Analysis of high-molecular-weight ribonucleic acid associated with intracisternal A particles

Involvement of human intracisternal A-type retroviral particles in autoimmunity

Prior studies have linked retroviruses to various arthropathies and autoimmune diseases. Sjögren's syndrome (SS), a systemic autoimmune disease, is characterized by aggressive infiltration of lymphocytes into the salivary and lacrimal glands, resulting in destruction of the glands and dry mouth and eyes (sicca syndrome). The infiltrating lymphocytes in SS may become overtly malignant, and thus, the incidence of lymphoma is greatly increased in SS patients. A human intracisternal A-type retroviral particle type I (HIAP-I) has been isolated from persons with SS. HIAP-I shares a limited number of antigenic epitopes with human immunodeficiency virus (HIV), but is distinguishable from HIV by morphological, physical, and biochemical criteria. A substantial majority of patients with SS or systemic lupus erythematosus (SLE) have serum antibodies to the proteins of this human retrovirus. Fewer than 3% of the normal blood donor population have antibodies to any HIAP-associated proteins. A second type of a human intracisternal A-type retrovirus, HIAP-II, was detected in a subset of patients with idiopathic CD4 lymphocytopenia (ICL), an AIDS-like immunodeficiency disease. Most HIAP-II positive ICL patients were also antinuclear antibody positive. Reviewed here are additional studies from several laboratories suggesting that HIAP or related viruses may be involved in SLE and other autoimmune conditions. Additionally, results of comprehensive surveys of autoimmune patients to determine seroreactivity to HIAP, and other human retroviruses, including HIV and human T-lymphotropic virus type I, are reported.

Antibodies against retroviral proteins and nuclear antigens in a subset of idiopathic CD4+ T lymphocytopenia patients

Idiopathic CD4+ T lymphocytopenia (ICL) is an immunodeficiency syndrome characterized by severe depletion of CD4+ T lymphocytes, but in which human immunodeficiency virus cannot be detected. Peripheral blood mononuclear cells (BPMCs) from an ICL patient were cocultured with HUT78 T-lymphoblastoid cells, and an acute cytopathic effect and formation of multinucleated cells were observed. A human intracisternal A-type retroviral particle designated HIAP-II was detected in cells surviving the acute cytopathic effect. Eight of 13 ICL patients in a blinded screen of a serological panel provided by the National Centers for Disease Control and Prevention (CDC) had serum antibodies that specifically reacted with HIAP-II associated proteins by Western immunoblotting. None of 19 control sera in the panel that were unreactive with HIV Gag proteins produced a positive result on HIAP-II immunoblots. Comparable results were obtained in a blinded screen of a second CDC serological panel. Sera from 8 of 14 ICL patients in the second serological panel were positive for antinuclear autoantibodies (ANAs) commonly observed in patients with systemic autoimmune diseases. These results suggest the possible involvement of an A-type retrovirus or autoimmunity in development of ICL in a subset of patients.

The expression of intracisternal A particle genes in the preimplantation mouse embryo

Intracisternal A particles (IAP), murine endogenous retrovirus, make up 0.3% of the mouse genome. They are expressed in some normal tissues, certain transformed cell lines, and show stage-specific patterns of expression in early embryos. We have used peptide-specific antisera and the polymerase chain reaction to explore type-specific expression of these IAP during preimplantation development. In this paper we show that the IAP core protein, p73, characteristic of type IIAP, is present throughout preimplantation development while the gag-pol fusion protein p120, characteristic of the variant type I delta 1, is synthesized and expressed only from the 8-cell stage onward. Type IIAP RNA is present at all stages and appearance of p120 at the 8-cell stage could represent new transcription or translation from a preexisting I delta 1 message. The presence of type II IAP RNA varies according to stage, with two sizes of type II transcripts present at all stages except the 2-cell stage at which time only the smaller of the two transcripts can be detected. The reappearance of the larger type II transcript subsequent to the 2-cell stage implies new transcription of this type II subspecies. The presence of type I, II, and p73 in the unfertilized egg strongly suggests maternal inheritance from the oocyte.

