Injury-induced apoptosis of neurons in adult brain is mediated by p53-dependent and p53-independent pathways and requires Bax

Multiple low-dose radiation-induced neuronal cysteine transporter expression and oxidative stress are rescued by N-acetylcysteine in neuronal SH-SY5Y cells

Recently, several studies have demonstrated that low-dose radiation (LDR) therapy has positively impacts on the treatment of Alzheimer's disease (AD). LDR suppresses the production of pro-neuroinflammation molecules and improves cognitive function in AD. However, it is unclear whether direct exposure to LDR causes beneficial effects and what mechanism is involved in neuronal cells. In this study, we first determined the effect of high-dose radiation (HDR) alone on C6 cells and SH-SY5Y cells. We found that SH-SY5Y cells were more vulnerable than C6 cells to HDR. Moreover, in neuronal SH-SY5Y cells exposed to single or multiple LDR, N-type cells showed decreased cell viability with increasing radiation exposure time and frequency, but S-type cells were unaffected. Multiple LDR increased proapoptotic molecules such as p53, Bax and cleaved caspase-3, and decreased anti-apoptotic molecule (Bcl₂). Multiple LDR also generated free radicals in neuronal SH-SY5Y cells. We detected a change in the expression of the neuronal cysteine transporter EAAC1. Pretreatment with N-acetylcysteine (NAC) rescued the increased in EAAC1 expression and the generation of ROS in neuronal SH-SY5Y cells after multiple LDR. Furthermore, we verified whether the increased in EAAC1 expression induces cell defense or cell death promotion signaling. We showed that transient overexpression of EAAC1 reduced the multiple LDR-induced p53 overexpression in neuronal SH-SY5Y cells. Our results indicate that neuronal cells can be injured by increased production of ROS not only by HDR but also by multiple LDR, which suggests that combination treatment with anti-free radical agents such as NAC may be useful in multiple LDR therapy.

Expression profile of the proapoptotic protein Bax in the human brain

Bax is a well-known universal proapoptotic protein. Bax protein is detected in almost all human organs, and its expression levels can be correlated with disease progression and therapeutic efficacy in certain settings. Interestingly, increasing evidence has shown that mature neuronal cell death is often not typical apoptosis. Most results on the expression of Bax proteins (predominantly Baxα) in the human brain come from disease-oriented studies, and the data on Bax protein expression in the normal brain are limited and lack consistency due to many variable factors. Here, we analyzed Bax RNA and protein expression data from multiple databases and performed immunostaining of over 80 samples from 25 healthy subjects across 7 different brain regions. We found that Bax protein expression was heterogeneous across brain regions and individual subjects. Both neurons and glial cells, such as astrocytes, could be Bax positive, but Bax positivity appeared to be highly selective, even within the same cell type in the same region. Furthermore, Bax proteins could be localized in the cytosol (evenly spread or concentrated to one region), nucleus or nucleolus depending on the cell type. Such variation and distribution in Bax expression suggest that Bax may function differently in the human brain than in other organs.

Mammalian target of rapamycin (mTOR) signaling pathway and traumatic brain injury: A novel insight into targeted therapy

Traumatic brain injury (TBI) is one of the most concerning health issues in which the normal brain function may be disrupted as a result of a blow, bump, or jolt to the head. Loss of consciousness, amnesia, focal neurological defects, alteration in mental state, and destructive diseases of the nervous system such as cognitive impairment, Parkinson's, and Alzheimer's disease. Parkinson's disease is a chronic progressive neurodegenerative disorder, characterized by the early loss of striatal dopaminergic neurons. TBI is a major risk factor for Parkinson's disease. Existing therapeutic approaches have not been often effective, indicating the necessity of discovering more efficient therapeutic targets. The mammalian target of rapamycin (mTOR) signaling pathway responds to different environmental cues to modulate a large number of cellular processes such as cell proliferation, survival, protein synthesis, autophagy, and cell metabolism. Moreover, mTOR has been reported to affect the regeneration of the injured nerves throughout the central nervous system (CNS). In this context, recent evaluations have revealed that mTOR inhibitors could be potential targets to defeat a group of neurological disorders, and thus, a number of clinical trials are investigating their efficacy in treating dementia, autism, epilepsy, stroke, and brain injury, as irritating neurological defects. The current review describes the interplay between mTOR signaling and major CNS-related disorders (esp. neurodegenerative diseases), as well as the mTOR signaling-TBI relationship. It also aims to discuss the promising therapeutic capacities of mTOR inhibitors during the TBI.

DNA damage accumulates and responses are engaged in human ALS brain and spinal motor neurons and DNA repair is activatable in iPSC-derived motor neurons with SOD1 mutations

DNA damage is implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). However, relationships between DNA damage accumulation, DNA damage response (DDR), and upper and lower motor neuron vulnerability in human ALS are unclear; furthermore, it is unknown whether epigenetic silencing of DNA repair pathways contributes to ALS pathogenesis. We tested the hypotheses that DNA damage accumulates in ALS motor neurons along with diminished DDR, and that DNA repair genes undergo hypermethylation. Human postmortem CNS tissue was obtained from ALS cases (N = 34) and age-matched controls without neurologic disease (N = 15). Compared to age-matched controls, abasic sites accumulated in genomic DNA of ALS motor cortex and laser capture microdissection-acquired spinal motor neurons but not in motor neuron mitochondrial DNA. By immunohistochemistry, DNA damage accumulated significantly in upper and lower motor neurons in ALS cases as single-stranded DNA and 8-hydroxy-deoxyguanosine (OHdG) compared to age-matched controls. Significant DDR was engaged in ALS motor neurons as evidenced by accumulation of c-Abl, nuclear BRCA1, and ATM activation. DNA damage and DDR were present in motor neurons at pre-attritional stages and throughout the somatodendritic attritional stages of neurodegeneration. Motor neurons with DNA damage were also positive for activated p53 and cleaved caspase-3. Gene-specific promoter DNA methylation pyrosequencing identified the DNA repair genes Ogg1, Apex1, Pnkp and Aptx as hypomethylated in ALS. In human induced-pluripotent stem cell (iPSC)-derived motor neurons with familial ALS SOD1 mutations, DNA repair capacity was similar to isogenic control motor neurons. Our results show that vulnerable neurons in human ALS accumulate DNA damage, and contrary to our hypothesis, strongly activate and mobilize response effectors and DNA repair genes. This DDR in ALS motor neurons involves recruitment of c-Abl and BRCA1 to the nucleus in vivo, and repair of DNA double-strand breaks in human ALS motor neurons with SOD1 mutations in cell culture.

Combination Treatment With Exogenous GDNF and Fetal Spinal Cord Cells Results in Better Motoneuron Survival and Functional Recovery After Avulsion Injury With Delayed Root Reimplantation

When spinal roots are torn off from the spinal cord, both the peripheral and central nervous system get damaged. As the motoneurons lose their axons, they start to die rapidly, whereas target muscles atrophy due to the denervation. In this kind of complicated injury, different processes need to be targeted in the search for the best treatment strategy. In this study, we tested glial cell-derived neurotrophic factor (GDNF) treatment and fetal lumbar cell transplantation for their effectiveness to prevent motoneuron death and muscle atrophy after the spinal root avulsion and delayed reimplantation. Application of exogenous GDNF to injured spinal cord greatly prevented the motoneuron death and enhanced the regeneration and axonal sprouting, whereas no effect was seen on the functional recovery. In contrast, cell transplantation into the distal nerve did not affect the host motoneurons but instead mitigated the muscle atrophy. The combination of GDNF and cell graft reunited the positive effects resulting in better functional recovery and could therefore be considered as a promising strategy for nerve and spinal cord injuries that involve the avulsion of spinal roots.

DNA Damage Response and Repair, DNA Methylation, and Cell Death in Human Neurons and Experimental Animal Neurons Are Different

Neurological disorders affecting individuals in infancy to old age elude interventions for meaningful protection against neurodegeneration, and preclinical work has not translated to humans. We studied human neuron responses to injury and death stimuli compared to those of animal neurons in culture under similar settings of insult (excitotoxicity, oxidative stress, and DNA damage). Human neurons were differentiated from a cortical neuron cell line and the embryonic stem cell-derived H9 line. Mouse neurons were differentiated from forebrain neural stem cells and embryonic cerebral cortex; pig neurons were derived from forebrain neural stem cells. Mitochondrial morphology was different in human and mouse neurons. Human and mouse neurons challenged with DNA-damaging agent camptothecin showed different chromatin condensation, cell death, and DNA damage sensor activation. DNA damage accumulation and repair kinetics differed among human, mouse, and pig neurons. Promoter CpG island methylation microarrays showed significant differential DNA methylation in human and mouse neurons after injury. Therefore, DNA damage response, DNA repair, DNA methylation, and autonomous cell death mechanisms in human neurons and experimental animal neurons are different.

