupirtine on the toxin (A beta, PrP or glutamate)-induced neuronal death the effect of the drug on the intracellular Ca2+ level [Ca2+]i, was measured. It is well established that incubation of neurons with glutamate causes an increase in [Ca2+]i. It was found that a simultaneous administration of Flupirtine and glutamate did not reduce the glutamate-induced high Ca2+ level. Only if the cells had been preincubated for approximately 30 min with the drug the intracellular Ca2+ level was significantly lower. Experimental evidence given here shows that the molecular basis for the antiapoptotic effect of Flupirtine against glutamate, triggered during pre-incubation, is an increased expression of the protooncogene bcl-2. The neuroprotective effect determined during coincubation with the inducer is attributed to a normalization of the glutathione level which dropped in the presence of the inducers. It is concluded that Flupirtine is a promising drug to treat neurodegenerative disorders occurring with age, e.g. Alzheimer's disease and prion based diseases, like Creutzfeldt-Jakob disease. This conclusion is corroborated by the favourable pharmacokinetic profile of Flupirtine.

Cell death/apoptosis: normal, chemically induced, and teratogenic effect

Cell death is an integral part of a variety of biological processes including cell proliferation, differentiation, and morphogenesis. We review here the morphological and biochemical nature as well as the genetic basis for cell death during normal and abnormal development. Most often referred to in normal development as programmed cell death, this controlled process determines the size, patterning, and function of many tissues. The importance of its proper genetic regulation is demonstrated by the discovery of cell death-specific genes and the several disorders including cancer and teratogenesis that result from repression or enhancement of cell death. In our studies we employed the developing mouse limb, which provides a defined window of active cell death, to elucidate mechanisms of cell death. We have developed markers that reveal in the developing normal limb an apoptotic morphology with phagocytosis and DNA fragmentation. In the limb deformity mutant Hammertoe there is a defective (restricted) cell death pattern, but the morphology remains apoptotic. By the use of these markers, we were able to observe that the teratogen retinoic acid produced enhanced apoptotic cell death. Most interestingly, retinoic acid-induced cell death in the Hammertoe mutant resulted in correction of the mutant phenotype. Future studies will determine the relationship between exogenous agents and endogenous signaling pathways as well as indicate how these interactions can alter the fate of a given cell and potentially ameliorate a genetic abnormality.

Cyclin-dependent kinase 5 is associated with apoptotic cell death during development and tissue remodeling

In a series of studies to more precisely localize the cellular sites of expression of the cyclin-dependent kinase (Cdk) family members in reproductive organs, we observed a striking expression of Cdk5 in atretic follicles in the ovary, particularly in granulosa cells that appeared to be dying. We determined that these granulosa cells were undergoing apoptotic cell death using the in situ DNA fragmentation assay. To extend the generality of the association of Cdk5 with apoptotic cells, we examined its expression as it correlated with the detection of apoptosis in a number of developmental paradigms, including regions of the embryonic nervous system, the developing eye, and the developing limb. Finally, the association of apoptosis and Cdk5 expression and associated kinase activity was examined in the limb and in an induced cell death system, that of androgen withdrawal-induced regression of the prostate gland in male mice. These observations provide new insight into the possible function of this novel Cdk during both differentiation and apoptotic cell death.

Improvement of cognitive deficits and decreased cholinergic neuronal cell loss and apoptotic cell death following neurotrophin infusion after experimental traumatic brain injury

This study explores the effects of infusion of nerve growth factor (NGF) on behavioral outcome and cell death in the septal region using the clinically relevant model of fluid-percussion brain injury in the rat. Animals were subjected to fluid-percussion brain injury and 24 hours later a miniosmotic pump was implanted to infuse NGF (12 animals) or vehicle (12 animals) directly into the region of maximum injury for 2 weeks. Four weeks postinjury the animals were tested for cognitive function using a Morris Water Maze paradigm. Neurological motor function was evaluated over a 4-week postinjury period. The rats receiving NGF infusions had significantly higher memory scores than vehicle-treated animals. Examination of the cholinergic neurons in the medial septal region using choline acetyltransferase immunohistochemistry demonstrated significant cell loss after injury. Infusion of NGF significantly attenuated loss of these cholinergic neurons. A second group of animals was subjected to fluid-percussion brain injury alone (23 rats) or injury followed by NGF infusion (18 rats). These animals were killed between 24 hours and 2 weeks postinjury and the septal region was examined for the presence of apoptotic cells using the terminal deoxynucleotidyl transferase-mediated biotinylated-deoxyuridinetriphosphate nick-end labeling technique. Apoptotic cells were identified as early as 24 hours postinjury; their numbers peaked at 4 and 7 days, and then declined by 14 days. The NGF-treated animals had some apoptotic cells; however, even at 7 days there were significantly fewer of these cells. No significant motor differences were observed between the NGF- and vehicle-treated groups. These data indicate that NGF administration beginning 24 hours after fluid-percussion brain injury has a beneficial effect on cognition and results in sparing of cholinergic septal neurons. These improvements persist after cessation of NGF administration. The beneficial effects of NGF may be related to its ability to attenuate traumatically induced apoptotic cell death.

