Aged SOD overexpressing mice exhibit enhanced spatial memory while lacking hippocampal neurogenesis

Intermittent Metabolic Switching and Vascular Cognitive Impairment

Intermittent fasting (IF), a dietary pattern alternating between eating and fasting periods within a 24-hour cycle, has garnered recognition for its potential to enhance both healthspan and lifespan in animal models and humans. It also shows promise in alleviating age-related diseases, including neurodegeneration. Vascular cognitive impairment (VCI) spans a severity range from mild cognitive deficits to severe cognitive deficits and loss of function in vascular dementia. Chronic cerebral hypoperfusion has emerged as a significant contributor to VCI, instigating vascular pathologies such as microbleeds, blood-brain barrier dysfunction, neuronal loss, and white matter lesions. Preclinical studies in rodents strongly suggest that IF has the potential to attenuate pathological mechanisms, including excitotoxicity, oxidative stress, inflammation, and cell death pathways in VCI models. Hence, this supports evaluating IF in clinical trials for both existing and at-risk VCI patients. This review compiles existing data supporting IF's potential in treating VCI-related vascular and neuronal pathologies, emphasizing the mechanisms by which IF may mitigate these issues. Hence providing a comprehensive overview of the available data supporting IF's potential in treating VCI by emphasizing the underlying mechanisms that make IF a promising intervention for VCI.

Oxidative Stress in Cognitive and Epigenetic Aging: A Retrospective Glance

Brain aging is the critical and common factor among several neurodegenerative disorders and dementia. Cellular, biochemical and molecular studies have shown intimate links between oxidative stress and cognitive dysfunction during aging and age-associated neuronal diseases. Brain aging is accompanied by oxidative damage of nuclear as well as mitochondrial DNA, and diminished repair. Recent studies have reported epigenetic alterations during aging of the brain which involves reactive oxygen species (ROS) that regulates various systems through distinct mechanisms. However, there are studies which depict differing roles of reactive oxidant species as a major factor during aging. In this review, we describe the evidence to show how oxidative stress is intricately linked to age-associated cognitive decline. The review will primarily focus on implications of age-associated oxidative damage on learning and memory, and the cellular events, with special emphasis on associated epigenetic machinery. A comprehensive understanding of these mechanisms may provide a perspective on the development of potential therapeutic targets within the oxidative system.

Citrulline prevents age-related LTP decline in old rats

The prevalence of cognitive decline is increasing as the ageing population is considerably growing. Restricting this age-associated process has become a challenging public health issue. The age-related increase in oxidative stress plays a major role in cognitive decline, because of its harmful effect on functional plasticity of the brain, such as long-term potentiation (LTP). Here, we show that citrulline (Cit) has powerful antioxidant properties that can limit ex vivo oxidative stress-induced LTP impairment in the hippocampus. We also illustrate that a three-month Cit supplementation has a protective effect on LTP in aged rats in vivo. The identification of a Cit oxidation byproduct in vitro suggests that the antioxidant properties of Cit could result from its own oxidation. Cit supplementation may be a promising preventive nutritional approach to limit age-related cognitive decline.

Young at heart: Insights into hippocampal neurogenesis in the aged brain

While the existence and importance of adult hippocampal neurogenesis in young adult rodents has been well-established, such qualities in aged animals and humans have remained poorly understood. Most evidence in humans has come from hippocampal volumetric changes that provide no direct proof of new neurons in adulthood. Here, we review the basic neurobiological evidence for adult hippocampal neurogenesis in the aged brain of experimental animals with short and long lifespans, and humans. The rate of cell cycling and addition of new hippocampal neurons to the existing hippocampal circuit undoubtedly decreases with age. Yet, neural stem/progenitor cells that persist into senescence may activate and produce a substantial number of functional new neurons that exhibit enhanced survival and integration given the right set of conditions. There thus exists remarkable potential for newly-generated neurons in the senescent hippocampus to make important circuit- and behavioral-level contributions, which may serve as a target for future therapeutics.