Up-regulation of FOS-like antigen 1 contributes to neuronal apoptosis in the cortex of rat following traumatic brain injury

Neuronal apoptosis is an important process of secondary brain injury which is induced by neurochemical signaling cascades after traumatic brain injury (TBI). Present study was designed to investigate whether FOS-like antigen 1 (Fra-1) is involved in the neuronal apoptosis. Western blot analysis and immunohistochemistry in a rat TBI model revealed a significant increase in the expression of Fra-1 in the ipsilateral brain cortex, which was in parallel with increase in the expression of active caspase-3. With immunofluorescence double-labeling, Fra-1 was colocalized with active caspase-3 and with NeuN, a neuronal marker. In an in vitro cell injury model, H₂O₂ exposure induced cell apoptosis and reduced cell viability and at the same time, a similar increased expression of active caspase-3, p53 and Fra-1 was found in PC12 cells. Down-regulation of Fra-1 through transfection with Fra-1 siRNA remarkably elevated cell viability, reduced the expression of active caspase-3 and p53, and decreased apoptosis of PC12 cells after H₂O₂ exposure. Taken together, present findings suggest that Fra-1 may be involved in the induction of neuronal apoptosis through up-regulating p53 signaling pathway and that this action may contribute to the secondary neuropathological process after TBI.

Enforced DNA repair enzymes rescue neurons from apoptosis induced by target deprivation and axotomy in mouse models of neurodegeneration

It is unknown whether DNA damage accumulation is an upstream instigator or secondary effect of the cell death process in different populations of adult postmitotic neurons in the central nervous system. In two different mouse models of injury-induced neurodegeneration characterized by relatively synchronous accumulation of mitochondria, oxidative stress, and DNA damage prior to neuronal apoptosis, we enforced the expression of human 8-oxoguanine DNA glycosylase (hOGG1) and human apurinic-apyrimidinic endonuclease-1/Ref1 (hAPE) using recombinant adenoviruses (Ad). Thalamic lateral geniculate neurons and lumbar spinal cord motor neurons were transduced by Ad-hOGG1 and Ad-hAPE injections into the occipital cortex and skeletal muscle, respectively, prior to their target deprivation- and axotomy-induced retrograde apoptosis. Enforced expression of hOGG1 and hAPE in thalamus and spinal cord was confirmed by western blotting and immunohistochemistry. In injured populations of neurons in thalamus and spinal cord, a DNA damage response (DDR) was registered, as shown by localization of phospho-activated p53, Rad17, and replication protein A-32 immunoreactivities, and this DDR was attenuated more effectively by enforced hAPE expression than by hOGG1 expression. Enforced expression of hOGG1 and hAPE significantly protected thalamic neurons and motor neurons from retrograde apoptosis induced by target deprivation and axotomy. We conclude that a DDR response is engaged pre-apoptotically in different types of injured mature CNS neurons and that DNA repair enzymes can regulate the survival of retrogradely dying neurons, suggesting that DNA damage and activation of DDR are upstream mechanisms for this form of adult neurodegeneration in vivo, thus identifying DNA repair as a therapeutic target for neuroprotection.

KHSRP participates in manganese-induced neurotoxicity in rat striatum and PC12 cells

Manganese (Mn) is an essential micronutrient. However, exposure to high doses of Mn may lead to a neurological disease known as manganism, which is characterized by marked brain neuronal loss. K-homology splicing regulator protein (KHSRP) is a multifunctional RNA-binding protein and has been implicated in the regulation of multiple cellular signaling associated with neuronal apoptosis and survival, such as p38 mitogen-activated protein kinase (MAPK), nuclear factor kappaB (NF-κB), and Wnt/β-catenin pathways. In the present study, the role of KHSRP in Mn-induced neurotoxicity was investigated in vivo using a rat model of chronic Mn exposure and in vitro using differentiated PC12 cell cultures. Western blot and immunohistochemical analyses showed a significant upregulation of KHSRP in rat striatum following Mn exposure. Immunofluorescent labeling indicated that KHSRP was localized mainly in neurons. Terminal deoxynucleotidyl transferase-mediated biotinylated-dUTP nick end labeling (TUNEL) assay showed that KHSRP was mainly distributed in apoptotic neurons. Increased KHSRP expression was positively correlated with the upregulation of several apoptosis-related proteins, such as p53, bax, and active caspase-3. In addition, significant co-localization of KHSRP and active caspase-3 in neurons after Mn exposure was also observed, suggesting a potential involvement of KHSRP in the regulation of Mn-induced striatal neuronal apoptosis. Importantly, interference with KHSRP apparently decreased the level of p53 and attenuated Mn-induced neuronal apoptosis. Taken together, these results indicate that upregulation of KHSRP may be involved in the pathological process underlying Mn neurotoxicity via the modulation of p53 signaling.

Remote neurodegeneration: multiple actors for one play

Remote neurodegeneration significantly influences the clinical outcome in many central nervous system (CNS) pathologies, such as stroke, multiple sclerosis, and traumatic brain and spinal cord injuries. Because these processes develop days or months after injury, they are accompanied by a therapeutic window of opportunity. The complexity and clinical significance of remote damage is prompting many groups to examine the factors of remote degeneration. This research is providing insights into key unanswered questions, opening new avenues for innovative neuroprotective therapies. In this review, we evaluate data from various remote degeneration models to describe the complexity of the systems that are involved and the importance of their interactions in reducing damage and promoting recovery after brain lesions. Specifically, we recapitulate the current data on remote neuronal degeneration, focusing on molecular and cellular events, as studied in stroke and brain and spinal cord injury models. Remote damage is a multifactorial phenomenon in which many components become active in specific time frames. Days, weeks, or months after injury onset, the interplay between key effectors differentially affects neuronal survival and functional outcomes. In particular, we discuss apoptosis, inflammation, oxidative damage, and autophagy-all of which mediate remote degeneration at specific times. We also review current findings on the pharmacological manipulation of remote degeneration mechanisms in reducing damage and sustaining outcomes. These novel treatments differ from those that have been proposed to limit primary lesion site damage, representing new perspectives on neuroprotection.

Downregulation of UBE2Q1 is associated with neuronal apoptosis in rat brain cortex following traumatic brain injury

Aberrant functionality of the ubiquitin proteasome system (UPS) has been implicated in the pathology of various neurological disorders. Although it has been reported that the expressions of various UPS components were altered significantly following traumatic brain injury (TBI), detailed information on the subject remains largely unclear. In the study, using microarray assay, we identified a gene encoding ubiquitin-conjugating enzyme E2Q1 (UBE2Q1) that was significantly downregulated during TBI. Western blot and immunohistochemical analyses verified the reduced expression of UBE2Q1 in ipsilateral brain cortex adjacent to the lesion site compared with the contralateral and sham-operated ones. Double-immunofluorescence staining indicated that UBE2Q1 was expressed mainly in the nucleus of neurons, with a minority in astrocytes in normal cortex. In addition, we observed a remarkable reduction in the number of UBE2Q1-positive neurons following brain trauma. Furthermore, we showed that TBI resulted in a significant increase in the levels of p53, bax, p21 and active caspase 3 in brain cortex, which was correlated with decreased expression of UBE2Q1. We also found that knockdown of UBE2Q1 apparently increased the level of p53, whereas overexpressing UBE2Q1 attenuated cellular p53 level in PC12 neuronal cells. Accordingly, interference with UBE2Q1 augmented H2O2-induced apoptosis of PC12 cells. Taken together, our findings indicate that UBE2Q1 might play an important role in the neuropathological process of TBI through modulating p53 signaling.

Thalamic atrophy in antero-medial and dorsal nuclei correlates with six-month outcome after severe brain injury

The primary and secondary damage to neural tissue inflicted by traumatic brain injury is a leading cause of death and disability. The secondary processes, in particular, are of great clinical interest because of their potential susceptibility to intervention. We address the dynamics of tissue degeneration in cortico-subcortical circuits after severe brain injury by assessing volume change in individual thalamic nuclei over the first six-months post-injury in a sample of 25 moderate to severe traumatic brain injury patients. Using tensor-based morphometry, we observed significant localized thalamic atrophy over the six-month period in antero-dorsal limbic nuclei as well as in medio-dorsal association nuclei. Importantly, the degree of atrophy in these nuclei was predictive, even after controlling for full-brain volume change, of behavioral outcome at six-months post-injury. Furthermore, employing a data-driven decision tree model, we found that physiological measures, namely the extent of atrophy in the anterior thalamic nucleus, were the most predictive variables of whether patients had regained consciousness by six-months, followed by behavioral measures. Overall, these findings suggest that the secondary non-mechanical degenerative processes triggered by severe brain injury are still ongoing after the first week post-trauma and target specifically antero-medial and dorsal thalamic nuclei. This result therefore offers a potential window of intervention, and a specific target region, in agreement with the view that specific cortico-thalamo-cortical circuits are crucial to the maintenance of large-scale network neural activity and thereby the restoration of cognitive function after severe brain injury.

