Atypical role of proximal caspase-8 in truncated Tau-induced neurite regression and neuronal cell death

Abnormal Tau protein is known to be closely associated with several neurodegenerative diseases. Previously, we showed that Tau was cleaved by caspase-3 to generate the cleavage product lacking the C-terminus (DeltaTau-1) during neuronal cell death. Here we characterized caspase-8-dependent neurotoxicity of the truncated Tau. Introduction of DeltaTau-1 into primary hippocampal neurons induced loss of neurites in a caspase-dependent manner. Caspase-8 and -6 were proteolytically activated during DeltaTau-1-triggered neuronal cell death, which was suppressed by IETD-fmk, caspase-8 inhibitor. Direct targeting of caspase-8 and its associated FADD with antisense approaches and transient expression of their dominant-negative mutants reduced DeltaTau-1-induced apopotosis. Cells deficient in caspase-8, but not caspase-3, became sensitized to DeltaTau-1-mediated toxicity upon reconstitution with caspase-8. In addition, ectopic expression of mitochondrial antiapoptotic Bcl-2, Bcl-X(L), or inactive caspase-9 short form suppressed DeltaTau-1 toxicity. These results suggest that the truncated Tau protein activates proximal caspase-8 through FADD as a necessary step leading to neuronal cell death and neurite regression, contributing to the progression of abnormal Tau-associated neurodegeneracy.     

Activation of calpain in cultured neurons overexpressing Alzheimer amyloid precursor protein

We have previously reported that overexpression of wild-type amyloid precursor protein (APP) in postmitotic neurons induces cleavage-dependent activation of caspase-3 both in vivo and in vitro. In this study, we investigated the mechanism underlying APP-induced caspase-3 activation using adenovirus-mediated gene transfer into postmitotic neurons derived from human embryonal carcinoma NT2 cells. Overexpression of wild-type APP significantly increased intracellular (45)Ca(2+) content prior to the activation of caspase-3 in NT2-derived neurons. Chelation of intracellular Ca(2+) markedly suppressed APP-induced activation of caspase-3. Furthermore, calpain, a Ca(2+)-dependent cysteine protease, was activated in neurons overexpressing APP as assessed by increased levels of calpain-cleaved alpha-fodrin and autolytic mu-calpain fragments. Neither calpain nor caspase-3 was activated in neurons expressing an APP mutant defective in the Abeta(1-20) domain. Calpain inhibitors almost completely suppressed APP-induced activation of neuronal caspase-3. E64d, a membrane permeable inhibitor of calpain, significantly suppressed APP-induced neuronal death. These results suggest that overexpression of wild-type APP activates calpain that mediates caspase-3 activation in postmitotic neurons.     

beta-Secretase cleavage of the amyloid precursor protein mediates neuronal apoptosis caused by familial Alzheimer's disease mutations.

The amyloid precursor protein (APP) is cleaved by two enzymes, beta-secretase and gamma-secretase, to generate the pathological amyloid beta (Abeta) peptide. Expression of familial Alzheimer's disease (FAD) mutants of APP in primary neurons causes both intracellular accumulation of the C-terminal beta-secretase cleavage product of APP and increased secretion of Abeta, and eventually results in apoptotic death of the cells. To determine whether either of these two processing products of APP is involved in this apoptotic pathway, we first modeled experimentally the accumulation of the beta-secretase cleavage product in neurons. The C-terminal 100 amino acids (C100) of APP, with and without a signal peptide, was expressed in cells via recombinant herpes simplex virus (HSV) vectors. Both transgene products were targeted to the membrane, and both caused apoptosis in the neurons, implicating the beta-secretase cleavage product of APP in apoptosis caused by FAD APPs. Expression in neurons of a mutant of FAD APP that inhibited beta-secretase cleavage inhibited its ability to cause apoptosis. However, expression in neurons of a mutant of FAD APP that inhibited gamma-secretase cleavage did not inhibit the ability of this mutant to cause apoptosis. These data suggested that the C-terminal beta-secretase cleavage product of APP, but not Abeta, mediates the apoptosis caused by FAD mutants of APP. Consistent with this hypothesis, C31, which is generated from the beta-secretase cleavage product, itself caused neuronal apoptosis. Inhibitors of caspases 3, 6 and 8, but not of caspase 9, inhibited the apoptosis caused by FAD mutants of APP. It may be inferred from these data that beta-secretase cleavage of FAD mutants of APP allows the appropriate caspase access to its site of action to produce C31, which directly causes neuronal apoptosis.     

