Brain Poly(A)RNA During Aging: Stability of Yield and Sequence Complexity in Two Rat Strains

Abstract

To evaluate whether major age-related changes occur during aging in the brain, we determined the sequence complexity of total poly(A)RNA and polysomal poly(A)RNA of brains from Sprague-Dawley and Fischer 344 male rats, aged 2-32 months. RNA complexity, determined by RNA-driven hybridization reactions with nonrepetitive DNA, was 6-7 X lox nucleotides for total poly(A)RNA, whereas polysomal poly(A)RNA had a sequence complexity of 2.3-2.8 x lo* nucleotides. Sprague-Dawley rats had polysomal poly(A)RNA with a yield per gram of brain and a complexity that was 20% greater than Fischer 344 rats.No age differences in total or polysomal poly(A)RNA complexity were detected in either strain. Mixtures of polysomal poly(A)RNA from different age groups gave complexity values that were indistinguishable from the complexity of each component, thereby indicating that most (>SO%) of the RNA sequences are common to all age groups studied. Analysis of total poly(A)RNA hybridization kinetics indicated that the major kinetic component had similar pseudo-first-order rate constants and complexity in all age groups. Consideration of interassay variation suggests that the limits of age changes in poly(A)RNA sequence complexity or content are ~1 0 % .Thus, age changes may be smaller than interstrain differences in the rat. We conclude that if genomic function is grossly impaired during aging, as presumed in many contemporary theories, the impairments are not expressed in most brain cells.Polysomal or total RNA (2-5 mg) was dissolved in buffer B (0.01 M-Tris-HC1, pH 7.4; 0.5 M -N~C~; 0.5% SDS) and bound to 1 g oligo(dT)-cellulose columns (type 7, P. L. Biochemicals, Milwaukee, Wisconsin), followed by washing with buffer B until the A260 of the eluant was negligible. Then, the bound poly(A)RNA-enriched fraction was eluted with H,O and rechromatographed twice to minimize contaminating ribosomal RNA (Kaplan et al., 1978, based on Aviv andLeder, 1972). The RNA was passed over Chelex (Biorad Laboratories, Richmond, California) (Davidson and Hough, 197 I) and precipitated the rat and mouse as a function of age and longevity. 1. Gerontol. 31, 405-408. Kurtz D. 1. and Sinex F. M. (1967) Age-related differences in the association of brain DNA and nuclear protein. Biochim. Biophys. Acia 145, 840-842. Mann D. M. A., Yates P. O., and Stamp J. E. (1978) The relationship between lipofuscin pigment and ageing in the human nervous system. J . Neurol. Sci. 37, 83 -93. Marmur J. (1961) A procedure for the isolation of DNA from microorganisms. . (1974) Thermal elution chromatography and the resolution of nucleic acids on hydroxylapatite. Anml. Biochetn. 61, 144-154. 487 -505. U S A 70, 3516-3520. idogia 12, 237-250. <'/?etil. 27, 77-82.

Age‐related increases in glial fibrillary acidic protein do not show proportionate changes in transcription rates or DNA methylation in the cerebral cortex and hippocampus of male rats

“…This finding supports earlier studies wherein we showed that the total inventory of gene expression in the brain did not change up to ages beyond the mean lifespan in rats (Colman et al, 1980). This conclusion, startling to many at that time, was based on data that showed the stability within +lo% of brain polysomal and nuclear poly(A) mRNA yield and sequence complexity in RNAdriven solution hybridization in rats up to 32 months old (Colman et al, 1980). Together, these findings contrast with many other examples of impaired gene expression during aging in the brain and in non-neural tissues (for review see Finch and Morgan, 1990;Van Remmen et al, 1994).…”

Plasticity of hippocampal corticosteroid receptors during aging in the rat

“…Although it is tempting to explain the reduced levels of MR and GR mRNA in the older animals in terms of age-associated hippocampal cell losses, this hypothesis becomes untenable in light of newer studies in which aged rats were found to have similar total numbers of pyramidal and dentate cells (22). Instances of ligand-mediated positive autoregulation of MR and GR have been reported (46), and it therefore seems likely that the age-related reductions in the levels of CORT may be responsible for the inefficient transcription of the MR and GR genes; indeed, steroid hormone effects on gene transcription are known to involve chromatin remodeling (47), and age has been shown to influence the transcriptional rate, stability, and processing of mRNA (48)(49)(50). The impaired transcription of the MR (but not GR) gene may be remedied by CORT replacement in old rats (41).…”

Gene Expression and Protein Degradation

“…Although there are only a few studies in which poly(A)+RNA synthesis has been measured as a function of age, there are several studies in which the poly(A)+RNA content of RNA has been measured in tissues of young and old rodents, for example, liver (22, 145,262) and brain (61). These studies reported n o agerelated change in the poly(A)+RNA content of the RNA; only the study by Semsei et al (343) found a significant decrease in poly(A)'RNA content with age.…”