Increased Intracellular Ionic Content Is Correlated with a Decreased Perichromatin Granule Density in Old Neurons

Abstract

The intracellular content of monovalent ions (Na(+), Cl(-), and K(+)) in brain cortical cells from young, adult, and old rats was measured by X-ray microanalysis. By referring the peak of each ion to the respective value of the dry mass, we obtained the intracellular content as a percentage of dry mass. A significant increase of the intracellular content of Na(+), Cl(-), and K(+) was found in old animals. In the same type of cells from rats of different ages, we estimated the density of perichromatin granules (PGs), the RNA structural correlates containing ribonucleoproteins (RNPs). PGs were discovered by Bernhard's method, which allows the preferential staining of RNPs. The PG density (i.e., the number of PGs/microm2 of nuclear area) was significantly lower (-27.8%) in the neurons of old animals. Taken together, the present findings document that in the nuclei of brain cortical cells of old animals the ionic strength is markedly increased and that this alteration may affect chromatin functions (e.g., DNA template activity). The ionic content of the nucleoplasm is reported to modulate the structure of chromatin molecules and regulate gene expression; thus, the increased content of the three ions found by us in old animals, by increasing the viscosity of the nucleoplasm, may affect gene expression by disturbing the inhibition (negative regulation) or potentiation (positive regulation) of RNA polymerase binding. In physiological aging, increased ionic strength may lead to age-related DNA dysfunction as well as to alterations in the processing of RNA structural constituents, such as PGs.