Assessment of Improvement in Oxidative Stress Indices with Resocialization in Memory Retrieval in Y-Maze in Male Rats

Introduction:

Memory deficit is an important issue in some psychiatric diseases either as a primary symptom or as a comorbid symptom. Factors that determine the decline or improvement of memory are an important subject to reduce the severity of these diseases.

Methods and materials:

In this study, 32 male Sprague-Dawley rats were randomly divided into 4 experimental groups: social (control), isolation, resocialization for 3 days, and resocialization for 7 days. Isolation occurred for 14 days. Resocialization groups were resocialized for 3 or 7 days after isolation. In the social group, there was no intervention with normal socializing among the rats. In the isolation group, rats were isolated with no resocialization. In all 4 groups, after performing the Y-maze, the rats’ brains were removed to assess oxidative stress status in the hippocampus and prefrontal cortex.

Results:

Y-maze performance improved after 3 and 7 days of resocialization. However, oxidative stress status for malondialdehyde, glutathione and nitrite/nitrate returned to normal levels except in 2 experiments after 7 days of resocialization. In addition, in 2 experiments, just glutathione in the prefrontal cortex and nitrite/nitrate in the hippocampus after 3 days of resocialization improved.

Conclusions:

A return to normal levels in all types of antioxidant markers in the resocialization groups is not the only factor for improving memory deficits resulting from isolation. Resocialization may also be activating other regulatory mechanisms besides an antioxidant defense.

Oxidative stress and mitochondrial dysfunction contributes to postoperative cognitive dysfunction in elderly rats

Postoperative cognitive dysfunction (POCD) is defined by cognitive impairment determined by neuropsychological tests from before to after surgery. Several mechanisms have been proposed in this bidirectional communication between the immune system and the brain after surgery. We aimed at understanding the mechanisms underlying POCD elderly rats in an experimental tibial fracture model. Elderly male Wistar rats were subjected to tibial fracture (TF) model. Control (sham) and fracture (TF) groups were followed to determine nitrite/nitrate concentration; oxidative damage to lipids and proteins; the activity of antioxidant enzymes (superoxide dismutase-SOD and catalase-CAT), mitochondrial respiratory chain enzymes, and creatine kinase (CK); and BDNF levels in the hippocampus and prefrontal cortex (at 24 h and at seven days) and cognitive function through habituation to the open field task and novel object recognition task (only at seven days). TF group presented increased concentration of nitrite/nitrate, hippocampal lipid peroxidation at seven days, protein oxidative damage in the prefrontal cortex and hippocampus at 24 h, decreased antioxidant activity in both structures on the first postoperative day and compromised function of the mitochondrial respiratory chain complexes as well as the CK enzyme. In addition, the levels of BDNF were reduced and memory function was impaired in the TF group. In conclusion, elderly rats submitted to an experimental model of tibial fracture displayed memory impairment accompanied by an increase in oxidative stress, mitochondrial dysfunction and reduced neurotrophin level.

Physical exercise, reactive oxygen species and neuroprotection

Regular exercise has systemic beneficial effects, including the promotion of brain function. The adaptive response to regular exercise involves the up-regulation of the enzymatic antioxidant system and modulation of oxidative damage. Reactive oxygen species (ROS) are important regulators of cell signaling. Exercise, via intensity-dependent modulation of metabolism and/or directly activated ROS generating enzymes, regulates the cellular redox state of the brain. ROS are also involved in the self-renewal and differentiation of neuronal stem cells and the exercise-mediated neurogenesis could be partly associated with ROS production. Exercise has strong effects on the immune system and readily alters the production of cytokines. Certain cytokines, especially IL-6, IL-1, TNF-α, IL-18 and IFN gamma, are actively involved in the modulation of synaptic plasticity and neurogenesis. Cytokines can also contribute to ROS production. ROS-mediated alteration of lipids, protein, and DNA could directly affect brain function, while exercise modulates the accumulation of oxidative damage. Oxidative alteration of macromolecules can activate signaling processes, membrane remodeling, and gene transcription. The well known neuroprotective effects of exercise are partly due to redox-associated adaptation.