Lentivirus-mediated APP695-RNAi reduces apoptosis in APP transgenic mouse neurons

Amyloid-β peptide (Aβ) is a cleavage product of the amyloid precursor protein (APP), which is thought to be important in the pathogenesis of Alzheimer's disease (AD). Recent evidence suggests that Aβ induces neuronal apoptosis in the brain and in primary neuronal cultures. If decreased Aβ whether could reduce the neuronal apoptosis? In this study, APP695-siRNA was delivered to hippocampal and cortical neurons of APP695 transgenic mice (AD model) in vitro using a recombinant lentivirus vector. The results show that lentivirus-mediated RNA interference of the APP695 gene could reduce neuronal apoptosis, possibly through the reduction of caspase-3 activity and the neuronal apoptosis pathway. These results suggest that lentivirus-mediated RNA interference may be a potential therapeutic for AD.     

Hyperphosphorylated truncated protein tau induces caspase-3 independent apoptosis-like pathway in the Alzheimer's disease cellular model

Neurofibrillary degeneration and neuronal loss represent key pathological hallmarks of Alzheimer's disease (AD). It has been demonstrated that the decrease of total neuronal numbers correlates with the presence of neurofibrillary degeneration in AD brain. In order to unravel the mechanism leading to the cell death in AD, we developed a stably transfected human neuroblastoma cellular model with doxycycline-regulatable expression of AD truncated tau protein (AT tau, 151-391 4R). Cells expressing the longest tau isoform (Tau 40) were used as a control. We found that more than 80% of the total amount of AT tau and Tau 40 were phosphorylated. Strikingly, both AT tau and Tau 40 reduced the metabolic activity of the cells in a time-dependent manner (p < 0.0001) suggesting that tau overexpression slows down cell proliferation. However, AT tau showed significantly higher toxicity than Tau 40 (p < 0.0001), which indicates that truncation leads to a toxic gain of function. The analysis of the type of the cell death revealed the characteristic features of apoptosis such as cell shrinkage, nuclear, and DNA fragmentation. However, we did not find either the activation of executive caspase (caspase-3) or the caspase cleavage products (PARP and fodrin). These results show that posttranslationally modified truncated tau protein induces caspase-3-independent apoptosis-like programmed cell death, a phenomenon we term tauoptosis.     

Caspase inhibitors protect against neuronal apoptosis induced by cerebrospinal fluid from multiple sclerosis patients

Neuronal apoptosis has recently been implicated in multiple sclerosis (MS). Apoptotic cell death of neurons is induced in cultures exposed to cerebrospinal fluid (CSF) from MS patients. Since caspases are essential in the regulation of apoptosis, direct evidence was sought linking caspases to CSF-induced neuronal death. Caspase activity was measured in cell extracts from MS CSF-treated cultured neurons by the cleavage of caspase-1 and caspase-3 substrates. Caspase-3 activity, but not caspase-1, was induced in neuronal cultures in response to MS CSF treatment. This caspase-3 activity was inhibited in vitro by Ac-YVAD-cmk and Ac-DEVD-cmk caspase inhibitors. Treatment of MS CSF-incubated neuronal cells with these caspase inhibitors completely preserved neuronal survival and largely attenuated DNA fragmentation detected in situ. These findings show that neuronal cells are rescued from MS CSF-induced death by caspase inhibitors and suggest ways to treat MS.     

