Verification of the membrane hypothesis of aging on the identified giant neurons of the snail Lymnaea stagnalis L. (Gastropoda, Pulmonata) by a combined application of intracellular electrophysiology and X-ray microanalysis

The Great Pond Snail (Lymnaea stagnalis) as a Model of Aging and Age-Related Memory Impairment: An Overview

With the increase of life span, normal aging and age-related memory decline are affecting an increasing number of people; however, many aspects of these processes are still not fully understood. Although vertebrate models have provided considerable insights into the molecular and electrophysiological changes associated with brain aging, invertebrates, including the widely recognized molluscan model organism, the great pond snail (Lymnaea stagnalis), have proven to be extremely useful for studying mechanisms of aging at the level of identified individual neurons and well-defined circuits. Its numerically simpler nervous system, well-characterized life cycle, and relatively long life span make it an ideal organism to study age-related changes in the nervous system. Here, we provide an overview of age-related studies on L. stagnalis and showcase this species as a contemporary choice for modeling the molecular, cellular, circuit, and behavioral mechanisms of aging and age-related memory impairment.

Screening for a suitable cell membrane anchoring tag for Pseudomonas aeruginosa and applying it in cell membrane real-time tracking to investigate membrane aging

Membrane proteins that have been widely used in drug delivery and cell labeling can localize onto the cell membrane by interacting with lipid bilayers. A membrane-binding tag fused with a fluorescent protein can enable tracking of the cell outline. However, numerous known membrane proteins have species preferences, and thus, a suitable membrane-binding tag for Pseudomonas aeruginosa has not been reported. In this study, we examined the membrane-binding effects of a series of endogenous and exogenous proteins (peptides) in P. aeruginosa; the proteins included LacY, WspA, tsr and its truncated mutant (tsrMut), exotoxin A signal peptide (ESP), and TAT. Among them, tsrMut exhibited a faster and steadier membrane positioning ability than others, and it also did not interfere with bacteria growth. In addition, tsrMut could be further applied for identifying and tracking cell membrane aging areas in real-time. By linking it with a tandem fluorescent timer (EGFP-Tdimer2), the aging areas of the cell membrane could easily be displayed and observed under the microscope. These findings suggest that tsrMut is a highly favorable binding tag for P. aeruginosa and integrating the tag with an aging timer may be a promising approach for studying bacterial membrane senescence at the single-cell level.

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.

Taste bud cells of adult mice are responsive to Wnt/β-catenin signaling: implications for the renewal of mature taste cells

Wnt/β-catenin signaling initiates taste papilla development in mouse embryos, however, its involvement in taste cell turnover in adult mice has not been explored. Here we used the BATGAL reporter mouse model, which carries an engineered allele in which the LacZ gene is expressed in the presence of activated β-catenin, to determine the responsiveness of adult taste bud cells to canonical Wnt signaling. Double immunostaining with markers of differentiated taste cells revealed that a subset of Type I, II, and III taste cells express β-galactosidase. Using in situ hybridization, we showed that β-catenin activates the transcription of the LacZ gene mainly in intragemmal basal cells that are immature taste cells, identified by their expression of Sonic Hedgehog (Shh). Finally, we showed that β-catenin activity is significantly reduced in taste buds of 25-week-old mice compared with 10-week-old animals. Our data suggest that Wnt/β-catenin signaling may influence taste cell turnover by regulating cell differentiation. Reduced canonical Wnt signaling in older mice could explain in part the loss of taste sensitivity with aging, implicating a possible deficiency in the rate of taste cell renewal. More investigations are now necessary to understand if and how Wnt signaling regulates adult taste cell turnover.