Oxidative Stress and Adult Neurogenesis

There is a growing evidence that adult neurogenesis is critical for brain function. The reactive oxygen species (ROS) is accumulated during adult neurogenesis as a physiological mechanism; while ROS overload impairs adult neurogenesis during ageing, neuroinflammation and neurodegeneration. Here we propose that targeting oxidative stress provides a novel way to regulate adult neurogenesis and manage different brain diseases.

Paradoxical Roles of Antioxidant Enzymes: Basic Mechanisms and Health Implications

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated from aerobic metabolism, as a result of accidental electron leakage as well as regulated enzymatic processes. Because ROS/RNS can induce oxidative injury and act in redox signaling, enzymes metabolizing them will inherently promote either health or disease, depending on the physiological context. It is thus misleading to consider conventionally called antioxidant enzymes to be largely, if not exclusively, health protective. Because such a notion is nonetheless common, we herein attempt to rationalize why this simplistic view should be avoided. First we give an updated summary of physiological phenotypes triggered in mouse models of overexpression or knockout of major antioxidant enzymes. Subsequently, we focus on a series of striking cases that demonstrate "paradoxical" outcomes, i.e., increased fitness upon deletion of antioxidant enzymes or disease triggered by their overexpression. We elaborate mechanisms by which these phenotypes are mediated via chemical, biological, and metabolic interactions of the antioxidant enzymes with their substrates, downstream events, and cellular context. Furthermore, we propose that novel treatments of antioxidant enzyme-related human diseases may be enabled by deliberate targeting of dual roles of the pertaining enzymes. We also discuss the potential of "antioxidant" nutrients and phytochemicals, via regulating the expression or function of antioxidant enzymes, in preventing, treating, or aggravating chronic diseases. We conclude that "paradoxical" roles of antioxidant enzymes in physiology, health, and disease derive from sophisticated molecular mechanisms of redox biology and metabolic homeostasis. Simply viewing antioxidant enzymes as always being beneficial is not only conceptually misleading but also clinically hazardous if such notions underpin medical treatment protocols based on modulation of redox pathways.

Oxidative stress and redox regulation on hippocampal-dependent cognitive functions

Hippocampal-dependent cognitive functions rely on production of new neurons and maintenance of dendritic structures to provide the synaptic plasticity needed for learning and formation of new memories. Hippocampal formation is exquisitely sensitive to patho-physiological changes, and reduced antioxidant capacity and exposure to low dose irradiation can significantly impede hippocampal-dependent functions of learning and memory by reducing the production of new neurons and alter dendritic structures in the hippocampus. Although the mechanism leading to impaired cognitive functions is complex, persistent oxidative stress likely plays an important role in the SOD-deficient and radiation-exposed hippocampal environment. Aging is associated with increased production of pro-oxidants and accumulation of oxidative end products. Similar to the hippocampal defects observed in SOD-deficient mice and mice exposed to low dose irradiation, reduced capacity in learning and memory, diminishing hippocampal neurogenesis, and altered dendritic network are universal in the aging brains. Given the similarities in cellular and structural changes in the aged, SOD-deficient, and radiation-exposed hippocampal environment and the corresponding changes in cognitive decline, understanding the shared underlying mechanism will provide more flexible and efficient use of SOD deficiency or irradiation to model age-related changes in cognitive functions and identify potential therapeutic or intervention methods.

Repeated administration of PEP-1-Cu,Zn-superoxide dismutase and PEP-1-peroxiredoxin-2 to senescent mice induced by D-galactose improves the hippocampal functions