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Performed acute and chronic structural MRI in 25 severe TBI patients

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Tensor brain morphometry (TBM) shows localized thalamic acute-to-chronic atrophy.

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Anterior, medio- and lateral-dorsal nuclei are the most significant.

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Atrophy in these nuclei predicts 6-month outcome scores (GOSe).

Degeneration of axotomized projection neurons in the rat dLGN: temporal progression of events and their mitigation by a single administration of FGF2

Removal of visual cortex in the rat axotomizes projection neurons in the dorsal lateral geniculate nucleus (dLGN), leading to cytological and structural changes and apoptosis. Biotinylated dextran amine was injected into the visual cortex to label dLGN projection neurons retrogradely prior to removing the cortex in order to quantify the changes in the dendritic morphology of these neurons that precede cell death. At 12 hours after axotomy we observed a loss of appendages and the formation of varicosities in the dendrites of projection neurons. During the next 7 days, the total number of dendrites and the cross-sectional areas of the dendritic arbors of projection neurons declined to about 40% and 20% of normal, respectively. The response of dLGN projection neurons to axotomy was asynchronous, but the sequence of structural changes in individual neurons was similar; namely, disruption of dendrites began within hours followed by cell soma atrophy and nuclear condensation that commenced after the loss of secondary dendrites had occurred. However, a single administration of fibroblast growth factor-2 (FGF2), which mitigates injury-induced neuronal cell death in the dLGN when given at the time of axotomy, markedly reduced the dendritic degeneration of projection neurons. At 3 and 7 days after axotomy the number of surviving dendrites of dLGN projection neurons in FGF-2 treated rats was approximately 50% greater than in untreated rats, and the cross-sectional areas of dendritic arbors were approximately 60% and 50% larger. Caspase-3 activity in axotomized dLGN projection neurons was determined by immunostaining for fractin (fractin-IR), an actin cleavage product produced exclusively by activated caspase-3. Fractin-IR was seen in some dLGN projection neurons at 36 hours survival, and it increased slightly by 3 days. A marked increase in reactivity was seen by 7 days, with the entire dLGN filled with dense fractin-IR in neuronal cell somas and dendrites.

Expressions of apoptosis-related proteins in rats with focal cerebral ischemia after Angong Niuhuang sticker point application

In this study, we extracted and purified components in the Angong Niuhuang pill. Then we applied transdermal enhancers to Angong Niuhuang stickers by modern technology. The Angong Niuhuang sticker includes extracts from curcuma, berberine hydrochloride, baicalin, geniposide, borneol, and musk. Angong Niuhuang stickers at different point application doses (1.35, 2.7, and 5.4 g/kg) were administered to Dazhui (DU14), Qihai (RN6) and Mingmen (DU4). Rats in the different dose point application and acupuncture groups were continuously administered for 7 days. Then a middle cerebral artery occlusion model was prepared for simulating human cerebral ischemia. Twelve hours later, expressions of Bcl-2, Bax and p53 protein in hippocampal CA1 were detected with immunohistochemistry. The expression level of Bcl-2, an anti-apoptotic protein, significantly increased in the high-, medium- and low-dose point application groups and the acupuncture group, while the expression of the pro-apoptotic proteins, Bax and p53, significantly decreased compared with the middle cerebral artery occlusion rats; and the Bcl-2/Bax ratio was significantly increased. The difference was noticeable for the high-dose point application group, which showed statistical difference compared with the low-dose point application group and the acupuncture group. Our experimental findings indicate that point application with Angong Niuhuang stickers promotes the expression of Bcl-2, and inhibits the expressions of Bax and p53 in the hippocampal CA1 area of rats after focal cerebral ischemia. Thus, point application of Angong Niuhang stickers protects brain tissues from cerebral ischemia.

Neuronal cell death in neonatal hypoxia-ischemia

Perinatal hypoxic-ischemic encephalopathy (HIE) is a significant cause of mortality and morbidity in infants and young children. Therapeutic opportunities are very limited for neonatal and pediatric HIE. Specific neural systems and populations of cells are selectively vulnerable in HIE; however, the mechanisms of degeneration are unresolved. These mechanisms involve oxidative stress, excitotoxicity, inflammation, and the activation of several different cell death pathways. Decades ago the structural and mechanistic basis of the cellular degeneration in HIE was thought to be necrosis. Subsequently, largely due to advances in cell biology and to experimental animal studies, emphasis has been switched to apoptosis or autophagy mediated by programmed cell death (PCD) mechanisms as important forms of degeneration in HIE. We have conceptualized based on morphological and biochemical data that this degeneration is better classified according to an apoptosis-necrosis cell death continuum and that programmed cell necrosis has prominent contribution in the neurodegeneration of HIE in animal models. It is likely that neonatal HIE evolves through many cell death chreodes influenced by the dynamic injury landscape. The relevant injury mechanisms remain to be determined in human neonatal HIE, though preliminary work suggests a complexity in the cell death mechanisms greater than that anticipated from experimental animal models. The accurate identification of the various cell death chreodes and their mechanisms unfolding within the immature brain matrix could provide fresh insight for developing meaningful therapies for neonatal and pediatric HIE.

Involvement of endoplasmic reticulum stress on neuronal cell death in the lateral geniculate nucleus in the monkey glaucoma model

We investigated whether endoplasmic reticulum (ER) stress was involved in the pathophysiological mechanisms underlying neuronal death of the lateral geniculate nucleus (LGN) after intraocular pressure (IOP) elevation. Five cynomolgus monkeys, four with a glaucomatous left eye after laser photocoagulation treatment and one normal monkey, were studied. At 4, 11, 15 and 24 weeks after the laser photocoagulation treatment, the numbers of LGN neurons and atrophy were immunohistochemically evaluated using anti-parvalbumin-antibody, which was used to specifically label relay neurons connecting to the visual cortex. In addition, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive cells, polyubiquitin, and production of ER stress-related proteins, such as the phosphorylation of eukaryotic initiation factor 2α (p-eIF2α) and C/EBP-homologous protein (CHOP), were also measured using in situ hybridization and immunostaining. Loss of neurons and/or neuronal atrophy in layers 1, 4 and 6 of the LGN on the contralateral side were observed at 4-24 weeks after the laser photocoagulation treatment. Furthermore, the retinal input from the high IOP eye projected to layers 2 (magnocellular layer), 3 and 5 (parvocellular layer) on the ipsilateral side. Neuronal damage was also confirmed in these layers. In the LGN region, TUNEL-positive cells, polyubiquitin, p-eIF2α and CHOP were also detected at 11-24 weeks after the laser photocoagulation treatment. These findings indicate that ER stress may play a pivotal role in neuronal death of the LGN after IOP elevation.

The mitochondrial permeability transition pore regulates nitric oxide-mediated apoptosis of neurons induced by target deprivation

Ablation of mouse occipital cortex induces precisely timed and uniform p53-modulated and Bax-dependent apoptosis of thalamocortical projection neurons in the dorsal lateral geniculate nucleus (LGN) by 7 d after lesion. We tested the hypothesis that this neuronal apoptosis is initiated by oxidative stress and the mitochondrial permeability transition pore (mPTP). Preapoptotic LGN neurons accumulate mitochondria, Zn(2+) and Ca(2+), and generate higher levels of reactive oxygen species (ROS), including superoxide, nitric oxide (NO), and peroxynitrite, than LGN neurons with an intact cortical target. Preapoptosis of LGN neurons is associated with increased formation of protein carbonyls, protein nitration, and protein S-nitrosylation. Genetic deletion of nitric oxide synthase 1 (nos1) and inhibition of NOS1 with nitroindazole protected LGN neurons from apoptosis, revealing NO as a mediator. Putative components of the mPTP are expressed in mouse LGN, including the voltage-dependent anion channel (VDAC), adenine nucleotide translocator (ANT), and cyclophilin D (CyPD). Nitration of CyPD and ANT in LGN mitochondria occurs by 2 d after cortical injury. Chemical cross-linking showed that LGN neuron preapoptosis is associated with formation of CyPD and VDAC oligomers, consistent with mPTP formation. Mice without CyPD are rescued from neuron apoptosis as are mice treated with the mPTP inhibitors TRO-19622 (cholest-4-en-3-one oxime) and TAT-Bcl-X(L)-BH4. Manipulation of the mPTP markedly attenuated the early preapoptotic production of reactive oxygen/nitrogen species in target-deprived neurons. Our results demonstrate in adult mouse brain neurons that the mPTP functions to enhance ROS production and the mPTP and NO trigger apoptosis; thus, the mPTP is a target for neuroprotection in vivo.