Detection of a human intracisternal A-type retroviral particle antigenically related to HIV

Sjögren's syndrome is an autoimmune disease that is characterized by dryness of the mouth and eyes. The loss of salivary and lacrimal gland function is accompanied by lymphocytic infiltration. Because similar symptoms and glandular pathology are observed in certain persons infected with human immunodeficiency virus (HIV), a search was initiated for a possible retroviral etiology in this syndrome. A human intracisternal A-type retroviral particle that is antigenically related to HIV was detected in lymphoblastoid cells exposed to homogenates of salivary tissue from patients with Sjögren's syndrome. Comparison of this retroviral particle to HIV indicates that they are distinguishable by several ultrastructural, physical, and enzymatic criteria.

Cytoplasmic inclusions within developing mouse retinal photoreceptor cells maintained in culture: possible type A retrovirus particles?

Unusual cytoplasmic inclusions were consistently observed in photoreceptor cells of explants of embryonic mouse retina maintained in culture for 21 days or longer. The inclusions were chiefly spherical (some appeared to be short cylinders) and about 76 nm in diameter. They consisted of a granular/amorphous electron-dense periphery and an electron-lucent core and were usually in clusters, often close to the Golgi apparatus or plasma membrane. They were also commonly present in photoreceptor cell synaptic terminals in the outer plexiform layer. They displayed close relationships with cisterns and tubules of the endoplasmic reticulum and many were located within dilated sacs of reticulum. Narrow, smooth-surfaced tubules appeared to run into or through the cores of some of the inclusion bodies linking them to others. Similar inclusions were not seen in any other cell types in the explants and have not been reported in previous in vivo and in vitro studies of developing or adult mouse retina. The possibility that these inclusions represent Type A retrovirus particles is considered.

Size and structure of the highly repetitive BAM HI element in mice

The BAM HI family of long interspersed DNAs in mice represent as much as 0.5% of the mouse genome. Cloned mouse DNA fragments which contain BAM HI/non-BAM HI junction sequences have been analyzed by restriction mapping and DNA sequencing. It has been found that BAM HI elements: (i) are approximately 7 kilobase pairs in size, (ii) are not bracketed by long repeated sequences analogous to the terminal repeats of proviruses and (iii) contain a poly-dA track at one end. The data strongly suggest that BAM HI elements arose by a process involving RNA intermediates. The beginning of the element, opposite the poly-dA track, contains a 22 base pair sequence exhibiting 65% homology to a ubiquitous mammalian sequence which may play a role in DNA replication (1). The poly-dA end of the element contains BAM5 and R sequences, both of which have been described previously (2,3).

Properties of an intracisternal A-particle-associated endonuclease activity which is stimulated by ATP

An endonuclease activity associated with purified proteinase K-treated intracisternal A-particles was identified and characterized. The activity required divalent cations, preferring Mn2+ to Mg2+. Salt concentrations above 50 mM inhibited the activity. The endonuclease was greatly stimulated by ATP, ADP, and dATP, whereas AMP appeared to produce a slight inhibition. GTP had no apparent effect on the activity. The enzyme introduced single-stranded nicks into DNA and nicked preferentially supercoiled DNA duplexes in the presence of ATP, although linear duplexes also functioned as substrates. Single-stranded DNA was not nicked to any great extent. The molecular weight of the enzyme was estimated to be about 40,000. The characteristics of this enzyme are very similar to those of the endonuclease found associated with Friend murine leukemia virus.

Intracisternal A-particle genes: identification in the genome of Mus musculus and comparison of multiple isolates from a mouse gene library

The genome of Mus musculus contains multiple copies (500 -1000) of DNA sequences related to the 35S RNA of intracisternal type A particles (IAPs). Using labeled IAP RNA as a probe in blot-hybridization experiments, we have identified a characteristic electrophoretic pattern of reactive fragments generated by restriction endonuclease cleavage of mouse DNA. From the genomic blots, we deduced a composite restriction map for a 6.5- to 7-kilobase (kb) DNA region containing sequences homologous to the IAP RNA. Units of this type appeared to be interspersed without obvious regularity in nonhomologous flanking regions. A 5.2-kb segment of this unit was inserted directly into plasmid pBR322 from HindIII/EcoRI digest of mouse DNA. The fragment was cloned and then labeled by nick-translation and used to scan a mouse embryo gene library (average 16-kb inserts in lambda Charon 4A); 1% of the library samples hybridized, confirming the extensive reiteration of IAP genetic units. Among six different library isolates containing 6.5- to 7-kb IAP units, some restriction sites were highly conserved whereas others varied in both occurrence and position. Despite this variation, heteroduplexes between the individual isolates showed continuous IAP homology regions of 7 kb. No flanking region homologies were seen in this limited sample. Some evidence suggests that mouse DNA may contain other dispersed sequence elements related to but smaller than the genetic unit defined above.

