Molecular mechanisms of neuronal cell death

Death-Associated Protein Kinase 1 Phosphorylation in Neuronal Cell Death and Neurodegenerative Disease

Regulated neuronal cell death plays an essential role in biological processes in normal physiology, including the development of the nervous system. However, the deregulation of neuronal apoptosis by various factors leads to neurodegenerative diseases such as ischemic stroke and Alzheimer’s disease (AD). Death-associated protein kinase 1 (DAPK1) is a calcium/calmodulin (Ca²⁺/CaM)-dependent serine/threonine (Ser/Thr) protein kinase that activates death signaling and regulates apoptotic neuronal cell death. Although DAPK1 is tightly regulated under physiological conditions, DAPK1 deregulation in the brain contributes to the development of neurological disorders. In this review, we describe the molecular mechanisms of DAPK1 regulation in neurons under various stresses. We also discuss the role of DAPK1 signaling in the phosphorylation-dependent and phosphorylation-independent regulation of its downstream targets in neuronal cell death. Moreover, we focus on the major impact of DAPK1 deregulation on the progression of neurodegenerative diseases and the development of drugs targeting DAPK1 for the treatment of diseases. Therefore, this review summarizes the DAPK1 phosphorylation signaling pathways in various neurodegenerative diseases.

Pink1 protects cortical neurons from thapsigargin-induced oxidative stress and neuronal apoptosis

Apoptosis mediates the precise and programmed natural death of neurons and is a physiologically important process in neurogenesis during maturation of the central nervous system. However, premature apoptosis and/or an aberration in apoptosis regulation are implicated in the pathogenesis of neurodegeneration. Thus, it is important to identify neuronal pathways/factors controlling apoptosis. Pink1 [phosphatase and tensin homologue (PTEN)-induced kinase 1] is a ubiquitously expressed gene and has been reported to have a physiological role in mitochondrial maintenance, suppressing mitochondrial oxidative stress, fission and autophagy. However, how Pink1 is involved in neuronal survival against oxidative stress remains not well understood. In the present paper, we demonstrate that thapsigargin, a specific irreversible inhibitor of endoplasmic reticulum (ER) calcium-ATPase, could lead to dramatic oxidative stress and neuronal apoptosis by ectopic calcium entry. Importantly, the neuronal toxicity of thapsigargin inhibits antioxidant gene Pink1 expression. Although Pink1 knockdown enhances the neuronal apoptosis by thapsigargin, its overexpression restores it. Our findings have established the neuronal protective role of Pink1 against oxidative stress and afford rationale for developing new strategy to the therapy of neurodegenerative diseases.

Pink1 (PTEN-induced kinase 1) have a physiological role in mitochondrial maintenance, suppressing mitochondrial oxidative stress, fission, and autophagy. Our findings indicated that thapsigargin induced oxidative stress and neuronal apoptosis in cultured neurons is at least partly mediated inactivation of Pink1.

Retinoic acid influences the embryoid body formation in mouse embryonic stem cells by induction of caspase and p38 MAPK/JNK-mediated apoptosis

Although all-trans retinoic acid (RA), the oxidative metabolite of vitamin A, is essential for normal development, high levels are teratogenic in many species. RA results in immediate effects on the preimplantation embryo and on blastocyst development in vitro and in vivo. To further elucidate the cellular mechanisms of early postimplantation embryo development induced by RA, we present an embryonic cell line, B5, as a candidate system for the investigation of these processes. We used undifferentiated ES cells as the model, which is from the undifferentiated status to differentiated status [embryoid body (EB) formation] mimicking postimplantation embryo development (egg-cylinder stage of embryo formation) to clarify the cellular mechanism of action of RA in the implanted blastocysts and cell apoptosis following the series of exposures to differing RA concentrations. Using an in vitro model, we identified the impact of RA on undifferentiated embryonic stem (ES) cells, including inhibition of cell proliferation and induction of cell apoptosis. JNK, P-38 and caspase activation were shown in the nature of RA-triggered apoptotic signaling in ES cells. The carry-on influences of RA on the ES cell were shown in the formation of EB from the pretreated ES cells. RA resulted in apparent impact on undifferentiated ES cells in vitro, with increased numbers of apoptotic cells initially and inhibited cell proliferation, which led to decreased size of EB. The process of EB formation (mimicking the early postimplantation embryo development) is regulated by RA-induced apoptosis through the activation of caspase and P38 MAPK/JNK pathway.