Experimental gerontology in Hungary

Gerontological research has some past and sporadically also some highlights in Hungary, but its present state can be easily deduced from the following data. During the last 12 years and more, well over 10,000 Hungarian scientific papers have been published in well-recognized national or international journals. Altogether approximately 1% of them have been classified as gerontological publications from Hungary. This low figure shows that gerontology has low priority and--unfortunately low support--in Hungary. This statement does not intend to downgrade Hungarian gerontologists, however points out that the Hungarian trends are not far from those of European or world wide interest in aging. Despite the recognition that we have to accept the inevitable fact that industrial societies will have (they already have) an aging population with all the social and medical problems arising, the focus of interest is wide from this significant and interesting (sub)population, which is neglected (sometimes even despised); yet everybody is absolutely eager to join this club. The average of the Hungarian research achievements and publication activities are among the better European achievements. There are some highlights and new trends even initiated by some outstanding Hungarian scientists, yet the overall weight of gerontology research is still an orphan in the Hungarian scientific life. We deal in this short and far from complete summary almost exclusively with experimental gerontology. We have to apologize if we have not included everybody, who also contributed even significantly to this field because the time for the preparation of this overview was short.

Age-dependent modification of aspecific cellular effects of the benzodiazepine flunitrazepam

The experiments presented here demonstrate an aspecific effect of the benzodiazepine derivative flunitrazepam (FNZ). It differs in sites and mechanisms of action, both from benzodiazepine (BZ) specific effects on gamma-aminobutyric acid (GABA) blocking transmission via central BZ receptors and from BZ effects mediated by peripheral BZ receptors. The aspecific effect of FNZ can suitably be examined on isolated and identified neurons of the mollusc Lymnaea stagnalis (pond snail). The physiological sites of action are outside the synaptic zone, on the neuron somatic membrane and affect 'intrinsic' properties of membrane, including calcium, calcium-activated potassium and chloride channels. The aspecific FNZ effect exerts an influence on the metabolism of the cell by decreasing the permeability of the calcium channel, diminishing the excitability of the neuron membrane, and hyperpolarizing the cell, thus potentiating the specific effect of FNZ. The senile alterations of the neuron function intensify the aspecific effects of FNZ to such a degree that it must be taken seriously in consideration in anesthesia of elderly patients.

The horizons of an interdisciplinary synthesis in experimental gerontology

This review summarizes the main points of an interdisciplinary, theoretically established approach to the problem of cell senescence, called membrane hypothesis of aging (MHA). The main knowledge and some new suggestions regarding the damaging and sometimes useful roles of the oxygen free radicals is outlined. The most important experimental results are listed, which harmonize with the MHA. It is emphasized that MHA is not an alternative to the other aging hypotheses but represents a synthesis of most of them. It is pointed out that the new drug design based on the MHA resulted in a useful new molecule which is able to improve the key cell parameters deteriorated by advancing age. The necessity of a closer international cooperation is emphasized, in order to achieve a breakthrough in experimental gerontology within this century.

Electron microscopic morphometric studies on the effects of idebenone on the synaptic remodelling activity in the hippocampus and cerebellum in normal old as well as in vitamin E-deficient adult rats

Electron microscopic morphometric investigation has been carried out on the synaptic junctions of the hippocampal dentate gyri and cerebellar glomeruli of normal, old female Wistar rats (29 months of age), and vitamin E-deficient, female adult rats (11 months of age) of the same strain. The vitamin E-deficient diet was maintained from the age of 1 month for the subsequent 10-month period. Both the normal old and the vitamin E-deficient rats were treated with a daily dose of 50 mg oxidized idebenone/kg body w/day or with its solvent (5% gum arabic) through a gastric tube during the last month before killing them. The following morphometric parameters were evaluated in the hippocampal dentate gyrus and cerebellar glomerulus: the average length of the synapses (L) the surface density (S(v)) and the numerical density (N(v)) of the synaptic contact zones. Although the idebenone treatment caused a tendency to improve these parameters in both brain compartments studied, these improvements did not reach statistical significance in the cerebellum, but did so in the case of hippocampal N(v). Vitamin E deprivation caused the usual, known alterations of the synaptic parameters. Idebenone treatment during the last month of this experiment compensated the decrease of S(v) in both the hippocampus and the cerebellum; however, its protective effect was significant only in the case of hippocampus. Idebenone effect manifests itself in the increase of L, contributing mainly to the increase of S(v), since N(v) remained practically invariate. Placebo treatments did not result in any significant alterations in the vitamin E-deficient group.