Characterization of infectious oncornaviruses from MOPC-460 plasmacytomas: their relation to A-type particles

MOPC-460 mouse plasmacytoma cells produce intracellular A-type particles and extracellular oncornavirus-like particles ("myeloma-associated virus," abbreviated MAV). The genomes of these two particles are closely related. During attempts to establish infections with MOPC-460 extracellular particles, we isolated ecotropic and xenotropic infectious forms of murine leukemia virus. We have investigated the relation of these isolates to A-type particles and to MAV by nucleic acid hybridization. Using complementary DNA probes prepared from the two isolates, we found that these infectious murine leukemia viruses differ from A-type particles and from MAV. Moreover, we found that MAV is the predominant extracellular component: the ecotropic and xenotropic forms of murine leukemia virus were present at only low levels (less than 5%) in MAV preparations. Neither the SC-1 cells infected with ectropic murine leukemia virus nor the mink cells infected with xenotropic murine leukemia virus showed any A-type particles in their cytoplasm when examined by electron microscopy. Our inability to demonstrate infection by the A-type particle-related component, MAV, suggests that these may be defective.

Characterization of the proteins of intracisternal type A and extracellular oncornavirus-like particles produced by MOPC-460 myeloma cells

The mouse plasmacytoma cell line, MOPC-460, produces both intracisternal and intracytoplasmic A-type particles when grown as a solid tumor. When these cells are grown either as an ascites tumor or in tissue culture, a third type of particle is produced extracellularly. This particle, the "myeloma-associated virus," is closely related to, and probably an alternate form of, the intracisternal A-type particle. The proteins present in these two types of particles were compared by tryptic peptide mapping. Both types of particles were found to contain essentially the same major proteins of 76,000 (p76), 68,000 to 70,000 (p68-70), and 45,000 (p45) daltons, in addition to varying amounts of smaller proteins. The relative proportions of all these proteins varied from preparation to preparation in an unpredictable way. The p45, p68, and p70 proteins all contained sequences found in p76, suggesting precursor-product relationships of p76 leads to p70 leads to p45 for solid tumor A-type particles and p76 leads to p68 leads to p45 for extracellular myeloma-associated virus. In addition, immune precipitation experiments have established that p76 contains at least some of the antigenic determinants characteristic of murine leukemia virus p30. This confirms earlier nucleic acid hybridization studies which indicated a moderate degree of relatedness between MOPC-460 A-type particles and several standard murine leukemia and sarcoma viruses. Taken together, our results provide evidence supporting the concept that MOPC-460 A-type particles may represent aberrant forms of C-type murine viruses.

Genetic individuality of intracisternal A-particles of Mus musculus

The nucleic acid sequence relationship between mouse intracisternal type A-particles and type C and B viruses was examined by reciprocal complementary DNA-RNA hybridization; complementary DNAs prepared from the RNAs of intracisternal A-particles were hybridized with high-molecular-weight RNAs from a variety of murine tumor viruses, and complementary DNAs representing a variety of RNA tumor virus genomes were hybridized with the high-molecular-weight RNAs from A-particles. The criterion for homology between two types of virus was that the heterologous hybridization reaction occurs over the same RNA concentration range as the homologous reacton. The results of these hybridizations indicate that there are no major sequence homologies between the RNA of intracisternal A-particles and the RNA of representative members of type B and C viruses of Mus musculus.

Nucleotide sequence relationship between intracisternal type A particles of Mus musculus and an endogenous retrovirus (M432) of Mus cervicolor

Intracisternal type A particles are retrovirus-like structures found in embryonic cells and many tumors of Mus musculus but having no clear relationship with other retroviruses of this mouse species. We have observed a partial nucleotide sequence homology between the high-molecular-weight (32S and 35S) RNA components of intracisternal A-particles from a neuroblastoma cell line and the 70S RNA fraction from M432, a type of retrovirus endogenous to the Asian mouse Mus cervicolor. M432 complementary DNA (cDNA) was hybridized to the extent of 30% by the A-particle RNAs. The hybrids showed a lower thermal stability (DeltaT(m), 7 degrees C) than those formed with homologous RNA. The reaction was commensurate with that found between M432 cDNA and divergent sequences in the M. musculus genome. The capacity to hybridize M432 cDNA was closely correlated with the concentration of A-particle sequences in the cytoplasmic RNA of several M. musculus cell types. The major RNA fraction of M432 virus showed a reciprocal partial reaction with the A-particle cDNA's; the virus, which was grown in NIH/3T3 (M. musculus) cells, also contained a small proportion of apparently authentic A-particle nucleotide sequences. A subset of A-particle sequences seemed to be almost totally lacking in the main M432 RNA. The A-particle cDNA's hybridized extensively with divergent sequences in M. cervicolor cellular DNA, indicating that this mouse species may contain not only the partially homologous M432 virogene, but also a more complete genetic equivalent of the intracisternal A-particle.