CaMKII in cerebral ischemia

Ischemic insults on neurons trigger excessive, pathological glutamate release that causes Ca²⁺ overload resulting in neuronal cell death (excitotoxicity). The Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a major mediator of physiological excitatory glutamate signals underlying neuronal plasticity and learning. Glutamate stimuli trigger autophosphorylation of CaMKII at T286, a process that makes the kinase "autonomous" (partially active independent from Ca²⁺ stimulation) and that is required for forms of synaptic plasticity. Recent studies suggested autonomous CaMKII activity also as potential drug target for post-insult neuroprotection, both after glutamate insults in neuronal cultures and after focal cerebral ischemia in vivo. However, CaMKII and other members of the CaM kinase family have been implicated in regulation of both neuronal death and survival. Here, we discuss past findings and possible mechanisms of CaM kinase functions in excitotoxicity and cerebral ischemia, with a focus on CaMKII and its regulation.

Characterization of intracellular translocation of Forkhead transcription factor O (FoxO) members induced by NGF in PC12 cells

Nuclear translocation of Forkhead transcription factors of the O class (FoxOs) is important for the action of growth factors. However it is not known if all members of the FOXO family have the same translocation properties. We examined the effects of nerve growth factor (NGF) on nuclear/cytoplasmic shuttling of FoxO1, FoxO3a and FoxO6 in PC12 cells and determined their translocation kinetics. Our data demonstrated that NGF could induce the nuclear exclusion of FoxO1-GFP and FoxO3a-GFP in PC12 cells with different properties, but had no effect on FoxO6-GFP's nuclear localization and FoxO6-GFP showed an exclusive nuclear localization. Translocat ould be blocked by K252a and LY294002 but not by PD98059. Moreover, FoxO3a returned to cytoplasm at a higher rate than FoxO1 after NGF stimulation and it was more sensitive than FoxO1 to NGF stimulation.

Retinal progenitor cells, differentiation, and barriers to cell cycle reentry

Neurogenesis in the retina occurs via the coordination of proliferation, cell cycle exit and differentiation of retinal progenitor cells. Until recently, it was widely assumed that once a retinal progenitor cell produced a postmitotic neuron, there was no possibility for cell-cycle re-entry. However, recent studies have shown that mature differentiated horizontal neurons with reduced Rb pathway function can re-enter the cell cycle and proliferate while maintaining their differentiated features. This chapter will explore the molecular and cellular mechanisms that help to keep differentiated retinal neurons and glia postmitotic. We propose that there are cell-type specific barriers to cell-cycle re-entry by differentiated neurons and these may include apoptosis, chromatin/epigenetics mechanisms, cellular morphology and/or metabolic demands that are distinct across cell populations. Our data suggest that differentiated neurons span a continuum of cellular properties related to their ability to re-enter the cell cycle and undergo cytokinesis while maintaining their differentiated features. A deeper understanding of these processes may allow us to begin to explain the cell type specificity of neuronal cell death and tumor susceptibility. For example, neurons that have more barriers to cell-cycle re-entry may be less likely to form tumors but more likely to undergo degeneration. Conversely, neurons that have fewer barriers to cell-cycle re-entry may be more likely to form tumors but less likely to undergo degeneration.

Androgens induce dopaminergic neurotoxicity via caspase-3-dependent activation of protein kinase Cdelta

Aged men have a greater incidence of Parkinson's disease (PD) than women. PD is a neurodegenerative condition associated with the loss of dopamine neurons in the nigrostriatal pathway. This study examined the neurotoxic effects of androgens in a dopaminergic cell line (N27 cells) and the downstream signaling pathways activated by androgens. Treatment of N27 cells with testosterone- and dihydrotestosterone-induced mitochondrial dysfunction, protein kinase C (PKC)-delta cleavage, and apoptosis in dopaminergic neuronal cells. Inhibition of caspase-3 prevented the cleavage of PKCdelta from the full-length element to the catalytic fragment and apoptosis in N27 cells, suggesting that androgen-induced apoptosis is mediated by caspase-3-dependent activation of PKCdelta. Androgen-induced apoptosis may be specific to dopamine neurons as evidenced by a lack of testosterone-induced apoptosis in GnRH neurons. These results support a neurotoxic consequence of testosterone on dopaminergic neurons and may provide insight into the gender bias found in PD.