Mitochondrial and Cell Death Mechanisms in Neurodegenerative Diseases

Alzheimer’s disease (AD), Parkinson’s disease (PD) and amyotrophic lateral sclerosis (ALS) are the most common human adult-onset neurodegenerative diseases. They are characterized by prominent age-related neurodegeneration in selectively vulnerable neural systems. Some forms of AD, PD, and ALS are inherited, and genes causing these diseases have been identified. Nevertheless, the mechanisms of the neuronal cell death are unresolved. Morphological, biochemical, genetic, as well as cell and animal model studies reveal that mitochondria could have roles in this neurodegeneration. The functions and properties of mitochondria might render subsets of selectively vulnerable neurons intrinsically susceptible to cellular aging and stress and overlying genetic variations, triggering neurodegeneration according to a cell death matrix theory. In AD, alterations in enzymes involved in oxidative phosphorylation, oxidative damage, and mitochondrial binding of Aβ and amyloid precursor protein have been reported. In PD, mutations in putative mitochondrial proteins have been identified and mitochondrial DNA mutations have been found in neurons in the substantia nigra. In ALS, changes occur in mitochondrial respiratory chain enzymes and mitochondrial cell death proteins. Transgenic mouse models of human neurodegenerative disease are beginning to reveal possible principles governing the biology of selective neuronal vulnerability that implicate mitochondria and the mitochondrial permeability transition pore. This review summarizes how mitochondrial pathobiology might contribute to neuronal death in AD, PD, and ALS and could serve as a target for drug therapy.

Knockdown of SMN by RNA interference induces apoptosis in differentiated P19 neural stem cells

Spinal muscular atrophy (SMA) is a common neurodegenerative disease that is caused by mutations in the survival of motor neuron gene (SMN), leading to reduced levels of the SMN protein in affected individuals. In SMA, motor neurons selectively degenerate, however, the mechanism of cell death and the precise role of SMN in this process are not completely understood. In this study, we apply RNA interference (RNAi) to knockdown Smn gene expression in the murine embryonal carcinoma stem cell line P19, which can be differentiated into neuronal cells. A direct effect of Smn loss on apoptotic cell death in differentiated P19 neuronal cells, and to a lesser extent in undifferentiated cells was observed. Apoptosis could be partly reversed by expression of an SMN rescue construct, was reversible by the addition of the caspase-inhibitor ZVAD-fmk and involved the cytochrome c pathway. This study shows for the first time that knockdown of SMN results in apoptosis in mammalian neuronal cells and has implications for understanding the cause of motor neuron-specific cell loss in SMA, and for identifying novel therapeutic targets for this disease.

Bax-dependent and -independent death of motoneurons after facial nerve injury in adult mice

Nerve injury-induced neuronal death may occur after accidental trauma or nerve inflammation. Although the response to facial root avulsion has been examined in rodent models, there are conflicting results as to whether motoneuron (MN) death is mediated by apoptosis or necrosis. We examined the response of MNs and proximal nerves after facial nerve avulsion in adult mice. Following facial nerve avulsion in 4-5-week-old mice, we observed a progressive reduction of MNs such that by 4 weeks less than 10% of avulsed MNs remained compared with the control side. The profile of MN degeneration was distinct from axotomy-induced responses. For example, the onset of MN death was more rapid, and the extent of MN loss was greater compared with axotomy. Furthermore, the degeneration of oligodendrocytes and the activation of microglia were increased in the proximal nerve after avulsion. Ultrastructural observations suggested that root avulsion mainly induces non-apoptotic neuronal death, although a small subset of neurons appeared to die via apoptosis. To evaluate the contribution of apoptotic death, we evaluated MN responses in Bax-knockout (KO) mice in which neurons are rescued from apoptotic death. Surprisingly, although the majority of Bax-KO mice exhibited only a moderate MN loss after avulsion, a subset of Bax-KO mice (25%) exhibited extensive MN death and injury-induced changes in the nerve that were indistinguishable from events in wild-type littermates. These results suggest that both Bax-dependent and -independent forms of cell death are evoked by root avulsion, and that programmed cell death may be involved in triggering a robust necrotic response.

Apoptotic changes in the cortex and hippocampus following minimal brain trauma in mice

Interpretation of the cellular and molecular pathogenic basis of post-minimal traumatic brain injury is a significant clinical and scientific problem, especially due to the high prevalence of motor vehicle--and other accidents. Pathogenetic brain mechanisms following traumatic impact are usually investigated by using models of severe or moderate trauma. Apoptotic neuronal degeneration after notable brain trauma is a well-known phenomenon, but the source of its activation is not clear, especially after mild, subclinical brain trauma. In the present study, we used a closed head weight-drop model to induce minimal brain injury in mice. Pellets of 5, 10, 15, 20, 25 and 30 g were dropped on the right side of mice's head kept under light ether anesthesia. No abnormal behavioral or neurophysiological changes were seen following the head trauma. Morphological assessment was done 72 h after the traumatic impact using TUNEL assay and silver staining. We found gradual increase of TUNEL-positive and silver-impregnated cells number in different cortical and hippocampal regions of both injured and contralateral hemispheres. The threshold of traumatic impact that caused a significant activation was 10-15 g pellets (evident by silver staining), and 15-20 g for apoptosis. The most sensitive zones for trauma were anterior cingulate cortex and CA3 area of hippocampus. No bilateral hemispheric differences were found. Our results demonstrate that even closed head minimal traumatic brain injury can cause diffused neuronal damage and apoptosis. This results correlate well with cognitive and behavioral deficits described for mice suffering similar mTBI and can also explain the wide variety of mental disturbances described for post-concussion syndrome in patients who suffered mild head injury.

Adult motor neuron apoptosis is mediated by nitric oxide and Fas death receptor linked by DNA damage and p53 activation

The mechanisms of injury- and disease-related degeneration of motor neurons (MNs) need clarification. Unilateral avulsion of the sciatic nerve in the mouse induces apoptosis of spinal MNs that is p53 and Bax dependent. We tested the hypothesis that MN apoptosis is Fas death receptor dependent and triggered by nitric oxide (NO)- and superoxide-mediated damage to DNA. MNs in mice lacking functional Fas receptor and Fas ligand were protected from apoptosis. Fas protein levels and cleaved caspase-8 increased in MNs after injury. Fas upregulation was p53 dependent. MNs in mice deficient in neuronal NO synthase (nNOS) and inducible NOS (iNOS) resisted apoptosis. After injury, MNs increased nNOS protein but decreased iNOS protein; however, iNOS contributed more than nNOS to basal and injury-induced levels of NADPH diaphorase activity in MNs. NO and peroxynitrite (ONOO-) fluorescence increased in injured MNs, as did nitrotyrosine staining of MNs. DNA damage, assessed as 8-hydroxy-2-deoxyguanosine and single-stranded DNA, accumulated within injured MNs and was attenuated by nNOS and iNOS deficiency. nNOS deficiency increased DNA repair protein oxoguanine DNA-glycosylase, whereas iNOS deficiency blocked diaphorase activity. MN apoptosis was blocked by the antioxidant Trolox and by overexpression of wild-type human superoxide dismutase-1 (SOD1). In contrast, injured MNs in mice harboring mutant human SOD1 had upregulated Fas and iNOS, escalated DNA damage, and accelerated and increased MN degeneration and underwent necrosis instead of apoptosis. Thus, adult spinal MN apoptosis is mediated by upstream NO and ONOO- genotoxicity and downstream p53 and Fas activation and is shifted to necrosis by mutant SOD1.

Evaluation of the apoptosis-related proteins of the BCL-2 family in the traumatic penumbra area of the rat model of cerebral contusion, treated by hyperbaric oxygen therapy: a quantitative immunohistochemical study

The growth and progression of traumatic brain injury (TBI) lesions depend significantly on developments in the traumatic penumbra area, perilesional region, where delayed neuronal death occurs. Recent data supports the important role of apoptosis in delayed cell death in TBI. Previously we demonstrated a significant reduction of apoptosis in traumatic penumbra in animals treated by hyperbaric oxygen (HBO). In this study we evaluate the expression of apoptosis-related proteins of the Bcl-2 family (Bcl-2, Bax and Bcl-xL) in the traumatic penumbra area in correlation with the extent of apoptosis in the rat model of focal cerebral contusion, treated by HBO. Sprague-Dawley rats underwent cortical dynamic deformation, some with subsequent hypoxemia. A group of both hypoxemic and non-hypoxemic animals was treated by HBO. The pathological study was based on immunohistochemical staining of the brain sections for Bcl-2, Bax and Bcl-xL with quantitative evaluation of staining by image analysis. The expression of Bcl-2 in hypoxemic animals was lower than in non-hypoxemic animals, but a significant increase in Bcl-2 expression was seen in both groups after HBO treatment. Bcl-xL also demonstrated an increase after HBO treatment but less significant. Staining for Bax protein did not demonstrate significant change after treatment. These data correlate well with the reduction of TUNEL-positive cells in traumatic penumbra after HBO treatment. We concluded that the apoptotic mechanisms are important in delayed cell death in TBI and that post-traumatic hypoxemia increases the intensity of apoptosis, probably through a decrease in Bcl-2 and Bcl-xL expression which normally repress apoptosis. The beneficial effect of HBO treatment in our model of brain contusion correlates well with the increased expression of anti-apoptotic proteins (Bcl-2 and Bcl-xL) following treatment and the appropriate decrease in the extent of apoptosis. In light of these results, the usage of HBO is justified as neuroprotective treatment in TBI.