The effects of idebenone on DNA and RNA contents as well as synthesis rates of total and poly(A)+ RNA in brain of normal, old C57BL/6J mice and in experimental partial cerebral ischemia of rats

Effects of idebenone on RNA and DNA contents as well as on synthesis rates of total and poly(A)(+) RNA in the brain were measured in two animal models: (1) Normal young and old, male C57BL/6J mice (6 and 32 months). Idebenone suspended in 5% gum arabic was applied in 50 mg/kg/day dose to old mice for 1 month through a gastric tube. (2) Adult female CFY rats (14-18 months) in which experimental partial cerebral ischemia was induced by bilateral common carotid artery occlusion. Idebenone was administered intraperitoneally in two dose (10mg/kg and 100 mg/kg body weight) 30 min before the interruption of carotid blood flow. DNA content remained invariate during aging in the brain; idebenone treatment did not exert any influence on this parameter. RNA content as well as total and poly(A)(+) RNA synthesis rates, which were measured by incorporation of tritiated uridine into RNA, decreased significantly with age in brain. Idebenone treatment did not cause any essential change of the metabolism of RNA under the given conditions. The RNA and DNA contents of brain were influenced neither by experimental partial cerebral ischemia nor by treatment with idebenone during the ischemia. Partial cerebral ischemia decreased the rate of total and poly(A)(+) RNA synthesis in the brain about 15-45% depending on the methods and basis of expression. This decline could totally be prevented by intraperitoneal application of 10 mg/kg idebenone 30 min before the onset of the partial ischemia. The dose of 100 mg/kg idebenone also elevated the rate of RNA synthesis; however, this increase remained statistically insignificant.

The effect of idebenone on the total content and solubility characteristics of proteins and osmometric behavior of the intracellular mass in brain of CFY and SHRsp rats

Female CFY rats (21 months old) and male spontaneously hypertensive, stroke-prone (SHRsp) rats (3 months old), in conventional housing conditions, received placebo (5% gum arabic solution) or 50 mg/kg bw/day idebenone suspended in 5% gum arabic, through a gastric tube for 5 weeks; then their brains were elaborated as follows: (1) Total proteins as well as water-soluble and water-insoluble proteins (WSP and WIP, respectively) were separated from the brain homogenate by centrifugation at 500 X g. The WIP fractions were tested also in vitro by heat denaturation at 64 degrees C (10 min) and by 3 M urea treatment. In the placebo group of CFY rats the total protein content was 113.9 mg per g fresh weight. WIP amounted to 27.2% of the total proteins. Idebenone-treatment did not alter the protein composition in these old rats. In the SHRsp rats the total protein content of the brain cortex was almost identical with that of the normal, Wistar-derived CFY rats of much more advanced age (about 2 years). The idebenone-treatment did not alter the protein content of the brain cortex, although the WIP content and the heat-resistant fraction of it increased significantly in this strain. (2) The osmotic potential of brain tissue was determined by measuring swelling or shrinkage velocities in Ringer solution, the osmotic concentration of which was rendered hypo- or hyperosmotic by dilution or addition of polythylene glycol (PEG 6000), respectively. Idebenone treatment exerted no effect on the osmometric properties of the brain tissue in either the normal old or the SHRsp rats.

