Ageing of Neurospora crassa. VI. Cytochemical and cytological correlates of senescence in three model systems

Abstract

Senescent cells from three model systems in Neurospora crassa exhibit excessive rates of non-enzymatic mitochondrial lipid peroxidation in vitro and release excessive amounts of material into either the growth medium or hypotonic salt solutions, indicating that both mitochondrial and plasma membranes are abnormal. Since culture with the antioxidant nordihydroguaiaretic acid not only alleviates senescence of growth rate in each of the systems, but also alleviates or prevents the occurence of a number of biochemical abnormalities related to membranes and lipid peroxidation, excessive lipid peroxidation in plasma membranes during senescence appears to lead to abnormality of their osmotic and permeability properties. In support of this hypothesis, we demonstrated that: (1) culture of cells which undergo senescence with nordihydroguaiaretic acid inhibits the excessive release of cellular material, either during culture or in hypotonic saline solutions, to a level nearly that of normal cells; and (2) incubation of normal cells in conditions favoring peroxidation of their plasma membrane lipids, i.e., with ferrous ascorbate, enhanced the release of cellular material and nordihydroguaiaretic acid inhibited this enhancement to a level equivalent to that of untreated cells. Additional studies of the natural-death mutant revealed an acceleration of rate of senescence by hypertonic medium and a premature age-dependent decline of cytochrome oxidase activity when grown in normal medium. Analyses of growth rates, osmotic fragility, and cellular respiratory competence of sub-clonal samples of this mutant indicated that senescence also occurs in post-mitotic cells; but that the youngest cells in time of origin are the most senescent in their physiology. These latter results indicate that there may be an age-dependent accumulation of molecular errors in mitotic cells, probably in replicating molecules such as nuclear and/or mitochondrial DNA. Deleterious reactions with the products of lipid peroxidation are the most probable source of such errors.