Nucleoli of diploid cell strains. Thei normal ultrastructure and the effects of toyocamycin and actinomycin D

Heat shock but not cold shock leads to disturbed intracellular zinc homeostasis

The heat shock response is a highly conserved process essential for surviving environmental stress, including extremes of temperature. To investigate whether heat shock has an impact on intracellular Zn(2+) homeostasis, cells were subjected to heat shock, and subsequently the intracellular free zinc concentration was investigated. Sublethal heat shock induced a temperature-dependent and transient intracellular Zn(2+) release that was repeatable after 24 h. The free zinc was localized in round-shaped nuclear bodies identified as nucleoli. Metallothionein, the main cellular zinc storing protein, was found to be not functionally essential for this heat-shock-induced effect. No significant oxidative stress within the cells was detected after heat shock. Cold shock and subsequent rewarming did not result in disturbed intracellular zinc homeostasis. These results show that heat shock and cold shock differ with respect to intracellular Zn(2+) release. A role for zinc as signaling ion during fever is conceivable.

Nucleolar antigens and autoantibodies in hepatocellular carcinoma and other malignancies

Patients with hepatocellular carcinoma (HCC), gastrointestinal, lung, and ovarian cancers were shown to have autoantibodies to nuclear and nucleolar antigens as detected by immunofluorescence on cell substrates. The frequency of antinuclear antibodies (ANAs) was significantly higher (P less than 0.001) in patients with HCC (57/184 = 31%) than in patients with chronic hepatitis or liver cirrhosis (25/187 = 13%). Although a range of fluorescence patterns was observed, a higher percentage of nucleolar fluorescence was detected in HCC, and three of these nucleolar antigens were identified. They were NOR-90, nucleolus organizer region doublet polypeptides of 93 and 89 kDa involved in RNA polymerase I transcription; fibrillarin, a 34 kDa protein of the nucleolar U3 ribonucleoprotein particle which is engaged in preribosomal RNA processing; and nucleophosmin/protein B23, a 37 kDa polypeptide which is associated with ribosome maturation and cellular proliferation. All these antigens are nucleolar components that are engaged in some aspect of ribosome biosynthesis. Since autoantibodies to these nucleolar antigens have also been found in systemic autoimmune diseases, they do not represent autoimmune reactions unique to cancer but might reflect reaction pathways related to immune responses that are antigen-driven. The ANA response in HCC appears to be dynamic reactions to this antigen-drive since some patients with chronic liver disease showed seroconversion to ANA positivity, marked increase in titer and/or change in antibody specificity preceding or coincident with clinical detection of HCC. These changes in ANA showed a close temporal relationship with transformation from long-established chronic liver disease to HCC.

Detection of fibrillarin in nucleolar remnants and the nucleolar matrix

In order to gain further insights into the fundamental structure of the nucleolus, nucleolar remnants of Xenopus and chickens were examined for the presence of fibrillarin and nucleolus organizer region (NOR) silver staining. Nucleolar remnants of Xenopus nucleated red blood cells were found to contain easily detectable amounts of fibrillarin and NOR silver staining. Upon examination of various tissues, fibrillarin and NOR silver staining were detected in nucleoli of Xenopus liver hepatocytes and within nucleoli of oocytes and follicle cells from ovaries of mature female toads. By comparison, nucleolar remnants of adult chicken nucleated red blood cells contained only trace amounts of fibrillarin and NOR silver staining, whereas red blood cell nucleolar remnants of immature chicks had easily detectable amounts of fibrillarin and NOR silver staining. Nucleoli from hepatocytes of both adult and immature chickens demonstrated comparable levels of fibrillarin and NOR silver staining. Since fibrillarin was found in nucleolar remnant structures, we tested for (and detected) its presence in residual nucleoli of in situ nuclear matrix derived from HeLa cells. These findings are discussed in terms of the basic structural and functional organization of the nucleolus.