RNA associated with murine intracisternal type A particles codes for the main particle protein

Intracisternal type A particles were isolated from MOPC-104E myeloma grown subcutaneously and from N 4 neuroblastoma cells in culture. Polyadenylated RNA was prepared from the particles and tested in a cell-free translation system derived from rabbit reticulocytes. RNA from the two sources directed the synthesis of multiple polypeptides with similar distributions of electrophoretic mobilities in sodium dodecyl sulfate-containing polyacrylamide gels, including one conponent of the same size as the major A-particle structural protein (73,000 daltons). Analysis of the RNAs by electrophoresis in methyl mercury-containing agarose gels revealed a 35S component common to A-particles from both cell types. This was a major component of the N4 preparations, whereas a 28S species predominated in the case of MOPC-104E. These two RNAs (35S from N4 cells and 28S from MOPC-104E), when isolated on isokinetic sucrose gradients, each directed the synthesis of a 73,000-molecular-weight polypeptide that comigrated on gels with authentic A-particle structural protein. Idnetity of the cell-free product was confirmed by two-dimensional analysis of the [35S]methionine-labeled tryptic peptides. The N4 RNA preparations also contained a major32S component which did not code effectively for the A-particle structural protein.

Purification and characterization of DNA polymerases from the plasmacytoma MOPC 104E and Abelson murine leukemia viruses

RNA-dependent DNA polymerases of intracisternal A particles from the mouse plasma cell tumor MOPC 104E and of Abelson murine leukemia virus (A-MuLV) were isolated from particle preparations by Nonidet P40 and ultrasonic treatment and purified by column chromatography on DEAE-cellulose and phosphocellulose, followed by centrifugation in linear sucrose gradients. Both DNA polymerases were very similar in their elution patterns from phospho and DEAE-cellulose, template specificities, requirements for optimum activity and inactivation by anti-(reverse transcriptase) antiserum. They are associated with ribonuclease H activity. For molecular weight determinations, antibody-precipitated enzymes were bound to staphylococcal-protein-A-Sepharose, solubilized and run on dodecylsulfate/polyacrylamide gels. Their apparent molecular weight was estimated to be 80000.

The development and degeneration of Purkinje cells in pcd mutant mice

Purkinje cell degeneration (pcd), an autosomal recessive mutation in the mouse, causes the postnatal death of virtually all cerebellar Purkinje cells during the third and fourth postnatal week. We have compared the postnatal development of normal and pcd mutant Purkinje cells. The early deviations from normal development involve primarily the perikaryonal polysomes and endoplasmic reticulum. Many of the mutant Purkinje cells retain abnormally the basal accumulation of polysomes, a finding which permits the identification of affected animals at postnatal day 15, one week prior to the onset of behavioral abnormalities. In addition, the affected Purkinje cells possess unusual configurations of endoplasmic reticulum with associated electron-dense particles similar to but larger than ribosomes, mature and forming intracisternal A particles and nematosomes. Before the pcd Purkinje cells degenerate they appear to receive all their appropriate synaptic contacts. Some disruption, however, of parallel fiber: Purkinje spine synaptogenesis occurs at late stages of development. Some spines lack presynaptic elements, postsynaptic thickenings are present along the dendritic shafts and parallel fibers appear to make synaptic contacts directly onto the shafts. The spectrum of early morphological changes that has been observed in pcd mutant Purkinje cells is thus far unique to this cerebellar abnormality.

