[The phenomenon of nucleolar segregation: specificity of action of certain antimetabolites]

A comparative study of the effect of aflatoxin B and actinomycin D on HeLa cells

1. The cytotoxic effects of aflatoxin B(1) on HeLa cells were examined and effects of short exposures of the cells to the toxin were found to be reversible. 2. Aflatoxin B(1) inhibited the synthesis of both ribosomal and heterodisperse RNA. It is proposed that the toxin's mechanism of action on ribosomal RNA synthesis is related to its inhibitory effect on the maturation of the 45s-ribosomal-RNA precursor. 3. Protein synthesis is inhibited to a greater extent by aflatoxin B(1) than by actinomycin D. In contrast with actinomycin D, aflatoxin B(1) was shown to disaggregate polyribosomes directly.

Effects of a pathogenic canine herpesvirus on established thyroid adenocarcinoma cell cultures

The effects of a pathogenic canine herpesvirus were studied sequentially in established canine thyroid adenocarcinoma tissue culture cells. The lesions were defined as early, middle, and late manifestations of viral effects based on the time a lesion first appeared. The early phase (3 to 10 hours) consisted of nucleolar swelling followed by disruption. The nucleolar changes were characterized by loss of RNA as determined with acridine orange and segregation of the nucleolar components as demonstrated by electron microscopy. The middle phase (10 to 34 hours) was characterized by the appearance of three types of eosinophilic intranuclear inclusions. One type of inclusion appeared to transform progressively into another. The appearance of viral antigen in the nucleus as demonstrated by immunofluorescence, correlated with ultrastructural evidence of viral replication. The late phase (34 to 72 hours) was manifested by cellular degeneration and viral release into the cytoplasm.

Reformation of nucleoli after ethione-induced fragmentation in the absence of significant protein synthesis

The rat liver nucleolus, after fragmentation induced by ethionine treatment, has been found to undergo complete reformation by adenine in the presence of a dose of cycloheximide sufficient to cause inhibition of protein synthesis by 90-95%. In contrast, actinomycin D given along with adenine was followed by the appearance of a small compact mass containing only the fibrillar component with no evident granules. This structure resembled pseudonucleoli seen in the anucleolate mutant of Xenopus laevis or in certain early stages of amphibian oocytes. Actinomycin D administered 2 hr after adenine induced a segregation of the fibrillar and granular components of nucleoli similar to that induced in the normal nucleolus. The implications of these findings in relation to nucleolar organization are briefly discussed.

Cytoplasmic structures associated with an arbovirus infection: loci of viral ribonucleic acid synthesis

Unique cytoplasmic structures, herein designated as type I cytopathic vacuoles (CPV-I), are found in chick embryo cells early in the logarithmic phase of Semliki Forest virus replication. High resolution autoradiography demonstrated that the CPV-I are loci of (3)H-uridine incorporation. This evidence correlates well with previous biochemical data and electron microscopy of the subcellular fractions active in Semliki Forest virus ribonucleic acid synthesis. Origin of the CPV-I within host cell cytoplasm is confirmed by the distribution of electron-dense tracer particles and sequential ultrastructural observations.

The disorganization of hepatic cell nucleoli induced by ethionine and its reversal by adenine

The structure of nuclei and nucleoli of hepatic cells after short-term ethionine administration was investigated with the electron microscope. By 1(1/2) hr after the injection, a distinct alteration occurred in the nucleoli which was characterized by the appearance of electron-opaque masses in the nucleolonema. After 6-8 hr, the nucleoli showed partial fragmentation into small, dense masses. Large aggregates of interchromatinic granules appeared in the nucleoplasm. Condensation of chromatin became prominent in the nucleoplasm particularly along the nuclear membrane. By 12 hr almost complete fragmentation of nucleoli had occurred. The administration of adenine or methionine at 4 hr prevented the development of nucleolar changes. Also, adenine administration at 8 hr after ethionine completely reversed the nucleolar lesion by 12 hr. After methionine administration at 8 hr, many nucleoli showed incomplete reconstruction with many twisted ropelike structures when viewed 4 hr later. Identical structures were found when adenine was given at 8 hr, and animals were sacrificed 2 hr later. On the basis of this observation, the simplified structures of nucleoli found 2 hr after adenine or 4 hr after methionine appeared to be precursors of the nucleolonema. It is suggested that nucleoli show at least two basic reaction patterns to inhibitors of RNA synthesis, one typified by actinomycin D and one by ethionine.