Cytochemical localization of actin and myosin aggregates in interphase nuclei in situ

Biochemical and ultrastructural studies on isolated nuclear compartments have previously shown actin and myosin to be constituents of interphase nuclei. In the present work, immunocytochemistry, in conjunction with confocal microscopy and ultrastructural immunogold techniques, shows that interphase nuclei of intact dorsal root ganglion neurons and of PC12 cells contain actin and myosin. Nuclear actin was observed to be distributed throughout the nucleoplasm occurring as distinct aggregates. Frequently, prominent actin aggregates were associated with the nucleolar periphery, often near nucleolar satellites. Ultrastructurally, actin was observed to be associated with linear, electron-dense structures, putatively identified as chromatin fibers, extending from nucleoli. Use of three antibodies against subclasses of alpha-actin isoforms revealed that nuclear actin is more closely related to alpha-sarcomeric actin than to alpha-smooth muscle actin. Those aggregates associated with the nucleolus were found to be in the polymerized F-actin form, in a small fraction of neurons, as assessed by FITC-phalloidin. A myosin-like antigen was also observed to occur as intranuclear aggregates. Quantitative assays of the distribution of actin and myosin aggregates by nearest neighbour analysis indicated a distribution characterized as uniform and failed to reveal statistically significant associations between any set of aggregates. The evidence presented herein indicates that actin and myosin are constituent proteins of interphase nuclei in situ of both normal mammalian and transformed mammalian cells.