A neuroprotective role for the DNA damage checkpoint in tauopathy

ATM and p53, effectors of the DNA damage checkpoint, are generally considered pro-apoptotic in neurons. We show that DNA damage and checkpoint activation occurs in postmitotic neurons in animal models of tauopathy, neurodegenerative disorders that include Alzheimer's disease. Surprisingly, checkpoint attenuation potently increases neurodegeneration through aberrant cell cycle re-entry of postmitotic neurons. These data suggest an unexpected neuroprotective role for the DNA damage checkpoint in tauopathies.     

Neurodegenerative disorders and ischemic brain diseases

Degeneration and death of neurons is the fundamental process responsible for the clinical manifestations of many different neurological disorders of aging, incuding Alzheimer's disease, Parkinson's disease and stroke. The death of neurons in such disorders involves apoptotic biochemical cascades involving upstream effectors (Par-4, p53 and pro-apoptotic Bcl-2 family members), mitochondrial alterations and caspase activation. Both genetic and environmental factors, and the aging process itself, contribute to intiation of such neuronal apoptosis. For example, mutations in the amyloid precursor protein and presenilin genes can cause Alzheimer's disease, while head injury is a risk factor for both Alzheimer's and Parkinson's diseases. At the cellular level, neuronal apoptosis in neurodegenerative disorders may be triggered by oxidative stress, metabolic compromise and disruption of calcium homeostasis. Neuroprotective (anti-apoptotic) signaling pathways involving neurotrophic factors, cytokines and conditioning responses can counteract the effects of aging and genetic predisposition in experimental models of neurodegenerative disorders. A better understanding of the molecular underpinnings of neuronal death is leading directly to novel preventative and therapeutic approaches to neurodegenerative disorders.     

SIRT1 deacetylase protects against neurodegeneration in models for Alzheimer's disease and amyotrophic lateral sclerosis

A progressive loss of neurons with age underlies a variety of debilitating neurological disorders, including Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS), yet few effective treatments are currently available. The SIR2 gene promotes longevity in a variety of organisms and may underlie the health benefits of caloric restriction, a diet that delays aging and neurodegeneration in mammals. Here, we report that a human homologue of SIR2, SIRT1, is upregulated in mouse models for AD, ALS and in primary neurons challenged with neurotoxic insults. In cell-based models for AD/tauopathies and ALS, SIRT1 and resveratrol, a SIRT1-activating molecule, both promote neuronal survival. In the inducible p25 transgenic mouse, a model of AD and tauopathies, resveratrol reduced neurodegeneration in the hippocampus, prevented learning impairment, and decreased the acetylation of the known SIRT1 substrates PGC-1alpha and p53. Furthermore, injection of SIRT1 lentivirus in the hippocampus of p25 transgenic mice conferred significant protection against neurodegeneration. Thus, SIRT1 constitutes a unique molecular link between aging and human neurodegenerative disorders and provides a promising avenue for therapeutic intervention.     

Neurotoxicity from glutathione depletion is mediated by Cu-dependent p53 activation

Loss of intracellular neuronal glutathione (GSH) is an important feature of neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. The consequences of GSH depletion include increased oxidative damage to proteins, lipids, and DNA and subsequent cytotoxic effects. GSH is also an important modulator of cellular copper (Cu) homeostasis and altered Cu metabolism is central to the pathology of several neurodegenerative diseases. The cytotoxic effects of Cu in cells depleted of GSH are not well understood. We have previously reported that depletion of neuronal GSH levels results in cell death from trace levels of extracellular Cu due to elevated Cu(I)-mediated free radical production. In this study we further examined the molecular pathway of trace Cu toxicity in neurons and fibroblasts depleted of GSH. Treatment of primary cortical neurons or 3T3 fibroblasts with the glutathione synthetase inhibitor buthionine sulfoximine resulted in substantial loss of intracellular GSH and increased cytotoxicity. We found that both neurons and fibroblasts revealed increased expression and activation of p53 after depletion of GSH. The increased p53 activity was induced by extracellular trace Cu. Furthermore, we showed that in GSH-depleted cells, Cu induced an increase in oxidative stress resulting in DNA damage and activation of p53-dependent cell death. These findings may have important implications for neurodegenerative disorders that involve GSH depletion and aberrant Cu metabolism.     