Caspase-3 activation and inflammatory responses in rat hippocampus inoculated with a recombinant adenovirus expressing the Alzheimer amyloid precursor protein

To elucidate the mechanism of neuronal death in Alzheimer's disease, we investigated the effects of overexpression of wild-type Alzheimer amyloid precursor protein (APP) on neuronal cells and glial cells in vivo. When an APP695-expressing adenovirus was injected into the dorsal hippocampal region, a number of neurons in remote areas were positively stained with anti-APP monoclonal antibody, and underwent severe degeneration from 3 to 7 days after viral inoculation. Most degenerating neurons were immunopositive with both APP and activated caspase-3, but some neurons that expressed activated caspase-3 were not expressing APP from 7 to 14 days after virus injection. In the neighborhood of the degenerating neurons, activated microglia/macrophages, which were identified by the phenotypic marker C3bi receptor (CD11b/c; OX-42), were observed, and some of them appeared to phagocytose the caspase-3-immunopositive degenerating neurons. In addition to microglia/macrophages, infiltrating leukocytes expressing CD45 or CD4 were also detected. These results suggest that the increased accumulation of APP induced not only caspase-3-mediated death machinery, but also inflammatory responses including microglial activation. These inflammatory responses might cause further neurodegeneration through the alternative pathway that might activate the caspase-3-mediated death machinery without APP expression.     

Activation of caspase-3 in beta-amyloid-induced apoptosis of cultured rat cortical neurons.

Amyloid beta protein (Abeta) has been thought to participate in the neurodegeneration associated with Alzheimer's disease. We here report on caspase-3 activation by Abeta-treatment of cultured neurons. Treatment of rat primary cortical culture with Abeta 25-35, an active fragment of Abeta, induced neuronal death as determined by a decrease in neuron-specific microtubule-associated protein 2 (MAP2)-like immunoreactivity and by the release of cellular lactate dehydrogenase (LDH). Abeta 25-35 also induced elevation of caspase-3-like Ac-DEVD-MCA cleavage activity in advance of neuronal death with similar concentration-dependency for neuronal death. Inhibitor sensitivity of the Abeta-induced proteolytic activity was similar to that of human recombinant caspase-3. Cleavage of pro-caspase-3 and cleavage of its endogenous substrates, poly (ADP-ribose) polymerase (PARP) and alpha-fodrin, were produced by Abeta-treatment. A caspase-3 inhibitor, Ac-DEVD-CHO, prevented Abeta-induced DNA fragmentation and cleavage of alpha-fodrin, but not of PARP. Caspase inhibitor of broad specificity, Z-VAD-CH(2)-DCB, additionally prevented Abeta-induced cleavage of PARP and some early loss of cell membrane integrity measured by LDH release. However, Abeta-induced condensation of nuclear chromatin and most of the late disintegration of cell membranes were not prevented in the presence of these caspase inhibitors. These results suggest that activation of both caspase-3 and caspase(s) other than caspase-3 play distinct roles in Abeta-induced apoptosis of rat cortical neurons. Furthermore, in the presence of caspase inhibitors, Abeta-induced neuronal death still occurred with different morphological features.     

Caspase regulation of neuronal progenitor cell apoptosis

Programmed cell death (apoptosis) of both proliferating neuroblasts and postmitotic neurons is essential for normal nervous system development. To study the molecular regulation of apoptosis in neuronal progenitor cells, we developed a flow cytometric assay capable of distinguishing between viable, apoptotic, and necrotic cell populations. Incubation of freshly dissociated telencephalic cells from gestational day 12-13 mouse embryos with either cytosine arabinoside (AraC) or staurosporine caused a marked increase in the percentage of apoptotic cells. Both drugs induced caspase-3 activation, as determined by in vitro cleavage of a caspase-3 substrate and immunocytochemical detection of activated caspase-3. Treatment of telencephalic cells with the broad caspase inhibitor BAF, blocked caspase-3 activation and protected cells against both AraC and staurosporine-induced apoptotic death. These results indicate that neuronal progenitors possess a caspase-dependent apoptotic pathway, the activation of which may regulate neuronal progenitor cell numbers in vivo.     