Description and partial characterization of a nucleolar RNA-associated autoantigen defined by a human monoclonal antibody

B lymphocytes from a patient with systemic lupus erythematosus (SLE) and several circulating autoantibodies (including antinucleolar antibodies) were immortalized by fusion with a hypoxanthine/guanine phosphoribosyl transferase (HGPRT)-deficient human B cell line. Multiple human monoclonal antibodies (mAb) were obtained which, in solid-phase enzyme immunoassay, were reactive with DNA. One mAb was of special interest because it reacted strongly with both single-stranded DNA and an extractable nuclear antigen found in rabbit thymus extract (RTE). In an immunofluorescent assay using fixed human cells, the latter mAb also bound predominantly to cell nucleoli. A combination of enzyme digestion and metabolic inhibitor studies of the target cells in this immunofluorescent assay suggested that the antigen(s) bound by the mAb was an RNA-associated protein or a ribonucleoprotein that is distinct from intact RNA polymerase I and not associated with the transcriptional units of the nucleolus. In other experiments, using fractions of RTE isolated by ion-exchange chromatography, the antigens bound by the mAb were shown to be highly negatively charged molecules. Immunoprecipitation and SDS-PAGE analyses of labeled cell extracts bound by the mAb revealed a doublet of 17 and 18 kD. Since the original patient's serum autoantibodies also bound to both an RNase-sensitive, acidic, extractable nuclear antigen and to nucleoli, and immunoprecipitated proteins of similar molecular masses in SDS-PAGE, it appears that the described mAb is a product of an immortalized autoantibody-producing B cell clone from the SLE patient's peripheral blood. This mAb probably defines a novel RNA-associated autoantigen residing predominantly in the nucleolus or, less likely, a variant of either RNA polymerase I or the ribosomal autoantigens (P proteins).

Correlation between rDNA transcription and distribution of a 100 kD nucleolar protein in CHO cells

A major nucleolar protein with a molecular weight of 100 kD is directly implicated in the transcription of pre-ribosomal RNA (pre-rRNA) and appears to be cleaved into specific maturation products during pre-ribosome biogenesis. Polyclonal antibodies which recognize the 100 kD protein and its products were used to determine the correlation between rDNA transcription and these proteins. Actinomycin D (AMD) was used to block selectively rDNA transcription (AMD 0.1 microgram/ml). Immunoperoxidase and immunogold staining were carried out in untreated and treated cells. Digitalization allowed the quantification of label according to the nucleolar components and the cellular areas. In exponentially growing cells, the dense fibrillar component was shown to contain more 100 kD protein than the granular RNP component but both nucleolar components were positively immunostained. The distribution of the 100 kD protein was rapidly modified by AMD: loss of label occurred first in the dense fibrillar zone of the nucleolus, demonstrating the correlation between rDNA transcription and the presence of this protein. However, one part of the protein remains in the segregated nucleolus after 1 h of AMD treatment, thus supporting the structural function of this protein.

The effect of actinomycin D on the nucleolus and on pigment synthesis in pigment cells of Xenopus laevis: an ultrastructural study

The effect of AMD on the nucleolus and on melanogenesis in differentiating pigment cells of Xenopus laevis was investigated in cultured neural crest cells. Cultures were treated with either 2 or 10 microgram/ml AMD for 41/2 hours. Following treatment the antibiotic was removed. Observations of the cells were made with both scanning and transmission electron microscopes. Actinomycin D almost entirely stopped pigment formation in neural crest cultures during treatment. The morphological sequence in the formation of melanin granules in the untreated pigment cells appears to be as follows: The earliest identifiable premelanosome is membrane-bound and contains very thin laminae and/or small vesciles. The premelanosomes become until they are approximately the size of mature melanin granules and there is thickening of the laminae, which appear to have a periodic substructure. The cells eventually become packed with electron-dense melanin granules. Compared with controls, the cytoplasm of treated cells showed a greater abundance of smooth- than of fuzzy-membraned vesicles, less rough endoplasmic reticulum, dilatation of the Golgi cisternae, and a smaller number of premelanosomes. The necleolus showed segregation and blebbing of its components, decrease in size and even disappearance; sometimes confluence of the components occurred. The most consistent morphological effect of AMD on the nucleolus was the separation of the fibrillar and granular areas. The granular component appeared to undergo marked changes in size and arrangement and is thought to be the source of ribosomal RNA precursors. The alteration in size of the outer component of the nucleolus went hand in hand with disappearance of free ribosomes from the cytoplasm of treated cells and inhibition of pigment synthesis.

