Proteasome inhibition: an early or late event in nitric oxide-induced neuronal death?

Nitric oxide (NO), ubiquitously expressed in the central nervous system, has been perceived to be a potential neuromodulator. Employing cultured murine primary cortical neurons, NO resulted in an inhibition of the ubiquitin-proteasome system (UPS) with a dose- and time-dependent decrease in cell viability. This is consistent with a previous study that reported a dysfunction of UPS with consequential apoptotic death in macrophage cell with NO treatment. However, it cannot be unclear if the drop in UPS efficiency is directly imposed on by NO. Therefore by using microarray analysis, our study revealed an early down-regulation or non-significant differential expression of genes encoding UPS proteins in NOC-18 (NO donor)-treated neurons as compared to an observed elevation of corresponding gene expression genes in lactacystin (classical proteasome inhibitor)-treated neurons (conducted earlier). Furthermore, time-course analysis of proteasome activity in NOC-18-treated neurons demonstrated a late onset of reduction. This is intriguing as it is well established that in an exclusive proteasome dysfunction-induced cell death, a compensatory feedback mechanism will be activated with an initial and concerted up-regulation of genes encoding proteins involved in UPS as seen when neurons were treated with lactacystin. Thus, it is highly suggestive that NO-triggered neuronal death takes on a different signaling cascade from that of a classical proteasome inhibitor, and that the late reduction of proteasome activity is a downstream event following the activation of apoptotic cellular signaling cascade. In intracellular condition, the proteasome is not NO preferred primary target responsible for the trigger of the cell death machinery. In conclusion, we presented novel findings that shed light into NO-induced cell death signaling cascade, which would be important in understanding the pathogenesis of neurodegenerative disorders such as Parkinson's disease.     

The role of VDAC in cell death: Friend or foe?

As the voltage-dependent anion channel (VDAC) forms the interface between mitochondria and the cytosol, its importance in metabolism is well understood. However, research on VDAC's role in cell death is a rapidly growing field, unfortunately with much confusing and contradictory results. The fact that VDAC plays a role in outer mitochondrial membrane permeabilization is undeniable, however, the mechanisms behind this remain very poorly understood. In this review, we will summarize the studies that show evidence of VDAC playing a role in cell death. To begin, we will discuss the evidence for and against VDAC's involvement in mitochondrial permeability transition (MPT) and attempt to clarify that VDAC is not an essential component of the MPT pore (MPTP). Next, we will evaluate the remaining literature on VDAC in cell death which can be divided into three models: proapoptotic agents escaping through VDAC, VDAC homo- or hetero-oligomerization, or VDAC closure resulting in outer mitochondrial membrane permeabilization through an unknown pathway. We will then discuss the growing list of modulators of VDAC activity that have been associated with induction/protection against cell death. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.     

Discrimination of the elderly: how guilty are you?

In the Netherlands a recent discussion on ‘the right to die’ and premature ending of life for people over the age of 70 has only highlighted one side of the story. The tale of a situation of bad health, depression and physical ailments, which progresses to, worst of all, a painful and loveless ending of life, has often been told and scares us literally to death. It may distract us from a far bigger and more complex problem.     

Cell size: a matter of life or death?

Proper growth and development of multicellular organisms require the tight regulation of cell growth, cell division and cell death. A recent study has identified a novel regulatory link between two of these processes: cell growth and cell death.     

Non-cell autonomous influence of the astrocyte system xc? on hypoglycaemic neuronal cell death