Oxidative stress initiates age-related reduction in hippocampal neurogenesis and the use of antioxidants has been proposed as an effective strategy to prevent or attenuate the reduction of neurogenesis in the hippocampus. In the present study, we investigated the effects of Cu,Zn-superoxide dismutase (SOD1) and/or peroxiredoxin-2 (PRX2) on cell proliferation and neuroblast differentiation in the dentate gyrus in a model of D-galactose-induced aging model. For this study, we constructed an expression vector, PEP-1, fused PEP-1 with SOD1 or PRX2, and generated PEP-1-SOD1 and PEP-1-PRX2 fusion protein. The aging model was induced by subcutaneous injection of D-galactose (100 mg/kg) to 6-week-old male mice for 10 weeks. PEP-1, PEP-1-SOD1 and/or PEP-1-PRX2 fusion protein was intraperitoneally administered to these mice at 13-week-old once a day for 3 weeks and sacrificed at 30 min after the last administrations. The administration of PEP-1-SOD1 and/or PEP-1-PRX2 significantly improved D-galactose-induced deficits on the escape latency, swimming speeds, platform crossings, spatial preference for the target quadrant in Morris water maze test. In addition, the administration of PEP-1-SOD1 and/or PEP-1-PRX2 ameliorated D-galactose-induced reductions of cell proliferation and neuroblast differentiation in the dentate gyrus and significantly reduced D-galactose-induced lipid peroxidation in the hippocampus. These effects were more prominent in the PEP-1-SOD1-treated group with PEP-1-PRX2. These results suggest that a SOD1 and/or PRX2 supplement to aged mice could improve the memory deficits, cell proliferation and neuroblast differentiation in the dentate gyrus of D-galactose induced aged mice by reducing lipid peroxidation.

FoxOs in neural stem cell fate decision

Neural stem cells (NSCs) persist over the lifespan of mammals to give rise to committed progenitors and their differentiated cells in order to maintain the brain homeostasis. To this end, NSCs must be able to self-renew and otherwise maintain their quiescence. Suppression of aberrant proliferation or undesired differentiation is crucial to preclude either malignant growth or precocious depletion of NSCs. The PI3K-Akt-FoxO signaling pathway plays a central role in the regulation of multiple stem cells including one in the mammalian brain. In particular, members of FoxO family transcription factors are highly expressed in these stem cells. As an important downstream effector of growth, differentiation, and stress stimuli, mammalian FoxO transcription factor family controls cellular proliferation, oxidative stress response, homeostasis, and eventual maintenance of long-term repopulating potential. The review will focus on the current understanding of FoxO function in NSCs as well as discuss their biological activities that contribute to determining neural stem cell fate.

Redox balance- and radiation-mediated alteration in hippocampal neurogenesis

Changes in the intracellular and extracellular redox balance have been correlated with cell fate decisions in terms of proliferation versus differentiation, entering versus existing cell cycle and survival versus cell death. Adult hippocampal neurogenesis has been correlated with neuronal plasticity of learning and memory; however, the process is exquisitely sensitive to changes in redox balance. Cranial irradiation is an effective modality in treating brain tumours but often leads to deficits in hippocampus-related learning and memory, which is most likely due to sustained elevation of oxygen free radical production and suppression of hippocampal neurogenesis. The subcellular redox environment affecting hippocampal neurogenesis is largely unknown. Using mutant mice deficient in each one of the three superoxide dismutase (SOD, EC 1.15.1.1) isoforms, we have begun to determine the consequences of SOD deficiency in hippocampal neurogenesis and the related functions of learning and memory under normal condition and following cranial irradiation.

Influence of viral vector-mediated delivery of superoxide dismutase and catalase to the hippocampus on spatial learning and memory during aging

AIMS

Studies employing transgenic mice indicate that overexpression of superoxide dismutase 1 (SOD1) improves memory during aging. It is unclear whether the improvement is due to a lifetime of overexpression, decreasing the accumulation of oxidized molecules, or if increasing antioxidant enzymes in older animals could reduce oxidative damage and improve cognitive function. We used adeno-associated virus to deliver antioxidant enzymes (SOD1, SOD2, catalase [CAT], and SOD1+CAT) to the hippocampus of young (4 months) and aged (19 months) F344/BN F1 male rats and examined memory-related behavioral performance 1 month and 4 months postinjection.

RESULTS

Overexpression of antioxidant enzymes reduced oxidative damage; however, memory function was not related to the level of oxidative damage. Increased expression of SOD1, initiated in advanced age, impaired learning. Increased expression of SOD1+CAT provided protection from impairments associated with overexpression of SOD1 alone and appears to guard against cognitive impairments in advanced age.

