Age-related changes in gene expression in the rat brain revealed by differential display

Induction of cell proliferation in old rat liver can reset certain gene expression levels characteristic of old liver to those associated with young liver

During the past decade, it has become increasingly clear that consistent changes in the levels of expression of a small cohort of genes accompany the aging of mammalian tissues. In many cases, these changes have been shown to generate features that are characteristic of the senescent phenotype. Previously, a small pilot study indicated that some of these changes might be reversed in rat liver, if the liver cells became malignant and were proliferating. The present study has tested the hypothesis that inducing proliferation in old rat liver can reset the levels of expression of these age-related genes to that observed in young tissue. A microarray approach was used to identify genes that exhibited the greatest changes in their expression during aging. The levels of expression of these markers were then examined in transcriptomes of both proliferating hepatomas from old animals and old rat liver lobes that had regenerated after partial hepatectomy but were again quiescent. We have found evidence that over 20 % of the aging-related genes had their levels of expression reset to young levels by stimulating proliferation, even in cells that had undergone a limited number of cell cycles and then become quiescent again. Moreover, our network analysis indicated alterations in MAPK/ERK and Jun-N-terminal kinase pathways and the potential important role of PAX3, VCAN, ARRB2, NR1H2, and ITGA5 that may provide insights into mechanisms involved in longevity and regeneration that are distinct from cancer.

Altered expression of trefoil factor 3 and cathepsin L gene in rat kidney during aging

Alterations in a wide array of physiological functions are normal consequences of aging. It is likely that, decline in cellular and physiological functions that occur during aging are the net result of age related differential gene expression and their consequent down stream effects. In this report we demonstrate that in aged kidney there is a decrease in the expression of trefoil factor 3 gene and an age-related increase in the expression of cathepsin L gene as revealed by differential display PCR (DD-PCR) and northern blot analysis. Trefoil factor 3 is mainly expressed in the alimentary canal and protects it from the degradative effect of HCl by stimulating the goblet cells to synthesize mucin. Though the exact role of trefoil factor 3 in kidney is not known, we speculate that it has a protective role in kidney. Cathepsin L is a cysteine protease which degrades connective tissue proteins like collagen, elastin and fibronectin. Increase in the expression of cathepsin L in aged kidney leading to considerable loss of organ function in old age. Down regulation of trefoil factor 3 and up regulation of cathepsin L may contribute to lack of protection and increased age related tissue damage to kidney in aging.

Glucose metabolism and insulin receptor signal transduction in Alzheimer disease

Nosologically, Alzheimer disease is not a single disorder in spite of a common clinical phenotype. Etiologically, two different types or even more exist. (1) In a minority of about 5% or less of all cases, Alzheimer disease is due to mutations of three genes, resulting in the permanent generation of betaA4. (2) The great majority (95% or more) of cases of Alzheimer disease are sporadic in origin, with old age as main risk factor, supporting the view that susceptibility genes and aging contribute to age-related sporadic Alzheimer disease. However, disturbances in the neuronal insulin signal transduction pathway may be of central pathophysiological significance. In early-onset familial Alzheimer disease, the inhibition of neuronal insulin receptor function may be due to competitive binding of amyloid beta (Abeta) to the insulin receptor. In late-onset sporadic Alzheimer disease, the neuronal insulin receptor may be desensitized by inhibition of receptor function at different sites by noradrenaline and/or cortisol, the levels of which both increase with increasing age. The consequences of the inhibition of neuronal insulin signal transduction may be largely identical to those of disturbances of oxidative energy metabolism and related metabolism, and of hyperphosphorylation of tau-protein. As far as the metabolism of amyloid precursor protein (APP) in late-onset sporadic Alzheimer disease is concerned, neuronal insulin receptor dysfunction may result in the intracellular accumulation of Abeta and in subsequent cellular damage. In this context, the desensitization of the neuronal insulin receptor in late-onset sporadic Alzheimer disease is different from that occurring in normal aging and early-onset familial Alzheimer disease. In late-onset sporadic Alzheimer disease changes in the brain are similar to those caused by non-insulin-dependent diabetes mellitus.