Leptin neuroprotection in the CNS: mechanisms and therapeutic potentials

Leptin is well known as a hormone important in the central control of appetitive behaviors via receptor-mediated actions in the hypothalamus, where leptin adjusts food intake to maintain homeostasis with the body's energy stores. Recent evidence has shown that leptin and its receptors are widespread in the CNS and may provide neuronal survival signals. This review summarizes our current knowledge of how leptin functions in the brain and then focuses on the ability of leptin to mitigate neuronal damage in experimental models of human neurological disorders. Damage to the brain by acute events such as stroke, or long-term loss of neurons associated with neurodegenerative diseases, including Parkinson's and Alzheimer's disease, may be amenable to treatment using leptin to limit death of susceptible cells. Leptin-mediated pro-survival signaling is now known to prevent the death of neurons in these models. The signaling cascades that leptin generates are shared by other neuroprotective molecules including insulin and erythropoietin, and are thus a component of the neurotrophic effects mediated by endogenous hormones. Coupled with evidence that leptin dysregulation in human disease also results in enhanced neuronal susceptibility to damage, development of leptin as a therapeutic methodology is an attractive and viable possibility.     

Neuronal life and death: an essential role for the p53 family

Recent evidence indicates that the p53 tumor suppressor protein, and its related family member, p73, play an essential role in regulating neuronal apoptosis in both the developing and injured, mature nervous system. In the developing nervous system, they do so by regulating naturally-occurring cell death in neural progenitor cells and in postmitotic neurons, acting to ensure the apoptosis of cells that either do not appropriately undergo the progenitor to postmitotic neuron transition, or that fail to compete for sufficient quantities of trophic support. Somewhat surprisingly, in developing postmitotic neurons, p53 plays a proapoptotic role, while a naturally-occurring, truncated form of p73, DeltaNp73, antagonizes p53 and plays an anti-apoptotic role. In the mature nervous system, numerous studies indicate that p53 is essential for the neuronal death in response to a variety of insults, including DNA damage, ischemia and excitotoxicity. It is likely that all of these insults culminate in DNA damage, which may well be a common trigger for neuronal apoptosis. In this regard, the signaling pathways that are responsible for triggering p53-dependent neuronal apoptosis are starting to be elucidated, and involve cell cycle deregulation and activation of the JNK pathway. Finally, accumulating evidence indicates that p53 is perturbed in the CNS in a number of neurodegenerative disorders, leading to the hypothesis that longterm oxidative damage and/or excitotoxicity ultimately trigger p53-dependent apoptosis in the chronically degenerating nervous system.     

Disruption of neuronal autophagy by infected microglia results in neurodegeneration

There is compelling evidence to support the idea that autophagy has a protective function in neurons and its disruption results in neurodegenerative disorders. Neuronal damage is well-documented in the brains of HIV-infected individuals, and evidence of inflammation, oxidative stress, damage to synaptic and dendritic structures, and neuronal loss are present in the brains of those with HIV-associated dementia. We investigated the role of autophagy in microglia-induced neurotoxicity in primary rodent neurons, primate and human models. We demonstrate here that products of simian immunodeficiency virus (SIV)-infected microglia inhibit neuronal autophagy, resulting in decreased neuronal survival. Quantitative analysis of autophagy vacuole numbers in rat primary neurons revealed a striking loss from the processes. Assessment of multiple biochemical markers of autophagic activity confirmed the inhibition of autophagy in neurons. Importantly, autophagy could be induced in neurons through rapamycin treatment, and such treatment conferred significant protection to neurons. Two major mediators of HIV-induced neurotoxicity, tumor necrosis factor-alpha and glutamate, had similar effects on reducing autophagy in neurons. The mRNA level of p62 was increased in the brain in SIV encephalitis and as well as in brains from individuals with HIV dementia, and abnormal neuronal p62 dot structures immunoreactivity was present and had a similar pattern with abnormal ubiquitinylated proteins. Taken together, these results identify that induction of deficits in autophagy is a significant mechanism for neurodegenerative processes that arise from glial, as opposed to neuronal, sources, and that the maintenance of autophagy may have a pivotal role in neuroprotection in the setting of HIV infection.     

