Changes in morphology and spatial position of coiled bodies during NGF-induced neuronal differentiation of PC12 cells

Interphase nuclei are organized into structural and functional domains. The coiled body, a nuclear organelle of unknown function, exhibits cell type-specific changes in number and morphology. Its association with nucleoli and with small nuclear ribonucleo-proteins (snRNPs) indicates that it functions in RNA processing. In cycling cells, coiled bodies are round structures not associated with nucleoli. In contrast, in neurons, they frequently present as nucleolar "caps." To test the hypothesis that neuronal differentiation is accompanied by changes in the spatial association of coiled bodies with nucleoli and in their morphology, PC12 cells were differentiated into a neuronal phenotype with nerve growth factor (NGF) and coiled bodies detected by immunocytochemical localization of p80-coilin and snRNPs. The fraction of cells that showed coiled bodies as nucleolar caps increased from 1.6 +/- 0.9% (mean +/- SEM) in controls to 16.5 +/- 1.6% in NGF-differentiated cultures. The fraction of cells with ring-like coiled bodies increased from 17.2 +/- 5.0% in controls to 57.8 +/- 4.4% in differentiated cells. This was accompanied by a decrease, from 81.2 +/- 5.7% to 25.7 +/- 3.1%, in the fraction of cells with small, round coiled bodies. SnRNPs remained associated with typical coiled bodies and with ring-like coiled bodies during NGF-induced recruitment of snRNPs to the nuclear periphery. Together with the observation that coiled bodies are also present as nucleolar caps in sensory neurons, the results indicate that coiled bodies alter their morphology and increase their association with nucleoli during NGF-induced neuronal differentiation.

Organization of centromeric domains in hepatocyte nuclei: rearrangement associated with de novo activation of the vitellogenin gene family in Xenopus laevis

The existence of a function-dependent, nonrandom organization of chromatin domains within interphase nuclei is supported by evidence which suggests that specific chromatin domains undergo spatial rearrangement under conditions which alter gene expression. Exposure to estrogen of male Xenopus laevis hepatocytes in vitro results in de novo activation of vitellogenin mRNA production and vitellogenin protein synthesis and provides an ideal model to study the association between chromatin organization and changes in gene expression. In a test of the hypothesis that the de novo induction of vitellogenesis in male X. laevis is associated with a spatial rearrangement of specific chromatin domains, centromeric regions were localized by immunofluorescent labeling of associated kinetochore proteins in naive and in estrogen-treated, vitellogenic cells. Analyses by confocal scanning laser microscopy of the three-dimensional spatial distribution of kinetochores in estrogen-treated male hepatocytes showed that a significantly greater proportion of signals was associated with the nuclear periphery than in non-estrogen-treated, naive male cells. In hepatocyte nuclei, quantification of kinetochore signal sizes using image analysis showed that these signals were fewer in number and showed greater variation in size than those of cells in metaphase, with larger signals exhibiting total normalized fluorescence intensities of two, three, four, and five times that associated with kinetochore signals of metaphase cells. These observations are taken to reflect the existence of clustering of kinetochores and, by extension, of centromeres in these cells. In summary, the results show that centromeric domains within interphase nuclei of Xenopus hepatocytes occur as clusters and that these domains undergo spatial rearrangement under conditions which alter the transcriptional state of the cell.