Changes in ultrastructure and transcription induced by elevated temperature in Zea mays embryonic root cells

A study has been made of the changes occurring in Zea mays kernels exposed to thermal shock at the 48th h of soaking at 16°C, i.e., just before the protrusion of the root. Heat shock of 5 h at 46°C temporarily inhibits the resumption of root growth and consequently retards the protrusion of the root. On the ultrastructural level, it is the nucleolus which undergoes the most dramatic changes. Total loss of the granular component occurs and new electron opaque corpuscles with diameters ranging from 80 to 140 nm appear. These corpuscles contain RNA and proteins. Microstereology shows that the vacuolation of the nucleolus is increased whereas its volume is decreased. Autoradiographical and biochemical studies of RNA synthesis show that the heat shock induces an inhibition of pre-rRNA synthesis. Only RNAs of low molecular weight are still synthesized. After 19 h of a return to 16°C, the ultrastructural changes of the nucleolus are reversed. Nevertheless, increased nucleolar vacuolation persists up to that time. Exposure of maize kernels to 46°C also produces the appearance of corpuscles in some areas of the cytoplasm and in the matrix of mitochondria. Disappearance of these structures is complete within 19 h, except in some mitochondria.

[The structure of the nucleolus during the inhibition of RNA-and protein synthesis (author's transl)]

In cells treated with antimetabolites to inhibit RNA- and protein synthesis, electron microscopic studies reveal structural alterations of the nucleolus. The morphological appearance of the nucleolus differs depending of the inhibitor used. If transcription is prevented, segregation of nucleolar components is observed. Inhibition of processing of newly synthesized RNA results in a degranulation and an increase in the amount of nucleolar fibrils. A disturbance of the release of nucleolar ribonucleoproteins into the cytoplasm leads to an enlargement and a hypergranulation of the nucleolus. On the other hand interruption of translation of mRNAs has no immediate effect on the appearance of the nucleolar structure. Only after longer treatment of the cells with the translation inhibitor the nucleolus shrinks and becomes degranulated. The use of inhibitors with clearly defined mechanisms of action in a morphological study should make it possible to interpret similar nucleolar alterations seen in cancer cells and virus-infected cells on a molecular biological basis.

The nucleolar fibrillar centres in various cell types in vivo or in vitro

Nucleoli were studied in chick fibroblasts cultured in vitro, under normal or under experimental conditions, and in several mammalian cell types in vivo. All these cells frequently contain nucleoli with fibrillar centres. The nucleolar fibrillar centres are composed of fibrous material of low electron density and are always intimately associated with the dense fibrillar component. Their morphology is very similar to that analysed cytochemically in Ehrlich tumour cells. It therefore appears that they could be related to the nucleolar organizers as suggested in Ehrlich tumour cells.

Nucleolar necklaces in chick embryo fibroblast cells. II. Microscope observations of the effect of adenosine analogues on nucleolar necklace formation

The round nucleoli of chick embryo fibroblast cells, when exposed to adenosine (2 mM)or to a number of adenosine analogues, lose material and unravel over a period of several hours to become beaded strands, 20 mu M in length, termed nucleolar necklaces (NN). Light microscope observations on this process are described. Biochemical experiments have revealed that most of these analogues interfere with both messenger RNA synthesis and ribosome synthesis, causing extensive degradation of the preribosome species containing 32S RNA although most of the preribosomes containing 18S RNA survive. We suggest that it is the depletion from the nucleolus of the adhesive 32S and 28S RNA preribosomes which allows the remaining nucleolar apparatus to spread apart into the NN configuration. Also required for the maintenance of the NN structure is the synthesis of some ribosomal RNA (rRNA) possibly present as rRNA "feathers" on the DNA. The addition of inhibitors of rRNA synthesis such as actinomycin D to the NN-containing cells causes loss of rRNA. Then a contraction and collapse of the NN structure into small dense spheres is observed.

