Effects of cycloheximide on the structural organization of the nucleolus and the coiled body in normal and stimulated supraoptic neurons of the rat

New Insights into Nucleolar Architecture and Activity

The nucleolus is the most obvious and clearly differentiated nuclear subcompartment. It is where ribosome biogenesis takes place and has been the subject of research over many decades. In recent years progress in our understanding of ribosome biogenesis has been rapid and is accelerating. This review discusses current understanding of how the biochemical processes of ribosome biosynthesis relate to an observable nucleolar structure. Emerging evidence is also described that points to other, unconventional roles for the nucleolus, particularly in the biogenesis of other RNA-containing cellular machinery, and in stress sensing and the control of cellular activity. Striking recent observations show that the nucleolus and its components are highly dynamic, and that the steady state structure observed by microscopical methods must be interpreted as the product of these dynamic processes. We still do not have detailed enough information to understand fully the organization and regulation of the various processes taking place in the nucleolus. However, the present power of light and electron microscopy (EM) techniques means that a description of nucleolar processes at the molecular level is now achievable, and the time is ripe for such an effort.

Nucleolar coefficient of granulocyte precursors and granulocytes after visualization of nucleoli by two different methods

To clarify differences in the incidence and number of nucleoli in the granulopoietic lineage, these nuclear components were studied in human and rabbit granulocyte precursors and granulocytes after visualization by 2 widely employed cytochemical procedures, i.e. a procedure for the demonstration of RNA and the silver reaction for the demonstration of nucleolar silver stainable proteins (SSPs). In early stages of the granulocyte proliferating compartment, substantial differences were not found between specimens in which nucleoli were visualized by both procedures. However, in contrast to specimens stained with the silver reaction, the number of cells without nucleoli was substantially larger in advanced stages of granulocyte development in specimens stained for RNA. The number of nucleoli per cell as expressed by the nucleolar coefficient was generally larger in specimens stained with the silver reaction for nucleolar SSPs. These differences were significant starting with the stage of myelocytes. Moreover, in specimens stained with the silver reaction, most of human mature granulocytes did not contain nucleoli but nucleoli were present in all mature granulocytes of rabbits. Such differences were not observed in specimens stained for RNA in which most granulocytes were without RNA-containing nucleoli. Thus, the evaluation of the presence or absence of nucleoli in specimens depended on the visualization procedure. It is likely that in micronucleoli which are characteristic for terminal differentiation of the granulocytic lineage, RNA-containing structures may be lost or are below the detection limit of the light microscope. In addition, differences in the presence of nucleoli exist apparently between human and rabbit granulocytes in specimens stained for SSPs but not in those stained for RNA.

Age-related changes of the nucleolar organizer regions without neuron loss in medial mamillary bodies (hypothalamus) in old rats

Age-related morphological and functional changes were studied in the medial mamillary nuclei of the hypothalamus (MMN). The number of nerve cells and the volume of MMN were estimated in both groups of Wistar rats, adult and old (3 and 22 months, respectively) using stereologic methods such as the optical fractionator and Cavalieri's method respectively. The number of neurons and glial cells remain inalterable with ageing but there was an age-dependent reduction in MMN volume. In the second experiment, functional changes in the MMN neurons were observed although there was no loss in neuron number. Several functional parameters of the Ag-NORs were quantified by a computer analysis system: area and number of Ag-NORs per neuron; number of neurons with one or several Ag-NORs and also surface of neuronal nucleus. The present study shows how ageing causes volumetric and functional changes in the MMN whereas the number of neurons and glial cells remain unchanged in the mamillary region. All these results confirm the effects of age on proteic synthesis activity in neurons of the MMN, showing a decreased neuronal activity in this region.

Microinjection of anti-coilin antibodies affects the structure of coiled bodies

The coiled body is a distinct subnuclear domain enriched in small nuclear ribonucleoprotein particles (snRNPs) involved in processing of pre-mRNA. Although the function of the coiled body is still unknown, current models propose that it may have a role in snRNP biogenesis, transport, or recycling. Here we describe that anti-coilin antibodies promote a specific disappearance of the coiled body in living human cells, thus providing a novel tool for the functional analysis of this structure. Monoclonal antibodies (mAbs) were raised against recombinant human coilin, the major structural protein of the coiled body. Four mAbs are shown to induce a progressive disappearance of coiled bodies within approximately 6 h after microinjection into the nucleus of HeLa cells. After their disappearance, coiled bodies are not seen to re-form, although injected cells remain viable for at least 3 d. Epitope mapping reveals that the mAbs recognize distinct amino acid motifs scattered along the complete coilin sequence. By 24 and 48 h after injection of antibodies that promote coiled body disappearance, splicing snRNPs are normally distributed in the nucleoplasm, the nucleolus remains unaffected, and the cell cycle progresses normally. Furthermore, cells devoid of coiled bodies for approximately 24 h maintain the ability to splice both adenoviral pre-mRNAs and transiently overexpressed human beta-globin transcripts. In conclusion, within the time range of this study, no major nuclear abnormalities are detected after coiled body disappearance.

