Chromatin motion in interphase nuclei, its modulation and its potential role in gene expression

Abstract

Nuclear Rotation (NR) refers to the rotatory motion of nuclei in cells in vitro, a motion measured as the displacement of nucleoli over time. NR occurs in cycling cells; however, its observation in neurons indicates that mechanisms related to mitosis are not a prerequisite. We have shown that NR includes motion of chromatin domains in addition to those represented by nucleoli, that movements are saltatory, with periods of stationarity and reversal of direction. The observation that NR occurs independently of concurrent motion of juxtanuclear, cytoplasmic structures, leads to the concept of a stationary outer nuclear membrane. Although traditionally perceived as rotation in a two-dimensional plane, NR represents a complex, three-dimensional motion of chromatin within the interphase nucleus, with nucleoli and DAPI-stained, fluorescent chromatin domains describing curvilinear trajectories extending throughout the nucleus. Based upon evidence that the rate of this motion changes with metabolic demands, we have postulated that NR functions in gene expression, by transposing chromatin domains to be transcribed to specific nuclear compartments. In a test of this hypothesis, Nerve Growth Factor (NGF), which alters gene expression, increased NR at a time post-NGF coincident with increased activity of RNA polymerases, while GABA, also postulated to alter transcription, increased NR with near instantaneous shifts of nucleolar positions within the nuclear space. The calcium ionophore A23187 and the chelator EGTA, agents which redistribute calcium (Ca), also increased NR, while additional Ca, in presence of EGTA, returned NR to control rates. It is difficult to link NR with the action of agents which alter transcription or ion balance. Nevertheless, in support of our hypothesis, available evidence indicates that agents which alter gene expression, alter NR and that they do so, probably through calcium dependent mechanisms.