Caspase-3-mediated cleavage of PHF-1 tau during apoptosis irrespective of excitotoxicity and oxidative stress: an implication to Alzheimer's disease

Excitotoxicity, oxidative stress, and apoptosis have been recognized as routes to neuronal death in various neurological diseases. We examined the possibility that PHF-1 tau, a substrate for various proteases, would be selectively cleaved depending upon routes of neuronal death. Cleavage form of PHF-1 tau was not observed in cortical cell cultures exposed to excitotoxins or oxidative stress that cause neuronal cell necrosis. PHF-1 tau was cleaved within 8 h following exposure of cortical cell cultures to apoptosis-inducing agents. This cleavage was blocked by inclusion of zDEVD-fmk, an inhibitor of caspase-3, and accompanied by activation of caspase-3. Levels and cleavage of PHF-1 tau were markedly increased in AD brain compared with control. Moreover, PHF-1 tau and active caspase-3 were colocalized mostly in tangle-bearing neurons. The current findings suggest that PHF-1 tau is cleaved by caspase-3 during apoptosis and neurodegenerative process in AD.     

Accumulation of caspase cleaved amyloid precursor protein represents an early neurodegenerative event in aging and in Alzheimer's disease

The activation of caspase-3 and possibly other caspases during apoptosis may lead to the cleavage of the amyloid precursor protein (APP) and subsequent accumulation of APP cleavage products (cAPP). We examined the association between activated caspase-3 and cAPP in human brain by qualitative and quantitative analysis of in situ immunohistochemistry and Western blots. Frontal cortex and hippocampal tissue from age-matched control and Alzheimer's brains (AD) was used. Both activated caspase-3 and cAPP are increased in AD [Braak and Braak (BB) stage IV-VI] compared to aged control (BB stage 0-1) and transitional (BB stage II-III) cases in the hippocampal and frontal cortex. Caspase-3 activation and the accumulation of APP cleavage fragments appear to either parallel or precede neurofibrillary tangle formation. These findings raise the possibility that the activation of caspase-3 and cleavage of APP may be involved with neuronal degeneration and that pathways characteristic of apoptosis are activated in AD.     

Tau cleavage at D421 by caspase-3 is induced in neurons and astrocytes infected with herpes simplex virus type 1

Herpes Simplex Virus Type 1 (HSV-1) is ubiquitous, neurotropic, and the most common pathogenic causes of sporadic acute encephalitis in humans. Herpes simplex encephalitis is associated with a high mortality rate and significant neurological, neuropsychological, and neurobehavioral sequelae, which afflict patients for life. HSV-1 infects limbic system structures in the central nervous system and has been suggested as an environmental risk factor for Alzheimer's disease. However, the possible mechanisms that link HSV-1 infection with the neurodegenerative process are still largely unknown. In a previous study we demonstrated that HSV-1 triggers hyperphosphorylation of tau epitopes serine202/threonine205 and serine396/serine404 in neuronal cultures, resembling what occurs in neurodegenerative diseases. Therefore, the aim of the present study was to evaluate at the cellular level if another event associated with neurodegeneration, such as caspase-3 induced cleavage of tau, could also be triggered by HSV-1 infection in primary neuronal and astrocyte cultures. As expected, induction of caspase-3 activation and cleavage of tau protein at its specific site (aspartic acid 421) was observed by Western blot and immunofluorescence analyses in mice neuronal primary cultures infected with HSV-1. In agreement with our previous study on tau hyperphosphorylation, tau cleavage was also observed during the first 4 hours of infection, before neuronal death takes place. This tau processing has been previously demonstrated to increase the kinetics of tau aggregation in vitro and has also been observed in neurodegenerative pathologies. In conclusion, our findings support the idea that HSV-1 could contribute to induce neurodegenerative processes in age-associated pathologies such as Alzheimer's disease.     

