Phospholipase A2 - nexus of aging, oxidative stress, neuronal excitability, and functional decline of the aging nervous system? Insights from a snail model system of neuronal aging and age-associated memory impairment.

The aging brain undergoes a range of changes varying from subtle structural and physiological changes causing only minor functional decline under healthy normal aging conditions, to severe cognitive or neurological impairment associated with extensive loss of neurons and circuits due to age-associated neurodegenerative disease conditions. Understanding how biological aging processes affect the brain and how they contribute to the onset and progress of age-associated neurodegenerative diseases is a core research goal in contemporary neuroscience. This review focuses on the idea that changes in intrinsic neuronal electrical excitability associated with (per)oxidation of membrane lipids and activation of phospholipase A2 (PLA2) enzymes are an important mechanism of learning and memory failure under normal aging conditions. Specifically, in the context of this special issue on the biology of cognitive aging we portray the opportunities offered by the identifiable neurons and behaviorally characterized neural circuits of the freshwater snail Lymnaea stagnalis in neuronal aging research and recapitulate recent insights indicating a key role of lipid peroxidation-induced PLA2 as instruments of aging, oxidative stress and inflammation in age-associated neuronal and memory impairment in this model system. The findings are discussed in view of accumulating evidence suggesting involvement of analogous mechanisms in the etiology of age-associated dysfunction and disease of the human and mammalian brain.

Oxidative-stress induced increase in circulating fatty acids does not contribute to phospholipase A2-dependent appetitive long-term memory failure in the pond snail Lymnaea stagnalis.

BACKGROUND: Reactive oxygen species (ROS) are essential for normal physiological functioning of the brain. However, uncompensated increase in ROS levels may results in oxidative stress. Phospholipase A2 (PLA2) is one of the key players activated by elevated ROS levels resulting in the hydrolysis of various products from the plasmamembrane such as peroxidized fatty acids. Free fatty acids (FFAs) and fatty acid metabolites are often implicated to the genesis of cognitive impairment. Previously we have shown that age-, and experimentally induced oxidative stress causes PLA2-dependent long-term memory (LTM) failure in an aversive operant conditioning model in Lymnaea stagnalis. In the present study, we investigate the effects of experimentally induced oxidative stress and the role of elevated levels of circulating FFAs on LTM function using a non-aversive appetitive classical conditioning paradigm. RESULTS: We show that intracoelomic injection of exogenous PLA2 or pro-oxidant induced PLA2 activation negatively affects LTM performance in our learning paradigm. In addition, we show that experimental induction of oxidative stress causes significant temporal changes in circulating FFA levels. Importantly, the time of training coincides with the peak of this change in lipid metabolism. However, intracoelomic injection with exogenous arachidonic acid, one of the main FFAs released by PLA2, does not affect LTM function. Moreover, sequestrating circulating FFAs with the aid of bovine serum albumin does not rescue pro-oxidant induced appetitive LTM failure. CONCLUSIONS: Our data substantiates previous evidence linking lipid peroxidation and PLA2 activation to age- and oxidative stress-related cognitive impairment, neuronal dysfunction and disease. In addition however, our data indicate that lipid peroxidation induced increased levels of circulating (per)oxidized FFAs are not a factor in oxidative stress induced LTM impairment.

Diminishing glutathione availability and age-associated decline in neuronal excitability.

Oxidative stress is frequently implicated in diminished electrical excitability of aging neurons yet the foundations of this phenomenon are poorly understood. This study explored links between alterations in cellular thiol-redox state and age-associated decline in electrical excitability in identified neurons (right pedal dorsal 1 [RPeD1]) of the gastropod Lymnaea stagnalis. Intracellular thiol redox state was modulated with either dithiothreitol or membrane permeable ethyl ester of the antioxidant glutathione (et-GSH). Neuronal antioxidant demand was manipulated through induction of lipid peroxidation with 2,2'-azobis-2-methyl-propanimidamide-dihydrochloride (AAPH). Glutathione synthesis was manipulated with buthionine sulfoximine (BSO). We show that; glutathione content of snail brains declines with age, whereas pyroglutamate content increases; treatment with AAPH and BSO alone aggravated the natural low excitability state of old RPeD1, but only the combination of AAPH + BSO affected electrical excitability of young RPeD1; et-GSH reversed this effect in young RPeD1; et-GSH and dithiothreitol treatment reversed age-associated low excitability of old RPeD1. Together, these data argue for a tight association between glutathione availability and the regulation of neuronal electrical excitability and indicate perturbation of cellular thiol-redox metabolism as a key factor in neuronal functional decline in this gastropod model of biological aging.

Using a virtual game system to innovate pulmonary rehabilitation: safety, adherence and enjoyment in severe chronic obstructive pulmonary disease.