On the role of intracellular physicochemistry in quantitative gene expression during aging and the effect of centrophenoxine. A review

The turnover of proteins in biological systems is due mostly to an ever-occurring damaging (cross-linking) effect of the OH. free radicals. The replacement of the damaged proteins requires a continuous gene expression. A key issue of experimental gerontology is why the gene expression maintains the fidelity but loses the speed during aging. The membrane hypothesis of aging (MHA) proposes a cellular mechanism based on the fact that the more compact cellular structures (e.g., membranes) are damaged faster than the more diluted ones (e.g., cytosol). In addition, the cell membrane is exposed also to the residual heat-induced damage deriving from a frequent discharge of its electric polarity. Therefore, one can assume that even an extremely small incompleteness of the replacement of the damaged membrane components per turnover cycle may result in an error accumulation, which may be responsible first for the inhibition of growth, then for aging of cells. In agreement with this hypothesis, neurons display a life-long, gradual loss of the passive potassium permeability of the cell membrane, resulting in a continuous dehydration of the intracellular mass, i.e., an increase of physical density. Theory and experimental models show that this latter process causes a slowing down of all enzyme functions including those realizing the gene expression and the elimination of the damaged components. Increase of the dry mass content of cells and tissues is an obligatory process for maturation; however, later on it leads to aging. The known nootropic effects of centrophenoxine (CPH) can be interpreted on the basis of the OH. radical scavenger properties of dimethylaminoethanol (DMAE) which is incorporated in the neuronal membranes of the brain in form of phosphatidyl-DMAE. The protective effects of CPH (and of similar, newly synthesized other drugs) on the membrane components can slow down the age-dependent deteriorations of the intracellular physicochemistry, in agreement with the predictions of the MHA.

Aging and ethanol alter neuronal electric membrane properties

The effect of age of donor mouse and acute ethanol exposure on the electrical membrane properties (EMP) of 825 freshly dissociated dorsal root ganglion (DRG) neurons was investigated. Both age and ethanol exerted a number of significant linear and independent effects. Age effects included a 52% increase in action potential overshoot, a 50% increase in a measure of afterhyperpolarization duration, a 31% increase in action potential duration due to decreased rate of repolarization and depolarization, and decreased electrical excitability. Also, specific membrane resistance increased and specific membrane capacitance decreased while the membrane time constant was not significantly affected by age. These EMP alterations with age for freshly dissociated DRG neurons are consistent with those reported previously for cultured DRG neurons and hence further support the hypothesis that the EMP of neurons in situ change significantly with age. Acute ethanol exposure caused relatively few EMP alterations including decreased electrical excitability, specific membrane capacitance, time constant, and overshoot; specific membrane resistance was increased. These changes are similar to those seen previously and are consistent with a membrane expansion effect of ethanol.

Alkaline phosphatase activity in striated muscle: the effect of aging and long-term training in female mice

Alkaline pNPPase (Alk'ase) activity was measured in bicarbonate buffer and in Triton buffer of striated muscles of aging mice. Between young age (4-9 months) and middle age (13-15 months) the enzyme specific activity increased by 41.8% in the bicarbonate buffer and by 22.6% in the Triton extraction. Thereafter, between middle age and old age (32 months), a marked decrease in enzyme activity took place in both the bicarbonate and Triton extractions (23.5% and 31.4%, respectively). Alk'ase activity in muscles of old mice that had been exposed for prolonged training (12-18 months), did not differ from that of age-matched controls in both types of extracts. However, cysteine, an Alk'ase inhibitor, affected the enzymatic activity in the various samples tested differently: in the bicarbonate buffer its inhibitory effect increased with age and training, whereas in the Triton extraction its effect decreased with age and following training. Thus, it seems conceivable that with age and following long-term physical exercise an overall functional change is taking place in muscle which makes Alk'ase more vulnerable. The membranous Alk'ase, however, appears more resistant to external inhibitors in the form of endurance training and to the aging process.