Neurodegeneration and peroxidases

Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS) are neurodegenerative diseases that affect different parts of the central nervous system. However, a review of the literature indicates that certain biochemical reactions involved in neurodegeneration in these three diseases are quite similar and could be partly identical. This article critically examines the similarities and, based on data from our own and other laboratories, proposes a novel explanation for neurodegeneration in these three diseases. We identified about 20 commonalities that exist in the neurodegenerative process of each disease. We hypothesize that there are two enzyme-catalyzed pathways that operate in affected neurons: an oxidative pathway leading to destruction of various neuronal proteins and lipids, and an apoptotic pathway which the body normally uses to remove unwanted and dysfunctional cells. Data from many laboratories indicate that oxidative reactions are primarily responsible for neurodegeneration, whereas apoptosis may well be a secondary response to the presence of neurons that have already been severely damaged by oxidative reactions. Attempts to inhibit apoptosis for the purpose of attenuating progression of these diseases may therefore be only of marginal benefit. Specific oxidative reactions within affected neurons led us to propose that one or more heme peroxidases may be the catalyst(s) involved in oxidation of proteins and lipids. Support for this proposal is provided by the recent finding that amyloi-beta peptide may act as a peroxidase in AD. Possible participation of the peroxidase activity of cytochrome c, herein designated as cytochrome c(px) to distinguish it from yeast cytochrome c peroxidase, is discussed. Of special interest is our recent finding that many compounds that cause attenuation of neurodegeneration are inhibitors of the peroxidase activity of cytochrome c. Several inhibitors were subsequently identified as suicide substrates. Such inhibitors could be ideally suited for targeted clinical approaches aimed at arresting progression of neurodegeneration. Finally, it is possible that immobilized yet still active peroxidase(s) may be present in protein aggregates in AD, PD, and ALS. This activity could be the catalyst for the slow, self-perpetuating and irreversible degeneration of affected neurons that occurs over long periods of time in these neurodegenerative diseases.

Translocation of cytochrome c during cerebellar degeneration in Lurcher and weaver mutant mice

Cytochrome c translocation from the inner mitochondrial membrane into the cytosol is the initial step of the intrinsic apoptotic pathway. As no evidence was ever presented for cytochrome c translocation during cerebellar degeneration in Lurcher (Lc/+) and weaver (wv/wv) mutant mice, we searched for the presence of such a process in cerebellar homogenates of mutant and wild-type mice from postnatal day (P)1 to P56. Here we present the first documented time course of cytochrome c translocation spanning the entire period of neurodegeneration in both mutant types. We identified cytochrome c with Western blotting and monitored cell loss in the cerebellum with Calbindin D-28k immunohistochemistry, Nissl-staining and morphometry. No cytochrome c translocation was ever detected in wild-types at any age investigated. Translocated cytochrome c appeared between P13 and P21 in Lc/+ and between P5 and P6 in wv/wv. These two intervals precisely coincide with the respective periods of maximal neuronal death in the cerebellum. Secondary translocation was also observed at a later stage between P42 and P49 in Lc/+ and from P22 onwards in wv/wv. Since no substantial neuronal loss has ever been observed in Lc/+ and wv/wv mutants at these postnatal ages, the delayed translocation may correspond to cytochrome c of extraneuronal, presumably glial origin. Observations of an increased expression of glial fibrillary acidic protein and sustained remodeling of the astrocytic network in the cerebellum of both mutants, long after the cessation of neuronal death make this assumption rather plausible.

Endoplasmic reticulum stress and trophic factor withdrawal activate distinct signaling cascades that induce glycogen synthase kinase-3 beta and a caspase-9-dependent apoptosis in cerebellar granule neurons