Zinc accumulation after target loss: an early event in retrograde degeneration of thalamic neurons

Accumulation of cytoplasmic zinc is linked with a cascade of events leading to neuronal death. In many in vivo models of zinc-induced cell death, toxic concentrations of synaptically released zinc enter vulnerable neurons via neurotransmitter- or voltage-gated ion channels. In vitro studies demonstrate, in addition, that zinc can be liberated from intracellular stores following oxidative stress and contribute to cell death processes, including apoptosis. Here we describe accumulation of intracellular zinc in an in vivo model of cell death in the absence of presynaptic zinc release. We focused on the lateral geniculate nucleus (LGN) because LGN neurons undergo apoptosis when separated from their target, the primary visual cortex (V1), and the LGN is mostly devoid of zinc-containing presynaptic terminals. Infant and adult rats and adult mice received unilateral ablation of V1, either by aspiration or kainate injection. One to 14 days later, brain sections were stained with selenium autometallography or fluorescently labeled to localize zinc, or stained immunochemically for activated caspase-3. V1 lesions led to zinc accumulation in LGN neurons in infant and adult subjects. Zinc-containing neurons were evident 1-3 days after aspiration lesions, depending on age, but not until 14 days after kainate injection. Zinc accumulation was followed rapidly by immunostaining for activated caspase-3. Our data indicate that like neurotrauma and excitotoxicity, target deprivation leads to accumulation of zinc in apoptotic neurons. Moreover, zinc accumulation in vivo can occur in the absence of presynaptic zinc release. Together these findings suggest that accumulation of intracellular zinc is a ubiquitous component of the cell death cascade in neurons.

Progesterone Differentially Regulates Pro- and Anti-Apoptotic Gene Expression in Cerebral Cortex Following Traumatic Brain Injury in Rats

Although the administration of progesterone has been shown to be neuroprotective in experimental models of traumatic brain injury (TBI), the mechanisms for this beneficial effect are still poorly understood. The present study examined the effects of progesterone on mRNA and protein levels of the Bcl-2 apoptosis regulatory genes, bax, bad, bcl-2, and bcl-x(L), in cerebral cortex after TBI. Male Sprague-Dawley rats were subjected to either sham surgery or lateral fluid percussion brain injury of moderate severity (2.4-2.6 atm). Within 1 h post-surgery, progesterone (4 mg/kg) or vehicle (corn oil) administration was initiated for 1-7 days postoperatively. Our results indicate that bax and bad mRNA levels and Bax and Bad protein expression in the ipsilateral, injured cerebral cortex were significantly elevated post-TBI, while mRNA levels of bcl-2 and bcl-x(L) or Bcl-2 and Bcl-x(L) protein expression were not changed. Under the sham-treated condition, progesterone significantly increased mRNA levels of the anti-apoptotic gene, bcl-2, but down-regulated pro-apoptotic gene expression (bax and bad) in cerebral cortex. After TBI, progesterone treatment reduced bax and bad mRNA levels in the ipsilateral cerebral cortex of TBI rats, and decreased Bax and Bad protein levels. In addition, bcl-2 and bcl-x(L) mRNA levels, as well as Bcl-2 and Bcl-x(L) protein expression, were increased by progesterone in TBI injured cortex. These data indicate that one of the neuroprotective mechanisms of progesterone may be related to its differential regulation of apoptotic signals.

The multiple roles of p53 in the pathogenesis of HIV associated dementia

The mechanism by which infection with the human immunodeficiency virus (HIV) leads to injury and dysfunction within the central nervous system (CNS) is not completely understood. Most studies support the hypothesis that neurons are impacted as bystander cells in a tissue environment made hostile by the innate and adaptive immune responses to chronic HIV infection within CNS tissue. The tumor suppressor transcription factor p53 participates in multiple cellular processes within the HIV infected CNS, and experimental evidence suggests that the resulting neurodegeneration occurs by induction of p53-mediated apoptotic pathways. Here we review the evidence for p53 as a participant in the responses of multiple CNS cell types to the presence of HIV and propose the hypothesis that HIV induced alterations in the CNS extracellular milieu converge at neuronal p53 activation.

Cyclooxygenase-2-specific inhibitor improves functional outcomes, provides neuroprotection, and reduces inflammation in a rat model of traumatic brain injury

OBJECTIVE

Increases in brain cyclooxygenase-2 (COX2) are associated with the central inflammatory response and with delayed neuronal death, events that cause secondary insults after traumatic brain injury. A growing literature supports the benefit of COX2-specific inhibitors in treating brain injuries.

METHODS

DFU [5,5-dimethyl-3(3-fluorophenyl)-4(4-methylsulfonyl)phenyl-2(5)H)-furanone] is a third-generation, highly specific COX2 enzyme inhibitor. DFU treatments (1 or 10 mg/kg intraperitoneally, twice daily for 3 d) were initiated either before or after traumatic brain injury in a lateral cortical contusion rat model.

RESULTS

DFU treatments initiated 10 minutes before injury or up to 6 hours after injury enhanced functional recovery at 3 days compared with vehicle-treated controls. Significant improvements in neurological reflexes and memory were observed. DFU initiated 10 minutes before injury improved histopathology and altered eicosanoid profiles in the brain. DFU 1 mg/kg reduced the rise in prostaglandin E2 in the brain at 24 hours after injury. DFU 10 mg/kg attenuated injury-induced COX2 immunoreactivity in the cortex (24 and 72 h) and hippocampus (6 and 72 h). This treatment also decreased the total number of activated caspase-3-immunoreactive cells in the injured cortex and hippocampus, significantly reducing the number of activated caspase-3-immunoreactive neurons at 72 hours after injury. DFU 1 mg/kg amplified potentially anti-inflammatory epoxyeicosatrienoic acid levels by more than fourfold in the injured brain. DFU 10 mg/kg protected the levels of 2-arachidonoyl glycerol, a neuroprotective endocannabinoid, in the injured brain.

CONCLUSION

These improvements, particularly when treatment began up to 6 hours after injury, suggest exciting neuroprotective potential for COX2 inhibitors in the treatment of traumatic brain injury and support the consideration of Phase I/II clinical trials.

Amphetamine induces apoptosis of medium spiny striatal projection neurons via the mitochondria‐dependent pathway

Amphetamine (AMPH) is a psychostimulant whose chronic abuse may cause impairments in attention and memory in humans. These cognitive deficits might be related to neurotoxic effects of the drug. One such toxic effect is the well-described destruction of striatal dopaminergic terminals in mammals. In the present study, we investigated the possibility that AMPH might also cause neuronal apoptosis in the rodent striatum. Administration of a dose of the drug (10 mg/kg, 4 times, every 2 h) that is toxic to dopaminergic terminals resulted in the appearance of striatal cells that were positive for cleaved caspase-3 and for terminal deoxynucleotidyl transferase-mediated biotin-dUTP nick-end labeling (TUNEL), observations that are indicative of an ongoing apoptotic process. Dual immunofluorescence staining revealed that cleaved caspase-3-positive cells express calbindin and DARPP-32, but not somatostatin, parvalbumin, or cholinergic markers. In addition, AMPH also caused increased expression of p53 and Bax at both transcript and protein levels; in contrast, Bcl-2 levels were decreased after the AMPH injections. Moreover, Bax knockout mice showed resistance to AMPH-induced apoptotic cell death but not to AMPH-induced destruction of dopaminergic terminals. When taken together, these observations indicate that injections of doses of AMPH that are known to destroy striatal dopamine terminals can also cause apoptotic death of postsynaptic medium spiny projection neurons via mitochondria-dependent mechanisms.