Death of dopaminergic neurons in vitro and in nigral grafts: reevaluating the role of caspase activation

Caspases are cysteine proteases involved in apoptotic cell death, and pharmacological caspase inhibition has been demonstrated to prevent neuronal cell death in certain experimental paradigms. In this study, the role of caspase-1 and -3 in the death of dopaminergic neurons derived from the E14 rat ventral mesencephalon (VM) has been examined in two model systems using peptide caspase inhibitors. First, cell death was induced in vitro by withdrawing serum after 2 days. Different doses of caspase-1 (IL-1beta converting enzyme) and caspase-3 inhibitors (Ac-DEVD-cmk) were added to the medium at the time of serum withdrawal, and the ability of the inhibitors to promote dopaminergic neuronal survival and prevent activation of caspase-3 was assessed at 7 days. Immunostaining using tyrosine hydroxylase (TH) and cleaved caspase-3 antibodies demonstrated that caspase-1 and -3 inhibitors reduce caspase-3 activation as well as overall cell death. This did not, however, improve the survival of TH-positive neurons, although it did appear to promote their maturation. The second paradigm investigated the effects of these inhibitors in the 6-hydroxydopamine rat model of PD, and similarly, addition of caspase-1 or -3 inhibitor during tissue preparation or immediately prior to grafting of VM tissue did not promote dopaminergic neuronal survival. These results demonstrate that the reduction of apoptotic cell death by pharmacological inhibition of caspase-1 and -3 does not increase dopaminergic neuronal survival in these paradigms and suggest either that caspase-3 activation is not the major determinant of dopaminergic neuronal death in vitro and in grafts or that the ability of caspase inhibitors to rescue cells depends upon the degree of apoptotic stress. This implies that strategies to improve dopaminergic cell survival in clinical programmes of transplantation for PD will need to target other pathways of cell death.     

Activation of caspase-3 in β-amyloid-induced apoptosis of cultured rat cortical neurons

Amyloid β protein (Aβ) has been thought to participate in the neurodegeneration associated with Alzheimer's disease. We here report on caspase-3 activation by Aβ-treatment of cultured neurons. Treatment of rat primary cortical culture with Aβ 25–35, an active fragment of Aβ, induced neuronal death as determined by a decrease in neuron-specific microtubule-associated protein 2 (MAP2)-like immunoreactivity and by the release of cellular lactate dehydrogenase (LDH). Aβ 25–35 also induced elevation of caspase-3-like Ac-DEVD-MCA cleavage activity in advance of neuronal death with similar concentration-dependency for neuronal death. Inhibitor sensitivity of the Aβ-induced proteolytic activity was similar to that of human recombinant caspase-3. Cleavage of pro-caspase-3 and cleavage of its endogenous substrates, poly (ADP-ribose) polymerase (PARP) and α-fodrin, were produced by Aβ-treatment. A caspase-3 inhibitor, Ac-DEVD-CHO, prevented Aβ-induced DNA fragmentation and cleavage of α-fodrin, but not of PARP. Caspase inhibitor of broad specificity, Z-VAD-CH₂-DCB, additionally prevented Aβ-induced cleavage of PARP and some early loss of cell membrane integrity measured by LDH release. However, Aβ-induced condensation of nuclear chromatin and most of the late disintegration of cell membranes were not prevented in the presence of these caspase inhibitors. These results suggest that activation of both caspase-3 and caspase(s) other than caspase-3 play distinct roles in Aβ-induced apoptosis of rat cortical neurons. Furthermore, in the presence of caspase inhibitors, Aβ-induced neuronal death still occurred with different morphological features.     

Wild-type alpha-synuclein interacts with pro-apoptotic proteins PKCdelta and BAD to protect dopaminergic neuronal cells against MPP+-induced apoptotic cell death

