Comparison of the lateral diffusion constant of hepatocyte membrane proteins in two wild mouse strains of considerably different longevity: FRAP studies on liver smears

What really declines with age? The Hayflick Lecture for 2006 35th American Aging Association

In order to understand the basic mechanisms underlying the organismic aging process, considerable efforts have been devoted in the last half-century to biochemical (enzyme activity) alterations in specific tissues and organs of various organisms associated with aging. When a decline in enzyme activities with age has been found in a study, especially for key enzymes such as antioxidant enzymes, the results have often been interpreted as a cause for the aging of the entire body. Retrospectively, however, these changes turned out to be so variable--depending on species, strains and sexes of animals--that the interpretation of these results in general terms of aging became invalid. Further, unlike the prediction for the whole human body, many enzyme activities in a vital organ, such as the liver, remained unchanged, as long as the old subjects remained healthy. However, enzyme activities in old animals and humans are often more susceptible to morbidities and frailties, which themselves are often accompanied by infections and malnutrition. Despite the rather stable enzyme functions in the liver with age, a distinct and progressive decline in the lateral diffusion coefficient of proteins of hepatocyte plasma membranes has been demonstrated by fluorescence recovery after photobleaching (FRAP), which was implicated as the cause for the decline of hepatocyte functions such as ouabain (and taurocholate) hepatic uptake and their eventual biliary excretion. Since a similar decline in protein diffusion coefficients was observed in brain and muscle cells, it is likely that these changes are occurring in common with many cell types of the body, thus causing a delay in transmembrane transport of endogenous and exogenous substances whose transports are mediated by membrane proteins. In attempts to prolong the life spans of animals other than by calorie restriction, but instead using deprenyl or tetrahydrocurcumin, works by the author and coworkers are introduced and discussed. Despite limited success along these lines thus far, further attempts are encouraged, primarily to understand the mechanisms underlying organismic aging processes and to find a practical way to prolong the health span of the elderly.

Aging and the liver: functional aspects

The drastic decline in the function of the hepatic microsomal cytochrome monooxygenase system, initially reported in male rat livers, was shown to be due to a feminization of male rat livers with aging. In female rat livers as well as in mouse livers, this system was found to stay unchanged with age. Phase II reactions which showed some decline with aging in male rat livers again stayed fairly stable with age in female rat and mouse livers. Glutathione S-transferase (GST) enzyme activities, which are very stable with age in female rat and mouse livers, demonstrated highly age-dependent changes when dietary conditions were manipulated, suggesting a potential age difference in the homeostatic regulation of this enzyme system. Using the fluorescence recovery after photobleaching (FRAP) technique, unique studies revealed an age-dependent decline in the lateral mobility of proteins in hepatocyte surface membranes. The protease inhibitor model of aging, initially proposed by Ivy for brain cells, has been validated in hepatocytes, demonstrating an accumulation of lipofuscin-like granules in young animals treated with i.p. infusion of leupeptin for only 2 weeks. Antioxidant enzyme activities such as superoxide dismutase (SOD) and catalase (CAT) in the liver were clearly demonstrated no to be reduced in general terms with aging. Rather, a clear increase in CAT enzyme activities with age was demonstrated in female rat livers, thus challenging the concept that intracellular enzyme activities generally decline with aging. In this paper, studies performed in Japan on aging and the liver over the past 30 years, with a focus on its functional aspects, are critically reviewed in terms of the clinical implications of these studies as well as on theories of aging in general.

Living fast, dying when? The link between aging and energetics

The idea that aging should be linked to energy expenditure has a long history that can be traced to the late 1800s and the industrial revolution. Machines that are run fast wear out more quickly, so the notion was born that humans and animals might experience similar fates: the faster they live (expressed as greater energy expenditure), the sooner they die. Evidence supporting the "rate-of-living" theory was gleaned from the scaling of resting metabolism and life span as functions of body mass. The product of these factors yields a mass-invariant term, equivalent to the "amount of living." There are at least four problems with this evidence, which are summarized and reviewed in this communication: 1) life span is a poor measure of aging, 2) resting metabolism is a poor measure of energy expenditure, 3) the effects are confounded by body mass and 4) the comparisons made are not phylogenetically independent. We demonstrate that there is a poor association between resting metabolic rate (RMR) and daily energy expenditure (DEE) measured using the doubly labeled water (DLW) method at the level of species. Nevertheless, the scaling relation between DEE and body mass still has the same scaling exponent as the RMR and body mass relationship. Thus, if we use DEE rather than RMR in the analysis, the rate-of-living ideas are still supported. Data for 13 species of small mammal were obtained, where energy demands by DLW and longevity were reliably known. In these species, there was a strong negative relationship between residual longevity and residual DEE, both with the effects of body mass removed (r(2) = 0.763, F = 32.1, P < 0.001). Hence, the association of energy demands and life span is not attributed to the confounding effects of body size. We subjected these latter data to an analysis that extracts phylogenetically independent contrasts, and the relationship remained significant (r(2) = 0.815, F = 39.74, P < 0.001). Small mammals that live fast really do die young. However, there are very large differences between species in the amounts of living that each enjoy and these disparities are even greater when other taxa are included in the comparisons. Such differences are incompatible with the "rate-of-living" theory. However, the link between energetics and aging across species is reconcilable within the framework of the "free-radical damage hypothesis" and the "disposable soma hypothesis." Within species one might anticipate the rate-of-living model would be more appropriate. We reviewed data generated from three different sources to evaluate whether this were so, studies in which metabolic rate is experimentally increased and impacts on life span followed, studies of caloric restriction and studies where links between natural variation in metabolism and life span are sought. This review reveals that there might be contrasting effects of resting and nonresting energy expenditure on aging, with increases in the former being protective and increases in the latter being harmful.

