The ultrastructural localization of 60-kDa Ro protein and human cytoplasmic RNAs: Association with novel electron-dense bodies

Abstract

The 60-kDa Ro ribonucleoprotein is an important target of humoral autoimmune responses. However, the ultrastructural locations of the 60-kDa Ro protein and its asso- Ro ribonucleoproteins (RNP) were first identified as targets of humoral autoimmune responses in patients with systemic lupus erythematosus and Sjögren syndrome. Antibodies to 60-kDa Ro have been linked to specific subsets of lupus, including ''ANA-negative'' systemic lupus erythematosus, subacute cutaneous lupus erythematosus, homozygous C 2 deficiency with systemic lupus erythematosus, and neonatal lupus (1). In all of these subsets, photosensitive skin disease is a prominent finding, whereas internal organs are often minimally affected. It appears that the autoantibodies may play a causative role, since women who have anti-Ro may have babies with transient subacute cutaneous lupus skin lesions (2).The Ro RNP family includes the 60-kDa Ro protein, which is associated with one of four human cytoplasmic RNAs (hY RNAs). Four distinct small cytoplasmic RNAs (Y RNAs) are immunoprecipitated from nucleated human cells with antibodies to 60-kDa Ro (hY1, hY3, hY4, and hY5); they range from 85 to 112 nucleotides in length and are products of RNA polymerase III transcription (3-6). Western blot analysis and DNA sequencing reveal a high conservation of the 60-kDa Ro protein among vertebrates, with a 78% identity between the human and Xenopus proteins (7,8). Like the 60-kDa Ro protein, the 60-kDa Roassociated Y RNAs are conserved among vertebrates by immunoprecipitation and by sequence, although the number of Y RNAs present is not conserved (3,6,(8)(9)(10)(11)(12)(13)(14).That the Ro RNP is highly conserved and is, in addition, present in every cell type tested suggests that it plays an important role in cellular metabolism. That role, however, remains unknown. Efforts to characterize the location of the Ro RNPs in cells have included numerous immunofluorescence studies that variably localized the 60-kDa Ro protein to the nucleus (15-17), the cytoplasm (6, 18), or both (7,19). Biochemical fractionation studies have suggested that the majority of Ro protein and Y RNAs reside in the cytoplasm of cells (5,8,20). Although one such study found an exclusively cytoplasmic location for the Y RNAs and the Ro RNP, a substantial amount of Y RNA-free Ro protein was detected in the nucleus (21). More recent studies include different approaches to determine the subcellular localization of the Ro RNP components. Microinjection of 60-kDa Ro into the cytoplasm of Xenopus oocytes resulted in redistribution of the antigen to both the nucleus and the cytoplasm, whereas microinjection of hY1 RNA into oocyte nuclei resulted in redistribution to the cytoplasm (22). Overexpression of recombinant 60-kDa Ro cDNA in transfected HEp-2 cells resulted in a nuclear speckled immunofluorescence pattern with prominently stained nucleoli and weak cytoplasmic staining when reacted with anti-60-kDa Ro-specific antisera (23). A study of the subcellular localization of hY RNAs at the optical...

Dynamic interaction of Y RNAs with chromatin and initiation proteins during human DNA replication

“…Our data show that hY RNAs associate rapidly and preferentially with euchromatin from a soluble pool before DNA replication initiates, and dissociate again from sites of replicated DNA after initiation has occurred. Consistent with earlier ultrastructural work (Matera et al, 1995;Farris et al, 1997), we have detected hY RNAs associated with nucleolar structures upon longer incubation in vitro. It is therefore likely that Y RNAs associate with euchromatin only transiently before the initiation of DNA replication at the G1 to S phase transition and persist longer in nucleolar heterochromatin.…”

“…Previous reports on the intracellular localisation of Y RNAs are controversial, suggesting cytoplasmic, nuclear and nucleolar locations (O'Brien et al, 1993;Peek et al, 1993;Simons et al, 1994;Matera et al, 1995;Farris et al, 1997;Pruijn et al, 1997). Our data show that hY RNAs associate rapidly and preferentially with euchromatin from a soluble pool before DNA replication initiates, and dissociate again from sites of replicated DNA after initiation has occurred.…”

“…In enucleation and cell fractionation experiments, Y RNAs were found to be almost exclusively soluble and cytoplasmic in Xenopus laevis oocytes and mammalian cells (O'Brien et al, 1993;Peek et al, 1993;Simons et al, 1994). By contrast, Y RNAs were detected by in situ hybridisation in both the cytoplasm and the nucleus of mammalian cells (Matera et al, 1995;Farris et al, 1997). It is thus unknown whether Y RNAs associate with nuclei during DNA replication, or whether they exert their function in solution.…”

Evidence for Multiple Shared Antigenic Determinants within Ro60 and Other Lupus-Related Ribonucleoprotein Autoantigens in Human Autoimmune Responses

“…Studies using polyclonal anti-Ro60 Abs isolated from the sera of SLE patients have previously localized Ro60 to the nucleus (29,30), the cytoplasm (31), or to both regions (32,33). In this regard, patterns of localization we obtained deserve some mention, in the context of the differential antigenic recognition of the mAbs.…”

Characterization and genomic sequence of the murine 60 kD Ro gene

“…60 kD Ro polypeptide is expressed in a wide variety of tissues and has been localized to both the nucleus and cytoplasm. 2 Animal models of autoimmunity often provide important insights into the mechanisms of pathogenesis. Autoantibodies binding 60 kD Ro have been reported in MRL lpr/lpr mice, 3 bacterial DNA immunized mice, 4 antiidiotype induced experimental SLE, non-obese diabetic mice 5 and, most recently, in rabbits immunized with peptides from 60 kD Ro.…”