INNOVATION

Viral vector gene delivery provides a novel approach to test the hypothesis that increased expression of antioxidant enzymes, specifically in hippocampal neurons, will provide protection from age-related cognitive decline. Further, expression of multiple vectors permits more detailed investigation of mechanistic pathways.

CONCLUSION

Oxidative stress is a likely component of aging; however, it is unclear whether increased production of reactive oxygen species or the accumulation of oxidative damage is the primary cause of functional decline. The results provide support for the idea that altered redox-sensitive signaling rather than the accumulation of damage may be of greater significance in the emergence of age-related learning and memory deficits.

Effects of Cu,Zn-superoxide dismutase on cell proliferation and neuroblast differentiation in the mouse dentate gyrus

Oxidative stress is one of the most important factors in reducing adult hippocampal neurogenesis in the adult brain. In this study, we observed the effects of Cu,Zn-superoxide dismutase (SOD1) on lipid peroxidation, cell proliferation, and neuroblast differentiation in the mouse dentate gyrus using malondialdehyde (MDA), Ki67, and doublecortin (DCX), respectively. We constructed an expression vector, PEP-1, fused PEP-1 with SOD1, and generated PEP-1-SOD1 fusion protein. We administered PEP-1 and 100 or 500 μg PEP-1-SOD1 intraperitoneally once a day for 3 weeks and sacrificed at 30 min after the last administrations. PEP-1 administration did not change the MDA levels compared to those in the vehicle-treated group, while PEP-1-SOD1 treatment significantly reduced MDA levels compared to the vehicle-treated group. In the PEP-1-treated group, the number of Ki67-positive nuclei was similar to that in the vehicle-treated group. In the 100 μg PEP-1-SOD1-treated group, the number of Ki67-positive nuclei was slightly decreased; however, in the 500 μg PEP-1-SOD1-treated group, Ki67-positive nuclei were decreased to 78.5% of the vehicle-treated group. The number of DCX-positive neuroblasts in the PEP-1-treated group was similar to that in the vehicle-treated group. However, the arborization of DCX-positive neuroblasts was significantly decreased in both the 100 and 500 μg PEP-1-SOD1-treated groups compared to that in the vehicle-treated group. The number of DCX-positive neuroblasts with tertiary dendrites was markedly decreased in the 500 μg PEP-1-SOD1-treated group. These results suggest that a SOD1 supplement to healthy mice may not be necessary to modulate cell proliferation and neuroblast differentiation in the dentate gyrus.

H2O2: a dynamic neuromodulator

Increasing evidence implicates hydrogen peroxide (H(2)O(2)) as an intracellular and intercellular signaling molecule that can influence processes from embryonic development to cell death. Most research has focused on relatively slow signaling, on the order of minutes to days, via second messenger cascades. However, H(2)O(2) can also mediate subsecond signaling via ion channel activation. This rapid signaling has been examined most thoroughly in the nigrostriatal dopamine (DA) pathway, which plays a key role in facilitating movement mediated by the basal ganglia. In DA neurons of the substantia nigra, endogenously generated H(2)O(2) activates ATP-sensitive K(+) (K-ATP) channels that inhibit DA neuron firing. In the striatum, H(2)O(2) generated downstream from glutamatergic AMPA receptor activation in medium spiny neurons acts as a diffusible messenger that inhibits axonal DA release, also via K-ATP channels. The source of dynamically generated H(2)O(2) is mitochondrial respiration; thus, H(2)O(2) provides a novel link between activity and metabolism via K-ATP channels. Additional targets of H(2)O(2) include transient receptor potential (TRP) channels. In contrast to the inhibitory effect of H(2)O(2) acting via K-ATP channels, TRP channel activation is excitatory. This review describes emerging roles of H(2)O(2) as a signaling agent in the nigrostriatal pathway and basal ganglia neurons.

