Does aging affect liver microtubules?

Abstract

Microtubules are essential for many cell processes, e.g., ligand-receptor endocytosis and the vectorial movement of endosomes. The cytoskeleton, particularly microtubules, may undergo age-related changes that are reflected in cell dysfunctions. For example, the translocation of 125I-IgA-containing vesicles from the sinusoidal surface to the pericanalicular cytoplasm is reduced (greater than 40%) in old versus young rats. Electron microscopic analysis demonstrated that the concentration of microtubule profiles in young animals is within 10-20% of that in old rats. The relative concentration of polymerized tubulin declines greater than 70% by 12 months of age, but the total tubulin content remains unchanged until later, i.e., declining 50% by 24 months. Concomitant increases occur in the free fractions of microtubule-associated proteins (MAP), i.e., MAP1 and heat-stable MAPS. These fractions are not associated with polymerized tubulin. The declines in total and polymerized tubulin, together with the increases in the MAPS' free fractions, may be indicative of fewer and/or shorter microtubules. These data lend credence to the supposition that aging is accompanied by perturbations of microtubule functions that ultimately are expressed as biomarkers characteristic of aging.