Chromatin motion in neuronal interphase nuclei: changes induced by disruption of intermediate filaments

Abstract

Motion of nucleoli within interphase nuclei, known as nuclear rotation, may be used as a measure of motion of chromatin domains within the global confines of the nucleus. Mechanisms by which chromatin domains are transposed remain enigmatic. It has been established that nuclei are anchored by a network of intermediate filaments, structural proteins which share epitopes with nuclear lamins and possibly representing a constraint on nuclear rotation. It is postulated that selective removal of this constraint, by acrylamide, would result in increased chromatin motion. Mean rates of nucleolar displacement were quantified in neurons, in vitro. Nuclear rotation increased from a mean control rate of 0.102 +/- 0.002 micron/min (n = 52) to a maximum mean rate of 0.207 +/- 0.026 micron/min (n = 11), after 23 hr of exposure to 4 mM acrylamide. Despite this significant increase in motion of intranuclear domains, cytoplasmic structures in the immediate juxtanuclear area did not exhibit increases in rates of motion. Immunocytochemistry was used to visualize cytoskeletal structures and to assay selective disruption of neurofilaments by acrylamide. Increased rates of chromatin motion coincided with breakdown of the intermediate filament network. Ultrastructural analyses showed that the increase in chromatin motion induced by acrylamide was also associated with a significant (P less than 0.005) change in the thickness of the nuclear lamina, decreasing from 20.9 +/- 5.10 nm (n = 159) in controls to 18.9 +/- 3.1 nm (n = 148), to 19.5 +/- 3.6 nm (n = 240) and to 16.1 +/- 4.4 nm (n = 103) at 4, 8 and 22 hr exposure, respectively. Moreover, the number of mitochondria per unit area changed significantly (P less than 0.0001) with exposure to acrylamide, increasing from 9.1 +/- 2.2 mitochondrial profiles in controls to 16.5 +/- 5.3 profiles after 22 hr exposure to acrylamide. Distribution of other cytoskeletal components, actin and microtubules, was not altered and does not appear to play a significant role in the observed increase in rates of nuclear rotation. We conclude that the removal of the damping effects on chromatin motion normally imposed by the nuclear lamina and by intermediate filaments results in increased chromatin motion.