Loss of trophic or activity-dependent survival signals is commonly recognized as a stimulus for neuronal apoptosis and may play a significant role in neurodegeneration. Recent data have also implicated endoplasmic reticulum (ER) stress as an important factor in some neurodegenerative conditions. However, whether shared or unique apoptotic cascades are activated by trophic factor withdrawal (TFW) versus ER stress in primary neurons has not previously been investigated. In primary cultures of rat cerebellar granule neurons (CGNs), the ER stressor brefeldin A activated a discrete pathway involving the following: (1) stimulation of the ER resident kinase PERK, (2) enhanced phosphorylation of the translation initiation factor eIF2alpha, and (3) increased expression and nuclear localization of the transcription factor Gadd153/CHOP. ER stress-induced CGN apoptosis was blocked by an antagonist of IP3 receptor-mediated Ca2+ release, 2-aminoethoxydiphenyl borate (2-APB), and by expression of ER-targeted Bcl-2. In contrast, CGN apoptosis elicited by TFW (i.e., removal of serum and depolarizing extracellular potassium) did not display any ER stress component nor was it blocked by either 2-APB or ER-Bcl-2. Despite these apparent differences, both brefeldin A and TFW induced dephosphorylation (activation) of glycogen synthase kinase-3beta (GSK-3beta). Moreover, inhibitors of GSK-3beta (IGF-I, lithium) and caspase-9 (LEHD-fmk) significantly protected CGNs from apoptosis induced by either ER stress or TFW. These data indicate that ER stress and TFW elicit distinct signals that activate GSK-3beta and intrinsic apoptosis in neurons.

Biophysical properties of the apoptosis-inducing plasma membrane voltage-dependent anion channel

Ion channels in the plasma membrane play critical roles in apoptosis. In a recent study we found that a voltage-dependent anion channel in the plasma membrane (VDACpl) of neuronal hippocampal cell line (HT22) cells was activated during apoptosis and that channel block prevented apoptosis. Whether or not VDACpl is identical to the mitochondrial VDACmt has been debated. Here, we biophysically characterize the apoptosis-inducing VDACpl and compare it with other reports of VDACpls and VDACmt. Excised membrane patches of apoptotic HT22 cells were studied with the patch-clamp technique. VDACpl has a large main-conductance state (400 pS) and occasionally subconductance states of approximately 28 pS and 220 pS. The small subconductance state is associated with long-lived inactivated states, and the large subconductance state is associated with excision of the membrane patch and subsequent activation of the channel. The open-probability curve is bell shaped with its peak around 0 mV and is blocked by 30 microM Gd3+. The gating can be described by a symmetrical seven-state model with one open state and six closed or inactivated states. These channel properties are similar to those of VDACmt and other VDACpls and are discussed in relation to apoptosis.

PKCdelta alternatively spliced isoforms modulate cellular apoptosis in retinoic acid-induced differentiation of human NT2 cells and mouse embryonic stem cells

NT2 cells are a human teratocarcinoma cell line that, upon treatment with retinoic acid (RA), begin differentiating into a neuronal phenotype. The transformation of undifferentiated NT2 cells into hNT neurons presents an opportunity to investigate the mechanisms involved in neurogenesis because a key component is cell apoptosis, which is essential for building neural networks. Protein kinase Cdelta (PKCdelta) plays an important role as a mediator of cellular apoptosis in response to various stimuli. PKCdelta (deltaI) is proteolytically cleaved at its hinge region (V3) by caspase 3 and the catalytic fragment is sufficient to induce apoptosis in various cell types. Mouse PKCdeltaII is rendered caspase resistant due to an insertion of 78 bp within the caspase recognition site in its V3 domain. No functional role has been attributed to these alternatively spliced variants of PKCdelta. We sought to find a correlation between the onset of apoptosis, neurogenesis, and the expression of PKCdelta isoforms. Our results indicate that RA regulates the expression of PKCdelta alternative splicing variants in NT2 cells. Further, overexpression of PKCdeltaI promotes apoptosis while PKCdeltaII overexpression shields the cells from apoptosis. This is the first report to attribute physiological function to PKCdeltaI and -deltaII isoforms. Next we demonstrated that mouse embryonic stem cells differentiate in vitro into dopaminergic neurons upon stimulation with RA and ciliary neurotrophic factor. These cells showed a simultaneous increase in tyrosine hydroxylase and PKCdeltaII expression. We suggest that the molecular mechanisms regulating differentiation and apoptosis could be understood by alternative expression of PKCdelta isoforms.

Opening of plasma membrane voltage-dependent anion channels (VDAC) precedes caspase activation in neuronal apoptosis induced by toxic stimuli

Apoptotic cell death is an essential process in the development of the central nervous system and in the pathogenesis of its degenerative diseases. Efflux of K(+) and Cl(-) ions leads to the shrinkage of the apoptotic cell and facilitates the activation of caspases. Here, we present electrophysiological and immunocytochemical evidences for the activation of a voltage-dependent anion channel (VDAC) in the plasma membrane of neurons undergoing apoptosis. Anti-VDAC antibodies blocked the channel and inhibited the apoptotic process. In nonapoptotic cells, plasma membrane VDAC1 protein can function as a NADH (-ferricyanide) reductase. Opening of VDAC channels in apoptotic cells was associated with an increase in this activity, which was partly blocked by VDAC antibodies. Hence, it appears that there might be a dual role for this protein in the plasma membrane: (1) maintenance of redox homeostasis in normal cells and (2) promotion of anion efflux in apoptotic cells.