Copper mediated neurological disorder: Visions into amyotrophic lateral sclerosis,Alzheimer and Menkes disease

Copper (Cu) is a vital redox dynamic metal that is possibly poisonous in superfluous. Metals can traditionally or intricately cause propagation in reactive oxygen species (ROS) accretion in cells and this may effect in programmed cell death. Accumulation of Cu causes necrosis that looks to be facilitated by DNA damage, followed by activation of P53. Cu dyshomeostasis has also been concerned in neurodegenerative disorders such as Alzheimer, Amyotrophic lateral sclerosis (ALS) or Menkes disease and is directly related to neurodegenerative syndrome that usually produces senile dementia. These mortal syndromes are closely related with an immense damage of neurons and synaptic failure in the brain. This review focuses on copper mediated neurological disorders with insights into amyotrophic lateral sclerosis, Alzheimer and Menkes disease.     

Relevance of protein nitration in brain injury: a key pathophysiological mechanism in neurodegenerative, autoimmune, or inflammatory CNS diseases and stroke

Summary. This review has focused on the evidence for the involvement of nitrative oxidation in certain neurodegenerative disorders (Parkinsons Disease, Alzheimers Disease, Amyotrophic Lateral Sclerosis), stroke, and inflammatory and autoimmune disorders (with particular attention devoted to multiple sclerosis).The relationship between protein peroxidation and pathological changes observed in the above disorders has been reported. Whereas many of the findings are from studies with animal models and autoptic specimens from human patients, few data are available from cerebrospinal fluid and blood samples of the patients at different times and disease stages.The participation of nitrative oxidation to the direct and indirect injury of neurons and other cells of the brain (i.e., oligodendrocytes, for multiple sclerosis) is clear; less evident is their relevance for the development and progression of these disorders.Further studies should be aimed to establish the clinical and prognostic value of peroxidative markers for the CNS diseases considered. This is fundamental for the development of therapeutic interventions antagonizing nitric oxide-related species damage.     

p53 at the crossroads between cancer and neurodegeneration

Aging, dementia, and cancer share a critical set of altered cellular functions in response to DNA damage, genotoxic stress, and other insults. Recent data suggest that the molecular machinery involved in maintaining neural function in neurodegenerative disease may be shared with oncogenic pathways. Cancer and neurodegenerative diseases may be influenced by common signaling pathways regulating the balance of cell survival versus death, a decision often governed by checkpoint proteins. This paper focuses on one such protein, p53, which represents one of the most extensively studied proteins because of its role in cancer prevention and which, furthermore, has been recently shown to be involved in aging and Alzheimer disease (AD). The contribution of a conformational change in p53 to aging and neurodegenerative processes has yet to be elucidated. In this review we discuss the multiple functions of p53 and how these correlate between cancer and neurodegeneration, focusing on various factors that may have a role in regulating p53 activity. The observation that aging and AD interfere with proteins controlling duplication and cell cycle may lead to the speculation that, in senescent neurons, aberrations in proteins generally dealing with cell cycle control and apoptosis could affect neuronal plasticity and functioning rather than cell duplication.     