Correlation of DNA fragmentation and chromatin condensation in apoptotic nuclei of the Ser 6 mouse retina

The form of cell death known as apoptosis was first described in thymocytes. The hallmarks of apoptosis include chromatin condensation, membrane blebbing, formation of apoptotic bodies, and DNA fragmentation. DNA fragmentation can be visualized morphologically by the TdT-mediated dUTP-biotin nick end labeling (TUNEL) method that labels the cut DNA ends. However, at the light microscopic (LM) level, TUNEL-positive nuclei cannot readily be correlated with the other hallmarks of apoptosis. In the retina, chromatin condensation and DNA fragmentation are the major features of developmental cell death as well as photoreceptor degeneration. We performed TUNEL at the electron microscopic (EM) level, which permitted correlation of DNA fragmentation with chromatin condensation. We studied the retinas of transgenic mice (Ser 6) expressing the Pro347Ser mutant rhodopsin gene during developmental cell death (age 7 days) and photoreceptor degeneration (age 21 days). We found that 90% of the nuclei showing chromatin condensation were TUNEL positive as well. Our results demonstrated DNA fragmentation and chromatin condensation in the same cells as they underwent apoptosis in vivo, confirming the notion that these processes are concomitant events, and by implication, that activation of an endogenous endonuclease is an important step in the death process of retinal neurons.

Increased ALZ-50 immunoreactivity in sudden infant death syndrome

Neuronal expression of the ALZ-50 epitope was investigated in hippocampus and medulla from infants dying of sudden infant death syndrome or known causes (controls). Hippocampal studies include data from 31 infants dying of known causes between 32 weeks' gestation and 16 months postpartum and 46 infants who died of sudden infant death syndrome. The medulla at the level of the mid olivary protuberance was investigated in 22 infants with sudden infant death syndrome and 11 controls matched for age and postmortem interval. Medullary sections were also examined using immunohistochemical methods to demonstrate reactivity to glial fibrillary acidic protein antibody. The density of ALZ-50-immunodecorated neurons in control hippocampus rises from the level observable in utero to a maximum between 1 and 4 months of age and declines thereafter. The density of ALZ-50-immunoreactive neurons in hippocampus is significantly increased in infants with sudden infant death syndrome at all ages. Significant regionally specific increases in the number of ALZ-50-immunoreactive neurons, and glial fibrillary acidic protein-reactive cells were found in sudden infant death syndrome medulla; coincidental increases were observed in only the solitary nucleus. Neurons exhibiting the ALZ-50 epitope may reflect apoptotic neuron death of normal development, and increased numbers of immunoreactive neurons may suggest enhanced neurodegeneration in sudden infant death syndrome.

Use of a multiparametric panel to target subpopulations in a heterogeneous solid tumor model for improved analytical accuracy

The exclusion of non-tumor and dead cells from the analysis of live tumor cells can significantly improve the accuracy of prognostic indicators such as proliferative and DNA indexes. To target live breast tumor cells in a heterogeneous breast tumor model, we have designed a panel consisting of the DNA-specific dye DAPI and epithelial tissue-specific (cytokeratin), tumor-associated (MC5), proliferation-associated (proliferating cell nuclear antigen), and viability-associated (tubulin) markers. The breast tumor model consisted of a mixture of equal numbers of live and dead MDA-MB-175-VII (breast tumor) cells, live CEM (leukemic) cells, and live peripheral blood mononuclear cells. Targeting the live MDA cells in the mixture by gating on tubulin, cytokeratin, and MC5 resulted in a sevenfold increase in PCNA positivity (from 3% ungated to 22.3%), a 60% decrease in the %S-phase fraction (from 37.2% ungated to 15%), and elimination of extraneous hypodiploid and diploid components, enriching the tetraploid MDAs. These results are consistent with those obtained from unmixed live MDA cells. The combined utilization of this panel and "cumulative" electronic gating of the targeted population increases the number of relevant parameters that can be analyzed per sample and the accuracy of the resultant data.

Comparison of cell viability probes compatible with fixation and permeabilization for combined surface and intracellular staining in flow cytometry