Despite longstanding evidence that hypoglycaemic neuronal injury is mediated by glutamate excitotoxicity, the cellular and molecular mechanisms involved remain incompletely defined. Here, we demonstrate that the excitotoxic neuronal death that follows GD (glucose deprivation) is initiated by glutamate extruded from astrocytes via system x_c⁻ – an amino acid transporter that imports l-cystine and exports l-glutamate. Specifically, we find that depriving mixed cortical cell cultures of glucose for up to 8 h injures neurons, but not astrocytes. Neuronal death is prevented by ionotropic glutamate receptor antagonism and is partially sensitive to tetanus toxin. Removal of amino acids during the deprivation period prevents – whereas addition of l-cystine restores – GD-induced neuronal death, implicating the cystine/glutamate antiporter, system x_c⁻. Indeed, drugs known to inhibit system x_c⁻ ameliorate GD-induced neuronal death. Further, a dramatic reduction in neuronal death is observed in chimaeric cultures consisting of neurons derived from WT (wild-type) mice plated on top of astrocytes derived from sut mice, which harbour a naturally occurring null mutation in the gene (Slc7a11) that encodes the substrate-specific light chain of system x_c⁻ (xCT). Finally, enhancement of astrocytic system x_c⁻ expression and function via IL-1β (interleukin-1β) exposure potentiates hypoglycaemic neuronal death, the process of which is prevented by removal of l-cystine and/or addition of system x_c⁻ inhibitors. Thus, under the conditions of GD, our studies demonstrate that astrocytes, via system x_c⁻, have a direct, non-cell autonomous effect on cortical neuron survival.     

Searching the point of no return in Helicobacter pylori life: necrosis and/or programmed death?

AIMS: Ultrastructural and molecular studies to support the hypothesis of programmed cell death in Helicobacter pylori were conducted. METHODS AND RESULTS: Evidence of programmed death in H. pylori is provided through electron microscopic detection and cytochemical labelling of electrondense bodies (EDB), containing packaged DNA in coccoid cells, resembling micronuclei of apoptotic eukaryotic cells. This morphological evidence is also supported by DNA cleavage in homogeneous fragments of about 100 base pairs. Programmed cell death was observed in H. pylori cultures at 37 degrees C, with a maximum of 37.5% of EDB coccoid cells after 7 days. The non-permissive temperature of 4 degrees C anticipated this process, with 40% of EDB coccoid forms within 3 days, and it remained substantially unaffected during the observation time of 14 days. CONCLUSION: In these experiments, deprivation of nutrients and a non-permissive temperature acted as a powerful trigger for programmed cell death. SIGNIFICANCE AND IMPACT OF THE STUDY: Helicobacter pylori bacterial populations, under stressing stimuli, can respond with programmed cell suicide as a means of species preservation.     

Daxx: death or survival protein?

The death domain-associated protein (Daxx) was originally cloned as a CD95 (FAS)-interacting protein and modulator of FAS-induced cell death. Daxx accumulates in both the nucleus and the cytoplasm; in the nucleus, Daxx is found associated with the promyelocytic leukaemia (PML) nuclear body and with alpha-thalassemia/mental retardation syndrome protein (ATRX)-positive heterochromatic regions. In the cytoplasm, Daxx has been reported to interact with various proteins involved in cell death regulation. Despite a significant number of studies attempting to determine Daxx function in apoptotic and non-apoptotic cell death, its precise role in this process is only partially understood. Here, we critically review the current understanding of Daxx function and shed new light on this interesting field.     

Apoptosis and cell death in neuronal cells: where does Ca2+ fit in?

Mounting evidence shows that neuronal death is an important and essential component of brain tissue homeostasis, with major forms of cell death occurring: necrosis and apoptosis. No general consensus exists as to whether these two forms of neuronal death represent separate cellular processes or just two different forms of a common 'death pathway'. One difference between them is the role played by intracellular Ca2+: central and obligatory, in necrosis and possible, but not always necessary in triggering apoptosis. Furthermore, the same assessment of the involvement of Ca2+ signalling could also distinguish between two possible apoptotic states in the nervous system: one, the 'developmental apoptosis', involving immature and developing neurons, in which Ca2+ plays mainly an apoprotector role, and another one, associated mainly with pathological instances and involving fully matured and established neurons, in which Ca2+ plays an apo-inducing role.     

Nitric oxide: promoter or suppressor of programmed cell death?