BDNF down-regulates the caspase 3 pathway in injured geniculo-cortical neurones

Visual cortex ablation in newborn rats causes a rapid and almost complete degeneration of neurones in the dorsal lateral geniculate nucleus (dLGN), as a consequence of the axotomy of geniculo-cortical fibres. Death of dLGN neurones occurs by apoptosis and is partially prevented (approximately 50%) by intraocular delivery of brain-derived neurotrophic factor (BDNF). Here we investigated the molecular mechanisms of BDNF-mediated neuroprotection. We found that exogenous administration of BDNF partially decreases (approximately 50%) the up-regulation of apoptotic proteins (phosphorylated c-Jun, cytochrome C and cleaved caspase 3), that occurs in dLGN neurones following visual cortex ablation at postnatal day 7. These results demonstrate that the neuroprotective action of BDNF on axotomised dLGN neurones involves the partial blockade of well-characterised apoptotic pathways.

Intraocular delivery of BDNF following visual cortex lesion upregulates cytosolic branched chain aminotransferase (BCATc) in the rat dorsal lateral geniculate nucleus

Visual cortex ablation in newborn rats determines the almost complete degeneration of neurons in the dorsal lateral geniculate nucleus (dLGN), as a consequence of the axotomy of the geniculo-cortical fibres. Death of dLGN neurons is massive and rapid, and occurs by apoptosis. We recently showed that exogenous administration of the neurotrophin brain-derived neurotrophic factor (BDNF) in the eye prevents the degeneration of dLGN neurons occurring after visual cortex lesion in newborn rats. To elucidate the molecular mechanisms of BDNF-mediated neuroprotection, we sought to identify novel genes regulated by BDNF in the rat dLGN after visual cortex lesion. By using mRNA fingerprinting, we isolated a cDNA fragment upregulated in the dLGN of lesioned rats treated with BDNF. This cDNA fragment shared 100% homology with the rat cytosolic branched chain aminotransferase (BCATc), a key enzyme of glutamate metabolism. Quantitative reverse transcription-polymerase chain reaction and in situ hybridization confirmed that BCATc mRNA is markedly overexpressed by exogenous supply of BDNF to axotomized dLGNs. Immunohistochemical analysis showed that upregulation of BCATc in the dLGN of lesioned rats treated with BDNF takes place in astrocytes. These results suggest that modulation of glutamate metabolism by astrocytes might play an important role in BDNF-mediated survival of axotomized dLGN neurons.

Bcl-2 and Bax proteins are increased in neocortical but not in thalamic apoptosis following devascularizing lesion of the cerebral cortex in the rat: an immunohistochemical study

The hypothesis that devascularization of somatosensory and motor cortex causes apoptosis in infarcted regions and in the linked thalamic nuclei was evaluated. To unravel whether Bcl-related proteins, known to regulate apoptosis, participate in neuronal and glial responses to devascularization, we analyzed immunohistochemically the distribution and intensity of staining of Bcl-2 and Bax proteins at different time points after lesion. Both early (up to 6 h) and late (1-7 days) responses were studied. Devascularization led to rapid (within hours) apoptosis in the cortex and to a delayed (within 3-7 days) apoptosis in thalamic nuclei. In control groups, Bcl-2 and Bax immunoreactivity (IR) was detected in neurons and oligodendrocytes but not in astrocytes or microglia. Following devascularization, Bcl-2 IR and Bax IR increased in neurons before the onset of the apoptosis. In the ischemic focus, the increase reached maximal values 3 h after the lesion. The increase was of slower onset in the penumbra zone (24 h and after), a region in which both proteins were induced in astrocytes also. The change of Bax IR intensity exceeded four times that of Bcl-2 at all time points investigated, indicating a diminution of Bcl-2/Bax ratio that may direct neurons to apoptotic pathway. In numerous neurons, an increase of IR in the cytoplasm was accompanied by induction of nuclear staining. No changes of Bcl-2 and Bax IR were found in thalamic nuclei. Our results point to different mechanisms underlying apoptosis of cortical and thalamic neurons. Nuclear appearance of Bcl-2 and Bax suggests they possess regulatory role of gene expression changes triggered by cortical infarct.

Molecular mechanisms of neuronal death in the dorsal lateral geniculate nucleus following visual cortical lesions

We investigated the molecular mechanisms of cell death in the dorsal lateral geniculate nucleus of the rat, following suction lesion of the visual cortex at birth or in the third postnatal week, using terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) technique and immunohistochemistry for caspase-3, -7, -8, and cleaved poly(ADP-ribose) polymerase. Following lesion at birth, TUNEL-positive neurons were found in the dorsal lateral geniculate nucleus between 24 h and 3 days after lesion, with a peak on the second day. Shorter survival times (12-18 h) resulted in labeling of very few neurons in dorsal lateral geniculate nucleus and of several neurons in the perilesional cortex. Activated caspase-3 was expressed from the first to the third days after lesion, whereas cleaved poly(ADP-ribose) polymerase and activated caspase-8 were expressed on the second and third day. Activated caspase-7 was expressed mainly in pretectal nuclei. Caspase-3 activation coincided with the appearance of TUNEL-positive profiles, but decreased earlier than TUNEL. In the ipsi- and contralateral cerebral cortex, all parameters were unchanged. In animals lesioned in the third week, rare apoptotic thalamic neurons were detected as TUNEL- and activated caspase-3-positive profiles 2 days after cortical ablation, and were still present 1 week after lesion.Thus, early target ablation has dramatic effects on neonatal thalamic neurons, which die following activation of caspases 3 and 8. In contrast, cortical neurons are relatively unaffected by target deprivation. Compared with early lesions, late lesions induce a limited thalamic cell death, that persists over time.

Early events of target deprivation/axotomy-induced neuronal apoptosis in vivo: oxidative stress, DNA damage, p53 phosphorylation and subcellular redistribution of death proteins

The mechanisms of injury- and disease-associated apoptosis of neurons within the CNS are not understood. We used a model of cortical injury in rat and mouse to induce retrograde neuronal apoptosis in thalamus. In this animal model, unilateral ablation of the occipital cortex induces apoptosis of corticopetal projection neurons in the dorsal lateral geniculate nucleus (LGN), by 7 days post-lesion, that is p53 modulated and Bax dependent. We tested the hypothesis that this degenerative process is initiated by oxidative stress and early formation of DNA damage and is accompanied by changes in the levels of pro-apoptotic mediators of cell death. Immunoblotting revealed that the protein profiles of Bax, Bak and Bad were different during the progression of neuronal apoptosis in the LGN. Bax underwent a subcellular redistribution by 1 day post-lesion, while Bak increased later. Bad showed an early sustained increase. Cleaved caspase-3 was elevated maximally at 5 and 6 days. Active caspase-3 underwent a subcellular translocation to the nucleus. A dramatic phosphorylation of p53 was detected at 4 days post-lesion. DNA damage was assessed immunocytochemically as hydroxyl radical adducts (8-hydroxy-2-deoxyguanosine) and single-stranded DNA. Both forms of DNA damage accumulated early in target-deprived LGN neurons. Transgenic overexpression of superoxide dismutase-1 provided significant protection against the apoptosis but antioxidant pharmacotreatments with trolox and ascorbate were ineffective. We conclude that overlapping and sequential signaling pathways are involved in the apoptosis of adult brain neurons and that DNA damage generated by superoxide derivatives is an upstream mechanism for p53-regulated, Bax-dependent apoptosis of target-deprived neurons.

Apoptosis and brain ischaemia

There is increasing evidence that some neuronal death after brain ischaemia is mediated by the action of cysteine-requiring aspartate-directed proteases (caspases), the proteases responsible for apoptosis in mammals, although this form of neuronal death is not always accompanied by the morphological changes that are typical of apoptosis in other tissues. Caspase-mediated neuronal death is more extensive after transient than permanent focal brain ischaemia and may contribute to delayed loss of neurons from the penumbral region of infarcts. The activation of caspases after brain ischaemia is largely consequent on the translocation of Bax, Bak, and other BH3-only members of the Bcl-2 family to the mitochondrial outer membrane and the release of cytochrome c, procaspase-9, and apoptosis activating factor-1 (Apaf-1) from the mitochondrial intermembrane space. How exactly ischaemia induces this translocation is still poorly understood. NF-kappaB, the c-jun N-terminal kinase-c-Jun pathway, p53, E2F1, and other transcription factors are probably all involved in regulating the expression of BH3-only proteins after brain ischaemia, and mitochondrial translocation of Bad from sequestering cytosolic proteins is promoted by inactivation of the serine-threonine kinase, Akt. Other processes that are probably involved in the activation of caspases after brain ischaemia include the mitochondrial release of the second mitochondrial activator of caspases (Smac) or direct inhibitor-of-apoptosis-binding protein with low pI (DIABLO), the accumulation of products of lipid peroxidation, a marked reduction in protein synthesis, and the aberrant reentry of neurons into the cell cycle. Non-caspase-mediated neuronal apoptosis may also occur, but there is little evidence to date that this makes a significant contribution to brain damage after ischaemia. The intracellular processes that contribute to caspase-mediated neuronal death after ischaemia are all potential targets for therapy. However, anti-apoptotic interventions in stroke patients will require detailed evaluation using a range of outcome measures, as some such interventions seem simply to delay neuronal death and others to preserve neurons but not neuronal function.