Role of peroxidases in Parkinson disease: a hypothesis

Extensive research has been done to elucidate the underlying molecular events causing neurodegenerative diseases such as Parkinson disease, yet the cause and the individual steps in the progression of such diseases are still unknown. Here we advance the hypothesis that, rather than or in addition to inorganic radical molecules, heme-containing peroxidase enzymes may play a major role in the etiology of Parkinson disease. This hypothesis is based on the following considerations: (1) several heme-containing enzymes with peroxidase activity are present in the substantia nigra pars compacta; (2) these peroxidases have the ability to catalyze the oxidation of proteins and lipids; (3) certain heme peroxidases are known to destroy cells in vivo; (4) heme peroxidases have the stability and specificity that could account for the fact that specific molecules and cells are subject to damage in Parkinson disease, rather than a random destruction; (5) heme peroxidase activity could account for certain reactions in connection with parkinsonism that thus far have not been adequately explained; and (6) the participation of a heme peroxidase could explain some recent observations that are inconsistent with the oxyradical theory. The peroxidase-catalyzed oxidative pathway proposed here does not preclude the participation of apoptosis as an additional mechanism for cell destruction.

Nuclear/cytoplasmic shuttling of the transcription factor FoxO1 is regulated by neurotrophic factors

FoxO1, a member of the FoxO subfamily of forkhead transcription factors, is an important target for insulin and growth factor signaling in the regulation of metabolism, cell cycle and proliferation, and survival in peripheral tissues. However, its role in the central nervous system is mostly unknown. In this study, we examined the effect of neurotrophic factors on nuclear/cytoplasmic shuttling of FoxO1. We showed that insulin-like growth factor-1 (IGF-1) and nerve growth factor (NGF) potently induced the nuclear exclusion of FoxO1-green fluorescent protein (GFP) while neurotrophin (NT)-3 and NT-4 were much weaker and brain-derived neurotrophic factor (BDNF) failed to induce FoxO1 translocation in PC12 cells. FoxO1 translocation was inhibited by LY294002, a well-established PI3K/Akt kinase inhibitor. Moreover, FoxO1 was phosphorylated at Thr24 and Ser256 residues by the above neurotrophic factors, with the exception of BDNF. Triple mutant FoxO1, in which three Akt/PKB phosphorylation sites (Thr24, Ser256 and Ser319) were mutated to alanine, resulted in the complete nuclear targeting of the expressed FoxO1-GFP fusion protein in the presence of the above neurotrophic factors in both PC12 cells and cultured hippocampal and cortical neurons. Taken together, these findings demonstrate that neurotrophic factors are able to regulate nuclear/cytoplasmic shuttling of FoxO1 via the PI3K/Akt pathway in neuronal cells.

Expression of a dominant negative form of Daxxin vivo rescues motoneurons from Fas (CD95)-induced cell death

Fas-induced death of motoneurons in vitro has been shown to involve two signaling cascades that act together to execute the death program: a Fas-Daxx-ASK-1-p38 kinase-nNOS branch, which controls transcriptional and post-translational events, and the second classical Fas-FADD-caspase-8 branch. To analyze the role of Daxx in the developmental motoneuron cell death, we studied Fas-dependent cell death in motoneurons from transgenic mice that overexpress a dominant-negative form of Daxx. Motoneurons purified from these transgenic mice are resistant to Fas-induced death. This protective effect is specific to Fas because ultraviolet irradiation-triggered death is not affected by the transgene. The Daxx and the FADD pathways work in parallel because only Daxx, but not FADD, is involved in the transcriptional control of neuronal nitric oxide synthase and nitric oxide production. Nevertheless, we do not observe involvement of Daxx in developmental motoneuronal cell death, as the pattern of naturally occurring programmed cell death in vivo is normal in transgenic mice overexpressing the dominant negative form of Daxx, suggesting that Daxx-independent pathways are used during development.