Sequences associated with intracisternal A particles are reiterated in the mouse genome

Using a 3H-cDNA for RNA sequences specifically associated with murine intracisternal type A particles, we have found multiple copies of this information in high molecular weight nuclear DNA from tissues of both Mus muscules (BALB/c, NIH Swiss, A/Jax and feral) and Mus cervicolor. Reiteration frequencies varied from 1050-1800 per haploid genome, except that fewer copies (450) were found in BALB/3T3 cells. In the series studied, the reiteration frequencies in the DNA of A particle-rich tumor cells (myeloma and neuroblastoma) were not higher than those in normal tissues (liver and sperm). Multiple copies were retained when cellular DNAs were sedimented through alkaline sucrose gradients, indicating that the sequences are integrated in the mouse genome. In situ hybridization with cDNA showed that the sequences were associated with many chromosomes and were concentrated over certain regions of some chromosomes. Only low levels of homologous sequences were detected in rat, hamster and guinea pig DNA under stringent conditions of hybridization. The presence of reiterated sequence transcripts in poly(A) RNA from a neuroblastoma A particle fraction was confirmed by direct hybridization of the RNA with cellular DNA.

DNA polymerases from intracisternal A-type particles on the mouse plasmacytoma MPC 11

Intracisternal A particles from mouse plasma cell tumors were isolated from microsomal vesiclels by detergent treatment and separated in linear sucrose gradients. Four peaks of DNA polymerase activities banding at densities of 1.30, 1.24, 1.20--1.22 and 1.13 g/cm3 were observed assaying on poly(rA).oligo(dT). Solubolized DNA polymerases of the 1.30 g/cm3...

Physicochemical analysis of the deoxyribonucleic acid product of murine intracisternal A particle RNA-directed DNA polymerase

The nascent DNA transcript of intracisternal A particle RNA-directed DNA polymerase appeared to be covalently linked to an RNA primer. Fidelity of transcription is evident since the DNA transcript hybridized specifically back to 35-70 S RNA of intracisternal A particles but not with heterologous RNAs. This DNA transcript has an approximate molecular weight of 145 000, estimating 350 polynucleotides and base ratios with an excess of dGMP.

Murine neuroblastoma clones with varying degrees of C-type virus expression

Three tissue culture clones of neuroblastoma cells derived from the C-1300 tumor in strain A/J mice were found to contain type-C virus-specific gas antigen. None of the clones spontaneously released mouse-tropic type-C viruses although one clone, N-4, spontaneously released a xenotropic virus. Two clones, NB-2A and N-4, could be induced by treatment with 5-iododeoxyuridine and dexamethasone phosphate to produce B-tropic type-C virus, but clone N-18 failed to release either ecotropic or xenotropic virus under several different induction conditions. Karyotype analysis did not reveal specific chromosome deletions in clone N-18.

Differential response of type C and intracisternal type A particle markers in cells treated with iododeoxyuridine and dexamethasone

Mouse neuroblastoma cells containing intracisternal type A particles were treated with iododeoxyuridine and dexamethasone to induce the release of type C oncornavirus particles. For 5 days after treatment, antigenic markers and DNA polymerase activities specific to particles of each of the two types were assayed in the cells and in pellets obtained by high-speed centrifugation of the culture fluid. There was a marked release of C-particle antigen (p30) and DNA polymerase activity in extracellular particulate form, reaching a maximum on day 3 after treatment and falling thereafter. In contrast, no extracellular A-particle antigen was detected, and A-particle-specific DNA polymerase activity in the medium pellets did not increase from the original very low level. Electron microscopy confirmed the presence of free type C virus particles, but not intracisternal type A particles, in the culture fluid. Although intracellular levels of C-particle antigen rose 20- to 30-fold per milligram of cell protein, intracellular A-particle antigen and DNA polymerase activity did not vary more than two-fold. The relative rate of A-particle synthesis in the treated cells, as judged by incorporation of radioactive amino acids into the major structural protein (P73), was also unchanged over the period of observation. Thus, the induction of type C virus particle formation in cultured neuroblastoma cells had no detectable effect on the quantity, synthesis rate, or location of intracisternal type A particles.

In vitro synthesis of A-particle structual protein by membrane-bound polyribosomes

On the basis of association with endoplasmic reticulum membranes, poyribosomes isolated from mouse myeloma MOPC-104E were separated into two classes, membrane bound and free. The membrane-bound and free polyribosomes were then compared for their capacity to incorporate [35S]methionine into A-particle proteins in vitro. As revealed by a radioimmunological assay method, labeling of A-particle protein occurred with the membrane-bound polyribosomes but not with the free polyribosomes. Peptide mapping of the immunoprecipitated, in vitro [35S]methionine-labeled product confirmed that A-particle protein had been synthesized in vitro.