Characterization of Bax and Bcl-2 in apoptosis after experimental traumatic brain injury in the rat

This study was undertaken to fulfill the need for additional data on the dynamics of Bax and Bcl-2 expression in conjunction to the cell death that ensues following experimental brain contusion. Adult Sprague-Dawley rats were subjected to a unilateral experimental controlled cortical contusion and killed at 1, 2, 4, 6 and 10 days post injury (dpi). Cell death was examined by the terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) method together with immunohistochemistry for cellular markers. Expression of Bax and Bcl-2 were analyzed by immunohistochemistry and in situ hybridization. The number of TUNEL-positive cells was highest at 1 dpi and decreased with time. At all time points, 10-16% of the TUNEL-positive cells showed an apoptotic nuclear morphology. The apoptotic features were restricted to neurons and some inflammatory cells. Immunohistochemistry for Bax revealed a translocation of Bax from a diffuse to a granular distribution in neurons. An up-regulation of Bax mRNA at 6 dpi was discernible. This increase was associated with a statistically significant increase in number of cells with up-regulated and translocated Bax protein. Moreover, a statistically significant increase of Bcl-2 mRNA was detected at 10 dpi. The potential window for anti-apoptotic treatment to salvage neurons is wide. The susceptibility of neurons to necrosis and apoptosis through different pathways during a prolonged post-traumatic period indicate that different pharmacological strategies may be required at different time points after trauma.

Projection neurons and interneurons in the lateral geniculate nucleus undergo distinct forms of degeneration ranging from retrograde and transsynaptic apoptosis to transient atrophy after cortical ablation in rat

The cytological responses of thalamic interneurons to selective degeneration of thalamocortical projection neurons after cortical damage in the adult brain are poorly understood. We used a unilateral neocortical lesion model (occipital cortex ablation) in the adult rat to test the hypothesis that interneurons and projection neurons in the lateral geniculate nucleus undergo distinct forms of degeneration. In situ nuclear DNA fragmentation in neurons in the lateral geniculate occurs maximally at 7 days postlesion. Geniculocortical projection neurons that are identified by the retrograde tracer Fluorogold die primarily with a morphology of endstage apoptosis prominent at 7 days postlesion. In contrast, interneurons, identified by their particular nuclear ultrastructure and by glutamic acid decarboxylase immunoreactivity, undergo an atrophic vacuolar pathology starting early during the period of projection neuron death and peaking after the projection neuron death is complete. This degeneration of interneurons is transient, because these neurons exhibit structural recovery and their numbers are not changed significantly postlesion. A rare subset of interneurons (less than one in 100 interneurons and less than one in 100 apoptotic cells) undergoes apoptosis concurrently with the projection neurons. We conclude that different types of neurons within the same thalamic nucleus respond differently to focal cortical target deprivation. Unlike the apoptosis-prone projection neurons, most interneurons undergo transient transsynaptic atrophy and recovery rather than cell death. Nevertheless, a small subset of lateral geniculate interneurons undergoes transsynaptic apoptosis in response to projection neuron apoptosis. The pathological responses of thalamic neurons to cortical trauma vary depending on cell type.

Provision of brain-derived neurotrophic factor via anterograde transport from the eye preserves the physiological responses of axotomized geniculate neurons

The neurotrophic factors of the nerve growth factor family (neurotrophins) have been shown to promote neuronal survival after brain injury and in various models of neurodegenerative conditions. However, it has not been determined whether neurotrophin treatment results in the maintenance of function of the rescued cells. Here we have used the retrograde degeneration of geniculate neurons as a model system to evaluate neuronal rescue and sparing of function after administration of brain-derived neurotrophic factor (BDNF). Death of geniculate neurons was induced by a visual cortex lesion in adult rats, and exogenous BDNF was delivered to the axotomized geniculate cells via anterograde transport after injection into the eye. By microelectrode recordings from the geniculate in vivo we have measured several physiological parameters such as contrast threshold, spatial resolution (visual acuity), signal-to-noise ratio, temporal resolution, and response latency. In control lesioned animals we found that geniculate cell dysfunction precedes the onset of neuronal death, indicating that an assessment of neuronal number per se is not predictive of functional performance. The administration of BDNF resulted in a highly significant cell-saving effect up to 2 weeks after the cortical damage and maintained nearly normal physiological responses in the geniculate. This preservation of function in adult axotomized neurons suggests possible therapeutic applications of BDNF.

p53-dependent cell death signaling in neurons

The p53 tumor suppressor gene is a sequence-specific transcription factor that activates the expression of genes engaged in promoting growth arrest or cell death in response to multiple forms of cellular stress. p53 expression is elevated in damaged neurons in acute models of injury such as ischemia and epilepsy and in brain tissue samples derived from animal models and patients with chronic neurodegenerative diseases. p53 deficiency or p53 inhibition protects neurons from a wide variety of acute toxic insults. Signal transduction pathways associated with p53-induced neuronal cell death are being characterized, suggesting that intervention may prove effective in maintaining neuronal viability and restoring function following neural injury and disease.

Thalamic neuron apoptosis emerges rapidly after cortical damage in immature mice

In adults and children, head trauma can have long-term neuropathological and functional consequences. The thalamus is a major site of remote neurodegeneration after cortical damage in adult humans and experimental animals, but less is known about thalamic responses to cortical injury in the immature brain. This study introduces an in vivo model of axotomy/target deprivation-induced neuronal apoptosis in the dorsal lateral geniculate nucleus of the thalamus produced by unilateral ablation of the occipital cortex in the immature mouse. We specifically examined whether occipital cortex ablation in the immature brain causes apoptotic death of projection neurons in the dorsal lateral geniculate nucleus. After unilateral occipital cortex aspiration, 10-day-old C57BL/6 mice were recovered for up to 28 days. Fluorogold-prelabeled thalamocortical projection neurons were apoptotic at 36-48 h after ablation. The structural progression of apoptosis in the immature lateral geniculate nucleus reveals typical chromatolytic morphology by 18-24 h, followed by cytoplasmic shrinkage and chromatin condensation characteristic of end-stage apoptosis after 36-48 h. Electron microscopy confirmed the presence of apoptosis. This study shows internucleosomal DNA fragmentation and expression of cleaved caspase-3 occurs rapidly, being noted first at 18 h, well before the peak of apoptotic cell death occurring at 36 h after cortical damage in the immature brain. From these data we suggest that axotomy/target deprivation-induced cell death in the immature brain may: (1) differ from that previously reported in adult mice with respect to the time required for progression to cell death; (2) be mediated by caspase-3 activation.

Immature and mature cortical neurons engage different apoptotic mechanisms involving caspase-3 and the mitogen-activated protein kinase pathway

The authors used cultured mouse cortical neurons to study mechanisms of DNA damage-induced apoptosis in immature and mature neurons. Neurons were maintained viably for 60 days in vitro (DIV60). The increased levels of glutamate receptors, synaptic proteins, and glycolytic enzyme were used to track maturation. Exposure of neurons to the DNA-damaging agent camptothecin induced apoptosis in immature (DIV5) and mature (DIV25-30) neurons. Internucleosomal fragmentation of DNA emerged more rapidly in mature neurons than in immature neurons. Immunoblotting revealed that cleaved caspase-3 increased in apoptotic DIV5 neurons but not in DIV30 neurons, but immunolocalization showed accumulation of cleaved caspase-3 in DIV5 and DIV30 neurons. A reversible caspase-3 inhibitor blocked apoptosis in DIV5 neurons but not in DIV30 neurons. Phosphorylation of extracellular signal-regulated kinase/mitogen-activated protein kinase (Erk/MAP kinase)-42/44 occurred preapoptotically in mature but not immature neurons, while Erk54 nuclear translocation and MAP kinase kinase kinase-1 cleavage into putative caspase-3-generated proapoptotic fragments occurred in DIV5 but not DIV30 neurons. Inhibition of Erk activation with MAP kinase kinase inhibitor blocked apoptosis at both ages. The results show that immature and mature cortical neurons engage different signaling mechanisms in MAP kinase and caspase pathways during apoptosis; thus, neuron age influences the mechanisms and progression of apoptosis.