Dead cells represent a significant source of interference in the flow cytometric analysis of viable cells primarily due to nonspecific uptake of probes, increased autofluorescence, and altered antigen expression and DNA content. Traditional methods of dead cell exclusion, based on light scatter or uptake of dyes such as propidium iodide (PI) or fluorescein diacetate (FDA), are appropriate for the analysis of fresh, relatively homogeneous samples. However, they are incompatible with the development in this laboratory of a solid tumor monoclonal antibody panel incorporating combined surface and intracellular staining: Light scatter is unreliable in heterogeneous samples such as solid tumors, and most of the widely used viability probes are incompatible, due to weak or reversible binding, with the use of permeabilizing agents for intracellular staining. To determine the best viability marker for inclusion in the solid tumor panel, we compared cultured cells held under hypoxic conditions for up to 15 days after harvest, stained with eight viability probes, and processed according to the solid tumor panel procedure (unprocessed cells from each day, stained with PI, were used as standards). The viability probes included PI (in processed and unprocessed samples); 7-aminoactinomycin D (7-AAD); TO-PRO-3; laser dye styryl (LDS)-751; ethidium monoazide (EMA); and actin, cytokeratin, and tubulin indirectly labelled with sheep-alpha-mouse-FITC (SAM-FITC). The selection criteria for the best viability probe included broad cell type specificity: low nonspecific staining of live cells, specific staining of dead cells strong enough to withstand the permeabilization procedure, high signal-to-noise ratio throughout the time course, and compatibility with the four other fluorescent probes making up the tumor antibody panel. TO-PRO-3, LDS-751, and PI (in processed cells) stained both live and dead cells indiscriminately. Actin-SAM-FITC, EMA, and 7-AAD did not display sufficiently high signal-to-noise ratios over the entire time course. Cytokeratin-SAM-FITC was acceptable in every respect other than its specificity only for cells of epithelial origin. Tubulin-SAM-FITC alone satisfied all the criteria and was selected for inclusion in the monoclonal antibody panel as a viability probe.

Ultraviolet B-induced apoptosis of keratinocytes in murine skin is reduced by mild local hyperthermia

Two components of sunlight, ultraviolet (UV) B (290-320 nm) and infrared (greater than 700 nm), each cause damage to the skin. However, we recently identified a protective response in which heat reduces UVB-induced killing of cultured keratinocytes. Here, this investigation is extended to the living epidermis. The effects of hyperthermic preconditioning upon UVB-induced apoptosis were studied morphologically with hematoxylin and eosin staining, and biochemically with TUNEL (terminal deoxynucleotide transferase nick-end labeling) to measure endonucleolytic cleavage of DNA in situ. Anesthetized SKH-1 hairless mice were exposed to UVB light (0 to 120 mJ/cm2), after which their skin was biopsied at 24 h and paraffin sections were stained with hematoxylin and eosin or with TUNEL. Apoptotic keratinocytes were found to increase after UVB in a dose-related manner. In contrast, if one flank of the mouse was pretreated at 40 degrees C for 1 h and both flanks subsequently were UVB-irradiated at 6 h, the resulting formation of apoptotic cells was reduced twofold or more in the heated flank. Protection appeared by 3 h, reached a maximum at 6 h, and disappeared by 12 h. In summary, heat induces a transient protective effect that reduces UVB-mediated death of keratinocytes in skin at physiologically attainable doses of heat and UVB.

Amyloid beta peptide induces necrosis rather than apoptosis

Amyloid beta peptide (A beta P), a major component of Alzheimer's disease plaques, is toxic to rat pheochromocytoma PC12 cells and to rat cortical neurons. A reduction in cell survival could be detected after 24 h incubation with 0.01 to 20 microM of the 25-35 peptide fragment (beta 25-35) of A beta P. To study the mechanism of cell death induced by A beta P, the morphological as well as the biochemical features of neuronal cell death were analyzed. To distinguish between necrosis and apoptosis, PC12 cell death caused by beta 25-35 was compared to that induced by serum deprivation, a process known to be apoptotic in these cells. The DNA-degradation pattern of A beta P treated cells appeared random rather than at distinct internucleosomal sites as with apoptosis. Electron microscopic studies of NGF-treated PC12 cells and cortical primary cultures exposed to 20 microM beta 25-35 revealed immediate cellular damage such as vacuolization of the cytoplasm, breakdown of Golgi-apparatus and other membrane systems, and neurite disintegration. This was followed by total collapse of the cytoplasm and cell lysis. These data show that A beta P toxicity occurs via a necrotic rather than an apoptotic pathway.

Programmed cell death, apoptosis and killer genes

A cursory examination of the literature reveals that the study of programmed cell death and apoptosis is increasing exponentially. Most contributors to this field have come either from developmental biology or immunology and view programmed cell death from different perspectives, leading both to confusion and an inability to fully appreciate the literature from other disciplines. Here, Lawrence Schwartz and Barbara Osborne define the terms and ideas relevant to the study of cell death in a way that will be accessible to investigators from all fields.

Apoptosis: final common pathway of photoreceptor death in rd, rds, and rhodopsin mutant mice

Mutations in the retinal degeneration, retinal degeneration slow(/peripherin) and rhodopsin genes cause photoreceptor degeneration in humans and mice. Although the phenotypes arising from these mutations are different, suggesting different mechanisms of pathogenesis, we present evidence that apoptosis may be the final common pathway of the disease process linking genotype to phenotype. We observed internucleosomal cleavage of retinal DNA by gel electrophoresis and fragmented DNA at the single cell level by labeling the nicked DNA ends with biotinylated poly(dU). In retinal degeneration mice, DNA fragmentation occurred during the period of photoreceptor degeneration. In retinal degeneration slow mice and in transgenic mice expressing a mutant (Pro347Ser) rhodopsin gene, DNA fragmentation occurred after normal histogenetic cell death (also apoptosis) had ceased. Since DNA fragmentation by internucleosomal cleavage is a cardinal feature of apoptosis, our data suggest that all three of these genetic mutations lead to apoptosis.