Nitric oxide (NO) is a short-lived gaseous free radical that predominantly functions as a messenger and effector molecule. It affects a variety of physiological processes, including programmed cell death (PCD) through cyclic guanosine monophosphate (cGMP)-dependent and-independent pathways. In this field, dominant discoveries are the diverse apoptosis networks in mammalian cells, which involve signals primarily via death receptors (extrinsic pathway) or the mitochondria (intrinsic pathway) that recruit caspases as effector molecules. In plants, PCD shares some similarities with animal cells, but NO is involved in PCD induction via interacting with pathways of phytohormones. NO has both promoting and suppressing effects on cell death, depending on a variety of factors, such as cell type, cellular redox status, and the flux and dose of local NO. In this article, we focus on how NO regulates the apoptotic signal cascade through protein S-nitrosylation and review the recent progress on mechanisms of PCD in both mammalian and plant cells.     

Mitochondria and aging: innocent bystanders or guilty parties?

There are many theories of aging and a number ofthem encompass the role of mitochondria in this process. Mitochondrial DNA mutations and deletions have been shown to accumulate in many tissues in mammals during aging. However, there is little evidence that these mutations could affect the functioning of aging tissues.     

Disintegration of the medial epithelial seam: Is cell death important in palatogenesis?

During palatogenesis, the palatal medial edge epithelium (MEE) forms the medial epithelial seam (MES) on adhesion of the opposing palatal shelves. The MES eventually disappears, leading to mesenchymal confluence of the palate and completion of palatogenesis. Failure of these processes results in cleft palate, one of the most common congenital anomalies in human affecting around one case in 500-2500 live births. The cell fate of MEE has been controversial for more than 20 years. Recent studies suggest that the disappearance of MES is a complex process involving cell death, epithelial-mesenchymal transition (EMT) and epithelial migration. Interestingly, transforming growth factor-β3 (Tgf β3) expression in MEE and the tip epithelium of the nasal septum begins just before palatal shelf reorientation and lasts until MES disruption, and several works including targeted disruption of the gene have indicated that the process appears to be regulated mainly by the TGFβ3-TGFβR signaling. However, how MEE cells choose their fate and how the cell fate is altered in response to cellular environment remains to be elucidated.     

Axon degeneration is key component of neuronal death in amyloid-β toxicity

Depending upon the stimulus, neuronal cell death can either be triggered from the cell body (soma) or the axon. We investigated the origin of the degeneration signal in amyloid β (Aβ) induced neuronal cell death in cultured in vitro hippocampal neurons. We discovered that Aβ_1–42 toxicity-induced axon degeneration precedes cell death in hippocampal neurons. Overexpression of Bcl-xl inhibited both axonal and cell body degeneration in the Aβ-42 treated neurons. Nicotinamide mononucleotide adenylyltransferase 1 (Nmnat1) blocks axon degeneration in a variety of paradigms, but it cannot block neuronal cell body death. Therefore, if the neuronal death signals in Aβ_1–42 toxicity originate from degenerating axons, we should be able to block neuronal death by inhibiting axon degeneration. To explore this possibility we over-expressed Nmnat1 in hippocampal neurons. We found that inhibition of axon degeneration in Aβ_1–42 treated neurons prevented neuronal cell death. Thus, we conclude that axon degeneration is the key component of Aβ_1–42 induced neuronal degeneration, and therapies targeting axonal protection can be important in finding a treatment for Alzheimer’s disease.     

Cell death in Leishmania induced by stress and differentiation: programmed cell death or necrosis?

Unicellular organisms, such as the protozoan parasite Leishmania, can be stimulated to show some morphological and biochemical features characteristic of mammalian apoptosis. This study demonstrates that under a variety of stress conditions such as serum deprivation, heat shock and nitric oxide, cell death can be induced leading to genomic DNA fragmentation into oligonucleosomes. DNA fragmentation was observed, without induction, in the infectious stages of the parasite, and correlated with the presence of internucleosomal nuclease activity, visualisation of 45 to 59 kDa nucleases and detection of TUNEL-positive nuclei. DNA fragmentation was not dependent on active effector downstream caspases nor on the lysosomal cathepsin L-like enzymes CPA and CPB. These data are consistent with the presence of a caspase-independent cell death mechanism in Leishmania, induced by stress and differentiation that differs significantly from metazoa.