Alpha-synuclein is a pre-synaptic protein of unknown function that has been implicated in the pathogenesis of Parkinson's disease (PD). Recently, we demonstrated that 1-methyl-4-phenylpyridinium (MPP+) induces caspase-3-dependent proteolytic activation of PKCdelta, which subsequently contributes to neuronal apoptotic cell death in mesencephalic dopaminergic neuronal cells. In the present study, we examined whether PKCdelta interacts with alpha-synuclein to modulate MPP+-induced dopaminergic degeneration. Over-expression of wild-type human alpha-synuclein in mesencephalic dopaminergic neuronal cells (N27 cells) attenuated MPP+-induced (300 microM) cytotoxicity, release of mitochondrial cytochrome c, and subsequent caspase-3 activation, without affecting reactive oxygen species (ROS) generation. Wild-type alpha-synuclein over-expression also dramatically reduced MPP+-induced caspase-3-mediated proteolytic cleavage of PKCdelta, whereas over-expression of the mutant human alpha-synucleinA53T did not alter the PKCdelta cleavage under similar conditions. Immunoprecipitation-kinase assay revealed reduced PKCdelta kinase activity in wild-type alpha-synuclein over-expressing cells in response to MPP+ treatment. Wild-type alpha-synuclein over-expression also rescued mesencephalic dopaminergic neuronal cells from MPP+-induced apoptotic cell death, while alpha-synucleinA53T exacerbated the MPP+-induced DNA fragmentation. Furthermore, co-immunoprecipitation studies revealed that alpha-synuclein interacts with the pro-apoptotic proteins PKCdelta and BAD, but not with the anti-apoptotic protein Bcl-2 following MPP+ treatment. We also observed that the interaction between PKCdelta and alpha-synuclein does not involve direct phosphorylation. Together, our results demonstrate that wild-type alpha-synuclein interacts with the pro-apoptotic molecules BAD and PKCdelta to protect dopaminergic neuronal cells against neurotoxic insults.     

Hypoxia induces caspase-9 and caspase-3 activation without neuronal death in gerbil brains

To investigate the in vivo apoptotic machinery in oxygen deprived brain, we examined the expression of caspase-9 and caspase-3 in the hippocampus of Mongolian gerbils subjected to either transient hypoxia (4% O2 for 6 min) or forebrain ischemia (10 min bilateral carotid artery occlusion) followed by 8 h to 7 days of reoxygenation or blood recirculation. Apoptotic death was characterized by isolating hippocampal genomic DNA and analysing DNA fragmentation as well as histological studies including TUNEL assay and toluidine blue staining of brain sections. The results showed that both hypoxic and ischemic gerbil brains exhibited an increase in caspase-9 and caspase-3 gene expression. However, no cell damage was detectable following hypoxia, while marked DNA fragmentation and extensive cell death was observed following ischemia. Moreover, although hypoxia did not lead to cell death, both hypoxia and ischemia were associated with cleavage of procaspase-9 and procaspase-3 and increases in their activities as well as cleavage of poly(ADP-ribose) polymerase-1 (PARP-1), a major caspase-3 substrate. These results indicate that, in vivo, even late apoptotic events such as caspase activation and PARP-1 cleavage in hypoxic brains do not necessarily induce an irreversible commitment to apoptotic neuronal death.     

Assessment of white matter injury following prolonged focal cerebral ischaemia in the rat

The ability of putative neuroprotective compounds to protect against white matter injury remains poorly investigated due to the lack of suitable methods for assessing white matter injury. This study was therefore designed to investigate the utility of Tau 1 (oligodendrocytes/axons), myelin basic protein (MBP; myelin) and amyloid precursor protein (APP; axons) immunohistochemistry in assessing white matter injury at various times following middle cerebral artery occlusion (MCAO) in the rat. Focal cerebral, ischaemia was induced in halothane-anaesthetised rats using an intraluminal thread model. At 24 h, 1 and 2 weeks following MCAO, white matter injury was assessed using Tau 1, APP, MBP and Luxol-fast blue staining and neuronal injury with cresyl fast violet (CFV). In histologically normal tissue MBP immunoreactivity was detected in myelinated fibre tracts, while Tau 1 and APP were axonally located. At 24 h following permanent MCAO, MBP, and Tau 1 staining remained relatively unchanged within the myelin and axonal compartments of the ischaemic region. In contrast, increased Tau 1 staining was apparent in oligodendrocytes within ischaemic tissue, while APP accumulated in axons surrounding the lesion. At 1 and 2 weeks following transient MCAO, Tau 1 and APP staining was markedly decreased within ischaemic tissue. Marked reduction in MBP levels within ischaemic tissue were not detected until 2 weeks following MCAO. The area of axonal injury as assessed by reduced Tau 1 or APP staining correlated with the area of neuronal damage as assessed by CFV staining. This study shows that MBP, Tau 1 and APP immunohistochemistry can be utilised to assess myelin and axonal integrity following sustained ischaemia using standard image analysis techniques.     