Rapid subcellular redistribution of Bax precedes caspase-3 and endonuclease activation during excitotoxic neuronal apoptosis in rat brain

Neuronal apoptosis is induced prominently in the newborn rodent brain by glutamate receptor excitotoxicity and related insults, including trauma and hypoxia-ischemia. However, the molecular mechanisms of this neurodegeneration are unclear. We tested the hypothesis that changes in the subcellular distribution of the proapoptotic protein Bax precede the activation of downstream apoptosis-effector mechanisms such as caspase-3 cleavage and endonuclease activation during the progression of excitotoxic neuronal apoptosis in the striatum of newborn rat. Kainic acid (4 nmol) was injected into striatum of anesthetized 7-day-old rats, and the animals were killed at 2, 6, 12, and 24 h postinsult. Controls were age-matched, vehicle-injected, or naive rats. Counts of ultrastructurally confirmed striatal neuron apoptosis in brain sections were highest at 24 h. Striatal tissue was microdissected and fractionated into cytosolic, mitochondrial-, and nuclear-enriched compartments. Immunoblots showed that Bax translocates from the cytosol fraction to the mitochondrial fraction, with maximal translocation by 2 h in the absence of changes in mitochondrial accumulation. Cleaved caspase-3 levels increase progressively in both cytosolic and mitochondrial fractions between 6 and 24 h. Cleaved caspase-3 accumulates in apoptotic striatal neurons as shown by immunolocalization. Internucleosomal fragmentation of DNA coincides with caspase-3 cleavage. We conclude that rapid translocation of Bax to mitochondria precedes caspase-3 and endonuclease activation during excitotoxic neuronal apoptosis in newborn rat brain and that initiation of this death cascade occurs within 2 h after glutamate receptor activation.

Ablation of the chemokine monocyte chemoattractant protein-1 delays retrograde neuronal degeneration, attenuates microglial activation, and alters expression of cell death molecules

The mechanisms regulating retrograde neuronal degeneration and subsequent death of thalamic neurons following cortical injury are not well understood. However, the delay in the onset of retrograde cell death and observed morphological changes are consistent with apoptosis. Our previous studies demonstrated that monocyte chemoattractant protein-1 (MCP-1), a beta-chemokine that attracts cells of monocytic origin to sites of injury, is rapidly and specifically expressed in the lateral geniculate nucleus following visual cortical lesions. To determine the potential role of MCP-1 in retrograde degeneration, the present study examined the effect of genetic deletion of MCP-1 (MCP-1 KO or -/-) or its high affinity receptor CCR2 (CCR2 KO or -/-) on thalamic microglial activation and neuronal cell death following aspiration lesions of the visual cortex in adult mice. Deletion of the MCP-1 gene delayed microglial activation and transiently improved the survival of thalamic neurons. Deletion of the CCR2 receptor resulted in a significant increase in apoptosis as measured by nucleosomal fragmentation after injury compared to wild-type mice, but did not alter neuron survival, suggesting that glial apoptosis is increased in the receptor knockout mice. Investigation of Bcl-2, Bax, Fas, Fas ligand (FasL) and activated caspase-3, key regulators of apoptosis that can be modulated by cytokines, revealed complex alterations of mRNA and protein levels in MCP-1(-/-) and CCR2(-/-) mice. As examples, Bcl-2 protein was detected in wild-type, but not in MCP-1(-/-) mice. Caspase-3 activity was higher in MCP-1(-/-) mice compared to wild-type and CCR2(-/-) mice at 5 days after injury. High levels of activated caspase-3 correlate with the beginning of a period of delayed, but rapid cell death in the thalami of MCP-1(-/-) mice. In summary, our data strongly suggest that MCP-1 is involved in early microglial response to axotomy and that modulation of this chemokine could provide a novel strategy for improved neuronal survival following injury to the central nervous system.

Expression of BCL-2 via adeno-associated virus vectors rescues thalamic neurons after visual cortex lesion in the adult rat

Lesions of the mammalian visual cortex cause the retrograde degeneration of the thalamic neurons projecting to the damaged cortex. The proto-oncogene bcl-2 is known to inhibit neuronal apoptosis induced by a variety of noxious stimuli and preserve the functional integrity of the injured cells. Here we have tested whether the overexpression of bcl-2 via adeno-associated virus (AAV) vectors is able to protect the neurons in the lateral geniculate nucleus after visual cortex ablation in adult rats. Recombinant AAV vectors encoding Bcl-2 (AAV-Bcl-2) or green fluorescent protein (AAV-GFP) as a control were stereotaxically injected into the geniculate. Three weeks after vector injection, the ipsilateral visual cortex was removed by aspiration, and cell survival was assessed 2 weeks later. We found that 20% of the geniculate neurons were transduced by the Bcl-2 vector. These cells were completely protected from death following cortical ablation. Delivery of AAV-GFP transduced an identical number of geniculate neurons but had no effect on cell survival after lesion. The total number of surviving geniculate neurons was found to be significantly higher in animals injected with AAV-Bcl-2 than in rats injected with AAV-GFP or in control lesioned rats. These data indicate that Bcl-2 gene therapy with AAV vectors represents an effective treatment to promote neuronal survival after central nervous system insults.

Long-term culture of mouse cortical neurons as a model for neuronal development, aging, and death

A long-term cell culture system was used to study maturation, aging, and death of cortical neurons. Mouse cortical neurons were maintained in culture in serum-free medium (Neurobasal supplemented with B27) for 60 days in vitro (DIV). The levels of several proteins were evaluated by immunoblotting to demonstrate that these neurons matured by developing dendrites and synapses and remained continuously healthy for 60 DIV. During their maturation, cortical neurons showed increased or stable protein expression of glycolytic enzyme, synaptophysin, synapsin IIa, alpha and beta synucleins, and glutamate receptors. Synaptogenesis was prominent during the first 15 days and then synaptic markers remained stable through DIV60. Very early during dendritic development at DIV3, beta-synuclein (but not alpha-synuclein) was localized at the base of dendritic growth cones identified by MAP2 and alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) receptor GluR1. In mature neurons, alpha and beta synucleins colocalized in presynaptic axon terminals. Expression of N-methyl-D-aspartate (NMDA) and AMPA receptors preceded the formation of synapses. Glutamate receptors continued to be expressed strongly through DIV60. Cortical neurons aging in vitro displayed a complex profile of protein damage as identified by protein nitration. During cortical neuron aging, some proteins showed increased nitration, while other proteins showed decreased nitration. After exposure to DNA damaging agent, young (DIV5) and old (DIV60) cortical neurons activated apoptosis mechanisms, including caspase-3 cleavage and poly(ADP)-ribose polymerase inactivation. We show that cultured mouse cortical neurons can be maintained for long term. Cortical neurons display compartmental changes in the localization of synucleins during maturation in vitro. These neurons sustain protein nitration during aging and exhibit age-related variations in the biochemistry of neuronal apoptosis.

Injury-induced spinal motor neuron apoptosis is preceded by DNA single-strand breaks and is p53- and Bax-dependent

The mechanisms of injury-induced apoptosis of neurons within the spinal cord are not understood. We used a model of peripheral nerve-spinal cord injury in the rat and mouse to induce motor neuron degeneration. In this animal model, unilateral avulsion of the sciatic nerve causes apoptosis of motor neurons. We tested the hypothesis that p53 and Bax regulate this neuronal apoptosis, and that DNA damage is an early upstream signal. Adult mice and rats received unilateral avulsions causing lumbar motor neurons to achieve endstage apoptosis at 7-14 days postlesion. This motor neuron apoptosis is blocked in bax(-/-) and p53(-/-) mice. Single-cell gel electrophoresis (comet assay), immunocytochemistry, and quantitative immunogold electron microscopy were used to measure molecular changes in motor neurons during the progression of apoptosis. Injured motor neurons accumulate single-strand breaks in DNA by 5 days. p53 accumulates in nuclei of motor neurons destined to undergo apoptosis. p53 is functionally activated by 4-5 days postlesion, as revealed by immunodetection of phosphorylated p53. Preapoptotically, Bax translocates to mitochondria, cytochrome c accumulates in the cytoplasm, and caspase-3 is activated. These results demonstrate that motor neuron apoptosis in the adult spinal cord is controlled by upstream mechanisms involving DNA damage and activation of p53 and downstream mechanisms involving upregulated Bax and cytochrome c and their translocation, accumulation of mitochondria, and activation of caspase-3. We conclude that adult motor neuron death after nerve avulsion is DNA damage-induced, p53- and Bax-dependent apoptosis.

Neuronal survival and cell death signaling pathways

Neuronal viability is maintained through a complex interacting network of signaling pathways that can be perturbed in response to a multitude of cellular stresses. A shift in the balance of signaling pathways after stress or in response to pathology can have drastic consequences for the function or the fate of a neuron. There is significant evidence that acutely injured and degenerating neurons may die by an active mechanism of cell death. This process involves the activation of discrete signaling pathways that ultimately compromise mitochondrial structure, energy metabolism and nuclear integrity. In this review we examine recent evidence pertaining to the presence and activation of anti- and pro-cell death regulatory pathways in nervous system injury and degeneration.