Intracisternal A particles from FLOPC-1 BALB/c myeloma: presence of high-molecular-weight RNA and RNA-dependent DNA polymerase

Intracisternal A particles from the FLOPC-1 line of BALB/c myeloma have been shown to contain high-molecular-weight RNA (60 to 70S) that is sensitive to RNase, alkali degradation, and heat but resistant to Pronase treatment. The intracisternal A-particle RNA contains tract of poly (A) approximately 180 nucleotides long. As shown in a reconstitution experiment, by antigenic analysis of A-particle preparation and the SC cytopathogenicity assay, the 70S RNA was not due to contamination by type C virus particles. The FLOPC-1 intracisternal A particles also possess an endogenous RNA-dependent DNA polymerase. The enzyme required Mn2+ or Mg2+, dithiothreitol, detergent, and four deoxyribonucleoside triphosphates for maximum activity. Enzymatic activity was maximally stimuated by poly (rC)-oligo (dG)12-18 and less with poly (rG)-oligo (dC)10 or poly (rA)-oligo (dT)12-18 as compare with synthetic DNA/DNA duplex templates such as poly (dA)-oligo (dT)12-18. The enzyme can utilize the A-particle endogenous RNA as template as shown by analysis of the early and late DNA products of the endogenous reaction by CsSO4 isopycnic gradient centrifuation and hybridization of purified 70S or 35S A-particle RNA with the purified complementary DNA product. Approximately 50% of the A-particle complementary DNA also hybridized with oncornavirus RNA.

Relationships between intracisternal type A and extracellular oncornavirus-like particles produced in murine MOPC-460 myeloma cells

Oncornavirus-like particles of the "A" (both intracisternal and intracytoplasmic) and "B" or "C" (extracellular) types are produced by murine MOPC-460 myeloma cells. This communication describes a comparative study on tracisternal A and extracellular particles. Both types of particles contain an RNA-dependent DNA polymerase activity, traces of 35S and 70 S RNA in addition to larger amounts of degraded RNA, and proteins of approximately 76,000 and 45, 000 daltons. The 76,000-dalton proteins from intracisternal A and extracellular particles have the same cyanogen bromide peptides. Hybridization kinetic analysis indicates that the RNAs in the two particles are identical or very closely related and share partial homology with Moloney leukemia virus RNA. In contrast, the particles appear to have little or no relationship to murine mammary tumor virus as judged by several different criteria. Electron microscope studies indicate that the extracellular particles arise from the budding of core components through the plasma membrane. These results suggest that the intracisternal A and extracellular oncornavirus-like particles produced by MOPC-460 cells are closely related.

Murine intracisternal type A particles: a biochemical characterization

Intracisternal A particle preparations from a murine neuroblastoma cell line (N18) and from a mineral oil-induced murine plasmacytoma (MOPC-104E) contain both an endogenous RNA-dependent DNA polymerase activity and high molecular-weight polyadenylic acid (poly[A])-containing RNA. The DNA polymerase activity is stimulated by oligo(dG)-poly(C) and oligo(dT)-poly(A) and to a lesser extent by oligo(dT)-poly(dA), in agreement with previous reports. The high-molecular-weight RNA is predominantly 35S and contains a poly(A) tract of approximately 220 nucleotides as judged by polyacrylamide gel electrophoresis. Small amounts of 70S RNA are also present. This RNA preparation contains RNA homologous to RNA from type-C particles, as judged by molecular hybridization experiments. However, since this RNA derives only in part from A-particles and in part from other cellular RNA, hybridization of A-particle endogenously synthesized DNA or reverse transcripts of A-particle RNA to purified type C viral 70S RNA may more accurately reflect the relationship of A-particle RNA to RNA from C-particles. None of these DNA transcripts hybridizes significantly to C-particle 70S RNA, although MOPC and N18 DNA transcripts share significant homology. Our interpretation of these results is that murine intracisternal A particles are not closely related genetically to the tested murine type C viruses, although an alternate possibility is that all the A-particle DNA transcripts are copied from only a small part of the genome, which is unrelated to C-particle RNA.