Decreased expression of the pancreatic secretory trypsin inhibitor II gene during aging of the rat liver

The genetic constitution and differential gene expression of an organism play important roles in controlling the species-specific rate of aging and the maximum life span potential. We utilized a differential-display polymerase chain reaction technique to identify the age-dependent expression of genes in the rat liver. We demonstrate in this report, for the first time, that expression of the pancreatic secretory trypsin inhibitor II (PSTI-II) gene declines drastically during aging. We confirmed this decrease by Northern blot analysis. Low PSTI-II levels in aged animals might result in a lack of protection from prematurely activated trypsin-like proteases, which would thus enhance inflammation.

Age-related effects on induction of DNA strand breaks by intermittent exposure to electromagnetic fields

Several studies indicating a decline of DNA repair efficiency with age raise the question, if senescence per se leads to a higher susceptibility to DNA damage upon environmental exposures. Cultured fibroblasts of six healthy donors of different age exposed to intermittent ELF-EMF (50 Hz sinus, 1 mT) for 1-24 h exhibited different basal DNA strand break levels correlating with age. The cells revealed a maximum response at 15-19 h of exposure. This response was clearly more pronounced in cells from older donors, which could point to an age-related decrease of DNA repair efficiency of ELF-EMF induced DNA strand breaks.

Genes, telomeres and mammalian ageing

Although there appear to be several influences, which contribute to the ageing of mammals, the role of DNA appears to be pivotal. There is increasing evidence that oxidative damage is an important factor in producing mutations in genes, shortening telomeres and damaging mitochondrial DNA. Accumulation of mutations in genomic DNA could result in the gradual decline in cellular function, which is exhibited in a variety of tissues. The random nature of these mutations, could also offer an explanation for differences in the degree and time of onset of age-related changes, exhibited by different individuals. Shortening of telomeres, caused by oxidative damage or the end-replication problem, could result in the accumulation of post-mitotic cells in-vivo during ageing. This might impair certain aspects of physiology, such as wound healing. Mutation of mitochondrial DNA may also be important in causing loss of cells in post-mitotic tissues such as muscle or brain. In addition changes in the redox state during the life of an animal may alter transcription factor activities, leading to consistent changes in the gene expression profiles of mammalian tissues. The latter could explain consistent age-related changes that have been observed in cell structure and physiology. Although all of these mechanisms may make a contribution to ageing, it is likely that it is the interplay between them that produces the most prominent effects.

Altered composition of the AP-1 transcription factor in immortalized compared to normal proliferating cells

We have investigated the expression of the AP-1 transcription factor proteins, fos, fosB, fra1, fra2, jun, junB, junD, using Western blot analysis, in several types of asynchronously proliferating cells. The latter included normal fibroblasts, immortalized but not tumourigenic fibroblasts, and two immortalized tumour cell lines. All cells expressed fos, fra1 and jun proteins and none expressed fosB. There were, however, interesting qualitative differences between the normal fibroblasts and the immortalized cells. Expression of fra2 was difficult to detect in normal cells, but was very evident in all of the immortalized cells. The normal cells only expressed a 44 kDa junB species, whereas the immortalized cells expressed both this and another 34 kDa species. All of the cells expressed the two junD proteins but the smaller 39 kDa species was more prominent in the normal cells, whereas the larger 44 kDa protein was more prominent in the immortalized cells. These data indicate that immortalized cells are not simply cells in which the ageing process has been prevented or reversed, but instead exhibit additional characteristics to those associated with young normal cells.

