Aging affects the distribution of the circadian CLOCK protein in rat hepatocytes

Several biochemical, physiological, and behavioral processes exhibit cyclic oscillations of about 24 h, which have been defined as circadian rhythms. In mammals, the primary circadian pacemaker resides in the suprachiasmatic nuclei; however, cell-autonomous circadian oscillators occur also in extraneural tissues, including the liver. CLOCK protein is a transcription factor essential for normal circadian rhythms and recent studies have demonstrated that it undergoes intranuclear redistribution in hepatocytes, along the daily cycle. It is known that aging leads to a progressive deterioration of the circadian rhythm at the behavioral, physiological, and cellular levels; in addition, aging affects the organization of nuclear structural components involved in transcription and splicing. In this view, we carried out ultrastructural immunocytochemical analyses on hepatocytes of adult and old rats, so as to investigate possible qualitative and quantitative modifications of CLOCK protein, in relation to the aging process. Our observations demonstrated that most CLOCK protein was always located in the cell nucleus, where it accumulated on perichromatin fibrils (the sites of premRNA transcription and early splicing); in addition, CLOCK showed daily oscillations in the different nuclear compartments, but these oscillations differed significantly between adult and old animals. This unusual distribution of CLOCK protein during aging could be related to the prolonged diurnal activity of old animals and/or to altered nuclear pathways.