Repopulation of the postmitotic nucleolus by preformed RNA

This study is concerned with the fate of the nucleolar contents, particularly nucleolar RNA, during mitosis Mitotic cells harvested from monolayer cultures of Chinese hamster embryonal cells, KB6 (human) cells, or L929 (mouse) cells were allowed to proceed into interphase in the presence or absence (control) of 0.04-0 08 microg/ml of actinomycin D, a concentration which preferentially inhibits nucleolar (ribosomal) RNA synthesis 3 hr after mitosis, control cells had large, irregularly shaped nucleoli which stained intensely for RNA with azure B and for protein with fast green. In cells which had returned to interphase in the presence of actinomycin D, nucleoli were segregated into two components easily resolvable in the light microscope, and one of these components stained intensely for RNA with azure B. Both nucleolar components stained for protein with fast green In parallel experiments, cultures were incubated with 0.04-0 08 microg/ml actinomycin D for 3 hr before harvesting of mitotic cells, then mitotic cells were washed and allowed to return to interphase in the absence of actinomycin D. 3 hr after mitosis, nuclei of such cells were devoid of large RNA-containing structures, though small, refractile nucleolus-like bodies were observed by phase-contrast microscopy or in material stained for total protein. These experiments indicate that nucleolar RNA made several hours before mitosis persists in the mitotic cell and repopulates nucleoli when they reform after mitosis

Cytological studies on the inhibition by 5-fluorouracil of ribosome synthesis and growth in jerusalem artichoke tuber slices

Rapid auxin-induced cell expansion in artichoke tuber slices is obtained by aerating the slices in water ("aging") prior to auxin treatment. 5-fluorouracil (5-FU), an inhibitor of ribosomal RNA synthesis in plant cells, markedly inhibits auxin-induced growth only if present in the pre-growth aging period. Autoradiographic studies show that 5-FU given in the aging and/or growth periods reduces the incorporation of RNA precursors into the cytoplasm. Pulse-chase experiments suggest that the reduced cytoplasmic incorporation is in large part due to decreased stability of ribosomal rNA, as nucleolar and chromatin label are only slightly depressed at the end of the pulse. Though the nucleoli continue to incorporate RNA precursors following 5-FU treatment, they lack a distinct granular zone, and appear as homogeneous fibrillar structures under the electron microscope. 5-FU has a parallel inhibitory effect on growth and protein synthesis as shown by (3)H-leucine studies during the growth period. Electron-microscope studies show that treatment with 5-FU causes decreased numbers of ribosomes and rough endoplasmic reticulum. The results suggest that the ribosomes and rough endoplasmic reticulum formed during aging are important in obtaining subsequent rapid auxin-induced expansion. The new ribosomes serve in part to replace pre-existing ribosomes present at the time of excision, which from electron microscopic evidence from 5-FU treated tissue, appear to slowly disappear.

Distinctive characteristics of nucleoli of two established cell lines

Nucleoli of cultured cells of the established lines KB and L were found to possess a distinctive fine structural organization. The major portion of the nucleolar volume was composed of compact, particulate material. Spheroidal fibrillar zones about 0.4 micro in diameter occurred within the particulate mass. These fibrillar zones had a central light area and a denser rim. Toyocamycin treatment, which sharply inhibited the appearance of newly synthesized RNA in the cytoplasm, caused the gradual disappearance of the fibrillar material from nucleoli. Actinomycin D treatment, which inhibited virtually all RNA synthesis, caused varying types of segregation of nucleolar components. The morphology of nucleoli of KB and L cells and the reorganization of these nucleoli in response to drugs appear to be different from those of nucleoli of freshly initiated Chinese hamster and mouse cell lines.