Pharmacological interventions against aging through the cell plasma membrane: a review of the experimental results obtained in animals and humans

As was shown in a recent review by this author (Ann. N.Y. Acad. Sci., 928: 187-199, 2001), oxyradicals cannot be considered only as harmful by-products of the oxidative metabolism, but living cells and organisms implicitly require their production. This idea is supported by numerous facts and arguments, the most important of which is that the complete inhibition of the oxyradical production by KCN (or by any block of respiration) kills the living organisms long before the energy reserves would be exhausted. This new theoretical approach not only helps our understanding of the normal functions of the living organisms, such as the basic memory mechanisms in the brain cells, but also helps in identifying the site-specific, radical-induced damaging mechanisms that represent the undesirable side effects of oxygen free radicals. First of all, these effects make the cell plasma membrane vulnerable and cause a series of intracellular functional disorders, as described by the membrane hypothesis of aging (MHA). The logical way for any antiaging intervention therefore should be to increase the available number of loosely bound electrons inside the plasma membrane that are easily accessible for OH(*) free radical scavenging. The present review summarizes the available knowledge regarding the theory of the use of membrane-related antiaging pharmaca, like centrophenoxine (CPH), tested in both animal experiments and human clinical trials. A modified, developed version of CPH coded as BCE-001 is also reported.

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.

Comparison of the lateral diffusion coefficient of hepatocyte plasma membrane proteins in three strains of sensescence accelerated mouse (SAM)

The lateral diffusion coefficients of proteins (D(p)) were measured in hepatocyte plasma membrane in freshly prepared liver smears by means of the fluorescence recovery after photobleaching (FRAP) method. D(p) was measured after development of peroxide-induced autofluorescence (PIAF) in a total of 115 senescence accelerated mice (SAM), distributed in three strains, at least five age-groups in each, as follows: (i) SAMR1TA (25 males and 22 females), medium life span (MLS) in months, under specific pathogen free (SPF) conditions, MLS(spf) 20.1 and 20.0, respectively, while under conventional conditions, MLS(Deltapf)=18.9 in average for both sexes; (ii) SAMP6/Ta (18 males and 17 females), MLS(spf)=17.1 and 15.3, respectively, and MLS(cc)=8.1 for both sexes; (iii) SAMP6/Ta (17 males and 16 females), MLS(spf)=15.6 and 14.7, respectively, and MLS(cc)=10.0 for both sexes. A highly significant negative linear age-correlation of D(p) (R=0.975 or higher) was found in each strain, being roughly proportional with the MLS(cc) values. Since the studied mice kept under SPF conditions survived longer, than under conventional conditions, the actual age-dependent decay rates of D(p) values did not differ significantly in two pairs of comparisons (female R1/P6 and female R1/P8), whereas they did in all other possible pairs, including also the normal C57BL/6 mice. The main conclusion can be drawn that the D(p) of hepatocyte membranes is a good biomarker of aging and survival also in SAM, as in all other inbred and outbred rodents, studied so far.

Lateral mobility of proteins and lipids of cell surface membranes during aging: do the data support 'The Membrane Hypothesis of Aging'?