Apoptotic characteristics of cell death and the neuroprotective effect of homocarnosine on pheochromocytoma PC12 cells exposed to ischemia

We recently improved an in vitro ischemic model, using PC12 neuronal cultures exposed to oxygen-glucose deprivation (OGD) for 3 hr in a special device, followed by 18 hr of reoxygenation. The cell death induced in this ischemic model was evaluated by a series of markers: lactate dehydrogenase (LDH) release, caspase-3 activation, presence of cyclin D1, cytochrome c leakage from the mitochondria, BAX cellular redistribution, cleavage of poly (ADP-ribose) polymerase (PARP) to an 85-kDa apoptotic fragment, and DNA fragmentation. The OGD insult, in the absence of reoxygenation, caused a strong activation of the mitogen-activated protein kinase (MAPK) isoforms extracellular regulated kinase (ERK), c-Jun NH2-terminal kinase (JNK), and stress-activated protein kinase (SAPK), also known as p-38. The detection of apoptotic markers and activation of MAPKs during the ischemic insult strongly suggest that apoptosis plays an important role in the PC12 cell death. Homocarnosine, a neuroprotective histidine dipeptide, present in high concentrations in the brain, was found to provide neuroprotection, as expressed by a 40% reduction in LDH release and caspase-3 activity at 1 mM. Homocarnosine reduced OGD activation of ERK 1, ERK 2, JNK 1, and JNK 2 by 40%, 46%, 55%, and 30%, respectively. These results suggest that apoptosis is an important characteristic of OGD-induced neuronal death and that antioxidants, such as homocarnosine, may prevent OGD-induced neuronal death by inhibiting the apoptotic process and/or in relation to the differential attenuation of activity of MAPKs.     

The role of caspase cleavage of tau in Alzheimer disease neuropathology

Alzheimer disease (AD) is characterized by the accumulation of amyloid plaques and neurofibrillary tangles within selective brain regions. In addition, cell death pathways become active leading to neurodegeneration. Caspase activation, a key step in the programmed cell death pathway known as apoptosis, occurs in AD and leads to the proteolytic cleavage of several neuronal proteins. Previously, it was hypothesized that the development of the classical hallmarks of AD, amyloid plaques and neurofibrillary tangles, occur independently and do not involve the activation of caspases. However, recent studies suggest that plaques, tangles, and caspase activation share a common pathway. Beta-amyloid, the main component of amyloid plaques, activates caspases. Activated caspases can in turn cleave tau, the main component of neurofibrillary tangles. Caspase-cleaved tau (deltatau) may initiate or accelerate the development of tangle pathology. Tau, when cleaved by caspases at Asp421, "seeds" filamentous aggregates in vitro. Caspase-cleaved tau also adopts the MC1 conformation, one of the earliest pathologic events in tangle formation. Importantly, deltatau occurs early in the development of tangle pathology within AD brains and in a transgenic mouse model of AD. This review summarizes recent evidence suggesting that caspase cleavage of tau plays an important role in the development of neurofibrillary tangle pathology. In addition, a model is presented whereby caspase cleavage of tau provides a mechanistic link between the development of amyloid and tangle pathologies.     

Caspase-3 activation in oligodendrocytes from the myelin-deficient rat

The myelin-deficient (MD) rat has a point mutation in its proteolipid protein (PLP) gene that causes severe dysmyelination and oligodendrocyte cell death. Using an in vitro model, we have shown that MD oligodendrocytes initially differentiate similarly to wild-type cells, expressing galactocerebroside, 2',3'-cyclic nucleotide 3'-phosphodiesterase, and myelin basic protein. However, at the time when PLP expression would normally begin, the MD oligodendrocytes die via an apoptotic pathway involving caspase activation. The active form of caspase-3 was detected, along with the cleavage products of poly-(ADP-ribose) polymerase (PARP) and spectrin, major targets of caspase-mediated proteolysis. A specific inhibitor of casapse-3, Ac-DEVD-CMK, reduced apoptosis in MD oligodendrocytes, but the rescued cells did not mature fully or express myelin-oligodendrocyte glycoprotein. These results suggest that mutant PLP affects not only cell death but also oligodendrocyte differentiation.