Altered composition and DNA binding activity of the AP-1 transcription factor during the ageing of human fibroblasts

We have investigated the expression of AP-1 transcription factor proteins during the in-vitro ageing of human fibroblasts. The numbers of these cells that are in the cell cycle gradually decreases up to 45 cumulative population doublings (cPD), thereafter the decline is steeper, until almost all cells enter a post-mitotic state by 60 cPD. We observed that a 34 kd junB species began to replace the 44 kd junB species after 41 cPD. This was followed, after 44 cPD, by a loss of fra1 and both junD species. After 49 cPD there was a gradual decline in the levels of fos and jun proteins, but disproportionately, so that the fos/jun protein ratio also declined. Although fos and jun proteins were still clearly present at 60 cPD, utilisation of the AP-1 DNA consensus sequence could not be demonstrated after 54 cPD. These data indicate that significant changes occur in the composition of the AP-1 transcription factor during ageing, but also that alterations in its DNA binding activity may involve other factors.

Brain glucose and energy metabolism abnormalities in sporadic Alzheimer disease. Causes and consequences: an update

It is discussed that Alzheimer disease does not form a nosologic entity. 5 to 10% of all Alzheimer cases are due to inherited abnormalities on chromosomes 1, or 14, or 21, whereas the majority of 90-95% is sporadic in origin. Age-related changes in the composition of membranes and in glucose/energy metabolism along with a sympathetic tone in the brain are assumed to be cellular/molecular risk factors for this disease. In its pathogenesis, the desensitization of the neuronal insulin receptor similar to non-insulin dependent diabetes mellitus may be of pivotal significance. This abnormality along with a reduction in insulin concentration is assumed to induce a cascade-like process of disturbances including decreases in cellular glucose, acetylcholine, cholesterol, and ATP, associated with changes in the metabolism of amino acids and fatty acids. There is evidence that the reductions in the availability of both glucose/energy and insulin contribute to the formation of amyloidogenic derivatives and hyperphosphorylated tau protein. This may indicate that the amyloid cascade hypothesis in not valid for sporadic Alzheimer disease but that the formation of both, amyloidogenic derivatives and hyperphosphorylated tau protein is downstream the origin of this neurodegenerative disease.

Telomerase and mammalian ageing: a critical appraisal

The telomeres that occur at the end of chromosomes are maintained by the activity of telomerase and are thought to be important protective factors in maintaining the integrity of chromosomes. It now appears that in vitro replicative senescence, which has been observed in cultured somatic cells, is due to a loss of telomere length in those cells, caused by inactivity of telomerase. This has led to the proposition that telomerase activity is an important determinant in organismal ageing. However, many cells in the body do not proliferate regularly and therefore will not lose telomere length. Cells that do proliferate frequently have now been shown to have active telomerase. Other cells, such as fibroblasts, that do not have telomerase activity but proliferate only occasionally may not reach the Hayflick limit during the lifetime of an animal. There is also no correlation between telomere length and the maximal lifespan exhibited by different species. Studies of telomerase knock-out mice have reported some aspects of accelerated ageing after three generations, but the relevance of these observations to normal ageing remains unconvincing. The role of telomerase in producing immortal tumour cells and the possibility that activation of telomerase is an important event in malignant transformation is similarly controversial and open to alternative interpretations. The significance of these and other observations, and how they define the role of telomerase in ageing, is discussed.

Differential display analysis can reveal patterns of gene expression in immortalised hepatoma cells which are similar to those observed in young adult but not old adult liver cells

We used the differential display technique to examine whether there were any patterns of gene expression which were characteristic of both young adult rat liver and of immortalised rat hepatoma cell lines, but not of old adult rat liver. No genes were detected which appeared to be clearly expressed in young liver and immortalised cell lines, but not in old liver. However, 14 genes were detected in old liver which were down-regulated in young liver and the hepatoma cell lines. This observation lends support to the idea that immortalisation of malignant cells may involve, at least in some aspects, a reversal of the ageing process in these cells and that the genes involved have a recessive action.