Comparison of intracytoplasmic A particles and intracisternal A particles

Intracytoplasmic A particles and intracisternal A particles are associated with mouse tumors of various types, and both can coexist in the cytoplasm of the same cell. The designation of both as A particles is based on the recognition that they share morphological similarities. A comparison of purified isolates of these two particles reveals that the structural protein profiles are different, but that there is some antigenic cross-reaction as demonstrated by immunodiffusion and complement fixation. This result does not appear to involve the major structural protein of intracisternal A particles, but may reflect the presence of common antigenic determinants located on minor proteins.

Intracisternal A-particles in transplantable murine teratomas

Intracisternal A-particles occur in experimental transplantable murine teratomas OTT 2466 and OTT 6050 derived from embryos of the A/He and 129/Sv strains respectively. They are present in stem cells of both tumors and in the more differentiated cell types in OTT 6050, consisting of primitive endodermal cells, cells of neuroepithelial rosettes, neuroblasts, primitive glial cells and respiratory epithelial cells. Both parent strains, A/He and 129/Sv, also contain these particles.

Characterization of DNA polymerase and RNA associated with A-type particles from murine myeloma cells

The RNA-dependent DNA polymerase present in intracisternal A-type particles from mouse myeloma tumor cells has been studied. This polymerase can use either endogenous A particle RNA or an exogenous synthetic polynucleotide [poly (rA)] as a template. The DNA reaction product is small (4S-10S) and over 90% of it hybridizes to A particle RNA, whereas up to 50% of it hybridizes to murine sarcoma-leukemia virus RNAs. The RNA isolated from purified A particles is generally of low molecular weight (5S-15S) but contains small amount of 70S and 35S components. These results suggest that A-type particles may be related to C-type oncornaviruses.

Studies on the intracisternal A-type particles in mouse plasma cell tumors: induction of maturation of the particles

Maturation of the intracisternal A-type particle found in two mouse plasma cell tumors was induced by treating the cells in culture with IDU-DMSO or with DMSO only. Morphologically, the mature particles with electron-dense nucleoids closely resembled the mature particles described in human tumor cell lines treated in a like manner. They also closely resembled the virus that has been described in guinea pig leukemias. It was not possible to demonstrate infectivity of the mature particle, as latent intracisternal A-type particles induced by IDU were found in the mouse cells presumed to be free of virus. The biochemical studies did not show distinct new peaks of virus-specific particles in sucrose density gradients when the particles in the treated cells were compared with the particles of the untreated cells. There was a difference in the density of the particles observed in the induced cells (1.2) and those of the control cells (1.185). This may reflect the difficulty of separating mature and immature particles. Analysis of the RNA present in the particles showed that the ratio of heavy-molecular-weight RNA in activated cells to the predominant species (21S) is much greater than that in control cells. Detectable levels of enzyme activity were not found in the induced particles. This could be due to too low a concentration of particles in the preparations.

Murine intracisternal A type particles fail to separate from the membrane of the endoplasmic reticulum

Analysis of serial sections of murine cells containing intracisternal A particles revealed that over 99% of all A particles remain in a budding configuration. This indicates that these particles fail to detach from the membrane of the endoplasmic reticulum. This observation explains how, despite their intracellular abundance in certain murine tumors, no extracellular A-type particles can be found.

Properties of intracytoplasmic A particles isolated from oncornavirus-producing human cells

A particles with the diameter of 70 to 80 nm were isolated from the cytoplasm of HEp-2, HeLa, and AO cells producing oncornavirus of Mason-Pfizer-like type. Most of the A particles banded at 1.23 to 1.24 g/ml, whereas 3 to 10% banded at 1.29 g/ml in equilibrium sucrose gradients. They banded at 1.30 g/ml in CsCl gradients suggesting that they contained 8% RNA. Individual A particles sedimented at 200 to 250S in velocity sucrose gradients, but their significant part was found aggregated and sedimented at more than 300S. They were resistant to RNase digestion. A particles possessed polymerase activity which was preferentially activated by Mn(2+) rather than by Mg(2+), the RNA template being 60S RNA. Cross-hybridization with two DNA products and immunoassay showed that A particles and Mason-Pfizer-like oncornavirus produced by the same cells contained neither homological RNA sequences nor common antigens, suggesting that A particles are not intracellular precursors of Mason-Pfizer-like oncornavirus but represent an independent oncornavirus. Hybridization of A particle RNA with excess of cellular DNA revealed about 20 proviral copies per HEp-2 cell genome and no proviral copies in human embryo and placenta cell genomes.