Expression of signaling molecules associated with apoptosis in human ischemic stroke tissue

There is growing evidence that, because of the highly significant differences in gene activation/protein expression between animal models of stroke and stroke patients, the current treatment strategies based on animal stroke models have been unsuccessful. Therefore, it is imperative that the pathobiology of human stroke be studied. As a first step here, Western blotting and immunohistochemistry were employed to examine expression and tissue localization of key apoptotic proteins in infarct and peri-infarcted (penumbra) from grey and white matter in human postmortem tissue of 18 patients who died between 2 and 37 d after stroke caused by large vessel disease. The contralateral hemisphere was used as a control. JNK1, JNK2, and p53 were upregulated in the majority of samples, whereas Bcl-2, caspase-3, active caspase-3, phosphorylated p53 (p-p53), phosphorylated JNk1 (p-JNK1), and phosphorylated JNK2 (p-JNK2) were upregulated in approximately half of the samples. JNK1 expression was positively correlated with JNK2 expression in grey and white matter infarct and penumbra, whereas active caspase-3 levels were positively correlated with p-JNK2 levels in grey and white matter infarct. Using indirect immunoperoxidase staining of paraffin-embedded sections, active caspase-3 was found in infarcted neurons that co-localized with TUNEL-positive cells. p-JNK localization in the nuclei of TUNEL-positive cells with the morphological appearance of neurons from infarct and penumbra was also demonstrated. The use of Kaplan Meier survival data demonstrated that the presence of Bcl-2 in penumbra of grey matter correlated significantly with shorter survival (p=0.006). In conclusion, the present study has identified significantly altered expression of apoptotic proteins in human stroke tissue and shown that the presence of Bcl-2 in penumbra of grey matter has prognostic value. It is tempting to suggest that further studies of apoptotic proteins in human stroke may lead to identification of novel targets for drug discovery.     

p53 accumulation due to down-regulation of ubiquitin: relevance for neuronal apoptosis

The p53 tumor suppressor protein is a major regulator of cell growth arrest and apoptosis in response to DNA damage. Both p53 function and stability are tightly controlled by Mdm2, which binds to the p53 N-terminus and targets p53 for ubiquitin-mediated proteolysis. Previous studies suggest that adrenalectomy-induced neuronal apoptosis is p53-dependent. Here we demonstrate both nuclear accumulation and functional activation of p53 protein in apoptotic hippocampal neurons from adrenalectomized rats. Increased p53 expression occurred despite the accumulation of its negative regulator, Mdm2, and the formation of p53-Mdm2 complexes. The persistence of p53 expression was explained by a striking decrease in free ubiquitin in p53-positive neurons. The addition of exogenous ubiquitin to p53-Mdm2 complexes from apoptotic neurons restored p53 degradation. These findings demonstrate a novel mechanism of p53 stabilization mediated by decreased ubiquitin levels. Regulation of free ubiquitin may therefore be an effective way to modulate p53-dependent apoptosis in certain cell types.     

Angiotensin II AT(1) receptor blockers ameliorate inflammatory stress: a beneficial effect for the treatment of brain disorders

Excessive allostatic load as a consequence of deregulated brain inflammation participates in the development and progression of multiple brain diseases, including but not limited to mood and neurodegenerative disorders. Inhibition of the peripheral and brain Renin-Angiotensin System by systemic administration of Angiotensin II AT(1) receptor blockers (ARBs) ameliorates inflammatory stress associated with hypertension, cold-restraint, and bacterial endotoxin administration. The mechanisms involved include: (a) decreased inflammatory factor production in peripheral organs and their release to the circulation; (b) reduced progression of peripherally induced inflammatory cascades in the cerebral vasculature and brain parenchyma; and (c) direct anti-inflammatory effects in cerebrovascular endothelial cells, microglia, and neurons. In addition, ARBs reduce bacterial endotoxin-induced anxiety and depression. Further pre-clinical experiments reveal that ARBs reduce brain inflammation, protect cognition in rodent models of Alzheimer's disease, and diminish brain inflammation associated with genetic hypertension, ischemia, and stroke. The anti-inflammatory effects of ARBs have also been reported in circulating human monocytes. Clinical studies demonstrate that ARBs improve mood, significantly reduce cognitive decline after stroke, and ameliorate the progression of Alzheimer's disease. ARBs are well-tolerated and extensively used to treat cardiovascular and metabolic disorders such as hypertension and diabetes, where inflammation is an integral pathogenic mechanism. We propose that including ARBs in a novel integrated approach for the treatment of brain disorders such as depression and Alzheimer's disease may be of immediate translational relevance.