Nopp140 functions as a molecular link between the nucleolus and the coiled bodies

Coiled bodies are small nuclear organelles that are highly enriched in small nuclear RNAs, and that have long been thought to be associated with the nucleolus. Here we use mutational analysis, transient transfections, and the yeast two-hybrid system to show that the nucleolar phosphoprotein Nopp140 functions as a molecular link between the two prominent nuclear organelles. Exogenous Nopp140 accumulated in the nucleolus rapidly, but only after a lag phase in coiled bodies, suggesting a pathway between the two organelles. The expression of partial Nopp140 constructs exerted dominant negative effects on the endogenous Nopp140 by chasing it and other antigens that were common to both organelles out of the nucleolus. The alternating positively and negatively charged repeat domain of Nopp140 was required for targeting to both organelles. In addition, partial Nopp140 constructs caused formation of novel structures in the nucleoplasm and, in the case of the conserved carboxy terminus, led to the dispersal of coiled bodies. As a final link, we identified the coiled body-specific protein p80 coilin in a yeast two-hybrid screen with Nopp140. The interaction of the two proteins was confirmed by coimmunoprecipitation. Taken together, Nopp140 appeared to shuttle between the nucleolus and the coiled bodies, and to chaperone the transport of other molecules.

The centromeric/nucleolar chromatin protein ZFP-37 may function to specify neuronal nuclear domains

Murine ZFP-37 is a member of the large family of C2H2 type zinc finger proteins. It is characterized by a truncated NH2-terminal Krüppel-associated box and is thought to play a role in transcriptional regulation. During development Zfp-37 mRNA is most abundant in the developing central nervous system, and in the adult mouse expression is restricted largely to testis and brain. Here we show that at the protein level ZFP-37 is detected readily in neurons of the adult central nervous system but hardly in testis. In brain ZFP-37 is associated with nucleoli and appears to contact heterochromatin. Mouse and human ZFP-37 have a basic histone H1-like linker domain, located between KRAB and zinc finger regions, which binds double-stranded DNA. Thus we suggest that ZFP-37 is a structural protein of the neuronal nucleus which plays a role in the maintenance of specialized chromatin domains.

Changes in the nucleolar organizer regions in the tuberomammillar region after dehydration

The existence of efferent connections between tuberomammillary (TM) and the supraoptic (SON) nuclei led us to study the morphological changes at these levels during dehydration induced in rats by repeated administration of hypertonic saline. The dehydration effects are evaluated by three parameters: nucleus area, argyrophilic nucleolar organizer regions (Ag-NOR) area, and mean number of Ag-NORs. These parameters reflect the level of transcriptional activity of the rRNA genes, which in turn is closely related to the protein synthesis activity of the cell. The technique used is argyrophilic staining which labels the nucleolar organizer regions (NORs) of the nucleoli in the neurons and their area, permitting their quantification with an image analysis system. The statistical analysis reflects the sensitivity to dehydration of these structures at TM level. Our results support the regulatory role of vasopressin secretion by SON through the histaminergic pathways arising from TM in rat dehydration processes.

Number of nucleoli and coiled bodies and distribution of fibrillar centres in differentiating Purkinje neurons of chick and rat cerebellum

We used differentiating chick and rat Purkinje cells to investigate in homologous neurons the influence of the number of nucleolar organizer regions (two in the chick and six in the rat) on the behaviour of the nucleolus and coiled bodies. We employed specific silver-staining methods on smear preparations and on semithin and ultrathin sections. In chick Purkinje cells the number of nucleolar silver-staining granules increased from 15.7 +/- 3 (mean +/- SD) at embryonic day 13 to 23.8 +/- 3 at post-hatching day 7. These nucleolar granules were unevenly distributed between the two nucleoli of binucleolated cells. Electron-microscopic cytochemistry showed that nucleolar granules are equivalent to the fibrillar centres with their associated shell of dense fibrillar component. A reduction in the number of nucleoli was found during the differentiation of both chick and rat Purkinje cells, although in mature cells the average number of nucleoli per cell was higher in the chick (1.60) than in the rat (1.07). The number of coiled bodies decreased from 1.33 in newborn rats to 0.47 at postnatal day 90 in the rat. Coiled bodies were not observed in homologous chick Purkinje cells. The dynamic behaviour of nucleoli and coiled bodies during neuronal differentiation and the relationship of these two nuclear organelles with the number of nucleolar organizer regions is discussed.