Many previous studies regarding the change with age in surface membrane fluidity of different cell types, including hepatocytes, as determined by the fluorescence anisotropy method, are in conflict, demonstrating decreased, unchanged or even increased fluidity with age. In contrast, the results of our series of works using the fluorescence recovery after photobleaching (FRAP) technique, which measures protein lateral diffusion coefficients of hepatocyte surface membranes (Dp), have demonstrated that Dp generally declines in a linear fashion with age in hepatocytes of all animal strains and species examined. The major coworker (I. Zs.-Nagy) of these studies insists that our observations support his original hypothesis, 'The Membrane Hypothesis of Aging' (MHA), the primary assumption of which is that changes in cell surface membranes with age cause a general decline in intracellular enzyme activities. However, while it seems clear that cell surface membrane changes do occur with age, a number of past observations including those from the laboratory of this author, provide strong evidence that intracellular enzyme activities do not generally decline with age. This paper presents the data in detail, along with the author's view that the results do not support the main assumption of the MHA, but are more likely related to alterations in membrane functions with age.

Age-dependence of the lateral diffusion coefficient of Con-A receptor protein in the skeletal muscle membrane of C57BL/6J mice

The lateral diffusion coefficient (Dp) of the Con-A receptor protein was measured in the sarcolemma of the quadriceps femoris (QF) muscle of male and female C57BL/6JNia mice in four age groups between 2 and 26 months. Freshly prepared, ex vivo taken muscle strips were stained with Con-A-FL conjugate for 10 min, and fluorescence recovery after photobleaching (FRAP) measurements were carried out on 20-30 cells per animal, at 37 degrees C. Using this technique, Dp, and the fractional recovery (mobile fraction = FR%) of these proteins can be measured. In the youngest male and female age groups, Dp values of 5.72E-10 and 5.43E-10 cm2/s, and FR% values of 43.3 and 36.3%, were found, respectively. Dp displayed a characteristic, significant, negative, linear correlation with age in both sexes. The slope of the linear regression line calculated per month of age was 1.06E-11 and 0.96E-11 cm2/s for males and females, respectively; both of them differ from zero highly significantly. FR% values tended to increase slightly with age, yet the estimated average Dp = D(FR), calculated for the total Con-A receptor pool, maintained its significant, negative, linear age-correlation. The physiological significance of these changes needs to be clarified in the future.

In K562 and HL60 cells membrane ageing during cell growth is associated with changes in cholesterol concentration

In a cell culture model of aging we have previously shown that there is an age-related decrease in the lipid dynamics of the proerythropoetic K562 cell membranes, as determined by the generalized polarization (GP) of the phase-sensitive lipid probe 2-dimethylamino-6-lauroylnaphthalene (Laurdan) (T. Parasassi, M. Di Stefano, G. Ravagnan, O. Sapora and E. Gratton. Exp. Cell Res., 202 (1992) 432-439). In the present study we also extended our observations to the lymphoblastoid HL60 cell line. In both K562 and HL60 cells during the four days after the last cell culture medium renewal the GP Laurdan value increased in a linear fashion indicating a time-dependent decrease in lipid dynamics. The initial membrane physical properties were almost completely restored upon renewal of the cell culture medium. We measured lipid composition, including individual and total phospholipids, free and esterified cholesterol at the first ('young') and at the fourth ('aged') day after culture medium renewal. We found that the decreased membrane lipid 'fluidity' at the fourth day of cell growth was associated with a 40% increase in cholesterol concentration in both cell lines. This increase in cholesterol concentration was reversible 24 h following the culture medium change. We conclude that in K562 and HL60 cells the 'age-related' decrease in membrane lipid dynamics is mediated by an 'age-related' increase in cell cholesterol content.

Age-dependence of the lateral mobility of lipids in hepatocyte plasma membrane of male rats and the effect of life-long dietary restriction

The lateral diffusion constant of lipids (D(1)) in hepatocyte plasma membranes was measured in liver smears by means of the fluorescence recovery after photobleaching (FRAP) method, applying the label, N-4-nitrobenzo-2-oxa-1,3-diazolyl phosphatidylethanolamine (NBD-PE). Nineteen ad libitum fed, male Fischer-344 rats in four age groups (2.1-29.8 months of age) were studied. A highly significant negative linear age-correlation of D(1) (cc = 0.958) was found. D(1) values were 1.39 x 10(-9) cm2/s in the young rats, and only 6.77 x 10(-10) cm2/s in the oldest rats. Lipid lateral mobility is changing in parallel with that of proteins, having been measured previously also with the FRAP method by the authors. Fractional recovery values (FR%) of the lipids were lower than those of proteins even in the young ages, but also decreased linearly with age, therefore, the parameter, D, x FR decreased even steeper with age than D(1) itself. D(1) was also measured in a group of six male Fischer 344 rats having been kept on dietary restriction (DR) since their age of 1 month until 30 months of age (applying the every-other-day (EOD) feeding). DR caused an increase of D(1), compared with the age-matched ad libitum fed animals: the mean was 9.24 x 10(-10) cm(2)/s. FR% and D(I), x FR again increased considerably under DR. The results are interpreted in terms of the increased protein and lipid turnover under DR.