Identification of different gene expression patterns in low and high grade non-Hodgkin's lymphomas by differential display

We have compared the patterns of gene expression in non-Hodgkin's lymphoma (NHL) biopsy samples from patients with either low grade or high grade disease, by the polymerase chain reaction (PCR) based technique of differential display. By using a combination of 30 primer pairs we estimate that we were able to survey over 3,000 genes expressed in these tissues. In this study we compared a group of three low grade follicular centre lymphomas with a group of two high grade diffuse large cell lymphomas and scored only those PCR products that were represented in all samples of each group. In doing so we were able to avoid many of the problems associated with the occurence of false PCR-positives. 139 differences were then scored as representing genes which may be differentially expressed during the transformation from low to high grade disease. However, as many of these might simply reflect changing populations of cells, we focused on only those genes which appeared to be expressed exclusively in either low grade or high grade disease. We have identified 14 such genes, of which 10 were low grade specific and 4 were high grade specific. This approach therefore appears to offer a systematic method for the identification and characterisation of differentially expressed genes, which are characteristic of different NHL sub-types.

Fluorescence in situ hybridization analysis of the fos/jun ratio in the ageing brain

We have examined the expression of the fos and jun genes in the cerebellum of the rat brain during ageing, by use of a semi-quantitative fluorescence in situ hybridization (FISH) method. In these experiments we have utilised the digital imaging capabilities of a cooled CCD camera system to store the fluorescence intensities of individual cells and to compare the data from each target (fos or jun) gene with that of a control (beta-actin) gene. In this way we have been able to obtain a relative quantitation of fos and jun mRNA levels. Purkinje cells were analysed in brain from Sprague-Dawley rats of 6, 13 and 23 months of age. Data obtained in this way demonstrated that the level of fos expression decreased significantly during ageing but, in contrast, that of jun increased between 6 and 13 months and thereafter remained constant. We subsequently carried out a further comparison of fos/jun ratios in purkinje cells in Wistar rats and also observed a highly significant fall in the ratio between 6 and 23 months. This change in the fos/jun ratio has important implications for the composition of the AP-1 transcription factor and for the expression of genes that it regulates.

Expression of seizure-related PTZ-17 is induced by potassium deprivation in cerebellar granule cells

The aim of this study was to identify changes in gene expression during neuronal apoptosis using the differential display (DD) technique. Potassium deprivation was used to induce neuronal apoptosis in cultured rat cerebellar granule cells. DD analysis of about 1600 transcripts resulted in 8 cDNA clones that confirmed differential expression in a slot blot analysis. One of these clones was homologous to the 3' end of seizure-related PTZ-17 RNA. Northern blot analysis showed a marked upregulation of a 2.2 kb RNA 24 hours after potassium withdrawal. This upregulation was prevented by the RNA synthesis inhibitor actinomycin D. The increase in PTZ-17 expression was specific for potassium deprivation induced apoptosis, since the other apoptosis inducers, okadaic acid and staurosporine, did not affect PTZ-17 expression. The level of PTZ-17 RNA was not significantly affected by aging in rat cerebellum. Our data suggest that the upregulation of the PTZ-17 RNA is a part of the steps leading to apoptosis during potassium deprivation in cerebellar granule cells.

Differential display analysis of gene expression indicates that age-related changes are restricted to a small cohort of genes

It is clear that there is a genetic component associated with the ageing process. Although evolutionary theory has suggested that the activity of certain genes may facilitate ageing by favouring resource utilisation by the germ cells at the expense of somatic cells, there is reason to believe that the senescent phenotype, which is the endpoint of the ageing process, may be due to alterations in the levels of expression of other genes. To investigate this situation we have used the differential display technique to survey gene expression during ageing of the rat brain, heart and liver. By optimising this technique it is possible to identify up to 10000-14000 PCR products, which represent genes expressed in the tissue under study. Interestingly, only a relatively small cohort (approximately 2%) of these genes appear to show significant changes in their levels of expression during ageing. Characterisation of the latter has so far revealed certain genes, such as glial fibrillary acidic protein, which are associated with the senescent phenotype. It has also revealed that the level of fos, a component of the AP-1 transcription factor, decreases with age, which has implications for AP-1 regulated genes. The differential display technique has also revealed an increase in mitochondrial RNA during ageing of the heart, which may be due to a gene dosage effect caused by the presence of increased numbers of mitochondrial genomes in myocytes in old age. The differential display technique therefore appears to offer a powerful tool for identifying genes which contribute to the emergence of a senescent phenotype.