Reactive oxygen species in the regulation of synaptic plasticity and memory

The brain is a metabolically active organ exhibiting high oxygen consumption and robust production of reactive oxygen species (ROS). The large amounts of ROS are kept in check by an elaborate network of antioxidants, which sometimes fail and lead to neuronal oxidative stress. Thus, ROS are typically categorized as neurotoxic molecules and typically exert their detrimental effects via oxidation of essential macromolecules such as enzymes and cytoskeletal proteins. Most importantly, excessive ROS are associated with decreased performance in cognitive function. However, at physiological concentrations, ROS are involved in functional changes necessary for synaptic plasticity and hence, for normal cognitive function. The fine line of role reversal of ROS from good molecules to bad molecules is far from being fully understood. This review focuses on identifying the multiple sources of ROS in the mammalian nervous system and on presenting evidence for the critical and essential role of ROS in synaptic plasticity and memory. The review also shows that the inability to restrain either age- or pathology-related increases in ROS levels leads to opposite, detrimental effects that are involved in impairments in synaptic plasticity and memory function.

Irradiation enhances hippocampus-dependent cognition in mice deficient in extracellular superoxide dismutase

The effects of ionizing irradiation on the brain are associated with oxidative stress. While oxidative stress following irradiation is generally viewed as detrimental for hippocampal function, it might have beneficial effects as part of an adaptive or preconditioning response to a subsequent challenge. Here we show that in contrast to what is seen in wild-type mice, irradiation enhances hippocampus- dependent cognitive measures in mice lacking extracellular superoxide dismutase. These outcomes were associated with genotype-dependent effects on measures of oxidative stress. When cortices and hippocampi were analyzed for nitrotyrosine formation as an index of oxidative stress, the levels were chronically elevated in mice lacking extracellular superoxide dismutase. However, irradiation caused a greater increase in nitrotyrosine levels in wild-type mice than mice lacking extracellular superoxide dismutase. These paradoxical genotype-dependent effects of irradiation on measures of oxidative stress and cognitive function underscore potential beneficial effects associated with chronic oxidative stress if it exists prior to a secondary insult such as irradiation.

Radiation-induced reductions in neurogenesis are ameliorated in mice deficient in CuZnSOD or MnSOD

Ionizing irradiation significantly affects hippocampal neurogenesis and is associated with cognitive impairments; these effects may be influenced by an altered microenvironment. Oxidative stress is a factor that has been shown to affect neurogenesis, and one of the protective pathways that deal with such stress involves the antioxidant enzyme superoxide dismutase (SOD). This study addressed what impact a deficiency in cytoplasmic (SOD1) or mitochondrial (SOD2) SOD has on radiation effects on hippocampal neurogenesis. Wild-type (WT) and SOD1 and SOD2 knockout (KO) mice received a single X-ray dose of 5 Gy, and quantification of the survival and phenotypic fate of newly generated cells in the dentate subgranular zone was performed 2 months later. Radiation exposure reduced neurogenesis in WT mice but had no apparent effect in KO mice, although baseline levels of neurogenesis were reduced in both SOD KO strains before irradiation. Additionally, there were marked and significant differences between WT and both KO strains in how irradiation affected newly generated astrocytes and activated microglia. The mechanism(s) responsible for these effects is not yet known, but a pilot in vitro study suggests a "protective" effect of elevated levels of superoxide. Overall, these data suggest that under conditions of SOD deficiency, there is a common pathway dictating how neurogenesis is affected by ionizing irradiation.

Neuroinflammation and memory: the role of prostaglandins

Neuroinflammation is a complex response to brain injury involving the activation of glia, release of inflammatory mediators within the brain, and recruitment of peripheral immune cells. Interestingly, memory deficits have been observed following many inflammatory states including infection, traumatic brain injury (TBI), normal aging, and Alzheimer's disease (AD). Prostaglandins (PGs), a class of lipid mediators which can have inflammatory actions, are upregulated by these inflammatory challenges and can impair memory. In this paper, we critically review the success of nonsteroidal anti-inflammatory drugs, which prevent the formation of PGs, in preventing neuroinflammation-induced memory deficits following lipopolysaccharide injection, TBI, aging, and experimental models of AD in rodents and propose a mechanism by which PGs could disrupt memory formation.