Age-related changes in the function of somatic membrane potassium channels of neurons in the mollusc Lymnaea stagnalis

The function of the membrane potassium channels of the identified neurons of the small parietal ganglion was investigated in the adult (10-12-month) and old (22-24-month) molluscs Lymnaea stagnalis. Old molluscan neurons showed a decrease in maximal amplitude of fast and delayed potassium currents, in maximal potassium conductance, and in kinetics of inactivation of the delayed outward current. These features may be conditioned by the age changes in phospholipid composition of the membranes, rather than by the shifts of neuronal surface area during aging. Age dynamics in the function of potassium channels is thought to cause changes in the neuronal function.

Dysdifferentiation hypothesis of aging and cancer: a comparison with the membrane hypothesis of aging

Our laboratories have been testing the basic concept that the age-dependent deterioration of the molecular components of living systems may be due in part to the biochemical effects of active oxygen species. The dysdifferentiation hypothesis of aging and cancer (DHAC) as well as the membrane hypothesis of aging (MHA) are discussed and compared to each other. These two hypotheses consider cellular mechanisms through which free radical-induced alterations may lead to the aging process. DHAC emphasizes the importance of the instability of the differentiated state of cells and how active oxygen species may interact with the genetic apparatus of cells, leading to improper gene regulation. The evidence supporting this hypothesis includes an age-dependent increase in the expression of specific genes that normally are expected to be repressed. Such evidence now includes the c-myc oncogene as well as an age-dependent decrease in the average methylation level of the entire genome in liver tissue of mice. The central concept of DHAC is that aging is a result of gene regulatory instability and that lifespan is governed by mechanisms acting to stabilize proper gene regulation. MHA is based on the concept that all cellular components are exposed to free-radical attacks, and that the damaging efficiency of the radicals is density-dependent. Compact structures like membranes are consequently more susceptible to damage than cytosolic components. In addition, the cell plasma membrane is exposed to another damaging effect called residual heat damage, which is due to the depolarization-induced discharge of the membrane during the action potential. MHA predicts that a key process of normal differentiation as well as aging is a continuous, age-dependent loss of the passive permeability of the cell membrane for potassium and probably also for water. This is due to a constant difference between the rates of damage and replacement of the membrane components and results in a gradual dehydration of the intracellular mass from the embryonic state to the aging state. The increasing intracellular density will eventually become rate-limiting for many different cellular functions, resulting in the cessation of growth and the beginning of aging. MHA also predicts an overall decrease of gene expression and protein turnover rate during aging. Pharmacological interventions on the cell membrane have supported the validity of MHA and have indicated specific mechanisms of how aging and dysdifferentiation may occur.

Age-dependent decrease of the lateral diffusion constant of proteins in the plasma membrane of hepatocytes as revealed by fluorescence recovery after photobleaching in tissue smears

When fresh liver is smeared on slides and incubated in Krebs-Henseleit Ringer solution containing 1 mM H2O2 at 37 degrees C, a yellowish-green autofluorescence develops in the hepatocyte plasma membrane. Indirect evidence shows that this peroxide-induced autofluorescence (PIAF) is due most probably to chemical reactions between proteins and the malondialdehyde produced by the membrane lipid peroxidation. Although the chemical nature of the PIAF has not been clarified yet, it is suitable under certain conditions for the measurement of the average lateral diffusion constant of the membrane proteins by means of the fluorescence recovery after photobleaching (FRAP) technique without the addition of any external fluorescent label. Analysis of four age groups for both sexes of Fischer 344 rats (3-5 rats per group, total 32) from 2 to 31 months of age revealed a significant negative linear correlation of the lateral diffusion constant of proteins with age in both sexes, with the slope of the females being somewhat smaller. Young males showed a diffusion constant about 2.8 X 10(-10) decreasing to 1.7 X 10(-10) cm2 X s-1 by 31 months of age at 37 degrees C, whereas the respective values in females were 2.7 X 10(-10) and 1.9 X 10(-10). The results are consistent with the predictions of the membrane hypothesis of aging, according to which an age-dependent loss of the passive permeability of the cell membrane for potassium (and probably for water) is the crucial point of the cellular aging.