OBJECTIVE: The present pilot study tested the use of a virtual game system (VGS) for exercise training in patients with moderate to very severe chronic obstructive pulmonary disease undergoing pulmonary rehabilitation (PR). Safety, feasibility, enjoyment and adherence were assessed. METHODS: VGS (Wii [2006], Nintendo, USA) games were prescreened and categorized into lower- and upper-body workouts. Patients admitted for a three- to four-week inpatient PR program exercised daily. They were provided an opportunity to individually engage in VGS sessions three times weekly, varying with length of stay. Dyspnea, oxygen saturation and heart rate were measured before, during and after game sessions. Patients were considered to be adherent if they attended at least 50% of VGS sessions. Adverse events and enjoyment were evaluated. RESULTS: Thirty-two patients with a mean (± SD) age of 66±9 years and a mean forced expiratory volume in 1 s of 0.72±0.40 L participated. Among the 25 patients completing the program, adherence was 76%, with a mean attendance rate of 64±35%. Mean dyspnea score was 1.5±1.1 before and 3.2±1.2 after exercise. Mean oxygen saturation changed from 94±3% to 91±5% (P<0.001), while heart rate increased from 88±15 beats/min to 102±18 beats/min (P<0.001). One patient reported chest pain requiring nitroglycerin spray and five experienced transient desaturation below 85% with play. Patients enjoyed the program (visual analogue score 8±2.6/10) and most would highly recommend it to others. CONCLUSIONS: Moderate exercise using a VGS was safe, feasible and enjoyed as an adjunct to inpatient PR. This modality may encourage patients to maintain physical activity after PR.

Phospholipase A2: the key to reversing long-term memory impairment in a gastropod model of aging.

Memory failure associated with changes in neuronal circuit functions rather than cell death is a common feature of normal aging in diverse animal species. The (neuro)biological foundations of this phenomenon are not well understood although oxidative stress, particularly in the guise of lipid peroxidation, is suspected to play a key role. Using an invertebrate model system of age-associated memory impairment that supports direct correlation between behavioral deficits and changes in the underlying neural substrate, we show that inhibition of phospholipase A(2) (PLA(2)) abolishes both long-term memory (LTM) and neural defects observed in senescent subjects and subjects exposed to experimental oxidative stress. Using a combination of behavioral assessments and electrophysiological techniques, we provide evidence for a close link between lipid peroxidation, provocation of phospholipase A(2)-dependent free fatty acid release, decline of neuronal excitability, and age-related long-term memory impairments. This supports the view that these processes suspend rather than irreversibly extinguish the aging nervous system's intrinsic capacity for plasticity.

Failure of delayed nonsynaptic neuronal plasticity underlies age-associated long-term associative memory impairment.

BACKGROUND: Cognitive impairment associated with subtle changes in neuron and neuronal network function rather than widespread neuron death is a feature of the normal aging process in humans and animals. Despite its broad evolutionary conservation, the etiology of this aging process is not well understood. However, recent evidence suggests the existence of a link between oxidative stress in the form of progressive membrane lipid peroxidation, declining neuronal electrical excitability and functional decline of the normal aging brain. The current study applies a combination of behavioural and electrophysiological techniques and pharmacological interventions to explore this hypothesis in a gastropod model (Lymnaea stagnalis feeding system) that allows pinpointing the molecular and neurobiological foundations of age-associated long-term memory (LTM) failure at the level of individual identified neurons and synapses. RESULTS: Classical appetitive reward-conditioning induced robust LTM in mature animals in the first quartile of their lifespan but failed to do so in animals in the last quartile of their lifespan. LTM failure correlated with reduced electrical excitability of two identified serotonergic modulatory interneurons (CGCs) critical in chemosensory integration by the neural network controlling feeding behaviour. Moreover, while behavioural conditioning induced delayed-onset persistent depolarization of the CGCs known to underlie appetitive LTM formation in this model in the younger animals, it failed to do so in LTM-deficient senescent animals. Dietary supplementation of the lipophilic anti-oxidant α-tocopherol reversed the effect of age on CGCs electrophysiological characteristics but failed to restore appetitive LTM function. Treatment with the SSRI fluoxetine reversed both the neurophysiological and behavioural effects of age in senior animals. CONCLUSIONS: The results identify the CGCs as cellular loci of age-associated appetitive learning and memory impairment in Lymnaea and buttress the hypothesis that lipid peroxidation-dependent depression of intrinsic excitability is a hallmark of normal neuronal aging. The data implicate both lipid peroxidation-dependent non-synaptic as well as apparently lipid peroxidation-independent synaptic mechanisms in the age-dependent decline in behavioural plasticity in this model system.

Redox agents modulate neuronal activity and reproduce physiological aspects of neuronal aging.

The high oxygen consumption and post-mitotic nature of the central nervous system (CNS) makes it particularly susceptible to oxidative stress, the impact of which is widely regarded as a root cause of functional impairment of the aging brain in vertebrates and invertebrates alike. Using an invertebrate model system we demonstrate that the lipid soluble antioxidant α-tocopherol can both reverse 2,2-azobis(2-methylpropion-amidine) dihydrochloride (AAPH) induced decline in excitability in young neurons as well as restore the electrical activity and excitability of aged neurons not unlike the level of their younger equivalents. Furthermore, using two analogs of α-tocopherol where either the acyl chain has been removed (Trolox) or the hydroxyl group of the chromanol ring has been methylated we were able to assert that the restorative effect of α-tocopherol requires both insertion into the plasma membrane as well as an active OH group. Thus, our results indicate peroxidation is an important modulator of neuronal excitability as well as support the growing body of evidence suggesting α-tocopherol's actions may extend well beyond its established role as a lipid domain preventative antioxidant.